Fluorescent Lipo-Beads for the Sensitive Detection of Phospholipase A2 and Its Inhibitors

Phospholipase A₂ (PLA₂) is a membrane lytic enzyme that is present in many organisms. Human PLA₂ has emerged as a potential biomarker as well as a therapeutic target for several diseases including cancer, cardiovascular diseases, and some inflammatory diseases. The current study focuses on the development of lipo-beads that are very reactive and highly sensitive to PLA₂. To develop the best supported lipid bilayer formulation, several lipid combinations were investigated using 10 μm porous silica beads. The reactivity of PLA₂ was monitored via the decrease in particle fluorescence because of the release of entrapped fluorescent dye from the particle pores or the disintegration of a fluorescent lipid constituted on the bilayer upon lipid hydrolysis using flow cytometry. The enzyme binding studies indicate that lipo-beads with bulky fluorescent tags in the lipid head group and anionic lipids produce a more pronounced response. The kinetic studies suggest that these lipo-beads are very reactive with PLA₂ and can generate a detectable signal in less than 5 min. The enzyme inhibition studies were also conducted with two known PLA₂ inhibitors, varespladib and quercetin. We find that quercetin can hydrolyze the supported membrane, and thus inhibition of PLA₂ is not observed; however, varespladib has shown significant PLA₂ inhibition on lipo-beads. We have demonstrated that our lipo-bead-based approach can detect annexin-3, a known disease biomarker, as low as 10 nM within 5 min after incubation.

Anti-Tumor Effects of Melittin and Its Potential Applications in Clinic

Melittin, a major component of bee venom, is a water-soluble toxic peptide of which a various biological effects have been identified to be useful in anti-tumor therapy. In addition, Melittin also has anti-parasitic, anti-bacterial, anti-viral, and anti-inflammatory activities. Therefore, it is a very attractive therapeutic candidate for human diseases. However, melittin induces extensive hemolysis, a severe side effect that dampens its future development and clinical application. Thus, studies of melittin derivatives and new drug delivery systems have been conducted to explore approaches for optimizing the efficacy of this compound, while reducing its toxicity. A number of reviews have focused on each side, respectively. In this review, we summarize the research progress on the anti-tumor effects of melittin and its derivatives, and discuss its future potential clinical applications.

A Microsphere-Supported Lipid Bilayer Platform for DNA Reactions on a Fluid Surface

We report a versatile microsphere-supported lipid bilayer system that can serve as a general-purpose platform for implementing DNA nanotechnologies on a fluid surface. To demonstrate our platform, we implemented both toehold-mediated strand displacement (TMSD) and DNAzyme reactions, which are typically performed in solution and which are the cornerstone of DNA-based molecular logic and dynamic DNA nanotechnology, on the surface. We functionalized microspheres bearing supported lipid bilayers (μSLBs) with membrane-bound nucleic acid components. Using functionalized μSLBs, we developed TMSD and DNAzyme reactions by optimizing reaction conditions to reduce nonspecific interactions between DNA and phospholipids and to enhance bilayer stability. Additionally, the physical and optical properties of the bilayer were tuned via lipid composition and addition of fluorescently tagged lipids to create stable and multiplexable μSLBs that are easily read out by flow cytometry. Multiplexed TMSD reactions on μSLBs enabled the successful operation of a Dengue serotyping assay that correctly identified all 16 patterns of target sequences to demonstrate detection of DNA strands derived from the sequences of all four Dengue serotypes. The limit of detection for this assay was 3 nM. Furthermore, we demonstrated DNAzyme reactions on a fluid lipid surface, which benefit from free diffusion on the surface. This work provides the basis for expansion of both TMSD and DNAzyme based molecular reactions on supported lipid bilayers for use in molecular logic and DNA nanotechnology. As our system is multiplexable and results in fluid surfaces, it may be of use in compartmentalization and improved kinetics of molecular logic reactions and as a useful building block in a variety of DNA nanotechnology systems.

Multiplexed Lipid Bilayers on Silica Microspheres for Analytical Screening Applications

Most druggable targets are membrane components, including membrane proteins and soluble proteins that interact with ligands or receptors embedded in membranes. Current target-based screening and intermolecular interaction assays generally do not include the lipid membrane environment in presenting these targets, possibly altering their native structure and leading to misleading or incorrect results. To address this issue, an ideal assay involving membrane components would (1) mimic the natural membrane environment, (2) be amenable to high-throughput implementation, and (3) be easily multiplexed. In a step toward developing such an ideal target-based analytical assay for membrane components, we present fluorescently indexed multiplexed biomimetic membrane assays amenable to high-throughput flow cytometric detection. We build fluorescently multiplexed biomimetic membrane assays by using varying amounts of a fluorescently labeled lipid, NBD-DOPE [1,2-dioleoyl-sn-glycero-3-phosphoethanolamine-N-(7-nitro-2-1,3-benzoxadiazol-4-yl)], incorporated into a phospholipid membrane bilayer supported on 3 μm silica microspheres. Using flow cytometry, we demonstrate this multiplexed approach by measuring specific affinity of two well-characterized systems, the fluorescently labeled soluble proteins cholera toxin B subunit-Alexa 647 and streptavidin-PE/Cy5, to membranes containing different amounts of ligand targets of these proteins, GM1 and biotin-DOPE, respectively. This work will enable future efforts in developing highly efficient biomimetic assays for interaction analysis and drug screening involving membrane components.

POPC Bilayers Supported on Nanoporous Substrates: Specific Effects of Silica-Type Surface Hydroxylation and Charge Density

Recent advances in nanotechnology bring to the forefront a new class of extrinsic constraints for remodeling lipid bilayers. In this next-generation technology, membranes are supported over nanoporous substrates. The nanometer-sized pores in the substrate are too small for bilayers to follow the substrate topology; consequently, the bilayers hang over the pores. Experiments demonstrate that nanoporous substrates remodel lipid bilayers differently from continuous substrates. The underlying molecular mechanisms, however, remain largely undetermined. Here we use molecular dynamics (MD) simulations to probe the effects of silica-type hydroxylation and charge densities on adsorbed palmitoyl-oleoylphosphatidylcholine (POPC) bilayers. We find that a 50% porous substrate decorated with a surface density of 4.6 hydroxyls/nm(2) adsorbs a POPC bilayer at a distance of 4.5 Å, a result consistent with neutron reflectivity experiments conducted on topologically similar silica constructs under highly acidic conditions. Although such an adsorption distance suggests that the interaction between the bilayer and the substrate will be buffered by water molecules, we find that the substrate does interact directly with the bilayer. The substrate modifies several properties of the bilayer-it dampens transverse lipid fluctuations, reduces lipid diffusion rates, and modifies transverse charge densities significantly. Additionally, it affects lipid properties differently in the two leaflets. Compared to substrates functionalized with sparser surface hydroxylation densities, this substrate adheres to bilayers at smaller distances and also remodels POPC more extensively, suggesting a direct correspondence between substrate hydrophilicity and membrane properties. A partial deprotonation of surface hydroxyls, as expected of a silica substrate under mildly acidic conditions, however, produces an inverse effect: it increases the substrate-bilayer distance, which we attribute to the formation of an electric double layer over the negatively charged substrate, and restores, at least partially, leaflet asymmetry and headgroup orientations. Overall, this study highlights the intrinsic complexity of lipid-substrate interactions and suggests the prospect of making two surface attributes-dipole densities and charge densities-work antagonistically toward remodeling lipid bilayer properties.

Infrared and fluorescence spectroscopic studies of a phospholipid bilayer supported by a soft cationic hydrogel scaffold

Polarized attenuated total reflection (ATR-IR) spectroscopy and fluorescence microscopy techniques were used to characterize a 1,2-diphytanoyl-sn-glycero-3-phosphocholine (DPhPC) membrane supported on porous, cationic hydrogel beads. Fluorescence microscopy images showed that the DPhPC coated the external surface of the hydrogel scaffold. In addition, a fluorescence assay of the emission intensity of the Tb(3+)/dipicolinic acid complex demonstrated that the DPhPC coating acted as a barrier to Tb(3+) efflux from the scaffolded vesicle and successfully sealed the porous hydrogel bead. Fluorescence quenching and ATR-IR spectroscopic measurements revealed that the lipid coating has a bilayer structure. The phytanoyl chains were found to exhibit significant trans-gauche isomerization, characteristic of the fluid liquid phase. However, no lipid lateral mobility was observed by fluorescence recovery after photobleaching (FRAP) measurements. The phosphocholine headgroup was found to be well hydrated and oriented such that the cationic choline group tucked in behind the anionic phosphate group, consistent with an electrostatic attraction between the cationic scaffold and zwitterionic lipid. The absence of lipid lateral mobility may be due to the strength of this attraction.

Spectroscopic and permeation studies of phospholipid bilayers supported by a soft hydrogel scaffold

Polarized attenuated total reflection infrared (ATR-IR) spectroscopy, fluorescence microscopy, and fluorescence spectroscopy were used to characterize a lipid coating composed of 70 mol % 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and 30 mol % cholesterol, supported on a spherical hydrogel scaffold. The fluorescence microscopy images show an association between the lipid coating and the hydrogel scaffold. Fluorescence permeability measurements revealed that the phospholipid coating acts as a permeability barrier, exhibiting characteristics of a lamellar bilayer coating structure. Variable evanescent wave penetration depth ATR-IR spectroscopy studies validated the determination of quantitative molecular orientation information for a lipid coating supported on a spherical scaffold. These polarized ATR-IR studies measured an average DMPC acyl chain tilt angle of ∼21-25°, with respect to the surface normal.

Membrane-mediated neuroprotection by curcumin from amyloid-β-peptide-induced toxicity

Amyloid-β peptide (Aβ)-membrane interactions have been implicated in the formation of toxic oligomers that permeabilize membranes, allowing an influx of calcium ions and triggering cell death in the pathogenesis of Alzheimer's disease (AD). Curcumin, a small dietary polyphenolic molecule, has been shown to reduce Aβ-induced toxicity and AD pathology. We investigate here the effect of curcumin on Aβ40-induced toxicity in cultured human neuroblastoma SH-SY5Y cells and test a novel neuroprotection mechanism in which curcumin reduces Aβ-membrane interactions and attenuates Aβ-induced membrane disruptions. Predominantly monomeric Aβ40 exerts toxicity toward SH-SY5Y cells and has been shown to insert spontaneously into anionic lipid monolayers at the air/water interface, resulting in the misfolding and assembly of Aβ into β-sheet-enriched oligomers. Concomitantly, membrane morphology and lipid packing are disrupted. Curcumin dose-dependently ameliorates Aβ-induced neurotoxicity and reduces either the rate or extent of Aβ insertion into anionic lipid monolayers. Moreover, curcumin reduces Aβ-induced dye leakage from lipid-bilayer-covered, dye-loaded, porous silica microspheres. Because curcumin neither affects the inherent surface activity of Aβ nor modifies the membrane properties, it reduces Aβ insertion by directly attenuating Aβ-membrane interactions and reducing Aβ-induced membrane disruption. Although the exact molecular mechanism of curcumin's membrane protective effect remains unclear, this effect could in part contribute to curcumin's neuroprotective effect with respect to Aβ-induced toxicity. Our work reveals a novel molecular mechanism by which curcumin reduces Aβ-related pathology and toxicity and suggests a therapeutic strategy for preventing or treating AD by targeting the inhibition of Aβ-induced membrane disruption.

A lipobead microarray assembled by particle entrapment in a microfluidic obstacle course and used for the display of cell membrane receptors

Platforms which can display cell membrane ligands and receptors as a microarray library of probes for screening against a target are essential tools in drug discovery, biomarker identification, and pathogen detection. Membrane receptors and ligands require their native bilayer environment to retain their selectivity and binding affinity, and this complicates displaying them in a microarray platform. In this study, a design is developed in which the probes are first incorporated in supported lipid bilayers formed around micron-sized particles (lipobeads), and the microbeads themselves are then arrayed on a surface by hydrodynamic capture in a microfluidic obstacle course of traps. The traps are "V" shaped open enclosures, which are arranged in a wide channel of a microfluidic device, and capture the lipobeads (slightly smaller than the channel height) as they are streamed through the course. Screening assays are undertaken directly in the device after assembly, by streaming a fluorescently labeled target through the device and detecting the bead fluorescence. Conditions are first established for which the supported bilayers on the bead surface remain intact during the capture and assay steps, using fluorescent tags in the bilayer to infer bilayer integrity. Numerical calculations of the hydrodynamic drag coefficient on the entrapped beads are presented in conjunction with the stability experiments to develop criteria for the bilayer stability as a function of the screening assay perfusion rate. Simulations of the flow streamlines are also presented to quantify the trapping efficiency of the obstacle course. Screening assays are illustrated, assaying fluorescently labeled NeutrAvidin with biotin, and labeled cholera toxin with its ganglioside binding ligand, GM1. Sequential capturing of sets of lipobeads (one at a time, and with each set bearing a different probe), followed by indexing the bead positions after each set is entrapped, allows for the construction of an indexed array of multiple probes without the need for particle encoding and is illustrated using the NeutrAvidin-biotin pair. Finally, the lipobead platform is used for quantitatively measuring the kinetic rate constants for the binding of a probe (biotin) to a target (NeutrAvidin).

Collagen microsphere production on a chip

We have developed an integrated microfluidic material processing chip and demonstrated the rapid production of collagen microspheres encapsulating cells with high uniformity and cell viability. The chip integrated three material processing steps. Monodisperse microdroplets were generated at a microfluidic T junction between aqueous and mineral oil flows. The flow was heated immediately to 37 °C to initiate collagen fiber assembly within a gelation channel. Gelled microspheres were extracted from the mineral oil phase into cell culture media within an extraction chamber. Collagen gelation immediately after microdroplet generation significantly reduced coalescence among microdroplets that led to non-uniform microsphere production. The microfluidic extraction approach led to higher microsphere recovery and cell viability than when a conventional centrifugation extraction approach was employed. These results indicate that chip-based material processing is a promising approach for cell-ECM microenvironment generation for applications such as tissue engineering and stem cell delivery.

Biomimetic silica microspheres in biosensing

Lipid vesicles spontaneously fuse and assemble into a lipid bilayer on planar or spherical silica surfaces and other substrates. The supported lipid bilayers (SLBs) maintain characteristics of biological membranes, and are thus considered to be biomembrane mimetic systems that are stable because of the underlying substrate. Examples of their shared characteristics with biomembranes include lateral fluidity, barrier formation to ions and molecules, and their ability to incorporate membrane proteins into them. Biomimetic silica microspheres consisting of SLBs on solid or porous silica microspheres have been utilized for different biosensing applications. The advantages of such biomimetic microspheres for biosensing include their increased surface area to volume ratio which improves the detection limits of analytes, and their amenability for miniaturization, multiplexing and high throughput screening. This review presents examples and formats of using such biomimetic solid or porous silica microspheres in biosensing.