Lipid-based systems like liposomes and solid supported lipid membranes (SLMs) provide a well-defined platform for mimicking specific features of the cell membrane including membrane-to-membrane interactions. In solid supported lipid membranes the distal leaflet is exposed to the bulk solution and the proximal leaflet is facing a substrate, e.g. a sensor surface. The advantages of SLMs comprise the availability of reproducible preparation protocols, the possibility to systematically vary their composition and distribution (see Kunze et al. J. Am. Chem. Soc. 2009 and Jing et al. J. Phys. Chem. B 2014), and the opportunity to apply a variety of high-resolution techniques to visualize lipid domains as well as to measure non-specific and specific interactions such as carbohydrate carbohydrate interactions (Figure 1).
Figure 1: (a) Schematic illustration of the interaction between fluorescently labeled liposomes containing glycosphingolipids and an SLB containing glycosphingolipids. TIRF-based illumination is used to track surface-bound liposomes. (b) Typical TIRF image of surface bound liposomes together with a kymograph and the intensity profile of a small image area containing a single liposome (from Kunze et al. Sci. Rep. 2013).
A further advantage of liposomes and solid supported lipid membranes is maintained fluidity/mobility of molecules within the membrane. These given features plus the bioinertness of lipid model membranes makes them a promising substrate for cell attachment studies (Figure 2).
Figure 2: Schematic illustration of cells interacting with a lipid membrane containing GSLs, phosphatidylcholine, sphingomyelin and cholesterol. Specific interaction between GSLs within the cell membrane and the lipid membrane are proposed to lead to cell attachment and increased cell motility.
Cell attachment or cell adhesion is the binding of a cell to a surface. Cell adhesion is not only essential in maintaining multicellular structure; it is also involved in signal transduction and most importantly it is crucial for pathogenesis, e.g. malignancy. The underlying mechanism of cell adhesion is a complex process that involves various reactions occurring at the cell interface. Thus, there is a great demand advancing in this field of research. In particular, the study of material?cell interactions using surface-based analytical techniques allows for a more detailed understanding of the properties of the interface between cells and the adjacent surface down to the nm-level. We have shown in several reports that acoustical sensing is a powerful method to monitor in real-time and label-free cell attachment and other biochemical processes taking place at the interface between cells, such as human derived chondrocytes and thrombocytes and a biomaterial surface (see Nilebäck et al. Analyst 2014 and Kunze et al. Colloids Surf., B. 2014).
Figure 3: Schematic illustration of the experimental design, using a combined set-up with a windowed QCM-D module mounted on a light microscope, and the immobilization of end-on biotinylated hyaluronic acid (HA). HA is immobilized onto a streptavidin (SA) layer bound to a biotinylated thiol self-assembly monolayer (biotin-SAM) on a gold-coated sensor surface. Image is not drawn to scale (from Nilebäck et al. Analyst 2014).