Microsporidia have attracted much attention because they infect a variety of species ranging from protists to mammals, including immunocompromised patients with AIDS or cancer. Aside from the study on Nosema ceranae, few works have focused on elucidating the mechanism in host response to microsporidia infection. Nosema bombycis is a pathogen of silkworm pébrine that causes great economic losses to the silkworm industry. Detailed understanding of the host (Bombyx mori) response to infection by N. bombycis is helpful for prevention of this disease. A genome-wide survey of the gene expression profile at 2, 4, 6 and 8 days post-infection by N. bombycis was performed and results showed that 64, 244, 1,328, 1,887 genes were induced, respectively. Up to 124 genes, which are involved in basal metabolism pathways, were modulated. Notably, B. mori genes that play a role in juvenile hormone synthesis and metabolism pathways were induced, suggesting that the host may accumulate JH as a response to infection. Interestingly, N. bombycis can inhibit the silkworm serine protease cascade melanization pathway in hemolymph, which may be due to the secretion of serpins in the microsporidia. N. bombycis also induced up-regulation of several cellular immune factors, in which CTL11 has been suggested to be involved in both spore recognition and immune signal transduction. Microarray and real-time PCR analysis indicated the activation of silkworm Toll and JAK/STAT pathways. The notable up-regulation of antimicrobial peptides, including gloverins, lebocins and moricins, strongly indicated that antimicrobial peptide defense mechanisms were triggered to resist the invasive microsporidia. An analysis of N. bombycis-specific response factors suggested their important roles in anti-microsporidia defense. Overall, this study primarily provides insight into the potential molecular mechanisms for the host-parasite interaction between B. mori and N. bombycis and may provide a foundation for further work on host-parasite interaction between insects and microsporidia.
The spore wall of Nosema bombycis plays an important role in microsporidian pathogenesis. Protein fractions from germinated spore coats were analysed by two-dimensional polyacrylamide gel electrophoresis and MALDI-TOF/TOF mass spectrometry. Three protein spots were identified as the hypothetical spore wall protein NbHSWP12. A BAR-2 domain (e-value: 1.35e-03) was identified in the protein, and an N-terminal protein-heparin interaction motif, a potential N-glycosylation site, and 16 phosphorylation sites primarily activated by protein kinase C were also predicted. The sequence analysis suggested that Nbhswp12 and its homologous genes are widely distributed among microsporidia. Additionally, Nbhswp12 gene homologues share similar sequence features. An indirect immunofluorescence analysis showed that NbHSWP12 localized to the spore wall, and thus we renamed it spore wall protein 12 (NbSWP12). Moreover, NbSWP12 could adhere to deproteinized N. bombycis chitin coats that were obtained by hot alkaline treatment. This novel N. bombycis spore wall protein may function in a structural capacity to facilitate microsporidial spore maintenance.
Microsporidia are obligate intracellular parasites with rigid spore walls that protect against various environmental pressures. Despite an extensive description of the spore wall, little is known regarding the mechanism by which it is deposited or the role it plays in cell adhesion and infection. In this study, we report the identification and characterization of two novel spore wall proteins, SWP7 and SWP9, in the microsporidian species Nosema bombycis. SWP7 and SWP9 are mainly localized to the exospore and endospore of mature spores and the cytoplasm of sporonts, respectively. In addition, a portion of SWP9 is targeted to the spore wall of sporoblasts earlier than SWP7 is. Both SWP7 and SWP9 are specifically colocalized to the spore wall in mature spores. Furthermore, immunoprecipitation, far-Western blotting, unreduced SDS-PAGE, and yeast two-hybrid data demonstrated that SWP7 interacted with SWP9. The chitin binding assay showed that, within the total spore protein, SWP9 and SWP7 can bind to the deproteinated chitin spore coats (DCSCs) of N. bombycis. However, binding of the recombinant protein rSWP7-His to the DCSCs is dependent on the combination of rSWP9 -glutathione S-transferase (GST) with the DCSCs. Finally, rSWP9-GST, anti-SWP9, and anti-SWP7 antibodies decreased spore adhesion and infection of the host cell. In conclusion, SWP7 and SWP9 may have important structural capacities and play significant roles in modulating host cell adherence and infection in vitro. A possible major function of SWP9 is as a scaffolding protein that supports other proteins (such as SWP7) that form the integrated spore wall of N. bombycis.M icrosporidia, which infect numerous vertebrate and invertebrate species, are a specific group of obligate spore-forming, intracellular, and unicellular eukaryotic protists (1). The microsporidian species Nosema bombycis causes pebrine, a disease that was common in Europe, America, and Asia during the mid19th century and still causes heavy losses in silk-producing countries, such as China and India (1). It was first recognized in 1857 by Nägeli (2). Recently, phylogenetic analyses based on conserved proteins (3-5), ribosomal DNA (rDNA) sequences (6, 7), and complete sequencing of the Encephalitozoon cuniculi genome (8) have suggested that microsporidia, which undergo a highly reductive evolution, are closely related to fungi (9). However, the N. bombycis genome has greatly expanded due to transposable elements and gene duplications (10).The microsporidian spore wall, which is usually comprised of an electron-dense proteinaceous outer layer (exospore) of 25 to 30 nm and an electron-transparent chitinous inner layer (endospore) of 30 to 35 nm, maintains the spore's morphology and helps the mature spore resist the outer environment (11-13). Although chitin is the major component of the spore wall, only one chitin-associated protein has been identified. The chitin deacetylase activity of E. cuniculi (EcCDA) has been confirmed to be expressed during sporogonic stages (14). Currently, the deprot...
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