Abstract
This study aims to explore the variety of previously unknown morphologies that brain lipids form in aqueous solutions. We study how these structures are dependent on cholesterol content, salt solution composition, and temperature. For this purpose, dispersions of porcine sphingomyelin with varying amounts of cholesterol as well as dispersions of porcine brain lipid extracts were investigated. We used cryo-TEM to investigate the dispersions at high-salt solution content together with small-angle (SAXD) and wide-angle X-ray diffraction (WAXD) and differential scanning calorimetry (DSC) for dispersions in the corresponding salt solution at high lipid content. Sphingomyelin forms multilamellar vesicles in large excess of aqueous salt solution. These vesicles appear as double rippled bilayers in the images and as split Bragg peaks in SAXD together with a very distinct lamellar phase pattern. These features disappear with increasing temperature, and addition of cholesterol as the WAXD data shows that the peak corresponding to the chain crystallinity disappears. The dispersions of sphingomyelin at high cholesterol content form large vesicular type of structures with smooth bilayers. The repeat distance of the lamellar phase depends on temperature, salt solution composition, and slightly with cholesterol content. The brain lipid extracts form large multilamellar vesicles often attached to assemblies of higher electron density. We think that this is probably an example of supra self-assembly with a multiple-layered vesicle surrounding an interior cubic microphase. This is challenging to resolve. DSC shows the presence of different kinds of water bound to the lipid aggregates as a function of the lipid content. Comparison with the effect of lithium, sodium, and calcium salts on the structural parameters of the sphingomyelin and the morphologies of brain lipid extract morphologies demonstrate that lithium has remarkable effects also at low content.
Highlights
The structure—function relationship of brain tissue is poorly understood
This study aims to explore the variety of morphologies that brain lipid extracts do form in aqueous dispersions
Dispersions of porcine brain lipid extracts were prepared with 70% lipid in 0.9 wt% NaCl and 1 mM CaCl2 for small-angle X-ray diffraction (SAXD) and differential scanning calorimetry (DSC)
Summary
The structure—function relationship of brain tissue is poorly understood. The brain structure is a complex design based on a rich variety of lipids, assembled into intriguing structures, crucial to their function in, e.g., signal transduction that sometimes involved coupled proteins (Agranoff et al, 1999). One is local curvature at the lipid—aqueous interface set by the balance of head group vs hydrocarbon tail forces This is pivotal in many biological systems (Israelachvili et al, 1976; Hyde et al, 1997). The packing parameter (v/al) considers the ratio between the volume of the hydrophobic chain (v) and the product of the cross-section head group area (a) and the chain length in its fully stretched conformation (l) This concept, originally mostly used to describe “simpler” surfactant and lipid self-assembly system structures (Ninham et al, 2017a), has recently been found to be useful to describe more complex biological systems (Lauwers et al, 2016; Ninham et al, 2017b). These include geometric and molecular forces between aggregates (Israelachvili et al, 1976; Hyde et al, 1997)
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