Hydrophobic compounds reshape membrane domains

Cell membranes have a complex lateral organization featuring domains with distinct composition, also known as rafts, which play an essential role in cellular processes such as signal transduction and protein trafficking. In vivo, perturbations of membrane domains (e.g., by drugs or lipophilic compounds) have major effects on the activity of raft-associated proteins and on signaling pathways, but they are difficult to characterize because of the small size of the domains, typically below optical resolution. Model membranes, instead, can show macroscopic phase separation between liquid-ordered and liquid-disordered domains, and they are often used to investigate the driving forces of membrane lateral organization. Studies in model membranes have shown that some lipophilic compounds perturb membrane domains, but it is not clear which chemical and physical properties determine domain perturbation. The mechanisms of domain stabilization and destabilization are also unknown. Here we describe the effect of six simple hydrophobic compounds on the lateral organization of phase-separated model membranes consisting of saturated and unsaturated phospholipids and cholesterol. Using molecular simulations, we identify two groups of molecules with distinct behavior: aliphatic compounds promote lipid mixing by distributing at the interface between liquid-ordered and liquid-disordered domains; aromatic compounds, instead, stabilize phase separation by partitioning into liquid-disordered domains and excluding cholesterol from the disordered domains. We predict that relatively small concentrations of hydrophobic species can have a broad impact on domain stability in model systems, which suggests possible mechanisms of action for hydrophobic compounds in vivo.
Author Summary Cell membranes consist of a variety of lipids and proteins with inhomogeneous lateral distribution, forming domains with distinct composition and properties. These domains play a fundamental role in a number of biological processes, and perturbing them can have important effects on cellular functions. Some chemicals with high affinity for lipid membranes perturb membrane domains, but the link between properties of the chemicals and domain perturbation is not understood. The mechanisms of domain perturbation are also not understood. In the present work we use molecular simulations of model membranes to understand the driving forces and the mechanisms of domain perturbation by different chemicals. We explore the effect of six hydrophobic compounds, all of them rather simple and common but with different size, shape, and properties. We find that all hydrophobic compounds alter the stability of domains, but not all of them in the same way. We identify two groups of compounds with opposite effects: aromatic compounds stabilize domains, while aliphatic compounds destabilize them. Simulations also allow us to visualize, for the first time, the mechanism of domain perturbation – which is very difficult to assess experimentally. Our findings on model membranes suggest possible mechanisms of action for hydrophobic chemicals in living cells.