Down-regulation of platelet endothelial cell adhesion molecule-1 results in thrombospondin-1 expression and concerted regulation of endothelial cell phenotype

Hybrid cardiovascular sourced extracellular matrix scaffolds as possible platforms for vascular tissue engineering

The aim when designing a scaffold is to provide a supportive microenvironment for the native cells, which is generally achieved by structurally and biochemically imitating the native tissue. Decellularized extracellular matrix (ECM) possesses the mechanical and biochemical cues designed to promote native cell survival. However, when decellularized and reprocessed, the ECM loses its cell supporting mechanical integrity and architecture. Herein, we propose dissolving the ECM into a polymer/solvent solution and electrospinning it into a fibrous sheet, thus harnessing the biochemical cues from the ECM and the mechanical integrity of the polymer. Bovine aorta and myocardium were selected as ECM sources. Decellularization was achieved using sodium dodecyl sulfate (SDS), and the ECM was combined with polycaprolactone and hexafluoro-2-propanol for electrospinning. The scaffolds were seeded with human umbilical vein endothelial cells (HUVECs). The study found that the inclusion of aorta ECM increased the scaffold's wettability and subsequently lead to increased HUVEC adherence and proliferation. Interestingly, the inclusion of myocardium ECM had no effect on wettability or cell viability. Furthermore, gene expression and mechanical changes were noted with the addition of ECM. The results from this study show the vast potential of electrospun ECM/polymer bioscaffolds and their use in tissue engineering.

VEGF/BMP-2 loaded three-dimensional model for enhanced angiogenic and odontogenic potential of dental pulp stem cells

AIM

To investigate the proliferation and differentiation potential of human dental pulp stem cells (DPSCs) in a three-dimensional culture model (TDM) by incorporation of VEGF and BMP-2.

METHODOLOGY

TDM was established using fibrin gel (fg) as a soft tissue matrix and demineralized dentine disc (dd) as a hard tissue matrix. DPSCs and vascular endothelial growth factor (VEGF) were encapsulated in fibrin gel (fg-VEGF) and then inserted into bone morphogenetic protein (BMP-2)-coated demineralized dentine discs (dd-BMP-2). DPSCs were incubated for 28 days in various fg/dd combinations in the absence or presence of VEGF and BMP-2. Proliferation and morphology of DPSCs in fibrin gel were analysed using MTT and Live&Dead assays. Release profiles of VEGF and BMP-2 from fibrin gel and dentine discs were quantified using ELISA, and the expressions of angiogenic and odontogenic differentiation markers were determined with RT-qPCR analysis. Data were analysed statistically using Wilcoxon signed rank tests, Kruskal-Wallis tests with Mann-Whitney U tests and Bonferroni adjustment. The level of significance was set at P < 0.05.

RESULTS

DPSCs were able to proliferate and showed interconnected cellular elongations in fibrin gel depending on fibrinogen concentration whilst monolayer control group showed typical fibroblast-like cell morphology. Encapsulating of VEGF in fibrin gel and BMP-2 in gelatin that was used to coat dentine discs allowed the controlled releases of growth factors, which induced angiogenic and odontogenic gene expressions by DPSCs. Higher expressions of PECAM as an angiogenic factor, and BSP, DMP-1, OCN and CBFA as odontogenic factors, were observed in TDM as compared to the other fg/dd combinations and the monolayer control group (P < 0.05).

CONCLUSIONS

TDM consisting of fibrin gel and dentine matrix allowed cell-cell interactions. TDM was highly effective in delivering both VEGF and BMP-2 that enhanced the angiogenic and odontogenic potential of DPSCs.

The Roles of Thrombospondins in Hemorrhagic Stroke

Hemorrhagic stroke is a devastating cerebrovascular disease with significant morbidity and mortality worldwide. Thrombospondins (TSPs), as matricellular proteins, belong to the TSP family which is comprised of five members. All TSPs modulate a variety of cellular functions by binding to various receptors. Recently, TSPs gained attention in the area of hemorrhagic stroke, especially TSP-1. TSP-1 participates in angiogenesis, the inflammatory response, apoptosis, and fibrosis after hemorrhagic stroke through binding to various molecules including but not limited to CD36, CD47, and TGF-β. In this review, we will discuss the roles of TSPs in hemorrhagic stroke and focus primarily on TSP-1.

Interleukin 2 Activates Brain Microvascular Endothelial Cells Resulting in Destabilization of Adherens Junctions

The pleiotropic cytokine interleukin 2 (IL2) disrupts the blood-brain barrier and alters brain microcirculation, underlying vascular leak syndrome that complicates cancer immunotherapy with IL2. The microvascular effects of IL2 also play a role in the development of multiple sclerosis and other chronic neurological disorders. The mechanism of IL2-induced disruption of brain microcirculation has not been determined previously. We found that both human and murine brain microvascular endothelial cells express constituents of the IL2 receptor complex. Then we established that signaling through this receptor complex leads to activation of the transcription factor, nuclear factor κB, resulting in expression of proinflammatory interleukin 6 and monocyte chemoattractant protein 1. We also discovered that IL2 induces disruption of adherens junctions, concomitant with cytoskeletal reorganization, ultimately leading to increased endothelial cell permeability. IL2-induced phosphorylation of vascular endothelial cadherin (VE-cadherin), a constituent of adherens junctions, leads to dissociation of its stabilizing adaptor partners, p120-catenin and β-catenin. Increased phosphorylation of VE-cadherin was also accompanied by a reduction of Src homology 2 domain-containing protein-tyrosine phosphatase 2, known to maintain vascular barrier function. These results unravel the mechanism of deleterious effects induced by IL2 on brain microvascular endothelial cells and may inform the development of new measures to improve IL2 cancer immunotherapy, as well as treatments for autoimmune diseases affecting the central nervous system.

PECAM-1 isoforms, eNOS and endoglin axis in regulation of angiogenesis

Vascular development and maintenance of proper vascular function through various regulatory mechanisms are critical to our wellbeing. Delineation of the regulatory processes involved in development of the vascular system and its function is one of the most important topics in human physiology and pathophysiology. Platelet endothelial cell adhesion molecule-1 (PECAM-1/CD31), a cell adhesion molecule with proangiogenic and proinflammatory activity, has been the subject of numerous studies. In the present review, we look at the important roles that PECAM-1 and its isoforms play during angiogenesis, and its molecular mechanisms of action in the endothelium. In the endothelium, PECAM-1 not only plays a role as an adhesion molecule but also participates in intracellular signalling pathways which have an impact on various cell adhesive mechanisms and endothelial nitric oxide synthase (eNOS) expression and activity. In addition, recent studies from our laboratory have revealed an important relationship between PECAM-1 and endoglin expression. Endoglin is an essential molecule during angiogenesis, vascular development and integrity, and its expression and activity are compromised in the absence of PECAM-1. In the present review we discuss the roles that PECAM-1 isoforms may play in modulation of endothelial cell adhesive mechanisms, eNOS and endoglin expression and activity, and angiogenesis.

Adiponectin is expressed in the pancreas of high-fat-diet-fed mice and protects pancreatic endothelial function during the development of type 2 diabetes

AIM

Adiponectin levels in skeletal muscle and adipose tissue have been reported to be involved in insulin resistance in rats fed with a high-fat diet (HFD). Our objective was to explore whether adiponectin is also expressed in the pancreas and what its potential role is during the development of type 2 diabetes (T2D) in outbred CD-1 mice.

METHODS

Male 4-week-old outbred CD-1 mice were fed an HFD to induce a polygenic model of human T2D. Adiponectin expression was examined in mouse pancreas by quantitative real-time polymerase chain reaction (qPCR), western blots and immunofluorescence analyses. Human umbilical vein endothelium cells (HUVECs) were transfected with an adiponectin-expressing lentivirus to determine the effect of adiponectin on angiogenic function in vitro.

RESULTS

Feeding mice an HFD for 9weeks resulted in constant hyperglycaemia, obesity, impaired glucose tolerance and insulin resistance. Additional hyperinsulinaemia emerged in mice fed an HFD for 18weeks. Interestingly, aberrant expression of adiponectin was detectable in the pancreatic vascular endothelial cells (VECs) of mice fed with an HFD, but not in mice fed with regular chow (RC). Expression levels of pancreatic adiponectin varied during the development of T2D. This extraordinary expression of adiponectin in pancreatic VECs played a role in protecting endothelial function against potential damage by HFD. Our in vitro study has demonstrated that adiponectin promotes angiogenic function.

CONCLUSION

These results reveal for the first time that adiponectin is expressed in pancreatic VECs of HFD-fed mice during the development of T2D as a protective adaptation in response to the HFD.

Dkk1 and MSX2-Wnt7b signaling reciprocally regulate the endothelial-mesenchymal transition in aortic endothelial cells

OBJECTIVE

Endothelial cells (ECs) can undergo an endothelial-mesenchymal transition with tissue fibrosis. Wnt- and Msx2-regulated signals participate in arteriosclerotic fibrosis and calcification. We studied the impact of Wnt7, Msx2, and Dkk1, a Wnt7 antagonist, on endothelial-mesenchymal transition in primary aortic ECs.

APPROACH AND RESULTS

Transduction of aortic ECs with vectors expressing Dkk1 suppressed EC differentiation and induced a mineralizing myofibroblast phenotype. Dkk1 suppressed claudin 5, PECAM, cadherin 5 (Cdh5), Tie1, and Tie2. Dkk1 converted the cuboidal cell monolayer into a spindle-shaped multilayer and inhibited EC cord formation. Myofibroblast and osteogenic markers, SM22, type I collagen, Osx, Runx2, and alkaline phosphatase, were upregulated by Dkk1 via activin-like kinase/Smad pathways. Dkk1 increased fibrotic mineralization of aortic ECs cultured under osteogenic conditions--the opposite of mesenchymal cell responses. Msx2 and Wnt7b maintained morphology and upregulated markers of differentiated ECs. Deleting EC Wnt7b with the Cdh5-Cre transgene in Wnt7b(fl/fl);LDLR(-/-) mice upregulated aortic osteogenic genes (Osx, Sox9, Runx2, and Msx2) and nuclear phospho-Smad1/5, and increased collagen and calcium accumulation.

CONCLUSIONS

Dkk1 enhances endothelial-mesenchymal transition in aortic ECs, whereas Wnt7b and Msx2 signals preserve EC phenotype. EC responses to Dkk1, Wnt7b, and Msx2 are the opposite of mesenchymal responses, coupling EC phenotypic stability with osteofibrogenic predilection during arteriosclerosis.

Metabolite control of angiogenesis: angiogenic effect of citrate

Endothelial cells respond to hypoxic changes with resultant accumulation of several metabolites and switch over to angiogenic phenotype. Although certain intermediates of glycolytic and oxidative metabolic pathways have been known to affect angiogenesis, the effect of citrate, which accumulates in certain tumors, on angiogenesis is not known. Therefore, the effect of citrate on angiogenesis was studied using different model systems. Increased vascularization in chorioallantoic membrane assay, increased endothelial sprouting in rat aortic rings, and increased expression of CD31, E-selectin in endothelial cells suggested a possible proangiogenic effect of citrate. Upregulation of angiogenic factors such as vascular endothelial growth factor and fibroblast growth factor suggested that the effect of citrate involves modulation of expression of angiogenic growth factors. LY 294002, an inhibitor of PI3K-Akt pathway, and wortmannin, an inhibitor of Akt pathway, reversed the effect of citrate in human umbilical vein endothelial cells. Citrate induced significant upregulation and activation of Akt in endothelial cells. Rapamycin, an inhibitor of mTOR, also reversed the effect of citrate in human umbilical vein endothelial cells and sprouting of aortic rings suggesting that the angiogenic effect of citrate involves activation of PI3K-Akt-mTOR pathway.

Role of high endothelial postcapillary venules and selected adhesion molecules in periodontal diseases: a review

Periodontitis is accompanied by the proliferation of small blood vessels in the gingival lamina propria. Specialized postcapillary venules, termed periodontal high endothelial-like venules, are also present, and demonstrate morphological and functional traits similar to those of high endothelial venules (HEVs) in lymphatic organs. The suggested role of HEVs in the pathogenesis of chronic periodontitis involves participation in leukocyte transendothelial migration and therefore proinflammatory effects appear. Recent observations suggest that chronic periodontitis is an independent risk factor for systemic vascular disease and may result in stimulation of the synthesis of acute phase protein by cytokines released by periodontal high endothelial cells (HECs). However, tissue expression of HEV-linked adhesion molecules has not been evaluated in the gingiva of patients with chronic periodontitis. This is significant in relation to potential therapy targeting expression of the adhesion molecules. In this review, current knowledge of HEV structure and the related expression of four surface adhesion molecules of HECs [CD34, platelet endothelial cell adhesion molecule 1, endoglin and intercellular adhesion molecule 1 (ICAM-1)], involved in the key steps of the adhesion cascade in periodontal diseases, are discussed. Most studies on the expression of adhesion molecules in the development and progression of periodontal diseases pertain to ICAM-1 (CD54). Studies by the authors demonstrated quantitatively similar expression of three of four selected surface markers in gingival HEVs of patients with chronic periodontitis and in HEVs of reactive lymph nodes, confirming morphological and functional similarity of HEVs in pathologically altered tissues with those in lymphoid tissues.

VEGF 165 b in the developing vasculatures of the fetal human eye

VEGF(165) b is an anti-angiogenic form of VEGF(165) produced by alternative splicing. The localization of pro-angiogenic VEGF(165) and anti-angiogenic VEGF(165) b was investigated during development of the vasculatures in fetal human eyes from 7 to 21 weeks gestation (WG). The fetal vasculature of vitreous, which includes tunica vasculosa lentis (TVL), had moderate VEGF(165) immunoreactivity at 7WG and very little VEGF(165) b. Both forms were elevated at 12WG. VEGF(165) then decreased around 17WG when the TVL regresses but VEGF(165) b remained elevated. In choroid, VEGF(165) was present in forming choriocapillaris (CC) and retinal pigment epithelium (RPE) at 7WG while VEGF165b was present in CC and mesenchymal precursors within the choroidal stroma. By 21WG, both forms were elevated in RPE and choroidal blood vessels but VEGF(165) b was apical and VEGF(165) basal in RPE. Diffuse VEGF(165) immunoreactivity was prominent in 12WG innermost retina where blood vessels will form while VEGF(165) b was present in most CXCR4(+) progenitors in the inner neuroblastic layer and migrating angioblasts in the putative nerve fiber layer. By 21WG, VEGF(165) was present in nerve fibers and VEGF(165) b in the inner Muller cell process. The localization of VEGF(165) b was distinctly different from VEGF(165) both spatially and temporally and it was often associated with nucleus in progenitors.

Alteration of thrombospondin-1 and -2 in rat brains following experimental intracerebral hemorrhage. Laboratory investigation

OBJECT

Spontaneous intracerebral hemorrhage (ICH) is among the most intractable forms of stroke. Angiogenesis, an orchestrated balance between proangiogenic and antiangiogenic factors, is a fundamental process to brain development and repair by new blood vessel formation from preexisting ones and can be induced by ICH. Thrombospondin (TSP)–1 and TSP-2 are naturally occurring antiangiogenic factors. The aim of this study was to observe their expression in rat brains with ICH.

METHODS

Intracerebral hemorrhage was induced in adult male Sprague-Dawley rats by stereotactic injection of collagenase VII or autologous blood into the right globus pallidus. The expression of TSP-1 and -2 was evaluated by immunohistochemistry and quantitative real-time reverse transcription–polymerase chain reaction analysis.

RESULTS

After the induction of ICH, some TSP1- or TSP2-immunoreactive microvessels resided around the hematoma for ~ 7 days and extended into a clot thereafter. Cerebral endothelial cells expressed the TSPs. The expression of TSP-1 and TSP-2 mRNA peaked at 4 and 14 days after collagenase-induced ICH, respectively.

CONCLUSIONS

Findings in this study suggest that ICH can alter the expression of TSP-1 and TSP-2, which may be involved in modulating angiogenesis in brains following ICH.

PECAM-1 regulates proangiogenic properties of endothelial cells through modulation of cell-cell and cell-matrix interactions

Platelet endothelial cell adhesion molecule-1 (PECAM-1/CD31) is a member of the immunoglobulin superfamily of cell adhesion molecules with important roles in angiogenesis and inflammation. However, the molecular and cellular mechanisms, and the role that specific PECAM-1 isoforms play in these processes, remain elusive. We recently showed attenuation of retinal vascular development and neovascularization in PECAM-1-deficient (PECAM-1-/-) mice. To gain further insight into the role of PECAM-1 in these processes, we isolated primary retinal endothelial cells (EC) from wild-type (PECAM-1+/+) and PECAM-1-/- mice. Lack of PECAM-1 had a significant impact on endothelial cell-cell and cell-matrix interactions, resulting in attenuation of cell migration and capillary morphogenesis. Mechanistically these changes were associated with a significant decrease in expression of endothelial nitric oxide synthase (eNOS) and nitric oxide (NO) bioavailability in PECAM-1-/- retinal EC. PECAM-1-/- retinal EC also exhibited a lower rate of apoptosis under basal and challenged conditions, consistent with their increased growth rate. Furthermore, reexpression of PECAM-1 was sufficient to restore migration and capillary morphogenesis of null cells in an isoform-specific manner. Thus PECAM-1 expression modulates proangiogenic properties of EC, and these activities are significantly influenced by alternative splicing of its cytoplasmic domain.

Loss of fibulin-5 binding to beta1 integrins inhibits tumor growth by increasing the level of ROS

Tumor survival depends in part on the ability of tumor cells to transform the surrounding extracellular matrix (ECM) into an environment conducive to tumor progression. Matricellular proteins are secreted into the ECM and impact signaling pathways that are required for pro-tumorigenic activities such as angiogenesis. Fibulin-5 (Fbln5) is a matricellular protein that was recently shown to regulate angiogenesis; however, its effect on tumor angiogenesis and thus tumor growth is currently unknown. We report that the growth of pancreatic tumors and tumor angiogenesis is suppressed in Fbln5-null (Fbln5(-/-)) mice compared with wild-type (WT) littermates. Furthermore, we observed an increase in the level of reactive oxygen species (ROS) in tumors grown in Fbln5(-/-) animals. Increased ROS resulted in elevated DNA damage, increased apoptosis of endothelial cells within the tumor, and represented the underlying cause for the reduction in angiogenesis and tumor growth. In vitro, we identified a novel pathway by which Fbln5 controls ROS production through a mechanism that is dependent on beta1 integrins. These results were validated in Fbln5(RGE/RGE) mice, which harbor a point mutation in the integrin-binding RGD motif of Fbln5, preventing its interaction with integrins. Tumor growth and angiogenesis was reduced in Fbln5(RGE/RGE) mice, however treatment with an antioxidant rescued angiogenesis and elevated tumor growth to WT levels. These findings introduce a novel function for Fbln5 in the regulation of integrin-induced ROS production and establish a rationale for future studies to examine whether blocking Fbln5 function could be an effective anti-tumor strategy, alone or in combination with other therapies.

A mechanosensitive transcriptional mechanism that controls angiogenesis

Angiogenesis is controlled by physical interactions between cells and extracellular matrix as well as soluble angiogenic factors, such as VEGF. However, the mechanism by which mechanical signals integrate with other microenvironmental cues to regulate neovascularization remains unknown. Here we show that the Rho inhibitor, p190RhoGAP (also known as GRLF1), controls capillary network formation in vitro in human microvascular endothelial cells and retinal angiogenesis in vivo by modulating the balance of activities between two antagonistic transcription factors, TFII-I (also known as GTF2I) and GATA2, that govern gene expression of the VEGF receptor VEGFR2 (also known as KDR). Moreover, this new angiogenesis signalling pathway is sensitive to extracellular matrix elasticity as well as soluble VEGF. This is, to our knowledge, the first known functional cross-antagonism between transcription factors that controls tissue morphogenesis, and that responds to both mechanical and chemical cues.

Maturation of the fetal human choriocapillaris

PURPOSE

The purpose of this study was to examine the structural and functional maturation of the choriocapillaris (CC) and to determine when fenestrations form, the capillaries are invested with pericytes, and the endothelial cells (ECs) became functional.

METHODS

Immunohistochemistry was performed on cryopreserved sections of embryonic/fetal human eyes from 7 to 22 weeks' gestation (WG), using antibodies against PAL-E, PV-1 (fenestrations), carbonic anhydrase IV (CA IV), eNOS, and alpha-smooth muscle actin (alphaSMA) and NG2 (two pericyte markers) and the EC marker (CD31). Alkaline phosphatase (APase) enzymatic activity was demonstrated by enzyme histochemistry. Transmission electron microscopy (TEM) was performed on eyes at 11, 14, 16, and 22 WG. Adult human eyes were used as the positive control.

RESULTS

All EC markers were present in the CC by 7 WG. PAL-E, CA IV, and eNOS immunoreactivities and APase activity were present in the CC by 7 to 9 WG. TEM analysis demonstrated how structurally immature this vasculature was, even at 11 WG: no basement membrane, absence of pericytes, and poorly formed lumens that were filled with filopodia. The few fenestrations that were observed were often present within the luminal space in the filopodia. Contiguous fenestrations and significant PV-1 were not observed until 21 to 22 WG. alphaSMA was prominent at 22 WG, and the maturation of pericytes was confirmed by TEM.

CONCLUSIONS

It appears that ECs and their precursors express enzymes present in adult CC well before they are structurally mature. Although ECs make tight junctions early in development, contiguous fenestrations and mature pericytes occur much later in development.

Relationship between retention of a vascular endothelial growth factor receptor 2 (VEGFR2)-targeted ultrasonographic contrast agent and the level of VEGFR2 expression in an in vivo breast cancer model

OBJECTIVE

The aim of this study was to characterize the relationship between retention of a vascular endothelial growth factor receptor 2 (VEGFR2)-targeted ultrasonographic contrast agent (UCA) and VEGFR2 expression in tumor vasculature of breast cancer.

METHODS

67NR breast cancer tumors implanted in mice were evaluated in vivo with both VEGFR2-targeted and nontargeted UCAs, and a high-frequency ultrasound system. A bolus of the UCA was injected and allowed to circulate for 4 minutes to allow binding of targeted microbubbles. After that, 2 sets of images before and after a high-power ultrasonic destruction sequence were acquired. The average video intensity of predestruction and postdestruction images was measured and used as a relative measure of retention of the UCA in the tumor. Levels of VEGFR2 expression and tumor vascular density were quantified by immunohistochemical staining and compared with retention of the VEGFR2-targeted UCA.

RESULTS

Retention of VEGFR2-targeted microbubbles in tumors was significantly higher than retention of nontargeted microbubbles (mean +/- SD, 47.75+/-9.85 versus 18.5+/-5.46 dB; P< .001). Retention of the VEGFR2-targeted UCA was found to correlate with the level of VEGFR2 expression in the studied tumors (r(2)=0.41). In contrast, retention of the nontargeted UCA was not correlated with the level of VEGFR2 expression (r(2)=0.08). Furthermore, retention of the VEGFR2-targeted UCA was not correlated with the level of tumor vascularity.

CONCLUSIONS

The magnitude of the molecular ultrasonographic signal from a VEGFR2-targeted UCA retained by tissue correlates with VEGFR2 expression. These results validate the use of molecular ultrasonography for in vivo detection and quantification of VEGFR2 expression in this breast cancer model.

A genomic comparison of in vivo and in vitro brain microvascular endothelial cells

The blood-brain barrier (BBB) is composed of uniquely differentiated brain microvascular endothelial cells (BMEC). Often, it is of interest to replicate these attributes in the form of an in vitro model, and such models are widely used in the research community. However, the BMEC used to create in vitro BBB models de-differentiate in culture and lose many specialized characteristics. These changes are poorly understood at a molecular level, and little is known regarding the consequences of removing BMEC from their local in vivo microenvironment. To address these issues, suppression subtractive hybridization (SSH) was used to identify 25 gene transcripts that were differentially expressed between in vivo and in vitro BMEC. Genes affected included those involved in angiogenesis, transport and neurogenesis, and real-time quantitative polymerase chain reaction (qPCR) verified transcripts were primarily and significantly downregulated. Since this quantitative gene panel represented those BMEC characteristics lost upon culture, we used it to assess how culture manipulation, specifically BMEC purification and barrier induction by hydrocortisone, influenced the quality of in vitro models. Puromycin purification of BMEC elicited minimal differences compared with untreated BMEC, as assessed by qPCR. In contrast, qPCR-based gene panel analysis after induction with hydrocortisone indicated a modest shift of 10 of the 23 genes toward a more 'in vivo-like' gene expression profile, which correlated with improved barrier phenotype. Genomic analysis of BMEC de-differentiation in culture has thus yielded a functionally diverse set of genes useful for comparing the in vitro and in vivo BBB.

PECAM-1 isoform-specific functions in PECAM-1-deficient brain microvascular endothelial cells

Platelet endothelial cell adhesion molecule-1 (PECAM-1) is alternatively spliced generating eight isoforms that only differ in the length of their cytoplasmic domain. Multiple isoforms of PECAM-1 are present in the endothelium and their expression levels are regulated during vascular development and angiogenesis. However, the functional significance of PECAM-1 isoforms during these processes remains largely unknown. We recently showed that mouse brain endothelial (bEND) cells prepared from PECAM-1-deficient (PECAM-1-/-) mice differ in their cell adhesive and migratory properties compared to PECAM-1+/+ bEND cells. Here we demonstrate that the restoration of PECAM-1 expression in these cells affects their adhesive and migratory properties in an isoform-specific manner. Expression of Delta14&15 PECAM-1, the predominant isoform present in the mouse endothelium, in PECAM-1-/- bEND cells activated MAPK/ERKs, disrupted adherens junctions, and enhanced cell migration and capillary morphogenesis in Matrigel. In contrast, expression of Delta15 PECAM-1 in PECAM-1-/- bEND cells had minimal effects on their activation of MAPK/ERKs, migration, and capillary morphogenesis. The effects of PECAM-1 on cell adhesive and migratory properties were mediated in an isoform-specific manner, at least in part, through its interactions with intracellular signaling proteins, including SHP-2 and Src. These results suggest that the impact of PECAM-1 on EC adhesion, migration, and capillary morphogenesis is modulated by alternative splicing of its cytoplasmic domain.

Soluble vascular endothelial growth factor receptor-1 in intrauterine growth restricted fetuses and neonates

BACKGROUND

Angiogenesis, a critical process for growth and development is altered in intrauterine growth restriction (IUGR). Vascular endothelial growth factor (VEGF) and its receptors VEGFR-1, soluble (s) VEGFR-1 and VEGFR-2 represent a regulatory system, essential for both physiological and pathological angiogenesis.

AIM

To study the implication of sVEGFR-1-a VEGF antagonist-in IUGR.

STUDY DESIGN

Prospective study.

METHODS

Twenty-five IUGR and 15 appropriate for gestational age (AGA) full-term fetuses and neonates with their mothers were included in the study.

OUTCOME MEASURES

sVEGFR-1 levels were determined by enzyme immunoassay in the serum of: mothers (MS), umbilical cords (UC)-representing fetal state - and neonates on day 1 (N1) and 4 (N4) of life.

RESULTS

MS, UC, N1 and N4 sVEGFR-1 levels in IUGR were significantly higher compared to respective AGA cases (p = 0.005, p = 0.026, p = 0.005 and p = 0.017, respectively). In IUGR and AGA groups, maternal sVEGFR-1 levels were significantly higher than fetal and neonatal levels (p in all cases < 0.001). The latter presented in both IUGR and AGA groups a significant decrease from UC to N4 (p in all cases < 0.01). MS, N1 and N4 sVEGFR-1 levels negatively correlated with the infants' customized centiles [(r = -0.489, p = 0.001), (r = -0.440, p = 0.004), (r = -0.431, p = 0.006), respectively].

CONCLUSIONS

Higher sVEGFR-1 levels in the IUGR as compared to the AGA group possibly reflect the predominance of antiangiogenic mechanisms present in IUGR. The decrease of sVEGFR-1 levels from UC to N4 may represent ex utero initiation of growth and development and therefore, prevalence of angiogenic mechanisms.

Thrombospondin-1 is induced in rat myocardial infarction and its induction is accelerated by ischemia/reperfusion

Thrombospondin-1 (TSP-1) is a multifunctional, rapid-turnover matricellular protein. Recent studies demonstrated that TSP-1 has a role in regulating inflammatory reactions. Myocardial infarction (MI) is associated with an inflammatory response, ultimately leading to healing and scar formation. In particular, an enhanced inflammatory reaction and a massive accumulation of monocytes/macrophages is seen with reperfusion after MI. To examine the role of TSP-1 in MI, we isolated rat TSP-1 complementary DNA (cDNA) and analyzed the level and distribution of the mRNA expression. In infarcted rat hearts, TSP-1 mRNA increased markedly at 6 and 12 hrs after coronary artery ligation (27.97 +/- 3.40-fold and 22.77 +/- 1.83-fold, respectively, compared with sham-operated hearts). Western blot analysis revealed that TSP-1 protein was transiently induced in the infarcted heart. Using in situ hybridization analysis, TSP-1 mRNA signals were observed in the infiltrating cells at the border area of infarction. We then examined the effect of ischemia/reperfusion (I/R) on TSP-1 mRNA induction in the rats with infarcted hearts. Quantitative reverse transcriptase polymerase chain reaction (RT-PCR) demonstrated that I/R enhanced the TSP-1 mRNA expression approximately 4-fold, as compared with the level in the permanently ligated heart. Finally, we examined the effect of TSP-1 on proinflammatory cytokine release in mononuclear cells. The releases of interleukin-6 (IL-6) and monocyte chemoattractant protein-1 (MCP-1) from human mononuclear cells were enhanced by TSP-1 in a dose-dependent manner. Thus, the immediate and marked increase of TSP-1 expression suggests that TSP-1 has an inflammatory-associated role in MI.

Polyoma virus middle-T-transformed PECAM-1 deficient mouse brain endothelial cells proliferate rapidly in culture and form hemangiomas in mice

Wild-type mouse brain endothelial (bEND) cells transformed with the polyoma virus middle-T proliferate rapidly in culture and form hemangiomas in mice. These cells express high levels of platelet/endothelial cell adhesion molecule-1 (PECAM-1), a molecule shown to be important during hemangioma formation. In this study, we have examined the ability of polyoma virus middle-T-transformed mouse bEND cells prepared from PECAM-1-/- mice to proliferate in culture and form hemangiomas in mice. We show that these cells express a number of endothelial cell markers and share a similar morphology with PECAM-1+/+ bEND cells. PECAM-1-/- bEND cells exhibit a limited ability to form tubes in Matrigel and rapidly form hemangioma when injected into nude mice, very similar to PECAM-1+/+ bEND cells. These cells, however, have increased proliferation, slower migration, altered endothelial cell adhesion molecule expression, and are less adherent when compared to PECAM-1+/+ bEND cells. Therefore, lack of PECAM-1 expression impacts polyoma middle-T-transformed endothelial cell proliferative, adhesive, and migratory properties without impacting their ability to rapidly form hemangiomas in mice or poorly organize to capillary-like structures in Matrigel.

Thrombospondin-1 deficient mice exhibit an altered expression pattern of alternatively spliced PECAM-1 isoforms in retinal vasculature and endothelial cells

We have previously shown that thrombosponsin-1 (TSP1) and PECAM-1 are components of a regulatory switch whose reciprocal regulation in the endothelial cells (EC) promotes an angiogenic or a differentiated, quiescent phenotype. The physiological role TSP1 plays in modulation of PECAM-1 expression and function during vascular development and angiogenesis remains largely unknown. Here we demonstrate that PECAM-1 undergoes alternative splicing in its cytoplasmic domain generating eight isoforms in the retinal vasculature of wild type and TSP1-/- mice. All PECAM-1 isoforms examined contained exon 13. The frequency of PECAM-1 isoform(s) containing exon 14 was significantly higher during early stages of retinal vascularization, which decreased during later stages of retinal vascularization in wild type mice. In contrast, the frequency of exon 14 containing PECAM-1 isoform(s) did not significantly change during retinal vascularization in TSP1-/- mice. They consistently expressed higher number of isoforms with exon 14 during later stages of retinal vascularization. The higher level of PECAM-1 isoforms with exon 14 was also observed in cultured TSP1-/- retinal EC compared to wild type retinal EC. This was consistent with increased amounts of Src and SHP-2 associated with PECAM-1, and enhanced migration and proliferation in TSP1-/- retinal EC. These data suggest PECAM-1 signaling in the endothelium is modulated by its alternative splicing during retinal vascular development and angiogenesis, which may be impacted by TSP1 expression.

Molecular pathways in bladder cancer

The aim of this review is to provide a contemporary outline of our current understanding of the molecular and genetic events associated with tumorigenesis and the progression of bladder cancer. A comprehensive review of the literature was performed on the molecular alterations associated with transitional cell carcinoma (TCC) of the bladder. Intense research efforts are being made to better identify and characterize various bladder cancers and their true biologic potential. The need to predict which superficial tumors will recur or progress, and which invasive tumors will metastasize has led to a much better understanding of the molecular pathways associated with bladder cancer. The molecular changes that occur in TCC of the bladder are numerous and can be categorized into: (1) chromosomal alterations leading to carcinogenesis, (2) loss of cell cycle regulation accounting for cellular proliferation, and (3) metastasis, guided by events such as angiogenesis. It is becoming apparent that the accumulation of genetic and molecular changes ultimately determines a tumors phenotype and subsequent clinical behavior. At the present time, conventional histopathologic evaluation of bladder cancer (tumor grade and stage) is inadequate to accurately predict the behavior of most bladder tumors. While new laboratory techniques have allowed us to better understand how bladder cancer develops and ultimately progresses, few of these techniques are currently available for use in the clinical setting. The ultimate goal is to develop reliable prognostic markers which will accurately predict not only the expected clinical course of an individual bladder tumor but also the response of that tumor to currently available therapies. More importantly, this information may be employed in the future to dictate altogether new treatments for the prevention and/or stabilization of the early molecular events that lead to the development of bladder cancer.

Modulation of VE-cadherin and PECAM-1 mediated cell-cell adhesions by mitogen-activated protein kinases

Endothelial cell transition from a differentiated, quiescent phenotype to a migratory, proliferative phenotype is essential during angiogenesis. This transition is dependent on alterations in the balanced production of stimulatory and inhibitory factors, which normally keep angiogenesis in check. Activation of MAPK/ERKs is essential for endothelial cell migration and proliferation. However, its role in regulation of endothelial cell adhesive mechanisms requires further delineation. Here, we show that sustained activation of MAPK/ERKs results in disruption of cadherin-mediated cell-cell adhesion, down-regulation of PECAM-1 expression, and enhanced cell migration in microvascular endothelial cells. Expression of a constitutively active MEK-1 in mouse brain endothelial (bEND) cells resulted in down-regulation of VE-cadherin and catenins expression concomitant with down-regulation of PECAM-1 expression. In contrast, inhibition of MEK-1 restored parental morphology, cadherin/catenins expression and localization. These data are further supported by our observation that sustained activation of MAPK/ERKs in phorbol myristate acetate incubated HUVEC lead to disruption of cadherin-mediate cell-cell interactions and enhanced capillary formation on Matrigel. Thus, sustained activation of MAPK/ERKs plays an important role in disruption of cell-cell adhesion and migration of endothelial cells.

Expression pattern of alternatively spliced PECAM-1 isoforms in hematopoietic cells and platelets

PECAM-1 (CD31) is a cell adhesion molecule that is highly expressed in the endothelium. Hematopoietic cells including platelets, monocytes, neutrophils, and some T cells also express moderate levels of PECAM-1. PECAM-1 undergoes alternative splicing generating a number of isoforms in the endothelium. However, the expression of PECAM-1 isoforms in hematopoietic cells and platelets has not been determined. Here, we examined the expression pattern of PECAM-1 isoforms in human and rodent hematopoietic cells and platelets by RT-PCR and DNA sequencing analysis. Our results showed that multiple PECAM-1 isoforms are expressed in a cell-type and species-specific pattern. We identified seven human PECAM-1 isoforms, six murine PECAM-1 isoforms, and four rat PECAM-1 isoforms. The full-length PECAM-1 was the predominant isoform detected in human cells. The PECAM-1 isoforms that lack exon 14 and 15 (delta14&15) or delta12,14&15 were the predominant isoform in rodent cells. In addition, we identified a novel PECAM-1 isoform, delta13&14, in human hematopoietic cells. Thus, hematopoietic cells express multiple isoforms of PECAM-1 in a pattern similar to that observed in the endothelium of the same species. The regulated expression of these isoforms may be important during hematopoiesis and transendothelial migration.

Significance of endothelial cell survival programs for renal transplantation

Initial and longer term kidney transplant function is determined in part by the renal allograft microcirculation because it provides a thromboresistant surface, regulates cellular infiltration, and elaborates paracrine and autocrine growth and survival factors. Loss of endothelial-derived signaling mediators accelerates vascular injury and endothelial cell (EC) death. EC apoptosis is implicated in accelerated allograft vasculopathy and premature loss of organ function. Renal allograft EC injury and replacement by recipient-derived repair mechanisms has long been proposed to influence allograft acceptance and function. Repair of cellular injury in allografts is linked with cell-survival mechanisms, but few precise indicators exist to predict recovery and repair in organ transplants. The significance of the growth phenotype of the microvascular endothelium for acute and longer term renal allograft survival is presented.

Modulation of PECAM-1 expression and alternative splicing during differentiation and activation of hematopoietic cells

PECAM-1 (CD31) is a member of immunoglobulin gene superfamily, which is highly expressed on the surface of endothelial cells and at moderate levels on hematopoietic cells. Hematopoietic cells and platelets, like endothelial cells, express multiple isoforms of PECAM-1. However, the identity and physiological role of these isoforms during hematopoiesis remains largely unknown. Here we demonstrate that PECAM-1 expression is dramatically up regulated upon phorbol myristate acetate (PMA) or transforming growth factor (TGF)-beta1-mediated differentiation of leukemic HEL and U937 cells. The level of PECAM-1 expression did not significantly change during activation of Jurkat T cells by PMA or phytohaemagglutinin (PHA). Utilizing RT-PCR and DNA sequencing analysis, we show that the expression of PECAM-1 isoforms changes in a cell-type and lineage specific manner during cellular differentiation and activation. We identified a number of novel PECAM-1 isoforms previously not detected in the endothelium. These results demonstrate that regulated expression of PECAM-1 and its exonic inclusion/exclusion occurs during differentiation and/or activation of hematopoietic cells. Thus, different PECAM-1 isoforms may play important roles in generation of hematopoietic cells and their potential interactions with vascular endothelium.

Active immunization against the vascular endothelial growth factor receptor flk1 inhibits tumor angiogenesis and metastasis

The vascular endothelial growth factor (VEGF) receptor fetal liver kinase 1 (flk1; VEGFR-2, KDR) is an endothelial cell-specific receptor tyrosine kinase that mediates physiological and pathological angiogenesis. We hypothesized that an active immunotherapy approach targeting flk1 may inhibit tumor angiogenesis and metastasis. To test this hypothesis, we first evaluated whether immune responses to flk1 could be elicited in mice by immunization with dendritic cells pulsed with a soluble flk1 protein (DC-flk1). This immunization generated flk1-specific neutralizing antibody and CD8+ cytotoxic T cell responses, breaking tolerance to self-flk1 antigen. Tumor-induced angiogenesis was suppressed in immunized mice as measured in an alginate bead assay. Development of pulmonary metastases was strongly inhibited in DC-flk1-immunized mice challenged with B16 melanoma or Lewis lung carcinoma cells. DC-flk1 immunization also significantly prolonged the survival of mice challenged with Lewis lung tumors. Thus, an active immunization strategy that targets an angiogenesis-related antigen on endothelium can inhibit angiogenesis and may be a useful approach for treating angiogenesis-related diseases.

Regulation of PECAM-1 in endothelial cells during cell growth and migration

Endothelial cells (EC) that form the inner lining of blood vessels remain quiescent in the normal adult vasculature except during angiogenesis and reendothelialization, which result in EC proliferation and migration. EC placed in culture at subconfluent density also undergo cell multiplication and movement. This report demonstrates that whereas in confluent EC in a compact monolayer, the EC-EC adhesion molecule platelet-endothelial cell adhesion molecule-1 (PECAM-1) is strongly expressed at cell borders, little or no PECAM-1 immunostaining is detected in sparse or migrating cultured EC. Consistent with this observation, steady-state PECAM-1 mRNA expression was much lower in subconfluent EC than in confluent EC. The absence of PECAM-1 expression in sparse EC appeared not to be linked to ability to proliferate, since PECAM-1 expression remained low even in the presence of nitric oxide (NO) or mitomycin C, agents that inhibit EC growth. However, another growth-inhibitory agent, TGF-beta 1, did not alter PECAM-1 staining. Based on these observations, it is hypothesized that cell-associated mechanical forces underlying cell tensegrity regulate PECAM-1 expression.

The splice variants of vascular endothelial growth factor (VEGF) and their receptors

Vascular endothelial growth factor (VEGF) is a secreted mitogen highly specific for cultured endothelial cells. In vivo VEGF induces microvascular permeability and plays a central role in both angiogenesis and vasculogenesis. VEGF is a promising target for therapeutic intervention in certain pathological conditions that are angiogenesis dependent, most notably the neovascularisation of growing tumours. Through alternative mRNA splicing, a single gene gives rise to several distinct isoforms of VEGF, which differ in their expression patterns as well as their biochemical and biological properties. Two VEGF receptor tyrosine kinases (VEGFRs) have been identified, VEGFR-1 (Flt-1) and VEGFR-2 (KDR/Flk-1). VEGFR-2 seems to mediate almost all observed endothelial cell responses to VEGF, whereas roles for VEGFR-1 are more elusive. VEGFR-1 might act predominantly as a ligand-binding molecule, sequestering VEGF from VEGFR-2 signalling. Several isoform-specific VEGF receptors exist that modulate VEGF activity. Neuropilin-1 acts as a co-receptor for VEGF(165), enhancing its binding to VEGFR-2 and its bioactivity. Heparan sulphate proteoglycans (HSPGs), as well as binding certain VEGF isoforms, interact with both VEGFR-1 and VEGFR-2. HSPGs have a wide variety of functions, such as the ability to partially restore lost function to damaged VEGF(165) and thereby prolonging its biological activity.

Angiopoietin-1 is an antipermeability and anti-inflammatory agent in vitro and targets cell junctions

Inflammation is a basic pathological mechanism that underlies many diseases. An important component of the inflammatory response is the passage of plasma components and leukocytes from the blood vessel into the tissues. The endothelial monolayer lining blood vessels reacts to stimuli such as thrombin or vascular endothelial growth factor by changes in cell-cell junctions, an increase in permeability, and the leakage of plasma components into tissues. Other stimuli, such as tumor necrosis factor-alpha (TNF-alpha), are responsible for stimulating the transmigration of leukocytes. Here we show that angiopoietin-1, a cytokine essential in fetal angiogenesis, not only supports the localization of proteins such as platelet endothelial cell adhesion molecule-1 (PECAM-1) into junctions between endothelial cells and decreases the phosphorylation of PECAM-1 and vascular endothelial cadherin, but it also strengthens these junctions, as evidenced by a decrease in basal permeability and inhibition of permeability responses to thrombin and vascular endothelial growth factor. Furthermore, angiopoietin-1 inhibits TNF-alpha-stimulated leukocyte transmigration. Angiopoietin-1 may thus have a major role in maintaining the integrity of endothelial monolayers.

Differential modulation of cadherin-mediated cell-cell adhesion by platelet endothelial cell adhesion molecule-1 isoforms through activation of extracellular regulated kinases

The role of platelet endothelial cell adhesion molecule-1 (PECAM-1) in endothelial cell-cell interactions and its contribution to cadherin-mediated cell adhesion are poorly understood. Such studies have been difficult because all known endothelial cells express PECAM-1. We have used Madin-Darby canine kidney (MDCK) cells as a model system in which to evaluate the role of PECAM-1 isoforms that differ in their cytoplasmic domains in cell-cell interactions. MDCK cells lack endogenous PECAM-1 but form cell-cell junctions similar to those of endothelial cells, in which PECAM-1 is concentrated. MDCK cells were transfected with two isoforms of murine PECAM-1, Delta15 and Delta14&15, the predominant isoforms expressed in vivo. Expression of the Delta15 isoform resulted in apparent dedifferentiation of MDCK cells concomitant with the loss of adherens junctions, down-regulation of E-cadherin, alpha- and beta-catenin expression, and sustained activation of extracellular regulated kinases. The Delta15 isoform was not concentrated at cell-cell contacts. In contrast, the Delta14&15 isoform localized to sites of cell-cell contact and had no effect on MDCK cell morphology, cadherin/catenin expression, or extracellular regulated kinase activity. Thus, the presence of exon 14 in the cytoplasmic domain of PECAM-1 has dramatic effects on the ability of cells to maintain adherens junctions and an epithelial phenotype. Therefore, changes in the expression of exon 14 containing PECAM-1 isoforms, which we have observed during development, may have profound functional consequences.

Thrombospondin-1, a natural inhibitor of angiogenesis, is present in vitreous and aqueous humor and is modulated by hyperglycemia

Negative regulators of angiogenesis play a major role in maintaining vascular homeostasis. Thrombospondin-1 (TSP1) is a natural inhibitor of angiogenesis. This report examines the presence of TSP1 in ocular samples and determines whether its production is altered in diabetes. Western blot analysis detected a 140 kDa antiangiogenic fragment of TSP1(gp140) in vitreous samples prepared from normal human and rat eyes. Intact TSP1 was detected in aqueous humor samples prepared from normal rat and bovine eyes. In contrast, TSP1 was virtually absent in vitreous and aqueous humor samples prepared from diabetic rat eyes. Furthermore, production of TSP1 by microvascular endothelial cells in culture was sensitive to high concentrations of glucose. Retinal blood vessels appeared nonuniform and dilated in diabetic animals when compared to control animals. These results demonstrate that TSP1 and its antiangiogenic fragment are present in aqueous humor and vitreous of normal rat eyes and are dramatically reduced in diabetes. Thus, TSP1 may play a role in ocular vascular homeostasis and its absence may contribute to vascular dysfunctions associated with diabetes.

Functional roles for PECAM-1 (CD31) and VE-cadherin (CD144) in tube assembly and lumen formation in three-dimensional collagen gels

Various in vitro models have been described that emulate one or more of the processes involved in angiogenesis in vivo. In the present study endothelial cells were cultured in three-dimensional type I collagen lattices in the presence of a mixture of basic fibroblast growth factor, vascular endothelial cell growth factor, and phorbol myristate acetate. Under these conditions, the endothelial cells rapidly assemble into an interconnected network of tube-like structures with a high frequency of intercellular canals or lumens. The formation of the networks and lumens was completely blocked by cycloheximide and by actinomycin D. Monoclonal antibodies directed against CD31 or vascular endothelial cadherin (VE-cadherin) inhibited the formation of endothelial tubes. A subtle difference in the morphology of cells treated with anti-CD31 versus anti-VE-cadherin was noted; namely, cells incubated in the presence of CD31 antibodies were rounded or formed attenuated tube-like structures, both of which were characterized by a single, large intra- or intercellular vacuole. In contrast, tube formation by cells incubated in the presence of VE-cadherin antibodies was also impaired and, most notably, demonstrated a reduction in either vacuole formation or vacuole fusion, depending upon the monoclonal antibody used. We suggest that the two endothelial-junction-associated proteins, CD31 and VE-cadherin, play different roles in the process of tube formation. CD31 appears to be required for cell elongation, migration, and/or invasion in the gels as well as for cell-cell association to form the network structures. VE-cadherin also appears to be required for cell-cell association, but additionally appears to play some role in the process of vacuolization or vacuole fusion leading to intercellular lumen formation.

Tissue specific expression of alternatively spliced murine PECAM-1 isoforms

PECAM-1 (CD31) is a cell adhesion molecule that is highly expressed at the sites of endothelial cell-cell contact and at lower levels on the surface of platelets and leukocytes. It is a member of the immunoglobulin gene superfamily and undergoes alternative splicing to generate several isoforms that differ only in their cytoplasmic domains. The tissue distribution of the expression of different PECAM-1 isoforms has not been previously defined. We have examined PECAM-1 expression in various mouse tissues and endothelial cells. PECAM-1 mRNA was highly expressed in lung, heart, and kidney, and to a lower extent in brain and liver. Most endothelial cells in culture expressed high levels of PECAM-1 mRNA; however, normal mouse brain endothelial cells rapidly lost PECAM-1 expression in culture. To examine the tissue distribution of PECAM-1 isoform expression, RT/PCR was performed on the RNA isolated from various mouse tissues and mouse endothelial cells. Cloning and sequencing of the cDNA products indicated that most tissues and endothelial cells expressed several PECAM-1 isoforms at different frequencies. The PECAM-1 isoform that lacks exons 14 and 15 was most frequently detected in all cases. A novel PECAM-1 isoform that lacks exons 12 and 14 was detected in brain. An antibody to the extracellular domain of PECAM-1 reacted with two major bands, at 130 kDa and 110-120 kDa, in lysates prepared from endothelial cells or kidneys at different stages of development. An antibody prepared against PECAM-1 exon 14, which reacts only with cytoplasmic domain of PECAM-1 isoforms that contain exon 14, failed to react with the major lower molecular weight form of PECAM-1 in these lysates. Therefore, PECAM-1 isoforms that lack exon 14 are expressed in endothelial cells and tissues in developmentally regulated fashion. These results illustrate that multiple PECAM-1 isoforms are expressed in various mouse tissues and endothelial cells. Understanding the distribution of PECAM-1 isoforms, and the identity of intracellular proteins with which they may interact, will help to elucidate the role of PECAM-1 in endothelial cell-cell interactions and morphogenesis.