Reorganization of contractile elements in the platelet during clot retraction

A comparative optical aggregometry study of antiplatelet activity of taxanes from Taxus cuspidata

Platelets are highly reactive components of the circulatory system. The cytoskeleton of a platelet is an important structure for platelet aggregation as stimulated by several agonists. An anticancer agent, taxol, has been suggested to exert platelet anti-aggregating activity by stabilizing microtubules during the aggregation process. An activity-guided fractionation was performed with a methanol extract of the leaves and twigs of Taxus cuspidata to isolate taxanes with platelet anti-aggregating effects. Compounds 1 to 7 - taxinine (1), taxinine A (2), taxinine B (3), 2-deacetoxytaxinine B (4), taxacin (5), taxchinin B (6), and taxol (7) - were obtained as the antiplatelet components of this plant. These taxane compounds present the possibility of securing new antiplatelet compounds which differ from currently available antiplatelet agents in chemical structure and possibly in mechanisms of action. All compounds showed stronger inhibitory effects than acetylsalicylic acid (ASA) on platelet aggregation induced by arachidonic acid (AA) (IC(50): 14.4, 64.5, 35.5, 16.0, 21.9, 28.6 and 48.2 versus 63.0microM) or U46619 (IC(50): 34.8, 24.9, 36.2, 35.0, 46.9, 71.9 and 68.7 versus 340microM). Compounds 1, 3, 4 and 5, with a cinnamoyl group at the C(5) position, showed strong inhibitory effects against AA-induced aggregation compared to compound 2 (with an -OH group at C(5)) or compounds with an oxetane ring at C(4),(5), such as compounds 6 and 7. All of the seven compounds were 5-13-fold more strongly inhibitory than ASA against U46619-induced aggregation.

A comparison of the ADC and T2 mapping in an assessment of blood-clot lysability

The structural characteristics of blood clots are associated with their susceptibility to thrombolysis. As their morphology can be characterized by MRI, several attempts have been made to link the lysability of blood clots with their MRI properties; however, so far no study has associated a clot's lysability with the diffusion properties of the water in the clot. The apparent diffusion coefficient (ADC) is highly sensitive to changes in serum mobility and may be used to distinguish between the non-retracted and the fully retracted regions of the blood clot. Therefore, the ADC may be a suitable, or even a better, marker for an assessment of the clot's retraction and consequently for its lysability than the relaxation time T(2). The purpose of this study was to evaluate whether it is possible to predict the outcome of clot thrombolysis by ADC mapping prior to treatment. After two hours of thrombolysis using a recombinant tissue plasminogen activator in plasma, whole-blood clots were efficiently dissolved in regions with ADC >or= 0.8 x 10(-9) m(2)/s or T(2) >or= 130 ms, whereas dissolution was poor and prolonged in regions with ADC < 0.8 x 10(-9) m(2)/s or T(2) < 130 ms. An analysis based on a comparison between the initial and final ADC and T(2) maps after two hours of thrombolysis showed that the ADC can more accurately detect the different grades of clot retraction than T(2) and predict the regions of a clot that are resistant to thrombolysis. Therefore, the ADC could be used as an efficient prognostic marker for the outcome of thrombolysis. However, in vivo studies are needed to test this idea.

Myosin: a noncovalent stabilizer of fibrin in the process of clot dissolution

Myosin modulates the fibrinolytic process as a cofactor of the tissue plasminogen activator and as a substrate of plasmin. We report now that myosin is present in arterial thrombi and it forms reversible noncovalent complexes with fibrinogen and fibrin with equilibrium dissociation constants in the micromolar range (1.70 and 0.94 microM, respectively). Competition studies using a peptide inhibitor of fibrin polymerization (glycl-prolyl-arginyl-proline [GPRP]) indicate that myosin interacts with domains common in fibrinogen and fibrin and this interaction is independent of the GPRP-binding polymerization site in the fibrinogen molecule. An association rate constant of 1.81 x 10(2) M(-1) x s(-1) and a dissociation rate constant of 3.07 x 10(-4) s(-1) are determined for the fibrinogen-myosin interaction. Surface plasmon resonance studies indicate that fibrin serves as a matrix core for myosin aggregation. The fibrin clots equilibrated with myosin are stabilized against dissolution initiated by plasminogen and tissue-type plasminogen activator (tPA) or urokinase (at fibrin monomer-myosin molar ratio as high as 30) and by plasmin under static and flow conditions (at fibrin monomer-myosin molar ratio lower than 15). Myosin exerts similar effects on the tPA-induced dissolution of blood plasma clots. Covalent modification involving factor XIIIa does not contribute to this stabilizing effect; myosin is not covalently attached to the clot by the time of complete cross-linking of fibrin. Thus, our in vitro data suggest that myosin detected in arterial thrombi binds to the polymerized fibrin, in the bound form its tPA-cofactor properties are masked, and the myosin fibrin clot is relatively resistant to plasmin.

Inhibition of cytoskeletal assembly by cytochalasin B prevents signaling through tyrosine phosphorylation and secretion triggered by collagen but not by thrombin

Activation of platelets leads to cytoskeletal assembly that is responsible for platelet motility and internal contraction. We have evaluated the involvement of the cytoskeleton in platelet activation by two strong agonists, collagen and thrombin. Activation was assessed by measuring changes in cytoskeletal assembly, externalization of activation-dependent markers and expression of procoagulant activity, and tyrosine phosphorylation of proteins, in both the absence and the presence of cytochalasin B. Activation of platelets with collagen and thrombin induced morphological changes and increased the expression of CD62P, CD63, glycoprotein IV, and binding of annexin V to platelets. Moreover, both activating agents induced actin polymerization, increased the association of other contractile proteins, and promoted tyrosine phosphorylation of multiple proteins, some of which were associated with the cytoskeleton. The presence of cytochalasin B blocked the previous events when collagen was used as the activating agent, although binding of annexin V still occurred. In contrast, platelet response to thrombin was not completely prevented by the presence of cytochalasin B. Thus, activation by collagen requires a functional cytoskeleton to trigger signaling through tyrosine phosphorylation and secretion. This is not the case for thrombin, which is capable of activating signaling mechanisms in the presence of strong inhibitors of cytoskeletal assembly. Moreover, the expression of a procoagulant surface in platelets still occurs even when platelet motility has been inhibited.

Reorganization of stress fiber-like structures in spreading platelets during surface activation

Alpha-Actinin and myosin were associated into reorganized actin cable networks and partly formed stress fiber-like structures in platelets during surface activation. Double-label immunofluorescence staining using antibodies against alpha-actinin and platelet myosin heavy chain (MHC) showed that alpha-actinin and myosin were colocalized in the cell center at the early stage of activation and dynamically redistributed with shape change. In the later stage, two proteins were colocalized around the granulomeres. alpha-Actinin was also seen beneath the surface membrane while myosin was not. Occasionally, both proteins were segregated, revealed granular staining in the cell body of flattened platelets and often aligned irregular alternate arrangement in the actin cables. Immunoelectron microscopy (immunogold) employing antibodies against MHC and myosin light chain (MLC) demonstrated that myosin, associated with actin cytoskeleton was precisely filamentous (328 nm in average length, 15 nm in width) and bipolar with a central bare zone, since MLCs were located at both ends. Myosin formed a cluster composed of several filaments with repeating alignment, suggesting each cluster corresponded to the granular staining pattern of immunofluorescence. These observations indicated that the organization of alpha-actinin and myosin in actin cables in activated platelets resembled that in stress fibers in various cultured cells.

Formation of new quasi-crystalline ordered aggregates by gizzard myosin

Turkey gizzard myosin was found to self-assemble into new polymorphic forms as detected by thin-section electron microscopy. In high ionic strength buffers (0.3 mM KCl, pH 6.0), aggregates of sidepolar filaments were produced. Electron microscopy of thin sections revealed individual filaments with a 13.5 nm axial repeat. Under a number of conditions, with varying ionic strength, pH, MgCl2 and ATP, the filaments assembled through the head region with the tail portion projecting out radially from the aggregate. The regions corresponding to heads and tails within the aggregates were established by immunoelectron microscopy using anti-S1 and anti-LMM antibodies coupled to gold. These filaments often interacted to produce bilayer sheets, which, when cut perpendicular to the plane of the sheet, appeared as ladders. A hitherto unreported structure was obtained at 0.2 M KCl (pH 8.0): myosin aggregated to generate a three-dimensional quasi-crystalline lattice with a 270 nm period. In these aggregates, myosin was arranged in an antiparallel fashion, stacked on one another, producing ribbon-like strips stabilized through non-covalent interactions between heads, thereby producing a crystalline lattice. Neither Mg2+ nor ATP were required for this form. Phosphorylation of the regulatory light chains or the cleavage of the heavy chains at a single site in the head region prevented myosin from assembling in the 3-D lattice form. Generally, unphosphorylated myosin produced periodic paracrystals at low ionic strength in the presence of 10 mM MgCl2, but as the ionic strength was increased the regular 3-D lattice became the predominant form. Some paracrystalline forms could be obtained at high ionic strength without magnesium with phosphorylated myosin.

Reorganization of myosin in surface-activated spreading platelets

To study the localization of myosin in platelets, we utilized polyclonal antibody to heavy chain of platelet myosin. Both immunofluorescence and indirect immunogold staining techniques were employed. (1) In the unactivated platelets, myosin was distributed homogeneously throughout the cytosol. The cytosolic myosin was removed after platelets were treated with Triton X-100. The association of myosin with actin microfilaments in unactivated platelets was minimal. (2) In the surface-activated platelets, myosin was unextractable by Triton X-100. The myosin antibody heavily decorated the actin cable-networks which characterized the activated platelets. (3) In the Triton-unextractable cytoskeleton of both unactivated and activated platelets, we found fine fibrils (about 1-nm wide) that were often associated with immunogolds. These fibrils were similar to purified myosin molecules observed in rotary-shadowed metal replicas and ultrathin sections. These results indicate that cytosolic myosin becomes associated with actin cable-networks after the activation of platelets.

Attachment of cytoskeletons to cell membranes in human blood platelets as revealed by the quick-freezing and deep-etching replica method

This study evaluated ultrastructures of cell membrane-cytoskeletal interactions in resting and activated human blood platelets. Some blood platelets were fixed with paraformaldehyde to inhibit activation and the others were activated with thrombin treatment. The replicas of inside-out cell membranes, cell membrane surfaces, and Triton shells were made by the quick-freezing and deep-etching replica method. A geometrical pattern consisting of main hexagons and partial pentagons was revealed to be formed with microfilaments which were attached laterally to cytoplasmic sides of cell membranes in resting platelets. After the activation, cytoskeletal meshworks in the cytoplasm were densely associated with cell membranes, and parallel microfilamentous bundles in filopodia were connected with small polygonal meshworks lying under the cell membrane. The cell membrane-cytoskeletal interactions are essential to keeping a discoid shape at the resting stage, and to changing the shape quickly after the activation.

Filamin inhibits actomyosin ATPase activity in platelet

Filamin, an actin cross-linker protein, has been shown to exist in platelet. The role of this protein in the platelet has remained unclear. In this report, we show that filamin inhibits the actin-activated Mg2+ -ATPase activity of platelet myosin. The activation caused by platelet actin is inhibited by 50% at the molar ratio of filamin to actin of 1/50. Platelet tropomyosin, which we showed to enhance the ATPase activity, does not abolish the effect of filamin. The results support the view that filamin stabilizes the actin network in the resting platelet.