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Published on: February 8, 2011
Properties of hyperpolarization-activated pacemaker current defined by coassembly of HCN1 and HCN2 subunits and basal
S Chen1, J Wang, S A Siegelbaum
1Department of Pharmacology, ColumbiaUniversity, New York, New York 10032, USA.
Insights
Hyperpolarization-activated cyclic nucleotide-gated (HCN) channel isoforms 1 and 2 coassemble in the brain to form novel heteromeric channels. These channels exhibit unique biophysical properties distinct from homomeric channels, influencing neuronal activity.
Area of Science:
- Neuroscience
- Molecular Biology
- Biophysics
Background:
- Hyperpolarization-activated cation currents (Ih) are generated by HCN channels, crucial for pacemaker activity in the heart and brain.
- Four HCN isoforms (HCN1-4) exhibit distinct tissue expression patterns.
- HCN1 and HCN2 isoforms are coexpressed in the neocortex and hippocampus and possess different biophysical properties.
Purpose of the Study:
- To investigate the functional properties of heteromeric channels formed by coexpressed HCN1 and HCN2 isoforms.
- To determine if HCN1 and HCN2 channels coassemble and form channels with novel properties.
- To compare the properties of heteromeric HCN channels with those of homomeric HCN1 and HCN2 channels and native Ih currents.
Main Methods:
- Expression of HCN1 and HCN2 isoforms in Xenopus oocytes.
- Electrophysiological recordings (two-electrode voltage clamp) in oocytes.
- Cell-free patch-clamp recordings.
- Analysis of channel kinetics, voltage dependence, and cAMP modulation.
Main Results:
- HCN1 channels activate faster and at more positive potentials than HCN2 channels.
- HCN1 channels show minimal cAMP-dependent shift, while HCN2 channels show a pronounced shift.
- Coexpressed HCN1 and HCN2 channels form heteromeric channels with intermediate kinetics and voltage dependence, and a significant cAMP-dependent shift.
- The properties of heteromeric channels cannot be explained by a simple sum of homomeric channels.
- Heteromeric channel properties closely resemble native Ih currents in hippocampal CA1 pyramidal neurons.
Conclusions:
- HCN1 and HCN2 isoforms coassemble to form functional heteromeric channels with unique biophysical properties.
- These heteromeric channels contribute to the native Ih currents observed in hippocampal neurons.
- The formation of heteromeric channels provides a mechanism for generating diverse Ih current properties within a single cell.
Abstract:
Members of the HCN channel family generate hyperpolarization-activated cation currents (Ih) that are directly regulated by cAMP and contribute to pacemaker activity in heart and brain. The four HCN isoforms show distinct but overlapping patterns of expression in different tissues. Here, we report that HCN1 and HCN2, isoforms coexpressed in neocortex and hippocampus that differ markedly in their biophysical properties, coassemble to generate heteromultimeric channels with novel properties. When expressed in Xenopus oocytes, HCN1 channels activate 5-10-fold more rapidly than HCN2 channels. HCN1 channels also activate at voltages that are 10-20 mV more positive than those required to activate HCN2. In cell-free patches, the steady-state activation curve of HCN1 channels shows a minimal shift in response to cAMP (+4 mV), whereas that of HCN2 channels shows a pronounced shift (+17 mV). Coexpression of HCN1 and HCN2 yields Ih currents that activate with kinetics and a voltage dependence that tend to be intermediate between those of HCN1 and HCN2 homomers, although the coexpressed channels do show a relatively large shift by cAMP (+14 mV). Neither the kinetics, steady-state voltage dependence, nor cAMP dose-response curve for the coexpressed Ih can be reproduced by the linear sum of independent populations of HCN1 and HCN2 homomers. These results are most simply explained by the formation of heteromeric channels with novel properties. The properties of these heteromeric channels closely resemble the properties of I(h) in hippocampal CA1 pyramidal neurons, cells that coexpress HCN1 and HCN2. Finally, differences in Ih channel properties recorded in cell-free patches versus intact oocytes are shown to be due, in part, to modulation of Ih by basal levels of cAMP in intact cells.
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