The stereoconfiguration of newly formed molecules of bis(monoacylglycero)phosphate in BHK cells

PLA2G15 is a Lysosomal BMP Hydrolase with Ester Position Specificity and its Targeting Ameliorates Lysosomal Disease

Lysosomes catabolize lipids and other biological molecules, a function essential for cellular and organismal homeostasis. Key to lipid catabolism in the lysosome is bis(monoacylglycero)phosphate (BMP), a major lipid constituent of intralysosomal vesicles (ILVs) and a stimulator of lipid-degrading enzymes. BMP levels are altered in a broad spectrum of human conditions, including neurodegenerative diseases. Although BMP synthase was recently discovered, it has long been thought that BMP's unique stereochemistry confers resistance to acid phospholipases, a requirement for its role in the lysosome. Here, we demonstrate that PLA2G15, a major lysosomal phospholipase, efficiently hydrolyzes BMP with primary esters regardless of stereochemistry. Interestingly, we discover that BMP's unique esterification position is what confers resistance to hydrolysis. Purified PLA2G15 catabolizes most BMP species derived from cell and tissue lysosomes under acidic conditions. Furthermore, PLA2G15 catalytic activity against synthesized BMP stereoisomers with primary esters was comparable to its canonical substrates. Conversely, BMP with secondary esters is intrinsically stable in vitro and requires acyl migration for hydrolysis in lysosomes. Consistent with our biochemical data, PLA2G15-deficient tissues and cells accumulate multiple BMP species, a phenotype reversible by supplementing wildtype PLA2G15 but not its catalytically dead mutant. Increasing BMP levels by targeting PLA2G15 reverses the cholesterol accumulation phenotype in Niemann Pick Disease Type C (NPC1) patient fibroblasts and significantly ameliorate disease pathologies in NPC1-deficient mice leading to extended lifespan. Our findings establish the rules that govern the stability of BMP in the lysosome and identify PLA2G15 as a lysosomal BMP hydrolase and as a potential target for modulating BMP levels for therapeutic intervention.

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.

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.

Effects of the endosomal lipid bis(monoacylglycero)phosphate on the thermotropic properties of DPPC: A 2H NMR and spin label EPR study

Bis(monoacylglycero)phosphate (BMP) is an endosomal lipid with a unique structure that is implicated in the formation of intraendosomal vesicular bodies. Here we have characterized the effects of dioleoyl-BMP (BMP(18:1)) at concentrations of 5, 10, 15 and 20mol% on the thermotropic behavior of dipalmitoyl phosphatidylcholine (DPPC) vesicles, and compared them to those of equimolar concentrations of dioleoyl phosphatidylglycerol (DOPG), a structural isoform of BMP(18:1). Because BMP is found in the acidic environments of the late endosome and intralysosomal vesicles, samples were prepared at pH 4.2 to mimic the pH of the lysosome. Both (2)H NMR of perdeuterated DPPC and spin-labeled EPR with 16-doxyl phosphatidylcholine were utilized in these investigations. NMR and EPR results show that BMP(18:1) induces a lowering in the main phase transition temperature of DPPC similar to that of DOPG. The EPR studies reveal that BMP(18:1) induced more disorder in the L(beta) phase when compared to equimolar concentrations of DOPG. Analysis from dePaked (2)H NMR spectra in the L(alpha) phase reveals that BMP(18:1) induces less disorder than equal concentrations of DOPG. Additionally, the results demonstrate that BMP mixes with other phospholipids as a phospholipid and not as a detergent molecule as once speculated.

Bis(monoacylglycero)phosphate and ganglioside GM1 spontaneously form small homogeneous vesicles at specific concentrations

The morphology and size of hydrated lipid dispersions of bis(monoacylglycero)phosphate (BMP) mixed with varying mole percentages of the ganglioside GM1 were investigated by dynamic light scattering (DLS) and transmission electron microscopy (TEM). Electron paramagnetic resonance (EPR) spectroscopy of these same mixtures, doped at 0.5 mol% with doxyl labeled lipids, was used to investigate acyl-chain packing. Results show that for 20-30% GM1, hydrated BMP:GM1 mixtures spontaneously form small spherical vesicles with diameters approximately 100 nm and a narrow size distribution profile. For other concentrations of GM1, hydrated dispersions with BMP have non-spherical shapes and heterogeneous size profiles, with average vesicle diameters>400 nm. All samples were prepared at pH 5.5 to mimic the lumen acidity of the late endosome where BMP is an essential component of intraendosomal vesicle budding, lipid sorting and trafficking. These findings indicate that GM1 and BMP under a limited concentration range spontaneously form small vesicles of homogeneous size in an energy independent manner without the need of protein templating. Because BMP is essential for intraendosomal vesicle formation, these results imply that lipid-lipid interactions may play a critical role in the endosomal process of lipid sorting and trafficking.

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.

Bis(monoacylglycero)phosphate forms stable small lamellar vesicle structures: insights into vesicular body formation in endosomes

Bis(monoacylglycero)phosphate (BMP) is an unusually shaped lipid found in relatively high percentage in the late endosome. Here, we report the characterization of the morphology and molecular organization of dioleoyl-BMP (DOBMP) with dynamic light scattering, transmission electron microscopy, nuclear magnetic resonance (NMR) spectroscopy, and electron paramagnetic resonance spectroscopy. The morphology of hydrated DOBMP dispersions varies with pH and ionic strength, and DOBMP vesicles are significantly smaller in diameter than phosphatidylcholine dispersions. At neutral pH, DOBMP forms highly structured, clustered dispersions 500 nm in size. On the other hand, at acidic pH, spherically shaped vesicles are formed. NMR and spin-labeled electron paramagnetic resonance demonstrate that DOBMP forms a lamellar mesophase with acyl-chain packing similar to that of other unsaturated phospholipids. (31)P NMR reveals an orientation of the phosphate group in DOBMP that differs significantly from that of other phospholipids. These macroscopic and microscopic structural characterizations suggest that the biosynthesis of BMP on the inner luminal membrane of maturing endosomes may possibly produce budded vesicles high in BMP content, which form small vesicular structures stabilized by the physical properties of the BMP lipid.

Exosome lipidomics unravels lipid sorting at the level of multivesicular bodies

Exosomes are part of the family of "bioactive vesicles" and appear to be involved in distal communications between cells. They vehiculate bioactive lipids and lipolytic enzymes and their biogenesis require specific lipids and a membrane reorganisation. Their biogenesis pathway could be a way to secrete enzymes involved in lipid signalling and to generate "particulate agonists". However, this pathway seems also to be used by pathogens such as HIV. This review will consider several aspects of lipidomics studies which might help to understand the fate and role of these fascinating vesicles.

Rapid access to synthetic lysobisphosphatidic acids using P(III) chemistry

An expeditious route to synthetic lysobisphosphatidic acid S,S-1, its enantiomer, and regioisomers is reported. Synthetic difficulties concerning lipid stability and stereochemistry are bypassed using a phosphite triester approach in combination with multiple silyl protection. Spectroscopic studies evidence that acyl group migration in S,S-1 is accelerated by nonpolar solvents and inhibited by pyridine.

A lipid associated with the antiphospholipid syndrome regulates endosome structure and function

Little is known about the structure and function of membrane domains in the vacuolar apparatus of animal cells. A unique feature of late endosomes, which are part of the pathway that leads to lysosomes, is that they contain a complex system of poorly characterized internal membranes in their lumen. These endosomes are therefore known as multivesicular or multilamellar organelles. Some proteins distribute preferentially within these internal membranes, whereas others are exclusively localized to the organelle's limiting membrane. The composition and function of this membrane system are poorly understood. Here we show that these internal membranes contain large amounts of a unique lipid, and thus form specialized domains within endosomes. These specialized domains are involved in sorting the multifunctional receptor for insulin-like growth factor 2 and ligands bearing mannose-6-phosphate, in particular lysosomal enzymes. We also show that this unique lipid is a specific antigen for human antibodies associated with the antiphospholipid syndrome. These antibodies may act intracellularly by altering the protein-sorting functions of endosomes.

A new synthesis of bis(diacylglycero)phosphate

The chemical synthesis of bis(diacylglycero) phosphate previously named bisphosphatidic acid, starting with a diacylglycerol and phosphatidic acid, is described. The phosphodiester bond formation is catalyzed by triisopropylbenzenesulfonylchloride. This simple approach allows the preparation of saturated as well as unsaturated bis(diacylglycero)phosphate species in one step without the use of any protecting group. The methods used until now yield only mono-acid species, or mixed-acid unsaturated species after many steps involving the introduction and the removal of protecting groups. The synthetic products have been characterized by component analysis and NMR-techniques.

Conversion of phosphatidylglycerol lipids to bis(monoacylglycero)phosphate in vivo

Liposomes containing either 32P-labeled diphosphatidylglycerol (cardiolipin) or 32P-labeled phosphatidyl-rac-(1)-glycerol were injected into the circulation of rats. Analysis of the liver lipids 2-3 after injection showed incorporation of the 32P label from both lipids to a lipid which had chromatographic properties identical with bis(monoacylglycero)phosphate. Stereochemical analysis of this lipid indicated that its backbone was sn-glycero-1-phospho-1'-glycerol. Cultured hamster fibroblasts (BHK cells) were incubated in a medium containing lyso[32P]phosphatidyl-rac-(1)-glycerol and the formation of radioactive lipids in the cells was followed. Bis(monoacylglycero) phosphate was the major 32P-labelled lipid formed: as much as 36.4% of the lyso[32P]phosphatidyl-rac-(1)-glycerol absorbed to the cells was converted to bis(monoacylglycero)phosphate. Similar results were obtained with lyso[32P]phosphatidyl-sn-1-glycerol as a precursor. Stereoanalysis of the bis(monoacylglycero)-[32P]-phosphate formed from either precursor indicated that this lipid was a derivative of sn-glycero-1-phospho-1'-glycerol. These results establish phosphatidylglycerol, diphosphatidylglycerol and lysophosphatidylglycerol as precursors of bis-(monoacyl-sn-glycero-1)phosphate in vivo. The mechanism of the conversion of lysophosphatidylglycerol to bis-(monoacyl-sn-glycero-1)phosphate was studied by using 32P,3H-labeled lysophosphatidyl-rac-(1)-glycerol as a precursor. Both labels were incorporated to bis(monoacylglycero)phosphate with similar efficiency, which suggests that rearrangement, rather than replacement, of the (originally acylated) sn-glycero-3-phospho moiety of the precursor is the essential reaction in the biosynthesis of the sn-glycero-1-phospho-1'-glycerol backbone of bis(monoacylglycero) phosphate.