Unmasking features of the auto-epitope essential for β1 -adrenoceptor activation by autoantibodies in chronic heart failure

AIMS: Chronic heart failure (CHF) can be caused by autoantibodies stimulating the heart via binding to first and/or second extracellular loops of cardiac β1 -adrenoceptors. Allosteric receptor activation depends on conformational features of the autoantibody binding site. Elucidating these features will pave the way for the development of specific diagnostics and therapeutics. Our aim was (i) to fine-map the conformational epitope within the second extracellular loop of the human β1 -adrenoceptor (β1 ECII ) that is targeted by stimulating β1 -receptor (auto)antibodies and (ii) to generate competitive cyclopeptide inhibitors of allosteric receptor activation, which faithfully conserve the conformational auto-epitope.

METHODS AND RESULTS: Non-conserved amino acids within the β1 ECII loop (compared with the amino acids constituting the ECII loop of the β2 -adrenoceptor) were one by one replaced with alanine; potential intra-loop disulfide bridges were probed by cysteine-serine exchanges. Effects on antibody binding and allosteric receptor activation were assessed (i) by (auto)antibody neutralization using cyclopeptides mimicking β1 ECII ± the above replacements, and (ii) by (auto)antibody stimulation of human β1 -adrenoceptors bearing corresponding point mutations. With the use of stimulating β1 -receptor (auto)antibodies raised in mice, rats, or rabbits and isolated from exemplary dilated cardiomyopathy patients, our series of experiments unmasked two features of the β1 ECII loop essential for (auto)antibody binding and allosteric receptor activation: (i) the NDPK211-214 motif and (ii) the intra-loop disulfide bond C209 ↔C215 . Of note, aberrant intra-loop disulfide bond C209 ↔C216 almost fully disrupted the functional auto-epitope in cyclopeptides.

CONCLUSIONS: The conformational auto-epitope targeted by cardio-pathogenic β1 -receptor autoantibodies is faithfully conserved in cyclopeptide homologues of the β1 ECII loop bearing the NDPK211-214 motif and the C209 ↔C215 bridge while lacking cysteine C216 . Such molecules provide promising tools for novel diagnostic and therapeutic approaches in β1 -autoantibody-positive CHF.