Metabolic and molecular evidence for long-chain PUFA biosynthesis capacity in the grass carp Ctenopharyngodon idella.

There is a growing interest to understand the capacity of farmed fish species to biosynthesise the physiologically important long-chain (≥C20) n-3 and n-6 polyunsaturated fatty acids (LC-PUFAs), eicosapentaenoic acid (EPA), docosahexaenoic acid (DHA) and arachidonic acid (ARA), from their C18 PUFA precursors available in the diet. In fish, the LC-PUFA biosynthesis pathways involve sequential desaturation and elongation reactions from α-linolenic acid (ALA) and linoleic acid (LA), catalysed by fatty acyl desaturases (Fads) and elongation of very long-chain fatty acids (Elovl) proteins. Our current understanding of the grass carp (Ctenopharyngodon idella) LC-PUFA biosynthetic capacity is limited despite representing the most farmed finfish produced worldwide. To address this knowledge gap, this study first aimed at characterising molecularly and functionally three genes (fads2, elovl5 and elovl2) with putative roles in LC-PUFA biosynthesis. Using an in vitro yeast-based system, we found that grass carp Fads2 possesses ∆8 and ∆5 desaturase activities, with ∆6 ability to desaturase not only the C18 PUFA precursors (ALA and LA) but also 24:5n-3 to 24:6n-3, a key intermediate to obtain DHA through the "Sprecher pathway". Additionally, the Elovl5 showed capacity to elongate C18 and C20 PUFA substrates, whereas Elovl2 was more active over C20 and C22. Collectively, the molecular cloning and functional characterisation of fads2, elovl5 and elovl2 demonstrated that the grass carp has all the enzymatic activities required to obtain ARA, EPA and DHA from LA and ALA. Importantly, the hepatocytes incubated with radiolabelled fatty acids confirmed the yeast-based results and demonstrated that these enzymes are functionally active.

Desaturases and elongases involved in long-chain polyunsaturated fatty acid biosynthesis in aquatic animals: From genes to functions.

Marine ecosystems are rich in "omega-3" long-chain (C20-24) polyunsaturated fatty acids (LC-PUFA). Their production has been historically accepted to derive mostly from marine microbes. This long-standing dogma has been challenged recently by the discovery that numerous invertebrates, mostly with an aquatic life-style, have the enzyme machinery necessary for the de novo biosynthesis of polyunsaturated fatty acids (PUFA) and, from them, LC-PUFA. The key breakthrough was the detection in these animals of enzymes called "methyl-end desaturases" enabling PUFA de novo biosynthesis. Moreover, other enzymes with pivotal roles in LC-PUFA biosynthesis, including front-end desaturases and elongation of very long- chain fatty acids proteins, have been characterised in several non-vertebrate animal phyla. This review provides a comprehensive overview of the complement and functions of these gene/protein families in aquatic animals, particularly invertebrates and fish. Therefore, we expand and re-define our previous revision of the LC-PUFA biosynthetic enzymes present in chordates to animals as a whole, discussing how key genomic events have determined the diversity and distribution of desaturase and elongase genes in different taxa. We conclude that both invertebrates and fish display active, but markedly different, LC-PUFA biosynthetic gene networks that result from a complex evolutionary path combined with functional diversification and plasticity.

Long-chain polyunsaturated fatty acid biosynthesis in chordates: Insights into the evolution of Fads and Elovl gene repertoire.

Long-chain polyunsaturated fatty acids (LC-PUFA) are major components of complex lipid molecules and are also involved in numerous critical biological processes. Studies conducted mainly in vertebrates have demonstrated that LC-PUFA can be biosynthesized through the concerted action of two sets of enzymes, namely fatty acyl desaturases (Fads) and elongation of very long-chain fatty acid (Elovl) proteins. While LC-PUFA research is a thriving field, mainly focused on human health, an integrated view regarding the evolution of LC-PUFA biosynthetic genetic machinery in chordates is yet to be produced. Particularly important is to understand whether lineage specific life history trajectories, as well as major biological transitions, or particular genomic processes such as genome duplications have impacted the evolution of LC-PUFA biosynthetic pathways. Here we review the gene repertoire of Fads and Elovl in chordate genomes and the diversity of substrate specificities acquired during evolution. We take advantage of the magnitude of genomic and functional data to show that combination duplication processes and functional plasticity have generated a wide diversity of physiological capacities in extant lineages. A clear evolutionary framework is provided, which will be instrumental for the full clarification of functional capacities between the various vertebrate groups.