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Control of exocytosis by synaptotagmins and otoferlin in auditory hair cells.

Maryline Beurg 1, * Nicolas Michalski 2 Saaid Safieddine 3 Yohan Bouleau 1 Ralf Schneggenburger 2 Edwin R Chapman 4 Christine Petit 3, * Didier Dulon 1, * 
* Corresponding author
1 Neurophysiologie de la Synapse Auditive
UB - Université de Bordeaux, INSERM - Institut National de la Santé et de la Recherche Médicale : U1120, CHU de Bordeaux Pellegrin [Bordeaux], Neuroscience Institute
3 Chaire Génétique et physiologie cellulaire
CdF (institution) - Collège de France
Abstract : In pre-hearing mice, vesicle exocytosis at cochlear inner hair cell (IHC) ribbon synapses is triggered by spontaneous Ca(2+) spikes. At the onset of hearing, IHC exocytosis is then exclusively driven by graded potentials, and is characterized by higher Ca(2+) efficiency and improved synchronization of vesicular release. The molecular players involved in this transition are still unknown. Here we addressed the involvement of synaptotagmins and otoferlin as putative Ca(2+) sensors in IHC exocytosis during postnatal maturation of the cochlea. Using cell capacitance measurements, we showed that Ca(2+)-evoked exocytosis in mouse IHCs switches from an otoferlin-independent to an otoferlin-dependent mechanism at postnatal day 4. During this early exocytotic period, several synaptotagmins (Syts), including Syt1, Syt2 and Syt7, were detected in IHCs. The exocytotic response as well as the release of the readily releasable vesicle pool (RRP) was, however, unchanged in newborn mutant mice lacking Syt1, Syt2 or Syt7 (Syt1(-/-), Syt2(-/-) and Syt7(-/-) mice). We only found a defect in RRP recovery in Syt1(-/-) mice which was apparent as a strongly reduced response to repetitive stimulations. In post-hearing Syt2(-/-) and Syt7(-/-) mutant mice, IHC synaptic exocytosis was unaffected. The transient expression of Syt1 and Syt2, which were no longer detected in IHCs after the onset of hearing, indicates that these two most common Ca(2+)-sensors in CNS synapses are not involved in mature IHCs. We suggest that otoferlin underlies highly efficient Ca(2+)-dependent membrane-membrane fusion, a process likely essential to increase the probability and synchrony of vesicle fusion events at the mature IHC ribbon synapse.
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Maryline Beurg, Nicolas Michalski, Saaid Safieddine, Yohan Bouleau, Ralf Schneggenburger, et al.. Control of exocytosis by synaptotagmins and otoferlin in auditory hair cells.. Journal of Neuroscience, 2010, 30 (40), pp.13281-90. ⟨10.1523/JNEUROSCI.2528-10.2010⟩. ⟨pasteur-01472847⟩



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