Long isoforms of Espin and Espin-Like proteins are essential for development and maintenance of mechanotransducing stereocilia

Stereocilia are giant, specialized microvilli on auditory sensory cells in the inner ear. Their precise size and tiered organization are essential for function and are strictly controlled by actin-regulating proteins during postnatal development. Among these proteins, Espins (ESPNs) and Espin-like (ESPNL) proteins contribute to normal stereocilia development; however, the potential overlapping function between these families has not been explored. We found that ESPNL exists as both a long isoform and a novel short isoform. The short isoform localized differently from the long isoform and was lost from stereocilia when mechanotransduction was blocked. To understand how long isoforms contribute to stereocilia architecture, we generated mutations causing their loss while sparing short isoforms. While mutating ESPN-1 had little effect on stereocilia morphology or auditory function, loss of the long ESPNL isoform caused degeneration of shorter, mechanotransducing stereocilia rows, resulting in progressive hearing loss. Double mutant mice lacking both ESPN and ESPNL long isoforms showed a more rapid onset of the same phenotype, demonstrating that these proteins both contribute to mechanotransducing stereocilia development and maintenance.