Effects of proline on growth performance, protein synthesis and cold resistance in white shrimp (Litopenaeus vannamei)

This study aimed to explore the effects of proline on growth, protein synthesis, and cold resistance of Pacific white shrimp (Litopenaeus vannamei). A total of 720 juvenile shrimp (0.37 ± 0.00 g) were randomly distributed into 18 tanks with 40 shrimp per tank, and the temperature was controlled at 30 ± 1 °C. Shrimp were fed six diets with proline levels of 2.21%, 2.40%, 2.51%, 2.70%, 2.80%, and 3.02%, respectively for 56 d, and then subjected to acute cold stress (20 °C) for 12 h. The results showed that appropriate proline could improve growth performance, feed utilization, free proline, hydroxyproline, glycine, total amino acids, and total non-essential amino acids (P < 0.05), as well as mRNA expression (tor, akt, and s6k) and protein level (protein kinase B, Akt) of key genes involved in the target of rapamycin (TOR) signal pathway (P < 0.05), suggesting that proline may regulate protein deposition through TOR pathway. Acute cold stress significantly increased hemocyte reactive oxygen species (ROS) level, cell apoptosis and the expression of genes involved in antioxidant (cat, sod, gpx, hsp70), apoptosis (caspase-3, caspase-9, p53, romo1), autophagy (lc3, atg4, becline-1, and atg1), and adenosine 5'-monophosphate (AMP)-activated protein kinase (AMPK)/TOR signaling pathway (raptor and ampk). Under cold stress, dietary proline significantly decreased ROS levels, cell apoptosis and the expression of apoptosis related genes (caspase-3, caspase-9, and romo1), but increased the expression of antioxidant related genes (gpx and hsp70) and autophagy related genes (lc3, atg4, becline-1, and atg1) (P < 0.05). In addition, dietary proline significantly up-regulated the mRNA expression of ampk and raptor and the protein level of phospho-raptor in muscle (P < 0.05), while down-regulated the mRNA expression of s6k (P = 0.024). Based on the quadratic regression analysis of feed conversion ratio, protein efficiency ratio, ROS, apoptosis, and broken line analysis of free proline in muscle, dietary proline content of 2.50% to 2.79% could effectively improve growth, protein synthesis and cold resistance of Pacific white shrimp.