Molecular Basis of Transduction in Auditory Sensory Orga

立体纤毛是内耳感觉细胞表面的特殊肌动蛋白突起，是机械到电转导过程的关键步骤。在每个细胞中，立体纤毛被精确地排列成一束，形成特有的阶梯状。立体纤毛的组织模式和尺寸与特定的刺激频率选择性相匹配。立体纤毛是非常机械敏感的，很容易被过度刺激破坏。随之而来的一个重要问题是：它们是如何保持有序的结构，如此敏感，并在一生中持续发挥作用的？答案可能是存在一个快速的本构转换和立体纤毛分子成分的再循环。我们以前已经表明，感觉立体纤毛的细胞骨架核心经历了持续的快速更新。然而，立体纤毛膜成分的更新仍然知之甚少。我们研究了质膜Ca2+- atp酶异构体2 （PMCA2）的空间分布、迁移和运输，作为模型系统来评估立体纤毛膜蛋白更新的程度和机制。PMCA2定位于整个立体纤毛膜，在那里它是高度丰富和均匀分布的。我们测量了组成表达PMCA2的内在化率，我们分析了GFP标记的PMCA2的回收和迁移，并表征了毛细胞顶端区域的膜运输机制。我们的研究结果显示，PMCA2具有非常高的横向迁移率的大的移动部分的存在。这个PMCA2的移动池介导了PMCA2在相邻立纤毛之间的均匀分布，以及立纤毛之间的交换和发生在顶端细胞表面外围的快速内化和递送。基于我们的研究结果，我们认为PMCA2和其他可能的立纤毛膜蛋白可以快速更新，与之前描述的肌动蛋白核心成分的快速更新相匹配，进一步表明这些细胞器经历快速更新和持续的动态调节。

Stereocilia, are specialized actin protrusions on the surface of the sensory cells of the inner ear where the key step in the process of mechanical to electrical transduction takes place. In each cell, stereocilia are organized in bundles of precisely adjusted graded lengths forming characteristic staircase patterns. The pattern of organization and dimensions of the stereocilia are matched to specific stimulus frequency selectivity. Stereocilia are extraordinarily mechanically sensitive and can be easily disrupted by over-stimulation. An important question that follows is: How they can maintain their ordered structure, be so sensitive, and function continuously for a lifetime? The answer may be in the existence of a rapid constitutive turnover and recycling of the stereocilia molecular components. We have previously shown that the cytoskeletal core of sensory stereocilia undergoes continuous rapid renewal. However, the renewal of stereocilia membrane components remains poorly understood. We studied the spatial distribution, mobility, and trafficking of plasma membrane Ca2+-ATPase isoform 2 (PMCA2) as a model system to evaluate the extent and the mechanisms for renewal of stereocilia membrane proteins. PMCA2 is localized along the entire stereocilia membrane where it is highly abundant and uniformly distributed. We measured the rate of internalization of constitutively expressed PMCA2, we analyzed the recycling and mobility of GFP tagged PMCA2, and characterized the membrane traffic machinery present at the apical region of the hair cells. Our results show the presence of a large mobile fraction of PMCA2 with a very high lateral mobility. This mobile pool of PMCA2 mediates the uniform distribution of PMCA2 between neighboring stereocilia, as well as the exchanges between the stereocilia and the rapid internalization and delivery that takes place at the periphery of the apical cell surface. Based on our results we argue that PMCA2 and likely other stereocilia membrane proteins can turnover at fast rates, matching the previously described rapid turnover of the actin core components further demonstrating that these organelles undergo rapid renewal and continuous dynamic regulation.