Cellular expression and functional characterization of four hyperpolarization-activated pacemaker channels in cardiac and neuronal tissues

Abstract

Hyperpolarization-activated cation currents (I(h)) have been identified in cardiac pacemaker cells and a variety of central and peripheral neurons. Four members of a gene family encoding hyperpolarization-activated, cyclic nucleotide-gated cation channels (HCN1--4) have been cloned recently. Native I(h) currents recorded from different cell types exhibit distinct activation kinetics. To determine if this diversity of I(h) currents may be caused by differential expression of HCN channel isoforms, we investigated the cellular distribution of the transcripts of HCN1--4 in the murine sinoatrial node, retina and dorsal root ganglion (DRG) by in situ hybridization. In the sinoatrial node, the most prominently expressed HCN channel is HCN4, whereas HCN2 and HCN1 are detected there at moderate and low levels, respectively. Retinal photoreceptors express high levels of HCN1, whereas HCN2, 3 and 4 were not found in these cells. In DRG neurons, the dominant HCN transcript is HCN1, followed by HCN2. We next determined the functional properties of recombinant HCN1--4 channels expressed in HEK293 cells. All four channel types gave rise to I(h) currents but displayed marked differences in their activation kinetics. Our results suggest that the heterogeneity of native I(h) currents is generated, at least in part, by the tissue-specific expression of HCN channel genes.

G i - and G s -coupled receptors up-regulate the cAMP cascade to modulate HCN2, but not HCN1 pacemaker channels

“…HCN4 channels activate very slowly and also respond strongly to cAMP [10,11], similar to I h in thalamic relay neurons. HCN3, only recently expressed functionally in HEK293 cells, activates with a time course intermediate between HCN2 and HCN4 [12]. Expression patterns have confirmed that HCN1 expression is restricted to neurons of the neocortex, hippocampus, cerebellar cortex and brainstem nuclei [7,13,14].…”

HCN channels: Structure, cellular regulation and physiological function

“…In the sinoatrial node, in all species analyzed so far (e.g. rabbit, guinea pig, mouse and dog) HCN4 is the major isoform, accounting for about 80 % of I h [11,109]. The remaining fraction of this current is species-dependent.…”

The Role of Shox2 in SAN Development and Function

“…This specific molecular marker for the pacemaker [39] is expressed as early as E7.5 in the cardiac crescent, becoming restricted to the sinus horns of the sinus venosus and eventually to the SAN region by E12.5 [16]. Hcn4-mutant mice die at midgestation stage due to the lack of formation of a ''mature'' pacemaker, whereas its overexpression mimics pacemaker properties in cell cultures [26,31,39,40,48]. Nppa (also known as ''atrial natriuretic factor'' [ANF]), which is expressed in atrial myocytes, is a chamber differentiation marker and is absent in the pacemaker; the T-box transcription factor Tbx3, has been reported to be specifically expressed in the cardiac conduction system, including the SAN [21].…”