A cell cycle model for the tachyzoite of Toxoplasma gondii using the Herpes simplex virus thymidine kinase

Phosphoproteomic Analysis Reveals the Predominating Cellular Processes and the Involved Key Phosphoproteins Essential for the Proliferation of Toxoplasma gondii

PURPOSE

To explore the essential roles of phosphorylation in mediating the proliferation of T. gondii in its cell lytic life.

METHODS

We profiled the phosphoproteome data of T. gondii residing in HFF cells for 2 h and 6 h, representing the early- and late-stages of proliferation (ESP and LSP) within its first generation of division.

RESULTS

We identified 70 phosphoproteins, among which 8 phosphoproteins were quantified with the phosphorylation level significantly regulated. While only two of the eight phosphoproteins, GRA7 and TGGT1_242070, were significantly down-regulated at the transcriptional level in the group of LSP vs. ESP. Moreover, GO terms correlated with host membrane component were significantly enriched in the category of cellular component, suggesting phosphoprotein played important roles in acquiring essential substance from host cell via manipulating host membrane. Further GO analysis in the categories of molecular function and biological process and pathway analysis revealed that the cellular processes of glucose and lipid metabolism were regulated by T. gondii phosphoproteins such as PMCAA1, LIPIN, Pyk1 and ALD. Additionally, several phosphoproteins were enriched at the central nodes in the protein-protein interaction network, which may have essential roles in T. gondii proliferation including GAP45, MLC1, fructose-1,6-bisphosphate aldolase, GRAs and so on.

CONCLUSION

This study revealed the main cellular processes and key phosphoproteins crucial for the intracellular proliferation of T. gondii, which would provide clues to explore the roles of phosphorylation in regulating the development of tachyzoites and new insight into the mechanism of T. gondii development in vitro.

A single-parasite transcriptional atlas of Toxoplasma Gondii reveals novel control of antigen expression

Toxoplasma gondii, a protozoan parasite, undergoes a complex and poorly understood developmental process that is critical for establishing a chronic infection in its intermediate hosts. Here, we applied single-cell RNA-sequencing (scRNA-seq) on >5,400 Toxoplasma in both tachyzoite and bradyzoite stages using three widely studied strains to construct a comprehensive atlas of cell-cycle and asexual development, revealing hidden states and transcriptional factors associated with each developmental stage. Analysis of SAG1-related sequence (SRS) antigenic repertoire reveals a highly heterogeneous, sporadic expression pattern unexplained by measurement noise, cell cycle, or asexual development. Furthermore, we identified AP2IX-1 as a transcription factor that controls the switching from the ubiquitous SAG1 to rare surface antigens not previously observed in tachyzoites. In addition, comparative analysis between Toxoplasma and Plasmodium scRNA-seq results reveals concerted expression of gene sets, despite fundamental differences in cell division. Lastly, we built an interactive data-browser for visualization of our atlas resource.

Alpha-Herpesvirus Thymidine Kinase Genes Mediate Viral Virulence and Are Potential Therapeutic Targets

Alpha-herpesvirus thymidine kinase (TK) genes are virulence-related genes and are nonessential for viral replication; they are often preferred target genes for the construction of gene-deleted attenuated vaccines and genetically engineered vectors for inserting and expressing foreign genes. The enzymes encoded by TK genes are key kinases in the nucleoside salvage pathway and have significant substrate diversity, especially the herpes simplex virus 1 (HSV-1) TK enzyme, which phosphorylates four nucleosides and various nucleoside analogues. Hence, the HSV-1 TK gene is exploited for the treatment of viral infections, as a suicide gene in antitumor therapy, and even for the regulation of stem cell transplantation and treatment of parasitic infection. This review introduces the effects of α-herpesvirus TK genes on viral virulence and infection in the host and classifies and summarizes the current main application domains and potential uses of these genes. In particular, mechanisms of action, clinical limitations, and antiviral and antitumor therapy development strategies are discussed.

Apicomplexa Cell Cycles: Something Old, Borrowed, Lost, and New

Increased parasite burden is linked to the severity of clinical disease caused by Apicomplexa parasites such as Toxoplasma gondii, Plasmodium spp, and Cryptosporidium. Pathogenesis of apicomplexan infections is greatly affected by the growth rate of the parasite asexual stages. This review discusses recent advances in deciphering the mitotic structures and cell cycle regulatory factors required by Apicomplexa parasites to replicate. As the molecular details become clearer, it is evident that the highly unconventional cell cycles of these parasites is a blending of many ancient and borrowed elements, which were then adapted to enable apicomplexan proliferation in a wide variety of different animal hosts.

Comparative studies of Toxoplasma gondii transcriptomes: insights into stage conversion based on gene expression profiling and alternative splicing

Background

Toxoplasma gondii is one of the most important apicomplexan parasites and infects one-third of the human population worldwide. Transformation between the tachyzoite and bradyzoite stages in the intermediate host is central to chronic infection and life-long risk. There have been some transcriptome studies on T. gondii; however, we are still early in our understanding of the kinds and levels of gene expression that occur during the conversion between stages.

Results

We used high-throughput RNA-sequencing data to assemble transcripts using genome-based and de novo strategies. The expression-level analysis of 6996 T. gondii genes showed that over half (3986) were significantly differentially expressed during stage conversion, whereas 2205 genes were upregulated, and 1778 genes were downregulated in tachyzoites compared with bradyzoites. Several important gene families were expressed at relatively high levels. Comprehensive functional annotation and gene ontology analysis revealed that stress response-related genes are important for survival of bradyzoites in immune-competent hosts. We compared Trinity-based de novo and genome-based strategies, and found that the de novo assembly strategy compensated for the defects of the genome-based strategy by filtering out several transcripts with low expression or those unannotated on the genome. We also found some inaccuracies in the ToxoDB gene models. In addition, our analysis revealed that alternative splicing can be differentially regulated in response to life-cycle change. In depth analysis revealed a 20-nt, AG-rich sequence, alternative splicing locus from alt_acceptor motif search in tachyzoite.

Conclusion

This study represents the first large-scale effort to sequence the transcriptome of bradyzoites from T. gondii tissue cysts. Our data provide a comparative view of the tachyzoite and bradyzoite transcriptomes to allow a more complete dissection of all the molecular regulation mechanisms during stage conversions. A better understanding of the processes regulating stage conversion may guide targeted interventions to disrupt the transmission of T. gondii.

Electronic supplementary material

The online version of this article (10.1186/s13071-018-2983-5) contains supplementary material, which is available to authorized users.

The Toxoplasma Centrocone Houses Cell Cycle Regulatory Factors

Our knowledge of cell cycle regulatory mechanisms in apicomplexan parasites is very limited. In this study, we describe a novel Toxoplasma gondii factor that has a vital role in chromosome replication and the regulation of cytoplasmic and nuclear mitotic structures, and we named this factor ECR1 for essential for chromosome replication 1. ECR1 was discovered by complementation of a temperature-sensitive (ts) mutant that suffers lethal, uncontrolled chromosome replication at 40°C similar to a ts mutant carrying a defect in topoisomerase. ECR1 is a 52-kDa protein containing divergent RING and TRAF-Sina-like zinc binding domains that are dynamically expressed in the tachyzoite cell cycle. ECR1 first appears in the unique spindle compartment of the Apicomplexa (centrocone) of the nuclear envelope in early S phase and then in the nucleus in late S phase where it reaches maximum expression. Following nuclear division, but before daughter parasites separate from the mother parasite, ECR1 is downregulated and is absent in new daughter parasites. The proteomics of ECR1 identified interactions with the ubiquitin-mediated protein degradation machinery and the minichromosome maintenance complex, and the loss of ECR1 led to increased stability of a key member of this complex, MCM2. ECR1 also forms a stable complex with the cyclin-dependent kinase (CDK)-related kinase, Tgondii Crk5 (TgCrk5), which displays a similar cell cycle expression and localization during tachyzoite replication. Importantly, the localization of ECR1/TgCrk5 in the centrocone indicates that this Apicomplexa-specific spindle compartment houses important regulatory factors that control the parasite cell cycle.IMPORTANCE Parasites of the apicomplexan family are important causes of human disease, including malaria, toxoplasmosis, and cryptosporidiosis. Parasite growth is the underlying cause of pathogenesis, yet despite this importance, the molecular basis for parasite replication is poorly understood. Filling this knowledge gap cannot be accomplished by mining recent whole-genome sequencing data because apicomplexan cell cycles differ substantially and lack many of the key regulatory factors of well-studied yeast and mammalian cell division models. We have utilized forward genetics to discover essential factors that regulate cell division in these parasites using the Toxoplasma gondii model. An example of this approach is described here with the discovery of a putative E3 ligase/protein kinase mechanism involved in regulating chromosome replication and mitotic processes of asexual stage parasites.

Checkpoints of apicomplexan cell division identified in Toxoplasma gondii

The unusual cell cycles of Apicomplexa parasites are remarkably flexible with the ability to complete cytokinesis and karyokinesis coordinately or postpone cytokinesis for several rounds of chromosome replication, and are well recognized. Despite this surprising biology, the molecular machinery required to achieve this flexibility is largely unknown. In this study, we provide comprehensive experimental evidence that apicomplexan parasites utilize multiple Cdk-related kinases (Crks) to coordinate cell division. We determined that Toxoplasma gondii encodes seven atypical P-, H-, Y- and L- type cyclins and ten Crks to regulate cellular processes. We generated and analyzed conditional tet-OFF mutants for seven TgCrks and four TgCyclins that are expressed in the tachyzoite stage. These experiments demonstrated that TgCrk1, TgCrk2, TgCrk4 and TgCrk6, were required or essential for tachyzoite growth revealing a remarkable number of Crk factors that are necessary for parasite replication. G1 phase arrest resulted from the loss of cytoplasmic TgCrk2 that interacted with a P-type cyclin demonstrating that an atypical mechanism controls half the T. gondii cell cycle. We showed that T. gondii employs at least three TgCrks to complete mitosis. Novel kinases, TgCrk6 and TgCrk4 were required for spindle function and centrosome duplication, respectively, while TgCrk1 and its partner TgCycL were essential for daughter bud assembly. Intriguingly, mitotic kinases TgCrk4 and TgCrk6 did not interact with any cyclin tested and were instead dynamically expressed during mitosis indicating they may not require a cyclin timing mechanism. Altogether, our findings demonstrate that apicomplexan parasites utilize distinctive and complex mechanisms to coordinate their novel replicative cycles.

Apicomplexan parasites are unicellular eukaryotes that replicate in unusual ways different from their multicellular hosts. From a single infection, different apicomplexans can produce as few as two or up to many hundreds of progeny. How these flexible division cycles are regulated is poorly understood. In the current study we have defined the major mechanisms controlling the growth of the Toxoplasma gondii acute pathogenic stage called the tachyzoite. We show that T. gondii tachyzoites require not only multiple protein kinases to coordinate chromosome replication and the assembly of new daughter parasites, but also each kinase has unique responsibilities. By contrast, the mammalian cell that T. gondii infects requires far fewer kinase regulators to complete cell division, which suggests that these parasites have unique vulnerabilities. The increased complexity in parasite cell cycle controls likely evolved from the need to adapt to different hosts and the need to construct the specialized invasion apparatus in order to invade those hosts.

Role of NETs in the difference in host susceptibility to Toxoplasma gondii between sheep and cattle

The main aim of this study was to compare extracellular traps (NETs) formation by polymorphonuclear neutrophils (PMNs) of cattle and sheep when exposed to T. gondii tachyzoites in vitro. The effects of parasite concentrations and different incubation periods on NETs development in cattle and sheep PMNs were studied. The effect of NET structures on host cell invasion by tachyzoites was also studied. This is the first report of NETs development by sheep and cattle PMNs against T. gondii in vitro. T. gondii-induced extracellular DNA production from PMNs was dependent on tachyzoite concentrations and incubation time in both sheep and cattle. Many nuclear and cytoplasmic changes were observed in sheep and cattle PMNs after exposure to T. gondii tachyzoites. The typical appearance of NETs, with MPO, NE and histone (H3) attached to extracellular DNA, was observed. Tachyzoites were entrapped within this structure. Myeloperoxidase (MPO) activity was higher in the cattle PMN-tachyzoite co-cultures than sheep. NETs structures released from sheep PMNs caused mechanical immobilisation of T. gondii tachyzoites, however, NET structures released from cattle PMNs may be lethal to tachyzoites. Bovine MPO may have a lethal effect on T. gondii tachyzoites in vitro during a 3h incubation. Besides other mechanisms that effect on host susceptibility to T. gondii in sheep and cattle, extracellular traps formation as a part of immunological reactions may be play a role in host susceptibility to T. gondii.

Toxoplasma gondii MAF1b Binds the Host Cell MIB Complex To Mediate Mitochondrial Association

Many diverse intracellular pathogens, such as Legionella pneumophila, Chlamydia psittaci, Encephalitozoon sp., and Toxoplasma gondii, manipulate and relocate host cell organelles, including mitochondria. Toxoplasma tachyzoites use a secreted protein, mitochondrial association factor 1b (MAF1b), to drive the association between the host mitochondria and the membrane of the parasitophorous vacuole, in which the parasites grow. The identity of the host partner in this interaction, however, has not previously been identified. By exogenously expressing tagged MAF1b in mouse embryonic fibroblasts, we were able to isolate host cell proteins that specifically interact with MAF1b. We then verified these interactions in the MAF1b-expressing fibroblasts, as well as in the context of parasite infection in human fibroblasts and HeLa cells. The results show that a host cell mitochondrial complex, the mitochondrial intermembrane space bridging (MIB) complex, specifically interacts with MAF1b. We further demonstrate that a version of MAF1b that is deficient in host-mitochondrial association does not efficiently coprecipitate the MIB complex. Validation of the importance of the MAF1b-MIB interaction came from showing that knockdown of two MIB complex components, MIC60 and SAM50, substantially reduces mitochondrial association with the parasitophorous vacuole membrane. This interaction between a secreted membrane-integral parasite protein and a membrane-bound complex of a host organelle represents the first instance of organelle relocalization in which both the host and pathogen molecules are known and provides the foundation for more detailed biochemical studies. IMPORTANCE Parasites interact intimately with their hosts, and the interactions shape both parties. The common human parasite Toxoplasma gondii replicates exclusively in a vacuole in a host cell and alters its host cell's environment through secreted proteins. One of these secreted proteins, MAF1b, acts to concentrate mitochondria around the parasite's vacuole, and this relocalization alters the host immune response. Many other intracellular pathogens also recruit host mitochondria, but the identities of the partners that mediate this interaction have not previously been described in any infection. Here, we show that Toxoplasma MAF1b binds to the multifunctional MIB protein complex on the host mitochondria. Reducing the levels of the proteins in this mitochondrial complex reduces the close association of host cell mitochondria and the parasite's vacuole. This work provides new insight into a key host-pathogen interaction and identifies possible targets for future therapeutic intervention as well as a more molecular understanding of important biology.

Toxoplasma gondii: Laboratory Maintenance and Growth

Toxoplasma gondii is a highly successful apicomplexan protozoan capable of infecting any warm-blooded animal worldwide. In humans, Toxoplasma infections are life-long, with approximately one-third of the world's population chronically infected. Although normally controlled by the host immune system, T. gondii infection can lead to a variety of clinical outcomes in individuals with immature or suppressed immune systems. After penetrating the intestine, parasites rapidly disseminate throughout the body and stimulate production of the cytokines interleukin (IL)-12, IL-18, and interferon (IFN)-γ by immune cells. These cytokines play a key role in host resistance to T. gondii by promoting a strong Th1 response. Recent reports show that gut commensal bacteria can act as molecular adjuvants during T. gondii infection. Thus, T. gondii is an excellent model system to study host-pathogen interactions. This unit outlines the protocols for in vitro and in vivo maintenance and growth of T. gondii. © 2017 by John Wiley & Sons, Inc.

The Ubiquitin Proteome of Toxoplasma gondii Reveals Roles for Protein Ubiquitination in Cell-Cycle Transitions

Protein ubiquitination plays key roles in protein turnover, cellular signaling, and intracellular transport. The genome of Toxoplasma gondii encodes ubiquitination machinery, but the roles of this posttranslational modification (PTM) are unknown. To examine the prevalence and function of ubiquitination in T. gondii, we mapped the ubiquitin proteome of tachyzoites. Over 500 ubiquitin-modified proteins, with almost 1,000 sites, were identified on proteins with diverse localizations and functions. Enrichment analysis demonstrated that 35% of ubiquitinated proteins are cell-cycle regulated. Unexpectedly, most classic cell-cycle regulators conserved in T. gondii were not detected in the ubiquitinome. Furthermore, many ubiquitinated proteins localize to the cytoskeleton and inner membrane complex, a structure beneath the plasma membrane facilitating division and host invasion. Comparing the ubiquitinome with other PTM proteomes reveals waves of PTM enrichment during the cell cycle. Thus, T. gondii PTMs are implicated as critical regulators of cell division and cell-cycle transitions.

Apicomplexan cell cycle flexibility: centrosome controls the clutch

The centrosome serves as a central hub coordinating multiple cellular events in eukaryotes. A recent study in Toxoplasma gondii revealed a unique bipartite structure of the centrosome, which coordinates the nuclear cycle (S-phase and mitosis) and budding cycle (cytokinesis) of the parasite, and deciphers the principle behind flexible apicomplexan cell division modes.

A novel bipartite centrosome coordinates the apicomplexan cell cycle

Apicomplexan parasites can change fundamental features of cell division during their life cycles, suspending cytokinesis when needed and changing proliferative scale in different hosts and tissues. The structural and molecular basis for this remarkable cell cycle flexibility is not fully understood, although the centrosome serves a key role in determining when and how much replication will occur. Here we describe the discovery of multiple replicating core complexes with distinct protein composition and function in the centrosome of Toxoplasma gondii. An outer core complex distal from the nucleus contains the TgCentrin1/TgSfi1 protein pair, along with the cartwheel protein TgSas-6 and a novel Aurora-related kinase, while an inner core closely aligned with the unique spindle pole (centrocone) holds distant orthologs of the CEP250/C-Nap protein family. This outer/inner spatial relationship of centrosome cores is maintained throughout the cell cycle. When in metaphase, the duplicated cores align to opposite sides of the kinetochores in a linear array. As parasites transition into S phase, the cores sequentially duplicate, outer core first and inner core second, ensuring that each daughter parasite inherits one copy of each type of centrosome core. A key serine/threonine kinase distantly related to the MAPK family is localized to the centrosome, where it restricts core duplication to once per cycle and ensures the proper formation of new daughter parasites. Genetic analysis of the outer core in a temperature-sensitive mutant demonstrated this core functions primarily in cytokinesis. An inhibition of ts-TgSfi1 function at high temperature caused the loss of outer cores and a severe block to budding, while at the same time the inner core amplified along with the unique spindle pole, indicating the inner core and spindle pole are independent and co-regulated. The discovery of a novel bipartite organization in the parasite centrosome that segregates the functions of karyokinesis and cytokinesis provides an explanation for how cell cycle flexibility is achieved in apicomplexan life cycles.

The apicomplexan parasite Toxoplasma gondii has a unique centrosome with two specialized compartments, potentially explaining the remarkable flexibility in life cycle that these organisms can show in diverse host cells.

Apicomplexan parasites infect many different hosts and tissues, causing numerous human diseases, including malaria. These important pathogens have a peculiar cell cycle in which chromosomes sometimes amplify to remarkable levels, followed by concerted cell division—providing an unusual proliferative capacity. This capacity for proliferation, combined with an ability to change the scale of replication when needed, are hallmarks of the cell cycles of these parasites. Yet the molecular mechanism responsible for these peculiar cell cycles remains one of the unsolved mysteries of Apicomplexa biology. Here we show that the centrosome—an organelle that orchestrates several aspects of the cell cycle—of the apicomplexan parasite Toxoplasma gondii contains specialized structures that coordinate parasite cell division. Our findings demonstrate that a two-part centrosomal architecture, comprising an inner and an outer core with distinct protein compositions, segregates the processes of mitosis from the assembly of new daughter parasites. The modular organization of the centrosome offers an explanation for how cell division can be suspended while the parasites amplify their genome to the biotic scale required for their life cycles. It is unknown whether these distinct centrosome core complexes evolved independently in Apicompexa. Another possibility is that the foundations for these mechanisms were present in the original eukaryote, which could explain how the distinct extranuclear centrosome of animal cells and the novel yeast spindle pole body of the nuclear envelope may have evolved from a common ancestor.

Identifying novel cell cycle proteins in Apicomplexa parasites through co-expression decision analysis

Hypothetical proteins comprise roughly half of the predicted gene complement of Toxoplasma gondii and Plasmodium falciparum and represent the largest class of uniquely functioning proteins in these parasites. Following the idea that functional relationships can be informed by the timing of gene expression, we devised a strategy to identify the core set of apicomplexan cell division cycling genes with important roles in parasite division, which includes many uncharacterized proteins. We assembled an expanded list of orthologs from the T. gondii and P. falciparum genome sequences (2781 putative orthologs), compared their mRNA profiles during synchronous replication, and sorted the resulting set of dual cell cycle regulated orthologs (744 total) into protein pairs conserved across many eukaryotic families versus those unique to the Apicomplexa. The analysis identified more than 100 ortholog gene pairs with unknown function in T. gondii and P. falciparum that displayed co-conserved mRNA abundance, dynamics of cyclical expression and similar peak timing that spanned the complete division cycle in each parasite. The unknown cyclical mRNAs encoded a diverse set of proteins with a wide range of mass and showed a remarkable conservation in the internal organization of ordered versus disordered structural domains. A representative sample of cyclical unknown genes (16 total) was epitope tagged in T. gondii tachyzoites yielding the discovery of new protein constituents of the parasite inner membrane complex, key mitotic structures and invasion organelles. These results demonstrate the utility of using gene expression timing and dynamic profile to identify proteins with unique roles in Apicomplexa biology.

Cell division in apicomplexan parasites

Toxoplasma gondii and Plasmodium falciparum are important human pathogens. These parasites and many of their apicomplexan relatives undergo a complex developmental process in the cells of their hosts, which includes genome replication, cell division and the assembly of new invasive stages. Apicomplexan cell cycle progression is both globally and locally regulated. Global regulation is carried out throughout the cytoplasm by diffusible factors that include cell cycle-specific kinases, cyclins and transcription factors. Local regulation acts on individual nuclei and daughter cells that are developing inside the mother cell. We propose that the centrosome is a master regulator that physically tethers cellular components and that provides spatial and temporal control of apicomplexan cell division.

Endothelial cell invasion by Toxoplasma gondii: differences between cell types and parasite strains

Toxoplasma gondii disseminates and causes congenital infection by invasion of the endothelial cells. The aim of this study was to analyze the ability of two strains to invade two endothelial cell types. Tachyzoites of the RH and ME49 strains were expanded in Balb/c and C57BL6-RAG2-/- mice, respectively. Tachyzoites were harvested from 72 h Vero cell cultures and incubated for 30 min to 4 h at 10:1 parasite/cell ratio in 24-well plates, containing monolayers of either HMEC-1 line or human umbilical cells (HUVECs). The number of infected cells and parasitic vacuoles per infected cell were counted in Wright stained slides. A slow increase in the proportion of infected cells occurred but varied according to cell type-parasite strain combination: ME49 tachyzoites invaded up to 63% HMEC-1 cells, while RH parasites infected up to 19% HUVECs. ME49 and RH tachyzoites invaded 49 and 46% HUVECs and HMEC-1 cells, respectively. Reinvasion and formation of new parasitophorous vacuoles of infected cells was more frequent than invasion of noninfected cells. The results support that the factors influencing invasion, and thus dissemination and vertical transmission, are parasite type, host cell type/subtype, and activation state. Interestingly, T. gondii virulence does not seem to relay on its invasion efficiency, but probably on replication speed.

Cell division in Apicomplexan parasites is organized by a homolog of the striated rootlet fiber of algal flagella

Apicomplexan parasites undergo cell division using an evolutionarily conserved mechanism first described in the positioning and assembly of flagella in algae.

Apicomplexa are intracellular parasites that cause important human diseases including malaria and toxoplasmosis. During host cell infection new parasites are formed through a budding process that parcels out nuclei and organelles into multiple daughters. Budding is remarkably flexible in output and can produce two to thousands of progeny cells. How genomes and daughters are counted and coordinated is unknown. Apicomplexa evolved from single celled flagellated algae, but with the exception of the gametes, lack flagella. Here we demonstrate that a structure that in the algal ancestor served as the rootlet of the flagellar basal bodies is required for parasite cell division. Parasite striated fiber assemblins (SFA) polymerize into a dynamic fiber that emerges from the centrosomes immediately after their duplication. The fiber grows in a polarized fashion and daughter cells form at its distal tip. As the daughter cell is further elaborated it remains physically tethered at its apical end, the conoid and polar ring. Genetic experiments in Toxoplasma gondii demonstrate two essential components of the fiber, TgSFA2 and 3. In the absence of either of these proteins cytokinesis is blocked at its earliest point, the initiation of the daughter microtubule organizing center (MTOC). Mitosis remains unimpeded and mutant cells accumulate numerous nuclei but fail to form daughter cells. The SFA fiber provides a robust spatial and temporal organizer of parasite cell division, a process that appears hard-wired to the centrosome by multiple tethers. Our findings have broader evolutionary implications. We propose that Apicomplexa abandoned flagella for most stages yet retained the organizing principle of the flagellar MTOC. Instead of ensuring appropriate numbers of flagella, the system now positions the apical invasion complexes. This suggests that elements of the invasion apparatus may be derived from flagella or flagellum associated structures.

Malaria, toxoplasmosis, and related diseases are caused by infection with unicellular parasites called Apicomplexa. Their name refers to the elaborate invasion machinery that occupies the apical end of the parasite cell. This apparatus allows the parasite to force its way into the cells of its host, and to deliver factors that will manipulate host cell structure, gene expression, and metabolism. Once in the host cell the parasite will begin to grow. The parasite replicates its genome and organelles numerous times and then loads these various elements into numerous daughter cells that will further spread the infection. Here we report a fiber that coordinates the daughter cell budding process. The fiber links the centrosome, which controls the mitotic spindle, and the genome with the microtubule organizing center of the budding daughter. Parasite mutants lacking the proteins that build the fiber fail to form daughter cells at the earliest step. The fiber and its components are remarkably similar to fibers that coordinate flagella in algae. While Apicomplexa are not flagellated (with the exception of certain gamete stages) they evolved from flagellated algae. We propose that elements of the invasion apparatus evolved from the flagellum or flagellum associated structures.

Screening for small molecule inhibitors of Toxoplasma gondii

INTRODUCTION

Toxoplasma gondii, the agent that causes toxoplasmosis, is an opportunistic parasite that infects many mammalian species. It is an obligate intracellular parasite that causes severe congenital neurological and ocular disease mostly in immunocompromised humans. The current regimen of therapy includes only a few medications that often lead to hypersensitivity and toxicity. In addition, there are no vaccines available to prevent the transmission of this agent. Therefore, safer and more effective medicines to treat toxoplasmosis are urgently needed.

AREAS COVERED

The author presents in silico and in vitro strategies that are currently used to screen for novel targets and unique chemotypes against T. gondii. Furthermore, this review highlights the screening technologies and characterization of some novel targets and new chemical entities that could be developed into highly efficacious treatments for toxoplasmosis.

EXPERT OPINION

A number of diverse methods are being used to design inhibitors against T. gondii. These include ligand-based methods, in which drugs that have been shown to be efficacious against other Apicomplexa parasites can be repurposed to identify lead molecules against T. gondii. In addition, structure-based methods use currently available repertoire of structural information in various databases to rationally design small-molecule inhibitors of T. gondii. Whereas the screening methods have their advantages and limitations, a combination of methods is ideally suited to design small-molecule inhibitors of complex parasites such as T. gondii.

Mechanisms of Toxoplasma gondii persistence and latency

Toxoplasma gondii is an obligate intracellular protozoan parasite that causes opportunistic disease, particularly in immunocompromised individuals. Central to its transmission and pathogenesis is the ability of the proliferative stage (tachyzoite) to convert into latent tissue cysts (bradyzoites). Encystment allows Toxoplasma to persist in the host and affords the parasite a unique opportunity to spread to new hosts without proceeding through its sexual stage, which is restricted to felids. Bradyzoite tissue cysts can cause reactivated toxoplasmosis if host immunity becomes impaired. A greater understanding of the molecular mechanisms orchestrating bradyzoite development is needed to better manage the disease. Here, we will review key studies that have contributed to our knowledge about this persistent form of the parasite and how to study it, with a focus on how cellular stress can signal for the reprogramming of gene expression needed during bradyzoite development.

Cactin is essential for G1 progression in Toxoplasma gondii

Toxoplasma gondii is an obligate intracellular protozoan parasite whose rapid lytic replication cycles define its pathogenicity. We identified a temperature-sensitive growth mutant, FV-P6, which irreversibly arrests before the middle of the G1 stage of the tachyzoite cell cycle. This arrest is caused by a point mutation in a gene conserved across eukaryotes, Cactin, whose product localizes to the nucleus. To elucidate the role of TgCactin we performed genome-wide expression profiling. Besides the expected G1 expression profile, many genes associated with the extracellular state as well as with the bradyzoite cyst stage were identified. Consistent with these profiles were the expression of AP2 transcription factors typically associated with extracellular and bradyzoite stage parasites. This suggests a role for TgCactin in control of gene expression. As TgCactin does not contain any functionally defined domains we reasoned TgCactin exerts its function through interactions with other proteins. In support of this model we demonstrated that TgCactin is present in a protein complex and can oligomerize. Taken together, these results suggest that TgCactin acts as a pivotal protein potentially regulating gene expression at several transition points in parasite development.

Optimized expression of dual reporter genes in transient transfection of purified Toxoplasma gondii using different promoters

Fluorescent protein and luciferase genes are valuable reporter genes and have been widely used for noninvasive monitoring of gene expression in living tissues and cells. We tested expression of the dual reporter genes in transient transfection of purified Toxoplasma gondii tachyzoites. Two copies of the enhanced yellow fluorescent protein (EYFP) gene were put under the control of 3 representative T. gondii promoters (GRA1, SAG1, and DHFR). Fluorescence from each EYFP reporter was significantly higher than that from a green fluorescent protein (GFP) reporter. The GRA1-EYFP reporter gave the highest fluorescence. Although both fluorescence and luciferase were expressed in the dual reporter system, the luciferase reporter was more efficient than either the EYFP or GFP reporters, and it required fewer parasites to be successfully used.

Nuclear actin-related protein is required for chromosome segregation in Toxoplasma gondii

Apicomplexa parasites use complex cell cycles to replicate that are not well understood mechanistically. We have established a robust forward genetic strategy to identify the essential components of parasite cell division. Here we describe a novel temperature sensitive Toxoplasma strain, mutant 13-20C2, which growth arrests due to a defect in mitosis. The primary phenotype is the mis-segregation of duplicated chromosomes with chromosome loss during nuclear division. This defect is conditional-lethal with respect to temperature, although relatively mild in regard to the preservation of the major microtubule organizing centers. Despite severe DNA loss many of the physical structures associated with daughter budding and the assembly of invasion structures formed and operated normally at the non-permissive temperature before completely arresting. These results suggest there are coordinating mechanisms that govern the timing of these events in the parasite cell cycle. The defect in mutant 13-20C2 was mapped by genetic complementation to Toxoplasma chromosome III and to a specific mutation in the gene encoding an ortholog of nuclear actin-related protein 4. A change in a conserved isoleucine to threonine in the helical structure of this nuclear actin related protein leads to protein instability and cellular mis-localization at the higher temperature. Given the age of this protist family, the results indicate a key role for nuclear actin-related proteins in chromosome segregation was established very early in the evolution of eukaryotes.

Synchronization of pathogenic protozoans

Protozoans are single-cell eukaryotes and many of the best studied protozoans are parasitic to humans (e.g., Plasmodium falciparum causing malaria and Trypanosoma brucei causing sleeping sickness). These organisms are distantly related to humans but with retained eukaryotic type of cellular processes, making them good model systems for studies of the evolution of basic processes like the cell cycle. Giardia intestinalis causes 250 million cases of diarrhea yearly and is one of the earliest diverging protozoans. It has recently been possible to synchronize its cell cycle using compounds that inhibit different steps of the cell cycle and the detailed protocol is described here.

T. gondii RP promoters & knockdown reveal molecular pathways associated with proliferation and cell-cycle arrest

Molecular pathways regulating rapid proliferation and persistence are fundamental for pathogens but are not elucidated fully in Toxoplasma gondii. Promoters of T. gondii ribosomal proteins (RPs) were analyzed by EMSAs and ChIP. One RP promoter domain, known to bind an Apetela 2, bound to nuclear extract proteins. Promoter domains appeared to associate with histone acetyl transferases. To study effects of a RP gene's regulation in T. gondii, mutant parasites (Δrps13) were engineered with integration of tetracycline repressor (TetR) response elements in a critical location in the rps13 promoter and transfection of a yellow fluorescent-tetracycline repressor (YFP-TetR). This permitted conditional knockdown of rps13 expression in a tightly regulated manner. Δrps13 parasites were studied in the presence (+ATc) or absence of anhydrotetracycline (-ATc) in culture. -ATc, transcription of the rps13 gene and expression of RPS13 protein were markedly diminished, with concomitant cessation of parasite replication. Study of Δrps13 expressing Myc-tagged RPL22, -ATc, showed RPL22 diminished but at a slower rate. Quantitation of RNA showed diminution of 18S RNA. Depletion of RPS13 caused arrest of parasites in the G1 cell cycle phase, thereby stopping parasite proliferation. Transcriptional differences ±ATc implicate molecules likely to function in regulation of these processes. In vitro, -ATc, Δrps13 persists for months and the proliferation phenotype can be rescued with ATc. In vivo, however, Δrps13 could only be rescued when ATc was given simultaneously and not at any time after 1 week, even when L-NAME and ATc were administered. Immunization with Δrps13 parasites protects mice completely against subsequent challenge with wildtype clonal Type 1 parasites, and robustly protects mice against wildtype clonal Type 2 parasites. Our results demonstrate that G1 arrest by ribosomal protein depletion is associated with persistence of T. gondii in a model system in vitro and immunization with Δrps13 protects mice against subsequent challenge with wildtype parasites.

Cell cycle-dependent, intercellular transmission of Toxoplasma gondii is accompanied by marked changes in parasite gene expression

Intracellular microbes have evolved efficient strategies for transitioning from one cell to another in a process termed intercellular transmission. Here we show that host cell transmission of the obligate intracellular parasite Toxoplasma gondii is closely tied to specific cell cycle distributions, with egress and reinvasion occurring most proficiently by parasites in the G1 phase. We also reveal that Toxoplasma undergoes marked changes in mRNA expression when transitioning from the extracellular environment to its intracellular niche. These mRNA level changes reflect a modal switch from expression of proteins involved in invasion, motility and signal transduction in extracellular parasites to expression of metabolic and DNA replication proteins in intracellular parasites. Host cell binding and signalling associated with the discharge of parasite secretory proteins was not sufficient to induce this switch in gene expression, suggesting that the regulatory mechanisms responsible are tied to the establishment of the intracellular environment. The genes whose expression increased after parasite invasion belong to a progressive cascade known to underlie the parasite division cycle indicating that the unique relationship between the G1 phase and invasion effectively synchronizes short-term population growth. This work provides new insight into how this highly successful parasite competently transits from cell to cell.

Quantitative trait loci mapping reveals candidate pathways regulating cell cycle duration in Plasmodium falciparum

Background

Elevated parasite biomass in the human red blood cells can lead to increased malaria morbidity. The genes and mechanisms regulating growth and development of Plasmodium falciparum through its erythrocytic cycle are not well understood. We previously showed that strains HB3 and Dd2 diverge in their proliferation rates, and here use quantitative trait loci mapping in 34 progeny from a cross between these parent clones along with integrative bioinformatics to identify genetic loci and candidate genes that control divergences in cell cycle duration.

Results

Genetic mapping of cell cycle duration revealed a four-locus genetic model, including a major genetic effect on chromosome 12, which accounts for 75% of the inherited phenotype variation. These QTL span 165 genes, the majority of which have no predicted function based on homology. We present a method to systematically prioritize candidate genes using the extensive sequence and transcriptional information available for the parent lines. Putative functions were assigned to the prioritized genes based on protein interaction networks and expression eQTL from our earlier study. DNA metabolism or antigenic variation functional categories were enriched among our prioritized candidate genes. Genes were then analyzed to determine if they interact with cyclins or other proteins known to be involved in the regulation of cell cycle.

Conclusions

We show that the divergent proliferation rate between a drug resistant and drug sensitive parent clone is under genetic regulation and is segregating as a complex trait in 34 progeny. We map a major locus along with additional secondary effects, and use the wealth of genome data to identify key candidate genes. Of particular interest are a nucleosome assembly protein (PFL0185c), a Zinc finger transcription factor (PFL0465c) both on chromosome 12 and a ribosomal protein L7Ae-related on chromosome 4 (PFD0960c).

Coordinated progression through two subtranscriptomes underlies the tachyzoite cycle of Toxoplasma gondii

BACKGROUND

Apicomplexan parasites replicate by varied and unusual processes where the typically eukaryotic expansion of cellular components and chromosome cycle are coordinated with the biosynthesis of parasite-specific structures essential for transmission.

METHODOLOGY/PRINCIPAL FINDINGS

Here we describe the global cell cycle transcriptome of the tachyzoite stage of Toxoplasma gondii. In dividing tachyzoites, more than a third of the mRNAs exhibit significant cyclical profiles whose timing correlates with biosynthetic events that unfold during daughter parasite formation. These 2,833 mRNAs have a bimodal organization with peak expression occurring in one of two transcriptional waves that are bounded by the transition into S phase and cell cycle exit following cytokinesis. The G1-subtranscriptome is enriched for genes required for basal biosynthetic and metabolic functions, similar to most eukaryotes, while the S/M-subtranscriptome is characterized by the uniquely apicomplexan requirements of parasite maturation, development of specialized organelles, and egress of infectious daughter cells. Two dozen AP2 transcription factors form a series through the tachyzoite cycle with successive sharp peaks of protein expression in the same timeframes as their mRNA patterns, indicating that the mechanisms responsible for the timing of protein delivery might be mediated by AP2 domains with different promoter recognition specificities.

CONCLUSION/SIGNIFICANCE

Underlying each of the major events in apicomplexan cell cycles, and many more subordinate actions, are dynamic changes in parasite gene expression. The mechanisms responsible for cyclical gene expression timing are likely crucial to the efficiency of parasite replication and may provide new avenues for interfering with parasite growth.

3-Methyladenine blocks Toxoplasma gondii division prior to centrosome replication

The apicomplexan Toxoplasma gondii replicates by endodyogeny, in which replicated organelles assemble into nascent daughter buds within the maternal parasite. The mechanisms governing this complex sequence are not understood. We now report that the kinase inhibitor 3-methlyadenine (3-MA) efficiently blocks T. gondii replication. The inhibition could not be attributed to the effects of 3-MA on mammalian phosphatidylinositol 3-kinase and host cell autophagy. Furthermore, we show that accumulation of host lysosomes around the parasitophorous vacuoles was unaffected. Most 3-MA-treated parasites failed to form daughter buds or replicate DNA, indicating arrest in G1 or early S-phase. Some 3-MA-treated parasites displayed abortive cell division, in which nuclear segregation to malformed daughter buds was incomplete or asymmetrical. Electron microscopy revealed the presence of residual body-like structures in many vacuoles, even in the absence of daughter buds. Most treated parasites had otherwise normal morphology and were able to resume replication upon drug removal. 3-MA-treated and control parasites were similar with respect to the extent of Golgi body division and apicoplast elongation; however, treated parasites rarely possessed replicated centrosomes or apicoplasts. These data are suggestive of a generalized blockade of T. gondii cell cycle progression at stages preceding centrosome replication, rather than arrest at a specific checkpoint. We hypothesize that 3-MA treatment triggers a cell cycle pause program that may serve to protect parasites during periods, such as subsequent to egress, when cell cycle progression might be deleterious. Elucidation of the mechanism of 3-MA inhibition may provide insight into the control of parasite growth.

Toxoplasma H2A variants reveal novel insights into nucleosome composition and functions for this histone family

Toxoplasma gondii is an obligate intracellular parasite. Toxoplasmosis is incurable because of its ability to differentiate from the rapidly replicating tachyzoite stage into a latent cyst form (bradyzoite stage). Gene regulation pertinent to Toxoplasma differentiation involves histone modification, but very little is known about the histone proteins in this early branching eukaryote. Here, we report the characterization of three H2A histones, variants H2AX and H2AZ, and a canonical H2A1. H2AZ is the minor parasite H2A member. H2A1 and H2AX both have an SQ motif, but only H2AX has a complete SQ(E/D)varphi (where varphi denotes a hydrophobic residue) known to be phosphorylated in response to DNA damage. We show that a novel H2B variant interacts with H2AZ and H2A1 but not with H2AX. Chromatin immunoprecipitation (ChIP) revealed that H2AZ and H2Bv are enriched at active genes while H2AX is enriched at repressed genes as well as the silent TgIRE repeat element. During DNA damage, we detected an increase in H2AX phosphorylation as well as increases in h2a1 and h2ax transcription. We found that expression of h2ax, but not h2a1 or h2az, increases in bradyzoites generated in vitro. Similar analysis performed on mature bradyzoites generated in vivo, which are arrested in G0, showed that h2az and h2ax are expressed but h2a1 is not, consistent with the idea that h2a1 is the canonical histone orthologue in the parasite. The increase of H2AX, which localizes to silenced areas during bradyzoite differentiation, is consistent with the quiescent nature of this stage of the life cycle. Our results indicate that the early-branching eukaryotic parasite Toxoplasma contains nucleosomes of novel composition, which is likely to impact multiple facets of parasite biology, including the clinically important process of bradyzoite differentiation.

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.

The cell cycle and Toxoplasma gondii cell division: tightly knit or loosely stitched?

The flexibility displayed by apicomplexan parasites to vary their mode of replication has intrigued biologists since their discovery by electron microscopy in the 1960s and 1970s. Starting in the 1990s we began to understand the cell biology of the cytoskeleton elements driving cytokinesis. By contrast, the molecular mechanisms that regulate the various division modes and how they translate into the budding process that uniquely characterizes this parasite family are much less understood. Although growth mechanisms are a neglected area of study, it is an important pathogenic parameter as fast division rounds are associated with fulminant infection whereas slower growth attenuates virulence, as is exploited in some vaccine strains. In this review we summarize a recent body of cell biological experiments that are rapidly leading to an understanding of the events that yield successful mitosis and cytokinesis in Toxoplasma. We place these observations within a cell cycle context with comments on how these events may be regulated by known eukaryotic checkpoints active in fission and budding yeasts as well as mammalian cells. The presence of cell cycle control mechanisms in the Apicomplexa is supported by our findings that identify several cell cycle checkpoints in Toxoplasma. The progress of the cell cycle is ultimately controlled by cyclin-Cdk pair activities, which are present throughout the Apicomplexa. Although many of the known controllers of cyclin-Cdk activity are present, several key controls cannot readily be identified, suggesting that apicomplexan parasites deviate at these points from the higher eukaryotic models. Altogether, new insights in Toxoplasma replication are reciprocally applied to hypothesize how other division modes in the Toxoplasma life cycle and in other Apicomplexa species could be controlled in terms of cell cycle checkpoint regulation.

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.

Organellar dynamics during the cell cycle of Toxoplasma gondii

The protozoan phylum Apicomplexa encompasses approximately 5000 species of obligate intracellular parasites, including those responsible for malaria and toxoplasmosis. Rather than dividing by binary fission, apicomplexans use a remarkable mechanism for replication, assembling daughters de novo within the cytoplasm. Here, we exploit time-lapse microscopy of fluorescent markers targeted to various subcellular structures in Toxoplasma gondii tachyzoites to determine how these unicellular eukaryotes efficiently package a complete set of organelles, maintaining the highly polarized organization necessary for host cell invasion and pathogenesis. Golgi division and elongation of the apicoplast are among the first morphologically observable events, associated with an unusual pattern of centriolar migration. Daughter parasites are assembled on cytoskeletal scaffolding, whose growth proceeds from the apical end, first encapsulating the divided Golgi. Further extension of the cytoskeletal scaffold results in partitioning of the apicoplast, nucleus, endoplasmic reticulum, and finally the mitochondrion, which enters the developing daughters rapidly, but only very late during the division cycle. The specialized secretory organelles (micronemes and rhoptries) form de novo. This distinctive pattern of replication -- in which organellar segregation spans approximately 75% of the cell cycle, completely encompassing S phase -- suggests an unusual mechanism of cell cycle regulation.

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.

Toxoplasma gondii: Virulence of tachyzoites in serum free media at different temperatures

Highly virulent Toxoplasma gondii tachyzoite multiplication was recorded on the 4th and 5th days post cultivation (dpc) in seven selected cell lines either with or without fetal calf serum (FCS) in the maintenance media. The multiplication rate was slightly lower in the absence of FCS. The cell line mono-layers collapsed dying by the 6th day of infection both in presence or absence of FCS at 37 degrees C. Carcinoma of human larynx (Hep2) and Madian Darby Bovine Kidney (MDBK) cell lines were the most suitable for in vitro multiplication, followed by that of African green monkey kidney cells (VERO), pooled kidney from 1-day-old hamster (BHK), rabbit kidney cells (RK13) and human rhabdomyosarcoma (RDA), while Chicken embryo cells (CER) were the least suitable. In absence of FCS, CER, BHK, Hep2, RDA and MDBK were able to maintain virulent tachyzoites at +4 degrees C for 14 days. The infectivity of the tachyzoites was however lower, killing 40% of the inoculated mice. Tachyzoites survived at room temperature, in the dark, for 14 days in Hep2, RDA and MDBK. However, Hep2 was the only one able to keep virulent tachyzoites until 21 dpc at room temperature and at +4 degrees C. Hep2 propagated tachyzoites were still alive but with low infectivity up to 28 dpc. The cell-lines failed to support the development of tachyzoites after 7 dpc at 37 degrees C and after the 35 dpc at lower temperatures.

New host nuclear functions are not required for the modifications of the parasitophorous vacuole of Toxoplasma

The obligate intracellular parasite Toxoplasma develops within a parasitophorous vacuole (PV) uniquely adapted for its survival in mammalian cells. Post-invasion events extensively modify the PV, resulting in interactions with host cell structures. Recent studies emphasized that Toxoplasma is able to co-opt host gene expression, suggesting that host transcriptional activities are required for parasite infection. By using an experimental enucleation model, we investigated the potential need for Toxoplasma to modify its PV by modulating gene expression in the cell wherein it resides. Unexpectedly, cytoplasts can be actively invaded by Toxoplasma and sustain its replication inside a vacuole until egress and transmission to neighbouring cells. Although randomly distributed in the cytoplast, the PV associates with host centrosomes and the Golgi, is surrounded by host microtubules, and recruits host endoplasmic reticulum and mitochondria. Parasites are proficient in diverting exogenous nutrients from the endocytic network of cytoplasts. In enucleated cells invaded by an avirulent strain of T. gondii, the PV can normally transform into cysts. These observations suggest that new host nuclear functions are not proximately required for the post-invasion events underlying the remodelling of the host cell in which the parasites are confined, and therefore for the generation of infectious parasites in vitro.

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.

Molecular genetic tools in Toxoplasma and Plasmodium: achievements and future needs

The recent awarding of the Nobel prize to Andrew Fire and Craig Mello for the discovery of RNA-interference (RNAi) in plants once more demonstrated the importance of basic science in understanding biological mechanisms. Importantly, this discovery led to the establishment of powerful approaches to study gene function in a wide array of organisms. While a robust RNAi-technology remains elusive in apicomplexan parasites, other molecular genetic technologies have been introduced in recent years. Now, in the post genomic era, the task is to apply these methods to validate and functionally dissect an ever-expanding list of putative vaccine and drug candidates. The ultimate aim of such studies is to transform our knowledge of the genome to the knowledge of the phenome and ultimately new intervention strategies in these important pathogenic organisms. However, substantial limitations remain to the current repertoire of available molecular tools, which limits a comprehensive analysis of these candidates, especially of essential genes. This review summarises the methodologies available for functional gene analysis in apicomplexan parasites and discusses further needs in tool development.

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.

Biochemical and genetic analysis of the distinct proliferating cell nuclear antigens of Toxoplasma gondii

The apicomplexa parasite Toxoplasma gondii expresses two distinct proliferating cell nuclear antigens (PCNA) that exhibit distinct patterns of subcellular localization during tachyzoite growth. In all cell cycle phases, TgPCNA1 is concentrated in the nucleus, while TgPCNA2 is only concentrated in the nucleus during S-phase and uniformly distributed throughout the cell during mitosis and early G1-phase. TgPCNA1-GFP and native TgPCNA2 display a punctate staining pattern that is consistent with assembly into replication foci during S-phase; however, TgPCNA2 disassociates from replication foci before TgPCNA1-GFP. Consistent with the distinct pattern of TgPCNA2 cellular localization, homotypic TgPCNA2 interactions were primarily observed by yeast two-hybrid or co-immunoprecipitation analysis. Transgenic parasites in which the TgPCNA2 gene was disrupted displayed a slower growth rate in vitro; however, no difference in DNA polymerase activity, response to chemical mutagens, or recombinational frequency was observed in these mutant clones demonstrating that TgPCNA2 is non-essential in the tachyzoite developmental stage. Heterologous expression of TgPCNA1, but not TgPCNA2, was able to complement a POL30 cold-sensitive yeast strain suggesting that this isoform may serve as a major replisomal factor in T. gondii and is consistent with the failure to disrupt this gene in tachyzoites.

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.

A change in the premitotic period of the cell cycle is associated with bradyzoite differentiation in Toxoplasma gondii

We have demonstrated that bradyzoites return to the tissue-cyst stage by a developmental pathway that is indistinguishable from that initiated by sporozoites. Mature bradyzoites, like sporozoites from oocysts, were non-proliferative as they contained uniform 1N DNA contents, and replication occurred only in parasites that de-differentiated back into tachyzoites. Moreover, tachyzoites emergent from the bradyzoite-initiated pathway underwent a spontaneous growth shift prior to the onset of tissue cyst formation in a timeframe that was identical to cultures infected with sporozoites. In sporozoite-infected cultures, a novel premitotic, near-diploid subpopulation was detected during bradyzoite differentiation that co-expressed tachyzoite and bradyzoite markers. These observations suggest that activation of a G2-related cell cycle mechanism is required during bradyzoite development, and indicates that equivalent cell cycle mechanisms may govern development in the intermediate life cycle regardless of the origin of infection.

KINETICS OF THE INITIAL ROUNDS OF CELL DIVISION OF TOXOPLASMA GONDII

RH strain Toxoplasma gondii tachyzoites that had naturally lysed their host cells were allowed to infect new host cells for a limited amount of time; subsequent parasite cell divisions were observed closely. On the basis of 4 independent trials, the estimated time to first cell division was 9.8 hr postinfection (PI) and was quite variable (95% confidence interval [CI]: 3.1-16.5 hr PI). The estimated time to second cell division was 14.9 hr PI and was less variable (95% CI: 12.1-17.7 hr PI). Few parasites divided before 6 hr PI in these 4 trials. When tachyzoites were derived by forced lysis (scraping an infected host cell culture and passing it through 27-gauge needles), the first parasite cell division occurred much more rapidly than had been observed in any of the trials with parasites derived by natural lysis. When parasites derived by forced lysis were held away from host cells for 3 hr PI, the first cell division was delayed in a manner similar to that seen in parasites derived by natural lysis. No differences were observed in the timing of the second cell division of parasites derived by forced lysis whether or not they had been held away from cells. These studies demonstrate that the conditions to which tachyzoites are exposed during transit from one host cell to another can affect the kinetics of parasite cell division in the new host cell.

Cyclin-dependent kinase TPK2 is a critical cell cycle regulator in Toxoplasma gondii

The Apicomplexan parasite Toxoplasma gondii replicates by endodyogeny, an unusual form of binary fission. We tested the role of TPK2, a homologue of the CDC2 cyclin-dependent kinases, in cell cycle regulation. TPK2 tagged with HA epitope (TPK2-HA-wt) was expressed in mammalian cells as confirmed by Western blot analysis using HA tag and PSTAIRE antibodies. TPK2-HA-wt phosphorylated a peptide from Histone H1, proving that TPK2 is a functional kinase. TPK2-HA-wt coimmunoprecipitated with mammalian cyclins A, B1, D3 and E. Despite being a functional kinase, TPK2 did not rescue Schizosaccharomyces pombe cdc2 and Saccharomyces cerevisiae cdc28 mutant strains. Overexpression of a dominant-negative mutant of TPK2 (TPK2-HA-dn) in T. gondii tachyzoites arrested replication. FACS analysis of tachyzoites expressing TPK2-HA-dn revealed an increase in the fraction of cells in S-phase when compared with TPK2-HA-wt transfected parasites. Expression of TPK2-HA-wt did not arrest tachyzoite replication. No discernable G2 cell cycle block was evident suggesting that cell cycle checkpoints differ in T. gondii from most other eukaryotic cells. These data suggest that TPK2 executes an essential function in T. gondii cell cycle and is likely to be the T. gondii CDC2 orthologue.

Isolation of a Toxoplasma gondii cyclin by yeast two-hybrid interactive screen

GAL4-based yeast two-hybrid cDNA libraries from Toxoplasma gondii RH strain were constructed and screened for interactors of a putative T. gondii cdc2-related kinase, TgCRK2. A screen of 3.2 million transformants yielded a single yeast clone that harbored a protein fusion capable of specifically interacting with TgCRK2. Sequencing revealed the cDNA insert (TgCYC1) had homology to the cyclin class of proteins. The TgCYC1 cDNA fragment was used to probe a conventional T. gondii cDNA library and a 2.65 kb cDNA coding for a predicted protein of 582 amino acids was obtained. Based on comparison with a 5'-RACE product from tachyzoite mRNA, the 2.65 kb cDNA for TgCYC1 appeared to be complete. TgCYC1 had the highest similarity to Plasmodium falciparum CYC1 and displayed sequence characteristics that place it in the cyclin H class of eukaryotic cyclins. In synchronous tachyzoite populations the level of TgCYC1 mRNA was unchanged indicating it is not cell cycle regulated at the mRNA level. TgCYC1 rescues the G(1)/S cyclin cell cycle defect in S. cerevisiae strain DL1 demonstrating that this apicomplexan cyclin can function in an established heterologous model system.

Toxoplasma gondii-infected human myeloid dendritic cells induce T-lymphocyte dysfunction and contact-dependent apoptosis

Dendritic cells ignite adaptive immunity by priming naïve T lymphocytes. Human monocyte-derived dendritic cells (MDDCs) infected with Toxoplasma gondii induce T-lymphocyte gamma interferon production and may thus activate T. gondii-specific immunity. However, we now demonstrate that T. gondii-infected MDDCs are poor at activating T lymphocytes and are unable to induce specific cytotoxic T lymphocytes. On the other hand, MDDCs acquiring nonviable T. gondii antigens directly, or indirectly through captured apoptotic or necrotic cell bodies, induce potent T-lymphocyte activation. T lymphocytes exposed to infected MDDCs are significantly impaired in upregulation of CD69 and CD28, are refractory to activation, and die through contact-dependent apoptosis mediated by an as-yet-unidentified mechanism not requiring Fas, tumor necrosis factor-related apoptosis-inducing ligand, leukocyte function antigen 1, intercellular adhesion molecule 1, tumor necrosis factor alpha, interleukin 10, alpha interferon, gamma interferon, prostaglandins, or reactive nitrogen intermediates. Bystander T lymphocytes that were neither infected nor apoptotic were refractory to activation, suggesting global dysfunction. Immunosuppression and T-lymphocyte unresponsiveness and apoptosis are typical of acute T. gondii infection. Our data suggest that infected dendritic cells contribute to these processes. On the other hand, host cells infected with T. gondii are resistant to multiple inducers of apoptosis. Thus, regulation of host cell and bystander cell apoptosis by viable T. gondii may be significant components of a strategy to evade immunity and enhance intracellular parasite survival.