Stable DNA transformation of Toxoplasma gondii using phleomycin selection

Dynamic imaging of T cell-parasite interactions in the brains of mice chronically infected with Toxoplasma gondii

The intracellular parasite Toxoplasma gondii can establish persistent infection in the brain of a mammalian host, a standoff that involves the active participation of host CD8 T cells to control infection. CD8 T cells generally protect against intracellular pathogens by local delivery of effector molecules upon recognition of specific pathogen Ags on invaded host cells. However, the interactions between CD8 T cells, T. gondii, and APCs in the brain have not yet been examined. In this study we have used a mouse infection model in conjunction with two-photon microscopy of living brain tissue and confocal microscopy of fixed brain sections to examine the interactions between CD8 T cells, parasites, and APCs from chronically infected mice. We found that Ag-specific CD8 T cells were recruited to the brains of infected mice and persisted there in the presence of ongoing Ag recognition. Cerebral CD8 T cells made transient contacts with granuloma-like structures containing parasites and with individual CD11b(+) APCs, including some that did not contain parasites. In contrast, T cells ignored intact Ag-bearing cysts and did not contact astrocytes or neurons, including neurons containing parasites or cysts. Our data represent the first direct observation of the dynamics of T cell-parasite interactions within living tissue and provide a new perspective for understanding immune responses to persistent pathogens in the brain.

Aldolase is essential for energy production and bridging adhesin-actin cytoskeletal interactions during parasite invasion of host cells

Apicomplexan parasites rely on actin-based motility to drive host cell invasion. Prior in vitro studies implicated aldolase, a tetrameric glycolytic enzyme, in coupling actin filaments to the parasite's surface adhesin microneme protein 2 (MIC2). Here, we test the essentiality of this interaction in host cell invasion. Based on in vitro studies and homology modeling, we generated a series of mutations in Toxoplasma gondii aldolase (TgALD1) that delineated MIC2 tail domain (MIC2t) binding function from its enzyme activity. We tested these mutants by complementing a conditional knockout of TgALD1. Mutations that affected glycolysis also reduced motility. Mutants only affecting binding to MIC2t had no motility phenotype, but were decreased in their efficiency of host cell invasion. Our studies demonstrate that aldolase is not only required for energy production but is also essential for efficient host cell invasion, based on its ability to bridge adhesin-cytoskeleton interactions in the parasite.

Drug inhibition of HDAC3 and epigenetic control of differentiation in Apicomplexa parasites

Plasmodium and Toxoplasma are parasites of major medical importance that belong to the Apicomplexa phylum of protozoa. These parasites transform into various stages during their life cycle and express a specific set of proteins at each stage. Although little is yet known of how gene expression is controlled in Apicomplexa, histone modifications, particularly acetylation, are emerging as key regulators of parasite differentiation and stage conversion. We investigated the anti-Apicomplexa effect of FR235222, a histone deacetylase inhibitor (HDACi). We show that FR235222 is active against a variety of Apicomplexa genera, including Plasmodium and Toxoplasma, and is more potent than other HDACi's such as trichostatin A and the clinically relevant compound pyrimethamine. We identify T. gondii HDAC3 (TgHDAC3) as the target of FR235222 in Toxoplasma tachyzoites and demonstrate the crucial role of the conserved and Apicomplexa HDAC-specific residue TgHDAC3 T99 in the inhibitory activity of the drug. We also show that FR235222 induces differentiation of the tachyzoite (replicative) into the bradyzoite (nonreplicative) stage. Additionally, via its anti-TgHDAC3 activity, FR235222 influences the expression of approximately 370 genes, a third of which are stage-specifically expressed. These results identify FR235222 as a potent HDACi of Apicomplexa, and establish HDAC3 as a central regulator of gene expression and stage conversion in Toxoplasma and, likely, other Apicomplexa.

Efficient gene replacements in Toxoplasma gondii strains deficient for nonhomologous end joining

A high frequency of nonhomologous recombination has hampered gene targeting approaches in the model apicomplexan parasite Toxoplasma gondii. To address whether the nonhomologous end-joining (NHEJ) DNA repair pathway could be disrupted in this obligate intracellular parasite, putative KU proteins were identified and a predicted KU80 gene was deleted. The efficiency of gene targeting via double-crossover homologous recombination at several genetic loci was found to be greater than 97% of the total transformants in KU80 knockouts. Gene replacement efficiency was markedly increased (300- to 400-fold) in KU80 knockouts compared to wild-type strains. Target DNA flanks of only approximately 500 bp were found to be sufficient for efficient gene replacements in KU80 knockouts. KU80 knockouts stably retained a normal growth rate in vitro and the high virulence phenotype of type I strains but exhibited an increased sensitivity to double-strand DNA breaks induced by treatment with phleomycin or gamma-irradiation. Collectively, these results revealed that a significant KU-dependent NHEJ DNA repair pathway is present in Toxoplasma gondii. Integration essentially occurs only at the homologous targeted sites in the KU80 knockout background, making this genetic background an efficient host for gene targeting to speed postgenome functional analysis and genetic dissection of parasite biology.

Selection at a single locus leads to widespread expansion of Toxoplasma gondii lineages that are virulent in mice

Pathogenicity differences among laboratory isolates of the dominant clonal North American and European lineages of Toxoplasma gondii are largely controlled by polymorphisms and expression differences in rhoptry secretory proteins (ROPs). However, the extent to which such differences control virulence in natural isolates of T. gondii, including those from more diverse genetic backgrounds, is uncertain. We elucidated the evolutionary history and functional consequences of diversification in the serine/threonine kinase ROP18, a major virulence determinant in the mouse model. We characterized the extent of sequence polymorphism and the evolutionary forces acting on ROP18 and several antigen-encoding genes within a large collection of natural isolates, comparing them to housekeeping genes and introns. Surprisingly, despite substantial genetic diversity between lineages, we identified just three principal alleles of ROP18, which had very ancient ancestry compared to other sampled loci. Expression and allelic differences between these three alleles of ROP18 accounted for much of the variation in acute mouse virulence among natural isolates. While the avirulent type III allele was the most ancient, intermediate virulent (type II) and highly virulent (type I) lineages predominated and showed evidence of strong selective pressure. Out-group comparison indicated that historical loss of an upstream regulatory element increased ROP18 expression, exposing it to newfound diversifying selection, resulting in greatly enhanced virulence in the mouse model and expansion of new lineages. Population sweeps are evident in many genomes, yet their causes and evolutionary histories are rarely known. Our results establish that up-regulation of expression and selection at ROP18 in T. gondii has resulted in three distinct alleles with widely different levels of acute virulence in the mouse model. Preservation of all three alleles in the wild indicates they are likely adaptations for different niches. Our findings demonstrate that sweeping changes in population structure can result from alterations in a single gene.

Current and emerging approaches to studying invasion in apicomplexan parasites

In this chapter, we outline the tools and techniques available to study the process of host cell invasion by apicomplexan parasites and we provide specific examples of how these methods have been used to further our understanding of apicomplexan invasive mechanisms. Throughout the chapter we focus our discussion on Toxoplasmagondii, because T. gondii is the most experimentally accessible model organism for studying apicomplexan invasion (discussed further in the section, "Toxoplasma as a Model Apicomplexan") and more is known about invasion in T. gondii than in any other apicomplexan.

A novel actin-related protein is associated with daughter cell formation in Toxoplasma gondii

Cell division in Toxoplasma gondii occurs by an unusual budding mechanism termed endodyogeny, during which twin daughters are formed within the body of the mother cell. Cytokinesis begins with the coordinated assembly of the inner membrane complex (IMC), which surrounds the growing daughter cells. The IMC is compiled of both flattened membrane cisternae and subpellicular filaments composed of articulin-like proteins attached to underlying singlet microtubules. While proteins that comprise the elongating IMC have been described, little is known about its initial formation. Using Toxoplasma as a model system, we demonstrate that actin-like protein 1 (ALP1) is partially redistributed to the IMC at early stages in its formation. Immunoelectron microscopy localized ALP1 to a discrete region of the nuclear envelope, on transport vesicles, and on the nascent IMC of the daughter cells prior to the arrival of proteins such as IMC-1. The overexpression of ALP1 under the control of a strong constitutive promoter disrupted the formation of the daughter cell IMC, leading to delayed growth and defects in nuclear and apicoplast segregation. Collectively, these data suggest that ALP1 participates in the formation of daughter cell membranes during cell division in apicomplexan parasites.

Microneme rhomboid protease TgROM1 is required for efficient intracellular growth of Toxoplasma gondii

Rhomboids are serine proteases that cleave their substrates within the transmembrane domain. Toxoplasma gondii contains six rhomboids that are expressed in different life cycle stages and localized to different cellular compartments. Toxoplasma rhomboid protein 1 (TgROM1) has previously been shown to be active in vitro, and the orthologue in Plasmodium falciparum processes the essential microneme protein AMA1 in a heterologous system. We investigated the role of TgROM1 to determine its role during in vitro growth of T. gondii. TgROM1 was localized in the secretory pathway of the parasite, including the Golgi apparatus and micronemes, which contain adhesive proteins involved in invasion of host cells. However, unlike other micronemal proteins, TgROM1 was not released onto the parasite surface during cell invasion, suggesting it does not play a critical role in cell invasion. Suppression of TgROM1 using the tetracycline-regulatable system revealed that ROM1-deficient parasites were outcompeted by wild-type T. gondii. ROM1-deficient parasites showed only modest decrease in invasion but replicated more slowly than wild-type cells. Collectively, these results indicate that ROM1 is required for efficient intracellular growth by T. gondii.

Molecular genetic transfection of the coccidian parasite Sarcocystis neurona

Sarcocystis neurona is an apicomplexan parasite that is the major cause of equine protozoal myeloencephalitis (EPM). The biology of this pathogen remains poorly understood in part due to unavailability of molecular genetic tools. Hence, with an objective to develop DNA transfection capabilities for S. neurona, the 5' flanking region of the SnSAG1 gene was isolated from a genomic library and used to construct expression plasmids. In transient assays, the reporter molecules beta-galactosidase (beta-gal) and yellow fluorescent protein (YFP) could be detected in electroporated S. neurona, thereby confirming the feasibility of transgene expression in this organism. Stable transformation of S. neurona was achieved using a mutant dihydrofolate reductase thymidylate synthase (DHFR-TS) gene of Toxoplasma gondii that confers resistance to pyrimethamine. This selection system was used to create transgenic S. neurona that stably express beta-gal and YFP. As shown in this study, these transgenic clones can be useful for analyzing growth rate of parasites in vitro and for assessing drug sensitivities. More importantly, the DNA transfection methods described herein should greatly facilitate studies examining intracellular parasitism by this important coccidian pathogen.

Nourseothricin acetyltransferease: a positive selectable marker for Toxoplasma gondii

Molecular analysis of parasite genomes will require new molecular genetic tools. The nat1 gene of Streptomyces noursei encodes nourseothricin acetyltransferase, conferring resistance to the aminoglycoside antibiotic nourseothricin. Electroporation of nat1 cassettes into RH or Prugniaud strains of Toxoplasma gondii allows for selection of stable nourseothricin-resistant clones.

Fluorescent protein tagging in Toxoplasma gondii: identification of a novel inner membrane complex component conserved among Apicomplexa

Toxoplasma gondii is an obligate intracellular parasite, and its sub-cellular organization shows clear adaptations to this life-style. In addition to organelles shared among all eukaryotes, the organism possesses a number of specialized compartments with important roles in host cell invasion and intra-cellular survival. These unique aspects of the parasite's biology are also reflected in its genome. The ongoing genome sequencing efforts for T. gondii and related apicomplexans predict a high proportion of genes unique to the phylum, which lack homologs in other model organisms. Knowing the sub-cellular localization of these gene products will be an important first step towards their functional characterization. We used a library approach wherein parasite genomic DNA was fused to the yellow fluorescent protein (YFP) gene. Parasites transformed with this library were screened by flow cytometry and fluorescence microscopy. Clones tagged in a wide variety of sub-cellular compartments (nucleus, mitochondria, ER, dense granules (secreted), spliceosome, plasma membrane, apicoplast, inner membrane complex) were isolated and confirmed using compartment specific markers. Clones with tags in parasite-specific localizations were subjected to insert rescue and phenotypic verification using an in vitro recombination system. Among the genes identified is a novel inner membrane complex gene (IMC3) conserved among Apicomplexa.

Toxoplasma gondii: the model apicomplexan

Toxoplasma gondii is an obligate intracellular protozoan parasite which is a significant human and veterinary pathogen. Other members of the phylum Apicomplexa are also important pathogens including Plasmodium species (i.e. malaria), Eimeria species, Neospora, Babesia, Theileria and Cryptosporidium. Unlike most of these organisms, T. gondii is readily amenable to genetic manipulation in the laboratory. Cell biology studies are more readily performed in T. gondii due to the high efficiency of transient and stable transfection, the availability of many cell markers, and the relative ease with which the parasite can be studied using advanced microscopic techniques. Thus, for many experimental questions, T. gondii remains the best model system to study the biology of the Apicomplexa. Our understanding of the mechanisms of drug resistance, the biology of the apicoplast, and the process of host cell invasion has been advanced by studies in T. gondii. Heterologous expression of apicomplexan proteins in T. gondii has frequently facilitated further characterisation of proteins that could not be easily studied. Recent studies of Apicomplexa have been complemented by genome sequencing projects that have facilitated discovery of surprising differences in cell biology and metabolism between Apicomplexa. While results in T. gondii will not always be applicable to other Apicomplexa, T. gondii remains an important model system for understanding the biology of apicomplexan parasites.

Characterisation of Toxoplasma gondii engineered to express mouse interferon-gamma

Recent studies have shown the feasibility of using Toxoplasma gondii as an expression system for heterologous protein. For better understanding of the mechanism of interferon-gamma (IFN-gamma) dependent immunity to T. gondii, the parasites were stably transfected with IFN-gamma gene, under control of the GRA1 promoter. Immunofluorescence analyses showed that recombinant mouse IFN-gamma localised to discrete punctuate structures consistent with dense granules and secreted into the vacuolar space. The production of IFN-gamma was detectable in both extracellular parasites and the parasite-infected cells. Growth of the recombinant parasites was inhibited in the mouse macrophage cell line (J774A.1 cells), but not in monkey kidney adherent fibroblasts (Vero cells), demonstrating the species-specificity of IFN-gamma. Addition of anti-mouse IFN-gamma antibody resulted in growth recovery of the recombinant parasites, suggesting that IFN-gamma, secreted from the parasitised host cells across the parasitophorous vacuole membrane, acted in a paracrine manner. Reverse transcription (RT)-PCR analysis revealed significant expression of inducible nitric oxide synthase mRNA and high levels of nitric oxide production in recombinant parasite-infected J774A.1 cells. A competitive inhibitor of the L-arginine-dependent effector pathway, N(G)-monomethyl-L-arginine, inhibited the reduction of recombinant parasite growth in J774A.1 cells. Taken together, our data suggest that the T. gondii expression system may provide a new tool for cytokine gene expression and that parasites engineered to express a cytokine gene may be rationally designed for use in studies on immune responses to T. gondii.

Biogenesis of nanotubular network in Toxoplasma parasitophorous vacuole induced by parasite proteins

The intracellular parasite Toxoplasma gondii develops within a nonfusogenic vacuole containing a network of elongated nanotubules that form connections with the vacuolar membrane. Parasite secretory proteins discharged from dense granules (known as GRA proteins) decorate this intravacuolar network after invasion. Herein, we show using specific gene knockout mutants, that the unique nanotubule conformation of the network is induced by the parasite secretory protein GRA2 and further stabilized by GRA6. The vacuolar compartment generated by GRA2 knockout parasites was dramatically disorganized, and the normally tubular network was replaced by small aggregated material. The defect observed in Deltagra2 parasites was evident from the initial stages of network formation when a prominent cluster of multilamellar vesicles forms at a posterior invagination of the parasite. The secretory protein GRA6 failed to localize properly to this posterior organizing center in Deltagra2 cells, indicating that this early conformation is essential to proper assembly of the network. Construction of a Deltagra6 mutant also led to an altered mature network characterized by small vesicles instead of elongated nanotubules; however, the initial formation of the posterior organizing center was normal. Complementation of the Deltagra2 knockout with mutated forms of GRA2 showed that the integrity of both amphipathic alpha-helices of the protein is required for correct formation of the network. The induction of nanotubues by the parasite protein GRA2 may be a conserved feature of amphipathic alpha-helical regions, which have also been implicated in the organization of Golgi nanotubules and endocytic vesicles in mammalian cells.

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."

Selection based on the expression of antisense hypoxanthine-xanthine-guanine-phosphoribosyltransferase RNA in Toxoplasma gondii

We have previously shown that an antisense RNA strategy can be used to inhibit the expression of hypoxanthine-xanthine-guanine-phosphoribosyltransferase (HXGPRT) in Toxoplasma gondii [Nakaar et al., J. Biol. Chem. 1999;274:5083-5087]. Here, we report that parasites rendered deficient in HXGPRT by antisense RNA are resistant to high doses of 6-thioxanthine (6-TX). We have exploited this finding to develop a selection procedure. In this scheme, parasites transfected with a chimeric construct harboring the bacterial chloramphenicol acetyl transferase (CAT) reporter gene linked to antisense HXGPRT gene were selected in 6-TX to inhibit the growth of tachyzoites expressing endogenous HXGPRT. Concomitant with a reduction in HXGPRT levels by antisense RNA, 6-TX(R) parasites displayed reporter CAT activity. These data indicate that transfection of antisense HXGPRT gene provides a means to select for parasites expressing foreign or altered genes in T. gondii. These findings also suggest, in principle, that antisense RNA can be used as a strategy to generate selectable markers employing genes that encode enzymes with known subversive substrates.

The development and biology of bradyzoites of Toxoplasma gondii

Toxoplasma gondii is a protozoan parasite of mammals and birds that is an important human pathogen. Infection with this Apicomplexan parasite results in its dissemination throughout its host via the tachyzoite life-stage. After dissemination these tachyzoites differentiate into bradyzoites within cysts that remain latent. These bradyzoites can transform back into tachyzoites and in immunosupressed individuals this often results in symptomatic disease. Both tachyzoites and bradyzoites develop in tissue culture and thus this crucial differentiation event can be studied. Recent advances in the genetic manipulation of T. gondii have expanded the molecular tools that can be applied to studies on bradyzoite differentiation. Evidence is accumulating that this differentiation event is stress mediated and may share common pathways with other stress-induced differentiation events in other eukaryotic organisms. Study of the stress response and signaling pathways are areas of active research in this organism. In addition, characterization of unique bradyzoite-specific structures, such as the cyst wall, should lead to a further understanding of T. gondii biology. This review focuses on the biology and development of bradyzoites and current approaches to the study of the tachyzoite to bradyzoite differentiation process.

Experimental approaches to understanding virulence in toxoplasmosis

Toxoplasma gondii is a widespread protozoan parasite that causes severe disease only in immunocompromised individuals. Equipped with excellent animal models and relatively advanced systems for genetics, T. gondii provides an excellent system for understanding pathogenesis. Resistance to toxoplasmosis is governed by rapid innate and adaptive immunity that is characterized by a Th1 type profile of cytokines. Despite this effective response, acute infections can cause considerable damage and the parasite effectively establishes a long-term chronic infection that predisposes the host to reactivation and provides a means of eventual transmission. This complex interaction is brought about by the differentiation of the parasite from a rapidly replicating, lytic form (known as the tachyzoite) to a slow-growing form (known as the bradyzoite) that gives rise to chronic infection. The population structure of T. gondii is remarkably clonal, consisting of just three predominant lineages that are geographically widespread and found in a variety of hosts including humans. Acute virulence is strongly associated with the type I genotype which exhibits an enhanced replication rate in vitro and higher tissue burdens in vivo relative to non-virulent lineages. The pathology associated with acute infection appears to be due to excessive production of acute inflammatory mediators, suggesting that disease is partly due to over-response of the host immune system. A combination of refined animal models and newly developed genetic tools for establishing the relative contribution of genes to pathogenesis will enable a comprehensive analysis of the molecular basis of virulence in toxoplasmosis.

Characterization of tGLP-1, a Golgi and lysosome-associated, transmembrane glycoprotein of African trypanosomes

Purification of endosomal/lysosomal vesicles of Trypanosoma brucei brucei bloodstream forms and generation of monoclonal antibodies led to the isolation of antibodies directed against an 85 kDa, Golgi and endocytic traffic-associated protein termed tGLP-1, Trypanosoma Golgi/lysosome protein-1. Preliminary immunoelectron microscopical analysis revealed that the protein is present in, but not restricted to, the limiting membrane of multivesicular lysosomes and is more abundant in bloodstream forms compared to the procyclic stage. The corresponding gene was cloned and is present as a single copy. Blast searches did not reveal any homologies to other proteins and genes published. The nucleotide sequence of the gene (1848 base pairs) predicted a type 1 membrane topology with an N-terminal signal sequence (20 aa), a luminal domain with 2 N-glycosylation sites (524 aa), a transmembrane domain (23 aa), and a long cytosolic tail domain (49 aa). Polyclonal antibodies raised against the cytosolic tail confirmed the localization of the gene product to multivesicular lysosomes but revealed that the majority of the protein was in the Golgi apparatus. Colabelling with an antibody against p67, a lysosomal glycoprotein of trypanosomes, revealed extensive overlap between the proteins with opposing relative abundance. Expression of the tGLP-1 open reading frame in Leishmania resulted in Golgi localization, and in Toxoplasma, in localization to both the Golgi and endoplasmic reticulum. These data indicate conservation in the functionality of the Golgi-targeting sequence of tGLP-1.

Identification of the pro-mature processing site of Toxoplasma ROP1 by mass spectrometry

The rhoptries are specialized secretory organelles that function during host cell invasion in the obligate intracellular parasite Toxoplasma gondii. All T. gondii rhoptry proteins studied to date are synthesized as pro-proteins that are then processed to their mature forms. To understand the role of the pro region in rhoptry protein function, we have precisely defined the processing site of the pro-region of the rhoptry protein ROP1. Efforts to determine such processing sites have been prevented by blocked N-termini of mature proteins isolated from T. gondii. To overcome this problem, we have used an engineered form of ROP1 and mass spectrometry to demonstrate that proROP1 is processed to its mature form between the glutamic acid at position 83 and alanine at position 84. These data also show that mature ROP1 lacks substantial post-translational modifications, a result which has important implications for targeting of rhoptry proteins.

Targeted disruption of the GRA2 locus in Toxoplasma gondii decreases acute virulence in mice

Following invasion into the host cell, the protozoan Toxoplasma gondii secretes a variety of proteins that modify the parasitophorous vacuole. Within the vacuole, the 28-kDa dense granule protein known as GRA2 is specifically targeted to the tubulovesicular network which forms connections with the vacuolar membrane. To investigate the importance of GRA2, we derived from strain RH a mutant T. gondii line in which GRA2 was disrupted by replacement with the marker Ble (selecting for phleomycin resistance). The Deltagra2 mutant invaded and grew normally in both fibroblasts and macrophages in vitro; however, it was less virulent during acute infection in mice. The survival rate of mice inoculated with Deltagra2 was significantly higher; some infected mice survived the acute infection, whereas all mice infected with the wild-type strain RH succumbed to early death. Chronic infection by Deltagra2 was detected by positive serology, immunohistochemical detection of parasites and cysts in the brain, and reisolation of parasites by bioassay at 6 weeks postinfection. Thus, absence of GRA2 partially attenuates the virulence of T. gondii during the acute phase of infection and allows for establishment of chronic infection by the otherwise highly virulent RH strain. These results establish that GRA2 plays an important role during in vivo infection and provide a potential model for examining acute pathogenesis by T. gondii.

The amphipathic alpha helices of the toxoplasma protein GRA2 mediate post-secretory membrane association

The Toxoplasma gondii protein GRA2 is secreted into the parasite-containing vacuole where it is rapidly and specifically targeted to a network of membranous tubules that connect with the vacuolar membrane. To examine the molecular basis of this association, we expressed an HA9 epitope-tagged form of GRA2 by stable transformation of Toxoplasma. GRA2-HA9 was correctly packaged inside the dense granules, secreted into the PV and targeted to the network, as shown by immunoelectron microscopy, immunofluorescence and cell fractionation. Expression of deletion mutants of GRA2-HA9 lacking either of two amphipathic alpha helices resulted in the production and secretion of soluble proteins which were unable to stably associate with the network. A mutant in which the amino acids of the first alpha helix were rearranged to a non-amphipathic pattern localized correctly to the network but failed to remained stably associated with the membrane. Collectively, these results demonstrate that targeting and membrane association occur by separate mechanisms and that the combination of both alpha helices is essential for stable localization of GRA2 to the network.

Expression, selection, and organellar targeting of the green fluorescent protein in Toxoplasma gondii

We have engineered a mutant version of the green fluorescent protein GFP (Cormack et al. Selected for bright fluorescence in E. coli. Gene 1996;173:33-38) for expression in the protozoan parasite Toxoplasma gondii. Although intact GFP was not expressed at any detectable level, GFP fusion proteins could be detected by fluorescence microscopy, flow cytometry (FACS), and immunoblotting. Both extracellular tachyzoites and T. gondii-infected host cells could readily be sorted by FACS, which should facilitate a variety of selection strategies. Several selectable markers were tested for their ability to produce stable green transgenic parasites. Fluorescence intensity was directly correlated with gene copy number and protein expression level. Weak selectable markers such as chloramphenicol acetyl transferase (CAT) driven by the SAG1 promoter, which yield multicopy insertions, are therefore most effective for selecting green fluorescent parasites-particularly when coupled to constructs which employ a strong promoter to drive GFP expression. Transformation vectors developed in the course of this work should be of general utility for the overexpression of heterologous transgenes in Toxoplasma. CAT-GFP fusion proteins were expressed in the parasite cytoplasm. GFP fusions to the P30 major surface antigen (linked on the same plasmid to a CAT selectable marker under control of various promoters) could be detected in dense granules within living cells, and were efficiently secreted into the parasitophorous vacuole. GFP fusions to the rhoptry protein ROP1 were targeted to rhoptries (specialized secretory organelles at the apical end of the parasite).

Gene knock-outs and allelic replacements in Toxoplasma gondii: HXGPRT as a selectable marker for hit-and-run mutagenesis

The hypoxanthine-xanthine-guanine phosphoribosyl transferase (HXGPRT) gene of the protozoan parasite Toxoplasma gondii encodes a safe, practical genetic marker suitable for both positive and negative selection. Taking advantage of the ability to control homologous versus nonhomologous recombination in haploid T. gondii tachyzoites by manipulating the length of homologous DNA sequence, we have explored the possibility of 'hit-and-run' mutagenesis to introduce gene knock-outs (or allelic replacements) at loci for which no known selection or screen is available. Using the uracil phosphoribosyl transferase (UPRT) locus as a target, a genomic clone containing approximately 8 kb encompassing the UPRT gene (but lacking essential coding sequence) was fused to a cDNA-derived HXGPRT 'minigene', which lacks sufficient contiguous genomic sequence for homologous recombination. After transfection of circular plasmid DNA, positive selection for HXGPRT activity identified stable transformants, > 30% of which were found to have integrated at the UPRT locus as 'pseudodiploids' (produced by single-site homologous recombination between the circular plasmid and genomic DNA). Upon removal of mycophenolic acid, resolution of pseudodiploids by spontaneous intrachromosomal homologous recombination was selected using 6-thioxanthine, yielding a 1:1 ratio of UPRT knock-out parasites to wild-type revertants, at frequencies of approximately 10(-6) per parasite doubling. Applications of 'hit-and-run' technology relative to other gene targeting strategies are discussed.

Targeting the secretory pathway of Toxoplasma gondii

Little is known about the extent of conservation in the organization of the secretory pathway in organisms as different as prokaryotes, eukaryotes, and humans. The protozoan parasite Toxoplasma gondii allows easy genetic manipulations, and numerous vectors for selection of transgenic parasites have been developed. One approach to study the molecular mechanism of protein sorting and trafficking is the expression of foreign proteins. Here we describe the design and application of a vector that targets proteins to the secretory pathway of T. gondii and yields high-level expression of Escherichia coli reporter proteins. The general strategies and potential problems in expressing foreign proteins in T. gondii are discussed.

Development of molecular genetics for Neospora caninum: A complementary system to Toxoplasma gondii

The development of molecular genetics has greatly enhanced the study of Toxoplasma gondii, and investigations into the biology and pathology associated with neosporosis will be similarly benefited by the development of molecular tools for Neospora caninum. We have demonstrated the feasibility of using the existing DNA vectors developed for T. gondii to transfect and transform the Nc-1 strain of Neospora. We have also shown that T. gondii proteins are faithfully expressed and targeted in N. caninum, indicating the suitability of using Neospora as a heterologous expression system for studying T. gondii. These studies provide the basis for initiating molecular genetic studies on N. caninum and will allow for a number of molecular comparisons of these two closely related, though phenotypically distinct, parasites. Here we describe the methods and reagents used to perform genetic manipulations of N. caninum, and we present some of the principles and potential utilities of these molecular studies, including the use of N. caninum as a heterologous system for the study of T. gondii proteins.

Baculovirus immediate-early promoter-mediated expression of the Zeocin resistance gene for use as a dominant selectable marker in dipteran and lepidopteran insect cell lines

The antibiotic Zeocin, a derivative of phleomycin, was evaluated for use as a selection system in both dipteran and lepidopteran insect cell lines. Growth of Drosophila cell lines, Kc1 and SL2, was inhibited at Zeocin concentrations of 50 and 75 microg/ml, respectively, while the Spodoptera cell line, Sf9, was inhibited at a concentration of 250 microg/ml Zeocin. The mammalian cytomegalovirus (CMV) and Simian virus 40 (SV40) early promoters did not function in these insect cell lines. Several baculovirus-derived immediate-early (IE) promoters from the Orgyia pseudotsugata multicapsid nucleopolyhedrovirus (OpMNPV) and Autographa californica multicapsid nucleopolyhedrovirus (AcMNPV) were used to drive expression of the Zeocin resistance gene (ble) in these cell lines. The resulting plasmid vectors enabled selection of Zeocin-resistant cell lines within 3-4 weeks. Gene amplification events in the presence of increasing Zeocin concentrations were not detected using Southern blot analysis. Furthermore, the function of the baculovirus IE promoters, as demonstrated by beta-galactosidase expression, was not detectable in a variety of mammalian cell lines tested. A cloning/shuttle vector, containing ten unique restriction sites, was constructed which allows for selection of Zeocin resistance in insect cell lines and in Escherichia coli.

Molecular genetic strategies in Toxoplasma gondii: close in on a successful invader

Toxoplasma gondii is an obligate intracellular parasite with an exceptional ability to invade, survive and replicate within nearly all nucleated cells. Upon differentiation into an encysted form (bradyzoites), the parasites escape the host immune defenses and thus persist long enough in man and other hosts to ensure maintenance of transmission. This protozoan parasite has long been known to cause severe congenital infections in humans and animals but has recently gained additional notoriety as an opportunistic pathogen associated with AIDS. Development of a DNA transfection system for T. gondii has provided a new tool for exploring molecular aspects of important processes such as invasion and differentiation. Additional strategies associated with genetic transformation have been devised and elaboration of even more desirable molecular tools is in progress.

Insertional tagging, cloning, and expression of the Toxoplasma gondii hypoxanthine-xanthine-guanine phosphoribosyltransferase gene. Use as a selectable marker for stable transformation

A nonhomologous integration vector was used to identify the Toxoplasma gondii hypoxanthine-xanthine-guanine phosphoribosyl transferase (HXGPRT) gene by insertional mutagenesis. Parasite mutants resistant to 6-thioxanthine arose at a frequency of approximately3 x 10(-7). Genomic DNA flanking the insertion sites was retrieved by marker rescue and used to identify molecular clones exhibiting unambiguous homology to H(X)GPRT genes from other species. Sequence analysis of vector/genome junction sites reveals that integration of the linearized vector occurred with minimal rearrangement of either vector or target sequences, although the addition of filler DNA and small duplications or deletions of genomic sequences at the transgene termini was observed. Two differentially spliced classes of cDNA clones were identified, both of which complement hpt and gpt mutations in Escherichia coli. Kinetic analysis of purified recombinant enzyme revealed no significant differences between the two isoforms. Internally deleted clones spanning the genomic locus were used to create "knock-out" parasites, which lack all detectable HXGPRT activity. Complete activity could be restored to these knock-out mutants by transient transformation with either genomic DNA or cDNA-derived minigenes encoding both enzyme isoforms. Stable HXGPRT+ transformants were isolated under selection with mycophenolic acid, demonstrating the feasibility of HXGPRT as both a positive and negative selectable marker for stable transformation of T. gondii.