Steady axonal cargo flow is central to the functioning of healthy neurons. However, a substantial fraction of cargo in axons remains stationary up to several minutes. We examine the transport of precursors of synaptic vesicles (pre-SVs), endosomes and mitochondria in Caenorhabditis elegans touch receptor neurons, showing that stationary cargo are predominantly present at actin-rich regions along the neuronal process. Stationary vesicles at actin-rich regions increase the propensity of moving vesicles to stall at the same location, resulting in traffic jams arising from physical crowding. Such local traffic jams at actin-rich regions are likely to be a general feature of axonal transport since they also occur in Drosophila neurons. Repeated touch stimulation of C. elegans reduces the density of stationary pre-SVs, indicating that these traffic jams can act as both sources and sinks of vesicles. This suggests that vesicles trapped in actin-rich regions are functional reservoirs that may contribute to maintaining robust cargo flow in the neuron. A video abstract of this article can be found at: Video S1; Video S2.

Steady axonal cargo flow is central to the functioning of healthy neurons. However, a substantial fraction of cargo in axons remains stationary across a broad distribution of times. We examine the transport of pre-synaptic vesicles (pre-SVs), endosomes and mitochondria in C. elegans touch receptor neurons (TRNs), showing that stalled cargo are predominantly present at actinrich regions along the neuronal process. Cargo stalled at actin-rich regions increase the propensity of moving cargo to stall at the same location, resulting in traffic jams. Such local traffic jams at actin-rich regions are likely to be a general feature of axonal transport since they occur in Drosophila neurons as well. These traffic jams can act as both sources and sinks of vesicles. We propose that they act as functional reservoirs that contribute to maintaining robust cargo flow in the neuron.

Stationary clusters of vesicles are a prominent feature of axonal transport, but little is known about their physiological and functional relevance to axonal transport. We investigate the role of vesicle motility characteristics in modulating the formation and lifetimes of such stationary clusters, and their effect on cargo flow. We develop a simulation model describing key features of axonal cargo transport, benchmarking the model against experiments in the Posterior Lateral Mechanosensory (PLM) neurons of C. elegans. Our simulations include multiple microtubule tracks, varied cargo motion states and accounts for dynamic cargo-cargo interactions. Our model also incorporates static obstacles to vesicle transport in the form of microtubule ends, stalled vesicles, and stationary mitochondria. We demonstrate, both in simulations and in an experimental system, that a reduction in reversal rates is associated with a higher proportion of long-lived stationary vesicle clusters and reduced net anterograde transport. Our simulations support the view that stationary clusters function as dynamic reservoirs of cargo vesicles, and reversals aid cargo in navigating obstacles and regulate cargo transport by modulating the proportion of stationary vesicle clusters along the neuronal process.

Molecular motors drive the directed transport of presynaptic vesicles along the narrow axons of nerve cells. Stationary clusters of such vesicles are a prominent feature of axonal transport, but little is known about their physiological and functional relevance. Here, we develop a simulation model describing key features of axonal cargo transport with a view to addressing this question, benchmarking the model against our experiments in the touch neurons of C. elegans. Our simulations provide for multiple microtubule tracks and varied cargo motion states while also incorporating cargo-cargo interactions. Our model also incorporates obstacles to vesicle transport in the form of microtubule ends, stalled vesicles, and stationary mitochondria. We devise computational methodologies to simulate both axonal bleaching and axotomy, showing that our results reproduce the properties of both moving as well as stationary cargo in vivo. Increasing vesicle numbers leads to larger and more long-lived stationary clusters of vesicular cargo. Vesicle clusters are dynamically stable, explaining why they are ubiquitously seen. Modulating the rates of cargo motion-state switching allows cluster lifetimes and flux to be tuned both in simulations and experiments. We demonstrate, both in simulations and in an experimental system, that suppressing reversals leads to larger stationary vesicle clusters being formed while also reducing flux. Our simulation results support the view that the physiological significance of clusters is located in their role as dynamic reservoirs of cargo vesicles, capable of being released or sequestered on demand.

No abstract