Swiprosin-1 Promotes Mitochondria-Dependent Apoptosis of Glomerular Podocytes via P38 MAPK Pathway in Early-Stage Diabetic Nephropathy

Background/Aims: Podocyte injury, especially podocyte apoptosis, plays a major role in early-stage diabetic nephropathy (DN). Swiprosin-1, also known as EF hand domain containing 2 (EFhd2), is a Ca2+-binding protein in different cell types. However, the function of swiprosin-1 in podocytes remains unknown. Methods: The expression and distribution of swiprosin-1 were investigated in the mouse renal glomerulus and conditionally immortalized mouse podocyte cell line MPC-5. The expression of swiprosin-1 was also detected in streptozotocin (STZ)-treated mice and MPC-5 cells treated with high glucose (HG). Nephrin and podocin were detected by immunohistochemistry and immunofluroscence. Collagen IV, transforming growth factor-β (TGF-β) and fibronectin mRNA expressions were assayed by real-time PCR. Apoptotic proteins and phosphorylation of p38 mitogen-activated protein kinase (MAPK) were detected by immunoblotting. Results: Swiprosin-1 was found to be expressed in podocytes of the mouse glomerulus and MPC-5 cells. Swiprosin-1 expression was increased in STZ-treated mice and MPC-5 cells treated with HG. In Swiprosin-1-/- diabetic mice, kidney/ body weight, urinary albumin, podocyte foot process effacement and glomerular basement membrane thickening were attenuated; the downregulation of nephrin and podocin expression in the glomerulus was inhibited; and the upregulation of collagen IV, TGF-β and fibronectin mRNA expression in the renal cortex was ameliorated as compared with those in diabetic swiprosin-1+/+ mice. In addition, the increased apoptosis of podocytes, proapoptotic protein expression and p38 phosphorylation in Swiprosin-1-/- diabetic mice were inhibited as compared with those in diabetic swiprosin-1+/+ mice. Knockdown of swiprosin-1 in MPC-5 cells reduced the apoptosis of podocytes, proapoptotic protein expression and p38 phosphorylation induced by HG. Targeted knockdown of p38 attenuated the increased apoptosis of MPC-5 cells over-expressing swiprosin-1. Conclusion: Swiprosin-1 expression in podocytes of the mouse glomerulus played a critical role in early-stage DN. Swiprosin-1 deficiency in early DN attenuated mitochondria-dependent podocyte apoptosis induced by hyperglycemia or HG via p38 MAPK signaling pathway.