Vascular endothelial cell lineage-specific promoter in transgenic mice

RNA-binding protein Csx1 mediates global control of gene expression in response to oxidative stress

Fission yeast Spc1 (Sty1), a stress-activated mitogen-activated protein kinase (MAPK) homologous to human p38, orchestrates global changes in gene expression in response to diverse forms of cytotoxic stress. This control is partly mediated through Atf1, a transcription factor homologous to human ATF2. How Spc1 controls Atf1, and how the cells tailor gene expression patterns to different forms of stress, are unknown. Here we describe Csx1, a novel protein crucial for survival of oxidative but not osmotic stress. Csx1 associates with and stabilizes atf1+ mRNA in response to oxidative stress. Csx1 controls expression of the majority of the genes induced by oxidative stress, including most of the genes regulated by Spc1 and Atf1. These studies reveal a novel mechanism controlling MAPK-regulated transcription factors and suggest how gene expression patterns can be customized to specific forms of stress. Csx1-like proteins in humans may perform similar tasks.

Presenilin 1 is essential for cardiac morphogenesis

Presenilin 1 (PS1) is the gene responsible for the development of early-onset familial Alzheimer's disease. PS1-deficient mice have been reported to show defects in neurogenesis, somitogenesis and angiogenesis. Here, we report cardiac anomaly in PS1-deficient mice: the mutant hearts exhibited ventricular septal defect, double outlet right ventricle, and stenosis in the pulmonary artery. Immunohistochemistry using anti-PS1 antibody revealed the prominent expression of PS1 in mesenchymal cells at the septal area of the wild-type heart. These results suggest that PS1 may play an essential role in heart development.

Ece1 and Tbx1 define distinct pathways to aortic arch morphogenesis

Pharyngeal arch artery (PAA) remodeling defects account for several cases of congenital heart disease. Mutations in the Endothelin-1 genetic pathway or Tbx1, a candidate gene for DiGeorge syndrome, cause similar aortic arch defects. Previous research suggests that Tbx1 may trigger diffusible signals from the pharyngeal arches to support the growth of the PAAs that contribute to the mature aortic arch. The demonstration of genetic interaction between Tbx1 and Fgf8 pointed to FGF signaling as a possible candidate. Because Fgf8 interacts with Endothelin-1 signaling and because Endothelin-1 signaling interacts with neural crest-derived cells in the pharyngeal apparatus, we hypothesized that Tbx1 and Endothelin-1 signaling may contribute to the same pathway required for aortic arch morphogenesis. Therefore, we have analyzed mice mutated for the endothelin converting enzyme (Ece1) or Tbx1 genes and compound mutants. Results show that the two genes have different roles in the remodeling of the PAAs and do not interact. We propose that Tbx1 is required for the formation and early growth and remodeling of the PAAs, whereas Ece1 is necessary for regression of the cranial arch arteries and growth of the most caudal arch arteries. The latter function is likely related to the known role of the Endothelin-1 pathway in neural crest function.

Peripheral mobilization of recipient bone marrow-derived endothelial progenitor cells enhances pancreatic islet revascularization and engraftment after intraportal transplantation

BACKGROUND

Pancreatic islet transplantation has been validated as a treatment for type 1 diabetes. However, a high number of islets is required to establish euglycemia. Transplantation of islets leads to loss of islet vasculature, which requires revascularization to ensure adequate survival. Islet vascular density in transplanted islets is markedly decreased compared with endogenous islets. The feasibility of revascularization of ischemic tissues by mobilizing endothelial progenitor cells or angioblasts has been demonstrated. Therefore, we investigated the therapeutic potential of angioblast mobilization for stimulation of islet revascularization and therefore engraftment after transplantation.

METHODS

FVB/NJ mice underwent bone marrow transplantation from transgenic mice constitutively expressing beta-galactosidase encoded by LacZ under regulation of the endothelial cell-specific promoter TIE-2 (FEV/NJ-TIE-2-LacZ). Three weeks after reconstitution, animals received an intrahepatic islet syngeneic infusion (FVB/NJ donors). The contribution of angioblasts into sites of islet revascularization was analyzed by reverse transcriptase-polymerase chain reaction (RT-PCR), beta-galactosidase (beta-gal) activity, and immunohistochemistry. Islet vascular density was assessed morphometrically followed by in situ BS-1 lectin staining and functional islet mass after transplantation by metabolic studies. Angioblasts were mobilized with murine granulocyte-macrophage colony-stimulating factor (GM-CSF) (0.5 microg/day/7 days).

RESULTS

An islet dose-dependent increase in beta-gal was demonstrated after transplantation. These results were confirmed by RT-PCR and immunohistochemistry. GM-CSF increased the number of peripheral angioblasts and their localization into sites of islet revascularization. A significant increase in islet vascular density was observed in animals treated with GM-CSF versus controls. Higher functional islet mass was demonstrated in animals treated with GM-CSF.

CONCLUSIONS

Augmentation of angioblasts in the peripheral circulation resulted in higher islet vascular density and engraftment. This novel strategy may improve the results in clinical islet transplantation.

E-Tmod capping of actin filaments at the slow-growing end is required to establish mouse embryonic circulation

Tropomodulins are a family of proteins that cap the slow-growing end of actin filaments. Erythrocyte tropomodulin (E-Tmod) stabilizes short actin protofilaments in erythrocytes and caps longer sarcomeric actin filaments in striated muscles. We report the knockin of the beta-galactosidase gene (LacZ) under the control of the endogenous E-Tmod promoter and the knockout of E-Tmod in mouse embryonic stem cells. E-Tmod(-/-) embryos die around embryonic day 10 and exhibit a noncontractile heart tube with disorganized myofibrils and underdevelopment of the right ventricle, accumulation of mechanically weakened primitive erythroid cells in the yolk sac, and failure of primary capillary plexuses to remodel into vitelline vessels, all required to establish blood circulation between the yolk sac and the embryo proper. We propose a hemodynamic "plexus channel selection" mechanism as the basis for vitelline vascular remodeling. The defects in cardiac contractility, vitelline circulation, and hematopoiesis reflect an essential role for E-Tmod capping of the actin filaments in both assembly of cardiac sarcomeres and of the membrane skeleton in erythroid cells that is not compensated for by other proteins.

Endothelial cell heterogeneity

OBJECTIVE

To review recent advances in the field of endothelial cell heterogeneity, and to apply this knowledge to an understanding of site-specific vasculopathy, including acute lung injury.

DATA SOURCES AND STUDY SELECTION

Published research and review articles in the English language related to endothelial cell biology and endothelial cell heterogeneity.

DATA EXTRACTION AND SYNTHESIS

The results of published studies have been used to provide a perspective of endothelial cell phenotypes in health and disease.

CONCLUSIONS

The structure and function of endothelial cells are differentially regulated in space and time. Far from being a giant monopoly of homogeneous cells, the endothelium represents a consortium of smaller enterprises of cells located within blood vessels of different tissues. Although united in certain functions, each enterprise is uniquely adapted to meet the demands of the underlying tissue. The endothelium may also vary in its response to pathophysiologic stimuli and therefore contribute to the focal nature of vasculopathic disease states. In acute lung injury, the unique properties of the endothelium may conspire with systemic imbalances to localize pathology to the pulmonary vasculature.

A mutant receptor tyrosine phosphatase, CD148, causes defects in vascular development

Vascularization defects in genetic recombinant mice have defined critical roles for a number of specific receptor tyrosine kinases. Here we evaluated whether an endothelium-expressed receptor tyrosine phosphatase, CD148 (DEP-1/PTPeta), participates in developmental vascularization. A mutant allele, CD148(DeltaCyGFP), was constructed to eliminate CD148 phosphatase activity by in-frame replacement of cytoplasmic sequences with enhanced green fluorescent protein sequences. Homozygous mutant mice died at midgestation, before embryonic day 11.5 (E11.5), with vascularization failure marked by growth retardation and disorganized vascular structures. Structural abnormalities were observed as early as E8.25 in the yolk sac, prior to the appearance of intraembryonic defects. Homozygous mutant mice displayed enlarged vessels comprised of endothelial cells expressing markers of early differentiation, including VEGFR2 (Flk1), Tal1/SCL, CD31, ephrin-B2, and Tie2, with notable lack of endoglin expression. Increased endothelial cell numbers and mitotic activity indices were demonstrated. At E9.5, homozygous mutant embryos showed homogeneously enlarged primitive vessels defective in vascular remodeling and branching, with impaired pericyte investment adjacent to endothelial structures, in similarity to endoglin-deficient embryos. Developing cardiac tissues showed expanded endocardial projections accompanied by defective endocardial cushion formation. These findings implicate a member of the receptor tyrosine phosphatase family, CD148, in developmental vascular organization and provide evidence that it regulates endothelial proliferation and endothelium-pericyte interactions.

Versatile inducible activation system of Akt/PKB signaling pathway in mice

We report a transgenic mouse line in which Akt/protein kinase B (PKB) pathway can be activated in an inducible manner in defined cell types. In this transgenic mouse line, Cre expression allows the expression of a tamoxifen-activatable form of Akt/PKB in a defined cell type. Subsequent injection of tamoxifen triggers the transient activation of Akt/PKB in mice. Thus, this transgenic line allows the transient activation of Akt/PKB pathway in a predefined cell type. We expect that this transgenic system will provide a unique tool to study the roles of Akt/PKB pathway in mice.

Differential regulation of the von Willebrand factor and Flt-1 promoters in the endothelium of hypoxanthine phosphoribosyltransferase-targeted mice

An important limitation of standard transgenic assays is that multiple copies of the transgene are inserted randomly into the mouse genome, resulting in line-to-line variation in expression. One way to control for these variables is to target a single copy of the transgene to a defined locus of the mouse genome by homologous recombination. In the present study, we have used such an approach to target the promoters of 2 different genes, namely von Willebrand factor (VWF) and Flt-1, to the hypoxanthine phosphoribosyltransferase (Hprt) gene locus. Consistent with previous findings in standard transgenic animals, we report that the VWF promoter contains information for expression in a subset of endothelial cells in the heart, skeletal muscle, and brain. In contrast, the Flt-1 promoter directs expression in all vascular beds except for the liver. The Flt-1 transgene was active in the endothelium of tumor xenografts, whereas the VWF promoter was not. Under in vitro conditions, conditioned medium from tumor cells resulted in a significant up-regulation of Flt-1 mRNA and promoter activity, but no change in VWF levels. Taken together, these results suggest that (1) Hprt locus targeting is a valuable tool for studying vascular bed-specific gene regulation, (2) the VWF and Flt-1 promoters are regulated by distinct transcriptional mechanisms in the intact endothelium, and (3) tumor angiogenesis results in the differential activation of endothelial cell-specific promoters.

An allelic series of mutations in Smad2 and Smad4 identified in a genotype-based screen of N-ethyl-N- nitrosourea-mutagenized mouse embryonic stem cells

Using selectable genes as proof of principle, a new high-throughput genotype-based mutation screen in mouse embryonic stem (ES) cells was developed [Chen et al. (2002) Nat. Genet. 24, 314-317]. If expanded to nonselectable genes, this approach would allow one to proceed quickly from sequence to whole-animal phenotypes. Here data are presented showing that a screen of a cryopreserved library of clonal, germ line competent, N-ethyl-N-nitrosurea (ENU) mutagenized ES cells can identify a large series of allelic mutations in Smad2 and Smad4, two nonselectable genes of the transforming growth factor beta superfamily of signaling molecules. Whole animal phenotypic analyses of some of these alleles provided evidence for novel developmental processes mediated by these components of transforming growth factor beta signaling, demonstrating the utility of non-null alleles created by chemical mutagens. The accurately assessed mutation load of the ES cell library indicates that it is a valuable resource for developing mouse lines for genetic and functional studies. This methodology can conceptually be applied for the generation of an allelic series of subtle mutations at any locus of interest in the mouse.

Inducible and selective transgene expression in murine vascular endothelium

We have developed a system utilizing the murine Tie2 promoter/enhancer coupled with the "tetracycline-on" regulatory elements to create a model that allows regulated and selective expression of a beta-galactosidase (betaGal) reporter transgene in the adult murine vascular endothelium. Two independent lines of viable and fertile mice were characterized, and they exhibit minimal betaGal expression under basal conditions. In response to exogenous doxycycline (Dox), selective expression of betaGal was demonstrated in the vascular endothelium of all tissues examined. En face analyses of the aorta and its principle branches indicate that the vast majority of lumenal endothelial cells express the transgene. Inducible betaGal expression also extends to the endocardium and the microvasculature of all organs. There is no evidence of specific transgene expression in nonendothelial cell types. Induction of the betaGal was effectively achieved after 3 days of oral Dox treatment and persisted for over 3 mo with continuous administration. This model can now be widely applied to study the role of specific genes in the phenotype of adult murine vasculature.

Orchestration of angiogenesis and arteriovenous contribution by angiopoietins and vascular endothelial growth factor (VEGF)

Multiple classes of factors contribute to angiogenesis. In past years, the primary focus has been to understand the functions of individual classes of angiogenic factors. However, few studies have focused on the combinatorial roles of multiple classes of factors in angiogenesis. In this report, we have investigated the in vivo angiogenic processes regulated by two major classes of angiogenic factors, the angiopoietins and vascular endothelial growth factor (VEGF). Here we show that angiopoietin-1, a factor previously considered to be proangiogenic, can offset VEGF-induced angiogenesis in vivo. We also provide direct in vivo evidence for the synergistic effect of angiopoietin-2 and VEGF on the induction of angiogenesis. Furthermore, we show that these two classes of factors control the ratio of arterial and venous blood vessel types during angiogenesis. We believe that our study is a step toward understanding how multiple classes of factors harmonize angiogenesis and blood vessel types.

The NFY transcription factor functions as a repressor and activator of the von Willebrand factor promoter

Human von Willebrand factor (VWF) gene sequences -487 to +247 function as an endothelial-specific promoter in vitro. Analysis of the activation mechanism of the VWF promoter has resulted in the identification of a number of cis-acting elements and trans-acting factors that regulate its activity. The GATA and Ets transcription factors were shown to function as activators of transcription, whereas NF1 and Oct1 were shown to repress transcription. We have reported the presence of another repressor element in exon 1 that interacted with a protein complex designated "R." In the absence of NF1 binding, inhibition of this interaction resulted in promoter activation in nonendothelial cells. We have now identified the "R" protein complex as the NFY transcription factor. Using DNA methylation interference assay and base substitution mutation analysis, we show that NFY interacts with a novel DNA sequence corresponding to nucleotides +226 to +234 in the VWF promoter that does not conform to the consensus NFY binding sequence CCAAT. The VWF gene does contain a CCAAT element that is located downstream of the TATA box and we show that the NFY factor also interacts with this CCAAT element. Using antibodies specific against the A, B, and C subunits of NFY, we demonstrate that the NFY complexes interacting with the CCAAT sequence have a composition similar to that of the repressor binding to the first exon sequences. The results of mutation analysis and transfection studies demonstrated that the interaction of NFY with the upstream CCAAT element is required for VWF promoter activation. Based on these results, we hypothesize that NFY can function both as a repressor and activator of transcription and its function may be modulated through its DNA binding sequences.

The membrane-anchored MMP inhibitor RECK is a key regulator of extracellular matrix integrity and angiogenesis

Matrix metalloproteinases (MMPs) are essential for proper extracellular matrix remodeling. We previously found that a membrane-anchored glycoprotein, RECK, negatively regulates MMP-9 and inhibits tumor invasion and metastasis. Here we show that RECK regulates two other MMPs, MMP-2 and MT1-MMP, known to be involved in cancer progression, that mice lacking a functional RECK gene die around E10.5 with defects in collagen fibrils, the basal lamina, and vascular development, and that this phenotype is partially suppressed by MMP-2 null mutation. Also, vascular sprouting is dramatically suppressed in tumors derived from RECK-expressing fibrosarcoma cells grown in nude mice. These results support a role for RECK in the regulation of MMP-2 in vivo and implicate RECK downregulation in tumor angiogenesis.

Chronic inflammation and protection from acute hepatitis in transgenic mice expressing TNF in endothelial cells

Endothelial activation is an important feature of many inflammatory diseases and has been implicated as the cause of vascular complications in disorders such as diabetes, atherosclerosis, and transplant rejection. One of the most potent activators of the endothelium is TNF, which can also be expressed by endothelial cells, causing a permanent, autocrine stimulatory signal. To establish a model of continuous endothelial activation and to elucidate the role of endothelial derived TNF in vivo, we generated transgenic mice expressing a noncleavable transmembrane form of TNF under the control of the endothelial-specific tie2 promoter. Adult tie2-transmembrane TNF-transgenic mice developed chronic inflammatory pathology in kidney and liver, characterized by perivascular infiltration of mononuclear cells into these organs. Along with the infiltrate, an up-regulation of the adhesion molecules ICAM-1 and VCAM-1, but not E-selectin, in the endothelium was observed. Despite predisposition to chronic inflammation these mice were protected from immune-mediated liver injury in a model of Con A-induced acute hepatitis. Although the blood levels of soluble TNF and IFN-gamma were increased in transgenic animals after challenge with Con A, no damage of hepatocytes could be detected, as assessed by the lack of increase in plasma transaminase activities and the absence of TUNEL staining in the liver. We conclude that expression of transmembrane TNF in the endothelium causes continuous endothelial activation, leading to both proinflammatory and protective events.

Role of thrombin signalling in platelets in haemostasis and thrombosis

Platelets are critical in haemostasis and in arterial thrombosis, which causes heart attacks and other events triggered by abnormal clotting. The coagulation protease thrombin is a potent activator of platelets ex vivo. However, because thrombin also mediates fibrin deposition and because multiple agonists can trigger platelet activation, the relative importance of platelet activation by thrombin in haemostasis and thrombosis is unknown. Thrombin triggers cellular responses at least in part through protease-activated receptors (PARs). Mouse platelets express PAR3 and PAR4 (ref. 9). Here we show that platelets from PAR4-deficient mice failed to change shape, mobilize calcium, secrete ATP or aggregate in response to thrombin. This result demonstrates that PAR signalling is necessary for mouse platelet activation by thrombin and supports the model that mouse PAR3 (mPAR3) does not by itself mediate transmembrane signalling but instead acts as a cofactor for thrombin cleavage and activation of mPAR4 (ref. 10). Importantly, PAR4-deficient mice had markedly prolonged bleeding times and were protected in a model of arteriolar thrombosis. Thus platelet activation by thrombin is necessary for normal haemostasis and may be an important target in the treatment of thrombosis.

A role for thrombin receptor signaling in endothelial cells during embryonic development

The coagulation protease thrombin triggers fibrin formation, platelet activation, and other cellular responses at sites of tissue injury. We report a role for PAR1, a protease-activated G protein-coupled receptor for thrombin, in embryonic development. Approximately half of Par1-/- mouse embryos died at midgestation with bleeding from multiple sites. PAR1 is expressed in endothelial cells, and a PAR1 transgene driven by an endothelial-specific promoter prevented death of Par1-/- embryos. Our results suggest that the coagulation cascade and PAR1 modulate endothelial cell function in developing blood vessels and that thrombin's actions on endothelial cells-rather than on platelets, mesenchymal cells, or fibrinogen-contribute to vascular development and hemostasis in the mouse embryo.

HMG-CoA reductase inhibitor mobilizes bone marrow--derived endothelial progenitor cells

Endothelial progenitor cells (EPCs) have been isolated from circulating mononuclear cells in peripheral blood and shown to incorporate into foci of neovascularization, consistent with postnatal vasculogenesis. These circulating EPCs are derived from bone marrow and are mobilized endogenously in response to tissue ischemia or exogenously by cytokine stimulation. We show here, using a chemotaxis assay of bone marrow mononuclear cells in vitro and EPC culture assay of peripheral blood from simvastatin-treated animals in vivo, that the HMG-CoA reductase inhibitor, simvastatin, augments the circulating population of EPCs. Direct evidence that this increased pool of circulating EPCs originates from bone marrow and may enhance neovascularization was demonstrated in simvastatin-treated mice transplanted with bone marrow from transgenic donors expressing beta-galactosidase transcriptionally regulated by the endothelial cell-specific Tie-2 promoter. The role of Akt signaling in mediating effects of statin on EPCs is suggested by the observation that simvastatin rapidly activates Akt protein kinase in EPCs, enhancing proliferative and migratory activities and cell survival. Furthermore, dominant negative Akt overexpression leads to functional blocking of EPC bioactivity. These findings establish that augmented mobilization of bone marrow-derived EPCs through stimulation of the Akt signaling pathway constitutes a novel function for HMG-CoA reductase inhibitors.

Nkx2.5 and Nkx2.6, homologs of Drosophila tinman, are required for development of the pharynx

Nkx2.5 and Nkx2.6 are murine homologs of Drosophila tinman. Their genes are expressed in the ventral region of the pharynx at early stages of embryogenesis. However, no abnormalities in the pharynges of embryos with mutations in either Nkx2.5 or Nkx2.6 have been reported. To examine the function of Nkx2.5 and Nkx2.6 in the formation of the pharynx, we generated and analyzed Nkx2.5 and Nkx2.6 double-mutant mice. Interestingly, in the double-mutant embryos, the pharynx did not form properly. Pharyngeal endodermal cells were largely missing, and the mutant pharynx was markedly dilated. Moreover, we observed enhanced apoptosis and reduced proliferation in pharyngeal endodermal cells of the double-mutant embryos. These results demonstrated a critical role of the NK-2 homeobox genes in the differentiation, proliferation, and survival of pharyngeal endodermal cells. Furthermore, the development of the atrium was less advanced in the double-mutant embryos, indicating that these two genes are essential for both pharyngeal and cardiac development.

Study of the murine allantois by allantoic explants

The murine allantois will become the umbilical artery and vein of the chorioallantoic placenta. In previous studies, growth and differentiation of the allantois had been elucidated in whole embryos. In this study, the extent to which explanted allantoises grow and differentiate outside of the conceptus was investigated. The explant model was then used to elucidate cell and growth factor requirements in allantoic development. Early headfold-stage murine allantoises were explanted directly onto tissue culture plastic or suspended in test tubes. Explanted allantoises vascularized with distal-to-proximal polarity, they exhibited many of the same signaling factors used by the vitelline and cardiovascular systems, and they contained at least three cell types whose identity, gene expression profiles, topographical associations, and behavior resembled those of intact allantoises. DiI labeling further revealed that isolated allantoises grew and vascularized in the absence of significant cell mingling, thereby supporting a model of mesodermal differentiation in the allantois that is position- and possibly age-dependent. Manipulation of allantoic explants by varying growth media demonstrated that the allantoic endothelial cell lineage, like that of other embryonic vasculatures, is responsive to VEGF(164). Although VEGF(164) was required for both survival and proliferation of allantoic angioblasts, it was not sufficient to induce appropriate epithelialization of these cells. Rather, other VEGF isoforms and/or the outer sheath of mesothelium, whose maintenance did not appear to be dependent upon endothelium, may also play important roles. On the basis of these findings, we propose murine allantoic explants as a new tool for shedding light not only on allantoic development, but for elucidating universal mechanisms of blood vessel formation, including vascular supporting cells, either in the intact organism or in existing in vitro systems.

Caspase-mediated cleavage of hematopoietic progenitor kinase 1 (HPK1) converts an activator of NFkappaB into an inhibitor of NFkappaB

Hematopoietic progenitor kinase 1 (HPK1), a mammalian Ste20-related protein kinase, is a potent stimulator of the stress-activated protein kinases (SAPKs/JNKs). Here we report activation of NFkappaB transcription factors by HPK1 that was independent of SAPK/JNK activation. Overexpression of a dominant-negative SEK1 significantly inhibited SAPK/JNK activation, whereas NFkappaB stimulation by HPK1 remained unaffected. Furthermore, activation of NFkappaB required the presence of full-length, kinase-active HPK1, whereas the isolated kinase domain of HPK1 was sufficient for activation of SAPK/JNK. We also demonstrate that overexpression of a dominant-negative IKKbeta blocks HPK1-mediated NFkappaB activation suggesting that HPK1 acts upstream of the IkappaB kinase complex. In apoptotic myeloid progenitor cells HPK1 was cleaved at a DDVD motif resulting in the release of the kinase domain and a C-terminal part. Although expression of the isolated HPK1 kinase domain led to SAPK/JNK activation, the C-terminal part inhibited NFkappaB activation. This dominant-negative effect was not only restricted to HPK1-mediated but also to NIK- and tumor necrosis factor alpha-mediated NFkappaB activation, suggesting an impairment of the IkappaB kinase complex. Thus HPK1 activates both the SAPK/JNK and NFkappaB pathway in hematopoietic cells but is converted into an inhibitor of NFkappaB activation in apoptotic cells.

Multiple roles of mouse Numb in tuning developmental cell fates

BACKGROUND

Notch signaling regulates multiple differentiation processes and cell fate decisions during both invertebrate and vertebrate development. Numb encodes an intracellular protein that was shown in Drosophila to antagonize Notch signaling at binary cell fate decisions of certain cell lineages. Although overexpression experiments suggested that Numb might also antagonize some Notch activity in vertebrates, the developmental processes in which Numb is involved remained elusive.

RESULTS

We generated mice with a homozygous inactivation of Numb. These mice died before embryonic day E11.5, probably because of defects in angiogenic remodeling and placental dysfunction. Mutant embryos had an open anterior neural tube and impaired neuronal differentiation within the developing cranial central nervous system (CNS). In the developing spinal cord, the number of differentiated motoneurons was reduced. Within the peripheral nervous system (PNS), ganglia of cranial sensory neurons were formed. Trunk neural crest cells migrated and differentiated into sympathetic neurons. In contrast, a selective differentiation anomaly was observed in dorsal root ganglia, where neural crest--derived progenitor cells had migrated normally to form ganglionic structures, but failed to differentiate into sensory neurons.

CONCLUSIONS

Mouse Numb is involved in multiple developmental processes and required for cell fate tuning in a variety of lineages. In the nervous system, Numb is required for the generation of a large subset of neuronal lineages. The restricted requirement of Numb during neural development in the mouse suggests that in some neuronal lineages, Notch signaling may be regulated independently of Numb.

Tie receptors: new modulators of angiogenic and lymphangiogenic responses

Angiogenesis is required for normal embryonic vascular development and aberrant angiogenesis contributes to several diseases, including cancer, diabetes and tissue ischaemia. What are the molecular mechanisms that regulate this important process? The Tie family of receptors and their ligands, the angiopoietins, are beginning to provide insight into how vessels make decisions such as whether to grow or regress--processes that are important not only during development but throughout an organism's life.

Tie2-Cre transgenic mice: a new model for endothelial cell-lineage analysis in vivo

Endocardial cells are thought to contribute at least in part to the formation of the endocardial cushion mesenchyme. Here, we created Tie2-Cre transgenic mice, in which expression of Cre recombinase is driven by an endothelial-specific promoter/enhancer. To analyze the lineage of Cre expressing cells, we used CAG-CAT-Z transgenic mice, in which expression of lacZ is activated only after Cre-mediated recombination. We detected pan-endothelial expression of the Cre transgene in Tie2-Cre;CAG-CAT-Z double-transgenic mice. This expression pattern is almost identical to Tie2-lacZ transgenic mice. However, interestingly, we observed strong and uniform lacZ expression in mesenchymal cells of the atrioventricular canal of Tie2-Cre;CAG-CAT-Z double-transgenic mice. We also detected lacZ expression in the mesenchymal cells in part of the proximal cardiac outflow tract, but not in the mesenchymal cells of the distal outflow tract and branchial arch arteries. LacZ staining in Tie2-Cre;CAG-CAT-Z embryos is consistent with endocardial-mesenchymal transformation in the atrioventricular canal and outflow tract regions. Our observations are consistent with previously reported results from Cx43-lacZ, Wnt1-Cre;R26R, and Pax3-Cre;R26R transgenic mice, in which lacZ expression in the cardiac outflow tract identified contributions in part from the cardiac neural crest. Tie2-Cre transgenic mice are a new genetic tool for the analyses of endothelial cell-lineage and endothelial cell-specific gene targeting.

Role of SCL/Tal-1, GATA, and Ets transcription factor binding sites for the regulation of Flk-1 expression during murine vascular development

The receptor tyrosine kinase Flk-1 is essential for embryonic blood vessel development and for tumor angiogenesis. To identify upstream transcriptional regulators of Flk-1, the gene regulatory elements that mediate endothelium-specific expression in mouse embryos were characterized. By mutational analysis, binding sites for SCL/Tal-1, GATA, and Ets transcription factors located in theFlk-1 enhancer were identified as critical elements for the endothelium-specific Flk-1 gene expression in transgenic mice. c-Ets1, a transcription factor that is coexpressed withFlk-1 during embryonic development and tumor angiogenesis, activated the Flk-1 promoter via 2 binding sites. One of these sites was required for Flk-1 promoter function in the embryonic vasculature. These results provide the first evidence that SCL/Tal-1, GATA, and Ets transcription factors act upstream ofFlk-1 in a combinatorial fashion to determine embryonic blood vessel formation and are key regulators not only of the hematopoietic program, but also of vascular development.

Role of SCL/Tal-1, GATA, and Ets transcription factor binding sites for the regulation of Flk-1 expression during murine vascular development

The receptor tyrosine kinase Flk-1 is essential for embryonic blood vessel development and for tumor angiogenesis. To identify upstream transcriptional regulators of Flk-1, the gene regulatory elements that mediate endothelium-specific expression in mouse embryos were characterized. By mutational analysis, binding sites for SCL/Tal-1, GATA, and Ets transcription factors located in theFlk-1 enhancer were identified as critical elements for the endothelium-specific Flk-1 gene expression in transgenic mice. c-Ets1, a transcription factor that is coexpressed withFlk-1 during embryonic development and tumor angiogenesis, activated the Flk-1 promoter via 2 binding sites. One of these sites was required for Flk-1 promoter function in the embryonic vasculature. These results provide the first evidence that SCL/Tal-1, GATA, and Ets transcription factors act upstream ofFlk-1 in a combinatorial fashion to determine embryonic blood vessel formation and are key regulators not only of the hematopoietic program, but also of vascular development.

Maintenance of vascular integrity in the embryo requires signaling through the fibroblast growth factor receptor

Basic fibroblast growth factor (FGF)-2 is important for vessel formation and/or maintenance of vascular integrity in the embryo. FGF signaling may be mediated through transmembrane tyrosine kinase receptors or directly through intracellular pathways that do not involve receptor activation. To determine the role of receptor-mediated signaling in endothelial cells, an adenovirus encoding truncated FGF receptor (FGFR)-1, under the control of the cytomegalovirus promoter, was expressed in endothelial cells. FGF signaling was impaired, as indicated by inhibition of MAPK phosphorylation. Functional consequences included inhibition of endothelial cell migration and induction of apoptosis. To address the role of endothelial FGFR signaling in vascular development, recombinant adenovirus encoding a dominant-negative FGFR was injected into the sinus venosus of embryonic day 9.0 cultured mouse embryos. Previous studies demonstrated that transgenes delivered via adenovirus, under the control of the cytomegalovirus promoter, are expressed selectively in the developing vasculature. Embryos expressing a control adenovirus developed normally, whereas those expressing the FGFR-1 mutant exhibited abnormal embryonic and extra-embryonic vascular development. These data demonstrate that FGF, by signaling through the FGFR, plays a pivotal role in the development and maintenance of a mature vascular network in the embryo.

Retinoid signaling is essential for patterning the endoderm of the third and fourth pharyngeal arches

The requirement of retinoic acid (RA) in the initial formation of the pharyngeal arches was investigated by treating headfold-stage mouse embryos with a pan-RAR antagonist in vitro and in vivo. This results in a complete absence of mesenchyme, arteries, nerves and epibranchial placodes of the 3rd and 4th pharyngeal arches, complete agenesis of the 3rd and 4th pouches and consistent lack of the 6th arch artery. Mesodermally derived endothelial cells are absent from the 3rd and 4th pharyngeal arch region and the distribution domain of EphA2 transcripts in mesodermal cells is shifted caudally. In situ hybridization with CRABPI, kreisler and EphA4 probes and the pattern of expression of a Wnt1-lacZ transgene show that neural crest cells (NCC) normally destined to the 3rd and 4th arches migrate ectopically. Most interestingly, the appearance of the 3rd and 4th arches is prevented by the antagonist only during a very narrow window of time, which does not correspond to the period of post-otic NCC migration. Both the timing of appearance and the nature of the defects in RAR antagonist-treated embryos indicate that migrating NCC and mesodermal cells destined to the caudal pharyngeal arches do not represent primary targets of RA action. Alterations in the endodermal expression pattern of Hoxa1, Hoxb1, Pax1, Pax9, Fgf3 and Fgf8 in response to the antagonist-induced block in RA signal transduction demonstrate for the first time that RA signaling is indispensable for the specification of the pharyngeal endoderm and suggest that this signaling is necessary to provide a permissive environment locally for the migration of NCC and mesodermal cells. Our study also indicates that the formation of the 2nd pharyngeal arch and that of the 3rd and 4th pharyngeal arches probably involve distinct RA-dependent developmental processes.

In vivo expression profile of an endothelial nitric oxide synthase promoter-reporter transgene

Endothelium-derived nitric oxide (NO) is primarily attributable to constitutive expression of the endothelial nitric oxide synthase (eNOS) gene. Although a more comprehensive understanding of transcriptional regulation of eNOS is emerging with respect to in vitro regulatory pathways, their relevance in vivo warrants assessment. In this regard, promoter-reporter insertional transgenic murine lines were created containing 5,200 bp of the native murine eNOS promoter directing transcription of nuclear-localized beta-galactosidase. Examination of beta-galactosidase expression in heart, lung, kidney, liver, spleen, and brain of adult mice demonstrated robust signal in large and medium-sized blood vessels. Small arterioles, capillaries, and venules of the microvasculature were notably negative, with the exception of the vasa recta of the medullary circulation of the kidney, which was strongly positive. Only in the brain was the reporter expressed in non-endothelial cell types, such as the CA1 region of the hippocampus. Epithelial cells of the bronchi, bronchioles, and alveoli were scored as negative, as was renal tubular epithelium. Cardiac myocytes, skeletal muscle, and smooth muscle of both vascular and nonvascular sources failed to demonstrate beta-galactosidase staining. Expression was uniform across multiple founders and was not significantly affected by genomic integration site. These transgenic eNOS promoter-reporter lines will be a valuable resource for ongoing studies addressing the regulated expression of eNOS in vivo in both health and disease.

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.

The basic helix-loop-helix transcription factor, dHAND, is required for vascular development

Reciprocal interactions between vascular endothelial cells and vascular mesenchymal cells are essential for angiogenesis. Here we show that the basic helix-loop-helix transcription factor, dHAND/Hand2, is expressed in the developing vascular mesenchyme and its derivative, vascular smooth muscle cells (VSMCs). Targeted deletion of the dHAND gene in mice revealed severe defects of embryonic and yolk sac vascular development by embryonic day 9.5. Vascular endothelial cells expressed most markers of differentiation. Vascular mesenchymal cells migrated appropriately but failed to make contact with vascular endothelial cells and did not differentiate into VSMCs. In a screen for genes whose expression was dependent upon dHAND (using subtractive hybridization comparing wild-type and dHAND-null hearts), the VEGF(165) receptor, neuropilin-1, was found to be downregulated in dHAND mutants. These results suggest that dHAND is required for vascular development and regulates angiogenesis, possibly through a VEGF signaling pathway.

SCL Expression in the Mouse Embryo Detected With a Targeted lacZ Reporter Gene Demonstrates Its Localization to Hematopoietic, Vascular, and Neural Tissues

The helix-loop-helix transcription factor SCL (TAL1) is indispensable for blood cell formation in the mouse embryo. We have explored the localization and developmental potential of cells fated to express SCL during murine development using SCL-lacZmutant mice in which the Escherichia coli lacZreporter gene was ‘knocked in’ to the SCL locus. In addition to the hematopoietic defect associated with SCL deficiency, the yolk sac blood vessels in SCLlacZ/lacZ embryos formed an abnormal primary vascular plexus, which failed to undergo subsequent remodeling and formation of large branching vessels. Intraembryonic vasculogenesis in precirculationSCLlacZ/lacZ embryos appeared normal but, in embryos older than embryonic day (E) 8.5 to E9, absolute anemia leading to severe hypoxia precluded an accurate assessment of further vascular development. In heterozygous SCLlacZ/w embryos, lacZ was expressed in the central nervous system, vascular endothelia, and primitive and definitive hematopoietic cells in the blood, aortic wall, and fetal liver. Culture of fetal liver cells sorted for high and low levels of β galactosidase activity fromSCLlacZ/w heterozygous embryos indicated that there was a correlation between the level of SCL expression and the frequency of hematopoietic progenitor cells.

Characterization of the Mouse von Willebrand Factor Promoter

Expression of the von Willebrand factor (vWF) gene is restricted to the endothelial and megakaryocyte lineages. Within the endothelium, expression of vWF varies between different vascular beds. We have previously shown that the human vWF promoter spanning a region between −2182 (relative to the start site of transcription) and the end of the first intron contains information for environmentally responsive, vascular bed-specific expression in the heart, skeletal muscle, and brain. In the present study, we cloned the mouse vWF (mvWF) promoter and studied its function in cultured endothelial cells and transgenic mice. In transient transfection assays, the mvWF gene was found to be regulated by distinct mechanisms in different endothelial cell subtypes. In independent lines of transgenic mice, an mvWF promoter fragment containing DNA sequences between −2645 and the end of the first intron directed endothelial cell-specific expression in the microvascular beds of the heart, brain, and skeletal muscle as well as the endothelial lining of the aorta. In 1 line of mice, reporter gene activity was also detected in bone marrow megakaryocytes. Taken together, these findings suggest that both the mouse and human vWF promoters are regulated by vascular bed-specific mechanisms.

A requirement for neuropilin-1 in embryonic vessel formation

Neuropilin-1 is a membrane protein that is expressed in developing neurons and functions as a receptor or a component of the receptor complex for the class 3 semaphorins, which are inhibitory axon guidance signals. Targeted inactivation of the neuropilin-1 gene in mice induced disorganization of the pathway and projection of nerve fibers, suggesting that neuropilin-1 mediates semaphorin-elicited signals and regulates nerve fiber guidance in embryogenesis. Neuropilin-1 is also expressed in endothelial cells and shown to bind vascular endothelial growth factor (VEGF), a potent regulator for vasculogenesis and angiogenesis. However, the roles of neuropilin-1 in vascular formation have been unclear. This paper reported that the neuropilin-1 mutant mouse embryos exhibited various types of vascular defects, including impairment in neural vascularization, agenesis and transposition of great vessels, insufficient aorticopulmonary truncus (persistent truncus arteriosus), and disorganized and insufficient development of vascular networks in the yolk sac. The vascular defects induced by neuropilin-1 deficiency in mouse embryos suggest that neuropilin-1 plays roles in embryonic vessel formation, as well as nerve fiber guidance.

Embryonic expression and function of the chemokine SDF-1 and its receptor, CXCR4

Directed cell movement is integral to both embryogenesis and hematopoiesis. In the adult, the chemokine family of secreted proteins signals migration of hematopoietic cells through G-coupled chemokine receptors. We detected embryonic expression of chemokine receptor messages by RT-PCR with degenerate primers at embryonic day 7.5 (E7.5) or by RNase protection analyses of E8.5 and E12.5 tissues. In all samples, the message encoding CXCR4 was the predominate chemokine receptor detected, particularly at earlier times (E7.5 and E8.5). Other chemokine receptor messages (CCR1, CCR4, CCR5, CCR2, and CXCR2) were found in E12.5 tissues concordant temporally and spatially with definitive (adult-like) hematopoiesis. Expression of CXCR4 was compared with that of its only known ligand, stromal cell-derived factor-1 (SDF-1), by in situ hybridization. During organogenesis, these genes have dynamic and complementary expression patterns particularly in the developing neuronal, cardiac, vascular, hematopoietic, and craniofacial systems. Defects in the first four of these systems have been reported in CXCR4- and SDF-1-deficient mice. Our studies suggest new potential mechanisms for some of these defects as well as additional roles beyond the scope of the reported abnormalities. Earlier in development, expression of these genes correlates with migration during gastrulation. Migrating cells (mesoderm and definitive endoderm) contain CXCR4 message while embryonic ectoderm cells express SDF-1. Functional SDF-1 signaling in midgastrula cells as well as E12.5 hematopoietic progenitors was demonstrated by migration assays. Migration occurred with an optimum dose similar to that found for adult hematopoietic cells and was dependent on the presence of SDF-1 in a gradient. This work suggests roles for chemokine signaling in multiple embryogenic events.

Octamer-dependent in vivo expression of the endothelial cell-specific TIE2 gene

The TIE2 gene, also known as TEK, encodes a tyrosine kinase receptor that is required for the normal development of the vascular system during embryogenesis. TIE2 is specifically expressed in endothelial cells; however, the transcriptional mechanisms that regulate this highly restricted pattern of expression remain unknown. Here we demonstrate that a consensus octamer element located in the 5'-flanking region of TIE2 is required for normal expression in embryonic endothelial cells. Transgenic embryos carrying a TIE2/LacZ construct spanning 2.1 kilobases of upstream regulatory sequences exhibit expression of the reporter transgene specifically in endothelial cells. Site-directed mutagenesis of a consensus octamer element located in this region results in the loss of enhancer activity and significantly impairs the endothelial expression of the reporter transgene. Consistent with the in vivo data, in vitro DNA-protein binding studies show that the consensus octamer element displays an endothelial cell-specific pattern of binding, suggesting an interaction with a protein complex consisting of Oct1 and an endothelial cell-restricted cofactor. These data identify a novel role for the octamer element as an essential regulator of TIE2 expression, define the first known transcriptional pathway that mediates the expression of a developmental endothelial cell gene, and provide insights into the transcriptional mechanisms that regulate development of the vasculature during embryogenesis.

The transcription factor MEF2C-null mouse exhibits complex vascular malformations and reduced cardiac expression of angiopoietin 1 and VEGF

The MEF2 family of transcription factors has been implicated in transcriptional regulation in a number of different cell types. Targeted deletion of the MEF2C gene, in particular, revealed its importance for early cardiogenesis (Q. Lin et al., 1997, Science 276, 1404-1407). We report here that this deletion also resulted in vascular anomalies characterized by extreme variability in lumen size and defects in remodeling. While primary vascular networks formed in the yolk sac of the mutants, they failed to remodel into more complex vascular structures. Likewise, although the primordia of the dorsal aortae formed normally, anomalies were observed in these vessels later in development. Dorsal and anterior to the heart, the aortae exhibited abnormally small lumens, as did the anterior cardinal veins and intersegmental arteries. In contrast, the dorsal aortae and intersegmental arteries caudal to the heart were grossly enlarged. Cranial vessels were also enlarged and less branched than normal. Endocardiogenesis in the mutant was abnormal with the endothelial cells exhibiting a number of aberrant phenotypes. These endocardial defects were accompanied by a notable reduction in angiopoietin 1 and VEGF mRNA production by the myocardium, indicating that MEF2C is required for myocardial expression of these important endothelial-directed cytokines and thus for correct endocardial morphogenesis.

Identification of Vascular Endothelial Growth Factor (VEGF) Receptor-2 (Flk-1) Promoter/Enhancer Sequences Sufficient for Angioblast and Endothelial Cell-Specific Transcription in Transgenic Mice

The vascular endothelial growth factor (VEGF) receptor-2 (Flk-1) is the first endothelial receptor tyrosine kinase to be expressed in angioblast precursors, and its function is essential for the differentiation of endothelial cells and hematopoietic precursors. We have identified cis-acting regulatory elements of the murineFlk-1 gene that mediate endothelium-specific expression of a LacZ reporter gene in transgenic mice. Sequences within the 5′-flanking region of the Flk-1 gene, in combination with sequences located in the first intron, specifically targeted transgene expression to angioblasts and endothelial cells of transgenic mice. The intronic regulatory sequences functioned as an autonomous endothelium-specific enhancer. Sequences of the 5′-flanking region contributed to a strong, uniform, and reproducible transgene expression and were stimulated by the transcription factor HIF-2. The Flk-1 gene regulatory elements described in this study should allow the elucidation of the molecular mechanisms involved in endothelial cell differentiation and angiogenesis.

Role of the Ets transcription factors in the regulation of the vascular-specific Tie2 gene

The Tie2 gene encodes a vascular endothelium-specific receptor tyrosine kinase that is required for normal vascular development and is also upregulated during angiogenesis. The regulatory regions of the Tie2 gene that are required for endothelium-specific gene expression in vivo have been identified. However, the transcription factors required for Tie2 gene expression remain largely unknown. We have identified highly conserved binding sites for Ets transcription factors in the Tie2 promoter. Mutations in 2 particular binding sites lead to a 50% reduction in the endothelium-specific activity of the promoter. We have compared the ability of several members of the Ets family to transactivate the Tie2 promoter. Our results demonstrate that 1 of 3 distinct isoforms of the novel Ets transcription factor NERF, NERF2, is expressed in endothelial cells and can strongly transactivate the regulatory regions of the Tie2 gene in comparison to other Ets factors, which have little or no effect. NERF2 can bind to the Tie2 promoter Ets sites in electrophoretic mobility shift assays. These studies support a role for Ets factors in the regulation of vascular-specific gene expression and suggest that the novel Ets factor NERF2 may be a critical transcription factor in specifying the expression of the Tie2 gene in vascular endothelial cells.

Role of vascular endothelial-cadherin in vascular morphogenesis

Vascular endothelial (VE)-cadherin is an adhesive transmembrane protein specifically expressed at interendothelial junctions. Its extracellular domain exhibits Ca2+-dependent homophilic reactivity, promoting cell-cell recognition. Mice deficient in VE-cadherin die at mid-gestation resulting from severe vascular defects. At the early phases of vascular development (E8.5) of VE-cadherin-deficient embryos, in situ differentiation of endothelial cells was delayed although their differentiation program appeared normal. Vascularization was defective in the anterior part of the embryo, while dorsal aortae and vitelline and umbilical arteries formed normally in the caudal part. At E9.25, organization of endothelial cells into large vessels was incomplete and angiogenesis was impaired in mutant embryos. Defects were more severe in extraembryonic vasculature. Blood islands of the yolk sac and clusters of angioblasts in allantois failed to establish a capillary plexus and remained isolated. This was not due to defective cell-cell recognition as endothelial cells formed intercellular junctions, as shown by electron microscopy. These data indicate that VE-cadherin is dispensable for endothelial homophilic adhesion but is required for vascular morphogenesis.

Distinct 5' SCL enhancers direct transcription to developing brain, spinal cord, and endothelium: neural expression is mediated by GATA factor binding sites

The SCL gene encodes a basic helix-loop-helix transcription factor with a pivotal role in the development of endothelium and of all hematopoietic lineages. SCL is also expressed in the central nervous system, although its expression pattern has not been examined in detail and its function in neural development is unknown. In this article we present the first analysis of SCL transcriptional regulation in vivo. We have identified three spatially distinct regulatory modules, each of which was both necessary and sufficient to direct reporter gene expression in vivo to three different regions within the normal SCL expression domain, namely, developing endothelium, midbrain, and hindbrain/spinal cord. In addition we have demonstrated that GATA factor binding sites are essential for neural expression of the SCL constructs. The midbrain element was particularly powerful and axonal lacZ expression revealed the details of axonal projections, thus implicating SCL in the development of occulomotor, pupillary, or retinotectal pathways. The neural expression pattern of the SCL gene was highly conserved in mouse, chicken, and zebrafish embryos and the 5' region of the chicken SCL locus exhibited a striking degree of functional conservation in transgenic mice. These data suggest that SCL performs critical functions in neural development. The regulatory elements identified here provide important tools for analyzing these functions.

Ischemia- and cytokine-induced mobilization of bone marrow-derived endothelial progenitor cells for neovascularization

Endothelial progenitor cells (EPCs) have been isolated from circulating mononuclear cells in human peripheral blood and shown to be incorporated into foci of neovascularization, consistent with postnatal vasculogenesis. We determined whether endogenous stimuli (tissue ischemia) and exogenous cytokine therapy (granulocyte macrophage-colony stimulating factor, GM-CSF) mobilize EPCs and thereby contribute to neovascularization of ischemic tissues. The development of regional ischemia in both mice and rabbits increased the frequency of circulating EPCs. In mice, the effect of ischemia-induced EPC mobilization was demonstrated by enhanced ocular neovascularization after cornea micropocket surgery in mice with hindlimb ischemia compared with that in non-ischemic control mice. In rabbits with hindlimb ischemia, circulating EPCs were further augmented after pretreatment with GM-CSF, with a corresponding improvement in hindlimb neovascularization. There was direct evidence that EPCs that contributed to enhanced corneal neovascularization were specifically mobilized from the bone marrow in response to ischemia and GM-CSF in mice transplanted with bone marrow from transgenic donors expressing beta-galactosidase transcriptionally regulated by the endothelial cell-specific Tie-2 promoter. These findings indicate that circulating EPCs are mobilized endogenously in response to tissue ischemia or exogenously by cytokine therapy and thereby augment neovascularization of ischemic tissues.

Developmental platelet endothelial cell adhesion molecule expression suggests multiple roles for a vascular adhesion molecule

Platelet endothelial cell adhesion molecule (PECAM) is used extensively as a murine vascular marker. PECAM interactions have been implicated in both vasculogenesis and angiogenesis. To better understand the role of PECAM in mammalian development, PECAM expression was investigated during differentiation of murine embryonic stem (ES) cells and in early mouse embryos. Undifferentiated ES cells express PECAM, and as in vitro differentiation proceeds previously unidentified PECAM-positive cells that are distinct from vascular endothelial cells appear. PECAM expression is gradually restricted to endothelial cells and some hematopoietic cells of differentiated blood islands. In embryos, the preimplantation blastocyst contains PECAM-positive cells. PECAM expression is next documented in the postimplantation embryonic yolk sac, where clumps of mesodermal cells express PECAM before the development of mature blood islands. The patterns of PECAM expression suggest that undifferentiated cells, a prevascular cell type, and vascular endothelial cells express this marker during murine development. PECAM expression in blastocysts and by ES cells suggests that PECAM may function outside the vascular/hematopoietic lineage.

The cardiac homeobox gene Csx/Nkx2.5 lies genetically upstream of multiple genes essential for heart development

Csx/Nkx2.5 is a vertebrate homeobox gene with a sequence homology to the Drosophila tinman, which is required for the dorsal mesoderm specification. Recently, heterozygous mutations of this gene were found to cause human congenital heart disease (Schott, J.-J., Benson, D. W., Basson, C. T., Pease, W., Silberbach, G. M., Moak, J. P., Maron, B. J., Seidman, C. E. and Seidman, J. G. (1998) Science 281, 108–111). To investigate the functions of Csx/Nkx2.5 in cardiac and extracardiac development in the vertebrate, we have generated and analyzed mutant mice completely null for Csx/Nkx2.5. Homozygous null embryos showed arrest of cardiac development after looping and poor development of blood vessels. Moreover, there were severe defects in vascular formation and hematopoiesis in the mutant yolk sac. Interestingly, TUNEL staining and PCNA staining showed neither enhanced apoptosis nor reduced cell proliferation in the mutant myocardium. In situ hybridization studies demonstrated that, among 20 candidate genes examined, expression of ANF, BNP, MLC2V, N-myc, MEF2C, HAND1 and Msx2 was disturbed in the mutant heart. Moreover, in the heart of adult chimeric mice generated from Csx/Nkx2.5 null ES cells, there were almost no ES cell-derived cardiac myocytes, while there were substantial contributions of Csx /Nkx2.5-deficient cells in other organs. Whole-mount β-gal staining of chimeric embryos showed that more than 20% contribution of Csx/Nkx2. 5-deficient cells in the heart arrested cardiac development. These results indicate that (1) the complete null mutation of Csx/Nkx2.5 did not abolish initial heart looping, (2) there was no enhanced apoptosis or defective cell cycle entry in Csx/Nkx2.5 null cardiac myocytes, (3) Csx/Nkx2.5 regulates expression of several essential transcription factors in the developing heart, (4) Csx/Nkx2.5 is required for later differentiation of cardiac myocytes, (5) Csx/Nkx2. 5 null cells exert dominant interfering effects on cardiac development, and (6) there were severe defects in yolk sac angiogenesis and hematopoiesis in the Csx/Nkx2.5 null embryos.

Role of ets factors in the activity and endothelial cell specificity of the mouse Tie gene promoter

The Tie gene encodes an endothelial cell receptor tyrosine kinase necessary for normal vascular development. The Tie gene promoter targets expression of heterologous genes specifically to endothelial cells in transgenic mice. Here we have characterized the promoter sequences critical for endothelial cell-specific activity in cultured cells and transgenic mice. Progressive deletions and site-directed mutations of the promoter showed that the critical endothelial cell-specific elements are an octamer transcription factor binding site and several Ets binding sites located in two clusters within 300 bp upstream of the major transcription initiation site. Among members of the Ets transcription factor family tested, NERF-2 (a novel transcription factor related to the ets factor ELF-1), which is expressed in endothelial cells, and ETS2 showed the strongest transactivation of the Tie promoter; ETS1 gave lower levels of stimulation and the other Ets factors gave little or no transactivation. Furthermore, the Tie promoter directed the production of high amounts of human growth hormone into the circulation of transgenic mice. The secreted amounts correlated with transgene copy number, being relatively insensitive to the effects of the transgene integration site. These properties suggest that Tie promoter activity is controlled by endothelial cell Ets factors and that it has potential for use in vectors for endothelial cell-specific gene expression.