Net Interactions That Push Cholesterol Away from Unsaturated Phospholipids Are Driven by Enthalpy

Chain ordering of phospholipids in membranes containing cholesterol: what matters?

Cholesterol (CHOL) drives lipid segregation and is thus a key player for the formation of lipid rafts and followingly for the ability of a cell to, e.g., enable selective agglomeration of proteins. The lipid segregation is driven by cholesterol's affinity for saturated lipids, which stands directly in relation to the ability of cholesterol to order the individual phospholipid (PL) acyl chains. In this work, molecular dynamics simulations of DPPC (dipalmitoylphosphatidylcholine, saturated lipid) and DLiPC (dilineoylphosphatidylcholine, unsaturated lipid) mixtures with cholesterol are used to elucidate the underlying mechanisms of the cholesterol ordering effect. To this end, all enthalpic contributions, experienced by the PL molecules, are recorded as a function of the PL's acyl chain order. This involves the PL-PL, the PL-cholesterol interaction, the interaction of the PLs with water, and the interleaflet interaction. This systematic analysis allows one to unravel differences of saturated and unsaturated lipids in terms of the different interaction factors. It turns out that cholesterol's impact on chain ordering stems not only from direct interactions with the PLs but is also indirectly present in the other energy contributions. Furthermore, the analysis sheds light on the relevance of the entropic contributions, related to the degrees of freedom of the acyl chain.

Sphingomyelin Acyl Chains Influence the Formation of Sphingomyelin- and Cholesterol-Enriched Domains

The segregation of lipids into lateral membrane domains has been extensively studied. It is well established that the structural differences between phospholipids play an important role in lateral membrane organization. When a high enough cholesterol concentration is present in the bilayer, liquid-ordered (L_o) domains, which are enriched in cholesterol and saturated phospholipids such as sphingomyelin (SM), may form. We have recently shown that such a formation of domains can be facilitated by the affinity differences of cholesterol for the saturated and unsaturated phospholipids present in the bilayer. In mammalian membranes, the saturated phospholipids are usually SMs with different acyl chains, the abundance of which vary with cell type. In this study, we investigated how the acyl chain structure of SMs affects the formation of SM- and cholesterol-enriched domains. From the analysis of trans-parinaric acid fluorescence emission lifetimes, we could determine that cholesterol facilitated lateral segregation most with the SMs that had 16 carbon-long acyl chains. Using differential scanning calorimetry and Förster resonance energy transfer techniques, we observed that the SM- and cholesterol-enriched domains with 16 carbon-long SMs were most thermally stabilized by cholesterol. The Förster resonance energy transfer technique also suggested that the same SMs also form the largest L_o domains. In agreement with our previously published data, the extent of influence that cholesterol had on the propensity of lateral segregation and the properties of L_o domains correlated with the relative affinity of cholesterol for the phospholipids present in the bilayers. Therefore, the specific SM species present in the membranes, together with unsaturated phospholipids and cholesterol, can be used by the cell to fine-tune the lateral structure of the membranes.

Impact of Acyl Chain Mismatch on the Formation and Properties of Sphingomyelin-Cholesterol Domains

Lateral segregation and the formation of lateral domains are well-known phenomena in ternary lipid bilayers composed of an unsaturated (low gel-to-liquid phase transition temperature (T_m)) phospholipid, a saturated (high-T_m) phospholipid, and cholesterol. The formation of lateral domains has been shown to be influenced by differences in phospholipid acyl chain unsaturation and length. Recently, we also showed that differential interactions of cholesterol with low- and high-T_m phospholipids in the bilayer can facilitate phospholipid segregation. Now, we have investigated phospholipid-cholesterol interactions and their role in lateral segregation in ternary bilayers composed of different unsaturated phosphatidylcholines (PCs) with varying acyl chain lengths, N-palmitoyl-D-erythro-sphingomyelin (PSM), and cholesterol. Using deuterium NMR spectroscopy, we determined how PSM was influenced by the acyl chain composition in surrounding PC environments and correlated this with the affinity of cholestatrienol (a fluorescent cholesterol analog) for PSM in the different PC environments. Results from a combination of time-resolved fluorescence measurements of trans-parinaric acid and Förster resonance energy transfer experiments showed that the relative affinity of cholesterol for phospholipids determined the degree to which the sterol promoted domain formation. From Förster resonance energy transfer, deuterium NMR, and differential scanning calorimetry results, it was clear that cholesterol also influenced both the thermostability of the domains and the degree of order in and outside the PSM-rich domains. The results of this study have shown that the affinity of cholesterol for both low-T_m and high-T_m phospholipids and the effects of low- and high-T_m phospholipids on each other influence both lateral structure and domain properties in complex bilayers. We envision that similar effects also contribute to lateral heterogeneity in even more complex biological membranes.

How to Determine Lipid Interactions in Membranes from Experiment Through the Ising Model

The determination and the meaning of interactions in lipid bilayers are discussed and interpreted through the Ising model. Originally developed to understand phase transitions in ferromagnetic systems, the Ising model applies equally well to lipid bilayers. In the case of a membrane, the essence of the Ising model is that each lipid is represented by a site on a lattice and that the interaction of each site with its nearest neighbors is represented by an energy parameter ω. To calculate the thermodynamic properties of the system, such as the enthalpy, the Gibbs energy, and the heat capacity, the partition function is derived. The calculation of the configurational entropy factor in the partition function, however, requires approximations or the use of Monte Carlo (MC) simulations. Those approximations are described. Ultimately, MC simulations are used in combination with experiment to determine the interaction parameters ω in lipid bilayers. Several experimental approaches are described, which can be used to obtain interaction parameters. They include nearest-neighbor recognition, differential scanning calorimetry, and Förster resonance energy transfer. Those approaches are most powerful when used in combination of MC simulations of Ising models. Lipid membranes of different compositions are discussed, which have been studied with these approaches. They include mixtures of cholesterol, saturated (ordered) phospholipids, and unsaturated (disordered) phospholipids. The interactions between those lipid species are examined as a function of molecular properties such as the degree of unsaturation and the acyl chain length. The general rule that emerges is that interactions between different lipids are usually unfavorable. The exception is that cholesterol interacts favorably with saturated (ordered) phospholipids. However, the interaction of cholesterol with unsaturated phospholipids becomes extremely unfavorable as the degree of unsaturation increases.