Pigment epithelium-derived factor supports normal Muller cell development and glutamine synthetase expression after removal of the retinal pigment epithelium

In conditions in which the retinal pigment epithelium (RPE) is dystrophic, carries a genetic mutation, or is removed physically, Muller cells undergo degenerative changes that contribute to the retinal pathology. We previousl y demonstrated that; pigment epithelium-derived factor (PEDF), a glycoprote in secreted by the RPE cells with neuroprotective and differentiation prope rties, protects against photoreceptor degeneration induced by RPE removal. The purpose of the present study was to analyze the putative gliosupportive activity of PEDF on Muller cells of RPE-deprived retinas and assess whethe r protection of Muller cells was correlated with improved photoreceptor out er segment assembly. Eyes were dissected from Xenopus laevis tadpoles, and the RPE was removed before culturing in medium containing purified PEDF, PE DF plus anti-PEDF, or medium alone. Control eyes matured with an adherent R PE or in medium containing PEDF plus nonimmune serum. Muller cell ultrastru cture was examined. Glial fibrillary acidic protein (GFAP) and glutamine sy nthetase were localized immunocytochemically, and the corresponding protein levels were quantified. In control retinas, Muller cells were structurally intact and. formed adherens junctions with neighboring photoreceptors. In addition, they did not express GFAP, whereas glutamine synthetase expressio n was high. RPE removal dramatically altered the ultrastructure and biosynt hetic activity of Muller cells; Muller cells failed to form adherens juncti ons with photoreceptors and glutamine synthetase expression was suppressed. PEDF prevented the degenerative glial response; Muller cells were ultrastr ucturally normal and formed junctional complexes with photoreceptors. PEDF also preserved the expression of glutamine synthetase at near-normal levels . The morphogenetic effects of PEDF were blocked by the anti-PEDF antibody. Our study documents the glioprotective effects of PEDF and suggests that m aintenance of the proper Muller cell ultrastructure and expression of gluta mine synthetase may be necessary to support the proper assembly of photorec eptor outer segments. GLIA 35:14-25, 2001. (C) 2001 Wiley-Liss, Inc.