Elovl2 But Not Elovl5 Is Essential for the Biosynthesis of Docosahexaenoic Acid (DHA) in Zebrafish: Insight from a Comparative Gene Knockout Study

Teleost fish can synthesize one of the major omega-3 long-chain polyunsaturated fatty acids (n-3 LC-PUFAs), docosahexaenoic acid (DHA, 22:6n-3), from dietary alpha-linolenic acid (ALA; 18:3n-3), via elongase of very long-chain fatty acid (Elovl) and fatty acid desaturase (Fads). However, it remains unclear which elongase is primarily responsible for the endogenous synthesis of DHA. Here, in this study, the knockout models of the two major elongases, Elovl2 and Elovl5, were generated by CRISPR/Cas9 approach in zebrafish and comparatively analyzed. The homozygous mutants were validated by Sanger sequencing, mutation-mediated PCR, and whole-mount in situ hybridization analysis of the endogenous target genes. Compared with wild-type (WT) counterparts, the content of DHA was significantly reduced by 67.1% (P < 0.05) in the adult liver and by 91.7% (P < 0.01) in the embryo at 3-day post-fertilization (dpf) of theelovl2mutant, but not of theelovl5mutant. Further study revealed thatelovl2andfads2was upregulated by 9.9-fold (P < 0.01) and 9.7-fold (P < 0.01) in theelovl5mutant, andelovl5andfads2were upregulated by 15.1-fold (P < 0.01) and 21.5-fold (P < 0.01) in theelovl2mutant. Our study indicates that although both Elovl2 and Elovl5 have the elongase activity toward C20, the upregulation ofelovl2could completely replace the genetic depletion ofelovl5, but upregulation ofelovl5could not compensate the endogenous deficiency ofelovl2in mediating DHA synthesis. In conclusion, the endogenous synthesis of DHA in is mediated by Elovl2 but not Elovl5 in zebrafish and a DHA-deficient genetic model of zebrafish has been generated.