pH modulation of transport properties of alamethicin oligomers inserted in zwitterionic-based artificial lipid membranes

Heterosynaptic plasticity in biomembrane memristors controlled by pH

Abstract

In biology, heterosynaptic plasticity maintains homeostasis in synaptic inputs during associative learning and memory, and initiates long-term changes in synaptic strengths that nonspecifically modulate different synapse types. In bioinspired neuromorphic circuits, heterosynaptic plasticity may be used to extend the functionality of two-terminal, biomimetic memristors. In this article, we explore how changes in the pH of droplet interface bilayer aqueous solutions modulate the memristive responses of a lipid bilayer membrane in the pH range 4.97-7.40. Surprisingly, we did not find conclusive evidence for pH-dependent shifts in the voltage thresholds (V*) needed for alamethicin ion channel formation in the membrane. However, we did observe a clear modulation in the dynamics of pore formation with pH in time-dependent, pulsed voltage experiments. Moreover, at the same voltage, lowering the pH resulted in higher steady-state currents because of increased numbers of conductive peptide ion channels in the membrane. This was due to increased partitioning of alamethicin monomers into the membrane at pH 4.97, which is below the pKa (~5.3-5.7) of carboxylate groups on the glutamate residues of the peptide, making the monomers more hydrophobic. Neutralization of the negative charges on these residues, under acidic conditions, increased the concentration of peptide monomers in the membrane, shifting the equilibrium concentrations of peptide aggregate assemblies in the membrane to favor greater numbers of larger, increasingly more conductive pores. It also increased the relaxation time constants for pore formation and decay, and enhanced short-term facilitation and depression of the switching characteristics of the device. Modulating these thresholds globally and independently of alamethicin concentration and applied voltage will enable the assembly of neuromorphic computational circuitry with enhanced functionality.

Impact statement

We describe how to use pH as a modulatory "interneuron" that changes the voltage-dependent memristance of alamethicin ion channels in lipid bilayers by changing the structure and dynamical properties of the bilayer. Having the ability to independently control the threshold levels for pore conduction from voltage or ion channel concentration enables additional levels of programmability in a neuromorphic system. In this article, we note that barriers to conduction from membrane-bound ion channels can be lowered by reducing solution pH, resulting in higher currents, and enhanced short-term learning behavior in the form of paired-pulse facilitation. Tuning threshold values with environmental variables, such as pH, provide additional training and learning algorithms that can be used to elicit complex functionality within spiking neural networks.

Graphical abstract

Supplementary information

The online version contains supplementary material available at 10.1557/s43577-022-00344-z.

Tunable sustained release drug delivery system based on mononuclear aqueous core-polymer shell microcapsules

Poly(d,l-lactide-co-glycolide) (PLGA) and poly(d,l-lactide) (PLA) polymers were used successfully in the preparation of polymer shell microcapsules with mononuclear aqueous cores by the internal phase separation method. These microcapsules were prepared with varying amounts of polymer and water and loaded with fluorescein sodium as a model water soluble drug. Evaluation of drug loading and encapsulation efficiency reveals an optimum polymer to water ratio of around 1:3. Prepared PLGA and PLA microcapsules exhibit sustained drug release over 7 and 49 days, respectively. Drug release from both microcapsule types follow zero order kinetics over the first 90% release. Further tuning of release rate is found possible by preparing microcapsules with mixtures of PLGA and PLA polymers at varying ratios. These results suggest that aqueous core-PLGA and PLA microcapsules would be promising platforms for a wide range of sustained drug delivery systems for many hydrophilic drugs.

pH regulation of amphotericin B channels activity in the bilayer lipid membrane

Background:

Amphotericin B (AmB) is a polyene antibiotic frequently applied in the treatment of systemic fungal infections in spite of its secondary effects. The pH plays a crucial role in modulating biophysical features of ion channels in the bilayer lipid membranes.

Aim:

In this study, the role of pH in the regulation of AmB channel was assessed by single channel recording of ion channel incorporated in the artificial membrane.

Materials and Methods:

Bilayer lipid membrane was formed by phosphatidylcholine in a 350 μm diameter aperture between two chambers, cis and trans contained 200/50 mMKCl solutions, respectively; then AmB was incorporated into the bilayer lipid membrane. Single channel recordings were used to indicate the effects of pH changes on AmB channels activity. The records were analyzed by Clamp fit 10 software.

Results:

A kinetic analysis of single channel currents indicated a cation ion channel with 500 pS conductance and voltage-dependence of the open probability of the AmB channel (P_o). A reduction of cis pH to 6 decreased P_o and conductance. This effect was also voltage-dependent, being greater at a more positive above −40. The pH changes in the range of 6-8 had no effect on the reversal potential and ion selectivity.

Conclusion:

Our data indicated that extracellular acidity can reduce AmB activity.

The influence of halogen derivatives of thyronine and fluorescein on the dipole potential of phospholipid membranes

The effects of halogen derivatives of thyronine (tetraiodotironine and triiodothyronine) and fluorescein (Rose Bengal, phloxine B, erythrosin, eosin Y, and fluorescein) on the dipole potential of membranes composed of diphytanoylphosphocholine, diphytanoylphosphoserine, and diphytanoylphosphoethanolamine were investigated. A quantitative description of the modifying action of the agents was presented as characteristic parameters of the Langmuir adsorption isotherm: the maximum changes in the dipole potential of the membrane at an infinitely high concentration of modifiers and the desorption constant, characterizing their inverse affinities to the lipid phase. It was shown that the iodine-containing hormones led to a less significant reduction in the dipole potential of phospholipid membranes compared to the xanthene dyes, Rose Bengal, phloxine B, and erythrosin. The latter were characterized by the highest affinity for the lipid membranes compared to tetraiodotironine and triiodothyronine. It was found that the effect of iodine-containing hormones and xanthene dyes on the membrane dipole potential was caused by their uncharged and charged forms, respectively.

The effect of pH on the electrical capacitance of phosphatidylcholine-phosphatidylserine system in bilayer lipid membrane

This paper reports measurements on the pH dependence of the electrical capacitance of lipid membranes formed by 1:1 phosphatidylcholine-phosphatidylserine mixtures. A theoretical model was developed to describe this dependence, in which the contributions of functional groups (as the active centers of adsorption of the hydrogen and hydroxide ions) to the overall membrane capacitance were assumed to be additive. The proposed model was verified experimentally using electrochemical impedance spectroscopy. The theoretical predictions agreed with the experimental results over the measured pH range. A minimum corresponding to the isoelectric point appeared in both the theoretical equation and the experimental data.

Investigation of Cu2+ binding to human and rat amyloid fragments Aβ (1-16) with a protein nanopore

Recent evidence shows that metal coordination by amyloid beta peptides (Aβ) determines structural alterations of peptides, and His-13 from Aβ is crucial for Cu(2+) binding. This study used the truncated, more soluble Aβ1-16 isoforms derived from human and rat amyloid peptides to explore their interaction with Cu(2+) by employing the membrane-immobilized α-hemolysin (α-HL) protein as a nanoscopic probe in conjunction with single-molecule electrophysiology techniques. Unexpectedly, the experimental data suggest that unlike the case of the human Aβ1-16 peptide, Cu(2+) complexation by its rat counterpart leads to an augmented association and dissociation kinetics of the peptide reversible interaction with the protein pore, as compared to the Cu(2+)-free peptide. Single-molecule electrophysiology data reveal that both human and rat Cu(2+)-complexed Aβ peptides induce a higher degree of current flow obstruction through the α-HL pore, as compared to the Cu(2+)-free peptides. It is suggested that morphology changes brought by Cu(2+) binding to such amyloidic fragments depend crucially upon the presence of the His-13 residue on the primary sequence of such peptide fragments, and the α-HL protein-based approach provides unique opportunities and challenges to probing metal-induced folding of peptides.

A thermodynamic approach to alamethicin pore formation

The structure and energetics of alamethicin Rf30 monomer to nonamer in cylindrical pores of 5 to 11Å radius are investigated using molecular dynamics simulations in an implicit membrane model that includes the free energy cost of acyl chain hydrophobic area exposure. Stable, low energy pores are obtained for certain combinations of radius and oligomeric number. The trimer and the tetramer formed 6Å pores that appear closed while the larger oligomers formed open pores at their optimal radius. The hexamer in an 8Å pore and the octamer in an 11Å pore give the lowest effective energy per monomer. However, all oligomers beyond the pentamer have comparable energies, consistent with the observation of multiple conductance levels. The results are consistent with the widely accepted "barrel-stave" model. The N terminal portion of the molecule exhibits smaller tilt with respect to the membrane normal than the C terminal portion, resulting in a pore shape that is a hybrid between a funnel and an hourglass. Transmembrane voltage has little effect on the structure of the oligomers but enhances or decreases their stability depending on its orientation. Antiparallel bundles are lower in energy than the commonly accepted parallel ones and could be present under certain experimental conditions. Dry aggregates (without an aqueous pore) have lower average effective energy than the corresponding aggregates in a pore, suggesting that alamethicin pores may be excited states that are stabilized in part by voltage and in part by the ion flow itself.

Observing a model ion channel gating action in model cell membranes in real time in situ: membrane potential change induced alamethicin orientation change

Ion channels play crucial roles in transport and regulatory functions of living cells. Understanding the gating mechanisms of these channels is important to understanding and treating diseases that have been linked to ion channels. One potential model peptide for studying the mechanism of ion channel gating is alamethicin, which adopts a split α/3(10)-helix structure and responds to changes in electric potential. In this study, sum frequency generation vibrational spectroscopy (SFG-VS), supplemented by attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), has been applied to characterize interactions between alamethicin (a model for larger channel proteins) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) lipid bilayers in the presence of an electric potential across the membrane. The membrane potential difference was controlled by changing the pH of the solution in contact with the bilayer and was measured using fluorescence spectroscopy. The orientation angle of alamethicin in POPC lipid bilayers was then determined at different pH values using polarized SFG amide I spectra. Assuming that all molecules adopt the same orientation (a δ distribution), at pH = 6.7 the α-helix at the N-terminus and the 3(10)-helix at the C-terminus tilt at about 72° (θ(1)) and 50° (θ(2)) versus the surface normal, respectively. When pH increases to 11.9, θ(1) and θ(2) decrease to 56.5° and 45°, respectively. The δ distribution assumption was verified using a combination of SFG and ATR-FTIR measurements, which showed a quite narrow distribution in the angle of θ(1) for both pH conditions. This indicates that all alamethicin molecules at the surface adopt a nearly identical orientation in POPC lipid bilayers. The localized pH change in proximity to the bilayer modulates the membrane potential and thus induces a decrease in both the tilt and the bend angles of the two helices in alamethicin. This is the first reported application of SFG to the study of model ion channel gating mechanisms in model cell membranes.

Trichoderma viride cellulase induces resistance to the antibiotic pore-forming peptide alamethicin associated with changes in the plasma membrane lipid composition of tobacco BY-2 cells

BACKGROUND

Alamethicin is a membrane-active peptide isolated from the beneficial root-colonising fungus Trichoderma viride. This peptide can insert into membranes to form voltage-dependent pores. We have previously shown that alamethicin efficiently permeabilises the plasma membrane, mitochondria and plastids of cultured plant cells. In the present investigation, tobacco cells (Nicotiana tabacum L. cv Bright Yellow-2) were pre-treated with elicitors of defence responses to study whether this would affect permeabilisation.

RESULTS

Oxygen consumption experiments showed that added cellulase, already upon a limited cell wall digestion, induced a cellular resistance to alamethicin permeabilisation. This effect could not be elicited by xylanase or bacterial elicitors such as flg22 or elf18. The induction of alamethicin resistance was independent of novel protein synthesis. Also, the permeabilisation was unaffected by the membrane-depolarising agent FCCP. As judged by lipid analyses, isolated plasma membranes from cellulase-pretreated tobacco cells contained less negatively charged phospholipids (PS and PI), yet higher ratios of membrane lipid fatty acid to sterol and to protein, as compared to control membranes.

CONCLUSION

We suggest that altered membrane lipid composition as induced by cellulase activity may render the cells resistant to alamethicin. This induced resistance could reflect a natural process where the plant cells alter their sensitivity to membrane pore-forming agents secreted by Trichoderma spp. to attack other microorganisms, and thus adding to the beneficial effect that Trichoderma has for plant root growth. Furthermore, our data extends previous reports on artificial membranes on the importance of lipid packing and charge for alamethicin permeabilisation to in vivo conditions.

Single-molecule investigation of the influence played by lipid rafts on ion transport and dynamic features of the pore-forming alamethicin oligomer

In this experimental work we employed single-molecule electrical recordings on alamethicin oligomers inserted in lipid bilayers made of brain sphingomyelin (bSM), palmitoyloleoylphosphatidylcholine (POPC) and cholesterol (chol) to unravel novel aspects regarding lipid raft interactions with pore-forming peptides. We probed the effect of lipid rafts on electrical properties of inserted alamethicin oligomers, and our data convincingly prove that the single-channel electrical conductance of various subconductance states of the alamethicin oligomer (1) increases in the presence of raft-containing ternary lipid mixtures (POPC-chol-bSM) compared to cases when bilayers were made of POPC-chol and POPC and (2) decreases in the presence of raft-containing ternary lipid mixtures compared to nonraft ternary mixtures which favor the fluid and liquid ordered phases alone. Our data demonstrate that the presence of lipid rafts leads to a slower association kinetics of alamethicin oligomers, seemingly reflecting a slower lateral diffusion process of such peptide aggregates compared to the case of nonraft, binary lipid mixtures. Furthermore, we show that the electrical capacitance of ternary lipid mixtures (POPC-chol-bSM) decreases in the presence of raft domains by comparison to nonraft binary phases (POPC-chol) or POPC alone, and this could constitute an additional mechanism via which macroscopic electrical manifestations of eukaryotic cells are modulated by the coexistence of gel and fluid domains of the plasma membrane.

Ion selectivity, transport properties and dynamics of amphotericin B channels studied over a wide range of acidity changes

Amphotericin B (AmB) is an antifungal antibiotic which, despite the severe side effects, is still used for the treatment of systemic fungal infections. Herein we studied the influence of pH upon the selectivity and the transport properties of AmB channels inserted in reconstituted, ergosterol-containing zwitterionic lipid membranes. Our electrophysiology experiments carried out on single and multiple AmB channels prove that at pH 2.8 these channels are anion selective, whereas at neutral and alkaline pH's (pH 7 and pH 11) they become cation selective. We attribute this to the pH-dependent ionization state of the carboxyl and amino groups present at the mouth of AmB molecules. Surprisingly, our data reveal that the single-molecule ionic conductance of AmB channels varies in a non-monotonic fashion with pH changes, which we attribute to the pH-dependent variation of the surface and dipole membrane potential. We demonstrate that when added only from one side of the membrane, in symmetrical salt solutions across the membrane and low pH values, AmB channels display a strong rectifying behavior, and their insertion is strongly favored when positive potentials are present on the side of their addition.

Altering the activity of syringomycin E via the membrane dipole potential

The membrane dipole potential is responsible for the modulation of numerous biological processes. It was previously shown (Ostroumova, O. S.; Kaulin, Y. A.; Gurnev, P. A.; Schagina, L. V. Langmuir 2007, 23, 6889-6892) that variations in the dipole potential lead to changes in the channel properties of the antifungal lipodepsipeptide syringomycin E (SRE). Here, data are presented demonstrating the effect of the membrane dipole potential on the channel-forming activity of SRE. A rise in the dipole potential is accompanied by both an increase in the minimum SRE concentration required for the detection of single channels at fixed voltage and a decrease in the steady-state number of open SRE channels at a given SRE concentration and voltage. These alterations are determined by several factors: gating charge, connected with translocations of lipid and SRE dipoles during channel formation, the bilayer-water solution partitioning of SRE, and the chemical work related to conformational changes during channel formation.