Identification of an insulin receptor mediating the ovarian insulin pathway in the swimming crab, Portunus trituberculatus

The insulin signaling pathway is a metazoan conserved pathway that plays a key role in the "nutrition-reproduction" transition. However, the role and mechanism of insulin pathway on female crustacean reproduction was poorly understood. In this study, we found that inhibitors targeting the insulin pathway significantly reduced gonadosomatic index (GSI), oocyte size, and the vitellogenin receptor (VgR) gene and protein levels in the Portunus trituberculatus, which mimicked the inhibitory effects caused by fasting treatment. Fasting led to a systematically decrease in the expression of several insulin pathway genes, among which a neuro-derived insulin-like peptide (PtILP) and an ovarian-derived insulin receptor (PtIR3). Using fluorescence co-localization and yeast two-hybrid hybridization, it was demonstrated that PtIR3 can interact with PtILP, suggesting that their binding may activate the ovarian insulin pathway. Further in vitro experiments showed that recombinant ILP (rILP) promoted the AKT phosphorylation levels in ovary explants, whereas RNA interference with PtIR3 had the opposite effect. By contrast, neither treatment affect the MAPK phosphorylation levels, suggesting that the PtILP/PtIR3 signaling may regulate the ovarian development mainly by targeting the AKT pathway. Moreover, it was also found that rILP treatment and PtIR3 knockdown promoted and inhibited the ovarian VgR gene expression and protein levels, respectively, suggesting the PtIR3-mediated ovarian insulin pathway may be involved in the vitellogenin uptake process. Our results suggest that the insulin pathway is critical for the ovarian development in P. trituberculatus, among which the ovarian-derived IR3 upon binding with ILP activates the Akt pathway in the ovary, which in turn, may target the vitellogenin uptake.