Molecular characterization of a slowly gating human hyperpolarization-activated channel predominantly expressed in thalamus, heart, and testis

Abstract

Rhythmic activity of neurons and heart cells is endowed by pacemaker channels that are activated by hyperpolarization and directly regulated by cyclic nucleotides (termed HCN channels). These channels constitute a multigene family, and it is assumed that the properties of each member are adjusted to fit its particular function in the cell in which it resides. Here we report the molecular and functional characterization of a human subtype hHCN4. hHCN4 transcripts are expressed in heart, brain, and testis. Within the brain, the thalamus is the predominant area of hHCN4 expression. Heterologous expression of hHCN4 produces channels of unusually slow kinetics of activation and inactivation. The mean potential of half-maximal activation (V 1/2 ) was ؊75.2 mV. cAMP shifted V 1/2 by 11 mV to more positive values. The hHCN4 gene was mapped to chromosome band 15q24-q25. The characteristic expression pattern and the sluggish gating suggest that hHCN4 controls the rhythmic activity in both thalamocortical neurons and pacemaker cells of the heart.Ion channels activated by hyperpolarization and directly regulated by cyclic nucleotides (dubbed HCN channels; see ref. 1) play a fundamental role in shaping the autonomous rhythmic activity of single neurons and the periodicity of network oscillations (for reviews, see refs. 1-3). Among the best studied examples are the pacemaker currents in cells of the sinoatrial node (I f ) (4) and in relay neurons of the thalamus (I h ) (2). The thalamus is the major gateway for the flow of information toward the cerebral cortex, and it is the first station at which incoming signals can be blocked. During sleep, the rapid activity patterns characteristic of the aroused state are replaced by low-frequency, synchronized rhythms of neuronal activity. The physiological significance of the oscillatory modes is uncertain, but they may play a role in controlling the flow of information through the thalamus.During early stage of quiescent sleep, thalamocortical neurons produce synchronized network oscillations of slow periodicity called spindle waves. The waves of electrical activity at 7-14 Hz wax and wane within a 1-to 3-s period and recur periodically once every 3-20 s (reviewed in refs. 2 and 5). Two mouse clones, mHCN1 and mHCN2, have been functionally characterized by heterologous expression. Whereas HCN1 and HCN2 are both expressed in the thalamus, although not exclusively, their time course of activation (7, 8) is significantly faster than that of native HCN channels in thalamocortical neurons (9-12). Furthermore, at resting potentials of Ϫ60 mV to Ϫ68 mV (3), native HCN channels in thalamocortical neurons have a finite open probability (P o ) (2), whereas mHCN1 and mHCN2, owing to their negatively shifted midpoint potentials of activation (V 1/2 Յ Ϫ105 mV), are practically closed at rest.To molecularly identify the HCN channel subtype responsible for oscillatory activity in the thalamus, we screened a thalamus-specific cDNA library. Here we report the cloning and functional characteri...

Immunohistochemical localization of Ih channel subunits, HCN1–4, in the rat brain

“…Activation of HCN3 homomeric channels is even slower (Moosmang et al, 2001), and that of HCN4 channels is the slowest (seconds) (Ishii et al, 1999). Homomeric channels of HCN4 also show a strong response to cAMP (Ishii et al, 1999;Seifert et al, 1999). In addition, different HCN subunits can coassemble to form functional heteromers (Chen et al, 2001;Ulens and Tytgat, 2001).…”

“…Four members of a gene family encoding the mammalian Hyperpolarization-activated and Cyclic-Nucleotide-gated nonselective cation channels (HCNs: HCN1-4), which generate I h , have been cloned (Ludwig et al, 1998;Santoro et al, 1998;Ishii et al, 1999;Seifert et al, 1999).…”

Gene Therapy for Cardiac Arrhythmias

“…If current, also called a "pacemaker current", is the inward current during the diastolic phase, resulting in spontaneous rhythmic activity. 19,20 The adenoviral gene transduction of HCN2 channel induced pacemaker ac- tivity in the atrium 21 and the left bundle branch. 22 These initial trials revealed that a gene transfer into cardiomyocytes evoked spontaneous activity, but the heart rate as a result of these biological pacemakers was relatively low.…”

Computation by oscillations: Implications of experimental data for theoretical models of grid cells

“…A cyclic-nucleotide binding-domain is located on the COOH termini (Santoro et al, 1997;Chen et al, 2001), which mediates the response of HCN channels to cAMP levels. Levels of cAMP can subsequently modulate the activation kinetics of HCN channels to different degrees, another potential molecular mechanism underlying the dorsal-ventral change in stellate cell biophysical properties (Ludwig et al, 1998;Santoro et al, 1998;Seifert et al, 1999). Several other substances can also cause changes in the kinetics of HCN channels, which could also affect the dorsal-ventral changes observed in stellate cells.…”