Atomic force microscopy of nonhydroxy galactocerebroside nanotubes and their self-assembly at the air-water interface, with applications to myelin

Myelin is one of the few biological membranes to contain the lipid galactocerebrosides, although their role in myelin is unclear. To explore its structural role, we used fluorescence and atomic force microscopy (AFM) to study nonhydroxy galactocerebrosides (NCer) at the air-water interface of a Langmuir-Blodgett trough. Fluorescence microscopy at the air-water interface indicated that NCer forms micrometer scale domains of varying radii with six fractal-like extensions. Atomic force microscopy using TappingMode in water on samples transferred to mica confirmed the fractal-like domain structure in the absence of dye and showed that the domains consisted of many aggregated nanotubes with a diameter of 30 nm. The Hausdorf fractal dimension was estimated to be 1.26 and 1.11 for two domains imaged with AFM. This evidence indicates that NCer forms a bulk phase of nanotubes at the air-water interface, unlike the liquid-condensed phase of a phospholipid monolayer. That NCer forms bilayer nanotubes that aggregate strongly suggests NCer helps maintain the stability of myelin by contributing to the curvature and adhesion of the membrane. We found that NCer appears to be decreased in myelin from multiple sclerosis normal appearing white matter, which could be an important event in the loss of myelin stability.