Endocannabinoids Control Platelet Activation and Limit Aggregate Formation under Flow

<div><p>Background</p><p>The endocannabinoid system has previously been implicated in the regulation of neurons and inflammatory cells. Additionally, it has been reported that endocannabinoid receptors are present on circulating platelets, but there has been conflicting evidence on their contribution to platelet function.</p><p>Objectives</p><p>Our aim was to examine the role of endocannabinoids in platelet function <i>in vitro</i> and <i>in vivo</i>.</p><p>Methods and Results</p><p>We studied the effects of the well-characterized endogenous endocannabinoid anandamide on platelet aggregation in suspension, α-granule release, calcium mobilization, Syk phosphorylation, as well as platelet spreading and aggregate formation under flow. Anandamide inhibits platelet aggregation and α-granule release by collagen, collagen-derived peptide CRP-XL, ADP, arachidonic acid and thromboxane A2 analogue U46619. However, activation via thrombin receptor PAR-1 stays largely unaffected. Calcium mobilization is significantly impaired when platelets are stimulated with collagen or CRP-XL, but remains normal in the presence of the other agonists. In line with this finding, we found that anandamide prevents collagen-induced Syk phosphorylation. Furthermore, anandamide-treated platelets exhibit reduced spreading on immobilized fibrinogen, have a decreased capacity for binding fibrinogen in solution and show perturbed platelet aggregate formation under flow over collagen. Finally, we investigated the influence of <i>Cannabis sativa</i> consumption by human volunteers on platelet activation. Similar to our <i>in vitro</i> findings with anandamide, <i>ex vivo</i> collagen-induced platelet aggregation and aggregate formation on immobilized collagen under flow were impaired in whole blood of donors that had consumed <i>Cannabis sativa</i>.</p><p>Conclusions</p><p>Endocannabinoid receptor agonists reduce platelet activation and aggregate formation both <i>in vitro</i> and <i>ex vivo</i> after <i>Cannabis sativa</i> consumption. Further elucidation of this novel regulatory mechanism for platelet function may prove beneficial in the search for new antithrombotic therapies.</p></div

Anandamide inhibits platelet α-granule secretion and limits platelet aggregate formation.

<p>Platelet α-granule release was studied by flow cytometric analysis of platelet P-selectin externalization. Washed platelets from healthy human donors were pre-exposed to a concentration series of anandamide for 40 min at 37°C and subsequently stimulated with 2.5 µg/mL CRP-XL (A), 2.5 µM U46619 (B) or 5 µM TRAP (C). Data were normalized to the agonist-induced P-selectin expression of vehicle-pretreated control platelets. Reconstituted blood, which was preincubated with anandamide, was perfused over a collagen-coated surface at a shear rate of 1600 s⁻¹ and snapshots were taken after 5 minutes (D). Subsequently, cover slips were rinsed with buffer for 1 minute to investigate aggregate stability (E). The left panel shows vehicle control, the right panel indicates a perfusion in the presence of 50 µM anandamide. Data are shown as mean ± SD and represent 3 or more individual experiments.</p

The endocannabinoid anandamide inhibits platelet aggregation.

<p>Washed platelets were exposed to a concentration series of anandamide for 40 minutes at 37°C. Subsequently, they were stimulated with 0.5 or 5 µg/mL collagen (A; representative aggregation curves of 0.5 µg/mL collagen stimulation are shown in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0108282#pone.0108282.s001" target="_blank">Figure S1A</a>); 0.1 or 1 µg/mL CRP-XL (B; representative aggregation curves of 0.1 µg/mL CRP-XL are shown in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0108282#pone.0108282.s001" target="_blank">Figure S1B</a>); 10 µM Arachidonic Acid, (AA) (C; representative aggregation curves are shown in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0108282#pone.0108282.s001" target="_blank">Figure S1C</a>); 10 or 100 µM ADP (D; representative curves of 10 µM are shown in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0108282#pone.0108282.s001" target="_blank">Figure S1D</a>); 1 or 10 µM U46619 (E; representative aggregation curves of 1 µM U46619 are shown in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0108282#pone.0108282.s001" target="_blank">Figure S1E</a>); 1, 2.5, 5 or 10 µM TRAP (F; representative aggregation curves of 2.5 µM are shown in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0108282#pone.0108282.s001" target="_blank">Figure S1F</a>), 0.1 U/mL thrombin or anandamide only (G; representative aggregation curves are shown in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0108282#pone.0108282.s001" target="_blank">Figure S1G</a>). Data were normalized to 100% platelet aggregation obtained with the agonists alone in the presence of vehicle. The time-dependence of anandamide inhibition was investigated by incubating washed platelets with 50 µM anandamide or vehicle (DMSO) at 37°C in a time series and subsequently stimulated with 0.5 µg/mL collagen (H). Data are mean ± SD and show 3 or more individual experiments.</p

The inhibitory effect of anandamide on platelet activation is not dependent on platelet preactivation.

<p>Washed platelets were pretreated with 25 µM FAAH-inhibitor URB597. Subsequently, platelet aggregation was induced by various agonists after a suboptimal exposure to anandamide (10 µM, preincubated for 40 minutes). 0.5 µg/mL collagen (A), 0.1 µg/mL CRP-XL (B), 10 µM AA (C), 10 µM ADP (D), 1 µM U46619 (E) or 1 µM TRAP (F). In further experiments, washed platelets were pretreated with 10 µM indomethacin, exposed to anandamide (10 µM, preincubated for 40 minutes) and stimulated with collagen (0.5 µg/mL: G).</p

Anandamide inhibits glycoprotein VI-dependent calcium mobilization and Syk-phosphorylation.

<p>Washed platelets were preincubated with 50 µM anandamide or vehicle for 40 minutes at 37°C. Subsequently, they were exposed to 1 µg/mL collagen, 500 ng/mL CRP-XL, 20 µM ADP, 10 µM U46619, 10 µM TRAP or 1 U/mL thrombin and calcium mobilization was monitored. The (residual) calcium mobilization in the presence of anandamide is expressed as a percentage of uninhibited vehicle control in the presence of the same platelet agonist (A). Western blot analysis of collagen-induced Syk phosphorylation in the presence of vehicle, 50 µM anandamide or control Syk phosphorylation inhibitor PP2 (B; upper lanes indicate phosphorylated Syk (Syk-P), the lower lanes indicate total Syk antigen as a loading control (Syk-T)). Densitometric quantification of collagen-induced Syk-phosphorylation in the presence or absence of anandamide or PP2 (C; n = 4: data are expressed as mean +/− SD).</p

Anandamide reduces glycoprotein IIb/IIIa activation and inhibits platelet spreading.

<p>Washed platelets from healthy human donors were exposed to a concentration series (0–50 µM) of anandamide for 40 minutes at 37°C and subsequently stimulated with 2.5 µg/mL CRP-XL (A), 2.5 µM U46619 (B) or 5 µM TRAP (C). Platelet GPIIb/IIIa activation was studied by flow cytometry. Data were normalized to the agonist-induced GPIIb/IIIa activation of vehicle-pretreated control platelets. Washed platelets were preincubated with 50 µM anandamide or vehicle for 40 minutes at 37°C and perfused over immobilized fibrinogen- at a shear rate of 25 s⁻¹. Subsequently, differential interference contrast microscopy images were taken (D; representative images after 15 minutes of perfusion, scale bars represent 10 µm). Quantification of the spreading behaviour of individual platelets on immobilized fibrinogen (E; spreading was quantified for 3 individual platelets per separate experiment). Quantification of the amount of adhering platelets per field (F). Data represent mean and standard deviation (SD) of 3 or more individual experiments.</p

<i>Cannabis sativa</i> consumption limits platelet aggregate formation under flow and reduces platelet responsiveness to collagen.

<p>Whole blood from <i>Cannabis sativa</i> consumers (n = 4; “Consumers”) or healthy control donors (“Controls”) was perfused for 5 minutes over immobilized collagen at a shear rate of 1600 s⁻¹ (A). In further experiments, collagen-induced platelet aggregation was investigated in platelet-rich plasma from these <i>Cannabis sativa</i> consumers and controls (B). Representative aggregation curves are shown in panel C. Finally, collagen-induced platelet aggregation was studied in platelet-rich plasma of three self-reported <i>Cannabis sativa</i> consumers on two separate instances: once after 10 days of daily consumption of <i>Cannabis sativa</i> (“Use”), as well as after a period of 10 days without consumption (“Withdrawal”).</p

Reactivity of SLE/APS patient samples from cohort 2 in two different anti-β₂GPI assays.

<p>172 patient samples with suspected APS (including 51 SLE patients) were tested for reactivity against coated β₂GPI in assay A and assay B. Results are shown for the 23 domain I-reactive samples. Samples were considered strong positive (+++), positive (++) and weak positive (+) according to the recommendations of the manufacturers.</p>1<p>Antibody reactivity against domain I of β₂GPI, as described previously <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0071402#pone.0071402-deLaat2" target="_blank">[6]</a>.</p

Influence of different assay reagents/protocols on the reactivity of antibodies P1-117 and P2-6 on coated plates of 2 commercial anti-β₂GPI IgG assays.

<p>The reactivity of antibodies P1-117 and P2-6 was tested as explained in Fig. 2 on coated ELISA plates of assay A and B, applying the reagents/protocol of each of the 5 separate assays. Results are expressed as the percentage of the maximum OD value of P2-6. A–B. Graphic representation of results obtained when the protocol and reagents of assay B were applied on a coated plate of assay A (A) and vice versa (B). C. Table showing the reactivity of 10 µg of antibody P1-117 on coated plates of assay A and B using the protocol and reagents of assay A–E.</p

Reactivity of patient samples of cohort 1 in two different anti-β₂GPI assays.

<p>20 selected patient samples, with the indicated characteristics, were tested for reactivity against coated β2GPI in assay A and assay B. Samples were considered strong positive (+++), positive (++) and weak positive (+) according to the recommendations of the manufacturers.</p>1<p>Arterial or venous thrombosis.</p>2<p>Activated partial thromboplastin time/<b><i>dilute Russell's Viper Venom time.</i></b></p>3<p>Antibody reactivity against epitope G40-R43 of domain I of β₂GPI, as described previously <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0071402#pone.0071402-deLaat1" target="_blank">[3]</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0071402#pone.0071402-Iverson1" target="_blank">[4]</a>, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0071402#pone.0071402-Ioannou1" target="_blank">[5]</a>.</p