Tuesday, April 24, 2007

Carbohydrate metabolism in the Toxoplasma gondii apicoplast

Eukaryot Cell. 2007 Apr 20; [Epub ahead of print]

Carbohydrate metabolism in the Toxoplasma gondii apicoplast: localization of three glycolytic isoenzymes, the single pyruvate dehydrogenase complex and a plastid phosphate translocator

Fleige T, Fischer K, Ferguson DJ, Gross U, Bohne W.

Institute of Medical Microbiology, University of Gottingen, Kreuzbergring 57, Gottingen D-37075; GERMANY; Institute for Biology, University of Tromso, 9037 Tromso, NORWAY; Nuffield Department of Pathology, University of Oxford, John Radcliffe Hospital, Headington, Oxford OX3 9DU; ENGLAND.

Many apicomplexan parasites such as Toxoplasma gondii and Plasmodium species possess a non-photosynthetic plastid, referred to as the apicoplast, which is essential for parasite viability and displays characteristics similar to those of non-green plastids in plants. In this study, we localized several key enzymes of the carbohydrate metabolism of T. gondii to either the apicoplast or the cytosol by engineering parasites which express epitope tagged fusion proteins. The cytosol contains a complete set of enzymes for glycolysis, which should enable the parasite to metabolize imported glucose into pyruvate. All glycolytic enzymes from phosphofructokinase up to pyruvate kinase are present in the T. gondii genome as duplicates and isoforms of triose phosphate isomerase, phosphoglycerate kinase and pyruvate kinase were found to localize to the apicoplast. The mRNA expression level of all glycolytic genes was compared between tachyzoites and bradyzoites, however, a strict bradyzoite specific expression pattern was only observed for enolase-I. The T. gondii genome encodes a single pyruvate dehydrogenase complex which was located in the apicoplast and absent in the mitochondrion, as shown by targeting of epitope tagged fusion proteins and by immunolocalization of the native pyruvate dehydrogenase complex. Exchange of metabolites between the cytosol and the apicoplast is likely to be mediated by a phosphate translocator which was localized to the apicoplast. Based on these localization studies, a model is proposed that explains the supply of the apicoplast with ATP and reduction power as well as the exchange of metabolites between the cytosol and the apicoplast.

PMID: 17449654 [PubMed - as supplied by publisher]

Tuesday, April 17, 2007

Toxo and Schizophrenia: Linkage Through Astrocyte-Derived Kynurenic Acid?

Schizophr Bull. 2007 Apr 13; [Epub ahead of print]

Toxoplasma gondii and Schizophrenia: Linkage Through Astrocyte-Derived Kynurenic Acid?

Schwarcz R, Hunter CA.

2Department of Pathobiology, School of Veterinary Medicine, University of Pennsylvania, Philadelphia, PA 19104.

PMID: 17434932 [PubMed - as supplied by publisher]

Epidemiological evidences from China assume that psychiatric-related diseases may be associated with Toxo infection

Neuro Endocrinol Lett. 2007 Apr 15;28(2) [Epub ahead of print]

Epidemiological evidences from China assume that psychiatric-related diseases may be associated with Toxoplasma gondii infection

Zhu S, Guo MF, Feng QC, Fan JM, Zhang F.

School of Basic Medicine, Zhengzhou University, Zhengzhou, China. zsha@zzu.edu.cn.

In recent years, the effect of Toxoplasma gondii infection on psychiatric-related aspects has been increasingly recognized. T. gondii has a high affinity for brain tissue where tachyzoites may form tissue cysts and persist life long. In recent years, 15 serological surveys about T. gondii infection and psychiatric diseases have been carried out in different areas in China. Studies showed that the prevalence of antibodies against T. gondii in psychotic patients was much higher than in normal persons; statistically differences were significant. Studies also reported that raising cats or enjoying the habit of eating raw or under cooked meet were potential risk factors for the infection of T. gondii. The epidemiological and serological evidence support the hypothesis that some psychiatric diseases such as schizophrenia or mental retardation might be linked to T. gondii infection.

PMID: 17435677 [PubMed - as supplied by publisher]

Higher perceived dominance in Toxoplasma infected men

Neuro Endocrinol Lett. 2007 Apr 15;28(2) [Epub ahead of print]

Higher perceived dominance in Toxoplasma infected men - a new evidence for role of increased level of testosterone in toxoplasmosis-associated changes in human behavior

Hodkova H, Kolbekova P, Skallova A, Lindova J, Flegr J.

Department of Parasitology, Charles University, Viniena 7, Prague 128 44, Czech Republic. flegr@cesnet.cz.

Toxoplasma is parasite of cats that uses any warm-blooded animals as intermediate hosts. It is known to induce shifts in behavior, physiology and even morphology of its intermediate hosts, including humans. The lower second to fourth digit ratio (2D:4D ratio) in infected man and women, and higher height in infected man suggest that sex steroid hormones like testosterone could play a role in these shifts. Here, we searched for another indirect indication for a higher postnatal testosterone level, i.e. increased perceived dominance and masculinity in infected men. We showed portrait pictures of 89 male students of which 18 were Toxoplasma-infected to 109 female students. When we statistically corrected for age, men with latent toxoplasmosis were perceived as more dominant (p=0.009) and masculine (p=0.052). These results support the idea that the higher level of testosterone could be responsible for at least some of the toxoplasmosis-associated shifts in human and animal behavior.

PMID: 17435678 [PubMed - as supplied by publisher]

Thursday, April 12, 2007

Inhibition of intracellular growth by human lactoferrin

Pol J Microbiol. 2007;56(1):25-32.

Toxoplasma gondii: inhibition of the intracellular growth by human lactoferrin

Dzitko K, Dziadek B, Dziadek J, Dlugonska H.

Department of Immunoparasitology, Institute of Microbiology and Immunology, University of Lodz, Lodz, Poland.

In the present paper we focused on the potential role of human lactoferrin for the intracellular replication rate of T. gondii BK tachyzoites following our earlier observation that Toxoplasma gondii was able to bind human lactoferrin but not serum transferrin. The study was performed in vitro on human CaCo-2 epithelial cells and mouse L929 fibroblasts. We found that the multiplication of the parasite was inhibited by lactoferrin in both cell lines used. However, the direct cytotoxic effect on the parasite and the host cells was not observed. The intracellular growth of T. gondii was not affected when tachyzoites or host cells were only pre-coated with human lactoferrin. The results suggest that lactoferrin does not influence parasites penetration into host cells but could trigger unknown antiparasitic mechanisms in the infected cells.

Publication Types:
Research Support, Non-U.S. Gov't

PMID: 17419186 [PubMed - in process]

Tuesday, April 10, 2007

Purine nucleoside phosphorylase: a potential target for the development of drugs to treat T-cell- and apicomplexan diseases

Curr Drug Targets. 2007 Mar;8(3):413-22.

Purine nucleoside phosphorylase: a potential target for the development of drugs to treat T-cell- and apicomplexan parasite-mediated diseases

Silva RG, Nunes JE, Canduri F, Borges JC, Gava LM, Moreno FB, Basso LA, Santos DS.

Centro de Pesquisas em Biologia Molecular e Funcional, Instituto de Pesquisas Biomedicas, Pontificia Universidade do Rio Grande do Sul, Porto Alegre, RS, Brazil.

Purine nucleoside phosphorylase (PNP) catalyzes the reversible phosphorolysis of nucleosides and deoxynucleosides, generating ribose 1-phosphate and the purine base, which is an important step of purine catabolism pathway. The lack of such an activity in humans, owing to a genetic disorder, causes T-cell impairment, and thus drugs that inhibit human PNP activity have the potential of being utilized as modulators of the immunological system to treat leukemia, autoimmune diseases, and rejection in organ transplantation. Besides, the purine salvage pathway is the only possible way for apicomplexan parasites to obtain the building blocks for RNA and DNA synthesis, which makes PNP from these parasites an attractive target for drug development against diseases such as malaria. Hence, a number of research groups have made efforts to elucidate the mechanism of action of PNP based on structural and kinetic studies. It is conceivable that the mechanism may be different for PNPs from diverse sources, and influenced by the oligomeric state of the enzyme in solution. Furthermore, distinct transition state structures can make possible the rational design of specific inhibitors for human and apicomplexan enzymes. Here, we review the current status of these research efforts to elucidate the mechanism of PNP-catalyzed chemical reaction, focusing on the mammalian and Plamodium falciparum enzymes, targets for drug development against, respectively, T-Cell- and Apicomplexan parasites-mediated diseases.

Publication Types:
Research Support, Non-U.S. Gov't

PMID: 17348834 [PubMed - indexed for MEDLINE]

GRA2 and GRA6 formulated in MPL adjuvant against Toxo chronic infection

Vaccine. 2007 Mar 9; [Epub ahead of print]

Evaluation of protective effect of recombinant dense granule antigens GRA2 and GRA6 formulated in monophosphoryl lipid A (MPL) adjuvant against Toxoplasma chronic infection in mice

Golkar M, Shokrgozar MA, Rafati S, Musset K, Assmar M, Sadaie R, Cesbron-Delauw MF, Mercier C.

CNRS UMR 5163 - Universite Joseph Fourier, UFR de Biologie, Institut Jean Roget, Campus Sante, Domaine de la Merci. BP 170, 38042 Grenoble cedex 09, France; Molecular Parasitology Laboratory, Department of Parasitology, Pasteur Institute of Iran, 69 Pasteur Ave, Tehran 1316943551, Iran.

To investigate the vaccine potential of both the Toxoplasma GRA2 and GRA6 antigens, the full length recombinant proteins were produced in Escherichia coli, formulated in MPL adjuvant, and used alone and in combination ("mix"), to immunize CBA/J mice. Although high ratios of specific IgG2a/IgG1 were measured against both proteins, only spleen cells from GRA2-immunized mice and mix-immunized mice produced high amounts of both IFN-gamma and IL-2 upon induction with Toxoplasma gondii Excretory-Secretory Antigens. Intra peritoneal challenge with Toxoplasma cysts resulted in significant reduction of brain cysts in GRA2- and in mix-vaccinated mice only. This study shows the protective efficacy of recombinant GRA2 against chronic infection by T. gondii and confirms the utility of MPL adjuvant in enabling a vaccine candidate to induce a protective Th1 immune response.

PMID: 17418457 [PubMed - as supplied by publisher]

GRA3 is not necessary in cell culture but does contribute to virulence

Mol Biochem Parasitol. 2007 Mar 4; [Epub ahead of print]

Increased efficiency of homologous recombination in Toxoplasma gondii dense granule protein 3 demonstrates that GRA3 is not necessary in cell culture but does contribute to virulence

Craver MP, Knoll LJ.

Department of Medical Microbiology and Immunology, University of Wisconsin-Madison, 1300 University Avenue, Madison, WI 53706, United States.

Toxoplasma gondii possesses unique secretory organelles, which synchronously release proteins during and after invasion. One of these organelles, the dense granules, secrete proteins after invasion which are thought to be important in development of the parasite throughout all stages of its life cycle. Dense granule protein 3 (GRA3) is a 30kDa protein localized to the intravacuolar network and parasitophorous vacuole membrane (PVM). Like many dense granule proteins, GRA3 has no homology to proteins with described functions. However, it has been hypothesized to be involved in nutrient acquisition for the parasite due to its localization on the PVM. To begin to investigate the importance of GRA3, the locus was disrupted by homologous replacement with a chloramphenicol resistance gene in a type II strain. Two DeltaGRA3 strains were obtained after two independent electroporations with efficiency greater than 80%. No differences between wild-type and DeltaGRA3 were detected in cell culture growth rate or bradyzoite formation. Location of other parasite dense granule proteins and association with host cell organelles were also not affected in DeltaGRA3. Interestingly, at an infectious dose approximately four-fold above the lethal dose 50% for wild-type parasites, all mice infected with DeltaGRA3-2 infected mice survived acute infection. Complementation of GRA3 expression in the DeltaGRA3-2 strain restored virulence to wild-type levels, and increased the virulence of the DeltaGRA3-1, confirming that the GRA3 protein plays a role during acute infection in a type II strain.

PMID: 17418907 [PubMed - as supplied by publisher]

Monday, April 09, 2007

Rapid genotyping of Toxo by pyrosequencing

Clin Microbiol Infect. 2007 Apr;13(4):424-9.

Rapid genotyping of Toxoplasma gondii by pyrosequencing

Edvinsson B, Darde ML, Pelloux H, Evengard B; ESCMID Study Group on Toxoplasmosis.

Karolinska Institutet, Department of Laboratory Medicine, Division of Clinical Bacteriology, Karolinska University Hospital Huddinge, Stockholm, Sweden. benjamin.edvinsson@ki.se

Most human infections with the protozoan parasite Toxoplasma gondii are asymptomatic, but severe symptoms can occur in immunocompromised patients, in developing foetuses, and in ocular infections in immunocompetent individuals. The majority of T. gondii strains can be divided into three main lineages, denoted types I, II and III, which are known to cause different clinical presentations. Simple molecular methods with the capacity to discriminate rapidly among strains may help to predict the course of infection and influence the choice of treatment. In the present study, real-time PCR followed by pyrosequencing was used to discriminate among types I, II and III by analysis of two single nucleotide polymorphisms in the GRA6 gene. Twenty-one isolates of T. gondii characterised previously were analysed. Three different GRA6 alleles detected by the pyrosequencing technique identified types I, II and III isolates correctly, while four atypical isolates possessed either the GRA6 allele 1 or the GRA6 allele 3. Reproducibility was 100%, and typeability, when including atypical strains, was 81%. It was also possible to discriminate a mixture of two genotypes. The method was used to identify GRA6 type II in blood and lung tissue from an allogeneic transplant recipient with toxoplasmosis.

PMID: 17410643 [PubMed - in process]

Ribosomal phosphoprotein P0 of Neospora caninum as a potential common vaccine candidate

Mol Biochem Parasitol. 2007 Mar 4; [Epub ahead of print]

Identification of ribosomal phosphoprotein P0 of Neospora caninum as a potential common vaccine candidate for the control of both neosporosis and toxoplasmosis

Zhang H, Lee EG, Liao M, Compaore MK, Zhang G, Kawase O, Fujisaki K, Sugimoto C, Nishikawa Y, Xuan X.

National Research Center for Protozoan Diseases, Obihiro University of Agriculture and Veterinary Medicine, Inada-cho, Obihiro, Hokkaido 080-8555, Japan.

The characterization of the cross-reactive antigens of two closely related apicomplexan parasites, Neospora caninum and Toxoplasma gondii, is important to elucidate the common mechanisms of parasite-host interactions. In this context, a gene encoding N. caninum ribosomal phosphoprotein P0 (NcP0) was identified by immunoscreening of a N. caninum tachyzoite cDNA expression library with antisera from mice immunized with T. gondii tachyzoites. The NcP0 was encoded by a gene with open reading frame of 936bp, which encoded a protein of 311 amino acids. The NcP0 gene existed as a single copy in the genome and was interrupted by a 432bp intron. The NcP0 showed 94.5% amino acid identity to T. gondii P0 (TgP0). Anti-recombinant NcP0 (rNcP0) sera recognized a native parasite protein with a molecular mass of 34kDa in Western blot analysis. Immunofluorescence analysis showed that the NcP0 was localized to the surface of N. caninum tachyzoites. A purified anti-rNcP0 IgG antibody inhibited the growth of N. caninum and T. gondii in vitro in a concentration-dependent manner. These results indicate that P0 is a cross-reactive antigen between N. caninum and T. gondii and a potential common vaccine candidate to control both parasites.

PMID: 17412435 [PubMed - as supplied by publisher]

Friday, April 06, 2007

Molecular tools for analysis of gene function in parasitic microorganisms

Appl Microbiol Biotechnol. 2007 Mar 31; [Epub ahead of print]

Molecular tools for analysis of gene function in parasitic microorganisms

Meissner M, Agop-Nersesian C, Sullivan WJ Jr.

Hygieneinstitut, Abteilung Parasitologie, Universitatsklinikum Heidelberg, Im Neuenheimer Feld 324, 69120, Heidelberg, Germany, markus.meissner@med.uni-heidelberg.de.

With the completion of several genome sequences for parasitic protozoa, research in molecular parasitology entered the "post-genomic" era. Accompanied by global transcriptome and proteome analysis, huge datasets have been generated that have added many novel candidates to the list of drug and vaccine targets. The challenge is now to validate these factors and to bring science back to the bench to perform a detailed characterization. In some parasites, like Trypanosoma brucei, high-throughput genetic screens have been established using RNA interference [for a detailed review, see Motyka and Englund (2004)]. In most protozoan parasites, however, more time-consuming approaches have to be employed to identify and characterize the function of promising candidates in detail. This review aims to summarize the status of molecular genetic tools available for a variety of protozoan pathogens and discuss how they can be implemented to advance our understanding of parasite biology.

PMID: 17401559 [PubMed - as supplied by publisher]

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Toxo Dysregulates IFN-{gamma}-Inducible Gene Expression in Human Fibroblasts

J Immunol. 2007 Apr 15;178(8):5154-65.

Toxoplasma gondii Dysregulates IFN-{gamma}-Inducible Gene Expression in Human Fibroblasts: Insights from a Genome-Wide Transcriptional Profiling

Kim SK, Fouts AE, Boothroyd JC.

Department of Microbiology and Immunology, Stanford University School of Medicine, Stanford, CA 94305.

Toxoplasma gondii is an obligate intracellular parasite that persists for the life of a mammalian host. The parasite's ability to block the potent IFN-gamma response may be one of the key mechanisms that allow Toxoplasma to persist. Using a genome-wide microarray analysis, we show here a complete dysregulation of IFN-gamma-inducible gene expression in human fibroblasts infected with Toxoplasma. Notably, 46 of the 127 IFN-gamma-responsive genes were induced and 19 were suppressed in infected cells before they were exposed to IFN-gamma, indicating that other stimuli produced during infection may also regulate these genes. Following IFN-gamma treatment, none of the 127 IFN-gamma-responsive genes could be significantly induced in infected cells. Immunofluorescence assays showed at single-cell levels that infected cells, regardless of which Toxoplasma strain was used, could not be activated by IFN-gamma to up-regulate the expression of IFN regulatory factor 1, a transcription factor that is under the direct control of STAT1, whereas uninfected cells in the same culture expressed IFN regulatory factor 1 normally in response to IFN-gamma. STAT1 trafficked to the nucleus normally and indistinguishably in all uninfected and infected cells treated with IFN-gamma, indicating that the inhibitory effects of Toxoplasma infection likely occur via blocking STAT1 transcriptional activity in the nucleus. In contrast, a closely related apicomplexan, Neospora caninum, was unable to inhibit IFN-gamma-induced gene expression. A differential ability to interfere with the IFN-gamma response may, in part, account for the differences in the pathogenesis seen among Toxoplasma and Neospora parasite strains.

PMID: 17404298 [PubMed - in process]

Behavioral changes induced by Toxo infection of rodents are highly specific to aversion of cat odors

Proc Natl Acad Sci U S A. 2007 Apr 2; [Epub ahead of print]

Behavioral changes induced by Toxoplasma infection of rodents are highly specific to aversion of cat odors

Vyas A, Kim SK, Giacomini N, Boothroyd JC, Sapolsky RM.

Department of Biological Sciences, Stanford University, Stanford, CA 94305.

The protozoan parasite Toxoplasma gondii blocks the innate aversion of rats for cat urine, instead producing an attraction to the pheromone; this may increase the likelihood of a cat predating a rat. This is thought to reflect adaptive, behavioral manipulation by Toxoplasma in that the parasite, although capable of infecting rats, reproduces sexually only in the gut of the cat. The "behavioral manipulation" hypothesis postulates that a parasite will specifically manipulate host behaviors essential for enhancing its own transmission. However, the neural circuits implicated in innate fear, anxiety, and learned fear all overlap considerably, raising the possibility that Toxoplasma may disrupt all of these nonspecifically. We investigated these conflicting predictions. In mice and rats, latent Toxoplasma infection converted the aversion to feline odors into attraction. Such loss of fear is remarkably specific, because infection did not diminish learned fear, anxiety-like behavior, olfaction, or nonaversive learning. These effects are associated with a tendency for parasite cysts to be more abundant in amygdalar structures than those found in other regions of the brain. By closely examining other types of behavioral patterns that were predicted to be altered we show that the behavioral effect of chronic Toxoplasma infection is highly specific. Overall, this study provides a strong argument in support of the behavioral manipulation hypothesis. Proximate mechanisms of such behavioral manipulations remain unknown, although a subtle tropism on part of the parasite remains a potent possibility.

PMID: 17404235 [PubMed - as supplied by publisher]

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Waterborne transmission of protozoan parasites: a worldwide review of outbreaks

J Water Health. 2007 Mar;5(1):1-38.

Waterborne transmission of protozoan parasites: a worldwide review of outbreaks and lessons learnt

Karanis P, Kourenti C, Smith H.

Unit for Molecular Epidemiology and Protozoan Pathogenetics, National Research Center for Protozoan Diseases, Obihiro University for Agriculture and Veterinary Medicine, Inada-cho, Obihiro 080-8555, Japan. karanis@obihiro.ac.jp

At least 325 water-associated outbreaks of parasitic protozoan disease have been reported. North American and European outbreaks accounted for 93% of all reports and nearly two-thirds of outbreaks occurred in North America. Over 30% of all outbreaks were documented from Europe, with the UK accounting for 24% of outbreaks, worldwide. Giardia duodenalis and Cryptosporidium parvum account for the majority of outbreaks (132; 40.6% and 165; 50.8%, respectively), Entamoeba histolytica and Cyclospora cayetanensis have been the aetiological agents in nine (2.8%) and six (1.8%) outbreaks, respectively, while Toxoplasma gondii and Isospora belli have been responsible for three outbreaks each (0.9%) and Blastocystis hominis for two outbreaks (0.6%). Balantidium coli, the microsporidia, Acanthamoeba and Naegleria fowleri were responsible for one outbreak, each (0.3%). Their presence in aquatic ecosystems makes it imperative to develop prevention strategies for water and food safety. Human incidence and prevalence-based studies provide baseline data against which risk factors associated with waterborne and foodborne transmission can be identified. Standardized methods are required to maximize public health surveillance, while reporting lessons learned from outbreaks will provide better insight into the public health impact of waterborne pathogenic protozoa.

PMID: 17402277 [PubMed - in process]

Prevention and treatment of congenital toxoplasmosis

Expert Rev Anti Infect Ther. 2007 Apr;5(2):285-93.

Prevention and treatment of congenital toxoplasmosis

Petersen E.

Aarhus University Hospital, Department of Infectious Diseases, DK8000 Aarhus, Denmark. epf@sks.aaa.dk

Infection with Toxoplasma gondii is transmitted to man by infected meat or meat products and by contact with soil or surface water. In theory, prevention by hygienic measures is possible, but this has never been proved to work in practice. Therefore, pre- and postnatal screening has been implemented in several countries aiming at early diagnosis. However, data on the effect of treatment are limited and no randomized, controlled trials have been performed. The risk of T. gondii infection in Europe is declining and studies using historical controls from earlier decades cannot be used for decision making. The screening of pregnant women or neonates makes the assumption that any children diagnosed can be offered an effective treatment. There is an urgent need to test new drugs and demonstrate, using randomized, controlled trials, that the currently used drugs are effective.

PMID: 17402843 [PubMed - in process]

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The methylerythritol phosphate pathway for isoprenoid biosynthesis in coccidia

Exp Parasitol. 2007 Feb 21; [Epub ahead of print]

The methylerythritol phosphate pathway for isoprenoid biosynthesis in coccidia: Presence and sensitivity to fosmidomycin.

Clastre M, Goubard A, Prel A, Mincheva Z, Viaud-Massuart MC, Bout D, Rideau M, Velge-Roussel F, Laurent F.

EA2106 Biomolecules et Biotechnologies Vegetales, UFR Sciences Pharmaceutiques, Universite de Tours, 37200 Tours, France.

The apicoplast is a recently discovered, plastid-like organelle present in most apicomplexa. The methylerythritol phosphate (MEP) pathway involved in isoprenoid biosynthesis is one of the metabolic pathways associated with the apicoplast, and is a new promising therapeutic target in Plasmodium falciparum. Here, we check the presence of isoprenoid genes in four coccidian parasites according to genome database searches. Cryptosporidium parvum and C. hominis, which have no plastid genome, lack the MEP pathway. In contrast, gene expression studies suggest that this metabolic pathway is present in several development stages of Eimeria tenella and in tachyzoites of Toxoplasma gondii. We studied the potential of fosmidomycin, an antimalarial drug blocking the MEP pathway, to inhibit E. tenella and T. gondii growth in vitro. The drug was poorly effective even at high concentrations. Thus, both fosmidomycin sensitivity and isoprenoid metabolism differs substantially between apicomplexan species.

PMID: 17399705 [PubMed - as supplied by publisher]

Human brain parasite alters fear

Human brain parasite alters fear
Scientists say discovery could shed light "on how fear is generated

By Charles Q. Choi
Updated: 6:34 p.m. ET April 2, 2007

Rats usually have an innate fear of cat urine. The fear extends to rodents that have never seen a feline and those generations removed from ever meeting a cat. After they get infected with the brain parasite Toxoplasma gondii, however, rats become attracted to cat pee, increasing the chance they'll become cat food.
This much researchers knew. But a new study shows the parasite, which also infects more half the world's human population, seems to target a rat's fear of cat urine with almost surgical precision, leaving other kinds of fear alone.
This discovery could shed light "on how fear is generated in the first place" and how people can potentially better manage phobias, researcher Ajai Vyas, a Stanford University neuroscientist, told LiveScience.
Hijacking the mindT. gondii is a parasitic germ whose primary hosts are cats. However, it can be found in most warm-blooded animals, including an estimated 50 million people in the United States. One study suggests the parasite has altered human behavior enough to shape entire cultures.
In cats, the protozoan reproduces sexually, while it reproduces asexually in other animals.
The germ seems to especially like infesting the brain — "parasites hijacking the mind," Vyas said. Although the disease it causes in humans is rarely dangerous, it is the reason that pregnant women are sometimes told to avoid cat litter boxes (toxoplasmosis is risky for infants and others with compromised immune systems). Some scientists have suspected it might be linked to mental disorders such as schizophrenia and even neuroticism.
In 2000, scientists revealed T. gondii could modify the brains of rats to make them attracted to cat urine instead of afraid of it. Researchers suspect the germ does so to make it easier for it to jump into cats to begin the sexual part of its life cycle.
Vyas and his colleagues now show how specific this brain reprogramming is when it comes to rats, findings detailed online April 2 in the Proceedings of the National Academy of Sciences.
Just cat peeRats infected with the parasite became mildly attracted to bobcat pee. However, they remained as fearful of open spaces as normal rats. They reacted normally to sound cues that suggested mild electrical shocks were coming. Normally rats are somewhat reticent when it comes to eating food that smells unfamiliar. And the infected rats were, just like the normal rats, reticent when it came to food scented with the unfamiliar odor of coriander.
"One would thus assume that if something messes up with fear to cat pee, it will also mess up a variety of related behaviors," Vyas said. "We do not see that. Toxoplasma affects fear to cat odors with almost surgical precision."
In addition, "we show that parasites are a little more likely to be found in amygdala [a region of the brain] than in other brain areas," Vyas said. "This is important because the amygdala is involved in a variety of fear-related behaviors."
Future investigations can explore how exactly the parasite modifies the brain in such a precise manner. Potential targets in the brain for research include the stress hormone corticosterone and the brain chemical dopamine. Scientists might also want to see whether infected rats become less afraid of pictures of cats or scents of different predators of rats.

© 2007 LiveScience.com. All rights reserved.

URL: http://www.msnbc.msn.com/id/17920003/wid/11915773?GT1=9246