Molecular Basis Of Transduction In Auditory Sensory Orga

静纤毛是负责听觉和前庭感觉功能的关键细胞器，它被组织成一束束高度递增的精确指定的行，形成特征性的楼梯图案。虽然静纤毛对机械振动非常敏感，结构整齐，容易受到过度刺激而受损，但它们一生都保持着正常的工作状态。每个静纤毛由数百个平行的、均匀极化的、规则交联的肌动蛋白丝组成的刚性准晶体阵列支撑。我们先前已经证明，在毛细胞的感觉静纤毛的核心处的看似静态的肌动蛋白类晶体通过在静纤毛尖端自我复制、向后铣削和在基部自我分解而经历连续的更新（Schneider et al. Nature，418：837，2002）。我们现在已经使用了相同的方法，用肌动蛋白绿色荧光蛋白（GFP），espin-GFP和肌球蛋白XVa-GFP转染毛细胞来表征周转过程。肌动蛋白和espin在次晶尖端结合，并以相同的速率向后流动。通量率（~0.002？0.04肌动蛋白亚基每秒）与静纤毛长度成正比，使得整个阶梯静纤毛束同步翻转。肌动蛋白聚合抑制细胞松弛素D引起静纤毛缩短率匹配旁晶体营业额。肌球蛋白Ic和VIIa定位在肌动蛋白旁晶旁，在那里它们可以驱动肌动蛋白逆行流动。肌球蛋白XVa在静纤毛尖端观察到，与称为尖端密度的结构一致，水平与静纤毛长度成比例。电子显微镜分析shaker 2（肌球蛋白XVa突变体）小鼠的异常短静纤毛未显示特征性尖端密度结构。我们还表明，静纤毛长度调制的espin和肌球蛋白XVa的表达水平，以及由当地的物理参数，如张力的静纤毛链接和封装膜。我们认为，调节肌动蛋白聚合和微铣削动态形状的静纤毛的功能结构，并在过度刺激的恢复中发挥了核心作用。

Stereocilia, the key cellular organelles responsible for auditory and vestibular sensory function, are organized into bundles of precisely specified rows of increasing heights forming characteristic staircase patterns. Although stereocilia are exquisitely sensitive to mechanical vibration, orderly structured, and easily damaged by over stimulation, they are maintained in proper working order for an entire lifetime. Each stereocilium is supported by a rigid paracrystalline array of several hundred parallel, uniformly polarized and regularly cross-linked actin filaments. We have previously shown that the seemingly static actin paracrystal at the core of sensory stereocilia of hair cells undergoes continuous renewal by reproducing itself at the stereocilia tips, treadmilling rearwards, and dismantling itself at the base (Schneider et al. Nature, 418: 837, 2002). We have now used the same approach of transfecting hair cells with actin-green fluorescent protein (GFP), espin-GFP, and myosin XVa-GFP to characterize the turnover process. Actin and espin are incorporated at the paracrystal tip and flow rearwards at the same rate. The flux rates (~0.002?0.04 actin subunits per second) were proportional to the stereocilia length so that the entire staircase stereocilia bundle was turned over synchronously. Actin polymerization inhibition by cytochalasin D caused stereocilia to shorten at rates matching paracrystal turnover. Myosins Ic and VIIa were localized alongside the actin paracrystal where they could drive retrograde actin flow. Myosin XVa was observed at the stereocilia tips, coinciding with a structure called the tip density, at levels proportional to stereocilia lengths. Electron microscopy analysis of the abnormally short stereocilia in the shaker 2 (myosin XVa mutant) mice did not show the characteristic tip density structure. We also show that stereocilia length is modulated by the expression levels of espin and myosin XVa as well as by local physical parameters, such as tension on the stereocilia links and on the encapsulating membrane. We argue that regulation of actin polymerization and treadmilling dynamically shapes the functional architecture of stereocilia and plays a central role in recovery from over-stimulation.