Anterior subcapsular cataracts in βB2-crystallin mutants: implications for possible non-refractive functions of β-crystallins

Abstract

Purpose: β crystallins are abundant proteins in the ocular lens that have been largely investigated in relationship to their roles in conferring the refractive properties of the lens. However, these proteins are expressed in numerous extralenticular locations, and their expression is often elevated during cellular stress responses. We have found that mice carrying the Philly mutant allele of βB2-crystallin develop anterior subcapsular cataracts. Here we characterize these abnormalities and test the hypothesis that this phenotype results from a dysregulation of TGFβ activation.

Methods: Mice carrying the crybb2<phil> allele were backcrossed for 10 generations onto a C57Bl/6 genetic background and the lens phenotype characterized by gross observation, H&E staining, and electron microscopy. The molecular phenotype of the lens epithelium was investigated by confocal immunolocalization, pathway profiler PCR arrays, and active TGFβ bioassays.

Results: Heterozgyous crybb2<phil> mice develop focal anterior subcapsular cataracts by 12 weeks of age, while the lens epithelium of homozygotes undergoes a complete transition to a fibrotic phenotype as measured by α SMA expression although this is an unconventional response with these cells co-expressing elevated levels of Pax6. These lenses have a four fold elevation in the levels of active TGFβ consistent with dramatic increases in pSMAD3 levels. These former lens epithelial cells also overexpress numerous integrin subunits, especially those of the αV-family.

Conclusions: Mice expressing a mutant form of βB2-crystalin in the lens which has been previously shown to result in precipitation of the other lens β-crystallins (Russell and Chambers, 1990) results in the development of unconventional anterior subcapsular cataracts associated with dramatic elevations in TGFβ signaling. As this is associated with elevated levels of αV-integrins which we recently reported are required for injury induced EMT responses in the lens (Mamuya et al, in press), these data suggest that β-crystallins play important roles in maintaining lens epithelial cell homeostasis.

Keywords: 445 cataract • 488 crystallins • 512 EMT (epithelial mesenchymal transition)