Control of the expression of key genes to spare protein and increase the use of dietary carbohydrates in gilthead sea bream (Sparus aurata)

[eng] Aquaculture raises several concerns about environmental welfare and sustainability. To increase the current knowledge of the control of glucose homeostasis in carnivorous fish, we studied the effect of metformin, an anti-diabetic drug for humans, on serum metabolites, and ratelimiting enzymes in key pathways and lipogenic factors in the liver of gilthead sea bream (Sparus aurata). Our findings showed that metformin improved glucose homeostasis in S. aurata by counteracting glucose-dependent activation on key enzymes in glycolysis and the citric acid cycle, and the expression of lipogenic factors. In addition to advance in the characterization of the intermediary metabolism of a carnivorous and glucose intolerant fish, the results of this study suggest that metformin reduced gluconeogenesis by decreasing hepatic transdeamination and amino acid entrance into the citric acid cycle and their subsequent use as gluconeogenic substrates. To induce a protein-sparing effect in S. aurata by boosting conversion of carbohydrates into lipids, an additional aim of this thesis was to study the effect of chitosan-TPP nanoparticles complexed with a plasmid expressing the N-terminus of hamster SREBP1a (pSG5-SREBP1a) by periodical intraperitoneal injection (every 4 weeks; 3 doses in total) to S. aurata fed diets differing in macronutrient composition. Following 70 days of treatment, chitosan-TPP-pSG5-SREBP1a nanoparticles hugely upregulated SREBP1a mRNA levels in the liver of S. aurata. Consistent with improved conversion of dietary carbohydrates into lipids, overexpression of SREBP1a in the liver increased serum triglycerides and cholesterol as well as hepatic glucose oxidation via glycolysis and the pentose phosphate pathway, while not affecting gluconeogenesis and transamination. Furthermore, upregulation of SREBP1a significantly increased weight gain, specific growth rate and protein efficiency ratio, while decreased feed conversion ratio even in fish fed a low protein-high carbohydrate diet. In addition to show that chitosan-TPP-DNA nanoparticles constitute an efficient method to express exogenous genes in fish avoiding the use of genetically modified organisms, the results of this study support that periodical administration of chitosan-TPP-DNA nanoparticles to overexpress SREBP1a in the liver enhanced growth performance of S. aurata through a mechanism that enabled protein sparing by enhancing metabolization of dietary carbohydrates in a carnivorous glucose intolerant fish.