Characterizing the chemical complexity of patterned biomimetic membranes

Defined 2-D microtissues on soft elastomeric silicone rubber using lift-off epoxy-membranes for biomechanical analyses

Surface patterning with complex molecules has become a valuable tool in cell biology and biotechnology, as it enables one to control cell shape and function in culture. However, this technique for micro-contact printing is normally performed on rigid substrates, e.g. Petri dishes or glass. Despite the fact that these substrates can easily be patterned they are artificially stiff environments for cells affecting their morphology and function. Those artifacts can be avoided on tissue elasticity resembling substrates, leading to a nature like cell morphology and behavior. However, reproducible patterning of very soft elastomeric substrates is challenging. Here, we describe a simple and highly accurate method through cavities of lift-off membranes for protein patterning of silicone rubber substrates in an elasticity range down to 1.5 kPa without altering their mechanical properties. Membranes are made of epoxy resin with feature sizes that can be chosen almost arbitrarily including widths down to 5 μm and aspect ratios of 100 and more. Different feature shapes were used to actively manipulate cell adhesion, cell morphology and the actin cytoskeleton on soft substrates. Manipulation of cytoskeletal organization furthermore allowed the comparison of myofibril alignment and cellular forces of cardiac myocytes. These data could show that cell forces are largely unaffected upon active disordering of overall myofibril alignment on a single cell level while aligned multicellular systems generate cell forces in an additive manner.

Supported lipid bilayer microarrays created by non-contact printing

Arrays of supported lipid bilayers (SLBs) provide great potential for future drug development and multiplexed biological research, but are difficult to prepare due to the sensitivity of both the lipid and protein structural arrangement to air exposure. A novel way to produce arrays of SLBs is presented based on non-contact dispensing of vesicles to a substrate through a thin surface confined water film. The approach presents many degrees of freedom since it is not limited to a specific substrate, lipid composition, linker or controlled environment. The method allows adjustment of spot size (180-360 μm) by repeated dispensing as well as control over the composition of the spots and subsequent analytes. SLB formation by vesicle adsorption and rupture allows for incorporation of membrane proteins through pre-formed proteoliposomes. Dispensing through a dip-and-rinse water film avoids contamination, disruptive drying and the need for complex buffer compositions. Furthermore, no humidity control is necessary which simplifies the production step and prolongs the life-time of the spotting system. We characterize the method with respect to control over spot size, bilayer mobility and the formation process as well as demonstrate the possibility to fuse bilayer spots with subsequently added vesicles. Since complex lipid compositions and multiple spotting nozzles can be used, this novel technique is expected to be a promising platform for future applications, e.g. patterning to monitor peptide/protein-lipid interactions, for glycomics using glycolipids or lipopolysaccharides, and to study mixing of spatially confined lipid membranes.

Peripheral protein organization and its influence on lipid diffusion in biomimetic membranes

Protein organization on biomembranes and their dynamics are essential for cellular function. It is not clear, however, how protein binding may influence the assembly of underlying lipids or how the membrane structure leads to functional protein organization. Toward this goal, we investigated the effects of annexin a5 binding to biomimetic membranes using fluorescence imaging and correlation spectroscopy. Annexin a5 (anx a5), a peripheral intracellular protein that plays a membrane remodeling role in addition to other functions, binds specifically and tightly to anionic (e.g., phosphatidylserine)-containing membranes in the presence of calcium ion. Our fluorescence microscopy reveals that annexin likely forms assemblies, along with a more dispersed population, upon binding to anionic biomembranes in the presence of calcium ion, which is reflected in its two-component Brownian motion. To investigate the effects of annexin binding on the underlying lipids, we used specific acyl chain labeled phospholipid analogues, NBD-phosphatidylcholine (NBD-PC) and NBD-phosphatidylserine (NBD-PS). We find that both NBD-labeled lipids cluster under anx a5 assemblies, as compared with when they are found under the dispersed annexin population, and NBD-PS exhibits two-component lateral diffusion under the annexin assemblies. In contrast, NBD-PC diffusion is slower by an order of magnitude under the annexin assemblies in contrast to its diffusion when not localized under anx a5 assemblies. Our results indicate that, upon binding to membranes, the peripheral protein annexin organizes the underlying lipids into domains, which may have functional implications in vivo.

Recent developments in fluorescence correlation spectroscopy for diffusion measurements in planar lipid membranes

Fluorescence correlation spectroscopy (FCS) is a single molecule technique used mainly for determination of mobility and local concentration of molecules. This review describes the specific problems of FCS in planar systems and reviews the state of the art experimental approaches such as 2-focus, Z-scan or scanning FCS, which overcome most of the artefacts and limitations of standard FCS. We focus on diffusion measurements of lipids and proteins in planar lipid membranes and review the contributions of FCS to elucidating membrane dynamics and the factors influencing it, such as membrane composition, ionic strength, presence of membrane proteins or frictional coupling with solid support.