Leishmania MAP kinases – Familiar proteins in an unusual context

Repurposing approved protein kinase inhibitors as potent anti-leishmanials targeting Leishmania MAP kinases

AIMS

Leishmaniasis, caused by the protozoan parasite poses a significant health burden globally. With a very few specific drugs, increased drug resistance it is important to look for drug repurposing along with the identification of pre-clinical candidates against visceral leishmaniasis. This study aims to identify potential drug candidates against visceral leishmaniasis by targeting leishmanial MAP kinases and screening FDA approved protein kinase inhibitors.

MATERIALS AND METHODS

MAP kinases were identified from the Leishmania genome. 12 FDA approved protein kinase inhibitors were screened against Leishmania MAP kinases. Binding affinity, ADME and toxicity of identified drug candidates were profiled. The anti-proliferative effects and mechanism of action were assessed in Leishmania, including changes in cell morphology, flagellar length, cell cycle progression, reactive oxygen species (ROS) generation, and intra-macrophage parasitic burden.

KEY FINDINGS

23 MAP kinases were identified from the Leishmania genome. Sorafenib and imatinib emerged as repurposable drug candidates and demonstrated excellent anti-proliferative effects in Leishmania. Treatment with these inhibitors resulted in significant changes in cell morphology, flagellar length, and cell cycle arrest. Furthermore, sorafenib and imatinib promoted ROS generation and reduced intra-macrophage parasitic burden, and elicited anti-leishmanial activity in in vivo experimental VL models.

SIGNIFICANCE

Collectively, these results imply involvement of MAP kinases in infectivity and survival of the parasite and can pave the avenue for repurposing sorafenib and imatinib as anti-leishmanial agents. These findings contribute to the exploration of new treatment options for visceral leishmaniasis, particularly in the context of emerging drug resistance.

Leishmania donovani mitogen-activated protein kinases as a host-parasite interaction interface

Leishmaniasis, a major globally re-emerging neglected tropical disease, has a restricted repertoire of chemotherapeutic options due to a narrow therapeutic index, drug resistance, or patient non-compliance due to toxicity. The disease is caused by the parasite Leishmania that resides in two different forms in two different environments: as sessile intracellular amastigotes within mammalian macrophages and as motile promastigotes in sandfly gut. As mitogen-activated protein kinases (MAPKs) play important roles in cellular differentiation and survival, we studied the expression of Leishmania donovani MAPKs (LdMAPKs). The homology studies by multiple sequence alignment show that excepting LdMAPK1 and LdMAPK2, all thirteen other LdMAPKs share homology with human ERK and p38 isoforms. Expression of LdMAPK4 and LdMAPK5 is less in avirulent promastigotes and amastigotes. Compared to miltefosine-sensitive L. donovani parasites, miltefosine-resistant parasites have higher LdMAPK1, LdMAPK3-5, LdMAPK7-11, LdMAPK13, and LdMAPK14 expression. IL-4-treatment of macrophages down-regulated LdMAPK11, in virulent amastigotes whereas up-regulated LdMAPK5, but down-regulated LdMAPK6, LdMAPK12-15, expression in avirulent amastigotes. IL-4 up-regulated LdMAPK1 expression in both virulent and avirulent amastigotes. IFN-γ-treatment down-regulated LdMAPK6, LdMAPK13, and LdMAPK15 in avirulent amastigotes but up-regulated in virulent amastigotes. This complex profile of LdMAPKs expression among virulent and avirulent parasites, drug-resistant parasites, and in amastigotes within IL-4 or IFN-γ-treated macrophages suggests that LdMAPKs are differentially controlled at the host-parasite interface regulating parasite survival and differentiation, and in the course of IL-4 or IFN-γ dominated immune response.

Transgenic overexpression of heat shock protein (HSP83) enhances protein kinase A activity, disrupts GP63 surface protease expression and alters promastigote morphology in Leishmania amazonensis

Leishmania parasites undergo morphological changes during their infectious life cycle, including developmental transitions within the sandfly vector, culminating in metacyclic stages that are pre-adapted for infection. Upon entering vertebrate host phagocytes, Leishmania differentiate into intracellular amastigotes, the form that is ultimately transmitted back to the vector to complete the life cycle. Although environmental conditions that induce these cellular transitions are well-established, molecular mechanisms governing Leishmania morphologic differentiation in response to these cues remain largely uncharacterized. Previous studies indicate a key role for HSP83 in both promastigote metacyclogenesis and amastigote differentiation. To further elucidate HSP83 functions in the Leishmania lifecycle, we examined the biological impact of experimentally elevating HSP83 gene expression in Leishmania. Significantly, HSP83 overexpression was associated with altered metacyclic morphology, increased protein kinase A (PKA) activity and decreased expression of the Leishmania major surface protease, GP63. Corroborating these findings, overexpression of the L. amazonensis PKA catalytic subunit resulted in a largely similar phenotype. Our findings demonstrate for the first time in Leishmania, a functional link between HSP83 and PKA in the control of Leishmania gene expression, replication and morphogenesis.

An integrative in silico and in vitro study identifies Leishmania donovani MAP kinase12 as a probable virulence factor

Visceral leishmaniasis (VL), a potentially fatal vector-borne disease caused by the intracellular protozoan parasite Leishmania donovani, remains a major health problem due to restricted repertoire of drugs, deleterious side effects, high cost and increasing drug resistance. Therefore, identifying newer drug targets and developing efficacious affordable treatments with minimal or no side effects are pressing needs. Being regulators of diverse cellular processes, Mitogen-Activated Protein Kinases (MAPKs) are potential drug targets. Herein, we report L.donovani MAPK12 (LdMAPK12) as a probable virulence factor implying it as a plausible target. LdMAPK12 sequence is distinct from human MAPKs and is highly conserved in different Leishmania species. LdMAPK12 is expressed in both promastigotes and amastigotes. In comparison with the avirulent and procyclic promastigotes, the virulent and metacyclic promastigotes have higher expression of LdMAPK12. Pro-inflammatory cytokines reduced, whereas anti-inflammatory cytokines increased LdMAPK12 expression in macrophages. These data suggest a probable novel role of LdMAPK12 in parasite virulence and identifies it as a plausible drug target.

Potential molecular targets of Leishmania pathways in developing novel antileishmanials

The illness known as leishmaniasis has not become a household name like malaria, although it stands as the second-largest parasitic disease, surpassed only by malaria. As no licensed vaccine is available, treatment for leishmaniasis mostly relies on chemotherapy. Inefficiency and drug resistance are the major impediments in current therapeutics. In this scenario, identification of novel molecular drug candidates is indispensable to develop robust antileishmanials. The exploration of structure-based drugs to target enzymes/molecules of Leishmania which differ structurally/functionally from their equivalents in mammalian hosts not only helps in developing a new class of antileishmanials, but also paves the way to understand Leishmania biology. This review provides a comprehensive overview on possible drug candidates relating to various Leishmania molecular pathways.

Pathogenic role of mitogen activated protein kinases in protozoan parasites

Protozoan parasites with complex life cycles have high mortality rates affecting billions of human lives. Available anti-parasitic drugs are inadequate due to variable efficacy, toxicity, poor patient compliance and drug-resistance. Hence, there is an urgent need for the development of safer and better chemotherapeutics. Mitogen Activated Protein Kinases (MAPKs) have drawn much attention as potential drug targets. This review summarizes unique structural and functional features of MAP kinases and their possible role in pathogenesis of obligate intracellular protozoan parasites namely, Leishmania, Trypanosoma, Plasmodium and Toxoplasma. It also provides an overview of available knowledge concerning the target proteins of parasite MAPKs and the need to understand and unravel unknown interaction network(s) of MAPK(s).

The Leishmania donovani SENP Protease Is Required for SUMO Processing but Not for Viability

The protozoan parasite Leishmania donovani is part of an early eukaryotic branch and depends on post-transcriptional mechanisms for gene expression regulation. This includes post-transcriptional protein modifications, such as protein phosphorylation. The presence of genes for protein SUMOylation, i.e., the covalent attachment of small ubiquitin-like modifier (SUMO) polypeptides, in the Leishmania genomes prompted us to investigate the importance of the sentrin-specific protease (SENP) and its putative client, SUMO, for the vitality and infectivity of Leishmania donovani. While SENP null mutants are viable with reduced vitality, viable SUMO null mutant lines could not be obtained. SUMO C-terminal processing is disrupted in SENP null mutants, preventing SUMO from covalent attachment to proteins and nuclear translocation. Infectivity in vitro is not affected by the loss of SENP-dependent SUMO processing. We conclude that SENP is required for SUMO processing, but that functions of unprocessed SUMO are critical for Leishmania viability.

Biochemical characterization and chemical validation of Leishmania MAP Kinase-3 as a potential drug target

Protozoan parasites of the Leishmania genus have evolved unique signaling pathways that can sense various environmental changes and trigger stage differentiation for survival and host infectivity. MAP kinase (MAPK) plays a critical role in various cellular activities like cell differentiation, proliferation, stress regulation, and apoptosis. The Leishmania donovani MAPK3 (LdMAPK3) is involved in the regulation of flagella length and hence plays an important role in disease transmission. Here, we reported the gene cloning, protein expression, biochemical characterizations, inhibition studies and cell proliferation assay of LdMAPK3. The recombinant purified LdMAPK3 enzyme obeys the Michaelis-Menten equation with K_m and V_max of LdMAPK3 was found to be 20.23 nM and 38.77 ± 0.71 nmoles ATP consumed/mg LdMAPK3/min respectively. The maximum kinase activity of LdMAPK3 was recorded at 35 °C and pH 7. The in-vitro inhibition studies with two natural inhibitors genistein (GEN) and chrysin (CHY) was evaluated against LdMAPK3. The K_i value for GEN and CHY were found to be 3.76 ± 0.28 µM and K_i = 8.75 ± 0.11 µM respectively. The IC₅₀ value for the compounds, GEN and CHY against L. donovani promastigotes were calculated as 9.9 µg/mL and 13 µg/mL respectively. Our study, therefore, reports LdMAPK3 as a new target for therapeutic approach against leishmaniasis.

Identification of lead molecules against potential drug target protein MAPK4 from L. donovani: An in-silico approach using docking, molecular dynamics and binding free energy calculation

Leishmaniasis caused by obligate intracellular parasites of genus Leishmania is one of the most neglected tropical diseases threatening 350 million people worldwide. Protein kinases have drawn much attention as potential drug targets due to their important role in various cellular processes. In Leishmania sp. mitogen-activated protein kinase 4 is essential for the parasite survival because of its involvement in various regulatory, apoptotic and developmental pathways. The current study reveals the identification of natural inhibitors of L. donovani mitogen-activated protein kinase-4 (LdMPK4). We have performed in silico docking of 110 natural inhibitors of Leishmania parasite that have been reported earlier and identified two compounds Genistein (GEN) and Chrysin (CHY). The homology model of LdMPK4 was developed, followed by binding affinity studies, and pharmacokinetic properties of the inhibitors were calculated by maintaining ATP as a standard molecule. The modelled structure was deposited in the protein model database with PMDB ID: PM0080988. Molecular dynamic simulation of the enzyme-inhibitor complex along with the free energy calculations over 50 ns showed that GEN and CHY are more stable in their binding. These two molecules, GEN and CHY, can be considered as lead molecules for targeting LdMPK4 enzyme and could emerge as potential LdMPK4 inhibitors.

Leishmania donovani 90 kD Heat Shock Protein - Impact of Phosphosites on Parasite Fitness, Infectivity and Casein Kinase Affinity

Leishmania parasites are thought to control protein activity at the post-translational level, e.g. by protein phosphorylation. In the pathogenic amastigote, the mammalian stage of Leishmania parasites, heat shock proteins show increased phosphorylation, indicating a role in stage-specific signal transduction. Here we investigate the impact of phosphosites in the L. donovani heat shock protein 90. Using a chemical knock-down/genetic complementation approach, we mutated 11 confirmed or presumed phosphorylation sites and assessed the impact on overall fitness, morphology and in vitro infectivity. Most phosphosite mutations affected the growth and morphology of promastigotes in vitro, but with one exception, none of the phosphorylation site mutants had a selective impact on the in vitro infection of macrophages. Surprisingly, aspartate replacements mimicking the negative charge of phosphorylated serines or threonines had mostly negative impacts on viability and infectivity. HSP90 is a substrate for casein kinase 1.2-catalysed phosphorylation in vitro. While several putative phosphosite mutations abrogated casein kinase 1.2 activity on HSP90, only Ser289 could be identified as casein kinase target by mass spectrometry. In summary, our data show HSP90 as a downstream client of phosphorylation-mediated signalling in an organism that depends on post-transcriptional gene regulation.

Slow pace of antileishmanial drug development

The protozoan parasiteLeishmaniais endemic in large parts of the world which causes leishmaniasis. Its visceral form is fatal if not treated and is caused mostly byLeishmania donovani,Leishmania infantumandLeishmania chagasi. Given the difficulties linked to vector (sandfly) control and the lack of an effective vaccine, the control of leishmaniasis relies mostly on chemotherapy. Unfortunately, the prevalence of parasites becoming resistant to the first-line drug pentavalent antimony (SbV) is increasing worldwide. Few alternative drugs are available that includes amphotericin B, pentamidine and miltefosine (oral). Already, decreases in efficacy, resistance and toxicity have been noted against these drugs. Dry antileishmanial pipeline further indicates the slow pace of drug discovery in this field where resistance as a major barrier. Full understanding of the genetic and molecular basis of the parasite is lagging. Since leishmaniasis is a neglected disease and occurs predominantly in the developing world largely, therefore, it is unaddressed. The pharma industry argues that development of the new drug is too costly and risky to invest in low return neglected diseases is very high. Research is also needed to identify new and effective drug targets. The lack of drug research and development for neglected diseases will require some new strategies. We have discussed the various cause of slow pace of antileishmanial drug discovery in this review to pay attention of researchers and also take the public and private initiative to make the process fast for new antileishmanial drug development.

Differential Metabolic Pathway Analysis of the Proteomes of Leishmania donovani and Leptomonas seymouri

PURPOSE

Although in trypanosomatids, monoxeny (Leptomonas) is ancestral to dixeny (Leishmania), however clinical cases of visceral leishmanisis with Leptomonas co-infection are increasingly being reported from India. Using a proteogenomic approach, a detailed proteome analysis of these two kinetoplastid parasites viz., Leishmania and its sister Leptomonas, to catalog the key proteins associated with and therefore possibly responsible for phenotype changes in Leptomonas evolution and domestication as co-infection with Leishmania is carried out.

EXPERIMENTAL DESIGN

LC-MS/MS is utilized for this proteomic purpose. One Leishmania donovani WHO reference strain and two Leptomonas seymouri isolates, which are originally isolated from clinical cases of kala azar patients with different inherent drug sensitivity viz., responsive and unresponsive, are used in this study.

RESULTS

A network analysis, leveraging protein-protein interaction data helped to find the roles of the proteins in carbon metabolism and biosynthesis of secondary metabolites which is seen to be altered under stress conditions like drug resistance.

CONCLUSIONS AND CLINICAL RELEVANCE

The information provided about the metabolic pathways modulated when contrasting these two phenotypes may lead to the development of new strategies to block parasite differentiation within the host and to also circumvent the problem of drug resistance. This proteomic study also offers new grounds for the investigation of novel hypothetical proteins potentially playing a role in evolutionary biology the knowledge of which is essential for treatment of patients co-infected with these two kinetoplastid parasites.

In silico Tools for Target Identification and Drug Molecular Docking in Leishmania

Neglected tropical diseases represent a significant health burden in large parts of the world. Drug discovery is currently a key bottleneck in the pipeline of these diseases. In this chapter, the in silico approaches used for the processes involved in drug discovery, identification and validation of druggable Leishmania targets, and design and optimisation of new anti-leishmanial drugs are discussed. We also provide a general view of the different computational tools that can be employed in pursuit of this aim, along with the most interesting cases found in the literature.

Joining forces: first application of a rapamycin-induced dimerizable Cre system for conditional null mutant analysis in Leishmania

Reverse genetics in Leishmania spp has gained importance beyond basic research as efforts increase to discover and validate new drug targets. Often, the most promising targets are essential for viability of the parasites, defying a genetic analysis by current gene replacement strategies. Duncan et al. demonstrate the applicability of DiCre recombination in Leishmania for induced replacement of the kinase CRK3 gene in promastigotes. DiCre gene replacement leads to the rapid loss of the gene and allows monitoring the phenotypic effects of the loss of function, eliminating the need for prolonged cultivation and selection. Implementation of the DiCre approach will allow functional genetics of the most important of Leishmania genes and is likely to boost genetic research and drug target discovery.

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.

A touch of Zen: post-translational regulation of the Leishmania stress response

Across bacterial, archaeal and eukaryotic kingdoms, heat shock proteins (HSPs) are defined as a class of highly conserved chaperone proteins that are rapidly induced in response to temperature increase through dedicated heat shock transcription factors. While this transcriptional response governs cellular adaptation of fungal, plant and animal cells to thermic shock and other forms of stress, early-branching eukaryotes of the kinetoplastid order, including trypanosomatid parasites, lack classical mechanisms of transcriptional regulation and show largely constitutive expression of HSPs, thus raising important questions on the function of HSPs in the absence of stress and the regulation of their chaperone activity in response to environmental adversity. Understanding parasite-specific mechanisms of stress-response regulation is especially relevant for protozoan parasites of the genus Leishmania that are adapted for survival inside highly toxic phagolysosomes of host macrophages causing the various immuno-pathologies of leishmaniasis. Here we review recent advances on the function and regulation of chaperone activities in these kinetoplastid pathogens and propose a new model for stress-response regulation through a reciprocal regulatory relationship between stress kinases and chaperones that may be relevant for parasite-adaptive differentiation and infectivity.

In silico elucidation and inhibition studies of selected phytoligands against Mitogen activated protein kinases of protozoan parasites

Parasitic MAPKs exhibiting significant divergence with humans and playing an imperative role in parasitic metabolic activities have been exploited from several years as important targets for development of novel therapeutics. In addition, the emergence of the drug resistant variants of parasitic diseases in the recent years has aroused a great need for the development of potent inhibitors against them. In the present study we selected the metabolically active MAPKs LmxMPK4, PfMAP2 and TbMAPK5 of the three parasitic protozoans Leishmania mexicana, Plasmodium falciparum and Trypanosoma brucei respectively. The homology modeling technique was used to develop the 3D structures of these proteins and the same was validated by PROCHECK, ERRAT, ProQ and ProSA web servers to check the reliability. Ten phytoligands were employed for molecular docking studies with these proteins to search for potent phytoligand as a broad spectrum inhibitor. In this regard two phytoligands (Aspidocarpine for LmxMPK4 & TbMAPK5 and Cubebin for PfMAP2) were found to be more effective inhibitors, in term of robust binding energy, strong inhibition constant and better interactions between protein-ligand complexes. Furthermore predicted ADME & Toxicity properties suggested that these identified phytoligands exhibited comparable results to control drugs potentiating them as persuasive therapeutic agents for Leishmania, Trypanosoma and Plasmodium sp.

In silico elucidation and inhibition studies of selected phytoligands against Mitogen activated protein kinases of protozoan parasites

Parasitic MAPKs exhibiting significant divergence with humans and playing an imperative role in parasitic metabolic activities have been exploited from several years as important targets for development of novel therapeutics. In addition, the emergence of the drug resistant variants of parasitic diseases in the recent years has aroused a great need for the development of potent inhibitors against them. In the present study we selected the metabolically active MAPKs LmxMPK4, PfMAP2 and TbMAPK5 of the three parasitic protozoans Leishmania mexicana, Plasmodium falciparum and Trypanosoma brucei respectively. The homology modeling technique was used to develop the 3D structures of these proteins and the same was validated by PROCHECK, ERRAT, ProQ and ProSA web servers to check the reliability. Ten phytoligands were employed for molecular docking studies with these proteins to search for potent phytoligand as a broad spectrum inhibitor. In this regard two phytoligands (Aspidocarpine for LmxMPK4 & TbMAPK5 and Cubebin for PfMAP2) were found to be more effective inhibitors, in term of robust binding energy, strong inhibition constant and better interactions between protein-ligand complexes. Furthermore predicted ADME & Toxicity properties suggested that these identified phytoligands exhibited comparable results to control drugs potentiating them as persuasive therapeutic agents for Leishmania, Trypanosoma and Plasmodium sp.

Linear B-cell epitope mapping of MAPK3 and MAPK4 from Leishmania braziliensis: implications for the serodiagnosis of human and canine leishmaniasis

The correct and early identification of humans and dogs infected with Leishmania are key steps in the control of leishmaniasis. Additionally, a method with high sensitivity and specificity at low cost that allows the screening of a large number of samples would be extremely valuable. In this study, we analyzed the potential of mitogen-activated protein kinase 3 (MAPK3) and mitogen-activated protein kinase 4 (MAPK4) proteins from Leishmania braziliensis to serve as antigen candidates for the serodiagnosis of human visceral and tegumentary leishmaniasis, as well as canine visceral disease. Moreover, we mapped linear B-cell epitopes in these proteins and selected those epitopes with sequences that were divergent in the corresponding orthologs in Homo sapiens, in Canis familiaris, and in Trypanosoma cruzi. We compared the performance of these peptides with the recombinant protein using ELISA. Both MAPK3 and MAPK4 recombinant proteins showed better specificity in the immunodiagnosis of human and canine leishmaniasis than soluble parasite antigens and the EIE-leishmaniose-visceral-canina-bio-manguinhos (EIE-LVC) kit. Furthermore, the performance of this serodiagnosis assay was improved using synthetic peptides corresponding to B-cell epitopes derived from both proteins.

Several human cyclin-dependent kinase inhibitors, structurally related to roscovitine, are new anti-malarial agents

In Africa, malaria kills one child each minute. It is also responsible for about one million deaths worldwide each year. Plasmodium falciparum, is the protozoan responsible for the most lethal form of the disease, with resistance developing against the available anti-malarial drugs. Among newly proposed anti-malaria targets, are the P. falciparum cyclin-dependent kinases (PfCDKs). There are involved in different stages of the protozoan growth and development but share high sequence homology with human cyclin-dependent kinases (CDKs). We previously reported the synthesis of CDKs inhibitors that are structurally-related to (R)-roscovitine, a 2,6,9-trisubstituted purine, and they showed activity against neuronal diseases and cancers. In this report, we describe the synthesis and the characterization of new CDK inhibitors, active in reducing the in vitro growth of P. falciparum (3D7 and 7G8 strains). Six compounds are more potent inhibitors than roscovitine, and three exhibited IC₅₀ values close to 1 µM for both 3D7 and 7G8 strains. Although, such molecules do inhibit P. falciparum growth, they require further studies to improve their selectivity for PfCDKs.

Transgenic analysis of the Leishmania MAP kinase MPK10 reveals an auto-inhibitory mechanism crucial for stage-regulated activity and parasite viability

Protozoan pathogens of the genus Leishmania have evolved unique signaling mechanisms that can sense changes in the host environment and trigger adaptive stage differentiation essential for host cell infection. The signaling mechanisms underlying parasite development remain largely elusive even though Leishmania mitogen-activated protein kinases (MAPKs) have been linked previously to environmentally induced differentiation and virulence. Here, we unravel highly unusual regulatory mechanisms for Leishmania MAP kinase 10 (MPK10). Using a transgenic approach, we demonstrate that MPK10 is stage-specifically regulated, as its kinase activity increases during the promastigote to amastigote conversion. However, unlike canonical MAPKs that are activated by dual phosphorylation of the regulatory TxY motif in the activation loop, MPK10 activation is independent from the phosphorylation of the tyrosine residue, which is largely constitutive. Removal of the last 46 amino acids resulted in significantly enhanced MPK10 activity both for the recombinant and transgenic protein, revealing that MPK10 is regulated by an auto-inhibitory mechanism. Over-expression of this hyperactive mutant in transgenic parasites led to a dominant negative effect causing massive cell death during amastigote differentiation, demonstrating the essential nature of MPK10 auto-inhibition for parasite viability. Moreover, phosphoproteomics analyses identified a novel regulatory phospho-serine residue in the C-terminal auto-inhibitory domain at position 395 that could be implicated in kinase regulation. Finally, we uncovered a feedback loop that limits MPK10 activity through dephosphorylation of the tyrosine residue of the TxY motif. Together our data reveal novel aspects of protein kinase regulation in Leishmania, and propose MPK10 as a potential signal sensor of the mammalian host environment, whose intrinsic pre-activated conformation is regulated by auto-inhibition.

Leishmaniasis is an important human disease caused by Leishmania parasites. A crucial aspect of Leishmania infectivity is its capacity to sense different environments and adapt for survival inside insect vector and vertebrate host by stage differentiation. This process is triggered by environmental changes encountered in these organisms, including temperature and pH shifts, which usually are sensed and transduced by signaling cascades including protein kinases and their substrates. In this study, we analyzed the regulation of the Leishmania mitogen-activated protein kinase MPK10 using protein purified from transgenic parasites and combining site-directed mutagenesis and activity tests. We demonstrate that this kinase is activated during parasite differentiation and regulated by an atypical mechanism involving auto-inhibition, which is essential for parasite viability.

Drug resistance in leishmaniasis: Newer developments

Leishmaniasis is a vector borne protozoan disease and it remains a major public health problem world-wide. Lack of an effective vaccine and vector control program makes the chemotherapy as the primary tool for leishmaniasis. Antimonials were used as the first line of treatment for many years. Emergence of resistance against this drug has become a major concern. Literatures and studies published on anti-leishmanial drug resistance, newer drug discovery for leishmanial resistance etc., in PubMed, Medline and Google search and reviewed thoroughly. Various newer drugs have been identified but, are in limited use because of high cost, toxicity, resistance etc., Recently, many newer mechanisms of drug resistance have been identified which may boost in future designing and development of drugs.

Catalytic activity of a novel serine/threonine protein phosphatase PP5 from Leishmania major

Leishmaniasis is a vector-borne disease caused by protozoan parasites of the genus Leishmania. Our knowledge of protein phosphatases (PPs) and their implication in signaling events is very limited. Here we report the expression, characterization and mutagenesis analysis of a novel protein phosphatase 5 (PP5) in Leishmania major. Recombinant PP5 is a bona fide phosphatase and is enzymatically active. Site-directed mutagenesis revealed auto-inhibitory roles of the N-terminal region. This is a rational first approach to understand the role of PP5 in the biology of the parasite better as well as its potential future applicability to anti-parasitic intervention.

Selection of molecular targets for drug development against trypanosomatids

Trypanosomatid parasites are a group of flagellated protozoa that includes the genera Leishmania and Trypanosoma, which are the causative agents of diseases (leishmaniases, sleeping sickness and Chagas disease) that cause considerable morbidity and mortality, affecting more than 27 million people worldwide. Today no effective vaccines for the prevention of these diseases exist, whereas current chemotherapy is ineffective, mainly due to toxic side effects of current drugs and to the emergence of drug resistance and lack of cost effectiveness. For these reasons, rational drug design and the search of good candidate drug targets is of prime importance. The search for drug targets requires a multidisciplinary approach. To this end, the completion of the genome project of many trypanosomatid species gives a vast amount of new information that can be exploited for the identification of good drug candidates with a prediction of "druggability" and divergence from mammalian host proteins. In addition, an important aspect in the search for good drug targets is the "target identification" and evaluation in a biological pathway, as well as the essentiality of the gene in the mammalian stage of the parasite, which is provided by basic research and genetic and proteomic approaches. In this chapter we will discuss how these bioinformatic tools and experimental evaluations can be integrated for the selection of candidate drug targets, and give examples of metabolic and signaling pathways in the parasitic protozoa that can be exploited for rational drug design.

Natural product based leads to fight against leishmaniasis

The growing incidence of parasitic resistance against generic pentavalent antimonials, specifically for visceral disease in Indian subcontinent, is a serious issue in Leishmania control. Notwithstanding the two treatment alternatives, that is amphotericin B and miltefosine are being effectively used but their high cost and therapeutic complications limit their use in endemic areas. In the absence of a vaccine candidate, identification, and characterization of novel drugs and targets is a major requirement of leishmanial research. This review describes current drug regimens, putative drug targets, numerous natural products that have shown promising antileishmanial activity alongwith some key issues and strategies for future research to control leishmaniasis worldwide.

Distinct roles of a mitogen-activated protein kinase in cytokinesis between different life cycle forms of Trypanosoma brucei

Mitogen-activated protein kinase (MAPK) modules are evolutionarily conserved signaling cascades that function in response to the environment and play crucial roles in intracellular signal transduction in eukaryotes. The involvement of a MAP kinase in regulating cytokinesis in yeast, animals, and plants has been reported, but the requirement for a MAP kinase for cytokinesis in the early-branching protozoa is not documented. Here, we show that a MAP kinase homolog (TbMAPK6) from Trypanosoma brucei plays distinct roles in cytokinesis in two life cycle forms of T. brucei. TbMAPK6 is distributed throughout the cytosol in the procyclic form but is localized in both the cytosol and the nucleus in the bloodstream form. RNA interference (RNAi) of TbMAPK6 results in moderate growth inhibition in the procyclic form but severe growth defects and rapid cell death in the bloodstream form. Moreover, TbMAPK6 appears to be implicated in furrow ingression and cytokinesis completion in the procyclic form but is essential for cytokinesis initiation in the bloodstream form. Despite the distinct defects in cytokinesis in the two forms, RNAi of TbMAPK6 also caused defective basal body duplication/segregation in a small cell population in both life cycle forms. Altogether, our results demonstrate the involvement of the TbMAPK6-mediated pathway in regulating cytokinesis in trypanosomes and suggest distinct roles of TbMAPK6 in cytokinesis between different life cycle stages of T. brucei.

Synthesis and biological evaluation of 2,3-diarylimidazo[1,2-a]pyridines as antileishmanial agents

A novel series of 2,3-diarylimidazo[1,2-a]pyridines was synthesized and evaluated for their antileishmanial activities. Four derivatives exhibited good activity against the promastigote and intracellular amastigote stages of Leishmania major, coupled with a low cytotoxicity against the HeLa human cell line. The impact of compound lipophilicity on antiparasitic activities was investigated by Log D comparison. Although LmCK1 could be the parasitic target for three compounds (13, 18, 21), the inhibition of another target is under study to explain the antileishmanial effect of the most promising compounds.

Biomarkers of antimony resistance: need for expression analysis of multiple genes to distinguish resistance phenotype in clinical isolates of Leishmania donovani

Resistance to antimony is a major cause of failure to therapy in a large proportion of visceral leishmaniasis cases. Methods to distinguish resistant and sensitive parasite are urgently needed as the standard in vitro intracellular drug susceptibility assays are cumbersome and time consuming. Differential expression profiling studies have led to the identification of several antimony resistance-associated genes; however, their efficacy as a potential biomarker for monitoring antimony resistance remains imprecise. We analysed the expression of eight genes [antimony metabolism-associated genes - multidrug resistance protein A (MRPA), γ-glutamylcysteine synthetase (γ-GCS) and aquaporin-1 (AQP1) - and genes identified by proteome/transcriptome profiling—heat shock protein 83, mitogen-activated protein kinase 1 and histones H1, H2A and H4) in antimony-resistant (n=10) and antimony-sensitive (n=4) clinical isolates of Leishmania donovani by quantitative real-time PCR, in comparison with a lab-generated resistant and a standard sensitive isolate. We observed a significant differential expression of MRPA, histone H1 (p<0.01), γ-GCS, HSP83 (p<0.005) and histone H2A and H4 (p<0.0001) in a group of sodium antimony gluconate-resistant isolates compared to sensitive isolates. Preferential AQP1 expression was observed in all the sensitive isolates (p<0.0001). Overall, expression profile in field isolates for all the genes studied showed altered expression in majority of isolates, while in some, the expression was static. All the isolates showed a mosaic of expression pattern of the genes analysed indicating constellation of genes contributes towards the drug susceptibility of parasite. As none of the genes exhibit an absolute correlation with phenotype, targeted expression analysis of a set of genes should be considered as biomarker for distinguishing the antimony-resistant and antimony-sensitive parasite.

MAP kinase kinase 1 (MKK1) is essential for transmission of Trypanosoma brucei by Glossina morsitans

MAP kinase kinase 1 (MKK1) is encoded by a single copy gene in Trypanosoma brucei. It has been shown recently that MKK1 is not essential for bloodstream forms [14]. To investigate the requirement for MKK1 in other life-cycle stages we generated null mutants in procyclic forms of a fly-transmissible strain. These grew normally in culture and were able to establish midgut infections in tsetse at normal rates and intensities, but were incapable of colonising the salivary glands. Transformation of null mutants with an ectopic copy of MKK1 enabled parasites to complete the life cycle in tsetse and infect mice. This is the first example of a gene that is indispensable for transmission of T. brucei. It also raises the possibility that activating the MKK1 signalling cascade in vitro might trigger the differentiation and proliferation of life-cycle stages of T. brucei that are currently refractory to culture.

Modulation of Leishmania major aquaglyceroporin activity by a mitogen-activated protein kinase

Leishmania major aquaglyceroporin (LmjAQP1) adventitiously facilitates the uptake of antimonite [Sb(III)], an active form of Pentostam® or Glucantime®, which are the first line of defence against all forms of leishmaniasis. The present paper shows that LmjAQP1 activity is modulated by the mitogen-activated protein kinase, LmjMPK2. Leishmania parasites coexpressing LmjAQP1 and LmjMPK2 show increased Sb(III) uptake and increased Sb(III) sensitivity. When subjected to a hypo-osmotic stress, these cells show faster volume recovery than cells expressing LmjAQP1 alone. LmjAQP1 is phosphorylated in vivo at Thr-197 and this phosphorylation requires LmjMPK2 activity. Lys-42 of LmjMPK2 is critical for its kinase activity. Cells expressing altered T197A LmjAQP1 or K42A LmjMPK2 showed decreased Sb(III) influx and a slower volume recovery than cells expressing wild-type proteins. Phosphorylation of LmjAQP1 led to a decrease in its turnover rate affecting LmjAQP1 activity. Although LmjAQP1 is localized to the flagellum of promastigotes, upon phosphorylation, it is relocalized to the entire surface of the parasite. Leishmania mexicana promastigotes with an MPK2 deletion showed reduced Sb(III) uptake and slower volume recovery than wild-type cells. This is the first report where a parasite aquaglyceroporin activity is post-translationally modulated by a mitogen-activated protein kinase.

Leishmaniasis: current status of available drugs and new potential drug targets

The control of Leishmania infection relies primarily on chemotherapy till date. Resistance to pentavalent antimonials, which have been the recommended drugs to treat cutaneous and visceral leishmaniasis, is now widespread in Indian subcontinents. New drug formulations like amphotericin B, its lipid formulations, and miltefosine have shown great efficacy to treat leishmaniasis but their high cost and therapeutic complications limit their usefulness. In addition, irregular and inappropriate uses of these second line drugs in endemic regions like state of Bihar, India threaten resistance development in the parasite. In context to the limited drug options and unavailability of either preventive or prophylactic candidates, there is a pressing need to develop true antileishmanial drugs to reduce the disease burden of this debilitating endemic disease. Notwithstanding significant progress of leishmanial research during last few decades, identification and characterization of novel drugs and drug targets are far from satisfactory. This review will initially describe current drug regimens and later will provide an overview on few important biochemical and enzymatic machineries that could be utilized as putative drug targets for generation of true antileishmanial drugs.

Intracellular growth and pathogenesis of Leishmania parasites

Parasitic protozoa belonging to the genus Leishmania are the cause of a spectrum of diseases in humans, as well as chronic long-term infections. These parasites exhibit a remarkable capacity to survive and proliferate within the phagolysosome compartment of host macrophages. Studies with defined Leishmania mutants in mouse models of infection have highlighted processes that are required for parasite survival in macrophages. Parasite mutants have been identified that (i) are poorly virulent when the insect (promastigote) stage is used to initiate infection, but retain wild-type virulence following transformation to the obligate intracellular amastigote stage, (ii) are highly attenuated when either promastigotes or amastigotes are used, and (iii) are unable to induce characteristic lesion granulomas, but can persist within macrophages in other tissues. From these analyses it can be concluded that promastigote stages of some species require the surface expression of lipophosphoglycan, but not other surface components. Survival and subsequent proliferation of Leishmania in macrophages requires the activation of heat-shock responses (involving the up-regulation and/or phosphorylation of heat-shock proteins), the presence of oxidative and nitrosative defence mechanisms, and uptake and catabolism of carbon sources (glycoproteins, hexoses and amino acids) and essential nutrients (purines, amino acids and vitamins). Parasite mutants with defects in specific kinase/phosphatase-dependent signalling pathways are also severely attenuated in amastigote virulence, highlighting the potential importance of post-translational regulatory mechanisms in parasite adaptation to this host niche.

What has proteomics taught us aboutLeishmaniadevelopment?

Leishmaniaare obligatory intracellular parasitic protozoa that cycle between sand fly mid-gut and phagolysosomes of mammalian macrophages. They have developed genetically programmed changes in gene and protein expression that enable rapid optimization of cell function according to vector and host environments. During the last two decades, host-free systems that mimic intra-lysosomal environments have been devised in which promastigotes differentiate into amastigotes axenically. These cultures have facilitated detailed investigation of the molecular mechanisms underlyingLeishmaniadevelopment inside its host. Axenic promastigotes and amastigotes have been subjected to transcriptome and proteomic analyses. Development had appeared somewhat variable but was revealed by proteomics to be strictly coordinated and regulated. Here we summarize the current understanding ofLeishmaniapromastigote to amastigote differentiation, highlighting the data generated by proteomics.

Metabolic pathways required for the intracellular survival of Leishmania

Leishmania spp. are sandfly-transmitted parasitic protozoa that cause a spectrum of important diseases and lifelong chronic infections in humans. In the mammalian host, these parasites proliferate within acidified vacuoles in several phagocytic host cells, including macrophages, dendritic cells, and neutrophils. In this review, we discuss recent progress that has been made in defining the nutrient composition of the Leishmania parasitophorous vacuole, as well as metabolic pathways required by these parasites for virulence. Analysis of the virulence phenotype of Leishmania mutants has been particularly useful in defining carbon sources and nutrient salvage pathways that are essential for parasite persistence and/or induction of pathology. We also review data suggesting that intracellular parasite stages modulate metabolic processes in their host cells in order to generate a more permissive niche.

Signal transduction in Trypanosoma cruzi

Signal transduction plays a key role in regulating important functions in both multicellular and unicellular organisms and largely controls the manner in which cells respond to stimuli. Signal transduction pathways coordinate the functions in different type of cells in animals and control the growth and differentiation in unicellular organisms. Intracellular signal transduction pathways are largely activated by second messenger molecules. Trypanosoma cruzi has a complex life cycle involving four morphogenetic stages with various second messenger systems able to regulate its growth and differentiation. Signal transduction often alters the status of phosphorylation in target proteins and thus alters the activities of these proteins. In this review, two major signal transduction pathways, cyclic AMP-dependent pathway and mitogen-activated protein kinase pathway, are discussed. Protein phosphatases are also discussed due to their importance in dephosphorylating target proteins and terminating signal transduction. Understanding of the unique pathways in this pathogen may lead to the development of novel therapeutic agents.

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.

Downregulation of mitogen-activated protein kinase 1 of Leishmania donovani field isolates is associated with antimony resistance

Emergence of resistance to pentavalent antimonials has become a severe obstacle in the treatment of visceral leishmaniasis (VL) on the Indian subcontinent. The mechanisms operating in laboratory-generated strains are somewhat known, but the determinants of clinical antimony resistance are not well understood. By utilizing a DNA microarray expression profiling approach, we identified a gene encoding mitogen-activated protein kinase 1 (MAPK1) for the kinetoplast protozoan Leishmania donovani (LdMAPK1) that was consistently downregulated in antimony-resistant field isolates. The expression level of the gene was validated by real-time PCR. Furthermore, decreased expression of LdMAPK1 was also confirmed at the protein level in resistant isolates. Primary structure analysis of LdMAPK1 revealed the presence of all of the characteristic features of MAPK1. When expressed in Escherichia coli, the recombinant enzyme showed kinase activity with myelin basic protein as the substrate and was inhibited by staurosporine. Interestingly, overexpression of this gene in a drug-sensitive laboratory strain and a resistant field isolate resulted in increased the sensitivity of the transfectants to potassium antimony tartrate, suggesting that it has a role in antimony resistance. Our results demonstrate that downregulation of LdMAPK1 may be in part correlated with antimony drug resistance in Indian VL isolates.