Traffic in life

Traffic  in  life

Ph.D. advisor: Jaume Casademunt

Dept. d’Estructura i Constituents de la Matèria, Facultat de Física (UB)

David Oriola SantandreuU

BUNIVERSITAT DE BARCELONA

Cell cover, 141 (2), 2010

Intracellular transport

Intracellular transportSoma

AxonSynapticterminal

Intracellular transportSoma

AxonSynapticterminal

Organelle transport

Intracellular transportSoma

AxonSynapticterminal

Organelle transport

Axonal transport is an essential process in neurons because of the extreme polarity and size of these cells. Indeed, despite having axons of more than 1 metre in length, human spinal motor neurons, like other types of neurons, require efficient communication between their cell body and axon tip. Axonal transport keeps axons and nerve terminals supplied with proteins, lipids and mitochondria, and clears recycled or mis-folded proteins to avoid the build-up of toxic aggre-gates1. Apart from its role in neuronal metabolism, axonal transport is crucial for intracellular neural transmission and allows the neuron to respond effectively to trophic signals or stress insults1.

Impairment of axonal transport has recently emerged as a common factor in several neurodegenerative disorders1. Here, we review the current state of knowl-edge about axonal transport defects that are associated with such disorders, with a specific focus on the mecha-nisms that can affect microtubule-based axonal transport. Before doing so, we outline the various components and mechanisms that control such transport.

!"#$%&'(')*+(,-*./,0%1,)/&$,1-2%$&Microtubules are the main component of the cytoskeleton. They have a tubular structure (25 nm in diameter) and are composed of many !- and "-tubulin heterodimers, which undergo continuous polymerization and depo-lymerization at the centrosome1. Microtubules are polar-ized in axons (but not in dendrites): their slower growing minus end (at which !-tubulin is exposed) faces the cell body, whereas their faster growing plus end (at which "-tubulin is exposed) points towards the axon tips. They

are stabilized by microtubule-associated proteins such as tau. Microtubules in the axon essentially form tracks along which various cargoes can be transported by various motor proteins.

The various cargoes that are transported along micro-tubules in axons (TABLE!1) move in a saltatory fashion, exhibiting periods of rapid movements, pauses and directional switches. Filamentous cargoes, such as neuro-filaments, exhibit long periods of rest (spending on aver-age 73% of the time pausing) and movements mainly in an anterograde direction (that is, towards the cell body) at 0.23 #m per second2,3. By contrast, vesicular cargoes, such as lysosomes, show frequent pausing and direc-tional switches, and other vesicular structures such as autophagosomes exhibit persistent movements (they only pause 12% of the time) in a mainly retrograde direction (that is, away from the cell body) at 0.46 #m per second4. Thus, the average transport velocity of a particular cargo depends on the time that the cargo spends pausing. As neurofilament proteins move at a faster transport rate when axons are devoid of pre-existing neurofilament structures in!vivo, one of the key determinants that curbs the axonal transport of cytoskeleton components is the density of the stationary cytoskeletal network in the axons5. The transport of mitochondria and lysosomes is also dependent on cytoskeletal organization6.

For convenience, axonal transport can be divided into two categories: fast axonal transport, which is responsible for moving membrane-bound organelles (vesicles and mitochondria), and slow axonal trans-port, which drives the movement of cytoplasmic pro-teins (including various enzymes) and cytoskeletal

NeurofilamentsNeurofilaments are components of the neuronal cytoskeleton. They are intermediate filaments with a diameter of 10 nm and are composed of three subunits: the neurofilament light, medium and heavy chains.

Axonal transport deficits and neurodegenerative diseasesStéphanie Millecamps1 and Jean-Pierre Julien2

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

1Centre de Recherche de l’Institut du Cerveau et de la Moelle épinière, INSERM UMR_S975, CNRS UMR7225, Université Pierre et Marie Curie, Hôpital de la Pitié-Salpêtrière, 47–83 boulevard de l’Hôpital, 75013 Paris, France.2Centre de Recherche du Centre Hospitalier Universitaire de Québec, Department of Psychiatry and Neuroscience, Laval University, 2705 Boulevard Laurier, Quebec, Quebec City, G1V4G2, Canada.Correspondence to J.-P.J.! e-mail: [email protected]:10.1038/nrn3380Published online 30 January 2013

!"#$"%&

NATURE REVIEWS | !"#$%&'("!'") VOLUME 14 | MARCH 2013 | 343

© 2013 Macmillan Publishers Limited. All rights reserved

Axonal transport is an essential process in neurons because of the extreme polarity and size of these cells. Indeed, despite having axons of more than 1 metre in length, human spinal motor neurons, like other types of neurons, require efficient communication between their cell body and axon tip. Axonal transport keeps axons and nerve terminals supplied with proteins, lipids and mitochondria, and clears recycled or mis-folded proteins to avoid the build-up of toxic aggre-gates1. Apart from its role in neuronal metabolism, axonal transport is crucial for intracellular neural transmission and allows the neuron to respond effectively to trophic signals or stress insults1.

Impairment of axonal transport has recently emerged as a common factor in several neurodegenerative disorders1. Here, we review the current state of knowl-edge about axonal transport defects that are associated with such disorders, with a specific focus on the mecha-nisms that can affect microtubule-based axonal transport. Before doing so, we outline the various components and mechanisms that control such transport.

!"#$%&'(')*+(,-*./,0%1,)/&$,1-2%$&Microtubules are the main component of the cytoskeleton. They have a tubular structure (25 nm in diameter) and are composed of many !- and "-tubulin heterodimers, which undergo continuous polymerization and depo-lymerization at the centrosome1. Microtubules are polar-ized in axons (but not in dendrites): their slower growing minus end (at which !-tubulin is exposed) faces the cell body, whereas their faster growing plus end (at which "-tubulin is exposed) points towards the axon tips. They

are stabilized by microtubule-associated proteins such as tau. Microtubules in the axon essentially form tracks along which various cargoes can be transported by various motor proteins.

The various cargoes that are transported along micro-tubules in axons (TABLE!1) move in a saltatory fashion, exhibiting periods of rapid movements, pauses and directional switches. Filamentous cargoes, such as neuro-filaments, exhibit long periods of rest (spending on aver-age 73% of the time pausing) and movements mainly in an anterograde direction (that is, towards the cell body) at 0.23 #m per second2,3. By contrast, vesicular cargoes, such as lysosomes, show frequent pausing and direc-tional switches, and other vesicular structures such as autophagosomes exhibit persistent movements (they only pause 12% of the time) in a mainly retrograde direction (that is, away from the cell body) at 0.46 #m per second4. Thus, the average transport velocity of a particular cargo depends on the time that the cargo spends pausing. As neurofilament proteins move at a faster transport rate when axons are devoid of pre-existing neurofilament structures in!vivo, one of the key determinants that curbs the axonal transport of cytoskeleton components is the density of the stationary cytoskeletal network in the axons5. The transport of mitochondria and lysosomes is also dependent on cytoskeletal organization6.

For convenience, axonal transport can be divided into two categories: fast axonal transport, which is responsible for moving membrane-bound organelles (vesicles and mitochondria), and slow axonal trans-port, which drives the movement of cytoplasmic pro-teins (including various enzymes) and cytoskeletal

NeurofilamentsNeurofilaments are components of the neuronal cytoskeleton. They are intermediate filaments with a diameter of 10 nm and are composed of three subunits: the neurofilament light, medium and heavy chains.

Axonal transport deficits and neurodegenerative diseasesStéphanie Millecamps1 and Jean-Pierre Julien2

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

1Centre de Recherche de l’Institut du Cerveau et de la Moelle épinière, INSERM UMR_S975, CNRS UMR7225, Université Pierre et Marie Curie, Hôpital de la Pitié-Salpêtrière, 47–83 boulevard de l’Hôpital, 75013 Paris, France.2Centre de Recherche du Centre Hospitalier Universitaire de Québec, Department of Psychiatry and Neuroscience, Laval University, 2705 Boulevard Laurier, Quebec, Quebec City, G1V4G2, Canada.Correspondence to J.-P.J.! e-mail: [email protected]:10.1038/nrn3380Published online 30 January 2013

!"#$"%&

NATURE REVIEWS | !"#$%&'("!'") VOLUME 14 | MARCH 2013 | 343

© 2013 Macmillan Publishers Limited. All rights reserved

Axonal transport is an essential process in neurons because of the extreme polarity and size of these cells. Indeed, despite having axons of more than 1 metre in length, human spinal motor neurons, like other types of neurons, require efficient communication between their cell body and axon tip. Axonal transport keeps axons and nerve terminals supplied with proteins, lipids and mitochondria, and clears recycled or mis-folded proteins to avoid the build-up of toxic aggre-gates1. Apart from its role in neuronal metabolism, axonal transport is crucial for intracellular neural transmission and allows the neuron to respond effectively to trophic signals or stress insults1.

Impairment of axonal transport has recently emerged as a common factor in several neurodegenerative disorders1. Here, we review the current state of knowl-edge about axonal transport defects that are associated with such disorders, with a specific focus on the mecha-nisms that can affect microtubule-based axonal transport. Before doing so, we outline the various components and mechanisms that control such transport.

!"#$%&'(')*+(,-*./,0%1,)/&$,1-2%$&Microtubules are the main component of the cytoskeleton. They have a tubular structure (25 nm in diameter) and are composed of many !- and "-tubulin heterodimers, which undergo continuous polymerization and depo-lymerization at the centrosome1. Microtubules are polar-ized in axons (but not in dendrites): their slower growing minus end (at which !-tubulin is exposed) faces the cell body, whereas their faster growing plus end (at which "-tubulin is exposed) points towards the axon tips. They

are stabilized by microtubule-associated proteins such as tau. Microtubules in the axon essentially form tracks along which various cargoes can be transported by various motor proteins.

The various cargoes that are transported along micro-tubules in axons (TABLE!1) move in a saltatory fashion, exhibiting periods of rapid movements, pauses and directional switches. Filamentous cargoes, such as neuro-filaments, exhibit long periods of rest (spending on aver-age 73% of the time pausing) and movements mainly in an anterograde direction (that is, towards the cell body) at 0.23 #m per second2,3. By contrast, vesicular cargoes, such as lysosomes, show frequent pausing and direc-tional switches, and other vesicular structures such as autophagosomes exhibit persistent movements (they only pause 12% of the time) in a mainly retrograde direction (that is, away from the cell body) at 0.46 #m per second4. Thus, the average transport velocity of a particular cargo depends on the time that the cargo spends pausing. As neurofilament proteins move at a faster transport rate when axons are devoid of pre-existing neurofilament structures in!vivo, one of the key determinants that curbs the axonal transport of cytoskeleton components is the density of the stationary cytoskeletal network in the axons5. The transport of mitochondria and lysosomes is also dependent on cytoskeletal organization6.

For convenience, axonal transport can be divided into two categories: fast axonal transport, which is responsible for moving membrane-bound organelles (vesicles and mitochondria), and slow axonal trans-port, which drives the movement of cytoplasmic pro-teins (including various enzymes) and cytoskeletal

NeurofilamentsNeurofilaments are components of the neuronal cytoskeleton. They are intermediate filaments with a diameter of 10 nm and are composed of three subunits: the neurofilament light, medium and heavy chains.

Axonal transport deficits and neurodegenerative diseasesStéphanie Millecamps1 and Jean-Pierre Julien2

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

1Centre de Recherche de l’Institut du Cerveau et de la Moelle épinière, INSERM UMR_S975, CNRS UMR7225, Université Pierre et Marie Curie, Hôpital de la Pitié-Salpêtrière, 47–83 boulevard de l’Hôpital, 75013 Paris, France.2Centre de Recherche du Centre Hospitalier Universitaire de Québec, Department of Psychiatry and Neuroscience, Laval University, 2705 Boulevard Laurier, Quebec, Quebec City, G1V4G2, Canada.Correspondence to J.-P.J.! e-mail: [email protected]:10.1038/nrn3380Published online 30 January 2013

!"#$"%&

NATURE REVIEWS | !"#$%&'("!'") VOLUME 14 | MARCH 2013 | 343

© 2013 Macmillan Publishers Limited. All rights reserved

R E V I EW S

NATURE REVIEWS | NEUROSCIENCE ADVANCE ONLINE PUBLICATION | 1

Nature Reviews Neuroscience | AOP, published online 15 February 2005; doi:10.1038/nrn1624

A neuron has a highly polarized structure.A typical neu-ron comprises a cell body, several short, thick, taperingdendrites and one long, thin axon. Most of the proteinsthat are needed in the axon and synaptic terminals aresynthesized in the cell body and transported along theaxon in membranous organelles or protein complexes1.Most dendritic proteins are also transported from the cellbody, but several specific mRNAs are transported intodendrites to support local protein synthesis2 (BOX 1).

In the axon and dendrites, microtubules run in alongitudinal orientation3,4, and serve as rails alongwhich membranous organelles and macromolecularcomplexes can be transported5. A microtubule is a long,hollow cylinder that is made of a polymer of !- and "-tubulins and has a diameter of 25 nm. It has intrinsicpolarity, with a fast-growing ‘plus end’ and an opposite,slow-growing ‘minus end’6. Microtubules in axons anddistal dendrites are unipolar, with the plus end pointingaway from the cell body7,8. However, the microtubulesin proximal dendrites are of mixed polarity8. The orga-nization of microtubules also differs between axons anddendrites (BOX 2).

MOLECULAR MOTORS of the kinesin and dynein super-families move along microtubules. Many kinesinsuperfamily proteins (KIFs) move towards the plusend of microtubules (‘plus-end-directed motors’) and

participate in ANTEROGRADE TRANSPORT, selectively trans-porting molecules from the cell body to axons anddendrites. By contrast, RETROGRADE TRANSPORT, from theaxonal or dendritic terminals to the cell body, is car-ried out mostly by cytoplasmic dyneins, which areminus-end-directed motors5,9–12.

Selective transport to axons and dendrites has beenstudied from several viewpoints, including whichsequences of selectively transported proteins function asselective targeting signals, and whether the basic mecha-nism is one of selective transport or selective retention(whereby cargoes would be transported to both axonsand dendrites, and selectively eliminated by endocytosisfrom the inappropriate destination). However, manyseemingly unrelated sequences have been identified astargeting signals, and the identification of the targetingsequences of specific proteins has not always clarified theunderlying sorting mechanisms. Both selective transportand selective retention seem to occur, depending on thecargoes involved, but it is not clear how some cargoes aretransported selectively, whereas others are transportednonselectively. Understanding the mechanisms of sort-ing, selective transport and recognition is an importantendeavour. This review focuses on recent developmentsthat relate to the mechanisms of selective transport, withparticular emphasis on the role of KIFs.

MOLECULAR MOTORS ANDMECHANISMS OF DIRECTIONALTRANSPORT IN NEURONSNobutaka Hirokawa* and Reiko Takemura‡

Abstract | Intracellular transport is fundamental for neuronal morphogenesis, function and survival.Many proteins are selectively transported to either axons or dendrites. In addition, some specificmRNAs are transported to dendrites for local translation. Proteins of the kinesin superfamilyparticipate in selective transport by using adaptor or scaffolding proteins to recognize and bindcargoes. The molecular components of RNA-transporting granules have been identified, and it isbecoming clear how cargoes are directed to axons and dendrites by kinesin superfamily proteins.Here we discuss the molecular mechanisms of directional axonal and dendritic transport withspecific emphasis on the role of motor proteins and their mechanisms of cargo recognition.

MOLECULAR MOTOR

SUPERFAMILIES

Kinesin and dynein superfamilyproteins move alongmicrotubules, and myosinsuperfamily proteins movealong actin filaments by ATPhydrolysis.

*Department of Cell Biologyand Anatomy, GraduateSchool of Medicine,University of Tokyo,Hongo 7-3-1, Bunkyo-ku,Tokyo 113-0033, Japan.‡Okinaka MemorialInstitute for MedicalResearch, Toranomon 2-2-2,Minato-ku, Tokyo 105-8470,Japan.Correspondence to N.H.e-mail: [email protected]:10.1038/nrn1624Published online 15 February 2005

©!!""#!Nature Publishing Group!

!

R E V I EW S

NATURE REVIEWS ! NEUROSCIENCE VOLUME 6 ! MARCH 2005 ! 201

A neuron has a highly polarized structure.A typical neu-ron comprises a cell body, several short, thick, taperingdendrites and one long, thin axon. Most of the proteinsthat are needed in the axon and synaptic terminals aresynthesized in the cell body and transported along theaxon in membranous organelles or protein complexes1.Most dendritic proteins are also transported from the cellbody, but several specific mRNAs are transported intodendrites to support local protein synthesis2 (BOX 1).

In the axon and dendrites, microtubules run in alongitudinal orientation3,4, and serve as rails alongwhich membranous organelles and macromolecularcomplexes can be transported5. A microtubule is a long,hollow cylinder that is made of a polymer of !- and "-tubulins and has a diameter of 25 nm. It has intrinsicpolarity, with a fast-growing ‘plus end’ and an opposite,slow-growing ‘minus end’6. Microtubules in axons anddistal dendrites are unipolar, with the plus end pointingaway from the cell body7,8. However, the microtubulesin proximal dendrites are of mixed polarity8. The orga-nization of microtubules also differs between axons anddendrites (BOX 2).

MOLECULAR MOTORS of the kinesin and dynein super-families move along microtubules. Many kinesinsuperfamily proteins (KIFs) move towards the plusend of microtubules (‘plus-end-directed motors’) and

participate in ANTEROGRADE TRANSPORT, selectively trans-porting molecules from the cell body to axons anddendrites. By contrast, RETROGRADE TRANSPORT, from theaxonal or dendritic terminals to the cell body, is car-ried out mostly by cytoplasmic dyneins, which areminus-end-directed motors5,9–12.

Selective transport to axons and dendrites has beenstudied from several viewpoints, including whichsequences of selectively transported proteins function asselective targeting signals, and whether the basic mecha-nism is one of selective transport or selective retention(whereby cargoes would be transported to both axonsand dendrites, and selectively eliminated by endocytosisfrom the inappropriate destination). However, manyseemingly unrelated sequences have been identified astargeting signals, and the identification of the targetingsequences of specific proteins has not always clarified theunderlying sorting mechanisms. Both selective transportand selective retention seem to occur, depending on thecargoes involved, but it is not clear how some cargoes aretransported selectively, whereas others are transportednonselectively. Understanding the mechanisms of sort-ing, selective transport and recognition is an importantendeavour. This review focuses on recent developmentsthat relate to the mechanisms of selective transport, withparticular emphasis on the role of KIFs.

MOLECULAR MOTORS ANDMECHANISMS OF DIRECTIONALTRANSPORT IN NEURONSNobutaka Hirokawa* and Reiko Takemura‡

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

MOLECULAR MOTOR

SUPERFAMILIES

Kinesin and dynein superfamilyproteins move alongmicrotubules, and myosinsuperfamily proteins movealong actin filaments by ATPhydrolysis.

*Department of Cell Biologyand Anatomy, GraduateSchool of Medicine,University of Tokyo,Hongo 7-3-1, Bunkyo-ku,Tokyo 113-0033, Japan.‡Okinaka MemorialInstitute for MedicalResearch, Toranomon 2-2-2,Minato-ku, Tokyo 105-8470,Japan.Correspondence to N.H.e-mail: [email protected]:10.1038/nrn1624Published online 15 February 2005

R E V I EW S

NATURE REVIEWS | NEUROSCIENCE ADVANCE ONLINE PUBLICATION | 1

Nature Reviews Neuroscience | AOP, published online 15 February 2005; doi:10.1038/nrn1624

A neuron has a highly polarized structure.A typical neu-ron comprises a cell body, several short, thick, taperingdendrites and one long, thin axon. Most of the proteinsthat are needed in the axon and synaptic terminals aresynthesized in the cell body and transported along theaxon in membranous organelles or protein complexes1.Most dendritic proteins are also transported from the cellbody, but several specific mRNAs are transported intodendrites to support local protein synthesis2 (BOX 1).

In the axon and dendrites, microtubules run in alongitudinal orientation3,4, and serve as rails alongwhich membranous organelles and macromolecularcomplexes can be transported5. A microtubule is a long,hollow cylinder that is made of a polymer of !- and "-tubulins and has a diameter of 25 nm. It has intrinsicpolarity, with a fast-growing ‘plus end’ and an opposite,slow-growing ‘minus end’6. Microtubules in axons anddistal dendrites are unipolar, with the plus end pointingaway from the cell body7,8. However, the microtubulesin proximal dendrites are of mixed polarity8. The orga-nization of microtubules also differs between axons anddendrites (BOX 2).

MOLECULAR MOTORS of the kinesin and dynein super-families move along microtubules. Many kinesinsuperfamily proteins (KIFs) move towards the plusend of microtubules (‘plus-end-directed motors’) and

participate in ANTEROGRADE TRANSPORT, selectively trans-porting molecules from the cell body to axons anddendrites. By contrast, RETROGRADE TRANSPORT, from theaxonal or dendritic terminals to the cell body, is car-ried out mostly by cytoplasmic dyneins, which areminus-end-directed motors5,9–12.

Selective transport to axons and dendrites has beenstudied from several viewpoints, including whichsequences of selectively transported proteins function asselective targeting signals, and whether the basic mecha-nism is one of selective transport or selective retention(whereby cargoes would be transported to both axonsand dendrites, and selectively eliminated by endocytosisfrom the inappropriate destination). However, manyseemingly unrelated sequences have been identified astargeting signals, and the identification of the targetingsequences of specific proteins has not always clarified theunderlying sorting mechanisms. Both selective transportand selective retention seem to occur, depending on thecargoes involved, but it is not clear how some cargoes aretransported selectively, whereas others are transportednonselectively. Understanding the mechanisms of sort-ing, selective transport and recognition is an importantendeavour. This review focuses on recent developmentsthat relate to the mechanisms of selective transport, withparticular emphasis on the role of KIFs.

MOLECULAR MOTORS ANDMECHANISMS OF DIRECTIONALTRANSPORT IN NEURONSNobutaka Hirokawa* and Reiko Takemura‡

Abstract | Intracellular transport is fundamental for neuronal morphogenesis, function and survival.Many proteins are selectively transported to either axons or dendrites. In addition, some specificmRNAs are transported to dendrites for local translation. Proteins of the kinesin superfamilyparticipate in selective transport by using adaptor or scaffolding proteins to recognize and bindcargoes. The molecular components of RNA-transporting granules have been identified, and it isbecoming clear how cargoes are directed to axons and dendrites by kinesin superfamily proteins.Here we discuss the molecular mechanisms of directional axonal and dendritic transport withspecific emphasis on the role of motor proteins and their mechanisms of cargo recognition.

MOLECULAR MOTOR

SUPERFAMILIES

Kinesin and dynein superfamilyproteins move alongmicrotubules, and myosinsuperfamily proteins movealong actin filaments by ATPhydrolysis.

*Department of Cell Biologyand Anatomy, GraduateSchool of Medicine,University of Tokyo,Hongo 7-3-1, Bunkyo-ku,Tokyo 113-0033, Japan.‡Okinaka MemorialInstitute for MedicalResearch, Toranomon 2-2-2,Minato-ku, Tokyo 105-8470,Japan.Correspondence to N.H.e-mail: [email protected]:10.1038/nrn1624Published online 15 February 2005

Intracellular transportSoma

AxonSynapticterminal

Organelle transport

Axonal transport is an essential process in neurons because of the extreme polarity and size of these cells. Indeed, despite having axons of more than 1 metre in length, human spinal motor neurons, like other types of neurons, require efficient communication between their cell body and axon tip. Axonal transport keeps axons and nerve terminals supplied with proteins, lipids and mitochondria, and clears recycled or mis-folded proteins to avoid the build-up of toxic aggre-gates1. Apart from its role in neuronal metabolism, axonal transport is crucial for intracellular neural transmission and allows the neuron to respond effectively to trophic signals or stress insults1.

Impairment of axonal transport has recently emerged as a common factor in several neurodegenerative disorders1. Here, we review the current state of knowl-edge about axonal transport defects that are associated with such disorders, with a specific focus on the mecha-nisms that can affect microtubule-based axonal transport. Before doing so, we outline the various components and mechanisms that control such transport.

!"#$%&'(')*+(,-*./,0%1,)/&$,1-2%$&Microtubules are the main component of the cytoskeleton. They have a tubular structure (25 nm in diameter) and are composed of many !- and "-tubulin heterodimers, which undergo continuous polymerization and depo-lymerization at the centrosome1. Microtubules are polar-ized in axons (but not in dendrites): their slower growing minus end (at which !-tubulin is exposed) faces the cell body, whereas their faster growing plus end (at which "-tubulin is exposed) points towards the axon tips. They

are stabilized by microtubule-associated proteins such as tau. Microtubules in the axon essentially form tracks along which various cargoes can be transported by various motor proteins.

The various cargoes that are transported along micro-tubules in axons (TABLE!1) move in a saltatory fashion, exhibiting periods of rapid movements, pauses and directional switches. Filamentous cargoes, such as neuro-filaments, exhibit long periods of rest (spending on aver-age 73% of the time pausing) and movements mainly in an anterograde direction (that is, towards the cell body) at 0.23 #m per second2,3. By contrast, vesicular cargoes, such as lysosomes, show frequent pausing and direc-tional switches, and other vesicular structures such as autophagosomes exhibit persistent movements (they only pause 12% of the time) in a mainly retrograde direction (that is, away from the cell body) at 0.46 #m per second4. Thus, the average transport velocity of a particular cargo depends on the time that the cargo spends pausing. As neurofilament proteins move at a faster transport rate when axons are devoid of pre-existing neurofilament structures in!vivo, one of the key determinants that curbs the axonal transport of cytoskeleton components is the density of the stationary cytoskeletal network in the axons5. The transport of mitochondria and lysosomes is also dependent on cytoskeletal organization6.

For convenience, axonal transport can be divided into two categories: fast axonal transport, which is responsible for moving membrane-bound organelles (vesicles and mitochondria), and slow axonal trans-port, which drives the movement of cytoplasmic pro-teins (including various enzymes) and cytoskeletal

NeurofilamentsNeurofilaments are components of the neuronal cytoskeleton. They are intermediate filaments with a diameter of 10 nm and are composed of three subunits: the neurofilament light, medium and heavy chains.

Axonal transport deficits and neurodegenerative diseasesStéphanie Millecamps1 and Jean-Pierre Julien2

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

1Centre de Recherche de l’Institut du Cerveau et de la Moelle épinière, INSERM UMR_S975, CNRS UMR7225, Université Pierre et Marie Curie, Hôpital de la Pitié-Salpêtrière, 47–83 boulevard de l’Hôpital, 75013 Paris, France.2Centre de Recherche du Centre Hospitalier Universitaire de Québec, Department of Psychiatry and Neuroscience, Laval University, 2705 Boulevard Laurier, Quebec, Quebec City, G1V4G2, Canada.Correspondence to J.-P.J.! e-mail: [email protected]:10.1038/nrn3380Published online 30 January 2013

!"#$"%&

NATURE REVIEWS | !"#$%&'("!'") VOLUME 14 | MARCH 2013 | 343

© 2013 Macmillan Publishers Limited. All rights reserved

Axonal transport is an essential process in neurons because of the extreme polarity and size of these cells. Indeed, despite having axons of more than 1 metre in length, human spinal motor neurons, like other types of neurons, require efficient communication between their cell body and axon tip. Axonal transport keeps axons and nerve terminals supplied with proteins, lipids and mitochondria, and clears recycled or mis-folded proteins to avoid the build-up of toxic aggre-gates1. Apart from its role in neuronal metabolism, axonal transport is crucial for intracellular neural transmission and allows the neuron to respond effectively to trophic signals or stress insults1.

Impairment of axonal transport has recently emerged as a common factor in several neurodegenerative disorders1. Here, we review the current state of knowl-edge about axonal transport defects that are associated with such disorders, with a specific focus on the mecha-nisms that can affect microtubule-based axonal transport. Before doing so, we outline the various components and mechanisms that control such transport.

!"#$%&'(')*+(,-*./,0%1,)/&$,1-2%$&Microtubules are the main component of the cytoskeleton. They have a tubular structure (25 nm in diameter) and are composed of many !- and "-tubulin heterodimers, which undergo continuous polymerization and depo-lymerization at the centrosome1. Microtubules are polar-ized in axons (but not in dendrites): their slower growing minus end (at which !-tubulin is exposed) faces the cell body, whereas their faster growing plus end (at which "-tubulin is exposed) points towards the axon tips. They

are stabilized by microtubule-associated proteins such as tau. Microtubules in the axon essentially form tracks along which various cargoes can be transported by various motor proteins.

The various cargoes that are transported along micro-tubules in axons (TABLE!1) move in a saltatory fashion, exhibiting periods of rapid movements, pauses and directional switches. Filamentous cargoes, such as neuro-filaments, exhibit long periods of rest (spending on aver-age 73% of the time pausing) and movements mainly in an anterograde direction (that is, towards the cell body) at 0.23 #m per second2,3. By contrast, vesicular cargoes, such as lysosomes, show frequent pausing and direc-tional switches, and other vesicular structures such as autophagosomes exhibit persistent movements (they only pause 12% of the time) in a mainly retrograde direction (that is, away from the cell body) at 0.46 #m per second4. Thus, the average transport velocity of a particular cargo depends on the time that the cargo spends pausing. As neurofilament proteins move at a faster transport rate when axons are devoid of pre-existing neurofilament structures in!vivo, one of the key determinants that curbs the axonal transport of cytoskeleton components is the density of the stationary cytoskeletal network in the axons5. The transport of mitochondria and lysosomes is also dependent on cytoskeletal organization6.

For convenience, axonal transport can be divided into two categories: fast axonal transport, which is responsible for moving membrane-bound organelles (vesicles and mitochondria), and slow axonal trans-port, which drives the movement of cytoplasmic pro-teins (including various enzymes) and cytoskeletal

NeurofilamentsNeurofilaments are components of the neuronal cytoskeleton. They are intermediate filaments with a diameter of 10 nm and are composed of three subunits: the neurofilament light, medium and heavy chains.

Axonal transport deficits and neurodegenerative diseasesStéphanie Millecamps1 and Jean-Pierre Julien2

!"#$%&'$()(*+,(-.$%&',//0/&%($%&.#12%$(23(2%4&.,//,#(&/2.4(&.(&52.(-#('%0'-&/(32%($+,(6&-.$,.&.',(&.7(30.'$-2.(23(&(.,0%2.8(!.$,%24%&7,(&52.&/($%&.#12%$(+&#(&(%2/,(-.(#011/9-.4(1%2$,-.#(&.7(/-1-7#($2($+,(7-#$&/(#9.&1#,(&.7(6-$2'+2.7%-&(32%(/2'&/(,.,%49(%,:0-%,6,.$#;(<+,%,&#(%,$%24%&7,($%&.#12%$(-#(-.=2/=,7(-.($+,('/,&%&.',(23(6-#32/7,7(&.7(&44%,4&$,7(1%2$,-.#(3%26($+,(&52.(&.7($+,(-.$%&',//0/&%($%&.#12%$(23(7-#$&/($%21+-'(#-4.&/#($2($+,(#26&8(!52.&/($%&.#12%$('&.(",(&33,'$,7("9(&/$,%&$-2.#($2(=&%-20#('2612.,.$#(23($+,($%&.#12%$(6&'+-.,%98(>,%,;(<,(%,=-,<($+,('0%%,.$(#$&$,(23(?.2</,74,(&"20$(&52.&/($%&.#12%$(7,3,'$#($+&$(6-4+$('2.$%-"0$,($2($+,(1&$+24,.,#-#(23(1&%$-'0/&%(.,0%27,4,.,%&$-=,(7-#,&#,#8

1Centre de Recherche de l’Institut du Cerveau et de la Moelle épinière, INSERM UMR_S975, CNRS UMR7225, Université Pierre et Marie Curie, Hôpital de la Pitié-Salpêtrière, 47–83 boulevard de l’Hôpital, 75013 Paris, France.2Centre de Recherche du Centre Hospitalier Universitaire de Québec, Department of Psychiatry and Neuroscience, Laval University, 2705 Boulevard Laurier, Quebec, Quebec City, G1V4G2, Canada.Correspondence to J.-P.J.! e-mail: [email protected]:10.1038/nrn3380Published online 30 January 2013

!"#$"%&

NATURE REVIEWS | !"#$%&'("!'") VOLUME 14 | MARCH 2013 | 343

© 2013 Macmillan Publishers Limited. All rights reserved

Axonal transport is an essential process in neurons because of the extreme polarity and size of these cells. Indeed, despite having axons of more than 1 metre in length, human spinal motor neurons, like other types of neurons, require efficient communication between their cell body and axon tip. Axonal transport keeps axons and nerve terminals supplied with proteins, lipids and mitochondria, and clears recycled or mis-folded proteins to avoid the build-up of toxic aggre-gates1. Apart from its role in neuronal metabolism, axonal transport is crucial for intracellular neural transmission and allows the neuron to respond effectively to trophic signals or stress insults1.

Impairment of axonal transport has recently emerged as a common factor in several neurodegenerative disorders1. Here, we review the current state of knowl-edge about axonal transport defects that are associated with such disorders, with a specific focus on the mecha-nisms that can affect microtubule-based axonal transport. Before doing so, we outline the various components and mechanisms that control such transport.

!"#$%&'(')*+(,-*./,0%1,)/&$,1-2%$&Microtubules are the main component of the cytoskeleton. They have a tubular structure (25 nm in diameter) and are composed of many !- and "-tubulin heterodimers, which undergo continuous polymerization and depo-lymerization at the centrosome1. Microtubules are polar-ized in axons (but not in dendrites): their slower growing minus end (at which !-tubulin is exposed) faces the cell body, whereas their faster growing plus end (at which "-tubulin is exposed) points towards the axon tips. They

are stabilized by microtubule-associated proteins such as tau. Microtubules in the axon essentially form tracks along which various cargoes can be transported by various motor proteins.

The various cargoes that are transported along micro-tubules in axons (TABLE!1) move in a saltatory fashion, exhibiting periods of rapid movements, pauses and directional switches. Filamentous cargoes, such as neuro-filaments, exhibit long periods of rest (spending on aver-age 73% of the time pausing) and movements mainly in an anterograde direction (that is, towards the cell body) at 0.23 #m per second2,3. By contrast, vesicular cargoes, such as lysosomes, show frequent pausing and direc-tional switches, and other vesicular structures such as autophagosomes exhibit persistent movements (they only pause 12% of the time) in a mainly retrograde direction (that is, away from the cell body) at 0.46 #m per second4. Thus, the average transport velocity of a particular cargo depends on the time that the cargo spends pausing. As neurofilament proteins move at a faster transport rate when axons are devoid of pre-existing neurofilament structures in!vivo, one of the key determinants that curbs the axonal transport of cytoskeleton components is the density of the stationary cytoskeletal network in the axons5. The transport of mitochondria and lysosomes is also dependent on cytoskeletal organization6.

For convenience, axonal transport can be divided into two categories: fast axonal transport, which is responsible for moving membrane-bound organelles (vesicles and mitochondria), and slow axonal trans-port, which drives the movement of cytoplasmic pro-teins (including various enzymes) and cytoskeletal

NeurofilamentsNeurofilaments are components of the neuronal cytoskeleton. They are intermediate filaments with a diameter of 10 nm and are composed of three subunits: the neurofilament light, medium and heavy chains.

Axonal transport deficits and neurodegenerative diseasesStéphanie Millecamps1 and Jean-Pierre Julien2

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

1Centre de Recherche de l’Institut du Cerveau et de la Moelle épinière, INSERM UMR_S975, CNRS UMR7225, Université Pierre et Marie Curie, Hôpital de la Pitié-Salpêtrière, 47–83 boulevard de l’Hôpital, 75013 Paris, France.2Centre de Recherche du Centre Hospitalier Universitaire de Québec, Department of Psychiatry and Neuroscience, Laval University, 2705 Boulevard Laurier, Quebec, Quebec City, G1V4G2, Canada.Correspondence to J.-P.J.! e-mail: [email protected]:10.1038/nrn3380Published online 30 January 2013

!"#$"%&

NATURE REVIEWS | !"#$%&'("!'") VOLUME 14 | MARCH 2013 | 343

© 2013 Macmillan Publishers Limited. All rights reserved

R E V I EW S

NATURE REVIEWS | NEUROSCIENCE ADVANCE ONLINE PUBLICATION | 1

Nature Reviews Neuroscience | AOP, published online 15 February 2005; doi:10.1038/nrn1624

A neuron has a highly polarized structure.A typical neu-ron comprises a cell body, several short, thick, taperingdendrites and one long, thin axon. Most of the proteinsthat are needed in the axon and synaptic terminals aresynthesized in the cell body and transported along theaxon in membranous organelles or protein complexes1.Most dendritic proteins are also transported from the cellbody, but several specific mRNAs are transported intodendrites to support local protein synthesis2 (BOX 1).

In the axon and dendrites, microtubules run in alongitudinal orientation3,4, and serve as rails alongwhich membranous organelles and macromolecularcomplexes can be transported5. A microtubule is a long,hollow cylinder that is made of a polymer of !- and "-tubulins and has a diameter of 25 nm. It has intrinsicpolarity, with a fast-growing ‘plus end’ and an opposite,slow-growing ‘minus end’6. Microtubules in axons anddistal dendrites are unipolar, with the plus end pointingaway from the cell body7,8. However, the microtubulesin proximal dendrites are of mixed polarity8. The orga-nization of microtubules also differs between axons anddendrites (BOX 2).

MOLECULAR MOTORS of the kinesin and dynein super-families move along microtubules. Many kinesinsuperfamily proteins (KIFs) move towards the plusend of microtubules (‘plus-end-directed motors’) and

participate in ANTEROGRADE TRANSPORT, selectively trans-porting molecules from the cell body to axons anddendrites. By contrast, RETROGRADE TRANSPORT, from theaxonal or dendritic terminals to the cell body, is car-ried out mostly by cytoplasmic dyneins, which areminus-end-directed motors5,9–12.

Selective transport to axons and dendrites has beenstudied from several viewpoints, including whichsequences of selectively transported proteins function asselective targeting signals, and whether the basic mecha-nism is one of selective transport or selective retention(whereby cargoes would be transported to both axonsand dendrites, and selectively eliminated by endocytosisfrom the inappropriate destination). However, manyseemingly unrelated sequences have been identified astargeting signals, and the identification of the targetingsequences of specific proteins has not always clarified theunderlying sorting mechanisms. Both selective transportand selective retention seem to occur, depending on thecargoes involved, but it is not clear how some cargoes aretransported selectively, whereas others are transportednonselectively. Understanding the mechanisms of sort-ing, selective transport and recognition is an importantendeavour. This review focuses on recent developmentsthat relate to the mechanisms of selective transport, withparticular emphasis on the role of KIFs.

MOLECULAR MOTORS ANDMECHANISMS OF DIRECTIONALTRANSPORT IN NEURONSNobutaka Hirokawa* and Reiko Takemura‡

Abstract | Intracellular transport is fundamental for neuronal morphogenesis, function and survival.Many proteins are selectively transported to either axons or dendrites. In addition, some specificmRNAs are transported to dendrites for local translation. Proteins of the kinesin superfamilyparticipate in selective transport by using adaptor or scaffolding proteins to recognize and bindcargoes. The molecular components of RNA-transporting granules have been identified, and it isbecoming clear how cargoes are directed to axons and dendrites by kinesin superfamily proteins.Here we discuss the molecular mechanisms of directional axonal and dendritic transport withspecific emphasis on the role of motor proteins and their mechanisms of cargo recognition.

MOLECULAR MOTOR

SUPERFAMILIES

Kinesin and dynein superfamilyproteins move alongmicrotubules, and myosinsuperfamily proteins movealong actin filaments by ATPhydrolysis.

*Department of Cell Biologyand Anatomy, GraduateSchool of Medicine,University of Tokyo,Hongo 7-3-1, Bunkyo-ku,Tokyo 113-0033, Japan.‡Okinaka MemorialInstitute for MedicalResearch, Toranomon 2-2-2,Minato-ku, Tokyo 105-8470,Japan.Correspondence to N.H.e-mail: [email protected]:10.1038/nrn1624Published online 15 February 2005

©!!""#!Nature Publishing Group!

!

R E V I EW S

NATURE REVIEWS ! NEUROSCIENCE VOLUME 6 ! MARCH 2005 ! 201

A neuron has a highly polarized structure.A typical neu-ron comprises a cell body, several short, thick, taperingdendrites and one long, thin axon. Most of the proteinsthat are needed in the axon and synaptic terminals aresynthesized in the cell body and transported along theaxon in membranous organelles or protein complexes1.Most dendritic proteins are also transported from the cellbody, but several specific mRNAs are transported intodendrites to support local protein synthesis2 (BOX 1).

In the axon and dendrites, microtubules run in alongitudinal orientation3,4, and serve as rails alongwhich membranous organelles and macromolecularcomplexes can be transported5. A microtubule is a long,hollow cylinder that is made of a polymer of !- and "-tubulins and has a diameter of 25 nm. It has intrinsicpolarity, with a fast-growing ‘plus end’ and an opposite,slow-growing ‘minus end’6. Microtubules in axons anddistal dendrites are unipolar, with the plus end pointingaway from the cell body7,8. However, the microtubulesin proximal dendrites are of mixed polarity8. The orga-nization of microtubules also differs between axons anddendrites (BOX 2).

MOLECULAR MOTORS of the kinesin and dynein super-families move along microtubules. Many kinesinsuperfamily proteins (KIFs) move towards the plusend of microtubules (‘plus-end-directed motors’) and

participate in ANTEROGRADE TRANSPORT, selectively trans-porting molecules from the cell body to axons anddendrites. By contrast, RETROGRADE TRANSPORT, from theaxonal or dendritic terminals to the cell body, is car-ried out mostly by cytoplasmic dyneins, which areminus-end-directed motors5,9–12.

Selective transport to axons and dendrites has beenstudied from several viewpoints, including whichsequences of selectively transported proteins function asselective targeting signals, and whether the basic mecha-nism is one of selective transport or selective retention(whereby cargoes would be transported to both axonsand dendrites, and selectively eliminated by endocytosisfrom the inappropriate destination). However, manyseemingly unrelated sequences have been identified astargeting signals, and the identification of the targetingsequences of specific proteins has not always clarified theunderlying sorting mechanisms. Both selective transportand selective retention seem to occur, depending on thecargoes involved, but it is not clear how some cargoes aretransported selectively, whereas others are transportednonselectively. Understanding the mechanisms of sort-ing, selective transport and recognition is an importantendeavour. This review focuses on recent developmentsthat relate to the mechanisms of selective transport, withparticular emphasis on the role of KIFs.

MOLECULAR MOTORS ANDMECHANISMS OF DIRECTIONALTRANSPORT IN NEURONSNobutaka Hirokawa* and Reiko Takemura‡

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

MOLECULAR MOTOR

SUPERFAMILIES

Kinesin and dynein superfamilyproteins move alongmicrotubules, and myosinsuperfamily proteins movealong actin filaments by ATPhydrolysis.

*Department of Cell Biologyand Anatomy, GraduateSchool of Medicine,University of Tokyo,Hongo 7-3-1, Bunkyo-ku,Tokyo 113-0033, Japan.‡Okinaka MemorialInstitute for MedicalResearch, Toranomon 2-2-2,Minato-ku, Tokyo 105-8470,Japan.Correspondence to N.H.e-mail: [email protected]:10.1038/nrn1624Published online 15 February 2005

R E V I EW S

NATURE REVIEWS | NEUROSCIENCE ADVANCE ONLINE PUBLICATION | 1

Nature Reviews Neuroscience | AOP, published online 15 February 2005; doi:10.1038/nrn1624

A neuron has a highly polarized structure.A typical neu-ron comprises a cell body, several short, thick, taperingdendrites and one long, thin axon. Most of the proteinsthat are needed in the axon and synaptic terminals aresynthesized in the cell body and transported along theaxon in membranous organelles or protein complexes1.Most dendritic proteins are also transported from the cellbody, but several specific mRNAs are transported intodendrites to support local protein synthesis2 (BOX 1).

In the axon and dendrites, microtubules run in alongitudinal orientation3,4, and serve as rails alongwhich membranous organelles and macromolecularcomplexes can be transported5. A microtubule is a long,hollow cylinder that is made of a polymer of !- and "-tubulins and has a diameter of 25 nm. It has intrinsicpolarity, with a fast-growing ‘plus end’ and an opposite,slow-growing ‘minus end’6. Microtubules in axons anddistal dendrites are unipolar, with the plus end pointingaway from the cell body7,8. However, the microtubulesin proximal dendrites are of mixed polarity8. The orga-nization of microtubules also differs between axons anddendrites (BOX 2).

MOLECULAR MOTORS of the kinesin and dynein super-families move along microtubules. Many kinesinsuperfamily proteins (KIFs) move towards the plusend of microtubules (‘plus-end-directed motors’) and

participate in ANTEROGRADE TRANSPORT, selectively trans-porting molecules from the cell body to axons anddendrites. By contrast, RETROGRADE TRANSPORT, from theaxonal or dendritic terminals to the cell body, is car-ried out mostly by cytoplasmic dyneins, which areminus-end-directed motors5,9–12.

Selective transport to axons and dendrites has beenstudied from several viewpoints, including whichsequences of selectively transported proteins function asselective targeting signals, and whether the basic mecha-nism is one of selective transport or selective retention(whereby cargoes would be transported to both axonsand dendrites, and selectively eliminated by endocytosisfrom the inappropriate destination). However, manyseemingly unrelated sequences have been identified astargeting signals, and the identification of the targetingsequences of specific proteins has not always clarified theunderlying sorting mechanisms. Both selective transportand selective retention seem to occur, depending on thecargoes involved, but it is not clear how some cargoes aretransported selectively, whereas others are transportednonselectively. Understanding the mechanisms of sort-ing, selective transport and recognition is an importantendeavour. This review focuses on recent developmentsthat relate to the mechanisms of selective transport, withparticular emphasis on the role of KIFs.

MOLECULAR MOTORS ANDMECHANISMS OF DIRECTIONALTRANSPORT IN NEURONSNobutaka Hirokawa* and Reiko Takemura‡

Abstract | Intracellular transport is fundamental for neuronal morphogenesis, function and survival.Many proteins are selectively transported to either axons or dendrites. In addition, some specificmRNAs are transported to dendrites for local translation. Proteins of the kinesin superfamilyparticipate in selective transport by using adaptor or scaffolding proteins to recognize and bindcargoes. The molecular components of RNA-transporting granules have been identified, and it isbecoming clear how cargoes are directed to axons and dendrites by kinesin superfamily proteins.Here we discuss the molecular mechanisms of directional axonal and dendritic transport withspecific emphasis on the role of motor proteins and their mechanisms of cargo recognition.

MOLECULAR MOTOR

SUPERFAMILIES

Kinesin and dynein superfamilyproteins move alongmicrotubules, and myosinsuperfamily proteins movealong actin filaments by ATPhydrolysis.

*Department of Cell Biologyand Anatomy, GraduateSchool of Medicine,University of Tokyo,Hongo 7-3-1, Bunkyo-ku,Tokyo 113-0033, Japan.‡Okinaka MemorialInstitute for MedicalResearch, Toranomon 2-2-2,Minato-ku, Tokyo 105-8470,Japan.Correspondence to N.H.e-mail: [email protected]:10.1038/nrn1624Published online 15 February 2005

Intracellular transportSoma

AxonSynapticterminal

Organelle transport

Axonal transport is an essential process in neurons because of the extreme polarity and size of these cells. Indeed, despite having axons of more than 1 metre in length, human spinal motor neurons, like other types of neurons, require efficient communication between their cell body and axon tip. Axonal transport keeps axons and nerve terminals supplied with proteins, lipids and mitochondria, and clears recycled or mis-folded proteins to avoid the build-up of toxic aggre-gates1. Apart from its role in neuronal metabolism, axonal transport is crucial for intracellular neural transmission and allows the neuron to respond effectively to trophic signals or stress insults1.

Impairment of axonal transport has recently emerged as a common factor in several neurodegenerative disorders1. Here, we review the current state of knowl-edge about axonal transport defects that are associated with such disorders, with a specific focus on the mecha-nisms that can affect microtubule-based axonal transport. Before doing so, we outline the various components and mechanisms that control such transport.

!"#$%&'(')*+(,-*./,0%1,)/&$,1-2%$&Microtubules are the main component of the cytoskeleton. They have a tubular structure (25 nm in diameter) and are composed of many !- and "-tubulin heterodimers, which undergo continuous polymerization and depo-lymerization at the centrosome1. Microtubules are polar-ized in axons (but not in dendrites): their slower growing minus end (at which !-tubulin is exposed) faces the cell body, whereas their faster growing plus end (at which "-tubulin is exposed) points towards the axon tips. They

are stabilized by microtubule-associated proteins such as tau. Microtubules in the axon essentially form tracks along which various cargoes can be transported by various motor proteins.

The various cargoes that are transported along micro-tubules in axons (TABLE!1) move in a saltatory fashion, exhibiting periods of rapid movements, pauses and directional switches. Filamentous cargoes, such as neuro-filaments, exhibit long periods of rest (spending on aver-age 73% of the time pausing) and movements mainly in an anterograde direction (that is, towards the cell body) at 0.23 #m per second2,3. By contrast, vesicular cargoes, such as lysosomes, show frequent pausing and direc-tional switches, and other vesicular structures such as autophagosomes exhibit persistent movements (they only pause 12% of the time) in a mainly retrograde direction (that is, away from the cell body) at 0.46 #m per second4. Thus, the average transport velocity of a particular cargo depends on the time that the cargo spends pausing. As neurofilament proteins move at a faster transport rate when axons are devoid of pre-existing neurofilament structures in!vivo, one of the key determinants that curbs the axonal transport of cytoskeleton components is the density of the stationary cytoskeletal network in the axons5. The transport of mitochondria and lysosomes is also dependent on cytoskeletal organization6.

For convenience, axonal transport can be divided into two categories: fast axonal transport, which is responsible for moving membrane-bound organelles (vesicles and mitochondria), and slow axonal trans-port, which drives the movement of cytoplasmic pro-teins (including various enzymes) and cytoskeletal

NeurofilamentsNeurofilaments are components of the neuronal cytoskeleton. They are intermediate filaments with a diameter of 10 nm and are composed of three subunits: the neurofilament light, medium and heavy chains.

Axonal transport deficits and neurodegenerative diseasesStéphanie Millecamps1 and Jean-Pierre Julien2

!"#$%&'$()(*+,(-.$%&',//0/&%($%&.#12%$(23(2%4&.,//,#(&/2.4(&.(&52.(-#('%0'-&/(32%($+,(6&-.$,.&.',(&.7(30.'$-2.(23(&(.,0%2.8(!.$,%24%&7,(&52.&/($%&.#12%$(+&#(&(%2/,(-.(#011/9-.4(1%2$,-.#(&.7(/-1-7#($2($+,(7-#$&/(#9.&1#,(&.7(6-$2'+2.7%-&(32%(/2'&/(,.,%49(%,:0-%,6,.$#;(<+,%,&#(%,$%24%&7,($%&.#12%$(-#(-.=2/=,7(-.($+,('/,&%&.',(23(6-#32/7,7(&.7(&44%,4&$,7(1%2$,-.#(3%26($+,(&52.(&.7($+,(-.$%&',//0/&%($%&.#12%$(23(7-#$&/($%21+-'(#-4.&/#($2($+,(#26&8(!52.&/($%&.#12%$('&.(",(&33,'$,7("9(&/$,%&$-2.#($2(=&%-20#('2612.,.$#(23($+,($%&.#12%$(6&'+-.,%98(>,%,;(<,(%,=-,<($+,('0%%,.$(#$&$,(23(?.2</,74,(&"20$(&52.&/($%&.#12%$(7,3,'$#($+&$(6-4+$('2.$%-"0$,($2($+,(1&$+24,.,#-#(23(1&%$-'0/&%(.,0%27,4,.,%&$-=,(7-#,&#,#8

1Centre de Recherche de l’Institut du Cerveau et de la Moelle épinière, INSERM UMR_S975, CNRS UMR7225, Université Pierre et Marie Curie, Hôpital de la Pitié-Salpêtrière, 47–83 boulevard de l’Hôpital, 75013 Paris, France.2Centre de Recherche du Centre Hospitalier Universitaire de Québec, Department of Psychiatry and Neuroscience, Laval University, 2705 Boulevard Laurier, Quebec, Quebec City, G1V4G2, Canada.Correspondence to J.-P.J.! e-mail: [email protected]:10.1038/nrn3380Published online 30 January 2013

!"#$"%&

NATURE REVIEWS | !"#$%&'("!'") VOLUME 14 | MARCH 2013 | 343

© 2013 Macmillan Publishers Limited. All rights reserved

Axonal transport is an essential process in neurons because of the extreme polarity and size of these cells. Indeed, despite having axons of more than 1 metre in length, human spinal motor neurons, like other types of neurons, require efficient communication between their cell body and axon tip. Axonal transport keeps axons and nerve terminals supplied with proteins, lipids and mitochondria, and clears recycled or mis-folded proteins to avoid the build-up of toxic aggre-gates1. Apart from its role in neuronal metabolism, axonal transport is crucial for intracellular neural transmission and allows the neuron to respond effectively to trophic signals or stress insults1.

Impairment of axonal transport has recently emerged as a common factor in several neurodegenerative disorders1. Here, we review the current state of knowl-edge about axonal transport defects that are associated with such disorders, with a specific focus on the mecha-nisms that can affect microtubule-based axonal transport. Before doing so, we outline the various components and mechanisms that control such transport.

!"#$%&'(')*+(,-*./,0%1,)/&$,1-2%$&Microtubules are the main component of the cytoskeleton. They have a tubular structure (25 nm in diameter) and are composed of many !- and "-tubulin heterodimers, which undergo continuous polymerization and depo-lymerization at the centrosome1. Microtubules are polar-ized in axons (but not in dendrites): their slower growing minus end (at which !-tubulin is exposed) faces the cell body, whereas their faster growing plus end (at which "-tubulin is exposed) points towards the axon tips. They

are stabilized by microtubule-associated proteins such as tau. Microtubules in the axon essentially form tracks along which various cargoes can be transported by various motor proteins.

The various cargoes that are transported along micro-tubules in axons (TABLE!1) move in a saltatory fashion, exhibiting periods of rapid movements, pauses and directional switches. Filamentous cargoes, such as neuro-filaments, exhibit long periods of rest (spending on aver-age 73% of the time pausing) and movements mainly in an anterograde direction (that is, towards the cell body) at 0.23 #m per second2,3. By contrast, vesicular cargoes, such as lysosomes, show frequent pausing and direc-tional switches, and other vesicular structures such as autophagosomes exhibit persistent movements (they only pause 12% of the time) in a mainly retrograde direction (that is, away from the cell body) at 0.46 #m per second4. Thus, the average transport velocity of a particular cargo depends on the time that the cargo spends pausing. As neurofilament proteins move at a faster transport rate when axons are devoid of pre-existing neurofilament structures in!vivo, one of the key determinants that curbs the axonal transport of cytoskeleton components is the density of the stationary cytoskeletal network in the axons5. The transport of mitochondria and lysosomes is also dependent on cytoskeletal organization6.

For convenience, axonal transport can be divided into two categories: fast axonal transport, which is responsible for moving membrane-bound organelles (vesicles and mitochondria), and slow axonal trans-port, which drives the movement of cytoplasmic pro-teins (including various enzymes) and cytoskeletal

NeurofilamentsNeurofilaments are components of the neuronal cytoskeleton. They are intermediate filaments with a diameter of 10 nm and are composed of three subunits: the neurofilament light, medium and heavy chains.

Axonal transport deficits and neurodegenerative diseasesStéphanie Millecamps1 and Jean-Pierre Julien2

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

1Centre de Recherche de l’Institut du Cerveau et de la Moelle épinière, INSERM UMR_S975, CNRS UMR7225, Université Pierre et Marie Curie, Hôpital de la Pitié-Salpêtrière, 47–83 boulevard de l’Hôpital, 75013 Paris, France.2Centre de Recherche du Centre Hospitalier Universitaire de Québec, Department of Psychiatry and Neuroscience, Laval University, 2705 Boulevard Laurier, Quebec, Quebec City, G1V4G2, Canada.Correspondence to J.-P.J.! e-mail: [email protected]:10.1038/nrn3380Published online 30 January 2013

!"#$"%&

NATURE REVIEWS | !"#$%&'("!'") VOLUME 14 | MARCH 2013 | 343

© 2013 Macmillan Publishers Limited. All rights reserved

Axonal transport is an essential process in neurons because of the extreme polarity and size of these cells. Indeed, despite having axons of more than 1 metre in length, human spinal motor neurons, like other types of neurons, require efficient communication between their cell body and axon tip. Axonal transport keeps axons and nerve terminals supplied with proteins, lipids and mitochondria, and clears recycled or mis-folded proteins to avoid the build-up of toxic aggre-gates1. Apart from its role in neuronal metabolism, axonal transport is crucial for intracellular neural transmission and allows the neuron to respond effectively to trophic signals or stress insults1.

Impairment of axonal transport has recently emerged as a common factor in several neurodegenerative disorders1. Here, we review the current state of knowl-edge about axonal transport defects that are associated with such disorders, with a specific focus on the mecha-nisms that can affect microtubule-based axonal transport. Before doing so, we outline the various components and mechanisms that control such transport.

!"#$%&'(')*+(,-*./,0%1,)/&$,1-2%$&Microtubules are the main component of the cytoskeleton. They have a tubular structure (25 nm in diameter) and are composed of many !- and "-tubulin heterodimers, which undergo continuous polymerization and depo-lymerization at the centrosome1. Microtubules are polar-ized in axons (but not in dendrites): their slower growing minus end (at which !-tubulin is exposed) faces the cell body, whereas their faster growing plus end (at which "-tubulin is exposed) points towards the axon tips. They

are stabilized by microtubule-associated proteins such as tau. Microtubules in the axon essentially form tracks along which various cargoes can be transported by various motor proteins.

The various cargoes that are transported along micro-tubules in axons (TABLE!1) move in a saltatory fashion, exhibiting periods of rapid movements, pauses and directional switches. Filamentous cargoes, such as neuro-filaments, exhibit long periods of rest (spending on aver-age 73% of the time pausing) and movements mainly in an anterograde direction (that is, towards the cell body) at 0.23 #m per second2,3. By contrast, vesicular cargoes, such as lysosomes, show frequent pausing and direc-tional switches, and other vesicular structures such as autophagosomes exhibit persistent movements (they only pause 12% of the time) in a mainly retrograde direction (that is, away from the cell body) at 0.46 #m per second4. Thus, the average transport velocity of a particular cargo depends on the time that the cargo spends pausing. As neurofilament proteins move at a faster transport rate when axons are devoid of pre-existing neurofilament structures in!vivo, one of the key determinants that curbs the axonal transport of cytoskeleton components is the density of the stationary cytoskeletal network in the axons5. The transport of mitochondria and lysosomes is also dependent on cytoskeletal organization6.

For convenience, axonal transport can be divided into two categories: fast axonal transport, which is responsible for moving membrane-bound organelles (vesicles and mitochondria), and slow axonal trans-port, which drives the movement of cytoplasmic pro-teins (including various enzymes) and cytoskeletal

NeurofilamentsNeurofilaments are components of the neuronal cytoskeleton. They are intermediate filaments with a diameter of 10 nm and are composed of three subunits: the neurofilament light, medium and heavy chains.

Axonal transport deficits and neurodegenerative diseasesStéphanie Millecamps1 and Jean-Pierre Julien2

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

1Centre de Recherche de l’Institut du Cerveau et de la Moelle épinière, INSERM UMR_S975, CNRS UMR7225, Université Pierre et Marie Curie, Hôpital de la Pitié-Salpêtrière, 47–83 boulevard de l’Hôpital, 75013 Paris, France.2Centre de Recherche du Centre Hospitalier Universitaire de Québec, Department of Psychiatry and Neuroscience, Laval University, 2705 Boulevard Laurier, Quebec, Quebec City, G1V4G2, Canada.Correspondence to J.-P.J.! e-mail: [email protected]:10.1038/nrn3380Published online 30 January 2013

!"#$"%&

NATURE REVIEWS | !"#$%&'("!'") VOLUME 14 | MARCH 2013 | 343

© 2013 Macmillan Publishers Limited. All rights reserved

R E V I EW S

NATURE REVIEWS | NEUROSCIENCE ADVANCE ONLINE PUBLICATION | 1

Nature Reviews Neuroscience | AOP, published online 15 February 2005; doi:10.1038/nrn1624

A neuron has a highly polarized structure.A typical neu-ron comprises a cell body, several short, thick, taperingdendrites and one long, thin axon. Most of the proteinsthat are needed in the axon and synaptic terminals aresynthesized in the cell body and transported along theaxon in membranous organelles or protein complexes1.Most dendritic proteins are also transported from the cellbody, but several specific mRNAs are transported intodendrites to support local protein synthesis2 (BOX 1).

In the axon and dendrites, microtubules run in alongitudinal orientation3,4, and serve as rails alongwhich membranous organelles and macromolecularcomplexes can be transported5. A microtubule is a long,hollow cylinder that is made of a polymer of !- and "-tubulins and has a diameter of 25 nm. It has intrinsicpolarity, with a fast-growing ‘plus end’ and an opposite,slow-growing ‘minus end’6. Microtubules in axons anddistal dendrites are unipolar, with the plus end pointingaway from the cell body7,8. However, the microtubulesin proximal dendrites are of mixed polarity8. The orga-nization of microtubules also differs between axons anddendrites (BOX 2).

MOLECULAR MOTORS of the kinesin and dynein super-families move along microtubules. Many kinesinsuperfamily proteins (KIFs) move towards the plusend of microtubules (‘plus-end-directed motors’) and

participate in ANTEROGRADE TRANSPORT, selectively trans-porting molecules from the cell body to axons anddendrites. By contrast, RETROGRADE TRANSPORT, from theaxonal or dendritic terminals to the cell body, is car-ried out mostly by cytoplasmic dyneins, which areminus-end-directed motors5,9–12.

Selective transport to axons and dendrites has beenstudied from several viewpoints, including whichsequences of selectively transported proteins function asselective targeting signals, and whether the basic mecha-nism is one of selective transport or selective retention(whereby cargoes would be transported to both axonsand dendrites, and selectively eliminated by endocytosisfrom the inappropriate destination). However, manyseemingly unrelated sequences have been identified astargeting signals, and the identification of the targetingsequences of specific proteins has not always clarified theunderlying sorting mechanisms. Both selective transportand selective retention seem to occur, depending on thecargoes involved, but it is not clear how some cargoes aretransported selectively, whereas others are transportednonselectively. Understanding the mechanisms of sort-ing, selective transport and recognition is an importantendeavour. This review focuses on recent developmentsthat relate to the mechanisms of selective transport, withparticular emphasis on the role of KIFs.

MOLECULAR MOTORS ANDMECHANISMS OF DIRECTIONALTRANSPORT IN NEURONSNobutaka Hirokawa* and Reiko Takemura‡

Abstract | Intracellular transport is fundamental for neuronal morphogenesis, function and survival.Many proteins are selectively transported to either axons or dendrites. In addition, some specificmRNAs are transported to dendrites for local translation. Proteins of the kinesin superfamilyparticipate in selective transport by using adaptor or scaffolding proteins to recognize and bindcargoes. The molecular components of RNA-transporting granules have been identified, and it isbecoming clear how cargoes are directed to axons and dendrites by kinesin superfamily proteins.Here we discuss the molecular mechanisms of directional axonal and dendritic transport withspecific emphasis on the role of motor proteins and their mechanisms of cargo recognition.

MOLECULAR MOTOR

SUPERFAMILIES

Kinesin and dynein superfamilyproteins move alongmicrotubules, and myosinsuperfamily proteins movealong actin filaments by ATPhydrolysis.

*Department of Cell Biologyand Anatomy, GraduateSchool of Medicine,University of Tokyo,Hongo 7-3-1, Bunkyo-ku,Tokyo 113-0033, Japan.‡Okinaka MemorialInstitute for MedicalResearch, Toranomon 2-2-2,Minato-ku, Tokyo 105-8470,Japan.Correspondence to N.H.e-mail: [email protected]:10.1038/nrn1624Published online 15 February 2005

©!!""#!Nature Publishing Group!

!

R E V I EW S

NATURE REVIEWS ! NEUROSCIENCE VOLUME 6 ! MARCH 2005 ! 201

A neuron has a highly polarized structure.A typical neu-ron comprises a cell body, several short, thick, taperingdendrites and one long, thin axon. Most of the proteinsthat are needed in the axon and synaptic terminals aresynthesized in the cell body and transported along theaxon in membranous organelles or protein complexes1.Most dendritic proteins are also transported from the cellbody, but several specific mRNAs are transported intodendrites to support local protein synthesis2 (BOX 1).

In the axon and dendrites, microtubules run in alongitudinal orientation3,4, and serve as rails alongwhich membranous organelles and macromolecularcomplexes can be transported5. A microtubule is a long,hollow cylinder that is made of a polymer of !- and "-tubulins and has a diameter of 25 nm. It has intrinsicpolarity, with a fast-growing ‘plus end’ and an opposite,slow-growing ‘minus end’6. Microtubules in axons anddistal dendrites are unipolar, with the plus end pointingaway from the cell body7,8. However, the microtubulesin proximal dendrites are of mixed polarity8. The orga-nization of microtubules also differs between axons anddendrites (BOX 2).

MOLECULAR MOTORS of the kinesin and dynein super-families move along microtubules. Many kinesinsuperfamily proteins (KIFs) move towards the plusend of microtubules (‘plus-end-directed motors’) and

participate in ANTEROGRADE TRANSPORT, selectively trans-porting molecules from the cell body to axons anddendrites. By contrast, RETROGRADE TRANSPORT, from theaxonal or dendritic terminals to the cell body, is car-ried out mostly by cytoplasmic dyneins, which areminus-end-directed motors5,9–12.

Selective transport to axons and dendrites has beenstudied from several viewpoints, including whichsequences of selectively transported proteins function asselective targeting signals, and whether the basic mecha-nism is one of selective transport or selective retention(whereby cargoes would be transported to both axonsand dendrites, and selectively eliminated by endocytosisfrom the inappropriate destination). However, manyseemingly unrelated sequences have been identified astargeting signals, and the identification of the targetingsequences of specific proteins has not always clarified theunderlying sorting mechanisms. Both selective transportand selective retention seem to occur, depending on thecargoes involved, but it is not clear how some cargoes aretransported selectively, whereas others are transportednonselectively. Understanding the mechanisms of sort-ing, selective transport and recognition is an importantendeavour. This review focuses on recent developmentsthat relate to the mechanisms of selective transport, withparticular emphasis on the role of KIFs.

MOLECULAR MOTORS ANDMECHANISMS OF DIRECTIONALTRANSPORT IN NEURONSNobutaka Hirokawa* and Reiko Takemura‡

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

MOLECULAR MOTOR

SUPERFAMILIES

Kinesin and dynein superfamilyproteins move alongmicrotubules, and myosinsuperfamily proteins movealong actin filaments by ATPhydrolysis.

*Department of Cell Biologyand Anatomy, GraduateSchool of Medicine,University of Tokyo,Hongo 7-3-1, Bunkyo-ku,Tokyo 113-0033, Japan.‡Okinaka MemorialInstitute for MedicalResearch, Toranomon 2-2-2,Minato-ku, Tokyo 105-8470,Japan.Correspondence to N.H.e-mail: [email protected]:10.1038/nrn1624Published online 15 February 2005

R E V I EW S

NATURE REVIEWS | NEUROSCIENCE ADVANCE ONLINE PUBLICATION | 1

Nature Reviews Neuroscience | AOP, published online 15 February 2005; doi:10.1038/nrn1624

A neuron has a highly polarized structure.A typical neu-ron comprises a cell body, several short, thick, taperingdendrites and one long, thin axon. Most of the proteinsthat are needed in the axon and synaptic terminals aresynthesized in the cell body and transported along theaxon in membranous organelles or protein complexes1.Most dendritic proteins are also transported from the cellbody, but several specific mRNAs are transported intodendrites to support local protein synthesis2 (BOX 1).

In the axon and dendrites, microtubules run in alongitudinal orientation3,4, and serve as rails alongwhich membranous organelles and macromolecularcomplexes can be transported5. A microtubule is a long,hollow cylinder that is made of a polymer of !- and "-tubulins and has a diameter of 25 nm. It has intrinsicpolarity, with a fast-growing ‘plus end’ and an opposite,slow-growing ‘minus end’6. Microtubules in axons anddistal dendrites are unipolar, with the plus end pointingaway from the cell body7,8. However, the microtubulesin proximal dendrites are of mixed polarity8. The orga-nization of microtubules also differs between axons anddendrites (BOX 2).

MOLECULAR MOTORS of the kinesin and dynein super-families move along microtubules. Many kinesinsuperfamily proteins (KIFs) move towards the plusend of microtubules (‘plus-end-directed motors’) and

participate in ANTEROGRADE TRANSPORT, selectively trans-porting molecules from the cell body to axons anddendrites. By contrast, RETROGRADE TRANSPORT, from theaxonal or dendritic terminals to the cell body, is car-ried out mostly by cytoplasmic dyneins, which areminus-end-directed motors5,9–12.

Selective transport to axons and dendrites has beenstudied from several viewpoints, including whichsequences of selectively transported proteins function asselective targeting signals, and whether the basic mecha-nism is one of selective transport or selective retention(whereby cargoes would be transported to both axonsand dendrites, and selectively eliminated by endocytosisfrom the inappropriate destination). However, manyseemingly unrelated sequences have been identified astargeting signals, and the identification of the targetingsequences of specific proteins has not always clarified theunderlying sorting mechanisms. Both selective transportand selective retention seem to occur, depending on thecargoes involved, but it is not clear how some cargoes aretransported selectively, whereas others are transportednonselectively. Understanding the mechanisms of sort-ing, selective transport and recognition is an importantendeavour. This review focuses on recent developmentsthat relate to the mechanisms of selective transport, withparticular emphasis on the role of KIFs.

MOLECULAR MOTORS ANDMECHANISMS OF DIRECTIONALTRANSPORT IN NEURONSNobutaka Hirokawa* and Reiko Takemura‡

Abstract | Intracellular transport is fundamental for neuronal morphogenesis, function and survival.Many proteins are selectively transported to either axons or dendrites. In addition, some specificmRNAs are transported to dendrites for local translation. Proteins of the kinesin superfamilyparticipate in selective transport by using adaptor or scaffolding proteins to recognize and bindcargoes. The molecular components of RNA-transporting granules have been identified, and it isbecoming clear how cargoes are directed to axons and dendrites by kinesin superfamily proteins.Here we discuss the molecular mechanisms of directional axonal and dendritic transport withspecific emphasis on the role of motor proteins and their mechanisms of cargo recognition.

MOLECULAR MOTOR

SUPERFAMILIES

Kinesin and dynein superfamilyproteins move alongmicrotubules, and myosinsuperfamily proteins movealong actin filaments by ATPhydrolysis.

*Department of Cell Biologyand Anatomy, GraduateSchool of Medicine,University of Tokyo,Hongo 7-3-1, Bunkyo-ku,Tokyo 113-0033, Japan.‡Okinaka MemorialInstitute for MedicalResearch, Toranomon 2-2-2,Minato-ku, Tokyo 105-8470,Japan.Correspondence to N.H.e-mail: [email protected]:10.1038/nrn1624Published online 15 February 2005

Intracellular transportSoma

AxonSynapticterminal

Organelle transport

Axonal transport is an essential process in neurons because of the extreme polarity and size of these cells. Indeed, despite having axons of more than 1 metre in length, human spinal motor neurons, like other types of neurons, require efficient communication between their cell body and axon tip. Axonal transport keeps axons and nerve terminals supplied with proteins, lipids and mitochondria, and clears recycled or mis-folded proteins to avoid the build-up of toxic aggre-gates1. Apart from its role in neuronal metabolism, axonal transport is crucial for intracellular neural transmission and allows the neuron to respond effectively to trophic signals or stress insults1.

Impairment of axonal transport has recently emerged as a common factor in several neurodegenerative disorders1. Here, we review the current state of knowl-edge about axonal transport defects that are associated with such disorders, with a specific focus on the mecha-nisms that can affect microtubule-based axonal transport. Before doing so, we outline the various components and mechanisms that control such transport.

!"#$%&'(')*+(,-*./,0%1,)/&$,1-2%$&Microtubules are the main component of the cytoskeleton. They have a tubular structure (25 nm in diameter) and are composed of many !- and "-tubulin heterodimers, which undergo continuous polymerization and depo-lymerization at the centrosome1. Microtubules are polar-ized in axons (but not in dendrites): their slower growing minus end (at which !-tubulin is exposed) faces the cell body, whereas their faster growing plus end (at which "-tubulin is exposed) points towards the axon tips. They

are stabilized by microtubule-associated proteins such as tau. Microtubules in the axon essentially form tracks along which various cargoes can be transported by various motor proteins.

The various cargoes that are transported along micro-tubules in axons (TABLE!1) move in a saltatory fashion, exhibiting periods of rapid movements, pauses and directional switches. Filamentous cargoes, such as neuro-filaments, exhibit long periods of rest (spending on aver-age 73% of the time pausing) and movements mainly in an anterograde direction (that is, towards the cell body) at 0.23 #m per second2,3. By contrast, vesicular cargoes, such as lysosomes, show frequent pausing and direc-tional switches, and other vesicular structures such as autophagosomes exhibit persistent movements (they only pause 12% of the time) in a mainly retrograde direction (that is, away from the cell body) at 0.46 #m per second4. Thus, the average transport velocity of a particular cargo depends on the time that the cargo spends pausing. As neurofilament proteins move at a faster transport rate when axons are devoid of pre-existing neurofilament structures in!vivo, one of the key determinants that curbs the axonal transport of cytoskeleton components is the density of the stationary cytoskeletal network in the axons5. The transport of mitochondria and lysosomes is also dependent on cytoskeletal organization6.

For convenience, axonal transport can be divided into two categories: fast axonal transport, which is responsible for moving membrane-bound organelles (vesicles and mitochondria), and slow axonal trans-port, which drives the movement of cytoplasmic pro-teins (including various enzymes) and cytoskeletal

NeurofilamentsNeurofilaments are components of the neuronal cytoskeleton. They are intermediate filaments with a diameter of 10 nm and are composed of three subunits: the neurofilament light, medium and heavy chains.

Axonal transport deficits and neurodegenerative diseasesStéphanie Millecamps1 and Jean-Pierre Julien2

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

1Centre de Recherche de l’Institut du Cerveau et de la Moelle épinière, INSERM UMR_S975, CNRS UMR7225, Université Pierre et Marie Curie, Hôpital de la Pitié-Salpêtrière, 47–83 boulevard de l’Hôpital, 75013 Paris, France.2Centre de Recherche du Centre Hospitalier Universitaire de Québec, Department of Psychiatry and Neuroscience, Laval University, 2705 Boulevard Laurier, Quebec, Quebec City, G1V4G2, Canada.Correspondence to J.-P.J.! e-mail: [email protected]:10.1038/nrn3380Published online 30 January 2013

!"#$"%&

NATURE REVIEWS | !"#$%&'("!'") VOLUME 14 | MARCH 2013 | 343

© 2013 Macmillan Publishers Limited. All rights reserved

Axonal transport is an essential process in neurons because of the extreme polarity and size of these cells. Indeed, despite having axons of more than 1 metre in length, human spinal motor neurons, like other types of neurons, require efficient communication between their cell body and axon tip. Axonal transport keeps axons and nerve terminals supplied with proteins, lipids and mitochondria, and clears recycled or mis-folded proteins to avoid the build-up of toxic aggre-gates1. Apart from its role in neuronal metabolism, axonal transport is crucial for intracellular neural transmission and allows the neuron to respond effectively to trophic signals or stress insults1.

Impairment of axonal transport has recently emerged as a common factor in several neurodegenerative disorders1. Here, we review the current state of knowl-edge about axonal transport defects that are associated with such disorders, with a specific focus on the mecha-nisms that can affect microtubule-based axonal transport. Before doing so, we outline the various components and mechanisms that control such transport.

!"#$%&'(')*+(,-*./,0%1,)/&$,1-2%$&Microtubules are the main component of the cytoskeleton. They have a tubular structure (25 nm in diameter) and are composed of many !- and "-tubulin heterodimers, which undergo continuous polymerization and depo-lymerization at the centrosome1. Microtubules are polar-ized in axons (but not in dendrites): their slower growing minus end (at which !-tubulin is exposed) faces the cell body, whereas their faster growing plus end (at which "-tubulin is exposed) points towards the axon tips. They

are stabilized by microtubule-associated proteins such as tau. Microtubules in the axon essentially form tracks along which various cargoes can be transported by various motor proteins.

The various cargoes that are transported along micro-tubules in axons (TABLE!1) move in a saltatory fashion, exhibiting periods of rapid movements, pauses and directional switches. Filamentous cargoes, such as neuro-filaments, exhibit long periods of rest (spending on aver-age 73% of the time pausing) and movements mainly in an anterograde direction (that is, towards the cell body) at 0.23 #m per second2,3. By contrast, vesicular cargoes, such as lysosomes, show frequent pausing and direc-tional switches, and other vesicular structures such as autophagosomes exhibit persistent movements (they only pause 12% of the time) in a mainly retrograde direction (that is, away from the cell body) at 0.46 #m per second4. Thus, the average transport velocity of a particular cargo depends on the time that the cargo spends pausing. As neurofilament proteins move at a faster transport rate when axons are devoid of pre-existing neurofilament structures in!vivo, one of the key determinants that curbs the axonal transport of cytoskeleton components is the density of the stationary cytoskeletal network in the axons5. The transport of mitochondria and lysosomes is also dependent on cytoskeletal organization6.

For convenience, axonal transport can be divided into two categories: fast axonal transport, which is responsible for moving membrane-bound organelles (vesicles and mitochondria), and slow axonal trans-port, which drives the movement of cytoplasmic pro-teins (including various enzymes) and cytoskeletal

NeurofilamentsNeurofilaments are components of the neuronal cytoskeleton. They are intermediate filaments with a diameter of 10 nm and are composed of three subunits: the neurofilament light, medium and heavy chains.

Axonal transport deficits and neurodegenerative diseasesStéphanie Millecamps1 and Jean-Pierre Julien2

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

1Centre de Recherche de l’Institut du Cerveau et de la Moelle épinière, INSERM UMR_S975, CNRS UMR7225, Université Pierre et Marie Curie, Hôpital de la Pitié-Salpêtrière, 47–83 boulevard de l’Hôpital, 75013 Paris, France.2Centre de Recherche du Centre Hospitalier Universitaire de Québec, Department of Psychiatry and Neuroscience, Laval University, 2705 Boulevard Laurier, Quebec, Quebec City, G1V4G2, Canada.Correspondence to J.-P.J.! e-mail: [email protected]:10.1038/nrn3380Published online 30 January 2013

!"#$"%&

NATURE REVIEWS | !"#$%&'("!'") VOLUME 14 | MARCH 2013 | 343

© 2013 Macmillan Publishers Limited. All rights reserved

Axonal transport is an essential process in neurons because of the extreme polarity and size of these cells. Indeed, despite having axons of more than 1 metre in length, human spinal motor neurons, like other types of neurons, require efficient communication between their cell body and axon tip. Axonal transport keeps axons and nerve terminals supplied with proteins, lipids and mitochondria, and clears recycled or mis-folded proteins to avoid the build-up of toxic aggre-gates1. Apart from its role in neuronal metabolism, axonal transport is crucial for intracellular neural transmission and allows the neuron to respond effectively to trophic signals or stress insults1.

Impairment of axonal transport has recently emerged as a common factor in several neurodegenerative disorders1. Here, we review the current state of knowl-edge about axonal transport defects that are associated with such disorders, with a specific focus on the mecha-nisms that can affect microtubule-based axonal transport. Before doing so, we outline the various components and mechanisms that control such transport.

!"#$%&'(')*+(,-*./,0%1,)/&$,1-2%$&Microtubules are the main component of the cytoskeleton. They have a tubular structure (25 nm in diameter) and are composed of many !- and "-tubulin heterodimers, which undergo continuous polymerization and depo-lymerization at the centrosome1. Microtubules are polar-ized in axons (but not in dendrites): their slower growing minus end (at which !-tubulin is exposed) faces the cell body, whereas their faster growing plus end (at which "-tubulin is exposed) points towards the axon tips. They

are stabilized by microtubule-associated proteins such as tau. Microtubules in the axon essentially form tracks along which various cargoes can be transported by various motor proteins.

The various cargoes that are transported along micro-tubules in axons (TABLE!1) move in a saltatory fashion, exhibiting periods of rapid movements, pauses and directional switches. Filamentous cargoes, such as neuro-filaments, exhibit long periods of rest (spending on aver-age 73% of the time pausing) and movements mainly in an anterograde direction (that is, towards the cell body) at 0.23 #m per second2,3. By contrast, vesicular cargoes, such as lysosomes, show frequent pausing and direc-tional switches, and other vesicular structures such as autophagosomes exhibit persistent movements (they only pause 12% of the time) in a mainly retrograde direction (that is, away from the cell body) at 0.46 #m per second4. Thus, the average transport velocity of a particular cargo depends on the time that the cargo spends pausing. As neurofilament proteins move at a faster transport rate when axons are devoid of pre-existing neurofilament structures in!vivo, one of the key determinants that curbs the axonal transport of cytoskeleton components is the density of the stationary cytoskeletal network in the axons5. The transport of mitochondria and lysosomes is also dependent on cytoskeletal organization6.

For convenience, axonal transport can be divided into two categories: fast axonal transport, which is responsible for moving membrane-bound organelles (vesicles and mitochondria), and slow axonal trans-port, which drives the movement of cytoplasmic pro-teins (including various enzymes) and cytoskeletal

NeurofilamentsNeurofilaments are components of the neuronal cytoskeleton. They are intermediate filaments with a diameter of 10 nm and are composed of three subunits: the neurofilament light, medium and heavy chains.

Axonal transport deficits and neurodegenerative diseasesStéphanie Millecamps1 and Jean-Pierre Julien2

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

1Centre de Recherche de l’Institut du Cerveau et de la Moelle épinière, INSERM UMR_S975, CNRS UMR7225, Université Pierre et Marie Curie, Hôpital de la Pitié-Salpêtrière, 47–83 boulevard de l’Hôpital, 75013 Paris, France.2Centre de Recherche du Centre Hospitalier Universitaire de Québec, Department of Psychiatry and Neuroscience, Laval University, 2705 Boulevard Laurier, Quebec, Quebec City, G1V4G2, Canada.Correspondence to J.-P.J.! e-mail: [email protected]:10.1038/nrn3380Published online 30 January 2013

!"#$"%&

NATURE REVIEWS | !"#$%&'("!'") VOLUME 14 | MARCH 2013 | 343

© 2013 Macmillan Publishers Limited. All rights reserved

two-headed kinesin

one-headed kinesin

R E V I EW S

NATURE REVIEWS | NEUROSCIENCE ADVANCE ONLINE PUBLICATION | 1

Nature Reviews Neuroscience | AOP, published online 15 February 2005; doi:10.1038/nrn1624

A neuron has a highly polarized structure.A typical neu-ron comprises a cell body, several short, thick, taperingdendrites and one long, thin axon. Most of the proteinsthat are needed in the axon and synaptic terminals aresynthesized in the cell body and transported along theaxon in membranous organelles or protein complexes1.Most dendritic proteins are also transported from the cellbody, but several specific mRNAs are transported intodendrites to support local protein synthesis2 (BOX 1).

In the axon and dendrites, microtubules run in alongitudinal orientation3,4, and serve as rails alongwhich membranous organelles and macromolecularcomplexes can be transported5. A microtubule is a long,hollow cylinder that is made of a polymer of !- and "-tubulins and has a diameter of 25 nm. It has intrinsicpolarity, with a fast-growing ‘plus end’ and an opposite,slow-growing ‘minus end’6. Microtubules in axons anddistal dendrites are unipolar, with the plus end pointingaway from the cell body7,8. However, the microtubulesin proximal dendrites are of mixed polarity8. The orga-nization of microtubules also differs between axons anddendrites (BOX 2).

MOLECULAR MOTORS of the kinesin and dynein super-families move along microtubules. Many kinesinsuperfamily proteins (KIFs) move towards the plusend of microtubules (‘plus-end-directed motors’) and

participate in ANTEROGRADE TRANSPORT, selectively trans-porting molecules from the cell body to axons anddendrites. By contrast, RETROGRADE TRANSPORT, from theaxonal or dendritic terminals to the cell body, is car-ried out mostly by cytoplasmic dyneins, which areminus-end-directed motors5,9–12.

Selective transport to axons and dendrites has beenstudied from several viewpoints, including whichsequences of selectively transported proteins function asselective targeting signals, and whether the basic mecha-nism is one of selective transport or selective retention(whereby cargoes would be transported to both axonsand dendrites, and selectively eliminated by endocytosisfrom the inappropriate destination). However, manyseemingly unrelated sequences have been identified astargeting signals, and the identification of the targetingsequences of specific proteins has not always clarified theunderlying sorting mechanisms. Both selective transportand selective retention seem to occur, depending on thecargoes involved, but it is not clear how some cargoes aretransported selectively, whereas others are transportednonselectively. Understanding the mechanisms of sort-ing, selective transport and recognition is an importantendeavour. This review focuses on recent developmentsthat relate to the mechanisms of selective transport, withparticular emphasis on the role of KIFs.

MOLECULAR MOTORS ANDMECHANISMS OF DIRECTIONALTRANSPORT IN NEURONSNobutaka Hirokawa* and Reiko Takemura‡

Abstract | Intracellular transport is fundamental for neuronal morphogenesis, function and survival.Many proteins are selectively transported to either axons or dendrites. In addition, some specificmRNAs are transported to dendrites for local translation. Proteins of the kinesin superfamilyparticipate in selective transport by using adaptor or scaffolding proteins to recognize and bindcargoes. The molecular components of RNA-transporting granules have been identified, and it isbecoming clear how cargoes are directed to axons and dendrites by kinesin superfamily proteins.Here we discuss the molecular mechanisms of directional axonal and dendritic transport withspecific emphasis on the role of motor proteins and their mechanisms of cargo recognition.

MOLECULAR MOTOR

SUPERFAMILIES

Kinesin and dynein superfamilyproteins move alongmicrotubules, and myosinsuperfamily proteins movealong actin filaments by ATPhydrolysis.

*Department of Cell Biologyand Anatomy, GraduateSchool of Medicine,University of Tokyo,Hongo 7-3-1, Bunkyo-ku,Tokyo 113-0033, Japan.‡Okinaka MemorialInstitute for MedicalResearch, Toranomon 2-2-2,Minato-ku, Tokyo 105-8470,Japan.Correspondence to N.H.e-mail: [email protected]:10.1038/nrn1624Published online 15 February 2005

©!!""#!Nature Publishing Group!

!

R E V I EW S

NATURE REVIEWS ! NEUROSCIENCE VOLUME 6 ! MARCH 2005 ! 201

A neuron has a highly polarized structure.A typical neu-ron comprises a cell body, several short, thick, taperingdendrites and one long, thin axon. Most of the proteinsthat are needed in the axon and synaptic terminals aresynthesized in the cell body and transported along theaxon in membranous organelles or protein complexes1.Most dendritic proteins are also transported from the cellbody, but several specific mRNAs are transported intodendrites to support local protein synthesis2 (BOX 1).

In the axon and dendrites, microtubules run in alongitudinal orientation3,4, and serve as rails alongwhich membranous organelles and macromolecularcomplexes can be transported5. A microtubule is a long,hollow cylinder that is made of a polymer of !- and "-tubulins and has a diameter of 25 nm. It has intrinsicpolarity, with a fast-growing ‘plus end’ and an opposite,slow-growing ‘minus end’6. Microtubules in axons anddistal dendrites are unipolar, with the plus end pointingaway from the cell body7,8. However, the microtubulesin proximal dendrites are of mixed polarity8. The orga-nization of microtubules also differs between axons anddendrites (BOX 2).

MOLECULAR MOTORS of the kinesin and dynein super-families move along microtubules. Many kinesinsuperfamily proteins (KIFs) move towards the plusend of microtubules (‘plus-end-directed motors’) and

participate in ANTEROGRADE TRANSPORT, selectively trans-porting molecules from the cell body to axons anddendrites. By contrast, RETROGRADE TRANSPORT, from theaxonal or dendritic terminals to the cell body, is car-ried out mostly by cytoplasmic dyneins, which areminus-end-directed motors5,9–12.

Selective transport to axons and dendrites has beenstudied from several viewpoints, including whichsequences of selectively transported proteins function asselective targeting signals, and whether the basic mecha-nism is one of selective transport or selective retention(whereby cargoes would be transported to both axonsand dendrites, and selectively eliminated by endocytosisfrom the inappropriate destination). However, manyseemingly unrelated sequences have been identified astargeting signals, and the identification of the targetingsequences of specific proteins has not always clarified theunderlying sorting mechanisms. Both selective transportand selective retention seem to occur, depending on thecargoes involved, but it is not clear how some cargoes aretransported selectively, whereas others are transportednonselectively. Understanding the mechanisms of sort-ing, selective transport and recognition is an importantendeavour. This review focuses on recent developmentsthat relate to the mechanisms of selective transport, withparticular emphasis on the role of KIFs.

MOLECULAR MOTORS ANDMECHANISMS OF DIRECTIONALTRANSPORT IN NEURONSNobutaka Hirokawa* and Reiko Takemura‡

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

MOLECULAR MOTOR

SUPERFAMILIES

Kinesin and dynein superfamilyproteins move alongmicrotubules, and myosinsuperfamily proteins movealong actin filaments by ATPhydrolysis.

*Department of Cell Biologyand Anatomy, GraduateSchool of Medicine,University of Tokyo,Hongo 7-3-1, Bunkyo-ku,Tokyo 113-0033, Japan.‡Okinaka MemorialInstitute for MedicalResearch, Toranomon 2-2-2,Minato-ku, Tokyo 105-8470,Japan.Correspondence to N.H.e-mail: [email protected]:10.1038/nrn1624Published online 15 February 2005

R E V I EW S

NATURE REVIEWS | NEUROSCIENCE ADVANCE ONLINE PUBLICATION | 1

Nature Reviews Neuroscience | AOP, published online 15 February 2005; doi:10.1038/nrn1624

A neuron has a highly polarized structure.A typical neu-ron comprises a cell body, several short, thick, taperingdendrites and one long, thin axon. Most of the proteinsthat are needed in the axon and synaptic terminals aresynthesized in the cell body and transported along theaxon in membranous organelles or protein complexes1.Most dendritic proteins are also transported from the cellbody, but several specific mRNAs are transported intodendrites to support local protein synthesis2 (BOX 1).

In the axon and dendrites, microtubules run in alongitudinal orientation3,4, and serve as rails alongwhich membranous organelles and macromolecularcomplexes can be transported5. A microtubule is a long,hollow cylinder that is made of a polymer of !- and "-tubulins and has a diameter of 25 nm. It has intrinsicpolarity, with a fast-growing ‘plus end’ and an opposite,slow-growing ‘minus end’6. Microtubules in axons anddistal dendrites are unipolar, with the plus end pointingaway from the cell body7,8. However, the microtubulesin proximal dendrites are of mixed polarity8. The orga-nization of microtubules also differs between axons anddendrites (BOX 2).

MOLECULAR MOTORS of the kinesin and dynein super-families move along microtubules. Many kinesinsuperfamily proteins (KIFs) move towards the plusend of microtubules (‘plus-end-directed motors’) and

participate in ANTEROGRADE TRANSPORT, selectively trans-porting molecules from the cell body to axons anddendrites. By contrast, RETROGRADE TRANSPORT, from theaxonal or dendritic terminals to the cell body, is car-ried out mostly by cytoplasmic dyneins, which areminus-end-directed motors5,9–12.

Selective transport to axons and dendrites has beenstudied from several viewpoints, including whichsequences of selectively transported proteins function asselective targeting signals, and whether the basic mecha-nism is one of selective transport or selective retention(whereby cargoes would be transported to both axonsand dendrites, and selectively eliminated by endocytosisfrom the inappropriate destination). However, manyseemingly unrelated sequences have been identified astargeting signals, and the identification of the targetingsequences of specific proteins has not always clarified theunderlying sorting mechanisms. Both selective transportand selective retention seem to occur, depending on thecargoes involved, but it is not clear how some cargoes aretransported selectively, whereas others are transportednonselectively. Understanding the mechanisms of sort-ing, selective transport and recognition is an importantendeavour. This review focuses on recent developmentsthat relate to the mechanisms of selective transport, withparticular emphasis on the role of KIFs.

MOLECULAR MOTORS ANDMECHANISMS OF DIRECTIONALTRANSPORT IN NEURONSNobutaka Hirokawa* and Reiko Takemura‡

Abstract | Intracellular transport is fundamental for neuronal morphogenesis, function and survival.Many proteins are selectively transported to either axons or dendrites. In addition, some specificmRNAs are transported to dendrites for local translation. Proteins of the kinesin superfamilyparticipate in selective transport by using adaptor or scaffolding proteins to recognize and bindcargoes. The molecular components of RNA-transporting granules have been identified, and it isbecoming clear how cargoes are directed to axons and dendrites by kinesin superfamily proteins.Here we discuss the molecular mechanisms of directional axonal and dendritic transport withspecific emphasis on the role of motor proteins and their mechanisms of cargo recognition.

MOLECULAR MOTOR

SUPERFAMILIES

Kinesin and dynein superfamilyproteins move alongmicrotubules, and myosinsuperfamily proteins movealong actin filaments by ATPhydrolysis.

*Department of Cell Biologyand Anatomy, GraduateSchool of Medicine,University of Tokyo,Hongo 7-3-1, Bunkyo-ku,Tokyo 113-0033, Japan.‡Okinaka MemorialInstitute for MedicalResearch, Toranomon 2-2-2,Minato-ku, Tokyo 105-8470,Japan.Correspondence to N.H.e-mail: [email protected]:10.1038/nrn1624Published online 15 February 2005

Intracellular transportSoma

AxonSynapticterminal

Organelle transport

Axonal transport is an essential process in neurons because of the extreme polarity and size of these cells. Indeed, despite having axons of more than 1 metre in length, human spinal motor neurons, like other types of neurons, require efficient communication between their cell body and axon tip. Axonal transport keeps axons and nerve terminals supplied with proteins, lipids and mitochondria, and clears recycled or mis-folded proteins to avoid the build-up of toxic aggre-gates1. Apart from its role in neuronal metabolism, axonal transport is crucial for intracellular neural transmission and allows the neuron to respond effectively to trophic signals or stress insults1.

Impairment of axonal transport has recently emerged as a common factor in several neurodegenerative disorders1. Here, we review the current state of knowl-edge about axonal transport defects that are associated with such disorders, with a specific focus on the mecha-nisms that can affect microtubule-based axonal transport. Before doing so, we outline the various components and mechanisms that control such transport.

!"#$%&'(')*+(,-*./,0%1,)/&$,1-2%$&Microtubules are the main component of the cytoskeleton. They have a tubular structure (25 nm in diameter) and are composed of many !- and "-tubulin heterodimers, which undergo continuous polymerization and depo-lymerization at the centrosome1. Microtubules are polar-ized in axons (but not in dendrites): their slower growing minus end (at which !-tubulin is exposed) faces the cell body, whereas their faster growing plus end (at which "-tubulin is exposed) points towards the axon tips. They

are stabilized by microtubule-associated proteins such as tau. Microtubules in the axon essentially form tracks along which various cargoes can be transported by various motor proteins.

The various cargoes that are transported along micro-tubules in axons (TABLE!1) move in a saltatory fashion, exhibiting periods of rapid movements, pauses and directional switches. Filamentous cargoes, such as neuro-filaments, exhibit long periods of rest (spending on aver-age 73% of the time pausing) and movements mainly in an anterograde direction (that is, towards the cell body) at 0.23 #m per second2,3. By contrast, vesicular cargoes, such as lysosomes, show frequent pausing and direc-tional switches, and other vesicular structures such as autophagosomes exhibit persistent movements (they only pause 12% of the time) in a mainly retrograde direction (that is, away from the cell body) at 0.46 #m per second4. Thus, the average transport velocity of a particular cargo depends on the time that the cargo spends pausing. As neurofilament proteins move at a faster transport rate when axons are devoid of pre-existing neurofilament structures in!vivo, one of the key determinants that curbs the axonal transport of cytoskeleton components is the density of the stationary cytoskeletal network in the axons5. The transport of mitochondria and lysosomes is also dependent on cytoskeletal organization6.

For convenience, axonal transport can be divided into two categories: fast axonal transport, which is responsible for moving membrane-bound organelles (vesicles and mitochondria), and slow axonal trans-port, which drives the movement of cytoplasmic pro-teins (including various enzymes) and cytoskeletal

NeurofilamentsNeurofilaments are components of the neuronal cytoskeleton. They are intermediate filaments with a diameter of 10 nm and are composed of three subunits: the neurofilament light, medium and heavy chains.

Axonal transport deficits and neurodegenerative diseasesStéphanie Millecamps1 and Jean-Pierre Julien2

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

1Centre de Recherche de l’Institut du Cerveau et de la Moelle épinière, INSERM UMR_S975, CNRS UMR7225, Université Pierre et Marie Curie, Hôpital de la Pitié-Salpêtrière, 47–83 boulevard de l’Hôpital, 75013 Paris, France.2Centre de Recherche du Centre Hospitalier Universitaire de Québec, Department of Psychiatry and Neuroscience, Laval University, 2705 Boulevard Laurier, Quebec, Quebec City, G1V4G2, Canada.Correspondence to J.-P.J.! e-mail: [email protected]:10.1038/nrn3380Published online 30 January 2013

!"#$"%&

NATURE REVIEWS | !"#$%&'("!'") VOLUME 14 | MARCH 2013 | 343

© 2013 Macmillan Publishers Limited. All rights reserved

Axonal transport is an essential process in neurons because of the extreme polarity and size of these cells. Indeed, despite having axons of more than 1 metre in length, human spinal motor neurons, like other types of neurons, require efficient communication between their cell body and axon tip. Axonal transport keeps axons and nerve terminals supplied with proteins, lipids and mitochondria, and clears recycled or mis-folded proteins to avoid the build-up of toxic aggre-gates1. Apart from its role in neuronal metabolism, axonal transport is crucial for intracellular neural transmission and allows the neuron to respond effectively to trophic signals or stress insults1.

Impairment of axonal transport has recently emerged as a common factor in several neurodegenerative disorders1. Here, we review the current state of knowl-edge about axonal transport defects that are associated with such disorders, with a specific focus on the mecha-nisms that can affect microtubule-based axonal transport. Before doing so, we outline the various components and mechanisms that control such transport.

!"#$%&'(')*+(,-*./,0%1,)/&$,1-2%$&Microtubules are the main component of the cytoskeleton. They have a tubular structure (25 nm in diameter) and are composed of many !- and "-tubulin heterodimers, which undergo continuous polymerization and depo-lymerization at the centrosome1. Microtubules are polar-ized in axons (but not in dendrites): their slower growing minus end (at which !-tubulin is exposed) faces the cell body, whereas their faster growing plus end (at which "-tubulin is exposed) points towards the axon tips. They

are stabilized by microtubule-associated proteins such as tau. Microtubules in the axon essentially form tracks along which various cargoes can be transported by various motor proteins.

The various cargoes that are transported along micro-tubules in axons (TABLE!1) move in a saltatory fashion, exhibiting periods of rapid movements, pauses and directional switches. Filamentous cargoes, such as neuro-filaments, exhibit long periods of rest (spending on aver-age 73% of the time pausing) and movements mainly in an anterograde direction (that is, towards the cell body) at 0.23 #m per second2,3. By contrast, vesicular cargoes, such as lysosomes, show frequent pausing and direc-tional switches, and other vesicular structures such as autophagosomes exhibit persistent movements (they only pause 12% of the time) in a mainly retrograde direction (that is, away from the cell body) at 0.46 #m per second4. Thus, the average transport velocity of a particular cargo depends on the time that the cargo spends pausing. As neurofilament proteins move at a faster transport rate when axons are devoid of pre-existing neurofilament structures in!vivo, one of the key determinants that curbs the axonal transport of cytoskeleton components is the density of the stationary cytoskeletal network in the axons5. The transport of mitochondria and lysosomes is also dependent on cytoskeletal organization6.

For convenience, axonal transport can be divided into two categories: fast axonal transport, which is responsible for moving membrane-bound organelles (vesicles and mitochondria), and slow axonal trans-port, which drives the movement of cytoplasmic pro-teins (including various enzymes) and cytoskeletal

NeurofilamentsNeurofilaments are components of the neuronal cytoskeleton. They are intermediate filaments with a diameter of 10 nm and are composed of three subunits: the neurofilament light, medium and heavy chains.

Axonal transport deficits and neurodegenerative diseasesStéphanie Millecamps1 and Jean-Pierre Julien2

!"#$%&'$()(*+,(-.$%&',//0/&%($%&.#12%$(23(2%4&.,//,#(&/2.4(&.(&52.(-#('%0'-&/(32%($+,(6&-.$,.&.',(&.7(30.'$-2.(23(&(.,0%2.8(!.$,%24%&7,(&52.&/($%&.#12%$(+&#(&(%2/,(-.(#011/9-.4(1%2$,-.#(&.7(/-1-7#($2($+,(7-#$&/(#9.&1#,(&.7(6-$2'+2.7%-&(32%(/2'&/(,.,%49(%,:0-%,6,.$#;(<+,%,&#(%,$%24%&7,($%&.#12%$(-#(-.=2/=,7(-.($+,('/,&%&.',(23(6-#32/7,7(&.7(&44%,4&$,7(1%2$,-.#(3%26($+,(&52.(&.7($+,(-.$%&',//0/&%($%&.#12%$(23(7-#$&/($%21+-'(#-4.&/#($2($+,(#26&8(!52.&/($%&.#12%$('&.(",(&33,'$,7("9(&/$,%&$-2.#($2(=&%-20#('2612.,.$#(23($+,($%&.#12%$(6&'+-.,%98(>,%,;(<,(%,=-,<($+,('0%%,.$(#$&$,(23(?.2</,74,(&"20$(&52.&/($%&.#12%$(7,3,'$#($+&$(6-4+$('2.$%-"0$,($2($+,(1&$+24,.,#-#(23(1&%$-'0/&%(.,0%27,4,.,%&$-=,(7-#,&#,#8

1Centre de Recherche de l’Institut du Cerveau et de la Moelle épinière, INSERM UMR_S975, CNRS UMR7225, Université Pierre et Marie Curie, Hôpital de la Pitié-Salpêtrière, 47–83 boulevard de l’Hôpital, 75013 Paris, France.2Centre de Recherche du Centre Hospitalier Universitaire de Québec, Department of Psychiatry and Neuroscience, Laval University, 2705 Boulevard Laurier, Quebec, Quebec City, G1V4G2, Canada.Correspondence to J.-P.J.! e-mail: [email protected]:10.1038/nrn3380Published online 30 January 2013

!"#$"%&

NATURE REVIEWS | !"#$%&'("!'") VOLUME 14 | MARCH 2013 | 343

© 2013 Macmillan Publishers Limited. All rights reserved

Axonal transport is an essential process in neurons because of the extreme polarity and size of these cells. Indeed, despite having axons of more than 1 metre in length, human spinal motor neurons, like other types of neurons, require efficient communication between their cell body and axon tip. Axonal transport keeps axons and nerve terminals supplied with proteins, lipids and mitochondria, and clears recycled or mis-folded proteins to avoid the build-up of toxic aggre-gates1. Apart from its role in neuronal metabolism, axonal transport is crucial for intracellular neural transmission and allows the neuron to respond effectively to trophic signals or stress insults1.

Impairment of axonal transport has recently emerged as a common factor in several neurodegenerative disorders1. Here, we review the current state of knowl-edge about axonal transport defects that are associated with such disorders, with a specific focus on the mecha-nisms that can affect microtubule-based axonal transport. Before doing so, we outline the various components and mechanisms that control such transport.

!"#$%&'(')*+(,-*./,0%1,)/&$,1-2%$&Microtubules are the main component of the cytoskeleton. They have a tubular structure (25 nm in diameter) and are composed of many !- and "-tubulin heterodimers, which undergo continuous polymerization and depo-lymerization at the centrosome1. Microtubules are polar-ized in axons (but not in dendrites): their slower growing minus end (at which !-tubulin is exposed) faces the cell body, whereas their faster growing plus end (at which "-tubulin is exposed) points towards the axon tips. They

are stabilized by microtubule-associated proteins such as tau. Microtubules in the axon essentially form tracks along which various cargoes can be transported by various motor proteins.

The various cargoes that are transported along micro-tubules in axons (TABLE!1) move in a saltatory fashion, exhibiting periods of rapid movements, pauses and directional switches. Filamentous cargoes, such as neuro-filaments, exhibit long periods of rest (spending on aver-age 73% of the time pausing) and movements mainly in an anterograde direction (that is, towards the cell body) at 0.23 #m per second2,3. By contrast, vesicular cargoes, such as lysosomes, show frequent pausing and direc-tional switches, and other vesicular structures such as autophagosomes exhibit persistent movements (they only pause 12% of the time) in a mainly retrograde direction (that is, away from the cell body) at 0.46 #m per second4. Thus, the average transport velocity of a particular cargo depends on the time that the cargo spends pausing. As neurofilament proteins move at a faster transport rate when axons are devoid of pre-existing neurofilament structures in!vivo, one of the key determinants that curbs the axonal transport of cytoskeleton components is the density of the stationary cytoskeletal network in the axons5. The transport of mitochondria and lysosomes is also dependent on cytoskeletal organization6.

For convenience, axonal transport can be divided into two categories: fast axonal transport, which is responsible for moving membrane-bound organelles (vesicles and mitochondria), and slow axonal trans-port, which drives the movement of cytoplasmic pro-teins (including various enzymes) and cytoskeletal

NeurofilamentsNeurofilaments are components of the neuronal cytoskeleton. They are intermediate filaments with a diameter of 10 nm and are composed of three subunits: the neurofilament light, medium and heavy chains.

Axonal transport deficits and neurodegenerative diseasesStéphanie Millecamps1 and Jean-Pierre Julien2

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

1Centre de Recherche de l’Institut du Cerveau et de la Moelle épinière, INSERM UMR_S975, CNRS UMR7225, Université Pierre et Marie Curie, Hôpital de la Pitié-Salpêtrière, 47–83 boulevard de l’Hôpital, 75013 Paris, France.2Centre de Recherche du Centre Hospitalier Universitaire de Québec, Department of Psychiatry and Neuroscience, Laval University, 2705 Boulevard Laurier, Quebec, Quebec City, G1V4G2, Canada.Correspondence to J.-P.J.! e-mail: [email protected]:10.1038/nrn3380Published online 30 January 2013

!"#$"%&

NATURE REVIEWS | !"#$%&'("!'") VOLUME 14 | MARCH 2013 | 343

© 2013 Macmillan Publishers Limited. All rights reserved

two-headed kinesin

one-headed kinesin

How do kinesins behave in front of obstacles?

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