Dietary exogenous supplementation of nucleotides strengthens the disease resistance, antioxidant capacity and immunity in the gill of on-growing grass carp (Ctenopharyngodon idella) following a challenge with Flavobacterium columnare

Low-molecular-weight nucleotides are very important for fish and terrestrial animals, which have been verified to possess multidirectional biological functions including anti-inflammatory, anti-microbial, anti-oxidant, anti-apoptotic effects. This research was designed to fully explore the possible impacts of dietary exogenous nucleotides supplementation on gill disease resistance, antioxidant capacity, antibacterial activity, inflammatory cytokines and the associated signaling mechanisms in on-growing grass carp (Ctenopharyngodon idella) (200.0 ± 1.0 g). Fish were fed six diets containing graded levels of nucleotides at 0, 200.0, 400.0, 600.0, 800.0 and 1000.0 mg/kg in triplicate for 60 days, followed by a challenge with Flavobacterium columnare for 3 days. Our results showed that compared to 0 mg/kg nucleotides diet group, 600.0 mg/kg nucleotides diet group markedly decreased the gill rot morbidity of fish induced by F. columnare (P < 0.05), and significantly enhanced antioxidant related enzymes activities and corresponding gene expressions partly ascribed to the modulation of Nrf2 signaling (P < 0.05). Meanwhile, 600.0 mg/kg nucleotides diet group dramatically elevated antimicrobial substances (lysozyme and IgM) productions and antimicrobial peptides (such as β-defensin) and Mucin-2 expression levels (P < 0.05) as well as altered the expression profiles of main inflammatory-related genes (such as interleukin-1β and IL-8) partly correlated with the action of NF-κB and TOR signaling (P < 0.05). However, nucleotides diet groups did not alter the expressions of cytokines TNF-α, IFN-γ, IL-12p35 and IL-4/13B (P > 0.05). Dietary exogenous nucleotides supplementation has potential benefits in strengthening gill disease resistance, antioxidant capacity and immunity in grass carp, the optimal levels of dietary exogenous nucleotides supplementation are recommended to be 526.4–639.8 mg/kg based on gill health-related indices.