Dietary protease inclusion with activity after pelleting affects growth performance, digestive enzyme activity, nutrient sensing signaling pathway and intestinal health of larval largemouth bass (Micropterus salmoides)

A 32-day feeding trial was conducted to explore the effects of dietary protease (with activity after pelleting) on growth performance, digestive enzyme activity, nutrient-sensing signaling pathways and intestinal health of larval largemouth bass. Four isonitrogenous and isolipidic micro-diets were formulated with graded levels of protease (neutral protease: keratinase = 2:1) at 0 % (PE-0), 1 % (PE-1), 2 % (PE-2), and 3 % (PE-3). The larvae were fed these diets to apparent satiety four times daily with three replicates. In the present study, the exogenous enzymes have been confirmed to function after pelleting. Dietary protease inclusion linearly reduced the deformity rate and enhanced the growth performance of largemouth bass larvae, with no significant difference was observed in growth between the PE-2 and PE-3 groups. The activities of digestive enzymes also increased linearly, with no significant differences between the PE-2 and PE-3 groups. Furthermore, protease supplementation resulted in a linear increase in the gene expressions of gastrointestinal peptides and amino acid transporters, with the highest expression of lat1 and lat2 observed in the PE-2 group. Meanwhile, dietary protease inclusion linearly elevated the TOR pathway by activating tor and akt1, with no significant differences between the PE-2 and PE-3 groups, while it also restricted 4ebp1. Protease inclusion also linearly enhanced the gene expressions of intestinal tight junction proteins (claudin4 and ocln). Moreover, dietary protease inclusion linearly inhibited the expression of pro-inflammatory factors (p38β-mapk11, p38α-mapk14, rela, tnf-α, il-1β, and il-8) and increased the expression of the anti-inflammatory factor tgf-β1, with no significant differences observed between the PE-2 and PE-3 groups. Additionally, protease inclusion elevated the expression of the non-specific immune factor lysozyme, with the highest value observed in the PE-2 group. Overall, the inclusion of protease with activity after pelleting improved the growth and development of larval largemouth bass, and the inclusion dose of 2 %–3 % was recommended for the micro-diet.