No abstract

Fibrinogen is a soluble plasma protein which, after cleavage by the specific proteolytic enzyme thrombin, polymerizes to form the filamentous fibrin network during blood clotting (see refs 1 and 2 for reviews). Fibrinogen has a molecular weight of 340,000 and is composed of two identical halves, each containing three peptide chains designated A alpha, B beta and gamma. Fibrin monomers are produced by thrombin which releases the small negatively charged fibrinopeptides A and B. The overall shape of the fibrinogen molecule has not been unequivocally established. The trinodular, elongated (approximately 450 A long) structure proposed by Hall and Slayter is the most widely accepted model and it has obtained additional support from recent work. Fibrin monomers are also about 450 A long and in fibres they probably have a half-staggered arrangement along the axis. The fibres are an assembly of protofibrils whose structure and packing are not reliably known. We report here that highly oriented fibrin gels are formed when polymerization takes place slowly in a strong magnetic field. It is shown that the protofibrils pack into a three-dimensional crystalline lattice. We introduce magnetically induced birefringence as a potential tool for studying polymerization and briefly speculate on the applications of strong magnetic fields.

SummaryOne of the frequently proposed mechanisms for pregnancy losses refers to uteroplacental thrombosis. However the contribution of classical thrombotic risk factors remains questionable and, if real, does not account for a large number of pregnancy losses. The aim of this study was to investigate the presence of circulating procoagulant microparticles, a new marker of cell activation already associated with various prothrombotic clinical settings. Microparticles were assessed by an original prothrombinase assay on platelet depleted plasma obtained from 74 women with a history of pregnancy loss without apparent cause and 50 controls. Patients were studied at least 2 months after the last obstetrical event and were classified into 2 groups: 49 women with at least 3 consecutive spontaneous abortions at or before the 10th postmenstrual week and 25 with at least one fetal death beyond the 10th postmenstrual week. Among the 74 patients, 41 had increased levels of circulating microparticles, 29 belonging to the group of early pregnancy loss (59%) and 12 to the group of late pregnancy loss (48%). The high prevalence of increased levels of procoagulant microparticles in both groups makes this new marker very promising for the understanding, follow up and therapeutical handling of pregnancy loss.

No abstract

Accurate birefringence measurements show that fibrinogen orients to a small degree in high magnetic fields. This effect can be explained as due to the molecule having about 30% (by weight) a-helix oriented relatively parallel to the long axis. Birefringence measurements on fully oriented fibrin suggest that aligned a-helical content is less than that estimated for fibrinogen. But because of limitations in the analysis this difference must be viewed with caution. Highly oriented fibrin results when polymerization takes place slowly in a strong magnetic field. Low-angle neutron diffraction patterns from oriented fibrin made in the presence of EDTA, made in the presence of calcium, or stabilized with factor XIIIa are very similar, showing that the packing of the molecules within the fibers is the same or very similar in these different preparations. The induced magnetic birefringence was used to follow fibrin formation under conditions in which thrombin was rate limiting. The fiber network formed by approximately the gelation point constitutes a kind of matrix or frame that is largely built upon during the remaining ==85% of the reaction. After gelation the reaction is pseudo-first order.The arrest of blood loss from an injured vessel, hemostasis, requires the participation of several plasma proteins and also platelets, cells that form occlusive aggregates at the site of the rupture. The last stage of the blood clotting process is the enzyme-catalyzed activation of a soluble plasma protein, fibrinogen, which then undergoes polymerization to form an insoluble fibrin gel, thus mechanically reinforcing the platelet plug. The limited cleavage of fibrinogen by thrombin, a serine proteinase, is the result of a series of steps involving many other clotting factors; much is known about this sequence of highly regulated events (for a recent and exhaustive review see ref. 1). Thrombin also converts factor XIII into factor XIIIa, the plasma transglutaminase which, in the presence of calcium, crosslinks adjacent fibrin monomers of a fiber by forming E-(y-glutamyl)lysyl pseudo peptide bonds (2).The trinodular elongated (450-A-long) structure for the fibrinogen molecule proposed by Hall and Slayter (3) is the most widely accepted model, and it has obtained additional support from recent work on native fibrinogen (4-8) or slightly modified fibrinogen (9-11). Fibrin monomers are produced by thrombin, which releases the small negatively charged fibrinopeptides A and B. The monomers associate in a longitudinal half-staggered arrangement to generate the two-stranded fibrin protofibril (12, 13), then these protofibrils associate laterally to form the thicker fibrin fibers (12). In a recent study, we have shown that when polymerization of fibrin takes place slowly in a high magnetic field one ends up with a highly oriented gel on which neutron low-angle diffraction studies demonstrate that the protofibrils pack with three-dimensional order, probably in a tetragonal unit cell with a = b = 185 A and c = 446 A and containing eigh...

SummaryAn anticoagulant activity was isolated from the plasma of a patient with a strong lupus-like anticoagulant using gel filtration by high performance liquid chromatography. IgM were detected in this anticoagulant fraction which exhibited specificity towards 50% phosphatidylcholine - 50% phosphatidylserine vesicles and cardiolipin. These phospholipids were able to produce an apparent 3-fold enhancement of purified human protein C activation by human a-thrombin in the presence of purified human placenta thrombomodulin. In the absence of phospholipid, the anticoagulant fraction had no effect on thrombomodulin activity. The anticoagulant fraction could neutralize the enhancement of thrombomodulin activity by phospholipid in a dose-dependent manner. This study suggests that the neutralization of phospholipid might result in a reduced activation of protein C which could be responsible for the occurrence of thrombotic complications in a proportion of patients with lupus anticoagulants.

Because expressed at a significant level at the membrane of human T cells, we made the hypothesis that the cellular prion protein (PrPc) could behave as a receptor, and be responsible for signal transduction. PrPc engagement by specific antibodies was observed to induce an increase in cytosolic calcium concentration and led to enhanced activity of Src protein tyrosine kinases. Antibodies to CD4 and CD59 did not influence calcium fluxes or signaling. The effect was maximal after the formation of a network involving avidin and biotinylated antibody to PrPc and was inhibited after raft disruption. PrPc localization was not restricted to rafts in resting cells but engagement was a prerequisite for signaling induction, with concomitant PrPc recruitment into rafts. These results suggest a role for PrPc in signaling pathways, and show that lateral redistribution of the protein into rafts is important for subsequent signal transduction.

Objectives-We tested the hypothesis that the antithrombotic and cytoprotective effects of recombinant human activated protein C (rhAPC) protect baboons against the lethal effects of heatstroke. Methods and Results-Fourteen anesthetized baboons assigned randomly to rhAPC (nϭ7) or control group (nϭ7) were heat-stressed in a prewarmed incubator at 44 to 47°C until systolic blood pressure fell below 90 mm Hg, which signaled severe heatstroke. rhAPC was administered intravenously (24 g/kg/h) for 12 hours at onset of heatstroke. Heat stress induced coagulation and fibrinolysis activation as evidenced by a significant increase from baseline levels in plasma levels of thrombin-antithrombin (TAT) complexes, tissue plasminogen activator, and D-dimer. Heat stress elicited cell activation/injury as assessed by the release of interleukin (IL)-6, soluble thrombomodulin, and procoagulant microparticles (MPs). rhAPC did not significantly reduce heatstroke-induced thrombin generation, and D-dimer and had no effect on fibrinolytic activity. In contrast, rhAPC infusion attenuated significantly the plasma rise of IL-6 and inhibited the release of soluble thrombomodulin and MPs as compared with control group. No difference in survival was observed between rhAPC-treated and control group. Conclusions-rhAPC given to heatstroke baboons provided cytoprotection, but had no effect on heatstroke-induced coagulation activation and fibrin formation.