Vascular bed-specific expression of an endothelial cell gene is programmed by the tissue microenvironment.

The endothelium is morphologically and functionally adapted to meet the unique demands of the underlying tissue. At the present time, little is known about the molecular basis of endothelial cell diversity. As one approach to this problem, we have chosen to study the mechanisms that govern differential expression of the endothelial cell-restricted von Willebrand factor (vWF) gene. Transgenic mice were generated with a fragment of the vWF gene containing 2,182 bp of 5' flanking sequence, the first exon and first intron coupled to the LacZ reporter gene. In multiple independent lines of mice, beta-galactosidase expression was detected within endothelial cells in the brain, heart, and skeletal muscle. In isogeneic transplantation models, LacZ expression in host-derived auricular blood vessels was specifically induced by the microenvironment of the heart. In in vitro coculture assays, expression of both the transgene and the endogenous vWF gene in cardiac microvascular endothelial cells (CMEC) was upregulated in the presence of cardiac myocytes. In contrast, endothelial cell levels of thrombomodulin protein and mRNA were unchanged by the addition of ventricular myocytes. Moreover, CMEC expression of vWF was not influenced by the addition of 3T3 fibroblasts or mouse hepatocytes. Taken together, the results suggest that the vWF gene is regulated by vascular bed-specific pathways in response to signals derived from the local microenvironment.

Evolutionary conservation of the allosteric activation of factor VIIa by tissue factor in lamprey.

Essentials Tissue factor (TF) enhances factor VIIa (FVIIa) activity through structural and dynamic changes. We analyzed conservation of TF-activated FVIIa allosteric networks in extant vertebrate lamprey. Lamprey Tf/FVIIa molecular dynamics show conserved Tf-induced structural/dynamic FVIIa changes. Lamprey Tf activation of FVIIa allosteric networks follows molecular pathways similar to human. SUMMARY: Background Previous studies have provided insight into the molecular basis of human tissue factor (TF) activation of activated factor VII (FVIIa). TF-induced allosteric networks of FVIIa activation have been rationalized through analysis of the dynamic changes and residue connectivities in the human soluble TF (sTF)/FVIIa complex structure during molecular dynamics (MD) simulation. Evolutionary conservation of the molecular mechanisms for TF-induced allosteric FVIIa activation between humans and extant vertebrate jawless fish (lampreys), where blood coagulation emerged more than 500 million years ago, is unknown and of considerable interest. Objective To model the sTf/FVIIa complex from cloned Petromyzon marinus lamprey sequences, and with comparisons to human sTF/FVlla investigate conservation of allosteric mechanisms of FVIIa activity enhancement by soluble TF using MD simulations. Methods Full-length cDNAs of lamprey tf and f7 were cloned and characterized. Comparative models of lamprey sTf/FVIIa complex and free FVIIa were determined based on constructed human sTF/FVIIa complex and free FVIIa models, used in full-atomic MD simulations, and characterized using dynamic network analysis approaches. Results Allosteric paths of correlated motion from Tf contact points in lamprey sTf/FVIIa to the FVIIa active site were determined and quantified, and were found to encompass residue-residue interactions along significantly similar paths compared with human. Conclusions Despite low conservation of residues between lamprey and human proteins, 30% TF and 39% FVII, the structural and protein dynamic effects of TF activation of FVIIa appear conserved and, moreover, present in extant vertebrate proteins from 500 million years ago when TF/FVIIa-initiated extrinsic pathway blood coagulation emerged.

Enhancement of murine cardiac chronotropy by the molecular transfer of the human beta2 adrenergic receptor cDNA.

Cardiac pacemaking offers a unique opportunity for direct gene transfer into the heart. An experimental system was developed to assay the effects of transferring the human beta2 adrenergic receptor (beta2AR) under in vitro, ex vivo, and finally in vivo conditions. Constructs encoding either beta2AR or LacZ were used in chronotropy studies with isolated myocytes, and transplanted as well as endogenous murine hearts. Murine embryonic cardiac myocytes were transiently transfected with plasmid constructs. The total percentage of myocytes spontaneously contracting was greater in beta2AR transfected cells, as compared with control cells (67 vs. 42+/-5%). In addition, the percentage of myocytes with chronotropic rates > 60 beats per minute (bpm) was higher in the beta2AR population, as compared with control cells (37 vs. 15+/-5%). The average contractile rate was greater in the beta2AR transfected myocytes at baseline (71+/-14 vs. 50+/-10 bpm; P < 0.001) as well as with the addition of 10(-)3 M isoproterenol (98+/-26 vs. 75+/-18 bpm; P < 0.05). Based on these results, a murine neonatal cardiac transplantation model was used to study the ex vivo effects of targeted expression of beta2AR. The constructs were transfected into the right atrium of transplanted hearts. Injection of the beta2AR construct increased the heart rate by approximately 40% (224+/-37 vs. 161+/-42 bpm; P < 0.005). Finally, the constructs were tested in vivo with injection into the right atrium of the endogenous heart. These results were similar to the ex vivo data with injection of the beta2AR constructs increasing the endogenous heart rates by approximately 40%, as compared with control injected hearts (550+/-42 vs. 390+/-37 bpm; P < 0.05). These studies demonstrate that local targeting of gene expression may be a feasible modality to regulate the cardiac pacemaking activity.

PDGF mediates cardiac microvascular communication.

The diversity of cellular and tissue functions within organs requires that local communication circuits control distinct populations of cells. Recently, we reported that cardiac myocytes regulate the expression of both von Willebrand factor (vWF) and a transgene with elements of the vWF promoter in a subpopulation of cardiac microvascular endothelial cells (J. Cell Biol. 138:1117). The present study explores this communication. Histological examination of the cardiac microvasculature revealed colocalization of the vWF transgene with the PDGF alpha-receptor. Transcript analysis demonstrated that in vitro cardiac microvascular endothelial cells constitutively express PDGF-A, but not B. Cardiac myocytes induced endothelial expression of PDGF-B, resulting in PDGF-AB. Protein measurement and transcript analysis revealed that PDGF-AB, but not PDGF-AA, induced endothelial expression of vWF and its transgene. Antibody neutralization of PDGF-AB blocked the myocyte-mediated induction. Immunostaining demonstrated that vWF induction is confined to PDGF alpha-receptor-positive endothelial cells. Similar experiments revealed that the PDGF-AB/alpha-receptor communication also induces expression of vascular endothelial growth factor and Flk-1, critical components of angiogenesis. The existence of this communication pathway was confirmed in vivo. Injection of PDGF-AB neutralizing antibody into the amniotic fluid surrounding murine embryos extinguished expression of the transgene. In summary, these studies suggest that environmental induction of PDGF-AB/alpha-receptor interaction is central to the regulation of cardiac microvascular endothelial cell hemostatic and angiogenic activity.

A vascular bed-specific pathway.

The endothelial nitric oxide synthase (eNOS) gene is induced by a variety of extracellular signals under both in vitro and in vivo conditions. To gain insight into the mechanisms underlying environmental regulation of eNos expression, transgenic mice were generated with the 1,600-bp 5' flanking region of the human eNos promoter coupled to the coding region of the LacZ gene. In multiple independent lines of mice, transgene expression was detected within the endothelium of the brain, heart, skeletal muscle, and aorta. beta-galactosidase activity was consistently absent in the vascular beds of the liver, kidney, and spleen. In stable transfection assays of murine endothelial progenitor cells, the 1,600-bp promoter region was selectively induced by conditioned media from cardiac myocytes, skeletal myocytes, and brain astrocytes. Cardiac myocyte-mediated induction was partly abrogated by neutralizing anti-platelet-derived growth factor (PDGF) antibodies. In addition, promoter activity was upregulated by PDGF-AB. Analysis of promoter deletions revealed that a PDGF response element lies between -744 and -1,600 relative to the start site of transcription, whereas a PDGF-independent cardiac myocyte response element is present within the first 166 bp of the 5' flanking region. Taken together, these results suggest that the eNos gene is regulated in the cardiac endothelium by both a PDGF-dependent and PDGF-independent microvascular bed-specific signaling pathway.

Vascular bed-specific thrombosis.

Hemostasis represents a finely tuned balance between procoagulant and anticoagulant forces. An imbalance of these forces may lead to clinically significant disease, including arterial, venous and/or microvascular thrombosis. The vast majority of hypercoagulable states are associated with local thrombus formation. The goal of this review is to discuss the mechanisms underlying site-specific thrombosis.

Spatial and temporal dynamics of the endothelium.

The endothelium is a highly metabolically active organ that is involved in many physiological processes, including the control of vasomotor tone, barrier function, leukocyte adhesion and trafficking, inflammation, and hemostasis. Endothelial cell phenotypes are differentially regulated in space and time. Endothelial cell heterogeneity has important implications for developing strategies in basic research, diagnostics and therapeutics. The goals of this review are to: (i) consider mechanisms of endothelial cell heterogeneity; (ii) discuss the bench-to-bedside gap in endothelial biomedicine; (iii) revisit definitions for endothelial cell activation and dysfunction; and (iv) propose new goals in diagnosis and therapy. Finally, these themes will be applied to an understanding of vascular bed-specific hemostasis.

Vascular endothelial growth factor induces manganese-superoxide dismutase expression in endothelial cells by a Rac1-regulated NADPH oxidase-dependent mechanism.

Vascular endothelial growth factor (VEGF) is a potent vascular endothelial cell-specific mitogen that modulates endothelial cell function. In the present study, we show that VEGF induces manganese-superoxide dismutase (MnSOD) mRNA and protein in human coronary artery endothelial cells (HCAEC) and pulmonary artery endothelial cells. VEGF-mediated induction of MnSOD mRNA was inhibited by pretreatment with the NADPH oxidase inhibitors, diphenyleneiodonium (DPI), and 4-(2-aminoethyl)-benzenesulfonyl fluoride, but not with the nitric oxide synthase inhibitor L-NAME (N-monomethyl-L-arginine) or the xanthine oxidase inhibitor allopurinol. VEGF stimulation of MnSOD was also inhibited by adenoviral-mediated overexpression of catalase Cu, Zn-SOD and a dominant-negative form of the small GTPase component of NADPH oxidase Rac1 (Rac1N17). Treatment of HCAEC with VEGF resulted in a transient increase in ROS production at 20 min, as measured by 2,7-dichlorodihydrofluorescein oxidation. This effect was abrogated by expression of Rac1N17. Taken together, these findings suggest that VEGF induces MnSOD by an NADPH oxidase-dependent mechanism and that VEGF signaling in the endothelium is coupled to the redox state of the cell.

Endothelium and haemostasis.

The endothelium is a widely distributed organ system that plays an important role in health and disease. The endothelium is remarkably heterogeneous in structure and function. One vital function of the endothelium is to maintain blood in its fluid state, and to provide controlled haemostasis at sites of vascular injury. In keeping with the theme of endothelial cell heterogeneity, endothelial cells from different sites of the vascular employ different strategies to mediate local haemostatic balance. These differences are sufficient to explain why systemic imbalances of haemostatic components invariably lead to local thrombotic phenotypes. An important goal for the future is to identify diagnostic markers that reflect phenotypic changes at the level of individual vascular beds, and to develop therapies that target one or another site of the vasculature.

Targeting the Hprt locus in mice reveals differential regulation of Tie2 gene expression in the endothelium.

To study the in vivo expression of the murine Tie2 gene, we have targeted the hypoxanthine phosphoribosyltransferase (Hprt) gene locus to generate two single-copy transgenic mice: T1, containing the 2,100-bp Tie2 promoter upstream from the beta-galactosidase (LacZ) gene, and T5, which also included an enhancing element originating from the first intron of the Tie2 gene. Comparing T1 and T5 embryos at day E10.5 revealed differential endothelial cell-specific expression of LacZ, whereas colocalization analyses showed that the expression was confined to endothelial cells. Moderate reporter gene activity was observed in the brain and kidney of T1 adults, whereas extensive LacZ gene expression was seen in the vasculature of most organs of the T5 adults. This study demonstrates the feasibility of targeting the Hprt locus with endothelial cell-specific sequences to analyze the spatial-temporal expression of transgenes. Of particular importance is the observation that the analysis of a single transgene copy in a defined locus allows for an accurate and rapid comparison of transcriptional activity among regulatory DNA sequences.

Targeting of human eNOS promoter to the Hprt locus of mice leads to tissue-restricted transgene expression.

Phenotypic heterogeneity of the endothelium arises from cell type-specific differences in gene expression. An understanding of the mechanisms that underlie differential gene expression would provide important insight into the molecular basis of vascular diversity. In standard transgenic assays, multiple copies of heterologous DNA cassettes are randomly integrated into the mouse genome, resulting in significant line-to-line variation in expression. To overcome these limitations, we have targeted a single copy of a transgene that contains 1,600 bp of the human endothelial nitric oxide synthase (eNOS) promoter coupled to the LacZ reporter gene to the X-linked hypoxanthine phosphoribosyltransferase (Hprt) locus of mice by homologous recombination. The transgene was inserted in either of the orientations relative to that of the Hprt gene. In mice derived from multiple embryonic stem (ES) cell clones, the expression pattern was limited to a subset of endothelial cells, cardiomyocytes, and vascular smooth muscle cells. These findings suggest that Hprt locus targeting is a feasible tool for studying endothelial cell-restricted gene regulation.

NADPH oxidase activity is required for endothelial cell proliferation and migration.

NADPH oxidase has been shown to play an important role in cardiovascular biology. The goal of the present study was to determine whether NADPH oxidase activity is important for endothelial cell growth and migration. In proliferation assays, growth factor- or serum-induced DNA synthesis in three different types of human endothelial cells was abrogated by inhibitors of NADPH oxidase, but not by inhibitors of xanthine oxidase or nitric oxide synthase. Moreover, vascular endothelial growth factor-induced migration of human endothelial cells was suppressed in the presence of NADPH oxidase inhibitors. These results support a potential role for NADPH oxidase in mediating angiogenesis.

Hemostasis and irreducible complexity.

Coagulation evolved as a means to stem the loss of blood and to defend against pathogens. The complexity of the clotting cascade has been cited as evidence for the existence of divine intervention. The objective of this review is to draw on the debate between creationists and evolutionary biologists to highlight important evolutionary principles that underlie the hemostatic mechanism. I propose the following: (a) as with all biological systems, the hemostatic mechanism displays non-linear complexity; (b) the cellular response represents primary hemostasis owing to its place in the evolutionary time scale and functional importance; and (c) the rapid evolution of the hemostatic mechanism in vertebrates is testimony to the power and versatility of gene duplications and exon shuffling.

Evolutionary origins of the blood vascular system and endothelium.

Every biological trait requires both a proximate and evolutionary explanation. The field of vascular biology is focused primarily on proximate mechanisms in health and disease. Comparatively little attention has been given to the evolutionary basis of the cardiovascular system. Here, we employ a comparative approach to review the phylogenetic history of the blood vascular system and endothelium. In addition to drawing on the published literature, we provide primary ultrastructural data related to the lobster, earthworm, amphioxus, and hagfish. Existing evidence suggests that the blood vascular system first appeared in an ancestor of the triploblasts over 600 million years ago, as a means to overcome the time-distance constraints of diffusion. The endothelium evolved in an ancestral vertebrate some 540-510 million years ago to optimize flow dynamics and barrier function, and/or to localize immune and coagulation functions. Finally, we emphasize that endothelial heterogeneity evolved as a core feature of the endothelium from the outset, reflecting its role in meeting the diverse needs of body tissues.

RUNX1, but not its familial platelet disorder mutants, synergistically activates PF4 gene expression in combination with ETS family proteins.

BACKGROUND: Familial platelet disorder (FPD) is a rare autosomal dominant disease characterized by thrombocytopenia and abnormal platelet function. Causal mutations have been identified in the gene encoding runt-related transcription factor 1 (RUNX1) of FPD patients. OBJECTIVES: To elucidate the role of RUNX1 in the regulation of expression of platelet factor 4 (PF4) and to propose a plausible mechanism underlying RUNX1-mediated induction of the FPD phenotype. METHODS: We assessed whether RUNX1 and its mutants, in combination with E26 transformation-specific-1 (ETS-1), Core-binding factor subunit beta (CBFß), and Friend leukemia virus integration 1 (FLI-1), cooperatively regulate PF4 expression during megakaryocytic differentiation. In an embryonic stem cell differentiation system, expression levels of endogenous and exogenous RUNX1 and PF4 were determined by real-time RT-PCR. Promoter activation by the transcription factors were evaluated by reporter gene assays with HepG2 cells. DNA binding activity and protein interaction were analyzed by electrophoretic mobility shift assay and immunoprecipitation assay with Cos-7 cells, respectively. Protein localization was analyzed by immunocytochemistry and Western blotting with Cos-7 cells. RESULTS: We demonstrated that RUNX1 activates endogenous PF4 expression in megakaryocytic differentiation. RUNX1, but not its mutants, in combination with ETS-1 and CBFß, or FLI-1, synergistically activated the PF4 promoter. Each RUNX1 mutant harbors various functional abnormalities, including loss of DNA-binding activity, abnormal subcellular localization, and/or alterations of binding affinities for ETS-1, CBFß, and FLI-1. CONCLUSIONS: RUNX1, but not its mutants, strongly and synergistically activates PF4 expression along with ETS family proteins. Furthermore, loss of the RUNX1 transcriptional activation function is induced by various functional abnormalities.

Discovery of the cardiovascular system: from Galen to William Harvey.

The goal of this review is to examine the events that led to discovery of blood circulation. The Ancient Greeks, including Hippocrates and Galen viewed the cardiovascular system as comprising two distinct networks of arteries and veins. Galen claimed that the liver produced blood that was then distributed to the body in a centrifugal manner, whereas air or pneuma was absorbed from the lung into the pulmonary veins and carried by arteries to the various tissues of the body. Arteries also contained blood, which passed from the venous side via invisible pores in the interventricular septum and peripheral anastomoses. This was an open-ended system in which blood and air simply dissipated at the ends of veins and arteries according to the needs of the local tissue. Blood was not seen to circulate but rather to slowly ebb and flow. This view would hold sway for 15 centuries until 1628 when William Harvey published his momentous 72-page book, On the Motion of the Heart and Blood in Animals. Harvey employed experiment and deductive logic to show that arteries and veins are functionally, if not structurally, connected in the lung and the peripheral tissues, and that blood circulates. The mechanical force of the heart replaced Galen's elusive attractive powers. Ultimately, Galenism would collapse under the weight of Harvey's evidence, and a new paradigm of blood circulation would prevail.

A +220 GATA motif mediates basal but not endotoxin-repressible expression of the von Willebrand factor promoter in Hprt-targeted transgenic mice.

BACKGROUND: The von Willebrand factor (VWF) gene is a marker for spatial and temporal heterogeneity of the endothelium. A GATA motif at +220 has been implicated in basal VWF expression in vitro. Other studies have shown that GATA3 and VWF are transcriptionally downregulated in response to inflammatory mediators. OBJECTIVES: Our goal was to determine the importance of the +220 GATA motif in mediating expression of VWF promoter in vivo, and to elucidate whether the GATA element plays a role in spatial and/or temporal regulation of VWF expression. METHODS: ChIP and electrophoretic mobility shift assays were carried out in human umbilical vein endothelial cells (HUVEC). Reporter gene constructs containing 3.6 kb of the human VWF promoter with and without a mutation of the +220 GATA element were transfected into cultured endothelial cells or targeted to the Hprt locus of mice. The Hprt-targeted mice were subjected to endotoxemia. RESULTS: In protein-DNA binding assays, the +220 GATA motif bound GATA-2, -3 and -6. Mutation of the GATA site resulted in reduced basal promoter activity in HUVEC. When targeted to the Hprt locus of mice, the GATA mutation resulted in a significant, proportionate reduction of promoter activity in LacZ expressing vascular beds. Systemic administration of lipopolysaccharide (LPS) resulted in a widespread reduction in VWF mRNA expression and promoter activity. LPS-mediated repression of the VWF promoter was unaffected by the GATA mutation. CONCLUSIONS: A region of the VWF promoter between -2182 and the end of the first intron contains information for LPS-mediated gene repression. The +220 GATA motif is important for basal, but not LPS-repressible expression of the VWF gene.

GAS6-induced signaling in human endothelial cells is mediated by FOXO1a.

BACKGROUND: Growth Arrest Specific gene product 6 (gas6) is a gamma-carboxylated protein that protects endothelial cells against apoptosis. Gas6 has previously been shown to induce phospatidyl-3-inositol-kinase (PI3K)/Akt signaling. Other studies have demonstrated a link between PI3K/Akt signaling and forkhead transcription factors in endothelial cells. OBJECTIVE: To test the hypothesis that gas6 promotes cell survival via a forkhead-dependent pathway. RESULTS AND CONCLUSIONS: Treatment of serum-starved human umbilical vein endothelial cells (HUVECs) with gas6 induced time-dependent phosphorylation and nuclear exclusion of FOXO1a. This effect was suppressed by the PI3K inhibitor wortmannin, demonstrating that FOXO1a phosphorylation is PI3-kinase dependent. Transduction of HUVECs with a phosphorylation-resistant form of FOXO1a [triple mutant (TM)-FOXO1a] abrogated the pro-survival effect of gas6 on serum-starved endothelial cells. Finally, treatment of serum-starved HUVECs with gas6 resulted in a reduction of FOXO1a transcriptional activity and downregulation of the pro-apoptotic gene, p27(kip1). Taken together, these findings suggest that gas6 protects endothelial cells from apoptosis by a mechanism that involves PI3K-Akt-dependent inactivation of FOXO1a.

Vascular bed-specific hemostasis: role of endothelium in sepsis pathogenesis.

OBJECTIVES: To examine the role of vascular bed-specific pathways in determining the hemostatic phenotype in sepsis. DATA SOURCES/STUDY SELECTION: Published research and review articles related to hemostasis and endothelial cell biology. DATA EXTRACTION AND SYNTHESIS: The results of published studies have been used to generate a hypothesis of vascular bed-specific hemostasis in sepsis. CONCLUSIONS: In sepsis, coagulation is initiated by the extrinsic pathway and is amplified through the intrinsic pathway. In addition, the body's natural anticoagulant mechanisms are significantly dampened. Together, these changes result in a net imbalance of hemostasis. The nature of this imbalance varies from one vascular bed to the next according to the local set point of the endothelium. These concepts lay an important foundation for understanding the pathophysiology of sepsis.