Abstract
To investigate the effects and potential mechanisms of dietary choline on immune function in the skin of juvenile grass carp (Ctenopharyngodon idella), fish were fed different diets containing different levels of choline (142. 2, 407.4, 821.6, 1215.8, 1589.3, and 1996.6 mg/kg) for 70 d and then sampled after a 6-d challenge test. The results exhibited that dietary choline (1) advanced the contents of phosphatidylcholine (PC), betaine, and choline in grass carp skin (P < 0.05) and upregulated the mRNA abundance of choline transporter high-affinity choline transporter 1 (CHT1), choline transporter-like protein 1 (CTL1), and choline transporter-like protein 5 (CTL5), indicating that dietary choline could increase the contents of choline which might be connected with choline transporters in the grass carp skin; (2) receded skin rot symptom after infection with A. hydrophila (Aeromonas hydrophila), increased the levels of IgM, C4, and C3 and the activities of acid phosphatase (ACP) and lysozyme (LZ), raised mucin2, β-defensin, hepcidin, and LEAP-2B mRNA abundance (rather than LEAP-2A), downregulated pro-inflammatory cytokine mRNA abundance (IFN-γ2, IL-15, TNF-α, IL-6, IL-12P40, and IL-1β) in skin of juvenile grass carp (P < 0.05), and upregulated anti-inflammatory cytokine mRNA abundance (IL-10, IL-4/13A, TGF-β1, IL-11, and IL-4/13B) in grass carp skin (P < 0.05), demonstrating that choline enhanced the skin immune function; and (3) downregulated the mRNA abundance of IKKγ, NF-κBp52, IKKβ, c-Rel, NF-κBp65, STAT3b2, STAT3b1, JAK1, and JAK2 as well as protein level of NF-κBp65, p-STAT3 Tyr705, and p-STAT3 Ser727 in nucleus and inhibited the mRNA and protein level of IkBα (P < 0.05), indicating that choline-enhanced immune function might be relevant to the JAK1, 2 /STAT3, and NF-κB signaling pathway in fish skin. In conclusion, choline enhanced the skin immune function which might be related to JAK1, 2/STAT3, and NF-κB signaling molecules in fish. Furthermore, based on immune indices of grass carp (9.28–108.97 g) skin (C3 and IgM contents as well as ACP activities), the choline requirements were estimated to be 1475.81, 1364.24, and 1574.37 mg/kg diet, respectively.
Highlights
Choline is an indispensable vitamin B for fish [1]
The choline transporter mRNA abundance of skin is exhibited in Figure 1; CTL2 and CTL5 mRNA abundance was raised with dietary choline by 1215.8 and 1589.3 mg/kg (P < 0.05), respectively, plateaued
CHT1 mRNA abundance was slowly increased with dietary choline addition (P < 0.05)
Summary
Choline is an indispensable vitamin B for fish [1]. Our previous study found that optimal choline supplementation promoted the growth performances of grass carp (Ctenopharyngodon idella) [2, 3]. Skin is a crucial immune organ in fish [4], whose health is important for fish growth and disease resistance [5]. No studies had reported the effect of choline on grass carp skin. Fish skin immunity is closely related to the cytokines. NF-κB and STAT3 each control the expression of a large number of downstream genes that control cell proliferation, survival, stress responses, and immune functions. Choline enhanced the acetylcholine (ACh) level which could activate the JAK2/STAT3 signaling pathway in mouse PC12 cells [14, 15]. There might be a relationship between immune function and choline, which may be related to the JAK/STAT3 and NF-κB signaling pathway in fish skin
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