Intrinsic versus extrinsic voltage sensitivity of blocker interaction with an ion channel pore

The mechanism of flecainide action in CPVT does not involve a direct effect on RyR2

RATIONALE

Flecainide, a class 1c antiarrhythmic, has emerged as an effective therapy in preventing arrhythmias in patients with catecholaminergic polymorphic ventricular tachycardia (CPVT) refractory to β-adrenergic receptor blockade. It has been proposed that the clinical efficacy of flecainide in CPVT is because of the combined actions of direct blockade of ryanodine receptors (RyR2) and Na(+) channel inhibition. However, there is presently no direct evidence to support the notion that flecainide blocks RyR2 Ca(2+) flux in the physiologically relevant (luminal-to-cytoplasmic) direction. The mechanism of flecainide action remains controversial.

OBJECTIVE

To examine, in detail, the effect of flecainide on the human RyR2 channel and to establish whether the direct blockade of physiologically relevant RyR2 ion flow by the drug contributes to its therapeutic efficacy in the clinical management of CPVT.

METHODS AND RESULTS

Using single-channel analysis, we show that, even at supraphysiological concentrations, flecainide did not inhibit the physiologically relevant, luminal-to-cytosolic flux of cations through the channel. Moreover, flecainide did not alter RyR2 channel gating and had negligible effect on the mechanisms responsible for the sarcoplasmic reticulum charge-compensating counter current. Using permeabilized cardiac myocytes to eliminate any contribution of plasmalemmal Na(+) channels to the observed actions of the drug at the cellular level, flecainide did not inhibit RyR2-dependent sarcoplasmic reticulum Ca(2+) release.

CONCLUSIONS

The principal action of flecainide in CPVT is not via a direct interaction with RyR2. Our data support a model of flecainide action in which Na(+)-dependent modulation of intracellular Ca(2+) handling attenuates RyR2 dysfunction in CPVT.

Divergence of Ca(2+) selectivity and equilibrium Ca(2+) blockade in a Ca(2+) release-activated Ca(2+) channel

The Ca²⁺ selectivity of CRAC channels depends on the kinetics of ion entry and exit as well as the steady-state Ca²⁺ binding affinity.

Prevailing models postulate that high Ca²⁺ selectivity of Ca²⁺ release-activated Ca²⁺ (CRAC) channels arises from tight Ca²⁺ binding to a high affinity site within the pore, thereby blocking monovalent ion flux. Here, we examined the contribution of high affinity Ca²⁺ binding for Ca²⁺ selectivity in recombinant Orai3 channels, which function as highly Ca²⁺-selective channels when gated by the endoplasmic reticulum Ca²⁺ sensor STIM1 or as poorly Ca²⁺-selective channels when activated by the small molecule 2-aminoethoxydiphenyl borate (2-APB). Extracellular Ca²⁺ blocked Na⁺ currents in both gating modes with a similar inhibition constant (K_i; ∼25 µM). Thus, equilibrium binding as set by the K_i of Ca²⁺ blockade cannot explain the differing Ca²⁺ selectivity of the two gating modes. Unlike STIM1-gated channels, Ca²⁺ blockade in 2-APB–gated channels depended on the extracellular Na⁺ concentration and exhibited an anomalously steep voltage dependence, consistent with enhanced Na⁺ pore occupancy. Moreover, the second-order rate constants of Ca²⁺ blockade were eightfold faster in 2-APB–gated channels than in STIM1-gated channels. A four-barrier, three–binding site Eyring model indicated that lowering the entry and exit energy barriers for Ca²⁺ and Na⁺ to simulate the faster rate constants of 2-APB–gated channels qualitatively reproduces their low Ca²⁺ selectivity, suggesting that ion entry and exit rates strongly affect Ca²⁺ selectivity. Noise analysis indicated that the unitary Na⁺ conductance of 2-APB–gated channels is fourfold larger than that of STIM1-gated channels, but both modes of gating show a high open probability (P_o; ∼0.7). The increase in current noise during channel activation was consistent with stepwise recruitment of closed channels to a high P_o state in both cases, suggesting that the underlying gating mechanisms are operationally similar in the two gating modes. These results suggest that both high affinity Ca²⁺ binding and kinetic factors contribute to high Ca²⁺ selectivity in CRAC channels.

K⁺ conduction and Mg²⁺ blockade in a shaker Kv-channel single point mutant with an unusually high conductance

Potassium channels exhibit a large diversity of single-channel conductances. Shaker is a low-conductance K-channel in which Pro475→Asp, a single-point mutation near the internal pore entrance, promotes 6- to 8-fold higher unitary current. To assess the mechanism for this higher conductance, we measured Shaker-P475D single-channel current in a wide range of symmetrical K(+) concentrations and voltages. Below 300 mM K(+), the current-to-voltage relations (i-V) showed inward rectification that disappeared at 1000 mM K(+). Single-channel conductance reached a maximum of ∼190 pS at saturating [K(+)], a value 4- to 5-fold larger than that estimated for the native channel. Intracellular Mg(2+) blocked this variant with ∼100-fold higher affinity. Near zero voltage, blockade was competitively antagonized by K(+); however, at voltages >100 mV, it was enhanced by K(+). This result is consistent with a lock-in effect in a single-file diffusion regime of Mg(2+) and K(+) along the pore. Molecular-dynamics simulations revealed higher K(+) density in the pore, especially near the Asp-475 side chains, as in the high-conductance MthK bacterial channel. The molecular dynamics also showed that K(+) ions bound distally can coexist with other K(+) or Mg(2+) in the cavity, supporting a lock-in mechanism. The maximal K(+) transport rate and higher occupancy could be due to a decrease in the electrostatic energy profile for K(+) throughout the pore, reducing the energy wells and barriers differentially by ∼0.7 and ∼2 kT, respectively.

The voltage-dependent gate in MthK potassium channels is located at the selectivity filter

Understanding how ion channels open and close their pores is crucial for understanding their physiological roles. We used intracellular quaternary ammonium blockers to locate the voltage-dependent gate in MthK potassium channels from Methanobacterium thermoautotrophicum with electrophysiology and X-ray crystallography. Blockers bind in an aqueous cavity between two putative gates, an intracellular gate and the selectivity filter. Thus, these blockers directly probe gate location: an intracellular gate will prevent binding when closed, whereas a selectivity filter gate will always allow binding. A kinetic analysis of tetrabutylammonium block of single MthK channels combined with X-ray crystallographic analysis of the pore with tetrabutylantimony unequivocally determined that the voltage-dependent gate, like the C-type inactivation gate in eukaryotic channels, is located at the selectivity filter. State-dependent binding kinetics suggests that MthK inactivation leads to conformational changes within the cavity and intracellular pore entrance.

Ouabain binding site in a functioning Na+/K+ ATPase

The Na(+)/K(+) ATPase is an almost ubiquitous integral membrane protein within the animal kingdom. It is also the selective target for cardiotonic derivatives, widely prescribed inhibitors for patients with heart failure. Functional studies revealed that ouabain-sensitive residues distributed widely throughout the primary sequence of the protein. Recently, structural work has brought some consensus to the functional observations. Here, we use a spectroscopic approach to estimate distances between a fluorescent ouabain and a lanthanide binding tag (LBT), which was introduced at five different positions in the Na(+)/K(+) ATPase sequence. These five normally functional LBT-Na(+)/K(+) ATPase constructs were expressed in the cell membrane of Xenopus laevis oocytes, operating under physiological internal and external ion conditions. The spectroscopic data suggest two mutually exclusive distances between the LBT and the fluorescent ouabain. From the estimated distances and using homology models of the LBT-Na(+)/K(+) ATPase constructs, approximate ouabain positions could be determined. Our results suggest that ouabain binds at two sites along the ion permeation pathway of the Na(+)/K(+) ATPase. The external site (low apparent affinity) occupies the same region as previous structural findings. The high apparent affinity site is, however, slightly deeper toward the intracellular end of the protein. Interestingly, in both cases the lactone ring faces outward. We propose a sequential ouabain binding mechanism that is consistent with all functional and structural studies.

Voltage profile along the permeation pathway of an open channel

For ion channels, the transmembrane potential plays a critical role by acting as a driving force for permeant ions. At the microscopic level, the transmembrane potential is thought to decay nonlinearly across the ion permeation pathway because of the irregular three-dimensional shape of the channel's pore. By taking advantage of the current structural and functional understanding of cyclic nucleotide-gated channels, in this study we experimentally explore the transmembrane potential's distribution across the open pore. As a readout for the voltage drop, we engineered cysteine residues along the selectivity filter and scanned the sensitivity of their modification rates by Ag(+) to the transmembrane potential. The experimental data, which indicate that the majority of the electric field drops across the selectivity filter, are in good agreement with continuum electrostatic calculations using a homology model of an open CNG channel. By focusing the transmembrane potential across the selectivity filter, the electromotive driving force is coupled with the movement of permeant ions in the filter, maximizing the efficiency of this process.