Demonstration of a new glycoprotein Ib-related component in platelet extracts prepared in the presence of leupeptin

Flow cytometry of platelets for clinical analysis

Platelets are small, non-homogenous cells with distinctive surface features important to their essential role in hemostasis. The surface membrane is dynamic, and changes remarkably in lipid asymmetry and receptor expression on triggering of the activation process. There are also extensive and rapid intracellular changes in platelets as a result of biochemical activation through calcium fluxes, phospholipase activity, kinase activity, and phosphorylation mechanisms that lead to release of storage granule contents and generation of fast-acting prostaglandins, all in a matter of seconds after stimulation with a strong agonist. These characteristics make the platelet an interesting but difficult cell to study, and the explosion of knowledge over the last two decades has been fueled in large part by the application of flow cytometry techniques. Clinical applications of flow cytometry analysis of platelets have been pursued in individual specialized medical centers, but have not found widespread practice in clinical laboratories, mostly because of difficulties in standardization of techniques and the inherent biovariability in comparing normal to abnormal platelets. Despite these hurdles, it seems certain that flow cytometry analysis of platelets in pathological states will continue to evolve into more practical and robust procedures that will eventually become standard hematologic assays rather than specialized research tools.

Homozygous truncation of the fibrinogen Aα chain within the coiled coil causes congenital afibrinogenemia

The molecular basis of a novel congenital afibrinogenemia has been determined. The proposita, the only affected member in a consanguineous Norwegian family, suffers from a moderate to severe bleeding disorder due to the total absence of any detectable fibrinogen. Dot blots of solubilized platelets revealed a small amount of γ chain but no A or Bβ chains, whereas no chains were detected in plasma dot blots. DNA sequencing of the A chain gene revealed a homozygous C→T transversion 557 nucleotides from the transcription initiation site. This nucleotide change predicts the nonsense mutation A 149 Arg (CGA)→stop (TGA). Early truncation of the A chain appears to result in defective assembly or secretion of fibrinogen, probably due to the removal of the C-terminal disulfide ring residues that are critically required for the formation of a stable 3-chained half molecule.

Homozygous truncation of the fibrinogen Aα chain within the coiled coil causes congenital afibrinogenemia

The molecular basis of a novel congenital afibrinogenemia has been determined. The proposita, the only affected member in a consanguineous Norwegian family, suffers from a moderate to severe bleeding disorder due to the total absence of any detectable fibrinogen. Dot blots of solubilized platelets revealed a small amount of γ chain but no A or Bβ chains, whereas no chains were detected in plasma dot blots. DNA sequencing of the A chain gene revealed a homozygous C→T transversion 557 nucleotides from the transcription initiation site. This nucleotide change predicts the nonsense mutation A 149 Arg (CGA)→stop (TGA). Early truncation of the A chain appears to result in defective assembly or secretion of fibrinogen, probably due to the removal of the C-terminal disulfide ring residues that are critically required for the formation of a stable 3-chained half molecule.

Studies on the dual effects on platelets of a monoclonal antibody to CD9, and on the properties of platelet CD9

The article describes effects on human platelets of a murine monoclonal antibody of the IgG2a subtype (clone FN99) directed against the membrane glycoprotein CD9. This antibody exerts a dual action on human platelets in plasma depending on whether the complement system can be activated or not, resulting either in membrane permeabilization or a true platelet aggregation. Secretion from the alpha-granules during permeabilisation was not observed in the sense that the granule-located protein thrombospondin was retained in the platelets, as opposed to what was seen with platelets that had undergone an antibody-induced aggregation. Only a small fraction of P-selectin was found on the surface of the permeabilised platelets. The cytoskeletal protein actin-binding protein (filamin) was profoundly degraded during membrane permeabilisation, however, and scanning electron microscopy showed platelets that were swollen with only a few pseudopodia. Preincubation of platelets with three different antibodies to CD9 showed strong inhibition of a subsequent binding of FITC-labelled Fab fragment of FN99 indicating that antibodies tend to bind in the same area of the CD9 molecule. No association of CD9 to the platelet actin-based cytoskeleton was observed. CD9 was present on the surface of microvesicles derived from calcium ionophore-treated platelets.

Fine structural and functional consequences of deglycosylation of the platelet adhesion receptor GPIb-IX (CD 42b)

To investigate the role of the glycosylation of the platelet receptor glycoprotein Ib (GPIb, CD 42b), platelets and purified GPIb were deglycosylated by neuraminidase, O- and N-glycosidases. N-deglycosylation and neuraminic-acid cleavage had little effect on ristocetin and botrocetin-induced platelet agglutination. However, O-deglycosylation reduced the response by approximately 50%, and total deglycosylation (the combination of all three glycosidases) fully abolished the response to ristocetin. Interestingly, binding of von Willebrand Factor (vWF) to purified GPIb in the presence of ristocetin and botrocetin in a standardized microtiter plate assay was not altered by partial or even by total deglycosylation. Electron microscopy indicated that the normally stretched approximately 50 nm long molecule was approximately 32 nm after N-deglycosylation, approximately 20 nm after O-deglycosylation, and reduced in a approximately 15 nm long collapse by total deglycosylation. These results suggest that deglycosylation has major structural impacts on GPIb, strongly impairing platelet-vWF interactions; however, vWF binding to isolated GPIb remains unaffected.

Role of actin-binding protein in insertion of adhesion receptors into the membrane

The goal of this study was to determine whether actin-binding protein (ABP) regulates membrane composition. ABP-deficient and ABP-containing cells were transfected with the cDNAs coding for glycoprotein (GP) Ib-IX, a platelet receptor that interacts with ABP. Most of the GP Ib-IX remained inside the ABP-deficient cells. When ABP was present, functional GP Ib-IX was inserted into the membrane. GP Ib-IX lacking the domain that interacts with ABP also showed increased membrane insertion in ABP-expressing cells. Furthermore, a fragment of ABP that lacks the dimerization and GP Ib-IX-binding sites restored the spreading of the cells and increased the amount of GP Ib-IX in the membrane. Finally, expression of ABP also increased endogenous beta1 integrin in the membrane. These results indicate that 1) ABP maintains the properties of the cell such that adhesion receptors can be efficiently expressed in the membrane; 2) increased receptor expression is accompanied by increased ability of the cell to spread; and 3) ABP exerts its effect by a mechanism that does not appear to involve direct cross-linking of actin filaments or direct interaction with receptors.

The cytoplasmic domain of the alpha-subunit of glycoprotein (GP) Ib mediates attachment of the entire GP Ib-IX complex to the cytoskeleton and regulates von Willebrand factor-induced changes in cell morphology

The glycoprotein (GP) Ib-IX complex is one of the major platelet membrane glycoproteins. Its extracellular domain binds von Willebrand factor at a site of injury, an interaction that leads to activation of intracellular pathways. Its intracellular domain associates tightly with the platelet cytoskeleton through actin-binding protein. The goal of the present study was to investigate the role of the cytoplasmic domain of the GP Ib-IX complex and its interaction with the cytoskeleton. Cultured cells were transfected with the cDNAs coding for GP Ib(beta), GP IX, and full-length or truncated forms of GP Ib(alpha). Western blots of detergent-insoluble fractions of Triton X-100-lysed cells showed that deletion of amino acids Trp-570 to Ser-590 from the cytoplasmic domain of GP IB(alpha) abolished the interaction of the entire GP Ib-IX complex with the cytoskeleton. Truncated GP Ib(alpha) that was unable to associate with the cytoskeleton retained its ability to associate with GP Ib(beta), to be inserted into the membrane, and to bind von Willebrand factor. Cells expressing GP Ib(alpha) changed their shape following adhesion to immobilized von Willebrand factor. Cells expressing truncated GP Ib(alpha) also changed their shape following adhesion but showed a very different morphology as compared to cells expressing full-length GP Ib(alpha). These results show that GP Ib-IX-von Willebrand factor interactions lead to cytoskeletal reorganizations, that the cytoplasmic domain of GP Ib(alpha) regulates these reorganizations, and that the cytoplasmic domain of GP Ib(alpha) is absolutely required for attachment of the GP Ib-IX complex to the cytoskeleton.

Cytoplasmic calcium level and membrane fluidity of platelets contacting poly(acrylamide-co-methacrylic acid) particles with different surface properties

Changes in cytoplasmic free calcium levels and membrane fluidity of platelets in contact with poly(acrylamide-co-methacrylic acid) (PAAmMAc) particles were examined to analyze the mechanistic aspect of regulating platelet function. Our previous studies demonstrated interesting features of PAAmMAc particles during interaction with platelets: (1) PAAmMAc particles induce no calcium increase but enhance membrane fluidity of platelets: (2) thrombin induces no calcium increase in platelets when the platelets were mixed previously with PAAmMAc particles; and (3) PAAmMAc particles induce a calcium increase in platelets when they were treated previously with sodium azide (NaN3). These results suggest the possibility that PAAmMAc surfaces may regulate the calcium level by influencing platelet metabolism. In this study, non-cross-linked PAAmMAc solution with the same chemical composition as the particles showed a suppressive effect on thrombin-induced calcium increase, but, no influence on membrane fluidity. This result indicates that aggregated macromolecular surface assemblies of PAAmMAc may dominate the increase in membrane fluidity of platelets although the calcium change is induced by discrete molecular level interaction between the PAAmMAc and platelet membranes. It was also revealed that the suppression of thrombin-induced calcium increase and the membrane fluidity increase in platelets by PAAmMAc particles were reduced by albumin-treatment of the particles. This result suggests that such phenomena may be due to a decrease in any physicochemical interaction of PAAmMAc surfaces with albumin, rather than platelet metabolic change. PAAmMAc particle surfaces with higher carboxyl groups exhibited a more suppressive effect on thrombin-induced calcium increase, whereas those with lower carboxyl groups derived a higher calcium increase when the platelets were treated previously with NaN3. These results suggest the importance of electrostatic and any other physicochemical interaction of PAAmMAc chains on regulating cytoplasmic calcium levels.

Complement-mediated permeabilization of platelets by monoclonal antibodies to CD9: inhibition by leupeptin, and effects on the GP Ib-actin-binding protein system

Two monoclonal antibodies to CD9 of the IgM and IgG2a categories (FN 52 and FN 99), reproducibly induced platelet alterations in platelet-rich plasma by activation of the complement system with membrane incorporation of the pore-forming C5b-9 complex. The permeabilization could be monitored by measurements of extracellular ATP and observed as a shape change followed by an increase in light transmission in the aggregometer, and was associated with formation of tiny platelet aggregates. This could be accomplished by only minor lysis observed as extracellular lactate dehydrogenase (LDH). When leupeptin was added prior to, or immediately after the antibody, a total inhibition of the platelet alterations could be obtained. When added soon after the shape change, leupeptin had little effect on the liberation of ATP. However, whereas the ability of the platelets to become agglutinated by ristocetin was lost during the complement-mediated platelet alterations, addition of leupeptin immediately after the shape change, prevented this loss. The lost ability of the permeabilized platelets to undergo ristocetin-induced agglutination is not ascribed to degradation of GP Ib as this was relatively little affected in these studies as compared to the actin-binding protein (ABP) which was profoundly degraded. This protein represents a link between GP Ib and the submembraneous cytoskeleton, and the inhibition of its degradation by leupeptin, was clearly demonstrated. Experiments with digitonin-induced permeabilization showed that leupeptin did not inhibit permeabilization as such, but it did prevent the loss of ristocetin-induced agglutination even with this inducer.

The effects of calpeptin (a calpain specific inhibitor) on agonist induced microparticle formation from the platelet plasma membrane

Platelets activated by various agonists produce formation of vesicles shed from the plasma membrane (microparticles). However, the mechanism of microparticle (MP) formation has not been clarified yet. The aim of the present study was to determine the possibility of involvement of calpain (a Ca(2+)-dependent thiol protease) in MP formation. Washed platelets preincubated with calpeptin, a cell permeable calpain specific inhibitor, or with a vehicle were activated by thrombin plus collagen or by calcium ionophore A23187. Flow cytometry was used to detect the amount of microparticle formation by using murine monoclonal antibodies against GP IIb-IIIa or GP IIb and fluorescein 5-isothiocyanate labeled goat anti-mouse IgG. MP formation stimulated either by thrombin plus collagen or by A23187 was inhibited by calpeptin in a dose dependent manner. The microparticle formation from platelets activated by A23187 reached a plateau in approximately 5 min after activation, whereas that from platelets activated by thrombin plus collagen reached a plateau at 30 min following the stimulation. These time sequences corresponded well with those of degradation of actin-binding protein (ABP), a well known substrate of calpain, of platelets activated by these two stimulations. However, the inhibition of MP formation by calpeptin was more marked in the early stage (within 10 min) than in the late stage (after 30 min) of platelet activation. At 30 min after platelet activation by either two stimulations, a significant amount of microparticle formation was observed in the presence of 30 microM calpeptin, which inhibited hydrolysis of ABP almost completely. Our data suggest the involvement of calpain in the early stage (especially within 10 min) of microparticle formation.

Correlation of light transmission changes to changes of platelet glycoprotein Ib during storage of platelet concentrates

The present study compares some properties of platelet concentrates, i.e. their ability to transmit light and the degree of platelet lysis and the deterioration of the platelet membrane glycoprotein Ib (GPIb). Extracellular levels of glycocalicin and lactate dehydrogenase (LDH) served as indicators for the proteolytic cleavage of GPIb and platelet lysis, respectively. The light transmission of 16 platelet apheresis concentrates was monitored continuously during storage using a specially designed apparatus. Determination of platelet counts and extracellular glycocalicin levels was carried out on day 1. In addition, after 7 days of storage extracellular concentrations of LDH and glycocalicin were determined. After 7 days highly significant correlations were found both when comparing light transmission changes and extracellular glycocalicin levels (r = 0.78; p < 0.001) and when comparing light transmission changes and extracellular concentrations of LDH (r = 0.83; p < 0.001).

Mechanism of cytoplasmic calcium changes in platelets in contact with polystyrene and poly(acrylamide-co-methacrylic acid) surfaces

Changes in cytoplasmic free calcium levels ([Ca2+]i) in platelets in contact with polystyrene (PSt) and poly(acrylamide-co-methacrylic acid) (PAAmMAc) particles were evaluated and results were compared with those from two representative biological calcium agonists; thrombin and calcium ionophore A23187. PSt particles stimulated a steep increase in cytoplasmic calcium levels in platelets as much as thrombin and A23187. Serratia protease-treated platelets showed a steep increase in [Ca2+]i by PSt particles, suggesting that PSt surfaces can initiate platelet activation independent of a glycoprotein Ib (GPIb)-mediated pathway. By contrast, dibucaine-treated platelets showed little increase in [Ca2+]i by PSt particles, indicating that microfilament assembly, including binding of GPIb with actin binding protein, should be required for platelet activation in contact with PSt surfaces. PAAmMAc particles induced little increase in cytoplasmic calcium levels in platelets. However, PAAmMAc particle-treated platelets demonstrated little response to thrombin in terms of an increase in [Ca2+]i and ATP release, suggesting the possibility that PAAmMAc surfaces may regulate [Ca2+]i by influencing platelet metabolism. Furthermore, sodium azide-treated platelets showed an increase in [Ca2+]i in platelets when contacting PAAmMAc particles, supporting the suggestion that PAAmMAc surfaces could regulate platelet functions. Fluorescence polarization measurements using 1,6-diphenyl-1,3,5-hexatriene-loaded platelets revealed that PAAmMAc particles increased membrane fluidity in platelets, which may be due to physicochemical interaction with PAAmMAc surfaces.

Effects of toluene on platelet membrane glycoprotein Ib and actin-binding protein

Effects of the organic solvent toluene on the platelet membrane receptor glycoprotein Ib (GP Ib) and the cytoskeletal protein, actin-binding protein (ABP), were studied and related to the effects of the local anesthetic dibucaine. The glycocalicin-region of GP Ib contains the binding site for von Willebrand factor; intracellularly GP Ib is linked to the cytoskeleton via ABP. Both GP Ib and ABP are substrates for a calcium-dependent protease, calpain. Washed platelets were incubated with toluene or dibucaine. The toluene concentration in the platelet suspension was analysed by gas chromatography. Using 1.5-2.8 mmol/L toluene, calpain was activated, leading to degradation of ABP and release of glycocalicin from GP Ib. The latter phenomenon was paralleled by a reduced von Willebrand factor-induced platelet agglutination. At lower toluene concentrations (0.3-1.4 mmol/L), degradation of ABP was not detected but an initial increased agglutination that declined to the control level with time was observed. These effects of toluene on the GP Ib-ABP complex are similar to those observed with 1 mmol/L dibucaine. The lowest toluene concentrations used correspond to those that have been found in blood from toluene abusers ("sniffers").

Activation of calpain I and hydrolysis of calpain substrates (actin-binding protein, glycoprotein Ib, and talin) are not a function of thrombin-induced platelet aggregation

Calcium-activated neutral proteinase (calpain) has been shown to cleave proteins involved in the maintenance of cell structure. In human platelets, substrates of calpain include glycoprotein Ib (GPIb), actin-binding protein (ABP), and talin. GPIb-ABP complexes can be isolated in detergent extracts and are thought to represent membrane-cytoskeleton attachment sites. It has been hypothesized that the hydrolysis of GPIb-ABP by calpain is regulated by the extent of binding of this proteinase to the plasma membrane-cytoskeleton interface with platelet activation. Recently, another calpain substrate (talin) has been shown to redistribute from the cytoplasm to the plasma membrane-cytoskeleton interface as the result of thrombin stimulation. To investigate the intracellular distribution of calpain I, we employed the monoclonal antibody B27D8, specific for the heavy chain (catalytic subunit) of calpain I. Indirect immunofluorescent staining of resting human platelets revealed undetectable surface antigen. Permeabilization with Triton X-100, however, revealed a diffuse intracellular antigen consistent with a cytosolic distribution. To determine whether this antigen distribution reflected the proenzyme or the activated form of calpain I and to assess the degree of hydrolysis of ABP, GPIb, and talin, we employed B27D8 and murine monoclonal antibodies against ABP (1B3 and 3D1), GPIb (LJIb10), and rabbit polyclonal antibodies against talin (A2 and B11) in a quantitative immunotransblot assay. Examination of resting platelets revealed that calpain I existed as the 85-kd proenzyme form and that ABP, GPIb, and talin existed in their native intact forms. When platelets were aggregated with thrombin, autoproteolysis of calpain I occurred within the 30 seconds required to completely solubilize platelet aggregates in sodium dodecyl sulfate-containing buffer and not as a direct result of thrombin-induced activation.(ABSTRACT TRUNCATED AT 250 WORDS)

The use of PhastSystem crossed immunoelectrophoresis with immunoblotting to demonstrate a complex between glycoprotein Ib and the actin-binding protein (ABP) of human platelets

The study shows how a technique described in an accompanying paper can be applied to solve a biological problem. The technique makes use of the observation that a monoclonal antibody that has been coprecipitated with its antigen during crossed immunoelectrophoresis can be transferred to a nitrocellulose membrane and visualized. Previous studies using crossed immunoelectrophoresis of Triton X-100 extracts of platelets have indicated that a particular immunoprecipitate (peak III) of the membrane receptor glycoprotein Ib (GP Ib) might contain a complex between the receptor and the actin-binding protein (filamin). When a monoclonal antibody (PM6/317) directed towards the actin-binding protein was added to a platelet extract prior to immunoelectrophoresis and blotting, this was visualized on the blot as a replica of the peak III immunoprecipitate. This demonstrates a colocalization of GP Ib and the actin-binding protein in the precipitate, and thus the existence of a complex between the membrane receptor and the cytoskeletal protein.

Human platelet glycoprotein IX: an adhesive prototype of leucine-rich glycoproteins with flank-center-flank structures

The glycoprotein (GP) Ib-IX complex on the surface of human platelets functions as the von Willebrand factor receptor and mediates von Willebrand factor-dependent platelet adhesion to blood vessels. GPIX is a relatively small (Mr, 17,000) protein that may provide for membrane insertion and orientation of the larger component of the complex, GPIb (Mr, 165,000). Using antibody screening, we cloned a cDNA encoding GPIX from a human erythroleukemia cell cDNA library constructed in phage lambda gt11. Lacking a 5' untranslated region and start codon, the cDNA sequence includes 604 nucleotides, beginning with 495 bases at the 5' end coding for 165 amino acids, followed by a stop codon and 106 noncoding bases at the 3' end. By Northern blot analysis, the GPIX cDNA hybridizes with a single 1.0-kilobase species of platelet poly(A)+ RNA. Translation of the cDNA sequence gives a predicted protein sequence beginning with a truncated putative signal sequence of 5 amino acid followed by a sequence of 17 amino acids matching that determined directly by Edman degradation of intact GPIX. The predicted amino acid sequence of mature GPIX includes an NH2-terminal extracytoplasmic domain of 134 residues, a transmembrane domain of 20 residues, 6 intracytoplasmic residues, and 1 N-linked glycosylation site. GPIX contains a leucine-rich glycoprotein (LRG) sequence of 24 amino acids similar to conserved LRG sequences in GPIb and other proteins from humans, Drosophila, and yeast. "Flanking" sequences of approximately 22 amino acids are present at the NH2 and/or COOH sides of the "central" LRG sequence(s) in GPIX, GPIb, and the other human and Drosophila members of the LRG family. The role of the flank-LRG center-flank structure in the evolution and function of the LRG proteins remains to be defined.

Isolation and characterization of human platelet glycoprotein IX

Human platelet glycoprotein IX is a small (Mr 17,000) surface glycoprotein present in the plasma membrane as a 1:1 non-covalent complex with glycoprotein Ib (Mr 165,000). Glycoprotein IX was purified to near homogeneity by sequential wheat germ agglutinin and immuno-affinity chromatography, followed by gel filtration chromatography under denaturing conditions. Purified glycoprotein IX was characterized by determination of its amino acid composition and its NH2-terminal amino acid sequence (thr-lys-asp-xxx-pro-ser-xxx-leu-thr-(thr)2-arg-ala-(leu)-(glu)-xxx-met- gly), and monospecific anti-glycoprotein IX antibody was prepared. The study outlines a useful approach for separating and characterizing glycoprotein IX, free from other members of the glycoprotein Ib complex.

Phosphorylation of platelet actin binding protein protects against proteolysis by calcium dependent sulfhydryl protease

When Actin Binding Protein (ABP) isolated from human blood platelets is phosphorylated in vitro with a cyclic AMP dependent kinase it becomes resistant to proteolysis by the Calcium Dependent Sulfhydryl Protease (CDSP). This protection against proteolytic cleavage is specific for CDSP since phosphorylation of ABP does not protect against proteolysis by trypsin, papain and thermolysin. Thus, there appears to be a distinct phosphorylation site on the ABP molecule which is essential for regulating the initial proteolytic degradation of ABP by CDSP.

The glycoprotein Ib complex of human blood platelets

Human glycoprotein Ib (GPIb) is a major integral membrane protein on human blood platelets responsible for the initial attachment of platelets to the exposed vascular subendothelium. In this report we describe an isolation method for a 'GPIb complex' as well as for its individual components. A three-step procedure involving Triton X-114 phase-partition, affinity chromatography on wheat germ agglutinin and ion-exchange chromatography on DEAE-Sephacel yielded milligram quantities of purified GPIb complex. The single components of the complex were further purified by gel filtration on AcA34 in 0.1% sodium dodecyl sulfate. As well as GPIb, the complex contains GP17, actin-binding protein, actin and a series of unidentified proteins with different molecular masses. In contrast to GPIb alpha, which is very rich in O-linked oligosaccharides, sugar analysis revealed that GPIb beta and GP17 seem to have only N-linked chains of the lactosamine type. The C-terminal alpha-chain remnant, which probably spans the plasma membrane, was identified and isolated for the first time. Western blotting with polyclonal rabbit anti-GPIb antibodies and silver-staining of one- or two-dimensional dodecyl sulfate/polyacrylamide gels revealed that it has an apparent molecular mass of 20 kDa and is linked to GPIb beta by a disulfide bridge close to the membrane. The thrombin-binding site on GPIb is located near the N-terminus on a 40-kDa fragment of GPIb alpha. A disulfide bridge in the N-terminal region is not essential for thrombin binding to GPIb.

Platelet surface glycoproteins. Studies on resting and activated platelets and platelet membrane microparticles in normal subjects, and observations in patients during adult respiratory distress syndrome and cardiac surgery

The accurate definition of surface glycoprotein abnormalities in circulating platelets may provide better understanding of bleeding and thrombotic disorders. Platelet surface glycoproteins were measured on intact platelets in whole blood and platelet membrane microparticles were assayed in cell-free plasma using 125I-monoclonal antibodies. The glycoproteins (GP) studied were: GP Ib and GP IIb-IIIa, two of the major intrinsic plasma membrane glycoproteins; GMP-140, an alpha-granule membrane glycoprotein that becomes exposed on the platelet surface following secretion; and thrombospondin (TSP), an alpha-granule secreted glycoprotein that rebinds to the platelet surface. Thrombin-induced secretion in normal platelets caused the appearance of GMP-140 and TSP on the platelet surface, increased exposure of GP IIb-IIIa, and decreased antibody binding to GP Ib. Patients with adult respiratory distress syndrome had an increased concentration of GMP-140 and TSP on the surface of their platelets, demonstrating in vivo platelet secretion, but had no increase of platelet microparticles in their plasma. In contrast, patients after cardiac surgery with cardiopulmonary bypass demonstrated changes consistent with membrane fragmentation without secretion: a decreased platelet surface concentration of GP Ib and GP IIb with no increase of GMP-140 and TSP, and an increased plasma concentration of platelet membrane microparticles. These methods will help to define acquired abnormalities of platelet surface glycoproteins.

Linkage of a membrane skeleton to integral membrane glycoproteins in human platelets. Identification of one of the glycoproteins as glycoprotein Ib

Experiments were performed to determine whether platelets contain a membrane skeleton. Platelets were labeled by a sodium periodate/sodium [3H]borohydride method and lysed with Triton X-100. Much of the filamentous actin could be sedimented at low g forces (15,600 g, 4 min), but some of the actin filaments required high-speed centrifugation for their sedimentation (100,000 g, 3 h). The latter filaments differed from those in the low-speed pellet in that they could not be depolymerized by Ca2+ and could not be sedimented at low g forces even from Triton X-100 lysates of platelets that had been activated with thrombin. Actin-binding protein sedimented with both types of filaments, but 3H-labeled membrane glycoproteins were recovered mainly with the high-speed filaments. The primary 3H-labeled glycoprotein recovered with this "membrane skeleton" was glycoprotein (GP) Ib. Approximately 70% of the platelet GP Ib was present in this skeleton. Several other minor glycoproteins, including greater than 50% of the GP Ia and small amounts of three unidentified glycoproteins of Mr greater than 200,000, were also recovered with the membrane skeleton. The Triton X-100 insolubility of GP Ib, GP Ia, a minor membrane glycoprotein of 250,000 Mr, and actin-binding protein resulted from their association with actin filaments as they were rendered Triton X-100-soluble when actin filaments were depolymerized with deoxyribonuclease I and co-isolated with actin filaments on sucrose gradients. When isolated platelet plasma membranes were extracted with Triton X-100, actin, actin-binding protein, and GP Ib were recovered as the Triton X-100 residue. These studies show that unstimulated platelets contain a membrane skeleton composed of actin filaments and actin-binding protein that is distinct from the rest of the cytoskeleton and is attached to GP Ib, GP Ia, and a minor glycoprotein of 250,000 Mr on the plasma membrane.

Thiol-specific probes indicate that the beta-chain of platelet glycoprotein Ib is a transmembrane protein with a reactive endofacial sulfhydryl group

We used two membrane-permeable fluorescent reagents, monobromobimane and N-[[5-(dimethylamino)-1-naphthalenyl]sulfonyl]aziridine (N-dansylaziridine), and one membrane-impermeable fluorescent probe, monobromo(trimethylammonio)bimane, all three of which react selectively with protein thiols, to assess the presence of reactive sulfhydryls in the platelet glycoprotein Ib (GPIb) molecule and establish the topology of any GPIb-reactive thiols in the platelet membrane. Intact platelets were reacted with 1-10 mM monobromobimane or monobromo(trimethylammonio)bimane or 50-100 microM N-dansylaziridine for 30-60 min at 37 degree C. The platelets were then washed, solubilized in 1% Triton X-100, and analyzed by nonreduced-reduced polyacrylamide gel electrophoresis either directly or indirectly after immunopurification of GPIb. Monobromobimane and N-dansylaziridine labeled GPIb beta but not GPIb alpha in intact platelets. This labeling could be inhibited by pretreating the platelets with either N-ethylmalemide or p-(chloromercuri)benzenesulfonic acid, confirming the specificity of these probes for thiol groups. Monobromo(trimethylammonio)bimane, the membrane-impermeable reagent, did not label GPIb beta in intact platelets. However, it did label GPIb beta in sonicated platelets, indicating that the thiol group of GPIb beta occupies an intracellular location. Since the carbohydrate moiety of GPIb beta can be labeled from the outside of intact platelets with membrane-impermeable reagents, we conclude that GPIb beta has a transmembrane orientation.

Purification and preliminary characterization of the glycoprotein Ib complex in the human platelet membrane

Human platelet glycoprotein Ib (GP Ib) is a major integral membrane protein that has been identified as the platelet-binding site mediating the factor VIII/von Willebrand-factor-dependent adhesion of platelets to vascular subendothelium. Recent evidence suggests that GP Ib is normally complexed with another platelet membrane protein, GP IX. In this study, human platelet plasma membranes were selectively solubilized with a buffer containing 0.1% (v/v) Triton X-100. The GP Ib complex (GP Ib plus GP IX) was purified to homogeneity in approximately 30% yield by immunoaffinity chromatography of the membrane extract using the anti-(glycoprotein Ib complex) murine monoclonal antibody, WM 23, coupled to agarose. GP Ib and GP IX were subsequently isolated as purified components by immunoaffinity chromatography of the GP Ib complex using a second anti-(glycoprotein Ib complex) monoclonal antibody, FMC 25, coupled to agarose. As assessed by dodecyl sulphate/polyacrylamide gel electrophoresis, purified GP Ib was identical to the molecule on intact platelets and had an apparent relative molecular mass of 170 000 under nonreducing conditions and 135 000 (alpha subunit) and 25 000 (beta subunit) under reducing conditions. GP IX had an apparent Mr of 22 000 under both nonreducing and reducing conditions. Purified Gb Ib complex and GP Ib inhibited the ristocetin-mediated, human factor VIII/von Willebrand-factor-dependent and bovine factor VIII/von Willebrand-factor-dependent agglutination of washed human platelets suggesting the proteins had been isolated in functionally active form. GP Ib alpha had a similar amino acid composition to that previously reported for its proteolytic degradation product, glycocalicin. The amino acid compositions of GP Ib beta and GP IX were similar but showed marked differences in the levels of glutamic acid, alanine, histidine and arginine. The N-termini of GP Ib alpha and GP IX were blocked; GP Ib beta had the N-terminal sequence, Ile-Pro-Ala-Pro-. On crossed immunoelectrophoresis, both GP Ib and GP IX were found to occur in the same immunoprecipitin arc(s) whether the platelets had been solubilized in the absence or presence of the calcium-dependent protease inhibitor, leupeptin. Binding studies in platelet-rich plasma indicated a similar number of binding sites (means +/- SD) for three anti-(glycoprotein Ib complex) monoclonal antibodies: AN 51, epitope on GP Ib alpha (22 000 +/- 2700, n = 3), WM 23, epitope on GP Ib alpha (21 000 +/- 3400, n = 3), FMC 25, epitope on GP IX (20 100 +/- 2700, n = 3), and FMC 25 (Fab')2 (27 100 +/- 800, n = 2).(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of diamide and dibucaine on platelet glycoprotein Ib, actin-binding protein and cytoskeleton

During extraction of platelets by 1% Triton X-100, the actin-binding protein (platelet filamin) and a 230 kDa protein are degraded by a calcium-activated thiol protease. Occurrence of degradation products of Mr 190 000 (HF-1) and 90 000 (HF-2) is a sensitive indicator of this proteolysis, and can be used to decide whether reduced amounts of the actin-binding protein in extracts are due to proteolysis or to incorporation in the Triton-insoluble (cytoskeletal) fraction. Diamide, which is a sulfhydryl-oxidizing protein cross-linker, inhibits the calcium-activated protease, polymerizes the actin-binding protein and the 230 kDa protein, increases the incorporation of glycoprotein Ib into the cytoskeletal fraction, and inhibits platelet agglutination induced by bovine von Willebrand factor. Inhibition of platelet agglutination by pretreatment with diamide is partly reversed by dibucaine which activates the calcium-activated protease. These observations are in accordance with a working hypothesis that interactions of glycoprotein Ib with cytoskeleton affect, and possibly regulate, its receptor function in the intact platelet.

Tandem separation of labelled human blood platelet membrane glycoproteins by anion-exchange and gel fast protein liquid chromatography

Studies were made of the separation of surface-labelled platelet glycoproteins, solubilized in a non-ionic detergent (Berol 185), on a Mono Q anion-exchange column, coupled to a Superose 12 gel fast protein liquid chromatographic column. Peaks eluted from the anion-exchange and gel columns were subjected to electrophoresis on sodium dodecyl sulphate-polyacrylamide gels in the presence (non-reducing conditions) or absence (reducing conditions) of disulphide bridges. Labelled electrophoresed glycoprotein bands were rendered visible by fluorography or indirect autoradiography. Platelet membrane glycoproteins Ib, IIb, IIIa, IIIb, V and IX were identified by their apparent molecular weights and their surface labelling characteristics. It was concluded that tandem chromatography can be used to separate platelet membrane proteins and glycoproteins rapidly and with good resolution.

On the association of glycoprotein Ib and actin-binding protein in human platelets

Glycoprotein (GP) Ib was purified from lysates of human platelets prepared in the presence or absence of inhibitors of the endogenous calcium-activated neutral protease (CANP) by immunoaffinity chromatography, employing the GPIb-specific murine monoclonal antibody, AP1, coupled to Sepharose CL4B. When derived from lysates prepared in the presence of EDTA or leupeptin, the eluate from the AP1-affinity column contained a 240,000-260,000-mol-wt protein in addition to GPIb. In SDS PAGE, this protein was stained by Coomassie Blue R, but not by the periodic acid-Schiff reagent, and it was not labeled with 125I in intact platelets by the lactoperoxidase-catalyzed method. When derived from lysates prepared in the absence of CANP inhibitors, the eluate contained only GPIb and its proteolytic derivative, glycocalicin. A change in the electrophoretic mobility of GPIb consistent with its association with the 240,000-260,000-mol-wt protein was confirmed by crossed immunoelectrophoresis. By an immunoblot technique involving transfer of proteins eluted from the AP1-affinity column and separated by SDS PAGE onto a nitrocellulose membrane, the 240,000-260,000-mol-wt protein bound polyclonal goat antibody raised against rabbit macrophage actin-binding protein (ABP). On the basis of these results, we conclude the GPIb is tightly associated with ABP under conditions in which the endogenous CANP is inhibited, and that this apparent transmembrane complex of GPIb-ABP can be isolated in lysates of nonactivated human platelets.

Distribution of platelet glycoproteins and phosphoproteins in hydrophobic and hydrophilic phases in Triton X-114 phase partition

Platelets, either unlabelled, surface-labelled by the periodate NaB3H4 method or metabolically labelled with 32P were solubilized in Triton X-114 and partitioned into aqueous and detergent phases. The phases were analysed by two-dimensional polyacrylamide gel electrophoresis followed by silver-staining, fluorography or indirect autoradiography. Each of the phases contains a distinct set of proteins. The surface-labelled glycoproteins partition into the hydrophobic phase with the notable exceptions of glycoproteins Ib and GP17(5.8-6.5) and minor amounts of a few others. The phosphoproteins which undergo increased phosphorylation on platelet activation in general separate in the hydrophobic phase, while higher molecular weight phosphoproteins were principally in the hydrophilic phase. This method might be used as a first step in purifying many platelet components.

Molecular defects in interactions of platelets with the vessel wall

The objectives of this review have been to summarize the recent research on inherited defects involving abnormal platelet function and to illustrate how studies of hemorrhagic syndromes have led to an increased understanding of the molecular events involved in platelet adhesion and aggregation. Emphasis has been placed on the two primary hemostatic reactions: the interaction of platelets with von Willebrand factor to promote adhesion to the subendothelium, and the interaction of platelets with fibrinogen to promote platelet aggregation. Even as these events are more clearly defined, new concepts of molecular structure, function, and heterogeneity are emerging, and the variety of recognized genetic defects is becoming more complex.

Quantitation of platelet membrane glycoproteins in Glanzmann's thrombasthenia and the Bernard-Soulier syndrome by electroimmunoassay

A rapid and precise electroimmunoassay for quantitation of glycoprotein Ib and IIb-IIIa in platelet lysates is described. Using this method, platelets from two patients with Type II thrombasthenia contained 8% and 12% of normal levels of IIb-IIIa, whereas IIb-IIIa was undetectable in platelets of one Type I thrombasthenic patient. The level of Ib in two patients with Bernard-Soulier syndrome was essentially nil. The increased precision of this assay, when compared to CIE, allows for estimations of glycoproteins levels varying by no more than a few percent.

Cleavage of fibrinogen by human platelet calcium-activated protease

In lysates of washed human platelets produced by sonication or by addition of nonionic detergent, fibrinogen (Mr 340,000) was rapidly degraded, under conditions favorable to activation of the endogenous calcium-activated protease (CAP), to a core derivative (Mr 280-290,000) composed of partially degraded A alpha chains (Mr 47,000, 46,000, and 34,000) and B beta chains (Mr 56,000), and apparently intact gamma chains (Mr 53-54,000). Extensive degradation occurred within one minute at 4 degrees C, ambient temperature or at 37 degrees C, and was inhibited by leupeptin, EDTA, EGTA, or N-Ethylmaleimide, but not by soybean trypsin inhibitor, hirudin, aprotonin, benzamidine, phenylmethylsulfonyl fluoride or epsilon-aminocaproic acid. Purified plasma fibrinogen exposed to lysates containing active protease was cleaved in an identical fashion. The cleavage pattern of A alpha chains produced by this platelet protease activity is different from that produced by plasmin in vitro or that found in fibrinogen catabolites in vivo, and is unlike that produced by any cellular fibrinolytic enzyme yet described. In view of this finding, as well as the striking differential inhibitory effect of the agents cited above, we conclude that the degradation of platelet fibrinogen observed in these studies is due to direct proteolysis by platelet CAP.

Evidence that glycocalicin circulates in normal plasma

By using a combination of a heterologous antiserum to GPIb/glycocalicin and a radiolabeled monoclonal antibody to GPIb/glycocalicin, we were able to develop a sensitive and specific radioimmunoelectrophoretic assay that can distinguish small amounts of glycocalicin from GPIb. Normal plasmas were found to contain glycocalicin, even in samples treated with protease inhibitors and centrifuged extensively to remove platelets and platelet fragments. Confirmation that the plasma antigen had a relative molecular weight similar or identical to glycocalicin was obtained from studies employing gel chromatography and affinity chromatography. An immunoradiometric assay was developed to quantify plasma glycocalicin, and normal plasma was found to contain approximately 1-3 micrograms/ml. The plasma of a patient with severe thrombocytopenia due to aplastic anemia had less than 12.5% of the normal level of glycocalicin, whereas two patients with thrombocytopenia due to diseases of increased platelet destruction (idiopathic thrombocytopenic purpura and hemolytic-uremic syndrome) had normal levels. Thus, there appears to be ongoing catabolism of platelet GPIb in vivo, and we postulate that the plasma level of glycocalicin reflects a complex function of factors, including platelet count, platelet turnover, and the site of platelet destruction.

Crosslinking of platelet glycoprotein Ib by N-succinimidyl(4-azidophenyldithio)propionate and 3,3'-dithiobis(sulfosuccinimidyl propionate)

To examine the relationship between glycoprotein Ib and other proteins in the platelet membrane and the interaction of this protein with thrombin, platelets were crosslinked by two cleavable reagents, SADP (N-succinimidyl(4-azidophenyldithio)propionate) and DTSSP (3,3'-dithiobis(sulfosuccinimidyl propionate]. Two-dimensional, unreduced-reduced sodium dodecyl sulphate (SDS)-polyacrylamide electrophoresis and staining by silver or wheat germ agglutinin-conjugated peroxidase, after protein transfer to nitrocellulose, demonstrated that SADP intramolecularly crosslinked glycoprotein Ib and formed intermolecular complexes of glycorprotein IIb and some high molecular weight proteins. DTSSP intermolecularly crosslinked glycoprotein Ib, glycoprotein IIb, and other high molecular weight proteins. With a low concentration of 125I-labeled TLCK-thrombin (6 nM), crosslinking with SADP yielded a 200 000 Da complex containing radioactive-labeled thrombin, and high TLCK-thrombin concentration (0.1 microM) gave the complex and a 167 000 band. alpha- and TLCK-thrombin crosslinking with DTSSP also yielded the 200 000 complex formed by reaction with SADP or DTSSP was markedly reduced by preincubation of platelets with excess unlabeled TLCK-thrombin and had a pI similar to glycoprotein Ib. These results suggest that glycoprotein Ib is one of the proteins composing the high affinity receptor for thrombin.