Department of Physiology, College of Medicine, University of Kentucky, Lexington, KY, 40536, USA.
J Assoc Res Otolaryngol. 2020 Apr;21(2):121-135. doi: 10.1007/s10162-020-00745-4. Epub 2020 Mar 9.
Mammalian hair cells develop their mechanosensory bundles through consecutive phases of stereocilia elongation, thickening, and retraction of supernumerary stereocilia. Many molecules involved in stereocilia elongation have been identified, including myosin-XVa. Significantly less is known about molecular mechanisms of stereocilia thickening and retraction. Here, we used scanning electron microscopy (SEM) to quantify postnatal changes in number and diameters of the auditory hair cell stereocilia in shaker-2 mice (Myo15) that lack both "long" and "short" isoforms of myosin-XVa, and in mice lacking only the "long" myosin-XVa isoform (Myo15). Previously, we observed large mechanotransduction current in young postnatal inner (IHC) and outer (OHC) hair cells of both these strains. Stereocilia counts showed nearly identical developmental retraction of supernumerary stereocilia in control heterozygous, Myo15, and Myo15 mice, suggesting that this retraction is largely unaffected by myosin-XVa deficiency. However, myosin-XVa deficiency does affect stereocilia diameters. In control, the first (tallest) and second row stereocilia grow in diameter simultaneously. However, the third row stereocilia in IHCs grow only until postnatal day 1-2 and then become thinner. In OHCs, they also grow slower than taller stereocilia, forming a stereocilia diameter gradation within a hair bundle. The sh2 mutation disrupts this gradation and makes all stereocilia nearly identical in thickness in both IHCs and OHCs, with only subtle residual diameter differences. All Myo15 stereocilia grow postnatally including the third row, which is not a part of normal development. Serial sections with focused ion beam (FIB)-SEM confirmed that diameter changes of Myo15 IHC and OHC stereocilia resulted from corresponding changes of their actin cores. In contrast to Myo15, Myo15 hair cells develop prominent stereocilia diameter gradation. Thus, besides building the staircase, the short isoform of myosin-XVa is essential for controlling the diameter of the third row stereocilia and formation of the stereocilia diameter gradation in a hair bundle.
哺乳动物毛细胞通过连续的阶段发育其机械敏感束，包括纤毛的伸长、加粗和多余纤毛的回缩。已经鉴定出许多参与纤毛伸长的分子，包括肌球蛋白-XVa。然而，对于纤毛加粗和回缩的分子机制知之甚少。在这里，我们使用扫描电子显微镜（SEM）来量化缺乏肌球蛋白-XVa 的“长”和“短”两种同工型的 shaker-2 小鼠（Myo15）以及仅缺乏“长”肌球蛋白-XVa 同工型的小鼠（Myo15）的听觉毛细胞纤毛在出生后的数量和直径的变化。此前，我们观察到这两种品系的年轻出生后内（IHC）和外（OHC）毛细胞中都有较大的机械转导电流。纤毛计数显示，在对照杂合子、Myo15 和 Myo15 小鼠中，多余纤毛的发育回缩几乎相同，这表明肌球蛋白-XVa 的缺乏对这种回缩的影响不大。然而，肌球蛋白-XVa 的缺乏确实会影响纤毛的直径。在对照品系中，第一（最高）和第二排纤毛同时加粗。然而，IHC 的第三排纤毛仅在出生后 1-2 天生长，然后变细。在 OHC 中，它们的生长速度也比较高的纤毛慢，在毛束内形成纤毛直径梯度。sh2 突变破坏了这种梯度，使得 IHC 和 OHC 中的所有纤毛几乎在厚度上都相同，只有细微的残余直径差异。所有 Myo15 纤毛在出生后都生长，包括第三排，这不是正常发育的一部分。使用聚焦离子束（FIB）-SEM 的连续切片证实，Myo15 IHC 和 OHC 纤毛的直径变化是由于其肌动蛋白核心的相应变化。与 Myo15 相反，Myo15 毛细胞发育出明显的纤毛直径梯度。因此，除了构建楼梯外，肌球蛋白-XVa 的短同工型对于控制第三排纤毛的直径和毛束中纤毛直径梯度的形成也是必不可少的。
