Characterization of a novel spexin gene (spx2) in the half-smooth tongue sole and regulation of its expression by nutritional status

Spexin (SPX) is a newly identified peptide hormone of 14 amino acids (aa) with pleiotropic functions in vertebrates, and recently a second paralog of SPX (named SPX2) has been only found in some non-mammalian species (the previous form of SPX is now designated as SPX1). Results of previous studies have revealed the existence of a functional SPX1 in the half-smooth tongue sole. To clarify the divergence of the spexin family, we further isolated the spx2 gene from the same flatfish species in the current study. The open reading frame of tongue sole spx2 was 378 nucleotides in size that encoded a 125-aa preprohormone, and the tongue sole SPX2 precursor included a 17-aa mature peptide that may be amidated at the C-terminus, rather than a tetradecapeptide reported in most other vertebrates. Tissue distribution analysis showed that spx2 transcripts were found in a wide array of tissues of both sexes with the highest expression in the ovary. In addition, the spx2 mRNA abundance was also ubiquitously detected in different brain regions. Food deprivation for 4 weeks stimulated spx2 mRNA levels at the brain-pituitary-ovary axis, and refeeding of starved fish restored the spx2 mRNA levels back to normal. Taken together, this study provides initial evidence for the existence of a novel spexin gene (spx2) in Pleuronectiformes and suggest its possible involvement in the regulation of energy balance in the female tongue sole.