Protein-lipid interactions in raft-exhibiting membranes probed by combined AFM and FCS

The cellular membrane is a complex biological entity, far from being an inert assembly of protein and lipids which separates cells from the surrounding environment. A multitude of biological processes, ranging from active transport of ions into and out of the cell, to the immune response, are regulated at the level of the plasma membrane. The understanding of their molecular basis is among the central goals of modern biological research. In order to dissect the complexity of actual cell membranes, which involves a very complex network of intermolecular interactions, a “divide and conquer” strategy proves very useful. To this end, researchers try to isolate molecules from complex biological contexts to understand their function in simple model systems under controlled conditions. A variety of model membranes have thus been developed in order to gain insight into membrane processes. This approach has resulted in a deeper knowledge on how lipids and proteins interact and how these interactions govern the function of cellular membranes. In the recent past in fact, a connection has been established between the lateral structure of the plasma membrane and its biological function. Furthermore, a large range of biophysical techniques have been used to characterize protein-lipid microdomains. For example, atomic force microscopy (AFM) is a powerful technique which allows a highly detailed topographical characterization of lipid domains in physiological conditions. While AFM imaging offers an extremely high spatial resolution, up to the nanometer scale, the limited image acquisition speed (minutes) can pose a severe drawback in adequately studying fast dynamic processes. On the other hand, fluorescence based imaging techniques are much faster (10-3-100 s), but certainly lack the high spatial resolution that AFM offers. FCS in particular can also provide information about dynamic processes, like diffusion of fluorescent membrane components. For these reasons, implementing a combination of the above mentioned techniques on the same sample (e.g. cell membrane models) would prove extremely beneficial in the complete dynamic and structural characterization of molecular interactions. . The work described in this thesis can be summarized in two main points: i) the development of a novel combined approach of atomic force microscopy (AFM), laser scanning imaging (LSM), and fluorescence correlation spectroscopy (FCS) and ii) the study of the effects of ceramide in the lateral organization of model plasma membranes. We described one of the first simultaneous applications of AFM and FCS on biologically relevant systems. More specifically, model membranes showing complex phase separation were investigated with a combined approach of AFM, confocal fluorescence imaging, force measurements and FCS, based on commercially available instruments. AFM conveys information about the structural and mechanical properties of the different lipid phases. Different membrane domains can be distinguished based on height difference, elastic properties and line tension as measured by the AFM tip. Simultaneous optical measurements offer the correlation of these data in real time with the partition behavior and diffusion of fluorescent lipids and proteins. We established a clear link between the local membrane viscosity, probed by FCS, and the lipid-lipid interactions involved in line tension, probed by AFM force measurements. An example of a significant drawback circumvented by the AFM-FCS approach involves the use of AFM micromanipulation to eliminate unwanted interactions between lipid particles — similar to intra-cellular vesicles found in vivo experiments — and the membrane, which usually result in distorted FCS autocorrelation curves. Finally, the combined application of AFM and FCS on membrane-anchored proteins reconstituted in lipid bilayers has been instrumental in clarifying inconsistencies that arose in work that focused solely on either AFM or fluorescence techniques. We have shown that, in the case of proteins diffusing in the plane of the membrane, AFM can unambiguously detect only a small immobile fraction. Furthermore, since AFM detection of proteins might be facilitated by high local membrane viscosity (e.g. in ordered lipid phases), the measurement of protein partition between disordered and ordered membrane domains might be biased toward the latter. In this case, the use of FCS as a complementary technique allows a more thorough investigation and deeper understanding of the system of interest. The second part of this thesis dealt with the study of complex lipid mixtures which are used to model the putative lipid/proteins domains in cells, called “rafts”. Firstly, we proved how the combined fluorescence imaging/AFM approach is useful in general for studying supported lipid membranes and the role of lipid domains in biological contexts. We investigated the effect of environmental stress on biological membranes and the protective effects of several substances. Our experimental approach was shown to be a new valuable method to visualize the dehydration damage and its effects on the lateral organization of lipid domains. Our results demonstrated that disaccharides like trehalose or sucrose are effective in protecting lipid membranes, not only on a macroscopic scale — preserving the overall integrity of the bilayer — but also on a microscopic scale, preventing the clustering of microdomains. These phenomena are interesting in the context of biological damage to living cells which need to be stored for long time, like organs to be transplanted or blood platelets. Finally, a large section of this thesis focused on the effects of a specific lipid called “ceramide” on the lateral organization of proteins and lipids in the plasma membrane. Ceramide is produced by cells in several situations, like bacterial infections or apoptosis. As consequence of ceramide production in vivo, the local concentration and the dynamic behavior of lipids and membrane receptors are supposed to exhibit strong variations. In order to understand the molecular mechanisms responsible for these effects, we applied a combination of AFM, FCS and fluorescence imaging on simple model membranes containing ceramide. We could show for the first time that, in presence of raft-like Lo/Ld phase separation, physiological quantities of ceramide induced the formation of a highly ordered gel phase, constituted of ceramide and sphingomyelin. The enzymatic production of ceramide was monitored both in supported and in free-standing bilayers. In the second case, ceramide production was connected to selective vesicle budding from the raft-like phase. Since short-chain analogues are often used in both medical applications and biochemical research to mimic the effect of long-chain ceramides, we investigated the effect of chain-length on ceramide-induced membrane reorganization. We could show that only long-chain ceramides (C18 and C16) form highly ordered domains. Interestingly, FCS measurements indicated that the physical properties of the Lo raft-like domains are hardly affected by the presence of ceramide domains. Furthermore, the increased thickness of the Ld phase — as measured by AFM — and its higher viscosity — as measured by FCS — strongly support the hypothesis of ceramide-induced cholesterol displacement from rafts. On the other hand, short-chain ceramides showed completely different biophysical properties that lead to a destabilization of the raft domains, possibly acting as surfactants between the different lipid phases. Our findings contribute to the explanation of in vivo experiments where short-chain ceramides inhibit cell signaling by disrupting the lipid order in the plasma membrane. We have so far demonstrated that ceramide plays a fundamental role in lipid-lipid interactions. In a physiological context, it is also known to produce dramatic effects in living cells. Since a majority of the processes in vivo are thought to be governed by the activity of proteins, it is highly likely that ceramide not only affects lipid organization but also modifies protein-protein and protein-lipid interactions to produce its effects. To test this hypothesis, we reconstituted several membrane proteins in lipid bilayers containing Ld, Lo, and ceramide-rich domains. We were able to show that some membrane proteins are sorted into ceramide-rich domains. More specifically, the raft-associated proteins we tested were enriched in the highly ordered ceramide-rich domains, while the Ld-associated components were excluded from them. Furthermore, the inclusion of any membrane component in ceramide-rich domains is directly connected to a dramatic reduction of its in-plane diffusion. In an in vivo context, such a reorganization of membrane receptors might be used by the cell to alter the signaling process, for example, by i) separating raft receptors from inhibitors with lower raft affinity, ii) bringing both raft-associated receptors and raft-associated signaling molecules into contact, or iii) stabilizing the interactions between a receptor and its ligand by decreasing their diffusion coefficients. In conclusion, this thesis describes a novel combination of AFM, LSM, and FCS for the investigation of the lateral organization of biological membranes. Our results show that this approach applied on model membranes of increasing complexity is an effective tool for understanding the molecular mechanisms behind the organization of biological membranes. This report opens up new possibilities for further investigation in living cell membranes using the same methodology we have described