Wnt ligands from the embryonic surface ectoderm regulate ‘bimetallic strip’ optic cup morphogenesis in mouse

Abstract

The Wnt/β-catenin response pathway is central to many developmental processes. Here, we assessed the role of Wnt signaling in early eye development using the mouse as a model system. We showed that the surface ectoderm region that includes the lens placode expressed 12 out of 19 possible Wnt ligands. When these activities were suppressed by conditional deletion of wntless (Le-cre; Wlsfl/fl) there were dramatic consequences that included a saucer-shaped optic cup, ventral coloboma, and a deficiency of periocular mesenchyme. This phenotype shared features with that produced when the Wnt/β-catenin pathway co-receptor Lrp6 is mutated or when retinoic acid (RA) signaling in the eye is compromised. Consistent with this, microarray and cell fate marker analysis identified a series of expression changes in genes known to be regulated by RA or by the Wnt/β-catenin pathway. Using pathway reporters, we showed that Wnt ligands from the surface ectoderm directly or indirectly elicit a Wnt/β-catenin response in retinal pigment epithelium (RPE) progenitors near the optic cup rim. In Le-cre; Wlsfl/fl mice, the numbers of RPE cells are reduced and this can explain, using the principle of the bimetallic strip, the curvature of the optic cup. These data thus establish a novel hypothesis to explain how differential cell numbers in a bilayered epithelium can lead to shape change.

Signaling and Gene Regulatory Networks in Mammalian Lens Development

“…We also lack a transcriptomic definition of the lens cell fate competence, commitment, and determination. While the majority of extracellular signaling plays positive roles in lens cell formation, the repressive mechanisms seem to be underepresented (Table 1), and various cross-talks between individual pathways remain poorly understood [131, 190, 236]. Other levels of unresolved complexities relate to the contradictory findings that optic cup formation requires ectoderm-derived signals [137], anopthalmia is caused by various mutations of genes either in the ectoderm or optic vesicle [3, 51, 54, 79, 82, 83, 237], and that optic cup formation proceeds without any rudimentary lens in retinal organoid cultures [125, 238].…”

“…In medaka, the formation of filopodia that tether the lens to the retina requires YAP via fibronectin 1 (Fn1) and β1-integrin, implicating the Hippo-Yap pathway in actomyosin mediated regulation of tissue tension through the Rho GTPase activating protein ARHGAP18 [130]. Further complexity of the optic cup morphogenesis and regulation of its shape is demonstrated by a conditional inactivation of wntless (Wls) in the presumptive lens ectoderm which elicits major structural changes that yield a soucer-shaped optic cup damaged by a ventral coloboma, perturbation of RA-signaling, and reduction of the number of RPE cells near the optic cup rim [131]. These findings prompted a novel hypothesis that differential cell numbers in a bilayered epithelium, in analogy with a “bimetallic strip”, also regulate the final shape of the optic cup [131].…”

WNT/β-Catenin Signaling in Vertebrate Eye Development

“…During lens development, WNT/β-catenin signaling is active in the periocular surface ectoderm and lens epithelium (Stump et al, 2003; Smith et al, 2005; Kreslova et al, 2007; Machon et al, 2010; Carpenter et al, 2015). Conditional deletion of β -catenin in the presumptive lens placode and surrounding head surface ectoderm results in abnormal lens morphogenesis due to cell-cell adhesion defects.…”

“…This indicates that lens suppression by the neural crest-derived TGFβ is dependent on WNT/β-catenin signaling (Grocott et al, 2011). WNT2b null mice display no ocular defects and multiple WNTs are expressed in the surface ectoderm, therefore additional WNTs are required for the process in mice (Tsukiyama and Yamaguchi, 2012; Carpenter et al, 2015). …”

“…The dorsal optic vesicle receives signals from the extraocular mesenchyme and the head surface ectoderm to differentiate into the RPE (Fuhrmann et al, 2000; Martínez-Morales et al, 2004; Bharti et al, 2006; Steinfeld et al, 2013; Carpenter et al, 2015). During retinal development, WNT/β-catenin signaling is active in the dorsal optic vesicle which gives rise to presumptive RPE at the optic vesicle stage and is subsequently restricted to the peripheral RPE (Liu et al, 2006; Fujimura et al, 2009; Westenskow et al, 2009; Hägglund et al, 2013).…”