Human Molecular Genetics Advance Access originally published online on September 10, 2009
Human Molecular Genetics 2009 18(23):4615-4628; doi:10.1093/hmg/ddp429
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Myosin VI is required for the proper maturation and function of inner hair cell ribbon synapses
1 Inserm UMRS587, Unité de Génétique et Physiologie de l'Audition, Institut Pasteur, 25 rue du Dr Roux, 75724 Paris cedex 15, France, 2 School of Life Sciences, University of Sussex, Falmer, Brighton BN1 9QG, UK, 3 UPMC Paris 06, F75015 Paris, France, 4 Plate-Forme Microscopie Électronique, Institut Pasteur, 25 Rue du Docteur Roux, 75724 Paris Cedex 15, France and 5 Collège de France, F75015 Paris, France
* To whom correspondence should be addressed. Tel: +33 145688892; Fax: +33 140613442; saaid.safieddine{at}pasteur.fr (S.S.); c.j.kros{at}sussex.ac.uk (C.J.K.)
Received July 8, 2009; Accepted September 7, 2009
The ribbon synapses of auditory inner hair cells (IHCs) undergo morphological and electrophysiological transitions during cochlear development. Here we report that myosin VI (Myo6), an actin-based motor protein involved in genetic forms of deafness, is necessary for some of these changes to occur. By using post-embedding immunogold electron microscopy, we showed that Myo6 is present at the IHC synaptic active zone. In Snell's waltzer mutant mice, which lack Myo6, IHC ionic currents and ribbon synapse maturation proceeded normally until at least post-natal day 6. In adult mutant mice, however, the IHCs displayed immature potassium currents and still fired action potentials, as normally only observed in immature IHCs. In addition, the number of ribbons per IHC was reduced by 30%, and 30% of the remaining ribbons were morphologically immature. Ca2+-dependent exocytosis probed by capacitance measurement was markedly reduced despite normal Ca2+ currents and the large proportion of morphologically mature synapses, which suggests additional defects, such as loose Ca2+-exocytosis coupling or inefficient vesicular supply. Finally, we provide evidence that Myo6 and otoferlin, a putative Ca2+ sensor of synaptic exocytosis also involved in a genetic form of deafness, interact at the IHC ribbon synapse, and we suggest that this interaction is involved in the recycling of synaptic vesicles. Our findings thus uncover essential roles for Myo6 at the IHC ribbon synapse, in addition to that proposed in membrane turnover and anchoring at the apical surface of the hair cells.
Present address: Department of Otolaryngology—Head and Neck Surgery, The Johns Hopkins, University School of Medicine, Baltimore, MD 21205, USA.
Present address: Department of Biomedical Science, University of Sheffield, Sheffield S10 2TN, UK.