Spleen Tyrosine Kinase Contributes to Müller Glial Expression of Proangiogenic Cytokines in Diabetes

PURPOSE. Neuroglial dysfunction occurs early in the progression of diabetic retinopathy. In response to diabetes or hypoxia, Muller glia secrete cytokines and growth factors that contribute to disease progression. This study was designed to examine common signaling pathways activated in Muller glia by both type 1 and pre-/type 2 diabetes. METHODS. RiboTag (Pdgfra-cre;HA-Rpl22) mice were used to compare the impact of streptozotocin (STZ) and a high-fat, high-sucrose (HFHS) diet on ribosome association of mRNAs in Muller glia by RNA sequencing analysis. Human MIO-M1 Muller cells were exposed to either hyperglycemic or hypoxic culture conditions. Genetic manipulation and pharmacologic inhibition were used to interrogate signaling pathways. RESULTS. Association of mRNAs encoding triggering receptor expressed on myeloid cells 2 (TREM2), DNAX-activating protein 12 kDa (DAP12), and colony stimulating factor 1 receptor (CSF1R) with ribosomes isolated from Muller glia was upregulated in both STZ diabetic mice and mice fed an HFHS diet. The TREM2/DAP12 receptor-adaptor complex signals in coordination with CSF1R to activate spleen tyrosine kinase (SYK). SYK activation was enhanced in the retina of diabetic mice and in human MIO-M1 Muller cell cultures exposed to hyperglycemic or hypoxic culture conditions. DAP12 knockdown reduced SYK autophosphorylation in Muller cells exposed to hyperglycemic or hypoxic conditions. SYK inhibition or DAP12 knockdown suppressed hypoxia-induced expression of the transcription factor hypoxia-inducible factor 1a (HIF1a), as well as expression of vascular endothelial growth factor and angiopoietin-like 4. CONCLUSIONS. The findings support TREM2/DAP12 receptor-adaptor complex signaling via SYK to promote HIF1a stabilization and increased angiogenic cytokine production by Muller glia.