BACKGROUND: Apoptosis has been described extensively in acute myocardial infarction and chronic heart failure. Because Daxx (death-associated protein) appears to be essential for stress-induced cell death and acts as an antisurvival molecule, we tested the hypothesis that Daxx is involved in myocardial ischemia/reperfusion-induced cell death in vivo. METHODS AND RESULTS: Transgenic mice overexpressing a dominant-negative form of Daxx (Daxx-DN) under the control of the beta-actin promoter and control wild-type mice underwent an ischemia/reperfusion protocol: 40 minutes of left coronary artery occlusion and 60 minutes of reperfusion. Area at risk and infarct size were measured after dual staining by triphenyltetrazolium chloride and phthalocyanine blue dye. Apoptosis was measured in the ischemic versus the nonischemic part of the left ventricle by terminal deoxynucleotidyl transferase-mediated dUTP biotin nick end labeling staining, enzyme-linked immunosorbent assay, and Western blotting of caspase-3, caspase-8, and poly(ADP-ribose) polymerase. The mitogen-activated protein kinase status was investigated by Western blot analysis. Comparison between groups was assessed by ANOVA or Student t test (statistical significance: P<0.05). Left ventricle tissues from transgenic mice expressed Daxx-DN at the protein level. Area at risk/left ventricle values were comparable among groups. Infarct size/area at risk was 45% reduced in Daxx-DN versus wild-type mice (P<0.001). This cardioprotection was maintained for a 4-hour reperfusion. Ischemia/reperfusion-induced apoptosis was significantly decreased and ERK1/2 prosurvival pathway was activated in ischemic Daxx-DN hearts. CONCLUSIONS: Our study clearly indicates that Daxx participates in myocardial ischemia/reperfusion proapoptotic signaling in vivo.