Establishing the link between D-mannose and juvenile grass carp (Ctenopharyngodon idella): Improved growth and intestinal structure associated with endoplasmic reticulum stress, mitophagy, and apical junctional complexes

D-mannose, essential for protein glycosylation, has been reported to have immunomodulatory effects and to maintain intestinal flora homeostasis. In addition to evaluating growth performance, we examined the impact of D-mannose on the structure of epithelial cells and apical junction complexes in the animal intestine. All 1800 grass carp (16.20 ± 0.01 g) were randomly divided into six treatments with six replicates of 50 fish each and fed with six different levels of D-mannose (0.52, 1.75, 3.02, 4.28, 5.50 and 6.78 g/kg diet) for 70 d. The study revealed that D-mannose increased feed intake ( < 0.001) but did not affect the percent weight gain (PWG), special growth rate, and feed conversion ratio ( > 0.05). D-mannose supplementation at 1.75 g/kg increased crude protein content in fish and lipid production value ( < 0.05). D-mannose supplementation at 4.28 g/kg increased intestinal length, intestinal weight and fold height of grass carp compared to the control group ( < 0.05). This improvement may be attributed to the phosphomannose isomerase (PMI)-mediated enhancement of glycolysis. This study found that D-mannose supplementation at 4.28 or 3.02 g/kg reduced serum diamine oxidase activity or D-lactate content ( < 0.05) and improved cellular and intercellular structures for the first time. The improvement of cellular redox homeostasis involves alleviating endoplasmic reticulum (ER) stress through the inositol-requiring enzyme 1 (IRE1), RNA-dependent protein kinase-like ER kinase (PERK), and activating transcription factor 6 (ATF6) signaling pathways. The alleviation of ER stress may be linked to the phosphomannomutase (PMM)-mediated enhancement of protein glycosylation. In addition, ubiquitin-dependent [PTEN-induced putative kinase 1 (PINK1)/Parkin] and ubiquitin-independent [BCL2-interacting protein 3-like (BNIP3L), BCL2-interacting protein 3 (BNIP3), and FUN14 domain containing 1 (FUNDC1)] mitophagy may play a role in maintaining cellular redox homeostasis. The enhancement of intercellular structures includes enhancing tight junction and adherent junction structures, which may be closely associated with the small Rho GTPase protein (RhoA)/the Rho-associated protein kinase (ROCK) signaling pathway. In conclusion, D-mannose improved intestinal cellular redox homeostasis associated with ER stress and mitophagy pathways, and enhanced intercellular structures related to tight junctions and adherent junctions. Furthermore, quadratic regression analysis of the PWG and intestinal reactive oxygen species content indicated that the optimal addition level of D-mannose for juvenile grass carp was 4.61 and 4.59 g/kg, respectively.