The organism: Toxoplasma gondii and related intracellular apicomplexan parasites
Parasites of the phylum Apicomplexa are single-celled eukaryotic organisms that are mostly intracellular pathogens of warm-blooded animals. The best known species (both scientifically as well as publicly) are Plasmodium falciparum, the causative agent of tropical malaria, and Toxoplasma gondii, causing human toxoplasmosis. Both organisms are the subject of extensive worldwide research efforts to understand their interaction with the human host cells they infect. Based on this knowledge it is hoped to develop potent and cheap new medications that help to prevent or combat disease in infected humans. For more information on these two parasites see the CDC information on malaria(externer Link) and toxoplasmosis(externer Link) and the various links on the Links page.
T. gondii can be easily grown in almost all vertebrate nucleated cells, possesses a clear morphology easily visible under the microscope and has a vast number of molecular tools being developed during the last decade, allowing its straight-forward genetic manipulation (see Toxoplasma. Molecular and Cellular Biology(externer Link)). This has resulted in T. gondii being a good model organism for cell biologists but also for certain features shared by Plasmodium and T. gondii and that can be easier studied in the latter.
The current questions: organellar interactions within the parasite and with those of the host
One of the similarities between T. gondii and Plasmodium is the possesion of an organelle of secondary endosymbiotic origin called the apicoplast (apicomplexan plastid). This plastid-derived organelle hosts a number of metabolic pathways and seems to be essential for the survival of those apicomplexa that possess it. What exactly the essential component(s) are the apicoplast provides is still ill defined and is one of my major interests. Parasite apicoplast and mitochondrion are always in close physical contact, implying substantial 'crosstalk' and exchange of metabolites between these two organelles (see Seeber et al. 2008(externer Link)).
Another striking and unique organellar association is that between the vacuolar compartment T. gondii resides in (so-called parasitophorous vacuole, PV, surrounded by the parasitophorous vacuolar membrane, PVM) and the host mitochondria and ER, which firmly adhere to the PVM soon after invasion of the host cell. Again, this is taken as evidence for the exchange of compounds (lipids, lipoic acid(externer Link) etc.), although it is likely a one-way street towards the PV to help the parasite scavenging essential nutrients from the host cell. Elucidation of the nature of the scavenged metabolites and the molecular components involved in the mobilization and transport of these compounds is another important focus of my current research.
The current projects: organelles, metabolic pathways and new drugs
The fact that a number of the known metabolic pathways in the apicoplast are of plant or cyanobacterial origin and have thus no direct structural homologs in the human hosts have made them attractive targets for the development of new drugs. This is because such compounds are believed to cause less side effects (see e.g. Seeber 2009(externer Link), Wiesner and Seeber 2005 ). Especially in the case of Plasmodium falciparum which constantly develops resistance against currently used drugs such new targets are urgently required. However, there is also no cure for the chronic phase of an T. gondii infection, and consequently drugs against this dormant stage would be very welcomed.
One such potential drug target is the plant-type ferredoxin redox system (ptFd/FNR) that we have extensively charcterized in previous years. It donates electrons to more than one essential metabolic pathway in the apicoplast and it is absent in humans (see Seeber et al. 2005(externer Link)). Our current efforts are aimed at developing means to inhibit its function in the parasite, based on the known 3D-structure of the plasmodial proteins (see Milani et al. 2007(externer Link)). This might not only allow the development of drug-like substances in the near future but will be also of great benefit for the eludication of the exact function of the ptFd/FNR redox system in the apicomplexa.
Essential compound for apicomplexa that have to come from the host are biotin and lipoic acid (LA), and although the latter is synthesized in the apicoplast it is apparently not available to the parasite's mitochondrion where it is also required. Interfering with the uptake of LA from host mitochondria (the only place where LA is synthesized and used) would therefore confront T. gondii with an unresolvable problem. We are currently in the process to analyze how protein-bound LA is released for transport through the various membranes to reach the parasite's mitochondrion and which host and parasite proteins are involved in this complex task. Since liver stages of Plasmodium have the same LA requirements and are faced with similar problems this project is also highly relevant for this apicomplexan.
More (unnamed) projects: improvement of methods
Defining or testing new drugs requires specific assay systems which allow the potency of a substance to be monitored easily and specifically. Since T. gondii is an obligate intracellular parasite residing within an actively metabolising host cell, discrimination between the host's versus the parasite's vital functions is therefor essential. We have previously established several assay formats based on lacZ-transgenic T. gondii for this (see McFadden et al. 1997(externer Link)) and are evaluating new parasite-specific enzymes for the same purpose.
Also, a genetic screen is being developed in bacteria for finding disruptors of protein-protein interactions of ptFd/FNR. This system will potentially be transferred to T. gondii as a general means to study this important aspect in apicomplexa.