Betaine Alleviated the Ammonia‐Induced Apoptosis and Inflammation in the Skin of Largemouth Bass Fed With High‐Carbohydrate Diet via Inhibiting MAPK/NFκB‐Myd88 Pathway

Fish skin provides vital protection against environmental stressors and pathogens, whose health is significantly affected by dietary components. In intensive aquaculture, accumulated ammonia seriously affected fish skin structure and immune responses. In this study, largemouth bass were fed with control diet (Con), high-carbohydrate (HC) diet and HC diet supplemented with betaine (HC + Bet) for 8 weeks before ammonia exposure. The skin structure, immune responses, programmed cell death (PCD), and status of mitogen-activated protein kinase (MAPK) pathways were evaluated. Results indicated ammonia stress increased epidermal thickness and skin mucus cell numbers, while betaine supplementation recovered the HC-restricted epidermal thickness at 7 days post-stress. Ammonia stress also induced the expression of inflammatory cytokines in skin, while betaine significantly inhibited NF-κB/myd88 pathway to alleviate the over-inflammation in skin of HC-fed largemouth bass. Further study identified the significantly increased TdT-mediated dUTP Nick-End Labeling⁺ (TUNEL⁺) cell numbers in bass skin after ammonia stress, which resulted from apoptosis rather than pyroptosis. Furthermore, p38 MAPK and c-Jun N-terminal kinase (JNK) signaling pathways mediated for the ammonia-induced apoptosis, while dietary betaine supplementation inhibited the over-activation of p38 MAPK and JNK pathways to reduce epidermal apoptosis during ammonia stress. Therefore, dietary betaine alleviated both over-inflammation and apoptosis in the skin of largemouth bass fed with HC diet during ammonia stress.