Investigating Novel Molecular Regulators of the Auditory Ribbon Synapses of Mammalian Inner Hair Cells.

The auditory ribbon synapse is highly specialised to regulate the release of glutamate from IHCs and generate action potentials in auditory afferent fibres in response to small and graded changes in the receptor potential of IHCs. This is essential for maintaining the fidelity of auditory stimuli over wide range of frequencies and intensities. This study was aimed at identifying possible novel molecular regulators for the development and function of auditory ribbon synapses. I have used patch-clamp electrophysiology to record calcium currents and changes in membrane capacitance from IHCs to monitor vesicle fusion at the auditory ribbon synapse. The ubiquitously expressed calcium-sensor for vesicle priming, DOC2B was considered to be a potential priming factor at the auditory ribbon synapse. I have found that it is not involved in fast, calcium-dependent exocytosis of synaptic vesicles at the auditory ribbon synapse. The inositol 5’-phosphatase SynJ2 is required for the survival of auditory hair cells and is thought to regulate clathrin-mediated endocytosis. My results show that IHCs from SynJ2 KO mice display normal endocytic responses after exocytotic events and normal replenishment of the vesicle pools. Therefore, SynJ2 is not required for endocytosis, or vesicle recycling in IHCs. Connexins 26 and 30 are subunits of heteromeric gap-junctions and hemichannels in supporting cells of the cochlea. Connexin mutations cause over 50% of cases of non-syndromic hereditary deafness in humans. IHCs from mice with severely reduced connexin expression display larger calcium currents and smaller exocytotic responses. Therefore expression of connexins in the cochlea is essential for presynaptic function at the auditory ribbon synapse. Finally I have found that the transcriptional co-activator Wbp2 is not required for presynaptic function of mature IHCs, therefore Wbp2 is not involved in the transcription of key presynaptic molecules in IHCs.