Endothelial signal integration in vascular assembly

TLR4 signaling induces TLR2 expression in endothelial cells via neutrophil NADPH oxidase

Interactions of polymorphonuclear neutrophils (PMNs) with endothelial cells may contribute to the activation of endothelial cell responses involved in innate immunity. We explored a novel function of PMN NADPH oxidase in the mechanism of Toll-like receptor-2 (TLR2) upregulation induced by LPS-TLR4 signaling in endothelial cells. We showed that LPS induced TLR2 up-regulation through TLR4- and MyD88-dependent signaling. In neutropenic mice, the LPS-induced NF-kB activation and TLR2 expression were significantly reduced, and both responses were restored upon repletion by PMN obtained from WT mice but not by PMNs from NADPH oxidase gp91pho(-/-) mice. These findings were recapitulated in mouse lung vascular endothelial cells cocultured with PMNs, indicating that the augmented NF-kB activation and the resultant TLR2 upregulation in endothelial cells were secondary to oxidant signaling generated by PMN NADPH oxidase. The functional relevance of NADPH oxidase in mediating TLR4-induced TLR2 expression in endothelial cells was evident by markedly elevated and stable ICAM-1 expression as well as augmented PMN migration in response to sequential challenge with LPS and peptidoglycan. Thus, PMN NADPH oxidase-derived oxidant signaling is an important determinant of the cross talk between TLR4 and TLR2 and the control of endothelial cell activation.

Hypoxia-Inducible Factors and Kidney Vascular Development

ABSTRACT. Among the genes strongly induced by hypoxia-inducible factors (HIF) and highly expressed during kidney microvascular development is vascular endothelial growth factor, which encodes a potent endothelial mitogen and chemoattractant critical for embryonic vasculogenesis and angiogenesis. In developing kidney, glomerular podocytes are particularly rich sources of vascular endothelial growth factor, which probably serves to attract endothelial precursors into vascular clefts of immature glomeruli, promote their mitosis and differentiation into glomerular endothelial cells, and assist with maintenance of their highly differentiated state through maturation. This article summarizes the structure, function, and expression of HIF and discusses HIF target genes expressed during kidney vascular development. Furthermore, it is speculated that different HIF heterodimers are stabilized in different cell populations, which may lead to cell-selective induction of HIF target genes important for renal vasculogenesis/angiogenesis. E-mail: dabrahamson@kumc.edu

Stem cell-derived endothelial cells/progenitors migrate and pattern in the embryo using the VEGF signaling pathway

Endothelial precursor cells respond to molecular cues to migrate and assemble into embryonic blood vessels, but the signaling pathways involved in vascular patterning are not well understood. We recently showed that avian vascular patterning cues are recognized by mammalian angioblasts derived from somitic mesoderm through analysis of mouse-avian chimeras. To determine whether stem cell-derived endothelial cells/progenitors also recognize global patterning signals, murine ES cell-derived embryoid bodies (EBs) were grafted into avian hosts. ES cell-derived murine endothelial cells/progenitors migrated extensively and colonized the appropriate host vascular beds. They also formed mosaic vessels with avian endothelial cells. Unlike somite derived-endothelial cells, ES cell-derived endothelial cells/progenitors migrated across the host embryonic midline to the contralateral side. To determine the role of VEGF signaling in embryonic vascular patterning, EBs mutant for a VEGF receptor (flk-1(-/-)) or a signal (VEGF-A(-/-)) were grafted into quail hosts. Flk-1(-/-) EB grafts produced only rare endothelial cells that did not migrate or assemble into vessels. In contrast, VEGF-A(-/-) EB grafts produced endothelial cells that resembled wild-type and colonized host vascular beds, suggesting that host-derived signals can partially rescue mutant graft vascular patterning. VEGF-A(-/-) graft endothelial cells/progenitors crossed the host midline with much lower frequency than wild-type EB grafts, indicating that graft-derived VEGF compromised the midline barrier when present. Thus, ES cell-derived endothelial cells/progenitors respond appropriately to global vascular patterning cues, and they require the VEGF signaling pathway to pattern properly. Moreover, EB-avian chimeras provide an efficient way to screen mutations for vascular patterning defects.

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.

Vascularization of the developing chick limb bud: role of the TGFbeta signalling pathway

The developing vertebrate limb has fascinated developmental biologists and theoreticians for decades as a model system for investigating cell fate, cell signalling and tissue interactions. We are beginning to understand the mechanisms and signalling pathways that control and regulate the outgrowth and formation of the limb bud into a differentiated identifiable limb by late embryogenesis. However, the mechanisms underlying the development and maintenance of the vasculature of the developing limb are far from being completely understood. The vasculature supplies oxygen, nutrients and signals to developing tissues, allowing them to develop and grow. Moreover, a lot of evidence recently points to molecules involved in morphological development also controlling vascular development. Thus understanding how the vasculature forms and is patterned in the developing limb may further our understanding of limb development. In this review I outline how blood vessels are formed and maintained and how the developing chick limb is vascularized. I also review the role of the TGFbeta superfamily signalling pathway in the development of the chick limb vasculature: in particular, how antagonizing TGFbeta signalling in the developing chick limb has shed new light on the role vascular smooth muscle cells play in vessel calibre control and how this work has added to our understanding of TGFbeta superfamily signal transduction.

Enhancement of sphingosine 1-phosphate-induced migration of vascular endothelial cells and smooth muscle cells by an EDG-5 antagonist

Sphingosine 1-phosphate (Sph-1-P), a bioactive lysophospholipid capable of inducing a wide spectrum of biological responses, acts as an intercellular mediator, through interaction with the endothelial differentiation gene (EDG)/S1P family of G protein-coupled receptors. In this study, the effects of JTE-013, a specific antagonist of the migration-inhibitory receptor EDG-5, on Sph-1-P-elicited responses were examined in human umbilical vein endothelial cells (HUVECs) and vascular smooth muscle cells (SMCs), which expressed EDG-5 protein weakly and abundantly, respectively. This pyrazolopyridine compound reversed the inhibitory effect of Sph-1-P on SMC migration and further enhanced Sph-1-P-stimulated HUVEC migration. In contrast, its effect on Sph-1-P-induced intracellular Ca(2+) mobilization was marginal. Our results indicate that specific regulation of Sph-1-P-modulated migration responses in vascular cells can be achieved by EDG-5 antagonists and that manipulation of Sph-1-P biological activities by each EDG antagonist may lead to a therapeutical application to control vascular diseases.

Parametric imaging of tumor perfusion using flow- and permeability-limited tracers

PURPOSE

To quantitatively evaluate the spatial distribution of flow- and permeability-limited perfusion in MCF7 human breast cancer tumors orthotopically implanted in CD1-NU mice.

MATERIALS AND METHODS

Flow-limited perfusion was derived from (2)H-MRI recorded before and after infusion of deuterated water. Permeability-limited perfusion was evaluated from GdDTPA-enhanced (1)H-MRI.

RESULTS

The dominant processes in tumor perfusion, namely blood flow and capillary permeability, were mapped in orthotopically implanted MCF7 human breast cancer tumors. The dynamic data were processed according to physiological models, yielding parametric maps of intravascular volume fraction, water perfusion rate, GdDTPA permeability rate constant, and extracellular volume fraction accessible to GdDTPA. The maps exhibited the heterogeneous distribution of each perfusion parameter. Most of the tumor tissue (> or =95%) was perfused with HDO, while GdDTPA was perfused in only about 50% of it. In most loci the perfusion rate was limited by capillary permeability to GdDTPA.

CONCLUSION

The results demonstrated the instructive value of tracers with different properties used in conjunction to achieve a deeper understanding of tumor perfusion capacity. This study offers tools for the accurate, noninvasive evaluation of drug delivery efficacy.

Absence of erythrogenesis and vasculogenesis in Plcg1-deficient mice

Mice nullizygous for Plcg1 cease growing at early to mid-gestation. An examination of carefully preserved wild-type embryos shows clear evidence of erythropoiesis, but erythropoiesis is not evident in Plcg1 nullizygous embryos at the same stage. The analyses of embryonic materials demonstrate that in the absence of Plcg1, erythroid progenitors cannot be detected in the yolk sac or embryo body by three different assays, burst-forming units, colony-forming units, and analysis for the developmental marker Ter119. However, non-erythroid granulocyte/macrophage colonies are produced by Plcg1 null embryos. Further analysis of these embryos demonstrates significantly diminished vasculogenesis in Plcg1 nullizygous embryos based on the lack of expression of the endothelial marker platelet endothelial cell adhesion molecule-1. In addition, Plcg1 nullizygous embryos express a greatly reduced level of vascular endothelial growth factor receptor-2/Flk-1, consistent with significantly impaired vasculogenesis and erythropoiesis. Interestingly, these early embryos do express phospholipase C-gamma2, however, it is unable to substitute for the absence of phospholipase C-gamma1, which can be detected in its tyrosine-phosphorylated state.

Stabilization of vascular endothelial growth factor mRNA by hypoxia-inducible factor 1

Hypoxia regulates expression of vascular endothelial growth factor (VEGF) by increasing its transcription and by stabilizing its mRNA. Despite the pivotal role of hypoxia-inducible factor 1 (HIF-1) in transcriptional activation of hypoxia-responsive genes, it is not known whether HIF-1 mediates hypoxia-induced stabilization of VEGF mRNA. We constructed adenoviral vectors expressing either the wild-type HIF-1 alpha (Ad2/HIF-1 alpha/FL), a constitutively stable hybrid form of HIF-1 alpha (Ad2/HIF-1 alpha/VP16), or no transgene (Ad2/CMVEV). In rat glioma (C6) cells and human cardiac, vascular smooth muscle, and endothelial cells, infection with Ad2/HIF-1 alpha/VP16 or Ad2/HIF-1 alpha/FL increased VEGF expression at both the mRNA and protein levels. Under normoxic conditions, the half-life of VEGF mRNA was 42 min in C6 cells. Hypoxia and Ad2/HIF-1 alpha/VP16 increased the half-life of VEGF mRNA to 3.3 and 2.7 h, respectively, while Ad2/CMVEV had no effect. These studies are the first to demonstrate that overexpression of HIF-1 alpha increases VEGF mRNA stability. Our results also suggest that stabilization of VEGF mRNA by hypoxia is mediated, at least in part, by HIF-1.

Smad7 misexpression during embryonic angiogenesis causes vascular dilation and malformations independently of vascular smooth muscle cell function

Numerous in vitro and in vivo studies implicate transforming growth factor-beta (TGFbeta) superfamily signaling in vascular development and maintenance. Mice and humans with mutations in TGFbeta superfamily signaling pathway genes exhibit a range of vascular defects that include dilated, fragile and hemorrhagic vessels, defective angiogenic remodeling, severe vascular malformations including arterio-venous malformations, and disrupted vascular smooth muscle cell recruitment and maintenance. Despite a wealth of data, the functions of TGFbeta superfamily signals during angiogenesis are poorly defined, since early embryonic lethality and difficulty distinguishing between primary and secondary defects frequently confound phenotypic interpretation. To perturb TGFbeta superfamily signaling during angiogenesis, we have misexpressed Smad7, an intracellular antagonist of TGFbeta superfamily signaling, in the developing chick limb and head. We find that the great vessels are strikingly dilated and frequently develop intra and intervascular shunts. Neither noggin nor dominant negative BMP receptor misexpression causes similar vascular phenotypes. However, simultaneous misexpression of constitutively active BMP receptors with Smad7 suppresses the Smad7-induced phenotype, suggesting that a BMP-like intracellular pathway is the target of Smad7 action. Despite the gross morphological defects, further analyses find no evidence of hemorrhage and vessel structure is normal. Furthermore, enlarged vessels and vascular malformations form in either the presence or absence of vascular smooth muscle, and vascular smooth muscle cell recruitment is unperturbed. Our data define the TGFbeta superfamily pathway as an integral regulator of vessel caliber that is also essential for appropriate vessel connectivity. They demonstrate that dilation need not result in vessel rupture or hemorrhage, and dissociate vessel maintenance from the presence of a vascular smooth muscle cell coat. Furthermore they uncouple vascular smooth muscle cell recruitment and differentiation from TGFbeta superfamily signaling.

Integrin and growth factor receptor crosstalk

Crosstalk between integrins and growth factor receptors are an important signaling mechanism to provide specificity during normal development and pathological processes in vascular biology. Evidence from several model systems demonstrates the physiological importance of the coordination of signals from growth factors and the extracellular matrix to support cell proliferation, migration, and invasion in vivo. Several examples of crosstalk between these two important classes of receptors indicate that integrin ligation is required for growth factor-induced biological processes. Furthermore, integrins can directly associate with growth factor receptors, thereby regulating the capacity of integrin/growth factor receptor complexes to propagate downstream signaling. Recent data suggest that antagonists of alpha(v) integrins can provide a therapeutic benefit in human cancer patients, whereas knockout mice lacking specific integrins can provide an interesting insight into the role of integrins during development. This review will focus on the biological importance of integrin and growth factor receptor crosstalk that occurs during cell growth, migration, and invasion as well as in endothelial cells during angiogenesis.

Temporally compartmentalized expression of ephrin-B2 during renal glomerular development

Glomerular development proceeds through the spatially ordered and sequential recruitment, proliferation, assembly, and differentiation of endothelial, mesangial, and epithelial progenitors. The molecular determinants of cell-cell recognition and targeting in this process have yet to be defined. The Eph/ephrin family of membrane receptors and counter-receptors are critical participants of developmental vascular assembly in extrarenal sites. Renal expression patterns of ephrin-B2 and EphB4 were investigated using mice expressing beta-galactosidase under control of ephrin-B2 or EphB4 promoters. The earliest glomerular expression of ephrin-B2 was identified in a subset of differentiating comma-stage glomerular epithelial cells (podocyte progenitors) adjacent to the vascular cleft where endothelial progenitors are subsequently recruited. Epithelial ephrin-B2 expression was accompanied by expression in endothelial and mesangial cells as capillary assembly progressed. At or near completion of glomerular maturation, epithelial ephrin-B2 expression was extinguished, with persistence in glomerular endothelial cells. Throughout development, one of several ephrin-B2 receptors, EphB4, was persistently and exclusively expressed in endothelial cells of venous structures. The findings show sequential ephrin-B2 expression across glomerular lineages, first in a distinct subset of podocyte progenitors and subsequently in endothelial cells of the developing glomerulus. Given targeting functions for Eph/ephrin family proteins, the findings suggest that ephrin-B2 expression marks podocyte progenitors at the site of vascular cleft formation, where expression may establish an "address" to which endothelial and mesangial progenitors are recruited. Thus, the present results suggest that ephrin-B2 and EphB interactions play an important role in glomerular microvascular assembly.

Matrix metalloproteinases as targets for therapy in Kaposi sarcoma

Acquired immune deficiency syndrome-associated Kaposi sarcoma is a progressive and occasionally fatal condition. The strong angiogenic component of this disease makes it particularly suitable for treatment with the emerging class of drugs that act as antiangiogenic agents. Matrix metalloproteinases have been shown to play prominent roles in the angiogenic process, and small molecule inhibitors of these enzymes are currently being tested as antiangiogenic agents in various malignancies. Given that matrix metalloproteinases contribute to multiple steps of the angiogenic process, inhibitors of these enzymes, either alone or in combination with other agents, may represent a particularly effective therapeutic approach for Kaposi sarcoma.

Akt-mediated phosphorylation of the G protein-coupled receptor EDG-1 is required for endothelial cell chemotaxis

The role of the protein kinase Akt in cell migration is incompletely understood. Here we show that sphingosine-1-phosphate (S1P)-induced endothelial cell migration requires the Akt-mediated phosphorylation of the G protein-coupled receptor (GPCR) EDG-1. Activated Akt binds to EDG-1 and phosphorylates the third intracellular loop at the T(236) residue. Transactivation of EDG-1 by Akt is not required for G(i)-dependent signaling but is indispensable for Rac activation, cortical actin assembly, and chemotaxis. Indeed, T236AEDG-1 mutant sequestered Akt and acted as a dominant-negative GPCR to inhibit S1P-induced Rac activation, chemotaxis, and angiogenesis. Transactivation of GPCRs by Akt may constitute a specificity switch to integrate rapid G protein-dependent signals into long-term cellular phenomena such as cell migration.

Expression and potential role of angiopoietins and Tie-2 in early development of the mouse metanephros

Angiopoietins (Ang) are secreted factors which bind the Tie-2 receptor and modulate endothelial growth. This signalling system is known to be expressed in later stages of maturation of the mouse metanephros, the adult kidney precursor. In this study, by using reverse transcription polymerase chain reaction and Northern and Western blotting, we demonstrated that Ang-1, Ang-2, and Tie-2 were expressed during early metanephrogenesis when interstitial and glomerular capillaries begin to form. By using immunohistochemistry, embryonic kidney capillaries in the interstitium and glomeruli expressed Tie-2 at a later stage of differentiation compared with vascular endothelial growth factor receptor-2 and platelet-endothelial cell adhesion molecule. Addition of 200 ng/ml Ang-1 to explanted embryonic day (E) 12.5 metanephroi increased the proportion of vascular glomeruli that formed during 1 week in culture. These results are consistent with the hypotheses that Tie-2 has a role in vascular growth in the early stages of mammalian nephrogenesis and that Tie-2 activation may maintain the integrity of recently formed interstitial and glomerular vessels.

Function and spatial distribution of ion channels and transporters in cell migration

Cell migration plays a central role in many physiological and pathophysiological processes, such as embryogenesis, immune defense, wound healing, or the formation of tumor metastases. Detailed models have been developed that describe cytoskeletal mechanisms of cell migration. However, evidence is emerging that ion channels and transporters also play an important role in cell migration. The purpose of this review is to examine the function and subcellular distribution of ion channels and transporters in cell migration. Topics covered will be a brief overview of cytoskeletal mechanisms of migration, the role of ion channels and transporters involved in cell migration, and ways by which a polarized distribution of ion channels and transporters can be achieved in migrating cells. Moreover, a model is proposed that combines ion transport with cytoskeletal mechanisms of migration.

Coexpression of neuropilin-1, Flk1, and VEGF(164) in developing and mature mouse kidney glomeruli

Neuropilin-1, a neuronal cell surface semaphorin III receptor protein important for axonal guidance in developing peripheral nervous system efferents, has also been identified as a vascular endothelial growth factor (VEGF) receptor on endothelial cells. To evaluate its expression in kidney, we carried out RT-PCR on newborn and adult total renal RNAs. A 403-bp product, which was predicted to be that from neuropilin-1 mRNA, was found in both samples. Nucleotide sequencing confirmed that these products encoded neuropilin-1. Northern analysis of newborn and adult kidney RNA showed specific hybridization to appropriately sized bands of approximately 6 kb. In situ hybridization with a mouse-specific antisense neuropilin-1 (35)S-cRNA probe showed distinct glomerular localization on sections from both newborns and adults. Similar patterns of hybridization were seen in sections treated with antisense cRNA probes against another VEGF receptor, Flk1, and with VEGF probes. However, the VEGF hybridization signal was markedly less in adult glomeruli than those for neuropilin-1 and Flk1. Because neuropilin-1 specifically binds VEGF(165) in humans, we carried out RT-PCR on mouse kidney RNA with primers that amplified the three alternatively spliced isoforms of VEGF mRNA. Our analysis showed that for both newborn and adult kidneys, the relative abundance of VEGF mRNA was VEGF(164) >> VEGF(120) > VEGF(188). We conclude that the expression of neuropilin-1, in conjunction with Flk1 and VEGF(164), jointly contributes to the development and maintenance of glomerular capillaries.