Induction of peptidylglycine alpha-amidating monooxygenase in N(18)TG(2) cells: a model for studying oleamide biosynthesis

Identification of potential drug targets for insomnia by Mendelian randomization analysis based on plasma proteomics

Introduction

Insomnia, a common clinical disorder, significantly impacts the physical and mental well-being of patients. Currently, available hypnotic medications are unsatisfactory due to adverse reactions and dependency, necessitating the identification of new drug targets for the treatment of insomnia.

Methods

In this study, we utilized 734 plasma proteins as genetic instruments obtained from genome-wide association studies to conduct a Mendelian randomization analysis, with insomnia as the outcome variable, to identify potential drug targets for insomnia. Additionally, we validated our results externally using other datasets. Sensitivity analyses entailed reverse Mendelian randomization analysis, Bayesian co-localization analysis, and phenotype scanning. Furthermore, we constructed a protein-protein interaction network to elucidate potential correlations between the identified proteins and existing targets.

Results

Mendelian randomization analysis indicated that elevated levels of TGFBI (OR = 1.01; 95% CI, 1.01-1.02) and PAM ((OR = 1.01; 95% CI, 1.01-1.02) in plasma are associated with an increased risk of insomnia, with external validation supporting these findings. Moreover, there was no evidence of reverse causality for these two proteins. Co-localization analysis confirmed that PAM (coloc.abf-PPH4 = 0.823) shared the same variant with insomnia, further substantiating its potential role as a therapeutic target. There are interactive relationships between the potential proteins and existing targets of insomnia.

Conclusion

Overall, our findings suggested that elevated plasma levels of TGFBI and PAM are connected with an increased risk of insomnia and might be promising therapeutic targets, particularly PAM. However, further exploration is necessary to fully understand the underlying mechanisms involved.

Synthesis, Quantification, and Characterization of Fatty Acid Amides from In Vitro and In Vivo Sources

Fatty acid amides are a diverse family of underappreciated, biologically occurring lipids. Herein, the methods for the chemical synthesis and subsequent characterization of specific members of the fatty acid amide family are described. The synthetically prepared fatty acid amides and those obtained commercially are used as standards for the characterization and quantification of the fatty acid amides produced by biological systems, a fatty acid amidome. The fatty acid amidomes from mouse N₁₈TG₂ cells, sheep choroid plexus cells, Drosophila melanogaster, Bombyx mori, Apis mellifera, and Tribolium castaneum are presented.

Glycine N-acyltransferase-like 3 is responsible for long-chain N-acylglycine formation in N18TG2 cells

Long-chain fatty acid amides are signaling lipids found in mammals and other organisms; however, details of the metabolic pathways for the N-acylglycines and primary fatty acid amides (PFAMs) have remained elusive. Heavy-labeled precursor and subtraction lipidomic experiments in mouse neuroblastoma N18TG2 cells, a model cell line for the study of fatty acid amide metabolism, establish the biosynthetic pathways for the N-acylglycines and the PFAMs. We provide evidence that the N-acylglycines are formed by a long-chain specific glycine-conjugating enzyme, glycine N-acyltransferase-like 3 (GLYATL3). siRNA knockdown of GLYATL3 in the N18TG2 cells resulted in a decrease in the levels of the N-acylglycines and the PFAMs. This is the first report of an enzyme responsible for long-chain N-acylglycine production in cellula. The production of the PFAMs in N18TG2 cells was reported to occur by the oxidative cleavage of the N-acylglycines, as catalyzed by peptidylglycine α-amidating monooxygenase (PAM). siRNA knockdown of PAM resulted in an accumulation of [(13)C18]N-oleoylglycine and decreased levels of [(13)C18]oleamide when the N18TG2 cells were grown in the presence of [(13)C18]oleic acid. The addition of [1-(13)C]palmitate to the N18TG2 cell growth media led to the production of a family of [1-(13)C]palmitoylated fatty acid amides, consistent with the biosynthetic pathways detailed herein.

Striking Oxygen Sensitivity of the Peptidylglycine α-Amidating Monooxygenase (PAM) in Neuroendocrine Cells

Interactions between biological pathways and molecular oxygen require robust mechanisms for detecting and responding to changes in cellular oxygen availability, to support oxygen homeostasis. Peptidylglycine α-amidating monooxygenase (PAM) catalyzes a two-step reaction resulting in the C-terminal amidation of peptides, a process important for their stability and biological activity. Here we show that in human, mouse, and insect cells, peptide amidation is exquisitely sensitive to hypoxia. Different amidation events on chromogranin A, and on peptides processed from proopiomelanocortin, manifest similar striking sensitivity to hypoxia in a range of neuroendocrine cells, being progressively inhibited from mild (7% O2) to severe (1% O2) hypoxia. In developing Drosophila melanogaster larvae, FMRF amidation in thoracic ventral (Tv) neurons is strikingly suppressed by hypoxia. Our findings have thus defined a novel monooxygenase-based oxygen sensing mechanism that has the capacity to signal changes in oxygen availability to peptidergic pathways.

Drosophila melanogaster as a model system to study long-chain fatty acid amide metabolism

Long-chain fatty acid amides are cell-signaling lipids identified in mammals and, recently, in invertebrates, as well. Many details regarding fatty acid amide metabolism remain unclear. Herein, we demonstrate that Drosophila melanogaster is an excellent model system for the study long-chain fatty acid amide metabolism as we have quantified the endogenous levels of N-acylglycines, N-acyldopamines, N-acylethanolamines, and primary fatty acid amides by LC/QTOF-MS. Growth of D. melanogaster on media supplemented with [1-(13)C]-palmitate lead to a family of (13)C-palmitate-labeled fatty acid amides in the fly heads. The [1-(13)C]-palmitate feeding studies provide insight into the biosynthesis of the fatty acid amides.

Primary fatty acid amide metabolism: conversion of fatty acids and an ethanolamine in N18TG2 and SCP cells

Primary fatty acid amides (PFAM) are important signaling molecules in the mammalian nervous system, binding to many drug receptors and demonstrating control over sleep, locomotion, angiogenesis, and many other processes. Oleamide is the best-studied of the primary fatty acid amides, whereas the other known PFAMs are significantly less studied. Herein, quantitative assays were used to examine the endogenous amounts of a panel of PFAMs, as well as the amounts produced after incubation of mouse neuroblastoma N(18)TG(2) and sheep choroid plexus (SCP) cells with the corresponding fatty acids or N-tridecanoylethanolamine. Although five endogenous primary amides were discovered in the N(18)TG(2) and SCP cells, a different pattern of relative amounts were found between the two cell lines. Higher amounts of primary amides were found in SCP cells, and the conversion of N-tridecanoylethanolamine to tridecanamide was observed in the two cell lines. The data reported here show that the N(18)TG(2) and SCP cells are excellent model systems for the study of PFAM metabolism. Furthermore, the data support a role for the N-acylethanolamines as precursors for the PFAMs and provide valuable new kinetic results useful in modeling the metabolic flux through the pathways for PFAM biosynthesis and degradation.

Fatty acid amide signaling molecules

Key studies leading to the discovery and definition of the role of endogenous fatty acid amide signaling molecules are summarized.

Identification of glycine N-acyltransferase-like 2 (GLYATL2) as a transferase that produces N-acyl glycines in humans

The discovery of glycine conjugates of long-chain fatty acids (N-acyl glycines) in the brain and other non-neuronal tissues has led to the identification of an emerging class of bioactive lipids. The biological activities of N-acyl glycines include antinociceptive, anti-inflammatory and antiproliferative effects, and activation of G-protein-coupled receptors. However, despite the fact that N-acyl glycines are emerging as a distinct lipid signaling family, pathways for their production are not fully elucidated. Here we report on the characterization of human glycine N-acyltransferase-like 2 (hGLYATL2), a member of a gene family of 4 putative glycine conjugating enzymes, and show that it synthesizes various N-acyl glycines. Recombinantly expressed hGLYATL2 efficiently conjugated oleoyl-CoA, arachidonoyl-CoA, and other medium- and long-chain acyl-CoAs to glycine. The enzyme was specific for glycine as an acceptor molecule, and preferentially produced N-oleoyl glycine. The hGLYATL2 enzyme is localized to the endoplasmic reticulum, and the mRNA shows highest expression in salivary gland and trachea, but is also detected in spinal cord and skin fibroblasts. The expression pattern and the identification of high levels of N-acyl glycines in skin and lung may indicate a role for N-acyl glycines in barrier function/immune response and the potential role of hGLYATL2 in this regard is discussed.

N-palmitoyl glycine, a novel endogenous lipid that acts as a modulator of calcium influx and nitric oxide production in sensory neurons

N-arachidonoyl glycine is an endogenous arachidonoyl amide that activates the orphan G protein-coupled receptor (GPCR) GPR18 in a pertussis toxin (PTX)-sensitive manner and produces antinociceptive and antiinflammatory effects. It is produced by direct conjugation of arachidonic acid to glycine and by oxidative metabolism of the endocannabinoid anandamide. Based on the presence of enzymes that conjugate fatty acids with glycine and the high abundance of palmitic acid in the brain, we hypothesized the endogenous formation of the saturated N-acyl amide N-palmitoyl glycine (PalGly). PalGly was partially purified from rat lipid extracts and identified using nano-high-performance liquid chromatography/hybrid quadrupole time-of-flight mass spectrometry. Here, we show that PalGly is produced after cellular stimulation and that it occurs in high levels in rat skin and spinal cord. PalGly was up-regulated in fatty acid amide hydrolase knockout mice, suggesting a pathway for enzymatic regulation. PalGly potently inhibited heat-evoked firing of nociceptive neurons in rat dorsal horn. In addition, PalGly induced transient calcium influx in native adult dorsal root ganglion (DRG) cells and a DRG-like cell line (F-11). The effect of PalGly on the latter cells was characterized by strict structural requirements, PTX sensitivity, and dependence on the presence of extracellular calcium. PalGly-induced calcium influx was blocked by the nonselective calcium channel blockers ruthenium red, 1-(beta-[3-(4-methoxyphenyl)propoxy]-4-methoxyphenethyl)-1H-imidazole (SK&F96365), and La3+. Furthermore, PalGly contributed to the production of NO through calcium-sensitive nitric-oxide synthase enzymes present in F-11 cells and was inhibited by the nitric-oxide synthase inhibitor 7-nitroindazole.

Biosynthesis, degradation and pharmacological importance of the fatty acid amides

The identification of two biologically active fatty acid amides, N-arachidonoylethanolamine (anandamide) and oleamide, has generated a great deal of excitement and stimulated considerable research. However, anandamide and oleamide are merely the best-known and best-understood members of a much larger family of biologically occurring fatty acid amides. In this review, we will outline which fatty acid amides have been isolated from mammalian sources, detail what is known about how these molecules are made and degraded in vivo, and highlight their potential for the development of novel therapeutics.

Identification of an unusual naturally occurring apolar fatty acid amide in mammalian brain and a method for its quantitative determination

Fatty acid amides (FAAs), such as the N-acylamides, N-acylethanolamides, N-acyldopamines and N-acylamino acids, are now emerging as an important new class of lipid-signalling molecules. This paper provides evidence, based on high-performance liquid chromatography/electrospray ionisation mass spectrometry (HPLC/ESI-MS/MS), gas chromatography/mass spectrometry (GC/MS) and 1H-NMR, of the occurrence in mouse and bovine brain extracts of a compound characterised by a mass spectrum attributable to a FAA not previously described, namely, the isopropyl-amide of stearic acid (SIPA). A highly sensitive GC/MS method was developed for quantification of naturally occurring SIPA and, also, for purposes of comparison, that of palmitoylethanolamide (PEA), a structurally related compound commonly determined in animal tissues. The results obtained show that SIPA levels in mouse brain are 8-10-fold higher than those of PEA. Moreover, SIPA was found in human neuroblastoma cell (SHSY-5Y) extracts, at significantly higher levels following exposure of the cells to the mitochondrial inhibitor rotenone. All this evidence not only shows surprisingly that SIPA may be found naturally in mammalian biological extracts despite the unusual functional group (i.e. isopropylamide) implicated, but also raises many important questions concerning its biological origin.

Fatty acid amide hydrolase in brain ventricular epithelium: mutually exclusive patterns of expression in mouse and rat

Fatty acid amides and fatty acid ethanolamides are novel signalling molecules exemplified by the sleep-inducing lipid oleamide and the endocannabinoid anandamide, respectively. These substances are inactivated by fatty acid amide hydrolase (FAAH), an enzyme that is expressed by neurons and non-neuronal cells in the brain. In the rat, FAAH-immunoreactivity has been detected in epithelial cells of the choroid plexus and, in accordance with this finding, here we report FAAH mRNA expression in rat choroid plexus epithelium using in situ hybridisation methods. Surprisingly, a comparative analysis of mouse brain did not reveal FAAH mRNA expression or FAAH-immunoreactivity in the choroid plexus of this species. FAAH-immunoreactivity was, however, detected in non-choroidal ventricular ependymal cells in the mouse brain and the specificity of this immunostaining was confirmed by analysis of FAAH-knockout mice. FAAH-immunoreactivity was detected in ependymal cells throughout the ventricles of the mouse brain but with regional variation in the intensity of immunostaining. Intriguingly, in rat brain, although FAAH expression is observed in choroid plexus epithelial cells, little or no FAAH-immunoreactivity is present in the ventricular ependyma. Thus, there are mutually exclusive patterns of FAAH expression in the ventricular epithelium of rat and mouse brain. Our observations provide the basis for an experimental analysis that exploits differences in FAAH expression in rat and mouse to investigate FAAH function in ventricular epithelial cells and, in particular, the role of FAAH in regulating the sleep-inducing agent oleamide in cerebrospinal fluid.

Routing of membrane proteins to large dense core vesicles in PC12 cells

Routing of membrane proteins to large dense core vesicles in neuroendocrine cells can depend on information in both the lumenal and cytoplasmic domains. This study in PC12 cells focuses on the routing, cleavage, and secretion of an integral membrane protein, peptidylglycine alpha-amidating monooxygenase (PAM), examining both endogenous and virally derived membrane PAM. The role of the lumenal catalytic domains in membrane PAM trafficking was examined by replacement with an epitope tag. Virally derived membrane PAM is localized to the perinuclear area and to slender processes where the large dense core vesicles are located. Expression of PAM along with a neuroendocrine-specific endoprotease liberates a soluble monooxygenase domain, yielding regulated secretion of both the monooxygenase and the prohormone convertase from large dense core vesicles. The subcellular distribution of the epitope-substituted version of PAM within the cells is similar to that of membrane PAM, and both proteins are internalized from the plasma membrane. When secretion is stimulated, Serine937 in the cytoplasmic domain of PAM is phosphorylated to a similar extent in endogenous membrane PAM, virally encoded membrane PAM, and epitope-substituted PAM. Thus, the lumenal PAM catalytic domains are not required for routing or phosphorylation of PAM in PC12 cells.

Proteins regulating the biosynthesis and inactivation of neuromodulatory fatty acid amides

Fatty acid amides (FAAs) represent a growing family of biologically active lipids implicated in a diverse range of cellular and physiological processes. At present, two general types of fatty acid amides, the N-acylethanolamines (NAEs) and the fatty acid primary amides (FAPAs), have been identified as potential physiological neuromodulators/neurotransmitters in mammals. Representative members of these two subfamilies include the endocannabinoid NAE anandamide and the sleep-inducing FAPA oleamide. In this Chapter, molecular mechanisms proposed for the biosynthesis and inactivation of FAAs are critically evaluated, with an emphasis placed on the biochemical and cell biological properties of proteins thought to mediate these processes.