Abalone Haliotis discus caspase 8 is an apoptosis effector and a pyroptosis activator

In mammals, caspase 8 (CASP8) is a well-known initiator caspase of apoptosis. In invertebrates, the function of CASP8 is poorly understood. Herein, we examined the function of abalone Haliotis discus CASP8 (HdCASP8). Compared to mammalian CASP8, HdCASP8 possesses the conserved DED and CASc domains but also has an extra death domain (DD). HdCASP8 induced marked apoptosis of HEK293T cells without activating CASP3/6/7. Consistently, HdCASP8 did not cleave H. discus CASP3 (HdCASP3). HdCASP8 exhibited CASP3/6-like cleavage specificity and cleaved the apoptotic substrate DFF45. HdCASP3 is known to activate abalone pyroptosis by cleaving H. discus gasdermin E (HdGSDME) at two sites, DQVD and DEID. In the present work, HdCASP8 was found to interact with HdGSDME at its C-terminal region and induce pyroptosis by cleaving HdGSDME at DQVD but not at DEID. During bacterial infection, the expressions of HdCASP8 and HdGSDME were significantly upregulated in multiple tissues of abalone in a time-dependent manner. Together these results indicate that, most likely owing to its unique structural feature, HdCASP8 differs from the classical CASP8 by acting as an apoptosis/pyroptosis-regulating CASP3 and from the classical CASP3 in certain aspects of substrate specificity. These findings provide new insights into CASP8-mediated programmed cell death in invertebrates.