Biomimetic approaches for studying membrane processes

Light-Induced Self-Assembled Polydiacetylene/Carbon Dot Functional "Honeycomb"

The design of functional supramolecular assemblies from individual molecular building blocks is a fundamental challenge in chemistry and material science. We report on the fabrication of "honeycomb" films by light-induced coassembly of diacetylene derivatives and carbon dots. Specifically, modulating noncovalent interactions between the carbon dots, macrocyclic diacetylene, and anthraquinone diacetylene facilitates formation of thin films exhibiting a long-range, uniform pore structure. We show that light irradiation at distinct wavelengths plays a key role in the assembly process and generation of unique macro-porous morphology, by both initiating interactions between the carbon dots and the anthraquinone moieties and giving rise to the topotactic polymerization of the polydiacetylene network. We further demonstrate utilization of the macro-porous film as a photocatalytic platform for water pollutant degradation and as potential supercapacitor electrodes, both applications taking advantage of the high surface area, hydrophobicity, and pore structure of the film.

Microfluidic Devices for Drug Delivery Systems and Drug Screening

Microfluidic devices present unique advantages for the development of efficient drug carrier particles, cell-free protein synthesis systems, and rapid techniques for direct drug screening. Compared to bulk methods, by efficiently controlling the geometries of the fabricated chip and the flow rates of multiphase fluids, microfluidic technology enables the generation of highly stable, uniform, monodispersed particles with higher encapsulation efficiency. Since the existing preclinical models are inefficient drug screens for predicting clinical outcomes, microfluidic platforms might offer a more rapid and cost-effective alternative. Compared to 2D cell culture systems and in vivo animal models, microfluidic 3D platforms mimic the in vivo cell systems in a simple, inexpensive manner, which allows high throughput and multiplexed drug screening at the cell, organ, and whole-body levels. In this review, the generation of appropriate drug or gene carriers including different particle types using different configurations of microfluidic devices is highlighted. Additionally, this paper discusses the emergence of fabricated microfluidic cell-free protein synthesis systems for potential use at point of care as well as cell-, organ-, and human-on-a-chip models as smart, sensitive, and reproducible platforms, allowing the investigation of the effects of drugs under conditions imitating the biological system.

Responsive Polydiacetylene Vesicles for Biosensing Microorganisms

Polydiacetylene (PDA) inserted in films or in vesicles has received increasing attention due to its property to undergo a blue-to-red colorimetric transition along with a change from non-fluorescent to fluorescent upon application of various stimuli. In this review paper, the principle for the detection of various microorganisms (bacteria, directly detected or detected through the emitted toxins or through their DNA, and viruses) and of antibacterial and antiviral peptides based on these responsive PDA vesicles are detailed. The analytical performances obtained, when vesicles are in suspension or immobilized, are given and compared to those of the responsive vesicles mainly based on the vesicle encapsulation method. Many future challenges are then discussed.

A clickable and photocleavable lipid analogue for cell membrane delivery and release

For drug delivery purposes, the ability to conveniently attach a targeting moiety that will deliver drugs to cells and then enable controlled release of the active molecule after localization is desirable. Toward this end, we designed and synthesized clickable and photocleavable lipid analogue 1 to maximize the efficiency of bioconjugation and triggered release. This compound contains a dibenzocyclooctyne group for bioorthogonal derivatization linked via a photocleavable 2-nitrobenzyl moiety at the headgroup of a synthetic lipid backbone for targeting to cell membranes. To assess delivery and release using this system, we report fluorescence-based assays for liposomal modification and photocleavage in solution as well as through surface immobilization to demonstrate successful liposome functionalization and photoinduced release. In addition, fluorophore delivery to and release from live cells was confirmed and characterized using fluorescence microscopy and flow cytometry analysis in which 1 was delivered to cells, derivatized, and photocleaved. Finally, drug delivery studies were performed using an azide-tagged analogue of camptothecin, a potent anticancer drug that is challenging to deliver due to poor solubility. In this case, the ester attachment of the azide tag acted as a caging group for release by intracellular esterases rather than through photocleavage. This resulted in a dose-dependent response in the presence of liposomes containing delivery agent 1, confirming the ability of this compound to stimulate delivery to the cytoplasm of cells.

Conditions for liposome adsorption and bilayer formation on BSA passivated solid supports

Planar solid supported lipid membranes that include an intervening bovine serum albumen (BSA) cushion can greatly reduce undesirable interactions between reconstituted membrane proteins and the underlying substrate. These hetero-self-assemblies reduce frictional coupling by shielding reconstituted membrane proteins from the strong surface charge of the underlying substrate, thereby preventing them from strongly sticking to the substrate themselves. The motivation for this work is to describe the conditions necessary for liposome adsorption and bilayer formation on these hetero-self-assemblies. Described here are experiments that show that the state of BSA is critically important to whether a lipid bilayer is formed or intact liposomes are adsorbed to the BSA passivated surface. It is shown that a smooth layer of native BSA will readily promote lipid bilayer formation while BSA that has been denatured either chemically or by heat will not. Atomic force microscopy (AFM) and fluorescence microscopy was used to characterize the surfaces of native, heat denatured, and chemically reduced BSA. The mobility of several zwitterionic and negatively charged lipid combinations has been measured using fluorescence recovery after photobleaching (FRAP). From these measurements diffusion constants and percent recoveries have been determined and tabulated. The effect of high concentrations of beta-mercaptoethanol (β-ME) on liposome formation as well as bilayer formation was also explored.

Exploiting bioorthogonal chemistry to elucidate protein-lipid binding interactions and other biological roles of phospholipids

Lipids play critical roles in a litany of physiological and pathophysiological events, often through the regulation of protein function. These activities are generally difficult to characterize, however, because the membrane environment in which lipids operate is very complex. Moreover, lipids have a diverse range of biological functions, including the recruitment of proteins to membrane surfaces, actions as small-molecule ligands, and covalent protein modification through lipidation. Advancements in the development of bioorthogonal reactions have facilitated the study of lipid activities by providing the ability to selectively label probes bearing bioorthogonal tags within complex biological samples. In this Account, we discuss recent efforts to harness the beneficial properties of bioorthogonal labeling strategies in elucidating lipid function. Initially, we summarize strategies for the design and synthesis of lipid probes bearing bioorthogonal tags. This discussion includes issues to be considered when deciding where to incorporate the tag, particularly the presentation within a membrane environment. We then present examples of the application of these probes to the study of lipid activities, with a particular emphasis on the elucidation of protein-lipid binding interactions. One such application involves the development of lipid and membrane microarray analysis as a high-throughput platform for characterizing protein-binding interactions. Here we discuss separate strategies for binding analysis involving the immobilization of either whole liposomes or simplified isolated lipid structures. In addition, we present the different strategies that have been used to derivatize membrane surfaces via bioorthogonal reactions, either by using this chemistry to produce functionalized lipid scaffolds that can be incorporated into membranes or through direct modification of intact membrane surfaces. We then provide an overview of the development of lipid activity probes to label and identify proteins that bind to a particular lipid from complex biological samples. This process involves the strategy of activity-based proteomics, in which proteins are collectively labeled on the basis of function (in this case, ligand binding) rather than abundance. We summarize strategies for designing and applying lipid activity probes that allow for the selective labeling and characterization of protein targets. Additionally, we briefly comment on applications other than studying protein-lipid binding. These include the generation of new lipid structures with beneficial properties, labeling of tagged lipids in live cells for studies involving fluorescence imaging, elucidation of covalent protein lipidation, and identification of biosynthetic lipid intermediates. These applications illustrate the early phase of the promising field of applying bioorthogonal chemistry to the study of lipid function.

Generation of phospholipid vesicle-nanotube networks and transport of molecules therein

We describe micromanipulation and microinjection procedures for the fabrication of soft-matter networks consisting of lipid bilayer nanotubes and surface-immobilized vesicles. These biomimetic membrane systems feature unique structural flexibility and expandability and, unlike solid-state microfluidic and nanofluidic devices prepared by top-down fabrication, they allow network designs with dynamic control over individual containers and interconnecting conduits. The fabrication is founded on self-assembly of phospholipid molecules, followed by micromanipulation operations, such as membrane electroporation and microinjection, to effect shape transformations of the membrane and create a series of interconnected compartments. Size and geometry of the network can be chosen according to its desired function. Membrane composition is controlled mainly during the self-assembly step, whereas the interior contents of individual containers is defined through a sequence of microneedle injections. Networks cannot be fabricated with other currently available methods of giant unilamellar vesicle preparation (large unilamellar vesicle fusion or electroformation). Described in detail are also three transport modes, which are suitable for moving water-soluble or membrane-bound small molecules, polymers, DNA, proteins and nanoparticles within the networks. The fabrication protocol requires ∼90 min, provided all necessary preparations are made in advance. The transport studies require an additional 60-120 min, depending on the transport regime.

A tethered bilayer assembled on top of immobilized calmodulin to mimic cellular compartmentalization

Background

Biomimetic membrane models tethered on solid supports are important tools for membrane protein biochemistry and biotechnology. The supported membrane systems described up to now are composed of a lipid bilayer tethered or not to a surface separating two compartments: a ”trans” side, one to a few nanometer thick, located between the supporting surface and the membrane; and a “cis” side, above the synthetic membrane, exposed to the bulk medium. We describe here a novel biomimetic design composed of a tethered bilayer membrane that is assembled over a surface derivatized with a specific intracellular protein marker. This multilayered biomimetic assembly exhibits the fundamental characteristics of an authentic biological membrane in creating a continuous yet fluid phospholipidic barrier between two distinct compartments: a “cis” side corresponding to the extracellular milieu and a “trans” side marked by a key cytosolic signaling protein, calmodulin.

Methodology/Principal Findings

We established and validated the experimental conditions to construct a multilayered structure consisting in a planar tethered bilayer assembled over a surface derivatized with calmodulin. We demonstrated the following: (i) the grafted calmodulin molecules (in trans side) were fully functional in binding and activating a calmodulin-dependent enzyme, the adenylate cyclase from Bordetella pertussis; and (ii) the assembled bilayer formed a continuous, protein-impermeable boundary that fully separated the underlying calmodulin (trans side) from the above medium (cis side).

Conclusions

The simplicity and robustness of the tethered bilayer structure described here should facilitate the elaboration of biomimetic membrane models incorporating membrane embedded proteins and key cytoplasmic constituents. Such biomimetic structures will also be an attractive tool to study translocation across biological membranes of proteins or other macromolecules.

Characterization of protein-membrane binding interactions via a microplate assay employing whole liposome immobilization

Protein-cell membrane binding interactions control numerous vital biological processes, many of which can go awry during disease onset. However, the study of these events is complicated by the complexity of the membrane bilayer. These efforts would benefit from a rapid and easily accessible method for characterizing protein-membrane recognition events. Herein, we describe a microplate-based method for the detection of protein-membrane binding that employs whole liposome immobilization using a biotin anchor. First, control studies are detailed to test for nonspecific liposome immobilization (fluorescence assay; see Subheading 3.2), and to ensure that liposomes remain intact on the microplate surface (dye leakage assay; see Subheading 3.3). Finally, a protein-membrane binding detection assay is described through the example of protein kinase Cα binding to surface-immobilized whole liposomes (see Subheading 3.4).

Phospholipid bilayer formation on a variety of nanoporous oxide and organic xerogel films

Lipid bilayers supported by nanoporous xerogel materials are being explored as models for cell membranes. In order to better understand and characterize the nature of the surface-bilayer interactions, several oxide and organic nanoporous xerogel films (alumina, titania, iron oxide, phloroglucinol-formaldehyde, resorcinol-formaldehyde and cellulose acetate) have been investigated as a scaffold for vesicle-fused 1,2-dioleoyl-glycero-3-phosphocholine (DOPC) lipid bilayer formation and mobility. The surface topography of the different substrates was analyzed using contact and tapping-mode atomic force microscopy and the surface energy of the substrates was determined using contact angle goniometry. Lipid bilayer formation has been observed with fluorescence microscopy and lateral lipid diffusion coefficients have been determined using fluorescence recovery after photobleaching. Titania xerogel films were found to be a robust and convenient support for formation of a two-phase DOPC/1,2-distearoyl-glycero-3-phosphocholine bilayer and domains were observed with this system. It was found that the cellulose acetate xerogel film support produced the slowest lipid lateral diffusion.

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.