MFE1, a member of the peroxisomal hydroxyacyl coenzyme A dehydrogenase family, affects fatty acid metabolism necessary for morphogenesis in Dictyostelium spp

Biotechnological production of omega-3 fatty acids: current status and future perspectives

Omega-3 fatty acids, including alpha-linolenic acids (ALA), eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA), have shown major health benefits, but the human body's inability to synthesize them has led to the necessity of dietary intake of the products. The omega-3 fatty acid market has grown significantly, with a global market from an estimated USD 2.10 billion in 2020 to a predicted nearly USD 3.61 billion in 2028. However, obtaining a sufficient supply of high-quality and stable omega-3 fatty acids can be challenging. Currently, fish oil serves as the primary source of omega-3 fatty acids in the market, but it has several drawbacks, including high cost, inconsistent product quality, and major uncertainties in its sustainability and ecological impact. Other significant sources of omega-3 fatty acids include plants and microalgae fermentation, but they face similar challenges in reducing manufacturing costs and improving product quality and sustainability. With the advances in synthetic biology, biotechnological production of omega-3 fatty acids via engineered microbial cell factories still offers the best solution to provide a more stable, sustainable, and affordable source of omega-3 fatty acids by overcoming the major issues associated with conventional sources. This review summarizes the current status, key challenges, and future perspectives for the biotechnological production of major omega-3 fatty acids.

Fat-containing cells are eliminated during Dictyostelium development

Triacylglycerol is a universal storage molecule for metabolic energy in living organisms. However, Dictyostelium amoebae, that have accumulated storage fat from added fatty acids do not progress through the starvation period preceding the development of the durable spore. Mutants deficient in genes of fat metabolism, such as fcsA, encoding a fatty acid activating enzyme, or dgat1 and dgat2, specifying proteins that synthesize triacylglycerol, strongly increase their chances to contribute to the spore fraction of the developing fruiting body, but lose the ability to produce storage fat efficiently. Dictyostelium seipin, an orthologue of a human protein that in patients causes the complete loss of adipose tissue when mutated, does not quantitatively affect fat storage in the amoeba. Dictyostelium seiP knockout mutants have lipid droplets that are enlarged in size but reduced in number. These mutants are as vulnerable as the wild type when exposed to fatty acids during their vegetative growth phase, and do not efficiently enter the spore head in Dictyostelium development.

Summary: In contrast to many living organisms, storage fat is not beneficial for Dictyostelium cells when progressing through starvation and subsequent development of a dormant stage.

Artesunate Activates the Intrinsic Apoptosis of HCT116 Cells through the Suppression of Fatty Acid Synthesis and the NF-κB Pathway

The artemisinin compounds, which are well-known for their potent therapeutic antimalarial activity, possess in vivo and in vitro antitumor effects. Although the anticancer effect of artemisinin compounds has been extensively reported, the precise mechanisms underlying its cytotoxicity remain under intensive study. In the present study, a high-throughput quantitative proteomics approach was applied to identify differentially expressed proteins of HCT116 colorectal cancer cell line with artesunate (ART) treatment. Through Ingenuity Pathway Analysis, we discovered that the top-ranked ART-regulated biological pathways are abrogation of fatty acid biosynthetic pathway and mitochondrial dysfunction. Subsequent assays showed that ART inhibits HCT116 cell proliferation through suppressing the fatty acid biosynthetic pathway and activating the mitochondrial apoptosis pathway. In addition, ART also regulates several proteins that are involved in NF-κB pathway, and our subsequent assays showed that ART suppresses the NF-κB pathway. These proteomic findings will contribute to improving our understanding of the underlying molecular mechanisms of ART for its therapeutic cytotoxic effect towards cancer cells.

Crosstalk between mitochondria and peroxisomes

Mitochondria and peroxisomes are small ubiquitous organelles. They both play major roles in cell metabolism, especially in terms of fatty acid metabolism, reactive oxygen species (ROS) production, and ROS scavenging, and it is now clear that they metabolically interact with each other. These two organelles share some properties, such as great plasticity and high potency to adapt their form and number according to cell requirements. Their functions are connected, and any alteration in the function of mitochondria may induce changes in peroxisomal physiology. The objective of this paper was to highlight the interconnection and the crosstalk existing between mitochondria and peroxisomes. Special emphasis was placed on the best known connections between these organelles: origin, structure, and metabolic interconnections.

Dictyostelium acetoacetyl-CoA thiolase is a dual-localizing enzyme that localizes to peroxisomes, mitochondria and the cytosol

Acetoacetyl-CoA thiolase is an enzyme that catalyses both the CoA-dependent thiolytic cleavage of acetoacetyl-CoA and the reverse condensation reaction. In Dictyostelium discoideum, acetoacetyl-CoA thiolase (DdAcat) is encoded by a single acat gene. The aim of this study was to assess the localization of DdAcat and to determine the mechanism of its cellular localization. Subcellular localization of DdAcat was investigated using a fusion protein with GFP, and it was found to be localized to peroxisomes. The findings showed that the targeting signal of DdAcat to peroxisomes is a unique nonapeptide sequence (15RMYTTAKNL23) similar to the conserved peroxisomal targeting signal-2 (PTS-2). Cell fractionation experiments revealed that DdAcat also exists in the cytosol. Distribution to the cytosol was caused by translational initiation from the second Met codon at position 16. The first 18 N-terminal residues also exhibited function as a mitochondrial targeting signal (MTS). These results indicate that DdAcat is a dual-localizing enzyme that localizes to peroxisomes, mitochondria and the cytosol using both PTS-2 and MTS signals, which overlap each other near the N-terminus, and the alternative utilization of start codons.

Lipid droplet dynamics at early stages of Mycobacterium marinum infection in Dictyostelium

Lipid droplets exist in virtually every cell type, ranging not only from mammals to plants, but also to eukaryotic and prokaryotic unicellular organisms such as Dictyostelium and bacteria. They serve among other roles as energy reservoir that cells consume in times of starvation. Mycobacteria and some other intracellular pathogens hijack these organelles as a nutrient source and to build up their own lipid inclusions. The mechanisms by which host lipid droplets are captured by the pathogenic bacteria are extremely poorly understood. Using the powerful Dictyostelium discoideum/Mycobacterium marinum infection model, we observed that, immediately after their uptake, lipid droplets translocate to the vicinity of the vacuole containing live but not dead mycobacteria. Induction of lipid droplets in Dictyostelium prior to infection resulted in a vast accumulation of neutral lipids and sterols inside the bacterium-containing compartment. Subsequently, under these conditions, mycobacteria accumulated much larger lipid inclusions. Strikingly, the Dictyostelium homologue of perilipin and the murine perilipin 2 surrounded bacteria that had escaped to the cytosol of Dictyostelium or microglial BV-2 cells respectively. Moreover, bacterial growth was inhibited in Dictyostelium plnA knockout cells. In summary, our results provide evidence that mycobacteria actively manipulate the lipid metabolism of the host from very early infection stages.

Dictyostelium discoideum Dgat2 can substitute for the essential function of Dgat1 in triglyceride production but not in ether lipid synthesis

Triacylglycerol (TAG), the common energy storage molecule, is formed from diacylglycerol and a coenzyme A-activated fatty acid by the action of an acyl coenzyme A:diacylglycerol acyltransferase (DGAT). In order to conduct this step, most organisms rely on more than one enzyme. The two main candidates in Dictyostelium discoideum are Dgat1 and Dgat2. We show, by creating single and double knockout mutants, that the endoplasmic reticulum (ER)-localized Dgat1 enzyme provides the predominant activity, whereas the lipid droplet constituent Dgat2 contributes less activity. This situation may be opposite from what is seen in mammalian cells. Dictyostelium Dgat2 is specialized for the synthesis of TAG, as is the mammalian enzyme. In contrast, mammalian DGAT1 is more promiscuous regarding its substrates, producing diacylglycerol, retinyl esters, and waxes in addition to TAG. The Dictyostelium Dgat1, however, produces TAG, wax esters, and, most interestingly, also neutral ether lipids, which represent a significant constituent of lipid droplets. Ether lipids had also been found in mammalian lipid droplets, but the role of DGAT1 in their synthesis was unknown. The ability to form TAG through either Dgat1 or Dgat2 activity is essential for Dictyostelium to grow on bacteria, its natural food substrate.

Dictyostelium lipid droplets host novel proteins

Across all kingdoms of life, cells store energy in a specialized organelle, the lipid droplet. In general, it consists of a hydrophobic core of triglycerides and steryl esters surrounded by only one leaflet derived from the endoplasmic reticulum membrane to which a specific set of proteins is bound. We have chosen the unicellular organism Dictyostelium discoideum to establish kinetics of lipid droplet formation and degradation and to further identify the lipid constituents and proteins of lipid droplets. Here, we show that the lipid composition is similar to what is found in mammalian lipid droplets. In addition, phospholipids preferentially consist of mainly saturated fatty acids, whereas neutral lipids are enriched in unsaturated fatty acids. Among the novel protein components are LdpA, a protein specific to Dictyostelium, and Net4, which has strong homologies to mammalian DUF829/Tmem53/NET4 that was previously only known as a constituent of the mammalian nuclear envelope. The proteins analyzed so far appear to move from the endoplasmic reticulum to the lipid droplets, supporting the concept that lipid droplets are formed on this membrane.

Farnesyl diphosphate synthase, the target for nitrogen-containing bisphosphonate drugs, is a peroxisomal enzyme in the model system Dictyostelium discoideum

NBP (nitrogen-containing bisphosphonate) drugs protect against excessive osteoclast-mediated bone resorption. After binding to bone mineral, they are taken up selectively by the osteoclasts and inhibit the essential enzyme FDPS (farnesyl diphosphate synthase). NBPs inhibit also growth of amoebae of Dictyostelium discoideum in which their target is again FDPS. A fusion protein between FDPS and GFP (green fluorescent protein) was found, in D. discoideum, to localize to peroxisomes and to confer resistance to the NBP alendronate. GFP was also directed to peroxisomes by a fragment of FDPS comprising amino acids 1–22. This contains a sequence of nine amino acids that closely resembles the nonapeptide PTS2 (peroxisomal targeting signal type 2): there is only a single amino acid mismatch between the two sequences. Mutation analysis confirmed that the atypical PTS2 directs FDPS into peroxisomes. Furthermore, expression of the D. discoideum FDPS–GFP fusion protein in strains of Saccharomyces cerevisiae defective in peroxisomal protein import demonstrated that import of FDPS into peroxisomes was blocked in a strain lacking the PTS2-dependent import pathway. The peroxisomal location of FDPS in D. discoideum indicates that NBPs have to cross the peroxisomal membrane before they can bind to their target.

Peroxisomal ABC transporters: structure, function and role in disease

ATP-binding cassette (ABC) transporters belong to one of the largest families of membrane proteins, and are present in almost all living organisms from eubacteria to mammals. They exist on plasma membranes and intracellular compartments such as the mitochondria, peroxisomes, endoplasmic reticulum, Golgi apparatus and lysosomes, and mediate the active transport of a wide variety of substrates in a variety of different cellular processes. These include the transport of amino acids, polysaccharides, peptides, lipids and xenobiotics, including drugs and toxins. Three ABC transporters belonging to subfamily D have been identified in mammalian peroxisomes. The ABC transporters are half-size and assemble mostly as a homodimer after posttranslational transport to peroxisomal membranes. ABCD1/ALDP and ABCD2/ALDRP are suggested to be involved in the transport of very long chain acyl-CoA with differences in substrate specificity, and ABCD3/PMP70 is involved in the transport of long and branched chain acyl-CoA. ABCD1 is known to be responsible for X-linked adrenoleukodystrophy (X-ALD), an inborn error of peroxisomal β-oxidation of very long chain fatty acids. Here, we summarize recent advances and important points in our advancing understanding of how these ABC transporters target and assemble to peroxisomal membranes and perform their functions in physiological and pathological processes, including the neurodegenerative disease, X-ALD.

Fusion and fission, the evolution of sterol carrier protein-2

Sterol carrier protein-2 (SCP-2) is an intracellular, small, basic protein domain that in vitro enhances the transfer of lipids between membranes. It is expressed in bacteria, archaea, and eukaryotes. There are five human genes, HSD17B4, SCPX, HSDL2 STOML1, and C20orf79, which encode SCP-2. HSD17B4, SCPX, HSDL2, and STOML1 encode fusion proteins with SCP-2 downstream of another protein domain, whereas C20orf79 encodes an unfused SCP-2. We have extracted SCP-2 domains from databases and analyzed the evolution of the eukaryotic SCP-2. We show that SCPX and HSDL2 are present in most animals from Cnidaria to Chordata. STOML1 are present in nematodes and more advanced animals. HSD17B4 which encodes a D-bifunctional protein (DBP) with domains for D-3-hydroxyacyl-CoA dehydrogenase, enoyl-CoA hydratase, and SCP-2 are found in animals from insects to mammals and also in fungi. Nematodes, amoebas, ciliates, apicomplexans, and oomycetes express an alternative DBP with the SCP-2 domain directly connected to the D-3-hydroxyacyl-CoA dehydrogenase. This fusion has not been retained in plant genomes, which solely express unfused SCP-2 domains. Proteins carrying unfused SCP-2 domains are also encoded in bacteria, archaea, ciliates, fungi, insects, nematodes, and vertebrates. Our results indicate that the fusion between D-3-hydroxyacyl-CoA dehydrogenase and SCP-2 was formed early during eukaryotic evolution. There have since been several gene fission events where genes encoding unfused SCP-2 domains have been formed, as well as gene fusion events placing the SCP-2 domain in novel protein domain contexts.

Defect in peroxisomal multifunctional enzyme MFE1 affects cAMP relay in Dictyostelium

We have previously reported that cells of Dictyostelium discoideum lacking the fatty acid oxidation enzyme MFE1 accumulate excess cyclopropane fatty acids from ingested bacteria. Cells in which mfeA(-) is disrupted fail to develop when grown in association with bacteria but form normal fruiting bodies when grown in axenic media. Bacterially grown mfeA(-) cells express the genes for the cyclic AMP (cAMP) receptor (carA) and adenylyl cyclase (acaA) but fail to respond to a cAMP pulse by synthesis of additional cAMP which normally relays the signal. Moreover, they do not accumulate the adhesion protein, gp80, which is encoded by the cAMP-induced gene, csaA. As a consequence, they do not acquire developmentally regulated EDTA-resistant cell-cell adhesion. When mutant cells are mixed with wild-type cells and allowed to develop together, they co-aggregate and differentiate into both spores and stalk cells. Thus, most of the developmental consequences of excess cyclopropane fatty acids appear to result from impaired cAMP relay.

Temperature adaptation in Dictyostelium: role of Delta5 fatty acid desaturase

Membrane fluidity is critical for proper membrane function and is regulated in part by the proportion of unsaturated fatty acids present in membrane lipids. The proportion of these lipids in turn varies with temperature and may contribute to temperature adaptation in poikilothermic organisms. The fundamental question posed in this study was whether the unsaturation of fatty acids contributes to the ability to adapt to temperature stress in Dictyostelium. First, fatty acid composition was analysed and it was observed that the relative proportions of dienoic acids changed with temperature. To investigate the role of dienoic fatty acids in temperature adaptation, null mutants were created in the two known Delta5 fatty acid desaturases (FadA and FadB) that are responsible for the production of dienoic fatty acids. The fadB null mutant showed no significant alteration in fatty acid composition or in phenotype. However, the disruption of fadA resulted in a large drop in dienoic fatty acid content from 51.2 to 4.1 % and a possibly compensatory increase in monoenoic fatty acids (40.9-92.4 %). No difference was detected in temperature adaptation with that of wild-type cells during the growth phase. However, surprisingly, mutant cells developed more efficiently than the wild-type at elevated temperatures. These results show that the fatty acid composition of Dictyostelium changes with temperature and suggest that the regulation of dienoic fatty acid synthesis is involved in the development of Dictyostelium at elevated temperatures, but not during the growth phase.

Plants Express a Lipid Transfer Protein with High Similarity to Mammalian Sterol Carrier Protein-2

This is the first report describing the cloning and characterization of sterol carrier protein-2 (SCP-2) from plants. Arabidopsis thaliana SCP-2 (AtSCP-2) consists of 123 amino acids with a molecular mass of 13.6 kDa. AtSCP-2 shows 35% identity and 56% similarity to the human SCP-2-like domain present in the human D-bifunctional protein (DBP) and 30% identity and 54% similarity to the human SCP-2 encoded by SCP-X. The presented structural models of apo-AtSCP-2 and the ligand-bound conformation of AtSCP-2 reveal remarkable similarity with two of the structurally known SCP-2s, the SCP-2-like domain of human DBP and the rabbit SCP-2, correspondingly. The AtSCP-2 models in both forms have a similar hydrophobic ligand-binding tunnel, which is extremely suitable for lipid binding. AtSCP-2 showed in vitro transfer activity of BODIPY-phosphatidylcholine (BODIPY-PC) from donor membranes to acceptor membranes. The transfer of BODIPY-PC was almost completely inhibited after addition of 1-palmitoyl 2-oleoyl phosphatidylcholine or ergosterol. Dimyristoyl phosphatidic acid, stigmasterol, steryl glucoside, and cholesterol showed a moderate to marginal ability to lower the BODIPY-PC transfer rate, and the single chain palmitic acid and stearoyl-coenzyme A did not affect transfer at all. Expression analysis showed that AtSCP-2 mRNA is accumulating in most plant tissues. Plasmids carrying fusion genes between green fluorescent protein and AtSCP-2 were transformed with particle bombardment to onion epidermal cells. The results from analyzing the transformants indicate that AtSCP-2 is localized to peroxisomes.

Disruption of the peroxisomal citrate synthase CshA affects cell growth and multicellular development in Dictyostelium discoideum

Non-mitochondrial citrate synthase catalyses citrate synthesis in the glyoxylate cycle in gluconeogenesis. Screening Dictyostelium discoideum mutants generated by insertional mutagenesis isolated a poor-growing mutant that displayed aberrant developmental morphology on bacterial lawns. Axenically grown mutants developed normally and formed mature fruiting bodies on buffered agar. The affected locus encoded a novel protein (CshA) that was homologous to glyoxysomal citrate synthase. cshA was expressed maximally during vegetative growth and gradually decreased through subsequent developmental stages. An in vitro citrate synthase assay revealed that cshA disruption resulted in a 50% reduction in enzyme activity, implicating CshA as an active citrate synthase. The amino-terminus of CshA was found to have an atypical mitochondrial targeting signal, instead containing a unique nonapeptide sequence (RINILANHL) that was homologous to the conserved peroxisomal targeting signal 2 (PTS2). CshA protein was shown to be localized in the peroxisomes, and the RINILANHL sequence only efficiently targeted the peroxisomal green fluorescent protein. The growth defect of cshA(-) cells was associated with the impairment of phagocytosis and fluid-phase endocytosis, independent from cytokinesis. Disrupted multicellular development on bacterial lawns resulted from the abnormal susceptibility to the environmental conditions, perhaps because of citrate insufficiency. Taken together, these results provide new insights into the function of peroxisomal citrate synthase in cell growth and multicellular development.