Cytoskeleton assembly in Toxoplasma gondii cell division

Cell division across members of the protozoan parasite phylum Apicomplexa displays a surprising diversity between different species as well as between different life stages of the same parasite. In most cases, infection of a host cell by a single parasite results in the formation of a polyploid cell from which individual daughters bud in a process dependent on a final round of mitosis. Unlike other apicomplexans, Toxoplasma gondii divides by a binary process consisting of internal budding that results in only two daughter cells per round of division. Since T. gondii is experimentally accessible and displays the simplest division mode, it has manifested itself as a model for apicomplexan daughter formation. Here, we review newly emerging insights in the prominent role that assembly of the cortical cytoskeletal scaffold plays in the process of daughter parasite formation.

Roles of Apicomplexan protein kinases at each life cycle stage

Inhibitors of cellular protein kinases have been reported to inhibit the development of Apicomplexan parasites, suggesting that the functions of protozoan protein kinases are critical for their life cycle. However, the specific roles of these protein kinases cannot be determined using only these inhibitors without molecular analysis, including gene disruption. In this report, we describe the functions of Apicomplexan protein kinases in each parasite life stage and the potential of pre-existing protein kinase inhibitors as Apicomplexan drugs against, mainly, Plasmodium and Toxoplasma.

An agent-based model for the transmission dynamics of Toxoplasma gondii

Toxoplasma gondii (T. gondii) is a unicellular protozoan that infects up to one-third of the world's human population. Numerous studies revealed that a latent infection of T. gondii can cause life-threatening encephalitis in immunocompromised people and also has significant effects on the behavior of healthy people and animals. However, the overall transmission of T. gondii has not been well understood although many factors affecting this process have been found out by different biologists separately. Here we synthesize what is currently known about the natural history of T. gondii by developing a prototype agent-based model to mimic the transmission process of T. gondii in a farm system. The present model takes into account the complete life cycle of T. gondii, which includes the transitions of the parasite from cats to environment through feces, from contaminated environment to mice through oocysts, from mice to cats through tissue cysts, from environment to cats through oocysts as well as the vertical transmission among mice. Although the current model does not explicitly include humans and other end-receivers, the effect of the transition to end-receivers is estimated by a developed infection risk index. The current model can also be extended to include human activities and thus be used to investigate the influences of human management on disease control. Simulation results reveal that most cats are infected through preying on infected mice while mice are infected through vertical transmission more often than through infection with oocysts, which clearly suggests the important role of mice during the transmission of T. gondii. Furthermore, our simulation results show that decreasing the number of mice on a farm can lead to the eradication of the disease and thus can lower the infection risk of other intermediate hosts on the farm. In addition, with the assumption that the relation between virulence and transmission satisfies a normal function, we show that intermediate virulent lineages (type II) can sustain the disease most efficiently, which can qualitatively agree with the fact that the evolution of the parasite favors intermediate virulence. The effects of other related factors on transmission, including the latent period and imprudent behavior of mice, and prevention strategies are also studied based on the present model.

A transmembrane domain containing pellicle protein of Toxoplasma gondii enhances virulence and invasion after extracellular stress

To identify Toxoplasma gondii genes important in the establishment of a persistent infection, we previously used signature-tagged mutagenesis to identify mutants with reduced cyst numbers in the brains of mice. One of the mutants, 95C5, has an insertion within a predicted six transmembrane domain protein, which localizes to the parasite pellicle, thus we named it transmembrane pellicle protein 1 (TgTPP1). Although the 95C5 mutant was found be reduced in its ability to form brain cysts, it is defective during acute infection. Addition of TgTPP1 expressed from its endogenous promoter restored the acute lethality of the 95C5 mutant to parental levels. The 95C5 mutant does not have a growth defect in standard tissue culture conditions; however, we found a significant defect in host cell penetration after extracellular stress. Overall, TgTPP1 may function during acute infection by enhancing the parasites ability to invade after extracellular stress.

Phosphatidylinositol 3-Monophosphate Is Involved in Toxoplasma Apicoplast Biogenesis

Apicomplexan parasites cause devastating diseases including malaria and toxoplasmosis. They harbour a plastid-like, non-photosynthetic organelle of algal origin, the apicoplast, which fulfils critical functions for parasite survival. Because of its essential and original metabolic pathways, the apicoplast has become a target for the development of new anti-apicomplexan drugs. Here we show that the lipid phosphatidylinositol 3-monophosphate (PI3P) is involved in apicoplast biogenesis in Toxoplasma gondii. In yeast and mammalian cells, PI3P is concentrated on early endosomes and regulates trafficking of endosomal compartments. Imaging of PI3P in T. gondii showed that the lipid was associated with the apicoplast and apicoplast protein-shuttling vesicles. Interference with regular PI3P function by over-expression of a PI3P specific binding module in the parasite led to the accumulation of vesicles containing apicoplast peripheral membrane proteins around the apicoplast and, ultimately, to the loss of the organelle. Accordingly, inhibition of the PI3P-synthesising kinase interfered with apicoplast biogenesis. These findings point to an unexpected implication for this ubiquitous lipid and open new perspectives on how nuclear encoded proteins traffic to the apicoplast. This study also highlights the possibility of developing specific pharmacological inhibitors of the parasite PI3-kinase as novel anti-apicomplexan drugs.

Phosphatidyinositol 3-monophosphate (PI3P) is important for endocytic fusion events in eukaryotic cells. Despite the importance of this lipid in cell biology, its localization and function in apicomplexan parasites has not yet been extensively explored. In this study, we attribute for the first time a role for PI3P in Toxoplasma and identify a function different from classical endosomal trafficking. We show that the perturbation of PI3P function in T. gondii induced a morphological alteration of vesicles containing proteins destined for the outermost apicoplast membrane, which accumulated abnormally around the organelle, resulting ultimately in the loss of apicoplasts. These findings suggest a new role for PI3P in a vesicular trafficking process necessary for apicoplast biogenesis and provide an attractive model in which PI3P allows the fusion of vesicles containing nuclear-encoded apicoplast proteins with the apicoplast. As the outermost membrane of the apicoplast is originally derived from the endocytic compartment during the ancestral secondary endosymbiosis event, a fascinating question arises about whether apicomplexan parasites have reshaped the classical PI3P-dependent endocytic machinery to target proteins to the apicoplast.

Self-mating in the definitive host potentiates clonal outbreaks of the apicomplexan parasites Sarcocystis neurona and Toxoplasma gondii

Tissue-encysting coccidia, including Toxoplasma gondii and Sarcocystis neurona, are heterogamous parasites with sexual and asexual life stages in definitive and intermediate hosts, respectively. During its sexual life stage, T. gondii reproduces either by genetic out-crossing or via clonal amplification of a single strain through self-mating. Out-crossing has been experimentally verified as a potent mechanism capable of producing offspring possessing a range of adaptive and virulence potentials. In contrast, selfing and other life history traits, such as asexual expansion of tissue-cysts by oral transmission among intermediate hosts, have been proposed to explain the genetic basis for the clonal population structure of T. gondii. In this study, we investigated the contributing roles self-mating and sexual recombination play in nature to maintain clonal population structures and produce or expand parasite clones capable of causing disease epidemics for two tissue encysting parasites. We applied high-resolution genotyping against strains isolated from a T. gondii waterborne outbreak that caused symptomatic disease in 155 immune-competent people in Brazil and a S. neurona outbreak that resulted in a mass mortality event in Southern sea otters. In both cases, a single, genetically distinct clone was found infecting outbreak-exposed individuals. Furthermore, the T. gondii outbreak clone was one of several apparently recombinant progeny recovered from the local environment. Since oocysts or sporocysts were the infectious form implicated in each outbreak, the expansion of the epidemic clone can be explained by self-mating. The results also show that out-crossing preceded selfing to produce the virulent T. gondii clone. For the tissue encysting coccidia, self-mating exists as a key adaptation potentiating the epidemic expansion and transmission of newly emerged parasite clones that can profoundly shape parasite population genetic structures or cause devastating disease outbreaks.

The parasites Toxoplasma gondii and Sarcocystis neurona have lifecycles that include a sexual stage in a definitive host and an asexual stage in intermediate hosts. For T. gondii, laboratory studies have demonstrated that the sexual stage can serve the dual purpose of producing new, virulent genotypes through recombination and promoting expansion of single clones via self-mating. Self-mating and other life history traits of T. gondii, including transmission of asexual stages among intermediate hosts, are assumed to account for the clonal population genetic structure of this organism. However, the relative contributions of sexual recombination and self-mating verses other life history traits in causing disease outbreaks or in shaping Toxoplasma's population genetic structure have not been verified in nature, nor have these traits been extensively examined in related parasites. To address this knowledge gap, we conducted population genetic analyses on T. gondii and S. neurona strains isolated from naturally occurring outbreaks affecting humans and sea otters, respectively. Our results identify self-mating as a key trait potentiating disease outbreaks through the rapid amplification of a single clone into millions of infectious units. Selfing is likely a key adaptation for enhancing transmission of recently emerged, recombinant clones and reshaping population genetic structures among the tissue-cyst coccidia.

[Formation mechanism and the function of parasitophorous vacuole of Toxoplasma gondii]

Toxoplasma gondii is a ubiquitous, obligate intracellular protozoan parasite in human and animals. Chronic infection with this parasite is likely one of the most common infection in human. Following invasion of host cells T gondii resides within membrane-bound vacuoles known as parasitophorous vacuole (PV), which can protect the parasite from the endosomal acidification and lysosomal fusion of host cells. It plays an essential role in the whole parasitic process of T. gondii. This review summarizes the mechanism of PV formation and its function in the host cells.

An insertional trap for conditional gene expression in Toxoplasma gondii: identification of TAF250 as an essential gene

Toxoplasmosis is characterized by fast lytic replication cycles leading to severe tissue lesions. Successful host cell invasion is essential for pathogenesis. The division cycle of Toxoplasma gondii is characterized by an unusual cell cycle progression and a distinct internal budding mechanism. To identify essential genes involved in the lytic cycle we devised an insertional gene trapping strategy using the Tet-transactivator system. In essence, a random, active promoter is displaced with a tetracycline regulatable promoter, which if in an essential gene, will result in a conditionally lethal phenotype upon tetracycline addition. We isolated eight mutants with growth defects, two of which displayed modest invasion defects, one of which had an additional cell cycle defect. The trapped loci were identified using expression microarrays, exploiting the tetracycline dependent expression of the trapped genes. In mutant 3.3H6 we identified TCP-1, a component of the chaperonin protein folding machinery under the control of the Tet promoter. However, this gene was not critical for growth of mutant 3.3H6. Subsequently, we identified a suppressor gene encoding a protein with a hypothetical function by guided cosmid complementation. In mutant 4.3B13, we identified TAF250, an RNA polymerase II complex component, as the trapped, essential gene. Furthermore, by mapping the plasmid insertion boundaries we identified multiple genomic rearrangements, which hint at a potential replication dependent DNA repair mechanism. Furthermore, these rearrangements provide an explanation for inconsistent locus rescue results observed by molecular biological approaches. Taken together, we have added an approach to identify and study essential genes in Toxoplasma.

Toxoplasma gondii: inhibitory activity and encystation effect of securinine and pyrrolidine derivatives on Toxoplasma growth

Securinine, an alkaloid originally isolated from Securinega suffruticosa, exhibits a wide range of biological activities, including anti-malarial activity. Along with securinine, 10 pyrrolidine derivatives, generated via the retrosynthesis of (-)-securinine, were selected and tested for their inhibitory activity against Toxoplasma gondii growth in vitro. Anti-Toxoplasma activity correlated to hydrophobicity of the tested compounds. Three pyrrolidine derivatives along with securinine inhibit Toxoplasma proliferation at the micromolar range. These compounds act on parasite proliferation in different capacities, either by slowing the growth rate or inhibiting invasion of host cells. Securinine induces bradyzoite differentiation at comparable levels to treatment with alkali media in vitro.

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.

[In vitro co-cultivation of Toxoplasma gondii tachyzoites with rat brain astrocytes]

Purified astrocytes were cultured in plates. When astrocytes grew over 80% of the plate, tachyzoites of Toxoplasma gondii RH strain were added for co-culture. In the period of 0-72 h, change of the astrocytes and tachyzoites was observed after Giemsa staining. In 0-48 h, monodansylcadaverine (MDC) was used to study the action of autophagy in the process of tachyzoites invading astrocytes. At 1 h co-culture, tachyzoites had entered in astrocytes and the autophagosomes appeared. At 4 h, the autophagosomes increased pronouncedly. However, after 12 h, number of autophagosomes considerably decreased and damage of the cells occurred. 48 h later, autophagosomes disappeared and more astrocytes were destroyed. At 72 h most cells destroyed and tachyzoites were released. The result showed that autophagy is inhibited when the astrocytes were in vitro infected by tachyzoites.

Comparative analysis of stage specific gene regulation of apicomplexan parasites: Plasmodium falciparum and Toxoplasma gondii

Apicomplexans comprise some of the most life threatening parasites infecting human and livestock and includes Plasmodium and Toxoplasma, the causative agents of malaria and toxoplasmosis respectively, in humans as well as Neospora caninum (abortion in livestock, neosporosis in dogs), Cryptosporidium (Diarrheal cryptosporidiosis and opportunistic infections in AIDS patients) and Eimeria (poultry coccidiosis). These parasites are characterized by a complex life cycle usually alternating between sexual and asexual cycles in different hosts. The need to adapt to different host environments demands a tight regulation of gene expression during parasite development. Therefore, the understanding of parasite biology will facilitate the control of the infection and the disease. In this review we emphasize the progress made so far in gene regulation in two medically important parasites, namely Plasmodium falciparum and Toxoplasma gondii, as well as other less known apicomplexan. The genome of both Plasmodium and Toxoplasma has been sequenced and since then there has been a significant progress in understanding the molecular mechanisms that control stage specific gene expression in the two parasites. In addition, the information gained in each of the parasite can be used in studying mechanisms that are still elusive in the other apicomplexans that are not readily available. Additionally, they can serve as model system for other disease causing Apicomplexan parasites.

[Establishment of an in vitro tachyzoite-bradyzoite interconversion system for Toxoplasma gondii]

OBJECTIVE

To establish an tachyzoite-brachyzoite interconversion system for Toxoplasma gondii RH strain in vitro.

METHODS

COS-7 cells were inoculated with purified tachyzoites of T.gondii RH strain and cultured in vitro. The morphology of the cultured cells and parasites was observed and the total cellular RNA extracted on days 1 to 6 following the inoculation for detecting the expression of tachyzoite-specific protein (SAG1) and bradyzoite-specific proteins (BAG1 and SAG2C) using RT-PCR.

RESULTS

With the passage of time, the number of parasites in COS-7 cells increased but the proliferation rate was lowered gradually. The intracellular tachyzoites proliferated by means of budding and binary fission, which led to the changes in the alignment of the parasites in the cells from curved pairs, rosette or clustered, and semi-circular patterns to spherical encapsulation-like structures. These changes indicated the gradual transformation of the tachyzoites into bradyzoites. The expressions of the tachyzoite-specific SAG1 gene were detected throughout the 6 days of in vitro culture. The expression of the bradyzoite-specific BAG1 gene had been detected since the second day after the inoculation and SAG2C gene since the fifth day. Alteration of the culture condition resulted in gradual transformation of the bradyzoites into tachyzoites.

CONCLUSION

An in vitro tachzoites-bradyzoite interconversion system for T.gondii has been successfully established, which provides the basis for further study of the mechanism of interconversion.

Environmental exposure of pregnant women to infection with Toxoplasma gondii--state of the art

Infection with T. gondii is particularly dangerous for pregnant women as it may lead to the transplacental passage of the parasite. Currently, congenital toxoplasmosis is the second most frequent intrauterine infection. The risk of transmission of T. gondii to the foetus varies throughout the world and ranges from 0.6-1.7/1,000 of pregnant women. The consequences of congenital toxoplasmosis are multifarious. On the basis of current literature review, the authors discuss the epidemiological and clinical aspects of toxoplasmosis in pregnant women, the influence of climatic and environmental factors that may lead to an increase in T. gondii infections in humans, particularly in pregnant women, and the principles of prophylactics against T. gondii infections in those women.

Role of immune response in Toxoplasma gondii tachyzoite-bradyzote stage interconversion: a janus in determining disease outcome

No doubt, Toxoplasma gondii and toxoplasmosis superimposed many zoonotic diseases from geographical and zoological distribution world wide.

Sexual recombination punctuated by outbreaks and clonal expansions predicts Toxoplasma gondii population genetics

The cosmopolitan parasitic pathogen Toxoplasma gondii is capable of infecting essentially any warm-blooded vertebrate worldwide, including most birds and mammals, and establishes chronic infections in one-third of the globe's human population. The success of this highly prevalent zoonosis is largely the result of its ability to propagate both sexually and clonally. Frequent genetic exchanges via sexual recombination among extant parasite lineages that mix in the definitive felid host produces new lines that emerge to expand the parasite's host range and cause outbreaks. Highly successful lines spread clonally via carnivorism and in some cases sweep to pandemic levels. The extent to which sexual reproduction versus clonal expansion shapes Toxoplasma's current, global population genetic structure is the central question this review will attempt to answer.

[In vitro culture of Toxoplasma gondii tachyzoites in HFF and HeLa cells]

OBJECTIVE

To study the proliferation of Toxoplasma gondii RH strain tachyzoites in human foreskin fibroblast (HFF) cells and HeLa cells.

METHODS

HFF cells and HeLa cells were cultured in 35 mm cell culture dishes with glass cover slip. Confluent cells were co-cultured with tachyzoites which purified by 3 microm filter membrane. At 0.5, 1, 2, 4, 8, 12, 24, 36, 48, 72, and 96 h after co-culture, the invasion of tachyzoites into the cells and proliferation in cells were observed with Giemsa staining.

RESULTS

In 4 h after co-culture, there were dozens of T. gondii tachyzoites in the HFF cells. At 24 h many pseudocysts emerged. At 72 h most of the cells were destroyed by tachyzoites. While cultured in HeLa cells for 8 h, there were only 3-5 tachyzoites in one cell, and pseudocysts emerged at 48 h. At 96 h after co-culture, most cells were destroyed.

CONCLUSION

Toxoplasma gondii tachyzoites can be cultured in HFF cells and HeLa cells. The proliferation of tachyzoites in HFF cells was quicker than that in HeLa cells.

A unique dual activity amino acid hydroxylase in Toxoplasma gondii

The genome of the protozoan parasite Toxoplasma gondii was found to contain two genes encoding tyrosine hydroxylase; that produces l-DOPA. The encoded enzymes metabolize phenylalanine as well as tyrosine with substrate preference for tyrosine. Thus the enzymes catabolize phenylalanine to tyrosine and tyrosine to l-DOPA. The catalytic domain descriptive of this class of enzymes is conserved with the parasite enzyme and exhibits similar kinetic properties to metazoan tyrosine hydroxylases, but contains a unique N-terminal extension with a signal sequence motif. One of the genes, TgAaaH1, is constitutively expressed while the other gene, TgAaaH2, is induced during formation of the bradyzoites of the cyst stages of the life cycle. This is the first description of an aromatic amino acid hydroxylase in an apicomplexan parasite. Extensive searching of apicomplexan genome sequences revealed an ortholog in Neospora caninum but not in Eimeria, Cryptosporidium, Theileria, or Plasmodium. Possible role(s) of these bi-functional enzymes during host infection are discussed.

[The immunosuppressant effect of T. lewisi (Kinetoplastidae) infection on the multiplication of Toxoplasma gondii (Sarcocystidae) on alveolar and peritoneal macrophages of the white rat]

The immunosuppressant effect of T. lewisi infection on the multiplication of T. gondii was compared in peritoneal (MP) and alveolar macrophages (MA) of white rat. Two animal groups were infected with T. lewisi and sacrificed after four days and seven days post infection. A group without infection was maintained as a control. The number of intracellular parasites (tachyzoites) (IT) was counted by light microscopy, calculating the rate infection rate per 100 total cells (TC) and per infected cells (IC) for each group of phagocyte cells. The relation quotient IT, TC or IC multiplied percent, provided a statistical ratio (RE) of the relative number of parasites in both cellular types for each time interval. MA as well as MP obtained after 4 days showed a significant increase in the multiplication of T. gondii with respect to the control. Unlike the MP (which had an increase in the multiplication of T. gondii the fourth day of infection with T. lewisi diminishing towards the seventh day), the MA had an increase in the multiplication of the parasite from the fourth to the seventh day. This difference can be related to the route of infection used for the experiments, that affect the MP directly with a greater effect in comparison with the MA of the lungs. Lung compartment will be affected later, when the infection becomes systemic between the fourth and sixth day of infection. The immunity against T. gondii is similar between both phagocytes, but the time of infection and the compartment where the cells are located, makes the difference in the response time against T. gondii. Supernatants from macrophage cultures or T. lewisi by rat did not induced any immunosuppression.

Development of forward genetics in Toxoplasma gondii

The development of forward genetics as a functional system in Toxoplasma gondii spanned more than three decades from the mid-1970s until now. The initial demonstration of experimental genetics relied on chemically induced drug-resistant mutants that were crossed by co-infecting cats, collecting oocysts, sporulating and hatching progeny in vitro. To capitalise on this, genetic markers were employed to develop linkage maps by tracking inheritance through experimental crosses. In all, three generations of genetic maps were developed to define the chromosomes, estimate recombination rates and provide a system for linkage analysis. Ultimately this genetic map would become the foundation for the assembly of the T. gondii genome, which was derived from whole genome shotgun sequencing, into a chromosome-centric view. Finally, application of forward genetics to multigenic biological traits showed the potential to map and identify specific genes that control complex phenotypes including virulence.

The role played by electron microscopy in advancing our understanding of Toxoplasma gondii and other apicomplexans

In many ways the history of the discovery of the life cycle of Toxoplasma gondii and the development of biological electron microscopy progressed in parallel through the 1950s and 1960s. Although Toxoplasma was discovered in 1908, it was only in the 1950s that the extent of the infection in humans and domestic animals was realised and work was undertaken to elucidate its life cycle (reviewed elsewhere in this edition). The development of ultrastructural techniques and their application to biological systems including Toxoplasma developed over the same period. This resulted in a synergistic effect with the re-classification of previously unrelated parasites within a single phylum, the Apicomplexa, which was based on the ultrastructural appearances of the infectious stages. This review will describe the central role played by electron microscopy and Toxoplasma in the developments associated with this progress.

Rab11A-controlled assembly of the inner membrane complex is required for completion of apicomplexan cytokinesis

The final step during cell division is the separation of daughter cells, a process that requires the coordinated delivery and assembly of new membrane to the cleavage furrow. While most eukaryotic cells replicate by binary fission, replication of apicomplexan parasites involves the assembly of daughters (merozoites/tachyzoites) within the mother cell, using the so-called Inner Membrane Complex (IMC) as a scaffold. After de novo synthesis of the IMC and biogenesis or segregation of new organelles, daughters bud out of the mother cell to invade new host cells. Here, we demonstrate that the final step in parasite cell division involves delivery of new plasma membrane to the daughter cells, in a process requiring functional Rab11A. Importantly, Rab11A can be found in association with Myosin-Tail-Interacting-Protein (MTIP), also known as Myosin Light Chain 1 (MLC1), a member of a 4-protein motor complex called the glideosome that is known to be crucial for parasite invasion of host cells. Ablation of Rab11A function results in daughter parasites having an incompletely formed IMC that leads to a block at a late stage of cell division. A similar defect is observed upon inducible expression of a myosin A tail-only mutant. We propose a model where Rab11A-mediated vesicular traffic driven by an MTIP-Myosin motor is necessary for IMC maturation and to deliver new plasma membrane to daughter cells in order to complete cell division.

Apicomplexan parasites are unusual in that they replicate by assembling daughter parasites within the mother cell. This involves the ordered assembly of an Inner Membrane Complex (IMC), a scaffold consisting of flattened membrane cisternae and a subpellicular network made up of microtubules and scaffold proteins. The IMC begins to form at the onset of replication, but its maturation occurs at the final stage of cytokinesis (the last step during cell division) upon the addition of motor (glideosome) components such as GAP45 (Glideosome Associated Protein), Myosin A (MyoA), and Myosin-Tail-Interacting-Protein (MTIP, also known as Myosin Light Chain 1) that are necessary to drive the gliding motility required for parasite invasion. We demonstrate that Rab11A regulates not only delivery of new plasmamembrane to daughter cells, but, importantly, also correct IMC formation. We show that Rab11A physically interacts with MTIP/MLC1, implicating unconventional myosin(s) in both cytokinesis and IMC maturation, and, consistently, overexpression of a MyoA tail-only mutant generates a default similar to that which we observe upon Rab11A ablation. We propose a model where Rab11A-mediated vesicular traffic is required for the delivery of new plasma membrane to daughter cells and for the maturation of the IMC in order to complete cell division.

Neuropsychiatric disease and Toxoplasma gondii infection

Toxoplasma gondii infects approximately 30% of the world's population, but causes overt clinical symptoms in only a small proportion of people. In recent years, the ability of the parasite to manipulate the behaviour of infected mice and rats and alter personality attributes of humans has been reported. Furthermore, a number of studies have now suggested T. gondii infection as a risk factor for the development of schizophrenia and depression in humans. As T. gondii forms cysts that are located in various anatomical sites including the brain during a chronic infection, it is well placed anatomically to mediate these effects directly. The T. gondii genome is known to contain 2 aromatic amino acid hydroxylases that potentially could directly affect dopamine and/or serotonin biosynthesis. However, stimulation of the immune response has also recently been associated with mood and behavioural alterations in humans, and compounds designed to alter mood, such as fluoxetine, have been demonstrated to alter aspects of immune function. Herein, the evidence for T.-gondii-induced behavioural changes relevant to schizophrenia and depression is reviewed. Potential mechanisms responsible for these changes in behaviour including the role of tryptophan metabolism and the hypothalamic-pituitary-adrenal axis are discussed.