A synergistic blocking effect of Mg²⁺ and spermine on the inward rectifier K⁺ (Kir2.1) channel pore

Genetically engineered HEK cells as a valuable tool for studying electroporation in excitable cells

Electric pulses used in electroporation-based treatments have been shown to affect the excitability of muscle and neuronal cells. However, understanding the interplay between electroporation and electrophysiological response of excitable cells is complex, since both ion channel gating and electroporation depend on dynamic changes in the transmembrane voltage (TMV). In this study, a genetically engineered human embryonic kidney cells expressing NaV1.5 and Kir2.1, a minimal complementary channels required for excitability (named S-HEK), was characterized as a simple cell model used for studying the effects of electroporation in excitable cells. S-HEK cells and their non-excitable counterparts (NS-HEK) were exposed to 100 µs pulses of increasing electric field strength. Changes in TMV, plasma membrane permeability, and intracellular Ca2+ were monitored with fluorescence microscopy. We found that a very mild electroporation, undetectable with the classical propidium assay but associated with a transient increase in intracellular Ca2+, can already have a profound effect on excitability close to the electrostimulation threshold, as corroborated by multiscale computational modelling. These results are of great relevance for understanding the effects of pulse delivery on cell excitability observed in context of the rapidly developing cardiac pulsed field ablation as well as other electroporation-based treatments in excitable tissues.

Nutrigenomics of inward rectifier potassium channels

Inwardly rectifying potassium (Kir) channels play a key role in maintaining the resting membrane potential and supporting potassium homeostasis. There are many variants of Kir channels, which are usually tetramers in which the main subunit has two trans-membrane helices attached to two N- and C-terminal cytoplasmic tails with a pore-forming loop in between that contains the selectivity filter. These channels have domains that are strongly modulated by molecules present in nutrients found in different diets, such as phosphoinositols, polyamines and Mg²⁺. These molecules can impact these channels directly or indirectly, either allosterically by modulation of enzymes or via the regulation of channel expression. A particular type of these channels is coupled to cell metabolism and inhibited by ATP (KATP channels, essential for insulin release and for the pathogenesis of metabolic diseases like diabetes mellitus). Genomic changes in Kir channels have a significant impact on metabolism, such as conditioning the nutrients and electrolytes that an individual can take. Thus, the nutrigenomics of ion channels is an important emerging field in which we are attempting to understand how nutrients and diets can affect the activity and expression of ion channels and how genomic changes in such channels may be the basis for pathological conditions that limit nutrition and electrolyte intake. In this contribution we briefly review Kir channels, discuss their nutrigenomics, characterize how different components in the diet affect their function and expression, and suggest how their genomic changes lead to pathological phenotypes that affect diet and electrolyte intake.

Non-trivial dynamics in a model of glial membrane voltage driven by open potassium pores

Despite the molecular evidence that a nearly linear steady-state current-voltage relationship in mammalian astrocytes reflects a total current resulting from more than one differentially regulated K+ conductance, detailed ordinary differential equation (ODE) models of membrane voltage Vm are still lacking. Various experimental results reporting altered rectification of the major Kir currents in glia, dominated by Kir4.1, have motivated us to develop a detailed model of Vm dynamics incorporating the weaker potassium K2P-TREK1 current in addition to Kir4.1, and study the stability of the resting state Vr. The main question is whether, with the loss of monotonicity in glial I-V curve resulting from altered Kir rectification, the nominal resting state Vr remains stable, and the cell retains the trivial, potassium electrode behavior with Vm after EK. The minimal two-dimensional model of Vm near Vr showed that an N-shape deformed Kir I-V curve induces multistability of Vm in a model that incorporates K2P activation kinetics, and nonspecific K+ leak currents. More specifically, an asymmetrical, nonlinear decrease of outward Kir4.1 conductance, turning the channels into inward rectifiers, introduces instability of Vr. That happens through a robust bifurcation giving birth to a second, more depolarized stable resting state Vdr > -10 mV. Realistic recordings from electrographic seizures were used to perturb the model. Simulations of the model perturbed by constant current through gap junctions and seizure-like discharges as local field potentials led to depolarization and switching of Vm between the two stable states, in a downstate-upstate manner. In the event of prolonged depolarizations near Vdr, such catastrophic instability would affect all aspects of the glial function, from metabolic support to membrane transport, and practically all neuromodulatory roles assigned to glia.

Anomalous enhancement of resurgent Na+ currents at high temperatures by SCN9A mutations underlies the episodic heat-enhanced pain in inherited erythromelalgia

Inherited erythromelalgia (IEM), caused by mutations in Na_v1.7 channel is characterized by episodic neuropathic pain triggered especially by warm temperature. However, the mechanism underlying the temperature–dependent episodic attacks of IEM remains elusive. We investigated the electrophysiological effect of temperature changes on Na_v1.7 channels with three different mutations, p.I136V, p. I848T, and p.V1316A, both in vitro and in vivo. In vitro biophysical studies of the mutant channels show consistent temperature-dependent enhancement of the relative resurgent currents if normalized to the transient currents, as well as temperature-dependent changes in the time to peak and the kinetics of decay of the resurgent currents, but no congruent temperature–dependent changes in steady–state parameters such as shift of activation/inactivation curves and changes of the absolute size of the window or resurgent currents. In vivo nerve excitability tests (NET) in IEM patients reveal the essentially normal indices of NET at a single stimulus. However, there are evident abnormalities if assessed with preconditioning pulses, such as the decrease of threshold elevation in hyperpolarizing threshold electrotonus (50–100 ms), the increase of inward rectification in current–voltage curve, and the increase of refractoriness at the interpulse interval of 2–6 ms in recovery cycle, probably also implicating derangements in temperature dependence of inactivation and of recovery from inactivation in the mutant channels. The pathogenesis of heat–enhanced pain in IEM could be attributed to deranged temperature dependence of Na_v1.7 channels responsible for the genesis of resurgent currents.

The immature electrophysiological phenotype of iPSC-CMs still hampers in vitro drug screening: Special focus on IK1

Preclinical drug screens are not based on human physiology, possibly complicating predictions on cardiotoxicity. Drug screening can be humanised with in vitro assays using human induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs). However, in contrast to adult ventricular cardiomyocytes, iPSC-CMs beat spontaneously due to presence of the pacemaking current I_f and reduced densities of the hyperpolarising current I_K1. In adult cardiomyocytes, I_K1 finalises repolarisation by stabilising the resting membrane potential while also maintaining excitability. The reduced I_K1 density contributes to proarrhythmic traits in iPSC-CMs, which leads to an electrophysiological phenotype that might bias drug responses. The proarrhythmic traits can be suppressed by increasing I_K1 in a balanced manner. We systematically evaluated all studies that report strategies to mature iPSC-CMs and found that only few studies report I_K1 current densities. Furthermore, these studies did not succeed in establishing sufficient I_K1 levels as they either added too little or too much I_K1. We conclude that reduced densities of I_K1 remain a major flaw in iPSC-CMs, which hampers their use for in vitro drug screening.

The Biophysical Basis Underlying Gating Changes in the p.V1316A Mutant Nav1.7 Channel and the Molecular Pathogenesis of Inherited Erythromelalgia

The Na_v1.7 channel critically contributes to the excitability of sensory neurons, and gain-of-function mutations of this channel have been shown to cause inherited erythromelalgia (IEM) with neuropathic pain. In this study, we report a case of a severe phenotype of IEM caused by p.V1316A mutation in the Na_v1.7 channel. Mechanistically, we first demonstrate that the Na_vβ4 peptide acts as a gating modifier rather than an open channel blocker competing with the inactivating peptide to give rise to resurgent currents in the Na_v1.7 channel. Moreover, there are two distinct open and two corresponding fast inactivated states in the genesis of resurgent Na⁺ currents. One is responsible for the resurgent route and practically existent only in the presence of Na_vβ4 peptide, whereas the other is responsible for the “silent” route of recovery from inactivation. In this regard, the p.V1316A mutation makes hyperpolarization shift in the activation curve, and depolarization shift in the inactivation curve, vividly uncoupling inactivation from activation. In terms of molecular gating operation, the most important changes caused by the p.V1316A mutation are both acceleration of the transition from the inactivated states to the activated states and deceleration of the reverse transition, resulting in much larger sustained as well as resurgent Na⁺ currents. In summary, the genesis of the resurgent currents in the Na_v1.7 channel is ascribable to the transient existence of a distinct and novel open state promoted by the Na_vβ4 peptide. In addition, S4–5 linker in domain III where V1316 is located seems to play a critical role in activation–inactivation coupling, chiefly via direct modulation of the transitional kinetics between the open and the inactivated states. The sustained and resurgent Na⁺ currents may therefore be correlatively enhanced by specific mutations involving this linker and relevant regions, and thus marked hyperexcitability in corresponding neural tissues as well as IEM symptomatology.

Mutations in the Na_v1.7 sodium channel cause idiopathic erythromelalgia. This study shows that the pathogenic resurgent sodium currents arise via modification of gating behavior rather than via competing pore block by the Na_vβ4 peptide.

The gain-of-function mutation (p.V1316A) of the Na_v1.7 channel causes inherited erythromelalgia (IEM), a disease characterized by extremely enhanced activity in relevant neural tissues that results in neuropathic pain. We found that the p.V1316A mutation alters the basic gating properties of the channel, leading to increased sustained currents during membrane depolarization and resurgent currents during repolarization. Neurons expressing these mutant channels are more difficult to maintain in a hyperpolarized state and are thus more excitable. We demonstrate that there is very likely a distinct set of open/inactivated (O/I) states responsible for the genesis of resurgent currents. We show that the p.V1316A mutation chiefly accelerates the I to O transition in this set, but also decelerates the transitions between different sets of O/I states, to cause the channel gating and cellular excitability changes. Contrary to the conventional view, we find that the Na_vβ4 peptide, a key element responsible for sizable resurgent currents, does not seem to act as a pore blocker that competes with the inactivation peptide. Instead, we show that it acts as a gating modifier of the Na_v1.7 channel. Thus, the DIII/S4–5 linker, where V1316 is located, may play a critical role not only in O/I coupling but also in the couplings between different sets of O/I in the Na_v1.7 channel.