Folic acid deficiency impairs the gill health status associated with the NF-κB, MLCK and Nrf2 signaling pathways in the gills of young grass carp (Ctenopharyngodon idella)

• Folic acid deficiency decreased fish gills immune and barrier function.
• Folic acid regulated gills cytokines associated with NF-κB pathway.
• Folic acid regulated gills tight junction proteins depended on MLCK pathway.
• Folic acid regulated gills antioxidase activities relied on gene expression.
• Folic acid increased antioxidase gene expression associated with Nrf2 pathway.

The aim of this study was to investigate the effect of dietary folic acid on fish growth, the immune and barrier functions of fish gills, and the potential mechanisms of these effects. Young grass carp (Ctenopharyngodon idella) were fed diets containing graded levels of folic acid at 0.10 (basal diet), 0.47, 1.03, 1.48, 1.88 and 3.12 mg kg−1 diet for 8 weeks. The results showed that acid phosphatase and lysozyme activities and the complement component 3 content in fish gills decreased with folic acid deficiency (P < 0.05). Folic acid deficiency up-regulated liver-expressed antimicrobial peptide 1, interleukin 1β, interleukin 8, tumor necrosis factor α, nuclear factor κB p65, IκB kinase α (IKK-α), IKK-β and IKK-γ gene expression. Folic acid deficiency down-regulated interleukin 10, transforming growth factor β, IκB and target of rapamycin gene expression in fish gills (P < 0.05). These results showed that limited folic acid decreased fish gill immune status. Furthermore, folic acid deficiency down-regulated claudin-b, claudin-c, claudin-3, occludin and zonula occludens 1 gene expression, whereas folic acid deficiency up-regulated claudin-12, claudin-15, myosin light chain kinase and p38 mitogen activated protein kinase gene expression in fish gills (P < 0.05). These results suggested that folic acid deficiency disrupted tight junction-mediated fish gill barrier function. Additionally, folic acid deficiency increased the content of reactive oxygen species, protein carbonyl and malondialdehyde (MDA); Mn superoxide dismutase activity and gene expression; and Kelch-like-ECH-associated protein 1a (Keap1a) and Keap1b gene expression (P < 0.05). Conversely, folic acid deficiency decreased Cu/Zn superoxide dismutase, catalase, glutathione peroxidase, glutathione s-transferases and glutathione reductase activities and gene expression as well as NF-E2-related factor 2 gene expression in fish gills (P < 0.05). All of these results indicated that folic acid deficiency impaired fish gill health status via regulating gene expression of cytokines, tight junction proteins, antioxidant enzymes, NF-κB p65, MLCK and Nrf2. Based on percent weight gain, LZ activity and MDA content in the gills, the dietary folic acid requirements for young grass carp were 1.60, 2.07 and 2.08 mg kg−1, respectively.