Abstract
The purpose of the experiment was to investigate the Toll-like receptor signaling pathway of quercetin regulating avian beta-defensin (AvBD) in the ileum of Arbor Acre (AA) broilers. Four hundred and eighty one-day-old Arbor Acre broilers with similar body weight, half male and female, were randomly allotted to four treatments; the control treatment and three dietary treatments were fed with the basal diets supplemented with 0, 0.02%, 0.04, and 0.06% quercetin, respectively. The results showed that dietary quercetin supplementation did not significantly influence growth performance (p > 0.05), but significantly decreased the mortality rate of broilers by 85.74%, 85.74, and 71.28%, respectively (p < 0.05, F = 9.06). Compared with control, dietary supplementation with 0.04 and 0.06% quercetin significantly upregulated mRNA expression of total AvBD (p < 0.05), and there were no significant differences in the mRNA expression of AvBD1, AvBD2, and AvBD14 in three quercetin supplementation groups in the ileum of AA broilers (p > 0.05). Dietary supplementation with 0.02 and 0.06% quercetin significantly downregulated the mRNA expression of total Toll-like receptors (p < 0.05). Dietary quercetin supplementation significantly downregulated the mRNA expression of TLR1A, TLR1B, and TLR2A (p < 0.05); however, there were no significant differences in the mRNA expression of TLR2B, TLR5, and TLR15 (p > 0.05). Dietary quercetin supplementation significantly downregulated the mRNA expression of myeloid differentiation primary response protein 88 (MyD88) and TIR domain-containing adaptor protein/MyD88-adaptor-like (TIRAP/MAL) (p < 0.05), 0.02% quercetin significantly downregulated the mRNA expression of tank-binding kinase1 (TBK1), IκB kinase complex-α (IKKα), IKKβ, IKKε, nuclear factor-kappa B (NF-κB), NF-κB inhibitor-alpha (IκBα), IκBα, IκBβ, TNF-receptor-associated factor 3 (TRAF3), and interferons regulatory factor 7 (IRF7) (p < 0.05), 0.04% quercetin significantly downregulated the mRNA expression of IKKβ, IKKε, NF-κB, IκBα, IκBβ, TRAF3, and TRAF6 (p < 0.05), and 0.06% quercetin significantly downregulated the mRNA expression of TBK1 and IKKα (p < 0.05). 0.02% quercetin significantly decreased the relative abundance of Escherichia, Staphylococcus (p < 0.05), and Salmonella (p < 0.01), 0.04% quercetin significantly decreased the relative abundance of Staphylococcus (p < 0.05), Escherichia, and Salmonella (p < 0.01), and 0.06% quercetin significantly decreased the relative abundance of Salmonella (p < 0.05) and Staphylococcus (p < 0.01) in the ileum of AA broilers. These findings suggested that dietary quercetin supplementation regulated the mRNA expression of AvBD, TLR, and the TLR signaling pathways and NF-κB signalling pathways, thereby maintaining the microecological balance of the intestinal tract and decreasing the mortality of broilers, and the optimum addition amount of quercetin is 0.04% under the test conditions.
Highlights
Toll-like receptors (TLRs)-mediated signaling pathways are involved in regulating intestinal epithelial barrier integrity and promoting the health of chickens (Nighot et al, 2017), which leads to a well-studied signaling cascade initiated by TIR domain-containing signaling adapters, including myeloid differentiation primary response protein 88 (MyD88), TIR domain-containing adaptor inducing IFN-β (TRIF), TIR domain-containing adaptor protein/MyD88adaptor-like (TIRAP/MAL), and TRIF-related adaptor molecule (TRAM), and these adapters interact with the TIR domain of the TLR (Hamerman et al, 2015)
Dietary supplementation with 0.04 and 0.06% quercetin significantly upregulated the mRNA expression of total avian beta-defensin (AvBD) (p < 0.05, F = 4.27); quercetin did not significantly affect the mRNA expression of AvBD1 (p = 0.12, F = 2.22), AvBD2 (p = 0.49, F = 0.84), and AvBD14 (p = 0.43, F = 0.96) in the ileum of Arbor Acre (AA) broilers, compared with control
0.02% quercetin significantly downregulated the mRNA expression of TLR4 and TLR21 (p < 0.05); 0.04% quercetin significantly upregulated the mRNA expression of TLR4 and TLR7 (p < 0.05), and 0.06% quercetin significantly downregulated the mRNA expression of TLR3 (p < 0.05). These results indicated that dietary quercetin supplementation regulated the mRNA expression of TLR in the ileum of AA broilers (Table 9)
Summary
The intestine plays a crucial role in maintaining body immunity that is the first line of defense from exogenous pathogens infecting host cells and tissues of chicken, which is the largest organ with immune properties (Fisinin and Surai, 2013). Toll-like receptors (TLRs) are evolutionarily conserved pattern recognition receptors that have been identified in chickens, which initiate innate immune responses and regulate the expression of proinflammatory and antiinflammatory cytokines and avian beta-defensin (AvBD) (St Paul et al, 2013; Yoshimura, 2015; Kang, et al, 2019; Shimizu et al, 2020; Terada et al, 2020). TLR-mediated signaling pathways are involved in regulating intestinal epithelial barrier integrity and promoting the health of chickens (Nighot et al, 2017), which leads to a well-studied signaling cascade initiated by TIR domain-containing signaling adapters, including myeloid differentiation primary response protein 88 (MyD88), TIR domain-containing adaptor inducing IFN-β (TRIF), TIR domain-containing adaptor protein/MyD88adaptor-like (TIRAP/MAL), and TRIF-related adaptor molecule (TRAM), and these adapters interact with the TIR domain of the TLR (Hamerman et al, 2015). There are few studies related to quercetin, AvBD, and TLRs in the ileum of broilers. The effects of quercetin on TLR and signal molecules related to the TLR signaling pathway were explored to elucidate the mechanism by which dietary quercetin supplementation in AA broilers improves health
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