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Ask1 regulates murine platelet granule secretion, thromboxane A2 generation, and thrombus formation

Meghna U. Naik, Pravin Patel, Randall Derstine, Ramya Turaga, Xi Chen, Kalyan Golla, Keith B. Neeves, Hidenori Ichijo and Ulhas P. Naik

Key Points

  • ASK1 regulates TxA2 generation through p38 MAPK-dependent phosphorylation of cPLA2.

  • Because of impaired platelet function, Ask1−/− mice are protected from arterial thrombosis and pulmonary thromboembolism.

Publisher's Note: There is an Inside Blood Commentary on this article in this issue.

Abstract

Mitogen-activated protein kinases (MAPKs) are expressed in platelets and are activated downstream of physiological agonists. Pharmacological and genetic evidence indicate that MAPKs play a significant role in hemostasis and thrombosis, but it is not well understood how MAPKs are activated upon platelet stimulation. Here, we show that apoptosis signal-regulating kinase 1 (ASK1), a member of the MAP3K family, is expressed in both human and murine platelets. ASK1 is rapidly and robustly activated upon platelet stimulation by physiological agonists. Disruption of Ask1 (Ask1−/−) resulted in a marked functional defect in platelets. Ask1−/− platelets showed an impaired agonist-induced integrin αIIbβ3 activation and platelet aggregation. Although there was no difference in Ca2+ rise, platelet granule secretion and thromboxane A2 (TxA2) generation were significantly attenuated in Ask1−/− platelets. The defective granule secretion observed in Ask1−/− platelets was a consequence of impaired TxA2 generation. Biochemical studies showed that platelet agonists failed to activate p38 MAPK in Ask1−/− platelets. On the contrary, activation of c-Jun N-terminal kinases and extracellular signal-regulated kinase 1/2 MAPKs was augmented in Ask1−/− platelets. The defect in p38 MAPK results in failed phosphorylation of cPLA2 in Ask1−/− platelets and impaired platelet aggregate formation under flow. The absence of Ask1 renders mice defective in hemostasis as assessed by prolonged tail-bleeding times. Deletion of Ask1 also reduces thrombosis as assessed by delayed vessel occlusion of carotid artery after FeCl3-induced injury and protects against collagen/epinephrine-induced pulmonary thromboembolism. These results suggest that the platelet Ask1 plays an important role in regulation of hemostasis and thrombosis.

  • Submitted July 21, 2016.
  • Accepted December 12, 2016.
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