Abstract 138: Platelet RIPK1 Promotes Platelet Activation And Aggregation In Arteriolar Thrombosis And Ischemic Stroke

Receptor-interacting protein Ser/Thr kinase 1 (RIPK1) is a molecular switch for inflammation and cell death in nucleated cells, and its function is controlled by phosphorylation at the Ser166 residue. However, it is unknown whether RIPK1 regulates platelet function in arterial thrombosis. Intravital microscopy using megakaryocyte-specific Ripk1 conditional knockout (CKO) mice ( Ripk1 fl/fl;Pf4-cre ) revealed that platelet Ripk1 promoted platelet thrombus formation at sites of laser-induced cremaster arteriolar thrombosis. Deletion of platelet Ripk1 prolonged the time to occlusion in FeCl 3 -induced carotid arterial thrombosis without affecting tail bleeding times. Further, compared to wild-type (WT) mice, Ripk1 CKO mice exhibited improved neurological deficits and reduced infarct volume in ischemic stroke induced by transient middle cerebral artery occlusion. Flow cytometry and aggregation assays showed that deletion of platelet Ripk1 reduced P-selectin exposure, αIIbβ3 integrin activation, and platelet aggregation induced by thrombin, collagen-related peptides (CRP), and U46619, but not ADP. Pretreatment of Ripk1-null platelets with ADP rescued the aggregation defect. Treatment of platelets with Nec-1s, a RIPK1 inhibitor that blocks RIPK1-Ser166 phosphorylation, did not affect platelet activation and aggregation. Instead, RIPK1-Ser320/321 was rapidly phosphorylated in thrombin- or CRP-stimulated human and mouse platelets. Treatment of platelets with an inhibitor of phospholipase C, p38, or MAPK-activated protein kinase 2 (MK2), but not a protein kinase C inhibitor, decreased RIPK1-Ser320/321 phosphorylation after thrombin or CRP stimulation, implying that the p38-MK2 axis phosphorylates RIPK1-Ser320/321. Using a genetic approach in human megakaryoblastic MEG-01 cells, we found that RIPK1 knockdown reduced granule secretion from stimulated MEG-01 cells, which was rescued by overexpressing WT RIPK1, but not mutant RIPK1-Ser320Ala, in the RIPK1 knockdown MEG-01 cells. These results suggest that platelet RIPK1 enhances granule secretion during platelet activation through p38-MK2-mediated phosphorylation of RIPK1-Ser320/321, thereby contributing to the pathology of thrombotic disease.