Cholesterol and Lipid Rafts in the Biogenesis of Amyloid-β Protein and Alzheimer's Disease

Cholesterol has been conjectured to be a modulator of the amyloid cascade, the mechanism that produces the amyloid-β (Aβ) peptides implicated in the onset of Alzheimer's disease. We propose that cholesterol impacts the genesis of Aβ not through direct interaction with proteins in the bilayer, but indirectly by inducing the liquid-ordered phase and accompanying liquid-liquid phase separations, which partition proteins in the amyloid cascade to different lipid domains and ultimately to different endocytotic pathways. We explore the full process of Aβ genesis in the context of liquid-ordered phases induced by cholesterol, including protein partitioning into lipid domains, mechanisms of endocytosis experienced by lipid domains and secretases, and pH-controlled activation of amyloid precursor protein secretases in specific endocytotic environments. Outstanding questions on the essential role of cholesterol in the amyloid cascade are identified for future studies.

Spontaneous Membrane Nanodomain Formation in the Absence or Presence of the Neurotransmitter Serotonin

Detailed knowledge on the formation of biomembrane domains, their structure, composition, and physical characteristics is scarce. Despite its frequently discussed importance in signaling, e.g., in obtaining localized non-homogeneous receptor compositions in the plasma membrane, the nanometer size as well as the dynamic and transient nature of domains impede their experimental characterization. In turn, atomistic molecular dynamics (MD) simulations combine both, high spatial and high temporal resolution. Here, using microsecond atomistic MD simulations, we characterize the spontaneous and unbiased formation of nano-domains in a plasma membrane model containing phosphatidylcholine (POPC), palmitoyl-sphingomyelin (PSM), and cholesterol (Chol) in the presence or absence of the neurotransmitter serotonin at different temperatures. In the ternary mixture, highly ordered and highly disordered domains of similar composition coexist at 303 K. The distinction of domains by lipid acyl chain order gets lost at lower temperatures of 298 and 294 K, suggesting a phase transition at ambient temperature. By comparison of domain ordering and composition, we demonstrate how the domain-specific binding of the neurotransmitter serotonin results in a modified domain lipid composition and a substantial downward shift of the phase transition temperature. Our simulations thus suggest a novel mode of action of neurotransmitters possibly of importance in neuronal signal transmission.

Fabrication of Asymmetric Phosphatidylserine-Containing Lipid Vesicles: A Study on the Effects of Size, Temperature, and Lipid Composition

The asymmetric distribution of lipids in plasma membranes is closely related to the physiological functions of cells. To improve our previous approach in fabricating asymmetric vesicles, we defined a parameter, asymmetric degree, in this work and investigated the effects of vesicle size, incubation temperature, and lipid composition on the formation process of asymmetric phosphatidylserine (PS)-containing lipid vesicles. The results indicate that all of the three factors have marked but different effects on the time-dependent asymmetric degree of the vesicles as well as the flip and flop rate constants of the PS lipids. However, only vesicle size and PS content show significant influence on the maximal asymmetric degree of the vesicles, while the incubation temperature exhibits negligible effect. This work not only deepens our understanding on the packing property of PS molecules in self-assembled membranes and the formation mechanism of asymmetric vesicles but also practically provides a solution to regulate the asymmetric degree of the PS-containing vesicles using the established kinetic equation. In addition, the method would facilitate researches related to asymmetric vesicles or reconstruction of biological membranes.

Evaluation of Molecular Ordering in Bicelle Bilayer Membranes Based on Induced Circular Dichroism Spectra

Bicelles are submicrometer-sized disc-shaped molecular self-assemblies that can be obtained in aqueous solution by dispersing mixtures of certain amphiphiles. Although phospholipid bicelle and phospholipid vesicle assemblies adopt similar lipid bilayer structures, the differences in bilayer characteristics, especially physicochemical properties such as bilayer fluidity, are not clearly understood. Herein, we report the lipid ordering properties of bicelle bilayer membranes based on induced circular dichroism (ICD) and fluorescence polarization analyses using 1,6-diphenyl-1,3,5-hexatriene (DPH) as a probe. Bicelles were prepared by using 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and 1,2-dihexanoyl-sn-glycero-3-phosphocholine (DHPC), while pure DMPC vesicles and pure DHPC micelles were used as references. At temperatures below the phase transition temperature of DMPC, the bicelles showed lower membrane fluidities, whereas DHPC micelles showed higher membrane fluidity, suggesting no significant differences in bilayer fluidity between the bicelle and vesicle assemblies. The ICD signals of DPH were induced only when the membrane was in ordered (solid-ordered or ripple-gel) phases. In the bicelle systems, the ICD of DPH was more significant than that of the DMPC vesicle. The induced chirality of DPH was dependent on the chirality of the bilayer lipid. Compared to that of the DMPC/DHPC bicelle, the ICD of the 1,2-dipalmitoyl-sn-glycero-3-phosphocholine/DHPC bicelle was higher, while that of the bovine sphingomyelin/DHPC bicelle was lower. Because the lipids are tightly packed in the ordered phase, the ICD intensity reflects the molecular ordering state of the lipids in the bicelle bilayer.

Effect of Imidazolium-Based Ionic Liquids on the Structure and Phase Behavior of Palmitoyl-oleoyl-phosphatidylethanolamine

Among various applications, ionic liquids (ILs) have been used as antimicrobial agents in laboratories, possibly through induction of the leakage of bacteria. A molecular-level understanding of the mechanism that describes how ILs enhance the permeation of membranes is still lacking. In this study, the effects of imidazolium-based ILs with different alky chain lengths on the structure and phase behavior of 1-palmitoyl-2-oleoyl-phosphatidylethanolamine (POPE), which is a representative bacteria-membrane-rich lipid, have been investigated. By employing differential scanning calorimetry and synchrotron small- and wide-angle X-ray scattering techniques, we found that ILs with longer alkyl chains influenced the phase behavior more effectively, and lower IL concentrations are needed to induce phase separation for both lamellar liquid crystalline phase and nonlamellar inverted hexagonal phase of POPE. Interestingly, the IL with an alkyl chain of 12 carbon atoms ([C₁₂mim]Cl) shows a difference. It exhibits a stronger disturbing effect on the POPE bilayer structure than [C₁₆mim]Cl, indicating that the ability of ILs to influence the membrane structures is dependent not only on the alkyl chain length of ILs, but also on the degree of matching of the alkyl chain lengths of ILs and lipids. The new lamellar and nonlamellar structures induced by ILs both have smaller repeat distances than that of pure POPE, implying thinner membrane structures. Data of the fluorescence-based vesicle dye leakage assay are consistent with these results, particularly the defects caused by IL-induced phase separation can enhance the membrane permeability markedly. The present work may shed light on our understanding of the antimicrobial mechanism of ILs.

Regimes of Complex Lipid Bilayer Phases Induced by Cholesterol Concentration in MD Simulation

Cholesterol is essential to the formation of phase-separated lipid domains in membranes. Lipid domains can exist in different thermodynamic phases depending on the molecular composition and play significant roles in determining structure and function of membrane proteins. We investigate the role of cholesterol in the structure and dynamics of ternary lipid mixtures displaying phase separation using molecular dynamics simulations, employing a physiologically relevant span of cholesterol concentration. We find that cholesterol can induce formation of three regimes of phase behavior: 1) miscible liquid-disordered bulk, 2) phase-separated, domain-registered coexistence of liquid-disordered and liquid-ordered domains, and 3) phase-separated, domain-antiregistered coexistence of liquid-disordered and newly identified nanoscopic gel domains composed of cholesterol threads we name "cholesterolic gel" domains. These findings are validated and discussed in the context of current experimental knowledge, models of cholesterol spatial distributions, and models of ternary lipid-mixture phase separation.