The Leishmania mexicana proteasome

Unraveling the interaction between a glycolytic regulator protein EhPpdk and an anaphase promoting complex protein EhApc10: yeast two hybrid screening, in vitro binding assays and molecular simulation study

The anaphase promoting complex (APC or cyclosome) is a major ubiquitin ligase that coordinates mitotic and G1 progression, acting as a major regulator of chromosome segregation. While the human APC contains fourteen subunits, it is yet to be explored in the pathogen Entamoeba histolytica. Our study reveals the existence of a single functional Apc10 homolog in E. histolytica, which acts as a processivity factor of ubiquitin ligase activity in human. A cDNA library generated from HM1:IMSS strain of E. histolytica was screened for interaction partners of EhApc10 in yeast two hybrid study. The novel interactor, a glycolytic enzyme, pyruvate phosphate dikinase (Ppdk) was found to interact with EhApc10 and further validated by in vitro assay. A comprehensive in silico study has emphasized the structural and functional aspects, encompassing physicochemical traits, predictive 3D structure modelling, validation of EhApc10-EhPpdk interaction through molecular docking and simulation. The interplay between a cell cycle protein and a glycolytic enzyme highlights the connection between cellular metabolism and the cell cycle regulatory mechanism. The study serves as the groundwork for future research on the non-mitotic role of APC beyond cell cycle.

Functional characterization of Cullin-1-RING ubiquitin ligase (CRL1) complex in Leishmania infantum

Cullin-1-RING ubiquitin ligases (CRL1) or SCF1 (SKP1-CUL1-RBX1) E3 ubiquitin ligases are the largest and most extensively investigated class of E3 ligases in mammals that regulate fundamental processes, such as the cell cycle and proliferation. These enzymes are multiprotein complexes comprising SKP1, CUL1, RBX1, and an F-box protein that acts as a specificity factor by interacting with SKP1 through its F-box domain and recruiting substrates via other domains. E3 ligases are important players in the ubiquitination process, recognizing and transferring ubiquitin to substrates destined for degradation by proteasomes or processing by deubiquitinating enzymes. The ubiquitin-proteasome system (UPS) is the main regulator of intracellular proteolysis in eukaryotes and is required for parasites to alternate hosts in their life cycles, resulting in successful parasitism. Leishmania UPS is poorly investigated, and CRL1 in L. infantum, the causative agent of visceral leishmaniasis in Latin America, is yet to be described. Here, we show that the L. infantum genes LINF_110018100 (SKP1-like protein), LINF_240029100 (cullin-like protein-like protein), and LINF_210005300 (ring-box protein 1 -putative) form a LinfCRL1 complex structurally similar to the H. sapiens CRL1. Mass spectrometry analysis of the LinfSkp1 and LinfCul1 interactomes revealed proteins involved in several intracellular processes, including six F-box proteins known as F-box-like proteins (Flp) (data are available via ProteomeXchange with identifier PXD051961). The interaction of LinfFlp 1-6 with LinfSkp1 was confirmed, and using in vitro ubiquitination assays, we demonstrated the function of the LinfCRL1(Flp1) complex to transfer ubiquitin. We also found that LinfSKP1 and LinfRBX1 knockouts resulted in nonviable L. infantum lineages, whereas LinfCUL1 was involved in parasite growth and rosette formation. Finally, our results suggest that LinfCul1 regulates the S phase progression and possibly the transition between the late S to G2 phase in L. infantum. Thus, a new class of E3 ubiquitin ligases has been described in L. infantum with functions related to various parasitic processes that may serve as prospective targets for leishmaniasis treatment.

Identification of new drug candidates against Trichomonas gallinae using high-throughput screening

Trichomonas gallinae is a protozoan parasite that is the causative agent of trichomoniasis, and infects captive and wild bird species throughout the world. Although metronidazole has been the drug of choice against trichomoniasis for decades, most Trichomonas gallinae strains have developed resistance. Therefore, drugs with new modes of action or targets are urgently needed. Here, we report the development and application of a cell-based CCK-8 method for the high-throughput screening and identification of new inhibitors of Trichomonas gallinae as a beginning point for the development of new treatments for trichomoniasis. We performed the high-throughput screening of 173 anti-parasitic compounds, and found 16 compounds that were potentially effective against Trichomonas gallinae. By measuring the median inhibitory concentration (IC50) and median cytotoxic concentration (CC50), we identified 3 potentially safe and effective compounds against Trichomonas gallinae: anisomycin, fumagillin, and MG132. In conclusion, this research successfully established a high-throughput screening method for compounds and identified 3 new safe and effective compounds against Trichomonas gallinae, providing a new treatment scheme for trichomoniasis.

Drug Discovery for Cutaneous Leishmaniasis: A Review of Developments in the Past 15 Years

Leishmaniasis is a group of vector-borne, parasitic diseases caused by over 20 species of the protozoan Leishmania spp. The three major disease classifications, cutaneous, visceral, and mucocutaneous, have a range of clinical manifestations from self-healing skin lesions to hepatosplenomegaly and mucosal membrane damage to fatality. As a neglected tropical disease, leishmaniasis represents a major international health challenge, with nearly 350 million people living at risk of infection a year. The current chemotherapeutics used to treat leishmaniasis have harsh side effects, prolonged and costly treatment regimens, as well as emerging drug resistance, and are predominantly used for the treatment of visceral leishmaniasis. There is an undeniable need for the identification and development of novel chemotherapeutics targeting cutaneous leishmaniasis (CL), largely ignored by concerted drug development efforts. CL is mostly non-lethal and the most common presentation of this disease, with nearly 1 million new cases reported annually. Recognizing this unaddressed need, substantial yet fragmented progress in early drug discovery efforts for CL has occurred in the past 15 years and was outlined in this review. However, further work needs to be carried out to advance early discovery candidates towards the clinic. Importantly, there is a paucity of investment in the translation and development of therapies for CL, limiting the emergence of viable solutions to deal with this serious and complex international health problem.

The critical role of mode of action studies in kinetoplastid drug discovery

Understanding the target and mode of action of compounds identified by phenotypic screening can greatly facilitate the process of drug discovery and development. Here, we outline the tools currently available for target identification against the neglected tropical diseases, human African trypanosomiasis, visceral leishmaniasis and Chagas' disease. We provide examples how these tools can be used to identify and triage undesirable mechanisms, to identify potential toxic liabilities in patients and to manage a balanced portfolio of target-based campaigns. We review the primary targets of drugs that are currently in clinical development that were initially identified via phenotypic screening, and whose modes of action affect protein turnover, RNA trans-splicing or signalling in these protozoan parasites.

Discovery of Leishmania Druggable Serine Proteases by Activity-Based Protein Profiling

Leishmaniasis are a group of diseases caused by parasitic protozoa of the genus Leishmania. Current treatments are limited by difficult administration, high cost, poor efficacy, toxicity, and growing resistance. New agents, with new mechanisms of action, are urgently needed to treat the disease. Although extensively studied in other organisms, serine proteases (SPs) have not been widely explored as antileishmanial drug targets. Herein, we report for the first time an activity-based protein profiling (ABPP) strategy to investigate new therapeutic targets within the SPs of the Leishmania parasites. Active-site directed fluorophosphonate probes (rhodamine and biotin-conjugated) were used for the detection and identification of active Leishmania serine hydrolases (SHs). Significant differences were observed in the SHs expression levels throughout the Leishmania life cycle and between different Leishmania species. Using iTRAQ-labelling-based quantitative proteomic mass spectrometry, we identified two targetable SPs in Leishmania mexicana: carboxypeptidase LmxM.18.0450 and prolyl oligopeptidase LmxM.36.6750. Druggability was ascertained by selective inhibition using the commercial serine protease inhibitors chymostatin, lactacystin and ZPP, which represent templates for future anti-leishmanial drug discovery programs. Collectively, the use of ABPP method complements existing genetic methods for target identification and validation in Leishmania.

Targeting Proteasomes in Cancer and Infectious Disease: A Parallel Strategy to Treat Malignancies and Microbes

Essential core pathways of cellular biology are preserved throughout evolution, highlighting the importance of these pathways for both bacteria and human cancer cells alike. Cell viability requires a proper balance between protein synthesis and degradation in order to maintain integrity of the proteome. Proteasomes are highly intricate, tightly regulated multisubunit complexes that are critical to achieve protein homeostasis (proteostasis) through the selective degradation of misfolded, redundant and damaged proteins. Proteasomes function as the catalytic core of the ubiquitin-proteasome pathway (UPP) which regulates a myriad of essential processes including growth, survival, differentiation, drug resistance and apoptosis. Proteasomes recognize and degrade proteins that have been marked by covalently attached poly-ubiquitin chains. Deregulation of the UPP has emerged as an essential etiology of many prominent diseases, including cancer. Proteasome inhibitors selectively target cancer cells, including those resistant to chemotherapy, while sparing healthy cells. Proteasome inhibition has emerged as a transformative anti-myeloma strategy that has extended survival for certain patient populations from 3 to 8 years. The structural architecture and functional activity of proteasomes is conserved from Archaea to humans to support the concept that proteasomes are actionable targets that can be inhibited in pathogenic organisms to improve the treatment of infectious diseases. Proteasomes have an essential role during all stages of the parasite life cycle and features that distinguish proteasomes in pathogens from human forms have been revealed. Advancement of inhibitors that target Plasmodium and Mycobacterial proteasomes is a means to improve treatment of malaria and tuberculosis. In addition, PIs may also synergize with current frontline agents support as resistance to conventional drugs continues to increase. The proteasome represents a highly promising, actionable target to combat infectious diseases that devastate lives and livelihoods around the globe.

Ubiquitination and the Proteasome as Drug Targets in Trypanosomatid Diseases

The eukaryotic pathogens Trypanosoma brucei, Trypanosoma cruzi and Leishmania are responsible for debilitating diseases that affect millions of people worldwide. The numbers of drugs available to treat these diseases, Human African Trypanosomiasis, Chagas' disease and Leishmaniasis are very limited and existing treatments have substantial shortcomings in delivery method, efficacy and safety. The identification and validation of novel drug targets opens up new opportunities for the discovery of therapeutic drugs with better efficacy and safety profiles. Here, the potential of targeting the ubiquitin-proteasome system in these parasites is reviewed. Ubiquitination is the posttranslational attachment of one or more ubiquitin proteins to substrates, an essential eukaryotic mechanism that regulates a wide variety of cellular processes in many different ways. The best studied of these is the delivery of ubiquitinated substrates for degradation to the proteasome, the major cellular protease. However, ubiquitination can also regulate substrates in proteasome-independent ways, and proteasomes can degrade proteins to some extent in ubiquitin-independent ways. Because of these widespread roles, both ubiquitination and proteasomal degradation are essential for the viability of eukaryotes and the proteins that mediate these processes are therefore attractive drug targets in trypanosomatids. Here, the current understanding of these processes in trypanosomatids is reviewed. Furthermore, significant recent progress in the development of trypanosomatid-selective proteasome inhibitors that cure mouse models of trypanosomatid infections is presented. In addition, the targeting of the key enzyme in ubiquitination, the ubiquitin E1 UBA1, is discussed as an alternative strategy. Important differences between human and trypanosomatid UBA1s in susceptibility to inhibitors predicts that the selective targeting of these enzymes in trypanosomatids may also be feasible. Finally, it is proposed that activating enzymes of the ubiquitin-like proteins SUMO and NEDD8 may represent drug targets in these trypanosomatids as well.

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

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

Naegleria fowleri and Naegleria gruberi 20S proteasome: identification and characterization

The Naegleria are ubiquitous free-living amoebae and are characterized by the presence of three phases in their biological cycle: trophozoite, cyst and flagellate. Of this genus, only Naegleria fowleri has been reported as pathogenic to humans. The proteasome is a multi-catalytic complex and is considered to be the most important structure responsible for the degradation of intracellular proteins. This structure is related to the maintenance of cellular homeostasis and, in pathogenic microorganisms, to the modulation of their virulence. Until now, the proteasome and its function have not been described for the Naegleria genus. In the current study, using bioinformatic analysis, protein sequences homologous to those reported for the subunits of the 20S proteasome in other organisms were found, and virtual modelling was used to determine their three-dimensional structure. The presence of structural and catalytic subunits of the 20S proteasome was detected by Western and dot blot assays. Its localization was observed by immunofluorescence microscopy to be mainly in the cytoplasm, and a leading role of the chymotrypsin-like catalytic activity was determined using fluorogenic peptidase assays and specific proteasome inhibitors. Finally, the role of the 20S proteasome in the proliferation and differentiation of Naegleria genus trophozoites was demonstrated.

Hypoxia-induced 26S proteasome dysfunction increases immunogenicity of mesenchymal stem cells

Bone marrow-derived allogeneic (donor derived) mesenchymal stem cells (MSCs) are immunoprivileged and are considered to be prominent candidates for regenerative therapy for numerous degenerative diseases. Even though the outcome of initial allogeneic MSCs based clinical trials was encouraging, the overall enthusiasm, of late, has dimmed down. This is due to failure of long-term survival of transplanted cells in the recipient. In fact, recent analyses of allogeneic MSC-based studies demonstrated that cells after transplantation turned immunogenic and were subsequently rejected by host immune system. The current study reveals a novel mechanism of immune switch in MSCs. We demonstrate that hypoxia, a common denominator of ischemic tissues, induces an immune shift in MSCs from immunoprivileged to immunogenic state. The immunoprivilege of MSCs is preserved by downregulation or the absence of major histocompatibility complex class II (MHC-II) molecules. We found that 26S proteasome-mediated intracellular degradation of MHC-II helps maintain the absence of MHC-II expression on cell surface in normoxic MSCs and preserves their immunoprivilege. The exposure to hypoxia leads to dissociation of 19S and 20S subunits, and inactivation of 26S proteasome. This prevented the degradation of MHC-II and, as a result, the MSCs became immunogenic. Furthermore, we found that hypoxia-induced decrease in the levels of a chaperon protein HSP90α is responsible for inactivation of 26S proteasome. Maintaining HSP90α levels in hypoxic MSCs preserved the immunoprivilege of MSCs. Therefore, hypoxia-induced inactivation of 26S proteasome assembly instigates loss of immunoprivilege of allogeneic mesenchymal stem cells. Maintaining 26S proteasome activity in mesenchymal stem cells preserves their immunoprivilege.

Leishmanicidal therapy targeted to parasite proteases

Leishmaniasis is considered a serious public health problem and the current available therapy has several disadvantages, which makes the search for new therapeutic targets and alternative treatments extremely necessary. In this context, this review focuses on the importance of parasite proteases as target drugs against Leishmania parasites, as a chemotherapy approach. Initially, we discuss about the current scenario for the treatment of leishmaniasis, highlighting the main drugs used and the problems related to their use. Subsequently, we describe the inhibitors of major proteases of Leishmania already discovered, such as Compound s9 (aziridine-2,3-dicarboxylate), Compound 1c (benzophenone derivative), Au2Phen (gold complex), AubipyC (gold complex), MDL 28170 (dipeptidyl aldehyde), K11777, Hirudin, diazo-acetyl norleucine methyl ester, Nelfinavir, Saquinavir, Nelfinavir, Saquinavir, Indinavir, Saquinavir, GNF5343 (azabenzoxazole), GNF6702 (azabenzoxazole), Benzamidine and TPCK. Next, we discuss the importance of the protease gene to parasite survival and the aspects of the validation of proteases as target drugs, with emphasis on gene disruption. Then, we describe novel important strategies that can be used to support the research of new antiparasitic drugs, such as molecular modeling and nanotechnology, whose main targets are parasitic proteases. And finally, we discuss possible perspectives to improve drug development. Based on all findings, proteases could be considered potential targets against leishmaniasis.

2'-Hydroxyflavanone activity in vitro and in vivo against wild-type and antimony-resistant Leishmania amazonensis

Background

To overcome the current problems in leishmaniasis chemotherapy, natural products have become an interesting alternative over the past few decades. Flavonoids have been studied as promising family of compounds for leishmaniasis treatment. 2’-Hydroxyflavanone (2HF) is a flavanone, a class of flavonoid that has shown promising results in cancer studies. In this study, we demonstrated the effects of 2HF in vitro and in vivo against wild-type and antimony-resistant Leishmania amazonensis promastigotes.

Methodology/Principal findings

2HF was effective against promastigotes and the intracellular amastigote form, decreasing the infection index in macrophages infected with wild-type and antimony-resistant promastigotes, but it was not toxic to macrophages. In silico analysis indicated 2HF as a good oral candidate for leishmaniasis treatment. In vivo, 2HF was able to reduce the lesion size and parasite load in a murine model of cutaneous leishmaniasis using wild-type and antimony-resistant promastigotes, demonstrating no cross-resistance with antimonials.

Conclusions/Significance

Taken together, these results suggest 2HF as a potential candidate for leishmaniasis chemotherapy for cutaneous leishmaniasis caused by both wild-type and antimony-resistant Leishmania species by oral administration. Furthermore, studies should be conducted to determine the ideal dose and therapeutic regimen.

Leishmaniasis is a parasitic disease endemic to 98 countries, affecting more than 12 million people globally, and there are more than 350 million people in risk areas. Although there are many drugs available as alternatives for leishmaniasis treatment, they remain mostly ineffective, expensive and longstanding, in addition to generating side effects and resistance. Antimonial resistance is currently one of the biggest obstacles in leishmaniasis chemotherapy. Due to the poor chemotherapy scenario and the need for a drug able to overcome resistance problems and therapeutic failures, natural products have become an important alternative for leishmaniasis treatment. Here, we evaluated the antileishmanicidal activity of 2HF in vitro and in vivo against wild-type and antimony-resistant L. amazonensis cells. 2HF inhibited the cellular proliferation of promastigotes and the intracellular amastigote form in a dose-dependent manner in both wild-type and antimony-resistant cells. Furthermore, 2HF reduced the lesion size and parasitic load in a murine model of cutaneous leishmaniasis using wild-type and antimony-resistant promastigotes without altering hematological parameters and serological toxicology markers. This is the first time that the activity of a flavonoid on the antimony-resistant L. amazonensis has been demonstrated in vitro and in vivo by the oral route.

The proteasome as a target to combat malaria: hits and misses

The proteasome plays a vital role throughout the life cycle as Plasmodium parasites quickly adapt to a new host and undergo a series of morphologic changes during asexual replication and sexual differentiation. Plasmodium carries 3 different types of protease complexes: typical eukaryotic proteasome (26S) that resides in the cytoplasm and the nucleus, a prokaryotic proteasome homolog ClpQ that resides in the mitochondria, and a caseinolytic protease complex ClpP that resides in the apicoplast. In silico prediction in conjunction with immunoprecipitation analysis of ubiquitin conjugates have suggested that over half of the Plasmodium falciparum proteome during asexual reproduction are potential targets for ubiquitination. The marked potency of multiple classes of proteasome inhibitors against all stages of the life cycle, synergy with the current frontline antimalarial, artemisinin, and recent advances identifying differences between Plasmodium and human proteasomes strongly support further drug development efforts.

Effects of proteasome inhibitor MG-132 on the parasite Schistosoma mansoni

Proteasome is a proteolytic complex responsible for intracellular protein turnover in eukaryotes, archaea and in some actinobacteria species. Previous work has demonstrated that in Schistosoma mansoni parasites, the proteasome inhibitor MG-132 affects parasite development. However, the molecular targets affected by MG-132 in S. mansoni are not entirely known. Here, we used expression microarrays to measure the genome-wide changes in gene expression of S. mansoni adult worms exposed in vitro to MG-132, followed by in silico functional analyses of the affected genes using Ingenuity Pathway Analysis (IPA). Scanning electron microscopy was used to document changes in the parasites’ tegument. We identified 1,919 genes with a statistically significant (q-value ≤ 0.025) differential expression in parasites treated for 24 h with MG-132, when compared with control. Of these, a total of 1,130 genes were up-regulated and 790 genes were down-regulated. A functional gene interaction network comprised of MG-132 and its target genes, known from the literature to be affected by the compound in humans, was identified here as affected by MG-132. While MG-132 activated the expression of the 26S proteasome genes, it also decreased the expression of 19S chaperones assembly, 20S proteasome maturation, ubiquitin-like NEDD8 and its partner cullin-3 ubiquitin ligase genes. Interestingly, genes that encode proteins related to potassium ion binding, integral membrane component, ATPase and potassium channel activities were significantly down-regulated, whereas genes encoding proteins related to actin binding and microtubule motor activity were significantly up-regulated. MG-132 caused important changes in the worm tegument; peeling, outbreaks and swelling in the tegument tubercles could be observed, which is consistent with interference on the ionic homeostasis in S. mansoni. Finally, we showed the down-regulation of Bax pro-apoptotic gene, as well as up-regulation of two apoptosis inhibitor genes, IAP1 and BRE1, and in contrast, down-regulation of Apaf-1 apoptotic activator, thus suggesting that apoptosis is deregulated in S. mansoni exposed to MG-132. A considerable insight has been gained concerning the potential of MG-132 as a gene expression modulator, and overall the data suggest that the proteasome might be an important molecular target for the design of new drugs against schistosomiasis.

Characterisation of 20S Proteasome in Tritrichomonas foetus and Its Role during the Cell Cycle and Transformation into Endoflagellar Form

Proteasomes are intracellular complexes that control selective protein degradation in organisms ranging from Archaea to higher eukaryotes. These structures have multiple proteolytic activities that are required for cell differentiation, replication and maintaining cellular homeostasis. Here, we document the presence of the 20S proteasome in the protist parasite Tritrichomonas foetus. Complementary techniques, such as a combination of whole genome sequencing technologies, bioinformatics algorithms, cell fractionation and biochemistry and microscopy approaches were used to characterise the 20S proteasome of T. foetus. The 14 homologues of the typical eukaryotic proteasome subunits were identified in the T. foetus genome. Alignment analyses showed that the main regulatory and catalytic domains of the proteasome were conserved in the predicted amino acid sequences from T. foetus-proteasome subunits. Immunofluorescence assays using an anti-proteasome antibody revealed a labelling distributed throughout the cytosol as punctate cytoplasmic structures and in the perinuclear region. Electron microscopy of a T. foetus-proteasome-enriched fraction confirmed the presence of particles that resembled the typical eukaryotic 20S proteasome. Fluorogenic assays using specific peptidyl substrates detected presence of the three typical peptidase activities of eukaryotic proteasomes in T. foetus. As expected, these peptidase activities were inhibited by lactacystin, a well-known specific proteasome inhibitor, and were not affected by inhibitors of serine or cysteine proteases. During the transformation of T. foetus to endoflagellar form (EFF), also known as pseudocyst, we observed correlations between the EFF formation rates, increases in the proteasome activities and reduced levels of ubiquitin-protein conjugates. The growth, cell cycle and EFF transformation of T. foetus were inhibited after treatment with lactacystin in a dose-dependent manner. Lactacystin treatment also resulted in an accumulation of ubiquitinated proteins and caused increase in the amount of endoplasmic reticulum membranes in the parasite. Taken together, our results suggest that the ubiquitin-proteasome pathway is required for cell cycle and EFF transformation in T. foetus.

Role of the Ubiquitin-Proteasome Systems in the Biology and Virulence of Protozoan Parasites

In eukaryotic cells, proteasomes perform crucial roles in many cellular pathways by degrading proteins to enforce quality control and regulate many cellular processes such as cell cycle progression, signal transduction, cell death, immune responses, metabolism, protein-quality control, and development. The catalytic heart of these complexes, the 20S proteasome, is highly conserved in bacteria, yeast, and humans. However, until a few years ago, the role of proteasomes in parasite biology was completely unknown. Here, we summarize findings about the role of proteasomes in protozoan parasites biology and virulence. Several reports have confirmed the role of proteasomes in parasite biological processes such as cell differentiation, cell cycle, proliferation, and encystation. Proliferation and cell differentiation are key steps in host colonization. Considering the importance of proteasomes in both processes in many different parasites such as Trypanosoma, Leishmania, Toxoplasma, and Entamoeba, parasite proteasomes might serve as virulence factors. Several pieces of evidence strongly suggest that the ubiquitin-proteasome pathway is also a viable parasitic therapeutic target. Research in recent years has shown that the proteasome is a valid drug target for sleeping sickness and malaria. Then, proteasomes are a key organelle in parasite biology and virulence and appear to be an attractive new chemotherapeutic target.

Molecular identification and characterization of Trichinella spiralis proteasome subunit beta type-7

Background

Previous study showed that Trichinella spiralis proteasome subunit beta type-7 (Tspst) gene is an up-regulated gene in intestinal infective larvae (IIL) compared to muscle larvae (ML), which was screened by using suppression subtractive hybridization (SSH) and confirmed by real-time PCR. Tspst may be related to the larval invasion of intestinal epithelial cells (IECs). The aim of this study was to identify Tspst and to investigate its immune protection against intestinal T. spiralis infection.

Methods

The Tspst gene encoding a 29 kDa protein from T. spiralis infective larvae was cloned, and recombinant Tspst protein (rTspst) was produced in an Escherichia coli expression system. The rTspst was used to immunize BALB/c mice. Anti-rTspst antibodies were used to determine the immunolocolization of Tspst in the parasite. Transcription and expression of Tspst at T. spiralis different developmental stages were observed by RT-PCR and immunofluorescence test (IFT). The in vitro or in vivo immune protection of anti-rTspst serum or rTspst against intestinal T. spiralis infection in BALB/c mice was evaluated.

Results

Anti-rTspst serum recognized the native Tspst protein with 29 kDa in ML crude antigens. Transcription and expression of gene was observed at all T. spiralis different developmental stages (IIL, adult worms, newborn larvae, and ML). An immunolocalization analysis identified Tspst in the cuticle and internal organs of the parasite. An in vitro invasion assay showed that, when anti-rTspst serum, serum of mice infected with T. spiralis or normal mouse serum were added to the medium, the invasion rate of the infective larvae in an IEC monolayer was 25.2%, 11.4%, and 79%, respectively (P < 0.05), indicating that anti-rTspst serum partially prevented the larval invasion of IECs. After a challenge infection with T. spiralis muscle larvae, mice immunized with rTspst conferred a 45.7% reduction in adult worm burden in intestines.

Conclusions

In the present study, Tspst was first identified and characterized. Tspst is an invasion-related protein of T. spiralis IIL and could be considered as a potential vaccine candidate antigen against intestinal T. spiralis infection that merits further study.

Profound activity of the anti-cancer drug bortezomib against Echinococcus multilocularis metacestodes identifies the proteasome as a novel drug target for cestodes

A library of 426 FDA-approved drugs was screened for in vitro activity against E. multilocularis metacestodes employing the phosphoglucose isomerase (PGI) assay. Initial screening at 20 µM revealed that 7 drugs induced considerable metacestode damage, and further dose-response studies revealed that bortezomib (BTZ), a proteasome inhibitor developed for the chemotherapy of myeloma, displayed high anti-metacestodal activity with an EC₅₀ of 0.6 µM. BTZ treatment of E. multilocularis metacestodes led to an accumulation of ubiquinated proteins and unequivocally parasite death. In-gel zymography assays using E. multilocularis extracts demonstrated BTZ-mediated inhibition of protease activity in a band of approximately 23 kDa, the same size at which the proteasome subunit beta 5 of E. multilocularis could be detected by Western blot. Balb/c mice experimentally infected with E. multilocularis metacestodes were used to assess BTZ treatment, starting at 6 weeks post-infection by intraperitoneal injection of BTZ. This treatment led to reduced parasite weight, but to a degree that was not statistically significant, and it induced adverse effects such as diarrhea and neurological symptoms. In conclusion, the proteasome was identified as a drug target in E. multilocularis metacestodes that can be efficiently inhibited by BTZ in vitro. However, translation of these findings into in vivo efficacy requires further adjustments of treatment regimens using BTZ, or possibly other proteasome inhibitors.

Tapeworms (cestodes) are a class of important human pathogens, causing very severe diseases in man such as alveolar echinococcosis (Echinococcus multilocularis), cystic echinococcosis (E. granulosus) and neurocysticercosis (Taenia solium). Current treatments are mainly based on benzimidazoles that show some limited activity against cestode larvae, but often do not kill them. These compounds have to be taken for extended periods of time, and can cause adverse side-effects. Cestode infections cause neglected diseases and the pharmaceutical industry is generally not interested in investments for developing novel bioactive compounds. In this study we focus on a panel of FDA-approved drugs and assessed them in E. multilocularis, which causes the most deadly of all helminth infections. One compound, the anti-cancer drug bortezomib, exhibits considerable in vitro activity against E. multilocularis metacestodes, and we provide evidence that it acts on the proteasome. In experimentally infected mice bortezomib activity was lower than the currently used albendazole and induced adverse effects. Bortezomib is therefore not a useful drug for treatment of Echinococcus larvae, but our results demonstrate that in future studies the cestode proteasome should gain more attention as a drug target.

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.

Investigation on the 19S ATPase proteasome subunits (Rpt1-6) conservation and their differential gene expression in Schistosoma mansoni

The ubiquitin-proteasome system is responsible for degradation of the majority of intracellular proteins in eukaryotic cells. The 26S proteasome proteolytic complex is composed of a 20S core particle responsible for protein degradation and the 19S lid which plays a role in the recognition of polyubiquitinated substrates. The 19S regulatory particle (Rps) is composed of ATPase (Rpt) and non-ATPase (Rpn) subunits. In this study, we analyzed the expression profile of 19S Rpt subunits in the larvae and adult stage of the Schistosoma mansoni life cycle. Conventional reverse transcriptase polymerase chain reaction (RT-PCR) revealed that the majority of the 19S Rpt subunits amplified at the expected molecular masses for various investigated stages. In addition, SmRpt1, SmRpt2, and SmRpt6 transcript levels were increased in 3 h-cultured schistosomula and reasonably maintained until 5 h in culture, as revealed by qRT-PCR. Phylogenetic analysis of 19S Rpt subunits showed high structural conservation in comparison to other Rpt orthologues. The mRNA expression profile of 19S Rpt subunits did not correlate with 26S proteasome proteolytic activity as judged by a (14)C-casein-degrading assay, in the early cultured schistosomula. Taken together, these results revealed a differential expression profile for 19S Rpt subunits whose transcript levels could not be directly associated to 26S proteasome activity.

Autophagy in trypanosomatids

Autophagy is a ubiquitous eukaryotic process that also occurs in trypanosomatid parasites, protist organisms belonging to the supergroup Excavata, distinct from the supergroup Opistokontha that includes mammals and fungi. Half of the known yeast and mammalian AuTophaGy (ATG) proteins were detected in trypanosomatids, although with low sequence conservation. Trypanosomatids such as Trypanosoma brucei, Trypanosoma cruzi and Leishmania spp. are responsible for serious tropical diseases in humans. The parasites are transmitted by insects and, consequently, have a complicated life cycle during which they undergo dramatic morphological and metabolic transformations to adapt to the different environments. Autophagy plays a major role during these transformations. Since inhibition of autophagy affects the transformation, survival and/or virulence of the parasites, the ATGs offer promise for development of drugs against tropical diseases. Furthermore, various trypanocidal drugs have been shown to trigger autophagy-like processes in the parasites. It is inferred that autophagy is used by the parasites in an-not always successful-attempt to cope with the stress caused by the toxic compounds.

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.

Leishmania donovani: proteasome-mediated down-regulation of methionine adenosyltransferase

Methionine adenosyltransferase (MAT) is an important enzyme for metabolic processes, to the extent that its product, S-adenosylmethionine (AdoMet), plays a key role intrans-methylation,trans-sulphuration and polyamine synthesis. Previous studies have shown that a MAT-overexpressing strain ofLeishmania donovanicontrols AdoMet production, keeping the intracellular AdoMet concentration at levels that are compatible with cell survival. This unexpected result, together with the fact that MAT activity and abundance changed with time in culture, suggests that different regulatory mechanisms acting beyond the post-transcriptional level are controlling this protein. In order to gain an insight into these mechanisms, several experiments were carried out to explain the MAT abundance during promastigote cell growth. Determination of MAT turnover in cycloheximide (CHX)-treated cultures resulted in a surprising 5-fold increase in MAT turnover compared to CHX-untreated cultures. This increase agrees with a stabilization of the MAT protein, whose integrity was maintained during culture. The presence of proteasome inhibitors, namely MG-132, MG-115, epoxomycin and lactacystin in the culture medium prevented MAT degradation in both MAT-overexpressing and ‘mock-transfected’ leishmanial strains. The role of the ubiquitin (Ub) pathway in MAT down-regulation was supported using immunoprecipitation experiments. Immunoprecipitated MAT cross-reacted with anti-Ub antibodies, which provides evidence of a proteasome-mediated down-regulation of the leishmanial MAT abundance.

HIV Protease Inhibitors: Effect on the Opportunistic Protozoan Parasites

The impact of highly active antiretroviral therapy (HAART) in the natural history of AIDS disease has been allowed to prolong the survival of people with HIV infection, particularly whose with increased HIV viral load. Additionally, the antiretroviral therapy could exert a certain degree of protection against parasitic diseases. A number of studies have been evidenced a decrease in the incidence of opportunistic parasitic infections in the era of HAART. Although these changes have been attributed to the restoration of cell-mediated immunity, induced by either non-nucleoside reverse transcriptase inhibitors or HIV protease inhibitors, in combination with at least two nucleoside reverse transcriptase inhibitors included in HAART, there are evidence that the control of these parasitic infections in HIV-positive persons under HAART, is also induced by the inhibition of the proteases of the parasites. This review focuses on the principal available data related with therapeutic HIV-protease inhibitors and their in vitro and in vivo effects on the opportunistic protozoan parasites.

Cell homeostasis in a Leishmania major mutant overexpressing the spliced leader RNA is maintained by an increased proteolytic activity

Although several stage-specific genes have been identified in Leishmania, the molecular mechanisms governing developmental gene regulation in this organism are still not well understood. We have previously reported an attenuation of virulence in Leishmania major and L. braziliensis carrying extra-copies of the spliced leader RNA gene. Here, we surveyed the major differences in proteome and transcript expression profiles between the spliced leader RNA overexpressor and control lines using two-dimensional gel electrophoresis and differential display reverse transcription PCR, respectively. Thirty-nine genes related to stress response, cytoskeleton, proteolysis, cell cycle control and proliferation, energy generation, gene transcription, RNA processing and post-transcriptional regulation have abnormal patterns of expression in the spliced leader RNA overexpressor line. The evaluation of proteolytic pathways in the mutant revealed a selective increase of cysteine protease activity and an exacerbated ubiquitin-labeled protein population. Polysome profile analysis and measurement of cellular protein aggregates showed that protein translation in the spliced leader RNA overexpressor line is increased when compared to the control line. We found that L. major promastigotes maintain homeostasis in culture when challenged with a metabolic imbalance generated by spliced leader RNA surplus through modulation of intracellular proteolysis. However, this might interfere with a fine-tuned gene expression control necessary for the amastigote multiplication in the mammalian host.

HIV aspartyl peptidase inhibitors interfere with cellular proliferation, ultrastructure and macrophage infection of Leishmania amazonensis

BACKGROUND

Leishmania is the etiologic agent of leishmanisais, a protozoan disease whose pathogenic events are not well understood. Current therapy is suboptimal due to toxicity of the available therapeutic agents and the emergence of drug resistance. Compounding these problems is the increase in the number of cases of Leishmania-HIV coinfection, due to the overlap between the AIDS epidemic and leishmaniasis.

METHODOLOGY/PRINCIPAL FINDINGS

In the present report, we have investigated the effect of HIV aspartyl peptidase inhibitors (PIs) on the Leishmania amazonensis proliferation, ultrastructure, interaction with macrophage cells and expression of classical peptidases which are directly involved in the Leishmania pathogenesis. All the HIV PIs impaired parasite growth in a dose-dependent fashion, especially nelfinavir and lopinavir. HIV PIs treatment caused profound changes in the leishmania ultrastructure as shown by transmission electron microscopy, including cytoplasm shrinking, increase in the number of lipid inclusions and some cells presenting the nucleus closely wrapped by endoplasmic reticulum resembling an autophagic process, as well as chromatin condensation which is suggestive of apoptotic death. The hydrolysis of HIV peptidase substrate by L. amazonensis extract was inhibited by pepstatin and HIV PIs, suggesting that an aspartyl peptidase may be the intracellular target of the inhibitors. The treatment with HIV PIs of either the promastigote forms preceding the interaction with macrophage cells or the amastigote forms inside macrophages drastically reduced the association indexes. Despite all these beneficial effects, the HIV PIs induced an increase in the expression of cysteine peptidase b (cpb) and the metallopeptidase gp63, two well-known virulence factors expressed by Leishmania spp.

CONCLUSIONS/SIGNIFICANCE

In the face of leishmaniasis/HIV overlap, it is critical to further comprehend the sophisticated interplays among Leishmania, HIV and macrophages. In addition, there are many unresolved questions related to the management of Leishmania-HIV-coinfected patients. For instance, the efficacy of therapy aimed at controlling each pathogen in coinfected individuals remains largely undefined. The results presented herein add new in vitro insight into the wide spectrum efficacy of HIV PIs and suggest that additional studies about the synergistic effects of classical antileishmanial compounds and HIV PIs in macrophages coinfected with Leishmania and HIV-1 should be performed.

Intracellular survival of Leishmania species that cause visceral leishmaniasis is significantly reduced by HIV-1 protease inhibitors

Visceral leishmaniasis is now recognized as an opportunistic disease in individuals infected with human immunodeficiency virus type 1 (HIV-1). Although the usefulness of HIV-1 protease inhibitors (PIs) in antiretroviral regimens is well documented, little is known about their potential impact in the setting of Leishmania/HIV-1 coinfections. We now report that, although selected PIs do not inhibit the growth of Leishmania infantum promastigotes alone in culture, these drugs significantly inhibit the intracellular survival of parasites in phorbol myristate acetate-differentiated THP-1 macrophages and human primary monocyte-derived macrophages (MDMs). Furthermore, a field isolate of Leishmania donovani resistant to sodium stibogluconate (SbV), one of the drugs most commonly used to treat leishmaniasis, is equally susceptible to the tested PIs compared with a sensitive strain, thus suggesting that resistance to SbV does not result in cross-resistance to PIs. Importantly, the efficacy of PIs to reduce the intracellular growth of Leishmania parasites is also observed in MDMs coinfected with HIV-1.

Novel tetra-acridine derivatives as dual inhibitors of topoisomerase II and the human proteasome

Acridine derivatives, such as amsacrine, represent a well known class of multi-targeted anti-cancer agents that generally interfere with DNA synthesis and inhibit topoisomerase II. But in addition, these tricyclic molecules often display secondary effects on other biochemical pathways including protein metabolism. In order to identify novel anti-cancer drugs, we evaluated the mechanism of action of a novel series of bis- and tetra-acridines. As expected, these molecules were found to interact with DNA and inhibit the topoisomerase II-mediated DNA decatenation. Interestingly when tested on human tumour cells either sensitive (HL-60) or resistant (HL-60/MX2) to topoisomerase II inhibitors, these molecules proved equicytotoxic against the two cell lines, suggesting that they do not only rely on topoisomerase II inhibition to exert their cytotoxic effects. In order to identify alternative targets, we tested the capacity of acridines 1-9 to inhibit the proteasome machinery. Four tetra-acridines inhibited the proteasome in vitro, with IC(50) values up to 40 times lower than that of the reference proteasome inhibitor lactacystin. Moreover, unlike peptide aldehydes used as reference inhibitors for the proteasome, these new acridine compounds demonstrated a good selectivity towards the proteasome, when tested against four unrelated proteases. A cellular assay based on the degradation of a proteasome protein substrate indicated that at least two of the tetra-acridines maintained this proteasome inhibition activity in a cellular context. This is the first report of tetra-acridines that demonstrate dual topoisomerase II and proteasome inhibition properties. This new dual activity could represent a novel anti-cancer approach to circumvent certain forms of tumour resistance.

Phenotypical characteristics, biochemical pathways, molecular targets and putative role of nitric oxide-mediated programmed cell death in Leishmania

Nitric oxide (NO) has been demonstrated to be the principal effector molecule mediating intracellular killing of Leishmania, both in vitro and in vivo. We investigated the type of cell death process induced by NO for the intracellular amastigote stage of the protozoa Leishmania. Specific detection methods revealed a rapid and extensive cell death with morphological features of apoptosis in axenic amastigotes exposed to NO donors, in intracellular amastigotes inside in vitro - activated mouse macrophages and also in activated macrophages of regressive lesions in a leishmaniasis-resistant mouse model. We extended our investigations to the dog, a natural host-reservoir of Leishmania parasites, by demonstrating that co-incubation of infected macrophages with autologous lymphocytes derived from dogs immunised with purified excreted-secreted antigens of Leishmania resulted in a significant NO-mediated apoptotic cell death of intracellular amastigotes. From the biochemical point of view, NO-mediated Leishmania amastigotes apoptosis did not seem to be controlled by caspase activity as indicated by the lack of effect of cell permeable inhibitors of caspases and cysteine proteases, in contrast to specific proteasome inhibitors, such as lactacystin or calpain inhibitor I. Moreover, addition of the products of two NO molecular targets, cis-aconitase and glyceraldehyde-3-phosphate dehydrogenase, also had an inhibitory effect on the cell death induced by NO. Interestingly, activities of these two enzymes plus 6-phosphogluconate dehydrogenase, parasitic enzymes involved in both glycolysis and respiration processes, are overexpressed in amastigotes selected for their NO resistance. This review focuses on cell death of the intracellular stage of the pathogen Leishmania induced by nitrogen oxides and gives particular attention to the biochemical pathways and the molecular targets potentially involved. Questions about the role of Leishmania amastigotes NO-mediated apoptosis in the overall infection process are raised and discussed.

Protein turnover and differentiation in Leishmania

Leishmania occurs in several developmental forms and thus undergoes complex cell differentiation events during its life-cycle. Those are required to allow the parasite to adapt to the different environmental conditions. The sequencing of the genome of L. major has facilitated the identification of the parasite’s vast arsenal of proteolytic enzymes, a few of which have already been carefully studied and found to be important for the development and virulence of the parasite. This review focuses on these peptidases and their role in the cellular differentiation of Leishmania through their key involvement in a variety of degradative pathways in the lysosomal and autophagy networks.

Degradation of pteridine reductase 1 (PTR1) enzyme during growth phase in the protozoan parasite Leishmania donovani

Pteridine reductase 1 (PTR1) is an essential enzyme of pterin and folate metabolism in the protozoan parasite Leishmania. The present work is focused on the degradation of PTR1 during growth phase in Leishmania donovani. Western blot analysis with PTR1-GFP transfected promastigotes revealed that PTR1 protein was degraded in the stationary phase of growth at the time when the parasites were undergoing metacyclogenesis. Fluorescence microscopy revealed cytoplasmic localization of GFP tagged protein extending to the flagellum in these stationary phase promastigotes, implying that degradation of the protein was not by the usual multivesicular tubule lysosome (MVT) pathway. A probable destruction box of nine amino acids Q63ADLSNVAK71 and possible lysine residue K156 was identified in L. donovani PTR1 to be the site for ubiquitin conjugation. This suggests that PTR1 degradation during the stationary phase of growth is mediated by the proteasome. This leads to lower levels of H4-biopterin, which favors metacyclogenesis, and subsequently results in a highly infective stage of the parasite. Therefore, this finding has importance to identify new target molecule like the proteasome for therapeutic intervention.

DNA topoisomerase I from parasitic protozoa: A potential target for chemotherapy

The growing occurrence of drug resistant strains of unicellular prokaryotic parasites, along with insecticide-resistant vectors, are the factors contributing to the increased prevalence of tropical diseases in underdeveloped and developing countries, where they are endemic. Malaria, cryptosporidiosis, African and American trypanosomiasis and leishmaniasis threaten human beings, both for the high mortality rates involved and the economic loss resulting from morbidity. Due to the fact that effective immunoprophylaxis is not available at present; preventive sanitary measures and pharmacological approaches are the only sources to control the undesirable effects of such diseases. Current anti-parasitic chemotherapy is expensive, has undesirable side effects or, in many patients, is only marginally effective. Under this point of view molecular biology techniques and drug discovery must walk together in order to find new targets for chemotherapy intervention. The identification of DNA topoisomerases as a promising drug target is based on the clinical success of camptothecin derivatives as anticancer agents. The recent detection of substantial differences between trypanosome and leishmania DNA topoisomerase IB with respect to their homologues in mammals has provided a new lead in the study of the structural determinants that can be effectively targeted. The present report is an up to date review of the new findings on type IB DNA topoisomerase in unicellular parasites and the role of these enzymes as targets for therapeutic agents.

Cell cycle-dependent expression regulation by the proteasome pathway and characterization of the nuclear targeting signal of a Leishmania major Kin-13 kinesin

The LmjF01.0030 gene of Leishmania major Friedlin, annotated as 'MCAK-like', was confirmed as a kinesin with an internally located motor domain and termed LmjKIN13-1. Both the native form of the protein and a green fluorescent protein (GFP)-fused recombinant version were shown to be exclusively intranuclear, and, more specifically, to localize to the spindle and spindle poles. Cell cycle-dependent regulation of the protein levels was demonstrated using synchronized Leishmania cells: LmjKIN13-1 was highly abundant in the G2+M phase and present at very low levels after mitosis. Altogether, these features suggest that this protein participates in mitosis. The construction of systematic deletion mutants allowed the localization of the primary sequence regions responsible for nuclear targeting on the one hand, and for cell cycle-dependent variations on the other hand. A 42-amino-acid region of the carboxy(C)-terminal domain mediates nuclear import and could be defined as an atypical nuclear localization signal. Protein level regulation during the cell cycle was shown to also depend upon the C-terminal domain, where apparently redundant degradation signals are present. Putative degradation signals appear to be present on both sides and inside the nuclear localization signal. Further experiments strongly suggest a role for the ubiquitin/proteasome pathway in this cell cycle-dependent regulation. These data underline the importance of post-translational regulation of protein abundance in this ancestral eukaryote where transcriptional regulation seems to be rare or near absent.

Antileishmanial activity of HIV protease inhibitors

The proteasomes of some protozoa are possible targets for chemotherapy. Leishmaniasis is a major health problem in human immunodeficiency virus (HIV) co-infected subjects. Two HIV protease inhibitors (PI), indinavir and saquinavir, have been shown to block proteasome functions; we therefore investigated their effects on the growth of two Leishmania spp. (Leishmania major and Leishmania infantum). After 24 h of treatment, both drugs exhibited a dose-dependent antileishmanial activity, with 50% lethal dose (LD50) values of, respectively, 8.3 microM and 7 microM on L. major; minor activity was observed on L. infantum. These results add new in vitro insights into the wide-spectrum efficacy of PI and suggest studying their action on amastigote forms of leishmania within macrophages in order to validate their potential contribution against opportunistic infections in treated seropositive patients.

Schistosoma mansoni: functional proteasomes are required for development in the vertebrate host

Proteasomes are multi-subunit proteases involved in several mechanisms and thought to contribute to the regulation of cellular homeostasis. Here, we report for the first time biochemical evidence for the existence of a ubiquitin-proteasome proteolytic pathway in this parasite. Proteasomes from both cercariae and adult worms exhibited a high preference for hydrolysis of the substrate Suc-LLVY-AMC, although in the cercariae extract the rate of hydrolysis was 50% lower when compared to adult worms extracts. The same difference in proteasome activities was observed when endogenous proteins were broken down in the presence of ATP and ubiquitin. Additionally, accumulation of high molecular weight conjugates was observed when cercariae were pre-incubated with proteasome inhibitors. Finally, we present evidence that during experimental schistosomiasis, proteasome inhibitors were able to reduce the number of lung stage schistosomula, reduce the worm burden and consequently decrease the egg output in infected mice.

Proteomic analysis of Leishmania mexicana differentiation

We have resolved the proteome of axenically differentiated Leishmania mexicana parasites by two-dimensional gel electrophoresis (2DE), employing optimised, robust and reproducible procedures, and visualised (by silver staining) approximately 2000 protein species in each of three developmental stages: procyclic promastigotes, metacyclic promastigotes and amastigotes. This analysis has used homogeneous populations of these parasite stages, characterised according to their morphology, protease and nuclease activity profiles and expression of stage-specific antigens. Following comparison of the whole proteome profiles between stages, 47 spots were found to be stage-specific, while a further 100 spots changed in intensity during differentiation. The majority of "unique" spots were expressed during the infective stages of parasite differentiation, metacyclic promastigotes and amastigotes. CapLC-QTOF mass spectrometry has allowed the identification of 47 protein species to date, including a number which are only detected in the amastigote stage. Proteins identified are members of eight functionally related groupings, some of which are implicated in infectivity and host-parasite interactions.

The Leishmania chagasi proteasome: role in promastigotes growth and amastigotes survival within murine macrophages

Proteasomes are multisubunit proteases that exist universally among eukaryotes. They have multiple proteolytic activities and are believed to have important roles in regulating cell cycle, selective intracellular proteolysis, and antigen presentation. Here we have partially purified Leishmania chagasi proteasome. The L. chagasi proteasome rich fraction displayed the typical features of eukaryotic 20S proteasome complexes, being active towards peptidyl substrates with hydrophobic and acidic residues, and sensitive to the proteasome-specific inhibitor lactacystin. We have shown that lactacystin, or its active form clasto-lactacystin beta-lactone, but not E-64, blocks the in vitro growth of L. chagasi promastigotes, demonstrating that the interference with parasite growth is due to the lack of proteasome activity. Furthermore, pre-treatment of L. chagasi promastigotes with lactacystin did not prevent parasite entry in host cells, but markedly restricted its intracellular survival. These results demonstrate that intact parasite proteasome function is required for replication of L. chagasi and for amastigotes survival inside the vertebrate host cell.

A Leishmania donovani gene that confers accelerated recovery from stationary phase growth arrest

We have isolated a gene, LdGF1, from the protozoan parasite Leishmania donovani. Overexpression of this gene confers a strong selective advantage in liquid culture after stationary phase growth arrest. We could show that recombinant L. donovani or Leishmania major, when overexpressing LdGF1, recover faster from a stationary phase growth arrest than control parasite strains. While no advantage of LdGF1 overexpression could be observed in log phase cultures or after a hydroxyurea-induced S-phase growth arrest, recovery from a cell cycle arrest due to serum deprivation was faster in LdGF1-overexpressing strains. This was found to be due to an accelerated release from a G(1) cell cycle arrest. By contrast, in a BALB/c mouse infection system, overexpression of LdGF1 in L. major resulted in reduced virulence. We conclude that increased levels of LdGF1 are beneficiary during recovery from G(1) cell cycle arrest, but pose a disadvantage inside a mammalian host. These results are discussed in the context of the observed loss of virulence during in vitro passage of Leishmania parasites.

Characterization and role of protozoan parasite proteasomes

The proteasome, a large non-lysosomal multi-subunit protease complex, is ubiquitous in eukaryotic cells. In protozoan parasites, the proteasome is involved in cell differentiation and replication, and could therefore be a promising therapeutic target. This article reviews the present knowledge of proteasomes in protozoan parasites of medical importance such as Giardia, Entamoeba, Leishmania, Trypanosoma, Plasmodium and Toxoplasma spp.

Eubacterial HslV and HslU subunits homologs in primordial eukaryotes

ATP-dependent protease complexes are present in all three kingdoms of life, where they rid the cell of misfolded or damaged proteins and control the level of certain regulatory proteins. They include the proteasome in Eukaryotes, Archea, and Actinomycetales and the HslVU (ClpQY) complex in other eubacteria. We showed that genes homologous to eubacterial HslV (ClpQ) and HslU (ClpY) are present in the genome of trypanosomatid protozoa and are expressed. The features of the cDNAs indicated that bona fide trypanosomatid messengers had been cloned and ruled out bacterial contamination as the source of the material. The N-terminal microsequence of HslV from Leishmania infantum (Protozoa: Kinetoplastida) permitted the identification of the propeptide cleavage site and indicated that an active protease is present. High similarities (> or =57.5%) with the prototypical HslV and HslU from Escherichia coli and conservation of residues essential for biochemical activity suggested that a functional HslVU complex is present in trypanosomatid protozoa. The structure of the N-termini of HslV and HslU further suggested mitochondrial localization. Phylogenetic analysis indicated that HslV and HslU from trypanosomatids clustered with eubacterial homologs but did not point to any particular bacterial lineage. Because typical eukaryotic 20S proteasomes are present in trypanosomatids, we concluded that the eubacterial HslVU and the eukaryotic multicatalytic protease are simultaneously present in these organisms. To our knowledge this is the first report of a eubacterial HslVU complex in eukaryotes and, consequently, of the simultaneous occurrence of both a proteasome and HslVU in living cells.

Biological roles of proteases in parasitic protozoa

Proteases from a variety of protozoan parasites have been characterized at the molecular and cellular levels, and the many roles that proteases play in these organisms are coming into focus. Central roles have been proposed for proteases in diverse processes such as host cell invasion and egress, encystation, excystation, catabolism of host proteins, differentiation, cell cycle progression, cytoadherence, and both stimulation and evasion of host immune responses. Detailed structural and functional characterization of parasite proteases has led to novel insights into the workings of these fascinating catalytic machines. The possibility of developing selective inhibitors of key proteases of pathogenic parasites into novel chemotherapeutic strategies is being vigorously explored.

Identification of mRNA decapping activities and an ARE-regulated 3' to 5' exonuclease activity in trypanosome extracts

mRNA turnover is a regulated process that contributes to the steady state level of cytoplasmic mRNA. The amount of each mRNA determines, to a large extent, the amount of protein produced by that particular transcript. In trypanosomes, there is little transcriptional regulation; therefore, differential mRNA stability significantly contributes to mRNA levels in each stage of the parasite life cycle. To investigate the enzymatic activities that contribute to mRNA turnover, we developed a cell-free system for mRNA turnover using the trypanosome Leptomonas seymouri. We identified a decapping activity that removed m(7)GDP from mRNAs that contain an m(7)GpppN cap at their 5' end. In yeast, the release of m(7)GDP by the pyrophosphatase Dcp1p/Dcp2p is a rate-limiting step in mRNA turnover. A secondary enzymatic activity, similar to the human cap scavenger activity, was identified in the trypanosome extracts. Both the human and trypanosome scavenger activities generate m(7)GMP from short capped RNA and are inhibited by addition in trans of m(7)GpppG. A third enzymatic activity uncovered in the parasite extracts functioned as a 3' to 5' exonuclease. Importantly, this exonuclease activity was stimulated by an AU-rich element present in the RNA. In summary, the cell-free system has defined several RNA turnover steps that likely contribute to regulated mRNA decay in trypanosomes.

Cloning and characterization of a leucyl aminopeptidase from three pathogenic Leishmania species

Aminopeptidases are emerging as exciting novel drug targets and vaccine candidates in parasitic infections. In this study, we describe for the first time an aminopeptidase from three highly pathogenic Leishmania species. Intronless genes encoding a leucyl aminopeptidase (lap) were cloned from Leishmania amazonensis, Leishmania donovani, and Leishmania major, which encoded 60-kDa proteins that displayed homology to leucyl aminopeptidases from Gram-negative bacteria, plants, and mammals. The lap genes were present as a single copy in each genome, and lap mRNA was detected by reverse transcription-PCR in all life-cycle stages of L. amazonensis. Lap assembled into catalytically competent 360-kDa hexamers and demonstrated potent amidolytic activity against synthetic aminopeptidase substrates containing leucine, methionine, and cysteine residues, representing the most restricted substrate specificity of any leucyl aminopeptidase described to date. Optimal activity was observed against L-leucyl-7-amido-4-methylcoumarin (k(cat)/K(m) approximately 63 s(-1) x mm(-1)) with a pH optimum of 8.5. Leishmania Lap activity was inhibited by metal ion chelators and enhanced by divalent manganese, cobalt, and nickel cations, although only zinc was detected in the purified Lap by inductively coupled plasma atomic emission spectroscopy, indicating that zinc is the natural Lap cofactor. Activity was potently inhibited by bestatin and apstatin in a slow binding competitive fashion, with K(i)* values of 3 and 44 nm, respectively. Actinonin was a tight binding competitive inhibitor (K(i) approximately 1 nm), whereas arphamenine A (K(i) approximately 70 microm) and L-leucinol (K(i) approximately 100 microm) were non-tight binding competitive inhibitors. Lap was not secreted by Leishmania in vitro and was localized to the parasite cytosol.

Nitric oxide-mediated proteasome-dependent oligonucleosomal DNA fragmentation in Leishmania amazonensis amastigotes

Resistance to leishmanial infections depends on intracellular parasite killing by activated host macrophages through the L-arginine-nitric oxide (NO) metabolic pathway. Here we investigate the cell death process induced by NO for the intracellular protozoan Leishmania amazonensis. Exposure of amastigotes to moderate concentrations of NO-donating compounds (acidified sodium nitrite NaNO(2) or nitrosylated albumin) or to endogenous NO produced by lipopolysaccharide or gamma interferon treatment of infected macrophages resulted in a dramatic time-dependent cell death. The combined use of several standard DNA status analysis techniques (including electrophoresis ladder banding patterns, YOPRO-1 staining in flow cytofluorometry, and in situ recognition of DNA strand breaks by TUNEL [terminal deoxynucleotidyltransferase-mediated dUTP-biotin nick end labeling] assay) revealed a rapid and extensive fragmentation of nuclear DNA in both axenic and intracellular NO-treated amastigotes of L. amazonensis. Despite some similarities to apoptosis, the nuclease activation responsible for characteristic DNA degradation was not under the control of caspase activity as indicated by the lack of involvement of cell-permeable inhibitors of caspases and cysteine proteases. In contrast, exposure of NO-treated amastigotes with specific proteasome inhibitors, such as lactacystin or calpain inhibitor I, markedly reduced the induction of the NO-mediated apoptosis-like process. These data strongly suggest that NO-induced oligonucleosomal DNA fragmentation in Leishmania amastigotes is, at least in part, regulated by noncaspase proteases of the proteasome. The determination of biochemical pathways leading up to cell death might ultimately allow the identification of new therapeutic targets.