Molecular Basis of Transduction in Auditory Sensory Orga

Deafness & Other Communication Disorders

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Abstract

Stereocilia, are specialized actin protrusions on the surface of the sensory cells of the inner ear where the key step in the process of mechanical to electrical transduction takes place. In each cell, stereocilia are organized in bundles of precisely adjusted graded lengths forming characteristic staircase patterns. Recent studies on actin incorporation and turnover in stereocilia, as well as the identification of novel proteins that when mutated cause deafness through stereocilia disruption, provide unique insights into the mechanisms and molecules involved in the regulation of their steady state length. Our current research focuses on the role played by several stereocilia proteins that are associated with the regulation of the different compartments of the stereocilia structure. Class III myosins share structural features with myosins XVa, VIIa, and Ic including a canonical motor domain and a short regulatory neck domain containing a variable number of IQ motifs. Myosin III proteins are unique in that they contain an N-terminal kinase domain that undergoes auto-phosphorylation, and a region in their tail domain that functions as a second actin-binding site. Myosin IIIa has been implicated in DFNB30, a form of inherited progressive hearing loss. We used immunolabeling to show that myosin IIIa is localized at stereocilia tips. Myosin IIIa progressively accumulates during stereocilia maturation in a thimble-like pattern around the stereocilia tip. This pattern is distinct from the cap-like localization of myosin XVa and the shaft localization of myosin Ic. Overexpression of deletion mutants for functional domains of GFP-myosin IIIa shows that the motor domain, but not the actin-binding tail domain, is required for stereocilia tip localization. Deletion of the kinase domain produces stereocilia elongation and bulging of the stereocilia tips. We also evaluated the distribution and effects of GFP-myosin IIIa and the various myosin IIIa mutants on filopodia in transfected COS-7 cells. The spatial and temporal patterns of myosin IIIa localization at stereocilia tips and the effects of deletion mutants of myosin IIIa on stereocilia and other actin protrusions suggest that the pathophysiology of DFNB30 is likely to involve disruption of stereocilia structure and function. The thimble-like localization and the influence myosin IIIa has on stereocilia shape reveal a previously unrecognized molecular and functional compartment at the distal end of stereocilia, the site of both actin polymerization and operation of the mechanoelectrical transduction apparatus.