Pathological Platelet Research Articles

Hydroxysafflor yellow A inhibits the hyperactivation of rat platelets by regulating the miR-9a-5p/SRC axis

Pathological platelet activation plays a vital role in the prevalence of cardiovascular diseases. Hydroxysafflor yellow A (HSYA) has been shown to have significant anti-platelet aggregation and anti-activation effects, but its mechanism of action is unclear. Our study showed that HSYA inhibited the expression of platelet surface glycoproteins IIβ/III α (GPIIβ/III α) and thromboxane A2 (TXA2) during platelet activation and reduced platelet Ca2+ accumulation. HSYA significantly reduced the number of platelets and inhibited adrenaline-induced platelet hyperaggregation in rats. Transcriptomic analysis of platelets suggested that HSYA significantly suppressed SRC and MAPK3 (ERK1/2) gene expression. YEEI peptide, an SRC activator, could significantly reverse the inhibition of HSYA on the phosphorylation of SRC/PLCγ2/PKCδ/MEK/ERK1/2 pathway proteins and reverse the effect of HSYA on platelet activation-related markers GPIIβ/IIIα protein, TXA2 and cAMP. The SRC genes were further predicted by transcriptome analysis of HSYA-regulated miRNAs combined with bioinformatics techniques. The results suggested that HSYA could significantly upregulate the expression level of the miR-9a-5p gene and further confirmed that miR-9a-5p had a targeted regulatory relationship with SRC by dual-luciferase activity reporter and cell transfection experiments. The inhibitory effect of HSYA on the SRC/PLCγ2/PKCδ/MEK/ERK1/2 pathway was significantly reversed after platelets were transfected with the miR-9a inhibitor, while SRC siRNA attenuated the effect of the miR-9a inhibitor. SRC siRNA was able to attenuate the effect of the miR-9a inhibitor. In conclusion, this study suggests that HSYA can inhibit the activation of the SRC/PLCγ2/PKC δ/MEK/ERK1/2 axis by upregulating platelet miR-9a-5p, thereby reducing the activation of platelets and inhibiting platelet aggregation.

Abstract 156: Role of Platelets in Maintaining Hemostasis and Coagulation Activation in Viral-induced Acute Respiratory Distress Syndrome (ards) in Mice

Platelets are crucial players during viral infections, as bridge hemostasis, inflammation, and immune response. RNA viruses like IAV are associated with pathologic platelet activation and thrombosis. We are currently investigating how platelet depletion before or post initial IAV infection can affect disease outcome. WT mice were infected with IAV (0.02HAU, intranasally) for 7 days to induce viral ARDS. Mice were injected with platelet depletion antibody R300 or control IgG at day 0 (prior) and day 3 (after infection). Mice treated with R300/control IgG were subjected to dsRNA mimetic poly IC intranasally for 3 days to induce virus-like ARDS. Global lung function was recorded, plasma and BALF were collected 7 or 3 dpi or final poly IC treatment, respectively. BALF cellularity and blood cellular composition was analyzed. Markers of lung injury and cytokines were measured. We observed that IAV infection leads to local lung inflammation, coagulation activation and ARDS-like lung dysfunction. Mice treated with R300 showed severe thrombocytopenia in blood. We observed more impaired alveolar hemostasis and increased hemorrhages into the airspace when platelet depletion was initiated 3 dpi compared to mice where platelets were depleted prior to infection. In addition, in dsRNA-induced ARDS model, platelet depletion resulted in increased signs of lung dysfunction, increased lung injury and local inflammation compared to mice with normal platelet levels. In conclusion, depletion of platelets after IAV infection resulted in increased alveolar hemorrhage and was associated with more pronounced lung inflammation, injury, and lung dysfunction. Our study suggests that interfering with platelet before viral infection can be beneficial as it limits the role of platelets as potential multiplicator of an initial pathologic immune reaction subsequently reducing collateral damage and organ dysfunction/failure. However, at later stages of viral infection, platelet activation is beneficial to facilitate tissue hemostasis as their absence later can result in increased hemorrhagic risk and more pronounced lung dysfunction. We aim to address the mechanism by which platelet contribute later to pathologic immune processes in dsRNA-induced or viral ARDS

Plasma Concentration of Cortisol Negatively Associates with Platelet Reactivity in Older Subjects.

The interaction of platelets with steroid hormones is poorly investigated. Age is one of the factors that increase the risk of pathological platelet reactivity and thrombosis. The aim of this study was to assess whether there were associations between platelet reactivity and plasma cortisol levels in volunteers aged 60-65 years. For this purpose, impedance aggregometry in whole blood measured after arachidonic acid, collagen, or ADP stimulation was used to estimate platelet reactivity and mass spectrometry was used to measure peripheral plasma cortisol concentration. Statistically significant negative correlations were observed between cortisol concentration and platelet reactivity in response to arachidonic acid and ADP, but not to collagen. The presented results suggest for the very first time that cortisol is a new endogenous modulator of platelet reactivity in the elderly population.

Platelet in thrombo-inflammation: Unraveling new therapeutic targets.

In the broad range of human diseases, thrombo-inflammation appears as a clinical manifestation. Clinically, it is well characterized in context of superficial thrombophlebitis that is recognized as thrombosis and inflammation of superficial veins. However, it is more hazardous when developed in the microvasculature of injured/inflamed/infected tissues and organs. Several diseases like sepsis and ischemia-reperfusion can cause formation of microvascular thrombosis subsequently leading to thrombo-inflammation. Thrombo-inflammation can also occur in cases of antiphospholipid syndrome, preeclampsia, sickle cell disease, bacterial and viral infection. One of the major contributors to thrombo-inflammation is the loss of normal anti-thrombotic and anti-inflammatory potential of the endothelial cells of vasculature. This manifest itself in the form of dysregulation of the coagulation pathway and complement system, pathologic platelet activation, and increased recruitment of leukocyte within the microvasculature. The role of platelets in hemostasis and formation of thrombi under pathologic and non-pathologic conditions is well established. Platelets are anucleate cells known for their essential role in primary hemostasis and the coagulation pathway. In recent years, studies provide strong evidence for the critical involvement of platelets in inflammatory processes like acute ischemic stroke, and viral infections like Coronavirus disease 2019 (COVID-19). This has encouraged the researchers to investigate the contribution of platelets in the pathology of various thrombo-inflammatory diseases. The inhibition of platelet surface receptors or their intracellular signaling which mediate initial platelet activation and adhesion might prove to be suitable targets in thrombo-inflammatory disorders. Thus, the present review summarizes the concept and mechanism of platelet signaling and briefly discuss their role in sterile and non-sterile thrombo-inflammation, with the emphasis on role of platelets in COVID-19 induced thrombo-inflammation. The aim of this review is to summarize the recent developments in deciphering the role of the platelets in thrombo-inflammation and discuss their potential as pharmaceutical targets.

Glabridin, a Bioactive Flavonoid from Licorice, Effectively Inhibits Platelet Activation in Humans and Mice.

Platelets are crucial for hemostasis and arterial thrombosis, which may lead to severe cardiovascular diseases (CVDs). Thus, therapeutic agents must be developed to prevent pathological platelet activation. Glabridin, a major bioalkaloid extracted from licorice root, improves metabolic abnormalities (i.e., obesity and diabetes) and protects against CVDs and neuronal disorders. To the best of our knowledge, no studies have focused on glabridin’s effects on platelet activation. Therefore, we investigated these effects in humans and mice. Glabridin exhibited the highest inhibitory effects on collagen-stimulated platelet aggregation and moderate effects on arachidonic-acid-stimulated activation; however, no effects were observed for any other agonists (e.g., thrombin or U46619). Glabridin evidently reduced P-selectin expression, ATP release, and intracellular Ca2+ ([Ca2+]i) mobilization and thromboxane A2 formation; it further reduced the activation of phospholipase C (PLC)γ2/protein kinase C (PKC), phosphoinositide 3-kinase (PI3K)/Akt/glycogen synthase kinase-3β (GSK3β), mitogen-activated protein kinase (MAPK), and NF-κB. In mice, glabridin reduced the mortality rate caused by acute pulmonary thromboembolism without altering bleeding time. Thus, glabridin effectively inhibits the PLCγ2/PKC cascade and prevents the activation of the PI3K/Akt/GSK3β and MAPK pathways; this leads to a reduction in [Ca2+]i mobilization, which eventually inhibits platelet aggregation. Therefore, glabridin may be a promising therapeutic agent for thromboembolic disorders.

Endogenous and Exogenous Natural Sources of Inhibition of Arachidonic acid-induced Platelet aggregation

Platelets are central in mediating cardiovascular diseases including thrombosis. Aggregation of platelets is an important process following the adhesion of platelets to the injury site which may lead to the formation of a thrombus. Various aspects of platelet activation and aggregation could be modulated with compounds or fractions isolated from natural sources. These may include crude extracts, solvent-based fractions, or phytocompounds isolated from medicinal plants. Endogenous inhibition of platelet aggregation has been reported from plasma, serum, and many other blood components. This review summarizes these natural sources of inhibition against the most relevant physiological and pathological platelet agonists-arachidonic acid. Reported literature on the subject is further explored to discuss phytocompounds isolated from crude extracts or polarity-based fractions. Structures of the active compounds and their structure-activity relationship are also discussed.

Foudroyant cerebral venous (sinus) thrombosis triggered through CLEC-2 and GPIIb/IIIa dependent platelet activation

Cerebral venous (sinus) thrombosis (CVT) is an unusual manifestation of venous thrombosis causing severe neurological impairment and seizures1,2. Molecular mechanisms underlying CVT, potentially involving pathological platelet activation, are unknown. Here we show that antibody-(INU1-fab)-induced cooperative signaling of two platelet receptors, C-type lectin-like receptor-2 (CLEC-2) and GPIIb/IIIa, triggers within minutes a CVT-like thrombotic syndrome in mice, characterized by tonic–myoclonic seizures, platelet consumption and death. Brain autopsy showed thrombi mainly in the cortical venules, but no intracranial hemorrhages or edema formation. Transcranial intravital microscopy revealed rapidly progressing thrombosis in the superior sagittal sinus, a main site of CVT in humans. Interfering with CLEC-2 signaling or inhibition of GPIIb/IIIa completely blocked platelet activation and CVT. Blocking GPIIb/IIIa after onset of neurological symptoms protected mice from platelet consumption, CVT and death, which was not seen after treatment with heparin. These results point to aberrant platelet activation as a major trigger of CVT and potential target for treatment.

The effect of turbulence modelling on the assessment of platelet activation

Pathological platelet activation by abnormal shear stresses is regarded as a main clinical complication in recipients of cardiovascular mechanical devices. In order to improve their performance computational fluid dynamics (CFD) are used to evaluate flow fields and related shear stresses. CFD models are coupled with mathematical models that describe the relation between fluid dynamics variables, and in particular shear stresses, and the platelet activation state (PAS). These models typically use a Lagrangian approach to compute the shear stresses along possible platelet trajectories. However, in the case of turbulent flow, the choice of the proper turbulence closure is still debated for both concerning its effect on shear stress calculation and Lagrangian statistics. In this study different numerical simulations of the flow through a mechanical heart valve were performed and then compared in terms of Eulerian and Lagrangian quantities: a direct numerical simulation (DNS), a large eddy simulation (LES), two Reynolds-averaged Navier-Stokes (RANS) simulations (SST k-ω and RSM) and a “laminar” (no turbulence modelling) simulation. Results exhibit a large variability in the PAS assessment depending on the turbulence model adopted. “Laminar” and RSM estimates of platelet activation are about 60% below DNS, while LES is 16% less. Surprisingly, PAS estimated from the SST k- ω velocity field is only 8% less than from DNS data. This appears more artificial than physical as can be inferred after comparing frequency distributions of PAS and of the different Lagrangian variables of the mechano-biological model of platelet activation. Our study indicates how much turbulence closures may affect platelet activation estimates, in comparison to an accurate DNS, when assessing blood damage in blood contacting devices.

Cancer is associated with inferior outcome in patients with ischemic stroke

BackgroundWhether patients with stroke and cancer exhibit specific characteristics has remained controversial.MethodsMedical records of patients with ischemic stroke in 2014 or 2015 registered in the Swiss Stroke Registry of Zurich were retrospectively analyzed and integrated with regional cancer registry data. Associations of clinical and outcome parameters with cancer diagnosed up to 5 years prior to stroke were tested.ResultsOf 753 patients with ischemic stroke, 59 patients with cancer were identified. History of venous thromboembolism (p < 0.001) was associated with cancer while age and cardiovascular risk factors were not. Higher levels of D-dimers (p = 0.001), erythrocyte sedimentation rate (p = 0.003), C-reactive protein (CRP) (p < 0.001), and lower levels of hemoglobin (p = 0.003) were associated with cancer. For platelets, pathologically low (p = 0.034) or high levels (p < 0.001) were linked to cancer. Modified Rankin scale (mRS) scores ≥ 4 on admission and at follow-up were more frequent in cancer patients (p = 0.038 and p = 0.001). Poor post-stroke survival was associated with cancer (HR 2.2, p < 0.001). Multivariable analysis identified venous thromboembolism (OR 5.1), pathologic platelet count (OR = 2.9), low hemoglobin (OR 2.5) and elevated CRP (OR 1.8) as independently associated with cancer. In multivariable Cox regression, risk for death was associated with cancer (HR 1.7), low hemoglobin (HR 2.6), mRS on admission ≥ 4 (HR 1.9), pathologic platelet count (HR 1.6), female sex (HR 1.7), and elevated CRP (HR 1.4).ConclusionsConsidering cancer as a cofactor for post-stroke outcome may impact clinical decision making.

HMGB1-Mediated Platelet Activation Is Independent of Platelet Mitochondrial Reactive Oxygen Species Generation

Background: Sickle cell disease (SCD) is a hemolytic disorder that exhibits pathologic platelet activation. Notably, hemolysis is tightly associated with platelet activation and thrombotic complications of SCD such as stroke, leg ulceration, and pulmonary hypertension. To this end, we and others have shown that free hemoglobin (Hb) released into the plasma via hemolysis directly activates healthy platelets ex vivo in a concentration-dependent manner. Treatment with Hb stimulates the production of mitochondrial reactive oxygen species (mtROS) within the platelet, resulting in thrombotic activation. Accordingly, scavenging mtROS prevents Hb-induced platelet activation. While it is established that hemolysis mediates platelet activation, the factors that modulate this response remain unknown. High mobility group box 1 (HMGB1) is an inflammatory mediator, which signals through toll-like receptor-4 (TLR4), to instigate thrombosis. Notably, we previously showed that HMGB1 is elevated in the plasma of SCD patients and augments platelet activation on its own. Importantly, complementary interactions between Hb and other pro-thrombotic plasma components such as HMGB1 have not been investigated. Preliminary data show that HMGB1, like Hb, stimulates platelet mtROS generation. Thus, we hypothesized that elevated levels of HMGB1 and Hb synergize in SCD to stimulate platelet mtROS production and downstream platelet activation. Methods: Whole blood samples were collected from healthy human control subjects or SCD subjects in steady state (n=5 per group). Plasma HMGB1 concentrations were quantified by ELISA. Platelets were isolated and treated with reagents including HMGB1, ferric Hb, or the mtROS scavenger MitoTEMPO. Platelet activation was measured by flow cytometry using PE antibody to GPIIb (CD41) to select for platelets, and two markers of platelet activation: PAC1 to detect the activation-dependent conformational change in integrin αIIbβ3 (GP IIb-IIIa), and APC to detect exposure of surface CD62P (P-selectin). Platelet mtROS were estimated using MitoSOX Red and fluorescence spectroscopy. Data was analyzed using FlowJo software and nonparametric statistical tests. Results: The baseline level of platelet activation in SCD was 7-fold higher than in controls (P=0.009). However, there was no difference in platelet activation levels between isolated SCD and control platelets in response to ex vivo treatment with agonists ADP (5 uM), collagen (50 ug/ml), or thrombin (0.1 U/ml) (P=0.38), suggesting that SCD platelets have the same agonist-induced response as control platelets when they are removed from their native plasma. We found that the concentration of HMGB1 in SCD plasma is 1.5 fold higher than controls (P=0.04), and that plasma HMGB1 concentrations positively correlate with increased platelet activation (P=0.04). Co-incubation of isolated healthy control platelets with HMGB1 (10 ug/ml) and major hemolytic byproduct Hb (10 uM, 30 uM, 50 uM) enhanced platelet activation at all doses of Hb by 3-fold compared to that of Hb alone. Mechanistically, platelets exposed to Hb alone generated mtROS which are necessary for Hb-induced platelet activation. We found that HMGB1 similarly induced platelet mtROS generation, but scavenging with MitoTEMPO did not prevent HMGB1-mediated platelet activation. However the addition of MitoTEMPO to platelets co-treated with HMGB1 + Hb attenuated platelet activation 2-fold, indicating that scavenging mtROS neutralizes the enhanced effect of Hb + HMGB1. Conclusions: We show that HMGB1 synergizes with Hb to enhance platelet activation in SCD, but unlike Hb, HMGB1-dependent platelet activation is independent of mtROS generation. Accordingly, we observed that MitoTEMPO partially attenuates the enhanced activation from co-stimulation by HMGB1 + Hb. These data characterize the synergy between HMGB1 and Hb for the first time and demonstrate differential platelet signaling between the two agonists. Disclosures No relevant conflicts of interest to declare.

Shear-Dependent Platelet Aggregation: Mechanisms and Therapeutic Opportunities.

Cardiovascular diseases (CVD) are the number one cause of morbidity and death worldwide. As estimated by the WHO, the global death rate from CVD is 31% wherein, a staggering 85% results from stroke and myocardial infarction. Platelets, one of the key components of thrombi, have been well-investigated over decades for their pivotal role in thrombus development in healthy as well as diseased blood vessels. In hemostasis, when a vascular injury occurs, circulating platelets are arrested at the site of damage, where they are activated and aggregate to form hemostatic thrombi, thus preventing further bleeding. However, in thrombosis, pathological activation of platelets occurs, leading to uncontrolled growth of a thrombus, which in turn can occlude the blood vessel or embolize, causing downstream ischemic events. The molecular processes causing pathological thrombus development are in large similar to the processes controlling physiological thrombus formation. The biggest challenge of anti-thrombotics and anti-platelet therapeutics has been to decouple the pathological platelet response from the physiological one. Currently, marketed anti-platelet drugs are associated with major bleeding complications for this exact reason; they are not effective in targeting pathological thrombi without interfering with normal hemostasis. Recent studies have emphasized the importance of shear forces generated from blood flow, that primarily drive platelet activation and aggregation in thrombosis. Local shear stresses in obstructed blood vessels can be higher by up to two orders of magnitude as compared to healthy vessels. Leveraging abnormal shear forces in the thrombus microenvironment may allow to differentiate between thrombosis and hemostasis and develop shear-selective anti-platelet therapies. In this review, we discuss the influence of shear forces on thrombosis and the underlying mechanisms of shear-induced platelet activation. Later, we summarize the therapeutic approaches to target shear-sensitive platelet activation and pathological thrombus growth, with a particular focus on the shear-sensitive protein von Willebrand Factor (VWF). Inhibition of shear-specific platelet aggregation and targeted drug delivery may prove to be much safer and efficacious approaches over current state-of-the-art antithrombotic drugs in the treatment of cardiovascular diseases.

Porphyromonas gingivalis lipopolysaccharide activates platelet Cdc42 and promotes platelet spreading and thrombosis.

Periodontitis confers an increased risk for cardiovascular diseases, including thrombosis. However, the molecular mechanisms that potentially link periodontitis with thrombosis are undefined. Here we test the hypothesis that Gram-negative periodontal infection promotes pathological platelet activation and amplifies shape change. We focus specifically on lipopolysaccharide (LPS) signaling to platelets. Platelets were isolated from blood samples and allowed to spread on coverslips in the presence or absence of LPS purified from the periodontal pathogen Porphyromonas gingivalis. Platelets were fixed and stained with Alexa-488-phalloidin to label the actin cytoskeleton. The degree of platelet spreading and shape change was quantified by confocal microscopy. In a translational pilot study, blood samples were obtained from human subjects exhibiting generalized severe periodontitis (SP) or healthy periodontium (HP). Rotational thromboelastometry was used to quantify the rate of clot formation via the intrinsic coagulation pathway. LPS-treated platelets exhibited significantly (P<0.05) greater spreading and higher numbers of actin-rich filopodia (cell extensions) than controls. We also found that LPS stimulation of platelets promoted the activation of Cdc42, the small GTPase responsible for filopodia formation. Exposure of whole blood samples to LPS significantly (P<0.05) reduced clotting times. Blood from SP patients clotted significantly (P<0.05) more rapidly and exhibited shorter partial thromboplastin times compared with HP controls. This is the first study to suggest a mechanism by which LPS stimulation drives Cdc42 activation and platelet spreading. These data are consistent with the notion that periodontitis promotes accelerated clot formation and an increased risk of thrombosis.

Update on pathological platelet activation in coronary thrombosis.

Although coronary thrombosis (CT) is integral to cardiovascular outcomes, the underlying pathophysiological mechanisms remain unclear. CT may occur in case of atherosclerotic plaque erosion/rupture, or even after stenting implantation. Platelets (PLT) activation is the keystone of atherothrombosis and depends on many dysregulated elements, including endothelial dysfunction, oxidized lipoproteins, and immune response. Besides the classical view of PLT as an effector of hemostatic response, a new repertoire of PLT activities is emerging. PLT lipidome oxidation is a self-maintaining process which promotes PLT reactivity, coagulation cascade, and inflammatory cell activation. PLT-innate immune cell interaction is also sustained by neutrophil extracellular traps and NLRP3 inflammasome pathways. Other noteworthy emerging mechanisms are implicated in the crosstalk between PLT and surrounding cells. Especially, microvesicles (MVs) released from PLT may extend their signaling network far beyond the classical cell-cell interactions. Moreover, the recognition of noncoding RNA in PLT MVs introduce another layer of complexity in terms of intercellular signaling by a direct regulation of messenger RNA profile and gene expression in the recipient cells. The aim of this narrative review is to update the recent advance in CT and intracoronary stent thrombosis, including causal factors and potential translation of experimental evidence into the clinical setting.

A novel flow cytometry assay using dihydroethidium as redox-sensitive probe reveals NADPH oxidase-dependent generation of superoxide anion in human platelets exposed to amyloid peptide β

Reactive oxygen species (ROS) generation is critical in the regulation of platelets, which has important implications in the modulation of hemostasis and thrombosis. Nonetheless, despite several assays have been described and successfully utilized in the past, the analysis of ROS generation in human platelets remains challenging.Here we show that dihydroethidium (DHE) allows the characterization of redox responses upon platelet activation by physiological and pathological stimuli. In particular, the flow cytometry assay that we describe here allowed us to confirm that thrombin, collagen-related peptide (CRP) and arachidonic acid but not adenosine diphosphate (ADP) stimulate superoxide anion formation in a concentration-dependent manner. 0.1unit/ml thrombin, 3 μg/ml CRP and 30 μM arachidonic acid are commonly used to stimulate platelets in vitro and here were shown to stimulate a significant increase in superoxide anion formation. The ROS scavenger N-acetylcysteine (NAC) abolished superoxide anion generation in response to all tested stimuli, but the pan-NADPH oxidase (NOX) inhibitor VAS2870 only inhibited superoxide anion formation in response to thrombin and CRP. The involvement of NOXs in thrombin and CRP-dependent responses was confirmed by the inhibition of platelet aggregation induced by these stimuli by VAS2870, while platelet aggregation in response to arachidonic acid was insensitive to this inhibitor.In addition, the pathological platelet stimulus amyloid β (Aβ) 1–42 peptide induced superoxide anion formation in a concentration-dependent manner. Aβ peptide stimulated superoxide anion formation in a NOX-dependent manner, as proved by the use of VAS2870. Aβ 1–42 peptide displayed only moderate activity as an aggregation stimulus, but was able to significantly potentiate platelet aggregation in response to submaximal agonists concentrations, such as 0.03 unit/ml thrombin and 10 μM arachidonic acid. The inhibition of NOXs by 10 μM VAS2870 abolished Aβ-dependent potentiation of platelet aggregation in response to 10 μM arachidonic acid, suggesting that the pro-thrombotic activity of Aβ peptides depends on NOX activity. Similar experiments could not be performed with thrombin or collagen, as NOXs are required for the signaling induced by these stimuli. These findings shed some new light on the pro-thrombotic activity of Aβ peptides.In summary, here we describe a novel and reliable assay for the detection of superoxide anion in human platelets. This is particularly important for the investigation of the pathophysiological role of redox stress in platelets, a field of research of increasing importance, but hindered by the absence of a reliable and easily accessible ROS detection methodology applicable to platelets.

Normal and pathological dynamics of platelets in humans.

We develop a mathematical model of platelet, megakaryocyte, and thrombopoietin dynamics in humans. We show that there is a single stationary solution that can undergo a Hopf bifurcation, and use this information to investigate both normal and pathological platelet production, specifically cyclic thrombocytopenia. Carefully estimating model parameters from laboratory and clinical data, we then argue that a subset of parameters are involved in the genesis of cyclic thrombocytopenia based on clinical information. We provide model fits to the existing data for both platelet counts and thrombopoietin levels by changing four parameters that have physiological correlates. Our results indicate that the primary change in cyclic thrombocytopenia is an interference with, or destruction of, the thrombopoietin receptor with secondary changes in other processes, including immune-mediated destruction of platelets and megakaryocyte deficiency and failure in platelet production. This study contributes to the understanding of the origin of cyclic thrombocytopenia as well as extending the modeling of thrombopoiesis.

Persistent Thrombopoietin Mimetic Induced Myelofibrosis in a Woman with Chronic Refractory Immune Thrombocytopenia

BackgroundImmune thrombocytopenia (ITP) is a disease caused by autoimmune destruction and impaired production of platelets. It can be primary or secondary in origin. It manifests as isolated thrombocytopenia with platelet counts less than 100 x109/L. Therapy is typically initiated when the platelet counts fall to less than 20 to 30 x 109/L in asymptomatic patients to reduce the risk of bleeding. ITP that persists for greater than 12 months is classified as chronic and ITP that endures despite splenectomy is defined as refractory. Most therapeutic approaches, such as corticosteroids, target the pathologic platelet destruction. Thrombopoietin (Tpo) mimetics, on the other hand, enhance platelet production via stimulation of the Tpo receptor on megakaryocytes. Studies suggest that Tpo mimetics increase reticulin deposition in the bone marrow. Tpo mimetics are postulated to promote reticulin deposition through stimulation of cytokine production by megakaryocytes, including transforming growth factor beta. There are reports of myelofibrosis, with abnormal deposition of collagen and reticulin in bone marrow trephine biopsies of ITP patients treated with these agents. Tpo mimetic induced myelofibrosis however, is described as reversible upon cessation of Tpo based therapy in most reports. Here we present a case of myelofibrosis induced by sequential therapy with two high dose TPO mimetics, which was not reversed six months after discontinuation of the medications.Case ReportAn 87-year-old woman with chronic ITP resistant to multiple lines of therapy (prednisone, dexamethasone, rituximab, dapsone, mycofenylate mofetil, and splenectomy) was treated consecutively with two thrombopoietin (Tpo) mimetics, romiplostim and then eltrombopag. Bone marrow evaluation at diagnosis, prior to treatment with Tpo mimetics, showed megakaryocyte hyperplasia without evidence of fibrosis. There was no definitive evidence of an underlying clonal hematologic disorder.. The patient's course was complicated by gastrointestinal and intracranial hemorrhage. Tpo mimetics were introduced as fourth line therapy. ITP response to romiplostim and then to eltrombopag was initially encouraging but rapidly fleeting.The peak dose of romiplostim used was 10 mcg/kg/week and eltrombopag was used at a dose of 75 mg per day. While on eltrombopag and within six months of romiplostim, therapy, she developed persistent left-shifted leukocytosis and a peripheral blood film showing leukoerythroblastosis and dacrocytic erythrocytes. This prompted a bone marrow evaluation that confirmed myelofibrosis, with a dense and diffuse increase in reticulin content and focal bundles of collagen (MF 2 of 3). There was associated marrow hypercellularity, increased numbers of enlarged megakaryocytes, clustered megakaryocyte topography and extensive bone resorption and remodelling Eltrombopag was promptly discontinued and peripheral blood leukoerythroblastic picture improved. Repeat bone marrow evaluation 6 months after stopping therapy showed partial reversal of the abnormalities seen in the previous marrow, including a reduction in the extent of reticulin fibrosis without any evidence of collagen fibrosis (MF 1-2 of 3). These changes are shown in Figure 1.DiscussionChronic, refractory ITP is a challenging syndrome to treat, and the Tpo mimetics have emerged as an important therapeutic option for patients who have failed traditional therapies. Clinical experience with the relatively novel Tpo mimetic drug class is maturing as we obtain longitudinal data and reports about adverse effects like myelofibrosis. While the myelofibrosis in our patient improved upon cessation of Tpo mimetic therapy, it did not resolve entirely and persisted 6 months after stopping therapy. We hypothesize that the sequential use of high dose Tpo mimetic therapy may have contributed to the development and persistence of myelofibrosis in this patient. This report adds to the body of evidence suggesting that myelofibrosis is a serious and, potentially chronic, adverse effect of treatment with Tpo mimetics.TPO mimetic treatment and B) Grade 1-2 of 3 Myelofibrosis after cessation of TPO mimetic.Semiquantitative grading is based on the 2008 WHO classification.Images from left to right represent hematoxylin andeosin, reticulin, and trichrome stains. [Display omitted] DisclosuresSholzberg:CSL Behring: Honoraria, Research Funding; Baxalta: Honoraria, Research Funding; Amgen: Honoraria.

G477 Identification of novel par2 mutation in thrombotic microangiopathy

Atypical Haemolytic Uraemic Syndrome (aHUS) is a rare cause of renal failure, occurring as a result of glomerular endothelial cell injury, with a prevalence of 3–5 per million. The clinical features consist of recurrent episodes of haemolytic anaemia, thrombocytopaenia and renal failure with biopsy evidence of thrombotic microangiopathy. Mutations in complement regulatory genes account for approximately 70% of cases. More recently, components of the coagulation cascade, such as DGKE, have been implicated in the aetiopathogenesis of aHUS. We have identified a novel mutation in protease–activated receptor 2 (PAR2) in a patient with an aHUS phenotype. Methods Whole Exome Sequencing (WES) was performed on a child presenting at the age of 3 years with nephrotic range proteinuria and biopsy evidence of thrombotic microangiopathy, thrombocytopaenia and haemolytic anaemia. One year from presentation, his renal function recovered to baseline but he continues to experience episodic haemolytic anaemia and thrombocytopaenia. Of note, there is a familial history of relapsing thrombocytopaenia affecting the patient’s father and aunt. Results WES did not demonstrate any mutations in known aHUS genes. A novel mutations in PAR2 (Y345H) was identified, in silico analysis of this classified this as deleterious. This mutation was not detected in databases of healthy controls. Sequence analysis demonstrated segregation in affected family members (Figure 1) Western blot analysis suggests reduced PAR2 expression in the patient’s serum compared to controls. We also show that PAR2 is expressed in glomerular cells (Figures 2 and 3) Discussion PAR2 is a G-protein coupled receptor, which is activated by a number of pro-inflammatory and coagulation mediators. Activation of this receptor causes a positive feedback loop, leading to a pro-thrombotic state that may result in pathological platelet activation and endothelial dysfunction. As there are high levels of PAR2 in the kidney, this pro-thrombotic state may lead to the thrombotic microangiopathy seen in aHUS. PAR2 has additional roles in complement regulation that are less well defined. It is known to cause down–regulation of DAF, which is an important regulator of the complement pathway. Further analysis of genetically undefined cases of TMA and MPGN are required for PAR2 mutational screening to determine whether PAR2 represents a unique pathway in these rare renal phenotypes.

Circulation : Clinical Summaries

<i>Circulation</i> : Clinical Summaries

Plasmin Cleavage of von Willebrand Factor as an Emergency Bypass for ADAMTS13 Deficiency in Thrombotic Microangiopathy

Von Willebrand factor (VWF) multimer size is controlled through continuous proteolysis by ADAMTS13 (a disintegrin and metalloproteinase with a thrombospondin type I motif, member 13). This prevents spontaneous platelet agglutination and microvascular obstructions. ADAMTS13 deficiency is associated with thrombotic thrombocytopenic purpura, in which life-threatening episodes of microangiopathy damage kidneys, heart, and brain. Enigmatically, a complete ADAMTS13 deficiency does not lead to continuous microangiopathy. We hypothesized that plasmin, the key enzyme of the fibrinolytic system, serves as a physiological backup enzyme for ADAMTS13 in the degradation of pathological platelet-VWF complexes. Using real-time microscopy, we determined that plasmin rapidly degrades platelet-VWF complexes on endothelial cells in absence of ADAMTS13, after activation by urokinase-type plasminogen activator or the thrombolytic agent streptokinase. Similarly, plasmin degrades platelet-VWF complexes in platelet agglutination studies. Plasminogen directly binds to VWF and its A1 domain in a lysine-dependent manner, as determined by enzyme-linked immunosorbent assay. Plasma levels of plasmin-α2-antiplasmin complexes increase with the extent of thrombocytopenia in patients with acute episodes of thrombotic thrombocytopenic purpura, independent of ADAMTS13 activity. This indicates that plasminogen activation takes place during microangiopathy. Finally, we show that the thrombolytic agent streptokinase has therapeutic value for Adamts13(-/-) mice in a model of thrombotic thrombocytopenic purpura. We propose that plasminogen activation on endothelial cells acts as a natural backup for ADAMTS13 to degrade obstructive platelet-VWF complexes. Our findings indicate that thrombolytic agents may have therapeutic value in the treatment of microangiopathies and may be useful to bypass inhibitory antibodies against ADAMTS13 that cause thrombotic thrombocytopenic purpura.

Platelet Functional and Morphological Abnormalities In a Case Of Arthrogryposis, Renal Dysfunction and Cholestasis (ARC) Syndrome With a Novel Genetic Profile

ARC syndrome is a severe autosomal recessive multisystem disorder characterized by arthrogryposis, neonatal cholestatic jaundice, renal tubular acidosis, failure to thrive, dysmorphic features, congenital heart disease, cerebral malformation, hypotonia, recurrent febrile illness and a bleeding disorder. It is associated with consanguinity and most patients do not survive beyond the first year of life. ARC syndrome is linked to loss of function in either VPS33B or VIPAS39, respectively encoding VPS33B and VPS16B, which we have shown to be interaction partners within a multiprotein complex involved in intracellular vesicular trafficking (Urban et al, Blood. 2012 Dec 13;120(25):5032-40). Earlier we determined that bleeding problems in ARC syndrome patients (where platelet aggregation abnormalities have been observed) arise from abnormal function of normally abundant platelets lacking α-granules (Lo et al, Blood. 2005;106(13):4159-66). Detailed studies of ARC syndrome platelets have been hampered by limited availability.A term 4-month old boy was observed to have significant epistaxis and petechiae requiring multiple transfusions of plasma, packed red blood cells, tranexamic acid, vitamin K and nasal packing twice, with final success over control of bleeding. INR was 1.8 and PTT was 40 with a normal platelet count of 319 during this episode. Intermittent episodes of bleeding continued throughout his course, with difficulty in stopping the bleeding. The patient was found to have jaundice with scleral icterus, cholestasis and giant cell hepatitis seen on liver biopsy. A metabolic work-up revealed significant aminoaciduria and proximal tubular acidosis. He also had failure to thrive, feeding intolerance, severe bilateral sensorineural hearing loss, limb contractures, right rocker bottom foot and clubbed left foot. MRI of the brain showed abnormal corpus callosum with non-visualization of the anterior commissure. On abdominal ultrasound, the liver was enlarged and mildly echogenic with no evidence of portal hypertension and patent vessels. Echogenic kidneys were present with nephrocalcinosis. He was referred for evaluation and possible liver transplantation.Blood film examination revealed pale-appearing platelets and transmission electron microscopy confirmed the complete absence of α-granules, making a diagnosis of ARC syndrome highly probable. Immunofluorescence confocal microscopy detected abnormalities in patient platelets with regards to: 1) size (larger than normal); 2) morphology of the tubulin ring cytoskeleton (loosely organized compared to normal); and 3) intracellular distribution of CD63 (more abundant and widely dispersed than normal). Sequencing of patient DNA detected normal VIPAS39 and apparent homozygosity for a single base pair deletion (c.1531delT, p.Tyr511ThrfsStop10) in exon 20 of VPS33B, predicted to cause a frame shift and premature translation stop codon. Gene sequencing of the parents detected the identical VPS33B mutation in the mother heterozygous to a normal allele; no mutation was found in the father. Array comparative genomic hybridization (aCGH) – which allows detection of deletions and amplification mutations of one or more exons within a given gene – uncovered an approximately 140 kb deletion mutation encompassing the entire VPS33B gene in our patient (g.91,478,004 – proximal to g.91,618.247 – distal), confirming compound heterozygosity. According to the online locus-specific ARC database (https://grenada.lumc.nl/LOVD2/ARC), this is the first association of this mutation combination with ARC syndrome.Unusually for ARC syndrome, bleeding indicative of platelet dysfunction was a major presenting symptom in this patient. While the genetic profile is unusual, the pathological platelet hallmarks of ARC syndrome were present: hypogranular platelets on blood film and absence of α-granules via electron microscopy. We were also able to detect other platelet abnormalities via immunofluorescence microscopy. These observations lead us to suspect that ARC syndrome may represent the most severe class within a possible spectrum of platelet function disorders associated with mutations in VPS33B and/or VIPAS39, a possibility that requires further study. Disclosures:No relevant conflicts of interest to declare.