Histologic examination and transcriptome analysis uncovered liver damage in largemouth bass from formulated diets

As a commercial carnassial fish, largemouth bass (Micropterus salmoides) consumes a large amount of forage fish as food annually. To reduce the use of forage fish, a formulated diet was utilized in a largemouth bass culture. Although they have been improved upon for 10 years, formulated diets still cannot meet the nutritional needs of largemouth bass. The formulated diet and forage fish were separately used to feed fish in an experimental group (FDG) and control group (FFG) to investigate the effects of a commercial formulated diet on the growth performance and liver histomorphology and transcriptomics of largemouth bass. After the 8-month feeding trial, fish fed the formulated diet exhibited a significantly lower weight gain rate (WGR) and specific growth rate (SGR) but a higher hepatosomatic index (HSI) and hepatocyte vacuolization score. A further transcriptomic difference analysis demonstrated all of the physiological changes in the process of liver injury caused by the formulated diet. Several key genes involved in the process of liver disease, including gk (glucokinase), pck (phosphoenolpyruvate carboxykinase), acetyl-CoA acetyltransferase cytosolic-like (acat cytosolic-like), type-2 ice-structuring protein (type-2 isp), tf (transferrin), col6a3 (collagen type VI alpha 3), and TBT-bp (tributyltin-binding protein), showed significant differences in their expression patterns between the two diet groups. KEGG analysis showed that the differentially expressed genes in both females and males were involved in important pathways, including glycolysis, the citrate cycle (the TCA cycle), pathogenic E. coli infection and fatty acid biosynthesis. The results suggest that the extra carbohydrates in the formulated diet were transformed into fatty acids and that the synthesized fatty acids with the lipids in the formulated diet accumulated heavily in the liver, leading to inflammation and liver injury in largemouth bass.