That chemical release allows nerve messages to be passed from one neuron to another. In inner hair cells, those neurotransmitter-filled vesicles are held in place at the cell membrane by tethers known as "ribbons."
The current study follows up a report by Petit's team several years ago that people with a recessive form of deafness harbor two abnormal copies of the otoferlin gene. They also had some evidence hinting that the gene might act as a calcium sensor with an important role in neurotransmitter release by the inner hair cells. For example, otoferlin resembles a calcium-sensing protein involved in release of chemicals by sensory neurons elsewhere in the body. Their current study provides additional evidence to confirm that notion.
They now report that otoferlin activity in the cochlea occurs only in the inner hair cells, where it concentrates in the ribbon-associated synaptic vesicles. They also found that the otoferlin protein binds calcium and interacts with other proteins known to play a role in neurotransmitter release.
To further examine the gene's role in a living animal, the researchers studied "knockout" mice completely lacking a functional otoferlin gene. When exposed to sounds of various frequencies, the mice showed no detectable activity in parts of the brain that normally process sound.
They further found that the profoundly deaf mice suffered a complete loss of neurotransmitter release from their inner hair cells, despite having an apparently normal "ribbon synapse" and calcium flow.
The findings led the researchers to conclude that "otoferlin is essential for a late step of [neurotransmitter release] and may act as the major [calcium] sensor triggering membrane fusion at the inner hair cell