Regulated membrane-cytoskeleton linkages in platelets

Integrins: dynamic scaffolds for adhesion and signaling in platelets

The major platelet integrin, alphaIIbbeta3, is required for platelet interactions with proteins in plasma and the extracellular matrices (ECMs) that are essential for platelet adhesion and aggregation during hemo stasis and arterial thrombosis. Lig and binding to alphaIIbbeta3 is controlled by inside-out signals that modulate receptor conformation and clustering. In turn, ligand binding triggers outside-in signals through alphaIIbbeta3 that, when disrupted, can cause a bleeding diathesis. In the past 5 years there has been an explosion of knowledge about the structure and function ofalphaIIbbeta3 and the related integrin, alphaVbeta3. These developments are discussed here, and current models of bidirectional alphaIIbbeta3 signaling are presented as frameworks for future investigations. An understanding that alphaIIbbeta3 functions as a dynamic molecular scaffold for extracellular and intracellular proteins has translated into diagnostic and therapeutic insights relevant to hematology and cardiovascular medicine, and further advances can be anticipated.

Association of thrombospondin-1 with the actin cytoskeleton of human thrombin-activated platelets through an alphaIIbbeta3- or CD36-independent mechanism

Thrombospondin-1 (TSP-1) is an adhesive glycoprotein which, when secreted from alpha-granules of activated platelets, can bind to the cell surface and participate in platelet aggregate formation. In this study, we show that thrombin activation leads to the rapid and specific association of a large amount of secreted alpha-granular TSP-1 with the actin cytoskeleton. This cytoskeletal association of TSP-1 was correlated with platelet secretion, but not aggregation, and was inhibited by cytochalasin D, an inhibitor of actin polymerization. Association of TSP-1 with the actin cytoskeleton was mediated by membrane receptors, as shown by using MAII, a TSP-1-specific monoclonal antibody that inhibited both TSP-1 surface binding to activated platelets and cytoskeletal association. TSP-1 and its potential membrane receptors, e.g. alphaIIbbeta3 integrin, CD36 and CD47, concomitantly associated with the actin cytoskeleton. However, studies on platelets from a patient with type I Glanzmann's thrombasthenia lacking alphaIIbbeta3 and another with barely detectable CD36 showed normal TSP-1 surface expression and association with the actin cytoskeleton. Likewise, no involvement of CD47 in TSP-1 association with the actin cytoskeleton could be inferred from experiments with control platelets using the function-blocking anti-CD47 antibody B6H12. Finally, assembly of signalling complexes, as observed through translocation of tyrosine-phosphorylated proteins and kinases to the actin cytoskeleton, was found to occur in concert with cytoskeletal association of TSP-1, in control platelets as well as in thrombasthenic and CD36-deficient platelets. Our results imply a role for the actin cytoskeleton in the membrane-surface expression process of TSP-1 molecules and suggest a possible coupling of TSP-1 receptors to signalling events occurring independently of alphaIIbbeta3 or CD36. These results provide new insights into the link between surface-bound TSP-1 and the contractile actin microfilament system which may promote platelet aggregate cohesion.

Myristoylated alanine-rich C kinase substrate phosphorylation is involved in thrombin-induced serotonin release from platelets

Stimulation of platelets by thrombin induces protein kinase C (PKC) activation, phosphorylation of pleckstrin, aggregation and serotonin release. Here, we demonstrate that, in human platelets, thrombin stimulation also induced phosphorylation of the myristoylated alanine-rich C kinase substrate (MARCKS) and serotonin release in intact and digitonin-permeabilized platelets. MARCKS is known to bind actin and cross-link actin filaments, and this is inhibited by PKC-evoked MARCKS phosphorylation. MARCKS phosphorylation and serotonin release in response to increasing concentrations of thrombin have a similar EC50 and time course and, in permeabilized platelets, peptide MPSD, with an amino acid sequence corresponding to the phosphorylation site domain of MARCKS, blocked both responses. However, pleckstrin and myosin light chain phosphorylations were not modified. Ala-MPSD, in which the four serine residues of MPSD were substituted by alanines was ineffective. The results suggest a role for MARCKS in platelet secretion. The fact that pleckstrin phosphorylation has a different time course and was not modified in the presence of MPSD when MARCKS phosphorylation and serotonin release were inhibited would suggest either that pleckstrin phosphorylation is unrelated to secretion or that it might only be involved upstream in the events leading to secretion.

Platelet secretion induced by phorbol esters stimulation is mediated through phosphorylation of MARCKS: a MARCKS-derived peptide blocks MARCKS phosphorylation and serotonin release without affecting pleckstrin phosphorylation*

Previous experiments suggest that actin disassembly, perhaps at a specific site, is required for platelet secretion. Platelet stimulation by phorbol 12-myristate 13-acetate (PMA) induced pleckstrin phosphorylation, platelet aggregation, and secretion. Inhibition of protein kinase C (PKC) is accompanied by inhibition of pleckstrin phosphorylation and serotonin secretion. Here, we demonstrate the presence of myristoylated alanine-rich C kinase substrate (MARCKS), another PKC substrate, in platelets and its phosphorylation during PMA stimulation. MARCKS is known to bind actin and to cross-link actin filaments; the latter is inhibited by PKC-induced MARCKS phosphorylation. MARCKS phosphorylation and serotonin release from permeabilized platelets have the same time course and were blocked by a peptide (MPSD) with the amino acid sequence corresponding to the phosphorylation site domain of MARCKS. Pleckstrin and myosin light chain phosphorylation was not modified. A peptide (Ala-MPSD) in which the four serine residues of MPSD were substituted by alanines was ineffective. These results provide the first evidence that MARCKS may play a role in platelet secretion. Moreover, pleckstrin phosphorylation has a different time course than that of MARCKS or serotonin release and was not modified when MARCKS phosphorylation and serotonin release were inhibited, suggesting that pleckstrin is either not directly involved in secretion or that it might only be involved upstream in the cascade of events leading to exocytosis.

Selective association of the tyrosine kinases Src, Fyn, and Lyn with integrin-rich actin cytoskeletons of activated, nonaggregated platelets

Integrin-mediated interactions between cytoskeletal proteins and extracellular fibrinogen are required for platelet adhesion. We have previously demonstrated that the major platelet integrin, alpha(IIb)beta(3), becomes incorporated into the actin cytoskeleton of platelets in an activation-dependent, aggregation-independent manner. To determine if regulatory molecules are also associated with these integrin-rich cytoskeletal complexes, we examined actin cytoskeletons for the presence of kinases and phosphoproteins. Western immunoblot analysis revealed that the tyrosine kinases Src, Fyn, and Lyn are specifically associated with actin cytoskeletons of activated, nonaggregated platelets. However, as noted by others, the cytoskeletal association of focal adhesion kinase depends on platelet aggregation. Actin cytoskeletons isolated from (32)P-labeled platelets also contain a number of phosphorylated proteins. Interestingly, an approximately 18-kDa phosphoprotein was uniquely present in activated platelet cytoskeletons. Collectively, our results demonstrate that actin cytoskeletons of activated, nonaggregated platelets contain not only integrins, but also kinases and phosphoproteins that could regulate platelet adhesion and transmembrane communication.

Modulatory effect of 8-iso-PGE2 on platelets

1. 8-Iso-PGE2 induced either reversible or irreversible aggregation of platelets in human platelet-rich plasma (PRP) or in the suspension of washed platelets (WP). The values of EC50 for irreversible aggregation in PRP and WP were 4 and 2 microM, respectively. 2. In rabbit PRP, 8-iso-PGE2 (0.1-100 microM) itself did not induce or induced only reversible aggregation. 3. 8-Iso-PGE2 (0.1-20 microM) potentiated adenosine diphosphate-(ADP) induced platelet aggregation in both human and rabbit. The same effect also was found for adrenaline-induced platelet aggregation in rabbit. 4. The lower concentrations (0.2-0.5 microM) of 8-iso-PGE2 decreased, and higher concentrations (1-2 microM) increased platelet aggregating factor- (PAF) induced aggregation in human PRP. In rabbit PRP, 8-iso-PGE2 (0.02-200 microM) had only a decreasing effect on PAF-induced aggregation. 5. The results suggest that low concentrations of 8-iso-PGE2 can amplify or weaken platelet aggregation induced by various aggregatory agents.

Cytoskeletal domains in the activated platelet

Platelets circulate in the blood as discoid cells which, when activated, change shape by polymerizing actin into various structures, such as filopodia and stress fibers. In order to understand this process, it is necessary to determine how many other proteins are involved. As a first step in defining the full complement of actin-binding proteins in platelets, filamentous (F)-actin affinity chromatography was used. This approach identified > 30 different proteins from ADP-activated human blood platelets which represented 4% of soluble protein. Although a number of these proteins are previously identified platelet actin-binding proteins, many others appeared to be novel. Fourteen different polyclonal antibodies were raised against these apparently novel proteins and used to sort them into nine categories based on their molecular weights and on their location in the sarcomere of striated muscle, in fibroblasts and in spreading platelets. Ninety-three percent of these proteins (13 of 14 proteins tested) were found to be associated with actin-rich structures in vivo. Four distinct actin filament structures were found to form during the initial 15 min of activation on glass: filopodia, lamellipodia, a contractile ring encircling degranulating granules, and thick bundles of filaments resembling stress fibers. Actin-binding proteins not localized in the discoid cell became highly concentrated in one or another of these actin-based structures during spreading, such that each structure contains a different complement of proteins. These results present crucial information about the complexity of the platelet cytoskeleton, demonstrating that four different actin-based structures form during the first 15 min of surface activation, and that there remain many as yet uncharacterized proteins awaiting further investigation that are differentially involved in this process.