Junctionally restricted RhoA activity is necessary for apical constriction during phase 2 inner ear placode invagination

Abstract

After induction, the inner ear is transformed from a superficially located otic placode into an epithelial vesicle embedded in the mesenchyme of the head. Invagination of this epithelium is biphasic: phase 1 involves the expansion of the basal aspect of the otic cells, and phase 2, the constriction of their apices. Apical constriction is important not only for otic invagination, but also the invagination of many other epithelia; however, its molecular basis is still poorly understood. Here we show that phase 2 otic morphogenesis, like phase 1 morphogenesis, results from the activation of myosin-II. However unlike the actin depolymerising activity observed basally, active myosin-II results in actomyosin contractility. Myosin-II activation is triggered by the accumulation of the planar cell polarity (PCP) core protein, Celsr1 in apical junctions (AJ). Apically polarized Celsr1 orients and recruits the Rho Guanine exchange factor (GEF) ArhGEF11 to apical junctions, thus restricting RhoA activity to the junctional membrane where it activates the Rho kinase ROCK. We suggest that myosin-II and RhoA activation results in actomyosin dependent constriction in an apically polarised manner driving otic epithelium invagination.

Proliferation-independent regulation of organ size by Fgf/Notch signaling

“…Other molecules that interact with this complex are also upregulated, such as rab25a , a Rab11 GTPase family member, as is celsr2, an atypical cadherin (Figure 7X–Y’). Celsr1, if reduced, causes otic defects because of loss of apical constrictions due to disturbed actomyosin recruitment to the apical junctional complex (Sai et al, 2014). RNASeq analysis also revealed the induction of cd9b by Notch, a Tetraspanin family member implicated in cell-matrix adhesion and Sdf1 (Cxcl12a) mediated migration (Figure 7L–M’, [Arnaud et al, 2015; Leung et al, 2011]).…”

Spatial integration of E-cadherin adhesion, signalling and the epithelial cytoskeleton

“…Junction shrinkage may occur by asymmetric, spatially restricted contraction of thin bundles positioned at different heights along the contacting neighbouring cells. This localized contraction enables apical constriction during invagination [73] or defines the topography of cell extrusion (apical or basal side of the epithelial monolayer). At the molecular level, cell extrusion directionality is driven by specific oncogenes (Ras, APC) [74,75] and the engagement of N-WASP or crosslinking proteins such as filamins or EPLIN [45,76,77].…”

Development of the inner ear

“…The morphogenetic changes that generate the otocyst from the otic placode have been investigated in the chick embryo [11]. Here, invagination to form the otic cup and otocyst involves two phases: an initial basal expansion of placodal cells, followed by their apical constriction.…”

“…For example, a microarray study using an over-expression assay in Xenopus has identified nearly 30 genes expressed in the otocyst that are possible Six1 targets [10]. This and similar studies will provide not only a more complete picture of the transcriptional profile of early otic cells, but also new candidate genes for auditory disorders such as Branchio-Oto-Renal syndrome.The morphogenetic changes that generate the otocyst from the otic placode have been investigated in the chick embryo [11]. Here, invagination to form the otic cup and otocyst involves two phases: an initial basal expansion of placodal cells, followed by their apical constriction.…”

“…Here, invagination to form the otic cup and otocyst involves two phases: an initial basal expansion of placodal cells, followed by their apical constriction. Sai and colleagues used a variety of inhibitory approaches to elucidate a pathway -triggered by activation of the planar cell polarity mediator Celsr1 and involving RhoA, ROCK and myosin-II activation -leading to actin-mediated apical constriction of otic placodal cells, driving the second phase of the invagination process [11]. This model has close similarities with the events leading to neural tube closure.…”