Divergent roles for Wnt/β-catenin signaling in epithelial maintenance and breakdown during semicircular canal formation

Abstract

SUMMARYThe morphogenetic program that shapes the three semicircular canals (SSCs) must be executed with extreme precision to satisfy their complex vestibular function. The SSCs emerge from epithelial outgrowths of the dorsal otocyst, the central regions of which fuse and resorb to leave three fluid-filled canals. The Wnt/β-catenin signaling pathway is active at multiple stages of otic development, including during vestibular morphogenesis. How Wnt/β-catenin functionally integrates with other signaling pathways to sculpt the SSCs and their sensory patches is unknown. We used a genetic strategy to spatiotemporally modulate canonical Wnt signaling activity during SSC development in mice. Our findings demonstrate that Wnt/β-catenin signaling functions in a multifaceted manner during SSC formation. In the early phase, Wnt/β-catenin signaling is required to preserve the epithelial integrity of the vertical canal pouch perimeter (presumptive anterior and posterior SSCs) by establishing a sensory-dependent signaling relay that maintains expression of Dlx5 and opposes expression of the fusion plate marker netrin 1. Without this Wnt signaling activity the sensory to non-sensory signaling cascade fails to be activated, resulting in loss of vestibular hair and support cells and the anterior and posterior SSCs. In the later phase, Wnt/β-catenin signaling becomes restricted to the fusion plate where it facilitates the timely resorption of this tissue. Mosaic recombination of β-catenin in small clusters of canal pouch cells prevents their resorption, causing instead the formation of ectopic SSCs. Together, these disparate functions of the Wnt/β-catenin pathway in epithelial maintenance and resorption help regulate the size, shape and number of SSCs.

Modulation of Wnt Signaling Enhances Inner Ear Organoid Development in 3D Culture

“…CHIR appears to have little effect on ESC-derived aggregates when it is applied to the aggregates before Pax8 expression (Fig 2E). This is consistent with inner ear development in vivo where Wnt signaling appears to be necessary for otic placode induction after specification of the broader Pax8+ otic-epibranchial progenitor domain [15, 34]. As in inner ear development, in which Wnt ligands are thought to diffuse from the neural tube between embryonic days 7.5–8.5, our model appears to have a temporal window for Wnt activation in which otic induction is optimal [6, 35–38].…”

Development of the inner ear

“…In mice, disruption of the RA-synthesising enzyme gene Raldh3 results in both morphological and functional deficits of the entire vestibular system [45]. Another study in mice has demonstrated the importance of Wnt/b-catenin signalling in sculpting the semicircular canal ducts via regulation of Netrin1-mediated cell resorption at the canal fusion plate [46].…”

The cochlear sensory epithelium derives from Wnt responsive cells in the dorsomedial otic cup

“…The anterior and posterior canals stained more uniformly when tamoxifen was administered at or prior to E10.5, compared to later stages when Wnt/β-catenin signaling activity becomes more restricted (Fig. 2A–F; Rakowiecki and Epstein, 2013). The lateral canal labeled sparsely throughout these experiments due to the limited expression of Cre on the dorsolateral side of the otocyst, the region from where progenitors of this canal originate (Morsli et al, 1999).…”

“…It is intriguing to speculate that the canonical Wnt signaling pathway, which is essential for vestibular morphogenesis and sensory cristae specification at early stages of otic development (Riccomagno et al, 2005; Rakowiecki and Epstein, 2013), was appropriated for additional use to facilitate the ventral displacement of prosensory progenitors along the medial wall of the cochlear duct. This model provides a compelling explanation for why Wnt1 −/− ; Wnt3a −/− double mutants exhibit cochlear outgrowth defects, despite the selective loss of Wnt signaling activity in dorsal otic regions (Riccomagno et al, 2005).…”

“…The Wnt/β-catenin signaling pathway is required for many facets of inner ear development, including otic placode induction, dorsal otic identity, vestibular morphogenesis, and the specification, proliferation and differentiation of prosensory progenitors (Ladher et al, 2000; Stevens et al, 2003; Riccomagno et al, 2005; Ohyama et al, 2006; Jacques et al, 2012; Rakowiecki and Epstein, 2013; Munnamalai and Fekete, 2013; Forristall et al, 2014; Shi et al, 2014). Previously, we showed that Wnt1 and Wnt3a secreted from the dorsal neural tube are required for the expression of several key dorsal otic determinants, including Dlx5 , Dlx6 and Gbx2 (Riccomagno et al, 2005).…”