SMAD6

SMAD6 is one of the two inhibitory SMADs (I-SMADs), a feedback antagonist that selectively dampens BMP signaling. Induced by BMP-SMAD1/5 activation itself, SMAD6 acts through multiple mechanisms: it competitively blocks SMAD1/5 phosphorylation by binding type I receptors (particularly ALK3/ALK6), recruits E3 ubiquitin ligases SMURF1/SMURF2 to degrade activated receptors, interferes with R-SMAD/SMAD4 heteromerization, and recruits the corepressor HOXC8 to repress transcription. Beyond BMP pathways, SMAD6 restrains TGF-β-induced SMAD2/3 signaling in endothelial cells and vascular smooth muscle, conferring atheroprotective and anti-inflammatory effects. SMAD6 deficiency in mice results in cardiovascular defects-outflow tract malformations and valve hyperplasia-because of unrestrained BMP signaling during heart development. In humans, SMAD6 mutations cause congenital heart and aortic valve disease, bicuspid aortic valve and thoracic aortic aneurysm, while SMAD6 variants are associated with osteoarthritis susceptibility and neurodevelopmental outcomes, cementing SMAD6's role as guardian of BMP pathway homeostasis.