Research Projects – Group S. Besteiro
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
Over the last few years, we have focused on the iron-sulfur cluster synthesis pathways, one of which is hosted by the apicoplast and is essential for tachyzoite viability, while being absent from the mammalian hosts of the parasite.
Latest publications related to this project:
Renaud EA, Pamukcu S, Cerutti A, Berry L, Lemaire-Vieille C, Yamaryo-Botté Y, Botté CY, Besteiro S. (2022) Disrupting the plastidic iron-sulfur cluster biogenesis pathway in Toxoplasma gondii has pleiotropic effects irreversibly impacting parasite viability. J Biol Chem. Aug;298(8):102243. doi: 10.1016/j.jbc.2022.102243.
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 have sought to determine the importance of the organelle for survival and persistence of this parasite stage. We have used 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-).
Through this research project we have:
► generated genetic tools to investigate the function of essential genes in bradyzoites
► shown the importance of 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 publications related to this project:
Sanchez SG, Bassot E, Cerutti A, Mai Nguyen H, Aïda A, Blanchard N, Besteiro S. (2023) The apicoplast is important for the viability and persistence of Toxoplasma gondii bradyzoites. Proc Natl Acad Sci U S A. Aug 22;120(34):e2309043120. doi: 10.1073/pnas.2309043120.
Cerutti A, Blanchard N, Besteiro S. (2020) The Bradyzoite: A Key Developmental Stage for the Persistence and Pathogenesis of Toxoplasmosis. Pathogens. Mar 21;9(3):234. doi: 10.3390/pathogens9030234.
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 labs of Ellen Yeh, Stanford Uni., USA and of Vern Carruthers, Univ. Michigan, USA)
► identifying novel parasite-specific functions for the autophagy-related machinery at the apicoplast
Latest publications related to this project:
Walczak M, Meister TR, Nguyen HM, Zhu Y, Besteiro S, Yeh E. (2023) Structure-Function Relationship for a Divergent Atg8 Protein Required for a Nonautophagic Function in Apicomplexan Parasites. mBio. Feb 28;14(1):e0364221. doi: 10.1128/mbio.03642-21.
Smith D, Kannan G, Coppens I, Wang F, Nguyen HM, Cerutti A, Olafsson EB, Rimple PA, Schultz TL, Mercado Soto NM, Di Cristina M, Besteiro S, Carruthers VB. (2021) Toxoplasma TgATG9 is critical for autophagy and long-term persistence in tissue cysts. Elife. Apr 27;10:e59384. doi: 10.7554/eLife.59384.