Role of Axonal Transport in Neurodegenerative Diseases

Presynaptic nerve terminals are formed from preassembled vesicles that are delivered to the prospective synapse by kinesin-mediated axonal transport. However, precisely how the various cargoes are linked to the motor proteins remains unclear. Here, we report a transport complex linking syntaxin 1a (Stx) and Munc18, two proteins functioning in synaptic vesicle exocytosis at the presynaptic plasma membrane, to the motor protein Kinesin-1 via the kinesin adaptor FEZ1. Mutation of the FEZ1 ortholog UNC-76 in Caenorhabditis elegans causes defects in the axonal transport of Stx. We also show that binding of FEZ1 to Kinesin-1 and Munc18 is regulated by phosphorylation, with a conserved site (serine 58) being essential for binding. When expressed in C. elegans, wild-type but not phosphorylationdeficient FEZ1 (S58A) restored axonal transport of Stx. We conclude that FEZ1 operates as a kinesin adaptor for the transport of Stx, with cargo loading and unloading being regulated by protein kinases. T he formation and maintenance of presynaptic boutons are intricate but highly efficient processes during which the machinery for exocytosis and recycling of synaptic vesicles is assembled from preformed units. These units are delivered to the nascent synapse via molecular motor proteins of the kinesin superfamily (reviewed in Ref. 1).Although Kinesin-3 appears to be the main motor transporting synaptic vesicle precursors, recent evidence suggests that Kinesin-1 (KIF5) is also involved. In Drosophila, deletion of UNC-76/ fasciculation and elongation protein zeta 1 (FEZ1), a specific adaptor for Kinesin-1, left synaptic vesicles stranded in the axon (2, 3), showing that Kinesin-1 is needed at least during later phases of axonal transport. Transport of the synaptic vesicle protein synaptotagmin by the UNC-76/Kinesin-1 complex requires phosphorylation of UNC-76 by the UNC-51/ATG1 kinase, a prerequisite for UNC-76 to bind synaptotagmin (3). Deletion of this kinase phenocopies deletion of UNC-76. Indeed, phosphorylation-regulated interactions between cargo, adaptors, and kinesins have also been observed for other transport complexes such as the kinesin light chain/JIP1 (c-Jun N-terminal kinase-interacting protein 1) complex (4). This suggests that phosphorylation is a common mechanism for the regulation of kinesin-based transport complexes (5).Less is known about the involvement of Kinesin-1 in the transport of other classes of synaptic precursor vesicles. Transport of syntaxin 1a (Stx), an essential component of the exocytotic release apparatus residing in the presynaptic plasma membrane, is clearly distinct from synaptic vesicle precursors and appears to involve a complex between Kinesin-1 and the Stx-binding protein syntabulin (6, 7). Down-regulation or expression of dominant-negative syntabulin reduces but does not abolish membrane delivery of Stx, indicating the existence of other transport mechanisms (6). Moreover, proper intracellular trafficking of Stx and its function in exocytosis depends on Munc18 coexpression (8-14). Stx tra...

Section: Discussionmentioning

confidence: 99%

Phosphorylation-regulated axonal dependent transport of syntaxin 1 is mediated by a Kinesin-1 adapter

Chua

Butkevich

Worseck

et al. 2012

“…Consistent with this idea, it has been shown that mutations in transport-related genes can result in neurodegenerative phenotypes in mice and humans (3,4). Because transport deficits have been reported in many neurodegenerative diseases, including Alzheimer's disease, Huntington's disease, and amyotrophic lateral sclerosis (ALS) (5)(6)(7)(8), it is commonly assumed that disturbances in axonal transport are key pathological events that contribute to neurodegeneration (9)(10)(11). However, the causal relationship of axonal transport disturbances to degeneration remains unclear.…”

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confidence: 93%

Axonal transport deficits and degeneration can evolve independently in mouse models of amyotrophic lateral sclerosis

Marinković

Reuter

Brill

et al. 2012

Axonal transport deficits have been reported in many neurodegenerative conditions and are widely assumed to be an immediate causative step of axon and synapse loss. By imaging changes in axonal morphology and organelle transport over time in several animal models of amyotrophic lateral sclerosis (ALS), we now find that deficits in axonal transport of organelles (mitochondria, endosomes) and axon degeneration can evolve independently. This conclusion rests on the following results: (i) Axons can survive despite long-lasting transport deficits: In the SOD G93A model of ALS, transport deficits are detected soon after birth, months before the onset of axon degeneration. (ii) Transport deficits are not necessary for axon degeneration: In the SOD G85R model of ALS, motor axons degenerate, but transport is unaffected. (iii) Axon transport deficits are not sufficient to cause immediate degeneration: In mice that overexpress wild-type superoxide dismutase-1 (SOD WT ), axons show chronic transport deficits, but survive. These data suggest that disturbances of organelle transport are not a necessary step in the emergence of motor neuron degeneration.

“…Several types of vesicles, including amyloid precursor protein (APP) vesicles, depend upon kinesin-1 (9, 13-17) for their anterograde movement in axons. These studies raised the possibility that APP transport and its influence on APP proteolytic processing play an important role in the development of Alzheimer's disease (1,3,9,(18)(19)(20).Although much progress has been made toward elucidating underlying principles of motor coordination (21-25), we lack a full understanding of how motor subunits are recruited to APP vesicles and, moreover, how levels of these subunits on vesicles are determined. Previous work raised the possibility that APP could bind to KLC and thus kinesin-1 either directly or indirectly via complex formation with JIP1 (9, 26, 27), a proposal that has remained controversial in the field.…”

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confidence: 99%

“…These motor proteins and their regulators play critical roles in long-distance signaling events in neurons, neuronal regeneration, and in the development of neurodegenerative disease (1)(2)(3). Several diseases in humans, such as hereditary spastic paraplegia (4), CharcotMarie-Tooth type 2 (5), and ALS-like motor degeneration (6), have been linked directly to mutations in genes encoding motor proteins.…”

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confidence: 99%

Subpixel colocalization reveals amyloid precursor protein-dependent kinesin-1 and dynein association with axonal vesicles

Szpankowski

Encalada

Goldstein

2012

Intracellular transport of vesicles and organelles along microtubules is powered by kinesin and cytoplasmic dynein molecular motors. Both motors can attach to the same cargo and thus must be coordinated to ensure proper distribution of intracellular materials. Although a number of hypotheses have been proposed to explain how these motors are coordinated, considerable uncertainty remains, in part because of the absence of methods for assessing motor subunit composition on individual vesicular cargos. We developed a robust quantitative immunofluorescence method based on subpixel colocalization to elucidate relative kinesin-1 and cytoplasmic dynein motor subunit composition of individual, endogenous amyloid precursor protein (APP) vesicles in mouse hippocampal cells. The resulting method and data allow us to test a key in vivo prediction of the hypothesis that APP can recruit kinesin-1 to APP vesicles in neuronal axons. We found that APP levels are well-correlated with the amount of the light chain of kinesin-1 (KLC1) and the heavy chain of cytoplasmic dynein (DHC1) on vesicles. In addition, genetic reduction of APP diminishes KLC1 and DHC1 levels on APP cargos. Finally, our data reveal that reduction of KLC1 leads to decreased levels of DHC1 on APP vesicles, suggesting that KLC1 is necessary for the association of DHC1 to these cargos, and help to explain previously reported retrograde transport defects generated when kinesin-1 is reduced.axonal transport | microtubule motor recruitment | Alzheimer's disease M icrotubule-based transport of vesicles and organelles in neurons is coordinated by kinesin (anterograde) and cytoplasmic dynein (retrograde) motor complexes. These motor proteins and their regulators play critical roles in long-distance signaling events in neurons, neuronal regeneration, and in the development of neurodegenerative disease (1-3). Several diseases in humans, such as hereditary spastic paraplegia (4), CharcotMarie-Tooth type 2 (5), and ALS-like motor degeneration (6), have been linked directly to mutations in genes encoding motor proteins. Additionally, disruption of transport is thought to be an early and perhaps causative event in Alzheimer's disease, Parkinson disease, and ALS, where axonal pathologies including abnormal accumulations of proteins and organelles have been routinely observed (1).Kinesin-1 is composed of heavy chain (KHC or KIF5) and light chain (KLC) subunits (7,8). Motor activity is executed by KHC, and cargo binding and regulatory activities are carried out by both KHC and KLC (9-11). Dynein is a large protein complex with six subunits: a large dynein heavy chain (DHC), an intermediate and light-intermediate chain (DIC and DLIC), and three dynein light chains (DLC) (12). Several types of vesicles, including amyloid precursor protein (APP) vesicles, depend upon kinesin-1 (9, 13-17) for their anterograde movement in axons. These studies raised the possibility that APP transport and its influence on APP proteolytic processing play an important role in the development of Alzheim...