New Aspects of Bilayer Lipid Membranes for the Analysis of Ion Channel Functions

The bilayer lipid membrane (BLM) is the main structural component of cell membranes, in which various membrane proteins are embedded. Artificially formed BLMs have been used as a platform in studies of the functions of membrane proteins, including various ion channels. In this review, we summarize recent advances that have been made on artificial BLM systems for the analysis of ion channel functions. We focus on two BLM-based systems, cell-membrane mimicry and four-terminal BLM systems. As a cell-membrane-mimicking system, an efficient screening platform for the evaluation of drug side effects that act on a cell-free synthesized channel has been developed, and its prospects for use in personalized medicine will be discussed. In the four-terminal BLMs, we introduce "lateral voltage" to BLM systems as a novel input to regulate channel activities, in addition to the traditional transmembrane voltages. Such state-of-the-art technologies and new system setups are predicted to pave the way for a variety of applications, in both fundamental physiology and in drug discovery.

Single-component nanodiscs via the thermal folding of amphiphilic graft copolymers with the adjusted flexibility of the main chain

Nanodiscs have attracted considerable attention as structural scaffolds for membrane-protein research and as biomaterials in e.g. drug-delivery systems. However, conventional disc-fabrication methods are usually laborious, and disc fabrication via the self-assembly of amphiphiles is difficult. Herein, we report the formation of polymer nanodiscs based on the self-assembly of amphiphilic graft copolymers by adjusting the persistence length of the main chain. Amphiphilic graft copolymers with a series of different main-chain persistence lengths were prepared and these formed, depending on the persistence length, either rods, discs, or vesicles. Notably, polymer nanodiscs were formed upon heating a chilled polymer solution without the need for any additives, and the thus obtained nanodiscs were used to solubilize a membrane protein during cell-free protein synthesis. Given the simplicity of this disc-fabrication method and the ability of these discs to solubilize membrane proteins, this study considerably expands the fundamental and practical scope of graft-copolymer nanodiscs and demonstrates their utility as tools for studying the structure and function of membrane proteins.

Formation and function of OmpG or OmpA-incorporated liposomes using an in vitro translation system

Outer membrane proteins (OMPs), located on the outer membrane of gram-negative bacteria, have a β-strand structure and form nanopores, which allow passage of ions, sugars, and small molecules. Recently, OMPs have been used as sensing elements to detect biological molecules. OMPs are normally expressed and purified from Escherichia coli (E. coli). Although the cell-free synthesis of OMPs, such as OmpA and OmpG, is achieved in the presence of liposomes and periplasmic chaperones, the amount of OmpA and OmpG incorporated into the nano-sized liposomes is not clear. In this study, after in vitro translation, the incorporation of OmpG into purified nano-sized liposomes with various lipid compositions was investigated. Liposomes containing the synthesized OmpG were purified using a stepwise sucrose density gradient. We report that liposomes prepared with the E. coli lipid extract (PE/PG) had the highest amount of OmpG incorporated compared to liposomes with other lipid compositions, as detected by recording the current across the OmpG containing liposomes using the patch clamp technique. This study reveals some of the requirements for the insertion and refolding of OMPs synthesized by the in vitro translation system into lipid membranes, which plays a role in the biological sensing of various molecules.

Cell-Free Expression to Probe Co-Translational Insertion of an Alpha Helical Membrane Protein

The majority of alpha helical membrane proteins fold co-translationally during their synthesis on the ribosome. In contrast, most mechanistic folding studies address refolding of full-length proteins from artificially induced denatured states that are far removed from the natural co-translational process. Cell-free translation of membrane proteins is emerging as a useful tool to address folding during translation by a ribosome. We summarise the benefits of this approach and show how it can be successfully extended to a membrane protein with a complex topology. The bacterial leucine transporter, LeuT can be synthesised and inserted into lipid membranes using a variety of in vitro transcription translation systems. Unlike major facilitator superfamily transporters, where changes in lipids can optimise the amount of correctly inserted protein, LeuT insertion yields are much less dependent on the lipid composition. The presence of a bacterial translocon either in native membrane extracts or in reconstituted membranes also has little influence on the yield of LeuT incorporated into the lipid membrane, except at high reconstitution concentrations. LeuT is considered a paradigm for neurotransmitter transporters and possesses a knotted structure that is characteristic of this transporter family. This work provides a method in which to probe the formation of a protein as the polypeptide chain is being synthesised on a ribosome and inserting into lipids. We show that in comparison with the simpler major facilitator transporter structures, LeuT inserts less efficiently into membranes when synthesised cell-free, suggesting that more of the protein aggregates, likely as a result of the challenging formation of the knotted topology in the membrane.

Functional Oligomeric Forms of Uncoupling Protein 2: Strong Evidence for Asymmetry in Protein and Lipid Bilayer Systems

Stoichiometry of uncoupling proteins (UCPs) and their coexistence as functional monomeric and associated forms in lipid membranes remain intriguing open questions. In this study, tertiary and quaternary structures of UCP2 were analyzed experimentally and through molecular dynamics (MD) simulations. UCP2 was overexpressed in the inner membrane of Escherichia coli, then purified and reconstituted in lipid vesicles. Structure and proton transport function of UCP2 were characterized by circular dichroism (CD) spectroscopy and fluorescence methods. Findings suggest a tetrameric functional form for UCP2. MD simulations conclude that tetrameric UCP2 is a dimer of dimers, is more stable than its monomeric and dimeric forms, is asymmetrical and induces asymmetry in the membrane's lipid structure, and a biphasic on-off switch between the dimeric units is its possible mode of transport. MD simulations also show that the water density inside the UCP2 monomer is asymmetric, with the cytoplasmic side having a higher water density and a wider radius. In contrast, the structurally comparable adenosine 5'-diphosphate (ADP)/adenosine 5'-triphosphate (ATP) carrier (AAC1) did not form tetramers, implying that tetramerization cannot be generalized to all mitochondrial carriers.

Modulation of Function, Structure and Clustering of K+ Channels by Lipids: Lessons Learnt from KcsA

KcsA, a prokaryote tetrameric potassium channel, was the first ion channel ever to be structurally solved at high resolution. This, along with the ease of its expression and purification, made KcsA an experimental system of choice to study structure–function relationships in ion channels. In fact, much of our current understanding on how the different channel families operate arises from earlier KcsA information. Being an integral membrane protein, KcsA is also an excellent model to study how lipid–protein and protein–protein interactions within membranes, modulate its activity and structure. In regard to the later, a variety of equilibrium and non-equilibrium methods have been used in a truly multidisciplinary effort to study the effects of lipids on the KcsA channel. Remarkably, both experimental and “in silico” data point to the relevance of specific lipid binding to two key arginine residues. These residues are at non-annular lipid binding sites on the protein and act as a common element to trigger many of the lipid effects on this channel. Thus, processes as different as the inactivation of channel currents or the assembly of clusters from individual KcsA channels, depend upon such lipid binding.

Escherichia coli Extract-Based Cell-Free Expression System as an Alternative for Difficult-to-Obtain Protein Biosynthesis

Before utilization in biomedical diagnosis, therapeutic treatment, and biotechnology, the diverse variety of peptides and proteins must be preliminarily purified and thoroughly characterized. The recombinant DNA technology and heterologous protein expression have helped simplify the isolation of targeted polypeptides at high purity and their structure-function examinations. Recombinant protein expression in Escherichia coli, the most-established heterologous host organism, has been widely used to produce proteins of commercial and fundamental research interests. Nonetheless, many peptides/proteins are still difficult to express due to their ability to slow down cell growth or disrupt cellular metabolism. Besides, special modifications are often required for proper folding and activity of targeted proteins. The cell-free (CF) or in vitro recombinant protein synthesis system enables the production of such difficult-to-obtain molecules since it is possible to adjust reaction medium and there is no need to support cellular metabolism and viability. Here, we describe E. coli-based CF systems, the optimization steps done toward the development of highly productive and cost-effective CF methodology, and the modification of an in vitro approach required for difficult-to-obtain protein production.

Preparation of cationic proteoliposomes using cell-free membrane protein synthesis: the chaperoning effect of cationic liposomes

Membrane protein reconstituted cationic liposomes are constructed using cell-free membrane protein synthesis in the presence of cationic liposomes. The chaperon effect of cationic liposomal membrane assists in folding the functional conformation of membrane protein. This preparation method enables the provision of the usage of proteoliposomes for drug delivery.

The preparation method of cationic proteoliposomes is established using a cell-free membrane protein synthesis in the presence of cationic liposomes.

Cell-free synthesis of connexin 43-integrated exosome-mimetic nanoparticles for siRNA delivery

Exosomes are naturally secreted nanovesicles that have emerged as a promising therapeutic nanodelivery platform, due to their specific composition and biological properties. However, challenges like considerable complexity, low isolation yield, drug payload, and potential safety concerns substantially reduce their pharmaceutical acceptability. Given that the nano-bio-interface is a crucial factor for nanocarrier behavior and function, modification of synthetic nanoparticles with the intrinsic hallmarks of exosomes' membrane to create exosome mimetics could allow for siRNA delivery in a safer and more efficient manner. Herein, connexin 43 (Cx43)-embedded, exosome-mimicking lipid bilayers coated chitosan nanoparticles (Cx43/L/CS NPs) were constructed by using cell-free (CF) synthesis systems with plasmids encoding Cx43 in the presence of lipid-coated CS NPs (L/CS NPs). The integration of de novo synthesized Cx43 into the lipid bilayers of L/CS NPs occurred cotranslationally during one-pot reaction and, more importantly, the integrated Cx43 was functionally active in transport. In addition to considerably lower cytotoxicity (<four-fold) than cationic Lipo 2000, the obtained Cx43/L/CS-siRNA NPs showed feasible cellular uptake and silencing efficacy that was significantly higher than free siRNA and CS-siRNA NPs. By using a gap junction (GJ) inhibitor, 18β-glycyrrhetinic acid, we demonstrated that Cx43 facilitated the delivery of siRNA into Cx43-expressing U87 MG cells. Additionally, the cellular entry of Cx43/L/CS-siRNA NPs may rely on different endocytic mechanisms, depending on the types of recipient cells. However, Cx43/L/CS-siRNA NPs still exhibited far from adequate delivery efficiency compared with transfection reagent Lipo 2000. Taken together, our study provides a brand new strategy to construct Cx43-functionalized, exosome-mimetic nanoparticles, which may further encourage the establishment of more biomimetic nanocarriers with higher biocompatibility and delivery efficiency. SIGNIFICANCE OF STATEMENT: The major issue to move RNA interference (RNAi) therapy from bench to bedside is the lack of safe and efficient delivery vehicles. Given the certain advantages and limitations of exosomes and synthetic nanocarriers, a promising strategy is to facilitate positive feedbacks between the two fields, in which the superiority of exosomes regarding special membrane composition beneficial for cytoplasmic delivery and the better pharmaceutical acceptance of synthetic nanocarriers could be combined. In this study, we reported to construct Cx43-integrated, exosome-mimetic lipid bilayers coated nanoparticles by using CF synthesis technique. The obtained Cx43/L/CS-siRNA NPs were characterized by desirable cytotoxicity profile and feasible delivery efficiency. This study provides a new avenue and insights for the synthesis of more biocompatible and effective bio-mimetic siRNA delivery platforms.

Proteoliposome Engineering with Cell-Free Membrane Protein Synthesis: Control of Membrane Protein Sorting into Liposomes by Chaperoning Systems

Integral membrane proteins (IMPs) modulate key cellular processes; their dysfunctions are closely related to disease. However, production of IMPs in active conformations for further study is hindered by aggregation and toxicity in living expression systems. IMPs are therefore produced in cell-free systems employing liposome chaperoning, but membrane integration of the nascent IMPs is suboptimal and orientation of the integrated proteins remains uncontrollable. Thus, an artificial membrane protein sorting system is developed, based on polyhistidine/nickel-chelate affinity, combined with cell-free membrane protein synthesis. Its proof of concept is demonstrated with a N-terminal hexahistadine-fused conexin-43 (NHis-Cx43) model IMP. Nickel-chelating liposomes efficiently incorporate twofold newly synthesized NHis-Cx43 compared with Cx43. NHis-Cx43, when synthesized in this system, forms dye-permeable hemichannels, similar to plasma membrane pores formed by Cx43 in cells. The topology of incorporated NHis-Cx43 indicates two orientations in the liposomal membranes. However, NHis-Cx43 orientation is controlled, resulting in single topology, by combination of the natural molecular chaperone DnaKJE. Successful synthesis and at least 4.5-fold increase lipid incorporation are also achieved with three other NHis-fused IMPs, including α-helix and β-barrel IMPs. Overall, this simple membrane protein sorting system is usable combined with molecular chaperones to prepare proteoliposomes for many applications.

Liposome-chaperoned cell-free synthesis for the design of proteoliposomes: Implications for therapeutic delivery

Cell-free (CF) protein synthesis has emerged as a powerful technique platform for efficient protein production in vitro. Liposomes have been widely studied as therapeutic carriers due to their biocompatibility, biodegradability, low toxicity, flexible surface manipulation, easy preparation, and higher cargo encapsulation capability. However, rapid immune clearance, insufficient targeting capacity, and poor cytoplasmic delivery efficiency substantially restrict their clinical application. The incorporation of functional membrane proteins (MPs) or peptides allows the transfer of biological properties to liposomes and imparts them with improved circulation, increased targeting, and efficient intracellular delivery. Liposome-chaperoned CF synthesis enables production of proteoliposomes in one-step reaction, which not only substantially simplifies the production procedure but also keeps protein functionality intact. Building off these observations, proteoliposomes with integrated MPs represent an excellent candidate for therapeutic delivery. In this review, we describe recent advances in CF synthesis with emphasis on detailing key factors for improving CF expression efficiency. Furthermore, we provide insights into strategies for rational design of proteoliposomal nanodelivery systems via CF synthesis.

STATEMENT OF SIGNIFICANCE

Liposome-chaperoned CF synthesis has emerged as a powerful approach for the design of recombinant proteoliposomes in one-step reaction. The incorporation of bioactive MPs or peptides into liposomes via CF synthesis can facilitate the development of proteoliposomal nanodelivery systems with improved circulation, increased targeting, and enhanced cellular delivery capacity. Moreover, by adapting lessons learned from natural delivery vehicles, novel bio-inspired proteoliposomes with enhanced delivery properties could be produced in CF systems. In this review, we first give an overview of CF synthesis with focus on enhancing protein expression in liposome-chaperoned CF systems. Furthermore, we intend to provide insight into harnessing CF-synthesized proteoliposomes for efficient therapeutic delivery.

Amphiphobic Septa Enhance the Mechanical Stability of Free-Standing Bilayer Lipid Membranes

Artificial bilayer lipid membranes (BLMs) provide well-defined systems for investigating the fundamental properties of membrane proteins, including ion channels, and for screening the effect of drugs that act on them. However, the application of this technique is limited due to the low stability and low reconstitution efficiency of the process. We previously reported on improving the stability of BLM based on the fabrication of microapertures having a tapered edge in SiO₂/Si₃N₄ septa and efficient ion channel incorporation based on vesicle fusion accelerated by a centrifugal force. Although the BLM stability and incorporation probability were dramatically improved when these approaches were used, some BLMs were ruptured when subjected to a centrifugal force. To further improve the BLM stability, we investigated the effect of modifying the surface of the SiO₂/Si₃N₄ septa on the stability of BLM suspended in the septa. The modified surfaces were characterized in terms of hydrophobicity, lipophobicity, and surface roughness. Diffusion coefficients of the lipid monolayers formed on the modified surfaces were also determined. Highly fluidic lipid monolayers were formed on the amphiphobic substrates that had been modified with long-chain perfluorocarbons. Free-standing BLMs formed in amphiphobic septa showed a much higher mechanical stability, including tolerance to water movement and applied centrifugal forces with and without proteoliposomes, than those formed in the septa that had been modified with a short alkyl chain. These results demonstrate that highly stable BLMs are formed when the surface of the septa has amphiphobic properties. Because highly fluidic lipid monolayers that are formed on the septa seamlessly connect with BLMs in a free-standing region, the high fluidity of the lipids contributes to decreasing potential damage to BLMs when mechanical stresses are applied. This approach to improve the BLM stability increases the experimental efficiency of the BLM systems and will contribute to the development of high-throughput platforms for functional assays of ion channel proteins.

Concurrent In Vitro Synthesis and Functional Detection of Nascent Activity of the KcsA Channel under a Membrane Potential

Processes involved in the functional formation of prokaryotic membrane proteins have remained elusive. Here, we developed a new in vitro membrane protein expression system to detect nascent activities of the KcsA potassium channel in lipid bilayers under an applied membrane potential. The channel was synthesized using a reconstituted Escherichia coli-based in vitro transcription/translation system (IVTT) in a water-in-oil droplet lined by a membrane. The synthesized channels spontaneously incorporated into the membrane even without the translocon machinery (unassisted pathway) and formed functional channels with the correct orientation. The single-channel current of the first appearing nascent channel was captured, followed by the subsequent appearance of multiple channels. Notably, the first appearance time shortened substantially as the membrane potential was hyperpolarized. Under a steadily applied membrane potential, this system serves as a production line of membrane proteins via the unassisted pathway, mimicking the bacterial synthetic membrane.

Mechanically stable solvent-free lipid bilayers in nano- and micro-tapered apertures for reconstitution of cell-free synthesized hERG channels

The self-assembled bilayer lipid membrane (BLM) is the basic component of the cell membrane. The reconstitution of ion channel proteins in artificially formed BLMs represents a well-defined system for the functional analysis of ion channels and screening the effects of drugs that act on them. However, because BLMs are unstable, this limits the experimental throughput of BLM reconstitution systems. Here we report on the formation of mechanically stable solvent-free BLMs in microfabricated apertures with defined nano- and micro-tapered edge structures. The role of such nano- and micro-tapered structures on the stability of the BLMs was also investigated. Finally, this BLM system was combined with a cell-free synthesized human ether-a-go-go-related gene channel, a cardiac potassium channel whose relation to arrhythmic side effects following drug treatment is well recognized. Such stable BLMs as these, when combined with a cell-free system, represent a potential platform for screening the effects of drugs that act on various ion-channel genotypes.

Glycan profiling analysis using evanescent-field fluorescence-assisted lectin array: Importance of sugar recognition for cellular uptake of exosomes from mesenchymal stem cells

Studies involving the functional analysis of exosomal contents including proteins, DNA, and RNA have been reported. Most membrane proteins and lipids are glycosylated, which controls their physical properties and functions, but little is known about glycans on exosomes owing to the difficulty of analysing them. To shed light on these issues, we collected exosomes from mesenchymal stem cells (MSCs) derived from human adipose tissue for glycan profiling using evanescent-field fluorescence-assisted lectin array as well as analysis of their uptake in vivo. Initial analyses showed that the mean diameter of the collected exosomes was 178 nm and they presented with typical exosomal and MSC markers. Regarding the glycan profiling, exosomes interacted more strongly than the membrane of the original MSCs did with a range of lectins, especially sialic acid-binding lectins. The findings also showed that cellular exosome uptake involved recognition by HeLa cell-surface-bound sialic acid-binding immunoglobulin (Ig)-like lectins (siglecs). Confirming this siglec-related uptake, in vivo experiments involving subcutaneous injection of the fluorescently labelled exosomes into mice showed their transport into lymph nodes and internalization by antigen-presenting cells, particularly those expressing CD11b. Closer analysis revealed the colocalization of the exosomes with siglecs, indicating their involvement in the uptake. These findings provide us with an improved understanding of the importance of exosomal transport and targeting in relation to glycans on exosomal surfaces, potentially enabling us to standardize exosomes when using them for therapeutic purposes.