Key Points• In vivo and in vitro thrombus formation is altered in MRP4-deficient mice.• MRP4 modulates the cAMPprotein kinase A platelet signaling pathway.Molecules that reduce the level of cyclic adenosine 59-monophosphate (cAMP) in the platelet cytosol, such as adenosine 59-diphosphate (ADP) secreted from dense granules, trigger platelet activation. Therefore, any change in the distribution and/or availability of cyclic nucleotides or ADP may interfere with platelet reactivity. In this study, we evaluated the role of multidrug resistance protein 4 (MRP4, or ABCC4), a nucleotide transporter, in platelet functions in vivo and in vitro by investigating MRP4-deficient mice. MRP4 deletion resulted in a slight increase in platelet count but had no impact on platelet ultrastructure. In MRP4-deficient mice, the arterial occlusion was delayed and the tail bleeding time was prolonged. In a model of platelet depletion and transfusion mimicking a platelet-specific knockout, mice injected with MRP4 2/2 platelets also showed a significant increase in blood loss compared with mice injected with wild-type platelets.Defective thrombus formation and platelet activation were confirmed in vitro by studying platelet adhesion to collagen in flow conditions, integrin aIIbb3 activation, washed platelet secretion, and aggregation induced by low concentrations of proteinase-activated receptor 4-activating peptide, U46619, or ADP. We found no role of MRP4 in ADP dense-granule storage, but MRP4 redistributed cAMP from the cytosol to dense granules, as confirmed by increased vasodilator-stimulated phosphoprotein phosphorylation in MRP4-deficient platelets. These data suggest that MRP4 promotes platelet aggregation by modulating the cAMP-protein kinase A signaling pathway, suggesting that MRP4 might serve as a target for novel antiplatelet agents. (Blood. 2015;126(15):1823-1830