Impact of galactosylceramides on the nanomechanical properties of lipid bilayer models: an AFM-force spectroscopy study

Galactosylceramides (GalCer) are glycosphingolipids bound to a monosaccharide group, responsible for inducing extensive hydrogen bonds that yield their alignment and accumulation in the outer leaflet of the biological membrane together with cholesterol (Chol) in rafts. In this work, the influence of GalCer on the nanomechanical properties of supported lipid bilayers (SLBs) based on DPPC (1,2-dipalmitoyl-sn-glycero-3-phosphocholine) and DLPC (1,2-didodecanoyl-sn-glycero-3-phosphocoline) as model systems was assessed. Phosphatidylcholine (PC):GalCer SLBs were characterized by means of differential scanning calorimetry (DSC) and atomic force microscopy (AFM), in both imaging and force spectroscopy (AFM-FS) modes. Comparing both PC systems, we determined that the behaviour of SLB mixtures is governed by the PC phase-like state at the working temperature. While a phase segregated system is observed for DLPC:GalCer SLBs, GalCer are found to be dissolved in DPPC SLBs for GalCer contents up to 20 mol%. In both systems, the incorporation of GalCer intensifies the nanomechanical properties of SLBs. Interestingly, segregated domains of exceptionally high mechanical stability are formed in DLPC:GalCer SLBs. Finally, the role of 20 mol% Chol in GalCer organization and function in the membranes was assessed. Both PC model systems displayed phase segregation and remarkable nanomechanical stability when GalCer and Chol coexist in SLBs.