The Thrombomodulin-Protein C System and Angiopathy

A role of endothelial dysfunction for the pathogenesis of diabetic microvascular complications has been generally assumed. This assumption is supported by the clinically established association of diabetic microvascular complications with markers of endothelial dysfunction, e.g., soluble Thrombomodulin (TM). However, experimental evidence for a causal link was scarce at best. We demonstrated a reduction of TM expression in renal glomeruli of diabetic mice, which is associated with impaired activation of protein C (PC). This is consistent with the assumption that increased plasma levels of soluble TM reflect a functional impairment of the endothelial TM-PC system. To experimentally address the pathophysiological importance of altered activity within the TM-PC system, we used mice with genetically altered properties of this endothelial system. Diabetic mice with elevated plasma levels of activated PC (APC^high mice) were protected against diabetic nephropathy. Activated PC mediates nephroprotection through inhibition of apoptosis in endothelial cells and podocytes, establishing a crosstalk between the endothelium and podocytes. Activated PC reduced markers of oxidative stress and inhibited mitochondrial-dependent apoptosis through a PAR-1 / EPCR-dependent mechanism. Using mice lacking the TM^Led/Led domain, we evaluated the role of the TM lectin-like domain, which mediates cytoprotection independent of PC-activation. Diabetic nephropathy was aggravated in diabetic TM^Led/Led mice. Enhancement of diabetic nephropathy in diabetic TM^Led/Led mice appears to be independent of AGE (advanced glycation end product) or HMGB-1 but is associated with increased markers of coagulation activation (e.g., TAT, D-Dimer). Anticoagulation of diabetic TM^Led/Led mice with LMWH was partially protective, e.g., it reduced the level of albuminuria to that observed in diabetic wild-type mice. In further experiments, we explored whether TM-dependent PC activation modulates microvascular diabetic complications other than nephropathy. Functional analyses of peripheral nerves demonstrate that TM-dependent PC activation modulates the progression of peripheral nerve sensory loss. In diabetic wild type mice, neuropathy was aggravated, while diabetic APC^high mice were protected. These results establish a mechanistic link between impairment of the endothelial TM-PC system and diabetic microvascular complications. The identification of an endothelial system that modulates diabetic glomerulopathy shifts the focus away from mesangial cells and podocytes towards the cell type primarily exposed to hyperglycemia and its toxic metabolites, the endothelial cell. These data provide experimental evidence for a causative role of endothelial dysfunction for microvascular diabetic complications.