A platelet alpha-granule membrane protein (GMP-140) is expressed on the plasma membrane after activation

We have previously characterized a monoclonal antibody, S12, that binds only to activated platelets (McEver, R.P., and M.N. Martin, 1984, J. Biol. Chem., 259:9799-9804). It identifies a platelet membrane protein of Mr 140,000, which we have designated as GMP-140. Using immunocytochemical techniques we have now localized this protein in unstimulated and thrombin-stimulated platelets. Polyclonal antibodies to purified GMP-140 were used to enhance the sensitivity of detection. Nonpermeabilized, unstimulated platelets, incubated with anti-GMP-140 antibodies, and then with IgG-gold probes, showed very little label for GMP-140 along their plasma membranes. In contrast, thrombin-stimulated platelets exhibited at least a 50-fold increase in the amount of label along the plasma membrane. On frozen thin sections of unstimulated platelets we observed immunogold label along the alpha-granule membranes. We also employed the more sensitive technique of permeabilizing with saponin unstimulated platelets in suspension, and then incubating the cells with polyclonal anti-GMP-140 antibodies and Fab-peroxidase conjugate. Alpha-granule membranes showed heavy reaction product, but no other intracellular organelles were specifically labeled. These results demonstrate that GMP-140 is an alpha-granule membrane protein that is expressed on the platelet plasma membrane during degranulation.     

The platelet fibrinogen receptor: an immunogold-surface replica study of agonist-induced ligand binding and receptor clustering

Platelet aggregation requires the binding of fibrinogen to its receptor, a heterodimer consisting of the plasma-membrane glycoproteins (GP) IIb and IIIa. Although the GPIIb-IIIa complex is present on the surface of unstimulated platelets, it binds fibrinogen only after platelet activation. We have used an immunogold-surface replica technique to study the distribution of GPIIb-IIIa and bound fibrinogen over broad areas of surface membranes in unstimulated, as well as thrombin-activated and ADP-activated human platelets. We found that the immunogold-labeled GPIIb-IIIa was monodispersed over the surface of unstimulated platelets, although the cell surface lacked immunoreactive fibrinogen. On thrombin-stimulated platelets, approximately 65% of the GPIIb-IIIa molecules were in clusters within the plane of the membrane. Fibrinogen, which had been released from the alpha-granules of these cells, bound to GPIIb-IIIa on the cell surface and was similarly clustered. To determine whether the receptors clustered before ligand binding, or as a consequence thereof, we studied the surface distribution of GPIIb-IIIa after stimulation with ADP, which causes activation of the fibrinogen receptor function of GPIIb-IIIa without inducing the release of fibrinogen. In the absence of added fibrinogen, the unoccupied, yet binding-competent receptors on ADP-stimulated platelets were monodispersed. The addition of fibrinogen caused the GPIIb-IIIa molecules to cluster on the cell surface. Clustering was also induced by the addition of the GPIIb-IIIa-binding domains of fibrinogen, namely the tetrapeptide Arg-Gly-Asp-Ser on the alpha-chain or the gamma-chain decapeptide gamma 402-411. These results show that receptor occupancy causes clustering of GPIIb-IIIa in activated platelets.     

Immunogold-surface replica study of ADP-induced ligand binding and fibrinogen receptor clustering in human platelets

Platelet cohesion requires the binding of fibrinogen to its receptor, a heterodimer consisting of the plasma-membrane glycoproteins GPIIb and GPIIIa. Although the GPIIb-IIIa complex is present on the surface of unstimulated platelets, it binds fibrinogen only after platelet activation. We have used an immunogold-surface replica technique to study the distribution of GPIIb-IIIa and bound fibrinogen over broad expanses of surface membranes in unstimulated and ADP-activated human platelets. We found that the gold prove was monodispersed over the surface of unstimulated platelets, although the cell surface lacked immunoreactive fibrinogen. To ascertain whether the receptors clustered prior to ligand binding or as a consequence thereof, we studied the surface distribution of GPIIb-IIIa after stimulation with ADP, which causes activation of the fibrinogen receptor function of GPIIb-IIIa without inducing the secretion of fibrinogen. In the absence of added fibrinogen, the unoccupied, yet binding-competent receptors on ADP-stimulated platelets were monodispersed. The addition of fibrinogen caused the GPIIb-IIIa molecules to cluster on the cell surface. Clustering was also induced by the addition of the GPIIb-IIIa binding domains of fibrinogen--namely, the tetrapeptide Arg-Gly-Asp-Ser on the alpha-chain or the gamma-chain decapeptide gamma 402-411. These results show that receptor occupancy causes clustering of GPIIb-IIIa in activated platelets.     

Downregulation of GMP-140 (CD62 or PADGEM) expression on platelets by N,N-dimethyl and N,N,N-trimethyl derivatives of sphingosine.

GMP-140 (CD62 or PADGEM), a member of the selectin family, is a membrane glycoprotein in secretory granules of platelets and endothelial cells. When these cells are activated by agonists such as thrombin or AMP, GMP-140 is rapidly redistributed to the cell surface. The carbohydrate epitope defined by GMP-140 was identified as sialosyl-Le(x) (as for ELAM-1), which may play an essential role in adhesion of leukocytes or tumor cells on endothelial cells, through aggregation with platelets. Redistribution of GMP-140 from alpha-granules of platelets to the cell surface, induced by thrombin and PMA, was strongly inhibited by preincubation of platelets with N,N-dimethylsphingosine (DMS) or N,N,N-trimethylsphingosine (TMS) at 10-20 microM concentration for a brief period (5 min). Inhibition of GMP-140 redistribution to the cell surface by DMS or TMS was also detected by a cell adhesion assay using HL60 cells, which highly express sialosyl-Le(x); i.e., HL60 cells adhered on platelets activated by thrombin or PMA but not on platelets which were briefly preincubated with DMS or TMS followed by activation. The inhibitory effect of DMS or TMS on GMP-140 redistribution is not due to cytotoxicity, since the TMS-treated platelets were fully capable of aggregating in the presence of ristocetin. Sphingosine (SPN) and protein kinase C inhibitors such as H-7 and calphostin C showed weaker inhibitory activity than DMS and TMS. Our results indicate that both DMS and TMS could be useful reagents to inhibit cell surface expression of crucial selectins which promote adhesion of Le(x-) or sialosyl-Le(x)-expressing cells with platelets and endothelial cells.(ABSTRACT TRUNCATED AT 250 WORDS)     

Structural and biosynthetic studies of the granule membrane protein, GMP-140, from human platelets and endothelial cells

GMP-140 is an integral membrane glycoprotein of apparent Mr = 140,000 located in secretory storage granules of platelets and vascular endothelial cells. When these cells are activated, GMP-140 redistributes from the membrane of the granules to the plasma membrane. To gain insight into the potential function of GMP-140, we examined aspects of its structure and biosynthesis. The amino acid composition of platelet GMP-140 revealed elevated numbers of cystinyl (6.1%), prolinyl (7.2%), and tryptophanyl (2.1%) residues. GMP-140 contained 28.8% carbohydrate by weight, distributed among N-acetylneuraminic acid, neutral sugar, and N-acetylglucosamine residues. Enzymatic removal of N-linked oligosaccarides reduced the protein's apparent Mr by more than 50,000. The biosynthesis of GMP-140 in HEL cells, which share biochemical features with megakaryocytes, was studied by pulse-chase labeling with [35S]cysteine followed by immunoprecipitation. HEL cells synthesized a heterogeneous GMP-140 precursor of 98-125 kDa which converted to a mature 140-kDa form within 40-60 min. Removal of high mannose oligosaccarides by endo-beta-N-acetylglucosaminidase H treatment reduced the apparent Mr of the precursor but not the mature protein. Tunicamycin-treated HEL cells synthesized three to four precursors of 80-92 kDa, suggesting the possibility of heterogeneity of GMP-140 at the protein level. Exposure of activated platelets to proteases followed by Western blotting indicated that most of the mass of GMP-140 was located on the extracytoplasmic side of the membrane. Our studies indicate that GMP-140 is a cysteine-rich, heavily glycosylated protein with a large extracytoplasmic domain. These features are compatible with a receptor function for the molecule when it is exposed on the surface of activated platelets and endothelial cells.     

GMP-140: a receptor for neutrophils and monocytes on activated platelets and endothelium.

GMP-140 is a membrane glycoprotein located in secretory granules of platelets and endothelium. When these cells are activated by agonists such as thrombin, GMP-140 is rapidly translocated to the plasma membrane. GMP-140, along with ELAM-1 and the peripheral lymph node homing receptor, defines the selectin family of structurally related molecules that regulate interactions of leukocytes with the blood vessel wall. Each of these molecules contains an N-terminal lectin-like domain, followed by an EGF-like region, a series of consensus repeats related to those in complement-binding proteins, a transmembrane domain, and a short cytoplasmic tail. The genomic structures of the selectins suggest that they arose by duplication and modification of exons encoding specific structural domains. GMP-140 is a receptor for neutrophils and monocytes when it is expressed on activated platelets and endothelium. This property facilitates rapid adhesion of leukocytes to endothelium at regions of tissue injury as well as platelet-leukocyte interactions at sites of inflammation and hemorrhage. Like other leukocyte adhesion molecules, GMP-140 may also participate in pathologic inflammation, thrombosis, and tumor metastasis. Confirmation of such pathologic roles may lead to design of new drugs that block adhesive receptor function in human disease.     

Cloning of GMP-140, a granule membrane protein of platelets and endothelium: sequence similarity to proteins involved in cell adhesion and inflammation

GMP-140 is an integral membrane glycoprotein found in secretory granules of platelets and endothelial cells. After cellular activation, it is rapidly redistributed to the plasma membrane. The cDNA-derived primary structure of GMP-140 predicts a cysteine-rich protein with multiple domains, including a "lectin" region, an "EGF" domain, nine tandem consensus repeats related to those in complement-binding proteins, a transmembrane domain, and a short cytoplasmic tail. Some cDNAs also predict a soluble protein with a deleted transmembrane segment. The domain organization of GMP-140 is similar to that of ELAM-1, a cytokine-inducible endothelial cell receptor that binds neutrophils. This similarity suggests that GMP-140 belongs to a new family of inducible receptors with related structure and function on vascular cells.     

Properties of GMP-140, an inducible granule membrane protein of platelets and endothelium.

GMP-140 is an integral membrane glycoprotein with an apparent Mr of 140,000 located in secretory granules of human platelets and endothelial cells. When these cells are stimulated, the protein is rapidly redistributed to the plasma membrane; therefore, monoclonal antibodies to GMP-140 are useful markers of activated platelets and endothelium. GMP-140 is cysteine-rich and heavily glycosylated. The cDNA-derived amino acid sequence indicates that it contains a number of modular domains that are likely to fold independently. Beginning at the N-terminus, these comprise a "lectin" domain, an "EGF" domain, nine tandem consensus repeats similar to those in complement-binding proteins, a transmembrane domain, and a cytoplasmic tail. Some cDNAs also predict variant forms of GMP-140, including a putative soluble form lacking the transmembrane domain that appears to arise from alternative splicing of mRNA. The domain organization of GMP-140 is strikingly similar to two other vascular cell surface structures: ELAM-1, a cytokine-inducible endothelial cell receptor that binds neutrophils, and a lymphocyte-homing receptor that mediates the adherence of lymphocytes to high endothelial venules of peripheral lymph nodes. These "selectins" constitute a new gene family of receptors with related structure and potentially related function.     

Co-localization of CD9 and GPIIb-IIIa (·IIb ·3 integrin) on activated platelet pseudopods and ·-granule membranes

Summary CD9 is a 24-kDa membrane glycoprotein expressed on the surface of human platelets and potentially involved in cellular activation and adhesion functions. This protein belongs to a recently delineated family of cell-surface antigens that span the membrane four times, called tetraspans, and found mainly in leucocytes and tumour cells. As a first approach to clarify the function of CD9, we used immunoelectron microscopy to determine the localization of this antigen in human platelets, and compared its distribution with that of the GPIIb-IIIa integrin, the platelet receptor for fibrinogen. Monoclonal antibodies against CD9 (MAb7) and GPIIb-IIIa (HP1-1D) coupled to colloidal gold of different sizes (5 and 15 nm) were incubated with intact platelets in suspension or on ultrathin sections of platelets embedded in LR white. CD9 was found in association with GPIIb-IIIa on the inner face of ·-granule membranes. These two antigens also co-localized on pseudopods of activated platelets and in contact regions between adjacent platelets. CD63, another member of the tetraspan family, was absent from ·-granules but was associated with lysosomal structures. Flow cytometric analysis of platelet CD9 with a series of monoclonal antibodies revealed an increased expression upon thrombin stimulation, confirming the presence of an intracellular granular pool. The observation that CD9 and GPIIb-IIIa are stored in the same intracellular structures and migrate to the same activation zones after platelet stimulation lends support to previous suggestions of a close association between CD9 and GPIIb-IIIa in human platelets and of a possible involvement of CD9 in adhesive functions of platelets. This revised version was published online in November 2006 with corrections to the Cover Date.     

GMP-140 binding to neutrophils is inhibited by sulfated glycans.

GMP-140 is a 140-kDa granule membrane glycoprotein localized to the alpha-granules of platelets and the Weibel-Palade bodies of endothelial cells. Expression of GMP-140 on the activated cell surface has been shown to mediate the adhesion of thrombin-activated platelets to neutrophils and monocytes and the transient adhesion of neutrophils to endothelium. In contrast, fluid-phase GMP-140 strongly inhibits the CD18-dependent adhesion of tumor necrosis factor alpha-activated neutrophils to endothelium suggesting that GMP-140 can also serve an anti-adhesive function. In the present report, it is demonstrated that fluid-phase GMP-140 which exists predominantly as a tetramer binds to a single class of high affinity receptor on neutrophils and HL60 cells. Binding of 125I-labeled GMP-140 to neutrophils and HL60 cells and the rosetting of neutrophils and HL60 cells by thrombin-activated platelets were inhibited by EDTA, excess unlabeled fluid-phase GMP-140, Fab fragments of an affinity-purified rabbit anti-GMP-140 antibody, and by the murine anti-GMP-140 monoclonal antibody, AK 4. Both neutrophil and HL60 GMP-140 binding and platelet rosetting were strongly inhibited by heparin, fucoidin, and dextran sulfate 500,000, were partially inhibited by dextran sulfate 5,000 and lambda- and kappa-carrageenan, but were not inhibited by chondroitins 4- and 6-sulfate. Since this sulfated glycan specificity is identical to that previously reported by us for GMP-140, the present results suggest that the sulfated glycan binding site and the neutrophil receptor binding site on GMP-140 are either identical or proximal.     

Ca2+-mediated association of glycoprotein G (thrombinsensitive protein, thrombospondin) with human platelets

Washed human platelets suspended in buffers containing either 1.8 mM Ca2+ and 0.49 mM Mg2+ or 1 mM EDTA were treated with human alpha-thrombin to induce secretion. Glycoprotein G, a major glycoprotein in alpha-granules, was quantitatively secreted from platelets activated in the EDTA-containing buffer but remained with the platelet in the presence of Ca2+ and Mg2+. Addition of Ca2+ to the platelets that were activated in the presence of EDTA caused glycoprotein G to bind to platelets. To determine if glycoprotein G is expressed on the membrane surface of the activated platelet, platelets were rapidly labeled by a method employing lactoperoxidase-catalyzed iodination. Although glycoprotein G was barely detected on the surface of unstimulated platelets, labveling 1 min after thrombin treatment showed that glycoprotein G rapidly became one of the prominent surface proteins. These findings show that an alpha-granule protein, glycoprotein G, is one of the major glycoproteins on the membrane surface of thrombin-activated platelets and that its binding is dependent on divalent cations.     

Co-localization of CD9 and GPIIb-IIIa (·IIb ·3 integrin) on activated platelet pseudopods and ·-granule membranes

Summary  CD9 is a 24-kDa membrane glycoprotein expressed on the surface of human platelets and potentially involved in cellular activation and adhesion functions. This protein belongs to a recently delineated family of cell-surface antigens that span the membrane four times, called tetraspans, and found mainly in leucocytes and tumour cells. As a first approach to clarify the function of CD9, we used immunoelectron microscopy to determine the localization of this antigen in human platelets, and compared its distribution with that of the GPIIb-IIIa integrin, the platelet receptor for fibrinogen. Monoclonal antibodies against CD9 (MAb7) and GPIIb-IIIa (HP1-1D) coupled to colloidal gold of different sizes (5 and 15 nm) were incubated with intact platelets in suspension or on ultrathin sections of platelets embedded in LR white. CD9 was found in association with GPIIb-IIIa on the inner face of ·-granule membranes. These two antigens also co-localized on pseudopods of activated platelets and in contact regions between adjacent platelets. CD63, another member of the tetraspan family, was absent from ·-granules but was associated with lysosomal structures. Flow cytometric analysis of platelet CD9 with a series of monoclonal antibodies revealed an increased expression upon thrombin stimulation, confirming the presence of an intracellular granular pool. The observation that CD9 and GPIIb-IIIa are stored in the same intracellular structures and migrate to the same activation zones after platelet stimulation lends support to previous suggestions of a close association between CD9 and GPIIb-IIIa in human platelets and of a possible involvement of CD9 in adhesive functions of platelets. This revised version was published online in November 2006 with corrections to the Cover Date.     

Stimulated secretion of endothelial von Willebrand factor is accompanied by rapid redistribution to the cell surface of the intracellular granule membrane protein GMP-140

We have examined the cell activation-dependent redistribution of the intracellular granule membrane protein GMP-140 of human endothelial cells. By dual-label immunofluorescence, the distribution of GMP-140 within cultured human umbilical vein endothelial cells was found to coincide with the distribution of von Willebrand factor (vWF), suggesting that GMP-140 is located in the membranes of vWF-containing storage granules. Stimulation of vWF secretion resulted in an increase in GMP-140 on the cell surface, as detected by increased binding of the monoclonal antibody S12 which recognizes the extracytoplasmic domain of GMP-140. For each agonist tested (histamine, thrombin, phorbol 12-myristate 13-acetate, and the calcium ionophore A23187) a dose-dependent redistribution of GMP-140 to the endothelial surface was observed which closely paralleled the dose-dependent secretion of vWF into the cell supernatant. When cells were maximally stimulated by histamine in the presence of antibody S12, a 4-fold increase in S12 uptake by the cells was observed. This increase occurred rapidly and reached a plateau by 10 min. In contrast, when histamine-stimulated cells were first fixed with paraformaldehyde or chilled to 4 degrees C before addition of antibody S12, only a transient increase in cell surface GMP-140 was detected. Under these conditions of arrested membrane turnover during antibody binding, cell surface GMP-140 was maximal 3 min after histamine stimulation and then declined to control levels by 20 min. These data suggest that stimulated secretion of vWF from endothelial cells entails fusion of vWF-containing storage granules with the plasma membrane. Once inserted into the plasma membrane, GMP-140 is subsequently removed from the endothelial surface, most likely by an endocytic mechanism.     

Alterations of wheat-germ agglutinin binding pattern on cell surface of blood platelets after thrombin stimulation

An attempt was made to demonstrate wheat-germ agglutinin (WGA) binding sites on platelet surfaces after thrombin stimulation, by means of a post-embedding cytochemical technique using colloidal gold as marker at an ultrastructural level. In unstimulated platelets washed with EDTA, an intense uniform labeling of WGA-gold complexes was found on the surface membrane. When washed platelets were stimulated by thrombin in the absence of Ca2+, only a release reaction was induced. WGA labeling on the surface membranes of these platelets decreased dramatically. However, the labeling intensity of WGA-gold complexes on the surface membrane of aggregated platelets induced by thrombin in the presence of Ca2+ increased significantly compared to that of thrombin-stimulated platelets in the absence of Ca2+. In contrast to the uniform labeling on the surface membranes of unstimulated platelets, clusters of gold label were often found on the surface membrane of the aggregated platelets, although there was no significant quantitative difference in the labeling intensity between these two groups. Thus, we present direct morphological evidence demonstrating qualitative and quantitative alterations of WGA labeling on the surface membrane of platelets after thrombin stimulation. The possibility is considered that WGA-binding glycoproteins in the surface membrane are involved in the aggregation response after thrombin stimulation.     

Electrophysiological evidence that glycoprotein IIb-IIIa complex is involved in calcium channel activation on human platelet plasma membrane.

The involvement of platelet glycoprotein (GP) IIb-IIIa complex in calcium channel activity on the plasma membrane was investigated using an electrophysiological approach. Plasma membrane vesicles were prepared from thrombin-stimulated platelets and incorporated into planar lipid bilayers. Voltage-independent Ca2+ channel currents with a conductance of about 10 pS (in 53 mM Ba2+) were observed, in membranes derived from thrombin-stimulated, but not unstimulated platelet membranes. These channel activities were markedly reduced by exposure of membranes to EGTA at 37 degrees C. This reduction was specifically related to the dissociation of the GPIIb-IIIa complex since preincubation of the membranes with a monoclonal antibody to the GPIIb-IIIa complex (AP-2) could protect the channel activities from the effect of EGTA. Thrombasthenic platelets, which lack the GPIIb-IIIa complex, showed impaired channel activities characterized by decreased open probability and lowered conductance states. Furthermore, when platelets were stimulated by thrombin in the presence of EGTA, AP2, or the synthetic peptide RGDS, to prevent fibrinogen binding to the GPIIb-IIIa complex, open probabilities of the channel currents in these membrane vesicles were also decreased. These results suggest that the GPIIb-IIIa complex is involved in platelet Ca2+ channel activation and that ligand binding to the complex during platelet activation may modify the activation of Ca2+ channels.     

Alterations of wheat-germ agglutinin binding pattern on cell surface of blood platelets after thrombin stimulation

Summary An attempt was made to demonstrate wheatgerm agglutinin (WGA) binding sites on platelet surfaces after thrombin stimulation, by means of a post-embedding cytochemical technique using colloidal gold as marker at an ultrastructural level. In unstimulated platelets washed with EDTA, an intense uniform labeling of WGA-gold complexes was found on the surface membrane. When washed platelets were stimulated by thrombin in the absence of Ca²⁺, only a release reaction was induced. WGA labeling on the surface membranes of these platelets decreased dramatically. However, the labeling intensity of WGA-gold complexes on the surface membrane of aggregated platelets induced by thrombin in the presence of Ca²⁺ increased significantly compared to that of thrombin-stimulated platelets in the absence of Ca²⁺. In contrast to the uniform labeling on the surface membranes of unstimulated platelets, clusters of gold label were often found on the surface membrane of the aggregated platelets, although there was no significant quantitative difference in the labeling intensity between these two groups. Thus, we present direct morphological evidence demonstrating qualitative and quantitative alterations of WGA labeling on the surface membrane of platelets after thrombin stimulation. The possibility is considered that WGA-binding glycoproteins in the surface membrane are involved in the aggregation response after thrombin stimulation.     

Changes in platelet membrane glycoproteins during blood bank storage.

The literature on membrane glycoprotein changes occurring during platelet storage is reviewed. Technical problems which may have biased reports are clarified, and the consensus of current studies is established. The membrane alterations that occur during blood bank storage are reminiscent of the surface changes which result from platelet secretion: an increased surface concentration of GP IIb-IIIa and GMP-140, and the appearance of platelet membrane microparticles in the supernatant plasma. The clinical importance of these changes is unknown. Future directions for research on the structure and function of stored platelets are discussed.     

Morphological evidence for the association of plasma membrane glycoprotein IIb/IIIa with the membrane skeleton in human platelets

We examined the association between glycoprotein (GP) IIb/IIIa, a receptor for fibrinogen, and membrane skeletons in both unstimulated and thrombin-activated human platelets. After a treatment with dithiobis succinimidyl propionate (DTSP), a cross-linker, unstimulated and activated platelets were simultaneously extracted and fixed with a fixing solution containing Triton X-100. Also, the localization of GPIIb/IIIa on the plasma membrane was observed by a preembedding staining method of unextracted platelets. In unstimulated platelets, 20-40% of the whole plasma membrane remained in the detergent-extracted samples. Amorphous structures with 10-70 nm in diameters are distributed at 20 to 100-nm intervals on the surface of plasma membrane. Similar structures also were identified in the intact platelets by the immunocytochemical method. By careful inspection, we found that most of the amorphous structures that contained gold particles were connected to the submembrane zone just beneath the plasma membrane. The submembrane zone was identified as the membrane skeleton because actin was detected in the zone. After activation, detergent-insoluble granules were surrounded by dense networks of microfilaments in the central part of platelets. The filaments were identified as actin and became associated with myosin. These results demonstrate that GPIIb/IIIa on the plasma membrane is connected to the membrane skeleton and suggest that, during activation, actin filaments which extend into the cytoplasm from the membrane skeleton increase and form dense networks around Triton-insoluble granules.     

Selectin GMP-140 (CD62; PADGEM) binds to sialosyl-Le(a) and sialosyl-Le(x), and sulfated glycans modulate this binding.

GMP-140 (CD62; PADGEM) is a member of the selectin family expressed highly at the surface of platelets and endothelial cells by agonists such as thrombin or phorbol esters. Previous studies indicate that the lectin domain of GMP-140 recognizes sialosyl-Le(x) (SLex) and to a lesser extent Le(x) (Polley MJ, et al., Proc Natl Acad Sci USA 88:6224, 1991). We now report that GMP-140 binds to sialosyl Lea (SLea) and to SLex, and that degree of binding to SLea is greater than that to SLex under our experimental conditions. Binding of activated platelets to SLea or SLex was inhibited to various degrees in the presence of sulfated glycans, suggesting that sulfated glycans induce conformational change in the lectin domain of GMP-140 and modulates its binding affinity to SLea and SLex.     

Evidence for the selective association of a subpopulation of GPIIb-IIIa with the actin cytoskeletons of thrombin-activated platelets

Activation of blood platelets triggers a series of responses leading to the formation and retraction of blood clots. Among these responses is the establishment of integrin-mediated transmembrane connections between extracellular matrix components and the actin cytoskeleton of the platelet. Here we report that a specific subpopulation of the major platelet integrin, glycoprotein IIb-IIIa (GPIIb-IIIa) (also referred to as alpha IIb beta 3 integrin), becomes incorporated into the detergent- insoluble actin cytoskeleton of platelets during the platelet activation response. The cytoskeletal association of GPIIb-IIIa is independent of platelet aggregation and fibrin sedimentation and is sensitive to cytochalasin D treatment. As determined by Western immunoblot analysis, approximately 22% of the total cellular GPIIb-IIIa becomes associated with the actin cytoskeleton upon thrombin activation in a manner that is independent of the detection of talin, alpha- actinin, or vinculin in the complex. We found that the cytoskeleton- associated GPIIb-IIIa is derived from an intracellular source since it is not available for lactoperoxidase-catalyzed radioiodination before platelet activation. Two intracellular sources of GPIIb-IIIa are present in resting platelets: GPIIb-IIIa associated with the alpha- granule secretory compartment as well as surface-inaccessible domains of the surface-connected canalicular system. Interestingly, alpha- granule secretion, which occurs in thrombin-activated platelets and results in the translocation of intracellular GPIIb-IIIa to the plasma membrane, appears to be required for the cytoskeleton incorporation of GPIIb-IIIa that we observe. Collectively, our data provide evidence that a subpopulation of GPIIb-IIIa derived from an intracellular source is selectively linked to the actin cytoskeleton of platelets upon thrombin activation in the absence of platelet aggregation.