Bone morphogenetic protein receptor IB signaling mediates apoptosis independently of differentiation in osteoblastic cells

Bone morphogenetic protein-2 (BMP-2) is an important regulator of osteoblast differentiation. However, the regulation of osteoblast apoptosis by BMP signaling remains poorly understood. Here we examined the role of type I BMP receptor (BMP-RI) in osteoblast apoptosis promoted by BMP-2. Despite undetectable BMP-RIB expression in OHS4 cells, BMP-2 or BMP-2 overexpression increased osteoblast differentiation similarly as in SaOS2 cells which express BMP-RIB, as shown by alkaline phosphatase and CBFA1/RUNX2 expression. In contrast to SaOS2 cells, however, BMP-2 or BMP-2 overexpression did not increase caspase-9 and caspases-3, -6, and -7 activity and DNA fragmentation in OHS4 cells. Consistently, BMP-2 increased protein kinase C (PKC) activity, and PKC inhibition suppressed BMP-2-induced caspase activity in SaOS2 but not in OHS4 cells that lack BMP-RIB. A dominant negative BMP-RIB inhibited BMP-2-induced caspase activity, whereas wild-type BMP-RIB promoted caspase activity induced by BMP-2 in SaOS2 and MC3T3-E1 cells. Wild-type BMP-RIB rescued the apoptotic response to BMP-2, and a constitutively active BMP-RIB restored the apoptotic signal in OHS4 cells, supporting an essential role for BMP-RIB in osteoblast apoptosis. We also assessed whether BMP-2-induced apoptosis occurred independently of osteoblast differentiation. General inhibition of caspases did not abolish BMP-2-induced alkaline phosphatase and CBFA1/RUNX2 expression in SaOS2 cells. Furthermore, broad caspases inhibition increased matrix mineralization but did not reverse the BMP-2 effect on mineralization in MC3T3-E1 cells. These results indicate that BMP-2-induced apoptosis was mediated by BMP-RIB in osteoblasts and occurred independently of BMP-2-induced osteoblast differentiation, which provides additional insights into the dual mechanism of BMP-2 action on osteoblast fate.     

A BMP-inducible gene, dlx5, regulates osteoblast differentiation and mesoderm induction

Bone morphogenetic proteins (BMPs), members of the transforming growth factor beta superfamily, have been identified by their ability to induce cartilage and bone from nonskeletal cells and have been shown to act as a ventral morphogen in Xenopus mesoderm. We isolated a murine homeobox-containing gene, distal-less 5 (mDlx5), as a BMP-inducible gene in osteoblastic MC3T3-E1 cells. Stable transfectants of MC3T3-E1 that overexpress mDlx5 mRNA showed increase in various osteogenic markers, a fourfold increase in alkaline phosphatase activity, a sixfold increase in osteocalcin production, and appearance in mineralization of extracellular matrix. Furthermore, mDlx5 was induced orthotopically in mouse embryos treated with BMP-4 and in fractured bone of adult mice. Consistent with these observations, we also found that injection of mDlx5 mRNA into dorsal blastomeres enhanced the ventralization of Xenopus embryos. These findings suggest that mDlx5 is a target gene of the BMP signaling pathway and acts as an important regulator of both osteogenesis and dorsoventral patterning of embryonic axis.     

Twisted gastrulation modulates bone morphogenetic protein-induced collagen II and X expression in chondrocytes in vitro and in vivo

Twisted gastrulation (TSG) is an extracellular modulator of bone morphogenetic protein (BMP) activity and regulates dorsoventral axis formation in early Drosophila and Xenopus development. Studies on tsg-deficient mice also indicated a role of this protein in skeletal growth, but the mechanism of TSG activity in this process has not yet been investigated. Here we show for the first time by in situ hybridization and immunohistochemistry that TSG is strongly expressed in bovine and mouse growth plate cartilage as well as in fetal ribs, vertebral cartilage, and cartilage anlagen of the skull. Furthermore we provide evidence that TSG is directly involved in BMP-regulated chondrocyte differentiation and maturation. In vitro, TSG impaired the dose-dependent BMP-2 stimulation of collagen II and X expression in cultures of MC615 chondrocytes and primary mouse chondrocytes. In the presence of chordin, a BMP antagonist, the inhibitory effect of TSG was further enhanced. TSG also inhibited BMP-2-stimulated phosphorylation of Smad factors in chondrocytes, confirming the role of TSG as a modulator of BMP signaling. For analysis of TSG functions in cartilage development in vivo, the gene was overexpressed in transgenic mice under the control of the cartilage-specific Col2a1 promoter. As a result, Col10a1 expression was significantly reduced in the growth plates of transgenic embryos and newborns in comparison with wild type littermates as shown by in situ hybridization and by real time PCR analysis. The data suggest that TSG is an important modulator of BMP-regulated cartilage development and chondrocyte differentiation.     

Regulation of MAP kinase by the BMP-4/TAK1 pathway in Xenopus ectoderm

Bone morphogenetic protein-4 (BMP-4) induces epidermis and represses neural fate in Xenopus ectoderm. Our previous findings implicate p42 Erk MAP kinase (MAPK) in the response to neural induction. We have examined the effects of BMP-4 on MAPK activity in gastrula ectoderm. Expression of a dominant negative BMP-4 receptor resulted in a 4.5-fold elevation in MAPK activity in midgastrula ectoderm. MAPK activity was reduced in ectoderm expressing a constitutively active BMP-4 receptor, or ectoderm treated with BMP-4 protein in the presence or absence of cycloheximide. Overexpression of TAK1 led to a reduction in MAPK activity in early gastrula ectoderm. The inhibitory effects of TAK1 could be reversed by 1 microM SB 203580, a p38 inhibitor. Treatment of isolated ectoderm with SB 203580 led to expression of otx2, NCAM, and noggin. Western blot analyses indicated that the BMP-4 pathway does not activate JNKs in ectoderm. Our findings indicate that BMP-4 inhibits ectodermal MAPK activity through a TAK1/p38-type pathway. MAPK has been shown to inactivate Smad1. Thus, our results suggest that BMP-4 and MAPK pathways are mutually antagonistic in Xenopus ectoderm, and that interactions between these pathways may govern the choice between epidermal and neural fate.     

Regulation of bone morphogenetic protein-4 by matrix GLA protein in vascular endothelial cells involves activin-like kinase receptor 1

Matrix GLA protein (MGP) has previously been shown to enhance expression of vascular endothelial growth factor (VEGF) through the activin-like kinase receptor 1 (ALK1) in bovine aortic endothelial cells. MGP has also been identified as an inhibitor of bone morphogenetic protein-2 (BMP-2). This study showed that the effect of MGP on ALK1 signaling and VEGF expression in bovine aortic endothelial cells was dose-dependent, that a progressive increase of MGP levels ceased to be stimulatory and instead turned inhibitory. We identified a new regulatory pathway involving BMP that may explain this response. BMP-2 and BMP-4 induced expression of ALK1 in a dose-dependent fashion as determined by real-time PCR and immunoblotting. Activation of ALK1 signaling induced expression of MGP in addition to that of VEGF, allowing for negative feedback regulation of BMP by MGP. MGP inhibited BMP-4 activity similarly to that of BMP-2 and interacted with BMP-4 on a protein level as determined by co-immunoprecipitation. The dose-dependent effect on ALK1 expression and the stimulation of MGP and VEGF expression were dependent on signaling by transforming growth factor-beta (TGF-beta) and ALK1. Inhibition of TGF-beta by neutralizing antibodies abolished the inhibitory effect of high BMP-4 levels on ALK1 expression and the induction of MGP and VEGF. Depletion of ALK1 by small interfering RNA abolished the induction of MGP and VEGF. MGP promoter activity was also stimulated by BMP-4 in a TGF-beta-dependent fashion. The results suggest that the effects of BMP on endothelial cells occur in part through induction of ALK1, an effect that may be limited by ALK1-induced MGP.     

BMP-4 is proteolytically activated by furin and/or PC6 during vertebrate embryonic development.

Bone morphogenetic protein-4 (BMP-4) is a multifunctional developmental regulator. BMP-4 is synthesized as an inactive precursor that is proteolytically activated by cleavage following the amino acid motif -Arg-Ser-Lys-Arg-. Very little is known about processing and secretion of BMPs. The proprotein convertases (PCs) are a family of seven structurally related serine endoproteases, at least one of which, furin, cleaves after the amino acid motif -Arg-X-Arg/Lys-Arg-. To examine potential roles of PCs during embryonic development we have misexpressed a potent protein inhibitor of furin, alpha1-antitrypsin Portland (alpha1-PDX) in early Xenopus embryos. Ectopic expression of alpha1-PDX phenocopies the effect of blocking endogenous BMP activity, leading to dorsalization of mesoderm and direct neural induction. alpha1-PDX-mediated neural induction can be reversed by co-expression of downstream components of the BMP-4 signaling pathway. Thus, alpha1-PDX can block BMP activity upstream of receptor binding, suggesting that it inhibits an endogenous BMP-4 convertase(s). Consistent with this hypothesis, alpha1-PDX prevents cleavage of BMP-4 in an oocyte translation assay. Using an in vitro digestion assay, we demonstrate that four members of the PC family have the ability to cleave BMP-4, but of these, only furin and PC6B are sensitive to alpha1-PDX. These studies provide the first in vivo evidence that furin and/or PC6 proteolytically activate BMP-4 during vertebrate embryogenesis.     

Action range of BMP is defined by its N-terminal basic amino acid core

During early development, cells receive positional information from neighboring cells to form tissue patterns in initially uniform germ layers. Ligands of the transforming growth factor (TGF-beta) superfamily are known to participate in this pattern formation. In particular, activin has been shown to act as a long-range dorsalizing signal to establish a concentration gradient in Xenopus. In contrast, BMP-2 and BMP-4, other members of the family, appear to influence and induce ventral fates only where they are expressed. This raises a question as to how the action of BMPs is tightly restricted to the region within and around the cells that produce them. Here, we have demonstrated that a basic core of only three amino acids in the N-terminal region of BMP-4 is required for its restriction to the non-neural ectoderm as its expression domain. Our results also suggest that heparan sulfate proteoglycans bind to this basic core and thus play a role in trapping BMP-4. The present study is the first to identify the critical domain of BMP that is responsible for its interaction with the extracellular environment that restricts its diffusion in vivo.     

Biological function and cellular mechanism of bone morphogenetic protein-6 in the ovary.

The process of ovarian folliculogenesis is composed of proliferation and differentiation of the constitutive cells in developing follicles. Growth factors emitted by oocytes integrate and promote this process. Growth differentiation factor-9 (GDF-9), bone morphogenetic protein (BMP)-15, and BMP-6 are oocyte-derived members of the transforming growth factor-beta superfamily. In contrast to the recent studies on GDF-9 and BMP-15, nothing is known about the biological function of BMP-6 in the ovary. Here we show that, unlike BMP-15 and GDF-9, BMP-6 lacks mitogenic activity on rat granulosa cells (GCs) and produces a marked decrease in follicle-stimulating hormone (FSH)-induced progesterone (P(4)) but not estradiol (E(2)) production, demonstrating not only the first identification of GCs as BMP-6 targets in the ovary but also its selective modulation of FSH action in steroidogenesis. This BMP-6 activity resembles BMP-15 but differs from GDF-9 activities. BMP-6 also exhibited similar action to BMP-15 by attenuating the steady state mRNA levels of FSH-induced steroidogenic acute regulatory protein (StAR) and P450 side-chain cleavage enzyme (P450scc), without affecting P450 aromatase mRNA level, supporting its differential function on FSH-regulated P(4) and E(2) production. However, unlike BMP-15, BMP-6 inhibited forskolin- but not 8-bromo-cAMP-induced P(4) production and StAR and P450scc mRNA expression. BMP-6 also decreased FSH- and forskolin-stimulated cAMP production, suggesting that the underlying mechanism by which BMP-6 inhibits FSH action most likely involves the down-regulation of adenylate cyclase activity. This is clearly distinct from the mechanism of BMP-15 action, which causes the suppression of basal FSH receptor (FSH-R) expression, without affecting adenylate cyclase activity. As assumed, BMP-6 did not alter basal FSH-R mRNA levels, whereas it inhibited FSH- and forskolin- but not 8-bromo-cAMP-induced FSH-R mRNA accumulation. These studies provide the first insight into the biological function of BMP-6 in the ovary and demonstrate its unique mechanism of regulating FSH action.     

Regulation of bone morphogenetic protein-4 activity by sequence elements within the prodomain

Bone morphogenetic protein-4 (BMP-4) is synthesized as a large precursor protein, which undergoes proprotein convertase-mediated proteolytic maturation along the secretory pathway to release the active ligand. Pro-BMP-4 is initially cleaved at a consensus furin motif adjacent to the mature ligand domain (the S1 site), and this allows for subsequent cleavage at an upstream motif (the S2 site). This sequential cleavage liberates a small, evolutionarily conserved, prodomain fragment (the linker peptide) of unknown fate and function. Here we show that the linker domain is essential for proper folding, exit from the endoplasmic reticulum, and thus cleavage of the BMP-4 precursor when overexpressed in Xenopus oocytes and embryos but not in cultured mammalian cells. Mature BMP-4 synthesized from a precursor in which the S1 site is non-cleavable, such that the linker domain remains covalently attached to the ligand, has little or no activity in vivo. Finally, analysis of folding, cleavage, and bioactivity of chimeric precursors containing the BMP-7 prodomain and BMP-4 mature domain, or vice versa, with or without the BMP-4 linker domain revealed that the linker domain is only functional in the context of the BMP-4 prodomain, and that differential cleavage around this domain can regulate the activity of a heterologous ligand.     

Antagonizing the Spemann organizer: role of the homeobox gene Xvent-1.

We have identified a novel homeobox gene, Xvent-1, that is differentially expressed in the ventral marginal zone of the early Xenopus gastrula. Evidence is presented from mRNA microinjection experiments for a role for this gene in dorsoventral patterning of mesoderm. First, Xvent-1 is induced by BMP-4, a gene known to be a key regulator of ventral mesoderm development. Second, Xvent-1 and the organizer-specific gene goosecoid are able to interact, directly or indirectly, in a cross-regulatory loop suppressing each other's expression, consistent with their mutually exclusive expression in the marginal zone. Third, microinjection of Xvent-1 mRNA ventralizes dorsal mesoderm. The results suggest that Xvent-1 functions in a ventral signaling pathway that maintains the ventral mesodermal state and antagonizes the Spemann organizer.     

Activation of Stat3 by cytokine receptor gp130 ventralizes Xenopus embryos independent of BMP-4

Stat3 is one of the main signaling components of cytokine receptors, including gp130. Here we show that activation of cytokine receptor gp130 resulted in a dramatic ventralization of Xenopus embryos and that the ventralization correlated well with Stat3 activation potential of the receptor. This finding led to identification of Xenopus Stat3 (Xstat3), which showed a 95% homology to its murine and human counterparts, at the amino acid level, and was expressed from the one-cell stage throughout development. The mechanism of gp130/XStat3-mediated ventralization proved to be independent of BMP-4. gp130/Xstat3 stimulation inhibited Smad2-induced ectopic axis formation in embryos and Smad2-dependent luciferase activity. A dominant-negative Stat3, in contrast, dorsalized Xenopus embryos, resulting in ectopic axis formation. We propose that Stat3-mediated signaling has the capacity to modify dorsoventral patterning in the early development of Xenopus.     

Bone Morphogenetic Proteins

Bone Morphogenetic Protein (BMP) refers to an activity derived from bone that induces the formation of cartilage and bone in vivo. This activity leads to a series of developmental processes including chemotaxis, proliferation, and differentiation, which result in the transient formation of cartilage and the production of living bone tissue, complete with hematopoietic marrow. The determination of the factor or factors responsible for this activity has clear significance both for our understanding of bone biology and for the clinical application of cartilage and bone replacement. Several newly discovered factors, BMP-1, BMP-2 (BMP-2A), BMP-3 (osteogenin), BMP-4 (BMP-2B), BMP-5, BMP-6, BMP-7, and osteoinductive factor (OIF) have been implicated in the BMP process. BMP-2 through BMP-7 are all in the TGF-β superfamily of molecules, and are closely related to two factors (Vg1 and dpp) which are involved in a variety of developmental processes during embryogenesis. A recently discovered factor, OIF, exhibits BMP activity only in the presence of TGF-β. BMP-2, expressed as a recombinant protein, is the only molecule described to date that has been shown to clearly induce by itself the entire cartilage and bone formation process seen with bone-derived BMP. Evidence is accumulating that the BMP effect is a result of the combined actions of a set of BMP-2-like molecules. Definitive examination of the activities of the other factors will require expression of the recombinant proteins and testing of these in vivo alone and in combinations.     

Cell-surface heparan sulfate proteoglycans potentiate chordin antagonism of bone morphogenetic protein signaling and are necessary for cellular uptake of chordin

Signaling by bone morphogenetic proteins (BMPs) plays a central role in early embryonic patterning, organogenesis, and homeostasis in a broad range of species. Chordin, an extracellular antagonist of BMP signaling, is thought to readily diffuse in tissues, thus forming gradients of BMP inhibition that result in reciprocal gradients of BMP signaling. The latter determine cell fates along the embryonic dorsoventral axis. The secreted protein Twisted Gastrulation (TSG) is thought to help shape BMP signaling gradients by acting as a cofactor that enhances Chordin inhibition of BMP signaling. Here, we demonstrate that mammalian Chordin binds heparin with an affinity similar to that of factors known to functionally interact with heparan sulfate proteoglycans (HSPGs) in tissues. We further demonstrate that Chordin binding in mouse embryonic tissues was dependent upon its interaction with cell-surface HSPGs and that Chordin bound to cell-surface HSPGs (e.g. syndecans), but not to basement membranes containing the HSPG perlecan. Surprisingly, mammalian TSG did not bind heparin unless prebound to Chordin and/or BMP-4, although Drosophila TSG has been reported to bind heparin on its own. Results are also presented that indicate that Chordin-HSPG interactions strongly potentiate the antagonism of BMP signaling by Chordin and are necessary for the retention and uptake of Chordin by cells. These data and others regarding Chordin diffusion have implications for the paradigm of how Chordin is thought to regulate BMP signaling in the extracellular space and how gradients of BMP signaling are formed.     

Gremlin negatively modulates BMP-4 induction of embryonic mouse lung branching morphogenesis

Bone morphogenetic protein-4 (BMP-4) is a key morphogen for embryonic lung development that is expressed at high levels in the peripheral epithelium, but the mechanisms that modulate BMP-4 function in early mouse lung branching morphogenesis are unclear. Here, we studied the BMP-4 antagonist Gremlin, which is a member of the DAN family of BMP antagonists that can bind and block BMP-2/4 activity. The expression level of gremlin in embryonic mouse lungs is highest in the early embryonic pseudoglandular stage [embryonic days (E) 11.5-14.5] and is reduced during fetal lung maturation (E18.5 to postnatal day 1). In situ hybridization indicates that gremlin is diffusely expressed in peripheral lung mesenchyme and epithelium, but relatively high epithelial expression occurs in branching buds at E11.5 and in large airways after E16.5. In E11.5 lung organ culture, we found that exogenous BMP-4 dramatically enhanced peripheral lung epithelial branching morphogenesis, whereas reduction of endogenous gremlin expression with antisense oligonucleotides achieved the same gain-of-function phenotype as exogenous BMP-4, including increased epithelial cell proliferation and surfactant protein C expression. On the other hand, adenoviral overexpression of gremlin blocked the stimulatory effects of exogenous BMP-4. Therefore, our data support the hypothesis that Gremlin is a physiologically negative regulator of BMP-4 in lung branching morphogenesis.     

Cleavages within the prodomain direct intracellular trafficking and degradation of mature bone morphogenetic protein-4

Pro bone morphogenetic protein-4 (BMP-4) is initially cleaved at a consensus furin motif adjacent to the mature ligand domain (the S1 site), and this allows for subsequent cleavage at an upstream motif (the S2 site). Previous studies have shown that S2 cleavage regulates the activity and signaling range of mature BMP-4, but the mechanism by which this occurs is unknown. Here, we show that the pro- and mature domains of BMP-4 remain noncovalently associated after S1 cleavage, generating a complex that is targeted for rapid degradation. Degradation requires lysosomal and proteosomal function and is enhanced by interaction with heparin sulfate proteoglycans. Subsequent cleavage at the S2 site liberates mature BMP-4 from the prodomain, thereby stabilizing the protein. We also show that cleavage at the S2, but not the S1 site, is enhanced at reduced pH, consistent with the possibility that the two cleavages occur in distinct subcellular compartments. Based on these results, we propose a model for how cleavage at the upstream site regulates the activity and signaling range of mature BMP-4 after it has been released from the prodomain.