A study on the function of methionine in growth, immunity, antioxidant, endoplasmic reticulum stress and apoptosis of largemouth bass (Micropterus salmoides)

An 8-week culture experiment was conducted to evaluate the effects of methionine (Met) on growth, immunity and antioxidant capacity in largemouth bass ( Micropterus salmoides). Six dietary Met levels were tested and formulated based on isonitrogenous and isoenergetic principles: 0.74 %, 0.91 %, 1.13 %, 1.37 %, 1.59 % and 1.75 %. Compared to the control group (0 % Met supplementation, diet level of 0.74 %), dietary Met supplementation significantly improved growth, as indicated by higher FW, WG, SGR and lower FCR (p< 0.05). Furthermore, liver Oil Red O staining revealed a significantly lower positive area for groups receiving 1.13-1.75 % Met (p< 0.05). Plasma TC and TG were also significantly lower in the 0.91 %-1.75 % dietary Met groups (p< 0.05). Met supplementation enhanced antioxidant capacity by increasing plasma levels of antioxidant parameters, including CAT, T-SOD and GSH, and by modulating antioxidant gene expression within the Nrf2 signalling pathway in the liver ( nrf2 , keap1, cat and sod) (p< 0.05), which was supported by significantly lower MDA level in plasma (p< 0.05). Met supplementation also alleviated apoptosis and endoplasmic reticulum stress (ERS) by upregulating anti-apoptotic genes ( bcl-2 , bcl-xl) and downregulating pro-apoptotic and ERS marker genes ( caspase 3 , bax, caspase 8 , atf4, caspase 9 , ask1, ire1, chop alpha, eif2 alpha, perk) (p< 0.05). Met supplementation also reduced inflammatory responses by modulating NF-kappa B signalling pathway gene expressions ( il-8 , nf-kappa b, il-10 and tnf-alpha) and lowering plasma TNF-alpha and IL-8 levels. Nonetheless, mRNA levels of il-1 beta and tgf-beta in the liver and plasma remained unchanged across different Met levels. Based on FCR and SGR, the optimal Met requirement for largemouth bass was determined to be 1.26 %-1.32 % (2.65 %-2.78 % of dietary protein) through quadratic regression analysis. Hence, Met plays a crucial role in regulating growth, immunity, antioxidant, ERS and apoptosis in largemouth bass.