Nitrite poses a serious threat to intensive aquaculture. Protein, as a major nutrient in animals, is vital for protecting animal tissues from damage. In this study, we investigated the protective effect of dietary protein on gill tissue structure and the underlying mechanisms in sub-adult grass carp (Ctenopharyngodon idella) exposed to nitrite stress. Six iso-energetic semi-purified diets containing different protein levels (16-31 %) were formulated, and fed to fish for 60 d. The fish were then exposed to a nitrite solution for 4 d. Histopathological observation and determination of related indices (serum glucose, serum cortisol, nitric oxide, peroxynitrite, reactive oxygen species, malondialdehyde, and protein carbonyl) showed that 22-25 % dietary protein significantly alleviated the nitrite-induced stress response, gill tissue damage and oxidative damage. Further research found that a suitable dietary protein suppressed the nitrite-induced endoplasmic reticulum stress (ERS) 78 kDa glucose-regulated protein (GRP78) related signaling pathway which possibly activated autophagy and apoptosis. Interestingly, we discovered that proper dietary protein reduced autophagy, probably through unc-51-like kinase 1 (Ulk1), BCL-2-interacting myosin-like coiled-coil protein (Beclin1), autophagy-related gene 5 (Atg5), Atg12, microtubule-associated protein1 light chain 3 (LC3), BCL-2 interacting protein 3 (BNIP3) and autophagy receptor P62 (p62). We also found that an appropriate dietary protein inhibited nitrite-induced apoptosis via mitochondrial and death receptor pathways. In summary, our findings are the first to demonstrate that 22-25 % of dietary protein levels can play a protective role against nitrite-induced gill injury.
Keywords: Autophagy; Endoplasmic reticulum stress (ERS); Gill; Grass carp (Ctenopharyngodon idella); Nitrite; Protein.
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