Endothelium-specific ablation of PDGFB leads to pericyte loss and glomerular, cardiac and placental abnormalities

Cellular source and amount of vascular endothelial growth factor and platelet-derived growth factor in tumors determine response to angiogenesis inhibitors

Vascular endothelial growth factor (VEGF), platelet-derived growth factor (PDGF), and their receptors are important targets in cancer therapy based on angiogenesis inhibition. However, it is unclear whether inhibition of VEGF and PDGF together is more effective than inhibition of either one alone. Here, we used two contrasting tumor models to compare the effects of inhibiting VEGF or PDGF alone, by adenovirally generated soluble receptors, to the effects of inhibiting both together. In RIP-Tag2 tumors, VEGF and PDGF inhibition together reduced tumor vascularity and abundance of pericytes. However, VEGF inhibition reduced tumor vascularity without decreasing pericyte density, and PDGF inhibition reduced pericytes without reducing tumor vascularity. By contrast, in Lewis lung carcinomas (LLC), inhibition of VEGF or PDGF reduced blood vessels and pericytes to the same extent as did inhibition of both together. Similar results were obtained using tyrosine kinase inhibitors AG-013736 and imatinib. In LLC, VEGF expression was largely restricted to pericytes and PDGF was largely restricted to endothelial cells, but, in RIP-Tag2 tumors, expression of both growth factors was more widespread and significantly greater than in LLC. These findings suggest that inhibition of PDGF in LLC reduced pericytes, and then tumor vessels regressed because pericytes were the main source of VEGF. The vasculature of RIP-Tag2 tumors, in which most VEGF is from tumor cells, was more resistant to PDGF inhibition. The findings emphasize the interdependence of pericytes and endothelial cells in tumors and the importance of tumor phenotype in determining the cellular effects of VEGF and PDGF inhibitors on tumor vessels.

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.

Molecular Pathology of the Genitourinary Tract: Molecular Pathology of Kidney and Testes

With the advent of newer molecular technologies, our knowledge of cellular mechanisms with tumors of the kidney and testis has grown exponentially. Molecular technologies have led to better understanding of interplay between the von Hippel-Lindau gene and angiogenic cytokines in renal cancer and isochromosome 12p in testicular neoplasms. The result has been development of antiangiogenic-targeted therapy within recent years that has become the mainstay treatment for metastatic renal cell cancer. In the near future, classification and diagnosis of renal and testicular tumors through morphologic analysis will be supplemented by molecular information correlating to prognosis and targeted therapy. This article outlines tumor molecular pathology of the kidney and testis encompassing current genomic, epigenomic, and proteonomic findings.

The pericyte: cellular regulator of microvascular blood flow

The vascular system - through its development, response to injury, and remodeling during disease - constitutes one of the key organ systems sustaining normal human physiology; conversely, its dysregulation also underlies multiple pathophysiologic processes. Regulation of vascular endothelial cell function requires the integration of complex signals via multiple cell types, including arterial smooth muscle, capillary and post-capillary pericytes, and other perivascular cells such as glial and immune cells. Here, we focus on the pericyte and its roles in microvascular remodeling, reviewing current concepts in microvascular pathophysiology and offering new insights into the specific roles that pericyte-dependent signaling pathways may play in modulating endothelial growth and microvascular tone during pathologic angiogenesis and essential hypertension.

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.

Morphological and molecular aspects of physiological vascular morphogenesis

The cardiovascular system plays a crucial role in vertebrate development and homeostasis. Several genetic and epigenetic mechanisms are involved in the early development of the vascular system. During embryonal life, blood vessels first appear as the result of vasculogenesis, whereas remodeling of the primary vascular plexus occurs by angiogenesis. Many tissue-derived factors are involved in blood vessel formation and evidence is emerging that endothelial cells themselves represent a source of instructive signals to non-vascular tissue cells during organ development. This review article summarizes our knowledge concerning the principal factors involved in the regulation of vascular morphogenesis.

Pericytes and perivascular fibroblasts are the primary source of collagen-producing cells in obstructive fibrosis of the kidney

Understanding the origin of scar-producing myofibroblasts is vital in discerning the mechanisms by which fibrosis develops in response to inflammatory injury. Using a transgenic reporter mouse model expressing enhanced green fluorescent protein (GFP) under the regulation of the collagen type I, alpha 1 (coll1a1) promoter and enhancers, we examined the origins of coll1a1-producing cells in the kidney. Here we show that in normal kidney, both podocytes and pericytes generate coll1a1 transcripts as detected by enhanced GFP, and that in fibrotic kidney, coll1a1-GFP expression accurately identifies myofibroblasts. To determine the contribution of circulating immune cells directly to scar production, wild-type mice, chimeric with bone marrow from coll-GFP mice, underwent ureteral obstruction to induce fibrosis. Histological examination of kidneys from these mice showed recruitment of small numbers of fibrocytes to the fibrotic kidney, but these fibrocytes made no significant contribution to interstitial fibrosis. Instead, using kinetic modeling and time course microscopy, we identified coll1a1-GFP-expressing pericytes as the major source of interstitial myofibroblasts in the fibrotic kidney. Our studies suggest that either vascular injury or vascular factors are the most likely triggers for pericyte migration and differentiation into myofibroblasts. Therefore, our results serve to refocus fibrosis research to injury of the vasculature rather than injury to the epithelium.

Integrins

Angiogenesis, the formation of new blood vessels from preexisting vasculature, contributes to the pathogenesis of many disorders, including ischemic diseases and cancer. Integrins are cell adhesion molecules that are expressed on the surface of endothelial cells and pericytes, making them potential targets for antiangiogenic therapy. Here we review the contribution of endothelial and mural cell integrins to angiogenesis and highlight their potential as antiangiogenesis targets.

Soluble receptor-mediated selective inhibition of VEGFR and PDGFRbeta signaling during physiologic and tumor angiogenesis

The simultaneous targeting of both endothelial cells and pericytes via inhibition of VEGF receptor (VEGFR) and PDGFbeta receptor (PDGFRbeta) signaling, respectively, has been proposed to enhance the efficacy of antiangiogenic tumor therapy. Clinical and preclinical modeling of combined VEGFR and PDGFRbeta signaling inhibition, however, has used small molecule kinase inhibitors with inherently broad substrate specificities, precluding detailed examination of this hypothesis. Here, adenoviral expression of a soluble VEGFR2/Flk1 ectodomain (Ad Flk1-Fc) in combination with a soluble ectodomain of PDGFRbeta (Ad sPDGFRbeta) allowed highly selective inhibition of these pathways. The activity of Ad sPDGFRbeta was validated in vitro against PDGF-BB and in vivo with near-complete blockade of pericyte recruitment in the angiogenic corpus luteum, resulting in prominent hemorrhage, thus demonstrating an essential function for PDGF signaling during ovarian angiogenesis. Combination therapy with Ad PDGFRbeta and submaximal doses of Ad Flk1-Fc produced modest additive antitumor effects; however, no additivity was observed with maximal VEGF inhibition in numerous s.c. models. Notably, VEGF inhibition via Ad Flk1-Fc was sufficient to strongly suppress tumor endothelial and pericyte content as well as intratumoral PDGF-B mRNA, obscuring additive Ad sPDGFRbeta effects on pericytes or tumor volume. These studies using highly specific soluble receptors suggest that additivity between VEGFR and PDGFRbeta inhibition depends on the strength of VEGF blockade and appears minimal under conditions of maximal VEGF antagonism.

Role of platelet-derived growth factors in physiology and medicine

Platelet-derived growth factors (PDGFs) and their receptors (PDGFRs) have served as prototypes for growth factor and receptor tyrosine kinase function for more than 25 years. Studies of PDGFs and PDGFRs in animal development have revealed roles for PDGFR-alpha signaling in gastrulation and in the development of the cranial and cardiac neural crest, gonads, lung, intestine, skin, CNS, and skeleton. Similarly, roles for PDGFR-beta signaling have been established in blood vessel formation and early hematopoiesis. PDGF signaling is implicated in a range of diseases. Autocrine activation of PDGF signaling pathways is involved in certain gliomas, sarcomas, and leukemias. Paracrine PDGF signaling is commonly observed in epithelial cancers, where it triggers stromal recruitment and may be involved in epithelial-mesenchymal transition, thereby affecting tumor growth, angiogenesis, invasion, and metastasis. PDGFs drive pathological mesenchymal responses in vascular disorders such as atherosclerosis, restenosis, pulmonary hypertension, and retinal diseases, as well as in fibrotic diseases, including pulmonary fibrosis, liver cirrhosis, scleroderma, glomerulosclerosis, and cardiac fibrosis. We review basic aspects of the PDGF ligands and receptors, their developmental and pathological functions, principles of their pharmacological inhibition, and results using PDGF pathway-inhibitory or stimulatory drugs in preclinical and clinical contexts.

PDGF-B signaling is important for murine cardiac development: its role in developing atrioventricular valves, coronaries, and cardiac innervation

We hypothesized that PDGF-B/PDGFR-beta-signaling is important in the cardiac contribution of epicardium-derived cells and cardiac neural crest, cell lineages crucial for heart development. We analyzed hearts of different embryonic stages of both Pdgf-b-/- and Pdgfr-beta-/- mouse embryos for structural aberrations with an established causal relation to defective contribution of these cell lineages. Immunohistochemical staining for alphaSMA, periostin, ephrinB2, EphB4, VEGFR-2, Dll1, and NCAM was performed on wild-type and knockout embryos. We observed that knockout embryos showed perimembranous and muscular ventricular septal defects, maldevelopment of the atrioventricular cushions and valves, impaired coronary arteriogenesis, and hypoplasia of the myocardium and cardiac nerves. The abnormalities correspond with models in which epicardial development is impaired and with neuronal neural crest-related innervation deficits. This implies a role for PDGF-B/PDGFR-beta-signaling specifically in the contribution of these cell lineages to cardiac development.

PDGF-A as an epicardial mitogen during heart development

In the developing heart, reciprocal interactions between the epicardium and myocardium drive further sublineage specification and ventricular chamber morphogenesis. Several observations suggest that the epicardium is a source of secreted factors that influence cardiomyocyte proliferation, and these factors may have other roles as well. However, the identity of these epicardial factors remains mostly unknown. We have identified platelet-derived growth factor-A (PDGF-A) as one of several mitogens expressed by the rat EMC epicardial cell line (epicardial mesothelial cells), by embryonic epicardium and myocardium during mouse heart development, and by adult epicardium. Expression of the cognate receptor gene Pdgfra was detected in the epicardium, although a low level of expression in myocardium could not be ruled out. To address the potential role of PDGF signaling in heart development, we mutated both PDGF receptor genes in the myocardial and mesodermal compartments of the heart; however, this did not result in an observable cardiac phenotype. This finding suggests that mesodermal PDGF signaling is not essential in heart development, although its role may be redundant with other signaling pathways. Indeed, our results demonstrate the presence of additional mitogens that may have such an overlapping role.

Use of proximity ligation to screen for inhibitors of interactions between vascular endothelial growth factor A and its receptors

BACKGROUND

Improved methods are required to screen drug candidates for their influences on protein interactions. There is also a compelling need for miniaturization of screening assays, with attendant reductions in reagent consumption and assay costs.

METHODS

We used sensitive, miniaturized proximity ligation assays (PLAs) to monitor binding of vascular endothelial growth factor A (VEGF-A) to 2 of its receptors, VEGFR-1 and VEGFR-2. We measured the effects of proteins and low molecular weight compounds capable of disrupting these interactions and compared the results with those obtained by immunoblot analysis. We analyzed 6 different inhibitors: a DNA aptamer, a mixed DNA/RNA aptamer, a monoclonal VEGF-A neutralizing antibody, a monoclonal antibody directed against VEGFR-2, a recombinant competitive protein, and a low molecular weight synthetic molecule.

RESULTS

The PLAs were successful for monitoring the formation and inhibition of VEGF-A-receptor complexes, and the results correlated well with those obtained by measuring receptor phosphorylation. The total PLA time is just 3 hours, with minimal manual work and reagent additions. The method allows evaluation of the apparent affinity [half-maximal inhibitory concentration (IC(50))] from a dose-response curve.

CONCLUSIONS

The PLA may offer significant advantages over conventional methods for screening the interactions of ligands with their receptors. The assay may prove useful for parallel analyses of large numbers of samples in the screening of inhibitor libraries for promising agents. The technique provides dose-response curves, allowing IC(50) values to be calculated.

The blood-brain barrier in health and chronic neurodegenerative disorders

The blood-brain barrier (BBB) is a highly specialized brain endothelial structure of the fully differentiated neurovascular system. In concert with pericytes, astrocytes, and microglia, the BBB separates components of the circulating blood from neurons. Moreover, the BBB maintains the chemical composition of the neuronal "milieu," which is required for proper functioning of neuronal circuits, synaptic transmission, synaptic remodeling, angiogenesis, and neurogenesis in the adult brain. BBB breakdown, due to disruption of the tight junctions, altered transport of molecules between blood and brain and brain and blood, aberrant angiogenesis, vessel regression, brain hypoperfusion, and inflammatory responses, may initiate and/or contribute to a "vicious circle" of the disease process, resulting in progressive synaptic and neuronal dysfunction and loss in disorders such as Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, multiple sclerosis, and others. These findings support developments of new therapeutic approaches for chronic neurodegenerative disorders directed at the BBB and other nonneuronal cells of the neurovascular unit.

How do mesangial and endothelial cells form the glomerular tuft?

The glomerular capillary tuft is a highly intricate and specialized microvascular bed that filters plasma water and solute to form urine. The mature glomerulus contains four cell types: Parietal epithelial cells that form Bowman's capsule, podocytes that cover the outermost layer of the glomerular filtration barrier, glycocalyx-coated fenestrated endothelial cells that are in direct contact with blood, and mesangial cells that sit between the capillary loops. Filtration begins only after the influx and organization of endothelial and mesangial cells in the developing glomerulus. Tightly coordinated movement and cross-talk between these cell types is required for the formation of a functional glomerular filtration barrier, and disruption of these processes has devastating consequences for early life. Current concepts of the role of mesangial and endothelial cells in formation of the capillary tuft are reviewed here.

Bmp in podocytes is essential for normal glomerular capillary formation

Bone morphogenetic protein (BMP) 4 exerts multiple biological effects on kidney and ureter development. To examine the role of BMP4 in glomerular morphogenesis, we generated transgenic mice with altered BMP4 function in podocytes by conferring tissue-specificity with the nephrin (Nphs1) promoter. At birth, Tg(Nphs1-Nog) mice, which had loss of BMP4 function in podocytes, were found to have glomerular microaneurysms, collapsed glomerular capillary tufts, enlarged Bowman's capsules, and fewer normal proximal tubules. Conversely, Tg(Nphs1-Bmp4) mice, which had increased BMP4 function in podocytes, demonstrated defects in glomerular capillary formation, but podocytes were not appreciably affected. The Tg(Nphs1-Nog) and Tg(Nphs1-Bmp4) mice shared morphological characteristics with the previously reported podocyte-specific Vegf-A over-expressing and knockout mice, respectively. Consistent with the morphological similarity, in situ hybridization revealed an intense signal for podocyte expression of Vegf in Tg(Nphs1-Nog) mice, whereas the signal was markedly suppressed in Tg(Nphs1-Bmp4) mice. However, in vitro studies with metanephroi failed to demonstrate a direct interaction between BMP4 or Noggin and VEGF in podocytes. Instead, immunostaining showed that phosphorylated Smads, the mediators of BMP signaling, are present in endothelial and/or mesangial cells, but not in podocytes, within the developing glomeruli. Therefore, this study suggests that podocyte-derived BMP plays an important role in glomerular capillary formation, perhaps by acting on non-podocyte glomerular cells in a paracrine fashion.

AMP-activated protein kinase inhibits transforming growth factor-beta-induced Smad3-dependent transcription and myofibroblast transdifferentiation

In wound healing, myofibroblast transdifferentiation (MFT) is a metaplastic change in phenotype producing profibrotic effector cells that secrete and remodel the extracellular matrix. Unlike pathways that induce MFT, the molecular mechanisms that negatively regulate MFT are poorly understood. Here, we report that AMP-activated protein kinase (AMPK) blocks MFT in response to transforming growth factor-beta (TGFbeta). Pharmacological activation of AMPK inhibited TGFbeta-induced secretion of extracellular matrix proteins collagen types I and IV and fibronectin. AMPK activation also prevented induction of the myofibroblast phenotype markers alpha-smooth muscle actin and the ED-A fibronectin splice variant. AMPK activators did not prevent MFT in cells transduced with an adenovirus expressing dominant negative, kinase-dead AMPKalpha2. Moreover, AMPK activators did not inhibit MFT induction in AMPK(alpha1,2)(-/-) fibroblasts, demonstrating a requirement for AMPK(alpha) expression. Adenoviral transduction of constitutively active AMPK(alpha2) was sufficient to prevent TGFbeta-induced collagen I, alpha-smooth muscle actin, and ED-A fibronectin. AMPK did not reduce TGFbeta-stimulated Smad3 COOH-terminal phosphorylation and nuclear translocation, which are necessary for MFT. However, AMPK activation inhibited TGFbeta-induced transcription driven by Smad3-binding cis-elements. Consistent with a role for AMPK in transcriptional regulation, nuclear translocation of AMPKalpha2 correlated with the appearance of active AMPKalpha in the nucleus. Collectively, these results demonstrate that AMPK inhibits TGFbeta-induced transcription downstream of Smad3 COOH-terminal phosphorylation and nuclear translocation. Furthermore, activation of AMPK is sufficient to negatively regulate MFT in vitro.

Beta1 integrin expression by podocytes is required to maintain glomerular structural integrity

Integrins are transmembrane heteromeric receptors that mediate interactions between cells and extracellular matrix (ECM). beta1, the most abundantly expressed integrin subunit, binds at least 12 alpha subunits. beta1 containing integrins are highly expressed in the glomerulus of the kidney; however their role in glomerular morphogenesis and maintenance of glomerular filtration barrier integrity is poorly understood. To study these questions we selectively deleted beta1 integrin in the podocyte by crossing beta1(flox/flox) mice with podocyte specific podocin-cre mice (pod-Cre), which express cre at the time of glomerular capillary formation. We demonstrate that podocyte abnormalities are visualized during glomerulogenesis of the pod-Cre;beta1(flox/flox) mice and proteinuria is present at birth, despite a grossly normal glomerular basement membrane. Following the advent of glomerular filtration there is progressive podocyte loss and the mice develop capillary loop and mesangium degeneration with little evidence of glomerulosclerosis. By 3 weeks of age the mice develop severe end stage renal failure characterized by both tubulointerstitial and glomerular pathology. Thus, expression of beta1 containing integrins by the podocyte is critical for maintaining the structural integrity of the glomerulus.

Development of the renal glomerulus: good neighbors and good fences

The glomerulus of the mammalian kidney is an intricate structure that contains an unusual filtration barrier that retains higher molecular weight proteins and blood cells in the circulation. Recent studies have changed our conception of the glomerulus from a relatively static structure to a dynamic one, whose integrity depends on signaling between the three major cell lineages: podocytes, endothelial and mesangial cells. Research into the signaling pathways that control glomerular development and then maintain glomerular integrity and function has recently identified several genes, such as the nephrin and Wilms' tumor 1 genes, that are mutated in human kidney disease.

Chapter 9. Development of coronary vessels

This chapter summarizes experimental techniques used to study coronary vessel development from its origins in the proepicardium (PE) to the final assembled network of arteries, veins, and capillaries present in the mature heart. Methods are described for microdissection and culture of the PE and embryonic epicardial cells, isolation of total RNA from single PE primordia and analysis by RT-PCR, imaging of the epicardium and coronary vessels by whole-mount confocal microscopy and by scanning electron microscopy, and the preparation of coronary vascular corrosion casts to visualize the entire coronary artery network structure. These techniques form the basic tools to study the cellular and molecular pathways that guide development and remodeling of coronary vessels.

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.

Prevalence of congenital heart defects in monochorionic/diamniotic twin gestations: a systematic literature review

OBJECTIVE

Congenital heart defects (CHDs) affect approximately 0.5% of all neonates. Recent literature points to a possible increase in the CHD prevalence among monochorionic/diamniotic (MC/DA) twin gestations. We hypothesized that MC/DA twin pregnancy is a risk factor for CHD.

METHODS

A systematic review of all published English literature was conducted on MEDLINE (Ovid and PubMed) from January 2000 through April 2007 using the medical subject heading terms "congenital heart defect" and "monozygotic twins." Four observational studies were included in the final analysis. Published historical data were used for the population background risk of CHD. Relative risk (RR) estimates with 95% confidence intervals (CIs) were calculated by fixed and random effect models.

RESULTS

We included a total of 40 fetuses with CHDs among 830 fetuses from MC/DA twin gestations. Compared with the population, CHDs were significantly more prevalent in MC/DA twins regardless of the presence of twin-twin transfusion syndrome (TTTS) (RR, 9.18; 95% CI, 5.51-15.29; P < .001). Monochorionic/diamniotic twin gestations affected by TTTS were more likely to be complicated by CHDs than those that did not have TTTS (RR, 2.78; 95% CI, 1.03-7.52; P = .04). Ventricular septal defects were the most frequent heart defects. Pulmonary stenosis and atrial septal defects were significantly more prevalent in pregnancies complicated with TTTS.

CONCLUSIONS

Monochorionic/diamniotic twin gestation appears to be a risk factor for CHDs. Conditions that lead to abnormal placentation may also contribute to abnormal heart development, especially in MC/DA twin pregnancies complicated with TTTS. Fetal echocardiography may be considered for all MC/DA twin gestations because ventricular septal defects and pulmonary stenosis are the most common defects.

Contribution of endothelial cells to organogenesis: a modern reappraisal of an old Aristotelian concept

It is well established that many tissue-derived factors are involved in blood vessel formation, but evidence is now emerging that endothelial cells themselves represent a crucial source of instructive signals to non-vascular tissue cells during organ development. Thus, endothelial cell signalling is currently believed to promote fundamental cues for cell fate specification, embryo patterning, organ differentiation and postnatal tissue remodelling. This review article summarizes some of the recent advances in our understanding of the role of endothelial cells as effector cells in organ formation.

Platelet-derived growth factor reorganizes the actin cytoskeleton through 3-phosphoinositide-dependent and 3-phosphoinositide-independent mechanisms in human mesangial cells

BACKGROUND

Platelet-derived growth factor (PDGF) is a potent activator of mesangial cell proliferation and migration. Although phosphoinositide 3-kinase (PI3K) enzymes are important downstream targets of the PDGF receptor, the contribution made by their 3-phosphoinositide products in the reorganization of actin cytoskeleton and focal adhesions has been questioned.

METHODS AND RESULTS

Pharmacological inhibition of the PI3K activity blocks PDGF-induced migration of human primary mesangial cells using an in vitro scrape wound healing assay. Acute (<10 min) inhibition of the PI3K activity did not alter the effect of PDGF on either stress fibre dissolution or reorganization of focal adhesions. However, at later times (>30 min), PDGF-stimulated responses were inhibited. In contrast, PDGF-stimulated membrane ruffling remained insensitive to PI3K inhibitors throughout. Inhibition of protein kinase C and Erk also attenuated PDGF-stimulated mesangial cell migration; however, neither signaling pathway was responsible for the initial effects on filamentous actin and focal adhesions.

CONCLUSIONS

We propose that following PDGF stimulation of mesangial cells, reorganization of the actin cytoskeleton occurs in a biphasic manner. The mechanism responsible for mesangial cell migration that occurs immediately following PDGF stimulation may serve to 'prime' for the subsequent 3-phosphoinositide-, protein-kinase-C-, and Erk-dependent migration.

Myocardium defects and ventricular hypoplasia in mice homozygous null for the Forkhead Box M1 transcription factor

The Forkhead Box m1 (Foxm1) transcription factor is expressed in cardiomyocytes and cardiac endothelial cells during heart development. In this study, we used a novel Foxm1 -/- mouse line to demonstrate that Foxm1-deletion causes ventricular hypoplasia and diminished DNA replication and mitosis in developing cardiomyocytes. Proliferation defects in Foxm1 -/- hearts were associated with a reduced expression of Cdk1-activator Cdc25B phosphatase and NFATc3 transcription factor, and with abnormal nuclear accumulation of the Cdk-inhibitor p21(Cip1) protein. Depletion of Foxm1 levels by siRNA caused altered expression of these genes in cultured HL-1 cardiomyocytes. Endothelial-specific deletion of the Foxm1 fl/fl allele in Tie2-Cre Foxm1 fl/fl embryos did not influence heart development and cardiomyocyte proliferation. Foxm1 protein binds to the -9,259/-9,288-bp region of the endogenous mouse NFATc3 promoter, indicating that Foxm1 is a transcriptional activator of the NFATc3 gene. Foxm1 regulates expression of genes essential for the proliferation of cardiomyocytes during heart development.

Vascular endothelial cell-specific NF-kappaB suppression attenuates hypertension-induced renal damage

Nuclear factor kappa B (NF-kappaB) participates in hypertension-induced vascular and target-organ damage. We tested whether or not endothelial cell-specific NF-kappaB suppression would be ameliorative. We generated Cre/lox transgenic mice with endothelial cell-restricted NF-kappaB super-repressor IkappaBalphaDeltaN (Tie-1-DeltaN mice) overexpression. We confirmed cell-specific IkappaBalphaDeltaN expression and reduced NF-kappaB activity after TNF-alpha stimulation in primary endothelial cell culture. To induce hypertension with target-organ damage, we fed mice a high-salt diet and N(omega)-nitro-l-arginine-methyl-ester (L-NAME) and infused angiotensin (Ang) II. This treatment caused a 40-mm Hg blood pressure increase in both Tie-1-DeltaN and control mice. In contrast to control mice, Tie-1-DeltaN mice developed a milder renal injury, reduced inflammation, and less albuminuria. RT-PCR showed significantly reduced expression of the NF-kappaB targets VCAM-1 and ICAM-1, compared with control mice. Thus, the data demonstrate a causal link between endothelial NF-kappaB activation and hypertension-induced renal damage. We conclude that in vivo NF-kappaB suppression in endothelial cells stops a signaling cascade leading to reduced hypertension-induced renal damage despite high blood pressure.

Ultrasensitive assays for proteins

Proteins are essential components of organisms and are involved in a wide range of biological functions. There are increasing demands for ultra-sensitive protein detection, because many important protein biomarkers are present at ultra-low levels, especially during the early stages of disease. Measuring proteins at low levels is also crucial for investigations of the protein synthesis and functions in biological systems. In this review, we summarize the recent developments of novel technology enabling ultrasensitive protein detection. We focus on two groups of techniques that involve either polymerase amplification of affinity DNA probes or signal amplification by the use of nano-/micro-materials. The polymerase-based amplification of affinity DNA probes indirectly improves the sensitivity of protein detection by increasing the number of detection molecules. The use of nano-/micro-materials conjugated to affinity probes enhances the measurement signals by using the unique electrical, optical, and catalytic properties of these novel materials. This review describes the basic principles, performances, applications, merits, and limitations of these techniques.

Lack of endothelial cell survivin causes embryonic defects in angiogenesis, cardiogenesis, and neural tube closure

We explored the physiologic role of endothelial cell apoptosis during development by generating mouse embryos lacking the inhibitor of apoptosis protein (IAP) survivin in endothelium. This was accomplished by intercrossing survivin(lox/lox) mice with mice expressing cre recombinase under the control of the endothelial cell specific tie1 promoter (tie1-cre mice). Lack of endothelial cell survivin resulted in embryonic lethality. Mutant embryos had prominent and diffuse hemorrhages from embryonic day 9.5 (E9.5) and died before E13.5. Heart development was strikingly abnormal. Survivin-null endocardial lineage cells could not support normal epithelial-mesenchymal transformation (EMT), resulting in hypoplastic endocardial cushions and in utero heart failure. In addition, 30% of mutant embryos had neural tube closure defects (NTDs) that were not caused by bleeding or growth retardation, but were likely due to alterations in the release of soluble factors from endothelial cells that otherwise support neural stem cell proliferation and neurulation. Thus, regulation of endothelial cell survival, and maintenance of vascular integrity by survivin are crucial for normal embryonic angiogenesis, cardiogenesis, and neurogenesis.

Lactate adversely affects the in vitro formation of endothelial cell tubular structures through the action of TGF-beta1

When lactate accumulation in a tumor microenvironment reaches an average concentration of 10-20 mM, it tends to reflect a high degree of malignancy. However, the hypothesis that tumor-derived lactate has a number of partially adverse biological effects on malignant and tumor-associated host cells requires further evidence. The present study attempted to evaluate the impact of lactate on the process of angiogenesis, in particular on the formation of tubular structures. The endothelial cell (EC) network in desmoplastic breast tumors is primarily located in areas of reactive fibroblastic stroma. We employed a fibroblast-endothelial cell co-culture model as in vitro angiogenesis system normally producing florid in vitro tubule formation to analyze this situation. In contrast to previous studies, we found that lactate significantly reduces EC network formation in a dose-dependent manner as quantified by semi-automated morphometric analyses following immunohistochemical staining. The decrease in CD31-positive tubular structures and the number of intersections was independent of VEGF supplementation and became more pronounced in the presence of protons. The number of cells, primarily of the fibroblast population, was reduced but cell loss could not be attributed to a decrease in proliferative activity or pronounced apoptotic cell death. Treatment with 10 mM lactate was accompanied by enhanced mRNA expression and release of TGF-beta1, which also shows anti-angiogenic activity in the model. Both TGF-beta1 and lactate induced myofibroblastic differentiation adjacent to the EC tubular structures. The lactate response on the EC network was diminished by TGF-beta1 neutralization, indicating a causal relationship between lactate and TGF-beta1 in the finely tuned processes of vessel formation and maturation which may also occur in vivo within tumor tissue.

Cooperation between the PDGF receptors in cardiac neural crest cell migration

Neural crest cells (NCCs) are essential components of the sympathetic nervous system, skin, craniofacial skeleton, and aortic arch. It has been known for many years that perturbation of migration, proliferation, and/or differentiation of these cells leads to birth defects such as cleft palate and persistent truncus arteriosus (PTA). Previously, we had shown that disruption of the platelet-derived growth factor receptor (PDGFR) alpha in NCCs resulted in defects in craniofacial and aortic arch development, the latter with variable penetrance. Because we observed ventricular septal defects in embryos that are null for the PDGFRbeta, we hypothesized that both PDGF receptors are involved in NCC formation. Here, we show that both receptors are expressed in cardiac NCCs and that the combined loss of the PDGFRalpha and PDGFRbeta in NCCs resulted in NCC-related heart abnormalities, including PTA and a ventricular septal defect (VSD). Using NCC lineage tracing, we observed that loss of PDGF receptor signaling resulted in reduced NCCs in the conotruncus region, leading to defects in aortic arch septation. These results indicate that while PDGFRalpha plays a predominant role in NCC development, the PDGFRbeta is expressed by and functions in cardiac NCCs. Combined PDGF receptor signaling is required for sufficient recruitment of cardiac NCCs into the conotruncal region and for formation of the aortico-pulmonary and ventricular septum.

Defective N-sulfation of heparan sulfate proteoglycans limits PDGF-BB binding and pericyte recruitment in vascular development

During vascular development, endothelial platelet-derived growth factor B (PDGF-B) is critical for pericyte recruitment. Deletion of the conserved C-terminal heparin-binding motif impairs PDGF-BB retention and pericyte recruitment in vivo, suggesting a potential role for heparan sulfate (HS) in PDGF-BB function during vascular development. We studied the participation of HS chains in pericyte recruitment using two mouse models with altered HS biosynthesis. Reduction of N-sulfation due to deficiency in N-deacetylase/N-sulfotransferase-1 attenuated PDGF-BB binding in vitro, and led to pericyte detachment and delayed pericyte migration in vivo. Reduced N-sulfation also impaired PDGF-BB signaling and directed cell migration, but not proliferation. In contrast, HS from glucuronyl C5-epimerase mutants, which is extensively N- and 6-O-sulfated, but lacks 2-O-sulfated L-iduronic acid residues, retained PDGF-BB in vitro, and pericyte recruitment in vivo was only transiently delayed. These observations were supported by in vitro characterization of the structural features in HS important for PDGF-BB binding. We conclude that pericyte recruitment requires HS with sufficiently extended and appropriately spaced N-sulfated domains to retain PDGF-BB and activate PDGF receptor beta (PDGFRbeta) signaling, whereas the detailed sequence of monosaccharide and sulfate residues does not appear to be important for this interaction.

Phenotypic heterogeneity of the endothelium: II. Representative vascular beds

Endothelial cells, which form the inner cellular lining of blood vessels and lymphatics, display remarkable heterogeneity in structure and function. This is the second of a 2-part review on the phenotypic heterogeneity of blood vessel endothelial cells. The first part discusses the scope, the underlying mechanisms, and the diagnostic and therapeutic implications of phenotypic heterogeneity. Here, these principles are applied to an understanding of organ-specific phenotypes in representative vascular beds including arteries and veins, heart, lung, liver, and kidney. The goal is to underscore the importance of site-specific properties of the endothelium in mediating homeostasis and focal vascular pathology, while at the same time emphasizing the value of approaching the endothelium as an integrated system.

The Glomerular Transcriptome and Proteome

Histopathology provides the current basis for classification and diagnosis of glomerular disorders. Molecular profiling methods, such as microarray analysis of mRNA expression, have rapidly emerged over the past years and are now applicable to minute amounts of tissue material, such as glomeruli from embryos or adult experimental animals, or from human renal needle biopsies. This review summarizes current efforts aiming at the determination of the glomerular transcriptome and proteome during development, in the healthy adult, and in disease. These studies are encouraging and show that comprehensive molecular profiling of the kidney glomerulus will most likely provide significant new insights into the normal structure and function of the glomerular filter, the molecular mechanisms of glomerular development, the diagnosis and classification of glomerular disease, and the pathogenic mechanisms underlying the stepwise breakdown of glomerular filter function that accompanies several common systemic disorders.

Molecular Biology of Renal Cell Cancer and the Identification of Therapeutic Targets

Renal cell cancer (RCC) is a heterogeneous disease consisting of different histologic types. Major advances have been accomplished during the last 15 years in our understanding of the genetic events that initiate RCC. These advances were greatly facilitated by meticulous clinical description and registration of patients with familial predisposition to RCC. The cloning of the susceptibility genes that underline familial predisposition to RCC has offered entry points into the signaling pathways that are also deregulated in sporadic RCC. Biochemical studies of these signaling pathways and target validation experiments have already culminated in the discovery and clinical application of small molecules with promising activity in RCC. In this article, we highlight the molecular genetic features of RCC that are more directly related to identification and validation of promising targets for molecular therapy.

The role of platelet-derived growth factor signaling in healing myocardial infarcts

OBJECTIVES

This study sought to examine the role of platelet-derived growth factor (PDGF) signaling in healing myocardial infarcts.

BACKGROUND

Platelet-derived growth factor isoforms exert potent fibrogenic effects through interactions with PDGF receptor (PDGFR)-alpha and PDGFR-beta. In addition, PDGFR-beta signaling mediates coating of developing vessels with mural cells, leading to the formation of a mature vasculature. We hypothesized that PDGFR activation may regulate fibrosis and vascular maturation in healing myocardial infarcts.

METHODS

Mice undergoing reperfused infarction protocols were injected daily with a neutralizing anti-PDGFR-beta antibody (APB5), an anti-PDGFR-alpha antibody (APA5), or control immunoglobulin G, and were killed after 7 days of reperfusion.

RESULTS

The PDGF-B, PDGFR-alpha, and PDGFR-beta mRNA expression was induced in reperfused mouse infarcts. Perivascular cells expressing phosphorylated PDGFR-beta were identified in the infarct after 7 days of reperfusion, indicating activation of the PDGF-BB/PDGFR-beta pathway. The PDGFR-beta blockade resulted in impaired maturation of the infarct vasculature, enhanced capillary density, and formation of dilated uncoated vessels. Defective vascular maturation in antibody-treated mice was associated with increased and prolonged extravasation of red blood cells and monocyte/macrophages, suggesting increased permeability. These defects resulted in decreased collagen content in the healing infarct. In contrast, PDGFR-alpha inhibition did not affect vascular maturation, but significantly decreased collagen deposition in the infarct.

CONCLUSIONS

Platelet-derived growth factor signaling critically regulates postinfarction repair. Both PDGFR-beta- and PDGFR-alpha-mediated pathways promote collagen deposition in the infarct. Activation of PDGF-B/PDGFR-beta is also involved in recruitment of mural cells by neovessels, regulating maturation of the infarct vasculature. Acquisition of a mural coat and maturation of the vasculature promotes resolution of inflammation and stabilization of the scar.

MCP-1 mediates TGF-beta-induced angiogenesis by stimulating vascular smooth muscle cell migration

Transforming growth factor-beta (TGF-beta) and its signaling mediators play crucial roles in vascular formation. Our previous microarray analysis identified monocyte chemoattractant protein-1 (MCP-1) as a TGF-beta target gene in endothelial cells (ECs). Here, we report that MCP-1 mediates the angiogenic effect of TGF-beta by recruiting vascular smooth muscle cells (VSMCs) and mesenchymal cells toward ECs. By using a chick chorioallantoic membrane assay, we show that TGF-beta promotes the formation of new blood vessels and this promotion is attenuated when MCP-1 activity is blocked by its neutralizing antibody. Wound healing and transwell assays established that MCP-1 functions as a chemoattractant to stimulate migration of VSMCs and mesenchymal 10T1/2 cells toward ECs. Furthermore, the conditioned media from TGF-beta-treated ECs stimulate VSMC migration, and inhibition of MCP-1 activity attenuates TGF-beta-induced VSMC migration toward ECs. Finally, we found that MCP-1 is a direct gene target of TGF-beta via Smad3/4. Taken together, our findings suggest that MCP-1 mediates TGF-beta-stimulated angiogenesis by enhancing migration of mural cells toward ECs and thus promoting the maturation of new blood vessels.

Molecular balance of capillary tube formation versus regression in wound repair: role of matrix metalloproteinases and their inhibitors

In this review, we discuss the identification of distinct matrix metalloproteinases (MMPs) and their inhibitors that differentially control the processes of capillary tube formation (morphogenesis) versus capillary tube regression in three-dimensional (3D) collagen matrices. This work directly relates to both granulation tissue formation and regression during wound repair. The membrane metalloproteinase, MT1-MMP (MMP-14), is required for endothelial cell (EC) tube formation using in vitro assays that mimic vasculogenesis or angiogenic sprouting in 3D collagen matrices. These events are markedly blocked by small interfering RNA (siRNA) suppression of MT1-MMP in ECs or by addition of tissue inhibitor of metalloproteinases (TIMPs)-2,-3, and -4 but not TIMP-1. In contrast, MMP-1 and MMP-10 are strongly induced during EC tube formation to regulate the process of tube regression (following activation by serine proteases) rather than formation. TIMP-1, which selectively inhibits soluble MMPs, blocks tube regression by inhibiting MMP-1 and MMP-10 while having no influence on EC tube formation. siRNA suppression of MMP-1 and MMP-10 markedly blocks tube regression without affecting tube formation. Furthermore, we discuss that pericyte-induced stabilization of EC tube networks in our model system appears to occur through EC-derived TIMP-2 and pericyte-derived TIMP-3 to block both the capillary tube formation and regression pathways.

Bone morphogenetic protein receptor 1A signaling is dispensable for hematopoietic development but essential for vessel and atrioventricular endocardial cushion formation

Bone morphogenetic protein 4 (BMP4) is crucial for the formation of FLK1-expressing (FLK1(+)) mesodermal cells. To further define the requirement for BMP signaling in the differentiation of blood, endothelial and smooth muscle cells from FLK1(+) mesoderm, we inactivated Alk3 (Bmpr1a) in FLK1(+) cells by crossing Alk3(floxed/floxed) and Flk1(+/Cre)Alk3(+/floxed) mice. Alk3 conditional knockout (CKO) mice died between E10.5 and E11.5. Unexpectedly, Alk3 CKO embryos did not show any hematopoietic defects. However, Alk3 CKO embryos displayed multiple abnormalities in vascular development, including vessel remodeling and maturation, which contributed to severe abdominal hemorrhage. Alk3 CKO embryos also displayed defects in atrioventricular canal (AVC) endocardial cushion formation in the heart. Collectively, our studies indicate a crucial role for ALK3 in vessel remodeling, vessel integrity and endocardial cushion formation during the development of the circulation system.

Detection of individual microbial pathogens by proximity ligation

BACKGROUND

Nucleic acid amplification allows the detection of single infectious agents. Protein-based assays, although they provide information on ongoing infections, have substantially less detection sensitivity.

METHODS

We used proximity ligation reactions to detect proteins on bacteria and virus particles via nucleic acid amplification. Antibodies recognizing viral or bacterial surface proteins were equipped with DNA strands that could be joined by ligation when several antibodies were bound in proximity to surface proteins of individual infectious agents.

RESULTS

Detection sensitivities similar to those of nucleic acid-based detection reactions were achieved directly in infected samples for a parvovirus and an intracellular bacterium.

CONCLUSIONS

This method enables detection of ligated DNA strands with good sensitivity by real-time PCR and could be of value for early diagnosis of infectious disease and in biodefense.

Endothelial-cardiomyocyte interactions in cardiac development and repair

Communication between endothelial cells and cardiomyocytes regulates not only early cardiac development but also adult cardiomyocyte function, including the contractile state. In the normal mammalian myocardium, each cardiomyocyte is surrounded by an intricate network of capillaries and is next to endothelial cells. Cardiomyocytes depend on endothelial cells not only for oxygenated blood supply but also for local protective signals that promote cardiomyocyte organization and survival. While endothelial cells direct cardiomyocytes, cardiomyocytes reciprocally secrete factors that impact endothelial cell function. Understanding how endothelial cells communicate with cardiomyocytes will be critical for cardiac regeneration, in which the ultimate goal is not simply to improve systolic function transiently but to establish new myocardium that is both structurally and functionally normal in the long term.

Rat glomerular mesangial cells require laminin-9 to migrate in response to insulin-like growth factor binding protein-5

Temporal and spatial differences in extracellular matrix play critical roles in cell proliferation, differentiation and migration. Different migratory stimuli use different substrates and receptors to achieve cell migration. To understand the mechanism of insulin-like growth factor binding protein-5 (IGFBP-5)-induced migration in mesangial cells, the roles of integrins and substrates were examined. IGFBP-5 induced an increase in mRNA expression for laminin (LN) chains lama4, lamb2, and lamc1, suggesting that LN-9 might be required for migration. Antibodies to the LNalpha(4) and LNbeta(2) chains, but not LNbeta(1), blocked IGFBP-5-induced migration. Anti-sense morpholino oligonucleotide inhibition of expression of LNalpha(4) substantially reduced expression of LN-8/9 (alpha(4)beta(1)gamma(1)/alpha(4)beta(2)gamma(1), 411/421) and prevented IGFBP-5-induced migration. Anti-sense inhibition of lamb2 reduced expression of LN-9. Absence of LN-9 prevented IGFBP-5-induced migration, which was not preserved by continued expression of LN-8. The requirement for LN-9 was further supported by studies of T98G cells, which express predominantly LN-8. IGFBP-5 had little effect on migration in these cells, but increased migration when T98G cells were plated on LN-8/9. IGFBP-5-mediated mesangial cell migration was inhibited by antibodies that block attachment to alpha(6)beta(1)-integrins but was unaffected by antibodies and disintegrins that block binding to other integrins. Furthermore, in cells with anti-sense inhibited expression of LN-9, integrin alpha(6)beta(1) was no longer detected on the cell surface. These studies suggest the specificity of mechanisms of migration induced by specific stimuli and for the first time demonstrate a unique function for LN-9 in mediating IGFBP-5-induced migration.

Large-scale identification of genes implicated in kidney glomerulus development and function

To advance our understanding of development, function and diseases in the kidney glomerulus, we have established and large-scale sequenced cDNA libraries from mouse glomeruli at different stages of development, resulting in a catalogue of 6053 different genes. The glomerular cDNA clones were arrayed and hybridized against a series of labeled targets from isolated glomeruli, non-glomerular kidney tissue, FACS-sorted podocytes and brain capillaries, which identified over 300 glomerular cell-enriched transcripts, some of which were further sublocalized to podocytes, mesangial cells and juxtaglomerular cells by in situ hybridization. For the earliest podocyte marker identified, Foxc2, knockout mice were used to analyze the role of this protein during glomerular development. We show that Foxc2 controls the expression of a distinct set of podocyte genes involved in podocyte differentiation and glomerular basement membrane maturation. The primary podocyte defects also cause abnormal differentiation and organization of the glomerular vascular cells. We surmise that studies on the other novel glomerulus-enriched transcripts identified in this study will provide new insight into glomerular development and pathomechanisms of disease.

Endothelial cells and VEGF in vascular development

The intricate patterning processes that establish the complex vascular system during development depend on a combination of intrinsic pre-patterning and extrinsic responses to environmental parameters. Mutational studies in mice and fish have shown that the vascular system is highly sensitive to genetic disruption and have identified potential targets for therapeutic interventions. New insights into non-vascular roles of vascular endothelial growth factor and the requirement for endothelial cells in adult organs and stem-cell niches highlight possible side effects of anti-angiogenic therapy and the need for new targets.

Pericytes and endothelial precursor cells: cellular interactions and contributions to malignancy

Tumor vasculature is irregular, abnormal, and essential for tumor growth. Pericytes and endothelial precursor cells (EPC) contribute to the formation of blood vessels under angiogenic conditions. As primary cells in culture, pericytes and EPC share many properties such as tube/network formation and response to kinase inhibitors selective for angiogenic pathways. Expression of cell surface proteins including platelet-derived growth factor receptor, vascular cell adhesion molecule, intercellular adhesion molecule, CD105, desmin, and neural growth proteoglycan 2 was similar between pericytes and EPC, whereas expression of P1H12 and lymphocyte function-associated antigen-1 clearly differentiates the cell types. Further distinction was observed in the molecular profiles for expression of angiogenic genes. Pericytes or EPC enhanced the invasion of MDA-MB-231 breast cancer cells in a coculture assay system. The s.c. coinjection of live pericytes or EPC along with MDA-MB-231 cells resulted in an increased rate of tumor growth compared with coinjection of irradiated pericytes or EPC. Microvessel density analysis indicated there was no difference in MDA-MB-231 tumors with or without EPC or pericytes. However, immunohistochemical staining of vasculature suggested that EPC and pericytes may stabilize or normalize vasculature rather than initiate vasculogenesis. In addition, tumors arising from the coinjection of EPC and cancer cells were more likely to develop lymphatic vessels. These results support the notion that pericytes and EPC contribute to malignancy and that these cell types can be useful as cell-based models for tumor vascular development and selection of agents that may provide therapeutic benefit.

Endothelial extracellular matrix: biosynthesis, remodeling, and functions during vascular morphogenesis and neovessel stabilization

The extracellular matrix (ECM) is critical for all aspects of vascular biology. In concert with supporting cells, endothelial cells (ECs) assemble a laminin-rich basement membrane matrix that provides structural and organizational stability. During the onset of angiogenesis, this basement membrane matrix is degraded by proteinases, among which membrane-type matrix metalloproteinases (MT-MMPs) are particularly significant. As angiogenesis proceeds, ECM serves essential functions in supporting key signaling events involved in regulating EC migration, invasion, proliferation, and survival. Moreover, the provisional ECM serves as a pliable scaffold wherein mechanical guidance forces are established among distal ECs, thereby providing organizational cues in the absence of cell-cell contact. Finally, through specific integrin-dependent signal transduction pathways, ECM controls the EC cytoskeleton to orchestrate the complex process of vascular morphogenesis by which proliferating ECs organize into multicellular tubes with functional lumens. Thus, the composition of ECM and therefore the regulation of ECM degradation and remodeling serves pivotally in the control of lumen and tube formation and, finally, neovessel stability and maturation.