Focal adhesion kinase (FAK) expression and activation during lens development

PURPOSE: Regulation of lens development involves an intricate interplay between growth factor (e.g. FGF and TGFβ) and extracellular matrix (ECM) signaling pathways. Focal adhesion kinase (FAK) is a cytoplasmic tyrosine kinase that plays key roles in transmitting ECM signals by integrins. In this study, we delineated patterns of FAK expression and tyrosine phosphorylation (Y(397)) in the developing lens and investigated its regulation by FGF2. We also examined FAK expression and activation during disrupted fiber differentiation in mice expressing a dominant-negative TGFβ receptor. METHODS: FAK expression and activation (phosphorylation on Y(397)) was studied in embryonic and postnatal rodent lenses by in situ hybridization, immunofluorescence, and western blotting. Rat lens explants were used to investigate the effects of FGF2 on FAK expression and activation. Immunofluorescence and western blotting were used to examine FAK expression and phosphorylation in transgenic mice that express a dominant-negative TGFβ receptor. RESULTS: FAK is widely expressed and phosphorylated during embryonic stages of lens morphogenesis and differentiation. However, in postnatal lenses its expression and activation becomes restricted to the posterior germinative zone and the transitional zone at the lens equator. While both NH(2)- and COOH-terminal antibodies revealed cytoplasmic and membrane-associated staining in lens cells, the NH(2)-terminal antibody also showed FAK was present in fiber cell nuclei. In vitro, FAK expression and phosphorylation on Y(397) were increased by concentrations of FGF2 that initiate lens epithelial cell migration (10 ng/ml) and differentiation (50 ng/ml) but not proliferation (5 ng/ml). Moreover, reactivity for Y(397)phosphorylated FAK is prominent in the nuclei of differentiating fibers both in vivo and in vitro. Disruption of TGFβ-like signals by ectopic expression of a dominant-negative TGFβ receptor (TβRII(D/N)) results in abnormal lens fiber differentiation in transgenic mice. While FAK expression is initiated normally in the posterior germinative zone of TβRII(D/N) transgenic lenses, as fiber differentiation proceeds, FAK becomes localized to a perinuclear compartment, decreases its association with the cytoskeleton and is poorly phosphorylated on Y(397). CONCLUSIONS: FAK is widely expressed and activated during early lens morphogenesis. During secondary lens fiber differentiation, FAK is expressed and phosphorylated on Y(397) as epithelial cells exit the cell cycle, initiate migration at the equator, and undergo differentiation in the transitional zone. During terminal fiber differentiation an NH(2)-terminal fragment of FAK, including Y(397), is translocated to the nucleus. The expression, activation, and nuclear localization of FAK are regulated, at least partly, by FGF2. FAK activity and subcellular localization are also modulated by TGFβ-like signals. In fiber cells of TβRII(D/N) transgenic lenses, FAK is abnormally retained in a perinuclear compartment, loses its association with the cytoskeleton, and is poorly phosphorylated. These data suggest that integrin signaling via FAK plays important roles during lens differentiation, mediated by FGFs and TGFβ-superfamily signals.