Fast Vesicle Transport Is Required for the Slow Axonal Transport of Synapsin

Abstract

Although it is known that cytosolic/soluble proteins synthesized in cell bodies are transported at much lower overall velocities than vesicles in fast axonal transport, the fundamental basis for this slow movement is unknown. Recently, we found that cytosolic proteins in axons of mouse cultured neurons are conveyed in a manner that superficially resembles diffusion, but with a slow anterograde bias that is energy-and motor-dependent (Scott et al., 2011). Here we show that slow axonal transport of synapsin, a prototypical member of this rate class, is dependent upon fast vesicle transport. Despite the distinct overall dynamics of slow and fast transport, experimentally induced and intrinsic variations in vesicle transport have analogous effects on slow transport of synapsin as well. Dynamic cotransport of vesicles and synapsin particles is also seen in axons, consistent with a model where higher-order assemblies of synapsin are conveyed by transient and probabilistic associations with vesicles moving in fast axonal transport. We posit that such dynamic associations generate the slow overall anterogradely biased flow of the population ("dynamic-recruitment model"). Our studies uncover the underlying kinetic basis for a classic cytosolic/soluble protein moving in slow axonal transport and reveal previously unknown links between slow and fast transport, offering a clearer conceptual picture of this curious phenomenon.

Fast axonal transport of the proteasome complex depends on membrane interaction and molecular motor function

“…KIF5B knockdown showed that proteasome dynamics have a partial dependency on kinesin-1 and are consistent with the idea that the 26S proteasome is 'hitch-hiking' on different cargos in order to move. Interestingly, our results fit a model in which cytosolic proteins are transported by different types of movement -including the contribution of vesicle carriers under fast axonal transport (Scott et al, 2011;Tang et al, 2013). Furthermore, by high-resolution imaging and single-particle tracking, we identified and characterized diffusive modes of transport and actively driven transport that mediate the distribution of proteasomes within axons.…”

“…The movement of these and many cytosolic proteins has been suggested to depend on the interaction with fast motors for their stochastic movement (Scott et al, 2011). Recently, the fast movement of a carrier, now identified as moving vesicles, has been suggested to drive the anterograde slow axonal transport of the synapsin proteins, suggesting that there are many modes for the movement for soluble proteins (Tang et al, 2013). In this study, we tested whether the neuronal proteasome complex relies on different axonal transport mechanisms for movement in axons.…”

“…This slow average speed of soluble proteins depends on a probabilistic association with carriers that move under fast transport (Lasek et al, 1984;Scott et al, 2011). In support of this view, recent data has shown an axonal diffusion, with an anterograde bias, of synapsin that was mediated by the flux of fast moving vesicles (Tang et al, 2013). However, little is known about the transport modes that mediate the movement of large cytoplasmic protein complexes, such as the proteasome, along axons.…”

α-Synuclein Multimers Cluster Synaptic Vesicles and Attenuate Recycling

“…We transfected α-syn null neurons with VN/VC:α-syn (or Venus:α-syn) and visualized the entry of newly-synthesized (somatically-derived) fluorescent molecules into boutons (4-5 hours after transfection), adopting an imaging strategy that we recently developed ([21], see schematic in fig. 1E ).…”

“…Cultured α-syn −/− neurons were co-transfected with VN/VC:α-syn's (or Venus:α-syn) + soluble mCherry, and kinetics of initial α-syn entry and synaptic accumulation was evaluated by long-term imaging (see “results” and [21] for more details).…”