Modulation of Inter- and Intramembrane Coupling in Lipid Bilayers via the Aqueous Phase

Living matter interacts with each other frequently through an aqueous medium. Prime examples for such systems are cell membranes, which are fundamental elements of life. All these aqueous media contain electrolytes, i.e. ions, which, depending on type, valency and concentration are able to modulate intermembrane interactions. Ion-specific effects are general, but not well understood phenomena, that occur in diverse matter and under diverse conditions and are known since the studies of Franz Hofmeister on the stability of proteins in different salt solutions about 130 years ago. With respect to cell membranes, we hypothesize that ions do not only affect membrane interactions, but also intramembrane structure and in particular the formation, size, connectivity, elasticity, and surface charge of lipid domains. Hence, ions may have a more significant influence of membrane function than conceived previously. Within the proposed work we seek answers to three major questions. (i) How does ion mediated attraction between membranes affect the fluctuation spectrum of membranes as compared to membranes that are condensed by an external (osmotic) pressure? (ii) How do salts change lateral phase separation occurring in membranes, including the physical properties of the coexisting domains? (iii) Do specific interactions of ions with membrane domains create specific charge distributions on the membrane surface? To achieve our goals we will combine the experimental expertise of the Pabst group (University of Graz, Austria) in biomembrane physics with the theoretical expertise of the Podgornik group (University of Ljubljana, Slovenia) in (bio)macromolecular interactions. We will study a set of charged and charge-neutral lipid membranes, including complex lipid mixtures, in different ionic solutions, following a Hofmeister series that comprises also polyvalent ions. From these studies, we will derive an analytical description of ion-mediated interactions based on an extension of Poisson-Boltzmann theory, and elucidate its coupling to lateral membrane structure. We expect to delineate unprecedented insight on the physics pertaining to ion-mediated inter- and intramembrane coupling, knowledge which is important for understanding and treating a multitude of diseases relating to lipid disorder.

We performed in the framework of the bilateral FWF project jointly conducted with the theory group of Rudolf Podgornik (Univ. Ljubljana, Slovenia) a detailed study on ion-specific effects on structure and interactions of lipid-only mimics of natural cell membranes. The work was motivated by the abundance of (poly)valent ions of different size, charge, type in the natural aqueous environment of biological membranes, whose effect on their physiological functions has been largely unexplored. Ion-specific effects are general, but not well understood phenomena, that occur in diverse matter and under diverse conditions and are known since the studies of Franz Hofmeister on the stability of proteins in different salt solutions about 130 years ago. Two major questions were addressed within the project. (i) How are fundamental interactions between membrane assemblies influenced by the presence of ions and how does this couple to membrane elasticity? (ii) How does the presence of ions influence the structure of lipid domains, which mimic membrane signalling and communication platforms in life cells commonly also known as rafts. To this end we measured the forces occurring between membranes. Specifically, we applied defined osmotic pressure and determined the distance between adjacent membranes and their fluctuations using small-angle X-ray scattering. This way the distance between the membranes can be resolved with high accuracy in the nanometer regime. The obtained data were analysed with a newly developed theory, which simultaneously takes into account the fluctuations and osmotic pressure behaviour. A coupling to membrane electrostatics showed that salts as simple as NaCl are able to reduce the bending rigidity of membranes by a factor of six without specifically binding to the lipid headgroups. Experiments on polyvalent ions showed even more complex results requiring further theoretical developments, which are currently undertaken. Regarding effects on membrane structure, performed research showed that ions specifically interact with lipid domains. In particular more loosely packed lipid domains are susceptible to the ions. Moreover, significant different effects for ions of the same valence, but different atomic number show that there is fine ion-dependent balance in membrane restructuring. This implies that the bilayers do not exhibit a uniformly charged aqueous layer, but a highly dynamic laterally structured ionic interface. The putative implications to live cells and cell physiology can be conceived to be enormous. For example it would open up the possibility to modulate cell to cell communication or intracellular signalling by rapidly or selectively exchanging the ionic environment of cell membranes. Such aspects will be addressed in follow up projects.