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Two pacemaker channels from human heart with profoundly different activation kinetics
1Institut für Pharmakologie und Toxikologie der Technischen Universität München, Biedersteiner Strasse 29, 80802 München, Germany.
Insights
Researchers identified two human hyperpolarization-activated and cyclic nucleotide-gated cation channel genes, hHCN2 and hHCN4, in the heart. These channels likely generate the fast and slow components of the cardiac pacemaker current (If), crucial for heart rhythm.
Area of Science:
- Molecular biology
- Cardiovascular physiology
- Ion channel research
Background:
- Cardiac pacemaking relies on the slow diastolic depolarization phase of action potentials.
- The hyperpolarization-activated cation current (If) contributes significantly to pacemaker depolarization, exhibiting fast and slow kinetic components.
- Hyperpolarization-activated and cyclic nucleotide-gated (HCN) channels (HCN1-3) were previously identified in the mouse brain.
Purpose of the Study:
- To identify the molecular components responsible for cardiac pacemaker currents.
- To characterize novel human HCN channel genes expressed in the heart.
Main Methods:
- Screening of a human heart cDNA library using a conserved neuronal HCN channel segment.
- Gene sequencing, localization (chromosome 19p13.3), and exon-intron structure determination for hHCN2.
- Northern blot and PCR analyses to determine tissue expression patterns (heart ventricle and atrium).
- Functional expression of hHCN2 and hHCN4 cDNAs in HEK 293 cells to record hyperpolarization-activated cation currents.
Main Results:
- Two human HCN cDNAs, hHCN2 and hHCN4, were identified and encoded proteins of 889 and 1203 amino acids, respectively.
- Both hHCN2 and hHCN4 genes are expressed in the human heart (ventricle and atrium).
- Expressed hHCN2 and hHCN4 channels produced hyperpolarization-activated cation currents in HEK 293 cells, mimicking native cardiac If.
- hHCN2 currents exhibited fast activation kinetics, while hHCN4 currents showed slow activation kinetics.
Conclusions:
- The identified human genes, hHCN2 and hHCN4, are likely the molecular basis for the cardiac pacemaker current (If).
- hHCN2 and hHCN4 channels correspond to the fast and slow kinetic components of cardiac If, respectively.
- These findings provide crucial insights into the molecular mechanisms underlying cardiac pacemaking.
Abstract:
Cardiac pacemaking is produced by the slow diastolic depolarization phase of the action potential. The hyperpolarization-activated cation current (If) forms an important part of the pacemaker depolarization and consists of two kinetic components (fast and slow). Recently, three full-length cDNAs encoding hyperpolarization-activated and cyclic nucleotide-gated cation channels (HCN1-3) have been cloned from mouse brain. To elucidate the molecular identity of cardiac pacemaker channels, we screened a human heart cDNA library using a highly conserved neuronal HCN channel segment and identified two cDNAs encoding HCN channels. The hHCN2 cDNA codes for a protein of 889 amino acids. The HCN2 gene is localized on human chromosome 19p13.3 and contains eight exons spanning approximately 27 kb. The second cDNA, designated hHCN4, codes for a protein of 1203 amino acids. Northern blot and PCR analyses showed that both hHCN2 and hHCN4 are expressed in heart ventricle and atrium. When expressed in HEK 293 cells, either cDNA gives rise to hyperpolarization-activated cation currents with the hallmark features of native If. hHCN2 and hHCN4 currents differ profoundly from each other in their activation kinetics, being fast and slow, respectively. We thus conclude that hHCN2 and hHCN4 may underlie the fast and slow component of cardiac If, respectively.