Otoferlin, a multi C₂ domain protein shown to be essential for a late step in exocytosis of the auditory hair cells, is currently discussed to replace synaptotagmin (syt) at this excitatory synapse. This hypothesis is based on (i) otoferlin having 6 or 7 C₂ domains, of which 3 are predicted to bind Ca²⁺, (ii) the absence of syt 1, 2 and 3 at the first auditory synapse (Safieddine and Wenthold, 1999), (iii) the interaction of otoferlin with syntaxin 1 and SNAP-25 in immunoprecipitation assays and (iv) the absence of fast vesicle release in Otof^-/- hair cells (Roux et al, 2006).

In this study, we transduced auditory inner hair cells of Otof^-/- mice with syt 1 and tested exocytosis by patch-clamp capacitance measurements, but could not restore Ca²⁺-triggered exocytosis. Next, we transfected the developing otocysts of Otof^-/- embryos at E12 with syt 1 and measured hearing by auditory brainstem response in 3 week old animals. In comparison to the untransfected ear, no difference in hearing could be detected. Further, we analyzed exocytosis in autaptic cultures of syt1-deficient hippocampal neurons, but found no effect when overexpressing otoferlin.

Together, this study suggests that the mechanism of otoferlin function is different from syt action.