N-carbamylglutamate (NCG) supplementation in low-protein diets improving egg quality by regulating protein metabolism pathway of oviduct in laying hens.

The objective of this study was to evaluate the effects of low-protein dietary supplementation with NCG on productivity, egg quality and possible regulatory mechanisms in laying Hens. A total of 150 Beijing Type 2 laying hens at 300 days of age were randomly assigned to five groups: a control group, a low-protein diet group (LP), and three low-protein diet groups supplemented with 0.06%, 0.12%, or 0.18% NCG. Daily feed intake, Egg number and egg weight were recorded daily on a replicate basis to calculate the egg production rate, average feed intake (ADFI), feed conversion ratio (FCR) and evaluate egg quality. Blood samples were collected from the jugular vein, and the serum was obtained to determine various biochemical parameters using an automatic biochemical analyzer. The protein secretory region of the magnum part of the oviduct was collected and immediately frozen in liquid nitrogen for RNA sequencing analysis. The RNA-seq results were subsequently validated by real-time quantitative PCR (RT-qPCR) using Pearson's correlation analysis. Supplementation with 0.06% and 0.18% NCG in low-protein diets significantly increased egg weight compared with the LP group (P < 0.05). Albumen height and Haugh unit were significantly increased in the 0.06% NCG group compared with the other groups (P < 0.05). The 0.12% NCG supplementation significantly increased serum BUN content (P < 0.05), whereas the 0.18% NCG supplementation significantly decreased serum phosphorus content (P < 0.05). Serum ALB and NO concentrations were significantly increased in the 0.06% and 0.18% NCG groups compared with the LP group (P < 0.05). Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analyses were conducted to identify differentially expressed genes and enriched signaling pathways. The results demonstrated that the genes suppressor of cytokine signaling 3 (SOCS3), integrin subunit alpha 8 (ITGA8), platelet-derived growth factor receptor alpha (PDGFRA), angiotensin-converting enzyme (ACE), A disintegrin and metalloproteinase with thrombospondin motifs 1 (ADAMTS-1), and apolipoprotein L domain containing 1 (APOLD1), which are associated with the PI3K/AKT/mTOR signaling pathway, were regulated by NCG supplementation under low-protein dietary conditions. In conclusion, supplementation with 0.06% NCG in a low-protein diet improved egg weight and egg quality by regulating protein synthesis and secretion in the magnum part of the hen's oviduct.