Metabolic Cooperation of Glucose and Glutamine Is Essential for the Lytic Cycle of Obligate Intracellular Parasite Toxoplasma gondii

Metabolic crosstalk between the mitochondrion and the nucleus is essential for Toxoplasma gondii infection

Toxoplasma gondii, an intracellular pathogenic protist with a remarkable ability to infect a wide range of host cells, displays an equally exceptional design of its carbon metabolism. There are, however, critical gaps in our understanding of the metabolic network in T. gondii. We characterized the mito-nuclear metabolism and organelle coupling during its acute infection (lytic cycle). The major enzymes of the TCA cycle, i.e., citrate synthase (CS1), succinyl-CoA synthase alpha subunit (SCSα), succinate dehydrogenase (SDHA) and FAD malate dehydrogenase (MDH-FAD) located in the parasite mitochondrion support its asexual reproduction but are not needed for its survival. The SCSα and SDHA mutants are nearly avirulent in a mouse model, and they can protect the host against a lethal challenge infection. Genetic deletion of MDH-FAD dysregulated glucose-derived carbon flux, leading to a collapse of the mitochondrial membrane potential. The parasite also harbors a cytosolic isoform of MDH and a nuclear malic enzyme (ME) contributing to malate oxidation; however, only the latter is essential for the lytic cycle. Expression of ME in the nucleus is crucial for the parasite development. Besides, conditional knockdown of ME impairs the histone acetylation and disrupts the expression of several genes in tachyzoites. Our work discloses novel network design features of T. gondii and highlights the therapeutic and vaccination potential of the parasite metabolism.

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.

TgLaforin, a glucan phosphatase, reveals the dynamic role of storage polysaccharides in Toxoplasma gondii tachyzoites and bradyzoites

The asexual stages of Toxoplasma gondii are defined by the rapidly growing tachyzoite during the acute infection and by the slow growing bradyzoite housed within tissue cysts during the chronic infection. These stages represent unique physiological states, each with distinct glucans reflecting differing metabolic needs. A defining feature of T. gondii bradyzoites is the presence of insoluble storage glucans known as amylopectin granules (AGs), the function of which remains largely unexplored during the chronic infection. The presence of storage glucans has more recently been established in tachyzoites, a finding corroborated by specific labeling with the anti-glycogen antibody IV58B6. The T. gondii genome encodes activities needed for glucan turnover inlcuding: a glucan phosphatase (TgLaforin; TGME49_205290) and a glucan kinase (TgGWD; TGME49_214260) that catalyze a cycle of reversible glucan phosphorylation required for glucan degradation by amylases. Disruption of TgLaforin in tachyzoites had no impact on growth under nutrient-replete conditions. Growth of TgLaforin-KO tachyzoites was however severely stunted when starved of glutamine despite being glucose replete. Loss of TgLaforin attenuated acute virulence in mice and was accompanied by a lower tissue cyst burden, without a direct impact on tissue cyst size. Quantification of relative AG levels using AmyloQuant, an imaging based application, revealed the starch-excess phenotype associated with the loss of TgLaforin is heterogeneous and linked to an emerging AG cycle in bradyzoites. Excessive AG accumulation TgLaforin-KO bradyzoites promoted intra-cyst bradyzoite death implicating reversible glucan phosphorylation as a legitimate target for the development of new drugs against chronic T. gondii infections.

An endoplasmic reticulum localized acetyl-CoA transporter is required for efficient fatty acid synthesis in Toxoplasma gondii

Toxoplasma gondii is an obligate intracellular parasite that can infect humans and diverse animals. Fatty acids are critical for the growth and proliferation of T. gondii, which has at least two pathways to synthesize fatty acids, including the type II de novo synthesis pathway in the apicoplast and the elongation pathway in the endoplasmic reticulum (ER). Acetyl-CoA is the key substrate for both fatty acid synthesis pathways. In the apicoplast, acetyl-CoA is mainly provided by the pyruvate dehydrogenase complex. However, how the ER acquires acetyl-CoA is not fully understood. Here, we identified a putative acetyl-CoA transporter (TgAT1) that localized to the ER of T. gondii. Deletion of TgAT1 impaired parasite growth and invasion in vitro and attenuated tachyzoite virulence in vivo. Metabolic tracing using 13C-acetate found that loss of TgAT1 reduced the incorporation of 13C into certain fatty acids, suggesting reduced activities of elongation. Truncation of AT1 was previously reported to confer resistance to the antimalarial compound GNF179 in Plasmodium falciparum. Interestingly, GNF179 had much weaker inhibitory effect on Toxoplasma than on Plasmodium. In addition, deletion of AT1 did not affect the susceptibility of Toxoplasma to GNF179, suggesting that this compound might be taken up differently or has different inhibitory mechanisms in these parasites. Together, our data show that TgAT1 has important roles for parasite growth and fatty acid synthesis, but its disruption does not confer GNF179 resistance in T. gondii.

The 6-phosphofructokinase reaction in Acetivibrio thermocellus is both ATP- and pyrophosphate-dependent

Acetivibrio thermocellus (formerly Clostridium thermocellum) is a potential platform for lignocellulosic ethanol production. Its industrial application is hampered by low product titres, resulting from a low thermodynamic driving force of its central metabolism. It possesses both a functional ATP- and a functional PPi-dependent 6-phosphofructokinase (PPi-Pfk), of which only the latter is held responsible for the low driving force. Here we show that, following the replacement of PPi-Pfk by cytosolic pyrophosphatase and transaldolase, the native ATP-Pfk is able to carry the full glycolytic flux. Interestingly, the barely-detectable in vitro ATP-Pfk activities are only a fraction of what would be required, indicating its contribution to glycolysis has consistently been underestimated. A kinetic model demonstrated that the strong inhibition of ATP-Pfk by PPi can prevent futile cycling that would arise when both enzymes are active simultaneously. As such, there seems to be no need for a long-sought-after PPi-generating mechanism to drive glycolysis, as PPi-Pfk can simply use whatever PPi is available, and ATP-Pfk complements the rest of the PFK-flux. Laboratory evolution of the ΔPPi-Pfk strain, unable to valorize PPi, resulted in a mutation in the GreA transcription elongation factor. This mutation likely results in reduced RNA-turnover, hinting at transcription as a significant (and underestimated) source of anabolic PPi. Together with other mutations, this resulted in an A. thermocellus strain with the hitherto highest biomass-specific cellobiose uptake rate of 2.2 g/gx/h. These findings are both relevant for fundamental insight into dual ATP/PPi Pfk-nodes, which are not uncommon in other microorganisms, as well as for further engineering of A. thermocellus for consolidated bioprocessing.

Metabolic changes in Toxoplasma gondii-infected host cells measured by autofluorescence imaging

Toxoplasma gondii, the causative agent of toxoplasmosis, is an obligate intracellular parasite that infects warm-blooded vertebrates across the world. In humans, seropositivity rates of T. gondii range from 10% to 90% across communities. Despite its prevalence, few studies address how T. gondii infection changes the metabolism of host cells. In this study, we investigate how T. gondii manipulates the host cell metabolic environment by monitoring the metabolic response over time using noninvasive autofluorescence lifetime imaging of single cells, metabolite analysis, extracellular flux analysis, and reactive oxygen species (ROS) production. Autofluorescence lifetime imaging indicates that infected host cells become more oxidized and have an increased proportion of bound NAD(P)H compared to uninfected controls. Over time, infected cells also show decreases in levels of intracellular glucose and lactate, increases in oxygen consumption, and variability in ROS production. We further examined changes associated with the pre-invasion "kiss and spit" process using autofluorescence lifetime imaging, which also showed a more oxidized host cell with an increased proportion of bound NAD(P)H over 48 hours compared to uninfected controls, suggesting that metabolic changes in host cells are induced by T. gondii kiss and spit even without invasion.IMPORTANCEThis study sheds light on previously unexplored changes in host cell metabolism induced by T. gondii infection using noninvasive, label-free autofluorescence imaging. In this study, we use optical metabolic imaging (OMI) to measure the optical redox ratio (ORR) in conjunction with fluorescence lifetime imaging microscopy (FLIM) to noninvasively monitor single host cell response to T. gondii infection over 48 hours. Collectively, our results affirm the value of using autofluorescence lifetime imaging to noninvasively monitor metabolic changes in host cells over the time course of a microbial infection. Understanding this metabolic relationship between the host cell and the parasite could uncover new treatment and prevention options for T. gondii infections worldwide.

Mechanisms of adaptation and evolution in Toxoplasma gondii

Toxoplasma has high host flexibility, infecting all nucleated cells of mammals and birds. This implies that during its infective process the parasite must constantly adapt to different environmental situations, which in turn leads to modifications in its metabolism, regulation of gene transcription, translation of mRNAs and stage specific factors. There are conserved pathways that support these adaptations, which we aim to elucidate in this review. We begin by exploring the widespread epigenetic mechanisms and transcription regulators, continue with the supportive role of Heat Shock Proteins (Hsp), the translation regulation, stress granules, and finish with the emergence of contingency genes in highly variable genomic domains, such as subtelomeres. Within epigenetics, the discovery of a new histone variant of the H2B family (H2B.Z), contributing to T. gondii virulence and differentiation, but also gene expression regulation and its association with the metabolic state of the parasite, is highlighted. Associated with the regulation of gene expression are transcription factors (TFs). An overview of the main findings on TF and development is presented. We also emphasize the role of Hsp90 and Tgj1 in T. gondii metabolic fitness and the regulation of protein translation. Translation regulation is also highlighted as a mechanism for adaptation to conditions encountered by the parasite as well as stress granules containing mRNA and proteins generated in the extracellular tachyzoite. Another important aspect in evolution and adaptability are the subtelomeres because of their high variability and gene duplication rate. Toxoplasma possess multigene families of membrane proteins and contingency genes that are associated with different metabolic stresses. Among them parasite differentiation and environmental stresses stand out, including those that lead tachyzoite to bradyzoite conversion. Finally, we are interested in positioning protozoa as valuable evolution models, focusing on research related to the Extended Evolutionary Synthesis, based on models recently generated, such as extracellular adaptation and ex vivo cyst recrudescence.

Multiple pathways for glucose phosphate transport and utilization support growth of Cryptosporidium parvum

Cryptosporidium parvum is an obligate intracellular parasite with a highly reduced mitochondrion that lacks the tricarboxylic acid cycle and the ability to generate ATP, making the parasite reliant on glycolysis. Genetic ablation experiments demonstrated that neither of the two putative glucose transporters CpGT1 and CpGT2 were essential for growth. Surprisingly, hexokinase was also dispensable for parasite growth while the downstream enzyme aldolase was required, suggesting the parasite has an alternative way of obtaining phosphorylated hexose. Complementation studies in E. coli support a role for direct transport of glucose-6-phosphate from the host cell by the parasite transporters CpGT1 and CpGT2, thus bypassing a requirement for hexokinase. Additionally, the parasite obtains phosphorylated glucose from amylopectin stores that are released by the action of the essential enzyme glycogen phosphorylase. Collectively, these findings reveal that C. parvum relies on multiple pathways to obtain phosphorylated glucose both for glycolysis and to restore carbohydrate reserves.

Mitochondrial dysfunction induced by bedaquiline as an anti-Toxoplasma alternative

Toxoplasma gondii is a zoonotic parasite that infects one-third of the world's population and nearly all warm-blooded animals. Due to the complexity of T. gondii's life cycle, available treatment options have limited efficacy. Thus, there is an urgent need to develop new compounds or repurpose existing drugs with potent anti-Toxoplasma activity. This study demonstrates that bedaquiline (BDQ), an FDA-approved diarylquinoline antimycobacterial drug for the treatment of tuberculosis, potently inhibits the tachyzoites of T. gondii. At a safe concentration, BDQ displayed a dose-dependent inhibition on T. gondii growth with a half-maximal effective concentration (EC50) of 4.95 μM. Treatment with BDQ significantly suppressed the proliferation of T. gondii tachyzoites in the host cell, while the invasion ability of the parasite was not affected. BDQ incubation shrunk the mitochondrial structure and decreased the mitochondrial membrane potential and ATP level of T. gondii parasites. In addition, BDQ induced elevated ROS and led to autophagy in the parasite. By transcriptomic analysis, we found that oxidative phosphorylation pathway genes were significantly disturbed by BDQ-treated parasites. More importantly, BDQ significantly reduces brain cysts for the chronically infected mice. These results suggest that BDQ has potent anti-T. gondii activity and may impair its mitochondrial function by affecting proton transport. This study provides bedaquiline as a potential alternative drug for the treatment of toxoplasmosis, and our findings may facilitate the development of new effective drugs for the treatment of toxoplasmosis.

Gene expression profile of HCT-8 cells following single or co-infections with Cryptosporidium parvum and bovine coronavirus

Among the causative agents of neonatal diarrhoea in calves, two of the most prevalent are bovine coronavirus (BCoV) and the intracellular parasite Cryptosporidium parvum. Although several studies indicate that co-infections are associated with greater symptom severity, the host-pathogen interplay remains unresolved. Here, our main objective was to investigate the modulation of the transcriptome of HCT-8 cells during single and co-infections with BCoV and C. parvum. For this, HCT-8 cells were inoculated with (1) BCoV alone, (2) C. parvum alone, (3) BCoV and C. parvum simultaneously. After 24 and 72 h, cells were harvested and analyzed using high-throughput RNA sequencing. Following differential expression analysis, over 6000 differentially expressed genes (DEGs) were identified in virus-infected and co-exposed cells at 72 hpi, whereas only 52 DEGs were found in C. parvum-infected cells at the same time point. Pathway (KEGG) and gene ontology (GO) analysis showed that DEGs in the virus-infected and co-exposed cells were mostly associated with immune pathways (such as NF-κB, TNF-α or, IL-17), apoptosis and regulation of transcription, with a more limited effect exerted by C. parvum. Although the modulation observed in the co-infection was apparently dominated by the virus, over 800 DEGs were uniquely expressed in co-exposed cells at 72 hpi. Our findings provide insights on possible biomarkers associated with co-infection, which could be further explored using in vivo models.

Metabolic changes to host cells with Toxoplasma gondii infection

Toxoplasma gondii, the causative agent of toxoplasmosis, is an obligate intracellular parasite that infects warm-blooded vertebrates across the world. In humans, seropositivity rates of T. gondii range from 10% to 90%. Despite its prevalence, few studies address how T. gondii infection changes the metabolism of host cells. Here, we investigate how T. gondii manipulates the host cell metabolic environment by monitoring metabolic response over time using non-invasive autofluorescence lifetime imaging of single cells, seahorse metabolic flux analysis, reactive oxygen species (ROS) production, and metabolomics. Autofluorescence lifetime imaging indicates that infected host cells become more oxidized and have an increased proportion of bound NAD(P)H with infection. These findings are consistent with changes in mitochondrial and glycolytic function, decrease of intracellular glucose, fluctuations in lactate and ROS production in infected cells over time. We also examined changes associated with the pre-invasion "kiss and spit" process using autofluorescence lifetime imaging, which similarly showed a more oxidized host cell with an increased proportion of bound NAD(P)H over 48 hours. Glucose metabolic flux analysis indicated that these changes are driven by NADH and NADP+ in T. gondii infection. In sum, metabolic changes in host cells with T. gondii infection were similar during full infection, and kiss and spit. Autofluorescence lifetime imaging can non-invasively monitor metabolic changes in host cells over a microbial infection time-course.

Multiple pathways for glucose phosphate transport and utilization support growth of Cryptosporidium parvum

Cryptosporidium parvum is an obligate intracellular parasite with a highly reduced mitochondrion that lacks the TCA cycle and the ability to generate ATP, making the parasite reliant on glycolysis. Genetic ablation experiments demonstrated that neither of the two putative glucose transporters CpGT1 and CpGT2 were essential for growth. Surprisingly, hexokinase was also dispensable for parasite growth while the downstream enzyme aldolase was required, suggesting the parasite has an alternative way of obtaining phosphorylated hexose. Complementation studies in E. coli support a role for direct transport of glucose-6-phosphate from the host cell by the parasite transporters CpGT1 and CpGT2, thus bypassing a requirement for hexokinase. Additionally, the parasite obtains phosphorylated glucose from amylopectin stores that are released by the action of the essential enzyme glycogen phosphorylase. Collectively, these findings reveal that C. parvum relies on multiple pathways to obtain phosphorylated glucose both for glycolysis and to restore carbohydrate reserves.

An unconventional SNARE complex mediates exocytosis at the plasma membrane and vesicular fusion at the apical annuli in Toxoplasma gondii

Exocytosis is a key active process in cells by which proteins are released in bulk via the fusion of exocytic vesicles with the plasma membrane. Soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) protein-mediated vesicle fusion with the plasma membrane is essential in most exocytotic pathways. In mammalian cells, the vesicular fusion step of exocytosis is normally mediated by Syntaxin-1 (Stx1) and SNAP25 family proteins (SNAP25 and SNAP23). However, in Toxoplasma gondii, a model organism of Apicomplexa, the only SNAP25 family protein, with a SNAP29-like molecular structure, is involved in vesicular fusion at the apicoplast. Here, we reveal that an unconventional SNARE complex comprising TgStx1, TgStx20, and TgStx21 mediates vesicular fusion at the plasma membrane. This complex is essential for the exocytosis of surface proteins and vesicular fusion at the apical annuli in T. gondii.

The Mitochondrial Pyruvate Carrier Coupling Glycolysis and the Tricarboxylic Acid Cycle Is Required for the Asexual Reproduction of Toxoplasma gondii

Toxoplasma gondii is an obligate intracellular parasite capable of infecting humans and animals. The organism has extraordinary metabolic resilience that allows it to establish parasitism in varied nutritional milieus of diverse host cells. Our earlier work has shown that, despite flexibility in the usage of glucose and glutamine as the major carbon precursors, the production of pyruvate by glycolytic enzymes is central to the parasite's growth. Pyruvate is metabolized in a number of subcellular compartments, including the mitochondrion, apicoplast, and cytosol. With the objective of examining the mechanism and importance of the mitochondrial pool of pyruvate imported from the cytosol, we identified the conserved mitochondrial pyruvate carrier (MPC) complex, consisting of two subunits, MPC1 and MPC2, in T. gondii. The two parasite proteins could complement a yeast mutant deficient in growth on leucine and valine. Genetic ablation of either one or both subunits reduced the parasite's growth, mimicking the deletion of branched-chain ketoacid dehydrogenase (BCKDH), which has been reported to convert pyruvate into acetyl-coenzyme A (CoA) in the mitochondrion. Metabolic labeling of the MPC mutants by isotopic glucose revealed impaired synthesis of acetyl-CoA, correlating with a global decrease in carbon flux through glycolysis and the tricarboxylic acid (TCA) cycle. Disruption of MPC proteins exerted only a modest effect on the parasite's virulence in mice, further highlighting its metabolic flexibility. In brief, our work reveals the modus operandi of pyruvate transport from the cytosol to the mitochondrion in the parasite, providing the missing link between glycolysis and the TCA cycle in T. gondii. IMPORTANCE T. gondii is a zoonotic parasite capable of infecting many warm-blooded organisms, including humans. Among others, a feature that allows it to parasitize multiple hosts is its exceptional metabolic plasticity. Although T. gondii can utilize different carbon sources, pyruvate homeostasis is critical for parasite growth. Pyruvate is produced primarily in the cytosol but metabolized in other organelles, such as the mitochondrion and apicoplast. The mechanism of import and physiological significance of pyruvate in these organelles remains unclear. Here, we identified the transporter of cytosol-derived pyruvate into the mitochondrion and studied its constituent subunits and their relevance. Our results show that cytosolic pyruvate is a major source of acetyl-CoA in the mitochondrion and that the mitochondrial pyruvate transporter is needed for optimal parasite growth. The mutants lacking the transporter are viable and virulent in a mouse model, underscoring the metabolic plasticity in the parasite's mitochondrion.

The Toxoplasma micropore mediates endocytosis for selective nutrient salvage from host cell compartments

Apicomplexan parasite growth and replication relies on nutrient acquisition from host cells, in which intracellular multiplication occurs, yet the mechanisms that underlie the nutrient salvage remain elusive. Numerous ultrastructural studies have documented a plasma membrane invagination with a dense neck, termed the micropore, on the surface of intracellular parasites. However, the function of this structure remains unknown. Here we validate the micropore as an essential organelle for endocytosis of nutrients from the host cell cytosol and Golgi in the model apicomplexan Toxoplasma gondii. Detailed analyses demonstrated that Kelch13 is localized at the dense neck of the organelle and functions as a protein hub at the micropore for endocytic uptake. Intriguingly, maximal activity of the micropore requires the ceramide de novo synthesis pathway in the parasite. Thus, this study provides insights into the machinery underlying acquisition of host cell-derived nutrients by apicomplexan parasites that are otherwise sequestered from host cell compartments.

Two apicoplast dwelling glycolytic enzymes provide key substrates for metabolic pathways in the apicoplast and are critical for Toxoplasma growth

Many apicomplexan parasites harbor a non-photosynthetic plastid called the apicoplast, which hosts important metabolic pathways like the methylerythritol 4-phosphate (MEP) pathway that synthesizes isoprenoid precursors. Yet many details in apicoplast metabolism are not well understood. In this study, we examined the physiological roles of four glycolytic enzymes in the apicoplast of Toxoplasma gondii. Many glycolytic enzymes in T. gondii have two or more isoforms. Endogenous tagging each of these enzymes found that four of them were localized to the apicoplast, including pyruvate kinase2 (PYK2), phosphoglycerate kinase 2 (PGK2), triosephosphate isomerase 2 (TPI2) and phosphoglyceraldehyde dehydrogenase 2 (GAPDH2). The ATP generating enzymes PYK2 and PGK2 were thought to be the main energy source of the apicoplast. Surprisingly, deleting PYK2 and PGK2 individually or simultaneously did not cause major defects on parasite growth or virulence. In contrast, TPI2 and GAPDH2 are critical for tachyzoite proliferation. Conditional depletion of TPI2 caused significant reduction in the levels of MEP pathway intermediates and led to parasite growth arrest. Reconstitution of another isoprenoid precursor synthesis pathway called the mevalonate pathway in the TPI2 depletion mutant partially rescued its growth defects. Similarly, knocking down the GAPDH2 enzyme that produces NADPH also reduced isoprenoid precursor synthesis through the MEP pathway and inhibited parasite proliferation. In addition, it reduced de novo fatty acid synthesis in the apicoplast. Together, these data suggest a model that the apicoplast dwelling TPI2 provides carbon source for the synthesis of isoprenoid precursor, whereas GAPDH2 supplies reducing power for pathways like MEP, fatty acid synthesis and ferredoxin redox system in T. gondii. As such, both enzymes are critical for parasite growth and serve as potential targets for anti-toxoplasmic intervention designs. On the other hand, the dispensability of PYK2 and PGK2 suggest additional sources for energy in the apicoplast, which deserves further investigation.

Study on the antagonistic effects of koumiss on Toxoplasma gondii infection in mice

Toxoplasma gondii is an important food-borne zoonotic parasite, and approximately one-third of people worldwide are positive for T. gondii antibodies. To date, there are no specific drugs or vaccines against T. gondii. Therefore, developing a new safe and effective method has become a new trend in treating toxoplasmosis. Koumiss is rich in probiotics and many components that can alleviate the clinical symptoms of many diseases via the functional characteristics of koumiss and its regulation of intestinal flora. To investigate the antagonistic effect of koumiss on T. gondii infection, the model of acute and chronic T. gondii infection was established in this study. The survival rate, SHIRPA score, serum cytokine levels, brain cyst counts, β-amyloid deposition and intestinal flora changes were measured after koumiss feeding. The results showed that the clinical symptoms of mice were improved at 6 dpi and that the SHIRPA score decreased after koumiss feeding (P < 0.05). At the same time, the levels of IL-4, IFN-γ and TNF-α decreased (P < 0.001, P < 0.001, P < 0.01). There was no significant difference of survival rate between koumiss treatment and the other groups. Surprisingly, the results of chronic infection models showed that koumiss could significantly reduce the number of brain cysts in mice (P < 0.05), improve β-amyloid deposition in the hippocampus (P < 0.01) and decrease the levels of IFN-γ and TNF-α (P < 0.01, P < 0.05). Moreover, koumiss could influence the gut microbiota function in resisting T. gondii infection. In conclusion, koumiss had a significant effect on chronic T. gondii infection in mice and could improve the relevant indicators of acute T. gondii infection in mice. The research provides new evidence for the development of safe and effective anti-T. gondii methods, as well as a theoretical basis and data support for the use of probiotics against T. gondii infection and broadened thoughts for the development and utilization of koumiss.

Metabolic flexibilities and vulnerabilities in the pentose phosphate pathway of the zoonotic pathogen Toxoplasma gondii

Metabolic pathways underpin the growth and virulence of intracellular parasites and are therefore promising antiparasitic targets. The pentose phosphate pathway (PPP) is vital in most organisms, providing a reduced form of nicotinamide adenine dinucleotide phosphate (NADPH) and ribose sugar for nucleotide synthesis; however, it has not yet been studied in Toxoplasma gondii, a widespread intracellular pathogen and a model protozoan organism. Herein, we show that T. gondii has a functional PPP distributed in the cytoplasm and nucleus of its acutely-infectious tachyzoite stage. We produced eight parasite mutants disrupting seven enzymes of the PPP in T. gondii. Our data show that of the seven PPP proteins, the two glucose-6-phosphate dehydrogenases (TgG6PDH1, TgG6PDH2), one of the two 6-phosphogluconate dehydrogenases (Tg6PGDH1), ribulose-5-phosphate epimerase (TgRuPE) and transaldolase (TgTAL) are dispensable in vitro as well as in vivo, disclosing substantial metabolic plasticity in T. gondii. Among these, TgG6PDH2 plays a vital role in defense against oxidative stress by the pathogen. Further, we show that Tg6PGDH2 and ribulose-5-phosphate isomerase (TgRPI) are critical for tachyzoite growth. The depletion of TgRPI impairs the flux of glucose in central carbon pathways, and causes decreased expression of ribosomal, microneme and rhoptry proteins. In summary, our results demonstrate the physiological need of the PPP in T. gondii while unraveling metabolic flexibility and antiparasitic targets.

Metabolic pathways are intimately associated with the survival and replication of parasitic Toxoplasma gondii and thus represent potential targets for antiparasitic strategies. Herein, we focused on the pentose phosphate pathway (PPP) in T. gondii and examined its roles in supporting the growth of this ubiquitous pathogen. We found that TgG6PDH1 and TgG6PDH2 were needed to defend oxidative stress but not for pentose synthesis. We revealed that inactivation of the Tg6PGDH2 and TgRPI severely impaired the asexual reproduction of tachyzoites. We also highlighted the remarkable metabolic plasticity in tachyzoites that enables them to acquire some of the PPP intermediates from multiple routes. This study provides significant insights into the carbon metabolism properties of Toxoplasma parasites, opening avenues for targeting this pathway to develop therapeutic interventions against toxoplasmosis.

Metabolomics study of cat small intestine during the early stage of Toxoplasma gondii oocyst formation identifies potential biomarkers

Toxoplasma gondii (T. gondii) is a zoonotic intracellular protozoan parasite that can invade, replicate and survive in almost all cells of warm-blooded animals. T. gondii infection threatens the life of the fetus or can cause morbidity in the infant. As the only definitive host of T. gondii, felids spread the pathogen mainly by forming oocysts in the small intestines and discharging the oocysts into the ambient environment, consequently polluting water, vegetables, and meat products. In this study, we used untargeted metabolomics technology to study the changes in metabolites that occurred during the early stage of oocyst formation in the cat small intestine following T. gondii infection and attempted to identify metabolic biomarkers that could potentially be used as diagnostic molecular markers in the future. Domestic cats (Felis catus) were infected with T. gondii Pru tissue cysts, and samples of their small intestinal epithelium were collected at 2 and 4 days post-infection (DPI) for metabolic analysis. LC-MS/MS and multivariate statistical analysis were employed to detect metabolomic signatures that discriminated between the infected and control groups. A total of 1673 ions and 1201 ions were obtained in the positive and negative modes, respectively. Of these ions, 175 were up-regulated and 127 were down-regulated in the positive ion mode; whereas, 123 were up-regulated and 81 were down-regulated in the negative ion mode. Three commonly altered ions (0.74_313.0414 m/z, 8.82_615.2621 m/z and 8.16_325.2362 m/z) were determined to have potential research value. Seventy common metabolic pathways were enriched at two time points, with arginine biosynthesis, pyrimidine metabolism, pantothenate and CoA biosynthesis being the three most significant pathways related to T. gondii. The area under the curve (AUC) of differential metabolites combined with relevant literature analysis showed that N-Methylpelletierine and 3,3-Difluoro-17-methyl-5alpha-androstan-17beta-ol have higher predictability and better potential application value than other metabolites. Our analysis of metabolic markers during the early stage of T. gondii oocyst formation in the small intestine of the definitive host (cat) provided novel insight for understanding oocyst development and a theoretical basis for the application of potential biomarkers.

Assessment of selected molecular factors and 17-β estradiol dosage in response to Toxoplasma gondii infection in swine

Background and Aim:

Toxoplasma gondii is a global zoonotic parasite infecting virtually all warm-blooded species, although a species-specific variability is evident referring to symptoms frame. Both the success of T. gondii and the outcome of infection depend on a delicate balance between host cellular pathways and the evasion or modulation strategies elicited by the parasite. The hormonal and molecular mechanisms involved in this delicate host-parasite balance are still unclear, especially when considering intermediate host species other than mouse. This study aimed to assess any correlation between T. gondii infection and selected molecular and hormonal factors involved in responses to infection in susceptible species such as swine. Moreover, blood counts and hematochemical assays (glucose, total cholesterol, and triglycerides dosage) were performed to evaluate the overall health condition of animals.

Materials and Methods:

Toxoplasmosis was diagnosed by enzyme-linked immunosorbent assay for antibodies determination and real-time polymerase chain reaction (RT-PCR) for T. gondii DNA detection. Target genes coding for key factors of cell responses to T. gondii infection were selected, and their transcription was assessed in various tissues by quantitative RT-PCR. 17-β estradiol concentrations were assessed by fluorimetric enzyme-linked immunoassay and the AIA-360 automated immunoassay analyzer. Blood count and hematochemical analyses were performed by a blood cell counter and a spectrophotometer, respectively.

Results:

The present research highlighted significant differences among infected and uninfected swine (control group) for both transcription profiles of some of the molecular factors considered and 17-β estradiol concentrations. Referring to the assessed hematological and biochemical parameters, no statistically significant differences were observed in infected swine compared to the control group.

Conclusion:

Our results contribute to the enrichment of data available about the subject and could be useful for a deeper knowledge of the interaction between this parasite and its hosts. However, more aspects are still unclear, such as the effective response of downstream molecules from the same pathways to the variation of factors observed in this study either assessing how the same factors respond to Toxoplasma gondii infection in other host speciesand further analyses should be performed on other host species.

Systematic identification of the lysine lactylation in the protozoan parasite Toxoplasma gondii

Background

Lysine lactylation (Kla) is a novelposttranslational modification (PTM) identified in histone and nonhistone proteins of several eukaryotic cells that directly activates gene expression and DNA replication. However, very little is known about the scope and cellular distribution of Kla in apicomplexan parasites despite its significance in public and animal health care.

Methods

Toxoplasma gondii, the causative agent of toxoplasmosis, is an obligate intracellular apicomplexan parasite that can infect different nucleated cell types of animals and humans. We used this parasite as a model organism and extracted the total protein of tachyzoites to produce the first global lysine lactylome profile of T. gondii through liquid chromatography–tandem mass spectrometry. We also investigated the level and localization of the Kla protein in T. gondii using western blotting and the indirect fluorescent antibody test (IFA), respectively.

Results

A total of 983 Kla sites occurring on 523 lactylated proteins were identified in the total protein extracted from Toxoplasma tachyzoites, the acute toxoplasmosis-causing stage. Bioinformatics analysis revealed that the lactylated proteins were evolutionarily conserved and involved in a wide variety of cellular functions, such as energy metabolism, gene regulation and protein biosynthesis. Subcellular localization analysis and IFA results further revealed that most of the lactylated T. gondii proteins were localized in the nucleus, indicating the potential impact of Kla on gene regulation in the T. gondii model. Notably, an extensive batch of parasite-specific proteins unique to phylum Apicomplexa is lactylated in T. gondii.

Conclusions

This study revealed that Kla is widespread in early dividing eukaryotic cells. Lactylated proteins, including a batch of unique parasite proteins, are involved in a remarkably diverse array of cellular functions. These valuable data will improve our understanding of the evolution of Kla and potentially provide the basis for developing novel therapeutic avenues.

Graphical Abstract

Supplementary Information

The online version contains supplementary material available at 10.1186/s13071-022-05315-6.

Toxoplasma gondii DNA methyltransferases regulate parasitic energy metabolism

The intracellular protozoan Toxoplasma gondii results in serious diseases such as encephalitis, and retinochoroiditis in immunocompromised patients. The interconversion between tachyzoites and bradyzoites under the host's immune pressure results in the interchange of acute infection and chronic infection. We previously reported two functional DNA methyltransferases (DNMT) in Toxoplasma gondii named TgDNMTa and TgDNMTb. In this research, proteomics analysis for T. gondii tachyzoites of ME49 WT, dnmta knockout (ME49-∆Tgdnmta), and dnmtb knockout (ME49-∆Tgdnmtb) strains, revealed 362 significantly regulated proteins for ME49-∆Tgdnmta, and 219 for ME49-∆Tgdnmtb, compared with the proteins of ME49 WT. TgDNMTa down regulated three glycolytic enzymes, one gluconeogenic enzyme and four pyruvate metabolic enzymes. Furthermore, TgDNMTb up regulated two proteins in the tricarboxylic acid (TCA) cycle. Glucose metabolic flux detection showed that TgDNMTa inhibited the glycolysis pathway, while TgDNMTb promoted the tricarboxylic acid (TCA) cycle so as to promote parasite's proliferation. These findings demonstrated that the functions of Toxoplasma gondii DNA methyltransferases extended beyond DNA methylation to the regulation of parasitic energy metabolism.

Essential role of pyrophosphate homeostasis mediated by the pyrophosphate-dependent phosphofructokinase in Toxoplasma gondii

Many biosynthetic pathways produce pyrophosphate (PPi) as a by-product, which is cytotoxic if accumulated at high levels. Pyrophosphatases play pivotal roles in PPi detoxification by converting PPi to inorganic phosphate. A number of apicomplexan parasites, including Toxoplasma gondii and Cryptosporidium parvum, express a PPi-dependent phosphofructokinase (PPi-PFK) that consumes PPi to power the phosphorylation of fructose-6-phosphate. However, the physiological roles of PPi-PFKs in these organisms are not known. Here, we report that Toxoplasma expresses both ATP- and PPi-dependent phosphofructokinases in the cytoplasm. Nonetheless, only PPi-PFK was indispensable for parasite growth, whereas the deletion of ATP-PFK did not affect parasite proliferation or virulence. The conditional depletion of PPi-PFK completely arrested parasite growth, but it did not affect the ATP level and only modestly reduced the flux of central carbon metabolism. However, PPi-PFK depletion caused a significant increase in cellular PPi and decreased the rates of nascent protein synthesis. The expression of a cytosolic pyrophosphatase in the PPi-PFK depletion mutant reduced its PPi level and increased the protein synthesis rate, therefore partially rescuing its growth. These results suggest that PPi-PFK has a major role in maintaining pyrophosphate homeostasis in T. gondii. This role may allow PPi-PFK to fine-tune the balance of catabolism and anabolism and maximize the utilization efficiency for carbon nutrients derived from host cells, increasing the success of parasitism. Moreover, PPi-PFK is essential for parasite propagation and virulence in vivo but it is not present in human hosts, making it a potential drug target to combat toxoplasmosis.

Different from classic ATP-dependent phosphofructokinases, PPi-PFKs use pyrophosphate consumption to power the conversion of fructose-6-phosphate to fructose-1,6-bisphosphate, the committed step of glycolysis. PPi-PFK is found in diverse organisms including archaea, bacteria, protists and plants. However, half a century after its first discovery, the physiological functions of PPi-PFK are still not well defined. Using the Toxoplasma gondii parasite as a model, here we show that PPi-PFK has a coordinator function to assure matched activities of anabolism and catabolism. This is achieved by maintaining the homeostasis of PPi, which is a byproduct, as well as an inhibitor of many biosynthetic reactions. PPi-PFK hydrolyzes PPi to promote anabolism, meanwhile being a glycolytic enzyme involved in catabolism. As such, it gauges the anabolic and catabolic activities in parasites to maximize the utilization efficiency of acquired nutrients. This work provides important insights to understand the physiological significance of PPi-PFK in Toxoplasma and other organisms.

Study on the effect of koumiss on the intestinal microbiota of mice infected with Toxoplasma gondii

Toxoplasma gondii is a worldwide food-borne parasite that can infect almost all warm-blooded animals, including humans. To date, there are no effective drugs to prevent or eradicate T. gondii infection. Recent studies have shown that probiotics could influence the relationship between the microbiota and parasites in the host. Koumiss has been used to treat many diseases based on its probiotic diversity. Therefore, we explored the effect of koumiss on T. gondii infection via its effect on the host intestinal microbiota. BALB/c mice were infected with T. gondii and treated with PBS, koumiss and mares' milk. Brain cysts were counted, and long-term changes in the microbiota and the effect of koumiss on gut microbiota were investigated with high-throughput sequencing technology. The results suggested that koumiss treatment significantly decreased the cyst counts in the brain (P < 0.05). Moreover, T. gondii infection changed the microbiota composition, and koumiss treatment increased the relative abundance of Lachnospiraceae and Akkermansia muciniphila, which were associated with preventing T. gondii infection. Moreover, koumiss could inhibit or ameliorate T. gondii infection by increasing the abundance of certain bacteria that control unique metabolic pathways. The study not only established a close interaction among the host, intracellular pathogens and intestinal microbiota but also provided a novel focus for drug development to prevent and eradicate T. gondii infection.

Contact and competition between mitochondria and microbes

Invading microbes occupy the host cytosol and take up nutrients on which host organelles are also dependent. Thus, host organelles are poised to interact with intracellular microbes. Despite the essential role of host mitochondria in cellular metabolic homeostasis and in mediating cellular responses to microbial infection, we know little of how these organelles interact with intracellular pathogens, and how such interactions affect disease pathogenesis. Here, we give an overview of the different classes of physical and metabolic interactions reported to occur between mitochondria and eukaryotic pathogens. Investigating the underlying molecular mechanisms and functions of such interactions will reveal novel aspects of infection biology.

Eimeria bovis Macromeront Formation Induces Glycolytic Responses and Mitochondrial Changes in Primary Host Endothelial Cells

Eimeria bovis is an intracellular apicomplexan parasite that causes considerable economic losses in the cattle industry worldwide. During the first merogony, E. bovis forms large macromeronts with >140,000 merozoites I in host endothelial cells. Because this is a high-energy demanding process, E. bovis exploits the host cellular metabolism to fulfill its metabolic requirements. We here analyzed the carbohydrate-related energetic metabolism of E. bovis–infected primary bovine umbilical vein endothelial cells during first merogony and showed that during the infection, E. bovis–infected culture presented considerable changes in metabolic signatures, glycolytic, and mitochondrial responses. Thus, an increase in both oxygen consumption rates (OCR) and extracellular acidification rates (ECAR) were found in E. bovis–infected host cells indicating a shift from quiescent to energetic cell status. Enhanced levels of glucose and pyruvate consumption in addition to increased lactate production, suggesting an important role of glycolysis in E. bovis–infected culture from 12 days p.i. onward. This was also tested by glycolytic inhibitors (2-DG) treatment, which reduced the macromeront development and diminished merozoite I production. As an interesting finding, we observed that 2-DG treatment boosted sporozoite egress. Referring to mitochondrial activities, intracellular ROS production was increased toward the end of merogony, and mitochondrial potential was enhanced from 12 d p. i. onward in E. bovis–infected culture. Besides, morphological alterations of membrane potential signals also indicated mitochondrial dysfunction in macromeront-carrying host endothelial culture.

Identifying the major lactate transporter of Toxoplasma gondii tachyzoites

Toxoplasma gondii and Plasmodium falciparum parasites both extrude l-lactate, a byproduct of glycolysis. The P. falciparum Formate Nitrite Transporter, PfFNT, mediates l-lactate transport across the plasma membrane of P. falciparum parasites and has been validated as a drug target. The T. gondii genome encodes three FNTs that have been shown to transport l-lactate, and which are proposed to be the targets of several inhibitors of T. gondii proliferation. Here, we show that each of the TgFNTs localize to the T. gondii plasma membrane and are capable of transporting l-lactate across it, with TgFNT1 making the primary contribution to l-lactate transport during the disease-causing lytic cycle of the parasite. We use the Xenopus oocyte expression system to provide direct measurements of l-lactate transport via TgFNT1. We undertake a genetic analysis of the importance of the tgfnt genes for parasite proliferation, and demonstrate that all three tgfnt genes can be disrupted individually and together without affecting the lytic cycle under in vitro culture conditions. Together, our experiments identify the major lactate transporter in the disease causing stage of T. gondii, and reveal that this transporter is not required for parasite proliferation, indicating that TgFNTs are unlikely to be targets for anti-Toxoplasma drugs.

Amino Acid Metabolism in Apicomplexan Parasites

Obligate intracellular pathogens have coevolved with their host, leading to clever strategies to access nutrients, to combat the host's immune response, and to establish a safe niche for intracellular replication. The host, on the other hand, has also developed ways to restrict the replication of invaders by limiting access to nutrients required for pathogen survival. In this review, we describe the recent advancements in both computational methods and high-throughput -omics techniques that have been used to study and interrogate metabolic functions in the context of intracellular parasitism. Specifically, we cover the current knowledge on the presence of amino acid biosynthesis and uptake within the Apicomplexa phylum, focusing on human-infecting pathogens: Toxoplasma gondii and Plasmodium falciparum. Given the complex multi-host lifecycle of these pathogens, we hypothesize that amino acids are made, rather than acquired, depending on the host niche. We summarize the stage specificities of enzymes revealed through transcriptomics data, the relevance of amino acids for parasite pathogenesis in vivo, and the role of their transporters. Targeting one or more of these pathways may lead to a deeper understanding of the specific contributions of biosynthesis versus acquisition of amino acids and to design better intervention strategies against the apicomplexan parasites.

Metabolic Signatures of Cryptosporidium parvum-Infected HCT-8 Cells and Impact of Selected Metabolic Inhibitors on C. parvum Infection under Physioxia and Hyperoxia

Cryptosporidium parvum is an apicomplexan zoonotic parasite recognized as the second leading-cause of diarrhoea-induced mortality in children. In contrast to other apicomplexans, C. parvum has minimalistic metabolic capacities which are almost exclusively based on glycolysis. Consequently, C. parvum is highly dependent on its host cell metabolism. In vivo (within the intestine) infected epithelial host cells are typically exposed to low oxygen pressure (1-11% O₂, termed physioxia). Here, we comparatively analyzed the metabolic signatures of C. parvum-infected HCT-8 cells cultured under both, hyperoxia (21% O₂), representing the standard oxygen condition used in most experimental settings, and physioxia (5% O₂), to be closer to the in vivo situation. The most pronounced effect of C. parvum infection on host cell metabolism was, on one side, an increase in glucose and glutamine uptake, and on the other side, an increase in lactate release. When cultured in a glutamine-deficient medium, C. parvum infection led to a massive increase in glucose consumption and lactate production. Together, these results point to the important role of both glycolysis and glutaminolysis during C. parvum intracellular replication. Referring to obtained metabolic signatures, we targeted glycolysis as well as glutaminolysis in C. parvum-infected host cells by using the inhibitors lonidamine [inhibitor of hexokinase, mitochondrial carrier protein (MCP) and monocarboxylate transporters (MCT) 1, 2, 4], galloflavin (lactate dehydrogenase inhibitor), syrosingopine (MCT1- and MCT4 inhibitor) and compound 968 (glutaminase inhibitor) under hyperoxic and physioxic conditions. In line with metabolic signatures, all inhibitors significantly reduced parasite replication under both oxygen conditions, thereby proving both energy-related metabolic pathways, glycolysis and glutaminolysis, but also lactate export mechanisms via MCTs as pivotal for C. parvum under in vivo physioxic conditions of mammals.

Metabolite salvage and restriction during infection - a tug of war between Toxoplasma gondii and its host

The apicomplexans, including the coccidian pathogen Toxoplasma gondii, are obligate intracellular parasites whose growth and development are intricately linked to the metabolism of their host. T. gondii depends on its host for the salvage of energy sources, building blocks, vitamins and cofactors to survive and replicate. Additionally, host metabolites directly impact on the parasite life cycle development by triggering or halting differentiation. Although T. gondii infects a wide range of host cells, it has evolved to modulate and maximally exploit its host's metabolism. In return the host has developed strategies to restrict parasite access to metabolites. Here we discuss recent findings which have shed light on the battle over metabolites between T. gondii and its host.

Structural, Functional, and Metabolic Alterations in Human Cerebrovascular Endothelial Cells during Toxoplasma gondii Infection and Amelioration by Verapamil In Vitro

Toxoplasma gondii (T. gondii), the causative agent of toxoplasmosis, is a frequent cause of brain infection. Despite its known ability to invade the brain, there is still a dire need to better understand the mechanisms by which this parasite interacts with and crosses the blood–brain barrier (BBB). The present study revealed structural and functional changes associated with infection and replication of T. gondii within human brain microvascular endothelial cells (BMECs) in vitro. T. gondii proliferated within the BMECs and disrupted the integrity of the cerebrovascular barrier through diminishing the cellular viability, disruption of the intercellular junctions and increasing permeability of the BMEC monolayer, as well as altering lipid homeostasis. Proton nuclear magnetic resonance (¹H NMR)-based metabolomics combined with multivariate data analysis revealed profiles that can be attributed to infection and variations in the amounts of certain metabolites (e.g., amino acids, fatty acids) in the extracts of infected compared to control cells. Notably, treatment with the Ca²⁺ channel blocker verapamil rescued BMEC barrier integrity and restricted intracellular replication of the tachyzoites regardless of the time of treatment application (i.e., prior to infection, early- and late-infection). This study provides new insights into the structural and functional changes that accompany T. gondii infection of the BMECs, and sheds light upon the ability of verapamil to inhibit the parasite proliferation and to ameliorate the adverse effects caused by T. gondii infection.

Multi-omics analysis delineates the distinct functions of sub-cellular acetyl-CoA pools in Toxoplasma gondii

BACKGROUND

Acetyl-CoA is a key molecule in all organisms, implicated in several metabolic pathways as well as in transcriptional regulation and post-translational modification. The human pathogen Toxoplasma gondii possesses at least four enzymes which generate acetyl-CoA in the nucleo-cytosol (acetyl-CoA synthetase (ACS); ATP citrate lyase (ACL)), mitochondrion (branched-chain α-keto acid dehydrogenase-complex (BCKDH)) and apicoplast (pyruvate dehydrogenase complex (PDH)). Given the diverse functions of acetyl-CoA, we know very little about the role of sub-cellular acetyl-CoA pools in parasite physiology.

RESULTS

To assess the importance and functions of sub-cellular acetyl-CoA-pools, we measured the acetylome, transcriptome, proteome and metabolome of parasites lacking ACL/ACS or BCKDH. We demonstrate that ACL/ACS constitute a synthetic lethal pair. Loss of both enzymes causes a halt in fatty acid elongation, hypo-acetylation of nucleo-cytosolic and secretory proteins and broad changes in gene expression. In contrast, loss of BCKDH results in an altered TCA cycle, hypo-acetylation of mitochondrial proteins and few specific changes in gene expression. We provide evidence that changes in the acetylome, transcriptome and proteome of cells lacking BCKDH enable the metabolic adaptations and thus the survival of these parasites.

CONCLUSIONS

Using multi-omics and molecular tools, we obtain a global and integrative picture of the role of distinct acetyl-CoA pools in T. gondii physiology. Cytosolic acetyl-CoA is essential and is required for the synthesis of parasite-specific fatty acids. In contrast, loss of mitochondrial acetyl-CoA can be compensated for through metabolic adaptations implemented at the transcriptional, translational and post-translational level.

Dual metabolomic profiling uncovers Toxoplasma manipulation of the host metabolome and the discovery of a novel parasite metabolic capability

The obligate intracellular parasite Toxoplasma gondii is auxotrophic for several key metabolites and must scavenge these from the host. It is unclear how T. gondii manipulates host metabolism to support its overall growth rate and non-essential metabolites. To investigate this question, we measured changes in the joint host-parasite metabolome over a time course of infection. Host and parasite transcriptomes were simultaneously generated to determine potential changes in expression of metabolic enzymes. T. gondii infection changed metabolite abundance in multiple metabolic pathways, including the tricarboxylic acid cycle, the pentose phosphate pathway, glycolysis, amino acid synthesis, and nucleotide metabolism. Our analysis indicated that changes in some pathways, such as the tricarboxylic acid cycle, were mirrored by changes in parasite transcription, while changes in others, like the pentose phosphate pathway, were paired with changes in both the host and parasite transcriptomes. Further experiments led to the discovery of a T. gondii enzyme, sedoheptulose bisphosphatase, which funnels carbon from glycolysis into the pentose phosphate pathway through an energetically driven dephosphorylation reaction. This additional route for ribose synthesis appears to resolve the conflict between the T. gondii tricarboxylic acid cycle and pentose phosphate pathway, which are both NADP+ dependent. Sedoheptulose bisphosphatase represents a novel step in T. gondii central carbon metabolism that allows T. gondii to energetically-drive ribose synthesis without using NADP+.

The obligate intracellular parasite T. gondii is commonly found among human populations worldwide and poses severe health risks to fetuses and individuals with AIDS. While some treatments are available they are limited in scope. A possible target for new therapies is T. gondii’s incomplete metabolism, which makes it heavily reliant on its host. In this study, we generated a joint host/parasite metabolome to better understand host manipulation by the parasite and to discover unique aspects of T. gondii metabolism that could serve as the next generation of drug targets. Metabolomic analysis of T. gondii infection over time found broad alterations to host metabolism by the parasite in both energetic and biosynthetic pathways. We discovered a new T. gondii enzyme, sedoheptulose bisphosphatase, which redirects carbon from glycolysis into the pentose phosphate pathway. The wholesale remodeling of host metabolism for optimal parasite growth is also of interest, although the mechanisms behind this host manipulation must be further studied before therapeutic targets can be identified.

Hydroxylamine and Carboxymethoxylamine Can Inhibit Toxoplasma gondii Growth through an Aspartate Aminotransferase-Independent Pathway

Toxoplasma gondii is an obligate intracellular protozoan parasite and a successful parasitic pathogen in diverse organisms and host cell types. Hydroxylamine (HYD) and carboxymethoxylamine (CAR) have been reported as inhibitors of aspartate aminotransferases (AATs) and interfere with the proliferation in Plasmodium falciparum Therefore, AATs are suggested as drug targets against Plasmodium The T. gondii genome encodes only one predicted AAT in both T. gondii type I strain RH and type II strain PLK. However, the effects of HYD and CAR, as well as their relationship with AAT, on T. gondii remain unclear. In this study, we found that HYD and CAR impaired the lytic cycle of T. gondii in vitro, including the inhibition of invasion or reinvasion, intracellular replication, and egress. Importantly, HYD and CAR could control acute toxoplasmosis in vivo Further studies showed that HYD and CAR could inhibit the transamination activity of rTgAAT in vitro However, our results confirmed that deficiency of AAT in both RH and PLK did not reduce the virulence in mice, although the growth ability of the parasites was affected in vitro HYD and CAR could still inhibit the growth of AAT-deficient parasites. These findings indicated that HYD and CAR inhibition of T. gondii growth and control of toxoplasmosis can occur in an AAT-independent pathway. Overall, further studies focusing on the elucidation of the mechanism of inhibition are warranted. Our study hints at new substrates of HYD and CAR as potential drug targets to inhibit T. gondii growth.

Characterization of the apicoplast-localized enzyme TgUroD in Toxoplasma gondii reveals a key role of the apicoplast in heme biosynthesis

Apicomplexan parasites such as Toxoplasma gondii possess an unusual heme biosynthesis pathway whose enzymes localize to the mitochondrion, cytosol, or apicoplast, a nonphotosynthetic plastid present in most apicomplexans. To characterize the involvement of the apicoplast in the T. gondii heme biosynthesis pathway, we investigated the role of the apicoplast-localized enzyme uroporphyrinogen III decarboxylase (TgUroD). We found that TgUroD knockdown impaired parasite proliferation, decreased free heme levels in the parasite, and decreased the abundance of heme-containing c-type cytochrome proteins in the parasite mitochondrion. We validated the effects of heme loss on mitochondrial cytochromes by knocking down cytochrome c/c1 heme lyase 1 (TgCCHL1), a mitochondrial enzyme that catalyzes the covalent attachment of heme to c-type cytochromes. TgCCHL1 depletion reduced parasite proliferation and decreased the abundance of c-type cytochromes. We further sought to characterize the overall importance of TgUroD and TgCCHL1 for both mitochondrial and general parasite metabolism. TgUroD depletion decreased cellular ATP levels, mitochondrial oxygen consumption, and extracellular acidification rates. By contrast, depletion of TgCCHL1 neither diminished ATP levels in the parasite nor impaired extracellular acidification rate, but resulted in specific defects in mitochondrial oxygen consumption. Together, our results indicate that the apicoplast has a key role in heme biology in T. gondii and is important for both mitochondrial and general parasite metabolism. Our study highlights the importance of heme and its synthesis in these parasites.

Functional and Computational Genomics Reveal Unprecedented Flexibility in Stage-Specific Toxoplasma Metabolism

To survive and proliferate in diverse host environments with varying nutrient availability, the obligate intracellular parasite Toxoplasma gondii reprograms its metabolism. We have generated and curated a genome-scale metabolic model (iTgo) for the fast-replicating tachyzoite stage, harmonized with experimentally observed phenotypes. To validate the importance of four metabolic pathways predicted by the model, we have performed in-depth in vitro and in vivo phenotyping of mutant parasites including targeted metabolomics and CRISPR-Cas9 fitness screening of all known metabolic genes. This led to unexpected insights into the remarkable flexibility of the parasite, addressing the dependency on biosynthesis or salvage of fatty acids (FAs), purine nucleotides (AMP and GMP), a vitamin (pyridoxal-5P), and a cofactor (heme) in both the acute and latent stages of infection. Taken together, our experimentally validated metabolic network leads to a deeper understanding of the parasite's biology, opening avenues for the development of therapeutic intervention against apicomplexans.

Vitamin and cofactor acquisition in apicomplexans: Synthesis versus salvage

The Apicomplexa phylum comprises diverse parasitic organisms that have evolved from a free-living ancestor. These obligate intracellular parasites exhibit versatile metabolic capabilities reflecting their capacity to survive and grow in different hosts and varying niches. Determined by nutrient availability, they either use their biosynthesis machineries or largely depend on their host for metabolite acquisition. Because vitamins cannot be synthesized by the mammalian host, the enzymes required for their synthesis in apicomplexan parasites represent a large repertoire of potential therapeutic targets. Here, we review recent advances in metabolic reconstruction and functional studies coupled to metabolomics that unravel the interplay between biosynthesis and salvage of vitamins and cofactors in apicomplexans. A particular emphasis is placed on Toxoplasma gondii, during both its acute and latent stages of infection.

Measuring Solute Transport in Toxoplasma gondii Parasites

The uptake of host-derived nutrients is key to the growth and survival of Toxoplasma gondii parasites. Nutrients are acquired via solute transporters that localize to the plasma membrane of the parasites. In this chapter, we describe methodology by which the uptake of solutes via plasma membrane transporters may be monitored and characterized. These assays, used here to investigate the uptake of amino acids into parasites, have broad applicability in measuring the uptake of a diverse range of solutes.

Global Lysine Crotonylation and 2-Hydroxyisobutyrylation in Phenotypically Different Toxoplasma gondii Parasites

Toxoplasma gondii is a unicellular protozoan parasite of the phylum Apicomplexa. The parasite repeatedly goes through a cycle of invasion, division and induction of host cell rupture, which is an obligatory process for proliferation inside warm-blooded animals. It is known that the biology of the parasite is controlled by a variety of mechanisms ranging from genomic to epigenetic to transcriptional regulation. In this study, we investigated the global protein posttranslational lysine crotonylation and 2-hydroxyisobutyrylation of two T. gondii strains, RH and ME49, which represent distinct phenotypes for proliferation and pathogenicity in the host. Proteins with differential expression and modification patterns associated with parasite phenotypes were identified. Many proteins in T. gondii were crotonylated and 2-hydroxyisobutyrylated, and they were localized in diverse subcellular compartments involved in a wide variety of cellular functions such as motility, host invasion, metabolism and epigenetic gene regulation. These findings suggest that lysine crotonylation and 2-hydroxyisobutyrylation are ubiquitous throughout the T. gondii proteome, regulating critical functions of the modified proteins. These data provide a basis for identifying important proteins associated with parasite development and pathogenicity.

Pyruvate Homeostasis as a Determinant of Parasite Growth and Metabolic Plasticity in Toxoplasma gondii

Toxoplasma gondii is a widespread intracellular pathogen infecting humans and a variety of animals. Previous studies have shown that Toxoplasma uses glucose and glutamine as the main carbon sources to support asexual reproduction, but neither nutrient is essential. Such metabolic flexibility may allow it to survive within diverse host cell types. Here, by focusing on the glycolytic enzyme pyruvate kinase (PYK) that converts phosphoenolpyruvate (PEP) into pyruvate, we found that Toxoplasma can also utilize lactate and alanine. We show that catabolism of all indicated carbon sources converges at pyruvate, and maintaining a constant pyruvate supply is critical to parasite growth. Toxoplasma expresses two PYKs: PYK1 in the cytosol and PYK2 in the apicoplast (a chloroplast relict). Genetic deletion of PYK2 did not noticeably affect parasite growth and virulence, which contrasts with the current model of carbon metabolism in the apicoplast. On the other hand, PYK1 was refractory to disruption. Conditional depletion of PYK1 resulted in global alteration of carbon metabolism, amylopectin accumulation, and reduced cellular ATP, leading to severe growth impairment. Notably, the attenuated growth of the PYK1-depleted mutant was partially rescued by lactate or alanine supplementation, and rescue by lactate required lactate dehydrogenase activity to convert it to pyruvate. Moreover, depletion of PYK1 in conjunction with PYK2 ablation led to accentuated loss of apicoplasts and complete growth arrest. Together, our results underline a critical role of pyruvate homeostasis in determining the metabolic flexibility and apicoplast maintenance, and they significantly extend our current understanding of carbon metabolism in T. gondii IMPORTANCE Toxoplasma gondii infects almost all warm-blooded animals, and metabolic flexibility is deemed critical for its successful parasitism in diverse hosts. Glucose and glutamine are the major carbon sources to support parasite growth. In this study, we found that Toxoplasma is also competent in utilizing lactate and alanine and, thus, exhibits exceptional metabolic versatility. Notably, all these nutrients need to be converted to pyruvate to fuel the lytic cycle, and achieving a continued pyruvate supply is vital to parasite survival and metabolic flexibility. Although pyruvate can be generated by two distinct pyruvate kinases, located in cytosol and apicoplast, respectively, the cytosolic enzyme is the main source of subcellular pyruvate, and cooperative usage of pyruvate among multiple organelles is critical for parasite growth and virulence. These findings expand our current understanding of carbon metabolism in Toxoplasma gondii and related parasites while providing a basis for designing novel antiparasitic interventions.

iTRAQ-based phosphoproteomic analysis reveals host cell's specific responses to Toxoplasma gondii at the phases of invasion and prior to egress

Protein phosphorylation plays a key role in host cell-T. gondii interaction. However, the phosphoproteome data of host cell at various phases of T. gondii infection has not been thoroughly described. In this study, we assessed the host phosphoproteome data with isobaric tags for relative and absolute quantification (iTRAQ) method during the phases of T. gondii invasion (30 min post infection, PI) and prior to egress (28 h PI). Our iTRAQ analysis revealed a total of 665 phosphoproteins, among which the significantly regulated phosphoproteins in different between-group comparisons were further analyzed. Functional analysis of these significantly regulated phosphoproteins suggested that T. gondii modulated host cell processes through phosphorylation including cell cycle regulation, inducing apoptosis, blocking the synthesis of some inflammatory factors, mediating metabolism to support its proliferation at the infection phase prior to egress, and utilizing membrane and energy from host cell, reorganizing cytoskeleton to favor its invasion and PV formation at the phase of invasion. The phosphorylation level of Smad2, CTNNA1, and HSPB1 identified with western blot revealed a consistent trend of change with iTRAQ result. These newly identified and significantly regulated phosphoproteins from our phosphoproteome data may provide new clues to unravel the host cell's complex reaction against T. gondii infection and the interaction between the host cell and T. gondii.

The effect of Toxoplasma gondii on plasma serotonin concentration in sheep

Background and Aim

Toxoplasma gondii is an intracellular parasite that commonly infects warm-blooded animals, including humans. Virtually all species can be infected, but a species-specific variability is evident, in terms of both type and severity of the symptoms encountered. As serotonin (5-hydroxytryptamine [5-HT]) plays an important regulatory role in both physiological and immune responses, the aim of this research was to assess whether toxoplasmosis disease could affect plasma 5-HT concentration and/or hematochemical parameters in a particularly susceptible species to infection as sheep.

Materials and Methods

5-HT plasma levels were analyzed in platelet-poor plasma fraction by enzyme-linked immunosorbent assay. Blood count and hematochemical parameters were evaluated. Total proteins (TPs), glucose (Glu), and lactate dehydrogenase were determined by a spectrophotometer.

Results

Results showed significantly higher levels in plasma 5-HT, monocytes, and TP and significantly lower levels of Glu, in infected sheep compared to the control group.

Conclusion

Results could support the hypothesis of an effect of toxoplasmosis infection on plasma 5-HT concentrations in sheep. More research is needed to assess the function of 5-HT in the regulation of infected sheep's immune responses.

Metabolic interactions between Toxoplasma gondii and its host

Toxoplasma gondii is an obligate intracellular parasite belonging to the phylum Apicomplexa that infects all warm-blooded animals, including humans. T. gondii can replicate in every nucleated host cell by orchestrating metabolic interactions to derive crucial nutrients. In this review, we summarize the current status of known metabolic interactions of T. gondii with its host cell and discuss open questions and promising experimental approaches that will allow further dissection of the host-parasite interface and discovery of ways to efficiently target both tachyzoite and bradyzoite forms of T. gondii, which are associated with acute and chronic infection, respectively.

Glycolysis is important for optimal asexual growth and formation of mature tissue cysts by Toxoplasma gondii

Toxoplasma gondii can grow and replicate using either glucose or glutamine as the major carbon source. Here, we have studied the essentiality of glycolysis in the tachyzoite and bradyzoite stages of T. gondii, using transgenic parasites that lack a functional hexokinase gene (Δhk) in RH (Type-1) and Prugniaud (Type-II) strain parasites. Tachyzoite stage Δhk parasites exhibit a fitness defect similar to that reported previously for the major glucose transporter mutant, and remain virulent in mice. However, although Prugniaud strain Δhk tachyzoites were capable of transforming into bradyzoites in vitro, they were severely compromised in their ability to make mature bradyzoite cysts in the brain tissue of mice. Isotopic labelling studies reveal that glucose-deprived tacyzoites utilise glutamine to replenish glycolytic and pentose phosphate pathway intermediates via gluconeogenesis. Interestingly, while glutamine-deprived intracellular Δhk tachyzoites continued to replicate, extracellular parasites were unable to efficiently invade host cells. Further, studies on mutant tachyzoites lacking a functional phosphoenolpyruvate carboxykinase (Δpepck1) revealed that glutaminolysis is the sole source of gluconeogenic flux in glucose-deprived parasites. In addition, glutaminolysis is essential for sustaining oxidative phosphorylation in Δhk parasites, while wild type (wt) and Δpepck1 parasites can obtain ATP from either glycolysis or oxidative phosphorylation. This study provides insights into the role of nutrient metabolism during asexual propagation and development of T. gondii, and validates the versatile nature of central carbon and energy metabolism in this parasite.

The intracellular parasite Toxoplasma gondii harbors three druggable FNT-type formate and l-lactate transporters in the plasma membrane

Toxoplasma gondii is a globally prevalent parasitic protist. It is well-known for its ability to infect almost all nucleated vertebrate cells, which is reflected by its unique metabolic architecture. Its fast-growing tachyzoite stage catabolizes glucose via glycolysis to yield l-lactate as a major by-product that must be exported from the cell to prevent toxicity; the underlying mechanism remains to be elucidated, however. Herein, we report three formate-nitrite transporter (FNT)-type monocarboxylate/proton symporters located in the plasma membrane of the T. gondii tachyzoite stage. We observed that all three proteins transport both l-lactate and formate in a pH-dependent manner and are inhibited by 2-hydroxy-chromanones (a class of small synthetic molecules). We also show that these compounds pharmacologically inhibit T. gondii growth. Using a chemical biology approach, we identified the critical residues in the substrate-selectivity region of the parasite transporters that determine differential specificity and sensitivity toward both substrates and inhibitors. Our findings further indicate that substrate specificity in FNT family proteins from T. gondii has evolved such that a functional repurposing of prokaryotic-type transporters helps fulfill a critical metabolic role in a clinically important parasitic protist. In summary, we have identified and characterized the lactate transporters of T. gondii and have shown that compounds blocking the FNTs in this parasite can inhibit its growth, suggesting that these transporters could have utility as potential drug targets.

Highly diverged novel subunit composition of apicomplexan F-type ATP synthase identified from Toxoplasma gondii

The mitochondrial F-type ATP synthase, a multisubunit nanomotor, is critical for maintaining cellular ATP levels. In T. gondii and other apicomplexan parasites, many subunit components necessary for proper assembly and functioning of this enzyme appear to be missing. Here, we report the identification of 20 novel subunits of T. gondii F-type ATP synthase from mass spectrometry analysis of partially purified monomeric (approximately 600 kDa) and dimeric (>1 MDa) forms of the enzyme. Despite extreme sequence diversification, key FO subunits a, b, and d can be identified from conserved structural features. Orthologs for these proteins are restricted to apicomplexan, chromerid, and dinoflagellate species. Interestingly, their absence in ciliates indicates a major diversion, with respect to subunit composition of this enzyme, within the alveolate clade. Discovery of these highly diversified novel components of the apicomplexan F-type ATP synthase complex could facilitate the development of novel antiparasitic agents. Structural and functional characterization of this unusual enzyme complex will advance our fundamental understanding of energy metabolism in apicomplexan species.

Brain proteomic differences between wild-type and CD44- mice induced by chronic Toxoplasma gondii infection

Chronic clinical Toxoplasma gondii (T. gondii) infection is the primary disease state that causes severe encephalitis. CD44 is a member of the cell adhesion molecule family and plays an important role in T. gondii infection. However, proteomic changes in CD44 during chronic T. gondii infection have rarely been reported. Thus, an iTRAQ-based proteomic study coupled with 2D-LC-MS/MS analysis was performed to screen CD44-related proteins during chronic T. gondii infection. As a result, a total of 2612 proteins were reliably identified and quantified. Subsequently, 259, 106, and 249 differentially expressed proteins (DEPs) were compared between CD44- mice (A) vs wild-type mice (B), B vs wild-type mice infected with T. gondii (C), and C vs CD44- mice infected with T. gondii (D). Gene ontology, KEGG pathway, and protein-protein interaction analyses were performed on the DEPs. According to the results, immune-related proteins were altered significantly among the A vs B, B vs C, and C vs D comparisons, which might indicate that chronic T.  gondii infection caused changes in the host immune response. Additionally, Ca²⁺- and metabolism-related proteins were upregulated in C vs D, which supported the hypothesis that CD44 mediated the production of host Ca²⁺ and IFN-γ and that the parasite preferentially invaded cells expressing high levels of CD44. The present findings validate and enable a more comprehensive knowledge of the role of CD44 in hosts chronically infected with T. gondii, thus providing new ideas for future studies on the specific functions of CD44 in latent toxoplasmosis.

Lipid analysis of Eimeria sporozoites reveals exclusive phospholipids, a phylogenetic mosaic of endogenous synthesis, and a host-independent lifestyle

Successful inter-host transmission of most apicomplexan parasites requires the formation of infective sporozoites within the oocysts. Unlike all other infective stages that are strictly intracellular and depend on host resources, the sporozoite stage develops outside the host cells, but little is known about its self-governing metabolism. This study deployed Eimeria falciformis, a parasite infecting the mouse as its natural host, to investigate the process of phospholipid biogenesis in sporozoites. Lipidomic analyses demonstrated the occurrence of prototypical phospholipids along with abundant expression of at least two exclusive lipids, phosphatidylthreonine (PtdThr) and inositol phosphorylceramide with a phytosphingosine backbone, in sporozoites. To produce them de novo, the parasite harbors nearly the entire biogenesis network, which is an evolutionary mosaic of eukaryotic-type and prokaryotic-type enzymes. Notably, many have no phylogenetic counterpart or functional equivalent in the mammalian host. Using Toxoplasma gondii as a gene-tractable surrogate to examine Eimeria enzymes, we show a highly compartmentalized network of lipid synthesis spread primarily in the apicoplast, endoplasmic reticulum, mitochondrion, and Golgi complex. Likewise, trans-genera complementation of a Toxoplasma mutant with the PtdThr synthase from Eimeria reveals a convergent role of PtdThr in fostering the lytic cycle of coccidian parasites. Taken together, our work establishes a model of autonomous membrane biogenesis involving significant inter-organelle cooperation and lipid trafficking in sporozoites. Phylogenetic divergence of certain pathways offers attractive drug targets to block the sporulation and subsequent transmission. Not least, our results vindicate the possession of an entire de novo lipid synthesis network in a representative protist adapted to an obligate intracellular parasitic lifestyle.

Toxoplasma gondii acetyl-CoA synthetase is involved in fatty acid elongation (of long fatty acid chains) during tachyzoite life stages

Apicomplexan parasites are pathogens responsible for major human diseases such as toxoplasmosis caused by Toxoplasma gondii and malaria caused by Plasmodium spp. Throughout their intracellular division cycle, the parasites require vast and specific amounts of lipids to divide and survive. This demand for lipids relies on a fine balance between de novo synthesized lipids and scavenged lipids from the host. Acetyl-CoA is a major and central precursor for many metabolic pathways, especially for lipid biosynthesis. T. gondii possesses a single cytosolic acetyl-CoA synthetase (TgACS). Its role in the parasite lipid synthesis is unclear. Here, we generated an inducible TgACS KO parasite line and confirmed the cytosolic localization of the protein. We conducted ¹³C-stable isotope labeling combined with mass spectrometry-based lipidomic analyses to unravel its putative role in the parasite lipid synthesis pathway. We show that its disruption has a minor effect on the global FA composition due to the metabolic changes induced to compensate for its loss. However, we could demonstrate that TgACS is involved in providing acetyl-CoA for the essential fatty elongation pathway to generate FAs used for membrane biogenesis. This work provides novel metabolic insight to decipher the complex lipid synthesis in T. gondii.