Dietary protein hydrolysate effects on growth, digestive enzymes activity, and expression of genes related to amino acid transport and metabolism of larval snakehead (Channa argus)

A feeding trial was conducted to investigate the effects of dietary protein hydrolysate on the growth, digestive enzyme activity, and expression of genes related to amino acid transport and metabolism in larval snakehead. Five isonitrogenous (crude protein, 59%) and isolipidic (crude lipid, 13%) diets were formulated using pre-mixed protein hydrolysates to replace approximately 0% (control, PH0), 25% (PH25), 50% (PH50), 75% (PH75) and 100% (PH100) un-hydrolyzed pre-mixed protein. Triplicate groups of snakehead larvae (17 days post-hatching, initial body weight 122 ± 0.10 mg) were randomly assigned to 15 fiberglass fish tanks with a density of 750 larvae per tank, and fed four times daily for 20 days. At the end of the feeding trial, the growth metric was monitored, and tissues, including intestine and muscle, were separated for physiology and molecular biology analysis. The results showed that dietary protein hydrolysate quadratically increased final body weight (FBW, 2.57–3.73 g) and survival rate (SR, 53.33–60.14%), with the maximum value being observed in the PH75 group (FBW, 3.73 g; SR, 60.14%). Meanwhile, the optimal level of dietary protein hydrolysate elevated the activity of digestive enzymes including trypsin (1558.24–2520.37 U/gpro), lipase (22.37–36.26 U/gpro) and alkaline phosphatase (3.05–3.98 U/mgpro). Gene expression results revealed that protein hydrolysate inclusion quadratically elevated the expression of intestinal amino acid transporter y+LAT2 (0.96–1.51) and peptide transporter PepT1 (0.68–1.22), with the maximum value observed in the PH75 (1.51) and PH50 (1.22) groups, respectively. Meanwhile, the expression of muscle PepT1 (0.97–1.73) was up-regulated linearly, and y+LAT2 expression (0.54–1.01) quadratically decreased with the inclusion of protein hydrolysate. Dietary protein hydrolysate appeared activate the target of rapamycin (TOR) pathway in the muscle through the up-regulation of TOR expression (1.00–1.20) and down-regulation of 4EBP1 expression (0.56–1.01). Additionally, the expression of genes involved in the amino acids response (AAR) pathway in muscle, eIF2α (0.50–1.03), CHOP (0.36–1.09) and REDD1 (0.27–0.98), were linearly inhibited with the inclusion of protein hydrolysate. In above, the optimal level of dietary protein hydrolysate (75%) elevated the growth performance and improved the digestive tract of snakehead larvae. The beneficial role of protein hydrolysates in growth and development is partly attributed to the regulation of genes related to amino acid transport and metabolism.