Lipid domain coarsening and fluidity in multicomponent lipid vesicles: A continuum based model and its experimental validation

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.

On Fusogenicity of Positively Charged Phased-Separated Lipid Vesicles: Experiments and Computational Simulations

This paper studies the fusogenicity of cationic liposomes in relation to their surface distribution of cationic lipids and utilizes membrane phase separation to control this surface distribution. It is found that concentrating the cationic lipids into small surface patches on liposomes, through phase-separation, can enhance liposome's fusogenicity. Further concentrating these lipids into smaller patches on the surface of liposomes led to an increased level of fusogenicity. These experimental findings are supported by numerical simulations using a mathematical model for phase-separated charged liposomes. Findings of this study may be used for design and development of highly fusogenic liposomes with minimal level of toxicity.

Charged Lipids Modulate the Phase Separation in Multicomponent Membranes

Phase separation in lipid membranes controls the organization of membrane components and thus regulates membrane-mediated processes. Membrane phase behavior is influenced by the molecular properties of its components and their relative concentrations. Charged lipid species are among the most essential components of lipid membranes, and their impact on the membrane phase behavior is yet to be fully understood. Aiming to provide insight into this impact, this paper investigates how the presence and amount of anionic and cationic lipids affect the phase behavior of multicomponent membranes. Membranes of ternary composition DOPC:DPPC:Chol with two distinct molar ratios were used to test the hypothesis that inclusion of charged lipids with saturated tails, beyond a certain concentration, would impede phase separation in an otherwise phase-separating membrane. Fluorescence microscopy examination of electroformed giant liposomes revealed that when more than half of DOPC in the examined mixtures was replaced with DOPA or DOTAP, phase separation in liposomes was somewhat suppressed, and this effect increased with increasing charged lipid content. This effect depended on the membrane surface charge density as the half-maximal effect was observed at around 0.0072 C Å-2 in all examined cases. The phase-separation suppressing effect of DOPA was neutralized when oppositely charged lipid DOTAP was included in the mixture. Likewise, presence of divalent cation Ca2+ in the solution neutralized the impact of negatively charged DOPA. These results underline the detrimental influence of surface charge density on membrane phase behavior. More importantly, these findings suggest that the charged lipid content in membranes may be a regulator of their phase behavior and open new opportunities for the design of synthetic lipid membranes.