Mouse Smad8 phosphorylation downstream of BMP receptors ALK-2, ALK-3, and ALK-6 induces its association with Smad4 and transcriptional activity

Smads are intracellular signaling mediators for TGF-beta superfamily. Smad1 and Smad5 are activated by BMP receptors, Here, we have cloned mouse Smad8 and functionally characterized its ability to transduce signals from BMP r eceptors. Constitutively active BMP type I receptors, ALK-3 and ALK-6, as w ell as ALK-2, were phosphorylated Smad8 and induced Smad8 interaction with Smad4. Nuclear translocation of Smad8 was stimulated by constitutively acti ve BMP type I receptors, In contrast, constitutively active TGF-beta type I receptor, ALK-5, did not exhibit any action on Smad8. Smad8 and Smad4 coop eratively induced the promoter of Xvent2, a homeobox gene that responds spe cifically to BMP signaling. Dominant-negative Smad8 was shown to inhibit th e increase of alkaline phosphatase activity induced by BMP-2 on pluripotent mesenchymal C3H10T1/2 and myoblastic C2C12 cell lines. The presence of Sma d8 mRNA in mouse calvaria cells and osteoblasts suggests a role of Smad8 in the osteoblast differentiation and maturation. (C) 2000 Academic Press.