Model Membranes

I. Köper, E.-K. Sinner

Biological membranes are highly complex architectures. Their central structure consists of a lipid bilayer, which fulfills many tasks and, in particular, serves as a barrier between different compartments of the cell. Membrane related processes have attracted an enormous interest, but systematic studies of such complex systems are difficult and limited to only a few examples. Therefore, the construction of an artificial model system, which mimics the natural bilayer lipid membrane, is an important task. Such a system opens new paths to investigate membrane-related processes such as cell adhesion, photosynthesis, respiration and drug-protein interactions.
One of the most promising model systems is the tethered bilayer lipid membrane (tBLM). It complements classical model systems such as Langmuir monolayers, liposomes and bilayer lipid membranes (BLMs).

As schematically depicted in the figure, a tBLM consists of a lipid bilayer that is tethered via a spacer group to a solid support. If this support can be used as an electrode, electrical characterization of the system is possible. The planar architecture allows for the application of various surface analytical techniques. The proximal layer of a tBLM is typically prepared by self assembly of spacer lipids while the distal layer is obtained by fusion of the monolayer with unilamelar vesicles or proteoliposomes. The spacer group of the proximal layer should provide a hydrophilic environment that mimics the function of the cytosol, e.g., serving as an ion reservoir. The bilayer can be functionalized by the incorporation of membrane proteins such as ion channels or transport proteins. The formation of the tBLM itself as well as the incorporation of proteins can be followed by surface analytical methods, e.g., surface plasmon resonance (SPR), surface plasmon enhanced fluorescence spectroscopy (SPFS), or quartz crystal microbalance (QCM) measurements. Electrical properties are characterized by using electrochemical methods, in particular electrical impedance spectroscopy. For the study of ion transport processes due to protein activity, a high background resistance of the membrane is essential. We were able to develop a variety of lipids that form insulating membranes on ultra-smooth gold electrodes. However lateral dilution of the spacer lipids using appropriate dilution molecules is essential, in order to incorporate larger channel proteins into tBLMs. Additionally, we could also show membrane formation on silicon oxide surfaces. This will help to couple our model membrane systems to many applications and micro-electronic read-out devices that are based on silicon-containing materials. Using a tBLM, either on gold or on silicon, a variety of membrane proteins has been incorporated in a functionally active form, including valinomycin, alpha hemolysin, gramicidin, melittin, the M2 subunit of the acetyl choline receptor nAChR, and the Maxi-K channel.

Cell adhesion is mediated by membrane proteins which are inserted in the plasma membrane. These proteins specifically recognize binding epitopes exposed on extracellular matrix proteins (ECM). These interactions are dominated by cell-adhesion receptors, among which integrins comprise a diverse and prominent family. Integrin-ligand interactions are considered to provide physical support for cells in order to permit cell adhesion and cell movement, as schematically depicted in Figure 3. The focus of this work is to develop of a model system to elucidate the processes involved in cell-surface interactions in more detail. Therefore, an artificial membrane system has been developed that allows for the functional incorporation of the molecules involved in cell adhesion. Thus we can investigate the interaction of adherent cells with a peptide supported lipid membrane.
Synthetic ECM-mimicking molecules – trimeric collagen peptides and the prominent RGD binding sequence – have been synthesized in the group of L. Moroder (MPI for Biochemistry, Martinsried) for detailed investigation using the peptide supported membrane.

R. Naumann, S.M. Schiller, F. Giess, B. Grohe, K.B. Hartman, I. Kärcher, I. Köper, J. Lübben, K. Vasilev, and W. Knoll, Tethered lipid bilayers on ultraflat gold surfaces. Langmuir, 19: 5435-5443 (2003).
V. Atanasov, N. Knorr, R.S. Duran, S. Ingebrandt, A. Offenhäuser, W. Knoll, and I. Köper, Membrane on a Chip. A functional tethered lipid bilayer membrane on silicon oxide surfaces. Biophysical Journal, 89 (2005).
F. Giess, M.G. Friedrich, J. Heberle, R. Naumann, and W. Knoll, The protein-tethered lipid bilayer: a novel mimic of the biological membrane. Biophysical Journal, 87: 3213-3220 (2004).
E.K. Sinner, U. Reuning, F. N. Kök, B. Saccà, L. Moroder, W. Knoll and D. Oesterhelt Incorporation of integrins into artificial planar lipid membranes: characterization by plasmon-enhanced fluorescence spectroscopy, Anal. Biochem. 333: 216-224 (2004).
M. Schütt, S. Krupka, A.G. Milbradt, S. Deindl, E.-K. Sinner, C. Renner and L. Moroder, Photocontrol of Cell Adhesion Processes. Model Studies with Cyclic Azobenzene-RGD Peptides, Chem Biol. 10: 487-490 (2003).