Scott syndrome dogs have impaired coated-platelet formation and calcein-release but normal mitochondrial depolarization

Increased procoagulant platelet levels are predictive of death in COVID-19

Prior research has identified abnormal platelet procoagulant responses in COVID-19. Coated-platelets, a form of procoagulant platelets, support thrombin formation and are elevated in ischemic stroke patients with increased risk for recurrent infarction. Our goal was to examine changes in coated-platelet levels over the course of COVID-19 infection and determine their association with disease severity, thrombosis, and death. Coated-platelet levels were assayed after admission and repeated weekly in COVID-19 patients, and in COVID-19 negative controls. Receiver operator characteristic (ROC) analysis was used to calculate area under the curve (AUC) values for a model including baseline coated-platelets to predict death. Kaplan–Meier and Cox proportional hazards analysis was used to predict risk for death at 90 days. We enrolled 33 patients (22 with moderate and 11 with severe infection) and 20 controls. Baseline coated-platelet levels were lower among moderate (mean ± SD; 21.3 ± 9.8%) and severe COVID-19 patients (28.5 ± 11.9%) compared to controls (38.1 ± 10.4%, p < 0.0001). Coated-platelet levels increased during follow-up in COVID-19 patients by 7% (relative) per day from symptom onset (95% CI 2–12%, p = 0.007). A cut-off of 33.9% for coated-platelet levels yielded 80% sensitivity and 96% specificity for death at 90 days, with resulting AUC of 0.880 (95% CI 0.680–1.0, p = 0.0002). The adjusted hazard ratio for death in patients with coated-platelet levels > 33.9% was 40.99 when compared to those with levels ≤ 33.9% (p < 0.0001). Platelet procoagulant potential is transiently decreased in most patients during COVID-19; however, increased baseline platelet procoagulant levels predict death. Defining the mechanisms involved and potential links with aging may yield novel treatment targets.

Laboratory Techniques Used to Diagnose Constitutional Platelet Dysfunction

Platelets play a major role in primary hemostasis, where activated platelets form plugs to stop hemorrhaging in response to vessel injuries. Defects in any step of the platelet activation process can cause a variety of platelet dysfunction conditions associated with bleeding. To make an accurate diagnosis, constitutional platelet dysfunction (CPDF) should be considered once von Willebrand disease and drug intake are ruled out. CPDF may be associated with thrombocytopenia or a genetic syndrome. CPDF diagnosis is complex, as no single test enables the analysis of all aspects of platelet function. Furthermore, the available tests lack standardization, and repeat tests must be performed in specialized laboratories especially for mild and moderate forms of the disease. In this review, we provide an overview of the laboratory tests used to diagnose CPDF, with a focus on light transmission platelet aggregation (LTA), flow cytometry (FC), and granules assessment. Global tests, mainly represented by LTA, are often initially performed to investigate the consequences of platelet activation on platelet aggregation in a single step. Global test results should be confirmed by additional analytical tests. FC represents an accurate, simple, and reliable test to analyze abnormalities in platelet receptors, and granule content and release. This technique may also be used to investigate platelet function by comparing resting- and activated-state platelet populations. Assessment of granule content and release also requires additional specialized analytical tests. High-throughput sequencing has become increasingly useful to diagnose CPDF. Advanced tests or external research laboratory techniques may also be beneficial in some cases.

Evaluation of cytokine concentrations in a trehalose-stabilised lyophilised canine platelet product: a preliminary study

Background

Platelet transfusion is indicated for haemorrhage due to severe thrombocytopenia and for trauma associated coagulopathy. Febrile non-haemolytic transfusion reactions are a common complication of platelet transfusions in people and may be due to accumulated inflammatory cytokines. The present study aimed to determine the cytokine profile of a novel canine lyophilised platelet product following reconstitution, to assess the lyophilised platelets’ activation response to physiological platelet agonists and to compare the cytokine profiles of basal and stimulated canine lyophilised platelets.

Methods

Cell counts and biochemical analyses were conducted following reconstitution. Cytokine concentrations were measured with a canine-specific multiplex immunocapture assay and with an electrochemiluminescent ELISA. Aliquots of reconstituted product from three separate vials were activated for 10 minutes under non-stirred conditions using adenosine diphosphate, thrombin or convulxin and their cytokine concentrations compared with unactivated samples. Flow cytometry and light-transmission aggregometry were used to evaluate the product’s ability to express a procoagulant surface, degranulate and aggregate. Fresh platelet-rich plasma was used as a positive control.

Results

The product had a mean±SD particle count of 1.23±0.2×10⁹/ml, contained platelets that expressed surface phosphatidylserine before agonist stimulation and was capable of aggregation in response to thrombin stimulation suggesting that the product may have haemostatic potential following in vivo administration. Cytokine concentrations measured by the immunocapture assay were generally low, while twofold to threefold increases relative to published intervals were noted for several cytokines using the ELISA. Concentrations of chemokine (C-X-C) motif ligand 8 and tumour necrosis factor-α were significantly increased as measured by the ELISA, but not by the immunocapture assay, while concentrations of KC-like were significantly increased as measured by the immunocapture assay. Stimulation with platelet agonists did not affect measured cytokine concentrations.

Conclusion

Further study of the effects of administration of this lyophilised platelet product is warranted.

Increased platelet procoagulant potential predicts recurrent stroke and TIA after lacunar infarction

BACKGROUND

Mean levels of coated-platelets, a subset of highly procoagulant platelets, are decreased in patients with lacunar as compared to those with non-lacunar stroke. Elevated coated-platelets are associated with increased risk for recurrent infarction in non-lacunar stroke and predict incident stroke after transient ischemic attack (TIA).

OBJECTIVE

We investigated if coated-platelet levels are predictive of recurrent cerebral ischemia following lacunar stroke.

METHODS

Coated-platelet levels were assayed in consecutive patients with acute lacunar stroke, who were followed for up to 12 months. Cox proportional hazards regression analysis was used to estimate the combined risk of stroke and TIA at 12 months according to initial coated-platelet levels.

RESULTS

We enrolled a total of 109 lacunar stroke patients. Eight events were recorded over a mean follow-up period of 10.8 months. A cut-off of 42.6% for coated-platelet levels yielded a sensitivity of 0.75 (0.35-0.97; 95% confidence interval [CI]), specificity of 0.92 (0.85-0.97), positive predictive value of 0.43 (0.26-0.62), and a negative predictive value of 0.98 (0.93-0.99) for recurrent stroke/TIA. The adjusted hazard ratio for recurrent stroke/TIA in patients with coated-platelet levels ≥ 42.6% was 23.9 (95% CI: 4.26-134.4) when compared to those with levels < 42.6%.

CONCLUSIONS

Identification of increased platelet procoagulant potential may improve our ability to identify patients at higher risk of recurrent stroke/TIA following a lacunar stroke. Further study of mechanisms involved is warranted and may yield novel targets for prevention and treatment.

Platelet Function and Therapeutic Applications in Dogs: Current Status and Future Prospects

Significant progress has been made in the functional characterization of canine platelets in the last two decades. The role of canine platelets in hemostasis includes their adhesion to the subendothelium, activation, and aggregation, leading to primary clot formation at the site of injury. Studies on canine platelet function and advancements in laboratory testing have improved the diagnosis and understanding of platelet-related disorders as well as the knowledge of the mechanisms behind these diseases. This review focuses on the most recent discoveries in canine platelet structure, function, and disorders; and discusses the efficacy of various tests in the diagnosis of platelet-related disorders. With the relatively recent discovery of angiogenetic and reparative effects of growth factors found in platelets, this review also summarizes the use of canine platelet-rich plasma (PRP) alone or in association with stem cells in regenerative therapy. The characterization of proteomic and lipidomic profiles and development of platelet gene therapy in veterinary species are areas of future study with potential for major therapeutic benefits.

Proteomic profiling of the thrombin-activated canine platelet secretome (CAPS)

Domestic dogs share the same environment as humans, and they represent a valuable animal model to study naturally-occurring human disease. Platelet proteomics holds promise for the discovery of biomarkers that capture the contribution of platelets to the pathophysiology of many disease states, however, canine platelet proteomic studies are lacking. Our study objectives were to establish a protocol for proteomic identification and quantification of the thrombin-activated canine platelet secretome (CAPS), and to compare the CAPS proteins to human and murine platelet proteomic data. Washed platelets were isolated from healthy dogs, and stimulated with saline (control) or gamma-thrombin (releasate). Proteins were separated by SDS-page, trypsin-digested and analyzed by liquid chromatography and tandem mass spectrometry (MS). CAPS proteins were defined as those with a MS1-abundance ratio of two or more for releasate vs. unstimulated saline control. A total of 1,918 proteins were identified, with 908 proteins common to all dogs and 693 characterized as CAPS proteins. CAPS proteins were similar to human and murine platelet secretomes and were highly represented in hemostatic pathways. Differences unique to CAPS included replacement of platelet factor 4 with other cleavage products of platelet basic protein (e.g. interleukin-8), novel proteins (e.g. C-C motif chemokine 14), and proteins in relatively high (e.g. protease nexin-1) or low (e.g. von Willebrand factor) abundance. This study establishes the first in-depth platelet releasate proteome from healthy dogs with a reference database of 693 CAPS proteins. Similarities between CAPS and the human secretome confirm the utility of dogs as translational models of human disease, but we also identify differences unique to canine platelets. Our findings provide a resource for further investigations into disease-related CAPS profiles, and for comparative pathway analyses of platelet activation among species.

Analytical validation of a flow cytometric protocol for quantification of platelet microparticles in dogs

BACKGROUND

Platelet microparticles (PMPs) are subcellular procoagulant vesicles released upon platelet activation. In people with clinical diseases, alterations in PMP concentrations have been extensively investigated, but few canine studies exist.

OBJECTIVES

This study aims to validate a canine flow cytometric protocol for PMP quantification and to assess the influence of calcium on PMP concentrations.

METHODS

Microparticles (MP) were quantified in citrated whole blood (WB) and platelet-poor plasma (PPP) using flow cytometry. Anti-CD61 antibody and Annexin V (AnV) were used to detect platelets and phosphatidylserine, respectively. In 13 healthy dogs, CD61⁺ /AnV^- concentrations were analyzed with/without a calcium buffer. CD61⁺ /AnV^- , CD61⁺ /AnV⁺ , and CD61^- /AnV⁺ MP quantification were validated in 10 healthy dogs. The coefficient of variation (CV) for duplicate (intra-assay) and parallel (inter-assay) analyses and detection limits (DLs) were calculated.

RESULTS

CD61⁺ /AnV^- concentrations were higher in calcium buffer; 841,800 MP/μL (526,000-1,666,200) vs without; 474,200 MP/μL (278,800-997,500), P < .05. In WB, PMP were above DLs and demonstrated acceptable (<20%) intra-assay and inter-assay CVs in 9/10 dogs: 1.7% (0.5-8.9) and 9.0% (0.9-11.9), respectively, for CD61⁺ /AnV^- and 2.4% (0.2-8.7) and 7.8% (0.0-12.8), respectively, for CD61⁺ /AnV⁺ . Acceptable CVs were not seen for the CD61^- /AnV⁺ MP. In PPP, quantifications were challenged by high inter-assay CV, overlapping DLs and hemolysis and lipemia interfered with quantification in 5/10 dogs.

CONCLUSIONS

Calcium induced higher in vitro PMP concentrations, likely due to platelet activation. PMP concentrations were reliably quantified in WB, indicating the potential for clinical applications. PPP analyses were unreliable due to high inter-CV and DL overlap, and not obtainable due to hemolysis and lipemia interference.

Endothelial alterations in a canine model of immune thrombocytopenia

Bleeding heterogeneity amongst patients with immune thrombocytopenia (ITP) is poorly understood. Platelets play a role in maintaining endothelial integrity, and variable thrombocytopenia-induced endothelial changes may influence bleeding severity. Platelet-derived endothelial stabilizers and markers of endothelial integrity in ITP are largely underexplored. We hypothesized that, in a canine ITP model, thrombocytopenia would lead to alterations in the endothelial ultrastructure and that the Von Willebrand factor (vWF) would serve as a marker of endothelial injury associated with thrombocytopenia. Thrombocytopenia was induced in healthy dogs with an antiplatelet antibody infusion; control dogs received an isotype control antibody. Cutaneous biopsies were obtained prior to thrombocytopenia induction, at platelet nadir, 24 hours after nadir, and on platelet recovery. Cutaneous capillaries were assessed by electron microscopy for vessel thickness, the number of pinocytotic vesicles, the number of large vacuoles, and the number of gaps between cells. Pinocytotic vesicles are thought to represent an endothelial membrane reserve that can be used for repair of damaged endothelial cells. Plasma samples were assessed for vWF. ITP dogs had significantly decreased pinocytotic vesicle numbers compared to control dogs (P = 0.0357) and the increase in plasma vWF from baseline to 24 hours correlated directly with the endothelial large vacuole score (R = 0.99103; P < 0.0001). This direct correlation between plasma vWF and the number of large vacuoles, representing the vesiculo-vacuolar organelle (VVO), a permeability structure, suggests that circulating vWF could serve as a biomarker for endothelial alterations and potentially a predictor of thrombocytopenic bleeding. Overall, our results indicate that endothelial damage occurs in the canine ITP model and variability in the degree of endothelial damage may account for differences in the bleeding phenotype among patients with ITP.

Microvesicles in vascular homeostasis and diseases. Position Paper of the European Society of Cardiology (ESC) Working Group on Atherosclerosis and Vascular Biology

Microvesicles are members of the family of extracellular vesicles shed from the plasma membrane of activated or apoptotic cells. Microvesicles were initially characterised by their pro-coagulant activity and described as "microparticles". There is mounting evidence revealing a role for microvesicles in intercellular communication, with particular relevance to hemostasis and vascular biology. Coupled with this, the potential of microvesicles as meaningful biomarkers is under intense investigation. This Position Paper will summarise the current knowledge on the mechanisms of formation and composition of microvesicles of endothelial, platelet, red blood cell and leukocyte origin. This paper will also review and discuss the different methods used for their analysis and quantification, will underline the potential biological roles of these vesicles with respect to vascular homeostasis and thrombosis and define important themes for future research.

Coated-platelet levels are persistently elevated in patients with mild traumatic brain injury

OBJECTIVE

Coated-platelets are procoagulant platelets that are elevated in patients with large-vessel ischemic stroke and are associated with stroke recurrence. Because of recent reports showing an increased risk for stroke following traumatic brain injury (TBI), we undertook a pilot study to investigate coated-platelet synthesis in veterans with TBI.

DESIGN

Cross-sectional study.

PARTICIPANTS

Forty patients with a diagnosis of mild TBI (mTBI) and 40 controls without a history of TBI and matched for age, gender, and ethnicity/race were enrolled in the study.

MAIN MEASURE

Coated-platelet levels were determined in patients with mTBI and controls. The time period since most recent injury ranged from 6 months to 9 years.

RESULTS

Coated-platelet levels were significantly higher for mTBI patients than for controls (mean ± SD = 52.0% ± 14.0% vs 35.4% ± 13.0%; P < .0001). No relationship between these levels and the length of time since the last injury was found (P = .5).

CONCLUSIONS

Coated-platelet levels are markedly and persistently elevated in individuals with mTBI. These data suggest a link to previous findings of increased stroke risk and chronic inflammation among individuals who sustained a TBI.

Acute hemorrhagic complications are associated with lower coated-platelet levels in non-lacunar brain infarction

BACKGROUND

Coated platelets are procoagulant platelets observed upon dual agonist stimulation with collagen and thrombin. Coated-platelet levels are elevated in patients with non-lacunar ischemic stroke and decreased in patients with spontaneous intracerebral hemorrhage as compared with controls.

OBJECTIVE

To investigate whether acute hemorrhagic complications occurring during the initial hospital admission for non-lacunar ischemic stroke are associated with lower coated-platelet levels.

PATIENTS/METHODS

Coated-platelet levels were determined in 385 consecutive patients with non-lacunar stroke. Hemorrhagic complications were defined as either intracranial hemorrhage or significant extracranial bleeding (drop in hemoglobin of ≥ 2 g dL(-1) ). The rate of acute hemorrhagic complication was compared among subjects categorized into tertiles of coated-platelet levels using an exact Cochrane-Armitage trend test. Logistic regression was used to estimate the adjusted odds of hemorrhagic complication associated with coated-platelet levels.

RESULTS

Hemorrhagic complications were present in 15 (3.9%) cases. Of these, four had intracranial hemorrhage and 11 had extracranial hemorrhage. The occurrence of hemorrhagic complications differed among the coated-platelet tertiles: 10.2% for the first tertile (coated-platelet levels < 35.5%), 1.5% for the second tertile and 0% for the third tertile (coated-platelet levels ≥ 47.5%, trend test). Logistic regression showed that the odds of hemorrhagic complication in those with levels < 35.5% were 14.59 times the odds for patients with levels ≥ 35.5% (95% CI: 3.24-65.7).

CONCLUSIONS

Lower levels of procoagulant platelets are associated with acute hemorrhagic complications following non-lacunar ischemic stroke. These results suggest a role for coated-platelets in risk/benefit assessment in the early stages of stroke.

A TMEM16F point mutation causes an absence of canine platelet TMEM16F and ineffective activation and death-induced phospholipid scrambling

BACKGROUND

TMEM16F is an ion channel and calcium-dependent lipid scramblase that mediates phosphatidylserine (PS) exposure on the plasma membrane. Two disparate disease phenotypes are associated with TMEM16F loss-of-function mutations: a rare bleeding disorder (Scott syndrome) and skeletal malformations due to aberrant bone mineralization in a TMEM16F knockout mouse. We therefore undertook comparative studies of TMEM16F expression in canine Scott syndrome (CSS), an autosomal recessive platelet defect.

OBJECTIVES

To define anoctamin proteins and scramblase response of CSS platelets and to determine whether TMEM16F is the CSS disease gene.

METHODS

CSS TMEM16F cDNA and gene were sequenced and mutation detection was performed in CSS pedigrees. Platelet fractions from CSS dogs were isolated for proteomic and immunologic characterization of TMEM16F. Annexin V was used as a flow cytometric marker of induced platelet PS externalization.

RESULTS

A TMEM16F splice site mutation segregated with the CSS trait and TMEM16F protein was undetectable in CSS platelet membranes; however, a second anoctamin, TMEM16K, was found. Proteomic analyses revealed a network of 32 proteins that differentially cosegregated with platelet plasma membrane TMEM16F. CSS platelets had profoundly impaired scramblase response to pharmacologic and physiologic agents that increase intraplatelet calcium and conditions that induce apoptotic and necrotic cell death.

CONCLUSIONS

CSS platelets represent a TMEM16F-null mutant model that demonstrates a central role for TMEM16F in mediating platelet PS externalization in response to activating and death signals. Platelet TMEM16F may prove a novel drug target for modulating platelet procoagulant activity and extending platelet life span.

Coated-Platelet Levels Increase with Number of Injuries in Patients with Mild Traumatic Brain Injury

Coated-platelets are procoagulant platelets that are elevated in stroke and are associated with stroke recurrence. In a previous study, prompted by data showing an increased risk for stroke following traumatic brain injury (TBI), we found that coated-platelet levels are elevated in patients with combat-related mild TBI (mTBI) several years after the injury, compared with controls. We now investigate in an expanded patient population whether parameters commonly recorded in mTBI are related to increased coated-platelet potential. Coated-platelet levels were assayed in 120 mTBI patients at intervals ranging from 6 months to 10 years from the last injury. Correlations were calculated between coated-platelet levels and age, gender, race/ethnicity, loss of consciousness, alteration in consciousness, post-traumatic amnesia, number of injuries, mechanism of injury, time since first and last injury, smoking, medications that may influence coated-platelet levels, and pertinent comorbid conditions. Significant correlations were detected between coated-platelet levels and number of injuries (p = 0.026), gender (p = 0.01), and time since last injury (p = 0.04). A multi-variable linear model analysis, including these three parameters and an additional three parameters (race/ethnicity, smoking, and mechanism of injury) that reached a p value of <0.2, showed that the number of injuries were predictive of coated-platelet levels (p = 0.004). These results support a mechanistic link between increased coated-platelet levels and repeated injuries in mTBI. Long-term studies will be required to determine the impact of increased prothrombotic potential in mTBI patients.

Lower Coated-Platelet Levels Are Associated With Increased Mortality After Spontaneous Intracerebral Hemorrhage

BACKGROUND AND PURPOSE

Coated-platelets are highly procoagulant platelets observed on dual-agonist stimulation with collagen and thrombin. Coated-platelet levels are decreased in patients with spontaneous intracerebral hemorrhage when compared with controls and inversely correlated with bleed volume. We sought to investigate whether coated-platelets are associated with increased mortality at 30 days after spontaneous intracerebral hemorrhage.

METHODS

Coated-platelet levels were assayed in 95 consecutive patients with spontaneous intracerebral hemorrhage. The main outcome was mortality at 30 days according to coated-platelet levels at enrollment. Subjects were grouped into tertiles of the observed coated-platelet level distribution. Groups defined by tertile of coated-platelet level were compared using either ANOVA or a Kruskal-Wallis test for small group size for continuous measures and an exact Cochrane-Armitage trend test for categorical measures. Logistic regression was used to estimate the adjusted odds of death within 30 days associated with coated-platelet levels.

RESULTS

Cumulative mortality at 30 days was 23% (22 subjects). Mortality at 30 days differed among the coated-platelet tertiles: 44% for the first tertile (lowest coated-platelet levels), 19% for the second tertile, and 6% for the third tertile (trend test; P=0.0004). Logistic regression examining the association between mortality and coated-platelet levels showed that the odds of death at 30 days in those with levels <27% (n=47) were 6.83× the odds for patients with levels ≥27% (95% confidence interval, 2.10-22.23).

CONCLUSIONS

These results support a link between impaired coated-platelet potential and outcome in intracerebral hemorrhage.

Platelet dysfunction in injured patients

A renewed understanding of Trauma Induced Coagulopathy (TIC) has implicated platelets as a crucial mediator and potential therapeutic target in hemostasis. While the importance of abnormal coagulation tests is well described in trauma, there is a paucity of data regarding the role of platelets in coagulopathy. New coagulation models, namely the cell-based-model of hemostasis, have refocused attention toward the platelet and endothelium as key regulators of clot formation. Although platelet dysfunction has been associated with worse outcomes in trauma, the mechanisms which platelet dysfunction contributes to coagulopathy are poorly understood. The goal of this review article is to outline recent advances in understanding hemostasis and the ensuing cellular dysfunction that contributes to the exsanguination of a critically injured patient.

Platelet dysfunction in injured patients

A renewed understanding of Trauma Induced Coagulopathy (TIC) has implicated platelets as a crucial mediator and potential therapeutic target in hemostasis. While the importance of abnormal coagulation tests is well described in trauma, there is a paucity of data regarding the role of platelets in coagulopathy. New coagulation models, namely the cell-based-model of hemostasis, have refocused attention toward the platelet and endothelium as key regulators of clot formation. Although platelet dysfunction has been associated with worse outcomes in trauma, the mechanisms which platelet dysfunction contributes to coagulopathy are poorly understood. The goal of this review article is to outline recent advances in understanding hemostasis and the ensuing cellular dysfunction that contributes to the exsanguination of a critically injured patient.

Mitochondrially mediated integrin αIIbβ3 protein inactivation limits thrombus growth

When platelets are strongly stimulated, a procoagulant platelet subpopulation is formed that is characterized by phosphatidylserine (PS) exposure and epitope modulation of integrin αIIbβ3 or a loss of binding of activation-dependent antibodies. Mitochondrial permeability transition pore (mPTP) formation, which is essential for the formation of procoagulant platelets, is impaired in the absence of cyclophilin D (CypD). Here we investigate the mechanisms responsible for these procoagulant platelet-associated changes in integrin αIIbβ3 and the physiologic role of procoagulant platelet formation in the regulation of platelet aggregation. Among strongly stimulated adherent platelets, integrin αIIbβ3 epitope changes, mPTP formation, PS exposure, and platelet rounding were closely associated. Furthermore, platelet mPTP formation resulted in a decreased ability to recruit additional platelets. In the absence of CypD, integrin αIIbβ3 function was accentuated in both static and flow conditions, and, in vivo, a prothrombotic phenotype occurred in mice with a platelet-specific deficiency of CypD. CypD-dependent proteolytic events, including cleavage of the integrin β3 cytoplasmic domain, coincided closely with integrin αIIbβ3 inactivation. Calpain inhibition blocked integrin β3 cleavage and inactivation but not mPTP formation or PS exposure, indicating that integrin inactivation and PS exposure are mediated by distinct pathways subsequent to mPTP formation. mPTP-dependent alkalinization occurred in procoagulant platelets, suggesting a possible alternative mechanism for enhancement of calpain activity in procoagulant platelets. Together, these results indicate that, in strongly stimulated platelets, mPTP formation initiates the calpain-dependent cleavage of integrin β3 and associated regulatory proteins, resulting in integrin αIIbβ3 inactivation, and demonstrate a novel CypD-dependent negative feedback mechanism that limits platelet aggregation and thrombotic occlusion.

Higher levels of coated-platelets are observed in patients with subarachnoid hemorrhage but lower levels are associated with increased mortality at 30 days

BACKGROUND

Coated-platelets are procoagulant platelets observed upon dual agonist stimulation with collagen and thrombin. Coated-platelet levels are elevated in non-lacunar ischemic stroke compared to either lacunar stroke or controls. In contrast, coated-platelet levels are decreased in spontaneous intracerebral hemorrhage (ICH) and inversely correlated with bleed size. We now report the first investigation of coated-platelets in patients with subarachnoid hemorrhage (SAH).

METHODS

Coated-platelet levels were determined in 40 consecutive patients with spontaneous SAH and in 40 controls. Results are reported as percent of cells converted to coated-platelets. Mortality at one month was recorded for all patients.

RESULTS

Coated-platelet levels (mean ± SD) were significantly higher in SAH patients compared to controls (41.8 ± 11.4% vs. 30.7 ± 12.2%, p<0.0001). Among all patients, mortality at 1 month was 20% (8 deaths). Patients were analyzed according to tertiles of coated-platelet levels (split at <36.7%, 36.7-46.2%, >46.2%). The 1-month mortality differed significantly between the coated-platelet tertiles (p=0.01) with 46% mortality (6/13) among patients in the lowest tertile (lowest levels) compared to 14.3% (2/14) among those in the middle tertile and 0% in the highest tertile.

CONCLUSIONS

Coated-platelet levels are higher in SAH patients compared to controls. However, lower coated-platelet levels are associated with increased 1-month mortality in SAH patients, a finding compatible with prior observations of an inverse relationship between coated-platelet levels and bleed volume in ICH. The current data support the role played by these prothrombotic platelets in thrombosis or hemorrhage and suggest a potential place for coated-platelet levels in predicting prognosis after SAH.

Superactivated Platelets

Platelets contribute to hemostasis by forming the platelet plug and then contributing to coagulation by providing a catalytic surface where thrombin generation occurs efficiently. This catalytic activity, known as the platelet procoagulant response, is being recognized as a nuanced response. This review examines platelets’ response to strong stimuli, which results in the formation of a platelet subpopulation (superactivated platelets) with several unique properties, including enhanced procoagulant activity. These platelets contribute uniquely to thrombus architecture and seem to have thrombus regulatory activity. Superactivated platelets’ role in diseases of thrombosis and hemostasis, as either potentiating or mitigating factors, is not currently known, but may be an important pharmacological target.

Flow cytometric detection and procoagulant activity of circulating canine platelet-derived microparticles

OBJECTIVE

To measure platelet membrane-derived microparticle (PMP) content and thrombin-generating capacity of canine plasma subjected to specific processing and storage conditions.

ANIMALS

31 clinically normal dogs (19 males and 12 females).

PROCEDURES

Citrate-anticoagulated blood samples obtained from each dog were centrifuged at 2,500 × g to isolate platelet-poor plasma (PPP), then PPP was centrifuged at 21,000 × g to isolate microparticle-free plasma (MPF) and microparticle-enriched plasma (MPEP). Whole blood and paired samples of fresh and frozen-thawed PPP, MPF, and MPEP were dual labeled for flow cytometric detection of membrane CD61 (constitutive platelet antigen) and annexin V (indicating phosphatidylserine externalization). Platelets and PMPs were enumerated with fluorescent, size-calibrated beads. Thrombin generation in fresh and frozen-thawed PPP, MPF, and MPEP was measured via kinetic fluorometric assays configured with low tissue factor and low phospholipid concentrations.

RESULTS

Initial centrifugation yielded PPP with < 0.5% the platelets of whole blood, with median counts of 413 PMPs/μL for males and 711 PMPs/μL for females. Sequential centrifugation resulted in a 10-fold concentration of PMPs in MPEP and virtually depleted PMPs from MPF. Thrombin generation depended on PMP content, with median endogenous thrombin potential of 0, 893, and 3,650 nmol•min for MPF, PPP, and MPEP, respectively. Freeze-thaw cycling caused significant increases in PMP counts and phosphatidylserine externalization.

CONCLUSIONS AND CLINICAL RELEVANCE

Canine PMPs were major determinants of thrombin-generating capacity; preanalytic variables influenced plasma PMP content. Processing conditions described here may provide a basis for characterization of PMPs in clinical studies of thrombosis in dogs.

Elevated coated-platelets in symptomatic large-artery stenosis patients are associated with early stroke recurrence

Coated-platelets are a subset of platelets with increased procoagulant potential observed upon dual agonist stimulation with collagen and thrombin. These prothrombotic platelets are elevated in patients with non-lacunar ischemic stroke and decreased in patients with spontaneous intracerebral hemorrhage compared to controls. We now investigated coated-platelet synthesis in patients with symptomatic large-artery stenosis and explored the association between coated-platelet levels and stroke recurrence at 3 months in this population. Coated-platelet levels were determined in 60 patients with either acute stroke or transient ischemic attack due to large-artery stenosis and 60 controls. Recurrent stroke incidence at 3 months was stratified by tertiles of coated-platelet levels and compared among groups using a log-rank test. Large-artery stenosis patients had significantly higher coated-platelet levels than controls (mean ± SD, 42.0 ± 15.5% vs. 29.4 ± 13.5%, p < 0.0001). The 3-month cumulative incidence of recurrent stroke was 41% for the highest, 6% for the middle, and 5% for the lowest tertile of coated-platelet levels (p = 0.0045). These results show that elevated coated-platelet levels in patients with symptomatic large-artery stenosis are associated with early stroke recurrence.

Both TMEM16F-dependent and TMEM16F-independent pathways contribute to phosphatidylserine exposure in platelet apoptosis and platelet activation

Scott syndrome, a bleeding disorder caused by defective phospholipid scrambling, has been associated with mutations in the TMEM16F gene. The role of TMEM16F in apoptosis- or agonist-induced phosphatidylserine (PS) exposure was studied in platelets from a Scott syndrome patient and control subjects. Whereas stimulation of control platelets with the BH3-mimetic ABT737 resulted in 2 distinct fractions with moderate and high PS exposure, the high PS-exposing fraction was markedly delayed in Scott platelets. High, but not moderate, PS exposure in platelets was suppressed by chelation of intracellular Ca(2+), whereas caspase inhibition completely abolished ABT737-induced PS exposure in both Scott and control platelets. On the other hand, high PS exposure induced by the Ca(2+)-mobilizing agonists convulxin/thrombin fully relied on mitochondrial depolarization and was virtually absent in Scott platelets. Finally, PS exposure induced by collagen/thrombin was partly affected in Scott platelets, and the residual PS positive fraction was insensitive to inhibition of caspases or mitochondrial depolarization. In conclusion, TMEM16F is not required for, but enhances, caspase-dependent PS exposure; convulxin-/thrombin-induced PS exposure is entirely dependent on TMEM16F, whereas collagen/thrombin-induced PS exposure results from 2 distinct pathways, one of which involves mitochondrial depolarization and is mediated by TMEM16F.

Higher coated-platelet levels are associated with stroke recurrence following nonlacunar brain infarction

Coated-platelets are procoagulant platelets observed upon dual-agonist stimulation with collagen and thrombin. Coated-platelet levels are elevated in patients with nonlacunar (large-vessel) ischemic stroke and decreased in patients with spontaneous intracerebral hemorrhage as compared with controls. The purpose of this study was to investigate a possible relationship between coated-platelet levels and stroke recurrence in patients with nonlacunar ischemic stroke. We assayed coated-platelet levels in 190 consecutive patients with nonlacunar stroke who were followed for up to 12 months; 20 subjects experienced recurrent stroke. Subjects were categorized into tertiles of coated-platelet levels. The distributions of time-to-recurrent stroke were estimated for each tertile using cumulative incidence curves and compared statistically using a log-rank test. The cumulative incidence of recurrent stroke at 12 months differed among the coated-platelet tertiles: 2% for the first tertile (lowest coated-platelet levels), 18% for the second tertile, and 17% for the third tertile (overall log-rank test, P=0.019). These data suggest that higher levels of coated-platelets, measured shortly after a nonlacunar stroke, are associated with an increased incidence of stroke recurrence. This observation offers an additional tool for identifying patients at highest risk for stroke recurrence following a nonlacunar (large-vessel) infarct.

Lipoxygenase and Cyclooxygenase Pathways and Colorectal Cancer Prevention

Colorectal cancer is one of the commonest malignancies in both men and women. In spite of significant progress in screening and in surgical and therapeutic interventions, colorectal cancer (CRC) is still a major public health problem. Accumulating evidence suggests that targeting inflammatory pathways may provide protection against the development of CRC. Eicosanoids derived from the enzymes cyclooxygenase (COX) and lipoxygenase (LOX) may contribute to CRC carcinogenesis. Approaches for targeting COX-1 and COX-2 with traditional nonsteroidal anti-inflammatory agents or targeting COX-2 with specific inhibitors are highly successful at the preclinical and clinical levels; however, large-scale clinical applicability of these agents is limited owing to unwanted side effects. Emerging studies suggests that 5-LOX-derived leukotrienes may contribute to colon tumor development and risk of thrombotic events. Thus, developing drugs that target both 5-LOX and COX-2 may provide a safer strategy. In this review, we discuss evidence for the involvement of 5-LOX in colon tumor development and targeting 5-LOX and COX-2 with synthetic and naturally occurring agents for CRC prevention.

Clinical characterization of canine platelet procoagulant deficiency (Scott syndrome)

BACKGROUND

Platelet function defects are rare causes of bleeding diatheses; however, disease prevalence might be underestimated because diagnosis requires assessment of specific parameters of platelet activation.

OBJECTIVES

The goal of this study was to characterize the clinical presentation of canine Scott syndrome (CSS), an intrinsic platelet function defect first identified in a closed colony of German Shepherds (GSD).

ANIMALS

Eleven (n = 6 female) client-owned GSD affected with CSS that sought veterinary care for one or more episodes of abnormal bleeding.

METHODS

Retrospective review of all cases of CSS diagnosed through the Comparative Coagulation Laboratory at Cornell University between 2005 and 2011. The diagnosis of CSS was based on 2 measures of platelet procoagulant activity: serum prothrombin consumption and flow cytometric detection of platelet phosphatidylserine externalization after in vitro activation.

RESULTS

Postoperative hemorrhage was the most common sign of CSS, whereas petechiae were not found in any dog. Although most GSD responded to platelet transfusion, refractory epistaxis in 2 GSD was managed by nasal arterial embolization. The CSS trait was not restricted to a single pedigree of related GSD or to a single geographic region.

CONCLUSIONS AND CLINICAL IMPORTANCE

Unlike thrombocytopenia and platelet aggregation defects, petechiae and other capillary hemorrhage are not typical features of CSS. After preliminary screening to rule out more common causes of hemorrhage, CSS should be considered in the differential diagnosis of recurrent hemorrhage in GSD, and potentially other breeds of dog. Definitive diagnosis of CSS requires specific tests of platelet procoagulant activity.

Two Types of Procoagulant Platelets Are Formed Upon Physiological Activation and Are Controlled by Integrin α IIb β 3

Objective—

Phosphatidylserine (PS) externalization by platelets upon activation is a key event in hemostasis and thrombosis. It is currently believed that strong stimulation of platelets forms 2 subpopulations, only 1 of which expresses PS.

Methods and Results—

Here, we demonstrate that physiological stimulation leads to the formation of not 1 but 2 types of PS-expressing activated platelets, with dramatically different properties. One subpopulation sustained increased calcium level after activation, whereas another returned to the basal low-calcium state. High-calcium PS-positive platelets had smaller size, high surface density of fibrin(ogen), no active integrin α IIb β 3 , depolarized mitochondrial membranes, gradually lost cytoplasmic membrane integrity, and were poorly aggregated. In contrast, the low-calcium PS-positive platelets had normal size, retained mitochondrial membrane potential and cytoplasmic membrane integrity, and combined retention of fibrin(ogen) with active α IIb β 3 and high proaggregatory function. Formation of low-calcium PS-positive platelets was promoted by platelet concentration increase or shaking and was decreased by integrin α IIb β 3 antagonists, platelet dilution, or in platelets from kindlin-3–deficient and Glanzmann thrombasthenia patients.

Conclusion—

Identification of a novel PS-expressing platelet subpopulation with low calcium regulated by integrin α IIb β 3 can be important for understanding the mechanisms of PS exposure and thrombus formation.

Inherited platelet disorders

OBJECTIVE

To present the latest information on inherited platelet disorders in domestic animals.

DATA SOURCES

Research articles and reviews spanning 40 years available on PubMed.

HUMAN DATA SYNTHESIS

Information regarding inherited platelet disorders in people is plentiful and often descriptions of human conditions have led to the identification of similar disorders in veterinary species. There are exceptions, however, in which specific inherited platelet disorders were first described in animals with subsequent identification in people.

VETERINARY DATA SYNTHESIS

Many inherited platelet disorders have been documented in animals at the functional and molecular level and that information is presented in this review.

CONCLUSIONS

Much progress has been made in the past 20 years in the characterization of inherited platelet disorders in animals at the functional, biochemical, and molecular level. The study of inherited platelet disorders has greatly enhanced the understanding of platelet physiology and has led in some instances to the development of platelet inhibitory medications. Characterization of inherited disorders at the molecular level greatly facilitates diagnosis and identification of affected and heterozygous animals thus avoiding propagation of the defect by breeders. When used with available functional and biochemical diagnostic tests, it significantly enhances the quality of care and case management.

Coated-platelet levels are elevated in patients with transient ischemic attack

Transient ischemic attack (TIA) presents a medical management dilemma because of the high frequency of subsequent ischemic strokes. The ABCD2 score is designed to stratify the short-term risk for stroke in TIA patients. Coated-platelets, which are a subset of procoagulant platelets observed after dual agonist stimulation with collagen and thrombin, are increased in cortical ischemic stroke and decreased in spontaneous intracerebral hemorrhage. Because TIA increases the risk of subsequent ischemic stroke, we conducted a pilot study to investigate whether coated-platelet production is increased in patients with TIA (n = 60) compared with controls (n = 60). Coated-platelet levels were higher in TIA patients versus controls [mean ± standard deviation (SD), 38.4 ± 15.1% vs 31.0 ± 13.2%, P = 0.005], and a significant linear correlation was detected between ABCD2 scores and coated-platelet levels in TIA patients (P = 0.0003, r = 0.45). These findings lend support to the hypothesis that TIA and ischemic stroke represent a spectrum of conditions dependent on similar prothrombotic processes.

Lower coated-platelet levels are associated with early hemorrhagic transformation in patients with non-lacunar brain infarction

BACKGROUND

Coated-platelets, representing a subset of platelets with procoagulant potential, are elevated in patients with non-lacunar ischemic stroke and decreased in patients with spontaneous intracerebral hemorrhage. However, within the non-lacunar patient population there are individuals with lower levels of coated-platelets, which raises the possibility that these individuals would be susceptible to early hemorrhagic transformation (HT) of ischemic stroke.

OBJECTIVE

Because extremes in coated-platelet potential may be associated with either thrombotic or hemorrhagic events, we undertook a pilot study to investigate whether there is an association between coated-platelet production and the presence of early HT in patients with non-lacunar ischemic stroke.

PATIENTS AND METHODS

Coated-platelet levels were determined in 115 consecutive eligible patients with a diagnosis of non-lacunar ischemic stroke. Early HT was determined on CT scan examination and confirmed by MRI studies. The distribution of coated-platelet levels was summarized using the median and interquartile range (25th-75th percentiles) and compared statistically between patients with and without early HT using the non-parametric Wilcoxon rank sum test.

RESULTS

The median coated-platelet level in all non-lacunar stroke patients was 38.0% (interquartile range 30.5-48.3%). Early HT was detected in 11 patients (9.6%), and these patients had significantly lower coated-platelet levels compared with those without early HT [median 25.1% (interquartile range 20.4-35.5%) vs. 39.2% (31.6-49.5%), P = 0.003].

CONCLUSIONS

Lower levels of coated-platelets are associated with the presence of early HT in patients with non-lacunar ischemic stroke.

Coated platelet levels correlate with bleed volume in patients with spontaneous intracerebral hemorrhage

BACKGROUND AND PURPOSE

Coated platelets are produced on dual agonist stimulation with collagen and thrombin. These highly procoagulant platelets are critical to normal hemostasis, and an earlier study demonstrated decreased coated platelet production in patients with spontaneous intracerebral hemorrhage. We have expanded this observation to investigate if coated platelet levels correlate with bleed volume in spontaneous intracerebral hemorrhage.

METHODS

Coated platelet levels and bleed volume were determined in 45 patients with a diagnosis of spontaneous intracerebral hemorrhage.

RESULTS

There was an inverse relationship between coated platelet levels and bleed volume (r=-0.38, P=0.01).

CONCLUSIONS

These data support a link between decreased coated platelet synthesis and the severity of spontaneous intracerebral hemorrhage.

A genome-wide linkage scan in German shepherd dogs localizes canine platelet procoagulant deficiency (Scott syndrome) to canine chromosome 27

Scott syndrome is a rare hereditary bleeding disorder associated with an inability of stimulated platelets to externalize the negatively charged phospholipid, phosphatidylserine (PS). Canine Scott syndrome (CSS) is the only naturally occurring animal model of this defect and therefore represents a unique tool to discover a disease gene capable of producing this platelet phenotype. We undertook platelet function studies and linkage analyses in a pedigree of CSS-affected German shepherd dogs. Based on residual serum prothrombin and flow cytometric assays, CSS segregates as an autosomal recessive trait. An initial genome scan, performed by genotyping 48 dogs for 280 microsatellite markers, suggested linkage with markers on chromosome 27. Genotypes ultimately obtained for a total of 56 dogs at 11 markers on chromosome 27 revealed significant LOD scores for 2 markers near the centromere, with multipoint linkage indicating a CSS trait locus spanning approximately 14 cm. These results provide the basis for fine mapping studies to narrow the disease interval and target the evaluation of putative disease genes.

Coated-platelet levels are low in patients with spontaneous intracerebral hemorrhage

BACKGROUND AND PURPOSE

Coated-platelets are a subset of platelets with high procoagulant potential observed on dual agonist stimulation with collagen and thrombin. Failure to produce coated-platelets in animals results in a bleeding diathesis. With this background, we undertook a pilot study to investigate coated-platelet production in patients with spontaneous intracerebral hemorrhage (SICH).

METHODS

Coated-platelet levels were determined in 26 patients with a diagnosis of SICH and 52 controls.

RESULTS

The patient population had significantly lower coated-platelet levels than the controls (mean+/-SD, 24.8+/-9.7% versus 32.9+/-12.6%, P=0.0035).

CONCLUSIONS

Decreased coated-platelet synthesis may be linked to the mechanisms involved in the events leading to SICH.

Cangrelor attenuates coated-platelet formation

P2Y(12) inhibitors were introduced clinically as effective inhibitors of adenosine-5'-diphosphate (ADP) mediated platelet activation and aggregation. This class of pharmacological agents has enjoyed considerable success. Cangrelor is a recently developed P2Y(12) inhibitor that has the advantage of being an active drug not requiring metabolic conversion, although it is not orally available. Coated-platelets are a subclass of activated platelets generated on dual agonist activation with collagen plus thrombin; the primary hallmark of coated-platelets is their ability to support prothrombinase activity. Interestingly, we recently observed that the relatively weak agonist ADP potentiates the production of coated-platelets by the very strong agonists collagen plus thrombin, a previously unknown role for ADP. The authors sought in this study to determine if P2Y(12) inhibitors, such as cangrelor, were capable of attenuating this augmentation of coated-platelet generation. Cangrelor, at physiologically relevant concentrations, was able to eliminate the ADP-dependent increase in coated-platelet production with an IC(50) of 1.4 nM. Cangrelor, however, had no effect on thrombin-dependent platelet activation as measured by P-selectin expression. Although this in vitro study does not address the question of whether the effectiveness of cangrelor in vivo is partially due to an attenuation of coated-platelet production in addition to its documented antiaggregatory effects, it does reveal an unexpected action of cangrelor. Additional studies will be required to determine if all P2Y(12) inhibitors are equally effective in attenuating coated-platelet production.

Coated-platelet levels in patients with Type 1 and with Type 2 diabetes mellitus

Coated-platelet levels were quantified in 58 people with Type 1 diabetes, 90 with Type 2 diabetes, and 54 non-diabetic controls. In diabetes high coated-platelet levels were related to smoking and glucose control drugs, but not to glycaemia or other drugs. Prospective studies should evaluate coated-platelets and complications and drug effects.

Coated-platelets in ischemic stroke: differences between lacunar and cortical stroke

BACKGROUND

Coated-platelets are a subset of platelets with procoagulant potential observed upon dual agonist stimulation with collagen and thrombin.

OBJECTIVE

The goal was to investigate if coated-platelet production differs between patients with lacunar ischemic stroke and non-lacunar (cortical) ischemic stroke as compared with controls.

PATIENTS AND METHODS

Blood samples from 60 patients with ischemic stroke (20 lacunar and 40 cortical) and 70 controls were analyzed for coated-platelet production.

RESULTS

Coated-platelet production was significantly lower in patients with lacunar stroke (21.8 +/- 11.4%, mean +/- 1 SD) as compared with either controls (31.6 +/- 13.2%, P = 0.008) or patients with cortical stroke (39.4 +/- 12.7%, P < 0.001). The increase in coated-platelets for patients with cortical stroke as compared with controls was also significant (P = 0.008).

CONCLUSIONS

Our results indicate a marked difference in coated-platelet synthesis in lacunar vs. non-lacunar stroke, thereby providing additional support for the existence of distinct pathological processes underlying these two subtypes of ischemic stroke. Further investigation of the role of coated-platelets in stroke, taking into account these preliminary findings, is warranted.