Characterization of Transient Receptor Potential Cation Channel subfamily Vanilloid 4 (TRPV4) in chub mackerel: Role in low-salinity acclimation and reproductive implications.

Salinity plays a crucial role in fish physiology, including osmoregulation, metabolism, growth, reproduction, and disease resistance. In this study, we characterized the Transient Receptor Potential Cation Channel Subfamily Vanilloid 4 (trpv4) gene and investigated the effects of low-salinity conditions on its expression in various tissues of chub mackerel (Scomber japonicus), an economically important aquaculture species. The trpv4 open reading frame (ORF) consists of 2616 nucleotides and encodes an 872-amino acid protein. It shares up to 94.01 % phylogenetic similarity with Thunnus maccoyii and possesses 6 transmembrane domains highly conserved with human TRPV4. Trpv4 was widely distributed across most organs and showed significant sex-biased expression. In situ hybridization data showed that trpv4 was explicitly expressed in various parts of the pituitary, from pars intermedia (PI) to rostral pars distalis (RPD) where it colocalized with prolactin 1 (prl1) in RPD. Furthermore, exposure to decreasing salinity from 35 ppt to 10 ppt demonstrated a significant upregulation of trpv4 transcription in the pituitary, gill, skin, and gonad under low-salinity conditions. Although prl1 was upregulated, follicle-stimulating hormone beta subunit (fshb) expression was decreased in the pituitaries of low-salinity groups. These findings suggest that trpv4 is regulated by salinity in chub mackerel and may play a role in reproductive responses to salinity changes. Overall, these results highlight the characterization of trpv4 in marine fish and its potential roles in osmoregulation, as well as in transmitting low-salinity signals to reproduction-related organs. The involvement of the brain-pituitary-gonad axis in this response warrants further investigation.