Metabolism of triacylglycerol in developing rat brain

Cellular Lipidome Changes during Retinoic Acid (RA)-Induced Differentiation in SH-SY5Y Cells: A Comprehensive In Vitro Model for Assessing Neurotoxicity of Contaminants

The SH-SY5Y, neuroblastoma cell line, is a common in vitro model used to study physiological neuronal function and the neuronal response to different stimuli, including exposure to toxic chemicals. These cells can be differentiated to neuron-like cells by administration of various reagents, including retinoic acid or phorbol-12-myristate-13-acetate. Despite their common use, there is an incomplete understanding of the molecular changes that occur during differentiation. Therefore, there is a critical need to fully understand the molecular changes that occur during differentiation to properly study neurotoxicity in response to various environmental exposures. Previous studies have investigated the proteome and transcriptome during differentiation; however, the regulation of the cellular lipidome in this process is unexplored. In this work, we conducted liquid chromatography-mass spectrometry (LC-MS)-based untargeted lipidomics in undifferentiated and differentiated SH-SY5Y cells, induced by retinoic acid. We show that there are global differences between the cellular lipidomes of undifferentiated and differentiated cells. Out of thousands of features detected in positive and negative electrospray ionization modes, 44 species were identified that showed significant differences (p-value ≤0.05, fold change ≥2) in differentiated cells. Identification of these features combined with targeted lipidomics highlighted the accumulation of phospholipids, sterols, and sphingolipids during differentiation while triacylglycerols were depleted. These results provide important insights into lipid-related changes that occur during cellular differentiation of SH-5YSY cells and emphasize the need for the detailed characterization of biochemical differences that occur during differentiation while using this in vitro model for assessing ecological impacts of environmental pollutants.

Metabolomics and neuroanatomical evaluation of post-mortem changes in the hippocampus

Understanding the human brain is the ultimate goal in neuroscience, but this is extremely challenging in part due to the fact that brain tissue obtained from autopsy is practically the only source of normal brain tissue and also since changes at different levels of biological organization (genetic, molecular, biochemical, anatomical) occur after death due to multiple mechanisms. Here we used metabolomic and anatomical techniques to study the possible relationship between post-mortem time (PT)-induced changes that may occur at both the metabolomics and anatomical levels in the same brains. Our experiments have mainly focused on the hippocampus of the mouse. We found significant metabolomic changes at 2 h PT, whereas the integrity of neurons and glia, at the anatomical/ neurochemical level, was not significantly altered during the first 5 h PT for the majority of histological markers.

Metabolomics studies in brain tissue: A review

Brain is still an organ with a composition to be discovered but beyond that, mental disorders and especially all diseases that curse with dementia are devastating for the patient, the family and the society. Metabolomics can offer an alternative tool for unveiling new insights in the discovery of new treatments and biomarkers of mental disorders. Until now, most of metabolomic studies have been based on biofluids: serum/plasma or urine, because brain tissue accessibility is limited to animal models or post mortem studies, but even so it is crucial for understanding the pathological processes. Metabolomics studies of brain tissue imply several challenges due to sample extraction, along with brain heterogeneity, sample storage, and sample treatment for a wide coverage of metabolites with a wide range of concentrations of many lipophilic and some polar compounds. In this review, the current analytical practices for target and non-targeted metabolomics are described and discussed with emphasis on critical aspects: sample treatment (quenching, homogenization, filtration, centrifugation and extraction), analytical methods, as well as findings considering the used strategies. Besides that, the altered analytes in the different brain regions have been associated with their corresponding pathways to obtain a global overview of their dysregulation, trying to establish the link between altered biological pathways and pathophysiological conditions.

Direct analysis of triacylglycerols from crude lipid mixtures by gold nanoparticle-assisted laser desorption/ionization mass spectrometry

Triacylglycerols (TAGs), essential energy storage lipids, are easily detected by conventional MALDI MS when occurring on their own. However, their signals are easily overwhelmed by other lipids, mainly phosphatidylcholines (PCs) and, therefore, require purification. In order to profile TAGs from crude lipid mixtures without prefractionation, we investigated alternative matrixes that can suppress phospholipid ion signals and enhance cationization of TAGs. We found that an aqueous solution of citrate-capped gold nanoparticles (AuNPs) with a diameter of 12 nm is a superior matrix for the laser desorption/ionization mass spectrometry (LDI MS) of TAGs in crude lipid mixtures. The AuNP matrix effectively suppressed other lipid signals such as phospholipids and also provided 100 times lower detection limit for TAGs than 2,5-dihydroxybenzoic acid (DHB), the best conventional MALDI matrix for TAGs. The AuNP-assisted LDI MS enabled us to obtain detailed TAG profiles including minor species directly from crude beef lipid extracts without phospholipid interference. In addition, we could detect TAGs at a trace level from a total brain lipid extract.

Ethanol-Induced Changes in the Content of Triglycerides, Ceramides, and Glucosylceramides in Cultured Neurons

BACKGROUND

Ethanol induces apoptosis in cultured neurons. To assess the involvement of sphingolipids and neutral lipids in the apoptotic process, ethanol-induced alterations in lipid content and metabolism were examined by using primary cultured rat cerebellar granule neurons (CGNs), human neuroblastoma SK-N-SH cells, and mouse neuroblastoma Neuro2a cells. Ethanol treatment conditions that induced apoptosis in CGNs and SK-N-SH cells but not in Neuro2a cells were used for these experiments.

METHODS

Cultured neurons were treated with and without 100 mM ethanol for one to three days, and the amounts of cellular sphingolipids [ceramide, glucosylceramide (GlcCer), and sphingomyelin] and neutral lipids [cholesterol, triglyceride (TG), and cholesterol ester (ChE)] were analyzed by high-performance thin-layer chromatography, using a Coomassie brilliant blue staining method. The incorporation of [C] acetate into each lipid fraction was measured in CGNs treated with and without ethanol. Also, the effect of delipidated serum, sterols, myriocin (a serine-palmitoyltransferase inhibitor), and desipramine (an acid sphingomyelinase inhibitor) on ethanol-induced lipid changes was studied by using Neuro2a cells.

RESULTS

The most prominent change common to CGN, SK-N-SH, and Neuro2a cells was ethanol-induced TG accumulation. Higher incorporation of radioactivity into TG was also observed in ethanol-treated cultures when cellular lipids were metabolically labeled with [C] acetate in CGNs. In addition, ethanol elevated ceramide levels in all these neurons. However, ethanol induced decreases in GlcCer along with the reduction of cell viability in SK-N-SH cells and CGNs, whereas it increased GlcCer in Neuro2a cells that remained viable. Myriocin, which reduced ceramide levels, attenuated ethanol-induced cell death in SK-N-SH cells. Ethanol-induced accumulation of TG was sterol-independent, whereas changes in ceramide and GlcCer were affected in Neuro2a cells by the presence of sterols in the medium. Staurosporine, which induced cell death in SK-N-SH cells, increased levels of TG, ChE, and ceramides and reduced the level of GlcCer.

CONCLUSIONS

The results showing that ethanol induces accumulation of TG and ceramide in cultured neurons suggest that ethanol enhances lipogenesis and/or reduces fatty acid degradation in neurons, as previously observed in other cell types. Further, ethanol-induced changes in lipid metabolism, specifically those of ceramide and GlcCer, may be related to the ethanol-induced apoptotic pathway.

Fatty acid composition of late embryonic and early postnatal rat brain

The fatty acid (FA) composition and distribution in a variety of phospholipids (PL) and neutral lipids (NL) at two discrete stages during the embryonic rat brain development were investigated. Over 96% of the FA were acylated into fetal brain PL at embryonic day 17 after the peak of neuronal proliferation and at embryonic day 20, one day prior to delivery. Phosphatidylcholine constituted approximately 60% of the total PL pool, phosphatidylethanolamine (PE) 30%, phosphatidylserine (PS) 6%, and phosphatidylinositol (PI) 4%. The diacylglycerols and triacylglycerols constituted 1-2% of the fetal brain lipids. alpha-Linolenic acid (18:3n-3) and linoleic acid (18:2n-6) were found in very low amounts in all fetal brain PL and NL. The percentage of the n-6 polyunsaturated FA, consisting of arachidonic acid (AA), 22:4n-6 and 22:5n-6, remained unchanged in all the fractions, except in PI, in which the proportion of AA increased. The concentration of docosahexaenoic acid (DHA) increased with age in all the fractions, with the bulk of accumulation accounted for by its increase in PE and, to a lesser extent, in PS. This finding suggests a "DHA accretion spurt" during the last three days of pregnancy.

Triacylglycerol in biomembranes

A better knowledge of the biochemical and biophysical properties of cell membranes has revealed fundamental concepts concerning the regulation of cell functions by intrinsic components of the lipid matrix. Membrane lipids exhibit high chemical heterogeneity, with hundreds of distinct chemical species; studies of structure-function relationships have unraveled new roles for an increasing number of these lipids as determinants of membrane structure, anchors for membrane-associated proteins or signalling agents. Recent observations have confirmed triacylglycerol (TG) as a quantitatively minor intrinsic membrane component which seems to play a specific role in important metabolic events such as cell stimulation or transformation and metastatic processes. The rapid turnover of the acyl chains into TG of cell membranes suggests an active metabolism. In the plasma membrane, TG appears to be implicated in the generation of transient non-bilayer domains suspected to be associated with specific cellular events. This paper summarizes the current information on TG metabolism and focuses on the potential role of this neutral lipid species on the structure and function of cell membranes.

Mouse alveolar surfactant: characterization of subtypes prepared by differential centrifugation

To characterize the properties of alveolar surfactant subfractions obtained from mouse lung by differential centrifugation, lavage fluid, following a preliminary centrifugation at 140 x g for 5 min to yield a cellular pellet (Pc), was sequentially centrifuged at 10,000 x g for 30 min, 60,000 x g for 60 min and 100,000 x g for 15 h; and the resultant pellets, respectively referred to as P10, P60 and P100, were harvested for electron microscopy, phospholipid analysis and surface tension measurements. Ultrastructural differences were observed, in that P10 contained large multilamellated structures which were typical of newly secreted surfactant, P100 contained small unilamellar vesicular structures, typical of catabolic end products of alveolar surfactant and P60 appeared to contain a mixture of structures present in P10 and P100 in addition to numerous, large unilamellar vesicles which were not present in either P10 or P100. Slight but significant differences were found in the phospholipid compositions of the three subfractions but not in the fatty acid composition of their phosphatidylcholine (PC) component. There were no significant differences in their disaturated PC/total PC ratios, but significant differences in their phospholipid/protein ratios. P60 had the highest proportion of phospholipid to protein. P10 and P60 demonstrated surface activity but P100 did not. Total alveolar surfactant phospholipid was evenly distributed among the three fractions. This pattern of distribution was significantly different from that observed in rabbit subfractions prepared by the same procedure. These data indicate that mouse alveolar surfactant consists of three distinct subfractions or subtypes which can be separately and quantitatively isolated by differential centrifugation.(ABSTRACT TRUNCATED AT 250 WORDS)

Linoleoyl-enriched triacylglycerol species increase in maternal liver during late pregnancy in the rat

In view of the previously reported changes in the fatty acid composition of maternal liver triacylglycerols in late pregnancy, changes in the composition of maternal liver triacylglycerol species were assessed in rats fed a semi-purified diet during pregnancy. Between day 18 and day 21 of pregnancy, total maternal liver triacylglycerols increased by 50%. Triacylglycerol species with a total acyl carbon number (C) of 50 or 60 (C50, C60) remained unchanged while C48 and C52-C58 were relatively increased. The individual triacylglycerol species containing one, two or three linoleoyl moieties were incompletely recovered using a polar high temperature gas-liquid chromatography (GLC) column. Nevertheless, at day 21 compared to day 18, the linoleoyl-containing species were relatively increased by 62-463%, while tripalmitin was decreased by 38%. Our data suggest that despite an adequate intake of linoleic acid (25 g/kg in the diet), maternal hepatic triacylglycerol content of linoleic acid decreased during mid-pregnancy but increased significantly toward term possibly in preparation for the transfer of linoleic acid to the neonate during lactation.

Sources for brain arachidonic acid uptake and turnover in glycerophospholipids

Brain arachidonic acid comes from linoleic acid and arachidonic acid in the blood. Part of the brain arachidonic acid is elongated to adrenic acid, 22:4 (n = 6), especially in higher animals. With labeled arachidonic acid injected into cerebral ventricles of mice, the highest specific radioactivity was in triacylglycerols. The highest labeling in PtdCho and PtdIns was found at 15 to 60 minutes. Labeling of PtdEtn was much less. The molecular species with 16:0 and 18:1 were labeled better than those with 18:0. Adrenic acid was preferred by alkylacyl-GroPEtn. The highest level of labeling by arachidonic acid was found in the choline plasmalogens and the alkylacyl-GroPCho at 24 hours after injection. The PtdCho arachidonic acid turned over several times within 24 hours. Part of this turnover probably represents the transfer of labeled arachidonic acid to unlabeled ether-linked choline glycerophospholipids, including 1-alkyl-2-lyso-GroPCho, also known as lyso platelet activating factor. The energy-independent transfer of arachidonic acid from PtdCho to ether-linked choline glycerophospholipids may follow removal of their arachidonic acid by phospholipase A2 due to receptor activation. The lack of pulse labeling of ether-linked choline glycerophospholipids complicates the study of their function.

4-Methylumbelliferyl lipase in human and mouse brain: a possible localization in myelin

4-Methylumbelliferyl (4-MU) lipase activity in human and mouse brain, measured with 4-MU palmitate, was characterized with respect to effects of pH and detergents, and subcellular and myelin localization. Purified myelin isolated by Norton's procedure [J. Neurochem. 21, 749-757 (1983)] contained higher specific activity of 4-MU lipase, particularly in alkaline pH. Myelin lipase activity was markedly affected by the addition of different types of detergents, the amount of detergents added, and substrate. The optimal pH in myelin was bimodal--pH 4.5 and up to 8.0, respectively. These data indicate that myelin possesses 4-MU lipase activity at alkaline pH, with lower levels at acidic pH.

Characterization and solubilization of membrane bound diacylglycerol lipases from bovine brain

Bovine brain contains two diacylglycerol lipases. One is localized in purified microsomes and the other is found in the plasma membrane fraction. The microsomal enzyme is markedly stimulated by the non-ionic detergent, Triton X-100, and Ca2+, whereas the plasma membrane diacylglycerol lipase is strongly inhibited by Triton X-100 and Ca2+ has no effect on its enzymic activity. Both enzymes were solubilized using 0.25% Triton X-100. The solubilized enzymes followed Michaelis-Menten kinetics. The apparent Km values for microsomal and plasma membrane enzymes are 30.5 and 12.0 microM respectively. Both lipases are strongly inhibited by RHC 80267, with Ki values for microsomal and plasma membrane diacylglycerol lipases of 70 and 43 microM, respectively. The retention of microsomal diacylglycerol lipase on a concanavalin A-Sepharose column and its elution by methyl alpha-D-mannoside indicates the glycoprotein nature of this enzyme.

Metabolic turnover of fatty acids and acylglycerols in rat sciatic nerve

To explain the discrepancy between the low level and high metabolic activity of endoneurial free fatty acids (FFAs) and triacylglycerol (TG), levels of de novo synthesized FFA and acylglycerols were measured in rat sciatic endoneurium at various intervals after endoneurial microinjection of [14C]acetate. Soon after injection (less than 10 min), the [14C]acetate was metabolized to FFA and incorporated into diacylglycerol (DG), TG, sterols, ceramides, and various phospholipids. The proportions of 14C-labeled FFA, DG, TG, and ceramides to total 14C-labeled lipids decreased, whereas those of phospholipids and cerebrosides increased with time after injection. These findings suggest that rapid turnover of FFA and TG may contribute to their low level in sciatic endoneurium. The de novo synthesized fatty acids were largely incorporated into phosphatidylcholine (approximately 50% of total 14C-labeled phospholipids), probably via the cytidine nucleotide pathway using 1,2-DG as a metabolic intermediate. Hydrolysis of [14C]phosphatidylcholine revealed that fatty acids were labeled at both the C-1 (approximately 43%) and C-2 (approximately 57%) positions. On the other hand, a temporal association between decreased amounts of 14C-label in ceramides and increased amounts of 14C-label in sphingomyelin and galactocerebrosides supports the hypothesis that peripheral nerve galactocerebroside is derived, in vivo, from ceramide via acylation of sphingosine. This exclusive labeling of endoneurial lipids by endoneurial microinjection of labeled precursor provides a unique model for studying synthesis and metabolic turnover of membrane lipids in experimental neuropathies.

Lipids of nervous tissue: composition and metabolism

As indicated in the Introduction, the many significant developments in the recent past in our knowledge of the lipids of the nervous system have been collated in this article. That there is a sustained interest in this field is evident from the rather long bibliography which is itself selective. Obviously, it is not possible to summarize a review in which the chemistry, distribution and metabolism of a great variety of lipids have been discussed. However, from the progress of research, some general conclusions may be drawn. The period of discovery of new lipids in the nervous system appears to be over. All the major lipid components have been discovered and a great deal is now known about their structure and metabolism. Analytical data on the lipid composition of the CNS are available for a number of species and such data on the major areas of the brain are also at hand but information on the various subregions is meagre. Such investigations may yet provide clues to the role of lipids in brain function. Compared to CNS, information on PNS is less adequate. Further research on PNS would be worthwhile as it is amenable for experimental manipulation and complex mechanisms such as myelination can be investigated in this tissue. There are reports correlating lipid constituents with the increased complexity in the organization of the nervous system during evolution. This line of investigation may prove useful. The basic aim of research on the lipids of the nervous tissue is to unravel their functional significance. Most of the hydrophobic moieties of the nervous tissue lipids are comprised of very long chain, highly unsaturated and in some cases hydroxylated residues, and recent studies have shown that each lipid class contains characteristic molecular species. Their contribution to the properties of neural membranes such as excitability remains to be elucidated. Similarly, a large proportion of the phospholipid molecules in the myelin membrane are ethanolamine plasmalogens and their importance in this membrane is not known. It is firmly established that phosphatidylinositol and possibly polyphosphoinositides are involved with events at the synapse during impulse propagation, but their precise role in molecular terms is not clear. Gangliosides, with their structural complexity and amphipathic nature, have been implicated in a number of biological events which include cellular recognition and acting as adjuncts at receptor sites. More recently, growth promoting and neuritogenic functions have been ascribed to gangliosides. These interesting properties of gangliosides wIll undoubtedly attract greater attention in the future.(ABSTRACT TRUNCATED AT 400 WORDS)

Biosynthesis of triglyceride and other fatty acyl esters by developing rat brain

The biosynthesis of triglyceride from 1,2-diglyceride and long-chain acyl coenzyme A (CoA) was studied in developing rat brain. Diglyceride acyltransferase activity was highest in a microsomal fraction, had a neutral pH optimum, and was stimulated by MgCl2. Palmitoyl CoA and oleoyl CoA served equally well as acyl donors. The enzyme catalyzed the acylation of both endogenous diglyceride and several naturally occurring and synthetic exogenous diglycerides. In addition, short-chain primary and secondary alcohols were found to be acylated under these conditions. A second acylation system, active at low pH, was found to catalyze esterification of ethanol and cholesterol, but not diglyceride, with free fatty acid. These results demonstrate that brain has the capacity to acylate a wide variety of physiological and nonphysiological hydroxyl compounds.

On the similarity of triacylglycerol and acylcholesterol lipases in rat brain

Previously, we described a similar stimulating effect of adrenocorticotrophic hormone (ACTH) and ACTH-related synthetic peptides on triacylglycerol (TAGL) and acylcholesterol (ACL) lipase activities of rat brain. In present study, TAGL and ACL activities from rat brain were further investigated and compared through the use of tri-[3H]oleoylglycerol and [14C]oleoylcholesterol, respectively, as the substrates. A number of specific properties proved to be similar for both activities: (a) comparable rates of activity decay were observed upon heat treatment, (b) similar reaction kinetics as determined at different substrate concentrations with and without ACTH (10(-4) M), (c) both activities showed a common susceptibility to the presence of sodium taurocholate (5 mM) or trace amounts (5 microliter/ml) of organic solvent in the assay medium, (d) the distributions of the two activities in various brain regions were comparable. Taken together, these data, along with the similar activability by ACTH, suggest a common structural basis for TAGL and ACL activities in rat brain.

The rapid incorporation of radioactive fatty acid into triacylglycerols during the in vitro acylation of native lipids of neuronal nuclei

Using neuronal nuclei (N1) and microsomes (P3) isolated from cerebral cortices of 15-day-old rabbits, the incorporation of [14C]oleate was followed in vitro, making use of fatty acid activation factors and endogenous membrane acyl acceptors. Of the lipids of N1, it was triacylglycerol which showed the highest rates of labelling and which represented 71-80% of the total incorporated radioactivity in this fraction. Specific rates of N1 triacylglycerol formation were 63-166 times those of P3 triacylglycerols (based upon membrane phospholipid content). In P3, phospholipids made up 85% of the total microsomal lipid labelling. The incorporation of oleate was dependent upon ATP and coenzyme A, and acyl-CoA synthetase activities were demonstrated in N1 and P3 (specific activity ratio, N1:P3 = 4.5). Using exogenous [14C]oleoyl-CoA, high rates of N1 triacylglycerol labelling were still seen relative to P3, but rates of diacylglycerol and phospholipid labelling were substantially elevated in both fractions in contrast to rates found using [14C]oleate. By increasing levels of endogenous diacylglycerol using preincubations with phospholipase C, a 3-fold increase was seen in specific rates of triacylglycerol formation in both fractions in subsequent assays with [14C]oleate. A 4.5-fold increase in N1 diacylglycerol concentrations was found when N1 was incubated for 10 min in the absence of fatty acid, ATP and coenzyme A. It is concluded that neuronal nuclei have a very active diacylglycerol acyltransferase as well as the ability to generate diacylglycerol substrates.

Inability of rabbit peritoneal polymorphonuclear leukocytes to synthesize arachidonic acid from linoleic acid

In PMN leukocytes isolated from rabbit peritoneal exudate the major phospholipids were choline phosphoglycerides (40%), ethanolamine phosphoglycerides (26%) and sphingomyelin (20%) with lesser amounts (3-6%) of serine and inositol phosphoglycerides. The essential fatty acid, linoleic acid, predominated (greater than 35%) in each phospholipid except in inositol phosphoglycerides where it was slightly less than arachidonate and in sphingomyelin where saturated acids predominated. However, on a total mass basis there was more arachidonate in ethanolamine and choline phosphoglycerides than in inositol phosphoglycerides. The uptake, incorporation and metabolism of [1-14C] fatty acids of varying chain length and degrees of unsaturation were examined. All fatty acids were taken up but incorporation of saturated acids varied inversely with chain length. Arachidic acid and trans-isomers of 18:1 and 18:2 were esterified primarily to triacylglycerol whereas phospholipids contained a large portion of the other acids. Icosatrienoic and arachidonic acids were esterified to ethanolamine, serine and inositol phosphoglycerides to a comparatively greater extent, reflecting the normal distribution of these fatty acids. PMN leukocytes had a low capacity for delta 9 desaturation and chain elongation and no delta 6 or delta 5 desaturation could be detected. Thus, PMN leukocytes lack the ability to form arachidonate from 18:2 precursor molecules available in the cellular neutral lipids and phospholipids and arachidonate per se is an essential fatty acid for these cells.