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

HCN4 in the atrioventricular node

Hyperpolarization-activated cyclic nucleotide-gated channel 4 (HCN4) drives the funny current in cardiac pacemaker regions. Its involvement in sinoatrial node pacemaker generation is well known, but its function in the atrioventricular (AV) node (AVN) has not intensively been studied. HCN4 is expressed in the AVN, and its expression within the AVN seems similar across mammalian species with HCN4 presence in the inferior nodal extensions, compact node, and AV bundle. The main direct regulators of HCN4 are cAMP and protein kinase A. In addition, indirect regulators may affect HCN4 via trafficking and localization. However, these effects are underexplored in the AVN. AVN-specific effects in knockout and knockin mice include reduced funny current density and increased AV block. HCN4 expression in the AVN could be affected by aging, exercise, heart failure, and diabetes. This could underlie changes in PR interval, atria-His interval, Wenckebach cycle length, and AVN effective refractory period. Clinical reports link the HCN4 variant G1097W to AV block. Other clinical data come from studies assessing ivabradine, an HCN4 inhibitor. In animals, ivabradine resulted in prolonged PR and atrial-his intervals. To date, uncertainty regarding the role of HCN4 in the AVN remains. However, AVN-focused studies suggest HCN4's importance for AVN function. This review summarizes recent findings and highlights the involvement of HCN4 in normal and pathological AVN function.

Subunit-specific conductance of single homomeric and heteromeric HCN pacemaker channels at femtosiemens resolution

In mammals, the four subunit isoforms HCN1-4 assemble to form functional homotetrameric and heterotetrameric hyperpolarization-activated cyclic nucleotide-modulated (HCN) ion channels. Despite the outstanding relevance of HCN channels for organisms, including generating electrical rhythmicity in cardiac pacemaker cells and diverse types of brain neurons, key channel properties are still elusive. In particular, the unitary conductance, γ, of HCN channels is highly controversial. We analyzed the unitary conductance at femtosiemens resolution of all four homotetrameric channels of the mouse, mHCN1-4. All conductance values are in the range of 1 pS which is exceptionally small compared to most other ion channels. Surprisingly, the conductance among the isoforms differs up to threefold (γmHCN2 = 1.54 pS > γmHCN1 = 0.84 pS > γmHCN3 = 0.54 pS ≈ γmHCN4 = 0.51 pS) though the residues in the two narrow parts of the pore, the selectivity filter and the inner gate, are conserved. Mutagenesis and all-atom molecular dynamics simulations demonstrate that the differences in the conductance are generated by different amounts of negative charges in the outer channel vestibule, which control ion accumulation. In line with these results, heterotetrameric channels exhibit intermediate unitary conductance values with respect to the homotetrameric channels. Our approach demonstrates how HCN channels can be functionally differentiated at the single-channel level, paving the way to target specific channels with selective drugs.

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.

Structural determinants of ivabradine block of the open pore of HCN4

HCN1-4 channels are the molecular determinants of the If/Ih current that crucially regulates cardiac and neuronal cell excitability. HCN dysfunctions lead to sinoatrial block (HCN4), epilepsy (HCN1), and chronic pain (HCN2), widespread medical conditions awaiting subtype-specific treatments. Here, we address the problem by solving the cryo-EM structure of HCN4 in complex with ivabradine, to date the only HCN-specific drug on the market. Our data show ivabradine bound inside the open pore at 3 Å resolution. The structure unambiguously proves that Y507 and I511 on S6 are the molecular determinants of ivabradine binding to the inner cavity, while F510, pointing outside the pore, indirectly contributes to the block by controlling Y507. Cysteine 479, unique to the HCN selectivity filter (SF), accelerates the kinetics of block. Molecular dynamics simulations further reveal that ivabradine blocks the permeating ion inside the SF by electrostatic repulsion, a mechanism previously proposed for quaternary ammonium ions.

Structural basis for hyperpolarization-dependent opening of human HCN1 channel

Hyperpolarization and cyclic nucleotide (HCN) activated ion channels are critical for the automaticity of action potentials in pacemaking and rhythmic electrical circuits in the human body. Unlike most voltage-gated ion channels, the HCN and related plant ion channels activate upon membrane hyperpolarization. Although functional studies have identified residues in the interface between the voltage-sensing and pore domain as crucial for inverted electromechanical coupling, the structural mechanisms for this unusual voltage-dependence remain unclear. Here, we present cryo-electron microscopy structures of human HCN1 corresponding to Closed, Open, and a putative Intermediate state. Our structures reveal that the downward motion of the gating charges past the charge transfer center is accompanied by concomitant unwinding of the inner end of the S4 and S5 helices, disrupting the tight gating interface observed in the Closed state structure. This helix-coil transition at the intracellular gating interface accompanies a concerted iris-like dilation of the pore helices and underlies the reversed voltage dependence of HCN channels.

Selective and brain-penetrant HCN1 inhibitors reveal links between synaptic integration, cortical function, and working memory

Hyperpolarization-activated and cyclic-nucleotide-gated 1 (HCN1) ion channels are proposed to be critical for cognitive function through regulation of synaptic integration. However, resolving the precise role of HCN1 in neurophysiology and exploiting its therapeutic potential has been hampered by minimally selective antagonists with poor potency and limited in vivo efficiency. Using automated electrophysiology in a small-molecule library screen and chemical optimization, we identified a primary carboxamide series of potent and selective HCN1 inhibitors with a distinct mode of action. In cognition-relevant brain circuits, selective inhibition of native HCN1 produced on-target effects, including enhanced excitatory postsynaptic potential summation, while administration of a selective HCN1 inhibitor to rats recovered decrement working memory. Unlike prior non-selective HCN antagonists, selective HCN1 inhibition did not alter cardiac physiology in human atrial cardiomyocytes or in rats. Collectively, selective HCN1 inhibitors described herein unmask HCN1 as a potential target for the treatment of cognitive dysfunction in brain disorders.

Cerebellar interneurons control fear memory consolidation via learning-induced HCN plasticity

While synaptic plasticity is considered the basis of learning and memory, modifications of the intrinsic excitability of neurons can amplify the output of neuronal circuits and consequently change behavior. However, the mechanisms that underlie learning-induced changes in intrinsic excitability during memory formation are poorly understood. In the cerebellum, we find that silencing molecular layer interneurons completely abolishes fear memory, revealing their critical role in memory consolidation. The fear conditioning paradigm produces a lasting reduction in hyperpolarization-activated cyclic nucleotide-gated (HCN) channels in these interneurons. This change increases intrinsic membrane excitability and enhances the response to synaptic stimuli. HCN loss is driven by a decrease in endocannabinoid levels via altered cGMP signaling. In contrast, an increase in release of cerebellar endocannabinoids during memory consolidation abolishes HCN plasticity. Thus, activity in cerebellar interneurons drives fear memory formation via a learning-specific increase in intrinsic excitability, and this process requires the loss of endocannabinoid-HCN signaling.

Structural Basis for Hyperpolarization-dependent Opening of the Human HCN1 Channel

Hyperpolarization and cyclic-nucleotide (HCN) activated ion channels play a critical role in generating self-propagating action potentials in pacemaking and rhythmic electrical circuits in the human body. Unlike most voltage-gated ion channels, the HCN channels activate upon membrane hyperpolarization, but the structural mechanisms underlying this gating behavior remain unclear. Here, we present cryo-electron microscopy structures of human HCN1 in Closed, Intermediate, and Open states. Our structures reveal that the inward motion of two gating charges past the charge transfer center (CTC) and concomitant tilting of the S5 helix drives the opening of the central pore. In the intermediate state structure, a single gating charge is positioned below the CTC and the pore appears closed, whereas in the open state structure, both charges move past CTC and the pore is fully open. Remarkably, the downward motion of the voltage sensor is accompanied by progressive unwinding of the inner end of S4 and S5 helices disrupting the tight gating interface that stabilizes the Closed state structure. This "melting" transition at the intracellular gating interface leads to a concerted iris-like displacement of S5 and S6 helices, resulting in pore opening. These findings reveal key structural features that are likely to underlie reversed voltage-dependence of HCN channels.

The Potential Antidepressant Compound Org 34167 Modulates HCN Channels Via a Novel Mode of Action

Org 34167 is a small molecule hyperpolarization-activated cyclic nucleotide-gated (HCN) channel modulator that has been trialed in humans for its potential antidepressant activity. The precise action of Org 34167 is not fully understood. Here we use two-electrode voltage clamp recordings and an allosteric model to explore the interaction of Org 34167 with human HCN1 channels. The impact of Org 34167 on channel function included a hyperpolarizing shift in activation voltage dependence and a slowing of activation kinetics. Furthermore, a reduction in the maximum open probability at extreme hyperpolarization argued for an additional voltage-independent mechanism. Org 34167 had a similar impact on a truncated HCN1 channel lacking the C-terminal nucleotide binding domain, thus ruling out an interaction with this domain. Fitting a gating model, derived from a 10-state allosteric scheme, predicted that Org 34167 strongly reduced the equilibrium constant for the voltage-independent pore domain to favor a closed pore, as well as reducing the voltage sensing domain-pore domain coupling and shifting the zero voltage equilibrium constant of the voltage sensing domain to favor the inactive state. SIGNIFICANCE STATEMENT: The brain penetrant small molecule Org 34167 has been reported to have an antidepressant action by targeting HCN channels; however, its mode of action is unknown. We used heterologously expressed human HCN1 channels to show that Org 34167 inhibits channel activity by modulating kinetic parameters associated with the channel pore domain, voltage sensing domain, and interdomain coupling.

Muscarinic Acetylcholine Receptors Modulate HCN Channel Properties in Vestibular Ganglion Neurons

Efferent modulation of vestibular afferent excitability is linked to muscarinic signaling cascades that close low-voltage-gated potassium channels (i.e., KCNQ). Here, we show that muscarinic signaling cascades also depolarize the activation range of hyperpolarization-activated cyclic-nucleotide gated (HCN) channels. We compared the voltage activation range and kinetics of HCN channels and induced firing patterns before and after administering the muscarinic acetylcholine receptor (mAChR) agonist oxotremorine-M (Oxo-M) in dissociated vestibular ganglion neurons (VGNs) from rats of either sex using perforated whole-cell patch-clamp methods. Oxo-M depolarized HCN channels' half-activation voltage (V 1/2) and sped up the rate of activation near resting potential twofold. HCN channels in large-diameter and/or transient firing VGN (putative cell bodies of irregular firing neuron from central epithelial zones) had relatively depolarized V 1/2 in control solution and were less sensitive to mAChR activation than those found in small-diameter VGN with sustained firing patterns (putatively belonging to regular firing afferents). The impact of mAChR on HCN channels is not a direct consequence of closing KCNQ channels since pretreating the cells with Linopirdine, a KCNQ channel blocker, did not prevent HCN channel depolarization by Oxo-M. Efferent signaling promoted ion channel configurations that were favorable to highly regular spiking in some VGN, but not others. This is consistent with previous observations that low-voltage gated potassium currents in VGN are conducted by mAChR agonist-sensitive and -insensitive channels. Connecting efferent signaling to HCN channels is significant because of the channel's impact on spike-timing regularity and nonchemical transmission between Type I hair cells and vestibular afferents.SIGNIFICANCE STATEMENT Vestibular afferents express a diverse complement of ion channels. In vitro studies identified low-voltage activated potassium channels and hyperpolarization-activated cyclic-nucleotide gated (HCN) channels as crucial for shaping the timing and sensitivity of afferent responses. Moreover, a network of acetylcholine-releasing efferent neurons controls afferent excitability by closing a subgroup of low-voltage activated potassium channels on the afferent neuron. This work shows that these efferent signaling cascades also enhance the activation of HCN channels by depolarizing their voltage activation range. The size of this effect varies depending on the endogenous properties of the HCN channel and on cell type (as determined by discharge patterns and cell size). Simultaneously controlling two ion-channel groups gives the vestibular efferent system exquisite control over afferent neuron activity.

The virtual sinoatrial node: What did computational models tell us about cardiac pacemaking?

Since its discovery, the sinoatrial node (SAN) has represented a fascinating and complex matter of research. Despite over a century of discoveries, a full comprehension of pacemaking has still to be achieved. Experiments often produced conflicting evidence that was used either in support or against alternative theories, originating intense debates. In this context, mathematical descriptions of the phenomena underlying the heartbeat have grown in importance in the last decades since they helped in gaining insights where experimental evaluation could not reach. This review presents the most updated SAN computational models and discusses their contribution to our understanding of cardiac pacemaking. Electrophysiological, structural and pathological aspects - as well as the autonomic control over the SAN - are taken into consideration to reach a holistic view of SAN activity.

Sedative Properties of Dexmedetomidine Are Mediated Independently from Native Thalamic Hyperpolarization-Activated Cyclic Nucleotide-Gated Channel Function at Clinically Relevant Concentrations

Dexmedetomidine is a selective α₂-adrenoceptor agonist and appears to disinhibit endogenous sleep-promoting pathways, as well as to attenuate noradrenergic excitation. Recent evidence suggests that dexmedetomidine might also directly inhibit hyperpolarization-activated cyclic-nucleotide gated (HCN) channels. We analyzed the effects of dexmedetomidine on native HCN channel function in thalamocortical relay neurons of the ventrobasal complex of the thalamus from mice, performing whole-cell patch-clamp recordings. Over a clinically relevant range of concentrations (1-10 µM), the effects of dexmedetomidine were modest. At a concentration of 10 µM, dexmedetomidine significantly reduced maximal I_h amplitude (relative reduction: 0.86 [0.78-0.91], n = 10, and p = 0.021), yet changes to the half-maximal activation potential V_1/2 occurred exclusively in the presence of the very high concentration of 100 µM (-4,7 [-7.5--4.0] mV, n = 10, and p = 0.009). Coincidentally, only the very high concentration of 100 µM induced a significant deceleration of the fast component of the HCN activation time course (τ_fast: +135.1 [+64.7-+151.3] ms, n = 10, and p = 0.002). With the exception of significantly increasing the membrane input resistance (starting at 10 µM), dexmedetomidine did not affect biophysical membrane properties and HCN channel-mediated parameters of neuronal excitability. Hence, the sedative qualities of dexmedetomidine and its effect on the thalamocortical network are not decisively shaped by direct inhibition of HCN channel function.

Two HCN4 Channels Play Functional Roles in the Zebrafish Heart

The HCN4 channel is essential for heart rate regulation in vertebrates by generating pacemaker potentials in the sinoatrial node. HCN4 channel abnormality may cause bradycardia and sick sinus syndrome, making it an important target for clinical research and drug discovery. The zebrafish is a popular animal model for cardiovascular research. They are potentially suitable for studying inherited heart diseases, including cardiac arrhythmia. However, it has not been determined how similar the ion channels that underlie cardiac automaticity are in zebrafish and humans. In the case of HCN4, humans have one gene, whereas zebrafish have two ortholog genes (DrHCN4 and DrHCN4L; ‘Dr’ referring to Danio rerio). However, it is not known whether the two HCN4 channels have different physiological functions and roles in heart rate regulation. In this study, we characterized the biophysical properties of the two zebrafish HCN4 channels in Xenopus oocytes and compared them to those of the human HCN4 channel. We found that they showed different gating properties: DrHCN4L currents showed faster activation kinetics and a more positively shifted G-V curve than did DrHCN4 and human HCN4 currents. We made chimeric channels of DrHCN4 and DrHCN4L and found that cytoplasmic domains were determinants for the faster activation and the positively shifted G-V relationship in DrHCN4L. The use of a dominant-negative HCN4 mutant confirmed that DrHCN4 and DrHCN4L can form a heteromultimeric channel in Xenopus oocytes. Next, we confirmed that both are sensitive to common HCN channel inhibitors/blockers including Cs⁺, ivabradine, and ZD7288. These HCN inhibitors successfully lowered zebrafish heart rate during early embryonic stages. Finally, we knocked down the HCN4 genes using antisense morpholino and found that knocking down either or both of the HCN4 channels caused a temporal decrease in heart rate and tended to cause pericardial edema. These findings suggest that both DrHCN4 and DrHCN4L play a significant role in zebrafish heart rate regulation.

Regulation of HCN Channels by Protein Interactions

Hyperpolarization-activated, cyclic nucleotide-sensitive (HCN) channels are key regulators of subthreshold membrane potentials in excitable cells. The four mammalian HCN channel isoforms, HCN1-HCN4, are expressed throughout the body, where they contribute to diverse physiological processes including cardiac pacemaking, sleep-wakefulness cycles, memory, and somatic sensation. While all HCN channel isoforms produce currents when expressed by themselves, an emerging list of interacting proteins shape HCN channel excitability to influence the physiologically relevant output. The best studied of these regulatory proteins is the auxiliary subunit, TRIP8b, which binds to multiple sites in the C-terminus of the HCN channels to regulate expression and disrupt cAMP binding to fine-tune neuronal HCN channel excitability. Less is known about the mechanisms of action of other HCN channel interaction partners like filamin A, Src tyrosine kinase, and MinK-related peptides, which have a range of effects on HCN channel gating and expression. More recently, the inositol trisphosphate receptor-associated cGMP-kinase substrates IRAG1 and LRMP (also known as IRAG2), were discovered as specific regulators of the HCN4 isoform. This review summarizes the known protein interaction partners of HCN channels and their mechanisms of action and identifies gaps in our knowledge.

Chinese natural compound decreases pacemaking of rabbit cardiac sinoatrial cells by targeting second messenger regulation of f-channels

Tongmai Yangxin (TMYX) is a complex compound of the Traditional Chinese Medicine (TCM) used to treat several cardiac rhythm disorders; however, no information regarding its mechanism of action is available. In this study we provide a detailed characterization of the effects of TMYX on the electrical activity of pacemaker cells and unravel its mechanism of action. Single-cell electrophysiology revealed that TMYX elicits a reversible and dose-dependent (2/6 mg/ml) slowing of spontaneous action potentials rate (−20.8/–50.2%) by a selective reduction of the diastolic phase (−50.1/–76.0%). This action is mediated by a negative shift of the I_f activation curve (−6.7/–11.9 mV) and is caused by a reduction of the cyclic adenosine monophosphate (cAMP)-induced stimulation of pacemaker channels. We provide evidence that TMYX acts by directly antagonizing the cAMP-induced allosteric modulation of the pacemaker channels. Noticeably, this mechanism functionally resembles the pharmacological actions of muscarinic stimulation or β-blockers, but it does not require generalized changes in cytoplasmic cAMP levels thus ensuring a selective action on rate. In agreement with a competitive inhibition mechanism, TMYX exerts its maximal antagonistic action at submaximal cAMP concentrations and then progressively becomes less effective thus ensuring a full contribution of I_f to pacemaker rate during high metabolic demand and sympathetic stimulation.

Mechanisms and Physiological Implications of Cooperative Gating of Ion Channels Clusters

Ion channels play a central role in the regulation of nearly every cellular process. Dating back to the classic 1952 Hodgkin-Huxley model of the generation of the action potential, ion channels have always been thought of as independent agents. A myriad of recent experimental findings exploiting advances in electrophysiology, structural biology, and imaging techniques, however, have posed a serious challenge to this long-held axiom as several classes of ion channels appear to open and close in a coordinated, cooperative manner. Ion channel cooperativity ranges from variable-sized oligomeric cooperative gating in voltage-gated, dihydropyridine-sensitive Ca_v1.2 and Ca_v1.3 channels to obligatory dimeric assembly and gating of voltage-gated Na_v1.5 channels. Potassium channels, transient receptor potential channels, hyperpolarization cyclic nucleotide-activated channels, ryanodine receptors (RyRs), and inositol trisphosphate receptors (IP₃Rs) have also been shown to gate cooperatively. The implications of cooperative gating of these ion channels range from fine tuning excitation-contraction coupling in muscle cells to regulating cardiac function and vascular tone, to modulation of action potential and conduction velocity in neurons and cardiac cells, and to control of pace-making activity in the heart. In this review, we discuss the mechanisms leading to cooperative gating of ion channels, their physiological consequences and how alterations in cooperative gating of ion channels may induce a range of clinically significant pathologies.

The Functional Role of Hyperpolarization Activated Current (I f) on Cardiac Pacemaking in Human vs. in the Rabbit Sinoatrial Node: A Simulation and Theoretical Study

The cardiac hyperpolarization-activated "funny" current (I f), which contributes to sinoatrial node (SAN) pacemaking, has a more negative half-maximal activation voltage and smaller fully-activated macroscopic conductance in human than in rabbit SAN cells. The consequences of these differences for the relative roles of I f in the two species, and for their responses to the specific bradycardic agent ivabradine at clinical doses have not been systematically explored. This study aims to address these issues, through incorporating rabbit and human I f formulations developed by Fabbri et al. into the Severi et al. model of rabbit SAN cells. A theory was developed to correlate the effect of I f reduction with the total inward depolarising current (I total) during diastolic depolarization. Replacing the rabbit I f formulation with the human one increased the pacemaking cycle length (CL) from 355 to 1,139 ms. With up to 20% I f reduction (a level close to the inhibition of I f by ivabradine at clinical concentrations), a modest increase (~5%) in the pacemaking CL was observed with the rabbit I f formulation; however, the effect was doubled (~12.4%) for the human I f formulation, even though the latter has smaller I f density. When the action of acetylcholine (ACh, 0.1 nM) was considered, a 20% I f reduction markedly increased the pacemaking CL by 37.5% (~27.3% reduction in the pacing rate), which is similar to the ivabradine effect at clinical concentrations. Theoretical analysis showed that the resultant increase of the pacemaking CL is inversely proportional to the magnitude of I total during diastolic depolarization phase: a smaller I f in the model resulted in a smaller I total amplitude, resulting in a slower pacemaking rate; and the same reduction in I f resulted in a more significant change of CL in the cell model with a smaller I total. This explained the mechanism by which a low dose of ivabradine slows pacemaking rate more in humans than in the rabbit. Similar results were seen in the Fabbri et al. model of human SAN cells, suggesting our observations are model-independent. Collectively, the results of study explain why low dose ivabradine at clinically relevant concentrations acts as an effective bradycardic agent in modulating human SAN pacemaking.

Do the functional properties of HCN1 mutants correlate with the clinical features in epileptic patients?

The altered function of the Hyperpolarization-activated Cyclic-Nucleotide-gated (HCN) ion channels plays an important role in the pathogenesis of epilepsy in humans. In particular, HCN1 missense mutations have been recently identified in patients with different epileptic phenotypes, varying from mild to severe. Their electrophysiological characterization shows that mutated channels can act both with loss-of-function and gain-of-function mechanisms of action, without an evident correlation with the phenotype. In search for a correlation between clinical features and biophysical properties of the mutations, in this work we considered sixteen HCN1 mutations, found in eighteen Early Infantile Epileptic Encephalopathy (EIEE) patients. Statistical analysis did not establish any significant correlation between the clinical parameters and the current properties of the mutant channels. The lack of significance of our results could depend on the small number of mutations analyzed, epilepsy-associated with certainty. With the progressive increase of Next Generation Sequencing in patients with early-onset epilepsy, it is expected that the number of patients with HCN1 mutations will grow steadily. Functional characterization of epilepsy-associated HCN1 mutations remains a fundamental tool for a better understanding of the pathogenetic mechanisms leading to the disease in humans.

Biophysical analysis of an HCN1 epilepsy variant suggests a critical role for S5 helix Met-305 in voltage sensor to pore domain coupling

Hyperpolarization-gated, cyclic nucleotide-activated (HCN1-4) channels are inwardly rectifying cation channels that display voltage dependent activation and de-activation. Pathogenic variants in HCN1 are associated with severe developmental and epileptic encephalopathies including the de novo HCN1 M305L variant. M305 is located in the S5 domain that is implicated in coupling voltage sensor domain movement to pore opening. This variant lacks voltage-dependent activation and de-activation and displays normal cation selectivity. To elucidate the impact of the mutation on the channel structure-function relations, molecular dynamics simulations of the wild type and mutant homotetramers were compared and identified a sulphur-aromatic interaction between M305 and F389 that contributes to the coupling of the voltage-sensing domain to the pore domain. To mimic the heterozygous condition as a heterotetrameric channel assembly, Xenopus oocytes were co-injected with various ratios of wild-type and mutant subunit cRNAs and the biophysical properties of channels with different subunit stoichiometries were determined. The results showed that a single mutated subunit was sufficient to significantly disrupt the voltage dependence of activation. The functional data were qualitatively consistent with predictions of a model that assumes independent activation of the voltage sensing domains allosterically controlling the closed to open transition of the pore. Overall, the M305L mutation results in an HCN1 channel that lacks voltage dependence and facilitates excitatory cation flow at membrane potentials that would normally close the channel. Our findings provide molecular insights into HCN1 channels and reveal the structural and biophysical basis of the severe epilepsy phenotype associated with the M305L mutation.

Gating movements and ion permeation in HCN4 pacemaker channels

The HCN1-4 channel family is responsible for the hyperpolarization-activated cation current If/Ih that controls automaticity in cardiac and neuronal pacemaker cells. We present cryoelectron microscopy (cryo-EM) structures of HCN4 in the presence or absence of bound cAMP, displaying the pore domain in closed and open conformations. Analysis of cAMP-bound and -unbound structures sheds light on how ligand-induced transitions in the channel cytosolic portion mediate the effect of cAMP on channel gating and highlights the regulatory role of a Mg2+ coordination site formed between the C-linker and the S4-S5 linker. Comparison of open/closed pore states shows that the cytosolic gate opens through concerted movements of the S5 and S6 transmembrane helices. Furthermore, in combination with molecular dynamics analyses, the open pore structures provide insights into the mechanisms of K+/Na+ permeation. Our results contribute mechanistic understanding on HCN channel gating, cyclic nucleotide-dependent modulation, and ion permeation.

Bidirectional flow of the funny current (If) during the pacemaking cycle in murine sinoatrial node myocytes

Sinoatrial node myocytes (SAMs) act as cardiac pacemaker cells by firing spontaneous action potentials (APs) that initiate each heartbeat. The funny current (If) is critical for the generation of these spontaneous APs; however, its precise role during the pacemaking cycle remains unresolved. Here, we used the AP-clamp technique to quantify If during the cardiac cycle in mouse SAMs. We found that If is persistently active throughout the sinoatrial AP, with surprisingly little voltage-dependent gating. As a consequence, it carries both inward and outward current around its reversal potential of -30 mV. Despite operating at only 2 to 5% of its maximal conductance, If carries a substantial fraction of both depolarizing and repolarizing net charge movement during the firing cycle. We also show that β-adrenergic receptor stimulation increases the percentage of net depolarizing charge moved by If, consistent with a contribution of If to the fight-or-flight increase in heart rate. These properties were confirmed by heterologously expressed HCN4 channels and by mathematical models of If Modeling further suggested that the slow rates of activation and deactivation of the HCN4 isoform underlie the persistent activity of If during the sinoatrial AP. These results establish a new conceptual framework for the role of If in pacemaking, in which it operates at a very small fraction of maximal activation but nevertheless drives membrane potential oscillations in SAMs by providing substantial driving force in both inward and outward directions.

Loss of HCN2 in Dorsal Hippocampus of Young Adult Mice Induces Specific Apoptosis of the CA1 Pyramidal Neuron Layer

Neurons inevitably rely on a proper repertoire and distribution of membrane-bound ion-conducting channels. Among these proteins, the family of hyperpolarization-activated and cyclic nucleotide-gated (HCN) channels possesses unique properties giving rise to the corresponding I_h-current that contributes to various aspects of neural signaling. In mammals, four genes (hcn1-4) encode subunits of HCN channels. These subunits can assemble as hetero- or homotetrameric ion-conducting channels. In order to elaborate on the specific role of the HCN2 subunit in shaping electrical properties of neurons, we applied an Adeno-associated virus (AAV)-mediated, RNAi-based knock-down strategy of hcn2 gene expression both in vitro and in vivo. Electrophysiological measurements showed that HCN2 subunit knock-down resulted in specific yet anticipated changes in I_h-current properties in primary hippocampal neurons and, in addition, corroborated that the HCN2 subunit participates in postsynaptic signal integration. To further address the role of the HCN2 subunit in vivo, we injected recombinant (r)AAVs into the dorsal hippocampus of young adult male mice. Behavioral and biochemical analyses were conducted to assess the contribution of HCN2-containing channels in shaping hippocampal network properties. Surprisingly, knock-down of hcn2 expression resulted in a severe degeneration of the CA1 pyramidal cell layer, which did not occur in mice injected with control rAAV constructs. This finding might pinpoint to a vital and yet unknown contribution of HCN2 channels in establishing or maintaining the proper function of CA1 pyramidal neurons of the dorsal hippocampus.

HCN4 current during human sinoatrial node-like action potentials

BACKGROUND

Despite the many studies carried out over the past 40 years, the contribution of the HCN4 encoded hyperpolarization-activated 'funny' current (I_f) to pacemaker activity in the mammalian sinoatrial node (SAN), and the human SAN in particular, is still controversial and not fully established.

OBJECTIVE

To study the contribution of I_f to diastolic depolarization of human SAN cells and its dependence on heart rate, cAMP levels, and atrial load.

METHODS

HCN4 channels were expressed in human cardiac myocyte progenitor cells (CMPCs) and HCN4 currents assessed using perforated patch-clamp in traditional voltage clamp mode and during action potential clamp with human SAN-like action potential waveforms with 500-1500 ms cycle length, in absence or presence of forskolin to mimic β-adrenergic stimulation and a -15 mV command potential offset to mimic atrial load.

RESULTS

Forskolin significantly increased the fully-activated HCN4 current density at -140 mV by 14% and shifted the steady-state activation curve by +7.4 mV without affecting its slope. In addition, forskolin significantly accelerated current activation but slowed deactivation. The HCN4 current did not completely deactivate before the subsequent diastolic depolarization during action potential clamp. The amplitude of HCN4 current increased with increasing cycle length, was significantly larger in the presence of forskolin at all cycle lengths, and was significantly increased upon the negative offset to the command potential.

CONCLUSIONS

I_f is active during a human SAN action potential waveform and its amplitude is modulated by heart rate, β-adrenergic stimulation, and diastolic voltage range, such that I_f is under delicate control.

The funny current in genetically modified mice

Since its first description in 1979, the hyperpolarization-activated funny current (I_f) has been the object of intensive research aimed at understanding its role in cardiac pacemaker activity and its modulation by the sympathetic and parasympathetic branches of the autonomic nervous system. I_f was described in isolated tissue strips of the rabbit sinoatrial node using the double-electrode voltage-clamp technique. Since then, the rabbit has been the principal animal model for studying pacemaker activity and I_f for more than 20 years. In 2001, the first study describing the electrophysiological properties of mouse sinoatrial pacemaker myocytes and those of I_f was published. It was soon followed by the description of murine myocytes of the atrioventricular node and the Purkinje fibres. The sinoatrial node of genetically modified mice has become a very popular model for studying the mechanisms of cardiac pacemaker activity. This field of research benefits from the impressive advancement of in-vivo exploration techniques of physiological parameters, imaging, genetics, and large-scale genomic approaches. The present review discusses the influence of mouse genetic on the most recent knowledge of the funny current's role in the physiology and pathophysiology of cardiac pacemaker activity. Genetically modified mice have provided important insights into the role of I_f in determining intrinsic automaticity in vivo and in myocytes of the conduction system. In addition, gene targeting of f-(HCN) channel isoforms have contributed to elucidating the current's role in the regulation of heart rate by the parasympathetic nervous system. This review is dedicated to Dario DiFrancesco on his retirement.

Promotion of Differentiating Bone Marrow Mesenchymal Stromal Cells (BMSCs) into Cardiomyocytes via HCN2 and HCN4 Cotransfection

AIM

Investigation of the influences HCN2 and HCN4 has on bone marrow mesenchymal stromal cells (BMSCs) on cardiomyocyte differentiation.

METHODS

Miniature adult pigs were used for bone marrow extraction and isolation of BMSCs. The identification of these BMSCs was done by using flow cytometry for the detection of expressed surface antigens CD45, CD11B, CD44, and CD90. Using HCN2 and HCN4 genes cotransfected into BMSCs as group HCN2+HCN4 while myocardial induction solution was used to induced BMSC differentiation in the BMSC induction group. Myocardial marker proteins α-actin and cTnT were detected by immunofluorescence staining, while α-actin, cTnT, and Desmin myocardial marker proteins expressed were detected by Western blot. The whole-cell patch-clamp technique was used to identify and detect cellular HCN2 channels, HCN4 channel current activation curve, and the inhibitory effect of CsCl on heterologous expression currents.

RESULTS

Flow cytometry results showed that CD45 and CD11B were expressed negatively while CD90 and CD44 were positive. Post HCN2 and HCN4 gene transfection, immunofluorescence staining, and Western blot showed significantly increased HCN2, HCN4, α-actin, and cTnT expressed in group HCN2+HCN4 were, which could be compared to the expression levels in the BMSC-induced group. The HCN2+HCN4 group was able to document cell membrane channel ion currents that were similar to If properties.

CONCLUSION

HCN2 and HCN4 overexpression can considerably enhance the MSC ability to differentiate into cardiomyocytes in vitro and restore the ionic current.

Dual role of miR-1 in the development and function of sinoatrial cells

miR-1, the most abundant miRNA in the heart, modulates expression of several transcription factors and ion channels. Conditions affecting the heart rate, such as endurance training and cardiac diseases, show a concomitant miR-1 up- or down-regulation. Here, we investigated the role of miR-1 overexpression in the development and function of sinoatrial (SAN) cells using murine embryonic stem cells (mESC). We generated mESCs either overexpressing miR-1 and EGFP (miR1OE) or EGFP only (EM). SAN-like cells were selected from differentiating mESC using the CD166 marker. Gene expression and electrophysiological analysis were carried out on both early mES-derived cardiac progenitors and SAN-like cells and on beating neonatal rat ventricular cardiomyocytes (NRVC) over-expressing miR-1. miR1OE cells increased significantly the proportion of CD166⁺ SAN precursors compared to EM cells (23% vs 12%) and the levels of the transcription factors TBX5 and TBX18, both involved in SAN development. miR1OE SAN-like cells were bradycardic (1,3 vs 2 Hz) compared to EM cells. In agreement with data on native SAN cells, EM SAN-like cardiomyocytes show two populations of cells expressing either slow- or fast-activating I_f currents; miR1OE SAN-like cells instead have only fast-activating I_f with a significantly reduced conductance. Western Blot and immunofluorescence analysis showed a reduced HCN4 signal in miR-1OE vs EM CD166+ precursors. Together these data point out to a specific down-regulation of the slow-activating HCN4 subunit by miR-1. Importantly, the rate and I_f alterations were independent of the developmental effects of miR-1, being similar in NRVC transiently overexpressing miR-1. In conclusion, we demonstrated a dual role of miR-1, during development it controls the proper development of sinoatrial-precursor, while in mature SAN-like cells it modulates the HCN4 pacemaker channel translation and thus the beating rate.

AAV-Mediated CRISPRi and RNAi Based Gene Silencing in Mouse Hippocampal Neurons

Uncovering the physiological role of individual proteins that are part of the intricate process of cellular signaling is often a complex and challenging task. A straightforward strategy of studying a protein's function is by manipulating the expression rate of its gene. In recent years, the Clustered Regularly Interspaced Short Palindromic Repeat (CRISPR)/Cas9-based technology was established as a powerful gene-editing tool for generating sequence specific changes in proliferating cells. However, obtaining homogeneous populations of transgenic post-mitotic neurons by CRISPR/Cas9 turned out to be challenging. These constraints can be partially overcome by CRISPR interference (CRISPRi), which mediates the inhibition of gene expression by competing with the transcription machinery for promoter binding and, thus, transcription initiation. Notably, CRISPR/Cas is only one of several described approaches for the manipulation of gene expression. Here, we targeted neurons with recombinant Adeno-associated viruses to induce either CRISPRi or RNA interference (RNAi), a well-established method for impairing de novo protein biosynthesis by using cellular regulatory mechanisms that induce the degradation of pre-existing mRNA. We specifically targeted hyperpolarization-activated and cyclic nucleotide-gated (HCN) channels, which are widely expressed in neuronal tissues and play essential physiological roles in maintaining biophysical characteristics in neurons. Both of the strategies reduced the expression levels of three HCN isoforms (HCN1, 2, and 4) with high specificity. Furthermore, detailed analysis revealed that the knock-down of just a single HCN isoform (HCN4) in hippocampal neurons did not affect basic electrical parameters of transduced neurons, whereas substantial changes emerged in HCN-current specific properties.

Attenuation of Native Hyperpolarization-Activated, Cyclic Nucleotide-Gated Channel Function by the Volatile Anesthetic Sevoflurane in Mouse Thalamocortical Relay Neurons

As thalamocortical relay neurons are ascribed a crucial role in signal propagation and information processing, they have attracted considerable attention as potential targets for anesthetic modulation. In this study, we analyzed the effects of different concentrations of sevoflurane on the excitability of thalamocortical relay neurons and hyperpolarization-activated, cyclic-nucleotide gated (HCN) channels, which play a decisive role in regulating membrane properties and rhythmic oscillatory activity. The effects of sevoflurane on single-cell excitability and native HCN channels were investigated in acutely prepared brain slices from adult wild-type mice with the whole-cell patch-clamp technique, using voltage-clamp and current-clamp protocols. Sevoflurane dose-dependently depressed membrane biophysics and HCN-mediated parameters of neuronal excitability. Respective half-maximal inhibitory and effective concentrations ranged between 0.30 (95% CI, 0.18–0.50) mM and 0.88 (95% CI, 0.40–2.20) mM. We witnessed a pronounced reduction of HCN dependent I_h current amplitude starting at a concentration of 0.45 mM [relative change at −133 mV; 0.45 mM sevoflurane: 0.85 (interquartile range, 0.79–0.92), n = 12, p = 0.011; 1.47 mM sevoflurane: 0.37 (interquartile range, 0.34–0.62), n = 5, p < 0.001] with a half-maximal inhibitory concentration of 0.88 (95% CI, 0.40–2.20) mM. In contrast, effects on voltage-dependent channel gating were modest with significant changes only occurring at 1.47 mM [absolute change of half-maximal activation potential; 1.47 mM: −7.2 (interquartile range, −10.3 to −5.8) mV, n = 5, p = 0.020]. In this study, we demonstrate that sevoflurane inhibits the excitability of thalamocortical relay neurons in a concentration-dependent manner within a clinically relevant range. Especially concerning its effects on native HCN channel function, our findings indicate substance-specific differences in comparison to other anesthetic agents. Considering the importance of HCN channels, the observed effects might mechanistically contribute to the hypnotic properties of sevoflurane.

Cellular and molecular landscape of mammalian sinoatrial node revealed by single-cell RNA sequencing

Bioelectrical impulses intrinsically generated within the sinoatrial node (SAN) trigger the contraction of the heart in mammals. Though discovered over a century ago, the molecular and cellular features of the SAN that underpin its critical function in the heart are uncharted territory. Here, we identify four distinct transcriptional clusters by single-cell RNA sequencing in the mouse SAN. Functional analysis of differentially expressed genes identifies a core cell cluster enriched in the electrogenic genes. The similar cellular features are also observed in the SAN from both rabbit and cynomolgus monkey. Notably, Vsnl1, a core cell cluster marker in mouse, is abundantly expressed in SAN, but is barely detectable in atrium or ventricle, suggesting that Vsnl1 is a potential SAN marker. Importantly, deficiency of Vsnl1 not only reduces the beating rate of human induced pluripotent stem cell - derived cardiomyocytes (hiPSC-CMs) but also the heart rate of mice. Furthermore, weighted gene co-expression network analysis (WGCNA) unveiled the core gene regulation network governing the function of the SAN in mice. Overall, these findings reveal the whole transcriptome profiling of the SAN at single-cell resolution, representing an advance toward understanding of both the biology and the pathology of SAN.

The spontaneous bioelectrical activity of pacemaker cells in sinoatrial node (SAN) triggers the heartbeats. Here, the authors perform single-cell RNA sequencing in the mouse SAN and identify molecular and cellular features of the SAN conserved in rabbit and cynomolgus monkey, identifying a new potential SAN marker.

Central Amygdala Projections to Lateral Hypothalamus Mediate Avoidance Behavior in Rats

Persistent avoidance of stress-related stimuli following acute stress exposure predicts negative outcomes such as substance abuse and traumatic stress disorders. Previous work using a rat model showed that the central amygdala (CeA) plays an important role in avoidance of a predator odor stress-paired context. Here, we show that CeA projections to the lateral hypothalamus (LH) are preferentially activated in male rats that show avoidance of a predator odor-paired context (termed Avoider rats), that chemogenetic inhibition of CeA-LH projections attenuates avoidance in male Avoider rats, that chemogenetic stimulation of the CeA-LH circuit produces conditioned place avoidance (CPA) in otherwise naive male rats, and that avoidance behavior is associated with intrinsic properties of LH-projecting CeA cells. Collectively, these data show that CeA-LH projections are important for persistent avoidance of stress-related stimuli following acute stress exposure.SIGNIFICANCE STATEMENT This study in rats shows that a specific circuit in the brain [i.e., neurons that project from the central amygdala (CeA) to the lateral hypothalamus (LH)] mediates avoidance of stress-associated stimuli. In addition, this study shows that intrinsic physiological properties of cells in this brain circuit are associated with avoidance of stress-associated stimuli. Further characterization of the CeA-LH circuit may improve our understanding of the neural mechanisms underlying specific aspects of stress-related disorders in humans.

Effect of Ginkgo biloba extract on pacemaker channels encoded by HCN gene

BACKGROUND

In the present study, the electropharmacological activity of traditional Chinese medicine, Ginkgo biloba extract (GBE), on human hyperpolarization-activated nucleotide-gated (HCN) channels and the underlying "funny" currents was investigated.

METHODS

Standard two-electrode voltage-clamp recordings were employed to examine the properties of cloned HCN subunit currents expressed in Xenopus oocytes under controlled conditions and GBE administration.

RESULTS

We found that GBE irreversibly inhibited the HCN2 and HCN4 channel currents in a concentration-dependent fashion and that the HCN4 current was more sensitive to GBE compared with HCN2. In addition, GBE inhibition of the current amplitudes of HCN2 and HCN4 currents was accompanied by a decrease in the activation and deactivation kinetics.

CONCLUSION

The results of this study contribute toward illustrating the antiarrhythmic mechanism of GBE, which might be useful for the treatment of arrhythmia.

Selective Blockade of HCN1/HCN2 Channels as a Potential Pharmacological Strategy Against Pain

A prominent role of hyperpolarization-activated, cyclic nucleotide-gated (HCN) channels has been suggested based on their expression and (dys)function in dorsal root ganglion (DRG) neurons, being likely involved in peripheral nociception. Using HCN blockers as antinociceptive drugs is prevented by the widespread distribution of these channels. However, tissue-specific expression of HCN isoforms varies significantly, HCN1 and HCN2 being considered as major players in DRG excitability. We characterized the pharmacological effect of a novel compound, MEL55A, able to block selectively HCN1/HCN2 isoforms, on DRG neuron excitability in-vitro and for its antiallodynic properties in-vivo. HEK293 cells expressing HCN1, HCN2, or HCN4 isoforms were used to verify drug selectivity. The pharmacological profile of MEL55A was tested on mouse DRG neurons by patch-clamp recordings, and in-vivo in oxaliplatin-induced neuropathy by means of thermal hypersensitivity. Results were compared to the non-isoform-selective drug, ivabradine. MEL55A showed a marked preference toward HCN1 and HCN2 isoforms expressed in HEK293, with respect to HCN4. In cultured DRG, MEL55A reduced I _h amplitude, both in basic conditions and after stimulation by forskolin, and cell excitability, its effect being quantitatively similar to that observed with ivabradine. MEL55A was able to relieve chemotherapy-induced neuropathic pain. In conclusion, selective blockade of HCN1/HCN2 channels, over HCN4 isoform, was able to modulate electrophysiological properties of DRG neurons similarly to that reported for classical I _h blockers, ivabradine, resulting in a pain-relieving activity. The availability of small molecules with selectivity toward HCN channel isoforms involved in nociception might represent a safe and effective strategy against chronic pain.

Transcription factor Tbx18 induces the differentiation of c-kit+ canine mesenchymal stem cells (cMSCs) into SAN-like pacemaker cells in a co-culture model in vitro

Bone mesenchymal stem cells (MSCs), as well as cardiomyocytes, are derived from early mesoderm, becoming committed to their fate under the influence of different differentiation factors. We examined whether the overexpression of Tbx18 can induce the differentiation of c-kit⁺ cMSCs into a phenotype similar to that of native pacemaker cells and whether these transfected cells can couple to adjacent atrial cells with functional consequences. The c-kit⁺ cMSCs were first sorted, then transfected with different lentiviral vectors. Tbx18-c-kit⁺ cMSCs represented the experimental group, while EYFP-c-kit⁺ cMSCs and canine sinoatrial node (SAN) cells were used as controls. Within days of transfection, the hyperpolarization-activated cyclic nucleotide-gated (HCN) channel HCN4 protein and gap junction protein Connexin 45 (Cx45) expression in Tbx18-c-kit⁺ cMSCs were 12-fold and 5.6-fold higher, respectively, than that in EYFP-c-kit⁺ cMSCs. After co-culture with canine atrial cells in vitro for three days, the funny currents (I_f) were recorded in the Tbx18-c-kit⁺ cMSCs, but not in EYFP-c-kit⁺ cMSCs. The trend of these I_f currents was highly similar to that of SAN cells, although the current density was smaller. The Tbx18-EYFP-c-kit⁺ cMSCs showed responsiveness to β-adrenergic stimulation, and the intracellular cyclic adenosine monophosphate (cAMP) level was higher than that in EYFP-c-kit⁺ cMSCs. The Tbx18-EYFP-c-kit⁺ cMSCs delivered fluorescent dye to neighboring atrial cells via gap junctions, thus these cell pairs could communicate as a pacemaker unit. We propose that the overexpression of Tbx18 in c-kit⁺ cMSCs induces their differentiation to SAN-like pacemaker cells.

Cell-Type Specific Development of the Hyperpolarization-Activated Current, Ih, in Prefrontal Cortical Neurons

H-current, also known as hyperpolarization-activated current (Ih), is an inward current generated by the hyperpolarization-activated cyclic nucleotide-gated (HCN) cation channels. Ih plays an essential role in regulating neuronal properties, synaptic integration and plasticity, and synchronous activity in the brain. As these biological factors change across development, the brain undergoes varying levels of vulnerability to disorders like schizophrenia that disrupt prefrontal cortex (PFC)-dependent function. However, developmental changes in Ih in PFC neurons remains untested. Here, we examine Ih in pyramidal neurons vs. gamma-aminobutyric acid (GABA)ergic parvalbumin-expressing (PV+) interneurons in developing mouse PFC. Our findings show that the amplitudes of Ih in these cell types are identical during the juvenile period but differ at later time points. In pyramidal neurons, Ih amplitude significantly increases from juvenile to adolescence and follows a similar trend into adulthood. In contrast, the amplitude of Ih in PV+ interneurons decreases from juvenile to adolescence, and does not change from adolescence to adulthood. Moreover, the kinetics of HCN channels in pyramidal neurons is significantly slower than in PV+ interneurons, with a gradual decrease in pyramidal neurons and a gradual increase in PV+ cells across development. Our study reveals distinct developmental trajectories of Ih in pyramidal neurons and PV+ interneurons. The cell-type specific alteration of Ih during the critical period from juvenile to adolescence reflects the contribution of Ih to the maturation of the PFC and PFC-dependent function. These findings are essential for a better understanding of normal PFC function, and for elucidating Ih's crucial role in the pathophysiology of neurodevelopmental disorders.

Activation gating in HCN2 channels

Hyperpolarization-activated cyclic nucleotide-modulated (HCN) channels control electrical rhythmicity in specialized brain and heart cells. We quantitatively analysed voltage-dependent activation of homotetrameric HCN2 channels and its modulation by the second messenger cAMP using global fits of hidden Markovian models to complex experimental data. We show that voltage-dependent activation is essentially governed by two separable voltage-dependent steps followed by voltage-independent opening of the pore. According to this model analysis, the binding of cAMP to the channels exerts multiple effects on the voltage-dependent gating: It stabilizes the open pore, reduces the total gating charge from ~8 to ~5, makes an additional closed state outside the activation pathway accessible and strongly accelerates the ON-gating but not the OFF-gating. Furthermore, the open channel has a much slower computed OFF-gating current than the closed channel, in both the absence and presence of cAMP. Together, these results provide detailed new insight into the voltage- and cAMP-induced activation gating of HCN channels.

Hyperpolarization-activated cyclic nucleotide-modulated (HCN) channels are tetrameric voltage-controlled ion channels in the cell membrane of specialized nerve and heart cells. Their main function is to generate a so-called pacemaker current which plays a key role in the generation of electrical rhythmicity. A special messenger molecule, cAMP, synthesized within these cells at sympathetic stimulation, can bind to these channels, thereby enhancing channel opening evoked by voltage. The mechanism of this dual activation is still poorly understood. Here we quantified this duality of activation for HCN2 channels by globally fitting hidden Markovian state models to extensive sets of data. We propose that activation of this tetrameric channel requires for a full description only two voltage-dependent steps that are followed by a voltage-independent opening step of the channel pore. According to this model analysis cAMP exerts multiple effects on channel activation: It notably accelerates the charge movement of the voltage-dependent steps and reduces the number of the involved electrical charges. Furthermore, it introduces an additional closed state and stabilizes the open pore. Together, our results provide new insight into the duality of voltage- and cAMP-induced activation of HCN channels.

HCN and K2P Channels in Anesthetic Mechanisms Research

The ability of a diverse group of agents to produce general anesthesia has long been an area of intense speculation and investigation. Over the past century, we have seen a paradigm shift from proposing that the anesthetized state arises from nonspecific interaction of anesthetics with the lipid membrane to the recognition that the function of distinct, and identifiable, membrane-embedded proteins is dramatically altered in the presence of intravenous and inhaled agents. Among proteinaceous targets, metabotropic and ionotropic receptors garnered much of the attention over the last 30 years, and it is only relatively recently that voltage-gated ion channels have clearly and rigorously been shown to be important molecular targets. In this review, we will consider the experimental issues relevant to two important ion channel anesthetic targets, HCN and K_2P.

Structural insights into the mechanisms of CNBD channel function

James and Zagotta discuss how recent cryoEM structures inform our understanding of cyclic nucleotide–binding domain channels.

Cyclic nucleotide-binding domain (CNBD) channels are a family of ion channels in the voltage-gated K⁺ channel superfamily that play crucial roles in many physiological processes. CNBD channels are structurally similar but functionally very diverse. This family includes three subfamilies: (1) the cyclic nucleotide-gated (CNG) channels, which are cation-nonselective, voltage-independent, and cyclic nucleotide-gated; (2) the hyperpolarization-activated cyclic nucleotide-gated (HCN) channels, which are weakly K⁺ selective, hyperpolarization-activated, and cyclic nucleotide-gated; and (3) the ether-à-go-go-type (KCNH) channels, which are strongly K⁺ selective, depolarization-activated, and cyclic nucleotide-independent. Recently, several high-resolution structures have been reported for intact CNBD channels, providing a structural framework to better understand their diverse function. In this review, we compare and contrast the recent structures and discuss how they inform our understanding of ion selectivity, voltage-dependent gating, and cyclic nucleotide–dependent gating within this channel family.

Pleiotropic effects of schizophrenia-associated genetic variants in neuron firing and cardiac pacemaking revealed by computational modeling

Schizophrenia patients have an increased risk of cardiac dysfunction. A possible factor underlying this comorbidity are the common variants in the large set of genes that have recently been discovered in genome-wide association studies (GWASs) as risk genes of schizophrenia. Many of these genes control the cell electrogenesis and calcium homeostasis. We applied biophysically detailed models of layer V pyramidal cells and sinoatrial node cells to study the contribution of schizophrenia-associated genes on cellular excitability. By including data from functional genomics literature to simulate the effects of common variants of these genes, we showed that variants of voltage-gated Na⁺ channel or hyperpolarization-activated cation channel-encoding genes cause qualitatively similar effects on layer V pyramidal cell and sinoatrial node cell excitability. By contrast, variants of Ca²⁺ channel or transporter-encoding genes mostly have opposite effects on cellular excitability in the two cell types. We also show that the variants may crucially affect the propagation of the cardiac action potential in the sinus node. These results may help explain some of the cardiac comorbidity in schizophrenia, and may facilitate generation of effective antipsychotic medications without cardiac side-effects such as arrhythmia.

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.

Cellular Computations Underlying Detection of Gaps in Sounds and Lateralizing Sound Sources

In mammals, acoustic information arises in the cochlea and is transmitted to the ventral cochlear nuclei (VCN). Three groups of VCN neurons extract different features from the firing of auditory nerve fibers and convey that information along separate pathways through the brainstem. Two of these pathways process temporal information: octopus cells detect coincident firing among auditory nerve fibers and transmit signals along monaural pathways, and bushy cells sharpen the encoding of fine structure and feed binaural pathways. The ability of these cells to signal with temporal precision depends on a low-voltage-activated K⁺ conductance (g_KL) and a hyperpolarization-activated conductance (g_h). This 'tale of two conductances' traces gap detection and sound lateralization to their cellular and biophysical origins.

Chronic cervical radiculopathic pain is associated with increased excitability and hyperpolarization-activated current ( Ih) in large-diameter dorsal root ganglion neurons

Cervical radiculopathic pain is a very common symptom that may occur with cervical spondylosis. Mechanical allodynia is often associated with cervical radiculopathic pain and is inadequately treated with current therapies. However, the precise mechanisms underlying cervical radiculopathic pain-associated mechanical allodynia have remained elusive. Compelling evidence from animal models suggests a role of large-diameter dorsal root ganglion neurons and plasticity of spinal circuitry attached with Aβ fibers in mediating neuropathic pain. Whether cervical radiculopathic pain condition induces plastic changes of large-diameter dorsal root ganglion neurons and what mechanisms underlie these changes are yet to be known. With combination of patch-clamp recording, immunohistochemical staining, as well as behavioral surveys, we demonstrated that upon chronic compression of C7/8 dorsal root ganglions, large-diameter cervical dorsal root ganglion neurons exhibited frequent spontaneous firing together with hyperexcitability. Quantitative analysis of hyperpolarization-activated cation current (I_h) revealed that I_h was greatly upregulated in large dorsal root ganglion neurons from cervical radiculopathic pain rats. This increased I_h was supported by the enhanced expression of hyperpolarization-activated, cyclic nucleotide-modulated channels subunit 3 in large dorsal root ganglion neurons. Blockade of I_h with selective antagonist, ZD7288 was able to eliminate the mechanical allodynia associated with cervical radiculopathic pain. This study sheds new light on the functional plasticity of a specific subset of large-diameter dorsal root ganglion neurons and reveals a novel mechanism that could underlie the mechanical allodynia associated with cervical radiculopathy.

Cyclic AMP reverses the effects of aging on pacemaker activity and If in sinoatrial node myocytes

Aging reduces pacemaker activity and shifts the voltage dependence of activation of the funny current, I_f, in sinoatrial node myocytes. Sharpe et al. find that these effects of aging can be reversed by application of exogenous cAMP but not by stimulation of endogenous cAMP.

Aerobic capacity decreases with age, in part because of an age-dependent decline in maximum heart rate (mHR) and a reduction in the intrinsic pacemaker activity of the sinoatrial node of the heart. Isolated sinoatrial node myocytes (SAMs) from aged mice have slower spontaneous action potential (AP) firing rates and a hyperpolarizing shift in the voltage dependence of activation of the “funny current,” I_f. Cyclic AMP (cAMP) is a critical modulator of both AP firing rate and I_f in SAMs. Here, we test the ability of endogenous and exogenous cAMP to overcome age-dependent changes in acutely isolated murine SAMs. We found that maximal stimulation of endogenous cAMP with 3-isobutyl-1-methylxanthine (IBMX) and forskolin significantly increased AP firing rate and depolarized the voltage dependence of activation of I_f in SAMs from both young and aged mice. However, these changes were insufficient to overcome the deficits in aged SAMs, and significant age-dependent differences in AP firing rate and I_f persisted in the presence of IBMX and forskolin. In contrast, the effects of aging on SAMs were completely abolished by a high concentration of exogenous cAMP, which restored AP firing rate and I_f activation to youthful levels in cells from aged animals. Interestingly, the age-dependent differences in AP firing rates and I_f were similar in whole-cell and perforated-patch recordings, and the hyperpolarizing shift in I_f persisted in excised inside-out patches, suggesting a limited role for cAMP in causing these changes. Collectively, the data indicate that aging does not impose an absolute limit on pacemaker activity and that it does not act by simply reducing the concentration of freely diffusible cAMP in SAMs.

Genetic perturbations suggest a role of the resting potential in regulating the expression of the ion channels of the KCNA and HCN families in octopus cells of the ventral cochlear nucleus

Low-voltage-activated K⁺ (g_KL) and hyperpolarization-activated mixed cation conductances (g_h) mediate currents, I_KL and I_h, through channels of the Kv1 (KCNA) and HCN families respectively and give auditory neurons the temporal precision required for signaling information about the onset, fine structure, and time of arrival of sounds. Being partially activated at rest, g_KL and g_h contribute to the resting potential and shape responses to even small subthreshold synaptic currents. Resting g_KL and g_h also affect the coupling of somatic depolarization with the generation of action potentials. To learn how these important conductances are regulated we have investigated how genetic perturbations affect their expression in octopus cells of the ventral cochlear nucleus (VCN). We report five new findings: First, the magnitude of g_h and g_KL varied over more than two-fold between wild type strains of mice. Second, average resting potentials are not different in different strains of mice even in the face of large differences in average g_KL and g_h. Third, I_KL has two components, one being α-dendrotoxin (α-DTX)-sensitive and partially inactivating and the other being α-DTX-insensitive, tetraethylammonium (TEA)-sensitive, and non-inactivating. Fourth, the loss of Kv1.1 results in diminution of the α-DTX-sensitive I_KL, and compensatory increased expression of an α-DTX-insensitive, tetraethylammonium (TEA)-sensitive I_KL. Fifth, I_h and I_KL are balanced at the resting potential in all wild type and mutant octopus cells even when resting potentials vary in individual cells over nearly 10 mV, indicating that the resting potential influences the expression of g_h and g_KL. The independence of resting potentials on g_KL and g_h shows that g_KL and g_h do not, over days or weeks, determine the resting potential but rather that the resting potential plays a role in regulating the magnitude of either or both g_KL and g_h.

Murine Electrophysiological Models of Cardiac Arrhythmogenesis

Cardiac arrhythmias can follow disruption of the normal cellular electrophysiological processes underlying excitable activity and their tissue propagation as coherent wavefronts from the primary sinoatrial node pacemaker, through the atria, conducting structures and ventricular myocardium. These physiological events are driven by interacting, voltage-dependent, processes of activation, inactivation, and recovery in the ion channels present in cardiomyocyte membranes. Generation and conduction of these events are further modulated by intracellular Ca2+ homeostasis, and metabolic and structural change. This review describes experimental studies on murine models for known clinical arrhythmic conditions in which these mechanisms were modified by genetic, physiological, or pharmacological manipulation. These exemplars yielded molecular, physiological, and structural phenotypes often directly translatable to their corresponding clinical conditions, which could be investigated at the molecular, cellular, tissue, organ, and whole animal levels. Arrhythmogenesis could be explored during normal pacing activity, regular stimulation, following imposed extra-stimuli, or during progressively incremented steady pacing frequencies. Arrhythmic substrate was identified with temporal and spatial functional heterogeneities predisposing to reentrant excitation phenomena. These could arise from abnormalities in cardiac pacing function, tissue electrical connectivity, and cellular excitation and recovery. Triggering events during or following recovery from action potential excitation could thereby lead to sustained arrhythmia. These surface membrane processes were modified by alterations in cellular Ca2+ homeostasis and energetics, as well as cellular and tissue structural change. Study of murine systems thus offers major insights into both our understanding of normal cardiac activity and its propagation, and their relationship to mechanisms generating clinical arrhythmias.

Hyperexcitability of Mesencephalic Trigeminal Neurons and Reorganization of Ion Channel Expression in a Rett Syndrome Model

People with Rett syndrome (RTT) have defects in motor function also seen in Mecp2-null mice. Motor function depends on not only central motor commands but also sensory feedback that is vulnerable to changes in excitability of propriosensory neurons. Here we report evidence for hyperexcitability of mesencephalic trigeminal (Me5) neurons in Mecp2-null mice and a novel cellular mechanism for lowering its impact. In in vitro brain slices, the Me5 neurons in both Mecp2^-/Y male and symptomatic Mecp2^+/- female mice were overly excitable showing increased firing activity in comparison to their wild-type (WT) male and asymptomatic counterparts. In Mecp2^-/Y males, Me5 neurons showed a reduced firing threshold. Consistently, the steady-state activation of voltage-gated Na⁺ currents (I_Na ) displayed a hyperpolarizing shift in the Mecp2-null neurons with no change in the I_Na density. This seems to be due to NaV1.1, SCN1B and SCN4B overexpression and NaV1.2 and SCN3B under-expression. In contrast to the hyperexcitability, the sag potential and postinhibitory rebound (PIR) were reduced in Mecp2-null mice. In voltage-clamp, the I_H density was deficient by ∼33%, and the steady-state half-activation had a depolarizing shift of ∼10 mV in the Mecp2-null mice. Quantitative PCR analysis indicated that HCN2 was decreased, HCN1 was upregulated with no change in HCN4 in Mecp2^-/Y mice compared to WT. Lastly, blocking I_H reduced the firing rate much more in WT than in Mecp2-null neurons. These data suggest that the Mecp2 defect causes an increase in Me5 neuronal excitability likely attributable to alterations in I_Na , meanwhile I_H is reduced likely altering neuronal excitability as well. J. Cell. Physiol. 232: 1151-1164, 2017. © 2016 Wiley Periodicals, Inc.