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Properties of hyperpolarization-activated pacemaker current defined by coassembly of HCN1 and HCN2 subunits and basal modulation by cyclic nucleotide.

Chen S, Wang J, Siegelbaum SA - J. Gen. Physiol. (2001)

Bottom Line: 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.

View Article: PubMed Central - PubMed

Affiliation: Department of Pharmacology, ColumbiaUniversity, New York, New York 10032, USA.

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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Activation kinetics for HCN1, HCN2, and coexpressed subunits from intact oocytes. (A) Ih currents during hyperpolarizing steps to −105 mV with superimposed fit of sum of two exponential functions (bottom traces) with residuals showing difference between data and fit (top traces). (left) HCN1 alone (10-s step); (middle) HCN1 + HCN2 (10-s step); and (right) HCN2 alone (30-s step). (B) Plot of two exponential constants as function of voltage. (left) Voltage dependence of fast exponential time constant (τf). (middle) Voltage dependence of slow exponential time constant (τs). (right) Relative amplitude of fast exponential component as function of voltage, Af/(Af + As), where Af and As are the amplitudes of the fast and slow exponential components, respectively. (open circles) HCN1; (open squares) HCN2; (closed diamonds) HCN1 + HCN2.
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Figure 2: Activation kinetics for HCN1, HCN2, and coexpressed subunits from intact oocytes. (A) Ih currents during hyperpolarizing steps to −105 mV with superimposed fit of sum of two exponential functions (bottom traces) with residuals showing difference between data and fit (top traces). (left) HCN1 alone (10-s step); (middle) HCN1 + HCN2 (10-s step); and (right) HCN2 alone (30-s step). (B) Plot of two exponential constants as function of voltage. (left) Voltage dependence of fast exponential time constant (τf). (middle) Voltage dependence of slow exponential time constant (τs). (right) Relative amplitude of fast exponential component as function of voltage, Af/(Af + As), where Af and As are the amplitudes of the fast and slow exponential components, respectively. (open circles) HCN1; (open squares) HCN2; (closed diamonds) HCN1 + HCN2.

Mentions: The time course of Ih upon coexpression of HCN1 and HCN2 subunits could not be reproduced by the algebraic sum of independent populations of homomeric HCN1 and HCN2 channel currents, suggesting the formation of heteromultimeric Ih currents with distinct properties (Fig. 1 B). To characterize the properties of the coexpressed channels, we fit the time course of Ih activation with two exponential components (Fig. 2 A), which were necessary and sufficient to describe adequately the activation kinetics of the coexpressed currents as well as the kinetics of HCN1 or HCN2 homomultimers (Santoro et al. 2000).


Properties of hyperpolarization-activated pacemaker current defined by coassembly of HCN1 and HCN2 subunits and basal modulation by cyclic nucleotide.

Chen S, Wang J, Siegelbaum SA - J. Gen. Physiol. (2001)

Activation kinetics for HCN1, HCN2, and coexpressed subunits from intact oocytes. (A) Ih currents during hyperpolarizing steps to −105 mV with superimposed fit of sum of two exponential functions (bottom traces) with residuals showing difference between data and fit (top traces). (left) HCN1 alone (10-s step); (middle) HCN1 + HCN2 (10-s step); and (right) HCN2 alone (30-s step). (B) Plot of two exponential constants as function of voltage. (left) Voltage dependence of fast exponential time constant (τf). (middle) Voltage dependence of slow exponential time constant (τs). (right) Relative amplitude of fast exponential component as function of voltage, Af/(Af + As), where Af and As are the amplitudes of the fast and slow exponential components, respectively. (open circles) HCN1; (open squares) HCN2; (closed diamonds) HCN1 + HCN2.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2233656&req=5

Figure 2: Activation kinetics for HCN1, HCN2, and coexpressed subunits from intact oocytes. (A) Ih currents during hyperpolarizing steps to −105 mV with superimposed fit of sum of two exponential functions (bottom traces) with residuals showing difference between data and fit (top traces). (left) HCN1 alone (10-s step); (middle) HCN1 + HCN2 (10-s step); and (right) HCN2 alone (30-s step). (B) Plot of two exponential constants as function of voltage. (left) Voltage dependence of fast exponential time constant (τf). (middle) Voltage dependence of slow exponential time constant (τs). (right) Relative amplitude of fast exponential component as function of voltage, Af/(Af + As), where Af and As are the amplitudes of the fast and slow exponential components, respectively. (open circles) HCN1; (open squares) HCN2; (closed diamonds) HCN1 + HCN2.
Mentions: The time course of Ih upon coexpression of HCN1 and HCN2 subunits could not be reproduced by the algebraic sum of independent populations of homomeric HCN1 and HCN2 channel currents, suggesting the formation of heteromultimeric Ih currents with distinct properties (Fig. 1 B). To characterize the properties of the coexpressed channels, we fit the time course of Ih activation with two exponential components (Fig. 2 A), which were necessary and sufficient to describe adequately the activation kinetics of the coexpressed currents as well as the kinetics of HCN1 or HCN2 homomultimers (Santoro et al. 2000).

Bottom Line: 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.

View Article: PubMed Central - PubMed

Affiliation: Department of Pharmacology, ColumbiaUniversity, New York, New York 10032, USA.

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.

Show MeSH
Related in: MedlinePlus