Canonical Transient Receptor Potential 6 (TRPC6) Dysregulation in Mesangial Cells and Glomerular Hyperfiltration during the Early Stages of Diabetes

Glomerular mesangial cells (MCs) regulate the glomerular filtration rate (GFR) by contracting and relaxing in response to agonists. The contractile function of MCs is controlled by intracellular Ca2+. Canonical Transient receptor potential 6 (TRPC6) contribute to Ca2+ signaling in a variety of cells. However, the physiological and pathological relevance of TRPC6 in MCs remains unknown. The present study was conducted to examine 1) if TRPC6 mediated an agonist-induced Ca2+ response in MCs and if this Ca2+ signaling mechanism was impaired in diabetes 2) the mechanism by which TRPC6 is dysregulated in diabetes. In the first study, angiotensin II (AngII)-stimulated membrane currents were significantly enhanced in TRPC6 overexpressing MCs, but significantly attenuated in cells with TRPC6 knockdown. AngII-induced calcium influx was suppressed in MCs with TRPC6 knockdown, as well as in MCs cultured in high glucose (HG). HG reduced both the mRNA and protein levels of TRPC6, but not other isoforms of TRPCs, in a time and dose dependent manner. Furthermore, in streptozotocin (STZ)-induced diabetic rat glomeruli, TRPC6 but not TRPC1 was downregulated. In the second study, we found that the diabetes-induced decrease in TRPC6 protein expression was specific for glomeruli and no change in TRPC6 expression was observed in the heart or aorta. In cultured MCs, hydrogen peroxide (H2O2) decreased TRPC6 protein expression in a dose and time dependent manner. Antioxidants prevented the inhibitory effect of HG on TRPC6 in MCs. Consistently, treatment of STZ-diabetic rats with tempol preserved TRPC6 in the glomeruli. Nox4 knockdown led to an increase in TRPC6 protein in MCs. Furthermore, PMA, but not its analog 4α-PDD, suppressed TRPC6 and this PMA effect was not altered by catalase. A PKC inhibitor, Gö6976, attenuated the downregulation of TRPC6 caused by both HG and H2O2. Taken together, these studies suggest that TRPC6 participates in Ca2+ signaling in MCs and hyperglycemia in diabetes downregulates TRPC6 protein expression through a Nox4-ROS-PKC pathway.