Prothrombotic Activation of Platelet Pannexin-1 Channels in Antiphospholipid Syndrome.

OBJECTIVE

ATP is released from platelets through both degranulation and pannexin-1 (PANX1) channels. ATP then activates P2X receptors to amplify platelet activation via calcium-dependent signaling. The objective of this study was to evaluate the role of platelet PANX1 channels in the pathophysiology of antiphospholipid syndrome (APS), an acquired thromboinflammatory disorder characterized by platelet-activating antiphospholipid antibodies.

METHODS

Extracellular ATP release was measured in platelets isolated from patients with persistently positive antiphospholipid antibodies (n = 36), patients with systemic lupus erythematosus (n = 10), and healthy controls (n = 16). In some assays, purified platelets were pretreated with the PANX inhibitor carbenoxolone. Platelet signaling, P-selectin exposure, and aggregation were assessed.

RESULTS

Basal ATP release was significantly higher in APS platelets than in controls (P < 0.0001) and normalized in the presence of carbenoxolone (P = 0.03). Carbenoxolone also reduced platelet ATP release and surface P-selectin expression induced by thrombin (P = 0.002 and P = 0.02, respectively) and convulxin (P = 0.0001 and P = 0.0012). Control platelets stimulated with IgG from patients with APS demonstrated enhanced phosphorylation of PANX1 at tyrosine-308 (P = 0.03), which was accompanied by enhanced ATP release (P = 0.0004). APS IgG boosted ADP-induced aggregation (P = 0.0011), which was reduced by carbenoxolone (P = 0.0007). The calcium chelator BAPTA-AM decreased APS IgG-mediated ATP release (P = 0.03), P-selectin expression (P = 0.0005), and aggregation (P = 0.0007). Desensitizing P2X1 receptors with α,β-MeATP decreased APS IgG-induced P-selectin expression (P = 0.0002) and aggregation (P = 0.01).

CONCLUSION

PANX1 channels appear to be important mediators of platelet activation in APS. Agents that block PANX1 may restore platelet homeostasis and limit thrombosis without impairing hemostasis.