Effect of l-Arginine Analogs and a Calcium Chelator on Nitric Oxide (NO˙) Production byLeishmaniasp

Unique thiol metabolism in trypanosomatids: Redox homeostasis and drug resistance

Trypanosomatids are mainly responsible for heterogeneous parasitic diseases: Leishmaniasis, Sleeping sickness, and Chagas disease and control of these diseases implicates serious challenges due to the emergence of drug resistance. Redox-active biomolecules are the endogenous substances in organisms, which play important role in the regulation of redox homeostasis. The redox-active substances like glutathione, trypanothione, cysteine, cysteine persulfides, etc., and other inorganic intermediates (hydrogen peroxide, nitric oxide) are very useful as defence mechanism. In the present review, the suitability of trypanothione and other essential thiol molecules of trypanosomatids as drug targets are described in Leishmania and Trypanosoma. We have explored the role of tryparedoxin, tryparedoxin peroxidase, ascorbate peroxidase, superoxide dismutase, and glutaredoxins in the anti-oxidant mechanism and drug resistance. Up-regulation of some proteins in trypanothione metabolism helps the parasites in survival against drug pressure (sodium stibogluconate, Amphotericin B, etc.) and oxidative stress. These molecules accept electrons from the reduced trypanothione and donate their electrons to other proteins, and these proteins reduce toxic molecules, neutralize reactive oxygen, or nitrogen species; and help parasites to cope with oxidative stress. Thus, a better understanding of the role of these molecules in drug resistance and redox homeostasis will help to target metabolic pathway proteins to combat Leishmaniasis and trypanosomiases.

Constitutive nitric oxide synthase-like enzyme in two species involved in cutaneous and mucocutaneous leishmaniasis

Leishmania is an obligate intracellular parasite that primarily inhabits macrophages. The destruction of the parasite in the host cell is a fundamental mechanism for infection control. In addition, inhibition of the leishmanicidal activity of macrophages seems to be related to the ability of some species to inhibit the production of nitric oxide (NO) by depleting arginine. Some species of Leishmania have the ability to produce NO from a constitutive nitric oxide synthase-like enzyme (cNOS-like). However, the localization of cNOS-like in Leishmania has not been described before. As such, this study was designed to locate cNOS-like enzyme and NO production in promastigotes of Leishmania (Leishmania) amazonensis and Leishmania (Viannia) braziliensis. NO production was initially quantified by flow cytometry, which indicated a significant difference in NO production between L. (L.) amazonensis (GMFC = 92.17 +/- 4.6) and L. (V.) braziliensis (GMFC = 18.89 +/- 2.29) (P < 0.05). Analysis of cNOS expression by immunoblotting showed more expression in L. (L.) amazonensis versus L. (V.) braziliensis. Subsequently, cNOS-like immunolabeling was observed in promastigotes in regions near vesicles, the flagellar pocket and mitochondria, and small clusters of particles appeared to be fusing with vesicles suggestive of glycosomes, peroxisome-like-organelles that compartmentalize the glycolytic pathway in trypanosomatid parasites. In addition, confocal microscopy analysis demonstrated colocalization of cNOS-like and GAPDH, a specific marker for glycosomes. Thus, L. (L.) amazonensis produces greater amounts of NO than L. (V.) braziliensis, and both species present the cNOS-like enzyme inside glycosomes.

Comparative analysis of biological aspects of Leishmania infantum strains

Leishmania infantum infantum (LII) is one of the species that causes visceral leishmaniasis (VL) in the Old World, while L. infantum chagasi (LIC) is present in the New World. Few studies address biological differences or the behavior of these strains during infection. These parasites live inside cells of their hosts, continuously evading microbicidal mechanisms and modulating the immune responses of these cells. One of the mechanisms used by these protozoa involves the L-arginine metabolism. Understanding the differences between Leishmania species and establishing an improved murine model for study of leishmaniasis are matters of extreme importance. Thereby, the objectives of this work were to analyze the biological and molecular differences between two Leishmania infantum strains (LII and LIC) and the degree of susceptibility to infection of mice with different genetic backgrounds. The infectivity in vivo and in vitro of LII and LIC strains was evaluated in BALB/c and Swiss Webster mice, as well the NOS and ARG activities. The LII strain was more infective than the LIC strain both in vivo and in vitro. In animals infected by the LII and LIC strains, differences in NOS and ARG activities occurred. In vitro, promastigotes of LII isolated from BALB/c and Swiss Webster mice showed higher ARG activity than LIC promastigotes during the growth curve. However, no difference was observed in intracellular NO production by promastigotes of these strains. The ARG gene sequences were compared, and those of both strains were identical. However, despite the similarity, the strains showed different expression levels of this gene. It can be concluded that although L. chagasi strains are considered identical to L. infantum strains from a molecular point of view, these strains have different biological behavior.

Leishmania Spp-Host Interaction: There Is Always an Onset, but Is There an End?

For a long time Leishmaniasis had been considered as a neglected tropical disease. Recently, it has become a priority in public health all over the world for different aspects such as geographic spread, number of population living at risk of infection as well as the potential lethality and/or the development of disfiguring lesions in the, respectively, visceral and tegumentary forms of the disease. As a result, several groups have been bending over this issue and many valuable data have been published. Nevertheless, parasite-host interactions are still not fully known and, consequently, we do not entirely understand the infection dynamics and parasite persistence. This knowledge may point targets for modulation or blockage, being very useful in the development of measures to interfere in the course of infection/ disease and to minimize the risks and morbidity. In the present review we will discuss some aspects of the Leishmania spp-mammalian host interaction in the onset of infection and after the clinical cure of the lesions. We will also examine the information already available concerning the parasite strategy to evade immune response mainly at the beginning of the infection, as well as during the parasite persistence. This knowledge can improve the conditions of treatment, follow-up and cure control of patients, minimizing the potential damages this protozoosis can cause to infected individuals.

Antileishmanial Activity and Inducible Nitric Oxide Synthase Activation by RuNO Complex

Parasites of the genus Leishmania are capable of inhibiting effector functions of macrophages. These parasites have developed the adaptive ability to escape host defenses; for example, they inactivate the NF-κB complex and suppress iNOS expression in infected macrophages, which are responsible for the production of the major antileishmanial substance nitric oxide (NO), favoring then its replication and successful infection. Metal complexes with NO have been studied as potential compounds for the treatment of certain tropical diseases, such as ruthenium compounds, known to be exogenous NO donors. In the present work, the compound cis-[Ru(bpy)₂SO₃(NO)]PF₆, or RuNO, showed leishmanicidal activity directly and indirectly on promastigote forms of Leishmania (Leishmania) amazonensis. In addition, treatment with RuNO increased NO production by reversing the depletion of NO caused by Leishmania. We also found increased expression of Akt, iNOS, and NF-κB in infected and treated macrophages. These results demonstrated that RuNO was able to kill the parasite by NO release and modulate the transcriptional capacity of the cell.

Escaping Deleterious Immune Response in Their Hosts: Lessons from Trypanosomatids

The Trypanosomatidae family includes the genera Trypanosoma and Leishmania, protozoan parasites displaying complex digenetic life cycles requiring a vertebrate host and an insect vector. Trypanosoma brucei gambiense, Trypanosoma cruzi, and Leishmania spp. are important human pathogens causing human African trypanosomiasis (HAT or sleeping sickness), Chagas' disease, and various clinical forms of Leishmaniasis, respectively. They are transmitted to humans by tsetse flies, triatomine bugs, or sandflies, and affect millions of people worldwide. In humans, extracellular African trypanosomes (T. brucei) evade the hosts' immune defenses, allowing their transmission to the next host, via the tsetse vector. By contrast, T. cruzi and Leishmania sp. have developed a complex intracellular lifestyle, also preventing several mechanisms to circumvent the host's immune response. This review seeks to set out the immune evasion strategies developed by the different trypanosomatids resulting from parasite-host interactions and will focus on: clinical and epidemiological importance of diseases; life cycles: parasites-hosts-vectors; innate immunity: key steps for trypanosomatids in invading hosts; deregulation of antigen-presenting cells; disruption of efficient specific immunity; and the immune responses used for parasite proliferation.

In vitro sodium nitroprusside-mediated toxicity towards Leishmania amazonensis promastigotes and axenic amastigotes

Leishmania parasites survive despite exposure to the toxic nitrosative oxidants during phagocytosis by the host cell. In this work, the authors investigated comparatively the resistance of Leishmania amazonensis promastigotes and axenic amastigotes to a relatively strong nitrosating agent that acts as a nitric oxide (NO) donor, sodium nitroprusside (SNP). Results demonstrate that SNP is able to decrease, in vitro, the number of L. amazonensis promastigotes and axenic amastigotes in a dose-dependent maner. Promastigotes, cultured in the presence of 0.25, 0.5, and 1 mmol L(-1) SNP for 24 h showed about 75% growth inhibition, and 97-100% when the cultures were treated with >2 mmol L(-1) SNP. In contrast, when axenic amastigotes were growing in the presence of 0.25-8 mM SNP added to the culture medium, 50% was the maximum of growth inhibition observed. Treated promastigotes presented reduced motility and became round in shape further confirming the leishmanicidal activity of SNP. On the other hand, axenic amastigotes, besides being much more resistant to SNP-mediated cytotoxicity, did not show marked morphological alteration when incubated for 24 h, until 8 mM concentrations of this nitrosating agent were used. The cytotoxicity toward L. amazonensis was attenuated by reduced glutathione (GSH), supporting the view that SNP-mediated toxicity triggered multiple oxidative mechanisms, including oxidation of thiols groups and metal-independent oxidation of biomolecules to free radical intermediates.

An intracellular calcium store is present in Leishmania donovani glycosomes

A fourth intracellular Ca2+ pool in Leishmania donovani was identified by permeabilizing plasma membrane with digitonin. In Fura 2 loaded cells Ca2+ was released synergistically when mitochondrial function was blocked by antimycin and oligomycin. Vanadate did not have any effect if applied before incorporation of these mitochondrial poisons. However, the same inhibitor which inhibits Ca2+-ATPase activity of endoplasmic reticulum was able to release Ca2+ at a slow rate when added after antimycin and oligomycin. Alkalization of cytoplasmic pH allowed further release of Ca2+ essentially from the acidocalcisome. Purified glycosomes could mediate Ca2+ uptake mechanism in presence of vanadate whereas bafilomycin, a specific and potent inhibitor of vacuolar proton pump did not have any effect. Glycosomal Ca2+-ATPase activity was optimum at pH 7.5. The apparent Km for calciumin presence of vanadate was 12 nM. Taken together, it may be suggested that a vanadate-insensitive Ca2+-ATPase is present in the membrane of this microbody. Presence of glycosomal Ca2+ was further confirmed by imaging of Ca2+ activity in the Fura 2 loaded purified organelle using confocal laser. Results reveal that newly localized glycosomal calcium may essentially be an effective candidate to play a significant role in cellular function.

Nitric Oxide Synthase (NOS) Characterization in Leishmania amazonensis Axenic Amastigotes

BACKGROUND

Although Leishmania virulence may be modulated by environmental and genetic factors of their mammalian hosts and sand fly vectors, molecular determinants of Leishmania sp. are the key elements. This work evidences that Leishmania amazonensis axenic amastigotes produce comparatively more NO than infective promastigotes.

METHODS

A soluble NOS was purified from L. amazonensis axenic amastigotes by affinity chromatography (2',5'-ADP-agarose), and on SDS-PAGE the enzyme migrates as a single protein band.

RESULTS

The presence of a constitutive NOS was detected through immunofluorescence using antibody against neuronal NOS (nNOS) and in NADPH consumption assays.

CONCLUSIONS

The present data show that NOS is prominent in axenic amastigote preparations, suggesting an association with the infectivity and/or an escaping mechanism of the parasite. The relationship between the NO-generating systems in the parasite and in their host cell warrants further investigation.

Nitric oxide biosynthesis by Leishmania amazonensis promastigotes containing a high percentage of metacyclic forms

Due to the diversity of its physiological and pathophysiological functions and general ubiquity, the study of nitric oxide (NO) has become of great interest. In this work, it was demonstrated that Leishmania amazonensis promastigotes produces NO, a free radical synthesized from L: -arginine by nitric oxide synthase (NOS). A soluble NOS was purified from L. amazonensis promastigotes by affinity chromatography (2', 5'-ADP-agarose) and on SDS-PAGE the enzyme migrates as a single protein band of 116.2 (+/-6) kDa. Furthermore, the presence of a constitutive NOS was detected through indirect immunofluorescence using anti-cNOS and in NADPH consumption assays. The present work show that NO production, detected as nitrite in culture supernatant, is prominent in promastigotes preparations with high number of metacyclic forms, suggesting an association with the differentiation and the infectivity of the parasite.

Phylogenesis of constitutively formed nitric oxide in non-mammals

It is widely recognized that nitric oxide (NO) in mammalian tissues is produced from L-arginine via catalysis by NO synthase (NOS) isoforms such as neuronal NOS (nNOS) and endothelial NOS (eNOS) that are constitutively expressed mainly in the central and peripheral nervous system and vascular endothelial cells, respectively. This review concentrates only on these constitutive NOS (cNOS) isoforms while excluding information about iNOS, which is induced mainly in macrophages upon stimulation by cytokines and polysaccharides. The NO signaling pathway plays a crucial role in the functional regulation of mammalian tissues and organs. Evidence has also been accumulated for the role of NO in invertebrates and non-mammalian vertebrates. Expression of nNOS in the brain and peripheral nervous system is widely determined by staining with NADPH (reduced nicotinamide adenine dinucleotide phosphate) diaphorase or NOS immunoreactivity, and functional roles of NO formed by nNOS are evidenced in the early phylogenetic stages (invertebrates and fishes). On the other hand, the endothelium mainly produces vasodilating prostanoids rather than NO or does not liberate endothelium-derived relaxing factor (EDRF) (fishes), and the ability of endothelial cells to liberate NO is observed later in phylogenetic stages (amphibians). This review article summarizes various types of interesting information obtained from lower organisms (invertebrates, fishes, amphibians, reptiles, and birds) about the properties and distribution of nNOS and eNOS and also the roles of NO produced by the cNOS as an important intercellular signaling molecule.