Beer-Hammer Sandra, Lee Sze Chim, Mauriac Stephanie A, Leiss Veronika, Groh Isabel A M, Novakovic Ana, Piekorz Roland P, Bucher Kirsten, Chen Chengfang, Ni Kun, Singer Wibke, Harasztosi Csaba, Schimmang Thomas, Zimmermann Ulrike, Pfeffer Klaus, Birnbaumer Lutz, Forge Andrew, Montcouquiol Mireille, Knipper Marlies, Nürnberg Bernd, Rüttiger Lukas
Department of Pharmacology and Experimental Therapy, and Interfaculty Center of Pharmacogenomics and Drug Research (ICePhA), University of Tübingen, Tübingen, Germany.
Molecular Physiology of Hearing, Tübingen Hearing Research Centre, Department of Otolaryngology, University of Tübingen, Tübingen, Germany.
Cell Physiol Biochem. 2018;47(4):1509-1532. doi: 10.1159/000490867. Epub 2018 Jun 21.
BACKGROUND/AIMS: From invertebrates to mammals, Gαi proteins act together with their common binding partner Gpsm2 to govern cell polarization and planar organization in virtually any polarized cell. Recently, we demonstrated that Gαi3-deficiency in pre-hearing murine cochleae pointed to a role of Gαi3 for asymmetric migration of the kinocilium as well as the orientation and shape of the stereociliary ("hair") bundle, a requirement for the progression of mature hearing. We found that the lack of Gαi3 impairs stereociliary elongation and hair bundle shape in high-frequency cochlear regions, linked to elevated hearing thresholds for high-frequency sound. How these morphological defects translate into hearing phenotypes is not clear.
Here, we studied global and conditional Gnai3 and Gnai2 mouse mutants deficient for either one or both Gαi proteins. Comparative analyses of global versus Foxg1-driven conditional mutants that mainly delete in the inner ear and telencephalon in combination with functional tests were applied to dissect essential and redundant functions of different Gαi isoforms and to assign specific defects to outer or inner hair cells, the auditory nerve, satellite cells or central auditory neurons.
Here we report that lack of Gαi3 but not of the ubiquitously expressed Gαi2 elevates hearing threshold, accompanied by impaired hair bundle elongation and shape in high-frequency cochlear regions. During the crucial reprogramming of the immature inner hair cell (IHC) synapse into a functional sensory synapse of the mature IHC deficiency for Gαi2 or Gαi3 had no impact. In contrast, double-deficiency for Gαi2 and Gαi3 isoforms results in abnormalities along the entire tonotopic axis including profound deafness associated with stereocilia defects. In these mice, postnatal IHC synapse maturation is also impaired. In addition, the analysis of conditional versus global Gαi3-deficient mice revealed that the amplitude of ABR wave IV was disproportionally elevated in comparison to ABR wave I indicating that Gαi3 is selectively involved in generation of neural gain during auditory processing.
We propose a so far unrecognized complexity of isoform-specific and overlapping Gαi protein functions particular during final differentiation processes.
背景/目的：从无脊椎动物到哺乳动物，Gαi蛋白与其共同结合伴侣Gpsm2共同作用，在几乎任何极化细胞中调控细胞极化和平面组织。最近，我们证明了听力发育前的小鼠耳蜗中Gαi3的缺失表明Gαi3在动纤毛的不对称迁移以及静纤毛（“毛”）束的方向和形状中发挥作用，这是成熟听力进展的必要条件。我们发现Gαi3的缺失会损害高频耳蜗区域的静纤毛伸长和毛束形状，这与高频声音的听力阈值升高有关。这些形态缺陷如何转化为听力表型尚不清楚。
在这里，我们研究了缺失一种或两种Gαi蛋白的全身性和条件性Gnai3和Gnai2小鼠突变体。对主要在内耳和端脑中缺失的全身性与Foxg1驱动的条件性突变体进行比较分析，并结合功能测试，以剖析不同Gαi亚型的基本和冗余功能，并将特定缺陷归因于外毛细胞或内毛细胞、听神经、卫星细胞或中枢听觉神经元。
我们在此报告，缺乏Gαi3而非普遍表达的Gαi2会提高听力阈值，并伴有高频耳蜗区域毛束伸长和形状受损。在未成熟内毛细胞（IHC）突触向成熟IHC的功能性感觉突触的关键重编程过程中，Gαi2或Gαi3的缺乏没有影响。相比之下，Gαi2和Gαi3亚型的双缺失会导致整个音频轴出现异常，包括与静纤毛缺陷相关的严重耳聋。在这些小鼠中，出生后IHC突触成熟也受损。此外，对条件性与全身性Gαi3缺陷小鼠的分析表明，ABR波IV的幅度与ABR波I相比不成比例地升高，这表明Gαi3在听觉处理过程中选择性地参与神经增益的产生。
我们提出了一种迄今未被认识到的Gαi蛋白亚型特异性和重叠功能的复杂性，特别是在最终分化过程中。
