Takahashi Satoe, Sun Willy, Zhou Yingjie, Homma Kazuaki, Kachar Bechara, Cheatham Mary Ann, Zheng Jing
Department of Otolaryngology - Head and Neck Surgery, Feinberg School of Medicine, Northwestern University, Chicago, IL, United States.
Section on Structural Cell Biology, National Institute on Deafness and Other Communication Disorders, National Institutes of Health, Bethesda, MD, United States.
Front Cell Neurosci. 2018 Jul 20;12:211. doi: 10.3389/fncel.2018.00211. eCollection 2018.
Outer hair cells (OHC) act as amplifiers and their function is modified by medial olivocochlear (MOC) efferents. The unique OHC motor protein, prestin, provides the molecular basis for somatic electromotility, which is required for sensitivity and frequency selectivity, the hallmarks of mammalian hearing. Prestin proteins are the major component of the lateral membrane of mature OHCs, which separates apical and basal domains. To investigate the contribution of prestin to this unique arrangement, we compared the distribution of membrane proteins in OHCs of wildtype (WT) and prestin-knockout (KO) mice. In WT, the apical protein PMCA2 was exclusively localized to the hair bundles, while it was also found at the lateral membrane in KOs. Similarly, a basal protein KCNQ4 did not coalesce at the base of OHCs but was widely dispersed in mice lacking prestin. Since the expression levels of PMCA2 and KCNQ4 remained unchanged in KOs, the data indicate that prestin is required for the normal distribution of apical and basal membrane proteins in OHCs. Since OHC synapses predominate in the basal subnuclear region, we also examined the synaptic architecture in prestin-KO mice. Although neurite densities were not affected, MOC efferent terminals in prestin-KO mice were no longer constrained to the basal pole as in WT. This trend was evident as early as at postnatal day 12. Furthermore, terminals were often enlarged and frequently appeared as singlets when compared to the multiple clusters of individual terminals in WT. This abnormality in MOC synaptic morphology in prestin-KO mice is similar to defects in mice lacking MOC pathway proteins such as α9/α10 nicotinic acetylcholine receptors and BK channels, indicating a role for prestin in the proper establishment of MOC synapses. To investigate the contribution of prestin's electromotility, we also examined OHCs from a mouse model that expresses non-functional prestin (499-prestin). We found no changes in PMCA2 localization and MOC synaptic morphology in OHCs from 499-prestin mice. Taken together, these results indicate that prestin, independent of its motile function, plays an important structural role in membrane compartmentalization, which is required for the formation of normal efferent-OHC synapses in mature OHCs.
外毛细胞（OHC）起到放大器的作用，其功能受内侧橄榄耳蜗（MOC）传出神经的调节。独特的OHC运动蛋白——预应力蛋白，为体细胞电运动提供了分子基础，而体细胞电运动是哺乳动物听力的标志——灵敏度和频率选择性所必需的。预应力蛋白是成熟OHC外侧膜的主要成分，外侧膜将顶端和基部区域分隔开来。为了研究预应力蛋白对这种独特排列的作用，我们比较了野生型（WT）和预应力蛋白敲除（KO）小鼠OHC中膜蛋白的分布。在WT小鼠中，顶端蛋白PMCA2仅定位于毛束，而在KO小鼠中，它也存在于外侧膜。同样，基部蛋白KCNQ4在OHC基部没有聚集，而是在缺乏预应力蛋白的小鼠中广泛分散。由于KO小鼠中PMCA2和KCNQ4的表达水平保持不变，数据表明预应力蛋白是OHC中顶端和基部膜蛋白正常分布所必需的。由于OHC突触主要位于核下基部区域，我们还检查了预应力蛋白KO小鼠的突触结构。虽然神经突密度没有受到影响，但预应力蛋白KO小鼠中的MOC传出神经末梢不再像WT小鼠那样局限于基部极。这种趋势早在出生后第12天就很明显。此外，与WT小鼠中单个末梢的多个簇相比，KO小鼠的末梢通常会增大，并且经常呈现为单个。预应力蛋白KO小鼠中MOC突触形态的这种异常类似于缺乏MOC通路蛋白（如α9/α10烟碱型乙酰胆碱受体和BK通道）的小鼠中的缺陷，表明预应力蛋白在MOC突触的正确形成中起作用。为了研究预应力蛋白电运动的作用，我们还检查了来自表达无功能预应力蛋白（499 - 预应力蛋白）的小鼠模型的OHC。我们发现499 - 预应力蛋白小鼠的OHC中PMCA2定位和MOC突触形态没有变化。综上所述，这些结果表明，预应力蛋白与其运动功能无关，在膜区室化中起着重要的结构作用，而膜区室化是成熟OHC中正常传出 - OHC突触形成所必需的。
