Diabetes enhances translation of Cd40 mRNA in murine retinal Muller glia via a 4E-BP1/2-dependent mechanism

Activation of the immune costimulatory molecule cluster of differentiation 40 (CD40) in Muller glia has been implicated in the initiation of diabetes-induced retinal inflammation. Results from previous studies support that CD40 protein expression is elevated in Muller glia of diabetic mice; however, the mechanisms responsible for this increase have not been explored. Here, we evaluated the hypothesis that diabetes augments translation of theCd40mRNA. Mice receiving thiamet G (TMG), an inhibitor of theO-GlcNAc hydrolaseO-GlcNAcase, exhibited enhanced retinal proteinO-GlcNAcylation and increasedCd40mRNA translation. TMG administration also promotedCd40mRNA association with Muller cell-specific ribosomes isolated from the retina of RiboTag mice. Similar effects onO-GlcNAcylation andCd40mRNA translation were also observed in the retina of a mouse model of type 1 diabetes. In cultured cells, TMG promoted sequestration of the cap-binding protein eIF4E (eukaryotic translation in initiation factor 4E) by 4E-BP1 (eIF4E-binding protein 1) and enhanced cap-independentCd40mRNA translation as assessed by a bicistronic reporter that contained the 5 '-UTR of theCd40mRNA. Ablation of 4E-BP1/2 prevented the increase inCd40mRNA translation in TMG-exposed cells, and expression of a 4E-BP1 variant that constitutively sequesters eIF4E promoted reporter activity. Extending on the cell culture results, we found that in contrast to WT mice, diabetic 4E-BP1/2-deficient mice did not exhibit enhanced retinalCd40mRNA translation and failed to up-regulate expression of the inflammatory marker nitric-oxide synthase 2. These findings support a model wherein diabetes-inducedO-GlcNAcylation of 4E-BP1 promotesCd40mRNA translation in Muller glia.