Fibrinogen interaction with platelet receptors

Gfi1b controls integrin signaling-dependent cytoskeleton dynamics and organization in megakaryocytes

Mutations in GFI1B are associated with inherited bleeding disorders called GFI1B-related thrombocytopenias. We show here that mice with a megakaryocyte-specific Gfi1b deletion exhibit a macrothrombocytopenic phenotype along a megakaryocytic dysplasia reminiscent of GFI1B-related thrombocytopenia. GFI1B deficiency increases megakaryocyte proliferation and affects their ploidy, but also abrogates their responsiveness towards integrin signaling and their ability to spread and reorganize their cytoskeleton. Gfi1b-null megakaryocytes are also unable to form proplatelets, a process independent of integrin signaling. GFI1B-deficient megakaryocytes exhibit aberrant expression of several components of both the actin and microtubule cytoskeleton, with a dramatic reduction of α-tubulin. Inhibition of FAK or ROCK, both important for actin cytoskeleton organization and integrin signaling, only partially restored their response to integrin ligands, but the inhibition of PAK, a regulator of the actin cytoskeleton, completely rescued the responsiveness of Gfi1b-null megakaryocytes to ligands, but not their ability to form proplatelets. We conclude that Gfi1b controls major functions of megakaryocytes such as integrin-dependent cytoskeleton organization, spreading and migration through the regulation of PAK activity whereas the proplatelet formation defect in GFI1B-deficient megakaryocytes is due, at least partially, to an insufficient α-tubulin content.

Hepatic fibrinogen storage disease: identification of two novel mutations (p.Asp316Asn, fibrinogen Pisa and p.Gly366Ser, fibrinogen Beograd) impacting on the fibrinogen γ-module

BACKGROUND

Quantitative fibrinogen deficiencies (hypofibrinogenemia and afibrinogenemia) are rare congenital disorders characterized by low/unmeasurable plasma fibrinogen antigen levels. Their genetic basis is invariably represented by mutations within the fibrinogen genes (FGA, FGB and FGG coding for the Aα, Bβ and γ chains). Currently, only four mutations (p.Gly284Arg, p.Arg375Trp, delGVYYQ 346-350, p.Thr314Pro), all affecting the fibrinogen γ chain, have been reported to cause fibrinogen storage disease (FSD), a disorder characterized by protein aggregation, endoplasmic reticulum retention and hypofibrinogenemia.

OBJECTIVES

To investigate the genetic basis of FSD in two hypofibrinogenemic patients.

METHODS

The mutational screening of the fibrinogen genes was performed by direct DNA sequencing. The impact of identified mutations on fibrinogen structure was investigated by in-silico molecular modeling. Liver histology was evaluated by light microscopy, electron microscopy and immunocytochemistry.

RESULTS

Here, we describe two hypofibrinogenemic children with persistent abnormal liver function parameters. Direct sequencing of the coding portion of fibrinogen genes disclosed two novel FGG missense variants (p.Asp316Asn, fibrinogen Pisa; p.Gly366Ser, fibrinogen Beograd), both present in the heterozygous state and affecting residues located in the fibrinogen C-terminal γ-module. Liver sections derived from biopsies of the two patients were examined by immunocytochemical analyses, revealing hepatocyte cytoplasmic inclusions immunoreactive to anti-fibrinogen antibodies.

CONCLUSIONS

Our work strongly confirms the clustering of mutations causing FSD in the fibrinogen γ chain between residues 284 and 375. Based on an in-depth structural analysis of all FSD-causing mutations and on their resemblance to mutations leading to serpinopathies, we also comment on a possible mechanism explaining fibrinogen polymerization within hepatocytes.

Adjustable bioadhesive control of PEGylated hyperbranch brushes on polystyrene microplate interface for the improved sensitivity of human blood typing

A PEGylated 96-well polystyrene (PS) microplate was first introduced for applications in high-throughput screening for selective blood typing to minimize the risks in blood transfusions. Herein, we present a hemocompatible PS 96-well microplate with adjustable PEGylated hyperbranch brush coverage prepared by ozone pretreated activation and thermally induced surface PEGylation. The grafting properties, hydration capacity, and blood compatibility of the PEGylated hyperbrush immobilized PS surfaces in human blood were illustrated by the combined chemical and physical properties of the surface, and the dependence of the specific absorption of human plasma fibrinogen onto the PEGylated surfaces on the grafting density was analyzed by monoclonal antibodies. The surface coverage of PEGylated brushes plays a major role in the bioadhesive properties of modified PS microplates, which in turn control the level of agglutination sensitivity in blood typing. The bioadhesive resistance toward proteins, platelets, and erythrocytes in human whole blood showed a correlation to the controlled hydration properties of the PEGylated hyperbrush-modified surfaces. Therefore, we suggested that the surface coverage of PEGylated hyperbrushes on PS surfaces can increase the sensitivity of cross-matching blood agglutination by up to 16-fold compared to that of the conventional 96-well virgin PS due to the regulated biorecognition of hematocrit and antibodies of the PEGylated hyperbrush-modified surfaces.

Interaction of fibrin(ogen) with the endothelial cell receptor VE-cadherin: localization of the fibrin-binding site within the third extracellular VE-cadherin domain

Interaction of fibrin with endothelial cells through their receptor VE-cadherin has been implicated in modulation of angiogenesis and inflammation. Previous studies identified the VE-cadherin-binding site in the fibrin betaN-domains formed by the NH(2)-terminal regions of fibrin beta chains and revealed that the recombinant dimeric (beta15-66)(2) fragment mimicking these domains preserves the VE-cadherin-binding properties of fibrin. To test if the other fibrin(ogen) regions/domains are involved in this interaction and localize the complementary fibrin-binding site in VE-cadherin, we prepared several recombinant fragments containing individual extracellular domains of VE-cadherin or combinations thereof, as well as several fragments corresponding to various fibrin(ogen) regions, and tested the interactions between them by ELISA and surface plasmon resonance. The experiments revealed that the betaN-domains are the only fibrin(ogen) regions involved in the interaction with VE-cadherin. They also localized the fibrin-binding site to the third extracellular domain of VE-cadherin and established that the fibrin-binding properties of this domain are not influenced by the presence or absence of the neighboring domains. In addition, the experiments confirmed that calcium ions, which are required to maintain proper conformation and adhesive properties of VE-cadherin, do not influence the fibrin-binding properties of the latter.

Molecular basis of biomaterial-mediated foreign body reactions

Despite being inert and nontoxic, implanted biomaterials often trigger adverse foreign body reactions such as inflammation, fibrosis, infection, and thrombosis. With regard to the inflammatory responses to biomaterial implants, it was previously found that a crucial precedent event was the spontaneous adsorption and denaturation of fibrinogen on implant surfaces. It was further found that interactions between the phagocyte integrin Mac-1 (CD11b/CD18) and one short sequence within the fibrinogen D domain (gamma 190-202; P1) at least partially explained phagocyte accumulation on implant surfaces. However, the reason that adsorbed fibrinogen is proinflammatory--while soluble fibrinogen clearly is not--remained obscure. In this study, therefore, the question of how fibrinogen is converted to a proinflammatory state when adsorbed to biomaterial surfaces is investigated. In soluble fibrinogen, the 13 amino acid P1 sequence was found to be hidden. However, the adsorption and denaturation of fibrinogen on the surfaces of commonly used biomaterials lead to the exposure of P1 and a second neo-epitope, gamma 377-395 (P2), which also interacts with Mac-1 and is similarly occult in the soluble protein. The extent of biomaterial-mediated P1 and P2 exposure appears directly related to the severity of inflammatory responses to a test panel of biomaterials. Finally, thrombin-mediated conversion of fibrinogen to fibrin also exposes both P1 and P2 epitopes. These observations may help explain both the inflammation caused by many types of implanted biomaterials and that which occurs naturally following thrombotic events. (Blood. 2001;98:1231-1238)

Testing of platelet deposition on polystyrene surface under flow conditions by the cone and plate(let) analyzer: role of platelet activation, fibrinogen and von Willebrand factor

Recently, we described a method of testing platelet deposition on extracellular matrix under flow conditions. The method was used for assessment of platelet function in various platelet disorders, for monitoring of replacement and anti-platelet therapy. In the present study, we investigated platelet deposition on a polystyrene surface compared with that on extracellular matrix, under defined shear rates, using the original Cone and Plate(let) Analyzer. A correlation of adhesion rate (surface coverage) and aggregate formation (average size) of platelets from normal citrated blood between polystyrene and extracellular matrix was observed. Blocking of von Willebrand factor binding to glycoprotein Ib by a recombinant von Willebrand factor fragment substantially decreased platelet adhesion to both surfaces. Blocking of GPIIb-IIIa by Arg-Gly-Asp-Ser peptide prevented platelet adhesion to the polystyrene while an extensive adhesion of single platelets to extracellular matrix was observed. Furthermore, platelet adhesion to polystyrene but not to extracellular matrix was completely inhibited by platelet inactivation with prostaglandin E(1). Platelets from patients with severe von Willebrand disease yielded very low adhesion to both polystyrene and extracellular matrix. The addition of von Willebrand factor to the blood of these patients or pre-coating of polystyrene surface with von Willebrand factor restored the ability of platelets to adhere and aggregate on the surface. Platelets from patients with Glanzmann's thrombasthenia and afibrinogenemia adhered to extracellular matrix (with defective aggregate formation), while they failed to adhere to the polystyrene. Fibrinogen added to afibrinogenemia blood or pre-coating of the polystyrene with fibrinogen restored the ability of platelets to adhere and aggregate on the surface. In conclusion, the polystyrene surface, like extracellular matrix, can be used to assess platelet function disorders taking in account that platelet deposition on polystyrene under flow is absolutely dependent on platelet activation and on the presence of fibrinogen, von Willebrand factor, and their receptors.

Platelet aggregation in flow: differential roles for adhesive receptors and ligands

This article addresses the flow-dependent differential roles of the platelet receptors, glycoprotein (GP) GPIb and GPIIb-IIIa, in platelet aggregation mediated by ristocetin and soluble von Willebrand factor (vWF), by adenosine diphosphate (ADP) and soluble fibrinogen (Fg), and by thrombin and ADP in absence of exogenous ligands. Platelet-rich plasma or "activated" washed platelets were sheared in a coaxial cylinder at 100 to 1000 sec(-1) or in tubular flow, with surface ligands monitored by flow cytometry, with fluorescently labeled soluble ligands or monoclonal antibodies against specific adhesive domains on receptors or ligands. Aggregation was quantitated by monitoring the change in particle concentration with time by particle counting, and expressed as Capture efficiencies (CE) = Experimental/calculated initial rates of aggregation. The contributions of adhesive domains on putative ligands or receptors mediating aggregation at any given flow condition were evaluated with monoclonal antibodies or peptides known to block these adhesive sites. Surprisingly, ristocetin, which "chemically activates" GPIb/vWF to mediate spontaneous binding of the ligand to its receptor, at low concentrations yielding <2000 platelet-bound vWF monomers, gave efficient aggregation even at 1000 sec(-1) (CE = 0.34 +/- 0.02, n = 11) with only GPIb required. The physiologic activators ADP and thrombin both supported efficient aggregation of washed platelets with no exogenous ligands at 1000 sec(-1) by surface-secreted vWF (CE = 0.08 +/- 0.01, n = 6), in contrast to poorer ADP and soluble Fg-mediated aggregation in the absence of secretion (CE = 0.05). The secreted, platelet-bound, vWF-mediated aggregation completely depends on GPIIb-IIIa but partially and increasingly requires GPIb with increasing shear. Molecular models for these interactions are presented in terms of "rolling" and "firm" capture. Flow conditions will be critically important in designing and selecting anti-thrombotic drugs directed against the appropriate adhesive domains on receptors and ligands, which likely include other members such as thrombospondin and P-selectin.

Structural and functional characterization of proteolytic fragments derived from the C-terminal regions of bovine fibrinogen

A number of new as well as previously described fragments derived from the D region of bovine fibrinogen by limited proteolysis have been characterized by sequence analysis, differential scanning calorimetry and circular dichroism. Determination of the extremities of the polypeptide chains forming individual fragments allowed the scheme of proteolysis and the borders between domains in the D region of fibrinogen to be established. It was also found that the most thermostable region of the D fragment (TSD) can be substantially reduced in size without loss of its compact structure. The alpha-helical content of the newly prepared 21-kDa TSD2 and 16-kDa TSD3 fragments were 82% and 75%, respectively, strongly supporting a coiled-coil structure for this region of the fibrinogen molecule. The DX and DZ fragments, prepared from a chymotryptic digest of the DLA fragment, were found to be similar to the DL and DY fragments, respectively, except for an internal cleavage at K393-T394 in their beta chains. This cleavage leads to destabilization of all thermolabile domains, indicating interaction between them. The DL and DY fragments, containing only one polymerization site in their beta chains, were able to inhibit fibrin polymerization at high concentration. However, these same fragments failed to bind to fibrin-Sepharose under conditions where their structural analogues, DX and DZ, were tightly bound, indicating that cleavage after K393 substantially increases the affinity of this site.

Dynamics of platelet glycoprotein IIb-IIIa receptor expression and fibrinogen binding. II. Quantal activation parallels platelet capture in stir-associated microaggregation

There is broad agreement that platelet aggregation is generally dependent on fibrinogen (Fg) binding to the glycoprotein (GP) IIb-IIIa receptor expressed on the activated platelet surface. We therefore compared rates and extents of aggregation and of fibrinogen receptor expression and specific Fg binding to activated platelets, as a function of ADP concentration. Human citrated platelet-rich plasma (diluted 10-fold) was stirred with adenosine diphosphate (ADP) for 10 s or 2 min to measure rates and extent of aggregation, respectively, determined from the decrease in the total number of particles. The number of fibrinogen receptors and bound Fg were measured from mean fluorescence values obtained with FITC-labeled IgM monoclonal antibody PAC1 and the IgG monoclonal antibody, 9F9, respectively, using flow cytometry as presented in part I (Frojmovic et al., 1994). Because flow cytometric and aggregation measurements were routinely determined at room temperature and 37 degrees C, respectively, we also compared and found temperature-independent initial rates of aggregation. The fraction of platelets with fluorescence values above one critical threshold value, corresponding to maximally "activated" platelets (P*), increased with increasing activator concentration and correlated linearly with the fraction of platelets recruited into aggregates for ADP (r > 0.9). Aggregation was not rate-limited by fibrinogen receptor expression or by Fg binding. It appears that each platelet expresses its maximal Fg receptors at a critical ADP concentration, i.e., occupancy of ADP receptors. This, in turn, leads to rapid Fg occupancy and capture of such "quantally activated" platelets into aggregates.

Dynamics of platelet glycoprotein IIb-IIIa receptor expression and fibrinogen binding. I. Quantal activation of platelet subpopulations varies with adenosine diphosphate concentration

We have previously reported that maximal platelet activation with adenosine diphosphate (100 microM ADP) causes rapid expression of all GPIIb-IIIa receptors for fibrinogen (FgR) (< 1-3 s), measured with FITC-labeled PAC1 by flow cytometry. We have extended these studies to examine the effects of ADP concentration on the graded expression and Fg occupancy of GPIIb-IIIa receptors. Human citrated platelet-rich plasma, diluted 10-fold with Walsh-albumin-Mg+2 (2 mM), was treated with ADP (0.1-100 microM). The rates of GPIIb-IIIa receptor expression or Fg binding were measured in unstirred samples by flow cytometry, using FITC-labeled monoclonal antibodies (mAb) PAC1 and 9F9, respectively, from on-rates, using increasing times between mAb and ADP additions. Fibrinogen receptors were all expressed rapidly at low (1 microM) or high (100 microM) ADP (few seconds), whereas Fg occupancy was 50% of maximal by about 2 min. The maximal extent of GPIIb-IIIa receptor expression and Fg occupancy was determined from maximal binding (Flmax) at 30 min incubation with PAC1 or 9F9. On-rates and maximal extents of binding for either PAC1 or 9F9 probes showed identical [ADP]-response profiles ("KD" approximately 1.4 +/- 0.1 microM). However, Flmax studies showed bimodal histograms consisting of "resting" (Po) and maximally "activated" (P*) platelets for both PAC1 and 9F9 binding, with the fraction of "activated" platelets increasing with ADP concentration. The data best fit a model where platelet subpopulations are "quantally" transformed from Po to P*, expressing all GPIIb-IIIa receptors, rapidly filled by Fg, but "triggered" at critical ADP concentrations. Larger, but not the largest, platelets appear to be the most sensitive subpopulation. The implications for clinical studies are discussed, and the relationship to dynamics of aggregation are described in a companion paper.

Effects of fibrinogens on phase transitions in lipid model membrane systems

An abnormal fibrinogen that caused aggregation of red blood cells (RBC) in a patient with gangrene was examined by real-time X-ray diffraction to determine its effects on dipalmitoylphosphatidylcholine (DPPC) and 1-palmitoyl-2-oleoylphosphatidylethanolamine (POPE) phase transitions. Similar studies were done with normal fibrinogen and results were compared. Both types of fibrinogen slightly increased the L alpha-->HII phase transition temperature and the HII phase parameters for POPE, while neither fibrinogen significantly affected the order-disordered acyl chain transitions in the lipid bilayer phase. However, fibrinogen differentially influenced the bilayer unit cell parameter of the gel and disordered bilayer and the gel state ripple phase. These results can be interpreted as indicating that fibrinogen has little effect on the balance of gel and disordered acyl chains in the lipid bilayer, but may influence membrane functions dependent on non-bilayer phases.

Aggregation efficiency of activated normal or fixed platelets in a simple shear field: effect of shear and fibrinogen occupancy

Shear rate can affect protein adsorption and platelet aggregation by regulating both the collision frequency and the capture efficiency (alpha). These effects were evaluated in well defined shear field in a micro-couette for shear rate G = 10 - 1000 s-1. The rate of protein binding was independent of G, shown for adsorption of albumin to latex beads and PAC1 to activated platelets. The initial aggregation rate for ADP-activated platelets in citrated platelet-rich plasma followed second order kinetics at the initial platelet concentrations between 20,000 and 60,000/microliters. alpha values, which dropped nearly fivefold for a 10-fold increase in G, were approximately proportional to G-1, contrary to a minor drop predicted by the theory that includes protein cross-bridging. Varying ADP concentration did not change alpha of maximally activated platelet subpopulations, suggesting that aggregation between unactivated and activated platelets is negligible. Directly blocking the unoccupied but activated GPIIb-IIIa receptors without affecting pre-bound Fg on "RGD"-activated, fixed platelets (AFP) by GRGDSP or Ro 43-5054 eliminated aggregation, suggesting that cross-bridging of GPIIb-IIIa on adjacent platelets by fibrinogen mediates aggregation. Alpha for AFP remained maximal (approximately 0.24) over 25-75% Fg occupancy, otherwise decreasing rapidly, with a half-maximum occurring at around 2% occupancy, suggesting that very few bound Fg were required to cause significant aggregation.

Structural difference between polymerized and non-polymerized fragment X, obtained by plasmin digest of fibrinogen

Fragment X (LMrFX) was obtained as low molecular weight preparations from a late stage 2 plasmin digest of human fibrinogen. The thrombin-treated LMrFX preparations, which resulted in impaired polymerization, were further subfractionated into polymerized and non-polymerized components. The fractions were examined by SDS-PAGE and immunochemical methods. In polymerized fractions, more peptide bands were observed on SDS-PAGE in the reduced state than in non-polymerized fractions. Both fractions contained a similar number of internal cleavages in the A alpha, B beta and gamma chains, which are linked by disulfide bonds. Thus, the partial deficiencies in polymerization sites of the carboxy terminal region of the gamma chain and the amino terminal portions of the B beta chain, as well as internal cleavage, were considered to participate in the impairment of the thrombin-induced polymerization of LMrFX.

Fibrinogen internalization by ADP-stimulated blood platelets. Ultrastructural studies with fibrinogen-colloidal gold probes

Interaction of gel filtered, ADP-stimulated human platelets with low (0.05 mg/ml) and high (1 mg/ml) fibrinogen (Fg) was examined by transmission electron microscopy. To visualize exogenous Fg in a course of its interaction with stimulated platelets, Fg coupled to 18-nm colloidal gold (Fg-Au) was employed. In the presence of either Fg or Fg-Au, rapid changes of platelets morphology indicative of resting to activated state transition, were observed. Without external ligands, stimulated platelet suspensions resemble rather control (untreated) samples. Using Fg-Au, it has been found that initial binding of gold labels to platelet surfaces is immediately followed by gold accumulation in plasmalemma pits subjected to further internalization. Serial sections proved that at 1 min, some of the labeled endocytic structures are already isolated in the platelet cytoplasm. After prolonged (20 min) incubations, different platelet subfractions have been found. Many single or loosely aggregated platelets with little or no surface labeling contained abundant stores of internal labels. In these cells, Fg-Au is localized in vacuole-like and/or granule-like structures. Some post-stimulated (discoidal) platelets are likely to release Fg-Au previously internalized. In the centers of platelet aggregates concentrated labels filled intercellular spaces and voluminous intraplatelet cavities, either open or occluded. These results indicate on different ultimate fates of exogenous Fg processed by the ADP-stimulated platelets. The data obtained suggest also that after initial binding, exogenous Fg may be implicated not only in aggregation, but in activation-related cellular responses as well.

Activation of the fibrinogen receptor on human platelets exposed to alpha chymotrypsin. Relationship with a major proteolytic cleavage at the carboxyterminus of the membrane glycoprotein IIb heavy chain

The serine proteinase alpha chymotrypsin from bovine pancreas (CT) is known to expose fibrinogen binding sites on the surface of human platelets in the absence of cell activation and granular secretion. This is accompanied by the appearance of membrane-bound chymotryptic fragments of both glycoprotein (GP) IIb and GPIIIa, the two subunits of the platelet fibrinogen receptor, the GPIIb-IIIa complex. However, no clear relationship between discrete proteolytic event(s) within GPIIb-IIIa and fibrinogen-binding-site expression has yet been established. We have now evaluated the proteolysis of GPIIb-IIIa by CT by Western blot analyses using a panel of polyclonal and monoclonal antibodies against GPIIb or GPIIIa. The different proteolytic events were then correlated with the kinetics of the expression of active fibrinogen binding sites on platelets, as measured through the binding of 125I-labelled purified fibrinogen and to the capacity of CT-treated platelets to aggregate. Treatment of platelets with CT at 22 degrees C resulted in the expression of fibrinogen binding sites prior to cleavage of GPIIIa (Mr approximately 90,000) into a previously described, major membrane-bound fragment with Mr 60,000. In contrast, fibrinogen receptor expression closely paralleled a proteolytic cleavage at the carboxy terminus of the GPIIb heavy chain (Mr approximately 120,000), which was converted into a faster migrating species with Mr approximately 115,000). This proteolysis resulted in the release of a soluble peptide with an expected molecular mass of less than 3.7 kDa. Quantitation of this peptide using a competitive immunoenzymatic assay, confirmed that its release from the platelet surface correlated with the expression of fibrinogen binding sites and aggregability. When platelets were exposed to CT at 37 degrees C, a prompt increase in fibrinogen binding sites and platelet aggregability was observed, whereas the GPIIb heavy chain was rapidly converted into the carboxy-terminal-cleaved form. However, incubation at 37 degrees C for longer than 10 min resulted in extensive and simultaneous degradation of both the GPIIb heavy and light chains and of GPIIIa, with the latter being converted into the 60-kDa fragment. These later events were associated with a sharp decline of platelet aggregability and a reduction in the number of fibrinogen binding sites. These data allow us to propose that an early and limited proteolytic processing of the GPIIb component of the platelet fibrinogen receptor is associated with a shift of this receptor complex into a state which expresses specific binding sites for fibrinogen. Further cleavage of GPIIIa to generate the 60-kDa fragment results in loss of receptor activity.

Fibrinogen-containing membrane-associated structures arising at the surfaces of ADP-stimulated blood platelets

Ultrastructural studies of ADP-stimulated gel-filtered human platelets incubated with different concentrations of fibrinogen reveal unusual extracellular structures composed at least partly of the aggregated fibrinogen. Development of these structures depends on the exogenous fibrinogen concentration and duration of the incubation. Fibrinogen-containing extracellular material exists in two different structural forms. As was shown earlier, one of them is represented by dense, amorphous intercellular matrix and is localized mainly in platelet microaggregates (Belitser et al., Thromb. Res., in press). Another one consists of huge sheet-like structures bearing individual or clumped platelets bound either to one or to both of their surfaces. After thrombin treatment, these structures could not be found anymore; sometimes they seem to be substituted by the fibrin-like fibers spatially connected to each other and/or to the platelet membranes. It has been suggested that soluble fibrinogen interacting with its specific membrane receptors undergoes conformational changes promoting intermolecular interactions and resulting in the fibrinogen aggregates formation at the surfaces of the activated platelets. A probable physiological significance of the structures described is discussed. It is supposed that under certain conditions in vivo they may play an important role, in particular as a highly concentrated substrate for the thrombin action, providing in such a way a possibility of rapid, effective consolidation of the initial platelet thrombi with the fibrin fibers.

Structural and functional characterization of major platelet membrane components derived by limited proteolysis of glycoprotein IIIa

The authors isolated a product of proteolytic degradation of glycoprotein IIIa (GPIIIa) which is formed on the surface of human platelets during incubation with chymotrypsin and which was previously described as the 66 kDa platelet membrane component. This component migrated with an apparent Mr 62,400 in a non-reduced system of sodium dodecyl sulfate polyacrylamide gel electrophoresis. In a reduced system it yielded two major subunits migrating with apparent Mr 14,000-17,000 and 65,000. The low-molecular weight component began with the NH2-terminal sequence of GPIIIa (GPNICTTR...) and the larger component with residue 348 of GPIIIa (GKIRSKKA...) as deduced from a cDNA clone of this glycoprotein. The two subunits appeared to be linked by one or more S-S bridges supporting the contention that GPIIIa is a highly folded molecule on the platelet membrane. In contrast to GPIIIa, the '66 kDa component' did not bind to GRGDSPK-agarose, to fibrinogen-agarose nor to insolubilized monoclonal antibody recognizing the GPIIb/IIIa complex. The exposure of fibrinogen receptors during the course of incubation of platelets with chymotrypsin preceded the formation of the '66 kDa component' characterized in this study. An intermediate product of GPIIIa proteolysis migrating with an apparent Mr 120,000 in a non-reduced system and Mr 80,000 in a reduced system was identified as a precursor of the '66 kDa component'. The '120 kDa component' was not retained on GRGDSPK-agarose or on fibrinogen-agarose but it was retained on insolubilized antibody recognizing the GPIIb/IIIa complex. Incubation of platelets with porcine pancreatic elastase or human granulocytic elastase resulted in the formation of similar proteolytic degradation fragments.

Modulation of polymorphonuclear leukocyte microbicidal activity and oxidative metabolism by fibrinogen degradation products D and E

Fibrinogen degradation products (FDP) D and E are typically present in blood of patients with disseminated intravascular coagulation and related conditions in which granulocyte (PMN) defense against bacterial infection may be compromised. This study was intended to determine whether FDP modify PMN functions critical to their bactericidal activity. Incubation of human PMN and Escherichia coli with 50-100 micrograms/ml FDP did not affect phagocytosis, but reduced by greater than 90% the cells' ability to inhibit bacterial colony growth compared with control PMN incubated with albumin or fibrinogen. FDP (10-100 micrograms/ml) inhibited PMN O2- release and chemotaxis stimulated by FMLP by 17-50% (P less than 0.005) and 41% (P less than 0.01), respectively. Fragment E3, and not fragment D1, was primarily responsible for inhibition of FMLP-induced PMN O2- release. Phorbol myristate acetate (10 ng/ml), 1-oleoyl-2-acetylglycerol (10(-6) M), AA (4.2 x 10(-5) M), and zymosan-activated serum-stimulated PMN O2- release were also decreased 37-63% by FDP compared with control protein. There are at least two mechanisms by which FDP may impair PMN responses. With respect to FMLP, FDP (16-100 micrograms/ml) inhibited specific binding to the cell surface over a ligand concentration range of 1.4-85 nM [3H]FMLP. In contrast, FDP did not effect the extent of phorbol ester binding to PMN but blocked activation of protein kinase C. These data suggest that elevated plasma FDP inhibit several PMN functions critical to the bactericidal role of these inflammatory cells.

Allosteric equilibria in the binding of fibrinogen to platelets

The binding of fibrinogen to platelets occurs according to the law of mass action. The platelet receptor binds reversibly a single fibrinogen molecule and undergoes a conformational transition between two allosteric states, T and R, that differ in their affinity for fibrinogen. The equilibrium between the two forms is shifted by ADP toward the R (high-affinity) state, thus promoting the aggregation process. This model opens the way to consideration of allosteric modulation of the binding of fibrinogen to its platelet receptor.

Human fibrinogen

The structure and physical properties of human fibrinogen and fibrin are reviewed along with methods for the detection of products of their metabolism. Interactions of human fibrinogen with thrombin, factor XIII, plasminogen, glycoprotein IIb/IIIa, and other proteins are related to their relevance to thrombosis and hemostasis. To the extent information is available, the structural determinants of these interactions are delineated, and kinetic and thermodynamic parameters associated with the interactions are listed. Individual steps in the reaction pathway for the conversion of fibrinogen to cross-linked fibrin are characterized. The altered hemostatic properties of mutational variants of fibrinogen are related to their altered structure. The structures of the genes coding for the polypeptide chains of fibrinogen are discussed along with the current state of knowledge of the control and regulation of fibrinogen synthesis. Fibrinogen catabolism and fibrinolysis are also reviewed.

Fibrinogen-induced erythrocyte aggregation: erythrocyte-binding site in the fibrinogen molecule

The effect of fibrinogen and fibrinogen-derived products on the velocity of rouleau formation of human erythrocytes was quantitatively examined with a rheoscope combined with a video-camera, an image analyzer and a computer. (i) The velocity of rouleau formation by naturally occurring low-molecular-weight fibrinogen of 305 kDa and by desialylated fibrinogen was the same as that by native fibrinogen of 340 kDa. (ii) Concerning fibrinogen degradation products by plasmin, the velocity of rouleau formation decreased upon going from fibrinogen greater than fragment X greater than fragment Y (the ratio of molar concentration of fibrinogen, fragment X and fragment Y for giving a certain velocity of rouleau formation was approx. 1:2:5). The effect of fragments X and Y on the fibrinogen-induced rouleau formation was additive. (iii) Fragments D and E could not induce rouleau formation and did not affect the fibrinogen-, fragment X- and fragment Y-induced rouleau formation. (iv) Fibrinopeptides A and B and artificial tetrapeptides (Gly-Pro-Arg-Pro and Gly-His-Arg-Pro) did not affect the fibrinogen-induced rouleau formation. (v) The possible erythrocyte-binding site in fibrinogen molecule for leading to rouleaux was proposed to be in A alpha-chain (probably, around residues No. 207-303) near the terminal domain of the trinodular structure of fibrinogen.

Quantitation and characterization of human platelet glycoprotein IIIa by radioimmunoassay

Glycoprotein IIIa was quantitated in human platelets by radioimmunoassay using antisera specific to platelet membranes and purified glycoprotein IIIa. Glycoprotein IIIa and glycoprotein IIb were isolated from washed platelets by Triton X-114 extraction followed by preparative sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Radioiodinated glycoprotein IIIa was further purified by affinity chromatography on Lentil lectin-Sepharose 4B. Purified glycoprotein IIb showed little crossreactivity with 125I-labeled glycoprotein IIIa using the anti-platelet membrane or anti-glycoprotein IIIa antisera on a competition inhibition radioimmunoassay. The expression of glycoprotein IIIa epitopes were the same for the purified glycoprotein IIIa and glycoprotein IIIa in Triton X-100 solubilized platelets. A 66 kDa protein derived from glycoprotein IIIa by limited proteolysis of platelet membranes also expressed the same epitopes as intact glycoprotein IIIa. Solubilized platelets contained approximately 16 micrograms of total glycoprotein IIIa antigen per 10(9) cells. The level of glycoprotein IIIa determined by radioimmunoassay in one patient with Glanzmann's thrombasthenia amounted to 6.7% of normal and it was close to the values obtained by other methods.

PAF-acether (1-O-hexadecyl/octadecyl-2-acetyl-sn-glycero-3-phosphocholine)-induced fibrinogen binding to platelets depends on metabolic energy

A combination of CN- and 2-deoxy-D-glucose decreases the binding of fibrinogen to platelets stimulated with PAF-acether (1-O-hexadecyl/octadecyl-2-acetyl-sn-glycero-3-phosphocholine). Decreased binding is found after pretreatment with metabolic inhibitors, thereby lowering the energy content before stimulation as well as at various stages after stimulation of undisturbed cells. Binding and ATP hydrolysis occur in parallel, suggesting tight coupling between both phenomena. Energy appears to be predominantly required for exposure and maintenance of accessible binding sites, whereas the interaction between fibrinogen and the exposed sites does not depend on metabolic energy.

The effect of fibrinogen sialic acid residues on ex vivo platelet deposition on biomaterials

The effect of fibrinogen sialic acid residues on platelet deposition onto polymer surfaces was examined using a canine ex vivo shunt model. To test the hypothesis that desialylated fibrinogen may enhance platelet deposition when precoated on biomaterials, canine fibrinogen was desialylated and precoated on polyvinyl chloride (PVC) shunts. When protein-coated PVC shunts were exposed to flowing whole blood, both the native and the desialylated fibrinogen elicited the same profile of platelet deposition. This study indicates that platelet deposition and thrombus formation on biomaterial surfaces is not mediated by a mechanism which involves the sialic acid residues of fibrinogen.

Two-dimensional electrophoretic studies of platelets from dogs affected with basset hound hereditary thrombopathy: a thrombasthenia-like aggregation defect

Because of a thrombasthenia-like platelet aggregation defect, platelets from dogs affected with Basset Hound Hereditary Thrombopathy were compared to normal control dog platelets by three different techniques in order to assess platelet membrane glycoprotein content. Crossed immunoelectrophoresis (CIE), two-dimensional nonreduced-reduced electrophoresis (NR-R), and O'Farrell two-dimensional electrophoresis were used for the assays. CIE and NR-R gels detected no differences between affected Basset Hound and control dog platelets. Gels run by the O'Farrell technique detected no differences in glycoprotein/protein content, however, there appear to be several constituents missing from BHT affected dog platelet samples. The missing components appear to be either lipids or sialoglycoproteins as they were detectable by silver staining but not by Coomassie Blue staining.

Platelet aggregation inhibitors from Agkistrodon acutus snake venom

Among all the purified components from A. acutus venom, including ADPase, 5'-nucleotidase, phospholipase A2 and fibrinogenases, only the venom ADPase (50-100 micrograms/ml) shows marked inhibitory action on ADP (10 microM)-, collagen (10 micrograms/ml)- and sodium arachidonate (100 microM)-induced platelet aggregations of rabbit platelet-rich plasma. The venom 5'-nucleotidase (100 micrograms/ml) inhibited ADP-induced platelet aggregation by 31 +/- 4% (n = 4, P less than 0.05). Fibrinogenolytic enzymes (fractions I and IX, 100 micrograms/ml) did not significantly inhibit platelet aggregation induced by ADP (10 microM), collagen (10 micrograms/ml) or sodium arachidonate (100 microM). However, when the fibrinogenase (fraction IX, 100 micrograms/ml) was preincubated with platelet-rich plasma for 30 min it inhibited collagen (20 micrograms/ml)- and ADP (10 microM)-induced platelet aggregations by 34 +/- 9% (n = 4, P less than 0.05) and 35 +/- 6% (n = 4, P less than 0.05), respectively. The phospholipase A2 (100 micrograms/ml) did not affect platelet aggregation. The venom ADPase is a single chain polypeptide with a molecular weight of 94,000. The specific ADPase activity is estimated to be 4.3 mu moles Pi/min/mg of protein. It also possesses phosphodiesterase and weak 5'-nucleotidase activities.

Fibrinogen and fibrin: biochemistry and pathophysiology

Fibrinogen is a thrombin-coagulable glycoprotein occurring in the blood of vertebrates. The primary structure of the alpha, beta, and gamma polypeptide chains of human fibrinogen is known from amino acid and nucleic acid sequencing. The intact molecule has a trinodular, dimeric structure and is functionally bivalent. Thrombin cleaves short peptides from the amino termini of the alpha and beta chains exposing polymerization sites that are responsible for the formation of fibrin fibers and appearance of a clot. The major physiological function of fibrinogen is the formation of fibrin that binds together platelets and some plasma proteins in a hemostatic plug. In pathological situations, the network entraps large numbers of erythrocytes and leukocytes forming a thrombus that may occlude a blood vessel. Fibrinogen and fibrin are multifunctional proteins. Fibrinogen is indispensable for platelet aggregation; it also binds to several plasma proteins, however, the biological function of this interaction is not completely understood. Fibrin is an essential matrix for regulation of fibrinolysis and for facilitation of cell attachment in wound healing.

A study of the kinetics and mechanism of ADP-triggered platelet aggregation

The time-course of ADP-triggered aggregation of human blood platelets has been followed by sensitive right-angle light scattering intensity measurements as a function of the platelet and fibrinogen concentrations. Rayleigh-Gans light scattering theory has been combined with the Smoluchowski aggregation model to predict the dependence of the right-angle scattering intensity on particle size and concentration as well as the time-dependent changes during aggregation. The validity of the calculations was confirmed by measuring the scattering intensity with suspensions of polystyrene microspheres of known radius, as well as the time-dependent changes in the 90 degrees scattering intensity during aggregation of these particles. However, in contrast to the predictions of the model, the time-course of the scattering intensity changes during platelet aggregation was characterized by single exponential decay with a rate constant which reached a limiting value of 0.017 s-1 at high platelet concentrations. The value of kagg was also independent of the fibrinogen concentration over a 30-fold range. Covalently cross-linked fibrinogen dimers and Fragment D-inhibited fibrin protofibrils yielded aggregation rates that agreed with those measured with fibrinogen. The results indicate that the rate of platelet aggregation is not limited by either the rate of fibrinogen binding or the frequency of platelet-platelet collisions under these conditions.

Alterations of blood platelet function induced by neutral proteases from human leukocytes

Washed human platelets are damaged by two neutral proteases from human leukocytes (elastase-like protease, ELP and chymotrypsin-like protease, CLP). The damage is manifested as inhibited aggregation by ristocetin and collagen, and enhanced aggregation by ADP in the presence of fibrinogen. Similarly to alpha-chymotrypsin (alpha-CT), CLP also increases binding of 125I-fibrinogen to platelets and renders platelets aggregable by human fibrinogen. ELP is less effective in this respect possibly due to damage to platelet receptors for fibrinogen. In the plasma medium platelets are not sensitive to leukocytic proteases added at concentrations that provoke some prolongation of the time of plasma clotting by thrombin.

Evidence that a collagen derived octapeptide inhibits fibrinogen binding to platelets stimulated by collagen and not by ADP

A synthetic octapeptide derived from type III collagen which specifically inhibits the activation and aggregation of platelets by collagen without affecting their adhesion was assayed on the collagen and ADP dependent fibrinogen binding to platelets. With 20 micrograms/ml collagen, the octapeptide (6 mM) inhibited by 68% the fibrinogen binding: this inhibition was correlated (p less than 0.01) to a decrease in the velocity of aggregation, suggesting that the fibrinogen binding might influence this parameter. The octapeptide did not affect the ADP-induced platelet aggregation and fibrinogen binding. This indicates that the octapeptide does not inhibit the binding of fibrinogen to its receptor directly, but interferes with some step(s) preceding the collagen-induced expression of the fibrinogen receptor.

Redistribution of the fibrinogen receptor of human platelets after surface activation

We investigated the whole cell distribution of the platelet membrane receptor for fibrinogen in surface-activated human platelets. Fibrinogen-labeled colloidal gold was used in conjunction with platelet whole mount preparations to visualize directly the fibrinogen receptor. Unstimulated platelets fail to bind fibrinogen, and binding was minimal in the stages of activation immediately following adhesion. The amount of fibrinogen bound per platelet increased rapidly during the shape changes associated with surface activation until 7,600 +/- 500 labels were present at saturation. Maximal binding of fibrinogen was followed by receptor redistribution. During the early stages of spreading, fibrinogen labels were uniformly distributed over the entire platelet surface, including pseudopodia, but the labels become progressively centralized as the spreading process continued. In well spread platelets, labels were found over the central regions, whereas peripheral areas were cleared of receptors. Receptor redistribution during spreading was accompanied by cytoskeletal reorganization such that a direct correlation was seen between the development of specific ultrastructural zones and the distribution of surface receptor sites suggesting a link between the surface receptors and the cytoskeleton. The association of fibrinogen receptors with contractile elements of the cytoskeleton, which permits coordinated receptor centralization, is important to the understanding of the role of fibrinogen in normal platelet aggregation and clot retraction.

Comparison of platelet fibrinogen receptors on intact and proteolytically-treated platelets by use of an anti-glycoprotein IIIa monoclonal antibody (MA 123)

A murine monoclonal antibody (MA 123) was selected by screening 153 supernatants of hybridoma cells secreting anti-human platelet antibodies for their ability to inhibit the fibrinogen-induced aggregation of chymotrypsin-treated platelets. MA 123 inhibited the binding of 125I-fibrinogen to ADP-stimulated intact human platelets and to platelets treated with chymotrypsin or pronase. Moreover, it inhibited the fibrinogen-induced aggregation of these platelet suspensions. The degree of inhibition was similar in each of the three types of platelets tested. The interactions of MA 123 with the 125I-labeled surface components of intact and chymotrypsin-treated platelets were studied by immunoprecipitation using Staphylococcus aureus coated with goat anti-mouse IgG, followed by SDS-polyacrylamide gel electrophoresis and autoradiography. MA 123 precipitated the glycoprotein IIb-glycoprotein IIIa (GPIIb-GPIIIa) complex from the surface of detergent solubilized intact human platelets; and it precipitated GPIIIa from the surface of chymotrypsin-treated platelets. Partially purified GPIIIa was also immunoprecipitated by MA 123. Our data suggest that the exposure of fibrinogen receptors by ADP, chymotrypsin or pronase, is associated with alterations of GPIIIa on the platelet surface.