Gating movements and ion permeation in HCN4 pacemaker channels

Area of Science:

• Structural Biology
• Molecular Physiology
• Biophysics

Background:

• The HCN1-4 channel family mediates the crucial hyperpolarization-activated cation current (I_f/I_h).
• This current is essential for controlling automaticity in cardiac and neuronal pacemaker cells.
• Understanding HCN channel gating and modulation is key to cardiac and neurological function.

Purpose of the Study:

• To elucidate the structural mechanisms of HCN4 channel gating.
• To investigate the role of cyclic adenosine monophosphate (cAMP) in modulating HCN4 channel activity.
• To provide atomic-level insights into ion permeation through HCN channels.

Main Methods:

• Cryo-electron microscopy (cryo-EM) to determine high-resolution structures of HCN4.
• Analysis of cAMP-bound and cAMP-unbound HCN4 structures in closed and open conformations.
• Molecular dynamics (MD) simulations to investigate ion permeation mechanisms.

Main Results:

• Presented cryo-EM structures of HCN4 in closed and open conformations, with and without cAMP.
• Identified a Mg²⁺ coordination site critical for cAMP-mediated regulation via C-linker and S4-S5 linker interactions.
• Revealed concerted movements of S5 and S6 transmembrane helices in the opening of the cytosolic gate.
• Provided insights into K⁺/Na⁺ permeation mechanisms through open pore structures.

Conclusions:

• The study provides a mechanistic understanding of HCN channel gating and cyclic nucleotide-dependent modulation.
• Structural insights explain how cAMP binding influences HCN4 channel function.
• The findings contribute to understanding ion permeation and the regulation of pacemaker cell automaticity.