Characterization of the chloroquine resistance transporter homologue in Toxoplasma gondii

Adaptations and metabolic evolution of myzozoan protists across diverse lifestyles and environments

SUMMARYMyzozoans encompass apicomplexans and dinoflagellates that manifest diverse lifestyles in highly varied environments. They show enormous propensity to employ different metabolic programs and exploit different nutrient resources and niches, and yet, they share much core biology that underlies this evolutionary success and impact. This review discusses apicomplexan parasites of medical significance and the traits and properties they share with non-pathogenic myzozoans. These include the versatility of myzozoan plastids, which scale from fully photosynthetic organelles to the site of very select key metabolic pathways. Pivotal evolutionary innovations, such as the apical complex, have allowed myzozoans to shift from predatory to parasitic and other symbiotic lifestyles multiple times in both apicomplexan and dinoflagellate branches of the myzozoan evolutionary tree. Such traits, along with shared mechanisms for nutrient acquisition, appear to underpin the prosperity of myzozoans in their varied habitats. Understanding the mechanisms of these shared traits has the potential to spawn new strategic interventions against medically and veterinary relevant parasites within this grouping.

A Toxoplasma gondii putative amino acid transporter localizes to the plant-like vacuolar compartment and controls parasite extracellular survival and stage differentiation

Toxoplasma gondii is a protozoan parasite that infects a broad spectrum of hosts and can colonize many organs and cell types. The ability to reside within a wide range of different niches requires substantial adaptability to diverse microenvironments. Very little is known about how this parasite senses various milieus and adapts its metabolism to survive, replicate during the acute stage, and then differentiate to the chronic stage. T. gondii possesses a lysosome-like organelle known as the plant-like vacuolar compartment (PLVAC), which serves various functions, including digestion, ion storage and homeostasis, endocytosis, and autophagy. Lysosomes are critical for maintaining cellular health and function by degrading waste materials and recycling components. To supply the cell with the essential building blocks and energy sources required for the maintenance of its functions and structures, the digested solutes generated within the lysosome are transported into the cytosol by proteins embedded in the lysosomal membrane. Currently, a limited number of PLVAC transporters have been characterized, with TgCRT being the sole potential transporter of amino acids and small peptides identified thus far. To bridge this knowledge gap, we used lysosomal amino acid transporters from other organisms as queries to search the T. gondii proteome. This led to the identification of four potential amino acid transporters, which we have designated as TgAAT1-4. Assessing their expression and sub-cellular localization, we found that one of them, TgAAT1, localized to the PLVAC and is necessary for normal parasite extracellular survival and bradyzoite differentiation. Moreover, we present preliminary data showing the possible involvement of TgAAT1 in the PLVAC transport of arginine.IMPORTANCEToxoplasma gondii is a highly successful parasite infecting a broad range of warm-blooded organisms, including about one-third of all humans. Although Toxoplasma infections rarely result in symptomatic disease in individuals with a healthy immune system, the incredibly high number of persons infected, along with the risk of severe infection in immunocompromised patients and the potential link of chronic infection to mental disorders, makes this infection a significant public health concern. As a result, there is a pressing need for new treatment approaches that are both effective and well tolerated. The limitations in understanding how Toxoplasma gondii manages its metabolism to adapt to changing environments and triggers its transformation into bradyzoites have hindered the discovery of vulnerabilities in its metabolic pathways or nutrient acquisition mechanisms to identify new therapeutic targets. In this work, we have shown that the lysosome-like organelle plant-like vacuolar compartment (PLVAC), acting through the putative arginine transporter TgAAT1, plays a pivotal role in regulating the parasite's extracellular survival and differentiation into bradyzoites.

A Toxoplasma gondii putative arginine transporter localizes to the plant-like vacuolar compartment and controls parasite extracellular survival and stage differentiation

Toxoplasma gondii is a protozoan parasite that infects a broad spectrum of hosts and can colonize many organs and cell types. The ability to reside within a wide range of different niches requires substantial adaptability to diverse microenvironments. Very little is known about how this parasite senses various milieus and adapts its metabolism to survive, replicate during the acute stage, and then differentiate to the chronic stage. Most eukaryotes, from yeast to mammals, rely on a nutrient sensing machinery involving the TORC complex as master regulator of cell growth and cell cycle progression. The lysosome functions as a signaling hub where TORC complex assembles and is activated by transceptors, which both sense and transport amino acids, including the arginine transceptor SLC38A9. While most of the TORC components are lost in T. gondii , indicating the evolution of a distinct nutrient sensing mechanism, the parasite's lysosomal plant-like vacuolar compartment (PLVAC) may still serve as a sensory platform for controlling parasite growth and differentiation. Using SLC38A9 to query the T. gondii proteome, we identified four putative amino acid transporters, termed TgAAT1-4, that structurally resemble the SLC38A9 arginine transceptor. Assessing their expression and sub-cellular localization, we found that one of them, TgAAT1, localized to the PLVAC and is necessary for normal parasite extracellular survival and bradyzoite differentiation. Moreover, we show that TgAAT1 is involved in the PLVAC efflux of arginine, an amino acid playing a key role in T. gondii differentiation, further supporting the hypothesis that TgAAT1 might play a role in nutrient sensing.

IMPORTANCE

T. gondii is a highly successful parasite infecting a broad range of warm-blood organisms including about one third of all humans. Although Toxoplasma infections rarely result in symptomatic disease in individuals with a healthy immune system, the incredibly high number of persons infected along with the risk of severe infection in immunocompromised patients and the potential link of chronic infection to mental disorders make this infection a significant public health concern. As a result, there is a pressing need for new treatment approaches that are both effective and well-tolerated. The limitations in understanding how Toxoplasma gondii manages its metabolism to adapt to changing environments and triggers its transformation into bradyzoites have hindered the discovery of vulnerabilities in its metabolic pathways or nutrient acquisition mechanisms to identify new therapeutic targets. In this work, we have shown that the lysosome-like organelle PLVAC, acting through the putative arginine transporter TgAAT1, plays a pivotal role in regulating the parasite's extracellular survival and differentiation into bradyzoites.

The Toxoplasma plant-like vacuolar compartment (PLVAC)

Toxoplasma gondii belongs to the phylum Apicomplexa and is an important cause of congenital disease and infection in immunocompromised patients. T. gondii shares several characteristics with plants including a nonphotosynthetic plastid termed apicoplast and a multivesicular organelle that was named the plant-like vacuole (PLV) or vacuolar compartment (VAC). The name plant-like vacuole was selected based on its resemblance in composition and function to plant vacuoles. The name VAC represents its general vacuolar characteristics. We will refer to the organelle as PLVAC in this review. New findings in recent years have revealed that the PLVAC represents the lysosomal compartment of T. gondii which has adapted peculiarities to fulfill specific Toxoplasma needs. In this review, we discuss the composition and functions of the PLVAC highlighting its roles in ion storage and homeostasis, endocytosis, exocytosis, and autophagy.

An Uninvited Seat at the Dinner Table: How Apicomplexan Parasites Scavenge Nutrients from the Host

Obligate intracellular parasites have evolved a remarkable assortment of strategies to scavenge nutrients from the host cells they parasitize. Most apicomplexans form a parasitophorous vacuole (PV) within the invaded cell, a replicative niche within which they survive and multiply. As well as providing a physical barrier against host cell defense mechanisms, the PV membrane (PVM) is also an important site of nutrient uptake that is essential for the parasites to sustain their metabolism. This means nutrients in the extracellular milieu are separated from parasite metabolic machinery by three different membranes, the host plasma membrane, the PVM, and the parasite plasma membrane (PPM). In order to facilitate nutrient transport from the extracellular environment into the parasite itself, transporters on the host cell membrane of invaded cells can be modified by secreted and exported parasite proteins to maximize uptake of key substrates to meet their metabolic demand. To overcome the second barrier, the PVM, apicomplexan parasites secrete proteins contained in the dense granules that remodel the vacuole and make the membrane permissive to important nutrients. This bulk flow of host nutrients is followed by a more selective uptake of substrates at the PPM that is operated by specific transporters of this third barrier. In this review, we recapitulate and compare the strategies developed by Apicomplexa to scavenge nutrients from their hosts, with particular emphasis on transporters at the parasite plasma membrane and vacuolar solute transporters on the parasite intracellular digestive organelle.

A single Na+-Pi cotransporter in Toxoplasma plays key roles in phosphate import and control of parasite osmoregulation

Inorganic ions such as phosphate, are essential nutrients required for a broad spectrum of cellular functions and regulation. During infection, pathogens must obtain inorganic phosphate (P_i) from the host. Despite the essentiality of phosphate for all forms of life, how the intracellular parasite Toxoplasma gondii acquires P_i from the host cell is still unknown. In this study, we demonstrated that Toxoplasma actively internalizes exogenous P_i by exploiting a gradient of Na⁺ ions to drive P_i uptake across the plasma membrane. The Na⁺-dependent phosphate transport mechanism is electrogenic and functionally coupled to a cipargarmin sensitive Na⁺-H⁺-ATPase. Toxoplasma expresses one transmembrane P_i transporter harboring PHO4 binding domains that typify the PiT Family. This transporter named TgPiT, localizes to the plasma membrane, the inward buds of the endosomal organelles termed VAC, and many cytoplasmic vesicles. Upon P_i limitation in the medium, TgPiT is more abundant at the plasma membrane. We genetically ablated the PiT gene, and ΔTgPiT parasites are impaired in importing P_i and synthesizing polyphosphates. Interestingly, ΔTgPiT parasites accumulate 4-times more acidocalcisomes, storage organelles for phosphate molecules, as compared to parental parasites. In addition, these mutants have a reduced cell volume, enlarged VAC organelles, defects in calcium storage and a slightly alkaline pH. Overall, these mutants exhibit severe growth defects and have reduced acute virulence in mice. In survival mode, ΔTgPiT parasites upregulate several genes, including those encoding enzymes that cleave or transfer phosphate groups from phosphometabolites, transporters and ions exchangers localized to VAC or acidocalcisomes. Taken together, these findings point to a critical role of TgPiT for P_i supply for Toxoplasma and also for protection against osmotic stresses.

Inorganic phosphate (P_i) is indispensable for the biosynthesis of key cellular components, and is involved in many metabolic and signaling pathways. Transport across the plasma membrane is the first step in the utilization of P_i. The import mechanism of P_i by the intracellular parasite Toxoplasma is unknown. We characterized a transmembrane, high-affinity Na⁺-P_i cotransporter, named TgPiT, expressed by the parasite at the plasma membrane for P_i uptake. Interestingly, TgPiT is also localized to inward buds of the endosomal VAC organelles and some cytoplasmic vesicles. Loss of TgPiT results in a severe reduction in P_i internalization and polyphosphate levels, but stimulation of the biogenesis of phosphate-enriched acidocalcisomes. ΔTgPiT parasites have a shrunken cell body, enlarged VAC organelles, poor release of stored calcium and a mildly alkaline pH, suggesting a role for TgPiT in the maintenance of overall ionic homeostasis. ΔTgPiT parasites are poorly infectious in vitro and in mice. The mutant appears to partially cope with the absence of TgPiT by up-regulating genes coding for ion transporters and enzymes catalyzing phosphate group transfer. Our data highlight a scenario in which the role of TgPiT in P_i and Na⁺ transport is functionally coupled with osmoregulation activities central to sustain Toxoplasma survival.

A key cytosolic iron-sulfur cluster synthesis protein localizes to the mitochondrion of Toxoplasma gondii

Iron-sulfur (Fe-S) clusters are prosthetic groups on proteins that function in a range of enzymatic and electron transfer reactions. Fe-S cluster synthesis is essential for the survival of all eukaryotes. Independent Fe-S cluster biosynthesis pathways occur in the mitochondrion, plastid, and cytosolic compartments of eukaryotic cells. Little is known about the cytosolic Fe-S cluster biosynthesis in apicomplexan parasites, the causative agents of diseases such as malaria and toxoplasmosis. NBP35 serves as a key scaffold protein on which cytosolic Fe-S clusters assemble, and has a cytosolic localization in most eukaryotes studied thus far. Unexpectedly, we found that the NBP35 homolog of the apicomplexan Toxoplasma gondii (TgNBP35) localizes to the outer mitochondrial membrane, with mitochondrial targeting mediated by an N-terminal transmembrane domain. We demonstrate that TgNBP35 is critical for parasite proliferation, but that, despite its mitochondrial localization, it is not required for Fe-S cluster synthesis in the mitochondrion. Instead, we establish that TgNBP35 is important for the biogenesis of cytosolic Fe-S proteins. Our data are consistent with TgNBP35 playing a central and specific role in cytosolic Fe-S cluster biosynthesis, and imply that the assembly of cytosolic Fe-S clusters occurs on the cytosolic face of the outer mitochondrial membrane in these parasites.

Role of Toxoplasma gondii Chloroquine Resistance Transporter in Bradyzoite Viability and Digestive Vacuole Maintenance

Toxoplasma gondii is a ubiquitous pathogen that can cause encephalitis, congenital defects, and ocular disease. T. gondii has also been implicated as a risk factor for mental illness in humans. The parasite persists in the brain as slow-growing bradyzoites contained within intracellular cysts. No treatments exist to eliminate this form of parasite. Although proteolytic degradation within the parasite lysosome-like vacuolar compartment (VAC) is critical for bradyzoite viability, whether other aspects of the VAC are important for parasite persistence remains unknown. An ortholog of Plasmodium falciparum chloroquine resistance transporter (CRT), TgCRT, has previously been identified in T. gondii To interrogate the function of TgCRT in chronic-stage bradyzoites and its role in persistence, we knocked out TgCRT in a cystogenic strain and assessed VAC size, VAC digestion of host-derived proteins and parasite autophagosomes, and the viability of in vitro and in vivo bradyzoites. We found that whereas parasites deficient in TgCRT exhibit normal digestion within the VAC, they display a markedly distended VAC and their viability is compromised both in vitro and in vivo Interestingly, impairing VAC proteolysis in TgCRT-deficient bradyzoites restored VAC size, consistent with a role for TgCRT as a transporter of products of digestion from the VAC. In conjunction with earlier studies, our current findings suggest a functional link between TgCRT and VAC proteolysis. This study provides further evidence of a crucial role for the VAC in bradyzoite persistence and a new potential VAC target to abate chronic Toxoplasma infection.IMPORTANCE Individuals chronically infected with the intracellular parasite Toxoplasma gondii are at risk of experiencing reactivated disease that can result in progressive loss of vision. No effective treatments exist for chronic toxoplasmosis due in part to a poor understanding of the biology underlying chronic infection and a lack of well-validated potential targets. We show here that a T. gondii transporter is functionally linked to protein digestion within the parasite lysosome-like organelle and that this transporter is necessary to sustain chronic infection in culture and in experimentally infected mice. Ablating the transporter results in severe bloating of the lysosome-like organelle. Together with earlier work, this study suggests the parasite's lysosome-like organelle is vital for parasite survival, thus rendering it a potential target for diminishing infection and reducing the risk of reactivated disease.

An ortholog of Plasmodium falciparum chloroquine resistance transporter (PfCRT) plays a key role in maintaining the integrity of the endolysosomal system in Toxoplasma gondii to facilitate host invasion

Toxoplasma gondii is an apicomplexan parasite with the ability to use foodborne, zoonotic, and congenital routes of transmission that causes severe disease in immunocompromised patients. The parasites harbor a lysosome-like organelle, termed the "Vacuolar Compartment/Plant-Like Vacuole" (VAC/PLV), which plays an important role in maintaining the lytic cycle and virulence of T. gondii. The VAC supplies proteolytic enzymes that contribute to the maturation of invasion effectors and that digest autophagosomes and endocytosed host proteins. Previous work identified a T. gondii ortholog of the Plasmodium falciparum chloroquine resistance transporter (PfCRT) that localized to the VAC. Here, we show that TgCRT is a membrane transporter that is functionally similar to PfCRT. We also genetically ablate TgCRT and reveal that the TgCRT protein plays a key role in maintaining the integrity of the parasite’s endolysosomal system by controlling morphology of the VAC. When TgCRT is absent, the VAC dramatically increases in volume by ~15-fold and overlaps with adjacent endosome-like compartments. Presumably to reduce aberrant swelling, transcription and translation of endolysosomal proteases are decreased in ΔTgCRT parasites. Expression of subtilisin protease 1 is significantly reduced, which impedes trimming of microneme proteins, and significantly decreases parasite invasion. Chemical or genetic inhibition of proteolysis within the VAC reverses these effects, reducing VAC size and partially restoring integrity of the endolysosomal system, microneme protein trimming, and invasion. Taken together, these findings reveal for the first time a physiological role of TgCRT in substrate transport that impacts VAC volume and the integrity of the endolysosomal system in T. gondii.

Toxoplasma gondii is an obligate intracellular protozoan parasite that belongs to the phylum Apicomplexa and that infects virtually all warm-blooded organisms. Approximately one-third of the human population is infected with Toxoplasma. Toxoplasma invades host cells using processed invasion effectors. A lysosome-like organelle (VAC) is involved in refining these invasion effectors to reach their final forms. A T. gondii ortholog of the malarial chloroquine resistance transporter protein (TgCRT) was found to be localized to the VAC membrane. Although the mutated version of the malarial chloroquine resistance transporter (PfCRT) has been shown to confer resistance to chloroquine treatment, its physiologic function remains poorly understood. Comparison between the related PfCRT and TgCRT facilitates definition of the physiologic role of CRT proteins. Here, we report that TgCRT plays a key role in affecting the integrity and proteolytic activity of the VAC and adjacent organelles, the secretion of invasion effectors, and parasite invasion and virulence. To relieve osmotic stress caused by VAC swelling when TgCRT is deleted, parasites repress proteolysis within this organelle to decrease solute accumulation, which then has secondary effects on parasite invasion. Our findings highlight a common function for PfCRT and TgCRT in mediating small solute transport to affect apicomplexan parasite vacuolar size and function.

The Vacuolar Zinc Transporter TgZnT Protects Toxoplasma gondii from Zinc Toxicity

Zinc (Zn2+) is the most abundant biological metal ion aside from iron and is an essential element in numerous biological systems, acting as a cofactor for a large number of enzymes and regulatory proteins. Zn2+ must be tightly regulated, as both the deficiency and overabundance of intracellular free Zn2+ are harmful to cells. Zn2+ transporters (ZnTs) play important functions in cells by reducing intracellular Zn2+ levels by transporting the ion out of the cytoplasm. We characterized a Toxoplasma gondii gene (TgGT1_251630, TgZnT), which is annotated as the only ZnT family Zn2+ transporter in T. gondii TgZnT localizes to novel vesicles that fuse with the plant-like vacuole (PLV), an endosome-like organelle. Mutant parasites lacking TgZnT exhibit reduced viability in in vitro assays. This phenotype was exacerbated by increasing zinc concentrations in the extracellular media and was rescued by media with reduced zinc. Heterologous expression of TgZnT in a Zn2+-sensitive Saccharomyces cerevisiae yeast strain partially restored growth in media with higher Zn2+ concentrations. These results suggest that TgZnT transports Zn2+ into the PLV and plays an important role in the Zn2+ tolerance of T. gondii extracellular tachyzoites.IMPORTANCEToxoplasma gondii is an intracellular pathogen of human and animals. T. gondii pathogenesis is associated with its lytic cycle, which involves invasion, replication, egress out of the host cell, and invasion of a new one. T. gondii must be able to tolerate abrupt changes in the composition of the surrounding milieu as it progresses through its lytic cycle. We report the characterization of a Zn2+ transporter of T. gondii (TgZnT) that is important for parasite growth. TgZnT restored Zn2+ tolerance in yeast mutants that were unable to grow in media with high concentrations of Zn2+ We propose that TgZnT plays a role in Zn2+ homeostasis during the T. gondii lytic cycle.

Non-Enzymatic Synthesis of Bioactive Isoprostanoids in the Diatom Phaeodactylum following Oxidative Stress

The ecological success of diatoms requires a remarkable ability to survive many types of stress, including variations in temperature, light, salinity, and nutrient availability. On exposure to these stresses, diatoms exhibit common responses, including growth arrest, impairment of photosynthesis, production of reactive oxygen species, and accumulation of triacylglycerol (TAG). We studied the production of cyclopentane oxylipins derived from fatty acids in the diatom Phaeodactylum tricornutum in response to oxidative stress. P. tricornutum lacks the enzymatic pathway for producing cyclopentane-oxylipins, such as jasmonate, prostaglandins, or thromboxanes. In cells subjected to increasing doses of hydrogen peroxide (H₂O₂), we detected nonenzymatic production of isoprostanoids, including six phytoprostanes, three F_2t-isoprostanes, two F_3t-isoprostanes, and three F_4t-neuroprostanes, by radical peroxidation of α-linolenic, arachidonic, eicosapentaenoic, and docosahexanoic acids, respectively. H₂O₂ also triggered photosynthesis impairment and TAG accumulation. F_1t-phytoprostanes constitute the major class detected (300 pmol per 1 million cells; intracellular concentration, ∼4 µm). Only two glycerolipids, phosphatidylcholine and diacylglycerylhydroxymethyl-trimethyl-alanine, could provide all substrates for these isoprostanoids. Treatment of P. tricornutum with nine synthetic isoprostanoids produced an effect in the micromolar range, marked by the accumulation of TAG and reduced growth, without affecting photosynthesis. Therefore, the emission of H₂O₂ and free radicals upon exposure to stresses can lead to glycerolipid peroxidation and nonenzymatic synthesis of isoprostanoids, inhibiting growth and contributing to the induction of TAG accumulation via unknown processes. This characterization of nonenzymatic oxylipins in P. tricornutum opens a field of research on the study of processes controlled by isoprostanoid signaling in various physiological and environmental contexts in diatoms.

Plasmodium falciparum chloroquine resistance transporter (PfCRT) isoforms PH1 and PH2 perturb vacuolar physiology

BACKGROUND

Recent work has perfected yeast-based methods for measuring drug transport by the Plasmodium falciparum chloroquine (CQ) resistance transporter (PfCRT).

METHODS

The approach relies on inducible heterologous expression of PfCRT in Saccharomyces cerevisiae yeast. In these experiments selecting drug concentrations are not toxic to the yeast, nor is expression of PfCRT alone toxic. Only when PfCRT is expressed in the presence of CQ is the growth of yeast impaired, due to inward transport of chloroquine (CQ) via the transporter.

RESULTS

During analysis of all 53 known naturally occurring PfCRT isoforms, two isoforms (PH1 and PH2 PfCRT) were found to be intrinsically toxic to yeast, even in the absence of CQ. Additional analysis of six very recently identified PfCRT isoforms from Malaysia also showed some toxicity. In this paper the nature of this yeast toxicity is examined. Data also show that PH1 and PH2 isoforms of PfCRT transport CQ with an efficiency intermediate to that catalyzed by previously studied CQR conferring isoforms. Mutation of PfCRT at position 160 is found to perturb vacuolar physiology, suggesting a fitness cost to position 160 amino acid substitutions.

CONCLUSION

These data further define the wide range of activities that exist for PfCRT isoforms found in P. falciparum isolates from around the globe.

Toxoplasma, or the discovery of a heterophage

In mammalian cells, the protozoan pathogen Toxoplasma resides in a nonfusiogenic vacuole that segregates it from host cell resources. How the parasite acquires nutrients and whether it is capable of internalizing host macromolecules have been long-standing mysteries. By exploiting a mutant of Toxoplasma lacking the cathepsin protease L, Dou et al. observed the accumulation of host cytosolic-derived proteins in a multivesicular post-Golgi compartment, which establishes the existence of a functional heterophagic pathway in Toxoplasma.