In high-energy metabolic tissues like the retina, creatine may play an important role in energy storage and in transmission of phosphate-bound energy substrates. To prove this, we investigated creatine synthesis in Müller glia. We also characterized the localization of the creatine synthetic enzyme, S-adenosyl-L-methionine:N-guanidinoacetate methyltransferase (GAMT) in the retina. Reverse transcription-polymerase chain reaction analysis revealed that L-arginine:glycine amidinotransferase and GAMT mRNAs were expressed in the retina and the Müller cell line, TR-MUL5. [14C]Creatine was detected after incubation of isolated rat retina or TR-MUL5 cells with [ 14C]glycine, L-arginine and L-methionine, suggesting creatine synthesis in Müller glia. Western blot analysis also revealed expression of GAMT protein in the rat retina and TR-MUL5 cells. Furthermore, confocal immunofluorescent microscopy of dual-labeled rat retinal sections demonstrated co-localization of GAMT with glutamine synthetase. Taken together, the results of the present study indicate creatine synthesis in Müller glia, implying an important role of creatine in energy metabolism in the retina.
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