Presynaptic Mechanisms for Cargo Recruitment

Abstract

Delivery and assembly of presynaptic material over long axonal distances is a daunting challenge for neurons. Many presynaptic proteins are thought to be delivered from the Golgi apparatus via precursor vesicles, the recruitment of which at nerve terminals is crucial for the maintenance of presynaptic function. But how do these transport vesicles know where to stop along the axon? In most cellular secretory pathways, Golgi-derived cargos are labeled with Rab GTPases that are recognized by tethering complexes at their fusion sites. Surprisingly, we know very little about what presynaptic tethers and cargo labels facilitate presynaptic cargo capture. I here address how transport vesicles are captured at nerve terminals. I find that Rab6 resides in the neuronal Golgi apparatus, and its active form also localizes to nerve terminals. Rab6 associates with highly mobile vesicles in axons and it binds to ELKS in the vesicle-associated form via a short sequence near the C-terminus of ELKS. Genetic ablation of Rab6B leads to a loss of synaptic vesicles and active zone material from nerve terminals together with an accumulation of uncaptured vesicles throughout the axon. ELKS1 is a large coiled coil protein that is localized to nerve terminals and is broadly distributed within them. Strikingly, conditional neuronal knockout of ELKS1 phenocopies Rab6B ablation with a similar loss of presynaptic material in the bouton and cargo accumulation in the axon, accompanied by loss of synaptic Rab6. Reversing these phenotypes by ELKS1 rescue requires Rab6-binding of ELKS1. Finally, mistargeting of ELKS1 or Rab6 onto mitochondria leads to aberrant cargo capture, indicating that ELKS1 and Rab6 are sufficient to mediate capture. I propose the new model that ELKS1 acts as a presynaptic cargo tether, capturing Rab6-labeled precursor vesicles transported from the Golgi apparatus. This mechanism is important for regulating the assembly and maintenance of presynaptic nerve terminals.