Chapter 8 Phospholipid biosynthesis in eukaryotes

Transcriptomics analysis of ethanol treatment of male Aedes aegypti reveals a small set of putative radioprotective genes

Introduction: Sterile Insect Technique (SIT) is based on releasing sterilized male insects into wild insect populations to compete for mating with wild females. Wild females mated with sterile males will produce inviable eggs, leading to a decline in population of that insect species. Sterilization with ionizing radiation (x-rays) is a commonly used mechanism for sterilization of males. Since irradiation can cause damage to both, somatic and germ cells, and can severely reduce the competitiveness of sterilized males relative to wild males, means to minimize the detrimental effects of radiation are required to produce sterile, competitive males for release. In an earlier study, we identified ethanol as a functional radioprotector in mosquitoes. Methods: Here, we used Illumina RNA-seq to profile changes in gene expression of male Aedes aegypti mosquitoes fed on 5% ethanol for 48 hours prior to receiving a sterilizing x-ray dose, compared to males fed on water prior to sterilization. Results: RNA-seq revealed a robust activation of DNA repair genes in both ethanol-fed and water-fed males after irradiation, but surprisingly few differences in gene expression between ethanol-fed and water-fed males regardless of radiation treatment. Discussion: While differences in gene expression due to ethanol exposure were minimal, we identified a small group of genes that may prime ethanol-fed mosquitoes for improved survivability in response to sterilizing radiation.

Localization and expression of CTP: Phosphocholine cytidylyltransferase in rat brain following cocaine exposure

Phosphatidylcholine (PC) is a primary phospholipid and major source of secondary lipid messengers and also serves as a biosynthetic precursor for other membrane phospholipids. Phosphocholine cytidylyltransferase (CCT) is the rate-limiting enzyme responsible for catalyzing the formation of PC. Changes in CCT activity have been associated with lipid dysregulation across various neurological disorders. Additionally, intermediates in PC synthesis, such as CDP-choline, have been suggested to attenuate drug craving during cocaine addiction. Recent work from our group demonstrated that cocaine exposure and conditioning alter the level of PC in the brain, specifically in the cerebellum and hippocampus. The present study examines the role of CCT expression in the brain and determines the effect of cocaine exposure on CCT expression. Immunohistochemical analysis (IHC) was performed to assess region-specific expression of CCT, including both of its isoforms; alpha (CCTα) and beta (CCTβ). IHC did not detect any staining of CCTα throughout the rat brain. In contrast, CCTβ expression was detected in the Purkinje cells of the cerebellum with decreases in expression following cocaine exposure. Collectively, these data demonstrate the region- and cell-specific localization of CCTα and CCTβ in the rat brain, as well as the altered expression of CCTβ in the cerebellum following cocaine exposure.

No up-regulation of the phosphatidylethanolamine N-methyltransferase pathway and choline production by sex hormones in cats

BACKGROUND

Feline hepatic lipidosis (FHL) is a common cholestatic disease affecting cats of any breed, age and sex. Both choline deficiency and low hepatic phosphatidylethanolamine N-methyltransferase (PEMT) activity are associated with hepatic lipidosis (HL) in humans, mice and rats. The PEMT expression is known to be upregulated by oestrogens, protecting the females in these species from the development of HL when exposed to choline deficient diets. The aim of the present study was to evaluate the influence of sex hormones on choline synthesis via the PEMT pathway in healthy male and female cats before and after spaying/neutering, when fed a diet with recommended dietary choline content.

RESULTS

From six female and six male cats PEMT activity was assayed directly in liver biopsies taken before and after spaying/neutering, and assessed indirectly by analyses of PEMT-specific hepatic phosphatidylcholine (PC) species and plasma choline levels. Hepatic PEMT activity did not differ between intact female and male cats and no changes upon spaying/neutering were observed. Likewise, no significant differences in liver PC content and PEMT-specific polyunsaturated PC species were found between the sexes and before or after spaying/neutering.

CONCLUSION

These results suggest that choline synthesis in cats differs from what is observed in humans, mice and rats. The lack of evident influence of sex hormones on the PEMT pathway makes it unlikely that spaying/neutering predisposes cats for HL by causing PC deficiency as suggested in other species.

Brain Natriuretic Peptide Stimulates Lipid Metabolism through Its Receptor NPR1 and the Glycerolipid Metabolism Pathway in Chicken Adipocytes

Brain natriuretic peptide (BNP) is related to lipid metabolism in mammals, but its effect and the molecular mechanisms underlying it in chickens are incompletely understood. We found that the level of natriuretic peptide precursor B (NPPB, which encodes BNP) mRNA expression in high-abdominal-fat chicken groups was significantly higher than that of low-abdominal-fat groups. Partial correlations indicated that changes in the weight of abdominal fat were positively correlated with NPPB mRNA expression level. In vitro, compared with the control group, preadipocytes with NPPB interference showed reduced levels of proliferation, differentiation, and glycerin in media. Treatments of cells with BNP led to enhanced proliferation and differentiation of cells and glycerin concentration, and mRNA expression of its receptor natriuretic peptide receptor 1 (NPR1) was upregulated significantly. In cells exposed to BNP, 482 differentially expressed genes were identified compared with controls without BNP. Four genes known to be related to lipid metabolism (diacylglycerol kinase; lipase, endothelial; 1-acylglycerol-3-phosphate O-acyltransferase 1; and 1-acylglycerol-3-phosphate O-acyltransferase 2) were enriched in the glycerolipid metabolism pathway and expressed differentially. In conclusion, BNP stimulates the proliferation, differentiation, and lipolysis of preadipocytes through upregulation of the levels of expression of its receptor NPR1 and key genes enriched in the glycerolipid metabolic pathway.

CTP:phosphocholine cytidylyltransferase: Function, regulation, and structure of an amphitropic enzyme required for membrane biogenesis

CTP:phosphocholine cytidylyltransferase (CCT) catalyzes a rate-limiting and regulated step in the CDP-choline pathway for the synthesis of phosphatidylcholine (PC) and PC-derived lipids. Control of CCT activity is multi-layered, and includes direct regulation by reversible membrane binding involving a built-in lipid compositional sensor. Thus CCT contributes to phospholipid compositional homeostasis. CCT also modifies the curvature of its target membrane. Knowledge of CCT structure and regulation of its catalytic function are relatively advanced compared to many lipid metabolic enzymes, and are reviewed in detail. Recently the genetic origins of two human developmental and lipogenesis disorders have been traced to mutations in the gene for CCTα.

An overview of inborn errors of complex lipid biosynthesis and remodelling

In a review published in 2012, we delineated 14 inborn errors of metabolism (IEM) related to defects in biosynthesis of complex lipids, particularly phospholipids and sphingolipids (Lamari et al 2013). Given the numerous roles played by these molecules in membrane integrity, cell structure and function, this group of diseases is rapidly expanding as predicted. Almost 40 new diseases related to genetic defects in enzymes involved in the biosynthesis and remodelling of phospholipids, sphingolipids and complex fatty acids are now reported. While the clinical phenotype associated with these defects is currently difficult to outline, with only a few patients identified to date, it appears that all organs and systems may be affected - central and peripheral nervous system, eye, muscle, skin, bone, liver, immune system, etc. This chapter presents an introductive overview of this new group of IEM. More broadly, this special issue provides an update on other IEM involving complex lipids, namely dolichol and isoprenoids, glycolipids and congenital disorders of glycosylation, very long chain fatty acids and plasmalogens. Likewise, more than 100 IEM may actually lead to primary or secondary defects of complex lipids synthesis and remodelling. Because of the implication of several cellular compartments, this new group of disorders affecting the synthesis and remodelling of complex molecules challenges our current classification of IEM still largely based on cellular organelles--i.e. mitochondrial, lysosomal, peroxisomal disorders. While most of these new disorders have been identified by next generation sequencing, we wish to emphasize the promising role of lipidomics in deciphering their pathophysiology and identifying therapeutic targets.

The mouse liver displays daily rhythms in the metabolism of phospholipids and in the activity of lipid synthesizing enzymes

The circadian system involves central and peripheral oscillators regulating temporally biochemical processes including lipid metabolism; their disruption leads to severe metabolic diseases (obesity, diabetes, etc). Here, we investigated the temporal regulation of glycerophospholipid (GPL) synthesis in mouse liver, a well-known peripheral oscillator. Mice were synchronized to a 12:12 h light-dark (LD) cycle and then released to constant darkness with food ad libitum. Livers collected at different times exhibited a daily rhythmicity in some individual GPL content with highest levels during the subjective day. The activity of GPL-synthesizing/remodeling enzymes: phosphatidate phosphohydrolase 1 (PAP-1/lipin) and lysophospholipid acyltransferases (LPLATs) also displayed significant variations, with higher levels during the subjective day and at dusk. We evaluated the temporal regulation of expression and activity of phosphatidylcholine (PC) synthesizing enzymes. PC is mainly synthesized through the Kennedy pathway with Choline Kinase (ChoK) as a key regulatory enzyme or through the phosphatidylethanolamine (PE) N-methyltransferase (PEMT) pathway. The PC/PE content ratio exhibited a daily variation with lowest levels at night, while ChoKα and PEMT mRNA expression displayed maximal levels at nocturnal phases. Our results demonstrate that mouse liver GPL metabolism oscillates rhythmically with a precise temporal control in the expression and/or activity of specific enzymes.

Choline deprivation: an overview of the major hepatic metabolic response pathways

Choline (Ch) is an important nutrient that is involved in many physiological functions. Deprivation of Ch (CD) may lead to hepatocellular modifications and/or even hepatic tumorigenesis and it can be a frequent problem in clinical settings; it can accompany various common pathological (alcoholism and malnutrition) or physiological states (pregnancy and lactation). The aim of this review is to provide an up-to-date overview of the major metabolic pathways involved in the hepatic response toward the experimentally or clinically induced CD, and to shed more light on the implicated (and probably interrelated) mechanisms responsible for the observed hepatocellular modifications and/or carcinogenesis.

MR evaluation of response to targeted treatment in cancer cells

The development of molecular technologies, together with progressive sophistication of molecular imaging methods, has allowed the further elucidation of the multiple mutations and dysregulatory effects of pathways leading to oncogenesis. Acting against these pathways by specifically targeted agents represents a major challenge for current research efforts in oncology. As conventional anatomically based pharmacological endpoints may be inadequate to monitor the tumor response to these targeted treatments, the identification and use of more appropriate, noninvasive pharmacodynamic biomarkers appear to be crucial to optimize the design, dosage and schedule of these novel therapeutic approaches. An aberrant choline phospholipid metabolism and enhanced flux of glucose derivatives through glycolysis, which sustain the redirection of mitochondrial ATP to glucose phosphorylation, are two major hallmarks of cancer cells. This review focuses on the changes detected in these pathways by MRS in response to targeted treatments. The progress and limitations of our present understanding of the mechanisms underlying MRS-detected phosphocholine accumulation in cancer cells are discussed in the light of gene and protein expression and the activation of different enzymes involved in phosphatidylcholine biosynthesis and catabolism. Examples of alterations induced in the MRS choline profile of cells exposed to different agents or to tumor environmental factors are presented. Current studies aimed at the identification in cancer cells of MRS-detected pharmacodynamic markers of therapies targeted against specific conditional or constitutive cell receptor stimulation are then reviewed. Finally, the perspectives of present efforts addressed to identify enzymes of the phosphatidylcholine cycle as possible novel targets for anticancer therapy are summarized.

A mathematical model for the determination of steady-state cardiolipin remodeling mechanisms using lipidomic data

Technical advances in lipidomic analysis have generated tremendous amounts of quantitative lipid molecular species data, whose value has not been fully explored. We describe a novel computational method to infer mechanisms of de novo lipid synthesis and remodeling from lipidomic data. We focus on the mitochondrial-specific lipid cardiolipin (CL), a polyglycerol phospholipid with four acyl chains. The lengths and degree of unsaturation of these acyl chains vary across CL molecules, and regulation of these differences is important for mitochondrial energy metabolism. We developed a novel mathematical approach to determine mechanisms controlling the steady-state distribution of acyl chain combinations in CL . We analyzed mitochondrial lipids from 18 types of steady-state samples, each with at least 3 replicates, from mouse brain, heart, lung, liver, tumor cells, and tumors grown in vitro. Using a mathematical model for the CL remodeling mechanisms and a maximum likelihood approach to infer parameters, we found that for most samples the four chain positions have an independent and identical distribution, indicating they are remodeled by the same processes. Furthermore, for most brain samples and liver, the distribution of acyl chains is well-fit by a simple linear combination of the pools of acyl chains in phosphatidylcholine (PC), phosphatidylethanolamine (PE), and phosphatidylglycerol (PG). This suggests that headgroup chemistry is the key determinant of acyl donation into CL, with chain length/saturation less important. This canonical remodeling behavior appears damaged in some tumor samples, which display a consistent excess of CL molecules having particular masses. For heart and lung, the “proportional incorporation” assumption is not adequate to explain the CL distribution, suggesting additional acyl CoA-dependent remodeling that is chain-type specific. Our findings indicate that CL remodeling processes can be described by a small set of quantitative relationships, and that bioinformatic approaches can help determine these processes from high-throughput lipidomic data.

Coordination of lipid metabolism in membrane biogenesis

Bilayer synthesis during membrane biogenesis involves the concerted assembly of multiple lipid species, requiring coordination of the level of lipid synthesis, uptake, turnover, and subcellular distribution. In this review, we discuss some of the salient conclusions regarding the coordination of lipid synthesis that have emerged from work in mammalian and yeast cells. The principal instruments of global control are a small number of transcription factors that target a wide range of genes encoding enzymes that operate in a given metabolic pathway. Critical in mammalian cells are sterol regulatory element binding proteins (SREBPs) that stimulate expression of genes for the uptake and synthesis of cholesterol and fatty acids. From work with Saccharomyces cerevisiae, much has been learned about glycerophospholipid and ergosterol regulation through Ino2p/Ino4p and Upc2p transcription factors, respectively. Lipid supply is fine-tuned through a multitude of negative feedback circuits initiated by both end products and intermediates of lipid synthesis pathways. Moreover, there is evidence that the diversity of membrane lipids is maintained through cross-regulatory effects, whereby classes of lipids activate the activity of enzymes operating in another metabolic branch.

Fundamental research is the basis for understanding and treatment of many human diseases

There are numerous examples of how fundamental research has been required to understand and treat human disease. This article focuses on three human diseases of lipid metabolism in which advancements in understanding and treatment would not have been possible without basic research. Fabry disease is an inherited metabolic disorder caused by the lack of a specific enzyme in glycosphingolipid catabolism. Cardiovascular disease is a complex and multifactorial disease but as many as half of the cases can be attributed to abnormal levels of plasma cholesterol. The incidence of liver disease is increasing due to the current epidemic of obesity. It is only recently that curiosity-driven research has yielded valuable insight into the mechanism by which liver disease evolves.

Macromolecular assemblies regulate nonvesicular phosphatidylserine traffic in yeast

PtdSer (phosphatidylserine) is synthesized in the endoplasmic reticulum and the related MAM (mitochondria-associated membrane), and transported to the PtdSer decarboxylases, Pds1p in the mitochondria, and Psd2p in the Golgi. Genetic and biochemical analyses of PtdSer transport are now revealing the role of specific protein and lipid assemblies on different organelles that regulate non-vesicular PtdSer transport. The transport of PtdSer from MAM to mitochondria is regulated by at least three genes: MET30 (encoding a ubiquitin ligase), MET4 (encoding a transcription factor), and one or more unknown genes whose transcription is regulated by MET4. MET30-dependent ubiquitination is required for the MAM to function as a competent donor membrane and for the mitochondria to function as a competent acceptor membrane. Non-vesicular transport of PtdSer to the locus of Psd2p is under the control of at least three genes, STT4 [encoding Stt4p (phosphatidylinositol 4-kinase)], PSTB2 (encoding the lipid-binding protein PstB2p) and PSD2 (encoding Psd2p). Stt4p is proposed to produce a pool of PtdIns4P that is necessary for lipid transport. PstB2p and Psd2p must be present on the acceptor membrane for PtdSer transport to occur. Psd2p contains a C2 (Ca2+ and phospholipid binding sequence) domain that is required for lipid transport. Reconstitution studies with chemically defined donor membranes demonstrate that membrane domains rich in the anionic lipids, PtdSer, PtdIns4P and phosphatidic acid function as the most efficient donors of PtdSer to Psd2p. The emerging view is that macromolecular complexes dependent on protein–protein and protein–lipid interactions form between donor and acceptor membranes and serve to dock the compartments and facilitate phospholipid transport.

Effects of hypoxia, glucose deprivation and acidosis on phosphatidylcholine synthesis in HL-1 cardiomyocytes. CTP:phosphocholine cytidylyltransferase activity correlates with sarcolemmal disruption

A decrease in [3H]Cho (choline) incorporation in to PtdCho (phos-phatidylcholine) preceded the onset of LDH (lactate dehydrogenase) release in HL-1 cardiomyocytes submitted to simulated ischaemia. This observation led us to examine the role of PtdCho synthesis in sarcolemmal disruption in HL-1 cardiomyocytes. To address this objective we analysed the individual effects of hypoxia, glucose deprivation and acidosis, three prominent components of ischaemia, on the different steps of the Kennedy pathway for the synthesis of PtdCho. Pulse and pulse-chase experiments with [3H]Cho, performed in whole HL-1 cells submitted to hypoxia or normoxia, in the presence or absence of glucose at different pHs indicated first, that CK (choline kinase) was inhibited by hypoxia and acidosis, whereas glucose deprivation exacerbated the inhibition caused by hypoxia. Second, the rate-limiting reaction in PtdCho synthesis, catalysed by CCT (CTP:phosphocholine cytidylyltransferase), was inhibited by hypoxia and glucose deprivation, but unexpectedly activated by acidosis. In cellfree system assays, acidosis inhibited both CK and CCT. In experiments performed in whole cells, the effect of acidosis was likely to be direct on CK, but indirect or intact-cell-dependent on CCT. Since hypoxia and glucose deprivation favoured membrane disruption, but acidosis prevented it, we hypothesized that the modulation of CCT could be an important determinant of cell survival. Supporting this hypothesis, we show that CCT activity in whole-cell experiments clearly correlated with LDH release, but not with ATP concentration. Altogether our results suggest a significant role for CCT activity in sarcolemmal disruption during ischaemia.

Protein and lipid motifs regulate phosphatidylserine traffic in yeast

Phosphatidylserine (PtdSer) is synthesized in the endoplasmic reticulum and its subdomains associated with the mitochondria [MAM (mitochondria-associated membrane)] and subsequently transported to the loci of the PtdSer decarboxylases, Pds1p (phosphatidylserine decarboxylase 1 encoded by the PSD1 gene that complements psd1 mutations) in the mitochondria, and Psd2p (PtdSer decarboxylase 2 encoded by the PSD2 gene that complements psd2 mutations) in the Golgi. Decarboxylation of PtdSer to PtdEtn (phosphatidylethanolamine) can be used as a biochemical indicator of transport to these organelles, which is regulated by specific lipid and protein motifs. PtdSer transport to mitochondria is controlled by ubiquitination via the action of the ubiquitin ligase subunit Met30p (a ubiquitin ligase subunit encoded by the MET30 gene that complements the met30 mutation affecting methionine biosynthesis). Mutant strains with lesions in the MET30 gene are defective in PtdSer transport and show altered ubiquitination of specific target proteins, such as the transcription factor Met4p (a transcription factor encoded by the MET4 gene that complements the met4 mutation affecting methionine biosynthesis). Mutations to MET30 cause defects in both the MAM as a donor of PtdSer, and the mitochondria as an acceptor of PtdSer in the transport reaction. PtdSer transport to the locus of Psd2p is controlled by specific protein and lipid motifs. The C2 (Ca2+ and phospholipid-binding sequence) domain of Psd2p, and the lipid-binding protein PstB2p (PtdSer transport B pathway protein encoded by the PSTB2 gene that complements the pstB2 mutation affecting PtdSer transport), must be present on acceptor membranes for PtdSer transport to occur. In addition, the action of the PtdIns 4-kinase, Stt4p (PtdIns 4-kinase encoded by the STT4 gene that complements the stt4 mutation causing staurosporine and temperature-sensitive growth) is also required for PtdSer transport to the locus of Psd2p. Reconstitution of PtdSer transport to Psd2p using liposomes demonstrates that PtdSer-rich domains present in vesicles are preferred substrates for transport. In addition, the incorporation of phosphatidic acid into donor membranes enhances the rate of PtdSer transport. Collectively, these data support a model for PtdSer transport in which specific proteins and lipids are required on donor and acceptor membranes.

Cloning and characterization of murine 1-acyl-sn-glycerol 3-phosphate acyltransferases and their regulation by PPARalpha in murine heart

AGPAT (1-acyl-sn-glycerol 3-phosphate acyltransferase) exists in at least five isoforms in humans, termed as AGPAT1, AGPAT2, AGPAT3, AGPAT4 and AGPAT5. Although they catalyse the same biochemical reaction, their relative function, tissue expression and regulation are poorly understood. Linkage studies in humans have revealed that AGPAT2 contributes to glycerolipid synthesis and plays an important role in regulating lipid metabolism. We report the molecular cloning, tissue distribution, and enzyme characterization of mAGPATs (murine AGPATs) and regulation of cardiac mAGPATs by PPARalpha (peroxisome-proliferator-activated receptor alpha). mAGPATs demonstrated differential tissue expression profiles: mAGPAT1 and mAGPAT3 were ubiquitously expressed in most tissues, whereas mAGPAT2, mAGPAT4 and mAGPAT5 were expressed in a tissue-specific manner. mAGPAT2 expressed in in vitro transcription and translation reactions and in transfected COS-1 cells exhibited specificity for 1-acyl-sn-glycerol 3-phosphate. When amino acid sequences of five mAGPATs were compared, three highly conserved motifs were identified, including one novel motif/pattern KX2LX6GX12R. Cardiac mAGPAT activities were 25% lower (P<0.05) in PPARalpha null mice compared with wild-type. In addition, cardiac mAGPAT activities were 50% lower (P<0.05) in PPARalpha null mice fed clofibrate compared with clofibrate fed wild-type animals. This modulation of AGPAT activity was accompanied by significant enhancement/reduction of the mRNA levels of mAGPAT3/mAGPAT2 respectively. Finally, mRNA expression of cardiac mAGPAT3 appeared to be regulated by PPARalpha activation. We conclude that cardiac mAGPAT activity may be regulated by both the composition of mAGPAT isoforms and the levels of each isoform.

Pharmacogenetic analysis of rosiglitazone-induced hepatosteatosis in new mouse models of type 2 diabetes

Although thiazolidinediones suppress hyperglycemia in diabetic (NON x NZO)F1 males, these mice exhibit unusual sensitivity to drug-induced exacerbation of an underlying hepatosteatosis only rarely experienced in human patients. To establish the pharmacogenetic basis for this sensitivity, a panel of recombinant congenic strains (RCSs) with varying degrees of obesity and diabetes was generated by fixing selected NZO HlLt alleles on the diabetes- and hepatosteatosis-resistant NON/Lt background. Four new strains in this panel were exposed to chronic rosiglitazone treatment. Only one, NONcNZO8 (designated RCS8), exhibited an F1-like hepatosteatotic response. In both the F1 and RCS8 males, this adverse effect correlated with rosiglitazone suppression of already impaired hepatic phosphatidylcholine biosynthetic enzymes in both arms of the biosynthetic pathway, the phosphatidylethanolamine methyl- transferase pathway, and the CDP-choline pathway, including choline kinase and CTP-cholinephosphate cytidylyltransferase. This adverse response was not reproduced by CL316,243, a beta3-adrenergic receptor agonist with potent antihyperlipemic effects. Genome comparison showed that RCS8 differed from the other strains in carrying NZO-derived genome on virtually all of chromosome 16 and in smaller segments on chromosomes 6, 14, and 17. Thus, these RCSs present a panel of new mouse models exhibiting differential levels of obesity and diabetes as well as different drug responses. This panel can be used to screen for treatments for type 2 diabetes and its complications.