Espin overexpression causes stereocilia defects and provides an anti‐capping effect on actin polymerization

Abstract

Stereocilia are actin-based projections of hair cells that are arranged in a step like array, in rows of increasing height, and that constitute the mechanosensory organelle used for the senses of hearing and balance. In order to function properly, stereocilia must attain precise sizes in different hair cell types and must coordinately form distinct rows with varying lengths. Espins are actin-bundling proteins that have a wellcharacterized role in stereocilia formation; loss of function mutations in Espin result in shorter stereocilia and deafness in the jerker mouse. Here we describe the generation of an Espin overexpressing transgenic mouse line that results in longer first row stereocilia and discoordination of second-row stereocilia length. Furthermore, Espin overexpression results in the misregulation of other stereocilia factors including GNAI3, GPSM2, EPS8, WHRN, and MYO15A, revealing that GNAI3 and GPSM2 are dispensable for stereocilia overgrowth. Finally, using an in vitro actin polymerization assay we show that espin provides an anti-capping function that requires both the Gactin binding WH2 domain as well as either the C-terminal F-actin binding domain or the internal xAB actin-binding domain. Our results provide a novel function for Espins at the barbed ends of actin filaments distinct from its previous known function of actin bundling that may account for their effects on stereocilia growth.

The role of Espin in the stereocilia regeneration and protection in Atoh1‐overexpressed cochlear epithelium

“…In addition, we found that in vivo Espin overexpression did not significantly increase the length of stereocilia in neonatal HCs. However, a previous study found that the length of stereocilia increased and became thicker at P60 after Espin overexpression, 20 which was inconsistent with our results. We increased the expression of Espin in regenerated HCs and in situ HCs with persistent overexpression of Atoh1, rather than HCs unaffected by Atoh1, where the expression of Atoh1 causes changes in HCs, so the stereocilia changes caused by Espin overexpression are inconsistent with the previous research.…”

“…Until P60, it was found that the stereocilia of HCs became thicker and the length of different rows of stereocilia increased differently, but no hearing function was tested. 20 Furthermore, an in vitro study found that Espin overexpression in regenerated vestibular HCs induced by Notch inhibition led to an increase in hair-bundle-like structures, suggesting that Espin overexpression might be involved in the injury or regeneration of stereocilia. 21 However, only in vitro experiments were conducted, and the effect on stereocilia in IHCs and OHCs was not studied in this study.…”