Incorporation of ionic channels from yeast plasma membranes into black lipid membranes

Characterizing the Structure and Interactions of Model Lipid Membranes Using Electrophysiology

The cell membrane is a protective barrier whose configuration determines the exchange both between intracellular and extracellular regions and within the cell itself. Consequently, characterizing membrane properties and interactions is essential for advancements in topics such as limiting nanoparticle cytotoxicity. Characterization is often accomplished by recreating model membranes that approximate the structure of cellular membranes in a controlled environment, formed using self-assembly principles. The selected method for membrane creation influences the properties of the membrane assembly, including their response to electric fields used for characterizing transmembrane exchanges. When these self-assembled model membranes are combined with electrophysiology, it is possible to exploit their non-physiological mechanics to enable additional measurements of membrane interactions and phenomena. This review describes several common model membranes including liposomes, pore-spanning membranes, solid supported membranes, and emulsion-based membranes, emphasizing their varying structure due to the selected mode of production. Next, electrophysiology techniques that exploit these structures are discussed, including conductance measurements, electrowetting and electrocompression analysis, and electroimpedance spectroscopy. The focus of this review is linking each membrane assembly technique to the properties of the resulting membrane, discussing how these properties enable alternative electrophysiological approaches to measuring membrane characteristics and interactions.

Electrodes for Membrane Surface Science. Bilayer Lipid Membranes Tethered by Commercial Surfactants on Electrochemical Sensors

Commercial surfactants, which are inexpensive and abundant, were covalently grafted to flat and transparent electrodes, and it appears to be a simple functionalization route to design biomembrane sensors at large-scale production. Sparsely tethered bilayer lipid membranes (stBLM) were stabilized using such molecular coatings composed of diluted anchor-harpoon surfactants that grab the membrane with an alkyl chain out of a PEGylated-hydrogel layer, which acts as a soft hydration cushion. The goal of avoiding the synthesis of complex organic molecules to scale up sensors was achieved here by grafting nonionic diblock oligomers (Brij58 = C _xH_{2 x+1}(OCH₂CH₂) _nOH with x = 16 and n = 23) and PEO short chains ((OCH₂CH₂) _nOH with n = 9 and n = 23) from their hydroxyl (-OH) end-moiety to a monolayer of -Ar-SO₂Cl groups, which are easy to form on electrodes (metals, semiconducting materials, ...) from aryl-diazonium salt reduction. A hybrid molecular coating on gold, with scarce Ar-SO₂-Brij58 and PEO oligomers, was used to monitor immobilization and fusion kinetics of DOPC small unilamellar vesicles (SUV) by both quartz crystal microbalance (QCM) and surface plasmon resonance (SPR) techniques. Using flat and transparent thin chromium film electrodes, we designed biosensors to couple surface sensitive techniques for membranes, including X-ray reflectivity (XRR), atomic force microscopy (AFM) with subnanometer resolution, and optical microscopy, such as fluorescence recovery after photobleaching measurements (FRAP), in addition to electrochemistry techniques (cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS)). The advantages of this biomembrane-sensing platform are discussed for research and applications.

Quantitative description of ion transport via plasma membrane of yeast and small cells

Modeling of ion transport via plasma membrane needs identification and quantitative understanding of the involved processes. Brief characterization of main ion transport systems of a yeast cell (Pma1, Ena1, TOK1, Nha1, Trk1, Trk2, non-selective cation conductance) and determining the exact number of molecules of each transporter per a typical cell allow us to predict the corresponding ion flows. In this review a comparison of ion transport in small yeast cell and several animal cell types is provided. The importance of cell volume to surface ratio is emphasized. The role of cell wall and lipid rafts is discussed in respect to required increase in spatial and temporary resolution of measurements. Conclusions are formulated to describe specific features of ion transport in a yeast cell. Potential directions of future research are outlined based on the assumptions.

Engineering lipid bilayer membranes for protein studies

Lipid membranes regulate the flow of nutrients and communication signaling between cells and protect the sub-cellular structures. Recent attempts to fabricate artificial systems using nanostructures that mimic the physiological properties of natural lipid bilayer membranes (LBM) fused with transmembrane proteins have helped demonstrate the importance of temperature, pH, ionic strength, adsorption behavior, conformational reorientation and surface density in cellular membranes which all affect the incorporation of proteins on solid surfaces. Much of this work is performed on artificial templates made of polymer sponges or porous materials based on alumina, mica, and porous silicon (PSi) surfaces. For example, porous silicon materials have high biocompatibility, biodegradability, and photoluminescence, which allow them to be used both as a support structure for lipid bilayers or a template to measure the electrochemical functionality of living cells grown over the surface as in vivo. The variety of these media, coupled with the complex physiological conditions present in living systems, warrant a summary and prospectus detailing which artificial systems provide the most promise for different biological conditions. This study summarizes the use of electrochemical impedance spectroscopy (EIS) data on artificial biological membranes that are closely matched with previously published biological systems using both black lipid membrane and patch clamp techniques.

Responses of Phanerochaete chrysosporium to toxic pollutants: physiological flux, oxidative stress, and detoxification

The white-rot fungus Phanerochaete chrysosporium has been widely used for the treatment of waste streams containing heavy metals and toxic organic pollutants. The development of fungal-based treatment technologies requires detailed knowledge of the relationship between bulk water quality and the physiological responses of fungi. A noninvasive microtest technique was used to quantify real-time changes in proton, oxygen, and cadmium ion fluxes following the exposure of P. chrysosporium to environmental toxic (2,4-dichlorophenol and cadmium). Significant changes in H(+) and O(2) flux occurred after exposure to 10 mg/L 2,4-dichlorophenol and 0.1 mM cadmium. Cd(2+) flux decreased with time. Reactive oxygen species formation and antioxidant levels increased after cadmium treatment. Superoxide dismutase activity correlated well with malondialdehyde levels (r(2) = 0.964) at low cadmium concentrations. However, this correlation diminished and malondialdehyde levels significantly increased at the highest cadmium concentration tested. Real-time microscale signatures of H(+), O(2), and Cd(2+) fluxes coupled with oxidative stress analysis can improve our understanding of the physiological responses of P. chrysosporium to toxic pollutants and provide useful information for the development of fungal-based technologies to improve the treatment of wastes cocontaminated with heavy metals and organic pollutants.

Natural and artificial ion channels for biosensing platforms

The single-molecule selectivity and specificity of the binding process together with the expected intrinsic gain factor obtained when utilizing flow through a channel have attracted the attention of analytical chemists for two decades. Sensitive and selective ion channel biosensors for high-throughput screening are having an increasing impact on modern medical care, drug screening, environmental monitoring, food safety, and biowarefare control. Even virus antigens can be detected by ion channel biosensors. The study of ion channels and other transmembrane proteins is expected to lead to the development of new medications and therapies for a wide range of illnesses. From the first attempts to use membrane proteins as the receptive part of a sensor, ion channels have been engineered as chemical sensors. Several other types of peptidic or nonpeptidic channels have been investigated. Various gating mechanisms have been implemented in their pores. Three technical problems had to be solved to achieve practical biosensors based on ion channels: the fabrication of stable lipid bilayer membranes, the incorporation of a receptor into such a structure, and the marriage of the modified membrane to a transducer. The current status of these three areas of research, together with typical applications of ion-channel biosensors, are discussed in this review.

A gigaseal obtained with a self-assembled long-lifetime lipid bilayer on a single polyelectrolyte multilayer-filled nanopore

A lipid bilayer with gigaohm resistance was fabricated over a single 800 nm pore in a Si3N4 chip using 50 nm liposomes. The nanopore was prefilled with a polyelectrolyte multilayer (PEM) that triggered the spontaneous fusion of the lipid vesicles. Pore-forming peptide melittin was incorporated in the bilayer, and single channel activities were monitored for a period of 2.5 weeks. The long lifetime of the system enabled the observation of the time-dependent stabilization effect of the melittin open state upon bias application.

Field-effect detection using phospholipid membranes

The application of field-effect devices to biosensors has become an area of intense research interest. An attractive feature of field-effect sensing is that the binding or reaction of biomolecules can be directly detected from a change in electrical signals. The integration of such field-effect devices into cell membrane mimics may lead to the development of biosensors useful in clinical and biotechnological applications. This review summarizes recent studies on the fabrication and characterization of field-effect devices incorporating model membranes. The incorporation of black lipid membranes and supported lipid monolayers and bilayers into semiconductor devices is described.

Solid supported lipid bilayers: From biophysical studies to sensor design

The lipid bilayer is one of the most eloquent and important self-assembled structures in nature. It not only provides a protective container for cells and sub-cellular compartments, but also hosts much of the machinery for cellular communication and transport across the cell membrane. Solid supported lipid bilayers provide an excellent model system for studying the surface chemistry of the cell. Moreover, they are accessible to a wide variety of surface-specific analytical techniques. This makes it possible to investigate processes such as cell signaling, ligand-receptor interactions, enzymatic reactions occurring at the cell surface, as well as pathogen attack. In this review, the following membrane systems are discussed: black lipid membranes, solid supported lipid bilayers, hybrid lipid bilayers, and polymer cushioned lipid bilayers. Examples of how supported lipid membrane technology is interfaced with array based systems by photolithographic patterning, spatial addressing, microcontact printing, and microfluidic patterning are explored. Also, the use of supported lipid bilayers in microfluidic devices for the development of lab-on-a-chip based platforms is examined. Finally, the utility of lipid bilayers in nanotechnology and future directions in this area are discussed.

Gating and conductance in an outward-rectifying K+ channel from the plasma membrane of Saccharomyces cerevisiae

The plasma membrane of the yeast Saccharomyces cerevisiae has been investigated by patch-clamp techniques, focusing upon the most conspicuous ion channel in that membrane, a K(+)-selective channel. In simple observations on inside-out patches, the channel is predominantly closed at negative membrane voltages, but opens upon polarization towards positive voltages, typically displaying long flickery openings of several hundred milliseconds, separated by long gaps (G). Elevating cytoplasmic calcium shortens the gaps but also introduces brief blocks (B, closures of 2-3 msec duration). On the assumption that the flickery open intervals constitute bursts of very brief openings and closings, below the time resolution of the recording system, analysis via the beta distribution revealed typical closed durations (interrupts, I) near 0.3 msec, and similar open durations. Overall behavior of the channel is most simply described by a kinetic model with a single open state (O), and three parallel closed states with significantly different lifetimes: long (G), short (B) and very short (I). Detailed kinetic analysis of the three open/closed transitions, particularly with varied membrane voltage and cytoplasmic calcium concentration, yielded the following stability constants for channel closure: K1 = 3.3 x e-zu in which u = eVm/kT is the reduced membrane voltage, and z is the charge number; KG = 1.9 x 10(-4) ([Ca2+].ezu)-1; and KB = 2.7 x 10(3)([Ca2+].ezu)2. Because of the antagonistic effects of both membrane voltage (Vm) and cytoplasmic calcium concentration ([Ca2+]cyt) on channel opening from the B state, compared with openings from the G state, plots of net open probability (Po) vs. either Vm or [Ca2+] are bell-shaped, approaching unity at low calcium (microM) and high voltage (+150 mV), and approaching 0.25 at high calcium (10 mM) and zero voltage. Current-voltage curves of the open channel are sigmoid vs. membrane voltage, saturating at large positive or large negative voltages; but time-averaged currents, along the rising limb of Po (in the range 0 to +150 mV, for 10 microM [Ca2+]) make this channel a strong outward rectifier. The overall properties of the channel suggest that it functions in balancing charge movements during secondary active transport in Saccharomyces.

Ionic channels in the plasma membrane of Schizosaccharomyces pombe: evidence from patch-clamp measurements

Patch-clamp studies of the yeast Schizosaccharomyces pombe reveal that the plasma membrane contains a voltage-gated channel mildly selective for potassium over sodium, lithium, and chloride. The channel exhibits several conductances with a maximum of 153 pS. The channel gates in the region of physiologically relevant voltages, being closed at hyperpolarizing and open at depolarizing voltages. It is not inhibited by tetraethylammonium, quinine, or quinidine applied from the cytoplasmic side of the membrane; similarly, ATP and stretch have no effect. The frequency of its occurrence in patches implies that about 35 channels of this kind are present in the plasma membrane of a single cell.

An energy-dependent efflux system for potassium ions in yeast

An efflux of potassium ions was demonstrated in mutants of yeast cells lacking a functional high affinity carrier system for monovalent cations. This efflux showed the following characteristics: (a) It was stimulated by the presence of a substrate, either glucose or ethanol. (b) It was stimulated by several cationic organic molecules, such as ethidium bromide, dihydrostreptomycin, diethylaminoethyldextran, and also by trivalent cations, such as Al3+ and lanthanides; this stimulation also depended on the presence of a substrate. (c) K+ efflux was decreased in yeast mutants with decreased ATPase activity, which generated a lower membrane potential. (d) Although the efflux appeared to be of an electrogenic nature, producing hyperpolarization of cells, it was accompanied by the efflux of phosphate, probably as an anion partially compensating for the large amount of cations leaving the cell. (e) K+ efflux was also accompanied by an uptake of protons. (f) The efflux appeared more clearly in cells grown in YPD medium, and not in more complex media nor in the same YPD medium if supplemented with Ca2+ or Mg2+. Efflux of monovalent cations produced by Tb3+ and organic cationic agents was also demonstrated in wild type strains. This efflux system appears to be, at least partially, electrogenic, but seems to be also an exchange system for protons and to function as a symport with phosphate; it may be involved in the regulation of the internal pH of the cell, and appears to be regulated by its link to the energetic status of the cell, probably through the membrane potential.