Alternations along gut-brain axis in response to hypo- and hyper-osmotic stress in yellowfin seabream (Acanthopagrus latus)

The fish gut plays an established role in osmoregulation, however, the effect of the gut-brain axis (GBA) under exposure to both hyper- and hypo-osmotic stress remains elusive. In this study, physiological, microbial community, transcriptomic, and histological analyses were used to examine the responses of immune functions and gene expression levels in gut and brain of yellowfin seabream ( Acanthopagrus latus ) after exposure to freshwater (FW, 0.2 ± 0.0 ppt), low-saline water (LW, 3.8 ± 0.7 ppt), isosmotic water (IW, 8.9 ± 0.5 ppt), and seawater (SW, 33.7 ± 0.6 ppt) for four weeks. The results showed that both hypo- and hyper-osmotic stress reshaped the gut microbial composition, increasing Proteobacteria and reducing Firmicutes . Pathogenic bacteria, including Vibrio , Aeromonas and Mycobacterium , were more abundant in FW and SW environments. Physiological analyses also showed that gut osmoregulation and the antioxidant defense system were significantly regulated under both hyper- and hypo-osmotic stress. At the transcriptional level, genes related to osmoregulation (e.g., AQP1 , NKCC2 , and ATPeV0A ), immune response (e.g., TLR2 , ZO2 , and IL20 ), and neurotransmitters (e.g., GCDH , HTR1 , SLC6A4 , and TPH1 ) play important roles in the response mechanism under hypo- and hyper-osmotic stress, and the tryptophan metabolism pathway may be a key factor in GBA communication. This research would be useful for the future understanding of the osmoregulatory mechanism in fish from the perspective of the GBA and provides a critical basis for studying the mechanism of osmoregulation. • Both hypo- and hyper-osmotic stress reshaped gut microbial community composition. • Intestinal osmoregulation and barrier integrity were regulated by osmotic stress. • Osmotic stress induced alternations of antioxidant defense and immune performance. • Tryptophan metabolism plays a key role in communications along gut-brain axis.