The murine HCN3 gene encodes a hyperpolarization-activated cation channel with slow kinetics and unique response to cyclic nucleotides

Cryo-EM structure of human HCN3 channel and its regulation by cAMP

HCN channels are important for regulating heart rhythm and nerve activity and have been studied as potential drug targets for treating depression, arrhythmia, nerve pain, and epilepsy. Despite possessing unique pharmacological properties, HCN channels share common characteristics in that they are activated by hyperpolarization and modulated by cAMP and other membrane lipids. However, the mechanisms of how these ligands bind and modulate HCN channels are unclear. In this study, we solved structures of full-length human HCN3 using cryo-EM and captured two different states, including a state without any ligand bound and a state with cAMP bound. Our structures reveal the novel binding sites for cholesteryl hemisuccinate in apo state and show how cholesteryl hemisuccinate and cAMP binding cause conformational changes in different states. These findings explain how these small modulators are sensed in mammals at the molecular level. The results of our study could help to design more potent and specific compounds to influence HCN channel activity and offer new therapeutic possibilities for diseases that lack effective treatment.

The enigmatic HCN channels: A cellular neurophysiology perspective

What physiological role does a slow hyperpolarization-activated ion channel with mixed cation selectivity play in the fast world of neuronal action potentials that are driven by depolarization? That puzzling question has piqued the curiosity of physiology enthusiasts about the hyperpolarization-activated cyclic nucleotide-gated (HCN) channels, which are widely expressed across the body and especially in neurons. In this review, we emphasize the need to assess HCN channels from the perspective of how they respond to time-varying signals, while also accounting for their interactions with other co-expressing channels and receptors. First, we illustrate how the unique structural and functional characteristics of HCN channels allow them to mediate a slow negative feedback loop in the neurons that they express in. We present the several physiological implications of this negative feedback loop to neuronal response characteristics including neuronal gain, voltage sag and rebound, temporal summation, membrane potential resonance, inductive phase lead, spike triggered average, and coincidence detection. Next, we argue that the overall impact of HCN channels on neuronal physiology critically relies on their interactions with other co-expressing channels and receptors. Interactions with other channels allow HCN channels to mediate intrinsic oscillations, earning them the "pacemaker channel" moniker, and to regulate spike frequency adaptation, plateau potentials, neurotransmitter release from presynaptic terminals, and spike initiation at the axonal initial segment. We also explore the impact of spatially non-homogeneous subcellular distributions of HCN channels in different neuronal subtypes and their interactions with other channels and receptors. Finally, we discuss how plasticity in HCN channels is widely prevalent and can mediate different encoding, homeostatic, and neuroprotective functions in a neuron. In summary, we argue that HCN channels form an important class of channels that mediate a diversity of neuronal functions owing to their unique gating kinetics that made them a puzzle in the first place.

HCN channels sense temperature and determine heart rate responses to heat

Heart rate increases with heat, [1-3] constituting a fundamental physiological relationship in vertebrates. Each normal heartbeat is initiated by an action potential generated in a sinoatrial nodal pacemaker cell. Pacemaker cells are enriched with hyperpolarization activated cyclic nucleotide-gated ion channels (HCN) that deliver cell membrane depolarizing inward current that triggers action potentials. HCN channel current increases due to cAMP binding, a mechanism coupling adrenergic tone to physiological 'fight or flight' heart rate acceleration. However, the mechanism(s) for heart rate response to thermal energy is unknown. We used thermodynamical and homology computational modeling, site-directed mutagenesis and mouse models to identify a concise motif on the S4-S5 linker of the cardiac pacemaker HCN4 channels (M407/Y409) that determines HCN4 current (If) and cardiac pacemaker cell responses to heat. This motif is required for heat sensing in cardiac pacemaker cells and in isolated hearts. In contrast, the cyclic nucleotide binding domain is not required for heat induced HCN4 current increases. However, a loss of function M407/Y409 motif mutation prevented normal heat and cAMP responses, suggesting that heat sensing machinery is essential for operating the cAMP allosteric pathway and is central to HCN4 modulation. The M407/Y409 motif is conserved across all HCN family members suggesting that HCN channels participate broadly in coupling heat to changes in cell membrane excitability.

Probucol is anti-hyperalgesic in a mouse peripheral nerve injury model of neuropathic pain

2,6-di-tert-butylphenol (2,6-DTBP) ameliorates mechanical allodynia and thermal hyperalgesia produced by partial sciatic nerve ligation in mice, and selectively inhibits HCN1 channel gating. We hypothesized that the clinically utilized non-anesthetic dimerized congener of 2,6-DTBP, probucol (2,6-di-tert-butyl-4-[2-(3,5-di-tert-butyl-4-hydroxyphenyl)sulfanylpropan-2-ylsulfanyl]phenol), would relieve the neuropathic phenotype that results from peripheral nerve damage, and that the anti-hyperalgesic efficacy in vivo would correlate with HCN1 channel inhibition in vitro. A single oral dose of probucol (800 mg/kg) relieved mechanical allodynia and thermal hyperalgesia in a mouse spared-nerve injury neuropathic pain model. While the low aqueous solubility of probucol precluded assessment of its possible interaction with HCN1 channels, our results, in conjunction with recent data demonstrating that probucol reduces lipopolysaccharide-induced mechanical allodynia and thermal hyperalgesia, support the testing/development of probucol as a non-opioid, oral antihyperalgesic albeit one of unknown mechanistic action.

An evolutionarily conserved pacemaker role for HCN ion channels in smooth muscle

Although hyperpolarization-activated cation (HCN) ion channels are well established to underlie cardiac pacemaker activity, their role in smooth muscle organs remains controversial. HCN-expressing cells are localized to renal pelvic smooth muscle (RPSM) pacemaker tissues of the murine upper urinary tract and HCN channel conductance is required for peristalsis. To date, however, the Ih pacemaker current conducted by HCN channels has never been detected in these cells, raising questions on the identity of RPSM pacemakers. Indeed, the RPSM pacemaker mechanisms of the unique multicalyceal upper urinary tract exhibited by humans remains unknown. Here, we developed immunopanning purification protocols and demonstrate that 96% of isolated HCN+ cells exhibit Ih . Single-molecule STORM to whole-tissue imaging showed HCN+ cells express single HCN channels on their plasma membrane and integrate into the muscular syncytium. By contrast, PDGFR-α+ cells exhibiting the morphology of ICC gut pacemakers were shown to be vascular mural cells. Translational studies in the homologous human and porcine multicalyceal upper urinary tracts showed that contractions and pacemaker depolarizations originate in proximal calyceal RPSM. Critically, HCN+ cells were shown to integrate into calyceal RPSM pacemaker tissues, and HCN channel block abolished electrical pacemaker activity and peristalsis of the multicalyceal upper urinary tract. Cumulatively, these studies demonstrate that HCN ion channels play a broad, evolutionarily conserved pacemaker role in both cardiac and smooth muscle organs and have implications for channelopathies as putative aetiologies of smooth muscle disorders. KEY POINTS: Pacemakers trigger contractions of involuntary muscles. Hyperpolarization-activated cation (HCN) ion channels underpin cardiac pacemaker activity, but their role in smooth muscle organs remains controversial. Renal pelvic smooth muscle (RPSM) pacemakers trigger contractions that propel waste away from the kidney. HCN+ cells localize to murine RPSM pacemaker tissue and HCN channel conductance is required for peristalsis. The HCN (Ih ) current has never been detected in RPSM cells, raising doubt whether HCN+ cells are bona fide pacemakers. Moreover, the pacemaker mechanisms of the unique multicalyceal RPSM of higher order mammals remains unknown. In total, 97% of purified HCN+ RPSM cells exhibit Ih . HCN+ cells integrate into the RPSM musculature, and pacemaker tissue peristalsis is dependent on HCN channels. Translational studies in human and swine demonstrate HCN channels are conserved in the multicalyceal RPSM and that HCN channels underlie pacemaker activity that drives peristalsis. These studies provide insight into putative channelopathies that can underlie smooth muscle dysfunction.

The Contribution of HCN Channelopathies in Different Epileptic Syndromes, Mechanisms, Modulators, and Potential Treatment Targets: A Systematic Review

Background

Hyperpolarization-activated cyclic nucleotide-gated (HCN) current reduces dendritic summation, suppresses dendritic calcium spikes, and enables inhibitory GABA-mediated postsynaptic potentials, thereby suppressing epilepsy. However, it is unclear whether increased HCN current can produce epilepsy. We hypothesized that gain-of-function (GOF) and loss-of-function (LOF) variants of HCN channel genes may cause epilepsy.

Objectives

This systematic review aims to summarize the role of HCN channelopathies in epilepsy, update genetic findings in patients, create genotype–phenotype correlations, and discuss animal models, GOF and LOF mechanisms, and potential treatment targets.

Methods

The review was conducted in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses statement, for all years until August 2021.

Results

We identified pathogenic variants of HCN1 (n = 24), HCN2 (n = 8), HCN3 (n = 2), and HCN4 (n = 6) that were associated with epilepsy in 74 cases (43 HCN1, 20 HCN2, 2 HCN3, and 9 HCN4). Epilepsy was associated with GOF and LOF variants, and the mechanisms were indeterminate. Less than half of the cases became seizure-free and some developed drug-resistant epilepsy. Of the 74 cases, 12 (16.2%) died, comprising HCN1 (n = 4), HCN2 (n = 2), HCN3 (n = 2), and HCN4 (n = 4). Of the deceased cases, 10 (83%) had a sudden unexpected death in epilepsy (SUDEP) and 2 (16.7%) due to cardiopulmonary failure. SUDEP affected more adults (n = 10) than children (n = 2). HCN1 variants p.M234R, p.C329S, p.V414M, p.M153I, and p.M305L, as well as HCN2 variants p.S632W and delPPP (p.719–721), were associated with different phenotypes. HCN1 p.L157V and HCN4 p.R550C were associated with genetic generalized epilepsy. There are several HCN animal models, pharmacological targets, and modulators, but precise drugs have not been developed. Currently, there are no HCN channel openers.

Conclusion

We recommend clinicians to include HCN genes in epilepsy gene panels. Researchers should explore the possible underlying mechanisms for GOF and LOF variants by identifying the specific neuronal subtypes and neuroanatomical locations of each identified pathogenic variant. Researchers should identify specific HCN channel openers and blockers with high binding affinity. Such information will give clarity to the involvement of HCN channelopathies in epilepsy and provide the opportunity to develop targeted treatments.

Review: HCN Channels in the Heart

Pacemaker cells are the basis of rhythm in the heart. Cardiovascular diseases, and in particular, arrhythmias are a leading cause of hospital admissions and have been implicated as a cause of sudden death. The prevalence of people with arrhythmias will increase in the next years due to an increase in the ageing population and risk factors. The current therapies are limited, have a lot of side effects, and thus, are not ideal. Pacemaker channels, also called hyperpolarizationactivated cyclic nucleotide-gated (HCN) channels, are the molecular correlate of the hyperpolarization- activated current, called Ih (from hyperpolarization) or If (from funny), that contribute crucially to the pacemaker activity in cardiac nodal cells and impulse generation and transmission in neurons. HCN channels have emerged as interesting targets for the development of drugs, in particular, to lower the heart rate. Nonetheless, their pharmacology is still rather poorly explored in comparison to many other voltage-gated ion channels or ligand-gated ion channels. Ivabradine is the first and currently the only clinically approved compound that specifically targets HCN channels. The therapeutic indication of ivabradine is the symptomatic treatment of chronic stable angina pectoris in patients with coronary artery disease with a normal sinus rhythm. Several other pharmacological agents have been shown to exert an effect on heart rate, although this effect is not always desired. This review is focused on the pacemaking process taking place in the heart and summarizes the current knowledge on HCN channels.

Ih from synapses to networks: HCN channel functions and modulation in neurons

Hyperpolarization-activated cyclic nucleotide gated (HCN) channels and the current they carry, I_h, are widely and diversely distributed in the central nervous system (CNS). The distribution of the four subunits of HCN channels is variable within the CNS, within brain regions, and often within subcellular compartments. The precise function of I_h can depend heavily on what other channels are co-expressed. In this review, we give an overview of HCN channel structure, distribution, and modulation by cyclic adenosine monophosphate (cAMP). We then discuss HCN channel and I_h functions, where we have parsed the roles into two main effects: a steady effect on maintaining the resting membrane potential at relatively depolarized values, and slow channel dynamics. Within this framework, we discuss I_h involvement in resonance, synaptic integration, transmitter release, plasticity, and point out a special case, where the effects of I_h on the membrane potential and its slow channel dynamics have dual roles in thalamic neurons.

HCN3 ion channels: roles in sensory neuronal excitability and pain

KEY POINTS

HCN ion channels conducting the Ih current control the frequency of firing in peripheral sensory neurons signalling pain. Previous studies have demonstrated a major role for the HCN2 subunit in chronic pain but the potential involvement of HCN3 in pain has not been investigated. HCN3 was found to be widely expressed in all classes of sensory neurons (small, medium, large) where it contributes to Ih . HCN3 deletion increased the firing rate of medium but not small, sensory neurons. Pain sensitivity both acutely and following neuropathic injury was largely unaffected by HCN3 deletion, with the exception of a small decrease of mechanical hyperalgesia in response to a pinprick. We conclude that HCN3 plays little role in either acute or chronic pain sensation.

ABSTRACT

HCN ion channels govern the firing rate of action potentials in the pacemaker region of the heart and in pain-sensitive (nociceptive) nerve fibres. Intracellular cAMP promotes activation of the HCN4 and HCN2 isoforms, whereas HCN1 and HCN3 are relatively insensitive to cAMP. HCN2 modulates action potential firing rate in nociceptive neurons and plays a critical role in all modes of inflammatory and neuropathic pain, although the role of HCN3 in nociceptive excitability and pain is less studied. Using antibody staining, we found that HCN3 is expressed in all classes of somatosensory neurons. In small nociceptive neurons, genetic deletion of HCN2 abolished the voltage shift of the Ih current carried by HCN isoforms following cAMP elevation, whereas the voltage shift was retained following deletion of HCN3, consistent with the sensitivity of HCN2 but not HCN3 to cAMP. Deletion of HCN3 had little effect on the evoked firing frequency in small neurons but enhanced the firing of medium-sized neurons, showing that HCN3 makes a significant contribution to the input resistance only in medium-sized neurons. Genetic deletion of HCN3 had no effect on acute thresholds to heat or mechanical stimuli in vivo and did not affect inflammatory pain measured with the formalin test. Nerve-injured HCN3 knockout mice exhibited similar levels of mechanical allodynia and thermal hyperalgesia to wild-type mice but reduced mechanical hyperalgesia in response to a pinprick. These results show that HCN3 makes some contribution to excitability, particularly in medium-sized neurons, although it has no major influence on acute or neuropathic pain processing.

Characterization of drug binding within the HCN1 channel pore

Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels mediate rhythmic electrical activity of cardiac pacemaker cells, and in neurons play important roles in setting resting membrane potentials, dendritic integration, neuronal pacemaking, and establishing action potential threshold. Block of HCN channels slows the heart rate and is currently used to treat angina. However, HCN block also provides a promising approach to the treatment of neuronal disorders including epilepsy and neuropathic pain. While several molecules that block HCN channels have been identified, including clonidine and its derivative alinidine, lidocaine, mepivacaine, bupivacaine, ZD7288, ivabradine, zatebradine, and cilobradine, their low affinity and lack of specificity prevents wide-spread use. Different studies suggest that the binding sites of these inhibitors are located in the inner vestibule of HCN channels, but the molecular details of their binding remain unknown. We used computational docking experiments to assess the binding sites and mode of binding of these inhibitors against the recently solved atomic structure of human HCN1 channels, and a homology model of the open pore derived from a closely related CNG channel. We identify a possible hydrophobic groove in the pore cavity that plays an important role in conformationally restricting the location and orientation of drugs bound to the inner vestibule. Our results also help explain the molecular basis of the low-affinity binding of these inhibitors, paving the way for the development of higher affinity molecules.

Minimal molecular determinants of isoform-specific differences in efficacy in the HCN channel family

Hyperpolarization-activated, cyclic nucleotide-gated (HCN) channels generate rhythmic activity in the heart and brain. Isoform-specific functional differences reflect the specializations required for the various roles that they play. Despite a high sequence and structural similarity, HCN isoforms differ greatly in their response to cyclic nucleotides. Cyclic AMP (cAMP) enhances the activity of HCN2 and HCN4 isoforms by shifting the voltage dependence of activation to more depolarized potentials, whereas HCN1 and HCN3 isoforms are practically insensitive to this ligand. Here, to determine the molecular basis for increased cAMP efficacy in HCN2 channels, we progressively mutate residues in the C-linker and cyclic nucleotide-binding domain (CNBD) of the mouse HCN2 to their equivalents in HCN1. We identify two clusters of mutations that determine the differences in voltage-dependent activation between these two isoforms. One maps to the C-linker region, whereas the other is in proximity to the cAMP-binding site in the CNBD. A mutant channel containing just five mutations (M485I, G497D, S514T, V562A, and S563G) switches cAMP sensitivity of full-length HCN2 to that of HCN1 channels. These findings, combined with a detailed analysis of various allosteric models for voltage- and ligand-dependent gating, indicate that these residues alter the ability of the C-linker to transduce signals from the CNBD to the pore gates of the HCN channel.

Disturbed Processing of Contextual Information in HCN3 Channel Deficient Mice

Hyperpolarization-activated cyclic nucleotide-gated channels (HCNs) in the nervous system are implicated in a variety of neuronal functions including learning and memory, regulation of vigilance states and pain. Dysfunctions or genetic loss of these channels have been shown to cause human diseases such as epilepsy, depression, schizophrenia, and Parkinson's disease. The physiological functions of HCN1 and HCN2 channels in the nervous system have been analyzed using genetic knockout mouse models. By contrast, there are no such genetic studies for HCN3 channels so far. Here, we use a HCN3-deficient (HCN3^−/−) mouse line, which has been previously generated in our group to examine the expression and function of this channel in the CNS. Specifically, we investigate the role of HCN3 channels for the regulation of circadian rhythm and for the determination of behavior. Contrary to previous suggestions we find that HCN3^−/− mice show normal visual, photic, and non-photic circadian function. In addition, HCN3^−/− mice are impaired in processing contextual information, which is characterized by attenuated long-term extinction of contextual fear and increased fear to a neutral context upon repeated exposure.

The Hyperpolarization-Activated Cyclic Nucleotide-Gated Channels: from Biophysics to Pharmacology of a Unique Family of Ion Channels

Hyperpolarization-activated, cyclic nucleotide-gated (HCN) channels are important members of the voltage-gated pore loop channels family. They show unique features: they open at hyperpolarizing potential, carry a mixed Na/K current, and are regulated by cyclic nucleotides. Four different isoforms have been cloned (HCN1-4) that can assemble to form homo- or heterotetramers, characterized by different biophysical properties. These proteins are widely distributed throughout the body and involved in different physiologic processes, the most important being the generation of spontaneous electrical activity in the heart and the regulation of synaptic transmission in the brain. Their role in heart rate, neuronal pacemaking, dendritic integration, learning and memory, and visual and pain perceptions has been extensively studied; these channels have been found also in some peripheral tissues, where their functions still need to be fully elucidated. Genetic defects and altered expression of HCN channels are linked to several pathologies, which makes these proteins attractive targets for translational research; at the moment only one drug (ivabradine), which specifically blocks the hyperpolarization-activated current, is clinically available. This review discusses current knowledge about HCN channels, starting from their biophysical properties, origin, and developmental features, to (patho)physiologic role in different tissues and pharmacological modulation, ending with their present and future relevance as drug targets.

HCN2 ion channels: basic science opens up possibilities for therapeutic intervention in neuropathic pain

Nociception — the ability to detect painful stimuli — is an invaluable sense that warns against present or imminent damage. In patients with chronic pain, however, this warning signal persists in the absence of any genuine threat and affects all aspects of everyday life. Neuropathic pain, a form of chronic pain caused by damage to sensory nerves themselves, is dishearteningly refractory to drugs that may work in other types of pain and is a major unmet medical need begging for novel analgesics. Hyperpolarisation-activated cyclic nucleotide (HCN)-modulated ion channels are best known for their fundamental pacemaker role in the heart; here, we review data demonstrating that the HCN2 isoform acts in an analogous way as a ‘pacemaker for pain’, in that its activity in nociceptive neurons is critical for the maintenance of electrical activity and for the sensation of chronic pain in pathological pain states. Pharmacological block or genetic deletion of HCN2 in sensory neurons provides robust pain relief in a variety of animal models of inflammatory and neuropathic pain, without any effect on normal sensation of acute pain. We discuss the implications of these findings for our understanding of neuropathic pain pathogenesis, and we outline possible future opportunities for the development of efficacious and safe pharmacotherapies in a range of chronic pain syndromes.

Hyperpolarization-Activated Currents and Subthreshold Resonance in Granule Cells of the Olfactory Bulb

An important contribution to neural circuit oscillatory dynamics is the ongoing activation and inactivation of hyperpolarization-activated currents (Ih). Network synchrony dynamics play an important role in the initial processing of odor signals by the main olfactory bulb (MOB) and accessory olfactory bulb (AOB). In the mouse olfactory bulb, we show that Ih is present in granule cells (GCs), the most prominent inhibitory neuron in the olfactory bulb, and that Ih underlies subthreshold resonance in GCs. In accord with the properties of Ih, the currents exhibited sensitivity to changes in extracellular K+ concentration and ZD7288 (4-ethylphenylamino-1,2-dimethyl-6-methylaminopyrimidin chloride), a blocker of Ih. ZD7288 also caused GCs to hyperpolarize and increase their input resistance, suggesting that Ih is active at rest in GCs. The inclusion of cAMP in the intracellular solution shifted the activation of Ih to less negative potentials in the MOB, but not in the AOB, suggesting that channels with different subunit composition mediate Ih in these regions. Furthermore, we show that mature GCs exhibit Ih-dependent subthreshold resonance in the theta frequency range (4-12 Hz). Another inhibitory subtype in the MOB, the periglomerular cells, exhibited Ih-dependent subthreshold resonance in the delta range (1-4 Hz), while principal neurons, the mitral cells, do not exhibit Ih-dependent subthreshold resonance. Importantly, Ih size, as well as the strength and frequency of resonance in GCs, exhibited a postnatal developmental progression, suggesting that this development of Ih in GCs may differentially contribute to their integration of sensory input and contribution to oscillatory circuit dynamics.

Two types of interneurons in the mouse lateral geniculate nucleus are characterized by different h-current density

Although hyperpolarization-activated cyclic nucleotide-gated cation (HCN) channels and the corresponding h-current (I_h) have been shown to fundamentally shape the activity pattern in the thalamocortical network, little is known about their function in local circuit GABAergic interneurons (IN) of the dorsal part of the lateral geniculate nucleus (dLGN). By combining electrophysiological, molecular biological, immunohistochemical and cluster analysis, we characterized the properties of I_h and the expression profile of HCN channels in IN. Passive and active electrophysiological properties of IN differed. Two subclasses of IN were resolved by unsupervised cluster analysis. Small cells were characterized by depolarized resting membrane potentials (RMP), stronger anomalous rectification, higher firing frequency of faster action potentials (APs), appearance of rebound bursting, and higher I_h current density compared to the large IN. The depolarization exerted by sustained HCN channel activity facilitated neuronal firing. In addition to cyclic nucleotides, I_h in IN was modulated by PIP₂ probably based on the abundant expression of the HCN3 isoform. Furthermore, only IN with larger cell diameters expressed neuronal nitric oxide synthase (nNOS). It is discussed that I_h in IN is modulated by neurotransmitters present in the thalamus and that the specific properties of I_h in these cells closely reflect their modulatory options.

Neuropeptide S modulates the amygdaloidal HCN activities (Ih) in rats: Implication in chronic pain

Neuropeptide S (NPS), an endogenous anxiolytic, has been shown to protect against chronic pain through interacting with its cognate NPS receptor (NPSR) in the brain. However, the cellular mechanism of this NPS action remains unclear. We report that NPS inhibits hyperpolarization-activated cyclic nucleotide-gated (HCN) channel current (Ih) in the rat's amygdala through activation of NPSR. This NPS effect is mediated through ERK1/2 phosphorylation in a subset of pyramidal-like neurons located in the medial amygdala. The characters of the recorded Ih suggest a major role for HCN1 activity in this process. Inhibition of Ih by NPS stimulates the glutamatergic drive onto fast spiking intra-amygdalolidal GABAergic interneurons, which in turn facilitates GABA release onto pyramidal-like neurons. Moreover, the HCN1 expression is increased in the amygdala of rats with peripheral nerve injury and intra-amygdaloidal administration of the HCN channel inhibitor ZD7288 attenuates nociceptive behavior in these rats. These results suggest that NPS-mediated modulation of intra-amygdaloidal HCN channel activities may be an important central inhibitory mechanism for regulation of chronic pain.

Functional Characterization of Cnidarian HCN Channels Points to an Early Evolution of Ih

HCN channels play a unique role in bilaterian physiology as the only hyperpolarization-gated cation channels. Their voltage-gating is regulated by cyclic nucleotides and phosphatidylinositol 4,5-bisphosphate (PIP₂). Activation of HCN channels provides the depolarizing current in response to hyperpolarization that is critical for intrinsic rhythmicity in neurons and the sinoatrial node. Additionally, HCN channels regulate dendritic excitability in a wide variety of neurons. Little is known about the early functional evolution of HCN channels, but the presence of HCN sequences in basal metazoan phyla and choanoflagellates, a protozoan sister group to the metazoans, indicate that the gene family predates metazoan emergence. We functionally characterized two HCN channel orthologs from Nematostella vectensis (Cnidaria, Anthozoa) to determine which properties of HCN channels were established prior to the emergence of bilaterians. We find Nematostella HCN channels share all the major functional features of bilaterian HCNs, including reversed voltage-dependence, activation by cAMP and PIP₂, and block by extracellular Cs⁺. Thus bilaterian-like HCN channels were already present in the common parahoxozoan ancestor of bilaterians and cnidarians, at a time when the functional diversity of voltage-gated K⁺ channels was rapidly expanding. NvHCN1 and NvHCN2 are expressed broadly in planulae and in both the endoderm and ectoderm of juvenile polyps.

Isoform dependent regulation of human HCN channels by cholesterol

Cholesterol has been shown to regulate numerous ion channels. HCN channels represent the molecular correlate of I_f or I_h in sinoatrial node (SAN) and neuronal cells. Previous studies have implicated a role for cholesterol in the regulation of rabbit HCN4 channels with effects on pacing in the rabbit SAN. Using electrophysiological and biochemical approaches, we examined the effect of cholesterol modulation on human HCN1, HCN2 and HCN4 isoforms. Patch-clamp experiments uncovered isoform specific differences in the effect of cholesterol on gating kinetics upon depletion by MβCD or mevastatin or enrichment using MβCD/cholesterol. Most dramatically cholesterol had isoform specific effects on mode-shifting, which has been suggested to play a key role in stabilizing firing rate and preventing arrhythmic firing in SAN cells and neurons. Mode-shifting in HCN1 channels was insensitive to cholesterol manipulation, while HCN2 and HCN4 were strongly affected. Trafficking of each isoform to the plasma membrane was also affected by cholesterol modulation differentially between isoforms, however, each isoform remained localized in lipid raft domains after cholesterol depletion. These effects may contribute to the side effects of cholesterol reducing therapies including disrupted heart rhythm and neuropathic pain, as well as the susceptibility of sinus dysfunction in patients with elevated cholesterol.

TET3 is recruited by REST for context-specific hydroxymethylation and induction of gene expression

Ten-eleven translocation hydroxylases (TET1-3) oxidize 5-methylcytosine (5mC) to 5-hydroxymethylcytosine (5hmC). In neurons, increased 5hmC levels within gene bodies correlate positively with gene expression. The mechanisms controlling TET activity and 5hmC levels are poorly understood. In particular, it is not known how the neuronal TET3 isoform lacking a DNA-binding domain is targeted to the DNA. To identify factors binding to TET3, we screened for proteins that co-precipitate with TET3 from mouse retina and identified the transcriptional repressor REST as a highly enriched TET3-specific interactor. REST was able to enhance TET3 hydroxylase activity after co-expression and overexpression of TET3-activated transcription of REST target genes. Moreover, we found that TET3 also interacts with NSD3 and two other H3K36 methyltransferases and is able to induce H3K36 trimethylation. We propose a mechanism for transcriptional activation in neurons that involves REST-guided targeting of TET3 to the DNA for directed 5hmC generation and NSD3-mediated H3K36 trimethylation.

cAMP control of HCN2 channel Mg2+ block reveals loose coupling between the cyclic nucleotide-gating ring and the pore

Hyperpolarization-activated cyclic nucleotide-regulated HCN channels underlie the Na⁺-K⁺ permeable I_H pacemaker current. As with other voltage-gated members of the 6-transmembrane K_V channel superfamily, opening of HCN channels involves dilation of a helical bundle formed by the intracellular ends of S6 albeit this is promoted by inward, not outward, displacement of S4. Direct agonist binding to a ring of cyclic nucleotide-binding sites, one of which lies immediately distal to each S6 helix, imparts cAMP sensitivity to HCN channel opening. At depolarized potentials, HCN channels are further modulated by intracellular Mg²⁺ which blocks the open channel pore and blunts the inhibitory effect of outward K⁺ flux. Here, we show that cAMP binding to the gating ring enhances not only channel opening but also the kinetics of Mg²⁺ block. A combination of experimental and simulation studies demonstrates that agonist acceleration of block is mediated via acceleration of the blocking reaction itself rather than as a secondary consequence of the cAMP enhancement of channel opening. These results suggest that the activation status of the gating ring and the open state of the pore are not coupled in an obligate manner (as required by the often invoked Monod-Wyman-Changeux allosteric model) but couple more loosely (as envisioned in a modular model of protein activation). Importantly, the emergence of second messenger sensitivity of open channel rectification suggests that loose coupling may have an unexpected consequence: it may endow these erstwhile “slow” channels with an ability to exert voltage and ligand-modulated control over cellular excitability on the fastest of physiologically relevant time scales.

A mechanism for the auto-inhibition of hyperpolarization-activated cyclic nucleotide-gated (HCN) channel opening and its relief by cAMP

Hyperpolarization-activated cyclic nucleotide-gated (HCN) ion channels control neuronal and cardiac electrical rhythmicity. There are four homologous isoforms (HCN1-4) sharing a common multidomain architecture that includes an N-terminal transmembrane tetrameric ion channel followed by a cytoplasmic "C-linker," which connects a more distal cAMP-binding domain (CBD) to the inner pore. Channel opening is primarily stimulated by transmembrane elements that sense membrane hyperpolarization, although cAMP reduces the voltage required for HCN activation by promoting tetramerization of the intracellular C-linker, which in turn relieves auto-inhibition of the inner pore gate. Although binding of cAMP has been proposed to relieve auto-inhibition by affecting the structure of the C-linker and CBD, the nature and extent of these cAMP-dependent changes remain limitedly explored. Here, we used NMR to probe the changes caused by the binding of cAMP and of cCMP, a partial agonist, to the apo-CBD of HCN4. Our data indicate that the CBD exists in a dynamic two-state equilibrium, whose position as gauged by NMR chemical shifts correlates with the V½ voltage measured through electrophysiology. In the absence of cAMP, the most populated CBD state leads to steric clashes with the activated or "tetrameric" C-linker, which becomes energetically unfavored. The steric clashes of the apo tetramer are eliminated either by cAMP binding, which selects for a CBD state devoid of steric clashes with the tetrameric C-linker and facilitates channel opening, or by a transition of apo-HCN to monomers or dimer of dimers, in which the C-linker becomes less structured, and channel opening is not facilitated.

Pathological α-synuclein impairs adult-born granule cell development and functional integration in the olfactory bulb

Although the role of noxious α-synuclein (α-SYN) in the degeneration of midbrain dopaminergic neurons and associated motor deficits of Parkinson’s disease is recognized, its impact on non-motor brain circuits and related symptoms remains elusive. Through combining in vivo two-photon imaging with time-coded labelling of neurons in the olfactory bulb of A30P α-SYN transgenic mice, we show impaired growth and branching of dendrites of adult-born granule cells (GCs), with reduced gain and plasticity of dendritic spines. The spine impairments are especially pronounced during the critical phase of integration of new neurons into existing circuits. Functionally, retarded dendritic expansion translates into reduced electrical capacitance with enhanced intrinsic excitability and responsiveness of GCs to depolarizing inputs, while the spine loss correlates with decreased frequency of AMPA-mediated miniature EPSCs. Changes described here are expected to interfere with the functional integration and survival of new GCs into bulbar networks, contributing towards olfactory deficits and related behavioural impairments.

Aggregation-prone forms of α-synuclein lead to degeneration of midbrain dopaminergic neurons, as seen in Parkinson’s disease, but less is known about the effects that the noxious protein has in other brain regions. Here, the authors investigate the effect of a pathological form of α-synuclein on the functional integration of new neurons into the olfactory bulb of adult mice.

Cyclic nucleotide mapping of hyperpolarization-activated cyclic nucleotide-gated (HCN) channels

Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels play a central role in the regulation of cardiac and neuronal firing rate, and these channels can be dually activated by membrane hyperpolarization and by binding of cyclic nucleotides. cAMP has been shown to directly bind HCN channels and modulate their activity. Despite this, while there are selective inhibitors that block the activation potential of the HCN channels, regulation by cAMP analogs has not been well investigated. A comprehensive screen of 47 cyclic nucleotides with modifications in the nucleobase, ribose moiety, and cyclic phosphate was tested on the three isoforms HCN1, HCN2, and HCN4. 7-CH-cAMP was identified to be a high affinity binder for HCN channels and crosschecked for its ability to act on other cAMP receptor proteins. While 7-CH-cAMP is a general activator for cAMP- and cGMP-dependent protein kinases as well as for the guanine nucleotide exchange factors Epac1 and Epac2, it displays the highest affinity to HCN channels. The molecular basis of the high affinity was investigated by determining the crystal structure of 7-CH-cAMP in complex with the cyclic nucleotide binding domain of HCN4. Electrophysiological studies demonstrate a strong activation potential of 7-CH-cAMP for the HCN4 channel in vivo. So, this makes 7-CH-cAMP a promising activator of the HCN channels in vitro whose functionality can be translated in living cells.

The h-current in periglomerular dopaminergic neurons of the mouse olfactory bulb

The properties of the hyperpolarization-activated cation current (I(h)) were investigated in rat periglomerular dopaminergic neurons using patch-clamp recordings in thin slices. A reliable identification of single dopaminergic neurons was made possible by use of a transgenic line of mice expressing eGFP under the tyrosine hydroxylase promoter. At 37 °C and minimizing the disturbance of the intracellular milieu with perforated patches, this current shows a midpoint of activation around -82.7 mV, with a significant level of opening already at rest, thereby giving a substantial contribution to the resting potential, and ultimately playing a relevant function in the control of the cell excitability. The blockage of I(h) has a profound influence on the spontaneous firing of these neurons, which result as strongly depressed. However the effect is not due to a direct role of the current in the pacemaker process, but to the I(h) influence on the resting membrane potential. I(h) kinetics is sensitive to the intracellular levels of cAMP, whose increase promotes a shift of the activation curve towards more positive potentials. The direct application of DA and 5-HT neurotransmitters, physiologically released onto bulbar dopaminergic neurons and known to act on metabotropic receptors coupled to the cAMP pathway, do not modifythe I(h) amplitude. On the contrary, noradrenaline almost halves the I(h) amplitude. Our data indicate that the HCN channels do not participate directly to the pacemaker activity of periglomerular dopaminergic neurons, but influence their resting membrane potential by controlling the excitability profile of these cells, and possibly affecting the processing of sensory information taking place at the entry of the bulbar circuitry.

Up-regulation of hyperpolarization-activated cyclic nucleotide-gated channel 3 (HCN3) by specific interaction with K+ channel tetramerization domain-containing protein 3 (KCTD3)

Most ion channels consist of the principal ion-permeating core subunit(s) and accessory proteins that are assembled with the channel core. The biological functions of the latter proteins are diverse and include the regulation of the biophysical properties of the ion channel, its connection to signaling pathways and the control of its cell surface expression. There is recent evidence that native hyperpolarization-activated cyclic nucleotide-gated channel complexes (HCN1-4) also contain accessory subunits, among which TRIP8b (tetratricopeptide repeat-containing Rab8b-interacting protein) has been most extensively studied. Here, we identify KCTD3, a so far uncharacterized member of the potassium channel tetramerization-domain containing (KCTD) protein family as an HCN3-interacting protein. KCTD3 is widely expressed in brain and some non-neuronal tissues and colocalizes with HCN3 in specific regions of the brain including hypothalamus. Within the HCN channel family, KCTD3 specifically binds to HCN3 and leads to a profound up-regulation of cell surface expression and current density of this channel. HCN3 can also functionally interact with TRIP8b; however, we found no evidence for channel complexes containing both TRIP8b and KCTD3. The C terminus of HCN3 is crucially required for functional interaction with KCTD3. Replacement of the cytosolic C terminus of HCN2 by the corresponding domain of HCN3 renders HCN2 sensitive to regulation by KCTD3. The C-terminal-half of KCTD3 is sufficient for binding to HCN3. However, the complete protein including the N-terminal tetramerization domain is needed for HCN3 current up-regulation. Together, our experiments indicate that KCTD3 is an accessory subunit of native HCN3 complexes.

The role of HCN channels in ventricular repolarization

Hyperpolarization-activated cyclic nucleotide gated (HCN) channels pass a cationic current (I(h)/I(f)) that crucially contributes to the slow diastolic depolarization (SDD) of sinoatrial pacemaker cells and, hence, is a key determinant of cardiac automaticity and the generation of the heartbeat. However, there is growing evidence that HCN channels are not restricted to the spontaneously active cells of the sinoatrial node and the conduction system but are also present in ventricular cardiomyocytes that produce an action potential lacking SDD. This observation raises the question of the principal function(s) of HCN channels in working myocardium. Our recent analysis of an HCN3-deficient (HCN3-/-) mouse line has shed new light on this central question. We propose that HCN channels contribute to the ventricular action potential waveform, specifically during late repolarization. In this review, we outline this new concept.

The h-current in the substantia Nigra pars compacta neurons: a re-examination

The properties of the hyperpolarization-activated cation current (I_h) were investigated in rat substantia nigra - pars compacta (SNc) principal neurons using patch-clamp recordings in thin slices. A reliable identification of single dopaminergic neurons was made possible by the use of a transgenic line of mice expressing eGFP under the tyrosine hydroxylase promoter. The effects of temperature and different protocols on the I_h kinetics showed that, at 37°C and minimizing the disturbance of the intracellular milieu with perforated patch, this current actually activates at potentials more positive than what is generally indicated, with a half-activation potential of −77.05 mV and with a significant level of opening already at rest, thereby substantially contributing to the control of membrane potential, and ultimately playing a relevant function in the regulation of the cell excitability. The implications of the known influence of intracellular cAMP levels on I_h amplitude and kinetics were examined. The direct application of neurotransmitters (DA, 5-HT and noradrenaline) physiologically released onto SNc neurons and known to act on metabotropic receptors coupled to the cAMP pathway modify the I_h amplitude. Here, we show that direct activation of dopaminergic and of 5-HT receptors results in I_h inhibition of SNc DA cells, whereas noradrenaline has the opposite effect. Together, these data suggest that the modulation of I_h by endogenously released neurotransmitters acting on metabotropic receptors –mainly but not exclusively linked to the cAMP pathway- could contribute significantly to the control of SNc neuron excitability.

Architecture of the HCN selectivity filter and control of cation permeation

Hyperpolarization-activated Cyclic Nucleotide-modulated (HCN) channels are similar in structure and function to voltage-gated potassium channels. Sequence similarity and functional analyses suggest that the HCN pore is potassium channel-like, consisting of a selectivity filter and an activation gate at the outer and inner ends, respectively. In GYG-containing potassium channels, the selectivity filter sequence is 'T/S-V/I/L/T-GYG', forming a row of four binding sites through which potassium ions flow. In HCNs, the equivalent residues are 'C-I-GYG', but whether they also form four cation binding sites is not known. Here, we focus on the anomalous filter residue of HCNs, the cysteine located at the inner side of the selectivity filter. In potassium channels, this position is occupied by threonine or serine and forms the fourth and most internal ion binding site of the selectivity filter. We find that this cysteine in HCNs does not contribute to permeation or form a fourth binding site.

Asymmetric divergence in structure and function of HCN channel duplicates in Ciona intestinalis

Hyperpolarization-activated Cyclic Nucleotide (HCN) channels are voltage-gated cation channels and are critical for regulation of membrane potential in electrically active cells. To understand the evolution of these channels at the molecular level, we cloned and examined two of three HCN homologs of the urochordate Ciona intestinalis (ciHCNa and ciHCNb). ciHCNa is like mammalian HCNs in that it possesses similar electrical function and undergoes N-glycosylation of a sequon near the pore. ciHCNb lacks the pore-associated N-glycosylation sequon and is predictably not N-glycosylated, and it also has an unusual gating phenotype in which the channel's voltage-sensitive gate appears to close incompletely. Together with previous findings, the data support an evolutionary trajectory in which an HCN ancestor underwent lineage-specific duplication in Ciona, to yield one HCN with most features that are conserved with the mammalian HCNs and another HCN that has been uniquely altered.

The distribution of HCN2-positive cells in the gastrointestinal tract of mice

HCN2 channels are involved in the spontaneous rhythmic activities of some CNS neurons and act by generating I(f) current. The gastrointestinal (GI) tract is known to be capable of spontaneous rhythmic activity; however, the possible role of HCN2 channels in this organ has not yet been elucidated. This study investigated the distribution of HCN2-positive cells in the mouse GI tract using immunohistochemistry. To identify the nature of these HCN2 cells, anti-ChAT and anti-Kit antibodies were used to co-label neurons and the interstitial cells of Cajal (ICCs), respectively. Additionally, differences in the distribution of HCN2-positive cells within the GI tract were also analyzed. Our results showed that HCN2 channels were mainly located within the myenteric neurons of the enteric nervous system in the GI tract. Double-staining revealed that HCN2-positive neurons were labeled by ChAT, indicating that these HCN2-positive cells are also cholinergic neurons. Although the HCN2-positive cells were not stained by the anti-Kit antibody, their processes were in close proximity to ICCs around the myenteric plexus region. Moreover, several differences in the distribution of HCN2 in the stomach, small intestine and colon were partly consistent with the regional differences in the spontaneous rhythmic activities of these organs. Basing on the role HCN2, we suggested that HCN2 channels facilitate the release of Ach from cholinergic neurons to affect the GI peristalsis by acting on M receptors on the ICCs. However, the HCN2 channels are not directly involved in spontaneous slow-wave initiation by ICCs.

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.

Distinct perinatal features of the hyperpolarization-activated non-selective cation current I(h) in the rat cortical plate

BACKGROUND

During neocortical development, multiple voltage- and ligand-gated ion channels are differentially expressed in neurons thereby shaping their intrinsic electrical properties. One of these voltage-gated ion channels, the hyperpolarization-activated cyclic nucleotide-gated (HCN) channel and its current I(h), is an important regulator of neuronal excitability. Thus far, studies on an early I(h) appearance in rodent neocortex are missing or conflicting. Therefore, we focused our study on perinatal neocortical I(h) and its properties.

RESULTS

In the perinatal rat neocortex we observed a rapid increase in the number of neurons exhibiting I(h). Perinatal I(h) had unique properties: first, a pronounced cAMP sensitivity resulting in a marked shift of the voltage sufficient for half-maximum activation of the current towards depolarized voltages and second, an up to 10 times slower deactivation at physiological membrane potentials when compared to the one at postnatal day 30. The combination of these features was sufficient to suppress membrane resonance in our in silico and in vitro experiments. Although all four HCN subunits were present on the mRNA level we only detected HCN4, HCN3 and HCN1 on the protein level at P0. HCN1 protein at P0, however, appeared incompletely processed. At P30 glycosilated HCN1 and HCN2 dominated. By in silico simulations and heterologous co-expression experiments of a 'slow' and a 'fast' I(h) conducting HCN channel subunit in HEK293 cells, we mimicked most characteristics of the native current, pointing to a functional combination of subunit homo- or heteromeres.

CONCLUSION

Taken together, these data indicate a HCN subunit shift initiated in the first 24 hours after birth and implicate a prominent perinatal role of the phylogenetically older HCN3 and/or HCN4 subunits in the developing neocortex.

An essential role for modulation of hyperpolarization-activated current in the development of binaural temporal precision

In sensory circuits of the brain, developmental changes in the expression and modulation of voltage-gated ion channels are a common occurrence, but such changes are often difficult to assign to clear functional roles. We have explored this issue in the binaural neurons of the medial superior olive (MSO), whose temporal precision in detecting the coincidence of binaural inputs dictates the resolution of azimuthal sound localization. We show that in MSO principal neurons of gerbils during the first week of hearing, a hyperpolarization-activated current (I(h)) progressively undergoes a 13-fold increase in maximal conductance, a >10-fold acceleration of kinetics, and, most surprisingly, a 30 mV depolarizing shift in the voltage dependence of activation. This period is associated with an upregulation of the hyperpolarization-activated and cyclic nucleotide-gated (HCN) channel subunits HCN1, HCN2, and HCN4 in the MSO, but only HCN1 and HCN4 were expressed strongly in principal neurons. I(h) recorded in nucleated patches from electrophysiologically mature MSO neurons (>P18) exhibited kinetics and an activation range nearly identical to the I(h) found in whole-cell recordings before hearing onset. These results indicate that the developmental changes in I(h) in MSO neurons can be explained predominantly by modulation from diffusible intracellular factors, and not changes in channel subunit composition. The exceptionally large modulatory changes in I(h), together with refinements in synaptic properties transform the coding strategy from one of summation and integration to the submillisecond coincidence detection known to be required for transmission of sound localization cues.

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.

Energetics of cyclic AMP binding to HCN channel C terminus reveal negative cooperativity

Cyclic AMP binds to the HCN channel C terminus and variably stabilizes its open state. Using isothermal titration calorimetry, we show that cAMP binds to one subunit of tetrameric HCN2 and HCN4 C termini with high affinity (∼0.12 μM) and subsequently with low affinity (∼1 μM) to the remaining three subunits. Changes induced by high affinity binding already exist in both a constrained HCN2 tetramer and the unconstrained HCN1 tetramer. Natural "preactivation" of HCN1 may explain both the smaller effect of cAMP on stabilizing its open state and the opening of unliganded HCN1, which occurs as though already disinhibited.

Cyclic AMP signaling control of action potential firing rate and molecular circadian pacemaking in the suprachiasmatic nucleus

Circadian pacemaking in suprachiasmatic nucleus (SCN) neurons revolves around transcriptional/posttranslational feedback loops, driven by protein products of "clock" genes. These loops are synchronized and sustained by intercellular signaling, involving vasoactive intestinal peptide (VIP) via its VPAC2 receptor, which positively regulates cAMP synthesis. In turn, SCN cells communicate circadian time to the brain via a daily rhythm in electrophysiological activity. To investigate the mechanisms whereby VIP/VPAC2/cAMP signaling controls SCN molecular and electrical pacemaking, we combined bioluminescent imaging of circadian gene expression and whole-cell electrophysiology in organotypic SCN slices. As a potential direct target of cAMP, we focused on hyperpolarization-activated, cyclic nucleotide-gated (HCN) channels. Mutations of VIP-ergic signaling compromised the SCN molecular pacemaker, diminishing the amplitude and intercellular synchrony of circadian gene expression. These deficits were transiently reversed by elevation of cAMP. Similarly, cellular synchrony in electrical firing rates was lost in SCN slices lacking the VPAC2 receptor for VIP. Whole-cell current-clamp recordings in wild-type (WT) slices revealed voltage responses shaped by the conductance I(h), which is mediated by HCN channel activity. The influence of I(h) on voltage responses showed a modest peak in early circadian day, identifying HCN channels as a putative mediator of cAMP-dependent circadian effects on firing rate. I(h), however, was unaffected by loss of VIP-ergic signaling in VPAC2-null slices, and inhibition of cAMP synthesis had no discernible effect on I(h) but did suppress gene expression and SCN firing rates. Moreover, only sustained but not acute, pharmacological blockade of HCN channels reduced action potential (AP) firing. Thus, our evidence suggests that in the SCN, cAMP-mediated signaling is not a principal regulator of HCN channel function and that HCN is not a determinant of AP firing rate. VIP/cAMP-dependent signaling sustains the SCN molecular oscillator and action potential firing via mechanisms yet to be identified.

Differential regulation of HCN channel isoform expression in thalamic neurons of epileptic and non-epileptic rat strains

Hyperpolarization-activated cyclic nucleotide-gated cation (HCN) channels represent the molecular substrate of the hyperpolarization-activated inward current (I(h)). Although these channels act as pacemakers for the generation of rhythmic activity in the thalamocortical network during sleep and epilepsy, their developmental profile in the thalamus is not yet fully understood. Here we combined electrophysiological, immunohistochemical, and mathematical modeling techniques to examine HCN gene expression and I(h) properties in thalamocortical relay (TC) neurons of the dorsal part of the lateral geniculate nucleus (dLGN) in an epileptic (WAG/Rij) compared to a non-epileptic (ACI) rat strain. Recordings of TC neurons between postnatal day (P) 7 and P90 in both rat strains revealed that I(h) was characterized by higher current density, more hyperpolarized voltage dependence, faster activation kinetics, and reduced cAMP-sensitivity in epileptic animals. All four HCN channel isoforms (HCN1-4) were detected in dLGN, and quantitative analyses revealed a developmental increase of protein expression of HCN1, HCN2, and HCN4 but a decrease of HCN3. HCN1 was expressed at higher levels in WAG/Rij rats, a finding that was correlated with increased expression of the interacting proteins filamin A (FilA) and tetratricopeptide repeat-containing Rab8b-interacting protein (TRIP8b). Analysis of a simplified computer model of the thalamic network revealed that the alterations of I(h) found in WAG/Rij rats compensate each other in a way that leaves I(h) availability constant, an effect that ensures unaltered cellular burst activity and thalamic oscillations. These data indicate that during postnatal developmental the hyperpolarizing shift in voltage dependency (resulting in less current availability) is compensated by an increase in current density in WAG/Rij thereby possibly limiting the impact of I(h) on epileptogenesis. Because HCN3 is expressed higher in young versus older animals, HCN3 likely does not contribute to alterations in I(h) in older animals.

HCN3 contributes to the ventricular action potential waveform in the murine heart

RATIONALE

The hyperpolarization-activated current I(h) that is generated by hyperpolarization-activated cyclic nucleotide-gated channels (HCNs) plays a key role in the control of pacemaker activity in sinoatrial node cells of the heart. By contrast, it is unclear whether I(h) is also relevant for normal function of cardiac ventricles.

OBJECTIVE

To study the role of the HCN3-mediated component of ventricular I(h) in normal ventricular function.

METHODS AND RESULTS

To test the hypothesis that HCN3 regulates the ventricular action potential waveform, we have generated and analyzed a HCN3-deficient mouse line. At basal heart rate, mice deficient for HCN3 displayed a profound increase in the T-wave amplitude in telemetric electrocardiographic measurements. Action potential recordings on isolated ventricular myocytes indicate that this effect was caused by an acceleration of the late repolarization phase in epicardial myocytes. Furthermore, the resting membrane potential was shifted to more hyperpolarized potentials in HCN3-deficient mice. Cardiomyocytes of HCN3-deficient mice displayed approximately 30% reduction of total I(h). At physiological ionic conditions, the HCN3-mediated current had a reversal potential of approximately -35 mV and displayed ultraslow deactivation kinetics.

CONCLUSIONS

We propose that HCN3 together with other members of the HCN channel family confer a depolarizing background current that regulates ventricular resting potential and counteracts the action of hyperpolarizing potassium currents in late repolarization. In conclusion, our data indicate that HCN3 plays an important role in shaping the cardiac action potential waveform.

Colocalization of hyperpolarization-activated, cyclic nucleotide-gated channel subunits in rat retinal ganglion cells

The current-passing pore of mammalian hyperpolarization-activated, cyclic nucleotide-gated (HCN) channels is formed by subunit isoforms denoted HCN1-4. In various brain areas, antibodies directed against multiple isoforms bind to single neurons, and the current (I(h)) passed during hyperpolarizations differs from that of heterologously expressed homomeric channels. By contrast, retinal rod, cone, and bipolar cells appear to use homomeric HCN channels. Here, we assess the generality of this pattern by examining HCN1 and HCN4 immunoreactivity in rat retinal ganglion cells, measuring I(h) in dissociated cells, and testing whether HCN1 and HCN4 proteins coimmunoprecipitate. Nearly half of the ganglion cells in whole-mounted retinae bound antibodies against both isoforms. Consistent with colocalization and physical association, 8-bromo-cAMP shifted the voltage sensitivity of I(h) less than that of HCN4 channels and more than that of HCN1 channels, and HCN1 coimmunoprecipitated with HCN4 from membrane fraction proteins. Finally, the immunopositive somata ranged in diameter from the smallest to the largest in rat retina, the dendrites of immunopositive cells arborized at various levels of the inner plexiform layer and over fields of different diameters, and I(h) activated with similar kinetics and proportions of fast and slow components in small, medium, and large somata. These results show that different HCN subunits colocalize in single retinal ganglion cells, identify a subunit that can reconcile native I(h) properties with the previously reported presence of HCN4 in these cells, and indicate that I(h) is biophysically similar in morphologically diverse retinal ganglion cells and differs from I(h) in rods, cones, and bipolar cells.

The hyperpolarization-activated cyclic nucleotide-gated HCN2 channel transports ammonium in the distal nephron

Recent studies have identified Rhesus proteins as important molecules for ammonia transport in acid-secreting intercalated cells in the distal nephron. Here, we provide evidence for an additional molecule that can mediate NH3/NH4 excretion, the subtype 2 of the hyperpolarization-activated cyclic nucleotide-gated channel family (HCN2), in collecting ducts in rat renal cortex and medulla. Chronic metabolic acidosis in rats did not alter HCN2 protein expression but downregulated the relative abundance of HCN2 mRNA. Its cDNA was identical to the homolog from the brain and the protein was post-translationally modified by N-type glycosylation. Electrophysiological recordings in Xenopus oocytes injected with HCN2 cRNA found that potassium was transported better than ammonium, each of which was transported significantly better than sodium, criteria that are compatible with a role for HCN2 in ammonium transport. In microperfused rat outer medullary collecting duct segments, the initial rate of acidification, upon exposure to a basolateral ammonium chloride pulse, was higher in intercalated than in principal cells. A specific inhibitor of HCN2 (ZD7288) decreased acidification only in intercalated cells from control rats. In rats with chronic metabolic acidosis, the rate of acidification doubled in both intercalated and principal cells; however, ZD7288 had no significant inhibitory effect. Thus, HCN2 is a basolateral ammonium transport pathway of intercalated cells and may contribute to the renal regulation of body pH under basal conditions.

PIP2-mediated HCN3 channel gating is crucial for rhythmic burst firing in thalamic intergeniculate leaflet neurons

Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels generate a pacemaking current, I(h), which regulates neuronal excitability and oscillatory activity in the brain. Although all four HCN isoforms are expressed in the brain, the functional contribution of HCN3 is unknown. Using immunohistochemistry, confocal microscopy, and whole-cell patch-clamp recording techniques, we investigated HCN3 function in thalamic intergeniculate leaflet (IGL) neurons, as HCN3 is reportedly preferentially expressed in these cells. We observed that I(h) recorded from IGL, but not ventral geniculate nucleus, neurons in HCN2(+/+) mice and rats activated slowly and were cAMP insensitive, which are hallmarks of HCN3 channels. We also observed strong immunolabeling for HCN3, with no labeling for HCN1 and HCN4, and only very weak labeling for HCN2. Deletion of HCN2 did not alter I(h) characteristics in mouse IGL neurons. These data together indicate that the HCN3 channel isoform generated I(h) in IGL neurons. Intracellular phosphatidylinositol-4,5-bisphosphate (PIP(2)) shifted I(h) activation to more depolarized potentials and accelerated activation kinetics. Upregulation of HCN3 function by PIP(2) augmented low-threshold burst firing and spontaneous oscillations; conversely, depletion of PIP(2) or pharmacologic block of I(h) resulted in a profound inhibition of excitability. The results indicate that functional expression of HCN3 channels in IGL neurons is crucial for intrinsic excitability and rhythmic burst firing, and PIP(2) serves as a powerful modulator of I(h)-dependent properties via an effect on HCN3 channel gating. Since the IGL is a major input to the suprachiasmatic nucleus, regulation of pacemaking function by PIP(2) in the IGL may influence sleep and circadian rhythms.

A transcriptomic analysis of type I-III neurons in the bed nucleus of the stria terminalis

The activity of neurons in the anterolateral cell group of the bed nucleus of the stria terminalis (BNST(ALG)) plays a critical role in anxiety- and stress-related behaviors. Histochemical studies have suggested that multiple distinct neuronal phenotypes exist in the BNST(ALG). Consistent with this observation, the physiological properties of BNST(ALG) neurons are also heterogeneous, and three distinct cell types can be defined (Types I-III) based primarily on their expression of four key membrane currents, namely I(h), I(A), I(T), and I(K(IR)). Significantly, all four channels are multimeric proteins and can comprise of more than one pore-forming α subunit. Hence, differential expression of α subunits may further diversify the neuronal population. However, nothing is known about the relative expression of these ion channel α subunits in BNST(ALG) neurons. We have addressed this lacuna by combining whole-cell patch-clamp recording together with single-cell reverse transcriptase polymerase chain reaction (scRT-PCR) to assess the mRNA transcript expression for each of the subunits for the four key ion channels in Type I-III neurons of the BNST(ALG.) Here, cytosolic mRNA from single neurons was probed for the expression of transcripts for each of the α subunits of I(h) (HCN1-HCN4), I(T) (Ca(v)3.1-Ca(v)3.3), I(A) (K(v)1.4, K(v)3.4, K(v)4.1-K(v) 4.3) and I(K(IR)) (Kir2.1-Kir2.4). An unbiased hierarchical cluster analysis followed by discriminant function analysis revealed that a positive correlation exists between the physiological and genetic phenotype of BNST(ALG) neurons. Thus, the analysis segregated BNST(ALG) neurons into 3 distinct groups, based on their α subunit mRNA expression profile, which positively correlated with our existing electrophysiological classification (Types I-III). Furthermore, analysis of mRNA transcript expression in Type I-Type III neurons suggested that, whereas Type I and III neurons appear to represent genetically homologous cell populations, Type II neurons may be further subdivided into three genetically distinct subgroups. These data not only validate our original classification scheme, but further refine the classification at the molecular level, and thus identifies novel targets for potential disruption and/or pharmacotherapeutic intervention in stress-related anxiety-like behaviors.

The cGMP-dependent protein kinase II Is an inhibitory modulator of the hyperpolarization-activated HCN2 channel

Opening of hyperpolarization-activated cyclic nucleotide-gated (HCN) channels is facilitated by direct binding of cyclic nucleotides to a cyclic nucleotide-binding domain (CNBD) in the C-terminus. Here, we show for the first time that in the HCN2 channel cGMP can also exert an inhibitory effect on gating via cGMP-dependent protein kinase II (cGKII)-mediated phosphorylation. Using coimmunoprecipitation and immunohistochemistry we demonstrate that cGKII and HCN2 interact and colocalize with each other upon heterologous expression as well as in native mouse brain. We identify the proximal C-terminus of HCN2 as binding region of cGKII and show that cGKII phosphorylates HCN2 at a specific serine residue (S641) in the C-terminal end of the CNBD. The cGKII shifts the voltage-dependence of HCN2 activation to 2–5 mV more negative voltages and, hence, counteracts the stimulatory effect of cGMP on gating. The inhibitory cGMP effect can be either abolished by mutation of the phosphorylation site in HCN2 or by impairing the catalytic domain of cGKII. By contrast, the inhibitory effect is preserved in a HCN2 mutant carrying a CNBD deficient for cGMP binding. Our data suggest that bidirectional regulation of HCN2 gating by cGMP contributes to cellular fine-tuning of HCN channel activity.

Structural basis for the cAMP-dependent gating in the human HCN4 channel

Hyperpolarization-activated cAMP-regulated (HCN) channels play important physiological roles in both cardiovascular and central nervous systems. Among the four HCN isoforms, HCN2 and HCN4 show high expression levels in the human heart, with HCN4 being the major cardiac isoform. The previously published crystal structure of the mouse HCN2 (mHCN2) C-terminal fragment, including the C-linker and the cyclic-nucleotide binding domain (CNBD), has provided many insights into cAMP-dependent gating in HCN channels. However, structures of other mammalian HCN channel isoforms have been lacking. Here we used a combination of approaches including structural biology, biochemistry, and electrophysiology to study cAMP-dependent gating in HCN4 channel. First we solved the crystal structure of the C-terminal fragment of human HCN4 (hHCN4) channel at 2.4 Å. Overall we observed a high similarity between mHCN2 and hHCN4 crystal structures. Functional comparison between two isoforms revealed that compared with mHCN2, the hHCN4 protein exhibited marked different contributions to channel function, such as a ∼3-fold reduction in the response to cAMP. Guided by structural differences in the loop region between β4 and β5 strands, we identified residues that could partially account for the differences in response to cAMP between mHCN2 and hHCN4 proteins. Moreover, upon cAMP binding, the hHCN4 C-terminal protein exerts a much prolonged effect in channel deactivation that could have significant physiological contributions.

cAMP sensitivity of HCN pacemaker channels determines basal heart rate but is not critical for autonomic rate control

BACKGROUND

HCN channels activate the pacemaker current I(f), which is thought to contribute significantly to generation and regulation of heart rhythm. HCN4 represents the dominant isotype in the sinoatrial node and binding of cAMP was suggested to be necessary for autonomic heart rate regulation.

METHODS AND RESULTS

In a candidate gene approach, a heterozygous insertion of 13 nucleotides in exon 6 of the HCN4 gene leading to a truncated cyclic nucleotide-binding domain was identified in a 45-year-old woman with sinus bradycardia. Biophysical properties determined by whole-cell patch-clamp recording of HEK293 cells demonstrated that mutant subunits (HCN4-695X) were insensitive to cAMP. Heteromeric channels composed of wild-type and mutant subunits failed to respond to cAMP-like homomeric mutant channels, indicating a dominant-negative suppression of cAMP-induced channel activation by mutant subunits. Pedigree analysis identified 7 additional living carriers showing similar clinical phenotypes, that is, sinus node dysfunction with mean resting heart rate of 45.9±4.6 bpm (n=8) compared with 66.5±9.1 bpm of unaffected relatives (n=6; P<0.01). Clinical evaluation revealed no ischemic or structural heart disease in any family member. Importantly, mutant carriers exhibited normal heart rate variance and full ability to accelerate heart rate under physical activity or pharmacological stimulation. Moreover, mutant carriers displayed distinctive sinus arrhythmias and premature beats linked to adrenergic stress.

CONCLUSIONS

In humans, cAMP responsiveness of I(f) determines basal heart rate but is not critical for maximum heart rate, heart rate variability, or chronotropic competence. Furthermore, cAMP-activated I(f) may stabilize heart rhythm during chronotropic response.

Distinct gene expression profiles directed by the isoforms of the transcription factor neuron-restrictive silencer factor in human SK-N-AS neuroblastoma cells

Neuron-restrictive silencer factor (NRSF) and its isoforms are differentially regulated in rodent models of self-sustaining status epilepticus (SSSE). NRSF isoforms regulate genes associated with SSSE, including the proconvulsant tachykinins, brain-derived neurotrophic factor and multiple ion channels. NRSF isoforms may direct distinct gene expression patterns during SSSE, and the ratio of each isoform may be a causative factor in traumatic damage to the central nervous system. Here, we analysed global gene expression changes by microarray in human SK-N-AS neuroblastoma cells following the over-expression of NRSF and a truncated isoform, HZ4. We used bioinformatics software to analyse the microarray dataset and correlated these data with epilepsy candidate gene pathways. Findings were validated by reverse transcriptase-polymerase chain reaction. We demonstrated that NRSF and HZ4 direct overlapping as well as distinct gene expression patterns, and that they differentially modulated gene expression patterns associated with epilepsy. Finally, we revealed that NRSF gene expression may be modulated by the anticonvulsant, phenytoin. We have interpreted our data to reflect altered gene expression directed by NRSF that might be relevant for SSSE.