Cryptosporidium parvum induces host cell actin accumulation at the host-parasite interface

Cryptosporidium and cryptosporidiosis

Cryptosporidium is one of the most common enteric protozoan parasites of vertebrates with a wide host range that includes humans and domestic animals. It is a significant cause of diarrhoeal disease and an ubiquitous contaminant of water which serves as an excellent vehicle for transmission. A better understanding of the development and life cycle of Cryptosporidium, and new insights into its phylogenetic relationships, have illustrated the need to re-evaluate many aspects of the biology of Cryptosporidium. This has been reinforced by information obtained from the recent successful Cryptosporidium genome sequencing project, which has emphasised the uniqueness of this organism in terms of its parasite life style and evolutionary biology. This chapter provides an up to date review of the biology, biochemistry and host parasite relationships of Cryptosporidium.

Morphological characterization of Cryptosporidium parvum life-cycle stages in an in vitro model system

Cryptosporidium parvum is a zoonotic protozoan parasite that mainly affects the ileum of humans and livestock, with the potential to cause severe enteric disease. We describe the complete life cycle of C. parvum in an in vitro system. Infected cultures of the human ileocecal epithelial cell line (HCT-8) were observed over time using electron microscopy. Additional data are presented on the morphology, development and behavioural characteristics of the different life-cycle stages as well as determining their time of occurrence after inoculation. Numerous stages of C. parvum and their behaviour have been visualized and morphologically characterized for the first time using scanning electron microscopy. Further, parasite-host interactions and the effect of C. parvum on host cells were also visualized. An improved understanding of the parasite's biology, proliferation and interactions with host cells will aid in the development of treatments for the disease.

Role of the parasite and host cytoskeleton in apicomplexa parasitism

The phylum Apicomplexa includes a large and diverse group of obligate intracellular parasites that rely on actomyosin-based motility to migrate, enter host cells, and egress from infected cells. To ensure their intracellular survival and replication, the apicomplexans have evolved sophisticated strategies for subversion of the host cytoskeleton. Given the properties in common between the host and parasite cytoskeleton, dissecting their individual contribution to the establishment of parasitic infection has been challenging. Nevertheless, recent studies have provided new insights into the mechanisms by which parasites subvert the dynamic properties of host actin and tubulin to promote their entry, development, and egress.

Ultrastructural localization of Cryptosporidium parvum antigen using human patients sera

The antigen location of Cryptosporidium parvum, which stimulates antibody formation in humans and animals, was investigated using infected human sera. Immuno-electron microscopy revealed that antigenicity-inducing humoral immunity was located at various developmental stages of parasites, including asexual, sexual stages, and oocysts. The amount of antigen-stimulating IgG antibodies was particularly high on the oocyst wall. The sporozoite surface was shown to give stimulation on IgG and IgM antibody formation. Trophozoites implicated the lowest antigenicity to humoral immunity, both IgG and IgM, by showing the least amount of gold labeling. Immunogold labeling also provided clues that antigens were presented to the host-cell cytoplasm via feeder organelles and host-parasite junctions.

Biphasic modulation of apoptotic pathways in Cryptosporidium parvum-infected human intestinal epithelial cells

The impact of Cryptosporidium parvum infection on host cell gene expression was investigated by microarray analysis with an in vitro model using human ileocecal HCT-8 adenocarcinoma cells. We found changes in 333 (2.6%) transcripts at at least two of the five (6, 12, 24, 48, and 72 h) postinfection time points. Fifty-one of the regulated genes were associated with apoptosis and were grouped into five clusters based on their expression patterns. Early in infection (6 and 12 h), genes with antiapoptotic roles were upregulated and genes with apoptotic roles were downregulated. Later in infection (24, 48, and 72 h), proapoptotic genes were induced and antiapoptotic genes were downregulated, suggesting a biphasic regulation of apoptosis: antiapoptotic state early and moderately proapoptotic state late in infection. This transcriptional profile matched the actual occurrence of apoptosis in the infected cultures. Apoptosis was first detected at 12 h postinfection and increased to a plateau at 24 h, when 20% of infected cells showed nuclear condensation. In contrast, experimental silencing of Bcl-2 induced apoptosis in 50% of infected cells at 12 h postinfection. This resulted in a decrease in the infection rate and a reduction in the accumulation of meront-containing cells. To test the significance of the moderately proapoptotic state late in the infection, we inhibited apoptosis using pancaspase inhibitor Z-VAD-FMK. This treatment also affected the progression of C. parvum infection, as reinfection, normally seen late (24 h to 48 h), did not occur and accumulation of mature meronts was impaired. Control of host apoptosis is complex and crucial to the life of C. parvum. Apoptosis control has at least two components, early inhibition and late moderate promotion. For a successful infection, both aspects appear to be required.

Hijacking of host cellular functions by the Apicomplexa

Intracellular pathogens such as viruses and bacteria subvert all the major cellular functions of their hosts. Targeted host processes include protein synthesis, membrane trafficking, modulation of gene expression, antigen presentation, and apoptosis. In recent years, it has become evident that protozoan pathogens, including members of the phylum Apicomplexa, also hijack their host cell's functions to access nutrients and to escape cellular defenses and immune responses. These obligate intracellular parasites provide superb illustrations of the subversion of host cell processes such as the recruitment and reorganization of host cell compartments without fusion around the parasitophorous vacuole of Toxoplasma gondii; the export of Plasmodium falciparum proteins on the surface of infected erythrocytes; and the induced transformation of the lymphocytes infected by Theileria parva, which leads to clonal extension.

Evidence for intestinal heterogenic expression of di-tripeptides transporter PepT1 during experimental cryptosporidiosis in neonatal rats

Cryptosporidium parvum is a protozoan parasite that causes intestinal malabsorptive syndrome and malnutrition. Considering the importance of di-tripeptide absorption for nutritional status, we previously investigated the regulation of PepT1 transporter in the suckling rat model of acute cryptosporidiosis and showed that PepT1 protein expression and activity were not modified in the parasitized intestine. Here we used confocal microscopy performed on intestinal villi to determine the subcellular localization of PepT1 together with f-actin and parasites. For this purpose, confocal microscopy using vibratome thick sections was developed on the distal small intestine, the preferential site of parasite implantation. Results showed major heterogeneity of apical PepT1 expression among enterocytes, which did not correlate with actin staining or parasite implantation. These results underscore the importance of considering the effect of C. parvum at the cellular scale and not only in the entire epithelium.

Active invasion and/or encapsulation? A reappraisal of host-cell parasitism by Cryptosporidium

Host-cell invasion by Cryptosporidium is a complex process that requires many different factors derived from both the parasite and the host cell. However, the exact natures of the processes have yet to be resolved. Here, research on different components of the invasion process is put in context, and the sequence of events and pathways associated with the establishment of Cryptosporidium in its unique niche is clarified. In addition, initial parasite-host contact, host-cell invasion and host-cell responses are described. The roles of parasite and host-cell-derived components in the invasion process are examined, as is the question of whether Cryptosporidium actively invades cells and to what extent host-cell responses are involved.

Host cell actin remodeling in response to Cryptosporidium

Cryptosporidium exhibits a complex strategy to invade and establish productive infection sites, involving complimentary parasite and host cell processes. While the work regarding host cell actin remodeling has greatly enhanced our understanding of the molecular pathways involved in the parasite induced actin reorganization, the specific function of host cell actin remodeling is still equivocal. We contend that host cell actin polymerization contributes to the development of productive C. parvum infection sites by generating membrane protrusion events, which may assist in the retention of the parasite at the apical surface within the unique extracytoplasmic niche. With our current understanding of the molecular pathways initiating actin remodeling upon C. parvum interactions with host cells, the next logical step is to determine the upstream events resulting in PI3K activation and the specific role of actin remodeling in parasite development, a process that may have implications beyond host-pathogen interactions.

Focal accumulation of defences at sites of fungal pathogen attack

Plants resist attack by haustorium-forming biotrophic and hemi-biotrophic fungi through fortification of the cell wall to prevent penetration through the wall and the subsequent establishment of haustorial feeding structures by the fungus. While the existence of cell wall-based defences has been known for many years, only recently have the molecular components contributing to such defences been identified. Forward genetic screens identified Arabidopsis mutants impaired in penetration resistance to powdery mildew fungi that were normally halted at the cell wall. Several loci contributing to penetration resistance have been identified and a common feature is the striking focal accumulation of proteins associated with penetration resistance at sites of interaction with fungal appressoria and penetration pegs. The focal accumulation of defence-related proteins and the deposition of cell wall reinforcements at sites of attempted fungal penetration represent an example of cell polarization and raise many questions of relevance, not only to plant pathology but also to general cell biology.

Quantitative tracking of Cryptosporidium infection in cell culture with CFSE

Immunofluorescence-based assays have been developed to detect and quantitate Cryptosporidium parvum infection in cell culture. Here, we describe a method that tracks and quantifies the early phase of attachment and invasion of C. parvum sporozoites using a fluorescent dye. Newly excysted sporozoites were labeled with the amine-reactive fluorescein probe carboxyfluorescein diacetate succinimidyl esters (CFSE) using an optimized protocol. The initial invasion of cells by labeled parasites was detected with fluorescent or confocal microscopy. The infection of cells was quantified by flow cytometry. Comparative analysis of infection of cells with CFSE-labeled and unlabeled sporozoites showed that the infectivity of C. parvum was not affected by CFSE labeling. Quantitative analysis showed that C. parvum Iowa and MD isolates were considerably more invasive than Cryptosporidium hominis isolate TU502. Unlike immunofluorescent assays, CFSE labeling permitted the tracking of the initial invasion of C. parvum. Such an assay may be useful for studying the dynamics of host cell-parasite interaction and possibly for drug screening.

Cryptosporidium parvum infects human cholangiocytes via sphingolipid-enriched membrane microdomains

Cryptosporidium parvum attaches to intestinal and biliary epithelial cells via specific molecules on host-cell surface membranes including Gal/GalNAc-associated glycoproteins. Subsequent cellular entry of this parasite depends on host-cell membrane alterations to form a parasitophorous vacuole via activation of phosphatidylinositol 3-kinase (PI-3K)/Cdc42-associated actin remodelling. How C. parvum hijacks these host-cell processes to facilitate its infection of target epithelia is unclear. Using specific probes to known components of sphingolipid-enriched membrane microdomains (SEMs), we detected aggregation of host-cell SEM components at infection sites during C. parvum infection of cultured human biliary epithelial cells (i.e. cholangiocytes). Activation and membrane translocation of acid-sphingomyelinase (ASM), an enzyme involved in SEM membrane aggregation, were also observed in infected cells. Pharmacological disruption of SEMs and knockdown of ASM via a specific small interfering RNA (siRNA) significantly decreased C. parvum attachment (by approximately 84%) and cellular invasion (by approximately 88%). Importantly, knockdown of ASM and disruption of SEMs significantly blocked C. parvum-induced accumulation of Gal/GalNAc-associated glycoproteins at infection sites by approximately 90%. Disruption of SEMs and knockdown of ASM also significantly blocked C. parvum-induced activation of host-cell PI-3K and subsequent accumulation of Cdc42 and actin by up to 75%. Our results suggest an important role of SEMs for C. parvum attachment to and entry of host cells, likely via clustering of membrane-binding molecules and facilitating of C. parvum-induced actin remodelling at infection sites through activation of the PI-3K/Cdc42 signalling pathway.

Influence of colostrum deprivation and concurrent Cryptosporidium parvum infection on the colonization and persistence of Escherichia coli O157 : H7 in young lambs

Escherichia coliO157 : H7 andCryptosporidium parvuminfections of man have been associated with direct contact with small ruminants. Colostrum protects neonates against gastrointestinal pathogens, and orphan lambs, which are common on petting farms, may be deprived of this protection. In a recent study, it was demonstrated that high shedding ofE. coliO157 : H7 by an 8-week-old goat kid was associated with coincidentalC. parvuminfection. Furthermore, both pathogens were co-located in the distal gastrointestinal tract. It was hypothesized that colostrum deprivation and pre-infection withC. parvumpredisposed young ruminants to colonization and increased shedding ofE. coliO157 : H7. To test this, 21 lambs 5 weeks of age were divided into four groups as follows: (A) colostrum-deprived and inoculated withE. coliO157 : H7, (B) colostrum-deprived and inoculated withC. parvumand thenE. coliO157 : H7, (C) conventionally reared and inoculated withE. coliO157 : H7, (D) conventionally reared and inoculated withC. parvumand thenE. coliO157 : H7.C. parvumwas detected between 8 and 12 days post-inoculation in most of the infected lambs. At 24 h post-inoculation withE. coliO157 : H7, all lambs were shedding between 5×104and 5×107 c.f.u.E. coliO157 : H7 per gram of faeces.E. coliO157 : H7 was shed in higher numbers in the groups pre-inoculated withC. parvum, whether conventionally reared or colostrum-deprived. Interestingly, for the colostrum-deprived lambs on day 3, a significant difference in shedding ofE. coliO157 : H7 was observed (P=0.038), with the lambs inoculated withE. colialone yielding higher counts than those pre-inoculated withC. parvum. From day 15 onwards, shedding ofE. coliO157 : H7 was highest from the colostrum-deprivedC. parvum-infected lambs, then (in descending order of shedding) the colostrum-deprived lambs, the conventionally reared lambs infected withC. parvum, and the conventionally reared animals. In total, four animals were euthanized, two at 24 h and two at 96 h post inoculation withE. coliO157 : H7 (two conventionally reared and two colostrum-deprived). All animals euthanized were from groups pre-inoculated withC. parvumprior to challenge withE. coliO157 : H7. On examination of tissues, in three of the four animals examined, multifocal attaching and effacing lesions were observed in the caecum, colon, rectum and at the recto-anal junction, and were confirmed by immunohistochemistry to be associated withE. coliO157 : H7.

Cellular Identity of a Novel Small Subunit rDNA Sequence Clade of Apicomplexans: Description of the Marine Parasite Rhytidocystis polygordiae n. sp. (Host: Polygordius sp., Polychaeta)

A new species of Rhytidocystis (Apicomplexa) is characterized from North American waters of the Atlantic Ocean using electron microscopy and phylogenetic analyses of small subunit (SSU) rDNA sequences. Rhytidocystis polygordiae n. sp. is a parasite of the polychaete Polygordius sp. and becomes the fourth described species within this genus. The trophozoites of R. polygordiae were relatively small oblong cells (L=35-55 microm; W=20-25 microm) and distinctive in possessing subterminal indentations at both ends of the cell. The surface of the trophozoites had six to eight longitudinal series of small transverse folds and several micropores arranged in short linear rows. The trophozoites of R. polygordiae were positioned beneath the brush border of the intestinal epithelium but appeared to reside between the epithelial cells within the extracellular matrix rather than within the cells. The trophozoites possessed a uniform distribution of paraglycogen granules, putative apicoplasts, mitochondria with tubular cristae, and a centrally positioned nucleus. The trophozoites were non-motile and lacked a mucron and an apical complex. Intracellular sporozoites of R. polygordiae had a conoid, a few rhoptries, micronemes, dense granules, and a posteriorly positioned nucleus. Phylogenies inferred from SSU rDNA sequences demonstrated a close relationship between R. polygordiae and the poorly known parasite reported from the hemolymph of the giant clam Tridacna crocea. The rhytidocystid clade diverged early in the apicomplexan radiation and showed a weak affinity to a clade consisting of cryptosporidian parasites, monocystids, and neogregarines.

Quantitative and qualitative comparisons of Cryptosporidium faecal purification procedures for the isolation of oocysts suitable for proteomic analysis

With the recent publication of the Cryptosporidium genome, investigation of the proteins expressed by Cryptosporidium parvum will provide complementary information on the biology of this complex organism. Proteomic studies on this apicomplexan parasite have been hampered due to the inability to culture or isolate high numbers required for 2D gel analysis. Neonatal calves are a common source of Cryptosporidium oocysts and we report on the development of a sucrose-Percoll purification procedure which produced the high yield and purity (free from faecal and bacterial contaminants) that is required for successful proteomic studies from neonatal calves. We report on the development of quantitative and qualitative flow cytometric methods which were confirmed by epifluorescence microscopy. A comparison of five common purification procedures was carried out to determine the efficiency of the sucrose-Percoll gradient. 2D-PAGE results strongly support the sucrose-Percoll procedure as the most suitable method for applications like proteomics which require the recovery of high numbers of isolated oocysts with minimal faecal and bacterial contaminants.

Accumulation of tropomyosin isoform 5 at the infection sites of host cells during Cryptosporidium invasion

The actin cytoskeleton of host cells has been implicated in Cryptosporidium invasion. However, the underlying mechanism of how actin filaments and associated proteins modulate this process remains unclear. In this study, we use in vitro cultured cell lines, human ileocecal adenocarcinoma HCT-8 and Chinese hamster ovary (CHO), and an in vivo mouse model to investigate the roles of tropomyosin isoforms in Cryptosporidium invasion. Using isoform-specific monoclonal antibodies, we found that the major human tropomyosin (hTM) isoforms expressed in HCT-8 cells are hTM4 and hTM5. HCT-8 cells also express hTM1 at low levels but not hTM2 and hTM3. During Cryptosporidium parvum infection, hTM5 colocalized to the infection sites with a novel parasite membrane protein, CP2. Neither hTM1 nor hTM4 accumulated at infection sites. Similarly, a high level of TM5 and varying amounts of TM4 accumulated at the C. parvum infection sites in CHO cells. CHO cells overexpressing hTM5 exhibit a significantly higher percent of mature meronts early in the infection process relative to CHO cells or CHO cells overexpressing a tropomyosin mutant, chimeric isoform hTM5/3. These results suggest that functional TM5 enhances Cryptosporidium invasion of host cells. In C. parvum-infected mice, accumulation and rearrangement of TM5 and TM4 were detected throughout the infected ileum. Similarly, in the Cryptosporidium muris-infected mice, TM5 accumulated in discrete regions of the epithelial cells of gastric glands and in the oocyst-laden stomach gland lumen. Cryptosporidium infection appears to rearrange and recruit host TM isoforms in both culture cells and in the mouse. Localized accumulation of tropomyosin at the infection sites may facilitate parasite invasion.

Interaction of Cryptosporidium hominis and Cryptosporidium parvum with primary human and bovine intestinal cells

Cryptosporidiosis in humans is caused by the zoonotic pathogen Cryptosporidium parvum and the anthroponotic pathogen Cryptosporidium hominis. To what extent the recently recognized C. hominis species differs from C. parvum is unknown. In this study we compared the mechanisms of C. parvum and C. hominis invasion using a primary cell model of infection. Cultured primary bovine and human epithelial intestinal cells were infected with C. parvum or C. hominis. The effects of the carbohydrate lectin galactose-N-acetylgalactosamine (Gal/GalNAc) and inhibitors of cytoskeletal function and signal transduction mechanisms on entry of the parasites into host cells were tested. HCT-8 cells (human ileocecal adenocarcinoma cells) were used for the purpose of comparison. Pretreatment of parasites with Gal/GalNAc inhibited entry of C. parvum into HCT-8 cells and primary bovine cells but had no effect on entry of either C. parvum or C. hominis into primary human cells or on entry of C. hominis into HCT-8 cells. Both Cryptosporidium species entered primary cells by a protein kinase C (PKC)- and actin-dependent mechanism. Staurosporine, in particular, attenuated infection, likely through a combination of PKC inhibition and induction of apoptosis. Diversity in the mechanisms used by Cryptosporidium species to infect cells of different origins has important implications for understanding the relevance of in vitro studies of Cryptosporidium pathogenesis.

Multiple TLRs are expressed in human cholangiocytes and mediate host epithelial defense responses to Cryptosporidium parvum via activation of NF-kappaB

Infection of epithelial cells by Cryptosporidium parvum triggers a variety of host-cell innate and adaptive immune responses including release of cytokines/chemokines and up-regulation of antimicrobial peptides. The mechanisms that trigger these host-cell responses are unclear. Thus, we evaluated the role of TLRs in host-cell responses during C. parvum infection of cultured human biliary epithelia (i.e., cholangiocytes). We found that normal human cholangiocytes express all known TLRs. C. parvum infection of cultured cholangiocytes induces the selective recruitment of TLR2 and TLR4 to the infection sites. Activation of several downstream effectors of TLRs including IL-1R-associated kinase, p-38, and NF-kappaB was detected in infected cells. Transfection of cholangiocytes with dominant-negative mutants of TLR2 and TLR4, as well as the adaptor molecule myeloid differentiation protein 88 (MyD88), inhibited C. parvum-induced activation of IL-1R-associated kinase, p-38, and NF-kappaB. Short-interfering RNA to TLR2, TLR4, and MyD88 also blocked C. parvum-induced NF-kappaB activation. Moreover, C. parvum selectively up-regulated human beta-defensin-2 in directly infected cells, and inhibition of TLR2 and TLR4 signals or NF-kappaB activation were each associated with a reduction of C. parvum-induced human beta-defensin-2 expression. A significantly higher number of parasites were detected in cells transfected with a MyD88 dominant-negative mutant than in the control cells at 48-96 h after initial exposure to parasites, suggesting MyD88-deficient cells were more susceptible to infection. These findings demonstrate that cholangiocytes express a variety of TLRs, and suggest that TLR2 and TLR4 mediate cholangiocyte defense responses to C. parvum via activation of NF-kappaB.

Gliding motility leads to active cellular invasion by Cryptosporidium parvum sporozoites

We examined gliding motility and cell invasion by an early-branching apicomplexan, Cryptosporidium parvum, which causes diarrheal disease in humans and animals. Real-time video microscopy demonstrated that C. parvum sporozoites undergo circular and helical gliding, two of the three stereotypical movements exhibited by Toxoplasma gondii tachyzoites. C. parvum sporozoites moved more rapidly than T. gondii sporozoites, which showed the same rates of motility as tachyzoites. Motility by C. parvum sporozoites was prevented by latrunculin B and cytochalasin D, drugs that depolymerize the parasite actin cytoskeleton, and by the myosin inhibitor 2,3-butanedione monoxime. Imaging of the initial events in cell entry by Cryptosporidium revealed that invasion occurs rapidly; however, the parasite does not enter deep into the cytosol but rather remains at the cell surface in a membrane-bound compartment. Invasion did not stimulate rearrangement of the host cell cytoskeleton and was inhibited by cytochalasin D, even in host cells that were resistant to the drug. Our studies demonstrate that C. parvum relies on a conserved actin-myosin motor for motility and active penetration of its host cell, thus establishing that this is a widely conserved feature of the Apicomplexa.

DISTRIBUTION OF CRYPTOSPORIDIUM PARVUM SPOROZOITE APICAL ORGANELLES DURING ATTACHMENT TO AND INTERNALIZATION BY CULTURED BILIARY EPITHELIAL CELLS

Although accumulating evidence supports an active role for host cells during Cryptosporidium parvum invasion of epithelia, our knowledge of the underlying parasite-specific processes triggering such events is limited. In an effort to better understand the invasion strategy of C. parvum, we characterized the presence and distribution of the apical organelles (micronemes, dense granules, and rhoptry) through the stages of attachment to, and internalization by, human biliary epithelia, using serial-section electron microscopy. Novel findings include an apparent organized rearrangement of micronemes upon host cell attachment. The apically segregated micronemes were apposed to a central microtubule-like filamentous structure, and the more distal micronemes localized to the periphery and apical region of the parasite during internalization, coinciding with the formation of the anterior vacuole. The morphological observations presented here extend our understanding of parasite-specific processes that occur during attachment to, and internalization by, host epithelial cells.

ELECTRON MICROSCOPIC OBSERVATION OF THE INVASION PROCESS OF CRYPTOSPORIDIUM PARVUM IN SEVERE COMBINED IMMUNODEFICIENCY MICE

Cryptosporidium parvum mainly invades the intestinal epithelium and causes watery diarrhea in humans and calves. However, the invasion process has not yet been clarified. In the present study, the invasion process of C. parvum in severe combined immunodeficiency (SCID) mice was examined. Infected mice were necropsied; the ilea were double-fixed routinely and observed by scanning and transmission electron microscopy. In addition, the microvillus membrane was observed by ruthenium red staining. Scanning electron micrographs showed elongation of the microvilli at the periphery of the parasite. The microvilli were shown to be along the surface of the parasite in higher magnification. Transmission electron microscopy confirmed that the invading parasites were located among microvilli. Parasites existed in the parasitophorous vacuole formed by the microvillus membrane. The parasite pellicle attached to the host cell membrane at the bottom of the parasite, and then the pellicle and host cell membrane became unclear. Subsequently, the pellicle became complicated and formed a feeder organelle. In addition, invasion of the parasite was not observed in either a microvillus or the cytoplasm of the host cell. Therefore, C. parvum invades among microvilli, is covered with membranes derived from numerous microvilli, and develops within the host cell.

Cryptosporidium excystation and invasion: getting to the guts of the matter

Cryptosporidium parvum excystation and host cell invasion have been characterized in some detail ultrastructurally. However, until recently, the biochemical and molecular basis of host-parasite interactions and parasite- and host-specific molecules involved in excystation, motility and host cell invasion have been poorly understood. This article describes our understanding of Cryptosporidium excystation and the events leading to host cell invasion, and draws from information available about these processes in other apicomplexans. Many questions remain but, once the specific mechanisms are identified, they could prove to be novel targets for drug delivery.

Trypanosoma cruzi infection disrupts vinculin costameres in cardiomyocytes

Chagas' disease cardiomyopathy is an important manifestation of Trypanosoma cruzi infection, leading to cardiac dysfunction and serious arrhythmias. We have here investigated by indirect immunofluorescence assay the distribution of vinculin, a focal adhesion protein with a major role in the transmission of contraction force, during the T. cruzi-cardiomyocyte infection in vitro and in vivo. No change in vinculin distribution was observed after 24 h of infection, where control and T. cruzi-infected cardiomyocytes displayed vinculin localized at costameres and intercalated discs. On the other hand, a clear disruption of vinculin costameric distribution was noted after 72 h of infection. A significant reduction in the levels of vinculin expression was observed at all times of infection. In murine experimental Chagas' disease, alteration in the vinculin distribution was also detected in the infected myocardium, with no costameric staining in infected myocytes and irregular alignment of intercalated discs in cardiac fibers. These data suggest that the disruption of costameric vinculin distribution and the enlargement of interstitial space due to inflammatory infiltration may contribute to the reduction of transmission of cardiac contraction force, leading to alterations in the heart function in Chagas' disease.

Localized glucose and water influx facilitates Cryptosporidium parvum cellular invasion by means of modulation of host-cell membrane protrusion

Dynamic membrane protrusions such as lamellipodia and filopodia are driven by actin polymerization and often hijacked by intracellular microbes to enter host cells. The overall rate of membrane protrusion depends on the actin polymerization rate and the increase of localized cell volume. Although the signaling pathways involving actin polymerization are well characterized, the molecular mechanisms regulating local cell volume associated with membrane protrusion are unclear. Cryptosporidium parvum, an intracellular parasite, depends on host-cell membrane protrusion to accomplish cell entry and form the parasitophorous vacuole. Here, we report that C. parvum infection of cholangiocytes recruits host-cell SGLT1, a Na+/glucose cotransporter, and aquaporin 1 (AQP1), a water channel, to the attachment site. SGLT1-dependent glucose uptake occurs at the attachment site. Concordantly, the region of attachment displays localized water influx that is inhibited by either suppression of AQP1 by means of AQP1-small interfering RNA (siRNA) or inhibition of SGLT1 by a specific pharmacologic inhibitor, phlorizin. Inhibition of SGLT1 does not affect actin accumulation but decreases the membrane protrusion at the attachment site. Moreover, functional inhibition of host-cell AQP1 and SGLT1 hampers C. parvum invasion of cholangiocytes. Thus, glucose-driven, AQP-mediated localized water influx is involved in the membrane protrusion during C. parvum cellular invasion, phenomena that may also be relevant to the mechanisms of cell membrane protrusion in general.

Survival of protozoan intracellular parasites in host cells

The most common human diseases are caused by pathogens. Several of these microorganisms have developed efficient ways in which to exploit host molecules, along with molecular pathways to ensure their survival, differentiation and replication in host cells. Although the contribution of the host cell to the development of many intracellular pathogens (particularly viruses and bacteria) has been unequivocally established, the study of host-cell requirements during the life cycle of protozoan parasites is still in its infancy. In this review, we aim to provide some insight into the manipulation of the host cell by parasites through discussing the hurdles that are faced by the latter during infection.

Requirement for the Rac GTPase in Chlamydia trachomatis invasion of non-phagocytic cells

Chlamydiae are gram-negative obligate intracellular pathogens to which access to an intracellular environment is paramount to their survival and replication. To this end, chlamydiae have evolved extremely efficient means of invading nonphagocytic cells. To elucidate the host cell machinery utilized by Chlamydia trachomatis in invasion, we examined the roles of the Rho GTPase family members in the internalization of chlamydial elementary bodies. Upon binding of elementary bodies on the cell surface, actin is rapidly recruited to the sites of internalization. Members of the Rho GTPase family are frequently involved in localized recruitment of actin. Clostridial Toxin B, which is a known enzymatic inhibitor of Rac, Cdc42 and Rho GTPases, significantly reduced chlamydial invasion of HeLa cells. Expression of dominant negative constructs in HeLa cells revealed that chlamydial uptake was dependent on Rac, but not on Cdc42 or RhoA. Rac but not Cdc42 was found to be activated by chlamydial attachment. The effect of dominant negative Rac expression on chlamydial uptake is manifested through the inhibition of actin recruitment to the sites of chlamydial entry. Studies utilizing Green Fluorescent Protein fusion constructs of Rac, Cdc42 and RhoA, showed Rac to be the sole member of the Rho GTPase family recruited to the site of chlamydial entry.

Host cell tropism underlies species restriction of human and bovine Cryptosporidium parvum genotypes

It has been recognized recently that human cryptosporidiosis is usually caused by Cryptosporidium parvum genotype I ("human" C. parvum), which is not found in animals. Compared to C. parvum genotype II, little is known of the biology of invasion of the human-restricted C. parvum genotype I. The aims of the present study were (i) to explore and compare with genotype II the pathogenesis of C. parvum genotype I infection by using an established in vitro model of infection and (ii) to examine the possibility that host-specific cell tropism determines species restriction among C. parvum genotypes by using a novel ex vivo small intestinal primary cell model of infection. Oocysts of C. parvum genotypes I and II were used to infect HCT-8 cells and primary intestinal epithelial cells in vitro. Primary cells were harvested from human endoscopic small-bowel biopsies and from bovine duodenum postmortem. C. parvum genotype I infected HCT-8 cells with lower efficiency than C. parvum genotype II. Actin colocalization at the host parasite interface and reduction in levels of invasion after treatment with microfilament inhibitors (cytochalasin B and cytochalasin D) were observed for both genotypes. C. parvum genotype II invaded primary intestinal epithelial cells, regardless of the species of origin. In contrast, C. parvum genotype I invaded only human small-bowel cells. The pathogenesis of C. parvum genotype I differs from C. parvum genotype II. C parvum genotype I does not enter primary bovine intestinal cells, suggesting that the species restriction of this genotype is due to host tissue tropism of the infecting isolate.

Phosphatidylinositol 3-Kinase and Frabin Mediate Cryptosporidium parvum Cellular Invasion via Activation of Cdc42

Cryptosporidium parvum invades target epithelia via a mechanism that involves host cell actin reorganization. We previously demonstrated that C. parvum activates the Cdc42/neural Wiskott-Aldrich syndrome protein network in host cells resulting in actin remodeling at the host cell-parasite interface, thus facilitating C. parvum cellular invasion. Here, we tested the role of phosphatidylinositol 3-kinase (PI3K) and frabin, a guanine nucleotide exchange factor specific for Cdc42 in the activation of Cdc42 during C. parvum infection of biliary epithelial cells. We found that C. parvum infection of cultured human biliary epithelial cells induced the accumulation of PI3K at the host cell-parasite interface and resulted in the activation of PI3K in infected cells. Frabin also was recruited to the host cell-parasite interface, a process inhibited by two PI3K inhibitors, wortmannin and LY294002. The cellular expression of either a dominant negative mutant of PI3K (PI3K-Deltap85) or functionally deficient mutants of frabin inhibited C. parvum-induced Cdc42 accumulation at the host cell-parasite interface. Moreover, LY294002 abolished C. parvum-induced Cdc42 activation in infected cells. Inhibition of PI3K by cellular overexpression of PI3K-Deltap85 or by wortmannin or LY294002, as well as inhibition of frabin by various functionally deficient mutants, decreased C. parvum-induced actin accumulation and inhibited C. parvum cellular invasion. In contrast, the overexpression of the p85 subunit of PI3K promoted C. parvum invasion. Our data suggest that an important component of the complex process of C. parvum invasion of target epithelia results from the ability of the organism to trigger host cell PI3K/frabin signaling to activate the Cdc42 pathway, resulting in host cell actin remodeling at the host cell-parasite interface.

Cdc42 and the actin-related protein/neural Wiskott-Aldrich syndrome protein network mediate cellular invasion by Cryptosporidium parvum

Cryptosporidium parvum invasion of epithelial cells involves host cell membrane alterations which require a remodeling of the host cell actin cytoskeleton. In addition, an actin plaque, possibly associated with the dense-band region, forms within the host cytoplasm at the host-parasite interface. Here we show that Cdc42 and RhoA, but not Rac1, members of the Rho family of GTPases, are recruited to the host-parasite interface in an in vitro model of human biliary cryptosporidiosis. Interestingly, activation of Cdc42, but not RhoA, was detected in the infected cells. Neural Wiskott-Aldrich syndrome protein (N-WASP) and p34-Arc, actin-regulating downstream effectors of Cdc42, were also recruited to the host-parasite interface. Whereas cellular expression of a constitutively active mutant of Cdc42 promoted C. parvum invasion, overexpression of a dominant negative mutant of Cdc42, or depletion of Cdc42 mRNA by short interfering RNA-mediated gene silencing, inhibited C. parvum invasion. Expression of the WA fragment of N-WASP to block associated actin polymerization also inhibited C. parvum invasion. Moreover, inhibition of host cell Cdc42 activation by dominant negative mutation inhibited C. parvum-associated actin remodeling, membrane protrusion, and dense-band formation. In contrast, treatment of cells with a Rho inhibitor, exoenzyme C3, or cellular overexpression of dominant negative mutants of RhoA and Rac1 had no effect on C. parvum invasion. These data suggest that C. parvum invasion of target epithelia results from the organism's ability to activate a host cell Cdc42 GTPase signaling pathway to induce host cell actin remodeling at the attachment site.

Cryptosporidium parvum regulation of human epithelial cell gene expression

Cryptosporidium parvum is an obligate intracellular protozoan capable of causing life-threatening diarrhoeal disease in immunocompromised individuals. Efforts to develop novel therapeutic strategies have been hampered by the lack of understanding of the pathogenesis of infection. To better understand the host response to C. parvum infection, gene expression profiles of infected human ileocecal adenocarcinoma cells were analysed by using Affymetrix oligonucleotide microarrays containing probe sets for 12,600 human genes. Statistical analysis of expression data from three independent experiments identified 223 genes whose expression was reproducibly regulated by C. parvum infection at 24 h post-inoculation (125 up-regulated and 98 down-regulated), 13 of which were validated by quantitative reverse transcriptase polymerase chain reaction analysis. This analysis revealed the consistent up-regulation of host heat-shock genes and genes for pro-inflammatory chemokines IL-8, RANTES, and SCYB5. Multiple genes for host actin and tubulin genes were up-regulated whereas genes for actin binding proteins were down-regulated, confirming previous observations of host cytoskeleton rearrangement in response to C. parvum infection. In addition, host genes associated with cell proliferation and apoptosis were differentially regulated, reflecting the complexity of host-parasite interaction. Together, this study demonstrated that C. parvum infection results in significant changes in host biochemical pathways and provides new insights into specific biological processes of infectious disease caused by an intracellular protozoan parasite.

Complete development of Cryptosporidium parvum in rabbit chondrocytes (VELI cells)

Cryptosporidium parvum is an intracellular protozoan parasite that causes severe infection in humans and animals. The great difficulties in treating people and animals suffering from cryptosporidiosis have prompted the development of in vitro experimental models. The aim of this study was to demonstrate that C. parvum can complete its entire life cycle-from sporozoite to infective oocyst-in VELI cells (a line derived from primary culture of rabbit auricular chondrocytes). Successful infections were produced by inoculating cell cultures. Infection of MDCK, HTC-8 and VELI cells with C. parvum closely paralleled in vivo infections with regard to host cell location and chronology of parasite development. Oocysts which were produced in VELI cells were infective for infant NMRI mice. The growth of C. parvum in VELI cells provides a model, both simple and inexpensive, for testing anticryptosporidial drugs and studying host-parasite interactions.

Intracellular Parasite Invasion Strategies

Intracellular parasites use various strategies to invade cells and to subvert cellular signaling pathways and, thus, to gain a foothold against host defenses. Efficient cell entry, ability to exploit intracellular niches, and persistence make these parasites treacherous pathogens. Most intracellular parasites gain entry via host-mediated processes, but apicomplexans use a system of adhesion-based motility called "gliding" to actively penetrate host cells. Actin polymerization-dependent motility facilitates parasite migration across cellular barriers, enables dissemination within tissues, and powers invasion of host cells. Efficient invasion has brought widespread success to this group, which includes Toxoplasma, Plasmodium, and Cryptosporidium.

Evidence for the absence of an intestinal adaptive mechanism to compensate for C. parvum-induced amino acid malabsorption in suckling rats

In order to assess the impact of Cryptosporidium parvum on host intestinal physiology, we investigated absorption of the two principal amino acids in dam's milk (leucine, glutamate), using Ussing chambers and RT-PCR analyses. Experiments were performed in both heavily (ileum) and mildly (duodenum) infected segments of the small intestine at the peak of infection [day 8 post-infection (PI)] and after spontaneous clearance of the parasite (day 17 PI). At day 8 PI, amino acid fluxes across the mucosa were decreased throughout the small intestine (P<0.01) and EAAT3 mRNA expression was reduced ( from -49% to -28%). At day 17 PI, leucine and glutamate fluxes were normalized but the decrease in EAAT3 mRNA levels persisted (from -31% to -46%). Our results demonstrate that cryptosporidiosis induces major amino acid malabsorption involving the entire small intestine which is not counterbalanced by any up-regulation, even after spontaneous clearance of the parasite.

Host cell fate on Cryptosporidium parvum egress from MDCK cells

Cryptosporidium parvum is an intracellular protozoan parasite that causes a severe diarrheal illness of unclear etiology. Also unclear is the fate of the host cell upon parasite egress. We show in an MDCK cell model that the host cell is killed upon parasite egress; this death is necrotic, rather than apoptotic, in nature.

Cryptosporidium parvum invasion of biliary epithelia requires host cell tyrosine phosphorylation of cortactin via c-Src

BACKGROUND & AIMS

Cryptosporidium parvum invasion of epithelia requires polymerization of host cell actin at the attachment site. We analyzed the role of host cell c-Src, a cytoskeleton-associated protein tyrosine kinase, in C. parvum invasion of biliary epithelia.

METHODS

In vitro models of biliary cryptosporidiosis using a human biliary epithelial cell line were used to assay the role of c-Src signaling pathway in C. parvum invasion.

RESULTS

c-Src and cortactin, an actin-binding protein and a substrate for c-Src, were recruited to the parasite-host cell interface during C. parvum invasion. Tyrosine phosphorylation of cortactin in infected cells was also detected. Inhibition of host cell c-Src significantly blocked C. parvum -induced accumulation and tyrosine phosphorylation of cortactin and actin polymerization at the attachment sites, thereby inhibiting C. parvum invasion of biliary epithelial cells. A triple mutation of tyrosine of cortactin in the epithelia also diminished C. parvum invasion. In addition, proteins originating from the parasite were detected within infected cells at the parasite-host cell interface. Antiserum against C. parvum membrane proteins blocked accumulation of c-Src and cortactin and significantly decreased C. parvum invasion. No accumulation of the endocytosis-related proteins, dynamin 2 and clathrin, was found at the parasite-host cell interface; also, inhibition of dynamin 2 did not block C. parvum invasion.

CONCLUSIONS

C. parvum invasion of biliary epithelial cells requires host cell tyrosine phosphorylation of cortactin by a c-Src-mediated signaling pathway to induce actin polymerization at the attachment site, a process associated with microbial secretion but independent of host cell endocytosis.

Chlamydia trachomatis induces remodeling of the actin cytoskeleton during attachment and entry into HeLa cells

To elucidate the host cell machinery utilized by Chlamydia trachomatis to invade epithelial cells, we examined the role of the actin cytoskeleton in the internalization of chlamydial elementary bodies (EBs). Treatment of HeLa cells with cytochalasin D markedly inhibited the internalization of C. trachomatis serovar L2 and D EBs. Association of EBs with HeLa cells induced localized actin polymerization at the site of attachment, as visualized by either phalloidin staining of fixed cells or the active recruitment of GFP-actin in viable infected cells. The recruitment of actin to the specific site of attachment was accompanied by dramatic changes in the morphology of cell surface microvilli. Ultrastructural studies revealed a transient microvillar hypertrophy that was dependent upon C. trachomatis attachment, mediated by structural components on the EBs, and cytochalasin D sensitive. In addition, a mutant CHO cell line that does not support entry of C. trachomatis serovar L2 did not display such microvillar hypertrophy following exposure to L2 EBs, which is in contrast to infection with serovar D, to which it is susceptible. We propose that C. trachomatis entry is facilitated by an active actin remodeling process that is induced by the attachment of this pathogen, resulting in distinct microvillar reorganization throughout the cell surface and the formation of a pedestal-like structure at the immediate site of attachment and entry.

The role of cytokines in the pathogenesis of Cryptosporidium infection

First described in 1912, the importance of the coccidian parasite Cryptosporidium parvum as an enteropathogen in humans was not recognized until the early 1980s, when it was found to be a common opportunistic infection in AIDS. Infection with this organism triggers a complex array of innate and cell-mediated immune responses within the intestinal mucosa. How cytokines and chemokines interact to regulate these responses in order to achieve clearance of the parasite yet preserve the integrity of the intestinal mucosa is still being unravelled. T helper type 1 cytokines, and particularly interferon-gamma, have long been considered to be the main orchestrators of the immune response to this infection, but recent studies suggest that T helper type 2 cytokines may also be involved. In addition, transforming growth factor-beta 1, although having little effect on parasite development, is an important modulator of the immune response and plays a role in protecting the epithelial integrity from the effects of the inflammatory process.

Cytoskeleton of apicomplexan parasites

The Apicomplexa are a phylum of diverse obligate intracellular parasites including Plasmodium spp., the cause of malaria; Toxoplasma gondii and Cryptosporidium parvum, opportunistic pathogens of immunocompromised individuals; and Eimeria spp. and Theileria spp., parasites of considerable agricultural importance. These protozoan parasites share distinctive morphological features, cytoskeletal organization, and modes of replication, motility, and invasion. This review summarizes our current understanding of the cytoskeletal elements, the properties of cytoskeletal proteins, and the role of the cytoskeleton in polarity, motility, invasion, and replication. We discuss the unusual properties of actin and myosin in the Apicomplexa, the highly stereotyped microtubule populations in apicomplexans, and a network of recently discovered novel intermediate filament-like elements in these parasites.

Update on protozoan parasites of the intestine

The last year has seen new approaches to the diagnosis, treatment, and prevention of protozoal infections of the gastrointestinal tract. Some of the news is not good: new foodborne and swimming pool outbreaks of cyclosporiasis and cryptosporidiosis, respectively, occurred in North America; paromomycin was shown to be ineffective treatment for cryptosporidiosis; and these parasitic diseases continued to have a worldwide impact on human health. On the bright side, there were important advances in the understanding of the pathogenesis of cryptosporidiosis and the diagnosis of amebiasis and giardiasis, and some new leads on the treatment of cryptosporidiosis and refractory giardiasis. Finally, evidence was found of acquired mucosal immunity to amebiasis in Bangladeshi children, offering a guide for the development of an amebiasis vaccine. This review is not intended to be comprehensive, but contains a variety of articles that the authors hope will be of interest to the reader.

Cryptosporidium parvum infection requires host cell actin polymerization

The intracellular protozoan parasite Cryptosporidium parvum accumulates host cell actin at the interface between the parasite and the host cell cytoplasm. Here we show that the actin polymerizing proteins Arp2/3, vasodilator-stimulated phosphoprotein (VASP), and neural Wiskott Aldrich syndrome protein (N-WASP) are present at this interface and that host cell actin polymerization is necessary for parasite infection.

Gut feelings: enteropathogenic E. coli (EPEC) interactions with the host

Enteropathogenic Escherichia coli (EPEC) is a gram-negative bacterial pathogen that adheres to human intestinal epithelial cells, resulting in watery, persistent diarrhea. It subverts the host cell cytoskeleton, causing a rearrangement of cytoskeletal components into a characteristic pedestal structure underneath adherent bacteria. In contrast to other intracellular pathogens that affect the actin cytoskeleton from inside the host cytoplasm, EPEC remains extracellular and transmits signals through the host cell plasma membrane via direct injection of virulence factors by a "molecular syringe," the bacterial type III secretion system. One injected factor is Tir, which functions as the plasma membrane receptor for EPEC adherence. Tir directly links extracellular EPEC through the epithelial membrane and firmly anchors it to the host cell actin cytoskeleton, thereby initiating pedestal formation. In addition to stimulating actin nucleation and polymerization in the host cell, EPEC activates several other signaling pathways that lead to tight junction disruption, inhibition of phagocytosis, altered ion secretion, and immune responses. This review summarizes recent developments in our understanding of EPEC pathogenesis and discusses similarities and differences between EPEC pedestals, focal contacts, and Listeria monocytogenes actin tails.