Lipid bilayer-mediated regulation of ion channel function by amphiphilic drugs

A one-dimensional continuum elastic model for membrane-embedded gramicidin dimer dissociation

Membrane elastic properties, which are subject to alteration by compounds such as cholesterol, lipid metabolites and other amphiphiles, as well as pharmaceuticals, can have important effects on membrane proteins. A useful tool for measuring some of these effects is the gramicidin A channels, which are formed by transmembrane dimerization of non-conducting subunits that reside in each bilayer leaflet. The length of the conducting channels is less than the bilayer thickness, meaning that channel formation is associated with a local bilayer deformation. Electrophysiological studies have shown that the dimer becomes increasingly destabilized as the hydrophobic mismatch between the channel and the host bilayer increases. That is, the bilayer imposes a disjoining force on the channel, which grows larger with increasing hydrophobic mismatch. The energetic analysis of the channel-bilayer coupling is usually pursued assuming that each subunit, as well as the subunit-subunit interface, is rigid. Here we relax the latter assumption and explore how the bilayer junction responds to changes in this disjoining force using a simple one-dimensional energetic model, which reproduces key features of the bilayer regulation of gramicidin channel lifetimes.

Screening for small molecules' bilayer-modifying potential using a gramicidin-based fluorescence assay

Many drugs and other small molecules used to modulate biological function are amphiphiles that adsorb at the bilayer/solution interface and thereby alter lipid bilayer properties. This is important because membrane proteins are energetically coupled to their host bilayer by hydrophobic interactions. Changes in bilayer properties thus alter membrane protein function, which provides a possible mechanism for "off-target" drug effects. We have previously shown that channels formed by the linear gramicidins are suitable probes for changes in lipid bilayer properties, as experienced by bilayer-spanning proteins. We now report a gramicidin-based fluorescence assay for changes in bilayer properties. The assay is based on measuring the time course of fluorescence quenching in fluorophore-loaded large unilamellar vesicles, due to entry of a gramicidin channel-permeable quencher. The method is scalable and suitable for both mechanistic studies and high-throughput screening for bilayer-perturbing, potential off-target effects, which we illustrate using capsaicin (Cap) and other compounds.

Unsupported planar lipid membranes formed from mycolic acids of Mycobacterium tuberculosis

The cell wall of mycobacteria includes a thick, robust, and highly impermeable outer membrane made from long-chain mycolic acids. These outer membranes form a primary layer of protection for mycobacteria and directly contribute to the virulence of diseases such as tuberculosis and leprosy. We have formed in vitro planar membranes using pure mycolic acids on circular apertures 20 to 90 μm in diameter. We find these membranes to be long lived and highly resistant to irreversible electroporation, demonstrating their general strength. Insertion of the outer membrane channel MspA into the membranes was observed indicating that the artificial mycolic acid membranes are suitable for controlled studies of the mycobacterial outer membrane and can be used in nanopore DNA translocation experiments.

N-acyl amino acids and N-acyl neurotransmitter conjugates: neuromodulators and probes for new drug targets

The myriad functions of lipids as signalling molecules is one of the most interesting fields in contemporary pharmacology, with a host of compounds recognized as mediators of communication within and between cells. The N-acyl conjugates of amino acids and neurotransmitters (NAANs) have recently come to prominence because of their potential roles in the nervous system, vasculature and the immune system. NAAN are compounds such as glycine, GABA or dopamine conjugated with long chain fatty acids. More than 70 endogenous NAAN have been reported although their physiological role remains uncertain, with various NAAN interacting with a low affinity at G protein coupled receptors (GPCR) and ion channels. Regardless of their potential physiological function, NAAN are of great interest to pharmacologists because of their potential as flexible tools to probe new sites on GPCRs, transporters and ion channels. NAANs are amphipathic molecules, with a wide variety of potential fatty acid and headgroup moieties, a combination which provides a rich source of potential ligands engaging novel binding sites and mechanisms for modulation of membrane proteins such as GPCRs, ion channels and transporters. The unique actions of subsets of NAAN on voltage-gated calcium channels and glycine transporters indicate that the wide variety of NAAN may provide a readily exploitable resource for defining new pharmacological targets. Investigation of the physiological roles and pharmacological potential of these simple lipid conjugates is in its infancy, and we believe that there is much to be learnt from their careful study.

Fatty acid modulation and polyamine block of GluK2 kainate receptors analyzed by scanning mutagenesis

RNA editing of kainate receptor subunits at the Q/R site determines their susceptibility to inhibition by cis-unsaturated fatty acids as well as block by cytoplasmic polyamines. Channels comprised of unedited (Q) subunits are strongly blocked by polyamines, but insensitive to fatty acids, such as arachidonic acid (AA) and docosahexaenoic acid (DHA), whereas homomeric edited (R) channels resist polyamine block but are inhibited by AA and DHA. In the present study, we have analyzed fatty acid modulation of whole-cell currents mediated by homomeric recombinant GluK2 (formerly GluR6) channels with individual residues in the pore-loop, M1 and M3 transmembrane helices replaced by scanning mutagenesis. Our results define three abutting surfaces along the M1, M2, and M3 helices where gain-of-function substitutions render GluK2(Q) channels susceptible to fatty acid inhibition. In addition, we identify four locations in the M3 helix (F611, L614, S618, and T621) at the level of the central cavity where Arg substitution increases relative permeability to chloride and eliminates polyamine block. Remarkably, for two of these positions, L614R and S618R, exposure to fatty acids reduces the apparent chloride permeability and potentiates whole-cell currents approximately 5 and 2.5-fold, respectively. Together, our results suggest that AA and DHA alter the orientation of M3 in the open state, depending on contacts at the interface between M1, M2, and M3. Moreover, our results demonstrate the importance of side chains within the central cavity in determining ionic selectivity and block by cytoplasmic polyamines despite the inverted orientation of GluK2 as compared with potassium channels and other pore-loop family members.

Lysophospholipids modulate voltage-gated calcium channel currents in pituitary cells; effects of lipid stress

Voltage-gated calcium channels (VGCCs) are osmosensitive. The hypothesis that this property of VGCCs stems from their susceptibility to alterations in the mechanical properties of the bilayer was tested on VGCCs in pituitary cells using cone-shaped lysophospholipids (LPLs) to perturb bilayer lipid stress. LPLs of different head group size and charge were used: lysophosphatidylcholine (LPC), lysophosphatidylinositol (LPI), lysophosphatidylserine (LPS) and lysophosphatidylethanolamine (LPE). Phosphatidylcholine (PC) and LPC (C6:0) were used as controls. We show that partition of both LPC and LPI into the membrane of pituitary cells suppressed L-type calcium channel currents (I(L)). This suppression of I(L) was slow in onset, reversible upon washout with BSA and associated with a depolarizing shift in activation ( approximately 8mV). In contrast to these effects of LPC and LPI on I(L), LPS, LPE, PC and LPC (C6:0) exerted minimal or insignificant effects. This difference may be attributed to the prominent conical shape of LPC and LPI compared to the shapes of LPS and LPE (which have smaller headgroups), and to PC (which is cylindrical). The similar effects of LPC and LPI on I(L), despite differences in the structure and charge of their headgroups suggest a common lipid stress dependent mechanism in their action on VGCCs.

RNA editing modulates the binding of drugs and highly unsaturated fatty acids to the open pore of Kv potassium channels

The time course of inactivation of voltage-activated potassium (Kv) channels is an important determinant of the firing rate of neurons. In many Kv channels highly unsaturated lipids as arachidonic acid, docosahexaenoic acid and anandamide can induce fast inactivation. We found that these lipids interact with hydrophobic residues lining the inner cavity of the pore. We analysed the effects of these lipids on Kv1.1 current kinetics and their competition with intracellular tetraethylammonium and Kvbeta subunits. Our data suggest that inactivation most likely represents occlusion of the permeation pathway, similar to drugs that produce 'open-channel block'. Open-channel block by drugs and lipids was strongly reduced in Kv1.1 channels whose amino acid sequence was altered by RNA editing in the pore cavity, and in Kv1.x heteromeric channels containing edited Kv1.1 subunits. We show that differential editing of Kv1.1 channels in different regions of the brain can profoundly alter the pharmacology of Kv1.x channels. Our findings provide a mechanistic understanding of lipid-induced inactivation and establish RNA editing as a mechanism to induce drug and lipid resistance in Kv channels.

Thiopental inhibits function of different inward rectifying potassium channel isoforms by a similar mechanism

Thiopental is a well-known intravenous barbiturate anesthetic with important cardiac side effects. The actions of thiopental on the transmembrane ionic currents that determine the resting potential and action potential duration in cardiomyocytes have been studied widely. We aimed at elucidating the characteristics and mechanism of inhibition by thiopental on members of the subfamily of inward rectifying Kir2.x (Kir2.1, 2.2 and 2.3), Kir1.1 and Kir6.2/SUR2A channels. These inward rectifier potassium channels were transfected in HEK-293 cells and macroscopic currents were recorded in the whole-cell and inside-out configurations of the patch-clamp technique. Thiopental inhibited Kir2.1, Kir2.2, Kir2.3, Kir1.1 and Kir6.2/SUR2A currents with similar potency; in whole-cell experiments 30 microM thiopental decreased Kir2.1, Kir2.2, Kir2.3 and Kir1.1 currents to 55+/-6, 39+/-8, 42+/-5 and 49+/-5% at -120 mV, respectively. Point mutations on Kir2.3 (I213L) or Kir2.1 (L222I) did not modify the potency of block. Thiopental inhibited all Kir channels in a concentration-dependent and voltage-independent manner. Also, the time course of thiopental inhibition was slow (T(1/2) approximately 4 min) and independent of external or internal drug application. However, in the presence of PIP(2), inhibition by thiopental on Kir2.1 was significantly decreased. Thiopental at clinically relevant concentrations significantly inhibited all Kir channels evaluated in this work. The reduction of thiopental effects during PIP(2) treatment suggests that thiopental inhibition on Kir2.1 channels is related to channel-PIP(2) interaction.

Phosphatidylinositol 4,5-bisphosphate regulates plant K+ channels

Phosphoinositides play an important role in both abiotic and biotic signalling in plants. The signalling cascade may include the production of second messengers by hydrolysis of PtdIns(4,5)P2. However, increasingly, PtdIns(4,5)P2 itself is shown to mediate signalling by regulating target proteins. The present mini-review summarizes the experimentally demonstrated effects of PtdIns(4,5)P2 on plant K+ channels and examines their structure for candidate sites of direct PtdIns(4,5)P2–protein interaction.

FM dyes enter via a store-operated calcium channel and modify calcium signaling of cultured astrocytes

The amphiphilic fluorescent styryl pyridinium dyes FM1-43 and FM4-64 are used to probe activity-dependent synaptic vesicle cycling in neurons. Cultured astrocytes can internalize FM1-43 and FM4-64 inside vesicles but their uptake is insensitive to the elevation of cytosolic calcium (Ca(2+)) concentration and the underlying mechanism remains unclear. Here we used total internal reflection fluorescence microscopy and pharmacological tools to study the mechanisms of FM4-64 uptake into cultured astrocytes from mouse neocortex. Our data show that: (i) endocytosis is not a major route for FM4-64 uptake into astrocytes; (ii) FM4-64 enters astrocytes through an aqueous pore and strongly affects Ca(2+) homeostasis; (iii) partitioning of FM4-64 into the outer leaflet of the plasma membrane results in a facilitation of store-operated Ca(2+) entry (SOCE) channel gating; (iv) FM4-64 permeates and competes with Ca(2+) for entry through a SOCE channel; (v) intracellular FM4-64 mobilizes Ca(2+) from the endoplasmic reticulum stores, conveying a positive feedback to activate SOCE and to sustain dye uptake into astrocytes. Our study demonstrates that FM dyes are not markers of cycling vesicles in astrocytes and calls for a careful interpretation of FM fluorescence.

Lipid bilayer regulation of membrane protein function: gramicidin channels as molecular force probes

Membrane protein function is regulated by the host lipid bilayer composition. This regulation may depend on specific chemical interactions between proteins and individual molecules in the bilayer, as well as on non-specific interactions between proteins and the bilayer behaving as a physical entity with collective physical properties (e.g. thickness, intrinsic monolayer curvature or elastic moduli). Studies in physico-chemical model systems have demonstrated that changes in bilayer physical properties can regulate membrane protein function by altering the energetic cost of the bilayer deformation associated with a protein conformational change. This type of regulation is well characterized, and its mechanistic elucidation is an interdisciplinary field bordering on physics, chemistry and biology. Changes in lipid composition that alter bilayer physical properties (including cholesterol, polyunsaturated fatty acids, other lipid metabolites and amphiphiles) regulate a wide range of membrane proteins in a seemingly non-specific manner. The commonality of the changes in protein function suggests an underlying physical mechanism, and recent studies show that at least some of the changes are caused by altered bilayer physical properties. This advance is because of the introduction of new tools for studying lipid bilayer regulation of protein function. The present review provides an introduction to the regulation of membrane protein function by the bilayer physical properties. We further describe the use of gramicidin channels as molecular force probes for studying this mechanism, with a unique ability to discriminate between consequences of changes in monolayer curvature and bilayer elastic moduli.

Tamoxifen inhibits inward rectifier K+ 2.x family of inward rectifier channels by interfering with phosphatidylinositol 4,5-bisphosphate-channel interactions

Tamoxifen, an estrogen receptor antagonist used in the treatment of breast cancer, inhibits the inward rectifier potassium current (I(K1)) in cardiac myocytes by an unknown mechanism. We characterized the inhibitory effects of tamoxifen on Kir2.1, Kir2.2, and Kir2.3 potassium channels that underlie cardiac I(K1). We also studied the effects of 4-hydroxytamoxifen and raloxifene. All three drugs inhibited inward rectifier K(+) 2.x (Kir2.x) family members. The order of inhibition for all three drugs was Kir2.3 > Kir2.1 approximately Kir2.2. The onset of inhibition of Kir2.x current by these compounds was slow (T(1/2) approximately 6 min) and only partially recovered after washout ( approximately 30%). Kir2.x inhibition was concentration-dependent but voltage-independent. The time course and degree of inhibition was independent of external or internal drug application. We tested the hypothesis that tamoxifen interferes with the interaction between the channel and the membrane-delimited channel activator, phosphatidylinositol 4,5-bisphosphate (PIP(2)). Inhibition of Kir2.3 currents was significantly reduced by a single point mutation of I213L, which enhances Kir2.3 interaction with membrane PIP(2). Pretreatment with PIP(2) significantly decreased the inhibition induced by tamoxifen, 4-hydroxytamoxifen, and raloxifene on Kir2.3 channels. Pretreatment with spermine (100 microM) decreased the inhibitory effect of tamoxifen on Kir2.1, probably by strengthening the channel's interaction with PIP(2). In cat atrial and ventricular myocytes, 3 microM tamoxifen inhibited I(K1), but the effect was greater in the former than the latter. The data strongly suggest that tamoxifen, its metabolite, and the estrogen receptor inhibitor raloxifene inhibit Kir2.x channels indirectly by interfering with the interaction between the channel and PIP(2).

Determining the effects of lipophilic drugs on membrane structure by solid-state NMR spectroscopy: the case of the antioxidant curcumin

Curcumin is the active ingredient of turmeric powder, a natural spice used for generations in traditional medicines. Curcumin's broad spectrum of antioxidant, anticarcinogenic, antimutagenic, and anti-inflammatory properties makes it particularly interesting for the development of pharmaceutical compounds. Because of curcumin's various effects on the function of numerous unrelated membrane proteins, it has been suggested that it affects the properties of the bilayer itself. However, a detailed atomic-level study of the interaction of curcumin with membranes has not been attempted. A combination of solid-state NMR and differential scanning calorimetry experiments shows curcumin has a strong effect on membrane structure at low concentrations. Curcumin inserts deep into the membrane in a transbilayer orientation, anchored by hydrogen bonding to the phosphate group of lipids in a manner analogous to cholesterol. Like cholesterol, curcumin induces segmental ordering in the membrane. Analysis of the concentration dependence of the order parameter profile derived from NMR results suggests curcumin forms higher order oligomeric structures in the membrane that span and likely thin the bilayer. Curcumin promotes the formation of the highly curved inverted hexagonal phase, which may influence exocytotic and membrane fusion processes within the cell. The experiments outlined here show promise for understanding the action of other drugs such as capsaicin in which drug-induced alterations of membrane structure have strong pharmacological effects.

Determining the effects of lipophilic drugs on membrane structure by solid-state NMR spectroscopy: the case of the antioxidant curcumin

Curcumin is the active ingredient of turmeric powder, a natural spice used for generations in traditional medicines. Curcumin's broad spectrum of antioxidant, anticarcinogenic, antimutagenic, and anti-inflammatory properties makes it particularly interesting for the development of pharmaceutical compounds. Because of curcumin's various effects on the function of numerous unrelated membrane proteins, it has been suggested that it affects the properties of the bilayer itself. However, a detailed atomic-level study of the interaction of curcumin with membranes has not been attempted. A combination of solid-state NMR and differential scanning calorimetry experiments shows curcumin has a strong effect on membrane structure at low concentrations. Curcumin inserts deep into the membrane in a transbilayer orientation, anchored by hydrogen bonding to the phosphate group of lipids in a manner analogous to cholesterol. Like cholesterol, curcumin induces segmental ordering in the membrane. Analysis of the concentration dependence of the order parameter profile derived from NMR results suggests curcumin forms higher order oligomeric structures in the membrane that span and likely thin the bilayer. Curcumin promotes the formation of the highly curved inverted hexagonal phase, which may influence exocytotic and membrane fusion processes within the cell. The experiments outlined here show promise for understanding the action of other drugs such as capsaicin in which drug-induced alterations of membrane structure have strong pharmacological effects.

Inhibition of human recombinant T-type calcium channels by the endocannabinoid N-arachidonoyl dopamine

BACKGROUND AND PURPOSE

N-arachidonoyl dopamine (NADA) has complex effects on nociception mediated via cannabinoid CB(1) receptors and the transient receptor potential vanilloid receptor 1 (TRPV1). Anandamide, the prototypic CB(1)/TRPV1 agonist, also inhibits T-type voltage-gated calcium channel currents (I(Ca)). These channels are expressed by many excitable cells, including neurons involved in pain detection and processing. We sought to determine whether NADA and the prototypic arachidonoyl amino acid, N-arachidonoyl glycine (NAGly) modulate T-type I(Ca)

EXPERIMENTAL APPROACH

Human recombinant T-type I(Ca) (Ca(V)3 channels) expressed in HEK 293 cells and native mouse T-type I(Ca) were examined using standard whole-cell voltage clamp electrophysiology techniques.

KEY RESULTS

N-arachidonoyl dopamine completely inhibited Ca(V)3 channels with a rank order of potency (pEC(50)) of Ca(V)3.3 (6.45) > or = Ca(V)3.1 (6.29) > Ca(V)3.2 (5.95). NAGly (10 micromol.L(-1)) inhibited Ca(V)3 I(Ca) by approximately 50% or less. The effects of NADA and NAGly were voltage- but not use-dependent, and both compounds produced significant hyperpolarizing shifts in Ca(V)3 channel steady-state inactivation relationships. By contrast with anandamide, NADA and NAGly had modest effects on Ca(V)3 channel kinetics. Both NAGly and NADA inhibited native T-type I(Ca) in mouse sensory neurons.

CONCLUSIONS AND IMPLICATIONS

N-arachidonoyl dopamine and NAGly increase the steady-state inactivation of Ca(V)3 channels, reducing the number of channels available to open during depolarization. These effects occur at NADA concentrations at or below to those affecting CB(1) and TRPV1 receptors. Together with anandamide, the arachidonoyl neurotransmitter amides, NADA and NAGly, represent a new family of endogenous T-type I(Ca) modulators.

Phosphatidylinositol (4,5)bisphosphate inhibits K+-efflux channel activity in NT1 tobacco cultured cells

In the animal world, the regulation of ion channels by phosphoinositides (PIs) has been investigated extensively, demonstrating a wide range of channels controlled by phosphatidylinositol (4,5)bisphosphate (PtdInsP2). To understand PI regulation of plant ion channels, we examined the in planta effect of PtdInsP2 on the K+-efflux channel of tobacco (Nicotiana tabacum), NtORK (outward-rectifying K channel). We applied a patch clamp in the whole-cell configuration (with fixed "cytosolic" Ca2+ concentration and pH) to protoplasts isolated from cultured tobacco cells with genetically manipulated plasma membrane levels of PtdInsP2 and cellular inositol (1,4,5)trisphosphate: "Low PIs" had depressed levels of these PIs, and "High PIs" had elevated levels relative to controls. In all of these cells, K channel activity, reflected in the net, steady-state outward K+ currents (IK), was inversely related to the plasma membrane PtdInsP2 level. Consistent with this, short-term manipulations decreasing PtdInsP2 levels in the High PIs, such as pretreatment with the phytohormone abscisic acid (25 microM) or neutralizing the bath solution from pH 5.6 to pH 7, increased IK (i.e. NtORK activity). Moreover, increasing PtdInsP2 levels in controls or in abscisic acid-treated high-PI cells, using the specific PI-phospholipase C inhibitor U73122 (2.5-4 microM), decreased NtORK activity. In all cases, IK decreases stemmed largely from decreased maximum attainable NtORK channel conductance and partly from shifted voltage dependence of channel gating to more positive potentials, making it more difficult to activate the channels. These results are consistent with NtORK inhibition by the negatively charged PtdInsP2 in the internal plasma membrane leaflet. Such effects are likely to underlie PI signaling in intact plant cells.

Voltage-dependent K+ channel gating and voltage sensor toxin sensitivity depend on the mechanical state of the lipid membrane

Voltage-dependent K(+) (Kv) channels underlie action potentials through gating conformational changes that are driven by membrane voltage. In this study of the paddle chimera Kv channel, we demonstrate that the rate of channel opening, the voltage dependence of the open probability, and the maximum achievable open probability depend on the lipid membrane environment. The activity of the voltage sensor toxin VsTx1, which interferes with voltage-dependent gating by partitioning into the membrane and binding to the channel, also depends on the membrane. Membrane environmental factors that influence channel function are divisible into two general categories: lipid compositional and mechanical state. The mechanical state can have a surprisingly large effect on the function of a voltage-dependent K(+) channel, including its pharmacological interaction with voltage sensor toxins. The dependence of VSTx1 activity on the mechanical state of the membrane leads us to hypothesize that voltage sensor toxins exert their effect by perturbing the interaction forces that exist between the channel and the membrane.

Structure, function, and modification of the voltage sensor in voltage-gated ion channels

Voltage-gated ion channels are crucial for both neuronal and cardiac excitability. Decades of research have begun to unravel the intriguing machinery behind voltage sensitivity. Although the details regarding the arrangement and movement in the voltage-sensor domain are still debated, consensus is slowly emerging. There are three competing conceptual models: the helical-screw, the transporter, and the paddle model. In this review we explore the structure of the activated voltage-sensor domain based on the recent X-ray structure of a chimera between Kv1.2 and Kv2.1. We also present a model for the closed state. From this we conclude that upon depolarization the voltage sensor S4 moves approximately 13 A outwards and rotates approximately 180 degrees, thus consistent with the helical-screw model. S4 also moves relative to S3b which is not consistent with the paddle model. One interesting feature of the voltage sensor is that it partially faces the lipid bilayer and therefore can interact both with the membrane itself and with physiological and pharmacological molecules reaching the channel from the membrane. This type of channel modulation is discussed together with other mechanisms for how voltage-sensitivity is modified. Small effects on voltage-sensitivity can have profound effects on excitability. Therefore, medical drugs designed to alter the voltage dependence offer an interesting way to regulate excitability.