Identification of a novel chitin-binding spore wall protein (NbSWP12) with a BAR-2 domain from Nosema bombycis (microsporidia)

Molecular characterization of turtle-like protein in whiteleg shrimp (Litopenaeus vannamei) and its role in Enterocytozoon hepatopenaei infection

Enterocytozoon hepatopenaei (EHP) is a microsporidian parasite that infects shrimp hepatopancreas, causing growth retardation and disease susceptibility. Knowledge of the host-pathogen molecular mechanisms is essential to understanding the microsporidian pathogenesis. Turtle-like protein (TLP) is part of the immunoglobulin superfamily of proteins, which is widely distributed in the animal kingdom. TLP has multiple functions, such as cell surface receptors and cell adhesion molecules. The spore wall proteins (SWPs) of microsporidia are involved in the infection mechanisms. Some SWPs are responsible for spore adherence, which is part of the activation and host cell invasion processes. Previous studies showed that TLP from silkworms (Bombyx mori) interacted with SWP26, contributing to the infectivity of Nosema bombycis to its host. In this study, we identified and characterized for the first time, the Litopenaeus vannamei TLP gene (LvTLP), which encodes an 827-aa protein (92.4 kDa) composed of five immunoglobulin domains, two fibronectin type III domains, and a transmembrane region. The LvTLP transcript was expressed in all tested tissues and upregulated in the hepatopancreas at 1 and 7 days post-cohabitation (dpc) and at 9 dpc in hemocytes. To identify the LvTLP binding counterpart, recombinant (r)LvTLP and recombinant (r)EhSWP1 were produced in Escherichia coli. Coimmunoprecipitation and enzyme-linked immunosorbent assays demonstrated that rLvTLP interacted with rEhSWP with high affinity (KD = 1.20 × 10-7 M). In EHP-infected hepatopancreases, LvTLP was clustered and co-localized with some of the developing EHP plasmodia. Furthermore, LvTLP gene silencing reduced the EHP copy numbers compared with those of the control group, suggesting the critical role of LvTLP in EHP infection. These results provide insight into the molecular mechanisms of the host-pathogen interactions during EHP infection.

Germination of Microsporidian Spores: The Known and Unknown

Microsporidia are a large group of mysterious obligate intracellular eukaryotic parasites. The microsporidian spore can survive in the absence of nutrients for years under harsh conditions and germinate within seconds under the stimulation of environmental changes like pH and ions. During germination, microsporidia experience an increase in intrasporal osmotic pressure, which leads to an influx of water into the spore, followed by swelling of the polaroplasts and posterior vacuole, which eventually fires the polar filament (PF). Infectious sporoplasm was transported through the extruded polar tube (PT) and delivered into the host cell. Despite much that has been learned about the germination of microsporidia, there are still several major questions that remain unanswered, including: (i) There is still a lack of knowledge about the signaling pathways involved in spore germination. (ii) The germination of spores is not well understood in terms of its specific energetics. (iii) Limited understanding of how spores germinate and how the nucleus and membranes are rearranged during germination. (iv) Only a few proteins in the invasion organelles have been identified; many more are likely undiscovered. This review summarizes the major resolved and unresolved issues concerning the process of microsporidian spore germination.

Generation of Resistance to Nosema bombycis (Dissociodihaplophasida: Nosematidae) by Degrading NbSWP12 Using the Ubiquitin-Proteasome Pathway in Sf9-III Cells

Nosema bombycis Naegeli (Dissociodihaplophasida: Nosematidae), an obligate intracellular parasite of the silkworm Bombyx mori, causes a devastating disease called pébrine. Every year pébrine will cause huge losses to the sericulture industry worldwide. Until now, there are no effective methods to inhibit the N. bombycis infection in silkworms. In this study, we first applied both the novel protein degradation Trim-Away technology and NSlmb (F-box domain-containing in the N-terminal part of supernumerary limbs from Drosophila melanogaster) to lepidopteran Sf9-III cells to check for specific degradation of a target protein in combination with a single-chain Fv fragment (scFv). Our results showed that the Trim-Away and NSlmb systems are both amenable to Sf9-III cells. We then created transgenic cell lines that overexpressed the protein degradation system and N. bombycis chimeric scFv targeting spore wall protein NbSWP12 and evaluated the effects of the insect transgenic cell lines on the proliferation of N. bombycis. Both methods could be applied to cell lines and both Trim-Away and NSlmb ubiquitin degradation systems effectively inhibited the proliferation of N. bombycis. Further, either of these degradation systems could be applied to individual silkworms through a transgenic platform, which would yield individual silkworms with high resistance to N. bombycis, thus greatly speeding up the process of acquiring resistant strains.

Expression of anti-NbHK single-chain antibody in fusion with NSlmb enhances the resistance to Nosema bombycis in Sf9-III cells

Nosema bombycis is a destructive and specific intracellular parasite of silkworm, which is extremely harmful to the silkworm industry. N. bombycis is considered as a quarantine pathogen of sericulture because of its long incubation period and horizontal and vertical transmission. Herein, two single-chain antibodies targeting N. bombycis hexokinase (NbHK) were cloned and expressed in fusion with the N-terminal of Slmb (a Drosophila melanogaster FBP), which contains the F-box domain. Western blotting demonstrated that Sf9-III cells expressed NSlmb-scFv-7A and NSlmb-scFv-6H, which recognized native NbHK. Subsequently, the NbHK was degraded by host ubiquitination system. When challenged with N. bombycis, the transfected Sf9-III cells exhibited better resistance relative to the controls, demonstrating that NbHK is a prospective target for parasite controls and this approach represents a potential solution for constructing N. bombycis-resistant Bombyx mori.

Heterologous Expressed NbSWP12 from Microsporidia Nosema bombycis Can Bind with Phosphatidylinositol 3-phosphate and Affect Vesicle Genesis

Microsporidia are a big group of single-celled obligate intracellular organisms infecting most animals and some protozoans. These minimalist eukaryotes lack numerous genes in metabolism and vesicle trafficking. Here, we demonstrated that the spore wall protein NbSWP12 of microsporidium Nosema bombycis belongs to Bin/Amphiphysin/Rvs (BAR) protein family and can specifically bind with phosphatidylinositol 3-phosphate [Ptdlns(3)P]. Since Ptdlns(3)P is involved in endosomal vesicle biogenesis and trafficking, we heterologous expressed NbSWP12 in yeast Saccharomyces cerevisiae and proved that NbSWP12 can target the cell membrane and endocytic vesicles. Nbswp12 transformed into Gvp36 (a BAR protein of S. cerevisiae) deletion mutant rescued the defect phenotype of vesicular traffic. This study identified a BAR protein function in vesicle genesis and sorting and provided clues for further understanding of how microsporidia internalize nutrients and metabolites during proliferation.

Proteomic Analysis of Spore Surface Proteins and Characteristics of a Novel Spore Wall Protein and Biomarker, EhSWP3, from the Shrimp Microsporidium Enterocytozoon hepatopenaei (EHP)

Enterocytozoon hepatopenaei, a spore-forming and obligate intracellular microsporidium, mainly infects shrimp and results in growth retardation and body length variation, causing huge economic losses to the Asian shrimp aquaculture industry. However, the lack of a full understanding of the surface proteins of spores associated with host infection has hindered the development of technologies for the detection of EHP. In this study, the surface proteins of EHP spores were extracted using the improved SDS method, and 130 proteins were identified via LC-MS/MS analysis. Bioinformatic analysis revealed that these proteins were enriched in biological processes (67), cellular components (62), and molecular functions (71) based on GO terms. KEGG pathway analysis showed that 20 pathways, including the proteasome (eight proteins) and the fatty acid metabolism (15 proteins), were enriched. Among 15 high-abundance surface proteins (HASPs), EhSWP3 was identified as a novel spore wall protein (SWP), and was localized on the endospore of the EHP spores with an indirect immunofluorescence and immunoelectron microscopy assay. Polyclonal antibodies against EhSWP3 showed strong species specificity and high sensitivity to the hepatopancreas of EHP-infected shrimp. As a specific high-abundance protein, EhSWP3 is therefore a promising target for the development of immunoassay tools for EHP detection, and may play a crucial role in the invasion of EHP into the host.

The Function and Structure of the Microsporidia Polar Tube

Microsporidia are obligate intracellular pathogens that were initially identified about 160 years ago. Current phylogenetic analysis suggests that they are grouped with Cryptomycota as a basal branch or sister group to the fungi. Microsporidia are found worldwide and can infect a wide range of animals from invertebrates to vertebrates, including humans. They are responsible for a variety of diseases once thought to be restricted to immunocompromised patients but also occur in immunocompetent individuals. The small oval spore containing a coiled polar filament, which is part of the extrusion and invasion apparatus that transfers the infective sporoplasm to a new host, is a defining characteristic of all microsporidia. When the spore becomes activated, the polar filament uncoils and undergoes a rapid transition into a hollow tube that will transport the sporoplasm into a new cell. The polar tube has the ability to increase its diameter from approximately 100 nm to over 600 nm to accommodate the passage of an intact sporoplasm and penetrate the plasmalemma of the new host cell. During this process, various polar tube proteins appear to be involved in polar tube attachment to host cell and can interact with host proteins. These various interactions act to promote host cell infection.

Ricin B lectin-like proteins of the microsporidian Encephalitozoon cuniculi and Anncaliia algerae are involved in host-cell invasion

Microsporidia are obligate intracellular pathogens capable of infecting a wide variety of hosts ranging from invertebrates to vertebrates. The infection process requires a step of prior adherence of Microsporidia to the surface of host cells. A few studies demonstrated the involvement of proteins containing a ricin-B lectin (RBL) domain in parasite infection. In this study Anncalia algerae and Encephalitozoon cuniculi genomes were screened by bioinformatic analysis to identify proteins with an extracellular prediction and possessing RBL-type carbohydrate-binding domains, being both potentially relevant factors contributing to host cell adherence. Three proteins named AaRBLL-1 and AaRBLL-2 from A. algerae and EcRBLL-1 from E. cuniculi, were selected and comparative analysis of sequences suggested their belonging to a multigenic family, with a conserved structural RBL domain despite a significant amino acid sequence divergence. The production of recombinant proteins and antibodies against the three proteins allowed their subcellular localization on the spore wall and/or the polar tube. Adherence inhibition assays based on pre-treatments with recombinant proteins or antibodies highlighted the significant decrease of the proliferation of both E. cuniculi and A. algerae, strongly suggesting that these proteins are involved in the infection process.

The First Report on the Transovarial Transmission of Microsporidian Nosema bombycis in Lepidopteran Crop Pests Spodoptera litura and Helicoverpa armigera

Microsporidia are ubiquitous fungi-related parasites infecting nearly all vertebrates and invertebrates. Microsporidian Nosema bombycis is a natural pathogen of multiple insects, including the silkworm and many agricultural and forest pests. N. bombycis can transovarially transmit in silkworm and cause huge economic losses to the sericulture. However, it remains unclear whether N. bombycis vertically transmits in the crop pests Spodoptera litura and Helicoverpa armigera. Here, we investigated the infection of N. bombycis in S. litura and H. armigera to illuminate its infectivity and transovarial transmission. In result, tissue examination with light microscopy revealed that the fat body, midgut, malpighian tubules, hemolymph, testis, and ovary were all infected in both pest pupae. Immunohistochemical analysis (IHA) of the ovariole showed that a large number of parasites in maturation and proliferation presented in follicle cell, nurse cell, and oocyte, suggesting that N. bombycis can infect and multiply in these cells and probably transovarially transmit to the next generations in both pests. Microscopic examination on the egg infection rate demonstrated that 50% and 38% of the S. litura and H. armigera eggs were congenitally infected, respectively. IHA of both eggs manifested numerous spores and proliferative pathogens in the oocyte, confirming that N. bombycis can invade into the female germ cell from the parent body. After hatching of the infected eggs, we detected the infection in offspring larvae and found large quantities of proliferative pathogens, confirming that N. bombycis can transovarially transmit in S. litura and H. armigera, and probably persists in both pest populations via congenital infection. In summary, our work, for the first time, proved that N. bombycis is able to vertically transmit in S. litura and H. armigera via infecting the oocyte in the parent, suggesting that N. bombycis could be a biological insecticide for controlling the population of crop pests.

Genome and Evolutionary Analysis of Nosema ceranae: A Microsporidian Parasite of Honey Bees

Microsporidia comprise a phylum of single cell, intracellular parasites and represent the earliest diverging branch in the fungal kingdom. The microsporidian parasite Nosema ceranae primarily infects honey bee gut epithelial cells, leading to impaired memory, suppressed host immune responses and colony collapse under certain circumstances. As the genome of N. ceranae is challenging to assembly due to very high genetic diversity and repetitive region, the genome was re-sequenced using long reads. We present a robust 8.8 Mbp genome assembly of 2,280 protein coding genes, including a high number of genes involved in transporting nutrients and energy, as well as drug resistance when compared with sister species Nosema apis. We also describe the loss of the critical protein Dicer in approximately half of the microsporidian species, giving new insights into the availability of RNA interference pathway in this group. Our results provided new insights into the pathogenesis of N. ceranae and a blueprint for treatment strategies that target this parasite without harming honey bees. The unique infectious apparatus polar filament and transportation pathway members can help to identify treatments to control this parasite.

Isolation of protein-free chitin spore coats of Nosema ceranae and its application to screen the interactive spore wall proteins

Nosema ceranae is the pathogen of nosemosis in the honey bee, which can bring great economic loss to apiculture. Chitin acts as a major component of the endospore of microsporidia and plays an essential role to form the bridges across the endospore. Here, Chitin Spore Coats (CSCs) of N. ceranae were successfully extracted by optimized hot alkaline treatment. SDS-PAGE and Calcofluor White Stain (CWS) staining indicated that the obtained CSCs were protein-free and the transmission electron microscopy analysis showed that CSCs performed the intact and loose chitin spore coats. Western blotting and indirect immunofluorescence analysis (IFA) demonstrated that CSCs could interact with three spore wall proteins (rNcSWP7, rNcSWP8, and rNcSWP12). Our method was effective to extract CSCs of N. ceranae and this could be very useful for screening spore wall proteins involved in endospore composition, which could be helpful to uncover the biological structure and pathogenesis of microsporidia.

Matrix-assisted laser desorption/ionization mass spectrometry biotyping, an approach for deciphering and assessing the identity of the honeybee pathogen Nosema

RATIONALE

The microsporidia are obligate intracellular pathogenic fungi that parasitize a wide range of invertebrate and vertebrate hosts and have important impacts on health, food security and the economy. In this paper, we focus on Nosema ceranae and N. apis, which chronically infect the digestive tract of honeybees, altering their physiology and lifespan.

METHODS

We applied matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) for rapid molecular profiling of extracts of Nosema spores in order to identify the species and the geographical origin, and assess the viability status of Nosema microsporidia in conjunction with a flow cytometric approach. Pure solutions of spores were prepared for flow cytometric analysis and MALDI-MS profiling. A mechanical extraction of viable or heat-killed Nosema spores was conducted to obtain mass fingerprints of peptides/proteins for samples of microsporidia from different geographical origins (MBO.NC01, MBO.NC02 and MBO.NA01).

RESULTS

A distinction in the peptide/protein profiles between two isolates with different geographical origins was observed. Mass fingerprints of viable and experimentally killed spores were also clearly distinguishable, regardless of Nosema species. Finally, using our computational models on the different Nosema species, we were able to classify five independent isolates of Nosema microsporidia.

CONCLUSIONS

We have shown that MALDI-MS is a rapid, cost-effective and simple method for identifying Nosema species. We demonstrated that MALDI Biotyping could represent a valuable surveillance tool of nosemosis in apiaries for sanitary services and beekeepers.

The shrimp microsporidian Enterocytozoon hepatopenaei (EHP): Biology, pathology, diagnostics and control

Disease is a major limiting factor in the global production of cultivated shrimp. The microsporidian parasite Enterocytozoon hepatopenaei (EHP) was formally characterized in 2009 as a rare infection of the black tiger shrimp Penaeus monodon. It remained relatively unstudied until mid-2010, after which infection with EHP became increasingly common in the Pacific whiteleg shrimp Penaeus vannamei, by then the most common shrimp species farmed in Asia. EHP infects the hepatopancreas of its host, causing hepatopancreatic microsporidiosis (HPM), a condition that has been associated with slow growth of the host in aquaculture settings. Unlike other infectious disease agents that have caused economic losses in global shrimp aquaculture, EHP has proven more challenging because too little is still known about its environmental reservoirs and modes of transmission during the industrial shrimp production process. This review summarizes our current knowledge of the EHP life cycle and the molecular strategies that it employs as an obligate intracellular parasite. It also provides an analysis of available and new methodologies for diagnosis since most of the current literature on EHP focuses on that topic. We summarize current knowledge of EHP infection and transmission dynamics and currently recommended, practical control measures that are being applied to limit its negative impact on shrimp cultivation. We also point out the major gaps in knowledge that urgently need to be bridged in order to improve control measures.

Invasion of Host Cells by Microsporidia

Microsporidia are found worldwide and both vertebrates and invertebrates can serve as hosts for these organisms. While microsporidiosis in humans can occur in both immune competent and immune compromised hosts, it has most often been seen in the immune suppressed population, e.g., patients with advanced HIV infection, patients who have had organ transplantation, those undergoing chemotherapy, or patients using other immune suppressive agents. Infection can be associated with either focal infection in a specific organ (e.g., keratoconjunctivitis, cerebritis, or hepatitis) or with disseminated disease. The most common presentation of microsporidiosis being gastrointestinal infection with chronic diarrhea and wasting syndrome. In the setting of advanced HIV infection or other cases of profound immune deficiency microsporidiosis can be extremely debilitating and carries a significant mortality risk. Microsporidia are transmitted as spores which invade host cells by a specialized invasion apparatus the polar tube (PT). This review summarizes recent studies that have provided information on the composition of the spore wall and PT, as well as insights into the mechanism of invasion and interaction of the PT and spore wall with host cells during infection.

A specific molecular label for identifying mature Nosema bombycis spores

Microsporidia are a fascinating phylum of obligate intracellular pathogens with unique infection processes and complicated life cycles. Microsporidian life cycles can be divided roughly into intracellular and extracellular stages. Currently, research on their life cycles were mainly explored by morphology because there are few molecular markers available with which to distinguish the different life stages. In this study, we generated H20, a monoclonal antibody (MAb) to label mature spores of Nosema bombycis. Immunofluorescence assays showed that the target protein of H20, which is highly stable and was barely affected by alkali and sodium dodecyl sulfate (SDS) treatments, was located on the mature spore surface. Western blot analysis showed that spore wall protein 26 (SWP26) was the likely target of H20. This MAb can specifically identify mature spores in a complex biological sample based on immunological detection of the parasite.

The role of Bombyx mori Bmtutl-519 protein in the infection of BmN cells by Nosema bombycis

Bmtutl-519 is an isoform of the Bombyx Turtle protein and a member of the immunoglobulin superfamily (IgSF). The relative expression level of Bmtutl-519 was significantly upregulated when BmN cells were infected by Nosema bombycis. The subcellular localization of Bmtutl-519 was studied using an indirect immunoinfluscent assay (IFA), Co-localization assay, Western blotting, and enhanced green fluorescent protein (EGFP) fusion constructs expressed in BmN cells transfected with a Bmtutl-519 expression plasmid. The results indicate that Bmtutl-519 is distributed in both the cytoplasm and the cell membrane of BmN cells. Bmtutl-519 may be involved in the infection process of N. bombycis as a cell surface receptor or regulatory factor. Interaction analysis of Bmtutl-519 with NbSWP26, a spore wall protein of N. bombycis involved in host cell adherence and infection, showed that the C-terminal heparin-binding motif (HBM) of NbSWP26 mediates the interaction between these two proteins. Mutation of the NbSWP26 HBM at K208G, K209G, K210G, and K213G led to a loss of the ability to bind the Bmtutl-519 protein. Spore adherence and infection assays showed that Bmtutl-519 enhances the binding ability of N. bombycis to the host cell surface, but this did not enhance host cell infection by N. bombycis. In contrast, the sustained high expression of Bmtutl-519 in BmN cells inhibited the proliferation of N. bombycis spores.

Spore morphology and ultrastructure of an Ascosphaera apis strain from the honeybees (Apis mellifera) in southwest China

Ascosphaera apis is an intestinally infective, spore-forming, filamentous fungus that infects honeybees and causes deadly chalkbrood disease. Although A. apis has been known for 60 y, little is known about the ultrastructure of the spores. In this study, the fine morphology and ultrastructure of an isolate, A. apis CQ1 from southwest China, was comprehensively identified by transmission electron microscopy, confocal laser scanning microscopy, scanning electron microscopy, and optical microscopy. The high sequence similarity and phylogenetic data based on nuc rDNA ITS1-5.8S-ITS2 (ITS) supported the hypothesis that the CQ1 strain is a new member of the A. apis species. Morphological observation indicated that the mature spores are long ovals with an average size of 2 × 1.2 µm and are tightly packed inside spherical spore balls. More than 10 spore balls that were 8-16 µm in diameter were wrapped and formed a spherical, nearly hyaline spore cyst of 50-60 µm in diameter. Ultrastructural analysis showed that mature spores have two nuclei with distinctly different sizes. A large nucleus with double nuclear membranes was found in the center of the spore, whereas the small nucleus was only one-fifth of the large nucleus volume and was located near the end of the spore. Numerous ribosomes filled the cytoplasm, and many mitochondria with well-defined structures were arranged along the inner spore wall. The spore wall consists of an electron-dense outer surface layer, an electron-lucent layer, and an inner plasma membrane. Chitin is the major component of the spore wall. The germinated spore was observed as an empty spore coat, whereas the protoplasts, including the nuclei, mitochondria, and ribosomes, had been discharged. In addition to these typical fungal spore organelles, an unknown electron-dense regular structure might be the growing mycelium, which was arranged close to the inner spore wall and almost covered the entire wall area.

Microsporidia: Obligate Intracellular Pathogens Within the Fungal Kingdom

Microsporidia are obligate intracellular pathogens related to Fungi. These organisms have a unique invasion organelle, the polar tube, which upon appropriate environmental stimulation rapidly discharges out of the spore, pierces a host cell's membrane, and serves as a conduit for sporoplasm passage into the host cell. Phylogenetic analysis suggests that microsporidia are related to the Fungi, being either a basal branch or sister group. Despite the description of microsporidia over 150 years ago, we still lack an understanding of the mechanism of invasion, including the role of various polar tube proteins, spore wall proteins, and host cell proteins in the formation and function of the invasion synapse. Recent advances in ultrastructural techniques are helping to better define the formation and functioning of the invasion synapse. Over the past 2 decades, proteomic approaches have helped define polar tube proteins and spore wall proteins as well as the importance of posttranslational modifications such as glycosylation in the functioning of these proteins, but the absence of genetic techniques for the manipulation of microsporidia has hampered research on the function of these various proteins. The study of the mechanism of invasion should provide fundamental insights into the biology of these ubiquitous intracellular pathogens that can be integrated into studies aimed at treating or controlling microsporidiosis.

Identification, characterization and heparin binding capacity of a spore-wall, virulence protein from the shrimp microsporidian, Enterocytozoon hepatopenaei (EHP)

Background

The microsporidian Enterocytozoon hepatopenaei (EHP) is a spore-forming, intracellular parasite that causes an economically debilitating disease (hepatopancreatic microsporidiosis or HPM) in cultured shrimp. HPM is characterized by growth retardation and wide size variation that can result in economic loss for shrimp farmers. Currently, the infection mechanism of EHP in shrimp is poorly understood, especially at the level of host-parasite interaction. In other microsporidia, spore wall proteins have been reported to be involved in host cell recognition. For the host, heparin, a glycosaminoglycan (GAG) molecule found on cell surfaces, has been shown to be recognized by many parasites such as Plasmodium spp. and Leishmania spp.

Results

We identified and characterized the first spore wall protein of EHP (EhSWP1). EhSWP1 contains three heparin binding motifs (HBMs) at its N-terminus and a Bin-amphiphysin-Rvs-2 (BAR2) domain at its C-terminus. A phylogenetic analysis revealed that EhSWP1 is similar to an uncharacterized spore wall protein from Enterospora canceri. In a cohabitation bioassay using EHP-infected shrimp with naïve shrimp, the expression of EhSWP1 was detected by RT-PCR in the naïve test shrimp at 20 days after the start of cohabitation. Immunofluorescence analysis confirmed that EhSWP1 was localized in the walls of purified, mature spores. Subcellular localization by an immunoelectron assay revealed that EhSWP1 was distributed in both the endospore and exospore layers. An in vitro binding assay, a competition assay and mutagenesis studies revealed that EhSWP1 is a bona fide heparin binding protein.

Conclusions

Based on our results, we hypothesize that EhSWP1 is an important host-parasite interaction protein involved in tethering spores to host-cell-surface heparin during the process of infection.

Electronic supplementary material

The online version of this article (10.1186/s13071-018-2758-z) contains supplementary material, which is available to authorized users.

The roles of microsporidia spore wall proteins in the spore wall formation and polar tube anchorage to spore wall during development and infection processes

Microsporidia are highly specialized obligate intracellular, spore forming divergent fungi with a wide variety host range that includes most vertebrates and invertebrates. The resistant spores are surrounded by a rigid cell wall which consists of three layers: the electron-lucent chitin and protein inner endospore, the outer-electron-dense and mainly proteinaceous exospore and plasma membrane. Interestingly, microsporidia owns a special invasion organelle, called polar tube, coiled within the interior of the spore wall and attached to anchoring disk at the anterior end of spore. Spore wall and polar tube are the major apparatuses for mature spores adhering and infecting to the host cells. In this review, we summarize the research advances in spore wall proteins (SWPs) related to spore adherence and infection, and SWPs and deproteinated chitin spore coats (DCSCs) interaction associated with SWPs deposit processes and spore wall assembly. Furthermore, we highlight the SWPs-polar tube proteins (PTPs) interaction correlated to polar tube orderly orientation, arrangement and anchorage to anchoring disk. Based on results obtained, it is helpful to improve understanding of the spore wall assembly and polar tube orderly arrangement mechanisms and molecular pathogenesis of microsporidia infection. Also, such information will provide a basis for developing effective control strategies against microporidia.

Engineered resistance to Nosema bombycis by in vitro expression of a single-chain antibody in Sf9-III cells

Nosema bombycis is a destructive, obligate intracellular parasite of the Bombyx mori. In this study, a single-chain variable fragment (scFv) dependent technology is developed for the purpose of inhibiting parasite proliferation in insect cells. The scFv-G4, which we prepared from a mouse G4 monoclonal antibody, can target the N. bombycis spore wall protein 12 (NbSWP12). Indirect immunofluorescence assays showed that NbSWP12 located mainly on the outside of the N. bombycis cytoskeleton, although some of it co-localized with β-tubulin in the meront-stage of parasites. When meront division began, NbSWP12 became concentrated at both ends of each meront. Western blotting showed that scFv-G4 could express in Sf9-III cells and recognized native NbSWP12. The transgenic Sf9-III cell line showed better resistance than the controls when challenged with N. bombycis, indicating that NbSWP12 is a promising target in this parasite and this scFv dependent strategy could be a solution for construction of N. bombycis-resistant Bombyx mori.

Pathological analysis of silkworm infected by two microsporidia Nosema bombycis CQ1 and Vairimorpha necatrix BM

Microsporidia Nosema bombycis CQ1 can be vertically transmitted in silkworm Bombyx mori but Vairimorpha necatrix BM cannot. Therefore, the pathological differences in silkworm infected with these two microsporidia required clarification. Here, we compared the virulence of N. bombycis CQ1 and V. necatrix BM against silkworm. The pathological characteristics in intestine, testis and ovary were surveyed using paraffin sections, scanning electron microscopy and transmission electron microscopy. Our data firstly showed that the virulence of V. necatrix BM was weaker than that of N. bombycis CQ1. Secondly, the typical symptom of V. necatrix BM infection is making xenomas, which are full of pathogens in different stages, at the posterior of intestine. However, no xenomas were formed surrounding intestines infected with N. bombycis CQ1. Thirdly, N. bombycis CQ1 can cluster spores near the trachea while infecting ovaries. It is worth noting that N. bombycis CQ1 infected epithelial cells and connective tissues of ovaries, while V. necatrix BM did not. Although silkworm ovaries can not be infected by V. necatrix BM in vivo, it can infect embryonic and ovarian cell lines in vitro. This study is the first report about comparing infection features of N. bombycis CQ1 and V. necatrix BM in silkworm tissues and it provided elaborate and visual information of pathological characteristics which can help to explain the different transmission strategies of these two microsporidia.

Easy labeling of proliferative phase and sporogonic phase of microsporidia Nosema bombycis in host cells

Microsporidia are eukaryotic, unicellular parasites that have been studied for more than 150 years. These organisms are extraordinary in their ability to invade a wide range of hosts including vertebrates and invertebrates, such as human and commercially important animals. A lack of appropriate labeling methods has limited the research of the cell cycle and protein locations in intracellular stages. In this report, an easy fluorescent labeling method has been developed to mark the proliferative and sporogonic phases of microsporidia Nosema bombycis in host cells. Based on the presence of chitin, Calcofluor White M2R was used to label the sporogonic phase, while β-tubulin antibody coupled with fluorescence secondary antibody were used to label the proliferative phase by immunofluorescence. This method is simple, efficient and can be used on both infected cells and tissue slices, providing a great potential application in microsporidia research.

A Novel Spore Wall Protein from Antonospora locustae (Microsporidia: Nosematidae) Contributes to Sporulation

Microsporidia are obligate intracellular parasites, existing in a wide variety of animal hosts. Here, we reported AlocSWP2, a novel protein identified from the spore wall of Antonospora locustae (formerly, Nosema locustae, and synonym, Paranosema locustae), containing four cysteines that are conserved among the homologues of several Microspodian pathogens in insects and mammals. AlocSWP2 was detected in the wall of mature spores via indirect immunofluorescence assay. In addition, immunocytochemistry localization experiments showed that the protein was observed in the wall of sporoblasts, sporonts, and meronts during sporulation within the host body, also in the wall of mature spores. AlocSWP2 was not detected in the fat body of infected locust until the 9th day after inoculating spores via RT‐PCR experiments. Furthermore, the survival percentage of infected locusts injected with dsRNA of AlocSWP2 on the 15th, 16th, and 17th days after inoculation with microsporidian were significantly higher than those of infected locusts without dsRNA treatment. Conversely, the amount of spores in locusts infected with A. locustae after treated with RNAi AlocSWP2 was significantly lower than those of infected locusts without RNAi of this gene. This novel spore wall protein from A. locustae may be involved in sporulation, thus contributing to host mortality.

Interaction between SWP9 and Polar Tube Proteins of the Microsporidian Nosema bombycis and Function of SWP9 as a Scaffolding Protein Contribute to Polar Tube Tethering to the Spore Wall

All microsporidia possess a unique, highly specialized invasion mechanism that involves the polar tube and spore wall. The interaction between spore wall proteins (SWPs) and polar tube proteins (PTPs) in the formation, arrangement, orderly orientation, and function of the polar tube and spore wall remains to be determined. This study was undertaken to examine the protein interactions of Nosema bombycis SWP7 (NbSWP7), NbSWP9, and PTPs. Coimmunoprecipitation, liquid chromatography-tandem mass spectrometry (LC-MS/MS), and yeast two-hybrid data demonstrated that NbSWP9, but not NbSWP7, interacts with NbPTP1 and NbPTP2. Furthermore, immunoelectron microscopy (IEM) showed that NbSWP9 was localized mainly in the developing polar tube of sporoblasts, while NbSWP7 was found randomly in the cytoplasm. However, both NbSWP9 and NbSWP7 were located in the polar tube and spore wall of N. bombycis mature spores. The reason why NbSWP7 was localized to the polar tube may be due to the interaction between NbSWP9 and NbSWP7. Interestingly, the majority of NbSWP9, but not NbSWP7, accumulated in the beginning part of the extruded polar tube and the ruptured spore wall called the anchoring disk (AD) when the mature spores germinated under weak-alkaline environmental stimulation. Additionally, anti-NbSWP9 antibody reduced spore germination in a dose-dependent manner. In conclusion, our study further confirmed that NbSWP9 is a scaffolding protein that not only anchors and holds the polar tube but also tethers the polar tube to the spore wall.

Microsporidia-host interactions

Microsporidia comprise one of the largest groups of obligate intracellular pathogens and can infect virtually all animals, but host response to these fungal-related microbes has been poorly understood. Several new studies of the host transcriptional response to microsporidia infection have found infection-induced regulation of genes involved in innate immunity, ubiquitylation, metabolism, and hormonal signaling. In addition, microsporidia have recently been shown to exploit host recycling endocytosis for exit from intestinal cells, and to interact with host degradation pathways. Microsporidia infection has also been shown to profoundly affect behavior in insect hosts. Altogether, these and other recent findings are providing much-needed insight into the underlying mechanisms of microsporidia interaction with host animals.

A monoclonal antibody that tracks endospore formation in the microsporidium Nosema bombycis

Nosema bombycis, the first identified microsporidium, is a destructive pathogen of the silkworm Bombyx mori and causes severe worldwide economic losses in sericulture. Major microsporidian structural proteins, such as the spore wall protein (SWP), are known to be involved in host invasion. In this study, the reactivity of the monoclonal antibody 2B10 was tested against an endospore protein of N. bombycis with a molecular weight size at 50-kDa, using Western blotting. The antigen was purified after immunoprecipitation and was further identified as EOB13320 according to MALDI-TOF MS assay. We found that EOB13320 locates to the surface of the different developmental stages of the parasite, mostly the sporoblast stage and the mature spore after immunoelectron microscopy examination. EOB13320 was also widely distributed in the developing endospore, especially at the sporoblast stage. This endospore protein also accumulated in the cytoplasm of both the merogony and sporoblast stages. These results imply that EOB13320 detected by monoclonal antibody 2B10 is expressed throughout the life cycle of the parasite, notably during the stage when the endospore is formed, and that this protein is important for spore-coat formation and parasite maintenance. Our study could be instrumental in the understanding of spore wall formation and will help to gain greater insight into the biology of this parasite.

Development of a strategy for the identification of surface proteins in the pathogenic microsporidian Nosema bombycis

Parasite–host interactions mediated by cell surface proteins have been implicated as a critical step in infections caused by the microsporidian Nosema bombycis. Such cell surface proteins are considered as promising diagnostic markers and targets for drug development. However, little research has specifically addressed surface proteome identification in microsporidia due to technical barriers. Here, a combined strategy was developed to separate and identify the surface proteins of N. bombycis. Briefly, following (1) biotinylation of the spore surface, (2) extraction of total proteins with an optimized method and (3) streptavidin affinity purification of biotinylated proteins, 22 proteins were identified based on LC-MS/MS analysis. Among them, 5 proteins were confirmed to be localized on the surface of N. bombycis. A total of 8 proteins were identified as hypothetical extracellular proteins, whereas 7 other hypothetical proteins had no available function annotation. Furthermore, a protein with a molecular weight of 18·5 kDa was localized on the spore surface by western blotting and immunofluorescence analysis, even though it was predicted to be a nuclear protein by bioinformatics. Collectively, our work provides an effective strategy for isolating microsporidian surface protein components for both drug target identification and further diagnostic research on microsporidian disease control.

Interaction and assembly of two novel proteins in the spore wall of the microsporidian species Nosema bombycis and their roles in adherence to and infection of host cells

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.

NbHSWP11, a microsporidia Nosema bombycis protein, localizing in the spore wall and membranes, reduces spore adherence to host cell BME

Microsporidia are obligate intracellular parasites, and a derivative of fungi, which harbor a rigid spore wall to resist adverse environmental pressures. The spore wall protein, which is thought to be the first and direct protein interacting with the host cell, may play a key role in the process of microsporidia infection. In this study, we report a protein, NbHSWP11, with a dnaJ domain. The protein also has 6 heparin-binding motifs which are known to interact with extracellular glycosaminoglycans. Syntenic analysis indicated that gene loci of Nbhswp11 are conserved and syntenic between Nosema bombycis and Nosema ceranae. Phylogenetic tree analysis showed that Nbhswp11 clusters with fungal dnaJ proteins and has 98% identity with an N. bombycis dnaJ protein. Nbhswp11 was transcribed throughout the entire life stages, and gradually increased during 1-7 days, in a silkworm that was infected by N. bombycis, as determined by reverse-transcription PCR (RT-PCR). The recombinant protein NbHSWP11 (rSWP11-HIS) was obtained and purified using gene cloning and prokaryotic expression. Western blotting analysis displayed NbHSWP11 expressed in the total mature spore proteins and spore coat proteins. Indirect immunofluorescence assay revealed NbHSWP11 located at the spore wall of mature spores and the spore coats. Furthermore, immune electron microscopy showed that NbHSWP11 localized in the cytoplasm of the sporont. Within the developmental process of N. bombycis, a portion of NbHSWP11 is targeted to the spore wall of sporoblasts and mature spores. However, most of NbHSWP11 distributes on the membraneous structures of the sporoblast and mature spore. In addition, using a host cell binding assay, native protein NbHSWP11 in the supernatant of total soluble mature spore proteins is shown to bind to the host cell BmE surface. Finally, an antibody blocking assay showed that purified rabbit antibody of NbHSWP11 inhibits spore adherence and decreases the adherence rate of spores by 20% compared to untreated spores. Collectively, the present results suggest that NbHSWP11 is involved in host cell adherence in vitro. Therefore NbHSWP11, which has a dnaJ domain, may modulate protein assembly, disassembly, and translocation in N. bombycis.