Bax functions as coelomocyte apoptosis regulator in the sea cucumber Apostichopus japonicus

Bcl-2-associated X (Bax) belongs to the Bcl-2 protein family and its pro-apoptotic function has been confirmed in many vertebrate species. However, the functional role of Bax in apoptosis in invertebrates is limited. Here, a Bax homologue (AjBax) in Apostichopus japonicas was cloned and characterized, and its pro-apoptotic function explored. In healthy sea cucumbers, AjBax was expressed in coelomocyte with the highest levels. AjBax mRNA and protein levels were significantly induced in coelomocytes post Vibrio splendidus challenge in vivo and LPS-exposed in vitro. Moreover, siRNA-mediated AjBax knockdown in coelomocyte significantly decreased AjBax mRNA and protein levels as well as the apoptosis levels of coelomocyte. Furthermore, AjBax protein levels and coelomocyte apoptosis levels could be partially recovered to their original levels after supplementation with recombinant AjBax. Our results support that AjBax has a similar function to Bax proteins in vertebrates and that it may serve as a pro-apoptotic regulator in sea cucumbers.