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

Molecular determinants of phospholipid treatment to reduce intracellular cholesterol accumulation in NPC1 deficiency

Niemann-Pick type C (NPC) disease, caused by mutations in the NPC1 or NPC2 genes, leads to abnormal intracellular cholesterol accumulation in late endosomes/lysosomes. Exogenous enrichment with lysobisphosphatidic acid (LBPA), also known as bis-monoacylglycerol phosphate, either directly or via the LBPA precursor phosphatidylglycerol (PG), has been investigated as a therapeutic intervention to reduce cholesterol accumulation in NPC disease. Here, we report the effects of stereoisomer configuration and acyl chain composition of LBPA on cholesterol clearance in NPC1-deficient cells. We find that S,R, S,S, and S,R LBPA stereoisomers behaved similarly, with all 3 compounds leading to comparable reductions in filipin staining in two NPC1-deficient human fibroblast cell lines. Examination of several LBPA molecular species containing one or two monounsaturated or polyunsaturated acyl chains showed that all LBPA species containing one 18:1 chain significantly reduced cholesterol accumulation, whereas the shorter chain species di-14:0 LBPA had little effect on cholesterol clearance in NPC1-deficient cells. Since cholesterol accumulation in NPC1-deficient cells can also be cleared by PG incubation, we used nonhydrolyzable PG analogs to determine whether conversion to LBPA is required for sterol clearance, or whether PG itself is effective. The results showed that nonhydrolyzable PG species were not appreciably converted to LBPA and showed virtually no cholesterol clearance efficacy in NPC1-deficient cells, supporting the notion that LBPA is the active agent promoting late endosome/lysosome cholesterol clearance. Overall these studies are helping to define the molecular requirements for potential therapeutic use of LBPA as an option for addressing NPC disease.

Review of Eukaryote Cellular Membrane Lipid Composition, with Special Attention to the Fatty Acids

Biological membranes, primarily composed of lipids, envelop each living cell. The intricate composition and organization of membrane lipids, including the variety of fatty acids they encompass, serve a dynamic role in sustaining cellular structural integrity and functionality. Typically, modifications in lipid composition coincide with consequential alterations in universally significant signaling pathways. Exploring the various fatty acids, which serve as the foundational building blocks of membrane lipids, provides crucial insights into the underlying mechanisms governing a myriad of cellular processes, such as membrane fluidity, protein trafficking, signal transduction, intercellular communication, and the etiology of certain metabolic disorders. Furthermore, comprehending how alterations in the lipid composition, especially concerning the fatty acid profile, either contribute to or prevent the onset of pathological conditions stands as a compelling area of research. Hence, this review aims to meticulously introduce the intricacies of membrane lipids and their constituent fatty acids in a healthy organism, thereby illuminating their remarkable diversity and profound influence on cellular function. Furthermore, this review aspires to highlight some potential therapeutic targets for various pathological conditions that may be ameliorated through dietary fatty acid supplements. The initial section of this review expounds on the eukaryotic biomembranes and their complex lipids. Subsequent sections provide insights into the synthesis, membrane incorporation, and distribution of fatty acids across various fractions of membrane lipids. The last section highlights the functional significance of membrane-associated fatty acids and their innate capacity to shape the various cellular physiological responses.

Cell density-dependent membrane distribution of ganglioside GM3 in melanoma cells

Monosialoganglioside GM3 is the simplest ganglioside involved in various cellular signaling. Cell surface distribution of GM3 is thought to be crucial for the function of GM3, but little is known about the cell surface GM3 distribution. It was shown that anti-GM3 monoclonal antibody binds to GM3 in sparse but not in confluent melanoma cells. Our model membrane study evidenced that monoclonal anti-GM3 antibodies showed stronger binding when GM3 was in less fluid membrane environment. Studies using fluorescent GM3 analogs suggested that GM3 was clustered in less fluid membrane. Moreover, fluorescent lifetime measurement showed that cell surface of high density melanoma cells is more fluid than that of low density cells. Lipidomics and fatty acid supplementation experiment suggested that monounsaturated fatty acid-containing phosphatidylcholine contributed to the cell density-dependent membrane fluidity. Our results indicate that anti-GM3 antibody senses GM3 clustering and the number and/or size of GM3 cluster differ between sparse and confluent melanoma cells.

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.

Intracellular cholesterol trafficking is dependent upon NPC2 interaction with lysobisphosphatidic acid

Unesterified cholesterol accumulation in the late endosomal/lysosomal (LE/LY) compartment is the cellular hallmark of Niemann-Pick C (NPC) disease, caused by defects in the genes encoding NPC1 or NPC2. We previously reported the dramatic stimulation of NPC2 cholesterol transport rates to and from model membranes by the LE/LY phospholipid lysobisphosphatidic acid (LBPA). It had been previously shown that enrichment of NPC1-deficient cells with LBPA results in cholesterol clearance. Here we demonstrate that LBPA enrichment in human NPC2-deficient cells, either directly or via its biosynthetic precursor phosphtidylglycerol (PG), is entirely ineffective, indicating an obligate functional interaction between NPC2 and LBPA in cholesterol trafficking. We further demonstrate that NPC2 interacts directly with LBPA and identify the NPC2 hydrophobic knob domain as the site of interaction. Together these studies reveal a heretofore unknown step of intracellular cholesterol trafficking which is critically dependent upon the interaction of LBPA with functional NPC2 protein.

Cholesterol is a type of fat that is essential for many processes in the body, such as repairing damaged cells and producing certain hormones. Normally, cholesterol enters cells from the bloodstream and is then moved to the parts of the cell that need it via a process known as ‘trafficking’.

When cholesterol trafficking goes wrong, abnormally large amounts of cholesterol and other fats accumulate within the cell. Over time, these fatty deposits become toxic to cells and eventually damage the affected tissues. Niemann-Pick type C disease (NPC) is a severe genetic disorder affecting cholesterol trafficking. It is characterized by cholesterol build-up in multiple tissues, including the brain, which ultimately causes degeneration and death of nerve cells.

Two proteins, NPC1 and NPC2, are involved in NPC disease. Both proteins normally help move cholesterol out of important trafficking compartments (known as the endosomal and lysosomal compartments) to other areas of the cell where it is needed. Patients with the disease can have mutations in either the gene for NPC1 or the gene for NPC2. This means that cells from NPC1 patients do not make enough functional NPC1 protein (but contain working NPC2), and vice versa.

Previous studies had shown that giving cells with NPC1 mutations large amounts of the small molecule lysobisphosphatidic acid (LBPA for short) could compensate for the loss of NPC1, and stop the toxic build-up of cholesterol. McCauliff, Langan, Li et al. therefore wanted to explore exactly how LBPA was doing this. They had shown that LBPA dramatically increased the ability of purified NPC2 protein to transport cholesterol, and wondered if the effect of LBPA in the cells without NPC1 depended on NPC2. They predicted that boosting LBPA levels would not work in cells lacking NPC2.

Biochemical experiments using purified protein showed that LBPA and NPC2 did indeed interact directly with each other. Systematically changing different building blocks of NPC2 revealed that a single region of the protein is sensitive to LBPA, and when this region was altered, LBPA could no longer interact with NPC2. Since LBPA is naturally produced by cells, they then stimulated cells grown in the laboratory to generate more LBPA using its precursor phosphatidylglycerol. They used cells from patients with mutations in either NPC1 or NPC2 and demonstrated that LBPA’s ability to reverse the accumulation of cholesterol was dependent on its interaction with NPC2. Thus, increasing LBPA levels in cells from patients with NPC1 mutations was beneficial, but had no effect on cells from patients with NPC2 mutations.

These results shed new light not only on how cells transport cholesterol, but also on potential methods to combat disorders of cellular cholesterol trafficking. In the future, LBPA could be developed as a genetically tailored, patient-specific therapy for diseases like NPC.

Separation and identification of phospholipids by hydrophilic interaction liquid chromatography coupled to tandem high resolution mass spectrometry with focus on isomeric phosphatidylglycerol and bis(monoacylglycero)phosphate

Changes in lipid composition of cells or tissue are often linked to various diseases. Studies indicate alterations of bis(monoacylglycero)phosphate (BMP) species in diseases such as cancer. Therefore, an extended phospholipid profiling method based on hydrophilic interaction liquid chromatography (HILIC) coupled to high-resolution mass spectrometry (MS) and data-dependent MS/MS acquisition was developed to separate and unambiguously identify BMP species. Lipid species identification was based on retention time, accurate mass and specific MS/MS fragments. The developed method was applied in a proof of concept study to lipid extracts of a cell culture model of conditional oncogene overexpression in MCF-7/NeuT breast cancer cells. Comparison of control and oncogene-induced MCF-7/NeuT breast cancer cells showed changes in BMP species distribution. Thereby, a shift from long-chain to shorter-chain fatty acid composition in BMP species was detected.

Endosomal Escape of Antisense Oligonucleotides Internalized by Stabilin Receptors Is Regulated by Rab5C and EEA1 During Endosomal Maturation

Second-generation (Gen 2) Antisense oligonucleotides (ASOs) show increased nuclease stability and affinity for their RNA targets, which has translated to improved potency and therapeutic index in the clinic. Gen 2 ASOs are typically modified using the phosphorothioate (PS) backbone modification, which enhances ASO interactions with plasma, cell surface, and intracellular proteins. This facilitates ASO distribution to peripheral tissues and also promotes cellular uptake after injection into animals. Previous work identified that Stabilin receptors specifically internalize PS-ASOs in the sinusoidal endothelial cells of the liver and the spleen. By modulating expression of specific proteins involved in the trafficking and maturation of the endolysosomal compartments, we show that Rab5C and EEA1 in the early endosomal pathway, and Rab7A and lysobisphosphatidic acid in the late endosomal pathway, are important for trafficking of PS-ASOs and facilitate their escape from endolysosomal compartments after Stabilin-mediated internalization. In conclusion, this work identifies key rate-limiting proteins in the pathway for PS-ASO translocation and escape from the endosome.

Identification of the phospholipid lysobisphosphatidic acid in the protozoan Entamoeba histolytica: An active molecule in endocytosis

Phospholipids are essential for vesicle fusion and fission and both are fundamental events for Entamoeba histolytica phagocytosis. Our aim was to identify the lysobisphosphatidic acid (LBPA) in trophozoites and investigate its cellular fate during endocytosis. LBPA was detected by TLC in a 0.5 R_f spot of total lipids, which co-migrated with the LBPA standard. The 6C4 antibody, against LBPA recognized phospholipids extracted from this spot. Reverse phase LC-ESI-MS and MS/MS mass spectrometry revealed six LBPA species of m/z 772.58–802.68. LBPA was associated to pinosomes and phagosomes. Intriguingly, during pinocytosis, whole cell fluorescence quantification showed that LBPA dropped 84% after 15 min incubation with FITC-Dextran, and after 60 min, it increased at levels close to steady state conditions. Similarly, during erythrophagocytosis, after 15 min, LBPA also dropped in 36% and increased after 60 and 90 min. EhRab7A protein appeared in some vesicles with LBPA in steady state conditions, but after phagocytosis co-localization of both molecules increased and in late phases of erythrophagocytosis they were found in huge phagosomes or multivesicular bodies with many intraluminal vacuoles, and surrounding ingested erythrocytes and phagosomes. The 6C4 and anti-EhADH (EhADH is an ALIX family protein) antibodies and Lysotracker merged in about 50% of the vesicles in steady state conditions and throughout phagocytosis. LBPA and EhADH were also inside huge phagosomes. These results demonstrated that E. histolytica LBPA is associated to pinosomes and phagosomes during endocytosis and suggested differences of LBPA requirements during pinocytosis and phagocytosis.

•

LBPA is identified for the first time in the protozoan Entamoeba histolytica.

•

LBPA is found in pinosomes and in 10–20 µm diameter phagosomes or multivesicular bodies.

•

LBPA appeared associated with EhRab7A protein, a late endosomes marker.

•

LBPA interacts with EhADH (an ALIX family protein) during phagocytosis.

Bis(monoacylglycero)phosphate as a Macrophage Enriched Phospholipid

Bis(monoacylglycero)phosphate (BMP) is a structural isomer of phosphatidylglycerol (PtdGro) with an unusual sn-1:sn-1' fatty acyl configuration and is found almost exclusively in late endosomes/lysosomes. BMP comprises only about 1-2% of the total phospholipids in most mammalian cells, but accumulates in tissues of humans and animals with lysosomal storage disorders including the gangliosidoses. Total BMP content was significantly greater in cells of macrophage/microglial origin than in cells of macroglial origin. BMP composition was similar in tumorigenic/metastatic macrophages and non-tumorigenic macrophages/microglia. Finally, BMP fatty acid composition differed between cells grown in culture and obtained in vivo suggesting an influence from growth environment.

Bis(monoacylglycero)phosphate: a secondary storage lipid in the gangliosidoses

Bis(monoacylglycero)phosphate (BMP) is a negatively charged glycerophospholipid with an unusual sn-1;sn-1' structural configuration. BMP is primarily enriched in endosomal/lysosomal membranes. BMP is thought to play a role in glycosphingolipid degradation and cholesterol transport. Elevated BMP levels have been found in many lysosomal storage diseases (LSDs), suggesting an association with lysosomal storage material. The gangliosidoses are a group of neurodegenerative LSDs involving the accumulation of either GM1 or GM2 gangliosides resulting from inherited deficiencies in β-galactosidase or β-hexosaminidase, respectively. Little information is available on BMP levels in gangliosidosis brain tissue. Our results showed that the content of BMP in brain was significantly greater in humans and in animals (mice, cats, American black bears) with either GM1 or GM2 ganglioside storage diseases, than in brains of normal subjects. The storage of BMP and ganglioside GM2 in brain were reduced similarly following adeno-associated viral-mediated gene therapy in Sandhoff disease mice. We also found that C22:6, C18:0, and C18:1 were the predominant BMP fatty acid species in gangliosidosis brains. The results show that BMP accumulates as a secondary storage material in the brain of a broad range of mammals with gangliosidoses.

Lipid oxidation and peroxidation in CNS health and disease: from molecular mechanisms to therapeutic opportunities

Reactive oxygen species (ROS) are produced at low levels in mammalian cells by various metabolic processes, such as oxidative phosphorylation by the mitochondrial respiratory chain, NAD(P)H oxidases, and arachidonic acid oxidative metabolism. To maintain physiological redox balance, cells have endogenous antioxidant defenses regulated at the transcriptional level by Nrf2/ARE. Oxidative stress results when ROS production exceeds the cell's ability to detoxify ROS. Overproduction of ROS damages cellular components, including lipids, leading to decline in physiological function and cell death. Reaction of ROS with lipids produces oxidized phospholipids, which give rise to 4-hydroxynonenal, 4-oxo-2-nonenal, and acrolein. The brain is susceptible to oxidative damage due to its high lipid content and oxygen consumption. Neurodegenerative diseases (AD, ALS, bipolar disorder, epilepsy, Friedreich's ataxia, HD, MS, NBIA, NPC, PD, peroxisomal disorders, schizophrenia, Wallerian degeneration, Zellweger syndrome) and CNS traumas (stroke, TBI, SCI) are problems of vast clinical importance. Free iron can react with H(2)O(2) via the Fenton reaction, a primary cause of lipid peroxidation, and may be of particular importance for these CNS injuries and disorders. Cholesterol is an important regulator of lipid organization and the precursor for neurosteroid biosynthesis. Atherosclerosis, the major risk factor for ischemic stroke, involves accumulation of oxidized LDL in the arteries, leading to foam cell formation and plaque development. This review will discuss the role of lipid oxidation/peroxidation in various CNS injuries/disorders.

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.

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.

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.