Grey platelet syndrome: studies on platelet alpha-granules, lysosomes and defective response to thrombin

Megakaryocytic emperipolesis and platelet function abnormalities in five patients with gray platelet syndrome

The gray platelet syndrome (GPS) is a rare congenital platelet disorder characterized by mild to moderate bleeding diathesis, macrothrombocytopenia and lack of azurophilic α-granules in platelets. Some platelet and megakaryocyte (MK) abnormalities have been described, but confirmative studies of the defects in larger patient cohorts have not been undertaken. We studied platelet function and bone marrow (BM) features in five GPS patients with NBEAL2 autosomal recessive mutations from four unrelated families. In 3/3 patients, we observed a defect in platelet responses to protease-activated receptor (PAR)1-activating peptide as the most consistent finding, either isolated or combined to defective responses to other agonists. A reduction of PAR1 receptors with normal expression of major glycoproteins on the platelet surface was also found. Thrombin-induced fibrinogen binding to platelets was severely impaired in 2/2 patients. In 4/4 patients, the BM biopsy showed fibrosis (grade 2-3) and extensive emperipolesis, with many (36-65%) MKs containing 2-4 leukocytes engulfed within the cytoplasm. Reduced immunolabeling for platelet factor 4 together with normal immunolabeling for CD63 in MKs of two patients demonstrated that GPS MKs display an alpha granule-specific defect. Increased immunolabeling for P-selectin and decreased immunolabeling for PAR1, PAR4 and c-MPL were also observed in MKs of two patients. Marked emperipolesis, specific defect of MK alpha-granule content and defect of PAR1-mediated platelet responses are present in all GPS patients that we could study in detail. These results help to further characterize the disease.

Inherited disorders of platelet alpha-granules

Platelet alpha-granules are the storage site for the internal membrane glycoprotein P-selectin and for a variety of megakaryocyte-synthesized and plasma-derived soluble proteins. Quantitative and/or qualitative abnormalities in alpha-granules are found in a number of inherited bleeding disorders, including gray platelet syndrome, alphadelta-storage pool deficiency, the Quebec platelet disorder, and in some patients with dysmegakaryopoietic thrombocytopenia. In addition, single alpha-granular protein deficiencies are seen in other bleeding disorders,including factor V deficiency, afibrinogenemia, Glanzmann's thrombasthenia, von Willebrand disease, and plasminogen-activator inhibitor-1 deficiency. The excessive bleeding that occurs in patients with inherited abnormalities of platelet alpha-granules indicates that the proteins stored within this compartment are important for normal hemostasis. The clinical and laboratory features of these different, inherited platelet storage pool disorders suggest unique molecular and biochemical defects are responsible for these conditions. However, the genetic causes of these disorders are largely unknown. This paper reviews our current knowledge of the inherited disorders of platelet alpha-granules.

Congenital Platelet Disorders

Congenital platelet disorders represent a rare group of diseases classified by either a qualitative or quantitative platelet defect. This article outlines the historical, clinical, laboratory, and genetic features of various inherited platelet disorders with attention given to updated information on disease classification, diagnosis, and genotypes. A separate discussion regarding management addresses the difficulty in treatment strategies, particularly in patients who develop alloimmunization to platelets.

Defective platelet responsiveness to thrombin and protease-activated receptors agonists in a novel case of gray platelet syndrome: correlation between the platelet defect and the alpha-granule content in the patient and four relatives

BACKGROUND

We report a novel case of gray platelet syndrome (GPS). A 14-year-old boy had bleeding diathesis, mild thrombocytopenia, giant platelets with severe defect of alpha-granule secretory proteins, myelofibrosis and splenomegaly.

METHODS AND RESULTS

Platelet function studies showed a marked reduction of aggregation and Ca(2+) mobilization by thrombin, protease-activated receptor 1 (PAR1)-activating peptide (AP) and PAR4-AP, PAR1 expression at 55% of normal levels, and a more than two hundred fold reduction of in vitro whole-blood thromboxane B(2) (TXB(2)) production. Sequencing of coding regions of the PAR1 gene failed to show abnormalities. This patient was initially classified as a sporadic case of GPS, as electron microscopy failed to identify giant platelets and/or alpha-granule deficiency in his relatives. However, further studies on the father and three other relatives showed a relative lack of platelet alpha-granule proteins by immunofluorescence microscopy, a defective platelet response to PAR4-AP, and severely reduced in vitro whole-blood TXB(2) production. On this basis, we suggest that in this family, GPS was transmitted in a dominant fashion with highly variable penetrance.

CONCLUSIONS

Our study suggests that current diagnostic criteria fail to identify some patients with a mild GPS phenotype and that such patients might be identified by the methods cited above. It also better characterizes the pathogenesis of defective platelet responses to thrombin, and raises interesting questions on the correlation between abnormal PAR function and the lack of alpha-granule content in GPS.

The gray platelet syndrome: Clinical spectrum of the disease

The gray platelet syndrome (GPS) is a rare inherited disorder of the megakaryocyte (MK) lineage. Thrombocytopenia and enlarged platelets are associated with a specific absence of alpha-granules and their contents. GPS patients exhibit much heterogeneity both in bleeding severity and in their response to platelet function testing. A unique feature is that proteins endogenously synthesised by megakaryocytes (MK) or endocytosed by MK or platelets fail to enter into the secretable storage pools that characterise alpha-granules of normal platelets. Although the molecular basis of the disease is unknown, evidence suggests that alpha-granules simply fail to mature during MK differentiation. One result is a continued leakage of growth factors and cytokines into the marrow causing myelofibrosis. While for some patients platelet function may be only moderately affected, for others thrombin and/or collagen-induced platelet aggregation is markedly modified and an acquired lack of the GPVI collagen receptor has been reported. In this review, we document the clinical and molecular heterogeneity in GPS, a unique disease of the biogenesis of platelet alpha-granules and of the storage of growth factors and secretable proteins.

Proteomic approaches to dissect platelet function: Half the story

Platelets play critical roles in diverse hemostatic and pathologic disorders and are broadly implicated in various biological processes that include inflammation, wound healing, and thrombosis. Recent progress in high-throughput mRNA and protein profiling techniques has advanced our understanding of the biological functions of platelets. Platelet proteomics has been adopted to decode the complex processes that underlie platelet function by identifying novel platelet-expressed proteins, dissecting mechanisms of signal or metabolic pathways, and analyzing functional changes of the platelet proteome in normal and pathologic states. The integration of transcriptomics and proteomics, coupled with progress in bioinformatics, provides novel tools for dissecting platelet biology. In this review, we focus on current advances in platelet proteomic studies, with emphasis on the importance of parallel transcriptomic studies to optimally dissect platelet function. Applications of these global profiling approaches to investigate platelet genetic diseases and platelet-related disorders are also addressed.

Inherited thrombocytopenias: toward a molecular understanding of disorders of platelet production

PURPOSE OF REVIEW

To review the defined syndromes of inherited thrombocytopenia and discuss new genetic data for several disorders that shed light on the process of megakaryopoiesis.

RECENT FINDINGS

The genes responsible for several inherited thrombocytopenias have been recently discovered, including congenital amegakaryocytic leukemia, amegakaryocytic thrombocytopenia with radio-ulnar synostosis, familial platelet syndrome with predisposition to acute myelogenous leukemia, Paris-Trousseau, Wiskott-Aldrich syndrome, and the May-Hegglin, Sebastian, Epstein, and Fechner syndromes. These clinical syndromes, combined with studies in mouse and in vitro models, reveal the importance of these genes for normal hematopoiesis.

SUMMARY

Although inherited syndromes of thrombocytopenia are rare, characterization of mutations in these disorders has contributed greatly to our understanding of megakaryocyte and platelet development. A systematic registry of congenitally thrombocytopenic individuals would almost certainly lead to new genetic discoveries.

Platelet Prothrombinase Activity and Intracellular Calcium Responses in Patients With Storage Pool Deficiency, Glycoprotein IIb-IIIa Deficiency, or Impaired Platelet Coagulant Activity — A Comparison With Scott Syndrome

The procoagulant activity of platelets induced by collagen, thrombin, and collagen plus thrombin, measured as their capacity to promote the conversion of prothrombin to thrombin in the presence of factors Va and Xa, was studied in patients with α, αδ, and δ storage pool deficiency (SPD), thrombasthenia, and in two new patients with isolated defects in platelet coagulant activity, and compared with that in Scott syndrome. The most significant abnormality in the new patients, whose defect may differ from that in Scott syndrome, is an impairment in collagen plus thrombin–induced prothrombinase activity in the absence of added factor Va. In one of these patients this may be caused by an abnormality in platelet α-granule factor V distinct from that described for factor V Quebec, αδ-SPD, or α-SPD (gray platelet syndrome). Prothrombinase activity in response to all agonists was impaired in δ-SPD and was associated with an inability of these platelets to maintain elevated intracellular calcium levels. Both the rapid decline in agonist-induced [Ca2+]i levels and the impaired prothrombinase activation in δ-SPD platelets were corrected by the addition of adenosine diphosphate (ADP) after stimulation. These findings suggest that secreted ADP may play an important role in the generation of prothrombinase activity by contributing to the maintenance of a critical [Ca2+]i level necessary to maintain aminophospholipids on the outer surface of the platelet membrane, and provide evidence that dense granules may be a major source of ADP which can contribute to calcium influx in stimulated platelets. Parallel alterations, including both increases and decreases, in the [Ca2+]i and prothrombinase responses were also observed in thrombasthenia, depending on the agonist and stirring conditions. Both responses were increased in collagen-stimulated, unstirred platelets, whereas an inability to maintain increased [Ca2+]i levels, associated with decreased prothrombinase activity in all but one atypical patient, was seen in stirred collagen plus thrombin-activated platelets. Although the parallel alterations in these responses in thrombasthenia, as in SPD, further show the close association between the generation of prothrombinase activity and the maintenance of increased intracellular Ca2+ levels, the specific role that GPIIb-IIIa may play in both these events remains unresolved. Our findings of both enhancement and inhibition of these activation-related events in thrombasthenic platelets may be related to previous conflicting reports on the promotion or inhibition of fibrin formation by GPIIb-IIIa, and could be relevant to the use of specific inhibitors of GPIIb-IIIa as antithrombotic agents. In addition, the study provides further support for the concept that the development of agents that could induce a Scott syndrome defect in normal platelets may provide a new approach to antithrombotic therapy.

Platelet alpha granule deficiency associated with decreased P-selectin and selective impairment of thrombin-induced activation in a new patient with gray platelet syndrome (alpha-storage pool deficiency)

We report studies on a new patient with gray platelet syndrome (GPS, alpha-storage pool deficiency). Her lifelong bleeding history is associated with platelet abnormalities characteristic of GPS including mild to moderate thrombocytopenia, a population of abnormally large platelets, and specific deficiencies of alpha-granule constituents and morphologically typical alpha-granules. Platelet function studies showed normal aggregation responses to adenosine diphosphate, epinephrine, collagen, arachidonate, and ristocetin but impaired activation responses to thrombin and a thrombin receptor-activating peptide (T1 peptide). These impaired responses included T1 peptide-induced aggregation, thrombin-induced adenine nucleotide secretion, and thrombin-induced (Ca2+)i increases. The impairment of the thrombin-induced (Ca2+)i increase was observed as a substantially slower initial rise in (Ca2+)i levels and a smaller maximum (Ca2+)i increase compared with the responses obtained in normal platelets and are thus similar to those reported previously in another patients with GPS. Flow cytometric measurements of the binding of two distinct monoclonal antibodies against the functional thrombin receptor indicated the presence of a normal number of receptors and normal receptor cleavage by thrombin in the GPS platelets, providing additional support for the hypothesis presented in previous studies that the thrombin activation defect in GPS platelets occurs subsequent to the interaction of thrombin with its receptor. The alpha-granule deficiency in this patient was associated with an approximately 50% decrease in the content and surface expression of the alpha-granule membrane-specific protein P-selectin in contrast to a previous report of normal amounts of P-selectin in the platelets of two related patients with GPS. This finding raises the possibility that the alpha-granule deficiency in GPS may be expressed in different phenotypes characterized by differences in the amount or constitution of residual alpha-granule membranes present in GPS platelets.

Platelet Prothrombinase Activity and Intracellular Calcium Responses in Patients With Storage Pool Deficiency, Glycoprotein IIb-IIIa Deficiency, or Impaired Platelet Coagulant Activity — A Comparison With Scott Syndrome

The procoagulant activity of platelets induced by collagen, thrombin, and collagen plus thrombin, measured as their capacity to promote the conversion of prothrombin to thrombin in the presence of factors Va and Xa, was studied in patients with α, αδ, and δ storage pool deficiency (SPD), thrombasthenia, and in two new patients with isolated defects in platelet coagulant activity, and compared with that in Scott syndrome. The most significant abnormality in the new patients, whose defect may differ from that in Scott syndrome, is an impairment in collagen plus thrombin–induced prothrombinase activity in the absence of added factor Va. In one of these patients this may be caused by an abnormality in platelet α-granule factor V distinct from that described for factor V Quebec, αδ-SPD, or α-SPD (gray platelet syndrome). Prothrombinase activity in response to all agonists was impaired in δ-SPD and was associated with an inability of these platelets to maintain elevated intracellular calcium levels. Both the rapid decline in agonist-induced [Ca2+]i levels and the impaired prothrombinase activation in δ-SPD platelets were corrected by the addition of adenosine diphosphate (ADP) after stimulation. These findings suggest that secreted ADP may play an important role in the generation of prothrombinase activity by contributing to the maintenance of a critical [Ca2+]i level necessary to maintain aminophospholipids on the outer surface of the platelet membrane, and provide evidence that dense granules may be a major source of ADP which can contribute to calcium influx in stimulated platelets. Parallel alterations, including both increases and decreases, in the [Ca2+]i and prothrombinase responses were also observed in thrombasthenia, depending on the agonist and stirring conditions. Both responses were increased in collagen-stimulated, unstirred platelets, whereas an inability to maintain increased [Ca2+]i levels, associated with decreased prothrombinase activity in all but one atypical patient, was seen in stirred collagen plus thrombin-activated platelets. Although the parallel alterations in these responses in thrombasthenia, as in SPD, further show the close association between the generation of prothrombinase activity and the maintenance of increased intracellular Ca2+ levels, the specific role that GPIIb-IIIa may play in both these events remains unresolved. Our findings of both enhancement and inhibition of these activation-related events in thrombasthenic platelets may be related to previous conflicting reports on the promotion or inhibition of fibrin formation by GPIIb-IIIa, and could be relevant to the use of specific inhibitors of GPIIb-IIIa as antithrombotic agents. In addition, the study provides further support for the concept that the development of agents that could induce a Scott syndrome defect in normal platelets may provide a new approach to antithrombotic therapy.

Megakaryocytes and platelets in alpha-granule disorders

This chapter summarizes research data contributing to current understanding of disorders affecting alpha-granules of megakaryocytes and platelets. Diagnostic features of the gray platelet syndrome are well defined. Combined evidence suggests a defect, specific to the megakaryocyte cell lineage, causing a cytoskeletal abnormality and defective targeting of endogenously synthesized proteins to the alpha-granule. The abnormalities linked by signal transduction pathways. von Willebrand disease and afibrinogenaemia are disorders which highlight the functional importance of platelet storage pools of von Willebrand factor and fibrinogen, essential ligands in the process of adhesion and aggregation. The abnormality in the factor V Quebec disorder leads to a degradation of most proteins contained within the alpha-granule. The familial platelet disorder Paris-Trousseau thrombocytopenia is the only alpha-granule disorder associated with a cytogenetic abnormality, and it presents a useful model for exploring the genetic influence on regulation of thrombopoiesis. Study of these syndromes has elucidated aspects of the physiology of normal megakaryocyte maturation and platelet formation, including storage organelle biosynthesis.

Inherited giant platelet disorders

Giant platelet disorders (GPD) refer to rare, usually inherited states characterized by abnormally large platelets, thrombocytopenia and bleeding tendency of variable severity. This review summarizes major clinical and laboratory features of three GPDs (Bernard-Soulier syndrome, May-Hegglin anomaly and gray platelet syndrome). Differential diagnosis between immunological thrombocytopenia and GPDs is important. Although rare, giant platelet disorders should be borne in mind, since bleeding tendency in some individuals may be severe and knowledge of bleeding diathesis is of importance before delivery or surgical procedures also in less symptomatic individuals.

Gray platelet syndrome with splenomegaly and signs of extramedullary hematopoiesis: a case report with review of the literature

Gray platelet syndrome (GPS) is a rare bleeding disorder characterized by thrombocytopenia, agranular gray platelets in blood films, and almost total absence of platelet alpha-granules and their constituents. We describe here a rare case of GPS with myelofibrosis and splenomegaly indicating extramedullary hematopoiesis. Splenectomy was followed by normalization of platelet count but the bleeding diathesis continued. Based on a follow-up of more than 15 years, the slight myelofibrosis in our patient seems to be non-progressive. We also summarize the major clinical and laboratory features of previously published cases of GPS in order to obtain more comprehensive understanding of this rare disorder. From the clinical point of view, the bleeding tendency in this syndrome generally varies from mild to moderate, and no specific treatment is usually needed. Careful examination of peripheral blood films is necessary for the diagnosis of this rare syndrome.

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.

Grey platelet syndrome: evidence for alpha-granule localization of the platelet plasminogen activator inhibitor-1 pool

The case of an 11-year-old boy with grey platelet syndrome is described. Platelets had the typical grey and ghostly appearance on May-Grünwald/Giemsa staining, caused by the absence of alpha granules confirmed by electron microscopy. Alpha granule protein content, i.e., beta-thromboglobulin and platelet factor 4, was less than 3% of normal and alpha granule secretion in response to thrombin was not detectable photometrically. The plasminogen activator inhibitor-1 pool in the patient's platelets was 5% of normal, confirming previous indirect evidence for the storage of this protein within the alpha-granule. Dense body secretion of adenosine triphosphate and 5-hydroxytryptamine was normal. Aggregation occurred normally in response to adenosine diphosphate and there was a slight delay in response to collagen.

Structural defects in inherited and giant platelet disorders

As diverse as the group of inherited structural defects and giant platelet disorders presented in this chapter may seem, there is a common thread that ties them together. All appear to represent some form of membrane aberration. Sometimes only a small inclusion identifies the membrane defect, sometimes a massive increase in size. In others, whole populations of organelles are missing or surface membranes lack specific glycoproteins essential for their function. All of them are born in the deep recesses of a hidden cell, the bone marrow megakaryocyte. Getting the megakaryocyte out into the light of day, or at least into a culture medium, should certainly lead to the solution of many, if not all, of the disorders of platelet membranes and membrane disorders. We have not been completely successful in our efforts to study the megakaryocyte in vitro. As a result, we do not yet understand the normal megakaryocyte, much less normal platelet. The megakaryocyte presents one of the greatest of challenges to our understanding of membrane biology. As our knowledge of how its cytoplasm fills with interiorly and exteriorly derived membranes, and the mechanisms underlying their organization into platelet surfaces, channels of the OCS and DTS, membrane complexes, and five kinds of organelles become clear, our ability to define the basic nature and inheritance of defects will improve rapidly. Within the next decade most aspects of platelet molecular genetics and cell biology will be solved.

Plasminogen activator inhibitor (PAI-1) in plasma and platelets

The distribution of PAI-1 in the plasma and platelets of normal individuals and of patients with platelet abnormalities was studied. An ELISA, capable of measuring PAI-1 in plasma at 1.5 ng/ml, and a functional assay of t-PA inhibition were used to assay platelet-free plasma (PFP), platelet-rich plasma in which the platelets were lysed (PRP) and serum. The PAI-1 concentration of normal PFP was 21.0 +/- 7.2 ng/ml (mean +/- SD) and those of PRP and serum were 282.6 +/- 68.0 and 270.3 +/- 71.9 ng/ml. The concentration of PAI-1 in PRP was proportional to the platelet count with 0.67 +/- 0.18 ng/10(6) platelets. Patients with thrombocytopenia had approximately normal PAI-1 concentrations in PFP; the extremely low concentrations in serum or PRP reflected the platelet count. A patient with grey platelet syndrome showed a comparable pattern, confirming that PAI-1 occurs in the platelet alpha-granules and indicating that the plasma concentration of PAI-1 is independent of the platelet pool of PAI-1. The median inhibitory activities towards t-PA were 1.6, 8.7 and 8.3 units/ml in normal PFP, PRP and serum respectively. PAI-1 in PFP had a median specific activity (units/mg PAI-1) about 5-fold higher than platelet PAI-1. Plasma and platelets represent two distinct pools of PAI-1, both of which should be considered in studies on the relationship between circulating PAI-1 and thrombotic disease.

Abnormal phosphoinositide metabolism and protein phosphorylation in platelets from a patient with the grey platelet syndrome

Washed platelets isolated from one patient suffering from the inherited grey platelet syndrome were studied during thrombin-induced activation. The agonist-induced changes in (i) morphology, (ii) typical functional cell responses, (iii) membrane phospholipid metabolism and protein phosphorylation were studied and compared with the changes obtained with normal platelets. The morphology of the platelets as visualized by electron microscopy confirmed the almost total absence of intracellular alpha-granules and marked vacuolization. During thrombin stimulation the morphological changes were clearly delayed as compared to normal platelets, the granule centralization and aggregation occurred only 15 s after thrombin addition instead of 5 s in normal platelets. After 15 s, however, even though no alpha-granules were observed, a ring-like structure occurred centrally, indicating that they are not a prerequisite for this reaction. The whole release reaction, i.e. liberation of [14C]serotonin from dense granules and beta-N-acetylglucosaminidase activity from lysosomes, and the thromboxane synthesis were delayed and remained lower than in normal platelets. No thrombin-induced phosphatidyl 4,5-bisphosphate breakdown was measurable on 32P-prelabelled platelets although [32P]phosphatidate formation occurred normally. Phosphorylation time courses of myosin light chain (P20) and of protein P43 (mol wt 43,000) markedly differed from those of controls, being less than half of the normal during the first 15 s and remaining subnormal even after complete aggregation. These results suggest that in platelets devoid of alpha-granules a deficient transmembrane signalling system is likely responsible for the impaired physiological responses.