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Objective: PECAM-1 (platelet endothelial cell adhesion molecule 1) is a 130 kDa member of the immunoglobulin (Ig) gene superfamily that is expressed on the surfaces of platelets and leukocytes and concentrated at the intercellular junctions of confluent endothelial cell monolayers. PECAM-1 Ig domains 1 and 2 (IgD1 and IgD2) engage in homophilic interactions that support a host of vascular functions, including support of leukocyte transendothelial migration and the maintenance of endothelial junctional integrity. The recently solved crystal structure of PECAM-1 IgD1 and IgD2 revealed a number of intermolecular interfaces predicted to play important roles in stabilizing PECAM-1/PECAM-1 homophilic interactions and in formation and maintenance of endothelial cell-cell contacts. We sought to determine whether the protein interfaces implicated in the crystal structure reflect physiologically important interactions. Approach and Results: We assessed the impact of single amino acid substitutions at the interfaces between opposing PECAM-1 molecules on homophilic binding and endothelial cell function. Substitution of key residues within the IgD1-IgD1 and IgD1-IgD2 interfaces but not those within the smaller IgD2-IgD2 interface, markedly disrupted PECAM-1 homophilic binding and its downstream effector functions, including the ability of PECAM-1 to localize at endothelial cell-cell borders, mediate the formation of endothelial tubes, and restore endothelial barrier integrity., Conclusions: Taken together, these results validate the recently described PECAM-1 IgD1/IgD2 crystal structure by demonstrating that specific residues visualized within the IgD1-IgD1 and IgD1-IgD2 interfaces of opposing molecules in the crystal are required for functionally important homophilic interactions. This information can now be exploited to modulate functions of PECAM-1 in vivo.

Objective: Thrombosis occurs at sites of vascular injury when platelets adhere to subendothelial matrix proteins and to each other. Platelets express many surface receptor proteins, the function of several of these remains poorly characterized. Cadherin 6 is expressed on the platelet surface and contains an arginine-glycine-aspartic acid motif, suggesting that it might have a supportive role in thrombus formation. The aim of this study was to characterize the role of cadherin 6 in platelet function., Methods and Results: Platelet aggregation was inhibited by both antibodies and exogenous soluble cadherin 6. Platelet adhesion to immobilized cadherin 6 was inhibited by arginine-glycine-aspartic acid-serine tetrapeptides. Antibodies to α(IIb)β(3) inhibited platelet adhesion to cadherin 6. Because platelet aggregation occurs in fibrinogen and von Willebrand factor double-deficient mice, we investigated whether cadherin 6 is an alternative ligand for the integrin α(IIb)β(3). Platelet aggregation in fibrinogen and von Willebrand factor double-deficient mice was significantly inhibited by an antibody to cadherin 6. In flow-based assays, inhibition of cadherin 6 caused a marked reduction in thrombus formation in both human and mouse blood., Conclusions: This study demonstrates the role of cadherin 6 as a novel ligand for α(IIb)β(3) and highlights its function in thrombus formation.

Objective: To better understand the mechanism of platelet recruitment and activation by Streptococcus gordonii. The oral bacterium Streptococcus gordonii, is amongst the most common pathogens isolated from infective endocarditis patients, and has the property of being able to activate platelets, leading to thrombotic complications. The mechanism of platelet recruitment and activation by S. gordonii is poorly understood., Methods and Results: Infective endocarditis is a bacterial infection of the heart valves that carries a high risk of morbidity and mortality. The oral bacterium, S gordonii, is among the most common pathogens isolated from patients with infective endocarditis and is able to activate platelets, leading to thrombotic complications. Platelets interact with S gordonii via glycoprotein Ibα- and α(IIb)β(3)-recognizing S gordonii surface proteins haemaglutitin salivary antigen (Hsa) and platelet adherence protein A, respectively. The inhibition of glycoprotein Ibα or α(IIb)β(3) using blocking antibodies or deletion of S gordonii Hsa or platelet adherence protein A significantly reduces platelet adhesion. Immunoreceptor tyrosine-based activation motif (ITAM)-containing proteins have recently played a role in transmitting activating signals into platelets. Platelet adhesion to immobilized S gordonii resulted in tyrosine phosphorylation of the ITAM-bearing receptor, FcγRIIa, and phosphorylation of downstream effectors (ie, spleen tyrosine kinase [Syk] and phospholipase C [PLC]-γ2). Tyrosine phosphorylation of FcγRIIa resulted in platelet-dense granule secretion, filopodial and lamellipodial extension, and platelet spreading. Inhibition of FcγRIIa ablated both dense granule release and platelet spreading., Conclusions: Streptococcus gordonii binding to the α(IIb)β(3)/FcγRIIa integrin/ITAM signaling complex results in platelet activation that likely contributes to the thrombotic complications of infective endocarditis.

Objective: Atherosclerosis is a vascular disease that involves lesion formation at sites of disturbed flow under the influence of genetic and environmental factors. Endothelial expression of adhesion molecules that enable infiltration of immune cells is important for lesion development. Platelet/endothelial cell adhesion molecule-1 (PECAM-1; CD31) is an adhesion and signaling receptor expressed by many cells involved in atherosclerotic lesion development. PECAM-1 transduces signals required for proinflammatory adhesion molecule expression at atherosusceptible sites; thus, it is predicted to be proatherosclerotic. PECAM-1 also inhibits inflammatory responses, on which basis it is predicted to be atheroprotective., Methods and Results: We evaluated herein the effect of PECAM-1 deficiency on development of atherosclerosis in LDL receptor-deficient mice. We found that PECAM-1 has both proatherosclerotic and atheroprotective effects, but that the former dominate in the inner curvature of the aortic arch whereas the latter dominate in the aortic sinus, branching arteries, and descending aorta. Endothelial cell expression of PECAM-1 was sufficient for its atheroprotective effects in the aortic sinus but not in the descending aorta, where the atheroprotective effects of PECAM-1 also required its expression on bone marrow-derived cells., Conclusions: We conclude that PECAM-1 influences initiation and progression of atherosclerosis both positively and negatively, and that it does so in a site-specific manner.

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.