Basic Fibroblast Growth Factor in Retinal Development: Differential Levels of bFGF Expression and Content in Normal and Retinal Degeneration (rd) Mutant Mice

AbstractThe spatial and temporal patterns of expression and content of bFGF during postnatal development of the retina were established in C57BL/6J mice. Western blot analysis, using an anti-rodent bFGF antibody, shows multiple molecular weights of 18, 20.5, and 22 kDa of bFGF protein isolated from the adult retina. A bioassay indicates that this putative basic fibroblast growth factor (bFGF) stimulates proliferation of BALB/c 3T3 fibroblasts in a dose-dependent manner identical to an authentic bFGF standard. Immunocytochemistry reveals that bFGF immunoreactivity is located primarily in the immature photoreceptors during postnatal development and is associated with the photoreceptor outer segment/interphotoreceptor matrix complex in the adult retina. bFGF mRNA expression pattern and levels were evaluated using mouse bFGF riboprobes with in situ hybridization and quantitative ribonuclease protection assay. bFGF mRNA expression is not detectable in the retina until Postnatal Day 10 (P10), although high levels of bFGF mRNA signals were consistently observed in astrocytes in the optic disc at all postnatal ages examined. From P10 to the adult stage, bFGF mRNA was localized mainly to the photoreceptor inner segments, and the bFGF mRNA levels were approximately the same at P10 and in the adult retina. The patterns of retinal bFGF expression and content during normal development established above were compared to these parameters in the retina of rd (C57BL/6J rd/rd), a spontaneous mouse mutant in which photoreceptors degenerate shortly after birth. More bFGF immunoreactivity was detected in the outer retina during photoreceptor degeneration than was present in normal photoreceptors at equivalent ages. Densitometry measurements indicate that the level of immunoreactivity is 56% to 1.8-fold higher in rd than in the normal retina between P6 and P10, respectively. This is at least partially due to elevated bFGF mRNA expression in rd retinas during photoreceptor degeneration. In situ hybridization showed more intense bFGF mRNA hybridization signals in rd photoreceptors from P10 to P15, and RNase protection assay demonstrated much higher hybridization signals in rd retinas from P6 to P10 than in the normal retinas at these stages. More bFGF mRNA hybridization signals were also present in some cells in the inner nuclear layer following photoreceptor cell death in the rd retina but were only weakly evident in the inner nuclear layer in the normal adult retina. These results provide the first evidence that a naturally occurring neuronal degeneration is accompanied by elevated expression of bFGF in degenerating neurons prior to cell death. Our observations suggest that up-regulation in the expression of this growth factor may reflect an endogenous protective mechanism which can be activated when neurons are subjected to metabolic stress