Background/Aims: From invertebrates to mammals, Gα_i 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 Gα_i3-deficiency in pre-hearing murine cochleae pointed to a role of Gα_i3 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 Gα_i3 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. Methods: Here, we studied global and conditional Gnai3 and Gnai2 mouse mutants deficient for either one or both Gα_i 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 Gα_i isoforms and to assign specific defects to outer or inner hair cells, the auditory nerve, satellite cells or central auditory neurons. Results: Here we report that lack of Gα_i3 but not of the ubiquitously expressed Gα_i2 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 Gα_i2 or Gα_i3 had no impact. In contrast, double-deficiency for Gα_i2 and Gα_i3 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 Gα_i3-deficient mice revealed that the amplitude of ABR wave IV was disproportionally elevated in comparison to ABR wave I indicating that Gα_i3 is selectively involved in generation of neural gain during auditory processing. Conclusion: We propose a so far unrecognized complexity of isoform-specific and overlapping Gα_i protein functions particular during final differentiation processes.

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Chorda tympani schwannoma is a very rare tumor, with only 12 reported cases in the English literature. There are few reports on the diagnosis of chorda tympani schwannoma, and it is easily misdiagnosed. At present, surgery is recognized as a treatment for chorda tympani schwannoma. We describe the clinical course of a patient presenting with a chorda tympani schwannoma treated with surgical resection using a transcanal endoscopic approach, and the results after this treatment were satisfactory. Therefore, we accordingly advocate the minimally invasive method of transcanal endoscopic resection to maximize intraoperative visualization and reduce postoperative morbidity for middle ear tumors.

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