Lo/Ld phase coexistence modulation induced by GM1

Lipid rafts are assumed to undergo biologically important size-modulations from nanorafts to microrafts. Due to the complexity of cellular membranes, model systems become important tools, especially for the investigation of the factors affecting raft-like L-o domain size and the search for L-o nanodomains as precursors in L-o microdomain formation. Because lipid compositional change is the primary mechanism by which a cell can alter membrane phase behavior, we studied the effect of the ganglioside GM1 concentration on the L-o/L-d lateral phase separation in PC/SM/Chol/GM1 bilayers. GM1 above 1 mol% abolishes the formation of the micrometer-scale L-o domains observed in GUVs. However, the apparently homogeneous phase observed in optical microscopy corresponds in fact, within a certain temperature range, to a L-o/L-d lateral phase separation taking place below the optical resolution. This nanoscale phase separation is revealed by fluorescence spectroscopy, including C12NBD-PC self-quenching and Laurdan GP measurements, and is supported by Gaussian spectral decomposition analysis. The temperature of formation of nanoscale L-o phase domains over an L-d phase is determined, and is shifted to higher values when the GM1 content increases. A morphological phase diagram could be made, and it displays three regions corresponding respectively to L-o/L-d micrometric phase separation, L-o/L-d nanometric phase separation, and a homogeneous L-d phase. We therefore show that a lipid only-based mechanism is able to control the existence and the sizes of phase-separated membrane domains. GM1 could act on the line tension, arresting domain growth and thereby stabilizing L-o nanodomains. (C) 2014 Elsevier B.V. All rights reserved.