Abstract: The direct cytoprotective activities of activated protein C (APC) on cells convey therapeutic relevant beneficial effects in injury and disease models in vivo and require the endothelial protein C receptor (EPCR) and protease activated receptor 1 (PAR1). Thrombin also activates PAR1 but its effects on cells contrast APC's cytoprotective effects. To gain insights into mechanisms for these contrasting cellular effects, PAR3 activation by APC and thrombin was studied. APC cleaved PAR3 on transfected and endothelial cells in the presence of EPCR. Remarkably, APC cleaved a synthetic PAR3 N-terminal peptide at Arg41, whereas thrombin cleaved at Lys38. On cells, APC failed to cleave R41Q-PAR3, whereas K38Q-PAR3 was still cleaved by APC but not by thrombin. PAR3 tethered-ligand peptides beginning at amino acid (aa) 42, but not those beginning at aa 39, conveyed endothelial barrier protective effects. In vivo, the APC-derived PAR3 tethered-ligand peptide, but not the thrombin-derived PAR3 peptide, blunted VEGF-induced vascular permeability. These data indicate that PAR3 cleavage by APC at Arg41 can initiate distinctive APC-like cytoprotective effects. These novel insights help explain the differentiation of APC's cytoprotective versus thrombin's proinflammatory effects on cells and suggest a unique contributory role for PAR3 in the complex mechanisms underlying APC cytoprotective effects.
Abstract: Multiple protective effects of pharmacological activated protein C (APC) are reported in several organ pathologies. To help evaluate the endogenous murine PC system, we characterized a rat monoclonal anti-mouse PC antibody, SPC-54, which inhibited the amidolytic and anticoagulant activities of murine APC by>95%. SPC-54 blocked active site titration of purified APC using the active site titrant, biotinylated FPR-chloromethylketone, showing that SPC-54 blocks access to APC's active site to inhibit all enzymatic activity. A single injection of SPC-54 (10mg/kg) neutralized circulating PC in mice for at least 7days, and immunoblotting and immuno-precipitation with protein G-agarose confirmed that SPC-54 in vivo was bound to PC in plasma. Pre-infusion of SPC-54 in tissue factor-induced murine acute thromboembolism experiments caused a major decrease in mean survival time compared to controls (7min vs. 42.5min, P=0.0016). SPC-54 decreased lung perfusion in this model by 54% when monitored by vascular perfusion methodologies using infrared fluorescence of Evans blue dye. In LD50 endotoxemia murine models, SPC-54 infused at 7hr after endotoxin administration increased mortality from 42% to 100% (P<0.001). In summary, monoclonal antibody SPC-54 ablates in vitro and in vivo APC protective functions and enzymatic activity. The ability of SPC-54 to block the endogenous PC/APC system provides a powerful tool to understand better the role of the endogenous PC system in murine injury models and in cell bioassays and also to neutralize the enzymatic activities of murine APC in any assay system.
Abstract: Activated protein C (APC) exerts endothelial cytoprotective actions that require protease-activated receptor 1 (PAR1), whereas thrombin acting via PAR1 causes endothelial disruptive, proinflammatory actions. APC's activities, but not thrombin's, require PAR1 located in caveolae. PAR1 is a biased 7-transmembrane receptor because G proteins mediate thrombin's signaling, whereas β-arrestin 2 mediates APC's signaling. Here we elucidate novel mechanisms for APC's initiation of signaling. Biochemical studies of APC's protease specificity showed that APC cleaved PAR1 sequences at both Arg41 and Arg46. That PAR1 cleavage at Arg46 can occur on cells was supported by APC's cleavage of N-terminal-SEAP-tagged R41Q-PAR1 but not R41Q/R46Q-PAR1 mutants transfected into cells and by anti-PAR1 epitope mapping of APC-treated endothelial cells. A synthetic peptide composing PAR1 residues 47-66, TR47, stimulated protective signaling in endothelial cells as reflected in Akt and glycogen synthase kinase 3β phosphorylation, Ras-related C3 botulinum toxin substrate 1 activation, and barrier stabilization effects. In mice, the TR47 peptide reduced VEGF-induced vascular leakage. These in vitro and in vivo data imply that the novel PAR1 N-terminus beginning at residue Asn47, which is generated by APC cleavage at Arg46, mediates APC's cytoprotective signaling and that this unique APC-generated N-terminal peptide tail is a novel biased agonist for PAR1.
Abstract: Background- Formation of platelet plug initiates hemostasis after vascular injury and triggers thrombosis in ischemic disease. However, the mechanisms leading to the formation of a stable thrombus are poorly understood. Connexins comprise a family of proteins that form gap junctions enabling intercellular coordination of tissue activity, a process termed gap junctional intercellular communication. Methods and Results- In the present study, we show that megakaryocytes and platelets express connexin37 (Cx37). Deletion of the Cx37 gene in mice shortens bleeding time and increases thrombus propensity. Aggregation is increased in murine Cx37(-/-) platelets or in murine Cx37(+/+) and human platelets treated with gap junction blockers. Intracellular microinjection of neurobiotin, a Cx37-permeant tracer, revealed gap junctional intercellular communication in platelet aggregates, which was impaired in Cx37(-/-) platelets and in human platelets exposed to gap junction blockers. Finally, healthy subjects homozygous for Cx37-1019C, a prognostic marker for atherosclerosis, display increased platelet responses compared with subjects carrying the Cx37-1019T allele. Expression of these polymorphic channels in communication-deficient cells revealed a decreased permeability of Cx37-1019C channels for neurobiotin. Conclusions- We propose that the establishment of gap junctional communication between Cx37-expressing platelets provides a mechanism to limit thrombus propensity. To our knowledge, these data provide the first evidence incriminating gap junctions in the pathogenesis of thrombosis.
Abstract: Connexins are a large family of proteins that form gap junction channels allowing exchange of ions and small metabolites between neighbouring cells. They have been implicated in pathological processes such as tumourogenesis in which they may act as tumour suppressors. A polymorphism in the human connexin37 (Cx37) gene (C1019T), resulting in a non-conservative amino acid change in the regulatory C-terminus of the Cx37 protein (P319S) has been suggested to be implicated in predisposition to angiosarcomas. In this study, we have used communication-deficient HeLa and SK-HEP-1 cells transfected with Cx37-319S, Cx37-319P or empty vector. We showed that the expression of Cx37-319P limited proliferation of HeLa and SK-HEP-1 cells, whereas Cx37-319S expression was without effect. Using an in vitro kinase assay, we demonstrated phosphorylation of Cx37 C-terminus (Cx37CT) by Glycogen Synthase Kinase-3 (GSK-3), a kinase known to be implicated in cell proliferation and cancer. GSK-3-induced phosphorylation was associated with reduced gap junctional intercellular communication (GJIC) as measured by microinjection of the tracer neurobiotin. Inhibition of GSK-3 by LiCl or SB415286 reduced phosphorylation of Cx37-319P and increased GJIC. This latter effect on GJIC involved the beta and not the alpha isoform of GSK-3. In contrast, GSK-3 inhibitors were without effect on HeLa cells expressing Cx37-319S. In conclusion, our data indicate functional effects of the Cx37 C1019T polymorphism on GJIC which might contribute to tumour cell growth.
Abstract: Connexins are members of a large family of transmembrane proteins that form hemichannels or gap junctions. These channels allow the exchange of ions and small metabolites between the cytosol and extracellular space or between neighboring cells. Connexins are important in vascular physiology; they support radial and longitudinal cell-to-cell communication in the vascular wall. Four connexins are expressed in the vascular wall: Cx37, Cx40, Cx43, and Cx45. Their expression is not uniform in all blood vessels and varies with vascular territory and species. Significant changes in the expression pattern of vascular connexins have been described during the development of atherosclerosis, a progressive inflammatory disease. In this review, we provide an overview of (1) the tools used to study the involvement of connexins in atherosclerosis, (2) the participation of connexins in atherogenesis, (3) the increasing interest of a polymorphism in the human connexin37 gene as marker of cardiovascular disease, and (4) the possible therapeutic implications of connexins.
Abstract: Recently, we showed that connexin37 (Cx37) protects against early atherosclerotic lesion development by regulating monocyte adhesion. The expression of this gap junction protein is altered in mouse and human atherosclerotic lesions; it is increased in macrophages newly recruited to the lesions and disappears from the endothelium of advanced plaques. To obtain more insight into the molecular role of Cx37 in advanced atherosclerosis, we used micro-array analysis for gene expression profiling in aortas of ApoE−/− and Cx37−/−ApoE−/− mice before and after 18 weeks of cholesterol-rich diet. Out of >15,000 genes, 106 genes were significantly differentially expressed in young mice before diet (P-value of <0.05, fold change of >0.7 or <−0.7, and intensity value >2.2 times background). Ingenuity pathway analysis (IPA) revealed differences in genes involved in cell-to-cell signaling and interaction, cellular compromise and nutritional disease. In addition, we identified 100 genes that were significantly perturbed after the cholesterol-rich diet. Similar to the analysis on 10-week-old mice, IPA revealed differences in genes involved in cell-to-cell signaling and interaction as well as to immuno-inflammatory disease. Furthermore, we found important changes in genes involved in vascular calcification and matrix degradation, some of which were confirmed at protein level by (immuno-)histochemistry. In conclusion, we suggest that Cx37 deficiency alters the global differential gene expression profiles in young mice towards a pro-inflammatory phenotype, which are then further influenced in advanced atherosclerosis. The results provide new insights into the significance of Cx37 in plaque calcification.
Abstract: Cell-to-cell communication is a process necessary for physiological tissue homeostasis and appears often altered during disease. Gap junction channels, formed by connexins, allow the direct intercellular communication between adjacent cells. After a brief review of the pathophysiology of atherosclerosis, we will discuss the role of connexins throughout the different stages of the disease.
Abstract: Considerable interest for cell-derived microparticles has emerged, pointing out their essential role in haemostatic response and their potential as disease markers, but also their implication in a wide range of physiological and pathological processes. They derive from different cell types including platelets - the main source of microparticles - but also from red blood cells, leukocytes and endothelial cells, and they circulate in blood. Despite difficulties encountered in analyzing them and disparities of results obtained with a wide range of methods, microparticle generation processes are now better understood. However, a generally admitted definition of microparticles is currently lacking. For all these reasons we decided to review the literature regarding microparticles in their widest definition, including ectosomes and exosomes, and to focus mainly on their role in haemostasis and vascular medicine.
Abstract: Many patients with anemia fail to respond to treatment with erythropoietin (Epo), a commonly used hormone that stimulates erythroid progenitor production and maturation by human BM or by murine spleen. The protein product of growth arrest-specific gene 6 (Gas6) is important for cell survival across several cell types, but its precise physiological role remains largely enigmatic. Here, we report that murine erythroblasts released Gas6 in response to Epo and that Gas6 enhanced Epo receptor signaling by activating the serine-threonine kinase Akt in these cells. In the absence of Gas6, erythroid progenitors and erythroblasts were hyporesponsive to the survival activity of Epo and failed to restore hematocrit levels in response to anemia. In addition, Gas6 may influence erythropoiesis via paracrine erythroblast-independent mechanisms involving macrophages. When mice with acute anemia were treated with Gas6, the protein normalized hematocrit levels without causing undesired erythrocytosis. In a transgenic mouse model of chronic anemia caused by insufficient Epo production, Gas6 synergized with Epo in restoring hematocrit levels. These findings may have implications for the treatment of patients with anemia who fail to adequately respond to Epo.
Abstract: Growth arrest-specific gene 6 (gas6) product enhances the formation of stable platelet macroaggregates in response to various agonists. To determine whether Gas6 amplifies the response to known platelet agonists through one or more of its receptor tyrosine kinases of the Tyro3 family, mice deficient in any one of the Gas6 receptors (Gas6-Rs: Tyro3, Axl, or Mer) were submitted to thrombosis challenge and their platelet function. The loss of any one of the Gas6-Rs protects mice against thromboembolism induced by collagen-epinephrine and stasis-induced thrombosis. Importantly, these mice do not suffer spontaneous bleeding and have a normal bleeding time but a tendency to repetitively re-bleed after transient hemostasis. Re-bleeding in mice lacking any one of the Gas6-Rs is not due to thrombocytopenia or coagulopathy but to a platelet dysfunction characterized by a lack of the second wave of platelet aggregation and an impaired clot retraction, at least in part by reducing outside-in alpha(IIb)beta(3) signaling and platelet granule secretion. The early release of Gas6 by agonists perpetuates platelet activation through its three receptors, reinforcing outside-in alpha(IIb)beta(3) signaling by activation of PI3K and Akt signaling and stimulation of tyrosine phosphorylation of the beta(3) integrin. Furthermore, "trapping" Gas6 prevents pathological thrombosis, which indicates that blocking this novel cross-talk between the Gas6-Rs and alpha(IIb)beta(3) integrin may constitute a novel target for antithrombotic therapy.
Abstract: Mechanisms regulating thrombus stabilization remain largely unknown. Here, we report that loss of any 1 of the Gas6 receptors (Gas6-Rs), i.e., Tyro3, Axl, or Mer, or delivery of a soluble extracellular domain of Axl that traps Gas6 protects mice against life-threatening thrombosis. Loss of a Gas6-R does not prevent initial platelet aggregation but impairs subsequent stabilization of platelet aggregates, at least in part by reducing "outside-in" signaling and platelet granule secretion. Gas6, through its receptors, activates PI3K and Akt and stimulates tyrosine phosphorylation of the beta3 integrin, thereby amplifying outside-in signaling via alphaIIbbeta3. Blocking the Gas6-R-alphaIIbbeta3 integrin cross-talk might be a novel approach to the reduction of thrombosis.
