Nephrin and CD2AP Associate with Phosphoinositide 3-OH Kinase and Stimulate AKT-Dependent Signaling

Abstract

Mutations of NPHS1 or NPHS2, the genes encoding nephrin and podocin, as well as the targeted disruption of CD2-associated protein (CD2AP), lead to heavy proteinuria, suggesting that all three proteins are essential for the integrity of glomerular podocytes, the visceral glomerular epithelial cells of the kidney. It has been speculated that these proteins participate in common signaling pathways; however, it has remained unclear which signaling proteins are actually recruited by the slit diaphragm protein complex in vivo. We demonstrate that both nephrin and CD2AP interact with the p85 regulatory subunit of phosphoinositide 3-OH kinase (PI3K) in vivo, recruit PI3K to the plasma membrane, and, together with podocin, stimulate PI3K-dependent AKT signaling in podocytes. Using two-dimensional gel analysis in combination with a phosphoserine-specific antiserum, we demonstrate that the nephrin-induced AKT mediates phosphorylation of several target proteins in podocytes. One such target is Bad; its phosphorylation and inactivation by 14-3-3 protects podocytes against detachment-induced cell death, suggesting that the nephrin-CD2AP-mediated AKT activity can regulate complex biological programs. Our findings reveal a novel role for the slit diaphragm proteins nephrin, CD2AP, and podocin and demonstrate that these three proteins, in addition to their structural functions, initiate PI3K/AKT-dependent signal transduction in glomerular podocytes.

ACKNOWLEDGMENTS

We thank Christina Engel, Birgit Schilling, and Stefanie Keller for excellent technical assistance and members of the Walz laboratory for helpful discussions. We are grateful to Lawrence B. Holzman for helpful discussions and providing immunoprecipitating nephrin antisera. We thank Karl Tryggvason, Lewis Cantley, David Fruman, Philip Tsichlis, Alex Toker, Silvio Gutkind, Brian Seed, Dusty Miller, Richard Mulligan, Jim Woodgett, and Roland Schüle for providing cDNAs.

This study was supported by DFG grants Be2212 and Wa517 and the Deutsche Nierenstiftung (T.B.H.).