Different CaV1.3 Channel Isoforms Control Distinct Components of the Synaptic Vesicle Cycle in Auditory Inner Hair Cells.

Philippe F.Y. Vincent, Yohan Bouleau, Gilles Charpentier, Alice Emptoz, Saaid Safieddine, Christine Petit, Didier Dulon
J. Neurosci.. 2017-02-13; 37(11): 2960-2975
DOI: 10.1523/jneurosci.2374-16.2017

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1. J Neurosci. 2017 Mar 15;37(11):2960-2975. doi: 10.1523/JNEUROSCI.2374-16.2017.
Epub 2017 Feb 13.

Different CaV1.3 Channel Isoforms Control Distinct Components of the Synaptic
Vesicle Cycle in Auditory Inner Hair Cells.

Vincent PF(1), Bouleau Y(1), Charpentier G(1), Emptoz A(2), Safieddine S(2),
Petit C(2)(3), Dulon D(4).

Author information:
(1)Université de Bordeaux, Institut des Neurosciences de Bordeaux, Equipe
Neurophysiologie de la Synapse Auditive, Inserm, Unité Mixte de Recherche en
Santé 1120, Centre Hospitalier Universitaire Hôpital Pellegrin, 33076 Bordeaux,
France.
(2)Institut Pasteur et Université Pierre et Marie Curie, Unité de Génétique et
Physiologie de l’Audition, Inserm, Unité Mixte de Recherche en Santé 1120, 75015
Paris, France, and.
(3)Collège de France, 75005 Paris, France.
(4)Université de Bordeaux, Institut des Neurosciences de Bordeaux, Equipe
Neurophysiologie de la Synapse Auditive, Inserm, Unité Mixte de Recherche en
Santé 1120, Centre Hospitalier Universitaire Hôpital Pellegrin, 33076 Bordeaux,
France, .

The mechanisms orchestrating transient and sustained exocytosis in auditory inner
hair cells (IHCs) remain largely unknown. These exocytotic responses are believed
to mobilize sequentially a readily releasable pool of vesicles (RRP) underneath
the synaptic ribbons and a slowly releasable pool of vesicles (SRP) at farther
distance from them. They are both governed by Cav1.3 channels and require
otoferlin as Ca2+ sensor, but whether they use the same Cav1.3 isoforms is still
unknown. Using whole-cell patch-clamp recordings in posthearing mice, we show
that only a proportion (∼25%) of the total Ca2+ current in IHCs displaying fast
inactivation and resistance to 20 μm nifedipine, a l-type Ca2+ channel blocker,
is sufficient to trigger RRP but not SRP exocytosis. This Ca2+ current is likely
conducted by short C-terminal isoforms of Cav1.3 channels, notably Cav1.342A and
Cav1.343S, because their mRNA is highly expressed in wild-type IHCs but poorly
expressed in Otof-/- IHCs, the latter having Ca2+ currents with considerably
reduced inactivation. Nifedipine-resistant RRP exocytosis was poorly affected by
5 mm intracellular EGTA, suggesting that the Cav1.3 short isoforms are closely
associated with the release site at the synaptic ribbons. Conversely, our results
suggest that Cav1.3 long isoforms, which carry ∼75% of the total IHC Ca2+ current
with slow inactivation and confer high sensitivity to nifedipine and to internal
EGTA, are essentially involved in recruiting SRP vesicles. Intracellular Ca2+
imaging showed that Cav1.3 long isoforms support a deep intracellular diffusion
of Ca2+SIGNIFICANCE STATEMENT Auditory inner hair cells (IHCs) encode sounds into
nerve impulses through fast and indefatigable Ca2+-dependent exocytosis at their
ribbon synapses. We show that this synaptic process involves long and short
C-terminal isoforms of the Cav1.3 Ca2+ channel that differ in the kinetics of
their Ca2+-dependent inactivation and their relative sensitivity to the l-type
Ca2+ channel blocker nifedipine. The short C-terminal isoforms, having fast
inactivation and low sensitivity to nifedipine, mainly control the fast fusion of
the readily releasable pool (RRP); that is, they encode the phasic exocytotic
component. The long isoforms, with slow inactivation and great sensitivity to
nifedipine, mainly regulate the vesicular replenishment of the RRP; that is, the
sustained or tonic exocytosis.

Copyright © 2017 the authors 0270-6474/17/372960-16$15.00/0.

DOI: 10.1523/JNEUROSCI.2374-16.2017
PMID: 28193694 [Indexed for MEDLINE]

Auteurs Bordeaux Neurocampus