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

TGF-beta regulates differentiation, growth, and apoptosis of podocytes and mediates podocyte depletion in glomerulosclerosis. TGF-beta 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-beta receptor-regulated Smad2 and Smad3. We found that the C-terminal region of CD2AP interacts directly with the cytoplasmic tail of the TGF-beta receptor type I (T beta RI) in a kinase-dependent manner and that the interaction between the T beta RI 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-beta-induced growth inhibition and apoptosis. In contrast, CD2AP-deficient cells were hypoproliferative and hypersensitive to TGF-beta-induced apoptosis. In vivo, to determine the effects of reduced Smad3 or CD2AP gene dosage on podocyte apoptosis and proteinuria characteristic of TGF-beta 1 transgenic mice, we generated TGF-beta 1 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 T beta RI and that mediate opposing signals for podocyte death or survival.