Rational metabolic engineering of transgenic plants for biosynthesis of omega-3 polyunsaturates

Towards establishing dietary reference intakes for eicosapentaenoic and docosahexaenoic acids

There is considerable interest in the impact of (n-3) long-chain PUFA in mitigating the morbidity and mortality caused by chronic diseases. In 2002, the Institute of Medicine concluded that insufficient data were available to define Dietary Reference Intakes (DRI) for eicosapentaenoic acid (EPA) or docosahexaenoic acid (DHA), noting only that EPA and DHA could contribute up to 10% toward meeting the Adequate Intake for alpha-linolenic acid. Since then, substantial new evidence has emerged supporting the need to reassess this recommendation. Therefore, the Technical Committee on Dietary Lipids of the International Life Sciences Institute North America sponsored a workshop on 4-5 June 2008 to consider whether the body of evidence specific to the major chronic diseases in the United States--coronary heart disease (CHD), cancer, and cognitive decline--had evolved sufficiently to justify reconsideration of DRI for EPA+DHA. The workshop participants arrived at these conclusions: 1) consistent evidence from multiple research paradigms demonstrates a clear, inverse relation between EPA+DHA intake and risk of fatal (and possibly nonfatal) CHD, providing evidence that supports a nutritionally achievable DRI for EPA+DHA between 250 and 500 mg/d; 2) because of the demonstrated low conversion from dietary ALA, protective tissue levels of EPA+DHA can be achieved only through direct consumption of these fatty acids; 3) evidence of beneficial effects of EPA+DHA on cognitive decline are emerging but are not yet sufficient to support an intake level different from that needed to achieve CHD risk reduction; 4) EPA+DHA do not appear to reduce risk for cancer; and 5) there is no evidence that intakes of EPA+DHA in these recommended ranges are harmful.

Enhancement of lipid production using biochemical, genetic and transcription factor engineering approaches

This paper compares three possible strategies for enhanced lipid overproduction in microalgae: the biochemical engineering (BE) approaches, the genetic engineering (GE) approaches, and the transcription factor engineering (TFE) approaches. The BE strategy relies on creating a physiological stress such as nutrient-starvation or high salinity to channel metabolic fluxes to lipid accumulation. The GE strategy exploits our understanding to the lipid metabolic pathway, especially the rate-limiting enzymes, to create a channelling of metabolites to lipid biosynthesis by overexpressing one or more key enzymes in recombinant microalgal strains. The TFE strategy is an emerging technology aiming at enhancing the production of a particular metabolite by means of overexpressing TFs regulating the metabolic pathways involved in the accumulation of target metabolites. Currently, BE approaches are the most established in microalgal lipid production. The TFE is a very promising strategy because it may avoid the inhibitive effects of the BE approaches and the limitation of "secondary bottlenecks" as commonly observed in the GE approaches. However, it is still a novel concept to be investigated systematically.

Transplastomic tobacco plants expressing a fatty acid desaturase gene exhibit altered fatty acid profiles and improved cold tolerance

The possibility of altering the unsaturation level of fatty acids in plant lipids by genetic transformation has implications for the stress tolerance of higher plants as well as for their nutritional value and industrial utilisation. While the integration and expression of transgenes in the plastome has several potential advantages over nuclear transformation, very few attempts have been made to manipulate fatty acid biosynthesis using plastid transformation. We produced transplastomic tobacco plants that express a Delta(9) desaturase gene from either the wild potato species Solanum commersonii or the cyanobacterium Anacystis nidulans, using PEG-mediated DNA uptake by protoplasts. Incorporation of chloroplast antibiotic-insensitive point mutations in the transforming DNA was used to select transformants. The presence of the transcript and the Delta(9) desaturase protein in transplastomic plants was confirmed by northern and western blot analyses. In comparison with control plants, transplastomic plants showed altered fatty acid profiles and an increase in their unsaturation level both in leaves and seeds. The two transgenes produced comparable results. The results obtained demonstrate the feasibility of using plastid transformation to engineer lipid metabolic pathways in both vegetative and reproductive tissues and suggest an increase of cold tolerance in transplastomic plants showing altered leaf fatty acid profiles. This is the first example of transplastomic plants expressing an agronomically relevant gene produced with the "binding-type" vectors, which do not contain a heterologous marker gene. In fact, the transplastomic plants expressing the S. commersonii gene contain only plant-derived sequences, a clear attraction from a public acceptability perspective.

Storage reserve accumulation in Arabidopsis: metabolic and developmental control of seed filling

In the life cycle of higher plants, seed development is a key process connecting two distinct sporophytic generations. Seed development can be divided into embryo morphogenesis and seed maturation. An essential metabolic function of maturing seeds is the deposition of storage compounds that are mobilised to fuel post-germinative seedling growth. Given the importance of seeds for food and animal feed and considering the tremendous interest in using seed storage products as sustainable industrial feedstocks to replace diminishing fossil reserves, understanding the metabolic and developmental control of seed filling constitutes a major focus of plant research. Arabidopsis thaliana is an oilseed species closely related to the agronomically important Brassica oilseed crops. The main storage compounds accumulated in seeds of A. thaliana consist of oil stored as triacylglycerols (TAGs) and seed storage proteins (SSPs). Extensive tools developed for the molecular dissection of A. thaliana development and metabolism together with analytical and cytological procedures adapted for very small seeds have led to a good description of the biochemical pathways producing storage compounds. In recent years, studies using these tools have shed new light on the intricate regulatory network controlling the seed maturation process. This network involves sugar and hormone signalling together with a set of developmentally regulated transcription factors. Although much remains to be elucidated, the framework of the regulatory system controlling seed filling is coming into focus.

Metabolic engineering of omega3-very long chain polyunsaturated fatty acid production by an exclusively acyl-CoA-dependent pathway

omega3-Very long chain polyunsaturated fatty acids (VLCPUFA) are essential for human development and brain function and, thus, are indispensable components of the human diet. The current main source of VLCPUFAs is represented by ocean fish stocks, which are in severe decline, and the development of alternative, sustainable sources of VLCPUFAs is urgently required. Our research aims at exploiting the powerful infrastructure available for the large scale culture of oilseed crops, such as rapeseed, to produce VLCPUFAs such as eicosapentaenoic acid in transgenic plants. VLCPUFA biosynthesis requires repeated desaturation and repeated elongation of long chain fatty acid substrates. In previous experiments the production of eicosapentaenoic acid in transgenic plants was found to be limited by an unexpected bottleneck represented by the acyl exchange between the site of desaturation, endoplasmic reticulum-associated phospholipids, and the site of elongation, the cytosolic acyl-CoA pool. Here we report on the establishment of a coordinated, exclusively acyl-CoA-dependent pathway, which avoids the rate-limiting transesterification steps between the acyl lipids and the acyl-CoA pool during VLCPUFA biosynthesis. The pathway is defined by previously uncharacterized enzymes, encoded by cDNAs isolated from the microalga Mantoniella squamata. The conceptual enzymatic pathway was established and characterized first in yeast to provide proof-of-concept data for its feasibility and subsequently in seeds of Arabidopsis thaliana. The comparison of the acyl-CoA-dependent pathway with the known lipid-linked pathway for VLCPUFA biosynthesis showed that the acyl-CoA-dependent pathway circumvents the bottleneck of switching the Delta6-desaturated fatty acids between lipids and acyl-CoA in Arabidopsis seeds.

High-value oils from plants

The seed oils of domesticated oilseed crops are major agricultural commodities that are used primarily for nutritional applications, but in recent years there has been increasing use of these oils for production of biofuels and chemical feedstocks. This is being driven in part by the rapidly rising costs of petroleum, increased concern about the environmental impact of using fossil oil, and the need to develop renewable domestic sources of fuel and industrial raw materials. There is also a need to develop sustainable sources of nutritionally important fatty acids such as those that are typically derived from fish oil. Plant oils can provide renewable sources of high-value fatty acids for both the chemical and health-related industries. The value and application of an oil are determined largely by its fatty acid composition, and while most vegetable oils contain just five basic fatty acid structures, there is a rich diversity of fatty acids present in nature, many of which have potential usage in industry. In this review, we describe several areas where plant oils can have a significant impact on the emerging bioeconomy and the types of fatty acids that are required in these various applications. We also outline the current understanding of the underlying biochemical and molecular mechanisms of seed oil production, and the challenges and potential in translating this knowledge into the rational design and engineering of crop plants to produce high-value oils in plant seeds.

Adipose tissue failure and mitochondria as a possible target for improvement by bioactive food components

PURPOSE OF REVIEW

Adipose tissue is an essential, highly dynamic and metabolically active tissue that vigorously communicates to support its primary function: the storage of lipids. It performs this function to secure energy supply and prevent lipotoxicity. Adipose tissue is essential for maintaining a healthy glucose and lipid homeostasis and failure results in disease. This review discusses causes of adipose tissue failure and four categories of bioactive food components that may help to prevent this.

RECENT FINDINGS

Based on recent findings, it is argued that initial adipose failure following long-term excess energy intake may be the result of reduced mitochondrial capacity associated with altered mitochondrial reactive oxygen species signaling and adipose tissue hypoxia. Current data suggest that different classes of bioactive food components, including vitamin B3, retinoids, fatty acids and polyphenols, may have the potential to modulate mitochondrial function and consequently prevent adipose dysfunction in obesity.

SUMMARY

It seems most attractive to aim nutritional intervention at the prevention of initial adipose dysfunction and hence to target dietary intervention at improvement of mitochondrial function.

Seed storage oil mobilization

Storage oil mobilization starts with the onset of seed germination. Oil bodies packed with triacylglycerol (TAG) exist in close proximity with glyoxysomes, the single membrane-bound organelles that house most of the biochemical machinery required to convert fatty acids derived from TAG to 4-carbon compounds. The 4-carbon compounds in turn are converted to soluble sugars that are used to fuel seedling growth. Biochemical analysis over the last 50 years has identified the main pathways involved in this process, including beta-oxidation, the glyoxylate cycle, and gluconeogenesis. In the last few years molecular genetic dissection of the overall process in the model oilseed species Arabidopsis has provided new insight into its complexity, particularly with respect to the specific role played by individual enzymatic steps and the subcellular compartmentalization of the glyoxylate cycle. Both abscisic acid (ABA) and sugars inhibit storage oil mobilization and a substantial degree of the control appears to operate at the transcriptional level.

A Molecular Caliper Mechanism for Determining Very Long-Chain Fatty Acid Length

Very long-chain fatty acids (VLCFAs) are essential lipids whose functional diversity is enabled by variation in their chain length. The full VLCFA biosynthetic machinery and how this machinery generates structural diversity remain elusive. Proteoliposomes reconstituted here from purified membrane components-an elongase protein (Elop), a novel dehydratase, and two reductases-catalyzed repeated rounds of two-carbon addition that elongated shorter FAs into VLCFAs whose length was dictated by the specific Elop homolog present. Mutational analysis revealed that the Elop active site faces the cytosol, whereas VLCFA length is determined by a lysine near the luminal end of an Elop transmembrane helix. By stepping the lysine residue along one face of the helix toward the cytosol, we engineered novel synthases with correspondingly shorter VLCFA outputs. Thus the distance between the active site and the lysine residue determines chain length. Our results uncover a mutationally adjustable, caliper-like mechanism that generates the repertoire of cellular VLCFAs.