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TβRI Independently Activates Smad- and CD2AP-Dependent Pathways in Podocytes

Sandhya Xavier^*,, Thiruvur Niranjan^*, Stefanie Krick^*,, Taoran Zhang^*,, Wenjun Ju^*,, Andrey S. Shaw, Mario Schiffer and Erwin P. Böttinger^*,

*Division of Nephrology, Department of Medicine, and
Charles R. Bronfman Institute for Personalized Medicine, Mount Sinai School of Medicine, New York, New York;
Department of Pathology and Immunology, Washington University School of Medicine, St. Louis, Missouri; and
Division of Nephrology, Department of Medicine, Hannover Medical School, Hannover, Germany

Correspondence: Dr. Erwin P. Böttinger, Mount Sinai School of Medicine, One Gustave L. Levy Place, Box 1003, New York, NY 10029. Phone: 212-241-0800; Fax: 212-849-2643; E-mail: erwin.bottinger{at}mssm.edu

Received for publication July 30, 2008. Accepted for publication May 18, 2009.

TGF-β regulates differentiation, growth, and apoptosis of podocytes and mediates podocyte depletion in glomerulosclerosis. TGF-β promotes proapoptotic signaling mediated by Smad3 but also activates prosurvival pathways such as phosphoinositide-3 kinase (PI3K)/AKT; the latter requires the CD2-associated adaptor protein (CD2AP) in podocytes. Whether the opposing activities mediated by Smad proteins and CD2AP involve molecular cross-talk is unknown. Here, we report that CD2AP-dependent early activation of the antiapoptotic PI3K/AKT pathway does not require TGF-β receptor–regulated Smad2 and Smad3. We found that the C-terminal region of CD2AP interacts directly with the cytoplasmic tail of the TGF-β receptor type I (TβRI) in a kinase-dependent manner and that the interaction between the TβRI and the p85 subunit of PI3K requires CD2AP. Consistent with the proapoptotic function of Smad signaling, Smad2/3-deficient podocytes were hyperproliferative and resistant to TGF-β–induced growth inhibition and apoptosis. In contrast, CD2AP-deficient cells were hypoproliferative and hypersensitive to TGF-β–induced apoptosis. In vivo, to determine the effects of reduced Smad3 or CD2AP gene dosage on podocyte apoptosis and proteinuria characteristic of TGF-β1 transgenic mice, we generated TGF-β1 transgenic mice deficient for Smad3 or heterozygous for CD2AP. Smad3 deficiency ameliorated podocyte apoptosis, and CD2AP heterozygosity increased both podocyte apoptosis and proteinuria. These data define distinct canonical (Smad) and noncanonical (CD2AP/PI3K/AKT) pathways that arise from direct, independent interactions with the TβRI and that mediate opposing signals for podocyte death or survival.

Copyright © 2009 by the American Society of Nephrology. Online ISSN: 1533-3450 Print ISSN: 1046-6673