Glucosylceramide Reorganizes Cholesterol-Containing Domains in a Fluid Phospholipid Membrane

Glucosylceramide (GlcCer), one of the simplest glycosphingolipids, plays key roles in physiology and pathophysiology. It has been suggested that GlcCer modulates cellular events by forming specialized domains. In this study, we investigated the interplay between GlcCer and cholesterol (Chol), an important lipid involved in the formation of liquid-ordered (l_o) phases. Using fluorescence microscopy and spectroscopy, and dynamic and electrophoretic light scattering, we characterized the interaction between these lipids in different pH environments. A quantitative description of the phase behavior of the ternary unsaturated phospholipid/Chol/GlcCer mixture is presented. The results demonstrate coexistence between l_o and liquid-disordered (l_d) phases. However, the extent of l_o/l_d phase separation is sparse, mainly due to the ability of GlcCer to segregate into tightly packed gel domains. As a result, the phase diagram of these mixtures is characterized by an extensive three-phase coexistence region of fluid (l_d-phospholipid enriched)/l_o (Chol enriched)/gel (GlcCer enriched). Moreover, the results show that upon acidification, GlcCer solubility in the l_o phase is increased, leading to a larger l_o/l_d coexistence region. Quantitative analyses allowed us to determine the differences in the composition of the phases at neutral and acidic pH. These results predict the impact of GlcCer on domain formation and membrane organization in complex biological membranes, and provide a background for unraveling the relationship between the biophysical properties of GlcCer and its biological action.