Expression and localization of GPR109A (PUMA-G/HM74A) mRNA and protein in mammalian retinal pigment epithelium.
OBJECTIVE
GPR109A has been identified as a G-protein-coupled receptor for niacin. beta-hydroxybutyrate (beta-HB) is a physiologic ligand for the receptor. beta-HB, the predominate ketone body in circulation, is an important energy source for neurons, including retinal neurons, under various physiologic and pathologic conditions. The identification of GPR109A as the receptor for beta-HB suggests additional, hitherto unknown, functions for this metabolite. The circulating levels of beta-HB increase in diabetes. Since retinopathy is a serious complication associated with diabetes, we investigated GPR109A expression in retina and in different retinal cell types to determine if the receptor may have a role in the pathophysiology of diabetic retinopathy.


METHODS
RT-PCR, fluorescent in situ hybridization, and immunofluorescent techniques were used to analyze GPR109A expression in mouse retina and in three transformed retinal cell lines: ARPE-19 (RPE), RGC-5 (ganglion), and rMC-1 (Müller). Activation of GPR109A by niacin and beta-HB was demonstrated in ARPE-19 cells by cAMP assay.


RESULTS
Studies conducted using mouse retinal tissues demonstrated that GPR109A is expressed in retina with its expression restricted to RPE, where it differentially polarizes to the basolateral membrane. These results were confirmed with cell lines, which demonstrated GPR109A expression in ARPE-19, but not in rMC-1 and RGC-5 cells. Primary cultures of mouse RPE also showed robust expression of GPR109A. cAMP assay demonstrated that GPR109A expressed in RPE is functional.


CONCLUSIONS
These data represent the first report on GPR109A expression in retina. The exclusive expression of GPR109A in RPE basolateral membrane, which has access to beta-HB in blood, may have biologic importance in diabetic retinopathy.
Martin
Pamela M
PM
1Department of Biochemistry and Molecular Biology, Medical College of Georgia, Augusta, GA 30912, USA. pmmartin@mcg.edu
Ananth
Sudha
S
Cresci
Gail
G
Roon
Penny
P
Smith
Sylvia
S
Ganapathy
Vadivel
V
D000818:Animals; D002461:Cell Line, Transformed; D000242:Cyclic AMP / Q000378:metabolism; D005455:Fluorescent Antibody Technique; D015870:Gene Expression / Q000502:physiology; D006801:Humans; D017404:In Situ Hybridization, Fluorescence; D051379:Mice; D008807:Mice, Inbred BALB C; D012333:RNA, Messenger / Q000235:genetics / Q000378:metabolism; D051381:Rats; D043562:Receptors, G-Protein-Coupled / Q000235:genetics* / Q000378:metabolism*; D011978:Receptors, Nicotinic / Q000235:genetics* / Q000378:metabolism*; D012160:Retina / Q000378:metabolism; D055213:Retinal Pigment Epithelium / Q000378:metabolism*; D020133:Reverse Transcriptase Polymerase Chain Reaction
C473652:Hcar2 protein, mouse; D012333:RNA, Messenger; D043562:Receptors, G-Protein-Coupled; D011978:Receptors, Nicotinic; D000242:Cyclic AMP