Synaptotagmin IV determines the linear Ca2+ dependence of vesicle fusion at auditory ribbon synapses

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Abstract

Mammalian cochlear inner hair cells (IHCs) are specialized for the dynamic coding of continuous and finely graded sound signals. This ability is largely conferred by the linear Ca2+ dependence of neurotransmitter release at their synapses, which is also a feature of visual and olfactory systems. The prevailing hypothesis is that linearity in IHCs occurs through a developmental change in the Ca2+ sensitivity of synaptic vesicle fusion from the nonlinear (high order) Ca2+ dependence of immature spiking cells. However, the nature of the Ca2+ sensor(s) of vesicle fusion at hair cell synapses is unknown. We found that synaptotagmin IV was essential for establishing the linear exocytotic Ca2+ dependence in adult rodent IHCs and immature outer hair cells. Moreover, the expression of the hitherto undetected synaptotagmins I and II correlated with a high-order Ca2+ dependence in IHCs. We propose that the differential expression of synaptotagmins determines the characteristic Ca2+ sensitivity of vesicle fusion at hair cell synapses.

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Figure 1: Exocytotic Ca2+ dependence and ribbon morphology in control and knockout adult Syt IV IHCs.
Figure 2: Synaptotagmin IV has no role at immature IHC ribbon synapses.
Figure 3: Role of synaptotagmin IV at immature OHC ribbon synapses.
Figure 4: Syt IV expression in the gerbil and mouse cochlea.
Figure 5: Electrical properties of IHCs from Syt IV mice.
Figure 6: Synaptotagmin I is expressed in cochlear IHCs.
Figure 7: Distribution of synaptotagmin II in immature and adult IHCs.

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Acknowledgements

We would like to thank M.C. Holley and E. Smythe for their critical feedback on an earlier version of the manuscript. We would also like to thank A. Catapang for help with genotyping, M. Cardwell and A. Davids for their assistance with the breeding of Syt IV animals, and K. Rohbock and S. Kasperek for their technical assistance with the immunolabeling experiments. The rSytIV pIE construct was obtained from Addgene. This work was supported by grants from the Wellcome Trust, Deafness Research UK, University of Sheffield Devolved Funds and The Royal Society to W.M., Deutsche Forschungsgameinschft 316-4-1 to M.K., and the Baden-Württemberg Graduate Programme of the University of Tübingen to M.K. and J.E. W.M. is a Royal Society University Research Fellow.

Author information

S.L.J. carried out the electrophysiological experiments and analysis, and helped to design the experiments and to write the paper. C.F. and J.E. performed the immunolabeling and in situ hybridization experiments. D.N.F. carried out the TEM experiments. L.R. performed hearing measurements. S.K., M.N.R., J.E., E.P.S., S.M. and H.R.H. were involved in study design and data interpretation. S.L.J. had the initial idea for the possible involvement of Syt IV in IHC synaptic activity. M.K. supervised and analyzed immunolabeling experiments. W.M. and M.K. designed the research. W.M. conceived and coordinated the study, supervised the experiments, participated in data collection and analysis, and wrote the paper. All authors discussed the results and commented on the manuscript.

Correspondence to Walter Marcotti.

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Johnson, S., Franz, C., Kuhn, S. et al. Synaptotagmin IV determines the linear Ca2+ dependence of vesicle fusion at auditory ribbon synapses. Nat Neurosci 13, 45–52 (2010) doi:10.1038/nn.2456

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