Evidence for pigment epithelium-derived factor receptors in the neural retina

PURPOSE. The neurotrophic activity of pigment epithelium-derived factor (PE DF), an extracellular factor present in the retina, is mediated by binding to cell-surface receptors in responsive cell cultures. In the present study , the expression of PEDF receptors in native neural retinas from adult stee rs was examined. METHODS. Binding reactions were performed with I-125-PEDF and fluoresceinat ed PEDF using plasma membranes, detergent-soluble membrane proteins, or cry osections of retina from adult bovine eves. Radioligand-binding and competi tion analyses were performed with a computer-assisted program. Ligand blot analysis of detergent-soluble membrane proteins was performed with I-125-PE DF followed by autoradiography. Ligand-affinity column chromatography of de tergent-soluble membrane proteins was performed with PEDF-coupled resin fol lowed by SDS-PAGE. Binding of fluoresceinated PEDF to retina cryosections w as detected by confocal microscopy. RESULTS. Radioligand-binding assays showed that I-125-PEDF bound in a speci fic and saturable fashion to one class of sites on retina membranes (K-d = 2.5-6.5 nM: maximum binding [B-max] = 1-48 X 10(10) sites/retina). A peptid e of 44 amino acids (44-mer), identified as the receptor-binding region of PEDF, competed efficiently for I-125-PEDF binding to retina membranes with kinetics similar to the full-length PEDF. Ligand blot analysis and ligand-a ffinity chromatography revealed a specific and high-affinity PEDF-binding p rotein of similar to 85 kDa in retina plasma membranes. Confocal microscopy showed that fluorescein-conjugated PEDF stained exclusively the inner segm ents of photoreceptors and cells of the ganglion cell layer in retinal cryo sections. CONCLUSIONS. Altogether, these data conclusively demonstrate the existence of PEDF receptors discretely distributed on the surface of cells from the a dult neural retina of bovine eyes. Furthermore, they provide evidence for t he direct action of PEDF on photoreceptor and ganglion cell neurons and an anatomic basis for studies to assess PEDF neurotrophic effects on the adult retina.