Glutamate-Induced Glutamine Synthetase Expression in Retinal Müller Cells after Short-term Ocular Hypertension in the Rat

Transformation of adult retina from the regenerative to the axonogenesis state activates specific genes in various subsets of neurons and glial cells

The purpose of this study was to identify the gene expression profile of the regenerating retina in vitro. To achieve this goal, three experimental groups were studied: (1) an injury control group (OC-LI group) that underwent open crush (OC) of the optic nerve and lens injury (LI) in vivo; (2) an experimental group (OC-LI-R group) that comprised animals treated like those in the OC-LI group except that retinal axons were allowed to regenerate (R) in vitro; and (3) an experimental group (OC-LI-NR group) that comprised animals treated as those in the OC-LI group, except that the retinas were cultured in vitro with the retinal ganglion cell (RGC) layer facing upwards to prevent axonal regeneration (NR). Gene expression in each treatment group was compared to that of untreated controls. Immunohistochemistry was used to examine whether expression of differentially regulated genes also occurred at the protein level and to localize these proteins to the respective retinal cells. Genes that were regulated belonged to different functional categories such as antioxidants, antiapoptotic molecules, transcription factors, secreted signaling molecules, inflammation-related genes, and others. Comparison of changes in gene expression among the various treatment groups revealed a relatively small cohort of genes that was expressed in different subsets of cells only in the OC-LI-R group; these genes can be considered to be regeneration-specific. Our findings demonstrate that axonal regeneration of RGC involves an orchestrated response of all retinal neurons and glia, and could provide a platform for the development of therapeutic strategies for the regeneration of injured ganglion cells.

Effect of glaucoma on the retinal glutamate/glutamine cycle activity

Glutamate-induced excitotoxicity has been proposed to mediate the death of retinal ganglion cells in glaucoma. The metabolic dependence of glutamatergic neurons upon glia via the glutamate/glutamine cycle to provide the precursor for neurotransmitter glutamate is well established. Thus, the aim of the present work was to study the retinal glutamate/glutamine activity in eyes with hypertension induced by intracameral injections of hyaluronic acid (HA). For this purpose, weekly injections of HA were performed unilaterally in the rat anterior chamber, whereas the contralateral eye was injected with saline solution. At 3 or 10 weeks of treatment, glutamate and glutamine uptake and release were assessed using [3H]-glutamate and [3H]-glutamine as radioligands, respectively. In addition, glutamine synthetase activity was assessed by a spectrophotometric assay, whereas glutaminase activity was measured through the conversion of [3H]-glutamine to [3H]-glutamate. At 3 weeks of treatment with HA, a significant decrease (P<0.01) in glutamate uptake and glutamine synthetase activity was observed. Glutamine uptake and release, as well as glutaminase activity, were significantly increased (P<0.01) in eyes injected with HA for 3 weeks compared with vehicle-injected eyes, whereas [3H]-glutamate release did not change in hypertensive eyes. Only the changes in glutamine synthetase activity persisted at 10 weeks of treatment with HA. These results indicate a significant alteration in the retinal glutamate/glutamine cycle activity in hypertensive eyes. Since these changes preceded both functional and histological alterations induced by ocular hypertension, these results support the involvement of glutamate in glaucomatous neuropathy.

Evidence for creatine biosynthesis in Müller glia

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. [(14)C]Creatine was detected after incubation of isolated rat retina or TR-MUL5 cells with [(14)C]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.