Identification and ovarian developmental regulation of Insulin-like growth factor 3 in spotted scat (Scatophagus argus)

Insulin-like growth factor 3 (IGF3), a newly discovered member of the IGF family in teleosts, plays a vital role in the reproduction system of fishes. Therefore, this study explored the spotted scat (Scatophagus argus), which is an aquaculture species of immense economic importance, but the regulatory mechanism of IGF3 in its ovarian development remains unknown. Here, we cloned igf3 cDNA in S. argus. Sequence analysis showed that the ORF of igf3 is 594 bp long and encodes 197 amino acid residues, possessing conserved characteristics of the IGF family. Tissue distribution revealed that igf3 is expressed in three different tissues: brain, gonad, and kidney. Following in situ hybridization, igf3 was detected in follicles, and its localization shifted during ovarian development. By qPCR analysis, the expression level of igf3 mRNA in the ovary increased as the ovary developed and was highest at stage Ⅳ which is approaching maturation. This study also produced recombinant S. argus IGF3 in bacteria to analyze the biological functions of IGF3. Incubating ovarian fragments with the recombinant IGF3 significantly promoted the cdc2, cyclinB1, 20β-HSD, egf, cyp11a1, cyp17a1, cyp19a1a, hsd17b1, and star1 expression levels, which are the key genes for ovarian development and maturation. The β-catenin signaling-sensitive genes, including cyclinD1 and axin2, were also promoted by IGF3 treatment. Collectively, our results demonstrate that IGF3 exerts functions on the development and maturation of ovaries in S. argus.