Identification and characterization of CDS2, a mammalian homolog of the Drosophila CDP-diacylglycerol synthase gene

Biosynthesis and Significance of Fatty Acids, Glycerophospholipids, and Triacylglycerol in the Processes of Glioblastoma Tumorigenesis

One area of glioblastoma research is the metabolism of tumor cells and detecting differences between tumor and healthy brain tissue metabolism. Here, we review differences in fatty acid metabolism, with a particular focus on the biosynthesis of saturated fatty acids (SFA), monounsaturated fatty acids (MUFA), and polyunsaturated fatty acids (PUFA) by fatty acid synthase (FASN), elongases, and desaturases. We also describe the significance of individual fatty acids in glioblastoma tumorigenesis, as well as the importance of glycerophospholipid and triacylglycerol synthesis in this process. Specifically, we show the significance and function of various isoforms of glycerol-3-phosphate acyltransferases (GPAT), 1-acylglycerol-3-phosphate O-acyltransferases (AGPAT), lipins, as well as enzymes involved in the synthesis of phosphatidylcholine (PC), phosphatidylethanolamine (PE), phosphatidylserine (PS), phosphatidylinositol (PI), and cardiolipin (CL). This review also highlights the involvement of diacylglycerol O-acyltransferase (DGAT) in triacylglycerol biosynthesis. Due to significant gaps in knowledge, the GEPIA database was utilized to demonstrate the significance of individual enzymes in glioblastoma tumorigenesis. Finally, we also describe the significance of lipid droplets in glioblastoma and the impact of fatty acid synthesis, particularly docosahexaenoic acid (DHA), on cell membrane fluidity and signal transduction from the epidermal growth factor receptor (EGFR).

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.

Modulation of the cell membrane lipid milieu by peroxisomal β-oxidation induces Rho1 signaling to trigger inflammatory responses

Phagocytosis, signal transduction, and inflammatory responses require changes in lipid metabolism. Peroxisomes have key roles in fatty acid homeostasis and in regulating immune function. We find that Drosophila macrophages lacking peroxisomes have perturbed lipid profiles, which reduce host survival after infection. Using lipidomic, transcriptomic, and genetic screens, we determine that peroxisomes contribute to the cell membrane glycerophospholipid composition necessary to induce Rho1-dependent signals, which drive cytoskeletal remodeling during macrophage activation. Loss of peroxisome function increases membrane phosphatidic acid (PA) and recruits RhoGAPp190 during infection, inhibiting Rho1-mediated responses. Peroxisome-glycerophospholipid-Rho1 signaling also controls cytoskeleton remodeling in mouse immune cells. While high levels of PA in cells without peroxisomes inhibit inflammatory phenotypes, large numbers of peroxisomes and low amounts of cell membrane PA are features of immune cells from patients with inflammatory Kawasaki disease and juvenile idiopathic arthritis. Our findings reveal potential metabolic markers and therapeutic targets for immune diseases and metabolic disorders.

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.

Anti-angiogenic effects of VEGF stimulation on endothelium deficient in phosphoinositide recycling

Anti-angiogenic therapies have generated significant interest for their potential to combat tumor growth. However, tumor overproduction of pro-angiogenic ligands can overcome these therapies, hampering success of this approach. To circumvent this problem, we target the resynthesis of phosphoinositides consumed during intracellular transduction of pro-angiogenic signals in endothelial cells (EC), thus harnessing the tumor’s own production of excess stimulatory ligands to deplete adjacent ECs of the capacity to respond to these signals. Using zebrafish and human endothelial cells in vitro, we show ECs deficient in CDP-diacylglycerol synthase 2 are uniquely sensitive to increased vascular endothelial growth factor (VEGF) stimulation due to a reduced capacity to re-synthesize phosphoinositides, including phosphatidylinositol-(4,5)-bisphosphate (PIP2), resulting in VEGF-exacerbated defects in angiogenesis and angiogenic signaling. Using murine tumor allograft models, we show that systemic or EC specific suppression of phosphoinositide recycling results in reduced tumor growth and tumor angiogenesis. Our results suggest inhibition of phosphoinositide recycling provides a useful anti-angiogenic approach.

Tumors can overproduce pro-angiogenic ligands overcoming currently approved anti-angiogenic therapies and hindering their success. Here, the authors show that targeting phosphoinositide recycling during tumor angiogenesis harnesses the tumor’s own production of angiogenic ligands to deplete adjacent endothelial cells of the capacity to respond to these signals.

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.

Enzymatic fluorometric assays for quantifying all major phospholipid classes in cells and intracellular organelles

Cell membrane phospholipids regulate various biological functions. We previously reported enzymatic fluorometric methods for quantifying phosphatidic acid, phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, sphingomyelin, phosphatidylglycerol and cardiolipin. In the present report, a new enzymatic fluorometric assay was developed for quantifying phosphatidylinositol. These simple, sensitive and high-throughput methods enabled us to quantify all major phospholipid classes in cultured cells and intracellular organelles. By conducting comprehensive quantitative analyses of major phospholipid classes, we demonstrated that the contents of phospholipid classes in HEK293 cells changed with cell density and that overexpression of phosphatidylinositol synthase or CDP-diacylglycerol synthase significantly affected the phospholipid compositions of microsomal and mitochondrial membranes. These enzymatic fluorometric assays for measuring all major phospholipid classes may be applicable to tissues, fluids, lipoproteins, extracellular vesicles and intracellular organelles of many organisms and will further our understanding of cellular, physiological and pathological processes.

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.

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.

Therapeutic applications of reconstituted HDL: When structure meets function

Reconstituted forms of HDL (rHDL) are under development for infusion as a therapeutic approach to attenuate atherosclerotic vascular disease and to reduce cardiovascular risk following acute coronary syndrome and ischemic stroke. Currently available rHDL formulations developed for clinical use contain apolipoprotein A-I (apoA-I) and one of the major lipid components of HDL, either phosphatidylcholine or sphingomyelin. Recent data have established that quantitatively minor molecular constituents of HDL particles can strongly influence their anti-atherogenic functionality. Novel rHDL formulations displaying enhanced biological activities, including cellular cholesterol efflux, may therefore offer promising prospects for the development of HDL-based, anti-atherosclerotic therapies. Indeed, recent structural and functional data identify phosphatidylserine as a bioactive component of HDL; the content of phosphatidylserine in HDL particles displays positive correlations with all metrics of their functionality. This review summarizes current knowledge of structure-function relationships in rHDL formulations, with a focus on phosphatidylserine and other negatively-charged phospholipids. Mechanisms potentially underlying the atheroprotective role of these lipids are discussed and their potential for the development of HDL-based therapies highlighted.

Cardiolipin metabolism and its causal role in the etiology of the inherited cardiomyopathy Barth syndrome

Cardiolipin (CL) is a phospholipid with many unique characteristics. CL is synthesized in the mitochondria and resides almost exclusively within the mitochondrial inner membrane. Unlike most phospholipids that have two fatty acyl chains, CL possesses four fatty acyl chains resulting in unique biophysical characteristics that impact several biological processes including membrane fission and fusion. In addition, several proteins directly bind CL including proteins within the electron transport chain, the ADP/ATP carrier, and proteins that mediate mitophagy. Tafazzin is an enzyme that remodels saturated fatty acyl chains within CL to unsaturated fatty acyl chains, loss of function mutations in the TAZ gene encoding tafazzin are causal for the inherited cardiomyopathy Barth syndrome. Cells from Barth syndrome patients as well as several models of Barth have reduced mitochondrial functions including impaired electron transport chain function and increased reactive oxygen species (ROS) production. Mitochondria in cells from Barth syndrome patients, as well as several model organism mimics of Barth syndrome, are large and lack cristae consistent with the recently described role of CL participating in the generation of mitochondrial membrane contact sites. Cells with an inactive TAZ gene have also been shown to have a decreased capacity to undergo mitophagy when faced with stresses such as increased ROS or decreased mitochondrial quality control. This review describes CL metabolism and how defects in CL metabolism cause Barth syndrome, the etiology of Barth syndrome, and known modifiers of Barth syndrome phenotypes some of which could be explored for their amelioration of Barth syndrome in higher organisms.

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.

Structure and mechanism of an intramembrane liponucleotide synthetase central for phospholipid biosynthesis

Phospholipids are elemental building-block molecules for biological membranes. Biosynthesis of phosphatidylinositol, phosphatidylglycerol and phosphatidylserine requires a central liponucleotide intermediate named cytidine-diphosphate diacylglycerol (CDP-DAG). The CDP-DAG synthetase (Cds) is an integral membrane enzyme catalysing the formation of CDP-DAG, an essential step for phosphoinositide recycling during signal transduction. Here we report the structure of the Cds from Thermotoga maritima (TmCdsA) at 3.4 Å resolution. TmCdsA forms a homodimer and each monomer contains nine transmembrane helices arranged into a novel fold with three domains. An unusual funnel-shaped cavity penetrates half way into the membrane, allowing the enzyme to simultaneously accept hydrophilic substrate (cytidine 5'-triphosphate (CTP)/deoxy-CTP) from cytoplasm and hydrophobic substrate (phosphatidic acid) from membrane. Located at the bottom of the cavity, a Mg(2+)-K(+) hetero-di-metal centre coordinated by an Asp-Asp dyad serves as the cofactor of TmCdsA. The results suggest a two-metal-ion catalytic mechanism for the Cds-mediated synthesis of CDP-DAG at the membrane-cytoplasm interface.

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.

Extraplastidial cytidinediphosphate diacylglycerol synthase activity is required for vegetative development in Arabidopsis thaliana

Cytidinediphosphate diacylglycerol synthase (CDS) catalyzes the activation of phosphatidic acid to cytidinediphosphate (CDP)-diacylglycerol, a central intermediate in glycerolipid biosynthesis in prokaryotic and eukaryotic organisms. Cytidinediphosphate-diacylglycerol is the precursor to phosphatidylinositol, phosphatidylglycerol (PG) and cardiolipin of eukaryotic phospholipids that are essential for various cellular functions. Isoforms of CDS are located in plastids, mitochondria and the endomembrane system of plants and are encoded by five genes in Arabidopsis. Two genes have previously been shown to code for the plastidial isoforms which are indispensable for the biosynthesis of plastidial PG, and thus biogenesis and function of thylakoid membranes. Here we have focused on the extraplastidial CDS isoforms, encoded by CDS1 and CDS2 which are constitutively expressed contrary to CDS3. We provide evidence that these closely related CDS genes code for membrane proteins located in the endoplasmic reticulum and possess very similar enzymatic properties. Development and analysis of Arabidopsis mutants lacking either one or both CDS1 and CDS2 genes clearly shows that these two genes have redundant functions. As reflected in the seedling lethal phenotype of the cds1cds2 double mutant, plant cells require at least one catalytically active microsomal CDS isoform for cell division and expansion. According to the altered glycerolipid composition of the double mutant in comparison with wild-type seedlings, it is likely that the drastic decrease in the level of phosphatidylinositol and the increase in phosphatidic acid cause defects in cell division and expansion.

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.

CDP-diacylglycerol synthetase-controlled phosphoinositide availability limits VEGFA signaling and vascular morphogenesis

Understanding the mechanisms that regulate angiogenesis and translating these into effective therapies are of enormous scientific and clinical interests. In this report, we demonstrate the central role of CDP-diacylglycerol synthetase (CDS) in the regulation of VEGFA signaling and angiogenesis. CDS activity maintains phosphoinositide 4,5 bisphosphate (PIP2) availability through resynthesis of phosphoinositides, whereas VEGFA, mainly through phospholipase Cγ1, consumes PIP2 for signal transduction. Loss of CDS2, 1 of 2 vertebrate CDS enzymes, results in vascular-specific defects in zebrafish in vivo and failure of VEGFA-induced angiogenesis in endothelial cells in vitro. Absence of CDS2 also results in reduced arterial differentiation and reduced angiogenic signaling. CDS2 deficit-caused phenotypes can be successfully rescued by artificial elevation of PIP2 levels, and excess PIP2 or increased CDS2 activity can promote excess angiogenesis. These results suggest that availability of CDS-controlled resynthesis of phosphoinositides is essential for angiogenesis.

Translocation of p53 to mitochondria is regulated by its lipid binding property to anionic phospholipids and it participates in cell death control

p53, can regulate cell apoptosis in both transcription-dependent and -independent manners. The transcription-independent pathway was demonstrated by the translocation of p53 to mitochondria. Our study showed that p53 mitochondrial translocation was found in mitomycin C (MMC)-treated HepG2. The p53 C-terminal domain is clustered with potential nuclear leading sequences and showed strong electrostatic ion-ion interactions with cardiolipin, phosphatidylglycerol and phosphatidic acid in vitro. Disruption of cardiolipin biosynthesis by phosphatidylglycero-phosphate synthase (PGS) or CDP-diacylglycerol synthase 2 (CDS-2) short hairpin RNA (shRNA) transfection eliminated the MMC-induced translocation of mitochondrial p53. The elimination of mitochondrial p53 translocation also reduced Bcl-xL and Bcl-2 mitochondrial distribution. In HEK 293T models with saturated p53 expression, the mitochondrial partition of p53, Bcl-xL, and Bcl-2 obviously decreased in their PGS shRNA- or CDS-2 shRNA-expressing stable clones. In p53-null H1299 models, both the mitochondrial partitions of Bcl-xL and Bcl-2 were strongly reduced in relation to the HEK 293T models. The Bcl-xL mitochondrial partition was elevated in H1299 models expressing pCEP4-p53wt suggesting the direct carrier role of p53 in transporting Bcl-xL to the mitochondria. We also found that the cytosolic pool of Bcl-xL and Bcl-2 remained unaffected in the low-dose MMC treatment but decreased in the high-dose MMC treatment. The cytosolic pool of Bcl-2 and Bcl-xL directly regulated their amounts in p53-dependent mitochondrial distribution. In the low-dose MMC treatment, the increased mitochondrial p53, Bcl-xL, and Bcl-2 could attenuate apoptosis. However, in the high-dose MMC treatment, only the p53 translocated to the mitochondria and resulted in apoptosis progression. On the basis of this study, we thought mitochondrial p53 might regulate apoptosis in a biphasic manner.

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.

Lipid metabolism in Trypanosoma brucei

Trypanosoma brucei membranes consist of all major eukaryotic glycerophospholipid and sphingolipid classes. These are de novo synthesized from precursors obtained either from the host or from catabolised endocytosed lipids. In recent years, substantial progress has been made in the molecular and biochemical characterisation of several of these lipid biosynthetic pathways, using gene knockout or RNA interference strategies or by enzymatic characterization of individual reactions. Together with the completed genome, these studies have highlighted several possible differences between mammalian and trypanosome lipid biosynthesis that could be exploited for the development of drugs against the diseases caused by these parasites.

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.

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.

Comparative genomics and evolution of eukaryotic phospholipid biosynthesis

Phospholipid biosynthetic enzymes produce diverse molecular structures and are often present in multiple forms encoded by different genes. This work utilizes comparative genomics and phylogenetics for exploring the distribution, structure and evolution of phospholipid biosynthetic genes and pathways in 26 eukaryotic genomes. Although the basic structure of the pathways was formed early in eukaryotic evolution, the emerging picture indicates that individual enzyme families followed unique evolutionary courses. For example, choline and ethanolamine kinases and cytidylyltransferases emerged in ancestral eukaryotes, whereas, multiple forms of the corresponding phosphatidyltransferases evolved mainly in a lineage specific manner. Furthermore, several unicellular eukaryotes maintain bacterial-type enzymes and reactions for the synthesis of phosphatidylglycerol and cardiolipin. Also, base-exchange phosphatidylserine synthases are widespread and ancestral enzymes. The multiplicity of phospholipid biosynthetic enzymes has been largely generated by gene expansion in a lineage specific manner. Thus, these observations suggest that phospholipid biosynthesis has been an actively evolving system. Finally, comparative genomic analysis indicates the existence of novel phosphatidyltransferases and provides a candidate for the uncharacterized eukaryotic phosphatidylglycerol phosphate phosphatase.

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.

Stimulation of cardiac cardiolipin biosynthesis by PPARα activation

The role of peroxisome proliferator-activated receptor alpha (PPARalpha)-stimulated phospholipase A2 (PLA2) in cardiac mitochondrial cardiolipin (CL) biosynthesis was examined in both in vivo and in vitro models. Treatment of rat heart H9c2 cells with clofibrate increased the expression and activity of 14 kDa PLA2 but did not affect the pool size of CL. Clofibrate treatment stimulated de novo CL biosynthesis via an increase in phosphatidylglycerolphosphate (PGP) synthase activity, accounting for the unaltered CL content. Cardiac PLA2, PGP synthase, and CDP-1,2-diacyl-sn-glycerol synthase (CDS-2) activities and CDS-2 mRNA levels were elevated in mice fed clofibrate for 14 days compared with controls. In PPARalpha-null mice, clofibrate feeding did not alter cardiac PLA2, PGP synthase activities, or CDS-2 activity and mRNA level, confirming that these enzymes are regulated by PPARalpha activation. In contrast to mouse heart, clofibrate treatment did not affect the activity or mRNA levels of CDS-2 in H9c2 cells, indicating that CDS-2 is regulated differently in rat heart H9c2 cells in vitro and in mouse heart in vivo. These results clearly indicate that cardiac CL de novo biosynthesis is stimulated by PPARalpha activation in responsive rodent models and that CDS-2 is an example of an enzyme that exhibits alternative regulation in vivo and in cultured cell lines. This study is the first to demonstrate that CL de novo biosynthesis is regulated by PPARalpha activation.

Biogenesis and cellular dynamics of aminoglycerophospholipids

Aminoglycerophospholipids phosphatidylserine (PtdSer), phosphatidylethanolamine (PtdEtn), and phosphatidylcholine (PtdCho) comprise about 80% of total cellular phospholipids in most cell types. While the major function of PtdCho in eukaryotes and PtdEtn in prokaryotes is that of bulk membrane lipids, PtdSer is a minor component and appears to play a more specialized role in the plasma membrane of eukaryotes, e.g., in cell recognition processes. All three aminoglycerophospholipid classes are essential in mammals, whereas prokaryotes and lower eukaryotes such as yeast appear to be more flexible regarding their aminoglycerophospholipid requirement. Since different subcellular compartments of eukaryotes, namely the endoplasmic reticulum and mitochondria, contribute to the biosynthetic sequence of aminoglycerophospholipid formation, intracellular transport, sorting, and specific function of these lipids in different organelles are of special interest.

Regulation of TRP channels via lipid second messengers

In Drosophila photoreceptors, the light-sensitive current is mediated downstream of phospholipase C by TRP (transient receptor potential) channels. Recent evidence suggests that Drosophila TRP channels are activated by diacylglycerol (DAG) or its metabolites (polyunsaturated fatty acids), possibly in combination with the reduction in phosphatidyl inositol 4,5 bisphosphate (PIP2). Consistent with this view, diacylglycerol kinase is identified as a key enzyme required for response termination. Signaling is critically dependent upon efficient PIP2 synthesis; mutants of this pathway in combination with genetically targeted PIP2 reporters provide unique insights into the kinetics and regulation of PIP2 turnover. Recent evidence indicates that a growing number of mammalian TRP homologues are also regulated by lipid messengers, including DAG, arachidonic acid, and PIP2.

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.