Insights into platelet pharmacology from a cryo-EM structure of the ABCC4 transporter.

Structural determination of the ABCC4 transporter is a major first step in providing crucial molecular insights into the transport of platelet substrates into granules, as well as drug transport from platelets. The findings provide a framework for understanding platelet interactions and potential design of specific platelet antagonists.

Aspirin thromboprophylaxis in joint replacement surgery.

BACKGROUND: Aspirin is commonly used as the only pharmacologic agent for prevention of venous thromboembolism (VTE) after joint replacement surgery in the United States. Despite this, prospective studies investigating VTE events after aspirin-only thromboprophylaxis in joint replacement surgery are lacking in the real-world setting. OBJECTIVES: The aim of this study was to estimate the risk of VTE with aspirin-only pharmacologic prophylaxis following joint replacement surgery. METHODS: We carried out a prospective observational study of 350 low-risk patients (no prior history of VTE and low cardiovascular risk factors) who underwent total knee and total hip arthroplasty and received only aspirin for thromboprophylaxis postoperatively. RESULTS: The observed risk of symptomatic VTE was 1.7% (95% confidence interval, 0.9%-3.3%) over 3 months of follow up, with only one major bleeding event and no surgical hematomas. CONCLUSION: The risk of VTE with aspirin monotherapy for thromboprophylaxis in joint replacement surgery in this real-world cohort was higher than previously reported.

Circulating Protein Disulfide Isomerase Is Associated with Increased Risk of Thrombosis in JAK2-Mutated Myeloproliferative Neoplasms.

PURPOSE: Thromboembolic events (TE) are the most common complications of myeloproliferative neoplasms (MPN). Clinical parameters, including patient age and mutation status, are used to risk-stratify patients with MPN, but a true biomarker of TE risk is lacking. Protein disulfide isomerase (PDI), an endoplasmic reticulum protein vital for protein folding, also possesses essential extracellular functions, including regulation of thrombus formation. Pharmacologic PDI inhibition prevents thrombus formation, but whether pathologic increases in PDI increase TE risk remains unknown. EXPERIMENTAL DESIGN: We evaluated the association of plasma PDI levels and risk of TE in a cohort of patients with MPN with established diagnosis of polycythemia vera (PV) or essential thrombocythemia (ET), compared with healthy controls. Plasma PDI was measured at enrollment and subjects followed prospectively for development of TE. RESULTS: A subset of patients, primarily those with JAK2-mutated MPN, had significantly elevated plasma PDI levels as compared with controls. Plasma PDI was functionally active. There was no association between PDI levels and clinical parameters typically used to risk-stratify patients with MPN. The risk of TE was 8-fold greater in those with PDI levels above 2.5 ng/mL. Circulating endothelial cells from JAK2-mutated MPN patients, but not platelets, demonstrated augmented PDI release, suggesting endothelial activation as a source of increased plasma PDI in MPN. CONCLUSIONS: The observed association between plasma PDI levels and increased risk of TE in patients with JAK2-mutated MPN has both prognostic and therapeutic implications.

VWF maturation and release are controlled by 2 regulators of Weibel-Palade body biogenesis: exocyst and BLOC-2.

von Willebrand factor (VWF) is an essential hemostatic protein that is synthesized in endothelial cells and stored in Weibel-Palade bodies (WPBs). Understanding the mechanisms underlying WPB biogenesis and exocytosis could enable therapeutic modulation of endogenous VWF, yet optimal targets for modulating VWF release have not been established. Because biogenesis of lysosomal related organelle-2 (BLOC-2) functions in the biogenesis of platelet dense granules and melanosomes, which like WPBs are lysosome-related organelles, we hypothesized that BLOC-2-dependent endolysosomal trafficking is essential for WPB biogenesis and sought to identify BLOC-2-interacting proteins. Depletion of BLOC-2 caused misdirection of cargo-carrying transport tubules from endosomes, resulting in immature WPBs that lack endosomal input. Immunoprecipitation of BLOC-2 identified the exocyst complex as a binding partner. Depletion of the exocyst complex phenocopied BLOC-2 depletion, resulting in immature WPBs. Furthermore, releasates of immature WPBs from either BLOC-2 or exocyst-depleted endothelial cells lacked high-molecular weight (HMW) forms of VWF, demonstrating the importance of BLOC-2/exocyst-mediated endosomal input during VWF maturation. However, BLOC-2 and exocyst showed very different effects on VWF release. Although BLOC-2 depletion impaired exocytosis, exocyst depletion augmented WPB exocytosis, indicating that it acts as a clamp. Exposure of endothelial cells to a small molecule inhibitor of exocyst, Endosidin2, reversibly augmented secretion of mature WPBs containing HMW forms of VWF. These studies show that, although BLOC-2 and exocyst cooperate in WPB formation, only exocyst serves to clamp WPB release. Exocyst function in VWF maturation and release are separable, a feature that can be exploited to enhance VWF release.

Dose-adjusted enoxaparin thromboprophylaxis in hospitalized cancer patients: a randomized, double-blinded multicenter phase 2 trial.

Hospitalized patients with cancer are at an increased risk of developing venous thromboembolism (VTE). The recommendation for routine pharmacologic thromboprophylaxis in hospitalized patients with cancer to prevent VTE is based on extrapolation of results from noncancer cohorts. There are limited data to support the efficacy and safety of fixed-dose low-molecular-weight heparin (LMWH) regimens in high-risk hospitalized patients with cancer. We conducted a randomized, double-blinded, phase 2 trial in hospitalized patients with active cancer at high risk of developing VTE based on Padua risk score. Patients were randomly assigned to fixed-dose enoxaparin (40 mg daily) vs weight-adjusted enoxaparin (1 mg/kg daily) during hospitalization. The primary objectives were to evaluate the safety of dose-adjusted enoxaparin and evaluate the incidence of VTE with fixed-dose enoxaparin. Blinded clinical assessments were performed at day 14, and patients randomly assigned to fixed-dose enoxaparin subsequently underwent a bilateral lower extremity ultrasound. A total of 50 patients were enrolled and randomized. The median weight of patients enrolled in weight-adjusted enoxaparin arm was 76 kg (range, 60.9-124.5 kg). There were no major hemorrhages or symptomatic VTE in either arm. At time of completion of the blinded clinical assessment, there was only 1 incidentally identified pulmonary embolus that occurred in the weight-adjusted arm. In the group randomly assigned to fixed-dose enoxaparin who subsequently underwent surveillance ultrasound, the cumulative incidence of DVT was 22% (90% binomial confidence interval, 0%-51.3%). This phase 2 trial confirms a high incidence of asymptomatic VTE among high-risk hospitalized patients with cancer and that weight-adjusted LMWH thromboprophylaxis is feasible and well-tolerated. This trial was registered at www.clinicaltrials.gov as #NCT02706249.

Detection of membrane-bound and soluble antigens by magnetic levitation.

Magnetic levitation is a technique for measuring the density and the magnetic properties of objects suspended in a paramagnetic field. We describe a novel magnetic levitation-based method that can specifically detect cell membrane-bound and soluble antigens by measurable changes in levitation height that result from the formation of antibody-coated bead and antigen complex. We demonstrate our method's ability to sensitively detect an array of membrane-bound and soluble antigens found in blood, including T-cell antigen CD3, eosinophil antigen Siglec-8, red blood cell antigens CD35 and RhD, red blood cell-bound Epstein-Barr viral particles, and soluble IL-6, and validate the results by flow cytometry and immunofluorescence microscopy performed in parallel. Additionally, employing an inexpensive, single lens, manual focus, wifi-enabled camera, we extend the portability of our method for its potential use as a point-of-care diagnostic assay.

Measurement of shear-activated platelet aggregate formation in non-anticoagulated blood: utility in detection of clopidogrel-aspirin-induced platelet dysfunction.

We studied the ability of a new instrument, the PlaCor PRT that measures shear-induced platelet aggregation in fingerstick, non-anticoagulated blood without added agonists, to detect platelet dysfunction ex vivo. Platelet reactivity time (PRT) and whole blood aggregation (WBA) were measured in 160 healthy volunteers, before and after aspirin and in 170 participants with established vascular disease or risk factors thereof treated with aspirin ± clopidogrel. Pretreatment PRT and WBA were significantly correlated (collagen r = -.63; arachidonate r = -.65; P < .0001). Following aspirin, the mean PRT increased from 82 to 142 seconds (P < .0001), and in participants treated with clopidogrel-aspirin, the mean PRT (286 seconds, n = 65) was significantly longer than with aspirin alone (166 seconds, n = 105; P < .001). Only 13% of PRTs of participants treated with clopidogrel and aspirin were within the normal range. We conclude that the PlaCor PRT is a simple, rapid, point-of-care instrument that compares favorably with published descriptions of other platelet function instruments.