Hereditary giant platelet syndrome. Absence of collagen-induced coagulant activity and deficiency of factor-XI binding to platelets

The GPIbα intracellular tail - role in transducing VWF- and collagen/GPVI-mediated signaling

The GPIbT-VWF A1 domain interaction is essential for platelet tethering under high shear. Synergy between GPIbα and GPVI signaling machineries has been suggested previously, however its molecular mechanism remains unclear. We generated a novel GPIbα transgenic mouse (GpIbαΔsig/Δsig) by CRISPR-Cas9 technology to delete the last 24 residues of the GPIbα intracellular tail that harbors the 14-3-3 and phosphoinositide-3 kinase binding sites. GPIbαΔsig/Δsig platelets bound VWF normally under flow. However, they formed fewer filopodia on VWF/botrocetin in the presence of a oIIbI3 blocker, demonstrating that despite normal ligand binding, VWF-dependent signaling is diminished. Activation of GpIbαΔsig/Δsig platelets with ADP and thrombin was normal, but GpIbαΔsig/Δsig platelets stimulated with collagen-related-peptide (CRP) exhibited markedly decreased P-selectin exposure and eIIbI3 activation, suggesting a role for the GpIbaaintracellular tail in GPVI-mediated signaling. Consistent with this, while haemostasis was normal in GPIbαΔsig/Δsig mice, diminished tyrosine-phosphorylation, (particularly pSYK) was detected in CRP-stimulated GpIbαΔsig/Δsig platelets as well as reduced platelet spreading on CRP. Platelet responses to rhodocytin were also affected in GpIbαΔsig/Δsig platelets but to a lesser extent than those with CRP. GpIbαΔsig/Δsig platelets formed smaller aggregates than wild-type platelets on collagen-coated microchannels at low, medium and high shear. In response to both VWF and collagen binding, flow assays performed with plasma-free blood or in the presence of bIIbI3- or GPVI-blockers suggested reduced bIIbI3 activation contributes to the phenotype of the GpIbαΔsig/Δsig platelets. Together, these results reveal a new role for the intracellular tail of GPIbiiin transducing both VWF-GPIbGGand collagen-GPVI signaling events in platelets.

Approach to the Assessment of Platelet Function: Comparison between Optical-based Platelet-rich Plasma and Impedance-based Whole Blood Platelet Aggregation Methods

Platelet aggregation studies play an important role in the assessment of hereditary and acquired platelet function defects. The first aggregation test introduced into laboratory practice used platelet-rich plasma (PRP) where aggregation was detected by an optical method. The assessment of platelet function using whole blood (WB) aggregation by an impedance method followed up nearly 20 years later. The WB impedance aggregation assay appears to be superior to the optical method because it 1) evaluates platelets in a physiologic milieu in the presence of red and white blood cells, which are known to modulate platelet function; 2) is faster; 3) has higher sensitivity; and 4) does not require centrifugation, thus avoiding injury to platelets and loss of giant thrombocytes. These two assays were compared. Clearly, the WB impedance aggregation methodology has many advantages over the optical PRP assay for the assessment of the hyperactive platelet syndrome and the effects of anti-platelet drugs.

8-BuS-ATP derivatives as specific NTPDase1 inhibitors

BACKGROUND AND PURPOSE

Ectonucleotidases control extracellular nucleotide levels and consequently, their (patho)physiological responses. Among these enzymes, nucleoside triphosphate diphosphohydrolase-1 (NTPDase1), -2, -3 and -8 are the major ectonucleotidases responsible for nucleotide hydrolysis at the cell surface under physiological conditions, and NTPDase1 is predominantly located at the surface of vascular endothelial cells and leukocytes. Efficacious inhibitors of NTPDase1 are required to modulate responses induced by nucleotides in a number of pathological situations such as thrombosis, inflammation and cancer.

EXPERIMENTAL APPROACH

Here, we present the synthesis and enzymatic characterization of five 8-BuS-adenine nucleotide derivatives as potent and selective inhibitors of NTPDase1.

KEY RESULTS

The compounds 8-BuS-AMP, 8-BuS-ADP and 8-BuS-ATP inhibit recombinant human and mouse NTPDase1 by mixed type inhibition, predominantly competitive with Ki values <1 μM. In contrast to 8-BuS-ATP which could be hydrolyzed by other NTPDases, the other BuS derivatives were resistant to hydrolysis by either NTPDase1, -2, -3 or -8. 8-BuS-AMP and 8-BuS-ADP were the most potent and selective inhibitors of NTPDase1 expressed in human umbilical vein endothelial cells as well as in situ in human and mouse tissues. As expected, as a result of their inhibition of recombinant human NTPDase1, 8-BuS-AMP and 8-BuS-ADP impaired the ability of this enzyme to block platelet aggregation. Importantly, neither of these two inhibitors triggered platelet aggregation nor prevented ADP-induced platelet aggregation, in support of their inactivity towards P2Y1 and P2Y12 receptors.

CONCLUSIONS AND IMPLICATIONS

The 8-BuS-AMP and 8-BuS-ADP have therefore potential to serve as drugs for the treatment of pathologies regulated by NTPDase1.

Identification of coagulation factor XI as a ligand for platelet apolipoprotein E receptor 2 (ApoER2)

OBJECTIVE

Factor XI (FXI) promotes hemostasis and thrombosis through enhancement of thrombin generation and has been shown to play a critical role in the formation of occlusive thrombi in arterial injury models. The aim of this study was to investigate the mechanisms governing interactions between FXI and platelets.

METHODS AND RESULTS

Platelet adhesion to immobilized FXI was abrogated in the presence of the low-density lipoprotein (LDL) receptor antagonist, receptor-associated protein (RAP), soluble recombinant apolipoprotein E receptor 2 (ApoER2), or the LDL-binding domain 1 or 2 of ApoER2. FXI supported wild-type murine platelet binding; in contrast, ApoER2-deficient murine platelets did not adhere to FXI. In the presence of shear, platelet aggregates formed on FXI or activated FXI (FXIa) surfaces, whereas the presence of RAP, binding domain 1 of ApoER2, or an anti-GPIb alpha mAb blocked platelet adhesion to FXI or FXIa under shear. Soluble FXI bound to immobilized ApoER2' with an affinity of 61 nmol/L.

CONCLUSIONS

This study has identified apolipoprotein E receptor 2 (ApoER2, LRP8), a member of the LDL receptor family, as a platelet receptor for FXI. The interaction of FXI with other cell types that express ApoER2 remains to be explored.

Receptors, rafts, and microvesicles in thrombosis and inflammation

Hemostasis at sites of blood vessel injury and its pathologic counterpart, thrombosis, involve a complex interplay between several blood elements: soluble proteins of the blood coagulation system, blood cells (most prominently platelets) and cell fragments, and elements of the vessel wall (endothelial cells and, at sites of injury, the exposed matrix and deeper cellular components). In this review, we focus on ways in which specialized membrane microdomains known as lipid rafts are involved in various phases of hemostasis and thrombosis.

Fibrin polymerization is crucial for thrombin generation in platelet-rich plasma in a VWF-GPIb-dependent process, defective in Bernard-Soulier syndrome

Defective prothrombin consumption has been reported in the proband case of Bernard-Soulier syndrome (BSS). There is no consensus, however, on whether the formation of platelet procoagulant activity (PPA) is impaired in BSS and, if so, whether this is due to the lack of GPIb-V-IX-dependent binding of thrombin or of von Willebrand factor (VWF). We show thrombin generation (TG) in platelet-rich plasma of BSS (BSS-PRP) to be defective provided that fibrin remains present in the reaction mixture and that the giant platelets are not damaged by frequent subsampling. In BSS-PRP addition of (thrombin-free) fibrin did not increase TG as in normal PRP, supporting our previous hypothesis that the interaction of fibrin, VWF and GPIb triggers PPA development. Fibrin formed during the lag phase of TG by a snake venom enzyme which only removed fibrinopeptide A induced an immediate burst of TG, that was inhibited by a monoclonal antibody against GPIb (6D1) that abolishes ristocetin-induced binding of VWF to platelets. Inversely, inhibition of polymerization decreased TG and the residual activity was insensitive to 6D1. We conclude that polymerizing fibrin interacts with VWF so as to activate GPIb.

RETRACTED: Factor XI binding to the platelet glycoprotein Ib-IX-V complex promotes factor XI activation by thrombin

Factor XI binds to high affinity sites on the surface of stimulated platelets where it is efficiently activated by thrombin. Here, we provide evidence that the factor XI binding site on platelets is in the glycoprotein (GP) Ibalpha subunit of the GP Ib-IX-V complex as follows. 1) Bernard-Soulier platelets, lacking the complex, are deficient in factor XI binding; 2) two GP Ibalpha ligands, SZ-2 (a monoclonal antibody) and bovine von Willebrand factor, inhibit factor XI binding to platelets; 3) by surface plasmon resonance, factor XI bound specifically to glycocalicin (the extracellular domain of GP Ibalpha) in Zn(2+)-dependent fashion (K(d)( app) approximately 52 nm). We then investigated whether glycocalicin could promote factor XI activation by thrombin, another GP Ibalpha ligand. In the presence of high molecular weight kininogen (45 nm), Zn(2+) and Ca(2+) ions, thrombin activated factor XI in the presence of glycocalicin at rates comparable with those seen in the presence of dextran sulfate (1 microg/ml). With higher high molecular weight kininogen concentrations (360 nm), the rate of thrombin-catalyzed factor XI activation in the presence of glycocalicin was comparable with that on activated platelets. Thus, factor XI binds to the GP Ib-IX-V complex, promoting its activation by thrombin.

Fibrin-Dependent Platelet Procoagulant Activity Requires GPIb Receptors and von Willebrand Factor

Thrombin generation in platelet-rich plasma (PRP) involves complex interactions between platelets and coagulation proteins. We previously reported that the addition of fibrin to PRP enhances tissue-factor initiated thrombin generation by ≈ 40%, and the current studies were designed to assess the mechanism(s) underlying thrombin generation in the absence and presence of fibrin. Blocking platelet GPIIb/IIIa + vβ3 receptors with a monoclonal antibody (MoAb) inhibited basal thrombin generation, but did not affect the enhancement produced by fibrin. In contrast, blocking GPIb with any of three different MoAbs had no effect on basal thrombin generation, but essentially eliminated fibrin enhancement of thrombin generation. When thrombin generation was tested in PRP deficient in von Willebrand factor (vWF), both basal and fibrin-enhanced thrombin generation were markedly reduced, and the addition of factor VIII did not normalize thrombin generation. Botrocetin, which induces the binding of vWF to GPIb, enhanced thrombin generation. In all studies, the ability of PRP to support thrombin generation correlated with the production of platelet-derived microparticles and serum platelet-derived procoagulant activity. Thus, two separate mechanisms, both of which depend on vWF, appear to contribute to platelet-derived procoagulant activity: one is independent of fibrin and relies primarily on GPIIb/IIIa, but with a minor contribution from vβ3; and the other is fibrin-dependent and relies on GPIb. These data may have implications for understanding the mechanisms of the abnormalities in serum prothrombin times reported in Bernard-Soulier syndrome, hemorrhage in von Willebrand disease (vWD), and the increased risk of thrombosis associated with elevated vWF levels.

Fibrin-Dependent Platelet Procoagulant Activity Requires GPIb Receptors and von Willebrand Factor

Thrombin generation in platelet-rich plasma (PRP) involves complex interactions between platelets and coagulation proteins. We previously reported that the addition of fibrin to PRP enhances tissue-factor initiated thrombin generation by ≈ 40%, and the current studies were designed to assess the mechanism(s) underlying thrombin generation in the absence and presence of fibrin. Blocking platelet GPIIb/IIIa + vβ3 receptors with a monoclonal antibody (MoAb) inhibited basal thrombin generation, but did not affect the enhancement produced by fibrin. In contrast, blocking GPIb with any of three different MoAbs had no effect on basal thrombin generation, but essentially eliminated fibrin enhancement of thrombin generation. When thrombin generation was tested in PRP deficient in von Willebrand factor (vWF), both basal and fibrin-enhanced thrombin generation were markedly reduced, and the addition of factor VIII did not normalize thrombin generation. Botrocetin, which induces the binding of vWF to GPIb, enhanced thrombin generation. In all studies, the ability of PRP to support thrombin generation correlated with the production of platelet-derived microparticles and serum platelet-derived procoagulant activity. Thus, two separate mechanisms, both of which depend on vWF, appear to contribute to platelet-derived procoagulant activity: one is independent of fibrin and relies primarily on GPIIb/IIIa, but with a minor contribution from vβ3; and the other is fibrin-dependent and relies on GPIb. These data may have implications for understanding the mechanisms of the abnormalities in serum prothrombin times reported in Bernard-Soulier syndrome, hemorrhage in von Willebrand disease (vWD), and the increased risk of thrombosis associated with elevated vWF levels.

Molecular cloning of platelet factor XI, an alternative splicing product of the plasma factor XI gene

Platelet factor XI is associated with the platelet plasma membrane and has an apparent Mr (220,000 nonreduced, 55,000 reduced) different from that of plasma factor XI. However, the site of synthesis and the nature of platelet factor XI are not known. Using reverse transcriptase polymerase chain reaction, 12 out of 13 exons (all except exon V) coding for mature plasma factor XI were amplified from human platelet mRNA. The sequence of each of these exons was identical to that of plasma factor XI. In situ amplification and hybridization of factor XI mRNA was positive for exon III and negative for exon V in platelets and negative for both exons in other blood cells. By Northern hybridization, a factor XI mRNA transcript of approximately 1.9 kilobases was detected in megakaryocytic cells, and one of approximately 2.1 kilobases was detected in liver cells. Factor XI cDNA was cloned from a megakaryocyte library and sequenced. Exon V was absent, and the splicing of exon IV to exon VI maintained the open reading frame without alteration of the amino acid sequence except for the deletion of amino acids Ala91-Arg144 within the amino-terminal portion of the Apple 2 domain. Thus, platelet factor XI is an alternative splicing product of the factor XI gene, localized to platelets and megakaryocytes but absent from other blood cells.

Evidence that a 210,000-molecular-weight glycoprotein (GP 210) serves as a platelet Fc receptor

We previously identified a 210,000-mol-wt platelet glycoprotein (GP 210) that is missing from Bernard-Soulier platelets, and found that an antibody against GP 210 inhibits ristocetin-induced platelet agglutination. We now show by immunoblotting that GP 210 binds heat-aggregated rabbit and human IgG, as well as keyhole limpet hemocyanin (KLH)-anti-KLH and ovalbumin (OA)-anti-OA immune complexes. Immune complex binding to GP 210 was preserved on chymotrypsin-treated platelets that lacked glycoprotein Ib (GP Ib). In contrast, ristocetin-induced platelet agglutination resulted in disappearance of immunologically detectable GP 210 and loss of immune complex binding, even though GP Ib remained intact. Purified Fc fragments inhibited binding of anti-GP 210 antibody to intact platelets and to GP 210 on immunoblots. The Fc fragments also blocked immune complex binding to GP 210. Conversely, anti-GP 210 antiserum and F(ab)2 fragments inhibited binding of fluorescein-labeled Fc fragments to intact platelets. We conclude that GP 210 functions as a platelet Fc receptor.

Role of shear rate and platelets in promoting fibrin formation on rabbit subendothelium. Studies utilizing patients with quantitative and qualitative platelet defects

The deposition of platelets on subendothelium of rabbit aortic segments exposed to non-anticoagulated human blood increased progressively with increasing wall shear rates (50-2,600 s-1), whereas fibrin deposition decreased. Studies in normal subjects and patients with platelet disorders suggested that, under the conditions used, platelets were essential for fibrin deposition at intermediate (650 s-1) but not low (50 s-1) shear rates. Fibrin deposition was markedly diminished in a patient with Scott syndrome whose platelets have a diminished capacity to bind Factor Xa and activate Factors IX and II. In glycoprotein IIb-IIIa deficiency, fibrin deposition was normal (or somewhat increased), whereas in glycoprotein Ib deficiency the association of fibrin with platelets, but not subendothelium, was decreased. The findings indicate that platelets, perhaps through surface localization of coagulation proteins, promote fibrin deposition on subendothelium at arterial shear rates and suggest that agents directed against platelet coagulant properties could be antithrombotic.

Branch artery occlusion in a young woman

A 31-year-old woman presented with signs and symptoms of anterior and posterior cerebral circulatory insufficiency. The differential diagnosis and therapeutic options are discussed. The most likely cause is a cardiogenic source of emboli.

Identification and function of the high affinity binding sites for Ca2+ on the surface of platelets

Extracellular Ca2+ is required for platelet aggregation and secretion in response to ADP or epinephrine. Recently, we reported that the platelet surface contains two classes of high affinity binding sites for extracellular Ca2+. To identify these sites and clarify their role in platelet function, we have now (a) studied platelets congenitally deficient in surface membrane glycoproteins and (b) examined the effect of removing surface-bound Ca2+ on platelet responses to ADP and epinephrine. Unstimulated normal platelets contained 86,000 Ca2+-binding sites/platelet with a dissociation constant (Kd) of 9 nM and 389,000 sites with a Kd of 400 nM. In contrast, thrombasthenic platelets, which lack glycoproteins IIb and IIIa, exhibited a 92% reduction in the number of higher affinity Ca2+-binding sites and a 63% reduction in the number of lower affinity sites. Bernard-Soulier platelets, which lack glycoprotein Ib, were not deficient in Ca2+-binding sites. After stimulation with ADP, both normal and thrombasthenic platelets developed approximately 138,000 new Ca2+-binding sites/platelet (Kd = 400 nM), while the larger Bernard-Soulier platelets developed 216,000 new sites. These data suggest that IIb and IIIa represent the major Ca2+-binding glycoproteins on unstimulated platelets, while neither these glycoproteins nor Ib represent the new Ca2+-binding sites on stimulated platelets. Removal of Ca2+ from the platelet surface inhibited platelet function. Despite the presence of 1 mM Mg2+, ADP- and epinephrine-induced aggregation and [14C]serotonin release were markedly decreased at free Ca2+ concentrations less than 7 nM, a value similar to the Kd of the higher affinity Ca2+-binding sites. Moreover, gadolinium, a lanthanide that competed for these Ca2+-binding sites, also inhibited aggregation and serotonin release. These studies demonstrate, therefore, that the binding of extracellular Ca2+ to glycoproteins IIb/IIIa on unstimulated platelets or to additional membrane proteins on stimulated platelets is necessary for maximal platelet responses to ADP and epinephrine. Thus, the requirement for extracellular Ca2+ during platelet activation by these agonists may actually represent a requirement for surface-bound Ca2+.

Coagulant activities of platelets in coronary artery disease

Platelets have been implicated in the pathogenesis of coronary artery disease, and a number of studies have examined platelet function and coagulation parameters in such patients. We have examined platelet coagulant activities, volumes, and aggregate ratios in 23 patients with chest pain, seven of whom had normal coronary angiograms (group I) and 16 of whom had angiographically proven coronary artery disease (group II). There were no significant differences in the mean values for platelet volume or platelet aggregate ratios between the two groups. The platelet coagulant activities concerned with initiation and the early stages of intrinsic coagulation were significantly increased in patients in group II as compared with those in group I. No significant differences were noted between the two groups with respect to prothrombin time, partial thromboplastin time, and plasma levels of fibrinogen and coagulation factors V and VIII. However, the mean activity in plasma of antithrombin III (but not the level of antithrombin III antigen) was significantly lower in patients of group II compared with group I. Overall, our observations provide evidence for an enhanced contribution of platelets to the intrinsic coagulation system in patients with coronary artery disease. The platelet coagulant hyperactivity noted in these patients may reflect a role of platelets in the pathogenesis of coronary artery disease or may be secondary to the underlying arterial disease.

Effect of intravenous carbenicillin, cefoxitin and cefamandole on ADP-induced platelet aggregation and shape change

The effect of a large daily dose of i.v. carbenicillin, cefoxitin and cefamandole on platelet aggregation with adenosine diphosphate (ADP) and the effect of carbenicillin on ADP induced shape change were evaluated. Six volunteers received carbenicillin 100 mg/kg every 4 hours. Three of them also received cefoxitin 3 gm every 6 hours and cefamandole 2 gm every 4 hours. All treatments were separated by 14 days. Prior to treatment, as well as 1 and 24 hours after the last dose of antibiotic, citrated platelet-rich plasma was tested for the extent of aggregation at a series of ADP concentrations. As previously reported by several groups, carbenicillin decreased the sensitivity of platelets in their aggregation response to ADP. Thus, the concentration of ADP needed to obtain 50% of maximal aggregation response was increased 3.5 +/- 1.7 (S.D.) fold. In addition, when measured separately, the sensitivity of platelets in their shape change response to ADP was decreased to a similar degree. The concentration of ADP needed to produce 50% of maximal shape change response was 2.8 +/- 1.6 (S.D.) fold higher after carbenicillin treatment. The results are in accordance with previous in vitro studies suggesting that carbenicillin interferes with the initial phase of platelet activation. Cefoxitin and cefamandole had no apparent effect on platelet aggregation.

Properties of human asialo-factor VIII. A ristocetin-independent platelet-aggregating agent

Human Factor VIII desialylated by treatment with Vibrio cholerae neuraminidase (ASVIII) aggregated human platelets in the absence of ristocetin in platelet-rich plasma and, to a lesser extent, in washed platelet suspensions. Aggregation is accompanied by thromboxane formation and is completely inhibited by EDTA. Aspirin blocks the second phase of aggregation and abolishes thromboxane production. Subaggregating doses of ASVIII and of either ADP, epinephrine, or collagen produce prompt and complete platelet aggregation. Bernard-Soulier syndrome platelets either did not aggregate with ASVIII (Two cases) or showed markedly decreased aggregation (one cases). Factor VIII complex was prepared from the plasma of two patients with variant von Willebrand's disease (sialic acid content 142 and 75 nmol/mg, respectively); neither protein generated platelet-aggregating activity upon desialylation. [3H]ASVIII binds rapidly to platelets and 37 degrees C, while tritiated, fully sialylated factor VIII binds to a negligible extent. As little as 1--2 micrograms ASVIII bound/10(9) platelets is capable of inducing platelet aggregation. ASVIII may be a useful tool for investigating platelet-Factor VIII interactions in the absence of ristocetin. Furthermore, desialylated Factor VIII might play a physiologic role in Factor VIII-mediated platelet reactions in vivo.

Bernard-Soulier disease: a study of four patients and their parents

Two families with Bernard-Soulier disease, including four patients and three of their parents, were studied and detailed clinical summaries are presented. One patient in each family has suffered severe bleeding problems while the other affected sibling is less severely affected. There has been no excessive bleeding in any of the parents or other family members. The patients demonstrated the abnormalities characteristic for Bernard-Soulier disease: thrombocytopenia, giant platelets, prolonged bleeding time, abnormal platelet aggregation to human FVIIIvWF and ristocetin or bovine FVIIIvWF alone, defective ristocetin-induced binding of human 125I-FVIIIvWF multimers, decreased platelet lysis by a drug-dependent antibody and complement, and a decreased concentration of membrane glycoprotein I. The parents had normal platelet counts, bleeding times, and FVIII-mediated aggregation. However, the parents had abnormally large platelets decreased sensitivity to lysis by a drug-dependent antibody and complement, and a decreased concentration of membrane glycoprotein I. Therefore the heterozygous state for Bernard-Soulier disease is recognizable by platelet membrane abnormalities although there is no defect of platelet function and no excessive bleeding. Red cell membrane proteins of one patient were normal, suggesting that phenotypic expression of the Bernard-Soulier disease defect is restricted to platelets.

Analysis of the glycoprotein and protein composition of Bernard-Soulier platelets by single and two-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis

Previous reports have described conflicting results concerning the glycoprotein (GP) and protein composition of Bernard-Soulier platelets. In view of this controversy we have analyzed the platelets of four Bernard-Soulier patients using improved single and two-dimensional sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis procedures. An absence of staining for carbohydrate of membrane GP Ib was characteristic for the platelets of each patient. Major periodate-Schiff staining bands corresponding to membrane GP IIb, IIIa, and IIIb were clearly detected and their presence was confirmed by two-dimensional SDS-polyacrylamide gel electrophoresis. The protein content of the Bernard-Soulier platelets was increased two- to fourfold. However, analysis of their protein composition using 7-12% acrylamide gradient gels showed normal polypeptide profiles. Lactoperoxidase-catalyzed 125I-labeling of the Bernard-Soulier platelet surface proteins was followed by SDS-polyacrylamide gel electrophoresis and autoradiography. No labeling in the Ib position was detected whereas the other major membrane GP, including Ia and IIa, were normally located. In contrast, GP Ib was clearly detected by periodate-Schiff staining and autoradiography when normal human platelets that had been exhaustively treated with neuraminidase before the lactoperoxidase-catalyzed iodination were analysed. No abnormalities were detected in the GP patterns of membranes isolated from the patients' erythrocytes. Only a severe molecular abnormality or possible deletion of GP Ib could account for this major platelet lesion in the Bernard-Soulier syndrome.

Platelets, thromboembolism and mitral valve prolapse

Cerebral and retinal ischemic events have been described in mitral valve prolapse. To determine whether platelets play a part in the pathogenesis of thromboembolism we studied 29 patients with mitral valve prolapse, including nine (group I) with thromboembolism (cerebral, retinal and deep venous), eight (group II) with transient visual obscurations and 12 (group III) with neither thromboembolism nor visual complaints, compared with 18 control patients and 38 normal subjects. Patients in groups I and II had increased platelet coagulant activities concerned with the initiation and early stages of intrinsic coagulation, and group I patients had an increased proportion of circulating platelet aggregates and platelets relatively insensitive top epinephrine in aggregation and secretion. The incidence of platelet coagulation hyperactivity in patients with mitral valve prolapse was 76% (100% in group I, 75% in group II, 58% in group III), compared with 6% in control patients. These results suggest that platelets play a role in the purported association of thromboembolism and mitral valve prolapse.

Retinal arterial obstruction in children and young adults

The records of 27 patients who developed retinal arterial obstruction (RAO) prior to the age of 30 years were studied to ascertain associated systemic and ocular findings as possible etiologic factors. A history of migraine was found in approximately one third of the patients, and coagulation abnormalities wer also common. Trauma, sickle cell hemoglobinopathies, cardiac disorders, use of oral contraceptives, pregnancy, systemic lupus erythematosus and intravenous drug abuse were less frequently encountered. Ocular abnormalities included increased intraocular pressure, subtle buried drusen of the optic nerve head and a congenital prepapillary arterial loop. In contrast to older patients with RAO, there was no clinical evidence of atheromatous disease. In most patients, one or more systemic or ocular etiologic factors could be discerned. Whereas etiologic relationships may be multifactorial and generally differ from those commonly found in older patients with RAO, the visual prognosis in younger and older patients appears to be similar.

Platelet coagulant activities in arterial occlusive disease of the eye

Ischemic optic neuropathy and retinal arterial occlusion are 2 forms of arterial occlusive disease affecting the eye. Reports in the literature suggest platelet hyperactivity in acute arterial occlusive diseases affecting other organ systems. Therefore, 14 patients with ischemic optic neuropathy and 17 patients with central or branch retinal artery occlusion were studied to determine whether platelets have a role in the pathogenesis of these vascular occlusive disorders. The results of the following investigations were no different in these patients compared with those in 18 control patients with non-vascular eye diseases: prothrombin times, partial thromboplastin times, plasma fibrinogen, factor V, factor VIII, platelet counts and threshold concentrations of ADP, epinephrine and collagen resulting in secondary platelet aggregation and serotonin release. In contrast, platelet coagulant activities concerned with the early stages of intrinsic coagulation were significantly increased in patients with retinal artery occlusion without hypertension or type IV hyperlipoproteinemia, but generally normal in patients with ischemic optic neuropathy and in patients with retinal artery occlusion associated with hypertension, type IV hyperlipoproteinemia, diabetes mellitus and generalized atherosclerosis. These results are consistent with a platelet contribution to retinal arterial occlusive disease in patients without other known contributing factors such as hypertension, serum lipid abnormalities, diabetes mellitus and generalized atherosclerosis and may have implications regarding prophylaxis.

Isolated deficiency of platelet procoagulant activity

This is a study of a 34 year old woman with a moderate to severe bleeding disorder in whom impaired platelet procoagulant activity (PPA) was found by several methods, including tests of factor 3 availability (PF-3a), prothrombin consumption and contact activation. No deficiencies of platelet adhesion, aggregation, secretion, metabolism or granule-bound substances were detectable. Under adequate platelet coverage, this woman underwent two surgical procedures without difficulty. These findings demonstrate the role of PPA in hemostasis and indicate that a defect in PPA can be an isolated occurrence. The abnormalities in PF-3a found in this patient could be due to the diminished number of factor V binding sites, resulting in impaired factor Xa binding, found in separate studies by Majerus et al.

Absence of the platelet receptor for drug-dependent antibodies in the Bernard-Soulier syndrome

The platelet membrane receptor for quinidine- and quinine-dependent antibodies was studied in three patients with the Bernard-Soulier syndrome (BSS) and in normal subjects with immunologic techniques based on the release of 51Cr from labeled platelets. The receptor could not be detected on BSS platelets but was present on platelets from each of 180 normal subjects. BSS platelets reacted normally with other allo- and autoantibodies. In confirmation of previous reports, BSS platelets were found to be deficient in glycoproteins Ib and Is. However, after apparently total cleavage of these proteins from the membrane of normal platelets by controlled hydrolysis with trypsin or chymotrypsin, 80% of the drug-dependent antibody receptor activity was retained. These observations suggest the existence of an additional, hitherto unrecognized membrane defect in Bernard-Soulier platelets.

Reduced thrombin binding and aggregation in Bernard-Soulier platelets

Platelets from two patients with Bernard-Soulier disease showed a reduction in their ability to bind human thrombin. Thrombin binding studies in the high affinity range showed 1,500 sites for the Bernard-Soulier platelets as against 4,000 for normal controls. However, the dissociation constant was the same for both normals and patients (4.4 nM) indicating identical affinity for thrombin at the available sites. In the low affinity range, the Bernard-Soulier platelets showed 8,800 thrombin binding sites as against 24,000 for the controls, but again with identical values of Kd (37 nM). In addition, platelets from these Bernard-Soulier patients showed a decreased rate of aggregation with thrombin at both optimal (300 mU/ml) and suboptimal (60 and 120 mU/ml) thrombin concentrations. The decreased amount of thrombin which can bind to Bernard-Soulier platelets and the decrease in thrombin-induced aggregation may partly explain the hemostatic defect in these patients. In addition, the identical ratios of high affinity and low affinity binding sites in normals and in patients (0.37 and 0.36, and 0.36, respectively) supports the idea of a single class of binding sites for thrombin on the platelet surface.

[Enzyme activities in platelets of different specific gravity in thrombocytosis of various etiology (author's transl)]

Platelets of patients with thrombocytosis following splenectomy, in chronic granulocytic leukaemia and in polycythaemia vera were separated into five fractions by centrifugation in discontinuous Ficoll density gradient. Platelet volume, content of protein and enzyme activities of lactic dehydrogenase, phosphoglycerate kinase and glyceraldehyde phosphate dehydrogenase were distinctly higher for the three groups in the heavy fraction IV compared with the light fraction I. With regard to the platelet volume, however, these differences were compensated almost completely like in the normal persons.

Metabolism and function of human platelets washed by albumin density gradient separation

A method for washing platelets by albumin density gradient separation, originally designed for the study of platelet coagulant activities, has been modified for platelet aggregation and metabolic studies. Platelets are sedimented into a continuous density gradient of isosmolar albumin containing apyrase to protect them from clumping and physical injury and are resuspended in calcium-free Tyrode's solution. The mean recovery of platelets after two separations relative to platelet-rich plasma (PRP) was 90.3%. When small amounts of plasma were added to washed platelet suspensions, aggregation and release of [14C]5-hydroxytryptamine (5HT) in response to adenosine diphosphate (adp) or 5HT were similar to results obtained with PRP. When fibrinogen was substituted for plasma, ADP-induced aggregation occurred but was feeble. Without added plasma or fibrinogen, platelets were refractory to ADP and insensitive to the cyclic endoperoxide analogue U44619. When both ADP and U44619 were added simultaneously, in low concentrations, to washed platelets without added plasma or fibrinogen, aggregation occurred immediately. Washed platelets were not aggregated by adrenaline, which potentiated ADP-induced aggregation. Several biochemical measurements which are sensitive indicators of cellular damage were normal in washed platelets, including [14C]adenine uptake, adenylate energy charge, hypoxanthine formation and the response of adenylate cyclase to stimulation by PGE1 or PGD2. Platelet coagulant activities were not made available and heparin-neutralizing activity (HNA) was not spontaneously released by the washing procedure, but the washed platelets responded normally to appropriate agents by developing coagulant activities and releasing HNA. The ultrastructure of washed platelets was similar to those in control PRP. Inclusion of apyrase in the first albumin gradient had a beneficial effect on platelet morphology, aggregation and metabolism, but washing at 37degreesC compared with 25degreesC did not. Albumin density gradient separation is a useful method for isolating platelets for aggregation and metabolic studies.

[Enzyme activities in normal platelets of different specific gravity (author's transl)]

Platelets of normal persons were separated into five fractions by centrifugation in discontinuous Ficoll density gradients. Enzyme activities of lactic dehydrogenase, phosphoglycerate kinase, glyceraldehyde phosphate dehydrogenase, pyruvate kinase, glucose-6-phosphate dehydrogenase and content of protein were constantly higher in the heavy fraction IV compared with the light fraction I. With regard to the platelet volume, however, these differences were compensated almost completely.

Platelet coagulant activities and serum lipids in transient cerebral ischemia

To determine whether platelets play a part in the pathogenesis of transient cerebrovascular ischemia, we studied 22 patients with transient ischemia, 18 control patients and 38 normal subjects. Platelet aggregation and [14C]-serotonin release by ADP, epinephrine and collagen were normal in all patients, as were plasma coagulation assays, except for shortened partial thromboplastin times in the patients with transient ischemia. Platelet coagulant activities concerned with initiation and early stages of intrinsic coagulation were increased two to three times in 12 patients with transient ischemic attacks with normal serum lipids and normal in the 10 others with Type IV hyperlipoproteinemia. These results indicate an association between platelet coagulant hyperactivity and transient ischemic attacks in a group of patients with normal serum lipids.

Comparison of the coagulant activities of platelets and phospholipids

The coagulant activities of various phospholipid preparations were compared with those of platelets. Folch phospholipid with maximal platelet factor 3 (PF3) activity produced long recalcified clotting times of relatively undiluted plasma in plastic tubes whereas untreated or ADP-treated platelets with minimal PF3 activity produced short clotting times in the same test system which is sensitive to activators of the contact system of intrinsic coagulation. Bell and Alton phospholipids with maximal PF3 activity produced recalcified clotting times similar to those in the presence of platelets. Bell and Alton phospholipids had tissue factor activity, but Folch phospholipid and platelets did not. Bell and Alton phospholipids and gum acacia (used as a vehicle in one of the preparations) activated factor XII as did platelets, but Folch phospholipid did not. The multiple coagulant activities of Bell and Alton phospholipids (i.e. PF3, tissue factor and contact activating) may account for the absence of coagulant superiority of platelets in the undiluted system in plastic tubes. The coagulant activities of platelets are also complex but different from Bell and Alton phospholipids whereas Folch phospholipid would appear to possess only PF3 activity.

The relationship of platelet coagulant activities to venous thrombosis following hip surgery

Platelets have recently been shown to trigger intrinsic coagulation by two alternative pathways, protect active clotting factors from inactivation by plasma inhibitors and catalyse intrinsic coagulation reactions on the platelet surface to form fibrin. To determine whether these platelet coagulant activities (PCA) might have a role in the pathogenesis of DVT, 29 patients have been studied before and after arthroplasty or other surgery for fractured hip or degenerative hip disease. The occurrence of DVT was detected by [125I]fibrinogen uptake in the legs and confirmed by venography. In patients who developed DVT, all PCA increased progressively and significantly on day 1 (mean rise, 146% of baseline), day 3 (228%) and day 5 (298%) after surgery before isotopic evidence of DVT appeared (mean 3.27 days postoperatively). In patients without DVT no changes in PCA were observed. Plasma coagulation factor assays were no different in patients with a without DVT. Platelet counts and total platelet antiheparin activity increased during the early postoperative period in DVT patients but not in patients without DVT. It is suggested that progressive increases in PCA concerned with triggering and catalysing intrinsic coagulation reactions may play a pathogenetic role in DVT after hip surgery.

Platelet membrane glycoproteins implicated in ristocetin-induced aggregation. Studies of the proteins on platelets from patients with Bernard-Soulier syndrome and von Willebrand's disease

The antibiotic ristocetin only aggregates platelets in the presence of plasma von Willebrand factor. Platelets from patients with Bernard-Soulier syndrome do not aggregate upon addition of ristocetin although, in contrast to von Willebrand's disease, plasma levels of factor VIII complex (factor VIII clotting activity, von Willebrand factor activity, and von Willebrand antigen) are normal. The membrane surface of normal platelets was modified and compared to the surface of platelets from a patient with Bernard-Soulier syndrome in an attempt to identify the receptor involved in von Willebrand factor-ristocetin-induced aggregation. After the incubation of washed normal platelets with a preparation of ristocetin previously shown to contain a proteolytic contaminant, the aggregation response is significantly decreased on addition or normal plasma. Analaysis by gel electrophoresis of such platelets when stained for carbohydrate revealed a decrease in the relative amounts of membrane glycopro-eins. Chymotrypsin-treated normal platelets had less membrane glycoproteins in addition to giving a reduced aggregation response in ristocetin-induced aggregation. Staining of gels for protein and carbohydrate indicated that there was an extensive change in the surface of Bernard-Soulier platelets, whereas those from patients with von Willebrand's disease appeared the same as normal. Platelets from patients were labeled by the lactoperoxidase iodination technique. Not only was the relative intensity of staining of platelet-specific proteins and glycoproteins changed in Bernard-Soulier platelets, but the iodination of the glycoproteins on the membrane surface relative to other membrane constituents was lower. In contrast, platelets from patients with von Willebrand's disease showed a normal exposure of membrane components. These data suggest therefore that membrane glycoproteins may play a functional role in ristocetin-induced aggregation.

The interaction of bovine factor VIII with human platelets

Treatment of human platelets with purified bovine Factor VIII caused three types of aggregation: (a) primary agglutination; (b) secondary aggregation involving the platelet release reaction; and (c) super-aggregation, in which the platelets were gathered into only a few large clumps. Removal of calcium ions or treatment with p-hydroxymercuiriphenyl sulfonate blocked the release reaction, but not primary agglutination or super-aggregation. Platelets treated with formalin were not aggregated by ADP, thrombin, or collagen, but were agglutinated by bovine Factor VIII, although they did not show super-aggregation. For malin-treated platelets were agglutinated by phytohemagglutinin P less extensively and less rapidly than by bovine Factor VIII. Treatment of platelets and Factor VIII with neuraminidase released 60 and 53%, respectively, of the sialic acid residues without affecting the agglutination reaction or the procoagulant activity of the Factor VIII. Agglutination was inhibited by high salt concentrations, dextran sulfate, and heparin. During agglutination, both the procoagulant and platelet-agglutinating activities of Factor VIII became bound to the platelet surface.