Effects of mTOR signalling pathway activation on growth performance, amino acid metabolism and intestinal health of sea bass (Lateolabrax maculatus) fed low-protein diets

This study examined how activation of the mTOR pathway influences growth, amino-acid metabolism, and intestinal health in juvenile spotted seabass (Lateolabrax maculatus) fed a low-protein diet. Triplicate groups received, for eight weeks, either a 50% crude-protein control diet (CON), a 48% crude-protein diet (P48), or the P48 diet supplemented with 0.1% of a specific mTOR activator (P48S). Fish (21.20 ± 0.20) g were reared in a recirculating aquaculture system under controlled conditions. Relative to CON, P48 depressed weight gain rate (WGR) and specific growth rate (SGR) and increased feed-conversion ratio (FCR). Supplementation in P48S reversed these deficits: WGR rose 25.12%, SGR 12.03%, and FCR fell 8.33% compared with P48. Whole-body crude protein increased 4.45%, while plasma total cholesterol and triglycerides declined 12.57% and 20.44%, respectively. Blood analysis revealed that the P48S group had significantly higher plasma levels of total free amino acids (TAA), total essential amino acids (EAA), and total non-essential amino acids (NEAA) compared to both the CON and P48 groups, coupled with elevated muscle free EAA content. Morphologically, villus height was 22.82% greater in P48S than in P48. Complementing these findings, further biochemical analysis uncovered enhanced activity of key intestinal digestive enzymes, specifically trypsin and lipase, in the P48S group. Additionally, gene expression analysis indicated upregulated levels of crucial amino acid and small peptide transporters (pept1, lat1, and asct2) within the intestines of the P48S group. In conclusion, while reducing dietary protein levels clearly suppresses growth performance in L. maculatus, the activation of the mTOR signaling pathway presents a viable strategy to counteract these effects. This activation improves protein synthesis by enhancing feed protein digestion, promoting amino acid absorption and metabolism, and thereby significantly restoring and enhancing growth performance, even under suboptimal protein dietary conditions.