Investigation des mécanismes physiologiques menant à la libération du BDNF par les plaquettes et leur susceptibilité aux médicaments antiplaquettaires

Platelets are considered one of the most important reservoirs not only of growth factors but also of neurotrophic factors that may contribute to the repair of vascular lesions and prevention of neurological deterioration. Among these factors, the Brain-Derived Neurotrophic Factor (BDNF), a protein belonging to the neurotrophin family, is largely expressed in both the hippocampus and platelets. In fact, platelets constitute an important reservoir of BDNF; however, little is known about the factors controlling its release into the circulation and whether antiplatelet drugs affect this secretion. Henceforth, the main hypothesis of this project is that platelet activation pathways lead to BDNF release which is affected by antiplatelet agents.  For this purpose, platelets were isolated from the blood of four groups of human subjects following their consent. Group 1 consisted of healthy volunteers; Group 2 and Group 3 consisted of  patients with stable cardiovascular disease on, respectively, dual antiplatelet therapy (aspirin + P2Y12 receptor antagonist) or monotherapy (aspirin) as secondary prevention; and Group 4 consisted of patients with valvular disease or cardiomyopathy who are not on antiplatelet therapy. Platelet aggregation was studied by optical aggregometry in response to the following agonists: adenosine diphosphate (ADP), arachidonic acid (AA), epinephrine, collagen, and thrombin-receptor activated peptide 6 (TRAP-6 amide). The antiplatelet agents that were tested antagonize cyclooxygenase-1 (COX-1) (acetylsalicylic acid (ASA) or aspirin), ADP P2Y12 receptor (AR-C), and fibrinogen receptor αIIbβ3 (Abciximab). BDNF release was quantified by ELISA. BDNF protein and its Tropomyosin-Related Kinase Receptor Type B (TrKB) receptor were detected by immunoblotting.  Our results show that platelet activation in response to several agonists tested induced 80% platelet aggregation and augmented BDNF release by 5 folds, from 2500 pg / 250 x 106 platelets at baseline to approximately 13000 pg / 250 x 106 after stimulation. Moreover, all the tested antiplatelet agents reduced the release of BDNF by stimulated platelets. However, the level of reduction varied differentially between platelet antagonists depending on the platelet agonist used. Indeed, ASA significantly reduced BDNF secretion in response to AA, epinephrine, and TRAP-6, whereas AR-C was more effective in response to ADP, AA, and epinephrine. Abciximab inhibited BDNF secretion as well as platelet aggregation in response to all agonists. Noteworthy, the release of BDNF in response to collagen was inhibited by ASA and AR-C, while platelet aggregation was not affected. Accordingly, no significant correlation between platelet aggregation and BDNF release could be obtained. Although intra-platelet expression was similar in the different groups, the presence of antiplatelet agents reduced the release of BDNF to varying degrees between groups. As such, antiplatelet agents reduced BDNF release more significantly in patients on dual or mono antiplatelet therapy (Groups 2 and 3) as compared to healthy volunteers (Group 1) and valvular disease patients (Group 4). We have also shown that exogenous BDNF activated isolated/washed platelets from healthy volunteers, inducing strong, stable, and irreversible aggregation. In contrast, exogenous BDNF could not induce aggregation of platelets in platelet-rich plasma. In addition, our results indicate that the truncated form of the BDNF receptor, TrKB, is expressed in platelets of healthy volunteers. Hence, the inhibition of TrKB kinase activity abolished BDNF-induced aggregation. These results suggest that the action of BDNF in washed platelets might ensue through TrKB.  We conclude from this study that BDNF is present in platelets and released following platelet activation, and its release is reduced by antiplatelet agents. However, platelet aggregation does not appear to be directly associated with BDNF secretion, suggesting that other underlying mechanisms may be involved in controlling its secretion. Antiplatelet agents reduce the release of BDNF, and it appears that the pro-aggregating action of BDNF on washed platelets ensues, non-exclusively, through TrKB, which means that other types of platelet receptors may also be involved in BDNF signaling. The pathophysiological implication of BDNF released following platelet activation or its bioavailability in the presence of antiplatelet agents in the cardiovascular system thus remain to be elucidated in order to reveal its diagnostic or therapeutic potential.

Keyphrases: BDNF, TrKB, Platelets, Antiplatelet agents, Release, Cardiovascular disease