Effects of thrombin on washed, human platelets: changes in the subcellular fractions

The pathobiology of platelet and megakaryocyte extracellular vesicles: A (c)lot has changed

Platelet-derived extracellular vesicles (PEVs) were originally studied for their potential as regulators of coagulation, a function redundant with that of their parent cells. However, as the understanding of the diverse roles of platelets in hemostasis and disease has developed, so has the understanding of PEVs. In addition, the more recent revelation of constitutively released megakaryocyte-derived extracellular vesicles (MKEVs) in circulation provides an interesting counterpoint and avenue for investigation. In this review, we highlight the historical link of PEVs to thrombosis and hemostasis and provide critical updates. We also expand our discussion to encompass the roles that distinguish PEVs and MKEVs from their parent cells. Furthermore, the role of extracellular vesicles in disease pathology, both as biomarkers and as exacerbators, has been of great interest in recent years. We highlight some of the key roles that PEVs and MKEVs play in autoimmune blood cell disorders, liver pathology, and cardiovascular disease. We then look at the future of PEVs and MKEVs as candidates for novel therapeutics.

Molecular and functional differences induced in thrombospondin-1 by the single nucleotide polymorphism associated with the risk of premature, familial myocardial infarction

A serine (Ser-700) amino acid rather than an asparagine (Asn-700) at residue 700 of thrombospondin-1 has been linked to an increased risk for development of premature, familial heart attacks. We now have identified both functional and structural differences between the Ser-700 and Asn-700 thrombospondin-1 variants. The Ser-700 variant increased the rate and extent of platelet aggregation and showed increased surface expression on platelets compared with the Asn-700 variant. These differences could be ascribed to an enhanced interaction of the Ser-700 variant with fibrinogen on the platelet surface and are consistent with a prothrombotic phenotype in Ser-700 individuals. The Ser-700 variant thrombospondin-1 was conformationally more labile than the Asn-700 variant as demonstrated by increased susceptibility to proteolytic digestion and enhanced susceptibility to unfolding by denaturants. These data suggest a potential molecular and cellular basis for a genetic risk factor associated with early onset myocardial infarction.

Thrombospondin as an agglutinin of platelets

To study a hypothesis that thrombospondin (TSP) might function as an agglutinin in platelet aggregation, we designed two experiments. First, we prepared fibrinogen-coated agarose beads (fbg-beads) as a model of platelets, and subjected them to aggregometry using TSP as an inducer. TSP induced agglutination of fbg-beads in a dose-dependent manner. Calcium (Ca) and Magnesium (Mg) were necessary for the agglutination, and the aggregability was dependent on the concentration of Ca. These results confirmed the function of TSP as an agglutinin, suggesting some characteristics of the fbg-TSP interaction as well. Secondly, a variety of platelets were subjected to TSP-induced aggregation assay. Both gel-filtrated and washed-platelets were aggregated by TSP in a dose dependent manner and dissociated with EDTA. The same aggregation was observed in formalin-fixed platelets. Both Ca and Mg were required for the aggregation, and the maximum aggregation rate was dependent on the Ca concentration. Ca seemed to regulate the capacity as well as the affinity of the binding sites for TSP on platelets. Fibrinogen and some aminosugars inhibited the aggregation. These data suggest TSP may function as an agglutinin of platelets, and Ca may regulate the interaction between platelets and TSP. As one of the candidates for the receptor for TSP on platelet, fbg-GPIIb/IIIa was suggested because of the similarity between fbg-beads and platelets aggregation induced by TSP, and the Ca-dependency in both the GPIIb/IIIa induction and the TSP-induced platelet aggregation.

Isolation of the thrombospondin membrane receptor

Thrombospondin (TSP), a 450-kD multifunctional glycoprotein with a broad tissue distribution, is secreted upon platelet stimulation, binds to the activated platelet surface, and supports platelet aggregation. We have identified and isolated an 88-kd membrane glycoprotein present in platelets, endothelial cells, monocytes, and a variety of human tumor cell lines that is the membrane binding site for TSP. Endogenous platelet TSP binding to thrombin- and ionophore-stimulated human platelets was inhibited in the presence of the monoclonal antibody OKM5. TSP binding to C32 melanoma cells and HT1080 fibrosarcoma cells was specific and also inhibitable with OKM5 Mab. Cell labeling followed by specific immunoprecipitation demonstrated biosynthesis of a single 88-kD glycoprotein. Binding of TSP to the isolated membrane protein was specific and saturable. These studies identify an 88-kD membrane glycoprotein that reacts with the monoclonal antibody, OKM5, and may function as the cellular TSP receptor.

Thrombospondin binds to monocytes-macrophages and mediates platelet-monocyte adhesion

Thrombospondin (TSP) is a multifunctional platelet glycoprotein synthesized by a variety of cells in culture including monocytes and macrophages. We now report that 125I-TSP binds specifically, saturably, and reversibly to mouse peritoneal macrophages and to cells of the monocyte-like human cell line U937 with dissociation constants of 6.7-14.5 X 10(-8) M and 3-4 X 10(5) binding sites per cell. TSP mediates an adhesive interaction between thrombin-stimulated platelets and both U937 cells and human blood monocytes. Using a sensitive rosetting assay, we found that monocytes were not rosetted by resting platelets whereas greater than 90% were rosetted by thrombin-stimulated platelets. Monoclonal and polyclonal anti-TSP antibodies markedly inhibited rosetting as did TSP itself. Neither control antibodies nor heparin, fibronectin, fibrinogen, nor the fibronectin adhesion tetrapeptide Arg-Gly-Asp-Ser inhibited rosetting. TSP may thus serve as a molecular bridge linking activated platelets with monocytes at sites of early vascular injury. Such interaction may be of critical importance in the regulation of thrombosis and the initiation of atherosclerosis.

Human brain glial cells synthesize thrombospondin

Thrombospondin, a 450-kDa multinodular glycoprotein with lectin-type activity, is found in human platelets, endothelial cells, fibroblasts, smooth muscle cells, monocytes, and granular pneumocytes. Thrombospondin interacts with heparin, fibrinogen, fibronectin, collagen, histidine-rich glycoprotein, and plasminogen. Recently, thrombospondin synthesis by smooth muscle cells has been reported to be augmented by platelet-derived growth factor. We present evidence that thrombospondin is present within and synthesized by astrocytic neuroglial cells. Heparin-Sepharose affinity chromatography of material derived from a human brain homogenate yielded a protein that, when reduced, had an apparent size of 180 kDa and comigrated with reduced platelet thrombospondin on NaDodSO4/PAGE. Immunoblot analysis with monospecific anti-thrombospondin confirmed the presence of immunoreactive thrombospondin. Indirect immunofluorescence of cultured human glial cells indicated the presence of thrombospondin. Metabolic labeling of glial cell cultures with [35S]methionine followed by immunoprecipitation with monospecific anti-thrombospondin revealed synthesis of a 180-kDa polypeptide that comigrated with platelet thrombospondin on NaDodSO4/PAGE. Cultured human glial cells were incubated for 48 hr in serum-free medium with purified platelet-derived growth factor at concentrations up to 50 ng/ml. Aliquots taken at intervals were analyzed by a quantitative double-antibody ELISA. The growth factor stimulated the release of thrombospondin into the culture medium by as much as 10-fold over control cultures. The presence of thrombospondin within glial cells of the central nervous system and the augmentation of its synthesis by platelet-derived growth factor suggest that thrombospondin may play an important role in regulating cell-cell and cell-matrix interactions during periods of cell division and growth.

Platelet thrombospondin forms a trimolecular complex with plasminogen and histidine-rich glycoprotein

Thrombospondin (TSP), a multifunctional alpha-granule glycoprotein of human platelets binds fibrinogen, fibronectin, heparin, histidine-rich glycoprotein (HRGP), and plasminogen (Plg), and thus, may play an important role in regulating thrombotic influences at vessel surfaces. In this study we have demonstrated that purified human platelet TSP formed a trimolecular complex with human Plg and HRGP. Complex formation was detected by a specific binding enzyme-linked immunosorbent assay (ELISA) which demonstrated simultaneous binding of fluid-phase Plg and HRGP to TSP adsorbed to microtitration wells. While neither ligand inhibited complex formation of the other with TSP, 10 mM epsilon-amino-n-caproic acid selectively blocked incorporation of Plg into the complex, suggesting that TSP contains independent binding sites for Plg and HRGP. Comparable extent of trimolecular complex formation was also detected when TSP monomer was substituted for whole TSP in the ELISA. HRGP covalently cross-linked to Sepharose 4B simultaneously bound both 125I-TSP and 131I-Plg, confirming trimolecular complex formation. Rocket immunoelectrophoresis of mixtures of the purified radiolabeled proteins into anti-Plg containing agarose also confirmed trimolecular complex formation. The TSP-HRGP-Plg complex bound a similar amount of heparin as the TSP-HRGP complex, demonstrating that the HRGP within the trimolecular complex maintained functional capability. Similarly, using a fluorometric plasmin substrate, the trimolecular complex was shown to be an effective substrate for tissue plasminogen activator. Significant amounts of plasmin were generated from the TSP-HRGP-Plg complex (equivalent to that from the TSP-Plg complex), but the rate of plasmin generation from the trimolecular complex was greater than from the bimolecular complex, suggesting an important interaction of HRGP with Plg when both are complexed to TSP. The macromolecular assembly of these three proteins on cellular surfaces, such as the platelet, may serve important regulatory functions, both prothrombotic at sites of active fibrin deposition and proteolytic in nonfibrin-containing microenvironments.

Comparison of secretion and subcellular localization of von Willebrand protein with that of thrombospondin and fibronectin in cultured human vascular endothelial cells

Cultured human vascular endothelial cells synthesize von Willebrand protein, thrombospondin and fibronectin. These proteins are secreted in the culture medium and incorporated into the extracellular matrix. We have compared the subcellular localization and the secretion of these proteins in response to stimulants in cultured human umbilical vein endothelial cells. Density gradient centrifugation using colloidal silica showed that the storage and secretion organelle with von Willebrand protein did not contain thrombospondin or fibronectin. Indirect immunofluorescence microscopy indicated that thrombospondin and fibronectin are not located in the rod-shaped organelles containing von Willebrand protein. Thrombin, ionophore A23187 and phorbol myristate acetate did not affect secretion of thrombospondin and fibronectin, while von Willebrand protein secretion was stimulated upon incubation of cells with these agents for 30 min. Prolonged incubation of cultured endothelial cells after a 1-h treatment with phorbol myristate acetate resulted in an increased secretion of von Willebrand protein into the conditioned medium; in contrast, accumulation of thrombospondin and fibronectin in endothelial cell-conditioned medium was decreased. These findings indicate that, unlike in platelets, these major endothelial proteins are not located in the same subcellular compartments. Von Willebrand protein is distinguished from thrombospondin and fibronectin both by its unique subcellular localization and its secretion rate in response to stimuli.

Complex formation of platelet thrombospondin with plasminogen. Modulation of activation by tissue activator

Thrombospondin (TSP), a multifunctional alpha-granule glycoprotein of platelets, binds fibrinogen, fibronectin, heparin, and histidine-rich glycoprotein and thus may play an important role in regulating thrombotic influences at vessel surfaces. In this study we have demonstrated that purified human platelet TSP formed a complex with purified human plasminogen (Plg). Complex formation was detected by rocket immunoelectrophoresis of mixtures of the purified radiolabeled proteins. Significant complex formation of fluid-phase Plg with adsorbed TSP was also demonstrated by enzyme-linked immunosorbent assay (ELISA). The complex formation was specific, saturable, and inhibited by excess fluid-phase TSP, with an apparent KD of approximately 35 nM. In both ELISA and rocket immunoelectrophoresis systems, complex formation was inhibited by 10 mM epsilon-amino-n-caproic acid, implying that there is a role for the lysine binding sites of Plg in mediating the interaction. TSP also formed a complex with plasmin as detected by ELISA but did not directly inhibit plasmin activity measured with a synthetic fluorometric substrate or with a 125I-fibrin plate assay. TSP, when incubated with Plg before addition to 125I-fibrin plates significantly inhibited the generation of plasmin activity by tissue plasminogen activator (TPA) in a manner that was calcium dependent. A kinetic study of Plg activation by TPA in the presence of TSP demonstrated that Michaelis-Menten kinetics were followed and that TSP acted as a noncompetitive inhibitor. These studies support the hypothesis that TSP, acting as a multifunctional regulator in focal areas of active hemostasis, could serve as a prothrombotic influence, leading to increased deposition of fibrin.

Role of thrombospondin in platelet aggregation

Thrombospondin (TSP), the major alpha-granule protein of human platelets, binds to the activated platelet surface upon platelet stimulation. TSP has hemagglutinating (lectin-like) activity and forms a specific complex with fibrinogen. Based on these observations, it was postulated that the interaction of TSP and fibrinogen on the activated platelet surface may be an important step in the platelet aggregation process. To test this hypothesis, monospecific, affinity-purified anti-TSP Fab fragments were prepared and their effects on platelet aggregation and platelet fibrinogen binding were studied. Anti-TSP Fab caused significant interference with thrombin- and collagen-induced platelet aggregation, as monitored by both turbidometric aggregometry and particle counting measuring the disappearance of single platelets. Phase-contrast microscopy revealed that anti-TSP Fab caused a marked decrease in platelet macroaggregates and an increase in microaggregates and nonaggregated single platelets. Anti-TSP Fab did not affect the initial phase of ADP-induced platelet aggregation but caused rapid platelet disaggregation with the abolition of the secondary phase of aggregation. The effect of anti-TSP Fab was not mediated by a direct inhibition of platelet secretion. The effect of anti-TSP Fab on specific binding of labeled fibrinogen to thrombin-stimulated platelets was also studied. Anti-TSP Fab caused a marked decrease in the affinity of fibrinogen binding to the receptors on the activated platelet surface. Kinetic analyses revealed significant displacement of labeled fibrinogen by unlabeled fibrinogen in the presence of anti-TSP Fab, suggesting that TSP serves to stabilize fibrinogen binding to the activated platelet surface and reinforces the strength of interplatelet interactions. It is proposed that platelet aggregation is a dynamic, multistep process, governed initially by the platelet membrane glycoprotein IIb/IIIa-fibrinogen interaction, with the TSP-fibrinogen interaction playing an important role in determining the size and reversibility of platelet aggregates.

Antibodies against a 23Kd heparin binding fragment of thrombospondin inhibit platelet aggregation

Antiserum against a 23Kd heparin binding fragment of thrombospondin inhibits the aggregation of platelets in response to ADP, collagen or thrombin. The antiserum inhibits the secretion-dependent second phase, but not the primary phase of aggregation of platelets responding to ADP. Although immune serum added during the second phase of ADP-induced aggregation causes some inhibition of secretion, it also causes reversal of aggregation to the level produced during primary aggregation. Since thrombospondin is the endogenous lectin of human platelets, these results support the conclusion that the endogenous lectin mediates, at least in part, the secretion-dependent aggregation of platelets. Our data suggest that the region of thrombospondin which contains the heparin binding domain(s) present in the 23Kd fragment play(s) a critical role in secretion-dependent aggregation of platelets.

Interactions of thrombospondin with extracellular matrix proteins: selective binding to type V collagen

Thrombospondin (TS), a protein first described in platelets, was recently shown to be synthesized and secreted by endothelial cells, fibroblasts, and smooth muscle cells. The presence of TS in the extracellular matrix of cultured cells has prompted us to examine the associations of this protein with matrix macromolecules. Interactions of TS with both matrix and serum proteins were tested using an enzyme-linked immunosorbent assay. With this assay we assessed the binding of TS in solution to proteins adsorbed to polystyrene microtiter plates. Among collagens, platelet TS bound to type V but not to types I, III, or IV. This selective interaction was confirmed in experiments using proteins linked to cyanogen bromide-activated Sepharose. TS released from platelets in response to thrombin activation, as well as that secreted by endothelial cells in culture, bound to type V but not to type I collagen-Sepharose. No binding was observed to denatured type V collagen-Sepharose. The binding region for type V collagen was located in a chymotrypsin-produced fragment of TS with chains of Mr = 70,000, after reduction. Interactions of TS with a number of other proteins, including fibronectin, fibrinogen, and laminin, could be demonstrated using the enzyme-linked immunosorbent assay technique but the interpretation of these findings is difficult since comparable binding to protein-Sepharose was not always observed. Our findings suggest that both the extravascular distribution and function of TS in vivo may involve an interaction with type V collagen.

Quantitative adsorption of platelet glycoprotein G (thrombin-sensitive protein, thrombospondin) to barium citrate

Glycoprotein G (thrombin-sensitive protein, thrombospondin) is a high-Mr calcium-sensitive protein secreted by activated platelets. We observed that this protein was precipitated with barium citrate, and this property was used to purify glycoprotein G. The simple and rapid purification procedure consisted of barium citrate adsorption followed by heparin-agarose affinity chromatography. Unlike other calcium-sensitive proteins that are precipitated by barium citrate, glycoprotein G does not contain gamma-carboxyglutamic acid. The ability of glycoprotein G to bind to both heparin and barium citrate is consistent with this protein possessing clusters of positive and negative charges.

Complex formation of platelet thrombospondin with histidine-rich glycoprotein

Thrombospondin and histidine-rich glycoprotein are two proteins with diverse biological activities which have been associated with human platelets and other cell systems. Using an enzyme-linked immunosorbent assay, we have demonstrated that purified human platelet thrombospondin formed a complex with purified human plasma histidine-rich glycoprotein. The formation of the thrombospondin-histidine-rich glycoprotein complex was specific, concentration dependent, and saturable. Significant binding was detected when histidine-rich glycoprotein was incubated with thrombospondin immobilized on anti-thrombospondin IgG-coated plates, indicating that the observed complex formation was not due to a thrombospondin interaction with the plastic surface. Sucrose-density-gradient ultracentrifugation of a mixture of thrombospondin and histidine-rich glycoprotein also revealed the formation of fluid-phase complexes, with an estimated stoichiometry of 1 thrombospondin: 3.5 histidine-rich glycoprotein. Fibrinogen, which has been previously shown to bind to absorbed thrombospondin, did not inhibit the formation of the thrombospondin-histidine-rich glycoprotein complex. Histidine-rich glycoprotein complexed with thrombospondin was capable of binding heparin and neutralizing the anticoagulant activity of heparin in plasma. Specific complex formation between thrombospondin and histidine-rich glycoprotein may play a significant role in influencing platelet blood vessel wall interactions as well as modulating the association of various cells with the extracellular matrix.

Secreted platelet thrombospondin binds monovalently to platelets and erythrocytes in the absence of free Ca2+

Washed human platelets suspended in Ca2+-free buffer bind thrombospondin secreted in response to stimulation by the calcium ionophore A23187. Under these conditions, the secreted thrombospondin binds to the surface of the platelets monovalently, that is, the thrombospondin does not agglutinate the platelets. In addition, the secreted thrombospondin can bind monovalently to the surface of the erythrocytes used to assay the endogenous lectin of human platelets. These findings resolve the contradictions resulting from the apparent requirement for free Ca2+ in the binding of secreted thrombospondin to the plasma membranes of platelets, the behavior of purified thrombospondin in hemagglutination assays and the characteristics of the endogenous lectin (thrombospondin) expressed by platelets stimulated with A23187 or gamma-thrombin.

Cultured human fibroblasts synthesize and secrete thrombospondin and incorporate it into extracellular matrix

Thrombospondin, a major glycoprotein released from alpha granules of thrombin-stimulated platelets, is a disulfide-bonded trimer of 160-kilodalton subunits. Cultured human foreskin and fetal lung fibroblasts secreted thrombospondin (determined by enzyme-linked immunosorbent assay) into the culture medium in a time-dependent manner (15.7 and 5.8 micrograms per 10(6) cells per 24 hr, respectively); secretion was blocked by cycloheximide. [3H]Thrombospondin was isolated from [3H]leucine-labeled fibroblast postculture medium and from cell layers with rabbit polyclonal or mouse monoclonal anti-thrombospondin coupled to staphylococcal protein A-Sepharose. The immunologically isolated [3H]thrombospondin migrated in NaDodSO4/polyacrylamide gels with purified marker platelet thrombospondin both with and without reduction. Immunofluorescence microscopy using rabbit polyclonal and mouse monoclonal anti-thrombospondin antibodies localized thrombospondin to the fibrillar extracellular matrix surrounding the cells. Thus, cultured human fibroblasts secrete thrombospondin and incorporate it into the extracellular matrix.

TFP inhibits the expression of the endogenous platelet lectin and secretion of alpha-granules

Activated platelets that have not undergone secretion do not express the endogenous platelet lectin whereas activated platelets that have undergone secretion do express this lectin. Expression of the lectin, secretion of beta-TG and PF-4, and secondary aggregation are inhibited by TFP. These results demonstrate that expression of the endogenous platelet lectin is apparently secretion dependent. Our results also reveal that inhibition of platelet aggregation by TFP apparently is not caused by inhibition of secretion and that there are TFP sensitive and TFP insensitive mechanisms of secretion.

Complex formation of platelet thrombospondin with fibrinogen

Using an enzyme-linked immunosorbent assay, we have demonstrated that purified human fibrinogen forms a complex with adsorbed platelet thrombospondin. The formation of the fibrinogen-thrombospondin complex was specific, saturable, and partially inhibited by mannosamine, glucosamine, and arginine. These same inhibitors have been previously shown to block thrombin-induced platelet lectin activity and platelet thrombospondin lectin activity. Adsorbed thrombospondin also formed a complex with fibronectin, although the extent of complex formation was significantly less than the extent of formation of the fibrinogen-thrombospondin complex. Platelet membrane glycoproteins IIb and IIIa, which have been previously shown to bind fibrinogen, did not inhibit the formation of the fibrinogen-thrombospondin complex. The present study supports the hypothesis that the interaction of fibrinogen with thrombospondin on the activated platelet surface may be an important step in the platelet aggregation process.

Synthesis and secretion of thrombospondin by cultured human endothelial cells

Thrombospondin, the major glycoprotein released from alpha-granules of thrombin-stimulated platelets, is a disulfide-bonded trimer of 160 kilodalton subunits and apparently functions as a platelet lectin. Because cultured human umbilical vein endothelial cells synthesize and secrete a glycoprotein (GP-160) which is a disulfide-bonded multimer of 160 kdalton subunits, the possibility that GP-160 is thrombospondin was investigated. Tritiated GP-160 could be immunoisolated from [3H]leucine-labeled endothelial cell postculture medium using a rabbit antiserum to human platelet thrombospondin. Thrombospondin and GP-160 comigrated in two different two-dimensional electrophoretic systems. Both proteins are disulfide-bonded trimers of acidic 160-kdalton subunits. A competitive radioimmunoassay for binding of 125I-thrombospondin to the rabbit antibodies indicated that 49 micrograms of thrombospondin antigen per 10(6) confluent endothelial cells accumulated in postculture medium over 24 h. Thus, endothelial cells secrete large amounts of a glycoprotein that is identical or very similar to platelet thrombospondin.

Biochemical studies of two patients with the gray platelet syndrome. Selective deficiency of platelet alpha granules

The biochemistry of platelets from two unrelated patients with the gray platelet syndrome, a deficiency of platelet alpha-granules, has been evaluated. Ultrastructural studies of their platelets revealed the number of alpha-granules to be less than 15% of normal, whereas the number of dense bodies was within normal limits. Platelets from both patients had severe deficiencies of platelet factor 4 and beta-thromboglobulin (less than 10% of normal). Sodium dodecyl sulfate-polyacrylamide gel electrophoresis showed a marked deficiency of thrombin-sensitive protein in both patients. Analysis of the platelet-derived growth factor in one patient showed it was also markedly reduced. Levels of lysosomal enzymes, adenine nucleotides, serotonin, and catalase, and conversion of arachidonic acid by the lipoxygenase and cyclo-oxygenase enzymes, were within normal limits. The results provide important evidence to define the contents of alpha-granules and to differentiate these contents from the contents of lysosomal granules, dense bodies, and peroxisomes. Functional studies of these platelets showed deficiencies in ADP, thrombin, and collagen aggregation. The results suggest that alpha-granules or their contents make a contribution to normal platelet aggregation.

A comparative study of the effect of modification of the surface of human platelets on the receptors for aggregated immunoglobulins and for ristocentin-von Willebrand factor

The receptors for aggregated immunoglobulin G (IgG) (an Fc receptor) and for ristocetin-von Willebrand factor on human platelets were studied by means of various modifications of the platelet surface. The expression of these receptors was measured by the agglutination of platelets to ristocetin in the presence of von Willebrand factor, which is part of the factor VIII complex, and by the binding of aggregated IgG coupled to 3H-labelled diazobenzene. Treatment of platelets with chymotrypsin, trypsin, papain and pronase which removed protein and glycoprotein from the platelet under conditions where the release reaction was inhibited caused loss of the expression of the receptor for ristocetin-von Willebrand factor and an enhancement of that for aggregated IgG. Induction of membrane changes with ADP and of the release reaction with the ionophore A23187 abolished agglutination to ristocentin-von Willebrand factor but did not alter the receptor for aggregated IgC. Possible contributions of unspecific membrane changes, produced by protease treatment of platelets, to the modification of receptor expression were eliminated by the use of formaldehyde-treated platelets. Trypsin, papain and pronase destroyed the ability of these platelets to agglutinate to ristocetin-von Willebrand factor but produced no change in the binding of aggregated IgC. Therefore, the receptor for ristocetin-von Willebrand factor is truly sensitive to proteolysis while the Fc receptor is not, but is partially masked by protease-sensitive material.

Studies on subcellular fractions of human platelets by the lactoperoxidase-iodination technique

Lactoperoxidase-catalyzed 125I iodination and sodium dodecyl sulphate-polyacrylamide gel electrophoresis have been performed on whole, washed platelets as well as on isolated platelet membranes and granules. Electrophoresis of the whole platelets demonstrated two major radioactive peaks, corresponding to glycopolypeptides of estimated molecular weights of 120 000 and 100 000. A small, but consistent amount of radioactivity was also associated with a 147 000 dalton glycopolypeptide. The membranes showed the same pattern of radioactivity as the whole platelets, whereas only negligible amounts of labeled material was found in the soluble and granule fractions. Practically all the polypeptides were labeled in membranes iodinated after their isolation. A glycopolypeptide of 147 000 molecular weight was observed also in the soluble and the granule fractions, but no radioactivity was associated with these substances. In unreduced form, the granule glycopolypeptide penetrated only slightly into the polyacrylamide gel. Thrombin induced the relase of this granule-located substance from whole platelets, as observed by gel electrophoresis of the supernatant after release reaction (secretion). The granule glycoproteins were only partly exposed on the granule membrane since about 50% of the acid-hydrolyzable sialic acid could be liberated by neuraminidase treatment of isolated granules. In whole, iodinated granules the bulk of the radioactivity was associated with a polypeptide of estimated molecular weight 46 000 (possibly actin). This polypeptide was not seen in the supernatant after removal of the thrombin-degranulated platelets by centrifugation, which indicates that the granule membrane is retained with the platelets during the secretion process.

Subcellular localization of the heparin-neutralizing factor in blood platelets

1. The distribution of the heparin-neutralizing factor (platelet factor 4, PF4) in subcellular organelles of blood platelets of rabbits and man was investigated. 2. In both species the organelles storing 5-hydroxytryptamine (5-HT storage organelles) contained only trivial amounts of PF4. 3. In contrast, the content of PF4 was highest in the subcellular fractions rich in alpha-granules. 4. In conclusion, PF4 is probably localized in the alpha-granules and therefore the platelets contain at least two types of organelles (5-HT organelles and alpha-granules) capable of releasing their contents in response to the same stimuli, such as exposure to collagen, thrombin, etc.