Anti-beta1-adrenoceptor autoantibodies with chronotropic activity from the serum of patients with dilated cardiomyopathy: mapping ofepitopes in the first and second extracellular loops

In a preceding communication (Wallukat et al., 1992, Z Kardiol 81 [Suppl. 4]: 79-83), it was reported that synthetic peptides, corresponding in amino acid sequence to either the first or the second extracellular loop of the human beta 1-adrenoceptor, selectively suppressed the metoprolol- and bisoprolol-sensitive positive chronotropic action exerted in cultures of beating neonatal rat cardiomyocytes by the serum immunoglobulin fraction of patients with myocarditis and idiopathic dilated cardiomyopathy (DCM) and by affinity-purified autoantibodies from that fraction. These observations added to existing evidence that these antibodies were directed against the beta 1-adrenoceptor and might thus contribute to the harmful chronic cardiac adrenergic drive to which patients with DCM are believed to be exposed. Specifically, they pointed to the putative first and second extracellular loops of this receptor (these loops are each identical in man and the rat) as the sites of epitopes recognized by the chronotropically active, beta 1-agonistic autoantibodies. Now we report on the mapping of these epitopes with the help of two series of short synthetic overlap peptides, one series forming part of the first and the other of the second extracellular loop of the beta 1-adrenoceptor. Inhibition of the positive chronotropic response of cultured rat cardiomyocytes to the anti-beta 1-receptor autoantibodies (EC50 = 0.14 +/- 0.01 nM) from the serum immunoglobulin fraction of patients with DCM was taken as reflecting the neutralization of these antibodies by a particular overlap peptide. In this way the sequences S-F-F-C-E-L (residues 129-134) and A-R-R-C-Y-N-D (residues 206-212) emerged as the dominant epitopes in the first and second extracellular loops, respectively, followed with respect to neutralizing ability by the first loop sequence E-Y-G-S-F-F (residues 126-131) and the second loop sequences H-W-W-R-A-E (residues 197-202) and P-K-C-C-D-F (residues 213-218). Synthetic peptides corresponding to the sequences of the third extracellular loop of the beta 1-receptor (residues 346-356) and of the second extracellular loop of the human beta 2-receptor (residues 172-197) failed to neutralize the beta 1-agonistic autoantibodies. Using dithiothreitol as a reducing agent a disulfide bridge between cysteine 132 in the first and cysteine 209 in the second extracellular loop was considered to be essential for the chronotropic action of these autoantibodies