HCN-related channelopathies

Not very funny: how a single mutation causes heritable bradycardia

HCN channels and their modulation by cAMP play a key role in cardiac pacemaking. In this issue of Structure, Xu and colleagues reveal that an arrhythmia-causing mutation of an HCN channel weakens cAMP binding to the channel by altering the local structure of its entry-exit pathway.

The role of HCN channels within the periaqueductal gray in neuropathic pain

Peripheral and spinal hyperpolarization-activated cyclic nucleotide-gated (HCN) channels play a key role in neuropathic pain by regulating neuronal excitability. HCN channels are expressed in the ventral-lateral periaqueductal gray (vlPAG), a region that is important for pain modulation. However, the role of vlPAG HCN channels in neuropathic pain remains poorly understood. In the present study, we investigated the impact of changes to vlPAG HCN channels on neural activity in neuropathic pain. First, sciatic nerve chronic constriction injury (CCI) was established as a neuropathic pain model. Then, changes in HCN channels and their influence on vlPAG neuronal activity were detected. Our results indicate that after CCI surgery the following changes occur in vlPAG neurons: the expression of HCN1 and HCN2 channels is increased, the amplitude of the hyperpolarization-activated current (Ih) is augmented and its activation curve is shifted to more positive potentials and there is an increase in the frequency of action potential (AP) firing and spontaneous EPSCs that is attenuated by ZD7288, a HCN channel blocker. In addition, forskolin, which can elevate intracellular cAMP, mimics the CCI induced changes in neuronal excitability in the vlPAG. The effects of forskolin were also reversed by ZD7288. Taken together, the present data indicate an important role for HCN channels in the vlPAG in neuropathic pain.

Ivabradine: an intelligent drug for the treatment of ischemic heart disease

Heart rate (HR) is a precisely regulated variable, which plays a critical role in health and disease. Elevated resting HR is a significant predictor of all-cause and cardiovascular mortality in the general population and patients with cardiovascular disease (CVD). β-blocking drugs exert negative effects on regional myocardial blood flow and function when HR reduction is eliminated by atrial pacing; calcium channel antagonists (CCAs) functionally antagonize coronary vasoconstriction mediated through α-adreno-receptors and are thus devoid of this undesired effect, but the compounds are nevertheless negative inotropes. From these observations derives the necessity to find alternative, more selective drugs to reduce HR through inhibition of specific electrical current (I(f)). Ivabradine (IVA) is a novel specific HR-lowering agent that acts in sinus atrial node (SAN) cells by selectively inhibiting the pacemaker I(f) current in a dose-dependent manner by slowing the diastolic depolarization slope of SAN cells, and by reducing HR at rest during exercise in humans. Coronary artery diseases (CAD) represent the most common cause of death in middle-aged and older adults in European Countries. Most ischemic episodes are triggered by an increase in HR, that induces an imbalance between myocardial oxygen delivery and consumption. IVA, a selective and specific inhibitor of the I(f) current which reduced HR without adverse hemodynamic effects, has clearly and unequivocally demonstrated its efficacy in the treatment of chronic stable angina pectoris (CSAP) and myocardial ischemia with optimal tolerability profile due to selective interaction with I(f) channels. The aim of this review is to point out the usefulness of IVA in the treatment of ischemic heart disease.

Local and global interpretations of a disease-causing mutation near the ligand entry path in hyperpolarization-activated cAMP-gated channel

Hyperpolarization-activated, cAMP-gated (HCN) channels sense membrane potential and intracellular cAMP levels. A mutation identified in the cAMP binding domain (CNBD) of the human HCN4 channel, S672R, severely reduces the heart rate, but the molecular mechanism has been unclear. Our biochemical binding assays on isolated CNBD and patch-clamp recordings on the functional channel show that S672R reduces cAMP binding. The crystal structure of the mutant CNBD revealed no global changes except a disordered loop on the cAMP entry path. To address this localized structural perturbation at a whole protein level, we studied the activity-dependent dynamic interaction between cAMP and the functional channel using the patch-clamp fluorometry technique. S672R reduces the binding of cAMP to the channels in the resting state and significantly increases the unbinding rate during channel deactivation. This study on a disease-causing mutation illustrates the important roles played by the structural elements on the ligand entry-exit path in stabilizing the bound ligand in the binding pocket.

Potassium channels in articular chondrocytes

Chondrocytes are the resident cells of cartilage, which synthesize and maintain the extracellular matrix. The range of known potassium channels expressed by these unique cells is continually increasing. Since chondrocytes are non-excitable, and do not need to be repolarized following action potentials, the function of potassium channels in these cells has, until recently, remained completely unknown. However, recent advances in both traditional physiology and “omic” technologies have enhanced our knowledge and understanding of the chondrocyte channelome. A large number of potassium channels have been identified and a number of putative, but credible, functions have been proposed. Members of each of the potassium channel sub-families (calcium activated, inward rectifier, voltage-gated and tandem pore) have all been identified. Mechanotransduction, cell volume regulation, apoptosis and chondrogenesis all appear to involve potassium channels. Since evidence suggests that potassium channel gene transcription is altered in osteoarthritis, future studies are needed that investigate potassium channels as potential cellular biomarkers and therapeutic targets for treatment of degenerative joint conditions.

Developing a gene expression model for predicting ventilator-associated pneumonia in trauma patients: a pilot study

Background

Ventilator-associated pneumonia (VAP) carries significant mortality and morbidity. Predicting which patients will become infected could lead to measures to reduce the incidence of VAP.

Methodology/Principal Findings

The goal was to begin constructing a model for VAP prediction in critically-injured trauma patients, and to identify differentially expressed genes in patients who go on to develop VAP compared to similar patients who do not. Gene expression profiles of lipopolysaccharide stimulated blood cells in critically injured trauma patients that went on to develop ventilator-associated pneumonia (n = 10) was compared to those that never developed the infection (n = 10). Eight hundred and ten genes were differentially expressed between the two groups (ANOVA, P<0.05) and further analyzed by hierarchical clustering and principal component analysis. Functional analysis using Gene Ontology and KEGG classifications revealed enrichment in multiple categories including regulation of protein translation, regulation of protease activity, and response to bacterial infection. A logistic regression model was developed that accurately predicted critically-injured trauma patients that went on to develop VAP (VAP+) and those that did not (VAP−). Five genes (PIK3R3, ATP2A1, PI3, ADAM8, and HCN4) were common to all top 20 significant genes that were identified from all independent training sets in the cross validation. Hierarchical clustering using these five genes accurately categorized 95% of patients and PCA visualization demonstrated two discernable groups (VAP+ and VAP−).

Conclusions/Significance

A logistic regression model using cross-validation accurately predicted patients that developed ventilator-associated pneumonia and should now be tested on a larger cohort of trauma patients.

Funny Current and Cardiac Rhythm: Insights from HCN Knockout and Transgenic Mouse Models

In the adult animal the sinoatrial node (SAN) rhythmically generates a depolarizing wave that propagates to the rest of the heart. However, the SAN is more than a simple clock; it is a clock that adjusts its pace according to the metabolic requirements of the organism. The Hyperpolarization-activated Cyclic Nucleotide-gated channels (HCN1–4) are the structural component of the funny (I_f) channels; in the SAN the I_f current is the main driving electrical force of the diastolic depolarization and the HCN4 is the most abundant isoform. The generation of HCN KO and transgenic mouse models has advanced the understanding of the role of these channels in cardiac excitability. The HCN4 KO models that were first developed allowed either global or cardiac-specific constitutive ablation of HCN4 channels, and resulted in embryonic lethality. A further progress was made with the development of three separate inducible HCN4 KO models; in one model KO was induced globally in the entire organism, in a second, ablation occurred only in HCN4-expressing cells, and finally in a third model KO was confined to cardiac cells. Unexpectedly, the three models yielded different results; similarities and differences among these models will be presented and discussed. The functional effects of HCN2 and HCN3 knockout models and transgenic HCN4 mouse models will also be discussed. In conclusion, HCN KO/transgenic models have allowed to evaluate the functional role of the I_f currents in intact animals as well as in single SAN cells isolated from the same animals. This opportunity is therefore unique since it allows (1) to verify the contribution of specific HCN isoforms to cardiac activity in intact animals, and (2) to compare these results to those obtained in single cell experiments. These combined studies were not possible prior to the development of KO models. Finally, these models represent critical tools to improve our understanding of the molecular basis of some inheritable arrhythmic human pathologies.

Ih current is necessary to maintain normal dopamine fluctuations and sleep consolidation in Drosophila

HCN channels are becoming pharmacological targets mainly in cardiac diseases. But apart from their well-known role in heart pacemaking, these channels are widely expressed in the nervous system where they contribute to the neuron firing pattern. Consequently, abolishing Ih current might have detrimental consequences in a big repertoire of behavioral traits. Several studies in mammals have identified the Ih current as an important determinant of the firing activity of dopaminergic neurons, and recent evidences link alterations in this current to various dopamine-related disorders. We used the model organism Drosophila melanogaster to investigate how lack of Ih current affects dopamine levels and the behavioral consequences in the sleep∶activity pattern. Unlike mammals, in Drosophila there is only one gene encoding HCN channels. We generated a deficiency of the DmIh core gene region and measured, by HPLC, levels of dopamine. Our data demonstrate daily variations of dopamine in wild-type fly heads. Lack of Ih current dramatically alters dopamine pattern, but different mechanisms seem to operate during light and dark conditions. Behaviorally, DmIh mutant flies display alterations in the rest∶activity pattern, and altered circadian rhythms. Our data strongly suggest that Ih current is necessary to prevent dopamine overproduction at dark, while light input allows cycling of dopamine in an Ih current dependent manner. Moreover, lack of Ih current results in behavioral defects that are consistent with altered dopamine levels.

HCN channels and heart rate

Hyperpolarization and Cyclic Nucleotide (HCN) -gated channels represent the molecular correlates of the "funny" pacemaker current (I(f)), a current activated by hyperpolarization and considered able to influence the sinus node function in generating cardiac impulses. HCN channels are a family of six transmembrane domain, single pore-loop, hyperpolarization activated, non-selective cation channels. This channel family comprises four members: HCN1-4, but there is a general agreement to consider HCN4 as the main isoform able to control heart rate. This review aims to summarize advanced insights into the structure, function and cellular regulation of HCN channels in order to better understand the role of such channels in regulating heart rate and heart function in normal and pathological conditions. Therefore, we evaluated the possible therapeutic application of the selective HCN channels blockers in heart rate control.

Evolutionary genomics reveals the premetazoan origin of opposite gating polarity in animal-type voltage-gated ion channels

Electrical signaling in animals ensures the rapid and accurate transmission of information, often carried by voltage-gated Na(+), Ca(2+) and K(+) channels that are activated by membrane depolarization. In heart and neurons, a distinct type of ion channel called the hyperpolarization-activated, cyclic nucleotide-regulated (HCN) channel is activated by membrane hyperpolarization. Recent genomic studies have revealed that animal-type voltage-gated Na(+) channels (Liebeskind BJ, et al. 2011. Proc Natl Acad Sci U S A. 108:9154) had evolved in choanoflagellates, one of the unicellular relatives of animals. To date, HCN channels have been considered to be animal-specific. Here, we demonstrate the presence of an HCN channel homolog (SroHCN) in the choanoflagellate protist Salpingoeca rosetta. SroHCN contains highly conserved functional domains and sequence motifs that are correlated with the unique biophysical activities of HCN channels. These findings provide novel genomic insights into the evolution of complex electrical signaling before the emergence of multicellular animals.

Exploring HCN channels as novel drug targets

Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels have a key role in the control of heart rate and neuronal excitability. Ivabradine is the first compound acting on HCN channels to be clinically approved for the treatment of angina pectoris. HCN channels may offer excellent opportunities for the development of novel anticonvulsant, anaesthetic and analgesic drugs. In support of this idea, some well-established drugs that act on the central nervous system - including lamotrigine, gabapentin and propofol - have been found to modulate HCN channel function. This Review gives an up-to-date summary of compounds acting on HCN channels, and discusses strategies to further explore the potential of these channels for therapeutic intervention.

The mechanism of ethanol action on midbrain dopaminergic neuron firing: a dynamic-clamp study of the role of I(h) and GABAergic synaptic integration

Hyperpolarization-activated and cyclic nucleotide-gated (HCN) channels are expressed in dopaminergic (DA) neurons of the ventral tegmental area (VTA) as well as in DA and GABAergic neurons of the substantia nigra (SN). The excitation of DA neurons induced by ethanol has been proposed to result from its enhancing HCN channel current, I(h). Using perforated patch-clamp recordings in rat midbrain slices, we isolated I(h) in these neurons by voltage clamp. We showed that ethanol reversibly increased the amplitude and accelerated the activation kinetics of I(h) and caused a depolarizing shift in its voltage dependence. Using dynamic-clamp conductance injection, we injected artificial I(h) and fluctuating GABAergic synaptic conductance inputs into neurons following block of intrinsic I(h). This demonstrated directly a major role of I(h) in promoting rebound spiking following phasic inhibition, which was enhanced as the kinetics and amplitude of I(h) were changed in the manner induced by ethanol. Similar effects of ethanol were observed on I(h) and firing rate in non-DA, putatively GABAergic interneurons, indicating that in addition to its direct effects on firing, ethanol will produce large changes in the inhibition and disinhibition (via GABAergic interneurons) converging on DA neurons. Thus the overall effects of ethanol on firing of DA cells of the VTA and SN in vivo, and hence on phasic dopamine release in the striatum, appear to be determined substantially by its action on I(h) in both DA cells and GABAergic interneurons.

Association of polymorphisms in HCN4 with mood disorders and obsessive compulsive disorder

Hyperpolarization activated cyclic nucleotide-gated (HCN) potassium channels are implicated in the control of neuronal excitability and are expressed widely in the brain. HCN4 is expressed in brain regions relevant to mood and anxiety disorders including specific thalamic nuclei, the basolateral amygdala, and the midbrain dopamine system. We therefore examined the association of HCN4 with a group of mood and anxiety disorders. We genotyped nine tag SNPs in the HCN4 gene using Sequenom iPLEX Gold technology in 285 Caucasian patients with DSM-IV mood disorders and/or obsessive compulsive disorder and 384 Caucasian controls. HCN4 polymorphisms were analyzed using single marker and haplotype-based association methods. Three SNPs showed nominal association in our population (rs12905211, rs3859014, rs498005). SNP rs12905211 maintained significance after Bonferroni correction, with allele T and haplotype CTC overrepresented in cases. These findings suggest HCN4 as a genetic susceptibility factor for mood and anxiety disorders; however, these results will require replication using a larger sample.

Advances in understanding basic mechanisms of epilepsy and seizures

Sixty years ago the clinical neurophysiology of epilepsy had progressed to the stage that it posed questions that could be addressed by major advances in cellular electrophysiology made around the that time. However, it took about 25-30 years to build up serious momentum in understanding the mechanisms of epileptic discharges. Over the past 2-3 decades developments in pharmacology and molecular biology have substantially increased the depth and complexity of our insights into the nervous system in general and the epileptic brain in particular. One of the biggest advances in our understanding of the brain is in its plasticity in the adult - that is its ability to modify its structure and function. The current state of play is that for most chronic epileptic foci it is possible to identify multiple differences from normal brain tissue in both the structure and function of neurons, neuronal networks and glia. This review will chart some of this progress to give an idea of the pace of advances over the decades.