Amphipathic helices support function of blood coagulation factor IXa

Procoagulant activities of skeletal and cardiac muscle myosin depend on contaminating phospholipid

Recent reports indicate that suspended skeletal and cardiac myosin, such as might be released during injury, can act as procoagulants by providing membrane-like support for factors Xa and Va in the prothrombinase complex. Further, skeletal myosin provides membrane-like support for activated protein C. This raises the question of whether purified muscle myosins retain procoagulant phospholipid through purification. We found that lactadherin, a phosphatidyl-l-serine-binding protein, blocked >99% of prothrombinase activity supported by rabbit skeletal and by bovine cardiac myosin. Similarly, annexin A5 and phospholipase A2 blocked >95% of myosin-supported activity, confirming that contaminating phospholipid is required to support myosin-related prothrombinase activity. We asked whether contaminating phospholipid in myosin preparations may also contain tissue factor (TF). Skeletal myosin supported factor VIIa cleavage of factor X equivalent to contamination by ∼1:100 000 TF/myosin, whereas cardiac myosin had TF-like activity >10-fold higher. TF pathway inhibitor inhibited the TF-like activity similar to control TF. These results indicate that purified skeletal muscle and cardiac myosins support the prothrombinase complex indirectly through contaminating phospholipid and also support factor X activation through TF-like activity. Our findings suggest a previously unstudied affinity of skeletal and cardiac myosin for phospholipid membranes.

Amphiphilic peptide-loaded nanofibrous calcium phosphate microspheres promote hemostasis in vivo

Most fatalities from trauma occur due to severe blood loss. There is a need for improved hemostatic biomaterials that can address this problem. The aim of this study was to determine the in vivo efficacy of nanofibrous microspheres (NFM) loaded with hemostatic peptides, specifically ideal amphipathic peptides (IAP) that have been demonstrated to possess both procoagulant and antifibrinolyic activities. We demonstrate that IAP can be coupled to NFM (IAP-NFM) to form matrices that exhibit substantial hemostatic activity. IAP-NFM matrices were compared to a commercial zeolitic hemostatic biomaterial (QuikClot) and have superior efficacy in reducing bleeding in vivo. In both a murine tail transection and a murine hepatic injury model, bleeding times were significantly reduced (P<0.05) with the use of IAP-NFM as compared with equal masses of either QuikClot or NFM alone, or no treatment. Importantly, histological examination revealed no tissue injury when IAP-NFM or NFM were applied to hepatic lacerations. In contrast, QuikClot caused widespread hepatocyte degeneration and necrosis, with higher average injury zone thickness as determined by semiquantitative analysis. In summary, NFM was able to maintain the pro-coagulant properties of IAP in our preclinical model, caused no observable tissue damage at the site of application and had better performance than QuikClot controls.

Characterization of In Vitro Hemostatic Peptide Effects by Thromboelastography

In this study, we validated a thromboelastography (TEG) method to evaluate the hemostatic effects of 3 peptides. The first peptide is an ideal amphipathic peptide composed of 22 leucine and lysine in a ratio of 2:1. At a very low concentration, the peptide had a procoagulant effect shown by decreases in reaction time (R) and coagulation time (K) but was impaired by a decrease in maximum amplitude (MA). At higher concentrations, the peptide had an anticoagulant effect. The α angle was minimally affected by the peptide. The second peptide is melittin derived from bee venom. Melittin showed procoagulant effects reflected by a decrease in clotting time but led to lower MA. The third peptide derived from fibrinogen γ chain promoted hemostasis only at an optimal concentration and became anticoagulant at a higher concentration. The hemostatic mechanisms of each peptide were discussed. Our study would facilitate further development of peptides for either hemorrhage control or thrombosis treatment.

Membrane-binding properties of the Factor VIII C2 domain

Factor VIII functions as a cofactor for Factor IXa in a membrane-bound enzyme complex. Membrane binding accelerates the activity of the Factor VIIIa-Factor IXa complex approx. 100000-fold, and the major phospholipid-binding motif of Factor VIII is thought to be on the C2 domain. In the present study, we prepared an fVIII-C2 (Factor VIII C2 domain) construct from Escherichia coli, and confirmed its structural integrity through binding of three distinct monoclonal antibodies. Solution-phase assays, performed with flow cytometry and FRET (fluorescence resonance energy transfer), revealed that fVIII-C2 membrane affinity was approx. 40-fold lower than intact Factor VIII. In contrast with the similarly structured C2 domain of lactadherin, fVIII-C2 membrane binding was inhibited by physiological NaCl. fVIII-C2 binding was also not specific for phosphatidylserine over other negatively charged phospholipids, whereas a Factor VIII construct lacking the C2 domain retained phosphatidyl-L-serine specificity. fVIII-C2 slightly enhanced the cleavage of Factor X by Factor IXa, but did not compete with Factor VIII for membrane-binding sites or inhibit the Factor Xase complex. Our results indicate that the C2 domain in isolation does not recapitulate the characteristic membrane binding of Factor VIII, emphasizing that its role is co-operative with other domains of the intact Factor VIII molecule.

Complex assemblies of factors IX and X regulate the initiation, maintenance, and shutdown of blood coagulation

Blood hemostasis is accomplished by a complex network of (anti-)coagulatory and fibrinolytic processes. These physiological processes are implemented by the assembly of multiprotein complexes involving both humoral and cellular components. Coagulation factor X, and particularly, factor IX, exemplify the dramatic enhancement that is obtained by the synergistic interaction of cell surface, inorganic and protein cofactors, protease, and substrate. With a focus on structure-function relationship, we review the current knowledge of activity modulation principles in the coagulation proteases factors IX and X and indicate future challenges for hemostasis research. This chapter is organized by describing the principles of hierarchical activation of blood coagulation proteases, including endogenous and exogenous protease activators, cofactor binding, substrate specificities, and protein inhibitors. We conclude by outlining pharmaceutical opportunities for unmet needs in hemophilia and thrombosis.

PEGylation of Melittin: Structural Characterization and Hemostatic Effects

To promote and understand the structure—property relationship for hemostasis, we modified melittin (MLT) using a four-arm poly(ethylene glycol) (PEG) with N-hydroxysuccinimide ester. The PEGylation was characterized by FTIR, MALDI-MS, NMR, a bicinchoninic acid assay, circular dichroism, hemolysis assay, and thromboelastography. Changes in the reaction conditions affected the extent of the modification, the numbers of MLT conjugated to PEG arms, and possible PEGylation sites. The reaction at pH 9.2 with a high MLT/PEG ratio, resulted in the highest modification. Reactions in dimethylsulfoxide (DMSO) resulted in more multi-arm coupled MLT, reaching a maximum of four MLT per PEG. The helicity of the modified peptide, relative to the native peptide, was essentially maintained in DMSO, but substantially lost at pH 9.2. PEGylation reduced the hemolytic effects of MLT and subsequently changed its coagulation profiles. The overall hemostatic effects of MLT modified in DMSO indicate that this may be a convenient approach to the PEGylation of biomolecules for biomedical applications.

Characterization of an ideal amphipathic peptide as a procoagulant agent

On the basis of previous evidence that amphipathic helical peptides accelerate Factor IXa activation of Factor X [Blostein, Rigby, Furie, Furie and Gilbert (2000) Biochemistry 39, 12000–12006], the present study was designed to assess the procoagulant activity of an IAP (ideal amphipathic peptide) of Lys7Leu15 composition. The results show that IAP accelerates Factor X activation by Factor IXa in a concentration-dependent manner and accelerates thrombin generation by Factor Xa with a comparable peptide- and substrate-concentration-dependence. A scrambled helical peptide with the same amino acid composition as IAP, but with its amphipathicity abolished, eliminated most of the aforementioned effects. The Gla (γ-carboxyglutamic acid)-rich domain of Factor X is required for IAP activity, suggesting that this peptide behaves as a phospholipid membrane. This hypothesis was confirmed, using fluorescence spectroscopy, by demonstrating direct binding between IAP and the Gla-rich domain of Factor X. In addition, the catalytic efficiencies of the tenase and prothrombinase enzymatic complexes, containing cofactors Factor VIIIa and Factor Va respectively, are enhanced by IAP. Finally, we show that IAP delays clot lysis in vitro. In summary, these observations demonstrate that IAP not only enhances essential procoagulant reactions required for fibrin generation, but also inhibits fibrinolysis, suggesting a potential role for IAP as a haemostatic agent.