Mobilization of steryl esters from lipid particles of the yeast Saccharomyces cerevisiae

TOR and heat shock response pathways regulate peroxisome biogenesis during proteotoxic stress

Peroxisomes are versatile organelles mediating energy homeostasis and redox balance. While peroxisome dysfunction is linked to numerous diseases, the molecular mechanisms and signaling pathways regulating peroxisomes during cellular stress remain elusive. Using yeast, we show that perturbations disrupting protein homeostasis including loss of ER or cytosolic chaperone function, impairments in ER protein translocation, blocking ER N-glycosylation, or reductive stress, cause peroxisome proliferation. This proliferation is driven by increased de novo biogenesis from the ER as well as increased fission of pre-existing peroxisomes, rather than impaired pexophagy. Notably, peroxisome biogenesis is essential for cellular recovery from proteotoxic stress. Through comprehensive testing of major signaling pathways, we determine this response to be mediated by activation of the heat shock response and inhibition of Target of Rapamycin (TOR) signaling. Finally, the effects of proteotoxic stress and TOR inhibition on peroxisomes are also captured in human fibroblasts. Overall, our findings reveal a critical and conserved role of peroxisomes in cellular response to proteotoxic stress.

Lipid droplets degradation mechanisms from microalgae to mammals, a comparative overview

Lipid droplets (LDs) are organelles composed of a hydrophobic core (mostly triacylglycerols and steryl esters) delineated by a lipid monolayer and found throughout the tree of life. LDs were seen for a long time as simple energy storage organelles but recent works highlighted their versatile roles in several fundamental cellular processes, particularly during stress response. LDs biogenesis occurs in the ER and their number and size can be dynamically regulated depending on their function, e.g. during development or stress. Understanding their biogenesis and degradation mechanisms is thus essential to better apprehend their roles. LDs degradation can occur in the cytosol by lipolysis or after their internalization into lytic compartments (e.g. vacuoles or lysosomes) using diverse mechanisms that depend on the considered organism, tissue, developmental stage or environmental condition. In this review, we summarize our current knowledge on the different LDs degradation pathways in several main phyla of model organisms, unicellular or pluricellular, photosynthetic or not (budding yeast, mammals, land plants and microalgae). We highlight the conservation of the main degradation pathways throughout evolution, but also the differences between organisms, or inside an organism between different organs. Finally, we discuss how this comparison can help to shed light on relationships between LDs degradation pathways and LDs functions.

Metabolic engineering strategies for de novo biosynthesis of sterols and steroids in yeast

Steroidal compounds are of great interest in the pharmaceutical field, with steroidal drugs as the second largest category of medicine in the world. Advances in synthetic biology and metabolic engineering have enabled de novo biosynthesis of sterols and steroids in yeast, which is a green and safe production route for these valuable steroidal compounds. In this review, we summarize the metabolic engineering strategies developed and employed for improving the de novo biosynthesis of sterols and steroids in yeast based on the regulation mechanisms, and introduce the recent progresses in de novo synthesis of some typical sterols and steroids in yeast. The remaining challenges and future perspectives are also discussed.

The metabolism of lipids in yeasts and applications in oenology

Lipids are valuable compounds present in all living organisms, which display an array of functions related to compartmentalization, energy storage and enzyme activation. Furthermore, these compounds are an integral part of the plasma membrane which is responsible for maintaining structure, facilitating the transport of solutes in and out of the cell and cellular signalling necessary for cell survival. The lipid composition of the yeast Saccharomyces cerevisiae has been extensively investigated and the impact of lipids on S. cerevisiae cellular functions during wine alcoholic fermentation is well documented. Although other yeast species are currently used in various industries and are receiving increasing attention in winemaking, little is known about their lipid metabolism. This review article provides an extensive and critical evaluation of our knowledge on the biosynthesis, accumulation, metabolism and regulation of fatty acids and sterols in yeasts. The implications of the yeast lipid content on stress resistance as well as performance during alcoholic fermentation are discussed and a particular emphasis is given on non-Saccharomyces yeasts. Understanding lipid requirements and metabolism in non-Saccharomyces yeasts may lead to a better management of these yeast to enhance their contributions to wine properties.

Anti-inflammatory, Antioxidant, Lung and Liver Protective Activity of Galaxaura oblongata as Antagonistic Efficacy against LPS using Hematological Parameters and Immunohistochemistry as Biomarkers

BACKGROUND

This study aimed to investigate the potential bioactivity and the ameliorative role of Galaxaura oblongata (G. oblongata) against LPS-induced toxicity by using hematological parameters.

OBJECTIVE

It is aimed also to examine its protective effect using the immunohistochemistry of liver and lungs as biomarkers in male BALB/C albino mice.

MATERIALS AND METHODS

The current study carried out using different in-vitro and in-vivo assays such as phytochemical, antioxidants, anti-inflammatory for in-vitro where the hematological and immunohistochemistry for lung and liver were investigated in vivo.

RESULTS

There are no previous studies were performed to investigate the in vivo and in vitro effects of the G. oblongata extracts as antioxidant and anti-inflammatory due to their rareness compared to other red algae. LPS treated mice revealed a significant decrease in total number of WBCs, RBCs, platelets, and HGB%, MPV, MCV and MCHC compared to the control group. On contrast, the HCT and MCHC were increased in the induction group which was treated with LPS compared to the control group. Furthermore, the immunohistochemistry results of the present study revealed the protective effect of G. oblongata compared to the induction group. G. oblongata can be used as protective marine natural products against the toxicity induced by LPS.

CONCLUSION

It exhibited a significant ameliorative role against the alterations in the hematological parameters and immunohistochemistry of liver and lungs, and helps to reduce as well as coordinate the acute inflammations caused by TNF.

Evaluation of ergosterol composition and esterification rate in fungi isolated from mangrove soil, long-term storage of broken spores, and two soils

Ergosterol is an important fungal-specific biomarker, but its use for fungal biomass estimation is still varied. It is important to distinguish between free and esterified ergosterols, which are mainly located on the plasma membrane and the cytosolic lipid particles, respectively. The present study analyzes free and esterified ergosterol contents in: (1) the fifty-nine strains of culturable fungi isolated from mangrove soil, (2) the broken spores of the fungus Ganoderma lucidum stored in capsule for more than 12 years, and (3) the mangrove soil and nearby campus wood soil samples by high performance liquid chromatography (HPLC). The results show that the contents of free and esterified ergosterols varied greatly in fifty-nine strains of fungi after 5 days of growth, indicating the diversity of ergosterol composition in fungi. The average contents of free and total ergosterols from the fifty-nine strains of fungi are 4.4 ± 1.5 mg/g and 6.1 ± 1.9 mg/g dry mycelia, respectively, with an average ergosterol esterification rate of 27.4%. The present study suggests that the fungi might be divided into two classes, one is fungi with high esterification rates (e.g., more than 27%) such as Nectria spp. and Fusarium spp., and the other is fungi with low esterification rates (e.g., less than 27%) such as Penicillium spp. and Trichoderma spp. Moreover, the ergosterol esterification rate in the spores of G. lucidum is 91.4% with a very small amount of free ergosterol (0.015 mg/g), compared with 41.9% with a higher level of free ergosterol (0.499 mg/g) reported in our previous study in 2007, indicating that free ergosterol degrades more rapidly than esterified ergosterol. In addition, the ergosterol esterification rates in mangrove soil and nearby campus wood soil samples range from 0 to 39.0%, compared with 80% in an old soil organic matter reported in a previous study, indicating the potential relationship between aging degree of fungi or soil and esterification rate. The present study proposes that both free and esterified ergosterols should be analyzed for fungal biomass estimation. When the ergosterol esterification rates in soils are higher, free ergosterol might be a better marker for fungal biomass. It is speculated that the ergosterol esterification rate in soils might contain some important information, such as the age of old-growth forests over time scales of centuries to millennia, besides the senescence degree of fungal mycelia in soils. KEY POINTS: • Fungi might be divided into two classes depending on ergosterol esterification rates. • Ergosterol esterification rate of broken spores stored for long time raised evidently. • Both free and esterified ergosterols should be analyzed for fungal biomass estimate. • Free ergosterol is a better marker for fungal biomass with a high esterification rate.

Environmental characteristics of a tundra river system in Svalbard. Part 1: Bacterial abundance, community structure and nutrient levels

The Arctic hosts a set of unique ecosystems, characterised by extreme environmental conditions and undergoing a rapid change resulting from the average temperature rising. We present a study on an aquatic ecosystem of the Revelva catchment (Spitsbergen), based on samples collected from the lake, river and their tributaries, in the summer of 2016. The landscape variety of the study site and the seasonal change in the hydrological regime modify the availability of nutrients. In general, the upper part of the catchment consists of the mountain rocky slopes which are especially abundant in iron minerals, sulphides and phosphorus minerals. The lower part of the catchment is covered by plants - lichens, saxifrages and bryophytes, which are a different source of nutrients. In the analysed water samples, the maximum concentrations of nutrients such as iron, boron and phosphorus were 0.28 μg L^-1, 4.52 μg L^-1 and 1.91 μg L^-1, respectively, in June, while in September, Fe and B reached the concentrations of 1.32 μg L^-1 and 2.71 μg L^-1, respectively. The concentration of P in September was below the detection limit of 1.00 μg L^-1, which may be explained by the necessity of bacteria to consume it immediately on current needs. We noted also an increase in TOC concentration between the June and September samples, which could originate both from the biomass accumulation in the catchment and the permafrost melting contributing to the hydrological regime of the river. The bacterial community developed in this environment consisted mainly of Proteobacteria, Actinobacteria, Bacteroidetes and Firmicutes phylum, while the presence of Acidobacteria was less pronounced than in other tundra-related environments. The described catchment shows that despite the relatively small amount of bioavailable nutrients, the Revelva system is biodiverse and one of the most significant biogeochemical changes occurs there in response to seasonally switching water sources.

A Genome-Wide Screen of Deletion Mutants in the Filamentous Saccharomyces cerevisiae Background Identifies Ergosterol as a Direct Trigger of Macrophage Pyroptosis

Phagocytic cells such as macrophages play an important role in the host defense mechanisms mounted in response to the common human fungal pathogen Candida albicans. In vitro, C. albicans triggers macrophage NLRP3-Casp1/11-mediated pyroptosis, an inflammatory programmed cell death pathway. Here, we provide evidence that Casp1/11-dependent pyroptosis occurs in the kidney of infected mice during the early stages of infection. We have also used a genome-wide screen of nonessential Σ1278b Saccharomyces cerevisiae genes to identify genes required for yeast-triggered macrophage pyroptosis. The set of genes identified by this screen was enriched for those with functions in lipid and sterol homeostasis and trafficking. These observations led us to discover that cell surface localization and/or total levels of ergosterol correlate with the ability of S. cerevisiae, C. albicans, and Cryptococcus neoformans to trigger pyroptosis. Since the mammalian sterol cholesterol triggers NLRP3-mediated pyroptosis, we hypothesized that ergosterol may also do so. Consistent with that hypothesis, ergosterol-containing liposomes but not ergosterol-free liposomes induce pyroptosis. Cell wall mannoproteins directly bind ergosterol, and we found that Dan1, an ergosterol receptor mannoprotein, as well as specific mannosyltransferases, is required for pyroptosis, suggesting that cell wall-associated ergosterol may mediate the process. Taken together, these data indicate that ergosterol, like mammalian cholesterol, plays a direct role in yeast-mediated pyroptosis.

Innate immune cells such as macrophages are key components of the host response to the human fungal pathogen Candida albicans. Macrophages undergo pyroptosis, an inflammatory, programmed cell death, in response to some species of pathogenic yeast. Prior to the work described in this report, yeast-triggered pyroptosis has been observed only in vitro; here, we show that pyroptosis occurs in the initial stages of murine kidney infection, suggesting that it plays an important role in the initial response of the innate immune system to invasive yeast infection. We also show that a key component of the fungal plasma membrane, ergosterol, directly triggers pyroptosis. Ergosterol is also present in the fungal cell wall, most likely associated with mannoproteins, and is increased in hyphal cells compared to yeast cells. Our data indicate that specific mannoproteins are required for pyroptosis. This is consistent with a potential mechanism whereby ergosterol present in the outer mannoprotein layer of the cell wall is accessible to the macrophage-mediated process. Taken together, our data provide the first evidence that ergosterol plays a direct role in the host-pathogen interactions of fungi.

The impact of nonpolar lipids on the regulation of the steryl ester hydrolases Tgl1p and Yeh1p in the yeast Saccharomyces cerevisiae

In the yeast Saccharomyces cerevisiae degradation of steryl esters is catalyzed by the steryl ester hydrolases Tgl1p, Yeh1p and Yeh2p. The two steryl ester hydrolases Tgl1p and Yeh1p localize to lipid droplets, a cell compartment storing steryl esters and triacylglycerols. In the present study we investigated regulatory aspects of these two hydrolytic enzymes, namely the gene expression level, protein amount, stability and enzyme activity of Tgl1p and Yeh1p in strains lacking both or only one of the two major nonpolar lipids, steryl esters and triacylglycerols. In a strain lacking both nonpolar lipids and consequently lipid droplets, Tgl1p as well as Yeh1p were present at low amount, became highly unstable compared to wild-type cells, and lost their enzymatic activity. Under these conditions both steryl ester hydrolases were retained in the endoplasmic reticulum. The lack of steryl esters alone was not sufficient to cause an altered intracellular localization of Tgl1p and Yeh1p. Surprisingly, the stability of Tgl1p and Yeh1p was markedly reduced in a strain lacking triacylglycerols, but their capacity to mobilize steryl esters remained unaffected. We also tested a possible cross-regulation of Tgl1p and Yeh1p by analyzing the behavior of each hydrolase in the absence of its counterpart steryl ester hydrolases. In summary, this study demonstrates a strong regulation of the two lipid droplet associated steryl ester hydrolases Tgl1p and Yeh1p due to the presence/absence of their host organelle.

Genome mining of fungal lipid-degrading enzymes for industrial applications

Lipases are interesting enzymes, which contribute important roles in maintaining lipid homeostasis and cellular metabolisms. Using available genome data, seven lipase families of oleaginous and non-oleaginous yeast and fungi were categorized based on the similarity of their amino acid sequences and conserved structural domains. Of them, triacylglycerol lipase (patatin-domain-containing protein) and steryl ester hydrolase (abhydro_lipase-domain-containing protein) families were ubiquitous enzymes found in all species studied. The two essential lipases rendered signature characteristics of integral membrane proteins that might be targeted to lipid monolayer particles. At least one of the extracellular lipase families existed in each species of yeast and fungi. We found that the diversity of lipase families and the number of genes in individual families of oleaginous strains were greater than those identified in non-oleaginous species, which might play a role in nutrient acquisition from surrounding hydrophobic substrates and attribute to their obese phenotype. The gene/enzyme catalogue and relevant informative data of the lipases provided by this study are not only valuable toolboxes for investigation of the biological role of these lipases, but also convey potential in various industrial applications.

Regulatory link between steryl ester formation and hydrolysis in the yeast Saccharomyces cerevisiae

Steryl esters and triacylglycerols are the major storage lipids of the yeast Saccharomyces cerevisiae. Steryl esters are formed in the endoplasmic reticulum by the two acyl-CoA:sterol acyltransferases Are1p and Are2p, whereas steryl ester hydrolysis is catalyzed by the three steryl ester hydrolases Yeh1p, Yeh2p and Tgl1p. To shed light on the regulatory link between steryl ester formation and hydrolysis in the maintenance of cellular sterol and free fatty acid levels we employed yeast mutants which lacked the enzymes catalyzing the degradation of steryl esters. These studies revealed feedback regulation of steryl ester formation by steryl ester hydrolysis although in a Δtgl1Δyeh1Δyeh2 triple mutant the gene expression levels of ARE1 and ARE2 as well as protein levels and stability of Are1p and Are2p were not altered. Nevertheless, the capacity of the triple mutant to synthesize steryl esters was significantly reduced as shown by in vitro and in vivo labeling of lipids with [(14)C]oleic acid and [(14)C]acetate. Enzymatic analysis revealed that inhibition of steryl ester formation occurred at the enzyme level. As the amounts and the formation of sterols and fatty acids were also decreased in the triple mutant we concluded that defects in steryl ester hydrolysis also caused feedback inhibition on the formation of sterols and fatty acids which serve as precursors for steryl ester formation. In summary, this study demonstrates a regulatory link within the steryl ester metabolic network which contributes to non-polar lipid homeostasis in yeast cells.

Lipid droplet dynamics in budding yeast

Eukaryotic cells store excess fatty acids as neutral lipids, predominantly triacylglycerols and sterol esters, in organelles termed lipid droplets (LDs) that bulge out from the endoplasmic reticulum. LDs are highly dynamic and contribute to diverse cellular functions. The catabolism of the storage lipids within LDs is channeled to multiple metabolic pathways, providing molecules for energy production, membrane building blocks, and lipid signaling. LDs have been implicated in a number of protein degradation and pathogen infection processes. LDs may be linked to prevalent human metabolic diseases and have marked potential for biofuel production. The knowledge accumulated on LDs in recent years provides a foundation for diverse, and even unexpected, future research. This review focuses on recent advances in LD research, emphasizing the diverse physiological roles of LDs in the model system of budding yeast.

ER stress induced by the OCH1 mutation triggers changes in lipid homeostasis in Kluyveromyces lactis

In Kluyveromyces lactis yeast, OCH1 encodes for the α-1,6-mannosyltrasferase that adds the initial α-1,6-mannose to the outer-chains of N-glycoproteins. Kloch1-1 mutant cells showed altered calcium homeostasis and endoplasmic reticulum (ER) stress. Since ER plays a major role in lipid biosynthesis and lipid droplet (LD) formation, herein the impact of Och1p depletion on lipid homeostasis was investigated. Transcriptional profiles of genes involved in biosynthesis of fatty acids, their amount and composition changed in mutant cells. An increased amount of ergosterol was determined in these cells. Enhanced transcription of genes involved in both synthesis and mobilization of LDs was also found in Kloch1-1 cells, accompanied by a reduced amount of LDs. We provide evidence that ER alterations, determined by protein misfolding as a result of reduced N-glycosylation, induced altered lipid homeostasis in Kloch1-1 cells. Chemical chaperone 4-phenyl butyrate (4-PBA) slightly alleviated the LD phenotype in cells depleted of Och1p. Remarkably, complete suppression of ER stress, via increased expression of plasma membrane calcium channel subunit Mid1, fully restored lipid homeostasis in mutant cells. To further reinforce this finding, low numbers of LDs were observed in wild type cells when ER stress was triggered by DTT treatment.

Storage lipids of yeasts: a survey of nonpolar lipid metabolism in Saccharomyces cerevisiae, Pichia pastoris, and Yarrowia lipolytica

Biosynthesis and storage of nonpolar lipids, such as triacylglycerols (TG) and steryl esters (SE), have gained much interest during the last decades because defects in these processes are related to severe human diseases. The baker's yeast Saccharomyces cerevisiae has become a valuable tool to study eukaryotic lipid metabolism because this single-cell microorganism harbors many enzymes and pathways with counterparts in mammalian cells. In this article, we will review aspects of TG and SE metabolism and turnover in the yeast that have been known for a long time and combine them with new perceptions of nonpolar lipid research. We will provide a detailed insight into the mechanisms of nonpolar lipid synthesis, storage, mobilization, and degradation in the yeast S. cerevisiae. The central role of lipid droplets (LD) in these processes will be addressed with emphasis on the prevailing view that this compartment is more than only a depot for TG and SE. Dynamic and interactive aspects of LD with other organelles will be discussed. Results obtained with S. cerevisiae will be complemented by recent investigations of nonpolar lipid research with Yarrowia lipolytica and Pichia pastoris. Altogether, this review article provides a comprehensive view of nonpolar lipid research in yeast.

Carbon accumulation in Rhodotorula glutinis induced by nitrogen limitation

BACKGROUND

Oleaginous microorganisms, such as bacterium, yeast and algal species, can represent an alternative oil source for biodiesel production. The composition of their accumulated lipid is similar to the lipid of an oleaginous plant with a predominance of unsaturated fatty acid. Moreover this alternative to conventional biodiesel production does not create competition for land use between food and oleo-chemical industry supplies. Despite this promising potential, development of microbial production processes are at an early stage. Nutritional limited conditions, such as nitrogen limitation, with an excess of carbon substrate is commonly used to induce lipid accumulation metabolism. Nitrogen limitation implies modification of the carbon-to-nitrogen ratio in culture medium, which impacts on carbon flow distribution in the metabolic network.

RESULTS

The goal of the present study is to improve our knowledge of carbon flow distribution in oleaginous yeast metabolism by focusing carbon distribution between carbohydrate and lipid pools in order to optimize microbial lipid production. The dynamic effects of limiting nitrogen consumption flux according to carbon flow were studied to trigger lipid accumulation in the oleaginous yeast Rhodotorula glutinis. With a decrease of the specific nitrogen consumption rate from 0.052 Nmol.CmolX (-1).h(-1) to 0.003 Nmol.CmolX (-1).h(-1), a short and transitory intracellular carbohydrate accumulation occurred before the lipid accumulation phase. This phenomenon was studied in fed-batch culture under optimal operating conditions, with a mineral medium and using glucose as carbon source. Two different strategies of decreasing nitrogen flow on carbohydrate accumulation were investigated: an instantaneous decrease and a progressive decrease of nitrogen flow.

CONCLUSIONS

Lipid production performance in these fed-batch culture strategies with R. glutinis were higher than those reported in the previous literature; the catalytic specific lipid production rate was 0.07 Cmollip.CmolX* (-1).h(-1). Experimental results suggested that carbohydrate accumulation was an intrinsic phenomenon connected to the limitation of growth by nitrogen when the nitrogen-to-carbon ratio in the feed flow was lower than 0.045 Nmol.Cmol(-1). Carbohydrate accumulation corresponded to a 440% increase of carbohydrate content. These results suggest that microbial lipid production can be optimized by culture strategy and that carbohydrate accumulation must be taken account for process design.

Lipid droplets and peroxisomes: key players in cellular lipid homeostasis or a matter of fat--store 'em up or burn 'em down

Lipid droplets (LDs) and peroxisomes are central players in cellular lipid homeostasis: some of their main functions are to control the metabolic flux and availability of fatty acids (LDs and peroxisomes) as well as of sterols (LDs). Both fatty acids and sterols serve multiple functions in the cell—as membrane stabilizers affecting membrane fluidity, as crucial structural elements of membrane-forming phospholipids and sphingolipids, as protein modifiers and signaling molecules, and last but not least, as a rich carbon and energy source. In addition, peroxisomes harbor enzymes of the malic acid shunt, which is indispensable to regenerate oxaloacetate for gluconeogenesis, thus allowing yeast cells to generate sugars from fatty acids or nonfermentable carbon sources. Therefore, failure of LD and peroxisome biogenesis and function are likely to lead to deregulated lipid fluxes and disrupted energy homeostasis with detrimental consequences for the cell. These pathological consequences of LD and peroxisome failure have indeed sparked great biomedical interest in understanding the biogenesis of these organelles, their functional roles in lipid homeostasis, interaction with cellular metabolism and other organelles, as well as their regulation, turnover, and inheritance. These questions are particularly burning in view of the pandemic development of lipid-associated disorders worldwide.

Neutral lipid metabolism influences phospholipid synthesis and deacylation in Saccharomyces cerevisiae

Establishment and maintenance of equilibrium in the fatty acid (FA) composition of phospholipids (PL) requires both regulation of the substrate available for PL synthesis (the acyl-CoA pool) and extensive PL turnover and acyl editing. In the present study, we utilize acyl-CoA synthetase (ACS) deficient cells, unable to recycle FA derived from lipid deacylation, to evaluate the role of several enzymatic activities in FA trafficking and PL homeostasis in Saccharomyces cerevisiae. The data presented show that phospholipases B are not contributing to constitutive PL deacylation and are therefore unlikely to be involved in PL remodeling. In contrast, the enzymes of neutral lipid (NL) synthesis and mobilization are central mediators of FA trafficking. The phospholipid:DAG acyltransferase (PDAT) Lro1p has a substantial effect on FA release and on PL equilibrium, emerging as an important mediator in PL remodeling. The acyl-CoA dependent biosynthetic activities of NL metabolism are also involved in PL homeostasis through active modulation of the substrate available for PL synthesis. In addition TAG mobilization makes an important contribution, especially in cells from stationary phase, to FA availability. Beyond its well-established role in the formation of a storage pool, NL metabolism could play a crucial role as a mechanism to uncouple the pools of PL and acyl-CoAs from each other and thereby to allow independent regulation of each one.

Endogenous sterol biosynthesis is important for mitochondrial function and cell morphology in procyclic forms of Trypanosoma brucei

Sterol biosynthesis inhibitors are promising entities for the treatment of trypanosomal diseases. Insect forms of Trypanosoma brucei, the causative agent of sleeping sickness, synthesize ergosterol and other 24-alkylated sterols, yet also incorporate cholesterol from the medium. While sterol function has been investigated by pharmacological manipulation of sterol biosynthesis, molecular mechanisms by which endogenous sterols influence cellular processes remain largely unknown in trypanosomes. Here we analyse by RNA interference, the effects of a perturbation of three specific steps of endogenous sterol biosynthesis in order to dissect the role of specific intermediates in proliferation, mitochondrial function and cellular morphology in procyclic cells. A decrease in the levels of squalene synthase and squalene epoxidase resulted in a depletion of cellular sterol intermediates and end products, impaired cell growth and led to aberrant morphologies, DNA fragmentation and a profound modification of mitochondrial structure and function. In contrast, cells deficient in sterol methyl transferase, the enzyme involved in 24-alkylation, exhibited a normal growth phenotype in spite of a complete abolition of the synthesis and content of 24-alkyl sterols. Thus, the data provided indicates that while the depletion of squalene and post-squalene endogenous sterol metabolites results in profound cellular defects, bulk 24-alkyl sterols are not strictly required to support growth in insect forms of T. brucei in vitro.

Genome-wide screens for gene products regulating lipid droplet dynamics

Lipid droplets (LDs) are emerging as dynamic cellular organelles that play a key role in lipid and membrane homeostasis. Abnormal lipid droplet dynamics are associated with the pathophysiology of many metabolic diseases, such as obesity, diabetes, atherosclerosis, fatty liver, and even cancer. Understanding the molecular mechanisms governing the dynamics of LDs, namely, their biogenesis, growth, maintenance, and degradation, will not only shed light on the cellular functions of LDs, but also provide additional clues to treatment of metabolic diseases. Genome-wide screen is a powerful approach to identify genetic factors that regulate lipid droplet dynamics. Here, we summarize recent genome-wide studies using yeast and Drosophila cells to understand the cellular dynamics of LDs. The results suggest that the genome-wide screens should be carried out in multiple organisms or cells, and using different nutritional conditions.

Multiple functions as lipase, steryl ester hydrolase, phospholipase, and acyltransferase of Tgl4p from the yeast Saccharomyces cerevisiae

Triacylglycerol (TAG) hydrolysis, membrane lipid biosynthesis, and lipid turnover are largely interlinked processes. In yeast, TAG is mobilized by three TAG lipases named Tgl3p, Tgl4p, and Tgl5p, which are localized to lipid particles/droplets. These TAG lipases posses a conserved GXSXG motif that is characteristic of hydrolytic enzymes. Here, we demonstrated that the yeast TAG lipase Tgl4p, the functional ortholog of the adipose TAG lipase, ATGL, catalyzes multiple functions in lipid metabolism. An extended domain and motif search analysis revealed that Tgl4p bears not only a lipase consensus domain but also a conserved motif for calcium-independent phospholipase A(2). We show that Tgl4p exhibits TAG lipase, steryl ester hydrolase, and phospholipase A(2) activities, but surprisingly it also catalyzed the acyl-CoA-dependent acylation of lysophosphatidic acid to phosphatidic acid (PA). Heterologous overexpression of Tgl4p in Pichia pastoris increased total phospholipid and specifically PA synthesis. Moreover, deletion of TGL4 in Saccharomyces cerevisiae showed an altered pattern of phosphatidylcholine and PA molecular species. Altogether, our data suggest that yeast Tgl4p functions as a hydrolytic enzyme in lipid degradation but also contributes to fatty acid channeling and phospholipid remodeling.

Efficient ER exit and vacuole targeting of yeast Sna2p require two tyrosine-based sorting motifs

SNA (Sensitive to Na(+)) proteins form a membrane protein family, which, in the yeast Saccharomyces cerevisiae, is composed of four members: Sna1p/Pmp3p, Sna2p, Sna3p and Sna4p. In this study, we focused on the 79 residue Sna2p protein. We found that Sna2p is localized in the vacuolar membrane. Directed mutagenesis showed that two functional tyrosine motifs YXXØ are present in the C-terminal region. Each of these is involved in a different Golgi-to-vacuole targeting pathway: the tyrosine 65 motif is involved in adaptor protein (AP-1)-dependent targeting, whereas the tyrosine 75 motif is involved in AP-3-dependent targeting. Moreover, our data suggest that these motifs also play a crucial role in the exit of Sna2p from the endoplasmic reticulum (ER). Directed mutagenesis of these tyrosines led to a partial redirection of Sna2p to lipid bodies, probably because of a decrease in ER exit efficiency. Sna2p is the first yeast protein in which two YXXØ motifs have been identified and both were shown to be functional at two different steps of the secretory pathway, ER exit and Golgi-to-vacuole transport.

Yeast lipid analysis and quantification by mass spectrometry

The systematic and quantitative analysis of the different lipid species within a cell or an organism has recently become possible and the general approach has been termed "lipidomics." Traditional methods of identification and quantification of lipid species were laborious processes and it was necessary to use a wide variety of techniques to analyse the different lipid species, especially concerning the assigning of particular acyl chain lengths, hydroxylations, and desaturations to the diverse lipid species. While it is still not possible to quantitatively analyze all lipid species in one fell swoop, great progress has been made with the intensive use of quantitative mass spectrometry approaches. It is now relatively simple to quantify most of the lipid species, including all of the major ones, in a yeast cell. Different degrees of sophistication of mass spectrometric analysis exist and the available techniques and instrumentation are evolving rapidly. Therefore, we have decided to present robust, simple methods to quantify the major yeast lipids by mass spectrometry that should be accessible to anyone who has access to a standard mass spectrometry equipment. The methods to identify and quantify yeast glycerophospholipids and sphingolipids involve electrospray ionization mass spectrometry using fragmentation to characterize the lipid species. A simplified gas chromatographic method is used to quantify the major sterols that occur in wild-type yeast cells and ergosterol biosynthesis mutants.

Investigating the effects of statins on cellular lipid metabolism using a yeast expression system

In humans, defects in lipid metabolism are associated with a number of severe diseases such as atherosclerosis, obesity and type II diabetes. Hypercholesterolemia is a primary risk factor for coronary artery disease, the major cause of premature deaths in developed countries. Statins are inhibitors of 3-hydroxy-3-methylglutaryl-CoA reductase (HMGR), the key enzyme of the sterol synthesis pathway. Since yeast Saccharomyces cerevisiae harbours many counterparts of mammalian enzymes involved in lipid-synthesizing pathways, conclusions drawn from research with this single cell eukaryotic organism can be readily applied to higher eukaryotes. Using a yeast strain with deletions of both HMG1 and HMG2 genes (i.e. completely devoid of HMGR activity) with introduced wild-type or mutant form of human HMGR (hHMGR) gene we investigated the effects of statins on the lipid metabolism of the cell. The relative quantification of mRNA demonstrated a different effect of simvastatin on the expression of the wild-type and mutated hHMGR gene. GC/MS analyses showed a significant decrease of sterols and enhanced conversion of squalene and sterol precursors into ergosterol. This was accompanied by the mobilization of ergosterol precursors localized in lipid particles in the form of steryl esters visualized by confocal microscopy. Changes in the level of ergosterol and its precursors in cells treated with simvastatin depend on the mutation in the hHMGR gene. HPLC/MS analyses indicated a reduced level of phospholipids not connected with the mevalonic acid pathway. We detected two significant phenomena. First, cells treated with simvastatin develop an adaptive response compensating the lower activity of HMGR. This includes enhanced conversion of sterol precursors into ergosterol, mobilization of steryl esters and increased expression of the hHMGR gene. Second, statins cause a substantial drop in the level of glycerophospholipids.