Molecular aspects of lipid metabolism, digestibility and antioxidant status of Atlantic bluefin tuna (T. thynnus L.) larvae during first feeding

Abstract

Atlantic bluefin tuna (Thunnus thynnus L.; ABT) larvae were fed on enriched rotifers Brachionus rotundiformis and copepod nauplii Acartia tonsa from first feeding to 15 days post hatching. Rotifers were enriched with five different commercial products: OG, MG, AG and RA plus selenium and vitamin E. Copepods (COP) were cultured with the algae Rhodomonas baltica. Metabolic processes were studied by determining the expression of 30 genes related to lipid metabolism (transcription factors, fatty acid metabolism and lipid homeostasis), antioxidant enzymes, myogenesis and digestive enzymes. Growth and development parameters and high expression of myogenesis genes myhc2 and tropo indicated that COP were better than enriched rotifers as live prey for first feeding ABT. COP and AG-fed larvae showed the lowest values for the transcription factors pparγ and srebp2. The expression of fas showed differences among treatments, with highest relative expression in COP-fed larvae and those fed with RA rotifers. In relation to fatty acid catabolism, larvae fed RA had the highest aco expression levels, with the lowest observed in those fed COP. The expression profiles of lipid homeostasis genes showed that larvae fed COP had higher fabp2 and 4 expressions. Larvae fed AG showed the lowest lpl expression levels, with highest values observed in larvae fed OG. Regarding antioxidant enzyme gene expression, sod showed highest values in larvae fed COP and RA, with larvae fed MG rotifers showing lowest expression levels. A similar pattern was observed for the expression of cat and gpx1 and 4. The expression of genes for digestive enzymes showed that tryp expression levels were highest in COPfed larvae but, in contrast, COP-fed larvae showed the lowest anpep and alp levels. ABT larvae fed AG displayed the lowest expression level of pla2. bal1 and bal2 presented similar expression patterns, with highest values in COP-fed ABT and lowest expression in larvae fed AG rotifers. Copepods were a superior live prey for first feeding ABT larvae compared to enriched rotifers, as indicated by the higher growth and flexion index achieved by COP-fed larvae, possibly reflecting the higher protein content of the copepods.

Performance, feed utilization, and hepatic metabolic response of weaned juvenile Atlantic bluefin tuna (Thunnus thynnus L.): effects of dietary lipid level and source

“…In red sea bream, both feeding conditions and dietary lipid and fatty acid levels were shown to alter lpl expression in liver, with lower expression in fish fed low lipid than in fish fed high lipid (Liang et al 2002a ). In the present study, the expression of lpl in liver of ABT juveniles was low and showed no clear response to diet, similar to previous studies where ABT larvae fed different live prey showed variable lpl expression (Betancor et al 2017a , b ). In contrast, hmgcl expression was upregulated in ABT juveniles fed higher lipid (20KO), while previous studies in larvae did not show any dietary regulation of this gene (Betancor et al 2017a , b ) although increased expression of this enzyme has been associated with enhanced fatty acid oxidation in rat liver (Ide et al 2009 ).…”

“…In the present study, the expression of lpl in liver of ABT juveniles was low and showed no clear response to diet, similar to previous studies where ABT larvae fed different live prey showed variable lpl expression (Betancor et al 2017a , b ). In contrast, hmgcl expression was upregulated in ABT juveniles fed higher lipid (20KO), while previous studies in larvae did not show any dietary regulation of this gene (Betancor et al 2017a , b ) although increased expression of this enzyme has been associated with enhanced fatty acid oxidation in rat liver (Ide et al 2009 ). Therefore, in the present study, the upregulation of hmgcl in fish fed the higher lipid content could be related to enhanced oxidation of lipids, consistent with the regulation of cpt1 .…”

“…In the present study, upregulation of fads2d6 expression was observed in juvenile ABT fed the test diets in relation to the reference diet (primarily in diet 15KO in trial A and diet 15KORO in trial B), while no clear pattern was observed in the expression of elovl5 , which was downregulated with test diets in trial A and slightly upregulated in 15KO-fed fish in trial B. In a previous trial with ABT larvae (Betancor et al 2017a ), no nutritional regulation of fads2d6 and elovl5 was observed and the authors hypothesized that this suggested all the diets provided sufficient n−3 LC-PUFA to satisfy minimum requirements and thus expression of the genes did not differ. Although differences in the DHA contents of the diets used in the previous trial (13.5–28.3 μg mg −1 ) and the present trial (15.5–29.5 μg mg −1 ) were similar, the observed difference in the regulation of the LC-PUFA genes between the studies may indicate an effect of life stage, with different requirements and/or basal biosynthetic activity/expression in larvae versus juveniles.…”

“…In the present study, fabp2 gene expression was higher in ABT fed the test diets containing 15% lipid (15KO and 15KORO) than in the fish fed higher dietary lipid (20KO). In a previous trial, fabp2 expression was higher when larvae were fed live prey containing 11% lipid and lower when fed prey with 5.6% lipid, which could indicate that lipid content regulates expression of this gene in larvae, rather than dietary fatty acid composition (Betancor et al 2017a ). However, Atlantic cod and gilthead sea bream juveniles fed diets containing vegetable oil showed upregulation in the expression of this gene (Lilleeng et al 2007 ; Betancor et al 2016 ).…”

Taurine metabolism and effects of inclusion levels in rotifer (Brachionus rotundiformis, Tschugunoff, 1921) on Atlantic bluefin tuna (Thunnus thynnus, L.) larvae

“…Taken together these results indicate a digestive promoting effect of taurine at an enrichment level of 3.7 mg taurine g -1 rotifer, which was entirely consistent with the impact of dietary taurine on ABT larval growth. However, it is worth noting that the expression levels of the digestive genes could be influenced by growth rather than dietary taurine levels, as previously suggested (Betancor et al, 2017b). Indeed, similar results were found by Saele et al (2010), where a relationship between bal genes and cod (Gadus morhua) larvae body size was shown.…”