Cytoskeletal roles in cardiac ion channel expression

The cytoskeleton and cardiac ion channel expression are closely linked. From the time that newly synthesized channels exit the endoplasmic reticulum, they are either traveling along the microtubule or actin cytoskeletons or likely anchored in the plasma membrane or in internal vesicular pools by those scaffolds. Molecular motors, small GTPases and even the dynamics of the cytoskeletons themselves influence the trafficking and expression of the channels. In some cases, the functioning of the channels themselves has profound influences on the cytoskeleton. Here we provide an overview of the current state of knowledge on the involvement of the actin and microtubule cytoskeletons in the trafficking, targeting and expression of cardiac ion channels and a few channels expressed elsewhere. We highlight, also, some of the many questions that remain about these processes. This article is part of a Special Issue entitled: Reciprocal influences between cell cytoskeleton and membrane channels, receptors and transporters. Guest Editor: Jean Claude Hervé.

Components of gating charge movement and S4 voltage-sensor exposure during activation of hERG channels

The human ether-á-go-go–related gene (hERG) K⁺ channel encodes the pore-forming α subunit of the rapid delayed rectifier current, I_Kr, and has unique activation gating kinetics, in that the α subunit of the channel activates and deactivates very slowly, which focuses the role of I_Kr current to a critical period during action potential repolarization in the heart. Despite its physiological importance, fundamental mechanistic properties of hERG channel activation gating remain unclear, including how voltage-sensor movement rate limits pore opening. Here, we study this directly by recording voltage-sensor domain currents in mammalian cells for the first time and measuring the rates of voltage-sensor modification by [2-(trimethylammonium)ethyl] methanethiosulfonate chloride (MTSET). Gating currents recorded from hERG channels expressed in mammalian tsA201 cells using low resistance pipettes show two charge systems, defined as Q₁ and Q₂, with V_1/2’s of −55.7 (equivalent charge, z = 1.60) and −54.2 mV (z = 1.30), respectively, with the Q₂ charge system carrying approximately two thirds of the overall gating charge. The time constants for charge movement at 0 mV were 2.5 and 36.2 ms for Q₁ and Q₂, decreasing to 4.3 ms for Q₂ at +60 mV, an order of magnitude faster than the time constants of ionic current appearance at these potentials. The voltage and time dependence of Q₂ movement closely correlated with the rate of MTSET modification of I521C in the outermost region of the S4 segment, which had a V_1/2 of −64 mV and time constants of 36 ± 8.5 ms and 11.6 ± 6.3 ms at 0 and +60 mV, respectively. Modeling of Q₁ and Q₂ charge systems showed that a minimal scheme of three transitions is sufficient to account for the experimental findings. These data point to activation steps further downstream of voltage-sensor movement that provide the major delays to pore opening in hERG channels.

The new antiarrhythmic drug vernakalant: ex vivo study of human atrial tissue from sinus rhythm and chronic atrial fibrillation

AIMS

Vernakalant is a newly developed antiarrhythmic drug against atrial fibrillation (AF). However, its electrophysiological actions on human myocardium are unknown.

METHODS AND RESULTS

Action potentials (APs) and ion currents were recorded in right atrial trabeculae and cardiomyocytes from patients in sinus rhythm (SR) and chronic AF. Vernakalant prolonged early repolarization in SR and AF, but late only in AF. AP amplitude (APA) and dV/dtmax were reduced in a concentration- and frequency-dependent manner with IC50 < 10 µM at >3 Hz. Effective refractory period was increased more than action potential duration (APD) in SR and AF. INa was blocked with IC50s of 95 and 84 µM for SR and AF, respectively (0.5 Hz). Vernakalant did not reduce outward potassium currents compared with time-matched controls. However, area under the current-time curve was reduced due to acceleration of current decline with IC50s of 19 and 12 µM for SR and AF, respectively. Vernakalant had less effect on APD than the IKr blocker E-4031, blocked IK,ACh, and had a small inhibitory effect on IK1 at 30 µM. L-Type Ca(2+) currents (SR) were reduced with IC50 of 84 µM.

CONCLUSION

Rate-dependent block of Na(+) channels represents the main antiarrhythmic mechanism of vernakalant in the fibrillating atrium. Open channel block of early transient outward currents and IK,ACh could also contribute.

Biolistic transfection of freshly isolated adult ventricular myocytes

Transfection of mammalian cells has long been an extremely powerful approach for the study of the effects of specific gene expression on cell function. Until recently, however, this approach has been unavailable for the study of gene function in adult cardiac myocytes. Here, an adaptation of the biolistic method to the transfection of adult cardiac myocytes is described. DNA is precipitated onto gold particles in the absence of PVP and the particles are biolistically delivered to freshly isolated adult rat cardiomyocytes via a Bio-Rad Helios System gene gun. The myocytes are cultured in the absence of bovine serum albumin and expression of the introduced genes, in phenotypically intact myocytes, is robust within 12-24 h.

External Ba(2+) block of Kv4.2 channels is enhanced in the closed-inactivated state

The effect of external barium ions on rat Kv4.2 channels expressed in HEK293 cells was investigated using whole cell, voltage-clamp recordings to determine its mechanism of action as well as its usefulness as a tool to probe the permeation pathway. Ba(2+) caused a concentration-dependent inhibition of current that was antagonized by increasing the external concentration of K(+) ([K(+)](o)), and the concentration and time dependence of the inhibition were well fitted by a model involving two binding sites aligned in series. Recovery from current inhibition was enhanced by increasing the intensity, duration, or frequency of depolarizing steps or by increasing [K(+)](o). These properties are consistent with the conclusion that Ba(2+) is a permeant ion that, by virtue of a stable interaction with a deep pore site, is able to block conduction. This blocking action was subsequently exploited to gain insights into the pore configuration in different channel states. In addition to blocking one or more states populated by brief depolarizing pulses to 80 mV, Ba(2+) blocked closed channels [the membrane voltage (V(m)) = -80 mV] and closed-inactivated channels (V(m) = -40 mV). Interestingly, the block of closed-inactivated channels was faster and more complete than for closed channels, which we interpret to mean that conformational changes underlying closed-state inactivation (CSI) enhance Ba(2+) binding and that the outer pore mouth remains patent during CSI. This provides the first direct evidence that an inactivation process involving a constriction of the outer pore mouth does not account for CSI in Kv4.2.

Basis for allosteric open-state stabilization of voltage-gated potassium channels by intracellular cations

The open state of voltage-gated potassium (Kv) channels is associated with an increased stability relative to the pre-open closed states and is reflected by a slowing of OFF gating currents after channel opening. The basis for this stabilization is usually assigned to intrinsic structural features of the open pore. We have studied the gating currents of Kv1.2 channels and found that the stabilization of the open state is instead conferred largely by the presence of cations occupying the inner cavity of the channel. Large impermeant intracellular cations such as N-methyl-d-glucamine (NMG⁺) and tetraethylammonium cause severe slowing of channel closure and gating currents, whereas the smaller cation, Cs⁺, displays a more moderate effect on voltage sensor return. A nonconducting mutant also displays significant open state stabilization in the presence of intracellular K⁺, suggesting that K⁺ ions in the intracellular cavity also slow pore closure. A mutation in the S6 segment used previously to enlarge the inner cavity (Kv1.2-I402C) relieves the slowing of OFF gating currents in the presence of the large NMG⁺ ion, suggesting that the interaction site for stabilizing ions resides within the inner cavity and creates an energetic barrier to pore closure. The physiological significance of ionic occupation of the inner cavity is underscored by the threefold slowing of ionic current deactivation in the wild-type channel compared with Kv1.2-I402C. The data suggest that internal ions, including physiological concentrations of K⁺, allosterically regulate the deactivation kinetics of the Kv1.2 channel by impairing pore closure and limiting the return of voltage sensors. This may represent a primary mechanism by which Kv channel deactivation kinetics is linked to ion permeation and reveals a novel role for channel inner cavity residues to indirectly regulate voltage sensor dynamics.

Dual effect of phosphatidylinositol (4,5)-bisphosphate PIP(2) on Shaker K(+) [corrected] channels

Phosphatidylinositol (4,5)-bisphosphate (PIP(2)) is a phospholipid of the plasma membrane that has been shown to be a key regulator of several ion channels. Functional studies and more recently structural studies of Kir channels have revealed the major impact of PIP(2) on the open state stabilization. A similar effect of PIP(2) on the delayed rectifiers Kv7.1 and Kv11.1, two voltage-gated K(+) channels, has been suggested, but the molecular mechanism remains elusive and nothing is known on PIP(2) effect on other Kv such as those of the Shaker family. By combining giant-patch ionic and gating current recordings in COS-7 cells, and voltage-clamp fluorimetry in Xenopus oocytes, both heterologously expressing the voltage-dependent Shaker channel, we show that PIP(2) exerts 1) a gain-of-function effect on the maximal current amplitude, consistent with a stabilization of the open state and 2) a loss-of-function effect by positive-shifting the activation voltage dependence, most likely through a direct effect on the voltage sensor movement, as illustrated by molecular dynamics simulations.

Trafficking of an endogenous potassium channel in adult ventricular myocytes

The roles of several small GTPases in the expression of an endogenous potassium current, I(to,f), in adult rat ventricular myocytes have been investigated. The results indicate that forward trafficking of newly synthesized Kv4.2, which underlies I(to,f) in these cells, requires both Rab1 and Sar1 function. Expression of a Rab1 dominant negative (DN) reduced I(to,f) current density by roughly one-half relative to control, mCherry-transfected myocytes. Similarly, expression of a Sar1DN nearly halved I(to,f) current density. Rab11 is not essential to trafficking of Kv4.2, as expression of a Rab11DN had no effect on I(to,f) over the time frames investigated here. In a process dependent on intact endoplasmic reticulum (ER)-to-Golgi transport, however, overexpression of wild-type Rab11 resulted in a doubling of I(to,f) density; block of ER-to-Golgi traffic by Brefeldin A completely abrogated the effect. Also implicated in the trafficking of Kv4.2 are Rab5 and Rab4. Rab5DN expression increased endogenous I(to,f) by two- to threefold, nonadditively with inhibition of dynamin-dependent endocytosis. And, in a phenomenon similar to that previously reported for myoblast-expressed Kv1.5, Rab4DN expression roughly doubled endogenous peak transient currents. Colocalization experiments confirmed the involvement of Rab4 in postinternalization trafficking of Kv4.2. There was little role evident for the lysosome in the degradation of internalized Kv4.2, as overexpression of neither wild-type nor DN isoforms of Rab7 had any effect on I(to,f). Instead, degradation may depend largely on the proteasome; the proteasome inhibitor MG132 significantly increased I(to,f) density.

Dynamic of Ion Channel Expression at the Plasma Membrane of Cardiomyocytes

Cardiac myocytes are characterized by distinct structural and functional entities involved in the generation and transmission of the action potential and the excitation-contraction coupling process. Key to their function is the specific organization of ion channels and transporters to and within distinct membrane domains, which supports the anisotropic propagation of the depolarization wave. This review addresses the current knowledge on the molecular actors regulating the distinct trafficking and targeting mechanisms of ion channels in the highly polarized cardiac myocyte. In addition to ubiquitous mechanisms shared by other excitable cells, cardiac myocytes show unique specialization, illustrated by the molecular organization of myocyte-myocyte contacts, e.g., the intercalated disc and the gap junction. Many factors contribute to the specialization of the cardiac sarcolemma and the functional expression of cardiac ion channels, including various anchoring proteins, motors, small GTPases, membrane lipids, and cholesterol. The discovery of genetic defects in some of these actors, leading to complex cardiac disorders, emphasizes the importance of trafficking and targeting of ion channels to cardiac function. A major challenge in the field is to understand how these and other actors work together in intact myocytes to fine-tune ion channel expression and control cardiac excitability.

Contributions of intracellular ions to kv channel voltage sensor dynamics

Voltage-sensing domains (VSDs) of Kv channels control ionic conductance through coupling of the movement of charged residues in the S4 segment to conformational changes at the cytoplasmic region of the pore domain, that allow K(+) ions to flow. Conformational transitions within the VSD are induced by changes in the applied voltage across the membrane field. However, several other factors not directly linked to the voltage-dependent movement of charged residues within the voltage sensor impact the dynamics of the voltage sensor, such as inactivation, ionic conductance, intracellular ion identity, and block of the channel by intracellular ligands. The effect of intracellular ions on voltage sensor dynamics is of importance in the interpretation of gating current measurements and the physiology of pore/voltage sensor coupling. There is a significant amount of variability in the reported kinetics of voltage sensor deactivation kinetics of Kv channels attributed to different mechanisms such as open state stabilization, immobilization, and relaxation processes of the voltage sensor. Here we separate these factors and focus on the causal role that intracellular ions can play in allosterically modulating the dynamics of Kv voltage sensor deactivation kinetics. These considerations are of critical importance in understanding the molecular determinants of the complete channel gating cycle from activation to deactivation.

Blockade of permeation by potassium but normal gating of the G628S nonconducting hERG channel mutant

G628S is a mutation in the signature sequence that forms the selectivity filter of the human ether-a-go-go-related gene (hERG) channel (GFG) and is associated with long-QT2 syndrome. G628S channels are known to have a dominant-negative effect on hERG currents, and the mutant is therefore thought to be nonfunctional. This study aims to assess the physiological mechanism that prevents the surface-expressing G628S channels from conducting ions. We used voltage-clamp fluorimetry along with two-microelectrode voltage clamping in Xenopus oocytes to confirm that the channels express well at the surface, and to show that they are actually functional, with activation kinetics comparable to that of wild-type, and that the mutation leads to a reduced selectivity to potassium. Although ionic currents are not detected in physiological solutions, removing extracellular K(+) results in the appearance of an inward Na(+)-dependent current. Using whole-cell patch clamp in mammalian transfected cells, we demonstrate that the G628S channels conduct Na(+), but that this can be blocked by both intracellular and higher-than-physiological extracellular K(+). Using solutions devoid of K(+) allows the appearance of nA-sized Na(+) currents with activation and inactivation gating analogous to wild-type channels. The G628S channels are functionally conducting but are normally blocked by intracellular K(+).

Fast and slow voltage sensor rearrangements during activation gating in Kv1.2 channels detected using tetramethylrhodamine fluorescence

The Kv1.2 channel, with its high resolution crystal structure, provides an ideal model for investigating conformational changes associated with channel gating, and fluorescent probes attached at the extracellular end of S4 are a powerful way to gain a more complete understanding of the voltage-dependent activity of these dynamic proteins. Tetramethylrhodamine-5-maleimide (TMRM) attached at A291C reports two distinct rearrangements of the voltage sensor domains, and a comparative fluorescence scan of the S4 and S3-S4 linker residues in Shaker and Kv1.2 shows important differences in their emission at other homologous residues. Kv1.2 shows a rapid decrease in A291C emission with a time constant of 1.5 +/- 0.1 ms at 60 mV (n = 11) that correlates with gating currents and reports on translocation of the S4 and S3-S4 linker. However, unlike any Kv channel studied to date, this fast component is dwarfed by a larger, slower quenching of TMRM emission during depolarizations between -120 and -50 mV (tau = 21.4 +/- 2.1 ms at 60 mV, V(1/2) of -73.9 +/- 1.4 mV) that is not seen in either Shaker or Kv1.5 and that comprises >60% of the total signal at all activating potentials. The slow fluorescence relaxes after repolarization in a voltage-dependent manner that matches the time course of Kv1.2 ionic current deactivation. Fluorophores placed directly in S1 and S2 at I187 and T219 recapitulate the time course and voltage dependence of slow quenching. The slow component is lost when the extracellular S1-S2 linker of Kv1.2 is replaced with that of Kv1.5 or Shaker, suggesting that it arises from a continuous internal rearrangement within the voltage sensor, initiated at negative potentials but prevalent throughout the activation process, and which must be reversed for the channel to close.

A novel mechanism for LQT3 with 2:1 block: a pore-lining mutation in Nav1.5 significantly affects voltage-dependence of activation

BACKGROUND

SCN5A mutations that cause a gain of function in the cardiac voltage-gated sodium channel (Nav1.5) lead to long QT syndrome and a higher risk for sudden cardiac death.

OBJECTIVE

Here we functionally characterize the biophysical properties of the LQT3 variant, V411M, found in a newborn with a QT interval of 640 ms and 2:1 atrioventricular block.

METHODS

Whole cell patch clamp was performed on wild-type and V411M Nav1.5 channels stably expressed in human embryonic kidney cells.

RESULTS

V411M channels showed hyperpolarizing shifts in both the conductance-voltage (V(1/2) = -48.5 ± 2.2 mV vs. -40.4 ± 1.6 mV for wild-type) and inactivation-voltage (-95.6 ± 1.9 mV vs. -87.7 ± 1.7 mV) relationships, and a two-fold increase in late (sustained) sodium current during voltage ramp repolarizations. While neither mexiletine nor lidocaine exhibited potency differences between WT and V411M, or shortened the QTc in vivo, increased mutant block was observed with 10 μM flecainide (71.4 ± 3.0% vs. 60.3 ± 2.8%), in a voltage-dependent manner. Incorporation of V411M kinetics into atrial and ventricular action potential models reproduced prolonged action potential repolarization.

CONCLUSIONS

Our data suggest a novel mechanism for LQT3, a result of a hyperpolarizing shift in the steady state activation relationship and re-activation of Nav1.5 towards a higher open probability during repolarization of the cardiac action potential. This results in an increased number of open-activated sodium channels, and so drugs that bind this state preferentially are expected to shorten the QTc more than those that favour the inactivated state.

Structure-based virtual screening and electrophysiological evaluation of new chemotypes of K(v)1.5 channel blockers

Atrial fibrillation (AF) is the most prevalent nonfatal cardiac rhythm disorder associated with an increased risk of heart failure and stroke. Considering the ventricular side effects induced by anti-arrhythmic agents in current use, K(v)1.5 channel blockers have attracted a great deal of deliberation owing to their selective actions on atrial electrophysiology. Herein we report new chemotypes of K(v)1.5 channel blockers that were identified through a combination of structure-based virtual screening and in silico druglike property prediction including six scoring functions, as well as electrophysiological evaluation. Among them, five of the 18 compounds exhibited >50 % blockade ratio at 10 microM, and have structural features different from conventional K(v)1.5 channel blockers. These novel scaffolds could serve as hits for further optimization and SAR studies for the discovery of selective agents to treat AF.

Research into the therapeutic roles of two-pore-domain potassium channels

The K(2P) potassium channels are responsible for the background conductance observed in several tissues. Their ubiquitous localization and thus their potential implications in diseases have led to increased research on these channels over the last few years. In this review, we outline different aspects of the research on K(2P) channels and highlight some of the latest discoveries in this area. We focus on research into K(2P) channels as potential therapeutic targets in ischemia/hypoxia, depression, memory disorders, pain, cardiovascular disease and disorders of the immune system. We address the challenge of developing novel pharmacological compounds to target these channels. We also discuss the regulation of expression of the K(2P) gene in health and disease, as well as the value of assessing the expression of K(2P) channels as potential biomarkers of disease.

Mechanistic basis for LQT1 caused by S3 mutations in the KCNQ1 subunit of IKs

Long QT interval syndrome (LQTS) type 1 (LQT1) has been reported to arise from mutations in the S3 domain of KCNQ1, but none of the seven S3 mutations in the literature have been characterized with respect to trafficking or biophysical deficiencies. Surface channel expression was studied using a proteinase K assay for KCNQ1 D202H/N, I204F/M, V205M, S209F, and V215M coexpressed with KCNE1 in mammalian cells. In each case, the majority of synthesized channel was found at the surface, but mutant I_Ks current density at +100 mV was reduced significantly for S209F, which showed ∼75% reduction over wild type (WT). All mutants except S209F showed positively shifted V_1/2’s of activation and slowed channel activation compared with WT (V_1/2 = +17.7 ± 2.4 mV and τ_activation of 729 ms at +20 mV; n = 18). Deactivation was also accelerated in all mutants versus WT (126 ± 8 ms at −50 mV; n = 27), and these changes led to marked loss of repolarizing currents during action potential clamps at 2 and 4 Hz, except again S209F. KCNQ1 models localize these naturally occurring S3 mutants to the surface of the helices facing the other voltage sensor transmembrane domains and highlight inter-residue interactions involved in activation gating. V207M, currently classified as a polymorphism and facing lipid in the model, was indistinguishable from WT I_Ks. We conclude that S3 mutants of KCNQ1 cause LQTS predominantly through biophysical effects on the gating of I_Ks, but some mutants also show protein stability/trafficking defects, which explains why the kinetic gain-of-function mutation S209F causes LQT1.

Kinetic analysis of the effects of H+ or Ni2+ on Kv1.5 current shows that both ions enhance slow inactivation and induce resting inactivation

External H+ and Ni2+ ions inhibit Kv1.5 channels by increasing current decay during a depolarizing pulse and reducing the maximal conductance. Although the former may be attributed to an enhancement of slow inactivation occurring from the open state, the latter cannot. Instead, we propose that the loss of conductance is due to the induction, by H+ or Ni2+, of a resting inactivation process. To assess whether the two inactivation processes are mechanistically related, we examined the time courses for the onset of and recovery from H+- or Ni2+-enhanced slow inactivation and resting inactivation. Compared to the time course of H+- or Ni2+-enhanced slow inactivation at +50 mV, the onset of resting inactivation induced at 80 mV with either ion involves a relatively slower process. Recovery from slow inactivation under control conditions was bi-exponential, indicative of at least two inactivated states. Recovery following H+- or Ni2+-enhanced slow inactivation or resting inactivation had time constants similar to those for recovery from control slow inactivation, although H+ and Ni2+ biased inactivation towards states from which recovery was fast and slow, respectively. The shared time constants suggest that the H+- and Ni2+-enhanced slow inactivated and induced resting inactivated states are similar to those visited during control slow inactivation at pH 7.4. We conclude that in Kv1.5 H+ and Ni2+ differentially enhance a slow inactivation process that involves at least two inactivated states and that resting inactivation is probably a close variant of slow inactivation.