Loss of <i>Dlg-1</i> in the Mouse Lens Impairs Fibroblast Growth Factor Receptor Signaling

<div><p>Coordination of cell proliferation, differentiation and survival is essential for normal development and maintenance of tissues in the adult organism. Growth factor receptor tyrosine kinase signaling pathways and planar cell polarity pathways are two regulators of many developmental processes. We have previously shown through analysis of mice conditionally null in the lens for the planar cell polarity gene (PCP), <i>Dlg-1,</i> that <i>Dlg-1</i> is required for fiber differentiation. Herein, we asked if <i>Dlg-1</i> is a regulator of the Fibroblast growth factor receptor (Fgfr) signaling pathway, which is known to be required for fiber cell differentiation. Western blot analysis of whole fiber cell extracts from control and <i>Dlg-1</i> deficient lenses showed that levels of the Fgfr signaling intermediates pErk, pAkt, and pFrs2α, the Fgfr target, Erm, and the fiber cell specific protein, Mip26, were reduced in the <i>Dlg-1</i> deficient fiber cells. The levels of Fgfr2 were decreased in <i>Dlg-1</i> deficient lenses compared to controls. Conversely, levels of Fgfr1 in <i>Dlg-1</i> deficient lenses were increased compared to controls. The changes in Fgfr levels were found to be specifically in the triton insoluble, cytoskeletal associated fraction of <i>Dlg-1</i> deficient lenses. Immunofluorescent staining of lenses from E13.5 embryos showed that expression levels of pErk were reduced in the transition zone, a region of the lens that exhibits PCP, in the <i>Dlg-1</i> deficient lenses as compared to controls. In control lenses, immunofluorescent staining for Fgfr2 was observed in the epithelium, transition zone and fibers. By E13.5, the intensity of staining for Fgfr2 was reduced in these regions of the <i>Dlg-1</i> deficient lenses. Thus, loss of <i>Dlg-1</i> in the lens impairs Fgfr signaling and leads to altered levels of Fgfrs, suggesting that <i>Dlg-1</i> is a modulator of Fgfr signaling pathway at the level of the receptors and that <i>Dlg-1</i> regulates fiber cell differentiation through its role in PCP.</p>