Synaptopodin Limits TRPC6 Podocyte Surface Expression and Attenuates Proteinuria

Abstract

Gain-of-function mutations of classic transient receptor potential channel 6 (TRPC6) were identified in familial FSGS, and increased expression of wild-type TRPC6 in glomeruli is observed in several human acquired proteinuric diseases. Synaptopodin, an actin binding protein that is important in maintaining podocyte function, is downregulated in various glomerular diseases. Here, we investigated whether 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. Overexpression of wild-type or FSGS-inducing mutant TRPC6 in synaptopodin-depleted podocytes enhanced TRPC6-mediated calcium influx and induced apoptosis. In vivo, knockdown of synaptopodin also caused increased podocyte surface expression of TRPC6. Administration of cyclosporin 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 cyclosporin 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. Reducing TRPC6 surface levels may be a new approach to restoring podocyte function.

The Use of Sildenafil for Glomerular Disease

“…For example, cGMP accumulation has been linked to podocyte contractility, mobility, and cytoskeletal structure 7 ; PKG-1 is associated with poor clinical outcome in renal cell carcinoma 8 ; PPAR-g agonists have been shown to protect podocytes from nephropathies 9 ; and overexpression of TRPC6 alone is sufficient to cause podocyte damage and subsequent glomerulopathies. 10,11 Collectively, these findings highlight the importance of crosstalk among cGMP, PKG-1, PPAR-g, and TRPC6, and demonstrate the importance of TRPC6 signaling in kidney disease.…”

Epigallocatechin-3-Gallate Attenuates Adriamycin-Induced Focal Segmental Glomerulosclerosis via Suppression of Oxidant Stress and Apoptosis by Targeting Hypoxia-Inducible Factor-1α/ Angiopoietin-Like 4 Pathway

“…A large number of studies have revealed that EGCG could inhibit the expression of HIF-1α [9,10]. HIF-1α is highly related with ROS production and cell apoptosis [34,35]. Over-expression of HIF-1α promotes fibrosis and contributes to CKD [13].…”

“…Over-expression of HIF-1α promotes fibrosis and contributes to CKD [13]. Prolonged activation of HIF-1α in kidney could lead to further tissue destruction [36] and blockade of HIF-1α may play a protective role in FSGS [34]. Importantly, it also has been reported that EGCG inhibit ROS production and cell apoptosis by inhibiting the expression of HIF-1α [10,15].…”

TRPC Channels in Proteinuric Kidney Diseases

“…Thus, TRPC6-induced calcineurin activation affects numerous downstream signaling pathways that may contribute to disease progression in CKD. For example, the podocyte protein synaptopodin associates with the actin cytoskeleton and plays a key role in maintaining the complex morphology of the glomerular podocyte, as well as inhibiting cell surface expression of TRPC6 [87]. As shown in Figure 3, Faul et al [88] found that synaptopodin was phosphorylated by either protein kinase A (PKA) or calcium-dependent protein kinase II (CaMKII).…”