Microglial polarization is independent of glucose levels and methylglyoxal

Microglia perform various functions, either inflammatory response or protection, depending on their current microenvironment. Diabetic Retinopathy is driven through local inflammations mediated by microglial cells. The question arises, whether high glucose levels or methylglyoxal are able to induce microglial polarization on their own.

Primary murine microglial BV2 cells were used for the stimulation, which was performed with DMEM containing different concentrations of glucose and methylglyoxal for 72 hours (5mmol/L; 25mmol/L; 50mmol/L; 100mmol/L of glucose or 500µmol/L; 5mmol/L; 10mmol/L of methylglyoxal). After incubation, immunofluorescence staining and western blots were performed, focusing on M1 and M2 polarization markers determined in previous studies.

Neither high glucose levels nor methylglyoxal led to an obvious polarization of the microglial cells. The immunofluorescence staining showed no expression of CD74 or CD206 as most prominent M1 and M2 markers. These results were confirmed in western blots. No changes in the expression of Iba1 were observed. The staining for RAGE showed a slight increase at the cell membrane at 100mmol/L of glucose, but no quantitative changes were detected in the western blots. For Gal3 we observed morphological changes towards a more M2 phenotypic pattern in the cells stimulated with 500µmol/L and 5mmol/L of methylglyoxal but not with 10mmol/L. The western blots for IL-1β revealed no differences among the groups.

Our results show that high glucose and methylglyoxal are not sufficient to induce microglial polarization. Activation of microglia in Diabetic Retinopathy therefore has to be a multimodal process, involving reactive metabolites but also different cell types and their interactions.