Endoglin, a TGF-beta binding protein of endothelial cells, is the gene for hereditary haemorrhagic telangiectasia type 1

Mapping epitopes to distinct regions of the extracellular domain of endoglin using bacterially expressed recombinant fragments

Endoglin (CD105) is a homodimeric cell surface component of the TGF-beta 1 receptor complex, which is expressed at high levels on vascular endothelium and at lower levels on activated monocytes. It is also the target gene for the dominantly inherited vascular disorder hereditary hemorrhagic telangiectasia type 1. To date, each family has a distinct endoglin mutation, most of which generate premature stop codons. The purpose of the current study was to identify monoclonal antibodies capable of binding to normal and mutated forms of the protein. We generated stable transfectants of full-length human endoglin in murine fibroblasts and engineered and expressed in bacteria several fragments of the extracellular domain. Relatively pure polypeptides were recovered with good yield from inclusion bodies and were tested by ELISA and Western blot; 11 monoclonal antibodies were shown to react specifically with the endoglin transfectants. Ten of these monoclonal antibodies reacted with the bacterial fragments, and their epitopes were assigned to 3 distinct regions of endoglin. Monoclonal antibodies P3D1, TEC4 and GRE reacted with the N-terminal region of 204 amino acids encoded by exons 1 to 5. Monoclonal antibodies P4A4, 44G4, E-9, MAEND3 and PN-E2 all bound to a region of 54 amino acids encoded mostly by exon 7. Monoclonal antibodies CLE4 and RMAC8 reacted with the C-terminal region of the extracellular domain, coded for by exons 8 to 12. Knowing the localization of these epitopes will facilitate the structural and functional analysis of normal and mutated forms of endoglin.

Growth/differentiation factor-5 induces angiogenesis in vivo

Bone morphogenetic proteins (BMPs) are multifunctional cytokines, which induce bone and cartilage formation and exert various other effects on many tissues. Since angiogenesis is involved in the bone formation process, certain members in the BMP family may induce angiogenesis. We examined the in vivo angiogenic activity of BMP family members, i.e., growth/differentiation factor (GDF)-5 and BMP-2. GDF-5 induced angiogenesis in both chick chorioallantoic membrane and rabbit cornea assays. In contrast, BMP-2 did not induce angiogenesis. In order to elucidate the mechanism of angiogenesis, we examined the effects of GDF-5 on cultured bovine aortic endothelial cells (BECs). GDF-5 induced plasminogen activator activity and accelerated the migration of BECs in a chemotactic fashion, which may contribute to the process of angiogenesis in vivo. These results suggest that GDF-5 is one of the molecules which induce angiogenesis in the bone formation process.

The use of recombinant vaccinia virus to generate monoclonal antibodies against the cell-surface glycoprotein endoglin

Characterization of novel cell-surface protein molecules, initially identified by cDNA cloning techniques, usually requires the generation of specific antibodies to further analyze their biochemical and/or functional properties. Here we report a simple method, using recombinant vaccinia virus, for the generation of monoclonal antibodies (mAb) to the cell-surface antigen endoglin. A recombinant vaccinia virus carrying a cDNA encoding human endoglin was inserted into the thymidine kinase locus under the control of the 7.5k vaccinia virus promoter. Infection of Balb/c mice with this recombinant virus led to the generation of specific polyclonal antibodies, as demonstrated by the antisera reactivity against human endoglin transfectants. The spleen cells of these infected animals were fused to myeloma cells, allowing efficient generation of several hybridomas which secrete mAbs to human endoglin, as evidenced by their reactivity with purified endoglin as well as with endoglin transfectants. Some of the mAbs selected seem to be specific for regions of endoglin conserved among different species as evidenced by their cross-reactivity with chicken endoglin. These results underline the utility of recombinant vaccinia virus to generate antibodies with novel properties to new cell surface proteins such as endoglin.

Expression of ALK-1, a type 1 serine/threonine kinase receptor, coincides with sites of vasculogenesis and angiogenesis in early mouse development

ALK-1 is a type I serine/threonine kinase receptor for members of the TGF-beta superfamily of growth factors; its endogenous ligand is not known. In this study, we have analyzed the temporal and spatial expression pattern of ALK-1 mRNA in mouse embryos from the one-cell zygote until 12.5 dpc using RT-PCR and in situ hybridization. ALK-1 mRNA was first detected in the embryo at 6.5 dpc. From 7.5-8.5 dpc expression was highest at sites of vasculogenesis in both the embryonic and extraembryonic part of the conceptus, in trophoblast giant cells, and in the endothelial lining of the blood vessels in the decidua. From 9.5-12.5 dpc, ALK-1 was found to be expressed in several different tissues and organs, but was highest in blood vessels, mesenchyme of the lung, submucosal layer of the stomach and intestines, and at specific sites of epithelial-mesenchymal interactions. Its expression pattern suggests that ALK-1 is a type I receptor for TGF-beta1 in the developing mouse.

Heterogeneity of endothelial cells. Specific markers

During embryonic development, endothelial cells differentiate from a common precursor called angioblast and acquire organ-specific properties. One of the important determinants of endothelial cell differentiation is the local environment, and especially the interaction with surrounding cells. This interaction may occur through the release of soluble cytokines, cell-to-cell adhesion and communication, and the synthesis of matrix proteins on which the endothelium adheres and grows. The acquisition and maintenance of specialized properties by endothelial cells is important in the functional homeostasis of the different organs. For instance, in the brain, alteration of the blood-brain barrier properties may have important consequences on brain functional integrity. One of the major limitations to the study of endothelial cell heterogeneity is the fact that these cells are still difficult to isolate and culture from the microcirculation of different organs, and once in culture, they tend to lose their specialized properties. This finding suggests that we have to develop new culture systems, which possibly include coculture with other cell types. An important issue is to develop tools that can help in recognizing endothelial cells and their differentiated phenotype both in vivo and in tissue culture. In this review we give a short overview of the differentiated properties of the endothelium, considering a few examples of highly specialized endothelial cells, such as the brain or bone marrow microcirculation or high endothelial venules. We made a particular effort to list the most common markers of endothelial cell phenotypes. These molecules and related antibodies may be valuable tools for endothelial cell isolation and characterization.

The activin receptor-like kinase 1 gene: genomic structure and mutations in hereditary hemorrhagic telangiectasia type 2

The activin receptor-like kinase 1 gene (ALK-1) is the second locus for the autosomal dominant vascular disease hereditary hemorrhagic telangiectasia (HHT). In this paper we present the genomic structure of the ALK-1 gene, a type I serine-threonine kinase receptor expressed predominantly in endothelial cells. The coding region is contained within nine exons, spanning < 15 kb of genomic DNA. All introns follow the GT-AG rule, except for intron 6, which has a TAG/gcaag 5' splice junction. The positions of introns in the intracellular domain are almost identical to those of the mouse serine-threonine kinase receptor TSK-7L. By sequencing ALK-1 from genomic DNA, mutations were found in six of six families with HHT either shown to link to chromosome 12q13 or in which linkage of HHT to chromosome 9q33 had been excluded. Mutations were also found in three of six patients from families in which available linkage data were insufficient to allow certainty with regard to the locus involved. The high rate of detection of mutations by genomic sequencing of ALK-1 suggests that this will be a useful diagnostic test for HHT2, particularly where preliminary linkage to chromosome 12q13 can be established. In two cases in which premature termination codons were found in genomic DNA, the mutant mRNA was either not present or present at barely detectable levels. These data suggest that mutations in ALK-1 are functionally null alleles.

Characterization of endoglin and identification of novel mutations in hereditary hemorrhagic telangiectasia

To identify mutations that cause hereditary hemorrhagic telangiectasia (HHT, or Rendu-Osler-Weber syndrome), clinical evaluations and genetic studies were performed on 32 families. Linkage studies in four of eight families indicated an endoglin (ENG) gene mutation. ENG sequences of affected members of the four linked families and probands from the 24 small families were screened for mutations, by Southern blot analyses and by cycle sequencing of PCR-amplified DNA. Seven novel mutations were identified in eight families. Two mutations (a termination codon in exon 4 and a large genomic deletion extending 3' of intron 8) did not produce a stable ENG transcript in lymphocytes. Five other mutations (two donor splice-site mutations and three deletions) produce altered mRNAs that are predicted to encode markedly truncated ENG proteins. Mutations in other families are predicted to lie in ENG-regulatory regions or in one of the additional genes that may cause HHT. These data suggest that the molecular mechanism by which ENG mutations cause HHT is haploinsufficiency. Furthermore, because the clinical manifestation of disease in these eight families was similar, we hypothesize that phenotypic variation of HHT is not related to a particular ENG mutation.

TGF-beta and the endothelium during immune injury

During immune injury, activation of endothelial cells by inflammatory cytokines stimulates leukocyte adhesion to the endothelium, turns the endothelium from an anticoagulant surface to one that is frankly procoagulant, and results in the release of vasoactive mediators and growth factors. Cytokine activation of endothelial cells also results in increased endothelial cell TGF-beta 1 synthesis and enhanced activation of latent TGF-beta, the latter involving a shift of plasmin production from the apical to subendothelial surface. In cytokine-stimulated endothelial cells, TGF-beta hinders leukocyte adhesion and transmigration via inhibition of IL-8 and E-selectin expression. TGF-beta also profoundly diminishes cytokine-stimulated inducible nitric oxide synthase production and instead augments endothelial nitric oxide synthase expression. Thus, some of the TGF-beta actions on endothelium during immune activation can viewed as immunosuppressive. TGF-beta also influences mechanisms of vascular remodeling during the healing phase of immune injury. It stimulates PDGF-B synthesis by endothelial cells, causes bFGF release from subendothelial matrix, and promotes VEGF synthesis by non-endothelial cells. Together these mediators control angiogenesis, a critical component of the vascular repair phenomenon. Further, endothelial cell derived PDGF-B and bFGF influence the proliferation and migration of neighboring cells. Thus, endothelial cells and TGF-beta actions on the endothelium play important roles both during the initial phase of immune injury and during the later remodeling phase.

Endoglin gene polymorphism as a risk factor for sporadic intracerebral hemorrhage

Intracerebral hemorrhage (ICH) is a common and serious type of stroke. Recent studies have shown that inherited factors that affect the development of the vessel wall can increase the risk of ICH. We studied endoglin as a candidate gene in patients with sporadic ICH, since mutations in this gene can cause telangiectasia formation. One hundred three patients with sporadic ICH and 202 controls were studied. The polymerase chain reaction and single-strand conformational polymorphism analysis were used to screen for mutations in exon 7 of the endoglin gene. No coding mutations in exon 7 were identified in the ICH patients or controls. A 6-base intronic insertion was found 26 bases beyond the 3' end of exon 7. The homozygous form of the insertion was present in 9 of 103 (8.7%) ICH patients compared with 4 of 202 (2.0%) controls, p = 0.012 (odds ratio 4.8 [95% confidence interval, 1.28, 21.60]). Analysis of the endoglin transcript around the insertion did not reveal any changes in the RNA sequence. There were no obvious clinical features that distinguished the ICH patients with the homozygous insertion from the other patients. The pathophysiologic mechanism underlying this association remains to be determined.

Biology of TGF-beta in knockout and transgenic mouse models

This paper reviews the basic biology and biochemistry of the TGF-beta isoforms including their unique serine-threonine receptors and signaling intermediates. Dysregulation of TGF-beta expression and/or receptor/signaling function have been implicated in a wide variety of pathologies. We will discuss mechanisms underlying some of these disease processes as gained from study of transgenic mice in which expression of TGF-beta 1 has either been lost by targeted deletion of its gene, is overexpressed in a tissue-specific manner, or blocked by its latency associated peptide.

Recurrent epistaxes in hereditary haemorrhagic telangiectasia

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TGF beta in prostate cancer: a growth inhibitor that can enhance tumorigenicity

A common feature of cancer cells is the autocrine production of growth promoters and the loss of function of tumor suppressors. In our search for such features of prostate cancer, we discovered that transforming growth factor beta 1 (TGF beta 1) levels are higher in prostate cancer than in normal prostate, and prostate cancer cells can activate endogenously-produced latent TGF beta to a bioactive form. Because TGF beta 1 is a potent growth inhibitor of epithelial cells, it seems paradoxical that malignant epithelial cells make high levels of a growth inhibitor. Even prostate cancer cells can be growth-inhibited by TGF beta 1, but only under specific conditions in vitro (plating at low cell density in serum-free medium), and this response is readily disrupted by growth factors, serum, and extracellular matrix, to all of which the cells are exposed in vivo. This explains why prostate cancer cells are resistant to the growth-inhibitory effect of TGF beta in vivo. In vivo, TGF beta 1 actually enhances prostate tumor growth and metastasis, but not by affecting tumor cell proliferation directly. One possibility is that TGF beta affects the host to allow increased numbers of tumor cells to survive and produce progeny. In addition, since prostate cancer cells can still respond to TGF beta, e.g., by increased cell motility, even under conditions that prevent growth inhibition, the ability of TGF beta to enhance tumorigenicity in vivo might also occur via direct effects on the tumor cells themselves. I will discuss new developments in our understanding of TGF beta action, which provide a framework for elucidating the mechanism by which prostate cancer cells have devised a way to protect themselves from being growth-inhibited by TGF beta 1 in vivo. Since the cells retain the ability to be growth-inhibited by TGF beta, indicating that the TGF beta receptors and signaling pathways for growth inhibition are intact, albeit inactive, it might be possible to reactivate this pathway to achieve a therapeutic benefit in vivo.

Transforming growth factor-beta: vasculogenesis, angiogenesis, and vessel wall integrity

Genetic studies have recently revealed a role for transforming growth factor-beta-1 (TGF-beta 1) and its receptors (TGF-beta Rs I and II as well as endoglin) in embryonic vascular assembly and in the establishment and maintenance of vessel wall integrity. The purpose of this review is threefold: first, to reassess previous studies on TGF-beta and endothelium in the light of these recent findings; second, to describe some of the well-established as well as controversial issues concerning TGF-beta and its regulatory role in angiogenesis; and third, to explore the notion of "context' with respect to TGF-beta and endothelial cell function. Although the focus of this review will be on the endothelium, other vascular wall cells are also likely to be important in the pathogenesis of the vascular lesions revealed by genetic studies.

Mapping of a major genetic modifier of embryonic lethality in TGF beta 1 knockout mice

The transforming growth factor beta 1 (TGF beta 1) signalling pathway is important in embryogenesis and has been implicated in hereditary haemorrhagic telangiectasia (HHT), atherosclerosis, tumorigenesis and immunomodulation. Therefore, identification of factors which modulate TGF beta 1 bioactivity in vivo is important. On a mixed genetic background, approximately 50% Tgfb1-/- conceptuses die midgestation from defective yolk sac vasculogenesis. The other half are developmentally normal but die three weeks postpartum. Intriguingly, the vascular defects of Tgfb1-/- mice share histological similarities to lesions seen in HHT patients. It has been suggested that dichotomy in Tgfb1-/- lethal phenotypes is due to maternal TGF beta 1 rescue of some, but not all, Tgfb1-/- embryos12. Here we show that the Tgfb1-/- phenotype depends on the genetic background of the conceptus. In NIH/Ola, C57BL/6J/Ola and F1 conceptuses, Tgfb1-/- lethality can be categorized into three developmental classes. A major codominant modifier gene of embryo lethality was mapped to proximal mouse chromosome 5, using a genome scan for non-mendelian distribution of alleles in Tgfb1-/- neonatal animals which survive prenatal lethality. This gene accounts for around three quarters of the genetic effect between mouse strains and can, in part, explain the distribution of the three lethal phenotypes. This approach, using neonatal DNA samples, is generally applicable to identification of loci that influence the effect of early embryonic lethal mutations, thus furthering knowledge of genetic interactions that occur during early mammalian development in vivo.

Hepatic vascular malformations in hereditary hemorrhagic telangiectasia: Doppler sonographic screening in a large family

BACKGROUND/AIMS

The prevalence of hepatic vascular malformations in hereditary hemorrhagic telangiectasia has been estimated in the literature on clinical criteria, thus giving unreliable data. In our study the presence of hepatic vascular malformations in hereditary hemorrhagic telangiectasia was evaluated in a large Italian family by using Doppler sonography findings were compared to computed tomography and angiography results. Clinical features were related to the severity of hepatic vascular malformations.

METHODS

Seventy-three relatives were checked for the presence of signs of hereditary hemorrhagic telangiectasia. Abdominal Doppler ultrasonography was performed in all of them. Every subject with a positive Doppler ultrasonography for hepatic vascular malformations underwent abdominal computed tomography and celiac angiography.

RESULTS

Forty family members proved to be affected by hereditary hemorrhagic telangiectasia. Of these, hepatic vascular malformations were evidenced by Doppler ultrasonography in 13 females. Doppler ultrasongraphy demonstrated minimal hepatic vascular abnormalities in three subjects, moderate in three, and severe in seven. Doppler study was diagnostic for arteriovenous shunt with hepatic veins in seven cases and with portal vein in two. Computed tomography failed to demonstrate hepatic vascular malformations in two cases, while angiography confirmed the Doppler sonographic findings in all cases. Cholestasis was present in subjects with moderate and severe hepatic vascular malformations.

CONCLUSIONS

Doppler sonography is the ideal imaging technique to screen hereditary hemorrhagic telangiectasia affected families for hepatic vascular malformations. These malformations do not appear to be age-dependent, but sex-dependent. Cholestasis is the main clinical sign, and it seems to correlate with the severity of hepatic vascular derangement.

Transforming growth factor beta s and wound healing

The Transforming Growth Factor beta superfamily (TGF beta) is one of the most complex groups of cytokines with widespread effects on many aspects of growth and development. The TGF beta isoforms and other family members, e.g. Activins and BMPs, have diverse effects in similar physiological situations. TGF beta is involved in the wound healing process. The three mammalian isoforms (TGF beta 1, 2 and 3) and recently other family members, e.g. Activin, have been localised in healing wounds. Manipulation of the ratios of TGF beta superfamily members, particularly the ratio of TGF beta 1 relative to TGF beta 3, reduces scarring and fibrosis. Such manipulations include reducing the levels of TGF beta 1/TGF beta 2 using neutralising antibodies or preventing the activation of TGF beta s. In chronic or impaired wounds the exogenous addition of TGF beta superfamily members accelerates aspects of the healing process. This review summarises evidence for the role of TGF beta superfamily members in wound healing and how modulation of TGF beta levels can prevent scarring and fibrosis.

Hereditary hemorrhagic telangiectasia: what the otolaryngologist should know

Hereditary hemorrhagic telangiectasia (HHT) or Rendu-Osler-Weber disease is a systemic autosomal dominant disorder involving blood vessels. Phenotypically, the disease presents with telangiectases that involve all areas of the body. Ninety percent of patients experience epistaxis and are referred to the otolaryngologist for evaluation. Because otolaryngologists may be the primary physicians caring for these patients, it is critical they be knowledgeable about high risk groups, screening protocols for arteriovenous malformations, antibiotic prophylaxis, and genetic screening. It is important that they be aware of the many therapeutic modalities available for the treatment of epistaxis. In this article, the diagnosis, screening, treatment, and molecular genetics of HHT will be discussed. In addition, our experience with 20 patients treated with the Nd:YAG laser for recurrent epistaxis will be reviewed.

Vascular dysmorphogenesis caused by an activating mutation in the receptor tyrosine kinase TIE2

Venous malformations (VMs), the most common errors of vascular morphogenesis in humans, are composed of dilated, serpiginous channels. The walls of the channels have a variable thickness of smooth muscle; some mural regions lack smooth muscle altogether. A missense mutation resulting in an arginine-to-tryptophan substitution at position 849 in the kinase domain of the receptor tyrosine kinase TIE2 segregates with dominantly inherited VM in two unrelated families. Using proteins expressed in insect cells, we demonstrate that the mutation results in increased activity of TIE2. We conclude that an activating mutation in TIE2 causes inherited VMs in the two families and that the TIE2 signaling pathway is critical for endothelial cell-smooth muscle cell communication in venous morphogenesis.

Signal transduction by members of the transforming growth factor-beta superfamily

Transforming growth factor-beta (TGF beta) superfamily members exert their diverse biological effects through their interaction with heteromeric receptor complexes of transmembrane serine/threonine kinases. Both components of the receptor complex, known as receptor I and receptor II are essential for signal transduction. The composition of these complexes can vary significantly due to the promiscuous nature of the ligands and the receptors, and this diversity of interactions can yield a variety of biological responses. Several receptor interacting proteins and potential mediators of signal transduction have now been identified. Recent advances, particularly in our understanding of the function of Mothers against dpp-related (MADR) proteins, are providing new insights into how the TGF beta superfamily signals its diverse biological activities.

Binding affinity of transforming growth factor-beta for its type II receptor is determined by the C-terminal region of the molecule

Transforming growth factor-beta (TGF-beta) isoforms have differential binding affinities for the TGF-beta type II receptor (TbetaRII). In most cells, TGF-beta1 and TGF-beta3 bind to TbetaRII with much higher affinity than TGF-beta2. Here, we report an analysis of the effect of TGF-beta structure on its binding to TbetaRII by using TGF-beta mutants with domain deletions, amino acid replacements, and isoform chimeras. Examination of the binding of TGF-beta mutants to the recombinant extracellular domain of TbetaRII by a solid-phase TGF-beta/TbetaRII assay demonstrated that only those TGF-beta mutants containing the C terminus of TGF-beta1 (TGF-beta1-(Delta69-73), TGF-beta1-(Trp71), and TGF-beta2/beta1-(83-112)) bind with high affinity to TbetaRII, similar to native TGF-beta1. Moreover, replacement of only 6 amino acids in the C terminus of TGF-beta1 with the corresponding sequence of TGF-beta2 (TGF-beta1/beta2-(91-96)) completely eliminated the high affinity binding of TGF-beta1. Proliferation of fetal bovine heart endothelial (FBHE) cells was inhibited to a similar degree by all of the TGF-beta mutants. However, recombinant soluble TbetaRII blocked the inhibition of FBHE cell proliferation induced by TGF-beta mutants retaining the C terminus of TGF-beta1, consistent with the high binding affinity between these TGF-beta molecules and TbetaRII. It was further confirmed that the TGF-beta2 mutant with its C terminus replaced by that of TGF-beta1 (TGF-beta2/beta1-(83-112)) competed as effectively as TGF-beta1 with 125I-TGF-beta1 for binding to membrane TbetaRI and TbetaRII on FBHE cells. These observations clearly indicate that the domain in TGF-beta1 responsible for its high affinity binding to TbetaRII, both the soluble and membrane-bound forms, is located at C terminus of the molecule.

Pulmonary arteriovenous malformations: diagnosis and transcatheter embolotherapy

The recent long-term studies from England, France, and the Netherlands, as well as our own, indicate that transcatheter embolotherapy is definitive treatment for PAVM. More recently, Puskas et al have questioned transcatheter embolotherapy as a primary treatment for patients with PAVM (4,56). Their opinion was based on two recurrences among five patients treated with transcatheter embolotherapy. It is not clear why one of the late recurrences in the series by Puskas et al happened, and the other recurrence could have been dut to early deflation of the balloon. Nevertheless, we believe that the collective experience in the larger series reporting on transcatheter embolotherapy of PAVM supports the use of embolotherapy as a primary modality of treatment. Because many patients have bilateral pulmonary malformations and many pulmonary malformations will grow with time, repeated surgical intervention is not ideal therapy. The recurrence rate of 8% reported by Remy et al using coils, and 2% reported by Pollak et al using balloons and coils supports our contention that transcatheter embolotherapy is durable and should be the initial treatment. Also, recurrences are easily retreated by transcatheter embolotherapy with durable results (54). We favor detachable balloons over coils for occluding PAVMs because immediate cross-sectional occlusion of the segmental artery is obtained in a position that preserves the most normal branches. The necessity for repeated introduction of coils, when using the coil method, contributes to longer procedure times with an increased risk of air introduction and, in our experience, a greater risk of postprocedure pleurisy. At the same time, we appreciate that approximately 70% of PAVMs can be occluded equally well with balloons or coils. We also believe that coils have unique advantages over balloons in specific anatomic situations including oversized arteries (where coils are the only option) and for occlusion of the aneurysm of a PAVM. As with all forms of embolotherapy, the interventionalist is best served by having more than one option of treatment, which for PAVM includes both balloons and coils. In summary, PAVMs are effectively managed by means of transcatheter embolotherapy. This therapy has been demonstrated to be safe and durable. Careful technique with modifications depending on the angioarchitecture of the PAVM is required. Patients with PAVMs require follow-up at 1 month and 1 year. While observations documenting serial growth of small PAVMs are somewhat limited, there is published evidence to support their growth with time (35,36). Because of these reports and our unpublished observations, we believe that patients with treated PAVM need long-term follow-up every 5 years to detect growth of small PAVMs that will ultimately reach a size where they may cause paradoxical embolization and stroke (1).

Endoglin modulates cellular responses to TGF-beta 1

Endoglin is a homodimeric membrane glycoprotein which can bind the beta 1 and beta 3 isoforms of transforming growth factor-beta (TGF-beta). We reported previously that endoglin is upregulated during monocyte differentiation. We have now observed that TGF-beta itself can stimulate the expression of endoglin in cultured human monocytes and in the U-937 monocytic line. To study the functional role of endoglin, stable transfectants of U-937 cells were generated which overexpress L- or S- endoglin isoforms, differing in their cytoplasmic domain. Inhibition of cellular proliferation and downregulation of c-myc mRNA which are normally induced by TGF-beta 1 in U-937 cells were totally abrogated in L-endoglin transfectants and much reduced in the S-endoglin transfectants. Inhibition of proliferation by TGF-beta 2 was not altered in the transfectants, in agreement with the isoform specificity of endoglin. Additional responses of U-937 cells to TGF-beta 1, including stimulation of fibronectin synthesis, cellular adhesion, platelet/endothelial cell adhesion molecule 1 (PECAM-1) phosphorylation, and homotypic aggregation were also inhibited in the endoglin transfectants. However, modulation of integrin and PECAM-1 levels and stimulation of mRNA levels for TGF-beta 1 and its receptors R-I, R-II, and betaglycan occurred normally in the endoglin transfectants. No changes in total ligand binding were observed in L-endoglin transfectants relative to mock, while a 1.5-fold increase was seen in S-endoglin transfectants. The degradation rate of the ligand was the same in all transfectants. Elucidating the mechanism by which endoglin modulates several cellular responses to TGF-beta 1 without interfering with ligand binding or degradation should increase our understanding of the complex pathways which mediate the effects of this factor.

Mutations in the activin receptor-like kinase 1 gene in hereditary haemorrhagic telangiectasia type 2

Hereditary haemorrhagic telangiectasia, or Osler-Rendu-Weber (ORW) syndrome, is an autosomal dominant vascular dysplasia. So far, two loci have been demonstrated for ORW. Linkage studies established an ORW locus at chromosome 9q3; endoglin was subsequently identified as the ORW1 gene. A second locus, designated ORW2, was mapped to chromosome 12. Here we report a new 4 cM interval for ORW2 that does not overlap with any previously defined. A 1.38-Mb YAC contig spans the entire interval. It includes the activin receptor like kinase 1 gene (ACVRLK1 or ALK1), a member of the serine-threonine kinase receptor family expressed in endothelium. We report three mutations in the coding sequence of the ALK1 gene in those families which show linkage of the ORW phenotype to chromosome 12. Our data suggest a critical role for ALK1 in the control of blood vessel development or repair.

Hereditary haemorrhagic telangiectasia with extensive liver involvement is not caused by either HHT1 or HHT2

Hereditary haemorrhagic telangiectasia (HHT) is a genetically heterogeneous dominant disorder. Two disease loci have been mapped to chromosomes 9q3 and 12q. In a large pedigree, with an unusually high number of patients with liver vascular malformations, both previously mapped loci have been excluded. The loci for two other inherited vascular malformation diseases, cerebral cavernous malformations and multiple cutaneous and mucosal venous malformations, have also been excluded. Thus we conclude that at least a third, as yet unmapped, HHT locus does exist, possibly associated with high frequency of liver involvement.

Molecular genetic insights into cardiovascular disease

The recent application of molecular genetic tools to inherited forms of cardiovascular disease has provided important insight into the molecular mechanisms underlying cardiac arrhythmias, cardiomyopathies, and vascular diseases. These studies point to defects in ion channels, contractile proteins, structural proteins, and signaling molecules as key players in disease pathogenesis. Genetic testing is now available for a subset of inherited cardiovascular diseases, and new mechanism-based therapies may be available in the near future. This remarkable progress and the implications it may have for more common forms of cardiovascular disease are reviewed here.

Expression of angiogenic factors and structural proteins in central nervous system vascular malformations

Little is known of the molecular mechanisms mediating the genesis and subsequent biological behavior of central nervous system vascular malformations. The role of angiogenic and permeability-inducing factors in the pathogenesis of these lesions has not bee previously explored. In this study, we subject specimens from 12 cases of excised vascular malformation to a battery of immunostaining for vascular endothelial growth factor, basic fibroblast growth factor, and selected structural and matrix proteins. The lesions consisted of seven arteriovenous malformations (AVMs), including one angiographically occult AVM, one arterialized vein from a dural AVM, and five cavernous malformations (CMs). Vascular endothelial growth factor was expressed by all lesions and was localized predominantly in the subendothelial layer and in perivascular spaces. Four of seven AVMs and four of five CMs demonstrated faint basic fibroblast growth factor expression that was localized to the media of AVM vessels and the subendothelial layer and intercavernous matrix of CMs. This pattern of angiogenic factor immunostaining was correlated with the expression of structural and matrix proteins in the same lesions. Laminin was not expressed in any of the CMs, confirming previous reports from our laboratory. By contrast, fibronectin expression was more prominent in CMs than in AVMs. Collagen Type IV and alpha smooth muscle actin expression occurred in every lesion. We conclude that angiogenic growth factors are expressed in all types of vascular malformations of the central nervous system. The pattern of expression suggests diffuse activation of angiogenesis without specific relation to individual vessel types or recent clinical behavior. Defining the role of angiogenesis in vascular malformations might provide insight into their pathogenesis and suggest novel strategies for modification of their behavior.

Hepatopulmonary syndrome. A pulmonary vascular complication of liver disease

Hepatopulmonary syndrome is part of the spectrum of pulmonary vascular disorders seen in advanced liver disease. The pathophysiology of these entities likely is dependent on the degree of pulmonary vasoconstriction or vasodilation that occurs. Our understanding of hepatopulmonary syndrome has helped further our knowledge of the interaction of the liver and the lung. Advances in the management of this disorder, especially liver transplantation, finally have allowed us to offer some hope to patients with this disease.

Clinical heterogeneity in hereditary haemorrhagic telangiectasia: are pulmonary arteriovenous malformations more common in families linked to endoglin?

Pulmonary arteriovenous malformations (PAVMs) occur in up to 27% of patients with hereditary haemorrhagic telangiectasia (HHT) and are associated with a rate of paradoxical cerebral embolism at presentation of up to 36%. At least two different loci have been shown for HHT. Mutations in endoglin have been found in some families and the locus designated ORW1. In other families this locus has been excluded. In this paper we confirm that in families linked to ORW1 there is a prevalence of PAVMs among affected members of 29.2%, compared to a prevalence of 2.9% in families in which this locus has been excluded (chi 2 = 19.2, p < 0.001). This information can be used to decide how to screen HHT patients for PAVMs.

Hepatocyte growth factor releases epithelial and endothelial cells from growth arrest induced by transforming growth factor-beta1

Human lung fibroblasts and Mv1Lu mink lung epithelial cells were used as a model to study the role of extracellular matrix in epithelial-mesenchymal interactions. Extracellular matrices of fibroblasts were found to contain growth promoting activity that reduced the sensitivity of Mv1Lu cells to the growth inhibitory effects of transforming growth factor-beta (TGF-beta). The majority of the activity was identified as hepatocyte growth factor/scatter factor (HGF) by inhibition with specific antibodies and by reconstitution of the effect by recombinant HGF. HGF induced cell proliferation when contact-inhibited Mv1Lu cells were trypsinized and plated in the presence of TGF-beta1. The effect was valid also in assays where Madin-Darby canine kidney epithelial cells or bovine capillary endothelial cells were used. The multiplication of chronically TGF-beta1 inhibited Mv1Lu cells was also induced by HGF. In addition, HGF induced anchorage independent growth of Mv1Lu cells that was refractory to TGF-beta1 growth inhibition. Immunoprecipitation analysis indicated that HGF prevented the suppression of Cdk4 and Cdk2, but not the induction of p21, by TGF-beta1. Since both TGF-beta1 and HGF require proteolysis for activation, the results imply that proteolytic activity of epithelial and endothelial cells directs their responses to signals from mesenchymal-type extracellular matrices, and that during development, matrix-bound growth and invasion promoting and suppressing factors are activated in a coordinated manner.

Serum levels of the TGF-beta receptor are increased in atherosclerosis

Murine monoclonal antibody E9 recognises a transforming growth factor (TGF) beta receptor, which is expressed in increased amounts by activated endothelial cells. In order to examine the biological role of this molecule in atherosclerosis, we have measured levels of the TGF-beta receptor alongside those of two other endothelial cell products (von Willebrand factor and soluble E-selectin) in the serum of 55 patients with atherosclerosis (29 with ischaemic heart disease and 26 with peripheral vascular disease), and in a cohort of 26 age- and sex-matched asymptomatic controls. There were increased levels of the TGF-beta receptor (P = 0.0079) and von Willebrand factor (P = 0.0001), but not soluble E-selection in patients' serum relative to the controls. In multivariate analysis of the endothelial cell products against total cholesterol, high density lipoprotein cholesterol and low density lipoprotein cholesterol, triglycerides, systolic and diastolic blood pressures, age, sex and smoking, both the TGF-beta receptor and von Willebrand factor correlated with total cholesterol (Spearman's r = 0.37 and r = 0.35, respectively, both P < 0.001). Lack of a correlation with a coarse endothelial damage marker von Willebrand factor or soluble E-selectin (produced by immunologically stimulated endothelial cells) implies other mechanisms are responsible for increased levels of the TGF-beta receptor in serum of patients with atherosclerosis.

Transforming growth factor-beta system and its regulation by members of the steroid-thyroid hormone superfamily

TGF-beta s and their receptors are expressed ubiquitously, and they act as key regulators of many aspects of cell growth, differentiation, and function. Steroid action on target tissues is often associated with increase in TGF-beta isoforms. Regulation of TGF-beta expression and activation is crucial for normal development and growth control. The loss of responsiveness of different tumor cells to the antiproliferative effects of TGF-beta is a common feature in carcinogenesis. Multiple changes are required for the cells to gain complete resistance to TGF-beta growth inhibition (Fynan and Reiss, 1993; Kimchi et al., 1988; Samuel et al., 1992). Although many tumor cells are not growth inhibited by TGF-beta, they respond to TGF-beta treatment by changes in the expression of matrix components and enhanced proteolytic activity (KeskiOja et al., 1988). Agents that induce TGF-beta production in target tissues can have a chemopreventive or chemotherapeutic value for the management of epithelial malignancies. Conversely, data supporting a positive role for TGF-beta in established tumor progression are beginning to emerge (Arteaga et al., 1993a,b; Barrett-Lee et al., 1990; Arrick et al., 1992 ; E. A. Thompson et al., 1991). In later stages of tumor development, cell proliferation is often not inhibited by TGF-beta, and tumor cells secrete large amounts of this growth factor (Fynan and Reiss, 1993). In vivo TGF-beta secreted by tumor or stromal cells can influence host responses such as a natural killer cell function and thus indirctly support tumor cell viability (Arteaga et al., 1993b). TGF-beta may also affect tumor growth indirectly by stromal effects and promotion of angiogenesis. TGF-beta may also be involved in the progression of breast tumors from the steroid-sensitive to steroid-insensitive state (King et al., 1989). Understanding of the net effect of TGF-beta in different stages of tumor development is critical for the evaluation of its therapeutic value in cancer treatment.

Irradiation induces up-regulation of E9 protein (CD105) in human vascular endothelial cells

The use of MAb E-9 raised against tissue-cultured endothelial cells (EC) has shown marked heterogeneity in vascular EC lining the blood vessels of normal and tumour tissues. MAb E-9 is human EC-specific and the protein recognized by it is a homodimer with a molecular mass of 97 kDa. The E-9 protein is resistant to treatment by 3 mM sodium periodate, but is sensitive to 10% trichloroacetic acid and 70% ethanol. E-9 protein has been assigned to a new cluster, CD105, and mapped to human chromosome 9q3. It has approximately 70% homology with type-III cell-surface receptor for transforming growth factor beta (TGF-beta). Recently CD105 has been reported to be the gene in patients with hereditary haemorrhagic telangiectasia. We have examined the effects of radiation on its expression in normal human umbilical-vein endothelial cells (HUVEC) and brain-tumour-derived endothelial cells (BTEC). Irradiation induced dose- and time-dependent up-regulation in the expression of the E-9 protein on the plasma membranes of EC, and also resulted in greater increase in the expression of the E-9 protein in semi-confluent (proliferating) as compared with confluent (non-proliferating) EC. It may well be that, following radiotherapy in cancer patients, E-9 protein is also up-regulated. The presence of increased amounts of E-9 protein in EC makes it an attractive target in the control of angiogenesis, especially after radiotherapy in cancer patients. The time scale involved in the up-regulation of E-9 protein following irradiation has led us to suggest that it may be a secondary event, the primary being the production and release of mitogenic factors (such as basic fibroblast growth factor) from irradiated EC.

A second locus for hereditary hemorrhagic telangiectasia maps to chromosome 12

Hereditary hemorrhagic telangiectasia (HHT) or Osler-Rendu-Weber (ORW) disease is an autosomal dominant vascular dysplasia. Initial linkage studies identified an ORW gene localized to 9q33-q34 but with some families clearly excluding this region. A probable correlation in clinical phenotype between the 9q3-linked families and unlinked families was described with a significantly lower incidence of pulmonary arteriovenous malformations observed in the unlinked families. In this study we examined four unrelated ORW families for which linkage to chromosome 9q33-q34 has been previously excluded. Linkage was established for all four families to markers on chromosome 12, with a combined maximum lod score of 10.77 (theta = 0.04) with D12S339. Mapping of crossovers using haplotype analysis indicated that the candidate region lies in an 11-CM interval between D12S345 and D12S339, in the pericentromeric region of chromosome 12. A map location for a second ORW locus is thus established that exhibits a significantly reduced incidence of pulmonary involvement.

Mapping a gene causing cerebral cavernous malformation to 7q11.2-q21

Cerebral cavernous malformation is a common disease of the brain vasculature of unknown cause characterized by dilated thin-walled sinusoidal vessels (caverns); these lesions cause varying clinical presentations which include headache, seizure, and hemorrhagic stroke. This disorder is frequently familial, with autosomal dominant inheritance. Using a general linkage approach in two extended cavernous malformation kindreds, we have identified linkage of this trait to chromosome 7q11.2-q21. Multipoint linkage analysis yields a peak logarithm of odds (lod) score of 6.88 with zero recombination with locus D7S669 and localizes the gene to a 7-cM region in the interval between loci ELN and D7S802.

Expression of transforming growth factor type III receptor in vascular endothelial cells increases their responsiveness to transforming growth factor beta 2

Bovine aortic endothelial cells (BAECs) express both type I and type II receptors for transforming growth factor beta (TGF beta). These cells respond to TGF beta 1 but are relatively refractory to another isoform of TGF beta, termed TGF beta 2. TGF beta s are thought to signal through receptor complexes composed of type I and/or type II receptors, both of which appear to be functional serine-threonine kinases. The TGF beta type III receptor, on the other hand, does not seem to have any direct signaling capacity. We have now stably transfected BAECs with the type III receptor cDNA. These cells displayed surface expression of the type III receptor protein, as determined by cross-linking with iodinated TGF beta 1 and immunoprecipitation with antibodies to the type III receptor protein. Transfected BAECs exhibit increased responsiveness to TGF beta 2 by several different criteria including an increase in plasminogen activator inhibitor-1 protein and inhibition of migration and proliferation. Thus, the type III receptor protein may play a role in presenting TGF beta 2 to the type II receptor and increase responsiveness to TGF beta 2 to a level comparable to that of TGF beta 1.

Specific binding of endocrine transforming growth factor-beta 1 to vascular endothelium

The presentation of recombinant biologically active 125I-TGF-beta 1 via the bloodstream to potential target cells in mice and rats was evaluated by quantitative light and electron microscope radioautography. Specificity was evaluated by in vivo competition with excess unlabeled TGF-beta 1, and integrity of the ligand at the binding site was demonstrated by trichloroacetic acid precipitation after extraction from tissues. The distribution of radiolabel at 2.5, 15, 30, 45, and 60 min after 125I-TGF-beta 1 injection revealed radiolabel principally over microvasculature endothelium but at times > 2.5 min over endothelial endocytic components indicative of internalization. Nonspecific binding of 125I-TGF-beta 1 to the apex of the proximal convoluted tubule of the kidney indicated it as the likely site of rapid clearance of TGF-beta 1 from the circulation, while a comparison of the binding of 125I-TGF-beta 1 (endothelial) to that of 125I-TGF-beta 1 complexed with alpha 2-macroglobulin-methylamine (liver parenchyma) indicated that clearance of TGF-beta 1 complexed alpha 2-macroglobulin was likely via the hepatic alpha 2-macroglobulin receptor. The endothelial TGF-beta receptors uncovered here are likely involved in the local regulatory mechanism of leukocyte and monocyte adhesion and tissue infiltration regulated by endocrine TGF-beta 1.