Sphingosine 1-phosphate receptor regulation of N-cadherin mediates vascular stabilization

Identification of a novel dual E- and N-cadherin antagonist

E- and N-cadherin are related calcium-dependent cell adhesion molecules that exert an influence over multiple biological and disease processes. Antagonists of these cadherins can therefore be envisaged as therapeutically useful drugs. We have used phage display technology to discover such antagonists. A peptide phage library was screened against a chimeric protein composed of the human E-cadherin ectodomain fused to the Fc fragment of human immunoglobulin G1 (E-cad/Fc). All of the phage clones that were isolated also bound a chimeric protein composed of the human N-cadherin ectodomain fused to the Fc fragment of human immunoglobulin G1 (N-cad/Fc). A peptide displayed by several of the isolated phage clones was synthesized (H-SWELYYPLRANL-NH2) and found to bind both E- and N-cad/Fc chimeric proteins with affinities (K(D)) of 9.4 microM and 323 nM, respectively, as judged by surface plasmon resonance spectroscopy. This peptide was also capable of blocking the aggregation of E- and N-cad/Fc chimeric protein-coated beads, as well as the aggregation of MCF-7 and MDA-MB435 human breast cancer cells (these cells express E- and N-cadherin, respectively). Finally, we showed that the peptide disrupted MCF-7 and MDA-MB435 cell monolayers. The peptide, H-SWELYYPLRANL-NH(2) thus proved to be a biologically active, dual E- and N-cadherin antagonist. Such an antagonist has application in a wide variety of biological contexts.

Hypoxic preconditioning-induced cerebral ischemic tolerance: role of microvascular sphingosine kinase 2

BACKGROUND AND PURPOSE

The importance of bioactive lipid signaling under physiological and pathophysiological conditions is progressively becoming recognized. The disparate distribution of sphingosine kinase (SphK) isoform activity in normal and ischemic brain, particularly the large excess of SphK2 in cerebral microvascular endothelial cells, suggests potentially unique cell- and region-specific signaling by its product sphingosine-1-phosphate. The present study sought to test the isoform-specific role of SphK as a trigger of hypoxic preconditioning (HPC)-induced ischemic tolerance.

METHODS

Temporal changes in microvascular SphK activity and expression were measured after HPC. The SphK inhibitor dimethylsphingosine or sphingosine analog FTY720 was administered to adult male Swiss-Webster ND4 mice before HPC. Two days later, mice underwent a 60-minute transient middle cerebral artery occlusion and at 24 hours of reperfusion, infarct volume, neurological deficit, and hemispheric edema were measured.

RESULTS

HPC rapidly increased microvascular SphK2 protein expression (1.7+/-0.2-fold) and activity (2.5+/-0.6-fold), peaking at 2 hours, whereas SphK1 was unchanged. SphK inhibition during HPC abrogated reductions in infarct volume, neurological deficit, and ipsilateral edema in HPC-treated mice. FTY720 given 48 hours before stroke also promoted ischemic tolerance; when combined with HPC, even greater (and dimethylsphingosine-reversible) protection was noted.

CONCLUSIONS

These findings indicate hypoxia-sensitive increases in SphK2 activity may serve as a proximal trigger that ultimately leads to sphingosine-1-phosphate-mediated alterations in gene expression that promote the ischemia-tolerant phenotype. Thus, components of this bioactive lipid signaling pathway may be suitable therapeutic targets for protecting the neurovascular unit in stroke.

Vascular defects in a mouse model of hypotrichosis-lymphedema-telangiectasia syndrome indicate a role for SOX18 in blood vessel maturation

Mutations in the transcription factor gene SOX18 cause vascular, lymphatic and hair follicle defects in humans with dominant and recessive forms of hypotrichosis-lymphedema-telangiectasia (HLT) syndrome. Here, we clarify the role of SOX18 in the vascular dysfunction in HLT by ultrastructural, immunofluorescence, molecular and functional analysis of vascular anomalies in embryos of the naturally occurring Sox18-mutant mouse strain ragged-opossum (Ra(Op)). Early genesis and patterning of vasculature was unimpaired in Ra(Op) embryos, but surface capillaries became enlarged from 12.5 dpc and embryos developed massive surface hemorrhage by 14.5 dpc. Large focal breaches in the endothelial barrier were observed, in addition to endothelial hyperplasia associated with impaired pericyte recruitment to the microvasculature. Expression of the genes encoding the endothelial factors MMP7, IL7R and N-cadherin was reduced in Ra(Op) embryos, suggesting that these are downstream targets of SOX18. Together, our results indicate that vascular anomalies in HLT arise from defects in regulation of genes required for the acquisition of structural integrity during microvascular maturation.

Regulation of vascular integrity

The integrity of blood vessels is critical to vascular homeostasis. Maintenance of vascular integrity has been conventionally regarded as a passive process that is largely dependent on continuous blood flow. Recent studies, however, have begun unveiling molecular processes essential for maintenance of vascular integrity and homeostasis under physiological conditions, leading to the notion that maintenance of the vasculature is an active biological process that requires continuous, basal cellular signaling. Failure of this system results in serious consequences such as hemorrhage, edema, inflammation, and tissue ischemia. In this review, we will discuss the emerging concepts in regulation of vascular integrity with the emphasis on structural components of blood vessels that are essential for vascular maintenance.

Endothelial cell lumen and vascular guidance tunnel formation requires MT1-MMP-dependent proteolysis in 3-dimensional collagen matrices

Here we show that endothelial cells (EC) require matrix type 1-metalloproteinase (MT1-MMP) for the formation of lumens and tube networks in 3-dimensional (3D) collagen matrices. A fundamental consequence of EC lumen formation is the generation of vascular guidance tunnels within collagen matrices through an MT1-MMP-dependent proteolytic process. Vascular guidance tunnels represent a conduit for EC motility within these spaces (a newly remodeled 2D matrix surface) to both assemble and remodel tube structures. Interestingly, it appears that twice as many tunnel spaces are created than are occupied by tube networks after several days of culture. After tunnel formation, these spaces represent a 2D migratory surface within 3D collagen matrices allowing for EC migration in an MMP-independent fashion. Blockade of EC lumenogenesis using inhibitors that interfere with the process (eg, integrin, MMP, PKC, Src) completely abrogates the formation of vascular guidance tunnels. Thus, the MT1-MMP-dependent proteolytic process that creates tunnel spaces is directly and functionally coupled to the signaling mechanisms required for EC lumen and tube network formation. In summary, a fundamental and previously unrecognized purpose of EC tube morphogenesis is to create networks of matrix conduits that are necessary for EC migration and tube remodeling events critical to blood vessel assembly.

Regulation of vascular physiology and pathology by the S1P2 receptor subtype

Sphingosine-1-phosphate (S1P) is now recognized as a lipid mediator that acts via G-protein-coupled receptors. S1P receptors couple to various heterotrimeric G-proteins and regulate downstream targets and ultimately cell behaviour. The prototypical S1P1 receptor is known to couple to Gi and regulates angiogenesis, vascular development, and immune cell trafficking. In this review, we focus our attention on the S1P2 receptor, which has a unique G-protein-coupling property in that it preferentially activates the G(12/13) pathway. Recent studies indicate that the S1P2 receptor regulates critical intracellular signalling pathways, such as Rho GTPase, the phosphatase PTEN, and VE-cadherin-based adherens junctions. Analysis of mutant mice has revealed the critical role of this receptor in inner ear physiology, heart and vascular development, vascular remodelling, and vascular tone, permeability, and angiogenesis in vertebrates. These studies suggest that selective modulation of S1P2 receptor function by pharmacological tools may be useful in a variety of pathological conditions.

Attenuation of proliferation and migration of retinal pericytes in the absence of thrombospondin-1

Perivascular supporting cells, including vascular smooth muscle cells (VSMCs) and pericytes (PCs), provide instructive signals to adjacent endothelial cells helping to maintain vascular homeostasis. These signals are provided through direct contact and by the release of soluble factors by these cells. Thrombospondin (TSP)1 is a matricellular protein and an autocrine factor for VSMCs. TSP1 activity, along with that of PDGF, regulates VSMC proliferation and migration. However, the manner in which TSP1 and PDGF impact retinal PC function requires further investigation. In the present study, we describe, for the first time, the isolation and culture of retinal PCs from wild-type (TSP1(+/+)) and TSP1-deficient (TSP1(-/-)) immortomice. We showed that these cells express early and mature markers of PCs, including NG2, PDGF receptor-beta, and smooth muscle actin as well as desmin, calbindin, and mesenchymal stem cell markers. These cells were successfully passaged and maintained in culture for several months without significant loss of expression of these markers. TSP1(+/+) PCs proliferated at a faster rate compared with TSP1(-/-) PCs. In addition, TSP1(+/+) PCs, like VSMCs, responded to PDGF-BB with enhanced migration and proliferation. In contrast, TSP1(-/-) PCs failed to respond to the promigratory and proliferative activity of PDGF-BB. This may be attributed, at least in part, to the limited interaction of PDGF-BB with TSP1 in null cells, which is essential for PDGF proliferative and migratory action. We observed no significant differences in the rates of apoptosis in these cells. TSP1(-/-) PCs were also less adherent, expressed increased levels of TSP2 and fibronectin, and had decreased amounts of N-cadherin and alpha(v)beta(3)-integrin on their surface. Thus, TSP1 plays a significant role in retinal PC proliferation and migration impacting retinal vascular development and homeostasis.

Endothelial-mural cell signaling in vascular development and angiogenesis

Mural cells are essential components of blood vessels and are necessary for normal development, homeostasis, and organ function. Alterations in mural cell density or the stable attachment of mural cells to the endothelium is associated with several human diseases such as diabetic retinopathy, venous malformation, and hereditary stroke. In addition mural cells are implicated in regulating tumor growth and have thus been suggested as potential antiangiogenic targets in tumor therapy. In recent years our knowledge of mural cell function and endothelial-mural cell signaling has increased dramatically, and we now begin to understand the mechanistic basis of the key signaling pathways involved. This is mainly thanks to sophisticated in vivo experiments using a broad repertoire of genetic technologies. In this review, we summarize the five currently best understood signaling pathways implicated in mural cell biology. We discuss PDGFB/PDGFRbeta- dependent pericyte recruitment, as well as the role of angiopoietins and Tie receptors in vascular maturation. In addition, we highlight the effects of sphingosine-1-phosphate signaling on adherens junction assembly and vascular stability, as well as the role of TGF-beta-signaling in mural cell differentiation. We further reflect recent data suggesting an important function for Notch3 signaling in mural cell maturation.

N-cadherin levels in endothelial cells are regulated by monolayer maturity and p120 availability

Endothelial cells (ECs) express VE-cadherin and N-cadherin, and recent data suggest that VE-cadherin levels are dependent on N-cadherin expression. While investigating changes in N-cadherin levels during endothelial monolayer maturation, the authors found that VE-cadherin levels are maintained in ECs despite a decrease in N-cadherin, suggesting that VE-cadherin levels may not depend on N-cadherin. Knockdown of N-cadherin did not affect VE-cadherin levels in ECs with low endogenous N-cadherin expression. Surprisingly, however, knockdown of N-cadherin in ECs with high endogenous N-cadherin expression increased VE-cadherin levels, suggesting an inverse relationship between the two. This was further supported by a decrease in VE-cadherin following overexpression of N-cadherin. Experiments in which p120, a catenin that binds N- and VE-cadherin, was knocked down or overexpressed indicate that these two cadherins compete for p120. These data demonstrate that VE-cadherin levels are not directly related to N-cadherin levels but may be inversely related due to competition for p120.

The role of adherens junctions and VE-cadherin in the control of vascular permeability

Endothelial cells control the passage of plasma constituents and circulating cells from blood to the underlying tissues. This specialized function is lost or impaired in several pathological conditions - including inflammation, sepsis, ischemia and diabetes - which leads to severe, and sometimes fatal, organ dysfunction. Endothelial permeability is regulated in part by the dynamic opening and closure of cell-cell adherens junctions (AJs). In endothelial cells, AJs are largely composed of vascular endothelial cadherin (VE-cadherin), an endothelium-specific member of the cadherin family of adhesion proteins that binds, via its cytoplasmic domain, to several protein partners, including p120, beta-catenin and plakoglobin. Endogenous pathways that increase vascular permeability affect the function and organization of VE-cadherin and other proteins at AJs in diverse ways. For instance, several factors, including vascular endothelial growth factor (VEGF), induce the tyrosine phosphorylation of VE-cadherin, which accompanies an increase in vascular permeability and leukocyte diapedesis; in addition, the internalization and cleavage of VE-cadherin can cause AJs to be dismantled. From the knowledge of how AJ organization can be modulated, it is possible to formulate several pharmacological strategies to control the barrier function of the endothelium. We discuss the possible use of inhibitors of SRC and other kinases, of agents that increase cAMP levels, and of inhibitors of lytic enzymes as pharmacological tools for decreasing endothelial permeability.

Lysophospholipid receptors in vertebrate development, physiology, and pathology

Lysophospholipid (LP) research has experienced a period of renaissance with the discovery of the lysophosphatidic acid (LPA) and sphingosine 1-phosphate (S1P) receptors in the late 1990s. Vertebrate LP receptors regulate embryogenesis, vascular development, neurogenesis, uterine development, oocyte survival, immune cell trafficking and inflammatory reactions. LP signaling is important in cancer, autoimmunity and inflammatory diseases. Research on LP biology has contributed to the development of a first-generation S1P receptor modulator that has entered phase III clinical trials for the treatment of multiple sclerosis. Further basic research on LP signaling is anticipated to lead to novel therapeutic tools to combat various human diseases.

Identification of a novel N-cadherin antagonist

The cell adhesion molecule, N-cadherin plays a pivotal role in many biological and disease processes. Drugs that modulate N-cadherin function should therefore be useful therapeutic agents. We have used phage display technology to identify amino acid sequences capable of binding to N-cadherin. All of these sequences harbor a Trp residue in the second position from the N-terminus. A synthetic linear peptide containing one of these sequences, H-SWTLYTPSGQSK-NH(2) was found to bind a chimeric protein composed of the N-cadherin ectodomain fused to the immunoglobulin G1 Fc fragment with an affinity (K(D)) of 10.7microM, as determined by surface plasmon resonance. It also blocked the aggregation of beads coated with this chimeric protein. Furthermore, this peptide disrupted adhesion and tube formation by N-cadherin-expressing human umbilical vein endothelial cells in vitro. These observations suggest that N-cadherin antagonists have the potential of serving as anti-angiogenic agents. The peptide, H-SWTLYTPSGQSK-NH(2) should prove useful for studies designed to evaluate N-cadherin function in various biological processes.

Organization and signaling of endothelial cell-to-cell junctions in various regions of the blood and lymphatic vascular trees

Adhesive intercellular junctions between endothelial cells are formed by tight junctions and adherens junctions. In addition to promoting cell-to-cell adhesion, these structures regulate paracellular permeability, contact inhibition of endothelial cell growth, cell survival, and maintenance of cell polarity. Furthermore, adherens junctions are required for the correct organization of new vessels during embryo development or during tissue proliferation in the adult. Extensive research on cultured epithelial and endothelial cells has resulted in the identification of many molecular components of tight junctions and adherens junctions. Such studies have revealed the complexity of these structures, which are formed by membrane-associated adhesion proteins and a network of several intracellular signaling partners. This review focuses on the structural organization of junctional structures and their functional interactions in the endothelium of blood vessels and lymphatics. We emphasize the way that these structures regulate endothelial cell homeostasis by transferring specific intracellular signals and by modulating activation and signaling of growth factor receptors.

Sphingosine-1-phosphate regulation of mammalian development

Sphingosine-1-phosphate (S1P) was first identified as a lysophospholipid metabolite whose formation is required for the irreversible degradation of sphingolipids. Years later, it was discovered that S1P is a bioactive lipid that provokes varied cell responses by acting through cell-surface receptors to drive cell signaling. More recent findings in model organisms have now established that S1P metabolism and signaling are integrated into many physiological systems. We describe here the surprising breadth of function of S1P in mammalian development and the underlying biologic processes that S1P regulates.

Homotypic and endothelial cell adhesions via N-cadherin determine polarity and regulate migration of vascular smooth muscle cells

Migration of smooth muscle cells from the arterial media to the intima is central to several vascular pathologies including restenosis. This study demonstrates that, like directional migration of other cells, smooth muscle migration is accompanied by a dramatic, polarized reorganization of the cell cytoskeleton that is accompanied by activation of the Rho GTPase Cdc42 and inactivation of glycogen synthase kinase-3beta. We also show, for the first time, that signals generated at the posterior-lateral aspects of wound edge cells by the cell-cell adhesion molecule N-cadherin are required for polarization and rapid migration of vascular smooth muscle. Importantly, when a cohort of migrating smooth muscle cells encounter CHO cells or the A10 smooth muscle cell line, neither of which expresses N-cadherin, polarity is only slightly suppressed. However, when smooth muscle cells encounter stably transfected, N-cadherin-expressing A10 cells or (N-cadherin-expressing) vascular endothelium, they rapidly lose their polarized phenotype. The latter finding indicates that endothelial signaling to innermost smooth muscle cells via N-cadherin may be critical to normal vessel wall stability. We infer that asymmetrical distribution of N-cadherin is necessary for the establishment of cell polarity during migration and that N-cadherin ligation is highly effective in abrogating polarized migration. Finally, we showed that endothelial cell polarity does not depend on N-cadherin; therefore, this molecule may be an attractive target for therapies to prevent restenosis without suppressing endothelial repair and risking late thrombosis.

Fibroblast growth factor regulation of neovascularization

PURPOSE OF REVIEW

Fibroblast growth factors are potent angiogenic inducers; however, their precise roles in angiogenesis have not been well understood. In this review, we will focus on specific roles played by fibroblast growth factors in neovascularization.

RECENT FINDINGS

Although fibroblast growth factors promote a strong angiogenic response, it has been suggested that FGF-induced angiogenesis requires activation of the vascular endothelial growth factor system. Recent findings have endorsed the view of indirect contribution of fibroblast growth factor signaling to vascular development. A study using embryoid bodies demonstrated a nonimmediate role played by fibrobalst growth factor receptor 1 in vasculogenesis as vascular endothelial growth factor supplementation was sufficient to promote vascular development in Fgfr1-/- embryoid bodies. Moreover, another line of evidence indicated that myocardial fibroblast growth factor signaling is essential for mouse coronary development. The key role of fibroblast growth factor signaling in this process is Hedgehog activation, which induces vascular endothelial growth factor expression and formation of the coronary vasculature. In addition to vascular endothelial growth factor interaction, fibroblast growth factors can control neovascularization by influencing other growth factors and chemokines such as platelet-derived growth factor, hepatocyte growth factor and monocyte chemoattractant protein-1, contributing to development of mature vessels and collateral arteries.

SUMMARY

Although fibroblast growth factors are potent angiogenic factors, they may indirectly control neovascularization in concert with other growth factors. Thus, the unique role played by fibroblast growth factors might be organization of various angiogenic pathways and coordination of cell-cell interactions in this process.

Attenuation of cell motility observed with high doses of sphingosine 1-phosphate or phosphorylated FTY720 involves RGS2 through its interactions with the receptor S1P

Sphingosine 1-phosphate (S1P) stimulation enhances cell motility via the G-protein coupled S1P receptor S1P1. This ligand-induced, receptor-mediated cell motility follows a typical bell-shaped dose-response curve, that is, stimulation with low concentrations of S1P enhances cell motility, whereas excess ligand stimulation does not enhance it. So far, the attenuation of the response at higher ligand concentrations has not been explained. We report here that S1P1 interacts with the regulator of G protein signaling (RGS)-2 protein, which is a GTPase-activating protein (GAP) for heterotrimeric G proteins, in a concentration dependent manner. The RGS2-S1P1 complex dissociated at higher ligand concentrations, yet it was unaffected at low concentrations, suggesting that the dissociated RGS2 is involved in the concurrent decrease of cell motility. In RGS2 knockdown cells, the decrease of cell motility induced by high ligand concentrations was rescued. S1P1 internalization was not implicated in the attenuation of the response. Similar results were observed upon stimulation with the phosphorylated form of FTY720 (FTYP), which is an S1P1 agonist. In conclusion, the suppressed response in cell motility induced by excess S1P or FTYP via S1P1 is regulated by RGS2 functioning through a mechanism that is independent of S1P1 internalization.

Sphingosine-1-phosphate signaling and biological activities in the cardiovascular system

The plasma lysophospholipid mediator sphingosine-1-phosphate (S1P) is produced exclusively by sphingosine kinase (SPHK) 1 and SPHK2 in vivo, and plays diverse biological and pathophysiological roles by acting largely through three members of the G protein-coupled S1P receptors, S1P1, S1P2 and S1P3. S1P1 expressed on endothelial cells mediates embryonic vascular maturation and maintains vascular integrity by contributing to eNOS activation, inhibiting vascular permeability and inducing endothelial cell chemotaxis via Gi-coupled mechanisms. By contrast, S1P2, is expressed in high levels on vascular smooth muscle cells (VSMCs) and certain types of tumor cells, inhibiting Rac and cell migration via a G(12/13)-and Rho-dependent mechanism. In rat neointimal VSMCs, S1P1 is upregulated to mediate local production of platelet-derived growth factor, which is a key player in vascular remodeling. S1P3 expressed on endothelial cells also mediates chemotaxis toward S1P and vasorelaxation via NO production in certain vascular bed, playing protective roles for vascular integrity. S1P3 expressed on VSMCs and cardiac sinoatrial node cells mediates vasopressor and negative chronotropic effect, respectively. In addition, S1P3, together with S1P2 and SPHK1, is suggested to play a protective role against acute myocardial ischemia. However, our recent work indicates that overexpressed SPHK1 is involved in cardiomyocyte degeneration and fibrosis in vivo, in part through S1P activation of the S1P3 signaling. We also demonstrated that exogenously administered S1P accelerates neovascularization and blood flow recovery in ischemic limbs, suggesting its usefulness for angiogenic therapy. These results provide evidence for S1P receptor subtype-specific pharmacological intervention as a novel therapeutic approach to cardiovascular diseases and cancer.

Phospholipase C beta3 deficiency leads to macrophage hypersensitivity to apoptotic induction and reduction of atherosclerosis in mice

Atherosclerosis is an inflammatory disease that is associated with monocyte recruitment and subsequent differentiation into lipid-laden macrophages at sites of arterial lesions, leading to the development of atherosclerotic plaques. PLC is a key member of signaling pathways initiated by G protein-coupled ligands in macrophages. However, the role of this enzyme in the regulation of macrophage function is not known. Here, we studied macrophages from mice lacking PLC beta2, PLC beta3, or both PLC isoforms and found that PLC beta3 is the major functional PLC beta isoform in murine macrophages. Although PLC beta3 deficiency did not affect macrophage migration, adhesion, or phagocytosis, it resulted in macrophage hypersensitivity to multiple inducers of apoptosis. PLC beta3 appeared to regulate this sensitivity via PKC-dependent upregulation of Bcl-XL. The significance of PLC beta signaling in vivo was examined using the apoE-deficient mouse model of atherosclerosis. Mice lacking both PLC beta3 and apoE exhibited fewer total macrophages and increased macrophage apoptosis in atherosclerotic lesions, as well as reduced atherosclerotic lesion size when compared with mice lacking only apoE. These results demonstrate what we believe to be a novel role for PLC activity in promoting macrophage survival in atherosclerotic plaques and identify PLC beta3 as a potential target for treatment of atherosclerosis.

Sustained release of sphingosine 1-phosphate for therapeutic arteriogenesis and bone tissue engineering

Sphingosine 1-phosphate (S1P) is a bioactive phospholipid that impacts migration, proliferation, and survival in diverse cell types, including endothelial cells, smooth muscle cells, and osteoblast-like cells. In this study, we investigated the effects of sustained release of S1P on microvascular remodeling and associated bone defect healing in vivo. The murine dorsal skinfold window chamber model was used to evaluate the structural remodeling response of the microvasculature. Our results demonstrated that 1:400 (w/w) loading and subsequent sustained release of S1P from poly(lactic-co-glycolic acid) (PLAGA) significantly enhanced lumenal diameter expansion of arterioles and venules after 3 and 7 days. Incorporation of 5-bromo-2-deoxyuridine (BrdU) at day 7 revealed significant increases in mural cell proliferation in response to S1P delivery. Additionally, three-dimensional (3D) scaffolds loaded with S1P (1:400) were implanted into critical-size rat calvarial defects, and healing of bony defects was assessed by radiograph X-ray, microcomputed tomography (muCT), and histology. Sustained release of S1P significantly increased the formation of new bone after 2 and 6 weeks of healing and histological results suggest increased numbers of blood vessels in the defect site. Taken together, these experiments support the use of S1P delivery for promoting microvessel diameter expansion and improving the healing outcomes of tissue-engineered therapies.

Bone marrow-derived mesenchymal stem cells facilitate engineering of long-lasting functional vasculature

Vascular tissue engineering requires a ready source of endothelial cells and perivascular cells. Here, we evaluated human bone marrow-derived mesenchymal stem cells (hMSCs) for use as vascular progenitor cells in tissue engineering and regenerative medicine. hMSCs expressed a panel of smooth muscle markers in vitro including the cardiac/smooth muscle-specific transcription coactivator, myocardin. Cell-cell contact between endothelial cells and hMSCs up-regulated the transcription of myocardin. hMSCs efficiently stabilized nascent blood vessels in vivo by functioning as perivascular precursor cells. The engineered blood vessels derived from human umbilical cord vein endothelial cells and hMSCs remained stable and functional for more than 130 days in vivo. On the other hand, we could not detect differentiation of hMSCs to endothelial cells in vitro, and hMSCs by themselves could not form conduit for blood flow in vivo. Similar to normal perivascular cells, hMSC-derived perivascular cells contracted in response to endothelin-1 in vivo. In conclusion, hMSCs are perivascular cell precursors and may serve as an attractive source of cells for use in vascular tissue engineering and for the study of perivascular cell differentiation.

Activity-induced protocadherin arcadlin regulates dendritic spine number by triggering N-cadherin endocytosis via TAO2beta and p38 MAP kinases

Synaptic activity induces changes in the number of dendritic spines. Here, we report a pathway of regulated endocytosis triggered by arcadlin, a protocadherin induced by electroconvulsive and other excitatory stimuli in hippocampal neurons. The homophilic binding of extracellular arcadlin domains activates TAO2beta, a splice variant of the thousand and one amino acid protein kinase 2, cloned here by virtue of its binding to the arcadlin intracellular domain. TAO2beta is a MAPKKK that activates the MEK3 MAPKK, which phosphorylates the p38 MAPK. Activation of p38 feeds-back on TAO2beta, phosphorylating a key serine required for triggering endocytosis of N-cadherin at the synapse. Arcadlin knockout increases the number of dendritic spines, and the phenotype is rescued by siRNA knockdown of N-cadherin. This pathway of regulated endocytosis of N-cadherin via protocadherin/TAO2beta/MEK3/p38 provides a molecular mechanism for transducing neuronal activity into changes in synaptic morphologies.

RGS5 expression is a quantitative measure of pericyte coverage of blood vessels

Pericytes play a key role in the process of vascular maturation and stabilization however, the current methods for quantifying pericyte coverage of the neovasculature are laborious and subjective in nature. In this study, we have developed an objective, sensitive, and high-throughput method for quantifying pericyte coverage of angiogenic vessels by analyzing the expression of the pericyte-specific gene, the regulator of G-protein signaling 5 (RGS5). We determined that RGS5 expression was up-regulated during a defined developmental time period in which nascent vessel sprouts acquired a pericyte covering. Furthermore, RGS5 expression was dramatically reduced in vessels with poor pericyte coverage compared to normal angiogenic vasculature. Finally, we determined that the susceptibility of nascent vessels to regression by vascular endothelial growth factor (VEGF) inhibition was significantly reduced following RGS5 up-regulation, further implicating RGS5 in pericyte-endothelial cell interactions and the vascular maturation process. These studies establish the use of RGS5 gene expression as a quantitative and robust measure of pericyte coverage of neovasculature. This method provides a tool for vascular biologists studying pericyte-endothelial cell interactions and vascular maturation in both normal and pathological conditions, such as diabetic retinopathy and cancer.

Pericytes and ocular diseases

Pericytes are vascular mural cells which play multiple roles in angiogenesis and maintenance of blood vessel morphology and stability. In this review, we analyze recent data on the participation of pericytes in the pathogenesis of proliferative and non-proliferative diabetic retinopathy, as well as an emerging role in other angiogenic ocular diseases such as retinopathy of prematurity. Ways to exploit pericytes as targets for treatment of ocular diseases are discussed.

Essential role of sphingosine 1-phosphate receptor 2 in pathological angiogenesis of the mouse retina

Sphingosine 1-phosphate (S1P), a multifunctional lipid mediator that signals via the S1P family of G protein-coupled receptors (S1PR), regulates vascular maturation, permeability, and angiogenesis. In this study, we explored the role of S1P 2 receptor (S1P2R) in normal vascularization and hypoxia-triggered pathological angiogenesis of the mouse retina. S1P2R is strongly induced in ECs during hypoxic stress. When neonatal mice were subjected to ischemia-driven retinopathy, pathologic neovascularization in the vitreous chamber was suppressed in S1p2-/- mice concomitant with reduction in endothelial gaps and inflammatory cell infiltration. In addition, EC patterning and normal revascularization into the avascular zones of the retina were augmented. Reduced expression of the proinflammatory enzyme cyclooxygenase-2 (COX-2) and increased expression of eNOS were observed in the S1p2-/- mouse retina. S1P2R activation in ECs induced COX-2 expression and suppressed the expression of eNOS. These data identify the S1P2R-driven inflammatory process as an important molecular event in pathological retinal angiogenesis. We propose that antagonism of the S1P2R may be a novel therapeutic approach for the prevention and/or treatment of pathologic ocular neovascularization.

Origin of periendothelial cells in microvessels derived from human microvascular endothelial cells

In microvessels, periendothelial cells expressing alpha smooth muscle actin (alphaSMA) interact with the endothelial cells and are essential for vessel maturation and stabilization. In adult tissues, the cellular origin of the periendothelial cells is still not clear, in particular in humans. To determine the origin of human periendothelial cells, we used a recently developed 3D co-culture system that mimics human skin connective tissue. This system is composed of normal human dermal fibroblasts (NHDF), human dermal microvascular endothelial cells (HMEC-1), and a collagen matrix. In this system, "microvessels" composed of an endothelial lumen associated with periendothelial cells develop. Using this co-culture system, we (i) labelled fibroblasts with the vital dye CFDA-SE, cultured them with unlabelled endothelial cells, and observed that only endothelium-associated CFDA-SE-labelled cells express alphaSMA; (ii) infected endothelial cells with a retrovirus stably expressing eGFP, cultured them with unlabelled fibroblasts, and observed that cells expressing alphaSMA did not co-express eGFP, but were associated with the eGFP-expressing endothelial cells of the microvessels. Together, these results indicate that periendothelial cells arise by differentiation from fibroblasts and that they require interaction with endothelial cells to do so.

Soluble cadherins as cancer biomarkers

Molecular activities, regulating a balanced tissue organisation, are frequently disturbed during cancer progression. These include protein ectodomain shedding, a post-translational process that substantially changes the functional properties of the substrate protein. In comparison with normal epithelia, cancer cells almost invariably show diminished cadherin-mediated intercellular adhesion. This review will address cadherin ectodomain shedding and its functional consequence in normal physiology and in the tumor environment. Soluble cadherin fragments may retain specific biological activities during cancer cell invasion, angiogenesis and perineural invasion. When diffusion barriers disappear, soluble cadherins are detected in sera from cancer patients. Soluble N-(neural) cadherin may represent a novel diagnosis/prognostic biomarker showing a correlation with PSA in sera of prostate cancer patients. Furthermore, therapeutic monitoring in pancreas adenomacarcinoma revealed a correlation between circulating soluble N-cadherin and CA 19-9. A better understanding of cadherin regulation in cancer progression will likely increase our awareness of the importance of the combinatorial signals that regulate tissue integrity and eventually result in the identification of new therapeutics targeting cadherins.

Sphingolipids and membrane biology as determined from genetic models

The importance of sphingolipids in membrane biology was appreciated early in the twentieth century when several human inborn errors of metabolism were linked to defects in sphingolipid degradation. The past two decades have seen an explosion of information linking sphingolipids with cellular processes. Studies have unraveled mechanistic details of the sphingolipid metabolic pathways, and these findings are being exploited in the development of novel therapies, some now in clinical trials. Pioneering work in yeast has laid the foundation for identifying genes encoding the enzymes of the pathways. The advent of the era of genomics and bioinformatics has led to the identification of homologous genes in other species and the subsequent creation of animal knock-out lines for these genes. Discoveries from these efforts have re-kindled interest in the role of sphingolipids in membrane biology. This review highlights some of the recent advances in understanding sphingolipids' roles in membrane biology as determined from genetic models.

Endothelial cell migration on RGD-peptide-containing PEG hydrogels in the presence of sphingosine 1-phosphate

Sphingosine 1-phosphate (S1P) is a potent chemokinetic agent for endothelial cells that is released by activated platelets. We previously developed Arg-Gly-Asp (RGD)-containing polyethylene glycol biomaterials for the controlled delivery of S1P to promote endothelialization. Here, we studied the effects of cell adhesion strength on S1P-stimulated endothelial cell migration in the presence of arterial levels of fluid shear stress, since an upward shift in optimal cell adhesion strengths may be beneficial for promoting long-term cell adhesion to materials. Two RGD peptides with different integrin-binding specificities were added to the polyethylene glycol hydrogels. A linear RGD bound primarily to beta(3) integrins, whereas a cyclic RGD bound through both beta(1) and beta(3) integrins. We observed increased focal adhesion formation and better long-term adhesion in flow with endothelial cells on linear RGD peptide, versus cyclic RGD, even though initial adhesion strengths were higher for cells on cyclic RGD. Addition of 100 nM S1P increased cell speed and random motility coefficients on both RGD peptides, with the largest increases found on cyclic RGD. For both peptides, much of the increase in cell migration speed was found for smaller cells (<1522 microm(2) projected area), although the large increases on cyclic RGD were also due to medium-sized cells (2288-3519 microm(2)). Overall, a compromise between high cell migration rates and long-term adhesion will be important in the design of materials that endothelialize after implantation.

COX-2 suppresses tissue factor expression via endocannabinoid-directed PPARdelta activation

Although cyclooxygenase (COX)-2 inhibitors (coxibs) are effective in controlling inflammation, pain, and tumorigenesis, their use is limited by the recent revelation of increased adverse cardiovascular events. The mechanistic basis of this side effect is not well understood. We show that the metabolism of endocannabinoids by the endothelial cell COX-2 coupled to the prostacyclin (PGI₂) synthase (PGIS) activates the nuclear receptor peroxisomal proliferator–activated receptor (PPAR) δ, which negatively regulates the expression of tissue factor (TF), the primary initiator of blood coagulation. Coxibs suppress PPARδ activity and induce TF expression in vascular endothelium and elevate circulating TF activity in vivo. Importantly, PPARδ agonists suppress coxib-induced TF expression and decrease circulating TF activity. We provide evidence that COX-2–dependent attenuation of TF expression is abrogated by coxibs, which may explain the prothrombotic side-effects for this class of drugs. Furthermore, PPARδ agonists may be used therapeutically to suppress coxib-induced cardiovascular side effects.

Molecular regulation of angiogenesis and lymphangiogenesis

Blood vessels and lymphatic vessels form extensive networks that are essential for the transport of fluids, gases, macromolecules and cells within the large and complex bodies of vertebrates. Both of these vascular structures are lined with endothelial cells that integrate functionally into different organs, acquire tissue-specific specialization and retain plasticity; thereby, they permit growth during tissue repair or in disease settings. The angiogenic growth of blood vessels and lymphatic vessels coordinates several biological processes such as cell proliferation, guided migration, differentiation and cell-cell communication.

The suppressive effect of sphingosine 1-phosphate on monocyte-endothelium adhesion may be mediated by the rearrangement of the endothelial integrins alpha(5)beta(1) and alpha(v)beta(3)

BACKGROUND

Sphingosine 1-phosphate (S1P), known to play important roles in vascular biology, is a bioactive lysophospholipid mediator that maintains endothelial integrity via its cell-surface receptors (S1Ps). In this in vitro study, we aimed to examine the role of S1P in monocyte-endothelium adhesion, which is an important event in the pathophysiology of atherosclerosis.

METHODS AND RESULTS

S1P pretreatment of human umbilical vein endothelial cells (ECs), but not U937 cells, effectively suppressed U937-EC adhesion independently from the expression of adhesion molecules, namely ICAM-1, VCAM-1, and E-selectin. This S1P-induced suppressive effect was inhibited by the blockage of S1P(1) and S1P(3) receptors and the specific inhibitors of G(i) protein, Src family proteins, phosphatidylinositol 3-kinase, and Rac1, indicating involvement of these key downstream pathways. Moreover, the RGD peptide and antibodies, which neutralize adhesion via alpha(5)beta(1) and alpha(v)beta(3), effectively inhibited U937-EC adhesion with a degree similar to S1P pretreatment. Both an adhesion assay and flow-cytometric analysis demonstrated that U937 cells adhered through integrins alpha(5)beta(1) and alpha(v)beta(3) expressed on the apical surface of monolayer ECs, and S1P shifted the localization of these integrins from the apical surface to the basal surface.

CONCLUSIONS

From the present results, we propose that S1P may contribute to the maintenance of vascular integrity and the regulation of atherogenesis through the rearrangement of endothelial integrins.

Enhanced capillary formation stimulated by a chimeric vascular endothelial growth factor/vascular endothelial growth factor-C silk domain fusion protein

Vascular endothelial growth factor (VEGF)-C and VEGF-D require proteolytic cleavage of the carboxy terminal silk-homology domain for activation. To study the functions of the VEGF-C propeptides, we engineered a chimeric growth factor protein, VEGF-CAC, composed of the amino- and carboxy-terminal propeptides of VEGF-C fused to the receptor-activating core domain of VEGF. Like VEGF-C, VEGF-CAC underwent proteolytic cleavage, and like VEGF, it bound to and activated VEGF receptor-1 and VEGF receptor-2, but not the VEGF-C receptor VEGF receptor-3. VEGF-CAC also bound to neuropilins in a heparin-dependent manner. Strikingly, when VEGF-CAC was expressed via an adenovirus vector in the ear skin of immunodeficient mice, it proved to be a more potent inducer of capillary angiogenesis than VEGF. The VEGF-CAC-induced vessels differed greatly from those induced by VEGF, as they formed a very dense and fine network of pericyte and basement membrane-covered capillaries that were functional, as shown by lectin perfusion experiments. VEGF-CAC could prove useful in proangiogenic therapies in patients experiencing tissue ischemia.

N-cadherin as a therapeutic target in cancer

During tumor progression, cancer cells undergo dramatic changes in the expression profile of adhesion molecules resulting in detachment from original tissue and acquisition of a highly motile and invasive phenotype. A hallmark of this change, also referred to as the epithelial-mesenchymal transition, is the loss of E- (epithelial) cadherin and the de novo expression of N- (neural) cadherin adhesion molecules. N-cadherin promotes tumor cell survival, migration and invasion, and a high level of its expression is often associated with poor prognosis. N-cadherin is also expressed in endothelial cells and plays an essential role in the maturation and stabilization of normal vessels and tumor-associated angiogenic vessels. Increasing experimental evidence suggests that N-cadherin is a potential therapeutic target in cancer. A peptidic N-cadherin antagonist (ADH-1) has been developed and has entered clinical testing. In this review, the authors discuss the biochemical and functional features of N-cadherin, its potential role in cancer and angiogenesis, and summarize the preclinical and clinical results achieved with ADH-1.

Immunosuppressive and Anti-angiogenic Sphingosine 1-Phosphate Receptor-1 Agonists Induce Ubiquitinylation and Proteasomal Degradation of the Receptor

Sphingosine 1-phosphate (S1P), a multifunctional lipid mediator, regulates lymphocyte trafficking, vascular permeability, and angiogenesis by activation of the S1P1 receptor. This receptor is activated by FTY720-P, a phosphorylated derivative of the immunosuppressant and vasoactive compound FTY720. However, in contrast to the natural ligand S1P, FTY720-P appears to act as a functional antagonist, even though the mechanisms involved are poorly understood. In this study, we investigated the fate of endogenously expressed S1P1 receptor in agonist-activated human umbilical vein endothelial cells and human embryonic kidney 293 cells expressing green fluorescent protein-tagged S1P1. We show that FTY720-P is more potent than S1P at inducing receptor degradation. Pretreatment with an antagonist of S1P1, VPC 44116, prevented receptor internalization and degradation. FTY720-P did not induce degradation of internalization-deficient S1P1 receptor mutants. Further, small interfering RNA-mediated down-regulation of G protein-coupled receptor kinase-2 and beta-arrestins abolished FTY720-P-induced S1P1 receptor degradation. These data suggest that agonist-induced phosphorylation of S1P1 and subsequent endocytosis are required for FTY720-P-induced degradation of the receptor. S1P1 degradation is blocked by MG132, a proteasomal inhibitor. Indeed, FTY720-P strongly induced polyubiquitinylation of S1P1 receptor, whereas S1P at concentrations that induced complete internalization was not as efficient, suggesting that receptor internalization is required but not sufficient for ubiquitinylation and degradation. We propose that the ability of FTY720-P to target the S1P1 receptor to the ubiquitinylation and proteasomal degradation pathway may at least in part underlie its immunosuppressive and anti-angiogenic properties.

Design of a Filamentous Polymeric Scaffold forin VivoGuided Angiogenesis

Angiogenesis is mandatory for reperfusion of viable tissues, and lack of vascularization may cause ischemia. The increasing disparity between the demand and availability of adequate substitutes for small-diameter human blood vessels has prompted an intensive search for artificial materials or biological allograft tissues, both of which usually fail in the long term. The objective of this study was to pioneer a novel model for in vivo guided angiogenesis based on a specific design process of a filamentous polymeric scaffold with endothelial cells in a 3-dimensional culture system. To our knowledge, this is the first report of an in vivo guided angiogenesis approach based on a 2-step model, composed of endothelial cells and a filamentous polymeric scaffold framework. Endothelial cells that had been cultured on a specifically designed filamentous polymeric scaffold within a regulated dynamic tissue culture system were shown in vivo to induce guided angiogenesis. Cells seeded on a biodegradable polymeric scaffold were implanted into mice. On day 28 after implantation, analysis revealed a guided angiogenic process along the path of the implanted polymeric scaffold as well as initial evidence for early maturation of engineered vessels, allowing red blood cells to flow through the forming lumina of new vessels as the polymer degraded. The authors conclude that in vivo guided angiogenesis can be achieved by combining endothelial cells with biodegradable filamentous polymeric scaffolds and that this model can lay the cornerstone for vascular engineering and future development of clinically available protocols aimed to treat life-threatening cardiovascular conditions.

Sphingosine 1-phosphate regulates cytoskeleton dynamics: implications in its biological response

The bioactive sphingolipid sphingosine 1-phosphate (S1P) elicits robust cytoskeletal rearrangement in a large variety of cell systems, mainly acting through a panel of specific cell surface receptors, named S1P receptors. Recent studies have begun to delineate the molecular mechanisms involved in the complex process responsible for cytoskeletal rearrangement following S1P ligation to its receptors. Notably, changes of cell shape and/or motility induced by S1P via cytoskeletal remodelling are functional to the biological action exerted by S1P which appears to be highly cell-specific. This review focuses on the current knowledge of the regulatory mechanisms of cytoskeleton dynamics elicited by S1P, with special emphasis on the relationship between cytoskeletal remodelling and the biological effects evoked by the sphingolipid in various cell types.

Sphingosine kinase: biochemical and cellular regulation and role in disease

Sphingolipids have emerged as molecules whose metabolism is regulated leading to generation of bioactive products including ceramide, sphingosine, and sphingosine-1-phosphate. The balance between cellular levels of these bioactive products is increasingly recognized to be critical to cell regulation; whereby, ceramide and sphingosine cause apoptosis and growth arrest phenotypes, and sphingosine-1-phosphate mediates proliferative and angiogenic responses. Sphingosine kinase is a key enzyme in modulating the levels of these lipids and is emerging as an important and regulated enzyme. This review is geared at mechanisms of regulation of sphingosine kinase and the coming to light of its role in disease.

Delivery of sphingosine 1-phosphate from poly(ethylene glycol) hydrogels

While protein growth factors promote therapeutic angiogenesis, delivery of lipid factors such as sphingosine 1-phosphate (S1P) may provide better stabilization of newly formed vessels. We developed a biomaterial for the controlled delivery of S1P, a bioactive lipid released from activated platelets. Multiarm poly(ethylene glycol)-vinyl sulfone was cross-linked with albumin, a lipid-transporting protein, to form hydrogels. The rate of S1P release from the materials followed Fickian kinetics and was dependent upon the presence of lipid carriers in the release solution. Delivery of S1P from RGD-modified hydrogels increased the cell migration speed of endothelial cells growing on the materials. The materials also induced angiogenesis in the chorioallantoic membrane assay. Our data demonstrate that the storage and release of lipid factors provides a new route for the induction of angiogenesis by artificial materials.

The multiple languages of endothelial cell-to-cell communication

Intercellular adhesion plays a key role during development and maintenance of tissue homeostasis. Within the vascular system, cell-cell adhesion is particularly important for the correct formation, networking, and remodeling of vessels. Although in vascular endothelial cells adhesive junctions account for the integrity of the vessel wall, they are not to be considered as static molecular structures that function as intercellular glue. This becomes evident during the remodeling of the endothelium in various physiological and pathological processes, requiring highly dynamic vascular adhesion complexes. Moreover, it has recently become evident that, besides their structural functions, adhesion molecules involved in endothelial cell-cell interaction play an important role in inducing and integrating intracellular signals that, in turn, impact on several aspects of vascular cell physiology. In this review, we describe these recent findings focusing on junctional proteins at adherens and tight junctions. The role of this adhesion molecule-mediated signaling is discussed in the context of developmental and pathological angiogenesis.

Antagonism of sphingosine-1-phosphate receptors by FTY720 inhibits angiogenesis and tumor vascularization

FTY720, a potent immunomodulator, becomes phosphorylated in vivo (FTY-P) and interacts with sphingosine-1-phosphate (S1P) receptors. Recent studies showed that FTY-P affects vascular endothelial growth factor (VEGF)-induced vascular permeability, an important aspect of angiogenesis. We show here that FTY720 has antiangiogenic activity, potently abrogating VEGF- and S1P-induced angiogenesis in vivo in growth factor implant and corneal models. FTY720 administration tended to inhibit primary and significantly inhibited metastatic tumor growth in a mouse model of melanoma growth. In combination with a VEGFR tyrosine kinase inhibitor PTK787/ZK222584, FTY720 showed some additional benefit. FTY720 markedly inhibited tumor-associated angiogenesis, and this was accompanied by decreased tumor cell proliferation and increased apoptosis. In transfected HEK293 cells, FTY-P internalized S1P1 receptors, inhibited their recycling to the cell surface, and desensitized S1P receptor function. Both FTY720 and FTY-P apparently failed to impede VEGF-produced increases in mitogen-activated protein kinase activity in human umbilical vascular endothelial cells (HUVEC), and unlike its activity in causing S1PR internalization, FTY-P did not result in a decrease of surface VEGFR2 levels in HUVEC cells. Pretreatment with FTY720 or FTY-P prevented S1P-induced Ca2+ mobilization and migration in vascular endothelial cells. These data show that functional antagonism of vascular S1P receptors by FTY720 potently inhibits angiogenesis; therefore, this may provide a novel therapeutic approach for pathologic conditions with dysregulated angiogenesis.

Sphingosine-1-phosphate agonists increase macrophage homing, lymphocyte contacts, and endothelial junctional complex formation in murine lymph nodes

The sphingosine-1-phosphate (S1P) receptor agonist, phosphorylated FTY720 (FTY-P), causes lymphopenia, lymphocyte sequestration in mesenteric lymph nodes (MLNs), and immunosuppression. Using multiple techniques to analyze MLN cells harvested from mice treated with S1P receptor agonists, we saw a redistribution of lymphocytes out of nodal sinuses and an expansion of follicles. Although changes in circulating monocytes were not observed with overnight exposure to FTY720, we saw a significant increase in S1P receptor 1 (S1P1)-expressing CD68+ macrophages in subcapsular sinuses of FTY-P-treated MLNs. This was confirmed by quantitative analysis of F4/80+ cells in MLN suspensions. The sinus volume and number of S1P1-positive cells within sinuses were also increased by FTY-P. High endothelial venules and lymphatic endothelium expressed high levels of S1P1, and treatment with FTY-P resulted in intense staining and colocalization of CD31, beta-catenin, and zona occludens 1 in junctions between sinus cells. Transmission electron microscopy showed that FTY-P greatly reduced lymphocyte microvilli and increased cell-cell contacts in the parenchyma. Immunoelectron microscopy revealed that intranodal lymphocytes lacked surface expression of S1P1, whereas S1P1 was evident on the surface and within the cytoplasm of macrophages, endothelial cells, and stromal cells. This subcellular pattern of intranodal receptor distribution was unchanged by treatment with FTY-P. We conclude that S1P1 agonists have profound effects on macrophages and endothelial cells, in addition to inducing lymphopenia.

Endothelial/pericyte interactions

Interactions between endothelial cells and mural cells (pericytes and vascular smooth muscle cells) in the blood vessel wall have recently come into focus as central processes in the regulation of vascular formation, stabilization, remodeling, and function. Failure of the interactions between the 2 cell types, as seen in numerous genetic mouse models, results in severe and often lethal cardiovascular defects. Abnormal interactions between the 2 cell types are also implicated in a number of human pathological conditions, including tumor angiogenesis, diabetic microangiopathy, ectopic tissue calcification, and stroke and dementia syndrome CADASIL. In the present review, we summarize current knowledge concerning the identity, characteristics, diversity, ontogeny, and plasticity of pericytes. We focus on the advancement in recent years of the understanding of intercellular communication between endothelial and mural cells with a focus on transforming growth factor beta, angiopoietins, platelet-derived growth factor, spingosine-1-phosphate, and Notch ligands and their respective receptors. We finally highlight recent important data contributing to the understanding of the role of pericytes in tumor angiogenesis, diabetic retinopathy, and hereditary lymphedema.