MYO7a is required for the functional integrity of the mechanoelectrical transduction complex in the outer hair cells of the adult cochlea.. MYO7a is required for the functional integrity of the mechanoelectrical transduction complex in the outer hair cells of the adult cochlea.

Myosin-VIIA (MYO7A) in an unconventional myosin responsible for syndromic (Usher 1B) or non-syndromic recessive deafness in humans when mutated. In the cochlea, MYO7A is expressed in the stereocilia of both inner and outer hair cells, where it is believed to act as the motor protein tensioning the mechanoelectrical transducer (MET) channel. Whether this tensioning role is a common feature among both types of cochlear hair cells is unknown. Here we show that MYO7A has a distinct role in adult outer hair cells (OHCs), being crucial for the structural integrity of the MET complex. Postnatal deletion of MYO7A did not affect the staircase structure of the hair bundles but caused a progressive reduction of the size of the MET current without affecting the resting open probability and calcium sensitivity of the MET channel. The hair bundle of OHCs deficient in MYO7A also showed reduced bundle stiffness and was highly susceptible to noise exposure. RNA-sequencing identified the down-regulation of several stereociliary proteins in the Myo7a-deficient cochlea. This study reveals that IHCs and OHCs use different mechanisms to maintain the MET channel in its most sensitive resting open position, and confirms that in OHCs, this mechanism is MYO7A independent. Overall design: For the conditional knockout mice (Myo7afl/flMyo15-cre+/-), the targeted tm1a allele for Myo7a (Myo7atm1a(EUCOMM)Wtsi allele ID: 4431921) was generated by the Mouse Genetics Programme at the Wellcome Trust Sanger Institute (Cambridge, UK). For Myo7a the critical exons 10 and 11 were floxed and the tm1c allele was obtained by crossing the tm1a mouse to a FLPeR carrying mouse (Rosa26Fki). The tm1d alleles, which were used for the experiments, were obtained by crossing the tm1c mouse (Myo7afl/fl) with the Myo15-cre mice. For experiments littermate control Myo7afl/flMyo15-cre+/+ (referred in the manuscript as Myo7afl/fl) and knockout mice (Myo7afl/flMyo15-cre+/-) were used.

肌球蛋白VIIA（Myosin-VIIA, MYO7A）属于非传统肌球蛋白，人类发生突变时可引发综合征性（乌谢尔综合征1B型，Usher 1B）或非综合征性隐性遗传性耳聋。在耳蜗中，MYO7A表达于内毛细胞（inner hair cells, IHCs）与外毛细胞（outer hair cells, OHCs）的静纤毛中，被认为是张紧机械电转导（mechanoelectrical transducer, MET）通道的动力蛋白。目前尚不明确该张紧作用是否为两类耳蜗毛细胞的共有特征。
本研究发现，MYO7A在成年外毛细胞中具有独特功能，对MET复合物的结构完整性至关重要。出生后敲除MYO7A不会影响毛细胞束的阶梯状结构，但会导致MET电流大小进行性降低，且不改变MET通道的静息开放概率与钙敏感性。MYO7A缺陷的外毛细胞毛束还表现出毛束刚度下降，且极易受到噪声暴露的损伤。RNA测序结果显示，Myo7a缺陷的耳蜗中多种静纤毛蛋白的表达水平显著下调。
本研究揭示，内毛细胞与外毛细胞维持MET通道处于最敏感的静息开放状态的机制存在差异，并证实外毛细胞的该维持机制不依赖MYO7A。
实验整体设计：本研究使用的条件性敲除小鼠（Myo7afl/flMyo15-cre+/-）的靶向tm1a等位基因（Myo7atm1a(EUCOMM)Wtsi，等位基因ID：4431921）由英国剑桥威康信托桑格研究所（Wellcome Trust Sanger Institute）小鼠遗传学项目构建。Myo7a的关键外显子10与11被loxP位点侧翼标记，通过将tm1a小鼠与携带FLPeR的小鼠（Rosa26Fki）杂交获得tm1c等位基因。本实验所用的tm1d等位基因通过将tm1c小鼠（Myo7afl/fl）与Myo15-cre小鼠杂交获得。实验对照为同窝野生型小鼠Myo7afl/flMyo15-cre+/+（本文中记为Myo7afl/fl）与敲除小鼠Myo7afl/flMyo15-cre+/-。