IGF/MIP promotes shell growth through activation of Runt-related transcription factors in the pearl oyster Pinctada fucata martensii

Biomineralization, the biologically controlled process of shell (exoskeleton) formation, is a critical indicator of growth in mollusks. In the pearl oyster Pinctada fucata martensii , the insulin-like growth factor (IGF) homolog MIP-1 and the Runt-related transcription factor PmRunt have been identified as key regulators of shell development. However, the phenotypic effects and regulatory mechanisms of these genes remain poorly understood. In this study, RNA interference (RNAi) was used to suppress PmRunt expression, resulting in significant reductions in shell length, height, width, and nacre thickness by 42.86 %, 42.86 %, 66.67 %, and 83.63 %, respectively, compared to that observed in the control. Similarly, pharmacological inhibition of PmRunt activation using RO5–3335 significantly decreased shell length, height, and width growth rates by 20 %, 33 %, and 66.67 %, respectively. Furthermore, overexpression of MIP-1 via recombinant protein injection significantly increased the transcriptional and post-translational activation of PmRunt, whereas RNAi-mediated knockdown of MIP-1 had the opposite effect (P < 0.05). Western blot analysis confirmed that MIP-1 overexpression significantly increased the expression and phosphorylation levels of PmRunt protein compared to that in the control, while MIP-1 knockdown significantly reduced these levels (P < 0.05). Dual-luciferase assays revealed that MIP-1 overexpression significantly enhanced the promoter activity of the PmRunt gene (P < 0.05). Together, these findings reveal that MIP-1 facilitates shell growth and nacre deposition in P. f. martensii by upregulating and activating PmRunt through transcriptional and phosphorylation-mediated mechanisms. This study highlights the pivotal role of the MIP-1/PmRunt axis signaling in regulating biomineralization, providing valuable insights into genetic strategies for enhancing pearl oyster shell formation and pearl quality.