Spherical nanoparticle supported lipid bilayers for the structural study of membrane geometry-sensitive molecules

Interaction between membrane curvature sensitive factors SpoVM and SpoIVA in Bicelle condition

SpoVM and SpoIVA are essential proteins for coat assembly in Bacillus subtilis. SpoVM is a membrane curvature sensor, specifically localized on the forespore membrane. SpoIVA is an ATP hydrolase that self-assembles by hydrolyzing ATP. In this work, SpoVM and its mutant SpoVMP9A were obtained by cyanogen bromide cleavage and reconstituted into bicelles. The purification of SpoIVA was achieved through a rigorous process involving Ni-NTA chromatography column and size exclusion chromatography. This study utilized Biacore to obtain a direct determination of the kinetic parameters of interaction between SpoVM (SpoVMP9A) and SpoIVA in Bicelle conditions.

Impact of poly(ethylene glycol) functionalized lipids on ordering and fluidity of colloid supported lipid bilayers

Colloid supported lipid bilayers (CSLBs) are highly appealing building blocks for functional colloids. In this contribution, we critically evaluate the impact on lipid ordering and CSLB fluidity of inserted additives. We focus on poly(ethylene glycol) (PEG) bearing lipids, which are commonly introduced to promote colloidal stability. We investigate whether their effect on the CSLB is related to the incorporated amount and chemical nature of the lipid anchor. To this end, CSLBs were prepared from lipids with a low or high melting temperature (T_m), DOPC, and DPPC, respectively. Samples were supplemented with either 0, 5 or 10 mol% of either a low or high T_m PEGylated lipid, DOPE-PEG₂₀₀₀ or DSPE-PEG₂₀₀₀, respectively. Lipid ordering was probed via differential scanning calorimetry and fluidity by fluorescence recovery after photobleaching. We find that up to 5 mol% of either PEGylated lipids could be incorporated into both membranes without any pronounced effects. However, the fluorescence recovery of the liquid-like DOPC membrane was markedly decelerated upon incorporating 10 mol% of either PEGylated lipids, whilst insertion of the anchoring lipids (DOPE and DSPE without PEG₂₀₀₀) had no detectable impact. Therefore, we conclude that the amount of incorporated PEG stabilizer, not the chemical nature of the lipid anchor, should be tuned carefully to achieve sufficient colloidal stability without compromising the membrane dynamics. These findings offer guidance for the experimental design of studies using CSLBs, such as those focusing on the consequences of intra- and inter-particle inhomogeneities for multivalent binding and the impact of additive mobility on superselectivity.

Lipid flip-flop and desorption from supported lipid bilayers is independent of curvature

Flip-flop of lipids of the lipid bilayer (LBL) constituting the plasma membrane (PM) plays a crucial role in a myriad of events ranging from cellular signaling and regulation of cell shapes to cell homeostasis, membrane asymmetry, phagocytosis, and cell apoptosis. While extensive research has been conducted to probe the lipid flip flop of planar lipid bilayers (LBLs), less is known regarding lipid flip-flop for highly curved, nanoscopic LBL systems despite the vast importance of membrane curvature in defining the morphology of cells and organelles and in maintaining a variety of cellular functions, enabling trafficking, and recruiting and localizing shape-responsive proteins. In this paper, we conduct molecular dynamics (MD) simulations to study the energetics, structure, and configuration of a lipid molecule undergoing flip-flop and desorption in a highly curved LBL, represented as a nanoparticle-supported lipid bilayer (NPSLBL) system. We compare our findings against those of a planar substrate supported lipid bilayer (PSSLBL). Our MD simulation results reveal that despite the vast differences in the curvature and other curvature-dictated properties (e.g., lipid packing fraction, difference in the number of lipids between inner and outer leaflets, etc.) between the NPSLBL and the PSSLBL, the energetics of lipid flip-flop and lipid desorption as well as the configuration of the lipid molecule undergoing lipid flip-flop are very similar for the NPSLBL and the PSSLBL. In other words, our results establish that the curvature of the LBL plays an insignificant role in lipid flip-flop and desorption.

Use of Superparamagnetic Iron Oxide Nanoparticles (SPIONs) via Multiple Imaging Modalities and Modifications to Reduce Cytotoxicity: An Educational Review

The aim of the present educational review on superparamagnetic iron oxide nanoparticles (SPIONs) is to inform and guide young scientists and students about the potential use and challenges associated with SPIONs. The present review discusses the basic concepts of magnetic resonance imaging (MRI), basic construct of SPIONs, cytotoxic challenges associated with SPIONs, shape and sizes of SPIONs, site-specific accumulation of SPIONs, various methodologies applied to reduce cytotoxicity including coatings with various materials, and application of SPIONs in targeted delivery of chemotherapeutics (Doxorubicin), biotherapeutics (DNA, siRNA), and positron emission tomography (PET) imaging applications.

Formation and Properties of a Self-Assembled Nanoparticle-Supported Lipid Bilayer Probed through Molecular Dynamics Simulations

We have carried out coarse-grained molecular dynamics (MD) simulations to study the self-assembly procedure of a system of randomly placed lipid molecules, water beads, and a nanoparticle (NP). The self-assembly results in the formation of the nanoparticle-supported lipid bilayer (NPSLBL), with the self-assembly mechanism being driven by events such as the formation of small lipid clusters, merging of the lipid clusters in the vicinity of the NP to form NP-embedded vesicle with a pore, and collapsing of that pore to eventually form the equilibrated NPSLBL system overcoming a large free-energy barrier. Subsequently, we quantify the properties and the configurations of this NPSLBL system. We reveal that unlike our proposition of an equal number of lipid molecules occupying the inner and outer leaflets in a recent report studying the properties of a preassembled lipid bilayer, the equilibrated self-assembled NPSLBL system demonstrates a much larger number of lipid molecules occupying the outer leaflet as compared to the inner leaflet. Second, the thickness of the water layer entrapped between the NP and the inner leaflet shows similar values as predicted by experiments and our previous study. Finally, we reveal that, similar to our previous study, the diffusivity of the lipid molecules in the outer leaflet is larger than that in the inner leaflet but, due to higher temperature employed during our simulations, are even larger than that predicted by our previous study.

Understanding the Nano-bio Interfaces: Lipid-Coatings for Inorganic Nanoparticles as Promising Strategy for Biomedical Applications

Inorganic nanoparticles (NPs) exhibit relevant physical properties for application in biomedicine and specifically for both the diagnosis and therapy (i.e. theranostic) of severe pathologies, such as cancer. The inorganic NP core is often not stable in aqueous suspension and can induce cytotoxic effects. For this reason, over the years, several coating strategies were suggested to improve the NP stability in aqueous solutions as well as the NP biocompatibility. Among the various components which can be used for NP coatings, lipids, and in particular phospholipids emerged as versatile molecular building blocks for the production of NP coatings suitable for biomedical application. The recent synthetic efforts in NP lipid coatings allows today to introduce on the NP surface a large variety of lipid molecules eventually in mixture with amphiphilic or hydrophobic drugs or active molecules for cell targeting. In this review, the most relevant examples of NP lipid-coatings are presented and grouped in two main categories: supported lipid bilayers (SLB) and hybrid lipid bilayers (HLB). The discussed scientific cases take into account the most commonly used inorganic NP for biomedical applications in cancer therapy and diagnosis.

Nanovesicles Versus Nanoparticle-Supported Lipid Bilayers: Massive Differences in Bilayer Structures and in Diffusivities of Lipid Molecules and Nanoconfined Water

We carry out molecular dynamics (MD) simulations to compare the equilibrium architecture and properties of nanoparticle-supported lipid bilayers (NPSLBLs) with the free vesicles of similar dimensions. Three key differences emerge. First, we witness that for a free vesicle, a much larger number of lipid molecules occupy the outer layer as compared to the inner layer; on the other hand, for the NPSLBL the number of lipid molecules occupying the inner and outer layers is identical. Second, we witness that the diffusivities of the lipid molecules occupying both the inner and the outer layers of the free vesicles are identical, whereas for the NPSLBLs the diffusivity of the lipid molecules in the outer layer is more than twice the diffusivity of the lipid molecules in the inner layer. Finally, the NPSLBLs entrap nanoscopic thin water film between the inner lipid layer and the NP and the diffusivity of this water film is nearly 1 order of magnitude smaller than the diffusivity of the bulk water; on the other hand, the water inside the free vesicles has a diffusivity that is only slightly lower than that of the bulk water. Our findings, possibly the first probing the atomistic details of the NPSLBLs, are anticipated to shed light on the properties of this important nanomaterial with applications in a large number of disciplines ranging from drug and gene delivery to characterizing curvature-sensitive molecules.

Osmotic Shock-Triggered Assembly of Highly Charged, Nanoparticle-Supported Membranes

Spherical nanoparticle-supported lipid bilayers (SSLBs) combine precision nanoparticle engineering with biocompatible interfaces for various applications, ranging from drug delivery platforms to structural probes for membrane proteins. Although the bulk, spontaneous assembly of vesicles and larger silica nanoparticles (>100 nm) robustly yields SSLBs, it will only occur with low charge density vesicles for smaller nanoparticles (<100 nm), a fundamental barrier in increasing SSLB utility and efficacy. Here, through whole mount and cryogenic transmission electron microscopy, we demonstrate that mixing osmotically loaded vesicles with smaller nanoparticles robustly drives the formation of SSLBs with high membrane charge density (up to 60% anionic lipid or 50% cationic lipid). We show that the osmolyte load necessary for SSLB formation is primarily a function of absolute membrane charge density and is not lipid headgroup-dependent, providing a generalizable, tunable approach toward bulk production of highly curved and charged SSLBs with various membrane compositions.

Quantification of Multivalency in Protein-Oligomer-Coated Nanoparticles Targeting Dynamic Membrane Glycan Receptors

Multivalent binding of proteins to glycan receptors on the host cell quantitatively controls the initial adhesion of most viruses. However, quantifying such multivalency in terms of binding valency has always been a challenge because of the hierarchy of multivalency involving multiple protein oligomers on the virus, limiting our understanding of virus adhesion and virulence. To address this challenge, we mimicked virus adhesion to cell surfaces by attaching protein-oligomer-coated nanoparticles (NPs) to fluidic glycolipid membranes with surface glycan density varying over 4 orders of magnitude. Using total internal reflection fluorescence microscopy to track single attached NPs, we show that the binding isotherms exhibit two regions, attributed to monovalent and multivalent protein/glycan interactions at low and high glycan densities, respectively. The bimodal binding curve allows the quantification of the different valency and binding constants of monovalent and multivalent interactions. In addition, the competitive inhibition of multivalency by the glycopolymer presenting multiple glycan moieties is quantitatively appreciated. This work is essential to mapping and understanding the complex binding specificities of glycan-binding proteins and inhibitory drug designs and applications.

Electric double layer electrostatics of lipid-bilayer-encapsulated nanoparticles: Toward a better understanding of protocell electrostatics

Lipid-bilayer-encapsulated nanoparticles (LBLENPs) or NP-supported LBL systems, such as protocells (which are lipid bilayer encapsulated mesoporous silica nanoparticles or MSNPs) have received extensive attention for applications like targeted drug and gene deliveries, multimodal diagnostics, characterization of membrane-geometry sensitive molecules, etc. Very often electrostatic-mediated interactions have been hypothesized to play key roles in the functioning of these LBLENPs. Despite that, very little has been done to theoretically quantify the fundamental electric double layer (EDL) electrostatics of such LBLENPs. In this study, we develop an EDL theory to describe the electrostatics of such LBLENPs. We show that the electrostatics is a manifestation of the charged/dielectric nature of the NP, LBL structure and charging, and the ionic environment in which the LBLENPs are present. We also establish that for certain conditions of charging of the NP one witnesses a most remarkable charge inversion like electrostatics within the LBL membrane or the NP itself. We anticipate that our findings will provide an extremely useful platform for better understanding the fabrication and functioning of such LBLENPs and discuss examples where our theory can be useful.

Spherical Nanoparticle Supported Lipid Bilayers: A Tool for Modeling Protein Interactions with Curved Membranes

Mechanistic studies of protein-membrane interactions can be complicated by the limitations of the membrane model system chosen. Many of these limitations can be overcome by using a spherical silica nanoparticle to support the membrane. In this chapter, we present a detailed protocol for the construction of spherical nanoparticle supported lipid bilayers (SSLBs), with discussion of methods to improve production.

Cell-free expression, purification, and membrane reconstitution for NMR studies of the nonstructural protein 4B from hepatitis C virus

We describe the expression of the hepatitis C virus nonstructural protein 4B (NS4B), which is an integral membrane protein, in a wheat germ cell-free system, the subsequent purification and characterization of NS4B and its insertion into proteoliposomes in amounts sufficient for multidimensional solid-state NMR spectroscopy. First spectra of the isotopically [(2)H,(13)C,(15)N]-labeled protein are shown to yield narrow (13)C resonance lines and a proper, predominantly α-helical fold. Clean residue-selective leucine, isoleucine and threonine-labeling is demonstrated. These results evidence the suitability of the wheat germ-produced integral membrane protein NS4B for solid-state NMR. Still, the proton linewidth under fast magic angle spinning is broader than expected for a perfect sample and possible causes are discussed.