Functional Divergence of adcyap1b Splice Variants in Regulating Pituitary Hormone Expression in the Chinese Tongue Sole (Cynoglossus semilaevis)

Sexual size dimorphism (SSD) refers to the phenomenon where males and females of the same species exhibit differences in overall or partial body size, and it is widespread among mammals, birds, reptiles, and fish. Notably, this dimorphism is significantly influenced by the sexually dimorphic secretion of growth hormone (<i>gh</i>), a key pituitary-derived growth regulator. Commonly, the secretion of <i>gh</i> is positively regulated by glucagon family members such as growth hormone-releasing hormone (<i>ghrh</i>) and adenylate cyclase-activating polypeptide 1 (<i>adcyap1</i>). To explore the stimulators for pituitary hormones (especially <i>gh</i>) in the teleost, we performed genome-wide identification and functional characterization of the glucagon family on Chinese tongue sole (<i>Cynoglossus semilaevis</i>) that exhibits typical female-biased sexual size dimorphism. Four members of <i>adcyap1</i>/vasoactive intestinal polypeptide(<i>vip</i>)/<i>ghrh</i> family and ten members of their receptor family were identified. Expression pattern analysis revealed high expression of adenylate cyclase-activating polypeptide 1b (<i>adcyap1b</i>) and its receptors in the brain. Moreover, two alternative splice variants for the <i>adcyap1b</i> gene were discovered, resulting from the skipping of exon 4. Following the acquisition of the two eukaryotic recombinant protein splice variants (ADCYAP1b_tv1 and ADCYAP_tv2) from HEK 293T cells, incubation experiments were conducted using <i>C. semilaevis</i> pituitary cell line. The results demonstrated that both variants promoted the expression of <i>gh</i>, pro-opiomelanocortin (<i>pomc</i>), and corticoliberin (<i>crh</i>), but ADCYAP1b_tv1 had a significantly stronger effect and uniquely stimulated prolactin (<i>prl</i>) and somatolactin (<i>sl</i>). This study demonstrates a functional divergence between the two ADCYAP1b splice variants in teleosts, with ADCYAP1b_tv1 acting as a more potent and versatile pituitary hormone stimulator. Further research on their receptor-binding affinity and downstream signaling pathways would be valuable for exploring the mechanism underlying sexual size dimorphism.