Novel insights into choline's role in promoting myofiber hypertrophy and mitochondrial function in muscle of sub-adult grass carp (Ctenopharyngodon idella).

Choline is an essential nutrient for fish, and its metabolites are components of all membrane systems. Membrane phospholipids play an important role in maintaining the morphology and function of mitochondria, which could affect the growth and development of myofiber. To evaluate the effects of choline on growth performance, muscle fiber development, and mitochondrial morphology and function in grass carp, six diets containing graded choline levels [154.0 (basal diet), 321.8, 672.2, 1024.9, 1373.2, and 1722.8 mg/kg] were fed to 450 sub-adult grass carp (583.44 ± 1.65 g) for 56 d. The fish were divided into six groups with three replicates of 25 fish each. The results showed as follows: 1) compared with the control group, choline increased the growth performance, muscle protein content and choline deposition in sub-adult grass carp (P < 0.05), while decreased adhesive force (P < 0.001) and cooking loss (P = 0.037), thus improving fish flesh quality. 2) Dietary choline levels of 1024.9 to 1373.2 mg/kg significantly increased the frequency of myofibers with diameter >100 μm (P = 0.002) and fusion coefficient (P < 0.001). The mRNA expression of MyoG (P < 0.001), MyoD (P < 0.001), Myf5 (P = 0.005), MRF4 (P = 0.028), MYHC (P = 0.005), and myomaker (P < 0.001) increased quadratically with dietary choline, whereas MSTN expression was non-significantly lower than controls at 1024.9 to 1373.2 mg/kg (P = 0.078). Furthermore, choline increased the mRNA expression of IGF1, PI3K, AKT, and TOR (P < 0.05) and decreased the mRNA expression of FoxO3a and MuRF1 (P < 0.05). 3) Muscle adenosine triphosphate (ATP) content (P < 0.001), protein levels of NDUFV1 (P = 0.004) and ATP5A1 (P = 0.046), and the mRNA levels of NRF1 (P = 0.017), TFAM (P = 0.014), and OPA1 (P < 0.001) were increased quadratically. Conversely, the mRNA levels of DRP1 (P < 0.001), PINK (P < 0.001), and PARKIN (P = 0.002) were decreased quadratically. 4) According to the quadratic regression analysis of percent weight gain (PWG) and muscle ATP content, the choline requirement of sub-adult grass carp was 951.2 and 1084.9 mg/kg diet, respectively. In summary, optimal levels of dietary choline improved the myofiber hypertrophy in grass carp, which was associated with upregulation of myogenesis regulator and the activation of the IGF1/PI3K/AKT signaling pathway, and maintained mitochondrial stability probably via regulating mitochondrial quality control.