Muscle-like nicotinic receptor accessory molecules in sensory hair cells of the inner ear

Nothing is known about the regulation of nicotinic acetylcholine receptors (nAChRs) in hair cells of the inner ear. MuSK, rapsyn and RIC-3 are accessory molecules associated with muscle and brain nAChR function. We demonstrate that these accessory molecules are expressed in the inner ear raising the possibility of a muscle-like mechanism for clustering and assembly of nAChRs in hair cells. We focused our investigations on rapsyn and RIC-3. Rapsyn interacts with the cytoplasmic loop of nAChR alpha9 subunits but not nAChR alpha10 subunits. Although rapsyn and RIC-3 increase nAChR alpha9 expression, rapsyn plays a greater role in receptor clustering while RIC-3 is important for acetylcholine-induced calcium responses. Our data suggest that RIC-3 facilitates receptor function, while rapsyn enhances receptor clustering at the cell surface.