Loss of BMPR2 leads to high bone mass due to increased osteoblast activity

The roles and regulatory mechanisms of TGF-β and BMP signaling in bone and cartilage development, homeostasis and disease

Transforming growth factor-βs (TGF-βs) and bone morphometric proteins (BMPs) belong to the TGF-β superfamily and perform essential functions during osteoblast and chondrocyte lineage commitment and differentiation, skeletal development, and homeostasis. TGF-βs and BMPs transduce signals through SMAD-dependent and -independent pathways; specifically, they recruit different receptor heterotetramers and R-Smad complexes, resulting in unique biological readouts. BMPs promote osteogenesis, osteoclastogenesis, and chondrogenesis at all differentiation stages, while TGF-βs play different roles in a stage-dependent manner. BMPs and TGF-β have opposite functions in articular cartilage homeostasis. Moreover, TGF-β has a specific role in maintaining the osteocyte network. The precise activation of BMP and TGF-β signaling requires regulatory machinery at multiple levels, including latency control in the matrix, extracellular antagonists, ubiquitination and phosphorylation in the cytoplasm, nucleus-cytoplasm transportation, and transcriptional co-regulation in the nuclei. This review weaves the background information with the latest advances in the signaling facilitated by TGF-βs and BMPs, and the advanced understanding of their diverse physiological functions and regulations. This review also summarizes the human diseases and mouse models associated with disordered TGF-β and BMP signaling. A more precise understanding of the BMP and TGF-β signaling could facilitate the development of bona fide clinical applications in treating bone and cartilage disorders.

Gene losses, parallel evolution and heightened expression confer adaptations to dedicated cleaning behaviour

BACKGROUND

Cleaning symbioses are captivating interspecific interactions in which a cleaner fish removes ectoparasites from its client, contributing to the health and diversity of natural fish communities and aquaculture systems. However, the genetic signatures underlying this specialized behaviour remain poorly explored. To shed light on this, we generated a high-quality chromosome-scale genome of the bluestreak cleaner wrasse Labroides dimidiatus, a dedicated cleaner with cleaning as primary feeding mechanism throughout its life.

RESULTS

Compared with facultative and non-cleaner wrasses, L. dimidiatus was found with notable contractions in olfactory receptors implying their limited importance in dedicated cleaning. Instead, given its distinct tactile pre-conflict strategies, L. dimidiatus may rely more heavily on touch sensory perception, with heightened gene expression in the brain in anticipation of cleaning. Additionally, a reduction in NLR family CARD domain-containing protein 3 might enhance innate immunity of L. dimidiatus, probably assisting to reduce the impacts from parasite infections. In addition, convergent substitutions for a taste receptor and bone development genes across cleaners (L. dimidiatus and facultative cleaners) may provide them with evolved food discrimination abilities and jaw morphology that differentiate them from non-cleaners. Moreover, L. dimidiatus may exhibit specialized neural signal transductions for cleaning, as evidenced by positive selection in genes related to the glutamatergic synapse pathway. Interestingly, numerous glutamate receptors also demonstrated significantly higher expression in L. dimidiatus not engaged in cleaning, as compared to those involved in cleaning. Besides, apparent contractions in L. dimidiatus for protocadherins, which are responsible for neuronal development, may further promote specialized neural signal transductions in this species.

CONCLUSIONS

This study reveals that L. dimidiatus harbours substantial losses in specific gene families, convergent evolutions across cleaners and a large-scale high gene expression in preparation for cleaning, allowing for adaptation to the dedicated cleaning behaviour.

Functional comparison of human ACVR1 and zebrafish Acvr1l FOP-associated variants in embryonic zebrafish

BACKGROUND

Fibrodysplasia ossificans progressiva (FOP), a rare disease characterized by progressive heterotopic ossification of muscle and connective tissues, is caused by autosomal dominant activating mutations in the type I receptor, ACVR1/ALK2. The classic human FOP variant, ACVR1R206H , shows increased bone morphogenetic protein (BMP) signaling and activation by activins.

RESULTS

Here, we performed in vivo functional characterization of human ACVR1R206H and orthologous zebrafish Acvr1lR203H using early embryonic zebrafish dorsoventral patterning as a phenotypic readout for receptor activity. Our results showed that human ACVR1R206H and zebrafish Acvr1lR203H exhibit functional differences in early embryonic zebrafish, and that human ACVR1R206H retained its signaling activity in the absence of a ligand-binding domain (LBD). We also showed, for the first time, that zebrafish Acvr2ba/Acvr2bb receptors are required for human ACVR1R206H signaling in early embryonic zebrafish.

CONCLUSIONS

Together, these data provide new insight into ACVR1R206H signaling pathways that may facilitate the design of new and effective therapies for FOP patients.

FKBP12 is a major regulator of ALK2 activity in multiple myeloma cells

BACKGROUND

The immunophilin FKBP12 binds to TGF-β family type I receptors, including the BMP type I receptor ALK2. FKBP12 keeps the type I receptor in an inactive state and controls signaling activity. Removal of FKBP12 with drugs such as the FKBP-ligand FK506 enhances BMP activity in various cell types. In multiple myeloma cells, activation of SMAD1/5/8 leads to apoptosis. We hypothesized that removing FKBP12 from ALK2 in myeloma cells would potentiate BMP-induced ALK2-SMAD1/5/8 activity and in consequence cell death.

METHODS

Multiple myeloma cell lines were treated with FK506, or other FKBP-binding compounds, combined with different BMPs before analyzing SMAD1/5/8 activity and cell viability. SMAD1/5/8 activity was also investigated using a reporter cell line, INA-6 BRE-luc. To characterize the functional signaling receptor complex, we genetically manipulated receptor expression by siRNA, shRNA and CRISPR/Cas9 technology.

RESULTS

FK506 potentiated BMP-induced SMAD1/5/8 activation and apoptosis in multiple myeloma cell lines. By using FKBP-binding compounds with different affinity profiles, and siRNA targeting FKBP12, we show that the FK506 effect is mediated by binding to FKBP12. Ligands that typically signal via ALK3 in myeloma cells, BMP2, BMP4, and BMP10, did not induce apoptosis in cells lacking ALK3. Notably, BMP10 competed with BMP6 and BMP9 and antagonized their activity via ALK2. However, upon addition of FK506, we saw a surprising shift in specificity, as the ALK3 ligands gained the ability to signal via ALK2 and induce apoptosis. This indicates that the receptor complex can switch from an inactive non-signaling complex (NSC) to an active one by adding FK506. This gain of activity was also seen in other cell types, indicating that the observed effects have broader relevance. BMP2, BMP4 and BMP10 depended on BMPR2 as type II receptor to signal, which contrasts with BMP6 and BMP9, that activate ALK2 more potently when BMPR2 is knocked down.

CONCLUSIONS

In summary, our data suggest that FKBP12 is a major regulator of ALK2 activity in multiple myeloma cells, partly by switching an NSC into an active signaling complex. FKBP12 targeting compounds devoid of immunosuppressing activity could have potential in novel treatment strategies aiming at reducing multiple myeloma tumor load. Video Abstract.

BMPR2 Variants Underlie Nonsyndromic Oligodontia

Oligodontia manifests as a congenital reduction in the number of permanent teeth. Despite the major efforts that have been made, the genetic etiology of oligodontia remains largely unknown. Bone morphogenetic protein receptor type 2 (BMPR2) variants have been associated with pulmonary arterial hypertension (PAH). However, the genetic significance of BMPR2 in oligodontia has not been previously reported. In the present study, we identified a novel heterozygous variant (c.814C > T; p.Arg272Cys) of BMPR2 in a family with nonsyndromic oligodontia by performing whole-exome sequencing. In addition, we identified two additional heterozygous variants (c.1042G > A; p.Val348Ile and c.1429A > G; p.Lys477Glu) among a cohort of 130 unrelated individuals with nonsyndromic oligodontia by performing Sanger sequencing. Functional analysis demonstrated that the activities of phospho-SMAD1/5/8 were significantly inhibited in BMPR2-knockout 293T cells transfected with variant-expressing plasmids, and were significantly lower in BMPR2 heterozygosity simulation groups than in the wild-type group, indicating that haploinsufficiency may represent the genetic mechanism. RNAscope in situ hybridization revealed that BMPR2 transcripts were highly expressed in the dental papilla and adjacent inner enamel epithelium in mice tooth germs, suggesting that BMPR2 may play important roles in tooth development. Our findings broaden the genetic spectrum of oligodontia and provide clinical and genetic evidence supporting the importance of BMPR2 in nonsyndromic oligodontia.

An intersectional analysis of LncRNAs and mRNAs reveals the potential therapeutic targets of Bi Zhong Xiao Decoction in collagen-induced arthritis rats

Background

Bi Zhong Xiao decoction (BZXD), a traditional Chinese herbal formula, has been used clinically for many years to treat rheumatoid arthritis (RA). Both clinical and experimental studies have revealed that BZXD is effective in treating RA, but the mechanism remains unclear. In this study, we aimed to explore the mechanism of efficacy of BZXD through transcriptomic analysis of lncRNA and mRNA.

Methods

The combination method of ultra-high performance liquid chromatography-mass spectrometry/mass spectrometry was used to assess the quality of BZXD. The efficacy of BZXD in treating collagen-induced arthritis (CIA) was evaluated by clinical assessment, weight changes, hematoxylin–eosin and safranin o-fast green staining, and Micro-CT. Arraystar rat lncRNA-mRNA chip technology was used to determine the lncRNA and mRNA expression profiles of the Control, CIA and BZXD groups, and to screen gene expression profiles related to the curative effect of BZXD. A lncRNA-mRNA co-expression network was constructed for the therapeutic efficacy genes. Through GO function and KEGG pathway enrichment analysis, the biological functions and signaling pathways of therapeutic efficacy genes were determined. Based on fold change and functional annotation, key differentially expressed lncRNAs and mRNAs were selected for reverse transcription-quantitative polymerase chain reaction (RT-qPCR) validation. The functions of lncRNAs targeting mRNAs were verified in vitro.

Results

We demonstrated that BZXD could effectively reverse bone erosion. After BZXD treatment, up to 33 lncRNAs and 107 mRNAs differentially expressed genes were reversely regulated by BZXD. These differentially expressed lncRNAs are mainly involved in the biological process of the immune response and are closely related to the ECM-receptor interaction, MAPK signaling pathway, Focal adhesion, Ras signaling pathway, Antigen processing and presentation, and Chemokine signaling pathway. We identified four lncRNAs (uc.361−, ENSRNOT00000092834, ENSRNOT00000089244, ENSRNOT00000084631) and three mRNAs (Acvr2a, Cbx2, Morc4) as potential therapeutic targets for BZXD and their microarray data consistent with the RT-qPCR. In vitro experiments confirmed that silencing the lncRNAs ENSRNOT00000092834 and ENSRNOT00000084631 reversed the expression of target mRNAs.

Conclusions

This study elucidates the possible mechanism of BZXD reversing bone erosion in CIA rats from the perspective of lncRNA and mRNA. To provide a basis and direction for further exploration of the mechanism of BZXD in treating RA.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13020-022-00670-z.

Impacts of microplastics and carbamazepine on the shell formation of thick-shell mussels and the underlying mechanisms of action

Forming calcareous exoskeletons is crucial for the health and survival of calcifiers such as bivalves. However, the impacts of waterborne emergent pollutants on this important process remain largely unknown. In this study, the effects of two types of emergent pollutants, microplastics (MPs) and carbamazepine (CBZ), which are ubiquitously present in ocean environments, on shell formation were assessed in the thick-shell mussel (Mytilus coruscus) with a shell regeneration experiment. In addition, their impacts on the in vivo contents of ATP, Ca²⁺, carbonic anhydrase (CA), and bone morphogenetic protein receptor type-2 (BMPR2), the activity of phosphofructokinase (PFK) and Ca²⁺-ATPase, and the expression of shell-formation related genes were analyzed. The data collected demonstrated that shell regeneration after mechanical injury was significantly arrested by CBZ and/or MPs. Besides, all the physiological and molecular parameters investigated were markedly suppressed by these two pollutants. Furthermore, synergistic impacts on most of the parameters examined were observed between CBZ and MPs. Our results indicate that these two pollutants may disrupt shell formation by constraining the availability of raw materials and energy, inhibiting the formation of the organic shell matrix, and interfering with the regulation of crystallization, which may have far-reaching impacts on the health of marine calcifiers.

The context-dependent, combinatorial logic of BMP signaling

Cell-cell communication systems typically comprise families of ligand and receptor variants that function together in combinations. Pathway activation depends on the complex way in which ligands are presented extracellularly and receptors are expressed by the signal-receiving cell. To understand the combinatorial logic of such a system, we systematically measured pairwise bone morphogenetic protein (BMP) ligand interactions in cells with varying receptor expression. Ligands could be classified into equivalence groups based on their profile of positive and negative synergies with other ligands. These groups varied with receptor expression, explaining how ligands can functionally replace each other in one context but not another. Context-dependent combinatorial interactions could be explained by a biochemical model based on the competitive formation of alternative signaling complexes with distinct activities. Together, these results provide insights into the roles of BMP combinations in developmental and therapeutic contexts and establish a framework for analyzing other combinatorial, context-dependent signaling systems.

Competition between type I activin and BMP receptors for binding to ACVR2A regulates signaling to distinct Smad pathways

Background

Activins and bone morphogenetic proteins (BMPs) play critical, sometimes opposing roles, in multiple physiological and pathological processes and diseases. They signal to distinct Smad branches; activins signal mainly to Smad2/3, while BMPs activate mainly Smad1/5/8. This gives rise to the possibility that competition between the different type I receptors through which activin and BMP signal for common type II receptors can provide a mechanism for fine-tuning the cellular response to activin/BMP stimuli. Among the transforming growth factor-β superfamily type II receptors, ACVR2A/B are highly promiscuous, due to their ability to interact with different type I receptors (e.g., ALK4 vs. ALK2/3/6) and with their respective ligands [activin A (ActA) vs. BMP9/2]. However, studies on complex formation between these full-length receptors situated at the plasma membrane, and especially on the potential competition between the different activin and BMP type I receptors for a common activin type II receptor, were lacking.

Results

We employed a combination of IgG-mediated patching-immobilization of several type I receptors in the absence or presence of ligands with fluorescence recovery after photobleaching (FRAP) measurements on the lateral diffusion of an activin type II receptor, ACVR2A, to demonstrate the principle of competition between type I receptors for ACVR2. Our results show that ACVR2A can form stable heteromeric complexes with ALK4 (an activin type I receptor), as well as with several BMP type I receptors (ALK2/3/6). Of note, ALK4 and the BMP type I receptors competed for binding ACVR2A. To assess the implications of this competition for signaling output, we first validated that in our cell model system (U2OS cells), ACVR2/ALK4 transduce ActA signaling to Smad2/3, while BMP9 signaling to Smad1/5/8 employ ACVR2/ALK2 or ACVR2/ALK3. By combining ligand stimulation with overexpression of a competing type I receptor, we showed that differential complex formation of distinct type I receptors with a common type II receptor balances the signaling to the two Smad branches.

Conclusions

Different type I receptors that signal to distinct Smad pathways (Smad2/3 vs. Smad1/5/8) compete for binding to common activin type II receptors. This provides a novel mechanism to balance signaling between Smad2/3 and Smad1/5/8.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-022-01252-z.

Receptor binding competition: A paradigm for regulating TGF-β family action

The transforming growth factor (TGF)-β family is a group of structurally related, multifunctional growth factors, or ligands that are crucially involved in the development, regulation, and maintenance of animal tissues. In humans, the family counts over 33 members. These secreted ligands typically form multimeric complexes with two type I and two type II receptors to activate one of two distinct signal transduction branches. A striking feature of the family is its promiscuity, i.e., many ligands bind the same receptors and compete with each other for binding to these receptors. Although several explanations for this feature have been considered, its functional significance has remained puzzling. However, several recent reports have promoted the idea that ligand-receptor binding promiscuity and competition are critical features of the TGF-β family that provide an essential regulating function. Namely, they allow a cell to read and process multi-ligand inputs. This capability may be necessary for producing subtle, distinctive, or adaptive responses and, possibly, for facilitating developmental plasticity. Here, we review the molecular basis for ligand competition, with emphasis on molecular structures and binding affinities. We give an overview of methods that were used to establish experimentally ligand competition. Finally, we discuss how the concept of ligand competition may be fundamentally tied to human physiology, disease, and therapy.

A study of Drynaria fortunei in modulation of BMP–2 signalling by bone tissue engineering

Background/aim

Drynaria fortunei (Gusuibu; GSB) is a popular traditional Chinese medicine used for bone repair. An increasing number of studies have reported that GSB induces osteogenic differentiation in bone marrow mesenchymal stem cells (BMSCs). These results provide insight into the application of GSB for bone tissue engineering techniques used to repair large bone defects. However, few studies have described the molecular mechanisms of GSB.

Materials and methods

In the present study, the effects of GSB and naringin, a marker compound, on the binding of BMP-2 to BMPR and BMP-2-derived signal transduction were investigated using surface plasmon resonance (SPR) and coculturing with BMPR-expressed cell line, C₂C₁₂, respectively. Furthermore, naringin was also used to prepare naringin contained scaffolds for bone tissue engineering. The physical and chemical properties of these scaffolds were analysed using scanning electron microscopy (SEM) and highperformance liquid chromatography (HPLC). These scaffolds were cocultured with rabbit BMSCs in vitro and implanted into rabbit calvarial defects for bone repair assessment.

Results

The results showed that GSB and naringin affect the binding of BMP and BMPR in SPR experiments. GSB is a subtle BMP modulator that simultaneously inhibits the binding of BMP-2 to BMPR-1A and enhances its binding to BMPR-1B. In contrast, naringin inhibited BMP-2 binding to BMPR-1A. In vitro studies involving the phosphorylation of signals downstream of BMPR and Smad showed that GSB and naringin affected stem cell differentiation by inhibiting BMPR-1A signalling. When using GSB for bone tissue engineering, naringin exhibited a higher capacity for slow and gradual release from the scaffold, which promotes bone formation via osteoinduction. Moreover, control and naringin scaffolds were implanted into rabbit calvarial defects for 4 weeks, and naringin enhanced bone regeneration in vivo significantly.

Conclusion

GSB and its marker compound (naringin) could inhibit the binding of BMP-2 and BMPR-1A to control cell differentiation by blocked BMPR-1A signalling and enhanced BMPR-1B signalling. GSB and naringin could be good natural BMP regulators for bone tissue engineering.

Loss of BMP signaling mediated by BMPR1A in osteoblasts leads to differential bone phenotypes in mice depending on anatomical location of the bones

Bone morphogenetic protein (BMP) signaling in osteoblasts plays critical roles in skeletal development and bone homeostasis. Our previous studies showed loss of function of BMPR1A, one of the type 1 receptors for BMPs, in osteoblasts results in increased trabecular bone mass in long bones due to an imbalance between bone formation and bone resorption. Decreased bone resorption was associated with an increased mature-to-immature collagen cross-link ratio and mineral-matrix ratios in the trabecular compartments, and increased tissue-level biomechanical properties. Here, we investigated the bone mass, bone composition and biomechanical properties of ribs and spines in the same genetically altered mouse line to compare outcomes by loss of BMPR1A functions in bones from different anatomic sites and developmental origins. Bone mass was significantly increased in both cortical and trabecular compartments of ribs with minimal to modest changes in compositions. While tissue-levels of biomechanical properties were not changed between control and mutant animals, whole bone levels of biomechanical properties were significantly increased in association with increased bone mass in the mutant ribs. For spines, mutant bones showed increased bone mass in both cortical and trabecular compartments with an increase of mineral content. These results emphasize the differential role of BMP signaling in osteoblasts in bones depending on their anatomical locations, functional loading requirements and developmental origin.

Bone marrow niche crosses paths with BMPs: a road to protection and persistence in CML

Chronic myeloid leukaemia (CML) is a paradigm of precision medicine, being one of the first cancers to be treated with targeted therapy. This has revolutionised CML therapy and patient outcome, with high survival rates. However, this now means an ever-increasing number of patients are living with the disease on life-long tyrosine kinase inhibitor (TKI) therapy, with most patients anticipated to have near normal life expectancy. Unfortunately, in a significant number of patients, TKIs are not curative. This low-level disease persistence suggests that despite a molecularly targeted therapeutic approach, there are BCR-ABL1-independent mechanisms exploited to sustain the survival of a small cell population of leukaemic stem cells (LSCs). In CML, LSCs display many features akin to haemopoietic stem cells, namely quiescence, self-renewal and the ability to produce mature progeny, this all occurs through intrinsic and extrinsic signals within the specialised microenvironment of the bone marrow (BM) niche. One important avenue of investigation in CML is how the disease highjacks the BM, thereby remodelling this microenvironment to create a niche, which enables LSC persistence and resistance to TKI treatment. In this review, we explore how changes in growth factor levels, in particular, the bone morphogenetic proteins (BMPs) and pro-inflammatory cytokines, impact on cell behaviour, extracellular matrix deposition and bone remodelling in CML. We also discuss the challenges in targeting LSCs and the potential of dual targeting using combination therapies against BMP receptors and BCR-ABL1.

Activin-A Induces Fewer, but Larger Osteoclasts From Monocytes in Both Healthy Controls and Fibrodysplasia Ossificans Progressiva Patients

Fibrodysplasia Ossificans Progressiva (FOP) is a rare genetic disease characterized by heterotopic ossification (HO) that occurs in muscle tissue, tendons, and ligaments. The disease is caused by mutations in the Activin receptor type I (ACVR1) gene resulting in enhanced responsiveness to Activin-A. Binding of this molecule to the mutated receptor induces HO. Bone metabolism normally requires the coupled action of osteoblasts and osteoclasts, which seems to be disturbed during HO. We hypothesize that Activin-A may also counteract the formation of osteoclasts in FOP patients. In this study we investigated the effect of Activin-A on osteoclast differentiation of CD14+ monocytes from FOP patients and healthy controls. The lymphocytic and monocytic cell populations were determined by FACS analysis. Expression of the mutated R206H receptor was assessed and confirmed by allele specific PCR. The effect of Activin-A on osteoclastogenesis was assessed by counting the number and size of multinucleated cells. Osteoclast activity was determined by culturing the cells on Osteo Assay plates. The influence of Activin-A on expression of various osteoclast related genes was studied with QPCR. Blood from FOP patients contained similar percentages of classical, intermediate, or non-classical monocytes as healthy controls. Addition of Activin-A to the osteoclastogenesis cultures resulted in fewer osteoclasts in both control and FOP cultures. The osteoclasts formed in the presence of Activin-A were, however, much larger and more active compared to the cultures without Activin-A. This effect was tempered when the Activin-A inhibitor follistatin was added to the Activin-A containing cultures. Expression of osteoclast specific genes Cathepsin K and TRAcP was upregulated, gene expression of osteoclastogenesis related genes M-CSF and DC-STAMP was downregulated by Activin-A. Since Activin-A is a promising target for inhibiting the formation of HO in FOP, it is important to know its effects on both osteoblasts and osteoclasts. Our study shows that Activin-A induces fewer, but larger and more active osteoclasts independent of the presence of the mutated ACVR1 receptor. When considering FOP as an Activin-A driven disease that acts locally, our findings suggest that Activin-A could cause a more pronounced local resorption by larger osteoclasts. Thus, when targeting Activin-A in patients with neutralizing antibodies, HO formation could potentially be inhibited, and osteoclastic activity could be slightly reduced, but then performed dispersedly by more and smaller osteoclasts.

Synergetic roles of TGF-β signaling in tissue engineering

Recent advances in tissue engineering highlight biomaterial designs with context-specific growth factors, cytokines and various small molecules to better mimic the natural extracellular matrix (ECM) microenvironments. These efforts have led to direct improvements in cell-cell and cell-ECM interactions while mitigating undesirable cellular and immunogenic responses. In this short review, we focus on the crucial roles and regulation of transforming growth factor β (TGF-β) signaling in biomaterial applications during tissue repair and regeneration.

Inflammation induces endothelial-to-mesenchymal transition and promotes vascular calcification through downregulation of BMPR2

Endothelial‐to‐mesenchymal transition (EndMT) has been unveiled as a common cause for a multitude of human pathologies, including cancer and cardiovascular disease. Vascular calcification is a risk factor for ischemic vascular disorders and slowing calcification may reduce mortality in affected patients. The absence of early biomarkers hampers the identification of patients at risk. EndMT and vascular calcification are induced upon cooperation between distinct stimuli, including inflammatory cytokines and transforming growth factor beta (TGF‐β) family members. However, how these signaling pathways interplay to promote cell differentiation and eventually vascular calcification is not well understood. Using in vitro and ex vivo analysis in animal models and patient‐derived tissues, we have identified that the pro‐inflammatory cytokines tumor necrosis factor alpha (TNF‐α) and interleukin‐1 beta (IL‐1β) induce EndMT in human primary aortic endothelial cells, thereby sensitizing them for BMP‐9‐induced osteogenic differentiation. Downregulation of the BMP type II receptor BMPR2 is a key event in this process. Rather than compromising BMP canonical signal transduction, loss of BMPR2 results in decreased JNK signaling in ECs, thus enhancing BMP‐9‐induced mineralization. Altogether, our results point at the BMPR2–JNK signaling axis as a key pathway regulating inflammation‐induced EndMT and contributing to calcification. © 2018 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of Pathological Society of Great Britain and Ireland.

The BMP Pathway and Its Inhibitors in the Skeleton

Bone morphogenetic proteins (BMPs) constitute the largest subdivision of the transforming growth factor-β family of ligands. BMPs exhibit widespread utility and pleiotropic, context-dependent effects, and the strength and duration of BMP pathway signaling is tightly regulated at numerous levels via mechanisms operating both inside and outside the cell. Defects in the BMP pathway or its regulation underlie multiple human diseases of different organ systems. Yet much remains to be discovered about the BMP pathway in its original context, i.e., the skeleton. In this review, we provide a comprehensive overview of the intricacies of the BMP pathway and its inhibitors in bone development, homeostasis, and disease. We frame the content of the review around major unanswered questions for which incomplete evidence is available. First, we consider the gene regulatory network downstream of BMP signaling in osteoblastogenesis. Next, we examine why some BMP ligands are more osteogenic than others and what factors limit BMP signaling during osteoblastogenesis. Then we consider whether specific BMP pathway components are required for normal skeletal development, and if the pathway exerts endogenous effects in the aging skeleton. Finally, we propose two major areas of need of future study by the field: greater resolution of the gene regulatory network downstream of BMP signaling in the skeleton, and an expanded repertoire of reagents to reliably and specifically inhibit individual BMP pathway components.

Transferrin receptor 2 controls bone mass and pathological bone formation via BMP and Wnt signaling

Transferrin receptor 2 (Tfr2) is mainly expressed in the liver and controls iron homeostasis. Here, we identify Tfr2 as a regulator of bone homeostasis that inhibits bone formation. Mice lacking Tfr2 display increased bone mass and mineralization independent of iron homeostasis and hepatic Tfr2. Bone marrow transplantation experiments and studies of cell-specific Tfr2 knockout mice demonstrate that Tfr2 impairs BMP-p38MAPK signaling and decreases expression of the Wnt inhibitor sclerostin specifically in osteoblasts. Reactivation of MAPK or overexpression of sclerostin rescues skeletal abnormalities in Tfr2 knockout mice. We further show that the extracellular domain of Tfr2 binds BMPs and inhibits BMP-2-induced heterotopic ossification by acting as a decoy receptor. These data indicate that Tfr2 limits bone formation by modulating BMP signaling, possibly through direct interaction with BMP either as a receptor or as a co-receptor in a complex with other BMP receptors. Finally, the Tfr2 extracellular domain may be effective in the treatment of conditions associated with pathological bone formation.

Bone morphogenetic protein receptor signal transduction in human disease

Bone morphogenetic proteins (BMPs) are secreted cytokines that were initially discovered on the basis of their ability to induce bone. Several decades of research have now established that these proteins function in a large variety of physiopathological processes. There are about 15 BMP family members, which signal via three transmembrane type II receptors and four transmembrane type I receptors. Mechanistically, BMP binding leads to phosphorylation of the type I receptor by the type II receptor. This activated heteromeric complex triggers intracellular signaling that is initiated by phosphorylation of receptor‐regulated SMAD1, 5, and 8 (also termed R‐SMADs). Activated R‐SMADs form heteromeric complexes with SMAD4, which engage in specific transcriptional responses. There is convergence along the signaling pathway and, besides the canonical SMAD pathway, BMP‐receptor activation can also induce non‐SMAD signaling. Each step in the pathway is fine‐tuned by positive and negative regulation and crosstalk with other signaling pathways. For example, ligand bioavailability for the receptor can be regulated by ligand‐binding proteins that sequester the ligand from interacting with receptors. Accessory co‐receptors, also known as BMP type III receptors, lack intrinsic enzymatic activity but enhance BMP signaling by presenting ligands to receptors. In this review, we discuss the role of BMP receptor signaling and how corruption of this pathway contributes to cardiovascular and musculoskeletal diseases and cancer. We describe pharmacological tools to interrogate the function of BMP receptor signaling in specific biological processes and focus on how these agents can be used as drugs to inhibit or activate the function of the receptor, thereby normalizing dysregulated BMP signaling. © 2018 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of Pathological Society of Great Britain and Ireland.

BMPR2 inhibits activin and BMP signaling via wild-type ALK2

TGF-β/BMP superfamily ligands require heteromeric complexes of type 1 and 2 receptors for ligand-dependent downstream signaling. Activin A, a TGF-β superfamily member, inhibits growth of multiple myeloma cells, but the mechanism for this is unknown. We therefore aimed to clarify how activins affect myeloma cell survival. Activin A activates the transcription factors SMAD2/3 through the ALK4 type 1 receptor, but may also activate SMAD1/5/8 through mutated variants of the type 1 receptor ALK2 (also known as ACVR1). We demonstrate that activin A and B activate SMAD1/5/8 in myeloma cells through endogenous wild-type ALK2. Knockdown of the type 2 receptor BMPR2 strongly potentiated activin A- and activin B-induced activation of SMAD1/5/8 and subsequent cell death. Furthermore, activity of BMP6, BMP7 or BMP9, which may also signal via ALK2, was potentiated by knockdown of BMPR2. Similar results were seen in HepG2 liver carcinoma cells. We propose that BMPR2 inhibits ALK2-mediated signaling by preventing ALK2 from oligomerizing with the type 2 receptors ACVR2A and ACVR2B, which are necessary for activation of ALK2 by activins and several BMPs. In conclusion, BMPR2 could be explored as a possible target for therapy in patients with multiple myeloma.This article has an associated First Person interview with the first author of the paper.

BMPRIA is required for osteogenic differentiation and RANKL expression in adult bone marrow mesenchymal stromal cells

Bone morphogenetic proteins (BMPs) activate the canonical Smad1/5/8 and non-canonical Tak1-MAPK pathways via BMP receptors I and II to regulate skeletal development and bone remodeling. Specific ablation of Bmpr1a in immature osteoblasts, osteoblasts, or osteocytes results in an increase in cancellous bone mass, yet opposite results have been reported regarding the underlying mechanisms. Moreover, the role for BMPRIA-mediated signaling in bone marrow mesenchymal stromal cells (BM-MSCs) has not been explored. Here, we specifically ablated Bmpr1a in BM-MSCs in adult mice to study the function of BMPR1A in bone remodeling and found that the mutant mice showed an increase in cancellous and cortical bone mass, which was accompanied by a decrease in bone formation rate and a greater decrease in bone resorption. Decreased bone formation was associated with a defect in BM-MSC osteogenic differentiation whereas decreased bone resorption was associated with a decrease in RANKL production and osteoclastogenesis. However, ablation of Tak1, a critical non-canonical signaling molecule downstream of BMP receptors, in BM-MSCs at adult stage did not affect bone remodeling. These results suggest that BMP signaling through BMPRIA controls BM-MSC osteogenic differentiation/bone formation and RANKL expression/osteoclastogenesis in adult mice independent of Tak1 signaling.

TGF-β Family Signaling in Mesenchymal Differentiation

Mesenchymal stem cells (MSCs) can differentiate into several lineages during development and also contribute to tissue homeostasis and regeneration, although the requirements for both may be distinct. MSC lineage commitment and progression in differentiation are regulated by members of the transforming growth factor-β (TGF-β) family. This review focuses on the roles of TGF-β family signaling in mesenchymal lineage commitment and differentiation into osteoblasts, chondrocytes, myoblasts, adipocytes, and tenocytes. We summarize the reported findings of cell culture studies, animal models, and interactions with other signaling pathways and highlight how aberrations in TGF-β family signaling can drive human disease by affecting mesenchymal differentiation.

miR-3065-5p regulates mouse odontoblastic differentiation partially through bone morphogenetic protein receptor type II

Illumination of the molecular mechanisms regulating odontoblastic differentiation of dental papilla cells is of great significance for proper dentinogenesis and dental pulp regeneration. In this study, we discovered that microRNA (miR)-3065-5p is up-regulated during odontoblastic differentiation. Overexpression of miR-3065-5p promoted odontoblastic differentiation in vitro. Dual luciferase report assay verified that miR-3065-5p could bind to the 3'UTR of bone morphogenetic protein receptor type II (BMPR2), which dramatically increased in the beginning of odontoblastic differentiation but decreased in the terminal differentiation stage. Inhibition of Bmpr2 in the early stage retarded odontoblastic differentiation while knockdown of Bmpr2 in the terminal stage enhanced odontoblastic differentiation, resembling the effect of miR-3065-5p. Taken together, our present study suggests that miR-3065-5p positively regulates odontoblastic differentiation by directly binding to Bmpr2 in the terminal differentiation stage.

Loss of the nutrient sensor TAS1R3 leads to reduced bone resorption

The taste receptor type 1 (TAS1R) family of heterotrimeric G protein-coupled receptors participates in monitoring energy and nutrient status. TAS1R member 3 (TAS1R3) is a bi-functional protein that recognizes amino acids such as L-glycine and L-glutamate or sweet molecules such as sucrose and fructose when dimerized with TAS1R member 1 (TAS1R1) or TAS1R member 2 (TAS1R2), respectively. It was recently reported that deletion of TAS1R3 expression in Tas1R3 mutant mice leads to increased cortical bone mass but the underlying cellular mechanism leading to this phenotype remains unclear. Here, we independently corroborate the increased thickness of cortical bone in femurs of 20-week-old male Tas1R3 mutant mice and confirm that Tas1R3 is expressed in the bone environment. Tas1R3 is expressed in undifferentiated bone marrow stromal cells (BMSCs) in vitro and its expression is maintained during BMP2-induced osteogenic differentiation. However, levels of the bone formation marker procollagen type I N-terminal propeptide (PINP) are unchanged in the serum of 20-week-old Tas1R3 mutant mice as compared to controls. In contrast, levels of the bone resorption marker collagen type I C-telopeptide are reduced greater than 60% in Tas1R3 mutant mice. Consistent with this, Tas1R3 and its putative signaling partner Tas1R2 are expressed in primary osteoclasts and their expression levels positively correlate with differentiation status. Collectively, these findings suggest that high bone mass in Tas1R3 mutant mice is due to uncoupled bone remodeling with reduced osteoclast function and provide rationale for future experiments examining the cell-type-dependent role for TAS1R family members in nutrient sensing in postnatal bone remodeling.

Combinatorial Signal Perception in the BMP Pathway

The bone morphogenetic protein (BMP) signaling pathway comprises multiple ligands and receptors that interact promiscuously with one another and typically appear in combinations. This feature is often explained in terms of redundancy and regulatory flexibility, but it has remained unclear what signal-processing capabilities it provides. Here, we show that the BMP pathway processes multi-ligand inputs using a specific repertoire of computations, including ratiometric sensing, balance detection, and imbalance detection. These computations operate on the relative levels of different ligands and can arise directly from competitive receptor-ligand interactions. Furthermore, cells can select different computations to perform on the same ligand combination through expression of alternative sets of receptor variants. These results provide a direct signal-processing role for promiscuous receptor-ligand interactions and establish operational principles for quantitatively controlling cells with BMP ligands. Similar principles could apply to other promiscuous signaling pathways.

Clinical Relevance and Mechanisms of Antagonism Between the BMP and Activin/TGF-β Signaling Pathways

The transforming growth factor β (TGF-β) superfamily is a large group of signaling molecules that participate in embryogenesis, organogenesis, and tissue homeostasis. These molecules are present in all animal genomes. Dysfunction in the regulation or activity of this superfamily's components underlies numerous human diseases and developmental defects. There are 2 distinct arms downstream of the TGF-β superfamily ligands-the bone morphogenetic protein (BMP) and activin/TGF-β signaling pathways-and these 2 responses can oppose one another's effects, most notably in disease states. However, studies have commonly focused on a single arm of the TGF-β superfamily, and the antagonism between these pathways is unknown in most physiologic and pathologic contexts. In this review, the authors summarize the clinically relevant scenarios in which the BMP and activin/TGF-β pathways reportedly oppose one another and identify several molecular mechanisms proposed to mediate this interaction. Particular attention is paid to experimental findings that may be informative to human pathology to highlight potential therapeutic approaches for future investigation.

The Role of the Nuclear Envelope Protein MAN1 in Mesenchymal Stem Cell Differentiation

Mutations in MAN1, a protein of the nuclear envelope, cause bone phenotypes characterized by hyperostosis. The mechanism of this pro-osteogenic phenotype remains unknown. We increased and decreased MAN1 expression in mesenchymal stem cells (MSC) upon which standard osteogenic and adipogenic differentiation were performed. MAN1 knockdown increased osteogenesis and mineralization. In contrast, osteogenesis remained stable upon MAN1 overexpression. Regarding a mechanism, we found that low levels of MAN1 facilitated the nuclear accumulation of regulatory smads and smads-related complexes, with a concurrently high expression of nuclear β-Catenin. In addition, we found adipogenesis to be decreased in both conditions, although predominantly affected by MAN1 overexpression. Finally, lamin A, a protein of the nuclear envelope that regulates MSC differentiation, was unaffected by changes in MAN1. In conclusion, our studies demonstrated that lower levels of MAN1 in differentiating MSC are associated with higher osteogenesis and lower adipogenesis. High levels of MAN1 only affected adipogenesis. These effects could have an important role in the understanding of the role of the proteins of the nuclear envelope in bone formation. J. Cell. Biochem. 118: 4425-4435, 2017. © 2017 Wiley Periodicals, Inc.

Soluble activin type IIB receptor improves fracture healing in a closed tibial fracture mouse model

Fractures still present a significant burden to patients due to pain and periods of unproductivity. Numerous growth factors have been identified to regulate bone remodeling. However, to date, only the bone morphogenetic proteins (BMPs) are used to enhance fracture healing in clinical settings. Activins are pleiotropic growth factors belonging to the TGF-β superfamily. We and others have recently shown that treatment with recombinant fusion proteins of activin receptors greatly increases bone mass in different animal models by trapping activins and other ligands thus inhibiting their signaling pathways. However, their effects on fracture healing are less known. Twelve-week old male C57Bl mice were subjected to a standardized, closed tibial fracture model. Animals were divided into control and treatment groups and were administered either PBS control or a soluble activin type IIB receptor (ActRIIB-Fc) intraperitoneally once a week for a duration of two or four weeks. There were no significant differences between the groups at two weeks but we observed a significant increase in callus mineralization in ActRIIB-Fc-treated animals by microcomputed tomography imaging at four weeks. Bone volume per tissue volume was 60%, trabecular number 55% and bone mineral density 60% higher in the 4-week calluses of the ActRIIB-Fc-treated mice (p<0.05 in all). Biomechanical strength of 4-week calluses was also significantly improved by ActRIIB-Fc treatment as stiffness increased by 64% and maximum force by 45% (p<0.05) compared to the PBS-injected controls. These results demonstrate that ActRIIB-Fc treatment significantly improves healing of closed long bone fractures. Our findings support the previous reports of activin receptors increasing bone mass but also demonstrate a novel approach for using ActRIIB-Fc to enhance fracture healing.

Activin receptor type 2A (ACVR2A) functions directly in osteoblasts as a negative regulator of bone mass

Bone and skeletal muscle mass are highly correlated in mammals, suggesting the existence of common anabolic signaling networks that coordinate the development of these two anatomically adjacent tissues. The activin signaling pathway is an attractive candidate to fulfill such a role. Here, we generated mice with conditional deletion of activin receptor (ACVR) type 2A, ACVR2B, or both, in osteoblasts, to determine the contribution of activin receptor signaling in regulating bone mass. Immunohistochemistry localized ACVR2A and ACVR2B to osteoblasts and osteocytes. Primary osteoblasts expressed activin signaling components, including ACVR2A, ACVR2B, and ACVR1B (ALK4) and demonstrated increased levels of phosphorylated Smad2/3 upon exposure to activin ligands. Osteoblasts lacking ACVR2B did not show significant changes in vitro However, osteoblasts deficient in ACVR2A exhibited enhanced differentiation indicated by alkaline phosphatase activity, mineral deposition, and transcriptional expression of osterix, osteocalcin, and dentin matrix acidic phosphoprotein 1. To investigate activin signaling in osteoblasts in vivo, we analyzed the skeletal phenotypes of mice lacking these receptors in osteoblasts and osteocytes (osteocalcin-Cre). Similar to the lack of effect in vitro, ACVR2B-deficient mice demonstrated no significant change in any bone parameter. By contrast, mice lacking ACVR2A had significantly increased femoral trabecular bone volume at 6 weeks of age. Moreover, mutant mice lacking both ACVR2A and ACVR2B demonstrated sustained increases in trabecular bone volume, similar to those in ACVR2A single mutants, at 6 and 12 weeks of age. Taken together, these results indicate that activin receptor signaling, predominantly through ACVR2A, directly and negatively regulates bone mass in osteoblasts.

The critical role of the ZNF217 oncogene in promoting breast cancer metastasis to the bone

Bone metastasis affects >70% of patients with advanced breast cancer. However, the molecular mechanisms underlying this process remain unclear. On the basis of analysis of clinical datasets, and in vitro and in vivo experiments, we report that the ZNF217 oncogene is a crucial mediator and indicator of bone metastasis. Patients with high ZNF217 mRNA expression levels in primary breast tumours had a higher risk of developing bone metastases. MDA-MB-231 breast cancer cells stably transfected with ZNF217 (MDA-MB-231-ZNF217) showed the dysregulated expression of a set of genes with bone-homing and metastasis characteristics, which overlapped with two previously described 'osteolytic bone metastasis' gene signatures, while also highlighting the bone morphogenetic protein (BMP) pathway. The latter was activated in MDA-MB-231-ZNF217 cells, and its silencing by inhibitors (Noggin and LDN-193189) was sufficient to rescue ZNF217-dependent cell migration, invasion or chemotaxis towards the bone environment. Finally, by using non-invasive multimodal in vivo imaging, we found that ZNF217 increases the metastatic growth rate in the bone and accelerates the development of severe osteolytic lesions. Altogether, the findings of this study highlight ZNF217 as an indicator of the emergence of breast cancer bone metastasis; future therapies targeting ZNF217 and/or the BMP signalling pathway may be beneficial by preventing the development of bone metastases. Copyright © 2017 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

MicroRNAs 223-3p and 93-5p in patients with chronic kidney disease before and after renal transplantation

Chronic kidney disease (CKD) is associated with a multifactorial dysregulation of bone and vascular calcification and closely linked to increased cardiovascular mortality and concomitant bone disease. We aimed to investigate specific microRNA (miRNA) signatures in CKD patients to find indicators for vascular calcification and/or bone mineralization changes during CKD and after kidney transplantation (KT). A miRNA array was used to investigate serum miRNA profiles in CKD patients, then selected miRNAs were quantified in a validation cohort comprising 73 patients in CKD stages 3 to 5, 67 CKD patients after KT, and 36 healthy controls. A spectrum of biochemical parameters including markers for kidney function, inflammation, glucose, and mineral metabolism was determined. The relative expression of miR-223-3p and miR-93-5p was down-regulated in patients with CKD stage 4 and 5 compared to healthy controls. This down-regulation disappeared after kidney transplantation even when lower glomerular filtration rates (eGFR) persisted. MiR-223-3p and miR-93-5p were associated with interleukin-6 (IL-6) and eGFR levels, and by trend with interleukin-8 (IL-8), C-peptide, hematocrit, and parathyroid hormone (PTH). This study contributes new knowledge of serum miRNA expression profiles in CKD, potentially reflecting pathophysiological changes of bone and calcification pathways associated with inflammation, vascular calcification, mineral and glucose metabolism. Identified miRNA signatures can contribute to future risk markers or future therapeutic targets in bone and kidney disease.

The ACVR1 R206H mutation found in fibrodysplasia ossificans progressiva increases human induced pluripotent stem cell-derived endothelial cell formation and collagen production through BMP-mediated SMAD1/5/8 signaling

Background

The Activin A and bone morphogenetic protein (BMP) pathways are critical regulators of the immune system and of bone formation. Inappropriate activation of these pathways, as in conditions of congenital heterotopic ossification, are thought to activate an osteogenic program in endothelial cells. However, if and how this occurs in human endothelial cells remains unclear.

Methods

We used a new directed differentiation protocol to create human induced pluripotent stem cell (hiPSC)-derived endothelial cells (iECs) from patients with fibrodysplasia ossificans progressiva (FOP), a congenital disease of heterotopic ossification caused by an activating R206H mutation in the Activin A type I receptor (ACVR1). This strategy allowed the direct assay of the cell-autonomous effects of ACVR1 R206H in the endogenous locus without the use of transgenic expression. These cells were challenged with BMP or Activin A ligand, and tested for their ability to activate osteogenesis, extracellular matrix production, and differential downstream signaling in the BMP/Activin A pathways.

Results

We found that FOP iECs could form in conditions with low or absent BMP4. These conditions are not normally permissive in control cells. FOP iECs cultured in mineralization media showed increased alkaline phosphatase staining, suggesting formation of immature osteoblasts, but failed to show mature osteoblastic features. However, FOP iECs expressed more fibroblastic genes and Collagen 1/2 compared to control iECs, suggesting a mechanism for the tissue fibrosis seen in early heterotopic lesions. Finally, FOP iECs showed increased SMAD1/5/8 signaling upon BMP4 stimulation. Contrary to FOP hiPSCs, FOP iECs did not show a significant increase in SMAD1/5/8 phosphorylation upon Activin A stimulation, suggesting that the ACVR1 R206H mutation has a cell type-specific effect. In addition, we found that the expression of ACVR1 and type II receptors were different in hiPSCs and iECs, which could explain the cell type-specific SMAD signaling.

Conclusions

Our results suggest that the ACVR1 R206H mutation may not directly increase the formation of mature chondrogenic or osteogenic cells by FOP iECs. Our results also show that BMP can induce endothelial cell dysfunction, increase expression of fibrogenic matrix proteins, and cause differential downstream signaling of the ACVR1 R206H mutation. This iPSC model provides new insight into how human endothelial cells may contribute to the pathogenesis of heterotopic ossification.

Electronic supplementary material

The online version of this article (doi:10.1186/s13287-016-0372-6) contains supplementary material, which is available to authorized users.

A Survey of Strategies to Modulate the Bone Morphogenetic Protein Signaling Pathway: Current and Future Perspectives

Bone morphogenetic proteins (BMPs) constitute the largest subdivision of the TGF-β family of ligands and are unequivocally involved in regulating stem cell behavior. Appropriate regulation of canonical BMP signaling is critical for the development and homeostasis of numerous human organ systems, as aberrations in the BMP pathway or its regulation are increasingly associated with diverse human pathologies. In this review, we provide a wide-perspective on strategies that increase or decrease BMP signaling. We briefly outline the current FDA-approved approaches, highlight emerging next-generation technologies, and postulate prospective avenues for future investigation. We also detail how activating other pathways may indirectly modulate BMP signaling, with a particular emphasis on the relationship between the BMP and Activin/TGF-β pathways.

ACVR1R206H receptor mutation causes fibrodysplasia ossificans progressiva by imparting responsiveness to activin A

Fibrodysplasia ossificans progressiva (FOP) is a rare genetic disorder characterized by episodically exuberant heterotopic ossification (HO), whereby skeletal muscle is abnormally converted into misplaced, but histologically normal bone. This HO leads to progressive immobility with catastrophic consequences, including death by asphyxiation. FOP results from mutations in the intracellular domain of the type I BMP (bone morphogenetic protein) receptor ACVR1; the most common mutation alters arginine 206 to histidine (ACVR1(R206H)) and has been thought to drive inappropriate bone formation as a result of receptor hyperactivity. We unexpectedly found that this mutation rendered ACVR1 responsive to the activin family of ligands, which generally antagonize BMP signaling through ACVR1 but cannot normally induce bone formation. To test the implications of this finding in vivo, we engineered mice to carry the Acvr1(R206H) mutation. Because mice that constitutively express Acvr1[R206H] die perinatally, we generated a genetically humanized conditional-on knock-in model for this mutation. When Acvr1[R206H] expression was induced, mice developed HO resembling that of FOP; HO could also be triggered by activin A administration in this mouse model of FOP but not in wild-type controls. Finally, HO was blocked by broad-acting BMP blockers, as well as by a fully human antibody specific to activin A. Our results suggest that ACVR1(R206H) causes FOP by gaining responsiveness to the normally antagonistic ligand activin A, demonstrating that this ligand is necessary and sufficient for driving HO in a genetically accurate model of FOP; hence, our human antibody to activin A represents a potential therapeutic approach for FOP.

BMP signalling in skeletal development, disease and repair

Since the identification in 1988 of bone morphogenetic protein 2 (BMP2) as a potent inducer of bone and cartilage formation, BMP superfamily signalling has become one of the most heavily investigated topics in vertebrate skeletal biology. Whereas a large part of this research has focused on the roles of BMP2, BMP4 and BMP7 in the formation and repair of endochondral bone, a large number of BMP superfamily molecules have now been implicated in almost all aspects of bone, cartilage and joint biology. As modulating BMP signalling is currently a major therapeutic target, our rapidly expanding knowledge of how BMP superfamily signalling affects most tissue types of the skeletal system creates enormous potential to translate basic research findings into successful clinical therapies that improve bone mass or quality, ameliorate diseases of skeletal overgrowth, and repair damage to bone and joints. This Review examines the genetic evidence implicating BMP superfamily signalling in vertebrate bone and joint development, discusses a selection of human skeletal disorders associated with altered BMP signalling and summarizes the status of modulating the BMP pathway as a therapeutic target for skeletal trauma and disease.

Transforming Growth Factor-β Family Ligands Can Function as Antagonists by Competing for Type II Receptor Binding

Transforming growth factor-β (TGF-β) family ligands are pleiotropic cytokines. Their physiological activities are not determined by a simple coupling of stimulus and response, but depend critically on context, i.e. the interplay of receptors, ligands, and regulators that form the TGF-β signal transduction system of a cell or tissue. How these different components combine to regulate signaling activities remains poorly understood. Here, we describe a ligand-mediated mechanism of signaling regulation. Based on the observation that the type II TGF-β family receptors ActRIIA, ActRIIB, and BMPRII interact with a large group of overlapping ligands at overlapping epitopes, we hypothesized high affinity ligands compete with low affinity ligands for receptor binding and signaling. We show activin A and other high affinity ligands directly inhibited signaling by the low affinity ligands BMP-2, BMP-7, and BMP-9. We demonstrate activin A functions as a competitive inhibitor that blocks the ligand binding epitope on type II receptors. We propose binding competition and signaling antagonism are integral functions of the TGF-β signal transduction system. These functions could help explain how activin A modulates physiological signaling during extraordinary cellular responses, such as injury and wound healing, and how activin A could elicit disease phenotypes such as cancer-related muscle wasting and fibrosis.