FGF Signaling During Lens Induction

Abstract

Purpose:: Fibroblast growth factors (FGFs) are implicated in lens development. There are 22 FGF ligands and four FGF receptors (FRs). FR1, 2 and 3 have been previously shown to be present in the lens. We previously showed that FR1 is not necessary for normal lens development (Zhao et al., 2006) and that FR2 maintains lens cell survival and contributes to the withdrawal of fiber cells from the cell cycle (Garcia et al., 2005). In the present study we found that FR4 is present in the lens and examined possible genetic interactions between FR1 and FR2 during lens induction.

Methods:: RNA was collected from wild type adult and E12.5 mice and RT-PCR was used to detect FR4 transcripts. Mice expressing LeCre were mated to mice which had critical exons of FR1, FR2 or both genes flanked by loxP sequences. Mice lacking FR4 (Weinstein et al., 1998) were mated to FR2 conditional knockout mice to generate FR2FR4 double knockout mice. Lens placodes (E9.5) and eyes from embryos (E12.5) were prepared for immunohistological studies to measure BrdU incorporation and for the TUNEL assay.

Results:: FR4 transcripts were detected by PCR in epithelia and peripheral fibers cells in adult and E12.5 lenses. Lenses lacking FR4 appeared normal in the adult. However, at E12.5, these lenses were smaller than wild type, with decreased in BrdU incorporation in the epithelial cells. Lens placodes lacking FR2 had increased cell death compared to their wildtype littermates. FR1FR2^DCKO animals usually did not form a lens; in cases when the lens formed, it was small and disorganized at E12.5. FR2FR4 double knockout lenses were smaller than FR2^CKO lenses. Like the FR2^CKO lenses, the double knockouts had increased cell death in lens epithelia and fiber cells and abnormal BrdU incorporation in fiber cells.

Conclusions:: FR4 is expressed in lens epithelial and peripheral fiber cells and mediates epithelial cell proliferation at early stages of lens differentiation. No single FR is required for lens induction or the formation of epithelial and primary fiber cells. FR2 signaling promotes lens cell survival starting at the placode stage. Although FR1 is not required for normal lens development, the enhanced phenotype of the double FR1FR2 knockout demonstrates that signaling from both of these receptors contributes to normal lens development.

Keywords: receptors • signal transduction • apoptosis/cell death