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Toxoplasma Research

Apicomplexan Metabolomics

In the metabolomics project, we are taking a metabolomics approach to characterize the different types (strains) of Toxoplasma gondii, which is a parasite related to the Malaria pathogen, infecting at least one quarter of the world population. The disease caused by this parasite is called Toxoplasmosis. It has been found worldwide and is one of the most common parasitic infections of humans and other warm-blooded animals. No sterilizing drug exists. In most adults Toxoplasmosis does not cause serious illness, but it can cause blindness and mental retardation in congenitally infected children and devastating disease in immunocompromised individuals, e.g. when receiving an organ transplant or being infected with HIV. Therefore, it is urgent to decipher potential drug targets. One therapeutic strategy is to find enzyme inhibitors targeting critical metabolic processes of the pathogen. Our goal is to determine those critical metabolic processes in the different types respectively strains of T. gondii.

There are at least three different types (I, II, and III) of T. gondii which differ in their virulence and metabolic strategies they are using. Our hypothesis is that the different types have evolved specific metabolic strategies critical for growth and survival during human infection. Preliminary analysis show that the abundance of metabolites is different, i.e. key metabolic enzymes might be different between strains and linked to their difference in virulence. To this end, we aim to find out which metabolic processes are critical for the survival of the different strains in the human body, while the parasite is actively replicating. We are using an in vitro model of T. gondii in the first instance, with the purpose of determining potential biomarkers or drug targets.

We used mass spectrometry to identify the frequency of metabolites present within different metabolic groups (Eicosanoids, Sphingolipids, Phosphatidylcholines (PC) and Lyso-PCs, some Amino Acids, Glycolysis, Pentose phosphate pathway, and the Citric acid cycle).

Metabolic Reconstruction and Flux Balance Analysis of Toxoplasma gondii

The Apicomplexan parasite Toxoplasma gondii has an amazingly broad host range due to its ability to infect virtually any nucleated cell.  Subsequently, T. gondii has partitioned into discrete strains displaying different virulence profiles across various hosts.  In this work, we hypothesize that differences in strain virulence are due at least in part to differences in metabolism in T. gondii.  To test this hypothesis, we have undertaken the first highly-curated genome-scale metabolic reconstruction of the parasite through literature review and by using a number of tools, such as BRENDA, ToxoDB, and DETECT.  The resulting model is called iCS382 and is available on the projects section of our website.

Growth within the parasite has been simulated using Flux Balance Analysis, an established method that has been and continues to be used to analyze a variety of systems.  Layering on expression data on top of the network for the strains RH and Me49, we are able to recapitulate the faster growth rate of RH.  In addition, we find that differences in utilization of energy-production pathways can explain differences in their growth rates.  We have performed in silico single and double reaction knockouts to explore differences in strategies for survival in strains RH and Me49.  Interestingly, we have been able to confirm certain essentiality predictions through drug inhibition assays performed by our collaborators at the NIH.

We anticipate that iCS382 will serve as a useful resource for further exploration of different aspects of the parasite Toxoplasma gondii.

Song, C, Chiasson MA, Nursimulu N, Hung SS, Wasmuth J, Grigg ME, Parkinson J. (2013) Metabolic reconstruction identifies strain-specific regulation of virulence in Toxoplasma gondii. Mol Syst Biol 9: 708

Identification of Novel Drug Targets Through Comparative Genomics

Toxoplasma gondii is a single cell parasite capable of infecting nearly all warm blooded animals, including humans. It infects at least a third of the world's population. In new-borns and immunocompromised individuals, T. gondii infection leads to a serious condition known as toxoplasmosis. Symptoms of toxoplasmosis can range from blindness to foetal death. Current treatment for toxoplasmosis has both limited effectiveness and serious side effects. Hence, there is an urgent need for new therapeutics. A key challenge to designing a new therapeutic is the identification of a therapeutic target – a component of the pathogen that the drug acts on. Therapeutic targets must be effective and specific to the pathogen in order to avoid high dosage and side effects. Given that many of life's essential processes are shared between humans and pathogens, this is often difficult.

We believe that the best approach to this problem is comparative genomics, where the genomes of different species or strains of an organism are compared. This  can identify both elements conserved across all the strains and elements unique to each strain. The conserved elements represent genes that are crucial to the organism's survival. The unique elements are associated with each strain's unique characteristics such as virulence and nutritional requirements. Taken together, they offer a wealth of potential targets, many of which are unique to the organism. Our project applies this technique to a large collection of T. gondii strains to identify potential therapeutic targets.

First, we will construct a computational pipeline to compare the genomes of different T. gondii strains. We will then apply this pipeline to 64 strains to identify conserved and unique genomic elements. From these results, we will select and verify potential therapeutic targets. Drugs can then be designed against the verified targets to create a safe and effective treatment against Toxoplasmosis.

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