Ionophore incorporated bilayer lipid membranes that selectively respond to metal ions and induce membrane permeability changes

Transmembrane Signaling with Lipid-Bilayer Assemblies as a Platform for Channel-Based Biosensing

Artificial and natural lipid membranes that elicit transmembrane signaling is are useful as a platform for channel-based biosensing. In this account we summarize our research on the design of transmembrane signaling associated with lipid bilayer membranes containing nanopore-forming compounds. Channel-forming compounds, such as receptor ion-channels, channel-forming peptides and synthetic channels, are embedded in planar and spherical bilayer lipid membranes to develop highly sensitive and selective biosensing methods for a variety of analytes. The membrane-bound receptor approach is useful for introducing receptor sites on both planar and spherical bilayer lipid membranes. Natural receptors in biomembranes are also used for designing of biosensing methods.

Sensing cisplatin-induced permeation of single live human bladder cancer cells by scanning electrochemical microscopy

Cisplatin is a widely used anti-cancer agent, which was believed to trigger apoptosis of cancer cells by forming DNA adducts. However, recent studies evidenced a cisplatin-induced extrinsic apoptotic pathway through interaction with plasma membranes. We present quantitative time-course imaging of cisplatin-induced permeation of ferrocenemethanol to single live human bladder cancer cells (T24) using scanning electrochemical microscopy (SECM). Simultaneous quantification of cellular topography and membrane permeability was realized by running SECM in the depth scan mode. It was demonstrated that the acute addition of cisplatin to the outer environment of T24 cells immediately induced membrane permeability change in 5 min, which indicated a loosened structure of the cellular membrane upon cisplatin dosage. The cisplatin-induced permeation of T24 cells might be a one-step action, an extrinsic mechanism, since the cell response was quick, and no continuous increase in the membrane permeability was observed. The time-lapse SECM depth scan method provided a simple and facile way of monitoring cisplatin-induced membrane permeability changes. Our study is anticipated to lead to a methodology of screening anti-cancer drugs through their interactions with live cells.

Action of Chicory Fructooligosaccharides on Biomimetic Membranes

Fructooligosaccharides from chicory (FOSC) are functional prebiotic foods recognized to exert several well-being effects in human health and animal production, as decreasing blood lipids, modulating the gut immune system, enhancing mineral bioavailability, and inhibiting microbial growth, among others. Mechanisms of actions directly on cell metabolism and structure are however little known. In this sense this work was targeted to investigate the interaction of FOSC with biomimetic membranes (liposomes and supported bilayer membrane; s-BLM) through cyclic voltammetry, impedance spectroscopy, spectrofluorimetry, and microscopy. FOSC was able to disrupt the membrane structure of liposomes and s-BLM from the onset of molecular pores induced on it. The mechanism of interaction of fructans with biomimetic membranes suggests hydrogen bonding between the polyhydroxylated structure of the oligosaccharides and the negative polar group of L-α-phosphatidylcholine (PC) present in both liposomes and s-BLM.

Potential analytical applications of lysenin channels for detection of multivalent ions

Transmembrane protein transporters possessing binding sites for ions, toxins, pharmaceutical drugs, and other molecules constitute excellent candidates for developing sensitive and selective biosensing devices. Their attractiveness for analytical purposes is enhanced by the intrinsic amplification capabilities shown when the binding event leads to major changes in the transportation of ions or molecules other than the analyte itself. The large-scale implementation of such transmembrane proteins in biosensing devices is limited by the difficulties encountered in inserting functional transporters into artificial bilayer lipid membranes and by the limitations in understanding and exploiting the changes induced by the interaction with the analyte for sensing purposes. Here, we show that lysenin, a pore-forming toxin extracted from earthworm Eisenia foetida, which inserts stable and large conductance channels into artificial bilayer lipid membranes, functions as a multivalent ion-sensing device. The analytical response consists of concentration and ionic-species-dependent macroscopic conductance inhibition most probably linked to a ligand-induced gating mechanism. Multivalent ion removal by chelation or precipitation restores, in most cases, the initial conductance and demonstrates reversibility. Changes in lipid bilayer membrane compositions leading to the absence of voltage-induced gating do not affect the analytical response to multivalent ions. Microscopic current analysis performed on individual lysenin channels in the presence of Cu(2+) revealed complex open-closed transitions characterized by unstable intermediate sub-conducting states. Lysenin channels provide an analytical tool with a built-in sensing mechanism for inorganic and organic multivalent ions, and the excellent stability in an artificial environment recommend lysenin as a potential candidate for single-molecule detection and analysis.

Voltammetric study on ion transport across a bilayer lipid membrane in the presence of a hydrophobic ion or an ionophore

This review describes voltammetric studies on ion transport from one aqueous phase (W1) to another (W2) across a bilayer lipid membrane (BLM) containing a hydrophobic ion, valinomycin (Val) or gramicidin A (GA). In particular, the ion transport mechanisms are discussed in terms of the distribution of a pair of ions between aqueous and BLM phases. By addition of a small amount of hydrophobic ion into W1 and/or W2 containing a hydrophilic salt as a supporting electrolyte, the hydrophobic ion was distributed into the BLM with the counter ion to maintain electroneutrality within the BLM phase. It was found that the counter ion was transferred between W1 and W2 across the BLM by applying a membrane potential. Facilitated transport of alkali ions across a BLM containing Val as an ion carrier compound, could be interpreted by considering not only the formation of the alkali metal ion-Val complex but also the distribution of both the objective cation and the counter ion. In the case of addition of GA as a channel-forming compound into the BLM, the facilitated transport of alkali ions across the BLM depended on the ionic species of the counter ions. It was discovered that the influence of the counter ion on the facilitated transport of alkali ions across the BLM could be explained in terms of the hydrophobicity and the ionic radius of the counter ion.

Glutamate Receptor Incorporated in a Mixed Hybrid Bilayer Lipid Membrane Array, as a Sensing Element of a Biosensor Working under Flowing Conditions

The realization of a reliable receptor biosensor requires stable, long-lasting, reconstituted biomembranes able to supply a suitable biomimetic environment where the receptor can properly work after incorporation. To this end, we developed a new method for preparing stable biological membranes that couple the biomimetic properties of BLMs (bilayer lipid membranes) with the high stability of HBMs (hybrid bilayer membranes); this gives rise to an innovative assembly, named MHBLM (mixed hybrid bilayer lipid membrane). The present work deals with the characterization of biosensors achieved by embedding an ionotropic glutamate receptor (GluR) on MHBLM. Thanks to signal (transmembrane current) amplification, which is typical of natural receptors, the biosensor here produced detects glutamate at a level of nmol L(-1). The transmembrane current changes linearly vs glutamate up to 100 nmol L(-1), while the limit of detection is 1 nmol L(-1). In addition, the biosensor response can be modulated both by receptor agonists (glycine) and antagonists (Mg(2+)) as well, and by exploiting the biosensor response, the distribution of different kinds of ionotropic GluR present in the purified sample, and embedded in MHBLM, was also evaluated. Finally, one of the most important aspects of this investigation is represented by the high stability of the biomimetic system, which allows the use of biosensor under flowing conditions, where the solutions flow on both biomembrane faces.

Arachidonic acid-induced channel- and carrier-type ion transport across planar bilayer lipid membranes

Transmembrane ion transport by arachidonic acid (AA) through bilayer lipid membranes (BLMs) was investigated by means of electrochemical measurements to provide a basis for designing a sensor membrane. We found that AA induces a channel-type current, in addition to a carrier-type current, across planar BLMs. A linear relation between the logarithmic value of the AA concentration and the current responses (given as integrated currents) was observed for a carrier-type current, while a sigmoid relation was found for a channel-type current. Although AA transports Na+, Ca2+ and Mg2+ and exhibits ion selectivity between Na+ and Mg2+ for the carrier-type current, ion transport for the channel-type current was non-selective. It was found that ion transport via the channel mechanism occurs frequently for AA, while channel-type currents were only occasionally observed for y-linolenic acid and prostaglandin D2. No channel-type currents were induced by other fatty acids (oleic, linoleic, stearic, myristic, eicosapentanoic and docosahexanoic acids) and metabolites of AA (12-HETE and 5-HETE). The carrier-type ion transport occurs selectively to these compounds if the concentration is below 1.0 microM. These results suggest that AA selectively facilitates an ion flux through the BLMs, generating channel-type and/or carrier-type currents, which can be used as a measure of the AA concentration.

A glass capillary ultramicroelectrode with an electrokinetic sampling ability

A glass capillary ultramicroelectrode (tip diameter approximately 1.2 microm) having an electrokinetic sampling ability is described. It is composed of a pulled glass capillary filled with an inner solution and three internal electrodes (Pt working and counter electrodes and an Ag/AgCl reference electrode). The voltammetric response of the capillary electrode is based on electrokinetic transport of analyte ions from the sample solution into the inner solution across the conical tip. It was found that the electrophoretic migration of analytes at the conical tip is faster than electroosmotic flow, enabling electrokinetic transport of analyte ions into the inner solution of the electrode. By using [Fe(CN)6]4- and (ferrocenylmethyl)trimethylammonium (FcTMA+) ions as model analytes, differential pulse voltammetric responses of the capillary electrode were investigated in terms of tip diameter of the capillary, sampling voltage, sampling time, detection limit and selectivity. The magnitude of the response depends on the size and charge of analyte ions. With a capillary electrode having a approximately 1.2-microm tip diameter, which minimizes non-selective diffusional entry of analytes, the response after 1 h sampling at +1.7 V is linearly related to [Fe(CN)6]4- concentration in the range of 0.50-5.0 mM with the detection limit of 30 microM. Application of a potential of the same sign as that of the analyte ion forces the analyte to move out from the electrode to the solution, enabling reuse of the same capillary electrode. The charge-selective detection of analytes with the capillary electrode is demonstrated for [Fe(CN)6]4- in the presence of FcTMA+.