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Non-invasive in vivo imaging of the vasculature is a powerful tool for studying disease mechanisms in rodents. To achieve high sensitivity imaging of the microvasculature using Doppler ultrasound methods, imaging modalities employing the concept of ultrafast imaging are preferred. By increasing the frame rate of the ultrasound scanner to thousands of frames per second, it becomes possible to improve the sensitivity of the blood flow down to 2 mm/s and to obtain functional information about the microcirculation in comparison to a sensitivity of around 1 cm/s in conventional Doppler modes. While Ultrafast Doppler ultrasound (UFUS) imaging has become adopted in neuroscience, where it can capture brain activity through neurovascular coupling, it presents greater challenges when imaging the vasculature of abdominal organs due to larger motions linked to breathing. The liver, positioned anatomically under the diaphragm, is particularly susceptible to out-of-plane movement and oscillating respiratory motion. These artifacts not only adversely affect Doppler imaging but also complicate the anatomical analysis of vascular structures and the computation of vascular parameters. Here, we present a qualitative and quantitative imaging analysis of the hepatic vasculature in mice by UFUS. We identify major anatomical vascular structures and provide graphical illustrations of the hepatic macroscopical anatomy, comparing it to an in-depth anatomical assessment of the hepatic vasculature based on Doppler readouts. Additionally, we have developed a quantification protocol for robust measurements of hepatic blood volume of the microvasculature over time. To contemplate further research, qualitative vascular analysis provides a comprehensive overview and suggests a standardized terminology for researchers working with mouse models of liver disease. Furthermore, it offers the opportunity to apply ultrasound as a non-invasive complementary method to inspect hepatic vascular defects in vivo and measure functional microvascular alterations deep within the organ before unraveling blood vessel anomalies at the micron scale levels using ex vivo staining on tissue sections.

Haploinsufficiency for Endoglin (ENG) and activin A receptor type II-like I ( ACVRL1 /ALK1) lead to the formation of weak and abnormal vessels in hereditary hemorrhagic telangiectasia (HHT). These cause epistaxis (nosebleeds) and/or gastrointestinal blood loss. In vitro in cultured endothelial cells, tacrolimus has been shown to increase ENG and ALK1 expression. It is, therefore, a potential treatment option. We report here a proof-of-concept study in patients with HHT and severe epistaxis and/or gastrointestinal bleeding who were treated daily with orally-administered tacrolimus for twenty weeks. Twenty-five patients with HHT (11 females (44%)) and median age of 59 years were enrolled. Five patients (20%) stopped the trial prematurely-four due to (serious) adverse events ((S)AE). Twenty patients were included in further analyses. Hemoglobin levels increased during tacrolimus treatment from 6.1 (IQR 5.2-6.9) mmol/L at baseline (9.8 g/dL) to 6.7 (6.5-7.1) mmol/L (10.8 g/dL), p = 0.003. The number of blood transfusions over the twenty weeks decreased from a mean of 5.0 (±9.2) to 1.9 (±3.5), p = 0.04. In 64% of the patients, at least one AE occurred. Oral tacrolimus, thus, significantly increased hemoglobin levels and decreased blood transfusion needs, epistaxis and/or gastrointestinal bleeding in patients with HHT. However, side-effects were common. Further investigation of the potential therapeutic benefit is justified by the outcome of the study.

Hereditary hemorrhagic telangiectasia (HHT) is a genetic disease characterized by weak blood vessels. HHT1 is caused by mutations in the ENDOGLIN (ENG) gene. Here, we generated induced pluripotent stem cells (hiPSCs) from a patient with rare mosaic HHT1 with tissues containing both mutant (ENG c.1678C>T ) and normal cells, enabling derivation of isogenic diseased and healthy hiPSCs, respectively. We showed reduced ENG expression in HHT1 endothelial cells (HHT1-hiPSC-ECs), reflecting haploinsufficiency. HHT1 c.1678C>T -hiPSC-ECs and the healthy isogenic control behaved similarly in two-dimensional (2D) culture, forming functionally indistinguishable vascular networks. However, when grown in 3D organ-on-chip devices under microfluidic flow, lumenized vessels formed in which defective vascular organization was evident: interaction between inner ECs and surrounding pericytes was decreased, and there was evidence for vascular leakage. Organs on chip thus revealed features of HHT in hiPSC-derived blood vessels that were not evident in conventional 2D assays., (Copyright © 2022 The Author(s). Published by Elsevier Inc. All rights reserved.)

Hereditary Hemorrhagic Telangiectasia type 1 (HHT1) is an autosomal dominant inherited disease characterized by arteriovenous malformations and hemorrhage. HHT1 is caused by mutations in ENDOGLIN , which encodes an ancillary receptor for Transforming Growth Factor-β/Bone Morphogenetic Protein-9 expressed in all vascular endothelial cells. Haploinsufficiency is widely accepted as the underlying mechanism for HHT1. However, it remains intriguing that only some, but not all, vascular beds are affected, as these causal gene mutations are present in vasculature throughout the body. Here, we have examined the endoglin expression levels in the blood vessels of multiple organs in mice and in humans. We found a positive correlation between low basal levels of endoglin and the general prevalence of clinical manifestations in selected organs. Endoglin was found to be particularly low in the skin, the earliest site of vascular lesions in HHT1, and even undetectable in the arteries and capillaries of heterozygous endoglin mice. Endoglin levels did not appear to be associated with organ-specific vascular functions. Instead, our data revealed a critical endoglin threshold compatible with the haploinsufficiency model, below which endothelial cells independent of their tissue of origin exhibited abnormal responses to Vascular Endothelial Growth Factor. Our results support the development of drugs promoting endoglin expression as potentially protective.

Renal microvascular rarefaction plays a pivotal role in progressive kidney disease. Therefore, modalities to visualize the microcirculation of the kidney will increase our understanding of disease mechanisms and consequently may provide new approaches for evaluating cell-based therapy. At the moment, however, clinical practice is lacking non-invasive, safe, and efficient imaging modalities to monitor renal microvascular changes over time in patients suffering from renal disease. To emphasize the importance, we summarize current knowledge of the renal microcirculation and discussed the involvement in progressive kidney disease. Moreover, an overview of available imaging techniques to uncover renal microvascular morphology, function, and behavior is presented with the associated benefits and limitations. Ultimately, the necessity to assess and investigate renal disease based on in vivo readouts with a resolution up to capillary level may provide a paradigm shift for diagnosis and therapy in the field of nephrology.

Background: A glycocalyx envelope consisting of proteoglycans and adhering proteins covers endothelial cells, both the luminal and abluminal surface. We previously demonstrated that short-term loss of integrity of the luminal glycocalyx layer resulted in perturbed glomerular filtration barrier function., Methods: To explore the role of the glycocalyx layer of the endothelial extracellular matrix in renal function, we generated mice with an endothelium-specific and inducible deletion of hyaluronan synthase 2 (Has2), the enzyme that produces hyaluronan, the main structural component of the endothelial glycocalyx layer. We also investigated the presence of endothelial hyaluronan in human kidney tissue from patients with varying degrees of diabetic nephropathy., Results: Endothelial deletion of Has2 in adult mice led to substantial loss of the glycocalyx structure, and analysis of their kidneys and kidney function showed vascular destabilization, characterized by mesangiolysis, capillary ballooning, and albuminuria. This process develops over time into glomerular capillary rarefaction and glomerulosclerosis, recapitulating the phenotype of progressive human diabetic nephropathy. Using a hyaluronan-specific probe, we found loss of glomerular endothelial hyaluronan in association with lesion formation in tissue from patients with diabetic nephropathy. We also demonstrated that loss of hyaluronan, which harbors a specific binding site for angiopoietin and a key regulator of endothelial quiescence and maintenance of EC barrier function results in disturbed angiopoietin 1 Tie2., Conclusions: Endothelial loss of hyaluronan results in disturbed glomerular endothelial stabilization. Glomerular endothelial hyaluronan is a previously unrecognized key component of the extracelluar matrix that is required for glomerular structure and function and lost in diabetic nephropathy., (Copyright © 2019 by the American Society of Nephrology.)

Background: Hereditary Hemorrhagic Telangiectasia type 2 (HHT2) is an inherited genetic disorder characterized by vascular malformations and hemorrhage. HHT2 results from ACVRL1 haploinsufficiency, the remaining wild-type allele being unable to contribute sufficient protein to sustain endothelial cell function. Blood vessels function normally but are prone to respond to angiogenic stimuli, leading to the development of telangiectasic lesions that can bleed. How ACVRL1 haploinsufficiency leads to pathological angiogenesis is unknown., Methods: We took advantage of Acvrl1 +/- mutant mice that exhibit HHT2 vascular lesions and focused on the neonatal retina and the airway system after Mycoplasma pulmonis infection, as physiological and pathological models of angiogenesis, respectively. We elucidated underlying disease mechanisms in vitro by generating Acvrl1 +/- mouse embryonic stem cell lines that underwent sprouting angiogenesis and performed genetic complementation experiments. Finally, HHT2 plasma samples and skin biopsies were analyzed to determine whether the mechanisms evident in mice are conserved in humans., Results: Acvrl1 +/- retinas at postnatal day 7 showed excessive angiogenesis and numerous endothelial "tip cells" at the vascular front that displayed migratory defects. Vascular endothelial growth factor receptor 1 (VEGFR1; Flt-1) levels were reduced in Acvrl1 +/- mice and HHT2 patients, suggesting similar mechanisms in humans. In sprouting angiogenesis, VEGFR1 is expressed in stalk cells to inhibit VEGFR2 (Flk-1, KDR) signaling and thus limit tip cell formation. Soluble VEGFR1 (sVEGFR1) is also secreted, creating a VEGF gradient that promotes orientated sprout migration. Acvrl1 +/- embryonic stem cell lines recapitulated the vascular anomalies in Acvrl1 +/- (HHT2) mice. Genetic insertion of either the membrane or soluble form of VEGFR1 into the ROSA26 locus of Acvrl1 +/- embryonic stem cell lines prevented the vascular anomalies, suggesting that high VEGFR2 activity in Acvrl1 +/- endothelial cells induces HHT2 vascular anomalies. To confirm our hypothesis, Acvrl1 +/- mice were infected by Mycoplasma pulmonis to induce sustained airway inflammation. Infected Acvrl1 +/- tracheas showed excessive angiogenesis with the formation of multiple telangiectases, vascular defects that were prevented by VEGFR2 blocking antibodies., Conclusions: Our findings demonstrate a key role of VEGFR1 in HHT2 pathogenesis and provide mechanisms explaining why HHT2 blood vessels respond abnormally to angiogenic signals. This supports the case for using anti-VEGF therapy in HHT2.

Neural crest (NC) cells are a migratory, multipotent population giving rise to numerous lineages in the embryo. Their plasticity renders attractive their use in tissue engineering-based therapies, but further knowledge on their in vivo behavior is required before clinical transfer may be envisioned. We here describe the isolation and characterization of a new mouse embryonic stem (ES) line derived from Wnt1-CRE-R26 Rosa TomatoTdv blastocyst and show that it displays the characteristics of typical ES cells. Further, these cells can be efficiently directed toward an NC stem cell-like phenotype as attested by concomitant expression of NC marker genes and Tomato fluorescence. As native NC progenitors, they are capable of differentiating toward typical derivative phenotypes and interacting with embryonic tissues to participate in the formation of neo-structures. Their specific fluorescence allows purification and tracking in vivo. This cellular tool should facilitate a better understanding of the mechanisms driving NC fate specification and help identify the key interactions developed within a tissue after in vivo implantation. Altogether, this novel model may provide important knowledge to optimize NC stem cell graft conditions, which are required for efficient tissue repair.

Objective: To determine the role of Gja5 that encodes for the gap junction protein connexin40 in the generation of arteriovenous malformations in the hereditary hemorrhagic telangiectasia type 2 (HHT2) mouse model., Approach and Results: We identified GJA5 as a target gene of the bone morphogenetic protein-9/activin receptor-like kinase 1 signaling pathway in human aortic endothelial cells and importantly found that connexin40 levels were particularly low in a small group of patients with HHT2. We next took advantage of the Acvrl1(+/-) mutant mice that develop lesions similar to those in patients with HHT2 and generated Acvrl1(+/-); Gja5(EGFP/+) mice. Gja5 haploinsufficiency led to vasodilation of the arteries and rarefaction of the capillary bed in Acvrl1(+/-) mice. At the molecular level, we found that reduced Gja5 in Acvrl1(+/-) mice stimulated the production of reactive oxygen species, an important mediator of vessel remodeling. To normalize the altered hemodynamic forces in Acvrl1(+/-); Gja5(EGFP/+) mice, capillaries formed transient arteriovenous shunts that could develop into large malformations when exposed to environmental insults., Conclusions: We identified GJA5 as a potential modifier gene for HHT2. Our findings demonstrate that Acvrl1 haploinsufficiency combined with the effects of modifier genes that regulate vessel caliber is responsible for the heterogeneity and severity of the disease. The mouse models of HHT have led to the proposal that 3 events-heterozygosity, loss of heterozygosity, and angiogenic stimulation-are necessary for arteriovenous malformation formation. Here, we present a novel 3-step model in which pathological vessel caliber and consequent altered blood flow are necessary events for arteriovenous malformation development., (© 2016 American Heart Association, Inc.)

Defective paracrine Transforming Growth Factor-β (TGF-β) signaling between endothelial cells and the neighboring mural cells have been thought to lead to the development of vascular lesions that are characteristic of Hereditary Hemorrhagic Telangiectasia (HHT). This review highlights recent progress in our understanding of TGF-β signaling in mural cell recruitment and vessel stabilization and how perturbed TGF-β signaling might contribute to defective endothelial-mural cell interaction affecting vessel functionalities. Our recent findings have provided exciting insights into the role of thalidomide, a drug that reduces both the frequency and the duration of epistaxis in individuals with HHT by targeting mural cells. These advances provide opportunities for the development of new therapies for vascular malformations.

Mutations affecting transforming growth factor-beta (TGF-β) superfamily receptors, activin receptor-like kinase (ALK)-1, and endoglin (ENG) occur in patients with pulmonary arterial hypertension (PAH). To determine whether the TGF-β/ALK1/ENG pathway was involved in PAH, we investigated pulmonary TGF-β, ALK1, ALK5, and ENG expressions in human lung tissue and cultured pulmonary-artery smooth-muscle-cells (PA-SMCs) and pulmonary endothelial cells (PECs) from 14 patients with idiopathic PAH (iPAH) and 15 controls. Seeing that ENG was highly expressed in PEC, we assessed the effects of TGF-β on Smad1/5/8 and Smad2/3 activation and on growth factor production by the cells. Finally, we studied the consequence of ENG deficiency on the chronic hypoxic-PH development by measuring right ventricular (RV) systolic pressure (RVSP), RV hypertrophy, and pulmonary arteriolar remodeling in ENG-deficient (Eng+/-) and wild-type (Eng+/+) mice. We also evaluated the pulmonary blood vessel density, macrophage infiltration, and cytokine expression in the lungs of the animals. Compared to controls, iPAH patients had higher serum and pulmonary TGF-β levels and increased ALK1 and ENG expressions in lung tissue, predominantly in PECs. Incubation of the cells with TGF-β led to Smad1/5/8 phosphorylation and to a production of FGF2, PDGFb and endothelin-inducing PA-SMC growth. Endoglin deficiency protected mice from hypoxic PH. As compared to wild-type, Eng+/- mice had a lower pulmonary vessel density, and no change in macrophage infiltration after exposure to chronic hypoxia despite the higher pulmonary expressions of interleukin-6 and monocyte chemoattractant protein-1. The TGF-β/ALK1/ENG signaling pathway plays a key role in iPAH and experimental hypoxic PH via a direct effect on PECs leading to production of growth factors and inflammatory cytokines involved in the pathogenesis of PAH.

ENDOGLIN (ENG) is a co-receptor for transforming growth factor-β (TGF-β) family members that is highly expressed in endothelial cells and has a critical function in the development of the vascular system. Mutations in Eng are associated with the vascular disease known as hereditary hemorrhagic telangiectasia type l. Using mouse embryonic stem cells we observed that angiogenic factors, including vascular endothelial growth factor (VEGF), induce vasculogenesis in embryoid bodies even when Eng deficient cells or cells depleted of Eng using shRNA are used. However, ENG is required for the stem cell-derived endothelial cells to organize effectively into tubular structures. Consistent with this finding, fetal metatarsals isolated from E17.5 Eng heterozygous mouse embryos showed reduced VEGF-induced vascular network formation. Moreover, shRNA-mediated depletion and pharmacological inhibition of ENG in human umbilical vein cells mitigated VEGF-induced angiogenesis. In summary, we demonstrate that ENG is required for efficient VEGF-induced angiogenesis.

The (pro)renin receptor (PRR) is a component of the renin-angiotensin system (RAS) that is believed to control blood pressure and salt homeostasis in mammals by favouring tissue activation of RAS. Genetic studies have recently provided novel and exciting insights into how PRR regulates embryonic development in Drosophila and Xenopus through RAS independent functions. By interacting with the H+ vacuolar ATPase (V-ATPase), PRR modulates Wnt signalling pathways. Signalling by Wnt family members governs many aspects of embryonic development and tissue homeostasis. In particular, in mammals, Wnt signalling plays essential roles in the control of stem cell fate decision and lineage commitment in tissues with high self-renewal capacities such as the intestine and the skin, in which we have found PRR to be strongly expressed. Here, we review recent data on how PRR is thought to function during development and place it in the broader context of wnt signalling in skin in general.

Hereditary hemorrhagic telangiectasia (HHT) is an inherited disorder characterized by vascular malformations. Many affected individuals develop recurrent nosebleeds, which can severely affect their quality of life and are clinically difficult to treat. We report here that treatment with thalidomide reduced the severity and frequency of nosebleeds (epistaxis) in the majority of a small group of subjects with HHT tested. The blood hemoglobin levels of the treated individuals rose as a result of reduced hemorrhage and enhanced blood vessel stabilization. In mice heterozygous for a null mutation in the Eng gene (encoding endoglin), an experimental model of HHT, thalidomide treatment stimulated mural cell coverage and thus rescued vessel wall defects. Thalidomide treatment increased platelet-derived growth factor-B (PDGF-B) expression in endothelial cells and stimulated mural cell activation. The effects of thalidomide treatment were partially reversed by pharmacological or genetic interference with PDGF signaling from endothelial cells to pericytes. Biopsies of nasal epithelium from individuals with HHT treated or not with thalidomide showed that similar mechanisms may explain the effects of thalidomide treatment in humans. Our findings demonstrate the ability of thalidomide to induce vessel maturation, which may be useful as a therapeutic strategy for the treatment of vascular malformations.

Background and Purpose: Unstable atherosclerotic plaques are characterized by increased macrophages and reduced smooth muscle cells (SMCs) and collagen content. Endoglin, an accessory transforming growth factor-beta (TGFbeta) receptor, is a modulator of TGFbeta signaling recently found to be expressed on SMCs in atherosclerotic plaques. Its function in plaque SMCs and plaque development is unknown. Early growth response-1 (EGR-1), a transcription factor downstream of TGFbeta, stimulates SMC proliferation and collagen synthesis. In atherosclerotic lesions, it is mainly expressed by SMCs. Therefore, we studied the TGFbeta, endoglin, and EGR-1 pathway in advanced atherosclerotic plaques in relation to plaque phenotype., Methods: Human carotid atherosclerotic plaques (n=103) were collected from patients undergoing carotid endarterectomy. Histologically, plaques were analyzed for plaque characteristics, ie, collagen, macrophage and SMC content, and intraplaque thrombus. Intraplaque endoglin, pSmad (indicative for TGFbeta signaling), EGR-1, and TGFbeta levels were analyzed using Western blots and enzyme-linked immunosorbent assays, respectively., Results: Higher endoglin and EGR-1 protein levels correlated positively with increased plaque collagen levels, increased smooth muscle cell content, and decreased intraplaque thrombi as well as TGFbeta signaling (pSmad). Although EGR-1 overexpression in vitro stimulated collagen synthesis, inhibiting endoglin resulted in lower EGR-1 levels, decreased SMC proliferation, and decreased collagen content., Conclusions: TGFbeta in human atherosclerotic plaques is active and signals through the TGFbeta/Smad pathway. For the first time, we show a strong association between endoglin and EGR-1, increased collagen and SMCs expression, decreased levels of intraplaque thrombosis, and a stable plaque phenotype.

Defined growth conditions are essential for many applications of human embryonic stem cells (hESC). Most defined media are presently used in combination with Matrigel, a partially defined extracellular matrix (ECM) extract from mouse sarcoma. Here, we defined ECM requirements of hESC by analyzing integrin expression and ECM production and determined integrin function using blocking antibodies. hESC expressed all major ECM proteins and corresponding integrins. We then systematically replaced Matrigel with defined medium supplements and ECM proteins. Cells attached efficiently to natural human vitronectin, fibronectin, and Matrigel but poorly to laminin + entactin and collagen IV. Integrin-blocking antibodies demonstrated that alphaVbeta5 integrins mediated adhesion to vitronectin, alpha5beta1 mediated adhesion to fibronectin, and alpha6beta1 mediated adhesion to laminin + entactin. Fibronectin in feeder cell-conditioned medium partially supported growth on all natural matrices, but in defined, nonconditioned medium only Matrigel or (natural and recombinant) vitronectin was effective. Recombinant vitronectin was the only defined functional alternative to Matrigel, supporting sustained self-renewal and pluripotency in three independent hESC lines.

Apelin has been identified as the endogenous ligand of the human orphan G protein-coupled receptor APJ. This peptide exerts a variety of cardiovascular effects and particularly acts as an activator of angiogenesis. Importantly, hypoxia has been reported to regulate apelin expression but the molecular mechanism underlying hypoxia-induced apelin expression and the relationship with the physiological response of the apelin/APJ system are still not established. Here, we demonstrate that apelin expression is induced by hypoxia in cultured endothelial and vascular smooth muscle cells as well as in lung from mice exposed to acute hypoxia. Transient transfection experiments show that hypoxia-inducible transcriptional activation of apelin requires an intact hypoxia-responsive element (+813/+826) located within the first intron of the human apelin gene. Chromatin immunoprecipitation assay reveals that hypoxia-inducible factor-1alpha binds to the endogenous hypoxia-responsive element site of the apelin gene. Moreover, overexpression of hypoxia-inducible factor-1alpha increases the transcriptional activity of a reporter construct containing this hypoxia-responsive element, whereas small interfering RNA-mediated hypoxia-inducible factor-1alpha knockdown abolishes hypoxia-induced apelin expression. Finally, microinterfering RNA-mediated apelin or APJ receptor knockdown inhibits both hypoxia-induced endothelial cell proliferation in vitro and hypoxia-induced vessel regeneration in the caudal fin regeneration of Fli-1 transgenic zebrafish. The hypoxia-induced apelin expression may, thus, provide a new mechanism involved in adaptive physiological and pathophysiological response of vascular cells to low oxygen level.

Endoglin is emerging as a pivotal component of the gateway for signaling by transforming growth factor-beta (TGF-beta) in vascular endothelial cells. Mutations in endoglin cause a rare vascular disorder in humans known as hereditary hemorrhagic telengiectasia (HHT). Although rare, in-depth analysis of mutant mice and mononuclear cells from the blood of patients with HHT have provided novel and exciting insights into how the vasculature is formed, maintained, and repaired during disease. Here, we review recent data on how endoglin is thought to function in endothelial cells and place it in the broader context of signaling by TGF-beta family members in vascular cells in general. We highlight where the controversies on underlying molecular mechanisms currently lie and indicate areas of present research focus.

Vascular development depends on transforming growth factor beta (TGFbeta), but whether signalling of this protein is required for the development of endothelial cells (ECs), vascular smooth muscle cells (VSMCs) or both is unclear. To address this, we selectively deleted the type I (ALK5, TGFBR1) and type II (TbetaRII, TGFBR2) receptors in mice. Absence of either receptor in ECs resulted in vascular defects in the yolk sac, as seen in mice lacking receptors in all cells, causing embryonic lethality at embryonic day (E)10.5. Deletion of TbetaRII specifically in VSMCs also resulted in vascular defects in the yolk sac; however, these were observed at later stages of development, allowing the embryo to survive to E12.5. Because TGFbeta can also signal in ECs via ALK1 (ACVRL1), we replaced ALK5 by a mutant defective in SMAD2 and SMAD3 (SMAD2/3) activation that retained the ability to transactivate ALK1. This again caused defects in the yolk sac vasculature with embryonic lethality at E10.5, demonstrating that TGFbeta/ALK1 signalling in ECs cannot compensate for the lack of TGFbeta/ALK5-induced SMAD2/3 signalling in vivo. Unexpectedly, SMAD2 phosphorylation and alpha-smooth muscle actin (SMAalpha, ACTA2) expression occurred in the yolk sacs of ALK5(-/-) embryos and ALK5(-/-) embryonic stem cells undergoing vasculogenesis, and these processes could be blocked by an ALK4 (ACVR1B)/ALK5 inhibitor. Together, the data show that ALK5 is required in ECs and VSMCs for yolk sac vasculogenesis; in the absence of ALK5, ALK4 mediates SMAD2 phosphorylation and consequently SMAalpha expression.

The International Stem Cell Initiative characterized 59 human embryonic stem cell lines from 17 laboratories worldwide. Despite diverse genotypes and different techniques used for derivation and maintenance, all lines exhibited similar expression patterns for several markers of human embryonic stem cells. They expressed the glycolipid antigens SSEA3 and SSEA4, the keratan sulfate antigens TRA-1-60, TRA-1-81, GCTM2 and GCT343, and the protein antigens CD9, Thy1 (also known as CD90), tissue-nonspecific alkaline phosphatase and class 1 HLA, as well as the strongly developmentally regulated genes NANOG, POU5F1 (formerly known as OCT4), TDGF1, DNMT3B, GABRB3 and GDF3. Nevertheless, the lines were not identical: differences in expression of several lineage markers were evident, and several imprinted genes showed generally similar allele-specific expression patterns, but some gene-dependent variation was observed. Also, some female lines expressed readily detectable levels of XIST whereas others did not. No significant contamination of the lines with mycoplasma, bacteria or cytopathic viruses was detected.

Caudal related homeobox (Cdx) genes have so far been shown to be important for embryonic axial elongation and patterning in several vertebrate species. We have generated a targeted mutation of mouse Cdx4, the third member of this family of transcription factor encoding genes and the last one to be inactivated genetically. Cdx4-null embryos were born healthy and appeared morphologically normal. A subtle contribution of Cdx4 to anteroposterior (AP) vertebral patterning was revealed in Cdx1/Cdx4 and Cdx2/Cdx4 compound mutants. Neither Cdx4-null nor Cdx1/Cdx4 double mutants are impaired in their axial elongation, but a redundant contribution of Cdx4 in this function was unveiled when combined with a Cdx2 mutant allele. In addition, inactivation of Cdx4 combined with heterozygous loss of Cdx2 results in embryonic death around E10.5 and reveals a novel function of Cdx genes in placental ontogenesis. In a subset of Cdx2/Cdx4 compound mutants, the fully grown allantois failed to fuse with the chorion. The remaining majority of these mutants undergo successful chorio-allantois fusion but fail to properly extend their allantoic vascular network into the chorionic ectoderm and do not develop a functional placental labyrinth. We present evidence that Cdx4 plays a crucial role in the ontogenesis of the allantoic component of the placental labyrinth when one Cdx2 allele is inactivated. The axial patterning role of Cdx transcription factors thus extends posteriorly to the epiblast-derived extra-embryonic mesoderm and, consequent upon the evolution of placental mammals, is centrally involved in placental morphogenesis. The relative contribution of Cdx family members in the stepwise ontogenesis of a functional placenta is discussed, with Cdx2 playing an obligatory part, assisted by Cdx4. The possible participation of Cdx1 was not documented but cannot be ruled out until allelic combinations further decreasing Cdx dose have been analyzed. Cdx genes thus operate in a redundant way during placentogenesis, as they do during embryonic axial elongation and patterning, and independently from the previously reported early Cdx2-specific role in the trophectoderm at implantation.

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.

Genetic studies in mice and humans have revealed the pivotal role of transforming growth factor-beta (TGF-beta) signaling during angiogenesis. Mice deficient for various TGF-beta signaling components present an embryonic lethality due to vascular defects. In patients, mutations in the TGF-beta type I receptor ALK1 or in the accessory TGF-beta receptor endoglin are linked to an autosomal dominant disorder of vascular dysplasia termed Hereditary Haemorrhagic Telangiectasia (HHT). It has puzzled researchers for years to explain the effects of TGF-beta being a stimulator and an inhibitor of angiogenesis in vitro and in vivo. Recently, a model has been proposed in which TGF-beta by binding to the TGF-beta type II receptor can activate two distinct type I receptors in endothelial cells (ECs), i.e., the EC-restricted ALK1 and the broadly expressed ALK-5, which have opposite effects on ECs behavior. ALK1 via Smad1/5 transcription factors stimulates EC proliferation and migration, whereas ALK5 via Smad2/3 inhibits EC proliferation and migration. Here, the new findings are presented concerning the molecular mechanisms that take place in ECs to precisely regulate and even switch between TGF-beta-induced biological responses. In particular, the role of the accessory TGF-beta receptor endoglin in the regulation of EC behavior is addressed and new insights are discussed concerning the possible mechanisms that are implicated in the development of HHT.

Endoglin is a transmembrane accessory receptor for transforming growth factor-beta (TGF-beta) that is predominantly expressed on proliferating endothelial cells in culture and on angiogenic blood vessels in vivo. Endoglin, as well as other TGF-beta signalling components, is essential during angiogenesis. Mutations in endoglin and activin receptor-like kinase 1 (ALK1), an endothelial specific TGF-beta type I receptor, have been linked to the vascular disorder, hereditary haemorrhagic telangiectasia. However, the function of endoglin in TGF-beta/ALK signalling has remained unclear. Here we report that endoglin is required for efficient TGF-beta/ALK1 signalling, which indirectly inhibits TGF-beta/ALK5 signalling. Endothelial cells lacking endoglin do not grow because TGF-beta/ALK1 signalling is reduced and TGF-beta/ALK5 signalling is increased. Surviving cells adapt to this imbalance by downregulating ALK5 expression in order to proliferate. The ability of endoglin to promote ALK1 signalling also explains why ectopic endoglin expression in endothelial cells promotes proliferation and blocks TGF-beta-induced growth arrest by indirectly reducing TGF-beta/ALK5 signalling. Our results indicate a pivotal role for endoglin in the balance of ALK1 and ALK5 signalling to regulate endothelial cell proliferation.

Transforming growth factor-beta (TGFbeta) regulates the activation state of the endothelium via two opposing type I receptor/Smad pathways. Activin receptor-like kinase-1 (ALK1) induces Smad1/5 phosphorylation, leading to an increase in endothelial cell proliferation and migration, while ALK5 promotes Smad2/3 activation and inhibits both processes. Here, we report that ALK5 is important for TGFbeta/ALK1 signaling; endothelial cells lacking ALK5 are deficient in TGFbeta/ALK1-induced responses. More specifically, we show that ALK5 mediates a TGFbeta-dependent recruitment of ALK1 into a TGFbeta receptor complex and that the ALK5 kinase activity is required for optimal ALK1 activation. TGFbeta type II receptor is also required for ALK1 activation by TGFbeta. Interestingly, ALK1 not only induces a biological response opposite to that of ALK5 but also directly antagonizes ALK5/Smad signaling.

Biochemical studies in endothelial cells (ECs) and genetic studies in mice and humans have yielded major insights into the role of transforming growth factor beta (TGF-beta) and its downstream Smad effectors in embryonic vascular morphogenesis and in the establishment and maintenance of vessel wall integrity. These studies showed that TGF-beta signaling is of critical importance for normal vascular development and physiology. They also indicated the involvement of two distinct TGF-beta signaling cascades within ECs, namely the activin receptor-like kinase 5 (ALK5)-Smad2/3 pathway and the ALK1-Smad1/5 pathway. Aberrant TGF-beta signaling forms the basis for several vascular disorders such as hereditary hemorrhagic telengiectasia and primary pulmonary hypertension as well as neovascularization during tumorigenesis. This review describes the role of TGF-beta in angiogenesis and some of the controversial issues concerning TGF-beta signaling through ALK1 and ALK5 in ECs.

Background: Bone morphogenetic proteins (BMPs) are multifunctional proteins that regulate the proliferation, differentiation, and migration of a large variety of cell types. Like other members of the transforming growth factor-beta family, BMPs elicit their cellular effects through activating specific combinations of type I and type II serine/threonine kinase receptors and their downstream effector proteins, which are termed Smads. In the present study, we investigated BMP receptor/Smad expression and signaling in endothelial cells (ECs) and examined the effects of BMP on EC behavior., Methods and Results: Immunohistochemical analysis of tissue sections of human colon and mouse heart and aorta showed that BMP receptors are expressed in ECs in vivo. Bovine aortic ECs and mouse embryonic ECs were found to express BMP receptors and their Smads. BMP receptor activation induced the phosphorylation of specific Smad proteins and promoted EC migration and tube formation. Id1 was identified as a BMP/Smad target in ECs. Ectopic expression of Id1 mimicked BMP-induced effects. Importantly, specific interference with Id1 expression blocked BMP-induced EC migration., Conclusions: The BMP/Smad pathway can potently activate the endothelium. Id1 expression is strongly induced by BMP in ECs. Ectopic expression of Id1 induces EC migration and tube formation. Moreover, Id1 played a critical role in mediating BMP-induced EC migration.

Protein kinase CK2 is an ubiquitous and pleiotropic Ser/Thr protein kinase composed of two catalytic (alpha and/or alpha') and two regulatory (beta) subunits generally combined to form alpha(2)beta(2), alphaalpha'beta(2), or alpha'(2)beta(2) heterotetramers. To gain more insight into the role of CK2 in the control of proliferation in mammalian cells, overexpression of isolated CK2 subunits alpha, alpha', or beta was carried out in two fibroblast cell lines: NIH3T3 and CCL39. To interfere with CK2 cellular functions, cells were also transfected with a kinase-inactive mutant of CK2alpha catalytic subunit: CK2alpha-K68A. In NIH3T3 cells, overexpression of either wild-type subunit (alpha, alpha' or beta) had no effect on cell proliferation. In contrast, overexpression of the CK2alpha kinase-deficient mutant induced a marked inhibition of cell proliferation. This resulted from a defect in G1/S progression as demonstrated in transient transfection experiments in both NIH3T3 and CCL39 cells using BrdU incorporation measurements and in CCL39 clones stably overexpressing the CK2alpha-K68A mutant by growth curve analysis. We demonstrated that the kinase-negative mutant has the capacity to integrate the endogenous CK2 subunit pool both as an isolated kinase-inactive alpha subunit and as associated to the beta subunit in a kinase-inactive tetramer. Finally we showed that expression of the kinase-inactive mutant interferes with phosphorylation of an endogenous CK2 substrate; we speculate that optimal phosphorylation of target proteins by CK2 is required to achieve optimal cell cycle progression.

Despite its wide range of known substrates, the signaling function of protein kinase CK2 is still enigmatic. Mounting evidence suggests that CK2alpha, the catalytic subunit of holoenzymic CK2, may exist free of its usual regulatory partner CK2beta, raising the possibility that 'free' CK2alpha has regulation and function distinct from those of the holoenzyme. We previously reported that CK2alpha could bind to the core dimer of protein phosphatase 2A, and indirectly cause down-regulation of the PP2A substrate MEK1, possibly via activation of PP2A and/or targeting of PP2A to some element of the Ras/Raf/MEK pathway. Here, these results are confirmed and extended. By using transfection experiments and immune kinase assays, we show that endogenous PP2Ac and CK2beta are the only major substrates associating with epitope-tagged CK2alpha, and that expression of activated Raf results in disruption of the CK2alpha-PP2A association. Such disruption might be a necessary step for maximal activation of the MAP kinase pathway by Raf. In keeping with this idea, overexpression ofCK2alpha dose-dependently inhibits the mitogen-induced activation of cotransfected, epitope-tagged MAP kinase. We suggest that the CK2beta free form of CK2alpha is both a target and a regulator of Raf/MAPK signaling.

Timely deactivation of kinase cascades is crucial to the normal control of cell signaling and is partly accomplished by protein phosphatase 2A (PP2A). The catalytic (alpha) subunit of the serine-threonine kinase casein kinase 2 (CK2) bound to PP2A in vitro and in mitogen-starved cells; binding required the integrity of a sequence motif common to CK2alpha and SV40 small t antigen. Overexpression of CK2alpha resulted in deactivation of mitogen-activated protein kinase kinase (MEK) and suppression of cell growth. Moreover, CK2alpha inhibited the transforming activity of oncogenic Ras, but not that of constitutively activated MEK. Thus, CK2alpha may regulate the deactivation of the mitogen-activated protein kinase pathway.