DNA replication and daughter cell budding are not tightly linked in the protozoan parasite Toxoplasma gondii

TgZFP2 is a novel zinc finger protein involved in coordinating mitosis and budding in Toxoplasma

Zinc finger proteins (ZFPs) are one of the most abundant groups of proteins with a wide range of molecular functions. We have characterised a Toxoplasma protein that we named TgZFP2, as it bears a zinc finger domain conserved in eukaryotes. However, this protein has little homology outside this region and contains no other conserved domain that could hint for a particular function. We thus investigated TgZFP2 function by generating a conditional mutant. We showed that depletion of TgZFP2 leads to a drastic arrest in the parasite cell cycle, and complementation assays demonstrated the zinc finger domain is essential for TgZFP2 function. More precisely, whereas replication of the nuclear material is initially essentially unaltered, daughter cell budding is seriously impaired: to a large extent newly formed buds fail to incorporate nuclear material. TgZFP2 is found at the basal complex in extracellular parasites and after invasion, but as the parasites progress into cell division, it relocalises to cytoplasmic punctate structures and, strikingly, accumulates in the pericentrosomal area at the onset of daughter cell elongation. Centrosomes have emerged as major coordinators of the budding and nuclear cycles in Toxoplasma, and our study identifies a novel and important component of this machinery.

Targeting Toxoplasma tubules: tubulin, microtubules, and associated proteins in a human pathogen

Toxoplasma gondii is an obligate intracellular parasite that causes serious opportunistic infections, birth defects, and blindness in humans. Microtubules are critically important components of diverse structures that are used throughout the Toxoplasma life cycle. As in other eukaryotes, spindle microtubules are required for chromosome segregation during replication. Additionally, a set of membrane-associated microtubules is essential for the elongated shape of invasive "zoites," and motility follows a spiral trajectory that reflects the path of these microtubules. Toxoplasma zoites also construct an intricate, tubulin-based apical structure, termed the conoid, which is important for host cell invasion and associates with proteins typically found in the flagellar apparatus. Last, microgametes specifically construct a microtubule-containing flagellar axoneme in order to fertilize macrogametes, permitting genetic recombination. The specialized roles of these microtubule populations are mediated by distinct sets of associated proteins. This review summarizes our current understanding of the role of tubulin, microtubule populations, and associated proteins in Toxoplasma; these components are used for both novel and broadly conserved processes that are essential for parasite survival.

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.

Actin-like protein 1 (ALP1) is a component of dynamic, high molecular weight complexes in Toxoplasma gondii

Apicomplexan parasites, such as Toxoplasma gondii, rely on actin-based motility for cell invasion, yet conventional actin does not appear to be required for cell division in these parasites. Apicomplexans also contain a variety of actin-related proteins (Arps); however, most of these not directly orthologous to Arps in well-studied systems. We recently identified an apicomplexan-specific member of this family called Actin-Like Protein 1, (ALP1), which plays a role in the assembly of vesicular components recruited to the inner membrane complex (IMC) of daughter cells during cell division. In addition to its enrichment at daughter cell membranes, ALP1 is localized throughout the cytoplasm both diffusely distributed and concentrated in clusters that are detected by fluorescence microscopy, suggesting it forms complexes. Using quantitative optical imaging methods, including fluorescence recovery after photobleaching (FRAP) and fluorescence loss in photobleaching (FLIP), we demonstrated that ALP1 is a component of a large complex, and that it readily exchanges between diffusible and complex-bound forms. Sedimentation and density gradient analyses revealed that ALP1 is found in a freely soluble state as well as high molecular weight complexes. During cell division, ALP1 was dynamically associated with the IMC, suggesting it rapidly cycles between freely diffusible and complex forms during daughter cell assembly.

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

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

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.

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.

Synchronisation of Giardia lamblia: identification of cell cycle stage-specific genes and a differentiation restriction point

The intestinal parasite Giardia lamblia undergoes cell differentiations that entail entry into and departure from the replicative cell cycle. The pathophysiology of giardiasis depends directly upon the ability of the trophozoite form to replicate in the host upper small intestine. Thus, cell proliferation is tightly linked to disease. However, studies of cell cycle regulation in Giardia have been hampered by the inability to synchronise cultures. Here we report that Giardia isolates of the major human genotypes A and B can be synchronised using aphidicolin, a mycotoxin that reversibly inhibits replicative DNA polymerases in eukaryotic cells. Aphidicolin arrests Giardia trophozoites in the early DNA synthesis (S) phase of the cell cycle. We identified a set of cell cycle orthologues in the Giardia genome using bioinformatic analyses and showed that synchronised parasites express these genes in a cell cycle stage-specific manner. The synchronisation method also showed that during encystation, exit from the ordinary cell cycle occurs preferentially in G(2) and defines a restriction point for differentiation. Synchronisation opens up possibilities for further molecular and cell biological studies of chromosome replication, mitosis and segregation of the complex cytoskeleton in Giardia.

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.

Building the perfect parasite: cell division in apicomplexa

Apicomplexans are pathogens responsible for malaria, toxoplasmosis, and crytposporidiosis in humans, and a wide range of livestock diseases. These unicellular eukaryotes are stealthy invaders, sheltering from the immune response in the cells of their hosts, while at the same time tapping into these cells as source of nutrients. The complexity and beauty of the structures formed during their intracellular development have made apicomplexans the darling of electron microscopists. Dramatic technological progress over the last decade has transformed apicomplexans into respectable genetic model organisms. Extensive genomic resources are now available for many apicomplexan species. At the same time, parasite transfection has enabled researchers to test the function of specific genes through reverse and forward genetic approaches with increasing sophistication. Transfection also introduced the use of fluorescent reporters, opening the field to dynamic real time microscopic observation. Parasite cell biologists have used these tools to take a fresh look at a classic problem: how do apicomplexans build the perfect invasion machine, the zoite, and how is this process fine-tuned to fit the specific niche of each pathogen in this ancient and very diverse group? This work has unearthed a treasure trove of novel structures and mechanisms that are the focus of this review.

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.

Variability and heritability of cell division pathways in Toxoplasma gondii

A histone 2b-YFP fusion protein stably expressed in Toxoplasma gondii has several advantages: it reveals previously hidden details of nuclear morphology; it makes it possible to observe cell-cycle events; it provides a basis for quantitative measurements of DNA content in living cells; and it enables sorting of live cells according to cell-cycle phase or ploidy. With this cell line it was possible to recognize and directly clone individual progeny arising from different patterns of cell division that produce two, three or four daughter cells. These experiments established that the progeny produced by all cell division pathways are viable and infective. Furthermore, the number of progeny produced by a mature parasite during cell division is not correlated with the number of its siblings. The complete repertoire of cell division pathways is therefore inherited by a single cell produced through any one of the individual paths. The results expand the range of what must be considered normal in T. gondii cell division and provide a useful tool for further study of nuclear structure and proliferation in this important human pathogen.

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.

Levels of circumsporozoite protein in the Plasmodium oocyst determine sporozoite morphology

The sporozoite stage of the Plasmodium parasite is formed by budding from a multinucleate oocyst in the mosquito midgut. During their life, sporozoites must infect the salivary glands of the mosquito vector and the liver of the mammalian host; both events depend on the major sporozoite surface protein, the circumsporozoite protein (CS). We previously reported that Plasmodium berghei oocysts in which the CS gene is inactivated do not form sporozoites. Here, we analyzed the ultrastructure of P.berghei oocyst differentiation in the wild type, recombinants that do not produce or produce reduced amounts of CS, and corresponding complemented clones. The results indicate that CS is essential for establishing polarity in the oocyst. The amounts of CS protein correlate with the extent of development of the inner membranes and associated microtubules underneath the oocyst outer membrane, which normally demarcate focal budding sites. This is a first example of a protein controlling both morphogenesis and infectivity of a parasite stage.

An rRNA mutation identifies the apicoplast as the target for clindamycin in Toxoplasma gondii

Toxoplasma gondii is a protozoan sensitive to several inhibitors of prokaryotic translation (e.g. clindamycin, macrolides and tetracyclines). A priori, two prokaryotic-like organelles, the 'apicoplast' (a non-photosynthetic plastid) and the mitochondrion, are likely targets for these drugs. Without using overt mutagenesis, we selected two independent clones (ClnR-4 and ClnR-21) with strong and stable clindamycin resistance. Several lines with substantial but lower levels of resistance were also isolated with (XR-46) or without (ClnR-23) overt mutagenesis. The ClnR-4 and ClnR-21 mutants uniquely possess a G-->U point mutation at position 1857 of the apicoplast large-subunit rRNA, whereas no mutation was identified in this region for ClnR-23 or XR-46. Position 1857 corresponds to position 2061 in Escherichia coli where it is predicted to bind clindamycin. The mutation is present in all the apicoplast rDNA copies (an estimated 12 per organelle), indicative of a strong selective advantage in the presence of clindamycin. In the absence of drug, however, such a mutation is unlikely to be neutral, as the G is a critical contributor to the transpeptidation reaction and absolutely conserved in all kingdoms. This may explain why ClnR-4 shows a slight growth defect in vitro. These mutants provide direct genetic evidence that apicoplast translation is the target for clindamycin in Toxoplasma. Further, their sensitivity profiles to other antibiotics specific for the large ribosomal subunit (macrolides and chloramphenicol) and, intriguingly, the small subunit (doxycycline) argue that these drugs also target the apicoplast ribosome.

Daughter cell assembly in the protozoan parasite Toxoplasma gondii

The phylum Apicomplexa includes thousands of species of obligate intracellular parasites, many of which are significant human and/or animal pathogens. Parasites in this phylum replicate by assembling daughters within the mother, using a cytoskeletal and membranous scaffolding termed the inner membrane complex. Most apicomplexan parasites, including Plasmodium sp. (which cause malaria), package many daughters within a single mother during mitosis, whereas Toxoplasma gondii typically packages only two. The comparatively simple pattern of T. gondii cell division, combined with its molecular genetic and cell biological accessibility, makes this an ideal system to study parasite cell division. A recombinant fusion between the fluorescent protein reporter YFP and the inner membrane complex protein IMC1 has been exploited to examine daughter scaffold formation in T. gondii. Time-lapse video microscopy permits the entire cell cycle of these parasites to be visualized in vivo. In addition to replication via endodyogeny (packaging two parasites at a time), T. gondii is also capable of forming multiple daughters, suggesting fundamental similarities between cell division in T. gondii and other apicomplexan parasites.