To convert the complicated temporal structure and intensity of sound into nerve transmission, the sensory inner hair cells (IHCs) of the cochlea must signal to the auditory nerves with extraordinary precision. The IHC has an unconventional synaptic structure; however, little is known about the molecular mechanisms underlying its specialized function. Now Christine Petit and colleagues, reporting in Cell, identify otoferlin as an essential component of IHCs that controls immediate neurotransmitter exocytosis in response to IHC depolarization.
Defects in the otoferlin gene cause a severe form of human deafness. To investigate the function of otoferlin, Petit's group first determined its localization in the mouse cochlea. By immunofluorescence analysis they showed that otoferlin was localized to the IHCs, and closer ultrastructural examination by immunogold electron microscopy revealed that otoferlin specifically localizes to IHC synaptic vesicles.
The IHC synaptic structure of mice that lack otoferlin forms normally. However, otoferlin-deficient mice were deaf and did not show auditory brain stem activity in response to sound stimulation. Given the localization of otoferlin, failure at the IHC synapse seemed most likely to blame.
The team found that the number of synaptic vesicles in otoferlin-deficient IHCs was similar to that of wild-type cells and, furthermore, the localization of these vesicles to the presynaptic plasma membrane was also unaffected. Exocytosis of these synaptic vesicles in response to IHC calcium influx, however, was almost completely absent in otoferlin-deficient mice. Otoferlin therefore seems to control the last step in the IHC exocytosis pathway.
The amino acid sequence of otoferlin predicts a transmembranous calcium binding protein, and the team showed that otoferlin was indeed able to bind calcium. In addition, immunoprecipitation and in vitro binding experiments revealed that otoferlin interacts with components of the synaptic secretory machinery in a calcium-dependent manner.
It has been reported that depolarization-induced exocytosis is linearly dependent on the amount of presynaptic calcium influx, which in IHCs means a direct relationship between sound intensity and exocytosis. The authors propose that the ability of otoferlin to bind calcium and the dependence on calcium binding for interaction with the synaptic secretory machinery allow otoferlin to act as a calcium trigger, inducing a rapid and precise exocytosis in response to sound vibrations that depolarize the IHC. The authors raise the interesting possibility that otoferlin substitutes for synaptotagmin I and II, which have so far not been detected in IHCs.