Analytical and Clinical Validation of a Non-Ristocetin Based VWF Assay on 2 Automated Analyzers in a Large Reference Laboratory.

BACKGROUND: Historically, von Willebrand factor (VWF) activity assays utilized ristocetin despite limitations including poor limits of detection and high imprecision. Newer VWF activity assays such as the INNOVANCE® VWF Ac assay, however, do not rely on ristocetin to measure platelet-dependent VWF function. The purpose of this study was to evaluate the analytical and clinical performance of the Siemens Healthineers INNOVANCE VWF Ac Assay on the Siemens BCS® XP and the Sysmex® CS-2500 systems in a large reference laboratory setting. METHODS: Performance indicators for the INNOVANCE VWF Ac assay were the limit of quantitation (LoQ), precision, and method comparison. Method comparison studies were performed using remnant plasma patient samples from routine coagulation tests and analyzed using both the INNOVANCE VWF Ac assay and the Siemens Healthineers ristocetin-dependent BC von Willebrand Reagent. RESULTS: Evaluation of the INNOVANCE VWF Ac assay on the BCS® XP and CS-2500 systems demonstrated good precision and a lower LoQ compared to the BC von Willebrand Reagent. Method comparisons support the use of the INNOVANCE VWF Ac assay on the BCS® XP and CS-2500 systems to measure platelet-dependent VWF function. The INNOVANCE VWF Ac assay was able to further assist in von Willebrand disease classification in 6/7 (86%) samples when the result was below the LoQ for the BC von Willebrand Reagent (ristocetin cofactor activity). CONCLUSIONS: These data are consistent with the 2021 American Society of Hematology/International Society on Thrombosis and Haemostasis/National Hemophilia Foundation/World Federation of Hemophilia von Willebrand disease guidelines that suggest using newer assays such as the INNOVANCE VWF Ac assay in place of ristocetin cofactor activity assays.

Reduction of ristocetin-induced platelet aggregation (RIPA) during storage despite plasma renewal: evidence for hemostatic importance of GPIbα shedding.

BACKGROUND: Platelet storage is complicated by deleterious changes, among which reduction of ristocetin-induced platelet aggregation (RIPA) has a poorly understood mechanism. The study elucidates the mechanistic roles of all the possible players in this process. RESEARCH DESIGN AND METHODS: PRP-platelet concentrates were subjected to RIPA, collagen-induced platelet aggregation (CIPA), and flowcytometric analysis of GPIbα and PAC-1 binding from days 0 to 5 of storage. Platelet-poor plasma was subjected to colorimetric assays for glucose/LDH evaluation and automatic analyzer to examine VWF antigen and activity. RESULTS: From day three of platelet storage, reducing CIPA but not RIPA was correlated with the reduction of both metabolic state and integrin activity. RIPA reduction was directly related to the decreased levels of total-content/expression of GPIbα, and inversely related to its shedding levels during storage. Re-suspension of 5-day stored platelet in fresh plasma compensated CIPA, but not RIPA. VWF concentration and its activity did not change during storage while they had no correlation with RIPA. CONCLUSIONS: This study identified the irreversible loss of platelet GPIbα, but not VWF status, as the primary cause of the storage-dependent decrease of RIPA. Unlike CIPA, this observation was not compensated by plasma refreshment, suggesting that some evidence of PSL may not be recovered after transfusion.

Investigation of the role of von Willebrand factor in shear-induced platelet activation and functional alteration under high non-physiological shear stress.

BACKGROUND: von Willebrand factor (vWF) plays a crucial role in physiological hemostasis through platelet and subendothelial collagen adhesion. However, its role in shear-induced platelet activation and functional alteration under non-physiological conditions common to blood-contacting medical devices (BCMDs) is not well investigated. METHODS: Fresh healthy human blood was treated with an anti-vWF antibody to block vWF-GPIbα interaction. Untreated blood was used as a control. They were exposed to three levels of non-physiological shear stress (NPSS) (75, 125, and 175 Pa) through a shearing device with an exposure time of 0.5 s to mimic typical shear conditions in BCMDs. Flow cytometric assays were used to measure the expression levels of PAC-1 and P-Selectin and platelet aggregates for platelet activation and the expression levels of GPIbα, GPIIb/IIIa, and GPVI for receptor shedding. Collagen/ristocetin-induced platelet aggregation capacity was characterized by aggregometry. RESULTS: The levels of platelet activation and aggregates increased with increasing NPSS in the untreated blood. More receptors were lost with increasing NPSS, resulting in a decreased capacity of collagen/ristocetin-induced platelet aggregation. In contrast, the increase in platelet activation and aggregates after exposure to NPSS, even at the highest level of NPSS, was significantly lower in treated blood. Nevertheless, there was no notable difference in receptor shedding, especially for GPIIb/IIIa and GPVI, between the two blood groups at the same level of NPSS. The block of vWF exacerbated the decreased capacity of collagen/ristocetin-induced platelet aggregation. CONCLUSIONS: High NPSS activates platelets mainly by enhancing the vWF-GPIbα interaction. Platelet activation and receptor shedding induced by high NPSS likely occur through different pathways.

Synergy by Ristocetin and CXCL12 in Human Platelet Activation: Divergent Regulation by Rho/Rho-Kinase and Rac.

CXCL12, belonging to the CXC chemokine family, is a weak agonist of platelet aggregation. We previously reported that the combination of CXCL12 and collagen at low doses synergistically activates platelets via not CXCR7 but CXCR4, a specific receptor for CXCL12 on the plasma membrane. Recently, we reported that not Rho/Rho kinase, but Rac is involved in the platelet aggregation induced by this combination. Ristocetin is an activator of the von Willebrand factor that interacts with glycoprotein (GP) Ib/IX/V, which generates thromboxane A2 via phospholipase A2 activation, resulting in the release of the soluble CD40 ligand (sCD40L) from human platelets. In the present study, we investigated the effects of a combination of ristocetin and CXCL12 at low doses on human platelet activation and its underlying mechanisms. Simultaneous stimulation with ristocetin and CXCL12 at subthreshold doses synergistically induce platelet aggregation. A monoclonal antibody against not CXCR7 but CXCR4 suppressed platelet aggregation induced by the combination of ristocetin and CXCL12 at low doses. This combination induces a transient increase in the levels of both GTP-binding Rho and Rac, followed by an increase in phosphorylated cofilin. The ristocetin and CXCL12-induced platelet aggregation as well as the sCD40L release were remarkably enhanced by Y27362, an inhibitor of Rho-kinase, but reduced by NSC23766, an inhibitor of the Rac-guanine nucleotide exchange factor interaction. These results strongly suggest that the combination of ristocetin and CXCL12 at low doses synergistically induces human platelet activation via Rac and that this activation is negatively regulated by the simultaneous activation of Rho/Rho-kinase.

Multicenter evaluation of light transmission platelet aggregation reagents: communication from the ISTH SSC Subcommittee on Platelet Physiology.

BACKGROUND: Light transmission aggregation (LTA) is used widely by the clinical and research communities. Although it is a gold standard, there is a lack of interlaboratory harmonization. OBJECTIVES: The primary objective was to assess whether sources of activators (mainly adenosine diphosphate [ADP], collagen, arachidonic acid, epinephrine, and thrombin receptor activating peptide6) and ristocetin contribute to poor LTA reproducibility. The secondary objective was to evaluate interindividual variability of results to appreciate the distribution of normal values and consequently better interpret pathologic results. METHODS: An international multicenter study involving 28 laboratories in which we compared LTA results obtained with center-specific activators and a comparator that we supplied. RESULTS: We report variability in the potency (P) of activators in comparison with the comparator. Thrombin receptor activating peptide 6 (P, 1.32-2.68), arachidonic acid (P, 0.87-1.43), and epinephrine (P, 0.97-1.34) showed the greatest variability. ADP (P, 1.04-1.20) and ristocetin (P, 0.98-1.07) were the most consistent. The data highlighted clear interindividual variability, notably for ADP and epinephrine. Four profiles of responses were observed with ADP from high-responders, intermediate-responders, and low-responders. A fifth profile corresponding to nonresponders (5% of the individuals) was observed with epinephrine. CONCLUSION: Based on these data, the establishment and adoption of simple standardization principles should mitigate variability due to activator sources. The observation of huge interindividual variability for certain concentrations of activators should lead to a cautious interpretation before reporting a result as abnormal. Confidence can be taken from the fact that difference between sources is not exacerbated in patients treated with antiplatelet agents.

Glanzmann Thrombasthenia in Pakistani Patients: Identification of 7 Novel Pathogenic Variants in the Fibrinogen Receptor αIIbß3.

Glanzmann thrombasthenia (GT) is a rare autosomal recessive inherited platelet disorder occurring frequently in populations with high incidence of consanguineous marriages. GT is characterized by quantitative and/or qualitative defect of the platelet αIIbß3 (GPIIb/IIIa) receptor caused by pathogenic variants of the encoding genes: ITGA2B and ITGB3. Patients present with a moderate to severe bleeding tendency with normal platelet count. Platelets show reduced/absent aggregation for all agonists except ristocetin in light transmission aggregometry and reduced/absent αIIbß3 expression in flow cytometry (FC). In this study, we investigated a cohort of 20 Pakistani patients and 2 families collected from the National Institute of Blood Disease, Karachi and Chughtai's Lab, Lahore. Platelet aggregation studies, FC (platelet CD41, CD61, CD42a, CD42b) and direct sequencing of the candidate genes were performed. All patients showed altered platelet aggregation, but normal agglutination after stimulation with ristocetin. Absent/reduced αIIbß3 receptor expression was present in the platelets of 16 patients, in 4 patients expression was borderline/normal. Candidate gene sequencing identified pathogenic/likely pathogenic variants in 15 patients. Seven variants are novel. One patient with absent receptor expression remained without genetic finding. 13 (86.7%) of 15 patients stated consanguinity reflected by homozygosity finding in 14 (93.3%) patients.

Platelet aggregation inhibitors from Bothrops alternatus snake venom.

Snake venoms are a complex mixture of proteins and peptides that can activate/inhibit platelet aggregation. Bothrops alternatus venom include three main families: metalloproteinases (SVMPs), serinoproteinases (SVSPs) and phospholipases A2 (PLA2s), among other minor components. In this work, we used inhibitor cocktails (containing Na2-EDTA, PMSF and/or pBPB) to investigate the effect of these three families and of baltergin (a PIII SVMP) on platelet aggregation by a turbidmetric method using a microplate reader. Cocktails 1 (active SVMPs) and 2 (active PLA2s) significantly reduced aggregation induced by ristocetin and collagen and by collagen and thrombin, respectively. Cocktail 3 (active SVSPs) showed a mild activation of aggregation, indicating the content of thrombin-like enzymes (TLEs) in this venom is low. Cocktail 4 (active minor components) displayed inhibitory effect with all agonists assayed (ristocetin, ADP, collagen and thrombin) but at higher IC50 values. Baltergin exhibited inhibitory effect when the catalytic domain was active for ristocetin-stimulated platelet aggregation and showed a non-enzymatic mechanism of inhibition when collagen was used as agonist. It was not able to disaggregate platelet thrombus. We conclude that B. alternatus venom is a source of natural inhibitors of platelet aggregation due to the action of SVMPs and PLA2s. Other minor components such as C-type lectins likely contribute to the antiplatelet effect. The interest in knowing the action of venom components on platelet function lies both in the understanding of the pathophysiology of snake bite envenomation and in their biotechnological application.

Application of microfluidic chip technology to study the inhibitory effect of tetramethylpyrazine on platelet aggregation, activation, and phosphatidylserine exposure mediated by pathological high shear rate.

OBJECTIVE: In order to study the antithrombotic effect and mechanism of tetramethylpyrazine (TMA). METHODS: In this study, we developed a microfluidic chip model that can mimic normal arteries and stenotic arterial vessels, and studied the inhibitory effects of TMA on platelet aggregation, activation (P-selectin, GPIIb/IIIa, monocyte-platelet aggregates) and phosphatidyl serine (PS) exposure. In addition, we also investigated the effect of TMA on ADP and ristocetin-induced platelet aggregation by turbidimetry. RESULTS: The results showed that TMA significantly inhibited the platelet aggregation, activation and PS exposure induced by pathological high shear rate. Under static conditions, TMA can inhibit ADP and ristocetin-induced platelet aggregation. CONCLUSION: The results indicated that TMA mainly inhibited platelet aggregation, activation and PS exposure by inhibiting the binding of von Willebrand factor (vWF) to the GPIb/IX/V complex, and partially inhibited platelet aggregation through the platelet P2Y 12 -ADP receptor pathway.

Platelet dysfunction persists after trauma despite balanced blood product resuscitation.

BACKGROUND: Platelet activation and aggregation are critical to the initiation of hemostasis after trauma with hemorrhage. Platelet dysfunction is a well-recognized phenomenon contributing to trauma-induced coagulopathy. The goal of this study was to evaluate the timing and severity of platelet dysfunction in massively transfused, traumatically injured patients during the first 72 hours after injury and its association with 30-day survival. METHODS: A retrospective secondary cohort study of platelet count and function was performed using samples from the Pragmatic Randomized Optimal Platelet and Plasma Ratios trial. Platelet characteristics were measured at 8 timepoints during the first 72 hours of hospitalization and compared between 30-day survivors and nonsurvivors. Platelet counts were assessed via flow cytometry. Platelet function was analyzed with the use of serial thrombelastography and impedance aggregometry with agonists arachidonic acid, adenosine diphosphate, collagen, thrombin receptor activating peptide, and ristocetin. RESULTS: In total, 680 patients were included for analysis. Platelet counts were significantly lower from baseline to 72 hours after hospital admission with further 1.3 to 2-fold reductions noted in nonsurvivors compared to survivor patients. Platelet aggregation via adenosine diphosphate, arachidonic acid, collagen, thrombin receptor activating peptide, and ristocetin was significantly lower in nonsurvivors at all time points. The nadir of platelet aggregation was 2 to 6 hours after admission with significant improvements in viscoelastic maximum clot formation and agonist-induced aggregation by 12 hours without concomitant improvement in platelet count. CONCLUSION: Platelet aggregability recovers 12 hours after injury independent of worsening thrombocytopenia. Failure of platelet function to recover portends a poor prognosis.

Diagnóstico de la enfermedad de von Willebrand / Diagnosis of von Willebrand disease

La enfermedad de von Willebrand (EVW) es el trastorno hemorrágico hereditario más común, y se caracteriza por presentar disminución de la capacidad del factor von Willebrand (FVW) de unirse a las plaquetas y al colágeno de la matriz extracelular durante la hemostasia primaria, debido a defectos cuantitativos o cualitativos. La EVW se clasifica en tres fenotipos principales: el 1 y el 3 que son trastornos cuantitativos, y el 2 que se subclasifica en 2A, 2B, 2M y 2N, y refleja los trastornos cualitativos. Para su diagnóstico son necesarios varios pasos: 1) la evaluación del historial de sangrado personal y familiar del paciente, 2) detección inicial de trastornos hemorrágicos, 3) pruebas para la detección de la EVW, 4) pruebas para la tipificación de la EVW, y 5) el análisis molecular. Tanto la subclasificación de la EVW como su diagnóstico continúan planteando desafíos importantes, motivo por el cual se realiza esta revisión, de manera que los profesionales de la salud tengan una guía que los oriente al momento de tener pacientes con algún trastorno hemorrágico que amerite descartar una EVW e implementar un tratamiento adecuado

von Willebrand disease (VWD) is the most common hereditary bleeding disorder, and is characterized by a decreased ability of the von Willebrand factor (VWF) to bind to platelets and extracellular matrix collagen during primary hemostasis, due to quantitative or qualitative defects. VWD is classified into three main phenotypes: 1 and 3, which are quantitative disorders, and 2 (2A, 2B, 2M and 2N) that reflects qualitative disorders. Several steps are necessary for its diagnosis: 1) evaluation of the patient's personal and family bleeding history, 2) initial screening tests for bleeding disorders, 3) tests for the detection of VWD, 4) tests for the classification of VWD, and 5) molecular analysis. Both the subclassification of VWD and its diagnosis continue to represent important challenges, which we aimed to describe in this review, so that health professionals have a guide to assist them when they have patients with a bleeding disorder that requires exclusion of VWD, and implementation of an appropriate treatment.

Analysis of College of American Pathologists von Willebrand Factor Proficiency Testing Program.

Von Willebrand factor (VWF) level and/or function is altered in von Willebrand disease (VWD), the most common heritable bleeding disorder worldwide. Laboratory assessment of VWF is continually evolving. Historically, the primary method for the assessment of VWF platelet-binding activity was the ristocetin cofactor assay (VWF:RCo). Contemporary alternative measures of VWF platelet-binding activity include VWF:GPIbR (recombinant; using ristocetin), VWF:GPIbM (recombinant; gain-of-function mutant), and monoclonal antibody. Recently, the American Society of Hematology, International Society on Thrombosis and Haemostasis, National Hemophilia Foundation, and World Federation of Hemophilia collaboration issued guidelines recommending the use of newer assays of VWF platelet-binding activity (VWF: GPIbM, VWF: GPIbR) over VWF:RCo, given known limitations of the VWF:RCo assay. Despite this recommendation, the newer VWF:GPIbM and VWF:GPIbR assays are not United States Food and Drug Administration cleared, limiting their availability in the United States. We sought to assess assay utilization trends, agreement of VWF testing methods, and imprecision of VWF testing (based on assigned sample type) from the College of American Pathologists Proficiency Testing Surveys. The analysis confirms that, while VWF antigen testing has low imprecision, the various VWF activity assays have significant interassay variability, with VWF:RCo showing greater imprecision than the newer GPIb-binding assays. The overall trends in assay utilization reflect the barriers to complete compliance with modern VWD diagnostic guidelines in North America.

Role of thrombin to non-physiological shear stress induced platelet activation and function alternation.

OBJECTIVE: Non-physiological shear stress (NPSS) and thrombin have two distinct mechanisms for activating platelets. NPSS in mechanically assisted circulation (MAC) devices can cause platelet dysfunction, e.g., by shedding its key receptors. In addition, patients with heart failure have increased levels of thrombin generation, which may further affect the NPSS-induced platelet dysfunction, resulting in device-associated complications. This study aimed to assess the combined effect of NPSS and thrombin in platelet activation, expression of adhesion receptors on the platelet surface, and alterations of platelet aggregation. METHODS: Fresh human blood from healthy donors was divided into two groups; one group was treated by adding 0.01 U/mL thrombin, and another group not treated with thrombin served as a control comparison. They were then pumped through a novel blood shearing device which produces similar shear stress conditions to those in the MAC devices. Three levels of NPSS (i.e., 75, 125, and 175 Pa) with a 1.0 s exposure time were selected for the shearing conditions. Expression of platelet activation markers (PAC-1, activated GPIIb/IIIa and CD62P, platelet surface P-selectin) were investigated along with the shedding of platelet receptors (GPIb, GPIIb/IIIa, and GPVI), generation of platelet microparticles, and Phosphatidylserine (PS)-positive platelets detected by flow cytometry. Platelet aggregation (induced by collagen/ristocetin) was measured by Lumi-aggregometry. RESULTS: Platelet receptors were shed after exposure to NPSS showing a positive correlation with the level of shear stress. The generation of platelet microparticles and PS-positive platelets also increased with greater NPSS. Elevated NPSS decreased the platelet aggregation capacity. Platelet activation level increased with greater NPSS. Being treated by thrombin can further exacerbate these characteristics under same level of NPSS, except that platelet activation level drastically dropped after the exposure to 175 Pa NPSS in the thrombin-treated blood. CONCLUSION: After being treated by thrombin, platelets became more susceptible to NPSS, resulting in more receptor shedding, platelet microparticles, and PS-positive platelets, thus limiting platelet aggregation capacity after exposure to NPSS. Platelet activation, in terms of PAC-1 and P-selectin, is an interim status competing between the expression and shedding of these makers/receptors. When platelets have reached a saturation level of activation, exposure to excessive NPSS can potentially impair activation.

N-Stearoylethanolamine Inhibits Integrin-Mediated Activation, Aggregation, and Adhesion of Human Platelets.

N-stearoylethanolamine (NSE), a lipid mediator that belongs to the N-acylethanolamine (NAE) family, has anti-inflammatory, antioxidant, and membranoprotective actions. In contrast to other NAEs, NSE does not interact with cannabinoid receptors. The exact mechanism of its action remains unclear. The aim of this study is to evaluate the action of NSE on activation, aggregation, and adhesion of platelets that were chosen as a model of cellular response. Aggregation of platelets was measured to analyze the action of NSE (10-6-10-10 M) on platelet reactivity. Changes in granularity and shape of resting platelets and platelets stimulated with ADP in the presence of NSE were monitored by flow cytometry, and platelet deganulation was monitored by spectrofluorimetry. In vivo studies were performed using obese insulin-resistant rats. Binding of fibrinogen to the GPIIb/IIIa receptor was estimated using indirect ELISA and a scanning electron microscopy (SEM). It was found that NSE inhibits the activation and aggregation of human platelets. Our results suggest that NSE may decrease the activation and subsequent aggregation of platelets induced by ristocetin, epinephrine, and low doses of ADP. NSE also reduced the binding of fibrinogen to GPIIb/IIIa on activated platelets. These effects could be explained by the inhibition of platelet activation mediated by integrin receptors: the GPIb-IX-V complex for ristocetin-induced activation and GPIIb/IIIa when epinephrine and low doses of ADP were applied. The anti-platelet effect of NSE complements its anti-inflammatory effect and allows us to prioritize studies of NSE as a potent anti-thrombotic agent. SIGNIFICANCE STATEMENT: N-stearoylethanolamine (NSE) was shown to possess inhibitory action on platelet activation, adhesion, and aggregation. The mechanism of inhibition possibly involves integrin receptors. This finding complements the known anti-inflammatory effects of NSE.

Elucidation of retention mechanism of dipeptides on a ristocetin A-based chiral stationary phase using a combination of chromatographic and molecular simulation techniques.

Two chiral stationary phases virtually reproducing the Nautilus-R column were modeled in silico to study the enantiorecognition mechanism of some selected dipeptides, taking into consideration the two different anchoring alternatives to the silica layer involving the two ristocetin A amino groups. A mobile phase composed of water-methanol (40:60, v/v) was included in the system. The analyses of the trajectories supported the experimental L(LL)

Navigating Hemostasis of Bleeding Among Children With ß-Thalassemia.

Bleeding phenotype is reported in ß-thalassemia patients. However, the underlying etiology remains elusive. We aimed to assess coagulation profile and the platelet aggregation in ß-thalassemia children with bleeding diathesis. Fifty ß-thalassemia children with a positive bleeding history were recruited. Bleeding phenotype was explored through full history taking and thorough clinical examination. Complete blood count, prothrombin time, international normalized ratio, and platelets aggregometry were performed for children with negative workup. Mucosal bleeding was manifest among most of our patients (96%). Two-third of patients had decreased aggregation with ristocetin (68%), adenine di-phosphate (64%), and arachidonic acid (64%). While half of the patients (48%) had deficient response to epinephrine. Collagen, ristocetin, and arachidonic acid induced aggregation were negatively correlated to frequency of blood transfusion (P=0.021, r=-0.325; P<0.001, r=-0.465; P=0.018, r=-0.333, respectively). Aggregation to collagen and epinephrine demonstrated a negative correlation with age (P=0.04, r=-0.287; P=0.03, r=-0.315). Deferiprone was associated with a deficient response to ristocetin and collagen when compared with deferasirox or no chelation (P=0.021 and 0.006, respectively). Impaired ristocetin response was linked to hydroxyurea (P=0.035). Platelets function defect should be considered in ß-thalassemia patients with bleeding symptoms.

Osteoprotegerin modulates platelet adhesion to von Willebrand factor during release from endothelial cells.

BACKGROUND: Platelet-binding Von Willebrand Factor (VWF) strings assemble upon stimulated secretion from endothelial cells. OBJECTIVES: To investigate the efficiency of platelet binding to multi-molecular VWF bundles secreted from endothelial cells and to investigate the role of osteoprotegerin, a protein located in Weibel-Palade bodies that interacts with the VWF platelet binding domain. METHODS: The nanobody VWF/AU-a11 that specifically binds to VWF in its active platelet-binding conformation was used to investigate the conformation of VWF. RESULTS: Upon stimulated secretion from endothelial cells, VWF strings were only partially covered with platelets, while a VWD-type 2B mutation or ristocetin enhanced platelet binding by 2-3-fold. Osteoprotegrin, reduces platelet adhesion to VWF by 40% ± 18% in perfusion assays. siRNA-mediated down-regulation of endothelial osteoprotegerin expression resulted in a 1.8-fold increase in platelet adhesion to VWF strings. Upon viral infection, there is a concordant rise in VWF and osteoprotegerin plasma levels. Unexpectedly, no such increase was observed in plasma of desmopressin-treated hemophilia A-patients. In a mouse model, osteoprotegerin expression was low in liver endothelial cells of vehicle-treated mice, and concanavalin A-treatment increased VWF and osteoprotegerin expression 4- and 40-fold, respectively. This increase was translated in a 30-fold increased osteoprotegerin/VWF ratio in plasma. CONCLUSIONS: Release of VWF from endothelial cells opens the platelet-binding site, irrespective of the presence of flow. However, not all available platelet-binding sites are being occupied, suggesting some extent of regulation. Part of this regulation involves endothelial proteins that are co-secreted with VWF, like osteoprotegerin. This regulatory mechanism may be of more relevance under inflammatory conditions.

The von Willebrand factor A-1 domain binding aptamer BT200 elevates plasma levels of von Willebrand factor and factor VIII: a first-in-human trial.

Von Willebrand factor (VWF) and factor VIII (FVIII) circulate in a noncovalent complex in blood and promote primary hemostasis and clotting, respectively. A new VWF A1-domain binding aptamer, BT200, demonstrated good subcutaneous bioavailability and a long half-life in non-human primates. This first-in-human, randomized, placebo-controlled, doubleblind trial tested the hypothesis that BT200 is well tolerated and has favorable pharmacokinetic and pharmacodynamic effects in 112 volunteers. Participants received one of the following: a single ascending dose of BT200 (0.18-48 mg) subcutaneously, an intravenous dose, BT200 with concomitant desmopressin or multiple doses. Pharmacokinetics were characterized, and the pharmacodynamic effects were measured by VWF levels, FVIII clotting activity, ristocetin-induced aggregation, platelet function under high shear rates, and thrombin generation. The mean half-lives ranged from 7-12 days and subcutaneous bioavailability increased dose-dependently exceeding 55% for doses of 6-48 mg. By blocking free A1 domains, BT200 dose-dependently decreased ristocetin-induced aggregation, and prolonged collagen-adenosine diphosphate and shear-induced platelet plug formation times. However, BT200 also increased VWF antigen and FVIII levels 4-fold (P<0.001), without increasing VWF propeptide levels, indicating decreased VWF/FVIII clearance. This, in turn, increased thrombin generation and accelerated clotting. Desmopressin-induced VWF/FVIII release had additive effects on a background of BT200. Tolerability and safety were generally good, but exaggerated pharmacology was seen at saturating doses. This trial identified a novel mechanism of action for BT200: BT200 dose-dependently increases VWF/FVIII by prolonging half-life at doses well below those which inhibit VWF-mediated platelet function. This novel property can be exploited therapeutically to enhance hemostasis in congenital bleeding disorders.

Enhancing Ristomycin A Production by Overexpression of ParB-Like StrR Family Regulators Controlling the Biosynthesis Genes.

Amycolatopsis sp. strain TNS106 harbors a ristomycin-biosynthetic gene cluster (asr) in its genome and produces ristomycin A. Deletion of the sole cluster-situated StrR family regulatory gene, asrR, abolished ristomycin A production and the transcription of the asr genes orf5 to orf39. The ristomycin A fermentation titer in Amycolatopsis sp. strain TNS106 was dramatically improved by overexpression of asrR and a heterologous StrR family regulatory gene, bbr, from the balhimycin-biosynthetic gene cluster (BGC) utilizing strong promoters and multiple gene copies. Ristomycin A production was improved by approximately 60-fold, resulting in a fermentation titer of 4.01 g/liter in flask culture, in one of the engineered strains. Overexpression of AsrR and Bbr upregulated transcription of tested asr biosynthetic genes, indicating that these asr genes were positively regulated by AsrR and Bbr. However, only the promoter region of the asrR operon and the intergenic region upstream of orf12 were bound by AsrR and Bbr in gel retardation assays, suggesting that AsrR and Bbr directly regulated the asrR operon and probably orf12 to orf14 but no other asr biosynthetic genes. Further assays with synthetic short probes showed that AsrR and Bbr specifically bound not only probes containing the canonical inverted repeats but also a probe with only one 7-bp element of the inverted repeats in its native context. AsrR and Bbr have an N-terminal ParB-like domain and a central winged helix-turn-helix DNA-binding domain. Site-directed mutations indicated that the N-terminal ParB-like domain was involved in activation of ristomycin A biosynthesis and did not affect the DNA-binding activity of AsrR and Bbr. IMPORTANCE This study showed that overexpression of either a native StrR family regulator (AsrR) or a heterologous StrR family regulator (Bbr) dramatically improved ristomycin A production by increasing the transcription of biosynthetic genes directly or indirectly. The conserved ParB-like domain of AsrR and Bbr was demonstrated to be involved in the regulation of asr BGC expression. These findings provide new insights into the mechanism of StrR family regulators in the regulation of glycopeptide antibiotic biosynthesis. Furthermore, the regulator overexpression plasmids constructed in this study could serve as valuable tools for strain improvement and genome mining for new glycopeptide antibiotics. In addition, ristomycin A is a type III glycopeptide antibiotic clinically used as a diagnostic reagent due to its side effects. The overproduction strains engineered in this study are ideal materials for industrial production of ristomycin A.

In vitro assessment and phase I randomized clinical trial of anfibatide a snake venom derived anti-thrombotic agent targeting human platelet GPIbα.

The interaction of platelet GPIbα with von Willebrand factor (VWF) is essential to initiate platelet adhesion and thrombosis, particularly under high shear stress conditions. However, no drug targeting GPIbα has been developed for clinical practice. Here we characterized anfibatide, a GPIbα antagonist purified from snake (Deinagkistrodon acutus) venom, and evaluated its interaction with GPIbα by surface plasmon resonance and in silico modeling. We demonstrated that anfibatide interferds with both VWF and thrombin binding, inhibited ristocetin/botrocetin- and low-dose thrombin-induced human platelet aggregation, and decreased thrombus volume and stability in blood flowing over collagen. In a single-center, randomized, and open-label phase I clinical trial, anfibatide was administered intravenously to 94 healthy volunteers either as a single dose bolus, or a bolus followed by a constant rate infusion of anfibatide for 24 h. Anfibatide inhibited VWF-mediated platelet aggregation without significantly altering bleeding time or coagulation. The inhibitory effects disappeared within 8 h after drug withdrawal. No thrombocytopenia or anti-anfibatide antibodies were detected, and no serious adverse events or allergic reactions were observed during the studies. Therefore, anfibatide was well-tolerated among healthy subjects. Interestingly, anfibatide exhibited pharmacologic effects in vivo at concentrations thousand-fold lower than in vitro, a phenomenon which deserves further investigation.Trial registration: Clinicaltrials.gov NCT01588132.

Impact of electric cardioversion on platelet activation.

INTRODUCTION: Atrial fibrillation (AF) comes along with high risk of stroke. This risk continues even after re-establishing sinus rhythm with cardioversion. Aim of this study is to evaluate the contribution of electric cardioversion (EC) to platelet activation and procoagulatory tendency. METHODS: Extent of platelet activation before and after electric cardioversion was quantified using flow cytometry, impedance aggregation measurements with Multiplate®, and quantification of serum levels of platelet factor 4 (PF4) and ß-thromboglobulin (ß-TG) in patients with AF (N = 10). RESULTS: No significant differences were observed in any of the measured parameters comparing the values from before and after cardioversion. Geometric means of P-selectin expression and integrin αIIbß3 activation were 0.27 (+/- 0.07) and 2.30 (+/- 2.61) before EC and 0.28 (+/- 0.17) and 1.67 (+/- 1.82) after EC. Levels of ß-TG were 110.11 ng/ml (+/- 3.78) before and 110.51 ng/ml (+/- 2.56) after EC, levels of PF4 were 35.64 ng/ml (+/- 12.94) before and 32.40 ng/ml (+/- 4.95) after EC. Platelet aggregation triggered with adenosine diphosphate (ADP), arachidonic acid, collagen, Ristocetin, or thrombin receptor activating peptide (TRAP) revealed results within the normally expected ranges without significant changes before and after EC. DISCUSSION: Electric cardioversion has no influence on platelet activation markers which is in agreement with other studies reporting electrical cardioversion to be safe.