Identification of four caspase genes from Spodoptera exigua (Lepidoptera: Noctuidae) and their regulations toward different apoptotic stimulations

Apoptosis plays critical roles in multiple biological processes in multicellular organisms. Caspases are known as important participators and regulators of apoptosis. Here, four novel caspase genes of Spodoptera exigua were cloned and characterized, which were designated as SeCasp-1, SeCasp-6, SeCasp-7 and SeCasp-8. Analysis of the putative encoded protein sequences of these SeCasps indicated that SeCasp-1 and SeCasp-7 were possible homologs of executor caspases; SeCasp-8 was a possible homolog of initiator caspases; and SeCasp-6 was a unique caspase of S. exigua that shares low similarity with all the identified insect caspases. Based on baculovirus expression system analyses, SeCasp-1 exhibited similar caspase activity to human caspase-1, -3, -4, -6, -8 and -9; SeCasp-6 presented similar caspase activity to human caspase-2, -3, -4, -6, -8 and -9; SeCasp-7 exhibited similar caspase activity to human caspase-2, -3 and -6; and SeCasp-8 presented similar caspase activity only to human caspase-8. Induction with different chemicals revealed that SeCasp-1 showed extreme upregulation after 24 h in the treated fat body cell line (IOZCAS-Spex-II) of S. exigua. Developmental expression analysis revealed that SeCasp-1 was highly transcribed in the larval stages, while SeCasp-6, SeCasp-7, SeCasp-8 were down-regulated. The in vivo detection of the relative expression levels of SeCasps in S. eixgua larvae inoculated with different pathogens suggested that SeCasp-1 was sensitive to Bacillus thuringiensis infection and that SeCasp-6 was sensitive to baculovirus infection. SeCasp-7 and SeCasp-8 showed slight changes under either in vitro chemical apoptosis induction or in vivo pathogen infection.