Differential requirement for β-catenin in epithelial and fiber cells during lens development

Recent studies implicate Wnt/β-catenin signaling in lens differentiation (Stump, R. J., et al., 2003. A role for Wnt/beta-catenin signaling in lens epithelial differentiation. Dev Biol;259:48–61). β-catenin is a component of adherens junctions and functions as a transcriptional activator in canonical Wnt signaling. We investigated the effects of Cre/LoxP-mediated deletion of β-catenin during lens development using two Cre lines that specifically deleted β-catenin in whole lens or only in differentiated fibers, from E13.5. We found that β-catenin was required in lens epithelium and during early fiber differentiation but appeared to be redundant in differentiated fiber cells. Complete loss of β-catenin resulted in an abnormal and deficient epithelial layer with loss of E-cadherin and Pax6 expression as well as abnormal expression of c-Maf and p57kip2 but not Prox1. There was also disrupted fiber cell differentiation, characterized by poor cell elongation, decreased β-crystallin expression, epithelial cell cycle arrest at G1–S transition and premature cell cycle exit. Despite cell cycle arrest there was no induction of apoptosis. Mutant fiber cells displayed altered apical–basal polarity as evidenced by altered distribution of the tight junction protein, ZO1, disruption of apical actin filaments and abnormal deposition of extracellular matrix, resulting in a deficient lens capsule. Loss of β-catenin also affected the formation of adhesion junctions as evidenced by dissociation of N-cadherin and F-actin localization in differentiating fiber cells. However, loss of β-catenin from terminally differentiating fibers had no apparent effects on adhesion junctions between adjacent embryonic fibers. These data indicate that β-catenin plays distinct functions during lens fiber differentiation and is involved in both Wnt signaling and adhesion-related mechanisms that regulate lens epithelium and early fiber differentiation.