Vivien M. Chen

Disulfide isomerization switches tissue factor from coagulation to cell signaling

Cell-surface tissue factor (TF) binds the serine protease factor VIIa to activate coagulation or, alternatively, to trigger signaling through the G protein-coupled, protease-activated receptor 2 (PAR2) relevant to inflammation and angiogenesis. Here we demonstrate that TF·VIIa-mediated coagulation and cell signaling involve distinct cellular pools of TF. The surface-accessible, extracellular Cys186–Cys209 disulfide bond of TF is critical for coagulation, and protein disulfide isomerase (PDI) disables coagulation by targeting this disulfide. A TF mutant (TF C209A) with an unpaired Cys186 retains TF·VIIa signaling activity, and it has reduced affinity for VIIa, a characteristic of signaling TF on cells with constitutive TF expression. We further show that PDI suppresses TF coagulant activity in a nitric oxide-dependent pathway, linking the regulation of TF thrombogenicity to oxidative stress in the vasculature. Furthermore, a unique monoclonal antibody recognizes only the noncoagulant, cryptic conformation of TF. This antibody inhibits formation of the TF·PAR2 complex and TF·VIIa signaling, but it does not prevent coagulation activation. These experiments delineate an upstream regulatory mechanism that controls TF function, and they provide initial evidence that TF·VIIa signaling can be specifically inhibited with minimal effects on coagulation.

Allosteric disulfide bonds in thrombosis and thrombolysis

Summary.  Allosteric disulfide bonds control protein function by mediating conformational change when they undergo reduction or oxidation. The known allosteric disulfide bonds are characterized by a particular bond geometry, the −RHStaple. A number of thrombosis and thrombolysis proteins contain one or more disulfide bonds of this type. Tissue factor (TF) was the first hemostasis protein shown to be controlled by an allosteric disulfide bond, the Cys186–Cys209 bond in the membrane‐proximal fibronectin type III domain. TF exists in three forms on the cell surface: a cryptic form that is inert, a coagulant form that rapidly binds factor VIIa to initiate coagulation, and a signaling form that binds FVIIa and cleaves protease‐activated receptor 2, which functions in inflammation, tumor progression and angiogenesis. Reduction and oxidation of the Cys186–Cys209 disulfide bond is central to the transition between the three forms of TF. The redox state of the bond appears to be controlled by protein disulfide isomerase and NO. Plasmin(ogen), vitronectin, glycoprotein 1bα, integrin β3 and thrombomodulin also contain −RHStaple disulfides, and there is circumstantial evidence that the function of these proteins may involve cleavage/formation of these disulfide bonds.

Laser-induced endothelial cell activation supports fibrin formation

Laser-induced vessel wall injury leads to rapid thrombus formation in an animal thrombosis model. The target of laser injury is the endothelium. We monitored calcium mobilization to assess activation of the laser-targeted cells. Infusion of Fluo-4 AM, a calcium-sensitive fluorochrome, into the mouse circulation resulted in dye uptake in the endothelium and circulating hematopoietic cells. Laser injury in mice treated with eptifibatide to inhibit platelet accumulation resulted in rapid calcium mobilization within the endothelium. Calcium mobilization correlated with the secretion of lysosomal-associated membrane protein 1, a marker of endothelium activation. In the absence of eptifibatide, endothelium activation preceded platelet accumulation. Laser activation of human umbilical vein endothelial cells loaded with Fluo-4 resulted in a rapid increase in calcium mobilization associated cell fluorescence similar to that induced by adenosine diphosphate (10 μM) or thrombin (1 U/mL). Laser activation of human umbilical vein endothelial cells in the presence of corn trypsin inhibitor treated human plasma devoid of platelets and cell microparticles led to fibrin formation that was inhibited by an inhibitory monoclonal anti-tissue factor antibody. Thus laser injury leads to rapid endothelial cell activation. The laser activated endothelial cells can support formation of tenase and prothrombinase and may be a source of activated tissue factor as well.

Encryption and decryption of tissue factor.

Tissue factor (TF) is a transmembrane cofactor that binds and promotes the catalytic activity of factor (F) VIIa. The TF/VIIa complex activates FX by limited proteolysis to initiate blood coagulation and helps provide the thrombin burst that is important for a stable thrombus. TF is present both in the extravascular compartment, where it functions as a hemostatic envelope, and the intravascular compartment, where it contributes to thrombus formation, particularly when endothelial disruption is minimal. The regulation of its cofactor function appears to differ in the two compartments. Intravascular TF derives predominately from leucocytes, with either monocytes or neutrophils implicated in different models of thrombosis. This TF exists mostly in a non‐coagulant or cryptic form and acute events lead to local decryption of TF and FX activation. A variety of experimental observations imply that decryption of leucocyte surface TF involves both a dithiol/disulfide switch and exposure of phosphatidylserine. The dithiol/disulfide switch appears to involve the Cys186‐Cys209 disulfide bond in the membrane‐proximal domain of TF, although this has not been demonstrated in vivo. Activation of a purinergic receptor or complement has recently been observed to decrypt TF on myeloid cells and a dithiol/disulfide switch and the oxidoreductase, protein disulfide isomerase, have been implicated in both systems. The molecular mechanism of action of protein disulfide isomerase in TF encryption/decryption, though, remains to be determined.

An immediate hemolytic reaction induced by repeated administration of oxaliplatin.

BACKGROUND:  Platinum‐based chemotherapy agents have been associated with potentially fatal acute immune‐mediated hemolytic anemia. The target antigen, cause of the positive direct antiglobulin test (DAT) and mechanism of hemolysis have been the subject of controversy.

Imaging fibrin formation and platelet and endothelial cell activation in vivo

Over the past six decades research employing in vitro assays has identified enzymes, cofactors, cell receptors and associated ligands important to the haemostatic process and its regulation. These studies have greatly advanced our understanding of the molecular and cellular bases of haemostasis and thrombosis. However, in vitro assays cannot simultaneously reproduce the interactions of all of the components of the haemostatic process that occur in vivo nor do they reflect the importance of haemodynamic factors resulting from blood flow. To overcome these limitations investigators have increasingly turned to animal models of haemostasis and thrombosis. In this article we describe some advances in the visualisation of platelet and endothelial cell activation and blood coagulation in vivo and review what we have learned from our intravital microscopy experiments using primarily the laser-induced injury model for thrombosis.

Necrotic platelets provide a procoagulant surface during thrombosis

A subpopulation of platelets fulfills a procoagulant role in hemostasis and thrombosis by enabling the thrombin burst required for fibrin formation and clot stability at the site of vascular injury. Excess procoagulant activity is linked with pathological thrombosis. The identity of the procoagulant platelet has been elusive. The cell death marker 4-[N-(S-glutathionylacetyl)amino]phenylarsonous acid (GSAO) rapidly enters a subpopulation of agonist-stimulated platelets via an organic anion-transporting polypeptide and is retained in the cytosol through covalent reaction with protein dithiols. Labeling with GSAO, together with exposure of P-selectin, distinguishes necrotic from apoptotic platelets and correlates with procoagulant potential. GSAO(+) platelets form in occluding murine thrombi after ferric chloride injury and are attenuated with megakaryocyte-directed deletion of the cyclophilin D gene. These platelets form a procoagulant surface, supporting fibrin formation, and reduction in GSAO(+) platelets is associated with reduction in platelet thrombus size and fibrin formation. Analysis of platelets from human subjects receiving aspirin therapy indicates that these procoagulant platelets form despite aspirin therapy, but are attenuated by inhibition of the necrosis pathway. These findings indicate that the major subpopulation of platelets involved in fibrin formation are formed via regulated necrosis involving cyclophilin D, and that they may be targeted independent of platelet activation.

Extramedullary presentation of acute leukaemia: a case of myeloid/natural killer cell precursor leukaemia.

Aim: Myeloid/natural killer (NK) cell leukaemia is characterised by coexpression of myeloid with natural killer cell antigens, a high incidence of extramedullary disease and an aggressive clinical course. Methods: We report a case of a 28‐year‐old woman with myeloid/NK cell precursor acute leukaemia. Clinical presentation was correlated with leukaemic blast morphology, immunophenotype, cytogenetic analysis, molecular studies for clonal rearrangements and histological review. Results: The patient had noted skin lesions and a breast infiltrate 4 months prior to the diagnosis. Bone marrow biopsy at the time of presentation revealed characteristic morphological features with a dense infiltrate of bizarre, pleomorphic blast cells with marked nuclear invagination and reniform shapes. Immunophenotypic analysis of the blasts displayed coexpression of myeloid and natural killer cell antigens with a relatively immature phenotype: CD34−, HLADR+, CD33+, CD56+, CD16−, CD57−, MPO−. Cytogenetic analysis revealed a complex karyotype: del(6)(q21);−12 and add(19)(p13). Histological review of the previous breast biopsy was consistent with granulocytic sarcoma of the breast with a phenotype corresponding to the circulating blasts (positive cytoplasmic staining for myeloid markers, CD68 and CD31, and the NK cell marker CD56, with negative staining for MPO). Skin biopsy confirmed leukaemia cutis. Conclusion: Although nodal extramedullary disease is common in the myeloid/NK cell leukaemias, this is the first description of myeloid/NK cell leukemia primarily involving breast and skin. We speculate that CD56 may predispose to extramedullary localisation of tumour.Abbreviations: NK, natural killer; AML, acute myeloid leukaemia; NCAM, neural cell adhesion molecule; RT‐PCR, reverse transcriptase polymerase chain reaction; CR, complete remission.

Enumeration of extracellular vesicles by a new improved flow cytometric method is comparable to fluorescence mode nanoparticle tracking analysis

UNLABELLED Extracellular vesicles (EVs) play a role in a variety of physiological and pathological processes. However, use of EVs as biomarkers has been hampered by limitations of current detection and enumeration methods. We compared fluorescence-threshold flow cytometry (FT-FC) to nanoparticle tracking analysis (NTA) for enumeration of cell culture-derived EVs. FT-FC and NTA utilising fluorescence mode (F-NTA) enumerated similar numbers of EVs stained with a membrane dye PKH67. Both methods were sufficiently sensitive to detect cell-derived EVs above the background of culture medium. Light scatter NTA (LS-NTA) detected 10-100× more particles than either fluorescence-based method but demonstrated poor specificity. F-NTA appeared to have better sensitivity for <100nm vesicles, however, the FT-FC method combined direct enumeration of EVs with high sensitivity and specificity in the >100nm range. Due to wider availability and higher degree of automation and standardisation, FT-FC is a reasonable surrogate to F-NTA for quantification of EVs. FROM THE CLINICAL EDITOR Extracellular vesicles are small particles, which can act as tools for intercellular communication. One recent area of interest in EVs is their potentials as biomarkers. In this article, the authors investigated and compared fluorescence-threshold flow cytometry (FT-FC) to nanoparticle tracking analysis (NTA) for the detection of EVs and showed that FT- FC method could be more advantageous. This technique should provide a new alternative for the future.

High mortality in patients presenting with acute pulmonary embolism and elevated INR not on anticoagulant therapy

The prognostic significance of patients presenting with pulmonary embolism (PE) and elevated International Normalised Ratio (INR) not on anticoagulant therapy has not been described. We investigated whether these patients had higher mortality compared to patients with normal INR. A retrospective study of patients admitted to a tertiary hospital with acute PE from 2000 to 2012 was undertaken, with study outcomes tracked using a state-wide death registry. Patients were excluded if they were taking anticoagulants or had inadequate documentation of their INR and medication status. Of the 1,039 patients identified, 94 (9 %) had an elevated INR (> 1.2) in the absence of anticoagulant use. These patients had higher mortality at six months follow-up (26 % vs 6 %, p< 0.001) compared to controls (INR ≤ 1.2). An INR > 1.2 at diagnosis was an independent predictor of death at six months post-PE (hazard ratio [HR] 2.9, 95 % confidence interval [CI] 1.8-4.7, p< 0.001). The addition of INR to a multivariable model that included the simplified pulmonary embolism severity index (sPESI), chest pain, and serum sodium led to a significant net reclassification improvement estimated at 8.1 %. The final models C statistic increased significantly by 0.04 (95 % CI 0.01-0.08, p=0.03) to 0.83 compared to sPESI alone (0.79). In summary, patients presenting with acute PE and elevated INR while not on anticoagulant therapy appear to be at high risk of death. Future validation studies in independent cohorts will clarify if this novel finding can be usefully incorporated into clinical decision making in patients with acute PE.