Glycine transporters (glycine transporter 1 and glycine transporter 2) are expressed in retina

Purinergic-Glycinergic Interaction in Neurodegenerative and Neuroinflammatory Disorders of the Retina

Neurodegenerative–neuroinflammatory disorders of the retina seriously hamper human vision. In searching for key factors that contribute to the development of these pathologies, we considered potential interactions among purinergic neuromodulation, glycinergic neurotransmission, and microglia activity in the retina. Energy deprivation at cellular levels is mainly due to impaired blood circulation leading to increased release of ATP and adenosine as well as glutamate and glycine. Interactions between these modulators and neurotransmitters are manifold. First, P2Y purinoceptor agonists facilitate reuptake of glycine by glycine transporter 1, while its inhibitors reduce reverse-mode operation; these events may lower extracellular glycine levels. The consequential changes in extracellular glycine concentration can lead to parallel changes in the activity of NR1/NR2B type NMDA receptors of which glycine is a mandatory agonist, and thereby may reduce neurodegenerative events in the retina. Second, P2Y purinoceptor agonists and glycine transporter 1 inhibitors may indirectly inhibit microglia activity by decreasing neuronal or glial glycine release in energy-compromised retina. These inhibitions may have a role in microglia activation, which is present during development and progression of neurodegenerative disorders such as glaucomatous and diabetic retinopathies and age-related macular degeneration or loss of retinal neurons caused by thromboembolic events. We have hypothesized that glycine transporter 1 inhibitors and P2Y purinoceptor agonists may have therapeutic importance in neurodegenerative–neuroinflammatory disorders of the retina by decreasing NR1/NR2B NMDA receptor activity and production and release of a series of proinflammatory cytokines from microglial cells.

microRNA regulatory circuits in a mouse model of inherited retinal degeneration

miRNA dysregulation is a hallmark of many neurodegenerative disorders, including those involving the retina. Up-regulation of miR-1/133 and miR-142, and down-regulation of miR-183/96/182 has been described in the RHO-P347S mouse retina, a model for a common form of inherited blindness. High-throughput LC-MS/MS was employed to analyse the protein expression of predicted targets for these miRNAs in RHO-P347S mouse retinas; 133 potential target genes were identified. Pathway over-representation analysis suggests G-protein signaling/visual transduction, and synaptic transmission for miR-1, and transmembrane transport, cell-adhesion, signal transduction and apoptosis for miR-183/96/182 as regulated functions in retina. Validation of miRNA-target mRNA interactions for miR-1, miR-96/182 and miR-96 targeting Ctbp2, Rac1 and Slc6a9, respectively, was demonstrated in vitro. In vivo interaction of miR-183/96/182 and Rac1 mRNA in retina was confirmed using miR-CATCH. Additional miRNAs (including miR-103-3p, miR-9-5p) were both predicted to target Rac1 mRNA and enriched by Rac1-miR-CATCH. Other Rac1-miR-CATCH-enriched miRNAs (including miR-125a/b-5p, miR-378a-3p) were not predicted to target Rac1. Furthermore, levels of ~25% of the retinal Rac1 interactors were determined by LC-MS/MS; expression of Rap1gds1 and Cav1 was elevated. Our data suggest significant utilisation of miRNA-based regulation in retina. Possibly more than 30 miRNAs interact with Rac1 in retina, targeting both UTRs and coding regions.

Some Operational Characteristics of Glycine Release in Rat Retina: The Role of Reverse Mode Operation of Glycine Transporter Type-1 (GlyT-1) in Ischemic Conditions

Rat posterior eyecups containing the retina were prepared, loaded with [(3)H]glycine and superfused in order to determine its release originated from glycinergic amacrine cells and/or glial cells. Deprivation of oxygen and glucose from the Krebs-bicarbonate buffer used for superfusion evoked a marked increase of [(3)H]glycine release, an effect that was found to be external Ca(2+)-independent. Whereas oxygen and glucose deprivation increased [(3)H]glycine release, its uptake was reduced suggesting that energy deficiency shifts glycine transporter type-1 operation from normal to reverse mode. The increased release of [(3)H]glycine evoked by oxygen and glucose deprivation was suspended by addition of the non-competitive glycine transporter type-1 inhibitor NFPS and the competitive inhibitor ACPPB further suggesting the involvement of this transporter in the mediation of [(3)H]glycine release. Oxygen and glucose deprivation also evoked [(3)H]glutamate release from rat retina and the concomitantly occurring release of the NMDA receptor agonist glutamate and the coagonist glycine makes NMDA receptor pathological overstimulation possible in hypoxic conditions. [(3)H]Glutamate release was suspended by addition of the excitatory amino acid transporter inhibitor TBOA. Sarcosine, a substrate inhibitor of glycine transporter type-1, also increased [(3)H]glycine release probably by heteroexchange shifting transporter operation into reverse mode. This effect of sarcosine was also external Ca(2+)-independent and could be suspended by NFPS. Energy deficiency in retina induced by ouabain, an inhibitor of the Na(+)-K(+)-dependent ATPase, and by rotenone, a mitochondrial complex I inhibitor added with the glycolytic inhibitor 2-deoxy-D-glucose, led to increase of retinal [(3)H]glycine efflux. These effects of ouabain and rotenone/2-deoxy-D-glucose could also be blocked by NFPS pointed to the preferential reverse mode operation of glycine transporter type-1 as a consequence of impaired cellular energy homeostasis. Immunohistochemical studies revealed that glycine transporter type-1, of which reverse mode operation assures [(3)H]glycine release, is expressed in amacrine cells in the inner nuclear and plexiform layers of the retina and also in Müller macroglia cells. We conclude that disruption of the balanced normal/reverse mode operation of glycine transporter type-1 is likely a significant factor contributing to neurotoxic processes of the retina. The possibility to inhibit glycine transporter type-1 mediated glycine efflux by drugs more potently than glycine uptake might offer some therapeutic potential for the treatment of various neurodegenerative disorders of the retina.

Insulin stimulated-glucose transporter Glut 4 is expressed in the retina

The vertebrate retina is a very metabolically active tissue whose energy demands are normally met through the uptake of glucose and oxygen. Glucose metabolism in this tissue relies upon adequate glucose delivery from the systemic circulation. Therefore, glucose transport depends on the expression of glucose transporters. Here, we show retinal expression of the Glut 4 glucose transporter in frog and rat retinas. Immunohistochemistry and in situ hybridization studies showed Glut 4 expression in the three nuclear layers of the retina: the photoreceptor, inner nuclear and ganglionar cell layers. In the rat retina immunoprecipitation and Western blot analysis revealed a protein with an apparent molecular mass of 45 kDa. ¹⁴C-glucose accumulation by isolated rat retinas was significantly enhanced by physiological concentrations of insulin, an effect blocked by inhibitors of phosphatidyl-inositol 3-kinase (PI3K), a key enzyme in the insulin-signaling pathway in other tissues. Also, we observed an increase in ³H-cytochalasin binding sites in the presence of insulin, suggesting an increase in transporter recruitment at the cell surface. Besides, insulin induced phosphorylation of Akt, an effect also blocked by PI3K inhibition. Expression of Glut 4 was not modified in retinas of a type 1 diabetic rat model. To our knowledge, our results provide the first evidence of Glut4 expression in the retina, suggesting it as an insulin- responsive tissue.

The specification of glycinergic neurons and the role of glycinergic transmission in development

Glycine's role as an inhibitory neurotransmitter in the adult vertebrate nervous system has been well characterized in a number of different model organisms. However, a full understanding of glycinergic transmission requires a knowledge of how glycinergic synapses emerge and the role of glycinergic signaling during development. Recent literature has provided a detailed picture of the developmental expression of many of the molecular components that comprise the glycinergic phenotype, namely the glycine transporters and the glycine receptor subunits; the transcriptional networks leading to the expression of this important neurotransmitter phenotype are also being elucidated. An equally important focus of research has revealed the critical role of glycinergic signaling in sculpting many different aspects of neural development. This review examines the current literature detailing the expression patterns of the components of the glycinergic phenotype in various vertebrate model organisms over the course of development and the molecular mechanisms governing the expression of the glycinergic phenotype. The review then surveys the recent work on the role of glycinergic signaling in the developing nervous system and concludes with an overview of areas for further research.

Characteristics of glycine transport across the inner blood-retinal barrier

Although glycine plays a pivotal role in neurotransmission and neuromodulation in the retina and is present in high concentration in the retina, the source of retinal glycine is still unclear. The purpose of the present study was to investigate glycine transport across the inner blood-retinal barrier (inner BRB). [(14)C]Glycine transport at the inner BRB was characterized using a conditionally immortalized rat retinal capillary endothelial cell line (TR-iBRB2 cells) as an in vitro model of the inner BRB and in vivo vascular injection techniques. [(14)C]Glycine uptake by TR-iBRB2 cells was Na(+)- and Cl(-)-dependent, and concentration-dependent with Michaelis-Menten constants of 55.4 microM and 8.02 mM, and inhibited by glycine transporter 1 (GlyT1) and system A inhibitors. These uptake studies suggest that GlyT1 and system A are involved in [(14)C]glycine uptake by TR-iBRB2 cells. RT-PCR analysis demonstrated that GlyT1 and system A (encoding ATA 1 and ATA2) mRNA are expressed in TR-iBRB2 cells. An in vivo study suggested that [(14)C]glycine is transported from blood to the retina whereas [(14)C]alpha-methylaminoisobutyric acid, a selective substrate for system A, is not. In conclusion, GlyT1 most likely mediates glycine transport at the inner BRB and is expected to play an important role in regulating the glycine concentration in the neural retina.