Pigment Epithelium-Derived Factor Supports Normal Mu ¨ ller Cell Development and Glutamine Synthetase Expression After Removal of the Retinal Pigment Epithelium MONICA M. JABLONSKI, 1,2 * JOYCE TOMBRAN-TINK, 3 DAVID A. MRAZEK, 3 AND ALESSANDRO IANNACCONE 1 1 Retinal Degeneration Research Center, Department of Ophthalmology, University of Tennessee Health Science Center, Memphis, Tennessee 2 Department of Anatomy and Neurobiology, University of Tennessee Health Science Center, Memphis, Tennessee 3 George Washington University Medical Center, Department of Pharmaceutical Sciences, University of Missouri at Kansas City, Kansas City, Missouri KEY WORDS Mu ¨ ller cells; pigment epithelium-derived factor; glioprotection; retina; degeneration; dysmorphogenesis ABSTRACT In conditions in which the retinal pigment epithelium (RPE) is dystro- phic, carries a genetic mutation, or is removed physically, Mu ¨ ller cells undergo degen- erative changes that contribute to the retinal pathology. We previously demonstrated that pigment epithelium-derived factor (PEDF), a glycoprotein secreted by the RPE cells with neuroprotective and differentiation properties, protects against photoreceptor de- generation induced by RPE removal. The purpose of the present study was to analyze the putative gliosupportive activity of PEDF on Mu ¨ ller cells of RPE-deprived retinas and assess whether protection of Mu ¨ ller cells was correlated with improved photoreceptor outer segment assembly. Eyes were dissected from Xenopus laevis tadpoles, and the RPE was removed before culturing in medium containing purified PEDF, PEDF plus anti- PEDF, or medium alone. Control eyes matured with an adherent RPE or in medium containing PEDF plus nonimmune serum. Mu ¨ ller cell ultrastructure was examined. Glial fibrillary acidic protein (GFAP) and glutamine synthetase were localized immu- nocytochemically, and the corresponding protein levels were quantified. In control retinas, Mu ¨ ller cells were structurally intact and formed adherens junctions with neigh- boring photoreceptors. In addition, they did not express GFAP, whereas glutamine synthetase expression was high. RPE removal dramatically altered the ultrastructure and biosynthetic activity of Mu ¨ ller cells; Mu ¨ ller cells failed to form adherens junctions with photoreceptors and glutamine synthetase expression was suppressed. PEDF pre- vented the degenerative glial response; Mu ¨ ller cells were ultrastructurally 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 maintenance of the proper Mu ¨ ller cell ultrastructure and expression of glutamine synthetase may be necessary to support the proper assembly of photoreceptor outer segments. GLIA 35:14 –25, 2001. © 2001 Wiley-Liss, Inc. INTRODUCTION The avascular retina is a complex structure composed of several unique neuronal cell types, the Mu ¨ ller glia and the overlying retinal pigment epithelium (RPE). The health and survival of retinal photoreceptors, the neuro- Grant sponsor: National Eye Institute; Grant number: EY10853; Grant spon- sor: Research to Prevent Blindness; Grant sponsor: University of Tennessee Medical Group. *Correspondence to: Monica M. Jablonski, Department of Ophthalmology, University of Tennessee Health Science Center, Memphis, 956 Court Avenue, Suite D228, Memphis, TN 38163. E-mail: mjablonski@mail.eye.utmem.edu Received 29 November 2000; Accepted 11 April 2001 Published online xx Month 2001. GLIA 35:14 –25 (2001) © 2001 Wiley-Liss, Inc.