Alternative splicing of a specific cytoplasmic exon alters the binding characteristics of murine platelet/endothelial cell adhesion molecule-1 (PECAM-1)

Identification of goldfish (Carassius auratus L.) leukocyte immune-type receptors shows alternative splicing as a potential mechanism for receptor diversification

Leukocyte immune-type receptors (LITRs) are a multigene family of teleost immunoregulatory proteins that share structural, phylogenetic, and likely functional relationships with several innate immune receptor proteins in other vertebrates, including mammals. Originally discovered in channel catfish (Ictalurus punctatus), representative IpLITR-types have been shown to regulate diverse innate immune cell effector responses including phagocytosis, degranulation, and cytokine secretion. To date, IpLITRs have been primarily characterized using mammalian cell line expression systems, therefore many unanswered questions remain regarding their actual regulatory roles in fish immunity. In the present study, we report on the preliminary molecular characterization of five goldfish (Carassius auratus) CaLITR-types and the identification of several putative splice variants of these receptors cloned from various goldfish tissues and primary myeloid cell cultures. In general, CaLITR mRNA transcripts were detected in all goldfish tissues tested, and also in primary kidney macrophage and neutrophil cultures. Specifically, CaLITR1 is a functionally ambiguous receptor with no charged amino acids in its transmembrane (TM) segment and is devoid of tyrosine-based signaling motifs in its short cytoplasmic tail (CYT) region. CaLITR2 is a putative activating receptor-type that contains immunotyrosine-based activation motifs (ITAMs) within its long CYT region, and CaLITR3 has a positively charged TM segment, suggesting that it may recruit intracellular stimulatory adaptor signaling molecules. CaLITR4 and CaLITR5 appear to have diverse signaling capabilities since they contain various immunoregulatory signaling motifs within their CYT regions including putative Nck and STAT recruitment motifs as well as ITAM-like and ITIM sequences. We also identified putative CaLITR splice variants with altered extracellular Ig-like domain compositions and variable CYT regions. Interestingly, this suggests that alternative splicing-mediated diversification of CaLITRs can generate receptor forms with possible variable binding and/or intracellular signaling abilities. Overall, these findings reveal new information about the teleost LITRs and sets the stage for exploring how alternative splicing leads to the functional diversification of this complex multigene immunoregulatory receptor family.

Influence of PECAM-1 ligand interactions on PECAM-1-dependent cell motility and filopodia extension

Platelet endothelial cell adhesion molecule (PECAM‐1) has been implicated in angiogenesis through processes that involve stimulation of endothelial cell motility. Previous studies suggest that PECAM‐1 tyrosine phosphorylation mediates the recruitment and then activation of the tyrosine phosphatase SHP‐2, which in turn promotes the turnover of focal adhesions and the extension of filopodia, processes critical to cell motility. While these studies have implicated PECAM‐1‐dependent signaling in PECAM‐1‐mediated cell motility, the involvement of PECAM‐1 ligand binding in cell migration is undefined. Therefore to investigate the role of PECAM‐1 binding interactions in cell motility, mutants of PECAM‐1 were generated in which either homophilic or heparin/glycosaminoglycan (GAG)‐mediated heterophilic binding had been disabled and then expressed in an endothelial cell surrogate. We found that the ability of PECAM‐1 to stimulate cell migration, promote filopodia formation and trigger Cdc42 activation were lost if PECAM‐1‐dependent homophilic or heparin/GAG‐dependent heterophilic ligand binding was disabled. We further observed that PECAM‐1 concentrated at the tips of extended filopodia, an activity that was diminished if homophilic, but not heparin/GAG‐mediated heterophilic binding had been disrupted. Similar patterns of activities were seen in mouse endothelial cells treated with antibodies that specifically block PECAM‐1‐dependent homophilic or heterophilic adhesion. Together these data provide evidence for the differential involvement of PECAM‐1‐ligand interactions in PECAM‐1‐dependent motility and the extension of filopodia.

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.

PECAM-1: regulator of endothelial junctional integrity

PECAM-1 (also known as CD31) is a cellular adhesion and signaling receptor comprising six extracellular immunoglobulin (Ig)-like homology domains, a short transmembrane domain and a 118 amino acid cytoplasmic domain that becomes serine and tyrosine phosphorylated upon cellular activation. PECAM-1 expression is restricted to blood and vascular cells. In circulating platelets and leukocytes, PECAM-1 functions largely as an inhibitory receptor that, via regulated sequential phosphorylation of its cytoplasmic domain, limits cellular activation responses. PECAM-1 is also highly expressed at endothelial cell intercellular junctions, where it functions as a mechanosensor, as a regulator of leukocyte trafficking and in the maintenance of endothelial cell junctional integrity. In this review, we will describe (1) the functional domains of PECAM-1 and how they contribute to its barrier-enhancing properties, (2) how the physical properties of PECAM-1 influence its subcellular localization and its ability to influence endothelial cell barrier function, (3) various stimuli that initiate PECAM-1 signaling and/or function at the endothelial junction and (4) cross-talk of PECAM-1 with other junctional molecules, which can influence endothelial cell function.

Alternative splicing of the RAGE cytoplasmic domain regulates cell signaling and function

The Receptor for Advanced Glycation End-products (RAGE) is a multi-ligand receptor present on most cell types. Upregulation of RAGE is seen in a number of pathological states including, inflammatory and vascular disease, dementia, diabetes and various cancers. We previously demonstrated that alternative splicing of the RAGE gene is an important mechanism which regulates RAGE signaling through the production of soluble ligand decoy isoforms. However, no studies have identified any alternative splice variants within the intracellular region of RAGE, a region critical for RAGE signaling. Herein, we have cloned and characterized a novel splice variant of RAGE that has a truncated intracellular domain (RAGEΔICD). RAGEΔICD is prevalent in both human and mouse tissues including lung, brain, heart and kidney. Expression of RAGEΔICD in C6 glioma cells impaired RAGE-ligand induced signaling through various MAP kinase pathways including ERK1/2, p38 and SAPK/JNK. Moreover, RAGEΔICD significantly affected tumor cell properties through altering cell migration, invasion, adhesion and viability in C6 glioma cells. Furthermore, C6 glioma cells expressing RAGEΔICD exhibited drastic inhibition on tumorigenesis in soft agar assays. Taken together, these data indicate that RAGEΔICD represents a novel endogenous mechanism to regulate RAGE signaling. Significantly, RAGEΔICD could play an important role in RAGE related disease states through down regulation of RAGE signaling.

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.

Collaborative enhancement of antibody binding to distinct PECAM-1 epitopes modulates endothelial targeting

Antibodies to platelet endothelial cell adhesion molecule-1 (PECAM-1) facilitate targeted drug delivery to endothelial cells by “vascular immunotargeting.” To define the targeting quantitatively, we investigated the endothelial binding of monoclonal antibodies (mAbs) to extracellular epitopes of PECAM-1. Surprisingly, we have found in human and mouse cell culture models that the endothelial binding of PECAM-directed mAbs and scFv therapeutic fusion protein is increased by co-administration of a paired mAb directed to an adjacent, yet distinct PECAM-1 epitope. This results in significant enhancement of functional activity of a PECAM-1-targeted scFv-thrombomodulin fusion protein generating therapeutic activated Protein C. The “collaborative enhancement” of mAb binding is affirmed in vivo, as manifested by enhanced pulmonary accumulation of intravenously administered radiolabeled PECAM-1 mAb when co-injected with an unlabeled paired mAb in mice. This is the first demonstration of a positive modulatory effect of endothelial binding and vascular immunotargeting provided by the simultaneous binding a paired mAb to adjacent distinct epitopes. The “collaborative enhancement” phenomenon provides a novel paradigm for optimizing the endothelial-targeted delivery of therapeutic agents.

Vascular endothelial platelet endothelial cell adhesion molecule 1 (PECAM-1) regulates advanced metastatic progression

Most patients who die from cancer succumb to treatment-refractory advanced metastatic progression. Although the early stages of tumor metastasis result in the formation of clinically silent micrometastatic foci, its later stages primarily reflect the progressive, organ-destructive growth of already advanced metastases. Early-stage metastasis is regulated by multiple factors within tumor cells as well as by the tumor microenvironment (TME). In contrast, the molecular determinants that control advanced metastatic progression remain essentially uncharacterized, precluding the development of therapies targeted against it. Here we show that the TME, functioning in part through platelet endothelial cell adhesion molecule 1 (PECAM-1), drives advanced metastatic progression and is essential for progression through its preterminal end stage. PECAM-1-KO and chimeric mice revealed that its metastasis-promoting effects are mediated specifically through vascular endothelial cell (VEC) PECAM-1. Anti-PECAM-1 mAb therapy suppresses both end-stage metastatic progression and tumor-induced cachexia in tumor-bearing mice. It reduces proliferation, but not angiogenesis or apoptosis, within advanced tumor metastases. Because its antimetastatic effects are mediated by binding to VEC rather than to tumor cells, anti-PECAM-1 mAb appears to act independently of tumor type. A modified 3D coculture assay showed that anti-PECAM-1 mAb inhibits the proliferation of PECAM-1-negative tumor cells by altering the concentrations of secreted factors. Our studies indicate that a complex interplay between elements of the TME and advanced tumor metastases directs end-stage metastatic progression. They also suggest that some therapeutic interventions may target late-stage metastases specifically. mAb-based targeting of PECAM-1 represents a TME-targeted therapeutic approach that suppresses the end stages of metastatic progression, until now a refractory clinical entity.

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.

PECAM-1: conflicts of interest in inflammation

Platelet endothelial cell adhesion molecule-1 (PECAM-1, CD31) is a cell adhesion and signaling receptor that is expressed on hematopoietic and endothelial cells. PECAM-1 is vital to the regulation of inflammatory responses, as it has been shown to serve a variety of pro-inflammatory and anti-inflammatory functions. Pro-inflammatory functions of PECAM-1 include the facilitation of leukocyte transendothelial migration and the transduction of mechanical signals in endothelial cells emanating from fluid shear stress. Anti-inflammatory functions include the dampening of leukocyte activation, suppression of pro-inflammatory cytokine production, and the maintenance of vascular barrier integrity. Although PECAM-1 has been well-characterized and studied, the mechanisms through which PECAM-1 regulates these seemingly opposing functions, and how they influence each other, are still not completely understood. The purpose of this review, therefore, is to provide an overview of the pro- and anti-inflammatory functions of PECAM-1 with special attention paid to mechanistic insights that have thus far been revealed in the literature in hopes of gaining a clearer picture of how these opposing functions might be integrated in a temporal and spatial manner on the whole organism level. A better understanding of how inflammatory responses are regulated should enable the development of new therapeutics that can be used in the treatment of acute and chronic inflammatory disorders.

Differential ICAM-1 isoform expression regulates the development and progression of experimental autoimmune encephalomyelitis

Intercellular adhesion molecule-1 (ICAM-1) functions in leukocyte trafficking, activation, and the formation of the immunological synapse. ICAM-1 is a member of the immunoglobulin superfamily of adhesion proteins, which share a similar structure of repeating Ig-like domains. Many genes in this family, including ICAM-1, show alternative splicing leading to the production of different protein isoforms, although little functional information is available regarding the expression patterns, ligand interactions, and functions of these isoforms, especially those arising from the ICAM-1 gene. In this study, we show using different lines of mutant mice (Icam1(tm1Jcgr) and Icam1(tm1Bay)) that alterations in the expression of the alternatively spliced ICAM-1 isoforms can significantly influence the disease course during the development of EAE. Icam1(tm1Jcgr) mutant mice, unlike Icam1(tm1Bay) mutants, do not express isoforms containing the Mac-1 binding domain and had significantly attenuated of EAE. In contrast, Icam1(tm1Bay) mice developed severe EAE in both active and adoptive transfer models compared to both Icam1(tm1Jcgr) and wild type mice. We also observed that T cells from Icam1(tm1Bay) mice displayed increased proliferation kinetics and produced higher levels of IFN-gamma compared to Icam1(tm1Jcgr) and wild type mice. Thus, our investigations show that the alternatively spliced ICAM-1 isoforms are functional, and play key roles during the progression of CNS inflammation and demyelination in EAE. Furthermore, our findings suggest that these isoforms may also play key roles in controlling the development of inflammatory diseases such as multiple sclerosis, possibly through differential engagement with ICAM-1 ligands such as Mac-1.

An alternatively spliced isoform of PECAM-1 is expressed at high levels in human and murine tissues, and suggests a novel role for the C-terminus of PECAM-1 in cytoprotective signaling

The Ig-ITIM family member PECAM-1 is expressed in vascular and endothelial cells, and its functions include suppression of mitochondria-dependent apoptosis. Previous studies have identified distinct PECAM-1 cytoplasmic domain splice variants at the mRNA, but not protein, level. Several relatively abundant mRNA isoforms lack exon 15 (Delta15) and would theoretically encode a protein with a truncated cytoplasmic domain and a unique C-terminal sequence. Using a novel rabbit polyclonal antibody that specifically recognizes Delta15 PECAM-1, we found that the Delta15 PECAM-1 isoform was expressed in human tissues, including brain, testes and ovary. This isoform was also expressed on the cell surface of human platelets, human umbilical vein endothelial cells (HUVECs) and the Jurkat T-cell leukemia, human erythroleukemia (HEL) and U937 histiocytic lymphoma cell lines. Furthermore, murine platelets and lung lysates demonstrated abundant amounts of exon-15-deficient PECAM-1. Functional studies revealed that Delta15 PECAM-1 retains both its homophilic binding capacity and its ability to signal by means of its immunoreceptor tyrosine-based inhibitory motif (ITIM) domains. Delta15 PECAM-1 was unable, however, to protect against apoptosis induced by overexpression of Bax or treatment with the chemotherapy agent etoposide. These studies suggest a novel role for the PECAM-1 C-terminus in cytoprotective signaling and highlight a need for further characterization of expression of PECAM-1 isoforms in normal and malignant tissues.

Platelet endothelial cell adhesion molecule 1 (PECAM-1) and its interactions with glycosaminoglycans: 2. Biochemical analyses

Platelet endothelial cell adhesion molecule 1 (PECAM-1) (CD31), a member of the immunoglobulin (Ig) superfamily of cell adhesion molecules with six Ig-like domains, has a range of functions, notably its contributions to leukocyte extravasation during inflammation and in maintaining vascular endothelial integrity. Although PECAM-1 is known to mediate cell adhesion by homophilic binding via domain 1, a number of PECAM-1 heterophilic ligands have been proposed. Here, the possibility that heparin and heparan sulfate (HS) are ligands for PECAM-1 was reinvestigated. The extracellular domain of PECAM-1 was expressed first as a fusion protein with the Fc region of human IgG1 fused to domain 6 and second with an N-terminal Flag tag on domain 1 (Flag-PECAM-1). Both proteins bound heparin immobilized on a biosensor chip in surface plasmon resonance (SPR) binding experiments. Binding was pH-sensitive but is easily measured at slightly acidic pH. A series of PECAM-1 domain deletions, prepared in both expression systems, were tested for heparin binding. This revealed that the main heparin-binding site required both domains 2 and 3. Flag-PECAM-1 and a Flag protein containing domains 1-3 bound HS on melanoma cell surfaces, but a Flag protein containing domains 1-2 did not. Heparin oligosaccharides inhibited Flag-PECAM-1 from binding immobilized heparin, with certain structures having greater inhibitory activity than others. Molecular modeling similarly identified the junction of domains 2 and 3 as the heparin-binding site and further revealed the importance of the iduronic acid conformation for binding. PECAM-1 does bind heparin/HS but by a site that is distinct from that required for homophilic binding.

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.

PECAM-1: a multi-functional molecule in inflammation and vascular biology

Platelet endothelial cell adhesion molecule-1 (PECAM-1 or CD31) is a molecule expressed on all cells within the vascular compartment, being expressed to different degrees on most leukocyte sub-types, platelets, and on endothelial cells where its expression is largely concentrated at junctions between adjacent cells. As well as exhibiting adhesive properties, PECAM-1 is an efficient signaling molecule and is now known to have diverse roles in vascular biology including roles in angiogenesis, platelet function, and thrombosis, mechanosensing of endothelial cell response to fluid shear stress, and regulation of multiple stages of leukocyte migration through venular walls. This review will focus on some new developments with respect to the role of PECAM-1 in inflammation and vascular biology, highlighting the emerging complexities associated with the functions of this unique molecule.

PECAM-1 isoform-specific regulation of kidney endothelial cell migration and capillary morphogenesis

Platelet endothelial cell adhesion molecule-1 (PECAM-1) has been implicated in angiogenesis through its involvement in endothelial cell-cell and cell-matrix interactions and signal transduction. Recent studies indicate that the cytoplasmic domain of PECAM-1 plays an important role in its cell adhesive and signaling properties. However, the role PECAM-1 isoforms play during angiogenic events such as cell adhesion and migration requires further delineation. To gain insight into the role PECAM-1 plays during vascular development and angiogenesis, we examined the expression pattern of PECAM-1 isoforms during kidney vascularization. We show that multiple isoforms of PECAM-1 are expressed during renal vascular development with different frequencies. The PECAM-1 that lacks exons 14 and 15 (Delta14&15) was the predominant isoform detected in the renal vasculature. To further study PECAM-1 isoform-specific functions we isolated kidney endothelial cells (EC) from wild-type and PECAM-1-deficient (PECAM-1-/-) mice with B(4)-lectin-coated magnetic beads. PECAM-1-/- kidney EC showed reduced migration, inability to undergo capillary morphogenesis in Matrigel, dense peripheral focal adhesions, and peripheral cortical actin distribution compared with wild-type cells. PECAM-1-/- kidney EC secreted increased amounts of fibronectin and decreased amounts of tenascin-C and thrombospondin-1. Reexpression of Delta14&15, but not full-length, PECAM-1 in PECAM-1-/- kidney EC restored cell migration and capillary morphogenesis defects. Thus PECAM-1 may regulate the adhesive and migratory properties of kidney EC in an isoform-specific fashion through modulation of integrin activity and extracellular matrix protein expression. Our results indicate that regulated expression of specific PECAM-1 isoforms may enable EC to accommodate the different stages of angiogenesis.

CD248/Endosialin is dynamically expressed on a subset of stromal cells during lymphoid tissue development, splenic remodeling and repair

Studies of stromal cell populations in lymphoid tissue (LT) have been hampered by a lack of selective markers. Here, we show that CD248 (Endosialin/TEM1) is a stromal marker that is differentially expressed on fibroblasts and pericytes in the thymus, lymph node and spleen. Expression is high during LT development but largely disappears in the adult. CD248 is re-expressed in a Salmonella-induced model of splenic enlargement; peak expression corresponding to the peak of splenic enlargement. These results suggest that CD248 expression helps define a subset of LT stromal cells which play a role in remodelling during tissue development, infection and repair.

CD31/PECAM-1 genotyping and haplotype analyses show population diversity

Using direct sequencing of complementary DNA products, the sequences of human CD31 from exon 1 through exon 16 of 179 individuals (139 unrelated) were systematically examined. Of the 14 biallelic single nucleotide polymorphic sites detected, 7 polymorphic sites involved amino acid substitution. These 14 polymorphic sites yielded 18 observed CD31 alleles and 9 predicted CD31 polypeptide sequences. Based on molecular haplotyping and family pedigree analysis, linkage disequilibrium among some single nucleotide polymorphic sites was observed. Single nucleotide polymorphism frequencies between populations were also measured using dot-blot hybridization with DNA or peptide nucleic acid probes.

PECAM-1, a key player in neuroinflammation

Platelet endothelial cell adhesion molecule-1 (PECAM-1, CD31) is a 130-kDa protein, which plays a significant role in the adhesion cascade. It is therefore involved in leucocyte endothelium interaction and in leucocyte transendothelial migration during inflammation. As neuroinflammation and subsequent blood brain barrier disruption are integral processes in many neurological disorders, PECAM-1 and its soluble form (sPECAM-1) have been investigated in a number of conditions, rising hopes as a potential marker of disease activity, a possible target in treatment and a prognostic factor. It has been shown that serum and CSF levels of PECAM-1 and sPECAM-1 are increased in patients in active stages of multiple sclerosis. Similarly, they rise in individuals after ischaemic stroke. PECAM-1 has also been shown to be involved in the pathogenesis of Abeta-related cerebral vascular disorders, such as Alzheimer disease. It participates in the pathomechanism of paraneoplastic neurological disorders and in neuroinflammation in NeuroAIDS. A number of experiments on animal models were carried out in order to investigate PECAM-1 role in the above-mentioned conditions and more, including brain trauma and nerve root injury. In this review most recent investigations on PECAM-1 biology and its role in neuroinflammation have been described and discussed from a multidisciplinary point of view.

Endothelial targeting of a recombinant construct fusing a PECAM-1 single-chain variable antibody fragment (scFv) with prourokinase facilitates prophylactic thrombolysis in the pulmonary vasculature

Means to prevent thrombus extension and local recurrence remain suboptimal, in part because of the limited effectiveness of existing thrombolytics. In theory, plasminogen activators could be used for this purpose if they could be anchored to the vascular lumen by targeting stably expressed, noninternalized determinants such as platelet-endothelial-cell adhesion molecule 1 (PECAM-1). We designed a recombinant molecule fusing low-molecular-weight single-chain prourokinase plasminogen activator (lmw-scuPA) with a single-chain variable fragment (scFv) of a PECAM-1 antibody to generate the prodrug scFv/lmw-scuPA. Cleavage by plasmin generated fibrinolytically active 2-chain lmw-uPA. This fusion protein (1) bound specifically to PECAM-1-expressing cells; (2) was rapidly cleared from blood after intravenous injection; (3) accumulated in the lungs of wild-type C57BL6/J, but not PECAM-1 null mice; and (4) lysed pulmonary emboli formed subsequently more effectively than lmw-scuPA, thereby providing support for the concept of thromboprophylaxis using recombinant scFv-fibrinolytic fusion proteins that target endothelium.

Kinetic expression of platelet endothelial cell adhesion molecule-1 (PECAM-1/CD31) during embryonic stem cell differentiation

Platelet endothelial cell adhesion molecule-1 (PECAM-1/CD31) is widely used as a marker during vasculogenesis and angiogenesis from embryonic stem (ES) cells. However, the expression of PECAM-1 isoforms in ES cells has not been determined. The present study was designed to determine the role of PECAM-1 isoforms during in vitro endothelial differentiation of ES cells. It was found that undifferentiated ES cells expressed high level of PECAM-1, which primarily located at cell-cell junction, but the expression of PECAM-1 was sharply down-regulated during early ES cell differentiation. In addition, undifferentiated ES cells were found the expressed all eight known alternatively spliced PECAM-1 isoforms, among them the expression of PECAM-1 isoforms lacking exon 15 or 14&15 was predominant. Quantitative analysis revealed a significant increase in the expression of PECAM-1 isoform lacking exon 12&14&15 as vascular development of ES cells. These results indicate a constitutive expression of PECAM-1 in undifferentiated murine ES cells and suggest a developmental role of PECAM-1 isoform changes during vasculogenesis and angiogenesis.

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.

Function of alternative splicing

Alternative splicing is one of the most important mechanisms to generate a large number of mRNA and protein isoforms from the surprisingly low number of human genes. Unlike promoter activity, which primarily regulates the amount of transcripts, alternative splicing changes the structure of transcripts and their encoded proteins. Together with nonsense-mediated decay (NMD), at least 25% of all alternative exons are predicted to regulate transcript abundance. Molecular analyses during the last decade demonstrate that alternative splicing determines the binding properties, intracellular localization, enzymatic activity, protein stability and posttranslational modifications of a large number of proteins. The magnitude of the effects range from a complete loss of function or acquisition of a new function to very subtle modulations, which are observed in the majority of cases reported. Alternative splicing factors regulate multiple pre-mRNAs and recent identification of physiological targets shows that a specific splicing factor regulates pre-mRNAs with coherent biological functions. Therefore, evidence is now accumulating that alternative splicing coordinates physiologically meaningful changes in protein isoform expression and is a key mechanism to generate the complex proteome of multicellular organisms.

Ultrastructural localization of platelet endothelial cell adhesion molecule (PECAM-1, CD31) in vascular endothelium

The distribution of platelet endothelial cell adhesion molecule (PECAM-1, CD31) in vascular endothelium has been disputed. Originally reported to be highly concentrated at interendothelial cell contacts, recent studies have claimed that CD31 is distributed evenly over the entire endothelial cell surface. We re-investigated this question with two different murine anti-CD31 antibodies (MEC 13.3 and M-20), using a pre-embedding immunonanogold electron microscopic procedure that allowed precise label quantitation. MEC 13.3 reacted strongly with the luminal and abluminal plasma membranes of the endothelial cells lining microvessels in normal tissues and in angiogenic vessels induced by a tumor and vascular endothelial growth factor (VEGF-A164). Lateral plasma membranes were significantly less labeled. Conversely, M-20 strongly labeled the cytoplasmic face of the lateral plasma membranes of endothelial cells, although sparing specialized junctions, and only weakly labeled the luminal and abluminal plasma membranes. Both antibodies stained a significant minority of vesicles and vacuoles comprising the vesiculovacuolar organelle (VVO). Neither antibody was reactive in CD31-null mice. We conclude that CD31 is distributed over the entire endothelial cell surface, exclusive of specialized junctions, and in VVOs, but is not equally accessible to different antibodies in all locations.

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.

Signal transduction pathways mediated by PECAM-1: new roles for an old molecule in platelet and vascular cell biology

Recent studies of platelet endothelial cell adhesion molecule-1 (PECAM-1 [CD31])-deficient mice have revealed that this molecule plays an important role in controlling the activation and survival of cells on which it is expressed. In this review, we focus on the complex cytoplasmic domain of PECAM-1 and describe what is presently known about its structure, posttranslational modifications, and binding partners. In addition, we summarize findings that implicate PECAM-1 as an inhibitor of cellular activation via protein tyrosine kinase-dependent signaling pathways, an activator of integrins, and a suppressor of cell death via pathways that depend on damage to the mitochondria. The challenge of future research will be to bridge our understanding of the functional and biochemical properties of PECAM-1 by establishing mechanistic links between signals transduced by the PECAM-1 cytoplasmic domain and discrete cellular responses.

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.

Tissue-specific distributions of alternatively spliced human PECAM-1 isoforms

Platelet endothelial cell adhesion molecule-1 (PECAM-1) is a cell adhesion molecule that is highly expressed on the surface of endothelial cells and some hematopoietic cells. Its cytoplasmic domain is encoded by multiple exons, which undergo alternative splicing. Here, we demonstrate that the human PECAM-1 cytoplasmic domain undergoes alternative splicing, generating six different isoforms. RT-PCR cloning and DNA sequence analysis indicated that human tissue and endothelial cells express multiple isoforms of PECAM-1, including the full-length PECAM-1 and five other isoforms, which lack exon 12, 13, 14, or 15 or exons 14 and 15. The full-length PECAM-1 is the predominant isoform detected in human tissue and endothelial cells. This is in contrast to murine endothelium, in which the PECAM-1 isoform lacking exons 14 and 15 is the predominant isoform. The PECAM-1 isoform lacking exon 13 detected in human tissue and endothelial cells is absent in murine endothelium. The expression pattern of PECAM-1 isoforms changes during tube formation of endothelial cells on Matrigel, which may indicate specialized roles for specific isoforms of PECAM-1 during angiogenesis. The data presented here demonstrate that human PECAM-1 undergoes alternative splicing, generating multiple isoforms in vascular beds of various tissues. Therefore, the regulated expression of these isoforms may influence endothelial cell adhesive properties during angiogenesis and/or vasculogenesis.

Platelet endothelial cell adhesion molecule-1 in neutrophil emigration during acute bacterial pneumonia in mice and rats

Platelet endothelial cell adhesion molecule-1 (PECAM-1) (CD31) is an adhesion molecule believed to mediate transendothelial migration of neutrophils and other leukocytes after CD11/CD18-mediated adhesion. Our study evaluated the role of PECAM-1 in neutrophil emigration across the pulmonary capillaries and the bronchial microvasculature using blocking anti-PECAM-1 antibodies in mice and rats. Neutrophil emigration was induced by Escherichia coli, a stimulus eliciting CD11/CD18-dependent emigration, or Streptococcus pneumoniae, a stimulus inducing CD11/CD18-independent emigration. Although anti-PECAM-1 antibodies partially inhibited glycogen-induced neutrophil emigration into the peritoneum, neutrophil emigration across either the pulmonary capillaries or the bronchial microvasculature in response to either E. coli or S. pneumoniae was not prevented when the function of PECAM-1 was inhibited in either mice or rats. There was also no increase in the number of intravascular neutrophils within the bronchial vessels after treatment with anti-PECAM-1 antibody. These studies indicate that either CD11/CD18-dependent or -independent adhesion pathways may lead to PECAM-1-independent transendothelial migration through the pulmonary or the bronchial endothelium.

Polymorphisms in the platelet-endothelial cell adhesion molecule-1 (PECAM-1) gene, Asn563Ser and Gly670Arg, associated with myocardial infarction in the Japanese

We examined three missense polymorphisms of platelet-endothelial cell adhesion molecule-1 (PECAM-1), Val125Leu, Asn563Ser, and Gly670Arg, in 136 Japanese patients with myocardial infarction and 235 healthy Japanese controls. We found that these polymorphisms were in linkage disequilibrium with each other and that frequencies of 125Leu, 563Ser, and 670Arg alleles were significantly increased in patients compared with controls (0.522 vs 0.447, p = 0.048; 0.585 vs 0.502, p = 0.030; and 0.577 vs 0.492, p = 0.032, respectively). The frequencies of homozygotes for 563Ser and 670Arg alleles were also significantly increased in the patients (33.1% vs 23.4%, odds risk [OR] = 1.62, p = 0.040, 95% confidence interval [95%CI] = 1.01-2.58; and 32.4% vs 23.0%, OR = 1.60, p = 0.048, 95%CI = 1.00-2.57, respectively). These observations suggest that the 563Ser/Ser genotype and 670Arg/Arg genotype of PECAM-1 are novel genetic risk factors of myocardial infarction in Japanese. Stratification analysis of the patients showed that the associations of these PECAM-1 genotypes with myocardial infarction were preferentially found in male and younger patients (age of onset of myocardial infarction less than 60 years). In addition, the associations were stronger in patients with three-vessel disease than in the others and appeared independent of conventional risk factors including smoking, hypertension, diabetes mellitus, hyperlipidemia, and obesity.

Eosinophil tissue recruitment to sites of allergic inflammation in the lung is platelet endothelial cell adhesion molecule independent

Platelet endothelial cell adhesion molecule (PECAM or CD31) is a cell adhesion molecule expressed on circulating leukocytes and endothelial cells that plays an important role in mediating neutrophil and monocyte transendothelial migration in vivo. In this study, we investigated whether eosinophils, like neutrophils and monocytes, utilize PECAM for tissue recruitment to sites of allergic inflammation in vivo. Eosinophils express similar levels of PECAM as neutrophils as assessed by FACS analysis. RT-PCR studies demonstrate that eosinophils like neutrophils express the six extracellular domains of PECAM. Eosinophils exhibit homophilic binding to recombinant PECAM as assessed in a single-cell micropipette adhesion assay able to measure the biophysical strength of adhesion of eosinophils to recombinant PECAM. The strength of eosinophil adhesion to recombinant PECAM is the same as that of neutrophil binding to recombinant PECAM and can be inhibited with an anti-PECAM Ab. Although eosinophils express functional PECAM, anti-PECAM Abs did not inhibit bronchoalveolar lavage eosinophilia, lung eosinophilia, and airway hyperreactivity to methacholine in a mouse model of OVA-induced asthma in vivo. Thus, in contrast to studies that have demonstrated that neutrophil and monocyte tissue recruitment is PECAM dependent, these studies demonstrate that eosinophil tissue recruitment in vivo in this model is PECAM independent.

Inner cell mass-specific expression of a cell adhesion molecule (PECAM-1/CD31) in the mouse blastocyst

Platelet/Endothelial Cell Adhesion Molecule-1 (PECAM-1 or CD31) is thought to be a vascular-specific protein, but its function has not been clearly defined. Here, we demonstrate by using confocal immunofluorescence microscopy that PECAM-1 is first detected in the mouse blastocyst, which contains no vascular cells, and its expression is restricted to the pluripotent inner cell mass (ICM) cells. Expression is localized to cell-cell borders of the ICM and is detected at the very first signs of blastocoel formation. Consistent with these observations is that embryonic transcripts of PECAM-1 mRNA, as detected by RT-PCR, greatly increase during the morula-to-blastocyst transition and seven of the eight known alternatively spliced isoforms of PECAM-1 are expressed in the blastocyst. The synthesis of PECAM-1 is independent of compaction, cytokinesis, and DNA replication, as it is detected in embryos that are chronologically at the blastocyst stage following culture of 8-cell embryos in Ca2+-free medium, or medium containing cytochalasin D or aphidicolin, respectively. By the late blastocyst stage, PECAM-1 expression is restricted to the pluripotent epiblast, at which point it has a mutually exclusive expression pattern to that of type IV collagen, a basement membrane marker. The reduction in PECAM-1 transcripts in retinoic acid-induced differentiation of F9 teratocarcinoma cells, a model of epiblast-to-primitive endoderm differentiation, confirmed the epiblast-specific expression of PECAM-1. By the egg cylinder stage of development, at which point the epiblast is no longer pluripotent, PECAM-1 is not detected. This ICM-specific pattern of expression suggests a novel developmental role of PECAM-1 that is independent of its function in vascular ontogeny.

PECAM-1/CD31 trans-homophilic binding at the intercellular junctions is independent of its cytoplasmic domain; evidence for heterophilic interaction with integrin alphavbeta3 in Cis

PECAM-1/CD31 is a cell adhesion and signaling molecule that is enriched at the endothelial cell junctions. Alternative splicing generates multiple PECAM-1 splice variants, which differ in their cytoplasmic domains. It has been suggested that the extracellular ligand-binding property, homophilic versus heterophilic, of these isoforms is controlled by their cytoplasmic tails. To determine whether the cytoplasmic domains also regulate the cell surface distribution of PECAM-1 splice variants, we examined the distribution of CD31-EGFPs (PECAM-1 isoforms tagged with the enhanced green fluorescent protein) in living Chinese hamster ovary cells and in PECAM-1-deficient endothelial cells. Our results indicate that the extracellular, rather than the cytoplasmic domain, directs PECAM-1 to the cell-cell borders. Furthermore, coculturing PECAM-1 expressing and deficient cells along with transfection of CD31-EGFP cDNAs into PECAM-1 deficient cells reveal that this PECAM-1 localization is mediated by homophilic interactions. Although the integrin alphavbeta3 has been shown to interact with PECAM-1, this trans-heterophilic interaction was not detected at the borders of endothelial cells. However, based on cocapping experiments performed on proT cells, we provide evidence that the integrin alphavbeta3 associates with PECAM-1 on the same cell surface as in a cis manner.

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.

Contributions of the extracellular and cytoplasmic domains of platelet-endothelial cell adhesion molecule-1 (PECAM-1/CD31) in regulating cell-cell localization

PECAM-1/CD31, a vascular cell adhesion/signaling molecule that has been implicated in a number of vascular functions (including angiogenesis and the transmigration of leukocytes through endothelium) is highly enriched at the cell-cell borders of adjacent endothelial cells. To identify the mechanisms responsible for this localization, a series of PECAM-1 mutants and chimeric PECAM-1 molecules were transfected into non-PECAM-expressing cells and the ability of the constructs to move to cell-cell borders of adjacent cells was determined using immunohistochemistry and confocal microscopy. Although neither the extracellular domain, by itself, nor the cytoplasmic domain, by itself, was sufficient to direct cell-cell localization, the combination of the extracellular and transmembrane domains with a small group of highly charged amino acids in a membrane proximal region of the cytoplasmic domain was sufficient to direct efficient localization of the molecule to cell-cell borders. Importantly, only constructs that supported PECAM-1 mediated adhesion localized to cell-cell borders. Our data are consistent with a ‘diffusion trapping’ model in which movement of PECAM-1 in the cell membrane occurs relatively freely until the ‘stablized’ extracellular domain of the molecule encounters its ligand on an adjacent cell. When this occurs, the complex is ‘captured’ at the cell-cell interface leading to localization at cell-cell borders.

Tissue engineering of vascular grafts: human cell seeding of decellularised porcine matrix

OBJECTIVES

To develop a biocompatible and mechanically stable vascular graft combining human cells and a xenogenic acellular matrix. DESIGN/MATERIALS: Decellularised matrix tubes were obtained by enzymatic cell extraction of native porcine aortas. Endothelial cells and myofibroblasts were isolated from human saphenous veins and grown in cell cultures. The inner surface of the tubes was seeded with endothelial cells or myofibroblasts and exposed to pulsatile flow.

RESULTS

After cell extraction, the absence of cellular components, as well as the maintenance of matrix integrity, was demonstrated by means of light microscopy and scanning electron microscopy. Furthermore, the porcine matrix was successfully seeded with human endothelial cells, which grew to a monolayer under flow conditions. Stable biomechanical properties were achieved at physiological perfusion pressures in vitro.

CONCLUSIONS

Cellular components can be extracted from native porcine blood vessels. Vascular grafts can be generated in vitro of animal acellular matrix and human cells.

Immunotargeting of glucose oxidase to endothelium in vivo causes oxidative vascular injury in the lungs

Vascular immunotargeting is a novel approach for site-selective drug delivery to endothelium. To validate the strategy, we conjugated glucose oxidase (GOX) via streptavidin with antibodies to the endothelial cell surface antigen platelet endothelial cell adhesion molecule (PECAM). Previous work documented that 1) anti-PECAM-streptavidin carrier accumulates in the lungs after intravenous injection in animals and 2) anti-PECAM-GOX binds to, enters, and kills endothelium via intracellular H(2)O(2) generation in cell culture. In the present work, we studied the targeting and effect of anti-PECAM-GOX in animals. Anti-PECAM-GOX, but not IgG-GOX, accumulated in the isolated rat lungs, produced H(2)O(2,) and caused endothelial injury manifested by a fourfold elevation of angiotensin-converting enzyme activity in the perfusate. In intact mice, anti-PECAM-GOX accumulated in the lungs (27 +/- 9 vs. 2.4 +/- 0.3% injected dose/g for IgG-GOX) and caused severe lung injury and 95% lethality within hours after intravenous injection. Endothelial disruption and blebbing, elevated lung wet-to-dry ratio, and interstitial and alveolar edema indicated that anti-PECAM-GOX damaged pulmonary endothelium. The vascular injury in the lungs was associated with positive immunostaining for iPF(2alpha)-III isoprostane, a marker for oxidative stress. In contrast, IgG-GOX caused a minor lung injury and little (5%) lethality. Anti-PECAM conjugated with inert proteins induced no death or lung injury. None of the conjugates caused major injury to other internal organs. These results indicate that an immunotargeting strategy can deliver an active enzyme to selected target cells in intact animals. Anti-PECAM-GOX provides a novel model of oxidative injury to the pulmonary endothelium in vivo.

Cytoplasmic domain is not essential for the cell adhesion activities of gicerin, an Ig-superfamily molecule

Gicerin is a cell adhesion molecule in the immunoglobulin (Ig) superfamily and is expressed abundantly during development in the nervous system. It has homophilic cell adhesion activity and also has heterophilic binding activity with NOF (neurite outgrowth factor) and mediates neurite extension. There are two isoforms of gicerin, one with a short (s-gicerin) and the other with a longer cytoplasmic domain (l-gicerin). We have reported that s-gicerin possesses stronger activities than l-gicerin during cell aggregation, in NOF-binding, and in neurite extension. In this study, we established cell lines which expressed a mutant-gicerin whose cytoplasmic domain was deleted and we compared the above three biological activities of the mutant-gicerin with those of s- and l-gicerin. We found that the mutant-gicerin retained all these activities, but the activities were weaker than those of s-gicerin and almost the same as those of l-gicerin. We concluded that the cytoplasmic domain of gicerin is not essential for optimal adhesive activities of gicerin, but might be involved in the regulation of its activities.

Expression of PECAM-1/CD31 isoforms in human brain gliomas

Microvascular proliferation is a histopathological hallmark of glioblastomas and anaplastic oligodendrogliomas. Platelet endothelial cell adhesion molecule 1 (PECAM-1/CD31) is involved in angiogenesis. PECAM-1 mediates homophilic and heterophilic interactions (with glycosaminoglycans and alphaVbeta3), but deletion of exon 14 results in a loss of heterophilic adhesion. Expression of various PECAM-1 isoforms was searched for in brain gliomas, showing microvascular proliferation (glioblastomas and anaplastic oligodendrogliomas) or not (oligodendrogliomas). In addition, expression of alphaVbeta3 in some tumors was studied by immunohistochemistry. Various tissues and the HUVEC primary cell line were used as controls. Immunohistochemistry showed that PECAM-1 was expressed by all endothelial cells in all tissues and by some tumor cells in glioblastomas and anaplastic oligodendrogliomas. Microvascular proliferation always expressed alphaVbeta3. In addition, some tumor cells in anaplastic oligodendroglioma and glioblastomas expressed it. In all samples examined, PECAM-1 exists under at least two transcriptional isoforms: the whole length molecule and an isoform made by the splicing of exon 14. Western blot analysis revealed in all cases 130 and 110 kDa bands corresponding to the mature form and its precursor respectively. These results suggest that splicing of exon 14 occurs in vivo in various normal and tumoral tissues and may modulate PECAM-1 adhesion according to the presence or not of other PECAM-1 ligands such as alphaVbeta3. Expression of PECAM-1 by tumor cells in glioblastomas and anaplastic oligodendrogliomas may favour angiogenesis by specific PECAM-1 interactions between glial and endothelial cells.

Irradiation induces upregulation of CD31 in human endothelial cells

Radiation-induced vascular injury is believed to be a major factor contributing to parenchymal atrophy, fibrosis and necrosis in normal tissue after radiotherapy. In this study irradiation of human umbilical vein endothelial cells (HUVECs) significantly increased adherence of U-937 cells in a time-dependent manner. Given the potential multifunctional role of CD31 in the vasculature we have examined the possible effects of irradiation on levels of CD31 expression in HUVECs. Irradiation upregulated CD31 expression on HUVECs, independently of initial plating density and radiation-induced changes such as cell number, cell cycle stage, or cell size. CD31 mRNA levels were raised in irradiated HUVECs relative to controls. Both CD31 mRNA and surface protein showed similar changes, suggesting that the increase in mRNA in irradiated HUVECs is responsible for the elevation in cell surface protein. A semi-quantitative study of tissue specimens from patients who had received radiotherapy indicated that CD31 staining in the blood vessels from irradiated tissues was increased compared with controls. Endothelial CD31 is important in the transmigration of leukocytes. We have demonstrated that the incorporation of monoclonal antibody to CD31 significantly inhibited the transmigration of human peripheral blood leukocytes through a monolayer of irradiated HUVECs. Taken together these data strongly suggest that irradiation induces a marked increase in CD31 expression on endothelial cells as part of a general response to irradiation. Its upregulation may play an important role in the development of radiation-induced normal tissue damage and thus is a possible target for therapeutic intervention.

Activity-dependent regulation of alternative splicing patterns in the rat brain

Alternative splicing plays an important role in the expression of genetic information. Among the best understood alternative splicing factors are transformer and transformer-2, which regulate sexual differentiation in Drosophila. Like the Drosophila genes, the recently identified mammalian homologues are subject to alternative splicing. Using an antibody directed against the major human transformer-2 beta isoform, we show that it has a widespread expression in the rat brain. Pilocarpine-induced neuronal activity changes the alternative splicing pattern of the human transformer-2-beta gene in the brain. After neuronal stimulation, a variant bearing high similarity to a male-specific Drosophila tra-2179 isoform is switched off in the hippocampus and is detectable in the cortex. In addition, the ratio of another short RNA isoform (htra2-beta2) to htra2-beta1 is changed. Htra2-beta2 is not translated into protein, and probably helps to regulate the relative amounts of htra2-beta1 to beta3. We also observe activity-dependent changes in alternative splicing of the clathrin light chain B, c-src and NMDAR1 genes, indicating that the coordinated change of alternative splicing patterns might contribute to molecular plasticity in the brain.

The carboxyl-terminal region of biliary glycoprotein controls its tyrosine phosphorylation and association with protein-tyrosine phosphatases SHP-1 and SHP-2 in epithelial cells

Biliary glycoprotein (Bgp, C-CAM, or CD66a) is an immunoglobulin-like cell adhesion molecule and functions as a tumor suppressor protein. We have previously shown that the Bgp1 isoform responsible for inhibition of colonic, liver, prostate, and breast tumor cell growth contains within its cytoplasmic domain two tyrosine residues positioned in immunoreceptor tyrosine-based inhibition motif (ITIM) consensus sequences. Moreover, we determined that these residues, upon phosphorylation, associate with the protein-tyrosine phosphatase SHP-1. In this report, we have further evaluated the structural bases of the association of Bgp1 with Tyr phosphatases. First, we demonstrate that Bgp1 also associates with the SHP-2 Tyr phosphatase, but not with an unrelated Tyr phosphatase, PTP-PEST. Association of Bgp1 and SHP-2 involves the Tyr residues within the Bgp1 ITIM sequences, Val at position +3 relative to the second Tyr (Tyr-515), and the SHP-2 N-terminal SH2 domain. In addition, our results indicate that residues +4, +5, and +6 relative to Tyr-515 in the Bgp1 cytoplasmic domain play a significant role in these interactions, as their deletion reduced Bgp1 Tyr phosphorylation and association with SHP-1 and SHP-2 by as much as 80%. Together, these results indicate that both SHP-1 and SHP-2 interact with the Bgp1 cytoplasmic domain via ITIM-like sequences. Furthermore, they reveal that the C-terminal amino acids of Bgp1 are critical for these interactions.

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.

Cell-cell interactions during transendothelial migration of tumor cells

A key event in cancer metastasis is the transendothelial migration of tumor cells. This process involves multiple adhesive interactions between tumor cells and the endothelium. After adhering to the surface of endothelial cells, tumor cells must penetrate the endothelial junction, which contains high concentrations of the cell adhesion molecules VE-cadherin and PECAM-1. Studies using an in vitro model system, consisting of melanoma cells which are seeded onto a monolayer of endothelial cells cultured on Matrigel, have revealed reorganization of the cytoskeleton and dynamic changes in the cell shape of both tumor and endothelial cells. The initial stages of transmigration are characterized by numerous membrane blebs protruding from the basolateral surfaces of the melanoma cells. Contact regions also show an abundance of microfilaments arising from the underlying endothelial cells. These adhesive interactions lead to the redistribution of both VE-cadherin and PECAM-1 and, consequently, a localized dissolution of the endothelial junction. The penetration of the endothelial junction is initiated by melanoma pseudopods. Despite the disappearance of VE-cadherin from the retracting endothelial junction, heterotypic contacts between the tumor cell and its surrounding endothelial cells show a high concentration of pan-cadherin staining, suggesting that transmigration of melanoma cells might yet be facilitated by interactions with another member of the cadherin family. Upon adhesion to the Matrigel, melanoma cells begin to spread and invade the matrix material, while the endothelial cells extend processes over the melanoma cells to reform the monolayer. Interestingly, the leading margins of these endothelial processes contain a high concentration ofN-cadherin. VE-cadherin and PECAM-1 reappear only when the advancing endothelial processes meet to reform the endothelial junction. Together, these observations suggest that endothelial cells actively participate in the transmigration of tumor cells and specific cadherins are involved in different steps of this complex process.

Increased endothelial cell expression of platelet-endothelial cell adhesion molecule-1 during hyperoxic lung injury

Lung injury is a frequent consequence of oxygen (O2) therapy administered to newborns and adults with respiratory distress. Acute exposure to hyperoxia results in a well-described pathophysiologic response in the lungs. Because inflammation is an important component of pulmonary O2 toxicity, we have an interest in identifying the inflammatory mediators that increase during hyperoxia. Platelet-endothelial cell adhesion molecule-1 (PECAM-1), a member of the immunoglobulin superfamily that is expressed at the junctions between endothelial cells, is essential to the transendothelial migration of leukocytes. We hypothesized that increased expression of PECAM-1 occurs in pulmonary endothelial cells during hyperoxic lung injury. Adult mice were exposed to 100% O2 for up to 96 h. We analyzed PECAM-1 expression by RNA blot hybridization, in situ hybridization, and immunohistochemistry. A increase in PECAM-1 mRNA was seen as soon as 2 d of hyperoxia relative to unexposed control mice. PECAM-1 mRNA and protein were found in endothelial cells of both large and small arteries. The expression of PECAM-1 in capillary vessels was further confirmed using in situ hybridization at the electron microscope level. This increase in PECAM-1 expression coincided with the appearance of leukocytes in lung tissue. These observations suggest that PECAM-1 expression is a relatively early step in the inflammation cascade, and intervention at this phase may be critical to the prevention of further damage.

Tissue engineering of heart valves--human endothelial cell seeding of detergent acellularized porcine valves

OBJECTIVE

Tissue engineering of heart valves represents a new experimental concept to improve current modes of therapy in valvular heart disease. Drawbacks of glutaraldehyde fixed tissue valves or mechanical valves include the short durability or the need for life-long anticoagulation, respectively. Both have in common the inability to grow, which makes valvular heart disease especially problematic in children. The aim of this study was to develop a new methodology for a tissue engineered heart valve combining human cells and a xenogenic acellularized matrix.

METHODS

Porcine aortic valves were acellularized by deterging cell extraction using Triton without tanning. Endothelial cells were isolated in parallel from human saphenous veins and expanded in vitro. Specimens of the surface of the acellular matrix were seeded with endothelial cells. Analysis of acellularity was performed by light microscopy and scanning electron microscopy. Cell viability following seeding was assayed by fluorescence staining of viable cells.

RESULTS

The acellularization procedure resulted in an almost complete removal of the original cells while the 3D matrix was loosened at interfibrillar zones. However the 3D arrangement of the matrix fibers was grossly maintained. The porcine matrix could be seeded with in vitro expanded human endothelial cells and was maintained in culture for up to 3 days to document the formation of confluent cultures.

CONCLUSIONS

Porcine aortic valves can be almost completely acellularized by a non-tanning detergent extraction procedure. The xenogenic matrix was reseeded with human endothelial cells. This approach may eventually lead to the engineering of tissue heart valves repopulated with the patients own autologous cells.

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

bEND.3 cells are polyoma middle T-transformed mouse brain endothelial cells that express very little or no thrombospondin-1, a natural inhibitor of angiogenesis, but express high levels of platelet endothelial cell adhesion molecule-1 (PECAM-1) that localizes to sites of cell-cell contact. Here, we have examined the role of PECAM-1 in regulation of bEND.3 cell proliferation, migration, morphogenesis, and hemangioma formation. We show that down-regulating PECAM-1 expression by antisense transfection of bEND. 3 cells has a dramatic effect on their morphology, proliferation, and morphogenesis on Matrigel. There is an optimal level for PECAM-1 expression such that high levels of PECAM-1 inhibit, whereas moderate levels of PECAM-1 stimulate, endothelial cell morphogenesis. The down-regulation of PECAM-1 in bEND.3 cells resulted in reexpression of endogenous thrombospondin-1 and its antiangiogenic receptor CD36. The expression of the vascular endothelial growth factor receptors flk-1 and flt-1, as well as integrins and metalloproteinases (which are involved in angiogenesis), were also affected. These observations are consistent with the changes observed in proliferation, migration, and adhesion characteristics of the antisense-transfected bEND.3 cells as well as with their lack of ability to form hemangiomas in mice. Thus, a reciprocal relationship exists between thrombospondin-1 and PECAM-1 expression, such that these two molecules appear to be constituents of a "switch" that regulates in concert many components of the angiogenic and differentiated phenotypes of endothelial cells.