Effect of glucosylation for the vertical movement of cholesterol in bilayer membranes

Cholesterol (Chol) in mammalian cell membranes facilitates the assembly of dynamic membrane domains that are involved in vital biological processes through lateral and transbilayer movements in the membranes. In the cell membranes, Chol undergoes glucose transglycosylation to produce cholesteryl-β-d-glucoside (ChoGlc). ChoGlc is involved in neurodegenerative diseases and accumulates in lysosomal storage disorders. However, the effects of glucosylation on membrane properties of Chol remain unclear. We investigated the membrane interaction of ChoGlc and its subsequent translocation between leaflets using fluorescent probes, such as 4-trimethylammoniumphenyl)-6-phenyl-1,3,5-hexatriene (TMA-DPH) and the newly synthesized 7-nitro-2,1,3-benzoxadiazole-labeled ChoGlc (NBD-ChoGlc) in dioleoylphosphatidylcholine (DOPC) membranes. The fluorescence of TMA-DPH, which selectively reported the order of the outer leaflet of the bilayer, indicated that ChoGlc added to the external solution, was mostly incorporated into the membranes and increased the DOPC membrane ordering. Furthermore, the anisotropy values reached a level similar to that of the ChoGlc-preloaded symmetric vesicle within approximately 5 min owing to the rapid distribution of ChoGlc in both leaflets. This was further confirmed by the selective fluorescence quenching of NBD-ChoGlc in the outer leaflet through irreversible quenching by dithionite. The similarity of the fluorescence decay curves of NBD-ChoGlc and NBD-Chol indicated that the glucosylation had little impact on the flip-flops of Chol in the DOPC bilayers. Our data demonstrates that some of the important membrane properties of Chol, such as fast flip-flop between leaflets and increased membrane order, were mostly maintained in ChoGlc despite hydrophilic glucose modification.