Detection of immune-related enzyme activity, gene expression and metabolomic profiling in Chinese mitten crab Eriocheir sinensis parasitized by Polyascus gregaria

Polyascus gregaria is a specialized crustacean parasite of Chinese mitten crab; however, only limited information on interactions between parasite and its host is available. This study examined interactions between P. gregaria and Chinese mitten crab with regard to immune-related enzyme activities and expression of respective genes in the hepatopancreas. To further elucidate the pathogen-host interactions, untargeted gas chromatography-mass spectrometry (GC-MS) metabolomics was used to analyse changes in metabolites in response to P. gregaria infection. Superoxide dismutase, acid phosphatase and alkaline phosphatase activities were significantly increased in parasitized crabs, whereas phenoloxidase activity was significantly decreased. Moreover, the expression of three immune-related genes, that is proPO, Toll1 and Toll2, was markedly downregulated in parasitized crabs. In the metabolomic assay, 73 differential metabolites were identified, and they were significantly enriched in 71 pathways. Among these metabolites, 58 were significantly upregulated and 15 were downregulated. Lysolipids lysoPE, lysoPC, lysoPG, LysoPI and lysoPS were the most strongly upregulated metabolites, and 9-cis-retinol, glycerophosphocholine and 5-thymidylic acid were the top three downregulated metabolites. Most of these metabolites were enriched in the glycerophospholipid metabolism pathway, which was one of the most significantly activated pathway responses to P. gregaria infection. This increase in lysoglycerophospholipids caused major structural and functional abnormalities in the membranes of host tissues, especially in the hepatopancreas. We propose that long-term infection with P. gregaria induces silence of the innate immune system, disturbs intracellular homeostasis and causes cell membrane immune-related dysfunction in Chinese mitten crab. These results may help us better understand P. gregaria infection in Chinese mitten crab.