BAMBI

Abstract: Members of the transforming growth factor-beta (TGF-beta) superfamily, including TGF-beta, bone morphogenetic proteins (BMPs), activins and nodals, are vital for regulating growth and differentiation. These growth factors transduce their signals through pairs of transmembrane type I and type II receptor kinases. Here, we have cloned a transmembrane protein, BAMBI, which is related to TGF-beta-family type I receptors but lacks an intracellular kinase domain. We show that BAMBI is co-expressed with the ventralizing morphogen BMP4 (refs 5, 6) during Xenopus embryogenesis and that it requires BMP signalling for its expression. The protein stably associates with TGF-beta-family receptors and inhibits BMP and activin as well as TGF-beta signalling. Finally, we provide evidence that BAMBI's inhibitory effects are mediated by its intracellular domain, which resembles the homodimerization interface of a type I receptor and prevents the formation of receptor complexes. The results indicate that BAMBI negatively regulates TGF-beta-family signalling by a regulatory mechanism involving the interaction of signalling receptors with a pseudoreceptor.

Abstract: Osteogenic differentiation of mesenchymal stem cells (MSCs) is a complex process, which is regulated by various factors including microRNAs. Our preliminary data showed that the expression of endogenous miR-20a was increased during the course of osteogenic differentiation. Simultaneously, the expression of osteoblast markers and regulators BMP2, BMP4, Runx2, Osx, OCN and OPN was also elevated whereas adipocyte markers PPARγ and osteoblast antagonist, Bambi and Crim1, were downregulated, thereby suggesting that miR-20a plays an important role in regulating osteoblast differentiation. To validate this hypothesis, we tested its effects on osteogenic differentiation by introducing miR-20a mimics and lentiviral-miR20a-expression vectors into hMSCs. We showed that miR-20a promoted osteogenic differentiation by the upregulation of BMP/Runx2 signaling. We performed bioinformatics analysis and predicted that PPARγ, Bambi and Crim1 would be potential targets of miR-20a. PPARγ is a negative regulator of BMP/Runx2 signaling whereas Bambi or Crim1 are antagonists of the BMP pathway. Furthermore, we confirmed that all these molecules were indeed the targets of miR-20a by luciferase reporter, quantitative RT-PCR and western blot assays. Similarly to miR-20a overexpression, the osteogenesis was enhanced by the silence of PPARγ, Bambi or Crim1 by specific siRNAs. Taken together, for the first time, we demonstrated that miR-20a promoted the osteogenesis of hMSCs in a co-regulatory pattern by targeting PPARγ, Bambi and Crim1, the negative regulators of BMP signaling.