Fast flip-flop of cholesterol and fatty acids in membranes: implications for membrane transport proteins

Transport and Organization of Cholesterol in a Planar Solid-Supported Lipid Bilayer Depend on the Phospholipid Flip-Flop Rate

Understanding the transport behavior of the cholesterol molecules within a cell membrane is a key challenge in cell biology at present. Here, we have applied sum frequency generation vibrational spectroscopy to characterize the transport and organization of cholesterol in different kinds of planar solid-supported lipid bilayers by combining achiral- and chiral-sensitive polarization measurements. This method allows us to distinguish the organization of cholesterol in tail-to-tail, head-to-tail, head-to-head, and side-by-side manners. It is found that the movement of cholesterol in the lipid bilayer largely depends on the flip-flop rate of the phospholipid. The flip-flop dynamics of the phospholipid and cholesterol are synchronous. In the solid-supported zwitterionic phosphocholine lipid bilayer, the cholesterol molecules flip quickly from the distal leaflet to the neutral proximal leaflet of the bilayer and form tail-to-tail organization on both leaflets. The phosphocholine lipid and cholesterol show the same flip-flop rate. However, when the proximal leaflet is prepared using negative glycerol phospholipids, cholesterol organizes itself by mainly forming an α-β structure on the distal leaflet. Because of the strong interaction between the glycerol phospholipid and the substrate, no or only partial cholesterol molecules flip from the distal leaflet to the negatively charged proximal leaflet. However, the cholesterol molecules undergo flip-flop in the presence of salt solution because the ions weaken the interaction between the negative phospholipid and the substrate.

FABP1: A Novel Hepatic Endocannabinoid and Cannabinoid Binding Protein

Endocannabinoids (ECs) and cannabinoids are very lipophilic molecules requiring the presence of cytosolic binding proteins that chaperone these molecules to intracellular targets. While three different fatty acid binding proteins (FABP3, -5, and -7) serve this function in brain, relatively little is known about how such hydrophobic ECs and cannabinoids are transported within the liver. The most prominent hepatic FABP, liver fatty acid binding protein (FABP1 or L-FABP), has high affinity for arachidonic acid (ARA) and ARA-CoA, suggesting that FABP1 may also bind ARA-derived ECs (AEA and 2-AG). Indeed, FABP1 bound ECs with high affinity as shown by displacement of FABP1-bound fluorescent ligands and by quenching of FABP1 intrinsic tyrosine fluorescence. FABP1 also had high affinity for most non-ARA-containing ECs, FABP1 inhibitors, EC uptake/hydrolysis inhibitors, and phytocannabinoids and less so for synthetic cannabinoid receptor (CBR) agonists and antagonists. The physiological impact was examined with liver from wild-type (WT) versus FABP1 gene-ablated (LKO) male mice. As shown by liquid chromatography and mass spectrometry, FABP1 gene ablation significantly increased hepatic levels of AEA, 2-AG, and 2-OG. These increases were not due to increased protein levels of EC synthetic enzymes (NAPEPLD and DAGL) or a decreased level of EC degradative enzyme (FAAH) but correlated with complete loss of FABP1, a decreased level of SCP2 (8-fold less prevalent than FABP1, but also binds ECs), and a decreased level of degradative enzymes (NAAA and MAGL). These data indicated that FABP1 not only is the most prominent endocannabinoid and cannabinoid binding protein but also impacts hepatic endocannabinoid levels.

Thermodynamics of cell-penetrating HIV1 TAT peptide insertion into PC/PS/CHOL model bilayers through transmembrane pores: the roles of cholesterol and anionic lipids

Efficient delivery of pharmaceutically active molecules across cellular membranes using cell penetrating peptides (CPPs), such as the cationic human immunodeficiency virus-1 trans-acting activator of transcription peptide (HIV-1 TAT), continues to attract scientific attention in drug design and disease treatment. Experimental results show that the TAT peptide is not only capable of directly penetrating the biological membrane in a passive manner, but also forming physical, membrane-spanning pores that may facilitate transport. Experiments further show that anionic lipids accelerate peptide permeation within a range of mole percentage composition. In this work, we explored the structures and translocation thermodynamics of the cationic TAT peptide across a series of DPPC/DPPS model membranes with the presence of 0-30 mol% cholesterol. We computed the potentials of the mean force by using umbrella sampling molecular dynamics simulations coupled to the Martini coarse-grained force field. We systematically investigated the roles of cholesterol and anionic lipids (membrane surface charge) in TAT peptide translocation. In qualitative agreement with experimental findings, the barrier heights were significantly reduced in the presence of anionic lipids. A toroidal hydrophilic pore was strongly suggested by membrane structure analysis. Cholesterol stabilizes the liquid-ordered (Lo) phase of membranes and increases the elastic stiffness of bilayers. Consequently, it hinders transmembrane pore formation and thus modulates solute permeability, since the liquid-ordered phase suppresses reorientation of the lipid molecules on simulation time scales. Though cholesterol contributes marginally to the total free energy associated with peptide permeation, the coordination of cholesterol to the peptide weakens more favorable peptide-lipid interactions. The addition of the anionic lipid DPPS to the neutral DPPC bilayer leads to the emergence and further enhancement of an interfacially stable state of the peptide due to the favorable peptide-anionic lipid interactions. Translocation free energy barriers decrease in lockstep with increasing DPPS composition in the model bilayers simulated. Finally, we investigated the size of hydrophilic pores emerging in our simulations, as well as the qualitative mobility of the peptide on the membrane surface.

Influenza viral membrane fusion is sensitive to sterol concentration but surprisingly robust to sterol chemical identity

Influenza virions are enriched in cholesterol relative to the plasma membrane from which they bud. Previous work has shown that fusion between influenza virus and synthetic liposomes is sensitive to the amount of cholesterol in either the virus or the target membrane. Here, we test the chemical properties of cholesterol required to promote influenza fusion by replacing cholesterol with other sterols and assaying viral fusion kinetics. We find that influenza fusion with liposomes is surprisingly robust to sterol chemical identity, showing no significant dependence on sterol identity in target membranes for any of the sterols tested. In the viral membrane, lanosterol slowed fusion somewhat, while polar sterols produced a more pronounced slowing and inhibition of fusion. No other sterols tested showed a significant perturbation in fusion rates, including ones previously shown to alter membrane bending moduli or phase behavior. Although fusion rates depend on viral cholesterol, they thus do not require cholesterol’s ability to support liquid-liquid phase coexistence. Using electron cryo-microscopy, we further find that sterol-dependent changes to hemagglutinin spatial patterning in the viral membrane do not require liquid-liquid phase coexistence. We therefore speculate that local sterol-hemagglutinin interactions in the viral envelope may control the rate-limiting step of fusion.

Orthogonal Click Conjugation to the Liposomal Surface Reveals the Stability of the Lipid Anchorage as Crucial for Targeting

Synthetic access to multiple surface decorations are a bottleneck in the development of liposomes for receptor mediated targeting. This opens a complex multiparameter space, exploration of which is severely limited in terms of sample numbers and turnaround times. Here, we unlock this technological barrier by a combination of a milligram-scale liposome formulation using dual centrifugation and orthogonal click chemistry on the liposomal surface. Application of these techniques to conceptually new amphiphilic compounds, which feature norbornene and alkyne groups at the apex of sterically stabilizing, hyperbranched polyglycerol moieties, revealed a particular influence of the membrane anchor of functional amphiphiles. Folic acid residues clicked to cholesterol-based amphiphiles were inefficient in folate-mediated cell targeting, while dialkyl-anchored amphiphiles remained stable in the liposomal membrane and imparted efficient targeting properties. These findings are of specific importance considering the popularity of cholesterol as a lipophilic anchor.

Structural Origins of Cholesterol Accelerated Lipid Flip-Flop Studied by Sum-Frequency Vibrational Spectroscopy

The unique structure of cholesterol and its role in modulating lipid translocation (flip-flop) were examined using sum-frequency vibrational spectroscopy (SFVS). Two structural analogues of cholesterol--cholestanol and cholestene--were examined to explore the influence of ring rigidity and amphiphilicity on controlling distearoylphosphocholine (DSPC) flip-flop. Kinetic rates for DSPC flip-flop were determined as a function of sterol concentration and temperature. All three sterols increased the rate of DSPC flip-flop in a concentration-dependent manner following the order cholestene > cholestanol > cholesterol. Rates of DSPC flip-flop were used to calculate the thermodynamic activation free energy barrier (ΔG(‡)) in the presence of cholesterol, cholestanol, and cholestene. The acyl chain gauche content of DSPC, mean lipid area, and membrane compressibility were correlated to observed trends in ΔG(‡). ΔG(‡) for DSPC flip-flop showed a strong positive correlation with the molar compression modulus (K*) of the membrane, influenced by the type and concentration of the sterol added. Interestingly, cholesterol is distinctive in maintaining invariant membrane compressibility over the range of 2-10 mol %. The results in this study demonstrate that the compression modulus of a membrane plays a significant role in moderating ΔG(‡) and the kinetics of native, protein-free, lipid translocation in membranes.

Detectors for evaluating the cellular landscape of sphingomyelin- and cholesterol-rich membrane domains

Although sphingomyelin and cholesterol are major lipids of mammalian cells, the detailed distribution of these lipids in cellular membranes remains still obscure. However, the recent development of protein probes that specifically bind sphingomyelin and/or cholesterol provides new information about the landscape of the lipid domains that are enriched with sphingomyelin or cholesterol or both. Here, we critically summarize the tools to study distribution and dynamics of sphingomyelin and cholesterol. This article is part of a Special Issue entitled: The cellular lipid landscape edited by Tim P. Levine and Anant K. Menon.

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.

Cholesterol behavior in asymmetric lipid bilayers: insights from molecular dynamics simulations

Asymmetric lipid composition of the cell membranes plays an important role in the multitude of important biological functions. Much less is known, however, about the distribution and dynamics of cholesterol in asymmetric biological membranes. In this work we show how this issue could be addressed computationally by molecular dynamics simulations. The influence of the lipid head group charge, acyl chain saturation, spontaneous membrane curvature and the surface tension of the membrane on cholesterol distribution in asymmetric lipid bilayers is investigated. Four asymmetric bilayers containing DOPC, DOPS, DSPC, or DSPS lipids, were simulated on the time scale extended to tens of microseconds. We show that cholesterol strongly prefers anionic lipids to neutral and saturated lipid tails to unsaturated with distribution ratio ~0.4-0.6. Multiple flip-flop transitions of cholesterol were observed directly and their mean times range from 350 to 2,000 ns. It was shown that the distribution of cholesterol in the asymmetric bilayer depends not only on the type of lipids but also on the local membrane curvature and the surface tension. The geometric shape of spontaneously curved asymmetric bilayer changes dramatically in the presence of cholesterol. The membrane curvature becomes less homogeneous with large patches of flattened regions interleaved by rather sharp bends.

Impact of DHA on metabolic diseases from womb to tomb

Long chain polyunsaturated fatty acids (LC-PUFAs) are important mediators in improving and maintaining human health over the total lifespan. One topic we especially focus on in this review is omega-3 LC-PUFA docosahexaenoic acid (DHA). Adequate DHA levels are essential during neurodevelopment and, in addition, beneficial in cognitive processes throughout life. We review the impact of DHA on societal relevant metabolic diseases such as cardiovascular diseases, obesity, and diabetes mellitus type 2 (T2DM). All of these are risk factors for cognitive decline and dementia in later life. DHA supplementation is associated with a reduced incidence of both stroke and atherosclerosis, lower bodyweight and decreased T2DM prevalence. These findings are discussed in the light of different stages in the human life cycle: childhood, adolescence, adulthood and in later life. From this review, it can be concluded that DHA supplementation is able to inhibit pathologies like obesity and cardiovascular disease. DHA could be a dietary protector against these metabolic diseases during a person’s entire lifespan. However, supplementation of DHA in combination with other dietary factors is also effective. The efficacy of DHA depends on its dose as well as on the duration of supplementation, sex, and age.

Acid sphingomyelinase activity is regulated by membrane lipids and facilitates cholesterol transfer by NPC2

During endocytosis, membrane components move to intraluminal vesicles of the endolysosomal compartment for digestion. At the late endosomes, cholesterol is sorted out mainly by two sterol-binding proteins, Niemann-Pick protein type C (NPC)1 and NPC2. To study the NPC2-mediated intervesicular cholesterol transfer, we developed a liposomal assay system. (Abdul-Hammed, M., B. Breiden, M. A. Adebayo, J. O. Babalola, G. Schwarzmann, and K. Sandhoff. 2010. Role of endosomal membrane lipids and NPC2 in cholesterol transfer and membrane fusion. J. Lipid Res. 51: 1747-1760.) Anionic lipids stimulate cholesterol transfer between liposomes while SM inhibits it, even in the presence of anionic bis(monoacylglycero)phosphate (BMP). Preincubation of vesicles containing SM with acid sphingomyelinase (ASM) (SM phosphodiesterase, EC 3.1.4.12) results in hydrolysis of SM to ceramide (Cer), which enhances cholesterol transfer. Besides SM, ASM also cleaves liposomal phosphatidylcholine. Anionic phospholipids derived from the plasma membrane (phosphatidylglycerol and phosphatidic acid) stimulate SM and phosphatidylcholine hydrolysis by ASM more effectively than BMP, which is generated during endocytosis. ASM-mediated hydrolysis of liposomal SM was also stimulated by incorporation of diacylglycerol (DAG), Cer, and free fatty acids into the liposomal membranes. Conversely, phosphatidylcholine hydrolysis was inhibited by incorporation of cholesterol, Cer, DAG, monoacylglycerol, and fatty acids. Our data suggest that SM degradation by ASM is required for physiological secretion of cholesterol from the late endosomal compartment, and is a key regulator of endolysosomal lipid digestion.

Designer lipids for drug delivery: from heads to tails

For four decades, liposomes composed of both naturally occurring and synthetic lipids have been investigated as delivery vehicles for low molecular weight and macromolecular drugs. These studies paved the way for the clinical and commercial success of a number of liposomal drugs, each of which required a tailored formulation; one liposome size does not fit all drugs! Instead, the physicochemical properties of the liposome must be matched to the pharmacology of the drug. An extensive biophysical literature demonstrates that varying lipid composition can influence the size, membrane stability, in vivo interactions, and drug release properties of a liposome. In this review we focus on recently described synthetic lipid headgroups, linkers and hydrophobic domains that can provide control over the intermolecular forces, phase preference, and macroscopic behavior of liposomes. These synthetic lipids further our understanding of lipid biophysics, promote targeted drug delivery and improve liposome stability. We further highlight the immune reactivity of novel synthetic headgroups as a key design consideration. For instance it was originally thought that synthetic PEGylated lipids were immunologically inert; however, it's been observed that under certain conditions PEGylated lipids induce humoral immunity. Such immune activation may be a limitation to the use of other engineered lipid headgroups for drug delivery. In addition to the potential immunogenicity of engineered lipids, future investigations on liposome drugs in vivo should pay particular attention to the location and dynamics of payload release.

Recent insights into the molecular pathophysiology of lipid droplet formation in hepatocytes

Triacyglycerols are a major energy reserve of the body and are normally stored in adipose tissue as lipid droplets (LDs). The liver, however, stores energy as glycogen and digested triglycerides in the form of fatty acids. In stressed condition such as obesity, imbalanced nutrition and drug induced liver injury hepatocytes accumulate excess lipids in the form of LDs whose prolonged storage leads to disease conditions most notably non-alcoholic fatty liver disease (NAFLD). Fatty liver disease has become a major health burden with more than 90% of obese, nearly 70% of overweight and about 25% of normal weight patients being affected. Notably, research in recent years has shown LD as highly dynamic organelles for maintaining lipid homeostasis through fat storage, protein sorting and other molecular events studied in adipocytes and other cells of living organisms. This review focuses on the molecular events of LD formation in hepatocytes and the importance of cross talk between different cell types and their signalling in NAFLD as to provide a perspective on molecular mechanisms as well as possibilities for different therapeutic intervention strategies.

Complex Interplay between the P-Glycoprotein Multidrug Efflux Pump and the Membrane: Its Role in Modulating Protein Function

Multidrug resistance in cancer is linked to expression of the P-glycoprotein multidrug transporter (Pgp, ABCB1), which exports many structurally diverse compounds from cells. Substrates first partition into the bilayer and then interact with a large flexible binding pocket within the transporter's transmembrane regions. Pgp has been described as a hydrophobic vacuum cleaner or an outwardly directed drug/lipid flippase. Recent X-ray crystal structures have shed some light on the nature of the drug-binding pocket and suggested routes by which substrates can enter it from the membrane. Detergents have profound effects on Pgp function, and several appear to be substrates. Biochemical and biophysical studies in vitro, some using purified reconstituted protein, have explored the effects of the membrane environment. They have demonstrated that Pgp is involved in a complex relationship with its lipid environment, which modulates the behavior of its substrates, as well as various functions of the protein, including ATP hydrolysis, drug binding, and drug transport. Membrane lipid composition and fluidity, phospholipid headgroup and acyl chain length all influence Pgp function. Recent studies focusing on thermodynamics and kinetics have revealed some important principles governing Pgp-lipid and substrate-lipid interactions, and how these affect drug-binding and transport. In some cells, Pgp is associated with cholesterol-rich microdomains, which may modulate its functions. The relationship between Pgp and cholesterol remains an open question; however, it clearly affects several aspects of its function in addition to substrate-membrane partitioning. The action of Pgp modulators appears to depend on their membrane permeability, and membrane fluidizers and surfactants reverse drug resistance, likely via an indirect mechanism. A detailed understanding of how the membrane affects Pgp substrates and Pgp's catalytic cycle may lead to new strategies to combat clinical drug resistance.

In Situ and Real-Time SFG Measurements Revealing Organization and Transport of Cholesterol Analogue 6-Ketocholestanol in a Cell Membrane

Cholesterol organization and transport within a cell membrane are essential for human health and many cellular functions yet remain elusive so far. Using cholesterol analogue 6-ketocholestanol (6-KC) as a model, we have successfully exploited sum frequency generation vibrational spectroscopy (SFG-VS) to track the organization and transport of cholesterol in a membrane by combining achiral-sensitive ssp (ppp) and chiral-sensitive psp polarization measurements. It is found that 6-KC molecules are aligned at the outer leaflet of the DMPC lipid bilayer with a tilt angle of about 10°. 6-KC organizes itself by forming an α-β structure at low 6-KC concentration and most likely a β-β structure at high 6-KC concentration. Among all proposed models, our results favor the so-called umbrella model with formation of a 6-KC cluster. Moreover, we have found that the long anticipated flip-flop motion of 6-KC in the membrane takes time to occur, at least much longer than previously thought. All of these interesting findings indicate that it is critical to explore in situ, real-time, and label-free methodologies to obtain a precise molecular description of cholesterol's behavior in membranes. This study represents the first application of SFG to reveal the cholesterol-lipid interaction mechanism at the molecular level.

Cholesterol translocation in a phospholipid membrane

Cholesterol (CHOL) molecules play a key role in modulating the rigidity of cell membranes and controlling intracellular transport and signal transduction. Using an all-atom molecular dynamics approach, we study the process of CHOL interleaflet transport (flip-flop) in a dipalmitoylphosphatidycholine (DPPC)-CHOL bilayer over a time period of 15 μs. We investigate the effect of the flip-flop process on mechanical stress across the bilayer and the role of CHOL in inducing molecular order in bilayer leaflets. The simulations are carried out at physiologically relevant CHOL concentration (30%), temperature (323 K), and pressure (1 bar). CHOL flip-flop events are observed with a rate constant of 3 × 10⁴s⁻¹. Once a flip-flop event is triggered, a CHOL molecule takes an average of 73 nanoseconds to migrate from one bilayer leaflet to the other.

Synaptic vesicle morphology: a case of protein sorting?

Synaptic vesicles (SVs) are the repositories of neurotransmitters. They are locally recycled at nerve terminals following exocytosis. A unique feature of these vesicles is the uniformity of their morphology, which is well maintained even after rounds of exocytosis and endocytosis. Several studies suggest that disruption of clathrin adaptor proteins leads to defects in sorting cargoes and alterations in SV morphology. Here, we review the links between adaptor proteins and SV size, and highlight how protein sorting may impact SV architecture. Molecular players such as clathrin, adaptor proteins, accessory proteins, SV cargoes and lipid composition may act together to establish a stable regulatory network to maintain SV morphology.

Molecular simulation of rapid translocation of cholesterol, diacylglycerol, and ceramide in model raft and nonraft membranes

The translocation of lipids across membranes (flip-flop) is an important biological process. Slow exchange on a physiological timescale allows the creation of asymmetric distributions of lipids across cellular membranes. The location of lipids and their rate of exchange have important biological consequences, especially for lipids involved in cellular signaling. We investigated the translocation of cholesterol, ceramide, and diacylglycerol in two model bilayers using molecular dynamics simulations. We estimate half times for flip-flop for cholesterol, diacylglycerol, and ceramide of 20 μs, 30 μs, and 10 ms in a POPC bilayer, compared with approximately 30 min, 30 ms, and 30 s in a model raft bilayer (1:1:1 PSM, POPC, and cholesterol). Cholesterol has a large (54 kJ/mol) free energy of exchange between the POPC and raft bilayer, and therefore, it strongly prefers the more ordered and rigid raft bilayer over the more liquid POPC bilayer. Ceramide and diacylglycerol have relatively small free energies of exchange, suggesting nearly equal preference for both bilayers. This unexpected result may have implications for ceramide and diacylglycerol signaling and membrane localization.

Proteins that bind and move lipids: MsbA and NPC1

Membrane proteins that bind and transport lipids face special challenges. Since lipids typically have low water solubility, both accessibility of the substrate to the protein and delivery to the desired destination are problematical. The amphipathic nature of membrane lipids, and their relatively large molecular size, also means that these proteins must possess substrate-binding sites of a different nature than those designed to handle small polar molecules. This review considers two integral proteins whose function is to bind and transfer membrane lipids within or across a membrane. The first protein, MsbA, is a putative lipid flippase that is a member of the ATP-binding cassette (ABC) superfamily. The protein is found in the inner (cytoplasmic) membrane (IM) of Gram-negative bacteria such as E. coli, where it is proposed to move lipid A from the inner to the outer membrane (OM) leaflet, an important step in the lipopolysaccharide biosynthetic pathway. Cholesterol is a major component of the plasma membrane in eukaryotic cells, where it regulates bilayer fluidity. The other lipid-binding protein discussed here, mammalian NPC1 (Niemann-Pick disease, Type C1), binds cholesterol inside late endosomes/lysosomes (LE/LY) and is involved in its transfer to the cytosol as part of a key intracellular sterol-trafficking pathway. Mutations in NPC1 lead to a devastating neurodegenerative condition, Niemann-Pick Type C disease, which is characterized by massive cholesterol accumulation in LE/LY. The accelerating pace of membrane protein structure determination over the past decade has allowed us a glimpse of how lipid binding and transfer by membrane proteins such as MsbA and NPC1 might be achieved.

Noninvasive neutron scattering measurements reveal slower cholesterol transport in model lipid membranes

Proper cholesterol transport is essential to healthy cellular activity and any abnormality can lead to several fatal diseases. However, complete understandings of cholesterol homeostasis in the cell remains elusive, partly due to the wide variability in reported values for intra- and intermembrane cholesterol transport rates. Here, we used time-resolved small-angle neutron scattering to measure cholesterol intermembrane exchange and intramembrane flipping rates, in situ, without recourse to any external fields or compounds. We found significantly slower transport kinetics than reported by previous studies, particularly for intramembrane flipping where our measured rates are several orders of magnitude slower. We unambiguously demonstrate that the presence of chemical tags and extraneous compounds employed in traditional kinetic measurements dramatically affect the system thermodynamics, accelerating cholesterol transport rates by an order of magnitude. To our knowledge, this work provides new insights into cholesterol transport process disorders, and challenges many of the underlying assumptions used in most cholesterol transport studies to date.

Structural studies of the monolayers and bilayers formed by a novel cholesterol-phospholipid chimera

Langmuir isotherm, neutron reflectivity, and small angle neutron scattering studies have been conducted to characterize the monolayers and vesicular bilayers formed by a novel chimeric phospholipid, ChemPPC, that incorporates a cholesteryl moeity and a C-16 aliphatic chain, each covalently linked via a glycerol backbone to phosphatidylcholine. The structures of the ChemPPC monolayers and bilayers are compared against those formed from pure dipalmitoylphoshatidylcholine (DPPC) and those formed from a 60:40 mol % mixture of DPPC and cholesterol. In accord with previous findings showing that very similar macroscopic properties were exhibited by ChemPPC and 60:40 mol % DPPC/cholesterol vesicles, it is found here that the chimeric lipid and lipid/sterol mixture have very similar monolayer structures (each having a monolayer thickness of ∼26 Å), and they also form vesicles with similar lamellar structure, each having a bilayer thickness of ∼50 Å and exhibiting a repeat spacing of ∼65 Å. The interfacial area of ChemPPC, however, is around 10 Å(2) greater than that of the combined DPPC/cholesterol unit in the mixed lipid monolayer (viz., 57 ± 1 vs 46 ± 1 Å(2), at 35 mN·m(-1)), and this difference in area is attributed to the succinyl linkage which joins the ChemPPC steroid and glyceryl moieties. The larger area of the ChemPPC is reflected in a slightly thicker monolayer solvent distribution width (9.5 vs 9 Å for the DPPC/cholesterol system) and by a marginal increase in the level of lipid headgroup hydration (16 vs 13 H(2)O per lipid, at 35 mN·m(-1)).

Decoding the membrane activity of the cyclotide kalata B1: the importance of phosphatidylethanolamine phospholipids and lipid organization on hemolytic and anti-HIV activities

Cyclotides, a large family of cyclic peptides from plants, have a broad range of biological activities, including insecticidal, cytotoxic, and anti-HIV activities. In all of these activities, cell membranes seem likely to be the primary target for cyclotides. However, the mechanistic role of lipid membranes in the activity of cyclotides remains unclear. To determine the role of lipid organization in the activity of the prototypic cyclotide, kalata B1 (kB1), and synthetic analogs, their bioactivities and affinities for model membranes were evaluated. We found that the bioactivity of kB1 is dependent on the lipid composition of target cell membranes. In particular, the activity of kB1 requires specific interactions with phospholipids containing phosphatidylethanolamine (PE) headgroups but is further modulated by nonspecific peptide-lipid hydrophobic interactions, which are favored in raft-like membranes. Negatively charged phospholipids do not favor high kB1 affinity. This lipid selectivity explains trends in antimicrobial and hemolytic activities of kB1; it does not target bacterial cell walls, which are negatively charged and lacking PE-phospholipids but can insert in the membranes of red blood cells, which have a low PE content and raft domains in their outer layer. We further show that the anti-HIV activity of kB1 is the result of its ability to target and disrupt the membranes of HIV particles, which are raft-like membranes very rich in PE-phospholipids.

Dynamic transbilayer lipid asymmetry

Cells have thousands of different lipids. In the plasma membrane, and in membranes of the late secretory and endocytotic pathways, these lipids are not evenly distributed over the two leaflets of the lipid bilayer. The basis for this transmembrane lipid asymmetry lies in the fact that glycerolipids are primarily synthesized on the cytosolic and sphingolipids on the noncytosolic surface of cellular membranes, that cholesterol has a higher affinity for sphingolipids than for glycerolipids. In addition, P4-ATPases, "flippases," actively translocate the aminophospholipids phosphatidylserine and phosphatidylethanolamine to the cytosolic surface. ABC transporters translocate lipids in the opposite direction but they generally act as exporters rather than "floppases." The steady state asymmetry of the lipids can be disrupted within seconds by the activation of phospholipases and scramblases. The asymmetric lipid distribution has multiple implications for physiological events at the membrane surface. Moreover, the active translocation also contributes to the generation of curvature in the budding of transport vesicles.

Interleaflet coupling and domain registry in phase-separated lipid bilayers

There is clear evidence of an interleaflet coupling in model lipid/cholesterol membranes exhibiting liquid-liquid phase separation. The strength of this coupling is quantified by the mismatch free energy, γ. We calculate it using a molecular mean-field model of a phase-separated lipid/cholesterol bilayer and obtain values that increase as the concentration of saturated lipids in the coexisting phases is increased. These values lie in the range 0.01-0.03 k(B)T/nm(2). We clarify the relationship between the interleaflet coupling and the extent of interleaflet alignment of liquid domains by analyzing a statistical mechanical model of coupled fluctuating domain interfaces. The model is solved exactly using the correspondence between statistical mechanics and quantum mechanics, yielding an expression for the characteristic size of fluctuations out of domain registry. This length scale depends only weakly on the strength of the interleaflet coupling and inevitably is only of the order of nanometers, which explains the experimental result that fluctuations out of domain registry have not been observed by optical microscopy.

Caveolae and lipid trafficking in adipocytes

The abundance of caveolae in adipocytes suggests a possible cell-specific role for these structures, and because these cells take up and release fatty acids as their quantitatively most robust activity, modulation of fatty acid movement is one such role that is supported by substantial in vitro and in vivo data. In addition, caveolae are particularly rich in cholesterol and sphingolipids, and indeed, fat cells harbor more cholesterol than any other tissue. In this article, we review the role of adipocyte caveolae with regard to these important lipid classes.

Fluorescent n-3 and n-6 very long chain polyunsaturated fatty acids: three-photon imaging in living cells expressing liver fatty acid-binding protein

Despite the considerable beneficial effects of n-3 and n-6 very long chain polyunsaturated fatty acids (VLC-PUFAs), very little is known about the factors that regulate their uptake and intracellular distribution in living cells. This issue was addressed in cells expressing liver-type fatty acid-binding protein (L-FABP) by real time multiphoton laser scanning microscopy of novel fluorescent VLC-PUFAs containing a conjugated tetraene fluorophore near the carboxyl group and natural methylene-interrupted n-3 or n-6 grouping. The fluorescent VLC-PUFAs mimicked many properties of their native nonfluorescent counterparts, including uptake, distribution, and metabolism in living cells. The unesterified fluorescent VLC-PUFAs distributed either equally in nuclei versus cytoplasm (22-carbon n-3 VLC-PUFA) or preferentially to cytoplasm (20-carbon n-3 and n-6 VLC-PUFAs). L-FABP bound fluorescent VLC-PUFA with affinity and specificity similar to their nonfluorescent natural counterparts. Regarding n-3 and n-6 VLC-PUFA, L-FABP expression enhanced uptake into the cell and cytoplasm, selectively altered the pattern of fluorescent n-6 and n-3 VLC-PUFA distribution in cytoplasm versus nuclei, and preferentially distributed fluorescent VLC-PUFA into nucleoplasm versus nuclear envelope, especially for the 22-carbon n-3 VLC-PUFA, correlating with its high binding by L-FABP. Multiphoton laser scanning microscopy data showed for the first time VLC-PUFA in nuclei of living cells and suggested a model, whereby L-FABP facilitated VLC-PUFA targeting to nuclei by enhancing VLC-PUFA uptake and distribution into the cytoplasm and nucleoplasm.

Fatty acid flux in adipocytes: the in's and out's of fat cell lipid trafficking

The trafficking of fatty acids into and out of adipocytes is regulated by a complex series of proteins and enzymes and is under control by a variety of hormonal and metabolic factors. The biochemical basis of fatty acid influx, despite its widespread appreciation, remains enigmatic with regard to the biophysical and biochemical properties that facilitate long-chain fatty acid uptake. Fatty acid efflux is initiated by hormonally controlled lipolysis of the droplet stores and produces fatty acids that must transit from their site of production to the plasma membrane and subsequently out of the cells. This review will focus on the "in's and out's" of fatty acid trafficking and summarize the current concepts in the field.

Membrane transport in primitive cells

Although model protocellular membranes consisting of monoacyl lipids are similar to membranes composed of contemporary diacyl lipids, they differ in at least one important aspect. Model protocellular membranes allow for the passage of polar solutes and thus can potentially support cell-to functions without the aid of transport machinery. The ability to transport polar molecules likely stems from increased lipid dynamics. Selectively permeable vesicle membranes composed of monoacyl lipids allow for many lifelike processes to emerge from a remarkably small set of molecules.

Cholesterol inhibits M-type K+ channels via protein kinase C-dependent phosphorylation in sympathetic neurons

M-type (KCNQ) potassium channels play an important role in regulating the action potential firing in neurons. Here, we investigated the effect of cholesterol on M current in superior cervical ganglion (SCG) sympathetic neurons, using the patch clamp technique. M current was inhibited in a dose-dependent manner by cholesterol loading with a methyl-beta-cyclodextrin-cholesterol complex. This effect was prevented when membrane cholesterol level was restored by including empty methyl-beta-cyclodextrin in the pipette solution. Dialysis of cells with AMP-PNP instead of ATP prevented cholesterol action on M currents. Protein kinase C (PKC) inhibitor, calphostin C, abolished cholesterol-induced inhibition whereas the PKC activator, PDBu, mimicked the inhibition of M currents by cholesterol. The in vitro kinase assay showed that KCNQ2 subunits of M channel can be phosphorylated by PKC. A KCNQ2 mutant that is defective in phosphorylation by PKC failed to show current inhibition not only by PDBu but also by cholesterol. These results indicate that cholesterol-induced inhibition of M currents is mediated by PKC phosphorylation. The inhibition of M currents by PDBu and cholesterol was completely blocked by PIP(2) loading, indicating that the decrease in PIP(2)-channel interaction underlies M channel inhibition by PKC-mediated phosphorylation. We conclude that cholesterol specifically regulates M currents in SCG neurons via PKC activation.

Fatty acids are rapidly delivered to and extracted from membranes by methyl-beta-cyclodextrin

We performed detailed biophysical studies of transfer of long-chain fatty acids (FAs) from methyl-beta-CD (MBCD) to model membranes (egg-PC vesicles) and cells and the extraction of FA from membranes by MBCD. We used i) fluorescein phosphatidylethanolamine to detect transfer of FA anions arriving in the outer membrane leaflet; ii) entrapped pH dyes to measure pH changes after FA diffusion (flip-flop) across the lipid bilayer; and iii) soluble fluorescent-labeled FA binding protein to measure the concentration of unbound FA in water. FA dissociated from MBCD, bound to the membrane, and underwent flip-flop within milliseconds. In the presence of vesicles, MBCD maintained the aqueous concentration of unbound FA at low levels comparable to those measured with albumin. In studies with cells, addition of oleic acid (OA) complexed with MBCD yielded rapid (seconds) dose-dependent OA transport into 3T3-L1 preadipocytes and HepG2 cells. MBCD extracted OA from cells and model membranes rapidly at concentrations exceeding those required for OA delivery but much lower than concentrations commonly used for extracting cholesterol. Compared with albumin, MBCD can transfer its entire FA load and is less likely to extract cell nutrients and to introduce impurities.

Flip-flop-induced relaxation of bending energy: implications for membrane remodeling

Cellular and organellar membranes are dynamic materials that underlie many aspects of cell biology. Biological membranes have long been thought of as elastic materials with respect to bending deformations. A wealth of theory and experimentation on pure phospholipid membranes provides abundant support for this idea. However, biological membranes are not composed solely of phospholipids--they also incorporate a variety of amphiphilic molecules that undergo rapid transbilayer flip-flop. Here we describe several experimental systems that demonstrate deformation-induced molecular flip-flop. First we use a fluorescence assay to track osmotically controlled membrane deformation in single component fatty acid vesicles, and show that the relaxation of the induced bending stress is mediated by fatty acid flip-flop. We then look at two-component phospholipid/cholesterol composite vesicles. We use NMR to show that the steady-state rate of interleaflet diffusion of cholesterol is fast relative to biological membrane remodeling. We then use a Förster resonance energy transfer assay to detect the transbilayer movement of cholesterol upon deformation. We suggest that our results can be interpreted by modifying the area difference elasticity model to account for the time-dependent relaxation of bending energy. Our findings suggest that rapid interleaflet diffusion of cholesterol may play a role in membrane remodeling in vivo. We suggest that the molecular characteristics of sterols make them evolutionarily preferred mediators of stress relaxation, and that the universal presence of sterols in the membranes of eukaryotes, even at low concentrations, reflects the importance of membrane remodeling in eukaryotic cells.

Fast diffusion of very long chain saturated fatty acids across a bilayer membrane and their rapid extraction by cyclodextrins: implications for adrenoleukodystrophy

Abnormalities in the transport of saturated very long chain fatty acids (VLCFA; >C18:0) contribute to their toxic levels in peroxisomal disorders of fatty acid metabolism, such as adrenoleukodystrophy and adrenomyeloneuropathy. We previously showed that VLCFA desorb much slower than normal dietary fatty acids from both albumin and protein-free lipid bilayers. The important step of transbilayer movement (flip-flop) was not measured directly as a consequence of this very slow desorption from donors, and the extremely low aqueous solubility of VLCFA precludes addition of unbound VLCFA to lipid membranes. We have overcome these limitations using methyl-beta-cyclodextrin to solubilize VLCFA for rapid delivery to "acceptor" phosphatidylcholine vesicles (small and large unilamellar) and to cells. VLCFA binding was monitored in real time with the fluorescent probe fluorescein-labeled phosphatidylethanolamine in the outer membrane leaflet, and entrapped pyranine was used to detect flip-flop across the membrane. The upper limit of the rate of flip-flop across the membrane was independent of temperature and media viscosity and was similar for model raft and non-raft membranes as well as living cells. We further showed that cyclodextrins can extract VLCFA rapidly (within seconds) from vesicles and cells, which have implications for the mechanism and potential alternative approaches to treat adrenoleukodystrophy. Because VLCFA diffuse through the lipid bilayer, proteins may not be required for their transport across the peroxisomal membrane.

Identification and characterization of small compound inhibitors of human FATP2

Fatty acid transport proteins (FATPs) are bifunctional proteins, which transport long chain fatty acids into cells and activate very long chain fatty acids by esterification with coenzyme A. In an effort to understand the linkage between cellular fatty acid transport and the pathology associated with excessive accumulation of exogenous fatty acids, we targeted FATP-mediated fatty acid transport in a high throughput screen of more than 100,000 small diverse chemical compounds in yeast expressing human FATP2 (hsFATP2). Compounds were selected for their ability to depress the transport of the fluorescent long chain fatty acid analogue, C(1)-BODIPY-C(12). Among 234 hits identified in the primary screen, 5 compounds, each representative of a structural class, were further characterized in the human Caco-2 and HepG2 cell lines, each of which normally expresses FATP2, and in 3T3-L1 adipocytes, which do not. These compounds were effective in inhibiting uptake with IC(50)s in the low micromolar range in both Caco-2 and HepG2 cells. Inhibition of transport was highly specific for fatty acids and there were no effects of these compounds on cell viability, trans-epithelial electrical resistance, glucose transport, or long chain acyl-CoA synthetase activity. The compounds were less effective when tested in 3T3-L1 adipocytes suggesting selectivity of inhibition. These results suggest fatty acid transport can be inhibited in a FATP-specific manner without causing cellular toxicity.

Activation of membrane cholesterol by 63 amphipaths

A few membrane-intercalating amphipaths have been observed to stimulate the interaction of cholesterol with cholesterol oxidase, saponin and cyclodextrin, presumably by displacing cholesterol laterally from its phospholipid complexes. We now report that this effect, referred to as cholesterol activation, occurs with dozens of other amphipaths, including alkanols, saturated and cis- and trans-unsaturated fatty acids, fatty acid methyl esters, sphingosine derivatives, terpenes, alkyl ethers, ketones, aromatics and cyclic alkyl derivatives. The apparent potency of the agents tested ranged from 3 microM to 7 mM and generally paralleled their octanol/water partition coefficients, except that relative potency declined for compounds with >10 carbons. Some small amphipaths activated cholesterol at a membrane concentration of approximately 3 mol per 100 mol of bilayer lipids, about equimolar with the cholesterol they displaced. Lysophosphatidylserine countered the effects of all these agents, consistent with its ability to reduce the pool of active membrane cholesterol. Various amphipaths stabilized red cells against the hemolysis elicited by cholesterol depletion, presumably by substituting for the extracted sterol. The number and location of cis and trans fatty acid unsaturations and the absolute stereochemistry of enantiomer pairs had only small effects on amphipath potency. Nevertheless, potency varied approximately 7-fold within a group of diverse agents with similar partition coefficients. We infer that a wide variety of amphipaths can displace membrane cholesterol by competing stoichiometrically but with only limited specificity for weak association with phospholipids. Any number of other drugs and experimental agents might do the same.

Cholesterol sensor ORP1L contacts the ER protein VAP to control Rab7-RILP-p150 Glued and late endosome positioning

Late endosomes (LEs) have characteristic intracellular distributions determined by their interactions with various motor proteins. Motor proteins associated to the dynactin subunit p150(Glued) bind to LEs via the Rab7 effector Rab7-interacting lysosomal protein (RILP) in association with the oxysterol-binding protein ORP1L. We found that cholesterol levels in LEs are sensed by ORP1L and are lower in peripheral vesicles. Under low cholesterol conditions, ORP1L conformation induces the formation of endoplasmic reticulum (ER)-LE membrane contact sites. At these sites, the ER protein VAP (VAMP [vesicle-associated membrane protein]-associated ER protein) can interact in trans with the Rab7-RILP complex to remove p150(Glued) and associated motors. LEs then move to the microtubule plus end. Under high cholesterol conditions, as in Niemann-Pick type C disease, this process is prevented, and LEs accumulate at the microtubule minus end as the result of dynein motor activity. These data explain how the ER and cholesterol control the association of LEs with motor proteins and their positioning in cells.