Seminar by Prof. Anna Moroni

University of Milano, Italy; 2TU-Darmstadt, Germany

Structural understanding of HCN4 channel permeation and gating

Hyperpolarization activated cyclic nucleotide gated subtype 4 (HCN4) channels, also known as pacemaker channels of the sinoatrial node, drive the mixed Na⁺–K⁺ inward current I_f that controls automaticity and adrenergic regulation of heart rate. We have solved the structure of the channel with and without cAMP bound to gain information on ligand-modulation of HCN gating. We will present evidence for a subtype-specific metal ion binding site that modulate cAMP effect on the voltage sensor and of two novel helices, aD and aE, that controls the affinity for cAMP in the CNBD. An unexpected finding was that the pore of the apo structure is open and MD simulations confirmed that is conductive. Here, we provide a molecular explanation for two peculiar biophysical properties of HCN channels, very low single channel conductance and weak K+ selectivity. By solving the structure of the complex with ivabradine, an antiarrhythmic drug, we further acquired insight into the mechanism of pore block. Finally, we will show novel therapeutic approaches emerging from HCN4 structural studies such as the isolation and validation of HCN4-specific nanobodies. We found that NB5 harnesses its target, HCN4, into a desired nonpathological conformation, setting the stage to develop new strategies for treating sinus node disease (SND).