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Published on: January 10, 2011
Molecular basis for the different activation kinetics of the pacemaker channels HCN2 and HCN4
Juliane Stieber1, Anna Thomer, Barbara Much
1Institut für Pharmakologie und Toxikologie der Technischen Universität München, Biedersteiner Strasse 29, 80802 Munich, Germany. stieber@ipt.med.tu-muenchen.de
Insights
Researchers identified key amino acid residues in HCN4 pacemaker channels that control activation speed. These findings are crucial for understanding cardiac rhythm regulation and developing new therapies.
Area of Science:
- Molecular biology
- Cardiovascular physiology
- Ion channel function
Background:
- Pacemaker channels HCN2 and HCN4 are vital for cardiac sino-atrial node function.
- These channels exhibit distinct activation kinetics, with HCN2 being fast and HCN4 being slow.
- Understanding the molecular basis of these kinetic differences is essential for cardiac electrophysiology.
Purpose of the Study:
- To identify specific amino acid residues responsible for the kinetic differences between HCN2 and HCN4 pacemaker channels.
- To elucidate the structural determinants of the activation time constant (tau act) in HCN channels.
- To investigate the role of specific channel segments and residues in regulating channel gating and cAMP modulation.
Main Methods:
- Construction and analysis of HCN2/4 chimeras and site-directed mutants.
- Electrophysiological recordings to measure activation time constants (tau act) under basal and cAMP-stimulated conditions.
- Investigated mutations in transmembrane segments (S1, S2) and the S1-S2 linker region.
Main Results:
- Single amino acid substitutions in HCN4's S1 segment (L272F) significantly decreased its activation time constant, mimicking HCN2 kinetics.
- Mutations in the S1-S2 linker of HCN4 (N291T, T293A) altered basal activation speed but not cAMP response.
- A mutation in the S2 segment of HCN4 (I308M) abolished cAMP-dependent acceleration and affected C-terminal cAMP binding site deletion response.
Conclusions:
- Specific residues within the S1, S1-S2 linker, and S2 segments of HCN4 are major determinants of its activation speed.
- The S2 segment residue I308 is critical for cAMP-mediated regulation of HCN4 channel activation kinetics.
- These findings provide critical insights into the structural basis of pacemaker channel function and regulation.
Abstract:
The pacemaker channels HCN2 and HCN4 have been identified in cardiac sino-atrial node cells. These channels differ considerably in several kinetic properties including the activation time constant (tau act), which is fast for HCN2 (144 ms at -140 mV) and slow for HCN4 (461 ms at -140 mV). Here, by analyzing HCN2/4 chimeras and mutants we identified single amino acid residues in transmembrane segments 1 and 2 and the connecting loop between S1 and S2 that are major determinants of this difference. Replacement of leucine 272 in S1 of HCN4 by the corresponding phenylalanine present in HCN2 decreased tau act of HCN4 to 149 ms. Conversely, activation of the fast channel HCN2 was decreased 3-fold upon the corresponding mutation of F221L in the S1 segment. Mutation of N291T and T293A in the linker between S1 and S2 of HCN4 shifted tau act to 275 ms. While residues 272, 291, and 293 of HCN4 affected the activation speed at basal conditions they had no obvious influence on the cAMP-dependent acceleration of activation kinetics. In contrast, mutation of I308M in S2 of HCN4 abolished the cAMP-dependent decrease in tau act. Surprisingly, this mutation also prevented the acceleration of channel activation observed after deletion of the C-terminal cAMP binding site. Taken together our results indicate that the speed of activation of the HCN4 channel is determined by structural elements present in the S1, S1-S2 linker, and the S2 segment.
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