A novel regulatory mechanism of the bone morphogenetic protein (BMP) signaling pathway involving the carboxyl-terminal tail domain of BMP type II receptor

Bone morphogenetic protein (BMP) signaling regulates many different biological processes, including cell growth, differentiation, and embryogenesis. BMPs bind to heterogeneous complexes of transmembrane serine/threonine (Ser/Thr) kinase receptors known as the BMP type I and II receptors (BMPRI and BMPRII). BMPRII phosphorylates and activates the BMPRI kinase, which in turn activates the Smad proteins. The cytoplasmic region of BMPRII contains a "tail" domain (BMPRII-TD) with no enzymatic activity or known regulatory function. The discovery of mutations associated with idiopathic pulmonary artery hypertension mapping to BMPRII-TD underscores its importance. Here, we report that Tribbles-like protein 3 (Trb3) is a novel BMPRII-TD-interacting protein. Upon BMP stimulation, Trb3 dissociates from BMPRII-TD and triggers degradation of Smad ubiquitin regulatory factor 1 (Smurf1), which results in the stabilization of BMP receptor-regulated Smads and potentiation of the Smad pathway. Downregulation of Trb3 inhibits BMP-mediated cellular responses, including osteoblast differentiation of C2C12 cells and maintenance of the smooth muscle phenotype of pulmonary artery smooth muscle cells. Thus, Trb3 is a critical component of a novel mechanism for regulation of the BMP pathway by BMPRII.     

Repulsive guidance molecule RGMa alters utilization of bone morphogenetic protein (BMP) type II receptors by BMP2 and BMP4

Bone morphogenetic proteins (BMPs) are members of the transforming growth factor-beta superfamily of multifunctional ligands that transduce their signals through type I and II serine/threonine kinase receptors and intracellular Smad proteins. Recently, we identified the glycosylphosphatidylinositol-anchored repulsive guidance molecules RGMa, DRAGON (RGMb), and hemojuvelin (RGMc) as coreceptors for BMP signaling (Babbit, J. L., Huang, F. W., Wrighting, D. W., Xia, Y., Sidis, Y., Samad, T. A., Campagna, J. A., Chung, R., Schneyer, A., Woolf, C. J., Andrews, N. C., and Lin, H. Y. (2006) Nat. Genet. 38, 531-539; Babbit, J. L., Zhang, Y., Samad, T. A., Xia, Y., Tang, J., Schneyer, A., Woolf, C. J., and Lin, H. Y. (2005) J. Biol. Chem. 280, 29820-29827; Samad, T. A., Rebbapragada, A., Bell, E., Zhang, Y., Sidis, Y., Jeong, S. J., Campagna, J. A., Perusini, S., Fabrizio, D. A., Schneyer, A. L., Lin, H. Y., Brivanlou, A. H., Attisano, L., and Woolf, C. J. (2005) J. Biol. Chem. 280, 14122-14129). However, the mechanism by which RGM family members enhance BMP signaling remains unknown. Here, we report that RGMa bound to radiolabeled BMP2 and BMP4 with Kd values of 2.4+/-0.2 and 1.4+/-0.1 nm, respectively. In KGN human ovarian granulosa cells and mouse pulmonary artery smooth muscle cells, BMP2 and BMP4 signaling required BMP receptor type II (BMPRII), but not activin receptor type IIA (ActRIIA) or ActRIIB, based on changes in BMP signaling by small interfering RNA inhibition of receptor expression. In contrast, cells transfected with RGMa utilized both BMPRII and ActRIIA for BMP2 or BMP4 signaling. Furthermore, in BmpRII-null pulmonary artery smooth muscle cells, BMP2 and BMP4 signaling was reduced by inhibition of endogenous RGMa expression, and RGMa-mediated BMP signaling required ActRIIA expression. These findings suggest that RGMa facilitates the use of ActRIIA by endogenous BMP2 and BMP4 ligands that otherwise prefer signaling via BMPRII and that increased utilization of ActRIIA leads to generation of an enhanced BMP signal.     

Molecular recognition in bone morphogenetic protein (BMP)/receptor interaction

Bone morphogenetic proteins (BMPs) and other members of the TGF-beta superfamily are secreted signalling proteins determining the development, maintenance and regeneration of tissues and organs. These dimeric proteins bind, via multiple epitopes, two types of signalling receptor chains and numerous extracellular modulator proteins that stringently control their activity. Crystal structures of free ligands and of complexes with type I and type II receptor extracellular domains and with the modulator protein Noggin reveal structural epitopes that determine the affinity and specificity of the interactions. Modelling of a ternary complex BMP/(BMPR-IA(EC))2 / (ActR-II(EC))2 suggests a mechanism of receptor activation that does not rely on direct contacts between extracellular domains of the receptors. Mutational and interaction analyses indicate that the large hydrophobic core of the interface of BMP-2 (wrist epitope) with the type I receptor does not provide a hydrophobic hot spot for binding. Instead, main chain amide and carbonyl groups that are completely buried in the contact region represent major binding determinants. The affinity between ligand and receptor chains is probably strongly increased by two-fold interactions of the dimeric ligand and receptor chains that exist as homodimers in the membrane (avidity effects). BMP muteins with disrupted epitopes for receptor chains or modulator proteins provide clues for drug design and development.     

Bone morphogenetic protein (BMP) type II receptor deletion reveals BMP ligand-specific gain of signaling in pulmonary artery smooth muscle cells

Bone morphogenetic protein (BMP) ligands signal by binding the BMP type II receptor (BMPR2) or the activin type II receptors (ActRIIa and ActRIIb) in conjunction with type I receptors to activate SMADs 1, 5, and 8, as well as members of the mitogen-activated protein kinase family. Loss-of-function mutations in Bmpr2 have been implicated in tumorigenesis and in the etiology of primary pulmonary hypertension. Because several different type II receptors are known to recognize BMP ligands, the specific contribution of BMPR2 to BMP signaling is not defined. Here we report that the ablation of Bmpr2 in pulmonary artery smooth muscle cells, using an ex vivo conditional knock-out (Cre-lox) approach, as well as small interfering RNA specific for Bmpr2, does not abolish BMP signaling. Disruption of Bmpr2 leads to diminished signaling by BMP2 and BMP4 and augmented signaling by BMP6 and BMP7. Using small interfering RNAs to inhibit the expression of other BMP receptors, we found that wild-type cells transduce BMP signals via BMPR2, whereas BMPR2-deficient cells transduce BMP signals via ActRIIa in conjunction with a set of type I receptors distinct from those utilized by BMPR2. These findings suggest that disruption of Bmpr2 leads to the net gain of signaling by some, but not all, BMP ligands via the activation of ActRIIa.     

Involvement of endogenous bone morphogenetic protein (BMP) 2 and BMP6 in bone formation

Although accumulated evidence has shown the bone anabolic effects of bone morphogenetic proteins (BMPs) that were exogenously applied in vitro and in vivo, the roles of endogenous BMPs during bone formation remain to be clarified. This study initially investigated expression patterns of BMPs in the mouse long bone and found that BMP2 and BMP6 were the main subtypes expressed in hypertrophic chondrocytes that induce endochondral bone formation. We then examined the involvement of the combination of these BMPs in bone formation in vivo by generating the compound-deficient mice (Bmp2+/-;Bmp6-/-). Under physiological conditions, these mice exhibited moderate growth retardation compared with the wild-type (WT) littermates during the observation period up to 52 weeks of age. Both the fetal and adult compound-deficient mice showed a reduction in the trabecular bone volume with suppressed bone formation, but normal bone resorption, whereas the single deficient mice (Bmp2+/- or Bmp6-/-) did not. When a fracture was created at the femoral midshaft and the bone healing was analyzed, the endochondral bone formation, but not intramembranous bone formation, was impaired by the compound deficiency. In the cultures of bone marrow cells, however, there was no difference in osteogenic differentiation between WT and compound-deficient cells in the presence or absence of the exogenous BMP2. We thus concluded that endogenous BMP2 and BMP6 cooperatively play pivotal roles in bone formation under both physiological and pathological conditions.     

Pleiotrophin regulates bone morphogenetic protein (BMP)-induced ectopic osteogenesis

We previously isolated pleiotrophin (PTN) from bovine bone as a protein and showed that it stimulated osteoblastic growth and differentiation. Further details of its function, however, have not been fully clarified. The aim of this paper was to elucidate the effects of PTN on bone morphogenetic protein (BMP)-induced ectopic osteogenesis. Recombinant human BMP (rhBMP)-2 (1.2 microg) was combined with a fibrous glass membrane, which had been established as an effective carrier. Various amounts of the purified bovine PTN (5, 10, 50, and 100 microg) or rhPTN (5 and 10 microg) were added to the rhBMP-2/carrier composites and implanted into rats subcutaneously as reported. It was found that the amount of bone induced in the system increased with the addition of 10 microg of either purified PTN or rhPTN. However, the amount of bone decreased with the addition of 50 or 100 microg of purified PTN dose-dependently, as judged by both alkaline phosphatase activity and calcium content in the retrieved implants. It was concluded that purified PTN or rhPTN, at ratios of concentration of 10-100 microg of PTN to 1.2 microg of rhBMP-2 in the carrier, regulated the ectopic bone-inducing activity of rhBMP-2.     

Formation of stable homomeric and transient heteromeric bone morphogenetic protein (BMP) receptor complexes regulates Smad protein signaling

The type I and type II bone morphogenetic protein receptors (BMPRI and BMPRII) are present at the plasma membrane as monomers and homomeric and heteromeric complexes, which are modulated by ligand binding. The complexes of their extracellular domains with ligand were shown to form heterotetramers. However, the dynamics of the oligomeric interactions among the full-length receptors in live cell membranes were not explored, and the roles of BMP receptor homodimerization were unknown. Here, we investigated these issues by combining patching/immobilization of an epitope-tagged BMP receptor at the cell surface with measurements of the lateral diffusion of a co-expressed, differently tagged BMP receptor by fluorescence recovery after photobleaching (FRAP). These studies led to several novel conclusions. (a) All homomeric complexes (without or with BMP-2) were stable on the patch/FRAP time scale (minutes), whereas the heterocomplexes were transient, a difference that may affect signaling. (b) Patch/FRAP between HA- and myc-tagged BMPRII combined with competition by untagged BMPRIb showed that the heterocomplexes form at the expense of homodimers. (c) Stabilization of BMPRII·BMPRIb heterocomplexes (but not homomeric complexes) by IgG binding to same-tag receptors elevated phospho-Smad formation both without and with BMP-2. These findings suggest two mechanisms that may suppress the tendency of preformed BMP receptor hetero-oligomers to signal without ligand: (a) competition between homo- and heterocomplex formation, which reduces the steady-state level of the latter, and (b) the transient nature of the heterocomplexes, which limits the time during which BMPRI can be phosphorylated by BMPRII in the heterocomplex.     

Downregulation of type II bone morphogenetic protein receptor in hypoxic pulmonary hypertension

Heterozygous mutations in the type II receptor for bone morphogenetic protein (BMPR-II) and dysfunction of BMPR-II have been implicated in patients with primary pulmonary hypertension (PH). To clarify the possible involvement of BMP and BMPR-II in the development of hypoxic PH, the expression of BMP-2, BMPR-II, and their downstream signals were investigated in rat lung under normal and hypoxic conditions by RT-PCR, immunoblot, and immunohistochemical methods. In rats under normal conditions, BMP-2 is localized in the endothelium of the pulmonary artery, whereas BMPR-II is abundantly expressed in the endothelium, smooth muscle cells, and adventitial fibroblasts. After 0.5 and 3 days of exposure to hypoxia, upregulation of BMP-2 was observed in the intrapulmonary arteries. The change was accompanied by activation of its downstream signaling, p38 MAPK, and Erk1/2 MAPK, and the apoptotic process, measured by caspase-3 activity and TdT-mediated dUTP nick end labeling-positive cells. In contrast, a significant decrease in the expression of BMPR-II and inactivation of p38 MAPK and caspase-3 were observed in the pulmonary vasculature after 7-21 days of hypoxia exposure. Because BMP-2 is known to inhibit proliferation of vascular smooth muscle cells and promote cellular apoptosis, disruption of BMP signaling pathway through downregulation of BMPR-II in chronic hypoxia may result in pulmonary vascular remodeling due to the failure of critical antiproliferative/differentiation programs in the pulmonary vasculature. These results suggest abrogation of BMP signaling may be a common molecular pathogenesis in the development of PH with various pathophysiological events, including primary and hypoxic PH.     

Bacterial expression and purification of the ovine type II bone morphogenetic protein receptor ectodomain

Bone morphogenetic proteins (BMPs) are essential for a wide range of developmental processes. They signal through type I and type II serine/threonine kinase receptors, and differ from other TGF-beta family members in that the type II receptor binds with a lower affinity than the type I. Here, we describe the development of various Escherichia coli expression systems for the extracellular domain of the ovine type II bone morphogenetic protein receptor. In order to facilitate disulfide bond formation and protein solubility, BMPRII was expressed fused to bacterial thioredoxin, which, following cleavage, could be purified using several chromatography steps. Although this material migrated as a single band in denaturing PAGE, native-PAGE indicated heterogeneity, and this protein could not be crystallised. When expressed alone, either containing a histidine tag or as an untagged protein, the receptor ectodomain was deposited as insoluble inclusion bodies. Protein subjected to in vitro refolding procedures exhibited multiple species following anion exchange chromatography and size exclusion chromatography, as visualised on native-PAGE. Separation of these species could be achieved using a MonoP chromatofocusing matrix. One of these separated fractions, representing about 5% of the starting material, was amenable to crystallisation, and furthermore exhibited biological activity. Crystals of the histidine-tagged form were shown to diffract weakly, whereas crystals of the native form grew in two different morphologies, and diffracted to a resolution of 1.2A.     

Type I collagen induces expression of bone morphogenetic protein receptor type II

The extracellular matrix regulates many fundamental cellular processes such as proliferation, migration, and differentiation. Among the ECM components, type I collagen induces endothelial tube formation in vitro. By analysing genes participating in this event, the bone morphogenetic protein receptor-II (BMPR-II) was detected to be upregulated in cells cultured on or within fibrillar type I collagen. Furthermore, the basement membrane type IV collagen or amorphous type I collagen did not show an induction of BMPR-II. Addition of the BMPR-II specific ligands, BMP2 and BMP4, in the culture medium of the endothelial cells seeded on type I collagen increased [(3)H]-thymidine incorporation into cellular DNA, indicating that endothelial cells were able to form a functional receptor. In addition, in the chick chorioallantoic membrane (CAM), an in vivo angiogenesis model, BMPR-II and BMPR-I were upregulated in the growing phase and ceased in the mature CAM.     

Structure-activity relationship study of bone morphogenetic protein (BMP) signaling inhibitors

A structure–activity relationship study of dorsomorphin, a previously identified inhibitor of SMAD 1/5/8 phosphorylation by bone morphogenetic protein (BMP) type 1 receptors ALK2, 3, and 6, revealed that increased inhibitory activity could be accomplished by replacing the pendent 4-pyridine ring with 4-quinoline. The activity contributions of various nitrogen atoms in the core pyrazolo[1,5-a]pyrimidine ring were also examined by preparing and evaluating pyrrolo[1,2-a]pyrimidine and pyrazolo[1,5-a]pyridine derivatives. In addition, increased mouse liver microsome stability was achieved by replacing the ether substituent on the pendent phenyl ring with piperazine. Finally, an optimized compound 13 (LDN-193189 or DM-3189) demonstrated moderate pharmacokinetic characteristics (e.g., plasma t_1/2 = 1.6 h) following intraperitoneal administration in mice. These studies provide useful molecular probes for examining the in vivo pharmacology of BMP signaling inhibition.     

Bone morphogenetic protein (BMP)1-3 enhances bone repair

Small molecules that exhibit biological activity have contributed to the understanding of the molecular mechanisms of various biological phenomena. 5-Bromodeoxyuridine (BrdU) is a thymidine analogue that modulates various biological phenomena such as cellular differentiation and cellular senescence in cultured mammalian cells. Although BrdU is thought to function through changing chromatin structure and gene expression, its precise molecular mechanisms are not understood. To study the molecular mechanism for the action of BrdU, we have employed the yeast Saccharomycescerevisiae as a model system, and screened multi-copy suppressor genes that confer resistance to BrdU. Our genetic screen has revealed that expression of the N-terminal short fragment of TUP1, and also disruption of HDA1 or HOS1, histone deacetylases that interact with TUP1, conferred resistance to BrdU. These results suggest the implication of the chromatin proteins in the function of BrdU, and would provide novel clues to answer the old question of how BrdU modulates various biological phenomena.     

Bone morphogenetic protein (BMP)1-3 enhances bone repair

Members of the astacin family of metalloproteinases such as human bone morphogenetic protein 1 (BMP-1) regulate morphogenesis by processing precursors to mature functional extracellular matrix (ECM) proteins and several growth factors including TGFβ, BMP2, BMP4 and GFD8. We have recently discovered that BMP1-3 isoform of the Bmp-1 gene circulates in the human plasma and is significantly increased in patients with acute bone fracture. We hypothesized that circulating BMP1-3 might have an important role in bone repair and serve as a novel bone biomarker. When administered systemically to rats with a long bone fracture and locally to rabbits with a critical size defect of the ulna, recombinant human BMP1-3 enhanced bone healing. In contrast, neutralization of the endogenous BMP1-3 by a specific polyclonal antibody delayed the bone union. Invitro BMP1-3 increased the expression of collagen type I and osteocalcin in MC3T3-E₁ osteoblast like cells, and enhanced the formation of mineralized bone nodules from bone marrow mesenchymal stem cells. We suggest that BMP1-3 is a novel systemic regulator of bone repair.     

High resolution structures of the bone morphogenetic protein type II receptor in two crystal forms: implications for ligand binding

BMPRII is a type II TGF-beta serine threonine kinase receptor which is integral to the bone morphogenetic protein (BMP) signalling pathway. It is known to bind BMP and growth differentiation factor (GDF) ligands, and has overlapping ligand specificity with the activin type II receptor, ActRII. In contrast to activin and TGF-beta type ligands, BMPs bind to type II receptors with lower affinity than type I receptors. Crystals of the BMPRII ectodomain were grown in two different forms, both of which diffracted to high resolution. The tetragonal form exhibited some disorder, whereas the entire polypeptide was seen in the orthorhombic form. The two structures retain the basic three-finger toxin fold of other TGF-beta receptor ectodomains, and share the main hydrophobic patch used by ActRII to bind various ligands. However, they present different conformations of the A-loop at the periphery of the proposed ligand-binding interface, in conjunction with rearrangement of a disulfide bridge within the loop. This particular disulfide (Cys94-Cys117) is only present in BMPRII and activin receptors, suggesting that it is important for their likely shared mode of binding. Evidence is presented that the two crystal forms represent ligand-bound and free conformations of BMPRII. Comparison with the solved structure of ActRII bound to BMP2 suggests that His87, unique amongst TGF-beta receptors, may play a key role in ligand recognition.     

The mode of bone morphogenetic protein (BMP) receptor oligomerization determines different BMP-2 signaling pathways.

Bone morphogenetic proteins (BMPs) are multifunctional proteins regulating cell growth, differentiation, and apoptosis. BMP-2 signals via two types of receptors (BRI and BRII) that are expressed at the cell surface as homomeric as well as heteromeric complexes. Prior to ligand binding, a low but measurable level of BMP-receptors is found in preformed hetero-oligomeric complexes. The major fraction of the receptors is recruited into hetero-oligomeric complexes only after ligand addition. For this, BMP-2 binds first to the high affinity receptor BRI and then recruits BRII into the signaling complex. However, ligand binding to the preformed complex composed of BRII and BRI is still required for signaling, suggesting that it may mediate activating conformational changes. Using several approaches we have addressed the following questions: (i) Are preformed complexes incompetent of signaling in the absence of BMP-2? (ii) Which domains of the BRII receptors are essential for this complex formation? (iii) Are there differences in signals sent from BMP-induced versus preformed receptor complexes? By measuring the activation of Smads, of p38 MAPK and of alkaline phosphatase, we show that the ability of kinase-deficient BRII receptor mutants to inhibit BMP signaling depends on their ability to form heteromeric complexes with BRI. Importantly, a BRII mutant that is incapable in forming preassembled receptor complexes but recruits into a BMP-induced receptor complex does not interfere with the Smad pathway but does inhibit the induction of alkaline phosphatase as well as p38 phosphorylation. These results indicate that signals induced by binding of BMP-2 to preformed receptor complexes activate the Smad pathway, whereas BMP-2-induced recruitment of receptors activates a different, Smad-independent pathway resulting in the induction of alkaline phosphatase activity via p38 MAPK.