Biosynthesis of very long-chain (>C24) polyunsaturated fatty acids in juveniles of gilthead seabream (Sparus aurata)

Abstract

Very long-chain polyunsaturated fatty acids (VLC-PUFA), compounds with unusually long acyl chains (>C24), have been scarcely investigated in fish. Studies in mammals have demonstrated that VLC-PUFA, biosynthesized from shorter-chain precursors by fatty acyl elongases termed Elovl4, play key roles in the function of eye, brain and gonads. Many of the physiological processes in which VLC-PUFA are involved are particularly crucial in fast-growing life stages of animals, when biomembranes in which VLC-PUFA accumulate are developing. The situation can further aggravate when, along with rapid growth rates, dietary deficiencies of metabolic precursors occur. In the context of fish farming, a combination of both factors (fast growth and dietary deficiencies) often applies to the culture of juveniles of marine species that are currently fed diets with low levels of fish oil (FO), potentially limiting precursors for VLC-PUFA biosynthesis. The present study aimed to characterize the Elovl4 complement and their nutritional regulation in juveniles of gilthead seabream (Sparus aurata) fed diets with varying levels of FO. Our results demonstrated that S. aurata possesses two elovl4 genes (elovl4a and elovl4b). The function of the S. aurata Elovl4 enzymes was assessed in yeast. Both Elovl4 enzymes showed the ability to elongate C20 (20:5n-3 and 20:4n-6) and C22 (22:5n-3 and 22:4n-6) substrates to longer products, which produced in some cases VLC-PUFA of up to C32. Tissue expression analysis of elovl4 confirmed that eye and brain are sites of VLC-PUFA biosynthesis in seabream juveniles. We further investigated the nutritional regulation of VLC-PUFA biosynthetic genes in seabream juveniles fed diets with different FO levels for four months. While elovl4b appeared not to be regulated in eye and brain, elovl4a was down-regulated (P<0.05) in both tissues with decreased dietary FO inclusion, possibly suggesting that expression may be related to precursor availability and that low dietary FO could limit VLC-PUFA biosynthesis