CD146 is a cell-surface molecule belonging to the immunoglobulin superfamily and expressed in all types of human endothelial cells. Confocal and electron microscopic analysis of confluent human umbilical vein endothelial cells (HUVECs) were used to demonstrate that CD146 is a component of the endothelial junction. Double immunolabeling with vascular endothelial cadherin showed that CD146 is localized outside the adherens junction.Moreover, CD146 expression is not restricted to the junction, since part of the labeling was detectable at the apical side of the HUVECs. Interestingly, cell-surface expression of CD146 increased when HUVECs reached confluence. In addition, the paracellular permeability of CD146-transfected fibroblast cells was decreased compared with that of control cells. Finally, CD146 colocalized with actin, was partly resistant to Triton X-100 extraction, and had its expression altered by actindisrupting agents, indicating that CD146 is associated with the actin cytoskeleton. These results show the regulated expression of CD146 at areas of cell-cell junction and strongly suggest involvement of CD146 as a mediator of cell-cell interaction. ( IntroductionThe vascular endothelium forms a continuous monolayer on the inner surface of the vessel wall and plays a pivotal role in regulating blood flow, vascular permeability, thrombogenesis, and hematogenous metastasis. 1 Positioned at the interface between blood and tissues, quiescent endothelial cells (ECs) generate an antithrombotic surface equipped to respond quickly to biologic needs. 2 The endothelial monolayer requires highly effective intercellular junctions that control the contact between adjacent cells and the trafficking of circulating blood cells. 3,4 At least 2 types of cell-cell junctional structures have been identified in the endothelium: adherens junctions (AJs) and tight junctions (TJs). These play a central part in the control of paracellular permeability and maintenance of cell polarity. [5][6][7] The junctions are tightly regulated structures composed of several adhesion molecules interacting with cytoskeletal proteins. Among the adhesive molecules, the endothelium-specific cadherin 5 or vascular endothelial cadherin (VEcadherin) 8,9 is localized in AJs, whereas the junctional adhesion molecule (JAM) 10 was reported to be present in TJs. Other molecules, such as platelet endothelial cell adhesion molecule 1 (PECAM-1)/CD31, are not restricted to one type of junctional structure, and their specific localization appears to be important to their vascular functions. 11,12 The S-Endo 1-associated antigen (CD146), also referred to as MelCAM or MUC18, 13 is a transmembrane glycoprotein that is constitutively expressed in the whole human endothelium, irrespective of its anatomical site or vessel caliber. 14,15 CD146 expression is not restricted to ECs; it has also been observed on several other cell types, including melanoma cells, 13 smooth muscle cells, and follicular dendritic cells. 14 Using optical microscopy, we previously showed that,...

The balance between lesion and regeneration of the endothelium is critical for the maintenance of vessel integrity. Exposure to cardiovascular risk factors (CRF) alters the regulatory functions of the endothelium that progresses from a quiescent state to activation, apoptosis and death. In the last 10 years, identification of circulating endothelial cells (CEC) and endothelial-derived microparticles (EMP) in the circulation has raised considerable interest as non-invasive markers of vascular dysfunction. Indeed, these endothelial-derived biomarkers were associated with most of the CRFs, were indicative of a poor clinical outcome in atherothrombotic disorders and correlated with established parameters of endothelial dysfunction. CEC and EMP also behave as potential pathogenic vectors able to accelerate endothelial dysfunction and promote disease progression. The endothelial response to injury has been enlarged by the discovery of a powerful physiological repair process based on the recruitment of circulating endothelial progenitor cells (EPC) from the bone marrow. Recent studies indicate that reduction of EPC number and function by CRF plays a critical role in the progression of cardiovascular diseases. This EPC-mediated repair to injury response can be integrated into a clinical endothelial phenotype defining the ‘vascular competence’ of each individual. In the future, provided that standardization of available methodologies could be achieved, multimarker strategies combining CEC, EMP and EPC levels as integrative markers of ‘vascular competence’ may offer new perspectives to assess vascular risk and to monitor treatment efficacy.

In the present study we investigated whether endothelial microparticles (

Thrombin exerts pleiotropic effects on endothelial cells, including the release of microparticles (EMPs) that disseminate and exchange information with vascular cells. Nevertheless, the mechanisms leading to their generation are not elucidated. We performed microarray analysis to identify genes involved in EMP release by the endothelial cell line HMEC-1 in response to thrombin. We identified a group of genes linked to the cytoskeleton reorganization family. Among these, the Rhokinase ROCK-II presented a high transcription rate. ROCK-I, another Rhokinase isoform, was not modulated by thrombin. Pharmacologic inhibition of Rho-kinases or specific depletion of ROCK-II by short interfering (si) RNA inhibited thrombin-induced EMP release. In contrast, ROCK-I mRNA silencing did not modify EMP generation by thrombin. Exposure of HMEC-1 to thrombin in presence of the caspase-2 selective inhibitor Z-VDVAD-FMK prevented ROCK-II cleavage and inhibited the thrombin-induced EMP release. These events were observed in absence of cell death. Our data clearly identified ROCK-II as a target of thrombin in EMP generation. They indicated that the 2 Rho-kinases did not share identical functions. The involvement of caspase-2 in ROCK-II activation independently of cell death points out a novel signaling pathway that emphasizes the proteolytic activity of caspase in EMP generation in response to cell activation. IntroductionThrombin is a serine protease playing a central role in the coagulation cascade and hemostasis. Generated at sites of vascular injury in the vicinity of a thrombus, thrombin promotes platelet activation, adhesion, and trafficking of inflammatory cells into sites of injury. 1 Moreover, thrombin exerts pleiotropic effects on the endothelium controlling the proliferative/reparative responses to injury and promoting both proinflammatory and procoagulant endothelial phenotypes. 2 Thrombin activities are mediated by the activation of the G-protein-linked protease-activated receptors (PARs), mainly PAR-1 on the endothelial cells. 3 Recently, the gene profiling of primary endothelial cells in response to thrombin indicates changes in the transcription of a multitude of genes, 2,4 which results in modifications of the endothelial cell shape, cytoskeleton rearrangement, and increased permeability. 5 Despite the large body of experiments describing the effects of thrombin on the endothelium, little is known on its capacity to generate endothelial cell microparticles 6 and the mechanisms involved in their release have never been reported.Microparticles found in the extracellular space derive from membrane blebs. Blebbing is a reversible dynamic event that occurs during cell activation or apoptosis. 7 Viable cells display plasma membrane blebbing when spreading, migrating, dividing, or under conditions of stress. Blebbing depends on intracellular forces generated by the actin-myosin cytoskeleton and is driven through the activation of actin-myosin contraction 8,9 by the small GTP-binding protein Rho and its major effectors, the ...

CD146 (S-Endo 1 Ag or MUC18) is a transmembraneCas tyrosine phosphorylation. Moreover, a complex association was observed between Pyk2, p130Cas , and paxillin. These results indicate that CD146 is coupled to a FYN-dependent pathway that triggers Ca 2؉ flux via phospholipase C-␥ activation leading subsequently to the tyrosine phosphorylation of downstream targets such as Pyk2, p130Cas , FAK, and paxillin. In addition to its role in cell-cell adhesion, CD146 is a signaling molecule involved in the dynamics of actin cytoskeleton rearrangement.

Endothelial microparticles (EMP) are complex vesicular structures that can be shed by activated or apoptotic endothelial cells. EMP are composed of a phospholipid bilayer that exposes transmembrane proteins and receptors and encloses cytosolic components such as enzymes, transcription factors and mRNA derived from their parent cells. Thus, EMP behave as biological conveyors playing a key role in the tuning of vascular homeostasis. This review focuses on the multifaceted roles of EMP, notably in coagulation, inflammation and angiogenesis and also on the mechanisms that trigger their formation. In this context, EMP could compromise vascular homeostasis and then represent key players in the pathogenesis of several inflammatory and thrombotic diseases. Consequently, elucidating their role and their mechanisms of formation will bring new insights into the understanding of endothelial-associated diseases. Moreover, in the future, it can open novel therapeutic perspectives based on the inhibition of EMP release.

The online version of this article has a Supplementary Appendix. BackgroundWe recently assigned a new fibrinolytic function to cell-derived microparticles in vitro. In this study we explored the relevance of this novel property of microparticles to the in vivo situation. Design and MethodsCirculating microparticles were isolated from the plasma of patients with thrombotic thrombocytopenic purpura or cardiovascular disease and from healthy subjects. Microparticles were also obtained from purified human blood cell subpopulations. The plasminogen activators on microparticles were identified by flow cytometry and enzyme-linked immunosorbent assays; their capacity to generate plasmin was quantified with a chromogenic assay and their fibrinolytic activity was determined by zymography. ResultsCirculating microparticles isolated from patients generate a range of plasmin activity at their surface. This property was related to a variable content of urokinase-type plasminogen activator and/or tissue plasminogen activator. Using distinct microparticle subpopulations, we demonstrated that plasmin is generated on endothelial and leukocyte microparticles, but not on microparticles of platelet or erythrocyte origin. Leukocyte-derived microparticles bear urokinase-type plasminogen activator and its receptor whereas endothelial microparticles carry tissue plasminogen activator and tissue plasminogen activator/inhibitor complexes. ConclusionsEndothelial and leukocyte microparticles, bearing respectively tissue plasminogen activator or urokinase-type plasminogen activator, support a part of the fibrinolytic activity in the circulation which is modulated in pathological settings. Awareness of this blood-borne fibrinolytic activity conveyed by microparticles provides a more comprehensive view of the role of microparticles in the hemostatic equilibrium.Key words: fibrinolytic microparticles, plasmin, plasminogen, uPA; tPA. Plawinski L, Robert S, Doeuvre L, Sabatier F, Martinez de Lizarrondo S, Mezzapesa A, Anfosso F, Leroyer AS, Poullin P, Jourde N, Njock M-S, Boulanger CM, Anglés-Cano E, and Dignat-George F. Leukocyte-and endothelial-derived microparticles: a circulating source for fibrinolysis. Haematologica 2012;97(12):1864-1872. doi:10.3324/haematol.2012 This is an open-access paper. Citation: Lacroix R, Leukocyte-and endothelial-derived microparticles: a circulating source for fibrinolysis ABSTRACT© F e r r a t a S t o r t i F o u n d a t i o n

S-Endo-1 antigen (CD146), a transmembrane receptor also known as MUC18/MCAM, is a member of the immunoglobulin superfamily and belongs to a group of cell adhesion molecules. CD146 is highly expressed on the whole vascular tree. We demonstrate here that engagement of CD146 on human endothelial cells isolated from cord blood results in tyrosine phosphorylation of a large panel of cellular proteins, although no tyrosine phosphorylation of CD146 was detected.