HCN2 overexpression in newborn and adult ventricular myocytes: distinct effects on gating and excitability

Control of cardiac rate by "funny" channels in health and disease

Activation of the "funny" (pacemaker, I f) current during the diastolic depolarization phase of an action potential is the main mechanism underlying spontaneous, rhythmic activity of cardiac pacemaker cells. In the past three decades, a wealth of evidence elucidating the function of the funny current in the generation and modulation of cardiac pacemaker activity has been gathered. The slope of early diastolic depolarization, and thus the heart rate, is controlled precisely by the degree of I f activation during diastole. I f is also accurately and rapidly modulated by changes of the cytosolic concentration of the second messenger cAMP, operated by the autonomous nervous system through beta-adrenergic, mainly beta2, and in the opposite way by muscarinic receptor, stimulation. Recently, novel in vivo data, both in animal models and humans, have been collected that confirm the key role of I f in pacemaking. In particular, an inheritable point mutation in the cyclic nucleotide-binding domain of human HCN4, the main hyperpolarization-activated cyclic nucleotide (HCN) isoform contributing to native funny channels of the sinoatrial node, was shown to be associated with sinus bradycardia in a large family. Because of their properties, funny channels have long been a major target of classical pharmacological research and are now target of innovative gene/cell-based therapeutic approaches aimed to exploit their function in cardiac rate control.

Overexpression of HCN-encoded pacemaker current silences bioartificial pacemakers

BACKGROUND

Current strategies of engineering bioartificial pacemakers from otherwise silent yet excitable adult atrial and ventricular cardiomyocytes primarily rely on either maximizing the hyperpolarization-activated I(f) or on minimizing its presumptive opponent, the inwardly rectifying potassium current I(K1).

OBJECTIVE

The purpose of this study was to determine quantitatively the relative current densities of I(f) and I(K1) necessary to induce automaticity in adult atrial cardiomyocytes.

METHODS

Automaticity of adult guinea pig atrial cardiomyocytes was induced by adenovirus (Ad)-mediated overexpression of the gating-engineered HCN1 construct HCN1-DeltaDeltaDelta with the S3-S4 linker residues EVY235-7 deleted to favor channel opening.

RESULTS

Whereas control atrial cardiomyocytes remained electrically quiescent and had no I(f), 18% of Ad-CMV-GFP-IRES-HCN1-DeltaDeltaDelta (Ad-CGI-HCN1-DeltaDeltaDelta)-transduced cells demonstrated automaticity (240 +/- 14 bpm) with gradual phase 4 depolarization (143 +/- 28 mV/s), a depolarized maximal diastolic potential (-45.3 +/- 2.2 mV), and substantial I(f) at -140 mV (I(f,-140 mV) = -9.32 +/- 1.84 pA/pF). In the remaining quiescent Ad-CGI-HCN1-DeltaDeltaDelta-transduced atrial cardiomyocytes, two distinct immediate phenotypes were observed: (1) 13% had a hyperpolarized resting membrane potential (-56.7 +/- 1.3 mV) with I(f,-140 mV) of -4.85 +/- 0.97 pA/pF; and (2) the remaining 69% displayed a depolarized resting membrane potential (-27.6 +/- 1.3 mV) with I(f,-140 mV) of -23.0 +/- 3.71 pA/pF. Upon electrical stimulation, both quiescent groups elicited a single action potential with incomplete phase 4 depolarization that was never seen in controls. Further electrophysiologic analysis indicates that an intricate balance of I(K1) and I(f) is necessary for induction of atrial automaticity.

CONCLUSION

Optimized pacing induction and modulation can be better achieved by engineering the I(f)/I(K1) ratio rather than the individual currents.

Down-regulation of miR-1/miR-133 contributes to re-expression of pacemaker channel genes HCN2 and HCN4 in hypertrophic heart

Cardiac hypertrophy is characterized by electrical remolding with increased risk of arrhythmogenesis. Enhanced abnormal automaticity of ventricular cells contributes critically to hypertrophic arrhythmias. The pacemaker current I(f), carried by the hyperpolarization-activated channels encoded mainly by the HCN2 and HCN4 genes in the heart, plays an important role in determining cardiac automaticity. Their expressions reportedly increase in hypertrophic and failing hearts, contributing to arrhythmogenesis under these conditions. We performed a study on post-transcriptional regulation of expression of HCN2 and HCN4 genes by microRNAs. We experimentally established HCN2 as a target for repression by the muscle-specific microRNAs miR-1 and miR-133 and established HCN4 as a target for miR-1 only. We unraveled robust increases in HCN2 and HCN4 protein levels in a rat model of left ventricular hypertrophy and in angiotensin II-induced neonatal ventricular hypertrophy. The up-regulation of HCN2/HCN4 was accompanied by pronounced reduction of miR-1/miR-133 levels. Forced expression of miR-1/miR-133 by transfection prevented overexpression of HCN2/HCN4 in hypertrophic cardiomyocytes. The serum-responsive factor protein level was found significantly decreased in hypertrophic hearts, and silencing of this protein by RNA interference resulted in increased levels of miR-1/miR-133 and concomitant increases in HCN2 and HCN4 protein levels. We conclude that down-regulation of miR-1 and miR-133 expression contributes to re-expression of HCN2/HCN4 and thereby the electrical remodeling process in hypertrophic hearts. Our study also sheds new light on the cellular function and pathological role of miR-1/miR-133 in the heart.

HCN212-channel biological pacemakers manifesting ventricular tachyarrhythmias are responsive to treatment with I(f) blockade

BACKGROUND

A potential concern about biological pacemakers is their possible malfunction, which might create ventricular tachycardias (VTs).

OBJECTIVE

The purpose of this study was to test our hypothesis that should VTs complicate implantation of HCN-channel-based biological pacemakers, they would be suppressed by inhibitors of the pacemaker current, I(f).

METHODS

We created a chimeric channel (HCN212) containing the N- and C-termini of mouse HCN2 and the transmembrane region of mouse HCN1 and implanted it in HEK293 cells. Forty-eight hours later, in whole-cell patch clamp recordings, mean steady state block induced by 3 microM ivabradine (IVB) showed HCN1 = HCN212 > HCN2 currents. The HCN212 adenoviral construct was then implanted into the canine left bundle branch in 11 dogs. Complete AV block was created via radiofrequency ablation, and a ventricular demand electronic pacemaker was implanted (VVI 45 bpm). Electrocardiogram, 24-hour Holter monitoring, and pacemaker log record check were performed for 11 days.

RESULTS

All dogs developed rapid VT (>120 bpm, maximum rate = 285 +/- 37 bpm) at 0.9 +/- 0.3 days after implantation that persisted through 5 +/- 1 days. IVB, 1 mg/kg over 5 minutes, was administered during rapid VT, and three dogs received a second dose 24 hours later. While VT terminated with IBV in all instances within 3.4 +/- 0.6 minutes, no effect of IVB on sinus rate was noted.

CONCLUSION

We conclude that (1) I(f)-associated tachyarrhythmias-if they occur with HCN-based biological pacemakers-can be controlled with I(f)-inhibiting drugs such as IVB; (2) in vitro, IVB appears to have a greater steady state inhibiting effect on HCN1 and HCN212 isoforms than on HCN4; and (3) VT originating from the HCN212 injection site is suppressed more readily than sinus rhythm. This suggests a selectivity of IVB at the concentration attained for ectopic over HCN4-based pacemaker function. This might confer a therapeutic benefit.

Novel approaches for gene-specific interference via manipulating actions of microRNAs: examination on the pacemaker channel genes HCN2 and HCN4

Recent evidence has suggested microRNAs as viable therapeutic targets for a wide range of human disease. However, lack of gene-specificity of microRNA actions may hinder this application. Here we developed two new approaches, the gene-specific microRNA mimic and microRNA-masking antisense approaches, to explore the possibility of using microRNA's principle of actions in a gene-specific manner. We examined the value of these strategies as rational approaches to develop heart rate-reducing agents and "biological pacemakers" by manipulating the expression of the cardiac pacemaker channel genes HCN2 and HCN4. We showed that the gene-specific microRNA mimics, 22-nt RNAs designed to target the 3'untranslated regions (3'UTRs) of HCN2 and HCN4, respectively, were efficient in abrogating expression and function of HCN2 and HCN4. The gene-specific microRNA mimics repressed protein levels, accompanied by depressed f-channel conductance and the associated rhythmic activity, without affecting mRNA levels of HCN2 and HCN4. Meanwhile, we also designed the microRNA-masking antisense based on the miR-1 and miR-133 target sites in the 3'UTRs of HCN2 and HCN4 and found that these antisense oligodeoxynucleotides markedly enhanced HCN2/HCN4 expression and function, as reflected by increased protein levels of HCN2/HCN4 and If conductance, by removing the repression of HCN2/HCN4 expression induced by endogenous miR-1/miR-133. The experimental examination of these techniques and the resultant findings not only indicate feasibility of interfering miRNA action in a gene-specific fashion but also may provide a new research tool for studying function of miRNAs. The new approaches also have the potential of becoming alternative gene therapy strategies.

HCN-encoded pacemaker channels: from physiology and biophysics to bioengineering

The depolarizing membrane ionic current I(h) (also known as I(f), "f" for funny), encoded by the hyperpolarization-activated cyclic-nucleotide-modulated (HCN1-4) channel gene family, was first discovered in the heart over 25 years ago. Later, I(h) was also found in neurons, retina, and taste buds. HCN channels structurally resemble voltage-gated K(+) (Kv) channels but the molecular features underlying their opposite gating behaviors (activation by hyperpolarization rather than depolarization) and non-selective permeation profiles (> or =25 times less selective for K(+) than Kv channels) remain largely unknown. Although I(h) has been functionally linked to biological processes from the autonomous beating of the heart to pain transmission, the underlying mechanistic actions remain largely inferential and, indeed, somewhat controversial due to the slow kinetics and negative operating voltage range relative to those of the bioelectrical events involved (e.g., cardiac pacing). This article reviews the current state of our knowledge in the structure-function properties of HCN channels in the context of their physiological functions and potential HCN-based therapies via bioengineering.

Mechanistic role of I(f) revealed by induction of ventricular automaticity by somatic gene transfer of gating-engineered pacemaker (HCN) channels

BACKGROUND

Although I(f), encoded by the hyperpolarization-activated cyclic-nucleotide-modulated (HCN) channel gene family, is known to be functionally important in pacing, its mechanistic action is largely inferential and indeed somewhat controversial. To dissect in detail the role of I(f), we investigated the functional consequences of overexpressing in adult guinea pig left ventricular cardiomyocytes (LVCMs) various HCN1 constructs that have been engineered to exhibit different gating properties.

METHODS AND RESULTS

We created the recombinant adenoviruses Ad-CMV-GFP-IRES (CGI), Ad-CGI-HCN1, Ad-CGI-HCN1-delta delta delta, and Ad-CGI-HCN1-Ins, which mediate ectopic expression of GFP alone, WT, EVY235-7delta delta delta, and Ins HCN1 channels, respectively; EVY235-7delta delta delta and Ins encode channels in which the S3-S4 linkers have been shortened and lengthened to favor and inhibit opening, respectively. Ad-CGI-HCN1, Ad-CGI-HCN1-delta delta delta, and Ad-CGI-HCN1-Ins, but not control Ad-CGI, transduction of LVCMs led to robust expression of I(f) with comparable densities when fully open (approximately = -22 pA/pF at -140 mV; P>0.05) but distinctive activation profiles (V(1/2) = -70.8+/-0.6, -60.4+/-0.7, and -87.7+/-0.7 mV; P<0.01, respectively). Whereas control (nontransduced or Ad-CGI-transduced) LVCMs were electrically quiescent, automaticity (206+/-16 bpm) was observed exclusively in 61% of Ad-HCN1-delta delta delta-transduced cells that displayed depolarized maximum diastolic potential (-60.6+/-0.5 versus -70.6+/-0.6 mV of resting membrane potential of control cells; P<0.01) and gradual phase 4 depolarization (306+/-32 mV/s) that were typical of genuine nodal cells. Furthermore, spontaneously firing Ad-HCN1-delta delta delta-transduced LVCMs responded positively to adrenergic stimulation (P<0.05) but exhibited neither overdrive excitation nor suppression. In contrast, the remaining 39% of Ad-HCN1-delta delta delta-transduced cells exhibited no spontaneous action potentials; however, a single ventricular action potential associated with a depolarized resting membrane potential and a unique, incomplete "phase 4-like" depolarization that did not lead to subsequent firing could be elicited on simulation. Such an intermediate phenotype, similarly observed in 100% of Ad-CGI-HCN- and Ad-CGI-HCN1-Ins-transduced LVCMs, could be readily reversed by ZD7288, hinting at a direct role of I(f). Correlation analysis revealed the specific biophysical parameters required for I(f) to function as an active membrane potential oscillator.

CONCLUSIONS

Our results not only contribute to a better understanding of cardiac pacing but also may advance current efforts that focus primarily on automaticity induction to the next level by enabling bioengineering of central and peripheral cells that make up the native sinoatrial node.

Virtual cell and tissue dynamics of ectopic activation of the ventricles

Cardiac ventricular cells and tissues are normally excitable, and are activated by propagating waves of excitation that are initiated in the specialized pacemaking region of the heart. However, isolated or repetitive activity can be initiated at abnormal (ectopic) sites in the ventricles. To trigger an endogenous ectopic beat, there must be a compact focus of cells with changed membrane excitation parameters and kinetics, which initiate activity by after-depolarizations triggered by propagating activity, or that have bifurcated into autorhythmicity. This ectopic focus needs to be large enough, and adequately coupled, to drive the surrounding tissue. We investigate the initiation of ectopic excitation in computational models of human ventricular cells triggered by after-depolarizations and by up/down regulation of specific membrane conductance systems, and the propagation and evolution of ectopic activity in homogeneous or heterogeneous and isotropic, anisotropic, or orthotropic tissues.

Pacemaking by HCN channels requires interaction with phosphoinositides

Hyperpolarization-activated, cyclic-nucleotide-gated (HCN) channels mediate the depolarizing cation current (termed I(h) or I(f)) that initiates spontaneous rhythmic activity in heart and brain. This function critically depends on the reliable opening of HCN channels in the subthreshold voltage-range. Here we show that activation of HCN channels at physiologically relevant voltages requires interaction with phosphoinositides such as phosphatidylinositol-4,5-bisphosphate (PIP(2)). PIP(2) acts as a ligand that allosterically opens HCN channels by shifting voltage-dependent channel activation approximately 20 mV toward depolarized potentials. Allosteric gating by PIP(2) occurs in all HCN subtypes and is independent of the action of cyclic nucleotides. In CNS neurons and cardiomyocytes, enzymatic degradation of phospholipids results in reduced channel activation and slowing of the spontaneous firing rate. These results demonstrate that gating by phospholipids is essential for the pacemaking activity of HCN channels in cardiac and neuronal rhythmogenesis.

Pathophysiology of HCN channels

Hyperpolarization-activated cation currents termed I (f/h) are observed in many neurons and cardiac cells. Four genes (HCN1-4) encode the channels underlying these currents. New insights into the pathophysiological significance of HCN channels have been gained recently from analyses of mice engineered to be deficient in HCN genes. Lack of individual subunits results in markedly different phenotypes. Disruption of HCN1 impairs motor learning but enhances spatial learning and memory. Deletion of HCN2 results in absence epilepsy, ataxia, and sinus node dysfunction. Mice lacking HCN4 die during embryonic development and develop no sinoatrial node-like action potentials. In the present review, we summarize the physiology and pathophysiology of HCN channel family members based primarily on information from the transgenic mouse models and on data from human patients carrying defects in HCN4 channels.

Biological pacing by gene and cell therapy

At present, cardiac rhythm disorders such as sick sinus syndrome (SSS) or AV nodal block (AVB) are usually treated by electronic pacemakers. These devices have significant shortcomings, including lack of autonomic modulation, and the need for repetitive procedures for battery replacement or lead repositioning. Biological pacemakers as replacement or complement to electronic pacemakers have been the subject of increasing research interest. This research has resulted in many encouraging preclinical studies. Various approaches in the field of gene and cell therapy have been developed by different groups and this combined effort makes it increasingly realistic that this therapy will eventually find its way to clinical applicability. (Neth Heart J 2007;15:318-22.).

Regulation of gating and rundown of HCN hyperpolarization-activated channels by exogenous and endogenous PIP2

The voltage dependence of activation of the HCN hyperpolarization-activated cation channels is shifted in inside-out patches by −40 to −60 mV relative to activation in intact cells, a phenomenon referred to as rundown. Less than 20 mV of this hyperpolarizing shift can be due to the influence of the canonical modulator of HCN channels, cAMP. Here we study the role of phosphatidylinositol 4,5-bisphosphate (PI(4,5)P₂) in HCN channel rundown, as hydrolysis of PI(4,5)P₂ by lipid phosphatases is thought to underlie rundown of several other channels. We find that bath application of exogenous PI(4,5)P₂ reverses the effect of rundown, producing a large depolarizing shift in HCN2 activation. A synthetic short chain analogue of PI(4,5)P₂, dioctanoyl phosphatidylinositol 4,5-bisphosphate, shifts the HCN2 activation curve to more positive potentials in a dose-dependent manner. Other dioctanoyl phosphatidylinositides with one or more phosphates on the lipid headgroup also shift activation, although phosphatidylinositol (PI) is ineffective. Several lines of evidence suggest that HCN2 is also regulated by endogenous PI(4,5)P₂: (a) blockade of phosphatases slows the hyperpolarizing shift upon patch excision; (b) application of an antibody that binds and depletes membrane PIP₂ causes a further hyperpolarizing shift in activation; (c) the shift in activation upon patch excision can be partially reversed by MgATP; and (d) the effect of MgATP is blocked by wortmannin, an inhibitor of PI kinases. Finally, recordings from rabbit sinoatrial cells demonstrate that diC₈ PI(4,5)P₂ delays the rundown of native HCN currents. Thus, both native and recombinant HCN channels are regulated by PI(4,5)P₂.

Gene therapy to create biological pacemakers

Old age and a variety of cardiovascular disorders may disrupt normal sinus node function. Currently, this is successfully treated with electronic pacemakers, which, however, leave room for improvement. During the past decade, different strategies to initiate pacemaker function by gene therapy were developed. In the search for a biological pacemaker, various approaches were explored, including beta(2)-adrenergic receptor overexpression, down regulation of the inward rectifier current, and overexpression of the pacemaker current. The most recent advances include overexpression of bioengineered ion channels and genetically modified stem cells. This review considers the strengths and the weaknesses of the different approaches and discusses some of the different viral vectors currently used.

Biological pacemakers based on I(f)

Biological pacemaking as a replacement for or adjunct to electronic pacemakers has been a subject of interest since the 1990s. In the following pages, we discuss the rational for and progress made using a hyperpolarization activated, cyclic nucleotide gated channel isoform to carry the I(f) pacemaker current in gene and cell therapy approaches. Both strategies have resulted in effective biological pacemaker function over a period of weeks in intact animals. Moreover, the use of adult human mesenchymal stem cells as a platform for carrying pacemaker genes has resulted in the formation of functional gap junctions with cardiac myocytes in situ leading to effective and persistent propagation of pacemaker current. The approaches described are encouraging, suggesting that biological pacemakers based on this strategy can be brought to clinical testing.

Physiology and pharmacology of the cardiac pacemaker (“funny”) current

First described over a quarter of a century ago, the cardiac pacemaker "funny" (I(f)) current has been extensively characterized since, and its role in cardiac pacemaking has been thoroughly demonstrated. A similar current, termed I(h), was later described in different types of neurons, where it has a variety of functions and contributes to the control of cell excitability and plasticity. I(f) is an inward current activated by both voltage hyperpolarization and intracellular cAMP. In the heart, as well as generating spontaneous activity, f-channels mediate autonomic-dependent modulation of heart rate: beta-adrenergic stimulation accelerates, and vagal stimulation slows, cardiac rate by increasing and decreasing, respectively, the intracellular cAMP concentration and, consequently, the f-channel degree of activation. Four isoforms of hyperpolarization-activated, cyclic nucleotide-gated (HCN) channels have been cloned more recently and shown to be the molecular correlates of native f-channels in the heart and h-channels in the brain. Individual HCN isoforms have kinetic and modulatory properties which differ quantitatively. A comparison of their biophysical properties with those of native pacemaker channels provides insight into the molecular basis of the pacemaker current properties and, together with immunolabelling and other detection techniques, gives information on the pattern of HCN isoform distribution in different tissues. Because of their relevance to cardiac pacemaker activity, f-channels are a natural target of drugs aimed at the pharmacological control of heart rate. Several agents developed for their ability to selectively reduce heart rate act by a specific inhibition of f-channel function; these substances have a potential for the treatment of diseases such as angina and heart failure. In the near future, devices based on the delivery of f-channels in situ, or of a cellular source of f-channels (biological pacemakers), will likely be developed for use in therapies for diseases of heart rhythm with the aim of replacing electronic pacemakers.

MiRP1 Modulates HCN2 Channel Expression and Gating in Cardiac Myocytes

MinK-related protein (MiRP1 or KCNE2) interacts with the hyperpolarization-activated, cyclic nucleotide-gated (HCN) family of pacemaker channels to alter channel gating in heterologous expression systems. Given the high expression levels of MiRP1 and HCN subunits in the cardiac sinoatrial node and the contribution of pacemaker channel function to impulse initiation in that tissue, such an interaction could be of considerable physiological significance. However, the functional evidence for MiRP1/HCN interactions in heterologous expression studies has been accompanied by inconsistencies between studies in terms of the specific effects on channel function. To evaluate the effect of MiRP1 on HCN expression and function in a physiological context, we used an adenovirus approach to overexpress a hemagglutinin (HA)-tagged MiRP1 (HAMiRP1) and HCN2 in neonatal rat ventricular myocytes, a cell type that expresses both MiRP1 and HCN2 message at low levels. HA-MiRP1 co-expression with HCN2 resulted in a 4-fold increase in maximal conductance of pacemaker currents compared with HCN2 expression alone. HCN2 activation and deactivation kinetics also changed, being significantly more rapid for voltages between -60 and -95 mV when HA-MiRP1 was co-expressed with HCN2. However, the voltage dependence of activation was not affected. Co-immunoprecipitation experiments demonstrated that expressed HA-MiRP1 and HCN2, as well as endogenous MiRP1 and HCN2, co-assemble in ventricular myocytes. The results indicate that MiRP1 acts as a beta subunit for HCN2 pacemaker channel subunits and alters channel gating at physiologically relevant voltages in cardiac cells.

Structural elements of instantaneous and slow gating in hyperpolarization-activated cyclic nucleotide-gated channels

Hyperpolarization-activated cyclic nucleotide-gated (HCN) subunits produce a slowly activating current in response to hyperpolarization (If) and an instantaneous voltage-independent current (Iinst) when expressed in Chinese hamster ovary (CHO) cells. Here we found that a mutation in the S4-S5 linker of HCN2 (Y331D) produced an additional mixed cationic instantaneous current. However, this current was inhibited by external Cs+ like If and unlike Iinst. Together with a concomitant reduction in If, the data suggest that the Y331D mutation disrupted channel closing placing the channel in a "If-like," and not an "Iinst-like," state. The "If-like" instantaneous current represented approximately 70% of total If over voltages ranging from +20 to -150 mV in high K+ solutions. If activated at more depolarized potentials and the activation curve was less steep, whereas deactivation was significantly slowed, consistent with the idea that the mutation inhibited channel closing. The data suggest that the mutation produced allosteric effects on the activation gate (S6 segment) and/or on voltage-sensing elements. We also found that decreases in the ratio of external K+/Na+ further disrupted channel closing in the mutant channel. Finally, our data suggest that the structures involved in producing Iinst are similar between the HCN1 and HCN2 isoforms and that excess HCN protein on the plasma membrane of CHO cells relative to native cells is not responsible for Iinst. The data are consistent with Iinst flowing through a "leaky" closed state but do not rule out flow through a second configuration of recombinant HCN channels or up-regulated endogenous channels/subunits.

Overexpression of a hyperpolarization-activated cation current (Ih) channel gene modifies the firing activity of identified motor neurons in a small neural network

The hyperpolarization-activated cation current (Ih) is widely distributed in excitable cells. Ih plays important roles in regulation of cellular excitability, rhythmic activity, and synaptic function. We previously showed that, in pyloric dilator (PD) neurons of the stomatogastric ganglion (STG) of spiny lobsters, Ih can be endogenously upregulated to compensate for artificial overexpression of the Shal transient potassium channel; this maintains normal firing properties of the neuron despite large increases in potassium current. To further explore the function of Ih in the pyloric network, we injected cRNA of PAIH, a lobster gene that encodes Ih, into rhythmically active PD neurons. Overexpression of PAIH produced a fourfold increase in Ih, although with somewhat different biophysical properties than the endogenous current. Compared with the endogenous Ih, the voltage for half-maximal activation of the PAIH-evoked current was depolarized by 10 mV, and its activation kinetics were significantly faster. This increase in Ih did not affect the expression of IA or other outward currents. Instead, it significantly altered the firing properties of the PD neurons. Increased Ih depolarized the minimum membrane potential of the cell, reduced the oscillation amplitude, decreased the time to the first spike, and increased the duty cycle and number of action potentials per burst. We used both dynamic-clamp experiments, injecting the modeled PAIH currents into PD cells in a functioning STG, and a theoretical model of a two-cell network to demonstrate that the increased Ih was sufficient to cause the observed changes in the PD activity.

Hallmarks of ion channel gene expression in end-stage heart failure

Electrical conductance is greatly altered in end-stage heart failure, but little is known about the underlying events. We therefore investigated the expression of genes coding for major inward and outward ion channels, calcium binding proteins, ion receptors, ion exchangers, calcium ATPases, and calcium/calmodulin-dependent protein kinases in explanted hearts (n=13) of patients diagnosed with end-stage heart failure. With the exception of Kv11.1 and Kir3.1 and when compared with healthy controls, major sodium, potassium, and calcium ion channels, ion transporters, and exchangers were significantly repressed, but expression of Kv7.1, HCN4, troponin C and I, SERCA1, and phospholamban was elevated. Hierarchical gene cluster analysis provided novel insight into regulated gene networks. Significant induction of the transcriptional repressor m-Bop and the translational repressor NAT1 coincided with repressed cardiac gene expression. The statistically significant negative correlation between repressors and ion channels points to a mechanism of disease. We observed coregulation of ion channels and the androgen receptor and propose a role for this receptor in ion channel regulation. Overall, the reversal of repressed ion channel gene expression in patients with implanted assist devices exemplifies the complex interactions between pressure load/stretch force and heart-specific gene expression.

Hyperpolarization-activated cation currents: from molecules to physiological function

Hyperpolarization-activated cation currents, termed If, Ih, or Iq, were initially discovered in heart and nerve cells over 20 years ago. These currents contribute to a wide range of physiological functions, including cardiac and neuronal pacemaker activity, the setting of resting potentials, input conductance and length constants, and dendritic integration. The hyperpolarization-activated, cation nonselective (HCN) gene family encodes the channels that underlie Ih. Here we review the relation between the biophysical properties of recombinant HCN channels and the pattern of HCN mRNA expression with the properties of native Ih in neurons and cardiac muscle. Moreover, we consider selected examples of the expanding physiological functions of Ih with a view toward understanding how the properties of HCN channels contribute to these diverse functional roles.

Heteromeric HCN1-HCN4 channels: a comparison with native pacemaker channels from the rabbit sinoatrial node

'Funny-' (f-) channels of cardiac sino-atrial node (SAN) cells are key players in the process of pacemaker generation and mediate the modulatory action of autonomic transmitters on heart rate. The molecular components of f-channels are the hyperpolarization-activated, cyclic nucleotide-gated (HCN) channels. Of the four HCN isoforms known, two (HCN4 and HCN1) are expressed in the rabbit SAN at significant levels. However, the properties of f-channels of SAN cells do not conform to specific features of the two isoforms expressed locally. For example, activation kinetics and cAMP sensitivity of native pacemaker channels are intermediate between those reported for HCN1 and HCN4. Here we have explored the possibility that both HCN4 and HCN1 isoforms contribute to the native If in SAN cells by co-assembling into heteromeric channels. To this end, we used heterologous expression in human embryonic kidney (HEK) 293 cells to investigate the kinetics and cAMP response of the current generated by co-transfected (HCN4 + HCN1) and concatenated (HCN4-HCN1 (4-1) tandem or HCN1-HCN4 (1-4) tandem) rabbit constructs and compared them with those of the native f-current from rabbit SAN. 4-1 tandem, but not co-transfected, currents had activation kinetics approaching those of If; however, the activation range of 4-1 tandem channels was more negative than that of the f-channel and their cAMP sensitivity were poorer (although that of 1-4 tandem channels was normal). Co-transfection of 4-1 tandem channels with minK-related protein 1(MiRP1) did not alter their properties. HCN1 and HCN4 may contribute to native f-channels, but a 'context'-dependent mechanism is also likely to modulate the channel properties in native tissues.