The Rho GTPase Cdc42 regulates hair cell planar polarity and cellular patterning in the developing cochlea

Abstract

Hair cells of the organ of Corti (OC) of the cochlea exhibit distinct planar polarity, both at the tissue and cellular level. Planar polarity at tissue level is manifested as uniform orientation of the hair cell stereociliary bundles. Hair cell intrinsic polarity is defined as structural hair bundle asymmetry; positioning of the kinocilium/basal body complex at the vertex of the V-shaped bundle. Consistent with strong apical polarity, the hair cell apex displays prominent actin and microtubule cytoskeletons. The Rho GTPase Cdc42 regulates cytoskeletal dynamics and polarization of various cell types, and, thus, serves as a candidate regulator of hair cell polarity. We have here induced Cdc42 inactivation in the late-embryonic OC. We show the role of Cdc42 in the establishment of planar polarity of hair cells and in cellular patterning. Abnormal planar polarity was displayed as disturbances in hair bundle orientation and morphology and in kinocilium/basal body positioning. These defects were accompanied by a disorganized cell-surface microtubule network. Atypical protein kinase C (aPKC), a putative Cdc42 effector, colocalized with Cdc42 at the hair cell apex, and aPKC expression was altered upon Cdc42 depletion. Our data suggest that Cdc42 together with aPKC is part of the machinery establishing hair cell planar polarity and that Cdc42 acts on polarity through the cell-surface microtubule network. The data also suggest that defects in apical polarization are influenced by disturbed cellular patterning in the OC. In addition, our data demonstrates that Cdc42 is required for stereociliogenesis in the immature cochlea.

Role of Polarity Proteins in the Generation and Organization of Apical Surface Protrusions

“…The INSC/GPSM2/GNAI domain may limit the PRKC zone, maintaining the asymmetry at the apical surface of the hair cell (Tarchini et al 2013). Furthermore, mutations in Cdc42 leads to misorientation of the stereocilia bundles similar to the reported effects for the kinocilia mutants described above (Kirjavainen et al 2015). Finally, in Cdc42 mutants, those microtubules surrounding and emanating from the kinocilium basal body are disorganized and malformed (Kirjavainen et al 2015).…”

“…Furthermore, mutations in Cdc42 leads to misorientation of the stereocilia bundles similar to the reported effects for the kinocilia mutants described above (Kirjavainen et al 2015). Finally, in Cdc42 mutants, those microtubules surrounding and emanating from the kinocilium basal body are disorganized and malformed (Kirjavainen et al 2015). Although the underlying mechanism of how INSC/ GPSM2/GNAI and the Par complex work, it is possible that these two modules may be exerting opposing pulling forces on basal body nucleated microtubules, helping to direct the movement and final positioning of the kinocilium, and by extension the stereociliary bundle.…”

The GPSM2/LGN GoLoco motifs are essential for hearing

“…Several mouse models carrying mutations in genes with similar effects on stereocilia contour have been described recently (Grimsley-Myers et al 2009; Sipe et al 2013; Ezan et al 2013; Tarchini et al 2013; Gegg et al 2014; Kirjavainen et al 2015), suggesting these genes are all involved in generating the hair bundle’s intrinsic polarity. The encoded proteins are involved in basal body/kinocilium positioning or function, critical processes in hair bundle morphogenesis (Schwander et al 2010).…”

Whole exome sequencing identified a second pathogenic variant in HOMER2 for autosomal dominant non‐syndromic deafness

“…Looking now at the role of HOMER2 in stereocilia maintenance, stereocilia are actin‐based protrusions on auditory and vestibular sensory cells that are required for hearing and balance . As a key regulator of dynamic actin organization, CDC42 is required for stereociliogenesis in the immature cochlea and structural differentiation of auditory supporting cells . In HeLa cells, overexpression of HOMER2 suppresses CDC42 inducing the formation of filopodia‐like protrusions through its CBD domain (aa202‐294) .…”