Mutational and phenotypic characterization of hereditary hemorrhagic telangiectasia.

Hereditary hemorrhagic telangiectasia (HHT) is an autosomal dominant vascular dysplasia. Care delivery for HHT patients is impeded by the need for laborious, repeated phenotyping and gaps in knowledge regarding the relationships between causal DNA variants in ENG, ACVRL1, SMAD4 and GDF2, and clinical manifestations. To address this, we analyzed DNA samples from 183 previously uncharacterized, unrelated HHT and suspected HHT cases using the ThromboGenomics high-throughput sequencing platform. We identified 127 rare variants across 168 heterozygous genotypes. Applying modified American College of Medical Genetics and Genomics Guidelines, 106 variants were classified as pathogenic/likely pathogenic and 21 as nonpathogenic (variant of uncertain significance/benign). Unlike the protein products of ACVRL1 and SMAD4, the extracellular ENG amino acids are not strongly conserved. Our inferences of the functional consequences of causal variants in ENG were therefore informed by the crystal structure of endoglin. We then compared the accuracy of predictions of the causal gene blinded to the genetic data using 2 approaches: subjective clinical predictions and statistical predictions based on 8 Human Phenotype Ontology terms. Both approaches had some predictive power, but they were insufficiently accurate to be used clinically, without genetic testing. The distributions of red cell indices differed by causal gene but not sufficiently for clinical use in isolation from genetic data. We conclude that parallel sequencing of the 4 known HHT genes, multidisciplinary team review of variant calls in the context of detailed clinical information, and statistical and structural modeling improve the prognostication and treatment of HHT.

Molecular basis of ALK1-mediated signalling by BMP9/BMP10 and their prodomain-bound forms.

Activin receptor-like kinase 1 (ALK1)-mediated endothelial cell signalling in response to bone morphogenetic protein 9 (BMP9) and BMP10 is of significant importance in cardiovascular disease and cancer. However, detailed molecular mechanisms of ALK1-mediated signalling remain unclear. Here, we report crystal structures of the BMP10:ALK1 complex at 2.3 Å and the prodomain-bound BMP9:ALK1 complex at 3.3 Å. Structural analyses reveal a tripartite recognition mechanism that defines BMP9 and BMP10 specificity for ALK1, and predict that crossveinless 2 is not an inhibitor of BMP9, which is confirmed by experimental evidence. Introduction of BMP10-specific residues into BMP9 yields BMP10-like ligands with diminished signalling activity in C2C12 cells, validating the tripartite mechanism. The loss of osteogenic signalling in C2C12 does not translate into non-osteogenic activity in vivo and BMP10 also induces bone-formation. Collectively, these data provide insight into ALK1-mediated BMP9 and BMP10 signalling, facilitating therapeutic targeting of this important pathway.

Mutational Analysis of the Putative Anti-Müllerian Hormone (AMH) Binding Interface on its Type II Receptor, AMHR2.

Anti-Müllerian hormone (AMH) or Müllerian inhibiting substance is a unique member of the TGF-ß family responsible for development and differentiation of the reproductive system. AMH signals through its own dedicated type II receptor, anti-Müllerian hormone receptor type II (AMHR2), providing an exclusive ligand-receptor pair within the broader TGF-ß family. In this study, we used previous structural information to derive a model of AMH bound to AMHR2 to guide mutagenesis studies to identify receptor residues important for AMH signaling. Nonconserved mutations were introduced in AMHR2 and characterized in an AMH-responsive cell-based luciferase assay and native PAGE. Collectively, our results identified several residues important for AMH signaling within the putative ligand binding interface of AMHR2. Our results show that AMH engages AMHR2 at a similar interface to how activin and BMP class ligands bind the type II receptor, ACVR2B; however, there are significant molecular differences at the ligand interface of these 2 receptors, where ACVR2B is mostly hydrophobic and AMHR2 is predominately charged. Overall, this study shows that although the location of ligand binding on the receptor is similar to ACVR2A, ACVR2B, and BMPR2; AMHR2 uses unique ligand-receptor interactions to impart specificity for AMH.

A Bone Morphogenetic Protein (BMP)-derived Peptide Based on the Type I Receptor-binding Site Modifies Cell-type Dependent BMP Signalling.

Bone morphogenetic proteins (BMPs) are multifunctional cytokines of the transforming growth factor ß (TGFß) superfamily with potential therapeutic applications due to their broad biological functionality. Designing BMP mimetics with specific activity will contribute to the translational potential of BMP-based therapies. Here, we report a BMP9 peptide mimetic, P3, designed from the type I receptor binding site, which showed millimolar binding affinities for the type I receptor activin receptor like kinase 1 (ALK1), ALK2 and ALK3. Although showing no baseline activity, P3 significantly enhanced BMP9-induced Smad1/5 phosphorylation as well as ID1, BMPR2, HEY1 and HEY2 gene expression in pulmonary artery endothelial cells (hPAECs), and this activity is dependent on its alpha helix propensity. However, in human dermal microvascular endothelial cells, P3 did not affect BMP9-induced Smad1/5 phosphorylation, but potently inhibited ALK3-dependent BMP4-induced Smad1/5 phosphorylation and gene expression. In C2C12 mouse myoblast cells, P3 had no effect on BMP9-induced osteogenic signalling, which is primarily mediated by ALK2. Interestingly, a previously published peptide from the knuckle region of BMP9 was found to inhibit BMP4-induced Smad1/5 phosphorylation. Together, our data identify a BMP9-derived peptide that can selectively enhance ALK1-mediated BMP9 signalling in hPAECs and modulate BMP9 and BMP4 signalling in a cell type-specific manner.

Crystal structure of the WFIKKN2 follistatin domain reveals insight into how it inhibits growth differentiation factor 8 (GDF8) and GDF11.

Growth differentiation factor 8 (GDF8; also known as myostatin) and GDF11 are closely related members of the transforming growth factor ß (TGF-ß) family. GDF8 strongly and negatively regulates skeletal muscle growth, and GDF11 has been implicated in various age-related pathologies such as cardiac hypertrophy. GDF8 and GDF11 signaling activities are controlled by the extracellular protein antagonists follistatin; follistatin-like 3 (FSTL3); and WAP, follistatin/kazal, immunoglobulin, Kunitz, and netrin domain-containing (WFIKKN). All of these proteins contain a follistatin domain (FSD) important for ligand binding and antagonism. Here, we investigated the structure and function of the FSD from murine WFIKKN2 and compared it with the FSDs of follistatin and FSTL3. Using native gel shift and surface plasmon resonance analyses, we determined that the WFIKKN2 FSD can interact with both GDF8 and GDF11 and block their interactions with the type II receptor activin A receptor type 2B (ActRIIB). Further, we solved the crystal structure of the WFIKKN2 FSD to 1.39 Å resolution and identified surface-exposed residues that, when substituted with alanine, reduce antagonism of GDF8 in full-length WFIKKN2. Comparison of the WFIKKN2 FSD with those of follistatin and FSTL3 revealed differences in both the FSD structure and position of residues within the domain that are important for ligand antagonism. Taken together, our results indicate that both WFIKKN and follistatin utilize their FSDs to block the type II receptor but do so via different binding interactions.

Antibody-based strategies for the detection of Luspatercept (ACE-536) in human serum.

Luspatercept (ACE-536, ACVR2B-Fc), a fusion protein consisting of the extracellular domain of ActRIIB receptor and the Fc-part of human immunoglobulin G1 (IgG1), is currently under clinical development (Phase III). It stimulates the formation of red blood cells and hence may be misused by athletes for doping purposes in the future. Several antibody-based strategies for the detection of Luspatercept and other ACVR2B-Fc fusion proteins in human serum were evaluated (ELISA; IEF-, SDS-, and SAR-PAGE followed by Western blotting; immunoprecipitation). Two methods led to useful results: a commercial "soluble" ACTR-IIB ELISA, which also detected Luspatercept and other ACVR2B-Fc's, but showed no cross-reactivity with Sotatercept/ACVR2A-Fc's. The ELISA might be applied as fast screening tool (100 µL serum; limit of detection (LOD) ca 15.6 ng/mL). The second method uses a polyclonal ACVR2B-antibody for immunoprecipitation followed by SAR-PAGE and Western blotting with a monoclonal detection antibody (50 µL serum; LOD ca 1.0 ng/mL). It can be used for initial as well as for confirmatory testing. Due to the high doses (mg/kg) and long serum half-life of Luspatercept, both strategies may be useful in anti-doping control in the future. Copyright © 2017 John Wiley & Sons, Ltd.

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.

Dual targeting of vascular endothelial growth factor and bone morphogenetic protein-9/10 impairs tumor growth through inhibition of angiogenesis.

Clinical development of anti-angiogenic agents has been a major landmark in cancer therapy for several types of cancers. Signals mediated by both vascular endothelial growth factor (VEGF) and bone morphogenetic protein (BMP)-9 and 10 have been implicated in tumor angiogenesis. However, previous studies have shown that targeting the individual signals was not sufficiently effective in retarding tumor growth in certain preclinical and clinical conditions. In the present study, we developed a novel decoy chimeric receptor that traps both VEGF and BMP-9/10. Single targeting of either VEGF or BMP-9/10 signals significantly reduced the formation of tumor vessels in a mouse xenograft model of human pancreatic cancer; however, it did not show significant therapeutic effects on tumor growth. In contrast, dual targeting of the angiogenic signals resulted in more significant inhibition of tumor angiogenesis, leading to delay of tumor growth. Our findings suggest that simultaneous blockade of VEGF and BMP-9/10 signals is a promising therapeutic strategy for the cancers that are resistant to anti-VEGF and BMP-9/10 therapies.

Identification of the First Selective Activin Receptor-Like Kinase 1 Inhibitor, a Reversible Version of L-783277.

We synthesized 1 (San78-130), a reversible version of L-783277, as a selective and potent ALK1 inhibitor. Our study showed that 1 possesses great kinase selectivity against a panel of 342 kinases and more potent activity against ALK1 than L-783277. Among the six ALK isotypes (ALK1-6), ALK1 is most significantly inhibited by compound 1. Compound 1 suppresses the BMP9-induced Smad1/5 pathway by mainly inhibiting ALK1 in C2C12 cells. Our molecular dynamics simulations suggest that H-bonding interaction between the C-4' hydroxyl group of 1 and Arg334 of ALK1 substantially contributes to the ALK1 inhibition. To the best of our knowledge, 1 is the first selective ALK1 inhibitor. Furthermore, compound 1 promoted angiogenesis in both endothelial tube formation and microfluidic chip based 3D angiogenesis assays, suggesting that 1 could be a lead compound for therapeutic angiogenesis agents. Our study may provide an insight into designing selective and potent inhibitors against ALK1.

Molecular characterization of Activin Receptor Type IIA and its expression during gonadal maturation and growth stages in rohu carp.

Activin receptor type IIA (ActRIIA), a transmembrane serine/threonine kinase receptor is an important regulator of physiological traits, viz., reproduction and body growth in vertebrates including teleosts. However, existing knowledge of its role in regulating fish physiology is limited. To address this, we have cloned and characterized the ActRIIA cDNA of Labeo rohita (rohu), an economically important fish species of the Indian subcontinent. Comparative expression profiling of the receptor gene at various reproductive and growth stages supports to its role in promoting oocyte maturation, spermatogenesis and skeletal muscle development via interaction with multiple ligands of transforming growth factor-ß (TGF-ß) family. The full-length cDNA of rohu ActRIIA was found to be of 1587bp length encoding 528 amino acids. The three-dimensional structure of the intracellular kinase domain of rohu ActRIIA has also been predicted. Phylogenetic relationship studies showed that the gene is evolutionarily conserved across the vertebrate lineage implicating that the functioning of the receptor is more or less similar in vertebrates. Taken together, these findings could be an initial step towards the use of ActRIIA as a potential candidate gene marker for understanding the complex regulatory mechanism of fish reproduction and growth.

Activin Receptor Type IIB Inhibition Improves Muscle Phenotype and Function in a Mouse Model of Spinal Muscular Atrophy.

Spinal muscular atrophy (SMA) is a devastating neurodegenerative disorder that causes progressive muscle atrophy and weakness. Using adeno-associated virus-mediated gene transfer, we evaluated the potential to improve skeletal muscle weakness via systemic, postnatal inhibition of either myostatin or all signaling via the activin receptor type IIB (ActRIIB). After demonstrating elevated p-SMAD3 content and differential content of ActRIIB ligands, 4-week-old male C/C SMA model mice were treated intraperitoneally with 1x1012 genome copies of pseudotype 2/8 virus encoding a soluble form of the ActRIIB extracellular domain (sActRIIB) or protease-resistant myostatin propeptide (dnMstn) driven by a liver specific promoter. At 12 weeks of age, muscle mass and function were improved in treated C/C mice by both treatments, compared to controls. The fast fiber type muscles had a greater response to treatment than did slow muscles, and the greatest therapeutic effects were found with sActRIIB treatment. Myostatin/activin inhibition, however, did not rescue C/C mice from the reduction in motor unit numbers of the tibialis anterior muscle. Collectively, this study indicates that myostatin/activin inhibition represents a potential therapeutic strategy to increase muscle mass and strength, but not neuromuscular junction defects, in less severe forms of SMA.

Testing for the erythropoiesis-stimulating agent Sotatercept/ACE-011 (ActRIIA-Fc) in serum by means of Western blotting and LC-HRMS.

Sotatercept (formerly ACE-011) is a glycosylated, dimeric fusion protein composed of the extracellular domain of the human activin receptor type IIA (ActRIIA) and the Fc region of human IgG1. The protein-based drug candidate acts as a ligand trap which competitively binds to activin A and other members of the transforming growth factor beta superfamily, thus blocking signalling through ActRIIA. Since the inhibition of activin A was found to significantly increase bone formation and quality, Sotatercept was originally developed for the treatment of diseases involving bone loss. But as the protein therapeutic also stimulates erythropoiesis by a mechanism independent of the EPO receptor, it has been evaluated for the treatment of anaemia in rare blood diseases such as beta thalassemia. Due to its positive effects on erythropoiesis and bone formation, Sotatercept may also be misused as performance-enhancing agent in sports. Within this study, two complementary detection assays for Sotatercept and related ActRIIA-Fc fusion proteins in serum samples were developed. While the first assay combines affinity purification and Western blotting to generically detect ActRIIA-Fc fusion proteins irrespective of their amino acid sequence, the liquid chromatography-high resolution mass spectrometry (LC-HRMS) method is highly specific for proteolytic peptides originating from the receptor and Fc domain of Sotatercept. Both approaches can readily be modified to include other pharmaceutical proteins such as therapeutic antibodies, and serve as proof-of-concept for the capability of the approach to detect TGF-ß inhibitors and Fc fusion proteins in doping control serum samples. Copyright © 2016 John Wiley & Sons, Ltd.

Rapid Activation of Bone Morphogenic Protein 9 by Receptor-mediated Displacement of Pro-domains.

By non-covalent association after proteolytic cleavage, the pro-domains modulate the activities of the mature growth factor domains across the transforming growth factor-ß family. In the case of bone morphogenic protein 9 (BMP9), however, the pro-domains do not inhibit the bioactivity of the growth factor, and the BMP9·pro-domain complexes have equivalent biological activities as the BMP9 mature ligand dimers. By using real-time surface plasmon resonance, we could demonstrate that either binding of pro-domain-complexed BMP9 to type I receptor activin receptor-like kinase 1 (ALK1), type II receptors, co-receptor endoglin, or to mature BMP9 domain targeting antibodies leads to immediate and complete displacement of the pro-domains from the complex. Vice versa, pro-domain binding by an anti-pro-domain antibody results in release of the mature BMP9 growth factor. Based on these findings, we adjusted ELISA assays to measure the protein levels of different BMP9 variants. Although mature BMP9 and inactive precursor BMP9 protein were directly detectable by ELISA, BMP9·pro-domain complex could only be measured indirectly as dissociated fragments due to displacement of mature growth factor and pro-domains after antibody binding. Our studies provide a model in which BMP9 can be readily activated upon getting into contact with its receptors. This increases the understanding of the underlying biology of BMP9 activation and also provides guidance for ELISA development for the detection of circulating BMP9 variants.

ACVR2A promoter polymorphism rs1424954 in the Activin-A signaling pathway in trophoblasts.

INTRODUCTION: Pre-eclampsia is a pregnancy-specific disorder and characterized by reduced trophoblast invasion and reduced spiral artery remodeling in the first trimester placenta. A polymorphism located in the promoter region of ACVR2A (rs1424954 (A > G)) has previously been shown to be significantly associated with pre-eclampsia. METHODS: The effects of this variant on ACVR2A expression and its function in the Activin-A signaling pathway were studied by transfections in SGHPL-5 extravillous trophoblasts followed by qRT-PCR. RESULTS: Here we show that the ACVR2A promoter susceptibility variant causes a downregulation of ACVR2A expression. We also provide evidence for transcription of a so-called PROMPT (PROMoter-uPstream-Transcript) in the opposite direction of ACVR2A, containing the polymorphism, and downregulated when the susceptibility allele is carried, which either shares the same promoter as ACVR2A or is a non-coding RNA that is able to enhance ACVR2A transcription. Furthermore, when the effect of the susceptibility variant is mimicked by knockdown of ACVR2A, physiologic concentrations of Activin-A cause a reduction in NODAL mRNA expression in the SGHPL-5 trophoblasts, indicative of a protective effect as reduction in NODAL expression is associated with an increase in trophoblast invasion. However, at pathologic levels of Activin-A, as found in pre-eclampsia, this effect is no longer seen, and we show this is potentially caused by a lack of downregulation of ACVR2B. DISCUSSION: The combined data suggest a double hit phenomenon in which the first hit, the promoter variant, together with the second hit, pathological levels of Activin-A, lead to high levels of NODAL, associated with reduced trophoblast invasion and observed in pre-eclamptic placentas.

RAP-011, an activin receptor ligand trap, increases hemoglobin concentration in hepcidin transgenic mice.

Over expression of hepcidin antimicrobial peptide is a common feature of iron-restricted anemia in humans. We investigated the erythroid response to either erythropoietin or RAP-011, a "murinized" ortholog of sotatercept, in C57BL/6 mice and in hepcidin antimicrobial peptide 1 over expressing mice. Sotatercept, a soluble, activin receptor type IIA ligand trap, is currently being evaluated for the treatment of anemias associated with chronic renal disease, myelodysplastic syndrome, ß-thalassemia, and Diamond Blackfan anemia and acts by inhibiting signaling downstream of activin and other Transforming Growth Factor-ß superfamily members. We found that erythropoietin and RAP-011 increased hemoglobin concentration in C57BL/6 mice and in hepcidin antimicrobial peptide 1 over expressing mice. While erythropoietin treatment depleted splenic iron stores in C57BL/6 mice, RAP-011 treatment did not deplete splenic iron stores in mice of either genotype. Bone marrow erythroid progenitors from erythropoietin-treated mice exhibited iron-restricted erythropoiesis, as indicated by increased median fluorescence intensity of transferrin receptor immunostaining by flow cytometry. In contrast, RAP-011-treated mice did not exhibit the same degree of iron-restricted erythropoiesis. In conclusion, we have demonstrated that RAP-011 can improve hemoglobin concentration in hepcidin antimicrobial peptide 1 transgenic mice. Our data support the hypothesis that RAP-011 has unique biologic effects which prevent or circumvent depletion of mouse splenic iron stores. RAP-011 may, therefore, be an appropriate therapeutic for trials in human anemias characterized by increased expression of hepcidin antimicrobial peptide and iron-restricted erythropoiesis.

Small molecules dorsomorphin and LDN-193189 inhibit myostatin/GDF8 signaling and promote functional myoblast differentiation.

GDF8, or myostatin, is a member of the TGF-ß superfamily of secreted polypeptide growth factors. GDF8 is a potent negative regulator of myogenesis both in vivo and in vitro. We found that GDF8 signaling was inhibited by the small molecule ATP competitive inhibitors dorsomorphin and LDN-193189. These compounds were previously shown to be potent inhibitors of BMP signaling by binding to the BMP type I receptors ALK1/2/3/6. We present the crystal structure of the type II receptor ActRIIA with dorsomorphin and demonstrate that dorsomorphin or LDN-193189 target GDF8 induced Smad2/3 signaling and repression of myogenic transcription factors. As a result, both inhibitors rescued myogenesis in myoblasts treated with GDF8. As revealed by quantitative live cell microscopy, treatment with dorsomorphin or LDN-193189 promoted the contractile activity of myotubular networks in vitro. We therefore suggest these inhibitors as suitable tools to promote functional myogenesis.

A miR-590/Acvr2a/Rad51b axis regulates DNA damage repair during mESC proliferation.

Embryonic stem cells (ESCs) enable rapid proliferation that also causes DNA damage. To maintain genomic stabilization during rapid proliferation, ESCs must have an efficient system to repress genotoxic stress. Here, we show that withdrawal of leukemia inhibitory factor (LIF), which maintains the self-renewal capability of mouse ESCs (mESCs), significantly inhibits the cell proliferation and DNA damage of mESCs and upregulates the expression of miR-590. miR-590 promotes single-strand break (SSB) and double-strand break (DSB) damage repair, thus slowing proliferation of mESCs without influencing stemness. miR-590 directly targets Activin receptor type 2a (Acvr2a) to mediate Activin signaling. We identified the homologous recombination-mediated repair (HRR) gene, Rad51b, as a downstream molecule of the miR-590/Acvr2a pathway regulating the SSB and DSB damage repair and cell cycle. Our study shows that a miR-590/Acvr2a/Rad51b signaling axis ensures the stabilization of mESCs by balancing DNA damage repair and rapid proliferation during self-renewal.

N-linked glycosylation of the bone morphogenetic protein receptor type 2 (BMPR2) enhances ligand binding.

The bone morphogenetic protein (BMP) signaling pathway is essential for normal development and tissue homeostasis. BMP signal transduction occurs when ligands interact with a complex of type 1 and type 2 receptors to activate downstream transcription factors. It is well established that a single BMP receptor may bind multiple BMP ligands with varying affinity, and this has been largely attributed to conformation at the amino acid level. However, all three type 2 BMP receptors (BMPR2, ACVR2A/B) contain consensus N-glycosylation sites in their extracellular domains (ECDs), which could play a role in modulating interaction with ligand. Here, we show a differential pattern of N-glycosylation between BMPR2 and ACVR2A/B. Site-directed mutagenesis reveals that BMPR2 is uniquely glycosylated near its ligand binding domain and at a position that is mutated in patients with heritable pulmonary arterial hypertension. We further demonstrate using a cell-free pulldown assay that N-glycosylation of the BMPR2-ECD enhances its ability to bind BMP2 ligand but has no impact on binding by the closely-related ACVR2B. Our results illuminate a novel aspect of BMP signaling pathway mechanics and demonstrate a functional difference resulting from post-translational modification of type 2 BMP receptors. Additionally, since BMPR2 is required for several aspects of normal development and defects in its function are strongly implicated in human disease, our findings are likely to be relevant in several biological contexts in normal and abnormal human physiology.

Endoglin-mediated suppression of prostate cancer invasion is regulated by activin and bone morphogenetic protein type II receptors.

Mortality from prostate cancer (PCa) is due to the formation of metastatic disease. Understanding how that process is regulated is therefore critical. We previously demonstrated that endoglin, a type III transforming growth factor ß (TGFß) superfamily receptor, suppresses human PCa cell invasion and metastasis. Endoglin-mediated suppression of invasion was also shown by us to be dependent upon the type I TGFß receptor, activin receptor-like kinase 2 (ALK2), and the downstream effector, Smad1. In this study we demonstrate for the first time that two type II TGFß receptors are required for endoglin-mediated suppression of invasion: activin A receptor type IIA (ActRIIA) and bone morphogenetic protein receptor type II (BMPRII). Downstream signaling through these receptors is predominantly mediated by Smad1. ActRIIA stimulates Smad1 activation in a kinase-dependent manner, and this is required for suppression of invasion. In contrast BMPRII regulates Smad1 in a biphasic manner, promoting Smad1 signaling through its kinase domain but suppressing it through its cytoplasmic tail. BMPRII's Smad1-regulatory effects are dependent upon its expression level. Further, its ability to suppress invasion is independent of either kinase function or tail domain. We demonstrate that ActRIIA and BMPRII physically interact, and that each also interacts with endoglin. The current findings demonstrate that both BMPRII and ActRIIA are necessary for endoglin-mediated suppression of human PCa cell invasion, that they have differential effects on Smad1 signaling, that they make separate contributions to regulation of invasion, and that they functionally and physically interact.