Platelet reactivity is a stable and global phenomenon in aspirin-treated cardiovascular patients

Do miRNAs Have a Role in Platelet Function Regulation?

MicroRNAs (miRNAs) are a class of non-coding RNAs known to repress mRNA translation and subsequent protein production. miRNAs are predicted to modulate many targets and are involved in regulating various cellular processes. Identifying their role in cell function regulation may allow circulating miRNAs to be used as diagnostic or prognostic markers of various diseases. Increasing numbers of clinical studies have shown associations between circulating miRNA levels and platelet reactivity or the recurrence of cardiovascular events. However, these studies differed regarding population selection, sample types used, miRNA quantification procedures, and platelet function assays. Furthermore, they often lacked functional validation of the miRNA identified in such studies. The latter step is essential to identifying causal relationships and understanding if and how miRNAs regulate platelet function. This review describes recent advances in translational research dedicated to identifying miRNAs' roles in platelet function regulation.

An Ex Vivo and In Silico Study Providing Insights into the Interplay of Circulating miRNAs Level, Platelet Reactivity and Thrombin Generation: Looking beyond Traditional Pharmacogenetics

Platelet reactivity (PR), a key pharmacodynamic (PD) component of the action of antiplatelet drugs in cardiovascular disease (CVD) patients, is highly variable. PR is associated with occurrence or recurrence of thrombotic and bleeding events, but this association is modulated by several factors. Conventional pharmacogenetics explains a minor part of this PR variability, and among determinants of PR, circulating microRNAs (miRNAs) have been the focus of attention during these last years as biomarkers to predict PR and clinical outcomes in CVD. This being said, the impact of miRNAs on platelet function and the mechanisms behind it are largely unknown. The level of a set of candidate miRNAs including miR-126-3p, miR-150-5p, miR-204-5p and miR-223-3p was quantified in plasma samples of stable CVD patients and correlated with PR as assessed by light-transmission aggregometry and in vivo thrombin generation markers. Finally, miRNA target networks were built based on genes involved in platelet function. We show that all candidate miRNAs were associated with platelet aggregation, while only miR-126-3p and miR-223-3p were positively correlated with in vivo thrombin generation markers. In silico analysis identified putative miRNA targets involved in platelet function regulation. Circulating miRNAs were associated with different aspects of platelet reactivity, including platelet aggregation and platelet-supported thrombin generation. This paves the way to a personalized antithrombotic treatment according to miRNA profile in CVD patients.

Pannexin1 Single Nucleotide Polymorphism and Platelet Reactivity in a Cohort of Cardiovascular Patients

Pannexin1 (Panx1), a membrane channel-forming protein permitting the passage of small-sized molecules, such as ATP, is expressed in human platelets. Recently, we showed that inhibiting Panx1 affects collagen-induced platelet aggregation but not aggregation triggered by other agonists. We also found that a single nucleotide polymorphism (SNP; rs1138800) in the Panx1 gene encoded for a gain-of-function channel (Panx1-400C) and was associated with enhanced collagen-induced platelet reactivity. Here, we assessed the association of this SNP with platelet reactivity in a cohort of 758 stable cardiovascular patients from the ADRIE study treated with aspirin and/or clopidogrel. We found that presence of the Panx1-400C allele was not associated with platelet reactivity in stable cardiovascular patients, irrespective of the platelet aggregation agonist used (collagen, ADP or arachidonic acid) or the anti-platelet drug regimen. Moreover, the Panx1-400A > C SNP did also not affect the re-occurrence of cardiac ischemic events in the same stable cardiovascular patient cohort.

Artificial MiRNA Knockdown of Platelet Glycoprotein lbα: A Tool for Platelet Gene Silencing

In recent years, candidate genes and proteins implicated in platelet function have been identified by various genomic approaches. To elucidate their exact role, we aimed to develop a method to apply miRNA interference in platelet progenitor cells by using GPIbα as a proof-of-concept target protein. After in silico and in vitro screening of siRNAs targeting GPIbα (siGPIBAs), we developed artificial miRNAs (miGPIBAs), which were tested in CHO cells stably expressing GPIb-IX complex and megakaryoblastic DAMI cells. Introduction of siGPIBAs in CHO GPIb-IX cells resulted in 44 to 75% and up to 80% knockdown of GPIbα expression using single or combined siRNAs, respectively. Conversion of siGPIBAs to miGPIBAs resulted in reduced silencing efficiency, which could however be circumvented by tandem integration of two hairpins targeting different regions of GPIBA mRNA where 72% GPIbα knockdown was achieved. CHO GPIb-IX cells transfected with the miGPIBA construct displayed a significant decrease in their ability to aggregate characterized by lower aggregate numbers and size compared to control CHO GPIb-IX cells. More importantly, we successfully silenced GPIbα in differentiating megakaryoblastic DAMI cells that exhibited morphological changes associated with actin organization. In conclusion, we here report the successful use of miRNA technology to silence a platelet protein in megakaryoblastic cells and demonstrate its usefulness in functional assays. Hence, we believe that artificial miRNAs are suitable tools to unravel the role of a protein of interest in stem cells, megakaryocytes and platelets, thereby expanding their application to novel fields of basic and translational research.

Functional role of a polymorphism in the Pannexin1 gene in collagen-induced platelet aggregation

Pannexin1 (Panx1) forms ATP channels that play a critical role in the immune response by reinforcing purinergic signal amplification in the immune synapse. Platelets express Panx1 and given the importance of ATP release in platelets, we investigated Panx1 function in platelet aggregation and the potential impact of genetic polymorphisms on Panx1 channels. We show here that Panx1 forms ATP release channels in human platelets and that inhibiting Panx1 channel function with probenecid, mefloquine or specific (10)Panx1 peptides reduces collagen-induced platelet aggregation but not the response induced by arachidonic acid or ADP. These results were confirmed using Panx1-/- platelets. Natural variations have been described in the human Panx1 gene, which are predicted to induce non-conservative amino acid substitutions in its coding sequence. Healthy subjects homozygous for Panx1-400C, display enhanced platelet reactivity in response to collagen compared with those bearing the Panx1-400A allele. Conversely, the frequency of Panx1-400C homozygotes was increased among cardiovascular patients with hyper-reactive platelets compared with patients with hypo-reactive platelets. Exogenous expression of polymorphic Panx1 channels in a Panx-deficient cell line revealed increased basal and stimulated ATP release from cells transfected with Panx1-400C channels compared with Panx1-400A expressing transfectants. In conclusion, we demonstrate a specific role for Panx1 channels in the signalling pathway leading to collagen-induced platelet aggregation. Our study further identifies for the first time an association between a Panx1-400A>C genetic polymorphism and collagen-induced platelet reactivity. The Panx1-400C variant encodes for a gain-of-function channel that may adversely affect atherothrombosis by specifically enhancing collagen-induced ATP release and platelet aggregation.

Antiplatelet drugs and platelet reactivity: is it time to halt clinical research on tailored strategies?

Personalized medicine of antiplatelet drugs in cardiovascular patients has led to a significant enthusiasm. Indeed, numerous longitudinal studies showed an association between high platelet reactivity and the recurrence of ischemic events. The first small randomized trials of P2Y12 blockers tailored to each patient's platelet reactivity yielded encouraging reductions of coronary stent thrombosis in high-risk populations. The discovery of genetic variants contributing to the pharmacodynamic effect of clopidogrel has then paved the way toward a personalized antiplatelet therapy based on reliable and stable genetic tests. This enthusiasm was soon tempered by large interventional trials demonstrating that a platelet function testing-based strategy did not improve clinical outcome and that genetic variants discovered up to now only explained a small part of the pharmacodynamic effect of clopidogrel, thus limiting its clinical use. Looking back to the most recent trials, their target populations and the type of clinical setting, it seems that the one-size-fits all policy regarding antiplatelet drugs may be well acceptable for low-risk patients. On the contrary, integration of the clinical setting as well as other risk factors may help to identify subgroups of patients who could derive a benefit from a truly personalized management of antiplatelet therapy.

Time-dependent changes in non-COX-1-dependent platelet function with daily aspirin therapy

To develop an integrated metric of non-COX-1-dependent platelet function (NCDPF) to measure the temporal response to aspirin in healthy volunteers and diabetics. NCDPF on aspirin demonstrates wide variability, despite suppression of COX-1. Although a variety of NCDPF assays are available, no standard exists and their reproducibility is not established. We administered 325 mg/day aspirin to two cohorts of volunteers (HV1, n = 52, and HV2, n = 96) and diabetics (DM, n = 74) and measured NCDPF using epinephrine, collagen, and ADP aggregometry and PFA100 (collagen/epi) before (Pre), after one dose (Post), and after several weeks (Final). COX-1 activity was assessed with arachidonic acid aggregometry (AAA). The primary outcome of the study, the platelet function score (PFS), was derived from a principal components analysis of NCDPF measures. The PFS strongly correlated with each measure of NCDPF in each cohort. After 2 or 4 weeks of daily aspirin the Final PFS strongly correlated (r > 0.7, P < 0.0001) and was higher (P < 0.01) than the Post PFS. The magnitude and direction of the change in PFS (Final–Post) in an individual subject was moderately inversely proportional to the Post PFS in HV1 (r = -0.45), HV2 (r = -0.54), DM (r = -0.68), P < 0.0001 for all. AAA remained suppressed during aspirin therapy. The PFS summarizes multiple measures of NCDPF. Despite suppression of COX-1 activity, NCDPF during aspirin therapy is predictably dynamic: those with heightened NCDPF continue to decline whereas those with low/normal NCDPF return to pre-aspirin levels over time.