Bone morphogenetic proteins signal through the transforming growth factor-beta type III receptor

Signaling via the Tgf-beta type I receptor Alk5 in heart development

Trophic factors secreted both from the endocardium and epicardium regulate appropriate growth of the myocardium during cardiac development. Epicardially-derived cells play also a key role in development of the coronary vasculature. This process involves transformation of epithelial (epicardial) cells to mesenchymal cells (EMT). Similarly, a subset of endocardial cells undergoes EMT to form the mesenchyme of endocardial cushions, which function as primordia for developing valves and septa. While it has been suggested that transforming growth factor-betas (Tgf-beta) play an important role in induction of EMT in the avian epi- and endocardium, the function of Tgf-betas in corresponding mammalian tissues is still poorly understood. In this study, we have ablated the Tgf-beta type I receptor Alk5 in endo-, myo- and epicardial lineages using the Tie2-Cre, Nkx2.5-Cre, and Gata5-Cre driver lines, respectively. We show that while Alk5-mediated signaling does not play a major role in the myocardium during mouse cardiac development, it is critically important in the endocardium for induction of EMT both in vitro and in vivo. Moreover, loss of epicardial Alk5-mediated signaling leads to disruption of cell-cell interactions between the epicardium and myocardium resulting in a thinned myocardium. Furthermore, epicardial cells lacking Alk5 fail to undergo Tgf-beta-induced EMT in vitro. Late term mutant embryos lacking epicardial Alk5 display defective formation of a smooth muscle cell layer around coronary arteries, and aberrant formation of capillary vessels in the myocardium suggesting that Alk5 is controlling vascular homeostasis during cardiogenesis. To conclude, Tgf-beta signaling via Alk5 is not required in myocardial cells during mammalian cardiac development, but plays an irreplaceable cell-autonomous role regulating cellular communication, differentiation and proliferation in endocardial and epicardial cells.

Expression, purification and characterization of BG(E)RII: a novel pan-TGFbeta inhibitor

Transforming growth factor beta (TGFbeta) isoforms are known to be upregulated during the progression of some diseases. They have been shown to stimulate invasion and metastasis during carcinogenesis and promote many pathological fibrotic diseases when overstimulated. This involvement in late-stage carcinoma and pathological fibrosis makes TGFbeta isoforms prime targets for therapeutic intervention. Although soluble ectodomains of TGFbeta type II (RII) and betaglycan (BG) have been utilized as TGFbeta inhibitors, their antagonistic potency against different TGFbeta isoforms varies considerably because RII does not appreciably bind to TGFbeta2 whereas BG binds weakly to TGFbeta1 and TGFbeta3. In this study, we have successfully constructed and expressed a recombinant fusion protein containing the endoglin domain of BG (BG(E)) and the extracellular domain of RII. The fusion protein (named BG(E)RII) was purified from bacterial inclusion bodies by immobilized metal ion chromatography, refolded and characterized. It bound with higher affinity to TGFbeta1 and TGFbeta3 than a commercially available soluble RII and to TGFbeta2 than a commercially available soluble BG. More significantly, whereas BG(E) or RII alone showed no antagonistic activity towards TGFbeta2, BG(E)RII inhibited the signaling of both TGFbeta1 and TGFbeta2 in cell-based assays including TGFbeta-induced phosphorylation of Smad2 and Smad3, and transcription from a TGFbeta-responsive promoter more effectively than equimolar concentrations of either RII or BG. After further purification by gel filtration chromatography, BG(E)RII was found to have greater activity than other potent TGFbeta inhibitors in blocking the signaling of TGFbeta1 and TGFbeta3. Thus, BG(E)RII is a potent pan-TGFbeta inhibitor in vitro and has potential for blocking TGFbeta-induced pathogenesis in vivo.

Inhibin and the regulation of bone mass

Inhibins A and B are gonadal peptide members of the transforming growth factor-beta superfamily that serve as negative feedback regulators of pituitary follicle-stimulating hormone (FSH). Accumulating evidence suggests that bone turnover and bone loss increase in women before menopause and the decrease in serum estradiol levels. Increased FSH levels have been correlated with some of these perimenopausal changes, whereas decreased inhibins strongly correlate with increases in bone formation and resorption across the menopause transition, and predict lumbar bone mass in perimenopausal women, likely resulting from the direct inhibin suppression of osteoblast and osteoclast development. Interestingly, continuous exposure of mice to inhibin A in vivo is anabolic and protective against gonadectomy-induced bone loss. Together, these data suggest inhibins contribute to the endocrine regulation of bone metabolism via a bimodal mechanism of action such that cycling inhibin exposure suppresses bone turnover, and continuous exposure to inhibins is anabolic.