Dietary threonine above the requirement for protein synthesis examined at the molecular level stimulates gill mucus cell production in juvenile Nile tilapia

Abstract This study investigates the hypothesis that dietary supplementation of threonine above the current recommendation for protein synthesis stimulates gill mucus cell production in juvenile Nile tilapia, Oreochromis niloticus. During our experiment, six isonitrogenous and isocaloric extruded diets were elaborated with graded threonine levels [8 (control), 12, 16, 20, 24, and 28 g kg−1]. Fish (n = 216; 10.78 ± 0.03 g) were randomly assigned to triplicate groups of 12 fish and hand-fed six times a day until apparent satiety. Maximum body weight gain and feed efficiency ratio occurred in fish fed up to 17.71 and 13.73 g kg−1 diet of threonine, respectively. However, gill mucus cell counts were significantly higher in fish fed up to 20 g kg−1 diet of threonine, than that fed diet control (8 g kg−1 diet threonine). The relative expressions of MyoD and MyoG genes were upregulated in fish fed threonine at 16 g kg−1. Second-order polynomial regression analysis of protein retention efficiency gain against dietary threonine levels estimates that the dietary threonine requirement for juvenile Nile tilapia is 16.30 g kg−1 diet (4.89 g 100 g−1 protein). In contrast, a higher threonine level of 20 g kg−1 diet (6.00 g100 g−1 protein) optimizes gill mucus cell counts. This study provides insight into threonine as functional amino acid in tilapia aquaculture.