Soft Materials

Overview

The experimental program in soft materials primarily focuses on problems of biological relevance, in particular the plasma membrane and technologies for delivering therapeutics across such membranes. Neutron beams are excellent probes of biological materials because these materials are rich in hydrogen. Further, neutrons not only differentiate between the different elements, but also the element's isotopes, such as between hydrogen and deuterium. At NRC-CNBC, neutron beams with fluxes and wavelengths suitable for such experiments are produced at the National Research Universal (NRU) reactor.

Research Highlight:

The Unusual Location of Cholesterol in a Polyunsaturated Lipid Membrane

T. A. Harroun, J. Katsaras and S. R. Wassall

Cholesterol is an essential component of mammalian cells and is either obtained from foods of animal origin (e.g., milk, cheese, meat, eggs) or synthesized in the endoplasmic reticulum, a subcellular organelle. Cholesterol is required for building and maintaining cell membranes, regulates their fluidity, and may act as an antioxidant to prevent or slow the oxidative damage to cells.

Figure 1. Schematic of the locations and orientations of cholesterol in a membrane. A gray background of model phospholipids is shown for illustration purposes. (A) Canonical location and orientation of cholesterol in a bilayer. (B) Location and orientation of cholesterol in 20:4-20:4 PC bilayers.

Polyunsaturated fatty acids (PUFA) constitute a biologically influential group of molecules whose physiological importance is now becoming well established. A multitude of disease states and chronic conditions are alleviated by dietary consumption of PUFA. While interest in the topic has centred on the omega-3 class of PUFA lipids, it has also spanned a variety of human health issues, including PUFA-associated effects on protein signalling in inflammation and cancer, arteriosclerosis, and suppressive effects on the immune system. The efficacy of PUFAs has been attributed to the formation of membrane domains enriched in PUFA-containing phospholipids. According to this model, the highly disordered PUFAs provide a local environment necessary for protein function.

In recent years, researchers from Brock University (St. Catharines, ON), Indiana University-Purdue University Indianapolis (IN, USA) and NRC-CNBC have studied the location of cholesterol in a number of model membrane systems. Using neutron diffraction [1], they found cholesterol aligning in the generally accepted "upright" orientation (See Figure 1A) in most membranes. The exception was found to be 20:4-20:4 (PUFA) phosphatidylcholine membranes, whereby cholesterol was unequivocally found to lie flat in the middle of the bilayer (See Figure 1B).

The findings from the neutron diffraction work suggest that the poor affinity for PUFA may affect the transmembrane, as well as the lateral, distribution of cholesterol. A tendency to sit at the centre of the PUFA-containing membranes would facilitate the sterol's flip-flop from one side of a membrane to the other. For example, the presence of PUFAs in the inner leaflet of plasma membranes is thought to enhance the transfer of cholesterol to the outer leaflet, potentially modifying raft composition and function. Indeed, enhanced rates of flip-flop were observed for cholesterol in recently published coarsegrained simulations that identified the presence of the sterol embedded between monolayers of arachidonic acid-containing PC bilayers [2].

Further study continues to show a functional significance of lipid diversity, that is, lipid species determines the orientation of cholesterol in bilayers [3]. We observed the same location for cholesterol in PUFA bilayers doped with small amounts of POPC. Figure 1A shows the NSLD difference profile corresponding to PUFA bilayers containing 30 mol % POPC, in which the cholesterol label was clearly observed in the bilayer center. This was the highest POPC concentration in which cholesterol was unambiguously observed in the bilayer center. However, the situation changed with increasing amounts of POPC. Figure 1B shows the NSLD difference profile for PUFA bilayers containing 50 mol % POPC, the lowest concentration at which we observed cholesterol to revert to its upright orientation. The deuterium label appears to be approximately 15 Å from the bilayer center, placing cholesterol's hydroxyl group within the bilayer's hydrophobic/hydrophilic interfacial region, in excellent agreement with previous data.

Figure 2. Neutron scattering length density difference profiles (deuterated minus nondeuterated cholesterol) showing the distribution of cholesterol’s deuterium label in (A) 30 and (B) 50 mol % POPC-doped PUFA bilayers. Cartoons depict a schematic view of the various bilayer components, with POPC in yellow, PUFA in green, cholesterol in purple, and deuterium label in black.

The situation when doping PUFA bilayers with DMPC is, however, dramatically different from that with POPC. While 50 mol % POPC was necessary to flip cholesterol into its upright position in PUFA bilayers, Figure 2 shows that only 5 mol % DMPC was necessary to achieve the same effect. This result clearly demonstrates cholesterol’s affinity for saturated chains.

Top of Page

[1] T. A. Harroun, J. Katsaras and S. R. Wassall. Biochemistry 45, 1227 - 1233 (2006).

T. A. Harroun, J. Katsaras and S. R. Wassall. Biochemistry 47, 7090 - 7096 (2008).

[2] S. J. Marrink, A. H. de Vries, T. A. Harroun, J. Katsaras and S. R. Wassall. J. Am. Chem. Soc. 130, 10 - 11 (2008).

[3] N. Kucerka, D. Marquardt, T.A. Harroun, M.P. Nieh, S.R. Wassall, J. Katsaras.  J. Am. Chem. Soc. 131, 16358 (2009).

Related Information

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CNBC Expands Capacity for Research on Nanotechnologies, Soft and Biologically Relevant Materials [News]

Strategic Research Alliance with MDS Nordion [News]

Institutes:

• NRC Canadian Neutron Beam Centre