Enoyl-CoA hydratase/3-hydroxyacyl CoA dehydrogenase is essential for the production of DHA in zebrafish.

Compared with other species, freshwater fish are more capable of synthesizing DHA via same biosynthetic pathways. Freshwater fish have a "Sprecher" pathway to biosynthesize DHA in a peroxisome-dependent manner. Enoyl-CoA hydratase/3-hydroxyacyl CoA dehydrogenase (Ehhadh) is involved in the hydration and dehydrogenation reactions of fatty acid ß-oxidation in peroxisomes. However, the role of Ehhadh in the synthesis of DHA in freshwater fish remains largely unclear. In this study, the knockout of Ehhadh significantly inhibited DHA synthesis in zebrafish. Liver transcriptome analysis showed that Ehhadh deletion significantly inhibited SREBF and PPAR signaling pathways and decreased the expression of PUFA synthesis-related genes. Our results from the analysis of transgenic zebrafish (Tg:Ehhadh) showed that Ehhadh overexpression significantly increased the DHA content in the liver and significantly upregulated the expression of genes related to PUFA synthesis. In addition, the DHA content in the liver of Tg:Ehhadh fed with linseed oil was significantly higher than that of wildtype, but the expression of PUFA synthesis-related genes fads2 and elovl2 were significantly lower, indicating that Ehhadh had a direct effect on DHA synthesis. In conclusion, our results showed that Ehhadh was essential for DHA synthesis in the "Sprecher" pathway, and Ehhadh overexpression could promote DHA synthesis. This study provides insight into the role of Ehhadh in freshwater fish.

scd knockout activates ß-oxidation of fatty acids via accumulating stearic acid (18:0) and induces anorexia in zebrafish.

Stearoyl-CoA desaturase (scd) is the rate-limiting enzyme for the biosynthesis of monounsaturated fatty acids (MUFA), and it plays a critical role in regulating hepatic lipogenesis and lipid oxidation. However, its role in teleosts remains unclear. In this study, we generated scd knockout zebrafish (scd-/-) to explore the role of Scd in regulating growth and metabolism in teleosts. The results showed that scd knockout reduces hepatic lipid deposition by down-regulating the expression of lipogenesis-related genes and up-regulating the expression of lipolysis-related genes. In addition, the knockout of scd suppressed food intake and reduced body weight. Further analysis confirmed that scd knockout suppressed the feeding behavior by decreasing expression of orexigenic peptide genes and increasing expression of anorexigenic peptide genes. The high-level stearic acid (18:0) feeding experiment results showed that the accumulation of 18:0 inhibited feeding behavior, reduced food intake, decreased body weight, and increased lipid ß-oxidation, which was essentially consistent with the phenotypes of scd deficiency. Taken together, our results indicate that the knockout of scd inhibited the food intake through the accumulation of 18:0. This study preliminarily reveals the role of Scd in regulating food intake of teleosts, which provides theoretical basis for the functional study of Scd.