Biosynthesis of long-chain polyunsaturated fatty acids in marine fish: Characterization of an Elovl4-like elongase from cobia Rachycentron canadum and activation of the pathway during early life stages

Marine fish, unlike freshwater species, have been generally considered to have a limited ability to biosynthesize long-chain polyunsaturated fatty acids (LC-PUFA) from C18 precursors due to apparent limited enzymatic activities involved in the pathway. Although LC-PUFA play important physiological roles throughout the entire life cycle, requirements for early life stages are especially high and provision of preformed LC-PUFA in egg lipids appears critical to support the formation of developing tissues where these compounds accumulate. No studies, however, have been conducted to explore the capability of marine fish embryos (here referring to life stages from zygote to the oesophagus opening) for de novo synthesis of the LC-PUFA required for normal growth and development. The present study aimed to investigate the activation of the LC-PUFA biosynthetic pathway during embryogenesis of the marine teleost cobia (Rachycentron canadum). First, a fatty acyl elongase with sequence similarity to mammalian elongase of very long-chain fatty acids 4 (Elovl4) was isolated, and its biochemical function characterized showing that it catalyzed the production of very long-chain fatty acids (VLC-FA) including both saturated and polyunsaturated fatty acids with chain lenghts ≥ 24 carbons. Notably, cobia Elovl4 was able to elongate 22:5n−3 to 24:5n−3 and thus could play a key role in the biosynthesis of docosahexaenoic acid (22:6n−3), a critical fatty acid in neural tissues. Subsequently, the fatty acid dynamics of embryos at different developmental stages and the temporal expression patterns of target genes including elovl4, and the formerly characterized elovl5 elongase and ∆6 fatty acyl desaturase, were analyzed in order to elucidate the overall activation of the LC-PUFA biosynthetic pathway in cobia embryos. Our results indicated that expression of the LC-PUFA biosynthetic pathway in cobia embryos is initiated at 12–18 h post-fertilization.