Structure, phase behaviour and membrane interactions of N-acylethanolamines and N-acylphosphatidylethanolamines

ATP-Assisted Protocellular Membrane Formation with Ethanolamine-Based Amphiphiles

Prebiotic membranes are one of the essential elements of the origin of life because they build compartments to keep genetic materials and metabolic machinery safe. Since modern cell membranes are made up of ethanolamine-based phospholipids, prebiotic membrane formation with ethanolamine-based amphiphiles and phosphates might act as a bridge between the prebiotic and contemporary eras. Here, we report the prebiotic synthesis of O-lauroyl ethanolamine (OLEA), O-lauroyl methyl ethanolamine (OLMEA), and O-lauroyl dimethylethanolamine (OLDMEA) under wet-dry cycles. Turbidimetric, NMR, DLS, fluorescence, microscopy, and glucose encapsulation studies highlighted that OLEA-ATP and OLMEA-ATP form protocellular membranes in a 3:1 ratio, where ATP acts as a template. OLDMEA with a dimethyl group did not form any membrane in the presence of ATP. ADP can also template OLEA to form vesicles in a 2:1 ratio, but the ADP-templated vesicles were smaller. This suggests the critical role of the phosphate backbone in controlling the curvature of supramolecular assembly. The mechanisms of hierarchical assembly and transient dissipative assembly are discussed based on templated-complex formation via electrostatic, hydrophobic, and H-bonding interactions. Our results suggest that N-methylethanolamine-based amphiphiles could be used to form prebiotic vesicles, but the superior H-bonding ability of the ethanolamine moiety likely provides an evolutionary advantage for stable protocell formation during the fluctuating environments of early earth.

Anandamide and other N-acylethanolamines: A class of signaling lipids with therapeutic opportunities

N-acylethanolamines (NAEs), including N-palmitoylethanolamine (PEA), N-oleoylethanolamine (OEA), N-arachidonoylethanolamine (AEA, anandamide), N-docosahexaenoylethanolamine (DHEA, synaptamide) and their oxygenated metabolites are a lipid messenger family with numerous functions in health and disease, including inflammation, anxiety and energy metabolism. The NAEs exert their signaling role through activation of various G protein-coupled receptors (cannabinoid CB₁ and CB₂ receptors, GPR55, GPR110, GPR119), ion channels (TRPV1) and nuclear receptors (PPAR-α and PPAR-γ) in the brain and periphery. The biological role of the oxygenated NAEs, such as prostamides, hydroxylated anandamide and DHEA derivatives, are less studied. Evidence is accumulating that NAEs and their oxidative metabolites may be aberrantly regulated or are associated with disease severity in obesity, metabolic syndrome, cancer, neuroinflammation and liver cirrhosis. Here, we comprehensively review NAE biosynthesis and degradation, their metabolism by lipoxygenases, cyclooxygenases and cytochrome P450s and the biological functions of these signaling lipids. We discuss the latest findings and therapeutic potential of modulating endogenous NAE levels by inhibition of their degradation, which is currently under clinical evaluation for neuropsychiatric disorders. We also highlight NAE biosynthesis inhibition as an emerging topic with therapeutic opportunities in endocannabinoid and NAE signaling.

Segregated functions of two cytosolic phospholipase A2 isoforms (cPLA2α and cPLA2ε) in lipid mediator generation

Among the phospholipase A₂ (PLA₂) superfamily, group IVA cytosolic PLA₂ (cPLA₂α) is currently attracting much attention as a central regulator of arachidonic acid (AA) metabolism linked to eicosanoid biosynthesis. Following cell activation, cPLA₂α selectively releases AA, a precursor of a variety of eicosanoids, from phospholipids in perinuclear membrane compartments. cPLA₂α-null mice display various phenotypes that could be largely explained by reduced eicosanoid signaling. In contrast, group IVE cPLA₂ε, another member of the cPLA₂ family, acts as a Ca²⁺-dependent N-acyltransferase rather than a PLA₂, thereby regulating the biosynthesis of N-acylethanolamines (NAEs), a unique class of lipid mediators with an anti-inflammatory effect. In response to Ca²⁺ signaling, cPLA₂ε translocates to phosphatidylserine-rich organelle membranes in the endocytic/recycling pathway. In vivo, cPLA₂ε is induced in keratinocytes of psoriatic skin, and its genetic deletion exacerbates psoriatic inflammation due to a marked reduction of NAE-related lipids. cPLA₂ε also contributes to NAE generation in several if not all mouse tissues. Thus, the two members of the cPLA₂ family, cPLA₂α and cPLA₂ε, catalyze distinct enzymatic reactions to mobilize distinct sets of lipid mediators, thereby differently regulating pathophysiological events in health and disease. Such segregation of the cPLA₂α-eicosanoid and cPLA₂ε-NAE pathways represents a new paradigm of research on PLA₂s and lipid mediators.

Head-Group Acylation of Chloroplast Membrane Lipids

Head group-acylated chloroplast lipids were discovered in the 1960s, but interest was renewed about 15 years ago with the discovery of Arabidopsides E and G, acylated monogalactosyldiacylglycerols with oxidized fatty acyl chains originally identified in Arabidopsis thaliana. Since then, plant biologists have applied the power of mass spectrometry to identify additional oxidized and non-oxidized chloroplast lipids and quantify their levels in response to biotic and abiotic stresses. The enzyme responsible for the head-group acylation of chloroplast lipids was identified as a cytosolic protein closely associated with the chloroplast outer membrane and christened acylated galactolipid-associated phospholipase 1 (AGAP1). Despite many advances, critical questions remain about the biological functions of AGAP1 and its head group-acylated products.

Structure, thermotropic phase behavior and membrane interaction of N-acyl-β-alaninols. Homologs of stress-combating N-acylethanolamines

β-Alaninol and its derivatives were reported to exhibit interesting biological and pharmacological activities and showed potential application in formulating drug delivery vehicles. In the present study, we report the synthesis and characterization of N-acyl-β-alaninols (NABAOHs) bearing saturated acyl chains (n = 8-20) with respect to thermotropic phase behavior, supramolecular organization and interaction with diacylphosphatidylcholine, a major membrane lipid. Results obtained from DSC and powder XRD studies revealed that the transition temperatures (T_t), transition enthalpies (ΔH_t), transition entropies (ΔS_t) and d-spacings of NABAOHs show odd-even alteration. A linear dependence was observed in the values of ΔH_t and ΔS_t on the acyl chain length, independently for even and odd acyl chains in both dry and hydrated states; further, the even chainlength molecules exhibited higher values than the odd chainlength series. The crystals structures of N-lauroyl-β-alaninol and N-palmitoyl-β-alaninol, solved in monoclinic system in the P21/c space group, show that the NABAOHs adopt a tilted bilayer structure. A number of NH⋯O, O-H⋯O, and C-H⋯O hydrogen bonds between the hydroxyl and amide moieties of the head groups of NABAOH molecules belonging to adjacent and opposite layers stabilize the overall supramolecular organization of the self-assembled bilayer system. DSC studies on the interaction of N-myristoyl-β-alaninol (NMBAOH) with dimyristoylphosphatidylcholine (DMPC) indicate that these two lipids mix well up to 45 mol% NMBAOH, whereas phase separation was observed at higher contents of NMBAOH. Transmission electron microscopic studies reveal that mixtures containing 20-50 mol% NMBAOH form stable ULVs of 90-150 nm diameter, suitable for use in drug delivery applications.

NAPE-specific phospholipase D regulates LRRK2 association with neuronal membranes

N-acylphosphatidylethanolamines (NAPEs) are glycerophospholipid precursors for bioactive lipid amides and potential regulators of membrane function. They are hydrolyzed by NAPE-specific phospholipase D (NAPE-PLD) and have been implicated in neurodegenerative disorders such as Parkinson's disease. Here, we used siRNA-mediated silencing of NAPE-PLD in human SH-SY5Y cells and NAPE-PLD^-/- mice to determine whether NAPEs influence the membrane association of LRRK2, a multifunctional protein kinase that is frequently mutated in persons with sporadic Parkinson's disease. NAPE-PLD deletion caused a significant accumulation of non-metabolized NAPEs, which was accompanied by a shift of LRRK2 from membrane to cytosol and a reduction in total LRRK2 content. Conversely, exposure of intact SH-SY5Y cells to bacterial PLD lowered NAPE levels and enhanced LRRK2 association with membranes. The results suggest that NAPE-PLD activity may contribute to the control of LRRK2 localization by regulating membrane NAPE levels.

Evaluation of the toxicities of silver and silver sulfide nanoparticles against Gram-positive and Gram-negative bacteria

In this study, the endogenous lipid signalling molecules, N-myristoylethanolamine, were explored as a capping agent to synthesise stable silver nanoparticles (AgNPs) and Ag sulphide NPs (Ag₂S NPs). Sulphidation of the AgNPs abolishes the surface plasmon resonance (SPR) maximum of AgNPs at 415 nm with concomitant changes in the SPR, indicating the formation of Ag₂S NPs. Transmission electron microscopy revealed that the AgNPs and Ag₂S NPs are spherical in shape with a size of 5-30 and 8-30 nm, respectively. AgNPs and Ag₂S NPs exhibit antimicrobial activity against Gram-positive and Gram-negative bacteria. The minimum inhibitory concentrations (MIC) of 25 and 50 μM for AgNPs and Ag₂S NPs, respectively, were determined from resazurin microtitre plate assay. At or above MIC, both AgNPs and Ag₂S NPs decrease the cell viability through the mechanism of membrane damage and generation of excess reactive oxygen species.

A protective role for N-acylphosphatidylethanolamine phospholipase D in 6-OHDA-induced neurodegeneration

N-acylphosphatidylethanolamine phospholipase D (NAPE-PLD) catalyzes the cleavage of membrane NAPEs into bioactive fatty-acid ethanolamides (FAEs). Along with this precursor role, NAPEs might also serve autonomous signaling functions. Here, we report that injections of 6-hydroxydopamine (6-OHDA) into the mouse striatum cause a local increase in NAPE and FAE levels, which precedes neuronal cell death. NAPE, but not FAE, accumulation is enhanced in mice lacking NAPE-PLD, which display a substantial reduction in 6-OHDA-induced neurotoxicity, as shown by increased survival of substantia nigra dopamine neurons, integrity of striatal dopaminergic fibers, and striatal dopamine metabolite content. Reduced damage is accompanied by attenuation of the motor response evoked by apomorphine. Furthermore, NAPE-PLD silencing protects cathecolamine-producing SH-SY5Y cells from 6-OHDA-induced reactive oxygen species formation, caspase-3 activation and death. Mechanistic studies in mice suggest the existence of multiple molecular contributors to the neuroprotective effects of NAPE-PLD deletion, including suppression of Rac1 activity and attenuated transcription of several genes (Cadps, Casp9, Egln1, Kcnj6, Spen, and Uchl1) implicated in dopamine neuron survival and/or Parkinson's disease. The findings point to a previously unrecognized role for NAPE-PLD in the regulation of dopamine neuron function, which may be linked to the control of NAPE homeostasis in membranes.

Phosphatidylserine-stimulated production of N-acyl-phosphatidylethanolamines by Ca2+-dependent N-acyltransferase

N-acyl-phosphatidylethanolamine (NAPE) is known to be a precursor for various bioactive N-acylethanolamines including the endocannabinoid anandamide. NAPE is produced in mammals through the transfer of an acyl chain from certain glycerophospholipids to phosphatidylethanolamine (PE) by Ca²⁺-dependent or -independent N-acyltransferases. The ε isoform of mouse cytosolic phospholipase A₂ (cPLA₂ε) was recently identified as a Ca²⁺-dependent N-acyltransferase (Ca-NAT). In the present study, we first showed that two isoforms of human cPLA₂ε function as Ca-NAT. We next purified both mouse recombinant cPLA₂ε and its two human orthologues to examine their catalytic properties. The enzyme absolutely required Ca²⁺ for its activity and the activity was enhanced by phosphatidylserine (PS). PS enhanced the activity 25-fold in the presence of 1 mM CaCl₂ and lowered the EC₅₀ value of Ca²⁺ >8-fold. Using a PS probe, we showed that cPLA₂ε largely co-localizes with PS in plasma membrane and organelles involved in the endocytic pathway, further supporting the interaction of cPLA₂ε with PS in living cells. Finally, we found that the Ca²⁺-ionophore ionomycin increased [¹⁴C]NAPE levels >10-fold in [¹⁴C]ethanolamine-labeled cPLA₂ε-expressing cells while phospholipase A/acyltransferase-1, acting as a Ca²⁺-independent N-acyltransferase, was insensitive to ionomycin for full activity. In conclusion, PS potently stimulated the Ca²⁺-dependent activity and human cPLA₂ε isoforms also functioned as Ca-NAT.

Comparative analyses of isoforms of the calcium-independent phosphatidylethanolamine N-acyltransferase PLAAT-1 in humans and mice

N-Acylphosphatidylethanolamines (NAPEs) are a class of glycerophospholipids, which are known as precursors for different bioactive N-acylethanolamines. We previously reported that phospholipase A/acyltransferase-1 (PLAAT-1), which was originally found in mammals as a tumor suppressor, catalyzes N-acylation of phosphatidylethanolamines to form NAPEs. However, recent online database suggested the presence of an uncharacterized isoform of PLAAT-1 with an extra sequence at the N terminus. In the present study, we examined the occurrence, intracellular localization, and catalytic properties of this longer isoform, as well as the original shorter isoform from humans and mice. Our results showed that human tissues express the longer isoform but not the short isoform at all. In contrast, mice expressed both isoforms with different tissue distribution. Unlike the cytoplasmic localization of the shorter isoform, the long isoform was found in both cytoplasm and nucleus, inferring that the extra sequence harbors a nuclear localization signal. As assayed with purified proteins, neither isoform required calcium for full activity. Moreover, the overexpression of each isoform remarkably increased cellular NAPE levels. These results conclude that the new long isoform of PLAAT-1 is a calcium-independent N-acyltransferase existing in both cytoplasm and nucleus and suggest a possible formation of NAPEs in various membrane structures including nuclear membrane. J. Lipid Res 2016. 57: 2051-2060.

Synthesis, characterization and thermotropic phase behavior of a homologous series of N-acyl-L-alaninols and interaction of N-myristoyl L-alaninol with dimyristoylphosphatidylcholine

N-acylethanolamines (NAEs) and their precursors, N-acylphosphatidylethanolamines are present in the cell membranes of a variety of species and exhibit interesting biological properties. N-acyl-L-alaninols (NAAOHs) are chiral homologues of NAEs and reduced forms of N-acyl-L-alanines (NAAs) and were reported to induce apoptosis in human lymphocytes. In the present study, we have synthesized and characterized a homologous series of N-acyl-L-alaninols (n=9-20). In DSC studies in the dry as well as hydrated states NAAOHs with different chain lengths showed single sharp transitions similar to N-acyl-L-alanines. Transition enthalpies (ΔHt) and entropies (ΔSt) of NAAOHs are linearly dependent on the acyl chain length in both dry and hydrated states. Powder X-ray diffraction studies showed that the d-spacings of NAAOHs exhibit linear dependence on the chain length and the incremental increase in the d values suggest that they may be packed in a tilted bilayer pattern. Studies on the interaction of N-myristoyl L-alaninol (NMAOH) with DMPC revealed that the two amphiphiles mix well up to 45 mol% of NMAOH, whereas phase separation is observed at higher contents of the alaninol. Transmission electron microscopic studies show that the NMAOH:DMPC (45:55, mol/mol) mixture forms unilamellar vesicles of about 120-150 nm in diameter.

Accumulation of N-Acylphosphatidylserines and N-Acylserines in the Frontal Cortex in Schizophrenia

BACKGROUND

While schizophrenia is generally considered a neurodevelopment disorder, our basic understanding of the biochemical processes involved in disease etiology and/or progression is limited. One class of biochemical mediators that has been suggested to play a role in the development of schizophrenia is N-acyl ethanolamine metabolites of N-acylphosphatidylethanolamines. However, no investigations of N-acylphosphatidylserines or their N-acylserine metabolites have been published.

METHODS

We undertook a targeted postmortem lipidomics analysis of N-acylphosphatidylserines (NAPS) and N-acylserines (NAS) in gray matter of the frontal cortex of schizophrenia subjects.

RESULTS

Our data are the first to demonstrate that NAPS and NAS are present in human brain. Furthermore, NAPS and their bioactive metabolites, N-acylserines (NAS), were found to be significantly elevated in the frontal cortex of schizophrenia subjects.

CONCLUSIONS

Elevated levels of NAPS lipid pools in schizophrenia may result in complex alterations in the structural function of neuronal membranes while increases in NAS may alter signal transduction pathways.

Synthesis, physicochemical characterization and membrane interactions of a homologous series of N-acylserotonins: Bioactive, endogenous conjugates of serotonin with fatty acids

N-Acylserotonins (NASTs), present in the mammalian gastro-intestinal tract and central nervous tissues, exhibit significant biological and pharmacological activities. In the present study, a homologous series of NASTs have been synthesized and characterized. Differential scanning calorimetric studies show that in the dry and hydrated states the transition temperatures, enthalpies, and entropies of NASTs exhibit odd-even alternation. Both odd and even chain length NASTs independently display linear dependence of the transition enthalpies and entropies on the chain length under dry as well as hydrated conditions, suggesting that the molecular packing and intermolecular interactions in each series (odd or even) are likely to be similar for NASTs with different acyl chain lengths in the dry state as well as in the hydrated state. Powder X-ray diffraction studies indicated that the incremental increase in the d-spacing per CH₂group is 1.023 Å, suggesting that the lipid acyl chains are most likely packed in an interdigitated fashion. Results of computational studies are consistent with this and suggest that the acyl chains of the NASTs are tilted with respect to the bilayer normal. Incorporation of N-myristoylserotonin (NMST) into dimyristoylphosphatidylcholine (DMPC) membranes did not significantly affect the phase transition properties at low mole fractions (1-5 mol%), although distinct decrease in the chain-melting transition temperature and increase in the pretransition temperature were observed at higher contents (7.5-30 mol%), suggesting that NMST increases the stability of the tilted gel phase (L(β)') but destabilizes the ripple phase (P(β)'). These observations provide a thermodynamic basis for understanding the functional role of NASTs in their parent tissues.

Lamellar self-assemblies of single-chain amphiphiles and sterols and their derived liposomes: distinct compositions and distinct properties

Typically, single-chain amphiphiles and sterols do not form fluid lamellar phases once hydrated individually. Most of the single-chain amphiphiles form actually micelles in aqueous environments, while sterols display a very limited solubility in water. However, under certain conditions, mixtures of single-chain amphiphiles and sterols lead to the formation of stable fluid bilayers. Over the past decade, several of these systems leading to fluid lamellar self-assemblies have been identified and this article reviews the current knowledge relative to these non-phospholipid bilayers made of single-chain amphiphiles and sterols. It presents an integrated view about the molecular features that are required for their stability, the properties they share, and the origin of these characteristics. It was also shown that these lamellar systems could lead to the formation of unilamellar vesicles, similar to phospholipid based liposomes. These vesicles display distinct properties that make them potentially appealing for technological applications; they display a limited permeability, they are stable, they are formed with molecules that are relatively chemically inert (and relatively cheap), and they can be readily functionalized. The features of these distinct liposomes and their technological applications are reviewed. Finally, the putative biological implications of these non-phospholipid fluid bilayers are also discussed.

N-Acylated phospholipid metabolism and seedling growth: insights from lipidomics studies in Arabidopsis

N-Acylphosphatidylethanolamines (NAPEs) are precursors of endogenous bioactive lipids, N-acylethanolamines (NAEs). NAPEs, which occur as a minor membrane lipid, are hydrolyzed in a single enzymatic step catalyzed by a type of phospholipase D (PLD) to generate fatty acid ethanolamides. Although, the occurrence of NAPE is widespread in the plant kingdom, the physiological roles remain under appreciated due to the lack of sensitive tools to quantify the pathway metabolites. In Kilaru et al. (2012, Planta, DOI 10.1007/s00425-012-1669-z), comprehensive mass spectrometry (MS)-based methods were developed to gain a clearer understanding of the complex network of metabolites that participate in NAE metabolic pathway. This targeted lipidomics approach allowed insights to be drawn into the implications of altered NAE levels on NAPE content and composition, and the overall regulation of PLD-mediated hydrolysis in Arabidopsis. Based on these results, we point out here the important need for the identification of the precise isoform(s) of PLD in plants that is (are) involved in the regulated hydrolysis of NAPE and formation of NAE lipid mediators in vivo.