MicrosporidianNosema bombycissecretes serine protease inhibitor to suppress host cell apoptosisviacaspase BmICE

Abstract

Microsporidia are a group of intracellular pathogens that actively manipulate host cell biological processes to facilitate their intracellular niche. Apoptosis is an important defense mechanism by which host cell control intracellular pathogens. Microsporidia modulating host cell apoptosis has been reported previously, however the molecular mechanism is not yet clear. In this report, we describe that the microsporidiaNosema bombycisinhibits apoptosis ofBombyx moricells through a secreted protein NbSPN14, which is a serine protease inhibitor (Serpin). An immunofluorescent assay demonstrated that upon infection withN. bombycis, NbSPN14 was initially found in theB. moricell cytoplasm and then became enriched in the host cell nucleus. Overexpression and RNA-interference (RNAi) of NbSPN14 inB. mori’embryo cells confirmed that NbSPN14 inhibited host cell apoptosis. Immunofluorescent and Co-IP assays verified the co-localization and interaction of NbSPN14 with the BmICE, the caspase 3 homolog inB. mori. Knocking out of BmICE or mutating the BmICE-interacting P1 site of NbSPN14, eliminated the localization of NbSPN14 into the host nucleus and prevented the apoptosis-inhibiting effect of NbSPN14, which also proved that the interaction between BmICE and NbSPN14 occurred in host cytoplasm and the NbSPN14 translocation into host cell nucleus is dependent on BmICE. These data elucidate thatN. bombycissecretory protein NbSPN14 inhibits host cell apoptosis by directly inhibiting the caspase protease BmICE, which provides an important insight for understanding pathogen-host interactions and a potential therapeutic target forN. bombycisproliferation.Author SummaryMicrosporidia constitute a class of eukaryotic pathogens that exclusively reside within host cells. The speciesNosema bombycisis the first microsporidian identified as the pathogen of silkworm Pébrine disease. In our research, we discovered howN. bombyciscleverly evades the host’s defenses. It has developed a strategy to survive inside host cells by manipulating host cell apoptosis, disarming the host cell’s self-destruct mechanism. In this study, we discovered that theN. bombycissecretes a serine protease inhibitor named NbSPN14, which infiltrates the cytoplasm of the host cell. The NbSPN14 interacts with the executioner Caspase protease BmICE within the silkworm’s apoptotic pathway, effectively neutralizing its apoptoic activity and thus curbing the apoptosis of the host cells.