α6 integrin is regulated with lens cell differentiation by linkage to the cytoskeleton and isoform switching

The developing chicken embryo lens provides a unique model for examining the relationship between alpha 6 integrin expression and cell differentiation, since multiple stages of differentiation are expressed concurrently at one stage of development. We demonstrate that ab integrin is likely to mediate the inductive effects of laminin on lens differentiation as well as to function in a matrix-independent manner along the cell-cell interfaces of the differentiating cortical lens fiber cells. Both alpha 6 isoform expression and its linkage to the cytoskeleton were regulated in a differentiation-specific manner. The association of alpha 6 integrin with the Triton-insoluble cytoskeleton increased as the lens cells differentiated, reaching its highest levels in the cortical fiber region where the lens fiber cells are formed. In this region of the lens alpha 6 integrin was uniquely localized along the cell-cell borders of the differentiating fiber cells, similar to beta 1. alpha 6 beta 4, the primary transmembrane protein of hemidesmosomes, is also expressed in the lens, but in the absence of hemidesmosomes. Differential expression of alpha 6A and alpha 6B isoforms with lens cell differentiation was seen at both the mRNA and the protein levels. RT-PCR studies demonstrated that alpha 6B was the predominant isoform expressed both early in development, embryonic day 4, and in the epithelial regions of the day 10 embryonic lens. Isoform switching, with alpha 6A now the predominant isoform, occurred in the fiber cell zones. Immunoprecipitation studies showed that alpha 6B, which is characteristic of undifferentiated cells, was expressed by the lens epithelial cells but was dramatically reduced in the lens fiber zones. Expression of alpha 6B began to drop as the cells initiated their differentiation and then dropped precipitously in the cortical fiber zone. In contrast, expression of the alpha 6A isoform remained high until the cells became terminally differentiated. alpha 6A was the predominant isoform expressed in the cortical fiber region. The down-regulation of alpha 6B relative to alpha 6A provides a developmental switch in the process of lens fiber cell differentiation. (C) 1999 Academic Press.