Molecular Basis of Transduction in Auditory Sensory Organs

哺乳动物耳蜗外毛细胞的显著特征是当其细胞内电势发生变化时，在声波频率下伸长和缩短。这种“电动”将这些细胞转变为活跃的装置，能够放大由声音引起的科尔蒂器官的机械反应。这些细胞的胞浆基本上不含细胞骨架元素，它们的圆柱形主要由细胞内的静水压力(膨胀力)保持，这使得电动成为可能。我们现在已经确定了外毛细胞的渗透性，并提供了水通过特殊的水通道进入细胞的证据。这些水通道被发现定位在外毛细胞“马达”蛋白所在的侧质膜上。通过电生理学测量，我们发现在高敏感性听力开始时，水通道和“马达”蛋白的表达同时发生。此外，我们还发现外毛细胞的水流入受细胞内电压的调节，这可能代表了一种重要的局部反馈机制，建立了OHC马达输出的有效性。我们还使用了电子显微镜技术的组合来描述听觉和前庭传导中的另一个关键元素-尖端链接的新特性。毛细胞的转导机制与其他特性良好的感觉系统的转导有很大的不同。与视觉和嗅觉转导不同，视觉和嗅觉转导依赖于G蛋白偶联酶放大步骤，而听觉和前庭系统依赖于转导通道的直接机械门控。尽管尖端环节几乎肯定是打开转导通道的机械链的一部分，但目前还不太清楚尖端环节本身是否是门控弹簧，即在毛细胞刺激期间拉伸的弹性元件。我们的结构实验更好地定义了尖端链节的结构--一种由螺旋双丝制成的僵硬结构，两端都有专门的薄膜链接器结构--并表明门控弹簧与尖端链节的螺旋段串联。

The distinguishing feature of the outer hair cells of the mammalian cochlea is to elongate and shorten at acoustic frequencies, when their intracellular potential is changed. This "electromotility" turns these cells into active devices that are able to amplify the sound-evoked mechanical responses of the organ of Corti. The cytosol of these cells is largely free of cytoskeletal elements and their cylindrical shape is maintained mainly by intracellular hydrostatic pressure (turgor), which makes the electromotility possible. We have now characterized the osmotic water permeability of outer hair cells and provided evidence that water is entering the cell through specialized water channels. These water channels were found to localize in the lateral plasma membrane where the outer hair cell "motor" proteins are located. Using electrophysiology measurements we showed that the expression of the water channels and the "motor" proteins occur simultaneously at the onset of high sensitivity hearing. In addition we found that water influx into the outer hair cell is regulated by intracellular voltage which may represent an important local feedback mechanism setting up the effectiveness of the OHC motor output. We also used a combination of electron microscopy techniques to describe novel properties of another key element in auditory and vestibular transduction, the tip link. The mechanism of transduction by hair cells is quite different from transduction in other well characterized sensory systems. Unlike visual and olfactory transduction, which rely on G-protein coupled enzyme amplification steps, the auditory and vestibular systems rely on a direct mechanical gating of the transduction channel. Although the tip link is almost certainly part of the mechanical chain that opens the transduction channel, it is less clear whether the tip link itself is the gating spring, the elastic element that stretches during hair-cell stimulation. Our structural experiments, better define the structure of the tip link -- a stiff structure made of a helical double filament with specialized membrane linker structures at either end-- and suggest that the gating spring instead lies in series with the helical segment of the tip link.