1. Structure and Occurrence
Bis(monoacylglycero)phosphate ('BMP') was first isolated from pig lung in 1967 and is now known to be a common if minor constituent of all animal tissues. It was first incorrectly termed ‘lysobisphosphatidic acid’, although it is only superficially related to phosphatidic acid per se and might better be considered a structural isomer of phosphatidylglycerol. It is an interesting lipid from several standpoints. For example, its stereochemical configuration differs from that of all other animal glycero-phospholipids in that the phosphodiester moiety is linked only to positions sn-1 and sn-1’ of glycerol, rather than to positions sn-3 and sn-3’. This has been established by many methods, including confirmation by 1H NMR spectroscopy with chiral shift reagents.
In the lipid that has been isolated from animal tissues, all the initial studies from the first publication until recently suggest that positions sn-3 and 3’ in the glycerol moieties are esterified with fatty acids. On the other hand, there is an increasing school of thought to the effect that the fatty acids are esterified to position sn-2 and sn-2’ in the native molecule. Certainly, fatty acids in a lipid with the latter structure would be expected to undergo rapid acyl migration when subjected to most extraction and isolation procedures, resulting in the most thermodynamically stable form with fatty acids in the primary positions.
Synthetic studies have shown how readily this can occur, and acyl migration might also be expected to take place under the acidic conditions in lysosomes (see below). Further evidence for this hypothesis comes from biosynthetic considerations, from the fact that a specific phospholipase A2 has been found that degrades bis(monoacylglycero)phosphate at the physiological pH in lysosomes, from physical chemical studies and from improved chromatographic conditions, which show three peaks for the lipid. It may also be relevant that aqueous dispersions containing the sn‑2,2'‑lipid, but not the sn-3,3'-form, produce multivesicular liposomes in vitro in a pH-dependent manner. While none of this is conclusive, if taken together with doubts about some of the early NMR data, a re-evaluation of the basic structure of this molecule may be necessary. Those most active in the study of this lipid appear to favour the 2,2'-structure, and it has been established that the 2,2’-dioleoyl form rather than the sn‑3,3’‑isomer is essential for the function of bis(monacylglycero)phosphate in cholesterol metabolism in lysosomes (see below).
Bis(monacylglycero)phosphate is usually a rather minor component of animal tissues (~1-2%), but it is highly enriched in the lysosomal membranes of liver and other tissues, where it can amount to 15% or more of the phospholipids; it is now recognized as a marker for this organelle. Lysosomes are the digestive organelles of the cell and are rich in hydrolytic enzymes at an acidic pH (4.6 to 5). Cellular constituents, including excess nutrients, growth factors and foreign antigens are captured by receptors on the cell surface, for uptake and delivery to lysosomes. Within the cell, receptors such as the mannose-6-phosphate receptor bind and divert hydrolytic enzymes from biosynthetic pathways to the lysosomes. These molecules pass through an intermediate heterogeneous set of organelles known as endosomes, which act as a kind of sorting station where the receptors are recycled before the hydrolases and other materials are directed to the lysosomes. There, the hydrolases are activated and the unwanted materials are digested. It is the internal membranes of mature or ‘late’ endosomes and the lysosomes that contain the unique lipid, bis(monacylglycero)phosphate. Indeed, there appear to be inner membranes of the late endosomes that contain as much as 70% of the phospholipids as this lipid. It can amount to 11% of macrophage/microglial cell lines, where it may reflect the increased endo-lysosomal capacity of these cells.
Exosomes shed from reticulocytes are nano-vesicles that carry a cargo of lipids and other materials into the circulation; they can be distinguished from secreted microvesicles of a different biological origin by their content of bis(monacylglycero)phosphate. The 22:6-22:6 species is reportedly a biomarker for phagocytizing macrophages/microglia cell during cerebral ischaemia and of urinary exosomes as a consequence of drug-induced endolysosomal dysfunction. In plasma, the lipid is found at low levels only where it is associated both with the lipoprotein fractions (40%) and the lipoprotein-deficient compartment (60%).
Whatever the positions of the fatty acids on the glycerol molecule, their compositions can be distinctive with 18:1(n-9) and 18:2(n-6), 20:4 and 22:6(n-3) being abundant, although this is highly dependent on the specific tissue, cell type or organelle (see Table 1). For example, the testis lipid contains more than 70% 22:5(n-6); to my knowledge, no other natural lipid contains so much of this fatty acid. Lung alveolar macrophages contain mainly C18 fatty acids, and baby hamster kidney (BHK) fibroblast cells are very different in that they contain more than 80% of oleate. In contrast, the metabolically important lysosomal lipid contains almost 70% 22:6(n-3). Such unusual compositions must confer distinctive properties in membranes and suggest quite specific functions, most of which have yet to be revealed.
Table 1. Fatty acid composition (wt% of the total) of bis(monoacylglycero)phosphate from various tissues.
|Fatty acid||Rat liver lysosomes||Human liver||Rabbit lung macrophages||Rat uterine stromal cells||Rat testis||BHK cells|
|22:5(n-6)||} 4||} 2||3||70|
|1, Wherrett, J.R. and Huterer, S. Lipids, 8, 531-533 (1973); DOI. 2, Huterer, S. and Wherrett, J. J. Lipid Res., 20, 966-973 (1979); DOI. 3, Luquain, C. et al., Biochem. J., 351, 795-804 (2000); DOI. 4, Brotherus, J. and Renkonen, O. Chem. Phys. Lipids, 13, 11-20 (1974); DOI.|
This lipid may not, however, be uniquely of animal origin, as the plant bacterial-pathogen Agrobacterium tumefaciens can take up lyso-phosphatidylglycerol and convert it to two distinct isoforms of BMP. It has also been reported from some alkalophilic strains of Bacillus species, although it is not known whether these have the distinctive stereochemistry of the animal equivalent.
2. Biochemistry and Function
Biosynthesis: There is good evidence that bis(monacylglycero)phosphate is synthesised from phosphatidylglycerol, primarily in the endosomal system. Although the later steps have still to be demonstrated experimentally and none of the required enzymes have been characterized, the scheme outlined below is believed to be the primary route. In the first step, a phospholipase A2 removes the fatty acid from position sn-2 of phosphatidylglycerol. In the second, the lysophosphatidylglycerol is acylated on the sn-2’ position of the head group glycerol moiety to yield sn-3:sn-1’ bis(monacylglycero)phosphate, by means of a transacylase reaction with lysophosphatidylglycerol as both the acyl donor and acyl acceptor. The third step leading to the stereospecific conversion of the precursor molecule to the unusual sn-1:sn-1' S-configuration has still to be adequately described but must involve removal of the fatty acid from position sn-1 of the primary glycerol unit and a rearrangement of the phosphoryl ester from the sn-3 to the sn-1 position. Finally position sn-2 of the primary glycerol unit is esterified, probably by a transacylation reaction with another phospholipid as donor (thence the distinctive fatty acid compositions).
The intracellular site of this synthesis has still to be confirmed. Bis(monacylglycero)phosphate with the sn-3:sn-1’ configuration has been isolated from BHK and rat uterine stromal cells, but it may be an intermediate in the biosynthetic pathway. While other biosynthetic routes may be possible, cardiolipin has been ruled out as a potential precursor.
Physical and chemical properties: The properties of bis(monacylglycero)phosphate in membranes will be highly dependent on fatty acid composition, but the function in lysosomes is of particular interest and is under active investigation. It certainly has a structural role in developing the complex intraluminal membrane system, aided by a tendency not to form a bilayer. Like cardiolipin, it is a cone-shaped molecule with a small but hydrated head group, which is negatively charged, and it encourages fusion of membranes or formation of internal vesicles (invagination) at the pH in the endosomes. It also may associate with specific proteins, which carry a positive charge under the acidic conditions in lysosomes.
The unique stereochemistry of bis(monacylglycero)phosphate means that it is resistant to most phospholipases, and this may hinder or prevent self digestion of the lysosomal membranes. Although the fatty acid constituents may turn over rapidly by transacylation, the glycerophosphate backbone is stable, and it is not touched by the main phospholipases that hydrolyse phosphatidylcholine and phosphatidylethanolamine. However, several phospholipases have been tentatively identified that may be involved in catabolism under acidic conditions and other local environmental factors, although the control mechanisms are not known (see below).
Function in lysosomes: Bis(monoacylglycero)phosphate is negatively charged at lysosomal pH and can form a stable docking station in the endosomal membranes for luminal acid hydrolases that are positively charged at acidic pH and require a water-lipid interface for activation. BMP-enriched vesicles serve in endosomal-lysosomal trafficking and activate these enzymes. For example, by binding in this way, it stimulates the activity of a number of lysosomal lipid-degrading enzymes, including acid sphingomyelinase, acid ceramidase, acid phospholipase A2, and an acid lipase with the capacity to hydrolyse triacylglycerols and cholesterol esters. For example, by binding to the heat-shock protein Hsp70, which promotes survival of stressed cells by inhibiting lysosomal membrane permeabilization on the inner lysosomal membrane, it activates the acid sphingomyelinase for the stabilization of lysosomes. It also has a dynamic role in the provision of arachidonate for eicosanoid production in alveolar macrophages. Thus, bis(monoacylglycero)phosphate plays a central role in cargo sorting by stimulating degradation and sorting of lipids.
The endosomal membranes are a continuation of the lysosomal membranes, and their function is also to sort and recycle material back to the plasma membrane and endoplasmic reticulum. Thus, low-density lipoproteins (LDL) internalized in the liver reach the late endosomes where the constituent cholesterol esters are hydrolysed by an acidic cholesterol ester hydrolase. The characteristic network of bis(monacylglycero)phosphate-rich membranes contained within multivesicular late endosomes is an important element of cholesterol homeostasis in that it regulates cholesterol transport by acting as a collection and re-distribution point for the free cholesterol generated in this way. For example, when lysosomal membranes are incubated with antibodies to bis(monacylglycero)phosphate, substantial amounts of cholesterol accumulate. The process is under the control of Alix/AlP1, a cytosolic protein that interacts specifically with this lipid and is involved in sorting into multivesicular endosomes. BMP is also involved in the regulation of intracellular cholesterol traffic in macrophages such as those in foam cells, where it is reported to have a protective effect by inhibiting the production of pro-apoptotic oxysterols.
In animal models, it has been demonstrated that different tissues have characteristic bis(monacylglycero)phosphate profiles that adapt to the nutritional and metabolic state, especially in hepatocytes, brown adipocytes, and pancreatic cells, suggesting that this lipid has a role in how these adapt to nutrient availability and ambient temperatures.
Disease: In consequence of its role in lysosomes, it has become evident that bis(monacylglycero)phosphate is involved in the pathology of lysosomal storage diseases such as Niemann-Pick C disease (cholesterol accumulation). Similarly, high levels of bis(monacylglycero)phosphate enriched in docosahexaenoic acid are found in the retinal pigment epithelium in Stargardt disease, which is characterized by juvenile onset retinal degeneration, and they are presumed to be a consequence of late endosomal/lysosomal dysfunction. It also accumulates as a secondary storage material in the brain of a broad range of mammals with gangliosidoses. In these circumstances, its concentration can increase substantially, probably as a secondary event, and its composition may change to favour molecular species that contain less of the polyunsaturated components. Dysregulation of bis(monacylglycero)phosphate metabolism and thence of cholesterol homeostasis may be relevant to atherosclerosis.
Bis(monacylglycero)phosphate is an antigen recognized by autoimmune sera from patients with a rare and poorly understood disease known as antiphospholipid syndrome, so it is obviously a factor in the pathological basis of this illness. In general in such diseases, bis(monoacylglycerol)phosphate levels are elevated in the circulation and its fatty acid composition changes, so that this can be used as a diagnostic biomarker that enables a clear distinction between lipid overload and drug-induced lysosomal storage diseases. For example, an elevated concentration of di-docosahexaenoyl bis(monoacylglycerol)phosphate in urine is considered to be a biomarker of drug-induced phospholipidosis. It may also be a biomarker for metastatic cancers of macrophage origin.
Non-enveloped viruses of the family Reoviridae, which include mammalian pathogens, enter cells without the aid of a limiting membrane and thus cannot fuse with host cell membranes. However, the bluetongue virus has been shown to use bis(monoacylglycero)phosphate in endosomes for membrane penetration and entry into host cells. There has been speculation that it may influence the infectivity of other viruses, including COVID-19, by enabling it to hijack the endosomal machinery leading to fusion of viral and endosomal membranes and release of the viral RNA into the cytosol.
Catabolism: Although bis(monoacylglycero)phosphate is resistant to hydrolysis by many of the common phospholipases because of its unique stereochemistry. it is now known to be hydrolysed with high specificity in liver by a hydrolase designated α/β hydrolase domain-containing 6 or ABHD6, once thought to be mainly a monoacylglycerol lipase, capable of degrading the endocannabinoid 2-arachidonoylglycerol. An enzyme designated ABHD12 may function in a similar manner to ABHD6 in brain. It can also be hydrolysed in vitro at least by the lysosomal acid sphingomyelinase.
3. Related Lipids
'Semilysobisphosphatidic acid', i.e. with three moles of fatty acid per mole of lipid, is occasionally found in tissues. In particular, it is concentrated in the Golgi membranes, where the relative amount varies in different regions, but can attain as much as 15% of the total phospholipids in those compartments that are most active biologically. It would not be at all surprising if this lipid were found to have a distinctive role in the Golgi complex, but at the moment this is a matter of speculation.
The fully acylated lipid, bis(diacylglycero)phosphate or 'bisphosphatidic acid', has been found in lysosomes from cultured hamster fibroblasts (BHK21 cells). In addition, it has been detected in bacteria, where it presumably has a different stereochemistry because of the mechanism of its synthesis from phosphatidylglycerol.
Although Archaeal glycerolipids also have the phosphate moiety linked to position sn-1 of the glycerol moiety, the biosynthetic mechanism (and function) of these lipids is entirely different from that of bis(monoacylglycerol)phosphate.
Bis(monoacylglycero)phosphate is easily misidentified as phosphatidic acid in many chromatographic systems. Modern mass spectrometric methods involving electrospray ionization now appear to be well suited to analysis, but especially when used in conjunction with liquid chromatography to ensure separation from phosphatidylglycerol with which it is isobaric.
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|Credits/disclaimer||Updated: October 11th, 2021||Author: William W. Christie|