Overlapping functions of the yeast oxysterol-binding protein homologues

The Saccharomyces cerevisiae genome encodes seven homologues of the mammalian oxysterol-binding protein (OSBP), a protein implicated in lipid trafficking and sterol homeostasis. To determine the functions of the yeast OSBP gene family (OSH1-OSH7), we used a combination of genetics, genomics, and sterol lipid analysis to characterize OSH deletion mutants. All 127 combinations and permutations of OSH deletion alleles were constructed. Individual OSH genes were not essential for yeast viability, but the elimination of the entire gene family was lethal. Thus, the family members shared an essential function. In addition, the in vivo depletion of all Osh proteins disrupted sterol homeostasis. Like mutants that affect ergosterol production, the viable combinations of OSH deletion alleles exhibited specific sterol-related defects. Although none of the single OSH deletion mutants was defective for growth, gene expression profiles revealed that each mutant had a characteristic molecular phenotype. Therefore, each gene performed distinct nonessential functions and contributed to a common essential function. Our findings indicated that OSH genes performed a multitude of nonessential roles defined by specific subsets of the genes and that most shared at least one essential role potentially linked to changes in sterol lipid levels.     

Multisite phosphorylation of oxysterol-binding protein regulates sterol binding and activation of sphingomyelin synthesis

The endoplasmic reticulum (ER)-Golgi sterol transfer activity of oxysterol-binding protein (OSBP) regulates sphingomyelin (SM) synthesis, as well as post-Golgi cholesterol efflux pathways. The phosphorylation and ER-Golgi localization of OSBP are correlated, suggesting this modification regulates the directionality and/or specificity of transfer activity. In this paper, we report that phosphorylation on two serine-rich motifs, S381-S391 (site 1) and S192, S195, S200 (site 2), specifically controls OSBP activity at the ER. A phosphomimetic of the SM/cholesterol-sensitive phosphorylation site 1 (OSBP-S5E) had increased in vitro cholesterol and 25-hydroxycholesterol-binding capacity, and cholesterol extraction from liposomes, but reduced transfer activity. Phosphatidylinositol 4-phosphate (PI(4)P) and cholesterol competed for a common binding site on OSBP; however, direct binding of PI(4)P was not affected by site 1 phosphorylation. Individual site 1 and site 2 phosphomutants supported oxysterol activation of SM synthesis in OSBP-deficient CHO cells. However, a double site1/2 mutant (OSBP-S381A/S3D) was deficient in this activity and was constitutively colocalized with vesicle-associated membrane protein-associated protein A (VAP-A) in a collapsed ER network. This study identifies phosphorylation regulation of sterol and VAP-A binding by OSBP in the ER, and PI(4)P as an alternate ligand that could be exchanged for sterol in the Golgi apparatus.     

Liver fatty acid binding protein enhances sterol transfer by membrane interaction

Among the large family of fatty acid binding proteins, the liver L-FABP is unique in that it not only binds fatty acids but also interacts with sterols to enhance sterol transfer between membranes. Nevertheless, the mechanism whereby L-FABP potentiates intermembrane sterol transfer is unknown. Both fluorescence and dialysis data indicate L-FABP mediated sterol transfer between L-cell fibroblast plasma membranes occurs by a direct membrane effect: First, dansylated-L-FABP (DNS-L-FABP) is bound to L-cell fibroblast plasma membranes as indicated by increased DNS-L-FABP steady state polarization and phase resolved limiting anisotropy. Second, coumarin-L-FABP (CPM-L-FABP) fluorescence lifetimes were significantly increased upon interaction with plasma membranes. Third, dialysis studies with³H-cholesterol loaded plasma membranes showed that L-FABP added to the donor compartment of the dialysis cell stimulated³H-cholesterol transfer whether or not the dialysis membrane was permeable to L-FABP. However, L-FABP mediated intermembrane sterol transfer did require a sterol binding site on L-FABP. Chemically blocking the ligand binding site also inhibited L-FABP activity in intermembrane sterol transfer. Finally, L-FABP did not act either as an aqueous carrier or in membrane fusion. The fact that L-FABP interacted with plasma membrane vesicles and required a sterol binding site was consistent with a mode of action whereby L-FABP binds to the membrane prior to releasing sterol from the bilayer.Abbreviations ³H-CHO [1,2-³H(N)]-cholesterol - ANTS 8-aminonaphthalene-1,3,6-trisulfonic acid - CF carboxyfluorescein - CHO cholesterol - CPM (coumarin maleimide) 7-diethylamino-3-(4-maleimidylphenyl)-4-methylcoumarin - cPNA cisparinaric acid - DHE (dehydroergosterol) ^5,7,9(11),22-ergostatetraen-3-ol - DMF dimethyl formamide - DMPOPOP 1,4-bis[4-methyl-5-phenyl-2-oxazolyl]benzene - DNS (dansyl chloride) 5-dimethylaminonaphthalene-1-sulfonylchloride - DPX p-xylene-bis-pyridinium bromide - FBS fetal bovine serum - fluorescamine 4-phenylspiro[furan-2(3H), 1 phthalan]-3,3-dione - L-FABP liver fatty acid binding protein - NPG p-nitrophenylglyoxal - PIPES piperazine-N,N-bis(2-ethanesulfonic acid) - POPC 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine - SUV small unilamellar vesicle(s) - TNM tetranitromethane This work was supported in part by the National Institutes of Health United States Public Health Service (GM31651 and DK41402) and the American Heart Association (Postdoctoral Fellowship to JKW). The helpful assistance of Dr. Scott M. Colles and Mr. Daniel R. Prows in isolating L-FABP was much appreciated.     

Phosphatidylinositol phosphates modulate interactions between the StarD4 sterol trafficking protein and lipid membranes

There is substantial evidence for extensive nonvesicular sterol transport in cells. For example, lipid transfer by the steroidogenic acute regulator-related proteins (StarD) containing a StarT domain has been shown to involve several pathways of nonvesicular trafficking. Among the soluble StarT domain–containing proteins, StarD4 is expressed in most tissues and has been shown to be an effective sterol transfer protein. However, it was unclear whether the lipid composition of donor or acceptor membranes played a role in modulating StarD4-mediated transport. Here, we used fluorescence-based assays to demonstrate a phosphatidylinositol phosphate (PIP)-selective mechanism by which StarD4 can preferentially extract sterol from liposome membranes containing certain PIPs (especially, PI(4,5)P₂ and to a lesser degree PI(3,5)P2). Monophosphorylated PIPs and other anionic lipids had a smaller effect on sterol transport. This enhancement of transport was less effective when the same PIPs were present in the acceptor membranes. Furthermore, using molecular dynamics (MD) simulations, we mapped the key interaction sites of StarD4 with PIP-containing membranes and identified residues that are important for this interaction and for accelerated sterol transport activity. We show that StarD4 recognizes membrane-specific PIPs through specific interaction with the geometry of the PIP headgroup as well as the surrounding membrane environment. Finally, we also observed that StarD4 can deform membranes upon longer incubations. Taken together, these results suggest a mechanism by which PIPs modulate cholesterol transfer activity via StarD4.     

Acidic phospholipids strikingly potentiate sterol carrier protein 2 mediated intermembrane sterol transfer

A liposomal membrane model system was developed to examine the mechanism of spontaneous and protein-mediated intermembrane cholesterol transfer. Rat liver sterol carrier protein 2 (SCP2) and fatty acid binding protein (FABP, also called sterol carrier protein) both bind sterol. However, only SCP2 mediates sterol transfer. The exchange of sterol between small unilamellar vesicles (SUV) containing 35 mol % sterol was monitored with a recently developed assay [Nemecz, G., Fontaine, R. N., & Schroeder, F. (1988) Biochim. Biophys. Acta 943, 511-541], modified to continuous polarization measurement and not requiring separation of donor and acceptor membrane vesicles. As compared to spontaneous sterol exchange, 1.5 microM rat liver SCP2 enhanced the initial rate of sterol exchange between neutral zwwitterionic phosphatidylcholine SUV 2.3-fold. More important, the presence of acidic phospholipids (2.5-30 mol %) stimulated the SCP2-mediated increase in sterol transfer approximately 35-42-fold. Thus, acidic phospholipids strikingly potentiate the effect of SCP2 by 15-18 times as compared to SUV without negatively charged lipids. Rat liver FABP (up to 60 microM) was without effect on sterol transfer in either neutral zwitterionic or anionic phospholipid containing SUV. The potentiation of SCP2 action by acidic phospholipids was suppressed by high ionic strength, neomycin, and low pH. The results suggest that electrostatic interaction between SCP2 and negatively charged membranes may play an important role in the mechanism whereby SCP2 enhances intermembrane cholesterol transfer.     

Facilitated transfer of cholesteryl ester between rough and smooth microsomal membranes by plasma lipid transfer protein

The accessibility of intracellular membrane cholesteryl esters to removal was tested with plasma lipid transfer protein as a tool. Incubation of a mixture of non-radioactive smooth microsomes + rough microsomes prelabeled with cholesteryl ester resulted in slight movement (2-4%) of radioactive cholesteryl ester into smooth microsomes. With the addition of increasing amounts of plasma lipid transfer protein to the mixture, the % transfer of cholesteryl ester into smooth microsomes progressively increased until a plateau was reached at 14%. Movement of cholesteryl ester in the reverse direction was examined with non-radioactive rough microsomes as an acceptor and smooth microsomes prelabeled with cholesteryl ester as a donor. The pattern of the % cholesteryl ester transferred in the reverse and forward direction was almost identical in the presence of plasma lipid transfer protein, showing bidirectional movement of cholesteryl ester between membranes.     

Regulation of sterol transport between membranes and NPC2

Niemann-Pick disease type C (NPC) is caused by defects in either the NPC1 or NPC2 gene and is characterized by accumulation of cholesterol and glycolipids in the late endosome/lysosome compartment. NPC2 is an intralysosomal protein that binds cholesterol in vitro. Previous studies demonstrated rapid rates of cholesterol transfer from NPC2 to model membranes [Cheruku, S. R., et al. (2006) J. Biol. Chem. 281, 31594-31604]. To model the potential role of NPC2 as a lysosomal cholesterol export protein, in this study we used fluorescence spectroscopic approaches to examine cholesterol transfer from membranes to NPC2, assessing the rate, mechanism, and regulation of this transport step. In addition, we examined the effect of NPC2 on the rate and kinetic mechanism of intermembrane sterol transport, to model the movement of cholesterol from internal lysosomal membranes to the limiting lysosomal membrane. The results support the hypothesis that NPC2 plays an important role in endo/lysosomal cholesterol trafficking by markedly accelerating the rates of cholesterol transport. Rates of sterol transfer from and between membranes were increased by as much as 2 orders of magnitude by NPC2. The transfer studies indicate that the mechanism of NPC2 action involves direct interaction of the protein with membranes. Such interactions were observed directly using FTIR spectroscopy and protein tryptophan spectral shifts. Additionally, cholesterol transfer by NPC2 was found to be greatly enhanced by the unique lysosomal phospholipid lyso-bisphosphatidic acid (LBPA), suggesting an important role for LBPA in NPC2-mediated cholesterol trafficking.     

A fluorescence and radiolabel study of sterol exchange between membranes

The fluorescent sterols delta 5,7,9(11),22-ergostatetraen-3 beta-ol (dehydroergosterol) and delta 5,7,9,(11)-cholestatrien-3 beta-ol (cholestatrienol) as well as [1,2-3H]cholesterol were utilized as cholesterol analogues to examine spontaneous exchange of sterol between 1-palmitoyl-2-oleoylphosphatidylcholine (POPC) small unilamellar vesicles (SUV). Exchange of fluorescent sterols was monitored at 24 degrees C by release from self-quenching of polarization from the time of mixing without separation of donor and acceptor vesicles. The polarization curve for 35 mol% sterol in POPC best fitted a two-exponential function, with a fast-exchange rate constant k1 = 0.0217 min-1, 1t1/2 = 32 min, size pool 1 = 12%, and a slow rate constant k2 = 2.91.10(-3) min-1, 2t1/2 = 238 min, size pool 2 = 88%. In addition to the above two exchangeable pools of sterol, the data were consistent with the presence of a slowly or nonexchangeable pool, 42% of total sterol, that was highly dependent on sterol content. These results were confirmed by simultaneous monitoring of [1,2-3H]cholesterol radioactivity and dehydroergosterol fluorescence intensity after separation of donor and acceptor vesicles by ion-exchange column chromatography. Thus, dehydroergosterol or cholestatrienol exchange as measured by fluorescence parameters (polarization and/or intensity) provides two new methods to follow cholesterol spontaneous exchange. These methods allow resolution and quantitation of a shorter exchange t1/2 near 30 min previously not reported. Thus, the cholesterol desorption rate from membranes may be faster than previously believed. In addition, the presence of a slowly non-exchangeable pool was confirmed.     

Single cell and neural process experimentation using laterally applied electrical fields between pairs of closely apposed microelectrodes with vertical sidewalls

As biomedical research has moved increasingly towards experimentation on single cells and subcellular structures, there has been a need for microscale devices that can perform manipulation and stimulation at a correspondingly small scale. We propose a microelectrode array (MEA) featuring thickened microelectrodes with vertical sidewalls (VSW) to focus electrical fields horizontally on targets positioned in between paired electrodes. These microelectrodes were fabricated using gold electroplating that was molded by photolithographically patterned SU-8 photoresist. Finite element modeling showed that paired VSW electrodes produce more uniform electrical fields compared to conventional planar microelectrodes. Using paired microelectrodes, 3 μm thick and spaced 10 μm apart, we were able to perform local electroporation of individual axonal processes, as demonstrated by entry of EGTA to locally chelate intra-axonal calcium, quenching the fluorescence of a pre-loaded calcium indicator dye. The same electrode configuration was used to electroporate individual cells, resulting in the targeted transfection of a transgene expressing a cytoplasmically soluble green fluorescent protein (GFP). In addition to electropration, our electrode configuration was also capable of precisely targeted field stimulation on individual neurons, resulting in action potentials that could be tracked by optical means. With its ability to deliver well-characterized electrical fields and its versatility, our configuration of paired VSW electrodes may provide the basis for a new tool for high-throughput and high-content experimentation in broad areas of neuroscience and biomedical research.     

Heme transfer between phospholipid membranes and uptake by apohemoglobin

The incorporation of CO-heme into single bilayer, egg lecithin vesicles was examined by following the spectral changes that occur when the porphyrin becomes embedded in the membranes. The rate of CO-heme uptake by liposomes is extremely fast (t1/2 less than or equal to 20 ms at 10 degrees C), and the maximum extent is roughly 1 heme/5 phospholipid molecules. This limiting stoichiometry is due to unfavorable electrostatic interactions between the propionate groups of the bound CO-heme. This effect was treated theoretically by attenuating the intrinsic heme partitioning equilibrium constant with an exponential term reflecting the surface potential of the membranes. The surface potential was assumed to be proportional to the concentration of CO-heme in the membranes, and the final expression is Kp = Kop exp[-AHb/VpCp], where Kp is the observed partition constant; Kop, the intrinsic constant; Hb, the concentration of bound heme in the suspension; Vp, the partial molar volume of egg lecithin; Cp, the concentration of lipid phosphate; and A, an empirical constant representing the capacitance of the membrane for heme. For the analysis of kinetic data, the electrostatic term is assumed to apply only to the membrane dissociation rate constant, k-1, and not the association rate constant, k1. The dissociation rate was measured independently either by following the transfer of CO-heme from one vesicle fraction to another or by monitoring heme efflux from the membranes and incorporation into apohemoglobin at high protein concentrations. The data for all three sets of experiments, heme uptake, transfer, and incorporation into globin at 10 degrees C, were fitted quantitatively to the partitioning mechanism using A = 15 M-1, Kop = 5 X 10(5), k1 = 2 X 10(6) s-1, and k0(-1) = 4 s-1. Thus, heme can spontaneously migrate across lipid-water interfaces and hence diffuse rapidly from the mitochondrial inner membrane where it is synthesized to the rough endoplasmic reticulum where it is incorporated into hemoglobin.     

Protein kinase D negatively regulates hepatitis C virus secretion through phosphorylation of oxysterol-binding protein and ceramide transfer protein

Hepatitis C virus (HCV) RNA replicates its genome on specialized endoplasmic reticulum modified membranes termed membranous web and utilizes lipid droplets for initiating the viral nucleocapsid assembly. HCV maturation and/or the egress pathway requires host sphingolipid synthesis, which occur in the Golgi. Ceramide transfer protein (CERT) and oxysterol-binding protein (OSBP) play a crucial role in sphingolipid biosynthesis. Protein kinase D (PKD), a serine/threonine kinase, is recruited to the trans-Golgi network where it influences vesicular trafficking to the plasma membrane by regulation of several important mediators via phosphorylation. PKD attenuates the function of both CERT and OSBP by phosphorylation at their respective Ser¹³² and Ser²⁴⁰ residues (phosphorylation inhibition). Here, we investigated the functional role of PKD in HCV secretion. Our studies show that HCV gene expression down-regulated PKD activation. PKD depletion by shRNA or inhibition by pharmacological inhibitor Gö6976 enhanced HCV secretion. Overexpression of a constitutively active form of PKD suppressed HCV secretion. The suppression by PKD was subverted by the ectopic expression of nonphosphorylatable serine mutant CERT S132A or OSBP S240A. These observations imply that PKD negatively regulates HCV secretion/release by attenuating OSBP and CERT functions by phosphorylation inhibition. This study identifies the key role of the Golgi components in the HCV maturation process.     

Sterol domains in phospholipid membranes: dehydroergosterol polarization measures molecular sterol transfer.

The domain structure of cholesterol in membranes and factors affecting it are not well understood. A method, based on kinetics of delta 5,7,9,(11),22-erogostatetraen-3 beta-ol (dehydroergosterol) fluorescence polarization change and not requiring separation of donor and acceptor membranes, was used to examine sterol domains in three-component cholesterol:dehydroergosterol:phospholipid small unilamellar vesicles (SUV). A new mathematical data treatment was developed to provide a direct correlation between molecular sterol exchange and steady-state dehydroergosterol fluorescence polarization measurements. The method identified multiple kinetic pools of sterol in SUV: a small but rapidly exchanging pool, a predominant slowly exchanging pool, and a very slowly exchangeable (nonexchangeable) pool. The relative sizes of the pools and half-times of exchange were highly dependent on the presence of acidic phospholipids and on cytosolic proteins involved in sterol transfer. Thus, the method provides a direct measure of molecular sterol transfer between membranes without separating donor and acceptor membranes.     

Myelin glycosphingolipids, galactosylceramide and sulfatide, participate in carbohydrate-carbohydrate interactions between apposed membranes and may form glycosynapses between oligodendrocyte and/or myelin membranes

Glycosphingolipids (GSLs) can interact with each other by homotypic or heterotypic trans carbohydrate-carbohydrate interactions across apposed membranes, resulting in cell-cell adhesion. This interaction can also provide an extracellular signal which is transmitted to the cytosolic side, thus forming a glycosynapse between two cells. The two major GSLs of myelin, galactosylceramide (GalC) and its sulfated form, galactosylceramide I(3)-sulfate (SGC), are an example of a pair of GSLs which can participate in these trans carbohydrate-carbohydrate interactions and trigger transmembrane signaling. These GSLs could interact across apposed oligodendrocyte membranes at high cell density or when a membranous process of a cell contacts itself as it wraps around the axon. GalC and SGC also face each other in the apposed extracellular surfaces of the multilayered myelin sheath. Communication between the myelin sheath and the axon regulates both axonal and myelin function and is necessary to prevent neurodegeneration. Participation of transient GalC and SGC interactions in glycosynapses between the apposed extracellular surfaces of mature myelin might allow transmission of signals throughout the myelin sheath and thus facilitate myelin-axonal communication.     

Structural basis of transfer between lipoproteins by cholesteryl ester transfer protein

Human cholesteryl ester transfer protein (CETP) mediates the net transfer of cholesteryl ester mass from atheroprotective high-density lipoproteins to atherogenic low-density lipoproteins by an unknown mechanism. Delineating this mechanism would be an important step toward the rational design of new CETP inhibitors for treating cardiovascular diseases. Using EM, single-particle image processing and molecular dynamics simulation, we discovered that CETP bridges a ternary complex with its N-terminal β-barrel domain penetrating into high-density lipoproteins and its C-terminal domain interacting with low-density lipoprotein or very-low-density lipoprotein. In our mechanistic model, the CETP lipoprotein-interacting regions, which are highly mobile, form pores that connect to a hydrophobic central cavity, thereby forming a tunnel for transfer of neutral lipids from donor to acceptor lipoproteins. These new insights into CETP transfer provide a molecular basis for analyzing mechanisms for CETP inhibition.     

An assay for the alpha-tocopherol binding protein mediated transfer of vitamin E between membranes

A model system consisting of donor membrane (egg lecithin liposomes) and acceptor membrane (human erythrocyte ghosts or rat liver mitochondria) were used to investigate the alpha-tocopherol binding protein (alpha TBP) mediated transfer of alpha-tocopherol. Liposomes containing RRR-[alpha-3H]tocopherol ([alpha-3H]T) were incubated with acceptor membrane at 37 degrees C for 0-45 min in the presence or absence of rat liver cytosol or a dialyzed 30-60% saturated ammonium sulfate precipitated fraction of rat liver cytosol (Fraction B). Erythrocyte ghosts and liver mitochondria were compared and found to behave similarly in the presence of Fraction B. alpha-Tocopherol transfer activity (alpha TTA) typically varied 0- to 27-fold greater than buffer blanks, depending upon type and concentration of protein preparation. Gel filtration of Fraction B yielded one alpha TTA peak (liver mitochondria as acceptor) with an estimated Mr of 39,000. [alpha-3H]T recovered from erythrocyte ghosts pellets by HPLC suggest that the [alpha-3H]T was transferred intact. alpha TTA of Fraction B in the presence of varying concentrations of erythrocyte ghosts and liposomal [alpha-3H]T followed saturation kinetics. Optimal concentrations gave alpha TTA responses directly proportional to rat liver cytosol concentration. alpha TTA was inhibited only 5% in the presence of a 32-fold excess of cold liposomal alpha-tocopheryl acetate suggesting that the free hydroxyl group on the chromanol ring of alpha-tocopherol is needed for transfer. Coefficient of variation of repeated measures of alpha TTA in rat liver cytosol was 2.9%. Thus, the intermembrane transfer phenomenon of alpha-tocopherol can be studied quantitatively and can be used to compare liver protein preparations exhibiting transfer activity.     

Complexing of glycolipids and their transfer between membranes by the activator protein for degradation of lysosomal ganglioside GM2

The lysosomal degradation of ganglioside GM2 by hexosaminidase A depends on the presence of the specific activator protein which mediates the interaction between micellar or membrane-bound ganglioside and water-soluble hydrolase. The mechanism and the glycolipid specificity of this activator were studied in more detail. 1. It could be shown with three different techniques (isoelectric focusing, centrifugation and electrophoresis) that the activator protein extracts glycolipid monomers from micelles or liposomes to give water-soluble complexes with a stoichiometry of 1 mol of glycolipid/mol of activator protein. Liposome-bound ganglioside GM2 is considerably more stable against extraction and degradation than micellar ganglioside. 2. In the absence of enzyme the activator acts in vitro as glycolipid transfer protein, transporting glycolipids from donor to acceptor membranes. 3. The activator protein is rather specific for ganglioside GM2. Other glycolipids (GM3 GM1, GD1a and GA2) form less stable complexes with the activator and are transferred at a slower rate (except for ganglioside GM1) than ganglioside GM2.     

Cholesterol metabolism and sterol carrier protein-2 (non-specific lipid transfer protein)

Hepatic sterol carrier protein-2 significantly enhances the microsomal conversion of cholesterol to 7 alpha-hydroxy-cholesterol. In the present work we have attempted to correlate the hepatic content of sterol carrier protein-2 with bile acid formation. We have determined the amount of this protein in a variety of physiological and experimental conditions, in which the rate of bile acid synthesis varies over a wide range, viz. during fetal development, in inbred strains of rats with different rates of bile acid synthesis, and in rats fed diets containing drugs which modify the rate of bile acid synthesis. The outcome of these experiments does not support the idea that sterol carrier protein-2 has any association with bile acid synthesis. From our data we further conclude that hepatic sterol carrier protein-2 is an adaptable protein because its level increases during development from the fetal to the post-weaning stage of the rat and since it can be modulated by oral administration of certain drugs. Furthermore, it is demonstrated that the level of sterol carrier protein-2 varies between six inbred strains of rats.