Dietary lysine deficiency causes intestinal immune dysfunction partially via Janus kinases (JAKs)/signal transducers and activators of transcription (STATs) signalling pathways of on-growing grass carp (Ctenopharyngodon idella)

Lysine is the limiting amino acid in aquatic animals, primarily derived from protein sources in feed, including fish meal, rapeseed meal, soybean meal, and casein. The intestinal immune function is crucial for maintaining the health and normal growth of organisms, particularly in fish. Further investigation was conducted to explore the potential impact of dietary lysine deficiency on intestinal disease resistance, immune response and signaling mechanisms in grass carp (Ctenopharyngodon idella) infected with Aeromonas hydrophila. A total of 540 fish (initial body weight: 164.85 ± 0.79 g) were allotted to 1 of 6 treatments with 3 replicates and 30 fish per replicate. The 30 fish were placed in a fishing net (140 cm × 140 cm × 140 cm). Fish were fed 6 isonitrogenous diets with varying lysine levels (5.6, 8.5, 11.6, 14.4, 17.5, and 20.7 g/kg diet) over a period of 60 days, followed by a 6-day challenge experiment. During the feeding trial, the fish were fed their corresponding diets 4 times daily. At the conclusion of the experiment, 24 fish from each treatment group (8 fish from each replicate) were selected for sampling based on their average weight. The research manifested that dietary lysine deficiency (5.6 g/kg diet) resulted in: (1) significantly (P < 0.05) decreased the growth performance (body length, body condition factor and feeding rate) and intestinal development (intestinal length, intestinal weight, intestinal length index and intestinal somatic index); (2) markedly (P < 0.05) decreased the resistance to pathogenic Aeromonas hydrophila infection, as evidenced by the highest rates of enteritis morbidity and notable histopathological alterations in the intestine; (3) significantly (P < 0.05) reduced the intestinal immune response through alterations in various immune components, including antibacterial peptides, complement 3 (C3) and immunoglobulin M (IgM); and (4) dramatically (P < 0.05) aggravated the intestinal inflammation through alterations in the expression profiles of inflammatory-related cytokines, which are linked to the regulation of Janus Kinase (JAK)/signal transducer and activator of transcription (STAT) signaling. In contrast, the intestinal transcriptional abundances of interleukin (IL)-12p35, STAT2, STAT3a and STAT6 were not regulated by dietary lysine. Furthermore, we estimated that the dietary lysine requirement levels for on-growing grass carp are between 13.93 and 14.51 g/kg of diet, based on various immune and physiological parameters.