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P.M. Martin, S. Ananth, P. Roon, S.B. Smith, V. Ganapathy; Expression of PUMA–G, a Novel Receptor for ß–Hydroxybutyrate, in Normal and Diabetic Mouse Retina . Invest. Ophthalmol. Vis. Sci. 2006;47(13):2572.
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PUMA–G (Protein Up–regulated in Macrophages by Interferon–Gamma) was identified less than two years ago as a novel G–protein coupled receptor for niacin. While small quantities of niacin can be obtained from the diet, endogenous levels of this compound are too low to significantly impact PUMA–G activity. We were intrigued by the recent discovery that ß–hydroxybutyrate (ß–HB), a metabolite whose circulatory levels increase several fold in diabetes, is the physiologic ligand for the receptor, since ketone bodies like ß–HB constitute primary energy substrates for neurons under various physiologic and pathologic conditions. However, PUMA–G is thought to be expressed primarily in adipocytes and immune cells. Its expression in retina has not been studied. In the present study we asked whether PUMA–G is expressed in retina, and if so whether its expression is altered under diabetic conditions.
Immunohistochemical techniques were used to localize PUMA–G in intact normal retina. RT– PCR was done to analyze the expression of PUMA–G mRNA in normal (age–matched) and 4–week streptozotocin–diabetic neural retina and RPE/eyecup. Expression of PUMA–G mRNA and protein in ARPE–19 (RPE), RGC–5 (ganglion) and RMC–1 (Müller) cells was also analyzed using similar techniques.
Immunolocalization of PUMA–G protein in normal retina was restricted primarily to the RPE, where it differentially polarizes to the basolateral (BLM) membrane. These results were confirmed by RT–PCR analysis which showed predominant expression of PUMA–G in RPE/eyecup. RT–PCR analysis of diabetic and age–matched control tissues revealed that expression of PUMA–G in RPE is not altered even after four weeks of diabetes. In vitro studies of PUMA–G expression revealed robust expression of PUMA–G mRNA and protein in ARPE–19 cells. No expression was detectable in RGC–5 or RMC–1 cells.
This represents the first report on the localization of PUMA–G in retina. In RPE, the principal site of PUMA–G expression in retina, the receptor is localized specifically to the BLM. This expression is not altered by short–term diabetes. Because the BLM of RPE is in direct contact with choroidal blood and hence has access to ß–HB present in the circulation, the differential localization of PUMA–G in this membrane suggests that the receptor may have biologic importance with respect to diabetes. Future studies will determine whether this receptor has any role in the biology of the retina and in the pathogenesis of diabetic retinopathy.
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