Dual function of Yap in the regulation of lens progenitor cells and cellular polarity

Abstract

Hippo-Yap signaling has been implicated in organ size determination via its regulation of cell proliferation, growth and apoptosis (Pan, 2007). The vertebrate lens comprises only two major cell types, lens progenitors and differentiated fiber cells, thereby providing a relatively simple system for studying size-controlling mechanisms. In order to investigate the role of Hippo-Yap signaling in lens size regulation, we conditionally ablated Yap in the developing mouse lens. Lens progenitor specific deletion of Yap led to near obliteration of the lens primarily due to hypocellularity in the lens epithelium (LE) and accompanying lens fiber (LF) defects. A significantly reduced LE progenitor pool resulted mainly from failed self-renewal and increased apoptosis. Additionally, Yap-deficient lens progenitor cells precociously exited the cell cycle and expressed the LF marker, β-Crystallin. The mutant progenitor cells also exhibited multiple cellular and subcellular alterations including cell and nuclear shape change, organellar polarity disruption, and disorganized apical polarity complex and junction proteins such as Crumbs, Pals1, Par3 and ZO-1. Yap-deficient LF cells failed to anchor to the overlying LE layer, impairing their normal elongation and packaging. Furthermore, our localization study results suggest that, in the developing LE, Yap participates in the cell context-dependent transition from the proliferative to differentiation-competent state by integrating cell density information. Taken together, our results shed new light on Yap’s indispensable and novel organizing role in mammalian organ size control by coordinating multiple events including cell proliferation, differentiation, and polarity.

Yap is essential for retinal progenitor cell cycle progression and RPE cell fate acquisition in the developing mouse eye

“…Yap’s function in maintaining progenitor pools by promoting cell proliferation has been well demonstrated in many tissues, including cardiac and lens (Heallen et al, 2011; Song et al, 2014). In addition, overexpression of Yap enhanced proliferation of progenitors in postnatal retinal explants, and RNA interference decreased proliferation (Zhang et al, 2012).…”

“…Destabilized Crumbs polarity complex proteins and cell-cell junctions in Yap CKO are consistent with previous reports, including a comprehensive proteomics study showing significant overrepresentation of proteins associated with cell-cell contacts, among them apical polarity complex proteins, as interacting partners of Yap (Kohli et al, 2014; Varelas et al, 2010). Unstable cell-cell attachment and destabilization of apical polarity complex proteins were independently observed in developing ependymal cells and lens deficient in Yap (Park et al, 2016; Song et al, 2014). We also noted that Yap functions cell-autonomously within retinal epithelia (Fig.…”

“…Rx-Cre line and Yap f/f allele were previously described (Swindell et al, 2006; Xin et al, 2011). PCR-based genotyping was performed using the primers previously described (Song et al, 2014). Yap CKO ( Yap f/f ; Rx-Cre ), heterozygote ( Yap +/− ; Rx-Cre ) and WT control littermates (mice not carrying Rx-Cre) used for the assays were generally obtained from the crosses of heterozygotes.…”

“…Yap is expressed in late-stage progenitor cells in the mouse retina, and RNAi mediated functional analysis of Yap in postnatal retinas has identified Yap’s essential role in promoting the proliferation of retinal progenitors and inhibiting their cell cycle exit (Zhang et al, 2012). Yap is also essential in the lens; it maintains lens progenitor cells in lens epithelium, where it is specifically expressed, and promotes epithelial integrity by stabilizing apical polarity/adhesion complexes (Song et al, 2014). Although most previous reports define Hippo-Yap pathway as a general regulator of cell proliferation and survival, other evidence indicates its function in cell-, region- and tissue-fate determination.…”

Signaling and Gene Regulatory Networks in Mammalian Lens Development

“…Interestingly, ablation of β-catenin does not cause lens herniation, but rather a disruption in polarity and adhesion of the lens epithelial cells, which are similar phenotypes to those observed in cadherin knockouts [119, 154, 201]. Genetic rescue experiments also demonstrated that the cadherin-binding protein NF2 negatively regulates YAP, which is required both to maintain lens epithelial cell polarity and to prevent premature differentiation [202, 203]. YAP has also been shown to be intricately involved in Wnt/β-catenin signaling in other systems.…”