Thesis defense – Thibault Peineau

Venue: Centre Broca Nouvelle-Aquitaine

Defense in french

Title

Synaptopathies des cellules ciliées auditives associées à un dysfonctionnement de l’exocytose calcium-dépendante

Abstract

Acoustic signals are encoded into electrical nerve impulses at the ribbon synapses of the cochlear inner hair cells (IHC). Exocytosis of the synaptic glutamatergic vesicles at these ribbon synapses is triggered by the activation of the voltage-dependent calcium channels CaV1.3 and required a specific calcium sensor: otoferlin. In the first part of my thesis, we studied the role of clarin-2, a newly identified tetraspan protein paralog of clarin-1, in these synapses. We know from previous studies that clarin-1, by forming a complex associating CaV1.3 channels, harmonin-b and actin-F, is essential for a compact spatial organization of the Ca1.3 channels at the active zone of the IHC ribbons. We show that clarin-2 is essential for a normal voltage-dependent activation of the IHC calcium channels and is likely part of the clarin-1 complex at the ribbon synapses. In the second part of my thesis, we studied the role of SNAP-25, a protein of the SNARE complex implicated in the docking, priming and fusion of the synaptic vesicles at neuronal central synapses but whose implication at the ribbon hair cell synapses remain still under debate. We show that mutant mice specifically deleted for SNAP-25 in hair cells are profoundly deaf and have a severe vestibular deficit. Electrophysiological patch-clamp recordings in IHCs and utricular hair cells indicated a severe defect in fast exocytosis. This synaptic functional defect was associated with a progressive degeneration of the ribbon synapses and the hair cells themselves, suggesting that SNAP-25 is also essential for the release of important neurotrophic factors. In the third part of my thesis, we investigated the functional changes of the aging ribbon synapses in C57BL/6J mice. We found that the aging synaptic IHC ribbons, while progressively degenerating, become larger with enlarged Ca2+ microdomains to produce a more sustained exocytosis of synaptic vesicles. The expression and the voltage dependence of the fast BK channels, essential for fast IHC repolarization, were also severely affected with aging. All these results suggested that the IHC ribbon synapses become potentiated with aging, a process that may partly explain the paradoxical hyperacusis phenomenon in presbyacusis.