Transcriptome, enzyme activity and histopathology analysis reveal the effects of dietary carbohydrate on glycometabolism in juvenile largemouth bass, Micropterus salmoides

Largemouth bass (Micropterus salmoides) is one of the most valuable freshwater aquatic species in China. However, M. salmoides appeared to use dietary carbohydrate more poorly than other carnivorous fish. Although carbohydrate metabolism of fish has been well studied for the past decades, studies regarding transcriptomic profiling of carbohydrate metabolism in fish are lacking. In this study, the transcriptomic profiles of M. salmoides liver were explored for the first time by feeding seven kinds of diets, and selectively analyzed the glycometabolism at different levels of dietary carbohydrate (ML-120 g kg−1, MM-240 g kg−1, and MH-360 g kg−1). Totally, the de novo assemblies yielded 50,743 unigenes with an average of 1169 bp. The assembled unigenes were evaluated and functionally annotated by comparing with sequences in major public databases including protein sequence (Nr), Clusters of Orthologous Groups (COG), Swiss-Prot, Kyoto Encyclopedia of Genes and Genomes (KEGG) and Gene Ontology database (GO). Comparison of gene expression levels revealed that differentially expressed genes (DEGs) (P < .05): a total of 9096 (613 up-regulated and 8483 down-regulated genes) was obtained between ML and MM; 5778 (414 up-regulated and 5365 down-regulated) was obtained between MH and MM. Moreover, 12 carbohydrate metabolism-related genes were selected for quantitative real-time PCR analysis to validate the RNA-Seq data. The results confirmed the consistency of the expression levels between RNA-Seq and qRT-PCR data. Meanwhile, present study also examined the related enzyme activities of carbohydrate metabolism. The results showed that high level of dietary carbohydrate (360 g kg−1) exhibited a strong inhibitory effect on the genes of M. salmoides. Dietary carbohydrate in MM up-regulated 93.3% of differential expression genes (DEGs) compared to ML, however, 92.9% of the DEGs in MH were depressed compared to MM. This trend is also reflected in the corresponding enzyme activities in general. With dietary carbohydrate increase, there were severe histological alterations in M. salmoides liver. Collectively, this study shows a systematic overview of the transcriptome analysis in M. salmoides, and provides valuable gene information for studying molecular mechanisms of carbohydrate metabolism in freshwater fish.