Degenerative retinal diseases are a group of human retinal disorders characterized by progressive photoreceptor cell death and blindness. Inspire of the advances made in their diagnosis, there is still no known cure for these diseases, and the exact mechanisms of photoreceptor cell loss remain unknown. In animal models of retinal degeneration, photoreceptor cell death is accompanied by migration of phagocytic cells into the photoreceptor cell layer; Muller cell changes such as migration, proliferation, and increased expression of glial fibrillary acidic protein [GFAP]; and degeneration and increased permeability of retinal vessels. Our studies suggest that the phagocytic cells are activated microglial cells and their close spatial and temporal relationships with the above events suggest that they may be involved in these changes. Moreover, our preliminary data show that activated microglial cells secrete soluble products that induce endothelial cell death in vitro. Morphological findings in degenerating endothelial cells resemble those observed in cells undergoing apoptosis. These findings lead us to hypothesize that in degenerative retinal diseases, activated microglial cells promote photoreceptor cell death, Muller cell gliosis, and degeneration of retinal microvessels. Studies are now proposed to test these hypotheses. First, to prove that the phagocytic cells in the subretinal space of dystrophic rat retinas are not derived from blood-borne macrophages or monocytes, labeled monocytes will be injected intravenously and shown to be present in punctured retinas but not in dystrophic retina. Second, photoreceptor or endothelial cell death (shown by uptake of ethidium homodimer, or release of lactate dehydrogenase); Muller cell proliferation, chemotaxis in blind- well chambers, and increased GFAP expression in Western blots will be tested in the presence of microglial conditioned medium. Third, the microglial effects on photoreceptors, Muller cells and retinal vessels in vivo will be studied following subretinal injections of microglial cells in normal rats using morphological techniques, electron microscopy and immunocytochemistry. Lastly, microglia-derived cytotoxic factor(s) [MDCF] which promote endothelial cell death will be characterized according to size, and trypsin, heat, or pH sensitivity. Apoptosis as a mechanism of MDCP-induced endothelial cell death in vitro will be determined using electron microscopy, 35S labeling for increased protein synthesis, and DNA "laddering". These studies will help us understand better the mechanisms involved in photoreceptor and endothelial cell death, and Muller cell alterations in degenerative retinal diseases as well as provide clues to their treatment. These studies could also result in the discovery of novel factor(s) which promote regression of pathologic angiogenesis in the eye (retinal angiogenesis), as well as in other parts of the human body (hemangiomas, tumor-associated angiogenesis).