Genome-wide analysis of the intrinsic apoptotic pathway in the tropical sea cucumber Holothuria leucospilota

To date, studies on apoptosis in echinoderms mainly focus on the caspase-dependent extrinsic apoptotic pathway, while knowledges regarding the intrinsic apoptotic pathway is still limited. In this study, 55 genes belong to 7 families of the intrinsic apoptotic pathway, namely, Bcl-2, caspase, AIF, Cyt c, Apaf-1, Smac, HTRA2 and Endo G, were identified by screening of the genomic database of the tropical sea cucumber Holothuria leucospilota. The Bcl-2 family showed significant gene expansion in H. leucospilota but not in other echinoderms, while the pro-apoptotic factor Endo G is generally lost in echinoderms. Transcriptomic data showed that the genes of the intrinsic apoptotic pathway were ubiquitously expressed in all tissues, and all embryonic and larval developmental stages of H. leucospilota. The gene numbers and functional domains of the caspase family and AIF family were further clarified and showed conserved with their counterparts in vertebrates. In primary coelomocytes, the transcript expression of different caspases showed different responses to pathogenic challenges of vibrio and pathogen-associated molecular patterns, and environmental challenges of heave metal and reactive oxygen species. Overall, this study indicates that the intrinsic apoptotic pathway is conserved between vertebrates and echinoderms by using the sea cucumber H. leucospilota as a model. This study may contribute in increasing the knowledge of development and innate immunity in ancestral deuterostomia species.