Phosphodiesterase 1 (PDE1) at the crossroads of calcium and cyclic nucleotide signaling in diabetic nephropathy

Indiana University-Purdue University Indianapolis (IUPUI)Diabetic Kidney Disease (DKD) is a major complication of diabetes. Incomplete understanding of its molecular mechanisms is highlighted by the limited treatments options. We hypothesized that inhibition of protective endogenous mechanisms plays major role in the pathogenesis of DKD. While renoprotection is mediated by cyclic nucleotides (cAMP and cGMP), phosphodiesterases (PDEs) lead to cyclic nucleotide degradation. Our investigation focused on the role of calcium/calmodulin activated PDE1 in DKD. Three isoforms of PDE1 are differentially expressed in vascular smooth muscle cells, renal tubular epithelial cells, podocytes, and mesangial cells. We used highly potent and selective PDE1 inhibitor LY1 to explore systemic hemodynamic and local renal role of PDE1. LY1 reduced systolic and diastolic blood pressure in normotensive and spontaneously hypertensive rats. Renal protection with PDE1 inhibition was tested in mouse model of DKD, featuring a combination of diabetes, nephron loss and arterial hypertension. In this model, a PDE1 inhibitor caused a significant improvement in renal function as evident by significant reduction of albuminuria, serum creatinine and several urine biomarkers of inflammation and injury. Histopathological analysis revealed substantial improvement in the pathology of DKD in the treated group that was associated with the reduction of gene expression related to inflammation and fibrosis. Thus, we revealed the role of calcium activated PDE1 in DKD. However, the source of calcium in this context remained obscure. Our bioinformatics analysis pointed out that calcium channel TRPC6 is likely to be involved. Further in vitro studies demonstrated that TRPC6 activation induced apoptosis in human mesangial cells and isolated rat glomeruli, which was attenuated by both TRPC6 and PDE1 inhibition, thereby suggesting a functional coupling between TRPC6 (as a source of calcium) and PDE1 activation. Moving upstream, we showed that several systemic risk factors of DKD (angiotensin II, endothelin 1 and glucose) activated TRPC6 in a different manner, through generation of either reactive oxygen species or diacylglycerol. The computational modeling to relate human transcriptomic and phenotype data demonstrated the pre-clinical findings of renal benefit upon PDE1 inhibition is translatable in human. Taken together, our results suggest mechanistic link among systemic risk factors, TRPC6, calcium flux and PDE1 activation in pathogenesis of DKD. As a corollary, PDE1 inhibition leads to direct and indirect renoprotective effects