New Mechanism for Podocyte Dysfunction: Regulation of Nephrin by SHP-1

Abstract

Diabetic nephropathy is characterized by dedifferentiation and apoptosis of podocytes, which are highly specialized epithelial cells involved in the glomerular filtration process. Nephrin, a transmembrane protein found in the slit diaphragm, has been found to play a role in the integrity of the podocytes. More recently, studies have shown that phosphorylation of tyrosine residues of nephrin participate in intracellular pathways regulating actin dynamics and podocyte survival. Clinical observations indicated that nephrin expression was reduced in kidney biopsy of diabetes patients. Our laboratory has recently published that the expression of SHP-1, a protein tyrosine phosphatase that contains 2 SH2 domains, is elevated in cultured podocytes exposed to high glucose concentrations. Since nephrin can interact with proteins that contain SH2 domains, we hypothesize that SHP-1 binds with nephrin and deregulates nephrin-mediated pathways in diabetes. In vivo, our data demonstrated that both nephrin expression and phosphorylation was reduced in 6 months of age Akita mice, a type 1 diabetes model, which correlated with elevated SHP-1 expression in the glomeruli. In vitro, co-immunoprecipitation assays in HEK cells expressing both nephrin and SHP-1 showed an interaction between the 2 proteins. Immunoblot analysis indicated that this interaction decreased tyrosine phosphorylation of nephrin. Moreover, SHP-1 overexpression reduced phosphorylation of nephrin tyrosine 1176/1193 and 1217 tyrosine residues that regulate actin dynamics and survival. In conclusion, our results suggest that diabetes triggered SHP-1 expression in podocytes, which reduced nephrin activity contributing to the development of diabetic nephropathy.