Diabetes enhances translation of Cd40 mRNA in murine retinal Müller 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; yet, the mechanisms responsible for this increase have not been explored. Here, we evaluated the hypothesis that diabetes augments translation of the Cd40 mRNA. Mice receiving thiamet G (TMG), an inhibitor of the O-GlcNAc hydrolase O-GlcNAcase, exhibited enhanced retinal protein O-GlcNAcylation and increased Cd40 mRNA translation. TMG administration also promoted Cd40 mRNA association with Muller-cell specific ribosomes isolated from the retina of RiboTag mice. Similar effects on O-GlcNAcylation and Cd40 mRNA 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 eukaryotic translation in initiation factor 4E (eIF4E) by eIF4E-binding protein 1 (4E-BP1) and enhanced cap-independent Cd40 mRNA translation as assessed by a bicistronic reporter that contained the 5'-UTR of the Cd40 mRNA. Ablation of 4E-BP1/2 prevented the increase in Cd40 mRNA 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 wild-type mice, diabetic 4E-BP1/2-deficient mice did not exhibit enhanced retinal Cd40 mRNA translation and failed to upregulate expression of the inflammatory marker nitric oxide synthase 2. These findings support a model wherein diabetes-induced O-GlcNAcylation of 4E-BP1 promotes Cd40 mRNA translation in Muller glia.