Transcriptomic Analysis and Histological Alteration of Black Sea Bream (Acanthopagrus schlegelii) Liver Fed Different Protein/Energy Ratio Diets

Abstract

To systematically investigate the effects of different protein/energy (P/E) ratio diets on the microstructure and metabolic response of black sea bream liver, fish (initial weight $11.20\pm 0.20$  g) were fed three isoenergetic (20.9 kJ/g) diets with varying protein/lipid levels (LR, 28.8% protein/26.3% lipid; MR, 37.5% protein/20.9% lipid; and HR, 42.8% protein/17.3% lipid, P/E ratio increased gradually) for 8 weeks. The lipid and glycogen contents in the liver decreased with increasing dietary P/E ratios ( $P<0.05$ ). Histological observation found that the liver of the LR group presented many adipose cells, along with polarized nucleus, massive lipid droplets, and unclear organelle structure. The liver cellular structures of MR and HR groups were normal with some small unmatured lipid droplets. RNA-Seq was applied to investigate the hepatic transcriptome response to different P/E ratio diets of black sea bream. Different levels of gene expressions were identified in the signaling pathways related to energy metabolism, amino acid metabolism, lipid metabolism, carbohydrate metabolism, immune system, etc. The LR diet inhibited the genes related to lipid oxidation, glycolysis, and protein metabolism, whereas the HR diet inhibited those of glycolysis and protein catabolism. The present study provides a comprehensive understanding of the molecular mechanisms behind the effects of different P/E ratio diets on the liver metabolism of black sea bream.