Platelet receptor recognition site on human fibrinogen. Synthesis and structure-function relationship of peptides corresponding to the carboxy-terminal segment of the gamma chain

Binding of fibrinogen to human platelets depends on the interaction of the gamma-chain carboxy-terminal segment with specific receptors exposed by different agonists such as ADP, epinephrine, and thrombin. The functions of a series of synthetic peptides encompassing the sequence of the 15 carboxy-terminal residues of the gamma chain were investigated in this study. Both pentadecapeptide (gamma 397-411) and dodecapeptide (gamma 400-411) inhibited binding of 125I-fibrinogen to ADP-treated platelets, with the concentration causing 50% inhibition (IC50) being 28 microM. In comparison, decapeptide (gamma 402-411) was almost 4 times less active (IC50 = 106 microM), thus suggesting that the two histidine residues (gamma 400-401) are required for a full inhibitory effect. A heptapeptide (gamma 405-411) had a similar effect (IC50 = 102 microM) whereas a pentapeptide (gamma 407-411) was even less inhibitory (IC50 = 190 microM), indicating that the lack of lysine (gamma 406) further diminishes the reactivity of the platelet recognition site on the gamma chain of human fibrinogen. The heptapeptide (gamma 400-406) containing two histidine residues and derived from the dodecapeptide by proteolytic degradation with trypsin had very low inhibitory activity. The synthetic peptides inhibited fibrinogen-supported platelet aggregation in the same order of decreasing reactivity: pentadecapeptide = dodecapeptide greater than decapeptide = heptapeptide greater than pentapeptide. Modified synthetic pentadecapeptides bearing tyrosine or cysteinyltyrosine at the amino terminal were prepared to provide a means for radiolabeling and for formation of molecules of higher valency.(ABSTRACT TRUNCATED AT 250 WORDS)

Interaction of fibrinogen with staphylococcal clumping factor and with platelets

Fibrinogen, a clottable plasma glycoprotein, participates in cell adhesion phenomena involving prokaryotic cells, e.g. staphylococci, and eukaryotic cell fragments, e.g. platelets. Among the three chains (alpha, beta, gamma) of human fibrinogen, the gamma chain bears the main site recognizing the staphylococcal clumping receptor and human platelet receptor induced by ADP. The platelet receptors are also recognized, albeit less avidly, by a site associated with the alpha chain. The gamma chain site recognizing staphylococcal clumping factor exists on the COOH-terminal segment of this chain encompassing the 15 residues (gamma 397-411) including the COOH-terminal valine. The location of the gamma chain site interacting with the human platelet receptor had been pinpointed to the 27 residue CNBr COOH-terminal segment (gamma 385-411). The results of enzymatic degradation of the 27-residue peptide indicate that the continuity of the last 15 amino acid residues at the COOH-terminal end of the gamma chain of human fibrinogen seems to be essential for its interaction with human platelets. The sequence of the gamma chain interacting with the platelet receptor (gamma 385-411) indicates that this segment is a unique region of fibrinogen endowed with three important functions: cross-linking of fibrin, clumping of staphylococci, and aggregation of platelets. [Note added in proof: Recently we obtained evidence that dodecapeptide gamma 393-411 fully retains platelet receptor recognition site (Kloczewiak et al. 1983. Clin. Res 31:534A.)]

Endogenous biosynthesis of prostacyclin and thromboxane and platelet function during chronic administration of aspirin in man

To assess the pharmacologic effects of aspirin on endogenous prostacyclin and thromboxane biosynthesis, 2,3-dinor-6-keto PGF1 alpha (PGI-M) and 2,3-dinor-thromboxane B2 (Tx-M) were measured in urine by mass spectrometry during continuing administration of aspirin. To define the relationship of aspirin intake to endogenous prostacyclin biosynthesis, sequential urines were initially collected in individuals prior to, during, and subsequent to administration of aspirin. Despite inter- and intra-individual variations, PGI-M excretion was significantly reduced by aspirin. However, full mass spectral identification confirmed continuing prostacyclin biosynthesis during aspirin therapy. Recovery of prostacyclin biosynthesis was incomplete 5 d after drug administration was discontinued. To relate aspirin intake to indices of thromboxane biosynthesis and platelet function, volunteers received 20 mg aspirin daily followed by 2,600 mg aspirin daily, each dose for 7 d in sequential weeks. Increasing aspirin dosage inhibited Tx-M excretion from 70 to 98% of pretreatment control values; platelet TxB2 formation from 4.9 to 0.5% and further inhibited platelet function. An extended study was performed to relate aspirin intake to both thromboxane and prostacyclin generation over a wide range of doses. Aspirin, in the range of 20 to 325 mg/d, resulted in a dose-dependent decline in both Tx-M and PGI-M excretion. At doses of 325-2,600 mg/d Tx-M excretion ranged from 5 to 3% of control values while PGI-M remained at 37-23% of control. 3 d after the last dose of aspirin (2,600 mg/d) mean Tx-M excretion had returned to 85% of control, whereas mean PGI-M remained at 40% of predosing values. Although the platelet aggregation response (Tmax) to ADP ex vivo was inhibited during administration of the lower doses of aspirin the aggregation response returned to control values during the final two weeks of aspirin administration (1,300 and 2,600 mg aspirin/d) despite continued inhibition of thromboxane biosynthesis. These results suggest that although chronic administration of aspirin results in inhibition of endogenous thromboxane and prostacyclin biosynthesis over a wide dose range, inhibition of thromboxane biosynthesis is more selective at 20 than at 2,600 mg aspirin/d. However, despite this, inhibition of platelet function is not maximal at the lower aspirin dosage. Doses of aspirin in excess of 80 mg/d resulted in substantial inhibition of endogenous prostacyclin biosynthesis. Thus, it is unlikely that any dose of aspirin can maximally inhibit thromboxane generation without also reducing endogenous prostacyclin biosynthesis. These results also indicate that recovery of endogenous prostacyclin biosynthesis is delayed following aspirin administration and that the usual effects of aspirin on platelet function ex vivo may be obscured during chronic aspirin administration in man.

Recognition site for the platelet receptor is present on the 15-residue carboxy-terminal fragment of the gamma chain of human fibrinogen and is not involved in the fibrin polymerization reaction

A pentadecapeptide, derived from a staphylococcal protease digest of the 27-residue carboxy-terminal cyanogen bromide fragment of human fibrinogen gamma chain, inhibits binding of 125I-fibrinogen to human platelet receptors and aggregation of platelets induced by ADP and fibrinogen. Amino acid composition and NH2 terminal analysis indicate that the isolated pentadecapeptide corresponds to residues 397 to 411 of the gamma chain. A synthetic peptide also inhibited binding of 125I-fibrinogen and aggregation of platelets. In contrast, the isolated pentadecapeptide and its parent 27-residue fragment lack inhibitory activity toward the polymerization reaction of fibrin monomer. Thus, the site recognizing the platelet receptor encompasses residues 397-411 of the gamma chain of fibrinogen and is distinct from the site(s) involved in polymerization of fibrin monomers.

Evidence that changes in platelet cyclic AMP levels regulate the fibrinogen receptor on human platelets

Fibrinogen binds to human platelets after specific receptor sites are exposed by thrombin, ADP, epinephrine, and other stimuli. Since prostaglandin I2 (PGI2), a potent activator of platelet adenylate cyclase, prevents mobilization of the fibrinogen receptor by aggregating agents, we investigated the relationship between platelet cAMP levels and fibrinogen receptor status in thrombin-stimulated human platelets. A dose-dependent rise in platelet cAMP in response to two adenylate cyclase agonists, PGI2 and forskolin, correlated with progressive inhibition of fibrinogen binding. Moreover, the receptor inhibition produced by either agonist was sustained up to 2 h and was associated with a persistent increase in cAMP levels. The phosphodiesterase inhibitor, 1-methyl-3-isobutylxanthine, in the presence of a subthreshold concentration of PGI2 also raised cAMP and inhibited fibrinogen binding. In contrast, the effects of PGI2 on both cAMP and fibrinogen binding were markedly attenuated by 9-(tetrahydro-2-furyl) adenine, an adenylate cyclase inhibitor. These results indicate that the inhibition of fibrinogen binding by PgI2 is linked to its effect on cAMP levels and suggest that elevation of platelet cAMP levels from any cause prevents exposure of the fibrinogen receptor.

Thrombin-induced exposure and prostacyclin inhibition of the receptor for factor VIII/von Willebrand factor on human platelets

The receptor for Factor VIII/von Willebrand factor (F. VIIIVWF) is readily available on circulating platelets. We have found that the stimulation of platelets with traces of thrombin at concentrations that are generated physiologically (0.008 U-0.05 U/ml) induced concentration-dependent binding of 125I-labeled F. VIIIVWF in a steady-state system. The binding induced by thrombin was specific because it was inhibited by a 100-fold molar excess of unlabeled F. VIIIVWF factor, by rabbit monospecific antibody against Factor VIII, and was not inhibited by an excess of fibrinogen or fibronectin. Binding induced by thrombin required metabolically active platelets, in contrast to a system with ristocetin that also prompted binding to glutaraldehyde-treated platelets. The thrombin effects on binding of 125I-F. VIIIVWF was not observed when platelets were washed with EDTA-containing buffers; EDTA and EGTA both inhibited thrombin-induced binding. Platelet membrane glycoproteins were required because enzymatic stripping od them from the platelet surface with chymotrypsin reduced binding 2.5-5.0-fold. Prostacyclin, in the concentration range of 1 to 50 nM, had two distinct effects on the receptor for F. VIIIWVF: (a) it prevented exposure of this receptor when added 10 min before thrombin, and (b) it reversed the binding of 125I-F. VIIIVWF to the platelet receptor when added 30 min after thrombin and the ligand, ie., when binding was at equilibrium. The dual effect of prostacyclin on the receptor for F. VIIIVWF was reproduced by dibutyryl cyclic AMP.

gamma and alpha chains of human fibrinogen possess sites reactive with human platelet receptors

Fibrinogen, a clottable plasma protein, agglutinates both prokaryotic cells (e.g., staphylococci) and eukaryotic cell fragments (e.g., platelets) through interaction with specific receptors. To identify the region of the fibrinogen molecule responsible for its interaction with human platelets, we prepared polypeptide chain subunits (alpha, beta, and gamma) of human fibrinogen by reduction and carboxymethylation. A mixture of the chains induced aggregation (clumping) of human platelets separated from plasma proteins and treated with ADP. When individual chains of fibrinogen were tested, gamma-chain multimers caused platelet aggregation at a molar concentration comparable with that of intact human fibrinogen. The beta chain remained inactive, and the alpha chain was 1/4th to 1/5th as reactive as the gamma chain. Monospecific antibody fragments against the gamma chain inhibited binding of 125I-labeled fibrinogen to the human platelet receptor and blocked aggregation of platelets induced by ADP in the presence of fibrinogen or gamma-chain multimers. These results indicate that the gamma chain of human fibrinogen bears the main site for interaction with the platelet receptor.

Platelet aggregates in peripheral and coronary-sinus blood in patients with spontaneous coronary-artery spasm

Five out of twenty patients with vasotonic angina had frequent spontaneous attacks with dramatic ST-segment alterations. These attacks were consistently associated with an increase in platelet aggregates in coronary-sinus blood but not arterial blood. This increase in platelet aggregates during coronary-artery spasm suggests a potential causal role for the platelet in this subset of patients with ischaemic heart-disease. However, the increase in platelet aggregates could be secondary to reduced coronary blood-flow.

Interaction of Histoplasma capsulatum with human platelets

Thrombocytopenia is a common accompaniment of disseminated histoplasmosis. The yeast form of Histoplasma capsulatum does not directly injure human platelets freed of plasma. Preincubation of H. capsulatum with plasma enabled it to induce prompt platelet aggregation and selective release of [3H]serotonin without release of lysosomal beta-glucuronidase and the cytoplasmic marker, lactate dehydrogenase. Platelet aggregation was mediated by adenosine diphosphate, as shown by the blocking of the reaction by apyrase. Indomethacin inhibited both aggregation and serotonin release, indicating their dependence on prostaglandin synthesis by platelets. Plasma IgG conferred [3H]serotonin-releasing activity after complexing with yeasts, and plasma fibrinogen was necessary for platelet aggregation; classical and alternative complement pathways were not involved. The interaction of H. capsulatum with human platelets, mediated by IgG and fibrinogen without complement, represents a new attribute of this fungal pathogen and may contribute to thrombocytopenia complicating disseminated histoplasmosis.

Prostacyclin inhibits mobilisation of fibrinogen-binding sites on human ADP- and thrombin-treated platelets

Prostacyclin (prostaglandin I2, PGI2), produced by the blood vessel wall is the most potent known inhibitor of platelet aggregation induced by such stimuli as ADP and thrombin. It binds to a specific platelet receptor and activates adenylate cyclase, raising the cyclic AMP level in platelets. This property can be important because platelets participate in several significant interactions. For example, the interaction with fibrinogen or fibrin contributes to the formation of the haemostatic plug. Intact plasma fibrinogen is required for the aggregation of platelets induced by ADP, and endogenous platelet fibrinogen influences thrombin-induced aggregation. We have therefore studied the effect of prostacyclin on the interaction of fibrinogen with human platelets. We now report that prostacyclin inhibits the mobilisation of specific binding sites ('receptors') for fibrinogen on human platelets and that this effect parallels the inhibition of ADP- or thrombin-induced aggregation. The inhibitory effect of prostacyclin may limit the extent of platelet-fibrinogen interaction in vivo and in extracorporeal circulation.

Staphylococci-induced human platelet injury mediated by protein A and immunoglobulin G Fc fragment receptor

Bloodstream infections with staphylococci are accompanied by thromboembolic complications. We have studied the mechanism of the interaction of staphylococci with human blood platelets. Staphylococci that possess protein A, a bacterial receptor for the Fc fragment of immunoglobulin G (IgG), caused aggregation of human platelets in whole plasma accompanied by release of [(3)H]serotonin. These reactions were time and concentration dependent, requiring two or more staphylococci per platelet to give maximal response within 5 min. The interaction between staphylococci and platelets required the presence of cell wall-bound protein A and of IgG with an intact Fc fragment. It did not require an intact complement system. Cell wall-bound protein A (solid phase) was capable of aggregating human platelets in whole plasma. In contrast, free, solubilized protein A (fluid phase) did not cause measurable aggregation, and release of [(3)H]serotonin was reduced. An excess of free, solubilized protein A blocked aggregation of human platelets induced by staphylococci in whole plasma. The role of the Fc fragment of IgG in the staphylococci-human platelet interaction was demonstrated by an experiment in which free, isolated Fc fragment blocked aggregation of platelets in whole plasma induced by staphylococci. Furthermore, binding of (125)I-protein A to human platelets was demonstrated in the presence of complete IgG with intact Fc fragment but not in the presence of the F(ab)(2) fragment. Binding of the protein A-IgG complex to the human platelet Fc receptor was paralleled by the release of [(3)H]serotonin. These results represent a novel example of the interaction of two phylogenetically different Fc receptors, one on prokaryotic staphylococci and the other on human platelets. Their common ligand, IgG, is amplified by one Fc receptor (protein A) to react with another Fc receptor present on human platelets, which results in membrane-mediated aggregation and release reaction occurring in whole plasma. This mechanism can be of significance in the pathomechanism of thromboembolic complications at the site(s) of intravascular staphylococcal infection.

Affinity isolation and characterization of immunoglobulin G Fc fragment-binding glycoprotein from human blood platelets

Immune complexes bind to several eukaryotic cell types including human blood platelets through the Fc fragment of immunoglobulin (Ig) G. Utilizing immobilized Fc fragment of IgG enabled us to isolate from human blood platelets a glycoprotein of an apparent Mr = 255,000 which, upon reduction, dissociated into sub-units of an apparent Mr = 50,000. This Fc fragment-binding glycoprotein has an isoelectric point between pH 6.3 and 6.9 and is composed of 34% hydrophobic, 25% acidic, and 14% basic amino acids. The Fc fragment-binding glycoprotein was also isolated from human platelet membrane preparations and was unaffected by prior treatment of platelets with thrombin. Isolated Fc fragment-binding glycoprotein formed an in vitro complex with aggregated immunoglobulin G. These results suggest that the isolated Fc fragment-binding component may prove useful in studies concerning the functional role of glycoproteins as cellular receptors for the Fc fragment of IgG.

Mobilization of Fibrinogen Receptor Sites on Human Platelets Stimulated with ADP is Prevented by Prostacyclin

Fibrinogen is a plasma factor required for aggregation of human platelets by ADP. The mechanism of platelet-ADP-fibrinogen interaction was studied by measuring the equilibrium binding of 125I-fibrinogen to human platelets separated from plasma proteins. Binding of 125I-fibrinogen to platelets not stimulated with ADP was low and unaffected by an excess of unlabel led fibrinogen. However, when platelets were stimulated with 4μM of ADP, there was an eightfold increase In the number of available binding sites for human fibrinogen, with affinity constant of 1.9 x 109M-1. This striking increase in fibrinogen receptor sites on human platelets was specific for ADP as contrasted to ATP, AMP, and adenosine. Prostacyclin (Prostaglandin I2, PGI2), a novel prostaglandin produced by the blood vessel wall, completely blocked this ADP-induced increase in fibrinogen receptor sites on human platelets. The effect of PGI2 was prompt and concentration dependent, reaching maximum at 10-9M. 6-keto PGF2 a stable derivative ot PGI2, did not have such an effect. Thus movement of fibrinogen receptor sites on human platelet membrane stimulated with ADP is prevented by PGI2. This represents a new biologic property of this vascular hormone and contributes to better understanding of its potent inhibitory effect in vitro and in vivo on ADP-induced platelet aggregation requiring mobilization of fibrinogen receptor.

Immune Injury to Human Platelets Mediated By IgG Fc Receptor is Prevented by Prostacyclin

Immune injury to human platelets by drugs, bacteria, and viruses which form antigenantibody complexes is mediated by the IgG Fc receptor on human platelets and results in thrombocytopenia. We studied whether immune injury to human platelets mediated by the IgG Fc receptor can be prevented by prostacyclin (Prostagland in 12, PGI2), a novel prost glandin generated by the blood vessel wall. Immune injury to human platelets in whole plasma was elicited by Protein A-bearing staphylococci. Protein A induces binding of I to the human platelet Fc receptor, which results in platelet aggregation and 3H-seroton release in whole plasma. Excess of isolated Fc fragment inhibits aggregation and serotonin release in this model of immune injury. Synthetic PGI2 protected human platelets from this IgG Fc fragment-mediated immune injury in whole plasma. Inhibition was prompt (1 to 5 min) and dose dependent, reaching maximum at 10-6M of PGI2. Removal of plasma proteins and use of IgG-coated cells did not change the inhibitory potency of PGI2, which was at least 1000-fold more active than 6-keto PGF1α. Electron microscopy revealed than PGI2 prevented binding of IgG- coated cel is to human platelet membrane. From comparison with anti-inflammatory steroids (methylprednisolone) and nonsteroidal prostaglandin synthetase inhibitors (ASA), it appears that prostacyclin is the most active agent known the date to protect human platelets from IgG Fc receptor-mediated immune injury in vitro.

Membrane changes in human platelets induced by lipopolysaccharide endotoxin

The unexplained occurrence of thrombocytopenia in cases of Gramnegative sepsis in man led us, in the light of animal experiments indicating the blood platelet as the target cell for endotoxin, to examine the effect of Salmonella enteritidis lipopolysaccharide B on human platelets. Human platelets were separated from a coat of plasma proteins by Sepharose 2B filtration or by a combined procedure of albumin gradient and Sepharose 2B filtration. The action of endotoxin on human platelets resulted in membrane changes manifested by dose-dependent release of [3H]serotonin and adenine nucleotides. Cytoplasmic marker, lactic dehydrogenase, and lysosomal marker, beta glucuronidase, were retained indicating that the release reaction was selective. Release of [3H]serotonin was specific for endotoxin since other particulates, zymosan and erythrocyte stroma, were without effect. Endotoxin, added to gel-filtered human platelets, induced a significant evolution of platelet factor 3 procoagulant activity. Preincubation of endotoxin with a membrane-rich homogenate of human platelets inhibited its labilizing effect on human platelets thus suggesting an interaction between endotoxin and the platelet membrane itself. Other plausible factors in this interaction [fibrinogen, adenine nucleotides, thrombin, sialic acid residues, and IgG] were eliminated on the basis of a series of control experiments. From the negligible effect of aspirin and indomethacin, we may infer that the interaction of endotoxin with platelets does not depend on the platelet prostaglandin synthesis pathway. The direct interaction of endotoxin with the human platelet membrane comprises a new mechanism which may help to clarify the pathogenesis of vascular and haemostatic disorders accompanying bloodstream infections due to Gram-negative bacteria.

Immunoassay of human platelet factor 4(PF4, antiheparin factor) by radial immunodiffusion

Human platelet factor 4 antigen (PF4 antigen) was measured in platelets and in plasma by means of single radial immunodiffusion. Anti-PF4 antibody obtained in rabbits by injecting highly purified human PF4 was monospecific in double immunodiffusion and in quantitative "rocket" immunoelectrophoresis. A high degree of correlation was observed between the precipitation zones in the radial immunodiffusion method and the amount of purified PF4 (in the range of 0.6 to 50.0 mug per milliliter) or the number of platelets in plasma (in the range of 5 x 10(6) to 1.6 x 10(8) platelets per milliliter applied. The sensitivity of the method was 30 to 125 times higher as compared with clotting assay (antiheparin activity) and the standard error of the method was 2.3 per cent. The method was specific for the antigen present in platelets since human leukocytes and erythrocytes gave negative results. Release of PF4 antigen from washed platelets challenged with thrombin, collagen, ADP, and antigen-antibody complexes was measured by the radial immunodiffusion assay. It usually paralleled the release of 3H-serotonin but PF4 antigen was a more sensitive marker for platelet release reaction. Release of PF4 antigen was usually 2 to 4 times higher than release of the antiheparin activity as measured by clotting assay when both were compared as percentage of total content in platelets. The level of PF4 antigen was determined in platelet-rich plasma (PRP) and platelet-free plasma (PFP) obtained from 12 healthy volunteers. While the mean level of extraplatelet pool of PF4 antigen in PFP was 0.72 +/- 0.92 mug per milliliter, PRP contained 80 +/- 22 mug of PF4 antigen per 10(9) platelets. Addition of thrombin (1 U. per milliliter) liberated all of the PF4 antigen (78 +/- 24 mug) present in PRP but ADP (50 muM) released only 31 +/- 22 mug of PF4 antigen per 10(9) platelets. The presence of heparin did not interfere with the assay of intraplatelet or extraplatelet PF4 by single radial immunodiffusion. The method described represents a simple, sensitive, quantitative, and specific assay for human PF4 antigen possessing antiheparin activity.