Role of a differentially expressed cAMP phosphodiesterase in regulating the induction of resistance against oxidative damage in Leishmania donovani

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.

Evaluation of Modulators of cAMP-Response in Terms of Their Impact on Cell Cycle and Mitochondrial Activity of Leishmania donovani

With the identification of novel cAMP binding effector molecules in Trypanosoma, the role of cAMP in kinetoplastida parasites gained an intriguing breakthrough. Despite earlier demonstrations of the role of cAMP in the survival of Leishmania during macrophage infection, there is essential need to specifically clarify the involvement of cAMP in various cellular processes in the parasite. In this context, we sought to gain a comprehensive understanding of the effect of cAMP analogs and cAMP-cyclic nucleotide phosphodiesterase (PDE) inhibitors on proliferation of log phase parasites. Administration of both hydrolyzable (8-pCPT-cAMP) and nonhydrolyzable analogs (Sp-8-pCPT-cAMPS) of cAMP resulted in a significant decrease of Leishmania proliferation. Among the various PDE inhibitors, etazolate was found to be potently antiproliferative. BrdU cell proliferation and K/N/F-enumeration microscopic study revealed that both cAMP analogs and selective PDE inhibitors resulted in significant cell cycle arrest at G1 phase with reduced S-phase population. Furthermore, careful examination of the flagellar motility patterns revealed significantly reduced coordinated forward flagellar movement of the promastigotes with a concomitant decrease in cellular ATP levels. Alongside, 8-pCPT-cAMP and PDE inhibitors etazolate and trequinsin showed marked reduction in mitochondrial membrane potential. Treatment of etazolate at subcytotoxic concentration to infected macrophages significantly reduced parasite burden, and administration of etazolate to Leishmania-infected BALB/c mice showed reduced liver and spleen parasite burden. Collectively, these results imply involvement of cAMP in various crucial processes paving the avenue for developing potent antileishmanial agent.

Intracellular cyclic AMP levels modulate differential adaptive responses on epimastigotes and cell culture trypomastigotes of Trypanosoma cruzi

Among the many environmental challenges the parasite Trypanosoma cruzi has to overcome to complete its life cycle through different hosts, oxidative stress plays a central role. Different stages of this parasite encounter distinct sources of oxidative stress, such as the oxidative burst of the immune system, or the Heme released from hemoglobin degradation in the triatomine's midgut. Also, the redox status of the surroundings functions as a signal to the parasite, triggering processes coupled to differentiation or proliferation. Intracellular second messengers, like cAMP, are responsible for the transduction of environmental queues and initiating cellular processes accordingly. In trypanosomatids cAMP is involved in a variety of processes, including proliferation, differentiation, osmoregulation and quorum sensing. Trypanosomatid phosphodiesterases (PDE) show atypical pharmacological properties and some have been involved in key processes for the survival of the parasites, which validates them as attractive therapeutic targets. Our work here shows that cAMP modulates different processes according to parasite stage. Epimastigotes become more resistant to oxidative stress when pre-treated with cAMP analogs, while in trypomastigotes an increase in intracellular cAMP doesn't seem to aid in this response, although it does increase the number of amastigotes obtained 48 h after infection, compared to the control group. Also, we show that TcrPDEA1, a functionally enigmatic phosphodiesterase with very high Km, is involved in the epimastigotes response to oxidative stress.

Signal Transduction Pathways as Therapeutic Target for Chagas Disease

Trypanosomatids are a group of flagellated unicellular eukaryotes, causing serious human diseases including Chagas disease (Trypanosoma cruzi), sleeping sickness (Trypanosoma brucei spp.) and Leishmaniasis (Leishmania spp.). The second messenger cAMP is involved in numerous and fundamental processes in these parasites including differentiation between stages, proliferation, osmoregulation, oxidative stress and quorum sensing. Interestingly, its signaling pathway is quite different from that of mammals, including structurally different adenylyl cyclases, the shortage of orthologous effector proteins and the absence of G-protein-coupled-receptors, among others. These characteristics make the proteins involved in these transduction pathways good candidates for therapeutic targets. However, the identification of new unknown druggable targets involves extensive research time and is economically very expensive, making difficult the transition from basic research to the clinical phase. Trypanosomatid PDEs have characteristic binding pockets that allow for a differential inhibition from their human orthologs. Modification in the approved drugs for human to convert them into trypanocidal treatments could lead to more effective therapies, shorter lab time and lower costs. In view of the fact that kinetoplastid PDEs are highly conserved with their mammalian counterparts, and since there are already numerous drugs on the market against human PDEs, the drug repositioning approach is highly promising. The development of new technologies, higher government and industrial involvement and more scientists committed to basic investigation, are the key to ultimately find an effective treatment and cure for the neglected tropical diseases.

Structure-guided approach to identify a novel class of anti-leishmaniasis diaryl sulfide compounds targeting the trypanothione metabolism

Leishmania protozoans are the causative agent of leishmaniasis, a neglected tropical disease consisting of three major clinical forms: visceral leishmaniasis (VL), cutaneous leishmaniasis, and mucocutaneous leishmaniasis. VL is caused by Leishmania donovani in East Africa and the Indian subcontinent and by Leishmania infantum in Europe, North Africa, and Latin America, and causes an estimated 60,000 deaths per year. Trypanothione reductase (TR) is considered to be one of the best targets to find new drugs against leishmaniasis. This enzyme is fundamental for parasite survival in the human host since it reduces trypanothione, a molecule used by the tryparedoxin/tryparedoxin peroxidase system of Leishmania to neutralize the hydrogen peroxide produced by host macrophages during infection. Recently, we solved the X-ray structure of TR in complex with the diaryl sulfide compound RDS 777 (6-(sec-butoxy)-2-((3-chlorophenyl)thio)pyrimidin-4-amine), which impairs the parasite defense against the reactive oxygen species by inhibiting TR with high efficiency. The compound binds to the catalytic site and engages in hydrogen bonds the residues more involved in the catalysis, namely Glu466', Cys57 and Cys52, thereby inhibiting the trypanothione binding. On the basis of the RDS 777-TR complex, we synthesized structurally related diaryl sulfide analogs as TR inhibitors able to compete for trypanothione binding to the enzyme and to kill the promastigote in the micromolar range. One of the most active among these compounds (RDS 562) was able to reduce the trypanothione concentration in cell of about 33% via TR inhibition. RDS 562 inhibits selectively Leishmania TR, while it does not inhibit the human homolog glutathione reductase.

Druggable Targets in Cyclic Nucleotide Signaling Pathways in Apicomplexan Parasites and Kinetoplastids against Disabling Protozoan Diseases in Humans

Cell signaling in eukaryotes is an evolutionarily conserved mechanism to respond and adapt to various environmental changes. In general, signal sensation is mediated by a receptor which transfers the signal to a cascade of effector proteins. The cyclic nucleotides 3′,5′-cyclic adenosine monophosphate (cAMP) and 3′,5′-cyclic guanosine monophosphate (cGMP) are intracellular messengers mediating an extracellular stimulus to cyclic nucleotide-dependent kinases driving a change in cell function. In apicomplexan parasites and kinetoplastids, which are responsible for a variety of neglected, tropical diseases, unique mechanisms of cyclic nucleotide signaling are currently identified. Collectively, cyclic nucleotides seem to be essential for parasitic proliferation and differentiation. However, there is no a genomic evidence for canonical G-proteins in these parasites while small GTPases and secondary effector proteins with structural differences to host orthologues occur. Database entries encoding G-protein-coupled receptors (GPCRs) are still without functional proof. Instead, signals from the parasite trigger GPCR-mediated signaling in the host during parasite invasion and egress. The role of cyclic nucleotide signaling in the absence of G-proteins and GPCRs, with a particular focus on small GTPases in pathogenesis, is reviewed here. Due to the absence of G-proteins, apicomplexan parasites and kinetoplastids may use small GTPases or their secondary effector proteins and host canonical G-proteins during infection. Thus, the feasibility of targeting cyclic nucleotide signaling pathways in these parasites, will be an enormous challenge for the identification of selective, pharmacological inhibitors since canonical host proteins also contribute to pathogenesis.

Role of leishmanial acidocalcisomal pyrophosphatase in the cAMP homeostasis in phagolysosome conditions required for intra-macrophage survival

Exposure of Leishmania donovani to macrophage phagolysosome conditions (PC) (37°C and pH 5.5) led to increased intracellular cAMP and cAMP-mediated responses, which help in intra-macrophage survival pre-requisite for infectivity. In the absence of typical orthologs for G-proteins and G-protein coupled receptors, we sought to study the precise mechanisms for positive modulation of cAMP production during exposure to PC. Amongst two promastigote-stage specific membrane bound receptor adenylate cyclases (LdRAC-A and LdRAC-B), LdRAC-A appeared to function as a major cAMP generator following PC exposure. Pyrophosphate (PPi), an energy storage compound as well as a by-product of cAMP biosynthesis by adenylate cyclise, was found to be decreased following PC exposure. This may be due to microtubule and microfilament-driven translocation of acidocalcisomes near plasma membrane vicinity with concomitant increase of acidocalcisome membrane pyrophosphatase (LdV-H⁺PPase) and acidocalcisomal soluble pyrophosphatase (LdVSP1). Episomal over-expression and conditional silencing demonstrated regulatory role of V-H⁺PPase on cAMP trigger and consequent induction of resistance to macrophage-derived pro-oxidants and parasite killing. Furthermore, immunofluorescence analysis revealed possible co-localization of LdV-H⁺PPase and LdRAC-A during PC exposure. Collectively, these results suggest that translocation of acidocalcisome in membrane vicinity functions as a trigger for LdRAC-A-driven cAMP generation through depletion of PPi pool by LdV-H⁺PPase.

Effects of linalool and eugenol on the survival of Leishmania (L.) infantum chagasi within macrophages

The most commonly used drugs against visceral leishmaniasis are based on pentavalent antimonial compounds, which have played a fundamental role in therapy for over 70 years. However, the treatment is painful and has severe toxic side effects that can be fatal. Antimonial resistance is spreading and reaching alarming proportions. Linalool and eugenol have been shown to kill Leishmania (L.) amazonensis and Trypanosoma cruzi at low doses. In the present study, we demonstrate the effects of linalool and eugenol, components of essential oils, on Leishmania (L.) infantum chagasi, one of the causative agents of visceral leishmaniasis. We compared the effects of those compounds to the effects of glucantime, a positive control. In L. infantum chagasi killing assays, the LD₅₀ for eugenol was 220μg/ml, and that for linalool was 550μg/ml. L. infantum chagasi was added to cultures of peritoneal mouse macrophages for four hours prior to drug treatment. Eugenol and linalool significantly decreased the number of parasites within the macrophages. Eugenol and linalool enhanced the activities of the L. infantum chagasi protein kinases PKA and PKC. Linalool also decreased L. infantum chagasi oxygen consumption. In conclusion, both linalool and eugenol promoted a decrease in the proliferation and viability of L. infantum chagasi. These effects were more pronounced during the interaction between the parasites and peritoneal mouse macrophages.

Probing the molecular mechanism of hypericin-induced parasite death provides insight into the role of spermidine beyond redox metabolism in Leishmania donovani

Hypericin, a natural compound from Hypericum perforatum (St. John's wort), has been identified as a specific inhibitor of Leishmania donovani spermidine synthase (LdSS) using integrated computational and biochemical approaches. Hypericin showed in vitro inhibition of recombinant LdSS enzyme activity. The in vivo estimation of spermidine levels in Leishmania promastigotes after hypericin treatment showed significant decreases in the spermidine pools of the parasites, indicating target specificity of the inhibitor molecule. The inhibitor, hypericin, showed significant antileishmanial activity, and the mode of death showed necrosis-like features. Further, decreased trypanothione levels and increased glutathione levels with elevated reactive oxygen species (ROS) levels were observed after hypericin treatment. Supplementation with trypanothione in the medium with hypericin treatment restored in vivo trypanothione levels and ROS levels but could not prevent necrosis-like death of the parasites. However, supplementation with spermidine in the medium with hypericin treatment restored in vivo spermidine levels and parasite death was prevented to a large extent. The data overall suggest that the parasite death due to spermidine starvation as a result of LdSS inhibition is not related to elevated levels of reactive oxygen species. This suggests the involvement of spermidine in processes other than redox metabolism in Leishmania parasites. Moreover, the work provides a novel scaffold, i.e., hypericin, as a potent antileishmanial molecule.

A soluble phosphodiesterase in Leishmania donovani negatively regulates cAMP signaling by inhibiting protein kinase A through a two way process involving catalytic as well as non-catalytic sites

Intracellular cAMP level and cAMP mediated responses are elevated when Leishmania are exposed to macrophage phagolysosome conditions (37 °C and pH 5.5). Phosphodiesterases play major role in cAMP regulation and in the present study we have cloned and characterized a 2.1 kb cytosolic isoform of phosphodiesterase from Leishmania donovani (LdPDED) which plays important role in cAMP homeostasis when the promastigotes are exposed to macrophage phagolysome conditions for converting to axenic amastigotes. Domain characterization suggested the presence of two pseudo-substrate sites similar to the ones present in the regulatory subunit of cAMP-dependent protein kinase A (PKA) and a putative PKA phosphorylation site at T(708) of C-terminus of LdPDED. Deletion constructs and site directed mutagenesis revealed the ability of LdPDED to interact with L. donovani PKA catalytic subunits (LdPKAC1 and LdPKAC2) resulting in inhibition of kinase activity in one hand and increase of phosphodiesterase activity through PKA mediated phosphorylation at putative phosphorylation site on the other hand. This study therefore identifies a unique phosphodiesterase in L. donovani which appears to regulate cAMP-dependent PKA signaling through a two way process.

Molecular mechanism underlying antileishmanial effect of oxabicyclo[3.3.1]nonanones: inhibition of key redox enzymes of the pathogen

We report oxabicyclo[3.3.1]nonanones as inhibitors of key redox enzymes, trypanothione synthetase (TryS), and trypanothione reductase (TryR) of Leishmania. Further, detailed cellular effects of 4-(4,4,8-Trimethyl-7-oxo-3-oxabicyclo[3.3.1]non-2-yl)-benzoic acid methyl ester, a oxabicyclo[3.3.1]nonanones, on the parasite were investigated. As these compounds inhibit key redox enzymes (TryR amd TryS), treatment of these compounds resulted in increased reactive oxygen species (ROS), mitochondrial membrane damage, activation of caspase like proteases, and DNA damage that finally leads to apoptosis. Although the compound has modest IC50 value against parasite (4.9±0.4 μM), they identify a novel chemical space to design and develop drugs based on these compounds against the Leishmania parasite. This is first report of oxabicyclo[3.3.1]nonanones as antileishmanial.

Identification of a protein kinase A regulatory subunit from Leishmania having importance in metacyclogenesis through induction of autophagy

cAMP-mediated responses act as modulators of environmental sensing and cellular differentiation of many kinetoplastidae parasites including Leishmania. Although cAMP synthesizing (adenylate cyclase) and degrading (phosphodiesterase) enzymes have been cloned and characterized from Leishmania, no cAMP-binding effector molecule has yet been identified from this parasite. In this study, a regulatory subunit of cAMP-dependent protein kinase (Ldpkar1), homologous to mammalian class I cAMP-dependent protein kinase regulatory subunit, has been identified from L. donovani. Further characterization suggested possible interaction of LdPKAR1 with PKA catalytic subunits and inhibition of PKA activity. This PKA regulatory subunit is expressed in all life cycle stages and its expression attained maximum level in stationary phase promastigotes, which are biochemically similar to the infective metacyclic promastigotes. Starvation condition, the trigger for metacyclogenesis in the parasite, elevates LdPKAR1 expression and under starvation condition promastigotes overexpressing Ldpkar1 attained metacyclic features earlier than normal cells. Furthermore, Ldpkar1 overexpression accelerates autophagy, a starvation-induced cytological event necessary for metacyclogenesis and amastigote formation. Conditional silencing of Ldpkar1 delays the induction of autophagy in the parasite. The study, for the first time, reports the identification of a functional cAMP-binding effector molecule from Leishmania that may modulate important cytological events affecting metacyclogenesis.

Whole genome sequencing of multiple Leishmania donovani clinical isolates provides insights into population structure and mechanisms of drug resistance

Visceral leishmaniasis is a potentially fatal disease endemic to large parts of Asia and Africa, primarily caused by the protozoan parasite Leishmania donovani. Here, we report a high-quality reference genome sequence for a strain of L. donovani from Nepal, and use this sequence to study variation in a set of 16 related clinical lines, isolated from visceral leishmaniasis patients from the same region, which also differ in their response to in vitro drug susceptibility. We show that whole-genome sequence data reveals genetic structure within these lines not shown by multilocus typing, and suggests that drug resistance has emerged multiple times in this closely related set of lines. Sequence comparisons with other Leishmania species and analysis of single-nucleotide diversity within our sample showed evidence of selection acting in a range of surface- and transport-related genes, including genes associated with drug resistance. Against a background of relative genetic homogeneity, we found extensive variation in chromosome copy number between our lines. Other forms of structural variation were significantly associated with drug resistance, notably including gene dosage and the copy number of an experimentally verified circular episome present in all lines and described here for the first time. This study provides a basis for more powerful molecular profiling of visceral leishmaniasis, providing additional power to track the drug resistance and epidemiology of an important human pathogen.

Role of cAMP Signaling in the Survival and Infectivity of the Protozoan Parasite, Leishmania donovani

Leishmania donovani, while invading macrophages, encounters striking shift in temperature and pH (from 22°C and pH 7.2 to 37°C and pH 5.5), which act as the key environmental trigger for differentiation, and increases cAMP level and cAMP-mediated responses. For comprehensive understanding of cAMP signaling, we studied the enzymes related to cAMP metabolism. A stage-specific and developmentally regulated isoform of receptor adenylate cyclase (LdRACA) showed to regulate differentiation-coupled induction of cAMP. The soluble acidocalcisomal pyrophosphatase, Ldvsp1, was the major isoform regulating cAMP level in association with LdRACA. A differentially expressed soluble cytosolic cAMP phosphodiesterase (LdPDEA) might be related to infection establishment by shifting trypanothione pool utilization bias toward antioxidant defense. We identified and cloned a functional cAMP-binding effector molecule from L. donovani (a regulatory subunit of cAMP-dependent protein kinase, LdPKAR) that may modulate metacyclogenesis through induction of autophagy. This study reveals the significance of cAMP signaling in parasite survival and infectivity.

Cyclic-nucleotide signalling in protozoa

Compared with the impressive progress in understanding signal transduction pathways and mechanisms in mammalian systems, advances in protozoan signalling processes, including cyclic nucleotide metabolism, have been very slow. This is in large part connected to the fact that the components of these pathways are very different in the protozoan parasites, as confirmed by the recently completed genome. For instance, kinetoplastids have no equivalents to the mammalian Class I adenylyl cyclases (ACs) in their genomes nor any of the subunits of the associated G-proteins. The cyclases in kinetoplastid parasites contain a single transmembrane domain, a conserved intracellular catalytic domain and a highly variable extracellular domain - consistent with the expression of multiple receptor-activated cyclases - but no receptor ligands, agonists or antagonists have been identified. Apicomplexan AC and guanylyl cyclase (GC) are even more unusual, potentially being bifunctional, harbouring either a putative ion channel (AC) or a P-type ATPase-like domain (GC) alongside the catalytic region. Phosphodiesterases (PDEs) and cyclic-nucleotide-activated protein kinases are essentially conserved in protozoa, although mostly insensitive to inhibitors of the mammalian proteins. Some of the PDEs have now been validated as promising drug targets. In the following manuscript, we will summarize the existing literature on cAMP and cGMP in protozoa: cyclases, PDEs and cyclic-nucleotide-dependent kinases.

Defining the role of a FYVE domain in the localization and activity of a cAMP phosphodiesterase implicated in osmoregulation in Trypanosoma cruzi

Intracellular levels of cyclic nucleotide second messengers are regulated predominantly by a large superfamily of phosphodiesterases (PDEs). Trypanosoma cruzi, the causative agent of Chagas disease, encodes four different PDE families. One of these PDEs, T. cruzi PDE C2 (TcrPDEC2) has been characterized as a FYVE domain containing protein. Here, we report a novel role for TcrPDEC2 in osmoregulation in T. cruzi and reveal the relevance of its FYVE domain. Our data show that treatment of epimastigotes with TcrPDEC2 inhibitors improves their regulatory volume decrease, whereas cells overexpressing this enzyme are unaffected by the same inhibitors. Consistent with these results, TcrPDEC2 localizes to the contractile vacuole complex, showing strong labelling in the region corresponding to the spongiome. Furthermore, transgenic parasites overexpressing a truncated version of TcrPDEC2 without the FYVE domain show a failure in its targeting to the contractile vacuole complex and a marked decrease in PDE activity, supporting the importance of this domain to the localization and activity of TcrPDEC2. Taking together, the results here presented are consistent with the importance of the cyclic AMP signalling pathway in regulatory volume decrease and implicate TcrPDEC2 as a specifically localized PDE involved in osmoregulation in T. cruzi.