Aberrant lens fiber differentiation in anterior subcapsular cataract formation: A process dependent on reduced levels of Pax6

PURPOSE. TGFbeta can induce development in lenses of opaque subcapsular fibrotic plaques that have many features of human subcapsular cataracts. To understand further the events associated with the onset and progression of TGFbeta-induced cataract, several different models for anterior subcapsular cataract (ASC) were used and characterized. METHODS. Anterior subcapsular plaques were induced in rat lenses cultured with TGFbeta and in transgenic mice overexpressing TGFbeta in the lens. ASC was also examined in lenses of mice haploinsufficient for Pax6, as well as in human biopsy specimens. Immunofluorescence and in situ hybridization labeling were used to examine changes in patterns of gene expression associated with cataract formation in these models. RESULTS. Examination of TGFbeta-induced cataract in transgenic mice established that the subcapsular plaques are composed of a heterogenous cell population: a population of myofibroblastic cells as well as a population of lens-fiber-like cells. Further support for phenotypic change comes from the observation that the cells in these plaques no longer expressed lens epithelial markers, such as Pax6 and Connexin43. Subsequent examination of human biopsy specimens of ASC, as well as lenses from Pax6-deficient mice, showed that the anterior subcapsular plaques in both cases were also composed of a heterogenous population of cells. In contrast, anterior subcapsular plaques that developed in vitro in response to TGFbeta did not have this same cellular heterogeneity, as no fiber-like cells were present. CONCLUSIONS. These findings suggest that in vivo, during TGFbeta-induced cataract formation, some lens epithelial cells transform into myofibroblastic cells, whereas others differentiate into fiber cells. As this pathologic change is accompanied by altered expression of genes characteristic of the normal lens epithelial cell phenotype and as lenses from Pax6-deficient mice exhibit development of anterior subcapsular plaques closely resembling those induced by TGFbeta in transgenic mice, the authors propose that a reduction in Pax6 levels may be essential for this pathologic process to progress. Furthermore, it is clear from these in vitro studies that TGFbeta alone cannot reproduce the same morphologic and molecular changes associated with ASC formation in vivo, indicating that additional molecule(s) in the eye are important in this process.