Glutamine Promotes Collagen Deposition via TGF-β/Smads Pathway in Swim Bladder of Chu's Croaker (Nibea coibor)

Glutamine (Gln), one of the most abundant free conditionally essential amino acids in animals, plays a pivotal role in protein and energy metabolism. Beyond its nutritional importance, Gln also modulates key cellular processes through diverse signaling pathways. Our previous studies have shown that appropriate Gln (0.4%) can promote collagen deposition in the swim bladder of Nibea coibor; however, the mechanism has not been investigated. Therefore, this study used transcriptomic analysis on the control group (0.0% Gln) and the 0.4% Gln group to screen for possible pathways and validate the function of candidate pathways in vitro. The results showed that the TGF-β/Smads pathway was significantly upregulated in the 0.4% Gln group compared with 0.0% Gln. At the same time, Gln could also promote the swim bladder cells' proliferation and increase the extracellular type I collagen content and gene expression (col1a1, col1a2) in vitro, accompanied by the activation of the TGF-β/Smads pathway (tgf-β, smad2, smad3, smad4 upregulated). Inhibition of the TGF-β/Smads pathway (tgf-β, smad2, smad3, smad4 downregulated) partially reversed the effect of Gln on collagen deposition (col1a1, col1a2 downregulated), thus proving that the TGF-β/Smads pathway is involved in the process of Gln on collagen deposition. In conclusion, the discovery not only expands our understanding of amino acid functions but establishes a foundational theory for further research on collagen regulation in fish.