LFR1 ferric iron reductase of Leishmania amazonensis is essential for the generation of infective parasite forms

Targeting and activation of macrophages in leishmaniasis. A focus on iron oxide nanoparticles

Macrophages play a pivotal role as host cells for Leishmania parasites, displaying a notable functional adaptability ranging from the proinflammatory, leishmanicidal M1 phenotype to the anti-inflammatory, parasite-permissive M2 phenotype. While macrophages can potentially eradicate amastigotes through appropriate activation, Leishmania employs diverse strategies to thwart this activation and redirect macrophages toward an M2 phenotype, facilitating its survival and replication. Additionally, a competition for iron between the two entities exits, as iron is vital for both and is also implicated in macrophage defensive oxidative mechanisms and modulation of their phenotype. This review explores the intricate interplay between macrophages, Leishmania, and iron. We focus the attention on the potential of iron oxide nanoparticles (IONPs) as a sort of immunotherapy to treat some leishmaniasis forms by reprogramming Leishmania-permissive M2 macrophages into antimicrobial M1 macrophages. Through the specific targeting of iron in macrophages, the use of IONPs emerges as a promising strategy to finely tune the parasite-host interaction, endowing macrophages with an augmented antimicrobial arsenal capable of efficiently eliminating these intrusive microbes.

Upregulation of ATP6V0D2 benefits intracellular survival of Leishmania donovani in erythrocytes-engulfing macrophages

Visceral leishmaniasis (VL) is the most severe type of leishmaniasis which is caused by infection of Leishmania donovani complex. In the BALB/c mouse model of VL, multinucleated giant cells (MGCs) with heavy parasite infection consist of the largest population of hemophagocytes in the spleen of L. donovani-infected mice, indicating that MGCs provide the parasites a circumstance beneficial for their survival. Although ATP6V0D2 is a demonstrated factor inducing the formation of hemophagocytic MGCs during L. donovani infection, functions of this protein in shaping the infection outcome in macrophages remain unclear. Here we evaluated the influence of upregulated ATP6V0D2 on intracellular survival of the parasites. L. donovani infection-induced hemophagocytosis of normal erythrocytes by macrophages was suppressed by RNAi-based knockdown of Atp6v0d2. The knockdown of Atp6v0d2 did not improve the survival of amastigotes within macrophages when the cells were cultured in the absence of erythrocytes. On the other hand, reduced intracellular survival of amastigotes in macrophages by the knockdown was observed when macrophages were supplemented with antibody-opsonized erythrocytes before infection. There, increase in cytosolic labile iron pool was observed in the L. donovani-infected knocked-down macrophages. It suggests that ATP6V0D2 plays roles not only in upregulation of hemophagocytosis but also in iron trafficking within L. donovani-infected macrophages. Superior access to iron in macrophages may be how the upregulated expression of the molecule brings benefit to Leishmania for their intracellular survival in the presence of erythrocytes.

Access and utilization of host-derived iron by Leishmania parasites

Iron is involved in many biochemical processes including oxygen transport, ATP production, DNA synthesis and antioxidant defense. The importance of iron also applies to Leishmania parasites, an intracellular protozoan pathogen causing leishmaniasis. Leishmania are heme-auxotrophs, devoid of iron storage proteins and the heme synthesis pathway. Acquisition of iron and heme from the surrounding niche is thus critical for the intracellular survival of Leishmania inside the host macrophages. Moreover, Leishmania parasites are also exposed to oxidative stress within phagolysosomes of macrophages in mammalian hosts, and they need iron superoxide dismutase for overcoming this stress. Therefore, untangling the strategy adopted by these parasites for iron acquisition and utilization can be good targets for the development of antileishmanial drugs. Here, in this review, we will address how Leishmania parasites acquire and utilize iron and heme during infection to macrophages.

Complete assembly, annotation of virulence genes and CRISPR editing of the genome of Leishmania amazonensis PH8 strain

We report the sequencing and assembly of the PH8 strain of Leishmania amazonensis one of the etiological agents of leishmaniasis. After combining data from long Pacbio reads, short Illumina reads and synteny with the Leishmania mexicana genome, the sequence of 34 chromosomes with 8317 annotated genes was generated. Multigene families encoding three virulence factors, A2, amastins and the GP63 metalloproteases, were identified and compared to their annotation in other Leishmania species. As they have been recently recognized as virulence factors essential for disease establishment and progression of the infection, we also identified 14 genes encoding proteins involved in parasite iron and heme metabolism and compared to genes from other Trypanosomatids. To follow these studies with a genetic approach to address the role of virulence factors, we tested two CRISPR-Cas9 protocols to generate L. amazonensis knockout cell lines, using the Miltefosine transporter gene as a proof of concept.

Iron Uptake Controls Trypanosoma cruzi Metabolic Shift and Cell Proliferation

(1) Background: Ionic transport in Trypanosoma cruzi is the object of intense studies. T. cruzi expresses a Fe-reductase (TcFR) and a Fe transporter (TcIT). We investigated the effect of Fe depletion and Fe supplementation on different structures and functions of T. cruzi epimastigotes in culture. (2) Methods: We investigated growth and metacyclogenesis, variations of intracellular Fe, endocytosis of transferrin, hemoglobin, and albumin by cell cytometry, structural changes of organelles by transmission electron microscopy, O₂ consumption by oximetry, mitochondrial membrane potential measuring JC-1 fluorescence at different wavelengths, intracellular ATP by bioluminescence, succinate-cytochrome c oxidoreductase following reduction of ferricytochrome c, production of H₂O₂ following oxidation of the Amplex^® red probe, superoxide dismutase (SOD) activity following the reduction of nitroblue tetrazolium, expression of SOD, elements of the protein kinase A (PKA) signaling, TcFR and TcIT by quantitative PCR, PKA activity by luminescence, glyceraldehyde-3-phosphate dehydrogenase abundance and activity by Western blotting and NAD⁺ reduction, and glucokinase activity recording NADP⁺ reduction. (3) Results: Fe depletion increased oxidative stress, inhibited mitochondrial function and ATP formation, increased lipid accumulation in the reservosomes, and inhibited differentiation toward trypomastigotes, with the simultaneous metabolic shift from respiration to glycolysis. (4) Conclusion: The processes modulated for ionic Fe provide energy for the T. cruzi life cycle and the propagation of Chagas disease.

Development of antibodies to the iron-binding proteins transferrin and ferritin in dogs and mice infected with Leishmania parasites

Most microorganisms including Leishmania parasites compete with the innate immune defenses of the infected hosts to acquire iron, an essential nutrient necessary for their growth and replication. In mammals, iron is predominantly bound to protein carriers such as transferrin and ferritin and the strategies adopted by the infected host to restrict its uptake by pathogens are still not elucidated. We compared herein the development of anti-transferrin and anti-ferritin antibodies in hosts that differs by their susceptibility to Leishmania infection. Results showed that Leishmania infantum naturally-infected dogs which have developed canine leishmaniasis (CanL) demonstrated higher titers of IgG antibodies anti-leishmanial antigens and anti-iron binding proteins than those infected without clinical signs. In the experimental mouse model, C57BL/6 mice resisted L. major infection, developed lower titers of Leishmania-specific IgG antibodies than BALB/c susceptible mice but demonstrated also the production of anti-transferrin and anti-ferritin IgG antibodies. Overall, results are in favor that mechanisms, other than the polyclonal activation of B cells associated-hypergammaglobulinemia, a characteristic of susceptible animals, are likely involved and require a replicating parasite for the limitation of iron uptake.

Hemoglobin Endocytosis and Intracellular Trafficking: A Novel Way of Heme Acquisition by Leishmania

Leishmania species are causative agents of human leishmaniasis, affecting 12 million people annually. Drugs available for leishmaniasis are toxic, and no vaccine is available. Thus, the major thrust is to identify new therapeutic targets. Leishmania is an auxotroph for heme and must acquire heme from the host for its survival. Thus, the major focus has been to understand the heme acquisition process by the parasites in the last few decades. It is conceivable that the parasite is possibly obtaining heme from host hemoprotein, as free heme is not available in the host. Current understanding indicates that Leishmania internalizes hemoglobin (Hb) through a specific receptor by a clathrin-mediated endocytic process and targets it to the parasite lysosomes via the Rab5 and Rab7 regulated endocytic pathway, where it is degraded to generate intracellular heme that is used by the parasite. Subsequently, intra-lysosomal heme is initially transported to the cytosol and is finally delivered to the mitochondria via different heme transporters. Studies using different null mutant parasites showed that these receptors and transporters are essential for the survival of the parasite. Thus, the heme acquisition process in Leishmania may be exploited for the development of novel therapeutics.

Elucidation of iron homeostasis in Acanthamoeba castellanii

Acanthamoeba castellanii is a ubiquitously distributed amoeba that can be found in soil, dust, natural and tap water, air conditioners, hospitals, contact lenses and other environments. It is an amphizoic organism that can cause granulomatous amoebic encephalitis, an infrequent fatal disease of the central nervous system, and amoebic keratitis, a severe corneal infection that can lead to blindness. These diseases are extremely hard to treat; therefore, a more comprehensive understanding of this pathogen's metabolism is essential for revealing potential therapeutic targets. To propagate successfully in human tissues, the parasites must resist the iron depletion caused by nutritional immunity. The aim of our study is to elucidate the mechanisms underlying iron homeostasis in A. castellanii. Using a comparative whole-cell proteomic analysis of cells grown under different degrees of iron availability, we identified the primary proteins involved in Acanthamoeba iron acquisition. Our results suggest a two-step reductive mechanism of iron acquisition by a ferric reductase from the STEAP family and a divalent metal transporter from the NRAMP family. Both proteins are localized to the membranes of acidified digestive vacuoles where endocytosed medium and bacteria are trafficked. The expression levels of these proteins are significantly higher under iron-limited conditions, which allows Acanthamoeba to increase the efficiency of iron uptake despite the observed reduced pinocytosis rate. We propose that excessive iron gained while grown under iron-rich conditions is removed from the cytosol into the vacuoles by an iron transporter homologous to VIT/Ccc1 proteins. Additionally, we identified a novel protein that may participate in iron uptake regulation, the overexpression of which leads to increased iron acquisition.

An Iron Transporter Is Involved in Iron Homeostasis, Energy Metabolism, Oxidative Stress, and Metacyclogenesis in Trypanosoma cruzi

The parasite Trypanosoma cruzi causes Chagas’ disease; both heme and ionic Fe are required for its optimal growth, differentiation, and invasion. Fe is an essential cofactor in many metabolic pathways. Fe is also harmful due to catalyzing the formation of reactive O₂ species; for this reason, all living systems develop mechanisms to control the uptake, metabolism, and storage of Fe. However, there is limited information available on Fe uptake by T. cruzi. Here, we identified a putative 39-kDa Fe transporter in T. cruzi genome, TcIT, homologous to the Fe transporter in Leishmania amazonensis and Arabidopsis thaliana. Epimastigotes grown in Fe-depleted medium have increased TcIT transcription compared with controls grown in regular medium. Intracellular Fe concentration in cells maintained in Fe-depleted medium is lower than in controls, and there is a lower O₂ consumption. Epimastigotes overexpressing TcIT, which was encountered in the parasite plasma membrane, have high intracellular Fe content, high O₂ consumption—especially in phosphorylating conditions, high intracellular ATP, very high H₂O₂ production, and stimulated transition to trypomastigotes. The investigation of the mechanisms of Fe transport at the cellular and molecular levels will assist in elucidating Fe metabolism in T. cruzi and the involvement of its transport in the differentiation from epimastigotes to trypomastigotes, virulence, and maintenance/progression of the infection.

Novel aspects of iron homeostasis in pathogenic bloodstream form Trypanosoma brucei

Iron is an essential regulatory signal for virulence factors in many pathogens. Mammals and bloodstream form (BSF) Trypanosoma brucei obtain iron by receptor-mediated endocytosis of transferrin bound to receptors (TfR) but the mechanisms by which T. brucei subsequently handles iron remains enigmatic. Here, we analyse the transcriptome of T. brucei cultured in iron-rich and iron-poor conditions. We show that adaptation to iron-deprivation induces upregulation of TfR, a cohort of parasite-specific genes (ESAG3, PAGS), genes involved in glucose uptake and glycolysis (THT1 and hexokinase), endocytosis (Phosphatidic Acid Phosphatase, PAP2), and most notably a divergent RNA binding protein RBP5, indicative of a non-canonical mechanism for regulating intracellular iron levels. We show that cells depleted of TfR by RNA silencing import free iron as a compensatory survival strategy. The TfR and RBP5 iron response are reversible by genetic complementation, the response kinetics are similar, but the regulatory mechanisms are distinct. Increased TfR protein is due to increased mRNA. Increased RBP5 expression, however, occurs by a post-transcriptional feedback mechanism whereby RBP5 interacts with its own, and with PAP2 mRNAs. Further observations suggest that increased RBP5 expression in iron-deprived cells has a maximum threshold as ectopic overexpression above this threshold disrupts normal cell cycle progression resulting in an accumulation of anucleate cells and cells in G2/M phase. This phenotype is not observed with overexpression of RPB5 containing a point mutation (F61A) in its single RNA Recognition Motif. Our experiments shed new light on how T. brucei BSFs reorganise their transcriptome to deal with iron stress revealing the first iron responsive RNA binding protein that is co-regulated with TfR, is important for cell viability and iron homeostasis; two essential processes for successful proliferation.

Nutrient sensing in Leishmania: Flagellum and cytosol

Parasites are by definition organisms that utilize resources from a host to support their existence, thus, promoting their ability to establish long-term infections and disease. Hence, sensing and acquiring nutrients for which the parasite and host compete is central to the parasitic mode of existence. Leishmania are flagellated kinetoplastid parasites that parasitize phagocytic cells, principally macrophages, of vertebrate hosts and the alimentary tract of sand fly vectors. Because nutritional supplies vary over time within both these hosts and are often restricted in availability, these parasites must sense a plethora of nutrients and respond accordingly. The flagellum has been recognized as an "antenna" that plays a core role in sensing environmental conditions, and various flagellar proteins have been implicated in sensing roles. In addition, these parasites exhibit non-flagellar intracellular mechanisms of nutrient sensing, several of which have been explored. Nonetheless, mechanistic details of these sensory pathways are still sparse and represent a challenging frontier for further experimental exploration.

Iron in infection and immunity

Iron is an essential micronutrient for virtually all living cells. In infectious diseases, both invading pathogens and mammalian cells including those of the immune system require iron to sustain their function, metabolism and proliferation. On the one hand, microbial iron uptake is linked to the virulence of most human pathogens. On the other hand, the sequestration of iron from bacteria and other microorganisms is an efficient strategy of host defense in line with the principles of 'nutritional immunity'. In an acute infection, host-driven iron withdrawal inhibits the growth of pathogens. Chronic immune activation due to persistent infection, autoimmune disease or malignancy however, sequesters iron not only from infectious agents, autoreactive lymphocytes and neoplastic cells but also from erythroid progenitors. This is one of the key mechanisms which collectively result in the anemia of chronic inflammation. In this review, we highlight the most important interconnections between iron metabolism and immunity, focusing on host defense against relevant infections and on the clinical consequences of anemia of inflammation.

Cytokines and metabolic regulation: A framework of bidirectional influences affecting Leishmania infection

Leishmania, a protozoan parasite inflicting the complex of diseases called Leishmaniases, resides and replicates as amastigotes within mammalian macrophages. As macrophages are metabolically highly active and can generate free radicals that can destroy this parasite, Leishmania also devise strategies to modulate the host cell metabolism. However, the metabolic changes can also be influenced by the anti-leishmanial immune response mediated by cytokines. This bidirectional, dynamic and complex metabolic coupling established between Leishmania and its host is the result of a long co-evolutionary process. Due to the continuous alterations imposed by the host microenvironment, such metabolic coupling continues to be dynamically regulated. The constant pursuit and competition for nutrients in the host-Leishmania duet alter the host metabolic pathways with major consequences for its nutritional reserves, eventually affecting the phenotype and functionality of the host cell. Altered phenotype and functions of macrophages are particularly relevant to immune cells, as perturbed metabolic fluxes can crucially affect the activation, differentiation, and functions of host immune cells. All these changes can deterministically direct the outcome of an infection. Cytokines and metabolic fluxes can bidirectionally influence each other through molecular sensors and regulators to dictate the final infection outcome. Our studies along with those from others have now identified the metabolic nodes that can be targeted for therapy.

Leishmania infection triggers hepcidin-mediated proteasomal degradation of Nramp1 to increase phagolysosomal iron availability

Natural resistance-associated macrophage protein 1 (Nramp1) was originally discovered as a genetic determinant of resistance against multiple intracellular pathogens, including Leishmania. It encodes a transmembrane protein of the phago-endosomal compartments, where it functions as an iron transporter. But the mechanism by which Nramp1 controls host-pathogen dynamics and determines final outcome of an infection is yet to be fully deciphered. Whether the expression of Nramp1 is altered in response to a pathogen attack is also unknown. To address these, Nramp1 status was examined in Leishmania major-infected murine macrophages. We observed that at 12 hrs post infection, there was drastic lowering of Nramp1 level accompanied by increased phagolysosomal iron content and enhanced intracellular parasite growth. Leishmania infection-induced Nramp1 downregulation was caused by ubiquitin-proteasome degradation pathway, which in turn was found to be mediated by the iron-regulatory peptide hormone hepcidin. Blocking of Nramp1 degradation with proteasome inhibitor or transcriptional agonist of hepcidin resulted in depletion of phagolysosomal iron pool that led to significant reduction of intracellular parasite burden. Interestingly, Nramp1 level was restored to normalcy after 30 hrs of infection with a concomitant drop in phagolysosomal iron, which is suggestive of a host counteractive response to deprive the pathogen of this essential micronutrient. Taken together, our study implicates Nramp1 as a central player in the host-pathogen battle for phagolysosomal iron. We also report Nramp1 as a novel target for hepcidin, and this 'hepcidin-Nramp1' axis may have a broader role in regulating macrophage iron homeostasis.

A ferric reductase of Trypanosoma cruzi (TcFR) is involved in iron metabolism in the parasite

Trypanosoma cruzi is a parasitic protozoan that infects various species of domestic and wild animals, triatomine bugs and humans. It is the etiological agent of American trypanosomiasis, also known as Chagas Disease, which affects about 17 million people in Latin America and is emerging elsewhere in the world. Iron (Fe) is a crucial micronutrient for almost all cells, acting as a cofactor for several metabolic enzymes. T. cruzi has a high requirement for Fe, using heminic and non-heminic Fe for growth and differentiation. Fe occurs in the oxidized (Fe³⁺) form in aerobic environments and needs to be reduced to Fe²⁺ before it enters cells. Fe-reductase, located in the plasma membranes of some organisms, catalyzes the Fe³⁺⇒ Fe²⁺ conversion. In the present study we found an amino acid sequence in silico that allowed us to identify a novel 35 kDa protein in T. cruzi with two transmembrane domains in the C-terminal region containing His residues that are conserved in the Ferric Reductase Domain Superfamily and are required for catalyzing Fe³⁺ reduction. Accordingly, we named this protein TcFR. Intact epimastigotes from the T. cruzi DM28c strain reduced the artificial Fe³⁺-containing substrate potassium ferricyanide in a cell density-dependent manner, following Michaelis-Menten kinetics. The TcFR activity was more than eightfold higher in a plasma membrane-enriched fraction than in whole homogenates, and this increase was consistent with the intensity of the 35 kDa band on Western blotting images obtained using anti-NOX5 raised against the human antigen. Immunofluorescence experiments demonstrated TcFR on the parasite surface. That TcFR is part of a catalytic complex allowing T. cruzi to take up Fe from the medium was confirmed by experiments in which DM28c was assayed after culturing in Fe-depleted medium: (i) proliferation during the stationary growth phase was five times slower; (ii) the relative expression of TcFR (qPCR) was 50% greater; (iii) intact cells had 120% higher Fe-reductase activity. This ensemble of results indicates that TcFR is a conserved enzyme in T. cruzi, and its catalytic properties are modulated in order to respond to external Fe fluctuations.

Activation of artemisinin and heme degradation in Leishmania tarentolae promastigotes: A possible link

Endoperoxides (EPs) appear to be promising drug candidates against protozoal diseases, including malaria and leishmaniasis. Previous studies have shown that these drugs need an intracellular activation to exert their pharmacological potential. The efficiency of these drugs is linked to the extensive iron demand of these intracellular protozoal parasites. An essential step of the activation mechanism of these drugs is the formation of radicals in Leishmania. Iron is a known trigger for intracellular radical formation. However, the activation of EPs by low molecular iron or by heme iron may strongly depend on the structure of the EPs themselves. In this study, we focused on the activation of artemisinin (Art) in Leishmania tarentolae promastigotes (LtP) in comparison to reference compounds. Viability assays in different media in the presence of different iron sources (hemin/fetal calf serum) showed that IC50 values of Art in LtP were modulated by assay conditions, but overall were within the low micromolar range. Low temperature electron paramagnetic resonance (EPR) spectroscopy of LtP showed that Art shifted the redox state of the labile iron pool less than the EP ascaridole questioning its role as a major activator of Art in LtP. Based on the high reactivity of Art with hemin in previous biomimetic experiments, we focused on putative heme-metabolizing enzymes in Leishmania, which were so far not well described. Inhibitors of mammalian heme oxygenase (HO; tin and chromium mesoporphyrin) acted antagonistically to Art in LtP and boosted its IC50 value for several magnitudes. By inductively coupled plasma methods (ICP-OES, ICP-MS) we showed that these inhibitors do not block iron (heme) accumulation, but are taken up and act within LtP. These inhibitors blocked the conversion of hemin to bilirubin in LtP homogenates, suggesting that an HO-like enzyme activity in LtP exists. NADPH-dependent degradation of Art and hemin was highest in the small granule and microsomal fractions of LtP. Photometric measurements in the model Art/hemin demonstrated that hemin requires reduction to heme and that subsequently an Art/heme complex (λmax 474 nm) is formed. EPR spin-trapping in the system Art/hemin revealed that NADPH, ascorbate and cysteine are suitable reductants and finally activate Art to acyl-carbon centered radicals. These findings suggest that heme is a major activator of Art in LtP either via HO-like enzyme activities and/or chemical interaction of heme with Art.

Iron and Heme Metabolism at the Leishmania-Host Interface

Species of the protozoan Leishmania are causative agents of human leishmaniasis, a disease that results in significant death, disability, and disfigurement around the world. The parasite is transmitted to a mammalian host by a sand fly vector where it develops as an intracellular parasite within macrophages. This process requires the acquisition of nutritional iron and heme from the host as Leishmania lacks the capacity for de novo heme synthesis and does not contain cytosolic iron-storage proteins. Proteins involved in Leishmania iron and heme transport and metabolism have been identified and shown to be crucial for the parasite's growth and replication within the host. Consequently, a detailed understanding of how these parasites harness host pathways for survival may lay the foundation for promising new therapeutic intervention against leishmaniasis.

Iron in parasitic protists – from uptake to storage and where we can interfere

A comprehensive review of iron metabolism in parasitic protists and its potential use as a drug target.

Hemophagocytosis induced by Leishmania donovani infection is beneficial to parasite survival within macrophages

Visceral leishmaniasis (VL) is caused by parasitic protozoa of the genus Leishmania and is characterized by clinical manifestations such as fever, hepatosplenomegaly and anemia. Hemophagocytosis, the phenomenon of phagocytosis of blood cells by macrophages, is found in VL patients. In a previous study we established an experimental model of VL, reproducing anemia in mice for the first time, and identified hemophagocytosis by heavily infected macrophages in the spleen as a possible cause of anemia. However, the mechanism for parasite-induced hemophagocytosis or its role in parasite survival remained unclear. Here, we established an in vitro model of Leishmania-induced hemophagocytosis to explore the molecules involved in this process. In contrast to naïve RAW264.7 cells (mouse macrophage cell line) which did not uptake freshly isolated erythrocytes, RAW264.7 cells infected with L. donovani showed enhanced phagocytosis of erythrocytes. Additionally, for hemophagocytes found both in vitro and in vivo, the expression of signal regulatory protein α (SIRPα), one of the receptors responsible for the ‘don’t-eat-me’ signal was suppressed by post-transcriptional control. Furthermore, the overlapped phagocytosis of erythrocytes and Leishmania parasites within a given macrophage appeared to be beneficial to the parasites; the in vitro experiments showed a higher number of parasites within macrophages that had been induced to engulf erythrocytes. Together, these results suggest that Leishmania parasites may actively induce hemophagocytosis by manipulating the expression of SIRPα in macrophages/hemophagocytes, in order to secure their parasitism.

Parasites can manipulate host immune responses to build favorable environment to them. Because this parasite-driven immune modulation is often linked to symptoms in infected individuals, not only parasiticidal compounds but also immunological interventions limiting such the parasites’ abilities will serve as treatment options. In this study, we studied the mechanism and its role of hemophagocytosis (the phenomenon whereby macrophages engulf erythrocytes) caused by Leishmania donovani, a causative agent of VL. In vitro experiments revealed parasites have ability to directly disrupt macrophage’s recognition of self-cells, and that the induced engulfment of erythrocytes by L. donovani infection is beneficial to the parasites for their intracellular survival. These results suggest that Leishmania parasites actively induce hemophagocytosis by manipulating the ‘don’t-eat-me’ signal in macrophages for their survival. Although it is still to be determined how Leishmania parasites change the ‘don’t-eat-me’ signal in macrophages, our study may facilitate development of an immunotherapy which limits the change and lead to improvement of anemia due to hemophagocytosis as well as control of parasite survival.

Leishmania amazonensis ferric iron reductase (LFR1) is a bifunctional enzyme: Unveiling a NADPH oxidase activity

Leishmania amazonensis is one of leishmaniasis' causative agents, a disease that has no cure and leads to the appearance of cutaneous lesions. Recently, our group showed that heme activates a Na⁺/K⁺ ATPase in these parasites through a signaling cascade involving hydrogen peroxide (H₂O₂) generation. Heme has a pro-oxidant activity and signaling capacity, but the mechanism by which this molecule increases H₂O₂ levels in L. amazonensis has not been elucidated. Here we investigated the source of H₂O₂ stimulated by heme, ruling out the participation of mitochondria and raising the possibility of a role for a NADPH oxidase (Nox) activity. Despite the absence of a classical Nox sequence in trypanosomatid genomes, L. amazonensis expresses a surface ferric iron reductase (LFR1). Interestingly, Nox enzymes are thought to have evolved from ferric iron reductases because they share same core domain and are very similar in structure. The main difference is that Nox catalyses electron flow from NADPH to oxygen, generating reactive oxygen species (ROS), while ferric iron reductase promotes electron flow to ferric iron, generating ferrous iron. Using L. amazonensis overexpressing or knockout for LFR1 and heterologous expression of LFR1 in mammalian embryonic kidney (HEK 293) cells, we show that this enzyme is bifunctional, being able to generate both ferrous iron and H₂O₂. It was previously described that protozoans knockout for LFR1 have their differentiation to virulent forms (amastigote and metacyclic promastigote) impaired. In this work, we observed that LFR1 overexpression stimulates protozoan differentiation to amastigote forms, reinforcing the importance of this enzyme in L. amazonensis life cycle regulation. Thus, we not only identified a new source of ROS production in Leishmania, but also described, for the first time, an enzyme with both ferric iron reductase and Nox activities.

The Leishmania Parasitophorous Vacuole Membrane at the Parasite-Host Interface

The Leishmania parasitophorous vacuole membrane (LPVM) sits at the interface of the parasite and its host. Evidence shows that molecules from the endocytic pathway as well as molecules from the secretory pathway are localized in the LPV and displayed on LPVM. In the review, we discuss our current understanding of the composition of the LPVM.

Intracellular iron availability modulates the requirement for Leishmania Iron Regulator 1 (LIR1) during macrophage infections

The Leishmania plasma membrane transporter Leishmania Iron Regulator 1 (LIR1) facilitates iron export and is required for parasite virulence. By modulating macrophage iron content, we investigated the host site where LIR1 regulates Leishmania amazonensis infectivity. In bone marrow-derived macrophages, LIR1 null mutants demonstrated a paradoxical increase in virulence during infections in heme-depleted media, while wild-type growth was inhibited under the same conditions. Loading the endocytic pathway of macrophages with cationized ferritin prior to infection reversed the effect of heme depletion on both strains. Thus, LIR1 contributes to Leishmania virulence by protecting the parasites from toxicity resulting from iron accumulation inside parasitophorous vacuoles.

Recent advances in trypanosomatid research: genome organization, expression, metabolism, taxonomy and evolution

Unicellular flagellates of the family Trypanosomatidae are obligatory parasites of invertebrates, vertebrates and plants. Dixenous species are aetiological agents of a number of diseases in humans, domestic animals and plants. Their monoxenous relatives are restricted to insects. Because of the high biological diversity, adaptability to dramatically different environmental conditions, and omnipresence, these protists have major impact on all biotic communities that still needs to be fully elucidated. In addition, as these organisms represent a highly divergent evolutionary lineage, they are strikingly different from the common 'model system' eukaryotes, such as some mammals, plants or fungi. A number of excellent reviews, published over the past decade, were dedicated to specialized topics from the areas of trypanosomatid molecular and cell biology, biochemistry, host-parasite relationships or other aspects of these fascinating organisms. However, there is a need for a more comprehensive review that summarizing recent advances in the studies of trypanosomatids in the last 30 years, a task, which we tried to accomplish with the current paper.

A MFS-like plasma membrane transporter required for Leishmania virulence protects the parasites from iron toxicity

Iron is essential for many cellular processes, but can generate highly toxic hydroxyl radicals in the presence of oxygen. Therefore, intracellular iron accumulation must be tightly regulated, by balancing uptake with storage or export. Iron uptake in Leishmania is mediated by the coordinated action of two plasma membrane proteins, the ferric iron reductase LFR1 and the ferrous iron transporter LIT1. However, how these parasites regulate their cytosolic iron concentration to prevent toxicity remains unknown. Here we characterize Leishmania Iron Regulator 1 (LIR1), an iron responsive protein with similarity to membrane transporters of the major facilitator superfamily (MFS) and plant nodulin-like proteins. LIR1 localizes on the plasma membrane of L. amazonensis promastigotes and intracellular amastigotes. After heterologous expression in Arabidopsis thaliana, LIR1 decreases the iron content of leaves and worsens the chlorotic phenotype of plants lacking the iron importer IRT1. Consistent with a role in iron efflux, LIR1 deficiency does not affect iron uptake by L. amazonensis but significantly increases the amount of iron retained intracellularly in the parasites. LIR1 null parasites are more sensitive to iron toxicity and have drastically impaired infectivity, phenotypes that are reversed by LIR1 complementation. We conclude that LIR1 functions as a plasma membrane iron exporter with a critical role in maintaining iron homeostasis and promoting infectivity in L. amazonensis.

Emerging therapeutic targets for treatment of leishmaniasis

INTRODUCTION

Parasitic diseases that pose a threat to human life include leishmaniasis - caused by protozoan parasite Leishmania species. Existing drugs have limitations due to deleterious side effects like teratogenicity, high cost and drug resistance. This calls for the need to have an insight into therapeutic aspects of disease. Areas covered: We have identified different drug targets via. molecular, imuunological, metabolic as well as by system biology approaches. We bring these promising drug targets into light so that they can be explored to their maximum. In an effort to bridge the gaps between existing knowledge and prospects of drug discovery, we have compiled interesting studies on drug targets, thereby paving the way for establishment of better therapeutic aspects. Expert opinion: Advancements in technology shed light on many unexplored pathways. Further probing of well established pathways led to the discovery of new drug targets. This review is a comprehensive report on current and emerging drug targets, with emphasis on several metabolic targets, organellar biochemistry, salvage pathways, epigenetics, kinome and more. Identification of new targets can contribute significantly towards strengthening the pipeline for disease elimination.

Quantification of Intracellular Growth Inside Macrophages is a Fast and Reliable Method for Assessing the Virulence of Leishmania Parasites

The lifecycle of Leishmania, the causative agent of leishmaniasis, alternates between promastigote and amastigote stages inside the insect and vertebrate hosts, respectively. While pathogenic symptoms of leishmaniasis can vary widely, from benign cutaneous lesions to highly fatal visceral disease forms depending on the infective species, all Leishmania species reside inside host macrophages during the vertebrate stage of their lifecycle. Leishmania infectivity is therefore directly related to its ability to invade, survive and replicate within parasitophorous vacuoles (PVs) inside macrophages. Thus, assessing the parasite's ability to replicate intracellularly serves as a dependable method for determining virulence. Studying leishmaniasis development using animal models is time-consuming, tedious and often difficult, particularly with the pathogenically important visceral forms. We describe here a methodology to follow the intracellular development of Leishmania in bone marrow-derived macrophages (BMMs). Intracellular parasite numbers are determined at 24 h intervals for 72 - 96 h following infection. This method allows for a reliable determination of the effects of various genetic factors on Leishmania virulence. As an example, we show how a single allele deletion of the Leishmania Mitochondrial Iron Transporter gene (LMIT1) impairs the ability of the Leishmania amazonensis mutant strain LMIT1/ΔLmit1 to grow inside BMMs, reflecting a drastic reduction in virulence compared to wild-type. This assay also allows precise control of experimental conditions, which can be individually manipulated to analyze the influence of various factors (nutrients, reactive oxygen species, etc.) on the host-pathogen interaction. Therefore, the appropriate execution and quantification of BMM infection studies provide a non-invasive, rapid, economical, safe and reliable alternative to conventional animal model studies.

The role of membrane transporters in Leishmania virulence

Leishmania are parasitic protozoa which infect humans and cause severe morbidity and mortality. Leishmania parasitise as extracellular promastigotes in the insect vector and as intracellular amastigotes in the mammalian host. Cycling between hosts involves implementation of stringent and co-ordinated responses to shifting environmental conditions. One of the key dynamic aspects of Leishmania biology is substrate acquisition and metabolism. Genomic analyses have revealed that Leishmania encode many putative membrane transporters, many of which are differentially expressed during the parasite life cycle. Only a small fraction of these transporters, however, have been functionally characterised. Currently, most information is available about nutrient transporters, mainly involved in carbohydrate, amino acid, nucleobase and nucleoside, cofactor, and ion acquisition. Several have apparent roles in Leishmania virulence and will be discussed in this perspective.

The iron-dependent mitochondrial superoxide dismutase SODA promotes Leishmania virulence

Leishmaniasis is one of the leading globally neglected diseases, affecting millions of people worldwide. Leishmania infection depends on the ability of insect-transmitted metacyclic promastigotes to invade mammalian hosts, differentiate into amastigotes, and replicate inside macrophages. To counter the hostile oxidative environment inside macrophages, these protozoans contain anti-oxidant systems that include iron-dependent superoxide dismutases (SODs) in mitochondria and glycosomes. Increasing evidence suggests that in addition to this protective role, Leishmania mitochondrial SOD may also initiate H₂O₂-mediated redox signaling that regulates gene expression and metabolic changes associated with differentiation into virulent forms. To investigate this hypothesis, we examined the specific role of SODA, the mitochondrial SOD isoform in Leishmania amazonensis Our inability to generate L. amazonensis SODA null mutants and the lethal phenotype observed following RNAi-mediated silencing of the Trypanosoma brucei SODA ortholog suggests that SODA is essential for trypanosomatid survival. L. amazonensis metacyclic promastigotes lacking one SODA allele failed to replicate in macrophages and were severely attenuated in their ability to generate cutaneous lesions in mice. Reduced expression of SODA also resulted in mitochondrial oxidative damage and failure of SODA/ΔsodA promastigotes to differentiate into axenic amastigotes. SODA expression above a critical threshold was also required for the development of metacyclic promastigotes, as SODA/ΔsodA cultures were strongly depleted in this infective form and more susceptible to reactive oxygen species (ROS)-induced stress. Collectively, our data suggest that SODA promotes Leishmania virulence by protecting the parasites against mitochondrion-generated oxidative stress and by initiating ROS-mediated signaling mechanisms required for the differentiation of infective forms.

The Gut Microbiome of the Vector Lutzomyia longipalpis Is Essential for Survival of Leishmania infantum

The vector-borne disease leishmaniasis, caused by Leishmania species protozoa, is transmitted to humans by phlebotomine sand flies. Development of Leishmania to infective metacyclic promastigotes in the insect gut, a process termed metacyclogenesis, is an essential prerequisite for transmission. Based on the hypothesis that vector gut microbiota influence the development of virulent parasites, we sequenced midgut microbiomes in the sand fly Lutzomyia longipalpis with or without Leishmania infantum infection. Sucrose-fed sand flies contained a highly diverse, stable midgut microbiome. Blood feeding caused a decrease in microbial richness that eventually recovered. However, bacterial richness progressively decreased in L. infantum-infected sand flies. Acetobacteraceae spp. became dominant and numbers of Pseudomonadaceae spp. diminished coordinately as the parasite underwent metacyclogenesis and parasite numbers increased. Importantly, antibiotic-mediated perturbation of the midgut microbiome rendered sand flies unable to support parasite growth and metacyclogenesis. Together, these data suggest that the sand fly midgut microbiome is a critical factor for Leishmania growth and differentiation to its infective state prior to disease transmission.

Leishmania infantum, a parasitic protozoan causing fatal visceral leishmaniasis, is transmitted to humans through the bite of the sand fly Lutzomyia longipalpis. Development of the parasite to its virulent metacyclic state occurs in the sand fly gut. In this study, the microbiota within the Lu. longipalpis midgut was delineated by 16S ribosomal DNA (rDNA) sequencing, revealing a highly diverse community composition that lost diversity as parasites developed to their metacyclic state and increased in abundance in infected flies. Perturbing sand fly gut microbiota with an antibiotic cocktail, which alone had no effect on either the parasite or the fly, arrested both the development of virulent parasites and parasite expansion. These findings indicate the importance of bacterial commensals within the insect vector for the development of virulent pathogens, and raise the possibility that impairing the microbial composition within the vector might represent a novel approach to control of vector-borne diseases.

Leishmania and its quest for iron: An update and overview

Parasites of genus Leishmania are the causative agents of complex neglected diseases called leishmaniasis and continue to be a significant health concern globally. Iron is a vital nutritional requirement for virtually all organisms, including pathogenic trypanosomatid parasites, and plays a crucial role in many facets of cellular metabolism as a cofactor of several enzymes. Iron acquisition is essential for the survival of parasites. Yet parasites are also vulnerable to the toxicity of iron and reactive oxygen species. The aim of this review is to provide an update on the current knowledge about iron acquisition and usage by Leishmania species. We have also discussed about host strategy to modulate iron availability and the strategies deployed by Leishmania parasites to overcome iron withholding defences and thus favour parasite growth within host macrophages. Since iron plays central roles in the host's response and parasite metabolism, a comprehensive understanding of the iron metabolism is beneficial to identify potential viable therapeutic opportunities against leishmaniasis.

Leishmania infection inhibits macrophage motility by altering F-actin dynamics and the expression of adhesion complex proteins

Leishmania is an intracellular protozoan parasite that causes a broad spectrum of clinical manifestations, ranging from self-healing skin lesions to fatal visceralizing disease. As the host cells of choice for all species of Leishmania, macrophages are critical for the establishment of infections. How macrophages contribute to parasite homing to specific tissues and how parasites modulate macrophage function are still poorly understood. In this study, we show that Leishmania amazonensis infection inhibits macrophage roaming motility. The reduction in macrophage speed is not dependent on particle load or on factors released by infected macrophages. L. amazonensis-infected macrophages also show reduced directional migration in response to the chemokine MCP-1. We found that infected macrophages have lower levels of total paxillin, phosphorylated paxillin, and phosphorylated focal adhesion kinase when compared to noninfected macrophages, indicating abnormalities in the formation of signaling adhesion complexes that regulate motility. Analysis of the dynamics of actin polymerization at peripheral sites also revealed a markedly enhanced F-actin turnover frequency in L. amazonensis-infected macrophages. Thus, Leishmania infection inhibits macrophage motility by altering actin dynamics and impairing the expression of proteins that function in plasma membrane-extracellular matrix interactions.

Alice in microbes' land: adaptations and counter-adaptations of vector-borne parasitic protozoa and their hosts

In the present review, we aim to provide a general introduction to different facets of the arms race between pathogens and their hosts/environment, emphasizing its evolutionary aspects. We focus on vector-borne parasitic protozoa, which have to adapt to both invertebrate and vertebrate hosts. Using Leishmania, Trypanosoma and Plasmodium as main models, we review successively (i) the adaptations and counter-adaptations of parasites and their invertebrate host, (ii) the adaptations and counter-adaptations of parasites and their vertebrate host and (iii) the impact of human interventions (chemotherapy, vaccination, vector control and environmental changes) on these adaptations. We conclude by discussing the practical impact this knowledge can have on translational research and public health.

Iron acquisition in Leishmania and its crucial role in infection

Iron is an essential cofactor for many basic metabolic pathways in pathogenic microbes and their hosts. It is also dangerous as it can catalyse the production of reactive free radicals. This dual character makes the host can either limit iron availability to invading microbes or exploit iron to induce toxicity to pathogens. Successful pathogens, including Leishmania species, must possess mechanisms to circumvent host's iron limitation and iron-induced toxicity in order to survive. In this review, we discuss the regulation of iron metabolism in the setting of infection and delineate the iron acquisition strategies used by Leishmania parasites and their subversions to host iron metabolism to overcome host's iron-related defences.

Understanding the importance of conservative hypothetical protein LdBPK_070020 in Leishmania donovani and its role in subsistence of the parasite

The genome of Leishmania donovani, the causative agent of visceral leishmaniasis, codes for approximately 65% of both conserved and non-conserved hypothetical proteins. Studies on 'conserved hypothetical' proteins are expected to reveal not only new and crucial aspects of Leishmania biochemistry, but it could also lead to discovery of novel drug candidates. Conserved hypothetical protein, LdBPK_070020, is a 31.14 kDa protein, encoded by an 810 bp gene. BLAST analysis of LdBPK_070020, performed against NCBI non-redundant database, showed 80-99% similarity with conserved hypothetical proteins of Leishmania belonging to other species. Using homologues recombination method, we have performed gene knockout of LdBPK_070020 and effects of the same were investigated on the parasite. The gene knocked out strain shows significant retardation in growth with respect to wild type. Detailed biochemical studies indicated towards important role of LdBPK_070020 in the parasite survival and growth.

A Trypanosomatid Iron Transporter that Regulates Mitochondrial Function Is Required for Leishmania amazonensis Virulence

Iron, an essential co-factor of respiratory chain proteins, is critical for mitochondrial function and maintenance of its redox balance. We previously reported a role for iron uptake in differentiation of Leishmania amazonensis into virulent amastigotes, by a mechanism that involves reactive oxygen species (ROS) production and is independent of the classical pH and temperature cues. Iron import into mitochondria was proposed to be essential for this process, but evidence supporting this hypothesis was lacking because the Leishmania mitochondrial iron transporter was unknown. Here we describe MIT1, a homolog of the mitochondrial iron importer genes mrs3 (yeast) and mitoferrin-1 (human) that is highly conserved among trypanosomatids. MIT1 expression was essential for the survival of Trypanosoma brucei procyclic but not bloodstream forms, which lack functional respiratory complexes. L. amazonensis LMIT1 null mutants could not be generated, suggesting that this mitochondrial iron importer is essential for promastigote viability. Promastigotes lacking one LMIT1 allele (LMIT1/Δlmit1) showed growth defects and were more susceptible to ROS toxicity, consistent with the role of iron as the essential co-factor of trypanosomatid mitochondrial superoxide dismutases. LMIT1/Δlmit1 metacyclic promastigotes were unable to replicate as intracellular amastigotes after infecting macrophages or cause cutaneous lesions in mice. When induced to differentiate axenically into amastigotes, LMIT1/Δlmit1 showed strong defects in iron content and function of mitochondria, were unable to upregulate the ROS-regulatory enzyme FeSOD, and showed mitochondrial changes suggestive of redox imbalance. Our results demonstrate the importance of mitochondrial iron uptake in trypanosomatid parasites, and highlight the role of LMIT1 in the iron-regulated process that orchestrates differentiation of L. amazonensis into infective amastigotes.

Leishmaniasis is a serious parasitic disease that affects 12 million people worldwide, with clinical manifestations ranging from self-healing cutaneous lesions to deadly visceralizing disease. A vaccine is not available, and new and less toxic drugs against this protozoan parasite are urgently needed. Following introduction into vertebrate hosts during a sand fly blood meal, Leishmania parasites undergo extensive changes in morphology and metabolism that are critical for adaptation to life inside host macrophages and replication as amastigotes. Earlier studies identified major events that occur during amastigote differentiation, but the signaling mechanism initiating this process remained poorly understood. Previously we demonstrated a novel role for the reactive oxygen species (ROS) H₂O₂ in initiating amastigote differentiation, a process proposed to be dependent on iron availability inside the parasite’s mitochondria. In this study we identify LMIT1, a Leishmania transmembrane protein that functions as a mitochondrial iron transporter and is conserved in other trypanosomatid protozoan parasites. Reduced LMIT1 expression impairs mitochondrial function in the infective amastigote stage, abolishing parasite virulence. Our findings identify LMIT1 as a promising new drug target, and support the conclusion that iron-dependent ROS signals generated in the mitochondria regulate differentiation of virulent Leishmania amastigotes.

Iron-associated biology of Trypanosoma brucei

BACKGROUND

Every eukaryote requires iron, which is also true for the parasitic protist Trypanosoma brucei, the causative agent of sleeping sickness in humans and nagana in cattle. T. brucei undergoes a complex life cycle during which its single mitochondrion is subject to major metabolic and morphological changes.

SCOPE OF REVIEW

This review covers what is known about processes associated with iron-sulfur clusters and heme metabolism in T. brucei. We discuss strategies by which iron and heme are acquired and utilized by this model parasite, emphasizing the differences between its two life cycle stages residing in the bloodstream of the mammalian host and gut of the insect vector. Finally, the role of iron in the host-parasite interactions is discussed along with their possible exploitation in fighting these deadly parasites.

MAJOR CONCLUSIONS

The processes associated with acquisition and utilization of iron, distinct in the two life stages of T. brucei, are fine tuned for the dramatically different host environment occupied by them. Although the composition and compartmentalization of the iron-sulfur cluster assembly seem to be conserved, some unique features of the iron acquisition strategies may be exploited for medical interventions against these parasites.

GENERAL SIGNIFICANCE

As early-branching protists, trypanosomes and related flagellates are known to harbor an array of unique features, with the acquisition of iron being another peculiarity. Thanks to intense research within the last decade, understanding of iron-sulfur cluster assembly and iron metabolism in T. brucei is among the most advanced of all eukaryotes.

Leishmania carbon metabolism in the macrophage phagolysosome- feast or famine?

A number of medically important microbial pathogens target and proliferate within macrophages and other phagocytic cells in their mammalian hosts. While the majority of these pathogens replicate within the host cell cytosol or non-hydrolytic vacuolar compartments, a few, including protists belonging to the genus Leishmania, proliferate long-term within mature lysosome compartments.  How these parasites achieve this feat remains poorly defined. In this review, we highlight recent studies that suggest that Leishmania virulence is intimately linked to programmed changes in the growth rate and carbon metabolism of the obligate intra-macrophage stages. We propose that activation of a slow growth and a stringent metabolic response confers resistance to multiple stresses (oxidative, temperature, pH), as well as both nutrient limitation and nutrient excess within this niche. These studies highlight the importance of metabolic processes as key virulence determinants in Leishmania.

MavN is a Legionella pneumophila vacuole-associated protein required for efficient iron acquisition during intracellular growth

Iron is essential for the growth and virulence of most intravacuolar pathogens. The mechanisms by which microbes bypass host iron restriction to gain access to this metal across the host vacuolar membrane are poorly characterized. In this work, we identify a unique intracellular iron acquisition strategy used by Legionella pneumophila. The bacterial Icm/Dot (intracellular multiplication/defect in organelle trafficking) type IV secretion system targets the bacterial-derived MavN (more regions allowing vacuolar colocalization N) protein to the surface of the Legionella-containing vacuole where this putative transmembrane protein facilitates intravacuolar iron acquisition. The ΔmavN mutant exhibits a transcriptional iron-starvation signature before its growth is arrested during the very early stages of macrophage infection. This intracellular growth defect is rescued only by the addition of excess exogenous iron to the culture medium and not a variety of other metals. Consistent with MavN being a translocated substrate that plays an exclusive role during intracellular growth, the mutant shows no defect for growth in broth culture, even under severe iron-limiting conditions. Putative iron-binding residues within the MavN protein were identified, and point mutations in these residues resulted in defects specific for intracellular growth that are indistinguishable from the ΔmavN mutant. This model of a bacterial protein inserting into host membranes to mediate iron transport provides a paradigm for how intravacuolar pathogens can use virulence-associated secretion systems to manipulate and acquire host iron.

Kharon1 null mutants of Leishmania mexicana are avirulent in mice and exhibit a cytokinesis defect within macrophages

In a variety of eukaryotes, flagella play important roles both in motility and as sensory organelles that monitor the extracellular environment. In the parasitic protozoan Leishmania mexicana, one glucose transporter isoform, LmxGT1, is targeted selectively to the flagellar membrane where it appears to play a role in glucose sensing. Trafficking of LmxGT1 to the flagellar membrane is dependent upon interaction with the KHARON1 protein that is located at the base of the flagellar axoneme. Remarkably, while Δkharon1 null mutants are viable as insect stage promastigotes, they are unable to survive as amastigotes inside host macrophages. Although Δkharon1 promastigotes enter macrophages and transform into amastigotes, these intracellular parasites are unable to execute cytokinesis and form multinucleate cells before dying. Notably, extracellular axenic amastigotes of Δkharon1 mutants replicate and divide normally, indicating a defect in the mutants that is only exhibited in the intra-macrophage environment. Although the flagella of Δkharon1 amastigotes adhere to the phagolysomal membrane of host macrophages, the morphology of the mutant flagella is often distorted. Additionally, these null mutants are completely avirulent following injection into BALB/c mice, underscoring the critical role of the KHARON1 protein for viability of intracellular amastigotes and disease in the animal model of leishmaniasis.

Modulation of Na+/K+ ATPase Activity by Hydrogen Peroxide Generated through Heme in L. amazonensis

Leishmania amazonensis is a protozoan parasite that occurs in many areas of Brazil and causes skin lesions. Using this parasite, our group showed the activation of Na⁺/K⁺ ATPase through a signaling cascade that involves the presence of heme and protein kinase C (PKC) activity. Heme is an important biomolecule that has pro-oxidant activity and signaling capacity. Reactive oxygen species (ROS) can act as second messengers, which are required in various signaling cascades. Our goal in this work is to investigate the role of hydrogen peroxide (H₂O₂) generated in the presence of heme in the Na⁺/K⁺ ATPase activity of L. amazonensis. Our results show that increasing concentrations of heme stimulates the production of H₂O₂ in a dose-dependent manner until a concentration of 2.5 μM heme. To confirm that the effect of heme on the Na⁺/K⁺ ATPase is through the generation of H₂O₂, we measured enzyme activity using increasing concentrations of H₂O₂ and, as expected, the activity increased in a dose-dependent manner until a concentration of 0.1 μM H₂O₂. To investigate the role of PKC in this signaling pathway, we observed the production of H₂O₂ in the presence of its activator phorbol 12-myristate 13-acetate (PMA) and its inhibitor calphostin C. Both showed no effect on the generation of H₂O₂. Furthermore, we found that PKC activity is increased in the presence of H₂O₂, and that in the presence of calphostin C, H₂O₂ is unable to activate the Na⁺/K⁺ ATPase. 100 μM of Mito-TEMPO was capable of abolishing the stimulatory effect of heme on Na⁺/K⁺ ATPase activity, indicating that mitochondria might be the source of the hydrogen peroxide production induced by heme. The modulation of L. amazonensis Na⁺/K⁺ ATPase by H₂O₂ opens new possibilities for understanding the signaling pathways of this parasite.

The Heme Transport Capacity of LHR1 Determines the Extent of Virulence in Leishmania amazonensis

Leishmania spp. are trypanosomatid parasites that replicate intracellularly in macrophages, causing serious human morbidity and mortality throughout the world. Trypanosomatid protozoa cannot synthesize heme, so must acquire this essential cofactor from their environment. Earlier studies identified LHR1 as a Leishmania amazonensis transmembrane protein that mediates heme uptake. Null mutants of LHR1 are not viable and single knockout strains have reduced virulence, but very little is known about the properties of LHR1 directly associated with heme transport. Here, we use functional assays in Saccharomyces cerevisiae to show that specific tyrosine residues within the first three predicted transmembrane domains of LHR1 are required for efficient heme uptake. These tyrosines are unique to LHR1, consistent with the low similarity between LHR1 and its corresponding homologs in C. elegans and human. Substitution of these tyrosines in LHR1 resulted in varying degrees of heme transport inhibition, phenotypes that closely mirrored the impaired ability of L. amazonensis to replicate as intracellular amastigotes in macrophages and generate cutaneous lesions in mice. Taken together, our results imply that the mechanism for heme transport by LHR1 is distinctive and may have adapted to secure heme, a limiting cofactor, inside the host. Since LHR1 is significantly divergent from the human heme transporter HRG1, our findings lay the groundwork for selective targeting of LHR1 by small molecule antagonists.

Leishmania are protozoan parasites that infect humans and replicate intracellularly in macrophages, cells normally engaged in protecting the host from pathogens. These parasites have several strategies to survive inside the hostile environment of the host macrophage, and one of these strategies involves heme acquisition. Heme is an iron-containing molecule that is essential for many cellular functions. Unlike mammalian cells, Leishmania parasites cannot synthesize heme, so must acquire it from the host cell. In earlier work we found that the parasites express a surface protein, LHR1, which transports heme into the parasites. In this study we identified specific amino acids in LHR1 that are required for heme transport. When expressed in yeast cells, LHR1 carrying these mutations had defects in heme transport that were equivalent to the inhibition in virulence observed when these proteins were expressed in Leishmania and tested in macrophage and mouse infection assays. These critical amino acids do not exist in the human heme transporter, indicating that LHR1 is a promising target for the development of specific drugs for the treatment of leishmaniasis and possibly other serious parasitic diseases, such as Chagas’ disease and sleeping sickness.

Strategies of Intracellular Pathogens for Obtaining Iron from the Environment

Most microorganisms are destroyed by the host tissues through processes that usually involve phagocytosis and lysosomal disruption. However, some organisms, called intracellular pathogens, are capable of avoiding destruction by growing inside macrophages or other cells. During infection with intracellular pathogenic microorganisms, the element iron is required by both the host cell and the pathogen that inhabits the host cell. This minireview focuses on how intracellular pathogens use multiple strategies to obtain nutritional iron from the intracellular environment in order to use this element for replication. Additionally, the implications of these mechanisms for iron acquisition in the pathogen-host relationship are discussed.

Leishmania and the macrophage: a multifaceted interaction

ABSTRACT 

Leishmania, the causative agent of leishmaniases, is an intracellular parasite of macrophages, transmitted to humans via the bite of its sand fly vector. This protozoan organism has evolved strategies for efficient uptake into macrophages and is able to regulate phagosome maturation in order to make the phagosome more hospitable for parasite growth and to avoid destruction. As a result, macrophage defenses such as oxidative damage, antigen presentation, immune activation and apoptosis are compromised whereas nutrient availability is improved. Many Leishmania survival factors are involved in shaping the phagosome and reprogramming the macrophage to promote infection. This review details the complexity of the host–parasite interactions and summarizes our latest understanding of key events that make Leishmania such a successful intracellular parasite.

Leishmania amazonensis amastigotes highly express a tryparedoxin peroxidase isoform that increases parasite resistance to macrophage antimicrobial defenses and fosters parasite virulence

Professional phagocytes generate a myriad of antimicrobial molecules to kill invading microorganisms, of which nitrogen oxides are integral in controlling the obligate intracellular pathogen Leishmania. Although reactive nitrogen species produced by the inducible nitric oxide synthase (iNOS) can promote the clearance of intracellular parasites, some Leishmania species/stages are relatively resistant to iNOS-mediated antimicrobial activity. The underlying mechanism for this resistance remains largely uncharacterized. Here, we show that the amastigote form of L. amazonensis is hyper-resistant to the antimicrobial actions of cytokine-activated murine and human macrophages as compared to its promastigote counterpart. Amastigotes exhibit a marked ability to directly counter the cytotoxicity of peroxynitrite (ONOO-), a leishmanicidal oxidant that is generated during infection through the combined enzymatic activities of NADPH oxidase and iNOS. The enhanced antinitrosative defense of amastigotes correlates with the increased expression of a tryparedoxin peroxidase (TXNPx) isoform that is also upregulated in response to iNOS enzymatic activity within infected macrophages. Accordingly, ectopic over-expression of the TXNPx isoform by L. amazonensis promastigotes significantly enhances parasite resistance against ONOO- cytotoxicity. Moreover, TXNPx-overexpressing parasites exhibit greater intra-macrophage survival, and increased parasite growth and lesion development in a murine model of leishmaniasis. Our investigations indicate that TXNPx isoforms contribute to Leishmania's ability to adapt to and antagonize the hostile microenvironment of cytokine-activated macrophages, and provide a mechanistic explanation for persistent infection in experimental and human leishmaniasis.

Leishmania-mediated inhibition of iron export promotes parasite replication in macrophages

Leishmania parasites infect macrophages, cells that play an important role in organismal iron homeostasis. By expressing ferroportin, a membrane protein specialized in iron export, macrophages release iron stored intracellularly into the circulation. Iron is essential for the intracellular replication of Leishmania, but how the parasites compete with the iron export function of their host cell is unknown. Here, we show that infection with Leishmania amazonensis inhibits ferroportin expression in macrophages. In a TLR4-dependent manner, infected macrophages upregulated transcription of hepcidin, a peptide hormone that triggers ferroportin degradation. Parasite replication was inhibited in hepcidin-deficient macrophages and in wild type macrophages overexpressing mutant ferroportin that is resistant to hepcidin-induced degradation. Conversely, intracellular growth was enhanced by exogenously added hepcidin, or by expression of dominant-negative ferroportin. Importantly, dominant-negative ferroportin and macrophages from flatiron mice, a mouse model for human type IV hereditary hemochromatosis, restored the infectivity of mutant parasite strains defective in iron acquisition. Thus, inhibition of ferroportin expression is a specific strategy used by L. amazonensis to inhibit iron export and promote their own intracellular growth.

Iron in intracellular infection: to provide or to deprive?

Due to their chemical versatility, transition metals were incorporated as cofactors for several basic metabolic pathways in living organisms. This same characteristic makes them potentially harmful, since they can be engaged in deleterious reactions like Fenton chemistry. As such, organisms have evolved highly specialized mechanisms to supply their own metal needs while keeping their toxic potential in check. This dual character comes into play in host-pathogen interactions, given that the host can either deprive the pathogen of these key nutrients or exploit them to induce toxicity toward the invading agent. Iron stands as the prototypic example of how a metal can be used to limit the growth of pathogens by nutrient deprivation, a mechanism widely studied in Mycobacterium infections. However, the host can also take advantage of iron-induced toxicity to control pathogen proliferation, as observed in infections caused by Leishmania. Whether we may harness either of the two pathways for therapeutical purposes is still ill-defined. In this review, we discuss how modulation of the host iron availability impacts the course of infections, focusing on those caused by two relevant intracellular pathogens, Mycobacterium and Leishmania.

Cellular growth and mitochondrial ultrastructure of leishmania (Viannia) braziliensis promastigotes are affected by the iron chelator 2,2-dipyridyl

Background

Iron is an essential element for the survival of microorganisms in vitro and in vivo, acting as a cofactor of several enzymes and playing a critical role in host-parasite relationships. Leishmania (Viannia) braziliensis is a parasite that is widespread in the new world and considered the major etiological agent of American tegumentary leishmaniasis. Although iron depletion leads to promastigote and amastigote growth inhibition, little is known about the role of iron in the biology of Leishmania. Furthermore, there are no reports regarding the importance of iron for L. (V.) braziliensis.

Methodology/Principal Findings

In this study, the effect of iron on the growth, ultrastructure and protein expression of L. (V.) braziliensis was analyzed by the use of the chelator 2,2-dipyridyl. Treatment with 2,2-dipyridyl affected parasites' growth in a dose- and time-dependent manner. Multiplication of the parasites was recovered after reinoculation in fresh culture medium. Ultrastructural analysis of treated promastigotes revealed marked mitochondrial swelling with loss of cristae and matrix and the presence of concentric membranar structures inside the organelle. Iron depletion also induced Golgi disruption and intense cytoplasmic vacuolization. Fluorescence-activated cell sorting analysis of tetramethylrhodamine ester-stained parasites showed that 2,2-dipyridyl collapsed the mitochondrial membrane potential. The incubation of parasites with propidium iodide demonstrated that disruption of mitochondrial membrane potential was not associated with plasma membrane permeabilization. TUNEL assays indicated no DNA fragmentation in chelator-treated promastigotes. In addition, two-dimensional electrophoresis showed that treatment with the iron chelator induced up- or down-regulation of proteins involved in metabolism of nucleic acids and coordination of post-translational modifications, without altering their mRNA levels.

Conclusions

Iron chelation leads to a multifactorial response that results in cellular collapse, starting with the interruption of cell proliferation and culminating in marked mitochondrial impairment in some parasites and their subsequent cell death, whereas others may survive and resume proliferating.

American tegumentary leishmaniasis (ATL) is a neglected disease that is widely distributed in the Americas. The protozoan parasite Leishmania (Viannia) braziliensis is one of the main causative agents of ATL, being responsible for the development of different clinical manifestations of the disease, which ranges from self-healing cutaneous lesions to disseminated and mucocutaneous forms. Because iron is essential for the survival and growth of Leishmania, as it is required for colonization of macrophages and development of lesions in mice, several chelating compounds have been tested for their effects on the growth of these parasites. In the present work, treatment of L. (V.) braziliensis with the iron chelator 2,2-dipyridyl inhibited the growth of promastigote forms in a dose- and time-dependent manner. However, multiplication of the parasites was recovered after reinoculation in fresh culture medium. The iron chelator also induced mitochondrial dysfunction and altered expression of proteins involved in metabolism of nucleic acids and coordination of post-translational modifications. The events described above ultimately caused the death of some parasites, most likely due to mitochondrial dysfunction, whereas others adapted and survived, suggesting a plasticity or resilience of the mitochondrion in this parasite.