Phosphatidylserine and Related Lipids

Phosphatidylserine or 1,2-diacyl-sn-glycero-3-phospho-L-serine is an anionic phospholipid, which brings essential physical properties to membranes in both eukaryotes and prokaryotes. Independently of this, it has many functions in cells, including effects on blood coagulation and apoptosis, and it is the biosynthetic precursor for phosphatidylethanolamine in prokaryotes and in eukaryote mitochondria, while its metabolite lysophosphatidylserine is a signalling mediator that operates through specific receptors. Phosphatidylthreonine, a structurally related lipid, and other phospholipids linked to amino acids are of interest for similar reasons. The 1‑octadecanoyl-2-docosahexaenoyl molecular species of phosphatidylserine, which has a special significance in brain tissue, is illustrated here.

Formula of phosphatidylserine

1. Phosphatidylserine - Structure and Occurrence

Formula of calcium-chelated phosphatidylserine Phosphatidylserine is an acidic (anionic) phospholipid with three ionizable groups, i.e., the phosphate, amino and carboxyl moieties. As with other acidic lipids, it exists in nature in salt form, but it has a high propensity to chelate to calcium via the charged oxygen atoms of both the carboxyl and phosphate moieties, so modifying the conformation of the polar head group, an interaction that may be necessary during bone formation.

Although phosphatidylserine is distributed widely among animals, plants and microorganisms, it is usually less than 10% of the total phospholipids, the greatest concentration being in myelin from brain tissue. In humans, it comprising 15 % of brain phospholipids with testes (6 %), liver (4 %), kidneys (6 %), lungs (7 %), heart (3 %),skeletal muscles (3 %) and plasma (0.2 %), but it may comprise 10 to 20 mol% of the total phospholipids in the plasma membrane of cells, where under normal conditions it is concentrated in the inner leaflet (and in the endoplasmic reticulum). In leaves of plants, it rarely amounts to more than 2% of the total lipids, and in the yeast Saccharomyces cerevisiae, it is a minor component of most cellular organelles other than the plasma membrane, where surprisingly it can amount to more than 30% of the total lipids. In most bacteria, it is a minor membrane constituent, although it is important as an intermediate in phosphatidylethanolamine biosynthesis.

In animal cells, the fatty acid composition of phosphatidylserine varies from tissue to tissue, but it does not appear to resemble the precursor phospholipids, either because of selective utilization of certain molecular species for biosynthesis or because of re-modelling of the lipid via deacylation-reacylation reactions with lysophosphatidylserine as an intermediate (see below). In human plasma, 1‑stearoyl-2-oleoyl and 1-stearoyl-2-arachidonoyl species predominate, but in brain (especially grey matter), retina and many other tissues, the 1‑stearoyl-2-docosahexaenoyl species is more abundant and appears to be necessary for normal operation of the nervous system. Indeed, the ratio of n-3 to n-6 fatty acids in brain phosphatidylserine is much higher than in most other lipids. The positional distribution of fatty acids in phosphatidylserine from rat liver and bovine brain are listed in Table 1, and as with most phospholipids, saturated fatty acids are concentrated in position sn-1 and polyunsaturated in position sn-2.

Position	Fatty acid
Table 1. Positional distribution of fatty acids in phosphatidylserine from rat liver and bovine brain
	16:0	18:0	18:1	18:2	20:4	22:6

Rat liver [1]
sn-1	5	93	1
sn-2	6	29	8	4	32	19

Bovine brain [2]
sn-1	3	81	13
sn-2	2	1	25	trace	1	60
1. Wood, R. and Harlow, R.D. Arch. Biochem. Biophys., 135, 272-281 (1969); DOI. 2. Yabuuchi, H. and O'Brien, J.S. J. Lipid Res., 9, 65-67 (1968); DOI.

In leaves of Arabidopsis thaliana, used as a 'model' plant in many studies, the fatty acid composition of phosphatidylserine resembles that of phosphatidylethanolamine. There is an intriguing report that the chain-lengths of the acyl groups increase with age and stress in phosphatidylserine quite specifically, and 22:0 and 24:0 fatty acids have been reported to occur in this lipid in the plasma membrane of some plant species.

In marked contrast to phosphatidylethanolamine, phosphatidylserines with ether-linked moieties (alkyl and alkenyl) are not common in animal tissues, although they are reported to be relatively abundant in human retina and macrophages (they were first found in rat lung). As a generality, the concentration of phosphatidylserine is highest in plasma membranes and endosomes, but it is very low in mitochondria. Because of its location entirely on the inner monolayer surface of the plasma membrane (and of other cellular membranes) as the most abundant anionic phospholipid, it may make the largest contribution to interfacial effects via non-specific electrostatic interactions in membranes. This normal distribution is disturbed during platelet activation and cellular apoptosis.

N-Acylphosphatidylserine is reportedly present in the frontal cortex of patients with schizophrenia and as a minor component of the lipids of sheep erythrocytes, bovine brain, bryozoans and the central nervous system of freshwater fish. The N-arachidonoyl form may be the precursor of the bioactive lipoamino acid N‑arachidonoylserine.

2. Biosynthesis of Phosphatidylserine

L-Serine is a non-essential amino acid that is synthesised by most organisms. In animals, it is produced in nearly all cell types, although in brain it is synthesised by astrocytes but not by neurons, which must be supplied with this amino acid for the biosynthesis of phosphatidylserine (and of sphingoid bases).

In animal tissues, phosphatidylserine is synthesised solely by calcium-dependent base-exchange reactions in which the polar head-group of an existing phospholipid is exchanged for L-serine. There are two routes utilizing distinct enzymes (PS synthase I and II) with 30% homology and several membrane-spanning domains that can utilize different substrates. Phosphatidylserine is synthesised by both enzymes on the cytosolic face of the endoplasmic reticulum (ER) of the cell, but mainly in a characteristic domain of this termed the mitochondria-associated membrane ('MAM'), because it is tethered transiently to the mitochondrial outer membrane, presumably by a protein bridge. In yeast, a complex of integrated proteins ('ERMES') has been characterized that act in a similar manner with exchange of L-serine with either phosphatidylcholine or phosphatidylethanolamine, catalysed by PS synthase I (although it was long thought that only phosphatidylcholine was a substrate for this enzyme), while PS synthase II catalyses a comparable exchange with diacyl-phosphatidylethanolamine and the plasmalogen form. Both enzymes are subject to feedback regulation by their product phosphatidylserine, thereby maintaining the correct amounts of this lipid.

Figure 1. Biosynthesis of phosphatidylserine in animal tissues.

Phosphatidylserine synthase I is expressed in all mouse tissues, but notably the kidney, liver and brain, while phosphatidylserine synthase II is the main enzyme in brain and testis, but much less in other tissues, and has a high specificity for molecular species containing docosahexaenoic acid. It is not known why there should be two enzymes, but one virtue is that the free ethanolamine and choline formed are rapidly re-utilized for phospholipid synthesis, so both phosphatidylserine and phosphatidylethanolamine are produced without a reduction in the amount of phosphatidylcholine. Elimination of both enzymes is embryonically lethal in knock-out mice, but each of them can be knocked out separately and the mice survive, even though they have substantially reduced levels of phosphatidylserine and phosphatidylethanolamine.

As with other phospholipids, the final fatty acid composition in animal tissues is attained by a process of remodelling known as the Lands’ cycle (see the web page on phosphatidylcholine). The first step is hydrolysis by a phospholipase A₂ to lysophosphatidylserine, followed by reacylation by various acyl-CoA:lysophospholipid acyltransferases. One membrane-bound O‑acyltransferase (LPCAT4 or MBOAT2) with a preference for oleoyl-CoA has been characterized, while a second (LPCAT3 or MBOAT5) incorporates linoleoyl and arachidonoyl chains (and utilizes lysophosphatidylcholine).

Following synthesis, export of phosphatidylserine from the endoplasmic reticulum to other compartments and its transbilayer asymmetry is tightly regulated by nonvesicular transport by lipid transfer proteins at membrane contact sites (discussed below) as is flip-flop between membrane leaflets by flippases and scramblases. Two main types of membrane protein are now known to take part in the latter process, six members of the phospholipid flipping family and three cell division control proteins (CDC), and together they mediate synergistically the transmembrane transport of phospholipid molecules from the outer leaflet to the inner leaflet of the cell membrane and thereby maintain the normal asymmetric distribution of phosphatidylserine, which is crucial in brain especially to support neurophysiology.

Metabolism: Some of the newly synthesised phosphatidylserine is transferred to the plasma membrane, while a proportion is transported to the mitochondria, probably again via transient membrane contact (MAM), where it is decarboxylated to produce phosphatidylethanolamine by a decarboxylase in the inner mitochondrial membrane.

Figure 2. Decarboxylation of phosphatidylserine in mitochondria.

All the phosphatidylethanolamine in mitochondria is formed in this way, but some can return to the endoplasmic reticulum where it may be converted back to phosphatidylserine by the action of the PS synthases. Mitochondrial production of phosphatidylethanolamine from phosphatidylserine is not fully complemented by the CDP-ethanolamine pathway, as mice lacking the enzyme do not survive for long. Evidently, cellular concentrations of these two lipids are intimately related and tightly regulated. In yeast, this process occurs at the Golgi/endosome membranes, and there is a preference for molecular species containing two monoenoic fatty acids for transport and metabolism. Much of the phosphatidylserine thus formed is decarboxylated to phosphatidylethanolamine, and this may be the major route to the latter in bacteria.

Biosynthesis of phosphatidylserine - mitochondria

Biosynthesis in bacteria, fungi and plants: In bacteria and other prokaryotic organisms and in yeast, phosphatidylserine is synthesised by a mechanism comparable to that of most other phospholipids, i.e., by reaction of L‑serine with CDP-diacylglycerol (see our web pages on phosphatidylglycerol, for example), and depends on Mg²⁺ or Mn²⁺. Phosphatidylserine synthases belong to two different families: type I (non-integral membrane form) in the phospholipase D-like family as in E. coli and type II (integral membrane form) in the CDP-alcohol phosphotransferase family as in Bacillus sp. and the yeast S. cerevisiae, although the latter shows no homology with the bacterial enzymes. As phosphatidylcholine in yeast is produced via methylation of phosphatidylethanolamine, phosphatidylserine is the primary precursor for this phospholipid also in these organisms.

Figure 3. Biosynthesis of phosphatidylserine in prokaryotes.

In many plants, including in the model plant Arabidopsis, much of the phosphatidylserine is produced by a calcium-dependent base-exchange reaction in which the head-group of an existing phospholipid is exchanged for L-serine in the luminal leaflet of the endoplasmic reticulum (i.e., resembling PS synthase I mechanistically). It is transferred to the cytoplasmic membrane leaflet by flippases and thence to the post-Golgi compartments before eventually accumulating at the plasma membrane, although some vesicular transport may occur or there may be direct transfer at membrane contact sites. A CDP-diacylglycerol (prokaryotic-like) biosynthetic pathway exists in some species, e.g., wheat.

3. Phosphatidylserine – Function

Membrane location: Phosphatidylserine modulates membrane charge locally, enabling the recruitment of soluble cations and proteins, and so it contributes to the organization of metabolic processes within cell membranes. Its distribution between leaflets is tightly controlled as it facilitates signalling within the various cellular compartments. Thus, it undergoes a transition from the lumenal leaflet of the endoplasmic reticulum to the cytosolic leaflet in the trans Golgi network by the action of ATP-independent flippases and scramblases, and it is highly enriched on the inner, compared to outer, leaflet of the plasma membrane. Transport to the plasma membrane against a concentration gradient is aided in part by proteins designated 'ORP5' and 'ORP8' in humans (Osh 6 and Osh7 in yeast) with a 'PH' binding domain for phosphatidylinositol 4,5-bisphosphate and an 'ORD' domain for phosphatidylserine. At a membrane contact site between the endoplasmic reticulum and plasma membrane, phosphatidylserine is exchanged for phosphatidylinositol 4-phosphate. Such transfer requires an input of energy, which can be supplied in the form of ATP or by phosphoinositides. As these transport proteins show some preference for unsaturated species, they may contribute to the variation in acyl chain compositions among different cellular compartments.

The asymmetric structure of the plasma membrane with high concentrations of anionic lipids such as phosphatidylserine in the cytosolic leaflet with zwitterionic lipids in the extracellular leaflet generates two surfaces with greatly different electrostatic potentials that influence the association of proteins at the membrane surface and the behaviour of integral membrane proteins. This distribution is maintained and can be altered by various flippases, but mainly the P4 subfamily of P‑type ATPases (transfer back into the cytoplasmic leaflet), floppases such as the ATP-binding cassette (ABC) transporter superfamily (transfer out of the cytoplasmic leaflet), and scramblases (bidirectional transfer), including ATP-dependent translocases selective for phosphatidylserine. Of these, scramblases have the greatest capacity and relative importance. Floppase ABCA1 is vital for lipid efflux and plasma membrane remodelling, where it is responsible for both phosphatidylserine and cholesterol transport and for retaining cholesterol in the cytosolic leaflet of the plasma membrane to shield it from cholesterol oxidase.

Enzyme activation: As well as being a component of cellular membranes and as a precursor for other phospholipids, phosphatidylserine is a cofactor that binds to and activates many proteins, including some with signalling properties, and dynamic clustering of phosphatidylserine molecules into nanodomains for signalling at the plasma membrane is well established. The negative charge on the lipid facilitates the binding to proteins through electrostatic interactions or Ca²⁺ bridges, and the presence of appreciable amounts of phosphatidylserine on the cytosolic leaflet of endosomes and lysosomes enables these compartments to dock with proteins with phosphatidylserine-binding domains that include several signalling and fusogenic effectors. The cytoskeletal protein spectrin binds to phosphatidylserine in this way, and it is required by enzymes such as the neutral sphingomyelinase and the Na⁺/K⁺ ATPase, where the 18:0/18:1 molecular species has a special significance. It is believed that the fatty acyl components of this species in the inner leaflet of the plasma membrane (and potentially other intracellular membranes) may interact (interdigitation or "handshake") with the very-long chains of sphingolipids in the outer leaflet in raft microdomains, resulting in a high local concentration of the anionic phospholipid and an accumulation of negative surface charge to which poly-cationic proteins in the membranes can bind to enable transfer of signals across the membrane to the cytosol.

Phosphatidylserine participates directly in signalling pathways in brain by binding to the cytosolic proteins for neuronal signalling. At least three major pathways are affected, including those utilizing phosphatidylinositol 3-kinase and protein kinase C. Most enzymes of the protein kinase C family contain a 'C2' calcium-dependent cysteine-rich region that recognizes phosphatidylserine, and in coordination with the 'C1' domain that binds to diacylglycerols, this activates and locates them to the plasma membrane of appropriately stimulated cells. In neurons, membrane domains containing phosphatidylserine enriched in docosahexaenoic acid (DHA or 22:6(n‑3)) facilitate the translocation and activation of several kinases and induce signalling pathways that promote neuronal development and survival. Within the inner nuclear membrane, phosphatidylserine regulates two enzymes for phosphatidylcholine biosynthesis.

Blood coagulation: Phosphatidylserine is an important mediator of the blood coagulation process in platelets, where it is transported from the inner to the outer surface of the plasma membrane in platelets exposed to fibrin-binding receptors. Here, the surface phosphatidylserine enhances the activation of prothrombin to thrombin, the key molecule in the blood clotting cascade, by triggering a cascade of reactions and providing the negatively charged platform that enables calcium ions to form bridges with γ-carboxyglutamic acid-containing domains on the coagulation factors. Membrane vesicles with phosphatidylserine exposed on the surface can also be released from platelets and promote the coagulation process. Apolipoprotein A-1 in high-density lipoproteins has a controlling function in that it neutralizes these procoagulant properties by arranging the phospholipid in surface areas that are too small to accommodate the prothrombinase complex. Blood coagulation is beneficial when it prevents the loss of blood from the circulatory system, but it is detrimental when it causes thrombosis, and the action of phosphatidylserine is necessary to the regulation of the process.

Scottish thistle Apoptosis: In response to calcium-dependent stimuli, phosphatidylserine is known to have a role in the regulation of apoptosis (programmed cell death or efferocytosis), the natural process by which aged or damaged cells are removed from tissues before they can be harmful. When cells are damaged, a mechanism is initiated in which the normal distribution of this lipid on the inner leaflet of the plasma membrane bilayer is disrupted when caspases stimulate scramblases, which are ATP-independent and can move the lipid across the membrane to the outer leaflet and collapse the membrane asymmetry. This occurs together with inhibition of aminophospholipid translocases, which return the lipid to the inner side of the membrane. In erythrocytes, phosphatidylserine is located in the inner leaflet of the membrane bilayer under low Ca²⁺ conditions when a phospholipid scramblase is suppressed by membrane cholesterol, but it is exposed to the outer leaflet under elevated Ca²⁺ concentrations, which activate the scramblase.

After the collapse of this asymmetry and transfer of phosphatidylserine to the outer leaflet of an effete cell, it is believed that it is recognized by a cohort of receptors, either directly or indirectly, through bridging ligands on the surface of macrophages and related scavenger cells. These activate a family of cysteine-dependent aspartate-specific proteases, the caspases, and other enzymes to facilitate the engulfment of the apoptotic cells and their potentially toxic or immunogenic contents in a non-inflammatory manner. Binding of phosphatidylserine to such proteins as apolipoprotein H (β2‑glycoprotein 1) enhances the recognition and clearance, and it binds to the anti-coagulant protein annexin V with high specificity, an interaction of value as a probe to detect apoptotic cells.

During apoptosis, generation of reactive oxygen species occurs, mainly hydrogen peroxide, which together with the enzyme cytochrome c bring about rapid oxidation of the fatty acids in phosphatidylserine before this lipid is externalized. Indeed, it is now apparent that molecular species of phosphatidylserine with an oxidatively truncated sn-2 acyl group that incorporates terminal γ‑hydroxy(or oxo)-α,β-unsaturated acyl moieties are potent signals for scavenger receptors in macrophages as a prerequisite for engulfment of apoptotic cells; such oxidized lipids are discussed in our web page dealing with oxidized phospholipids.

Externalization of phosphatidylserine has been described as "a dominant and evolutionarily conserved immunosuppressive signal that promotes tolerance and prevents local and systemic immune activation" or more succinctly as an "eat-me signal" that is found in the nematode Caenorhabditis elegans, the fruit fly Drosophila melanogaster and mammals. This process is crucial for the development of lung and brain, and it is relevant to clinical situations where apoptosis is involved, such as cancer, ischemic stroke, chronic autoimmunity and infections. The change in lipid distribution acts then as a signalling mechanism to modulate the behaviour of several membrane proteins, and for example, phosphatidylserine is a necessary component of the TAM family of receptor tyrosine kinases and the receptor-ligand complex of particular importance in cancer cells, where phosphatidylserine-TAM signalling regulates many aspects of inflammation and immune resolution. Exposure of phosphatidylserine is increased substantially by scramblases on the surface of tumour cells or tumour cell-derived microvesicles, but in this case instead of apoptosis, it triggers immunosuppressive signalling events leading to inhibition of dendritic and natural killer cells and conversion of tumour-associated macrophages into anti-inflammatory or M2 macrophages. Pharmacological inhibition of PSS1 has been shown to suppress tumorigenesis. Externalized phosphatidylinositol 3,4,5-trisphosphate (PI(3,4,5)P) and oxidized phospholipids may have some comparable functions.

In relation to atherosclerosis, phosphatidylserine is believed to be anti-inflammatory and protective as a component of the high-density lipoproteins, probably mediated by the apoptosis mechanism. As this mechanism is required for the turnover of erythrocytes, it is relevant to thrombus formation and the stabilization of blood clots. Unfortunately, viruses such as Ebola, HIV and SARS-CoV-2 can hijack the apoptosis machinery by incorporating phosphatidylserine into their viral envelopes so conning cells into engulfing them; the viral glycoprotein/cellular receptor complex may then facilitate the entry of foreign organisms into other cells.

Phagocytosis: Exposure of phosphatidylserine on the cell surface is reported to be a factor in other forms of regulated inflammatory cell death, such as necroptosis, pyroptosis, ferroptosis and phagocytosis. The last is a related process in which exposure of phosphatidylserine is a molecular trigger for clearance of apoptotic lymphocytes by macrophages, although it is now defined as the uptake of large (ø > 1 µm) particles by cells, resulting in a transition from a pro-inflammatory to an anti-inflammatory state. The mechanism involves translocation of appreciable amounts of phosphatidylserine to the surface of T lymphocytes that express low levels of the trans-membrane enzyme tyrosine phosphatase for uptake by macrophages. In retinal pigment epithelial cells, this operates to remove the large amounts of photoreceptor cell debris that are generated continuously. It is noteworthy that phosphatidylserine is a major component of the membranes of microvesicles in animal cells, and translocation to the outer leaflet upon cellular activation is necessary for their biogenesis. An analogous process operates in erythrocytes infected with the malaria parasite Plasmodium where there is disruption in erythrocyte membrane asymmetry and an increased exposure of phosphatidylserine that results in phagocytosis and destruction of infected erythrocytes.

Brain: The high concentrations of DHA in brain and retinal phosphatidylserine are certainly required for the development and function of these tissues, and it be a reservoir for protectin formation. Accumulation of phosphatidylserine in neuronal membranes is promoted by this fatty acid, and this is critical for the maintenance of neuronal survival. Phosphatidylserine can bind directly to neurotransmitters to induce membrane signalling pathways, reduce neuroinflammation and promote neurotransmission. It has a high affinity for binding to serotonin and dopamine, and it participates in the stimulation of pro-survival signalling molecules, such as protein kinase B (AKT) and C, which might contribute to the alleviation of neurodegenerative diseases. On the other hand, the Food and Drug Administration in the USA considers that there is insufficient scientific evidence to support claims that dietary supplements of phosphatidylserine reduce the risk of dementia or cognitive dysfunction in the elderly, and other nutritional claims appear to be dubious. That said, there is increasing research in this area as commercial products derived from soybean and marine sources have become available, and there are numerous reports of beneficial effects, which may merit scrutiny by clinical experts (of whom I am not one).

Disease: Antibodies to phosphatidylserine are formed in several disease states, including thrombosis and recurrent spontaneous pregnancy loss. The rare genetic disease Lenz-Majewski syndrome characterized by bone malformation and often osteosclerosis is caused by a mutation in the gene encoding the phosphatidylserine synthase I in osteoclasts that greatly increases the activity of the enzyme while preventing feedback inhibition and resulting in more general changes in phospholipid compositions. In B cell lymphoma cell lines, inhibition of phosphatidylserine synthase 1 causes an imbalanced phospholipid metabolism that leads to aberrant hyperactivation of the B cell receptor, a B cell-specific survival mechanism, and ultimately cell death.

In yeasts such as Candida albicans, phosphatidylserine and the enzyme phosphatidylserine decarboxylase, which generates phosphatidylethanolamine, are both essential for the virulence of the organism towards host species. Parasites, including Leishmania, Trypanosoma and Toxoplasma species, utilize host phosphatidylserine to establish infections and facilitate disease progression as they do not then elicit production of proinflammatory cytokines. This mechanism has been termed 'apoptotic mimicry' and is critical for survival of parasites within the macrophage. However, Toxoplasma gondi can also synthesise phosphatidylserine de novo in the endoplasmic reticulum and convert it to phosphatidylethanolamine.

Other functions: Phosphatidylserine is required for the transmembrane movement of excess cholesterol, derived initially from the lysosomal degradation of low-density lipoproteins, from the plasma membrane to the endoplasmic reticulum thereby maintaining membrane integrity and ensuring cell survival. It is therefore an element in cholesterol homeostasis via a mechanism believed to use proteins known as GRAMD1s embedded in the endoplasmic reticulum membrane at sites in contact with the plasma membrane. These have two functional domains: the StART-like domain that binds cholesterol and the GRAM domain that binds anionic lipids, such as phosphatidylserine, and so forms a link between the two membranes that enables the transfer of cholesterol. Selective PSS1 inhibitors may have the potential to lower blood cholesterol levels.

Phosphatidylserine is a component of the lipid-calcium-phosphate complexes, which act as nucleation centres for hydroxyapatite formation and initiate mineral deposition during the formation of bone. It has been established that phosphatidylserine and inorganic phosphate must be present before calcium ions are introduced, because of the high affinity of phosphatidylserine for calcium ions with nucleation facilitated by the protein annexin V. During bone repair and maintenance, the fusion of osteoclasts requires the non-apoptotic exposure of phosphatidylserine at the surface of fusion-committed cells with the aid of a transmembrane protein (DC-STAMP) expressed in dendrocytes. This is relevant to cardiovascular disease and to the phenomenon of "hardening of the arteries", where atherosclerotic plaques can undergo mineralization with the deposition of hydroxyapatite.

Among many other metabolic processes in which phosphatidylserine participates, it is believed to be a vital surface membrane component for the fusion of cell types other than osteoclasts, including the formation of fibres in muscle cells, and fusion of macrophages into inflammatory giant cells and myoblasts into myotubes. Such cell fusions need the non-apoptotic exposure of phosphatidylserine at the surface of fusing cells, where it interacts with phosphatidylserine-recognizing proteins to regulate the time and place of cell-fusion. Phosphatidylserine provides stable membrane domains in spermatozoa for fertilization, and it is a component of the plasma membrane microdomains known as caveolae, where it is required both for their formation and stability possibly through binding to the cavin proteins, mainly caveolin-1, although it does not take part in membrane raft formation.

4. Lysophosphatidylserine

Structural formula of 2-lysophosphatidylserine Lysophosphatidylserine, i.e., with a fatty acid in one position only of the glycerol moiety, is known to be a mediator of several metabolic processes in animal tissues, especially in the context of the immune system. It has been found in the thymus, peripheral lymphoid tissues, central nervous system and colon, but is barely detectable in plasma. Deacylation of the diacyl lipid by phospholipases is the primary source, and a secreted isoform that is phosphatidylserine-specific (PLA1A) removes the sn-1 acyl group to generate sn‑2‑lysophosphatidylserine containing unsaturated fatty acids. This extracellular enzyme is upregulated greatly by various inflammatory stimuli and utilizes phosphatidylserine exposed on the cell membrane as a substrate, although other phospholipases may operate intracellularly and produce sn‑1‑lysophosphatidylserine. In some species, platelets secrete a phospholipase A₂ group IIA (ABHD16A), which generates saturated sn‑1‑lysophosphatidylserine and other lysophospholipids, but this is not a significant reaction in humans). ABHD12, a membrane-bound serine hydrolase, is a lysophosphatidylserine lipase in mammalian brain.

Lysophosphatidylserine has been detected after injury to animal tissues (tumour growth, graft rejection, burns), and it resembles lysophosphatidic acid in cell signalling by regulating calcium flux and stimulating immune cells through G protein-coupled receptors of which three (GPR34, P2Y10 and GPR174, or LPS1 to 3) have been detected in mice and humans. Of these, GPR174 mediates the suppression of T-cell proliferation induced in vitro by lysophosphatidylserine. In damaged cells, lysophosphatidylserine is generated by a reaction dependent on activation of the NADPH oxidase, and it can then diffuse and transmit the information to other cells, including mast cells, to enhance clearance of neutrophils, so it has a role in the resolution of inflammation. Only one molecular species, i.e., 1‑(11Z‑eicosenoyl)-glycero-3-phosphoserine, is reported to be a true agonist of the Toll-like receptor 2/6 heterodimer of importance to the immune response to pathogens with both its polar head group and the length of the acyl chain required. On the other hand, sn-2-lysophosphatidylserine is the ligand for GPR34, although it is the less stable isomer. It has proinflammatory reactions in that it augments mast cell degranulation and mast cell-dependent anaphylactic shock; most other lysophospholipids do not act in this way.

Deregulated lysophosphatidylserine metabolism has been linked to certain cancers, cardio-metabolic disorders, night blindness and the human genetic neurological disorder PHARC, while high serum levels of PLA1A are associated with such autoimmune disorders as Graves' disease and systemic lupus erythematosus, and there is increased expression of the enzyme in metastatic melanomas. In Schistosome infections, lysophosphatidylserine from the parasite is believed to participate in host metabolism. It is necessary for assembly of the hepatitis C virus, and it takes part in the antivirus innate immune response.

Negatively charged lysophosphatidylserine species tend to organize in non-bilayer structures and are believed to facilitate folding of certain membrane proteins in situ better than bilayer-forming lipids.

5. Phosphatidylthreonine and Other Amino Acid-Containing Phospholipids

Formulae of phosphatidyl-L-threonine Phosphatidyl-L-threonine, which is closely related structurally and metabolically to phosphatidylserine, was first detected in animal brain and tuna muscle, before it was characterized definitively as a minor component of polyoma virus-transformed embryo fibroblasts in hamsters, cultured hippocampal neurons and macrophages. Biosynthetic studies with microsomes from rat brain suggest that it is synthesised by the same base-exchange enzyme as for phosphatidylserine synthesis but at a much lower level. In laboratory animals, it is barely detectable in normal tissues such as brain, and it is decarboxylated in mitochondria in vitro to phosphatidylisopropanolamine. On the other hand, it is present in human blood where it is a pro-coagulant, and its concentration is elevated in coronary artery disease.

It has been detected in some bacterial species and especially Bdellovibrio bacteriovorus. Phosphatidylthreonine is now known to be a major phospholipid, with 20:1 and 20:4 as the fatty acid constituents, of the protozoan parasite Toxoplasma gondii, which can infect animals and humans. It is produced in the endoplasmic reticulum by a novel phosphatidylthreonine synthase, which has evolved from the well-known phosphatidylserine synthase, and it is required for asexual reproduction and virulence of the parasite in vivo. Targeted inhibition of this enzyme leads to dysregulation of calcium metabolism to influence many metabolic events as well as virulence. As a mutant strain of the organism lacking phosphatidylthreonine was able to protect vaccinated mice against acute and currently incurable chronic infection, there are obvious pharmacological implications. It is produced in the sporozoite stage of the apicomplexan parasite Eimeria falciformis.

Lysophosphatidylthreonine, with a fatty acid in position sn-1 only, acts in much the same way as lysophosphatidylserine in vitro, although it is not known whether this is true in vivo.

Other amino acid-linked phospholipids: Phosphatidyl-L-aspartate and phosphatidyl-L-glutamate with unique carboxylate-phosphate anhydride bonds have been detected in rat brain, and other phospholipids related to phosphatidylthreonine include phosphatidyl-O-[N-(2-hydroxyethyl)glycine], which was isolated from brown algae of the family Phaeophyceae such as Fucus serratus, where it can amount to as much as 25% of the total lipids. The fatty acid composition is distinctive among algae in that arachidonic acid comprises about 80% of the total. A minor phospholipid component from the bacterium E. coli contains a dipeptide unit, i.e., phosphatidylserylglutamate, while Porphyromas gingivalis, a Gram-negative anaerobic periodontal pathogen, and Bacteroides ovatus from the human gut contain phosphatidylserylglycine together with a form in which the dipeptide component is further acylated (and could be classified as a lipopeptide). This acylated form is a ligand for Toll-like receptor 2 (TLR2) and is believed to be relevant to chronic periodontitis in humans. sphingolipid analogues are known.

Structural formula of phosphatidylserylglutamate and phosphatidyl-O-[N-(2-hydroxyethyl) glycine]

Phosphatidylethanolamineglutamate has been detected in the bacterium Peredibacter starrii. Some other amino acid-containing phospholipids (the complex lipoamino acids) are more closely related to phosphatidylglycerol in structure and the mechanism of biosynthesis.

6. Analysis

As with other acidic lipids, the metal ions associated with phosphatidylserine hamper analysis, although the problem can be solved by an acid wash. It is easily separated from other phospholipids by two-dimensional thin-layer chromatography, but poorly shaped peaks are often seen with high-performance liquid chromatography. Mass spectrometry is being used increasingly for molecular species analysis and quantification.

	© Author: William W. Christie
	Contact/credits/disclaimer	Updated: December 11^th, 2024