An Ion-exchange Bone Demineralization Method for Improved Time, Expense, and Tissue Preservation

Here, we describe an ethylenediaminetetraacetic acid (EDTA)-based bone demineralization procedure that uses cation-exchange resin and dialysis tubing. This method does not require solution changes or special equipment, is faster than EDTA alone, is cost-effective, and is environmentally friendly. Like other EDTA-based methods, this procedure yields superior tissue preservation than formic acid demineralization. Greater protein antigenicity using EDTA as opposed to formic acid has been described, but we also find significant improvements in carbohydrate-based histological staining. Histological staining using this method reveals cartilage layers that are not distinguishable with formic acid demineralization. Carbohydrate preservation is relevant to many applications of bone demineralization, including the assessment of osteoarthritis from bone biopsies and the use of demineralized bone powder for tissue culture and surgical implants. The improvements in time, expense, and tissue quality indicate this method is a practical and often superior alternative to formic acid demineralization.

Forward genetic analysis of the apicomplexan cell division cycle in Toxoplasma gondii

Apicomplexa are obligate intracellular pathogens that have fine-tuned their proliferative strategies to match a large variety of host cells. A critical aspect of this adaptation is a flexible cell cycle that remains poorly understood at the mechanistic level. Here we describe a forward genetic dissection of the apicomplexan cell cycle using the Toxoplasma model. By high-throughput screening, we have isolated 165 temperature sensitive parasite growth mutants. Phenotypic analysis of these mutants suggests regulated progression through the parasite cell cycle with defined phases and checkpoints. These analyses also highlight the critical importance of the peculiar intranuclear spindle as the physical hub of cell cycle regulation. To link these phenotypes to parasite genes, we have developed a robust complementation system based on a genomic cosmid library. Using this approach, we have so far complemented 22 temperature sensitive mutants and identified 18 candidate loci, eight of which were independently confirmed using a set of sequenced and arrayed cosmids. For three of these loci we have identified the mutant allele. The genes identified include regulators of spindle formation, nuclear trafficking, and protein degradation. The genetic approach described here should be widely applicable to numerous essential aspects of parasite biology.

Inhibition of Toxoplasma gondii growth by pyrrolidine dithiocarbamate is cell cycle specific and leads to population synchronization

Successful completion of the Toxoplasma cell cycle requires the coordination of a series of complex and ordered processes that results in the formation of two daughters by internal budding. Although we now understand the order and timing of intracellular events associated with the parasite cell cycle, the molecular details of the checkpoints that regulate each step in Toxoplasma gondii division is still uncertain. In other eukaryotic cells, the use of cytostatic inhibitors that are able to arrest replication at natural checkpoints have been exploited to induce synchronization of population growth. Herein, we describe a novel method to synchronize T. gondii tachyzoites based on the reversible growth inhibition by the drug and pyrrolidine dithiocarbamate. This method is an improvement over other strategies developed for this parasites as no prior genetic manipulation of the parasite was required. RH tachyzoites blocked by pyrrolidine dithiocarbamate exhibited a near uniform haploid DNA content and single centrosome indicating that this compound arrests parasites in the G1 phase of the tachyzoite cell cycle with a minor block in late cytokinesis. Thus, these studies support the existence of a natural checkpoint that regulates passage through the G1 period of the cell cycle. Populations released from pyrrolidine dithiocarbamate inhibition completed progression through G1 and entered S phase approximately 2 h post-drug release. The transit of drug-synchronized populations through S phase and mitosis followed a similar timeframe to previous studies of the tachyzoite cell cycle. Tachyzoites treated with pyrrolidine dithiocarbamate were fully viable and completed two identical division cycles post-drug release demonstrating that this is a robust method for synchronizing population growth in Toxoplasma.

Toxoplasma co-opts host gene expression by injection of a polymorphic kinase homologue

Toxoplasma gondii, an obligate intracellular parasite of the phylum Apicomplexa, can cause severe disease in humans with an immature or suppressed immune system. The outcome of Toxoplasma infection is highly dependent on the strain type, as are many of its in vitro growth properties. Here we use genetic crosses between type II and III lines to show that strain-specific differences in the modulation of host cell transcription are mediated by a putative protein kinase, ROP16. Upon invasion by the parasite, this polymorphic protein is released from the apical organelles known as rhoptries and injected into the host cell, where it ultimately affects the activation of signal transducer and activator of transcription (STAT) signalling pathways and consequent downstream effects on a key host cytokine, interleukin (IL)-12. Our findings provide a new mechanism for how an intracellular eukaryotic pathogen can interact with its host and reveal important differences in how different Toxoplasma lineages have evolved to exploit this interaction.

Changes in the expression of human cell division autoantigen-1 influence Toxoplasma gondii growth and development

Toxoplasma is a significant opportunistic pathogen in AIDS, and bradyzoite differentiation is the critical step in the pathogenesis of chronic infection. Bradyzoite development has an apparent tropism for cells and tissues of the central nervous system, suggesting the need for a specific molecular environment in the host cell, but it is unknown whether this environment is parasite directed or the result of molecular features specific to the host cell itself. We have determined that a trisubstituted pyrrole acts directly on human and murine host cells to slow tachyzoite replication and induce bradyzoite-specific gene expression in type II and III strain parasites but not type I strains. New mRNA synthesis in the host cell was required and indicates that novel host transcripts encode signals that were able to induce parasite development. We have applied multivariate microarray analyses to identify and correlate host gene expression with specific parasite phenotypes. Human cell division autoantigen-1 (CDA1) was identified in this analysis, and small interfering RNA knockdown of this gene demonstrated that CDA1 expression causes the inhibition of parasite replication that leads subsequently to the induction of bradyzoite differentiation. Overexpression of CDA1 alone was able to slow parasite growth and induce the expression of bradyzoite-specific proteins, and thus these results demonstrate that changes in host cell transcription can directly influence the molecular environment to enable bradyzoite development. Investigation of host biochemical pathways with respect to variation in strain type response will help provide an understanding of the link(s) between the molecular environment in the host cell and parasite development.

Genetic rescue of a Toxoplasma gondii conditional cell cycle mutant

Growth rate is a major pathogenesis factor in the parasite Toxoplasma gondii; however, how cell division is controlled in this protozoan is poorly understood. Herein, we show that centrosomal duplication is an indicator of S phase entry while centrosome migration marks mitotic entry. Using the pattern of centrosomal replication, we confirmed that mutant ts11C9 undergoes a bimodal cell cycle arrest that is characterized by two subpopulations containing either single or duplicated centrosomes which correlate with the bipartite genome distribution observed at the non-permissive temperature. Genetic rescue of ts11C9 was performed using a parental RH strain cDNA library, and the cDNA responsible for conferring temperature resistance (growth at 40 degrees C) was recovered by recombination cloning. A single T. gondii gene encoding the protein homologue of XPMC2 was responsible for genetic rescue of the temperature-sensitive defect in ts11C9 parasites. This protein is a known suppressor of mitotic defects, and in tachyzoites, TgXPMC2-YFP localized to the parasite nucleus and nucleolus which is consistent with the expected subcellular localization of critical mitotic factors. Altogether, these results demonstrate that ts11C9 is a conditional mitotic mutant containing a single defect which influences two distinct control points in the T. gondii tachyzoite cell cycle.

Identification of a sporozoite-specific member of the Toxoplasma SAG superfamily via genetic complementation

Toxoplasma gondii sporozoites possess an array of stage-specific antigens that are localized to the membrane and internal cellular space, as well as secreted into the primary parasitophorous vacuole. Specific labelling of viable sporozoites excysted from oocysts reveals a complex admixture of surface proteins partially shared with tachyzoites. SAG1, SRS3 and SAG3 were detected on sporozoites as well as numerous minor antigens. In contrast, tachyzoite SAG2A and B were completely absent whereas a dominant 25 kDa protein was unique to the sporozoite surface. The sporozoite gene encoding this protein was identified in tachyzoites genetically complemented with a sporozoite cDNA library and cloned via site-specific recombination into a bacterial shuttle vector. The sporozoite cDNA identified in these experiments encoded a protein with conserved structural features of the prototypical T. gondii SAG1 (P30) and shared sequence identity with surface proteins from Sarcocystis spp. This new member of the SAG superfamily was designated SporoSAG. Expression of SporoSAG in tachyzoites conferred enhanced invasion on transgenic parasites suggesting a role for this protein in oocyst/sporozoite transmission to susceptible hosts.

Gene discovery in the apicomplexa as revealed by EST sequencing and assembly of a comparative gene database

Large-scale EST sequencing projects for several important parasites within the phylum Apicomplexa were undertaken for the purpose of gene discovery. Included were several parasites of medical importance (Plasmodium falciparum, Toxoplasma gondii) and others of veterinary importance (Eimeria tenella, Sarcocystis neurona, and Neospora caninum). A total of 55192 ESTs, deposited into dbEST/GenBank, were included in the analyses. The resulting sequences have been clustered into nonredundant gene assemblies and deposited into a relational database that supports a variety of sequence and text searches. This database has been used to compare the gene assemblies using BLAST similarity comparisons to the public protein databases to identify putative genes. Of these new entries, approximately 15%-20% represent putative homologs with a conservative cutoff of p < 10(-9), thus identifying many conserved genes that are likely to share common functions with other well-studied organisms. Gene assemblies were also used to identify strain polymorphisms, examine stage-specific expression, and identify gene families. An interesting class of genes that are confined to members of this phylum and not shared by plants, animals, or fungi, was identified. These genes likely mediate the novel biological features of members of the Apicomplexa and hence offer great potential for biological investigation and as possible therapeutic targets.

Defining the cell cycle for the tachyzoite stage of Toxoplasma gondii

Tachyzoite endodyogeny is characterized by a three phase cell cycle comprised of major G1 and S phases with mitosis following immediately upon the conclusion of DNA replication. Cytokinesis, which begins with the formation of daughter apical complexes, initiates in late S phase and overlaps mitosis. There is no evidence to support an extended G2 period in these parasites. In all strains, parasites with a 2 N DNA content are a relatively small subpopulation and when tachyzoites expressing a fluorescent nuclear marker (green-fluorescent-protein fused to proliferating-cell-nuclear-antigen) were observed by time-lapse microscopy, there appeared to be little delay between S phase and mitosis. Measurements of the DNA content of RH parasites by flow cytometry demonstrated that the G1 and S periods were approximately 60 and approximately 30% of a single division cycle, although these phases were longer in strains that display a slower growth rate. The overall length of S phase was determined by [3H]-thymidine autoradiography using transgenic parasites expressing herpes simplex thymidine kinase and validated by Northern analysis of S phase specific genes during synchronous growth. The fraction of S phase parasites by flow cytometry paralleled autoradiography, however, within S phase, the distribution of parasites was bimodal in all strains examined. Parasites containing a 1-1.7 N DNA complement were a small fraction when compared to the major S phase population which contained a near-diploid ( approximately 1.8 N) complement, suggesting parasites in late S phase have a slower rate of DNA replication. In lieu of a short or missing G2, where checkpoints are thought to operate in other eukaryotes, the bimodal replication of tachyzoite chromosomes may represent a distinct premitotic checkpoint associated with endodyogeny.

Toxoplasma gondii bradyzoites form spontaneously during sporozoite-initiated development

Tachyzoites (VEG strain) that emerge from host cells infected with Toxoplasma gondii sporozoites proliferate relatively fast and double their number every 6 h. This rate of growth is intrinsic, as neither the number of host cells invaded nor host cell type appears to influence emergent tachyzoite replication. Fast tachyzoite growth was not persistent, and following approximately 20 divisions, the population uniformly shifted to slower growth. Parasites 10 days post-sporozoite infection doubled only once every 15 h and, unlike emergent tachyzoites, they grew at this slower rate over several months of continuous cell culture. The spontaneous change in tachyzoite growth rate preceded the expression of the bradyzoite-specific marker, BAG1. Within 24 h of the growth shift, 2% of the population expressed BAG1, and by 15 days post-sporozoite infection, 50% of the parasites were positive for this marker. Spontaneous BAG1 expression was not observed in sporozoites or in tachyzoites during fast growth (through day 6 post-sporozoite inoculation), although these tachyzoites could be induced to express BAG1 earlier by culturing sporozoite-infected cells at pH 8.3. However, alkaline treatment also reduced the replication of emergent tachyzoites to the rate of growth-shifted parasites, supporting a link between reduced parasite growth and bradyzoite differentiation. The shift to slower growth was closely correlated with virulence in mice, as the initially fast-growing emergent tachyzoites were avirulent (100% lethal dose, >10(4) parasites), while a mutant VEG strain (MS-J) that is unable to growth shift caused 100% mortality in mice inoculated with 10 parasites. Parasites recovered from gamma interferon knockout mice inoculated with emergent tachyzoites grew at a slow rate and expressed BAG1, confirming that the replication switch occurs in animals and in the absence of a protective immune response.

Toxoplasma gondii sporozoites form a transient parasitophorous vacuole that is impermeable and contains only a subset of dense-granule proteins

Toxoplasma gondii sporozoites form two parasitophorous vacuoles during development within host cells, the first (PV1) during host cell invasion and the second (PV2) 18 to 24 h postinoculation. PV1 is structurally distinctive due to its large size, yet it lacks a tubulovesicular network (C. A. Speer, M. Tilley, M. Temple, J. A. Blixt, J. P. Dubey, and M. W. White, Mol. Biochem. Parasitol. 75:75-86, 1995). Confirming the finding that sporozoites have a different electron-dense-granule composition, we have now found that sporozoites within oocysts lack the mRNAs encoding the 5' nucleoside triphosphate hydrolases (NTPase). NTPase first appears 12 h postinfection. Other tachyzoite dense-granule proteins, GRA1, GRA2, GRA4, GRA5, and GRA6, were detected in oocyst extracts, and antibodies against these proteins stained granules in the sporozoite cytoplasm. In contrast to tachyzoite invasion of host cells, however, sporozoites did not exocytose the dense-granule proteins GRA1, GRA2, or GRA4 during PV1 formation. Even after NTPase induction, these proteins were retained within cytoplasmic granules rather than being secreted into PV1. Only GRA5 was secreted by the sporozoite during host cell invasion, becoming associated with the membrane surrounding PV1. Microinjection of sporozoite-infected cells with fluorescent dyes showed that PV1 is impermeable to fluorescent dyes with molecular masses as small as 330 Da, indicating that PV1 lacks channels through which molecules can pass from the host cytoplasm into the vacuole. By contrast, lucifer yellow rapidly diffused into PV2, demonstrating the presence of molecular channels. These studies indicate that PV1 and PV2 are morphologically, immunologically, and functionally distinct, and that PV2 appears to be identical to the tachyzoite vacuole. The inaccessibility of PV1 to host cell nutrients may explain why parasite replication does not occur in this vacuole.