Characterization of phosphotyrosine binding motifs in the cytoplasmic domain of platelet/endothelial cell adhesion molecule-1 (PECAM-1) that are required for the cellular association and activation of the protein-tyrosine phosphatase, SHP-2

CD31 as a probable responding and gate-keeping protein of the blood-brain barrier and the risk of Alzheimer's disease

Several studies have shown that an abnormal vascular-immunity link could increase Alzheimer's disease (AD) risk; however, the mechanism is unclear. CD31, also named platelet endothelial cell adhesion molecule (PECAM), is a surface membrane protein of both endothelial and immune cells and plays important roles in the interaction between the vascular and immune systems. In this review, we focus on research regarding CD31 biological actions in the pathological process that may contribute to AD based on the following rationales. First, endothelial, leukocyte and soluble forms of CD31 play multi-roles in regulating transendothelial migration, increasing blood-brain barrier (BBB) permeability and resulting in neuroinflammation. Second, CD31 expressed by endothelial and immune cells dynamically modulates numbers of signaling pathways, including Src family kinases, selected G proteins, and β-catenin which in turn affect cell-matrix and cell-cell attachment, activation, permeability, survival, and ultimately neuronal cell injury. In endothelia and immune cells, these diverse CD31-mediated pathways act as a critical regulator in the immunity-endothelia-brain axis, thereby mediating AD pathogenesis in ApoE4 carriers, which is the major genetic risk factor for AD. This evidence suggests a novel mechanism and potential drug target for CD31 in the background of genetic vulnerabilities and peripheral inflammation for AD development and progression.

The Innate Cellular Immune Response in Xenotransplantation

Xenotransplantation is very attractive strategy for addressing the shortage of donors. While hyper acute rejection (HAR) caused by natural antibodies and complement has been well defined, this is not the case for innate cellular xenogeneic rejection. An increasing body of evidence suggests that innate cellular immune responses contribute to xenogeneic rejection. Various molecular incompatibilities between receptors and their ligands across different species typically have an impact on graft outcome. NK cells are activated by direct interaction as well as by antigen dependent cellular cytotoxicity (ADCC) mechanisms. Macrophages are activated through various mechanisms in xenogeneic conditions. Macrophages recognize CD47 as a "marker of self" through binding to SIRPα. A number of studies have shown that incompatibility of porcine CD47 against human SIRPα contributes to the rejection of xenogeneic target cells by macrophages. Neutrophils are an early responder cell that infiltrates xenogeneic grafts. It has also been reported that neutrophil extracellular traps (NETs) activate macrophages as damage-associated pattern molecules (DAMPs). In this review, we summarize recent insights into innate cellular xenogeneic rejection.

Molecular features underlying differential SHP1/SHP2 binding of immune checkpoint receptors

A large number of inhibitory receptors recruit SHP1 and/or SHP2, tandem-SH2-containing phosphatases through phosphotyrosine-based motifs immunoreceptor tyrosine-based inhibitory motif (ITIM) and immunoreceptor tyrosine-based switch motif (ITSM). Despite the similarity, these receptors exhibit differential effector binding specificities, as exemplified by the immune checkpoint receptors PD-1 and BTLA, which preferentially recruit SHP2 and SHP1, respectively. The molecular basis by which structurally similar receptors discriminate SHP1 and SHP2 is unclear. Here, we provide evidence that human PD-1 and BTLA optimally bind to SHP1 and SHP2 via a bivalent, parallel mode that involves both SH2 domains of SHP1 or SHP2. PD-1 mainly uses its ITSM to prefer SHP2 over SHP1 via their C-terminal SH2 domains (cSH2): swapping SHP1-cSH2 with SHP2-cSH2 enabled PD-1:SHP1 association in T cells. In contrast, BTLA primarily utilizes its ITIM to prefer SHP1 over SHP2 via their N-terminal SH2 domains (nSH2). The ITIM of PD-1, however, appeared to be de-emphasized due to a glycine at pY+1 position. Substitution of this glycine with alanine, a residue conserved in BTLA and several SHP1-recruiting receptors, was sufficient to induce PD-1:SHP1 interaction in T cells. Finally, structural simulation and mutagenesis screening showed that SHP1 recruitment activity exhibits a bell-shaped dependence on the molecular volume of the pY+1 residue of ITIM. Collectively, we provide a molecular interpretation of the SHP1/SHP2-binding specificities of PD-1 and BTLA, with implications for the mechanisms of a large family of therapeutically relevant receptors.

The Regulation of Neutrophil Extracellular Trap-induced Tissue Damage by Human CD177

Background

Neutrophil-induced tissue damage contributes to the rejection in xenotransplantation. Therefore, suppressing neutrophil function could be effective in suppressing xenogeneic rejection. In a previous study, we demonstrated that the ectopic expression of human cluster of differentiation 31 (CD31) on porcine endothelial cells (PEC) significantly suppressed neutrophil-mediated cytotoxicity through the homophilic binding of CD31. Cluster of differentiation 177 (CD177) was recently reported to be a high-affinity heterophilic binding partner for CD31 on endothelial cells. Thus, we hypothesized that human CD177 on PEC might induce a stronger suppression in neutrophil-mediated cytotoxicity compared with CD31. In this study, the inhibitory function of human CD177 on PEC in neutrophil-mediated cytotoxicity was investigated.

Methods

PEC were transfected with a cloning plasmid containing cDNA inserts that encoded for hCD177 and hCD31 genes. Neutrophil-induced cytotoxicity was evaluated by flow cytometry after coculturing with PEC or PEC/CD177 in the presence of phorbol 12-myristate 13-acetate. To elucidate the mechanisms responsible for hCD177-induced suppression, the phosphorylation of src homology region 2 domain containing phosphatase 1 was measured by immunoblot analysis.

Results

Human CD177 on PEC induced a significant reduction in neutrophil-induced cytotoxicity. In addition, CD177 on PEC induced a significant increase in the phosphorylation of src homology region 2 domain-containing phosphatase 1 in neutrophils and suppressed NETosis.

Conclusions

These findings suggest that human CD177 suppresses neutrophil-mediated cytotoxicity through the inhibition of NETosis.

Targeting platelet inhibition receptors for novel therapies: PECAM-1 and G6b-B

While current oral antiplatelet therapies benefit many patients, they deregulate the hemostatic balance leaving patients at risk of systemic side-effects such as hemorrhage. Dual antiplatelet treatment is the standard approach, combining aspirin with P2Y₁₂ blockers. These therapies mainly target autocrine activation mechanisms (TxA₂, ADP) and, more recently, the use of thrombin or thrombin receptor antagonists have been added to the available approaches. Recent efforts to develop new classes of anti-platelet drugs have begun to focus on primary platelet activation pathways such as through the immunoreceptor tyrosine-based activation motif (ITAM)-containing collagen receptor GPVI/FcRγ-chain complex. There are already encouraging results from targeting GPVI, with reduced aggregation and smaller arterial thrombi, without major bleeding complications, likely due to overlapping activation signaling pathways with other receptors such as the GPIb-V-IX complex. An alternative approach to reduce platelet activation could be to inhibit this signaling pathway by targeting the inhibitory pathways intrinsic to platelets. Stimulation of endogenous negative modulators could provide more specific inhibition of platelet function, but is this feasible? In this review, we explore the potential of the two major platelet immunoreceptor tyrosine-based inhibitory motif (ITIM)-containing inhibitory receptors, G6b-B and PECAM-1, as antithrombotic targets.

PECAM-1 directed re-targeting of exogenous mRNA providing two orders of magnitude enhancement of vascular delivery and expression in lungs independent of apolipoprotein E-mediated uptake

Systemic administration of lipid nanoparticle (LNP)-encapsulated messenger RNA (mRNA) leads predominantly to hepatic uptake and expression. Here, we conjugated nucleoside-modified mRNA-LNPs with antibodies (Abs) specific to vascular cell adhesion molecule, PECAM-1. Systemic (intravenous) administration of Ab/LNP-mRNAs resulted in profound inhibition of hepatic uptake concomitantly with ~200-fold and 25-fold elevation of mRNA delivery and protein expression in the lungs compared to non-targeted counterparts. Unlike hepatic delivery of LNP-mRNA, Ab/LNP-mRNA is independent of apolipoprotein E. Vascular re-targeting of mRNA represents a promising, powerful, and unique approach for novel experimental and clinical interventions in organs of interest other than liver.

Immobilization of Nonactivated Unfixed Platelets for Real-Time Single-Cell Analysis

Existing methods for measuring the response of individual platelets to stimulation are limited. They either measure each platelet at one discrete time-point (flow cytometry) or rely on adhesive ligands to immobilize platelets that concomitantly generate activation signals (microscopy). Such methods of immobilization make it impossible to assess resting platelets, the changes that occur as platelets transition from resting to active states, or the signals generated by soluble agonists, such as ADP and thrombin, or by mechanical stimulus, independently from those generated by the adhesive ligand. Here we describe a microscopy method that allows the immobilization of platelets to a glass cover slip without triggering platelet activation. This method makes use of specific antibodies that bind platelet PECAM-1 without activating it. Platelets can therefore be immobilized to PECAM-1 antibody coated biochips without causing activation and perfused with agonists or inhibitors. Using this method, platelets can be stimulated by an array of soluble agonists at any concentration or combination, in the presence or absence of inhibitors or shear forces. This chapter describes in detail this PECAM-1 mediated immobilized platelet method and its use for measuring changes in Ca²⁺ signaling in individual platelets under a number of different conditions. While we focus on the measurement of Ca²⁺ dynamics in this chapter, it is important to consider that the basic method we describe will easily lend its self to other measures of platelet activation (integrin activation, shape change, actin dynamics, degranulation), and may, therefore, be used to measure almost any facet of platelet activation.

Involvement of TIMP-1 in PECAM-1-mediated tumor dissemination

Platelet endothelial cell adhesion molecule-1 (PECAM-1) is expressed on the vascular endothelium and has been implicated in the late progression of metastatic tumors. The activity of PECAM-1 appears to be mediated by modulation of the tumor microenvironment (TME) and promotion of tumor cell proliferation, rather than through the stimulation of tumor angiogenesis. The present study aimed to extend those initial findings by indicating that the presence of functional PECAM-1 on the endothelium promotes a proliferative tumor cell phenotype in vivo, as well as in tumor cell (B16-F10 melanoma and 4T1 breast cancer cell lines) co-culture assays with mouse endothelial cells (ECs) or a surrogate EC line (REN-MP). The pro-proliferative effects were mediated by soluble endothelial-derived factors that were dependent on PECAM-1 homophilic ligand interactions, but which were independent of PECAM-1-dependent signaling. Further analysis of the conditioned media obtained from tumor/EC and tumor/REN-MP co-cultures identified TIMP metallopeptidase inhibitor-1 (TIMP-1) as a PECAM-1-regulated factor, the targeting of which in the tumor cell/REN-MP system inhibited tumor cell proliferation. In addition, TIMP-1 expression was decreased in metastatic tumors from the lungs of PECAM-1-null mice, thus providing evidence of the in vivo significance of co-culture studies. Taken together, these studies indicated that endothelial PECAM-1, through PECAM-1-dependent homophilic binding interactions, may induce release of TIMP-1 from the endothelium into the TME, thus leading to increased tumor cell proliferation.

Human CD31 on porcine cells suppress xenogeneic neutrophil-mediated cytotoxicity via the inhibition of NETosis

BACKGROUND

Xenotransplantation is one of the promising strategies for overcoming the shortage of organs available for transplant. However, many immunological obstructions need to be overcome for practical use. Increasing evidence suggests that neutrophils contribute to xenogeneic cellular rejection. Neutrophils are regulated by activation and inhibitory signals to induce appropriate immune reactions and to avoid unnecessary immune reactivity. Therefore, we hypothesized that the development of neutrophil-targeted therapies may have the potential for increased graft survival in xenotransplantation.

METHODS

A plasmid containing a cDNA insert encoding the human CD31 gene was transfected into swine endothelial cells (SEC). HL-60 cells were differentiated into neutrophil-like cells by culturing them in the presence of 1.3% dimethyl sulfoxide for 48 hours. The cytotoxicity of the differentiated HL-60 cells (dHL-60) and peripheral blood-derived neutrophils was evaluated by WST-8 assays. To investigate the mechanism responsible for hCD31-induced immunosuppression, citrullinated histone 3 (cit-H3) and phosphorylation of SHP-1 were detected by a cit-H3 enzyme-linked immunosorbent assay (ELISA) and Western blotting, respectively.

RESULTS

A significant decrease in dHL-60 and neutrophil-mediated cytotoxicity in SEC/hCD31 compared with SEC was seen, as evidenced by a cytotoxicity assay. Furthermore, the suppression of NETosis and the induction of SHP-1 phosphorylation in neutrophils that had been co-cultured with SEC/CD31 were confirmed by cit-H3 ELISA and Western blotting with an anti-phosphorylated SHP-1.

CONCLUSION

These data suggest that human CD31 suppresses neutrophil-mediated xenogenic cytotoxicity via the inhibition of NETosis. As CD31 is widely expressed in a variety of inflammatory cells, human CD31-induced suppression may cover the entire xenogeneic cellular rejection, thus making the generation of human CD31 transgenic pigs very attractive for use in xenografts.

Molecular mechanisms for enhancement of stromal cell-derived factor 1-induced chemotaxis by platelet endothelial cell adhesion molecule 1 (PECAM-1)

Platelet endothelial cell adhesion molecule 1 (PECAM-1) is a cell adhesion protein involved in the regulation of cell adhesion and migration. Interestingly, several PECAM-1-deficient hematopoietic cells exhibit impaired chemotactic responses to stromal cell-derived factor 1 (SDF-1), a chemokine essential for B lymphopoiesis and bone marrow myelopoiesis. However, whether PECAM-1 is involved in SDF-1-regulated chemotaxis is unknown. We report here that SDF-1 induces tyrosine phosphorylation of PECAM-1 at its immunoreceptor tyrosine-based inhibition motifs in several hematopoietic cell lines via the Src family kinase Lyn, Bruton's tyrosine kinase, and JAK2 and that inhibition of these kinases reduced chemotaxis. Overexpression and knockdown of PECAM-1 enhanced and down-regulated, respectively, SDF-1-induced Gα_i-dependent activation of the PI3K/Akt/mTORC1 pathway and small GTPase Rap1 in hematopoietic 32Dcl3 cells, and these changes in activation correlated with chemotaxis. Furthermore, pharmacological or genetic inhibition of the PI3K/Akt/mTORC1 pathway or Rap1, respectively, revealed that these pathways are independently activated and required for SDF-1-induced chemotaxis. When coexpressed in 293T cells, PECAM-1 physically associated with the SDF-1 receptor CXCR4. Moreover, PECAM-1 overexpression and knockdown reduced and enhanced SDF-1-induced endocytosis of CXCR4, respectively. Furthermore, when expressed in 32Dcl3 cells, an endocytosis-defective CXCR4 mutant, CXCR4-S324A/S325A, could activate the PI3K/Akt/mTORC1 pathway as well as Rap1 and induce chemotaxis in a manner similar to PECAM-1 overexpression. These findings suggest that PECAM-1 enhances SDF-1-induced chemotaxis by augmenting and prolonging activation of the PI3K/Akt/mTORC1 pathway and Rap1 and that PECAM-1, at least partly, exerts its activity by inhibiting SDF-1-induced internalization of CXCR4.

Platelets modulate endothelial cell response to dynamic shear stress through PECAM-1

INTRODUCTION

Both vascular endothelial cells and platelets are sensitive to blood flow induced shear stress. We have recently reported that platelet-endothelial cell interaction could greatly affect platelet activation under flow. In the present study, we aimed to investigate how platelet-endothelial cell interaction affected endothelial cell inflammatory responses under flow.

MATERIALS AND METHODS

Human coronary artery endothelial cells were exposed to normal or low pulsatile shear stress with or without the presence of platelets. Following shear exposure, endothelial cell ICAM-1 expression was measured using ELISA, Western blot and PCR; cell surface PECAM-1 expression/phosphorylation was measured using ELISA. Platelet adhesion to endothelial cells was quantified using immunofluorescence microscopy. To determine the role of PECAM-1 in platelet-endothelial cell interaction, endothelial cell PECAM-1 expression was suppressed using siRNA.

RESULTS

Pathological low shear stress induced a significant increase in endothelial cell ICAM-1 expression, at both protein and mRNA levels. Platelet adhesion to endothelial cells increased significantly under low shear stress, co-localizing with PECAM-1 at endothelial cell junctions. The presence of platelets inhibited low shear stress-induced ICAM-1 upregulation. When endothelial cell PECAM-1 expression was suppressed, platelet adhesion to endothelial cells under low shear stress decreased significantly; endothelial cell ICAM-1 expression was not affected by shear stress, with or without platelets.

CONCLUSIONS

These results suggested that PECAM-1 could mediate platelet adhesion to endothelial cells under shear stress. Platelets binding to endothelial cells interfered with endothelial cell mechanotransduction through PECAM-1, affecting endothelial cell inflammatory responses towards pathological shear flow.

Hypobaric hypoxia down-regulated junctional protein complex: Implications to vascular leakage

Acute mountain sickness (AMS) can cause capillary hyper-permeability and vasogenic edema. However, its underlying mechanisms remained unclear and there is no previous in vitro study on AMS. We therefore conducted an in vitro study and examined whether continuous hypobaric hypoxia (CHH) could alter expression of junctional protein complex of vascular endothelial cells, causing hyper-permeabilization. EA.hy926 human endothelial cells were exposed to either CHH or normoxia for up to 24 h. Flow cytometry using annexin V/propidium iodide co-staining demonstrated that cell death had no significant difference at 12-h, but was increased by CHH at 24-h. Transendothelial resistance (TER) of endothelial cell monolayer was progressively decreased by CHH from 1-h to 24-h. Western blot analysis and immunofluorescence study demonstrated decreased expression levels of VE-cadherin, PECAM-1 and ZO-1 junctional proteins at both 12-h and 24-h exposure time-points. Interestingly, while the main form of ZO-1 (220 kDa) was decreased, its degraded form (100 kDa) was increased by 24-h CHH that might be linked to the increased cell death. Our data have demonstrated that CHH caused vascular endothelial hyper-permeability and defective junctional protein complex by reducing expression levels of VE-cadherin, PECAM-1, and ZO-1. Taken together, these data may explain pathophysiology underlying vascular hyper-permeability in AMS.

CD31 signals confer immune privilege to the vascular endothelium

Constitutive resistance to cell death induced by inflammatory stimuli activating the extrinsic pathway of apoptosis is a key feature of vascular endothelial cells (ECs). Although this property is central to the maintenance of the endothelial barrier during inflammation, the molecular mechanisms of EC protection from cell-extrinsic, proapoptotic stimuli have not been investigated. We show that the Ig-family member CD31, which is expressed by endothelial but not epithelial cells, is necessary to prevent EC death induced by TNF-α and cytotoxic T lymphocytes in vitro. Combined quantitative RT-PCR array and biochemical analysis show that, upon the engagement of the TNF receptor with TNF-α on ECs, CD31 becomes activated and, in turn, counteracts the proapoptotic transcriptional program induced by TNF-α via activation of the Erk/Akt pathway. Specifically, Akt activation by CD31 signals prevents the localization of the forkhead transcription factor FoxO3 to the nucleus, thus inhibiting transcription of the proapoptotic genes CD95/Fas and caspase 7 and de-repressing the expression of the antiapoptotic gene cFlar. Both CD31 intracellular immunoreceptor tyrosine-based inhibition motifs are required for its prosurvival function. In vivo, CD31 gene transfer is sufficient to recapitulate the cytoprotective mechanisms in CD31(-) pancreatic β cells, which become resistant to immune-mediated rejection when grafted in fully allogeneic recipients.

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.

Flow shear stress differentially regulates endothelial uptake of nanocarriers targeted to distinct epitopes of PECAM-1

Targeting nanocarriers (NC) to endothelial cell adhesion molecules including Platelet-Endothelial Cell Adhesion Molecule-1 (PECAM-1 or CD31) improves drug delivery and pharmacotherapy of inflammation, oxidative stress, thrombosis and ischemia in animal models. Recent studies unveiled that hydrodynamic conditions modulate endothelial endocytosis of NC targeted to PECAM-1, but the specificity and mechanism of effects of flow remain unknown. Here we studied the effect of flow on endocytosis by human endothelial cells of NC targeted by monoclonal antibodies Ab62 and Ab37 to distinct epitopes on the distal extracellular domain of PECAM. Flow in the range of 1-8dyn/cm(2), typical for venous vasculature, stimulated the uptake of spherical Ab/NC (~180nm diameter) carrying ~50 vs 200 Ab62 and Ab37 per NC, respectively. Effect of flow was inhibited by disruption of cholesterol-rich plasmalemma domains and deletion of PECAM-1 cytosolic tail. Flow stimulated endocytosis of Ab62/NC and Ab37/NC via eliciting distinct signaling pathways mediated by RhoA/ROCK and Src Family Kinases, respectively. Therefore, flow stimulates endothelial endocytosis of Ab/NC in a PECAM-1 epitope specific manner. Using ligands of binding to distinct epitopes on the same target molecule may enable fine-tuning of intracellular delivery based on the hemodynamic conditions in the vascular area of interest.

Platelet endothelial cell adhesion molecule-1 inhibits platelet response to thrombin and von Willebrand factor by regulating the internalization of glycoprotein Ib via AKT/glycogen synthase kinase-3/dynamin and integrin αIIbβ3

OBJECTIVE

Platelet endothelial cell adhesion molecule-1 (PECAM-1) regulates platelet response to multiple agonists. How this immunoreceptor tyrosine-based inhibitory motif-containing receptor inhibits G protein-coupled receptor-mediated thrombin-induced activation of platelets is unknown.

APPROACH AND RESULTS

Here, we show that the activation of PECAM-1 inhibits fibrinogen binding to integrin αIIbβ3 and P-selectin surface expression in response to thrombin (0.1-3 U/mL) but not thrombin receptor-activating peptides SFLLRN (3×10(-7)-1×10(-5) mol/L) and GYPGQV (3×10(-6)-1×10(-4) mol/L). We hypothesized a role for PECAM-1 in reducing the tethering of thrombin to glycoprotein Ibα (GPIbα) on the platelet surface. We show that PECAM-1 signaling regulates the binding of fluorescein isothiocyanate-labeled thrombin to the platelet surface and reduces the levels of cell surface GPIbα by promoting its internalization, while concomitantly reducing the binding of platelets to von Willebrand factor under flow in vitro. PECAM-1-mediated internalization of GPIbα was reduced in the presence of both EGTA and cytochalasin D or latrunculin, but not either individually, and was reduced in mice in which tyrosines 747 and 759 of the cytoplasmic tail of β3 integrin were mutated to phenylalanine. Furthermore, PECAM-1 cross-linking led to a significant reduction in the phosphorylation of glycogen synthase kinase-3β Ser(9), but interestingly an increase in glycogen synthase kinase-3α pSer(21). PECAM-1-mediated internalization of GPIbα was reduced by inhibitors of dynamin (Dynasore) and glycogen synthase kinase-3 (CHIR99021), an effect that was enhanced in the presence of EGTA.

CONCLUSIONS

PECAM-1 mediates internalization of GPIbα in platelets through dual AKT/protein kinase B/glycogen synthase kinase-3/dynamin-dependent and αIIbβ3-dependent mechanisms. These findings expand our understanding of how PECAM-1 regulates nonimmunoreceptor signaling pathways and helps to explains how PECAM-1 regulates thrombosis.

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.

Junctional adhesion molecule-A suppresses platelet integrin αIIbβ3 signaling by recruiting Csk to the integrin-c-Src complex

Fibrinogen binding to activated integrin induces outside-in signaling that results in stable platelet aggregates and clot retraction. How integrin αIIbβ3 is discouraged from spontaneous activation is not known. We have recently shown that junctional adhesion molecule-A (JAM-A) renders protection from thrombosis by suppressing integrin outside-in signaling. In this study, we show that JAM-A associates with integrin αIIbβ3 in resting platelets and dissociates upon platelet activation by agonists. We also show that integrin-associated JAM-A is tyrosine phosphorylated and is rapidly dephosphorylated upon platelet activation. C-terminal Src kinase (Csk) binds to tyrosine phosphorylated JAM-A through its Src homology 2 domain. Thus, JAM-A recruits Csk to the integrin-c-Src complex in resting platelets. Csk, in turn, keeps integrin-associated c-Src in an inactive state by phosphorylating Y(529) in its regulatory domain. Absence of JAM-A results in impaired c-SrcY(529) phosphorylation and augmentation of outside-in signaling-dependent c-Src activation. Our results strongly suggest that tyrosine-phosphorylated JAM-A is a Csk-binding protein and functions as an endogenous inhibitor of integrin signaling. JAM-A recruits Csk to the integrin-c-Src complex, where Csk negatively regulates c-Src activation, thereby suppressing the initiation of outside-in signaling. Upon agonist stimulation, JAM-A is dephosphorylated on the tyrosine, allowing the dissociation of Csk from the integrin complex, and thus facilitating outside-in signaling.

Protein-tyrosine phosphatases: a new frontier in platelet signal transduction

Platelet activation must be tightly controlled in order to allow platelets to respond rapidly to vascular injury and prevent thrombosis from occurring. Protein-tyrosine phosphorylation is one of the main ways in which activation signals are transmitted in platelets. Although much is known about the protein-tyrosine kinases (PTKs) that initiate and propagate activation signals, relatively little is known about the protein-tyrosine phosphatases (PTPs) that modulate these signals in platelets. PTPs are a family of enzymes that dephosphorylate tyrosine residues in proteins and regulate signals transmitted within cells. PTPs have been implicated in a variety of pathological conditions, including cancer, diabetes and autoimmunity, but their functions in hemostasis and thrombosis remain largely undefined. Exciting new findings from a number of groups have revealed that PTPs are in fact critical regulators of platelet activation and thrombosis. The primary aim of this review is to highlight the unique and important functions of PTPs in regulating platelet activity. Establishing the functions of PTPs in platelets is essential to better understand the molecular basis of thrombosis and may lead to the development of improved antithrombotic therapies.

Harnessing the platelet signaling network to produce an optimal hemostatic response

Once released into the circulation by megakaryocytes, circulating platelets can undergo rapid activation at sites of vascular injury and resist unwarranted activation, which can lead to heart attacks and strokes. Historically, the signaling mechanisms underlying the regulation of platelet activation have been approached as a collection of individual pathways unique to agonist. This review takes a different approach, casting platelet activation as the product of a signaling network, in which activating and restraining mechanisms interact in a flexible network that regulates platelet adhesiveness, cohesion between platelets, granule secretion, and the formation of a stable hemostatic thrombus.

Immunoreceptor tyrosine-based inhibitory motif (ITIM)-mediated inhibitory signaling is regulated by sequential phosphorylation mediated by distinct nonreceptor tyrosine kinases: a case study involving PECAM-1

The activation state of many blood and vascular cells is tightly controlled by a delicate balance between receptors that contain immunoreceptor tyrosine-based activation motifs (ITAMs) and those that contain immunoreceptor tyrosine-based inhibitory motifs (ITIMs). Precisely how the timing of cellular activation by ITAM-coupled receptors is regulated by ITIM-containing receptors is, however, poorly understood. Using platelet endothelial cell adhesion molecule 1 (PECAM-1) as a prototypical ITIM-bearing receptor, we demonstrate that initiation of inhibitory signaling occurs via a novel, sequential process in which Src family kinases phosphorylate the C-terminal ITIM, thereby enabling phosphorylation of the N-terminal ITIM of PECAM-1 by other Src homology 2 domain-containing nonreceptor tyrosine kinases (NRTKs). NRTKs capable of mediating the second phosphorylation event include C-terminal Src kinase (Csk) and Bruton's tyrosine kinase (Btk). Btk and Csk function downstream of phosphatidylinositol 3-kinase (PI3K) activation during ITAM-dependent platelet activation. In ITAM-activated platelets that were treated with a PI3K inhibitor, PECAM-1 was phosphorylated but did not bind the tandem SH2 domain-containing tyrosine phosphatase SHP-2, indicating that it was not phosphorylated on its N-terminal ITIM. Csk bound to and phosphorylated PECAM-1 more efficiently than did Btk and required its SH2 domain to perform these functions. Additionally, the phosphorylation of the N-terminal ITIM of Siglec-9 by Csk is enhanced by the prior phosphorylation of its C-terminal ITIM, providing evidence that the ITIMs of other dual ITIM-containing receptors are also sequentially phosphorylated. On the basis of these findings, we propose that sequential ITIM phosphorylation provides a general mechanism for precise temporal control over the recruitment and activation of tandem SH2 domain-containing tyrosine phosphatases that dampen ITAM-dependent signals.

PECAM-1 is involved in BCR/ABL signaling and may downregulate imatinib-induced apoptosis of Philadelphia chromosome-positive leukemia cells

PECAM-1 (CD31) is an immunoreceptor tyrosine-based inhibitory motif (ITIM)-containing surface glycoprotein expressed on various hematopoietic cells as well as on endothelial cells. PECAM-1 has been shown to play roles in regulation of adhesion, migration and apoptosis. The BCR/ABL fusion tyrosine kinase is expressed in chronic myeloid leukemia and Philadelphia-positive (Ph⁺) acute lymphoblastic leukemia cells, and its inhibition by the clinically used tyrosine kinase inhibitors imatinib or dasatinib induces apoptosis of these cells. In the present study, we demonstrate that PECAM-1 is tyrosine phospho rylated in its ITIM motifs in various BCR/ABL-expressing cells including primary leukemia cells. Studies using imatinib and dasatinib as well as transient expression experiments in 293T cells revealed that PECAM-1 was phosphorylated directly by BCR/ABL, which was enhanced by the imatinib-resistant E255K and T315I mutations, or partly by the Src family tyrosine kinases, including Lyn, which were activated dependently or independently on BCR/ABL. We also demonstrate by using a substrate trapping mutant of SHP2 that tyrosine phosphorylated PECAM-1 binds SHP2 and is a major substrate for this tyrosine phosphatase in BCR/ABL-expressing cells. Overexpression of PECAM-1 in BCR/ABL-expressing cells, including K562 human leukemia cells, enhanced cell adhesion and partially inhibited imatinib-induced apoptosis involving mitochondria depolarization and caspase-3 cleavage, at least partly, in an ITIM-independent manner. These data suggest that PECAM-1 may play a role in regulation of apoptosis as well as adhesion of BCR/ABL-expressing cells to modulate their imatinib sensitivity and would be a possible candidate for therapeutic target in Ph⁺ leukemias.

An investigation of hierachical protein recruitment to the inhibitory platelet receptor, G6B-b

Platelet activation is regulated by both positive and negative signals. G6B-b is an inhibitory platelet receptor with an immunoreceptor tyrosine-based inhibitory motif (ITIM) and an immunoreceptor tyrosine-based switch motif (ITSM). The molecular basis of inhibition by G6B-b is currently unknown but thought to involve the SH2 domain-containing tyrosine phosphatase SHP-1. Here we show that G6B-b also associates with SHP-2, as well as SHP-1, in human platelets. Using a number of biochemical approaches, we found these interactions to be direct and that the tandem SH2 domains of SHP-2 demonstrated a binding affinity for G6B-b 100-fold higher than that of SHP-1. It was also observed that while SHP-1 has an absolute requirement for phosphorylation at both motifs to bind, SHP-2 can associate with G6B-b when only one motif is phosphorylated, with the N-terminal SH2 domain and the ITIM being most important for the interaction. A number of other previously unreported SH2 domain-containing proteins, including Syk and PLCγ2, also demonstrated specificity for G6B-b phosphomotifs and may serve to explain the observation that G6B-b remains inhibitory in the absence of both SHP-1 and SHP-2. In addition, the presence of dual phosphorylated G6B-b in washed human platelets can reduce the EC(50) for both CRP and collagen.

CCL2 disrupts the adherens junction: implications for neuroinflammation

Alterations to blood-brain barrier (BBB) adhesion molecules and junctional integrity during neuroinflammation can promote central nervous system (CNS) pathology. The chemokine CCL2 is elevated during CNS inflammation and is associated with endothelial dysfunction. The effects of CCL2 on endothelial adherens junctions (AJs) have not been defined. We demonstrate that CCL2 transiently induces Src-dependent disruption of human brain microvascular endothelial AJ. β-Catenin is phosphorylated and traffics from the AJ to PECAM-1 (platelet endothelial cell adhesion molecule-1), where it is sequestered at the membrane. PECAM-1 is also tyrosine-phosphorylated, an event associated with recruitment of the phosphatase SHP-2 (Src homology 2 domain-containing protein phosphatase) to PECAM-1, β-catenin release from PECAM-1, and reassociation of β-catenin with the AJ. Surface localization of PECAM-1 is increased in response to CCL2. This may enable the endothelium to sustain CCL2-induced alterations in AJ and facilitate recruitment of leukocytes into the CNS. Our novel findings provide a mechanism for CCL2-mediated disruption of endothelial junctions that may contribute to BBB dysfunction and increased leukocyte recruitment in neuroinflammatory diseases.

Endogenous inhibitory mechanisms and the regulation of platelet function

The response of platelets to changes in the immediate environment is always a balance between activatory and inhibitory signals, the cumulative effect of which is either activation or quiescence. This is true of platelets in free flowing blood and of their regulation of haemostasis and thrombosis. In this review, we consider the endogenous inhibitory mechanisms that combine to regulate platelet activation. These include those derived from the endothelium (nitric oxide, prostacyclin, CD39), inhibitory receptors on the surface of platelets (platelet endothelial cell adhesion molecule-1, carcinoembryonic antigen cell adhesion molecule 1, G6b-B - including evidence for the role of Ig-ITIM superfamily members in the negative regulation of ITAM-associated GPVI platelet-collagen interactions and GPCR-mediated signalling and in positive regulation of "outside-in" integrin α(IIb)β(3)-mediated signalling), intracellular inhibitory receptors (retinoic X receptor, glucocorticoid receptor, peroxisome proliferator-activated receptors, liver X receptor), and emerging inhibitory pathways (canonical Wnt signalling, Semaphorin 3A, endothelial cell specific adhesion molecule, and junctional adhesion molecule-A).

Platelet signaling

This chapter summarizes current ideas about the intracellular signaling that drives platelet responses to vascular injury. After a brief overview of platelet activation intended to place the signaling pathways into context, the first section considers the early events of platelet activation leading up to integrin activation and platelet aggregation. The focus is on the G protein-mediated events utilized by agonists such as thrombin and ADP, and the tyrosine kinase-based signaling triggered by collagen. The second section considers the events that occur after integrin engagement, some of which are dependent on close physical contact between platelets. A third section addresses the regulatory events that help to avoid unprovoked or excessive platelet activation, after which the final section briefly considers individual variations in platelet reactivity and the role of platelet signaling in the innate immune response and embryonic development.

Regulation of platelet biology by platelet endothelial cell adhesion molecule-1

Platelet endothelial cell adhesion molecule-1 (PECAM-1), an immunoreceptor tyrosine-based inhibitory motif containing receptor, plays diverse and apparently contradictory roles in regulating the response of platelets to stimuli; inhibiting platelet response to immunoreceptor tyrosine-based activation motif and G protein-coupled receptor signalling following stimulation with collagen, adenosine diphosphate, and thrombin, as well as enhancing integrin outside-in signalling. These dual, and opposing, roles suggest an important and complex role for PECAM-1 in orchestrating platelet response to vascular damage. Indeed, during thrombus formation, the influence of PECAM-1 on the multiple signalling pathways combines leading to a relatively large inhibitory effect on thrombus formation.

Syk protein tyrosine kinase involves PECAM-1 signaling through tandem immunotyrosine inhibitory motifs in human THP-1 macrophages

Although recent evidence supports a functional relationship between platelet endothelial cell adhesion molecule (PECAM-1) and Syk tyrosine kinase, little is known about the interaction of Syk with PECAM-1. We report that down-regulation of Syk inhibits the spreading of human THP-1 macrophage cells. Moreover, our data indicate that Syk binds PECAM-1 through its immune tyrosine-based inhibitory motif (ITIM), and dual phosphorylation of the ITIM domain of PECAM-1 leads to activation of Syk. Our results indicate that the distance between the phosphotyrosines could be up to 22 amino acids in length, depending on the conformational flexibility, and that the dual ITIM tyrosine motifs of PECAM-1 facilitate immunoreceptor tyrosine-based activation motif-like signaling. The preferential binding of PECAM-1 to Src homology region 2 domain-containing phosphatase-2 or Syk may depend on their relative affinities, and could provide a mechanism by which signal transduction from PECAM-1 is internally regulated by both positive and negative signaling enzymes.

CD31 is required on CD4+ T cells to promote T cell survival during Salmonella infection

Hematopoietic cells constitutively express CD31/PECAM1, a signaling adhesion receptor associated with controlling responses to inflammatory stimuli. Although expressed on CD4(+) T cells, its function on these cells is unclear. To address this, we have used a model of systemic Salmonella infection that induces high levels of T cell activation and depends on CD4(+) T cells for resolution. Infection of CD31-deficient (CD31KO) mice demonstrates that these mice fail to control infection effectively. During infection, CD31KO mice have diminished numbers of total CD4(+) T cells and IFN-γ-secreting Th1 cells. This is despite a higher proportion of CD31KO CD4(+) T cells exhibiting an activated phenotype and an undiminished capacity to prime normally and polarize to Th1. Reduced numbers of T cells reflected the increased propensity of naive and activated CD31KO T cells to undergo apoptosis postinfection compared with wild-type T cells. Using adoptive transfer experiments, we show that loss of CD31 on CD4(+) T cells alone is sufficient to account for the defective CD31KO T cell accumulation. These data are consistent with CD31 helping to control T cell activation, because in its absence, T cells have a greater propensity to become activated, resulting in increased susceptibility to become apoptotic. The impact of CD31 loss on T cell homeostasis becomes most pronounced during severe, inflammatory, and immunological stresses such as those caused by systemic Salmonella infection. This identifies a novel role for CD31 in regulating CD4 T cell homeostasis.

Regulating thrombus growth and stability to achieve an optimal response to injury

An optimal platelet response to injury can be defined as one in which blood loss is restrained and haemostasis is achieved without the penalty of further tissue damage caused by unwarranted vascular occlusion. This brief review considers some of the ways in which thrombus growth and stability can be regulated so that an optimal platelet response can be achieved in vivo. Three related topics are considered. The first focuses on intracellular mechanisms that regulate the early events of platelet activation downstream of G protein coupled receptors for agonists such as thrombin, thromboxane A(2) and ADP. The second considers the ways in which signalling events that are dependent on stable contacts between platelets can influence the state of platelet activation and thus affect thrombus growth and stability. The third focuses on the changes that are experienced by platelets as they move from their normal environment in freely-flowing plasma to a very different environment within the growing haemostatic plug, an environment in which the narrowing gaps and junctions between platelets not only facilitate communication, but also increasingly limit both the penetration of plasma and the exodus of platelet-derived bioactive molecules.

Residues within a lipid-associated segment of the PECAM-1 cytoplasmic domain are susceptible to inducible, sequential phosphorylation

Immunoreceptor tyrosine-based inhibitory motif (ITIM)-containing receptors inhibit cellular responsiveness to immunoreceptor tyrosine-based activation motif (ITAM)-linked receptors. Although tyrosine phosphorylation is central to the initiation of both inhibitory ITIM and stimulatory ITAM signaling, the events that regulate receptor phosphorylation are incompletely understood. Previous studies have shown that ITAM tyrosines engage in structure-inducing interactions with the plasma membrane that must be relieved for phosphorylation to occur. Whether ITIM phosphorylation is similarly regulated and the mechanisms responsible for release from plasma membrane interactions to enable phosphorylation, however, have not been defined. PECAM-1 is a dual ITIM-containing receptor that inhibits ITAM-dependent responses in hematopoietic cells. We found that the PECAM-1 cytoplasmic domain is unstructured in an aqueous environment but adopts an α-helical conformation within a localized region on interaction with lipid vesicles that mimic the plasma membrane. The lipid-interacting segment contains the C-terminal ITIM tyrosine and a serine residue that undergo activation-dependent phosphorylation. The N-terminal ITIM is excluded from the lipid-interacting segment, and its phosphorylation is secondary to phosphorylation of the membrane-interacting C-terminal ITIM. On the basis of these findings, we propose a novel model for regulation of inhibitory signaling by ITIM-containing receptors that relies on reversible plasma membrane interactions and sequential ITIM phosphorylation.

Modulators of endothelial cell filopodia: PECAM-1 joins the club

Filopodia are an important feature of actively motile cells, probing the pericellular environment for chemotactic factors and other molecular cues that enable and direct the movement of the cell. They also act as points of attachment to the extracellular matrix for the cell, generating tension that may act to pull the cell forward and/or stabilize the cell as it moves. Endothelial cell motility is a critical aspect of angiogenesis, but only a limited number of molecules have been identified as specific regulators of endothelial cell filopodia. Recent reports, however, provide evidence for the involvement of PECAM-1, an endothelial cell adhesion and signaling molecule, in the formation of endothelial cell filopodia. This commentary will focus on these studies and their suggestion that at least two PECAM-1-regulated pathways are involved in the processes that enable filopodial protrusions by endothelial cells. Developing a more complete understanding of the role of PECAM-1 in mediating various endothelial cell activities, such as the extension of filopodia, will be essential for exploiting the therapeutic potential of targeting PECAM-1.

Platelet endothelial cell adhesion molecule-1 regulates collagen-stimulated platelet function by modulating the association of phosphatidylinositol 3-kinase with Grb-2-associated binding protein-1 and linker for activation of T cells

BACKGROUND

Platelet activation by collagen depends on signals transduced by the glycoprotein (GP)VI-Fc receptor (FcR)γ-chain collagen receptor complex, which involves recruitment of phosphatidylinositol 3-kinase (PI3K) to phosphorylated tyrosines in the linker for activation of T cells (LAT). An interaction between the p85 regulatory subunit of PI3K and the scaffolding molecule Grb-2-associated binding protein-1 (Gab1), which is regulated by binding of the Src homology 2 domain-containing protein tyrosine phosphatase-2 (SHP-2) to Gab1, has been shown in other cell types to sustain PI3K activity to elicit cellular responses. Platelet endothelial cell adhesion molecule-1 (PECAM-1) functions as a negative regulator of platelet reactivity and thrombosis, at least in part by inhibiting GPVI-FcRγ-chain signaling via recruitment of SHP-2 to phosphorylated immunoreceptor tyrosine-based inhibitory motifs in PECAM-1.

OBJECTIVE

To investigate the possibility that PECAM-1 regulates the formation of the Gab1-p85 signaling complexes, and the potential effect of such interactions on GPVI-mediated platelet activation in platelets.

METHODS

The ability of PECAM-1 signaling to modulate the LAT signalosome was investigated with immunoblotting assays on human platelets and knockout mouse platelets.

RESULTS

PECAM-1-associated SHP-2 in collagen-stimulated platelets binds to p85, which results in diminished levels of association with both Gab1 and LAT and reduced collagen-stimulated PI3K signaling. We therefore propose that PECAM-1-mediated inhibition of GPVI-dependent platelet responses result, at least in part, from recruitment of SHP-2-p85 complexes to tyrosine-phosphorylated PECAM-1, which diminishes the association of PI3K with activatory signaling molecules, such as Gab1 and LAT.

Ig gene-like molecule CD31 plays a nonredundant role in the regulation of T-cell immunity and tolerance

CD31 is an Ig-like molecule expressed by leukocytes and endothelial cells with an established role in the regulation of leukocyte trafficking. Despite genetic deletion of CD31 being associated with exacerbation of T cell-mediated autoimmunity, the contribution of this molecule to T-cell responses is largely unknown. Here we report that tumor and allograft rejection are significantly enhanced in CD31-deficient mice, which are also resistant to tolerance induction. We propose that these effects are dependent on an as yet unrecognized role for CD31-mediated homophilic interactions between T cells and antigen-presenting cells (APCs) during priming. We show that loss of CD31 interactions leads to enhanced primary clonal expansion, increased killing capacity, and diminished regulatory functions by T cells. Immunomodulation by CD31 signals correlates with a partial inhibition of proximal T-cell receptor (TCR) signaling, specifically Zap-70 phosphorylation. However, CD31-deficient mice do not develop autoimmunity due to increased T-cell death following activation, and we show that CD31 triggering induces Erk-mediated prosurvival activity in T cells either in conjunction with TCR signaling or autonomously. We conclude that CD31 functions as a nonredundant comodulator of T-cell responses, which specializes in sizing the ensuing immune response by setting the threshold for T-cell activation and tolerance, while preventing memory T-cell death.

Phosphorylation of leukocyte PECAM and its association with detergent-resistant membranes regulate transendothelial migration

Leukocyte migration across the endothelial lining is a critical step in the body's response to infection and inflammation. The homophilic interaction between endothelial PECAM and leukocyte PECAM is essential for this process. The molecular events that are triggered in the endothelial cell by PECAM engagement have been well characterized; however, the function of leukocyte PECAM remains to be elucidated. To study this, we first blocked leukocyte transmigration using anti-PECAM Ab and then specifically activated leukocyte PECAM. This was sufficient to overcome the block and promote transmigration, suggesting an active signaling role for leukocyte PECAM. Consistent with this, we found that ligation of leukocyte PECAM induces phosphorylation of two tyrosine residues on its cytoplasmic tail. By performing RNA interference-rescue experiments, we demonstrate that these phosphorylation events are indispensable for transendothelial migration. Finally, we show that leukocyte PECAM translocates to a detergent-resistant membrane (DRM) during transmigration. PECAM localized in DRMs displays reduced phosphorylation and does not support transmigration. Together, these data support a model whereby engagement of leukocyte PECAM induces its transient tyrosine phosphorylation and induction of downstream signals that drive transmigration. These signals are then downregulated following PECAM translocation to DRMs.

PECAM-1 expression and activity negatively regulate multiple platelet signaling pathways

Platelet endothelial cell adhesion molecule-1 (PECAM-1) inhibits platelet response to collagen and may also inhibit two other major platelet agonists ADP and thrombin although this has been less well explored. We hypothesized that the combined effect of inhibiting these three platelet activating pathways may act to significantly inhibit thrombus formation. We demonstrate a negative relationship between PECAM-1 surface expression and platelet response to cross-linked collagen related peptide (CRP-XL) and ADP, and an inhibitory effect of PECAM-1 clustering on platelet response to CRP-XL, ADP and thrombin. This combined inhibition of multiple signaling pathways results in a marked reduction in thrombus formation.

Mechanisms of PECAM-1-mediated cytoprotection and implications for cancer cell survival

Defects in apoptotic pathways can promote cancer development and cause cancers to become resistant to chemotherapy. The cell adhesion and signaling molecule PECAM-1 has been shown to potently suppress apoptosis in a variety of cellular systems. PECAM-1 expression has been reported on a variety of human malignancies-especially hematopoietic and vascular cell cancers-but the significance of this expression has not been fully explored. The ability of PECAM-1 to inhibit apoptosis makes it an attractive candidate as a molecule that may promote cancer development and/or confer resistance to chemotherapeutic treatment. The exact mechanisms by which PECAM-1 mediates its cytoprotection have not been fully defined, but its anti-apoptotic effects have been shown to require both homophilic binding and intracellular signaling via its immunoreceptor tyrosine-based inhibitory motif (ITIM) domains. In this review, we will discuss the data regarding PECAM-1's anti-apoptotic effects and ways in which this cytoprotection may be clinically relevant to the development and/or treatment of hematologic malignancies that express this vascular cell-specific surface molecule.

Role of Ceacam1 in VEGF induced vasculogenesis of murine embryonic stem cell-derived embryoid bodies in 3D culture

CEACAM1 (carcinoembryonic antigen-related cell adhesion molecule 1), a type I transmembrane glycoprotein involved in cell-cell adhesion has been shown to act as an angiogenic factor for mouse and human endothelial cells. Based on the ability of CEACAM1 to initiate lumen formation in human mammary epithelial cells grown in 3D culture (Matrigel), we hypothesized that murine CEACAM1 may play a similar role in vasculogenesis. In order to test this hypothesis, murine embryonic stem (ES) cells stimulated with VEGF were differentiated into embryoid bodies (EB) for 8 days (-8-0 d) and transferred to Matrigel in the presence or absence of anti-CEACAM1 antibody for an additional 12 days (0-12 d). In the absence of anti-CEACAM1 antibody or in the presence of an isotype control antibody, the EB in Matrigel underwent extensive sprouting, generating lengthy vascular structures with well-defined lumina as demonstrated by confocal microscopy, electron microscopy, and immunohistochemical analysis. Both the length and architecture of the vascular tubes were inhibited by anti-CEACAM1 mAb CC1, a mAb that blocks the cell-cell adhesion functions of CEACAM1, thus demonstrating a critical role for this cell-cell adhesion molecule in generating and maintaining vasculogenesis. QRT-PCR analysis of the VEGF treated ES cells grown under conditions that convert them to EB revealed expression of Ceacam1 as early as -5 to -3 d reaching a maximum at day 0 at which time EBs were transferred to Matrigel, thereafter levels at first declined and then increased over time. Other markers of vasculogenesis including Pecam1, VE-Cad, and Tie-1 were not detected until day 0 when EBs were transferred to Matrigel followed by a steady increase in levels, indicating later roles in vasculogenesis. In contrast, Tie-2 and Flk-1 (VEGFR2) were detected on day five of EB formation reaching a maximum at day 0 on transfer to Matrigel, similar to Ceacam1, but after which Tie-2 declined over time, while Flk-1 increased over time. QRT-PCR analysis of the anti-CEACAM1 treated ES cells revealed a significant decrease in the expression of Ceacam1, Pecam1, Tie-1, and Flk-1, while VE-Cad and Tie-2 expression were unaffected. These results suggest that the expression and signaling of CEACAM1 may affect the expression of other factors known to play critical roles in vasculogenesis. Furthermore this 3D model of vasculogenesis in an environment of extracellular matrix may be a useful model for comparison to existing models of angiogenesis.

Downregulation of platelet responsiveness upon contact with LDL by the protein-tyrosine phosphatases SHP-1 and SHP-2

OBJECTIVE

The sensitivity of platelets to aggregating agents increases when low-density lipoprotein (LDL) binds to apolipoprotein E receptor 2' (apoER2'), triggering activation of p38MAPK and formation of thromboxane A2. LDL signaling is terminated by PECAM-1 through recruitment and activation of the Ser/Thr protein phosphatase PP2A, but platelets remain unresponsive to LDL when PECAM-1 activation disappears. We report a second mechanism that halts LDL signaling and in addition lowers platelet responsiveness to aggregating agents.

METHODS AND RESULTS

After a first stimulation with LDL, platelets remain unresponsive to LDL for 60 minutes, despite normal apoER2' activation by a second dose of LDL. A possible cause is persistent activation of the tyrosine phosphatases SHP-1 and SHP-2, which may not only block a second activation of p38MAPK, PECAM-1, and PP2A by LDL but also seem to reduce aggregation by TRAP, collagen, and ADP.

CONCLUSION

These findings reveal that p38MAPK phosphorylation and platelet activation by LDL are suppressed by two mechanisms: (1) short activation of PECAM-1/PP2A, and (2) prolonged activation of SHP-1 and SHP-2. Activation of SHP-1 and SHP-2 is accompanied by reduced responsiveness to aggregating agents, which--if present in vivo--would make LDL an aggregation inhibitor during prolonged contact with platelets.

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.

RhoA activation and actin reorganization involved in endothelial CAM-mediated endocytosis of anti-PECAM carriers: critical role for tyrosine 686 in the cytoplasmic tail of PECAM-1

Platelet-endothelial cell adhesion molecule-1 (PECAM-1), a transmembrane glycoprotein involved in leukocyte transmigration, represents a good target for endothelial drug delivery (eg, using antibody-directed nanocarriers, anti-PECAM/NCs). Although endothelial cells do not internalize PECAM antibodies, PECAM-1 engagement by multivalent anti-PECAM conjugates and nanocarriers causes endocytosis via a nonclassic CAM-mediated pathway. We found that endothelial uptake of multivalent anti-PECAM complexes is associated with PECAM-1 phosphorylation. Using model REN cells expressing a series of PECAM-1 deletion and point mutants, we found that the PECAM-1 cytoplasmic domain and, more precisely, PECAM-1 tyrosine 686, is critical in mediating RhoA activation and recruitment of EGFP-RhoA to anti-PECAM/NC binding sites at the plasmalemma, actin polymerization into phalloidin-positive stress fibers, and finally CAM endocytosis of anti-PECAM/NCs. Endothelial targeting and endocytosis of anti-PECAM/NCs were markedly efficient and did not compromise endothelial barrier function in vitro (determined by immunostaining of VE-cadherin and (125)I-albumin transport across endothelial monolayers) or in vivo (determined by electron microscopy imaging of pulmonary capillaries and (125)I-albumin transport from the blood into the lung tissue after intravenous injection of anti-PECAM/NCs in mice). These results reveal PECAM-1 signaling and interactions with the cytoskeleton, which are required for CAM-endocytosis, and may provide safe intra-endothelial drug delivery by anti-PECAM/NCs.

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.