Dietary isoleucine improved flesh quality, muscle antioxidant capacity, and muscle growth associated with AKT/TOR/S6K1 and AKT/FOXO3a signaling in hybrid bagrid catfish (Pelteobagrus vachelli♀ × Leiocassis longirostris♂)

Abstract

Background: Muscle is the complex and heterogeneous tissue, which comprises the primary edible part of the trunk of fish and mammals. Previous studies have shown that dietary isoleucine (Ile) exerts beneficial effects on growth in aquatic animals. However, there were limited studies regarding the benefits of Ile on fish muscle and their effects on flesh quality and muscle growth. Thus, this study was conducted to explore whether dietary Ile had affected flesh quality and muscle growth in hybrid bagrid catfish (Pelteobagrus vachelli♀ × Leiocassis longirostris♂).

Methods: A total of 630 hybrid fish, with an initial average body weight of 33.11 ± 0.09 g, were randomly allotted into seven experimental groups with three replicates each, and respectively fed seven diets with 5.0, 7.5, 10.0, 12.5, 15.0, 17.5, and 20.0 g Ile/kg diets for 8 weeks.

Results: In the present study, we demonstrated that Ile significantly: (1) increased muscle protein and lipid contents and the frequency distribution of myofibers with ≤ 20 μm and ≥ 50 μm of diameter; (2) improved pH value, shear force, cathepsin B and L activities, hydroxyproline content, resilience, cohesiveness, and decreased cooking loss, lactate content, hardness, springiness, gumminess, and chewiness; (3) decreased reactive oxygen species (ROS), malondialdehyde (MDA), and protein carbonyl (PC) contents, GCLC and Keap1 mRNA levels, and up-regulated CuZnSOD, CAT, GPX1a, GST, and Nrf2 mRNA levels; (4) up-regulated the insulin-like growth factor 1, 2 (IGF-1, IGF-2), insulin-like growth factor 1 receptor (IGF-1R), proliferating cell nuclear antigen (PCNA), Myf5, Myod, Myog, Mrf4, and MyHC mRNA levels, and decreased MSTN mRNA level; (5) increased muscle protein deposition by activating AKT-TOR-S6K1 and AKT-FOXO3a signaling pathways.

Conclusion: These results revealed that dietary Ile improved flesh quality, which might be due to increasing nutritional content, physicochemical, texture parameters, and antioxidant ability; promoting muscle growth by affecting myocytes hyperplasia and hypertrophy, and muscle protein deposition associated with protein synthesis and degradation signaling pathways. Finally, the quadratic regression analysis of chewiness, ROS, and protein contents against dietary Ile levels suggested that the optimal dietary Ile levels for hybrid bagrid catfish was estimated to be 14.19, 12.36, and 12.78 g/kg diet, corresponding to 36.59, 31.87, and 32.96 g/kg dietary protein, respectively.

Keywords: Antioxidant capacity; Flesh quality; Hybrid bagrid catfish; Isoleucine; Myogenesis; Protein deposition.

Fig. 1

The mRNA levels of CuZnSOD, CAT, GST, GPX1a, and GCLC in muscle of hybrid bagrid catfish fed diets containing graded levels of Ile. Data represent means ± SEM, of six replicates. Values having different letters are significantly different (P < 0.05)

Fig. 2

The mRNA levels of Nrf2 and Keap1 in muscle of hybrid bagrid catfish fed diets containing graded levels of Ile. Data represent means ± SEM, of six replicates. Values having different letters are significantly different (P < 0.05)

Fig. 3

The mRNA levels of IGF1, IGF2, and IGF1R in muscle of hybrid bagrid catfish fed diets containing graded levels of Ile. Values are means ± SEM, of three replicates, with six fish in each replicate, and different letter denotes significant difference (P < 0.05)

Fig. 4

Effects of dietary Ile on muscle growth related gene mRNA levels in muscle of hybrid bagrid catfish fed diets containing graded levels of Ile. Values are means ± SEM, of three replicates, with six fish in each replicate, and different letter denotes significant difference (P < 0.05)

Fig. 5

Effect of dietary Ile on the PI3K-AKT-TOR signaling pathway in hybrid bagrid catfish muscle fed diets containing graded levels of Ile. (a) The mRNA levels of PI3K, AKT, TOR, S6K1, and 4E-BP1; (b) Protein abundances of p-AKT (Ser473), AKT, and ratio of p-AKT and AKT; (c) Protein abundances of p-TOR (Ser2448), TOR, and ratio of p-TOR and TOR; (d) Protein abundances of p-S6K1 (Thr421/Ser424), S6K1, and ratio of p-S6K1 and S6K1. Data represent means ± SEM, of three replicates. Values having different letters are significantly different (P < 0.05)

Fig. 6

Effects of dietary Ile on the protein degradation related signaling pathway in hybrid bagrid catfish muscle fed diets containing graded levels of Ile. (a) The mRNA levels of FOXO3a, MAFBX, and MURF-1; (b) Protein abundances of p-FOXO3a (Ser253), FOXO3a, and ratio of p-FOXO3a and FOXO3a; Protein abundances of MURF-1 (c) and MAFBX (d). Data represent means ± SEM, of three replicates. Values having different letters are significantly different (P < 0.05)