Cloning and characterization of a novel caspase-8-like gene in Crassostrea gigas

• CgCaspase8-2 contains two death effector domains and a caspase domain.
• CgCaspase8-2 induces cell death but may be not part of the extrinsic apoptotic pathway.
• CgCaspase8-2 involvement in immune response is specific to viruses.

Cysteine-dependent aspartate-directed proteases, or caspases, play key roles in apoptosis and immune defense. In this study, we cloned the first caspase-8-like gene (CgCaspase8-2) identified in the pacific oyster, Crassostrea gigas. The 2572-bp cDNA encodes a putative protein of 714 amino acids that contains two tandem death effector domains (DEDs) at the N-terminal, and P20 and P10 domains at the C-terminal. The conserved pentapeptide motif QACQG was also identified in the deduced CgCaspase8-2 protein. Phylogenetic analysis indicated that CgCaspase8-2 was clustered with initiator caspases in the invertebrate subgroup, but the similarity between CgCaspase8-2 and other invertebrate caspase-8s was low. CgCaspase8-2 protein was localized in the cytoplasm, and over-expression of CgCaspase8-2 in HEK293T cells induced cell death, suggesting a role in apoptosis. Quantitative real-time PCR results demonstrated that CgCaspase8-2 was widely expressed in various tissues and developmental stages, with the highest CgCaspase8-2 expression levels detected in hemolymph and the blastula stage. Furthermore, CgCaspase8-2 transcripts showed no change in response to a bacterial challenge but exhibited notable up-regulation post-poly (I:C) challenge, suggesting that CgCaspase8-2 is specifically involved in immune responses against viruses. In summary, CgCaspase8-2 is involved in both apoptotic and immune function.