Nutritional digestion and absorption, metabolism fates alteration was associated with intestinal function improvement by dietary threonine in juvenile grass carp (Ctenopharyngodon Idella)

Abstract

This study was designed to examine the effects of threonine (Thr) on digestion and absorption, serum biochemical parameters, genes expression of Thr metabolism related enzymes and intestinal mucins as well as potential regulation in juveniles grass carp (Ctenopharyngodon idella). Juveniles (9.53 ± 0.02 g) were reared for eight weeks with graded Thr levels (3.99, 7.70, 10.72, 14.10, 17.96 and 21.66 g/kg). Results showed that, comparing with Thr-insufficient diet (3.99 g/kg), optimal dietary Thr group (14.10 g/kg) significantly (P < 0.05) increased the digestive enzymes (trypsin, lipase and amylase) in the hepatopancreas and intestine, absorptive related enzymes Na+, K+-ATPase (NKA), alkaline phosphatase (AKP), creatine kinase (CK) and gamma-glutamyl transpeptidase (γ-GT) in intestines and contents of total protein (TP), albumin (ALB) and globulin (GLB) in the serum of juvenile grass carp. The notable decreases of serum total cholesterol (TC) content and activities of aspartate transaminase (AST) as well as alanine transaminase (ALT) were observed in juveniles fed 14.10 g/kg Thr diet (P < 0.05). Moreover, L-threonine dehydrogenase (tdh) was the main responsible for Thr catabolism enzyme with highly expressed in the hepatopancreas, which could be upregulated by increment dietary Thr in both hepatopancreas and intestines significantly (P < 0.05); L-threonine ammonia-lyase (thal) mRNA abundance was upregulated by dietary Thr in the hepatopancreas but not intestines; L-threonine aldolase (tha) mRNA abundance was not affected by dietary Thr in hepatopancreas and intestines (P > 0.05). Besides, the optimal Thr level promoted intestinal mucin synthesis involved enzyme N-acetyl-galactosaminyltransferase 2 (galnt2), mucin2 (muc-2) and muc-3 mRNA transcript abundances as well as phosphorylated protein levels of epidermal growth factor receptor (EGFR) (Tyr992), extracellular regulating kinase 1/2 (ERK1/2) (Thr202/Tyr204) and transcription factor SP1 (Thr453), thus enhancing the intestinal function. Additionally, the optimal dietary Thr could reverse the intestinal villus morphology structure destroyed with villus twist and exfoliation by dietary Thr deficiency as well as fusion by excess level of dietary Thr, which was in accordance with the improvement of intestinal function.