Cyclic dinucleotides bind the C-linker of HCN4 to control channel cAMP responsiveness

cAMP mediates autonomic regulation of heart rate by means of
hyperpolarization-activated cyclic nucleotide-gated (HCN) channels,
which underlie the pacemaker current I-f. cAMP binding to the C-terminal
cyclic nucleotide binding domain enhances HCN open probability through a
conformational change that reaches the pore via the C-linker. Using
structural and functional analysis, we identified a binding pocket in
the C-linker of HCN4. Cyclic dinucleotides, an emerging class of second
messengers in mammals, bind the C-linker pocket (CLP) and antagonize
cAMP regulation of the channel. Accordingly, cyclic dinucleotides
prevent cAMP regulation of I-f in sinoatrial node myocytes, reducing
heart rate by 30\%. Occupancy of the CLP hence constitutes an efficient
mechanism to hinder beta-adrenergic stimulation on I-f. Our results
highlight the regulative role of the C-linker and identify a potential
drug target in HCN4. Furthermore, these data extend the signaling scope
of cyclic dinucleotides in mammals beyond their first reported role in
innate immune system.