Platelet retraction force measurements using flexible post force sensors

Platelets play an important role in hemostasis by forming a thrombotic plug that seals the vessel wall and promotes vascular healing. After platelets adhere and aggregate at the wound site, their next step is to generate contractile forces through the coordination of physicochemical interactions between actin, myosin, and alpha(IIb)beta(3) integrin receptors that retract the thrombus' size and strengthen its adhesion to the exposed matrix. Although platelet contractile forces (PCF) are a definitive feature of hemostasis and thrombosis, there are few approaches that can directly measure them. In this study, we describe the development of an approach to measure PCF in microthrombi using a microscopic flexible post force sensor array. Quasi-static measurements and live microscopic imaging of thrombin-activated platelets on the posts were conducted to assay the development of PCF to various hemostatic conditions. Microthrombi were observed to produce forces that monotonically increased with thrombin concentration and activation time, but forces subsided when thrombin was removed. PCF results were statistically similar on arrays of posts printed with fibronectin or fibrinogen. PCF measurements were combined with clot volume measurements to determine that the average force per platelet was 2.1 +/- 0.1 nN after 60 min, which is significantly higher than what has been measured with previous approaches. Overall, the flexible post arrays for PCF measurements are a promising approach for evaluating platelet functionality, platelet physiology and pathology, the impacts of different matrices or agonists on hemostatic responses, and in providing critical information regarding platelet activity that can guide new hemostatic or thrombotic strategies.

Surface-dependent fibrinopeptide A accessibility to thrombin

Fibrinogen adsorption and more recently fibrin formation at interfaces has been reported to depend on surface properties of the underlying substrate. To provide insight into the surface-dependent mechanism of fibrinopeptide A (FpA) release and fibrin formation, the accessibility and susceptibility of FpA to thrombin-catalyzed fibrinopeptide cleavage were examined using polyclonal anti-FpA IgG binding and surface plasmon resonance (SPR). The amount of accessible FpA on adsorbed fibrinogen was significantly influenced by surface properties of the underlying substrate (methyl- and carboxyl-terminated self-assembled monolayers). Roughly 2.7 times more FpA was available on fibrinogen adsorbed at the hydrophobic vs. negatively charged surface. Upon exposure of adsorbed fibrinogen to thrombin, 100% of the available FpA was enzymatically cleaved at both surfaces, indicating that the extent of FpA release and fibrin formation is a function of the surface-dependent FpA availability. The results presented herein suggest negatively charged surfaces impair FpA accessibility, and therefore lead to reduced FpA release and subsequent fibrin formation. As such, negatively charged surfaces may be useful in minimizing surface-induced thrombosis initiated via fibrin formation thereby aiding in the development of more biocompatible blood-contacting devices.

Quantum and molecular dynamics study for binding of macrocyclic inhibitors to human alpha-thrombin

Molecular dynamics simulations followed by quantum mechanical calculation and Molecular Mechanics Poisson-Boltzmann Surface Area (MM-PBSA) analysis have been carried out to study binding of proline- and pyrazinone-based macrocyclic inhibitors (L86 and T76) to human alpha-thrombin. Detailed binding interaction energies between these inhibitors and individual protein fragments are calculated using DFT method based on a new quantum mechanical approach for computing protein-ligand interaction energy. The analysis of detailed interaction energies provides insight on the protein-ligand binding mechanism. Study shows that T76 and L86 bind to thrombin in a very similar "inhibition mode" except that T76 has relatively weaker binding interaction with Glu(217). The analysis from quantum calculation of binding interaction is consistent with the MM-PBSA calculation of binding free energy, and the calculated free energies for L86/T76-thrombin binding agree well with the experimental data.

Thrombin allostery

Thrombin is a Na(+)-activated, allosteric serine protease that plays multiple functional roles in blood pathophysiology. Binding of Na(+) is the major driving force behind the procoagulant, prothrombotic and signaling functions of the enzyme. This review summarizes our current understanding of the molecular basis of thrombin allostery with special emphasis on the kinetic aspects of Na(+) activation. The molecular mechanism of thrombin allostery is a remarkable example of long-range communication that offers a paradigm for many other biological systems.

In vitro procoagulant and anticoagulant properties of Naja naja naja venom

Bites by the Indian cobra (Naja naja naja) are common in India and Sri Lanka because of its close association with humans. Cobra venoms are complex and contain several toxic components, including neurotoxins that cause post-synaptic neuromuscular blockade with respiratory paralysis and even death. Bites may also cause extensive local necrosis by mechanisms not fully elucidated. Although no significant coagulopathy has been reported, N.n. naja venom can form blood clots in vitro by activating prothrombin as demonstrated by thrombin-specific chromogenic substrate. Scanning electron microscopy demonstrates that the clots formed by venom lack the thin fibrin strands of normal blood clots formed by thromboplastin or glass contact. Rheometry shows that clots formed by venom have abnormally low elasticity over an extended period and then, as the platelets contract, a retarded and more feeble increase in elasticity. Purified N.n. naja venom PLA2 inhibits platelet aggregation in PRP and explains the decreased clot retraction and retarded and compromised elasticity build up. The present study shows that the PLA2 and the prothrombin activator from N.n. naja venom have effects on haemostasis and blood clotting, although such effects are not observed systemically in envenomed humans.

Crystal structures of thrombin with thiazole-containing inhibitors: probes of the S1' binding site

Structures of the blood clotting enzyme thrombin complexed with hirugen and two active site inhibitors, RWJ-50353 10080(N-methyl-D-phenylalanyl-N-[5-[(aminoiminomethyl)amino]-1- [[(2-benzothiazolyl)carbonyl]butyl]-L-prolinamide trifluoroacetate hydrate) and RWJ-50215 (N-[4-(aminoiminomethyl)amino-1-[2- (thiazol-2-ylcarbonylethyl)piperidin- 1-ylcarbonyl]butyl]-5-(dimethylamino)naphthalenesulfonamide trifluoroacetate hydrate), were determined by x-ray crystallography. The refinements converged at R values of 0.158 in the 7.0-2.3-A range for RWJ-50353 and 0.155 in the 7.0-1.8-A range for RWJ-50215. Interactions between the protein and the thiazole rings of the two inhibitors provide new valuable information about the S1' binding site of thrombin. The RWJ-50353 inhibitor consists of an S1'-binding benzothiazole group linked to the D-Phe-Pro-Arg chloromethyl ketone motif. Interactions with the S1-S3 sites are similar to the D-phenylalanyl-prolyl-arginyl chloromethylketone structure. In RWJ-50215, a S1'-binding 2-ketothiazole group was added to the thrombin inhibitor-like framework of dansylarginine N-(3-ethyl-1,5-pentanediyl)amide. The geometry at the S1-S3 sites here is also similar to that of the parent compound. The benzothiazole and 2-ketothiazole groups bind in a cavity surrounded by His57, Tyr60A, Trp60D, and Lys60F. This location of the S1' binding site is consistent with previous structures of thrombin complexes with hirulog-3, CVS-995, and hirutonin-2 and -6. The ring nitrogen of the RWJ-50353 benzothiazole forms a hydrogen bond with His57, and Lys60F reorients because of close contacts. The oxygen and nitrogen of the ketothiazole of RWJ-50215 hydrogen bond with the NZ atom of Lys60F.

The Na+ binding site of thrombin

Thrombin is an allosteric serine protease existing in two forms, slow and fast, targeted toward anticoagulant and procoagulant activities. The slow --> fast transition is induced by Na+ binding to a site contained within a cylindrical cavity formed by three antiparallel beta-strands of the B-chain (Met180-Tyr184a, Lys224-Tyr228, and Val213-Gly219) diagonally crossed by the Glu188-Glu192 strand. The site is shaped further by the loop connecting the last two beta-strands and is located more than 15 A away from the catalytic triad. The cavity traverses through thrombin from the active site to the opposite surface and contains Asp189 of the primary specificity site near its midpoint. The bound Na+ is coordinated octahedrally by the carbonyl oxygen atoms of Tyr184a, Arg221a, and Lys224, and by three highly conserved water molecules in the D-Phe-Pro-Arg chloromethylketone thrombin. The sequence in the Na+ binding loop is highly conserved in thrombin from 11 different species and is homologous to that found in other serine proteases involved in blood coagulation. Mutation of two Asp residues flanking Arg221a (D221A/D222K) almost abolishes the allosteric properties of thrombin and shows that the Na+ binding loop is also involved in direct recognition of protein C and antithrombin.

Geometry of binding of the benzamidine- and arginine-based inhibitors N alpha-(2-naphthyl-sulphonyl-glycyl)-DL-p-amidinophenylalanyl-pipe ridine (NAPAP) and (2R,4R)-4-methyl-1-[N alpha-(3-methyl-1,2,3,4-tetrahydro-8- quinolinesulphonyl)-L-arginyl]-2-piperidine carboxylic acid (MQPA) to human alpha-thrombin. X-ray crystallographic determination of the NAPAP-trypsin complex and modeling of NAPAP-thrombin and MQPA-thrombin

The X-ray crystal structure of the trypsin complex formed with N alpha-(2-naphthyl-sulphonyl-glycyl)-DL-p-amidinophenylalanyl-piper idine (NAPAP) was determined with X-ray data to 0.18-nm resolution and crystallographically refined. NAPAP binds into the active site of trypsin in a quite compact form: the p-amidinophenylalanine moiety of the D-stereoisomer binds into the specificity pocket; the glycyl group is hydrogen bonded with Gly216; the naphthyl group stands perpendicular to the indole moiety of Trp215; the piperidine ring is tightly packed between this naphthyl moiety and His57; in consequence the carboxy-terminal amido bond of NAPAP is located in such a way that it is not susceptible to the active-site Ser195. NAPAP and (2R,4R)-4-methyl-1-[N alpha-(3-methyl-1,2,3,4-tetrahydro-8- quinolinesulphonyl)-L-arginyl]-2-piperidine carboxylic acid (MQPA) [Matzusaki, T., Sasaki, C., Okumura, C. & Umeyama (1989) J. Biochem. (Tokyo) 105, 949-952] were transferred in their trypsin-binding conformations to human alpha-thrombin [Bode, W., Mayr, I., Baumann, U., Huber, R., Stone, S. R. & Hofsteenge, J. (1989) EMBO J. 8. 3467 - 3475] and energy minimized. Both synthetic inhibitors fit perfectly into the much more restricted active site of thrombin. The accommodation of the S-aryl moieties in the 'aryl-binding site' and of the piperidine rings in the S2 subsite of thrombin are particularly favorable. The preference of thrombin for distinctly substituted piperidine derivatives and its generally higher (compared with trypsin) affinity for benzamidine and arginine-based inhibitors can be accounted for by these thrombin inhibitor models.

Affinity labeling of lysine-149 in the anion-binding exosite of human alpha-thrombin with an N alpha-(dinitrofluorobenzyl)hirudin C-terminal peptide

In order to define structural regions in thrombin that interact with hirudin, the N alpha-dinitrofluorobenzyl analogue of an undecapeptide was synthesized corresponding to residues 54-64 of hirudin [GDFEEIPEEY(O35SO3)L (DNFB-[35S]Hir54-64)]. DNFB-[35S]Hir54-64 was reacted at a 10-fold molar excess with human alpha-thrombin in phosphate-buffered saline at pH 7.4 and 23 degrees C for 18 h. Autoradiographs of the product in reducing SDS-polyacrylamide gels revealed a single 35S-labeled band of Mr approximately 32,500. The labeled product was coincident with a band on Coomassie Blue stained gels migrating slightly above an unlabeled thrombin band at Mr approximately 31,000. Incorporation of the 35S affinity reagent peptide was found markedly reduced when reaction with thrombin was performed in the presence of 5- and 20-fold molar excesses of unlabeled hirudin peptide, showing that a specific site was involved in complex formation. The human alpha-thrombin-DNFB-Hir54-64 complex was reduced, S-carboxymethylated, and treated with pepsin. Peptic fragments were separated by reverse-phase HPLC revealing two major peaks containing absorbance at 310 nm. Automated Edman degradation of the peptide fragments allowed identification of Lys-149 of human thrombin as the major site of DNFB-Hir54-64 derivatization. These data suggest that the anionic C-terminal tail of hirudin interacts with an anion-binding exosite in human thrombin removed 18-20 A from the catalytic apparatus.

Native fibrin gel networks and factors influencing their formation in health and disease

Hydrated fibrin gels were studied by confocal laser 3D microscopy, liquid permeation and turbidity. The gels from normal fibrinogen were found to be composed of straight rod-like fiber elements which sometimes originated from denser nodes. In gels formed at increasing thrombin or fibrinogen concentrations, the gel networks became tighter and the porosity decreased. The fiber strands also became shorter. Gel porosity of the network decreased dramatically in gels formed at increasing ionic strengths. Shortening of the fibers were observed and fiber swelling occurred at ionic strength above 0.24. Albumin and dextran, when present in the gel forming system, affected the formation of more porous structures with strands of larger mass-length ratio and fiber thickness. This type of gels were also formed in plasma. Albumin and lipoproteins may be among the determinants for the formation of this type of gel structure in plasma. Gels formed when factor XIIIa instead of thrombin was used as catalyst for gelation showed a completely different structure in which lumps of polymeric material were held together by a network of fine fiber strands. Our studies have also shown that the methodologies employed may be useful in studies of gel structures in certain dysfibrinogenemias as well as in other diseases. We give examples of two patients with abnormal fibrinogen and of patients with ischaemic heart disease.