Dynamic Cellular Interactions with Extracellular Matrix Triggered by Biomechanical Tuning of Low-Rigidity, Supported Lipid Membranes

Exploration of lipid bilayer mechanical properties using molecular dynamics simulation

Important biological structures known for their exceptional mechanical qualities, lipid bilayers are essential to many cellular functions. Fluidity, elasticity, permeability, stiffness, tensile strength, compressibility, shear viscosity, line tension, and curvature elasticity are some of the fundamental characteristics affecting their behavior. The purpose of this review is to examine these characteristics in more detail by molecular dynamics simulation, elucidating their importance and the elements that lead to their appearance in lipid bilayers. Comprehending the mechanical characteristics of lipid bilayers is critical for creating medications, drug delivery systems, and biomaterials that interact with biological membranes because it allows one to understand how these materials respond to different stresses and deformations. The influence of mechanical characteristics on important lipid bilayer properties is examined in this review. The mechanical properties of lipid bilayers were clarified through the use of molecular dynamics simulation analysis techniques, including bilayer thickness, stress-strain analysis, lipid bilayer area compressibility, membrane bending rigidity, and time- or ensemble-averaged the area per lipid evaluation. We explain the significance of molecular dynamics simulation analysis methods, providing important new information about the stability and dynamic behavior of the bilayer. In the end, we hope to use molecular dynamics simulation to provide a comprehensive understanding of the mechanical properties and behavior of lipid bilayers, laying the groundwork for further studies and applications. Taken together, careful investigation of these mechanical aspects deepens our understanding of the adaptive capacities and functional roles of lipid bilayers in biological environments.

Surface Viscosity-Dependent Neurite Initiation in Cortical Neurons

Dynamic extracellular environments profoundly affect the behavior and function of cells both biochemically and mechanically. Neurite initiation is the first step for neurons to establish intricate neuronal networks. How such a process is modulated by mechanical factors is not fully understood. Particularly, it is unknown whether the molecular clutch model, which has been used to explain cell responses to matrix rigidity, also holds for neurite initiation. To study how mechanical properties modulate neurite initiation, substrates with various well-defined surface viscosities using supported lipid bilayers (SLBs) are synthesized. The results show that ligands with intermediate viscosity greatly maximize neurite initiation in primary neurons, while neurite initiation is drastically limited on substrates with higher or lower viscosity. Importantly, biochemical characterizations reveal altered focal adhesion and calpain activity are associated with distinct neurite initiation patterns. Collectively, these results indicate that neurite initiation is surface viscosity-dependent; there is an optimal range of surface viscosities to drive neurite initiation. Upon binding to ligands of varying viscosities, calpain activity is differentially triggered and leads to distinct levels of neurite outgrowth. These findings not only enhance the understanding of how extracellular environments regulate neurons, but also demonstrate the potential utility of SLBs for neural tissue engineering applications.

Advances in Cell-Conductive Polymer Biointerfaces and Role of the Plasma Membrane

The plasma membrane (PM) is often described as a wall, a physical barrier separating the cell cytoplasm from the extracellular matrix (ECM). Yet, this wall is a highly dynamic structure that can stretch, bend, and bud, allowing cells to respond and adapt to their surrounding environment. Inspired by shapes and geometries found in the biological world and exploiting the intrinsic properties of conductive polymers (CPs), several biomimetic strategies based on substrate dimensionality have been tailored in order to optimize the cell-chip coupling. Furthermore, device biofunctionalization through the use of ECM proteins or lipid bilayers have proven successful approaches to further maximize interfacial interactions. As the bio-electronic field aims at narrowing the gap between the electronic and the biological world, the possibility of effectively disguising conductive materials to "trick" cells to recognize artificial devices as part of their biological environment is a promising approach on the road to the seamless platform integration with cells.

Probing the effect of clustering on EphA2 receptor signaling efficiency by subcellular control of ligand-receptor mobility

Clustering of ligand:receptor complexes on the cell membrane is widely presumed to have functional consequences for subsequent signal transduction. However, it is experimentally challenging to selectively manipulate receptor clustering without altering other biochemical aspects of the cellular system. Here, we develop a microfabrication strategy to produce substrates displaying mobile and immobile ligands that are separated by roughly 1 µm, and thus experience an identical cytoplasmic signaling state, enabling precision comparison of downstream signaling reactions. Applying this approach to characterize the ephrinA1:EphA2 signaling system reveals that EphA2 clustering enhances both receptor phosphorylation and downstream signaling activity. Single-molecule imaging clearly resolves increased molecular binding dwell times at EphA2 clusters for both Grb2:SOS and NCK:N-WASP signaling modules. This type of intracellular comparison enables a substantially higher degree of quantitative analysis than is possible when comparisons must be made between different cells and essentially eliminates the effects of cellular response to ligand manipulation.

Lipid-Bicelle-Coated Microfluidics for Intracellular Delivery with Reduced Fouling

Innovative technologies for intracellular delivery are ushering in a new era for gene editing, enabling the utilization of a patient's own cells for stem cell and immunotherapies. In particular, cell-squeezing platforms provide unconventional forms of intracellular delivery, deforming cells through microfluidic constrictions to generate transient pores and to enable effective diffusion of biomolecular cargo. While these devices are promising gene-editing platforms, they require frequent maintenance due to the accumulation of cellular debris, limiting their potential for reaching the throughputs necessary for scalable cellular therapies. As these cell-squeezing technologies are improved, there is a need to develop next-generation platforms with higher throughput and longer lifespan, importantly, avoiding the buildup of cell debris and thus channel clogging. Here, we report a versatile strategy to coat the channels of microfluidic devices with lipid bilayers based on noncovalent lipid bicelle technology, which led to substantial improvements in reducing cell adhesion and protein adsorption. The antifouling properties of the lipid bilayer coating were evaluated, including membrane uniformity, passivation against nonspecific protein adsorption, and inhibition of cell attachment against multiple cell types. This surface functionalization approach was applied to coat constricted microfluidic channels for the intracellular delivery of fluorescently labeled dextran and plasmid DNA, demonstrating significant reductions in the accumulation of cell debris. Taken together, our work demonstrates that lipid bicelles are a useful tool to fabricate antifouling lipid bilayer coatings in cell-squeezing devices, resulting in reduced nonspecific fouling and cell clogging to improve performance.

Enhanced avidin binding to lipid bilayers using PDP-PE lipids with PEG-biotin linkers

Two of the most important aspects of lipid bilayers that have increased their popularity in the field of nanotechnology and biosensors are their fluid nature, which is highly beneficial in ensuring the spatial organization of attached molecules, and the relative ease in which they can be manipulated to change the surface chemistry. Here we have used two different types of functionalized lipids to study the interaction of avidin, which is a common approach to attach further ligands for study. We have tested the commonly used Biotinyl-Cap-PE lipids at different molar percentages and reveal that avidin is not evenly distributed, but forms what looks like clusters even at low percentage occupancy which hampers the level of avidin that can be associated with the surface. We have then successfully employed the novel strategy of using PDP-PE lipids which contain a reducible disulphide to which we added maleamide-PEG-biotin spacers of different lengths. There is a more even distribution of avidin on these layers and thereby increasing the amount and efficiency of avidin association. The reduced levels of avidin that was being associated with the Biotinyl-Cap-PE layers as compared to the PDP-PE lipids could be analysed with QCM-D and interferometry approaches, but it was only with SEEC microscopy that the reason for the reduced occupancy was resolved.

Supported Lipid Bilayer Formation: Beyond Vesicle Fusion

Supported lipid bilayers (SLBs) are cell-membrane-mimicking platforms that can be formed on solid surfaces and integrated with a wide range of surface-sensitive measurement techniques. SLBs are useful for unravelling details of fundamental membrane biology and biophysics as well as for various medical, biotechnology, and environmental science applications. Thus, there is high interest in developing simple and robust methods to fabricate SLBs. Currently, vesicle fusion is a popular method to form SLBs and involves the adsorption and spontaneous rupture of lipid vesicles on a solid surface. However, successful vesicle fusion depends on high-quality vesicle preparation, and it typically works with a narrow range of material supports and lipid compositions. In this Feature Article, we summarize current progress in developing two new SLB fabrication techniques termed the solvent-assisted lipid bilayer (SALB) and bicelle methods, which have compelling advantages such as simple sample preparation and compatibility with a wide range of material supports and lipid compositions. The molecular self-assembly principles underpinning the two strategies and important experimental parameters are critically discussed, and recent application examples are presented. Looking forward, we envision that these emerging SLB fabrication strategies can be widely adopted by specialists and nonspecialists alike, paving the way to enriching our understanding of lipid membrane properties and realizing new application possibilities.

Probing Multidimensional Mechanical Phenotyping of Intracellular Structures by Viscoelastic Spectroscopy

Mechanical phenotyping of complex cellular structures gives insight into the process and function of mechanotransduction in biological systems. Several methods have been developed to characterize intracellular elastic moduli, while direct viscoelastic characterization of intracellular structures is still challenging. Here, we develop a needle tip viscoelastic spectroscopy method to probe multidimensional mechanical phenotyping of intracellular structures during a mini-invasive penetrating process. Viscoelastic spectroscopy is determined by magnetically driven resonant vibration (about 15 kHz) with a tiny amplitude. It not only detects the unique dynamic stiffness, damping, and loss tangent of the cell membrane-cytoskeleton and nucleus-nuclear lamina but also bridges viscoelastic parameters between the mitotic phase and interphase. Self-defined dynamic mechanical ratios of these two phases can identify two malignant cervical cancer cell lines (HeLa-HPV18+, SiHa-HPV16+) whose membrane or nucleus elastic moduli are indistinguishable. This technique provides a quantitative method for studying mechanosensation, mechanotransduction, and mechanoresponse of intracellular structures from a dynamic mechanical perspective. This technique has the potential to become a reliable quantitative measurement method for dynamic mechanical studies of intracellular structures.

Composite Lipid Bilayers from Cell Membrane Extracts and Artificial Mixes as a Cell Culture Platform

An artificial lipid bilayer is the closest possible model for the cell membrane. Despite that, current methods of lipid bilayer assembly and functionalization do not provide a satisfactory mimic of the cell-cell contact due to the inability to recreate an asymmetrical multicomponent system. In the current work, a method to produce an integrated solid-supported lipid bilayer combining natural extracts from cell membranes and artificially made lipid vesicles is proposed. This simple method allows delivery of transmembrane proteins and components of the extracellular matrix into the substrate. Biocompatibility of the composite natural/artificial lipid bilayers is evaluated by their interactions with the cardiomyocyte-like HL-1 cell line. Compared with fully artificial mixes, composite lipid bilayers allow cells to adhere and develop a morphologically more normal cytoskeleton.

Hybrid Live Cell–Supported Membrane Interfaces for Signaling Studies

A wide range of cell–microenvironmental interactions are mediated by membrane-localized receptors that bind ligands present on another cell or the extracellular matrix. This situation introduces a number of physical effects including spatial organization of receptor–ligand complexes and development of mechanical forces in cells. Unlike traditional experimental approaches, hybrid live cell–supported lipid bilayer (SLB) systems, wherein a live cell interacts with a synthetic substrate supported membrane, allow interrogation of these aspects of receptor signaling. The SLB system directly offers facile control over the identity, density, and mobility of ligands used for engaging cellular receptors. Further, application of various nano- and micropatterning techniques allows for spatial patterning of ligands. In this review, we describe the hybrid live cell–SLB system and its application in uncovering a range of spatial and mechanical aspects of receptor signaling. We highlight the T cell immunological synapse, junctions formed between EphA2- and ephrinA1-expressing cells, and adhesions formed by cadherin and integrin receptors.

Tissue-Inspired Interfacial Coatings for Regenerative Medicine

Biomedical devices have come a long way since they were first introduced as a medically interventional methodology in treating various types of diseases. Different techniques were employed to make the devices more biocompatible and promote tissue repair; such as chemical surface modifications, using novel materials as the bulk of a device, physical topological manipulations and so forth. One of the strategies that recently gained a lot of attention is the use of tissue-inspired biomaterials that are coated on the surface of biomedical devices via different coating techniques, such as the use of extracellular matrix (ECM) coatings, extracted cell membrane coatings, and so on. In this chapter, we will give a general overview of the different types of tissue-inspired coatings along with a summary of recent studies reported in this scientific arena.

Membrane Reconstitution of Monoamine Oxidase Enzymes on Supported Lipid Bilayers

Monoamine oxidase A and B (MAO-A and B) are mitochondrial outer membrane enzymes that are implicated in a number of human diseases, and the pharmacological inhibition of these enzymes is a promising therapeutic strategy to alleviate disease symptoms. It has been suggested that optimal levels of enzymatic activity occur in the membrane-associated state, although details of the membrane association process remain to be understood. Herein, we have developed a supported lipid bilayer platform to study MAO-A and B binding and evaluate the effects of known pharmacological inhibitors on the membrane association process. By utilizing the quartz crystal microbalance-dissipation (QCM-D) technique, it was determined that both MAOs exhibit tight binding to negatively and positively charged bilayers with distinct concentration-dependent binding profiles while only transiently binding to neutral bilayers. Importantly, in the presence of known inhibitors, the MAOs showed increased binding to negatively charged bilayers, although there was no effect of inhibitor treatment on binding to positively charged bilayers. Taken together, our findings establish that the membrane association of MAOs is highly dependent on membrane surface charge, and we outline an experimental platform to support the in vitro reconstitution of monoamine oxidases on synthetic membranes, including the evaluation of pharmacological drug candidates.

Fabrication of Multicomponent, Spatially Segregated DNA and Protein-Functionalized Supported Membrane Microarray

Deoxyribonucleic acid (DNA) has been used as a material for a variety of applications, including surface functionalization for cell biological or in vitro reconstitution studies. Use of DNA-based surface functionalization eliminates limitations of multiplexing posed by traditionally used methods in applications requiring spatially segregated surface functionalization. Recently, we have reported a stochastic, membrane fusion-based strategy to fabricate multicomponent membrane array substrates displaying spatially segregated protein ligands using biotin-streptavidin and Ni-NTA-polyhistidine interactions. Here, we report the delivery of DNA oligonucleotide-conjugated lipid molecules to membrane corrals, allowing spatially segregated membrane corral functionalization in a membrane microarray. Incubation of microbeads coated with the supported membrane resulted in an exchange of lipid contents with planar membrane corrals present on a micropatterned substrate. Increases in the system temperature and membrane corral size resulted in alterations in the rate constant of lipid exchange, which are in agreement with our previously developed analytical model and further confirm that lipid exchange is a diffusion-based process that takes place after the formation of a long "fusion-stalk" between the two membranes. We take advantage of the physical dimensions of the fusion-stalk with a large aspect ratio to deliver DNA oligonucleotide-conjugated lipid molecules to membrane corrals. We believe that the ability to functionalize membrane corrals with DNA oligonucleotides significantly increases the utility of the stochastic fusion-mediated lipid delivery strategy in the functionalization of biomolecules such as DNA or DNA-conjugated protein ligands.

About Chemical Strategies to Fabricate Cell-Instructive Biointerfaces with Static and Dynamic Complexity

Properly functioning cell-instructive biointerfaces are critical for healthy integration of biomedical devices in the body and serve as decisive tools for the advancement of our understanding of fundamental cell biological phenomena. Studies are reviewed that use covalent chemistries to fabricate cell-instructive biointerfaces. These types of biointerfaces typically result in a static presentation of predefined cell-instructive cues. Chemically defined, but dynamic cell-instructive biointerfaces introduce spatiotemporal control over cell-instructive cues and present another type of biointerface, which promises a more biomimetic way to guide cell behavior. Therefore, strategies that offer control over the lateral sorting of ligands, the availability and molecular structure of bioactive ligands, and strategies that offer the ability to induce physical, chemical and mechanical changes in situ are reviewed. Specific attention is paid to state-of-the-art studies on dynamic, cell-instructive 3D materials. Future work is expected to further deepen our understanding of molecular and cellular biological processes investigating cell-type specific responses and the translational steps toward targeted in vivo applications.

Hybrid Biomimetic Interfaces Integrating Supported Lipid Bilayers with Decellularized Extracellular Matrix Components

This paper describes the functionalization of solid supported phospholipid bilayer with decellularized extracellular matrix (dECM) components, toward the development of biomimetic platforms that more closely mimic the cell surface environment. The dECM was obtained through a combination of chemical and enzymatic treatments of mouse adipose tissue that contains collagen, fibronectin, and glycosaminoglycans (GAGs). Using amine coupling chemistry, the dECM components were attached covalently to the surface of a supported lipid bilayer containing phospholipids with reactive carboxylic acid headgroups. The bilayer formation and the kinetics of subsequent dECM conjugation were monitored by quartz crystal microbalance with dissipation (QCM-D). Fluorescence recovery after photobleaching (FRAP) confirmed the fluidity of the membrane after functionalization with dECM. The resulting hybrid biomimetic interface supports the attachment and survival of the human hepatocyte Huh 7.5 and maintains the representative hepatocellular function. Importantly, the platform is suitable for monitoring the lateral organization and clustering of cell-binding ligands and growth factor receptors in the presence of the rich biochemical profile of tissue-derived ECM components.

Nanostructured Lipid-based Films for Substrate Mediated Applications in Biotechnology

Amphiphilic in nature, lipids spontaneously self-assemble into a range of nanostructures in the presence of water. Among lipid self-assembled structures, liposomes and supported lipid bilayers have long held scientific interest for their main applications in drug delivery and plasma membrane models, respectively. In contrast, lipid-based multi-layered membranes on solid supports only recently begun drawing scientists' attention. New studies on lipid films show that the stacking of multiple bilayers on a solid support yields interestingly complex features to these systems. Namely, multiple layers exhibit cooperative structural and dynamic behavior. In addition, the materials enable compartmentalization, templating, and enhanced release of several molecules of interest. Importantly, supported lipid phases exhibit long-range periodic nano-scale order and orientation that is tunable in response to a changing environment. Herein, we summarize current and pertinent understanding of lipid-based film research focusing on how unique structural characteristics enable the emergence of new applications in biotechnology including label-free biosensors, macroscale drug delivery, and substrate-mediated gene delivery. Our very recent contributions to lipid-based films, focusing on the structural characterization at the meso, nano, and molecular-scale, using Small-Angle X-ray Scattering, Atomic Force Microscopy, Photothermal Induced Resonance, and Solid-State NMR will be also highlighted.

Supported lipid bilayer platforms to probe cell mechanobiology

Mammalian and bacterial cells sense and exert mechanical forces through the process of mechanotransduction, which interconverts biochemical and physical signals. This is especially important in contact-dependent signaling, where ligand-receptor binding occurs at cell-cell or cell-ECM junctions. By virtue of occurring within these specialized junctions, receptors engaged in contact-dependent signaling undergo oligomerization and coupling with the cytoskeleton as part of their signaling mechanisms. While our ability to measure and map biochemical signaling within cell junctions has advanced over the past decades, physical cues remain difficult to map in space and time. Recently, supported lipid bilayer (SLB) technologies have emerged as a flexible platform to mimic and perturb cell-cell and cell-ECM junctions, allowing one to study membrane receptor mechanotransduction. Changing the lipid composition and underlying substrate tunes bilayer fluidity, and lipid and ligand micro- and nano-patterning spatially control positioning and clustering of receptors. Patterning metal gridlines within SLBs confines lipid mobility and introduces mechanical resistance. Here we review fundamental SLB mechanics and how SLBs can be engineered as tunable cell substrates for mechanotransduction studies. Finally, we highlight the impact of this work in understanding the biophysical mechanisms of cell adhesion. This article is part of a Special Issue entitled: Interactions between membrane receptors in cellular membranes edited by Kalina Hristova.