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Increased Retinal Synthesis of Heparan Sulfate Proteoglycan and HNK-1 Glycoproteins Following Photoreceptor Degeneration


Address correspondence and reprint requests to R. A. Landers at Cullen Eye Institute, Baylor College of Medicine, Houston, TX 77030, U.S.A.


Abstract: In an earlier analysis of the retinal biosynthesis of proteoglycan, we noted that, following photoreceptor degeneration in the rd (retinal degeneration) mouse, the remaining inner retina exhibited a marked elevation in synthesis of heparan sulfate proteoglycan (HSPG), well above the level observed in the normal (nondegenerate) retina, as well as a pronounced increase in sulfation of protein substrates. Biochemical and autoradiographic results of 35S-amino acid utilization reported here confirm that the 35SO42− differences seen previously are accompanied by increased protein synthesis in the rd retina. An intact photoreceptor cell layer is neither a barrier to nor a sink for the amino acid precursor. Further, we have examined sulfate utilization in four other rodent strains with photoreceptor degenerations. In each of the models examined, an increase in retinal synthesis of 35SO42−-labeled HSPG and glycoproteins occurs following photoreceptor degeneration. We have metabolically labeled with Na235SO4 isolated retinal cultures from the following: (a) mice with light-induced photoreceptor degeneration; (b) rd mice; (c) transgenic mice with photoreceptor degeneration; (d) RCS rats; and (e) rats with light-induced photoreceptor degeneration. Comparisons were made with concurrent cultures of control nondegenerate retinal tissues. Protein and proteoglycan-enriched fractions were prepared from the incubation media and guanidine HCI/detergent extracts of the retinas by ion-exchange chromatography. The 35SO42−-proteoglycans were identified by chondroitinase ABC and nitrous acid treatments. Retinas lacking photoreceptors produced at least five times the amount of 35SO42−-HSPG found in control incubations. The RCS and light-damaged rats also showed increased synthesis of 35SO42−-chondroitin sulfate proteoglycan relative to the control, though the increase was of lesser magnitude than the HSPG effect. 35SO42−-protein in degenerate and light-damaged retinas always contained at least twice the radioactivity found in comparable control preparations. The bulk of the increased radiolabeling was found in N-linked oligosaccharides, including several recognized by the HNK-1 antibody. These data suggest that a sustained increase in HSPG and HNK-1 glycoprotein synthesis is a consistent response of inner retinal cells following loss of photoreceptors and is independent of the cause of photoreceptor degeneration.