Tyrosine kinase inhibition differentially regulates heterologously expressed HCN channels

5-Hydroxytryptamine Enhances the Pacemaker Activity of Interstitial Cells of Cajal in Mouse Colon

We examined the localization of the 5-hydroxytryptamine (5-HT) receptor and its effects on mouse colonic interstitial cells of Cajal (ICCs) using electrophysiological techniques. Treatment with 5-HT increased the pacemaker activity in colonic ICCs with depolarization of membrane potentials in a dose-dependent manner. Hyperpolarization-activated cyclic nucleotide-gated (HCN) channel blockers blocked pacemaker activity and 5-HT-induced effects. Moreover, an adenylate cyclase inhibitor inhibited 5-HT-induced effects, and cell-permeable 8-bromo-cAMP increased the pacemaker activity. Various agonists of the 5-HT receptor subtype were working in colonic ICCs, including the 5-HT4 receptor. In small intestinal ICCs, 5-HT depolarized the membrane potentials transiently. Adenylate cyclase inhibitors or HCN blockers did not show any influence on 5-HT-induced effects. Anoctamin-1 (ANO1) or T-type Ca2+ channel blockers inhibited the pacemaker activity of colonic ICCs and blocked 5-HT-induced effects. A tyrosine protein kinase inhibitor inhibited pacemaker activity in colonic ICCs under controlled conditions but did not show any influence on 5-HT-induced effects. Among mitogen-activated protein kinase (MAPK) inhibitors, a p38 MAPK inhibitor inhibited 5-HT-induced effects on colonic ICCs. Thus, 5-HT's effect on pacemaker activity in small intestinal and colonic ICCs has excitatory but variable patterns. ANO1, T-type Ca2+, and HCN channels are involved in 5-HT-induced effects, and MAPKs are involved in 5-HT effects in colonic ICCs.

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.

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.

The mineralocorticoid receptor leads to increased expression of EGFR and T-type calcium channels that support HL-1 cell hypertrophy

The EGF receptor (EGFR) has been extensively studied in tumor biology and recently a role in cardiovascular pathophysiology was suggested. The mineralocorticoid receptor (MR) is an important effector of the renin-angiotensin-aldosterone-system and elicits pathophysiological effects in the cardiovascular system; however, the underlying molecular mechanisms are unclear. Our aim was to investigate the importance of EGFR for MR-mediated cardiovascular pathophysiology because MR is known to induce EGFR expression. We identified a SNP within the EGFR promoter that modulates MR-induced EGFR expression. In RNA-sequencing and qPCR experiments in heart tissue of EGFR KO and WT mice, changes in EGFR abundance led to differential expression of cardiac ion channels, especially of the T-type calcium channel CACNA1H. Accordingly, CACNA1H expression was increased in WT mice after in vivo MR activation by aldosterone but not in respective EGFR KO mice. Aldosterone- and EGF-responsiveness of CACNA1H expression was confirmed in HL-1 cells by Western blot and by measuring peak current density of T-type calcium channels. Aldosterone-induced CACNA1H protein expression could be abrogated by the EGFR inhibitor AG1478. Furthermore, inhibition of T-type calcium channels with mibefradil or ML218 reduced diameter, volume and BNP levels in HL-1 cells. In conclusion the MR regulates EGFR and CACNA1H expression, which has an effect on HL-1 cell diameter, and the extent of this regulation seems to depend on the SNP-216 (G/T) genotype. This suggests that the EGFR may be an intermediate for MR-mediated cardiovascular changes and that SNP analysis can help identify subgroups of patients that will benefit most from MR antagonists.

Constant-potential environment for activating and synchronizing cardiomyocyte colonies with on-chip ion-depleting perm-selective membranes

In this study, an ion depleted zone created by an ion-selective membrane was used to impose a high and uniform constant extracellular potential over an entire ∼1000 cell rat cardiomyocyte (rCM) colony on-a-chip to trigger synchronized voltage-gated ion channel activities while preserving cell viability, thus extending single-cell voltage-clamp ion channel studies to an entire normalized colony. Image analysis indicated that rCM beating was strengthened and accelerated (by a factor of ∼2) within minutes of ion depletion and the duration of contraction and relaxation phases was significantly reduced. After the initial synchronization, the entire colony responds collectively to external potential changes such that beating over the entire colony can be activated or deactivated within 0.1 s. These newly observed collective dynamic responses, due to simultaneous ion channel activation/deactivation by a uniform constant-potential extracellular environment, suggest that perm-selective membrane modules on cell culture chips can facilitate studies of extracellular cardiac cell electrical communication and how ion-channel related pathologies affect cardiac cell synchronization. The future applications of this new technology can lead to better drug screening platforms for cardiotoxicity as well as platforms that can facilitate synchronized maturation of pluripotent stem cells into colonies with high electrical connectivity.

Regulation of HCN2 Current by PI3K/Akt Signaling

It has long been known that heart rate is regulated by the autonomic nervous system. Recently, we demonstrated that the pacemaker current, I_f, is regulated by phosphoinositide 3-kinase (PI3K) signaling independently of the autonomic nervous system. Inhibition of PI3K in sinus node (SN) myocytes shifts the activation of I_f by almost 16 mV in the negative direction. I_f in the SN is predominantly mediated by two members of the HCN gene family, HCN4 and HCN1. Purkinje fibers also possess I_f and are an important secondary pacemaker in the heart. In contrast to the SN, they express HCN2 and HCN4, while ventricular myocytes, which do not normally pace, express HCN2 alone. In the current work, we investigated PI3K regulation of HCN2 expressed in HEK293 cells. Treatment with the PI3K inhibitor PI-103 caused a negative shift in the activation voltage and a dramatic reduction in the magnitude of the HCN2 current. Similar changes were also seen in cells treated with an inhibitor of the protein kinase Akt, a downstream effector of PI3K. The effects of PI-103 were reversed by perfusion of cells with phosphatidylinositol 3,4,5-trisphosphate (the second messenger produced by PI3K) or active Akt protein. We identified serine 861 in mouse HCN2 as a putative Akt phosphorylation site. Mutation of S861 to alanine mimicked the effects of Akt inhibition on voltage dependence and current magnitude. In addition, the Akt inhibitor had no effect on the mutant channel. These results suggest that Akt phosphorylation of mHCN2 S861 accounts for virtually all of the observed actions of PI3K signaling on the HCN2 current. Unexpectedly, Akt inhibition had no effect on I_f in SN myocytes. This result raises the possibility that diverse PI3K signaling pathways differentially regulate HCN-induced currents in different tissues, depending on the isoforms expressed.

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.

Acetylcholine-dependent upregulation of TASK-1 channels in thalamic interneurons by a smooth muscle-like signalling pathway

KEY POINTS

The ascending brainstem transmitter acetylcholine depolarizes thalamocortical relay neurons while it induces hyperpolarization in local circuit inhibitory interneurons. Sustained K⁺ currents are modulated in thalamic neurons to control their activity modes; for the interneurons the molecular nature of the underlying ion channels is as yet unknown. Activation of TASK-1 K⁺ channels results in hyperpolarization of interneurons and suppression of their action potential firing. The modulation cascade involves a non-receptor tyrosine kinase, c-Src. The present study identifies a novel pathway for the activation of TASK-1 channels in CNS neurons that resembles cholinergic signalling and TASK-1 current modulation during hypoxia in smooth muscle cells.

ABSTRACT

The dorsal part of the lateral geniculate nucleus (dLGN) is the main thalamic site for state-dependent transmission of visual information. Non-retinal inputs from the ascending arousal system and inhibition provided by γ-aminobutyric acid (GABA)ergic local circuit interneurons (INs) control neuronal activity within the dLGN. In particular, acetylcholine (ACh) depolarizes thalamocortical relay neurons by inhibiting two-pore domain potassium (K_2P ) channels. Conversely, ACh also hyperpolarizes INs via an as-yet-unknown mechanism. By using whole cell patch-clamp recordings in brain slices and appropriate pharmacological tools we here report that stimulation of type 2 muscarinic ACh receptors induces IN hyperpolarization by recruiting the G-protein βγ subunit (Gβγ), class-1A phosphatidylinositol-4,5-bisphosphate 3-kinase, and cellular and sarcoma (c-Src) tyrosine kinase, leading to activation of two-pore domain weakly inwardly rectifying K⁺ channel (TWIK)-related acid-sensitive K⁺ (TASK)-1 channels. The latter was confirmed by the use of TASK-1-deficient mice. Furthermore inhibition of phospholipase Cβ as well as an increase in the intracellular level of phosphatidylinositol-3,4,5-trisphosphate facilitated the muscarinic effect. Our results have uncovered a previously unknown role of c-Src tyrosine kinase in regulating IN function in the brain and identified a novel mechanism by which TASK-1 channels are activated in neurons.

Protein kinase C bidirectionally modulates Ih and hyperpolarization-activated cyclic nucleotide-gated (HCN) channel surface expression in hippocampal pyramidal neurons

KEY POINTS

Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels, particularly that of the HCN1 isoform, are enriched in the distal dendrites of hippocampal CA1 pyramidal neurons; these channels have physiological functions with respect to decreasing neuronal excitability. In the present study, we aimed to investigate phosphorylation as a mechanism controlling Ih amplitude and HCN1 surface expression in hippocampal principal neurons under normal physiological conditions. Tyrosine phosphorylation decreased Ih amplitude at maximal activation (maximal Ih ), without altering HCN1 surface expression, in two classes of hippocampal principal neurons. Inhibition of serine/threonine protein phosphatases 1 and 2A decreased maximal Ih and HCN1 surface expression in hippocampal principal neurons. Protein kinase C (PKC) activation irreversibly diminished Ih and HCN1 surface expression, whereas PKC inhibition augmented Ih and HCN1 surface expression. PKC activation increased HCN1 channel phosphorylation. These results demonstrate the novel finding of a phosphorylation mechanism, dependent on PKC activity, which bidirectionally modulates Ih amplitude and HCN1channel surface expression in hippocampal principal neurons under normal physiological conditions.

ABSTRACT

Hyperpolarization-activated, cyclic nucleotide-gated (HCN) ion channels attenuate excitability in hippocampal pyramidal neurons. Loss of HCN channel-mediated current (Ih ), particularly that mediated by the HCN1 isoform, occurs with the development of epilepsy. Previously, we showed that, following pilocarpine-induced status epilepticus, there are two independent changes in HCN function in dendrites: decreased Ih amplitude associated with a loss of HCN1 surface expression and a hyperpolarizing shift in voltage-dependence of activation (gating). The hyperpolarizing shift in gating was attributed to decreased phosphorylation as a result of a loss of p38 mitogen-activated protein kinase activity and increased calcineurin activity; however, the mechanisms controlling Ih amplitude and HCN1 surface expression under epileptic or normal physiological conditions are poorly understood. We aimed to investigate phosphorylation as a mechanism regulating Ih amplitude and HCN1 surface expression (i.e. as is the case for HCN gating) in hippocampal principal neurons under normal physiological conditions. We discovered that inhibition of either tyrosine phosphatases or the serine/threonine protein phosphatases 1 and 2A decreased Ih at maximal activation in hippocampal CA1 pyramidal dendrites and pyramidal-like principal neuron somata from naïve rats. Furthermore, we found that inhibition of PP1/PP2A decreased HCN1 surface expression, whereas tyrosine phosphatase inhibition did not. Protein kinase C (PKC) activation reduced Ih amplitude and HCN1 surface expression, whereas PKC inhibition produced the opposite effect. Inhibition of protein phosphatases 1 and 2A and activation of PKC increased the serine phosphorylation state of the HCN1 protein. The effect of PKC activation on Ih was irreversible. These results indicate that PKC bidirectionally modulates Ih amplitude and HCN1 surface expression in hippocampal principal neurons.

HCN channels--modulators of cardiac and neuronal excitability

Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels comprise a family of cation channels activated by hyperpolarized membrane potentials and stimulated by intracellular cyclic nucleotides. The four members of this family, HCN1-4, show distinct biophysical properties which are most evident in the kinetics of activation and deactivation, the sensitivity towards cyclic nucleotides and the modulation by tyrosine phosphorylation. The four isoforms are differentially expressed in various excitable tissues. This review will mainly focus on recent insights into the functional role of the channels apart from their classic role as pacemakers. The importance of HCN channels in the cardiac ventricle and ventricular hypertrophy will be discussed. In addition, their functional significance in the peripheral nervous system and nociception will be examined. The data, which are mainly derived from studies using transgenic mice, suggest that HCN channels contribute significantly to cellular excitability in these tissues. Remarkably, the impact of the channels is clearly more pronounced in pathophysiological states including ventricular hypertrophy as well as neural inflammation and neuropathy suggesting that HCN channels may constitute promising drug targets in the treatment of these conditions. This perspective as well as the current therapeutic use of HCN blockers will also be addressed.

A di-arginine ER retention signal regulates trafficking of HCN1 channels from the early secretory pathway to the plasma membrane

Hyperpolarization-activated cyclic nucleotide-gated 1 (HCN1) channels carry I_h, which contributes to neuronal excitability and signal transmission in the nervous system. Controlling the trafficking of HCN1 is an important aspect of its regulation, yet the details of this process are poorly understood. Here, we investigated how the C-terminus of HCN1 regulates trafficking by testing for its ability to redirect the localization of a non-targeted reporter in transgenic Xenopus laevis photoreceptors. We found that HCN1 contains an ER localization signal and through a series of deletion constructs, identified the responsible di-arginine ER retention signal. This signal is located in the intrinsically disordered region of the C-terminus of HCN1. To test the function of the ER retention signal in intact channels, we expressed wild type and mutant HCN1 in HEK293 cells and found this signal negatively regulates surface expression of HCN1. In summary, we report a new mode of regulating HCN1 trafficking: through the use of a di-arginine ER retention signal that monitors processing of the channel in the early secretory pathway.

The possible roles of hyperpolarization-activated cyclic nucleotide channels in regulating pacemaker activity in colonic interstitial cells of Cajal

BACKGROUND

Hyperpolarization-activated cyclic nucleotide (HCN) channels are pacemaker channels that regulate heart rate and neuronal rhythm in spontaneously active cardiac and neuronal cells. Interstitial cells of Cajal (ICCs) are also spontaneously active pacemaker cells in the gastrointestinal tract. Here, we investigated the existence of HCN channel and its role on pacemaker activity in colonic ICCs.

METHODS

We performed whole-cell patch clamp, RT-PCR, and Ca(2+)-imaging in cultured ICCs from mouse mid colon.

RESULTS

SQ-22536 and dideoxyadenosine (adenylate cyclase inhibitors) decreased the frequency of pacemaker potentials, whereas both rolipram (cAMP-specific phosphodiesterase inhibitor) and cell-permeable 8-bromo-cAMP increased the frequency of pacemaker potentials. CsCl, ZD7288, zatebradine, clonidine (HCN channel blockers), and genistein (a tyrosine kinase inhibitor) suppressed the pacemaker activity. RT-PCR revealed expression of HCN1 and HCN3 channels in c-kit and Ano1 positive colonic ICCs. In recordings of spontaneous intracellular Ca(2+) [Ca(2+)]i oscillations, rolipram and 8-bromo-cAMP increased [Ca(2+)]i oscillations, whereas SQ-22536, CsCl, ZD7288, and genistein decreased [Ca(2+)]i oscillations.

CONCLUSIONS

HCN channels in colonic ICCs are tonically activated by basal cAMP production and participate in regulation of pacemaking activity.

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.

The fast and slow ups and downs of HCN channel regulation

Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels (h channels) form the molecular basis for the hyperpolarization-activated current, I(h), and modulation of h channels contributes to changes in cellular properties critical for normal functions in the mammalian brain and heart. Numerous mechanisms underlie h channel modulation during both physiological and pathological conditions, leading to distinct changes in gating, kinetics, surface expression, channel conductance or subunit composition of h channels. Here we provide a focused review examining mechanisms of h channel regulation, with an emphasis on recent findings regarding interacting proteins such as TRIP8b. This review is intended to serve as a comprehensive resource for physiologists to provide potential molecular mechanisms underlying functionally important changes in I(h) in different biological models, as well as for molecular biologists to delineate the predicted h channel changes associated with complex regulatory mechanisms in both normal function and in disease states.

HCN channel activity-dependent modulation of inhibitory synaptic transmission in the rat basolateral amygdala

Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels are expressed in the central nervous system and play a regulatory role in neuronal excitability. In the present study, we examined a physiological role of HCN channels in the rat basolateral amygdala (BLA). In vitro electrophysiological studies showed that ZD7288 decreased spontaneous inhibitory postsynaptic current (sIPSC) without changing miniature IPSC (mIPSC). HCN channel blockade also attenuated feedback inhibitions in BLA principal neurons. However, blockade of HCN channel had little effects on spontaneous excitatory postsynaptic current (sEPSC) and mEPSC. Therefore, HCN channel appeared to decrease BLA excitability by increasing the action potential-dependent inhibitory control over the BLA principal neurons. Anxiety is reported to be influenced by neuronal excitability in the BLA and inhibitory synaptic transmission is thought to play a pivotal role in regulating overall excitability of the amygdala. As expected, blockade of HCN channels by targeted injection of ZD7288 to the BLA increased anxiety-like behavior under elevated plus maze test. Our results suggest that HCN channel activity can modulate the GABAergic synaptic transmission in the BLA, which in turn control the amygdala-related emotional behaviors such as anxiety.

Downregulation of dendritic HCN channel gating in epilepsy is mediated by altered phosphorylation signaling

The onset of spontaneous seizures in the pilocarpine model of epilepsy causes a hyperpolarized shift in the voltage-dependent activation of hyperpolarization-activated cyclic nucleotide-gated (HCN) channel-mediated current (Ih) in CA1 hippocampal pyramidal neuron dendrites, contributing to neuronal hyperexcitability and possibly to epileptogenesis. However, the specific mechanisms by which spontaneous seizures cause downregulation of HCN channel gating are yet unknown. We asked whether the seizure-dependent downregulation of HCN channel gating was due to altered phosphorylation signaling mediated by the phosphatase calcineurin (CaN) or the kinase p38 mitogen-activated protein kinase (p38 MAPK). We first found that CaN inhibition upregulated HCN channel gating and reduced neuronal excitability under normal conditions, showing that CaN is a strong modulator of HCN channels. We then found that an in vitro model of seizures (1 h in 0 Mg2+ and 50 microM bicuculline at 35-37 degrees C) reproduced the HCN channel gating change seen in vivo. Pharmacological inhibition of CaN or activation of p38 MAPK partially reversed the in vitro seizure-induced hyperpolarized shift in HCN channel gating, and the shift was fully reversed by the combination of CaN inhibition and p38 MAPK activation. We then demonstrated enhanced CaN activity as well as reduced p38 MAPK activity in vivo in the CA1 hippocampal area of chronically epileptic animals. Pharmacological reversal of these phosphorylation changes restored HCN channel gating downregulation and neuronal hyperexcitability in epileptic tissue to control levels. Together, these results suggest that alteration of two different phosphorylation pathways in epilepsy contributes to the downregulation of HCN channel gating, which consequently produces neuronal hyperexcitability and thus may be a target for novel antiepileptic therapies.

HCN channels are not required for mechanotransduction in sensory hair cells of the mouse inner ear

The molecular composition of the hair cell transduction channel has not been identified. Here we explore the novel hypothesis that hair cell transduction channels include HCN subunits. The HCN family of ion channels includes four members, HCN1-4. They were orginally identified as the molecular correlates of the hyperpolarization-activated, cyclic nucleotide gated ion channels that carry currents known as I_f, I_Q or I_h. However, based on recent evidence it has been suggested that HCN subunits may also be components of the elusive hair cell transduction channel. To investigate this hypothesis we examined expression of mRNA that encodes HCN1-4 in sensory epithelia of the mouse inner ear, immunolocalization of HCN subunits 1, 2 and 4, uptake of the transduction channel permeable dye, FM1-43 and electrophysiological measurement of mechanotransduction current. Dye uptake and transduction current were assayed in cochlear and vestibular hair cells of wildtype mice exposed to HCN channel blockers or a dominant-negative form of HCN2 that contained a pore mutation and in mutant mice that lacked HCN1, HCN2 or both. We found robust expression of HCNs 1, 2 and 4 but little evidence that localized HCN subunits in hair bundles, the site of mechanotransduction. Although high concentrations of the HCN antagonist, ZD7288, blocked 50–70% of the transduction current, we found no reduction of transduction current in either cochlear or vestibular hair cells of HCN1- or HCN2- deficient mice relative to wild-type mice. Furthermore, mice that lacked both HCN1 and HCN2 also had normal transduction currents. Lastly, we found that mice exposed to the dominant-negative mutant form of HCN2 had normal transduction currents as well. Taken together, the evidence suggests that HCN subunits are not required for mechanotransduction in hair cells of the mouse inner ear.

Rescue of a trafficking defective human pacemaker channel via a novel mechanism: roles of Src, Fyn, and Yes tyrosine kinases

Therapeutic strategies such as using channel blockers and reducing culture temperature have been used to rescue some long QT-associated voltage-gated potassium Kv trafficking defective mutant channels. A hyperpolarization-activated cyclic nucleotide-gated HCN4 pacemaker channel mutant (D553N) has been recently found in a patient associated with cardiac arrhythmias including long QT. D553N showed the defective trafficking to the cell surface, leading to little ionic current expression (loss-of-function). We show in this report that enhanced tyrosine phosphorylation mediated by Src, Fyn, and Yes kinases was able to restore the surface expression of D553N for normal current expression. Src or Yes, but not Fyn, significantly increased the current density and surface expression of D553N. Fyn accelerated the activation kinetics of the rescued D553N. Co-expression of D553N with Yes exhibited the slowest activation kinetics of D553N. Src, Fyn, and Yes significantly enhanced the tyrosine phosphorylation of D553N. A combination of Src, Fyn, and Yes rescued the current expression and the gating of D553N comparable with those of wild-type HCN4. In conclusion, we demonstrate a novel mechanism using three endogenous Src kinases to rescue a trafficking defective HCN4 mutant channel (D553N) by enhancing the tyrosine phosphorylation of the mutant channel protein.

Hyperpolarization-activated cation channels: from genes to function

Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels comprise a small subfamily of proteins within the superfamily of pore-loop cation channels. In mammals, the HCN channel family comprises four members (HCN1-4) that are expressed in heart and nervous system. The current produced by HCN channels has been known as I(h) (or I(f) or I(q)). I(h) has also been designated as pacemaker current, because it plays a key role in controlling rhythmic activity of cardiac pacemaker cells and spontaneously firing neurons. Extensive studies over the last decade have provided convincing evidence that I(h) is also involved in a number of basic physiological processes that are not directly associated with rhythmicity. Examples for these non-pacemaking functions of I(h) are the determination of the resting membrane potential, dendritic integration, synaptic transmission, and learning. In this review we summarize recent insights into the structure, function, and cellular regulation of HCN channels. We also discuss in detail the different aspects of HCN channel physiology in the heart and nervous system. To this end, evidence on the role of individual HCN channel types arising from the analysis of HCN knockout mouse models is discussed. Finally, we provide an overview of the impact of HCN channels on the pathogenesis of several diseases and discuss recent attempts to establish HCN channels as drug targets.

Voltage-dependent opening of HCN channels: Facilitation or inhibition by the phytoestrogen, genistein, is determined by the activation status of the cyclic nucleotide gating ring

Investigation of the mechanistic bases and physiological importance of cAMP regulation of HCN channels has exploited an arginine to glutamate mutation in the nucleotide-binding fold, an approach critically dependent on the mutation selectively lowering the channel's nucleotide affinity. In apparent conflict with this, in intact Xenopus oocytes, HCN and HCN-RE channels exhibit qualitatively and quantitatively distinct responses to the tyrosine kinase inhibitor, genistein -- the estrogenic isoflavonoid strongly depolarizes the activation mid-point of HCN1-R538E, but not HCN1 channels (+9.8 mV + or - 0.9 versus +2.2 mV + or - 0.6) and hyperpolarizes gating of HCN2 (-4.8 mV + or - 1.0) but depolarizes gating of HCN2-R591E (+13.2 mV + or - 2.1). However, excised patch recording, X-ray crystallography and modeling reveal that this is not due to either a fundamental effect of the mutation on channel gating per se or of genistein acting as a mutation-sensitive partial agonist at the cAMP site. Rather, we find that genistein equivalently moves both HCN and HCN-RE channels closer to the open state (rendering the channels inherently easier to open but at a cost of decreasing the coupling energy of cAMP) and that the anomaly reflects a balance of these energetic effects with the isoform-specific inhibition of activation by the nucleotide gating ring and relief of this by endogenous cAMP. These findings have specific implications with regard to findings based on HCN-RE channels and kinase antagonists and general implications with respect to interpretation of drug effects in mutant channel backgrounds.

Associated changes in HCN2 and HCN4 transcripts and I(f) pacemaker current in myocytes

The time- and voltage-dependent inward current generated by the hyperpolarization-activated cyclic nucleotide-gated (HCN) channels contributes to the tissue-specific rhythmic activities in the brain and heart. Four isoforms (HCN1-HCN4) have been identified. Previous studies showed that different HCN isoforms may form functional heteromeric channels. We report here that when HCN2 and HCN4 mRNA were injected into Xenopus oocytes with various ratios of HCN2 over HCN4 at 1:1, 10:1, and 1:10, respectively, the resultant channels showed a depolarized current activation and significantly faster activation kinetics near the midpoint of activation compared with HCN4 homomeric channels. In adult rat myocytes overexpressing HCN4, there was an associated increase in HCN2 mRNA. In neonatal rat myocytes in which HCN2 was knocked down, there was also a simultaneous decrease in HCN4 mRNA. Coimmunoprecipitation experiments showed that HCN2 and HCN4 channel proteins can associate with each other in adult rat ventricles. Finally, in adult myocytes overexpressing HCN4, the hyperpolarization-activated inward current activation, I(f), was shifted to physiological voltages from non-physiological voltages, associated with faster activation kinetics. These data suggested that different ratios of HCN2 and HCN4 transcripts overlapping in different tissues also contribute to the tissue-specific properties of I(f).

Light responses in the mouse retina are prolonged upon targeted deletion of the HCN1 channel gene

Hyperpolarization-activated and cyclic nucleotide-gated (HCN) channels contribute to pacemaker activity, and co-determine the integrative behaviour of neurons and shape their response to synaptic stimulation. Four channel isoforms, HCN1-4, have been described in mammals. Recent studies showed particularly strong expression of HCN1 channels in rods and cones of the rat retina, suggesting that HCN1 channels are involved in the shaping of light responses in both types of photoreceptors. Therefore, the loss of HCN1 channels should lead to pronounced changes in light-induced electrical responses under both scotopic and photopic conditions. This was tested using a mouse transgenic approach. We used immunohistochemistry and patch-clamp recording to study the distribution of HCN1 channels in the mouse retina. HCN1 channels were strongly expressed in rod and cone photoreceptors, as well as in some bipolar, amacrine and ganglion cell types. In electroretinograms (ERGs) from animals in which the HCN1 channel gene had been knocked out, the b-wave amplitudes were unaltered (scotopic conditions) or somewhat reduced (photopic conditions), whereas the duration of both scotopic and photopic ERG responses was strikingly prolonged. Our data suggest that in visual information processing, shortening and shaping of light responses by activation of HCN1 at the level of the photoreceptors is an important step in both scotopic and photopic pathways.

Novel mechanism for suppression of hyperpolarization-activated cyclic nucleotide-gated pacemaker channels by receptor-like tyrosine phosphatase-alpha

We have previously reported an important role of increased tyrosine phosphorylation activity by Src in the modulation of hyperpolarization-activated cyclic nucleotide-gated (HCN) channels. Here we provide evidence showing a novel mechanism of decreased tyrosine phosphorylation on HCN channel properties. We found that the receptor-like protein-tyrosine phosphatase-alpha (RPTPalpha) significantly inhibited or eliminated HCN2 channel expression in HEK293 cells. Biochemical evidence showed that the surface expression of HCN2 was remarkably reduced by RPTPalpha, which was in parallel to the decreased tyrosine phosphorylation of the channel protein. Confocal imaging confirmed that the membrane surface distribution of the HCN2 channel was inhibited by RPTPalpha. Moreover, we detected the presence of RPTPalpha proteins in cardiac ventricles with expression levels changed during development. Inhibition of tyrosine phosphatase activity by phenylarsine oxide or sodium orthovanadate shifted ventricular hyperpolarization-activated current (I(f), generated by HCN channels) activation from nonphysiological voltages into physiological voltages associated with accelerated activation kinetics. In conclusion, we showed a critical role RPTPalpha plays in HCN channel function via tyrosine dephosphorylation. These findings are also important to neurons where HCN and RPTPalpha are richly expressed.

Age-dependent differences in the inhibition of HCN2 current in rat ventricular myocytes by the tyrosine kinase inhibitor erbstatin

Previously, we have shown that murine HCN2 channels over-expressed in newborn and adult cardiac myocytes produce currents with different biophysical characteristics. To investigate the role of tyrosine kinase modulation in these age-dependent differences, we employed the broad spectrum tyrosine kinase inhibitor erbstatin. Our results demonstrated distinct and separable effects of erbstatin on channel gating and current amplitude and a marked age dependence to these effects. In newborn myocytes, erbstatin decreased current amplitude, shifted the activation relation negative, and slowed activation kinetics. The effect on activation voltage but not that on amplitude was absent when expressing a cAMP-insensitive mutant (HCN2R/E), while a C-terminal truncated form of HCN2 (HCN2DeltaCx) exhibited only the voltage dependent but not the amplitude effect of erbstatin. Thus, the action of erbstatin on the activation relation and current amplitude are distinct and separable in newborn myocytes, and the effect on activation voltage depends on the cAMP status of HCN2 channels. In contrast to newborn myocytes, erbstatin had no effect on HCN2 under control conditions in adult myocytes but induced a negative shift with no change in amplitude when saturated cAMP was added to the pipette solution. We conclude that erbstatin's effects on HCN2 current magnitude and voltage dependence are distinct and separable, and there are fundamental developmental differences in the heart that affect channel function and its modulation by the tyrosine kinase inhibitor erbstatin.

Regulation of two-pore-domain (K2P) potassium leak channels by the tyrosine kinase inhibitor genistein

BACKGROUND AND PURPOSE

Two-pore-domain potassium (K2P) channels mediate potassium background (or 'leak') currents, controlling excitability by stabilizing membrane potential below firing threshold and expediting repolarization. Inhibition of K2P currents permits membrane potential depolarization and excitation. As expected for key regulators of excitability, leak channels are under tight control from a plethora of stimuli. Recently, signalling via protein tyrosine kinases (TKs) has been implicated in ion channel modulation. The objective of this study was to investigate TK regulation of K2P channels.

EXPERIMENTAL APPROACH

The two-electrode voltage clamp technique was used to record K2P currents in Xenopus oocytes. In addition, K2P channels were studied in Chinese hamster ovary (CHO) cells using the whole-cell patch clamp technique.

KEY RESULTS

Here, we report inhibition of human K2P3.1 (TASK-1) currents by the TK antagonist, genistein, in Xenopus oocytes (IC50=10.7 microM) and in CHO cells (IC50=12.3 microM). The underlying molecular mechanism was studied in detail. hK2P3.1 was not affected by genistin, an inactive analogue of genistein. Perorthovanadate, an inhibitor of tyrosine phosphatase activity, reduced the inhibitory effect of genistein. Current reduction was voltage independent and did not require channel protonation at position H98 or phosphorylation at the single TK phosphorylation site, Y323. Among functional hK2P family members, genistein also reduced K2P6.1 (TWIK-2), K2P9.1 (TASK-3) and K2P13.1 (THIK-1) currents, respectively.

CONCLUSIONS AND IMPLICATIONS

Modulation of K2P channels by the TK inhibitor, genistein, represents a novel molecular mechanism to alter background K+ currents.

Src tyrosine kinase alters gating of hyperpolarization-activated HCN4 pacemaker channel through Tyr531

We recently discovered that the constitutively active Src tyrosine kinase can enhance hyperpolarization-activated, cyclic nucleotide-gated (HCN) 4 channel activity by binding to the channel protein. To investigate the mechanism of modulation by Src of HCN channels, we studied the effects of a selective inhibitor of Src tyrosine kinase, 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine (PP2), on HCN4 and its mutant channels expressed in HEK 293 cells by using a whole cell patch-clamp technique. We found that PP2 can inhibit HCN4 currents by negatively shifting the voltage dependence of channel activation, decreasing the whole cell channel conductance, and slowing activation and deactivation kinetics. Screening putative tyrosine residues subject to phosphorylation yielded two candidates: Tyr(531) and Tyr(554). Substituting HCN4-Tyr(531) with phenylalanine largely abolished the effects of PP2 on HCN4 channels. Replacing HCN4-Tyr(554) with phenylalanine did not abolish the effects of PP2 on voltage-dependent activation but did eliminate PP2-induced slowing of channel kinetics. The inhibitory effects of HCN channels associated with reduced Src tyrosine activity is confirmed in HL-1 cardiomyocytes. Finally, we found that PP2 can decrease the heart rate in a mouse model. These results demonstrate that Src tyrosine kinase enhances HCN4 currents by shifting their activation to more positive potentials and increasing the whole cell channel conductance as well as speeding the channel kinetics. The tyrosine residue that mediates most of Src's actions on HCN4 channels is Tyr(531).

Hyperpolarization-activated currents control the excitability of principal neurons in the basolateral amygdala

Anxiety is thought to be influenced by neuronal excitability in basolateral nucleus of the amygdala (BLA). However, molecules that are critical for regulating excitability of BLA neurons are yet to be determined. In the present study, we have examined whether hyperpolarization-activated cyclic-nucleotide-gated (HCN) channels, which mediate the depolarizing cation current, can control the neuronal excitability. HCN channel-like activity appeared to be detected in BLA principal neurons. ZD7288, a specific blocker for HCN channels, increased the input resistance of membrane, hyperpolarized resting membrane potential, and enhanced action potential firing in BLA principal neurons. The blockade of HCN channels facilitated temporal summation of repetitively evoked excitatory postsynaptic potentials, suggesting that suppression of HCN channel activity in principal neurons can accelerate the propagation of synaptic responses onto the axon hillock. Thus, our findings have laid foundation for studies to reveal how HCN channel activity in BLA principal neurons regulates anxiety in vivo.

The role of pacemaker currents in neuropathic pain

Hyperpolarization-activated cation nonselective cyclic nucleotide-gated (HCN) channels mediate pacemaker currents that control basic rhythmic processes including heartbeat. Alterations in HCN channel expression or function have been described in both epilepsy and cardiac arrhythmias. Recent evidence suggests that pacemaker currents may also play an important role in ectopic neuronal activity that manifests as neuropathic pain. Pacemaker currents are subject to endogenous regulation by cyclic nucleotides, pH and perhaps phosphorylation. In addition, a number of neuromodulators with known roles in pain affect current density and kinetics. The pharmacology of a number of drugs that are commonly used to treat neuropathic pain includes effects on pacemaker currents. Altered pacemaker currents in injured tissues may be an important mechanism underlying neuropathic pain, and drugs that modulate these currents may offer new therapeutic options.

Modulation of cyclic nucleotide-regulated HCN channels by PIP(2) and receptors coupled to phospholipase C

Recent results indicate that phosphoinositides, including phosphatidylinositol 4,5-bisphosphate (PI(4,5)P(2)), directly enhance the opening of hyperpolarization-activated, cyclic nucleotide-regulated (HCN) channels by shifting their activation gating to more positive voltages. This contrasts with the action of phosphoinositides to inhibit the opening of the related cyclic nucleotide-gated (CNG) channels involved in sensory signaling. We both review previous studies and present new experiments that investigate whether HCN channels may be regulated by dynamic changes in PI(4,5)P(2) levels caused by the receptor-mediated activation of phospholipase C (PLC). We coexpressed HCN1 or HCN2 channels in Xenopus oocytes with the PLC-coupled bradykinin BK(2) receptor, the muscarinic M1 receptor, or the TrkA receptor. Activation of all three receptors produced a positive shift in HCN channel voltage gating, the opposite of the effect expected for PI(4,5)P(2) depletion. This action was not caused by alterations in cAMP as the effect was preserved in HCN mutant channels that fail to bind cAMP. The receptor effects were mediated by PLC activity, but did not depend on signaling through the downstream products of PI(4,5)P(2) hydrolysis: IP(3) or diacylglycerol (DAG). Importantly, the modulatory effects on gating were blocked by inhibitors of phosphatidylinositol (PI) kinases, suggesting a role for increased PI(4,5)P(2) synthesis. Finally, we found that bradykinin exerted a similar PI kinase-dependent effect on the gating of native HCN channels in cardiac sinoatrial node cells, suggesting that this pathway may represent a novel, physiologically relevant mechanism for enhancing HCN channel function.

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

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

Modulation of h-channels in hippocampal pyramidal neurons by p38 mitogen-activated protein kinase

Hyperpolarization-activated cyclic nucleotide-gated ion channels (h-channels; I(h); HCN) modulate intrinsic excitability in hippocampal and neocortical pyramidal neurons, among others. Whereas I(h) mediated by the HCN2 isoform is regulated by cAMP, there is little known about kinase modulation of I(h), especially for the HCN1 isoform predominant in pyramidal neurons. We used a computational method to identify a novel kinase modulator of h-channels, p38 mitogen-activated protein kinase (p38 MAPK). Inhibition of p38 MAPK in hippocampal pyramidal neurons caused a approximately 25 mV hyperpolarization of I(h) voltage-dependent activation. This downregulation of I(h) produced hyperpolarization of resting potential, along with increased input resistance and temporal summation of excitatory inputs. Activation of p38 MAPK caused a approximately 11 mV depolarizing shift in I(h) activation, along with depolarized resting potential, and decreased input resistance and temporal summation. Inhibition of related MAPKs, ERK1/2 (extracellular signal-related kinase 1/2) and JNK (c-Jun N-terminal kinase), produced no effect on I(h). These results show that p38 MAPK is a strong modulator of h-channel biophysical properties and may deserve additional exploration as a link between altered I(h) and pathological conditions such as epilepsy.

CNG and HCN channels: two peas, one pod

Cyclic nucleotide-activated ion channels play a fundamental role in a variety of physiological processes. By opening in response to intracellular cyclic nucleotides, they translate changes in concentrations of signaling molecules to changes in membrane potential. These channels belong to two families: the cyclic nucleotide-gated (CNG) channels and the hyperpolarization-activated cyclic nucleotide-modulated (HCN) channels. The two families exhibit high sequence similarity and belong to the superfamily of voltage-gated potassium channels. Whereas HCN channels are activated by voltage and CNG channels are virtually voltage independent, both channels are activated by cyclic nucleotide binding. Furthermore, the channels are thought to have similar channel structures, leading to similar mechanisms of activation by cyclic nucleotides. However, although these channels are structurally and behaviorally similar, they have evolved to perform distinct physiological functions. This review describes the physiological roles and biophysical behavior of CNG and HCN channels. We focus on how similarities in structure and activation mechanisms result in common biophysical models, allowing CNG and HCN channels to be viewed as a single genre.

Insensitivity of cardiac delayed-rectifier I(Kr) to tyrosine phosphorylation inhibitors and stimulators

1. The rapidly activating delayed-rectifying K+ current (I(Kr)) in heart cells is an important determinant of repolarisation, and decreases in its density are implicated in acquired and inherited long QT syndromes. The objective of the present study on I(Kr) in guinea-pig ventricular myocytes was to evaluate whether the current is acutely regulated by tyrosine phosphorylation. 2. Myocytes configured for ruptured-patch or perforated-patch voltage-clamp were depolarised with 200-ms steps to 0 mV for measurement of I(Kr) tail amplitude on repolarisations to -40 mV. 3. I(Kr) in both ruptured-patch and perforated-patch myocytes was only moderately (14-20%) decreased by 100 microM concentrations of protein tyrosine kinase (PTK) inhibitors tyrphostin A23, tyrphostin A25, and genistein. However, similar-sized decreases were induced by PTK-inactive analogues tyrphostin A1 and daidzein, suggesting that they were unrelated to inhibition of PTK. 4. Ruptured-patch and perforated-patch myocytes were also treated with promoters of tyrosine phosphorylation, including phosphotyrosyl phosphatase (PTP) inhibitor orthovanadate, exogenous c-Src PTK, and four receptor PTK activators (insulin, insulin-like growth factor-1, epidermal growth factor, and basic fibroblast growth factor). None of these treatments had a significant effect on the amplitude of I(Kr). 5. We conclude that Kr channels in guinea-pig ventricular myocytes are unlikely to be regulated by PTK and PTP.

Constitutively active Src tyrosine kinase changes gating of HCN4 channels through direct binding to the channel proteins

Cardiac pacemaker current, if, is generated by hyperpolarization-activated cyclic nucleotide-gated (HCN) channels. Our previous studies demonstrated that altered tyrosine phosphorylation can modulate the properties of both if and HCN channels. To assess a hypothesis that the intracellular tyrosine kinase Src may play a role in modulation by tyrosine phosphorylation of if, we cotransfected HEK293 cells with HCN4 and Src proteins. When HCN4 was cotransfected with a constitutively activated Src protein (Src529), the resultant voltage-dependent HCN4 activation was positively shifted (HCN4: V1/2 = -93 mV; Src529: V1/2 = -80 mV). The activation kinetics were accelerated at some potentials but not over the entire voltage range tested (eg, at -95 mV, tau_act(HCN4) = 3,243 ms; tau_act(Src529) = 1,113 ms). When HCN4 was cotransfected with a dominant negative Src protein (Src296), the HCN4 activation was shifted more negative to a smaller degree (HCN4: V1/2 = -93 mV; Src296: V1/2 = -98 mV; statistically insignificant) and the activation kinetics were slowed at most test potentials (eg, at -95 mV, tau_act(Src296) = 7,396 ms). Neither Src529 nor Src296 significantly altered HCN4 current density. Coimmunoprecipitation experiments revealed that Src forms a complex with HCN4 in HEK293 cells and in rat ventricular myocytes. Our data provide a novel mechanism of if regulation by Src tyrosine phosphorylation.

Effect of protein tyrosine kinase inhibitors on the current through the CaV3.1 channel

In the present study, we have investigated the effects of protein tyrosine kinase (PTK) inhibitors on the Ca(V)3.1 calcium channel stably transfected in HEK293 cells using the whole-cell configuration of the patch-clamp technique. We have tested two different tyrosine kinase inhibitors, genistein and tyrphostin AG213, and their inactive analogs, genistin and tyrphostin AG9. Bath application of genistein, but not genistin, decreased the T-type calcium current amplitude in a concentration-dependent manner with an IC(50) of 24.7+/-2.0 microM. This effect of genistein was accompanied by deceleration of channel activation and acceleration of channel inactivation. Intracellular application of neither genistein nor genistin had a significant effect on the calcium current. Extracellular application of 50 microM tyrphostin AG213 and its inactive analogue, tyrphostin AG9, did not affect the current through the Ca(V)3.1 channel. The effect of genistein on the channel was also not affected by the presence of catalytically active PTK, p60(c-src) inside the cell. We have concluded that genistein directly inhibited the channel. This mechanism does not involve a PTK-dependent pathway. The alteration of the channel kinetics by genistein suggests an interaction with the voltage sensor of the channel together with the channel pore occlusion.

Pacemaker current and automatic rhythms: toward a molecular understanding

The ionic basis of automaticity in the sinoatrial node and His-Purkinje system, the primary and secondary cardiac pacemaking regions, is discussed. Consideration is given to potential targets for pharmacologic or genetic therapies of rhythm disorders. An ideal target would be an ion channel that functions only during diastole, so that action potential repolarization is not affected, and one that exhibits regional differences in expression and/or function so that the primary and secondary pacemakers can be selectively targeted. The so-called pacemaker current, If, generated by the HCN gene family, best fits these criteria. The biophysical and molecular characteristics of this current are reviewed, and progress to date in developing selective pharmacologic agents targeting If and in using gene and cell-based therapies to modulate the current are reviewed.

Direct inhibition of the pacemaker (If) current in rabbit sinoatrial node cells by genistein

Genistein is a tyrosine kinase inhibitor which interferes with the activity of several ionic channels either by altering modulatory phosphorylating processes or by direct binding. In whole-cell conditions, genistein induces a partial inhibition of the pacemaker (I(f)) current recorded in cardiac sinoatrial and ventricular myocytes. We investigated the mechanism of action of genistein (50 microM) on the I(f) current in whole-cell, cell-attached, and inside-out configurations, and the measured fractional inhibitions were similar: 26.6, 27.2, and 33.6%, respectively. When ATP was removed from the whole-cell pipette solution no differences were revealed in the effect of the drug when compared to metabolically active cells. Genistein fully maintained its blocking ability even when herbimycin, a tyrosine kinase inhibitor, was added to the whole-cell ATP-free pipette solution. Genistein-induced block was independent of the gating state of the channel and did not display voltage or current dependence; this independence distinguishes genistein from all other f-channel blockers. When inside-out experiments were performed to test for a direct interaction with the channel, genistein, superfused on the intracellular side of the membrane, decreased the maximal I(f) conductance, and slightly shifted the current-activation curve to the left. Furthermore, the effect of genistein was independent of cAMP modulation. We conclude that, in addition to its tyrosine kinase-inhibitory properties, genistein also blocks I(f) by directly interacting with the channel, and thus cannot be considered a valuable pharmacological tool to investigate phosphorylation-dependent modulatory pathways of the I(f) current and of cardiac rhythm.

A novel mechanism of modulation of hyperpolarization-activated cyclic nucleotide-gated channels by Src kinase

Hyperpolarization-activated cyclic nucleotide-gated channels (HCN1-4) play a crucial role in the regulation of cell excitability. Importantly, they contribute to spontaneous rhythmic activity in brain and heart. HCN channels are principally activated by membrane hyperpolarization and binding of cAMP. Here, we identify tyrosine phosphorylation by Src kinase as another mechanism affecting channel gating. Inhibition of Src by specific blockers slowed down activation kinetics of native and heterologously expressed HCN channels. The same effect on HCN channel activation was observed in cells cotransfected with a dominant-negative Src mutant. Immunoprecipitation demonstrated that Src binds to and phosphorylates native and heterologously expressed HCN2. Src interacts via its SH3 domain with a sequence of HCN2 encompassing part of the C-linker and the cyclic nucleotide binding domain. We identified a highly conserved tyrosine residue in the C-linker of HCN channels (Tyr476 in HCN2) that confers modulation by Src. Replacement of this tyrosine by phenylalanine in HCN2 or HCN4 abolished sensitivity to Src inhibitors. Mass spectrometry confirmed that Tyr476 is phosphorylated by Src. Our results have functional implications for HCN channel gating. Furthermore, they indicate that tyrosine phosphorylation contributes in vivo to the fine tuning of HCN channel activity.

Phosphorylation of a tyrosine at the N-terminus regulates the surface expression of GIRK5 homomultimers

The G protein-coupled inwardly rectifying GIRK5 and Delta5GIRK5 splicing variants do not express functional potassium channels. In contrast, Delta25GIRK5 forms functional homomultimers in Xenopus laevis oocytes. A tyrosine is present at the N-term of the non-functional isoforms. We studied the effect of endogenous tyrosine phosphorylation on the GIRK5 surface and functional expression. Unlike wild type channels, GIRK5Y16A and Delta5GIRK5Y16A mutants displayed inwardly rectifying currents and inhibitors of Src tyrosine kinase promoted the traffiking of GIRK5 to the cell surface. This is the first evidence that endogenous phosphorylation of a tyrosine residue in a GIRK channel inhibits its surface expression.

Temperature Affects Voltage-Sensitive Conductances Differentially in Octopus Cells of the Mammalian Cochlear Nucleus

Temperature is an important physiological variable the influence of which on macroscopic electrophysiological measurements in slices is not well documented. We show that each of three voltage-sensitive conductances of octopus cells of the mammalian ventral cochlear nucleus (VCN) is affected differently by changes in temperature. As expected, the kinetics of the currents were faster at higher than at lower temperature. Where they could be measured, time constants of activation, deactivation, and inactivation had Q10 values between 1.8 and 4.6. The magnitude of the peak conductances was differentially affected by temperature. While the peak magnitude of the high-voltage-activated K+ conductance, gKH, was unaffected by changes in temperature, the peak of the low-voltage-activated K+ conductance, gKL, was reduced by half when the temperature was lowered from 33 to 23°C ( Q10 = 2). Changing the temperature changed the kinetics and the magnitude of the hyperpolarization-activated mixed cation conductance, gh, but the changes in magnitude were transient. The voltage sensitivity of the three conductances was unaffected by temperature. The action of temperature on these conductances is reflected in the resting potentials and in the shapes of action potentials.

Regulation of recombinant and native hyperpolarization-activated cation channels

Ionic currents generated by hyperpolarization-activated cation-nonselective (HCN) channels have been principally known as pacemaker h-currents (Ih), because they allow cardiac and neuronal cells to be rhythmically active over precise intervals of time. Presently, these currents are implicated in numerous additional cellular functions, including neuronal integration, synaptic transmission, and sensory reception. These roles are accomplished by virtue of the regulation of Ih by both voltage and ligands. The article summarizes recent developments on the properties and allosteric interactions of these two regulatory pathways in cloned and native channels. Additionally, it discusses how the expression and properties of native channels may be controlled via regulation of the transcription of the HCN channel gene family and the assembly of channel subunits. Recently, several cardiac and neurological diseases were found to be intimately associated with a dysregulation of HCN gene transcription, suggesting that HCN-mediated currents may be involved in the pathophysiology of excitable systems. As a starting point, we briefly review the general characteristics of Ih and the regulatory mechanisms identified in heterologously expressed HCN channels.

An arginine residue in the pore region is a key determinant of chloride dependence in cardiac pacemaker channels

The modulation of ion channel activity by extracellular ions plays a central role in the control of heart function. Here, we show that the sinoatrial pacemaker current I(f) is strongly affected by the extracellular Cl- concentration. We investigated the molecular basis of the Cl- dependence in heterologously expressed hyperpolarization-activated cyclic nucleotide-gated (HCN) channels that represent the molecular correlate of I(f). Currents carried by the two cardiac HCN channel isoforms (HCN2 and HCN4) showed the same strong Cl- dependence as the sinoatrial I(f) and decreased to about 10% in the absence of external Cl-. In contrast, the neuronal HCN1 current was reduced to only 50% under the same conditions. Depletion of Cl- did not affect the voltage dependence of activation or the ion selectivity of the channels, indicating that the reduction of I(f) was caused by a decrease of channel conductance. A series of chimeras between HCN1 and HCN2 was constructed to identify the structural determinants underlying the different Cl- dependence of HCN1 and HCN2. Exchange of the ion-conducting pore region was sufficient to switch the Cl- dependence from HCN1- to HCN2-type and vice versa. Replacement of a single alanine residue in the pore of HCN1 (Ala-352) by an arginine residue present in HCN2 at equivalent position (Arg-405) induced HCN2-type chloride sensitivity in HCN1. Our data indicate that Arg-405 is a key component of a domain that allosterically couples Cl- binding with channel activation.