Live Observation of Two Parallel Membrane Degradation Pathways at Axon Terminals

Abstract

SUMMARY Neurons are highly polarized cells that require continuous turnover of membrane proteins at axon terminals to develop, function, and survive. Yet, it is still unclear whether membrane protein degradation requires transport back to the cell body or whether degradation also occurs locally at the axon terminal, where live observation of sorting and degradation has remained a challenge. Here, we report direct observation of two cargo-specific membrane protein degradation mechanisms at axon terminals based on a live-imaging approach in intact Drosophila brains. We show that different acidification-sensing cargo probes are sorted into distinct classes of degradative “hub” compartments for synaptic vesicle proteins and plasma membrane proteins at axon terminals. Sorting and degradation of the two cargoes in the separate hubs are molecularly distinct. Local sorting of synaptic vesicle proteins for degradation at the axon terminal is, surprisingly, Rab7 independent, whereas sorting of plasma membrane proteins is Rab7 dependent. The cathepsin-like protease CP1 is specific to synaptic vesicle hubs, and its delivery requires the vesicle SNARE neuronal synaptobrevin. Cargo separation only occurs at the axon terminal, whereas degradative compartments at the cell body are mixed. These data show that at least two local, molecularly distinct pathways sort membrane cargo for degradation specifically at the axon terminal, whereas degradation can occur both at the terminal and en route to the cell body.

The Virtuous Cycle of Axon Growth: Axonal Transport of Growth‐Promoting Machinery as an Intrinsic Determinant of Axon Regeneration

“…There is evidence that as axons lose the necessity for growth and instead become geared for neurotransmission, there is a change in the type of endosomal recycling machinery present, with a decline in receptor and membrane protein recycling and a shift toward synaptic vesicle recycling (Bonanomi et al, ). Recent evidence suggests that membrane proteins and synaptic vesicle proteins are also degraded through separate pathways in axon terminals, in support of the hypothesis that the machinery for synaptic vesicle turnover is distinct from that which is required for membrane protein turnover (Jin et al, ). Moreover, the development of synapses has recently been associated with regenerative decline (Tedeschi et al, ).…”

Autophagy-dependent filopodial kinetics restrict synaptic partner choice during Drosophila brain wiring

“…7a-c). Previous work in primary vertebrate neuronal culture as well as Drosophila R1-R6 photoreceptors has shown that autophagosomes formed at axon terminals traffic retrogradely to the cell body 29,44 . We therefore analyzed photoreceptor cell bodies and found large Atg8-positive multivesicular bodies containing Syd-1 (Supplementary Fig.…”

Vps54 regulatesDrosophilaneuromuscular junction development and controls postsynaptic density composition via a Rab7-dependent mechanism

“…Rab5 (EEs), Rab7 (LEs), and Rab11 (REs) all traffic in vertebrate axons and have been implicated in the control of axon growth and guidance (Falk et al, 2014; Ponomareva et al, 2016; van Bergeijk et al, 2015). Membrane and transmembrane proteins are transported to axon terminals via a Rab11-dependent mechanism whereas Rab5 and Rab7 are involved in local recycling and retrograde transport back to the soma (Jin et al, 2018; Khodosh et al, 2006). In Drosophila , a loss-of-function mutation in Rab5 causes defects in axon elongation in olfactory projection neurons and sensory neurons (Sakuma et al, 2014; Satoh et al, 2008).…”