JNK1^−/− platelets show impaired aggregation responses, associated with defective activation of αIIbβ3. (A) The effect of JNK1 deletion on platelet aggregation. Aggregation of washed platelets (10⁸), monitored by measuring light transmission through stirred suspensions of WT (+/+) or JNK1^−/− (−/−) platelets, was initiated by adding the PAR4 agonist peptide (PAR4-AP; 100 and 150μM), thrombin (40 and 100 mU/mL), collagen (0.8-1.2 μg/mL), or ADP (0.3 and 0.5μM). Aggregation was measured and expressed as the percentage change in light transmission, with the value for the blank (buffer without platelet) set at 100%. Traces are representative of at least 3 independent experiments. (B) Integrin αIIbβ3 activation assessed by flow cytometry of WT platelets or JNK1^−/− platelets, activated by PAR4-AP (75-150μM), thrombin (10 and 50 mU/mL), or ADP (0.3-10μM). Platelets were incubated with a phycoerythrin-labeled rat anti–mouse integrin αIIbβ3 mAb (JON/A) specific for the activated conformation of the mouse integrin. Flow cytometry was performed, without stirring, to prevent platelet aggregation. The level of activated integrin is indicated by the mean fluorescence intensities (MFI) plus or minus SEM, in arbitrary units, from at least 3 experiments. **P < .01, ***P < .001 (unpaired Student t test).