Alkylacyl glycerophosphoinositol in human and bovine erythrocytes. Molecular species composition and comparison with glycosyl-inositolphospholipid anchors of erythrocyte acetylcholinesterases

Lipid remodelling of glycosylphosphatidylinositol (GPI) glycoconjugates in procyclic-form trypanosomes: biosynthesis and processing of GPIs revisited

The African trypanosome, Trypanosoma brucei, has been used as a model to study the biosynthesis of GPI (glycosylphosphatidylinositol) anchors. In mammalian (bloodstream)-form parasites, diacyl-type GPI precursors are remodelled in their lipid moieties before attachment to variant surface glycoproteins. In contrast, the GPI precursors of insect (procyclic)-form parasites, consisting of lyso-(acyl)PI (inositol-acylated acyl-lyso-phosphatidylinositol) species, remain unaltered before protein attachment. By using a combination of metabolic labelling, cell-free assays and complementary MS analyses, we show in the present study that GPI-anchored glycoconjugates in T. congolense procyclic forms initially receive tri-acylated GPI precursors, which are subsequently de-acylated either at the glycerol backbone or on the inositol ring. Chemical and enzymatic treatments of [3H]myristate-labelled lipids in combination with ESI-MS/MS (electrospray ionization-tandem MS) and MALDI-QIT-TOF-MS3 (matrix-assisted laser-desorption ionization-quadrupole ion trap-time-of-flight MS) analyses indicate that the structure of the lipid moieties of steady-state GPI lipids from T. congolense procyclic forms consist of a mixture of lyso-(acyl)PI, diacyl-PI and diacyl-(acyl)PI species. Interestingly, some of these species are myristoylated at the sn-2 position. To our knowledge, this is the first demonstration of lipid remodelling at the level of protein- or polysaccharide-linked GPI anchors in procyclic-form trypanosomes.

Analysis of glycerophosphoinositol by liquid chromatography–electrospray ionisation tandem mass spectrometry using a β-cyclodextrin-bonded column

Glycerophosphoinositol (GroPIns) has been demonstrated to have important roles in many intracellular regulatory processes. GroPIns has been analysed for many years by anion-exchange HPLC after radiolabelling of cells in culture, but no method has been developed, to our knowledge, for the direct detection and quantitation of the unlabelled compound in such biological samples. Here is reported a liquid chromatography-tandem mass spectrometry (LC-MS/MS) method for the direct quantitative analysis of GroPIns that can indeed be applied to cell extracts. Analyses were performed on a beta-cyclodextrin-bonded HPLC column using a binary mobile phase of acetonitrile and 20 mM ammonium formate in water, which allowed direct on-line detection by tandem mass spectrometry in negative electrospray ionisation (ESI) mode. The method was applied to the quantitative analysis of GroPIns in selected rat cell lines after a two-phase acid extraction of cultured cells using external calibration. The potential matrix signal suppression effects were investigated by the parallel quantitation of GroPIns in extracts of selected cultured cell lines with both external calibration and the standard additions method. The accuracy data obtained demonstrated the feasibility of external calibration, so allowing a simpler and less time-consuming approach than that of the standard additions method.

The diacylglycerol and protein kinase C pathways are not involved in insulin signalling in primary rat hepatocytes

Diacylglycerol (DAG) and protein kinase C (PKC) isoforms have been implicated in insulin signalling in muscle and fat cells. We evaluated the involvement of DAG and PKC in the action of insulin in adult rat hepatocytes cultured with dexamethasone, but in the absence of serum, for 48 h. Our results show that although insulin stimulated glycolysis and glycogen synthesis, it had no effect on DAG mass or molecular species composition. Epidermal growth factor showed the expected insulin-mimetic effect on glycolysis, whereas ATP and exogenous phospholipase C acted as antagonists and abolished the insulin signal. Similarly to insulin, epidermal growth factor had no effect on DAG mass or molecular species composition. In contrast, both ATP and phospholipase C induced a prominent increase in several DAG molecular species, including 18:0/20:4, 18:0/20:5, 18:0/22:5 and a decrease in 18:1/18:1. These changes were paralleled by an increase in phospholipase D activity, which was absent in insulin-treated cells. By immunoblotting or by measuring PKC activity, we found that neither insulin nor ATP translocated the PKCalpha, -delta, -epsilon or -zeta isoforms from the cytosol to the membrane in cells cultured for six or 48 h. Similarly, insulin had no effect on immunoprecipitable PKCzeta. Suppression of the glycogenic insulin signal by phorbol 12-myristate 13-acetate, but not by ATP, could be completely alleviated by bisindolylmaleimide. Finally, insulin showed no effect on DAG mass or translocation of PKC isoforms in the perfused liver, although it reduced the glucagon-stimulated glucose output by 75%. Together these results indicate that phospholipases C and D or multiple PKC isoforms are not involved in the hepatic insulin signal chain.

Analysis of phospholipid molecular species by liquid chromatography--atmospheric pressure chemical ionisation mass spectrometry of diacylglycerol nicotinates

A method using liquid chromatography - atmospheric pressure chemical ionisation mass spectrometry was evaluated for determining the molecular species composition of phospholipids (phosphatidylcholines from soybean, egg yolk and bovine liver) after conversion to diacylglycerol nicotinate derivatives. The structures could be deduced from pseudo-molecular ions ([MH-123](+)) and three pairs of monoacyl containing fragment ions. All molecular species in mixed peaks were readily identified and many minor components, earlier not encountered in the samples under investigation, were identified. Acyl chain regioisomers were readily distinguished by the ratio of the [MH-RCHCO](+) ions. Molecular species differing only in the position of the double bonds in one polyunsaturated acyl chain were separated on the basis of retention times. A half quantitative estimation of the molecular species composition of complex samples was achieved by a combination of UV detection and, for mixed peaks, the areas of [MH-123](+) ions.

Recent developments in the cell biology and biochemistry of glycosylphosphatidylinositol lipids (review)

Glycosylphosphatidylinositols (GPIs) represent an abundant and ubiquitous class of eukaryotic glycolipids. Although these structures were originally discovered in the form of GPI-anchored cell surface glycoproteins, it is becoming increasingly clear that a significant proportion of the GPI synthetic output of a cell is not directed to protein anchoring. Indeed, pools of non-protein-linked GPIs can approach 10(7) molecules per cell in some cell types, especially the protozoa, with a large proportion of these molecules being displayed at the cell surface. Recent studies which form the subject of this review indicate that there is (a) considerable diversity in the range of structural modifications found on GPI glycolipids within and between species and cell types, (b) complexity in the topological arrangement of the GPI biosynthetic pathway in the endoplasmic reticulum, and (c) spatial restriction of the biosynthetic pathway within the endoplasmic reticulum. Furthermore, consistent with additional functional roles for these lipids beyond serving as protein anchor precursors, products of the GPI biosynthetic pathway appear to be widely distributed in the cellular endomembrane system. These studies indicate that there is still much to learn about the organization of glycolipid biosynthetic pathways in eukaryotic cells, the nature and subcellular distribution of the lipid products of these pathways, and the function of these lipids within cells.

Lipid metabolism in peroxisomes in relation to human disease

Peroxisomes were long believed to play only a minor role in cellular metabolism but it is now clear that they catalyze a number of important functions. The importance of peroxisomes in humans is stressed by the existence of a group of genetic diseases in man in which one or more peroxisomal functions are impaired. Most of the functions known to take place in peroxisomes have to do with lipids. Indeed, peroxisomes are capable of 1. fatty acid beta-oxidation 2. fatty acid alpha-oxidation 3. synthesis of cholesterol and other isoprenoids 4. ether-phospholipid synthesis and 5. biosynthesis of polyunsaturated fatty acids. In Chapters 2-6 we will discuss the functional organization and enzymology of these pathways in detail. Furthermore, attention is paid to the permeability properties of peroxisomes with special emphasis on recent studies which suggest that peroxisomes are closed structures containing specific membrane proteins for transport of metabolites. Finally, the disorders of peroxisomal lipid metabolism will be discussed.

Delineation of three pathways of glycosylphosphatidylinositol biosynthesis in Leishmania mexicana. Precursors from different pathways are assembled on distinct pools of phosphatidylinositol and undergo fatty acid remodeling

Glycosylphosphatidylinositol (GPI) glycolipids are major cell surface constituents in the Leishmania parasites. Distinct classes of GPI are present as membrane anchors for several surface glycoproteins and an abundant lipophosphoglycan as well as being the major glycolipids (GIPLs) in the plasma membrane. In this study we have identified putative precursors for the protein and lipophosphoglycan anchors and delineated the complete pathway for GIPL biosynthesis in Leishmania mexicana promastigotes. Based on the structural analyses of these GPI intermediates and their kinetics of labeling in vivo and in cell-free systems, we provide evidence that the GIPLs are the products of an independent biosynthetic pathway rather than being excess precursors of the anchor pathways. First, we show that the similar glycan head groups of the GIPL and protein/lipophosphoglycan anchor precursors are assembled on two distinct pools of PI corresponding to 1-O-(C18:0)alkyl-2-stearoyl-PI and 1-O-(C24:0/C26:0)-2-stearoyl-PI, respectively. These PI species account for 20 and 1% of the total PI pool, respectively, indicating a remarkable specificity in their selection. Second, analysis of the flux of intermediates through these pathways in vivo and in a cell-free system suggests that the GIPL and anchor pathways are independently regulated. We also show that GIPL biosynthesis requires fatty acid remodeling, in which the sn-2 stearoyl chains are replaced with myristoyl or lauroyl chains. Fatty acid remodeling is dependent on CoA and ATP and occurs on pre-existing but not on de novo synthesized GIPLs. We suggest that the compartmentalization of different GPI pathways may be important in regulating the species and stage-specific expression of different GPI structures in these parasites.

Alternative lipid remodelling pathways for glycosylphosphatidylinositol membrane anchors in Saccharomyces cerevisiae

Glycosylphosphatidylinositol (GPI)-anchored membrane proteins of Saccharomyces cerevisiae exist with two types of lipid moiety--diacylglycerol or ceramide--both of which contain 26:0 fatty acids. To understand at which stage of biosynthesis these long-chain fatty acids become incorporated into diacylglycerol anchors, we compared the phosphatidylinositol moieties isolated from myo-[2-(3)H]inositol-labelled protein anchors and from GPI intermediates. There is no evidence for the presence of long-chain fatty acids in any intermediate of GPI biosynthesis. However, GPI-anchored proteins contain either the phosphatidylinositol moiety characteristic of the precursor lipids or a version with a long-chain fatty acid in the sn-2 position of glycerol. The introduction of long-chain fatty acids into sn-2 occurs in the endoplasmic reticulum (ER) and is independent of the sn-2-specific acyltransferase SLC1. Analysis of ceramide anchors revealed the presence of two types of ceramide, one added in the ER and another more polar molecule which is found only on proteins which have reached the mid Golgi. In summary, the lipid of GPI-anchored proteins can be exchanged by at least three different remodelling pathways: (i) remodelling from diacylglycerol to ceramide in the ER as proposed previously; (ii) remodelling from diacylglycerol to a more hydrophobic diacylglycerol with a long-chain fatty acid in sn-2 in the ER; and (iii) remodelling to a more polar ceramide in the Golgi.

Microheterogeneity of the hydrophobic and hydrophilic part of the glycosylphosphatidylinositol anchor of alkaline phosphatase from calf intestine

Digestion of calf intestine alkaline phosphatase with pronase and subsequent dephosphorylation of the released peptidyl-(Etn-P)2-glycosyl-PtdIns with HF generated 8 glycosyl-Ins species the largest of which (G1 and G2) have the following proposed structures: [sequence: see text] G3 and G5 are lower homologues of G1 and G2, respectively, being one alpha 1-2 linked mannopyranosyl residue shorter. G4 is analogous to G2 lacking the N-acetylgalactosaminyl residue and G6 is the next lower homologue of G4. Most of G4 and G6 occur substituted with a palmitoyl (G4, G6) or a myristoyl residue (G6) probably attached to the inositol moiety. Thus, the basic ManxGlc-Ins species are either substituted with an N-acetylgalactosaminyl residue or a fatty acid ester. The structures were deduced from compositional analysis, molecular-mass determination by matrix-assisted laser desorption MS, sequential hydrolysis with appropriate exoglycosidases and treatment with CrO3. Purification of the glycosylinositol species was achieved by a novel reverse-phase HPLC technique using fluorescent fluoren-9-yl-methoxy-carbonyl (Fmoc) derivatives. These stable derivatives were susceptible to hydrolysis with exoglycosidases which allowed sequential cleavages to be carried out and kinetics to be followed at the picomole level. We observed recently that native alkaline phosphatase separates on octyl-Sepharose into four distinct fractions of increasing hydrophobicity (F1-F4). Here we show that all four fractions contain G1-G6. The acylated species G4 and G6 were restricted to F2 and F4 which had been shown earlier to contain, on average, 2.5 and 3 fatty acid residues/subunit, respectively. In all four fractions the diradylglycerol moiety was predominantly diacylglycerol, alkylacylglycerol being less than 10% which is in contrast to most glycosyl-PtdIns--anchored proteins of mammalian origin.

Compositional analysis of glucosaminyl(acyl)phosphatidylinositol accumulated in HeLa S3 cells

GlcN(acyl)PtdIns, a derivative of phosphatidylinositol (PtdIns) in which glucosamine and a fatty acid are linked to inositol hydroxyl groups, has been proposed to be an intermediate in the mammalian biosynthetic pathway for glycosylphosphatidylinositol (glycosyl-PtdIns) anchors of membrane proteins. In this report, GlcN(acyl)PtdIns metabolically labeled with [3H]inositol is shown to accumulate in a HeLa S3 cell subline. The amount of GlcN(acyl)PtdIns in these HeLa S3 cells is about 10(7) molecules/cell, a level comparable to those of the most abundant glycosyl-PtdIns-containing molecules reported to date. GlcN(acyl)PtdIns was purified by a two-step procedure involving octyl-Sepharose and thin-layer chromatography. Octyl-Sepharose separated phospholipids according to their number of hydrocarbon chains: one in 2-lysoPtdIns, two in PtdIns, and three in GlcN(acyl)PtdIns. Purification also was aided by prior treatment of lipid extracts with bee venom phospholipase A2, an enzyme that did not cleave GlcN(acyl)PtdIns. The GlcN-inositol head group in purified GlcN(acyl)PtdIns was confirmed by a number of procedures, including cation-exchange chromatography and mass spectrometry; after radiomethylation, an equal molar ratio of GlcN(Me)2/inositol was measured. Fatty acid analysis indicated an overall stoichiometry of 2.3 mol fatty acid/mol inositol with palmitic (16:0), stearic (18:0) and oleic (18:1) acids being predominant. Analysis of GlcN(acyl)inositol produced by HF fragmentation showed that palmitate was the acyl group attached to inositol and indicated that stearic and oleic acids were in the glycerolipid. Base methanolysis revealed that about 15% of the purified GlcN(acyl)PtdIns contained alkylglycerol. A substantial conversion of GlcN(acyl)PtdIns to a slightly more polar lipid occurred after overnight incubation in even mildly alkaline buffers. Although the current data do not allow proposal of a structure for this lipid, its formation from GlcN(acyl)PtdIns may be important because the conversion appeared to occur in vivo.