Rho GTPase regulation of reactive oxygen species generation and signalling in platelet function and disease.

Platelets are master regulators and effectors of haemostasis with increasingly recognized functions as mediators of inflammation and immune responses. The Rho family of GTPase members Rac1, Cdc42 and RhoA are known to be major components of the intracellular signalling network critical to platelet shape change and morphological dynamics, thus playing a major role in platelet spreading, secretion and thrombus formation. Initially linked to the regulation of actomyosin contraction and lamellipodia formation, recent reports have uncovered non-canonical functions of platelet RhoGTPases in the regulation of reactive oxygen species (ROS), where intrinsically generated ROS modulate platelet function and contribute to thrombus formation. Platelet RhoGTPases orchestrate oxidative processes and cytoskeletal rearrangement in an interconnected manner to regulate intracellular signalling networks underlying platelet activity and thrombus formation. Herein we review our current knowledge of the regulation of platelet ROS generation by RhoGTPases and their relationship with platelet cytoskeletal reorganization, activation and function.

Clinical and laboratory phenotypes associated with the aspirin-like defect: a study in 17 unrelated families.

Aspirin-like defect (ALD) is a rare, mostly autosomal dominant inherited dysfunction of the intraplatelet arachidonic acid (AA) pathway leading to impaired thromboxane A2 signalling. We aimed to establish diagnostic criteria for ALD diagnosis and present clinical and laboratory phenotypes of 52 individuals from 17 unrelated families. Platelet in vitro function was determined on the basis of platelet aggregation response (PAR) to AA, adenosine diphosphate, collagen and ristocetin as well as PFA-100 closure times (CT). Using impaired PAR to AA (< or =10%) as the mandatory diagnostic criterion, ALD could be confirmed in 17 patients. Subsequently, family members were investigated and among 35 individuals an additional 13 ALD patients as well as 4 individuals with mild ALD (PAR to AA: 19-32%) were identified. At least one bleeding symptom was reported by 25 (74%) ALD patients and prolonged CT was detected in 24 (71%) of the cases, both significantly correlated with impaired PAR to AA (P = 0.001 and P = 0.002, respectively). An estimated 0.6% prevalence was determined for ALD in our paediatric patients with suspected coagulation disorders. Due to the mild bleeding symptoms, ALD is probably underdiagnosed. If ALD is suspected, PAR to AA is suitable for the identification of individuals at risk of increased haemorrhage.

[Antiplatelet therapy can unmask an inherited bleeding disorder. Aspirin-like defect of platelets does not protect against atherosclerosis]. / Leczenie przeciwplytkowe moze ujawnic wrodzona skaze krwotoczna. Aspirynopodobny defekt plytek krwi nie chroni przed miazdzyca

Some inherited platelet disorders may be revealed late, as in the presented case of a 68-year-old-man. Recurrent epistaxis following peri-interventional antiplatelet therapy (after three elective percutaneous coronary interventions) and an episode of upper gastrointestinal haemorrhage required aspirin withdrawal and less frequent clopidogrel use. Platelet studies showed an aspirin-like defect resulting in a lack of arachidonate-induced platelet aggregation. During dose-reduced (2-3 times a week) clopidogrel administration ADP-induced platelet aggregation was effectively inhibited and neither important bleeding nor stent thrombosis occurred. The inherited defect of cyclooxygenase-1, responsible for platelet thromboxane synthesis, did not protect the patient against coronary and extra-cardiac atherosclerosis.

Localization of a lysosomal enzyme in platelets from patients with the White platelet syndrome.

The White platelet syndrome (WPS) is an autosomal dominant platelet disorder. Platelet structural abnormalities include the presence of Golgi complexes in up to 13% of their cells, frequently accompanied by centrioles, deficient numbers or absent alpha granules in 30-40% of their platelets and masses of dense tubular system (DTS) channels often forming areas of cytoplasmic sequestration. The degradation of cytoplasm and organelles in the sequestered areas suggested the possibility that hydrolytic enzymes remained in the DTS and were being transferred to sequestration vacuoles. The present study has used ultrastructural cytochemistry to localize the sites of a lysosomal enzyme, aryl sulfatase, in normal and WPS platelets. Enzyme reaction product, lead sulfide, was localized to lysosomal organelles in normal platelets, and only in rare examples appeared in the DTS. Aryl sulfatase activity was found in the Golgi complexes, a few lysosomes, much of the DTS and areas of cytoplasmic sequestration in WPS platelets. The findings indicate that aryl sulfatase, and, most likely, proteins destined for alpha granules in WPS platelets are not completely transferred from the endoplasmic reticulum to Golgi complexes, then to Golgi vesicles and finally to lysosomal and alpha granules in the parent cell before the platelets are delivered to the circulation.

Platelets from patients with the Quebec platelet disorder contain and secrete abnormal amounts of urokinase-type plasminogen activator.

The Quebec platelet disorder (QPD) is an autosomal dominant platelet disorder associated with delayed bleeding and alpha-granule protein degradation. The degradation of alpha-granule, but not plasma, fibrinogen in patients with the QPD led to the investigation of their platelets for a protease defect. Unlike normal platelets, QPD platelets contained large amounts of fibrinolytic serine proteases that had properties of plasminogen activators. Western blot analysis, zymography, and immunodepletion experiments indicated this was because QPD platelets contained large amounts of urokinase-type plasminogen activator (u-PA) within a secretory compartment. u-PA antigen was not increased in all QPD plasmas, whereas it was increased more than 100-fold in QPD platelets (P <.00009), which contained increased u-PA messenger RNA. Although QPD platelets contained 2-fold more plasminogen activator inhibitor 1 (PAI-1) (P <.0008) and 100-fold greater u-PA-PAI-1 complexes (P <.0002) than normal platelets, they contained excess u-PA activity, predominantly in the form of two chain (tcu-PA), which required additional PAI-1 for full inhibition. There was associated proteolysis of plasminogen in QPD platelets, to forms that comigrated with plasmin. When similar amounts of tcu-PA were incubated with normal platelet secretory proteins, many alpha-granule proteins were proteolyzed to forms that resembled degraded QPD platelet proteins. These data implicate u-PA in the pathogenesis of alpha-granule protein degradation in the QPD. Although patients with the QPD have normal to increased u-PA levels in their plasma, without evidence of systemic fibrinogenolysis, their increased platelet u-PA could contribute to bleeding by accelerating fibrinolysis within the hemostatic plug. QPD is the only inherited bleeding disorder in humans known to be associated with increased u-PA.

Arg60 Leu mutation in the first cytoplasmic loop of the platelet thromboxane A2 receptor is not essential for mediating inhibitory coupling between the receptor and adenylyl cyclase.

Human platelet thromboxane A2 (TXA2) receptor (TXR) has been reported to functionally couple to the inhibitory GTP-binding protein for adenylyl cyclase (Gi). However, it still remains unclear which portions of the TXR structure are critical determinants in that coupling. We have previously reported several patients with platelet dysfunction, whose platelets showed impaired coupling between TXR and phospholipase C caused by an Arg60 to Leu mutation in the first cytoplasmic loop. To investigate whether this portion is essential for mediating inhibitory coupling between TXR and adenylyl cyclase, we analyzed the inhibition by the TXA2 analog of the PGE1 or forskolin-induced platelet cAMP increase in patients' platelets, and found that the inhibition occurred normally. This suggests that Arg60 in the first cytoplasmic loop of the TXR is not involved in TXR-Gi coupling.

Aminophospholipid exposure, microvesiculation and abnormal protein tyrosine phosphorylation in the platelets of a patient with Scott syndrome: a study using physiologic agonists and local anaesthetics.

The Scott syndrome is a rare inherited haemorrhagic disorder characterized by the inability of blood cells to expose aminophospholipids and to shed microparticles. We have had the opportunity to study a recently reported French patient with this syndrome and have confirmed by means of a fluorescence assay for transbilayer lipid movement a reduced aminophospholipid exposure when platelets were stimulated with the calcium-ionophore ionomycin, in spite of a normal elevation of intracellular Ca2+. Secretion and calpain activation were also shown to be normal. Significantly, the level of phosphotyrosine-labelled proteins in platelets treated with thrombin or a thrombin + collagen mixture and in particular the phosphorylation of a 40 kD band were severely reduced. Furthermore, inhibition of thiol-containing enzymes. including tyrosine-phosphatases, by N-ethyl maleimide did not lead to aminophospholipid exposure in the patient's platelets, in spite of increased tyrosine protein phosphorylation. In contrast, amphiphilic membrane drugs such as tetracaine and propranolol induced both surface aminophospholipid exposure in Scott platelets and the shedding of microparticles, thereby showing that membrane perturbation can lead to loss of phospholipid asymmetry in this syndrome. Our results provide the first insight that the lack of expression of procoagulant phospholipids and microparticle formation in Scott syndrome platelets is associated with a defect of intracellular signalling.

Platelet lipoxygenase defect (Wien-Penzing defect) in two patients with myocardial infarction.

In 2 male patients (35 and 38 years) presenting with myocardial infarction an abnormal conversion of exogenous 14C-arachidonic acid by the patients' platelets, incubated in vitro, was observed. Neither patient's platelets showed evidence of a lipoxygenase pathway. Platelet thromboxane formation from exogenous and endogenous substrate was high, while the platelet aggregation responses were normal. A myeloproliferative syndrome was excluded by bone marrow puncture. Similar defects have only been described so far in patients with myeloproliferative syndrome. This defect may be causative for the onset of clinical thrombotic events. It is speculative whether in vivo therapy with r-IFN alpha 1c might be able to eradicate the pathological platelet clone.

Coamplification of human acetylcholinesterase and butyrylcholinesterase genes in blood cells: correlation with various leukemias and abnormal megakaryocytopoiesis.

To study the yet unknown role of the ubiquitous family of cholinesterases (ChoEases) in developing blood cells, the recently isolated cDNAs encoding human acetylcholinesterase (AcChoEase; acetylcholine acetylhydrolase, EC 3.1.1.7) and butyrylcholinesterase (BtChoEase; cholinesterase; acylcholine acylhydrolase, EC 3.1.1.8) were used in blot hybridization with peripheral blood DNA from various leukemic patients. Hybridization signals (10- to 200-fold intensified) and modified restriction patterns were observed with both cDNA probes in 4 of the 16 leukemia DNA preparations examined. These reflected the amplification of the corresponding AcChoEase and BtChoEase genes (ACHE and CHE) and alteration in their structure. Parallel analysis of 30 control samples revealed nonpolymorphic, much weaker hybridization signals for each of the probes. In view of previous reports on the effect of acetylcholine analogs and ChoEase inhibitors in the induction of megakaryocytopoiesis and production of platelets in the mouse, we further searched for such phenomena in nonleukemic patients with platelet production disorders. Amplifications of both ACHE and CHE genes were found in 2 of the 4 patients so far examined. Pronounced coamplification of these two related but distinct genes in correlation with pathological production of blood cells suggests a functional role for members of the ChoEase family in megakaryocytopoiesis and raises the question whether the coamplification of these genes could be causally involved in the etiology of hemocytopoietic disorders.

Specific correction of impaired acid hydrolase secretion in storage pool-deficient platelets by adenosine diphosphate.

Storage pool-deficient (SPD) platelets, which have decreased amounts of dense-granule and/or alpha-granule constituents, contain normal amounts of lysosomal acid hydrolases, but in some cases exhibit impaired secretion of these enzymes. We examined this impaired secretion response in SPD patients with varying extents of granule deficiencies, and determined the effects of added dense-granule constituents. Acid hydrolase secretion was impaired in patients with severe dense-granule deficiencies, but not in patients with lesser dense-granule deficiencies, including those with alpha-granule deficiencies as well. When dense-granule constituents (ADP, ATP, serotonin, Ca+2, pyrophosphate) were added to gel-filtered platelets, ADP, but none of the other constituents, completely corrected the impairment of thrombin and A23187-induced secretion in SPD platelets. The concentration of ADP required to normalize thrombin-induced secretion varied markedly, from 0.01 to 10 microM, among the individual patients. Fixation of platelets with formaldehyde before centrifugation did not prevent the enhancement of secretion by ADP. Excess ATP, which acts as a specific antagonist of ADP-mediated responses, completely blocked this enhancement of secretion in SPD platelets by ADP, and partially inhibited acid hydrolase secretion induced by low, but not high, concentrations of thrombin in normal platelets as well. Treatment of normal platelets with acetylsalicylic acid in vivo, but not in vitro, produced an impairment of acid hydrolase secretion similar in extent to that in SPD platelets, but which could not be completely corrected by added ADP. One possible explanation of these results is that the impairment of acid hydrolase secretion may be secondary to the dense-granule deficiency in SPD platelets, and that secreted ADP may potentiate the lysosomal secretion response in normal platelets as well.

Enzymological and immunological studies on a clinical case of platelet cyclooxygenase abnormality.

A peroxidase-linked immunoassay of the sandwich type was developed for a quantitative determination of the amount of human cyclooxygenase. Two species of monoclonal antibodies (hPES01 against the human enzyme and PES-5 against the bovine enzyme) were utilized, which recognized different epitopes on the cyclooxygenase of human platelets. The peroxidase activity of the immunoprecipitate was correlated with the amount of cyclooxygenase. The enzyme immunoassay was applied to platelets from 15 normal subjects and a clinical case of platelet cyclooxygenase abnormality with a prolonged bleeding time. Almost the same level of immunoreactive protein was found in platelets of both normal subjects and the patient. However, the solubilized enzyme from the patient's platelets did not transform arachidonic acid to prostaglandin H2 (PGH2) while thromboxane production from PGH2 was observed at a normal level.

Thioredoxin reductase activity in Hermansky-Pudlak syndrome: a method for identification of putative heterozygotes.

Recent studies indicate that membrane-associated thioredoxin reductase (TR) is a possible regulator of melanin biosynthesis via the inhibition of tyrosinase by reduced thioredoxin. In normal individuals, the levels of TR activity in skin correlate linearly to the Fitzpatrick classification of skin type, being lowest in type I skin and highest in skin type VI. In this study, TR was measured in 3-mm skin biopsies in Hermansky-Pudlak syndrome (HPS) patients and their relatives. Forty-five individuals from seven Puerto Rican kindreds were tested, including 12 homozygotes, nine obligate heterozygotes, and 24 unclassified individuals. In addition, seven separate nonkindred HPS patients were tested. With one exception, TR activity was markedly decreased in 18 homozygotes. TR activity was decreased in eight obligate heterozygotes and in 12 unclassified kindred members, whereas 10 subjects had normal TR activity when compared to the expected activity of their skin type. Four individuals were excluded from the analysis because of inadequate controls for their age group or immunosuppressive treatment for kidney transplant. The results indicate that decreased TR activity assayed in 3-mm skin punch biopsies is a useful method for detecting carriers of the HPS gene.

Abnormal calcium transport into microsomes of grey platelet syndrome.

Calcium uptake into isolated membrane vesicles from two patients with a grey platelet syndrome has been investigated. An increase in calcium transport appears in both patients when compared to controls. Determination of the kinetic parameters of the calcium transport system gave similar apparent affinity for calcium and an increase in the calcium uptake velocity. This increase in calcium transport is correlated with the increase of the associated Ca2+ activated ATPase activity. The results would suggest a new relationship between the ultrastructural and functional abnormalities of the grey platelet syndrome.

Evidence for regulatory control of canine platelet phosphodiesterase.

Forskolin, epinephrine, and prostaglandin I2 were used to examine the adenylate cyclase-phosphodiesterase system of intact thrombopathic and normal canine platelets. The results provide indirect support for the hypothesis that the elevation of intraplatelet c-AMP in this unique hereditary defect is due to impaired phosphodiesterase activity. The inhibitory (Nj) and stimulatory (Ns) components of adenylate cyclase appeared functionally intact. Cytosolic fractions of normal and thrombopathic platelets had similar cAMP hydrolytic activities. The failure of intact forskolin-stimulated thrombopathic platelets to return elevated cAMP to non-stimulated levels after 15 min, despite significant phosphodiesterase activity in cytosolic fractions, implies that the platelet isoenzymes are under regulatory control.

Reduced platelet sialyltransferase activity in patients with primary release disorder.

Platelet sialyltransferase activity was determined in four patients with primary platelet release disorder. Basal enzyme activity was significantly reduced in all. Moreover, enzyme activity in response to stimulation by collagen or sodium arachidonate was also reduced. Platelet total and surface sialic-acid content was normal. The results indicate that defective membrane sialytransferase may be involved in the pathogenesis of primary platelet release disorder.

[Enzyme activities in platelets of different specific gravity in thrombocytosis of various etiology (author's transl)]. / Enzymaktivitäten und Proteingehalt von Thrombozyten verschiedener Dichte bei Thrombozytosen unterschiedlicher Atiologie.

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.