Cystathionine γ-lyase, an Enzyme Related to the Reverse Transsulfuration Pathway, is Functional in Leishmania spp

Deciphering Host-Parasite Interplay in Leishmania Infection through a One Health View of Proteomics Studies on Drug Resistance

Recent efforts in the study of vector-borne parasitic diseases (VBPDs) have emphasized an increased consideration for preventing drug resistance and promoting the environmental safety of drugs, from the beginning of the drug discovery pipeline. The intensive use of the few available antileishmanial drugs has led to the spreading of hyper-resistant Leishmania infantum strains, resulting in a chronic burden of the disease. In the present work, we have investigated the biochemical mechanisms of resistance to antimonials, paromomycin, and miltefosine in three drug-resistant parasitic strains from human clinical isolates, using a whole-cell mass spectrometry proteomics approach. We identified 14 differentially expressed proteins that were validated with their transcripts. Next, we employed functional association networks to identify parasite-specific proteins as potential targets for novel drug discovery studies. We used SeqAPASS analysis to predict susceptibility based on the evolutionary conservation of protein drug targets across species. MATH-domain-containing protein, adenosine triphosphate (ATP)-binding cassette B2, histone H4, calpain-like cysteine peptidase, and trypanothione reductase emerged as top candidates. Overall, this work identifies new biological targets for designing drugs to prevent the development of Leishmania drug resistance, while aligning with One Health principles that emphasize the interconnected health of people, animals, and ecosystems.

Metabolic control analysis of the transsulfuration pathway and the compensatory role of the cysteine transport in Trypanosoma cruzi

Trypanosoma cruzi is the causal agent of American Trypanosomiasis or Chagas Disease in humans. The current drugs for its treatment benznidazole and nifurtimox have inconveniences of toxicity and efficacy; therefore, the search for new therapies continues. Validation through genetic strategies of new drug targets against the parasite metabolism have identified numerous essential genes. Target validation can be further narrowed by applying Metabolic Control Analysis (MCA) to determine the flux control coefficients of the pathway enzymes. That coefficient is a quantitative value that represents the degree in which an enzyme/transporter determines the flux of a metabolic pathway; those with the highest coefficients can be promising drug targets. Previous studies have demonstrated that cysteine (Cys) is a key precursor for the synthesis of trypanothione, the main antioxidant metabolite in the parasite. In this research, MCA was applied in an ex vivo system to the enzymes of the reverse transsulfuration pathway (RTP) for Cys synthesis composed by cystathionine beta synthase (CBS) and cystathionine gamma lyase (CGL). The results indicated that CGL has 90% of the control of the pathway flux. Inhibition of CGL with propargylglycine (PAG) decreased the levels of Cys and trypanothione and depleted those of glutathione in epimastigotes (proliferative stage in the insect vector); these metabolite changes were prevented by supplementing with Cys, suggesting a compensatory role of the Cys transport (CysT). Indeed, Cys supplementation (but not PAG treatment) increased the activity of the CysT in epimastigotes whereas in trypomastigotes (infective stage in mammals) CysT was increased when they were incubated with PAG. Our results suggested that CGL could be a potential drug target given its high control on the RTP flux and its effects on the parasite antioxidant defense. However, the redundant Cys supply pathways in the parasite may require inhibition of the CysT as well. Our findings also suggest differential responses of the Cys supply pathways in different parasite stages.

Two phylogenetically divergent isocitrate dehydrogenases are encoded in Leishmania parasites. Molecular and functional characterization of Leishmania mexicana isoenzymes with specificity towards NAD+ and NADP

Leishmania parasites are of great relevance to public health because they are the causative agents of various long-term and health-threatening diseases in humans. Dependent on the manifestation, drugs either require difficult and lengthy administration, are toxic, expensive, not very effective or have lost efficacy due to the resistance developed by these pathogens against clinical treatments. The intermediary metabolism of Leishmania parasites is characterized by several unusual features, among which whether the Krebs cycle operates in a cyclic and/or in a non-cyclic mode is included. Our survey of the genomes of Leishmania species and monoxenous parasites such as those of the genera Crithidia and Leptomonas (http://www.tritrypdb.org) revealed that two genes encoding putative isocitrate dehydrogenases (IDHs) -with distantly related sequences- are strictly conserved among these parasites. Thus, in this study, we aimed to functionally characterize the two leishmanial IDH isoenzymes, for which we selected the genes LmxM10.0290 (Lmex_IDH-90) and LmxM32.2550 (Lmex_IDH-50) from L. mexicana. Phylogenetic analysis showed that Lmex_IDH-50 clustered with members of Subfamily I, which contains mainly archaeal and bacterial IDHs, and that Lmex_IDH-90 was a close relative of eukaryotic enzymes comprised within Subfamily II IDHs. 3-D homology modeling predicted that both IDHs exhibited the typical folding motifs recognized as canonical for prokaryotic and eukaryotic counterparts, respectively. Both IDH isoforms displayed dual subcellular localization, in the cytosol and the mitochondrion. Kinetic studies showed that Lmex_IDH-50 exclusively catalyzed the reduction of NAD⁺, while Lmex_IDH-90 solely used NADP⁺ as coenzyme. Besides, Lmex_IDH-50 differed from Lmex_IDH-90 by exhibiting a nearly 20-fold lower apparent K_m value towards isocitrate (2.0 μM vs 43 μM). Our findings showed, for the first time, that the genus Leishmania differentiates not only from other trypanosomatids such as Trypanosoma cruzi and Trypanosoma brucei, but also from most living organisms, by exhibiting two functional homo-dimeric IDHs, highly specific towards NAD⁺ and NADP⁺, respectively. It is tempting to argue that any or both types of IDHs might be directly or indirectly linked to the Krebs cycle and/or to the de novo synthesis of glutamate. Our results about the biochemical and structural features of leishmanial IDHs show the relevance of deepening our knowledge of the metabolic processes in these pathogenic parasites to potentially identify new therapeutic targets.

Mapping the metabolism of five amino acids in bloodstream form Trypanosoma brucei using U-13C-labelled substrates and LC-MS

The metabolism of the parasite Trypanosoma brucei has been the focus of numerous studies since the 1940s. Recently it was shown, using metabolomics coupled with heavy-atom isotope labelled glucose, that the metabolism of the bloodstream form parasite is more complex than previously thought. The present study also raised a number of questions regarding the origin of several metabolites, for example succinate, only a proportion of which derives from glucose. In order to answer some of these questions and explore the metabolism of bloodstream form T. brucei in more depth we followed the fate of five heavy labelled amino acids – glutamine, proline, methionine, cysteine and arginine – using an LC–MS based metabolomics approach. We found that some of these amino acids have roles beyond those previously thought and we have tentatively identified some unexpected metabolites which need to be confirmed and their function determined.

The Uptake and Metabolism of Amino Acids, and Their Unique Role in the Biology of Pathogenic Trypanosomatids

Trypanosoma brucei, as well as Trypanosoma cruzi and more than 20 species of the genus Leishmania, form a group of flagellated protists that threaten human health. These organisms are transmitted by insects that, together with mammals, are their natural hosts. This implies that during their life cycles each of them faces environments with different physical, chemical, biochemical, and biological characteristics. In this work we review how amino acids are obtained from such environments, how they are metabolized, and how they and some of their intermediate metabolites are used as a survival toolbox to cope with the different conditions in which these parasites should establish the infections in the insects and mammalian hosts.

Molecular and functional characterization of two malic enzymes from Leishmania parasites

Leishmania parasites cause a broad spectrum of clinical manifestations in humans and the available clinical treatments are far from satisfactory. Since these pathogens require large amounts of NADPH to maintain intracellular redox homeostasis, oxidoreductases that catalyze the production of NADPH are considered as potential drug targets against these diseases. In the sequenced genomes of most Leishmania spp. two putative malic enzymes (MEs) with an identity of about 55% have been identified. In this work, the ME from L. major (LmjF24.0770, Lmj_ME-70) and its less similar homolog from L. mexicana (LmxM.24.0761, Lmex_ME-61) were cloned and functionally characterized. Both MEs specifically catalyzed NADPH production, but only Lmex_ME-61 was activated by l-aspartate. Unlike the allosterically activated human ME, Lmex_ME-61 exhibited typical hyperbolic curves without any sign of cooperativity in the absence of l-aspartate. Moreover, Lmex_ME-61 and Lmj_ME-70 differ from higher eukaryotic homologs in that they display dimeric instead of tetrameric molecular organization. Homology modeling analysis showed that Lmex_ME-61 and Lmj_ME-70 notably differ in their surface charge distribution; this feature encompasses the coenzyme binding pockets as well. However, in both isozymes, the residues directly involved in the coenzyme binding exhibited a good degree of conservation. Besides, only Lmex_ME-61 and its closest homologs were immunodetected in cell-free extracts from L. mexicana, L. amazonensis and L. braziliensis promastigotes. Our findings provide a first glimpse into the biochemical properties of leishmanial MEs and suggest that MEs could be potentially related to the metabolic differences among the species of Leishmania parasites.

Interaction between cysteine synthase and serine O-acetyltransferase proteins and their stage specific expression in Leishmania donovani

Leishmania possess a unique trypanothione redox metabolism with undebated roles in protection from oxidative damage and drug resistance. The biosynthesis of trypanothione depends on l-cysteine bioavailability which is regulated by cysteine biosynthesis pathway. The de novo cysteine biosynthesis pathway is comprised of serine O-acetyltransferase (SAT) and cysteine synthase (CS) enzymes which sequentially mediate two consecutive steps of cysteine biosynthesis, and is absent in mammalian host. However, despite the apparent dependency of redox metabolism on cysteine biosynthesis pathway, the role of SAT and CS in redox homeostasis has been unexplored in Leishmania parasites. Herein, we have characterized CS and SAT to investigate their interaction and relative abundance of these proteins in promastigote vs. amastigote growth stages of L. donovani. CS and SAT genes of L. donovani (LdCS and LdSAT) were cloned, expressed, and fusion proteins purified to homogeneity with affinity column chromatography. Purified LdCS contains PLP as cofactor and showed optimum enzymatic activity at pH 7.5. Enzyme kinetics showed that LdCS catalyses the synthesis of cysteine using O-acetylserine and sulfide with a K_m of 15.86 mM and 0.17 mM, respectively. Digitonin fractionation and indirect immunofluorescence microscopy showed that LdCS and LdSAT are localized in the cytoplasm of promastigotes. Size exclusion chromatography, co-purification, pull down and immuno-precipitation assays demonstrated a stable complex formation between LdCS and LdSAT proteins. Furthermore, LdCS and LdSAT proteins expression/activity was upregulated in amastigote growth stage of the parasite. Thus, the stage specific differential expression of LdCS and LdSAT suggests that it may have a role in the redox homeostasis of Leishmania.

Upregulation of Cysteine Synthase and Cystathionine β-Synthase Contributes to Leishmania braziliensis Survival under Oxidative Stress

Cysteine metabolism is considered essential for the crucial maintenance of a reducing environment in trypanosomatids due to its importance as a precursor of trypanothione biosynthesis. Expression, activity, functional rescue, and overexpression of cysteine synthase (CS) and cystathionine β-synthase (CβS) were evaluated in Leishmania braziliensis promastigotes and intracellular amastigotes under in vitro stress conditions induced by hydrogen peroxide (H2O2), S-nitroso-N-acetylpenicillamine, or antimonial compounds. Our results demonstrate a stage-specific increase in the levels of protein expression and activity of L. braziliensis CS (LbrCS) and L. braziliensis CβS (LbrCβS), resulting in an increment of total thiol levels in response to both oxidative and nitrosative stress. The rescue of the CS activity in Trypanosoma rangeli, a trypanosome that does not perform cysteine biosynthesis de novo, resulted in increased rates of survival of epimastigotes expressing the LbrCS under stress conditions compared to those of wild-type parasites. We also found that the ability of L. braziliensis promastigotes and amastigotes overexpressing LbrCS and LbrCβS to resist oxidative stress was significantly enhanced compared to that of nontransfected cells, resulting in a phenotype far more resistant to treatment with the pentavalent form of Sb in vitro. In conclusion, the upregulation of protein expression and increment of the levels of LbrCS and LbrCβS activity alter parasite resistance to antimonials and may influence the efficacy of antimony treatment of New World leishmaniasis.