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M. K. Duncan, C. E. Decker, Y. Wang; Mutation of βB2-Crystallin Results in Lens Epithelial Cell Fibrosis. Invest. Ophthalmol. Vis. Sci. 2008;49(13):3729.
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Both α and taxon-specific crystallins have functions beyond those associated with light refraction in the lens and other tissues. However, while the β/γ crystallins evolved from ancestral genes involved in cellular stress responses and are expressed widely outside of the lens, their non-refractive function is not known. Here we reevaluate the lens phenotype of mice harboring the Philly mutation of the βB2 crystallin gene.
Mice harboring the Crybb2Phil mutation were backcrossed to the inbred strain C57Bl/6, and lenses were evaluated histologically using conventional methods and whole mount phalloidin imaging. The differentiation status of the lens epithelium was evaluated by a variety of molecular markers including those for the normal lens and epithelial-mesenchymal transition.
Adult lenses from Crybb2phil homozygotes are much smaller than normal and develop cataracts associated with a loss of F-actin from lens fiber cells and profound fibrosis of the adult lens epithelium including a reorganization of the F-actin cytoskeleton and the expression of abnormally high levels of αsmooth muscle actin (αSMA). A developmental study of this phenotype demonstrated that newborn and 2 week-old Crybb2Phil mice exhibited normal lens morphology consistent with the initiation of CryβB2 expression at birth while the loss of F-actin and upregulation of αSMA was apparent in 4 week-old homozyogotes. Lenses from heterozygous animals are of near normal size although they develop lens fiber cell opacities at 4-6 weeks postnatal and small foci of lens epithelial cell fibrosis is seen in older animals. Molecular characterization of the fibrotic cells observed in homozygous Crybb2 mutant lenses reveal a large upregulation of αSMA expression coupled with the downregulation of the epithelial markers Connexin-43 and E-cadherin. However, these cells appear to be quite different from those observed in more classic TGFβ mediated EMT in that Pax6 expression is maintained, collagen I expression is not induced, and aberrant expression of fiber cell markers was not detected.
Mice and humans heterozygous for mutations in βB2-crystallin develop lens fiber cell opacities that appear to be caused by aggregation of mutant βB2-crystallin. However, lens epithelial cells from Crybb2phil homozygotes develop the morphological features of epithelial mesenchymal transition (EMT) although only some of the molecular features of classic lens EMT are present. These data suggest that βB2-crystallin plays a role in maintaining the lens epithelial cell phenotype distinct from its refractive role in lens fiber cells.
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