Apoptosis-Inducing Factor 2 (AIF-2) Mediates a Caspase-Independent Apoptotic Pathway in the Tropical Sea Cucumber ( Holothuria leucospilota ).

Apoptosis, also known as programmed cell death, is a biological process that is critical for embryonic development, organic differentiation, and tissue homeostasis of organisms. As an essential mitochondrial flavoprotein, the apoptosis-inducing factor (AIF) can directly mediate the caspase-independent mitochondrial apoptotic pathway. In this study, we identified and characterized a novel AIF-2 ( Hl AIF-2) from the tropical sea cucumber Holothuria leucospilota . Hl AIF-2 contains a conserved Pyr_redox_2 domain and a putative C-terminal nuclear localization sequence (NLS) but lacks an N-terminal mitochondrial localization sequence (MLS). In addition, both NADH- and FAD-binding domains for oxidoreductase function are conserved in Hl AIF-2. Hl AIF-2 mRNA was ubiquitously detected in all tissues and increased significantly during larval development. The transcript expression of Hl AIF-2 was significantly upregulated after treatment with CdCl 2 , but not the pathogen-associated molecular patterns (PAMPs) in primary coelomocytes. In HEK293T cells, Hl AIF-2 protein was located in the cytoplasm and nucleus, and tended to transfer into the nucleus by CdCl 2 incubation. Moreover, there was an overexpression of Hl AIF-2-induced apoptosis in HEK293T cells. As a whole, this study provides the first evidence for heavy metal-induced apoptosis mediated by AIF-2 in sea cucumbers, and it may contribute to increasing the basic knowledge of the caspase-independent apoptotic pathway in ancient echinoderm species.