Biosynthetic conversion of phosphatidylglycerol to sn-1:sn-1' bis(monoacylglycerol) phosphate in a macrophage-like cell line

PLD3 and PLD4 synthesize S,S-BMP, a key phospholipid enabling lipid degradation in lysosomes

Bis(monoacylglycero)phosphate (BMP) is an abundant lysosomal phospholipid required for degradation of lipids, in particular gangliosides. Alterations in BMP levels are associated with neurodegenerative diseases. Unlike typical glycerophospholipids, lysosomal BMP has two chiral glycerol carbons in the S (rather than the R) stereo-conformation, protecting it from lysosomal degradation. How this unusual and yet crucial S,S-stereochemistry is achieved is unknown. Here we report that phospholipases D3 and D4 (PLD3 and PLD4) synthesize lysosomal S,S-BMP, with either enzyme catalyzing the critical glycerol stereo-inversion reaction in vitro. Deletion of PLD3 or PLD4 markedly reduced BMP levels in cells or in murine tissues where either enzyme is highly expressed (brain for PLD3; spleen for PLD4), leading to gangliosidosis and lysosomal abnormalities. PLD3 mutants associated with neurodegenerative diseases, including Alzheimer's disease risk, diminished PLD3 catalytic activity. We conclude that PLD3/4 enzymes synthesize lysosomal S,S-BMP, a crucial lipid for maintaining brain health.

LRRK2 and GBA1 variant carriers have higher urinary bis(monacylglycerol) phosphate concentrations in PPMI cohorts

We quantified concentrations of three isoforms of the endolysosomal lipid, bis(monoacylglycerol) phosphate (BMP) in the urine of deeply phenotyped cohorts in the Parkinson's Progression Markers Initiative: LRRK2 G2019S PD (N = 134) and non-manifesting carriers (NMC) (G2019S+ NMC; N = 182), LRRK2 R1441G PD (N = 15) and R1441G+ NMC (N = 15), GBA1 N409S PD (N = 76) and N409S+ NMC (N = 178), sporadic PD (sPD, N = 379) and healthy controls (HC) (N = 190). The effects of each mutation and disease status were analyzed using nonparametric methods. Longitudinal changes in BMP levels were analyzed using linear mixed models. At baseline, all LRRK2 carriers had 3-7× higher BMP levels compared to HC, irrespective of the disease status. GBA1 N409S carriers also showed significant, albeit smaller, elevation (~30-40%) in BMP levels compared to HC. In LRRK2 G2019S PD, urinary BMP levels remained stable over two years. Furthermore, baseline BMP levels did not predict disease progression as measured by striatal DaT imaging, MDS-UPDRS III Off, or MoCA in any of the cohorts. These data support the utility of BMP as a target modulation biomarker in therapeutic trials of genetic and sPD but not as a prognostic or disease progression biomarker.

Synthesis of Phosphatidyl Glycerol Containing Unsymmetric Acyl Chains Using H-Phosphonate Methodology

Naturally occurring phospholipids, such as phosphatidyl glycerol (PG), are gaining interest due to the roles they play in disease mechanisms. To elucidate the metabolism of PG, an optically pure material is required, but this is unfortunately not commercially available. Our previous PG synthesis route utilized phosphoramidite methodology that addressed issues surrounding fatty acid substrate scope and glycerol backbone modifications prior to headgroup phosphorylation, but faltered in the reproducibility of the overall pathway due to purification challenges. Herein, we present a robust pathway to optically pure PG in fewer steps, utilizing H-phosphonates that features a chromatographically friendly and stable triethyl ammonium H-phosphonate salt. Our route is also amendable to the simultaneous installation of different acyl chains, either saturated or unsaturated, on the glycerol backbone.

Life in the lumen: The multivesicular endosome

The late endosomes/endo‐lysosomes of vertebrates contain an atypical phospholipid, lysobisphosphatidic acid (LBPA) (also termed bis[monoacylglycero]phosphate [BMP]), which is not detected elsewhere in the cell. LBPA is abundant in the membrane system present in the lumen of this compartment, including intralumenal vesicles (ILVs). In this review, the current knowledge on LBPA and LBPA‐containing membranes will be summarized, and their role in the control of endosomal cholesterol will be outlined. Some speculations will also be made on how this system may be overwhelmed in the cholesterol storage disorder Niemann‐Pick C. Then, the roles of intralumenal membranes in endo‐lysosomal dynamics and functions will be discussed in broader terms. Likewise, the mechanisms that drive the biogenesis of intralumenal membranes, including ESCRTs, will also be discussed, as well as their diverse composition and fate, including degradation in lysosomes and secretion as exosomes. This review will also discuss how intralumenal membranes are hijacked by pathogenic agents during intoxication and infection, and what is the biochemical composition and function of the intra‐endosomal lumenal milieu. Finally, this review will allude to the size limitations imposed on intralumenal vesicle functions and speculate on the possible role of LBPA as calcium chelator in the acidic calcium stores of endo‐lysosomes.

Bis(monoacylglycero)phosphate, an important actor in the host endocytic machinery hijacked by SARS-CoV-2 and related viruses

Viruses, including the novel coronavirus SARS-CoV-2, redirect infected cell metabolism to their own purposes. After binding to its receptor angiotensin-converting enzyme 2 (ACE2) on the cell surface, the SARS-CoV-2 is taken up by receptor-mediated endocytosis ending in the acidic endolysosomal compartment. The virus hijacks the endosomal machinery leading to fusion of viral and endosomal membranes and release of the viral RNA into the cytosol. This mini-review specifically highlights the membrane lipid organization of the endosomal system focusing on the unconventional and late endosome/lysosome-specific phospholipid, bis(monoacylglycero)phosphate (BMP). BMP is enriched in alveolar macrophages of lung, one of the target tissue of SARS-CoV-2. This review details the BMP structure, its unsaturated fatty acid composition and fusogenic properties that are essential for the highly dynamic formation of the intraluminal vesicles inside the endosomes. Interestingly, BMP is necessary for infection and replication of enveloped RNA virus such as SARS-CoV-1 and Dengue virus. We also emphasize the role of BMP in lipid sorting and degradation, especially cholesterol transport in cooperation with Niemann Pick type C proteins (NPC 1 and 2) and with some oxysterol-binding protein (OSBP)-related proteins (ORPs) as well as in sphingolipid degradation. Interestingly, numerous virus infection required NPC1 as well as ORPs along the endocytic pathway. Furthermore, BMP content is increased during pathological endosomal lipid accumulation in various lysosomal storage disorders. This is particularly important knowing the high percentage of patients with metabolic disorders among the SARS-CoV-2 infected patients presenting severe forms of COVID-19.

Higher Urine bis(Monoacylglycerol)Phosphate Levels in LRRK2 G2019S Mutation Carriers: Implications for Therapeutic Development

Background

LRRK2 mutations are a common cause of dominantly inherited PD. Previous studies showed decreases in urine levels of didocohexaenoyl (22:6) bis(monoacylglycerol)phosphate in LRRK2‐knockout mice and in non‐human primates treated with LRRK2 kinase inhibitors. We hypothesized that urine levels of bis(monoacylglycerol)phosphate isoforms will be higher in individuals with a PD‐causing gain‐of‐kinase function mutation, LRRK2 G2019S. The objective of this study was to investigate alterations in urinary phospholipids as biomarkers of LRRK2 mutations and Parkinson's disease status/phenotypes.

Methods

Ultra‐performance liquid chromatography–tandem mass spectrometry (UPLC‐MS/MS) was used to assess 54 bioactive phospholipids in urine from the LRRK2 Cohort Consortium (n = 80). To confirm and extend the findings, urine from an independent LRRK2 cohort from Columbia University Irving Medical Center (n = 116) was used. Both cohorts were composed of LRRK2 G2019S carriers and non‐carriers with and without PD.

Results

In each cohort, 4 bis(monoacylglycerol)phosphate isoforms (di‐18:1‐bis[monoacylglycerol]phosphate, didocohexaenoyl [22:6] bis[monoacylglycerol] phosphate, 2,2′‐di‐22:6‐bis[monoacylglycerol]phosphate, and 2,2′‐di‐18:1‐bis[monoacylglycerol]phosphate) were significantly higher (2.5‐ to 4.3‐fold) in G2019S carriers compared with non‐carriers. Interestingly, 2,2′‐di‐18:1‐bis(monoacylglycerol)phosphate levels were marginally higher in LRRK2 carriers with PD than in those without PD (P = 0.045). Moreover, increased 2,2′ and total di‐22:6‐bis(monoacylglycerol)phosphate were associated with worse cognitive status assessed by the Montreal Cognitive Assessment (P = 0.0033 and 0.0144, respectively).

Conclusions

The observed association of bis(monoacylglycerol)phosphate isoforms with LRRK2 G2019S mutation, PD status among G2019S carriers, and correlation with cognitive decline suggest the potential use of urinary bis(monoacylglycerol)phosphate isoforms as biomarkers for clinical trials of LRRK2‐targeted therapies. © 2019 The Authors. Movement Disorders published by Wiley Periodicals, Inc. on behalf of International Parkinson and Movement Disorder Society.

Drug-induced increase in lysobisphosphatidic acid reduces the cholesterol overload in Niemann-Pick type C cells and mice

Most cells acquire cholesterol by endocytosis of circulating low-density lipoproteins (LDLs). After cholesteryl ester de-esterification in endosomes, free cholesterol is redistributed to intracellular membranes via unclear mechanisms. Our previous work suggested that the unconventional phospholipid lysobisphosphatidic acid (LBPA) may play a role in modulating the cholesterol flux through endosomes. In this study, we used the Prestwick library of FDA-approved compounds in a high-content, image-based screen of the endosomal lipids, lysobisphosphatidic acid and LDL-derived cholesterol. We report that thioperamide maleate, an inverse agonist of the histamine H3 receptor HRH3, increases highly selectively the levels of lysobisphosphatidic acid, without affecting any endosomal protein or function that we tested. Our data also show that thioperamide significantly reduces the endosome cholesterol overload in fibroblasts from patients with the cholesterol storage disorder Niemann-Pick type C (NPC), as well as in liver of Npc1-/- mice. We conclude that LBPA controls endosomal cholesterol mobilization and export to cellular destinations, perhaps by fluidifying or buffering cholesterol in endosomal membranes, and that thioperamide has repurposing potential for the treatment of NPC.

Major roles for minor bacterial lipids identified by mass spectrometry

Mass spectrometry of lipids, especially those isolated from bacteria, has ballooned over the past two decades, affirming in the process the complexity of the lipidome. With this has come the identification of new and interesting lipid structures. Here is an overview of several novel lipids, from both Gram-negative and Gram-positive bacteria with roles in health and disease, whose structural identification was facilitated using mass spectrometry. This article is part of a Special Issue entitled: Bacterial Lipids edited by Russell E. Bishop.

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.

Lipid sorting and multivesicular endosome biogenesis

Intracellular organelles, including endosomes, show differences not only in protein but also in lipid composition. It is becoming clear from the work of many laboratories that the mechanisms necessary to achieve such lipid segregation can operate at very different levels, including the membrane biophysical properties, the interactions with other lipids and proteins, and the turnover rates or distribution of metabolic enzymes. In turn, lipids can directly influence the organelle membrane properties by changing biophysical parameters and by recruiting partner effector proteins involved in protein sorting and membrane dynamics. In this review, we will discuss how lipids are sorted in endosomal membranes and how they impact on endosome functions.

Bis(monoacylglycero)phosphate, a peculiar phospholipid to control the fate of cholesterol: Implications in pathology

Bis(monoacylglycero)phosphate (BMP) is a structural isomer of phosphatidylglycerol that exhibits an unusual sn1:sn1' stereoconfiguration, based on the position of the phosphate moiety on its two glycerol units. Early works have underlined the high concentration of BMP in the lysosomal compartment, especially during some lysosomal storage disorders and drug-induced phospholipidosis. Despite numerous studies, both biosynthetic and degradative pathways of BMP remained not completely elucidated. More recently, BMP has been localized in the internal membranes of late endosomes where it forms specialized lipid domains. Its involvement in both dynamics and lipid/protein sorting functions of late endosomes has started to be documented, especially in the control of cellular cholesterol distribution. BMP also plays an important role in the late endosomal/lysosomal degradative pathway. Another peculiarity of BMP is to be naturally enriched in docosahexaenoic acid and/or to specifically incorporate this fatty acid compared to other polyunsaturated fatty acids, which may confer specific biophysical and functional properties to this phospholipid. This review summarizes and updates our knowledge on BMP with an emphasis on its possible implication in human health and diseases, especially in relation to cholesterol homeostasis.

ALCAT1 is a polyglycerophospholipid acyltransferase potently regulated by adenine nucleotide and thyroid status

Acyl-CoA:lysocardiolipin acyltransferase-1 (ALCAT1) catalyzes acylation of lysocardiolipin back to cardiolipin, an important step in cardiolipin remodeling. The present study reports the catalytic properties of ALCAT1 in vitro and its regulation by thyroid hormone status in mouse liver and heart. Recombinant ALCAT1 expressed in Sf9 cells preferred basic pH conditions and did not require divalent cations or integrity of the subcellular membrane for its enzymatic activity. Recombinant ALCAT1 was potently inhibited by ADP and ATP, but not by adenosine nucleotide analogs or other nucleotides, such as UTP and GTP, suggesting that ALCAT1 does not require ATP hydrolysis for its enzyme activity. In addition to cardiolipin, ALCAT1 also catalyzed acylation of other members of the polyglycerophospholipid family, including phosphatidylglycerol, a precursor for cardiolipin synthesis, and bis(monoacylglycero)phosphate, a structural isomer of lysophosphatidylglycerol and a metabolic intermediate of cardiolipin. These findings suggest that ALCAT1 plays a role in the remodeling of other polyglycerophospholipids. In support of a regulatory role of ALCAT1 in cardiolipin remodeling in response to oxidative stress, ALCAT1 expression in liver and heart was significantly downregulated in mice with hypothyroidism and upregulated in mice treated with thyroid hormone, which is known to stimulate mitochondrial activity, oxidative stress, and cardiolipin remodeling.

Selective decrease of bis(monoacylglycero)phosphate content in macrophages by high supplementation with docosahexaenoic acid

Bis(monoacylglycero)phosphate (BMP) is a unique phospholipid (PL) preferentially found in late endosomal membranes, where it forms specialized lipid domains. Recently, using cultured macrophages treated with anti-BMP antibody, we showed that BMP-rich domains are involved in cholesterol homeostasis. We had previously stressed the high propensity of BMP to accumulate docosahexaenoic acid (DHA), compared with other PUFAs. Because phosphatidylglycerol (PG) was reported as a precursor for BMP synthesis in RAW macrophages, we examined the effects of PG supplementation on both FA composition and amount of BMP in this cell line. Supplementation with dioleoyl-PG (18:1/18:1-PG) induced BMP accumulation, together with an increase of oleate proportion. Supplementation with high concentrations of didocosahexaenoyl-PG (22:6/22:6-PG) led to a marked enrichment of DHA in BMP, resulting in the formation of diDHA molecular species. However, the amount of BMP was selectively decreased. Similar effects were observed after supplementation with high concentrations of nonesterified DHA. Addition of vitamin E prevented the decrease of BMP and further increased its DHA content. Supplementation with 22:6/22:6-PG promoted BMP accumulation with an enhanced proportion of 22:6/22:6-BMP. DHA-rich BMP was significantly degraded after cell exposure to oxidant conditions, in contrast to oleic acid-rich BMP, which was not affected. Using a cell-free system, we showed that 22:6/22:6-BMP is highly oxidizable and partially protects cholesterol oxidation, compared with 18:1/18:1-BMP. Our data suggest that high DHA content in BMP led to specific degradation of this PL, possibly through the diDHA molecular species, which is very prone to peroxidation and, as such, a potential antioxidant in its immediate vicinity.

Selective incorporation of docosahexaenoic acid into lysobisphosphatidic acid in cultured THP-1 macrophages

Lysobisphosphatidic acid (LBPA) is highly accumulated in specific domains of the late endosome and is involved in the biogenesis and function of this organelle. Little is known about the biosynthesis and metabolism of this lipid. We examined its FA composition and the incorporation of exogenous FA into LBPA in the human monocytic leukemia cell line THP-1. The LBPA FA composition in THP-1 cells exhibits an elevated amount of oleic acid (18:1n-9) and enrichment of PUFA, especially DHA (22:6n-3). DHA supplemented to the medium was efficiently incorporated into LBPA. In contrast, arachidonic acid (20:4n-6) was hardly esterified to LBPA under the same experimental conditions. The turnover of DHA in LBPA was similar to that in other phospholipids. Specific incorporation of DHA into LBPA was also observed in baby hamster kidney fibroblasts, although LBPA in these cells contains very low endogenous levels of DHA in normal growth conditions. Our resuIts, together with published observations, suggest that the specific incorporation of DHA into LBPA is a common phenomenon in mammalian cells. The physiological significance of DHA-enriched LBPA is discussed.

De novo biosynthesis of the late endosome lipid, bis(monoacylglycero)phosphate

Bis(monoacylglycero)phosphate (BMP) is a unique lipid enriched in the late endosomes participating in the trafficking of lipids and proteins through this organelle. The de novo biosynthesis of BMP has not been clearly demonstrated. We investigated whether phosphatidylglycerol (PG) and cardiolipin (CL) could serve as precursors of de novo BMP synthesis using two different cellular models: CHO cells deficient in phosphatidylglycerophosphate (PGP) synthase, the enzyme responsible for the first step of PG synthesis; and human lymphoblasts from patients with Barth syndrome (BTHS), characterized by mutations in tafazzin, an enzyme implicated in the deacylation-reacylation cycle of CL. The biosynthesis of both PG and BMP was reduced significantly in the PGP synthase-deficient CHO mutants. Furthermore, overexpression of PGP synthase in the deficient mutants induced an increase of BMP biosynthesis. In contrast to CHO mutants, BMP biosynthesis and its fatty acid composition were not altered in BTHS lymphoblasts. Our results thus suggest that in mammalian cells, PG, but not CL, is a precursor of the de novo biosynthesis of BMP. Despite the decrease of de novo synthesis, the cellular content of BMP remained unchanged in CHO mutants, suggesting that other pathway(s) than de novo biosynthesis are also used for BMP synthesis.

pH-dependent formation of membranous cytoplasmic body-like structure of ganglioside G(M1)/bis(monoacylglycero)phosphate mixed membranes

Membrane structures of the mixtures of ganglioside G(M1) and endosome specific lipid, bis (monoacylglycero) phosphate (BMP, also known as lysobisphosphatidic acid) were examined at various pH conditions by freeze-fracture electron microscopy and small-angle x-ray scattering. At pH 8.5-6.5, a G(M1)/BMP (1:1 mol/mol) mixture formed small vesicular aggregates, whereas the mixture formed closely packed lamellar structures under acidic conditions (pH 5.5, 4.6) with the lamellar repeat distance of 8.06 nm. Since BMP alone exhibits a diffuse lamellar structure at a broad range of pH values and G(M1) forms a micelle, the results indicate that both G(M1) and BMP are required to produce closely stacked multilamellar vesicles. These vesicles resemble membranous cytoplasmic bodies in cells derived from patients suffering from G(M1) gangliosidosis. Similar to G(M1) gangliosidosis, cholesterol was trapped in BMP vesicles in G(M1)- and in a low pH-dependent manner. Studies employing different gangliosides and a G(M1) analog suggest the importance of sugar chains and a sialic acid of G(M1) in the pH-dependent structural change of G(M1)/BMP membranes.

Identification and Characterization of a Gene Encoding Human LPGAT1, an Endoplasmic Reticulum-associated Lysophosphatidylglycerol Acyltransferase

Phosphatidylglycerol (PG) is an important membrane polyglycerolphospholipid required for the activity of a variety of enzymes and is a precursor for synthesis of cardiolipin and bis(monoacylglycerol) phosphate. PG is subjected to remodeling subsequent to its de novo biosynthesis to incorporate appropriate acyl content for its biological functions and to prevent the harmful effect of lysophosphatidylglycerol (LPG) accumulation. The enzymes involved in the remodeling process have not yet been identified. We report here the identification and characterization of a human gene encoding an acyl-CoA: lysophosphatidylglycerol acyltransferase (LPGAT1). Expression of the LPGAT1 cDNA in Sf9 insect and COS-7 cells led to a significant increase in LPG acyltransferase activity. In contrast, no significant acyltransferase activities were detected against glycerol 3-phosphate or a variety of lysophospholipids, including lysophosphatidylcholine, lysophosphatidylethanolamine, lysophosphatidylinositol, and lysophosphatidylserine. The recombinant human LPGAT1 enzyme recognized various acyl-CoAs and LPGs as substrates but demonstrated clear preference to long chain saturated fatty acyl-CoAs and oleoyl-CoA as acyl donors, which is consistent with the lipid composition of endogenous PGs identified from different tissues. Kinetic analyses of LPGAT1 expressed in COS-7 cells showed that oleoyl-LPG was preferred over palmitoyl-LPG as an acyl receptor, whereas oleoyl-CoA was preferred over lauroyl-CoA as an acyl donor. Consistent with its proposed microsomal origin, LPGAT1 was localized to the endoplasmic reticulum by subcellular fractionation and immunohistochemical analyses. Northern blot analysis indicated that the human LPGAT1 was widely distributed, suggesting a dynamic functional role of the enzyme in different tissues.

Purification and properties of a phospholipase A2/lipase preferring phosphatidic acid, bis(monoacylglycerol) phosphate, and monoacylglycerol from rat testis

Phospholipase A(2) (PLA(2)) was purified to homogeneity from the supernatant fraction of rat testis homogenate. The purified 63-kDa enzyme did not require Ca(2+) ions for activity and exhibited both phosphatidic acid-preferring PLA(2) and monoacylglycerol lipase activities with a modest specificity toward unsaturated acyl chains. Anionic detergents enhanced these activities. Serine-modifying irreversible inhibitors, (p-amidinophenyl) methanesulfonyl fluoride and methylarachidonyl fluorophosphonate, inhibited both activities to a similar extent, indicating a single active site is involved in PLA(2) and lipase activities. The sequence of NH(2)-terminal 12 amino acids of purified enzyme was identical to that of a carboxylesterase from rat liver. The optimal pH for PLA(2) activity (around 5.5) differed from that for lipase activity (around 8.0). At pH 5.5 the enzyme also hydrolyzed bis(monoacylglycerol) phosphate, or lysobisphosphatidic acid (LBPA), that has been hitherto known as a secretory PLA(2)-resistant phospholipid and a late endosome marker. LBPA-enriched fractions were prepared from liver lysosome fractions of chloroquine-treated rats, treated with excess of pancreatic PLA(2), and then used for assaying LBPA-hydrolyzing activity. LBPA and the reaction products were identified by microbore normal phase high performance liquid chromatography/electrospray ionization ion-trap mass spectrometry. These enzymatic properties suggest that the enzyme can metabolize phosphatidic and lysobisphosphatidic acids in cellular acidic compartments.

High-performance liquid chromatography determination of bis(monoacylglycerol) phosphate and other lysophospholipids

Bis(monoacylglycerol) phosphate (BMP) is a very minor component of the phospholipid (PL) fraction in rat uterine stromal cell cultures (U(III) cells). Under several culture conditions, including the addition of (n-3) or (n-6) polyunsaturated fatty acids, BMP selectively accumulates docosahexaenoic acid (DHA). We have recently described the structure of this PL, but its biological function is still largely unknown, except for a role in late endosomes trafficking. In order to further investigate this function, we have developed a sensitive assay for accurate determination of BMP in small biological samples. Total PL from cells, labeled or not with trace amount of [3H]DHA, were extracted and PL classes separated by thin-layer chromatography. After extraction of the gel corresponding to the BMP area, a known amount of an internal standard was added. The free hydroxyl groups of PL were totally derivatized with naproxen. Derivatized PL were separated by normal-phase high-pressure liquid chromatography and quantified using UV absorption at 231 nm. Since the sensitivity of the proposed method was about 0.1 nmol for BMP, samples of only 3 x 10(5) cells were required. The BMP level was found to be 616 +/- 46 pmol for 10(6) control cells. It was increased threefold in starved cells and significantly increased in cells cultured in the presence of exogenous phosphatidylglycerol.

Lipid domains in the endocytic pathway

Whereas endosomes connect with both exocytic and endocytic organelle via extensive lipid and protein traffic, each endosome has a distinct lipid and protein composition. Recent observations suggest that different lipid membrane domains exist even in the same endosome. These lipid domains, together with low pH milieu, may present a variety of micro-environments to cargo molecules. Evidence is accumulating which suggests that the alteration of these lipid microdomains may be involved in a number of pathological conditions.

Transacylase formation of bis(monoacylglycerol)phosphate

Recent work within our laboratory has focused on the enzymes we hypothesize are involved in the biosynthesis of bis(monoacylglycerol)phosphate from phosphatidylglycerol. Here we describe a transacylase, active at acidic pH values, isolated from a macrophage-like cell line, RAW 264.7. This enzyme acylates the head group glycerol of sn-3:sn-1' lysophosphatidylglycerol to form sn-3:sn-1' bis(monoacylglycerol)phosphate. Here we demonstrate that this enzyme uses two lysophosphatidylglycerol molecules, one as an acyl donor and another as an acyl acceptor, and that the acyl contributions from all other lipids tested are comparatively minor. This enzyme prefers saturated acyl chains to monounsaturates, 16 and 18 carbon fatty acids over 14 carbon fatty acids, and saturated acyl chains at the sn-1 position to monounsaturated acyl chains on the sn-2 carbon of lysophosphatidylglycerol. We present data which show the transacylase activity depends on the presence of a lipid-water interface and the lipid polymorphic state.

The envelope of vaccinia virus reveals an unusual phospholipid in Golgi complex membranes

We isolated forms of enveloped vaccinia virus from infected HeLa cells to obtain membranes for the analysis of lipids of the cis-Golgi network and trans-Golgi network. The intracellular mature virus obtains its envelope by wrapping itself in the membranes of the cis-Golgi network. A fraction of these virions then acquires a second envelope by enwrapping trans-Golgi network membranes to form the intracellular enveloped virus. Lipids were analyzed by high performance thin layer chromatography and digital densitometry to establish a steady-state lipid profile of viral membranes, which should reflect the compositions of the cis-Golgi network and trans-Golgi network. Phosphatidyl-inositol was slightly enriched in the cis-Golgi network of HeLa cells, whereas the trans-Golgi network showed a minor increase in phosphatidylserine and sphingomyelin. Similarly, cholesterol was only slightly more abundant in the trans-Golgi compared to the cis-Golgi. An unusual lipid, semilysobisphosphatidic acid, was present in significant amounts in vaccinia envelopes. Semilysobisphosphatidic acid was present in similar levels in infected and uninfected cells, and was therefore not induced by vaccinia infection. Subcellular fractionation of HeLa cells indicated that the recovery of semilysobisphosphatidic acid paralleled the recovery of a Golgi marker. Furthermore, a lipid species that comigrated with semilysobisphosphatidic acid was also present in lipids extracted from highly purified, intact Golgi complexes from rat liver. Together, these results suggest that semilysobisphosphatidic acid is a normal component of Golgi membranes.