Disrupting the plastidic iron-sulfur cluster biogenesis pathway in Toxoplasma gondii has pleiotropic effects irreversibly impacting parasite viability

Iron depletion has different consequences on the growth and survival of Toxoplasma gondii strains

Toxoplasma gondii is an obligate intracellular parasite responsible for a pathology called toxoplasmosis, which primarily affects immunocompromised individuals and developing foetuses. The parasite can scavenge essential nutrients from its host to support its growth and survival. Among them, iron is one of the most important elements needed to sustain basic cellular functions as it is involved in a number of key metabolic processes, including oxygen transport, redox balance, and electron transport. We evaluated the effects of an iron chelator on the development of several parasite strains and found that they differed in their ability to tolerate iron depletion. The growth of parasites usually associated with a model of acute toxoplasmosis was strongly affected by iron depletion, whereas cystogenic strains were less sensitive as they were able to convert into persisting developmental forms that are associated with the chronic form of the disease. Ultrastructural and biochemical characterization of the impact of iron depletion on parasites also highlighted striking changes in both their metabolism and that of the host, with a marked accumulation of lipid droplets and perturbation of lipid homoeostasis. Overall, our study demonstrates that although acute iron depletion has an important effect on the growth of T. gondii, it has a more profound impact on actively dividing parasites, whereas less metabolically active parasite forms may be able to avoid some of the most detrimental consequences.

Complex Endosymbiosis II: The Nonphotosynthetic Plastid of Apicomplexa Parasites (The Apicoplast) and Its Integrated Metabolism

Chloroplasts are essential organelles that are responsible for photosynthesis in a wide range of organisms that have colonized all biotopes on Earth such as plants and unicellular algae. Interestingly, a secondary endosymbiotic event of a red algal ancestor gave rise to a group of organisms that have adopted an obligate parasitic lifestyle named Apicomplexa parasites. Apicomplexa parasites are some of the most widespread and poorly controlled pathogens in the world. These infectious agents are responsible for major human diseases such as toxoplasmosis, caused by Toxoplasma gondii, and malaria, caused by Plasmodium spp. Most of these parasites harbor this relict plastid named the apicoplast, which is essential for parasite survival. The apicoplast has lost photosynthetic capacities but is metabolically similar to plant and algal chloroplasts. The apicoplast is considered a novel and important drug target against Apicomplexa parasites. This chapter focuses on the apicoplast of apicomplexa parasites, its maintenance, and its metabolic pathways.

F-actin and myosin F control apicoplast elongation dynamics which drive apicoplast-centrosome association in Toxoplasma gondii

IMPORTANCE

Toxoplasma gondii and most other parasites in the phylum Apicomplexa contain an apicoplast, a non-photosynthetic plastid organelle required for fatty acid, isoprenoid, iron-sulfur cluster, and heme synthesis. Perturbation of apicoplast function results in parasite death. Thus, parasite survival critically depends on two cellular processes: apicoplast division to ensure every daughter parasite inherits a single apicoplast, and trafficking of nuclear encoded proteins to the apicoplast. Despite the importance of these processes, there are significant knowledge gaps in regards to the molecular mechanisms which control these processes; this is particularly true for trafficking of nuclear-encoded apicoplast proteins. This study provides crucial new insight into the timing of apicoplast protein synthesis and trafficking to the apicoplast. In addition, this study demonstrates how apicoplast-centrosome association, a key step in the apicoplast division cycle, is controlled by the actomyosin cytoskeleton.

The apicoplast is important for the viability and persistence of Toxoplasma gondii bradyzoites

Toxoplasma gondii is responsible for toxoplasmosis, a disease that can be serious when contracted during pregnancy, but can also be a threat for immunocompromised individuals. Acute infection is associated with the tachyzoite form that spreads rapidly within the host. However, under stress conditions, some parasites can differentiate into cyst-forming bradyzoites, residing mainly in the central nervous system, retina and muscle. Because this latent form of the parasite is resistant to all currently available treatments, and is central to persistence and transmission of the parasite, specific therapeutic strategies targeting this developmental stage need to be found. T. gondii contains a plastid of endosymbiotic origin called the apicoplast, which is an appealing drug target because it is essential for tachyzoite viability and contains several key metabolic pathways that are largely absent from the mammalian host. Its function in bradyzoites, however, is unknown. Our objective was thus to study the contribution of the apicoplast to the viability and persistence of bradyzoites during chronic toxoplasmosis. We have used complementary strategies based on stage-specific promoters to generate conditional bradyzoite mutants of essential apicoplast genes. Our results show that specifically targeting the apicoplast in both in vitro or in vivo-differentiated bradyzoites leads to a loss of long-term bradyzoite viability, highlighting the importance of this organelle for this developmental stage. This validates the apicoplast as a potential area to look for therapeutic targets in bradyzoites, with the aim to interfere with this currently incurable parasite stage.

F-actin and Myosin F control apicoplast elongation dynamics which drive apicoplast-centrosome association in Toxoplasma gondii

Toxoplasma gondii contains an essential plastid organelle called the apicoplast that is necessary for fatty acid, isoprenoid, and heme synthesis. Perturbations affecting apicoplast function or inheritance lead to parasite death. The apicoplast is a single copy organelle and therefore must be divided so that each daughter parasite inherits an apicoplast during cell division. In this study we identify new roles for F-actin and an unconventional myosin motor, TgMyoF, in this process. First, loss of TgMyoF and actin lead to an accumulation of apicoplast vesicles in the cytosol indicating a role for this actomyosin system in apicoplast protein trafficking or morphological integrity of the organelle. Second, live cell imaging reveals that during division the apicoplast is highly dynamic, exhibiting branched, U-shaped and linear morphologies that are dependent on TgMyoF and actin. In parasites where movement was inhibited by the depletion of TgMyoF, the apicoplast fails to associate with the parasite centrosomes. Thus, this study provides crucial new insight into mechanisms controlling apicoplast-centrosome association, a vital step in the apicoplast division cycle, which ensures that each daughter inherits a single apicoplast.

Mitochondrial [2Fe-2S] ferredoxins: new functions for old dogs

Ferredoxins (FDXs) comprise a large family of iron-sulfur proteins that shuttle electrons from NADPH and FDX reductases into diverse biological processes. This review focuses on the structure, function and specificity of mitochondrial [2Fe-2S] FDXs that are related to bacterial FDXs due to their endosymbiotic inheritance. Their classical function in cytochrome P450-dependent steroid transformations was identified around 1960, and is exemplified by mammalian FDX1 (aka adrenodoxin). Thirty years later the essential function in cellular Fe/S protein biogenesis was discovered for the yeast mitochondrial FDX Yah1 that is additionally crucial for the formation of haem a and ubiquinone CoQ₆ . In mammals, Fe/S protein biogenesis is exclusively performed by the FDX1 paralog FDX2, despite the high structural similarity of both proteins. Recently, additional and specific roles of human FDX1 in haem a and lipoyl cofactor biosyntheses were described. For lipoyl synthesis, FDX1 transfers electrons to the radical S-adenosyl methionine-dependent lipoyl synthase to kickstart its radical chain reaction. The high target specificity of the two mammalian FDXs is contained within small conserved sequence motifs, that upon swapping change the target selection of these electron donors.

The ferredoxin redox system - an essential electron distributing hub in the apicoplast of Apicomplexa

The apicoplast, a relict plastid found in most species of the phylum Apicomplexa, harbors the ferredoxin redox system which supplies electrons to enzymes of various metabolic pathways in this organelle. Recent reports in Toxoplasma gondii and Plasmodium falciparum have shown that the iron-sulfur cluster (FeS)-containing ferredoxin is essential in tachyzoite and blood-stage parasites, respectively. Here we review ferredoxin's crucial contribution to isoprenoid and lipoate biosynthesis as well as tRNA modification in the apicoplast, highlighting similarities and differences between the two species. We also discuss ferredoxin's potential role in the initial reductive steps required for FeS synthesis as well as recent evidence that offers an explanation for how NADPH required by the redox system might be generated in Plasmodium spp.