A Myosin Nanomotor Essential for Stereocilia Maintenance Expands the Etiology of Hereditary Hearing Loss DFNB3

Cochlear hair cells transduce sound using stereocilia, and disruption to these delicate mechanosensors is a significant cause of hearing loss. Stereocilia architecture is dependent upon the nanomotor myosin 15. A short isoform (MYO15A-2) drives stereocilia development by delivering an elongation-promoting complex (EC) to stereocilia tips, and an alternatively spliced long isoform (MYO15A-1) tunes postnatal size in shorter stereocilia, which possess mechanosensitive ion channels. Disruption of these functions causes two distinct stereocilia pathologies, which underly human autosomal recessive non-syndromic hearing loss DFNB3. Here, we characterize a new isoform, MYO15A-3, that increases expression in postnatal hair cells as the developmental MYO15A-2 isoform wanes reciprocally. We show the critical EC complex is initially delivered by MYO15A-2, followed by a postnatal handover to MYO15A-3, which continues to deliver the EC. In a Myo15a-3 mutant mouse, stereocilia develop normally with correct EC targeting, but lack the EC postnatally and do not maintain their adult architecture, leading to progressive hearing loss. We conclude MYO15A-3 delivers the EC in postnatal hair cells and that the EC and MYO15A-3 are both required to maintain stereocilia integrity. Our results add to the spectrum of stereocilia pathology underlying DFNB3 hearing loss and reveal new molecular mechanisms necessary for resilient hearing during adult life.