Probing Nanoscale Domain Formation in Artificial Membranes
Exploring nanoscale heterogeneity in biological membranes using artificial mimics
Biological membranes such as those around cells are integral to life. These selectively permeable membranes consist of lipid bilayers with embedded proteins. Biological membranes are heterogeneous with structures that are controlled by the components present. Interactions between cholesterol and lipids in membranes lead to the formation of nanoscale raft structures that are relatively lipid-rich or -poor with varying degrees of disorder. Lipid rafts may help to bring together proteins in cell membranes, although their role is not clearly understood.
An international collaboration led by researchers at Osaka University, together with colleagues in Japan and Finland, recently investigated the small transient domains in artificial membranes, which mimic these raft structures, in the hope of providing information about the nanoscale structure of cell membranes.
The researchers used fluorescence spectroscopy, and solid-state deuterium nuclear magnetic resonance (NMR) spectroscopy to investigate the formation of nanoscale domains in artificial membranes composed of cholesterol and a lipid, either stearoyl sphingomyelin (SSM) or 1-palmitoyl-2-stearoyl-sn-glycero-3-phosphocholine (PSPC), which have similar structures to aid data comparison.
“We measured fluorescence lifetimes to determine the nanosecond dynamics of probe molecules in the artificial membranes, while deuterium NMR spectroscopy provided microsecond information of the environment surrounding labeled atoms,” says Michio Murata of the Osaka University Graduate School of Science, Department of Chemistry. “The data obtained from these methods had not yet been effectively integrated.”
Ordering of the chains of the lipids in artificial membranes containing different contents of cholesterol was investigated by fluorescence measurements. The results indicated that both membranes contained multiple phase states, the distributions of which were influenced by cholesterol content and temperature. Fluorescence lifetime measurements revealed the presence of components with two different lifetimes for both types of membranes. The long-lived component was attributed to more-ordered domains (low-cholesterol gel-like phase) and the short-lived one to less-ordered domains (liquid-ordered phase). These two kinds of domains with different order and ratios of lipid to cholesterol existed in the membranes on a nanosecond time scale. In both membranes, the content of gel-like domains decreased with increasing cholesterol content and temperature.
“We believe that the cholesterol-poor gel-like domains help to increase the order in the liquid-ordered domains of the membrane,” explains J. Peter Slotte of Finland’s Åbo Akademi University. Deuterium NMR spectroscopy was then used to corroborate the fluorescence results by investigating the gel-like domains in the systems. The NMR data showed that the SSM membrane was less affected by temperature than the PSPC one, revealing that the mobility of the lipid chains affects the overall ordering of lipid membranes. The NMR data also indicated that the gel-like and liquid-ordered domains were very small because of the rapid transitioning between them. The domain size of SSM was much smaller than that of PSPC, which was caused by the higher thermal stability of SSM than that of PSPC. The higher thermal stability of SSM was attributed to its ability to form intermolecular hydrogen bonds.
The teamʼs findings indicate that the interaction of cholesterol with lipids in complex bilayers influences the formation of ordered nanoscale domains. The research expands knowledge of nanoscale behavior in biological membranes.
Professor Michio Murata
Graduate School of Science