title: Exocytotic machineries of vestibular type I and cochlear ribbon synapses display similar intrinsic otoferlin-dependent Ca2+ sensitivity but a different coupling to Ca2+ channels.
abstract: The hair cell ribbon synapses of the mammalian auditory and vestibular systems differ greatly in their anatomical organization and firing properties. Notably, vestibular Type I hair cells (VHC-I) are surrounded by a single calyx-type afferent terminal that receives input from several ribbons, whereas cochlear inner hair cells (IHCs) are contacted by several individual afferent boutons, each facing a single ribbon. The specificity of the presynaptic molecular mechanisms regulating transmitter release at these different sensory ribbon synapses is not well understood. Here, we found that exocytosis during voltage activation of Ca(2+) channels displayed higher Ca(2+) sensitivity, 10 mV more negative half-maximum activation, and a smaller dynamic range in VHC-I than in IHCs. VHC-I had a larger number of Ca(2+) channels per ribbon (158 vs 110 in IHCs), but their Ca(2+) current density was twofold smaller because of a smaller open probability and unitary conductance. Using confocal and stimulated emission depletion immunofluorescence microscopy, we showed that VHC-I had fewer synaptic ribbons (7 vs 17 in IHCs) to which Cav1.3 channels are more tightly organized than in IHCs. Gradual intracellular Ca(2+) uncaging experiments revealed that exocytosis had a similar intrinsic Ca(2+) sensitivity in both VHC-I and IHCs (KD of 3.3 ± 0.6 μM and 4.0 ± 0.7 μM, respectively). In otoferlin-deficient mice, exocytosis was largely reduced in VHC-I and IHCs. We conclude that VHC-I and IHCs use a similar micromolar-sensitive otoferlin Ca(2+) sensor and that their sensory encoding specificity is essentially determined by a different functional organization of Ca(2+) channels at their synaptic ribbons.
authors.affiliation: Université de Bordeaux, Institut des Neurosciences de Bordeaux, Equipe Neurophysiologie de la Synapse Auditive, Inserm, Unité Mixte de Recherche 1120, Centre Hospitalier Universitaire hôpital Pellegrin, 33076 Bordeaux, France, didier.dulon@inserm.fr philippe.vincent@inserm.fr.
authors.forename: Philippe F Y
authors.initials: PF
authors.lastname: Vincent
authors.affiliation: Université de Bordeaux, Institut des Neurosciences de Bordeaux, Equipe Neurophysiologie de la Synapse Auditive, Inserm, Unité Mixte de Recherche 1120, Centre Hospitalier Universitaire hôpital Pellegrin, 33076 Bordeaux, France.
authors.forename: Yohan
authors.initials: Y
authors.lastname: Bouleau
authors.affiliation: Institut Pasteur et Université Pierre et Marie Curie, Unité de Génétique et Physiologie de l'Audition, Inserm Unité Mixte de Recherche 1120, 75015 Paris, France, and.
authors.forename: Saaid
authors.initials: S
authors.lastname: Safieddine
authors.affiliation: Institut Pasteur et Université Pierre et Marie Curie, Unité de Génétique et Physiologie de l'Audition, Inserm Unité Mixte de Recherche 1120, 75015 Paris, France, and Collège de France, 75005 Paris, France.
authors.forename: Christine
authors.initials: C
authors.lastname: Petit
authors.affiliation: Université de Bordeaux, Institut des Neurosciences de Bordeaux, Equipe Neurophysiologie de la Synapse Auditive, Inserm, Unité Mixte de Recherche 1120, Centre Hospitalier Universitaire hôpital Pellegrin, 33076 Bordeaux, France, didier.dulon@inserm.fr philippe.vincent@inserm.fr.
authors.forename: Didier
authors.initials: D
authors.lastname: Dulon
mesh_terms: D000818:Animals; D002118:Calcium / Q000378:metabolism*; D015220:Calcium Channels / Q000378:metabolism*; D003051:Cochlea / Q000378:metabolism / Q000502:physiology; D005089:Exocytosis / Q000502:physiology*; D018069:Hair Cells, Vestibular / Q000378:metabolism / Q000502:physiology*; D008565:Membrane Proteins / Q000378:metabolism*; D051379:Mice; D009925:Organ of Corti / Q000378:metabolism / Q000502:physiology*; D013569:Synapses / Q000502:physiology*
chemical_list: D015220:Calcium Channels; D008565:Membrane Proteins; C516233:otoferlin protein, mouse; D002118:Calcium
keywords: auditory; exocytosis; hair cell; otoferlin; synapse; vestibular