Simultaneous optical and electrical recording of a single ion-channel

Confirming Silent Translocation through Nanopores with Simultaneous Single-Molecule Fluorescence and Single-Channel Electrical Recordings

Most of what is known concerning the luminal passage of materials through nanopores arises from electrical measurements. Whether nanopores are biological, solid-state, synthetic, hybrid, glass-capillary-based, or protein ion channels in cells and tissues, characteristic signatures embedded in the flow of ionic current are foundational to understanding functional behavior. In contrast, this work describes passage through a nanopore that occurs without producing an electrical signature. We refer to the phenomenon as "silent translocation." By definition, silent translocations are invisible to the standard tools of electrophysiology and fundamentally require a simultaneous ancillary measurement technique for positive identification. As a result, this phenomenon has been largely unexplored in the literature. Here, we report on a derivative of Cyanine 5 (sCy5a) that passes through the α-hemolysin (αHL) nanopore silently. Simultaneously acquired single-molecule fluorescence and single-channel electrical recordings from bilayers formed over a closed microcavity demonstrate that translocation does indeed take place, albeit infrequently. We report observations of silent translocation as a function of time, dye concentration, and nanopore population in the bilayer. Lastly, measurement of the translocation rate as a function of applied potential permits estimation of an effective energy barrier for transport through the pore as well as the effective charge on the dye, all in the absence of an information-containing electrical signature.

Emerging Diamond Quantum Sensing in Bio-Membranes

Bio-membranes exhibit complex but unique mechanical properties as communicative regulators in various physiological and pathological processes. Exposed to a dynamic micro-environment, bio-membranes can be seen as an intricate and delicate system. The systematical modeling and detection of their local physical properties are often difficult to achieve, both quantitatively and precisely. The recent emerging diamonds hosting quantum defects (i.e., nitrogen-vacancy (NV) center) demonstrate intriguing optical and spin properties, together with their outstanding photostability and biocompatibility, rendering them ideal candidates for biological applications. Notably, the extraordinary spin-based sensing enable the measurements of localized nanoscale physical quantities such as magnetic fields, electrical fields, temperature, and strain. These nanoscale signals can be optically read out precisely by simple optical microscopy systems. Given these exclusive properties, NV-center-based quantum sensors can be widely applied in exploring bio-membrane-related features and the communicative chemical reaction processes. This review mainly focuses on NV-based quantum sensing in bio-membrane fields. The attempts of applying NV-based quantum sensors in bio-membranes to investigate diverse physical and chemical events such as membrane elasticity, phase change, nanoscale bio-physical signals, and free radical formation are fully overviewed. We also discuss the challenges and future directions of this novel technology to be utilized in bio-membranes.

A Lipid Bilayer Formed on a Hydrogel Bead for Single Ion Channel Recordings

Ion channel proteins play important roles in various cell functions, making them attractive drug targets. Artificial lipid bilayer recording is a technique used to measure the ion transport activities of channel proteins with high sensitivity and accuracy. However, the measurement efficiency is low. In order to improve the efficiency, we developed a method that allows us to form bilayers on a hydrogel bead and record channel currents promptly. We tested our system by measuring the activities of various types of channels, including gramicidin, alamethicin, α-hemolysin, a voltage-dependent anion channel 1 (VDAC1), a voltage- and calcium-activated large conductance potassium channel (BK channel), and a potassium channel from Streptomyces lividans (KcsA channel). We confirmed the ability for enhanced measurement efficiency and measurement system miniaturizion.

Single-molecule imaging of PI(4,5)P2 and PTEN in vitro reveals a positive feedback mechanism for PTEN membrane binding

PTEN, a 3-phosphatase of phosphoinositide, regulates asymmetric PI(3,4,5)P3 signaling for the anterior-posterior polarization and migration of motile cells. PTEN acts through posterior localization on the plasma membrane, but the mechanism for this accumulation is poorly understood. Here we developed an in vitro single-molecule imaging assay with various lipid compositions and use it to demonstrate that the enzymatic product, PI(4,5)P2, stabilizes PTEN's membrane-binding. The dissociation kinetics and lateral mobility of PTEN depended on the PI(4,5)P2 density on artificial lipid bilayers. The basic residues of PTEN were responsible for electrostatic interactions with anionic PI(4,5)P2 and thus the PI(4,5)P2-dependent stabilization. Single-molecule imaging in living Dictyostelium cells revealed that these interactions were indispensable for the stabilization in vivo, which enabled efficient cell migration by accumulating PTEN posteriorly to restrict PI(3,4,5)P3 distribution to the anterior. These results suggest that PI(4,5)P2-mediated positive feedback and PTEN-induced PI(4,5)P2 clustering may be important for anterior-posterior polarization.

New light on ion channel imaging by total internal reflection fluorescence (TIRF) microscopy

Ion channels play pivotal roles in a wide variety of cellular functions; therefore, their physiological characteristics, pharmacological responses, and molecular structures have been extensively investigated. However, the mobility of an ion channel itself in the cell membrane has not been examined in as much detail. A total internal reflection fluorescence (TIRF) microscope allows fluorophores to be imaged in a restricted region within an evanescent field of less than 200 nm from the interface of the coverslip and plasma membrane in living cells. Thus the TIRF microscope is useful for selectively visualizing the plasmalemmal surface and subplasmalemmal zone. In this review, we focused on a single-molecule analysis of the dynamic movement of ion channels in the plasma membrane using TIRF microscopy. We also described two single-molecule imaging techniques under TIRF microscopy: fluorescence resonance energy transfer (FRET) for the identification of molecules that interact with ion channels, and subunit counting for the determination of subunit stoichiometry in a functional channel. TIRF imaging can also be used to analyze spatiotemporal Ca(2+) events in the subplasmalemma. Single-molecule analyses of ion channels and localized Ca(2+) signals based on TIRF imaging provide beneficial pharmacological and physiological information concerning the functions of ion channels.

Patch-clamp fluorometry: electrophysiology meets fluorescence

Ion channels and transporters are membrane proteins whose functions are driven by conformational changes. Classical biophysical techniques provide insight into either the structure or the function of these proteins, but a full understanding of their behavior requires a correlation of both these aspects in time. Patch-clamp and voltage-clamp fluorometry combine spectroscopic and electrophysiological techniques to simultaneously detect conformational changes and ionic currents across the membrane. Since its introduction, patch-clamp fluorometry has been responsible for invaluable advances in our knowledge of ion channel biophysics. Over the years, the technique has been applied to many different ion channel families to address several biophysical questions with a variety of spectroscopic approaches and electrophysiological configurations. This review illustrates the strength and the flexibility of patch-clamp fluorometry, demonstrating its potential as a tool for future research.

Molecular assembly and dynamics of fluorescent protein-tagged single KCa1.1 channel in expression system and vascular smooth muscle cells

The large-conductance Ca(2+)-activated K(+) (K(Ca)1.1, BK) channel has pivotal roles in the regulation of vascular tone. To clarify the molecular dynamics of BK channels and their functionally coupled protein on the membrane surface, we examined single-molecule imaging of fluorescent-labeled BK subunits in the plasma membrane using total internal reflection fluorescence (TIRF) microscopy. The dynamic mobility of yellow fluorescent protein (YFP)-tagged BKα subunit (BKα-YFP) expressed in human embryo kidney 293 (HEK) cells was detected in TIRF regions at the level of individual channels and their clusters on the plasma membrane with a diffusion coefficient of 6.7 × 10(3) nm(2)/s. When BKα-YFP was coexpressed with cyan fluorescent protein (CFP)-tagged BKβ1 subunit (BKβ1-CFP) in HEK cells, the mobility was reduced by ∼50%. Fluorescent image analyses suggest that green fluorescent protein (GFP)-tagged BKα subunit (BKα-GFP) expressed in vascular smooth muscle cells (VSMCs), at low density, preferentially formed a heterotetrameric molecular assembly with native BKα subunits, rather than homotetrameric BKα-GFP. Movement of BKα-YFP in VSMCs (0.29 × 10(3) nm(2)/s) was far more restricted than BKα-YFP/BKβ1-CFP in HEK cells (2.5 × 10(3) nm(2)/s). Actin disruption by pretreatment with cytochalasin D in VSMCs appeared to increase the mobile behavior of BKα-YFP, which was then significantly reduced by addition of jasplakinolide. Most BKα-YFP colocalized with caveolin 1 (Cav1)-CFP in VSMCs, but unexpectedly not frequently in HEK cells. Fluorescence resonance energy transfer analyses showed the direct interaction between BKα-YFP and Cav1-CFP, particularly in VSMCs. These results, obtained by single molecule imaging in living cells, indicate that the dynamics of BKα molecules on the membrane surface are strongly restricted or regulated by its auxiliary β-subunit, cytoskeleton, and direct interaction with Cav1 in VSMCs.

Storable droplet interface lipid bilayers for cell-free ion channel studies

An artificially created lipid bilayer is an important platform in studying ion channels and engineered biosensor applications. However, a lipid bilayer created using conventional techniques is fragile and short-lived, and the measurement of ion channels requires expertise and laborious procedures, precluding practical applications. Here, we demonstrate a storable droplet lipid bilayer precursor frozen with ion channels, resulting in a droplet interface bilayer upon thawing. A small vial with an aqueous droplet in organic solution was flash frozen in -80 °C methanol immediately after an aqueous droplet was introduced into the organic solution and gravity draws the droplet down to the interface upon thawing. A lipid bilayer created along the interface using this method had giga-ohm resistance and typical specific capacitance values. The noise level of this system is favorably comparable to the conventional system. The subsequent incorporation of ion channels, alpha-hemolysin and gramicidin A, showed typical conductance values consistent with those in previous literatures. This novel system to create a lipid bilayer as a whole can be automated from its manufacture to use and indefinitely stored when frozen. As a result, ion channel measurements can be carried out in any place, increasing the accessibility of ion channel studies as well as a number of applications, such as biosensors, ion channel drug screening, and biophysical studies.

Monitoring ion-channel function in real time through quantum decoherence

In drug discovery, there is a clear and urgent need for detection of cell-membrane ion-channel operation with wide-field capability. Existing techniques are generally invasive or require specialized nanostructures. We show that quantum nanotechnology could provide a solution. The nitrogen-vacancy (NV) center in nanodiamond is of great interest as a single-atom quantum probe for nanoscale processes. However, until now nothing was known about the quantum behavior of a NV probe in a complex biological environment. We explore the quantum dynamics of a NV probe in proximity to the ion channel, lipid bilayer, and surrounding aqueous environment. Our theoretical results indicate that real-time detection of ion-channel operation at millisecond resolution is possible by directly monitoring the quantum decoherence of the NV probe. With the potential to scan and scale up to an array-based system, this conclusion may have wide-ranging implications for nanoscale biology and drug discovery.

Simultaneous optical and electrical single channel recordings on a PEG glass

Single molecule imaging of working ion-channels is much more difficult than that of water-soluble proteins because of the fragile nature of membranes and lateral diffusion of particles in the membranes, which does not allow fluorescent contamination for optical single channel recording. In this report, we reconstituted maxi-potassium channels from porcine uterine smooth muscle into artificial planar bilayers formed on poly(ethylene glycol) (PEG) modified glass and performed simultaneous optical and electrical recording of the single channels. The channels were immobilized in the membranes by anchoring to PEG molecules on the glass. The technique developed in this study should pave the way for single molecule pharmacology of ion-channels.

Visualization of cargo concentration by COPII minimal machinery in a planar lipid membrane

Selective protein export from the endoplasmic reticulum is mediated by COPII vesicles. Here, we investigated the dynamics of fluorescently labelled cargo and non-cargo proteins during COPII vesicle formation using single-molecule microscopy combined with an artificial planar lipid bilayer. Single-molecule analysis showed that the Sar1p-Sec23/24p-cargo complex, but not the Sar1p-Sec23/24p complex, undergoes partial dimerization before Sec13/31p recruitment. On addition of a complete COPII mixture, cargo molecules start to assemble into fluorescent spots and clusters followed by vesicle release from the planar membrane. We show that continuous GTPase cycles of Sar1p facilitate cargo concentration into COPII vesicle buds, and at the same time, non-cargo proteins are excluded from cargo clusters. We propose that the minimal set of COPII components is required not only to concentrate cargo molecules, but also to mediate exclusion of non-cargo proteins from the COPII vesicles.

An improved synthesis of a fluorescent gabapentin-choline conjugate for single molecule detection

Voltage-gated calcium ion channels are comprised of pore-forming α(1) and auxiliary α(2)δ, β and γ subunits. They are important molecular devices involved in a variety of cell functions. Fluorescently labeled acylcholine analogues are important in studies such as ion channel regulation. Cy3-3-acetylcholine has recently been synthesized for single molecule detection studies; albeit in an extremely low overall yield (0.06 %). In this work, an alternative route to that used in the previous Cy3-3-acetylcholine synthesis was developed with a 90 % yield at a significantly lower material cost.

A membrane interferometer

Freestanding phospholipid bilayers have been assembled spanning shallow, micrometer-sized wells etched into a Si wafer substrate so that the bilayers are near (within hundreds of nanometers) but not in contact with the wafer surface. The proximity of the bilayers to the highly reflective Si/SiO(2) interface allows them to be probed by using fluorescence-interference techniques. These interferometry measurements show that the bilayers are curved and that the curvature can be varied by changes in osmotic pressure. Furthermore, the ionophore gramicidin can be incorporated into the bilayers, making them selectively permeable to monovalent cations. This freestanding architecture may overcome surface-interaction problems that occur when cell membrane proteins are introduced into solid supported bilayers, while also allowing for high-precision measurements of changes in fluorophore position by interferometry.

Analysing steroid modulation of BK(Ca) channels reconstituted into planar lipid bilayers

A number of recent studies have described the activation of BK(Ca) channels by steroid hormones such as estrogen. The proposed mechanisms are diverse and include both the direct interaction with the ion channel subunits and the stimulation via receptor activation and cell signalling pathways. To investigate the activation of BK(Ca) channels by estrogen we devised a cell-free system by incorporating recombinant channels of known subunit composition into artificial bilayers and recorded single channel currents. This chapter describes the methods used to prepare purified membrane fractions from cultured cells and the construction of artificial phospholipids bilayers for the incorporation and recording of ion channels.

Single-molecule imaging techniques to visualize chemotactic signaling events on the membrane of living Dictyostelium cells

In this chapter, we describe methods to monitor signaling events at the single-molecule level on the membrane of living cells by using total internal reflection fluorescence microscopy (TIRFM). The techniques provide a powerful tool for elucidating the stochastic properties of signaling molecules involved in chemotaxis of the cellular slime mold Dictyostelium discoideum. Taking cAMP receptor 1 (cAR1) as an example of a target protein for single-molecule imaging, we describe the experimental setup of TIRFM, a method for labeling cAR1 with a fluorescent dye, and a method for investigating the receptor's lateral mobility. We discuss how the developmental progression of cells modulates both cAR1 behavior and the phenotypic variability in cAR1 mobility for different cell populations.

Droplet interface bilayers

Droplet interface bilayers (DIBs) provide a superior platform for the biophysical analysis of membrane proteins. The versatile DIBs can also form networks, with features that include built-in batteries and sensors.

Stochastic sensing on a modular chip containing a single-ion channel

Engineered protein channels have many potential applications in biosensing at the single-molecule level. A future generation of biosensor could be an array of target-specific ion channels, where each protein pore acts as a sensor element. An important step toward this goal is to create a portable, durable, single-protein channel-integrated chip device. Here we report a versatile, modular chip that contains a single-ion channel for single-molecular biosensing. The core of the device is a long-lived lipid membrane that has been sandwiched between two air-insulated agarose layers which gel in situ. A single-protein pore embedded in the membrane serves as the sensor element. The modular device is highly portable, allowing a single-ion channel to continuously function following detachment of the chip from the instrument and independent transportation of the device. The chip also exhibits high durability, which is evidenced from long-duration continuous observation of single-channel dynamics. Once engineered protein pores are installed, the chip becomes a robust stochastic sensor for real-time targeting such as detection of the second messenger IP3. This pluggable biochip could be incorporated with many applicable devices, such as a microfluidic system, and be made into a microarray for both biomedical detection and membrane protein research.

Stochastic signal inputs for chemotactic response in Dictyostelium cells revealed by single molecule imaging techniques

Chemotactic cells can exhibit extreme sensitivity to chemical gradients. Theoretical estimations of the signal inputs required for chemotaxis suggest that the response can be achieved under the strong influence of stochastic input noise generated by the receptors during the transmembrane signaling. This arises a fundamental question regarding the mechanisms for directional sensing: how do cells obtain reliable information regarding gradient direction by using stochastically operating receptors and the downstream molecules? To address this question, we have developed single molecule imaging techniques to visualize signaling molecules responsible for chemotaxis in living Dictyostelium cells, allowing us to monitor the stochastic signaling processes directly. Single molecule imaging of a chemoattractant bound to a receptor demonstrates that signal inputs fluctuate with time and space. Downstream signaling molecules, such as PTEN and a PH domain-containing protein that are constituent parts of chemotactic signaling system, can also be followed at single molecule level in living cells, illuminating the stochastic nature of chemotactic signaling processes. In this report, we start with a brief introduction of chemotactic response of the eukaryotic cells, followed by an explanation for single molecule imaging techniques, and finally discuss these applications to chemotactic signaling system of Dictyostelium cells.

Controlled delivery of membrane proteins to artificial lipid bilayers by nystatin-ergosterol modulated vesicle fusion

The study of ion channels and other membrane proteins and their potential use as biosensors and drug screening targets require their reconstitution in an artificial membrane. These applications would greatly benefit from microfabricated devices in which stable artificial lipid bilayers can be rapidly and reliably formed. However, the amount of protein delivered to the bilayer must be carefully controlled. A vesicle fusion technique is investigated where composite ion channels of the polyene antibiotic nystatin and the sterol ergosterol are employed to render protein-carrying vesicles fusogenic. After fusion with an ergosterol-free artificial bilayer, the nystatin-ergosterol channels do not dissociate immediately and thus cause a transient current signal that marks the vesicle fusion event. Experimental pitfalls of this method were identified, the influence of the nystatin and ergosterol concentration on the fusion rate and the shape of the fusion event marker was explored, and the number of different lipid species was reduced. Under these conditions, the -amyloid peptide could be delivered in a controlled manner to a standard planar bilayer. Additionally, electrical recordings were obtained of vesicles fusing with a planar lipid bilayer in a microfabricated device, demonstrating the suitability of nystatin-ergosterol modulated vesicle fusion for protein delivery within microsystems.

Lysenin forms a voltage-dependent channel in artificial lipid bilayer membranes

Lysenin, a hemolytic protein derived from the body fluid of earthworm, was incorporated into artificial bilayer membranes. Upon insertion, it formed a voltage-dependent large conductance channel in asolectin bilayers in a sphingomyelin-dependent manner. The channel had low ion-selectivity. Single-channel conductance was calculated as approximately 550 pS in 100 mM KCl. The channel in asolectin bilayers closed when the membrane was held at a positive potential. In contrast, the channel showed no voltage dependency in membranes made of pure phosphatidylcholine and sphingomyelin, suggesting some lipid contents included in the asolectin membranes affected channel gating.

A novel method for artificial lipid-bilayer formation

Many proposals have been made regarding the development of biosensors using single-channel recording with an artificial planar bilayer. The fragile nature of bilayer membranes is the major difficulty for the application of the artificial bilayer technique to the development of biosensors. We have developed an apparatus that promptly forms artificial bilayers. This technique is more efficient than other techniques for forming artificial bilayers. Bilayer membranes could be formed within 10s requiring 1 microl of analyte solution to record single-channel currents using our apparatus. A bilayer was formed by pressing the membrane on an agarose layer with hydraulic pressure. With this novel apparatus, we have recorded single-channel currents of various types of channels such as the BK-channel, the nicotinic receptor channel and the ryanodine receptor channel. The properties of the channels determined with this novel technique agreed well with those determined with conventional techniques.

Single-channel recordings of gramicidin at agarose-supported bilayer lipid membranes formed by the tip-dip and painting methods

Agarose-supported BLMs were prepared by the tip-dip and painting methods, and single-channel recordings of gramicidin were examined for the development of an ion-channel sensor. The supported BLMs formed by the tip-dip method had an electric resistance of >1.0 x 10(11) omega and a longer lifetime as compared with unsupported ones, which enabled single-channel recordings of gramicidin. The supported BLMs formed by the painting method also enabled single-channel recordings, but the lifetime was shorter than that of unsupported planar BLMs formed by the monolayer folding method.