Very high proportion of disaturated molecular species in rat platelet diacyl-glycerophosphocholine: involvement of CoA-dependent transacylation reactions

Mast cell- and dendritic cell-derived exosomes display a specific lipid composition and an unusual membrane organization

Exosomes are small vesicles secreted from multivesicular bodies, which are able to stimulate the immune system leading to tumour cell eradication. We have analysed lipids of exosomes secreted either upon stimulation from rat mast cells (RBL-2H3 cells), or constitutively from human dendritic cells. As compared with parent cells, exosomes displayed an enrichment in sphingomyelin, but not in cholesterol. Phosphatidylcholine content was decreased, but an enrichment was noted in disaturated molecular species as in phosphatidylethanolamines. Lyso(bis)phosphatidic acid was not enriched in exosomes as compared with cells. Fluorescence anisotropy demonstrated an increase in exosome-membrane rigidity from pH 5 to 7, suggesting their membrane reorganization between the acidic multivesicular body compartment and the neutral outer cell medium. NMR analysis established a bilayer organization of exosome membrane, and ESR studies using 16-doxyl stearic acid demonstrated a higher flip-flop of lipids between the two leaflets as compared with plasma membrane. In addition, the exosome membrane exhibited no asymmetrical distribution of phosphatidylethanolamines. Therefore exosome membrane displays a similar content of the major phospholipids and cholesterol, and is organized as a lipid bilayer with a random distribution of phosphatidylethanolamines. In addition, we observed tight lipid packing at neutral pH and a rapid flip-flop between the two leaflets of exosome membranes. These parameters could be used as a hallmark of exosomes.

Rafts promote assembly and atypical targeting of a nonenveloped virus, rotavirus, in Caco-2 cells

Rotavirus follows an atypical pathway to the apical membrane of intestinal cells that bypasses the Golgi. The involvement of rafts in this process was explored here. VP4 is the most peripheral protein of the triple-layered structure of this nonenveloped virus. High proportions of VP4 associated with rafts within the cell as early as 3 h postinfection. In the meantime a significant part of VP4 was targeted to the Triton X-100-resistant microdomains of the apical membrane, suggesting that this protein possesses an autonomous signal for its targeting. At a later stage the other structural rotavirus proteins were also found in rafts within the cells together with NSP4, a nonstructural protein required for the final stage of virus assembly. Rafts purified from infected cells were shown to contain infectious particles. Finally purified VP4 and mature virus were shown to interact with cholesterol- and sphingolipid-enriched model lipid membranes that changed their phase preference from inverted hexagonal to lamellar structures. Together these results indicate that a direct interaction of VP4 with rafts promotes assembly and atypical targeting of rotavirus in intestinal cells.

Transacylase-mediated alkylacyl-GPC synthesis and its hydrolysis by phospholipase D occur in separate cell compartments in the human neutrophil

Subcellular localizations of CoA-independent transacylase and phospholipase D enzymes have been investigated in human neutrophils performing a two-step gradient system to separate plasma membranes from internal membranes and from the bulk of granules. The internal membranes were constituted by endoplasmic reticulum and by a subpopulation of specific and tertiary granules. The enzymes activities were assayed in vitro on gradient fractions using exogenous substrates. Following cell prelabelling with [3H]alkyllyso-GPC, we also analyzed the in situ localization of labelled products involving the action of both enzymes. The CoA-independent transacylase activity, together with the CoA-dependent transacylase and acyltransferase activities were only located in the internal membranes. Following 15 min cell labelling, part of the [3H]alkylacyl-GPC was recovered in plasma membranes indicating a rapid redistribution of the acylated compound. Very high contents in arachidonate containing [3H]alkylacyl-GPC were recovered both in plasma membranes and internal membranes. Phospholipase D activity being assayed in the presence of cytosol, GTP gamma S and gradient fractions, only the plasma membrane fractions from resting or stimulated cells allowed the enzyme to be active. The [3H]alkylacyl-GP and [3H]alkylacyl-GPethanol, phospholipase D breakdown products from [3H]alkylacyl-GPC, obtained after neutrophil prelabelling and activation by phorbol myristate acetate, were exclusively present in the plasma membranes. In contrast, the secondary generated [3H]alkylacylglycerols were equally distributed between plasma and internal membranes. No labelled product was recovered on azurophil granules. These data demonstrate that internal membranes are the site of action of the CoA-independent transacylase and plasma membranes are the site of action of the phospholipase D. This topographical separation between CoA-independent transacylase which generated substrate and phospholipase D which degraded it, suggested that subcellular localisation and traffic of substrates within the cell can be important to regulate the enzymes.

Coenzyme A-dependent cleavage of membrane phospholipids in several rat tissues: ATP-independent acyl-CoA synthesis and the generation of lysophospholipids

Substantial amounts of acyl-CoA were formed when microsomes from several rat tissues were incubated with varying concentrations of free CoA and bovine serum albumin even in the absence of ATP and Mg2+. For instance, 86 nmol of acyl-CoA was produced when microsomes (5 mg protein) were incubated with 300 microM CoA for 30 min. It was calculated that 1.8% of total fatty acyl residues were converted to acyl-CoA during the incubation. No appreciable amount of acyl-CoA was formed from free fatty acid or from boiled microsomes under the same experimental conditions. These observations indicate that acyl-CoA is formed from microsomal lipids by an enzyme activity distinct from previously known long-chain fatty acyl-CoA synthetase. The apparent Km value for CoA and Vmax were 180 microM and 20 nmol/30 min per mg protein, respectively. We found that several species of acyl-CoA such as arachidonoyl-CoA were preferentially synthesized through the reaction and that several types of phospholipids actually act as acyl donors in the formation of acyl-CoA. Phosphatidylinositol and phosphatidylcholine appear to be preferred substrates. We confirmed that lysophosphatidylinositol and lysophosphatidylcholine were generated along with the formation of acyl-CoA. It seems very likely that CoA-mediated cleavage of phospholipids/ATP-independent acyl-CoA synthesis is implicated in the metabolism of certain types of fatty acyl residues of membranous phospholipids in mammalian cells.

Biosynthesis of platelet-activating factor in cultured mast cells. Involvement of the CoA-independent transacylase demonstrated by analysis of the molecular species of platelet-activating factor

We have recently demonstrated that arachidonate [20:4(5,8,11,14)] was primarily linked to the hexadecyl (16:0) and octadecenyl (18:1) species of alkylacyl derivatives of glycerolphosphocholine (GroPCho). Consistent with the involvement of arachidonate-specific CoA-independent transacylase in the synthesis of platelet-activating factor (PAF; 1-O-alkyl-2-acetyl-GroPCho), 16:0 and 18:1 PAF species were formed upon antigen stimulation [Joly, F., Breton, M., Wolf, C., Ninio, E. & Colard, O. (1992) Biochim. Biophys. Acta 1125, 305-312]. In the present work, addition of lyso-PAF to mast cells resulted in PAF production. We analyzed the PAF species formed in the presence of a defined lyso-PAF molecular species in order to differentiate between either direct acetylation or involvement of the membrane precursor. The 18:1 lyso-PAF was more effective than the 16:0 in producing PAF which was composed of 95% 18:1 PAF, the balance being 16:0, indicating that part of the acetylated lyso-PAF originated from the cellular pool of alkyl-arachidonyl-GroPCho in resting cells. Consistent with alkyl-arachidonyl-GroPCho species content and acetyltransferase specificity, similar amounts of 16:0 and 18:1 PAF species were formed when mast cells were stimulated with antigen. Supplemented with 16:0 or 18:1 lyso-PAF, antigen-stimulated mast cells responded by 230% and 125% increase in PAF synthesis, respectively. As expected, the amount of the PAF species corresponding to the added lyso-PAF was increased. More interestingly, addition of 16:0 lyso-PAF almost doubled the amount of 18:1 PAF content as compared to antigen alone, thus indicating that the lyso-PAF formed via the CoA-independent transacylase was significantly used for PAF synthesis, despite a large excess of exogenous lyso-PAF. The CoA-independent transacylase, measured using [3H]lyso-PAF as a substrate in sonicates from antigen-stimulated cells, was decreased concurrently with PAF formation. In conclusion, we show that when lyso-PAF is added to mast cells, a direct acetylation may occur. However, PAF is preferentially synthesized through a mechanism involving the CoA-independent transacylase reaction.

The activation of rat platelets increases the exposure of polyunsaturated fatty acid enriched phospholipids on the external leaflet of the plasma membrane

Rat platelets have been hydrogenated in the presence of colloidal palladium adsorbed on the surface of the non water-soluble polymer polyvinylpolypyrrolidone. This non-permeating catalyst restricts hydrogenation of the fatty acyl double bonds of phospholipids only in the outer half of the plasma membrane. The pattern of hydrogenation of the molecular species present on the external side of the membrane is determined using desorption-chemical soft ionization-mass spectrometry (DCI-MS) before and after cell activation by the calcium ionophore A23187. The accessibility to the catalyst of the polyunsatured molecular species within each phospholipid class is compared for resting and activated cells. The abundance of polyunsaturated species of phosphatidyl-ethanolamine and -serine in the inner half of the resting biomembrane is confirmed in rat platelets. Phosphatidylcholine is especially rich in disaturated species in this membrane. The induced exposure of the polyunsaturated species of diacyl- and ether-phosphatidylethanolamine, and of phosphatidylserine on the external side of the membrane appears after activation by the calcium ionophore. A detailed quantitative analysis within a phospholipid class shows an unequal scrambling for diacyl-, alkyl-, alkenyl-phosphatidylethanolamine, and a variable involvement in the transmembrane redistribution following cell activation of the various molecular species as a function of the acyl moities.

Heterogeneity of arachidonate and paf-acether precursor pools in mast cells

In mammalian cells, arachidonate release and paf-acether formation are frequently associated. The alkyl-acyl-GPC has been proposed as an important source for released arachidonic acid and arachidonate-containing alkylacyl-GPC species as unique precursor for paf-acether. However, the specificity of precursor pools either concerning arachidonic acid or paf-acether is still a matter of controversy. We studied the relationship between the precursor pools for both autacoids in antigenically-stimulated cultured mast cells. We took advantage of the particular arachidonate turnover rate in each phospholipid to investigate the role of alkyl-arachidonyl-GPC in the supply of arachidonic acid by using newly and previously [14C]arachidonate-labeled cells. The specific activity of the released arachidonate was reduced 2-fold following overnight cell incubation, whereas labeling in alkyl-arachidonoyl-GPC was only slightly modified and never corresponded to that of released arachidonate when newly or previously labeled cells were triggered with the antigen. These results are not in favor of a major role for alkyl-arachidonoyl-GPC in supplying arachidonate. In contrast, by using previously labeled cells, we demonstrated that all arachidonate-containing phospholipids were involved in the release of arachidonic acid. The pattern of alkyl chains in alkyl-arachidonoyl-GPC, as well as in total alkylacyl-GPC, is unique since it consists mainly of 18:1 (more than 55%), whereas the 16:0 represents only about 30% of total alkyl chains. Therefore, we analyzed paf-acether molecular composition in order to compare it to the alkyl composition of the precursor pools. The content in 18:1 species of paf-acether, as measured by bioassay (aggregation of rabbit platelets), was always lower than that of 16:0 species and then did not correspond to the alkyl composition of the precursor. These data suggest that the enzymes involved in paf synthesis might be specific for 16:0 alkyl chains of precursor pool.

Phospholipid acylation by mouse sciatic nerve microsomes

The partition of 0.3 nmol of [1-14C]oleoyl-CoA in the microsomes (10 micrograms proteins) from mouse sciatic nerves is unaffected by the presence of lysophospholipids and is about 45% of the total oleoyl-CoA (77% of the acylglycerophosphocholine partition in the membrane). The concentration of both oleoyl-CoA and acylglycerophosphocholine is over 1 mM in the membrane. There is a selective acyl transfer from acyl-CoA to lysolipid acceptors (oleoyl greater than myristoyl, palmitoyl, stearoyl much greater than eicosanoyl greater than docosanoyl, tetracosanoyl). The exogenous acyl acceptors are acylglycerophosphocholine and acylglycerophosphoinositol and to a lesser extent acylglycerophosphoethanolamine, but not acylglycerophosphoserine. A PC formation from acylGPC in the absence of exogenous acyl donors or from oleoyl-CoA in the absence of exogenous acyl acceptor was also observed.

Biosynthesis of paf-acether. XVII. Regulation by the CoA-independent transacylase in human neutrophils

Treatment of intact human polymorphonuclear neutrophils (PMN) with low concentrations of phorbol myristate acetate (PMA, 1-10 ng/ml) induced paf-acether (paf) and lyso paf formation, arachidonate release, and simultaneous inhibition of CoA-independent lyso paf: transacylase as assayed in a cell-free system. Inhibition of [3H]lyso paf reacylation was also observed when it was exogenously added to the PMA-treated intact PMN. When higher concentrations of PMA (40-100 ng/ml) were used, paf biosynthesis was severely impaired and the level of the CoA-independent transacylase activity returned to basal level. Since lyso paf appears to be the substrate for PMA-activated paf formation (remodeling pathway), we showed that [14C]acetate was incorporated into the paf molecule. By contrast, labeling with [3H]choline was not appropriate in this model. The presented results are against the involvement of a de novo route in paf synthesis initiated by PMA and open a new possibility of an important role for the CoA-independent transacylase in controlling the level of lyso paf availability for paf formation.