Molecular and Functional Heterogeneity of Hyperpolarization-Activated Pacemaker Channels in the Mouse CNS

Abstract

The hyperpolarization-activated cation current (termed I h , I q , or I f ) was recently shown to be encoded by a new family of genes, named HCN for hyperpolarization-activated cyclic nucleotidesensitive cation nonselective. When expressed in heterologous cells, each HCN isoform generates channels with distinct activation kinetics, mirroring the range of biophysical properties of native I h currents recorded in different classes of neurons. To determine whether the functional diversity of I h currents is attributable to different patterns of HCN gene expression, we determined the mRNA distribution across different regions of the mouse CNS of the three mouse HCN genes that are prominently expressed there (mHCN1, 2 and 4). We observe distinct patterns of distribution for each of the three genes. Whereas mHCN2 shows a widespread expression throughout the CNS, the expression of mHCN1 and mHCN4 is more limited, and generally complementary. mHCN1 is primarily expressed within neurons of the neocortex, hippocampus, and cerebellar cortex, but also in selected nuclei of the brainstem. mHCN4 is most highly expressed within neurons of the medial habenula, thalamus, and olfactory bulb, but also in distinct neuronal populations of the basal ganglia. Based on a comparison of mRNA expression with an electrophysiological characterization of native I h currents in hippocampal and thalamic neurons, our data support the idea that the functional heterogeneity of I h channels is attributable, in part, to differential isoform expression. Moreover, in some neurons, specific functional roles can be proposed for I h channels with defined subunit composition.

Formation of heteromeric hyperpolarization-activated cyclic nucleotide-gated (HCN) channels in the hippocampus is regulated by developmental seizures

“…The enhanced appearance of HCN1/HCN2 coassociation after seizures was not attributable to cross-reactivity of their respective antisera for several reasons: first, IP using anti-HCN2 followed by probing for the same isoform resulted in an HCN2-immunoreactive band with the expected molecular weight of 96 kDa. As described for HCN2, this protein was more abundant in thalamic homogenates compared to those from equal-weight hippocampal tissue (Santoro and Baram, 2003;Santoro et al, 2000) (Fig. 1C, top).…”

“…As described for other members of the voltage-gated cation channel family, four HCN subunits assemble to form a channel (for recent reviews, see Robinson and Siegelbaum, 2003;Santoro and Baram, 2003). Homomeric HCN1 channels conduct fastkinetic currents with modest cAMP gating, consistent with currents recorded in hippocampal pyramidal cells and CA1 interneurons where HCN1 expression is high (Bender et al, 2001;Brewster et al, 2002;Santoro et al, 2000). By contrast, homomeric HCN2 channels conduct I h currents with slower kinetics and robust cAMP-evoked shifts in voltage dependence (Ludwig et al, 1998;Robinson and Siegelbaum, 2003).…”

“…HCN4 is expressed almost exclusively in subcortical regions, whereas levels of HCN3 expression within neurons are generally low (Bender et al, 2001;Moosmang et al, 1999;Santoro et al, 2000). As described for other members of the voltage-gated cation channel family, four HCN subunits assemble to form a channel (for recent reviews, see Robinson and Siegelbaum, 2003;Santoro and Baram, 2003).…”

“…Of the four characterized genes encoding HCN channels, two (HCN1 and 2) are substantially expressed in rodent hippocampus (Bender et al, 2001;Moosmang et al, 1999;Santoro et al, 2000) and both isoforms can reside in a single neuron (Brewster et al, 2002;Franz et al, 2000). HCN4 is expressed almost exclusively in subcortical regions, whereas levels of HCN3 expression within neurons are generally low (Bender et al, 2001;Moosmang et al, 1999;Santoro et al, 2000).…”

“…Because the isoform composition of both homomeric and heteromeric HCN channels determines their physiological characteristics (Robinson and Siegelbaum, 2003;Santoro and Baram, 2003), the relative abundance of the HCN isoforms, as well as the degree to which heteromerization occurs, will contribute to the properties of the I h of an individual neuron (Franz et al, 2000;Santoro et al, 2000;Vasilyev and Barish, 2002), and thus to its intrinsic firing patterns and network responses. Differential regulation of HCN1 and HCN2 mRNA expression has been found in pathological states (Bender et al, 2003;Bräuer et al, 2001;Brewster et al, 2002), including experimental febrile and kainate-induced seizures, and has been associated with altered properties of the I h (Chen et al, 2001a).…”