Supported lipid bilayer platforms to probe cell mechanobiology

Lipid membranes supported by polydimethylsiloxane substrates with designed geometry

The membrane curvature of cells and intracellular compartments continuously adapts to enable cells to perform vital functions, from cell division to signal trafficking. Understanding how membrane geometry affects these processes in vivo is challenging because of the biochemical and geometrical complexity as well as the short time and small length scales involved in cellular processes. By contrast, in vitro model membranes with engineered curvature would provide a versatile platform for this investigation and applications to biosensing and biocomputing. Here, we present a strategy that allows fabrication of lipid membranes with designed shape by combining 3D micro-printing and replica-molding lithography with polydimethylsiloxane to create curved micrometer-sized scaffolds with virtually any geometry. The resulting supported lipid membranes are homogeneous and fluid. We demonstrate the versatility of the system by fabricating structures of interesting combinations of mean and Gaussian curvature. We study the lateral phase separation and how local curvature influences the effective diffusion coefficient. Overall, we offer a bio-compatible platform for understanding curvature-dependent cellular processes and developing programmable bio-interfaces for living cells and nanostructures.

Nanomaterials Boost CAR-T Therapy for Solid Tumors

T cell engineering, particularly via chimeric antigen receptor (CAR) modifications for enhancing tumor specificity, has shown efficacy in treating hematologic malignancies. The extension of CAR-T cell therapy to solid tumors, however, is impeded by several challenges: The absence of tumor-specific antigens, antigen heterogeneity, a complex immunosuppressive tumor microenvironment, and physical barriers to cell infiltration. Additionally, limitations in CAR-T cell manufacturing capacity and the high costs associated with these therapies restrict their widespread application. The integration of nanomaterials into CAR-T cell production and application offers a promising avenue to mitigate these challenges. Utilizing nanomaterials in the production of CAR-T cells can decrease product variability and lower production expenses, positively impacting the targeting and persistence of CAR-T cells in treatment and minimizing adverse effects. This review comprehensively evaluates the use of various nanomaterials in the production of CAR-T cells, genetic modification, and in vivo delivery. It discusses their underlying mechanisms and potential for clinical application, with a focus on improving specificity and safety in CAR-T cell therapy.

Adhesion-induced cortical flows pattern E-cadherin-mediated cell contacts

Metazoan development relies on the formation and remodeling of cell-cell contacts. Dynamic reorganization of adhesion receptors and the actomyosin cell cortex in space and time plays a central role in cell-cell contact formation and maturation. Nevertheless, how this process is mechanistically achieved when new contacts are formed remains unclear. Here, by building a biomimetic assay composed of progenitor cells adhering to supported lipid bilayers functionalized with E-cadherin ectodomains, we show that cortical F-actin flows, driven by the depletion of myosin-2 at the cell contact center, mediate the dynamic reorganization of adhesion receptors and cell cortex at the contact. E-cadherin-dependent downregulation of the small GTPase RhoA at the forming contact leads to both a depletion of myosin-2 and a decrease of F-actin at the contact center. At the contact rim, in contrast, myosin-2 becomes enriched by the retraction of bleb-like protrusions, resulting in a cortical tension gradient from the contact rim to its center. This tension gradient, in turn, triggers centrifugal F-actin flows, leading to further accumulation of F-actin at the contact rim and the progressive redistribution of E-cadherin from the contact center to the rim. Eventually, this combination of actomyosin downregulation and flows at the contact determines the characteristic molecular organization, with E-cadherin and F-actin accumulating at the contact rim, where they are needed to mechanically link the contractile cortices of the adhering cells.

On the interactions between RNA and titrateable lipid layers: implications for RNA delivery with lipid nanoparticles

Characterising the interaction between cationic ionisable lipids (CIL) and nucleic acids (NAs) is key to understanding the process of RNA lipid nanoparticle (LNP) formation and release of NAs from LNPs. Here, we have used different surface techniques to reveal the effect of pH and NA type on the interaction with a model system of DOPC and the CIL DLin-MC3-DMA (MC3). At only 5% MC3, differences in the structure and dynamics of the lipid layer were observed. Both pH and %MC3 were shown to affect the absorption behaviour of erythropoietin mRNA, polyadenylic acid (polyA) and polyuridylic acid (polyU). The adsorbed amount of all studied NAs was found to increase with decreasing pH and increasing %MC3 but with different effects on the lipid layer, which could be linked to the NA secondary structure. For polyA at pH 6, adsorption to the surface of the layer was observed, whereas for other conditions and NAs, penetration of the NA into the layer resulted in the formation of a multilayer structure. By comparison to simulations excluding the secondary structure, differences in adsorption behaviours between polyA and polyU could be observed, indicating that the NA's secondary structure also affected the MC3-NA interactions.

Epithelial cell spreading assay on E-cadherin-coated glass or PDMS substrates for microscopy-based analysis of cadherin adhesions

Adherens junctions (AJs) are multi-protein adhesion structures that couple contractile actomyosin networks of epithelial cells within a tissue. Here, we present an epithelial cell spreading assay on E-cadherin-coated glass or polydimethylsiloxane (PDMS) substrates for detailed microscopy-based analysis of cadherin adhesions. We describe steps for preparation of glass coverslips and PDMS gels, E-cadherin coating, and epithelial cell spreading. Epithelial cells can be seeded on E-cadherin-coated surfaces, thereby mimicking AJ formation in X-Y dimension, making it suitable for microscopy analysis. For complete details on the use and execution of this protocol, please refer to Noordstra et al. (2023).1.

WASP facilitates tumor mechanosensitivity in T lymphocytes

Cytotoxic T lymphocytes (CTLs) carry out immunosurveillance by scanning target cells of diverse physical properties for the presence of antigens. While the recognition of cognate antigen by the T cell receptor is the primary signal for CTL activation, it has become increasingly clear that the mechanical stiffness of target cells plays an important role in antigen-triggered T cell responses. However, the molecular machinery within CTLs that transduces the mechanical information of tumor cells remains unclear. We find that CTL's mechanosensitive ability requires the activity of the actin-organizing protein Wiskott-Aldrich Syndrome Protein (WASP). WASP activation is modulated by the mechanical properties of antigen-presenting contexts across a wide range of target cell stiffnesses and activated WASP then mediates mechanosensitive activation of early TCR signaling markers in the CTL. Our results provide a molecular link between antigen mechanosensing and CTL immune response and suggest that CTL-intrinsic cytoskeletal organizing principles enable the processing of mechanical information from diverse target cells.

Gold-Supported Lipid Membranes Formed by Redox-Triggered Vesicle Fusion on Binary Self-Assembled Monolayers: Ion-Pairing Association and Surface Hydrophilicity

The assembly of biomimetic, planar supported lipid bilayers (SLBs) by the popular vesicle fusion method, which relies on the spontaneous adsorption and rupture of small unilamellar vesicles from aqueous solution on a solid surface, typically works with a limited range of support materials and lipid systems. We previously reported a conceptual advance in the formation of SLBs from vesicles in the gel or fluid phase using the interfacial ion-pairing association of charged phospholipid headgroups with electrochemically generated cationic ferroceniums bound to a self-assembled monolayer (SAM) chemisorbed to gold. This redox-driven approach lays down a single bilayer membrane on the SAM-modified gold surface at room temperature within minutes and is compatible with both anionic and zwitterionic phospholipids. The present work explores the effects of the surface ferrocene concentration and hydrophobicity/hydrophilicity on the formation of continuous SLBs of dialkyl phosphatidylserine, dialkyl phosphatidylglycerol, and dialkyl phosphatidylcholine using binary SAMs of ferrocenylundecanethiolate (FcC₁₁S) and dodecanethiolate (CH₃C₁₁S) or hydroxylundecanethiolate (HOC₁₁S) comprising different surface mole fractions of ferrocene (χ_Fc^surf). An increase in the surface hydrophilicity and surface free energy of the FcC₁₁S/HOC₁₁S SAM mitigates the decrease in the attractive ion-pairing interactions resulting from a reduced χ_Fc^surf. SLBs of ≳80% area coverage form on the FcC₁₁S/HOC₁₁S SAM for all the phospholipid types down to χ_Fc^surf of at least 0.2, composition yielding a water contact angle (θ_W) of 44 ± 4°. By contrast, a greater number of ion-pairing interactions is required on the hydrophobic FcC₁₁S/CH₃C₁₁S surface to drive the vesicle fusion process; bilayers or bilayer patches form at χ_Fc^surf ≳ 0.6 (θ_W = 97 ± 3°). These findings will aid in tailoring the surface chemistry of redox-active modified surfaces to widen the conditions that yield supported lipid membranes.

DNA tension assays reveal that force-dependent integrin activation regulates neurite outgrowth in primary cortical neurons

Biomechanical inputs are ubiquitously present in biological systems and are known to regulate various cell functions. In particular, neural cell development is sensitive to mechanical regulation, as these cells reside in one of the softest microenvironments in the body. To fully characterize and comprehend how mechanical force modulates early neuronal processes, we prepared substrates functionalized with DNA probes displaying integrin ligands, including cRGD and laminin, to quantify integrin-mediated molecular tension during neurite initiation in primary cortical neurons. Our live-cell imaging analysis reveals that integrin-mediated tension force is highly dynamic and distributed across the cell body, with the overall tension signal gradually increasing during neurite outgrowth. Notably, we detected a consistent level of mechanical force (amplitude = 4.7-12 piconewtons, pN) for cell integrin-ligand interactions. Further quantifications reveal that neurons exhibit faster cell spreading and neurite outgrowth upon interacting with ligands functionalized with 4.7 pN relative to 12 pN probes. These findings indicate that the magnitude of integrin-mediated mechanical feedback regulates neuronal activity during early neuritogenesis. Additionally, we observed that mechanical tension is correlated with calcium signaling, since inhibiting calcium influx substantially reduced mechanical tension. Thus, our findings support that the magnitude of integrin-mediated mechanical feedback regulates neuronal activity during early neuritogenesis and that mechanical force is an essential element complementing well-known biochemical regulatory mechanisms orchestrating the integrin activation machinery and controlled neurite outgrowth in cortical neurons.

The T Cell Journey: A Tour de Force

T cells act as the puppeteers in the adaptive immune response, and their dysfunction leads to the initiation and progression of pathological conditions. During their lifetime, T cells experience myriad forces that modulate their effector functions. These forces are imposed by interacting cells, surrounding tissues, and shear forces from fluid movement. In this review, a journey with T cells is made, from their development to their unique characteristics, including the early studies that uncovered their mechanosensitivity. Then the studies pertaining to the responses of T cell activation to changes in antigen-presenting cells' physical properties, to their immediate surrounding extracellular matrix microenvironment, and flow conditions are highlighted. In addition, it is explored how pathological conditions like the tumor microenvironment can hinder T cells and allow cancer cells to escape elimination.

Reversible Self-Assembled Monolayers with Tunable Surface Dynamics for Controlling Cell Adhesion Behavior

Cells adhering onto surfaces sense and respond to chemical and physical surface features. The control over cell adhesion behavior influences cell migration, proliferation, and differentiation, which are important considerations in biomaterial design for cell culture, tissue engineering, and regenerative medicine. Here, we report on a supramolecular-based approach to prepare reversible self-assembled monolayers (rSAMs) with tunable lateral mobility and dynamic control over surface composition to regulate cell adhesion behavior. These layers were prepared by incubating oxoacid-terminated thiol SAMs on gold in a pH 8 HEPES buffer solution containing different mole fractions of ω-(ethylene glycol)2-4- and ω-(GRGDS)-, α-benzamidino bolaamphiphiles. Cell shape and morphology were influenced by the strength of the interactions between the amidine-functionalized amphiphiles and the oxoacid of the underlying SAMs. Dynamic control over surface composition, achieved by the addition of inert filler amphiphiles to the RGD-functionalized rSAMs, reversed the cell adhesion process. In summary, rSAMs featuring mobile bioactive ligands offer unique capabilities to influence and control cell adhesion behavior, suggesting a broad use in biomaterial design, tissue engineering, and regenerative medicine.

Endothelial cell spreading on lipid bilayers with combined integrin and cadherin binding ligands

Endothelial cells play a central role in the vascular system, where their function is tightly regulated by both cell-extracellular matrix (e.g., via integrins) and cell-cell interactions (e.g., via cadherins). In this study, we incorporated cholesterol-modified integrin and N-cadherin peptide binding ligands in fluid supported lipid bilayers. Human umbilical vein endothelial cell adhesion, spreading and vinculin localization in these cells were dependent on ligand density. One composition led to observe a higher extent of cell spreading, where cells exhibited extensive lamellipodia formation and a qualitatively more distinct N-cadherin localization at the cell periphery, which is indicative of N-cadherin clustering and a mimic of cell-cell contact formation. The results can be used to reconstitute the endothelial-pericyte interface on biomedical devices and materials.

Super-Resolution Imaging of Fas/CD95 Reorganization Induced by Membrane-Bound Fas Ligand Reveals Nanoscale Clustering Upstream of FADD Recruitment

Signaling through the TNF-family receptor Fas/CD95 can trigger apoptosis or non-apoptotic cellular responses and is essential for protection from autoimmunity. Receptor clustering has been observed following interaction with Fas ligand (FasL), but the stoichiometry of Fas, particularly when triggered by membrane-bound FasL, the only form of FasL competent at inducing programmed cell death, is not known. Here we used super-resolution microscopy to study the behavior of single molecules of Fas/CD95 on the plasma membrane after interaction of Fas with FasL on planar lipid bilayers. We observed rapid formation of Fas protein superclusters containing more than 20 receptors after interactions with membrane-bound FasL. Fluorescence correlation imaging demonstrated recruitment of FADD dependent on an intact Fas death domain, with lipid raft association playing a secondary role. Flow-cytometric FRET analysis confirmed these results, and also showed that some Fas clustering can occur in the absence of FADD and caspase-8. Point mutations in the Fas death domain associated with autoimmune lymphoproliferative syndrome (ALPS) completely disrupted Fas reorganization and FADD recruitment, confirming structure-based predictions of the critical role that these residues play in Fas–Fas and Fas–FADD interactions. Finally, we showed that induction of apoptosis correlated with the ability to form superclusters and recruit FADD.

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.

Mechanosurveillance: Tiptoeing T Cells

Efficient scanning of tissue that T cells encounter during their migratory life is pivotal to protective adaptive immunity. In fact, T cells can detect even a single antigenic peptide/MHC complex (pMHC) among thousands of structurally similar yet non-stimulatory endogenous pMHCs on the surface of antigen-presenting cells (APCs) or target cells. Of note, the glycocalyx of target cells, being composed of proteoglycans and bulky proteins, is bound to affect and even modulate antigen recognition by posing as a physical barrier. T cell-resident microvilli are actin-rich membrane protrusions that puncture through such barriers and thereby actively place the considerably smaller T-cell antigen receptors (TCRs) in close enough proximity to APC-presented pMHCs so that productive interactions may occur efficiently yet under force. We here review our current understanding of how the plasticity of T-cell microvilli and physicochemical properties of the glycocalyx may affect early events in T-cell activation. We assess insights gained from studies on T-cell plasma membrane ultrastructure and provide an update on current efforts to integrate biophysical aspects such as the amplitude and directionality of TCR-imposed mechanical forces and the distribution and lateral mobility of plasma membrane-resident signaling molecules into a more comprehensive view on sensitized T-cell antigen recognition.

Molecular packing of lipid membranes and action mechanisms of membrane-active peptides

Biomembranes are involved in diverse cellular activities. How membranes and proteins interact in the activities might hinge on the former's physical characteristics, which in turn are influenced by packing of lipid molecules. Yet, the validity of this understanding and its mechanism are unclear. By varying chain saturation of membranes, we explored correlations between lipid packing and peptide-mediated membrane disruption for the antimicrobial peptide, melittin, and amyloidogenic peptide, β-amyloid (1-42). Remarkably, reducing molecular packing flexibility enhanced the membrane disruption, possibly due to a shift from membrane perforation to micellization. A theoretical analysis suggested the energetic basis of this shift. This mechanistically shows that a peptide's mechanism might be dictated not only by its intrinsic properties but also by physical characteristics of membranes.

The magnitude of LFA-1/ICAM-1 forces fine-tune TCR-triggered T cell activation

T cells defend against cancer and viral infections by rapidly scanning the surface of target cells seeking specific peptide antigens. This key process in adaptive immunity is sparked upon T cell receptor (TCR) binding of antigens within cell-cell junctions stabilized by integrin (LFA-1)/intercellular adhesion molecule-1 (ICAM-1) complexes. A long-standing question in this area is whether the forces transmitted through the LFA-1/ICAM-1 complex tune T cell signaling. Here, we use spectrally encoded DNA tension probes to reveal the first maps of LFA-1 and TCR forces generated by the T cell cytoskeleton upon antigen recognition. DNA probes that control the magnitude of LFA-1 force show that F>12 pN potentiates antigen-dependent T cell activation by enhancing T cell-substrate engagement. LFA-1/ICAM-1 mechanical events with F>12 pN also enhance the discriminatory power of the TCR when presented with near cognate antigens. Overall, our results show that T cells integrate multiple channels of mechanical information through different ligand-receptor pairs to tune function.

Multivalent Noncovalent Interfacing and Cross-Linking of Supramolecular Tubes

Natural supramolecular filaments have the ability to cross-link with each other and to interface with the cellular membrane via biomolecular noncovalent interactions. This behavior allows them to form complex networks within as well as outside the cell, i.e., the cytoskeleton and the extracellular matrix, respectively. The potential of artificial supramolecular polymers to interact through specific noncovalent interactions has so far only seen limited exploration due to the dynamic nature of supramolecular interactions. Here, a system of synthetic supramolecular tubes that cross-link forming supramolecular networks, and at the same time bind to biomimetic surfaces by the aid of noncovalent streptavidin-biotin linkages, is demonstrated. The architecture of the networks can be engineered by controlling the density of the biotin moiety at the exterior of the tubes as well as by the concentration of the streptavidin. The presented strategy provides a pathway for designing adjustable artificial supramolecular superstructures, which can potentially yield more complex biomimetic adaptive materials.

Topographically smooth and stable supported lipid bilayer for high-resolution AFM studies

The cellular membrane has been identified to play a critical role in various biological processes including the assembly of biological systems. Membranes are complex, primarily two-dimensional assemblies with varied lipid compositions depending on the particular region of the cell. Supported lipid bilayers are considered as appropriate models for physio-chemical studies of membranes including numerous single molecule techniques. Atomic force microscopy (AFM) as a topographic technique is a fully appropriate single molecule technique capable of direct observation of molecular processes on membranes. However, reliable experimental AFM studies require the preparation of the bilayer with a sub-nanometer smooth morphology, which remains stable over long-time observation. Here we present the methodology, which allows one to prepare a smooth, stable, structurally homogeneous lipid bilayer without the presence of any trapped vesicles. We described the application of such lipid bilayers to probe time-dependent early stages of aggregation of monomeric amyloid proteins. Importantly, the proposed methodology can be extended to bilayers with various compositions, by incorporating different lipids for on-membrane aggregation study including cholesterol. Furthermore, this methodology development allowed us to monitor the aggregation of amyloid protein at its physiologically relevant low protein concentration. The flexibility of altering the membrane composition allows to identify the specific role of a particular lipid towards the aggregation kinetics, revealing the plausible mechanism of disease development.

Dual Mode Sensing of Binding and Blocking of Cancer Exosomes to Biomimetic Human Primary Stem Cell Surfaces

Cancer-derived exosomes (cEXOs) facilitate transfer of information between tumor and human primary stromal cells, favoring cancer progression. Although the mechanisms used during this information exchange are still not completely understood, it is known that binding is the initial contact established between cEXOs and cells. Hence, studying binding and finding strategies to block it are of great therapeutic value. However, such studies are challenging for a variety of reasons, including the need for human primary cell culture, the difficulty in decoupling and isolating binding from internalization and cargo delivery, and the lack of techniques to detect these specific interactions. In this work, we created a supported biomimetic stem cell membrane incorporating membrane components from human primary adipose-derived stem cells (ADSCs). We formed the supported membrane on glass and on multielectrode arrays to offer the dual option of optical or electrical detection of cEXO binding to the membrane surface. Using our platform, we show that cEXOs bind to the stem cell membrane and that binding is blocked when an antibody to integrin β1, a component of ADSC surface, is exposed to the membrane surface prior to cEXOs. To test the biological outcome of blocking this interaction, we first confirm that adding cEXOs to cultured ADSCs leads to the upregulation of vascular endothelial growth factor, a measure of proangiogenic activity. Next, when ADSCs are first blocked with anti-integrin β1 and then exposed to cEXOs, the upregulation of proangiogenic activity and cell proliferation are significantly reduced. This biomimetic membrane platform is the first cell-free label-free in vitro platform for the recapitulation and study of cEXO binding to human primary stem cells with potential for therapeutic molecule screening as it is compatible with scale-up and multiplexing.

Direct-Write Patterning of Biomimetic Lipid Membranes In Situ with FluidFM

The creation of biologically inspired artificial membranes on substrates with custom size and in close proximity to each other not only provides a platform to study biological processes in a simplified manner, but they also constitute building blocks for chemical or biological sensors integrated in microfluidic devices. Scanning probe lithography tools such as dip-pen nanolithography (DPN) have opened a new paradigm in this regard, although they possess some inherent drawbacks like the need to operate in air environment or the limited choice of lipids that can be patterned. In this work, we propose the use of the fluid force microscopy (FluidFM) technology to fabricate biomimetic membranes without losing the multiplexing capability of DPN but gaining flexibility in lipid inks and patterning environment. We shed light on the driving mechanisms of the FluidFM-mediated lithography processes in air and liquid. The obtained results should prompt the creation of more realistic biomimetic membranes with arbitrary complex phospholipid mixtures, cholesterol, and potential functional membrane proteins directly patterned in physiological environment.

Multiplexed patterning of hybrid lipid membrane and protein arrays for cell signaling study

The supported lipid bilayer (SLB) is a powerful tool for studying dynamic cell-environment interactions and has been widely used for biosensing applications. Using a reusable microfluidic chip, we present here a strategy to fabricate highly multiplexed SLB and protein arrays for cell signaling research. This approach allows for the rapid patterning of hundreds of highly reproducible and size-tunable SLB arrays with distinct lipid composition and mobility. Using fluorescence microscopy and fluorescence correlation spectroscopy, the lipid mobility is found to play a central role for patterning this membrane assay. Adding protein rings as diffusion barriers extends the accessible mobility range and maintains long-term stability of the hybrid array. Subsequent protein functionalizations on the SLB could be conducted using standard conjugation methods. The utility of the hybrid array for cell signaling experiments is demonstrated by studying the immune NF-κB signaling, whose activity is triggered by the binding of the membrane receptor, toll-like-receptor 4 (TLR 4), to its ligand, lipopolysaccharide (LPS), that is functionalized on the SLB. The patterned array allows cells to adhere and spread on areas without LPS before migrating to interact with membrane-bound LPS to initiate NF-κB activation. Overall, the strategy offers an efficient route to rapidly generate easily controllable and multiplexed molecular arrays that can serve as versatile platforms for biosensing and cell signaling research.

Reconstitution of Functional Integrin αIIbβ3 and Its Activation in Plasma Membrane-Mimetic Lipid Environments

The study of the platelet receptor integrin αIIbβ3 in a membrane-mimetic environment without interfering signalling pathways is crucial to understand protein structure and dynamics. Our understanding of this receptor and its sequential activation steps has been tremendously progressing using structural and reconstitution approaches in model membranes, such as liposomes or supported-lipid bilayers. For most αIIbβ3 reconstitution approaches, saturated short-chain lipids have been used, which is not reflecting the native platelet cell membrane composition. We report here on the reconstitution of label-free full-length αIIbβ3 in liposomes containing cholesterol, sphingomyelin, and unsaturated phosphatidylcholine mimicking the plasma membrane that formed supported-lipid bilayers for quartz-crystal microbalance with dissipation (QCM-D) experiments. We demonstrate the relevance of the lipid environment and its resulting physicochemical properties on integrin reconstitution efficiency and its conformational dynamics. We present here an approach to investigate αIIbβ3 in a biomimetic membrane system as a useful platform do dissect disease-relevant integrin mutations and effects on ligand binding in a lipid-specific context, which might be applicable for drug screening.

Single-molecule imaging of pore-forming toxin dynamics in droplet interface bilayers

Single-channel recording from pore-forming toxins (PFTs) provides a clear and direct molecular readout of toxin action. However to complete any mechanistic understanding of PFT behavior, this functional kinetic readout must be linked to the underlying changes in toxin structure, binding, conformation, or stoichiometry. Here we review how single-molecule imaging methods might be used to further our understanding of PFTs, and provide detailed practical guidance on the use of droplet interface bilayers as a method capable of examining both single-molecule fluorescence and single-channel electrical signals from PFTs.

Spectroscopic Analysis of a Library of DNA Tension Probes for Mapping Cellular Forces at Fluid Interfaces

Oligonucleotide-based probes offer the highest spatial resolution, force sensitivity, and molecular specificity for cellular tension sensing and have been developed to measure a variety of molecular forces mediated by individual receptors in T cells, platelets, fibroblasts, B-cells, and immortalized cancer cell lines. These fluorophore-oligonucleotide conjugate probes are designed with a stem-loop structure that engages cell receptors and reversibly unfolds due to mechanical strain. With the growth of recent work bridging molecular mechanobiology and biomaterials, there is a need for a detailed spectroscopic analysis of DNA tension probes that are used for cellular imaging. In this manuscript, we conducted an analysis of 19 DNA hairpin-based tension probe variants using molecular dynamics simulations, absorption spectroscopy, and fluorescence imaging (epifluorescence and fluorescence lifetime imaging microscopy). We find that tension probes are highly sensitive to their molecular design, including donor and acceptor proximity and pairing, DNA stem-loop structure, and conjugation chemistry. We demonstrate the impact of these design features using a supported lipid bilayer model of podosome-like adhesions. Finally, we discuss the requirements for tension imaging in various biophysical contexts and offer a series of experimental recommendations, thus providing a guide for the design and application of DNA hairpin-based molecular tension probes.

Bioengineering tools for probing intracellular events in T lymphocytes

T lymphocytes are the central coordinator and executor of many immune functions. The activation and function of T lymphocytes are mediated through the engagement of cell surface receptors and regulated by a myriad of intracellular signaling network. Bioengineering tools, including imaging modalities and fluorescent probes, have been developed and employed to elucidate the cellular events throughout the functional lifespan of T cells. A better understanding of these events can broaden our knowledge in the immune systems biology, as well as accelerate the development of effective diagnostics and immunotherapies. Here we review the commonly used and recently developed techniques and probes for monitoring T lymphocyte intracellular events, following the order of intracellular events in T cells from activation, signaling, metabolism to apoptosis. The techniques introduced here can be broadly applied to other immune cells and cell systems. This article is categorized under: Immune System Diseases > Molecular and Cellular Physiology Immune System Diseases > Biomedical Engineering Infectious Diseases > Biomedical Engineering.

Structure, Formation, and Biological Interactions of Supported Lipid Bilayers (SLB) Incorporating Lipopolysaccharide

Biomimetic membrane systems play a crucial role in the field of biosensor engineering. Over the years, significant progress has been achieved creating artificial membranes by various strategies from vesicle fusion to Langmuir transfer approaches to meet an ever-growing demand for supported lipid bilayers on various substrates such as glass, mica, gold, polymer cushions, and many more. This paper reviews the diversity seen in the preparation of biologically relevant model lipid membranes which includes monolayers and bilayers of phospholipid and other crucial components such as proteins, characterization techniques, changes in the physical properties of the membranes during molecular interactions and the dynamics of the lipid membrane with biologically active molecules with special emphasis on lipopolysaccharides (LPS).

Supported Membrane Platform to Assess Surface Interactions between Extracellular Vesicles and Stromal Cells

Extracellular vesicles (EVs) are membrane-encapsulated particles secreted by eukaryotic cells that stimulate cell communication and horizontal cargo exchange. EV interactions with stromal cells can result in molecular changes in the recipient cell and, in some cases, lead to disease progression. However, mechanisms leading to these changes are poorly understood. A few model systems are available for studying the outcomes of surface interactions between EV membranes with stromal cells. Here, we created a hybrid supported bilayer incorporating EVs membrane material, called an extracellular vesicle supported bilayer, EVSB. Using EVSBs, we investigated the surface interactions between breast cancer EVs and adipose-derived stem cells (ADSCs) by culturing ADSCs on EVSBs and analyzing cell adhesion, spreading, viability, vascular endothelial growth factor (VEGF) secretion, and myofibroblast differentiation. Results show that cell viability, adhesion, spreading, and proangiogenic activity were enhanced, conditions that promote oncogenic activity, but cell differentiation was not. This model system could be used to develop therapeutic strategies to limit EV-ADSC interactions and proangiogenic conditions. Finally, this model system is not limited to the study of cancer but can be used to study surface interactions between EVs from any origin and any target cell to investigate EV mechanisms leading to cellular changes in other diseases.

Enhanced Stability of Lipid Structures by Dip-Pen Nanolithography on Block-Type MPC Copolymer

Biomimetic lipid membranes on solid supports have been used in a plethora of applications, including as biosensors, in research on membrane proteins or as interfaces in cell experiments. For many of these applications, structured lipid membranes, e.g., in the form of arrays with features of different functionality, are highly desired. The stability of these features on a given substrate during storage and in incubation steps is key, while at the same time the substrate ideally should also exhibit antifouling properties. Here, we describe the highly beneficial properties of a 2-methacryloyloxyethyl phosphorylcholine (MPC) copolymer for the stability of supported lipid membrane structures generated by dip-pen nanolithography with phospholipids (L-DPN). The MPC copolymer substrates allow for more stable and higher membrane stack structures in comparison to other hydrophilic substrates, like glass or silicon oxide surfaces. The structures remain highly stable under immersion in liquid and subsequent incubation and washing steps. This allows multiplexed functionalization of lipid arrays with antibodies via microchannel cantilever spotting (µCS), without the need of orthogonal binding tags for each antibody type. The combined properties of the MPC copolymer substrate demonstrate a great potential for lipid-based biomedical sensing and diagnostic platforms.

Recruitment of receptors at supported lipid bilayers promoted by the multivalent binding of ligand-modified unilamellar vesicles

The development of model systems that mimic biological interactions and allow the control of both receptor and ligand densities, is essential for a better understanding of biomolecular processes, such as the recruitment of receptors at interfaces, at the molecular level. Here we report a model system based on supported lipid bilayers (SLBs) for the investigation of the clustering of receptors at their interface. Biotinylated SLBs, used as cell membrane mimics, were functionalized with streptavidin (SAv), used here as receptor. Subsequently, biotinylated small (SUVs) and giant (GUVs) unilamellar vesicles were bound to the SAv-functionalized SLBs by multivalent interactions and found to induce the recruitment of both SAv on the SLB surface and the biotin moieties in the vesicles. The recruitment of receptors was investigated with quartz crystal microbalance with dissipation monitoring (QCM-D), which allowed the identification of the biotin and SAv densities necessary to obtain receptor recruitment. At approx. 0.6% of biotin in the vesicles, a transition between dense and low vesicle packing was observed, which coincided with the transitions between recruitment in the vesicles vs. recruitment in the SLB and between full and partial use of the biotin moieties in the vesicle. Direct optical visualization of the clustering at the interface of individual GUVs with the SLB platform was achieved with fluorescence microscopy, showing recruitment of SAv at the contact area as well as the deformation of the vesicles upon binding. Different vesicle binding regimes were observed for lower and higher biotin densities in the vesicles and at the SLBs. A more quantitative analysis of the molecular parameters implied in the interaction, indicated that approx. 10% of the vesicle area constitutes the contact area. Moreover, the SUV binding and recruitment appeared to be fast on the analysis time scale, whereas the binding of GUVs is slower due to the larger SLB area over which SAv recruitment needs to occur. The mechanisms revealed in this study may provide insight in biological processes in which recruitment occurs.

The development of model systems that mimic biological interactions and allow the control of both receptor and ligand densities, is essential for a molecular understanding of biomolecular processes, such as the recruitment of receptors at interfaces.

Lipid membranes supported by planar porous substrates

Biological membranes play key roles in cell life, but their intrinsic complexity motivated the study and development of artificial lipid membranes with the primary aim to reconstitute and understand the natural functions in vitro. Porous-supported lipid membrane (pSLM) has emerged as a flexible platform for studying the surface chemistry of the cell due to their high stability and fluidity, and their ability to study the transmembrane process of the molecules. In this review, the pSLM, for the first time, to our knowledge, was divided into three types according to the way of the porous materials support the lipid membrane, containing the lipid membrane on the pores of the porous materials, the lipid membrane on both sides of the porous materials, the lipid membrane in the pores of the porous materials. All of these pSLMs were systematically elaborated from several aspects, including the substrates, formation, and characterization. Meanwhile, the advantages and disadvantages of each model membranes were summarized. Finally, suggestions for selecting appropriate pSLM and future directions in this area are discussed.

RGD-functionalized supported lipid bilayers modulate pre-osteoblast adherence and promote osteogenic differentiation

Biomaterial integration into bone requires optimal surface conditions to promote osteoprogenitor behavior, which is affected by integrin-binding via arginine-glycine-aspartate (RGD). RGD-functionalized supported lipid bilayers (SLBs) might be interesting as biomaterial coating in bone regeneration, because they allow integration of proteins, for example, growth factors, cytokines, and/or antibacterial agents. Since it is unknown whether and how they affect osteoprogenitor adhesion and differentiation, the aim was to investigate adhesion, focal adhesion formation, morphology, proliferation, and osteogenic potential of pre-osteoblasts cultured on RGD-functionalized SLBs compared to unfunctionalized SLBs and poly-l-lysine (PLL). After 17 hr, pre-osteoblast density on SLBs without or with RGD was similar, but lower than on PLL. Cell surface area, elongation, and number and size of phospho-paxillin clusters were also similar. Cells on SLBs without or with RGD were smaller, more elongated, and had less and smaller phospho-paxillin clusters than on PLL. OPN expression was increased on SLBs with RGD compared to PLL. Moreover, after 1 week, COL1a1 expression was increased on SLBs without or with RGD. In conclusion, pre-osteoblast adhesion and enhanced differentiation were realized for the first time on RGD-functionalized SLBs, pointing to a new horizon in the management of bone regeneration using biomaterials. Together with SLBs nonfouling nature and the possibility of adjusting SLB fluidity and peptide content make SLBs highly promising as substrate to develop innovative biomimetic coatings for biomaterials in bone regeneration.

Effect of electrostatic interaction on the leaflet-specific diffusion in a supported lipid bilayer revealed by fluorescence lifetime correlation analysis

Lipid–support electrostatic interaction determines the lipid dynamics in the proximal leaflet of a SLB.

Supported Solid Lipid Bilayers as a Platform for Single-Molecule Force Measurements

Biocompatible surfaces are important for basic and applied research in life science with experiments ranging from the organismal to the single-molecule level. For the latter, examples include the translocation of kinesin motor proteins along microtubule cytoskeletal filaments or the study of DNA-protein interactions. Such experiments often employ single-molecule fluorescence or force microscopy. In particular for force measurements, a key requirement is to prevent nonspecific interactions of biomolecules and force probes with the surface, while providing specific attachments that can sustain loads. Common approaches to reduce nonspecific interactions include supported lipid bilayers or PEGylated surfaces. However, fluid lipid bilayers do not support loads and PEGylation may require harsh chemical surface treatments and have limited reproducibility. Here, we developed and applied a supported solid lipid bilayer (SSLB) as a platform for specific, load bearing attachments with minimal nonspecific interactions. Apart from single-molecule fluorescence measurements, anchoring molecules to lipids in the solid phase enabled us to perform force measurements of molecular motors and overstretch DNA. Furthermore, using a heating laser, we could switch the SSLB to its fluid state allowing for manipulation of anchoring points. The assay had little nonspecific interactions, was robust, reproducible, and time-efficient, and required less hazardous and toxic chemicals for preparation. In the long term, we expect that SSLBs can be widely employed for single-molecule fluorescence microscopy, force spectroscopy, and cellular assays in mechanobiology.

Self-Assembled Fluid Phase Floating Membranes with Tunable Water Interlayers

We present a reliable method for the fabrication of fluid phase, unsaturated lipid bilayers by self-assembly onto charged Self-Assembled Monolayer (SAM) surfaces with tunable membrane to surface aqueous interlayers. Initially, the formation of water interlayers between membranes and charged surfaces was characterized using a comparative series of bilayers deposited onto charged, self-assembled monolayers by sequential layer deposition. Using neutron reflectometry, a bilayer to surface water interlayer of ∼8 Å was found between the zwitterionic phospholipid 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) membrane and an anionic carboxyl terminated grafted SAM with the formation of this layer attributed to bilayer repulsion by hydration water on the SAM surface. Furthermore, we found we could significantly reduce the technical complexity of sample fabrication through self-assembly of planar membranes onto the SAM coated surfaces. Vesicle fusion onto carboxyl-terminated monolayers yielded high coverage (>95%) bilayers of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) which floated on a 7-11 Å solution interlayer between the membrane and the surface. The surface to membrane distance was then tuned via the addition of 200 mM NaCl to the bulk solution immersing a POPC floating membrane, which caused the water interlayer to swell reversibly to ∼33 Å. This study reveals that biomimetic membrane models can be readily self-assembled from solution onto functionalized surfaces without the use of polymer supports or tethers. Once assembled, surface to membrane distance can be tailored to the experimental requirements using physiological concentrations of electrolytes. These planar bilayers only very weakly interact with the substrate and are ideally suited for use as biomimetic models for accurate in vitro biochemical and biophysical studies, as well as for technological applications, such as biosensors.

DNA mechanotechnology reveals that integrin receptors apply pN forces in podosomes on fluid substrates

Podosomes are ubiquitous cellular structures important to diverse processes including cell invasion, migration, bone resorption, and immune surveillance. Structurally, podosomes consist of a protrusive actin core surrounded by adhesion proteins. Although podosome protrusion forces have been quantified, the magnitude, spatial distribution, and orientation of the opposing tensile forces remain poorly characterized. Here we use DNA nanotechnology to create probes that measure and manipulate podosome tensile forces with molecular piconewton (pN) resolution. Specifically, Molecular Tension-Fluorescence Lifetime Imaging Microscopy (MT-FLIM) produces maps of the cellular adhesive landscape, revealing ring-like tensile forces surrounding podosome cores. Photocleavable adhesion ligands, breakable DNA force probes, and pharmacological inhibition demonstrate local mechanical coupling between integrin tension and actin protrusion. Thus, podosomes use pN integrin forces to sense and respond to substrate mechanics. This work deepens our understanding of podosome mechanotransduction and contributes tools that are widely applicable for studying receptor mechanics at dynamic interfaces.

A DNA Nanodevice Simultaneously Activating the EGFR and Integrin for Enhancing Cytoskeletal Activity and Cancer Cell Treatment

Cell-surface receptors (e.g., EGFR and integrin) and their interactions play determining roles in signal transduction and cytoskeletal activation, which affect cell attachment/detachment, invasion, motility, metastasis (intracellular), and cell-cell signaling. For instance, the interactions between the EGFR and integrin (α6β4) may cause increased mechanical force and shear stress via enhanced cytoskeleton activation. Here, we design a DNA nanodevice (DNA-ND) that can simultaneously target the EGFR and integrin receptors on the caveolae. The piconewton (pN) forces in response to the EGFR-integrin coactivation can be sensed upon the unfolding of the DNA hairpin structure on the side arm of the device via changes of the fluorescence and plasmonic signals. We find that simultaneous activation of EGFR-integrin receptors causes enhanced signal transduction, contractions of the cells, and initiation of the biochemical pathways, thus resulting in a change of the cell division and endocytosis/exocytosis processes that affect the cell proliferation/apoptosis. The DNA-ND further enables us to visualize the cointernalization and degradation of the receptors by lysosomes, providing a novel approach toward bioimaging and mechano-pharmacology.

Protein Translocation Activity in Surface-Supported Lipid Bilayers

Surface-supported lipid bilayers are used widely throughout the nanoscience community as cellular membrane mimics. For example, they are frequently employed in single-molecule atomic force microscopy (AFM) studies to shed light on membrane protein conformational dynamics and folding. However, in AFM as well as in other surface-sensing techniques, the close proximity of the supporting surface raises questions about preservation of the biochemical activity. Employing the model translocase from the general secretory (Sec) system of Escherichia coli, here we quantify the activity via two biochemical assays in surface-supported bilayers. The first assesses ATP hydrolysis and the second assesses polypeptide translocation across the membrane via protection from added protease. Hydrolysis assays revealed distinct levels of activation ranging from medium (translocase-activated) to high (translocation-associated) that were similar to traditional solution experiments and further identified an adenosine triphosphatase population exhibiting characteristics of conformational hysteresis. Translocation assays revealed turn over numbers that were comparable to solution but with a 10-fold reduction in apparent rate constant. Despite differences in kinetics, the chemomechanical coupling (ATP hydrolyzed per residue translocated) only varied twofold on glass compared to solution. The activity changed with the topographic complexity of the underlying surface. Rough glass coverslips were favored over atomically flat mica, likely due to differences in frictional coupling between the translocating polypeptide and surface. Neutron reflectometry and AFM corroborated the biochemical measurements and provided structural characterization of the submembrane space and upper surface of the bilayer. Overall, the translocation activity was maintained for the surface-adsorbed Sec system, albeit with a slower rate-limiting step. More generally, polypeptide translocation activity measurements yield valuable quantitative metrics to assess the local environment about surface-supported lipid bilayers.

Reduction of glass-surface charge density slows the lipid diffusion in the proximal leaflet of a supported lipid bilayer

Understanding the effect of a solid support on the dynamical properties of a supported lipid bilayer (SLB) is a prerequisite for the applications of SLB as a model biomembrane. Here, we applied two-dimensional fluorescence lifetime correlation spectroscopy to examine the effect of solution pH on the diffusion of lipids in the proximal/distal leaflets of a zwitterionic SLB. Leaflet-specific diffusion analyses at various pH revealed that the diffusion of lipids in the proximal leaflet facing a glass surface becomes slower by decreasing pH with the transition pH of ∼7.4. We attributed it to the reduction of the surface charge density of a glass support. Furthermore, the data clearly showed that the lipid diffusion in the distal leaflet facing a bulk solution is insensitive to the change in the diffusion property of the proximal leaflet. This reflects a weak interleaflet coupling between the proximal and distal leaflets of the SLB.

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.

DNA Nanotechnology as an Emerging Tool to Study Mechanotransduction in Living Systems

The ease of tailoring DNA nanostructures with sub-nanometer precision has enabled new and exciting in vivo applications in the areas of chemical sensing, imaging, and gene regulation. A new emerging paradigm in the field is that DNA nanostructures can be engineered to study molecular mechanics. This new development has transformed the repertoire of capabilities enabled by DNA to include detection of molecular forces in living cells and elucidating the fundamental mechanisms of mechanotransduction. This Review first describes fundamental aspects of force-induced melting of DNA hairpins and duplexes. This is then followed by a survey of the currently available force sensing DNA probes and different fluorescence-based force readout modes. Throughout the Review, applications of these probes in studying immune receptor signaling, including the T cell receptor and B cell receptor, as well as Notch and integrin signaling, are discussed.

Sensing Ability and Formation Criterion of Fluid Supported Lipid Bilayer Coated Graphene Field-Effect Transistors

Supported lipid bilayers (SLBs) have been widely used to provide native environments for membrane protein studies. In this study, we utilized graphene field-effect transistors (GFETs) coated with a fluid SLB to perform label-free detection of membrane-associated ligand-receptor interactions in their native lipid bilayer environment. It is known that the analyte-binding event needs to occur within the Debye length for it to be significantly sensed by an FET sensor. However, the thickness of a lipid bilayer is around 4-5-nm-thick, which is larger than the Debye length of a solution with physiologically relevant ionic strength. There is thus a question of whether an FET sensor can detect the binding event above the bilayer. In this study, we show how the existence of an SLB can influence the effective detection distance and the formation criterion of a fluid and continuous SLB on a graphene surface. We discovered that the water intercalation between the graphene and the underlying silica substrate hinders the SLB formation but is required for the stable electrical recording by a GFET. To verify the existence of a fluid SLB on graphene, which was previously complicated by the graphene fluorescence quenching effect, we developed a modified fluorescence recovery after photobleaching method. In addition, our results showed that SLB coated GFETs can quantitatively detect ligand binding onto the receptors embedded in the SLBs. The comparison of our experimental data with a theoretical model shows that the contribution of the SLB acyl chain hydrophobic region to the screening effect can be negligible and, therefore, that the effective detection region can extend beyond the SLB.

Combination of Roll Grinding and High-Pressure Homogenization Can Prepare Stable Bicelles for Drug Delivery

To improve the solubility of the drug nifedipine (NI), NI-encapsulated lipid-based nanoparticles (NI-LNs) have been prepared from neutral hydrogenated soybean phosphatidylcholine and negatively charged dipalmitoylphosphatidylglycerol at a molar ratio of 5/1 using by roll grinding and high-pressure homogenization. The NI-LNs exhibited high entrapment efficiency, long-term stability, and enhanced NI bioavailability. To better understand their structures, cryo transmission electron microscopy and atomic force microscopy were performed in the present study. Imaging from both instruments revealed that the NI-LNs were bicelles. Structures prepared with a different drug (phenytoin) or with phospholipids (dimyristoylphosphatidylcholine, dipalmitoylphosphatidylcholine, and distearoylphosphatidylcholine) were also bicelles. Long-term storage, freeze-drying, and high-pressure homogenization did not affect the structures; however, different lipid ratios, or the presence of cholesterol, did result in liposomes (5/0) or micelles (0/5) with different physicochemical properties and stabilities. Considering the result of long-term stability, standard NI-LN bicelles (5/1) showed the most long-term stabilities, providing a useful preparation method for stable bicelles for drug delivery.