Streaming potentials in gramicidin channels measured with ion-selective microelectrodes

Geometrical and electrophysiological data of the moving membrane method for the osmotic water permeability of a lipid bilayer

Data of the osmotic water permeability of a lipid bilayer (diphytanoylphosphaticylcholin) in the presence of cholesterol (30 mole%) are shown under the simultaneous measurement of bilayer tension. Detailed methods and procedures for evaluating the water permeability using the moving membrane method (K. Yano, M. Iwamoto, T. Koshiji & S. Oiki: Visualizing the Osmotic Water Permeability of a Lipid Bilayer under Measured Bilayer Tension Using a Moving Membrane Method. Journal of Membrane Science, 627 (2021) 119231) are presented. The planar lipid bilayer is formed in a glass capillary, separating two aqueous compartments with different osmolarities, and osmotically-driven water flux is visualized as membrane movements along the capillary. The water permeability was evaluated under constant membrane area and tension after correcting for the unstirred layer effect. In these measurements, geometrical features, such as the edge of the planar lipid bilayer and the contact angle between bilayer and monolayer, were image-analyzed. The unstirred layer was evaluated electrophysiologically, in which gramicidin A channel was employed. In the presence of an osmotic gradient, the gramicidin channel generates the streaming potential, and the measured streaming potential data and the derived water-ion coupling ratio (water flux/ion flux) are shown. Detailed descriptions of the integrated method of the moving membrane allow researchers to reproduce the experiment and give opportunities to examine water permeability of various types of membranes, including those containing aquaporins. The present data of osmotic water permeability are compared with the previously published data, while they neglected the bilayer tension.

Electrolyte transport pathways induced in the midgut epithelium of Drosophila melanogaster larvae by commensal gut microbiota and pathogens

KEY POINTS

The digestive tract of larval and adult Drosophila is an excellent analogue of the mammalian gut. Enterocytes of the posterior midgut are separated by septa, with no paracellular path, and therefore perform both immune and transport functions. Using microperfusion electrophysiology, we show that larvae emerging from the embryo into sterile medium have symmetrical apical and basal membrane conductances while larvae emerging into non-sterile medium have apical membranes fivefold more conductive than basal membranes. The channels inserted into the apical membranes could originate in microbiata or host and mediate recognition of microbes. Entomopathogenic cyclic peptide toxins deplete intracellular ions reversibly, forming transient ion channels that do not conduct water, unlike an ionophore like nystatin that depletes ions irreversibly. We show the feasibility of studying the interaction of a single microbial species, or tractable combinatorials of microbial species, with only enterocytes in the primary epithelial barrier.

ABSTRACT

Microbiota colonizing exposed epithelial surfaces are vital for sustenance of metazoan life, but communication between microbiota, epithelial cells and the host immune system is only beginning to be understood. We address this issue in the posterior midgut epithelium of Drosophila larvae where nutrient transport and immune functions are exclusively transcellular. We showed that larvae emerging into a sterile post-embryonic environment have symmetrical apical and basal membranes. In contrast, larvae emerging into non-sterile media, the source of microbiota, have markedly asymmetrical membranes, with apical membrane conductance more than fivefold higher than the basal membrane. As an example of pathogen action, we showed that the entomopathogenic fungal toxin destruxin A (Dx) depleted intracellular ions. Reversibility of action of Dx was verified by bilayer reconstitution in forming transient non-specific channels that conduct ions but not water. Dx was also less effective from the apical side as compared to the basal side of the epithelium. We also showed that intercellular septa are not conductive in non-sterile cells, even though most cells are isopotential. Luminal microbiota therefore impart asymmetry to the epithelium, by activation of apical membrane conductance, enhancing inter-enterocyte communication, separated by insulating septa, via the gut lumen. These results also open the possibility of studying the basis of bidirectional molecular conversation specifically between enterocytes and microbiota that enables discrimination between commensals and pathogens, establishment of the former, and elimination of the latter.

Noncontact dipole effects on channel permeation. III. Anomalous proton conductance effects in gramicidin

Proton transport on water wires, of interest for many problems in membrane biology, is analyzed in side-chain analogs of gramicidin A channels. In symmetrical 0.1N HCl solutions, fluorination of channel Trp(11), Trp-(13), or Trp(15) side chains is found to inhibit proton transport, and replacement of one or more Trps with Phe enhances proton transport, the opposite of the effects on K(+) transport in lecithin bilayers. The current-voltage relations are superlinear, indicating that some membrane field-dependent process is rate limiting. The interfacial dipole effects are usually assumed to affect the rate of cation translocation across the channel. For proton conductance, however, water reorientation after proton translocation is anticipated to be rate limiting. We propose that the findings reported here are most readily interpreted as the result of dipole-dipole interactions between channel waters and polar side chains or lipid headgroups. In particular, if reorientation of the water column begins with the water nearest the channel exit, this hypothesis explains the negative impact of fluorination and the positive impact of headgroup dipole on proton conductance.

Stochastic study of the effect of ionic strength on noncovalent interactions in protein pores

Salt plays a critical role in the physiological activities of cells. We show that ionic strength significantly affects the kinetics of noncovalent interactions in protein channels, as observed in stochastic studies of the transfer of various analytes through pores of wild-type and mutant alpha-hemolysin proteins. As the ionic strength increased, the association rate constant of electrostatic interactions was accelerated, whereas those of both hydrophobic and aromatic interactions were retarded. Dramatic decreases in the dissociation rate constants, and thus increases in the overall reaction formation constants, were observed for all noncovalent interactions studied. The results suggest that with the increase of salt concentration, the streaming potentials for all the protein pores decrease, whereas the preferential selectivities of the pores for either cations or anions drop. Furthermore, results also show that the salt effect on the rate of association of analytes to a pore differs significantly depending on the nature of the noncovalent interactions occurring in the protein channel. In addition to providing new insights into the nature of analyte-protein pore interactions, the salt-dependence of noncovalent interactions in protein pores observed provides a useful means to greatly enhance the sensitivity of the nanopore, which may find useful application in stochastic sensing.

Framework models of ion permeation through membrane channels and the generalized King-Altman method

A modern approach to studying the detailed dynamics of biomolecules is to simulate them on computers. Framework models have been developed to incorporate information from these simulations in order to calculate properties of the biomolecules on much longer time scales than can be achieved by the simulations. They also provide a simple way to think about the simulated dynamics. This article develops a method for the solution of framework models, which generalizes the King-Altman method of enzyme kinetics. The generalized method is used to construct solutions of two framework models which have been introduced previously, the single-particle and Grotthuss (proton conduction) models. The solution of the Grotthuss model is greatly simplified in comparison with direct integration. In addition, a new framework model is introduced, generalizing the shaking stack model of ion conduction through the potassium channel.

Proton transfer in gramicidin water wires in phospholipid bilayers: attenuation by phosphoethanolamine

The transfer of protons in water wires was studied in native gramicidin A (gA), and in the SS- and RR-diastereoisomers of dioxolane-linked gA channels (SS and RR channels). These peptides were incorporated into membranes comprised of distinct combinations of phospholipid headgroups and acyl chains. Quantitative relationships between single channel conductances to H+ (g(H)) and [H+] were determined in distinct phospholipid membranes, and are in remarkable contrast with results previously obtained in monoglyceride membranes. In particular: 1), g(H)-[H+] relationships for the various gA channels in distinct phospholipid membranes are well fitted by single adsorption isotherms. A simple kinetic model assuming mono-occupancy of channels by protons fits said relationships. This does not occur with monoglyceride membranes. 2), Under nonsaturating [H+], g(H) is approximately 1 order of magnitude larger in phospholipid than in monoglyceride membranes. 3), Differences between rates of H+ transfer in various gA channels are still present but considerably attenuated in phospholipid relative to monoglyceride membranes. 4), Charged phospholipid headgroups affect g(H) via changes in [H+] at the membrane/solution interfaces. 5), Phosphoethanolamine groups caused a marked attenuation of g(H) relative to membranes with other phospholipid headgroups. This attenuation is voltage-dependent and tends to saturate H+ currents at voltages larger than 250 mV. This effect is likely to occur by limiting the access and exit of H+ in and out of the channel due to relatively strong oriented H-bonds between waters and phosphoethanolamine groups at channel interfaces. The differential effects of phospholipids on proton transfer could be reasoned by considering solvation effects of side chain residues of gramicidin channels by double acyl chains and by the presence of polar headgroups facilitating the entrance/exit of protons through the channel mouths.

Et tu, Grotthuss! and other unfinished stories

This review article is divided into three sections. In Section 1, a short biographical note on Freiherr von Grotthuss is followed by a detailed summary of the main findings and ideas present in his 1806 paper. Attempts to place Grotthuss contribution in the context of the science done at his time were also made. In Section 2, the modern version of the Grotthuss mechanism is reviewed. The classical Grotthuss model has been recently questioned and new mechanisms and ideas regarding proton transfer are briefly discussed. The last section discusses the significance of a classical Grotthuss mechanism for proton transfer in water chains inside protein cavities. This has been an interesting new twist in the ongoing history of the Grotthuss mechanism. A summary and discussion of what was learned from probably the simplest currently available experimental models of proton transfer in water wires in semi-synthetic ion channels are critically presented. This review ends discussing some of the questions that need to be addressed in the near future.

Coupled K+-water flux through the HERG potassium channel measured by an osmotic pulse method

The streaming potential (V(stream)) is a signature feature of ion channels in which permeating ions and water molecules move in a single file. V(stream) provides a quantitative measure of the ion and water flux (the water-ion coupling ratio), the knowledge of which is a prerequisite for elucidating the mechanisms of ion permeation. We have developed a method to measure V(stream) with the whole-cell patch-clamp configuration. A HEK293 cell stably expressing the HERG potassium channel was voltage clamped and exposed to hyperosmotic solutions for short periods of time (<1 s) by an ultrafast solution switching system (the osmotic pulse [quick jump-and-away] method). The reversal potentials were monitored by a series of voltage ramps before, during, and after the osmotic pulse. The shifts of the reversal potentials immediately after the osmotic jump gave V(stream). In symmetrical K+ solutions (10 mM), the V(stream)s measured at different osmolalities showed a linear relationship with a slope of -0.7 mV/DeltaOsm, from which the water-ion coupling ratio (n, the ratio of the flux of water to the flux of cations; Levitt, D.G., S.R. Elias, and J.M. Hautman. 1978. Biochim. Biophys. Acta. 512:436-451) was calculated to be 1.4. In symmetrical 100 mM K+ solutions, the coupling ratio was decreased significantly (n = 0.9), indicating that the permeation process through states with increased ion occupancy became significant. We presented a diagrammatic representation linking the water-ion coupling ratio to the mode of ion permeation and suggested that the coupling ratio of one may represent the least hydrated ion flux in the single-file pore.

Electroosmotic enhancement of the binding of a neutral molecule to a transmembrane pore

The flux of solvent water coupled to the transit of ions through protein pores is considerable. The effect of this electroosmotic solvent flow on the binding of a neutral molecule [beta-cyclodextrin (betaCD)] to sites within the staphylococcal alpha-hemolysin pore was investigated. Mutant alpha-hemolysin pores were used to which betaCD can bind from either entrance and through which the direction of water flow can be controlled by choosing the charge selectivity of the pore and the polarity of the applied potential. The Kd values for betaCD for individual mutant pores varied by >100-fold with the applied potential over a range of -120 to +120 mV. In all cases, the signs of the changes in binding free energy and the influence of potential on the association and dissociation rate constants for betaCD were consistent with an electroosmotic effect.

Water permeation through gramicidin A: desformylation and the double helix: a molecular dynamics study

Multinanosecond molecular dynamics simulations of gramicidin A embedded in a dimyristoylphosphatidylcholine bilayer show a remarkable structural stability for both experimentally determined conformations: the head-to-head helical dimer and the double helix. Water permeability was found to be much higher in the double helical conformation, which is explained by lower hydrogen bond-mediated enthalpic barriers at the channel entrance and its larger pore size. Free-energy perturbation calculations show that the double helical structure is stabilized by the positive charges at the N termini introduced by the desformylation, whereas the helical dimer is destabilized. Together with the recent experimental observation that desformyl gramicidin conducts water hundredfold better than gramicidin, this suggests that desformyl gramicidin A predominantly occurs in the double helical conformation.

Desformylgramicidin: a model channel with an extremely high water permeability

The water conductivity of desformylgramicidin exceeds the permeability of gramicidin A by two orders of magnitude. With respect to its single channel hydraulic permeability coefficient of 1.1.10(-12) cm(3) s(-1), desformylgramicidin may serve as a model for extremely permeable aquaporin water channel proteins (AQP4 and AQPZ). This osmotic permeability exceeds the conductivity that is predicted by the theory of single-file transport. It was derived from the concentration distributions of both pore-impermeable and -permeable cations that were simultaneously measured by double barreled microelectrodes in the immediate vicinity of a planar bilayer. From solvent drag experiments, approximately five water molecules were found to be transported by a single-file process along with one ion through the channel. The single channel proton, potassium, and sodium conductivities were determined to be equal to 17 pS (pH 2.5), 7 and 3 pS, respectively. Under any conditions, the desformyl-channel remains at least 10 times longer in its open state than gramicidin A.

Scanning electrochemical microscopy. 40. Voltammetric ion-selective micropipet electrodes for probing ion transfer at bilayer lipid membranes

Voltammetric ion-selective micropipet electrodes for use in scanning electrochemical microscopy (SECM) for detection of potassium ion were fabricated. These used pulled borosilicate capillaries with tip orifice radii of 0.7-20 microm with silanized inner walls filled with a solution of 10 mM valinomycin and 10 mM ETH 500 in dichloroethane. The electrodes were characterized by determining the steady-state tip current for K+ concentrations of 0.05-0.3 mM. The tips were used in the SECM feedback and generation-collection modes to study K+ transfer through gramicidin channels in a horizontal bilayer lipid membrane (glycerol monooleate).

Solvent drag across gramicidin channels demonstrated by microelectrodes

The competition of ion and water fluxes across gramicidin channels was assessed from the concentration distributions of both pore-impermeable and -permeable cations that were simultaneously measured by double-barreled microelectrodes in the immediate vicinity of a planar bilayer. Because water movement across the membrane led to accumulation of solutes on one side of the membrane and depletion on the other, the permeable cation was not only pushed by water across the channel (true solvent drag); it also flowed along its concentration gradient (pseudo-solvent drag). For the demonstration of true solvent drag, a difference between the bulk concentrations on the hypertonic and the hypotonic sides of the membrane was established. It was adjusted to get equal cation concentrations at both membrane/water interfaces. From the sodium and potassium fluxes measured along with membrane conductivity under these conditions, approximately five water molecules were found to be transported simultaneously with one ion through the channel. In diphytanoyl phosphatidylcholine membranes, a single-channel hydraulic permeability coefficient of 1.6 x 10(-14) cm(3) s(-1) was obtained.

Gramicidin D conformation, dynamics and membrane ion transport

The linear pentadecapeptide antibiotic, gramicidin D, a heterogeneous mixture of six components, is a naturally occurring product of Bacillus brevis known to form ion channels in synthetic and natural membranes. The conformation of gramicidin A in the solid state, in organic solvents, and in planar lipid bilayers and the relationship between the composition and the conformation of gramicidin and its selective transport of ions across membranes has been the subject of intense investigation for over 50 years. The x-ray crystal structure and nmr solution spectroscopy agree fully with one another and reveal that entirely different conformations of gramicidin are present in uncomplexed and ion complexed forms. Precise refinements of the three-dimensional structures of naturally occurring gramicidin D in crystals obtained from methanol, ethanol, and n-propanol demonstrate the unexpected presence of stable left-handed antiparallel double-helical heterodimers that vary with the crystallization solvent. The side chains of Trp residues in the three structures exhibit sequence-specific patterns of conformational preference. Tyr substitution for Trp at position 11 appears to favor beta ribbon formation and stabilization of the antiparallel double helix. This conformation acts as a template for gramicidin folding and nucleation of the different crystal forms. The fact that a minor component in a heterogeneous mixture influences aggregation and crystal nucleation has potential applications to other systems in which anomalous behavior is exhibited by aggregation of apparently homogeneous materials, such as the enigmatic behavior of prion proteins. The crystallographically determined structures of cesium, potassium, rubidium, and hydronium ion complexes of gramicidin A are in excellent agreement with the nmr structure determination of the cesium ion gramicidin complex in a methanol chloroform mixture (50 : 50). The right-handed antiparallel double stranded double helical structures (DSDHR) also exhibit geometric features compatible with the solid-state 15N and 2H nmr data recorded for gramicidin in planar lipid bilayers and attributed to the active form of gramicidin A. The DSDHR crystal structures reveal an ion channel with a single partially solvated cation distributed over three ion binding sites. The channel lumen is relatively smooth and electrostatically negative as required for cation passage, while the exterior is electrostatically neutral, a requirement for membrane insertion. The "coordination" of the Cs+ ion is achieved by interaction with the pi orbitals of the carbonyls which do not point toward the ions. The K+ binding sites, which are similar in position to Cs+ binding sites, are shifted off center slightly toward the wall of the channel.

The conducting form of gramicidin A is a right-handed double-stranded double helix

The linear pentadecapeptide antibiotic, gramicidin D, is a naturally occurring product of Bacillus brevis known to form ion channels in synthetic and natural membranes. The x-ray crystal structures of the right-handed double-stranded double-helical dimers (DSDH) reported here agree with 15N-NMR and CD data on the functional gramicidin D channel in lipid bilayers. These structures demonstrate single-file ion transfer through the channels. The results also indicate that previous crystal structure reports of a left-handed double-stranded double-helical dimer in complex with Cs+ and K+ salts may be in error and that our evidence points to the DSDH as the major conformer responsible for ion transport in membranes.