Adoptive passive transfer of rabbit β1-adrenoceptor peptide immune cardiomyopathy into the Rag2−/− mouse: Participation of the ER stress

Auto-antibodies against the β1-adrenoceptors are present in 30–40% of patients with dilated cardiomyopathy. Recently, a synthetic peptide corresponding to a sequence of the second extracellular loop of the human β1-adrenoceptor (β1-ECII) has been shown to produce endoplasmic reticulum (ER) stress, myocyte apoptosis and cardiomyopathy in immunized rabbits. To study the direct cardiac effects of anti-β1-ECII antibody in intact animals and if they are mediated via β1-adrenoceptor stimulation, we administered IgG purified from β1-ECII-immunized rabbits to recombination activating gene 2 knock-out (Rag2−/−) mice every 2 weeks with and without metoprolol treatment. Serial echocardiography and cardiac catheterization showed that β1-ECII IgG reduced cardiac systolic function after 3 months. This was associated with increase in heart weight, myocyte apoptosis, activation of caspase-3, -9 and -12, and increased ER stress as evidenced by upregulation of GRP78 and CHOP and cleavage of ATF6. The Rag2−/− mice also exhibited increased phosphorylation of CaMKII and p38 MAPK. Metoprolol administration, which attenuated the phosphorylation of CaMKII and p38 MAPK, reduced the ER stress, caspase activation and cell death. Finally, we employed the small-interfering RNA technology to reduce caspase-12 in cultured rat cardiomyocytes. This reduced not only the increase of cleaved caspase-12 but also of the number of myocyte apoptosis produced by β1-ECII IgG. Thus, we conclude that ER stress plays an important role in cell death and cardiac dysfunction in β1-ECII IgG cardiomyopathy, and the effects of β1-ECII IgG are mediated via the β1-adrenergic receptor.