Gene cloning and characterization of CDP-diacylglycerol synthase from rat brain

Metabolic routing maintains the unique fatty acid composition of phosphoinositides

Phosphoinositide lipids (PPIn) are enriched in stearic- and arachidonic acids (38:4) but how this enrichment is established and maintained during phospholipase C (PLC) activation is unknown. Here we show that the metabolic fate of newly synthesized phosphatidic acid (PA), the lipid precursor of phosphatidylinositol (PI), is influenced by the fatty acyl-CoA used with preferential routing of the arachidonoyl-enriched species toward PI synthesis. Furthermore, during agonist stimulation the unsaturated forms of PI(4,5P)₂ are replenished significantly faster than the more saturated ones, suggesting a favored recycling of the unsaturated forms of the PLC-generated hydrolytic products. Cytidine diphosphate diacylglycerol synthase 2 (CDS2) but not CDS1 was found to contribute to increased PI resynthesis during PLC activation. Lastly, while the lipid transfer protein, Nir2 is found to contribute to rapid PPIn resynthesis during PLC activation, the faster re-synthesis of the 38:4 species does not depend on Nir2. Therefore, the fatty acid side-chain composition of the lipid precursors used for PI synthesis is an important determinant of their metabolic fates, which also contributes to the maintenance of the unique fatty acid profile of PPIn lipids.

Chicken CDS2 isoforms presented distinct spatio-temporal expression pattern and regulated by insulin in a breed-specific manner

The cytidine diphosphate diacylglycerol synthases (CDSs) gene encodes the cytidine diphosphate-diacylglycerol (CDP-DAG) synthase enzyme that catalyzes the formation of CDP-diacylglycerol from phosphatidic acid. At present, there are no reports of CDS2 in birds. Here, we identified chicken CDS2 transcripts by combining conventional RT-PCR amplification, 5′ rapid amplification of cDNA ends (RACE), and 3′ RACE, explored the spatio-temporal expression profiles of total CDS2 and the longest transcript variant CDS2-4, and investigated the effect of exogenous insulin on the mRNA level of total CDS2 via quantitative RT-PCR. Four transcripts of chicken CDS2 (CDS2-1, -2, -3, and -4) were identified, which were alternatively spliced at the 3′-untranslated region (UTR). Both total CDS2 and CDS2-4 were prominently expressed in adipose tissue, and exhibited low expression in liver and pectoralis of 49-day-old chickens. Regarding the spatio-temporal expression patterns of CDS2 in chicken, total CDS2 exhibited a similar temporal expression tendency with a high level in the later period of incubation (embryonic day 19 [E19] or 1-day-old) in the brain, liver, and pectoralis. While CDS2-4 presented a distinct temporal expression pattern in these tissues, CDS2-4 levels peaked at 21 d in the brain and pectoralis, while liver CDS2-4 mRNA levels were highest at the early stage of hatching (E10). Total CDS2 (P < 0.001) and CDS2-4 (P = 0.0090) mRNA levels in the liver were differentially regulated throughout the development of the chicken. Total CDS2 levels in the liver of Silky chickens were higher than that of the broiler in the basal state and after insulin stimulation. Exogenous insulin significantly down-regulated the level of total CDS2 at 240 min in the pectoralis of Silky chickens (P < 0.01). In conclusion, chicken CDS2 isoforms with variation at the 3′-UTR were identified, which was prominently expressed in adipose tissue. Total CDS2 and CDS2-4 presented distinct spatio-temporal expression patterns, that is they were differentially regulated with age in brain, liver, and pectoralis. Insulin could regulate chicken CDS2 levels in a breed- and tissue-specific manner.

Specificity of Acyl Chain Composition of Phosphatidylinositols

Phosphatidylinositol (PI) lipids have a predominance of a single molecular species present through the organism. In healthy mammals this molecular species is 1-stearoyl-2-arachidonoyl (18:0/20:4) PI. Although the importance of PI lipids for cell physiology has long been appreciated, less is known about the biological role of enriching PI lipids with 18:0/20:4 acyl chains. In conditions with dysfunctional lipid metabolism, the predominance of 18:0/20:4 acyl chains is lost. Recently, molecular mechanisms underpinning the enrichment or alteration of these acyl chains in PI lipids have begun to emerge. In the majority of the cases a common feature is the presence of enzymes bearing substrate acyl chain specificity. However, in cancer cells, it has been shown that one (not the only) of the mechanisms responsible for the loss in this acyl chain enrichment is mutation on the transcription factor p53 gene, which is one of the most highly mutated genes in cancers. There is a compelling need for a global picture of the specificity of the acyl chain composition of PIs. This can be possible once high-resolution spatio-temporal information is gathered in a cellular context; which can ultimately lead to potential novel targets to combat conditions with altered PI acyl chain profiles.

CDP-diacylglycerol, a critical intermediate in lipid metabolism

The roles of lipids expand beyond the basic building blocks of biological membranes. In addition to forming complex and dynamic barriers, the thousands of different lipid species in the cell contribute to essentially all the processes of life. Specific lipids are increasingly identified in cellular processes, including signal transduction, membrane trafficking, metabolic control and protein regulation. Tight control of their synthesis and degradation is essential for homeostasis. Most of the lipid molecules in the cell originate from a small number of critical intermediates. Thus, regulating the synthesis of intermediates is essential for lipid homeostasis and optimal biological functions. Cytidine diphosphate diacylglycerol (CDP-DAG) is an intermediate which occupies a branch point in lipid metabolism. CDP-DAG is incorporated into different synthetic pathways to form distinct phospholipid end-products depending on its location of synthesis. Identification and characterization of CDP-DAG synthases which catalyze the synthesis of CDP-DAG has been hampered by difficulties extracting these membrane-bound enzymes for purification. Recent developments have clarified the cellular localization of the CDP-DAG synthases and identified a new unrelated CDP-DAG synthase enzyme. These findings have contributed to a deeper understanding of the extensive synthetic and signaling networks stemming from this key lipid intermediate.

How is the acyl chain composition of phosphoinositides created and does it matter?

The phosphoinositide (PIPn) family of signalling phospholipids are central regulators in membrane cell biology. Their varied functions are based on the phosphorylation pattern of their inositol ring, which can be recognized by selective binding domains in their effector proteins and be modified by a series of specific PIPn kinases and phosphatases, which control their interconversion in a spatial and temporal manner. Yet, a unique feature of PIPns remains largely unexplored: their unusually uniform acyl chain composition. Indeed, while most phospholipids present a range of molecular species comprising acyl chains of diverse length and saturation, PIPns in several organisms and tissues show the predominance of a single hydrophobic backbone, which in mammals is composed of arachidonoyl and stearoyl chains. Despite evolution having favoured this specific PIPn configuration, little is known regarding the mechanisms and functions behind it. In this review, we explore the metabolic pathways that could control the acyl chain composition of PIPns as well as the potential roles of this selective enrichment. While our understanding of this phenomenon has been constrained largely by the technical limitations in the methods traditionally employed in the PIPn field, we believe that the latest developments in PIPn analysis should shed light onto this old question.

CDP-Diacylglycerol Synthases (CDS): Gateway to Phosphatidylinositol and Cardiolipin Synthesis

Cytidine diphosphate diacylglycerol (CDP-DAG) is a key intermediate in the synthesis of phosphatidylinositol (PI) and cardiolipin (CL). Both PI and CL have highly specialized roles in cells. PI can be phosphorylated and these phosphorylated derivatives play major roles in signal transduction, membrane traffic, and maintenance of the actin cytoskeletal network. CL is the signature lipid of mitochondria and has a plethora of functions including maintenance of cristae morphology, mitochondrial fission, and fusion and for electron transport chain super complex formation. Both lipids are synthesized in different organelles although they share the common intermediate, CDP-DAG. CDP-DAG is synthesized from phosphatidic acid (PA) and CTP by enzymes that display CDP-DAG synthase activities. Two families of enzymes, CDS and TAMM41, which bear no sequence or structural relationship, have now been identified. TAMM41 is a peripheral membrane protein localized in the inner mitochondrial membrane required for CL synthesis. CDS enzymes are ancient integral membrane proteins found in all three domains of life. In mammals, they provide CDP-DAG for PI synthesis and for phosphatidylglycerol (PG) and CL synthesis in prokaryotes. CDS enzymes are critical for maintaining phosphoinositide levels during phospholipase C (PLC) signaling. Hydrolysis of PI (4,5) bisphosphate by PLC requires the resynthesis of PI and CDS enzymes catalyze the rate-limiting step in the process. In mammals, the protein products of two CDS genes (CDS1 and CDS2) localize to the ER and it is suggested that CDS2 is the major CDS for this process. Expression of CDS enzymes are regulated by transcription factors and CDS enzymes may also contribute to CL synthesis in mitochondria. Studies of CDS enzymes in protozoa reveal spatial segregation of CDS enzymes from the rest of the machinery required for both PI and CL synthesis identifying a key gap in our understanding of how CDP-DAG can cross the different membrane compartments in protozoa and in mammals.

Integrated regulation of the phosphatidylinositol cycle and phosphoinositide-driven lipid transport at ER-PM contact sites

Among the structural phospholipids that form the bulk of eukaryotic cell membranes, phosphatidylinositol (PtdIns) is unique in that it also serves as the common precursor for low-abundance regulatory lipids, collectively referred to as polyphosphoinositides (PPIn). The metabolic turnover of PPIn species has received immense attention because of the essential functions of these lipids as universal regulators of membrane biology and their dysregulation in numerous human pathologies. The diverse functions of PPIn lipids occur, in part, by orchestrating the spatial organization and conformational dynamics of peripheral or integral membrane proteins within defined subcellular compartments. The emerging role of stable contact sites between adjacent membranes as specialized platforms for the coordinate control of ion exchange, cytoskeletal dynamics, and lipid transport has also revealed important new roles for PPIn species. In this review, we highlight the importance of membrane contact sites formed between the endoplasmic reticulum (ER) and plasma membrane (PM) for the integrated regulation of PPIn metabolism within the PM. Special emphasis will be placed on non-vesicular lipid transport during control of the PtdIns biosynthetic cycle as well as toward balancing the turnover of the signaling PPIn species that define PM identity.

Sustained phospholipase C stimulation of H9c2 cardiomyoblasts by vasopressin induces an increase in CDP-diacylglycerol synthase 1 (CDS1) through protein kinase C and cFos

Chronic stimulation (24 h) with vasopressin leads to hypertrophy in H9c2 cardiomyoblasts and this is accompanied by continuous activation of phospholipase C. Consequently, vasopressin stimulation leads to a depletion of phosphatidylinositol levels. The substrate for phospholipase C is phosphatidylinositol (4, 5) bisphosphate (PIP2) and resynthesis of phosphatidylinositol and its subsequent phosphorylation maintains the supply of PIP2. The resynthesis of PI requires the conversion of phosphatidic acid to CDP-diacylglycerol catalysed by CDP-diacylglycerol synthase (CDS) enzymes. To examine whether the resynthesis of PI is regulated by vasopressin stimulation, we focussed on the CDS enzymes. Three CDS enzymes are present in mammalian cells: CDS1 and CDS2 are integral membrane proteins localised at the endoplasmic reticulum and TAMM41 is a peripheral protein localised in the mitochondria. Vasopressin selectively stimulates an increase CDS1 mRNA that is dependent on protein kinase C, and can be inhibited by the AP-1 inhibitor, T-5224. Vasopressin also stimulates an increase in cFos protein which is inhibited by a protein kinase C inhibitor. We conclude that vasopressin stimulates CDS1 mRNA through phospholipase C, protein kinase C and cFos and provides a potential mechanism for maintenance of phosphatidylinositol levels during long-term phospholipase C signalling.

Phosphatidylinositol synthesis at the endoplasmic reticulum

Phosphatidylinositol (PI) is a minor phospholipid with a characteristic fatty acid profile; it is highly enriched in stearic acid at the sn-1 position and arachidonic acid at the sn-2 position. PI is phosphorylated into seven specific derivatives, and individual species are involved in a vast array of cellular functions including signalling, membrane traffic, ion channel regulation and actin dynamics. De novo PI synthesis takes place at the endoplasmic reticulum where phosphatidic acid (PA) is converted to PI in two enzymatic steps. PA is also produced at the plasma membrane during phospholipase C signalling, where hydrolysis of phosphatidylinositol (4,5) bisphosphate (PI(4,5)P₂) leads to the production of diacylglycerol which is rapidly phosphorylated to PA. This PA is transferred to the ER to be also recycled back to PI. For the synthesis of PI, CDP-diacylglycerol synthase (CDS) converts PA to the intermediate, CDP-DG, which is then used by PI synthase to make PI. The de novo synthesised PI undergoes remodelling to acquire its characteristic fatty acid profile, which is altered in p53-mutated cancer cells. In mammals, there are two CDS enzymes at the ER, CDS1 and CDS2. In this review, we summarise the de novo synthesis of PI at the ER and the enzymes involved in its subsequent remodelling to acquire its characteristic acyl chains. We discuss how CDS, the rate limiting enzymes in PI synthesis are regulated by different mechanisms. During phospholipase C signalling, the CDS1 enzyme is specifically upregulated by cFos via protein kinase C.

Mitochondrial CDP-diacylglycerol synthase activity is due to the peripheral protein, TAMM41 and not due to the integral membrane protein, CDP-diacylglycerol synthase 1

CDP diacylglycerol synthase (CDS) catalyses the conversion of phosphatidic acid (PA) to CDP-diacylglycerol, an essential intermediate in the synthesis of phosphatidylglycerol, cardiolipin and phosphatidylinositol (PI). CDS activity has been identified in mitochondria and endoplasmic reticulum of mammalian cells apparently encoded by two highly-related genes, CDS1 and CDS2. Cardiolipin is exclusively synthesised in mitochondria and recent studies in cardiomyocytes suggest that the peroxisome proliferator-activated receptor γ coactivator 1 (PGC-1α and β) serve as transcriptional regulators of mitochondrial biogenesis and up-regulate the transcription of the CDS1 gene. Here we have examined whether CDS1 is responsible for the mitochondrial CDS activity. We report that differentiation of H9c2 cells with retinoic acid towards cardiomyocytes is accompanied by increased expression of mitochondrial proteins, oxygen consumption, and expression of the PA/PI binding protein, PITPNC1, and CDS1 immunoreactivity. Both CDS1 immunoreactivity and CDS activity were found in mitochondria of H9c2 cells as well as in rat heart, liver and brain mitochondria. However, the CDS1 immunoreactivity was traced to a peripheral p55 cross-reactive mitochondrial protein and the mitochondrial CDS activity was due to a peripheral mitochondrial protein, TAMM41, not an integral membrane protein as expected for CDS1. TAMM41 is the mammalian equivalent of the recently identified yeast protein, Tam41. Knockdown of TAMM41 resulted in decreased mitochondrial CDS activity, decreased cardiolipin levels and a decrease in oxygen consumption. We conclude that the CDS activity present in mitochondria is mainly due to TAMM41, which is required for normal mitochondrial function.

Unravelling the genetics of inherited retinal dystrophies: Past, present and future

The identification of the genes underlying monogenic diseases has been of interest to clinicians and scientists for many years. Using inherited retinal dystrophies as an example of monogenic disease we describe the history of molecular genetic techniques that have been pivotal in the discovery of disease causing genes. The methods that were developed in the 1970's and 80's are still in use today but have been refined and improved. These techniques enabled the concept of the Human Genome Project to be envisaged and ultimately realised. When the successful conclusion of the project was announced in 2003 many new tools and, as importantly, many collaborations had been developed that facilitated a rapid identification of disease genes. In the post-human genome project era advances in computing power and the clever use of the properties of DNA replication has allowed the development of next-generation sequencing technologies. These methods have revolutionised the identification of disease genes because for the first time there is no need to define the position of the gene in the genome. The use of next generation sequencing in a diagnostic setting has allowed many more patients with an inherited retinal dystrophy to obtain a molecular diagnosis for their disease. The identification of novel genes that have a role in the development or maintenance of retinal function is opening up avenues of research which will lead to the development of new pharmacological and gene therapy approaches. Neither of which can be used unless the defective gene and protein is known. The continued development of sequencing technologies also holds great promise for the advent of truly personalised medicine.

Decreased 16:0/20:4-phosphatidylinositol level in the post-mortem prefrontal cortex of elderly patients with schizophrenia

The etiology of schizophrenia includes phospholipid abnormalities. Phospholipids are bioactive substances essential for brain function. To analyze differences in the quantity and types of phospholipids present in the brain tissue of patients with schizophrenia, we performed a global analysis of phospholipids in multiple brain samples using liquid chromatography electrospray ionization mass/mass spectrometry (LC-ESI/MS/MS) and imaging mass spectrometry (IMS). We found significantly decreased 16:0/20:4-phosphatidylinositol (PI) levels in the prefrontal cortex (PFC) in the brains from patients with schizophrenia in the LC-ESI/MS/MS, and that the 16:0/20:4-PI in grey matter was most prominently diminished according to the IMS experiments. Previous reports investigating PI pathology of schizophrenia did not identify differences in the sn-1 and sn-2 fatty acyl chains. This study is the first to clear the fatty acid composition of PI in brains from patients with schizophrenia. Alteration in the characteristic fatty acid composition of PI may also affect neuronal function, and could play a role in the etiology of schizophrenia. Although further studies are necessary to understand the role of reduced 16:0/20:4-PI levels within the prefrontal cortex in the etiology of schizophrenia, our results provide insight into the development of a novel therapy for the clinical treatment of schizophrenia.

Lipid topogenesis--35years on

Glycerophospholipids are the principal fabric of cellular membranes. The pathways by which these lipids are synthesized were elucidated mainly through the work of Kennedy and colleagues in the late 1950s and early 1960s. Subsequently, attention turned to cell biological aspects of lipids: Where in the cell are lipids synthesized? How are lipids integrated into membranes to form a bilayer? How are they sorted and transported from their site of synthesis to other cellular destinations? These topics, collectively termed 'lipid topogenesis', were the subject of a review article in 1981 by Bell, Ballas and Coleman. We now assess what has been learned about early events of lipid topogenesis, i.e. "lipid synthesis, the integration of lipids into membranes, and lipid translocation across membranes", in the 35 years since the publication of this important review. We highlight the recent elucidation of the X-ray structures of key membrane enzymes of glycerophospholipid synthesis, progress on identifying lipid scramblase proteins needed to equilibrate lipids across membranes, and new complexities in the subcellular location and membrane topology of phosphatidylinositol synthesis revealed through a comparison of two unicellular model eukaryotes. This article is part of a Special Issue entitled: The cellular lipid landscape edited by Tim P. Levine and Anant K. Menon.

Assay for CDP-Diacylglycerol Generation by CDS in Membrane Fractions

CDP-DAG is a liponucleotide formed by the condensation of CTP with the phospholipid phosphatidic acid in a reaction catalyzed by CDP-DAG synthase (CDS). CDP-DAG is required for the synthesis of phosphatidylinositol; the parent molecule whence all seven phosphoinositides including the signaling molecules PI4P, PI(4,5)P2, and PI(3,4,5)P3 are derived. This protocol describes a highly sensitive radiometric assay to detect the generation of CDP-DAG on isolated biological membrane fractions.

Enzymatic measurement of phosphatidylglycerol and cardiolipin in cultured cells and mitochondria

Phosphatidylglycerol (PG) and cardiolipin (CL) are synthesized in mitochondria and regulate numerous biological functions. In this study, a novel fluorometric method was developed for measuring PG and CL using combinations of specific enzymes and Amplex Red. This assay quantified the sum of PG and CL (PG + CL) regardless of the species of fatty acyl chain. The calibration curve for PG + CL measurement was linear, and the detection limit was 1 μM (10 pmol in the reaction mixture). This new method was applied to the determinations of PG + CL content in HEK293 cells and CL content in purified mitochondria, because the mitochondrial content of PG is negligible compared with that of CL. We demonstrated that the PG+CL content was greater at low cell density than at high cell density. The overexpression of phosphatidylglycerophosphate synthase 1 (PGS1) increased the cellular contents of PG + CL and phosphatidylcholine (PC), and reduced that of phosphatidic acid. PGS1 overexpression also elevated the mitochondrial contents of CL and PC, but had no effect on the number of mitochondria per cell. In addition to the enzymatic measurements of other phospholipids, this simple, sensitive and high-throughput assay for measuring PG + CL can be used to understand cellular, physiological and pathological processes.

Arabidopsis DOK1 encodes a functional dolichol kinase involved in reproduction

Dolichol phosphate (Dol-P) serves as a carrier of complex polysaccharides during protein glycosylation. Dol-P is synthesized by the phosphorylation of dolichol or the monodephosphorylation of dolichol pyrophosphate (Dol-PP); however, the enzymes that catalyze these reactions remain unidentified in Arabidopsis thaliana. We performed a genome-wide search for cytidylyltransferase motif-containing proteins in Arabidopsis, and found that At3g45040 encodes a protein homologous with Sec59p, a dolichol kinase (DOK) in Saccharomyces cerevisiae. At3g45040, designated AtDOK1, complemented defects in the growth and N-linked glycosylation of the S. cerevisiae sec59 mutant, suggesting that AtDOK1 encodes a functional DOK. To characterize the physiological roles of AtDOK1 in planta, we isolated two independent lines of T-DNA-tagged AtDOK1 mutants, dok1-1 and dok1-2. The heterozygous plants showed developmental defects in male and female gametophytes, including an aberrant pollen structure, low pollen viability, and short siliques. Additionally, the mutations had incomplete penetrance. These results suggest that AtDOK1 is a functional DOK required for reproductive processes in Arabidopsis.

Distinct properties of the two isoforms of CDP-diacylglycerol synthase

CDP-diacylglycerol synthases (CDS) are critical enzymes that catalyze the formation of CDP-diacylglycerol (CDP-DAG) from phosphatidic acid (PA). Here we show in vitro that the two isoforms of human CDS, CDS1 and CDS2, show different acyl chain specificities for its lipid substrate. CDS2 is selective for the acyl chains at the sn-1 and sn-2 positions, the most preferred species being 1-stearoyl-2-arachidonoyl-sn-phosphatidic acid. CDS1, conversely, shows no particular substrate specificity, displaying similar activities for almost all substrates tested. Additionally, we show that inhibition of CDS2 by phosphatidylinositol is also acyl chain-dependent, with the strongest inhibition seen with the 1-stearoyl-2-arachidonoyl species. CDS1 shows no acyl chain-dependent inhibition. Both CDS1 and CDS2 are inhibited by their anionic phospholipid end products, with phosphatidylinositol-(4,5)-bisphosphate showing the strongest inhibition. Our results indicate that CDS1 and CDS2 could create different CDP-DAG pools that may serve to enrich different phospholipid species with specific acyl chains.

Increase in cellular triacylglycerol content and emergence of large ER-associated lipid droplets in the absence of CDP-DG synthase function

Cds1 is an evolutionarily conserved CDP-DG synthase. Fission yeast are used to demonstrate that cells deficient in its function exhibit markedly increased triacylglycerol content and assemble unusual ER-associated lipid droplets that recruit the triacylglycerol synthesis machinery and grow by expansion.

Excess fatty acids and sterols are stored as triacylglycerols and sterol esters in specialized cellular organelles, called lipid droplets. Understanding what determines the cellular amount of neutral lipids and their packaging into lipid droplets is of fundamental and applied interest. Using two species of fission yeast, we show that cycling cells deficient in the function of the ER-resident CDP-DG synthase Cds1 exhibit markedly increased triacylglycerol content and assemble large lipid droplets closely associated with the ER membranes. We demonstrate that these unusual structures recruit the triacylglycerol synthesis machinery and grow by expansion rather than by fusion. Our results suggest that interfering with the CDP-DG route of phosphatidic acid utilization rewires cellular metabolism to adopt a triacylglycerol-rich lifestyle reliant on the Kennedy pathway.

Dictyostelium uses ether-linked inositol phospholipids for intracellular signalling

Inositol phospholipids are critical regulators of membrane biology throughout eukaryotes. The general principle by which they perform these roles is conserved across species and involves binding of differentially phosphorylated inositol head groups to specific protein domains. This interaction serves to both recruit and regulate the activity of several different classes of protein which act on membrane surfaces. In mammalian cells, these phosphorylated inositol head groups are predominantly borne by a C38:4 diacylglycerol backbone. We show here that the inositol phospholipids of Dictyostelium are different, being highly enriched in an unusual C34:1e lipid backbone, 1-hexadecyl-2-(11Z-octadecenoyl)-sn-glycero-3-phospho-(1'-myo-inositol), in which the sn-1 position contains an ether-linked C16:0 chain; they are thus plasmanylinositols. These plasmanylinositols respond acutely to stimulation of cells with chemoattractants, and their levels are regulated by PIPKs, PI3Ks and PTEN. In mammals and now in Dictyostelium, the hydrocarbon chains of inositol phospholipids are a highly selected subset of those available to other phospholipids, suggesting that different molecular selectors are at play in these organisms but serve a common, evolutionarily conserved purpose.

Differentially localized acyl-CoA synthetase 4 isoenzymes mediate the metabolic channeling of fatty acids towards phosphatidylinositol

The acyl-CoA synthetase 4 (ACSL4) has been implicated in carcinogenesis and neuronal development. Acyl-CoA synthetases are essential enzymes of lipid metabolism, and ACSL4 is distinguished by its preference for arachidonic acid. Two human ACSL4 isoforms arising from differential splicing were analyzed by ectopic expression in COS cells. We found that the ACSL4_v1 variant localized to the inner side of the plasma membrane including microvilli, and was also present in the cytosol. ACSL4_v2 contains an additional N-terminal hydrophobic region; this isoform was located at the endoplasmic reticulum and on lipid droplets. A third isoform was designed de novo by appending a mitochondrial targeting signal. All three ACSL4 variants showed the same specific enzyme activity. Overexpression of the isoenzymes increased cellular uptake of arachidonate to the same degree, indicating that the metabolic trapping of fatty acids is independent of the subcellular localization. Remarkably, phospholipid metabolism was changed by ACSL4 expression. Labeling with arachidonate showed that the amount of newly synthesized phosphatidylinositol was increased by all three ACSL4 isoenzymes but not by ACSL1. This was dependent on the expression level and the localization of the ACSL4 isoform. We conclude that in our model system exogenous fatty acids are channeled preferentially towards phosphatidylinositol by ACSL4 overexpression. The differential localization of the endogenous isoenzymes may provide compartment specific precursors of this anionic phospholipid important for many signaling processes.

The essential roles of cytidine diphosphate-diacylglycerol synthase in bloodstream form Trypanosoma brucei

Lipid metabolism in Trypanosoma brucei, the causative agent of African sleeping sickness, differs from its human host in several fundamental ways. This has lead to the validation of a plethora of novel drug targets, giving hope of novel chemical intervention against this neglected disease. Cytidine diphosphate diacylglycerol (CDP‐DAG) is a central lipid intermediate for several pathways in both prokaryotes and eukaryotes, being produced by CDP‐DAG synthase (CDS). However, nothing is known about the single T. brucei CDS gene (Tb927.7.220/EC 2.7.7.41) or its activity. In this study we show TbCDS is functional by complementation of a non‐viable yeast CDS null strain and that it is essential in the bloodstream form of the parasite via a conditional knockout. The TbCDS conditional knockout showed morphological changes including a cell‐cycle arrest due in part to kinetoplast segregation defects. Biochemical phenotyping of TbCDS conditional knockout showed drastically altered lipid metabolism where reducing levels of phosphatidylinositol detrimentally impacted on glycoylphosphatidylinositol biosynthesis. These studies also suggest that phosphatidylglycerol synthesized via the phosphatidylglycerol‐phosphate synthase is not synthesized from CDP‐DAG, as was previously thought. TbCDS was shown to localized the ER and Golgi, probably to provide CDP‐DAG for the phosphatidylinositol synthases.

Phosphoinositides: tiny lipids with giant impact on cell regulation

Phosphoinositides (PIs) make up only a small fraction of cellular phospholipids, yet they control almost all aspects of a cell's life and death. These lipids gained tremendous research interest as plasma membrane signaling molecules when discovered in the 1970s and 1980s. Research in the last 15 years has added a wide range of biological processes regulated by PIs, turning these lipids into one of the most universal signaling entities in eukaryotic cells. PIs control organelle biology by regulating vesicular trafficking, but they also modulate lipid distribution and metabolism via their close relationship with lipid transfer proteins. PIs regulate ion channels, pumps, and transporters and control both endocytic and exocytic processes. The nuclear phosphoinositides have grown from being an epiphenomenon to a research area of its own. As expected from such pleiotropic regulators, derangements of phosphoinositide metabolism are responsible for a number of human diseases ranging from rare genetic disorders to the most common ones such as cancer, obesity, and diabetes. Moreover, it is increasingly evident that a number of infectious agents hijack the PI regulatory systems of host cells for their intracellular movements, replication, and assembly. As a result, PI converting enzymes began to be noticed by pharmaceutical companies as potential therapeutic targets. This review is an attempt to give an overview of this enormous research field focusing on major developments in diverse areas of basic science linked to cellular physiology and disease.

The role of phospholipids in the biological activity and structure of the endoplasmic reticulum

The endoplasmic reticulum (ER) is an interconnected network of tubular and planar membranes that supports the synthesis and export of proteins, carbohydrates and lipids. Phospholipids, in particular phosphatidylcholine (PC), are synthesized in the ER where they have essential functions including provision of membranes required for protein synthesis and export, cholesterol homeostasis, and triacylglycerol storage and secretion. Coordination of these biological processes is essential, as highlighted by findings that link phospholipid metabolism in the ER with perturbations in lipid storage/secretion and stress responses, ultimately contributing to obesity/diabetes, atherosclerosis and neurological disorders. Phospholipid synthesis is not uniformly distributed in the ER but is localized at membrane interfaces or contact zones with other organelles, and in dynamic, proliferating ER membranes. The topology of phospholipid synthesis is an important consideration when establishing the etiology of diseases that arise from ER dysfunction. This review will highlight our current understanding of the contribution of phospholipid synthesis to proper ER function, and how alterations contribute to aberrant stress responses and disease. This article is part of a Special Issue entitled: Functional and structural diversity of endoplasmic reticulum.

Diacylglycerol kinase epsilon in bovine and rat photoreceptor cells. Light-dependent distribution in photoreceptor cells

The present study shows the selective light-dependent distribution of 1,2-diacylglycerol kinase epsilon (DAGKɛ) in photoreceptor cells from bovine and albino rat retina. Immunofluorescence microscopy in isolated rod outer segments from bleached bovine retinas (BBROS) revealed a higher DAGKɛ signal than that found in rod outer segments from dark-adapted bovine retinas (BDROS). The light-dependent outer segment localization of DAGKɛ was also observed by immunohistochemistry in retinas from albino rats. DAGK activity, measured in terms of phosphatidic acid formation from a) [(3)H]DAG and ATP in the presence of EGTA and R59022, a type I DAGK inhibitor, or b) [γ-(32)P]ATP and 1-stearoyl, 2-arachidonoylglycerol (SAG), was found to be significantly higher in BBROS than in BDROS. Higher light-dependent DAGK activity (condition b) was also found when ROS were isolated from dark-adapted rat retinas exposed to light. Western blot analysis of isolated ROS proteins from bovine and rat retinas confirmed that illumination increases DAGKɛ content in the outer segments of these two species. Light-dependent DAGKɛ localization in the outer segment was not observed when U73122, a phospholipase C inhibitor, was present prior to the exposure of rat eyecups (in situ model) to light. Furthermore, no increased PA synthesis from [(3)H]DAG and ATP was observed in the presence of neomycin prior to the exposure of bovine eyecups to light. Interestingly, when BBROS were pre-phosphorylated with ATP in the presence of 1,2-dioctanoyl sn-glycerol (di-C8) or phorbol dibutyrate (PDBu) as PKC activation conditions, higher DAGK activity was observed than in dephosphorylated controls. Taken together, our findings suggest that the selective distribution of DAGKɛ in photoreceptor cells is a light-dependent mechanism that promotes increased SAG removal and synthesis of 1-stearoyl, 2-arachidonoyl phosphatidic acid in the sensorial portion of this cell, thus demonstrating a novel mechanism of light-regulated DAGK activity in the photoreceptors of two vertebrate species.

Phosphatidylethanolamine deficiency in Mammalian mitochondria impairs oxidative phosphorylation and alters mitochondrial morphology

Mitochondrial dysfunction is implicated in neurodegenerative, cardiovascular, and metabolic disorders, but the role of phospholipids, particularly the nonbilayer-forming lipid phosphatidylethanolamine (PE), in mitochondrial function is poorly understood. Elimination of mitochondrial PE (mtPE) synthesis via phosphatidylserine decarboxylase in mice profoundly alters mitochondrial morphology and is embryonic lethal (Steenbergen, R., Nanowski, T. S., Beigneux, A., Kulinski, A., Young, S. G., and Vance, J. E. (2005) J. Biol. Chem. 280, 40032-40040). We now report that moderate <30% depletion of mtPE alters mitochondrial morphology and function and impairs cell growth. Acute reduction of mtPE by RNAi silencing of phosphatidylserine decarboxylase and chronic reduction of mtPE in PSB-2 cells that have only 5% of normal phosphatidylserine synthesis decreased respiratory capacity, ATP production, and activities of electron transport chain complexes (C) I and CIV but not CV. Blue native-PAGE analysis revealed defects in the organization of CI and CIV into supercomplexes in PE-deficient mitochondria, correlated with reduced amounts of CI and CIV proteins. Thus, mtPE deficiency impairs formation and/or membrane integration of respiratory supercomplexes. Despite normal or increased levels of mitochondrial fusion proteins in mtPE-deficient cells, and no reduction in mitochondrial membrane potential, mitochondria were extensively fragmented, and mitochondrial ultrastructure was grossly aberrant. In general, chronic reduction of mtPE caused more pronounced mitochondrial defects than did acute mtPE depletion. The functional and morphological changes in PSB-2 cells were largely reversed by normalization of mtPE content by supplementation with lyso-PE, a mtPE precursor. These studies demonstrate that even a modest reduction of mtPE in mammalian cells profoundly alters mitochondrial functions.

LPIAT1 regulates arachidonic acid content in phosphatidylinositol and is required for cortical lamination in mice

Arachidonic acid (AA) is remarkably enriched in phosphatidylinositol (PI). Studies using knockout mice of lysophosphatidylinositol acyltransferase 1, which selectively incorporates AA into PI, reveal that AA-containing PI plays a crucial role in cortical lamination and neuronal migration during brain development.

Dietary arachidonic acid (AA) has roles in growth, neuronal development, and cognitive function in infants. AA is remarkably enriched in phosphatidylinositol (PI), an important constituent of biological membranes in mammals; however, the physiological significance of AA-containing PI remains unknown. In an RNA interference–based genetic screen using Caenorhabditis elegans, we recently cloned mboa-7 as an acyltransferase that selectively incorporates AA into PI. Here we show that lysophosphatidylinositol acyltransferase 1 (LPIAT1, also known as MBOAT7), the closest mammalian homologue, plays a crucial role in brain development in mice. Lpiat1^−/− mice show almost no LPIAT activity with arachidonoyl-CoA as an acyl donor and show reduced AA contents in PI and PI phosphates. Lpiat1^−/− mice die within a month and show atrophy of the cerebral cortex and hippocampus. Immunohistochemical analysis reveals disordered cortical lamination and delayed neuronal migration in the cortex of E18.5 Lpiat1^−/− mice. LPIAT1 deficiency also causes disordered neuronal processes in the cortex and reduced neurite outgrowth in vitro. Taken together, these results demonstrate that AA-containing PI/PI phosphates play an important role in normal cortical lamination during brain development in mice.

LYCAT, a homologue of C. elegans acl-8, acl-9, and acl-10, determines the fatty acid composition of phosphatidylinositol in mice

Mammalian phosphatidylinositol (PI) has a unique fatty acid composition in that 1-stearoyl-2-arachidonoyl species is predominant. This fatty acid composition is formed through fatty acid remodeling by sequential deacylation and reacylation. We recently identified three Caenorhabditis elegans acyltransferases (ACL-8, ACL-9, and ACL-10) that incorporate stearic acid into the sn-1 position of PI. Mammalian LYCAT, which is the closest homolog of ACL-8, ACL-9, and ACL-10, was originally identified as a lysocardiolipin acyltransferase by an in vitro assay and was subsequently reported to possess acyltransferase activity toward various anionic lysophospholipids. However, the in vivo role of mammalian LYCAT in phospholipid fatty acid metabolism has not been well elucidated. In this study, we generated LYCAT-deficient mice and demonstrated that LYCAT determined the fatty acid composition of PI in vivo. LYCAT-deficient mice were outwardly healthy and fertile. In the mice, stearoyl-CoA acyltransferase activity toward the sn-1 position of PI was reduced, and the fatty acid composition of PI, but not those of other major phospholipids, was altered. Furthermore, expression of mouse LYCAT rescued the phenotype of C. elegans acl-8 acl-9 acl-10 triple mutants. Our data indicate that LYCAT is a determinant of PI molecular species and its function is conserved in C. elegans and mammals.

CDP-diacylglycerol phospholipid synthesis in detergent-soluble, non-raft, membrane microdomains of the endoplasmic reticulum

Phosphatidylinositol (PI) is essential for numerous cell functions and is generated by consecutive reactions catalyzed by CDP-diacylglycerol synthase (CDS) and PI synthase. In this study, we investigated the membrane organization of CDP-diacylglycerol synthesis. Separation of mildly disrupted A431 cell membranes on sucrose density gradients revealed cofractionation of CDS and PI synthase activities with cholesterol-poor, endoplasmic reticulum (ER) membranes and partial overlap with plasma membrane caveolae. Cofractionation of CDS activity with caveolae was also observed when low-buoyant density caveolin-enriched membranes were prepared using a carbonate-based method. However, immunoisolation studies determined that CDS activity localized to ER membrane fragments containing calnexin and type III inositol (1,4,5)-trisphosphate receptors but not to caveolae. Membrane fragmentation in neutral pH buffer established that CDP-diacylglycerol and PI syntheses were restricted to a subfraction of the calnexin-positive ER. In contrast to lipid rafts enriched for caveolin, cholesterol, and GM1 glycosphingolipids, the CDS-containing ER membranes were detergent soluble. In cell imaging studies, CDS and calnexin colocalized in microdomain-sized patches of the ER and also unexpectedly at the plasma membrane. These results demonstrate that key components of the PI pathway localize to nonraft, phospholipid-synthesizing microdomains of the ER that are also enriched for calnexin.

Insulin modifies aging-related inhibition of 1-stearoyl, 2-arachidonoylglycerol phosphorylation in rat synaptic terminals

The purpose of the present study was to analyze diacylglycerol kinase (DAGK) activity in synaptic terminals from cerebral cortex (CC) and hippocampus (Hp) from adult (3-4 month-old) and aged (26-28 month-old) rats. The effect of insulin through DAGK activity on synaptosomes from adult and aged rats was also analyzed under conditions favoring saturated or unsaturated phosphatidic acid (PA) formation, using exogenous di-palmitoil glycerol (DPG) or 1-stearoyl-2-arachidonoylglycerol (SAG) as substrates. Results showed that the enzymatic activity preferentially uses SAG as substrate, thus indicating the presence of ɛ-type DAGK. A significant decrease in DAGK activity transforming SAG into PA was also observed in both tissues from aged rats. Western blot detection of DAGKɛ showed that enzyme content undergoes no changes with aging. [3H] inositol incorporation into phosphoinosites was also analyzed to evaluate the role of DAGKɛ in their synthesis. Data obtained from 3H-inositol incorporation into phosphoinositides revealed that in synaptosomes from aged rats phosphatidylinositol (PI) synthesis is lower than in adult animals. Interestingly, in the presence of SAG, PI synthesis was restored to adult values. DAGK activity over SAG was more highly stimulated by insulin in CC and Hp synaptosomes of aged rats with respect to adult rats. On the other hand, insulin exerted a stimulatory effect on PI and phosphatidylinositol 4 phosphate (PI(4)P) synthesis in synaptosomal CC from aged rats. Taken together, our findings indicate that in aged rats insulin triggers a stimulatory mechanism that reverts the diminished synaptosomal ability to synthesize arachidonoyl phosphatidic acid (20:4 PA). The recovery of this PA species indicates that insulin positively regulates phosphoinositide synthesis.

Plasmodium CDP-DAG synthase: an atypical gene with an essential N-terminal extension

Cytidine diphosphate diacylglycerol synthase (CDS) diverts phosphatidic acid towards the biosynthesis of CDP-DAG, an obligatory liponucleotide intermediate in anionic phospholipid biosynthesis. The 78kDa predicted Plasmodium falciparum CDS (PfCDS) is recovered as a 50 kDa conserved C-terminal cytidylyltransferase domain (C-PfCDS) and a 28kDa fragment that corresponds to the unusually long hydrophilic asparagine-rich N-terminal extension (N-PfCDS). Here, we show that the two fragments of PfCDS are the processed forms of the 78 kDa pro-form that is encoded from a single transcript with no alternate translation start site for C-PfCDS. PfCDS, which shares 54% sequence identity with Plasmodium knowlesi CDS (PkCDS), could substitute for PkCDS in P. knowlesi. Experiments to disrupt either the full-length or the N-terminal extension of PkCDS indicate that not only the C-terminal cytidylyltransferase domain but also the N-terminal extension is essential to Plasmodium spp. PkCDS and PfCDS introduced in P. knowlesi were processed in the parasite, suggesting a conserved parasite-dependent mechanism. The N-PfCDS appears to be a peripheral membrane protein and is trafficked outside the parasite to the parasitophorous vacuole. Although the function of this unusual N-PfCDS remains enigmatic, the study here highlights features of this essential gene and its biological importance during the intra-erythrocytic cycle of the parasite.

Immunohistochemical detection of transgene expression in the brain using small epitope tags

Background

In vivo overexpression of proteins is a powerful approach to study their biological function, generate disease models or evaluate gene therapy approaches. In order to investigate an exogenously expressed protein, specific and sensitive detection is essential. Unfortunately, antibodies that allow histological detection of the protein of interest are not always readily available. The use of an epitope tag fused to the protein can circumvent this problem as well as provide the possibility to discriminate endogenous from overexpressed proteins. In order to minimize impact on the bioactivity and biodistribution of the overexpressed protein, preference is given to small tags.

Results

In the present study, we evaluated several small epitope tags together with corresponding anti-tag antibodies for the detection of overexpressed proteins in rat brain, using eGFP as a reference. We generated several lentiviral vectors encoding eGFP with different N-terminally fused small epitope tags (AU1, flag, 3flag, HA, myc and V5). After confirmation of their functionality in cell culture, we injected these lentiviral vectors stereotactically into the striatum of rats and prepared paraformaldehyde fixed floating sections for immunohistochemical analysis. Using multiple antibodies and antibody dilutions per epitope tag, we extensively assessed the efficiency of several anti-tag antibodies for chromogenic immunohistochemical detection of the epitope tagged eGFPs by determining the proportion of immunoreactivity detected by anti-tag antibodies compared to anti-GFP antibody. Using fluorescence immunohistochemistry and confocal microscopy, we also quantified the proportion of eGFP-positive cells detected by anti-tag antibodies. Our results show that all the examined small epitope tags could be detected by anti-tag antibodies both in cell extracts as well as in vivo, although to varying degrees depending on the tag and antibody used. Using the presented protocol, V5/anti-V5 and HA/HA11 tag/antibody combinations provided the most sensitive detection in brain tissue. We confirmed the applicability of these optimized in vivo tag detection conditions for a difficult to detect protein, firefly luciferase (fLuc), using lentiviral vector constructs expressing V5 tagged and 3flag tagged fLuc protein.

Conclusions

We show here that several small epitope tags are useful for immunohistochemical detection of exogenous proteins in vivo. Our study also provides a generic methodology which is broadly applicable for the detection of overexpressed transgenes in mammalian brain tissue.

Antidepressant stimulation of CDP-diacylglycerol synthesis does not require monoamine reuptake inhibition

BACKGROUND

Recent studies demonstrate that diverse antidepressant agents increase the cellular production of the nucleolipid CDP-diacylglycerol and its synthetic derivative, phosphatidylinositol, in depression-relevant brain regions. Pharmacological blockade of downstream phosphatidylinositide signaling disrupted the behavioral antidepressant effects in rats. However, the nucleolipid responses were resistant to inhibition by serotonin receptor antagonists, even though antidepressant-facilitated inositol phosphate accumulation was blocked. Could the neurochemical effects be additional to the known effects of the drugs on monoamine transmitter transporters? To examine this question, we tested selected agents in serotonin-depleted brain tissues, in PC12 cells devoid of serotonin transporters, and on the enzymatic activity of brain CDP-diacylglycerol synthase - the enzyme that catalyzes the physiological synthesis of CDP-diacylglycerol.

RESULTS

Imipramine, paroxetine, and maprotiline concentration-dependently increased the levels of CDP-diacylglycerol and phosphatidylinositides in PC12 cells. Rat forebrain tissues depleted of serotonin by pretreatment with p-chlorophenylalanine showed responses to imipramine or maprotiline that were comparable to respective responses from saline-injected controls. With fluoxetine, nucleolipid responses in the serotonin-depleted cortex or hippocampus were significantly reduced, but not abolished. Each drug significantly increased the enzymatic activity of CDP-diacylglycerol synthase following incubations with cortical or hippocampal brain tissues.

CONCLUSION

Antidepressants probably induce the activity of CDP-diacylglycerol synthase leading to increased production of CDP-diacylglycerol and facilitation of downstream phosphatidylinositol synthesis. Phosphatidylinositol-dependent signaling cascades exert diverse salutary effects in neural cells, including facilitation of BDNF signaling and neurogenesis. Hence, the present findings should strengthen the notion that modulation of brain phosphatidylinositide signaling probably contributes to the molecular mechanism of diverse antidepressant medications.

Regulation of cardiolipin biosynthesis by fatty acid transport protein-1 IN HEK 293 cells

Cardiolipin (CL) is a major phospholipid involved in energy metabolism mammalian mitochondria and fatty acid transport protein-1 (FATP-1) is a fatty acid transport protein that may regulate the intracellular level of fatty acyl-Coenzyme A's. Since fatty acids are required for oxidative phosphorylation via mitochondrial oxidation, we examined the effect of altering FATP-1 levels on CL biosynthesis. HEK-293 mock- and FATP-1 siRNA transfected cells or mock and FATP-1 expressing cells were incubated for 24 h with 0.1 mM oleic acid bound to albumin (1:1 molar ratio) then incubated for 24 h with 0.1 mM [1,3-(3)H]glycerol and radioactivity incorporated into CL determined. FATP-1 siRNA transfected cells exhibited reduced FATP-1 mRNA and increased incorporation of [1,3-(3)H]glycerol into CL (2-fold, p<0.05) compared to controls indicating elevation in de novo CL biosynthesis. The reason for this was an increase in [1,3-(3)H]glycerol uptake and increase in activity and mRNA expression of the CL biosynthetic enzymes. In contrast, expression of FATP-1 resulted a reduction in incorporation of [1,3-(3)H]glycerol into CL (65%, p<0.05) indicating reduced CL synthesis. [1,3-(3)H]Glycerol uptake was unaltered whereas activity of cytidine-5'-diphosphate-1,2-diacyl-sn-glycerol synthetase (CDS) and CDS-2 mRNA expression were reduced in FATP-1 expressing cells compared to control. In addition, in vitro CDS activity was reduced by exogenous addition of oleoyl-Coenzyme A. The data indicate that CL de novo biosynthesis may be regulated by FATP-1 through CDS-2 expression in HEK 293 cells.

Insulin action on polyunsaturated phosphatidic acid formation in rat brain: an "in vitro" model with synaptic endings from cerebral cortex and hippocampus

The highly efficient formation of phosphatidic acid from exogenous 1-stearoyl-2-arachidonoyl-sn-glycerol (SAG) in rat brain synaptic nerve endings (synaptosomes) from cerebral cortex and hippocampus is reported. Phosphatidic acid synthesized from SAG or 1,2-dipalmitoyl-sn-glycerol (DPG) was 17.5 or 2.5 times higher, respectively, than from endogenous synaptosomal diacylglycerides. Insulin increased diacylglycerol kinase (DAGK) action on endogenous substrate in synaptic terminals from hippocampus and cerebral cortex by 199 and 97%, respectively. Insulin preferentially increased SAG phosphorylation from hippocampal membranes. In CC synaptosomes insulin increased phosphatidic acid (PA) synthesis from SAG by 100% with respect to controls. Genistein (a tyrosine kinase inhibitor) inhibited this stimulatory insulin effect. Okadaic acid or cyclosporine, used as Ser/Threo protein phosphatase inhibitors, failed to increase insulin effect on PA formation. GTP gamma S and particularly NaF were potent stimulators of PA formation from polyunsaturated diacylglycerol but failed to increase this phosphorylation when added after 5 min of insulin exposure. GTP gamma S and NaF increased phosphatidylinositol 4,5 bisphosphate (PIP2) labeling with respect to controls when SAG was present. On the contrary, they decreased polyphosphoinositide labeling with respect to controls in the presence of DPG. Our results indicate that a DAGK type 3 (DAGKepsilon) which preferentially, but not selectively, utilizes 1-acyl-2-arachidonoyl-sn-glycerol and which could be associated with polyphosphoinositide resynthesis, participates in synaptic insulin signaling. GTP gamma S and NaF appear to be G protein activators related to insulin and the insulin receptor, both affecting the signaling mechanism that augments phosphatidic acid formation.

Characterization of mouse lysophosphatidic acid acyltransferase 3: an enzyme with dual functions in the testis

Glycerophospholipids are structural and functional components of cellular membranes as well as precursors of various lipid mediators. Using acyl-CoAs as donors, glycerophospholipids are formed by the de novo pathway (Kennedy pathway) and modified in the remodeling pathway (Lands' cycle). Various acyltransferases, including two lysophosphatidic acid acyltransferases (LPAATs), have been discovered from a 1-acylglycerol-3-phosphate O-acyltransferase (AGPAT) family. Proteins of this family contain putative acyltransferase motifs, but their biochemical properties and physiological roles are not completely understood. Here, we demonstrated that mouse LPAAT3, previously known as mouse AGPAT3, possesses strong LPAAT activity and modest lysophosphatidylinositol acyltransferase activity with a clear preference for arachidonoyl-CoA as a donor. This enzyme is highly expressed in the testis, where CDP-diacylglycerol synthase 1 preferring 1-stearoyl-2-arachidonoyl-phosphatidic acid as a substrate is also highly expressed. Since 1-stearoyl-2-arachidonoyl species are the main components of phosphatidylinositol, mouse LPAAT3 may function in both the de novo and remodeling pathways and contribute to effective biogenesis of 1-stearoyl-2-arachidonoyl-phosphatidylinositol in the testis. Additionally, the expression of this enzyme in the testis increases significantly in an age-dependent manner, and beta-estradiol may be an important regulator of this enzyme's induction. Our findings identify this acyltransferase as an alternative important enzyme to produce phosphatidylinositol in the testis.

Mechanism of the elevation in cardiolipin during HeLa cell entry into the S-phase of the human cell cycle

CL (cardiolipin) is a key phospholipid involved in ATP generation. Since progression through the cell cycle requires ATP we examined regulation of CL synthesis during S-phase in human cells and investigated whether CL or CL synthesis was required to support nucleotide synthesis in S-phase. HeLa cells were made quiescent by serum depletion for 24 h. Serum addition resulted in substantial stimulation of [methyl-3H]thymidine incorporation into cells compared with serum-starved cells by 8 h, confirming entry into the S-phase. CL mass was unaltered at 8 h, but increased 2-fold by 16 h post-serum addition compared with serum-starved cells. The reason for the increase in CL mass upon entry into S-phase was an increase in activity and expression of CL de novo biosynthetic and remodelling enzymes and this paralleled the increase in mitochondrial mass. CL de novo biosynthesis from D-[U-14C]glucose was elevated, and from [1,3-3H]glycerol reduced, upon serum addition to quiescent cells compared with controls and this was a result of differences in the selection of precursor pools at the level of uptake. Triascin C treatment inhibited CL synthesis from [1-14C]oleate but did not affect [methyl-3H]thymidine incorporation into HeLa cells upon serum addition to serum-starved cells. Barth Syndrome lymphoblasts, which exhibit reduced CL, showed similar [methyl-3H]thymidine incorporation into cells upon serum addition to serum-starved cells compared with cells from normal aged-matched controls. The results indicate that CL de novo biosynthesis is up-regulated via elevated activity and expression of CL biosynthetic genes and this accounted for the doubling of CL seen during S-phase; however, normal de novo CL biosynthesis or CL itself is not essential to support nucleotide synthesis during entry into S-phase of the human cell cycle.

Caenorhabditis elegans mboa-7, a member of the MBOAT family, is required for selective incorporation of polyunsaturated fatty acids into phosphatidylinositol

Phosphatidylinositol (PI) is a component of membrane phospholipids, and it functions both as a signaling molecule and as a compartment-specific localization signal in the form of polyphosphoinositides. Arachidonic acid (AA) is the predominant fatty acid in the sn-2 position of PI in mammals. LysoPI acyltransferase (LPIAT) is thought to catalyze formation of AA-containing PI; however, the gene that encodes this enzyme has not yet been identified. In this study, we established a screening system to identify genes required for use of exogenous polyunsaturated fatty acids (PUFAs) in Caenorhabditis elegans. In C. elegans, eicosapentaenoic acid (EPA) instead of AA is the predominant fatty acid in PI. We showed that an uncharacterized gene, which we named mboa-7, is required for incorporation of PUFAs into PI. Incorporation of exogenous PUFA into PI of the living worms and LPIAT activity in the microsomes were greatly reduced in mboa-7 mutants. Furthermore, the membrane fractions of transgenic worms expressing recombinant MBOA-7 and its human homologue exhibited remarkably increased LPIAT activity. mboa-7 encodes a member of the membrane-bound O-acyltransferase family, suggesting that mboa-7 is LPIAT. Finally, mboa-7 mutants had significantly lower EPA levels in PI, and they exhibited larval arrest and egg-laying defects.

Characterization and Physical Mapping of the PorcineCDS1andCDS2Genes

CDP-diacylglycerol synthase (CDS) catalyzes the conversion of phosphatidic acid to CDP-diacylglycerol, an important precursor for the synthesis of phosphatidylinositol, phosphatidylglycerol, and cardiolipin. We amplified and sequenced 2,053 bp of the pig CDS1 mRNA. The structure of the pig CDS1 gene was determined, being very similar to that of the human, rat, and mouse genes with respect size and organization of the 13 exons. In addition, we identified three polymorphic positions in exons 10 and 11. One of them, the A/C1006, was genotyped in samples belonging to Iberian, Landrace, Large White, Pietrain, and Meishan pig breeds. Expression of this gene was also analyzed by real-time polymerase chain reaction (PCR) in different tissues showing a high CDS1 expression in testis. Moreover, a 1240-bp fragment of the pig CDS2 mRNA was amplified and sequenced. Finally, the CDS1 and CDS2 genes were physically mapped to porcine chromosomes 8 and 17, respectively, by using the INRA, University of Minnesota porcine Radiation Hybrid panel (IMpRH).

Human dolichol kinase, a polytopic endoplasmic reticulum membrane protein with a cytoplasmically oriented CTP-binding site

Dolichol kinase (DK) catalyzes the CTP-dependent phosphorylation of dolichol in the biosynthesis de novo and possibly the recycling of dolichyl monophosphate in yeast and mammals. A cDNA clone from human brain encoding the mammalian homologue, hDKp, of the yeast enzyme has recently been identified. In this study hDK has been overexpressed in Chinese hamster ovary cells and shown to be a polytopic membrane protein localized in the endoplasmic reticulum with an N terminus extended into the lumen and a cytoplasmically oriented C terminus. A conserved sequence, DXXAXXXGXXXGX(8)KKTXEG, found in several enzymes utilizing CTP as substrate including DKs, phytol kinases, and several CDP-diacylglycerol synthetases has been identified, and the possibility that it is part of the CTP-binding domain of hDKp has been investigated. Topological studies indicate that the loop between transmembrane domains (TMD) 11 and TMD12 of hDKp, containing the putative CTP binding domain, faces the cytoplasm. Deletion of the loop between TMD11-12, hDK(Delta459-474), or mutation of selected conserved residues within the cytoplasmic loop results in either a partial or total loss of activity and significant reductions in the affinity for CTP. In addition, the SEC59 gene in the yeast DK mutant was sequenced, and a G420D substitution was found. Conversion of the corresponding residue Gly-443 in hDKp to aspartic acid resulted in inactivation of the mammalian enzyme. These results extend the information on the topological arrangement of hDKp and indicate that the cytoplasmic loop between TMDs 11-12, containing the critical conserved residues, lysine 470 and lysine 471 in the (470)KKTXEG(475) motif, is part of the CTP-binding site in hDK.

Isolation and characterization of murine Cds (CDP-diacylglycerol synthase) 1 and 2

Phototransduction in Drosophila is a phosphoinositide-mediated signalling pathway. Phosphatidylinositol 4,5-bisphosphate (PIP2) plays a central role in this process, and its levels are tightly regulated. A photoreceptor-specific form of the enzyme CDP-diacylglycerol synthase (CDS), which catalyzes the formation of CDP-diacylglycerol from phosphatidic acid, is a key regulator of the amount of PIP2 available for signalling. cds mutants develop light-induced retinal degeneration. We report here the isolation and characterization of two murine genes encoding this enzyme, Cds1 and Cds2. The genes encode proteins that are 73% identical and 92% similar but exhibit very different expression patterns. Cds1 shows a very restricted expression pattern but is expressed in the inner segments of the photoreceptors whilst Cds2 shows a ubiquitous pattern of expression. Using fluorescent in situ hybridization we have mapped Cds1 and Cds2 to chromosomes 5E3 and 2G1 respectively. These are regions of synteny with the corresponding human gene localization (4q21 and 20p13). Transient transfection experiments with epitope tagged proteins have also demonstrated that both are associated with the endoplasmic reticulum.

Effects of thyroxine and 1-methyl, 2-mercaptoimidazol on phosphoinositides synthesis in rat liver

BACKGROUND

Phosphoinositides mediate one of the intracellular signal transduction pathways and produce a class of second messengers that are involved in the action of hormones and neurotransmitters on target cells. Thyroid hormones are well known regulators of lipid metabolism and modulators of signal transduction in cells. However, little is known about phosphoinositides cycle regulation by thyroid hormones. The present paper deals with phosphoinositides synthesis de novo and acylation in liver at different thyroid status of rats.

RESULTS

The experiments were performed in either the rat liver or hepatocytes of 90- and 720-day-old rats. Myo-[3H]inositol, [14C]CH3COONa, [14C]oleic and [3H]arachidonic acids were used to investigate the phosphatidylinositol (PtdIns), phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate (PtdInsP2) synthesis. 1-methyl, 2-mercaptoimidazol-induced hypothyroidism was associated with the decrease of myo-[3H]inositol and [3H]arachidonic acids incorporation into liver phosphoinositides and total phospholipids, respectively. The thyroxine (L-T4) injection to hypothyroid animals increased the hormones contents in blood serum and PtdInsP2 synthesis de novo as well as [3H]arachidonic acids incorporation into the PtdIns and PtdInsP2. Under the hormone action, the [14C]oleic acid incorporation into PtdIns reduced in the liver of hypothyroid animals. A single injection of L-T4 to the euthyroid [14C]CH3COONa-pre-treated animals or addition of the hormone to a culture medium of hepatocytes was accompanied by the rapid prominent increase in the levels of the newly synthesized PtdIns and PtdInsP2 and in the mass of phosphatidic acid in the liver or the cells.

CONCLUSIONS

The data obtained have demonstrated that thyroid hormones are of vital importance in the regulation of arachidonate-containing phosphoinositides metabolism in the liver. The drug-induced malfunction of thyroid gland noticeably changed the phosphoinositides synthesis de novo. The L-T4 injection to the animals was followed by the time-dependent increase of polyphosphoinositide synthesis in the liver. The both long-term and short-term hormone effects on the newly synthesized PtdInsP2 have been determined.

Phospholipid biosynthesis in mammalian cells

Identification of the genes and gene products involved in the biosynthesis of phosphatidylcholine, phosphatidylethanolamine, and phosphatidylserine has lagged behind that in many other fields because of difficulties encountered in purifying the respective proteins. Nevertheless, most of these genes have now been identified. In this review article, we have highlighted important new findings on the individual enzymes and the corresponding genes of phosphatidylcholine synthesis via its two major biosynthetic pathways: the CDP-choline pathway and the methylation pathway. We also review recent studies on phosphatidylethanolamine biosynthesis by two pathways: the CDP-ethanolamine pathway, which is active in the endoplasmic reticulum, and the phosphatidylserine decarboxylase pathway, which operates in mitochondria. Finally, the two base-exchange enzymes, phosphatidylserine synthase-1 and phosphatidylserine synthase-2, that synthesize phosphatidylserine in mammalian cells are also discussed.

Mechanisms of accumulation of arachidonate in phosphatidylinositol in yellowtail. A comparative study of acylation systems of phospholipids in rat and the fish species Seriola quinqueradiata

It is known that phosphatidylinositol (PtdIns) contains abundant arachidonate and is composed mainly of 1-stearoyl-2-arachidonoyl species in mammals. We investigated if this characteristic of PtdIns applies to the PtdIns from yellowtail (Seriola quinqueradiata), a marine fish. In common with phosphatidylcholine (PtdCho), phosphatidylethanolamine (PtdEtn) and phosphatidylserine (PtdSer) from brain, heart, liver, spleen, kidney and ovary, the predominant polyunsaturated fatty acid was docosahexaenoic acid, and levels of arachidonic acid were less than 4.5% (PtdCho), 7.5% (PtdEtn) and 3.0% (PtdSer) in these tissues. In striking contrast, arachidonic acid made up 17.6%, 31.8%, 27.8%, 26.1%, 25.4% and 33.5% of the fatty acid composition of PtdIns from brain, heart, liver, spleen, kidney and ovary, respectively. The most abundant molecular species of PtdIns in all these tissues was 1-stearoyl-2-arachidonoyl. Assay of acyltransferase in liver microsomes of yellowtail showed that arachidonic acid was incorporated into PtdIns more effectively than docosahexaenoic acid and that the latter inhibited incorporation of arachidonic acid into PtdCho without inhibiting the utilization of arachidonic acid for PtdIns. This effect of docosahexaenoic acid was not observed in similar experiments using rat liver microsomes and is thought to contribute to the exclusive utilization of arachidonic acid for acylation to PtdIns in yellowtail. Inositolphospholipids and their hydrolysates are known to act as signaling molecules in cells. The conserved hydrophobic structure of PtdIns (the 1-stearoyl-2-arachidonoyl moiety) may have physiological significance not only in mammals but also in fish.

Adhesion, migration, transcriptional, interferon-inducible, and other signaling molecules newly implicated in cancer susceptibility and resistance of JB6 cells by cDNA microarray analyses

Relative expression levels of 9500 genes were determined by cDNA microarray analyses in mouse skin JB6 cells susceptible (P+) and resistant (P-) to 12-O-tetradecanoyl phorbol-13 acetate (TPA)-induced neoplastic transformation. Seventy-four genes in 6 functional classes were differentially expressed: (I) extracellular matrix (ECM) and basement membrane (BM) proteins (20 genes). P+ cells express higher levels than P- cells of several collagens and proteases, and lower levels of protease inhibitors. Multiple genes encoding adhesion molecules are expressed preferentially in P- cells, including six genes implicated in axon guidance and adhesion. (II) Cytoskeletal proteins (13 genes). These include actin isoforms and regulatory proteins, almost all preferentially expressed in P- cells. (III) Signal transduction proteins (12 genes). Among these are Ras-GTPase activating protein (Ras-GAP), the deleted in oral cancer-1 and SLIT2 tumor suppressors, and connexin 43 (Cx43) gap junctional protein, all expressed preferentially in P- cells. (IV) Interferon-inducible proteins (3 genes). These include interferon-inducible protein (IFI)-16, an Sp1 transcriptional regulator expressed preferentially in P- cells. (V) Other transcription factors (4 genes). Paired related homeobox gene 2 (Prx2)/S8 homeobox, and retinoic acid (RA)-regulated nur77 and cellular retinoic acid-binding protein II (CRABPII) transcription factors are expressed preferentially in P- cells. The RIN-ZF Sp-transcriptional suppressor exhibits preferential P+ expression. (VI) Genes of unknown functions (22 sequences). Numerous mesenchymal markers are expressed in both cell types. Data for multiple genes were confirmed by real-time PCR. Overall, 26 genes were newly implicated in cancer. Detailed analyses of the functions of the genes and their interrelationships provided converging evidence for their possible roles in implementing genetic programs mediating cancer susceptibility and resistance. These results, in conjunction with cell wounding and phalloidin staining data, indicated that concerted genetic programs were implemented that were conducive to cell adhesion and tumor suppression in P- cells and that favored matrix turnover, cell motility, and abrogation of tumor suppression in P+ cells. Such genetic programs may in part be orchestrated by Sp-, RA-, and Hox-transcriptional regulatory pathways implicated in this study.

Metabolic characterization of sciadonic acid (5c,11c,14c-eicosatrienoic acid) as an effective substitute for arachidonate of phosphatidylinositol

Sciadonic acid (20:3 Delta-5,11,14) is an n-6 series trienoic acid that lacks the Delta8 double bond of arachidonic acid. This fatty acid is not converted to arachidonic acid in higher animals. In this study, we characterized the metabolic behavior of sciadonic acid in the process of acylation to phospholipid of HepG2 cells. One of the characteristics of fatty acid compositions of phospholipids in sciadonic acid-supplemented cells is a higher proportion of sciadonic acid in phosphatidylinositol (PtdIns) (27.4%) than in phosphatidylethanolamine (PtdEtn) (23.2%), phosphatidylcholine (PtdCho) (17.3%) and phosphatidylserine (PtdSer) (20.1%). Similarly, the proportion of arachidonic acid was higher in PtdIns (35.8%) than in PtdEtn (29.1%), PtdSer (18.2%) and PtdCho (20.2%) in arachidonic-acid-supplemented cells. The extensive accumulation of sciadonic acid in PtdIns resulted in the enrichment of newly formed 1-stearoyl-2-sciadonoyl molecular species (38%) in PtdIns and caused the reduction in the level of pre-existing arachidonic-acid-containing molecular species. The kinetics of incorporation of sciadonic acid to PtdEtn, PtdSer and PtdIns of cells were similar to those of arachidonic acid. In contrast to sciadonic acid, neither eicosapentaenoic acid (20:5 Delta-5,8,11,14,17) nor juniperonic acid (20:4 Delta-5,11,14,17) accumulated in the PtdIns fraction. Rather, these n-3 series polyunsaturated fatty acids, once incorporated into PtdIns, tended to be excluded from PtdIns. In addition, the level of arachidonic-acid-containing PtdIns molecular species remained unchanged by eicosapentaenoic-acid-supplementation. These results suggest that sciadonic acid or sciadonic-acid-containing glycerides are metabolized in a similar manner to arachidonic acid or arachidonic-acid-containing glyceride in the biosynthesis of PtdIns and that sciadonic acid can effectively modify the molecular species composition of PtdIns in HepG2 cells. In this regard, sciadonic acid will be an interesting experimental tool to clarify the significance of arachidonic acid-residue of PtdIns-origin bioactive lipids.

Regulation of mammalian cell membrane biosynthesis

This review explores current information on the interrelationship between phospholipid biochemistry and cell biology. Phosphatidylcholine is the most abundant phospholipid and it biosynthesis has been studied extensively. The choline cytidylyltransferase regulates phosphatidylcholine production, and recent advances in our understanding of the mechanisms that govern cytidylyltransferase include the discovery of multiple isoforms and a more complete understanding of the lipid regulation of enzyme activity. Similarities between phosphatidylcholine formation and the phosphatidylethanolamine and phosphatidylinositol biosynthetic pathways are discussed, together with current insight into control mechanisms. Membrane phospholipid doubling during cell cycle progression is a function of periodic biosynthesis and degradation. Membrane homeostasis is maintained by a phospholipase A-mediated degradation of excess phospholipid, whereas insufficient phosphatidylcholine triggers apoptosis in cells.

Requirement of phosphatidylglycerol for photosynthetic function in thylakoid membranes

To investigate the role of phosphatidylglycerol (PG) in photosynthesis, we constructed a mutant defective in the CDP-diacylglycerol synthase gene from a cyanobacterium, Synechocystis sp. PCC6803. The mutant, designated as SNC1, required PG supplementation for growth. Growth was repressed in PG-free medium concomitantly with the decrease in cellular content of PG. These results indicate that PG is essential, and that SNC1 is defective in PG synthesis. Decrease in PG content was accompanied by a reduction in the cellular content of chlorophyll, but with little effect on the contents of phycobilisome pigments, which showed that levels of chlorophyll-protein complexes decreased without alteration of those of phycobilisomes. Regardless of the decrease in the PG content, CO(2)-dependent photosynthesis by SNC1 was similar to that by the wild type on a chlorophyll basis, but consequently became lower on a cell basis. Simultaneously, the ratio of oxygen evolution of photosystem II (PSII) measured with p-benzoquinone to that of CO(2)-dependent photosynthesis, which ranged between 1.3 and 1.7 in the wild type. However, it was decreased in SNC1 from 1.3 to 0.4 during the early growth phase where chlorophyll content and CO(2)-dependent photosynthesis were little affected, and then finally to 0.1, suggesting that PSII first lost its ability to reduce p-benzoquinone and then decreased in its level and actual activity. These results indicate that PG contributes to the accumulation of chlorophyll-protein complexes in thylakoid membranes, and also to normal functioning of PSII.

Characterization of Plasmodium falciparum CDP-diacylglycerol synthase, a proteolytically cleaved enzyme

Cytidine diphosphate-diacylglycerol (CDP-DAG), an obligatory intermediate compound in the biosynthesis of the major anionic and zwitterionic phospholipids, is synthesized by CDP-DAG synthase (CDS). The gene encoding CDS was isolated from the human malaria parasite Plasmodium falciparum, based on sequence conservation to CDS from other organisms. The P. falciparum gene is located as a single copy on chromosome 14. The open reading frame (ORF) of PfCDS gene encodes a putative protein of 667 amino acids and 78 kDa. Only the C-terminal 422 amino acids share 40% homology with eukaryotic CDSs. The very long and non-conserved N-terminal region of 245 amino acids is hydrophilic and contains asparagine-rich and repetitive sequences. Two mRNA of 3.5 and 4 kb were detected. Transcription is developmentally regulated during the asexual intraerythrocytic cycle, being the weakest in the ring-stage. PfCDS enzyme activities in infected erythrocytes correlates with the transcription pattern, consistent with an increased synthesis of phospholipids in trophozoites and schizonts. Antisera raised against two synthetic peptides from the C-terminal region of PfCDS detected a single protein of 51 kDa in Western blot analysis, specific for parasitized erythrocytes. A protein of 28 kDa was recognized by an antiserum against an N-terminal peptide, indicating that PfCDS is proteolytically processed. Expression of 51- and 28-kDa proteins was developmentally regulated similar to regulation of the transcripts and the enzyme activity. The conserved C-terminal region of PfCDS, cloned into a eukaryote expression vector and transfected in COS-7 cells, showed a two-fold increase CDP-DAG synthase activities, indicating that the isolated gene most likely encoded the P. falciparum CDS enzyme.