Loss Of β-catenin In Lens Fibres Disrupts Adherens Junctions And Affects Na+/K+ ATPase Expression

Abstract

Purpose: : We previously showed a requirement for β-catenin in the embryonic lens (Cain et al., 2008, Dev Biol, 321:420-433). Conditional inactivation of Catnb in the whole lens (BCat10 mice) resulted in failure of epithelial cells to enter the cell cycle and aberrant differentiation from E13.5. By contrast, loss of Catnb from the lens fibres only (BCat39 mice) did not result in an apparent developmental phenotype. However, BCat39 mice were subsequently found to develop lens opacities from about 3 weeks of age. Here we report studies investigating this phenotype further.

Methods: : Histology of lens fiber cells was examined morphometrically (Axiovision software) in H&E stained sections from wild-type and BCat39 lenses on postnatal days 3, 21 and 65. Localization of adherens junction (β-catenin, N-cadherin, cadherin-11), gap junction (Cx50) and Na⁺/K⁺ ATPase proteins were examined by immunofluorescence. Expression of Na⁺/K⁺ ATPase mRNA was examined by RT-PCR.

Results: : BCat39 mice had distinct nuclear cataracts by P65. In dissected lenses at P21, but not at P3, the presence of a mild cataract was evident in the anterior lens fiber mass. Histological examination of fiber cross-sections at the equator showed progressive swelling and disruption of lens fibers from P3 to P65. Morphometry confirmed that BCat39 fibres, which lack β-catenin expression, lost the characteristic elongated hexagonal structure and became enlarged and spherical in shape. In BCat39 lenses, the characteristic localization of cadherin proteins to membrane domains along the short sides of hexagonal fiber profiles was disrupted with reactivity uniformly localized around the membrane. In contrast, Cx50 reactivity in its distinct domain in the long side of the hexagon remained intact. Expression of Na⁺/K⁺ ATPase in BCat39 lenses was virtually absent from fibres but not epithelial cells by immunofluorescence and RT-PCR. Analysis of the Atp1a1 promoter region indicated the presence of 5 putative Lef/Tcf binding sites in 3kb sequence upstream of the transcription initiation site.

Conclusions: : These data indicate that in addition to a role for β-catenin in regulating lens fibre cell adherens junctions, it may also act as a transcriptional activator of the Atp1a1 gene.

Keywords: cataract • cell adhesions/cell junctions • NaK ATPase