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      Real-Time Imaging of the Intracellular Glutathione Redox Potential in the Malaria Parasite Plasmodium falciparum

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          In the malaria parasite Plasmodium falciparum, the cellular redox potential influences signaling events, antioxidant defense, and mechanisms of drug action and resistance. Until now, the real-time determination of the redox potential in malaria parasites has been limited because conventional approaches disrupt sub-cellular integrity. Using a glutathione biosensor comprising human glutaredoxin-1 linked to a redox-sensitive green fluorescent protein (hGrx1-roGFP2), we systematically characterized basal values and drug-induced changes in the cytosolic glutathione-dependent redox potential ( E GSH) of drug-sensitive (3D7) and resistant (Dd2) P. falciparum parasites. Via confocal microscopy, we demonstrated that hGrx1-roGFP2 rapidly detects E GSH changes induced by oxidative and nitrosative stress. The cytosolic basal E GSH of 3D7 and Dd2 were estimated to be −314.2±3.1 mV and −313.9±3.4 mV, respectively, which is indicative of a highly reducing compartment. We furthermore monitored short-, medium-, and long-term changes in E GSH after incubation with various redox-active compounds and antimalarial drugs. Interestingly, the redox cyclers methylene blue and pyocyanin rapidly changed the fluorescence ratio of hGrx1-roGFP2 in the cytosol of P. falciparum, which can, however, partially be explained by a direct interaction with the probe. In contrast, quinoline and artemisinin-based antimalarial drugs showed strong effects on the parasites' E GSH after longer incubation times (24 h). As tested for various conditions, these effects were accompanied by a drop in total glutathione concentrations determined in parallel with alternative methods. Notably, the effects were generally more pronounced in the chloroquine-sensitive 3D7 strain than in the resistant Dd2 strain. Based on these results hGrx1-roGFP2 can be recommended as a reliable and specific biosensor for real-time spatiotemporal monitoring of the intracellular E GSH in P. falciparum. Applying this technique in further studies will enhance our understanding of redox regulation and mechanisms of drug action and resistance in Plasmodium and might also stimulate redox research in other pathogens.

          Author Summary

          In the malaria parasite Plasmodium falciparum, cellular redox reactions play important roles not only in redox-regulatory processes and antioxidant defense but also in the mechanisms of drug action and drug resistance. The tripeptide glutathione is present in malaria parasites in millimolar concentrations and represents the most important low molecular weight antioxidant. In this work, we describe the application of a genetically encoded glutathione biosensor comprising human glutaredoxin-1 linked to a redox-sensitive green fluorescent protein. By targeting this probe to the parasites' cytosol, we were able to verify its stability and responsiveness to the redox environment. Using this probe in living parasites, we further studied the effects of various redox active compounds and antimalarial drugs on the cytosolic glutathione redox potential in short, medium, and long-term experiments. Interestingly, after 24 h most antimalarial drugs, including quinolines and artemisinin derivatives, induced pronounced changes in the redox potential in pharmacologically meaningful concentrations. The chloroquine-sensitive P. falciparum 3D7 strain was generally more susceptible to these changes than the resistant Dd2 strain. We propose the hGrx1-roGFP2 probe as a reliable and most valuable tool for studying redox metabolism in living malaria parasites.

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          Most cited references 66

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            Plasmodium falciparum can now be maintained in continuous culture in human erythrocytes incubated at 38 degrees C in RPMI 1640 medium with human serum under an atmosphere with 7 percent carbon dioxide and low oxygen (1 or 5 percent). The original parasite material, derived from an infected Aotus trivirgatus monkey, was diluted more than 100 million times by the addition of human erythrocytes at 3- or 4-day intervals. The parasites continued to reproduce in their normal asexual cycle of approximately 48 hours but were no longer highly synchronous. The have remained infective to Aotus.
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              Synchronization of Plasmodium falciparum erythrocytic stages in culture.

              Synchronous development of the erythrocytic stages of a human malaria parasite, Plasmodium falciparum, in culture was accomplished by suspending cultured parasites in 5% D-sorbitol and subsequent reintroduction into culture. Immediately after sorbitol treatment, cultures consisted mainly of single and multiple ring-form infections. At the same time, varying degrees of lysis of erythrocytes infected with the more mature stages of the parasite was evident. Approximately 95% of the parasites were in the ring stage of development at 48 and 96 hr after sorbitol treatment-likewise, a high percentage of trophozoite and schizont stages was observed at 24, 72, and 120 hr. D-Mannitol produced similar, selective, lytic effects.

                Author and article information

                Role: Editor
                PLoS Pathog
                PLoS Pathog
                PLoS Pathogens
                Public Library of Science (San Francisco, USA )
                December 2013
                December 2013
                5 December 2013
                : 9
                : 12
                [1 ]Biochemistry and Molecular Biology, Interdisciplinary Research Center, Justus Liebig University Giessen, Giessen, Germany
                [2 ]INRES-Chemical Signaling, University of Bonn, Bonn, Germany
                The Hebrew University of Jerusalem, Israel
                Author notes

                The authors have declared that no competing interests exist.

                Conceived and designed the experiments: DK FM SR AJM KB. Performed the experiments: DK FM SR AJM. Analyzed the data: DK FM AJM KB. Contributed reagents/materials/analysis tools: AJM KB. Wrote the paper: DK FM SR AJM KB.


                This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

                Pages: 18
                This work was supported by the Deutsche Forschungsgemeinschaft, grant BE 1540/15-1/SPP 1710 to KB ( and by the German Academic Exchange Service, scholarship to DK ( The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
                Research Article

                Infectious disease & Microbiology


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