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 delved into the impact of the mTOR signaling pathway on key physiological aspects, including growth performance, amino acid metabolism, and intestinal health, in Lateolabrax maculatus maintained on low-protein diets. To investigate this, we designed a comparative trial involving three distinct dietary groups over an eight-week period. The control group (CON) received a standard diet containing 50% crude protein. This was contrasted with two experimental groups: a low-protein group (P48) receiving a diet with 48% crude protein, and a third experimental group (P48S) which received the same low-protein diet but supplemented with 0.1% of an mTOR signaling pathway activator. Juvenile L. maculatus, initially weighing (21.20 ± 0.20) g, were carefully reared within an indoor recirculating aquaculture system to ensure controlled environmental conditions throughout the feeding trial. The results demonstrated that the low-protein diet (P48) had a detrimental effect, significantly reducing weight gain rate (WGR) and specific growth rate (SGR) while concomitantly increasing the feed conversion ratio (FCR), indicating poorer feed utilization. However, supplementation with the 0.1% mTOR activator in the P48S group proved effective, as it successfully activated the mTOR signaling pathway, leading to notable improvements in both growth performance and feed utilization metrics. Specifically, compared to the P48 group, the P48S group exhibited a substantial 25.12% increase in WGR, a 12.03% increase in SGR, and an 8.33% reduction in FCR. Furthermore, the P48S group displayed a 4.45% increase in whole-body crude protein content, alongside healthier lipid profiles marked by 12.57% and 20.44% decreases in plasma total cholesterol and triglyceride levels, 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. At the intestinal level, morphological analysis showed that the P48S group had significantly greater intestinal villus height than the P48 group, with a 22.82% increase. 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.