Molecular Mechanism of Conductance Enhancement in Narrow Cation-Selective Membrane Channels

The significance of bioelectricity on all levels of organization of an organism. Part 1: From the subcellular level to cells

Bioelectricity plays an essential role in the structural and functional organization of biological organisms. In this first article of our three-part series, we summarize the importance of bioelectricity for the basic structural level of biological organization, i.e. from the subcellular level (charges, ion channels, molecules and cell organelles) to cells.

Blocking effect of ferritin on the ryanodine receptor-isoform 2

Iron, an essential element for most living organism, participates in a wide variety of physiological processes. Disturbance in iron homeostasis has been associated with numerous pathologies, particularly in the heart and brain, which are the most susceptible organs. Under iron-overload conditions, the generation of reactive oxygen species leads to impairment in Ca2+ signaling, fundamentally implicated in cardiac and neuronal physiology. Since iron excess is accompanied by increased expression of iron-storage protein, ferritin, we examined whether ferritin has an effect on the ryanodine receptor - isoform 2 (RYR2), which is one of the major components of Ca2+ signaling. Using the method of planar lipid membranes, we show that ferritin induced an abrupt, permanent blockage of the RYR2 channel. The ferritin effect was strongly voltage dependent and competitively antagonized by cytosolic TEA+, an impermeant RYR2 blocker. Our results collectively indicate that monomeric ferritin highly likely blocks the RYR2 channel by a direct electrostatic interaction within the wider region of the channel permeation pathway.

Direct proof of soft knock-on mechanism of ion permeation in a voltage gated sodium channel

Sodium channels selectively conduct Na⁺ ions across cellular membrane with extraordinary efficiency, which is essential for initiating action potentials. However, how Na⁺ ions permeate the ionic channels remains obscure and ambiguous. With more than 40 conductance events from microsecond molecular dynamics simulation, the soft knock-on ion permeation mediated by water molecules was observed and confirmed by the free energy profile and electrostatic potential calculation in this study. During the soft knock-on process, the change of average distance between four oxygen atoms in Glu177-Glu177 plays a very important role for the permeation of Na⁺ ion. Exploration of the ionic conductance mechanism could provide a guideline for designing ion channel targeted drug.

Nonselective cation permeation in an AMPA-type glutamate receptor

Fast excitatory synaptic transmission in the central nervous system relies on the AMPA-type glutamate receptor (AMPAR). This receptor incorporates a nonselective cation channel, which is opened by the binding of glutamate. Although the open pore structure has recently became available from cryo-electron microscopy (Cryo-EM), the molecular mechanisms governing cation permeability in AMPA receptors are not understood. Here, we combined microsecond molecular dynamic (MD) simulations on a putative open-state structure of GluA2 with electrophysiology on cloned channels to elucidate ion permeation mechanisms. Na⁺, K⁺, and Cs⁺ permeated at physiological rates, consistent with a structure that represents a true open state. A single major ion binding site for Na⁺ and K⁺ in the pore represents the simplest selectivity filter (SF) structure for any tetrameric cation channel of known structure. The minimal SF comprised only Q586 and Q587, and other residues on the cytoplasmic side formed a water-filled cavity with a cone shape that lacked major interactions with ions. We observed that Cl^- readily enters the upper pore, explaining anion permeation in the RNA-edited (Q586R) form of GluA2. A permissive architecture of the SF accommodated different alkali metals in distinct solvation states to allow rapid, nonselective cation permeation and copermeation by water. Simulations suggested Cs⁺ uses two equally populated ion binding sites in the filter, and we confirmed with electrophysiology of GluA2 that Cs⁺ is slightly more permeant than Na⁺, consistent with serial binding sites preferentially driving selectivity.

Calcium Release Channels: Structure and Function of IP3 Receptors and Ryanodine Receptors

Ca²⁺-release channels are giant membrane proteins that control the release of Ca²⁺ from the endoplasmic and sarcoplasmic reticulum. The two members, ryanodine receptors (RyRs) and inositol-1,4,5-trisphosphate Receptors (IP₃Rs), are evolutionarily related and are both activated by cytosolic Ca²⁺. They share a common architecture, but RyRs have evolved additional modules in the cytosolic region. Their massive size allows for the regulation by tens of proteins and small molecules, which can affect the opening and closing of the channels. In addition to Ca²⁺, other major triggers include IP₃ for the IP₃Rs, and depolarization of the plasma membrane for a particular RyR subtype. Their size has made them popular targets for study via electron microscopic methods, with current structures culminating near 3Å. The available structures have provided many new mechanistic insights int the binding of auxiliary proteins and small molecules, how these can regulate channel opening, and the mechanisms of disease-associated mutations. They also help scrutinize previously proposed binding sites, as some of these are now incompatible with the structures. Many questions remain around the structural effects of post-translational modifications, additional binding partners, and the higher-order complexes these channels can make in situ. This review summarizes our current knowledge about the structures of Ca²⁺-release channels and how this informs on their function.

Atomistic Details of Charge/Space Competition in the Ca2+ Selectivity of Ryanodine Receptors

Ryanodine receptors (RyRs) are ion channels responsible for the fast release of Ca²⁺ from the sarco/endoplasmic reticulum to the cytosol and show a selectivity of Ca²⁺ over monovalent cations. By utilizing a recently developed multisite Ca²⁺ model in molecular dynamic simulations, we show that multiple cations accumulate in the upper selectivity filter of RyRs, and the small size and high valence of Ca²⁺ make it preferable to K⁺ in competition for space in this confined region of negative electrostatic potential. The presence of Ca²⁺ in the upper selectivity filter significantly increases the energy barrier of K⁺ permeation, while the presence of K⁺ has little impact on the Ca²⁺ permeation. Our results provide the atomistic details of the charge/space competition mechanism for the ion selectivity of RyRs, which ensures the robustness of their Ca²⁺ release function. The mechanism could be utilized in protein- and nanoengineering for valence selectivity of ion species.

Structural Mechanism of ω-Currents in a Mutated Kv7.2 Voltage Sensor Domain from Molecular Dynamics Simulations

Activation of voltage-gated ion channels is regulated by conformational changes of the voltage sensor domains (VSDs), four water- and ion-impermeable modules peripheral to the central, permeable pore domain. Anomalous currents, defined as ω-currents, have been recorded in response to mutations of residues on the VSD S4 helix and associated with ion fluxes through the VSDs. In humans, gene defects in the potassium channel Kv7.2 result in a broad range of epileptic disorders, from benign neonatal seizures to severe epileptic encephalopathies. Experimental evidence suggests that the R207Q mutation in S4, associated with peripheral nerve hyperexcitability, induces ω-currents at depolarized potentials, but the fine structural details are still elusive. In this work, we use atom-detailed molecular dynamics simulations and a refined model structure of the Kv7.2 VSD in the active conformation in a membrane/water environment to study the effect of R207Q and four additional mutations of proven clinical importance. Our results demonstrate that the R207Q mutant shows the most pronounced increase of hydration in the internal VSD cavity, a feature favoring the occurrence of ω-currents. Free energy and kinetics calculations of sodium permeation through the native and mutated VSD indicate as more favorable the formation of a cationic current in the latter. Overall, our simulations establish a mechanistic linkage between genetic variations and their physiological outcome, by providing a computational description that includes both thermodynamic and kinetic features of ion permeation associated with ω-currents.

The Ca2+ permeation mechanism of the ryanodine receptor revealed by a multi-site ion model

Ryanodine receptors (RyR) are ion channels responsible for the release of Ca²⁺ from the sarco/endoplasmic reticulum and play a crucial role in the precise control of Ca²⁺ concentration in the cytosol. The detailed permeation mechanism of Ca²⁺ through RyR is still elusive. By using molecular dynamics simulations with a specially designed Ca²⁺ model, we show that multiple Ca²⁺ ions accumulate in the upper selectivity filter of RyR1, but only one Ca²⁺ can occupy and translocate in the narrow pore at a time, assisted by electrostatic repulsion from the Ca²⁺ within the upper selectivity filter. The Ca²⁺ is nearly fully hydrated with the first solvation shell intact during the whole permeation process. These results suggest a remote knock-on permeation mechanism and one-at-a-time occupation pattern for the hydrated Ca²⁺ within the narrow pore, uncovering the basis underlying the high permeability and low selectivity of the RyR channels.

Luminal addition of non-permeant Eu3+ interferes with luminal Ca2+ regulation of the cardiac ryanodine receptor

Dysregulation of the cardiac ryanodine receptor (RYR2) by luminal Ca²⁺ has been implicated in a life-threatening, stress-induced arrhythmogenic disease. The mechanism of luminal Ca²⁺-mediated RYR2 regulation is under debate, and it has been attributed to Ca²⁺ binding on the cytosolic face (the Ca²⁺ feedthrough mechanism) and/or the luminal face of the RYR2 channel (the true luminal mechanism). The molecular nature and location of the luminal Ca²⁺ site is unclear. At the single-channel level, we directly probed the RYR2 luminal face by Eu³⁺, considering the non-permeant nature of trivalent cations and their high binding affinities for Ca²⁺ sites. Without affecting essential determinants of the Ca²⁺ feedthrough mechanism, we found that luminal Eu³⁺ competitively antagonized the activation effect of luminal Ca²⁺ on RYR2 responsiveness to cytosolic caffeine, and no appreciable effect was observed for luminal Ba²⁺ (mimicking the absence of luminal Ca²⁺). Importantly, luminal Eu³⁺ caused no changes in RYR2 gating. Our results indicate that two distinct Ca²⁺ sites (available for luminal Ca²⁺ even when the channel is closed) are likely involved in the true luminal mechanism. One site facing the lumen regulates channel responsiveness to caffeine, while the other site, presumably positioned in the channel pore, governs the gating behavior.

Bases of Bacterial Sodium Channel Selectivity Among Organic Cations

Hille’s (1971) seminal study of organic cation selectivity of eukaryotic voltage-gated sodium channels showed a sharp size cut-off for ion permeation, such that no ion possessing a methyl group was permeant. Using the prokaryotic channel, NaChBac, we found some similarity and two peculiar differences in the selectivity profiles for small polyatomic cations. First, we identified a diverse group of minimally permeant cations for wildtype NaChBac, ranging in sizes from ammonium to guanidinium and tetramethylammonium; and second, for both ammonium and hydrazinium, the charge-conserving selectivity filter mutation (E191D) yielded substantial increases in relative permeability (P_X/P_Na). The relative permeabilities varied inversely with relative K_d calculated from 1D Potential of Mean Force profiles (PMFs) for the single cations traversing the channel. Several of the cations bound more strongly than Na⁺, and hence appear to act as blockers, as well as charge carriers. Consistent with experimental observations, the E191D mutation had little impact on Na⁺ binding to the selectivity filter, but disrupted the binding of ammonium and hydrazinium, consequently facilitating ion permeation across the NaChBac-like filter. We concluded that for prokaryotic sodium channels, a fine balance among filter size, binding affinity, occupancy, and flexibility seems to contribute to observed functional differences.

Conduction through a narrow inward-rectifier K+ channel pore

G-protein–gated inwardly rectifying potassium channels are important mediators of inhibitory neurotransmission. Based on microsecond-scale molecular dynamics simulations, Bernsteiner et al. propose novel gating details that may enable K⁺ flux via a direct knock-on mechanism.

Inwardly rectifying potassium (Kir) channels play a key role in controlling membrane potentials in excitable and unexcitable cells, thereby regulating a plethora of physiological processes. G-protein–gated Kir channels control heart rate and neuronal excitability via small hyperpolarizing outward K⁺ currents near the resting membrane potential. Despite recent breakthroughs in x-ray crystallography and cryo-EM, the gating and conduction mechanisms of these channels are poorly understood. MD simulations have provided unprecedented details concerning the gating and conduction mechanisms of voltage-gated K⁺ and Na⁺ channels. Here, we use multi-microsecond–timescale MD simulations based on the crystal structures of GIRK2 (Kir3.2) bound to phosphatidylinositol-4,5-bisphosphate to provide detailed insights into the channel’s gating dynamics, including insights into the behavior of the G-loop gate. The simulations also elucidate the elementary steps that underlie the movement of K⁺ ions through an inward-rectifier K⁺ channel under an applied electric field. Our simulations suggest that K⁺ permeation might occur via direct knock-on, similar to the mechanism recently shown for K_v channels.