A second FADD mediates coelomocyte apoptosis response to Vibrio splendidus infection in sea cucumber Apostichopus japonicus

Fas-associated protein with death domain (FADD) is a pivotal adaptor protein that functions in mediating cell death, cell cycle regulation, and particular in innate immunity by the main death receptors. In this study, a second FADD gene in sea cucumber Apostichopus japonicus (termed AjFADD-2) was cloned and its potential function in the innate responses was analyzed. The full-length cDNA of AjFADD-2 consists of 2405 bp and contains a 47 bp 5′-untranslated region (UTR), a 1629 bp 3′-UTR, and a 729 bp ORF encoding 242 amino acids. AjFADD-2 possesses two conserved domains of intracellular N-terminal death effector domain and an extracellular C-terminal death domain, which is different from the first cloned FADD gene in A. japonicus that only possesses the death domain. AjFADD-2 was examined in all sampled six tissues and was significantly induced in V. splendidus-challenged sea cucumbers and LPS-exposed coelomocytes. Subcellular localization detection showed that AjFADD-2 was primarily observed in the coelomocyte cytoplasm, and transferred to the nucleus post V. splendidus challenge. Consistently, AjFADD-2 knockdown significantly inhibited apoptosis in V. splendidus-challenged sea cucumbers and LPS-exposed coelomocytes. Taken together, our results provided evidence that AjFADD functioned as a positive regulator of coelomocytes apoptosis in response to pathogen V. splendidus challenge.