Toxoplasma gondii is a parasitic protist infecting a wide variety of warm-blooded animals and about one third of the human population. Although it remains essentially asymptomatic in immunocompetent individuals, this parasite can cause a life-threatening disease called toxoplasmosis in fetuses, newborns, and individuals with weakened immune systems. Our group studies how these obligate intracellular parasites multiply and persist in the host. We are targeting essential cellular processes, which are thus sometimes conserved among eukaryotes, but bearing enough differences to make them interesting potential therapeutic targets in the parasites.

 

Research project #1. The apicoplast as a drug target in the context of acute toxoplasmosis

As several other medically important parasites of the phylum Apicomplexa (ie malaria-causing Plasmodium), T. gondii contains an unusual organelle called the apicoplast. This organelle is a startling peculiarity that not only highlights the diversity of eukaryotic cell biology, but can also potentially be leveraged for therapeutic development. This four-membrane plastid was acquired by an unusual secondary endosymbiosis, in which an alga was engulfed by another eukaryote forming a new secondary plastid in the host. Although the apicoplast has lost its photosynthetic function, it houses several important metabolic pathways for generating: iron/sulfur cluster, hem, fatty acids (FASII), isoprenoids (Fig. 2).

 

Figure 2. Schematic representation of a Toxoplasma tachyzoite, with a four-membrane apicoplast and the main biochemical pathways it hosts.

Although the apicoplast is known as an important metabolic hub for many species of apicomplexan parasites, the metabolic pathways which are absolutely essential for parasite viability may vary depending on the parasite or the developmental stage. For instance, T. gondii has a complex life cycle involving several developmental stages that develop in felids (the definitive hosts, where sexual reproduction occurs), but also divide asexually in the many species of warm-blooded animals that can act as intermediate hosts. The two developmental forms found in intermediate hosts are the tachyzoite and the bradyzoite. Tachyzoites are rapidly dividing forms associated with the acute phase of toxoplasmosis. Upon control by a competent immune system, however, the parasites can differentiate into slowly-growing bradyzoites, establishing within tissue cysts and responsible for the chronic phase of toxoplasmosis.

The apicoplast is already targeted by anti-parasitic drugs (for instance through several prokaryotic translation inhibitors) and apicoplast-hosted hem, fatty acid and isoprenoid synthesis pathways have all been shown to contribute to the fitness of the tachyzoite stage. However, several metabolic pathways remain unexplored and unexploited as potential drug targets.  

This research project aims at:

► identifying novel apicoplast-hosted parasite-specific functions

► assessing if they can be exploited to target the tachyzoite stage

Latest publication related to this project:

Pamukcu S, Cerutti A, Bordat Y, Hem S, Rofidal V, Besteiro S. (2021) Differential contribution of two organelles of endosymbiotic origin to iron-sulfur cluster synthesis and overall fitness in Toxoplasma.  PLoS Pathog. 17(11):e1010096. doi: 10.1371/journal.ppat.1010096.

Research project #2. Contribution of the apicoplast to Toxoplasma persistence

As mentioned above, acute infection of intermediate hosts by T. gondii is associated with the rapid replication and spread of the tachyzoite forms within the body. This infection phase is often readily contained by the immune system. However, the parasites can differentiate into slowly growing bradyzoites, establishing within tissue cysts, primarily in the central nervous system and muscle. Although there are effective medicines available against tachyzoites, the persistent chronic form of the pathogen remains in the host throughout its life and can convert repeatedly back into tachyzoites, and hence lead to a severe pathology (i.e. encephalitis or retinitis) in the event of a weakened immune system. These bradyzoites forms are thus central to the pathology, yet there are no effective drugs against them so far.

In bradyzoites, where apicoplast function has been largely overlooked, we wish to determine the importance of the organelle for survival and persistence of this parasite stage. We are using stage-specific conditional knock-down or knock-out approaches to deplete apicoplast proteins in bradyzoites (Fig. 3).

Figure 3.  Stage-specific depletion of an apicoplast marker (green) in in vitro-differentiated bradyzoites (arrowhead, cyst labelled by the Dolichos biflorus lectin –DBL, red-).

This research project aims at:

► generating genetic tools to investigate the function of essential genes in bradyzoites

► elucidating the contribution of the apicoplast to the persistence and reactivation of bradyzoites in vitro and in vivo (collaboration with the lab of Nicolas Blanchard, Université de Toulouse, France)

Latest publication related to this project:

Cerutti A, Blanchard N, Besteiro S. Pathogens. (2020) The Bradyzoite: A Key Developmental Stage for the Persistence and Pathogenesis of Toxoplasmosis. Cerutti A, Blanchard N, Besteiro S. Pathogens. 2020 Mar 21;9(3):234. doi: 10.3390/pathogens9030234. Pubmed

Other research projects

We are also interested in other aspects of the cell biology of Toxoplasma, including specific aspects of the cell division process, the cytoskeleton, as well as canonical and non-canonical functions of the autophagy machinery.

This includes:

► elucidating the physiological roles of parasite autophagy in the context of acute and chronic toxoplasmosis (collaboration with the lab of Vern Carruthers, Univ. Michigan, USA)

► identifying novel parasite-specific functions for the autophagy-related machinery at the apicoplast