PI(3)P signaling regulates endosomal flux underlying developmental synaptic remodeling via Rab4

Rab4 GTPase, essential for endosomal sorting and trafficking, is implicated in synaptic atrophy and dementia. To uncover the underlying mechanism, we studied the correlation between Rab4 vesicle transport in axons and episodic remodeling of synapses in the central nervous system (CNS) of Drosophila larvae. We found that synapse-bound traffic and presynaptic enrichment of Rab4 vesicles increase during the programmed, transient contraction of synapses in the ventral neuropil region at a specific larval stage. This coincides with the episodic activation of insulin and Vps34-mediated signaling, which elevates phosphatidylinositol-3-phosphate levels on Rab4 vesicles. The presence of this phospholipid on Rab4-associated vesicles recruit a PX-domain-containing motor protein, Klp98A, accelerating synapse-directed traffic. This, in turn, increases presynaptic enrichment of Rab4 during the developmentally programmed synapse contraction phase. Our findings elucidate the molecular mechanism that regulates developmental synaptic plasticity in the CNS via insulin signaling and directed axonal transport of endosomes. Author summaryHealthy brain function relies on the growth and remodeling of synaptic connections between nerve cells. This process involves carefully orchestrated movements of proteins and membranes inside the neurites. For instance, endosome movement in the nerve cell extensions helps guide neuronal growth, recycle parts of the synapse, and alter signal transmissions between neurons. Rab GTPases, a group of small G-proteins, are crucial for these processes. Here, we aimed to understand what triggers these processes and how the supply chain within nerve cell extensions is connected to changes at the synapse. In fruit fly larvae, we observed that the movement of Rab4 vesicles in the nerve cell extensions is inversely related to gross synapse content. A further examination revealed that onset of insulin signaling accelerates the Rab4 vesicles towards synapse through a lipid kinase called Vps34, which makes a signaling lipid called phospatidylinnositol-3-phosphate. Higher levels this lipid molecule on Rab4 vesicles attract a motor protein, Klp98A, speeding up their movement during the synaptic contraction phase. Our findings reveal a new mechanism that regulates Rab4 vesicle transport and its impact on synapse stability. The study further suggests potential targets for controlling age-related dementia likely to be caused by excessive insulin signaling in neurons.