Identification of a Nosema bombycis (Microsporidia) spore wall protein corresponding to spore phagocytosis

Nosema bombycis: A remarkable unicellular parasite infecting insects

Microsporidia are opportunistic fungal-like pathogens that cause microsporidiosis, which results in significant economic losses and threatens public health. Infection of domesticated silkworms by the microsporidium Nosema bombycis causes pébrine disease, for which this species of microsporidia has received much attention. Research has been conducted extensively on this microsporidium over the past few decades to better understand its infection, transmission, host-parasite interaction, and detection. Several tools exist to study this species including the complete genome sequence of N. bombycis. In addition to the understanding of N. bombycis being important for the silkworm industry, this species has become a model organism for studying microsporidia. Research on biology of N. bombycis will contribute to the development of knowledge regarding microsporidia and potential antimicrosporidia drugs. Furthermore, this will provide insight into the molecular evolution and functioning of other fungal pathogens.

New insights into Microsporidia polar tube function and invasion mechanism

Microsporidia comprise a large phylum of single-cell and obligate intracellular parasites that can infect a wide range of invertebrate and vertebrate hosts including humans. These fungal-related parasites are characterized by a highly reduced genome, a strong energy dependence on their host, but also by their unique invasion organelle known as the polar tube which is coiled within the resistant spore. Upon appropriate environmental stimulation, the long hollow polar tube (ranging from 50 to 500 μm in length) is extruded at ultra-fast speeds (300 μm/s) from the spore acting as a harpoon-like organelle to transport and deliver the infectious material or sporoplasm into the host cell. To date, seven polar tube proteins (PTPs) with distinct localizations along the extruded polar tube have been described. For example, the specific location of PTP4 and PTP7 at the tip of the polar tube supports their role in interacting with cellular receptor(s). This chapter provides a brief overview on the current understanding of polar tube structure and dynamics of extrusion, primarily through recent advancements in cryo-tomography and 3D reconstruction. It also explores the various mechanisms used for host cell invasion. Finally, recent studies on the structure and maturation of sporoplasm and its moving through the tube are discussed.

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.

In-depth investigation of microRNA-mediated cross-kingdom regulation between Asian honey bee and microsporidian

Asian honey bee Apis cerana is the original host for Nosema ceranae, a unicellular fungal parasite that causes bee nosemosis throughout the world. Currently, interaction between A. cerana and N. ceranae is largely unknown. Our group previously prepared A. c. cerana workers’ midguts at 7 days post inoculation (dpi) and 10 dpi with N. ceranae spores as well as corresponding un-inoculated workers’ midguts, followed by cDNA library construction and a combination of RNAs-seq and small RNA-seq. Meanwhile, we previously prepared clean spores of N. ceranae, which were then subjected to cDNA library construction and deep sequencing. Here, based on the gained high-quality transcriptome datasets, N. ceranae differentially expressed mRNAs (DEmiRNAs) targeted by host DEmiRNAs, and A. c. cerana DEmRNAs targeted by microsporidian DEmiRNAs were deeply investigated, with a focus on targets involved in N. ceranae glycolysis/glyconeogenesis as well as virulence factors, and A. c. cerana energy metabolism and immune response. In A. c. cerana worker’s midguts at 7 (10) dpi (days post inoculation), eight (seven) up-regulated and six (two) down-regulated miRNAs were observed to target 97 (44) down-regulated and 60 (15) up-regulated N. ceranae mRNAs, respectively. Additionally, two up-regulated miRNAs (miR-60-y and miR-676-y) in host midgut at 7 dpi could target genes engaged in N. ceranae spore wall protein and glycolysis/gluconeogenesis, indicating potential host miRNA-mediated regulation of microsporidian virulence factor and energy metabolism. Meanwhile, in N. ceranae at 7 (10) dpi, 121 (110) up-regulated and 112 (104) down-regulated miRNAs were found to, respectively, target 343 (247) down-regulated and 138 (110) down-regulated mRNAs in A. c. cerana workers’ midguts. These targets in host were relevant to several crucial cellular and humoral immune pathways, such as phagasome, endocytosis, lysosomes, regulation of autophagy, and Jak–STAT signaling pathway, indicative of the involvement of N. ceranae DEmiRNAs in regulating these cellular and humoral immune pathways. In addition, N. ceranae miR-21-x was up-regulated at 7 dpi and had a target relative to oxidative phosphorylation, suggesting that miR-21-x may be used as a weapon to modulate this pivotal energy metabolism pathway. Furthermore, potential targeting relationships between two pairs of host DEmiRNAs-microsporidian DEmRNAs and two pairs of microsporidian DEmiRNAs-host DEmRNAs were validated using RT-qPCR. Our findings not only lay a foundation for exploring the molecular mechanism underlying cross-kingdom regulation between A. c. cerana workers and N. ceranae, but also offer valuable insights into Asian honey bee-microsporidian interaction.

Identification and Characterization of Three Spore Wall Proteins of Enterocytozoon Bieneusi

Enterocytozoon bieneusi is the most common microsporidian pathogen in farm animals and humans. Although several spore wall proteins (SWPs) of other human-pathogenic microsporidia have been identified, SWPs of E. bieneusi remain poorly characterized. In the present study, we identified the sequences of three E. bieneusi SWPs from whole genome sequence data, expressed them in Escherichia coli, generated a monoclonal antibody (mAb) against one of them (EbSWP1), and used the mAb in direct immunofluorescence detection of E. bieneusi spores in fecal samples. The amino acid sequence of EbSWP1 shares some identity to EbSWP2 with a BAR2 domain, while the sequence of EbSWP3 contains a MICSWaP domain. No cross-reactivity among the EbSWPs was demonstrated using the polyclonal antibodies generated against them. The mAb against EbSWP1 was shown to react with E. bieneusi spores in fecal samples. Using chromotrope 2R staining-based microscopy as the gold standard, the sensitivity and specificity of the direct immunofluorescence for the detection of E. bieneusi were 91.4 and 73.7%. Data generated from the study could be useful in the characterization of E. bieneusi and immunological detection of the pathogen.

Two new species of Bacillidium (Microsporidia) from coelomocytes of Branchiura sowerbyi (Oligochaeta: Naididae) in China

Bacillidium spp. exclusively infect oligochaetes and these microsporidian pathogens are typically characterized by their rod-shaped spores. Seven Bacillidium spp. are presently reported from different organs of oligochaetes. Here, we describe two new Bacillidium species, Bacillidium sinensis n. sp. and Bacillidium branchilis n. sp., from coelomocytes of Branchiura sowerbyi. This is the first report of Bacillidium spp. in oligochaetes from China. Both species of Bacillidium elicit the formations of opaque xenoma-like lesions in coelomocytes of the host. A diplokaryotic nucleus occurs in all life stages of these two new Bacillidium species. Mature spores of B. sinensis are 15.9 ±0.6 (14.7-17.1) μm long (average ± standard error, range, n = 50) and 2.5 ±0.1 (2.3-2.7) μm wide in fresh preparations. A new type of exospore (sixteen-layered exospore) is discovered from B. sinensis n. sp. which is distinctly different from B. branchilis n. sp., and other Bacillidium spp. (double-layered exospore) reported previously. These two Bacillidium species are morphologically distinguished from each other and all Bacillidium spp. described previously in terms of hosts, infection sites, spore size, spore wall or polar filament thickness. BLASTn searches indicated that these two new microsporidian parasites are surprisingly most similar to Janacekia tainanus (94.76% for B. sinensis and 90. 2% for B. branchilis) isolated from the fat body of midge larva (Kiefferulus tainanus). Phylogenetic analysis demonstrates that the two novel taxons cluster with J. debaisieuxi, J. tainanus, and Bacillidium sp. within the Jirovecia-Bacillidium-Janacekia clade. Other available 18S rRNA gene sequences for microsporidia that infect oligochaetes include J. sinensis, B. vesiculoformis, Neoflabelliforma aurantiae, and Bacillidium sp., but these do not form a single cluster with B. sinensis and B. branchilis, but are instead dispersed through the clade. Based on the ultrastructural features and molecular characteristics, two new species within the genus Bacillidium, B. sinensis n. sp. and B. branchilis n. sp., are designated.

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.

Microsporidiosis in Humans

Microsporidia are obligate intracellular pathogens identified ∼150 years ago as the cause of pébrine, an economically important infection in silkworms. There are about 220 genera and 1,700 species of microsporidia, which are classified based on their ultrastructural features, developmental cycle, host-parasite relationship, and molecular analysis. Phylogenetic analysis suggests that microsporidia are related to the fungi, being grouped with the Cryptomycota as a basal branch or sister group to the fungi. Microsporidia can be transmitted by food and water and are likely zoonotic, as they parasitize a wide range of invertebrate and vertebrate hosts. Infection in humans occurs in both immunocompetent and immunodeficient hosts, e.g., in patients with organ transplantation, patients with advanced human immunodeficiency virus (HIV) infection, and patients receiving immune modulatory therapy such as anti-tumor necrosis factor alpha antibody. Clusters of infections due to latent infection in transplanted organs have also been demonstrated. Gastrointestinal infection is the most common manifestation; however, microsporidia can infect virtually any organ system, and infection has resulted in keratitis, myositis, cholecystitis, sinusitis, and encephalitis. Both albendazole and fumagillin have efficacy for the treatment of various species of microsporidia; however, albendazole has limited efficacy for the treatment of Enterocytozoon bieneusi. In addition, immune restoration can lead to resolution of infection. While the prevalence rate of microsporidiosis in patients with AIDS has fallen in the United States, due to the widespread use of combination antiretroviral therapy (cART), infection continues to occur throughout the world and is still seen in the United States in the setting of cART if a low CD4 count persists.

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.

Identification and characterization of three microsporidian genera concurrently infecting a silkworm, Bombyx mori, in Brazil

Microsporidia are important entomopathogens known for infecting insects such as the silkworm (Bombyx mori) thus impairing global silk production. This study aimed to identify and characterize the microsporidia isolated from a diseased larva of silkworm, collected from a sericulture farm in southern Brazil. Identification was performed by phylogenetic analysis of the nucleotide sequences of the SSU rRNA genes. Characterization was performed by analyzing spore sizes, tissue tropism, internal and external symptoms, and pathogenicity against B. mori. Microsporidia belonging to three different genera were identified, namely, Endoreticulatus, Nosema and Tubulinosema. After inoculation of the mixed spores of the microsporidian isolates into B. mori larvae, a high prevalence of Tubulinosema spp. was observed. This isolate showed high prevalence on the silk glands and a late mortality, initially of around 10% until the 20th day post-inoculation but reaching 91.5% upon pupation. Therefore, we demonstrated that Tubulinosema spp. causes chronic infection with slow pathogenicity. We identified for the first time three different microsporidians concurrently infecting B. mori in Brazil. Tubulinosema is of particular interest because of its potential threat to silk production; it affects the formation of silk glands in B. mori while not presenting distinguishable external symptoms or causing the immediate death of these insects. Further studies focusing on this species, mainly regarding its life cycle within the host and the sublethal effects of surviving individuals, demonstrate the importance of describing it as a new species and improving the characterization of the disease in order to prevent its spread.

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.

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.

Comparative Analysis of the Proteins with Tandem Repeats from 8 Microsporidia and Characterization of a Novel Endospore Wall Protein Colocalizing with Polar Tube from Nosema bombycis

As a common feature of eukaryotic proteins, tandem amino acid repeat has been studied extensively in both animal and plant proteins. Here, a comparative analysis focusing on the proteins having tandem repeats was conducted in eight microsporidia, including four mammal-infecting microsporidia (Encephalitozoon cuniculi, Encephalitozoon intestinalis, Encephalitozoon hellem and Encephalitozoon bieneusi) and four insect-infecting microsporidia (Nosema apis, Nosema ceranae, Vavraia culicis and Nosema bombycis). We found that the proteins with tandem repeats were abundant in these species. The quantity of these proteins in insect-infecting microsporidia was larger than that of mammal-infecting microsporidia. Additionally, the hydrophilic residues were overrepresented in the tandem repeats of these eight microsporidian proteins and the amino acids residues in these tandem repeat sequences tend to be encoded by GC-rich codons. The tandem repeat position within proteins of insect-infecting microsporidia was randomly distributed, whereas the tandem repeats within proteins of mammal-infecting microsporidia rarely tend to be present in the N terminal regions, when compared with those present in the C terminal and middle regions. Finally, a hypothetical protein EOB14572 possessing four tandem repeats was successfully characterized as a novel endospore wall protein, which colocalized with polar tube of N. bombycis. Our study provided useful insight for the study of the proteins with tandem repeats in N. bombycis, but also further enriched the spore wall components of this obligate unicellular eukaryotic parasite.

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.

Ricin-B-lectin enhances microsporidia Nosema bombycis infection in BmN cells from silkworm Bombyx mori

Nosema bombycis is an obligate intracellular parasitic fungus that utilizes a distinctive mechanism to infect Bombyx mori Spore germination can be used for host cell invasion; however, the detailed mechanism remains to be elucidated. The ricin-B-lectin (RBL) gene is significantly differentially regulated after N. bombycis spore germination, and NbRBL might play roles in spore germination and infection. In this study, the biological function of NbRBL was examined. Protein sequence analysis showed that NbRBL is a secreted protein that attaches to carbohydrates. The relative expression level of the NbRBL gene was low during the first 30 h post-infection (hpi) in BmN cells, and high expression was detected from 42 hpi. Gene cloning, prokaryotic expression, and antibody preparation for NbRBL were performed. NbRBL was detected in total and secreted proteins using western blot analysis. Subcellular localization analysis showed that NbRBL is an intracellular protein. Spore adherence and infection assays showed that NbRBL could enhance spore adhesion to BmN cells; the proliferative activities of BmN cells incubated with anti-NbRBL were higher than those in negative control groups after N. bombycis infection; and the treatment groups showed less damage from spore invasion. We therefore, propose that NbRBL is released during spore germination, enhances spore adhesion to BmN cells, and contributes to spore invasion.

Quantitative Proteomic Analysis of Germination of Nosema bombycis Spores under Extremely Alkaline Conditions

The microsporidian Nosema bombycis is an obligate intracellular pathogen of the silkworm Bombyx mori, causing the epidemic disease Pebrine and extensive economic losses in sericulture. Although N. bombycis forms spores with rigid spore walls that protect against various environmental pressures, ingested spores germinate immediately under the extremely alkaline host gut condition (Lepidoptera gut pH > 10.5), which is a key developmental turning point from dormant state to infected state. However, to date this process remains poorly understood due to the complexity of the animal digestive tract and the lack of genetic tools for microsporidia. Here we show, using an in vitro spore germination model, how the proteome of N. bombycis changes during germination, analyse specific metabolic pathways employed in detail, and validate key functional proteins in vivo in silkworms. By a label-free quantitative proteomics approach that is directly based on high-resolution mass spectrometry (MS) data, a total of 1136 proteins were identified with high confidence, with 127 proteins being significantly changed in comparison to non-germinated spores. Among them, structural proteins including polar tube protein 1 and 3 and spore wall protein (SWP) 4 and 30 were found to be significantly down-regulated, but SWP9 significantly up-regulated. Some nucleases like polynucleotide kinase/phosphatase and flap endonucleases 1, together with a panel of hydrolases involved in protein degradation and RNA cleavage were overrepresented too upon germination, which implied that they might play important roles during spore germination. The differentially regulated trends of these genes were validated, respectively, by quantitative RT-PCR and 3 proteins of interest were confirmed by Western blotting analyses in vitro and in vivo. Furthermore, the pathway analysis showed that abundant up- and down-regulations appear involved in the glycolysis, pentose phosphate pathway, purine, and pyrimidine metabolism, suggesting preparations of energy generation and substance synthesis for the following invasion and proliferation inside the host. This report, to our knowledge, provides the first proteomic landscape of N. bombycis spores, and also a stepping stone on the way to further study of the unique infection mode of this economically important pathogen and other microsporidia in general.

Transcriptome sequencing and characterization of ungerminated and germinated spores of Nosema bombycis

Nosema bombycis is an obligate intracellular parasitic fungus that utilizes a distinctive mechanism to infect Bombyx mori. Germination, an indispensible process through which microsporidia infect the host cells, is regarded as a key developmental turning point for microsporidia from dormant state to reproduction state. Thus, elucidating the transcriptome changes before and after germination is crucial for parasite control. However, the molecular basis of germination of microsporidia remains unknown. To investigate this germination process, the transcriptome of N. bombycis ungerminated spores and germinated spores were sequenced and analyzed. More than 60 million high-quality transcript reads were generated from these two groups using RNA-Seq technology. After assembly, 2756 and 2690 unigenes were identified, respectively, and subsequently annotated based on known proteins. After analysis of differentially expressed genes, 66 genes were identified to be differentially expressed (P ≤ 0.05) between these two groups. A protein phosphatase-associated gene was first identified to be significantly up-regulated as determined by RNA-Seq and immunoblot analysis, indicating that dephosphorylation might potentially contribute to microsporidia germination. The DEGs that encode proteins involved in glycometabolism, spore wall proteins and ricin B lectin of N. bombycis were also analyzed. Gene Ontology and Kyoto Encyclopedia of Genes and Genomes analyses revealed genes responsible for some specific biological functions and processes. The datasets generated in this study provide a basic characterization of the transcriptome changes in N. bombycis during germination. The analysis of transcriptome data and identification of certain functional genes which are robust candidate genes related to germination will help to provide a deep understanding of spore germination and invasion.

Quantitative PCR for detection of Nosema bombycis in single silkworm eggs and newly hatched larvae

Pebrine disease is the only mandatory quarantine item in sericultural production due to its destructive consequences. So far, the mother moth microscopic examination method established by Pasteur (1870) remains the only detection method for screening for the causative agent Nosema bombycis (N. bombycis). Because pebrine is a horizontal and vertical transmission disease, it is better to inspect silkworm eggs and newly hatched larvae to investigate the infection rate, vertical transmission rate and spore load of the progenies. There is a rising demand for a more direct, effective and accurate detection approach in the sericultural industry. Here, we developed a molecular detection approach based on real-time quantitative PCR (qPCR) for pebrine inspection in single silkworm eggs and newly hatched larvae. Targeting the small-subunit rRNA gene of N. bombycis, this assay showed high sensitivity and reproducibility. Ten spores in a whole sample or 0.1 spore DNA (1 spore DNA represents the DNA content of one N. bombycis spore) in a reaction system was estimated as the detection limit of the isolation and real-time qPCR procedure. Silkworm egg tissues impact the detection sensitivity but are not significant in single silkworm egg detection. Of 400 samples produced by infected moths, 167 and 195 were scored positive by light microscopy and real-time qPCR analysis, respectively. With higher accuracy and the potential capability of high-throughput screening, this method is anticipated to be adaptable for pebrine inspection and surveillance in the sericultural industry. In addition, this method can be applied to ecology studies of N. bombycis-silkworm interactions due to its quantitative function.

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.

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.

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

Identification of a protein interacting with the spore wall protein SWP26 of Nosema bombycis in a cultured BmN cell line of silkworm

Nosema bombycis is a silkworm parasite that causes severe economic damage to sericulture worldwide. It is the first microsporidia to be described in the literature, and to date, very little molecular information is available regarding microsporidian physiology and their relationships with their hosts. Therefore, the interaction between the microsporidia N. bombycis and its host silkworm, Bombyx mori, was analyzed in this study. The microsporidian spore wall proteins (SWPs) play a specific role in spore adherence to host cells and recognition by the host during invasion. In this study, SWP26 fused with enhanced green fluorescence protein (EGFP) was expressed in BmN cells by using a Bac-to-Bac expression system. Subsequently, the turtle-like protein of B. mori (BmTLP) was determined to interact with SWP26 via the use of anti-EGFP microbeads. This interaction was then confirmed by yeast two-hybrid analysis. The BmTLP cDNA encodes a polypeptide of 447 amino acids that includes a putative signal peptide of 27 amino acid residues. In addition, the BmTLP protein contains 2 immunoglobulin (IG) domains and 2 IGc2-type domains, which is the typical domain structure of IG proteins. The results of this study indicated that SWP26 interacts with the IG-like protein BmTLP, which contributes to the infectivity of N. bombycis to its host silkworm.

SWP5, a spore wall protein, interacts with polar tube proteins in the parasitic microsporidian Nosema bombycis

Microsporidia are a group of eukaryotic intracellular parasites that infect almost all vertebrates and invertebrates. The microsporidian invasion process involves the extrusion of a unique polar tube into host cells. Both the spore wall and the polar tube play an important role in microsporidian pathogenesis. So far, five spore wall proteins (SWP1, SWP2, Enp1, Enp2, and EcCDA) from Encephalitozoon intestinalis and Encephalitozoon cuniculi and five spore wall proteins (SWP32, SWP30, SWP26, SWP25, and NbSWP5) from the silkworm pathogen Nosema bombycis have been identified. Here we report the identification and characterization of a spore wall protein (SWP5) with a molecular mass of 20.3 kDa in N. bombycis. This protein has low sequence similarity to other eukaryotic proteins. Immunolocalization analysis showed SWP5 localized to the exospore and the region of the polar tube in mature spores. Immunoprecipitation, mass spectrometry, and immunofluorescence analyses revealed that SWP5 interacts with the polar tube proteins PTP2 and PTP3. Anti-SWP5 serum pretreatment of mature spores significantly decreased their polar tube extrusion rate. Taken together, our results show that SWP5 is a spore wall protein localized to the spore wall and that it interacts with the polar tube, may play an important role in supporting the structural integrity of the spore wall, and potentially modulates the course of infection of N. bombycis.