Unmasking features of the auto-epitope essential for β -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 β -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 β -adrenoceptor (β EC ) that is targeted by stimulating β -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 β EC loop (compared with the amino acids constituting the EC loop of the β -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 β EC  ± the above replacements, and (ii) by (auto)antibody stimulation of human β -adrenoceptors bearing corresponding point mutations. With the use of stimulating β -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 β EC loop essential for (auto)antibody binding and allosteric receptor activation: (i) the NDPK motif and (ii) the intra-loop disulfide bond C ↔C . Of note, aberrant intra-loop disulfide bond C ↔C almost fully disrupted the functional auto-epitope in cyclopeptides.

Conclusions: The conformational auto-epitope targeted by cardio-pathogenic β -receptor autoantibodies is faithfully conserved in cyclopeptide homologues of the β EC loop bearing the NDPK motif and the C ↔C bridge while lacking cysteine C . Such molecules provide promising tools for novel diagnostic and therapeutic approaches in β -autoantibody-positive CHF.