Effects of external acidosis on HERG current expressed in Xenopus oocytes

hERG1 channel subunit composition mediates proton inhibition of rapid delayed rectifier potassium current (IKr) in cardiomyocytes derived from hiPSCs

The voltage-gated channel, hERG1, conducts the rapid delayed rectifier potassium current (IKr) and is critical for human cardiac repolarization. Reduced IKr causes long QT syndrome and increases the risk for cardiac arrhythmia and sudden death. At least two subunits form functional hERG1 channels, hERG1a and hERG1b. Changes in hERG1a/1b abundance modulate IKr kinetics, magnitude, and drug sensitivity. Studies from native cardiac tissue suggest that hERG1 subunit abundance is dynamically regulated, but the impact of altered subunit abundance on IKr and its response to external stressors is not well understood. Here, we used a substrate-driven human-induced pluripotent stem cell-derived cardiomyocyte (hiPSC-CM) maturation model to investigate how changes in relative hERG1a/1b subunit abundance impact the response of native IKr to extracellular acidosis, a known component of ischemic heart disease and sudden infant death syndrome. IKr recorded from immatured hiPSC-CMs displays a 2-fold greater inhibition by extracellular acidosis (pH 6.3) compared with matured hiPSC-CMs. Quantitative RT-PCR and immunocytochemistry demonstrated that hERG1a subunit mRNA and protein were upregulated and hERG1b subunit mRNA and protein were downregulated in matured hiPSC-CMs compared with immatured hiPSC-CMs. The shift in subunit abundance in matured hiPSC-CMs was accompanied by increased IKr. Silencing hERG1b's impact on native IKr kinetics by overexpressing a polypeptide identical to the hERG1a N-terminal Per-Arnt-Sim domain reduced the magnitude of IKr proton inhibition in immatured hiPSC-CMs to levels comparable to those observed in matured hiPSC-CMs. These data demonstrate that hERG1 subunit abundance is dynamically regulated and determines IKr proton sensitivity in hiPSC-CMs.

Fentanyl-Induced Block of hERG Channels Is Exacerbated by Hypoxia, Hypokalemia, Alkalosis, and the Presence of hERG1b

Human ether-a-go-go-related gene (hERG) encodes the pore-forming subunit of the rapidly activating delayed rectifier potassium current (I_Kr) important for repolarization of cardiac action potentials. Drug-induced disruption of hERG channel function is a main cause of acquired long QT syndrome, which can lead to ventricular arrhythmias and sudden death. Illicit fentanyl use is associated with sudden death. We have demonstrated that fentanyl blocks hERG current (I_hERG) at concentrations that overlap with the upper range of postmortem blood concentrations in fentanyl-related deaths. Since fentanyl can cause respiratory depression and electrolyte imbalances, in the present study we investigated whether certain pathologic circumstances exacerbate fentanyl-induced block of I_hERG Our results show that chronic hypoxia or hypokalemia additively reduced I_hERG with fentanyl. As well, high pH potentiated the fentanyl-mediated block of hERG channels, with an IC₅₀ at pH 8.4 being 7-fold lower than that at pH 7.4. Furthermore, although the full-length hERG variant, hERG1a, has been widely used to study hERG channels, coexpression with the short variant, hERG1b (which does not produce current when expressed alone), produces functional hERG1a/1b channels, which gate more closely resembling native I_Kr Our results showed that fentanyl blocked hERG1a/1b channels with a 3-fold greater potency than hERG1a channels. Thus, in addition to a greater susceptibility due to the presence of hERG1b in the human heart, hERG channel block by fentanyl can be exacerbated by certain conditions, such as hypoxia, hypokalemia, or alkalosis, which may increase the risk of fentanyl-induced ventricular arrhythmias and sudden death. SIGNIFICANCE STATEMENT: This work demonstrates that heterologously expressed human ether a-go-go-related gene (hERG) 1a/1b channels, which more closely resemble rapidly activating delayed rectifier potassium current in the human heart, are blocked by fentanyl with a 3-fold greater potency than the previously studied hERG1a expressed alone. Additionally, chronic hypoxia, hypokalemia, and alkalosis can increase the block of hERG current by fentanyl, potentially increasing the risk of cardiac arrhythmias and sudden death.

Modulation of hERG K+ Channel Deactivation by Voltage Sensor Relaxation

The hERG (human-ether-à-go-go-related gene) channel underlies the rapid delayed rectifier current, I_kr, in the heart, which is essential for normal cardiac electrical activity and rhythm. Slow deactivation is one of the hallmark features of the unusual gating characteristics of hERG channels, and plays a crucial role in providing a robust current that aids repolarization of the cardiac action potential. As such, there is significant interest in elucidating the underlying mechanistic determinants of slow hERG channel deactivation. Recent work has shown that the hERG channel S4 voltage sensor is stabilized following activation in a process termed relaxation. Voltage sensor relaxation results in energetic separation of the activation and deactivation pathways, producing a hysteresis, which modulates the kinetics of deactivation gating. Despite widespread observation of relaxation behaviour in other voltage-gated K⁺ channels, such as Shaker, Kv1.2 and Kv3.1, as well as the voltage-sensing phosphatase Ci-VSP, the relationship between stabilization of the activated voltage sensor by the open pore and voltage sensor relaxation in the control of deactivation has only recently begun to be explored. In this review, we discuss present knowledge and questions raised related to the voltage sensor relaxation mechanism in hERG channels and compare structure-function aspects of relaxation with those observed in related ion channels. We focus discussion, in particular, on the mechanism of coupling between voltage sensor relaxation and deactivation gating to highlight the insight that these studies provide into the control of hERG channel deactivation gating during their physiological functioning.

Extracellular protons accelerate hERG channel deactivation by destabilizing voltage sensor relaxation

The human ether-à-go-go–related gene (hERG) encodes a delayed rectifier K⁺ channel with slow deactivation gating. Shi et al. find that acidic residues on S3 contribute to slow deactivation kinetics by stabilizing the relaxed state of the voltage sensor, which can be mitigated by extracellular protons.

hERG channels underlie the delayed-rectifier K⁺ channel current (I_Kr), which is crucial for membrane repolarization and therefore termination of the cardiac action potential. hERG channels display unusually slow deactivation gating, which contributes to a resurgent current upon repolarization and may protect against post-depolarization–induced arrhythmias. hERG channels also exhibit robust mode shift behavior, which reflects the energetic separation of activation and deactivation pathways due to voltage sensor relaxation into a stable activated state. The mechanism of relaxation is unknown and likely contributes to slow hERG channel deactivation. Here, we use extracellular acidification to probe the structural determinants of voltage sensor relaxation and its influence on the deactivation gating pathway. Using gating current recordings and voltage clamp fluorimetry measurements of voltage sensor domain dynamics, we show that voltage sensor relaxation is destabilized at pH 6.5, causing an ∼20-mV shift in the voltage dependence of deactivation. We show that the pH dependence of the resultant loss of mode shift behavior is similar to that of the deactivation kinetics acceleration, suggesting that voltage sensor relaxation correlates with slower pore gate closure. Neutralization of D509 in S3 also destabilizes the relaxed state of the voltage sensor, mimicking the effect of protons, suggesting that acidic residues on S3, which act as countercharges to S4 basic residues, are involved in stabilizing the relaxed state and slowing deactivation kinetics. Our findings identify the mechanistic determinants of voltage sensor relaxation and define the long-sought mechanism by which protons accelerate hERG deactivation.

External protons destabilize the activated voltage sensor in hERG channels

Extracellular acidosis shifts hERG channel activation to more depolarized potentials and accelerates channel deactivation; however, the mechanisms underlying these effects are unclear. External divalent cations, e.g., Ca(2+) and Cd(2+), mimic these effects and coordinate within a metal ion binding pocket composed of three acidic residues in hERG: D456 and D460 in S2 and D509 in S3. A common mechanism may underlie divalent cation and proton effects on hERG gating. Using two-electrode voltage clamp, we show proton sensitivity of hERG channel activation (pKa = 5.6), but not deactivation, was greatly reduced in the presence of Cd(2+) (0.1 mM), suggesting a common binding site for the Cd(2+) and proton effect on activation and separable effects of protons on activation and deactivation. Mutational analysis confirmed that D509 plays a critical role in the pH dependence of activation, as shown previously, and that cooperative actions involving D456 and D460 are also required. Importantly, neutralization of all three acidic residues abolished the proton-induced shift of activation, suggesting that the metal ion binding pocket alone accounts for the effects of protons on hERG channel activation. Voltage-clamp fluorimetry measurements demonstrated that protons shifted the voltage dependence of S4 movement to more depolarized potentials. The data indicate a site and mechanism of action for protons on hERG activation gating; protonation of D456, D460 and D509 disrupts interactions between these residues and S4 gating charges to destabilize the activated configuration of S4.

External pH modulates EAG superfamily K+ channels through EAG-specific acidic residues in the voltage sensor

The Ether-a-go-go (EAG) superfamily of voltage-gated K⁺ channels consists of three functionally distinct gene families (Eag, Elk, and Erg) encoding a diverse set of low-threshold K⁺ currents that regulate excitability in neurons and muscle. Previous studies indicate that external acidification inhibits activation of three EAG superfamily K⁺ channels, Kv10.1 (Eag1), Kv11.1 (Erg1), and Kv12.1 (Elk1). We show here that Kv10.2, Kv12.2, and Kv12.3 are similarly inhibited by external protons, suggesting that high sensitivity to physiological pH changes is a general property of EAG superfamily channels. External acidification depolarizes the conductance–voltage (GV) curves of these channels, reducing low threshold activation. We explored the mechanism of this high pH sensitivity in Kv12.1, Kv10.2, and Kv11.1. We first examined the role of acidic voltage sensor residues that mediate divalent cation block of voltage activation in EAG superfamily channels because protons reduce the sensitivity of Kv12.1 to Zn²⁺. Low pH similarly reduces Mg²⁺ sensitivity of Kv10.1, and we found that the pH sensitivity of Kv11.1 was greatly attenuated at 1 mM Ca²⁺. Individual neutralizations of a pair of EAG-specific acidic residues that have previously been implicated in divalent block of diverse EAG superfamily channels greatly reduced the pH response in Kv12.1, Kv10.2, and Kv11.1. Our results therefore suggest a common mechanism for pH-sensitive voltage activation in EAG superfamily channels. The EAG-specific acidic residues may form the proton-binding site or alternatively are required to hold the voltage sensor in a pH-sensitive conformation. The high pH sensitivity of EAG superfamily channels suggests that they could contribute to pH-sensitive K⁺ currents observed in vivo.

hERG K+ Channels: Structure, Function, and Clinical Significance

The human ether-a-go-go related gene (hERG) encodes the pore-forming subunit of the rapid component of the delayed rectifier K+ channel, Kv11.1, which are expressed in the heart, various brain regions, smooth muscle cells, endocrine cells, and a wide range of tumor cell lines. However, it is the role that Kv11.1 channels play in the heart that has been best characterized, for two main reasons. First, it is the gene product involved in chromosome 7-associated long QT syndrome (LQTS), an inherited disorder associated with a markedly increased risk of ventricular arrhythmias and sudden cardiac death. Second, blockade of Kv11.1, by a wide range of prescription medications, causes drug-induced QT prolongation with an increase in risk of sudden cardiac arrest. In the first part of this review, the properties of Kv11.1 channels, including biogenesis, trafficking, gating, and pharmacology are discussed, while the second part focuses on the pathophysiology of Kv11.1 channels.

Proton block of the pore underlies the inhibition of hERG cardiac K+ channels during acidosis

Human ether-a-go-go-related gene (hERG) potassium channels are critical determinants of cardiac repolarization. Loss of function of hERG channels is associated with Long QT Syndrome, arrhythmia, and sudden death. Acidosis occurring as a result of myocardial ischemia inhibits hERG channel function and may cause a predisposition to arrhythmias. Acidic pH inhibits hERG channel maximal conductance and accelerates deactivation, likely by different mechanisms. The mechanism underlying the loss of conductance has not been demonstrated and is the focus of the present study. The data presented demonstrate that, unlike in other voltage-gated potassium (Kv) channels, substitution of individual histidine residues did not abolish the pH dependence of hERG channel conductance. Abolition of inactivation, by the mutation S620T, also did not affect the proton sensitivity of channel conductance. Instead, voltage-dependent channel inhibition (δ = 0.18) indicative of pore block was observed. Consistent with a fast block of the pore, hERG S620T single channel data showed an apparent reduction of the single channel current amplitude at low pH. Furthermore, the effect of protons was relieved by elevating external K(+) or Na(+) and could be modified by charge introduction within the outer pore. Taken together, these data strongly suggest that extracellular protons inhibit hERG maximal conductance by blocking the external channel pore.

Regulation of the voltage-insensitive step of HERG activation by extracellular pH

Human ether-à-go-go-related gene (HERG, Kv11.1, KCNH2) voltage-gated K(+) channels dominate cardiac action potential repolarization. In addition, HERG channels play a role in neuronal and smooth cell excitability as well as cancer pathology. Extracellular pH (pH(o)) is modified during myocardial ischemia, inflammation, and respiratory alkalosis, so understanding the response of HERG channels to changes in pH is of clinical significance. The relationship between pH(o) and HERG channel gating appears complex. Acidification has previously been reported to speed, slow, or have no effect on activation. We therefore undertook comprehensive analysis of the effect of pH(o) on HERG channel activation. HERG channels have unique and complex activation gating characteristics with both voltage-sensitive and voltage-insensitive steps in the activation pathway. Acidosis decreased the activation rate, suppressed peak current, and altered the sigmoidicity of gating near threshold potentials. At positive voltages, where the voltage-insensitive transition is rate limiting, pH(o) modified the voltage-insensitive step with a pK(a) similar to that of histidine. Hill coefficient analysis was incompatible with a coefficient of 1 but was well described by a Hill coefficient of 4. We derived a pH(o)-sensitive term for a five-state Markov model of HERG channel gating. This model demonstrates the mechanism of pH(o) sensitivity in HERG channel activation. Our experimental data and mathematical model demonstrate that the pH(o) sensitivity of HERG channel activation is dominated by the pH(o) sensitivity of the voltage-insensitive step, in a fashion that is compatible with the presence of at least one proton-binding site on each subunit of the channel tetramer.

Extracellular potassium effects are conserved within the rat erg K+ channel family

The biophysical properties of native cardiac erg1 and recombinant HERG1 channels have been shown to be influenced by the extracellular K(+) concentration ([K(+)](o)). The erg1 conductance, for example, increases dramatically with a rise in [K(+)](o). In the brain, where local [K(+)](o) can change considerably with the extent of physiological and pathophysiological neuronal activity, all three erg channel subunits are expressed. We have now investigated and compared the effects of an increase in [K(+)](o) from 2 to 10 mm on the three rat erg channels heterologously expressed in CHO cells. Upon increasing [K(+)](o), the voltage dependence of activation was shifted to more negative potentials for erg1 (DeltaV(0.5) = -4.0 +/- 1.1 mV, n = 28) and erg3 (DeltaV(0.5) = -8.4 +/- 1.2 mV, n = 25), and was almost unchanged for erg2 (DeltaV(0.5) = -2.0 +/- 1.3 mV, n = 6). For all three erg channels, activation kinetics were independent of [K(+)](o), but the slowing of inactivation by increased [K(+)](o) was even more pronounced for erg2 and erg3 than for erg1. In addition, with increased [K(+)](o), all three erg channels exhibited significantly slower time courses of recovery from inactivation and of deactivation. Whole-cell erg-mediated conductance was determined at the end of 4 s depolarizing pulses as well as with 1 s voltage ramps starting from the fully activated state. The rise in [K(+)](o) resulted in increased conductance values for all three erg channels which were more pronounced for erg2 (factor 3-4) than for erg1 (factor 2.5-3) and erg3 (factor 2-2.5). The data demonstrate that most [K(+)](o)-dependent changes in the biophysical properties are well conserved within the erg K(+) channel family, despite gradual differences in the magnitude of the effects.

Modulation of homomeric and heteromeric KCNQ1 channels by external acidification

The I(KS) K(+) channel plays a major role in repolarizing the cardiac action potential. It consists of an assembly of two structurally distinct alpha and beta subunits called KCNQ1 and KCNE1, respectively. Using two different expression systems, Xenopus oocytes and Chinese hamster ovary cells, we investigated the effects of external protons on homomeric and heteromeric KCNQ1 channels. External acidification (from pH 7.4 to pH 5.5) markedly decreased the homomeric KCNQ1 current amplitude and caused a positive shift (+25 mV) in the voltage dependence of activation. Low external pH (pH(o)) also slowed down the activation and deactivation kinetics and strongly reduced the KCNQ1 inactivation process. In contrast, external acidification reduced the maximum conductance and the macroscopic inactivation of the KCNQ1 mutant L273F by only a small amount. The heteromeric I(KS) channel complex was weakly affected by low pH(o), with minor effects on I(KS) current amplitude. However, substantial current inhibition was produced by protons with the N-terminal KCNE1 deletion mutant Delta11-38. Low pH(o) increased the current amplitude of the pore mutant V319C when co-expressed with KCNE1. The slowing of I(KS) deactivation produced by low pH(o) was absent in the KCNE1 mutant Delta39-43, suggesting that the residues lying at the N-terminal boundary of the transmembrane segment are involved in this process. In all, our results suggest that external acidification acts on homomeric and heteromeric KCNQ1 channels via multiple mechanisms to affect gating and maximum conductance. The external pH effects on I(Kr) versus I(KS) may be important determinants of arrhythmogenicity under conditions of cardiac ischaemia and reperfusion.