Axonal Transport Flashcards
Axonal Transport
transport of proteins and organelles essential in all cells
more extreme in neurons
highly polarised
syntheisied in cell body and required in different locations
Challenge in Axonal transport
Big journey – long distances to cover
- Microtubules
Rails at which long distance axonal transport occurs
Composed of tubulin dimers of alpha and beta tubulin
Polymerise to assemble micro tubule
Plus end quick growth minus end slow
Protofilament composed of head-to-tail arrays of tubulin dimers, are arranged in parallel.
Polarity-
In axons MT uniformly orientated plus end pointing away from cell body and toward growth cone and synapse
Anterograde
Toward plus end
Retrograde
Towards minus end
MT base Molecular motors
Mechanincal enz ATPases – ATP > ADP to generate force 2 families : Kinesin Cytoplasmic Dynamin
Kinesin 1
Anterograde
Plus end directed towards the synapse
Heterotetromeric protein tail stalk and motor domain
2 heavy chains and 2 light chain
Light chain regulates heavy chain – inhibits activity of motor unless carrying something
Vesicles huge and motor proteins tiny
Kinesin superfamily :
14 kinesin families
41 in humans
Majority plus end directed
Cytoplasmic Dyneins-
MT based
Retrograde – minus end directed
Multi subunit complex
Dynactin component involved in binding cargo
Axon transport is divided into what two categories?- how was it discovered
Slow and fast
-Found using radioactive methionine – shows any new protein synthesis – radioactive methionine shown in the protein
slice up
Gel
See which proteins labelled and how far have they travelled along optic nerve or sciatic nerve
Fast
40Cm per day
Membrane bound organelles
Slow
1mm per day
Proteins
How does the difference in speed arise ?
Both fast and slow axonal transport are mediated by the same “fast” molecular motors kinesin and cytoplasmic dynein. The slower overall rate of slow axonal transport is due to prolonged pauses between movements – “stop and go” model (express vs. local train)
Molecular components of MT base axonal transport
- Engines- Dynein and Kinesins
- Fuel – mitochondria
- Carriages – Neurofilament
- Rails- Microtubules
Defects in neurodegenerative disease :
Length dependent neuronal disease – degeneration of long motor neruons e.g CMT/ MND
Pathologies – Axonal and cell body accumulation of organelles and proteins
Axonal swelling filled with card (should be transported down axons )
Axonal transport is disrupted
Neurodegeneration examples and pathologies
Soma filled with neurofilament in ALS < Pathology
TDP-4 accumulation in MND < things not being trafficked
Incorrect accumulation – incorrect location
Shown using transfection – GFP labelled mitochondria accumulate in the cell body
Less mitochondria in the axon in mutant
HSP-
affect upper motor neuron s
Cell bodies in the motor cortex cross the spinal cord – connects brain to LMN
Large accumulation of proteins that should be transported
Prime evidence comes from MND
- ALs
- Distal hereditary motor neuropathy
- Spinal Muscular atrophy
- Hereditary Spastic paraplegia (HSP)
Upstream effect
Axonal transport defects are observed before the onset of neurodegeneration/ suggesting that they are the cause rather than consequence
Failure to supply essential components
Molecular mechanisms of Axonal transport defects
Issues with
- Motor proteins
- MT
- cargo
- Mitochondria
Mutations in what part of the molecular mechanism have been identified ?
Mutations in molecular motor identified in a number of disease
LOF mutations
HSP – mutation in Kinesin
ALS- part of dynactin complex
Hallmark pathologies for many human neurodegenerative diseases:
Axonal and cell body accumulations of organelles and proteins
Axonal swellings filled with cargo
How Do Axonal Transport Defects Cause/Contribute to Neurodegeneration?
Axonal transport defects are observed before the onset of neurodegeneration/disease suggesting that they are part of the cause of disease rather than a consequence!
Failure to supply essential components/energy to the axon/synapse
Failure to clear waste (e.g. misfolded proteins, non-functional mitochondria)
Impaired signalling – pro-survival and/or stress signals
mutations in molecular motors :
All LOF
Motor neuron disorders
HSP (SPG10): Kinesin-1 Reid, et al. Am. J. Hum. Genet. 2002; J. Med. Genet. 2003
ALS: p150Glued (a component of the dynactin complex) Puls, et al. Nat. Genet. 2003
CMT: KIF1Bß (Kinesin-3 family member); transports vesicles. Zhao, et al. Cell 2001
Phosphorylation of molecular motors
Molecular motor activity and interaction with cargo can be regulated by phosphorylation
Abnormal/Excessive activation of kinases is a hallmark of neurodegenerative diseases
Stress kinases: p38 MAPkinase, JNK
ALS, Huntington’s Disease
GSK3ß
Alzheimer’s Disease
Stress kinases: p38 MAPkinase
P38 MAPK is activated in ALS
Phosphorylation of kinesin light chain by p38 MAPkinase halts kinesin-1 mediated transport of mitochondria.
De Vos et.al
Stress kinases: c-Jun N-terminal kinase (JNK)
Huntington’s Disease, SBMA (poly-Q androgen receptor)
Phosphorylation of the kinesin-1 motor domain by JNK prevents its interaction with microtubules
Alzheimer’s Disease: GSK3ß and CK2
- Phosphorylation of kinesin light chain by GSK3ß prevents attachment of kinesin-1/KLC to vesicles
- Casein kinase 2 (CK2) activation by amyloid beta induces phosphorylation of KLC and disrupts axonal transport
Steric hindrance by Tau
Main MT binding protein
High levels of tau in aLzheimers
MT bound tau prevents kinesin 1 transport
Trap and causes release
kinesin molecules detach upon encountering tau
Stabilization of Microtubules: Role of Tau
Tau stabilizes axonal microtubules
Phosphorylation of tau by e.g. GSK3ß (activated in AD) releases tau from microtubules -> destabilisation
MT Bundling -
MT bundling – mutations in spastin – cause Mt to bundle so don’t form usual network
Spastin
Spastin most common gene mutated in HSP
LOF of spastin which usually severs MT into pieces
Causes reduction in anterograde transport of mitochondria
PTM of Tubulin
- Acetylation of tubulin cause increase binding of kinesin 1 and transport
- Histone deacetylase 6 inhibition compensated for transport deficit in HD – increase rates of transport
Are hallmark for pathology of ALS
Phosphorylation of Neurofilaments
Phosphorylation of Neurofilaments
Accumulation axons not in the cell body – disease \NF heavy phosphorylated in cell body
Mitochondria
Mitochondria are selectively targeted in many neurodegenerative disease that involve axonal transport defects:
Motor neuron disorders
ALS: SOD1
HSP: paraplegin
Alzheimer’s Disease: Aβ
Parkinson’s Disease: Pink, DJ1
Huntington’s Disease
CMT: mitofusin 2
Reduced mitochondrial function leads to ATP deficit which will affect axonal transport
Dysfunctional mitochondria show reduced anterograde transport (Miller and Sheetz, JCS, 2004)
Miss-assembly of Neurofilaments
Mutant NFL in CMT
Mutant HSPB1 in Distal Hereditary Motor Neuropathy
De Vos
Familial amyotrophic lateral sclerosis-linked SOD1mutants perturb fast axonal transport to reduce axonal mitochondria content
Mitochondria in parkinsons
Drosophila pink1 is required for mitochondrial function and interacts genetically with parkin
- Mitochondrial dysfunction in Parkinson’s disease
- The Parkinson’s disease genes pink1 and parkin promote mitochondrial fission and/or inhibit fusion in Drosophila
ALS mutations inhibit what?
ALS mutations inhibit anterograde transport of mitochondria
Defective axonal transport leads to what??
Defective axonal transport leads to depletion of axonal mitochondria
Defective axonal transport leads to what??
Defective axonal transport leads to depletion of axonal mitochondria And increased mitochondrial spacing
The Mitochondrial Hypothesis
Dysfunction of mitochondria leads to reduced axonal ATP.
Depletion of mitochondria from axons causes/exacerbates axonal transport defects
Age-related damage to mitochondria may amplify any primary defects to axonal transport. In this way, disruption to mitochondria may explain why many neurodegenerative diseases are diseases of old age.
