Background Implantable cardiovascular therapeutic devices, while hemodynamically effective, remain limited by thrombosis. A driver of device-associated thrombosis is shear-mediated platelet activation (SMPA). Underlying mechanisms of SMPA, as well as useful biomarkers able to detect and discriminate mechanical versus biochemical platelet activation, are poorly defined. We hypothesized that SMPA induces a differing pattern of biomarkers compared with biochemical agonists. Methods Gel-filtered human platelets were subjected to mechanical activation via either uniform constant or dynamic shear; or to biochemical activation by adenosine diphosphate (ADP), thrombin receptor-activating peptide 6 (TRAP-6), thrombin, collagen, epinephrine, or arachidonic acid. Markers of platelet activation (P-selectin, integrin αIIbβ3 activation) and apoptosis (mitochondrial membrane potential, caspase 3 activation, and phosphatidylserine externalization [PSE]) were examined using flow cytometry. Platelet procoagulant activity was detected by chromogenic assay measuring thrombin generation. Contribution of platelet calcium flux in SMPA was tested employing calcium chelators, ethylenediaminetetraacetic acid (EDTA), and BAPTA-AM. Results Platelet exposure to continuous shear stress, but not biochemical agonists, resulted in a dramatic increase of PSE and procoagulant activity, while no integrin αIIbβ3 activation occurred, and P-selectin levels remained barely elevated. SMPA was associated with dissipation of mitochondrial membrane potential, but no caspase 3 activation was observed. Shear-mediated PSE was significantly decreased by chelation of extracellular calcium with EDTA, while intracellular calcium depletion with BAPTA-AM had no significant effect. In contrast, biochemical agonists ADP, TRAP-6, arachidonic acid, and thrombin were potent inducers of αIIbβ3 activation and/or P-selectin exposure. This differing pattern of biomarkers seen for SMPA for continuous uniform shear was replicated in platelets exposed to dynamic shear stress via circulation through a ventricular assist device-propelled circulatory loop. Conclusion Elevated shear stress, but not biochemical agonists, induces a differing pattern of platelet biomarkers—with enhanced PSE and thrombin generation on the platelet surface. This differential biomarker phenotype of SMPA offers the potential for early detection and discrimination from that mediated by biochemical agonists.

Objective: Mechanical circulatory support has emerged as lifesaving therapy for patients with advanced heart failure. However, mechanical circulatory support remains limited by a paradoxical coagulopathy accompanied by both thrombosis and bleeding. While mechanisms of mechanical circulatory support thrombosis are increasingly defined, mechanical circulatory support-related bleeding, as related to shear-mediated alteration of platelet function, remains poorly understood. We tested the hypothesis that platelet exposure to elevated shear stress, while a defined prothrombotic activator of platelets, coordinately induces downregulation of key platelet adhesion receptors GPIb (glycocalicin)-IX-V, α IIb β 3 , and P-selectin, thus decreasing platelet functional responsiveness to physiological stimuli. Approach and Results: Human gel-filtered platelets were exposed to continuous or pulsatile shear stress in vitro. Surface expression of platelet receptors and platelet-derived microparticle generation were quantified by flow cytometry. Shedding of receptor soluble forms were assessed via ELISA, and platelet aggregation was measured by optical aggregometry. We demonstrate that platelet exposure to elevated shear stress led to a downregulation of GPIb and α IIb β 3 receptors on platelets with a progressive increase in the generation of platelet-derived microparticles expressing elevated levels of α IIb β 3 and GPIb on their surface. No shear-mediated shedding of GPIb and β 3 subunit soluble fragments was detected. Soluble P-selectin was extensively shed from platelets, while surface expression of P-selectin on platelets and microparticles was not significantly altered by shear. Shear-mediated downregulation of GPIb, α IIb β 3 , and P-selectin on platelets was associated with an evident decrease of platelet aggregatory response induced by ADP and TRAP 6 (thrombin receptor activating peptide 6). Conclusions: Our data clearly indicate that accumulation of shear stress, consistent with supraphysiologic conditions characterizing device-supported circulation (1) induces adequate platelet degranulation, yet (2) causes downregulation of primary platelet adhesion receptors via ejection of receptor-enriched platelet-derived microparticles, thus mechanistically limiting platelet activation and the aggregatory response.

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Background:We systematically analyzed the synergistic effect of: i) cytokine-mediated inflammatory activation of endothelial cells (ECs), and ii) shear-mediated platelet activation (SMPA) as a potential contributory mechanism to intraventricular thrombus formation in the setting of Left Ventricular Assist Device (LVAD) support.Methods: Intact and shear-activated human platelets were exposed to nonactivated and cytokine-activated ECs. To modulate the level of LVAD-related shear activation, platelets were exposed to shear stress patterns of varying magnitude (30, 50, 70 dynes/cm 2 , 10min) via a Hemodynamic Shearing Device. ECs were activated via exposure to inflammatory Tumor Necrosis Factor-α (TNF-α, 10 and 100 ng/mL, 24h), consistent with inflammatory activation recorded in patients on LVAD circulatory support.Results: Adhesivity of shear-activated platelets to ECs was significantly higher than that of intact/unactivated platelets, regardless of the initial activation level (70 dynes/cm 2 shearactivated platelets vs. intact platelets: +80%, p<0.001). Importantly, inflammatory activation of ECs amplified platelet prothrombinase activity progressively with increasing shear stress magnitude and TNF-α concentration: thrombin generation of 70 dynes/cm 2 shear-activated platelets was 2.6-fold higher following exposure and adhesion to 100 ng/mL TNF-α−activated ECs (p<0.0001). Conclusions:We demonstrated synergistic effect of SMPA and cytokine-mediated EC inflammatory activation to enhance EC-platelet adhesion and platelet prothrombotic function. These mechanisms might potentially contribute to intraventricular thrombosis in the setting of mechanical circulatory support.

Mechanical circulatory support (MCS) devices continue to be hampered by thrombotic adverse events (AEs), a consequence of device-imparted supraphysiologic shear stresses, leading to shear-mediated platelet activation (SMPA). In advancing MCS devices from design to clinical use, in vitro circulatory loops containing the device under development and testing are utilized as a means of assessing device thrombogenicity. Physical characteristics of these test circulatory loops may also contribute to inadvertent platelet activation through imparted shear stress, adding inadvertent error in evaluating MCS device thrombogenicity. While investigators normally control for the effect of a loop, inadvertent addition of what are considered innocuous connectors may impact test results. Here, we tested the effect of common, additive components of in vitro circulatory test loops, that is, connectors and loop geometry, as to their additive contribution to shear stress via both in silico and in vitro models. A series of test circulatory loops containing a ventricular assist device (VAD) with differing constituent components, were established in silico including: loops with 0~5 Luer connectors, a loop with a T-connector creating 90° angulation, and a loop with 90° angulation. Computational fluid dynamics (CFD) simulations were performed using a k − shear stress transport turbulence model to platelet activation index (PAI) based on a power law model. VAD-operated loops replicating in silico designs were assembled in vitro and gel-filtered human platelets were recirculated within (1 hour) and SMPA was determined. CFD simulations demonstrated high shear being introduced at non-smooth regions such as edge-connector boundaries, tubing, and at Luer holes. Noticeable peaks' shifts of scalar shear stress (sss) distributions toward high shear-region existed with increasing loop complexity. Platelet activation also increased with increasing shear exposure time, being statistically higher when platelets were exposed to connector-employed loop designs.The extent of platelet activation in vitro could be successfully predicted by CFD simulations. Loops employing additional components (non-physiological flow pattern connectors) resulted in higher PAI. Loops with more components (5-connector loop and 90° T-connector) showed 63% and 128% higher platelet activation levels, respectively, versus those with fewer (0-connector (P = .023) and a 90° heat-bend