Matthew A. Holden

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.

Droplet networks with incorporated protein diodes show collective properties

Recently, we demonstrated that submicrolitre aqueous droplets submerged in an apolar liquid containing lipid can be tightly connected by means of lipid bilayers to form networks. Droplet interface bilayers have been used for rapid screening of membrane proteins and to form asymmetric bilayers with which to examine the fundamental properties of channels and pores. Networks, meanwhile, have been used to form microscale batteries and to detect light. Here, we develop an engineered protein pore with diode-like properties that can be incorporated into droplet interface bilayers in droplet networks to form devices with electrical properties including those of a current limiter, a half-wave rectifier and a full-wave rectifier. The droplet approach, which uses unsophisticated components (oil, lipid, salt water and a simple pore), can therefore be used to create multidroplet networks with collective properties that cannot be produced by droplet pairs.

Asymmetric Droplet Interface Bilayers

In cell membranes, the lipid compositions of the inner and outer leaflets differ. Therefore, a robust model system that enables single-channel electrical recording with asymmetric bilayers would be very useful. We and others recently developed the droplet interface bilayer (DIB), which is formed by connecting lipid monolayer-encased aqueous droplets submerged in an oil-lipid mixture. Here, we incorporate lipid vesicles of different compositions into aqueous droplets and immerse them in an oil bath to form asymmetric DIBs (a-DIBs). Both alpha-helical and beta-barrel membrane proteins insert readily into a-DIBs, and their activity can be measured by single-channel electrical recording. We show that the gating behavior of outer membrane protein G (OmpG) from Escherichia coli differs depending on the side of insertion in an asymmetric DIB with a positively charged leaflet opposing a negatively charged leaflet. The a-DIB system provides a general platform for studying the effects of bilayer leaflet composition on the behavior of ion channels and pores.

Screening Blockers Against a Potassium Channel with a Droplet Interface Bilayer Array

Droplet interface bilayers (DIBs) form between two lipid monolayer-encased aqueous droplets submerged in oil. Both major structural classes of membrane proteins, alpha-helix bundles and beta barrels, represented by channels and pores, respectively, spontaneously insert into DIBs when freshly expressed by cell-free transcription and translation. Electrodes embedded within the droplets allow the measurement of transmembrane ionic currents carried by individual channels and pores. On the basis of these findings, we have devised a chip-based approach for the rapid screening of blockers against ion channels. The technique is demonstrated here with the viral potassium channel, Kcv.

Constructing droplet interface bilayers from the contact of aqueous droplets in oil

We describe a protocol for forming an artificial lipid bilayer by contacting nanoliter aqueous droplets in an oil solution in the presence of phospholipids. A lipid monolayer forms at each oil-water interface, and when two such monolayers touch, a bilayer is created. Droplet interface bilayers (DIBs) are a simple way to generate stable bilayers suitable for single-channel electrophysiology and optical imaging from a wide variety of preparations, ranging from purified proteins to reconstituted eukaryotic cell membrane fragments. Examples include purified proteins from the α-hemolysin pore from Staphylococcus aureus, the anthrax toxin pore and the 1.2-MDa mouse mechanosensitive channel MmPiezo1. Ion channels and ionotropic receptors can also be reconstituted from membrane fragments without further purification. We describe two approaches for forming DIBs. In one approach, a lipid bilayer is created between two aqueous droplets submerged in oil. In the other approach, a membrane is formed between an aqueous droplet and an agarose hydrogel, which allows imaging in addition to electrical recordings. The protocol takes <30 min, including droplet generation, monolayer assembly and bilayer formation. In addition to the main protocol, we also describe the preparation of Ag/AgCl electrodes and sample preparation.

Microfluidic diffusion diluter: bulging of PDMS microchannels under pressure-driven flow*

The bulging of microfluidic systems during pressure-driven flow is potentially a major consideration for polydimethylsiloxane (PDMS)-based devices. Microchannel cross-sectional areas can change drastically as a function of flow rate and downstream microchannel position. Such geometrical flexibility leads to difficulties in predicting convective/diffusive transport for these systems. We have previously introduced a non-dimensional parameter, κ, for characterizing convection and diffusion behavior for pressure-driven flow in rigid all-glass systems. This paper describes a modification of that concept for application to non-rigid systems, which is accomplished by incorporating an experimental step to account for the bulging in PDMS/glass microsystems. Specifically, an experimental measurement of channel height by fluorescence microscopy is combined with the aforementioned theory to characterize convective/diffusive behavior at a single location in the device. This allowed the parameter κ to be determined at that point and applied to predict fluid flow in the subsequent portion of the PDMS microsystem. This procedure was applied to a PDMS/glass microfluidic diffusion dilution (μDD) device designed for generating concentration gradients. Theoretically predicted and experimentally measured distributions of concentrations within the microsystem matched well.

Ultrasensitive detection of protein translocated through toxin pores in droplet-interface bilayers

Many bacterial toxins form proteinaceous pores that facilitate the translocation of soluble effector proteins across cellular membranes. With anthrax toxin this process may be monitored in real time by electrophysiology, where fluctuations in ionic current through these pores inserted in model membranes are used to infer the translocation of individual protein molecules. However, detecting the minute quantities of translocated proteins has been a challenge. Here, we describe use of the droplet-interface bilayer system to follow the movement of proteins across a model membrane separating two submicroliter aqueous droplets. We report the capture and subsequent direct detection of as few as 100 protein molecules that have translocated through anthrax toxin pores. The droplet-interface bilayer system offers new avenues of approach to the study of protein translocation.

Selective Detection of Protein Homologues in Serum Using an OmpG Nanopore.

Outer membrane protein G is a monomeric β-barrel porin that has seven flexible loops on its extracellular side. Conformational changes of these labile loops induce gating spikes in current recordings that we exploited as the prime sensing element for protein detection. The gating characteristics, open probability, frequency, and current decrease, provide rich information for analyte identification. Here, we show that two antibiotin antibodies each induced a distinct gating pattern, which allowed them to be readily detected and simultaneously discriminated by a single OmpG nanopore in the presence of fetal bovine serum. Our results demonstrate the feasibility of directly profiling proteins in real-world samples with minimal or no sample pretreatment.

Building interconnected membrane networks

Reconstituted replica cell membranes are easily created by contacting two lipid-monolayer-encased aqueous droplets under an oil phase. Called the droplet interface bilayer (DIB), this technique has been used to study a wide range of membrane processes. Importantly, this method is compatible with electrical measurements, meaning that membrane protein activities are easily observed in DIBs. By positioning droplets in two- and three-dimensional networks, sophisticated interconnected systems can be created that possess collective properties. The methods described here summarize the approaches used to create DIB networks and how to operate the devices that have been constructed so far.

Protein transport across membranes: Comparison between lysine and guanidinium-rich carriers

The mechanism(s) by which certain small peptides and peptide mimics carry large cargoes across membranes through exclusively non-covalent interactions has been difficult to resolve. Here, we use the droplet-interface bilayer as a platform to characterize distinct mechanistic differences between two such carriers: Pep-1 and a guanidinium-rich peptide mimic we call D9. While both Pep-1 and D9 can carry an enzyme, horseradish peroxidase (HRP) across a lipid bilayer, we found that they do so by different mechanisms. Specifically, Pep-1 requires voltage or membrane asymmetry while D9 does not. In addition, D9 can facilitate HRP transport without pre-forming a complex with HRP. By contrast, complex formation is required by Pep-1. Both carriers are capable of forming pores in membranes but our data hints that these pores are not responsible for cargo transport. Overall, D9 appears to be a more potent and versatile transporter when compared with Pep-1 because D9 does not require an applied voltage or other forces to drive transport. Thus, D9 might be used to deliver cargo across membranes under conditions where Pep-1 would be ineffective.