Otoferlin, Defective in a Human Deafness Form, Is Essential for Exocytosis at the Auditory Ribbon Synapse

Abstract

The auditory inner hair cell (IHC) ribbon synapse operates with an exceptional temporal precision and maintains a high level of neurotransmitter release. However, the molecular mechanisms underlying IHC synaptic exocytosis are largely unknown. We studied otoferlin, a predicted C2-domain transmembrane protein, which is defective in a recessive form of human deafness. We show that otoferlin expression in the hair cells correlates with afferent synaptogenesis and find that otoferlin localizes to ribbon-associated synaptic vesicles. Otoferlin binds Ca(2+) and displays Ca(2+)-dependent interactions with the SNARE proteins syntaxin1 and SNAP25. Otoferlin deficient mice (Otof(-/-)) are profoundly deaf. Exocytosis in Otof(-/-) IHCs is almost completely abolished, despite normal ribbon synapse morphogenesis and Ca(2+) current. Thus, otoferlin is essential for a late step of synaptic vesicle exocytosis and may act as the major Ca(2+) sensor triggering membrane fusion at the IHC ribbon synapse.

Differential dependence of phasic transmitter release on synaptotagmin 1 at GABAergic and glutamatergic hippocampal synapses

“…As a single synaptic vesicle contains Ϸ15 synaptotagmin molecules (35), the formation of heterooligomeric assemblies may be an additional complication. Finally, we cannot exclude that an entirely different protein triggers release at the BC-GC synapse in knockout mice, as recently suggested for auditory hair cells (36) and the calyx of Held (37).…”

Otoferlin is a calcium sensor that directly regulates SNARE-mediated membrane fusion

“…These mice also have functional afferent neurons, suggesting that otoferlin's role in the auditory pathway lies within the hair cell between the mechanoelectrical transduction and afferent neuron activation steps. In support of this idea, hair cells in otoferlin knockout mice do not release neurotransmitter in response to membrane depolarization and calcium influx, despite having apparently normal synapse morphology (Roux et al, 2006). O toferlin is a large multi-C2 domain protein pro posed to act as a calcium sensor that regulates synaptic vesicle exocytosis in cochlear hair cells.…”

Exocytosis at the hair cell ribbon synapse apparently operates without neuronal SNARE proteins

“…The IHC synaptic machinery differs from conventional synapses. For example, IHC synapses lack synaptotagmins I-III 1 and complexins 2,3 , but express the ribbon-protein RIBEYE 4 and the multi-C 2 -domain protein otoferlin 5 . Previous studies indicated that the neuronal SNAREs (SNAP [Soluble NSF…”