Project A2: Investigation of dynamic nanostructures in biomimetic membranes

Principal Investigators: S. Schiller (Freiburg) / C. Marques (Strasbourg) / W. Meier (Basel)
Collaborators: T. Schmatko, A. Schröder (Strasbourg)
PhD Students: Nehrukumar Matahiyan, Andreas Weinberger

Current state of the research

Biological membranes are one of nature’s most complex systems; they exhibit a fascinating range of dynamic phenomena as interfaces in biology. These self-assembled bilayers are comprised of several hundred lipid species, allowing the membrane to change its shape and behavior, thus forming nano- to micrometer sized structures that span several orders of magnitude in space and timescales. The control of the relation between the chemical properties and the geometric nature of the membrane is a key to understand those dynamic superstructures.

We aim to develop a new class of reconstituted membranes, self-assembled from well-defined lipid and block copolymer mixtures in various geometries, such as supported membranes, liposomes or Giant Unilamellar Vesicles (GUVs). Such mixtures open new perspectives for tuning the process of membrane structure formation, and allow in particular an in-depth inspection of the separation mechanisms leading to different membrane domains. Phase separation phenomena, which is biologically important for several functional states of membrane proteins, need to be dealt with by a novel combination of physical and chemical methods. We will explore physical methods related to the control of osmotic pressure, shear stress and temperature and will study the membrane structure and geometry as a function of these parameters. Chemically we will synthesize novel glycolipids and block copolymers. Glycolipids are known to contribute to the complex phase behavior of living cells due to their multifunctional character: amphiphilic, plus multiple hydrogen donor and acceptor functions and charged/noncharged states. Similar behavior can be expected for appropriately designed amphiphilic block copolymers that will also confer stability and robustness to the system. Taken together these combined approaches will allow us to manipulate and investigate complex membrane architectures with unprecedented precision and control. The yielded information will be of great value for the understanding of biological membrane behavior and will serve to design novel ways to arrange cell epitopes such as glycolipids in defined and dynamically structured cell-like scenarios.

Contributions of the participating groups

Three groups are involved in this project:

• The group of Stefan Schiller in Freiburg is specialized in the synthesis of lipid conjugates and proteins modified specifically with synthetic amino-acids to build up complex structural assemblies to control biological function and exert new chemical reactions [1,2]. The new glycolipids will be key components in our project.
• The group of Carlos Marques in Strasbourg has developed a number of new techniques and methods [3,4] to observe and micro-manipulate many bilayer structures ranging from small liposomes to supported membranes and Giant Unilamellar Vesicles. The group has also expertise to form, observe and manipulate bio-mimetic Giant Vesicles and to control their interactions with many bio-polymeric species including DNA molecules.
• The group of Wolfgang Meier in Basel is specialized in the synthesis and characterization of polymer amphiphiles and biomimetic block copolymer membranes [5,6]. The group has expertise in characterizing the mixing behavior of polymer with lipid mono- and bilayers, and in the investigation of vesicular structures thereof.

This project aims at the development of a new class of self-assembled bio-mimetic membranes combining new lipids and new block copolymers. The Freiburg partner will bring in to this project chemical synthetic capabilities, and a well recognized expertise in the design and synthesis of fluorescent probes, glycolipids and biomimetic systems. The Basel partner will contribute with its well-recognized experience in the synthesis of bio-relevant block copolymer systems and the formation of polymersomes. The Strasbourg group will add to the project the tools of a physicist group dedicated to the formation and micromanipulation of biomimetic bilayer systems.

Research project and collaborations

The project is divided into two parts:

(i) Synthesis and membrane characterization of a new charged glycolipid. These molecules will constitute a launching pad for a family of molecules that we will finely adjust to obtain the desired membrane nanostructure. Furthermore, glycolipids constitute important epitopes for cellular recognition processes. In this context, the properties of block copolymer based membranes in parallel will allow for novel conditions in terms of membrane stability and structuring of the glycolipid which will be revealed by a new fluorescent probe also to be synthesized as a part of this project (S. Schiller, W. Meier).

(ii) These components will be used to assemble novel membrane systems with advanced properties. Liposomes, GUVs and supported membranes will be formed and studied by a range of structural and dynamical techniques available in Strasbourg (C. Marques). Our project is highly interdisciplinary and relies on the complementary expertise of the three groups: expertise in the synthesis of the membrane components and modified proteins (S. Schiller), experience in the synthesis and selfassembly of amphiphilic block copolymers (W. Meier), and an environment designed for studying the assembly of biomimetic membranes and their physical characterization (C. Marques), each of these three elements being essential for the success of the project.

References
[1] S. M. Schiller et al., Angew Chem. Int. Ed. 42, 208 (2003).
[2] S. M. Schiller et al., Angew Chem. Int. Ed. 48, 6896 (2009).
[3] O. Mertins, N. P. da Silveira, A. R. Pohlmann, A. P. Schroder, C. M. Marques, Biophysical J. 96, 2719 (2009).
[4] M-L. Hisette, P. Haddad, T. Gisler, C. M. Marques, A. P. Schröder, Soft Matter 4, 828 (2008).
[5] K. Kita-Tokarczyk, F. Itel, M. Grzelakowski, S. Egli, P. Rossbach, W. Meier, Langmuir 25, 9847 (2009).
[6] R. Nehring, C.G. Palivan, O. Casse, P. Tanner, J. Tüxen, W. Meier, Langmuir 25, 1122 (2009).