Alternative splicing of Spodoptera exigua caspase-5 is involved in regulating host-cell apoptosis induced by AcMNPV infection

• SeCaspase-5 has alternative splicing which forms four splicing variants: SeCaspase-5a, SeCaspase-5b, SeCaspase-5c and SeCaspase-5d. • SeCaspase-5a has a proapoptotic function, while SeCaspase-5b, SeCaspase-5c and SeCaspase-5d have nonapoptotic functions and increase AcMNPV production. • SeCaspase-5b, SeCaspase-5c and SeCaspase-5d can inhibit the proapoptotic function of SeCaspase-5a through direct protein interactions. Caspases, which play key roles in cell apoptosis, undergo alternative splicing to form different splicing variants that can regulate the apoptotic process. Lepidopteran insect caspases undergo alternative splicing, although the functions of their splicing variants are still unclear. The Spodoptera exigua caspase-5 ( SeCaspase-5 ) gene was cloned and found to produce four different splicing variants with different gene sequences and protein functional domains, which were named SeCaspase-5a, SeCaspase-5b, SeCaspase-5c and SeCaspase-5d . Overexpression of these variants in S . exigua cells (Se-3) showed that SeCaspase-5a had a proapoptotic function, whereas SeCaspase-5b, SeCaspase-5c and SeCaspase-5d did not. Semi-qPCR analysis revealed that the expression of the SeCaspase-5 variants significantly differed during Autographa californica multiple nucleopolyhedrovirus (AcMNPV) infection. Furthermore, the SeCaspase-5 variants were constructed into the AcMNPV bacmid and transfected into Se-3 cells, which revealed that SeCaspase-5a promoted cell apoptosis and reduced virus production, whereas SeCaspase-5b, SeCaspase-5c and SeCaspase-5d did not promote cell apoptosis but instead increased virus production. Moreover, an analysis of the interactions between the SeCaspase-5 variants revealed that SeCaspase-5a directly interacted with SeCaspase-5b, SeCaspase-5c and SeCaspase-5d. Coexpression of these variants in Se-3 cells also revealed that SeCaspase-5b, SeCaspase-5c and SeCaspase-5d inhibited the proapoptotic function of SeCaspase-5a, resulting in a reduction in the percentage of apoptotic cells by about 20%. These results indicate that SeCaspase-5 undergoes alternative splicing and is involved in regulating the apoptosis induced by baculovirus infection. These findings increase our understanding of the functions of lepidopteran insect caspases and provide new insights into the mechanism of host-cell apoptosis induced by baculoviruses.