Molecular Pathway and Regulatory Mechanism of the Saponin Biosynthesis in Sea Cucumber Apostichopus japonicus

Sea cucumber Apostichopus japonicus is one of the few animals capable of synthesizing saponins, which are critical components of its nutritional quality and health-beneficial properties. However, the specific mechanism underlying saponin biosynthesis in sea cucumbers remains unclear despite previous investigations. This study aimed to characterize the molecular pathway and regulatory mechanism of saponin biosynthesis in A. japonicus. Thirteen candidate genes involved in de novo saponin skeleton synthesis were identified from the A. japonicus genome, and their full-length cDNAs were obtained via PCR-RACE. Sequence analysis predicted the intracellular localization of these genes. Combined in situ hybridization and quantitative real-time PCR analyses revealed their high expression in coelomocytes, indicating coelomocytes as the primary saponin synthesis sites. Knockdown of mevalonate kinase (AjMVK) and two oxidosqualene cyclases (AjPS and AjLS) caused a more obvious decrease in saponin levels, identifying them as key biosynthetic enzymes. Yeast two-hybrid assays revealed that AjPS and AjLS interact with ficolins, complement component 3-2, O-linked β-N-acetylglucosamine transferase, and α-L-fucosidase, whose regulatory effects were further validated by RNA interference and saponin content measurements. These results suggest that saponin biosynthesis in A. japonicus is regulated by the complement lectin pathway and modulated by glycosylation enzymes, providing a molecular foundation for enhancing bioactive saponin production for pharmaceutical and nutraceutical applications.