Nck Adaptor Proteins Link Nephrin at the Podocyte Slit Diaphragm to the Hippo Regulator WTIP
Podocytes are specialized epithelial cells that contribute to the kidney blood filtration barrier. Their unique cytoskeletal architecture and other complex biological signals are largely maintained by a modified adherens junction known as the slit diaphragm (SD). A major component of the SD is the transmembrane protein nephrin, which upon tyrosine phosphorylation of the intracellular domain regulates actin remodeling and cell survival. Nck adaptor proteins are a critical component of the filtration barrier and connect nephrin to actin-remodeling proteins by binding phosphotyrosine residues and proline-rich motifs. Herein we identify a novel Nck binding partner, Wilm’s tumor interacting protein (WTIP). WTIP is a transcription regulator in podocytes, though Nck is not nuclear localized under conditions known to induce WTIP nuclear accumulation. However, we demonstrate that WTIP is recruited to nephrin, upon nephrin tyrosine phosphorylation by Src Family Kinases, in an Nck-dependent manner. WTIP is an evolutionarily conserved negative regulator of the Hippo kinase pathway, which inhibits the transcription factor Yap. Yap activity promotes podocyte survival. We show in mice that cannot recruit Nck to nephrin, and by extension WTIP, that Yap protein levels are downregulated. Similarly, Yap downregulation is demonstrated in wildtype mice subjected the nephrotoxic serum model of podocyte injury, which is known to induce robust decrease in nephrin tyrosine phosphorylation. Overall, through identification of a novel Nck binding partner, WTIP, we have identified a novel mechanism through which Nephrin tyrosine phosphorylation is linked to the Hippo signaling cascade, which may in turn promote podocyte survival.