Knockout of the peroxiredoxin 5 homologue PFAOP does not affect the artemisinin susceptibility of Plasmodium falciparum

The cytosolic hyperoxidation-sensitive and -robust Leishmania peroxiredoxins cPRX1 and cPRX2 are both dispensable for parasite infectivity

Typical two-cysteine peroxiredoxins (2-Cys-PRXs) are H2O2-metabolizing enzymes whose activity relies on two cysteine residues. Protists of the family Trypanosomatidae invariably express one cytosolic 2-Cys-PRX (cPRX1). However, the Leishmaniinae sub-family features an additional isoform (cPRX2), almost identical to cPRX1, except for the lack of an elongated C-terminus with a Tyr-Phe (YF) motif. Previously, cytosolic PRXs were considered vital components of the trypanosomatid antioxidant machinery. Here, we shed new light on the properties, functions and relevance of cPRXs from the human pathogen Leishmania infantum. We show first that LicPRX1 is sensitive to inactivation by hyperoxidation, mirroring other YF-containing PRXs participating in redox signaling. Using genetic fusion constructs with roGFP2, we establish that LicPRX1 and LicPRX2 can act as sensors for H2O2 and oxidize protein thiols with implications for signal transduction. Third, we show that while disrupting the LicPRX-encoding genes increases susceptibility of L. infantum promastigotes to external H2O2in vitro, both enzymes are dispensable for the parasites to endure the macrophage respiratory burst, differentiate into amastigotes and initiate in vivo infections. This study introduces a novel perspective on the functions of trypanosomatid cPRXs, exposing their dual roles as both peroxidases and redox sensors. Furthermore, the discovery that Leishmania can adapt to the absence of both enzymes has significant implications for our understanding of Leishmania infections and their treatment. Importantly, it questions the conventional notion that the oxidative response of macrophages during phagocytosis is a major barrier to infection and the suitability of cPRXs as drug targets for leishmaniasis.

Characterization of the glutathione-dependent reduction of the peroxiredoxin 5 homolog PfAOP from Plasmodium falciparum

Peroxiredoxins use a variety of thiols to rapidly reduce hydroperoxides and peroxynitrite. While the oxidation kinetics of peroxiredoxins have been studied in great detail, enzyme-specific differences regarding peroxiredoxin reduction and the overall rate-limiting step under physiological conditions often remain to be deciphered. The 1-Cys peroxiredoxin 5 homolog PfAOP from the malaria parasite Plasmodium falciparum is an established model enzyme for glutathione/glutaredoxin-dependent peroxiredoxins. Here, we reconstituted the catalytic cycle of PfAOP in vitro and analyzed the reaction between oxidized PfAOP and reduced glutathione (GSH) using molecular docking and stopped-flow measurements. Molecular docking revealed that oxidized PfAOP has to adopt a locally unfolded conformation to react with GSH. Furthermore, we determined a second-order rate constant of 6 × 105  M-1  s-1 at 25°C and thermodynamic activation parameters ΔH‡ , ΔS‡ , and ΔG‡ of 39.8 kJ/mol, -0.8 J/mol, and 40.0 kJ/mol, respectively. The gain-of-function mutant PfAOPL109M had almost identical reaction parameters. Taking into account physiological hydroperoxide and GSH concentrations, we suggest (a) that the reaction between oxidized PfAOP and GSH might be even faster than the formation of the sulfenic acid in vivo, and (b) that conformational changes are likely rate limiting for PfAOP catalysis. In summary, we characterized and quantified the reaction between GSH and the model enzyme PfAOP, thus providing detailed insights regarding the reactivity of its sulfenic acid and the versatile chemistry of peroxiredoxins.

Protein abundance and folding rather than the redox state of Kelch13 determine the artemisinin susceptibility of Plasmodium falciparum

Decreased susceptibilities of the human malaria parasite Plasmodium falciparum towards the endoperoxide antimalarial artemisinin are linked to mutations of residue C580 of PfKelch13, a homologue of the redox sensor Keap1 and other vertebrate BTB-Kelch proteins. Here, we addressed whether mutations alter the artemisinin susceptibility by modifying the redox properties of PfKelch13 or by compromising its native fold or abundance. Using selection-linked integration and the glmS ribozyme, efficient down-regulation of PfKelch13 resulted in ring-stage survival rates around 40%. While the loss of the thiol group of C469 or of the potential disulfide bond between residues C580 and C532 had no effect on the artemisinin susceptibility, the thiol group of C473 could not be replaced. Furthermore, we detected two different forms of PfKelch13 with distinct electrophoretic mobilities around 85 and 95 kDa, suggesting an unidentified post-translational modification. We also established a protocol for the production of recombinant PfKelch13 and produced an antibody against the protein. Recombinant PfKelch13 adopted alternative oligomeric states and only two of its seven cysteine residues, C469 and C473, reacted with Ellman's reagent. While common field mutations resulted in misfolded and completely insoluble recombinant PfKelch13, cysteine-to-serine replacements had no effect on the solubility except for residue C473. In summary, in contrast to residues C469, C532, and C580, the surface-exposed thiol group of residue C473 appears to be essential. However, not the redox properties but impaired folding of PfKelch13, resulting in a decreased PfKelch13 abundance, alters the artemisinin susceptibility and is the central parameter for mutant selection.

Stochastic Protein Alkylation by Antimalarial Peroxides

Antimalarial peroxides such as the phytochemical artemisinin or the synthetic ozonides arterolane and artefenomel undergo reductive cleavage of the pharmacophoric peroxide bond by ferrous heme, released by parasite hemoglobin digestion. The generated carbon-centered radicals alkylate heme in an intramolecular reaction and proteins in an intermolecular reaction. Here, we determine the proteinaceous alkylation signatures of artemisinin and synthetic ozonides in Plasmodium falciparum using alkyne click chemistry probes to identify target proteins by affinity purification and mass spectrometry-based proteomics. Using stringent controls and purification procedures, we identified 25 P. falciparum proteins that were alkylated by the antimalarial peroxides in a peroxide-dependent manner, but the alkylation patterns were more random than we had anticipated. Moreover, there was little overlap in the alkylation signatures identified in this work and those disclosed in previous studies. Our findings suggest that alkylation of parasite proteins by antimalarial peroxides is likely to be a nonspecific, stochastic process.

Deciphering genetic regulation of CD14 by SP1 through characterization of peripheral blood mononuclear transcriptome of P. faiciparum and P. vivax infected malaria patients

BACKGROUND

Plasmodium falciparum and Plasmodium vivax are two major parasites responsible for malaria which remains a threat to almost 50% of world's population despite decade-long eradication program. One possible reason behind this conundrum is that the bases of clinical variability in malaria caused by either species are complex and poorly understood.

METHODS

Whole-genome transcriptome was analyzed to identify the active and predominant pathways in the PBMC of P. falciparum and P. vivax infected malaria patients. Deregulated genes were identified and annotated using R Bioconductor and DAVID/KEGG respectively. Genetic and functional regulation of CD14, a prioritized candidate, were established by quantitative RT-PCR, genotyping using RFLP and resequencing, mapping of transcription factor binding using CONSITE and TFBIND, dual luciferase assay, western blot analysis, RNAi- mediated gene knockdown and chromatin-immunoprecipation.

FINDINGS

The study highlighted that deregulation of host immune and inflammatory genes particularly CD14 as a key event in P. falciparum malaria. An abundance of allele-C of rs5744454, located in CD14 promoter, in severe malaria motivated us to establish an allele-specific regulation of CD14 by SP1. An enhancement of SP1 and CD14 expression was observed in artemisinin treated human monocyte cell line.

INTERPRETATION

Our data not only reinstates that CD14 of TLR pathway plays a predominant role in P. falciparum malaria, it establishes a functional basis for genetic association of rs5744454 with P. falciparum severe malaria by demonstrating a cis-regulatory role of this promoter polymorphism. Moreover, the study points towards a novel pharmacogenetic aspect of artemisinin-based anti-malarial therapy. FUND: DST-SERB, Govt. of India, SR/SO/HS-0056/2013.

Redox-sensitive GFP fusions for monitoring the catalytic mechanism and inactivation of peroxiredoxins in living cells

Redox-sensitive green fluorescent protein 2 (roGFP2) is a valuable tool for redox measurements in living cells. Here, we demonstrate that roGFP2 can also be used to gain mechanistic insights into redox catalysis in vivo. In vitro enzyme properties such as the rate-limiting reduction of wild type and mutant forms of the model peroxiredoxin PfAOP are shown to correlate with the ratiometrically measured degree of oxidation of corresponding roGFP2 fusion proteins. Furthermore, stopped-flow kinetic measurements of the oxidative half-reaction of PfAOP support the interpretation that changes in the roGFP2 signal can be used to map hyperoxidation-based inactivation of the attached peroxidase. Potential future applications of our system include the improvement of redox sensors, the estimation of absolute intracellular peroxide concentrations and the in vivo assessment of protein structure-function relationships that cannot easily be addressed with recombinant enzymes, for example, the effect of post-translational protein modifications on enzyme catalysis.