ALK1 belongs to the type I receptor family for transforming growth factor-beta family ligands. Heterozygous ALK1 mutations cause hereditary hemorrhagic telangiectasia type 2 (HHT2), a multisystemic vascular disorder. Based largely on in vitro studies, TGF-beta1 has been considered as the most likely ALK1 ligand related to HHT, yet the identity of the physiologic ALK1 ligand remains controversial. In cultured endothelial cells, ALK1 and another TGF-beta type I receptor, ALK5, regulate angiogenesis by controlling TGF-beta signal transduction, and ALK5 is required for ALK1 signaling. However, the extent to which such interactions between these 2 receptors play a role in pathogenesis of HHT is unknown. We directly addressed these issues in vivo by comparing the phenotypes of mice in which the Alk1, Alk5, or Tgfbr2 gene was conditionally deleted in restricted vascular endothelia using a novel endothelial Cre transgenic line. Alk1-conditional deletion resulted in severe vascular malformations mimicking all pathologic features of HHT. Yet Alk5- or Tgfbr2-conditional deletion in mice, or Alk5 inhibition in zebrafish, did not affect vessel morphogenesis. These data indicate that neither ALK5 nor TGFBR2 is required for ALK1 signaling pertinent to the pathogenesis of HHT and suggest that HHT might not be a TGF-beta subfamily disease.

Dysfunctional bone morphogenetic protein receptor-2 (BMPR2) signaling is implicated in the pathogenesis of pulmonary arterial hypertension (PAH). We used a transcriptional high-throughput luciferase reporter assay to screen 3,756 FDA-approved drugs and bioactive compounds for induction of BMPR2 signaling. The best response was achieved with FK506 (tacrolimus), via a dual mechanism of action as a calcineurin inhibitor that also binds FK-binding protein-12 (FKBP12), a repressor of BMP signaling. FK506 released FKBP12 from type I receptors activin receptor-like kinase 1 (ALK1), ALK2, and ALK3 and activated downstream SMAD1/5 and MAPK signaling and ID1 gene regulation in a manner superior to the calcineurin inhibitor cyclosporine and the FKBP12 ligand rapamycin. In pulmonary artery endothelial cells (ECs) from patients with idiopathic PAH, low-dose FK506 reversed dysfunctional BMPR2 signaling. In mice with conditional Bmpr2 deletion in ECs, low-dose FK506 prevented exaggerated chronic hypoxic PAH associated with induction of EC targets of BMP signaling, such as apelin. Low-dose FK506 also reversed severe PAH in rats with medial hypertrophy following monocrotaline and in rats with neointima formation following VEGF receptor blockade and chronic hypoxia. Our studies indicate that low-dose FK506 could be useful in the treatment of PAH.

Arteriovenous malformations (AVMs) are vascular anomalies where arteries and veins are directly connected through a complex, tangled web of abnormal arteries and veins instead of a normal capillary network. AVMs in the brain, lung, and visceral organs, including the liver and gastrointestinal tract, result in considerable morbidity and mortality. AVMs are the underlying cause of three major clinical symptoms of a genetic vascular dysplasia termed hereditary hemorrhagic telangiectasia (HHT), which is characterized by recurrent nosebleeds, mucocutaneous telangiectases, and visceral AVMs and caused by mutations in one of several genes, including activin receptor-like kinase 1 (ALK1). It remains unknown why and how selective blood vessels form AVMs, and there have been technical limitations to observing the initial stages of AVM formation. Here we present in vivo evidence that physiological or environmental factors such as wounds in addition to the genetic ablation are required for Alk1-deficient vessels to develop to AVMs in adult mice. Using the dorsal skinfold window chamber system, we have demonstrated for what we believe to be the first time the entire course of AVM formation in subdermal blood vessels by using intravital bright-field images, hyperspectral imaging, fluorescence recordings of direct arterial flow through the AV shunts, and vascular casting techniques. We believe our data provide novel insights into the pathogenetic mechanisms of HHT and potential therapeutic approaches.

Major discoveries have been obtained within the last decade in the field of hereditary predisposition to pulmonary arterial hypertension (PAH). Among them, the identification of bone morphogenetic protein receptor type 2 (BMPR2) as the major predisposing gene and activin A receptor type II-like kinase-1 (ACVRL1, also known as ALK1) as the major gene when PAH is associated with hereditary hemorrhagic telangiectasia. The mutation detection rate for the known genes is approximately 75% in familial PAH, but the mutation shortfall remains unexplained even after careful molecular investigation of these genes. To identify additional genetic variants predisposing to PAH, investigators harnessed the power of next-generation sequencing to successfully identify additional genes that will be described in this report. Furthermore, common genetic predisposing factors for PAH can be identified by genome-wide association studies and are detailed in this paper. The careful study of families and routine genetic diagnosis facilitated natural history studies based on large registries of PAH patients to be set up in different countries. These longitudinal or cross-sectional studies permitted the clinical characterization of PAH in mutation carriers to be accurately described. The availability of molecular genetic diagnosis has opened up a new field for patient care, including genetic counseling for a severe disease, taking into account that the major predisposing gene has a highly variable penetrance between families. Molecular information can be drawn from the genomic study of affected tissues in PAH, in particular, pulmonary vascular tissues and cells, to gain insight into the mechanisms leading to the development of the disease. High-throughput genomic techniques, on the basis of next-generation sequencing, now allow the accurate quantification and analysis of ribonucleic acid, species, including micro-ribonucleic acids, and allow for a genome-wide investigation of epigenetic or regulatory mechanisms, which include deoxyribonucleic acid methylation, histone methylation, and acetylation, or transcription factor binding.

Bone morphogenetic proteins (BMPs) are multifunctional growth factors belonging to the transforming growth factor beta (TGFbeta) superfamily. Recent observations clearly emphasize the emerging role of BMPs in angiogenesis: (i) two genetic vascular diseases (hereditary hemorrhagic telangiectasia (HHT) and pulmonary arterial hypertension (PAH)) are caused by mutations in genes encoding components of the BMP signalling pathway (endoglin, ALK1 and BMPRII). (ii) BMP9 has been identified as the physiological ligand of the endothelial receptor ALK1 in association with BMPRII. This review will focus on the diverse functions of BMPs in angiogenesis. We will propose a model that distinguishes the BMP2, BMP7 and GDF5 subgroups from the BMP9 subgroup on the basis of their functional implication in the two phases of angiogenesis (activation and maturation).

ALK1 is a type I receptor of the TGF-β family that is involved in angiogenesis. Circulating BMP9 was identified as a specific ligand for ALK1 inducing vascular quiescence. In this work, we found that blocking BMP9 with a neutralizing antibody in newborn mice significantly increased retinal vascular density. Surprisingly, Bmp9-KO mice did not show any defect in retinal vascularization. However, injection of the extracellular domain of ALK1 impaired retinal vascularization in Bmp9-KO mice, implicating another ligand for ALK1. Interestingly, we detected a high level of circulating BMP10 in WT and Bmp9-KO pups. Further, we found that injection of a neutralizing anti-BMP10 antibody to Bmp9-KO pups reduced retinal vascular expansion and increased vascular density, whereas injection of this antibody to WT pups did not affect the retinal vasculature. These data suggested that BMP9 and BMP10 are important in postnatal vascular remodeling of the retina and that BMP10 can substitute for BMP9. In vitro stimulation of endothelial cells by BMP9 and BMP10 increased the expression of genes involved in the Notch signaling pathway (Jagged1, Dll4, Hey1, Hey2, Hes1) and decreased apelin expression, suggesting a possible cross-talk between these pathways and the BMP pathway.

Hereditary hemorrhagic telangiectasia (HHT) is an autosomal dominant vascular disorder characterized by epistaxis, mucocutaneous telangiectases, and arteriovenous malformations (AVM). Two genes are linked to HHT: endoglin (ENG) in HHT1 and activin receptor-like kinase 1 (ACVRL1; ALK1) in HHT2. Although both genes are involved in the transforming growth factor beta signaling pathways, the pathogenetic mechanisms for HHT remain elusive. It was shown that mutations in the Alk1 gene in mice and zebrafish resulted in an embryonic lethal phenotype due to severe dilation of blood vessels. We created a novel null mutant mouse line for Alk1 (Alk1lacZ) by replacing its exons, including the one that encodes the transmembrane domain, with the beta-galactosidase gene. Using Alk1lacZ mice, we show that Alk1 is predominantly expressed in developing arterial endothelium. Alk1 expression is greatly diminished in adult arteries, but is induced in preexisting feeding arteries and newly forming arterial vessels during wound healing and tumor angiogenesis. We also show that hemodynamic changes, which require vascular remodeling, may regulate Alk1 expression. Our studies suggest the role of Alk1 signaling in arterialization and remodeling of arteries. Contrary to the current view of HHT as venous disease, our findings suggest that the arterioles rather than the venules are the primary vessels affected by the loss of an Alk1 allele, and that blood vessels with reduction in Alk1 expression may harbor defects in responding to demands for vascular remodeling.

BACKGROUND

Accurate, cost-effective methods for testing anaplastic lymphoma kinase gene rearrangement (ALK+) are needed to select patients with non-small cell lung carcinoma for ALK-inhibitor therapy. Fluorescent in situ hybridization (FISH) is used to detect ALK+, but it is expensive and not routinely available. We explored the potential of an immunohistochemistry (IHC) scoring system as an affordable, accessible approach.

METHODS

One hundred one samples were obtained from an enriched cohort of never-smokers with adenocarcinoma from the Mayo Clinic Lung Cancer Cohort. IHC was performed using the ALK1 monoclonal antibody with ADVANCE detection system (Dako) and FISH with dual-color, break-apart probe (Abbott Molecular) on formalin-fixed, paraffin-embedded tissue.

RESULTS

Cases were assessed as IHC score 0 (no staining; n = 69), 1+ (faint cytoplasmic staining, n = 21), 2+ (moderate, smooth cytoplasmic staining; n = 3), or 3+ (intense, granular cytoplasmic staining in ≥10% of tumor cells; n = 8). All IHC 3+ cases were FISH+, whereas 1 of 3 IHC 2+ and 1 of 21 IHC 1+ cases were FISH+. All 69 IHC 0 cases were FISH-. Considering FISH a gold-standard reference in this study, sensitivity and specificity of IHC were 90 and 97.8%, respectively, when 2+ and 3+ were regarded as IHC positive and 0 and 1+ as IHC negative.

CONCLUSIONS

IHC scoring correlates with FISH and may be a useful algorithm in testing ALK+ by FISH in non-small cell lung carcinoma, similar to human epidermal growth factor-2 testing in breast cancer. Further study is needed to validate this approach.

The t(2;5)(p23;q35) translocation, associated with anaplastic large-cell lymphoma (ALCL), results in the production of the nucleolar protein nucleophosmin-anaplastic lymphoma kinase (NPM-ALK) protein. This report describes an immunocytochemical study of the distribution of ALK and NPM-ALK proteins using a new monoclonal antibody, ALK1, that recognizes a formalin resistant epitope in both the 80-kD NPM-ALK chimeric and the 200-kD normal human ALK proteins. Cytoplasmic and nuclear labeling was seen in the t(2;5)+ SU-DHL-1 and Karpas 299 cell lines. Normal ALK protein expression was restricted to the central nervous system (in scattered neurons, glial cells, and endothelial cells). Two hundred and thirty-nine cases of lymphoma and 80 nonhematopoietic tumors were immunostained. Antibody ALK1 labeled 53.4% (39 of 73 cases) of CD30+ ALCL. A case of ALCL with a t(1;2) translocation was ALK1+. Three cases of CD30- ALCL with prominent nucleoli showed a unique pattern of coarse granular cytoplasmic labeling. All other tumors, including Hodgkin's disease and lymphomatoid papulosis, were ALK1-. These results indicate that reliable immunostaining of routine biopsy material for NPM-ALK and ALK proteins is feasible. Such analysis is of diagnostic importance, especially because t(2;5)+ ALCL cases have a good prognosis with appropriate treatment.

During osteoarthritis (OA) chondrocytes show deviant behavior resembling terminal differentiation of growth-plate chondrocytes, characterized by elevated MMP-13 expression. The latter is also a hallmark for OA. TGF-beta is generally thought to be a protective factor for cartilage, but it has also displayed deleterious effects in some studies. Recently, it was shown that besides signaling via the ALK5 (activin-like kinase 5) receptor, TGF-beta can also signal via ALK1, thereby activating Smad1/5/8 instead of Smad2/3. The Smad1/5/8 route can induce chondrocyte terminal differentiation. Murine chondrocytes stimulated with TGF-beta activated the ALK5 receptor/Smad2/3 route as well as the ALK1/Smad1/5/8 route. In cartilage of mouse models for aging and OA, ALK5 expression decreased much more than ALK1. Thus, the ALK1/ALK5 ratio increased, which was associated with changes in the respective downstream markers: an increased Id-1 (inhibitor of DNA binding-1)/PAI-1 (plasminogen activator inhibitor-1) ratio. Transfection of chondrocytes with adenovirus overexpressing constitutive active ALK1 increased MMP-13 expression, while small interfering RNA against ALK1 decreased MMP-13 expression to nondetectable levels. Adenovirus overexpressing constitutive active ALK5 transfection increased aggrecan expression, whereas small interfering RNA against ALK5 resulted in increased MMP-13 expression. Moreover, in human OA cartilage ALK1 was highly correlated with MMP-13 expression, whereas ALK5 correlated with aggrecan and collagen type II expression, important for healthy cartilage. Collectively, we show an age-related shift in ALK1/ALK5 ratio in murine cartilage and a strong correlation between ALK1 and MMP-13 expression in human cartilage. A change in balance between ALK5 and ALK1 receptors in chondrocytes caused changes in MMP-13 expression, thereby causing an OA-like phenotype. Our data suggest that dominant ALK1 signaling results in deviant chondrocyte behavior, thereby contributing to age-related cartilage destruction and OA.

Transforming growth factor (TGF)-beta is a multifunctional protein that initiates its diverse cellular responses by binding to and activating specific type I and type II serine/threonine kinase receptors. TGF-beta can act as a regulator of proliferation, migration, survival, differentiation, and extracellular matrix synthesis in endothelial cells and vascular smooth muscle cells, as well as in the maintenance of vascular homeostasis. Importantly, genetic studies in humans have revealed the pivotal role of TGF-beta as well as its signaling components in angiogenesis. Mutations in two TGF-beta receptors (ie, the activin receptor-like kinase (ALK) 1 and the accessory TGF-beta receptor endoglin) have been linked to vascular disorders named hereditary hemorrhagic telangiectasia. In addition, knockout mice for the different components of the TGF-beta signaling pathway have shown that TGF-beta is indispensable for angiogenesis. Recent studies have revealed that TGF-beta can regulate vascular homeostasis by balancing the signaling between two distinct TGF-beta type I receptors (ie, the endothelial-restricted ALK1 and the broadly expressed ALK5 receptors). The activation of these receptors has been shown to induce opposite effects on endothelial cell behavior and angiogenesis. In this review, we will present recent advances in understanding the role of TGF-beta signaling in endothelial cells as well as the underlying molecular mechanisms by which perturbation of this pathway can lead to vascular disorders.

Mutations in transforming growth factor-beta (TGF-beta) receptor superfamily members underlie conditions characterized by vascular dysplasia. Mutations in endoglin and activin-like kinase receptor 1 (ALK1) cause hereditary hemorrhagic telangiectasia, whereas bone morphogenetic protein type II receptor (BMPR-II) mutations underlie familial pulmonary arterial hypertension. To understand the functional roles of these receptors, we examined their relative contributions to BMP signaling in human pulmonary artery endothelial cells (HPAECs). BMP9 potently and selectively induced Smad1/5 phosphorylation and Id gene expression in HPAECs. Contrary to expectations, BMP9 also stimulated Smad2 activation. Furthermore, BMP9 induced the expression of interleukin 8 and E-selectin. Using small interfering RNA, we demonstrate that the type I receptor, ALK1, is essential for these responses. However, small interfering RNA and inhibitor studies showed no involvement of ALK5 or endoglin. We further demonstrate that, of the candidate type II receptors, BMPR-II predominantly mediated IL-8 and E-selectin induction and mitogenic inhibition by BMP9. Conversely, activin receptor type II (ActR-II) contributed more to BMP9-mediated Smad2 activation. Only abolition of both type II receptors significantly reduced the Smad1/5 and Id responses. Both ALK1 and BMPR-II contributed to growth inhibition of HPAECs, whereas ActR-II was not involved. Taken together, our findings demonstrate the critical role of type II receptors in balancing BMP9 signaling via ALK1 and emphasize the essential role for BMPR-II in a subset of BMP9 responses (interleukin 8, E-selectin, and proliferation). This differential signaling may contribute to the contrasting pathologies of hereditary hemorrhagic telangiectasia and pulmonary arterial hypertension.

Hereditary hemorrhagic telangiectasia (Osler-Weber-Rendu syndrome) is a disorder of development of the vasculature characterized by telangiectases and arteriovenous malformations in specific locations. It is one of most common monogenic disorders, but affected individuals are frequently not diagnosed. The most common features of the disorder, nosebleeds, and telangiectases on the lips, hands, and oral mucosa are often quite subtle. Optimal management requires an understanding of the specific presentations of these vascular malformations, especially their locations and timing during life. Telangiectases in the nasal and gastrointestinal mucosa and brain arteriovenous malformations generally present with hemorrhage. However, complications of arteriovenous malformations in the lungs and liver are generally the consequence of blood shunting through these abnormal blood vessels, which lack a capillary bed and thus result in a direct artery-to-vein connection. Mutations in at least five genes are thought to result in hereditary hemorrhagic telangiectasia, but mutations in two genes (ENG and ACVRL1/ALK1) cause approximately 85% of cases. The frequency of arteriovenous malformations in particular organs and the occurrence of certain rare symptoms are dependent on the gene involved. Molecular genetic testing is used to establish the genetic subtype of hereditary hemorrhagic telangiectasia in a clinically affected individual and family, and for early diagnosis to allow for appropriate screening and preventive treatment.

BACKGROUND

Diabetes mellitus is frequently complicated by cardiovascular disease, such as vascular calcification and endothelial dysfunction, which have been associated with bone morphogenetic proteins (BMPs).

OBJECTIVE

To determine whether hyperglycemia in vitro and diabetes in vivo promote vascular BMP activity and correlate with vascular calcification.

RESULTS

Increased glucose augmented expression of BMP-2 and BMP-4; the BMP inhibitors matrix Gla protein (MGP) and Noggin; activin-like kinase receptor (ALK)1, -2, -3 and -6; the BMP type 2 receptor; and the vascular endothelial growth factor in human aortic endothelial cells (HAECs). Diabetes induced expression of the same factors in the aortic wall of 3 animal models of diabetes, Ins2(Akita/+) mice, db/db mice, and HIP rats (rats transgenic for human islet amyloid polypeptide), representative of types 1 and 2 diabetes. Conditioned media from glucose-treated HAECs increased angiogenesis in bovine aortic endothelial cells, as mediated by BMP-4, and osteogenesis in calcifying vascular cells, as mediated by BMP-2. BMP-4, MGP, ALK1, and ALK2 were predominantly expressed on the endothelial side of the aorta, and small interfering RNA experiments showed that these genes were regulated as a group. Diabetic mice and rats showed a dramatic increase in aortic BMP activity, as demonstrated by SMAD1/5/8 phosphorylation. This was associated with increased osteogenesis and calcium accumulation. These changes were prevented in the Ins2(Akita/+) mice by breeding them with MGP transgenic mice, which increased aortic BMP inhibition.

CONCLUSIONS

Hyperglycemia and diabetes activate vascular BMP activity, which is instrumental in promoting vascular calcification and may be limited by increasing BMP inhibition.

Mesenchymal stromal cells (MSC) possess immunosuppressive properties, yet when treated with IFN-gamma they acquire APC functions. To gain insight into MSC immune plasticity, we explored signaling pathways induced by IFN-gamma required for MHC class II (MHC II)-dependent Ag presentation. IFN-gamma-induced MHC II expression in mouse MSC was enhanced by high cell density or serum deprivation and suppressed by TGF-beta. This process was regulated by the activity of the type IV CIITA promoter independently of STAT1 activation and the induction of the IFN regulatory factor 1-dependent B7H1/PD-L1 encoding gene. The absence of direct correlation with the cell cycle suggested that cellular connectivity modulates IFN-gamma responsiveness for MHC II expression in mouse MSC. TGF-beta signaling in mouse MSC involved ALK5 and ALK1 TGF-beta RI, leading to the phosphorylation of Smad2/Smad3 and Smad1/Smad5/Smad8. An opposite effect was observed in human MSC where IFN-gamma-induced MHC II expression occurred at the highest levels in low-density cultures; however, TGF-beta reduced IFN-gamma-induced MHC II expression and its signaling was similar as in mouse MSC. This suggests that the IFN-gamma-induced APC features of MSC can be modulated by TGF-beta, serum factors, and cell density in vitro, although not in the same way in mouse and human MSC, via their convergent effects on CIITA expression.

The transforming growth factor (TGF)-beta/Smad3 signaling pathway is considered a central mediator of pathological organ fibrosis; however, contribution of Smad2/3-independent TGF-beta signaling has not been fully explored. The present study utilized previously a described model of scleroderma (SSc) fibrosis based on forced expression of the TGF-betaRI (ALK5) (Pannu, J., Gardner, H., Shearstone, J. R., Smith, E., and Trojanowska, M. (2006) Arthritis Rheum. 54, 3011-3021). This study was aimed at determining the molecular mechanisms underlying the profibrotic program in this model. We demonstrate that the TGF-betaRI-dependent up-regulation of collagen and CCN2 (CTGF) does not involve Smad2/3 activation but is mediated by ALK1/Smad1 and ERK1/2 pathways. The following findings support this conclusion: (i) Smad2 and -3 were not phosphorylated in response to TGF-betaRI, (ii) a TGF-betaRI mutant defective in Smad2/3 activation, ALK5(3A), potently stimulated collagen production, (iii) elevation of TGF-betaRI triggered sustained association of ALK5 with ALK1 and high levels of Smad1 phosphorylation, (iv) blockade of Smad1 via small interfering RNA abrogated collagen and CCN2 up-regulation in this model, (v) elevated TGF-betaRI led to a prolonged activation of ERK1/2, (vi) the pharmacologic inhibitor of ERK1/2 inhibited Smad1 phosphorylation and abrogated profibrotic effects of elevated TGFbeta-RI. Additional experiments demonstrated that a GC-rich response element located -6 to -16 (upstream of the transcription start site) in the CCN2 promoter mediated Smad1-dependent increased promoter activity in this model. This element was shown previously to mediate up-regulation of the CCN2 promoter in SSc fibroblasts. In conclusion, this study defines a novel ALK1/Smad1- and ERK1/2-dependent, Smad3-independent mode of TGF-beta signaling that may operate during chronic stages of fibrosis in SSc.

The t(2;5)(p23;q35) translocation, associated with anaplastic large-cell lymphoma (ALCL), results in the expression of a chimeric NPM-ALK protein that can be detected by the ALK1 monoclonal antibody. This report describes the morphologic and phenotypic spectrum of 123 cases of lymphoma that all express ALK protein. The results provide strong evidence that the morphologic patterns of ALCL described in previous reports as representing possible subtypes of ALCL, eg, common type, lymphohistiocytic, or small cell patterns, are morphologic variants of the same disease entity. All of these morphologic patterns could be found within this series, and in some patients different subtypes coexisted in a single biopsy or were found in successive biopsies from a single patient. The link between these morphologic subtypes is further reinforced by the presence in all cases of a highly characteristic large cell, with an eccentric nucleus and an eosinophilic paranuclear region. We suggest that this cell can be considered as a major distinguishing feature of ALK-positive lymphomas. Another characteristic of these tumors was the perivascular pattern of neoplastic cell infiltration seen in a significant number of cases. In addition to ALK protein, all tumors expressed epithelial membrane antigen and lacked CD15, features that may be of value in differentiating ALCL from Hodgkin's disease. In the majority of cases (84%), malignant cells showed both a cytoplasmic and nuclear staining for ALK1 and thus presumably carried the 2;5 translocation, but staining was restricted to the cytoplasm in a few cases, suggesting that translocations other than t(2;5) may induce expression of ALK protein. We conclude from this study that ALK-positive neoplasms represent a distinct entity. Because their morphology is often neither anaplastic nor large cell, we suggest that they should henceforward be referred to as ALK lymphomas.

Arteriovenous malformations (AVMs) are fragile direct connections between arteries and veins that arise during times of active angiogenesis. To understand the etiology of AVMs and the role of blood flow in their development, we analyzed AVM development in zebrafish embryos harboring a mutation in activin receptor-like kinase I (alk1), which encodes a TGFβ family type I receptor implicated in the human vascular disorder hereditary hemorrhagic telangiectasia type 2 (HHT2). Our analyses demonstrate that increases in arterial caliber, which stem in part from increased cell number and in part from decreased cell density, precede AVM development, and that AVMs represent enlargement and stabilization of normally transient arteriovenous connections. Whereas initial increases in endothelial cell number are independent of blood flow, later increases, as well as AVMs, are dependent on flow. Furthermore, we demonstrate that alk1 expression requires blood flow, and despite normal levels of shear stress, some flow-responsive genes are dysregulated in alk1 mutant arterial endothelial cells. Taken together, our results suggest that Alk1 plays a role in transducing hemodynamic forces into a biochemical signal required to limit nascent vessel caliber, and support a novel two-step model for HHT-associated AVM development in which pathological arterial enlargement and consequent altered blood flow precipitate a flow-dependent adaptive response involving retention of normally transient arteriovenous connections, thereby generating AVMs.

Abnormalities of chromosome 2p23 with expression of ALK1 and p80 occur in both inflammatory myofibroblastic tumor (IMT) and anaplastic large cell lymphoma. This immunohistochemical study investigates whether the ALK family of neoplasms includes fibroblastic-myofibroblastic, myogenic, and spindle cell tumors. Formalin-fixed paraffin-embedded archival tissues from 10 IMTs and 125 other soft tissue tumors were stained for ALK1 and p80 with standard immunohistochemistry. ALK1 and/or p80 reactivity was observed in a cytoplasmic pattern in IMT (4/10; 40%), malignant peripheral nerve sheath tumor (4/10; 40%), rhabdomyosarcoma (6/31; 19%), leiomyosarcoma (1/10; 10%), and malignant fibrous histiocytoma (1/11; 9%). No staining was observed in nodular fasciitis, desmoid, infantile myofibromatosis, infantile fibrosarcoma, synovial sarcoma, leiomyoma, or myofibrosarcoma. Alveolar rhabdomyosarcomas (4/16; 25%) displayed a distinctive dot-like cytoplasmic positivity. No cases displayed nuclear reactivity. Fluorescent in situ hybridization on 12 of the positive cases revealed a combination of abnormalities including ALK break-apart signals, nucleophosmin (NPM)/ALK fusions, or extra copies of 2p23. This study demonstrates that in addition to IMT, abnormalities of ALK1 and p80 expression with a variety of structural chromosomal changes are found in several sarcomas, especially rhabdomyosarcoma and malignant peripheral nerve sheath tumor. Although immunoreactivity in non-IMTs cannot distinguish between structural abnormalities involving 2p23 or additional copies of 2p23, it supports the concept of ALK involvement in a larger group of neoplasms, some of which have other documented clonal abnormalities. In IMT, immunohistochemistry for ALK1 and p80 is useful as an indicator of a 2p23 abnormality, but it must be interpreted in the context of histologic and other clinicopathologic data if used as an adjunct to differential diagnosis.

Mesenchymal stem cells (MSCs) are bone marrow stromal cells that can differentiate into multiple lineages. We previously demonstrated that BMP9 is one of the most potent BMPs to induce osteogenic differentiation of MSCs. BMP9 is one of the least studied BMPs. Whereas ALK1, ALK5, and/or endoglin have recently been reported as potential BMP9 type I receptors in endothelial cells, little is known about type I receptor involvement in BMP9-induced osteogenic differentiation in MSCs. Here, we conduct a comprehensive analysis of the functional role of seven type I receptors in BMP9-induced osteogenic signaling in MSCs. We have found that most of the seven type I receptors are expressed in MSCs. However, using dominant-negative mutants for the seven type I receptors, we demonstrate that only ALK1 and ALK2 mutants effectively inhibit BMP9-induced osteogenic differentiation in vitro and ectopic ossification in MSC implantation assays. Protein fragment complementation assays demonstrate that ALK1 and ALK2 directly interact with BMP9. Likewise, RNAi silencing of ALK1 and ALK2 expression inhibits BMP9-induced BMPR-Smad activity and osteogenic differentiation in MSCs both in vitro and in vivo. Therefore, our results strongly suggest that ALK1 and ALK2 may play an important role in mediating BMP9-induced osteogenic differentiation. These findings should further aid us in understanding the molecular mechanism through which BMP9 regulates osteogenic differentiation of MSCs.

BACKGROUND

Pulmonary arterial hypertension (PAH) is a progressive disorder characterised by raised pulmonary artery pressures with pathological changes in small pulmonary arteries. Previous studies have shown that approximately 70% of familial PAH and also 11-40% of idiopathic PAH (IPAH) cases have mutations in the bone morphogenetic protein receptor type II (BMPR2) gene. In addition, mutations in the activin receptor-like kinase 1 (ALK1) gene have been reported in PAH patients. Since both the BMPR2 and ALK1 belonging to the transforming growth factor (TGF)-beta superfamily are known to predispose to PAH, mutations in other genes of the TGF-beta/BMP signalling pathways may also predispose to PAH.

METHODS

We screened for mutations in ENDOGLIN(ENG), SMAD1, SMAD2, SMAD3, SMAD4, SMAD5, SMAD6 and SMAD8 genes, which are involved in the TGF-beta/BMP signallings, in 23 patients with IPAH who had no mutations in BMPR2 or ALK1.

RESULTS

A nonsense mutation in SMAD8 designated c.606 C>A, p.C202X was identified in one patient. The father of this patient was also identified as having the same mutation. Functional analysis showed the truncated form of the SMAD8 C202X protein was not phosphorylated by constitutively active ALK3 and ALK1. The SMAD8 mutant was also unable to interact with SMAD4. The response to BMP was analysed using promoter-reporter activities with SMAD4 and/or ca-ALK3. The transcriptional activation of the SMAD8 mutant was inefficient compared with the SMAD8 wild type.

CONCLUSIONS

We describe the first mutation in SMAD8 in a patient with IPAH. Our findings suggest the involvement of SMAD8 in the pathogenesis of PAH.

Members of the transforming growth factor beta (TGF-beta) family have been genetically linked to vascular formation during embryogenesis. However, contradictory studies about the role of TGF-beta and other family members with reported vascular functions, such as bone morphogenetic protein (BMP) 9, in physiological and pathological angiogenesis make the need for mechanistic studies apparent. We demonstrate, by genetic and pharmacological means, that the TGF-beta and BMP9 receptor activin receptor-like kinase (ALK) 1 represents a new therapeutic target for tumor angiogenesis. Diminution of ALK1 gene dosage or systemic treatment with the ALK1-Fc fusion protein RAP-041 retarded tumor growth and progression by inhibition of angiogenesis in a transgenic mouse model of multistep tumorigenesis. Furthermore, RAP-041 significantly impaired the in vitro and in vivo angiogenic response toward vascular endothelial growth factor A and basic fibroblast growth factor. In seeking the mechanism for the observed effects, we uncovered an unexpected signaling synergy between TGF-beta and BMP9, through which the combined action of the two factors augmented the endothelial cell response to angiogenic stimuli. We delineate a decisive role for signaling by TGF-beta family members in tumor angiogenesis and offer mechanistic insight for the forthcoming clinical development of drugs blocking ALK1 in oncology.

Transforming growth factor-beta (TGF-beta) signaling in endothelial cells is able to modulate angiogenesis and vascular remodeling, although the underlying molecular mechanisms remain poorly understood. Endoglin and ALK-1 are components of the TGF-beta receptor complex, predominantly expressed in endothelial cells, and mutations in either endoglin or ALK-1 genes are responsible for the vascular dysplasia known as hereditary hemorrhagic telangiectasia. Here we find that the extracellular and cytoplasmic domains of the auxiliary TGF-beta receptor endoglin interact with ALK-1 (a type I TGF-beta receptor). In addition, endoglin potentiates TGF-beta/ALK1 signaling, with the extracellular domain of endoglin contributing to this functional cooperation between endoglin and ALK-1. By contrast, endoglin appears to interfere with TGF-beta/ALK-5 signaling. These results suggest that the functional association of endoglin with ALK-1 is critical for the endothelial responses to TGF-beta.

Mutations in two receptors of the transforming growth factor-beta family have recently been shown to be present in the majority of cases of inherited (familial) pulmonary arterial hypertension (PAH). Study of the biology of these receptors, bone morphogenetic protein receptor type-2 (BMPR2), and activin-like kinase type-1 (ALK-1) will certainly reveal pathogenic mechanisms of disease. Exonic mutations in BMPR2 are found in about 50% of patients with familial PAH, and ALK1 mutations are found in a minority of patients with hereditary hemorrhagic telangiectasia and co-existent PAH. Because familial PAH is highly linked to chromosome 2q33, it is likely that the remaining 50% of family cases without exonic mutations have either intronic BMPR2 abnormalities or alterations in the promoter or regulatory genes. Also, only about 10% of patients with "sporadic" idiopathic PAH have identifiable BMPR2 mutations. Mutations in BMPR2 confer a 15% to 20% chance of developing PAH in a carrier's lifetime. Thus, there must be gene-gene or gene-environment interactions that either enhance or prevent the development of the vascular disease in persons carrying a mutation, and there must be other patterns of susceptibility based on genetic makeup. To elucidate the genetic basis of PAH further, investigations are needed, including genome scanning for major and minor genes, analysis of genetic profiles of patients for candidate genes likely to modify risk for disease (e.g., serotonin transporter alleles, nitric oxide-synthases), proteomics, transgenic mice, and altered signal transduction. Advances in genetic testing, presymptomatic screening, and biomarkers should permit early detection of disease in those at risk of PAH and allow trials of preventive therapy in carriers.