A novel negative regulatory mechanism of Smurf2 in BMP/Smad signaling in bone

Transforming growth factor-β(TGF-β)and bone morphogenetic protein(BMP)play important roles in bone metabolism.Smad ubiquitination regulatory factors(Smurfs)regulate TGF-β/BMP signaling via ubiquitination,resulting in degradation of signaling molecules to prevent excessive activation of TGF-β/BMP signaling.Though Smurf2 has been shown to negatively regulate TGF-β/Smad signaling,its involvement in BMP/Smad signaling in bone metabolism has not been thoroughly investigated.In the present study,we sought to evaluate the role of Smurf2 in BMP/Smad signaling in bone metabolism.Absorbable collagen sponges containing 3μg of recombinant human BMP2(rhBMP2)were implanted in the dorsal muscle pouches of wild type(WT)and Smurf2−/−mice.The rhBMP2-induced ectopic bone in Smurf2−/−mice showed greater bone mass,higher mineral apposition and bone formation rates,and greater osteoblast numbers than the ectopic bone in WT mice.In WT mice,the ectopic bone consisted of a thin discontinuous outer cortical shell and scant inner trabecular bone.In contrast,in Smurf2−/−mice,the induced bone consisted of a thick,continuous outer cortical shell and abundant inner trabecular bone.Additionally,rhBMP2-stimulated bone marrow stromal cells(BMSCs)from Smurf2−/−mice showed increased osteogenic differentiation.Smurf2 induced the ubiquitination of Smad1/5.BMP/Smad signaling was enhanced in Smurf2−/−BMSCs stimulated with rhBMP2,and the inhibition of BMP/Smad signaling suppressed osteogenic differentiation of these BMSCs.These findings demonstrate that Smurf2 negatively regulates BMP/Smad signaling,thereby identifying a new regulatory mechanism in bone metabolism.