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Purpose: Apoptosis is the final common death pathway of photoreceptors in light-induced retinal degeneration and in several animal models for retinal dystrophy. To date, little is known about gene regulation of apoptosis in the retina. The expression of the immediate early gene c-fos is upregulated concomitant with apoptosis in light-induced photoreceptor degeneration and in the rd mouse, an animal model for inherited retinal degeneration. In a recent study it was shown that c-Fos is essential for light-induced apoptosis of photoreceptors in vivo. To determine whether c-Fos is also involved in the apoptotic pathway of inherited retinal degeneration, rd/rd, c-fos -/- double-mutant mice have been generated., Methods: Double-mutant mice (rd/rd, c-fos -/-) were crossbred from c-fos+/- mice and rd/rd mice. Their genotype was determined by polymerase chain reaction analysis of genomic DNA. Wild-type control mice and homozygous rd mice were killed at 2-day intervals from postnatal day (P)9 through P21. Double-mutant mice were killed at postnatal days P9, P11, P13, P15, and P21. To determine levels of apoptosis in the retina, eyes were enucleated and processed for light microscopy and in situ nick-end labeling. Total retinal DNA was extracted from isolated retinas for DNA fragmentation analysis., Results: Morphologic, histochemical, and biochemical analyses showed that the time course of apoptosis and the outcome of photoreceptor degeneration in rd/rd, c-fos-/- double-mutant mice was indistinguishable from that in rd mice carrying functional c-fos., Conclusions: These data suggest that in contrast to its role in light-induced photoreceptor degeneration, c-Fos is not essential for apoptosis in the rd mouse.

Apoptotic cell death in the retina was recently demonstrated in animal models of the hereditary human retinal dystrophy known as retinitis pigmentosa. Although recent evidence indicates that the proto-oncogene c-fos is a mediator of apoptosis, its precise role is unclear. In fact, under some conditions, c-fos may even protect against apoptotic cell death. In the retina, c-fos is physiologically expressed in a diurnal manner and is inducible by light. We previously observed a light-elicited, dose-dependent apoptotic response in rat photoreceptors. To determine whether c-fos is involved in the light-induced apoptotic pathway we have used control mice and mice lacking c-fos. We found that following dark adaptation and two hours of light exposure both groups of animals exhibited only a few apoptotic cells. However, at 12 and 24 additional hours after light exposure, apoptosis increased dramatically in controls but was virtually absent in those mice lacking c-fos. Therefore, c-fos is essential for light-induced apoptosis of photoreceptors. Notably, c-fos is continuously upregulated concomitant with apoptotic photoreceptor death in our system and in animal models of retinitis pigmentosa (Agarwal, N. et al., Invest. Ophthalmol. Vis.Sci. Suppl. 36, S638 and Rich, K.A. et al., Invest. Ophthalmol. Vis. Sci. Suppl. 35, 1833). Inhibition of c-fos expression might therefore represent a novel therapeutic strategy to retard the time course of retinal dystrophies and light-induced retinal degeneration.

Low intensity diffuse white fluorescent light (1,000 lx for 2 h) exclusively induced photoreceptor damage in the inferior retina of albino rats; the temporal retina showed extensive damage, whereas the nasal retina revealed threshold lesions prior to recovery. To expand our morphological data, further experiments were undertaken to determine if glial fibrillary acidic protein (GFAP) expression was associated with the regions of photoreceptor damage. To follow the time course of GFAP expression, immunoblot analysis was carried out on retinal homogenates of dark-adapted (control) rats and light-exposed rats returned to cyclic light for 0 h, 1, 2, 3 and 6 days. A significant twofold increase in GFAP immunoreactivity over controls was observed in the retinas of light-exposed rats returned to cyclic light for 6 days. Using an indirect immunohistochemical method, retinal sections of the control and light-exposed rats allowed to recover for 6 days were stained for GFAP. GFAP immunoreactivity was localised to the astrocytes and Müller cells. Moreover, GFAP staining in Müller cells in the retinas of control animals was uniformly restricted to rare end-feet. In contrast, a gradient of GFAP immunoreactivity was observed in experimental rats, rising from the superior retina to the inferior temporal quadrant; the GFAP staining in the inferior nasal quadrant was intermediate. Thus, GFAP immunoreactivity was proportional to photoreceptor damage. Interestingly, no GFAP induction could be demonstrated in the pineal glands of light-exposed rats.