In-vitro cultivation of Nosema bombycis sporoplasms: A method for potential genetic engineering of microsporidia

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.

Microsporidia dressing up: the spore polaroplast transport through the polar tube and transformation into the sporoplasm membrane

Microsporidia are obligate intracellular parasites that infect a wide variety of hosts including humans. Microsporidian spores possess a unique, highly specialized invasion apparatus involving the polar filament, polaroplast, and posterior vacuole. During spore germination, the polar filament is discharged out of the spore forming a hollow polar tube that transports the sporoplasm components including the nucleus into the host cell. Due to the complicated topological changes occurring in this process, the details of sporoplasm formation are not clear. Our data suggest that the limiting membrane of the nascent sporoplasm is formed by the polaroplast after microsporidian germination. Using electron microscopy and 1,1'-dioctadecyl-3,3,3',3' tetramethyl indocarbocyanine perchlorate staining, we describe that a large number of vesicles, nucleus, and other cytoplasm contents were transported out via the polar tube during spore germination, while the posterior vacuole and plasma membrane finally remained in the empty spore coat. Two Nosema bombycis sporoplasm surface proteins (NbTMP1 and NoboABCG1.1) were also found to localize in the region of the polaroplast and posterior vacuole in mature spores and in the discharged polar tube, which suggested that the polaroplast during transport through the polar tube became the limiting membrane of the sporoplasm. The analysis results of Golgi-tracker green and Golgi marker protein syntaxin 6 were also consistent with the model of the transported polaroplast derived from Golgi transformed into the nascent sporoplasm membrane.IMPORTANCEMicrosporidia, which are obligate intracellular pathogenic organisms, cause huge economic losses in agriculture and even threaten human health. The key to successful infection by the microsporidia is their unique invasion apparatus which includes the polar filament, polaroplast, and posterior vacuole. When the mature spore is activated to geminate, the polar filament uncoils and undergoes a rapid transition into the hollow polar tube that transports the sporoplasm components including the microsporidian nucleus into host cells. Details of the structural difference between the polar filament and polar tube, the process of cargo transport in extruded polar tube, and the formation of the sporoplasm membrane are still poorly understood. Herein, we verify that the polar filament evaginates to form the polar tube, which serves as a conduit for transporting the nucleus and other sporoplasm components. Furthermore, our results indicate that the transported polaroplast transforms into the sporoplasm membrane during spore germination. Our study provides new insights into the cargo transportation process of the polar tube and origin of the sporoplasm membrane, which provide important clarification of the microsporidian infection mechanism.

Bleomycin reduces Vairimorpha (Nosema) ceranae infection in honey bees with some evident host toxicity

Microsporidia cause disease in many beneficial insects, including honey bees, yet few pathogen control tools are available for protecting these important organisms against infection. Some evidence suggests that microsporidia possess a reduced number of genes encoding DNA repair proteins. We hypothesized that microsporidia would thus be susceptible to treatment with DNA-damaging agents and tested this hypothesis using a novel, rapid method for achieving robust and homogenous experimental infection of large numbers of newly emerged honey bees with one of its microsporidia pathogens, Vairimorpha (Nosema) ceranae. In carrying out these experiments, we found this novel V. ceranae inoculation method to have similar efficacy as other traditional methods. We show that the DNA-damaging agent bleomycin reduces V. ceranae levels, with minimal but measurable effects on honey bee survival and increased expression of midgut cellular stress genes, including those encoding SHSP. Increased expression of UpdlC suggests the occurrence of epithelial regeneration, which may contribute to host resistance to bleomycin treatment. While bleomycin does reduce infection levels, host toxicity issues may preclude its use in the field. However, with further work, bleomycin may provide a useful tool in the research setting as a potential selection agent for genetic modification of microsporidia.IMPORTANCEMicrosporidia cause disease in many beneficial insects, yet there are few tools available for control in the field or laboratory. Based on the reported paucity of DNA repair enzymes found in microsporidia genomes, we hypothesized that these obligate intracellular parasites would be sensitive to DNA damage. In support of this, we observed that the well-characterized DNA damage agent bleomycin can reduce levels of the microsporidia Vairimorpha (Nosema) ceranae in experimental infections in honey bees. Observation of slightly reduced honey bee survival and evidence of sublethal toxicity likely preclude the use of bleomycin in the field. However, this work identifies bleomycin as a compound that merits further exploration for use in research laboratories as a potential selection agent for generating genetically modified microsporidia.

Proteomic profile of polar filament and polar tube from fungal pathogen microsporidium Nosema bombycis provides new insights into its unique invasion organelle

Microsporidium is a kind of intracellular fungal pathogen that greatly threatens the human health, breeding industry, and food security. All members of microsporidia possess a unique, highly specialized invasion organelle, described as the polar filament. Like "reversing a finger of gloves", the polar filament discharges out of mature spores to transform as the polar tube, and pathogenic sporoplasm is transported to host cell through polar tube to complete infection. During the invasion process, the structure of polar filament and polar tube has changed, so does the protein composition on them? In this study, we firstly proposed a purification method for polar filament and polar tube from microsporidium Nosema bombycis which was infected silkworm Bombyx mori, and it was also found that the structure of polar filament and polar tube was obviously different. Therefore, the proteome of these two structures was comparatively analyzed. A total of 881 and 1216 proteins were respectively identified from the polar filament and polar tube. Ten potential novel polar tube proteins (PTPs) were screened, providing a reference for the novel PTPs identification. Compared with the polar filament, there were 35 upregulated and 41 downregulated proteins on the polar tube. GO and KEGG pathway analysis of all proteins from the polar filament and polar tube provided us with a profound understanding for the microsporidian germination process, which was of great significance for clarifying the infection mechanism of microsporidia. SIGNIFICANCE: Microsporidia are obligate intracellular parasites that infect a wide variety of hosts, including humans. The polar filament is a unique invasion organelle for microsporidia, and it is also one of the important indexes of microsporidian taxonomy. The polar tube is deformed from the primitive polar filament in mature spores. During the germination, the polar filament turns into a polar tube, like "reversing a finger of gloves", through which pathogenic sporoplasm is transported to host cells to complete infection. Since the structure of the polar filament and polar tube has changed, what about their protein composition? In this study, it was the first time to purify the polar filament and the polar tube from microsporidium Nosema bombycis that was infected silkworm Bombyx mori, which provided new insights for studying the invasion organelle of microsporidia. Comparing the fine structure of polar filament and polar tube, we found that their structure was obviously different. Therefore, the protein composition of these two structures is supposed to be varied. In this case, the proteome of these two structures was comparatively analyzed. A total of 881 and 1216 proteins were respectively identified from the polar filament and polar tube. Ten potential novel polar tube proteins (PTPs) were screened, providing a reference for the novel PTPs identification. Compared with the polar filament, there were 35 upregulated and 41 downregulated proteins on the polar tube. GO and KEGG pathway analysis of all proteins from the polar filament and polar tube provided us with a profound understanding for the microsporidian germination process, which was of great significance for clarifying the infection mechanism of microsporidia.

Stable transformation of fluorescent proteins into Nosema bombycis by electroporation

Background

Microsporidia are a group of intracellular parasitic eukaryotes, serious pathogens that cause widespread infection in humans, vertebrates, and invertebrates. Because microsporidia have a thick spore wall structure, the in vitro transformation, cell culture, and genetic operation technology of microsporidia are far behind that of other parasites.

Methods

In this study, according to an analysis of the life-cycle of microsporidia, Nosema bombycis, and different electro-transformation conditions, the transduction efficiency of introducing foreign genes into N. bombycis was systematically determined.

Results

We analyzed the direct electro-transformation of foreign genes into germinating N. bombycis using reporters under the regulation of different characteristic promoters. Furthermore, we systematically determined the efficiency of electro-transformation into N. bombycis under different electro-transformation conditions and different developmental stages through an analysis of the whole life-cycle of N. bombycis. These results revealed that foreign genes could be effectively introduced through a perforation voltage of 100 V pulsed for 15 ms during the period of N. bombycis sporeplasm proliferation.

Conclusions

We present an effective method for electro-transformation of a plasmid encoding a fluorescent protein into N. bombycis, which provides new insight for establishing genetic modifications and potential applications in these intracellular parasites.

Graphical Abstract

Advances in the Genetic Manipulation of Nosema bombycis

The microsporidium Nosema bombycis can infect and transmit both vertically and horizontally in multiple lepidopteran insects including silkworms and crop pests. While there have been several studies on the N. bombycis spore, there have been only limited studies on the N. bombycis sporoplasm. This chapter reviews what is known about this life cycle stage as well as published studies on purification of the N. bombycis sporoplasm and its survival in an in vitro cell culture system. Genetic transformation techniques have revolutionized the study of many pathogenic organisms. While progress has been made on the development of such systems for microsporidia, this critical problem has not been solved for these pathogens. This chapter provides a summary of the latest research progress on genetic manipulation of N. bombycis.

Nosema apis and N. ceranae Infection in Honey bees: A Model for Host-Pathogen Interactions in Insects

There has been increased focus on the role of microbial attack as a potential cause of recent declines in the health of the western honey bee, Apis mellifera. The Nosema species, N. apis and N. ceranae, are microsporidian parasites that are pathogenic to honey bees, and infection by these species has been implicated as a key factor in honey bee losses. Honey bees infected with both Nosema spp. display significant changes in their biology at the cellular, tissue, and organismal levels impacting host metabolism, immune function, physiology, and behavior. Infected individuals lead to colony dysfunction and can contribute to colony disease in some circumstances. The means through which parasite growth and tissue pathology in the midgut lead to the dramatic physiological and behavioral changes at the organismal level are only partially understood. In addition, we possess only a limited appreciation of the elements of the host environment that impact pathogen growth and development. Critical for answering these questions is a mechanistic understanding of the host and pathogen machinery responsible for host-pathogen interactions. A number of approaches are already being used to elucidate these mechanisms, and promising new tools may allow for gain- and loss-of-function experiments to accelerate future progress.

Identification and characterization a novel polar tube protein (NbPTP6) from the microsporidian Nosema bombycis

BACKGROUND

Microsporidians are opportunistic pathogens with a wide range of hosts, including invertebrates, vertebrates and even humans. Microsporidians possess a highly specialized invasion structure, the polar tube. When spores encounter an appropriate environmental stimulation, the polar tube rapidly everts out of the spore, forming a 50-500 µm hollow tube that serves as a conduit for sporoplasm passage into host cells. The polar tube is mainly composed of polar tube proteins (PTPs). So far, five major polar tube proteins have been isolated from microsporidians. Nosema bombycis, the first identified microsporidian, infects the economically important insect silkworm and causes heavy financial loss to the sericulture industry annually.

RESULTS

A novel polar tube protein of N. bombycis (NbPTP6) was identified. NbPTP6 was rich in histidine (H) and serine (S), which contained a signal peptide of 16 amino acids at the N-terminus. NbPTP6 also had 6 potential O-glycosylation sites and 1 potential N-glycosylation site. The sequence alignment analysis revealed that NbPTP6 was homologous with uncharacterized proteins from other microsporidians (Encephalitozoon cuniculi, E. hellem and N. ceranae). Additionally, the NbPTP6 gene was expressed in mature N. bombycis spores. Indirect immunofluorescence analysis (IFA) result showed that NbPTP6 is localized on the whole polar tube of the germinated spores. Moreover, IFA, enzyme-linked immunosorbent (ELISA) and fluorescence-activated cell sorting (FACS) assays results revealed that NbPTP6 had cell-binding ability.

CONCLUSIONS

Based on our results, we have confirmed that NbPTP6 is a novel microsporidian polar tube protein. This protein could adhere with the host cell surface, so we speculated it might play an important role in the process of microsporidian infection.