Cx43 Mediates TGF- Signaling through Competitive Smads Binding to Microtubules□D □V

Transforming growth factor(TGF) superfamily members play an important role in growth, differentiation, adhesion, apoptosis, and development in many species from insects and worms to vertebrates. Recently, TGFsignaling has been demonstrated to be negatively regulated by microtubules (MTs), which anchor endogenous Smad2/3 to cytosol and also directly interact with connexin43 (Cx43), and the activity of TGFis mediated by Cx43. However, the mechanism underlying the intracellular regulation of TGFactivity by Cx43 remains unknown. Here, we found that the functional link between TGFactivation and Cx43 is mediated by interactions among Smad2/3, MTs, and Cx43. We confirmed that Cx43 competes with Smad2/3 for binding to MTs, which Cx43 specifically induces release of Smad2/3 from MTs and increases phospho-Smad2 and which, as a result, Smad2/3 and Smad4 are accumulated in the nucleus, leading to activation of the transcription of target genes. Consistently, knockdown of the endogenous Cx43 activity with doublestrand RNA (dsRNA) in HL1 cardiomyocytes and Cx43 knockout mice cardiomyocytes consistently show the opposite effect. Our findings demonstrate a novel mechanism for Cx43 positive regulation of TGFfunction.