The transforming growth factor beta (TGF-β/Smads) pathway regulates collagen synthesis and deposition in swim bladder of Chu's croaker (Nibea coibor) stimulated by proline

Abstract

The swim bladder performs various functions in fish, is consumed by humans and can be processed into high-grade fish glue (traditional high-value tonic). However, little is known on the optimum amount of proline (Pro) required for promoting collagen synthesis and deposition in fish swim bladder and the regulatory mechanism. This study explored the effects and optimum amount of dietary Pro supplementation on collagen synthesis and deposition in Chu's croaker (Nibea coibor) swim bladder and its possible molecular mechanism. A total of 450 juvenile fish (8.64 ± 0.14 g) were randomly stocked into 18 cages (25 fish per cage) in triplicate and fed Pro supplemented diets (0, 5, 10, 15, 20 and 25 g kg−1) for eight weeks. Another 225 fish (11.62 ± 0.15 g) were distributed randomly into control, Pro and Pro diet plus injected with specific inhibitor of Smad3 (SIS3) in order to inhibit the transforming growth factor beta (TGF-β)/Smads pathway. Pro supplementation up-regulated significantly the expression of genes involved in TGF-β/Smads pathway such as collagen, type I, alpha 1 (col1a1) and 2 (col1a2), TGF-β and Smad2/3, and increased collagen content in the Chu's croaker swim bladder (P < 0.05). The optimum dietary Pro supplementation for deposition of collagen in Chu's croaker swim bladder was 19.36 g kg−1. The collagen content in the swim bladder positively correlated significantly with the mRNA expression of col1a1, col1a2, TGF-β and Smad2/3 genes (P < 0.05). The optimum dietary Pro supplementation for maximum synthesis of collagen in Chu's croaker swim bladder was 13.25, 13.32 and 15.94 g kg−1 based on mRNA expression of Smads2, Smads3 and col1a2 genes, respectively. SIS3 down-regulated the expression of col1a1, col1a2, TGF-β and Smad2/3 genes, subsequently decreased collagen deposition in the swim bladder (P < 0.05). These results suggest that 13.25 to 19.36 g kg−1 Pro supplementation improve collagen synthesis and deposition in Chu's croaker swim bladder. The TGF-β/Smad signaling pathway regulates collagen synthesis and deposition in Chu's croaker swim bladder. Our results provide an understanding on the molecular mechanism of collagen synthesis and deposition in fish for producing collagen required for human food and fish glue.