TGFbeta signaling is initiated when the type I receptor phosphorylates the MAD-related protein, Smad2, on C-terminal serine residues. This leads to Smad2 association with Smad4, translocation to the nucleus, and regulation of transcriptional responses. Here we demonstrate that Smad7 is an inhibitor of TGFbeta signaling. Smad7 prevents TGFbeta-dependent formation of Smad2/Smad4 complexes and inhibits the nuclear accumulation of Smad2. Smad7 interacts stably with the activated TGFbeta type I receptor, thereby blocking the association, phosphorylation, and activation of Smad2. Furthermore, mutations in Smad7 that interfere with receptor binding disrupt its inhibitory activity. These studies thus define a novel function for MAD-related proteins as intracellular antagonists of the type I kinase domain of TGFbeta family receptors.

Vascular endothelium is an important transducer and integrator of both humoral and biomechanical stimuli within the cardiovascular system. Utilizing a differential display approach, we have identified two genes, Smad6 and Smad7,

Summary Toll-Like Receptor (TLR) signaling is a key component of innate immunity. Aberrant TLR activation leads to immune disorders via dysregulation of cytokine production, such as IL-12/23. Herein we identify and characterize PIKfyve, a lipid kinase, as a critical player in TLR signaling using apilimod as an affinity tool. Apilimod is a potent small molecular inhibitor of IL-12/23 with an unknown target and has been evaluated in clinical trials for patients with Crohn’s disease or rheumatoid arthritis. Using a chemical genetics approach, we show that it binds to PIKfyve and blocks its phosphotransferase activity, leading to selective inhibition of IL-12/23p40. Pharmacological or genetic inactivation of PIKfyve is necessary and sufficient for suppression of IL-12/23p40 expression. Thus, we have uncovered a novel phosphoinositide-mediated regulatory mechanism that controls TLR signaling.