Targeted disruption of Muller cell metabolism induces photoreceptor dysmorphogenesis

Within the retina, the Muller cells and photoreceptors are in close physica l proximity and are metabolically coupled. It is unknown, however, whether Muller cells affect photoreceptor differentiation and outer segment membran e assembly. The objective of this study was to determine whether targeted d isruption of Muller cell metabolism would induce photoreceptor dysmorphogen esis. Intact isolated Xenopus laevis embryonic eyes were cultured in medium with or without Muller cell-specific inhibitors (i.e., alpha -aminoadipic acid and fluorocitrate). To assess Muller cell injury, the gross retinal mo rphology was examined along with immunocytochemical assessment of Muller ce ll-specific protein expression patterns. The steady-state levels of opsin w ere quantified to determine whether the Muller cell inhibitors negatively a ffected photoreceptor protein synthesis. Muller and photoreceptor cell ultr astructure was scrutinized and the organization of the outer segment membra nes was graded. In control retinas, there was no swelling of Mailer cell cy toplasm. Glial fibrillary acidic protein (GFAP) was undetectable, whereas g lutamine synthetase was abundant. The steady-state level of opsin was high and photoreceptors elaborated properly folded outer segments. Exposure to b oth Muller cell-specific inhibitors induced swelling of Muller cell endfeet , cytoplasmic paling and alterations of Muller cell-specific protein expres sion patterns. The steady-state level of opsin in retinas exposed to alpha -aminoadipic acid was unchanged compared with control eyes, whereas, in eye s exposed to fluorocitrate, opsin levels were slightly reduced. The most si gnificant finding was that targeted disruption of Muller cell metabolism ad versely affected photoreceptor outer segment membrane assembly, causing dys morphogenesis of nascent outer segments. These results suggest that the ter mination signal(s) necessary for proper outer segment folding were disrupte d by targeted inhibition of Muller cells and support the hypothesis that Mu ller cells interact with photoreceptors through mechanisms that may regulat e, at least in part, the assembly of photoreceptor outer segment membranes. (C) 2000 Wiley-Liss, Inc.