Apical spore phagocytosis is not a significant route of infection of differentiated enterocytes by Encephalitozoon intestinalis

Human microsporidian pathogen Encephalitozoon intestinalis impinges on enterocyte membrane trafficking and signaling

Microsporidia are a large phylum of obligate intracellular parasites. Approximately a dozen species of microsporidia infect humans, where they are responsible for a variety of diseases and occasionally death, especially in immunocompromised individuals. To better understand the impact of microsporidia on human cells, we infected human colonic Caco2 cells with Encephalitozoon intestinalis, and showed that these enterocyte cultures can be used to recapitulate the life cycle of the parasite, including the spread of infection with infective spores. Using transmission electron microscopy, we describe this lifecycle and demonstrate nuclear, mitochondrial and microvillar alterations by this pathogen. We also analyzed the transcriptome of infected cells to reveal host cell signaling alterations upon infection. These high-resolution imaging and transcriptional profiling analysis shed light on the impact of the microsporidial infection on its primary human target cell type.This article has an associated First Person interview with the first authors of the paper.

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.

Cyclophosphamide Treatment Mimics Sub-Lethal Infections With Encephalitozoon intestinalis in Immunocompromised Individuals

Microsporidia, including Encephalitozoon intestinalis, are emerging pathogens which cause opportunistic infections in immunocompromised patients, such as those with AIDS, cancer, the elderly and people on immunosuppressive drugs. Intestinal mucosa (IM) is crucial for developing an efficient adaptive immune response against pathogenic micro-organisms, thereby preventing their colonization and subsequent infection. As immunosuppressive drugs affect the intestinal immune response is little known. In the present study, we investigated the immune response to E. intestinalis infection in the IM and gut-associated lymphoid tissue (GALT) in cyclophosphamide (Cy) immunosuppressed mice, to mimic an immunocompromised condition. Histopathology revealed lymphoplasmacytic enteritis at 7 and 14 days-post-infection (dpi) in all infected groups, however, inflammation diminished at 21 and 28 dpi. Cy treatment also led to a higher number of E. intestinalis spores and lesions, which reduced at 28 dpi. In addition, flow cytometry analysis demonstrated CD4⁺ and CD8⁺ T cells to be predominant immune cells, with up-regulation in both Th1 and Th2 cytokines at 7 and 14 dpi, as demonstrated by histopathology. In conclusion, Cy treatment reduced GALT (Peyer’s plaques and mesenteric lymph nodes) and peritoneum populations but increased the T-cell population in the intestinal mucosa and the production of pro-and anti-inflammatory cytokines, which were able to eliminate this opportunistic fungus and reduced the E. intestinalis infection.

Genome analysis and polar tube firing dynamics of mosquito-infecting microsporidia

Microsporidia are highly divergent fungi that are obligate intracellular pathogens of a wide range of host organisms. Here we review recent findings from the genome sequences of mosquito-infecting microsporidian species Edhazardia aedis and Vavraia culicis, which show large differences in genome size, although similar numbers of predicted genes. We also show a video of E. aedis polar tube firing, which is the dramatic mechanism used by microsporidia to deliver the germ cell (sporoplasm) into the host cell to initiate intracellular infection.

Basement membrane influences intestinal epithelial cell growth and presents a barrier to the movement of macromolecules

This work examines the potential drug delivery barrier of the basement membrane (BM) by assessing the permeability of select macromolecules and nanoparticles. The study further extends to probing the effect of BM on intestinal epithelial cell attachment and monolayer characteristics, including cell morphology. Serum-free cultured Caco-2 cells were grown on BM-containing porous supports, which were obtained by prior culture of airway epithelial cells (Calu-3), shown to assemble and deposit a BM on the growth substrate, followed by decellularisation. Data overall show that the attachment capacity of Caco-2 cells, which is completely lost in serum-free culture, is fully restored when the cells are grown on BM-coated substrates, with cells forming intact monolayers with high electrical resistance and low permeability to macromolecules. Caco-2 cells cultured on BM-coated substrates displayed strikingly different morphological characteristics, suggestive of a higher level of differentiation and closer resemblance to the native intestinal epithelium. BM was found to notably hinder the diffusion of macromolecules and nanoparticles in a size dependent manner. This suggests that the specialised network of extracellular matrix proteins may have a significant impact on transmucosal delivery of certain therapeutics or drug delivery systems.

Caco-2 cell acquisition of dietary iron(III) invokes a nanoparticulate endocytic pathway

Dietary non-heme iron contains ferrous [Fe(II)] and ferric [Fe(III)] iron fractions and the latter should hydrolyze, forming Fe(III) oxo-hydroxide particles, on passing from the acidic stomach to less acidic duodenum. Using conditions to mimic the in vivo hydrolytic environment we confirmed the formation of nanodisperse fine ferrihydrite-like particles. Synthetic analogues of these (~ 10 nm hydrodynamic diameter) were readily adherent to the cell membrane of differentiated Caco-2 cells and internalization was visualized using transmission electron microscopy. Moreover, Caco-2 exposure to these nanoparticles led to ferritin formation (i.e., iron utilization) by the cells, which, unlike for soluble forms of iron, was reduced (p=0.02) by inhibition of clathrin-mediated endocytosis. Simulated lysosomal digestion indicated that the nanoparticles are readily dissolved under mildly acidic conditions with the lysosomal ligand, citrate. This was confirmed in cell culture as monensin inhibited Caco-2 utilization of iron from this source in a dose dependent fashion (p<0.05) whilet soluble iron was again unaffected. Our findings reveal the possibility of an endocytic pathway for acquisition of dietary Fe(III) by the small intestinal epithelium, which would complement the established DMT-1 pathway for soluble Fe(II).

Microsporidia and 'the art of living together'

Parasitism, aptly defined as one of the 'living-together' strategies (Trager, 1986), presents a dynamic system in which the parasite and its host are under evolutionary pressure to evolve new and specific adaptations, thus enabling the coexistence of the two closely interacting partners. Microsporidia are very frequently encountered obligatory intracellular protistan parasites that can infect both animals and some protists and are a consummate example of various aspects of the 'living-together' strategy. Microsporidia, relatives of fungi in the superkingdom Opisthokonta, belong to the relatively small group of parasites for which the host cell cytoplasm is the site of both reproduction and maturation. The structural and physiological reduction of their vegetative stage, together with the manipulation of host cell physiology, enables microsporidia to live in the cytosolic environment for most of their life cycle in a way resembling endocytobionts. The ability to form structurally complex spores and the invention and assembly of a unique injection mechanism enable microsporidia to disperse within host tissues and between host organisms, resulting in long-lasting infections. Microsporidia have adapted their genomes to the intracellular way of life, evolved strategies how to obtain nutrients directly from the host and how to manipulate not only the infected cells, but also the hosts themselves. The enormous variability of host organisms and their tissues provide microsporidian parasites a virtually limitless terrain for diversification and ecological expansion. This review attempts to present a general overview of microsporidia, emphasising some less known and/or more recently discovered facets of their biology.

Invasion and intracellular survival by protozoan parasites

Intracellular parasitism has arisen only a few times during the long ancestry of protozoan parasites including in diverse groups such as microsporidians, kinetoplastids, and apicomplexans. Strategies used to gain entry differ widely from injection (e.g. microsporidians), active penetration of the host cell (e.g. Toxoplasma), recruitment of lysosomes to a plasma membrane wound (e.g. Trypanosoma cruzi), to host cell-mediated phagocytosis (e.g. Leishmania). The resulting range of intracellular niches is equally diverse ranging from cytosolic (e.g. T. cruzi) to residing within a non-fusigenic vacuole (e.g. Toxoplasma, Encephalitozoon) or a modified phagolysosome (e.g. Leishmania). These lifestyle choices influence access to nutrients, interaction with host cell signaling pathways, and detection by pathogen recognition systems. As such, intracellular life requires a repertoire of adaptations to assure entry-exit from the cell, as well as to thwart innate immune mechanisms and prevent clearance. Elucidating these pathways at the cellular and molecular level may identify key steps that can be targeted to reduce parasite survival or augment immunologic responses and thereby prevent disease.

Germination of phagocytosed E. cuniculi spores does not significantly contribute to parasitophorous vacuole formation in J774 cells

The obligate intracellular microsporidia have developed a unique invasion mechanism to infect their host cells. Spores explosively evert a tube-like structure and extrude the infectious spore content through this organelle into the host cell. Spores from species of the genus Encephalitozoon were also shown to be efficiently internalized by phagocytosis, which led to the hypothesis that spore germination from inside a phagosome might contribute to the infection process. Here, we challenge this hypothesis by quantifying Encephalitozoon cuniculi infection rates of J774 cells that were incubated with the phagocytosis inhibitor cytochalasin D. We demonstrate that the invasion rate in cytochalasin D-treated cells is identical to untreated controls, although phagocytic uptake of E. cuniculi spores was less than 10% of control samples. This study suggests that germination of phagocytosed spores is not a significant infection mode for E. cuniculi.

Structural and functional characterization of a putative polysaccharide deacetylase of the human parasite Encephalitozoon cuniculi

The microsporidian Encephalitozoon cuniculi is an intracellular eukaryotic parasite considered to be an emerging opportunistic human pathogen. The infectious stage of this parasite is a unicellular spore that is surrounded by a chitin containing endospore layer and an external proteinaceous exospore. A putative chitin deacetylase (ECU11_0510) localizes to the interface between the plasma membrane and the endospore. Chitin deacetylases are family 4 carbohydrate esterases in the CAZY classification, and several bacterial members of this family are involved in evading lysis by host glycosidases, through partial de-N-acetylation of cell wall peptidoglycan. Similarly, ECU11_0510 could be important for E. cuniculi survival in the host, by protecting the chitin layer from hydrolysis by human chitinases. Here, we describe the biochemical, structural, and glycan binding properties of the protein. Enzymatic analyses showed that the putative deacetylase is unable to deacetylate chitooligosaccharides or crystalline beta-chitin. Furthermore, carbohydrate microarray analysis revealed that the protein bound neither chitooligosaccharides nor any of a wide range of other glycans or chitin. The high resolution crystal structure revealed dramatic rearrangements in the positions of catalytic and substrate binding residues, which explain the loss of deacetylase activity, adding to the unusual structural plasticity observed in other members of this esterase family. Thus, it appears that the ECU11_0510 protein is not a carbohydrate deacetylase and may fulfill an as yet undiscovered role in the E. cuniculi parasite.

Innate immune responses to Encephalitozoon species infections

Microsporidia are obligate intracellular, eukaryotic fungi, which have gained recognition as opportunistic parasites in immunocompromised patients. Resistance to lethal microsporidia infections requires a Th1 immune response; how this protection is initiated against Encephalitozoon species is the focus of this review article.

A role for antimicrobial peptides in intestinal microsporidiosis

SUMMARY

Clinical isolates from 3 microsporidia species, Encephalitozoon intestinalis and Encephalitozoon hellem, and the insect parasite Anncaliia (Brachiola, Nosema) algerae, were used in spore germination and enterocyte-like (C2Bbe1) cell infection assays to determine the effect of a panel of antimicrobial peptides. Spores were incubated with lactoferrin (Lf), lysozyme (Lz), and human beta defensin 2 (HBD2), human alpha defensin 5 (HD5), and human alpha defensin 1 (HNP1), alone and in combination with Lz, prior to germination. Of the Encephalitozoon species only E. hellem spore germination was inhibited by HNP1, while A. algerae spore germination was inhibited by Lf, HBD2, HD5 and HNP1, although HBD2 and HD5 inhibition required the presence of Lz. The effects of HBD2 and HD5 on microsporidia enterocyte infection paralleled their effects on spore germination. Lysozyme alone only inhibited infection with A. algerae, while Lf inhibited infection by E. intestinalis and A. algerae. HNP1 significantly reduced enterocyte infection by all 3 parasite species and a combination of Lf, Lz and HNP1 caused a further reduced infection with A. algerae. These data suggest that intestinal antimicrobial peptides contribute to the defence of the intestine against infection by luminal microsporidia spores and may partially determine which parasite species infects the intestine.

Effects of host temperature and gastric and duodenal environments on microsporidia spore germination and infectivity of intestinal epithelial cells

Approximately 14 of the more than 1,000 species of microsporidia infect humans, only two of which, Enterocytozoon bieneusi and Encephalitozoon intestinalis, cause intestinal microsporidiosis. Clinical isolates of three microsporidia species, E. intestinalis, Encephalitozoon hellem, and the insect parasite, Anncaliia (Brachiola, Nosema) algerae were used in a spore germination assay, and enterocyte attachment and infection assays were performed to model the potential roles of gastric and duodenal environments and host temperature in determining why only one of these microsporidia species causes intestinal microsporidiosis. Enterocyte infection with A. algerae spores was 10% that of the Encephalitozoon species, a difference not attributable to differences in spore attachment to host cells. Prior spore treatment with pepsin in HCl, pancreatic enzymes, or ox bile did not inhibit germination or enterocyte infection by the three microsporidia species. While the Encephalitozoon species differentiated to mature spores within 3 days, the time taken for many enterocytes to turn over, A. algerae took 3-5 days to produce mature spores, near the upper limit for enterocyte turnover in vivo. Thus, host temperature may contribute to A. algerae not causing human intestinal microsporidiosis, but none of the factors tested account for the inability of E. hellem to cause such an infection.

A proteomic-based approach for the characterization of some major structural proteins involved in host–parasite relationships from the silkworm parasiteNosema bombycis (Microsporidia)

Nosema bombycis is the causative agent of the silkworm Bombyx mori pebrine disease which inflicts severe worldwide economical losses in sericulture. Little is known about host-parasite interactions at the molecular level for this spore-forming obligate intracellular parasite which belongs to the fungi-related Microsporidia phylum. Major microsporidian structural proteins from the spore wall (SW) and the polar tube (PT) are known to be involved in host invasion. We developed a proteomic-based approach to identify few N. bombycis proteins belonging to these cell structures. Protein extraction protocols were optimized and four N. bombycis spore protein extracts were compared by SDS-PAGE and 2-DE to establish complementary proteomic profiles. Three proteins were shown to be located at the parasite SW. Moreover, 17 polyclonal antibodies were raised against major N. bombycis proteins from all extracts, and three spots were shown to correspond to polar tube proteins (PTPs) by immunofluorescent assay and transmission electron microscopy immunocytochemistry on cryosections. Specific patterns for each PTP were obtained by MALDI-TOF-MS and MS/MS. Peptide sequence tags were deduced by de novo sequencing using Peaks Online and DeNovoX, then evaluated by MASCOT and SEQUEST searches. Identification parameters were higher than false-positive hits, strengthening our strategy that could be enlarged to a nongenomic context.

Augmentation of microsporidia adherence and host cell infection by divalent cations

The infection process of intracellular opportunistic microsporidia involves the forcible eversion of a coiled hollow polar filament that pierces the host cell membrane, allowing the passage of infectious sporoplasm into the host cell cytoplasm. Although the exact mechanism of spore activation leading to polar filament discharge is unknown, we have shown that spore adherence to host cells, which is mediated by sulfated glycosaminoglycans, may play a vital role. When adherence is inhibited, host cell infection decreases, indicating a direct link between adherence and infection. The goal of this study was to evaluate the effects of exogenous divalent cations on microsporidia spore adherence and infection. Data generated using an in vitro spore adherence assay show that spore adherence is augmented by manganese (Mn2+) and magnesium (Mg2+), but not by calcium (Ca2+). However, each of the three divalent cations contributed to increased host cell infection when included in the assay. Finally, we show that Mn2+ and Mg2+ may activate a constituent on the microsporidia spore, not on the host cell, leading to higher infection efficiency. This report further supports recent evidence that spore adherence to the host cell surface is an important aspect of the microsporidial infection process.