Toxoplasma gondii: redistribution of monoclonal antibodies on tachyzoites during host cell invasion

A circular zone of attachment to the extracellular matrix provides directionality to the motility of Toxoplasma gondii in 3D

Toxoplasma gondii is a protozoan parasite that infects 30-40% of the world's population. Infections are typically subclinical but can be severe and, in some cases, life threatening. Central to the virulence of T. gondii is an unusual form of substrate-dependent motility that enables the parasite to invade cells of its host and to disseminate throughout the body. A hetero-oligomeric complex of proteins that functions in motility has been characterized, but how these proteins work together to drive forward motion of the parasite remains controversial. A key piece of information needed to understand the underlying mechanism(s) is the directionality of the forces that a moving parasite exerts on the external environment. The linear motor model of motility, which has dominated the field for the past two decades, predicts continuous anterior-to-posterior force generation along the length of the parasite. We show here using three-dimensional traction force mapping that the predominant forces exerted by a moving parasite are instead periodic and directed in toward the parasite at a fixed circular location within the extracellular matrix. These highly localized forces, which are generated by the parasite pulling on the matrix, create a visible constriction in the parasite's plasma membrane. We propose that the ring of inward-directed force corresponds to a circumferential attachment zone between the parasite and the matrix, through which the parasite propels itself to move forward. The combined data suggest a closer connection between the mechanisms underlying parasite motility and host cell invasion than previously recognized. In parasites lacking the major surface adhesin, TgMIC2, neither the inward-directed forces nor the constriction of the parasite membrane are observed. The trajectories of the TgMIC2-deficient parasites are less straight than those of wild-type parasites, suggesting that the annular zone of TgMIC2-mediated attachment to the extracellular matrix normally constrains the directional options available to the parasite as it migrates through its surrounding environment.

Nanos gigantium humeris insidentes: old papers informing new research into Toxoplasma gondii

Since Nicolle, Manceaux and Splendore first described Toxoplasma gondii as a parasite of rodents and rabbits in the early 20th century, a diverse and vigorous research community has been built around studying this fascinating intracellular parasite. In addition to its importance as a pathogen of humans, livestock and wildlife, modern researchers are attracted to T. gondii as a facile experimental system to study many aspects of evolutionary biology, cellular biology, host-microbe interactions, and host immunity. For new researchers entering the field, the extensive literature describing the biology of the parasite, and the interactions with its host, can be daunting. In this review, we examine four foundational studies that describe various aspects of T. gondii biology, presenting a 'journal club'-style analysis of each. We have chosen a paper that established the beguiling life cycle of the parasite (Hutchison et al., 1971), a paper that described key features of its cellular biology that the parasite shares with related organisms (Gustafson et al., 1954), a paper that characterised the origin of the unique compartment in which the parasite resides within host cells (Jones and Hirsch, 1972), and a paper that established a key mechanism in the host immune response to parasite infection (Pfefferkorn, 1984). These interesting and far-reaching studies set the stage for subsequent research into numerous facets of parasite biology. As well as providing new researchers with an entry point into the literature surrounding the parasite, revisiting these studies can remind us of the roots of our discipline, how far we have come, and the new directions in which we might head.

Engineering and Functional Evaluation of Neutralizing Antibody Fragments Against Congenital Toxoplasmosis

Maternal-fetal transmission of Toxoplasma gondii tachyzoites acquired during pregnancy has potentially dramatic consequences for the fetus. Current reference-standard treatments are not specific to the parasite and can induce severe side effects. In order to provide treatments with a higher specificity against toxoplasmosis, we developed antibody fragments-single-chain fragment variable (scFv) and scFv fused with mouse immunoglobulin G2a crystallizable fragment (scFv-Fc)-directed against the major surface protein SAG1. After validating their capacity to inhibit T. gondii proliferation in vitro, the antibody fragments' biological activity was assessed in vivo using a congenital toxoplasmosis mouse model. Dams were treated by systemic administration of antibody fragments and with prevention of maternal-fetal transmission being used as the parameter of efficacy. We observed that both antibody fragments prevented T. gondii dissemination and protected neonates, with the scFv-Fc format having better efficacy. These data provide a proof of concept for the use of antibody fragments as effective and specific treatment against congenital toxoplasmosis and provide promising leads.

TRIM21 is critical for survival of Toxoplasma gondii infection and localises to GBP-positive parasite vacuoles

Interferon gamma (IFNγ) is the major proinflammatory cytokine conferring resistance to the intracellular vacuolar pathogen Toxoplasma gondii by inducing the destruction of the parasitophorous vacuole (PV). We previously identified TRIM21 as an IFNγ-driven E3 ubiquitin ligase mediating the deposition of ubiquitin around pathogen inclusions. Here, we show that TRIM21 knockout mice were highly susceptible to Toxoplasma infection, exhibiting decreased levels of serum inflammatory cytokines and higher parasite burden in the peritoneum and brain. We demonstrate that IFNγ drives recruitment of TRIM21 to GBP1-positive Toxoplasma vacuoles, leading to Lys63-linked ubiquitination of the vacuole and restriction of parasite early replication without interfering with vacuolar disruption. As seen in vivo, TRIM21 impacted the secretion of inflammatory cytokines. This study identifies TRIM21 as a previously unknown modulator of Toxoplasma gondii resistance in vivo thereby extending host innate immune recognition of eukaryotic pathogens to include E3 ubiquitin ligases.

Eimeria tenella protein trafficking: differential regulation of secretion versus surface tethering during the life cycle

Eimeria spp. are intracellular parasites that have a major impact on poultry. Effective live vaccines are available and the development of reverse genetic technologies has raised the prospect of using Eimeria spp. as recombinant vectors to express additional immunoprotective antigens. To study the ability of Eimeria to secrete foreign antigens or display them on the surface of the sporozoite, transiently transfected populations of E. tenella expressing the fluorescent protein mCherry, linked to endogenous signal peptide (SP) and glycophosphatidylinositol-anchor (GPI) sequences, were examined. The SP from microneme protein EtMIC2 (SP2) allowed efficient trafficking of mCherry to cytoplasmic vesicles and following the C-terminal addition of a GPI-anchor (from surface antigen EtSAG1) mCherry was expressed on the sporozoite surface. In stable transgenic populations, mCherry fused to SP2 was secreted into the sporocyst cavity of the oocysts and after excystation, secretion was detected in culture supernatants but not into the parasitophorous vacuole after invasion. When the GPI was incorporated, mCherry was observed on the sporozites surface and in the supernatant of invading sporozoites. The proven secretion and surface exposure of mCherry suggests that antigen fusions with SP2 and GPI of EtSAG1 may be promising candidates to examine induction of protective immunity against heterologous pathogens.

Optogenetic monitoring identifies phosphatidylthreonine-regulated calcium homeostasis in Toxoplasma gondii

Toxoplasma gondii is an obligate intracellular parasite, which inflicts acute as well as chronic infections in a wide range of warm-blooded vertebrates. Our recent work has demonstrated the natural occurrence and autonomous synthesis of an exclusive lipid phosphatidylthreonine in T. gondii. Targeted gene disruption of phosphatidylthreonine synthase impairs the parasite virulence due to unforeseen attenuation of the consecutive events of motility, egress and invasion. However, the underlying basis of such an intriguing phenotype in the parasite mutant remains unknown. Using an optogenetic sensor (gene-encoded calcium indicator, GCaMP6s), we show that loss of phosphatidylthreonine depletes calcium stores in intracellular tachyzoites, which leads to dysregulation of calcium release into the cytosol during the egress phase of the mutant. Consistently, the parasite motility and egress phenotypes in the mutant can be entirely restored by ionophore-induced mobilization of calcium. Collectively, our results suggest a novel regulatory function of phosphatidylthreonine in calcium signaling of a prevalent parasitic protist. Moreover, our application of an optogenetic sensor to monitor subcellular calcium in a model intracellular pathogen exemplifies its wider utility to other entwined systems.

Metabolic Cooperation of Glucose and Glutamine Is Essential for the Lytic Cycle of Obligate Intracellular Parasite Toxoplasma gondii

Toxoplasma gondii is a widespread protozoan parasite infecting nearly all warm-blooded organisms. Asexual reproduction of the parasite within its host cells is achieved by consecutive lytic cycles, which necessitates biogenesis of significant energy and biomass. Here we show that glucose and glutamine are the two major physiologically important nutrients used for the synthesis of macromolecules (ATP, nucleic acid, proteins, and lipids) in T. gondii, and either of them is sufficient to ensure the parasite survival. The parasite can counteract genetic ablation of its glucose transporter by increasing the flux of glutamine-derived carbon through the tricarboxylic acid cycle and by concurrently activating gluconeogenesis, which guarantee a continued biogenesis of ATP and biomass for host-cell invasion and parasite replication, respectively. In accord, a pharmacological inhibition of glutaminolysis or oxidative phosphorylation arrests the lytic cycle of the glycolysis-deficient mutant, which is primarily a consequence of impaired invasion due to depletion of ATP. Unexpectedly, however, intracellular parasites continue to proliferate, albeit slower, notwithstanding a simultaneous deprivation of glucose and glutamine. A growth defect in the glycolysis-impaired mutant is caused by a compromised synthesis of lipids, which cannot be counterbalanced by glutamine but can be restored by acetate. Consistently, supplementation of parasite cultures with exogenous acetate can amend the lytic cycle of the glucose transport mutant. Such plasticity in the parasite's carbon flux enables a growth-and-survival trade-off in assorted nutrient milieus, which may underlie the promiscuous survival of T. gondii tachyzoites in diverse host cells. Our results also indicate a convergence of parasite metabolism with cancer cells.

Mitochondrial metabolism of glucose and glutamine is required for intracellular growth of Toxoplasma gondii

Toxoplasma gondii proliferates within host cell vacuoles where the parasite relies on host carbon and nutrients for replication. To assess how T. gondii utilizes these resources, we mapped the carbon metabolism pathways in intracellular and egressed parasite stages. We determined that intracellular T. gondii stages actively catabolize host glucose via a canonical, oxidative tricarboxylic acid (TCA) cycle, a mitochondrial pathway in which organic molecules are broken down to generate energy. These stages also catabolize glutamine via the TCA cycle and an unanticipated γ-aminobutyric acid (GABA) shunt, which generates GABA and additional molecules that enter the TCA cycle. Chemically inhibiting the TCA cycle completely prevents intracellular parasite replication. Parasites lacking the GABA shunt exhibit attenuated growth and are unable to sustain motility under nutrient-limited conditions, suggesting that GABA functions as a short-term energy reserve. Thus, T. gondii tachyzoites have metabolic flexibility that likely allows the parasite to infect diverse cell types.

Optogenetic modulation of an adenylate cyclase in Toxoplasma gondii demonstrates a requirement of the parasite cAMP for host-cell invasion and stage differentiation

BACKGROUND

cAMP research in intracellular parasites remains underappreciated, and it requires a specific method for cyclic nucleotide regulation.

RESULTS

Optogenetic induction of cAMP in T. gondii affects host-cell invasion, stage-specific expression, and parasite differentiation. The underlying method allows a versatile control of parasite cAMP.

CONCLUSIONS

Optogenetic parasite strains offer valuable tools for dissecting cAMP-mediated processes.

SIGNIFICANCE

The method is applicable to other gene-tractable intertwined systems. Successful infection and transmission of the obligate intracellular parasite Toxoplasma gondii depends on its ability to switch between fast-replicating tachyzoite (acute) and quiescent bradyzoite (chronic) stages. Induction of cAMP in the parasitized host cells has been proposed to influence parasite differentiation. It is not known whether the parasite or host cAMP is required to drive this phenomenon. Other putative roles of cAMP for the parasite biology also remain to be identified. Unequivocal research on cAMP-mediated signaling in such intertwined systems also requires a method for an efficient and spatial control of the cAMP pool in the pathogen or in the enclosing host cell. We have resolved these critical concerns by expressing a photoactivated adenylate cyclase that allows light-sensitive control of the parasite or host-cell cAMP. Using this method, we reveal multiple roles of the parasite-derived cAMP in host-cell invasion, stage-specific expression, and asexual differentiation. An optogenetic method provides many desired advantages such as: (i) rapid, transient, and efficient cAMP induction in extracellular/intracellular and acute/chronic stages; (ii) circumvention of the difficulties often faced in cultures, i.e. poor diffusion, premature degradation, steady activation, and/or pleiotropic effects of cAMP agonists and antagonists; (iii) genetically encoded enzyme expression, thus inheritable to the cell progeny; and (iv) conditional and spatiotemporal control of cAMP levels. Importantly, a successful optogenetic application in Toxoplasma also illustrates its wider utility to study cAMP-mediated signaling in other genetically amenable two-organism systems such as in symbiotic and pathogen-host models.

The cell biology of cryptosporidium infection

Cryptosporidiosis remains a significant cause of enteric disease worldwide. Basic investigations of host: pathogen interactions have revealed the intricate processes mediating infection. The following summarizes the interactions that mediate infection and the host responses that both permit and ultimately clear the infection.

The C-terminus of Toxoplasma RON2 provides the crucial link between AMA1 and the host-associated invasion complex

Host cell invasion by apicomplexan parasites requires formation of the moving junction (MJ), a ring-like apposition between the parasite and host plasma membranes that the parasite migrates through during entry. The Toxoplasma MJ is a secreted complex including TgAMA1, a transmembrane protein on the parasite surface, and a complex of rhoptry neck proteins (TgRON2/4/5/8) described as host cell-associated. How these proteins connect the parasite and host cell has not previously been described. Here we show that TgRON2 localizes to the MJ and that two short segments flanking a hydrophobic stretch near its C-terminus (D3 and D4) independently associate with the ectodomain of TgAMA1. Pre-incubation of parasites with D3 (fused to glutathione S-transferase) dramatically reduces invasion but does not prevent injection of rhoptry bulb proteins. Hence, the entire C-terminal region of TgRON2 forms the crucial bridge between TgAMA1 and the rest of the MJ complex but this association is not required for rhoptry protein injection.

Invasion by the obligate intracellular parasites, Toxoplasma and Plasmodium, requires the formation of a ring of contact between parasite and host plasma membranes, the so-called moving junction (MJ), that the parasite migrates through during entry. The MJ is a complex of secreted parasite proteins including AMA1, on the parasite surface, and several rhoptry neck proteins (RONs), which are reported to associate with the host plasma membrane. The precise nature of the interaction that causes these two membranes to be so tightly apposed has not yet been elucidated. Here we report that the carboxy-terminal region of Toxoplasma (Tg)RON2 is exposed to the extracytosolic face of the MJ and that two short domains (D3 and D4) within this region independently and efficiently interact with the exposed ectodomain of TgAMA1. As recombinant D3, representing just 54 amino acids from TgRON2, efficiently blocks invasion, this interaction represents the crucial linkage for the MJ complex. Interestingly, D3 does not prevent injection of a rhoptry reporter protein demonstrating that invasion, and specifically a functional MJ, is not required for such injection. Our results suggest that the D3–D4 subregion of RON2, which is conserved across the Apicomplexa, will be a potent addition to current, AMA1-based control strategies for malaria.

Biogenesis of and activities at the Toxoplasma gondii parasitophorous vacuole membrane

Apicomplexan parasites like Toxoplasma gondii are distinctive in their utilization of para site encoded motor systems to invade cells. Invasion results in the establishment of the parasitophorous vacuole (PV) within the infected cell. Most apicomplexans complete their intracellular tenure within the infected cell in the PV that is demarcated from the host cytoplasm by the parasitophorous vacuole membrane (PVM). In this chapter I focus on the events surrounding the formation of the PVM and selected activities attributed to it. Its central role as the interface between the parasite and its immediate environment, the host cytoplasm, is validated by the diversity of functions attributed to it. While functions in structural organization, nutrient acquisitions and signaling have been defined their molecular bases remain largely unknown. Several recent studies and the decoding of the Toxoplasma genome have set the stage for a rapid expansion in our understanding of the role of the PVM in parasite biology. Toxoplasma gondii, like all apicomplexan parasites are obligate intracellular pathogens. This family of parasites utilize their own actin-myosin based motor systems to gain entry into susceptible cells establishing themselves, in some cases transiently (e.g., Theileria spp) in specialized vacuolar compartment, the parasitophorous vacuole (PV). The T. gondii PV is highly dynamic compartment defining the replication permissive niche for the parasite. The delimiting membrane defining the parasitophorous vacuole, the parasitophorous vacuole membrane or PVM is increasingly being recognized as a specialized "organelle" that in the context of the infected cell is extracorporeal to the parent organism, the parasite. A systematic study of this enigmatic organelle has been severely limited by several issues. Primary among these is the fact that it is formed only in the context of the infected cell thereby limiting the amount of material. Secondly, unlike other cellular organelles that can often be purified by conventional approaches, the PVM, cannot be purified away from host cell organelles (see below). In spite of these significant obstacles considerable progress has been made in recent years toward understanding the biogenesis of the PVM, identification of its protein complement and the characterization of activities within it. These studies demonstrate that the PVM, on its own and by virtue of its interactions with cellular components, plays critical functions in the structural integrity of the vacuole, nutrient acquisition and the manipulation of cellular functions. In addition it appears that the repertoire of activities at the PVM is likely to be plastic reflecting temporal changes associated with the replicative phase of parasite growth. Finally, the PVM likely forms the foundation for the cyst wall as the parasite differentiates in the establishment of latent infection. As the critical border crossing between the parasite and invaded cell the study of the PVM provides a fertile area for new investigation aided by the recent decoding of the Toxoplasma genome (available at wwww.ToxoDB.org) and the application of proteomic analyses to basic questions in parasite biology.

Plasmodium falciparum AMA1 binds a rhoptry neck protein homologous to TgRON4, a component of the moving junction in Toxoplasma gondii

Plasmodium falciparum apical membrane antigen 1 (PfAMA1) coimmunoprecipitates with the Plasmodium homologue of TgRON4, a secreted rhoptry neck protein of Toxoplasma gondii that migrates at the moving junction in association with TgAMA1 during invasion. PfRON4 also originates in the rhoptry necks, suggesting that this unusual collaboration of micronemes and rhoptries is a conserved feature of Apicomplexa.

Laser scanning cytometer-based assays for measuring host cell attachment and invasion by the human pathogen Toxoplasma gondii

BACKGROUND

Toxoplasma gondii is among the most common protozoan parasites of humans. Both attachment to and invasion of host cells by T. gondii are necessary for infection, yet little is known about the molecular mechanisms underlying these processes. T. gondii's etiological importance and its role as a model organism for studying invasion in related parasites necessitate a means to quantitatively assay host cell attachment and invasion.

METHODS

We present here Laser Scanning Cytometer (LSC)-based assays of T. gondii invasion and attachment. The invasion assay involves automated counting of invaded and non-invaded parasites, differentially labeled with distinct fluorochromes. The attachment assay compares the relative binding of differentially labeled parasites. The assays were evaluated using treatments known to decrease invasion or attachment.

RESULTS

The LSC-based assays are robust and reproducible, remove operator bias, and significantly increase the sample size that can be feasibly counted compared to other currently available microscope-based methods. In the first application of the new assays, we have shown that parasites attach to fixed and unfixed host cells using different mechanisms.

CONCLUSIONS

The LSC-based assays represent useful new methods for quantitatively measuring attachment and invasion by T. gondii, and can be readily adapted to study similar processes in other host-pathogen systems.

Identification of the moving junction complex of Toxoplasma gondii: a collaboration between distinct secretory organelles

Apicomplexan parasites, including Toxoplasma gondii and Plasmodium sp., are obligate intracellular protozoa. They enter into a host cell by attaching to and then creating an invagination in the host cell plasma membrane. Contact between parasite and host plasma membranes occurs in the form of a ring-shaped moving junction that begins at the anterior end of the parasite and then migrates posteriorly. The resulting invagination of host plasma membrane creates a parasitophorous vacuole that completely envelops the now intracellular parasite. At the start of this process, apical membrane antigen 1 (AMA1) is released onto the parasite surface from specialized secretory organelles called micronemes. The T. gondii version of this protein, TgAMA1, has been shown to be essential for invasion but its exact role has not previously been determined. We identify here a trio of proteins that associate with TgAMA1, at least one of which associates with TgAMA1 at the moving junction. Surprisingly, these new proteins derive not from micronemes, but from the anterior secretory organelles known as rhoptries and specifically, for at least two, from the neck portion of these club-shaped structures. Homologues for these AMA1-associated proteins are found throughout the Apicomplexa strongly suggesting that this moving junction apparatus is a conserved feature of this important class of parasites. Differences between the contributing proteins in different species may, in part, be the result of selective pressure from the different niches occupied by these parasites.

A new release on life: emerging concepts in proteolysis and parasite invasion

Cell invasion by apicomplexan pathogens such as the malaria parasite and Toxoplasma is accompanied by extensive proteolysis of zoite surface proteins (ZSPs) required for attachment and penetration. Although there is still little known about the proteases involved, a conceptual framework is emerging for the roles of proteolysis in cell invasion. Primary processing of ZSPs, which includes the trimming of terminal peptides or segmentation into multiple fragments, is proposed to activate these adhesive ligands for tight binding to host receptors. Secondary processing, which occurs during penetration, results in the shedding of ZSPs by one of two mechanistically distinct ways, shaving or capping. Resident surface proteins are typically shaved from the surface whereas adhesive ligands mobilized from intracellular secretory vesicles are capped to the posterior end of the parasite before being shed during the final steps of penetration. Intriguingly, recent studies have revealed that ZSPs can be released either by being cleaved adjacent to the membrane anchor or actually within the membrane itself. Mounting evidence suggests that intramembrane cleavage is catalysed by one or more integral membrane serine proteases of the Rhomboid family and we propose that several malaria adhesive ligands may be potential substrates for these enzymes. We also discuss the evidence that the key reason for ZSP shedding during invasion is to break the connection between parasite surface ligands and host receptors. The sequential proteolytic events associated with invasion by pathogenic protozoa may represent vulnerable pathways for the future development of synergistic anti-protozoal therapies.

Conditional expression of Toxoplasma gondii apical membrane antigen-1 (TgAMA1) demonstrates that TgAMA1 plays a critical role in host cell invasion

Toxoplasma gondii is an obligate intracellular parasite and an important human pathogen. Relatively little is known about the proteins that orchestrate host cell invasion by T. gondii or related apicomplexan parasites (including Plasmodium spp., which cause malaria), due to the difficulty of studying essential genes in these organisms. We have used a recently developed regulatable promoter to create a conditional knockout of T. gondii apical membrane antigen-1 (TgAMA1). TgAMA1 is a transmembrane protein that localizes to the parasite's micronemes, secretory organelles that discharge during invasion. AMA1 proteins are conserved among apicomplexan parasites and are of intense interest as malaria vaccine candidates. We show here that T. gondii tachyzoites depleted of TgAMA1 are severely compromised in their ability to invade host cells, providing direct genetic evidence that AMA1 functions during invasion. The TgAMA1 deficiency has no effect on microneme secretion or initial attachment of the parasite to the host cell, but it does inhibit secretion of the rhoptries, organelles whose discharge is coupled to active host cell penetration. The data suggest a model in which attachment of the parasite to the host cell occurs in two distinct stages, the second of which requires TgAMA1 and is involved in regulating rhoptry secretion.

The SAG5 locus of Toxoplasma gondii encodes three novel proteins belonging to the SAG1 family of surface antigens

We have identified three novel Toxoplasma gondii proteins showing close structural similarity to molecules of the SAG1 family, a group of glycosylphosphatidylinositol-anchored surface antigens expressed by the invasive stages of T. gondii. The novel proteins, denominated SAG5A, SAG5B and SAG5C, are encoded by tandemly arrayed and tightly clustered genes containing no introns. The 367 amino acid-long SAG5B and SAG5C are 97.5% identical to each other, whereas SAG5A (362 amino acids) consists of a C-terminal domain sharing 98% identity with SAG5B and SAG5C, and an N-terminal domain whose identity to the other SAG5 polypeptides is only 42%. Expression analysis of the T. gondii strains RH (virulent) and 76 K (avirulent) showed that all members of the SAG5 cluster are transcribed in T. gondii tachyzoites and bradyzoites. However, immunoblot studies on the RH strain revealed that the synthesis of SAG5A does not occur in tachyzoites and is possibly controlled at the post-transcriptional level. On the contrary, SAG5B and SAG5C were detected by immunoblot in tachyzoite lysates and found to migrate in the 40-45 kDa range under reducing conditions or at approximately 34 kDa under unreduced conditions. Triton X-114 partitioning of tachyzoite protein lysates treated with phosphatidylinositol-specific phospholipase C indicated that SAG5B and SAG5C are glycosylphosphatidylinositol-anchored membrane-associated molecules. Consistently, immunofluorescence analysis of transformed tachyzoites over-expressing SAG5B or SAG5C showed that these molecules are targeted to the parasite surface. The characterisation of the SAG5 locus sheds further light on the complex repertoire of SAG1-related genes in T. gondii, that now comprises 14 highly homologous members and five distantly related genes belonging to the SAG2 family.

Binding of a monoclonal antibody to sporozoites of Sarcocystis singaporensis enhances escape from the parasitophorous vacuole, which is necessary for intracellular development

Early intracellular development in vitro of the cyst-forming protozoon Sarcocystis singaporensis and the influence of a monoclonal antibody on invasion, intracellular localization, and development of sporozoites were studied. As revealed by immunofluorescence using parasite-specific antibodies which labeled the parasitophorous vacuole membrane (PVM) and by ultrastructural analysis, sporozoites invaded pneumonocytes of the rat via formation of a parasitophorous vacuole (PV). About half of the sporozoites left this compartment within the first 8 h postinfection to enter the host cell cytosol. By semiquantitative analysis of acetyl-histone H4 expression of sporozoites, a marker linked to early gene expression of eukaryotic cells, we show (supported by ultrastructural analysis) that escape from the PV appears to be necessary for early intracellular development. More than 90% of sporozoites located in the cytosol expressed high levels of acetylated histone H4 in the nucleus, whereas only a quarter of the intravacuolar sporozoites exhibited a similar signal. As revealed by ultrastructural analysis, young schizonts all resided in the cytosol. Specific binding of a monoclonal antibody (11D5/H3) to sporozoites before invasion significantly enhanced their escape from the PV, whereas cell invasion itself remained unaffected. The antibody actually increased proliferation of the parasites in vitro, providing a further link between residence in the cytosol and successful intracellular development. Monoclonal antibody 11D5/H3 precipitated a major 58-kDa antigen from oocyst-sporocyst extracts and reacted with the cytoplasm and the surface of sporozoites in immunofluorescence assays. Collectively, the observed antibody-parasite interaction suggests the existence of a signaling event that influences intracellular development of Sarcocystis.

Gliding motility and cell invasion by Apicomplexa: insights from the Plasmodium sporozoite

Apicomplexa constitute one of the largest phyla of protozoa. Most Apicomplexa, including those pathogenic to humans, are obligate intracellular parasites. Their extracellular forms, which are highly polarized and elongated cells, share two unique abilities: they glide on solid substrates without changing their shape and reach an intracellular compartment without active participation from the host cell. There is now ample ultrastructural evidence that these processes result from the backward movement of extracellular interactions along the anteroposterior axis of the parasite. Recent work in several Apicomplexa, including genetic studies in the Plasmodium sporozoite, has provided molecular support for this 'capping' model. It appears that the same machinery drives both gliding motility and host cell invasion. The cytoplasmic motor, a transmembrane bridge and surface ligands essential for cell invasion are conserved among the main apicomplexan pathogens.

Lytic cycle of Toxoplasma gondii

Toxoplasma gondii is an obligate intracellular pathogen within the phylum Apicomplexa. This protozoan parasite is one of the most widespread, with a broad host range including many birds and mammals and a geographic range that is nearly worldwide. While infection of healthy adults is usually relatively mild, serious disease can result in utero or when the host is immunocompromised. This sophisticated eukaryote has many specialized features that make it well suited to its intracellular lifestyle. In this review, we describe the current knowledge of how the asexual tachyzoite stage of Toxoplasma attaches to, invades, replicates in, and exits the host cell. Since this process is closely analogous to the way in which viruses reproduce, we refer to it as the Toxoplasma "lytic cycle."

Conservation of a gliding motility and cell invasion machinery in Apicomplexan parasites

Most Apicomplexan parasites, including the human pathogens Plasmodium, Toxoplasma, and Cryptosporidium, actively invade host cells and display gliding motility, both actions powered by parasite microfilaments. In Plasmodium sporozoites, thrombospondin-related anonymous protein (TRAP), a member of a group of Apicomplexan transmembrane proteins that have common adhesion domains, is necessary for gliding motility and infection of the vertebrate host. Here, we provide genetic evidence that TRAP is directly involved in a capping process that drives both sporozoite gliding and cell invasion. We also demonstrate that TRAP-related proteins in other Apicomplexa fulfill the same function and that their cytoplasmic tails interact with homologous partners in the respective parasite. Therefore, a mechanism of surface redistribution of TRAP-related proteins driving gliding locomotion and cell invasion is conserved among Apicomplexan parasites.

Analysis of Toxoplasma gondii stably transfected with a transmembrane variant of its major surface protein, SAG1

We have genetically engineered Toxoplasma gondii so that its major surface antigen SAG1 is anchored by a human transmembrane domain (SAG1-TM) instead of its natural GPI anchor (SAG1-GPI) in order to initiate studies to address the function of this protein anchor in parasitic protozoa as well as to get insights into the functional role of SAG1. Our results show that SAG1-TM is correctly folded (at least as judged by the presence of conformationally dependent epitopes) and targeted to the surface of the parasite, indicating that the GPI anchor does not determine its localization nor overall three-dimensional structure. No significant difference was seen in any aspect of the growth of the SAG1-TM mutant. However, compared to the natural SAG1-GPI, SAG1-TM does not form strong associations with itself and/or other molecules in high molecular weight complexes suggesting that allowing such complexes to form may be one role of the GPI anchor. The in vitro half-life of SAG1-TM of extracellular parasites is significantly lower than that of SAG1-GPI suggesting a stabilizing function of the glycolipid anchor against degradation and/or membrane release. Antibodies to SAG1 are shed from SAG1-TM parasites as they invade, just as they are stripped from SAG1-GPI bearing parasites. The stripping, therefore, is unlikely to be driven by the action of lipases.

Safe haven: the cell biology of nonfusogenic pathogen vacuoles

Our understanding of both membrane traffic in mammalian cells and the cell biology of infection with intracellular pathogens has increased dramatically in recent years. In this review, we discuss the cell biology of the host-microbe interaction for four intracellular pathogens: Chlamydia spp., Legionella pneumophila, Mycobacterium spp., and the protozoan parasite Toxoplasma gondii. All of these organisms reside in vacuoles inside cells that have restricted fusion with host organelles of the endocytic cascade. Despite this restricted fusion, the vacuoles surrounding each pathogen display novel interactions with other host cell organelles. In addition to the effect of infection on host membrane traffic, we focus on these novel interactions and relate them where possible to nutrient acquisition by the intracellular organisms.

TRAP is necessary for gliding motility and infectivity of plasmodium sporozoites

Many protozoans of the phylum Apicomplexa are invasive parasites that exhibit a substrate-dependent gliding motility. Plasmodium (malaria) sporozoites, the stage of the parasite that invades the salivary glands of the mosquito vector and the liver of the vertebrate host, express a surface protein called thrombospondin-related anonymous protein (TRAP) that has homologs in other Apicomplexa. By gene targeting in a rodent Plasmodium, we demonstrate that TRAP is critical for sporozoite infection of the mosquito salivary glands and the rat liver, and is essential for sporozoite gliding motility in vitro. This suggests that in Plasmodium sporozoites, and likely in other Apicomplexa, gliding locomotion and cell invasion have a common molecular basis.

Subpellicular microtubules associate with an intramembranous particle lattice in the protozoan parasite Toxoplasma gondii

Application of Fourier analysis techniques to images of isolated, frozen-hydrated subpellicular microtubules from the protozoan parasite Toxoplasma gondii demonstrates a distinctive 32 nm periodicity along the length of the microtubules. A 32 nm longitudinal repeat is also observed in the double rows of intramembranous particles seen in freeze-fracture images of the parasite's pellicle; these rows are thought to overlie the subpellicular microtubules. Remarkably, the 32 nm intramembranous particle periodicity is carried over laterally to the single rows of particles that lie between the microtubule-associated double rows. This creates a two-dimensional particle lattice, with the second dimension at an angle of approximately 75 degrees to the longitudinal rows (depending on position along the length of the parasite). Drugs that disrupt known cytoskeletal components fail to destroy the integrity of the particle lattice. This intramembranous particle organization suggests the existence of multiple cytoskeletal filaments of unknown identity. Filaments associated with the particle lattice provide a possible mechanism for motility and shape change in Toxoplasma: distortion of the lattice may mediate the twirling motility seen upon host-cell lysis, and morphological changes observed during invasion.

A ubiquitous intracellular parasite: the cellular biology of Toxoplasma gondii

Toxoplasma gondii shares many features with other apicomplexan parasites but is unusual in its extremely broad host and tissue specificity. The parasite exhibits typical 'zoite' morphology, its highly polar structure being dictated by the complex cytoskeleton. Molecules on the surface of the zoite are prime candidates for interaction with the host cell and in vitro assays have implicated 2 of the 5 tachyzoite surface molecules in invasion: SAG1 as a ligand mediating host cell invasion, and SAG2 in enabling reorientation prior to invasion. The functional roles of other molecules, secreted from internal organelles during invasion and intracellular development, are also becoming clear through immuno-EM and biochemical studies, and from sequence data. Molecules from the rhoptries including the penetration enhancing factor ROP1 are secreted at the point of invasion and are integral to the newly formed parasitophorous vacuole membrane. Release of the dense granule molecules GRA 1-6, appears to be calcium regulated and occurs within 10 min of invasion leading to formation of the tubular membranous network and stabilization of the vacuole. The interaction between Toxoplasma and the host cell is stage specific. The tachyzoite divides rapidly and synchronously forming rosettes and causing host cell lysis, while the bradyzoite exhibits slow asynchronous division secreting a granular matrix and becoming enclosed within a cyst wall. This altered phenotype is a reflection of changes in gene expression. Bradyzoite specific molecules are found internally, on the parasite surface, and in the cyst matrix while important tachyzoite proteins such as SAG1 and SAG2 are downregulated. Differentiation between the 2 stages is reversible and is influenced by immunomodulatory agents. However a strong genetic element is involved and it is notable that virulent strains show a very low frequency of cyst production.

Survival of immunoglobulin G-opsonized Toxoplasma gondii in nonadherent human monocytes

Toxoplasma gondii is a protozoan parasite that is able to penetrate human monocytes by either passive uptake during phagocytosis or active penetration. It is expected that immunoglobulin G (IgG) opsonization will target the parasite to macrophage Fc gamma receptors for phagocytic processing and subsequent degradation. Antibody-opsonized T. gondii tachyzoites were used to infect nonadherent and adherent human monocytes obtained from the peripheral blood of seronegative individuals. The infected monocytes were evaluated for the presence of intracellular parasites and the degree of parasiticidal activity. A marked difference in both the numbers of infected macrophages and numbers of parasites per 100 macrophages was observed in the nonadherent cells when compared with those of the adherent cell population. When macrophage Fc gamma receptors were down-modulated, opsonized tachyzoites retained their ability to penetrate the host cell at a rate similar to that observed for unopsonized parasites. These results suggest that antibody opsonization of T. gondii does not prevent active penetration of human monocytes by the parasite and, furthermore, has little effect on intracellular replication of the parasite.

A selector of transcription initiation in the protozoan parasite Toxoplasma gondii

The recent development of an efficient transfection system for the apicomplexan Toxoplasma gondii allows a comprehensive dissection of the elements involved in gene transcription in this obligate intracellular parasite. We demonstrate here that for the SAG1 gene, a stretch of six repeated sequences in the region 35 to 190 bp upstream of the first of two transcription start sites is essential for efficient and accurate transcription initiation. This repeat element shows characteristics of a selector in determining the position of the transcription start sites.

Toxoplasma gondii: redistribution of tachyzoite surface protein during host cell invasion and intracellular development

Immunoperoxidase localisation of antigen at the electron microscope level confirms that parasite surface proteins, in association with membrane, are shed from the surface of the zoite on invasion, while varying amounts are also internalised. SAG 1 is stable on intracellular zoites for up to 48 h, although new protein is also synthesised. SAG1 is present on the surface of daughter zoites and is found throughout the infected cell in distinct vacuoles; these vacuoles represent either direct extensions of the parasitophorous vacuole or true export of parasite surface material. Conflicting reports exist concerning the presence of SAG1 on the developing intraphagosomal membrane (IPM) network immediately post-invasion (Sibley et al. 1986; Dubremetz et al. 1993). It is not known whether the molecule continues to be expressed during intracellular development. The current study follows the fate of SAG1 during invasion and over the first 48 h of parasite multiplication within the host cell, using pre- and postinvasion labeling techniques at the electron microscope level.

Mucin-like glycoproteins linked to the membrane by glycosylphosphatidylinositol anchor are the major acceptors of sialic acid in a reaction catalyzed by trans-sialidase in metacyclic forms of Trypanosoma cruzi

We have previously shown that 35- and 50-kDa glycoconjugates of cultured metacyclic trypomastigotes participate in the attachment of parasites to mammalian cells. Here we show that when metacyclic trypomastigotes are incubated with [3H]sialyllactose, most of the sialic acid is transferred to these 35/50-kDa molecules in a reaction catalyzed by a parasite transsialidase. The sialic acid is incorporated in oligosaccharides of about 10 glucose units in size that are released from the glycoconjugate by mild alkaline hydrolysis. Compositional analysis reveals that the 35/50-kDa molecules are highly glycosylated proteins rich in threonine, galactose, N-acetyl-glucosamine and sialic acid. These glycoproteins can be labeled in vivo with [3H]palmitate, and the labeled fatty acid is released by glycosylphosphatidylinositol specific phospholipases C. This result, associated with the fact that they contain mannose, ethanolamine, myo-inositol, and lipid, indicate that these glycoproteins are anchored to the membrane by glycosylphosphatidylinositol. During cell invasion, these molecules appear to be capped and locally released by the parasite.

Toxoplasma gondii: the role of a 30-kDa surface protein in host cell invasion

C1E3, a monoclonal antibody recognizing protein P30, a major surface antigen of Toxoplasma gondii tachyzoites, was shown to have a consistent effect on invasion in adult bovine kidney cells. In 10 replicate assays, the overall invasion was reduced to 37% of control values (P less than 0.0001). These results support the role of a functional role for P30 in mediating invasion.

Automated microparticle enzyme immunoassays for IgG and IgM antibodies to Toxoplasma gondii

Fully automated microparticle enzyme immunoassays (MEIA) for the IMx immunoassay analyser were developed to detect IgG and IgM antibodies to Toxoplasma gondii. The IgG MEIA results are expressed in International Units (IU) of IgG antibody interpolated from a six point calibration curve covering the range from 0 to 300 IU/ml. Reproducible results were obtained from a calibration curve stored in the instrument for at least one month. The qualitative IgM MEIA expresses results as an index using a single calibrator included in each run. The Toxo IgG MEIA and Toxo IgM MEIA were in 98% and 97% agreement, respectively, with the reference assays used. Twenty four sera can be completely processed in about 35 minutes.

Attachment of Trypanosoma cruzi to mammalian cells requires parasite energy, and invasion can be independent of the target cell cytoskeleton

We have previously shown that the binding of Trypanosoma cruzi trypomastigotes to glutaraldehyde-fixed mammalian cells has the characteristics of a receptor-mediated process and that it mimics the attachment step of the invasion of live cells by this parasite. In this study we examined the metabolic requirements for the attachment of trypomastigotes to glutaraldehyde-fixed fibroblasts. The attachment of trypomastigotes to fixed cells is prevented when the energy conservation mechanisms are inhibited with the drugs 2-deoxyglucose, sodium azide, antimycin, crystal violet, oligomycin, N,N'-dicyclohexylcarbodiimide, and carbonyl cyanide 3-chlorophenylhydrazone. However, under the same experimental conditions, the movement of parasites is not significantly affected. Several of these drugs totally inhibit the penetration of the parasite into live target cells. We conclude that the attachment of trypomastigotes to mammalian cells is an active process that requires trypomastigote energy. In addition, we present evidence that penetration into nonphagocytic cells can also be an active process. Trypomastigotes can be seen in scanning electron micrographs traversing extended lamellipodia and entering paraformaldehyde-fixed epithelial cells. Cytochalasin D, a drug that disrupts microfilaments and prevents the formation of plasma membrane extensions mediated by actin, had little or no effect on trypomastigote invasion, while it inhibited Salmonella entry into epithelial cells.

Toxoplasma gondii: immunocytochemistry of four immunodominant antigens with monoclonal antibodies

Four monoclonal antibodies in which diagnostic usefulness has been observed, concerning congenital, acquired, and reactivated toxoplasmosis, were raised against Toxoplasma gondii tachyzoïtes in order to localize immunodominant antigens. On immunoblots, it appears that McAb IV47, McAB GII9, McAb II38, and McAb IE10 identify families of proteins with estimated molecular weights of 28-30 kDa, 30 kDa, 45-50 kDa, and 66-70 kDa, respectively. By immunogold preembedding techniques one can observe an homogeneous labeling of the outer pellicle of the tachyzoïtes with the McAb GII9 and IV47 and a light labeling with the McAb II38 and IE10. The three-dimensional observation of cell surface antigens is performed by applying a modified metal extraction replica method, i.e., A plasma polymerization method of glow discharge by Tanaka (1979). By immunogold preembedding techniques [with saponin permeabilization (0.1%)], and by immunogold postembedding techniques, a labeling of the rhoptries is observed with McAb GII9 and McAb IV47 but essentially all label is found with McAb II38 and IE10. With McAb GII9 a uniform labeling is observed on the cell surface. By immunoenzymatic techniques (peroxidase) a cell surface labeling is observed with the four McAb. Intracellular Toxoplasma, the outer pellicle, and the vesicles of the network (elaborated by Toxoplasma in parasitophorous vacuole) are also labeled with McAb IE10. These results indicate that McAb GII9 recognizes antigens of the antigen family (P 30) located on the cell surface and in the rhoptries. The antigen recognized by McAb IV47 is essentially located on and beneath the Toxoplasma cell surface membrane, and McAb II38 and IE10 identify preferentially rhoptry proteins.

Toxoplasma gondii: fusion competence of parasitophorous vacuoles in Fc receptor-transfected fibroblasts

After actively entering its host cells, the protozoan parasite Toxoplasma gondii resides in an intracellular vacuole that is completely unable to fuse with other endocytic or biosynthetic organelles. The fusion blocking requires entry of viable organisms but is irreversible: fusion competence of the vacuole is not restored if the parasite is killed after entry. The fusion block can be overcome, however, by altering the parasite's route of entry. Thus, phagocytosis of viable antibody-coated T. gondii by Chinese hamster ovary cells transfected with macrophage-lymphocyte Fc receptors results in the formation of vacuoles that are capable of both fusion and acidification. Phagocytosis and fusion appear to involve a domain of the Fc receptor cytoplasmic tail distinct from that required for localization at clathrin-coated pits. These results suggest that the mechanism of fusion inhibition is likely to reflect a modification of the vacuole membrane at the time of its formation, as opposed to the secretion of a soluble inhibitor by the parasite.

Evidence for glycosyl-phosphatidylinositol anchoring of Toxoplasma gondii major surface antigens

The four major surface antigens of Toxoplasma gondii tachyzoites (P43, P35, P30, and P22) were made water soluble by phosphatidylinositol-specific phospholipase C (PI-PLC). These antigens were biosynthetically labeled with 3H-fatty acids, [3H]ethanolamine, and [3H]carbohydrates. Treatment of 3H-fatty-acid-labeled parasite lysates with PI-PLC removed the radioactive label from these antigens. A cross-reacting determinant was exposed on these antigens after PI-PLC treatment.

Evidence for glycosyl-phosphatidylinositol anchoring of Toxoplasma gondii major surface antigens

The four major surface antigens of Toxoplasma gondii tachyzoites (P43, P35, P30, and P22) were made water soluble by phosphatidylinositol-specific phospholipase C (PI-PLC). These antigens were biosynthetically labeled with 3H-fatty acids, [3H]ethanolamine, and [3H]carbohydrates. Treatment of 3H-fatty-acid-labeled parasite lysates with PI-PLC removed the radioactive label from these antigens. A cross-reacting determinant was exposed on these antigens after PI-PLC treatment.

Localization of a 23,000 MW antigen of Cryptosporidium by immunoelectron microscopy

Rabbit antiserum was raised against a 23,000 molecular weight (MW) antigen prepared from Cryptosporidium oocysts by electro-elution from polyacrylamide gels. The antiserum was tested for specificity by immunoblotting against solubilized oocyst preparations. Several antigens including the 23,000 MW antigen were recognized suggesting that it shared common epitopes with higher MW proteins. The antiserum was then used in conjunction with a protein A-colloidal gold conjugate to locate antigenic sites within exogenous and endogenous developmental stages of Cryptosporidium. The pellicles of both sporozoites and merozoites exhibited specific labelling, particularly around their anterior ends. No specific labelling was observed for any other membrane determinants or organelles in these or other life cycle stages.

Characterization of a surface antigen of Eimeria nieschulzi (Apicomplexa, Eimeriidae) sporozoites

A monoclonal antibody (McAb 3C3) reacting with a pellicular antigen of Eimeria nieschulzi sporozoites has been selected among hybridomas produced against this organism by immunofluorescence assay. This antigen has been shown to be located on the zoite surface by immunofluorescence on living organisms. Capping and shedding of antigen-monoclonal antibody immune complexes was observed upon incubation at 37 degrees C. On western immunoblotting, two polypeptides at 22 and 26 kDa were recognized by McAb 3C3, whereas only one polypeptide of 22 kDa was immunoprecipitated by the same antibody after lactoperoxidase surface radio-iodination of sporozoites.

Membrane proteins and their antigenicity of Toxoplasma gondii

Surface membrane proteins of virulent RH strain and tissue cyst-forming Fukaya strain of Toxoplasma gondii were analyzed by SDS-polyacrylamide gel electrophoresis after LPO-catalyzed surface iodination and lectin blotting, then identified the zoite-specific antigens. Prior to the analyses, purification of RH tachyzoites from mouse peritoneal exudate and of Fukaya bradyzoites from mouse brain tissues were performed by centrifugation on the discontinuous Percoll density-gradient. Tachyzoites were obtained at the interface of 50 per cent and 60 per cent Percoll solution and brain cysts were harvested at the interfaces of 40-50 per cent and 50-60 per cent, then bradyzoites were obtained by treating the cysts with hypertonic solution. The LPO-catalyzed iodination detected 15 KDa and 14 KDa proteins of bradyzoites and 30 KDa protein of tachyzoites as major bands with several other minor bands. But Con A blotting revealed some bands of 200 K-50 KDa glycoproteins of bradyzoites and 52 KDa band as major and minor bands of 33 K-20 KDa of tachyzoites. Phytohemagglutinin did not detect any band in the two forms. EITB with anti-Fukaya antibody and anti-RH antibody revealed cross-reactivities between the two forms. Despite the cross-reactivity, anti-Fukaya antibody reacted with 15 KDa band of bradyzoites specifically and, anti-RH antibody with 52 KDa, 30 KDa, and 25 KDa bands of tachyzoites, respectively. It was identified that 15 KDa protein in bradyzoite, which was not a glycoprotein, was a major membrane protein with sufficient antigenicity, and in the case of tachyzoite, 52 KDa surface glycoprotein (gp52) with specific antigenicity might be added to the major surface protein, p30.

Surface antigens of Toxoplasma gondii

Monoclonal antibodies reacting with pellicular antigens of Toxoplasma gondii tachyzoites have been selected among hybridomas produced against this organism by immunofluorescence assay. These antigens have been further characterized by immunofluorescence on living zoites, Western immunoblotting and immunoprecipitation of lactoperoxidase surface radio-iodinated tachyzoite lysates. The simultaneous characterization of 5 different surface antigens (P43, P35, P30, P23, P22) some of which have already been studied individually allowed a better definition of these antigens and the characterization of a yet undescribed surface molecule (P23).

Toxoplasma antigens recognized by naturally occurring human antibodies

Sera of most adults have high agglutination test titers to Toxoplasma gondii whether or not the adults have other serological evidence of the infection. This finding has been attributed to the presence of naturally occurring antibodies to T. gondii. Consistent with this observation, we have recently noted that protein blots (PB) of sera of individuals not previously infected with T. gondii had immunoglobulin G (IgG) and IgM antibodies to antigens of the parasite. To further define the antigens recognized by these naturally occurring antibodies, we studied PB of sera of 44 adults and 9 children who had no serological evidence of the infection. Multiple antigens of T. gondii with molecular weights of 15,000 to greater than 205,000 were recognized by IgG and IgM natural antibodies of each of the sera. Although a relatively consistent pattern was noted on the IgM PB of the sera of the adults in the molecular weight range of 48,000 to 85,000, greater heterogeneity was noted on the IgG PB. The most common bands noted on the latter were of approximately 30,000 and 92,000 molecular weight. All of the PB obtained with the serial sera collected at yearly intervals from the children revealed bands; in some cases, new bands had appeared with time, and in others the pattern was constant. In children older than 8 years, the patterns of the PB were similar to those noted in PB of sera of the adults.