LC-PUFA biosynthesis in fisha phylogenetic and/or environmental question?

Resumen

The study of long-chain (≥C20) polyunsaturated fatty acids (LC-PUFA) biosynthesis in farmed fish has been a topic of interest within the scientific community. Investigate the ability to produce the physiologically important arachidonic acid (20:4n-6, ARA), eicosa- pentaenoic acid (20:5n-3, EPA) and docosahexaenoic acid (22:6n-3, DHA) is crucial in the aquaculture industry to know the essential fatty acid requirements of farmed species. Knowledge of the biosynthetic capacity to obtain LC-PUFA from C18 PUFA allows the proper use of vegetable-based diets that lack the former. Several studies have shown that the endogenous capacity to produce LC-PUFA depends on the complement and function of fatty acyl desaturase (fads) and elongase of very long chain fatty acid (elovl) genes that differ between species. Furthermore, the biosynthetic capacity can be modulated by environmental factors such as salinity or by the LC-PUFA supply in the diet. The present study aims to elucidate how the phylogenetic and environmental parameters influence the LC-PUFA biosynthetic capacities of different teleost species of interest in fish farming. This aim was addressed by, first, characterising the LC-PUFA biosynthesis in a relatively low-trophic level teleost, the grass carp Ctenopharyngodon idella, and second, establish- ing how both dietary fatty acid composition and salinity can modulate the LC-PUFA bi- osynthetic capacity of two marine species occupying intermediate trophic levels, namely Solea senegalensis and Chelon labrosus. The capacity to biosynthesise LC-PUFA in the grass carp (C. idella) was determined through the study of the molecular and functional characterization of fads and elovl genes involved in LC-PUFA biosynthesis and the in vitro activity of these enzymes in isolated hepatocytes. The combination of the functional characterisation and enzymatic activity assays demonstrated that C. idella shows all the desaturase and elongase activities re- quired to convert LA into ARA, and ALA into EPA and DHA. These results strongly suggest that grass carp can satisfy its essential LC-PUFA requirements with an adequate supply of C18 PUFA in the diet. To assess the effect of salinity and dietary LC-PUFA supply on the metabolic response of Solea senegalensis and Chelon labrosus we studied the changes in lipid composition of muscle, hepatocytes and enterocytes, and its relationship with the pattern of expression of fads and elovl genes in liver and intestine and the in vitro activity of these enzymes in isolated hepatocytes and enterocytes. Lipid composition of muscle demonstrated that Solea senegalensis is able to compensate for the limited provision of dietary LC-PUFA. FO replacement prevailed over salinity regulating the hepatic fads2 expression while salinity did over diet in elovl5, suggesting that partial substitution of FO by VO and the combination with a lower salinity can be a sustainable farming strategy to obtain commercial S. senegalensis rich in DHA. C. labrosus trial evidence that dietary DHA deficiencies can be partially compensated. Furthermore, increased expression of fads2 and elovl5 in liver and intestine reared at re- duced salinity (20 ppt) shows a key role of ambient salinity in the regulation of LC-PUFA biosynthesis in this species. Therefore, the combination of a moderate dietary supply of n-3 LC-PUFA and a reduced salinity is a suitable strategy to obtain C. labrosus specimens rich in DHA. Moreover, fatty acid profiles and in vitro assays confirm the biosynthesis of LC-PUFA from C18 precursors through the “Sprecher pathway” in C. labrosus.