Synaptic endosomes as sorting stations for synaptic vesicle proteins. Valerie Uytterhoeven, Ine Maes, Sabine Kuenen, Jaroslaw Kasprowicz, Katarzyna Miskiewicz, Patrik Verstreken. Center for human Genetics, KU Leuven, Center for the Biology of Disease, VIB, Leuven, Vlaams-Brabant, Belgium.

   Neuronal terminals that are located far away from their cell bodies largely operate as independent units during long periods of stimulation. While many proteins in vesicle fusion and reformation are characterized, it is not known how synaptic terminals replace dysfunctional proteins and lipids and incorporate fresh ones to protect against synaptic ageing. Recently, we uncovered a protein we named Skywalker (Sky) that controls the sorting and degradation of dysfunctional proteins. Sky contains a TBC domain that is commonly found in GTPase Activating Proteins (GAPs). Our work indicates that Sky accelerates the GTPase activity of Rab35, a member of the Rab GTPase family that controls specific vesicle trafficking events, in vitro and in vivo at synapses. Furthermore, in sky mutants or in animals with constitutive active Rab35, newly formed synaptic vesicles are forced to travel excessively via an endosomal compartment at the nerve terminal. At these stations, dysfunctional, ubiquitinated synaptic vesicle proteins are recognized and sorted for degradation in the lysosome. As a consequence, vesicles that leave the endosome in sky mutants or in animals that express GTP-bound active Rab35, harbor a larger percentage of functional proteins in their membranes and as a result, sky mutants (or active Rab35) display increased neurotransmitter release. Hence, Sky controls synaptic ageing and loss of Sky function results in a more performant synaptic release apparatus. Sky defines a novel molecular mechanism that is used in neurons to control ageing and synaptic plasticity and our ongoing work is geared towards further elucidating the synaptic Sky-pathway using genetic modifier screens based on electrophysiology as well as using yeast two hybrid interaction screens.