Abstract
Mutations of classic Transient Receptor Potential Channel 6 (TRPC6) were identified in hereditary FSGS, most of which are gain-of function. Increased expression of wild-type TRPC6 in glomeruli was observed in several human acquired proteinuric diseases. Synaptopodin, an actin binding protein that is important in maintaining podocyte function, is down-regulated in various glomerular diseases. Here, we provided evidence that synaptopodin maintains podocyte function by regulating podocyte surface expression and activity of TRPC6. We show indirect interaction and nonrandom association of synaptopodin and TRPC6 in podocytes. Knockdown of synaptopodin in cultured mouse podocytes increased the expression of TRPC6 at the plasma membrane, whereas overexpression of synaptopodin decreased it. Mechanistically, synaptopodin–dependent TRPC6 surface expression required functional actin and microtubule cytoskeletons. Functionally, overexpression of wild–type TRPC6 or the FSGS–inducing mutant TRPC6M131T in synaptopodin-depleted podocytes enhanced TRPC6–mediated calcium influx and induced apoptosis. In vivo, knockdown of synaptopodin by tail vain injection of siRNAs caused increased podocyte surface expression of TRPC6. Administration of cyclosporine A, which stabilizes synaptopodin, reduced LPS-induced proteinuria significantly in wild-type mice but to a lesser extent in TRPC6 knockout mice. Furthermore, administration of cyclosporine A reversed the LPS-induced increase in podocyte surface expression of TRPC6 in wild-type mice. Our findings suggest that alteration in synaptopodin levels under disease conditions may modify intracellular TRPC6 channel localization and activity, which further contribute to podocyte dysfunction. Therefore, reducing TRPC6 surface levels may be considered to be a new approach to restoring podocyte function.