Metabolic Engineering of Mortierella alpina for Enhanced Arachidonic Acid Production through the NADPH-Supplying Strategy

AMP deaminase: A crucial regulator in nitrogen stress and lipid metabolism in Mucor circinelloides

Lipid biosynthesis is a significant metabolic response to nitrogen starvation in oleaginous fungi. The oleaginous fungus Mucor circinelloides copes with nitrogen stress by degrading AMP through AMP deaminase (AMPD). However, the mechanism of AMPD in regulating lipogenesis remains largely unclear. To elucidate the mechanism of AMPD in lipid synthesis in this M. circinelloides, we identified two genes (ampd1 and ampd2) encoding AMPD and constructed an ampd double knockout mutant. The engineered M. circinelloides strain elevated cell growth and lipid accumulation, as well as the content of oleic acid (OA) and gamma-linolenic acid (GLA). In addition to the expected increase in transcription levels of genes associated with lipid and TAG synthesis, we observed suppression of lipid degradation and reduced amino acid biosynthesis. This suggested that the deletion of AMPD genes induces the redirection of carbon towards lipid synthesis pathways. Moreover, the pathways related to nitrogen metabolism, including nitrogen assimilation and purine metabolism (especially energy level), were also affected in order to maintain homeostasis. Further analysis discovered that the transcription factors (TFs) related to lipid accumulation were also regulated. This study provides new insights into lipid biosynthesis in M. circinelloides, indicating that the trigger for lipid accumulation is not entirely AMPD-dependent and suggest that there may be additional mechanisms involved in the initiation of lipogenesis.

NADP+-dependent isocitrate dehydrogenase as a novel target for altering carbon flux to lipid accumulation and enhancing antioxidant capacity in Tetradesmus obliquus

Regulatory complexities in lipogenesis hinder the harmonization of metabolic carbon precursors towards lipid synthesis. Exploring regulatory complexities in lipogenesis, this study identifies NADP+-dependent isocitrate dehydrogenase (IDH) in Tetradesmus obliquus as a key factor. Overexpression IDH in strains ToIDH-1 and ToIDH-2 resulted in a 1.69 and 1.64-fold increase in neutral lipids, respectively, compared to the wild type, with lipid yield reaching 234.56 and 227.17 mg/L. Notably, despite slower growth, the cellular biomass augmented to 790.67 mg/L. Metabolite analysis indicated a shift in carbon precursors from protein to lipid and carbohydrate synthesis. Morphological observations revealed increases in the volume and number of lipid droplets, alongside a change in the fatty acid profile favoring monounsaturated and saturated fatty acids. Furthermore, IDH overexpression enhanced NADPH production and antioxidant activity, thereby further boosting lipid accumulation when combined with salt stress. This study suggests a pathway for improved lipogenesis and algal growth via metabolic engineering.

Fructose corn syrup induces inflammatory injury and obesity by altering gut microbiota and gut microbiota-related arachidonic acid metabolism

Excessive fructose corn syrup (FCS) intake brings a series of health problems. The aim of the present study was to explore the mechanism of FCS-induced metabolic disorders from the perspective of gut microbiota. Mice were fed for 16 weeks with normal or 30% FCS drinking water. Compared to the control group, FCS caused significantly higher fat deposition, hepatic steatosis, liver and intestinal inflammatory damages (P<.05). FCS increased the abundance of Muribaculaceae in vivo and in vitro, which was positively correlated with the indices of metabolic disorders (P<.05). In vivo and in vitro data indicated that FCS enhanced the microbial function involved in pentose phosphate pathway and arachidonic acid metabolism, metabolomics further demonstrated that FCS led to an increase in prostaglandins (the catabolites of arachidonic acid) (P<.05). Our study confirmed that FCS can directly promote gut microbiota to synthesize inflammatory factor prostaglandins, which provides new insights and directions for the treatment of FCS-induced metabolic disorders and inflammation.

Role of AMP Deaminase in Mucor circinelloides: Implications for Nitrogen Utilization and Lipid Biosynthesis

Lipid accumulation in oleaginous organisms is initiated by AMP deaminase (AMPD) after nitrogen depletion because it mediates the concentration of intracellular adenosine monophosphate (AMP). However, the role of AMPD in lipogenesis in the oleaginous fungus Mucor circinelloides is largely unknown. Therefore, we identified the genes (ampd1 and ampd2) encoding AMPD and investigated the role of AMPD in lipid synthesis in this fungus by overexpressing and deleting ampd genes. Deletion of ampd1 and ampd2 caused 21 and 28% increments in lipid contents under N-limited conditions, respectively. These increases were correlated with the activation of enzymes involved in lipogenesis and the alteration of energy balance. Unexpectedly, overexpression of ampd genes affected nitrogen consumption in both N-limited and N-excess media, which resulted in an increase in cell growth and lipid accumulation compared with the control strain when nitrogen was available. Furthermore, the increased lipid accumulation in the ampd-overexpressing mutants in N-excess media was accompanied by enhanced activities of lipid biosynthetic enzymes. These data suggested that nitrogen metabolism and energy metabolism are affected by AMPD, and overexpression of ampd genes induced lipid accumulation under nitrogen-rich conditions by mimicking the nitrogen limitation response. This highlights an intriguing function of AMPD in M. circinelloides.

Modular Metabolic Engineering of Mortierella alpina by the 2A Peptide Platform to Improve Arachidonic Acid Production

Arachidonic acid (ARA) is an essential fatty acid in human nutrition. Mortierella alpina, a filamentous fungus, has been widely used for the production of ARA. Here, we report a modular engineering approach that systematically eliminates metabolic bottlenecks in the multigene elongase/desaturase pathway and has led to significant improvements of the ARA titer. The elongase/desaturase pathway in Mortierella alpina was recast into two modules, namely, push and pull modules, based on its function in the ARA synthesis. Combinatorial optimization of these two modules has balanced the production and consumption of intermediate metabolites. A 2A peptide-based facile assembly platform that can achieve multigene expression as a polycistron was first established. The platform was then applied to express the push and pull modules in Mortierella alpina. In the shake-flask fermentation, the lipid and ARA contents of the engineered strain MA5 were increased by 1.2-fold and 77.6%, respectively, resulting in about fivefold increase of the ARA yield. The final ARA titer reached 4.4 g L-1 in shake-flask fermentation. The modular engineering strategies presented in this study demonstrate a generalized approach for the engineering of cell factories in the production of valuable metabolites.

Redistributing Carbon Flux by Impairing Saccharide Synthesis to Enhance Lipid Yield in Oleaginous Fungus Mortierella alpina

Increasing carbon flux toward target metabolites is important in improving microbial productivity and economic value. To improve the efficiency of lipid production in Mortierella alpina, we knocked down genes for trehalose-6-phosphate synthetase (Matps) and phosphoenolpyruvate carboxykinase (Mapepck) in the major pathways for saccharide synthesis. The knockdown of Matps reduced trehalose content by an average of 31.87%, while the knockdown of Mapepck reduced the total saccharide content by 28.6%, and both recombinant strains showed more than 20% increased lipid yield. Trehalose plays a vital role in stress resistance, but a higher polyunsaturated fatty acid-rich lipid content was found to partly compensate for the loss of trehalose after Matps knockdown. As compared with Matps knockdown, the knockdown of Mapepck gave better lipid production by bringing forward the time to maximum lipid yield by three days in a scale-up test. The arachidonic acid yield after the Mapepck knockdown reached 1.23 g/L, which was 39.9% higher than that of the original strain. The present research provided an efficient strategy for redistributing carbon flux among different metabolites and therefore promoted microbial lipid yield in a shorter fermentation period.

Overexpression of Citrate-Malate Carrier Promoted Lipid Accumulation in Oleaginous Filamentous Fungus Mortierella alpina

The mitochondrial citrate-malate carrier is responsible for the transport of citrate and malate between the mitochondria and cytosol, ensuring citrate supply substrate for fatty acid synthesis. In this study, we investigated the overexpression of the citrate-malate carrier coded by three genes (MaCT1/MaCT2/MaTCT) in Mortierella alpina to enhance lipid accumulation. Our results showed that the overexpression of MaCT1, MaCT2, and MaTCT increased the fatty acid content by up to 21.7, 29.5, and 12.8%, respectively, compared with the control strain, but had no effect on the growth. Among them, the MaCT2-overexpressing strain performed the best, and its total fatty acid yield was increased by 51.6% compared to the control. Furthermore, the relative transcription level of MaCT2 indeed increased significantly in the recombinant strains. These findings are beneficial to understanding the citrate transport system and improve the industrial applications of the oleaginous filamentous fungus M. alpina.

Potential of Mortierellaceae for polyunsaturated fatty acids production: mini review

The health benefits of polyunsaturated fatty acids (PUFAs) have encouraged the search for rich sources of these compounds. However, the supply chain of PUFAs from animals and plants presents environmental concerns, such as water pollution, deforestation, animal exploitation and interference in the trophic chain. In this way, a viable alternative has been found in microbial sources, mainly in single cell oil (SCO) production by yeast and filamentous fungi. Mortierellaceae is a filamentous fungal family world-renowned for PUFA-producing strains. For example, Mortierella alpina can be highlighted due to be industrially applied to produce arachidonic acid (20:4 n6), an important component of infant supplement formulas. Thus, the state of the art of strategies to increase PUFAs production by Mortierellaceae strains is presented in this review. Firstly, we have discussed main phylogenetic and biochemical characteristics of these strains for lipid production. Next, strategies based on physiological manipulation, using different carbon and nitrogen sources, temperature, pH and cultivation methods, which can increase PUFA production by optimizing process parameters are presented. Furthermore, it is possible to use metabolic engineering tools, controlling the supply of NADPH and co-factors, and directing the activity of desaturases and elongase to the target PUFA. Thus, this review aims to discuss the functionality and applicability of each of these strategies, in order to support future research for PUFA production by Mortierellaceae species.

Construction of Glucose-6-Phosphate Dehydrogenase Overexpression Strain of Schizochytrium sp. H016 to Improve Docosahexaenoic Acid Production

Docosahexaenoic acid (DHA) is an important omega-3 polyunsaturated fatty acid (PUFA) that plays a critical physiological role in human health. Schizochytrium sp. is considered an excellent strain for DHA production, but the synthesis of DHA is limited by the availability of nicotinamide adenine dinucleotide phosphate (NADPH). In this study, the endogenous glucose-6-phosphate dehydrogenase (G6PD) gene was overexpressed in Schizochytrium sp. H016. Results demonstrated that G6PD overexpression increased the availability of NADPH, which ultimately altered the fatty acid profile, resulting in a 1.91-fold increase in DHA yield (8.81 g/L) and increased carbon flux by shifting it from carbohydrate and protein synthesis to lipid production. Thus, G6PD played a vital role in primary metabolism. In addition, G6PD significantly increased DHA content and lipid accumulation by 31.47% and 40.29%, respectively. The fed-batch fermentation experiment results showed that DHA production reached 17.01 g/L in the overexpressing G6PD strain. These results elucidated the beneficial effects of NADPH on the synthesis of PUFA in Schizochytrium sp. H016, which may be a potential target for metabolic engineering. Furthermore, this study provides a promising regulatory strategy for the large-scale production of DHA in Schizochytrium sp.

The Lipid Profile of the Endomyces magnusii Yeast upon the Assimilation of the Substrates of Different Types and upon Calorie Restriction

The study analyzes the dynamics in the lipid profile of the Endomyces magnusii yeast during the long-lasting cultivation using the substrates of “enzymatic” or “oxidative” type. Moreover, we studied its changes upon calorie restriction (CR) (0.5% glucose) and glucose depletion (0.2% glucose). Di-(DAGs), triacylglycerides (TAGs) and free fatty acids (FFAs) dominate in the storage lipid fractions. The TAG level was high in all the cultures tested and reached 80% of the total lipid amount. While being cultured on 2% substrates, the level of storage lipids decreased at the four-week stage, whereas upon CR their initially low amount doubled. Phosphatidylethanolamines (PE), sterols (St) (up to 62% of total lipids), phosphatidylcholines (PC), and phosphatidic acids (PA) (more than 40% of total lipids) were dominating in the membrane lipids of E magnusii. Upon CR at the late stationary growth stages (3–4 weeks), the total level of membrane lipid was two-fold higher than those on glycerol and 2% glucose. The palmitic acid C16:0 (from 10 to 23%), the palmitoleic acid C16:1 (from 4.3 to 15.9%), the oleic acid C18:1 (from 23.4 to 59.2%), and the linoleic acid C18:2 (from 10.8 to 49.2%) were the dominant fatty acids (FAs) of phospholipids. Upon glucose depletion (0.2% glucose), the total amount of storage and membrane lipids in the cells was comparable to that in the cells both on 2% and 0.5% glucose. High levels of PC and sphingolipids (SL) at the late stationary growth stages and an increased PA level throughout the whole experiment were typical for the membrane lipids composition upon the substrate depletion. There was shown a crucial role of St, PA, and a high share of the unsaturated FAs in the membrane phospholipids upon the adaptation of the E. magnusii yeast to the long-lasting cultivation upon the substrate restriction is shown. The autophagic processes in some fractions of the cell population provide the support of high level of lipid components at the late stages of cultivation upon substrate depletion under the CR conditions. CR is supposed to play the key role in regulating the lipid synthesis and risen resistance to oxidative stress, as well as its possible biotechnological application.

Recent Development of Advanced Biotechnology in the Oleaginous Fungi for Arachidonic Acid Production

Arachidonic acid is an essential ω-6 polyunsaturated fatty acid, which plays a significant role in cardiovascular health and neurological development, leading to its wide use in the food and pharmaceutical industries. Traditionally, ARA is obtained from deep-sea fish oil. However, this source is limited by season and is depleting the already threatened global fish stocks. With the rapid development of synthetic biology in recent years, oleaginous fungi have gradually attracted increasing attention as promising microbial sources for large-scale ARA production. Numerous advanced technologies including metabolic engineering, dynamic regulation of fermentation conditions, and multiomics analysis were successfully adapted to increase ARA synthesis. This review summarizes recent advances in the bioengineering of oleaginous fungi for ARA production. Finally, perspectives for future engineering approaches are proposed to further improve the titer yield and productivity of ARA.

Production, Biosynthesis, and Commercial Applications of Fatty Acids From Oleaginous Fungi

Oleaginous fungi (including fungus-like protists) are attractive in lipid production due to their short growth cycle, large biomass and high yield of lipids. Some typical oleaginous fungi including Galactomyces geotrichum, Thraustochytrids, Mortierella isabellina, and Mucor circinelloides, have been well studied for the ability to accumulate fatty acids with commercial application. Here, we review recent progress toward fermentation, extraction, of fungal fatty acids. To reduce cost of the fatty acids, fatty acid productions from raw materials were also summarized. Then, the synthesis mechanism of fatty acids was introduced. We also review recent studies of the metabolic engineering strategies have been developed as efficient tools in oleaginous fungi to overcome the biochemical limit and to improve production efficiency of the special fatty acids. It also can be predictable that metabolic engineering can further enhance biosynthesis of fatty acids and change the storage mode of fatty acids.

Characterization and Molecular Mechanism of a Novel Cytochrome b5 Reductase with NAD(P)H Specificity from Mortierella alpina

The electron-transfer capabilities of cytochrome b₅ reductase (Cyt b₅R) and NADPH supply have been shown to be critical factors in microbial fatty acid synthesis. Unfortunately, Cyt b₅R substrate specificity is limited to the coenzyme NADH. In this study, we discovered that a novel Cyt b₅R from Mortierella alpina (MaCytb5RII) displays affinity for NADPH and NADH. The enzymatic characteristics of high-purity MaCytb5RII were determined with the K_m,NADPH and K_m,NADH being 0.42 and 0.07 mM, respectively. MaCytb5RII shows high specific activity at 4 °C and pH 9.0. We anchored the residues that interacted with the coenzymes using the homology models of MaCytb5Rs docking NAD(P)H and FAD. The enzyme activity analysis of the purified mutants MaCytb5RII_[S230N], MaCytb5RII_[Y242F], and MaCytb5RII_[S272A] revealed that Ser230 is essential for MaCytb5RII to have dual NAD(P)H dependence, whereas Tyr242 influences MaCytb5RII's NADPH affinity and Ala272 greatly decreases MaCytb5RII's NADH affinity.

NADPH–Cytochrome P450 Reductase Mediates the Fatty Acid Desaturation of ω3 and ω6 Desaturases from Mortierella alpina

Fatty acid desaturases play an important role in maintaining the appropriate structure and function of biological membranes. The biochemical characterization of integral membrane desaturases, particularly ω3 and ω6 desaturases, has been limited by technical difficulties relating to the acquisition of large quantities of purified proteins, and by the fact that functional activities of these proteins were only tested in an NADH-initiated reaction system. The main aim of this study was to reconstitute an NADPH-dependent reaction system in vitro and investigate the kinetic properties of Mortierella alpina ω3 and ω6 desaturases in this system. After expression and purification of the soluble catalytic domain of NADPH–cytochrome P450 reductase, the NADPH-dependent fatty acid desaturation was reconstituted for the first time in a system containing NADPH, NADPH–cytochrome P450 reductase, cytochrome b5, M. alpina ω3 and ω6 desaturase and detergent. In this system, the maximum activity of ω3 and ω6 desaturase was 213.4 ± 9.0 nmol min⁻¹ mg⁻¹ and 10.0 ± 0.5 nmol min⁻¹ mg⁻¹, respectively. The highest k_cat/K_m value of ω3 and ω6 desaturase was 0.41 µM⁻¹ min⁻¹ and 0.09 µM⁻¹ min⁻¹ when using linoleoyl CoA (18:2 ω6) and oleoyl CoA (18:1 ω9) as substrates, respectively. M. alpina ω3 and ω6 desaturases were capable of using NADPH as reductant when mediated by NADPH–cytochrome P450 reductase; although, their efficiency is distinguishable from NADH-dependent desaturation. These results provide insights into the mechanisms underlying ω3 and ω6 fatty acid desaturation and may facilitate the production of important fatty acids in M. alpina.

Enhancement of lipid accumulation and docosahexaenoic acid synthesis in Schizochytrium sp. H016 by exogenous supplementation of sesamol

Schizochytrium sp. is one of the most promising marine oleaginous microorganisms for industrial production of docosahexaenoic acid (DHA). In this study, the exogenous supplementation of 1 mM sesamol to the fermentation medium effectively prevented the peroxidation of polyunsaturated fatty acids in the fermentation process, which thereby significantly increasing the lipid and DHA yield by 53.52% and 78.30%, respectively. The addition of sesamol also increased the total antioxidant capacity of cells and induce the gene expression of polyketide synthase and antioxidant enzyme system. Moreover, the supply of nicotinamide adenine dinucleotide phosphate was regulated by sesamol by inhibiting the malic enzyme activity and promoting the glucose-6-phosphate dehydrogenase activity. Finally, fed-batch fermentation showed that the addition of sesamol significantly enhanced the DHA yield by 90.76%. This study provides an important reference for enhancing the DHA productivity of Schizochytrium sp. in industrial fermentation.

Recent Molecular Tools for the Genetic Manipulation of Highly Industrially Important Mucoromycota Fungi

Mucorales is the largest and most well-studied order of the phylum Mucormycota and is known for its rapid growth rate and various industrial applications. The Mucorales fungi are a fascinating group of filamentous organisms with many uses in research and the industrial and medical fields. They are widely used biotechnological producers of various secondary metabolites and other value-added products. Certain members of Mucorales are extensively used as model organisms for genetic and molecular investigation and have extended our understanding of the metabolisms of other members of this order as well. Compared with other fungal species, our understanding of Mucoralean fungi is still in its infancy, which could be linked to their lack of effective genetic tools. However, recent advancements in molecular tools and approaches, such as the construction of recyclable markers, silencing vectors, and the CRISPR-Cas9-based gene-editing system, have helped us to modify the genomes of these model organisms. Multiple genetic modifications have been shown to generate valuable products on a large scale and helped us to understand the morphogenesis, basic biology, pathogenesis, and host–pathogen interactions of Mucoralean fungi. In this review, we discuss various conventional and modern genetic tools and approaches used for efficient gene modification in industrially important members of Mucorales.

Characterization of NAD+/NADP+-Specific Isocitrate Dehydrogenases From Oleaginous Fungus Mortierella alpina Involved in Lipid Accumulation

Mortierella alpina has a strong capacity for lipid accumulation. Isocitrate dehydrogenase (IDH) plays an important role in affecting the flow of intracellular carbon sources and reducing power NADPH for lipid biosynthesis. In this study, the effect of various IDHs (NAD+- and NADP+-specific) in M. alpina on the lipid accumulation was investigated through homologous overexpression. The results showed that the transcription level and enzyme activity of the IDHs from M. alpina (MaIDHs) in homologous overexpressing strains were higher than those of the control strain, but that their biomass was not significantly different. Among the various NAD+-specific MaIDH1/2/3 overexpression, NAD+-MaIDH3 reduced total lipid content by 12.5%, whereas overexpression NAD+-MaIDH1 and NAD+-MaIDH2 had no effect on fatty acid content. Intracellular metabolites analysis indicated that the overexpression NAD+-MaIDH3 strain had reduced the fatty acid accumulation, due to its greater carbon flux with the tricarboxylic acid cycle and less carbon flux with fatty acid biosynthesis. For the NADP+-MaIDH4/5/6 recombinant strains overexpressing only NADP+-MaIDH4 enhanced the total fatty acid content by 8.2%. NADPH analysis suggested that this increase in lipid accumulation may have been due to the great reducing power NADPH is produced in this recombinant strain. This study provides theoretical basis and guidance for the analysis of the mechanism of IDH function and the potential to improve lipid production in M. alpina.

Advances in improving the biotechnological application of oleaginous fungus Mortierella alpina

Mortierella alpina is an oleaginous filamentous fungus with considerable lipid productivity, and it has been widely used for industrial production of arachidonic acid. The fermentation process of M. alpina is complicated and can be affected by various factors; therefore, a comprehensive knowledge of its metabolic characteristics and key factors governing lipid biosynthesis is required to further improve its industrial performance. In this review, we discuss the metabolic features and extracellular factors that affect lipid biosynthesis in M. alpina. The current progress in fermentation optimisation and metabolic engineering to improve lipid yield are also summarised. Moreover, we review the applications of M. alpina in the food industry and propose fermentation strategies for better utilisation of this genus in the future. In our opinion, the economic performance of M. alpina should be enhanced from multiple levels, including strains with ideal traits, efficient fermentation strategies, controllable fermentation costs, and competitive products of both high value and productivity. By reviewing the peculiarities of M. alpina and current progress to improve its suitability for biotechnological production, we wish to provide more efficient strategies for future development of M. alpina as a high-value lipid cell factory. KEY POINTS: • Understanding M. alpina metabolism is helpful for rational design of its fermentation processes. • Nitrogen source is a key point that affects PUFA's component and fermentation cost in M. alpina. • Dynamic fermentation strategy combined with breeding is needed to increase lipid yield in M. alpina.

Function of ORFC of the polyketide synthase gene cluster on fatty acid accumulation in Schizochytrium limacinum SR21

BACKGROUND

As a potential source of polyunsaturated fatty acids (PUFA), Schizochytrium sp. has been widely used in industry for PUFA production. Polyketide synthase (PKS) cluster is supposed to be the primary way of PUFA synthesis in Schizochytrium sp. As one of three open reading frames (ORF) in the PKS cluster, ORFC plays an essential role in fatty acid biosynthesis. However, the function of domains in ORFC in the fatty acid synthesis of Schizochytrium sp. remained unclear.

RESULTS

In this study, heterologous expression and overexpression were carried out to study the role of ORFC and its domains in fatty acid accumulation. Firstly, ORFC was heterologously expressed in yeast which increased the PUFA content significantly. Then, the dehydratase (DH) and enoyl reductase (ER) domains located on ORFC were overexpressed in Schizochytrium limacinum SR21, respectively. Fatty acids profile analysis showed that the contents of PUFA and saturated fatty acid were increased in the DH and ER overexpression strains, respectively. This indicated that the DH and ER domains played distinct roles in lipid accumulation. Metabolic and transcriptomic analysis revealed that the pentose phosphate pathway and triacylglycerol biosynthesis were enhanced, while the tricarboxylic acid cycle and fatty acids oxidation were weakened in DH-overexpression strain. However, the opposite effect was found in the ER-overexpression strain.

CONCLUSION

Therefore, ORFC was required for the biosynthesis of fatty acid. The DH domain played a crucial role in PUFA synthesis, whereas the ER domain might be related to saturated fatty acids (SFA) synthesis in Schizochytrium limacinum SR21. This research explored the role of ORFC in the PKS gene cluster in Schizochytrium limacinum and provided potential genetic modification strategies for improving lipid production and regulating PUFA and SFA content.

Lipid metabolism research in oleaginous fungus Mortierella alpina: Current progress and future prospects

The oleaginous fungus Mortierella alpina has distinct advantages in long-chain PUFAs production, and it is the only source for dietary arachidonic acid (ARA) certificated by FDA and European Commission. This review provides an overall introduction to M. alpina, including its major research methods, key factors governing lipid biosynthesis, metabolic engineering and omics studies. Currently, the research interests in M. alpina focus on improving lipid yield and fatty acid desaturation degree by enhancing fatty acid precursors and the reducing power NADPH, and genetic manipulation on PUFAs synthetic pathways is carried to optimise fatty acid composition. Besides, multi-omics studies have been applied to elucidate the global regulatory mechanism of lipogenesis in M. alpina. However, research challenges towards achieving a lipid cell factory lie in strain breeding and cost control due to the coenocytic mycelium, long fermentation period and insufficient conversion rate from carbon to lipid. We also proposed future research goals based on a multilevel regulating strategy: obtaining ideal chassis by directional evolution and high-throughput screening; rewiring central carbon metabolism and inhibiting competitive pathways by multi-gene manipulation system to enhance carbon to lipid conversion rate; optimisation of protein function based on post-translational modification; application of dynamic fermentation strategies suitable for different fermentation phases. By reviewing the comprehensive research progress of this oleaginous fungus, we aim to further comprehend the fungal lipid metabolism and provide reference information and guidelines for the exploration of microbial oils from the perspectives of fundamental research to industrial application.

The Role of Metabolic Engineering Technologies for the Production of Fatty Acids in Yeast

Simple Summary

Metabolic engineering involves the sustainable production of high-value products. E. coli and yeast, in particular, are used for such processes. Using metabolic engineering, the biosynthetic pathways of these cells are altered to obtain a high production of desired products. Fatty acids (FAs) and their derivatives are products produced using metabolic engineering. However, classical methods used for engineering yeast metabolic pathways for the production of fatty acids and their derivatives face problems such as the low supply of key precursors and product tolerance. This review introduces the different ways FAs are being produced in E. coli and yeast and the genetic manipulations for enhanced production of FAs. The review also summarizes the latest techniques (i.e., CRISPR–Cas and synthetic biology) for developing FA-producing yeast cell factories.

Abstract

Metabolic engineering is a cutting-edge field that aims to produce simple, readily available, and inexpensive biomolecules by applying different genetic engineering and molecular biology techniques. Fatty acids (FAs) play an important role in determining the physicochemical properties of membrane lipids and are precursors of biofuels. Microbial production of FAs and FA-derived biofuels has several advantages in terms of sustainability and cost. Conventional yeast Saccharomyces cerevisiae is one of the models used for FA synthesis. Several genetic manipulations have been performed to enhance the citrate accumulation and its conversation into acetyl-CoA, a precursor for FA synthesis. Success has been achieved in producing different chemicals, including FAs and their derivatives, through metabolic engineering. However, several hurdles such as slow growth rate, low oleaginicity, and cytotoxicity are still need to be resolved. More robust research needs to be conducted on developing microbes capable of resisting diverse environments, chemicals, and cost-effective feed requirements. Redesigning microbes to produce FAs with cutting-edge synthetic biology and CRISPR techniques can solve these problems. Here, we reviewed the technological progression of metabolic engineering techniques and genetic studies conducted on S. cerevisiae, making it suitable as a model organism and a great candidate for the production of biomolecules, especially FAs.

The role of MTHFDL in mediating intracellular lipogenesis in oleaginous Mortierella alpina

The oleaginous fungus Mortierella alpina can synthesize a variety of polyunsaturated fatty acids, which are used extensively in industry for the production of arachidonic acid (AA). NADPH is the limiting factor and critical reducing agent in lipid biosynthesis. In the folate cycle, methylenetetrahydrofolate dehydrogenase (MTHFDL) catalyzes the conversion of methylene tetrahydrofolate into 10-formyl-tetrahydrofolate with the reduction of NADP⁺ to NADPH. MTHFDL RNAi was used to investigate the role of the folate cycle in lipogenesis. Gene knockdown decreased the transcript levels of MTHFDL by about 50 % and attenuated cell fatty acid synthesis. The observation of decreased NADPH levels and downregulated NADPH-producing genes in response to MTHFDL RNAi indicates a novel aspect of the NADPH regulatory mechanism. Thus, our study demonstrates that MTHFDL plays key role in the mediation of NADPH in lipogenesis in M. alpina.

Role of beta-isopropylmalate dehydrogenase in lipid biosynthesis of the oleaginous fungus Mortierella alpina

Branched-chain amino acids (BCAAs) play an important role in lipid metabolism by serving as signal molecules as well as a potential acetyl-CoA source. Our previous study found that in the oleaginous fungus Mucor circinelloides, beta-isopropylmalate dehydrogenase (IPMDH), an important enzyme participating in the key BCAA leucine biosynthesis, was differentially expressed during lipid accumulation phase and has a positive role on lipogenesis. To further analyze its effects on lipogenesis in another oleaginous fungus Mortierella alpina, the IPMDH-encoding gene MaLeuB was homologously expressed. It was found that the total fatty acid content in the recombinant strain was increased by 20.2% compared with the control strain, which correlated with a 4-fold increase in the MaLeuB transcriptional level. Intracellular metabolites analysis revealed significant changes in amino acid biosynthesis and metabolism, tricarboxylic acid cycle and butanoate metabolism; specifically, leucine and isoleucine levels were upregulated by 6.4-fold and 2.2-fold, respectively. Our genetic engineering approach and metabolomics study demonstrated that MaLeuB is involved in fatty acid metabolism in M. alpina by affecting BCAAs metabolism, and this newly discovered role of IPMDH provides a potential bypass route to increase lipogenesis in oleaginous fungi.

Coproduction of polymalic acid and liamocins from two waste by-products from the xylitol and gluconate industries by Aureobasidium pullulans

The coproduction of polymalic acid (PMA) and liamocins, two important metabolites secreted by Aureobasidium pullulans, from two waste by-products from the xylitol and gluconate industries was investigated in shake flasks and fermentors, confirming that waste xylose mother liquor (WXML) could be utilized as an economical feedstock without any pretreatment. Gluconate could strengthen carbon flux and NADPH supply for the synergetic biosynthesis of PMA and liamocins. High PMA and liamocin titers of 82.9 ± 2.1 and 28.3 ± 2.7 g/L, respectively, were obtained from the coupled WXML and waste gluconate mother liquor (WGML) in batch fermentation, with yields of 0.84 and 0.25 g/g, respectively. These results are comparable to those obtained from renewable feedstocks. Economic assessment of the process revealed that PMA and liamocins could be coproduced from two by-products at costs of $1.48/kg or $0.67/kg (with liamocins credit), offering an economic and sustainable process for the application of waste by-products.

Role of the mitochondrial citrate-oxoglutarate carrier in lipid accumulation in the oleaginous fungus Mortierella alpina

OBJECTIVES

The transport of citrate from the mitochondria to the cytoplasm is essential during lipid accumulation. This study aimed to explore the role of mitochondrial citrate-oxoglutarate carrier in lipid accumulation in the oleaginous fungus Mortierella alpina.

RESULTS

Homologous MaYHM (the gene encoding the mitochondrial citrate-oxoglutarate carrier) was overexpressed in M. alpina. The fatty acid content of MaYHM-overexpressing recombinant strains was increased by up to 30% compared with the control. Moreover, the intracellular α-ketoglutarate level in recombinant strains was increased by 2.2 fold, together with a 23-35% decrease in NAD⁺-isocitrate dehydrogenase activity compared with the control. The overexpression of MaYHM altered the metabolic flux in the glutamate dehydrogenase shunt and 4-aminobutyric acid shunt during metabolic reprogramming, supplying more carbon to synthesize fatty acids.

CONCLUSIONS

Overexpression of MaYHM resulted in more efflux of citrate from mitochondria to the cytoplasm and enhanced lipid accumulation. These findings provide new perspectives for the improvement of industrial lipid production in M. alpina.

The Potential of Single-Cell Oils Derived From Filamentous Fungi as Alternative Feedstock Sources for Biodiesel Production

Microbial lipids, also known as single-cell oils (SCOs), are highly attractive feedstocks for biodiesel production due to their fast production rates, minimal labor requirements, independence from seasonal and climatic changes, and ease of scale-up for industrial processing. Among the SCO producers, the less explored filamentous fungi (molds) exhibit desirable features such as a repertoire of hydrolyzing enzymes and a unique pellet morphology that facilitates downstream harvesting. Although several oleaginous filamentous fungi have been identified and explored for SCO production, high production costs and technical difficulties still make the process less attractive compared to conventional lipid sources for biodiesel production. This review aims to highlight the ability of filamentous fungi to hydrolyze various organic wastes for SCO production and explore current strategies to enhance the efficiency and cost-effectiveness of the SCO production and recovery process. The review also highlights the mechanisms and components governing lipogenic pathways, which can inform the rational designs of processing conditions and metabolic engineering efforts for increasing the quality and accumulation of lipids in filamentous fungi. Furthermore, we describe other process integration strategies such as the co-production with hydrogen using advanced fermentation processes as a step toward a biorefinery process. These innovative approaches allow for integrating upstream and downstream processing units, thus resulting in an efficient and cost-effective method of simultaneous SCO production and utilization for biodiesel production.

The Role of Glyceraldehyde-3-Phosphate Dehydrogenases in NADPH Supply in the Oleaginous Filamentous Fungus Mortierella alpina

Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) is a highly conserved enzyme within the glycolytic pathway. GAPDH catalyzes the transformation of glyceraldehyde 3-phosphate to glycerate-1, 3-biphosphate, a process accompanied by the production of NADH. Its role in the NADPH production system of the oleaginous filamentous fungus Mortierella alpina was explored. Two copies of genes encoding GAPDH were characterized, then endogenously overexpressed and silenced through Agrobacterium tumefaciens-mediated transformation methods. The results showed that the lipid content of the overexpression strain, MA-GAPDH1, increased by around 13%. RNA interference of GAPDH1 and GAPDH2 (MA-RGAPDH1 and MA-RGAPDH2) greatly reduced the biomass of the fungus. The lipid content of MA-RGAPDH2 was found to be about 23% higher than that of the control. Both of the lipid-increasing transformants showed a higher NADPH/NADP ratio. Analysis of metabolite and enzyme expression levels revealed that the increased lipid content of MA-GAPDH1 was due to enhanced flux of glyceraldehyde-3-phosphate to glycerate-1, 3-biphosphate. MA-RGAPDH2 was found to strengthen the metabolic flux of dihydroxyacetone phosphate to glycerol-3-phosphate. Thus, GAPDH1 contributes to NADPH supply and lipid accumulation in M. alpina, and has a distinct role from GAPDH2.

Production of tailor-made enzymes to facilitate lipid extraction from the oleaginous yeast Schwanniomyces occidentalis

Due to the depletion of fossil fuel resources and concern about increasing atmospheric CO₂ levels, the production of microbial oil as source for energy and chemicals is considered as a sustainable alternative. A promising candidate strain for the production of microbial oil is the oleaginous yeast Schwanniomyces occidentalis CBS 2864. To compete with fossil resources, cultivation and processing of S. occidentalis requires improvement. Currently, different cell wall disruption techniques based on mechanical, chemical, physiological, and biological methods are being investigated using a variety of oil producing yeasts and microalgae. Most of these techniques are not suitable for upscaling because they are technically or energetically unfavorable. Therefore, new techniques have to be developed to overcome this challenge. Here, we demonstrate an effective mild enzymatic approach for cell disruption to facilitate lipid extraction from the oleaginous yeast S. occidentalis. Most oil was released by applying 187 mg L⁻¹ tailor-made enzymes from Trichoderma harzianum CBS 146429 against the yeast cell wall of S. occidentalis at pH 5.0 and 40 °C with 4 h of incubation time after applying 1 M NaOH as a pretreatment step.

Ultra Performance Liquid Chromatography-Q Exactive Orbitrap/Mass Spectrometry-Based Lipidomics Reveals the Influence of Nitrogen Sources on Lipid Biosynthesis of Mortierella alpina

The objective of the present study was to reveal the effects of four types of nitrogen sources (soymeal, yeast extract, KNO₃, and ammonium tartrate) on the lipid metabolism of the oleaginous fungus Mortierella alpina using untargeted lipidomics, targeted fatty acid, and reverse transcription quantitative polymerase chain reaction (RT-qPCR) analysis. Our results showed clear differences in the contents and compositions of lipids between four types of nitrogen sources. Soymeal and ammonium tartrate supplementation favored the accumulation of triglycerides with arachidonic acid (ARA) and C_16-18 fatty acids, respectively. These results were further validated by our targeted fatty acid analysis. RT-qPCR analysis of related genes in M. alpina between the four nitrogen source conditions found that soymeal supplementation dramatically increased the expression of GPAT, ELOVL, and Δ12/Δ6 desaturase. Our findings provided new insights into the regulation of lipid biosynthesis in M. alpina and potential avenues for genetic manipulation and highlighted the importance of an optimal nitrogen source for ARA-rich oil production.

Overexpression of delta-12 desaturase in the yeast Schwanniomyces occidentalis enhances the production of linoleic acid

The oleaginous yeast Schwanniomyces occidentalis was previously isolated because of its excellent suitability to convert lignocellulosic hydrolysates into triacyl glycerides: it is able to use a broad range of sugars and is able to tolerate high concentrations of lignocellulosic hydrolysate inhibitors. Compared to other oleaginous yeasts S. occidentalis however produces a low content of unsaturated fatty acids. We show here that the linoleic acid content can be significantly improved by (over)expression Δ12-desaturases derived from S. occidentalis and Fusarium moniliforme. Expression was stable for the homologous expression but decreased during heterologous expression. Both homologous and heterologous expression of mCherry-Δ12-desaturase led to a 4-fold increase in linoleic acid from 0.02 g/g biomass to 0.08 g/g biomass resulting in the production of 2.23 g/L and 2.05 g/L of linoleic acid.

Role of Adenosine Monophosphate Deaminase during Fatty Acid Accumulation in Oleaginous Fungus Mortierella alpina

In oleaginous micro-organisms, nitrogen limitation activates adenosine monophosphate deaminase (AMPD) and promotes lipogenesis via the inhibition of isocitrate dehydrogenase. We found that the overexpression of homologous AMPD in Mortierella alpina favored lipid synthesis over cell growth. Total fatty acid content in the recombinant strain was 15.0-34.3% higher than that in the control, even though their biomass was similar. During the early fermentation stage, the intracellular AMP level reduced by 40-60%, together with a 1.9-2.7-fold increase in citrate content compared with the control, therefore provided more precursors for fatty acid synthesis. Moreover, the decreased AMP level resulted in metabolic reprogramming, reflected by the blocked TCA cycle and reduction of amino acids, distributing more carbon to lipid synthesis pathways. By coupling the energy balance with lipogenesis, this study provides new insights into cell metabolism under nitrogen-limited conditions and targets the regulation of fatty acid accumulation in oleaginous micro-organisms.

Metabolic engineering of l-leucine production in Escherichia coli and Corynebacterium glutamicum: a review

l-Leucine, as an essential branched-chain amino acid for humans and animals, has recently been attracting much attention because of its potential for a fast-growing market demand. The applicability ranges from flavor enhancers, animal feed additives and ingredients in cosmetic to specialty nutrients in pharmaceutical and medical fields. Microbial fermentation is the major method for producing l-leucine by using Escherichia coli and Corynebacterium glutamicum as host bacteria. This review gives an overview of the metabolic pathway of l-leucine (i.e. production, import and export systems) and highlights the main regulatory mechanisms of operons in E. coli and C. glutamicum l-leucine biosynthesis. We summarize here the current trends in metabolic engineering techniques and strategies for manipulating l-leucine producing strains. Finally, future perspectives to construct industrially advantageous strains are considered with respect to recent advances in biology.

Characterization of an Omega-3 Desaturase From Phytophthora parasitica and Application for Eicosapentaenoic Acid Production in Mortierella alpina

Omega-3 long-chain polyunsaturated fatty acids (LC-PUFAs) have important therapeutic and nutritional benefits in humans. In the biosynthesis pathways of these LC-PUFAs, omega-3 desaturase plays a critical role. In this study, we report a new omega-3 desaturase (PPD17) from Phytophthora parasitica. This desaturase shares high similarities with the known omega-3 desaturases and was expressed in Saccharomyces cerevisiae for the activity and substrate specificity research. The desaturase has a wide omega-6 fatty acid substrate, containing both 18C and 20C fatty acids, and exhibits a strong activity of delta-17 desaturase but a weak activity of delta-15 desaturase. The new desaturase converted the omega-6 arachidonic acid (AA, C20:4) to EPA (an omega-3 LC-PUFA, C20:5) with a substrate conversion rate of 70%. To obtain a high EPA-producing strain, we transformed PPD17 into Mortierella alpina, an AA-producing filamentous fungus. The EPA content of the total fatty acids in reconstruction strains reached 31.5% and was followed by the fermentation optimization of the EPA yield of up to 1.9 g/L. This research characterized a new omega-3 desaturase and provides a possibility of industrially producing EPA using M. alpina.

Redox cofactor engineering in industrial microorganisms: strategies, recent applications and future directions

NAD and NADP, a pivotal class of cofactors, which function as essential electron donors or acceptors in all biological organisms, drive considerable catabolic and anabolic reactions. Furthermore, they play critical roles in maintaining intracellular redox homeostasis. However, many metabolic engineering efforts in industrial microorganisms towards modification or introduction of metabolic pathways, especially those involving consumption, generation or transformation of NAD/NADP, often induce fluctuations in redox state, which dramatically impede cellular metabolism, resulting in decreased growth performance and biosynthetic capacity. Here, we comprehensively review the cofactor engineering strategies for solving the problematic redox imbalance in metabolism modification, as well as their features, suitabilities and recent applications. Some representative examples of in vitro biocatalysis are also described. In addition, we briefly discuss how tools and methods from the field of synthetic biology can be applied for cofactor engineering. Finally, future directions and challenges for development of cofactor redox engineering are presented.

Future insights in fungal metabolic engineering

Filamentous fungi exhibit versatile abilities, including organic acid fermentation, protein production, and secondary metabolism, amongst others, and thus have applications in the medical and food industries. Previous genomic analyses of several filamentous fungi revealed their further potential as host microorganisms for bioproduction. Recent advancements in molecular genetics, marker recycling, and genome editing could be used to alter transformation and metabolism, based on optimized design carbolated with computer science. In this review, we detail the current applications of filamentous fungi and describe modern molecular genetic tools that could be used to expand the role of these microorganisms in bioproduction. The present review shed light on the possibility of filamentous fungi as host microorganisms in the field of bioproduction in the future.

A silver bullet in a golden age of functional genomics: the impact of Agrobacterium-mediated transformation of fungi

The implementation of Agrobacterium tumefaciens as a transformation tool revolutionized approaches to discover and understand gene functions in a large number of fungal species. A. tumefaciens mediated transformation (AtMT) is one of the most transformative technologies for research on fungi developed in the last 20 years, a development arguably only surpassed by the impact of genomics. AtMT has been widely applied in forward genetics, whereby generation of strain libraries using random T-DNA insertional mutagenesis, combined with phenotypic screening, has enabled the genetic basis of many processes to be elucidated. Alternatively, AtMT has been fundamental for reverse genetics, where mutant isolates are generated with targeted gene deletions or disruptions, enabling gene functional roles to be determined. When combined with concomitant advances in genomics, both forward and reverse approaches using AtMT have enabled complex fungal phenotypes to be dissected at the molecular and genetic level. Additionally, in several cases AtMT has paved the way for the development of new species to act as models for specific areas of fungal biology, particularly in plant pathogenic ascomycetes and in a number of basidiomycete species. Despite its impact, the implementation of AtMT has been uneven in the fungi. This review provides insight into the dynamics of expansion of new research tools into a large research community and across multiple organisms. As such, AtMT in the fungi, beyond the demonstrated and continuing power for gene discovery and as a facile transformation tool, provides a model to understand how other technologies that are just being pioneered, e.g. CRISPR/Cas, may play roles in fungi and other eukaryotic species.

Expression of Vitreoscilla hemoglobin enhances production of arachidonic acid and lipids in Mortierella alpina

BACKGROUND

Arachidonic acid (ARA, C20:4, n-6), which belongs to the omega-6 series of polyunsaturated fatty acids and has a variety of biological activities, is commercially produced in Mortierella alpina. Dissolved oxygen or oxygen utilization efficiency is a critical factor for Mortierella alpina growth and arachidonic acid production in large-scale fermentation. Overexpression of the Vitreoscilla hemoglobin gene is thought to significantly increase the oxygen utilization efficiency of the cells.

RESULTS

An optimized Vitreoscilla hemoglobin (VHb) gene was introduced into Mortierella alpina via Agrobacterium tumefaciens-mediated transformation. Compared with the parent strain, the VHb-expressing strain, termed VHb-20, grew faster under both limiting and non-limiting oxygen conditions and exhibited dramatic changes in cell morphology. Furthermore, VHb-20 produced 4- and 8-fold higher total lipid and ARA yields than those of the wild-type strain under a microaerobic environment. Furthermore, ARA production of VHb-20 was also 1.6-fold higher than that of the wild type under normal conditions. The results demonstrated that DO utilization was significantly increased by expressing the VHb gene in Mortierella alpina.

CONCLUSIONS

The expression of VHb enhances ARA and lipid production under both lower and normal dissolved oxygen conditions. This study provides a novel strategy and an engineered strain for the cost-efficient production of ARA.