Toxoplasma gondii catalase: are there peroxisomes in toxoplasma?

Toxoplasma profilin is essential for host cell invasion and TLR11-dependent induction of an interleukin-12 response

Apicomplexan parasites exhibit actin-dependent gliding motility that is essential for migration across biological barriers and host cell invasion. Profilins are key contributors to actin polymerization, and the parasite Toxoplasma gondii possesses a profilin-like protein that is recognized by Toll-like receptor TLR11 in the host innate immune system. Here, we show by conditional disruption of the corresponding gene that T.gondii profilin, while not required for intracellular growth, is indispensable for gliding motility, host cell invasion, active egress from host cells, and virulence in mice. Furthermore, parasites lacking profilin are unable to induce TLR11-dependent production in vitro and in vivo of the defensive host cytokine interleukin-12. Thus, profilin is an essential element of two aspects of T. gondii infection. Like bacterial flagellin, profilin plays a role in motility while serving as a microbial ligand recognized by the host innate immune system.

The putative peroxisomal gene Pxt1 is exclusively expressed in the testis

Genes reported to be crucial for spermatogenesis are often exclusively expressed in the testis. We have identified a novel male germ cell-specific expressed gene named peroxisomal testis specific 1 (Pxt1) with expression starting at the spermatocyte stage during mouse spermatogenesis. The putative amino acid sequence encoded by the cDNA of the Pxt1 gene contains a conserved Asn-His-Leu (NHL)-motif at its C-terminal end, which is characteristic for peroxisomal proteins. Pxt1-EGFP fusion protein is co-localized with known peroxisomal marker proteins in transfected NIH3T3 cells. In addition, we could demonstrate that the peroxisomal targeting signal NHL is functional and responsible for the correct subcellular localization of the Pxt1-EGFP fusion protein. In male germ cells peroxisomes were reported only in spermatogonia. The Pxt1 gene is so far the first gene coding for a putative peroxisomal protein which is expressed in later steps of spermatogenesis, namely in pachytene spermatocytes.

Niche metabolism in parasitic protozoa

Complete or partial genome sequences have recently become available for several medically and evolutionarily important parasitic protozoa. Through the application of bioinformatics complete metabolic repertoires for these parasites can be predicted. For experimentally intractable parasites insight provided by metabolic maps generated in silico has been startling. At its more extreme end, such bioinformatics reckoning facilitated the discovery in some parasites of mitochondria remodelled beyond previous recognition, and the identification of a non-photosynthetic chloroplast relic in malarial parasites. However, for experimentally tractable parasites, mapping of the general metabolic terrain is only a first step in understanding how the parasite modulates its streamlined, yet still often puzzlingly complex, metabolism in order to complete life cycles within host, vector, or environment. This review provides a comparative overview and discussion of metabolic strategies used by several different parasitic protozoa in order to subvert and survive host defences, and illustrates how genomic data contribute to the elucidation of parasite metabolism.

Identification and expression analysis of ABC protein-encoding genes in Toxoplasma gondii. Toxoplasma gondii ATP-binding cassette superfamily

The ATP-binding cassette (ABC) transporters are one of the largest evolutionarily conserved families of proteins. They are characterized by the presence of nucleotide-binding domains (NBDs), which are highly conserved among organisms. In the present study, we used human and protozoan ABC sequences, and ATP-binding consensus motifs to screen the Toxoplasma gondii TwinScan2 predicted proteins database. We identified 24 ABC open reading frames (ORFs), whose deduced amino acid sequences exhibited all the typical biochemical features of the ABC family members. Fifteen of them clustered into five of the seven families of human ABC proteins: six ABCBs (drug, peptides and lipid export), two ABCCs (organic anion conjugates and drug export), one ABCE (Rnase L inhibitor, RLI, antibiotic resistance and translation regulation), one ABCF (drug resistance and regulation of gene expression) and five ABCGs (drug export and resistance). The nine other ORFs were represented by four ABCHs (energy-generating subunits), four SMCs (structural maintenance of chromosomes) and one member of unclear origin, whose closest homologue was the yeast Elf1 protein (mRNA export factor). A notable feature of the Toxoplasma ABC superfamily seems to be the absence of genes encoding ABCA and ABCD members. Expression analysis of ABC genes in tachyzoite and bradyzoite stages revealed the presence of ABC transcripts for all genes studied. Further research on the implication of these ABC proteins will increase our knowledge of the basic biology of Toxoplasma and provide the opportunity to identify novel therapeutic targets. To our knowledge, this is the first report of ABC transporters in T. gondii.

Fusion and fission, the evolution of sterol carrier protein-2

Sterol carrier protein-2 (SCP-2) is an intracellular, small, basic protein domain that in vitro enhances the transfer of lipids between membranes. It is expressed in bacteria, archaea, and eukaryotes. There are five human genes, HSD17B4, SCPX, HSDL2 STOML1, and C20orf79, which encode SCP-2. HSD17B4, SCPX, HSDL2, and STOML1 encode fusion proteins with SCP-2 downstream of another protein domain, whereas C20orf79 encodes an unfused SCP-2. We have extracted SCP-2 domains from databases and analyzed the evolution of the eukaryotic SCP-2. We show that SCPX and HSDL2 are present in most animals from Cnidaria to Chordata. STOML1 are present in nematodes and more advanced animals. HSD17B4 which encodes a D-bifunctional protein (DBP) with domains for D-3-hydroxyacyl-CoA dehydrogenase, enoyl-CoA hydratase, and SCP-2 are found in animals from insects to mammals and also in fungi. Nematodes, amoebas, ciliates, apicomplexans, and oomycetes express an alternative DBP with the SCP-2 domain directly connected to the D-3-hydroxyacyl-CoA dehydrogenase. This fusion has not been retained in plant genomes, which solely express unfused SCP-2 domains. Proteins carrying unfused SCP-2 domains are also encoded in bacteria, archaea, ciliates, fungi, insects, nematodes, and vertebrates. Our results indicate that the fusion between D-3-hydroxyacyl-CoA dehydrogenase and SCP-2 was formed early during eukaryotic evolution. There have since been several gene fission events where genes encoding unfused SCP-2 domains have been formed, as well as gene fusion events placing the SCP-2 domain in novel protein domain contexts.

Plasmodium falciparum: Discovery of peroxidase active organelles

Staining with 3,3' diaminobenzidine tetrahydrochloride (DAB) is a common method used for the detection of peroxidases. Using this histochemical staining method in conjunction with transmission electron microscopy, we observed oxidation of DAB that was localized to a discrete set of organelles displaying morphological similarity to small (75-90 nm diameter) versions of higher eukaryotic microbodies or peroxisomes. These single membrane bounded organelles were characterized by an asymmetrical matrix capable of oxidizing DAB to an electron dense inclusion. Oxidation of DAB was further found to be dependent upon hydrogen peroxide (H2O2) as a substrate. Given a lack of peroxisomal import proteins and enzymes, it is unlikely that these represent conventional peroxisomes. Rather, they likely represent specialized organelles containing endogenous peroxidase or pseudo-peroxidase activity.

Peroxiredoxin-linked Detoxification of Hydroperoxides in Toxoplasma gondii

The apicomplexan parasite Toxoplasma gondii is highly susceptible to oxidative stress caused by tert-butyl-hydroperoxide, juglone, and phenazine methylsulfate with IC(50) in the nanomolar range. Using dichlorofluorescein diacetate, a detector of endogenous oxidative stress, it was shown that juglone and phenazine methylsulfate are potentially toxic to the parasites without affecting the host cells. These results demonstrate that T. gondii is vulnerable to oxidative challenge that results from disruption of its redox balance and so this could be an effective approach to therapeutic intervention. This study has characterized redox active and antioxidant peroxidases belonging to the class of 1-Cys and 2-Cys peroxiredoxins that play crucial roles in maintaining redox balance. The tachyzoite stages of T. gondii express thioredoxin (TgTrx), 1-Cys peroxiredoxin (TgTrx-Px2), and a 2-Cys peroxiredoxin (TgTrx-Px1) and immunofluorescent studies revealed that all three proteins are located in the cytosol of the parasite confirming previous studies on TgTrx-Px1 (Kwok, L.Y., Schluter, D., Clayton, C., and Soldati, D. (2004) Mol. Microbiol. 51, 47-61). TgTrx-Px1 showed K(m) values for H(2)O(2) and tert-butyl hydroperoxide in the nanomolar range, emphasizing the great affinity of the protein for theses substrates. Moreover, the catalytic efficiency of TgTrx-Px1 for these substrates at 10(6)-10(7) M(-1) s(-1) is unusually high, which qualifies the enzyme as an extremely potent antioxidant. Kinetic analyses revealed that TgTrx-Px1 is inhibited by tert-butyl hydroperoxide, and apparent inhibition constants were determined to be between 33 and 35.6 microm depending on the concentration of the non-inhibitory substrate thioredoxin. TgTrx-Px2 protected glutamine synthetase from inactivation by Fe(3+)/DTT, showing that it is an active peroxiredoxin.

The antioxidant systems in Toxoplasma gondii and the role of cytosolic catalase in defence against oxidative injury

Superoxide dismutase, catalase, glutathione peroxidase and peroxiredoxins form an antioxidant network protecting cells against reactive oxygen species (ROS). Catalase is a potent H2O2-detoxifying enzyme, which is unexpectedly absent in some members of the Kinetoplastida and Apicomplexa, but present in Toxoplasma gondii. In T. gondii, catalase appears to be cytosolic. In addition, T. gondii also possesses genes coding for other types of peroxidases, including glutathione/thioredoxin-like peroxidases and peroxiredoxins. This study presents a detailed analysis of the role of catalase in the parasite and reports the existence of antioxidant enzymes localized in the cytosol and the mitochondrion of T. gondii. The catalase gene was disrupted and, in addition, T. gondii cell lines overexpressing either catalase or a cytosolic 1-cys peroxiredoxin, TgPrx2, under the control of a strong promoter were created. Analysis of these mutants confirmed that the catalase activity is cytosolic and is encoded by a unique gene in T. gondii. Furthermore, the catalase confers protection against H2O2 exposure and contributes to virulence in mice. The overexpression of Prx2 also increases protection against H2O2 treatment, suggesting that catalase and other peroxidases function as a defence mechanism against endogenously produced reactive oxygen intermediates and the oxidative stress imposed by the host.

Identification of a type 1 peroxisomal targeting signal in a viral protein and demonstration of its targeting to the organelle

Peroxisomes are unimembrane, respiratory organelles of the cell. Transport of cellular proteins to the peroxisomal matrix requires a type 1 peroxisomal targeting signal (PTS1) which essentially constitutes a tripeptide from the consensus sequence S/T/A/G/C/N-K/R/H-L/I/V/M/A/F/Y. Although PTS-containing proteins have been identified in eukaryotes, prokaryotes, and parasites, viral proteins with such signals have not been identified so far. We report here the first instance of a virus, the rotavirus, which causes infantile diarrhea worldwide, containing a functional C-terminal PTS1 in one of its proteins (VP4). Analysis of 153 rotavirus VP4-deduced amino acid sequences identified five groups of conserved C-terminal PTS1 tripeptide sequences (SKL, CKL, GKL, CRL, and CRI), of which CRL is represented in approximately 62% of the sequences. Infection of cells by a CRL-containing representative rotavirus (SA11 strain) and confocal immunofluorescence analysis revealed colocalization of VP4 with peroxisomal markers and morphological changes of peroxisomes. Further, transient cellular expression of green fluorescent protein (GFP)-fused VP4CRL resulted in transport of VP4 to peroxisomes, whereas the chimera lacking the PTS1 signal, GFP-VP4DeltaCRL, resulted in diffuse cytoplasmic staining, suggesting a CRL-dependent targeting of the protein. The present study therefore demonstrates hitherto unreported organelle involvement, specifically of the peroxisomes, in rotaviral infections as demonstrated by using the SA11 strain of rotavirus and opens a new line of investigation toward understanding viral pathogenesis and disease mechanisms.

Toxoplasma gondii myosins B/C: one gene, two tails, two localizations, and a role in parasite division

In apicomplexan parasites, actin-disrupting drugs and the inhibitor of myosin heavy chain ATPase, 2,3-butanedione monoxime, have been shown to interfere with host cell invasion by inhibiting parasite gliding motility. We report here that the actomyosin system of Toxoplasma gondii also contributes to the process of cell division by ensuring accurate budding of daughter cells. T. gondii myosins B and C are encoded by alternatively spliced mRNAs and differ only in their COOH-terminal tails. MyoB and MyoC showed distinct subcellular localizations and dissimilar solubilities, which were conferred by their tails. MyoC is the first marker selectively concentrated at the anterior and posterior polar rings of the inner membrane complex, structures that play a key role in cell shape integrity during daughter cell biogenesis. When transiently expressed, MyoB, MyoC, as well as the common motor domain lacking the tail did not distribute evenly between daughter cells, suggesting some impairment in proper segregation. Stable overexpression of MyoB caused a significant defect in parasite cell division, leading to the formation of extensive residual bodies, a substantial delay in replication, and loss of acute virulence in mice. Altogether, these observations suggest that MyoB/C products play a role in proper daughter cell budding and separation.

Biogenesis and function of peroxisomes and glycosomes

Peroxisomes of higher eukaryotes, glycosomes of kinetoplastids, and glyoxysomes of plants are related microbody organelles that perform differing metabolic functions tailored to their cellular environments. The close evolutionary relationship of these organelles is most clearly evidenced by the conservation of proteins involved in matrix protein import and biogenesis. The glycosome can be viewed as an offshoot of the peroxisomal lineage with additional metabolic functions, specifically glycolysis and purine salvage. Within the parasitic protozoa, only kinetoplastids have been conclusively demonstrated to possess glycosomes or indeed any peroxisome-like organelle. The importance of glycosomal pathways and their compartmentation emphasizes the potential of the glycosome and glycosomal proteins as drug targets.

Differential expression of two plant-like enolases with distinct enzymatic and antigenic properties during stage conversion of the protozoan parasite Toxoplasma gondii

The precise molecular mechanisms underlying the switch between the two developmental stages of Toxoplasma gondii, and the metabolic adaptations occurring during this stage conversion are poorly understood. Because inhibitors of mitochondrial respiration are known to trigger differentiation from tachyzoite into bradyzoite stages, we believe that some of the switch components may be sought in the regulation of central carbohydrate metabolism. We have previously described a cDNA encoding a bradyzoite-specific enolase, ENO1. We now report the isolation and characterization of another enolase-encoding cDNA (ENO2) that is expressed preferentially in the tachyzoite stage. The deduced amino acid sequences of ENO1 and ENO2 share 73.65 % identity. They both display significant homologies to plant enolases with the presence of two plant-like peptide insertions, a pentapeptide EWGW(Y)C(S) and a dipeptide EK (or DK). We demonstrate that deletions of the ENO1 pentapeptide motif on its own or together with the dipeptide reduce drastically the affinity for the 2PGA substrate, suggesting that the evolutionary acquisition of these peptides in enolases of land plants and apicomplexan parasites contribute a specific function to their enzymatic activities. T. gondii ENO1 and ENO2 were also expressed as active recombinant enzymes in Escherichia coli. While ENO1 and ENO2 display similar K(m) values, the pure tachyzoite-specific enzyme (ENO2) has a threefold specific activity at V(max) compared with that of the bradyzoite-specific enolase (ENO1). Moreover, immunoblot analyses performed using polyclonal antibodies raised against the recombinant enzymes revealed that the native enolase in tachyzoite and bradyzoite are also antigenically distinct. Taken together, our results indicate that the differences witnessed between the two activities may be instrumental in maintaining glycolysis in pace with the distinct stage-specific requirements of carbohydrate metabolism.

Molecular cloning and characterization of peroxiredoxin from Toxoplasma gondii

A cDNA of 1.1 kb comprising the gene encoding the peroxiredoxin of Toxoplasma gondii (TgPrx) has been cloned. The open reading frame of 591 bp was translated into a protein of 196 amino acids with a molecular mass of 25 kDa. Conserved 2 cysteine domains of Phe-Val-Cys-Pro and Glu-Val-Cys-Pro indicated TgPrx belonged to 2-Cys Prx families. TgPrx showed the highest homology with that of Arabidopsis thaliana by 53.9% followed by Entamoeba histolytica with 39.5% by the amino acid sequence alignment. Polyclonal antibody against recombinant TgPrx detected 25 kDa band in T. gondii without binding to host cell proteins. TgPrx was located in the cytoplasm of T. gondii extracellularly or intracellularly by immunofluorescence assay. The expression of TgPrx was increased as early as 30 min after the treatment with artemisinin in the intracellular stage, while no changes in those of host Prx I and TgSOD. This result implies that TgPrx may function as an antioxidant protecting the cell from the attack of reactive oxygen intermediates. It is also suggested that TgPrx is a possible target of chemotherapy.

Large amounts of apicoplast nucleoid DNA and its segregation in Toxoplasma gondii

Apicoplasts (apicomplexan plastids) are nonphotosynthetic, secondary endosymbiotic plastids that are found in most apicomplexans. Although these organelles are essential for parasite survival, their functions, activities, and structures are not well understood. We examined the apicoplast nucleoid of Toxoplasma gondii from a morphological aspect by high-resolution epifluorescence microscopy and electron microscopy. We found unexpectedly large amounts of DNA in the nucleoid and the presence of several division-related structures. Initially, we identified the organellar nucleoids by staining with the DNA-specific dye 4',6-diamidino-2-phenylindole. A single nucleoid was observed per apicoplast, and the fluorescent spot representing the nucleoid was bright and spherical in contrast to the weak and filamentous spot representing the mitochondrial nucleoid. We also measured the DNA content of each apicoplast nucleoid by a video-intensified microscope photon-counting system and determined that the genomic copy number was at least 25, a figure over four times greater than that reported previously. Moreover, several groups of apicoplasts had significantly higher genomic copy numbers. The DNA molecules were accurately divided into two daughter apicoplasts just before nuclear division. In addition, we examined nucleoid segregation and the division apparatus using electron microscopy. However, we failed to observe nucleoid structures, suggesting that the apicoplasts are predominantly composed of nucleoid material. In addition, we observed "cap" structures at the termini of dividing apicoplasts, a possible plastid-dividing ring, and a microbody-like granule around the constriction. These structures may be involved in apicoplast division.