Molecular organization of cholesterol in polyunsaturated membranes: microdomain formation

Cholesterol-dependent amyloid β production: space for multifarious interactions between amyloid precursor protein, secretases, and cholesterol

Amyloid β is considered a key player in the development and progression of Alzheimer's disease (AD). Many studies investigating the effect of statins on lowering cholesterol suggest that there may be a link between cholesterol levels and AD pathology. Since cholesterol is one of the most abundant lipid molecules, especially in brain tissue, it affects most membrane-related processes, including the formation of the most dangerous form of amyloid β, Aβ42. The entire Aβ production system, which includes the amyloid precursor protein (APP), β-secretase, and the complex of γ-secretase, is highly dependent on membrane cholesterol content. Moreover, cholesterol can affect amyloidogenesis in many ways. Cholesterol influences the stability and activity of secretases, but also dictates their partitioning into specific cellular compartments and cholesterol-enriched lipid rafts, where the amyloidogenic machinery is predominantly localized. The most complicated relationships have been found in the interaction between cholesterol and APP, where cholesterol affects not only APP localization but also the precise character of APP dimerization and APP processing by γ-secretase, which is important for the production of Aβ of different lengths. In this review, we describe the intricate web of interdependence between cellular cholesterol levels, cholesterol membrane distribution, and cholesterol-dependent production of Aβ, the major player in AD.

Membrane Models and Experiments Suitable for Studies of the Cholesterol Bilayer Domains

Cholesterol (Chol) is an essential component of animal cell membranes and is most abundant in plasma membranes (PMs) where its concentration typically ranges from 10 to 30 mol%. However, in red blood cells and Schwann cells, PMs Chol content is as high as 50 mol%, and in the PMs of the eye lens fiber cells, it can reach up to 66 mol%. Being amphiphilic, Chol molecules are easily incorporated into the lipid bilayer where they affect the membrane lateral organization and transmembrane physical properties. In the aqueous phase, Chol cannot form free bilayers by itself. However, pure Chol bilayer domains (CBDs) can form in lipid bilayer membranes with the Chol content exceeding 50 mol%. The range of Chol concentrations surpassing 50 mol% is less frequent in biological membranes and is consequently less investigated. Nevertheless, it is significant for the normal functioning of the eye lens and understanding how Chol plaques form in atherosclerosis. The most commonly used membrane models are unilamellar and multilamellar vesicles (MLVs) and supported lipid bilayers (SLBs). CBDs have been observed directly using confocal microscopy, X-ray reflectometry and saturation recovery electron paramagnetic resonance (SR EPR). Indirect evidence of CBDs has also been reported by using atomic force microscopy (AFM) and fluorescence recovery after photobleaching (FRAP) experiments. The overall goal of this review is to demonstrate the advantages and limitations of the various membrane models and experimental techniques suitable for the detection and investigation of the lateral organization, function and physical properties of CBDs.

Cholesterol solubility in mixed DMPE/DMPC bilayers as determined by small angle X-ray scattering

Small angle X-ray scattering measurements under ambient conditions (T ≈ 294 K) provide evidence for the formation of separate domains in a ternary, mixed phospholipid ([DMPE]/[DMPC] = 3/1) / cholesterol model bilayer membrane. As we interpret these results, the domains contain cholesterol and DMPC, with which cholesterol is known to preferentially interact in a binary model membrane (solubility limit, mol fraction cholesterol 0.5), as compared to DMPE (solubility limit, mol fraction cholesterol 0.45). The solubility limit for the ternary system is mol fraction cholesterol 0.2-0.3. Although literature EPR spectra find that non-crystalline, cholesterol bilayer domains may be present even prior to the observation of cholesterol crystal diffraction, X-ray scattering cannot detect their presence.

Biophysics of Membrane Stiffening by Cholesterol and Phosphatidylinositol 4,5-bisphosphate (PIP2)

Cell membranes regulate a wide range of phenomena that are implicated in key cellular functions. Cholesterol, a critical component of eukaryotic cell membranes, is responsible for cellular organization, membrane elasticity, and other critical physicochemical parameters. Besides cholesterol, other lipid components such as phosphatidylinositol 4,5-bisphosphate (PIP2) are found in minor concentrations in cell membranes yet can also play a major regulatory role in various cell functions. In this chapter, we describe how solid-state deuterium nuclear magnetic resonance (²H NMR) spectroscopy together with neutron spin-echo (NSE) spectroscopy can inform synergetic changes to lipid molecular packing due to cholesterol and PIP2 that modulate the bending rigidity of lipid membranes. Fundamental structure-property relations of molecular self-assembly are illuminated and point toward a length and time-scale dependence of cell membrane mechanics, with significant implications for biological activity and membrane lipid-protein interactions.

Cholesterol crystals and atherosclerotic plaque instability: Therapeutic potential of Eicosapentaenoic acid

Atherosclerotic plaques associated with acute coronary syndromes (ACS), i.e. culprit lesions, frequently feature a ruptured fibrous cap with thrombotic complications. On imaging, these plaques exhibit a low attenuation, lipid-rich, necrotic core containing cholesterol crystals and are inherently unstable. Indeed, cholesterol crystals are causally associated with plaque vulnerability in vivo; their formation results from spontaneous self-assembly of cholesterol molecules. Cholesterol homeostasis is a central determinant of the physicochemical conditions leading to crystal formation, which are favored by elevated membrane free cholesterol content in plaque endothelial cells, smooth muscle cells, monocyte-derived macrophages, and foam cells, and equally by lipid oxidation. Emerging evidence from imaging trials in patients with coronary heart disease has highlighted the impact of intervention involving the omega-3 fatty acid, eicosapentaenoic acid (EPA), on vulnerable, low attenuation atherosclerotic plaques. Thus, EPA decreased features associated with unstable plaque by increasing fibrous cap thickness in statin-treated patients, by reducing lipid volume and equally attenuating intraplaque inflammation. Importantly, atherosclerotic plaques rapidly incorporate EPA; indeed, a high content of EPA in plaque tissue is associated with decreased plaque inflammation and increased stability. These findings are entirely consistent with the major reduction seen in cardiovascular events in the REDUCE-IT trial, in which high dose EPA was administered as its esterified precursor, icosapent ethyl (IPE); moreover, clinical benefit was proportional to circulating EPA levels. Eicosapentaenoic acid is efficiently incorporated into phospholipids, where it modulates cholesterol-enriched domains in cell membranes through physicochemical lipid interactions and changes in rates of lipid oxidation. Indeed, biophysical analyses indicate that EPA exists in an extended conformation in membranes, thereby enhancing normal cholesterol distribution while reducing propagation of free radicals. Such effects mitigate cholesterol aggregation and crystal formation. In addition to its favorable effect on cholesterol domain structure, EPA/IPE exerts pleiotropic actions, including antithrombotic, antiplatelet, anti-inflammatory, and proresolving effects, whose plaque-stabilizing potential cannot be excluded. Docosahexaenoic acid is distinguished from EPA by a higher degree of unsaturation and longer carbon chain length; DHA is thus predisposed to changes in its conformation with ensuing increase in membrane lipid fluidity and promotion of cholesterol aggregation into discrete domains. Such distinct molecular effects between EPA and DHA are pronounced under conditions of high cellular cholesterol content and oxidative stress. This review will focus on the formation and role of cholesterol monohydrate crystals in destabilizing atherosclerotic plaques, and on the potential of EPA as a therapeutic agent to attenuate the formation of deleterious cholesterol membrane domains and of cholesterol crystals. Such a therapeutic approach may translate to enhanced plaque stability and ultimately to reduction in cardiovascular risk.

Cholesterol Stiffening of Lipid Membranes

Biomembrane order, dynamics, and other essential physicochemical parameters are controlled by cholesterol, a major component of mammalian cell membranes. Although cholesterol is well known to exhibit a condensing effect on fluid lipid membranes, the extent of stiffening that occurs with different degrees of lipid acyl chain unsaturation remains an enigma. In this review, we show that cholesterol locally increases the bending rigidity of both unsaturated and saturated lipid membranes, suggesting there may be a length-scale dependence of the bending modulus. We review our published data that address the origin of the mechanical effects of cholesterol on unsaturated and polyunsaturated lipid membranes and their role in biomembrane functions. Through a combination of solid-state deuterium NMR spectroscopy and neutron spin-echo spectroscopy, we show that changes in molecular packing cause the universal effects of cholesterol on the membrane bending rigidity. Our findings have broad implications for the role of cholesterol in lipid–protein interactions as well as raft-like mixtures, drug delivery applications, and the effects of antimicrobial peptides on lipid membranes.

A biological rationale for the disparate effects of omega-3 fatty acids on cardiovascular disease outcomes

The omega-3 fatty acids (n3-FAs) eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) rapidly incorporate into cell membranes where they modulate signal transduction pathways, lipid raft formation, and cholesterol distribution. Membrane n3-FAs also form specialized pro-resolving mediators and other intracellular oxylipins that modulate inflammatory pathways, including T-cell differentiation and gene expression. Cardiovascular (CV) trials have shown that EPA, administered as icosapent ethyl (IPE), reduces composite CV events, along with plaque volume, in statin-treated, high-risk patients. Mixed EPA/DHA regimens have not shown these benefits, perhaps as the result of differences in formulation, dosage, or potential counter-regulatory actions of DHA. Indeed, EPA and DHA have distinct, tissue-specific effects on membrane structural organization and cell function. This review summarizes: (1) results of clinical outcome and imaging trials using n3-FA formulations; (2) membrane interactions of n3-FAs; (3) effects of n3-FAs on membrane oxidative stress and cholesterol crystalline domain formation during hyperglycemia; (4) n3-FA effects on endothelial function; (5) role of n3-FA-generated metabolites in inflammation; and (6) ongoing and future clinical investigations exploring treatment targets for n3-FAs, including COVID-19.

Order vs. Disorder: Cholesterol and Omega-3 Phospholipids Determine Biomembrane Organization

Lipid structural diversity strongly affects biomembrane chemico-physical and structural properties in addition to membrane-associated events. At high concentrations, cholesterol increases membrane order and rigidity, while polyunsaturated lipids are reported to increase disorder and flexibility. How these different tendencies balance in composite bilayers is still controversial. In this study, electron paramagnetic resonance spectroscopy, small angle neutron scattering, and neutron reflectivity were used to investigate the structural properties of cholesterol-containing lipid bilayers in the fluid state with increasing amounts of polyunsaturated omega-3 lipids. Either the hybrid 1-stearoyl-2-docosahexaenoyl-sn-glycero-3-phosphocholine or the symmetric 1,2-docosahexaenoyl-sn-glycero-3-phosphocholine were added to the mixture of the naturally abundant 1-palmitoyl-2-oleyl-sn-glycero-3-phosphocholine and cholesterol. Our results indicate that the hybrid and the symmetric omega-3 phospholipids affect the microscopic organization of lipid bilayers differently. Cholesterol does not segregate from polyunsaturated phospholipids and, through interactions with them, is able to suppress the formation of non-lamellar structures induced by the symmetric polyunsaturated lipid. However, this order/disorder balance leads to a bilayer whose structural organization cannot be ascribed to either a liquid ordered or to a canonical liquid disordered phase, in that it displays a very loose packing of the intermediate segments of lipid chains.

EPA and DHA containing phospholipids have contrasting effects on membrane structure

Omega-3 FAs EPA and DHA influence membrane fluidity, lipid rafts, and signal transduction. A clinical trial, Reduction of Cardiovascular Events with Icosapent Ethyl—Intervention Trial, demonstrated that high-dose EPA (4 g/d icosapent ethyl) reduced composite cardiovascular events in statin-treated high-risk patients. EPA benefits correlated with on-treatment levels, but similar trials using DHA-containing formulations did not show event reduction. We hypothesized that differences in clinical efficacy of various omega-3 FA preparations could result from differential effects on membrane structure. To test this, we used small-angle X-ray diffraction to compare 1-palmitoyl-2-eicosapentaenoyl-sn-glycero-3-phosphocholine (PL-EPA), 1-palmitoyl-2-docosahexaenoyl-sn-glycero-3-phosphocholine (PL-DHA), and 1-palmitoyl-2-arachidonoyl-sn-glycero-3-phosphocholine (PL-AA) in membranes with and without 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and cholesterol. Electron density profiles (electrons/ų vs. Å) were used to determine membrane structure, including membrane width (d-space). PL-EPA and PL-DHA had similar membrane structures without POPC and/or cholesterol but had contrasting effects in the presence of POPC and cholesterol. PL-EPA increased membrane hydrocarbon core electron density over an area of ±0–10 Å from the center, indicating an extended orientation. PL-DHA increased electron density in the phospholipid head group region, concomitant with disordering in the hydrocarbon core and a similar d-space (58 Å). Adding equimolar amounts of PL-EPA and PL-DHA produced changes that were attenuated compared with their separate effects. PL-AA increased electron density centered ±12 Å from the membrane center. The contrasting effects of PL-EPA, PL-DHA, and PL-AA on membrane structure may contribute to differences observed in the biological activities and clinical actions of various omega-3 FAs.

Omega-3 PUFA profoundly affect neural, physiological, and behavioural competences - implications for systemic changes in trophic interactions

In recent decades, much conceptual thinking in trophic ecology has been guided by theories of nutrient limitation and the flow of elements, such as carbon and nitrogen, within and among ecosystems. More recently, ecologists have also turned their attention to examining the value of specific dietary nutrients, in particular polyunsaturated fatty acids (PUFA), among which the omega-3 PUFA, especially eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) play a central role as essential components of neuronal cell membranes in many organisms. This review focuses on a new neuro-ecological approach stemming from the biochemical (mechanistic) and physiological (functional) role of DHA in neuronal cell membranes, in particular in conjunction with G-protein coupled receptors (GPCRs). We link the co-evolution of these neurological functions to metabolic dependency on dietary omega-3 PUFA. We outline ways in which deficiencies in dietary DHA supply may affect, cognition, vision, and behaviour, and ultimately, the biological fitness of consumers. We then review emerging evidence that changes in access to dietary omega-3 PUFA may ultimately have profound impacts on trophic interactions leading to potential changes in community structure and ecosystem functioning that, in turn, may affect the supply of DHA within and across ecosystems, including the supply for human consumption.

Hypothetical Pathway for Formation of Cholesterol Microcrystals Initiating the Atherosclerotic Process

Major factors leading to the development of atherosclerosis are a high cholesterol (Chol) level in the blood and oxidative stress. Both promote the formation of Chol microcrystals in blood vessel walls. Deposition of Chol microcrystals in arterial intima causes inflammation, which initiates and accompanies the atherosclerotic process in all its phases. One of the possible sources of Chol in the blood vessel walls is oxidized low-density lipoproteins-this atherosclerotic plaque formation pathway has already been described in the literature. Here, we hypothesize that initiation of the atherosclerotic process may involve Chol domains in the plasma membranes of arterial cells. Increased Chol content and the presence of polyunsaturated phospholipids in these membranes together with oxidative stress (phospholipid peroxidation) may lead to the formation of pure Chol bilayer domains that, with further peroxidation and increased Chol content, may collapse in the form of Chol seed crystals. Independent of their origin, Chol microcrystals activate inflammasomes, thereby stimulate immune responses, and initiate inflammation that may lead to the development of atherosclerosis. This new, hypothetical pathway has not yet been investigated in depth; however, data from the literature and our own results support its feasibility.

Why Is Very High Cholesterol Content Beneficial for the Eye Lens but Negative for Other Organs?

The plasma membranes of the human lens fiber cell are overloaded with cholesterol that not only saturates the phospholipid bilayer of these membranes but also leads to the formation of pure cholesterol bilayer domains. Cholesterol level increases with age, and for older persons, it exceeds the cholesterol solubility threshold, leading to the formation of cholesterol crystals. All these changes occur in the normal lens without too much compromise to lens transparency. If the cholesterol content in the cell membranes of other organs increases to extent where cholesterol crystals forma, a pathological condition begins. In arterial cells, minute cholesterol crystals activate inflammasomes, induce inflammation, and cause atherosclerosis development. In this review, we will indicate possible factors that distinguish between beneficial and negative cholesterol action, limiting cholesterol actions to those performed through cholesterol in cell membranes and by cholesterol crystals.

Nonlamellar-Phase-Promoting Colipids Enhance Segregation of Palmitoyl Ceramide in Fluid Bilayers

Ceramide is an important intermediate in sphingolipid homeostasis. We examined how colipids, with negative intrinsic curvature and which may induce curvature stress in the bilayers, affected the segregation of palmitoyl ceramide (PCer). Such colipids include 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE), 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE), and tetra-linoleoyl cardiolipin (CL). In 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) bilayers, PCer formed ordered, gel-like domains at concentrations above 10 mol% at 23°C, as evidenced by the change in the average lifetime of the trans-parinaric acid emission. When POPE or DOPE were included in the DOPC bilayer (at 20:80 or 40:60 POPE or DOPE to DOPC, by mol), the lateral segregation of PCer was facilitated in a concentration-dependent manner, and less PCer was required for the formation of the ordered ceramide-rich domains. Inclusion of CL in the DOPE bilayer (at 10:90 or 20:80 CL to PC, by mol) also caused a similar facilitation of the lateral segregation of PCer. The PCer-rich domains formed in the presence of POPE, DOPE, or CL in DOPC bilayers were slightly more thermostable (by 2-10°C) when compared to PCer-rich domains in DOPC-only bilayers. Nonlamellar phases were not present in bilayers in which the effects of POPE or DOPE on PCer segregation were the largest, as verified by ³¹P NMR. When palmitoyl sphingomyelin was added to the different bilayer compositions at 5 mol%, relative to the phospholipids, PCer segregated into gel domains at lower concentrations (2-3 mol% PCer), and the effect of POPE on PCer segregation was eliminated. We suggest that the effects of POPE, DOPE, and CL on PCer segregation was in part influenced by their effects on membrane curvature stress and in part because of unfavorable interactions with PCer due to their unsaturated acyl chains. These lipids are abundant in mitochondrial membranes and are likely to affect functional properties of saturated ceramides in them.

The Production of Matchout-Deuterated Cholesterol and the Study of Bilayer-Cholesterol Interactions

The deuteration of biomolecules provides advanced opportunities for neutron scattering studies. For low resolution studies using techniques such as small-angle neutron scattering and neutron reflection, the level of deuteration of a sample can be varied to match the scattering length density of a specific D₂O/H₂O solvent mixture. This can be of major value in structural studies where specific regions of a complex system can be highlighted, and others rendered invisible. This is especially useful in analyses of the structure and dynamics of membrane components. In mammalian membranes, the presence of cholesterol is crucial in modulating the properties of lipids and in their interaction with proteins. Here, a protocol is described for the production of partially deuterated cholesterol which has a neutron scattering length density that matches that of 100% D₂O solvent (hereby named matchout cholesterol). The level of deuteration was determined by mass spectrometry and nuclear magnetic resonance. The cholesterol match-point was verified experimentally using small angle neutron scattering. The matchout cholesterol was used to investigate the incorporation of cholesterol in various phosphatidylcholine supported lipid bilayers by neutron reflectometry. The study included both saturated and unsaturated lipids, as well as lipids with varying chain lengths. It was found that cholesterol is distributed asymmetrically within the bilayer, positioned closer to the headgroups of the lipids than to the middle of the tail core, regardless of the phosphatidylcholine species.

Cholesterol in phospholipid bilayers: positions and orientations inside membranes with different unsaturation degrees

Cholesterol is an essential component of all animal cell membranes and plays an important role in maintaining the membrane structure and physical-chemical properties necessary for correct cell functioning. The presence of cholesterol is believed to be responsible for domain formation (lipid rafts) due to different interactions of cholesterol with saturated and unsaturated lipids. In order to get detailed atomistic insight into the behaviour of cholesterol in bilayers composed of lipids with varying degrees of unsaturation, we have carried out a series of molecular dynamics simulations of saturated and polyunsaturated lipid bilayers with different contents of cholesterol, as well as well-tempered metadynamics simulations with a single cholesterol molecule in these bilayers. From these simulations we have determined distributions of cholesterol across the bilayer, its orientational properties, free energy profiles, and specific interactions of molecular groups able to form hydrogen bonds. Both molecular dynamics and metadynamics simulations showed that the most unsaturated bilayer with 22:6 fatty acid chains shows behaviour which is most different from other lipids. In this bilayer, cholesterol is relatively often found in a "flipped" configuration with the hydroxyl group oriented towards the membrane middle plane. This bilayer has also the highest (least negative) binding free energy among liquid phase bilayers, and the lowest reorientation barrier. Furthermore, cholesterol molecules in this bilayer are often found to form head-to-tail contacts which may lead to specific clustering behaviour. Overall, our simulations support ideas that there can be a subtle interconnection between the contents of highly unsaturated fatty acids and cholesterol, deficiency or excess of each of them is related to many human afflictions and diseases.

Permeability and Line-Tension-Dependent Response of Polyunsaturated Membranes to Osmotic Stresses

The lipidome of plant plasma membranes-enriched in cellular phospholipids containing at least one polyunsaturated fatty acid tail and a variety of phytosterols and phytosphingolipids-is adapted to significant abiotic stresses. But how mesoscale membrane properties of these membranes such as permeability and deformability, which arise from their unique molecular compositions and corresponding lateral organization, facilitate response to global mechanical stresses is largely unknown. Here, using giant vesicles reconstituting mixtures of polyunsaturated lipids (soy phosphatidylcholine), glucosylceramide, and sitosterol common to plant membranes, we find that the membranes adopt "janus-like" domain morphologies and display anomalous solute permeabilities. The former textures the membrane with a single sterol-glucosylceramide-enriched, liquid-ordered domain separated from a liquid-disordered phase consisting primarily of soy phosphatidylcholine. When subject to osmotic downshifts, the giant unilamellar vesicles (GUVs) respond by transiently producing well-known swell-burst cycles. In each cycle, the influx of water swells the GUV, rendering the membrane tense. Subsequent rupture of the membrane through transient poration, which localizes in the liquid-disordered phase or at the domain boundaries, reduces the osmotic stress by expelling some of the excess osmolytes (and solvent) before sealing. When subject to abrupt hypertonic stress, they deform by nucleating buds at the domain phase boundaries. Remarkably, this incipient vesiculation is reversed in a statistically significant fraction of GUVs because of the interplay with solute permeation timescales, which render osmotic stresses short-lived. This, then, suggests a novel control mechanism in which an interplay of permeability and deformability regulates osmotically induced membrane deformation and limits vesiculation-induced loss of membrane material. Interestingly, recapitulation of such dynamic morphological reconfigurability-switching between budded and nonbudded morphologies-due to the interplay of membrane permeability, which temporally reverses the osmotic gradient, and domain boundaries, which select modes of deformations, might prove valuable in endowing synthetic cells with novel morphological responsiveness.

Eicosapentaenoic acid and docosahexaenoic acid have distinct membrane locations and lipid interactions as determined by X-ray diffraction

Eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) differentially influence lipid oxidation, signal transduction, fluidity, and cholesterol domain formation, potentially due in part to distinct membrane interactions. We used small angle X-ray diffraction to evaluate the EPA and DHA effects on membrane structure. Membrane vesicles composed of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) and cholesterol (C) (0.3C:POPC mole ratio) were prepared and treated with vehicle, EPA, or DHA (1:10 mol ratio to POPC). Electron density profiles generated from the diffraction data showed that EPA increased membrane hydrocarbon core electron density over a broad area, up to ± 20 Å from the membrane center, indicating an energetically favorable extended orientation for EPA likely stabilized by van der Waals interactions. By contrast, DHA increased electron density in the phospholipid head group region starting at ± 12 Å from the membrane center, presumably due to DHA-surface interactions, with coincident reduction in electron density in the membrane hydrocarbon core centered ± 7-9 Å from the membrane center. The membrane width (d-space) decreased by 5 Å in the presence of vehicle as the temperature increased from 10 °C to 30 °C due to increased acyl chain trans-gauche isomerizations, which was unaffected by addition of EPA or DHA. The influence of DHA on membrane structure was modulated by temperature changes while the interactions of EPA were unaffected. The contrasting EPA and DHA effects on membrane structure indicate distinct molecular locations and orientations that may contribute to observed differences in biological activity.

Myocyte membrane and microdomain modifications in diabetes: determinants of ischemic tolerance and cardioprotection

Cardiovascular disease, predominantly ischemic heart disease (IHD), is the leading cause of death in diabetes mellitus (DM). In addition to eliciting cardiomyopathy, DM induces a ‘wicked triumvirate’: (i) increasing the risk and incidence of IHD and myocardial ischemia; (ii) decreasing myocardial tolerance to ischemia–reperfusion (I–R) injury; and (iii) inhibiting or eliminating responses to cardioprotective stimuli. Changes in ischemic tolerance and cardioprotective signaling may contribute to substantially higher mortality and morbidity following ischemic insult in DM patients. Among the diverse mechanisms implicated in diabetic impairment of ischemic tolerance and cardioprotection, changes in sarcolemmal makeup may play an overarching role and are considered in detail in the current review. Observations predominantly in animal models reveal DM-dependent changes in membrane lipid composition (cholesterol and triglyceride accumulation, fatty acid saturation vs. reduced desaturation, phospholipid remodeling) that contribute to modulation of caveolar domains, gap junctions and T-tubules. These modifications influence sarcolemmal biophysical properties, receptor and phospholipid signaling, ion channel and transporter functions, contributing to contractile and electrophysiological dysfunction, cardiomyopathy, ischemic intolerance and suppression of protective signaling. A better understanding of these sarcolemmal abnormalities in types I and II DM (T1DM, T2DM) can inform approaches to limiting cardiomyopathy, associated IHD and their consequences. Key knowledge gaps include details of sarcolemmal changes in models of T2DM, temporal patterns of lipid, microdomain and T-tubule changes during disease development, and the precise impacts of these diverse sarcolemmal modifications. Importantly, exercise, dietary, pharmacological and gene approaches have potential for improving sarcolemmal makeup, and thus myocyte function and stress-resistance in this ubiquitous metabolic disorder.

Concepts and Methods of Solid-State NMR Spectroscopy Applied to Biomembranes

Concepts of solid-state NMR spectroscopy and applications to fluid membranes are reviewed in this paper. Membrane lipids with ²H-labeled acyl chains or polar head groups are studied using ²H NMR to yield knowledge of their atomistic structures in relation to equilibrium properties. This review demonstrates the principles and applications of solid-state NMR by unifying dipolar and quadrupolar interactions and highlights the unique features offered by solid-state ²H NMR with experimental illustrations. For randomly oriented multilamellar lipids or aligned membranes, solid-state ²H NMR enables direct measurement of residual quadrupolar couplings (RQCs) due to individual C-²H-labeled segments. The distribution of RQC values gives nearly complete profiles of the segmental order parameters S_CD⁽ⁱ⁾ as a function of acyl segment position (i). Alternatively, one can measure residual dipolar couplings (RDCs) for natural abundance lipid samples to obtain segmental S_CH order parameters. A theoretical mean-torque model provides acyl-packing profiles representing the cumulative chain extension along the normal to the aqueous interface. Equilibrium structural properties of fluid bilayers and various thermodynamic quantities can then be calculated, which describe the interactions with cholesterol, detergents, peptides, and integral membrane proteins and formation of lipid rafts. One can also obtain direct information for membrane-bound peptides or proteins by measuring RDCs using magic-angle spinning (MAS) in combination with dipolar recoupling methods. Solid-state NMR methods have been extensively applied to characterize model membranes and membrane-bound peptides and proteins, giving unique information on their conformations, orientations, and interactions in the natural liquid-crystalline state.

The Affinity of Cholesterol for Different Phospholipids Affects Lateral Segregation in Bilayers

Saturated and unsaturated phospholipids (PLs) can segregate into lateral domains. The preference of cholesterol for saturated acyl chains over monounsaturated, and especially polyunsaturated ones, may also affect lateral segregation. Here we have studied how cholesterol influenced the lateral segregation of saturated and unsaturated PLs, for which cholesterol had a varying degree of affinity. The fluorescence lifetime of trans-parinaric acid reported the formation of ordered domains (gel or liquid-ordered (lo)) in bilayers composed of different unsaturated phosphatidylcholines, and dipalmitoyl-phosphatidylcholine or n-palmitoyl-sphingomyelin, in the presence or absence of cholesterol. We observed that cholesterol facilitated lateral segregations and the degree of facilitation correlated with the relative affinity of cholesterol for the different PLs in the bilayers. Differential scanning calorimetry and (2)H nuclear magnetic resonance showed that cholesterol increased the thermostability of both the gel and lo-domains. Increased number of double bonds in the unsaturated PL increased the order in the lo-domains, likely by enriching the ordered domains in saturated lipids and cholesterol. This supported the conclusions from the trans-parinaric acid experiments, and offers insight into how cholesterol facilitated lateral segregation. In conclusion, the relative affinity of cholesterol for different PLs appears to be an important determinant for the formation of ordered domains. Our data suggests that knowledge of the affinity of cholesterol for the different PLs in a bilayer allows prediction of the degree to which the sterol promotes lo-domain formation.

Molecular Insight into the Line Tension of Bilayer Membranes Containing Hybrid Polyunsaturated Lipids

Line tension (γ) is a key parameter for the structure and dynamics of membrane domains. It was proposed that hybrid lipids, with mixed saturated and unsaturated acyl chains, participate in the relaxation of γ through different mechanisms. In this work, we used molecular dynamics simulations of the coarse-grained MARTINI model to measure γ in liquid-ordered-liquid-disordered (Lo-Ld) membranes, with increasingly larger relative proportion of the hybrid polyunsaturated lipid PAPC (4:0-5:4PC) to DAPC (di5:4PC) (i.e., X_H). We also calculated an elastic contribution to γ by the Lo-Ld thickness mismatch, tilt moduli, and bending moduli, as predicted by theory. We found that an increase in X_H decreased the overall γ value and the elastic contribution to line tension. The effect on the elastic line tension is driven by a reduced hydrophobic mismatch. Changes in the elastic constants of the phases due to an increase in X_H produced a slightly larger elastic γ term. In addition to this elastic energy, other major contributions to γ are found in these model membranes. Increasing X_H decreases both elastic and nonelastic contributions to γ. Finally, PAPC also behaves as a linactant, relaxing γ through an interfacial effect, as predicted by theoretical results. This study gives insight into the actual contribution of distinct energy terms to γ in bilayers containing polyunsaturated hybrid lipids.

Depletion with Cyclodextrin Reveals Two Populations of Cholesterol in Model Lipid Membranes

Recent results provide evidence that cholesterol is highly accessible for removal from both cell and model membranes above a threshold concentration that varies with membrane composition. Here we measured the rate at which methyl-β-cyclodextrin depletes cholesterol from a supported lipid bilayer as a function of cholesterol mole fraction. We formed supported bilayers from two-component mixtures of cholesterol and a PC (phosphatidylcholine) lipid, and we directly visualized the rate of decrease in area of the bilayers with fluorescence microscopy. Our technique yields the accessibility of cholesterol over a wide range of concentrations (30-66 mol %) for many individual bilayers, enabling fast acquisition of replicate data. We found that the bilayers contain two populations of cholesterol, one with low surface accessibility and the other with high accessibility. A larger fraction of the total membrane cholesterol appears in the more accessible population when the acyl chains of the PC-lipid tails are more unsaturated. Our findings are most consistent with the predictions of the condensed-complex and cholesterol bilayer domain models of cholesterol-phospholipid interactions in lipid membranes.

Lipid bilayer thickness determines cholesterol's location in model membranes

Cholesterol is an essential biomolecule of animal cell membranes, and an important precursor for the biosynthesis of certain hormones and vitamins. It is also thought to play a key role in cell signaling processes associated with functional plasma membrane microdomains (domains enriched in cholesterol), commonly referred to as rafts. In all of these diverse biological phenomena, the transverse location of cholesterol in the membrane is almost certainly an important structural feature. Using a combination of neutron scattering and solid-state ²H NMR, we have determined the location and orientation of cholesterol in phosphatidylcholine (PC) model membranes having fatty acids of different lengths and degrees of unsaturation. The data establish that cholesterol reorients rapidly about the bilayer normal in all the membranes studied, but is tilted and forced to span the bilayer midplane in the very thin bilayers. The possibility that cholesterol lies flat in the middle of bilayers, including those made from PC lipids containing polyunsaturated fatty acids (PUFAs), is ruled out. These results support the notion that hydrophobic thickness is the primary determinant of cholesterol's location in membranes.

Domain Stability in Biomimetic Membranes Driven by Lipid Polyunsaturation

Biological membranes contain a broad variety of lipid species whose individual physicochemical properties and collective interactions ultimately determine membrane organization. A key aspect of the organization of cellular membranes is their lateral subdivision into domains of distinct structure and composition. The most widely studied membrane domains are lipid rafts, which are the biological manifestations of liquid-ordered phases that form in sterol-containing membranes. Detailed studies of biomimetic membrane mixtures have yielded wide-ranging insights into the physical principles behind lipid rafts; however, these simplified models do not fully capture the diversity and complexity of the mammalian lipidome, most notably in their exclusion of polyunsaturated lipids. Here, we assess the role of lipid acyl chain unsaturation as a driving force for phase separation using coarse-grained molecular dynamics (CGMD) simulations validated by model membrane experiments. The clear trends in our observations and good qualitative agreements between simulations and experiments support the conclusions that highly unsaturated lipids promote liquid-liquid domain stability by enhancing the differences in cholesterol content and lipid chain order between the coexisting domains. These observations reveal the important role of noncanonical biological lipids in the physical properties of membranes, showing that lipid polyunsaturation is a driving force for liquid-liquid phase separation.

Effect of Sterol Structure on the Physical Properties of 1-Palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine Membranes Determined Using (2)H Nuclear Magnetic Resonance

The effect of a series of phytosterols on lipid chain ordering in 1-palmitoyl((2)H31)-2-oleoyl-sn-glycero-3-phosphocholine (POPC-d31) multibilayer vesicles was examined by (2)H NMR spectroscopy at 25 °C. These results, along with existing data for other sterols, indicate that the ordering power of sterols in POPC-d31 depends on subtle aspects of sterol structure. Cholesterol, 7-dehydrocholesterol (7-DHC), campesterol, β-sitosterol, ergosterol, brassicasterol, and stigmasterol all increase the lipid chain order as sterol concentration is increased. However, saturation of the ordering occurs at different sterol concentrations for ergosterol (as previously reported), brassicasterol, β-sitosterol, and stigmasterol. Here our interest lies in finding which part of the sterol structure is responsible for the observed saturation of the palmitoyl chain order as a function of sterol concentration. In particular, we propose that the saturation of the ordering of POPC-d31/brassicasterol and POPC-d31/stigmasterol membranes at quite low sterol concentrations is due to the presence of a double bond at C22. We also discuss how the structural differences between the sterols affect their ability to intercalate between the POPC acyl chains. Furthermore, the effective solubility of sterols in POPC is discussed in relation to the dependence of maximum POPC-d31 chain order vs sterol concentration.

Effect of Phosphatidylcholine Unsaturation on the Lateral Segregation of Palmitoyl Ceramide and Palmitoyl Dihydroceramide in Bilayer Membranes

To better understand the interactions of saturated ceramides with unsaturated glycerophospholipids in bilayer membranes, we measured how palmitoyl ceramide (PCer) and dihydroceramide (dihydro-PCer, lacking the trans 4 double bond of the sphingoid base of ceramide) can interact with phosphatidylcholines (PCs) with palmitic acid in the sn-1 position and increasingly unsaturated acyl chains in the sn-2 position. The PCs were 16:0/18:1 (POPC), 16:0/18:2 (PLPC), 16:0/20:4 (PAPC), and 16:0(22:6 (PDPC). We also included di-18:1-PC (DOPC) to compare it with POPC. Because the ceramides were expected to segregate laterally to an ordered ceramide-rich phase, we determined the formation of the ordered phase using lifetime analysis of trans-parinaric acid (tPA) fluorescence. The presence of ordered domains, as indicated by tPA lifetime analysis, was verified by an analysis of tPA anisotropy as a function of temperature. The interaction between PCer and POPC was clearly more favored than interactions with DOPC, as seen from a more thermostable gel phase in POPC than in DOPC at equal ceramide content. The concentration needed for PCer gel phase formation was also lower in POPC than in the DOPC bilayers, suggesting that POPC had better miscibility in the ordered phase. The increased unsaturation of the sn-2 acyl chains of the PCs had more clear effects of dihydro-PCer segregation than on PCer segregation, and the dihydro-PCer gel phase became more thermostable as the unsaturation in the PC increased. We conclude that the interactions between ceramides and PCs were complex and affected both by the trans 4 double bond of PCer by the palmitoyl acyl in the sn-1 position and by the overall degree of unsaturation of the sn-2 acyl chain of the PCs.

Metabolic Precursor of Cholesterol Causes Formation of Chained Aggregates of Liquid-Ordered Domains

7-Dehydrocholesterol, an immediate metabolic predecessor of cholesterol, can accumulate in tissues due to some metabolic abnormalities, causing an array of symptoms known as Smith-Lemli-Opitz syndrome. Enrichment of cellular membranes with 7-dehydrocholesterol interferes with normal cell-signaling processes, which involve interaction between rafts and formation of the so-called signaling platforms. In model membranes, cholesterol-based ordered domains usually merge upon contact. According to our experimental data, ordered domains in the model systems where cholesterol is substituted for 7-dehydrocholesterol never merge on the time scale of the experiment, but clusterize into necklace-like aggregates. We attribute such different dynamical behavior to altered properties of the domain boundary. In the framework of thickness mismatch model, we analyzed changes of interaction energy profiles of two approaching domains caused by substitution of cholesterol by 7-dehydrocholesterol. The energy barrier for domain merger is shown to increase notably, with simultaneous appearance of another distinct local energy minimum. Such energy profile is in perfect qualitative agreement with the experimental observations. The observed change of domain dynamics can impair proper interaction between cellular rafts underlying pathologies associated with deviations in cholesterol metabolism.

Phase diagram of a polyunsaturated lipid mixture: Brain sphingomyelin/1-stearoyl-2-docosahexaenoyl-sn-glycero-3-phosphocholine/cholesterol

Phospholipids having a polyunsaturated acyl chain are abundant in biological membranes, but their behavior in lipid mixtures is difficult to study. Here we elucidate the nature of such mixtures with this report of the first ternary phase diagram containing the polyunsaturated lipid SDPC in mixtures of BSM/SDPC/Chol (brain sphingomyelin/1-stearoyl-2-docosahexaenoyl-sn-glycero-3-phosphocholine/cholesterol). These mixtures show coexisting macroscopic liquid-disordered (Ld) and liquid-ordered (Lo) phase separation, with phase boundaries determined by FRET and by fluorescence microscopy imaging of giant unilamellar vesicles (GUVs). Surprisingly, SDPC mixes with BSM/Chol similarly to how DOPC and POPC mix with BSM/Chol. Notably, intermediate states are produced within the Ld+Lo liquid-liquid immiscibility region upon addition of fourth component POPC. These mixtures of BSM/SDPC/POPC/Chol exhibit nanoscopic Ld+Lo domains over a very large volume of composition space, possibly because Ld/Lo line tension is not high.

α-Tocopherol Is Well Designed to Protect Polyunsaturated Phospholipids: MD Simulations

The presumptive function for alpha-tocopherol (αtoc) in membranes is to protect polyunsaturated lipids against oxidation. Although the chemistry of the process is well established, the role played by molecular structure that we address here with atomistic molecular-dynamics simulations remains controversial. The simulations were run in the constant particle NPT ensemble on hydrated lipid bilayers composed of SDPC (1-stearoyl-2-docosahexaenoylphosphatidylcholine, 18:0-22:6PC) and SOPC (1-stearoyl-2-oleoylphosphatidylcholine, 18:0-18:1PC) in the presence of 20 mol % αtoc at 37°C. SDPC with SA (stearic acid) for the sn-1 chain and DHA (docosahexaenoic acid) for the sn-2 chain is representative of polyunsaturated phospholipids, while SOPC with OA (oleic acid) substituted for the sn-2 chain serves as a monounsaturated control. Solid-state (2)H nuclear magnetic resonance and neutron diffraction experiments provide validation. The simulations demonstrate that high disorder enhances the probability that DHA chains at the sn-2 position in SDPC rise up to the bilayer surface, whereby they encounter the chromanol group on αtoc molecules. This behavior is reflected in the van der Waals energy of interaction between αtoc and acyl chains, and illustrated by density maps of distribution for acyl chains around αtoc molecules that were constructed. An ability to more easily penetrate deep into the bilayer is another attribute conferred upon the chromanol group in αtoc by the high disorder possessed by DHA. By examining the trajectory of single molecules, we found that αtoc flip-flops across the SDPC bilayer on a submicrosecond timescale that is an order-of-magnitude greater than in SOPC. Our results reveal mechanisms by which the sacrificial hydroxyl group on the chromanol group can trap lipid peroxyl radicals within the interior and near the surface of a polyunsaturated membrane. At the same time, water-soluble reducing agents that regenerate αtoc can access the chromanol group when it locates at the surface.

Comment on "Cholesterol solubility limit in lipid membranes probed by small angle neutron scattering and MD simulations" by S. Garg et al., Soft Matter, 2014, 10, 9313

In a recent article, Garg et al. used neutron scattering techniques to determine the limiting amount of cholesterol which vesicles of either POPS or POPC can accommodate. This amount was called "the cholesterol solubility limit". In light of extensive literature on cholesterol phase separation in phospholipid bilayers, the way in which "solubility limit" is defined in this article and the conclusions derived are misleading and require some clarification.

Studying antibiotic-membrane interactions via X-ray diffraction and fluorescence microscopy

Antibiotic drug resistance is a serious issue for the treatment of bacterial infection. Understanding the resistance to antibiotics is a key issue for developing new drugs. We used penicillin and sulbactam as model antibiotics to study their interaction with model membranes. Cholesterol was used to target the membrane for comparison with the well-known insertion model. Lamellar X-ray diffraction (LXD) was used to determine membrane thickness using successive drug-to-lipid molar ratios. The aspiration method for a single giant unilamellar vesicle (GUV) was used to monitor the kinetic binding process of antibiotic-membrane interactions in an aqueous solution. Both penicillin and sulbactam are found positioned outside the model membrane, while cholesterol inserts perpendicularly into the hydrophobic region of the membrane in aqueous solution. This result provides structural insights for understanding the antibiotic-membrane interaction and the mechanism of antibiotics.

Novel Raman-tagged sphingomyelin that closely mimics original raft-forming behavior

Three Raman probes of sphingomyelin (SM) were synthesized and evaluated for their applicability to imaging experiments. One probe containing a hydroxymethyl-1,3-butadiyne moiety in the polar head group showed strong scattering. The solid-state (2)H NMR spectra of this probe in oriented bilayer membrane revealed excellent compatibility with natural SM in phase behavior since the probe undergoes phase separation to form raft-like liquid ordered (Lo) domains in the raft-mimicking mixed bilayers.

Docosahexaenoic acid promotes micron scale liquid-ordered domains. A comparison study of docosahexaenoic versus oleic acid containing phosphatidylcholine in raft-like mixtures

The understanding of the functional role of the lipid diversity in biological membranes is a major challenge. Lipid models have been developed to address this issue by using lipid mixtures generating liquid-ordered (Lo)/liquid-disordered (Ld) immiscibility. The present study examined mixtures comprising Egg sphingomyelin (SM), cholesterol (chol) and phosphatidylcholine (PC) either containing docosahexaenoic (PDPC) or oleic acid (POPC). The mixtures were examined in terms of their capability to induce phase separation at the micron- and nano-scales. Fluorescence microscopy, electron spin resonance (ESR), X-ray diffraction (XRD) and calorimetry methods were used to analyze the lateral organization of the mixtures. Fluorescence microscopy of giant vesicles could show that the temperature of the micron-scale Lo/Ld miscibility is higher for PDPC than for POPC ternary mixtures. At 37°C, no micron-scale Lo/Ld phase separation could be identified in the POPC containing mixtures while it was evident for PDPC. In contrast, a phase separation was distinguished for both PC mixtures by ESR and XRD, indicative that PDPC and POPC mixtures differed in micron vs nano domain organization. Compared to POPC, the higher line tension of the Lo domains observed in PDPC mixtures is assumed to result from the higher difference in Lo/Ld order parameter rather than hydrophobic mismatch.

Cholesterol solubility limit in lipid membranes probed by small angle neutron scattering and MD simulations

The solubility limits of cholesterol in small unilamellar vesicles made of POPS and POPC were probed using Small Angle Neutron Scattering (SANS) and coarse grained (CG) molecular dynamics (MD) simulations. SANS, being non-invasive, allowed the direct and quantitative measurement of cholesterol in intact vesicles. Our experimental measurements reveal a 61% mole fraction solubility limit of cholesterol in POPC, consistent with previous studies. However, in POPS the solubility limit of cholesterol is found to be 73% mole fraction. Previous work reports solubility limits of cholesterol in POPS varying significantly, ranging from 36% up to 66%. The CG MD simulations are in remarkable quantitative agreement with our experimental results showing similar solubility limits. Further, neither experiments nor simulations show evidence of stable nanodomains of cholesterol in POPS membranes as suggested in some previous reports.

n-3 polyunsaturated fatty acids and mechanisms to mitigate inflammatory paracrine signaling in obesity-associated breast cancer

Globally, the prevalence of obesity is increasing which subsequently increases the risk of the development of obesity-related chronic diseases. Low-grade chronic inflammation and dysregulated adipose tissue inflammatory mediator/adipokine secretion are well-established in obesity, and these factors increase the risk of developing inflammation-associated cancer. Breast cancer is of particular interest given that increased inflammation within the subcutaneous mammary adipose tissue depot can alter the local tissue inflammatory microenvironment such that it resembles that of obese visceral adipose tissue. Therefore, in obese women with breast cancer, increased inflammatory mediators both locally and systemically can perpetuate inflammation-associated pro-carcinogenic signaling pathways, thereby increasing disease severity. Herein, we discuss some of these inflammation-associated pro-carcinogenic mechanisms of the combined obese breast cancer phenotype and offer evidence that dietary long chain n-3 polyunsaturated fatty acids (PUFA) may have utility in mitigating the severity of obesity-associated inflammation and breast cancer.

Mechanisms by Which Pleiotropic Amphiphilic n-3 PUFA Reduce Colon Cancer Risk

Colorectal cancer is one of the major causes of cancer-related mortality in both men and women worldwide. Genetic susceptibility and diet are primary determinants of cancer risk and tumor behavior. Experimental, epidemiological, and clinical data substantiate the beneficial role of n-3 polyunsaturated fatty acids (PUFA) in preventing chronic inflammation and colon cancer. From a mechanistic perspective, n-3 PUFA are pleiotropic and multifaceted with respect to their molecular mechanisms of action. For example, this class of dietary lipid uniquely alters membrane structure/ cytoskeletal function, impacting membrane receptor function and downstream signaling cascades, including gene expression profiles and cell phenotype. In addition, n-3 PUFA can synergize with other potential anti-tumor agents, such as fermentable fiber and curcumin. With the rising prevalence of diet-induced obesity, there is also an urgent need to elucidate the link between chronic inflammation in adipose tissue and colon cancer risk in obesity. In this review, we will summarize recent developments linking n-3 PUFA intake, membrane alterations, epigenetic modulation, and effects on obesity-associated colon cancer risk.

Essentially all excess fibroblast cholesterol moves from plasma membranes to intracellular compartments

It has been shown that modestly increasing plasma membrane cholesterol beyond its physiological set point greatly increases the endoplasmic reticulum and mitochondrial pools, thereby eliciting manifold feedback responses that return cell cholesterol to its resting state. The question arises whether this homeostatic mechanism reflects the targeting of cell surface cholesterol to specific intracellular sites or its general equilibration among the organelles. We now show that human fibroblast cholesterol can be increased as much as two-fold from 2-hydroxypropyl-β-cyclodextrin without changing the size of the cell surface pool. Rather, essentially all of the added cholesterol disperses rapidly among cytoplasmic membranes, increasing their overall cholesterol content by as much as five-fold. We conclude that the level of plasma membrane cholesterol is normally at capacity and that even small increments above this physiological set point redistribute essentially entirely to intracellular membranes, perhaps down their chemical activity gradients.

How polyunsaturated fatty acids modify molecular organization in membranes: insight from NMR studies of model systems

Marine long chain n-3 polyunsaturated fatty acids (PUFA), eicosapentaenoic (EPA) and docosahexaenoic acid (DHA), are bioactive molecules with clinical applications for the treatment of several diseases. In order to effectively translate these molecules into clinical trials, it is essential to establish the underlying mechanisms for n-3 PUFA. This review focuses on efforts to understand how EPA and DHA, upon incorporation into plasma membrane phospholipids, remodel the molecular organization of cholesterol-enriched lipid microdomains. We first give an overview of results from studies on cells. Paradoxical data generated from mouse studies indicate that EPA and DHA incorporate into lipid microdomains, yet in spite of their high disorder increase molecular order within the domain. We then spotlight the utility of solid state (2)H NMR spectroscopy of model bilayers as a tool for elucidating underlying mechanisms by which n-3 PUFA-containing phospholipids can regulate molecular organization of lipid microdomains. Evidence is presented demonstrating that n-3 PUFA exert differential structural effects when incorporated into phosphatidylethanolamines (PE) compared to phosphatidylcholines (PC), which explains some of the conflicting results observed in vivo. Recent studies that reveal differences between the interactions of EPA and DHA with lipid microdomains, potentially reflecting a differential in bioactivity, are finally described. Overall, we highlight the notion that NMR experiments on model membranes suggest a complex model by which n-3 PUFA reorganize lipid microdomains in vivo.

Key molecular requirements for raft formation in lipid/cholesterol membranes

The lipid mixture of DPPC (saturated lipid)/DUPC (unsaturated lipid)/CHOL (cholesterol) is studied with respect to its ability to form liquid-ordered and liquid-disordered phases. We employ coarse-grained simulations with MARTINI force field. All three components are systematically modified in order to explore the relevant molecular properties, leading to phase separation. Specifically, we show that the DPPC/DUPC/CHOL system unmixes due to enthalpic DPPC-DPPC and DPPC-CHOL interactions. The phase separation remains unchanged, except for the formation of a gel phase at long times after decreasing the conformational degrees of freedom of the unsaturated DUPC. In contrast, the phase separation can be suppressed by softening the DPPC chains. In an attempt to mimic the ordering and unmixing effect of CHOL the latter is replaced by a stiff and shortened DPPC-like lipid. One still observes phase separation, suggesting that it is mainly the rigid and planar structure of CHOL which is important for raft formation. Addition of an extra bead to the head of CHOL has no notable impact on the phase separation of the system, supporting the irrelevance of the Umbrella model for the phase separation. Reduction of the conformational entropy of CHOL by stiffening its last bead results in a significant increase of the order of the DPPC/CHOL domain. This suggests that the conformational entropy of CHOL is important to prohibit the gelation process. The interleaflet interactions as mediated by the terminal molecular groups seem to have a strong impact on the possibility of a subsequent gelation process after phase separation.

Using neurolipidomics to identify phospholipid mediators of synaptic (dys)function in Alzheimer's Disease

Not all of the mysteries of life lie in our genetic code. Some can be found buried in our membranes. These shells of fat, sculpted in the central nervous system into the cellular (and subcellular) boundaries of neurons and glia, are themselves complex systems of information. The diversity of neural phospholipids, coupled with their chameleon-like capacity to transmute into bioactive molecules, provides a vast repertoire of immediate response second messengers. The effects of compositional changes on synaptic function have only begun to be appreciated. Here, we mined 29 neurolipidomic datasets for changes in neuronal membrane phospholipid metabolism in Alzheimer's Disease (AD). Three overarching metabolic disturbances were detected. We found that an increase in the hydrolysis of platelet activating factor precursors and ethanolamine-containing plasmalogens, coupled with a failure to regenerate relatively rare alkyl-acyl and alkenyl-acyl structural phospholipids, correlated with disease severity. Accumulation of specific bioactive metabolites [i.e., PC(O-16:0/2:0) and PE(P-16:0/0:0)] was associated with aggravating tau pathology, enhancing vesicular release, and signaling neuronal loss. Finally, depletion of PI(16:0/20:4), PI(16:0/22:6), and PI(18:0/22:6) was implicated in accelerating Aβ₄₂ biogenesis. Our analysis further suggested that converging disruptions in platelet activating factor, plasmalogen, phosphoinositol, phosphoethanolamine (PE), and docosahexaenoic acid metabolism may contribute mechanistically to catastrophic vesicular depletion, impaired receptor trafficking, and morphological dendritic deformation. Together, this analysis supports an emerging hypothesis that aberrant phospholipid metabolism may be one of multiple critical determinants required for Alzheimer disease conversion.