|dc.description.abstracteng||Within the mammalian inner ear, sensory inner hair cells (IHCs) of the organ of Corti are required to transduce sound waves into electrical signals. Specialized synapses at the IHC basal pole, so-called ribbon synapses, facilitate the vivid release of the neurotransmitter glutamate over long time periods. Not only the exocytic release machinery but also the organization of the synaptic vesicle (SV) pools are adapted in these cells. SV release in IHCs is further balanced by robust membrane retrieval and SV reformation. Even though fast and slow forms of endocytosis have been described, to date, little is known about the molecular entities regulating SV recycling in IHCs.
In the here presented two studies, I examined the roles of two key players in neuronal endocytosis, namely endophilin-A1-3 and AP180, in IHC synaptic transmission. Combining various methodologies, I could show together with my collaborators, that both proteins seem to have several functions along the SV cycle. Both, endophilin-A and AP180, are involved in clathrin-dependent processes in IHCs: while AP180 recruits clathrin, endophilin-A regulates the fission and uncoating of clathrin-coated pits and vesicles. Especially for the clathrin-dependent reformation of SVs from endosome-like vacuoles (ELVs), the actions of those two proteins appear rate-limiting. The two proteins endophilin-A and AP180 seem further required for the tight coupling of exo- and endocytosis. We could show that absence of endophilin-A results in impaired sustained exocytosis, which points towards a deficit in SV replenishment or SV recruitment to the ribbon, or to impaired release site clearance. In either of these processes, endophilin-A may interact with otoferlin. Absence of AP180 resulted in a defect downstream of docking, which we attribute to a function of AP180 in release site clearance. Finally, we could for the first time show that endophilin-A functionally modulates presynaptic Ca2+-channels and promotes Ca2+-channel clustering at IHC active zones.
Taken together, these two studies on the endocytic proteins endophilin-A and AP180 demonstrate that the different processes mediating the SV cycle in IHCs are tightly coupled and balanced. However, they also illustrate the robustness of the SV recycling and transmitter release of the first auditory synapse towards molecular disruption of genes coding for endocytic proteins, as none of the examined mouse mutants exhibited severe hearing deficits.||de