Gαi Proteins are Indispensable for Hearing

Abstract

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.

Intrinsic planar polarity mechanisms influence the position-dependent regulation of synapse properties in inner hair cells

“…Downregulating Gαi activity in HCs with PTXa, inactivating both Gαi2 + Gαi3 or inactivating LGN resulted in profound deafness in young adults already (22)(23)(24)(25)(26). To explore the possibility that the disruption of the modiolar-pillar gradient of ribbon size is consecutive to the loss of sound-evoked synaptic activity, we turned to Myo15 sh2 mice.…”

“…Although it is well established that PTXa specifically ADPribosylates and impairs Gαi function(35), PTXa might affect IHC development in more ways than strictly disrupting planar polarity mechanisms. While the enlarged synaptic Ca 2+ influx observed upon PTXa argues against decreased IHC fitness or impaired IHC activity, simultaneous disruption of Gαi2 and Gαi3 was reported to impair IHC maturation(26). This conclusion was based on the absence of large-conductance Ca 2+ -activated K + (BK) channels at the IHC neck and the persistence of small-conductance Ca 2+ -activated K + (SK2) channels present before hearing onset and juxtaposed to efferent boutons.…”

Wnts regulate planar cell polarity via heterotrimeric G protein and PI3K signaling

“…Several planar polarized iPCP effector modules including Rac1 and Cdc42 GTPases, cell polarity proteins, and microtubule motors act in concert to couple hair bundle orientation with kinocilium positioning (Ezan et al, 2013;Grimsley-Myers et al, 2009;Kirjavainen et al, 2015;Landin Malt et al, 2019;Siletti et al, 2017;Sipe et al, 2013;Sipe and Lu, 2011;Tarchini et al, 2013). Of particular interest, Gαi proteins and their regulators LGN/G protein signaling modulator 2 (Gpsm2) and Daple are essential for iPCP in the OC (Beer-Hammer et al, 2018;Ezan et al, 2013;Siletti et al, 2017;Tarchini et al, 2013). Here, we investigate the unsolved questions regarding the function of heterotrimeric G protein signaling and its regulation by Wnts during PCP establishment in the OC.…”

Multiple PDZ Domain Protein Maintains Patterning of the Apical Cytoskeleton in Sensory Hair Cells

“…We and others showed that inhibitory Guanine nucleotide-binding proteins (G proteins) of the alpha family (GNAI1-3, collectively GNAI) bind the scaffold protein G Protein Signaling Modulator 2 (GPSM2) to form a complex essential for the polarized morphogenesis of the apical cytoskeleton in HCs [5–11]. Post-mitotic HCs are initially covered uniformly with microvilli, but planar polarized enrichment of GNAI-GPSM2 and Inscuteable (INSC) at the apical membrane defines a lateral/abneural region without protrusions, the bare zone [8].…”

“…At stereocilia tips, GNAI-GPSM2 promotes enrichment of MYO15A and other partners, conferring the first stereocilia row with its unique tallest identity [10]. Loss of GNAI or GPSM2 in mouse leads to stereocilia stunting, and the resulting immature hair bundle morphology in adults coincides with profound deafness [5, 7, 9]. In humans, mutations in GPSM2 are responsible for the Chudley-McCullough syndrome where hearing loss is the principal affliction [12, 13].…”