Lecture Unit 3: Endocytosis
Modes'of'endocytosis'
Macropino>cytosis'
Clathrin)Mediated)endocytosis)
Pinocytosis'
Caveolar'medidated'
Phagocytosis'
Phagocytosis'of'erythrocytes'by'lung'macrophage.'
SEM'image.'
Arrows'indicate'edge'of'engulging'phagocy3c'cup.''
Neutrophil'phagocytosing'a'bacteria;'note'the'bacteria'is'coated'with'an3>bug'an3bodies'which'are'recognized'by'IgG'FC'receptors'on'neutrophil'cell'surface.''
Lysosomes'
Note'meshwork'of'ac3n'filaments'underlying'phagocy3c'cup;'Beads'can'be'mechanically'isolated'with'aQached'membrane.'
coated'latex'bead'
Steps&in&phagocytosis:&1. Phagocyte'
recogni3on'of'foreign'par3cle'(e.g.'bacterium).'
2. Recruitment'of'ac3n'filament'machinery'to'extend'membrane'around'par3cle.''
3. Forma3on'of'a'phagosome'vesicle;'removal'/disassembly'of'ac3n'
4.''Primary'and'secondary'Lysosome'fusion'with'phagosome'and'degrada3on'of'contents.''Note,'unlike'other'endocy3c'vesicles,'phagosomes'do'not'first'fuse'with'the'endosome.'''
Organelles'of'the'Endocy3c'Pathway'(post'endocytosis/internaliza3on):'
Endocy3c'vesicle'Early''late'endosome'Lyososome'Trans>Golgi'network'
1. Fluid'phase'2. Endocy3c'membrane'3. Receptor'4. Ligand'
Components'of'an'Endocy3c'Vesicle'
1'
2'3'
4'
'Endosome'
Early'late'endosome'matura3on'Note'decrease'in'pH'during'endosome'matura3on.''Note'that'recycling'of'membrane'can'occur'directly'from'endosome'or'through'TGN/recycling'endosome'compartments'
Pseudo>colored'TEM'of'early'(EE)'and''late'(LE)'endosomes,'lysosomes'(LY)'and'clathrin'coated'pits'(CP).'
Experimental'demonstra3on'that'internalized'endocy3c'membrane'components'are'recycled:''
1. Prepare'beads'with'covalently'linked'lactoperoxidase.'
2. “Feed”'beads'to'two'aliquots'of'macrophages'and'wait'10'minutes'to'allow'endocy3c'internaliza3on;''
3. chill'cells'to'4oC;'add'125>I'and'H2O2;''(This'covalently'links'125>I'(iodina3on)'to'any'proteins'in'contact'with'the'bead'surface)'
4. Fix'one'aliquot'and''process'for'EM'autoradiography.'
5. Warm'the'other'batch'of'cells'to'37o'and'incubate'for'10'minutes;'fix'and'process'for'EM'AR.'
'0o'cell' Cell'warmed'to'37o'for'10'min.'''
Result:''Iodinated,'internalized'membrane'proteins'have'recycled'back'to'plasma'membrane'ader'warming.''
Acid'ac3vated'hydrolases'of'the'lysosome'
Imagine)the)consequences)of)secre7ng)these)enzymes)into)the)extracellular)compartments)of)your)body.'
The'“machinery”'involved'in'clathrin'mediated'endocytosis:'
Cytoplasmic'adaptor'targe3ng'domains'of'receptors'Adaptors'that'bind'to'receptor'target'domains'Clathrin'triskelions'to'form'clathrin'“basket”/'coated'pits'Dynamin'to'effect'scission'of'coated'vesicle''Uncoa3ng'chaparones'to'remove'clathrin'coat'
Coated'vesicle'uptake'of'yolk'protein'(from'blood'meal)'by'mosquito'oocytes:'The'first'visualiza3on'of'clathrin>coated'pits/vesicles'(early'60’s)'
Coated'vesicles'can'be'readily'purified'by'cell'frac3ona3on/'differen3al'sedimenta3on'allowing'purifica3on'of'clathrin.'
Clathrin'triskelion'structure;'heavy'chain'triskelions'are'stabilized'by'the'light'chains.'
The'polygonal'clathrin'triskelion'cage'surrounds'the'endocy3c'pit/vesicle'
1.'Adap3ns/adaptors'recognize'receptors,2.Adap3ns'recruit'clathrin'to'assemble'coated'pit'3.'Dynamin>GTPase''effects'vesicle'closure.'4.''HSC'70'(chaparone'ATPase)'plus'coat'binding'protein'auxilin'mediate'vesicle'uncoa3ng.'
Stages'of'coated'pit'forma3on,'closure,'coated'vesicle'release'and'vesicle'uncoa3ng.'
Clathrin'is'recruited'to'different'compartments'by'compartment'specific'adaptors.'
Note,'for'this'unit,'we'will'only'consider'clathrin’s'role'in'receptor'mediated'endocytosis'
Adaptors'recognize'specific'signals'on'cytoplasmic'domain'of'receptors.'
/adaptor'
Sor3ng'signals'using'contain'at'least'one'tyrosine'residue'
Example'of'an'internaliza3on'“signal”'recognized'by'adap3n'complex:''The'tyrosine'residue'is'crucial.'''
Forma3on'of'dynamin>GTP>gamma>S'(nonhydrolyzable')'induced'collars'in'isolated'synap3c'membranes'in'vitro.'
This'demonstrates'that'dynamin>GTP'binds'to'the'pit'“neck”'to'form'the'constric3on'collar'but'closure'of'the'collar'to''pinch'of'the'vesicle'requires'GTP'hydrolysis.'
Classes'of'receptors'internalized'by'clathrin'mediated'endocytosis.'
>>Non'recycled'receptors'(e.g.'growth'factor'receptors);'Internaliza3on'of'these'receptors'requires'ligand'binding.'
>>Recycled'receptors'(e.g.'“nutrient”'receptors);'these'receptors'are'internalized'with'or'without'bound'ligand.''
Other'components'internalized'by'clathrin'mediated'endocytosis:''exocytosed'membrane,'certain'viruses.'
Why'might'some'receptors'recycle'back'to'the'cell'surface'(e.g.'LDL'receptor)''while'others'(e.g.'growth'factor'receptors)'are'not?'
pH&dependence&of&receptor4ligand&binding:'pH'resistant'binding'results'in'targe3ng'of'receptor'and'ligand'to'late'endosome/lysosome;'pH'sensi3ve'(acid'dissocia3on)'binding'results'in'ligand'dissocia3on'and'recycling'of'receptor'to'plasma'membrane.''
pH'sensi3ve'receptors:''LDL'receptor','Fe2+'bound'to'transferrin'on'transferrin'receptor.'
pH'insensi3ve'receptors:''EGF'receptor'
Transcytosis:''Endocy3c'movement'of'cargo'from'one'plasma'membrane'(e.g.'apical'domain)'to'another'plasma'membrane'domain'(e.g.'basolateral'domain'of'an'epithelial'cell'
pH'6'
pH'7.4:'IgG'dissociates'from'FC'receptor''
IgG'binds'to'Fc'receptor'at'~'acid'pH'of'gut'lumen.'
Uptake'of'maternal'an3bodies'in'neonate'intes3ne'via'clathrin'mediated'transcytosis'of'the'FC'receptor.''
coated'vesicle>FC'receptor'mediated'uptake'of'maternal'IgG'in'intes3ne'of'neonate.''Note'the'numerous'coated'pits'(far'fewer'in'adult)'
Experimental'dissec3on'of'the'LDL'uptake'pathway:'From'clinical'studies'of'pa3ents'with'severe'heart'disease'to'dissec3on'of'the'cell'and'molecular'mechanics'of'clathrin/receptor'mediated'endocytosis.'Brown,'Goldstein'(Anderson):'Nobel'Prize'in'Medicine.'1985.'
Low'density'lipoprotein'par3cle'(LDL):'
“bad”'serum'cholesterol.'LDL'receptor'binds'to'the'protein'component,'apoprotein'B'
LDL'serum'concentra3ons'and'onset'of'heart'aQacks'in'pa3ents'homozygous'and'heterozygous'for''a'LDL'receptor'null'muta3on.'
Familial'Hypercholesterolemia'
From'the'studies'of'Brown,'Goldstein'and'Anderson'
Chronic'uptake'of'LDL'results'in''down'regula3on'of'receptor'and'enzymes'for'cholesterol'synthesis;'up'regula3on'of'enzymes'for'cholesterol'storage.'
target'for'cholesterol'reduc3on'drugs'like'Lipitor.'
From'the'studies'of'Brown,'Goldstein'and'Anderson'
N>linked'glycosyl'groups'
N>linked'glycosyl'groups'
O>linked'glycosyl'groups'
Structure'of'the'LDL'Receptor'
Analysis'of'LDL'cell'surface'binding'at'4o'and'kine3cs'of'uptake'ader'warming'to'37o'
1. Iodinate'LDL'(on'tyrosine'residues'of'apoprotein'B)'with'125'I'(radioac3ve).'
2. Add'125>I>LDL'to'cells'at'4o.''Wash'out'unbound'LDL.'3. Release'surface'bound'LDL'with'heparin'(a'compe33ve'
inhibitor)'and'quan3fy'releasable'125>I'LDL'par3cles'(=surface'bound)'
4. Warm'cells'to'37o'and'quan3fy'at'various'3mes'ader'warming''surface'bound'(heparin'releasible)'and'nonreleasible'(internalized),'TCA'precipitable'and'TCA'soluble'125'I.'''TCA'ppt='intact'protein''TCA'soluble=digested'small'125'I'tyrosine'contac3ng'pep3des'and'free'125>I'tyrosine.''
Low'density'lipoprotein'par3cle'(LDL):'
“bad”'serum'cholesterol.'LDL'receptor'binds'to'the'protein'component,'apoprotein'B'
125>I>'
LDL'cell'surface'binding'and'kine3cs'of'LDL'uptake'by'cells'with'wild'type'LDL'receptor.'''Why'is'there'a'gradual'increase'in'TCA'soluble'125>I'and'a'gradual'decrease'in'TCA'insoluble'(heparin'resistant)'125>I.''
Heparin'releases'LDL'from'receptors'on'cell'surface'but'not'internalized'LDL'(heparin'resistant)'
Uptake'of'ferri3n>labeled'LDL'by'coated'pits'(7,'8)'and'subsequent'targe3ng'to'lysosomes'(9).''This'was'performed'as'a'pulse>chase'experiment:'Cells'in'7,8'were'fixed'for'EM'1,2'minutes'ader''LDL'addi3on'at'4o'
Cell'in'9'was'fixed'ader'warming'cells'to'37o'for'~'10'minutes'to'allow'internaliza3on'and'targe3ng'to'lyosome.'
Experimental'demonstra3on'that'LDL'is'internalized'by'clathrin'coated'pits/vesicles'
Stereo'view'of'LDL'par3cles'on'outside'surface'of'coated'pit.'Quick'freeze'deep'etch'rotary'shadowed'replica'
Classes&of&LDL&receptor&(homozygous)&muta?ons&leading&to&familial&cholesterolemia:&
1. null'muta3on:''no'LDL'surface'binding;'no'internaliza3on'
2. Mutated'ligand'binding'domain:''No'LDL'binding'or'internaliza3on;'receptor'localizes'to'coated'pits;'cycles'through'endosome>>cell'surface'
3. Truncated'cytoplasmic'domain:''Cell'surface'binding'of'LDL;'no'internaliza3on'
4. Stop'codon'muta3on'before'transmembrane'domain:''no'binding'no'internaliza3on;'receptor'is'secreted/degraded.'
5. TyrosineX&muta?on&in&cytoplasmic&domain;&normal&surface&binding;&no&recruitment&to&coated&pits;&no&internaliza?on.&&
WT'''#1'''''#2'''''#3'''''''''''''''#4'''''''#5'
Classes'of'LDL'receptor'muta3ons'iden3fied'by'Brown,'Goldstein'and'Anderson'
Muta3ons'in'LDL'receptor'result'in'high'levels'of'serum'LDL'and'early'onset'of'heart'disease.'
WT'
Class'5'muta3on'
Classes'of'LDL'receptor'muta3ons:'
1. null'muta3on:''no'LDL'surface'binding;'no'internaliza3on'
2. Mutated'ligand'binding'domain:''No'LDL'binding'or'internaliza3on;'receptor'localizes'to'coated'pits;'cycles'
3. Truncated'cytoplasmic'domain:''Cell'surface'binding'of'LDL;'no'internaliza3on'
4. Stop'codon'muta3on'before'transmembrane'domain:''no'binding,'no'internaliza3on'
5. YX'muta3on'in'cytoplasmic'domain;'normal'surface'binding;'no'recruitment'to'coated'pits;'no'internaliza3on.''
Diagram'the'kine3cs'of'LDL'surface'binding'(at'4oC),''and'uptake,'degrada3on'ader'warming'cells'with'Class'1>5'LDL'receptor'muta3ons.'''