A supported lipid bilayer to model solid-ordered membrane domains

Membrane models are widely used to mimic the behaviour of native plasma membranes and to simulate interactions occurring at their interface. Such models can be built up with different molecular compositions, ranging from single phospholipids to more complex, heterogeneous mixtures of phospho- and sphingo-lipids, possibly enriched with cholesterol and proteins. In particular, mixing different lipids and cholesterol is instrumental to promote the formation of phase-separated, ordered domains, which resemble the structure of lipid rafts, specialized functional domains of real membranes. According to the specific lipid composition, physical characteristics of the rafts can be tuned, such as fluidity, strongly related to membrane biological activity. Here, we introduce a novel three-component membrane model constituted by the mixing of a saturated phospholipid, 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC), sphingomyelin and cholesterol to mimic the presence of solid ordered rafts and to study their behaviour. Differential scanning calorimetry, neutron reflectometry, and atomic force microscopy were synergistically applied to gain information on the membrane's transverse and lateral organization, as well as on its thermotropic behaviour. The membrane model benefits from the use of DMPC, a lipid (i) characterized by an accessible transition temperature; (ii) saturated; (iii) fluid at physiological temperature and (iv) commercially available in both protiated and deuterated forms. The proposed model, along with the wide range of biophysical techniques employed, constitutes an ideal system to study the molecular mechanisms and the physical properties that govern membrane functions, such as molecular signalling and membrane trafficking.