Molecular insights into ion channel function (Review)

Water is probably the most important molecule in biology. It solvates molecules, all biochemical reactions occur in it and it is a major driving force in protein folding. Phospholipid membranes separate different water environments, but connections do exist between the different compartments. The integral membrane proteins (IMPs) form these connections. In the case of ions, IMPs form the passageways that regulate ion movement across the membrane. Structural information from three ion distinct channels are examined to see how these channels first select for and then control the movement of their target ions. This review focuses on how these channels select for target ions and control their movement while taking into account and using different properties of water. This includes the use of hydrophobic gates, mimicking the water environment, and controlling ions indirectly by controlling water.     

Effects of membrane lipids on ion channel structure and function

Biologic membranes are not simply inert physical barriers, but complex and dynamic environments that affect membrane protein structure and function. Residing within these environments, ion channels control the flux of ions across the membrane through conformational changes that allow transient ion flux through a central pore. These conformational changes may be modulated by changes in transmembrane electrochemical potential, the binding of small ligands or other proteins, or changes in the local lipid environment. Ion channels play fundamental roles in cellular function and, in higher eukaryotes, are the primary means of intercellular signaling, especially between excitable cells such as neurons. The focus of this review is to examine how the composition of the bilayer affects ion channel structure and function. This is an important consideration because the bilayer composition varies greatly in different cell types and in different organellar membranes. Even within a membrane, the lipid composition differs between the inner and outer leaflets, and the composition within a given leaflet is both heterogeneous and highly dynamic. Differential packing of lipids (and proteins) leads to the formation of microdomains, and lateral diffusion of these microdomains or "lipid rafts" serve as mobile platforms for the clustering and organization of bilayer constituents including ion channels. The structure and function of these channels are sensitive to specific chemical interactions with neighboring components of the membrane and also to the biophysical properties of their membrane microenvironment (e.g., fluidity, lateral pressure profile, and bilayer thickness). As specific examples, we have focused on the K+ ion channels and the ligand-gated nicotinicoid receptors, two classes of ion channels that have been well-characterized structurally and functionally. The responsiveness of these ion channels to changes in the lipid environment illustrate how ion channels, and more generally, any membrane protein, may be regulated via cellular control of membrane composition.     

The hidden potential of lysosomal ion channels: A new era of oncogenes

Lysosomes serve as the control centre for cellular clearance. These membrane-bound organelles receive biomolecules destined for degradation from intracellular and extracellular pathways; thus, facilitating the production of energy and shaping the fate of the cell. At the base of their functionality are the lysosomal ion channels which mediate the function of the lysosome through the modulation of ion influx and efflux. Ion channels form pores in the membrane of lysosomes and allow the passage of ions, a seemingly simple task which harbours the potential of overthrowing the cell’s stability. Considered the master regulators of ion homeostasis, these integral membrane proteins enable the proper operation of the lysosome. Defects in the structure or function of these ion channels lead to the development of lysosomal storage diseases, neurodegenerative diseases and cancer. Although more than 50 years have passed since their discovery, lysosomes are not yet fully understood, with their ion channels being even less well characterized. However, significant improvements have been made in the development of drugs targeted against these ion channels as a means of combating diseases. In this review, we will examine how Ca²⁺, K⁺, Na⁺ and Cl⁻ ion channels affect the function of the lysosome, their involvement in hereditary and spontaneous diseases, and current ion channel-based therapies.     

The ion channels to cytoskeleton connection as potential mechanism of mechanosensitivity

As biological force-sensing systems mechanosensitive (MS) ion channels present the best example of coupling molecular dynamics of membrane proteins to the mechanics of the surrounding cell membrane. In animal cells MS channels have over the past two decades been very much in focus of mechanotransduction research. In recent years this helped to raise awareness of basic and medical researchers about the role that abnormal MS channels may play in the pathophysiology of diseases, such as cardiac hypertrophy, atrial fibrillation, muscular dystrophy or polycystic kidney disease. To date a large number of MS channels from organisms of diverse phylogenetic origins have been identified at the molecular level; however, the structure of only few of them has been determined. Although their function has extensively been studied in a great variety of cells and tissues by different experimental approaches it is, with exception of bacterial MS channels, very little known about how these channels sense mechanical force and which cellular components may contribute to their function. By focusing on MS channels found in animal cells this article discusses the ways in which the connections between cytoskeleton and ion channels may contribute to mechanosensory transduction in these cells. This article is part of a Special Issue entitled: Reciprocal influences between cell cytoskeleton and membrane channels, receptors and transporters. 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.     

A two-channel electrostatic model of an ionic counterport

An alternative model is presented for an ionic counterport that depends upon electrostatic rather than steric forces. It consists of two passive ion channels, one selective for I-type ions and the other for J-type ions. The ions interact electrostatically such that the presence of one type of ion within its channel affects the motion of the second type of ion within its channel. In these circumstances it is possible to arrange that the spontaneous flow of I ions across the membrane, down their electrochemical potential gradient, pumps J ions in the opposite direction across the membrane, against their electrochemical gradient. To illustrate this type of model, a particular example of interionic coupling is described in which both types of ion interact with the electric dipole moments of some membrane-spanning alpha-helical sections of the counterport protein complex. By assuming that a group of four alpha-helices is free to rotate slightly about an axis perpendicular to the membrane, the desired form of coupling is obtained. Making simplifying assumptions, it is possible to calculate the kinetics of the model and to compare these with those expected in real counterports. Finally it is shown that, if the helix group rotation is powered by an external energy source, the pair of coupled passive ion channels can mimic a primary exchange pump such as Na+-K+ ATPase. Here both types of ion are propelled in opposite directions across the membrane and simultaneously against their electrochemical potential gradients.     

Complex functional interaction between integrin receptors and ion channels

Integrin receptors mediate adhesion of the cell to the extracellular matrix and thereby regulate cell motility, proliferation, differentiation and apoptosis. These processes are frequently accompanied by alterations in ion flow. Recent evidence suggests that integrins can regulate ion channels and form macromolecular complexes, thus contributing to the localization of the channel onto the plasma membrane. The integrin-channel complex regulates downstream signaling proteins, such as tyrosine kinases and GTPases. This process could occur in plasma membrane microdomains, such as caveolae. It seems that ion channels sometimes transmit their signals through conformational coupling, instead of change in ion fluxes. Finally, the channel protein is not merely a final target, because it often feeds back by controlling integrin activation and/or expression. These findings have important implications for the physiology of normal and neoplastic cells and suggest interesting perspectives for studies of synaptic plasticity.     

Keeping active channels in their place: Membrane phosphoinositides regulate TRPM channel activity in a compartment-selective manner

We have long appreciated that the controlled movement of ions and solutes across the cell surface or plasma membrane affects every aspect of cell function, ranging from membrane excitability to metabolism to secretion, and is also critical for the long-term maintenance of cell viability. Studies examining these physiological transport processes have revealed a vast array of ion channels, transporters and ATPase-driven pumps that underlie these transmembrane ionic movements and how acquired or genetic disruption of these processes are linked to disease. More recently, it has become evident that the ongoing function of intracellular organelles and subcellular compartments also depends heavily on the controlled movement of ions to establish distinct pH or ionic environments. However, limited experimental access to these subcellular domains/structures has hampered scientific progress in this area, due in large part to the difficulty of applying proven functional assays, such as patch clamp and radiotracer methodologies, to these specialized membrane locations. Using both functional and immune-labeling assays, we now know that the types and complement of channels, transporters and pumps located within intracellular membranes and organelles often differ from those present on the plasma membrane. Moreover, it appears that this differential distribution is due to the presence of discrete tags/signals present within these transport proteins that dictate their sorting/trafficking to spatially discrete membrane compartments, where they may also interact with scaffolding proteins that help maintain their localization. Such targeting signals may thus operate in a manner analogous to the way a postal code is used to direct the delivery of a letter.     

Two decades with dimorphic Chloride Intracellular Channels (CLICs)

Plasma membrane channels have been extensively studied, and their physiological roles are well established. In contrast, relatively little information is available about intracellular ion channels. Chloride Intracellular Channel (CLICs) proteins are a novel class of putative intracellular ion channels. They are widely expressed in different intracellular compartments, and possess distinct properties such as the presence of a single transmembrane domain, and a dimorphic existence as either a soluble or membranous form. How these soluble proteins unfold, target to, and auto-insert into the intracellular membranes to form functional integral ion channels is a complex biological question. Recent information from studies of their crystal structures, biophysical characterization and functional roles has provoked interest in these unusual channels.     

Predicting ion channels and their types by the dipeptide mode of pseudo amino acid composition

Ion channels are integral membrane proteins that control movement of ions into or out of cells. They are key components in a wide range of biological processes. Different types of ion channels have different biological functions. With the appearance of vast proteomic data, it is highly desirable for both basic research and drug-target discovery to develop a computational method for the reliable prediction of ion channels and their types. In this study, we developed a support vector machine-based method to predict ion channels and their types using primary sequence information. A feature selection technique, analysis of variance (ANOVA), was introduced to remove feature redundancy and find out an optimized feature set for improving predictive performance. Jackknife cross-validated results show that the proposed method can discriminate ion channels from non-ion channels with an overall accuracy of 86.6%, classify voltage-gated ion channels and ligand-gated ion channels with an overall accuracy of 92.6% and predict four types (potassium, sodium, calcium and anion) of voltage-gated ion channels with an overall accuracy of 87.8%, respectively. These results indicate that the proposed method can correctly identify ion channels and provide important instructions for drug-target discovery. The predictor can be freely downloaded from http://cobi.uestc.edu.cn/people/hlin/tools/IonchanPred/.     

Functional Consequences of Oxidative Membrane Damage

The interaction of reactive oxygen species with biological membranes is known to produce a great variety of different functional modifications. Part of these modifications may be classified as direct effects. They are due to direct interaction of the reactive species with the molecular machinery under study with a subsequent chemical and functional modification of these molecules. An important part of the observed functional modifications are, however, indirect effects. They are the consequence of an oxidative modification of the environment of biological macromolecules. Lipid peroxidation—via its generation of chemically reactive products—contributes to the loss of cellular functions through the inactivation of membrane enzymes and even of cytoplasmic (i.e., water soluble) proteins. Oxidation of membrane lipids may, however, also increase the efficiency of membrane functions. This was observed for a series of transport systems. Lipid peroxidation was accompanied by activation of certain types of ion channels and ion carriers. The effect is due to an increase of the polarity of the membrane interior by accumulation of polar oxidation products. The concomitant change of the dielectric constant, which may be detected via the increase of the membrane capacitance, facilitates the opening of membrane channels and lowers the inner membrane barrier for the movement of ions across the membrane. The predominant effect, however, at least at a greater extent of lipid peroxidation, is the inhibition of membrane functions. The strong increase of the leak conductance contributes to the depolarization of the membrane potential, it destroys the barrier properties of the membrane and it may finally lead, via an increase of cytoplasmic Ca²⁺ concentration, to cell death. The conclusions were derived from experiments performed with different systems: model systems in planar lipid membranes, native ion channels either reconstituted in lipid membranes or investigated in their natural environment by the patch-clamp method, and two important ion pumps, the Na/K-ATPase and the sarcoplasmic reticulum (SR) Ca-ATPase.     

A marriage of convenience: beta-subunits and voltage-dependent K+ channels

The movement of ions across cell membranes is essential for a wide variety of fundamental physiological processes, including secretion, muscle contraction, and neuronal excitation. This movement is possible because of the presence in the cell membrane of a class of integral membrane proteins dubbed ion channels. Ion channels, thanks to the presence of aqueous pores in their structure, catalyze the passage of ions across the otherwise ion-impermeable lipid bilayer. Ion conduction across ion channels is highly regulated, and in the case of voltage-dependent K(+) channels, the molecular foundations of the voltage-dependent conformational changes leading to the their open (conducting) configuration have provided most of the driving force for research in ion channel biophysics since the pioneering work of Hodgkin and Huxley (Hodgkin, A. L., and Huxley, A. F. (1952) J. Physiol. 117, 500-544). The voltage-dependent K(+) channels are the prototypical voltage-gated channels and govern the resting membrane potential. They are responsible for returning the membrane potential to its resting state at the termination of each action potential in excitable membranes. The pore-forming subunits (alpha) of many voltage-dependent K(+) channels and modulatory beta-subunits exist in the membrane as one component of macromolecular complexes, able to integrate a myriad of cellular signals that regulate ion channel behavior. In this review, we have focused on the modulatory effects of beta-subunits on the voltage-dependent K(+) (Kv) channel and on the large conductance Ca(2+)- and voltage-dependent (BK(Ca)) channel.     

Members of the Arabidopsis AtTPK/KCO family form homomeric vacuolar channels in planta

The Arabidopsis thaliana K⁺ channel family of AtTPK/KCO proteins consists of six members including a 'single-pore' (K_ir-type) and five 'tandem-pore' channels. AtTPK4 is currently the only ion channel of this family for which a function has been demonstrated in planta . The protein is located at the plasma membrane forming a voltage-independent K⁺ channel that is blocked by extracellular calcium ions. In contrast, AtTPK1 is a tonoplast-localized protein, that establishes a K⁺-selective, voltage-independent ion channel activated by cytosolic calcium when expressed in a heterologous system, i.e. yeast. Here, we provide evidence that other AtTPK/KCO channel subunits, i.e. AtTPK2, AtTPK3, AtTPK5 and AtKCO3, are also targeted to the vacuolar membrane, opening the possibility that they interact at the target membrane to form heteromeric ion channels. However, when testing the cellular expression patterns of AtTPK / KCO genes we observed distinct expression domains that overlap in only a few tissues of the Arabidopsis plant, making it unlikely that different channel subunits interact to form heteromeric channels. This conclusion was substantiated by in planta expression of combinations of selected tonoplast AtTPK/KCO proteins. Fluorescence resonance energy transfer assays indicate that protein interaction occurs between identical channel subunits (most efficiently between AtTPK1 or AtKCO3) but not between different channel subunits. The finding could be confirmed by bimolecular fluorescence complementation assays. We conclude that tonoplast-located AtTPK/KCO subunits form homomeric ion channels in vivo .     

The ultrastructural effects of β-amyloid peptide on cultured PC12 cells: changes in cytoplasmic and intramembranous features.

Summary The fine structural features of cultured PC12 cells were investigated after treatment for 1, 3, or 5 days with different concentrations of the vascular form of β- 1–40 (β-AP). PC12 cells treated with β-AP showed time- and concentration-dependent lysosomal system activation and cell toxicity. We observed increases in the number and size of cytoplasmic lysosomes as indicated by increased acid phosphatase reactivity. Some lysosomes were in the form of multivesicular bodies or large residual bodies that appeared to arise by autophagia or by endocytotic uptake. Double-sided plasma membrane invaginations were observed to give rise to increasingly extensive intracytoplasmic vacuolization that was correlated with duration of β-AP treatment. Freeze-fracture studies of the intramembranous particle (IMP) population in the plasma membrane P-face showed that both control and β-AP treated cells had two major P-face IMP populations, small-diameter (4–8 nm) IMPs, and large-diameter (≤ 9nm) IMPs. The larger category of IMPs was found to possess a greater average diameter in the β-AP treated cells than in the control cells. These IMPs could represent modifications to existing transmembranous receptors, channels, or transducing molecules by the β-AP. These results demonstrate that β-AP can induce time- and concentration-dependent ultrastructural changes in PC12 cell membranes.     

The effect of ion channels on cell membrane motion

Cell membranes possess ion channels and pumps which allow them to pass electrical currents. In this paper it is shown that the ion channels and pumps should induce electric fields in the area surrounding the membrane. The magnitude of this field is computed. It is shown that when the channels and pumps are separated by a sufficient distance, then the electric fields which they induce will cause the solution which surrounds the membrane to move. This movement of the fluid should cause motion of the membrane. Thus it is demonstrated that under certain conditions the conductance of ions across cell membranes will give rise to cell membrane motion. It is suggested that this phenomenon may play a role in processes such as chemotaxis, cell division, and pinocytosis.     

Avenues of the membrane transport system in adaptation of plants to abiotic stresses

Abstract

Abiotic stress imposed by many factors such as: extreme water regimes, adverse temperatures, salinity, and heavy metal contamination result in severe crop yield losses worldwide. Plants must be able to quickly respond to these stresses in order to adapt to their growing conditions and minimize metabolic losses. In this context, transporter proteins play a vital role in regulating stress response mechanisms by facilitating movement of a variety of molecules and ions across the plasma membrane in order to maintain fundamental cellular processes such as ion homeostasis, osmotic adjustment, signal transduction, and detoxification. Aquaporins play a crucial role in alleviating abiotic stress by transporting water and other small molecules to maintain cellular homeostasis. Similarly, other transporter families such as CDF, ZIP, ABC, NHX, HKT, SWEETs, TMTs, and ion channels also contribute to abiotic stress tolerance. Hormones and other signaling molecules are necessary to coordinate responses across different tissues and to precisely regulate molecular trafficking. The present review highlights the current understanding of how membrane transporters orchestrate stress responses in plants. It also provides insights about the importance of these sensing and adaptive mechanisms for ensuring improved sustainable crop production during unfavorable conditions. Finally, this review discusses future prospects for the use of computational tools in constructing signaling networks to improve our understanding of the behavior of transporters under abiotic stress.     

Determinants of voltage-gated potassium channel surface expression and localization in Mammalian neurons

Neurons strictly regulate expression of a wide variety of voltage-dependent ion channels in their surface membranes to achieve precise yet dynamic control of intrinsic membrane excitability. Neurons also exhibit extreme morphological complexity that underlies diverse aspects of their function. Most ion channels are preferentially targeted to either the axonal or somatodendritic compartments, where they become further localized to discrete membrane subdomains. This restricted accumulation of ion channels enables local control of membrane signaling events in specific microdomains of a given compartment. Voltage-dependent K+ (Kv) channels act as potent modulators of diverse excitatory events such as action potentials, excitatory synaptic potentials, and Ca2+ influx. Kv channels exhibit diverse patterns of cellular expression, and distinct subtype-specific localization, in mammalian central neurons. Here we review the mechanisms regulating the abundance and distribution of Kv channels in mammalian neurons and discuss how dynamic regulation of these events impacts neuronal signaling.     

Cl? and K+ channels and their role in primary brain tumour biology

Profound cell volume changes occur in primary brain tumours as they proliferate, invade surrounding tissue or undergo apoptosis. These volume changes are regulated by the flux of Cl⁻ and K⁺ ions and concomitant movement of water across the membrane, making ion channels pivotal to tumour biology. We discuss which specific Cl⁻ and K⁺ channels are involved in defined aspects of glioma biology and how these channels are regulated. Cl⁻ is accumulated to unusually high concentrations in gliomas by the activity of the NKCC1 transporter and serves as an osmolyte and energetic driving force for volume changes. Cell volume condensation is required as cells enter M phase of the cell cycle and this pre-mitotic condensation is caused by channel-mediated ion efflux. Similarly, Cl⁻ and K⁺ channels dynamically regulate volume in invading glioma cells allowing them to adjust to small extracellular brain spaces. Finally, cell condensation is a hallmark of apoptosis and requires the concerted activation of Cl⁻ and Ca²⁺-activated K⁺ channels. Given the frequency of mutation and high importance of ion channels in tumour biology, the opportunity exists to target them for treatment.     

Characterization of Eag1 Channel Lateral Mobility in Rat Hippocampal Cultures by Single-Particle-Tracking with Quantum Dots

Voltage-gated ion channels are main players involved in fast synaptic events. However, only slow intracellular mechanisms have so far been described for controlling their localization as real-time visualization of endogenous voltage-gated channels at high temporal and spatial resolution has not been achieved yet. Using a specific extracellular antibody and quantum dots we reveal and characterize lateral mobility as a faster mechanism to dynamically control the number of endogenous ether-a-go-go (Eag)1 ion channels inside synapses. We visualize Eag1 entering and leaving synapses by lateral diffusion in the plasma membrane of rat hippocampal neurons. Mathematical analysis of their trajectories revealed how the motion of Eag1 gets restricted when the channels diffuse into the synapse, suggesting molecular interactions between Eag1 and synaptic components. In contrast, Eag1 channels switch to Brownian movement when they exit synapses and diffuse into extrasynaptic membranes. Furthermore, we demonstrate that the mobility of Eag1 channels is specifically regulated inside synapses by actin filaments, microtubules and electrical activity. In summary, using single-particle-tracking techniques with quantum dots nanocrystals, our study shows for the first time the lateral diffusion of an endogenous voltage-gated ion channel in neurons. The location-dependent constraints imposed by cytoskeletal elements together with the regulatory role of electrical activity strongly suggest a pivotal role for the mobility of voltage-gated ion channels in synaptic activity.     

Determinants of Voltage-Gated Potassium Channel Surface Expression and Localization in Mammalian Neurons

Neurons strictly regulate expression of a wide variety of voltage-dependent ion channels in their surface membranes to achieve precise yet dynamic control of intrinsic membrane excitability. Neurons also exhibit extreme morphological complexity that underlies diverse aspects of their function. Most ion channels are preferentially targeted to either the axonal or somatodendritic compartments, where they become further localized to discrete membrane subdomains. This restricted accumulation of ion channels enables local control of membrane signaling events in specific microdomains of a given compartment. Voltage-dependent K⁺ (Kv) channels act as potent modulators of diverse excitatory events such as action potentials, excitatory synaptic potentials, and Ca²⁺ influx. Kv channels exhibit diverse patterns of cellular expression, and distinct subtype-specific localization, in mammalian central neurons. Here we review the mechanisms regulating the abundance and distribution of Kv channels in mammalian neurons and discuss how dynamic regulation of these events impacts neuronal signaling.