Mepacrine Labelling Test and Uranaffin Cytochemical Reaction in Human Megakaryocytes

5-HT storage organelles were observed by electron microscope analysis in human megakaryocytes. They were less numerous per unit of surface than in platelets. Their number depended on the visualization technique employed. Thus after fixation with calcium-enriched glutaraldehyde a higher number of very opaque organelles was observed than of uranaffin-positive organelles after the cytochemical uranaffin reaction. With conventional electron microscopy deep black granules characteristic of dense bodies were not observed.

Fluorescent microscopy showed greenish-yellow granules distributed throughout the whole cytoplasm in 96 ± 1.4% of normal megacaryocytes incubated with mepacrine. In 85.6 ± 5%, 5.28 ± 1.28 granules per 10 ¼m2 were observed.

With the mepacrine labelling test, 74% of the megakaryocytes of a patient with Hermansky-Pudlak syndrome contained no granules. A similar finding was made in the platelets of the same patient. This suggests that mepacrine also stains the dense bodies in the megakaryocytes and that in the Hermansky-Pudlak syndrome the platelet anomaly is secondary to a megakaryocyte anomaly.

Megakaryocytes from the Marrow of a Patient with Glanzmann’s Thrombasthenia Lacked GPII b-III a Complexes

Although it is recognized that glycoprotein (GP) IIb-IIIa complexes are deficient in platelets in Glanzmann’s thrombasthenia, little is known of the origin of the defect. We have examined the megakaryocytes in a bone marrow aspirate obtained from a thrombasthenia patient during surgery. Analysis of platelet proteins by SDS-polyacrylamide gel electrophoresis confirmed the patient to be of the type I subgroup. The megakaryocytes were examined by immunofluorescence or by immunocytochemical procedures combined with electron microscopy. Antibodies used included the murine monoclonal antibody, AP-2 and the human allo-antibody, IgG L, both of which recognize determinants on GP IIb-IIIa complexes. Bound antibody was detected by anti-IgG antibodies coupled to fluorescein isothiocyanate or adsorbed on gold particles. In the immunofluorescence studies, permeabilized megakaryocytes were identified by double staining using an antibody to von Willebrand factor (vWF). Whereas mature megakaryocytes and their small precursor cells from normal individuals were strongly fluorescent with AP-2 and IgG L, most vWF positive cells from the Glanzmann’s thrombasthenia patient were negative and the remainder gave but a weak background fluorescence. Immunogold staining on the surface of marrow cells was severely reduced. Our results confirm a deficiency of GP IIb-IIIa complexes in megakaryocytes in thrombasthenia.

Platelet activation in thrombotic disorders

Recent advances have resulted in the elucidation of the principal molecular pathways of platelet function. Parallel studies have led to the identification of glycoprotein antigens whose presence at the platelet surface indicates an activated state. Such markers include GMP-140 and other glycoproteins of intracellular membranes whose translocation requires secretion and fusion of granule membranes with those joined to the surface. Other markers include activation-dependent epitopes on the GP IIb-IIIa complex and adhesive proteins bound to the activated GP IIb-IIIa receptor. Such epitopes can be detected by specific monoclonal antibodies. Quantification of their binding by flow cytometry allows an estimation of epitope expression within the whole platelet population. Our studies are designed to answer the question of whether measuring these epitopes is useful for predicting thrombosis in patients with acute cardiovascular disease. For this, we have examined three states where platelet function may be modified and where the risk of thrombosis and/or bleeding is increased. These include (i) patients with severe burns, (ii) patients who have undergone coronary angioplasty, and (iii) patients receiving fibrinolytic therapy following myocardial infarction. Our results show that activated platelets can be detected in the circulation and that their level reflects the degree of the lesion. Nonetheless, we have as yet failed to show a direct correlation between their presence and a future pathological event.

A defect of platelet aggregation associated with an abnormal distribution of glycoprotein IIb-IIIa complexes within the platelet: the cause of a lifelong bleeding disorder

A young Italian man (A.P.) has a lifelong history of bleeding from gums and mucocutaneous tissue. Electron microscopy showed a wide diversity of platelet size including giant forms. In citrated platelet-rich plasma (PRP), platelet aggregation induced by adenosine diphosphate (ADP) and other agonists was much reduced. Both secretion and clot retraction were normal. The aggregation of washed platelets with ADP was improved but remained subnormal, as was aggregation with collagen and thrombin. Fibrinogen-binding was analyzed by flow cytometry using platelets in whole blood or PRP and was markedly decreased. Crossed immunoelectrophoresis of Triton X-100 extracts of (A.P.) platelets showed that GP IIb-IIIa levels were 40% to 50% of normal. Glycoprotein (GP) IIb and GP IIIa were of usual migration in sodium dodecyl sulfate-polyacrylamide gel electrophoresis, but their labeling was much reduced during lactoperoxidase-catalyzed iodination. Binding to (A.P.) platelets of four different 125I-labeled monoclonal antibodies to GP IIb-IIIa complexes was reduced to 12% to 20% of normal levels. However, when the patient's platelets were stimulated with alpha-thrombin, monoclonal antibody binding showed the same increase (approximately 20,000 sites) as normal platelets. Both flow cytometry and immunocytochemical studies showed that the distribution of residual surface GP IIb-IIIa within the total (A.P.) platelet population was heterogeneous and not related to platelet size. Staining of ultrathin sections confirmed the presence of an internal pool of GP IIb-IIIa. Monoclonal antibodies to other membrane glycoproteins bound normally to (A.P.) platelets. The patient has a selective deficiency of the surface pool of GP IIb-IIIa complexes that is manifested clinically by a mild Glanzmann's thrombasthenia-like syndrome.

von Willebrand factor bound to glycoprotein Ib is cleared from the platelet surface after platelet activation by thrombin

We recently reported that after activation of human platelets by thrombin, glycoprotein (GP) Ib-IX complexes are translocated to the surface-connected canalicular system (SCCS) (Blood 76:1503, 1990). As GPIb is a major receptor for von Willebrand factor (vWF) in platelet adhesion, we have now examined the consequences of thrombin activation on the organization of vWF bound to GPIb on the platelet surface. Studies were performed using monoclonal or polyclonal antibodies in either immunogold staining and electron microscopy (Au-EM) or in flow cytometry. When unstirred platelet-rich plasma was incubated with ristocetin, bound vWF was located by Au-EM as discrete masses regularly distributed over the cell surface. Platelets from a patient with Glanzmann's thrombasthenia, lacking GPIIb-IIIa complexes, gave a similar pattern, confirming that this represented binding to GPIb. That ristocetin was not precipitating vWF before their binding to the platelets was shown by the detection of similar masses on the surface of platelets of a patient with type IIB von Willebrand disease. Experiments were continued using washed normal platelets incubated in Tyrode-EDTA, the purpose of the EDTA being to limit the surface expression of endogenous vWF after platelet stimulation. Under these conditions, platelets were treated with ristocetin for 5 minutes at 37 degrees C in the presence of increasing amounts of purified vWF. This was followed by incubation with thrombin (0.5 U/mL) for periods of up to 10 minutes. Flow cytometry showed a time-dependent loss in the surface expression of vWF bound to GPIb and these changes were confirmed by Au-EM. In particular, immunogold staining performed on ultrathin sections showed that the bulk of the vWF was being cleared to internal membrane systems. Surface clearance of vWF during thrombin-induced platelet activation is a potential mechanism for regulating platelet adhesivity.

Thrombin-induced platelet aggregates have a dynamic structure. Time-dependent redistribution of glycoprotein IIb-IIIa complexes and secreted adhesive proteins

The role of glycoprotein (GP) IIb-IIIa complexes and of adhesive proteins in mediating platelet aggregation is now well defined. However, less is known of the changes that occur once aggregation has begun. We report immunogold staining of thin sections of platelets or platelet aggregates, embedded in Lowicryl K4M, after the use of polyclonal antibodies to GP IIb or GP IIIa, fibrinogen (Fg), von Willebrand factor (vWF), and thrombospondin (TSP). Bound immunoglobulin G (IgG) was located by species-specific anti-IgG coupled to 5-nm gold particles and by electron microscopy. Initial experiments with platelet-rich plasma confirmed the feasibility of visualizing adhesive proteins between platelets in aggregates. Experiments then continued, using stirred suspensions of washed platelets incubated with alpha-thrombin. After 20 seconds, platelets were in contact without detectable release, although giant secretory vesicles containing adhesive proteins were seen. Internal pools of GP IIb-IIIa were progressively externalized within the aggregate. Secreted Fg was readily detected between platelets at 40 seconds. After 3 minutes, when most of the secretion had occurred, Fg had a patchwork-like distribution within the aggregate. After 6 minutes, zones with closely interspaced surface membranes, usually representing pseudopods, were dominant and Fg free. Results for vWF and TSP were similar to those for Fg. Nonetheless, GP IIb-IIIa complexes continued to be located between adjacent surface membranes throughout the aggregate. Thrombin-induced platelet aggregates were isolated, and sodium dodecyl sulfate-soluble extracts were obtained. Western blot experiments showed that, although fibrinopeptide A had been cleaved, degradation of adhesive proteins by platelet proteases had not occurred. These results emphasize that a platelet aggregate is a dynamic structure and suggest that not all surface-contact interactions are mediated by Fg or the other adhesive proteins tested in this study.

Studies on the megakaryocytes of a patient with the Bernard-Soulier syndrome

We have used monoclonal antibodies AP-1 (anti-GP Ib alpha). AP-2 (anti-GP IIb-IIIa) and FMC 25 (anti-GP IX) in immunofluorescence and immunocytochemical studies on megakaryocytes (MK) isolated from a Bernard-Soulier syndrome (BSS) patient whose giant platelets were characteristically deficient in GP Ib-IX complexes. Electron microscopy showed that the patient's MK were similar in size to normal MK. However, a striking feature was the variable and intermittent nature of the demarcation membrane system which was often vacuolar in appearance. Permeabilized mature MK from normal individuals were strongly positive with AP-2, AP-1 and FMC 25. Those from the BSS patient were normal for AP-2, negative for AP-1 but weakly positive with FMC 25. Binding of the monoclonal antibodies to the patient's platelets was evaluated using flow cytometry. The results confirmed the absence of GP Ib alpha from the surface membranes, but showed the presence of small amounts of GP IX distributed throughout the platelet population. Our findings confirm that the membrane lesion in BSS is also to be found in MK and further show that the defect may affect differently individual constituents of the GP-Ib-IX complex.

Thrombin induces a rapid redistribution of glycoprotein Ib-IX complexes within the membrane systems of activated human platelets

Previous studies have shown a decreased binding of monoclonal antibodies (MoAbs) to glycoprotein (GP) Ib-IX complexes on thrombin-stimulated platelets, but the reason for this is poorly understood. We have used (1) immunofluorescence procedures and flow cytometry, and (2) immunogold staining and electron microscopy to investigate this phenomenon. Washed platelets were incubated with alpha-thrombin, adenosine diphosphate, or ionophore A23187 for increasing lengths of time. For alpha-thrombin, but not the other agonists, flow cytometry confirmed a dose- and time-dependent decrease in the binding of MoAbs specific for GP Ib alpha (AP-1, Bx-1), GP IX (FMC 25), or to the complex itself (SZ 1). Immunoglold staining performed using standard transmission or scanning electron microscopy high-lighted surface areas devoid of bound antibody. However, a quantitatively normal immunofluorescence was restored if paraformaldehyde-fixed, thrombin-stimulated platelets were permeabilized with Triton X-100 (Sigma Chemical Co, St Louis, MO) before MoAb addition, while immunogold staining was now seen to be concentrated within the interior of the platelet. Glutaraldehyde-fixed samples were then embedded in the resin Lowicryl K4M (Taab Laboratories Equipment Ltd, Aldermaston, England) and immunogold staining performed on thin sections using a polyclonal antibody to glycocalicin. An increased presence of GP Ib-IX complexes within surface-connected membrane systems of the thrombin-stimulated platelets was confirmed. Interestingly, GP Ib-IX movement was opposite to the thrombin-induced externalization of internal pools of GP IIb-IIIa complexes and of the alpha-granule membrane GP, GMP-140.

Megakaryocytes from the marrow of a patient with Glanzmann's thrombasthenia lacked GP IIb-IIIa complexes

Although it is recognized that glycoprotein (GP) IIb-IIIa complexes are deficient in platelets in Glanzmann's thrombasthenia, little is known of the origin of the defect. We have examined the megakaryocytes in a bone marrow aspirate obtained from a thrombasthenia patient during surgery. Analysis of platelet proteins by SDS-polyacrylamide gel electrophoresis confirmed the patient to be of the type I subgroup. The megakaryocytes were examined by immunofluorescence or by immunocytochemical procedures combined with electron microscopy. Antibodies used included the murine monoclonal antibody, AP-2 and the human allo-antibody, IgG L, both of which recognize determinants on GP IIb-IIIa complexes. Bound antibody was detected by anti-IgG antibodies coupled to fluorescein isothiocyanate or absorbed on gold particles. In the immunofluorescence studies, permeabilized megakaryocytes were identified by double staining using an antibody to von Willebrand factor (vWF). Whereas mature megakaryocytes and their small precursor cells from normal individuals were strongly fluorescent with AP-2 and IgG L, most vWF positive cells from the Glanzmann's thrombasthenia patient were negative and the remainder gave but a weak background fluorescence. Immunogold staining on the surface of marrow cells was severely reduced. Our results confirm a deficiency of GP IIb-IIIa complexes in megakaryocytes in thrombasthenia.

Immunocytochemical study of the binding of fibrinogen and thrombospondin to ADP- and thrombin-stimulated human platelets

We have used immunogold staining to locate thrombospondin (TSP) on thrombin-activated human platelets, and have compared its distribution with that of fibrinogen (or fibrin) on thrombin- and ADP-stimulated platelets. To do this, isolated platelets were incubated with monospecific antibodies to TSP or fibrinogen (fib) and the bound IgG located with a second antibody adsorbed to gold particles. Thrombin-induced secretion in Tyrode-Ca2+ was followed by both anti-TSP and anti-fib binding, with large clusters of gold particles observed on the platelet surface. Little or no labeling was observed on unstimulated platelets with either antibody. When secretion was effected in Tyrode-EDTA, anti-TSP IgG still bound to the activated platelets, but the number of particle clusters was significantly reduced. Little binding of anti-fib IgG now occurred. Platelets activated with ADP in the presence of added fib, and subsequently incubated with anti-fib IgG, showed small particle clusters over the whole platelet surface. Thrombin-stimulated platelets from two patients with thrombasthenia bound anti-TSP IgG similarly to normal platelets activated in Tyrode-EDTA. No anti-fib binding occurred. Our results suggest that fib and TSP bind to specific domains on the stimulated platelet membrane. Such sites may be responsible for the mediation of platelet surface contact interactions.

Mepacrine labelling test and uranaffin cytochemical reaction in human megakaryocytes

5-HT storage organelles were observed by electron microscope analysis in human megakaryocytes. They were less numerous per unit of surface than in platelets. Their number depended on the visualization technique employed. Thus after fixation with calcium-enriched glutaraldehyde a higher number of very opaque organelles was observed than of uranaffin-positive organelles after the cytochemical uranaffin reaction. With conventional electron microscopy deep black granules characteristic of dense bodies were not observed. Fluorescent microscopy showed greenish-yellow granules distributed throughout the whole cytoplasm in 96 +/- 1.4% of normal megakaryocytes incubated with mepacrine. In 85.6 +/- 5%, 5.28 +/- 1.28 granules per 10 microns 2 were observed. With the mepacrine labelling test, 74% of the megakaryocytes of a patient with Hermansky-Pudlak syndrome contained no granules. A similar finding was made in the platelets of the same patient. This suggests that mepacrine also stains the dense bodies in the megakaryocytes and that in the Hermansky-Pudlak syndrome the platelet anomaly is secondary to a megakaryocyte anomaly.

Occurence and Evolution of in Vivo Platelet Release Phenomenon. Study in Severe Burns

During the first post burn period (first and second week) the most severe cases showed evidence of a platelet storage pool deficiency, i.e. : no aggregation with ADP, decreased nucleotide content, reduced number of granules loaded with mepacrine (presence of non granulated platelets) and increase in the percentage of light platelets. During the second post burn period (3rd to 5th week) on the same patients and for the least severe cases, the abnormalities were found heterogeneous, suggesting evidence of a partial release phenomenon, Altough the platelet nucleotide content was reduced, quite a few “empty” platelets were found, there was no significant increase of light platelets.

These data indicate, with regard to the platelets, the presence of a partial release phenomenon during extensive peripheral destruction of platelets which can sometimes reach the state of a Storage Pool Dasease.