Mutant cDNAs coding for dominant-negative forms of the fibroblast growth factor receptors 1 (FGFR-I) and 2 (FGFR-2) that lack tyrosine kinase activity were ligated to a 2.2 kb DNA fragment containing the bovine rhodopsin promoter and used to generate transgenic mice. Six independent lines were generated with the FGFR-1 construct, and five were generated with the FGFR-2 construct. Five of the six FGFR-1 mutant lines and all five FGFR-2 mutant lines showed transgene expression in the retina by reverse transcription-PCR.By both in situ hybridization and immunohistochemistry, mutant FGFRs were found to be expressed specifically in photoreceptors of transgenepositive FGFR-1 and FGFR-2 mice. Lines expressing the FGFR-2 mutant showed progressive photoreceptor degeneration; the retinas showed minimal or no abnormalities at 1 month, but by 2 months they showed focal areas of thinning of the outer nuclear layer and disruption of photoreceptors. By 2-4 months, areas of complete loss of photoreceptors were seen. These abnormalities were not seen in control littermates not expressing the transgene. Mice from two FGFR-1 mutant lines showed focal areas of thinning of the outer nuclear layer and numerous photoreceptors with fragmented chromatin, whereas the other FGFR-1 lines showed minimal or no abnormalities. These data indicate that perturbation of FGF signaling in photoreceptors is associated with progressive photoreceptor degeneration, suggesting that one or more of the FGFs may act as a survival factor for photoreceptor cells.

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Basic fibroblast growth factor (FGF) protein has been recognized as a potent factor for angiogenesis and as a mitogen. The sites of basic FGF in the mammalian retina have varied from report to report. On the other hand, only the inner segments of the photoreceptor cells have been reported to synthesize basic FGF as revealed by the presence of mRNA for basic FGF by in situ hybridization. To define the sites of basic FGF and its mRNA in the human retina, we immunohistochemically localized basic FGF protein and mRNA for basic FGF by in situ hybridization in normal human retinas. Basic FGF protein was found in the ganglion cell layer, the inner and outer nuclear layers, and the basement membranes of Müller cells (the inner limiting membrane), blood vessels, and the retinal pigment epithelium (Bruch’s membrane). The mRNA for basic FGF was found in the cells of the ganglion cell layer, the inner nuclear layer, and the outer nuclear layer, and the inner segments of the photoreceptor cells. These findings suggest that basic FGF in the human retina functions in both an auto-crine as well as a paracrine fashion.

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