Interfacing living cells and spherically supported bilayer lipid membranes

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.

Geometric pinning and antimixing in scaffolded lipid vesicles

Previous studies on the phase behaviour of multicomponent lipid bilayers found an intricate interplay between membrane geometry and its composition, but a fundamental understanding of curvature-induced effects remains elusive. Thanks to a combination of experiments on lipid vesicles supported by colloidal scaffolds and theoretical work, we demonstrate that the local geometry and global chemical composition of the bilayer determine both the spatial arrangement and the amount of mixing of the lipids. In the mixed phase, a strong geometrical anisotropy can give rise to an antimixed state, where the lipids are mixed, but their relative concentration varies across the membrane. After phase separation, the bilayer organizes in multiple lipid domains, whose location is pinned in specific regions, depending on the substrate curvature and the bending rigidity of the lipid domains. Our results provide critical insights into the phase separation of cellular membranes and, more generally, two-dimensional fluids on curved substrates.

Lipid exchange enhances geometric pinning in multicomponent membranes on patterned substrates

Experiments on supported lipid bilayers featuring liquid ordered/disordered domains have shown that the spatial arrangement of the lipid domains and their chemical composition are strongly affected by the curvature of the substrate. Furthermore, theoretical predictions suggest that both these effects are intimately related with the closed topology of the bilayer. In this work, we test this hypothesis by fabricating supported membranes consisting of colloidal particles of various shapes lying on a flat substrate. A single lipid bilayer coats both colloids and substrate, allowing local lipid exchange between them, thus rendering the system thermodynamically open, i.e. able to exchange heat and molecules with an external reservoir in the neighborhood of the colloid. By reconstructing the Gibbs phase diagram for this system, we demonstrate that the free-energy landscape is directly influenced by the geometry of the colloid. In addition, we find that local lipid exchange enhances the pinning of the liquid disordered phase in highly curved regions. This allows us to provide estimates of the bending moduli difference of the domains. Finally, by combining experimental and numerical data, we forecast the outcome of possible experiments on catenoidal and conical necks and show that these geometries could greatly improve the precision of the current estimates of the bending moduli.

Colloid supported lipid bilayers for self-assembly

The use of colloid supported lipid bilayers (CSLBs) has recently been extended to create colloidal joints, that enable the assembly of structures with internal degrees of flexibility, and to study lipid membranes on curved and closed geometries. These novel applications of CSLBs rely on previously unappreciated properties: the simultaneous fluidity of the bilayer, lateral mobility of inserted (linker) molecules and colloidal stability. Here we characterize every step in the manufacturing of CSLBs in view of these requirements using confocal microscopy and fluorescence recovery after photobleaching (FRAP). Specifically, we have studied the influence of different particle properties (roughness, surface charge, chemical composition, polymer coating) on the quality and mobility of the supported bilayer. We find that the insertion of lipopolymers in the bilayer can affect its homogeneity and fluidity. We improve the colloidal stability by inserting lipopolymers or double-stranded inert DNA into the bilayer. We include surface-mobile DNA linkers and use FRAP to characterize their lateral mobility both in their freely diffusive and bonded state. Finally, we demonstrate the self-assembly of flexibly linked structures from the CSLBs modified with surface-mobile DNA linkers. Our work offers a collection of experimental tools for working with CSLBs in applications ranging from controlled bottom-up self-assembly to model membrane studies.

Artificial Lipid Membranes: Past, Present, and Future

The multifaceted role of biological membranes prompted early the development of artificial lipid-based models with a primary view of reconstituting the natural functions in vitro so as to study and exploit chemoreception for sensor engineering. Over the years, a fair amount of knowledge on the artificial lipid membranes, as both, suspended or supported lipid films and liposomes, has been disseminated and has helped to diversify and expand initial scopes. Artificial lipid membranes can be constructed by several methods, stabilized by various means, functionalized in a variety of ways, experimented upon intensively, and broadly utilized in sensor development, drug testing, drug discovery or as molecular tools and research probes for elucidating the mechanics and the mechanisms of biological membranes. This paper reviews the state-of-the-art, discusses the diversity of applications, and presents future perspectives. The newly-introduced field of artificial cells further broadens the applicability of artificial membranes in studying the evolution of life.

Phase Composition Control in Microsphere-Supported Biomembrane Systems

The popularization of studies in membrane protein lipid phase coexistence has prompted the development of new techniques to construct and study biomimetic systems with cholesterol-rich lipid microdomains. Here, microsphere-supported biomembranes with integrated α-helical peptides, referred to as proteolipobeads (PLBs), were used to model peptide/protein partitioning within DOPC/DPPC/cholesterol phase-separated membranes. Due to the appearance of compositional heterogeneity and impurities in the formation of model PLB assemblies, fluorescence-activated cell sorting (FACS) was used to characterize and sort PLB populations on the basis of disordered phase (Ld) content. In addition, spectral imaging was used to assess the partitioning of FITC-labeled α-helical peptide between fluorescently labeled Ld phase and unlabeled ordered phase (Lo) phase lipid microdomains. The apparent peptide partition coefficient, Kp,app, was measured to be 0.89 ± 0.06, indicating a slight preference of the peptide for the Lo phase. A biomimetic motif of the Lo phase concentration enhancement of the biotinyl-peptide ligand display in proteolipobeads was also observed. Finally, peptide mobility was measured by FRAP separately in each lipid phase, yielding diffusivities of 0.036 ± 0.005 and 0.014 ± 0.003 μm2/s in the Ld and Lo phases, respectively.

Lipid Microdomains in Synapse Formation

Membrane lipid rafts (i.e., cholesterol/sphingolipids domains) exhibit functional roles in both healthy and pathological states of the nervous system. However, due to their highly dynamic nature, it remains a challenge to characterize the fundamental aspects of lipid rafts that are important for specific neuronal processes. An experimental approach is presented here that allows for the interfacing of living neurons with an experimentally accessible model membrane where lipid order in cellular rafts can be reproducibly mimicked. It is demonstrated that coexisting lipid microdomains in model membranes can regulate axonal guidance and establish stable presynaptic contacts when interfaced with neurons in vitro. Experimental evidence is provided where specific functional groups and lateral organizations are favored by neurons in establishing synaptic connections. The model membrane platform presented in this work provides an accessible and direct means to investigate how lipid rafts regulate synapse formation. This experimental platform can similarly be extended to explore a variety of other cellular events where lipid lateral organization is believed to be important.