Targeting of Porcine Pancreatic Phospholipase A2 to Human Platelets: Introduction of an RGD Sequence by Genetic Engineering

The possibility to induce specific disruption of activated platelets by binding of porcine pancreatic phospholipase A2 (PLA2) was tested by constructing a set of PLA2-mutants containing an Arg-Gly-Asp (RGD) sequence. One mutant was made with RGD as part of a surface-exposed loop (RGDloop). Four mutants were made with RGD as part of a C-terminal extension: one with RGD directly coupled to the C-terminus (RGDc) and three mutants (CRSx) with x = 22,42 and 82 hydrophylic non-charged amino acids between RGD and the enzyme. All mutants retained 20-80% activity of native PLA2 and showed little binding to resting platelets. The binding of the native enzyme and RGDloop was not increased following stimulation. In contrast, the mutants RGDc and CRSx showed stimulation-dependent binding to the platelet receptor GPIIb/IIIa, since GRGDS-peptide and a monoclonal antibody against the complex interfered with binding. In α-thrombin-stimulated platelets, CRS42 and CRS82 induced about 5% hydrolysis of [3H]-arachidonic acid-labeled phospholipids. Stimulation with a combination of a-thrombin and collagen (known to expose phosphatidylserine) increased hydrolysis to 11%. Despite the membrane disruption, the cells did not leak lactate dehydrogenase. We conclude that PLA2 can be targeted to activated platelets by introducing RGD in a C-terminal extension with a minimum distance (42 amino acids) between RGD and the enzyme. However, more hydrolytic activity is required to eliminate activated platelets among a suspension of resting platelets and other blood cells.

Increased glutamine synthetase but normal EAAT2 expression in platelets of ALS patients

Amyotrophic lateral sclerosis is a fatal neurodegenerative disease and glutamate excitotoxicity has been implicated in its pathogenesis. Platelets contain a glutamate uptake system and express components of the glutamate-glutamine cycle, such as the predominant glial excitatory amino acid transporter 2 (EAAT2). In several neurological diseases platelets have proven to be systemic markers for the disease. We compared properties of key components of the glutamate-glutamine cycle in blood platelets of ALS patients and healthy controls. Platelets were analyzed for (3)H-glutamate uptake in the presence or absence of thrombin and for EAAT2 and glutamine synthetase protein expression by Western blotting. Platelets of ALS patients showed a 37% increase in expression of glutamine synthetase, but normal expression of glutamate transporter EAAT2. Glutamate uptake in resting or thrombin-stimulated platelets did not differ significantly between platelets from ALS patients and controls. Thrombin-stimulation resulted in about a seven-fold increase in glutamate uptake. Our data suggest that glutamine synthetase may be a peripheral marker of ALS and encourage further investigation into the role of this enzyme in ALS.

Thrombin-stimulated glutamate uptake in human platelets is predominantly mediated by the glial glutamate transporter EAAT2

Glutamate toxicity has been implicated in the pathogenesis of various neurological diseases. Glial glutamate transporters play a key role in the regulation of extracellular glutamate levels in the brain by removing glutamate from the extracellular fluid. Since human blood platelets possess an active glutamate uptake system, they have been used as a peripheral model of glutamate transport in the central nervous system (CNS). The present study is aimed at identifying the glutamate transporter on blood platelets, and to asses the influence of platelet activation on glutamate uptake. Platelets from healthy donors showed Na+-dependent glutamate uptake (Km, 3.5+/-0.9 microM; Vmax, 2.8+/-0.2 pmol glutamate/75 x 10(6)platelets/30 min), which could be blocked dose-dependently by the EAAT specific inhibitors DL-threo-E-benzyloxyaspartate (TBOA), L-trans-pyrrolidine-2,4-dicarboxylic acid (tPDC) and high concentrations of the EAAT2 inhibitor dihydrokainate (DHK). Analysis of platelet homogenates on Western blots showed EAAT2 as the predominant glutamate transporter. Platelet activation by thrombin caused an increase in glutamate uptake, which could be inhibited by TBOA and the EAAT2 inhibitor DHK. Kinetic analysis showed recruitment of new transporters to the membrane. Indeed, Western blot analysis of subcellular fractions revealed that alpha-granules, which fuse with the membrane upon thrombin stimulation, contained significant EAAT2 immunoreactivity. Inhibition of the second messengers involved in alpha-granule secretion (protein kinase C, phosphatidylinositol-3-kinase) inhibited thrombin-stimulated uptake, but not basal uptake. These data show that the glial EAAT2 is the predominant glutamate transporter on blood platelets and suggest, that thrombin increases glutamate uptake capacity by recruiting new transporters (EAAT2) from alpha-granules.

Involvement of the beta3 E749ATSTFTN756 region in stabilizing integrin alphaIIbbeta3-ligand interaction

Platelet integrin alphaIIbbeta3 must be activated via intracellular mechanisms before it binds soluble ligands, and it is thought to be activated at its extracellular site by surface-bound ligands. Integrin activation is associated with rearrangement of the cytoskeleton and phosphorylation of proteins that become localized in focal contacts. In these processes, the cytoplasmic tail of the beta-subunit plays a central role. We introduced peptides homologous to the E749ATSTFTN756 domain (E-N peptide) and the T755NITYRGT762 domain (T-T peptide) of beta3 in streptolysin O-permeabilized platelets and analyzed the initial interaction with soluble fibronectin, fibrinogen and PAC-1 after stimulation with thrombin. E-N peptide left the initial binding of fibronectin intact but interfered with stable receptor occupancy. E-N peptide also inhibited fibrinogen binding, thereby reducing the formation of large aggregates. Strikingly, E-N peptide did not disturb the binding of PAC-1, which is known to reflect activation of the integrin. E-N peptide also inhibited tyrosine phosphorylation of focal adhesion kinase, a response known to be dependent on alphaIIbbeta3. T-T peptide did not affect these processes. In a model for outside-in integrin activation, E-N peptide disrupted the binding of CHO cells expressing alphaIIbbeta3 to surface-bound ligand. Again, T-T peptide had no effect. We conclude that the E749ATSTFTN756 region of the beta3-tail stabilizes the binding of soluble and surface-bound ligand to integrin alphaIIbbeta3 via a mechanism that involves the phosphorylation of FAK.

Epinephrine--via activation of p38-MAPK--abolishes the effect of aspirin on platelet deposition to collagen

The mechanism by which epinephrine enhances experimental thrombosis in the presence of aspirin is poorly understood. In this study, we set to explore, in aspirinised platelet-rich plasma (PRP), the effect of epinephrine (100 nmol/l) on platelet deposition to immobilised collagen and the subsequent involvement of several intracellular pathways. Under these experimental conditions, which allow platelet aggregation on top of the collagen-adherent platelets, epinephrine increased platelet deposition by 55-86%. This enhancement could be specifically prohibited by the alpha(2A)-adrenoceptor antagonist, atipamezole, the p38 mitogen-activated protein kinase (p38MAPK) inhibitor SB203580, and the cytosolic phospholipase A(2) (cPLA(2)) inhibitor, mepacrine. The effect of epinephrine coincided with increased phosphorylation of p38MAPK and cPLA(2) and with arachidonic acid (AA) release from platelet membrane. We conclude that epinephrine enhanced platelet deposition on collagen in aspirinised PRP via a mechanism dependent on both free AA in platelet cytosol (released by cPLA(2)) and p38MAPK.

Negative regulation of the platelet Na+/H+ exchanger by trimeric G-proteins

Human platelets contain a Na+/H+ exchanger (NHE) that regulates the cytosolic pH. The role of trimeric G-proteins in NHE control was investigated in plasma membrane vesicles by measuring exchange of intravesicular protons for extravesicular Na+. Exchange was saturable, independent of membrane potential and inhibited by ethylisopropyl amiloride (Ki 0.05 micromol.L-1), demonstrating the involvement of NHE-1. The G-protein activators AlF4- and GMP-P(NH)P reduced exchange by increasing the Km for Na+ from 11.3 +/- 2.1 mM to 21.6 +/- 1.4 mM (AlF4-) and 19.8 +/- 1.1 mM (GMP-P(NH)P), leaving Vmax and the Hill coefficient unchanged. This effect was abolished by inhibitors of Gi-proteins (N-ethylmaleimide, holoenzyme- and A-protomer of pertussis toxin) and by an anti-Galpha Ig and GDP(beta)S. Activation of Gi-proteins by mastoparan and its synthetic analogue Mas7 also strongly reduced NHE activity. These data show that in platelets NHE-1 is under negative control of the Gi-family of trimeric G-proteins.

Platelets release thrombopoietin (Tpo) upon activation: another regulatory loop in thrombocytopoiesis?

Thrombopoietin is produced at a constant rate by the liver and kidney and is removed from the circulation upon binding and subsequent uptake via the Tpo receptor, c-Mpl, expressed by platelets and mega-karyocytes. Apart from uptake, this study shows that platelets can also function as a storage pool for Tpo. Upon stimulation with various platelet agonists, full-length biologically active Tpo was released by platelets. Platelet fractionation experiments indicated that this Tpo most likely is contained in the granules. When platelets were preincubated with Tpo-peptide mimetic or truncated Tpo prior to maximal activation, a three- to fivefold increment in Tpo release was seen. whereas, the release of other granule proteins such as vWF-propeptide or serotonin remained unchanged. Therefore, the Mpl agonists might compete with Mpl-bound Tpo, thereby releasing Tpo into the platelet supernatant. Intravascular release of Tpo by platelets might occur in patients with massive platelet activation, as occurs in patients with disseminated intravascular coagulation. The Tpo concentration in these patients is elevated (p <0.01) and correlates with markers for thrombin generation, TAT complexes and F1+2(r(p)= 0.8 and 0.9; p <0.01). This suggests that the increment in Tpo concentration was attributed to Tpo release by activated platelets in vivo, which might be instrumental in subsequent stimulation of thrombocytopoiesis.

Inhibition of platelet integrin alpha(IIb)beta(3) by peptides that interfere with protein kinases and the beta(3) tail

alpha-Thrombin stimulation of human platelets initiates inside-out signaling to integrin alpha(IIb)beta(3) (glycoprotein IIb/IIIa), resulting in the exposure of ligand binding sites. In the present study, the regulation of alpha(IIb)beta(3) via protein kinases was investigated in platelets permeabilized with streptolysin O by introducing peptides that interfere with these enzymes and with possible regulatory domains in the cytosolic tail of the beta(3) subunit. Compared with intact platelets, the permeabilized platelets preserved >80% of the aggregation, secretion, and alpha(IIb)beta(3) ligand binding capacity. The peptide YIYGSFK, a substrate for Src kinases, inhibited alpha-thrombin-induced ligand binding to alpha(IIb)beta(3), but a reversed peptide with Y-->F substitutions (KFSGFIF) had no effect. Ligand binding to alpha(IIb)beta(3) was also inhibited by the peptide RKRCLRRL, which binds irreversibly to the catalytic domain of protein kinase C. Peptides corresponding to parts of the protein C inhibitor and beta(2)-glycoprotein I were used as negative controls and failed to interfere with ligand binding. Possible target domains for protein kinases are present in the cytoplasmic tail of the beta(3) subunit. The LLITIHDR peptide, matching the membrane-proximal domain of beta(3) (residues 717 to 724), had no effect, but NNPLYKEA (residues 743 to 750), EATSTFTN (residues 749 to 756), and TNITYRGT (residues 755 to 762), which mimicked overlapping domains of the carboxy-terminal part of beta(3), reduced alpha-thrombin-induced ligand binding by 60+/-4%, 97+/-1%, and 97+/-2% (n=3) at 500 micromol/L peptide, respectively. These observations indicate that Src kinases and protein kinase C take part in inside-out signaling to integrin alpha(IIb)beta(3) and identify target domains in beta(3) that contribute to the regulation of this integrin.

Thrombopoietin potentiates collagen receptor signaling in platelets through a phosphatidylinositol 3-kinase-dependent pathway

Collagen activates platelets through a tyrosine kinase-dependent pathway, involving phospholipase Cgamma2. Functional responses such as aggregation and secretion induced by collagen are potentiated by preincubation with thrombopoietin (TPO). In this study, we show that collagen and thrombopoietin activate the phosphatidylinositol 3-kinase (PI 3-kinase) pathway and that this contributes to their respective actions. The structurally distinct inhibitors of PI 3-kinase, wortmannin, and LY294002, completely inhibit formation of phosphatidylinositol 3,4,5-trisphosphate by collagen. This leads to a substantial reduction in the formation of inositol phosphates and phosphatidic acid, 2 indices of PLC activity, and the consequent inhibition of intracellular Ca(++) [Ca(++)](i), aggregation and secretion. Potentiation of the collagen response by TPO is prevented in the presence of wortmannin and LY294002. However, when the 2 PI 3-kinase inhibitors are given after the addition of TPO but before the collagen, recovery of potentiation is observed. This suggests that potentiation is mediated through activation of PI 3-kinase. TPO stimulates aggregation of platelets from a low percentage of donors and this is also blocked by wortmannin. These results suggest that the PI 3-kinase pathway plays an important role in signaling by collagen and in the priming action of TPO.

Thrombopoietin increases platelet sensitivity to alpha-thrombin via activation of the ERK2-cPLA2 pathway

Thrombopoietin (TPO) regulates stem cell proliferation and maturation of megakaryocytes by activating the c-Mp1-receptor, a member of the hematopoietic cytokine family. As human platelets possess c-Mp1-receptors and supraphysiological concentrations of TPO trigger platelet aggregation and secretion, we searched for the signalling pathways through which the c-Mp1-receptor might activate platelets. A physiological concentration of TPO (20 ng/mL) did not trigger platelet functions, but increased their sensitivity to alpha-thrombin resulting in a 4-fold faster dense granule secretion. The effect of TPO was abolished by indomethacin and caused by synergism with signal generation by alpha-thrombin at the level of the cytosolic phospholipase A2 (cPLA2) pathway resulting in more arachidonate release, cPLA2 phosphorylation and thromboxane A2 formation. A similar synergism was seen at the level of extracellular signal-regulated kinase 2 (ERK2 or p42-MAPK). These data suggest, that TPO increases the sensitivity of platelets to alpha-thrombin by enhancing cPLA2 activation via the ERK2-cPLA2 pathway.

Sequential regulation of the small GTPase Rap1 in human platelets

Rap1, a small GTPase of the Ras family, is ubiquitously expressed and particularly abundant in platelets. Previously we have shown that Rap1 is rapidly activated after stimulation of human platelets with alpha-thrombin. For this activation, a phospholipase C-mediated increase in intracellular calcium is necessary and sufficient. Here we show that thrombin induces a second phase of Rap1 activation, which is mediated by protein kinase C (PKC). Indeed, the PKC activator phorbol 12-myristate 13-acetate induced Rap1 activation, whereas the PKC-inhibitor bisindolylmaleimide inhibited the second, but not the first, phase of Rap1 activation. Activation of the integrin alpha(IIb)beta(3), a downstream target of PKC, with monoclonal antibody LIBS-6 also induced Rap1 activation. However, studies with alpha(IIb)beta(3)-deficient platelets from patients with Glanzmann's thrombasthenia type 1 show that alpha(IIb)beta(3) is not essential for Rap1 activation. Interestingly, induction of platelet aggregation by thrombin resulted in the inhibition of Rap1 activation. This downregulation correlated with the translocation of Rap1 to the Triton X-100-insoluble, cytoskeletal fraction. We conclude that in platelets, alpha-thrombin induces Rap1 activation first by a calcium-mediated pathway independently of PKC and then by a second activation phase mediated by PKC and, in part, integrin alpha(IIb)beta(3). Inactivation of Rap1 is mediated by an aggregation-dependent process that correlates with the translocation of Rap1 to the cytoskeletal fraction.

Characterization of beta2 (CD18) integrin phosphorylation in phorbol ester-activated T lymphocytes

Integrins are transmembrane proteins involved in cell-cell and cell-extracellular-matrix interactions. The affinity and avidity of integrins for their ligands change in response to cytoplasmic signals. This 'inside-out' activation has been reported to occur also with beta2 integrins (CD18). The beta2 integrin subunit has previously been shown to become phosphorylated in T lymphocytes on cytoplasmic serine and the functionally important threonine residues after treatment with phorbol esters or on triggering of T-cell receptors. We have now characterized the phosphorylation of beta2 integrins in T-cells in more detail. When T-cells were activated by phorbol esters the phosphorylation was mainly on Ser756. After inhibition of serine/threonine phosphatases, phosphorylation was also found in two of the threonine residues in the threonine triplet 758-760 of the beta2 cytoplasmic domain. Activation of T-cells by phorbol esters resulted in phosphorylation in only approx. 10% of the integrin molecules. Okadaic acid increased this phosphorylation to approx. 30% of the beta2 molecules, assuming three phosphorylation sites. This indicates that a strong dynamic phosphorylation exists in serine and threonine residues of the beta2 integrins.

Recruitment of the LIM protein hic-5 to focal contacts of human platelets

Platelets are anuclear, membrane-bounded fragments derived from megakaryocytes which, upon stimulation, assemble an actin skeleton including stress fibres and focal contacts. The focal contacts resemble those of tissue culture cells. However, they lack paxillin, a conspicuous component of these organelles. We found that instead of paxillin, platelets contain a related protein with a molecular mass of 55 kDa that crossreacts with a monoclonal antibody against paxillin. The gene for the 55 kDa protein was cloned from a bone marrow cDNA library and turned out to be identical to a recently discovered gene encoding hic-5. Like paxillin, hic-5 is a cytoskeletal protein containing four carboxy-terminal LIM domains and LD motifs in the amino-terminal half. The LIM domains of both hic-5 and paxillin are capable of targetting green fluorescent protein to focal contacts. In addition, GST-hic-5 precipitates the focal adhesion kinase pp125(FAK) and talin from platelet extracts. Only trace amounts of hic-5 occur in DAMI cells, a megakaryocytic cell line, and in megakaryocytes cultured from CD34+ cells obtained from umbilical cord blood. However, RT-polymerase chain reactions performed with RNA obtained from platelets gave a positive result when primers specific for hic-5 were used, but were negative with paxillin-specific primers, indicating that a switch from paxillin expression to hic-5 expression must occur late in the maturation of megakaryocytes into platelets.

Maturation of megakaryoblastic cells is accompanied by upregulation of G(s)alpha-L subtype and increased cAMP accumulation

In platelets and megakaryoblasts Gs, the trimeric G-protein that stimulates adenylyl cyclase, is present in a short, 45 kDa, and a long, 52 kDa isoform termed G(s)alpha-S and G(s)alpha-L, respectively. To assess the relative contribution of these isoforms in the cellular synthesis of cAMP, the ratio G(s)alpha-S/G(s)alpha-L was changed in the megakaryoblastic cell line DAMI by inducing cell maturation with recombinant human thrombopoietin (TPO) or the phorbol ester PMA. Flow cytometric analysis confirmed that this treatment induced a moderate (TPO) and extensive (PMA) increase in nuclear ploidy and expression of the glycoproteins-IIIa and -Ib. Northern blot analysis revealed downregulation of total Gs-mRNA after treatment of DAMI-cells with TPO and PMA. Western blot analysis showed significant (P < 0.05) upregulation of Gs-L with respective amounts of 27 +/- 4% of total Gs in untreated cells, 35 +/- 1% in TPO- and 41 +/- 3% in PMA-treated DAMI cells (n = 3-4). DAMI cells contained 6 +/- 1 pmol cAMP/10(6) cells, which was not changed by treatment with TPO or PMA. In untreated cells this level increased to 70 +/- 9 pmol cAMP/10(6) cells after 10 min stimulation with 1 micromol/l of the stable prostacyclin analog iloprost. The same stimulation with iloprost resulted in 165 +/- 32 pmol cAMP/10(6) in TPO-treated cells and in 588 +/- 100 pmol cAMP/10(6) in cells treated with PMA. Thus, a shift from G(s)alpha-S to G(s)alpha-L during megakaryoblast maturation strongly potentiates the production of cAMP. A similar shift may occur during normal megakaryocyte maturation and may explain the extreme sensitivity to prostacyclin of platelets, which contain G(s)alpha-S and G(s)alpha-L in approximately equal amounts.

Differential involvement of tyrosine and serine/threonine kinases in platelet integrin alphaIIbbeta3 exposure

The relative contributions of protein tyrosine kinases (PTKs) and protein kinase C isoenzymes (PKCs), a family of serine/threonine kinases, in integrin alpha(IIb)beta3 (glycoprotein IIb/IIIa) exposure are the subject of much controversy. In the present study we measured the effect of the PTK inhibitor herbimycin A and the PKC inhibitor bisindolylmaleimide I on 125I-fibrinogen binding to alpha(IIb)beta3 and on aggregation/secretion induced by different agonists. Dose-response studies showed complete inhibition of alpha(IIb)beta3 exposure by 30 micromol/L (ADP stimulation) and 35 to 40 micromol/L (alpha-thrombin stimulation) herbimycin A. In contrast, inhibition of exposure by bisindolylmaleimide I varied from none (for ADP and epinephrine), to 30% (for platelet-activating factor), and to approximately 80% (for alpha-thrombin). Studies with a submaximal dose of herbimycin A (approximately 50% inhibition of the ADP-response) and a maximal dose of bisindolylmaleimide I showed that optical aggregation had a similar sensitivity to the inhibitors as alpha(IIb)beta3 exposure with minimal interference by secreted ADP. Thus, the relative contributions of tyrosine and serine/threonine kinases in alpha(IIb)beta3 exposure and aggregation differ among the different agonists, with an exclusive role for PTKs in ADP- and epinephrine-induced responses and a role for both PTKs and PKCs in responses induced by platelet-activating factor and alpha-thrombin.

Signal transduction through trimeric G proteins in megakaryoblastic cell lines

The biogenesis of trimeric G proteins was investigated by measurement of the expression of alpha-subunits in the megakaryoblastic cell lines MEG-01, DAMI, and CHRF-288-11, representing stages of increasing maturation, and compared with platelets. Megakaryoblasts and platelets contained approximately equal amounts of Gi alpha-1/2, Gi alpha-3, Gq alpha, and G12 alpha protein. Maturation was accompanied by (1) downregulation of mRNA for Gs alpha and disappearance of iloprost-induced Ca2+ mobilization, (2) upregulation of the long form of Gs alpha protein (Gs alpha-L) and an increase in iloprost-induced cAMP formation, and (3) upregulation of G16 alpha mRNA and G16 alpha protein and appearance of thromboxane A2-induced signaling (Ca2+ mobilization and stimulation of prostaglandin I2-induced cAMP formation). Gz alpha protein was absent in the megakaryoblasts despite weak expression of Gz alpha mRNA in DAMI and relatively high levels of Gz alpha mRNA and Gz alpha protein in platelets. These findings reveal major changes in G protein-mediated signal transduction during megakaryocytopoiesis and indicate that G16 alpha couples the thromboxane receptor to phospholipase C beta.

Subcellular localization of alpha-subunits of trimeric G-proteins in human platelets

Following subcellular fractionation of platelet homogenates and Western blotting, two groups of alpha-subunits of trimeric G-proteins could be distinguished. Group 1 consisted of alpha(i)-2, alpha(i)-3, alpha(z), alpha(s), and alpha(q) and was predominantly localized in membranes. Group 2 consisted of alpha16 and alph12 and was predominantly localized in the cytosol. Plasma membranes and dense tubular system (DTS)-membranes showed the same distribution of Group 1 alpha-subunits. An exception was alpha(q), which was virtually absent in the DTS as were Group 2 subunits. In addition, this compartment showed a doublet for alpha(z). Group 1 alpha-subunits were also found in fractions with the combined secretory granules and in separate dense granules. In addition, alpha16 was found in these granule fractions, but secretion granules were devoid of alpha12. These data reveal a heterogeneous distribution of alpha-subunits in platelet compartments and may indicate that G12 and G16 play different roles in platelets than members of the G(i) and G(s) classes and other members of the G(q) class.

Lateral mobility of integrin alpha IIb beta 3 (glycoprotein IIb/IIIa) in the plasma membrane of a human megakaryocyte

The migration of integrins to sites of cell-cell and cell-matrix contact is thought to be important for adhesion strengthening. We studied the lateral diffusion of integrin alpha IIb beta 3 (glycoprotein IIb/IIIa) in the plasma membrane of a cultured human megakaryocyte by fluorescence recovery after photobleaching of FITC-labelled monovalent Fab fragments directed against the beta 3 subunit. The diffusion of beta 3 on the unstimulated megakaryocyte showed a lateral diffusion coefficient (D) of 0.37 x 10(-9) cm2/s and a mobile fraction of about 50%. Stimulation with ADP (20 microM) or alpha-thrombin (10 U/ml) at 22 degrees C induced transient decreases in both parameters reducing D to 0.21 x 10(-9) cm2/s and the mobile fraction to about 25%. The fall in D was observed within 1 min after stimulation but the fall in mobile fraction showed a lag phase of 5 min. The lag phase was absent in the presence of Calpain I inhibitor, where-as cytochalasin D completely abolished the decreased in mobile fraction. The data are compatible with the concept that cell activation induces anchorage of 50% of the mobile alpha IIb beta 3 (25% of the whole population of receptor) to the cytoplasmic actin filaments, although, as discussed, other rationals are not ruled out.

Targeting of porcine pancreatic phospholipase A2 to human platelets. Introduction of an RGD sequence and acyl-group by chemical modification

In the present study we prepared by chemical modification a series of porcine pancreatic phospholipase A2 (PLA) derivatives, that bind to the activated glycoprotein (GP) IIb/IIIa complex and hydrolyse phospholipids in the outer leaflet of the platelet membrane. To the native enzyme, an RGD-containing peptide was coupled to introduce affinity for GPIIb/IIIa in combination with lauric acid to improve binding to the membrane. As controls, derivatives containing only one of these modifications were prepared. Acylation of the enzyme improved the affinity for densely packed phospholipids, as deduced by kinetic analyses. After stimulation of platelets, the RGD-containing PLAs bound to GPIIb/IIIa since GRGDS peptide and a monoclonal antibody against the complex interfered with binding. No binding was found with native PLA. The binding seen with lauric acid PLA was not mediated by GPIIb/IIIa. All modified PLAs induced 1-3% hydrolysis of [3H]arachidonic-acid-labelled phospholipids in resting platelets. After activation with alpha-thrombin, hydrolysis increased to 17%, corresponding to about 90% of [3H]arachidonate-labelled phospholipids in the outer leaflet of the plasma membrane. RGD-containing PLAs were more active than lauroyl PLA, and their activity was mediated via GPIIb/IIIa since GRGDS inhibited release of [3H]arachidonic acid. Acylation of the RGD-containing PLAs did not further improve the hydrolytic properties. We conclude that chemical modification of PLA leads to a targetted hydrolytic action and could be a basis for the design of enzymes that specifically destroy activated platelets.

Exposure of ligand-binding sites on platelet integrin alpha IIB/beta 3 by phosphorylation of the beta 3 subunit

The exposure of ligand-binding sites for adhesive proteins on platelet integrin alpha IIB/beta 3 (glycoprotein IIB/IIIA) by platelet-activating (PAF) is transient, whereas sites exposed by alpha-thrombin remain accessible. The same difference is seen in the phosphorylation of the beta 3 subunit. Inhibition of protein kinases (1 microM staurosporine) and protein kinase C (10 microM bisindolylmaleimide) closes binding sites exposed by both agonists and induces dephosphorylation of beta 3. Inhibition of Tyr-kinases (20 microM Herbimycin A) has only a slight effect. Inhibition of Ser/Thr-phosphatases (1 microM okadaic acid, 30 s preincubation) changes the transient exposure and beta phosphorylation by PAF into the 'permanent' patterns induced by alpha-thrombin. Inhibition of Tyr-phosphatases (100 microM vanadate) has little effect. Preincubation with okadaic acid makes exposed binding sites and phosphorylated beta 3 insensitive to staurosporine, resulting in exposed alpha IIB/beta 3 independent of concurrent phosphorylation/dephosphorylation. The stoichiometry of beta 3 phosphorylation by alpha-thrombin is 0.80+/-0.10. Thus, one of the mechanisms that regulates exposure and closure of ligand-binding sites on the alpha IIb/beta 3 is phosphorylation/dephosphorylation of a Ser/Thr-residue in the beta 3 subunit.

Targeting of porcine pancreatic phospholipase A2 to human platelets: introduction of an RGD sequence by genetic engineering

The possibility to induce specific disruption of activated platelets by binding of porcine pancreatic phospholipase A2 (PLA2) was tested by constructing a set of PLA2-mutants containing an Arg-Gly-Asp (RGD) sequence. One mutant was made with RGD as part of a surface-exposed loop (RGDloop). Four mutants were made with RGD as part of a C-terminal extension: one with RGD directly coupled to the C-terminus (RGDc) and three mutants (CRSx) with x= 22, 42 and 82 hydrophylic non-charged amino acids between RGD and the enzyme. All mutants retained 20-80% activity of native PLA2 and showed little binding to resting platelets. The binding of the native enzyme and RGDloop was not increased following stimulation. In contrast, the mutants RGDc and CRSx showed stimulation-dependent binding to the platelet receptor GPIIb/IIIa, since GRGDS-peptide and a monoclonal antibody against the complex interfered with binding. In alpha-thrombin-stimulated platelets, CRS42 and CRS82 induced about 5% hydrolysis of [3H]-arachidonic acid-labeled phospholipids. Stimulation with a combination of alpha-thrombin and collagen (known to expose phosphatidylserine) increased hydrolysis to 11%. Despite the membrane disruption, the cells did not leak lactate dehydrogenase. We conclude that PLA2 can be targeted to activated platelets by introducing RGD in a C-terminal extension with a minimum distance (42 amino acids) between RGD and the enzyme. However, more hydrolytic activity is required to eliminate activated platelets among a suspension of resting platelets and other blood cells.

Identification of alpha-subunits of trimeric GTP-binding proteins in human platelets by RT-PCR

In a search for new alpha-subunits of trimeric GTP-binding proteins in human platelets, we prepared leucocyte-free platelet concentrates and analyzed total RNA for areas homologous to known alpha-subunits. RT-PCR based on two degenerate primers revealed the expected band of 495 base pairs and an additional band of 540 base pairs reflecting the alternative splice product of Gs alpha. Following subcloning in pGEM-T vector and sequencing, we identified the alpha-subunits Gi alpha-2 and Gs alpha-S of the regulating GTP-binding proteins of adenyl cyclase as well as Gz alpha whose function is unknown, confirming earlier immunological identification. In addition, we identified Gs alpha-L (differing from Gs alpha-S by an insertion of 45 base pairs), G16 alpha, (a member of the pertussis toxin insensitive Gq-family), and two new variants of both Gs alpha-S and Gs alpha-L each containing a C-A-G triplet. With G16 we have identified another candidate for pertussis-toxin insensitive signal transduction in platelets. The C-A-G containing sequences of Gs alpha lead to an insertion of a Ser-residue, which results in the consensus sequence of a phosphorylation site for protein kinase C (Ser-X-Lys), making these variants candidates for protein kinase C-sensitive cyclic AMP formation.

Regulation of platelet glycoprotein IIb/IIIa (integrin alpha IIB beta 3) function via the thrombin receptor

Binding sites on glycoprotein (GP) IIb/IIIa exposed by 0.5 unit/ml alpha-thrombin are insensitive to prostaglandin I2 (PGI2), in contrast with sites exposed by ADP or platelet-activating factor. Here we show that the thrombin receptor agonist peptide (TRAP) (SFLLRN; 15 microM) opens almost the same number of GPIIb/IIIa molecules as 0.5 unit/ml alpha-thrombin (64840 +/- 8920 compared with 81050 +/- 6030 molecules of fibronectin bound/platelet), but these sites rapidly close on addition of PGI2. To investigate whether alpha-thrombin and TRAP initiate different signalling pathways, we measured phospholipase C (PLC)-mediated control of GPIIb/IIIa and its sensitivity to cyclic AMP. Optimal concentrations of alpha-thrombin and TRAP activated PLC maximally, but TRAP induced only about 50% protein kinase C PKC) activation after 10 min stimulation compared with alpha-thrombin. These concentrations also suppressed PGI2-induced cyclic AMP accumulation, with alpha-thrombin inducing complete inhibition and TRAP about 10% less. Direct activation of PKC by phorbol 12-myristate 13-acetate confirmed earlier observations that PGI2-induced cyclic AMP accumulation is partly inhibited via PKC. Applying different concentration of alpha-thrombin, TRAP or a combination of alpha-thrombin and the thrombin receptor inhibitory peptide (TRIP) (Mpr-F-Cha-Cha-RKPNDK-NH2; 800 microM) (Mpr, 3-mercaptopropionic acid; Cha, cyclohexylalanine), we show that the different means of stimulating the thrombin receptor all suppressed PGI2-induced cyclic AMP accumulation via (i) activation of PKC and (ii) activation of the heterotrimeric G-protein, Gi. We conclude that complete inhibition of cyclic AMP accumulation requires activation of both PKC and Gi, as observed with 0.5 unit/ml alpha-thrombin. Although TRAP almost fully exposes GPIIb/IIIa, its activation of PKC is incomplete, enabling PGI2 to raise cyclic AMP concentration from 1.4 +/- 0.7 to 4.1 +/- 1.3 nmol/10(11) platelets (P < 0.005) which is sufficient to close exposed GPIIb/IIIa molecules.

Suitability of low-molecular-weight heparin(oid)s and a pentasaccharide for an in vitro human thrombosis model

There is much interest in in vitro thrombosis systems using exclusively human materials for evaluating new drugs. We have previously developed such a model using a perfusion chamber in which whole blood anticoagulated with low-molecular-weight heparin (LMWH) was circulated over the extracellular matrix of endothelial cells that had been stimulated with phorbol myristate acetate to cause tissue factor formation. Here we studied various LMWHs and a pentasaccharide to find out which was most useful in an in vitro thrombosis model. We compared unfractionated heparin, two commercial LMWHs (Fragmin and Fraxiparine), one commercial heparinoid (Orgaran), and a chemically synthesized derivative of the natural pentasaccharide (Org 31550). Blood was anticoagulated with the concentration of each glycosaminoglycan that prevented thrombin formation for at least 3 hours in the test tube (Fragmin, 20 anti-Xa U/mL; Fraxiparine, 40 anti-Xa Institute Choay U/mL; Orgaran, 15 anti-Xa U/mL; Org 31550, 200 anti-Xa U/mL; unfractionated heparin, 5 IU/mL) and recirculated over the matrix of unstimulated cells (no tissue factor in the matrix) and phorbol-stimulated endothelial cells (tissue factor in the matrix). Platelet adhesion, aggregate formation, and fibrin deposition were evaluated. In perfusions over the extracellular matrix of unstimulated cells, the highest platelet adhesion rates were observed with Orgaran. Fibrin deposition was absent with unfractionated heparin in phorbol-stimulated matrix. Increasing amounts of fibrin were observed with Orgaran, Fragmin, and Fraxiparine. Most fibrin was found with the pentasaccharide Org 31550.(ABSTRACT TRUNCATED AT 250 WORDS)

Alpha 2A-adrenergic receptors activate protein kinase C in human platelets via a pertussis toxin-sensitive G-protein

4,4'-Diisothiocyanato-stilbene-2,2'-disulfonic acid (DIDS) stimulates human platelets via alpha 2A-adrenergic receptor-mediated activation of protein kinase C (PKC) independent of the phospholipase C pathway. Here we show, that in permeabilized platelets activation of PKC by DIDS (20 microM), measured as 32P incorporation in pleckstrin, is completely inhibited by guanosine 5'-(2-O-thio)diphosphate (200 microM), an inhibitor of heterotrimeric G-proteins. Also pertussin toxin (4 micrograms/ml), which ADP-ribosylates the alpha-subunits of Gi's and Go, prevents pleckstrin phosphorylation by DIDS. N-Ethylmaleimide (50 microM), which uncouples Gi from alpha 2A-adrenoceptors, inhibits pleckstrin phosphorylation by DIDS in intact platelets. Activation of PKC by 55 nM phorbol 12-myristate 13-acetate and 500 nM platelet-activating factor are not disturbed by NEM. DIDS inhibits by 40 +/- 5% (n = 4) the pertussis toxin-catalyzed [32P]ADP-ribosylation of a 41 kDa protein fraction previously shown to contain the alpha-subunits of Gi alpha-1, Gi alpha-2 and Gi alpha-3. Thus, the alpha 2A-adrenergic receptor activates PKC via a G-protein of the Gi-family.

LDLs increase the exposure of fibrinogen binding sites on platelets and secretion of dense granules

Because previous studies show that lipoproteins affect platelet aggregation, we studied the effect of low-density lipoprotein (LDL) and high-density lipoprotein (HDL) on the binding of fibrinogen, which mediates platelet-platelet contact. Neither LDL nor HDL induced 125I-fibrinogen binding at concentrations up to 2 g protein/L. In contrast, platelets stimulated with 10 mumol/L ADP bound 63 734 +/- 2453 molecules of fibrinogen per platelet. A 5-minute preincubation with LDL (0.5 to 2 g/L protein) induced a dose-dependent increase to 91 307 +/- 2164 molecules of fibrinogen per platelet at 1.5 g/L, which is in the range found after optimal stimulation with alpha-thrombin. The increased fibrinogen binding in the presence of LDL resulted in faster aggregation with a 16% increase in single platelet disappearance and a faster optical aggregation at 5 mumol/L ADP and 1.5 g protein/L LDL. Inhibition of prostaglandin G2/H2-thromboxane A2 formation with indomethacin (30 mumol/L) did not change the stimulation by LDL. In contrast, modification of lysine residues of LDL, which is known to prevent specific binding to platelets, completely abolished the effect of LDL. Under the same conditions HDL did not change fibrinogen binding or aggregation. LDL also enhanced alpha-thrombin-induced [14C]serotonin secretion, but this property was not affected by lysine modification of LDL. These data indicate that LDL enhances platelet aggregation by stimulating the mechanisms that control exposure of fibrinogen binding sites on the glycoprotein IIB/IIIA complex via a mechanism that differs from the effect of LDL on secretion.

Different pathways for control of Na+/H+ exchange via activation of the thrombin receptor

The aim of the present study was to clarify the control of Na+/H+ exchange in platelets activated via the thrombin receptor. When human BCECF-loaded platelets were stimulated with the thrombin-receptor-activating peptide (TRAP; amino acid sequence SFLLRN), which activates the receptor independently of proteolysis, the cytosolic pH (pHi) rose from 7.13 +/- 0.04 (n = 6) to 7.27 +/- 0.04 (n = 5), followed by a rapid decrease to resting values. Trypsin, which cleaves the receptor, induced a rapid and irreversible rise in pHi to 7.31 +/- 0.06 (n = 5). gamma-Thrombin, which cleaves the receptor but is unable to bind to the hirudin-like domain, induced a slow and irreversible rise in pHi to 7.31 +/- 0.04 (n = 14). alpha-Thrombin, which cleaves the receptor and binds to its hirudin-like domain, induced a rapid and irreversible rise in pHi to 7.31 +/- 0.04 (n = 22). Changes in pHi induced by TRAP, trypsin, gamma- and alpha-thrombin were accompanied by similar changes in cytosolic Ca2+ concentration ([Ca2+]i) and 32P-pleckstrin, a substrate of protein kinase C (PKC). The separate chelation of Ca2+i (30 microM BAPTA-AM) or inhibition of PKC (1 microM staurosporine) induced about 50% inhibition of the pHi responses triggered by TRAP, trypsin, gamma- and alpha-thrombin, but the combination induced complete inhibition. Thus the different types of activation of the thrombin receptor control Na+/H+ exchange via the same mechanism. Binding of thrombin to the hirudin-like domain accelerates exchange activation, whereas proteolysis of the receptor is essential for a sustained increase in pHi.

Regulation of glycoprotein IIB/IIIA exposure on platelets stimulated with alpha-thrombin

Previous studies have shown that binding sites for fibrinogen on platelets stimulated with platelet-activating factor (PAF), adenosine diphosphate or epinephrine rapidly close in the absence of fibrinogen. In the present study we investigated whether alpha-thrombin induced similar changes in the glycoprotein (GP) IIB/IIIA-complex. Whereas 80% of binding sites exposed by PAF closed within 30 minutes (22 degrees C), alpha-thrombin (0.1 U/mL) triggered long-lasting exposure of binding sites for [125I]-fibrinogen and [125I]-fibronectin. Even removal of alpha-thrombin with an excess of hirudin failed to close the binding sites. Similar to PAF, alpha-thrombin-exposed sites rapidly closed after addition of the protein kinase C inhibitor staurosporine (1 mumol/L) or dibutyryl cyclic adenosine monophosphate (250 mumol/L). In contrast, prostacyclin (PGI2, 10 ng/mL), which induced rapid closure of binding sites in platelets stimulated with PAF, failed to close the sites in alpha-thrombin-treated platelets. Removal of alpha-thrombin from the platelets restored the PGI2-sensitivity. These data indicate that a short interaction between alpha-thrombin and platelets triggers long-lasting exposure of GPIIB/IIIA. Furthermore, as long as alpha-thrombin remains bound to the platelets, agonists that activate the PGI2-receptor are unable to close GPIIB/IIIA.

Protein kinase C and cyclic AMP regulate reversible exposure of binding sites for fibrinogen on the glycoprotein IIB-IIIA complex of human platelets

Platelet aggregation is mediated via binding of fibrinogen to sites on the membrane glycoprotein IIB-IIIA complex which become exposed when the cells are stimulated. We report here evidence of a dynamic and reversible exposure of binding sites for fibrinogen. In the absence of fibrinogen, exposed sites (B*) gradually lose their capacity to bind fibrinogen and close (Bo). On stimulation with platelet-activating factor (PAF, 500 nM) at 22 degrees C, closing of B* is enhanced by agents that raise cyclic AMP levels (10 ng of prostaglandin I2/ml; 5 mM-theophylline), inhibit protein kinase C (PKC; 25 microM-sphingosine; 1 microM-staurosporine), or disrupt the energy supply (30 mM-2-deoxy-D-glucose + 1 mM-CN-), or by raising the temperature to 37 degrees C. Conversely, activation of PKC 1 microM-1,2-dioctanoyl-sn-glycerol; 55 nM-phorbol 12-myristate 13-acetate) and an increase in intracellular [Ca2+] (100 nM-ionomycin + extracellular Ca2+) oppose the disappearance of B*. Phosphorylation of the 47 kDa protein illustrates the tight coupling between PKC and B* under all conditions tested, except when the cyclic AMP level is raised, and B* is converted to Bo without affecting PKC activity. Although the increase in PKC activity is much smaller with ADP or even absent upon stimulation with adrenaline, the control of B* is equally sensitive to modulation of cyclic AMP and PKC activity. We conclude that PAF, ADP and adrenaline regulate exposure of fibrinogen binding sites through a common mechanism consisting of two independent pathways, one dominated by PKC and the other by an as yet unidentified cyclic AMP-sensitive step.

Different evolution rates within the lens-specific beta-crystallin gene family

We have determined the sequence of a rat beta A3/A1-crystallin complementary DNA (cDNA) clone and the (partial) sequence of the human beta B3-crystallin gene. Calculation of the ratio of silent to nonsynonymous substitution between orthologous beta A3/A1-, beta B3-, and other beta- and gamma-crystallin sequences revealed that the region encoding the two globular domains of the beta A3/A1-crystallin sequence is the best conserved during evolution, much better than the corresponding region of the beta B1-, beta B3-, or the gamma-crystallin sequences, and even better (at least in the rodent/frog comparison) than the well-conserved alpha A-crystallin sequence. Remarkably, the rate of change of the beta A3/A1-crystallin coding sequence does not differ in the rodent and primate lineages, in contrast with previous findings concerning the evolution rates of the alpha A- or gamma-crystallin sequences in these two lineages. Comparison of the regions that encode the four motifs of the beta-crystallin between orthologous mammalian sequences showed that the extent of nonsynonymous substitution in each of these four homologous motif regions is the same. However, when the orthologous beta-crystallin genes of more distantly related species (mammals vs chicken or frog) are compared, the extent of non-synonymous substitution is higher in the regions encoding the external motifs I and III than in the regions encoding the internal motifs II and IV. This phenomenon is also observed when paralogous members of the beta/gamma-crystallin supergene family are compared.

Attachment of origins of replication to the nuclear matrix and the chromosomal scaffold

We have investigated the attachment of DNA to the nuclear matrix and chromosomal scaffold in synchronized bovine liver cells. Label incorporated at the onset of the S phase remained preferentially associated with the matrix during the subsequent G1 phase and with a residual protein structure from dehistonized chromosomes during mitosis. On the other hand label incorporated during mid or late S phase was about equally distributed over the DNA molecule after a chase into the G1 phase. These results suggest that DNA is attached to the nuclear matrix and chromosome scaffolds by the origins of replication.