Protein-Misfolding Diseases PHY6940 April 8 th 2009 Jessica Nasica
Dec 23, 2015
Protein-Misfolding DiseasesPHY6940April 8th 2009
Jessica Nasica
Overview of the presentation
• General facts on protein folding• Causes of misfolding and protein aggregation• Cellular consequences of protein aggregation• Protein-misfolding diseases• Amyloidoses• Amyloid fibril formation• Neurodegenerative diseases and Alzheimer’s disease• Therapy solutions• Curing Alzheimer’s disease
General facts on protein folding
General facts on protein folding
The “new view”
• A stochastic search of the many conformations accessible to one particular sequence
• A polypeptide chain searches for the lowest-energy state by a process of trial and error
• Native-like interactions between residues are more stable than non-native ones
• To undergo correct folding, the native structure must go through a transition state
Possible starting configurations : 10^16
Compact configurations : 10^10
Transition states : 10^3
Native conformation : 1Lowest energy state
only 1 amino acid sequence
General facts on protein folding
The Protein Quality Control (PQC) system
found in ER and cytosol
General facts on protein folding
Role of chaperones• To help proteins in their folding process• To unfold misfolded proteins before their degradation by
the proteasome unit• To protect proteins from interfering interactions during
folding
General facts on protein folding
Chaperones
The concentration of chaperones is genetically self-regulated and increases with the presence of misfolded proteins.
They possess an ATPase domain that reversibly binds with the hydrophobic parts of partially folded proteins
General facts on protein folding
• Important elements for protein folding:
– The amino acid sequence
– The right cellular environment
– The perfect balance between the various folding states
– A fully functional Protein Quality Control (PQC) system
(T ,P ,pH , etc …)
(Chaperones, proteasome unit )
Causes of misfolding and protein aggregation
3 main causes leading to aggregation :• Genetic mutations resulting in a substitution, deletion or
addition of amino acids
• Changes in environmental conditions of the cell leading to dysfunction of the PQC system or the mitochondria and/or the creation of unwanted interactions with folding proteins
• Post-translational accidental changes to the polypeptide after RNA-translation into amino-acids
α-synuclein gene mutations (Parkinson’s)
Causes of misfolding and protein aggregation
How protein aggregates form
• Change in cellular conditions more misfolded proteins the PQC system is overwhelmed -> aggregation is favored
• Aggregation is thought to be set in by protein segments containing hydrophobic amino acids residues, β-sheet predisposition and low net charge.
Causes of misfolding and protein aggregation
States accessible to a protein molecule Free-energy folding landscape for chaperone-mediated protein folding
Causes of misfolding and protein aggregation
Protein aggregation is a 2-stage event
1. The nucleation proteins start attaching reversibly to a growing nucleus
2. Proteins attach irreversibly to the nucleus until it becomes a larger aggregate.
Cellular consequences of protein aggregation
• Loss-of-function pathogenesis: if misfolded proteins are prematurely degraded by PQC system
protein deficiency disease
• Gain-of-function pathogenesis: if misfolded proteins are not eliminated but accumulated instead
disease pathology
toxicity
Some diseases display both pathogenic mechanisms.
Cellular consequences of protein aggregation
the slowing down of polypeptides translation+
Protein-misfolding diseasesinclude conditions where a protein:
• fails to fold correctly (cystic fibrosis, Marfan syndrome, amyotonic lateral sclerosis)
• is not stable enough to perform its normal function (many forms of cancer)
• fails to be correctly trafficked (familial hypercholesterolemia, α1-antitrypsin deficiency)
• forms insoluble aggregates that deposit toxically (neurodegenerative diseases: Alzheimer’s, type II diabetes, Parkinson’s and many more)
Protein-misfolding diseases
Protein-misfolding diseases
Conditions may be :
• Familial the disorder is genetically inherited and symptoms appear during childhood ( e.g., Huntington )
• Sporadic patternless and characterized by a late onset. Primarily due to aging or to an incorrect lifestyle.
Not associated with gene mutations. (e.g., most of Alzheimer’s and Parkinson’s cases and many more)
• Transmissible (e.g., prion disease, spongiform encephalopathies and fatal familial insomnia)
A closer look at amyloidoses
Main family of protein-misfolding diseases most clinically relevant due to the high occurrence of neurodegenerative diseases and type II diabetes
2 types:
• Systemic amyloidoses large amounts of fibrils accumulate everywhere.
• Organ-limited amyloidoses fibrils accumulate locally in one organ ( e.g., brain )
A closer look at amyloidoses
Characterized by the formation, accumulation and deposition of similar highly-organized insoluble fibrillar aggregates amyloid fibrils
AFM image of β2-microglobulin amyloid fibrils
Causes of the formation of amyloid fibrils
Amyloid fibril formation
Structural properties• Insoluble fibrous aggregates• Specific optical behavior (binds to dye “Congo red”) • Highly organized macrostructure a few nm in diameter• Characterized by a cross-β quaternary structure• Cross-β composed of 2-β sheets facing each other,
closely interacting• β-sheets have their β-strands perpendicular to fibril axis
Laser scanning confocal microscopy
Amyloid fibril formation
Mechanism of formation 3 steps:1. Alignment of the molecules to form β-sheets fastest
stage involves H-bonds
2. Formation of the cross-β structure slower than step 1 involves Van-der-Waals forces interdigitation of residues side chains “steric zipper” structure
3. Fibril formation involves non-covalent bonds
Amyloid fibril formation
Amyloid fibril formation is a nucleated-growth process presence of a pre-formed nucleus
rapid growth: additional proteins added more easily once the large entropy barrier is
overcome
Amyloid fibrils are stabilized by the protein concentration and by the formation of steric zippers
Aggregation rates depend on the charge, secondary structure propensities, hydrophobicity and length of the proteins
The efficiency of the PQC system is also very important
Toxicity of amyloid fibrils
annular pores
Schematic representation of structural species during amyloid formation.Protofibrils may form spherical, chain-like or annular pore-like structures to go through the cell membranes
Consequences of amyloid fibril formation
Neurodegenerative diseases
They are localized amyloidoses affecting the brain
& the most-occurring type of age-related PMDs
Neurodegenerative diseases
Aggregation happens in neurons in the brain & spinal cord
deterioration of neurons or their myelin sheath
dysfunction
movement coordination memory loss
impairedSchematic view of a neuron
Alzheimer’s disease most common progressive neurodegenerative disordermassive loss of neurons
Accumulation of Aβ-amyloid protein (extracellular)Accumulation of tau protein (intracellular)
Formation of amyloid plaques (Aβ )Formation of neurofibrillar tangles or NFTs ( tau protein )
Plaques
NFTs
Alzheimer’s disease
Effects on the brain
Brain shrinkage
Alzheimer’s disease
Resulting symptoms
• Memory impairment• Inability to think by themselves• Inability to function independently
Parkinson’s disease
2nd most common neurodegenerative disease due to aging
Degeneration of specific dopaminergic neurons in the substantia nigra
Accumulation of α-synuclein protein in presence of dopamine
Other neurons are not affectedΑ-synuclein accumulating neurons
Parkinson’s disease
Resulting symptoms
• Muscular rigidity extreme pain• Postural instability• Resting tremor
Huntington’s disease
• Polyglutamine disease mutation encoding for an addition of Q amino-acids
• Accumulation of the misfolded protein Huntingtin• Formation of toxic inclusions in brain cells• Degeneration of glutamatergic striatal neurons
polyQ inclusionIn neocortex
Therapeutic solutions
3 main approaches:
1. Inhibition of protein aggregation
2. Interference with post-translational peptide changes before the misfolding/aggregation step
3. Upregulation of molecular chaperones or aggregate-clearance mechanisms
Therapeutic solutions
1.
Inhibition of amyloid ß (Aß) fibril neurotoxicity by laminin.
Therapeutic solutions
2.
• Targeting Aβ-formation by inhibiting β- and γ- secretase proteins responsible for the formation of amyloid plaques
• Inhibition of tau protein phosphorylation (hyperphosphorylation of tau protein is responsible for its aggregation)
Therapeutic solutions
3.
Clonidine and Minoxidil enhance the clearance of aggregate prone proteins, including mutant Huntingtin and mutants of α-synuclein
Curing Alzheimer’s disease
Disease affecting 37 million people worldwide
In 40 years, 1 in 85 people will develop AD
Current drug research aims at preventing Aβ-formation and blocking the formation of amyloid plaques to slow the progression of the disease
A few programs targeting the tau protein
Curing Alzheimer’s disease
Preventing Alzheimer’s disease
Thank you !