Rapid induction of microsomal delta 12(omega 6)-desaturase activity in chilled Acanthamoeba castellanii

Functional expression of Δ12 fatty acid desaturase modulates thermoregulatory behaviour in Drosophila

Polyunsaturated fatty acids (PUFAs) play crucial roles in adaptation to cold environments in a wide variety of animals and plants. However, the mechanisms by which PUFAs affect thermoregulatory behaviour remain elusive. Thus, we investigated the roles of PUFAs in thermoregulatory behaviour of Drosophila melanogaster. To this end, we generated transgenic flies expressing Caenorhabditis elegans Δ12 fatty acid desaturase (FAT-2), which converts mono-unsaturated fatty acids to PUFAs such as linoleic acid [C18:2 (n-6)] and linolenic acid [C18:3 (n-3)]. Neuron-specific expression of FAT-2 using the GAL4/UAS expression system led to increased contents of C18:2 (n-6)-containing phospholipids in central nerve system (CNS) and caused significant decreases in preferred temperature of third instar larvae. In genetic screening and calcium imaging analyses of thermoreceptor-expressing neurons, we demonstrated that ectopic expression of FAT-2 in TRPA1-expressing neurons led to decreases in preferred temperature by modulating neuronal activity. We conclude that functional expression of FAT-2 in a subset of neurons changes the thermoregulatory behaviour of D. melanogaster, likely by modulating quantities of PUFA-containing phospholipids in neuronal cell membranes.

Endoplasmic reticulum retention signaling and transmembrane channel proteins predicted for oilseed ω3 fatty acid desaturase 3 (FAD3) genes

Oilseed crop oils contain a variety of unsaturated fatty acids that are synthesized and regulated by fatty acid desaturases (FADs). In this study, 14 FAD3 (ω3 desaturase) protein sequences from oilseeds are analyzed and presented through the application of several computational tools. The results indicated a close relationship between Brassica napus and Camelina sativa, as well as between Salvia hispanica and Perilla frutescens FAD3s, due to a high similarity in codon preferences in codon usage clusters and the phylogenetic tree. The cis-acting element results reveal that the seed-specific promoter region of BnFAD3 contains the critical conserved boxes such as HSE and ABRE, which are involved in responsiveness to heat stress and abscisic acid. The presence of the aforementioned conserved boxes may increase cold acclimation as well as tolerance to drought and high salinity. Omega(ω)3 desaturases contain a Skn-1 motif which is a cis-acting regulatory element required involved in endosperm development. In oilseed FAD3s, leucine is the most repeated amino acid in FAD3 proteins. The study conveyed that B. napus, Camelina sativa, Linum usitatissimum, Vernicia fordii, Gossypium hirsutum, S. hispanica, Cannabis sativa, and P. frutescens have retention signal KXKXX/XKXX at their c-terminus sites, which is one of the most important characteristics of FADs. Additionally, it was found that BnFAD3 is a transmembrane protein that can convert ω6 to ω3 fatty acids and may simultaneously act as a potassium ion channel in the ER.

Free-living amoebae and squatters in the wild: ecological and molecular features

Free-living amoebae are protists frequently found in water and soils. They feed on other microorganisms, mainly bacteria, and digest them through phagocytosis. It is accepted that these amoebae play an important role in the microbial ecology of these environments. There is a renewed interest for the free-living amoebae since the discovery of pathogenic bacteria that can resist phagocytosis and of giant viruses, underlying that amoebae might play a role in the evolution of other microorganisms, including several human pathogens. Recent advances, using molecular methods, allow to bring together new information about free-living amoebae. This review aims to provide a comprehensive overview of the newly gathered insights into (1) the free-living amoeba diversity, assessed with molecular tools, (2) the gene functions described to decipher the biology of the amoebae and (3) their interactions with other microorganisms in the environment.

Inspired by lipids: the Morton Lecture Award Presentation

Lipids are key molecules for membranes, energy storage and signalling. I have been privileged to have worked in such a diverse field and in organisms from microbes to humans. Here I will describe some of those contrasting areas which range from environmental impacts to food production and on to human health. It has been a fascinating journey which still continues to excite me.

Classification and substrate head-group specificity of membrane fatty acid desaturases

Membrane fatty acid desaturases are a diverse superfamily of enzymes that catalyze the introduction of double bonds into fatty acids. They are essential in a range of metabolic processes, such as the production of omega-3 fatty acids. However, our structure-function understanding of this superfamily is still developing and their range of activities and substrate specificities are broad, and often overlapping, which has made their systematic characterization challenging. A central issue with characterizing these proteins has been the lack of a structural model, which has been overcome with the recent publication of the crystal structures of two mammalian fatty acid desaturases. In this work, we have used sequence similarity networks to investigate the similarity among over 5000 related membrane fatty acid desaturase sequences, leading to a detailed classification of the superfamily, families and subfamilies with regard to their function and substrate head-group specificity. This work will facilitate rapid prediction of the function and specificity of new and existing sequences, as well as forming a basis for future efforts to manipulate the substrate specificity of these proteins for biotechnology applications.

Environmental chlamydiae alter the growth speed and motility of host acanthamoebae

Symbiosis between living beings is an important driver of evolutionary novelty and ecological diversity; however, understanding the mechanisms underlying obligate mutualism remains a significant challenge. Regarding this, we have previously isolated two different Acanthamoeba strains harboring endosymbiotic bacteria, Protochlamydia (R18 symbiotic amoebae: R18WT) or Neochlamydia (S13 symbiotic amoebae; S13WT). In this study, we treated the symbiotic amoebae R18WT and S13WT with doxycycline (DOX) and rifampicin (RFP), respectively, to establish the aposymbiotic amoebae R18DOX and S13RFP, respectively. Subsequently, we compared the growth speed, motility, phagocytosis, pinocytosis, and morphology of the symbiotic and aposymbiotic amoebae. The growth speed of R18DOX was decreased, although that of S13RFP was increased. A marked change in motility was observed only for R18DOX amoebae. There was no difference in phagocytic and pinocytic activities between the symbiotic and aposymbiotic amoebae. Meanwhile, we observed a significant change in the phalloidin staining pattern and morphological changes in R18DOX (but not S13RFP) aposymbiotic amoebae, indicating a change in actin accumulation upon removal of the Protochlamydia. Infection of C3 (a reference strain) or S13RFP amoebae with Protochlamydia had a harmful effect on the host amoebae, but R18DOX amoebae re-infected with Protochlamydia showed recovery in both growth speed and motility. Taken together, we conclude that endosymbiont environmental chlamydiae alter the growth speed and/or motility of their host Acanthamoeba, possibly implying an close mutual relationship between amoebae and environmental chlamydiae.

Temperature Stress: Reacting and Adapting: Lessons from Poikilotherms

Acanthamoeba castellanii (A. castellanii) is a common soil- or water-borne protozoon that feeds on bacteria by phagocytosis. A. castellanii can grow between 4 and 32 degrees C and has to adapt quickly to chilling in order to survive. We have identified a Delta12-fatty acid desaturase as key to low temperature adaptation. The activity of this enzyme is mainly increased through gene expression and new protein synthesis. Interestingly, the activity can also be altered independently by dissolved oxygen levels. In addition, we have identified a gene for the Delta12-desaturase, which, when expressed in yeast, catalyses Delta15-desaturation also. Moreover, it is also capable of producing very unusual n-1 polyunsaturated products.

A Bifunctional Δ12,Δ15-Desaturase from Acanthamoeba castellanii Directs the Synthesis of Highly Unusual n-1 Series Unsaturated Fatty Acids

The free-living soil protozoon Acanthamoeba castellanii synthesizes a range of polyunsaturated fatty acids, the balance of which can be altered by environmental changes. We have isolated and functionally characterized in yeast a microsomal desaturase from A. castellanii, which catalyzes the sequential conversion of C(16) and C(18) Delta9-monounsaturated fatty acids to di- and tri-unsaturated forms. In the case of C(16) substrates, this bifunctional A. castellanii Delta12,Delta15-desaturase generated a highly unusual fatty acid, hexadecatrienoic acid (16:3Delta(9,12,15)(n-1)). The identification of a desaturase, which can catalyze the insertion of a double bond between the terminal two carbons of a fatty acid represents a new addition to desaturase functionality and plasticity. We have also co-expressed in yeast the A. castellanii bifunctional Delta12,Delta15-desaturase with a microsomal Delta6-desaturase, resulting in the synthesis of the highly unsaturated C(16) fatty acid hexadecatetraenoic acid (16:4Delta(6,9,12,15)(n-1)), previously only reported in marine microorganisms. Our work therefore demonstrates the feasibility of the heterologous synthesis of polyunsaturated fatty acids of the n-1 series. The presence of a bifunctional Delta12,Delta15-desaturase in A. castellanii is also considered with reference to the evolution of desaturases and the lineage of this protist.

Mechanisms of temperature adaptation in poikilotherms

For good function, membrane lipids have to be arranged appropriately and be in the correct physical state. In poikilotherms, exposure to cold stress or heat shock can alter membrane properties such that, unless they are corrected quickly, damage and, possibly, death can result. Low temperature stress is countered by modifying membrane lipids such that their average transition temperature is lowered. There are various ways in which this can be achieved but an increase in fatty acid unsaturation is the most common. For heat shock, various changes in lipids have been noted and some defensive strategies involving heat shock proteins noted. In this short review, we will describe recent results where adaptive lipid changes, as a result of temperature stress, have been found. Mechanisms for bringing about such alterations are discussed, together with the contrasting data for different organisms.

The alternative pathway C20 Delta8-desaturase from the non-photosynthetic organism Acanthamoeba castellanii is an atypical cytochrome b5-fusion desaturase

A cDNA encoding a C20 Delta8-desaturase was isolated from the free-living soil amoeba, Acanthamoeba castellanii and functionally characterised by heterologous expression. The open reading frame of the A. castellanii C20 Delta8-desaturase showed similarity to other microsomal front-end desaturases, but the N-terminal domain contained a variant form of the conserved heme-binding motif in which H-P-G-G is replaced by H-P-A-G. Co-expression of the A. castellani Delta8-desaturase with the Isochrysis galbana Delta9-elongase in transgenic Arabidopsis plants confirmed the activity observed in yeast and its role in the alternative pathway for C20 polyunsaturated fatty acid synthesis. Acyl-CoA profiles of these transgenic plants revealed an unexpected accumulation of C20 fatty acids in the acyl-CoA pool. This is the first report of an alternative pathway C20 Delta8-desaturase from a non-photosynthetic organism, and also the first report of a front-end desaturase lacking the canonical cytochrome b5 domain.

Production of gamma-linolenic acid by disrupted mycelia of Mortierella isabellina

AIMS

To optimize the production of linolenic acid by disrupted mycelia of Mortierella isabellina.

METHODS AND RESULTS

Effects of incubation conditions such as incubation time, pH of reaction mixture, concentration of Mg2+ or malate and incubation temperature on production of linolenic acid were studied. The production of gamma-linolenic acid reached 224 mg g-1 dry cells when the reaction mixture was composed of 1.0 g (dry mycelial mass) of disrupted mycelia of M. isabellina, 50 ml (50 mmol l(-1)) potassium phosphate buffer supplemented with 0.312 mmol l(-1) of Mg2+ and 10 mmol l(-1) of malate, pH 7.0 and incubated at 5 degrees C for 1 day.

CONCLUSIONS

Incubation temperature, concentration of Mg2+ and malate showed major effects on the increased linolenic acid production.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study highlights conditions for increasing gamma-linolenic acid production by cell-free mycelia of M. isabellina and an insight into rapidly gaining high production of polyunsaturated fatty acids.

Oxygen induction of a novel fatty acid n-6 desaturase in the soil protozoon, Acanthamoeba castellanii

Induction of fatty acid desaturation is very important for the temperature adaptation of poikilotherms. However, in oxygen-limited late-exponential-phase Acanthamoeba castellanii cultures, oxygen alone was able to induce increased activity of a fatty acid desaturase that converts oleate into linoleate and which has been implicated in the temperature adaptation of this organism. Experiments with Delta(10)-nonadecenoate showed that the enzyme is an n -6 desaturase rather than a Delta(12)-desaturase. It also used preferentially 1-acyl-2-oleoyl-phosphatidylcholine as substrate and NAD(P)H as electron donor. The involvement of cytochrome b (5) as an intermediate electron carrier was shown by difference spectra measurements and anti-(cytochrome b (5)) antibody experiments. Of the three protein components of the desaturase complex, oxygen only increased the activity of the terminal (cyanide-sensitive) protein during n -6 desaturase induction. The induction of this terminal protein paralleled well the increase in overall oleate n -6 desaturation. The ability of oxygen to induce oleate desaturase independently of temperature in this lower eukaryotic animal model is of novel intrinsic interest, as well as being important for the design of future experiments to determine the molecular mechanism of temperature adaptation in poikilotherms.

Effects of developmental age, ambient temperature, and dietary alterations on delta(12) desaturase activity in the house cricket, Acheta domesticus

Double bond formation in polyunsaturated fatty acids (PUFA) is mediated by desaturase enzymes. Certain insect species have been found to possess a Delta(12) desaturase, previously thought to occur exclusively in plants. We have begun to characterize this enzyme to determine its relatedness to those found in plants and animals. Desaturase activity can be altered significantly by a number of environmental factors in protozoa, cyanobacteria, plants, fish, and rats. We present evidence here that Delta(12) desaturase activity in Acheta domesticus is affected by developmental stage, starvation, dietary alterations, and fluctuations in ambient temperature. Highest activity is observed during the middle of the penultimate instar and 3 to 6 days after adult emergence. Starvation markedly decreases Delta(12) activity, whereas resumption of feeding on fat-free or low fat diets increases activity.

Structure and expression of fatty acid desaturases

Fatty acid desaturases are enzymes that introduce double bonds into fatty acyl chains. They are present in all groups of organisms, i.e., bacteria, fungi, plants and animals, and play a key role in the maintenance of the proper structure and functioning of biological membranes. The desaturases are characterized by the presence of three conserved histidine tracks which are presumed to compose the Fe-binding active centers of the enzymes. Recent findings on the structure and expression of different types of fatty acid desaturase in cyanobacteria, plants and animals are reviewed in this article. Roles of individual desaturases in temperature acclimation and principles of regulation of the desaturase genes are discussed.

Oxygen induces fatty acid (n-6)-desaturation independently of temperature in Acanthamoeba castellanii

Induction of a microsomal oleate delta12 (n-6) desaturase which is mainly responsible for an increase in membrane lipid unsaturation at low temperature has been observed in the free-living amoeba Acanthamoeba castellanii. In this study we show that the enzyme can also be regulated by oxygen independently of temperature in batch cultures grown to O2-limitation. Raising the oxygen concentration from below the lower limit of detection (< 0.1 microM) to approximately air-saturation (230 microM), whilst maintaining the growth temperature constant (30 degrees C), increased lipid unsaturation and elevated n-6-desaturase activity 2.3-fold. Addition of the protein synthesis inhibitor, anisomycin, showed that increased desaturase activity was due to new protein synthesis rather than activation of pre-existing enzyme. These observations are important for future studies of the mechanism of temperature adaptation in poikilotherms.

Biochemical characterization of a delta12 acyl-lipid desaturase after overexpression of the enzyme in Escherichia coli

The Delta12 acyl-lipid desaturase of Synechocystis sp. PCC 6803 was overexpressed in Escherichia coli as an active enzyme. The overexpressed protein was associated with cell membranes; it represented about 10% of the total cellular protein and 25% of the total membrane protein. The enzyme in the membrane fraction exhibited strong fatty-acid desaturase activity. The desaturase in salt-washed membranes was stabilized by the presence of sorbitol. Storage of salt-washed membranes in 2 M sorbitol at 4 degrees C and at pH 7-8 for six days resulted in the loss of less than 10% of the desaturase activity. The desaturase activity had a positive temperature coefficient, a result that suggests that the increase in the desaturation of fatty acids at low temperature might not be caused by the activation of desaturases at low temperature but, rather, by the increased synthesis of desaturases de novo.

The biosynthesis of omega 3 fatty acids from 18:2 omega 6 in Artemia spp

To examine the biosynthesis of omega 3 fatty acids from 18:2 omega 6, reference Artemia (RAC III, Artemia Reference Center, Gent, Belgium) were fed rice bran supplemented with [1(-14)C]18:2 omega 6 for 2 days following 48 hours starvation since cyst hydration and 1, 2, or 3 days following 72 hours of starvation, under axenic conditions. Artemia fatty acids were analyzed by gas chromatography, AgNO3 thin layer chromatography and high performance liquid chromatography and fatty acid fractions were collected for radioisotope counts. No significant differences were observed in the omega 3 fatty acid content of Artemia cultured under axenic and xenic conditions. Radioisotope studies showed that radioactivity from [1(-14)C]18:2 omega 6 was incorporated into other fatty acids, including 18:3 omega 3, 18:4 omega 3 and 20:5 omega 3. The conversion rate was less than 5% for the two day feeding period. In the three day feeding experiment, the amount of radioactivity recovered in both 18:3 omega 3 and 20:5 omega 3 increased by 2.2- and 1.8-fold, respectively, over the three day feeding period. These results demonstrated that Artemia synthesized these omega 3 polyunsaturated fatty acids from 18:2 omega 6.

Temperature-dependent changes in plasma-membrane lipid order and the phagocytotic activity of the amoeba Acanthamoeba castellanii are closely correlated

The relationship between temperature-dependent changes in phagocytotic activity of Acanthamoeba castellanii and the fatty acid composition and physical properties of plasma membrane-enriched fractions were determined in cultures acclimated to 30 degrees C and 15 degrees C. Chilling (from 30 degrees C to 15 degrees C) had a very pronounced short-term inhibitory effect on phagocytosis only in stationary-phase cultures, which displayed a low degree of fatty acid unsaturation. A subsequent increase in phagocytosis by these cells was correlated with a low-temperature-induced increase in fatty acid unsaturation (shown previously [Jones, Lloyd and Harwood (1993) Biochem. J. 296, 183-188] to be due to n-6 desaturase induction). Plasma membrane-enriched fractions from 15 degrees C-acclimated cells also showed a marked increase in the relative proportion of polyunsaturated fatty acids. Steady-state fluorescence depolarization studies, using the membrane probe diphenylhexatriene, revealed increases in plasma membrane order with decreasing assay temperature. Over the upper assay-temperature range (25-40 degrees C), fluorescence anisotropy values were higher in membranes from 30 degrees C-acclimated cells; a 3.3 degrees C relative displacement of plots indicated that temperature-induced changes in membrane lipid composition compensated for approx. 22% of the ordering effect of low temperature. Changes in the temperature-dependence of fluorescence anisotropy, possibly corresponding to lateral phase separations or alterations in other bulk physical properties of membranes, occurred between 20 and 25 degrees C in membranes from 30 degrees C-acclimated cells and between 15 and 20 degrees C in membranes from 15 degrees C-acclimated cells. Fluorescence anisotropy plots were superimposed at assay temperatures between 5 and 15 degrees C. Short-term phagocytotic rates in whole cells decreased with assay temperature. Arrhenius discontinuities in rates of phagocytosis occurred at approx. 25.0 degrees C and 17.5 degrees C in 30 degrees C- and 15 degrees C-acclimated cells respectively, and in each case were thus within the temperature ranges of slope-change in the corresponding fluorescence anisotropy plots. The results show a direct correlation between plasma membrane fatty acid unsaturation, membrane physical properties and phagocytotic activity in A. castellanii. Therefore, a specific integrated physiological process has been correlated with fatty acid desaturase induction for the first time.

Acyl-lipid desaturases and their importance in the tolerance and acclimatization to cold of cyanobacteria

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