Mechanisms of Dynamic Recruitment of the ESCRT Pathway in Axons

Clearance of molecularly damaged and misfolded synaptic vesicle (SV) proteins is vital for the maintenance of healthy, functional synapses. However, this process poses significant trafficking challenges for neurons, as the majority of degradative organelles and machinery are localized in the somatodendritic compartment, far from SV pools in presynaptic terminals. Our previous work showed that SV protein degradation is mediated by the endosomal sorting complex required for transport (ESCRT) pathway in an activity-dependent manner. Moreover, we found that neuronal activity increased ESCRT protein recruitment to axons and SV pools, suggesting a novel mechanism for regulating the trafficking of this critical degradative machinery, whose localization and transport in neurons has been unexplored. Here, we characterize the axonal transport of ESCRT-0 proteins Hrs and STAM1, the first components of the ESCRT pathway, which are critical for initiating SV protein degradation. We find that Hrs- and STAM1-positive transport vesicles exhibit increased anterograde and bidirectional motility in response to neuronal activity, as well as frequent contact with SV pools. ESCRT-0 vesicles typically colocalize with early endosome marker Rab5, but their transport dynamics do not mirror those of the total Rab5 vesicle pool. Moreover, other ESCRT pathway components and effectors do not show activity-dependent changes to motility, indicating that neuronal firing specifically regulates the motility of the ESCRT-0+ subset of Rab5+ structures in axons. Finally, we identify kinesin-3 motor protein KIF13A as essential for the activity-dependent transport of ESCRT-0 vesicles as well as the degradation of SV membrane proteins. Altogether, these studies demonstrate a novel activity-dependent mechanism for mobilizing the axonal transport of a newly characterized endosomal subtype carrying ESCRT machinery. This activity-induced transport is necessary for ESCRT-mediated degradation of synaptic vesicle proteins.