Mechanism of potassium ion uptake by the Na(+)/K(+)-ATPase

Nernst Equilibrium, Rectification, and Saturation: Insights into Ion Channel Behavior

The dissipation of electrochemical gradients through ion channels plays a central role in biology. Herein we use voltage responsive kinetic models of ion channels to explore how electrical and chemical potentials differentially influence ion transport properties. These models demonstrate how electrically driven flux is greater than the Nernstian equivalent chemically driven flux, yet still perfectly cancels when the two gradients oppose each other. We find that the location and relative stability of ion binding sites dictates rectification properties by shifting the location of the most voltage sensitive transitions. However, these rectification properties invert when bulk concentrations increase relative to the binding site stabilities, moving the rate limiting steps from uptake into a relatively empty channel to release from an ion-blocked full channel. Additionally, the origin of channel saturation is shown to depend on the free energy of uptake relative to bulk concentrations. Collectively these insights provide framework for interpreting and predicting how channel properties manifest in electrochemical transport behavior.

Multiscale Responsive Kinetic Modeling: Quantifying Biomolecular Reaction Flux under Varying Electrochemical Conditions

Attaining a complete thermodynamic and kinetic characterization for processes involving multiple interconnected rare-event transitions remains a central challenge in molecular biophysics. This challenge is amplified when the process must be understood under a range of reaction conditions. Herein, we present a condition-responsive kinetic modeling framework that can combine the strengths of bottom-up rate quantification from multiscale simulations with top-down solution refinement using experimental data. Although this framework can be applied to any process, we demonstrate its use for electrochemically driven transport through channels and transporters. Using the Cl- /H+ antiporter ClC-ec1 as a model system, we show how robust and predictive kinetic solutions can be obtained when the solution space is grounded by thermodynamic constraints, seeded through multiscale rate quantification, and further refined with experimental data, such as electrophysiology assays. Turning to the Shaker K+ channel, we demonstrate that robust solutions and biophysical insights can also be obtained with sufficient experimental data. This multi-pathway method proves capable of identifying single-pathway dominant mechanisms but also highlights that competing and off-pathway flux is still essential to replicate experimental findings and to describe concentration-dependent channel rectification.

Genome-wide identification and expression analysis of the HAK/KUP/KT gene family in Moso bamboo

The K+ uptake permease/high-affinity K+/K+ transporter (KUP/HAK/KT) family is the most prominent group of potassium (K+) transporters, playing a key role in K+ uptake, transport, plant growth and development, and stress tolerance. However, the presence and functions of the KUP/HAK/KT family in Moso bamboo (Phyllostachys edulis (Carriere) J. Houzeau), the fastest-growing plant, have not been studied. In this study, we identified 41 KUP/HAK/KT genes (PeHAKs) distributed across 18 chromosomal scaffolds of the Moso bamboo genome. PeHAK is a typical membrane protein with a conserved structural domain and motifs. Phylogenetic tree analysis classified PeHAKs into four distinct clusters, while collinearity analysis revealed gene duplications resulting from purifying selection, including both tandem and segmental duplications. Enrichment analysis of promoter cis-acting elements suggested their plausible role in abiotic stress response and hormone induction. Transcriptomic data and STEM analyses indicated that PeHAKs were involved in tissue and organ development, rapid growth, and responded to different abiotic stress conditions. Subcellular localization analysis demonstrated that PeHAKs are predominantly expressed at the cell membrane. In-situ PCR experiments confirmed that PeHAK was mainly expressed in the lateral root primordia. Furthermore, the involvement of PeHAKs in potassium ion transport was confirmed by studying the potassium ion transport properties of a yeast mutant. Additionally, through homology modeling, we revealed the structural properties of HAK as a transmembrane protein associated with potassium ion transport. This research provides a solid basis for understanding the classification, characterization, and functional analysis of the PeHAK family in Moso bamboo.

Pumping Small Molecules Selectively through an Energy-Assisted Assembling Process at Nonequilibrium States

In living organisms, precise control over the spatial and temporal distribution of molecules, including pheromones, is crucial. This level of control is equally important for the development of artificial active materials. In this study, we successfully controlled the distribution of small molecules in the system at nonequilibrium states by actively transporting them, even against the apparent concentration gradient, with high selectivity. As a demonstration, in the aqueous solution of acid orange (AO7) and TMC10COOH, we found that AO7 molecules can coassemble with transient anhydride (TMC10CO)2O to form larger assemblies in the presence of chemical fuel 1-ethyl-3-(3-(dimethylamino)propyl) carbodiimide hydrochloride (EDC). This led to a decrease in local free AO7 concentration and caused AO7 molecules from other locations in the solution to move toward the assemblies. Consequently, AO7 accumulates at the location where EDC was injected. By continuously injecting EDC, we could maintain a stable high value of the apparent AO7 concentration at the injection point. We also observed that this process which operated at nonequilibrium states exhibited high selectivity.

Simultaneous Increases in Intracellular Sodium and Tonicity Boost Antimicrobial Activity of Macrophages

Inflamed and infected tissues can display increased local sodium (Na+) levels, which can have various effects on immune cells. In macrophages, high salt (HS) leads to a Na+/Ca2+-exchanger 1 (NCX1)-dependent increase in intracellular Na+ levels. This results in augmented osmoprotective signaling and enhanced proinflammatory activation, such as enhanced expression of type 2 nitric oxide synthase and antimicrobial function. In this study, the role of elevated intracellular Na+ levels in macrophages was investigated. Therefore, the Na+/K+-ATPase (NKA) was pharmacologically inhibited with two cardiac glycosides (CGs), ouabain (OUA) and digoxin (DIG), to raise intracellular Na+ without increasing extracellular Na+ levels. Exposure to HS conditions and treatment with both inhibitors resulted in intracellular Na+ accumulation and subsequent phosphorylation of p38/MAPK. The CGs had different effects on intracellular Ca2+ and K+ compared to HS stimulation. Moreover, the osmoprotective transcription factor nuclear factor of activated T cells 5 (NFAT5) was not upregulated on RNA and protein levels upon OUA and DIG treatment. Accordingly, OUA and DIG did not boost nitric oxide (NO) production and showed heterogeneous effects toward eliminating intracellular bacteria. While HS environments cause hypertonic stress and ionic perturbations, cardiac glycosides only induce the latter. Cotreatment of macrophages with OUA and non-ionic osmolyte mannitol (MAN) partially mimicked the HS-boosted antimicrobial macrophage activity. These findings suggest that intracellular Na+ accumulation and hypertonic stress are required but not sufficient to mimic boosted macrophage function induced by increased extracellular sodium availability.

Transcriptome analysis reveals differences in developmental neurotoxicity mechanism of methyl-, ethyl-, and propyl- parabens in zebrafish embryos

Studies on the comparison of developmental (neuro) toxicity of parabens are currently limited, and unharmonized concentrations between phenotypic observations and transcriptome analysis hamper the understanding of their differential molecular mechanisms. Thus, developmental toxicity testing was conducted herein using the commonly used methyl- (MtP), ethyl- (EtP), and propyl-parabens (PrP) in zebrafish embryos. With a benchmark dose of 5%, embryonic-mortality-based point-of-departure (M-POD) values of the three parabens were determined, and changes in locomotor behavior were evaluated at concentrations of 0, M-POD/50, M-POD/10, and M-POD, where transcriptome analysis was conducted to explore the underlying neurotoxicity mechanism. Higher long-chained parabens were more toxic than short-chained parabens, as determined by the M-POD values of 154.1, 72.6, and 24.2 µM for MtP, EtP, and PrP, respectively. Meanwhile, exposure to EtP resulted in hyperactivity, whereas no behavioral effect was observed with MtP and PrP. Transcriptome analysis revealed that abnormal behaviors in the EtP-exposed group were associated with distinctly enriched pathways in signaling, transport, calcium ion binding, and metal binding. In contrast, exposure to MtP and PrP mainly disrupted membranes and transmembranes, which are closely linked to abnormal embryonic development rather than neurobehavioral changes. According to the changes in the expressions of signature mRNAs, tentative transcriptome-based POD values for each paraben were determined as MtP (2.68 µM), EtP (3.85 µM), and PrP (1.4 µM). This suggests that different molecular perturbations initiated at similar concentrations determined the extent and toxicity outcome differently. Our findings provide insight into better understanding the differential developmental neurotoxicity mechanisms of parabens.

Diet, cellular, and systemic homeostasis control the cycling of potassium stable isotopes in endothermic vertebrates

The naturally occurring stable isotopes of potassium (41K/39K, expressed as δ41K) have the potential to make significant contributions to vertebrate and human biology. The utility of K stable isotopes is, however, conditioned by the understanding of the dietary and biological factors controlling natural variability of δ41K. This paper reports a systematic study of K isotopes in extant terrestrial endothermic vertebrates. δ41K has been measured in 158 samples of tissues, biofluids, and excreta from 40 individuals of four vertebrate species (rat, guinea pig, pig and quail) reared in two controlled feeding experiments. We show that biological processing of K by endothermic vertebrates produces remarkable intra-organism δ41K variations of ca. 1.6‰. Dietary δ41K is the primary control of interindividual variability and δ41K of bodily K is +0.5-0.6‰ higher than diet. Such a trophic isotope effect is expected to propagate throughout trophic chains, opening promising use for reconstructing dietary behaviors in vertebrate ecosystems. In individuals, cellular δ41K is related to the intensity of K cycling and effectors of K homeostasis, including plasma membrane permeability and electrical potential. Renal and intestinal transepithelial transports also control fractionation of K isotopes. Using a box-modeling approach, we establish a first model of K isotope homeostasis. We predict a strong sensitivity of δ41K to variations of intracellular and renal K cycling in normal and pathological contexts. Thus, K isotopes constitute a promising tool for the study of K dyshomeostasis.

Membrane protein synthesis: no cells required

Despite advances in membrane protein (MP) structural biology and a growing interest in their applications, these proteins remain challenging to study. Progress has been hindered by the complex nature of MPs and innovative methods will be required to circumvent technical hurdles. Cell-free protein synthesis (CFPS) is a burgeoning technique for synthesizing MPs directly into a membrane environment using reconstituted components of the cellular transcription and translation machinery in vitro. We provide an overview of CFPS and how this technique can be applied to the synthesis and study of MPs. We highlight numerous strategies including synthesis methods and folding environments, each with advantages and limitations, to provide a survey of how CFPS techniques can advance the study of MPs.

Stimuli-responsive ultra-small vanadate prodrug nanoparticles with NIR photothermal properties to precisely inhibit Na/K-ATPase for enhanced cancer therapy

Inhibition of Na/K-ATPase is a promising cancer treatment owing to the essential role of Na/K-ATPase in maintaining various cellular functions. The potent Na/K-ATPase inhibitor, vanadate(V) (termed as V(V)), has exhibited efficient anticancer effects. However, nonspecific inhibition using V(V) results in serious side effects, which hinder its clinical application. Here, bovine serum albumin (BSA)-modified ultra-small vanadate prodrug nanoparticles (V(IV) NPs) were synthesized via a combined reduction-coordination strategy with a natural polyphenol tannic acid (TA). A lower systemic toxicity of V(IV) NPs is achieved by strong metal-polyphenol coordination interactions. An efficient V(V) activation is realized by reactive oxygen species (ROS) at the tumor site. Furthermore, V(IV) NPs show excellent photothermal properties in the near-infrared (NIR) region. By NIR irradiation at the tumor site for mild hyperthermia, selective enhancement of the interactions between V(V) and Na/K-ATPase achieves stronger inhibition of Na/K-ATPase for robust cell killing effect. Altogether, V(IV) NPs specifically inhibit Na/K-ATPase in cancer cells with negligible toxicity to normal tissues, thus making them a promising candidate for clinical applications of Na/K-ATPase inhibition.

Norepinephrine May Exacerbate Septic Acute Kidney Injury: A Narrative Review

Sepsis, the most serious complication of infection, occurs when a cascade of potentially life-threatening inflammatory responses is triggered. Potentially life-threatening septic shock is a complication of sepsis that occurs when hemodynamic instability occurs. Septic shock may cause organ failure, most commonly involving the kidneys. The pathophysiology and hemodynamic mechanisms of acute kidney injury in the case of sepsis or septic shock remain to be elucidated, but previous studies have suggested multiple possible mechanisms or the interplay of multiple mechanisms. Norepinephrine is used as the first-line vasopressor in the management of septic shock. Studies have reported different hemodynamic effects of norepinephrine on renal circulation, with some suggesting that it could possibly exacerbate acute kidney injury caused by septic shock. This narrative review briefly covers the updates on sepsis and septic shock regarding definitions, statistics, diagnosis, and management, with an explanation of the putative pathophysiological mechanisms and hemodynamic changes, as well as updated evidence. Sepsis-associated acute kidney injury remains a major burden on the healthcare system. This review aims to improve the real-world clinical understanding of the possible adverse outcomes of norepinephrine use in sepsis-associated acute kidney injury.

Red ginseng extract improves skeletal muscle energy metabolism and mitochondrial function in chronic fatigue mice

Background: Skeletal muscles are organs with high energy requirements, especially during vigorous exercise. Adequate mitochondrial function is essential to meet the high energy needs of skeletal muscle cells. Recent studies have reported that red ginseng can significantly improve chronic fatigue; however, the specific mechanism of action is still not clear. Methods: A chronic fatigue syndrome mouse model was developed using C57BL/6J mice through long-term compound stimulation of stress factors. Following this, the animals were orally administered 200, 400, or 600 mg/kg red ginseng extracts for 28 days. Skeletal muscle lactate acid, serum lactate dehydrogenase, urea concentrations, ATP level, mitochondrial membrane potential, activities of Na⁺-K⁺-ATPase and cytochrome c oxidase were determined using assay kits or an automatic biochemical analyser detection system. Skeletal muscle mitochondria morphology was observed using electron microscopy and the expression of p-AMPK, PGC-1α, ACO2 and complex I in skeletal muscle protein was determined by western blotting. Results: Oral administration of 400 or 600 mg/kg red ginseng extract in mice with chronic fatigue reduced lactic acid, lactate dehydrogenase and urea, rescued the density and morphology of skeletal muscle mitochondria, increased the activities of Na⁺-K⁺-ATPase and cytochrome c oxidase, and activated the AMPK/PGC-1α cascade pathway, resulting in improved skeletal muscle mitochondrial function by restoring ATP level, mitochondrial membrane potential, complex I and mitochondrial biogenesis. Conclusion: The anti-fatigue effects of red ginseng are partly related to its potent mitochondrial improving activity, including decreasing mitochondrial swelling and mitochondrial membrane permeability, increasing mitochondrial biogenesis, thus ameliorating mitochondrial dysfunction.

Measurements of Na+-occluded intermediates during the catalytic cycle of the Na+/K+-ATPase provide novel insights into the mechanism of Na+ transport

The Na⁺/K⁺-ATPase is an integral plasma membrane glycoprotein of all animal cells that couples the exchange of intracellular Na⁺ for extracellular K⁺ to the hydrolysis of ATP. The asymmetric distribution of Na⁺ and K⁺ is essential for cellular life and constitutes the physical basis of a series of fundamental biological phenomena. The pumping mechanism is explained by the Albers-Post model. It involves the presence of gates alternatively exposing Na⁺/K⁺-ATPase transport sites to the intracellular and extracellular sides and includes occluded states in which both gates are simultaneously closed. Unlike for K⁺, information is lacking about Na⁺-occluded intermediates, as occluded Na⁺ was only detected in states incapable of performing a catalytic cycle, including two Na⁺-containing crystallographic structures. The current knowledge is that intracellular Na⁺ must bind to the transport sites and become occluded upon phosphorylation by ATP to be transported to the extracellular medium. Here, taking advantage of epigallocatechin-3-gallate to instantaneously stabilize native Na⁺-occluded intermediates, we isolated species with tightly bound Na⁺ in an enzyme able to perform a catalytic cycle, consistent with a genuine occluded state. We found that Na⁺ becomes spontaneously occluded in the E1 dephosphorylated form of the Na⁺/K⁺-ATPase, exhibiting positive interactions between binding sites. In fact, the addition of ATP does not produce an increase in Na⁺ occlusion as it would have been expected; on the contrary, occluded Na⁺ transiently decreases, whereas ATP lasts. These results reveal new properties of E1 intermediates of the Albers-Post model for explaining the Na⁺ transport pathway.

The Na,K-ATPase in complex with beryllium fluoride mimics an ATPase phosphorylated state

The Na⁺,K⁺-ATPase generates electrochemical gradients of Na⁺ and K⁺ across the plasma membrane via a functional cycle that includes various phosphoenzyme intermediates. However, the structure and function of these intermediates and how metal fluorides mimick them require further investigation. Here, we describe a 4.0 Å resolution crystal structure and functional properties of the pig kidney Na⁺,K⁺-ATPase stabilized by the inhibitor beryllium fluoride (denoted E2-BeF_x). E2-BeF_x is expected to mimic properties of the E2P phosphoenzyme, yet with unknown characteristics of ion and ligand binding. The structure resembles the E2P form obtained by phosphorylation from inorganic phosphate (P_i) and stabilized by cardiotonic steroids, including a low-affinity Mg²⁺ site near ion binding site II. Our anomalous Fourier analysis of the crystals soaked in Rb⁺ (a K⁺ congener) followed by a low-resolution rigid-body refinement (6.9-7.5 Å) revealed pre-occlusion transitions leading to activation of the dephosphorylation reaction. We show that the Mg²⁺ location indicates a site of initial K⁺ recognition and acceptance upon binding to the outward-open E2P state after Na⁺ release. Furthermore, using binding and activity studies, we find that the BeF_x-inhibited enzyme is also able to bind ADP/ATP and Na⁺. These results relate the E2-BeF_x complex to a transient K⁺- and ADP-sensitive E*P intermediate of the functional cycle of the Na⁺,K⁺-ATPase, prior to E2P.

Activity Coefficients and Solubilities of NaCl in Water-Methanol Solutions from Molecular Dynamics Simulations

We obtain activity coefficients and solubilities of NaCl in water-methanol solutions at 298.15 K and 1 bar from molecular dynamics (MD) simulations with the Joung-Cheatham, SPC/E, and OPLS-AA force fields for NaCl, water, and methanol, respectively. The Lorentz-Berthelot combining rules were adopted for the unlike-pair interactions of Na⁺, Cl^-, and the oxygen site in SPC/E water, and geometric combining rules were utilized for the remainder of the cross interactions. We found that the selection of appropriate combining rules is important in obtaining physically realistic solubilities. The solvent compositions studied range from pure water to pure methanol. Several salt concentrations were investigated at each solvent composition, from the lowest concentrations permitted by the system size used up to the experimental solubilities. We first calculated individual ion activity coefficients (IIACs) for Na⁺ and Cl^- from the free energy change due to the gradual insertion of a single cation or anion into the solution, accompanied by a neutralizing background. We obtained the salt solubilities by comparing the chemical potentials in solution with solid NaCl chemical potentials calculated previously using the Einstein crystal method. Mean ionic activity coefficients obtained from the IIACs are in reasonable agreement with experimental data, with deviations increasing for solutions of higher methanol content. Predictions for the salt solubility are in surprisingly good agreement with experimental data, despite well-known challenges in the simultaneous calculation of activity coefficients and solubilities with classical MD simulations. The present study demonstrates that good predictions for these two important phase equilibrium properties can be obtained for mixed-solvent electrolyte solutions using existing nonpolarizable models and further suggests that the previously proposed single ion insertion technique can be extended to complex mixed-solvent solutions as well.

A Na+/K+ ATPase Pump Regulates Chondrocyte Differentiation and Bone Length Variation in Mice

The genetic and developmental mechanisms involved in limb formation are relatively well documented, but how these mechanisms are modulated by changes in chondrocyte physiology to produce differences in limb bone length remains unclear. Here, we used high throughput RNA sequencing (RNAseq) to probe the developmental genetic basis of variation in limb bone length in Longshanks, a mouse model of experimental evolution. We find that increased tibia length in Longshanks is associated with altered expression of a few key endochondral ossification genes such as Npr3, Dlk1, Sox9, and Sfrp1, as well reduced expression of Fxyd2, a facultative subunit of the cell membrane-bound Na⁺/K⁺ ATPase pump (NKA). Next, using murine tibia and cell cultures, we show a dynamic role for NKA in chondrocyte differentiation and in bone length regulation. Specifically, we show that pharmacological inhibition of NKA disrupts chondrocyte differentiation, by upregulating expression of mesenchymal stem cell markers (Prrx1, Serpina3n), downregulation of chondrogenesis marker Sox9, and altered expression of extracellular matrix genes (e.g., collagens) associated with proliferative and hypertrophic chondrocytes. Together, Longshanks and in vitro data suggest a broader developmental and evolutionary role of NKA in regulating limb length diversity.

The conformational cycle of prestin underlies outer-hair cell electromotility

The voltage-dependent motor protein prestin (also known as SLC26A5) is responsible for the electromotive behaviour of outer-hair cells and underlies the cochlear amplifier1. Knockout or impairment of prestin causes severe hearing loss2-5. Despite the key role of prestin in hearing, the mechanism by which mammalian prestin senses voltage and transduces it into cellular-scale movements (electromotility) is poorly understood. Here we determined the structure of dolphin prestin in six distinct states using single-particle cryo-electron microscopy. Our structural and functional data suggest that prestin adopts a unique and complex set of states, tunable by the identity of bound anions (Cl- or SO42-). Salicylate, a drug that can cause reversible hearing loss, competes for the anion-binding site of prestin, and inhibits its function by immobilizing prestin in a new conformation. Our data suggest that the bound anion together with its coordinating charged residues and helical dipole act as a dynamic voltage sensor. An analysis of all of the anion-dependent conformations reveals how structural rearrangements in the voltage sensor are coupled to conformational transitions at the protein-membrane interface, suggesting a previously undescribed mechanism of area expansion. Visualization of the electromotility cycle of prestin distinguishes the protein from the closely related SLC26 anion transporters, highlighting the basis for evolutionary specialization of the mammalian cochlear amplifier at a high resolution.

Enantioselective Antiport in Asymmetric Nanochannels

Enantioselective sensing and separation are major challenges. Nanochannel technologies are energy-saving and efficient for membrane separation. Herein, inspired by biological antiporter proteins, artificial nanochannels with antiporter behavior were fabricated for chiral sensing and separation. Tyrosine enantiomers were incorporated into hourglass-shaped nanochannels via stepwise modifications to fabricating multiligand-modified asymmetric channels. Chiral distinction of naproxen enantiomers was amplified in the l-Tyr/d-Tyr channels, with an enantioselectivity coefficient of 524, which was over 100-fold that of one-ligand-modified nanochannels. Furthermore, transport experiments evidenced the spontaneous antiport of naproxen enantiomers in the l-Tyr/d-Tyr channels. The racemic naproxen sample was separated via the chiral antiport process, with an enantiomeric excess of 71.2%. Further analysis using electro-osmotic flow experiments and finite-element simulations confirmed that the asymmetric modified multiligand was key to achieving separation of the naproxen enantiomers. We expect these multiligand-modified asymmetric nanochannels to provide insight into mimicking biological antiporter systems and offer an approach to energy-efficient and robust enantiomer separation.

The transferred translocases: An old wine in a new bottle

The role of translocases was underappreciated and was not included as a separate class in the enzyme commission until August 2018. The recent research interests in proteomics of orphan enzymes, ionomics, and metallomics along with high-throughput sequencing technologies generated overwhelming data and revamped this enzyme into a separate class. This offers a great opportunity to understand the role of new or orphan enzymes in general and specifically translocases. The enzymes belonging to translocases regulate/permeate the transfer of ions or molecules across the membranes. These enzyme entries were previously associated with other enzyme classes, which are now transferred to a new enzyme class 7 (EC 7). The entries that are reclassified are important to extend the enzyme list, and it is the need of the hour. Accordingly, there is an upgradation of entries of this class of enzymes in several databases. This review is a concise compilation of translocases with reference to the number of entries currently available in the databases. This review also focuses on function as well as dysfunction of translocases during normal and disordered states, respectively.

Geometry and water accessibility of the inhibitor binding site of Na+-pump: Pulse- and CW-EPR study

Spin labels based on cinobufagin, a specific inhibitor of the Na,K-ATPase, have proved valuable tools to characterize the binding site of cardiotonic steroids (CTSs), which also constitutes the extracellular cation pathway. Because existing literature suggests variations in the physiological responses caused by binding of different CTSs, we extended the original set of spin-labeled inhibitors to the more potent bufalin derivatives. Positioning of the spin labels within the Na,K-ATPase site was defined and visualized by molecular docking. Although the original cinobufagin labels exhibited lower affinity, continuous-wave electron paramagnetic resonance spectra of spin-labeled bufalins and cinobufagins revealed a high degree of pairwise similarity, implying that these two types of CTS bind in the same way. Further analysis of the spectral lineshapes of bound spin labels was performed with emphasis on their structure (PROXYL vs. TEMPO), as well as length and rigidity of the linkers. For comparable structures, the dynamic flexibility increased in parallel with linker length, with the longest linker placing the spin label at the entrance to the binding site. Temperature-related changes in spectral lineshapes indicate that six-membered nitroxide rings undergo boat-chair transitions, showing that the binding-site cross section can accommodate the accompanying changes in methyl-group orientation. D₂O-electron spin echo envelope modulation in pulse-electron paramagnetic resonance measurements revealed high water accessibilities and similar polarity profiles for all bound spin labels, implying that the vestibule leading to steroid-binding site and cation-binding sites is relatively wide and water-filled.

Binding of cardiotonic steroids to Na+,K+-ATPase in the E2P state

The sodium pump (Na⁺, K⁺-ATPase, NKA) is vital for animal cells, as it actively maintains Na⁺ and K⁺ electrochemical gradients across the cell membrane. It is a target of cardiotonic steroids (CTSs) such as ouabain and digoxin. As CTSs are almost unique strong inhibitors specific to NKA, a wide range of derivatives has been developed for potential therapeutic use. Several crystal structures have been published for NKA-CTS complexes, but they fail to explain the largely different inhibitory properties of the various CTSs. For instance, although CTSs are thought to inhibit ATPase activity by binding to NKA in the E2P state, we do not know if large conformational changes accompany binding, as no crystal structure is available for the E2P state free of CTS. Here, we describe crystal structures of the BeF₃ ^- complex of NKA representing the E2P ground state and then eight crystal structures of seven CTSs, including rostafuroxin and istaroxime, two new members under clinical trials, in complex with NKA in the E2P state. The conformations of NKA are virtually identical in all complexes with and without CTSs, showing that CTSs bind to a preformed cavity in NKA. By comparing the inhibitory potency of the CTSs measured under four different conditions, we elucidate how different structural features of the CTSs result in different inhibitory properties. The crystal structures also explain K⁺-antagonism and suggest a route to isoform specific CTSs.

Na+/K+-ATPase Revisited: On Its Mechanism of Action, Role in Cancer, and Activity Modulation

Maintenance of Na⁺ and K⁺ gradients across the cell plasma membrane is an essential process for mammalian cell survival. An enzyme responsible for this process, sodium-potassium ATPase (NKA), has been currently extensively studied as a potential anticancer target, especially in lung cancer and glioblastoma. To date, many NKA inhibitors, mainly of natural origin from the family of cardiac steroids (CSs), have been reported and extensively studied. Interestingly, upon CS binding to NKA at nontoxic doses, the role of NKA as a receptor is activated and intracellular signaling is triggered, upon which cancer cell death occurs, which lies in the expression of different NKA isoforms than in healthy cells. Two major CSs, digoxin and digitoxin, originally used for the treatment of cardiac arrhythmias, are also being tested for another indication—cancer. Such drug repositioning has a big advantage in smoother approval processes. Besides this, novel CS derivatives with improved performance are being developed and evaluated in combination therapy. This article deals with the NKA structure, mechanism of action, activity modulation, and its most important inhibitors, some of which could serve not only as a powerful tool to combat cancer, but also help to decipher the so-far poorly understood NKA regulation.

CG4928 Is Vital for Renal Function in Fruit Flies and Membrane Potential in Cells: A First In-Depth Characterization of the Putative Solute Carrier UNC93A

The number of transporter proteins that are not fully characterized is immense. Here, we used Drosophila melanogaster and human cell lines to perform a first in-depth characterization of CG4928, an ortholog to the human UNC93A, of which little is known. Solute carriers regulate and maintain biochemical pathways important for the body, and malfunctioning transport is associated with multiple diseases. Based on phylogenetic analysis, CG4928 is closely related to human UNC93A and has a secondary and a tertiary protein structure and folding similar to major facilitator superfamily transporters. Ubiquitous knockdown of CG4928 causes flies to have a reduced secretion rate from the Malpighian tubules; altering potassium content in the body and in the Malpighian tubules, homologous to the renal system; and results in the development of edema. The edema could be rescued by using amiloride, a common diuretic, and by maintaining the flies on ion-free diets. CG4928-overexpressing cells did not facilitate the transport of sugars and amino acids; however, proximity ligation assay revealed that CG4928 co-localized with TASK¹ channels. Overexpression of CG4928 resulted in induced apoptosis and cytotoxicity, which could be restored when cells were kept in high-sodium media. Furthermore, the basal membrane potential was observed to be disrupted. Taken together, the results indicate that CG4928 is of importance for generating the cellular membrane potential by an unknown manner. However, we speculate that it most likely acts as a regulator or transporter of potassium flows over the membrane.

Exploring the Therapeutic Potential of Membrane Transport Proteins: Focus on Cancer and Chemoresistance

Improving the therapeutic efficacy of conventional anticancer drugs represents the best hope for cancer treatment. However, the shortage of druggable targets and the increasing development of anticancer drug resistance remain significant problems. Recently, membrane transport proteins have emerged as novel therapeutic targets for cancer treatment. These proteins are essential for a plethora of cell functions ranging from cell homeostasis to clinical drug toxicity. Furthermore, their association with carcinogenesis and chemoresistance has opened new vistas for pharmacology-based cancer research. This review provides a comprehensive update of our current knowledge on the functional expression profile of membrane transport proteins in cancer and chemoresistant tumours that may form the basis for new cancer treatment strategies.

Transient Electrical Currents Mediated by the Na+/K+-ATPase: A Tour from Basic Biophysics to Human Diseases

The Na⁺/K⁺-ATPase is a chemical molecular machine responsible for the movement of Na⁺ and K⁺ ions across the cell membrane. These ions are moved against their electrochemical gradients, so the protein uses the free energy of ATP hydrolysis to transport them. In fact, the Na⁺/K⁺-ATPase is the single largest consumer of energy in most cells. In each pump cycle, the protein sequentially exports 3Na⁺ out of the cell, then imports 2K⁺ into the cell at an approximate rate of 200 cycles/s. In each half cycle of the transport process, there is a state in which ions are stably trapped within the permeation pathway of the protein by internal and external gates in their closed states. These gates are required to open alternately; otherwise, passive ion diffusion would be a wasteful end of the cell's energy. Once one of these gates open, ions diffuse from their binding sites to the accessible milieu, which involves moving through part of the electrical field across the membrane. Consequently, ions generate transient electrical currents first discovered more than 30 years ago. They have been studied in a variety of preparations, including native and heterologous expression systems. Here, we review three decades' worth of work using these transient electrical signals to understand the kinetic transitions of the movement of Na⁺ and K⁺ ions through the Na⁺/K⁺-ATPase and propose the significance that this work might have to the understanding of the dysfunction of human pump orthologs responsible for some newly discovered neurological pathologies.

Curcumin Attenuates Colistin-Induced Peripheral Neurotoxicity in Mice

Peripheral neurotoxicity often occurs in patients receiving parenteral polymyxin therapy (i.e., colistin methanesulfonate or polymyxin B). The present study aimed to investigate the protective effect of curcumin on colistin-induced peripheral neurotoxicity using a murine model. Female C57BL/6 mice (n = 10 in each group) were randomly divided into the following: (1) control group (saline), (2) curcumin only group (200 mg/kg/day; orally), (3) colistin only group (18 mg/kg/day; i.p.), (4) colistin (18 mg/kg/day) plus curcumin 50 mg/kg/day group, (5) colistin (18 mg/kg/day) plus curcumin 100 mg/kg/day group, (6) colistin (18 mg/kg/day) plus curcumin 200 mg/kg/day group; all mice were treated for 7 days. Orally applied curcumin was detected in the brain, cerebellum, and sciatic nerve. Co-administration of oral curcumin markedly improved colistin-induced impaired sensory and motor dysfunctions in a dose-dependent manner. Curcumin supplementation at 100 and 200 mg/kg significantly decreased lipid peroxidation and upregulated catalase (CAT) and superoxide dismutase (SOD) activities, ATP levels, and Na⁺/K⁺-ATPase activity in sciatic nerve tissue, compared to the colistin alone group. Curcumin supplementation at 200 mg/kg upregulated the levels of AKT, NGF, mTOR, Nrf2, and HO-1 mRNA and concomitantly downregulated Bax, caspases-3, and -9 mRNA; it also decreased caspase-3 and caspase-9 activity. In summary, for the first time, our study reveals that the protective effect of oral curcumin on colistin induced peripheral neurotoxicity is associated with the activation of NGF/Akt and Nrf2/HO-1 pathways and inhibition of oxidative stress. This study highlights the potential clinical application of curcumin as an oral neuroprotective agent coadministered during colistin therapy.

A Model for the Homotypic Interaction between Na+,K+-ATPase β1 Subunits Reveals the Role of Extracellular Residues 221-229 in Its Ig-Like Domain

The Na⁺, K⁺-ATPase transports Na⁺ and K⁺ across the membrane of all animal cells. In addition to its ion transporting function, the Na⁺, K⁺-ATPase acts as a homotypic epithelial cell adhesion molecule via its β₁ subunit. The extracellular region of the Na⁺, K⁺-ATPase β₁ subunit includes a single globular immunoglobulin-like domain. We performed Molecular Dynamics simulations of the ectodomain of the β₁ subunit and a refined protein-protein docking prediction. Our results show that the β₁ subunit Ig-like domain maintains an independent structure and dimerizes in an antiparallel fashion. Analysis of the putative interface identified segment Lys221-Tyr229. We generated triple mutations on YFP-β₁ subunit fusion proteins to assess the contribution of these residues. CHO fibroblasts transfected with mutant β₁ subunits showed a significantly decreased cell-cell adhesion. Association of β₁ subunits in vitro was also reduced, as determined by pull-down assays. Altogether, we conclude that two Na⁺, K⁺-ATPase molecules recognize each other by a large interface spanning residues 221–229 and 198–207 on their β₁ subunits.

Colistin induced peripheral neurotoxicity involves mitochondrial dysfunction and oxidative stress in mice

Polymyxin is a critical antibiotic against the infection caused by multidrug-resistant gram-negative bacteria. Neurotoxicity is one of main dose-limiting factors. The present study aimed to investigate the underlying molecular mechanism on colistin induced peripheral neurotoxicity using a mouse model. Forty mice were divided into control, colistin 1-, 3- and 7-day groups, the mice were intravenously injected with saline or colistin (sulfate) at the dose of 15 mg/kg/day for 1, 3 and 7 days, respectively. The results showed that, colistin treatment for 7 days markedly resulted in the demyelination, axonal degeneration and mitochondria swelling in the mice's sciatic tissues. Colistin treatment induces oxidative stress as well as the increases of mitochondrial permeability transition, decreases of membrane potential (ΔΨ_m) and activities of mitochondrial respiratory chain in the mice's sciatic nerve tissues. Furthermore, in the colistin-7 day group, adenosine-triphosphate (ATP) level Na⁺/K⁺-ATPase activity decreased to 75.2% (p < 0.01) and 80.1% (p < 0.01), respectively. Meanwhile, colistin treatment down-regulates the expression of protein kinase B (Akt) and mammalian target of rapamycin (mTOR) mRNAs and up-regulates the expression of Bax and caspase-3 mRNAs. Our results reveal that colistin induced sciatic nerves damage involves oxidative stress, mitochondrial dysfunction and the inhibition of Akt/mTOR pathway.

Construction of K+ responsive surface on SEBS to reduce the hemolysis of preserved erythrocytes

Hemolysis of stored erythrocytes is a big obstacle for the development of new plasticizer-free polymer containers. Hemolysis is mainly caused by cell membrane oxidation and cation leaks from the intracellular fluid during storage. To construct an anti-hemolytic surface for a plasticizer-free polymer, we fabricated 2-O-α-d-glucopyranosyl-l-ascorbic acid (AA-2G)-loaded polycaprolactone (PCL)-crown ether micro/nanofibers on the surface of styrene-b-(ethylene-co-butylene)-b-styrene (SEBS). Our strategy is based on the sensitive response of the crown ether to leaked potassium, causing the release of AA-2G, the AA-2G can then remove the excess ROS, maintaining the Na/K-pump activity and the cell integrity. We demonstrated that the PCL-crown ether micro/nanofibers have been well prepared on the surface of SEBS; the micro/nanofibers provide a sensitive response to excess K+ and trigger the rapid release of AA-2G. AA-2G then acts as an antioxidant to reduce the excess ROS and maintain the Na/K-pump activity to mitigate cation leaks, resulting in the reduced hemolysis of the preserved erythrocytes. Our work thus provides a novel method for the development of plasticizer-free polymers for the storage of erythrocytes, and has the potential to be used to fabricate long-term anti-hemolytic biomaterials for in vivo use.

Influences: The Cell Physiology Laboratory in Montemar, Chile

Bezanilla recalls the profound influence that the Laboratory of Cell Physiology in Montemar, with its investigators and visitors, have had in shaping his scientific career.

Proton Countertransport and Coupled Gating in the Sarcoplasmic Reticulum Calcium Pump

The calcium pump of the sarcoplasmic reticulum (SERCA) is an ATP-driven active transporter of Ca²⁺ ions that functions via an "alternating-access" cycle mechanism. In each cycle, SERCA transports two Ca²⁺ ions toward the lumen of the sarcoplasmic reticulum and two to three protons to the cytoplasm. How the latter conformational transition is coupled to cytoplasmic release of protons remains poorly understood. The present computational study shows how the mechanism of proton countertransport is coupled to the alternating access gating process in SERCA. Molecular dynamics simulation trajectories are generated starting from a series of configurations taken along the E2 to E1 transition pathway determined by the string method with swarms-of-trajectories. Simulations of different protonation configurations at the binding sites reveal how deprotonation events affect the opening of the cytoplasmic gate. The results show that there is a strong coupling between the chronological order of deprotonation, the entry of water molecules into the TM region, and the opening of the cytoplasmic gate. Deprotonation of E309 and E771 is sequential with E309 being the first to lose the proton. The deprotonation promotes the opening of the cytoplasmic gate but leads to a productive gating transition only if it occurs after the transmembrane domain has reached an intermediate conformation. Deprotonation of E309 and E771 is unproductive when it occurs too early because it causes the re-opening of the luminal gate.

K+ binding and proton redistribution in the E2P state of the H+, K+-ATPase

The H+, K+-ATPase (HKA) uses ATP to pump protons into the gastric lumen against a million-fold proton concentration gradient while counter-transporting K+ from the lumen. The mechanism of release of a proton into a highly acidic stomach environment, and the subsequent binding of a K+ ion necessitates a network of protonable residues and dynamically changing protonation states in the cation binding pocket dominated by five acidic amino acid residues E343, E795, E820, D824, and D942. We perform molecular dynamics simulations of spontaneous K+ binding to all possible protonation combinations of the acidic amino acids and carry out free energy calculations to determine the optimal protonation state of the luminal-open E2P state of the pump which is ready to bind luminal K+. A dynamic pKa correlation analysis reveals the likelihood of proton transfer events within the cation binding pocket. In agreement with in-vitro measurements, we find that E795 is likely to be protonated, and that E820 is at the center of the proton transfer network in the luminal-open E2P state. The acidic residues D942 and D824 are likely to remain protonated, and the proton redistribution occurs predominantly amongst the glutamate residues exposed to the lumen. The analysis also shows that a lower number of K+ ions bind at lower pH, modeled by a higher number of protons in the cation binding pocket, in agreement with the 'transport stoichiometry variation' hypothesis.

Gating currents

A voltage change across a membrane protein moves charges or dipoles producing a gating current that is an electrical expression of a conformational change.

Many membrane proteins sense the voltage across the membrane where they are inserted, and their function is affected by voltage changes. The voltage sensor consists of charges or dipoles that move in response to changes in the electric field, and their movement produces an electric current that has been called gating current. In the case of voltage-gated ion channels, the kinetic and steady-state properties of the gating charges provide information of conformational changes between closed states that are not visible when observing ionic currents only. In this Journal of General Physiology Milestone, the basic principles of voltage sensing and gating currents are presented, followed by a historical description of the recording of gating currents. The results of gating current recordings are then discussed in the context of structural changes in voltage-dependent membrane proteins and how these studies have provided new insights on gating mechanisms.

Volumetric, Viscosity and Conductance Studies of Solute–Solute and Solute–Solvent Interactions of Some Alkali Metal Chlorides in Aqueous Citric Acid at Different Temperatures

The present study aims for the structure-making and structure-breaking behavior of some electrolytes in aqueous citric acid solution. The density, viscosity and conductance of some alkali metal chlorides lithium chloride (LiCl), sodium chloride (NaCl) and potassium chloride (KCl) in 0.01 m aqueous citric acid have been measured in the concentration range 0.01–0.12 m at 303.15, 308.15, 313.15 and 318.15 K. From these measurements, molar volume, viscosity parameters and molar conductance have been deliberated. Debye Hückel limiting law is used for the assessment of the contributions of various types of solute–solvent interactions. Jones–Dole viscosity equation is used to calculate viscosity B-coefficient for these salts in aqueous citric acid, which is known to provide information concerning the solvation of ions and their effects on the structure of the solvent in the near environment of the solute particles. The free energies of activation of viscous flow per mole of solvent, Δ μ 1 0 ‡ $\Delta \mu _1^{0\ddagger }$ and solute, Δ μ 2 0 ‡ , $\Delta \mu _2^{0\ddagger },$ have also been evaluated by using viscosity data. Using molar volume, the transfer volume Фv o tr has also been computed. The structure making/ breaking behavior of LiCl, NaCl and KCl is inferred from the sign of second derivative of partial molar volume with respect to temperature at constant pressure (d2φv o/dT2)p, Temperature coefficient of B. dB/dT and temperature coefficient of Walden product i.e. d(Λm oηo)/dT values. It has been found from these studies that LiCl, NaCl and KCl behave as structure-breaker in 0.01 m aqueous citric acid solution. The results have been qualitatively used to explain the molecular interaction and structural changes between the components of these mixtures.

Invited review: Mineral absorption mechanisms, mineral interactions that affect acid-base and antioxidant status, and diet considerations to improve mineral status

Several minerals are required for life to exist. In animals, 7 elements (Ca, P, Mg, Na, K, Cl, and S) are required to be present in the diet in fairly large amounts (grams to tens of grams each day for the dairy cow) and are termed macrominerals. Several other elements are termed microminerals or trace minerals because they are required in much smaller amounts (milligrams to micrograms each day). In most cases the mineral in the diet must be absorbed across the gastrointestinal mucosa and enter the blood if it is to be of value to the animal. The bulk of this review discusses the paracellular and transcellular mechanisms used by the gastrointestinal tract to absorb each of the various minerals needed. Unfortunately, particularly in ruminants, interactions between minerals and other substances within the diet can occur within the digestive tract that impair mineral absorption. The attributes of organic or chelated minerals that might permit diet minerals to circumvent factors that inhibit absorption of more traditional inorganic forms of these minerals are discussed. Once absorbed, minerals are used in many ways. One focus of this review is the effect macrominerals have on the acid-base status of the animal. Manipulation of dietary cation and anion content is commonly used as a tool in the dry period and during lactation to improve performance. A section on how the strong ion theory can be used to understand these effects is included. Many microminerals play a role in the body as cofactors of enzymes involved in controlling free radicals within the body and are vital to antioxidant capabilities. Those same minerals, when consumed in excess, can become pro-oxidants in the body, generating destructive free radicals. Complex interactions between minerals can compromise the effectiveness of a diet in promoting health and productivity of the cow. The objective of this review is to provide insight into some of these mechanisms.

Intracellular Requirements for Passive Proton Transport through the Na+,K+-ATPase

The Na⁺,K⁺-ATPase (NKA or Na/K pump) hydrolyzes one ATP to exchange three intracellular Na+ (Na⁺_i) for two extracellular K+ (K⁺_o) across the plasma membrane by cycling through a set of reversible transitions between phosphorylated and dephosphorylated conformations, alternately opening ion-binding sites externally (E2) or internally (E1). With subsaturating [Na⁺]_o and [K⁺]_o, the phosphorylated E2P conformation passively imports protons generating an inward current (I_H), which may be exacerbated in NKA-subunit mutations associated with human disease. To elucidate the mechanisms of I_H, we studied the effects of intracellular ligands (transported ions, nucleotides, and beryllium fluoride) on I_H and, for comparison, on transient currents measured at normal Na⁺_o (Q_Na). Utilizing inside-out patches from Xenopus oocytes heterologously expressing NKA, we observed that 1) in the presence of Na⁺_i, I_H and Q_Na were both activated by ATP, but not ADP; 2) the [Na⁺]_i dependence of I_H in saturating ATP showed K_0.5,Na = 1.8 ± 0.2 mM and the [ATP] dependence at saturating [Na⁺]_i yielded K_0.5,ATP = 48 ± 11 μM (in comparison, Na⁺_i-dependent Q_Na yields K_0.5,Na = 0.8 ± 0.2 mM and K_0.5,ATP = 0.43 ± 0.03 μM; 3) ATP activated I_H in the presence of K⁺_i (∼15% of the I_H observed in Na⁺_i) only when Mg²⁺_i was also present; and 4) beryllium fluoride induced maximal I_H even in the absence of nucleotide. These data indicate that I_H occurs when NKA is in an externally open E2P state with nucleotide bound, a conformation that can be reached through forward Na/K pump phosphorylation of E1, with Na⁺_i and ATP, or by backward binding of K⁺_i to E1, which drives the pump to the occluded E2(2K), where free P_i (at the micromolar levels found in millimolar ATP solutions) promotes external release of occluded K⁺ by backdoor NKA phosphorylation. Maximal I_H through beryllium-fluorinated NKA indicates that this complex mimics ATP-bound E2P states.

Biophysical comparison of ATP-driven proton pumping mechanisms suggests a kinetic advantage for the rotary process depending on coupling ratio

ATP-driven proton pumps, which are critical to the operation of a cell, maintain cytosolic and organellar pH levels within a narrow functional range. These pumps employ two very different mechanisms: an elaborate rotary mechanism used by V-ATPase H⁺ pumps, and a simpler alternating access mechanism used by P-ATPase H⁺ pumps. Why are two different mechanisms used to perform the same function? Systematic analysis, without parameter fitting, of kinetic models of the rotary, alternating access and other possible mechanisms suggest that, when the ratio of protons transported per ATP hydrolyzed exceeds one, the one-at-a-time proton transport by the rotary mechanism is faster than other possible mechanisms across a wide range of driving conditions. When the ratio is one, there is no intrinsic difference in the free energy landscape between mechanisms, and therefore all mechanisms can exhibit the same kinetic performance. To our knowledge all known rotary pumps have an H⁺:ATP ratio greater than one, and all known alternating access ATP-driven proton pumps have a ratio of one. Our analysis suggests a possible explanation for this apparent relationship between coupling ratio and mechanism. When the conditions under which the pump must operate permit a coupling ratio greater than one, the rotary mechanism may have been selected for its kinetic advantage. On the other hand, when conditions require a coupling ratio of one or less, the alternating access mechanism may have been selected for other possible advantages resulting from its structural and functional simplicity.

Computer modelling reveals new conformers of the ATP binding loop of Na+/K+-ATPase involved in the transphosphorylation process of the sodium pump

Hydrolysis of ATP by Na⁺/K⁺-ATPase, a P-Type ATPase, catalyzing active Na⁺ and K⁺ transport through cellular membranes leads transiently to a phosphorylation of its catalytical α-subunit. Surprisingly, three-dimensional molecular structure analysis of P-type ATPases reveals that binding of ATP to the N-domain connected by a hinge to the P-domain is much too far away from the Asp³⁶⁹ to allow the transfer of ATP’s terminal phosphate to its aspartyl-phosphorylation site. In order to get information for how the transfer of the γ-phosphate group of ATP to the Asp³⁶⁹ is achieved, analogous molecular modeling of the M₄–M₅ loop of ATPase was performed using the crystal data of Na⁺/K⁺-ATPase of different species. Analogous molecular modeling of the cytoplasmic loop between Thr³³⁸ and Ile⁷⁶⁰ of the α₂-subunit of Na⁺/K⁺-ATPase and the analysis of distances between the ATP binding site and phosphorylation site revealed the existence of two ATP binding sites in the open conformation; the first one close to Phe⁴⁷⁵ in the N-domain, the other one close to Asp³⁶⁹ in the P-domain. However, binding of Mg²⁺•ATP to any of these sites in the “open conformation” may not lead to phosphorylation of Asp³⁶⁹. Additional conformations of the cytoplasmic loop were found wobbling between “open conformation” <==> “semi-open conformation <==> “closed conformation” in the absence of 2Mg²⁺•ATP. The cytoplasmic loop’s conformational change to the “semi-open conformation”—characterized by a hydrogen bond between Arg⁵⁴³ and Asp⁶¹¹—triggers by binding of 2Mg²⁺•ATP to a single ATP site and conversion to the “closed conformation” the phosphorylation of Asp³⁶⁹ in the P-domain, and hence the start of Na⁺/K⁺-activated ATP hydrolysis.

Plasma membrane ion channels and epithelial to mesenchymal transition in cancer cells

A variety of studies have suggested that epithelial to mesenchymal transition (EMT) may be important in the progression of cancer in patients through metastasis and/or therapeutic resistance. A number of pathways have been investigated in EMT in cancer cells. Recently, changes in plasma membrane ion channel expression as a consequence of EMT have been reported. Other studies have identified specific ion channels able to regulate aspects of EMT induction. The utility of plasma membrane ion channels as targets for pharmacological modulation make them attractive for therapeutic approaches to target EMT. In this review, we provide an overview of some of the key plasma membrane ion channel types and highlight some of the studies that are beginning to define changes in plasma membrane ion channels as a consequence of EMT and also their possible roles in EMT induction.

The selectivity of the Na(+)/K(+)-pump is controlled by binding site protonation and self-correcting occlusion

The Na⁺/K⁺-pump maintains the physiological K⁺ and Na⁺ electrochemical gradients across the cell membrane. It operates via an 'alternating-access' mechanism, making iterative transitions between inward-facing (E₁) and outward-facing (E₂) conformations. Although the general features of the transport cycle are known, the detailed physicochemical factors governing the binding site selectivity remain mysterious. Free energy molecular dynamics simulations show that the ion binding sites switch their binding specificity in E₁ and E₂. This is accompanied by small structural arrangements and changes in protonation states of the coordinating residues. Additional computations on structural models of the intermediate states along the conformational transition pathway reveal that the free energy barrier toward the occlusion step is considerably increased when the wrong type of ion is loaded into the binding pocket, prohibiting the pump cycle from proceeding forward. This self-correcting mechanism strengthens the overall transport selectivity and protects the stoichiometry of the pump cycle.

DOI:http://dx.doi.org/10.7554/eLife.16616.001

A protein called the sodium-potassium pump resides in the membrane that surrounds living cells. The role of this protein is to 'pump' sodium and potassium ions across the membrane to help restore their concentration inside and outside of the cell. About 25% of the body's energy is used to keep the pump going, rising to nearly 70% in the brain. Problems that affect the pump have been linked to several disorders, including heart, kidney and metabolic diseases, as well as severe neurological conditions.

The sodium-potassium pump must be able to effectively pick out the correct ions to transport from a mixture of many different types of ions. However, it was not clear how the pump succeeds in doing this efficiently. Rui et al. have now used a computational method called molecular dynamics simulations to model how the sodium-potassium pump transports the desired ions across the cell membrane.

The pump works via a so-called 'alternating-access' mechanism, repeatedly transitioning between inward-facing and outward-facing conformations. In each cycle, it binds three sodium ions from the cell’s interior and exports them to the outside. Then, the pump binds to two potassium ions from outside the cell and imports them inside. Although the bound sodium and potassium ions interact with similar binding sites in the pump, the pump sometimes preferentially binds sodium, and sometimes potassium. The study performed by Rui et al. shows that this preference is driven by how protons (hydrogen ions) bind to the amino acids that make up the binding site.

The simulations also suggest that the pump uses a ‘self-correcting’ mechanism to prevent the pump from transporting the wrong types of ions. When incorrect ions are present at the binding sites, the pump cycle pauses temporarily until the ions detach from the pump. Only when the correct ions are bound will the pump cycle continue again.

In the future, Rui et al. hope to use long time-scale molecular dynamics simulations to show the conformational transition in action. In addition, the 'self-correcting' mechanism will be directly tested by letting the wrong and correct ions compete for the binding sites to see whether the pump will transport only the correct ions.

DOI:http://dx.doi.org/10.7554/eLife.16616.002

Combined effects of seawater acidification and salinity changes in Ruditapes philippinarum

Due to human activities, predictions for the coming years indicate increasing frequency and intensity of extreme weather events (rainy and drought periods) and pollution levels, leading to salinity shifts and ocean acidification. Therefore, several authors have assessed the effects of seawater salinity shifts and pH decrease on marine bivalves, but most of these studies evaluated the impacts of both factors independently. Since pH and salinity may act together in the environment, and their impacts may differ from their effects when acting alone, there is an urgent need to increase our knowledge when these environmental changes act in combination. Thus, the present study assessed the effects of seawater acidification and salinity changes, both acting alone and in combination, on the physiological (condition index, Na and K concentrations) and biochemical (oxidative stress related biomarkers) performance of Ruditapes philippinarum. For that, specimens of R. philippinarum were exposed for 28days to the combination of different pH levels (7.8 and 7.3) and salinities (14, 28 and 35). The results obtained showed that under control pH (7.8) and low salinity (14) the physiological status and biochemical performance of clams was negatively affected, revealing oxidative stress. However, under the same pH and at salinities 28 and 35 clams were able to maintain/regulate their physiological status and biochemical performance. Moreover, our findings showed that clams under low pH (7.3) and different salinities were able to maintain their physiological status and biochemical performance, suggesting that the low pH tested may mask the negative effects of salinity. Our results further demonstrated that, in general, at each salinity, similar physiological and biochemical responses were found in clams under both tested pH levels. Also, individuals under low pH (salinities 14, 28 and 25) and exposed to pH 7.8 and salinity 28 (control) tend to present a similar response pattern. These results indicate that pH may have a lower impact on clams than salinity. Thus, our findings point out that the predicted increase of CO2 in seawater and consequent seawater acidification will have fewer impacts on physiological and biochemical performance of R. philippinarum clams than salinity shifts.

Down-regulated Na(+)/K(+)-ATPase activity in ischemic penumbra after focal cerebral ischemia/reperfusion in rats

This study was aimed to examine whether the Na(+)/K(+) adenosine triphosphatase (Na(+)/K(+)-ATPase) activity in ischemic penumbra is associated with the pathogenesis of ischemia/reperfusion-induced brain injury. An experimental model of cerebral ischemia/reperfusion was made by transient middle cerebral artery occlusion (tMCAO) in rats and the changes of Na(+)/K(+)-ATPase activity in the ischemic penumbra was examined by Enzyme Assay Kit. Extensive infarction was observed in the frontal and parietal cortical and subcortical areas at 6 h, 24 h, 48 h, 3 d and 7 d after tMCAO. Enzyme Assay analyses revealed the activity of Na(+)/K(+)-ATPase was decreased in the ischemic penumbra of model rats after focal cerebral ischemia/reperfusion compared with sham-operated rats, and reduced to its minimum at 48 h, while the infarct volume was enlarged gradually. In addition, accompanied by increased brain water content, apoptosis-related bcl-2 and Bax proteins, apoptotic index and neurologic deficits Longa scores, but fluctuated the ratio of bcl-2/Bax. Correlation analysis showed that the infarct volume, apoptotic index, neurologic deficits Longa scores and brain water content were negatively related with Na(+)/K(+)-ATPase activity, while the ratio of bcl-2/Bax was positively related with Na(+)/K(+)-ATPase activity. Our results suggest that down-regulated Na(+)/K(+)-ATPase activity in ischemic penumbra might be involved in the pathogenesis of cerebral ischemia/reperfusion injury presumably through the imbalance ratio of bcl-2/Bax and neuronal apoptosis, and identify novel target for neuroprotective therapeutic intervention in cerebral ischemic disease.