BMP-3 is a novel inhibitor of both activin and BMP-4 signaling in Xenopus embryos

In Xenopus, the biological effects of BMP-3 oppose those of ventralizing BMPs, but the mechanism for this antagonism remains unclear. Here, we demonstrate that BMP-3 is a dorso-anteriorizing factor in Xenopus embryos that interferes with both activin and BMP signaling. BMP-3 acts by binding to ActRIIB, the common type II receptor for these proteins. Once BMP-3 binds to ActRIIB, it cannot be competed off by excess ligand making a receptor complex that is unable to activate R-Smads and transduce signal. Consistent with a model where BMP-3 interferes with activin and BMPs through a shared receptor, we show that overexpression of BMP-3 can only be rescued by co-injection of xActRIIB. Our results identify BMP-3 as a novel antagonist of both activin and BMPs and uncover how some of the diverse developmental processes that are regulated by both activin and BMP signaling can be modulated during embryogenesis.     

Crystal structure of the BMP-2-BRIA ectodomain complex

Bone morphogenetic proteins (BMPs) belong to the large transforming growth factor-beta (TGF-beta) superfamily of multifunctional cytokines. BMP-2 can induce ectopic bone and cartilage formation in adult vertebrates and is involved in central steps in early embryonal development in animals. Signaling by these cytokines requires binding of two types of transmembrane serine/threonine receptor kinase chains classified as type I and type II. Here we report the crystal structure of human dimeric BMP-2 in complex with two high affinity BMP receptor IA extracellular domains (BRIAec). The receptor chains bind to the 'wrist' epitopes of the BMP-2 dimer and contact both BMP-2 monomers. No contacts exist between the receptor domains. The model reveals the structural basis for discrimination between type I and type II receptors and the variability of receptor-ligand interactions that is seen in BMP-TGF-beta systems.     

A surface component on GH3 pituitary cells that recognizes transforming growth factor-beta, activin, and inhibin

We have examined the ability of various forms of activin and inhibin, which are structurally related to transforming growth factor-beta (TGF-beta), to interact with various types of cell surface TGF-beta binding sites. Activin AB, inhibin A, and inhibin B were unable to compete with 125I-TGF-beta 1 for binding to the TGF-beta receptor types I, II, or III that coexist in human skin fibroblasts, rat liver epithelial cells, and mink lung epithelial cells. In contrast, activins and inhibins effectively competed for TGF-beta 1 binding to GH3 rat pituitary tumor cells. Binding of TGF-beta 1 to GH3 cells was mediated by about 2700 sites/cell with a Kd = 90 pM. Affinity labeling of these GH3 binding sites by cross-linking to 125I-TGF-beta 1 yielded 70-74-kDa labeled complexes distinct from previously identified TGF-beta binding components. Labeling of these 70-74-kDa components with 125I-TGF-beta 1 was inhibited by TGF-beta 1, TGF-beta 2, activin AB, and inhibin B at concentrations in the high picomolar to low nanomolar range, but it was not significantly affected by other polypeptide hormones and growth factors tested. The 70-74-kDa labeled GH3 components represent a novel type of cell surface TGF-beta binding protein that is unique in its ability to recognize various other members of the TGF-beta family of bioactive polypeptides.     

A new model for growth factor activation: type II receptors compete with the prodomain for BMP-7

Bone morphogenetic proteins (BMPs) are morphogens with long-range signaling activities. BMP-7 is secreted as a stable complex consisting of a growth factor noncovalently associated with two propeptides. In other transforming growth factor-β-like growth factor complexes, the prodomain (pd) confers latency to the complex. However, we detected no difference in signaling capabilities between the growth factor and the BMP-7 complex in multiple in vitro bioactivity assays. Biochemical and biophysical methods elucidated the interaction between the BMP-7 complex and the extracellular domains of its type I and type II receptors. Results showed that type II receptors, such as BMP receptor II, activin receptor IIA, and activin receptor IIB, competed with the pd for binding to the growth factor and displaced the pd from the complex. In contrast, type I receptors interacted with the complex without displacing the pd. These studies suggest a new model for growth factor activation in which proteases or other extracellular molecules are not required and provide a molecular mechanism consistent with a role for BMP receptors in the establishment of early morphogen gradients.     

Identification of the ligand-binding site of the BMP type IA receptor for BMP-4

Bone morphogenetic proteins (BMPs) belong to the transforming growth factor-beta (TGF-beta) superfamily of multifunctional cytokines. BMP induces its signal to regulate growth, differentiation, and apoptosis of various cells upon trimeric complex formation with two distinct type I and type II receptors on the cell surface: both are single-transmembrane serine/threonine kinase receptors. To identify the amino acid residues on BMP type I receptor responsible for its ligand binding, the structure-activity relationship of the extracellular ligand-binding domain of the BMP type IA receptor (sBMPR-IA) was investigated by alanine-scanning mutagenesis. The mutant receptors, as well as sBMPR-IA, were expressed as fusion proteins with thioredoxin in Escherichia coli, and purified using reverse phase high performance liquid chromatography (RP-HPLC) after digestion with enterokinase. Structural analysis of the parent protein and representative mutants in solution by CD showed no detectable differences in their folding structures. The binding affinity of the mutants to BMP-4 was determined by surface plasmon resonance biosensor. All the mutant receptors examined, with the exception of Y70A, displayed reduced affinities to BMP-4 with the rank order of decreases: I52A (17-fold) approximately F75A (15-fold) > T64A (4-fold) = T62A (4-fold) approximately E54A (3-fold). The decreases in binding affinity observed for the latter three mutants are mainly due to decreased association rate constants while alterations in rate constants both, for association and dissociation, result in the drastically reduced affinities for the former two mutants. These results allow us to conclude that sBMPR-IA recognizes the ligand using the concave face of the molecule. The major ligand-binding site of the BMP type IA receptor consists of Phe75 in loop 2 and Ile52, Glu54, Thr62 and Thr64 on the three-stranded beta-sheet. These findings should provide a general basis for the ligand/type I receptor recognition in the TGF-beta superfamily.     

Activin A/bone morphogenetic protein (BMP) chimeras exhibit BMP-like activity and antagonize activin and myostatin

Activins and bone morphogenetic proteins (BMPs) are members of the transforming growth factor-beta family of growth and differentiation factors that induce signaling in target cells by assembling type II and type I receptors at the cell surface. Ligand residues involved in type II binding are located predominantly in the C-terminal region that forms an extended beta-sheet, whereas residues involved in type I binding are located in the alpha-helical and preceding loop central portion of the molecule. To test whether the central residues are sufficient to determine specificity toward type I receptors, activin A/BMP chimeras were constructed in which the central residues (45-79) of activin A were replaced with corresponding residues of BMP2 and BMP7. The chimeras were assessed for activin type II receptor (Act RII) binding, activin-like bioactivity, and BMP-like activity as well as antagonistic properties toward activin A and myostatin. ActA/BMP7 chimera retained Act RII binding affinity comparable with wild type activin A, whereas ActA/BMP2 chimera showed a slightly reduced affinity toward Act RII. Both the chimeras were devoid of significant activin bioactivity in 293T cells in the A3 Lux reporter assay up to concentrations 10-fold higher than the minimal effective activin A concentration (approximately 4 nM). In contrast, these chimeras showed BMP-like activity in a BRE-Luc assay in HepG2 cells as well as induced osteoblast-like phenotype in C2C12 cells expressing alkaline phosphatase. Furthermore, both the chimeras activated Smad1 but not Smad2 in C2C12 cells. Also, both the chimeras antagonized ligands that signal via activin type II receptor, such as activin A and myostatin. These data indicate that activin residues in the central region determine its specificity toward type I receptors. ActA/BMP chimeras can be useful in the study of receptor specificities and modulation of transforming growth factor-beta members, activins, and BMPs.     

BMP-2/4 and BMP-6/7 differentially utilize cell surface receptors to induce osteoblastic differentiation of human bone marrow-derived mesenchymal stem cells

Bone morphogenetic proteins (BMPs) are members of the transforming growth factor-beta superfamily of growth factors and are used clinically to induce new bone formation. The purpose of this study was to evaluate receptor utilization by BMP-2, BMP-4, BMP-6, and BMP-7 in primary human mesenchymal stem cells (hMSC), a physiologically relevant cell type that probably mediates the in vivo effects of BMPs. RNA interference-mediated gene knockdown revealed that osteoinductive BMP activities in hMSC are elicited through the type I receptors ACVR1A and BMPR1A and the type II receptors ACVR2A and BMPR2. BMPR1B and ACVR2B were expressed at low levels and were not found to play a significant role in signaling by any of the BMPs evaluated in this study. Type II receptor utilization differed significantly between BMP-2/4 and BMP-6/7. A greater reliance on BMPR2 was observed for BMP-2/4 relative to BMP-6/7, whereas ACVR2A was more critical to signaling by BMP-6/7 than BMP-2/4. Significant differences were also observed for the type I receptors. Although BMP-2/4 used predominantly BMPR1A for signaling, ACVR1A was the preferred type I receptor for BMP-6/7. Signaling by both BMP-2/4 and BMP-6/7 was mediated by homodimers of ACVR1A or BMPR1A. A portion of BMP-2/4 signaling also required concurrent BMPR1A and ACVR1A expression, suggesting that BMP-2/4 signal in part through ACVR1A/BMPR1A heterodimers. The capacity of ACVR1A and BMPR1A to form homodimers and heterodimers was confirmed by bioluminescence resonance energy transfer analyses. These results suggest different mechanisms for BMP-2/4- and BMP-6/7-induced osteoblastic differentiation in primary hMSC.     

Type I receptor binding of bone morphogenetic protein 6 is dependent on N-glycosylation of the ligand

Bone morphogenetic proteins (BMPs), together with transforming growth factor (TGF)-beta and activins/inhibins, constitute the TGF-beta superfamily of ligands. This superfamily is formed by more than 30 structurally related secreted proteins. The crystal structure of human BMP-6 was determined to a resolution of 2.1 A; the overall structure is similar to that of other TGF-beta superfamily ligands, e.g. BMP-7. The asymmetric unit contains the full dimeric BMP-6, indicating possible asymmetry between the two monomeric subunits. Indeed, the conformation of several loops differs between both monomers. In particular, the prehelix loop, which plays a crucial role in the type I receptor interactions of BMP-2, adopts two rather different conformations in BMP-6, indicating possible dynamic flexibility of the prehelix loop in its unbound conformation. Flexibility of this loop segment has been discussed as an important feature required for promiscuous binding of different type I receptors to BMPs. Further studies investigating the interaction of BMP-6 with different ectodomains of type I receptors revealed that N-glycosylation at Asn73 of BMP-6 in the wrist epitope is crucial for recognition by the activin receptor type I. In the absence of the carbohydrate moiety, activin receptor type I-mediated signaling of BMP-6 is totally diminished. Thus, flexibility within the binding epitope of BMP-6 and an unusual recognition motif, i.e. an N-glycosylation motif, possibly play an important role in type I receptor specificity of BMP-6.     

An activin mutant with disrupted ALK4 binding blocks signaling via type II receptors

Activins control many physiologic and pathophysiologic processes in multiple tissues and, like other TGF-beta superfamily members, signal via type II (ActRII/IIB) and type I (ALK4) receptor serine kinases. ActRII/IIB are promiscuous receptors known to bind at least a dozen TGF-beta superfamily ligands including activins, myostatin, several BMPs, and nodal. Here we utilize a new screening procedure to rapidly identify activin-A mutants with loss of signaling activity. Our goal was to identify activin-A mutants able to bind ActRII but unable to bind ALK4 and which would be, therefore, candidate type II activin receptor antagonists. Using the structure of BMP-2 bound to its type I receptor (ALK3) as a guide, we introduced mutations in the context of the inhibin betaA cDNA and assessed the signaling activity of the resulting mutant proteins. We identified several mutants in the finger (M91E, I105E, M108A) and wrist (activin A/activin C chimera, S60P, I63P) regions of activin-A with reduced signaling activity. Of these the M108A mutant displayed the lowest signaling activity while retaining wild-type-like affinity for ActRII. Unlike wild-type activin-A, the M108A mutant was unable to form a cross-linked complex with ALK4 in the presence of ActRII indicating that its ability to bind ALK4 was disrupted. This data suggested that the M108A mutant might be capable of modulating signaling of activin and related ligands. Indeed, the M108A mutant antagonized activin-A and myostatin, but not TGF-beta, signaling in 293T cells, indicating it may be generally capable of blocking ligands that signal via ActRII/IIB.     

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 cysteine-rich domain protein KCP is a suppressor of transforming growth factor beta/activin signaling in renal epithelia

The transforming growth factor beta (TGF-beta) superfamily, including the bone morphogenetic protein (BMP) and TGF-beta/activin A subfamilies, is regulated by secreted proteins able to sequester or present ligands to receptors. KCP is a secreted, cysteine-rich (CR) protein with similarity to mouse Chordin and Xenopus laevis Kielin. KCP is an enhancer of BMP signaling in vertebrates and interacts with BMPs and the BMP type I receptor to promote receptor-ligand interactions. Mice homozygous for a KCP null allele are hypersensitive to developing renal interstitial fibrosis, a disease stimulated by TGF-beta but inhibited by BMP7. In this report, the effects of KCP on TGF-beta/activin A signaling are examined. In contrast to the enhancing effect on BMPs, KCP inhibits both activin A- and TGF-beta1-mediated signaling through the Smad2/3 pathway. These inhibitory effects of KCP are mediated in a paracrine manner, suggesting that direct binding of KCP to TGF-beta1 or activin A can block the interactions with prospective receptors. Consistent with this inhibitory effect, primary renal epithelial cells from KCP mutant cells are hypersensitive to TGF-beta and exhibit increased apoptosis, dissociation of cadherin-based cell junctions, and expression of smooth muscle actin. Furthermore, KCP null animals show elevated levels of phosphorylated Smad2 after renal injury. The ability to enhance BMP signaling while suppressing TGF-beta activation indicates a critical role for KCP in modulating the responses between these anti- and profibrotic cytokines in the initiation and progression of renal interstitial fibrosis.     

Increased reactivity of cultured chicken blastodermal cells to anti-stage-specific embryonic antigen-1 antibody after exposure to bone morphogenetic proteins

We evaluated whether bone morphogenetic proteins (BMPs) increased the reactivity of chicken stage X blastodermal cells to the germ cell marker, anti-stage-specific embryonic antigen (SSEA)-1 antibody. In Experiment 1, blastodermal cells cultured on a feeder layer of SIM mouse embryo-derived thioguanine and ouabain resistant (STO) cells were treated with different doses of BMP-2 and/or BMP-4, and the anti-SSEA-1 antibody reactivity of cultured cells was examined 48 h later. A significant (P < 0.05) increase in the number of anti-SSEA-1 antibody-positive cells was detected after the addition of 75 or 100 ng/ml BMP-2. Neither 0-20 ng/ml BMP-4 nor the combined addition of 75 ng/ml BMP-2 with either 10 or 15 ng/ml BMP-4 increased reactivity more than that induced by 75 ng/ml BMP-2 alone. Results of the qualification and quantification of BMP receptor kinase (BRK)-1, BRK-2, and BRK-3 using RT-PCR and real-time PCR showed that all three receptors were detected in blastodermal cells treated with BMPs, intact stage X embryos and 5.5-day-old embryonic gonads, but no expression was detected in STO feeder cells. In Experiment 2, the treatment of stage X embryos with different doses of BMP-2 (0.15-3 ng/embryo) or BMP-4 (0.02-0.4 ng/embryo) did not affect the reactivity of 5.5-day-old embryonic gonadal cells to the anti-SSEA-1 antibody. BRK-1 expression was selectively increased in stage X embryos after the infusion of 3ng BMP-2 than after no infusion, but no changes in other BRKs' expression were detected. In conclusion, the addition of BMP-2 to culture medium in the presence of STO feeder cells promoted the reactivity of blastodermal cells to anti-SSEA-1 antibody, which might contribute to the generation of chicken primordial germ cell precursor or germ cell-like cells. The relationship between BMP action and BRK expression was further discussed.     

Molecular recognition of BMP-2 and BMP receptor IA

Bone morphogenetic protein-2 (BMP-2) and other members of the TGF-beta superfamily regulate the development, maintenance and regeneration of tissues and organs. Binding epitopes for these extracellular signaling proteins have been defined, but hot spots specifying binding affinity and specificity have so far not been identified. In this study, mutational and structural analyses show that epitopes of BMP-2 and the BRIA receptor form a new type of protein-protein interface. The main chain atoms of Leu 51 and Asp53 of BMP-2 represent a hot spot of binding to BRIA. The BMP-2 variant L51P was deficient in type I receptor binding only, whereas its overall structure and its binding to type II receptors and modulator proteins, such as noggin, were unchanged. Thus, the L51P substitution converts BMP-2 into a receptor-inactive inhibitor of noggin. These results are relevant for other proteins of the TGF-beta superfamily and provide useful clues for structure-based drug design.     

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

The bone morphogenetic protein (BMP) family, the largest subfamily of the structurally conserved transforming growth factor-beta (TGF-beta) superfamily of growth factors, are multifunctional regulators of development, proliferation, and differentiation. The TGF-beta type III receptor (TbetaRIII or betaglycan) is an abundant cell surface proteoglycan that has been well characterized as a TGF-beta and inhibin receptor. Here we demonstrate that TbetaRIII functions as a BMP cell surface receptor. TbetaRIII directly and specifically binds to multiple members of the BMP subfamily, including BMP-2, BMP-4, BMP-7, and GDF-5, with similar kinetics and ligand binding domains as previously identified for TGF-beta. TbetaRIII also enhances ligand binding to the BMP type I receptors, whereas short hairpin RNA-mediated silencing of endogenous TbetaRIII attenuates BMP-mediated Smad1 phosphorylation. Using a biologically relevant model for TbetaRIII function, we demonstrate that BMP-2 specifically stimulates TbetaRIII-mediated epithelial to mesenchymal cell transformation. The ability of TbetaRIII to serve as a cell surface receptor and mediate BMP, inhibin, and TGF-beta signaling suggests a broader role for TbetaRIII in orchestrating TGF-beta superfamily signaling.     

Repulsive guidance molecule (RGMa), a DRAGON homologue, is a bone morphogenetic protein co-receptor

Bone morphogenetic proteins (BMPs) are members of the transforming growth factor beta (TGF-beta) superfamily of ligands, which regulate many mammalian physiologic and pathophysiologic processes. BMPs exert their effects through type I and type II serine/threonine kinase receptors and the Smad intracellular signaling pathway. Recently, the glycosylphosphatidylinositol (GPI)-anchored protein DRAGON was identified as a co-receptor for BMP signaling. Here, we investigate whether a homologue of DRAGON, repulsive guidance molecule (RGMa), is similarly involved in the BMP signaling pathway. We show that RGMa enhances BMP, but not TGF-beta, signals in a ligand-dependent manner in cell culture. The soluble extracellular domain of RGMa fused to human Fc (RGMa.Fc) forms a complex with BMP type I receptors and binds directly and selectively to radiolabeled BMP-2 and BMP-4. RGMa mediates BMP signaling through the classical BMP signaling pathway involving Smad1, 5, and 8, and it up-regulates endogenous inhibitor of differentiation (Id1) protein, an important downstream target of BMP signals. Finally, we demonstrate that BMP signaling occurs in neurons that express RGMa in vivo. These data are consistent with a role for RGMa as a BMP co-receptor.     

BMP-3 and BMP-6 structures illuminate the nature of binding specificity with receptors

Bone morphogenetic proteins (BMPs) are extracellular messenger ligands involved in controlling a wide array of developmental and intercellular signaling processes. To initiate their specific intracellular signaling pathways, the ligands recognize and bind two structurally related serine/threonine kinase receptors, termed type I and type II, on the cell surface. Here, we present the crystal structures of BMP-3 and BMP-6, of which BMP-3 has remained poorly understood with respect to its receptor identity, affinity, and specificity. Using surface plasmon resonance (BIAcore) we show that BMP-3 binds Activin Receptor type II (ActRII) with Kd approximately 1.8 microM but ActRIIb with 30-fold higher affinity at Kd approximately 53 nM. This low affinity for ActRII may involve Ser-28 and Asp-33 of BMP-3, which are found only in BMP-3's type II receptor-binding interfaces. Point mutations of either residue to alanine results in up to 20-fold higher affinity to either receptor. We further demonstrate by Smad-based whole cell luciferase assays that the increased affinity of BMP-3S28A to ActRII enables the ligand's signaling ability to a level comparable to that of BMP-6. Focusing on BMP-3's preference for ActRIIb, we find that Lys-76 of ActRII and the structurally equivalent Glu-76 of ActRIIb are distinct between the two receptors. We demonstrate that ActRIIbE76K and ActRII bind BMP-3 with similar affinity, indicating BMP-3 receptor specificity is controlled by the interaction of Lys-30 of BMP-3 with Glu-76 of ActRIIb. These studies illustrate how a single amino acid can regulate the specificity of ligand-receptor binding and potentially alter biological signaling and function in vivo.