Inherited defects in platelet signaling mechanisms

Serotonin secretion by blood platelets: accuracy of high-performance liquid chromatography-electrochemical technique compared with the isotopic test and use in a clinical laboratory

Background

Mild secretion defects are the most frequent and challenging blood platelet disorders to diagnose. Most δ-granule secretion tests lack validation, are not quantitative, or have unreliable response to weak platelet agonists.

Objectives

To compare platelet serotonin secretion by HPLC-electrochemical detection technique (HPLC-ECD) with the reference isotopic test (3H-5-HT), evaluating its performance in clinical laboratories.

Methods

The assay validation followed STARD-2015 recommendations. HPLC-ECD measured the nonsecreted serotonin remaining in platelet pellets after aggregation, comparing it with the reference 3H-5-HT assay. We studied subjects with inherited and aspirin-induced blood platelet disorders and assessed the HPLC-ECD operation for routine clinical diagnosis.

Results

Calibration curves were linear (R2 = 0.997), with SD for residuals of 3.91% and analytical sensitivity of 5ng/mL. Intra- and interassay imprecision bias ranged between -8.5% and 2.1% and -9% and 3.1%, respectively. Serotonin recovery and stability were >95%, and the variability range of measurements was -5.5% to 4.6%. Statistical differences detected between tests were biologically irrelevant, with bias of 1.48% (SD, 8.43) and CI agreement of -18% to 15%. Both assays distinctly detected platelet secretion induced by 10 μM epinephrine and 4 μmM adenosine diphosphate. However, HPLC-ECD is quantitative and more sensitive to low serotonin content in blood platelets. Reference cutoffs for each agonist were determined in 87 subjects. Initially, the HPLC-ECD requires relatively expensive equipment and trained operators but has remarkably cheap running costs and a turn-around time of 24-36 hours. We have used this diagnostic tool routinely for >8 years.

Conclusion

HPLC-ECD assay for platelet serotonin secretion is highly accurate, has advantages over the reference 3H-5-HT test, and is suitable as a clinical laboratory technique.

Inherited platelet diseases with normal platelet count: phenotypes, genotypes and diagnostic strategy

Inherited platelet disorders resulting from platelet function defects and a normal platelet count cause a moderate or severe bleeding diathesis. Since the description of Glanzmann thrombasthenia resulting from defects of ITGA2B and ITGB3, new inherited platelet disorders have been discovered, facilitated by the use of high throughput sequencing and genomic analyses. Defects of RASGRP2 and FERMT3 responsible for severe bleeding syndromes and integrin activation have illustrated the critical role of signaling molecules. Important are mutations of P2RY12 encoding the major ADP receptor causal for an inherited platelet disorder with inheritance characteristics that depend on the variant identified. Interestingly, variants of GP6 encoding the major subunit of the collagen receptor GPVI/FcRγ associate only with mild bleeding. The numbers of genes involved in dense granule defects including Hermansky-Pudlak and Chediak Higashi syndromes continue to progress and are updated. The ANO6 gene encoding a Ca2+-activated ion channel required for phospholipid scrambling is responsible for the rare Scott syndrome and decreased procoagulant activity. A novel EPHB2 defect in a familial bleeding syndrome demonstrates a role for this tyrosine kinase receptor independent of the classical model of its interaction with ephrins. Such advances highlight the large diversity of variants affecting platelet function but not their production, despite the difficulties in establishing a clear phenotype when few families are affected. They have provided insights into essential pathways of platelet function and have been at the origin of new and improved therapies for ischemic disease. Nevertheless, many patients remain without a diagnosis and requiring new strategies that are now discussed.

Platelet Disorders

After vascular injury and exposure of subendothelial matrix proteins to the intravascular space, mediators of hemostasis are triggered and allow for clot formation and restoration of vascular integrity. Platelets are the mediators of primary hemostasis, creating a platelet plug and allowing for initial cessation of bleeding. Platelet disorders, qualitative and quantitative, may result in bleeding signs and symptoms, particularly mucocutaneous bleeding such as epistaxis, bruising, petechiae, and heavy menstrual bleeding. Increasing evidence suggests that platelets have functional capabilities beyond hemostasis, but this review focuses solely on platelet hemostatic properties. Herein, normal platelet function as well as the effects of abnormal function and thrombocytopenia are reviewed.

An integrated approach to inherited platelet disorders: results from a research collaborative, the Sydney Platelet Group

Inherited disorders of platelet function (IPFD) and/or number (IPND) are heterogeneous conditions that result in variable mucocutaneous bleeding symptoms as a result of deranged primary haemostasis caused by platelet dysfunction or thrombocytopenia. Diagnosis is important to guide post-operative bleeding prophylactic strategies, to avoid treatment with inappropriate medications, and inform prognosis. Achieving an accurate diagnosis has traditionally been hampered by the requirement of multiple, often complex, laboratory tests that are not always available at single centres. To improve the diagnosis of these disorders a research collaborative was established, the Sydney Platelet Group, that explored an integrated approach combining traditional and contemporary platelet phenotypic and genetic diagnostic platforms available at four Sydney tertiary hospitals. Herein we report the outcomes of the first 50 patients evaluated using this approach. The cohort included 22 individuals with suspected IPFD and 28 with thrombocytopenia. Bleeding scores were higher in individuals with IPFD (mean 5.75; SD 4.83) than those with IPNDs (mean 2.14; SD 2.45). In cases with suspected IPFD, diagnosis to the level of the defective pathway was achieved in 71% and four individuals were found not to have a definitive platelet function defect. Dense granule secretion disorders were the most common platelet pathway abnormality detected (n=5). Mean bleeding scores in these individuals were not significantly different to individuals with defects in other commonly detected platelet pathways (dense granules, signal transduction and 'undetermined'). A molecular diagnosis was achieved in 52% of individuals with IPNDs and 5% with IPFD. Likely pathogenic and pathogenic variants detected included variants associated with extra-haematological complications (DIAPH1, MYH9) and potential for malignancy (ANKRD26 and RUNX1). The level of platelet investigation undertaken by this initiative is currently not available elsewhere in Australia and initial results confirm the utility of this integrated phenotypic-genetic approach.

Congenital Disorders of Platelet Function and Number

Mucocutaneous bleeding symptoms and/or persistent thrombocytopenia occur in individuals with congenital disorders of platelet function and number. Apart from bleeding, these disorders are often associated with additional hematologic and clinical manifestations, including auditory, immunologic, and oncologic disease. Autosomal recessive, dominant, and X-linked inheritance patterns have been demonstrated. Precise delineation of the molecular cause of the platelet disorder can aid the pediatrician in the detection and prevention of specific disorder-associated manifestations and guide appropriate treatment and anticipatory care for the patient and family.

OP9 bone marrow stroma cells differentiate into megakaryocytes and platelets

Platelets are essential for hemostatic plug formation and thrombosis. The mechanisms of megakaryocyte (MK) differentiation and subsequent platelet production from stem cells remain only partially understood. The manufacture of megakaryocytes (MKs) and platelets from cell sources including hematopoietic stem cells and pluripotent stem cells have been highlighted for studying the platelet production mechanisms as well as for the development of new strategies for platelet transfusion. The mouse bone marrow stroma cell line OP9 has been widely used as feeder cells for the differentiation of stem cells into MK lineages. OP9 cells are reported to be pre-adipocytes. We previously reported that 3T3-L1 pre-adipocytes differentiated into MKs and platelets. In the present study, we examined whether OP9 cells differentiate into MKs and platelets using MK lineage induction (MKLI) medium previously established to generate MKs and platelets from hematopoietic stem cells, embryonic stem cells, and pre-adipocytes. OP9 cells cultured in MKLI medium had megakaryocytic features, i.e., positivity for surface markers CD41 and CD42b, polyploidy, and distinct morphology. The OP9-derived platelets had functional characteristics, providing the first evidence for the differentiation of OP9 cells into MKs and platelets. We then analyzed gene expressions of critical factors that regulate megakaryopoiesis and thrombopoiesis. The gene expressions of p45NF-E2, FOG, Fli1, GATA2, RUNX1, thrombopoietin, and c-mpl were observed during the MK differentiation. Among the observed transcription factors of MK lineages, p45NF-E2 expression was increased during differentiation. We further studied MK and platelet generation using p45NF-E2-overexpressing OP9 cells. OP9 cells transfected with p45NF-E2 had enhanced production of MKs and platelets. Our findings revealed that OP9 cells differentiated into MKs and platelets in vitro. OP9 cells have critical factors for megakaryopoiesis and thrombopoiesis, which might be involved in a mechanism of this differentiation. p45NF-E2 might also play important roles in the differentiation of OP9 cells into MK lineages cells.

Is my patient a bleeder? A diagnostic framework for mild bleeding disorders

Congenital mild bleeding disorders (MBDs) are very prevalent and are the source of frequent diagnostic problems. Most MBDs are categorized as disorders of primary hemostasis (ie, type 1 VWD and platelet function disorders), but mild or moderate deficiencies of clotting factors and some rare hyperfibrinolytic disorders are also included. These patients have abnormal bleeding from the skin and mucous membranes, menorrhagia, and disproportionate hemorrhages after trauma, invasive procedures, and surgery. This review addresses the main problems that physicians and hemostasis laboratories confront with the diagnosis of these patients, including: discerning normal/appropriate from pathological bleeding, the role and yield of screening tests, the lack of distinctive bleeding pattern among the different diseases, the inherent difficulties in the diagnosis of type 1 VWD and the most common platelet functional disorders, improvements in assays to measure platelet aggregation and secretion, and the evidence that most of the patients with MBDs end up without a definite diagnosis after exhaustive and repeated laboratory testing. Much research is needed to determine the pathogenesis of bleeding in MBD patients. Better standardization of current laboratory assays, progress in the knowledge of fibrinolytic mechanisms and their laboratory evaluation, and new understanding of the factors contributing to platelet-vessel wall interaction, along with the corresponding development of laboratory tools, should improve our capacity to diagnose a greater proportion of patients with MBDs.

Advances in our understanding of the molecular basis of disorders of platelet function

Genetic defects of platelet function give rise to mucocutaneous bleeding of varying severity because platelets fail to fulfil their haemostatic role after vessel injury. Abnormalities of pathways involving glycoprotein (GP) mediators of adhesion (Bernard-Soulier syndrome, platelet-type von Willebrand disease) and aggregation (Glanzmann thrombasthenia) are the most studied and affect the GPIb-IX-V complex and integrin αIIbβ3, respectively. Leukocyte adhesion deficiency-III combines Glanzmann thrombasthenia with infections and defects of kindlin-3, a mediator of integrin activation. Agonist-specific deficiencies in platelet aggregation relate to mutations of primary receptors for ADP (P2Y(12)), thromboxane A(2) (TXA2R) and collagen (GPVI); however, selective abnormalities of intracellular signalling pathways remain better understood in mouse models. Defects of secretion from δ-granules are accompanied by pigment defects in the Hermansky-Pudlak and Chediak-Higashi syndromes; they concern multiple genes and protein complexes involved in secretory organelle biogenesis and function. Quebec syndrome is linked to a tandem duplication of the urokinase plasminogen activator (PLAU) gene while locus assignment to chromosome 3p has advanced the search for the gene(s) responsible for α-granule deficiency in the gray platelet syndrome. Defects of α-granule biosynthesis also involve germline VPS33B mutations in the ARC (arthrogryposis, renal dysfunction and cholestasis) syndrome. A mutation in transmembrane protein 16F (TMEM16F) has been linked to a defective procoagulant activity and phosphatidylserine expression in the Scott syndrome. Cytoskeletal dysfunction (with platelet anisotrophy) occurs not only in the Wiskott-Aldrich syndrome but also in filamin A deficiency or MYH9-related disease while GATA1 mutations or RUNX1 haploinsufficiency can affect expression of multiple platelet proteins.

Diagnosis and Management of Inherited Platelet Disorders

In clinical daily practice the definition of a bleeding tendency is rather subjective. Clinical manifestations usually include hematoma, epistaxis, menorrhagia, and severe bleeding episodes after surgery or injuries. The most common causes are disorders of primary hemostasis that occur sometimes due to platelet function disorders. Inherited thrombocytopathies are much less frequent in comparison to acquired platelet function disorders. However, congenital disorders can lead to severe bleeding tendency and are often not diagnosed. They are induced by different platelet defects based on disorders of platelet adhesion, receptors, secretion, and signal transduction. In some cases, they are associated with thrombocytopenias, giant platelets, and various comorbidities. This article gives an overview of the different defects, their diagnosis, and treatment options.

Inherited disorders of platelets: an update

PURPOSE OF REVIEW

To overview inherited syndromes that affect platelets and to discuss current data on the molecular origin and management of these rare diseases.

RECENT FINDINGS

An increasing number of genes responsible for inherited thrombocytopenias have been identified and these now extend to glycosylation defects. Although Glanzmann thrombasthenia remains the predominant disorder of platelet function, knowledge is increasing of pathologies concerning primary receptors for adhesion and signalling, the activation and secretory pathways, and even the development of procoagulant activity.

SUMMARY

These syndromes affect cell adhesion, cell activation, and cell-to-cell contact interactions fundamental in cell biology. Studies on the pathophysiology of alphaIIbbeta3 in platelets have helped unravel the molecular mechanisms of integrin function, and the information gained has resulted in improved antithrombotic therapy. The establishment of national registries and the use of state-of-the-art genomic and proteomic technologies will accelerate progress and help to define how mutations affecting a much larger range of proteins contribute alone or in combination to defining specific platelet phenotypes.

Leukocyte count and leukocyte ecto-nucleotidase are major determinants of the effects of adenosine triphosphate and adenosine diphosphate on platelet aggregation in human blood

ADP induces platelet aggregation in human whole blood and platelet-rich plasma (PRP). ATP induces aggregation in whole blood only; this involves leukocytes and is mediated by ADP. Here we studied ATP- and ADP-induced aggregation in patients with raised leukocyte counts (mean 46.2x10(3) leukocytes/microl). Platelet aggregation was measured by platelet counting. ATP, ADP and metabolites were measured by HPLC. Aggregation to ADP (1-10 microM) and ATP (10-100 microM) was markedly reduced, but to ATP (1000 microM) was enhanced (all p<0.001). Aggregation to ADP in PRP was normal. Increasing the leukocyte count in normal blood reproduced the findings in the patients. Adding leukocytes (either MNLs or PMNLs) to normal PRP enabled a response to ATP and caused marked inhibition of ADP-induced aggregation. Breakdown of ATP or ADP to AMP and adenosine in leukocyte-rich plasma was rapid (t1/2=4 min) and far higher than in cell-free plasma or PRP. With ATP there was also formation of ADP, maximal at 4 min. The presence of the ectonucleotidase NTPDase1 (CD39) was demonstrated on MNLs (all of the monocytes and a proportion of the lymphocytes) and all PMNLs by flow cytometry. We conclude that leukocytes provide a means of dephosphorylating ATP which enables ATP-induced aggregation via conversion to ADP, but also convert ADP to AMP and adenosine. Platelet aggregation extent is a balance between these activities, and high white cell counts influence this balance.

Adenine nucleotide metabolism in human blood--important roles for leukocytes and erythrocytes

Adenosine diphosphate (ADP) released into blood induces platelet aggregation and contributes to hemostasis and thrombosis. Released ATP can also induce platelet aggregation and there is evidence that blood leukocytes and also erythrocytes play important roles in this. Rapid metabolism of ADP and ATP by endothelial cells is important in protecting platelets from their effects. Here we have performed a systematic investigation of adenine nucleotide metabolism in human blood and the involvement of blood cells. Conversion of ATP to ADP in blood was due almost exclusively to the presence of leukocytes; plasma, platelets and erythrocytes made little or no contribution. Mononuclear leukocytes (MNLs) and polymorphonuclear leukocytes (PMNLs) were equally effective. Conversion of ADP to AMP was also promoted by leukocytes, with no involvement of platelets or erythrocytes. Some ADP was also converted to ATP in blood, apparently via an enzyme present in plasma, but ATP was then rapidly removed by the leukocytes. Conversion of AMP to adenosine occurred via a plasma enzyme with little or no contribution from any cellular element. As expected, in blood the adenosine produced was removed very rapidly by erythrocytes and then converted to inosine and then hypoxanthine. In the absence of erythrocytes plasma supported only a slow conversion of adenosine to inosine and hypoxanthine, which was not influenced by platelets or leukocytes. This study has demonstrated that leukocytes and erythrocytes play a major role in adenine nucleotide metabolism in blood and that these cells, as well as endothelial cells, may be important determinants of the effects of ATP and ADP on platelets.

Qualitative disorders of platelets and megakaryocytes

Qualitative disorders of platelet function and production form a large group of rare diseases which cover a multitude of genetic defects that by and large have as a common symptom, excessive mucocutaneous bleeding. Glanzmann thrombasthenia, is enabling us to learn much about the pathophysiology of integrins and of how alphaIIb beta3 functions. Bernard-Soulier syndrome, an example of macrothrombocytopenia, combines the production of large platelets with a deficit or non-functioning of the major adhesion receptor of platelets, the GPIb-IX-V complex. Amino acid substitutions in GPIb alpha, may lead to up-regulation and spontaneous binding of von Willebrand factor as in Platelet-type von Willebrand disease. In disorders with defects in the MYH9 gene, macrothrombocytopenias are linked to modifications in kidney, eye or ear, whereas other inherited thrombocytopenias variously link a low platelet count with a propensity to leukemia, skeletal defects, learning impairment, and abnormal red cells. Defects of secretion from platelets include an abnormal alpha-granule formation as in the gray platelet syndrome (with marrow myelofibrosis), and of organelle biogenesis in the Hermansky-Pudlak and Chediak-Higashi syndromes where platelet dense body defects are linked to abnormalities of other lysosomal-like organelles including melanosomes. Finally, defects involving surface receptors (P2Y(12), TPalpha) for activating stimuli, of proteins essential for signaling pathways (including Wiskott-Aldrich syndrome), and of platelet-derived procoagulant activity (Scott syndrome) show how studies on platelet disorders are helping unravel the pathways of primary hemostasis.

Antiplatelet Agents: Current Drugs and Future Trends

Antiplatelet drugs in clinical use are discussed in terms of their mechanisms of action and the relevancy of that to the physiology of platelets and the pathophysiology of arterial thrombosis. Current clinical usage is outlined in detail for each drug. Experimental antiplatelet drugs also are discussed.

Inherited Platelet Disorders

The inherited platelet disorders are a heterogeneous collection of rare diseases that are infrequently encountered in clinical practice. They are, however, fascinating abnormalities, which have taught us a great deal about normal platelet biochemistry and physiology. In this section of the presentation we will review disorders of the platelet membrane, platelet granule packaging disorders, the hereditary macrothrombocytopenias, platelet signaling disorders and disorders of platelet coagulant function. The molecular basis of the disorders, the cardinal features of their clinical presentation and best methods to make their diagnosis and the latest information regarding therapy will be presented.

A platelet secretion pathway mediated by cGMP-dependent protein kinase

Platelet secretion (exocytosis) is critical in amplifying platelet activation, in stabilizing thrombi, and in arteriosclerosis and vascular remodeling. The signaling mechanisms leading to secretion have not been well defined. We have shown previously that cGMP-dependent protein kinase (PKG) plays a stimulatory role in platelet activation via the glycoprotein Ib-IX pathway. Here we show that PKG also plays an important stimulatory role in mediating aggregation-dependent platelet secretion and secretion-dependent second wave platelet aggregation, particularly those induced via Gq-coupled agonist receptors, the thromboxane A2 (TXA2) receptor, and protease-activated receptors (PARs). PKG I knock-out mouse platelets and PKG inhibitor-treated human platelets showed diminished aggregation-dependent secretion and also showed a diminished secondary wave of platelet aggregation induced by a TXA2 analog and thrombin receptor-activating peptides that were rescued by the granule content ADP. Low dose collagen-induced platelet secretion and aggregation were also reduced by PKG inhibitors. Furthermore PKG I knockout and PKG inhibitors significantly attenuated activation of the Gi pathway that is mediated by secreted ADP. These data unveil a novel PKG-dependent platelet secretion pathway and a mechanism by which PKG promotes platelet activation.

Mechanisms involved in adenosine triphosphate--induced platelet aggregation in whole blood

OBJECTIVE

Effects on platelet aggregation of adenosine triphosphate (ATP) released from damaged cells and from platelets undergoing exocytosis have not been clearly established. In this study we report on the effects of ATP on platelet aggregation in whole blood.

METHODS AND RESULTS

Aggregation, measured using a platelet-counting technique, occurred in response to ATP and was maximal at 10 to 100 micromol/L. It was abolished by MRS2179, AR-C69931, and creatine phosphate/creatine phosphokinase, implying that conversion to adenosine diphosphate (ADP) is required. ATP did not induce aggregation in platelet-rich plasma, but aggregation did occur when apyrase or hexokinase was added. Aggregation also occurred after addition of leukocytes to platelet-rich plasma (as a source of ecto-ATPase), and this was potentiated on removal of adenosine by adenosine deaminase, indicating that adenosine production modulates the response. Dipyridamole, which inhibits adenosine uptake into erythrocytes, inhibited aggregation induced by ATP in whole blood, and adenosine deaminase reversed this. DN9693 and forskolin synergized with dipyridamole to inhibit ATP-induced aggregation.

CONCLUSIONS

ATP induces aggregation in whole blood via conversion of ATP to ADP by ecto-ATPases on leukocytes. This is inhibited by agents that prevent adenosine removal. Reduced aggregation at high concentrations of ATP (>100 micromol/L) may be a consequence of inhibition by ATP of ADP action at ADP receptors.