Comparison of methods probing the intracellular redox milieu in Plasmodium falciparum

Let it glow: genetically encoded fluorescent reporters in Plasmodium

The use of fluorescent proteins (FPs) in Plasmodium parasites has been key to understand the biology of this obligate intracellular protozoon. FPs like the green fluorescent protein (GFP) enabled to explore protein localization, promoter activity as well as dynamic processes like protein export and endocytosis. Furthermore, FP biosensors have provided detailed information on physiological parameters at the subcellular level, and fluorescent reporter lines greatly extended the malariology toolbox. Still, in order to achieve optimal results, it is crucial to know exactly the properties of the FP of choice and the genetic scenario in which it will be used. This review highlights advantages and disadvantages of available landing sites and promoters that have been successfully applied for the ectopic expression of FPs in Plasmodium berghei and Plasmodium falciparum. Furthermore, the properties of newly developed FPs beyond DsRed and EGFP, in the visualization of cells and cellular structures as well as in the sensing of small molecules are discussed.

Neospora caninum peroxiredoxin 1 is an essential virulence effector with antioxidant function

Neospora caninum, an obligate intracellular parasitic protozoan discovered by Dubey in 1988, is the pathogen of neosporosis, which causes neurological symptoms in dogs and abortions in cows. Since there is no effective drug or vaccine against N. caninum, a deeper understanding of the molecules critical to parasite survival inside host cells is necessary. This study aimed to determine the role of N. caninum peroxiredoxin 1 (NcPrx1) in maintaining redox homeostasis and virulence of N. caninum. By determining the localization of NcPrx1 protein and establishing NcPrx1 gene knockout strain (ΔNcPrx1), the roles of NcPrx1 in N. caninum for invasion, replication, growth, oxidative stress, as well as pathogenicity were investigated. Our results showed that a predicted Alkyl Hydroperoxide1 (AHP1) domain was found in the amino acid sequence of NcPrx1, which displayed a high degree of similarity to homologs of several protozoa. Immunofluorescence assay (IFA) indicated that NcPrx1 was a cytoplasmic protein in N. caninum tachyzoites. Compared to wild type (WT) strain, ΔNcPrx1 strain showed reduced plaque area, invasion and egress rates. Reactive oxygen species (ROS) and malondialdehyde (MDA) were accumulated, and total antioxidant capacity (T-AOC) was attenuated in ΔNcPrx1 tachyzoites, which indicated that ΔNcPrx1 strain was more sensitive to oxidative stress. Furthermore, ΔNcPrx1 strain-infected C57BL/6 mice showed improved survival rate, reduced parasite burden, alleviated pathological changes in tissues, and decreased secretions of IL-6, IL-12, TNF-α, and IFN-γ in serum compared to the WT strain group. These findings suggested that NcPrx1 was a virulence factor of N. caninum which played an important role in maintaining the redox homeostasis of the parasite.

Genetically Encoded Biosensors for the Fluorescence Detection of O2 and Reactive O2 Species

The intracellular concentrations of oxygen and reactive oxygen species (ROS) in living cells represent critical information for investigating physiological and pathological conditions. Real-time measurement often relies on genetically encoded proteins that are responsive to fluctuations in either oxygen or ROS concentrations. The direct binding or chemical reactions that occur in their presence either directly alter the fluorescence properties of the binding protein or alter the fluorescence properties of fusion partners, mostly consisting of variants of the green fluorescent protein. Oxygen sensing takes advantage of several mechanisms, including (i) the oxygen-dependent hydroxylation of a domain of the hypoxia-inducible factor-1, which, in turn, promotes its cellular degradation along with fluorescent fusion partners; (ii) the naturally oxygen-dependent maturation of the fluorophore of green fluorescent protein variants; and (iii) direct oxygen binding by proteins, including heme proteins, expressed in fusion with fluorescent partners, resulting in changes in fluorescence due to conformational alterations or fluorescence resonance energy transfer. ROS encompass a group of highly reactive chemicals that can interconvert through various chemical reactions within biological systems, posing challenges for their selective detection through genetically encoded sensors. However, their general reactivity, and particularly that of the relatively stable oxygen peroxide, can be exploited for ROS sensing through different mechanisms, including (i) the ROS-induced formation of disulfide bonds in engineered fluorescent proteins or fusion partners of fluorescent proteins, ultimately leading to fluorescence changes; and (ii) conformational changes of naturally occurring ROS-sensing domains, affecting the fluorescence properties of fusion partners. In this review, we will offer an overview of these genetically encoded biosensors.

Ascaridole exerts the leishmanicidal activity by inhibiting parasite glycolysis

BACKGROUND

The global burden of leishmaniasis is exacerbated by the limited repertoire of drugs, resulting in an urgent need to develop new therapeutic alternatives. Endoperoxides like ascaridole have emerged as promising anti-parasitic candidates, and its effectiveness was established in an animal model of cutaneous leishmaniasis (CL). However, its impact on Leishmania donovani parasites, causative of visceral leishmaniasis (VL) remains to be established.

PURPOSE

This study aimed to delineate the underlying mechanisms contributing towards the leishmanicidal effect of ascaridole in terms of its impact on the cellular redox status and metabolic bioenergetics of L. donovani parasites.

METHODOLOGY

The anti-promastigote activity of ascaridole was established by a cell viability assay in L. donovani [MHOM/IN/1983/AG83] and anti-amastigote activity by microscopy and ddPCR (droplet digital polymerase chain reaction). The cellular redox status, mitochondrial membrane potential (MMP), annexin V positivity and cell cycle arrest was evaluated by flow cytometry, while cellular and mitochondrial bioenergetics was assessed using Agilent XFp Analyzer, and the levels of ATP was measured by chemiluminescence.

RESULTS

Ascaridole demonstrated strong anti-promastigote and anti-amastigote activities in l. donovani, IC₅₀ (half maximal Inhibitory concentration) being 2.47 ± 0.18 µM and 2.00±0.34 µM respectively, while in J774.A1 and murine peritoneal macrophages, the CC₅₀ (half maximal cytotoxic concentration) was 41.47 ± 4.89 µM and 37.58 ± 5.75 µM respectively. Ascaridole disrupted the redox homeostasis via an enhanced generation of reactive oxygen species (ROS), lipid peroxidation and concomitant depletion of thiols. However, it failed to increase the generation of mitochondrial superoxide, which minimally impacted on mitochondrial respiration and was corroborated by energy metabolism studies. Instead, ascaridole inhibited glycolysis of promastigotes, caused a loss in MMP, which translated into ATP depletion. In promastigotes, ascaridole enhanced annexin-V positivity and caused a cell cycle arrest at sub- G₀/G₁ phase.

CONCLUSION

In summary, ascaridole displays its leishmanicidal activity possibly due to its ability to auto-generate free radicals following cleavage of its endoperoxide bridge that led to disruption of the redox homeostasis, inhibition of glycolysis and culminated in an apoptotic like cell death.

Peroxide Antimalarial Drugs Target Redox Homeostasis in Plasmodium falciparum Infected Red Blood Cells

Plasmodium falciparum causes the most lethal form of malaria. Peroxide antimalarials based on artemisinin underpin the frontline treatments for malaria, but artemisinin resistance is rapidly spreading. Synthetic peroxide antimalarials, known as ozonides, are in clinical development and offer a potential alternative. Here, we used chemoproteomics to investigate the protein alkylation targets of artemisinin and ozonide probes, including an analogue of the ozonide clinical candidate, artefenomel. We greatly expanded the list of proteins alkylated by peroxide antimalarials and identified significant enrichment of redox-related proteins for both artemisinins and ozonides. Disrupted redox homeostasis was confirmed by dynamic live imaging of the glutathione redox potential using a genetically encoded redox-sensitive fluorescence-based biosensor. Targeted liquid chromatography-mass spectrometry (LC-MS)-based thiol metabolomics also confirmed changes in cellular thiol levels. This work shows that peroxide antimalarials disproportionately alkylate proteins involved in redox homeostasis and that disrupted redox processes are involved in the mechanism of action of these important antimalarials.

Identification and Function of Apicoplast Glutaredoxins in Neospora caninum

Glutaredoxins (GRXs), important components of the intracellular thiol redox system, are involved in multiple cellular processes. In a previous study, we identified five GRXs in the apicomplexan parasite, Neospora caninum. In the present study, we confirmed that the GRXs S14 and C5 are located in the apicoplast, which suggests unique functions for these proteins. Although single-gene deficiency did not affect the growth of parasites, a double knockout (Δgrx S14Δgrx C5) significantly reduced their reproductive capacity. However, there were no significant changes in redox indices (GSH/GSSG ratio, reactive oxygen species and hydroxyl radical levels) in double-knockout parasites, indicating that grx S14 and grx C5 are not essential for maintaining the redox balance in parasite cells. Key amino acid mutations confirmed that the Cys²⁰³ of grx S14 and Cys^253/256 of grx C5 are important for parasite growth. Based on comparative proteomics, 79 proteins were significantly downregulated in double-knockout parasites, including proteins mainly involved in the electron transport chain, the tricarboxylic acid cycle and protein translation. Collectively, GRX S14 and GRX C5 coordinate the growth of parasites. However, considering their special localization, the unique functions of GRX S14 and GRX C5 need to be further studied.

In Vivo Imaging with Genetically Encoded Redox Biosensors

Redox reactions are of high fundamental and practical interest since they are involved in both normal physiology and the pathogenesis of various diseases. However, this area of research has always been a relatively problematic field in the context of analytical approaches, mostly because of the unstable nature of the compounds that are measured. Genetically encoded sensors allow for the registration of highly reactive molecules in real-time mode and, therefore, they began a new era in redox biology. Their strongest points manifest most brightly in in vivo experiments and pave the way for the non-invasive investigation of biochemical pathways that proceed in organisms from different systematic groups. In the first part of the review, we briefly describe the redox sensors that were used in vivo as well as summarize the model systems to which they were applied. Next, we thoroughly discuss the biological results obtained in these studies in regard to animals, plants, as well as unicellular eukaryotes and prokaryotes. We hope that this work reflects the amazing power of this technology and can serve as a useful guide for biologists and chemists who work in the field of redox processes.

Glutaredoxin 1 Deficiency Leads to Microneme Protein-Mediated Growth Defects in Neospora caninum

Neospora caninum is an obligate intracellular protozoan parasite that infects a wide range of mammalian species and causes spontaneous abortion in cattle. N. caninum is exposed to oxidative stress during its life cycle. Oxidoreductase is crucial for parasite response to the environmental stresses. Glutaredoxins (Grxs) are small oxidoreductases of the thioredoxin family proteins that catalyze thiol-disulfide exchange reactions by utilizing electrons from the tripeptide glutathione (γGlu-Cys-Gly; GSH). Grxs are key elements in redox signaling and cell signal transduction. However, Grxs are an unexplored set of oxidoreductases in N. caninum. Here, we identified two cytoplasm located glutaredoxin domain-containing proteins (NcGrx1 and NcGrx3) in N. caninum. To better understand the functions of these Grx proteins, we generated NcGrx1 and NcGrx3 deficiency and overexpression strains. The deletion or overexpression of NcGrx3 had no significant effect on the growth of N. caninum in vitro and in vivo. NcGrx1 knockout parasites displayed a significant growth defect, which was due to the influence on invasion and egress abilities. Moreover, NcGrx1 deficiency decreased the ratio of reduced glutathione (GSH) to oxidized glutathione (GSSG) (GSH/GSSG ratio), caused a significant accumulation of hydroxyl radical in parasites, and an increase in apoptotic cells under oxidative stress (H₂O₂) condition. To determine the cause of growth defects in ΔNcGrx1, we examined the transcription levels of various invasion-egress related genes as measured by qPCR. We found a significant decrease in MIC1, MIC4, and MIC6 genes. Further investigation found that the secretion of MIC1, MIC4, and MIC6 proteins was significantly affected. Collectively, Ncgrx1 is important for microneme protein-mediated parasite growth, and maybe a potential intervention target for the N. caninum.

Insights into the intracellular localization, protein associations and artemisinin resistance properties of Plasmodium falciparum K13

The emergence of artemisinin (ART) resistance in Plasmodium falciparum intra-erythrocytic parasites has led to increasing treatment failure rates with first-line ART-based combination therapies in Southeast Asia. Decreased parasite susceptibility is caused by K13 mutations, which are associated clinically with delayed parasite clearance in patients and in vitro with an enhanced ability of ring-stage parasites to survive brief exposure to the active ART metabolite dihydroartemisinin. Herein, we describe a panel of K13-specific monoclonal antibodies and gene-edited parasite lines co-expressing epitope-tagged versions of K13 in trans. By applying an analytical quantitative imaging pipeline, we localize K13 to the parasite endoplasmic reticulum, Rab-positive vesicles, and sites adjacent to cytostomes. These latter structures form at the parasite plasma membrane and traffic hemoglobin to the digestive vacuole wherein artemisinin-activating heme moieties are released. We also provide evidence of K13 partially localizing near the parasite mitochondria upon treatment with dihydroartemisinin. Immunoprecipitation data generated with K13-specific monoclonal antibodies identify multiple putative K13-associated proteins, including endoplasmic reticulum-resident molecules, mitochondrial proteins, and Rab GTPases, in both K13 mutant and wild-type isogenic lines. We also find that mutant K13-mediated resistance is reversed upon co-expression of wild-type or mutant K13. These data help define the biological properties of K13 and its role in mediating P. falciparum resistance to ART treatment.

NIR-triggered thermo-responsive biodegradable hydrogel with combination of photothermal and thermodynamic therapy for hypoxic tumor

Hypoxia is a typical feature of solid tumors, which highly limits the application of the oxygen-dependent therapy. Also, the dense and hyperbaric tumor tissues impede the penetration of nanoparticles into the deep tumor. Thereby, we designed a novel localized injectable hydrogel combining the photothermal therapy (PTT) and the thermodynamic therapy (TDT), which is based on the generation of free radicals even in the absence of oxygen for hypoxic tumor therapy. In our study, gold nanorods (AuNRs) and 2,2'-Azobis[2-(2-imidazalin-2-yl)propane] dihydrochlaride (AIPH) were incorporated into the hydrogel networks, which were formed by the copolymerization of hydrophobic N-isopropyl acrylamide (NIPAM) and hydrophilic glycidyl methacrylate modified hyaluronic acid (HA-GMA) to fabricate an injectable and near-infrared (NIR) responsive hydrogel. The crosslinked in situ forming hydrogel could not only realize PTT upon the NIR laser irradiation, but also generate free radicals even in hypoxic condition. Meanwhile the shrink of hydrogels upon thermal could accelerate the generation of free radicals to further damage the tumors, achieving the controlled drug release on demand. The designed hydrogel with a sufficient loading capacity, excellent biocompatibility and negligible systemic toxicity could serve as a long-acting implant for NIR-triggered thermo-responsive free radical generation. The in vitro cytotoxicity result and the in vivo antitumor activity illustrated the excellent therapeutic effect of hydrogels even in the absence of oxygen. Therefore, this innovative oxygen-independent platform combining the antitumor effects of PTT and TDT would bring a new insight into hypoxic tumor therapy by the application of alkyl free radical.

Live and Let Dye: Visualizing the Cellular Compartments of the Malaria Parasite Plasmodium falciparum

Malaria remains one of the deadliest diseases worldwide and it is caused by the protozoan parasite Plasmodium spp. Parasite visualization is an important tool for the correct detection of malarial cases but also to understand its biology. Advances in visualization techniques promote new insights into the complex life cycle and biology of Plasmodium parasites. Live cell imaging by fluorescence microscopy or flow cytometry are the foundation of the visualization technique for malaria research. In this review, we present an overview of possibilities in live cell imaging of the malaria parasite. We discuss some of the state-of-the-art techniques to visualize organelles and processes of the parasite and discuss limitation and advantages of each technique. © 2019 International Society for Advancement of Cytometry.

Stable Integration and Comparison of hGrx1-roGFP2 and sfroGFP2 Redox Probes in the Malaria Parasite Plasmodium falciparum

Studying redox metabolism in malaria parasites is of great interest for understanding parasite biology, parasite-host interactions, and mechanisms of drug action. Genetically encoded fluorescent redox sensors have recently been described as powerful tools for determining the glutathione-dependent redox potential in living parasites. In the present study, we genomically integrated and expressed the ratiometric redox sensors hGrx1-roGFP2 (human glutaredoxin 1 fused to reduction-oxidation sensitive green fluorescent protein) and sfroGFP2 (superfolder roGFP2) in the cytosol of NF54- attB blood-stage Plasmodium falciparum parasites. Both sensors were evaluated in vitro and in cell culture with regard to their fluorescence properties and reactivity. As genomic integration allows for the stable expression of redox sensors in parasites, we systematically compared single live-cell imaging with plate reader detection. For these comparisons, short-term effects of redox-active compounds were analyzed along with mid- and long-term effects of selected antimalarial agents. Of note, the single components of the redox probes themselves did not influence the redox balance of the parasites. Our analyses revealed comparable results for both the hGrx1-roGFP2 and sfroGFP2 probes, with sfroGFP2 exhibiting a more pronounced fluorescence intensity in cellulo. Accordingly, the sfroGFP2 probe was employed to monitor the fluorescence signals throughout the parasites' asexual life cycle. Through the use of stable genomic integration, we demonstrate a means of overcoming the limitations of transient transfection, allowing more detailed in-cell studies as well as high-throughput analyses using plate reader-based approaches.

Growth inhibitory effects of standard pro- and antioxidants on the human malaria parasite Plasmodium falciparum

The redox metabolism of the malaria parasite Plasmodium falciparum and its human host has been suggested to play a central role for parasite survival and clearance. A common approach to test hypotheses in redox research is to challenge or rescue cells with pro- and antioxidants. However, quantitative data on the susceptibility of infected erythrocytes towards standard redox agents is surprisingly scarce. Here we determined the IC₅₀ values of P. falciparum strains 3D7 and Dd2 for a set of redox agents using a SYBR green-based growth assay. Parasite killing in this assay required extremely high concentrations of hydrogen peroxide with a millimolar IC₅₀ value, whereas IC₅₀ values for tert-butyl hydroperoxide and diamide were between 67 and 121 μM. Thus, in contrast to tert-butyl hydroperoxide and the disulfide-inducing agent diamide, the host-parasite unit appears to be very robust against challenges with hydrogen peroxide with implications for host defense mechanisms. N-acetylcysteine, ascorbate, and dithiothreitol also had antiproliferative instead of growth-promoting effects with IC₅₀ values around 12, 3 and 0.4 mM, respectively. So-called antioxidants can therefore also inhibit parasite growth with implications for clinical trials and studies on 'oxidative stress'. Furthermore, the addition of reductants to parasite cultures resulted in the gelation of albumin, the formation of methemoglobin and hemolysis. These effects can alter the fluorescence in SYBR green assays and have to be taken into account for the determination of IC₅₀ values. In summary, standard oxidants and reductants both inhibit the growth of P. falciparum with IC₅₀ values differing by three orders of magnitude.