Spexin and its receptors in the yellowtail kingfish (Seriola lalandi): identification, expression profiles and reproductive function.

Spexin (SPX1) is a neuropeptide of 14 amino acids (aa), originally identified by bioinformatics, which has been implicated in various physiological functions in vertebrates via galanin receptors 2 and 3 (GALR2/3). To clarify the biological role of SPX1 in the control of reproduction in yellowtail kingfish, which is regarded as a promising species for offshore aquaculture worldwide, cDNA sequences of spx1 and six potential receptors were identified in the current study. The open reading frame of yellowtail kingfish spx1 was 363 nucleotides in size that encoded a 120-aa preprohormone, and its mature peptide was highly conserved among other species. The cDNA sequences of six GALRs (galr1a, galr1b, galr2a, galr2b, galr type 1, and galr type 2) were 1053 base pairs (bp), 1068 bp, 981 bp, 1137 bp, 1038 bp, 924 bp, which encoded G protein-coupled receptors of 350 aa, 355 aa, 326 aa, 378 aa, 345 aa, 307 aa, respectively. Tissue distribution analysis showed that spx1, galr1b, and galr2b transcripts were mainly detected in the brain. The highest mRNA levels of galr1a and galr2a were observed in the pituitary, followed by the brain and ovary. Both galr type 1 and galr type 2 were widely expressed in various tissues, with a peak level in the kidney. Moreover, all spx1 and galr genes significantly fluctuated during early ontogeny, exhibiting different expression patterns. Intraperitoneal injection of SPX1 significantly increased brain gnrh1, gnih, spx1, gal, and tac3 expression, while it inhibited gnrh2, kiss1r, and kiss2r mRNA levels. In the pituitary, SPX1 injection reduced transcript levels of gh, lhβ, and fshβ. Overall, our results have revealed the involvement of SPX1 in the reproductive functions in yellowtail kingfish.