The Loss of Endothelial Glycocalyx in Diabetic Retinopathy

INTRODUCTION: The vascular endothelium is lined with the endothelial glycocalyx, composed of core proteins and glycosaminoglycans (GAGs). The loss of glycocalyx can cause endothelial injury and could contribute to the progression of retinopathy. The glycocalyx thickness has been shown to decrease in the diabetic retina; however, changes in individual components of the retinal glycocalyx have yet to be determined. Therefore, the present study examined hyperglycemia-induced changes in retinal glycocalyx components. METHODS: Type-1 diabetic rats and primary rat retinal microvascular endothelial cells (RRMECs) exposed to high glucose were used to mimic diabetes. Western blots and qRT-PCR were used to analyze the expression of core proteins. Further, GAG composition was studied using LC-MS. RESULTS: Both retinal endothelial cells and rat retina exhibited a significant decrease in mRNA transcripts and protein levels of syndecan-3 under hyperglycemic condition, whereas syndecan-1 expression was significantly increased. Further, we observed a substantial loss of glypican-1 in RRMECs with high glucose, but a significant increase in the retina of diabetic rats. RRMECs in high glucose exhibited a significant decrease in chondroitin sulfate (CS) and hyaluronic acid (HA), but no change in heparan sulfate (HS) levels. In addition, 4-O-sulfated CS residues and 2-O-N-sulfated HS residues were reduced, whereas N-sulfated and 6-O-N-sulfated HS residues were increased in retinal endothelial cells treated with high glucose. Lastly, we observed a significant increase in media levels of syndecan-1, HA, HS, and CS, indicating increased shedding of glycocalyx components under hyperglycemia. CONCLUSIONS: Our findings suggest that the loss of endothelial glycocalyx in the diabetic retina could be a result of a loss of the proteoglycan syndecan-3 (and possibly glypican-1) as well as the GAGs CS and HA.