Cathepsin B-like cysteine proteinase-deficient mutants of Leishmania mexicana

Crystal structure of Leishmania mexicana cysteine protease B in complex with a high-affinity azadipeptide nitrile inhibitor

Leishmania mexicana is an obligate intracellular protozoan parasite that causes the cutaneous form of leishmaniasis affecting South America and Mexico. The cysteine protease LmCPB is essential for the virulence of the parasite and therefore, it is an appealing target for antiparasitic therapy. A library of nitrile-based cysteine protease inhibitors was screened against LmCPB to develop a treatment of cutaneous leishmaniasis. Several compounds are sufficiently high-affinity LmCPB inhibitors to serve both as starting points for drug discovery projects and as probes for target validation. A 1.4 Å X ray crystal structure, the first to be reported for LmCPB, was determined for the complex of this enzyme covalently bound to an azadipeptide nitrile ligand. Mapping the structure-activity relationships for LmCPB inhibition revealed superadditive effects for two pairs of structural transformations. Therefore, this work advances our understanding of azadipeptidyl and dipeptidyl nitrile structure-activity relationships for LmCPB structure-based inhibitor design. We also tested the same series of inhibitors on related cysteine proteases cathepsin L and Trypanosoma cruzi cruzain. The modulation of these mammalian and protozoan proteases represents a new framework for targeting papain-like cysteine proteases.

Detection of Metalloproteases and Cysteine Proteases RNA Transcripts of Leishmania (Leishmania) infantum in Ear Edge Skin of Naturally Infected Dogs

Leishmania spp. proteases have been proposed as virulence factors contributing to adaptive success these parasites to the mammalian hosts. Since these enzymes are poorly studied in naturally infected dogs, this work aims to show the differences in metalloprotease and cysteine proteases gene expression in ear edge skin of dogs naturally infected by Leishmania (Leishmania) infantum. A cohort of dogs (n = 20) naturally infected by L. (L.) infantum was clinically classified as asymptomatic, oligosymptomatic, and polysymptomatic and the parasite load range estimated. The analysis of proteases expression by RT-PCR in the ear edge skin was also assessed, suggesting more transcripts of proteases in cDNA samples from polysymptomatic dogs than oligosymptomatic and asymptomatic ones. Metalloprotease RT-PCR assays yielded products (202 bp) in all assessed cDNA dog samples. In contrast, cysteine proteases transcripts (227 bp) had shown to be better detected in cDNA samples of polysymptomatic dogs, compared with cDNA samples from asymptomatic and oligosymptomatic dogs. Predictive in silico assays suggested that secondary structures of metalloproteasee mRNAs can be more stable than cysteine proteases at the skin temperature of dogs. Evidence is presented that during natural infection of dogs by L. (L.) infantum, this parasite produces transcripts of metalloprotease and cysteine protease RNA in the skin from asymptomatic, oligosymptomatic, and polysymptomatic dogs.

Genetically modified live attenuated vaccine: A potential strategy to combat visceral leishmaniasis

Visceral leishmaniasis (VL) is caused by a protozoan parasite Leishmania donovani mainly influencing the population of tropical and subtropical regions across the globe. The arsenal of drugs available is limited, and prolonged use of such drugs makes parasite to become resistant. Therefore, it is very imperative to develop a safe, cost-effective and inexpensive vaccine against VL. Although in recent years, many strategies have been pursued by researchers, so far only some of the vaccine candidates reached for clinical trial and more than half of them are still in pipeline. There is now a broad consent among Leishmania researchers that the perseverance of parasite is very essential for eliciting a protective immune response and may perhaps be attained by live attenuated parasite vaccination. For making a live attenuated parasite, it is very essential to ensure that the parasite is deficient of virulence and should further study genetically modified parasites to perceive the mechanism of pathogenesis. So it is believed that in the near future, a complete understanding of the Leishmania genome will explore clear strategies to discover a novel vaccine. This review describes the need for a genetically modified live attenuated vaccine against VL, and obstacles associated with its development.

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.

Cysteine proteases in protozoan parasites

Cysteine proteases (CPs) play key roles in the pathogenesis of protozoan parasites, including cell/tissue penetration, hydrolysis of host or parasite proteins, autophagy, and evasion or modulation of the host immune response, making them attractive chemotherapeutic and vaccine targets. This review highlights current knowledge on clan CA cysteine proteases, the best-characterized group of cysteine proteases, from 7 protozoan organisms causing human diseases with significant impact: Entamoeba histolytica, Leishmania species (sp.), Trypanosoma brucei, T. cruzi, Cryptosporidium sp., Plasmodium sp., and Toxoplasma gondii. Clan CA proteases from three organisms (T. brucei, T. cruzi, and Plasmodium sp.) are well characterized as druggable targets based on in vitro and in vivo models. A number of candidate inhibitors are under development. CPs from these organisms and from other protozoan parasites should be further characterized to improve our understanding of their biological functions and identify novel targets for chemotherapy.

Leishmanicidal and Immunomodulatory Activities of the Palladacycle Complex DPPE 1.1, a Potential Candidate for Treatment of Cutaneous Leishmaniasis

The present study focused on the activity of the palladacycle complex DPPE 1.1 on Leishmania (Leishmania) amazonensis. Promastigotes of L. (L.) amazonensis were destroyed in vitro by nanomolar concentrations of DPPE 1.1, whereas intracellular amastigotes were killed at drug concentrations fivefold less toxic than those harmful to macrophages. L. (L.) amazonensis-infected BALB/c mice were treated by intralesional injection of DPPE 1.1. Animals treated with 3.5 and 7.0 mg/kg of DPPE 1.1 showed a significant decrease of foot lesion sizes and a parasite load reduction of 93 and 99%, respectively, when compared to untreated controls. Furthermore, DPPE 1.1 was non-toxic to treated animals. The cathepsin B activity of L. (L.) amazonensis amastigotes was inhibited by DPPE 1.1 as demonstrated spectrofluorometrically by use of a specific fluorogenic substrate. Analysis of T-cells populations in mice treated with DPPE 1.1 and untreated controls was performed by fluorescence-activated cell sorter (FACS). IFN-γ was measured in supernatants of lymphocytes from popliteal and inguinal lymph nodes isolated from treated and untreated mice and stimulated with L. (L.) amazonensis amastigotes extract and active TGF-β was evaluated in supernatants of foot lesions; both dosages were carried out by means of a double-sandwich ELISA assay. A significant increase of TCD4+ and TCD8+ lymphocytes and IFN-γ secretion was displayed in mice treated with DPPE 1.1 compared to untreated animals, whereas a significant reduction of active TGF-β was observed in treated mice. These findings open perspectives for further investment in DPPE 1.1 as an alternative option for the chemotherapy of cutaneous leishmaniasis.

Increasing in cysteine proteinase B expression and enzymatic activity during in vitro differentiation of Leishmania (Viannia) braziliensis: First evidence of modulation during morphological transition

Leishmania (Viannia) braziliensis presents adaptive protease-dependent mechanisms, as cysteine proteinases B (CPB). This study investigates the expression of three cpb gene isoforms and CPB enzymatic activity during the parasite differentiation. Relative expression levels of LbrM.08.0810 gene were assessed, exhibiting a higher quantity of transcripts in the logarithmic promastigotes phase than in the stationary promastigotes phase (>1.5 times). The cbp gene tends to decrease during acid pH shock and increases when the temperature rises (>1.3 times). LbrM.08.0820 and LbrM.08.0830 genes exhibited similar expression profiles to LbrM.08.0810 gene, with lower levels being observed overall. The proteolytic activity exhibits a gradual increase during the parasite's differentiation with low levels in samples of logarithmic promastigotes phase (3.2 ± 0.08 mmol min^-1 mg protein^-1) to a peak of activity after 72 h of incubation at 32 °C (4.2 ± 0.026 mmol min^-1 mg protein^-1) followed by a subsequent decrease of 68 % of peak activity levels after 96 h of incubation at 32 °C (2.8 ± 0.37 mmol min^-1 mg protein^-1). These activities were also measured in the presence of selective inhibitors for cysteine proteinases, such as Z-Phe-Phe-fluoromethyl ketone and trans-epoxysuccinyl-L-leucylamido(4-guanidino)butane, demonstrating their source as cathepsin-like proteinases. To the best of our knowledge, this report presents the first description of a modulation of cathepsin L-like expression during the L. (V.) braziliensis in vitro differentiation induced by acid pH and high temperature.

Novel cathepsin B and cathepsin B-like cysteine protease of Naegleria fowleri excretory-secretory proteins and their biochemical properties

Naegleria fowleri causes a lethal primary amoebic meningoencephalitis (PAM) in humans and experimental animals, which leads to death within 7-14 days. Cysteine proteases of parasites play key roles in nutrient uptake, excystment/encystment, host tissue invasion, and immune evasion. In this study, we cloned N. fowleri cathepsin B (nfcpb) and cathepsin B-like (nfcpb-L) genes from our cDNA library of N. fowleri. The full-length sequences of genes were 1,038 and 939 bp (encoded 345 and 313 amino acids), and molecular weights were 38.4 and 34 kDa, respectively. Also, nfcpb and nfcpb-L showed a 56 and 46 % identity to Naegleria gruberi cathepsin B and cathepsin B-like enzyme, respectively. Recombinant NfCPB (rNfCPB) and NfCPB-L (rNfCPB-L) proteins were expressed by the pEX5-NT/TOPO vector that was transformed into Escherichia coli BL21, and they showed 38.4 and 34 kDa bands on sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) and Western blot analysis using their respective antibodies. Proteolytic activity of refolded rNfCPB and rNfCPB-L was maximum at a pH of 4.5, and the most effective substrate was Z-LR-MCA. rNfCPB and rNfCPB-L showed proteolytic activity for several proteins such as IgA, IgG, IgM, collagen, fibronectin, hemoglobin, and albumin. These results suggested that NfCPB and NfCPB-L cysteine protease are important components of the N. fowleri ESP, and they may play important roles in host tissue invasion and immune evasion as pathogens that cause N. fowleri PAM.

Cathepsin B gene disruption induced Leishmania donovani proteome remodeling implies cathepsin B role in secretome regulation

Leishmania cysteine proteases are potential vaccine candidates and drug targets. To study the role of cathepsin B cysteine protease, we have generated and characterized cathepsin B null mutant L. donovani parasites. L. donovani cathepsin B null mutants grow normally in culture, but they show significantly attenuated virulence inside macrophages. Quantitative proteome profiling of wild type and null mutant parasites indicates cathepsin B disruption induced remodeling of L. donovani proteome. We identified 83 modulated proteins, of which 65 are decreased and 18 are increased in the null mutant parasites, and 66% (55/83) of the modulated proteins are L. donovani secreted proteins. Proteins involved in oxidation-reduction (trypanothione reductase, peroxidoxins, tryparedoxin, cytochromes) and translation (ribosomal proteins) are among those decreased in the null mutant parasites, and most of these proteins belong to the same complex network of proteins. Our results imply virulence role of cathepsin B via regulation of Leishmania secreted proteins.

Effect of the silencing of the Ehcp112 gene on the in vitro virulence of Entamoeba histolytica

Background

Entamoeba histolytica is an intestinal protozoan parasite that causes amoebiasis in humans, affecting up to 50 million people worldwide each year and causing 40,000 to 100,000 deaths annually. EhCP112 is a cysteine proteinase of E. histolytica able to disrupt cell monolayers and digest extracellular matrix proteins, it is secreted by trophozoites and it can be active in a wide range of temperature and pH. These characteristics have encouraged the use of EhCP112 in the design and production of possible vaccines against amoebiasis, obtaining promising results. Nevertheless, we have no conclusive information about the role of EhCP112 in the E. histolytica pathogenesis.

Methods

A set of three specific siRNA sequences were used to silence the Ehcp112 gene via the soaking system. Silencing was evaluated by Western blot using an antibody against the EhCP112 recombinant protein. Finally, we analyzed the protease activity, the phagocytosis rate and the ability to destroy MDCK cells of the EhCP112-silenced trophozoites.

Results

The highest silencing effect on EhCP112 was detected at 16 h of treatment; time enough to perform the in vitro virulence assays, which showed that EhCP112 silencing produces a significant reduction in cytolysis and phagocytosis of target cells, indicating the participation of this proteinase in these events.

Conclusions

EhCP112 is involved in the in vitro virulence of E. histolytica.

Proteinases as virulence factors in Leishmania spp. infection in mammals

Leishmania parasites cause human tegumentary and visceral infections that are commonly referred to as leishmaniasis. Despite the high incidence and prevalence of cases, leishmaniasis has been a neglected disease because it mainly affects developing countries. The data obtained from the analysis of patients' biological samples and from assays with animal models confirm the involvement of an array of the parasite's components in its survival inside the mammalian host. These components are classified as virulence factors. In this review, we focus on studies that have explored the role of proteinases as virulence factors that promote parasite survival and immune modulation in the mammalian host. Additionally, the direct involvement of proteinases from the host in lesion evolution is analyzed. The gathered data shows that both parasite and host proteinases are involved in the clinical manifestation of leishmaniasis. It is interesting to note that although the majority of the classes of proteinases are present in Leishmania spp., only cysteine-proteinases, metalloproteinases and, to a lesser scale, serine-proteinases have been adequately studied. Members from these classes have been implicated in tissue invasion, survival in macrophages and immune modulation by parasites. This review reinforces the importance of the parasite proteinases, which are interesting candidates for new chemo or immunotherapies, in the clinical manifestations of leishmaniasis.

In vitro and in vivo activity of a palladacycle complex on Leishmania (Leishmania) amazonensis

Background

Antitumor cyclopalladated complexes with low toxicity to laboratory animals have shown leishmanicidal effect. These findings stimulated us to test the leishmanicidal property of one palladacycle compound called DPPE 1.2 on Leishmania (Leishmania) amazonensis, an agent of simple and diffuse forms of cutaneous leishmaniasis in the Amazon region, Brazil.

Methodology/Principal Findings

Promastigotes of L. (L.) amazonensis and infected bone marrow-derived macrophages were treated with different concentrations of DPPE 1.2. In in vivo assays foot lesions of L. (L.) amazonensis-infected BALB/c mice were injected subcutaneously with DPPE 1.2 and control animals received either Glucantime or PBS. The effect of DPPE 1.2 on cathepsin B activity of L. (L.) amazonensis amastigotes was assayed spectrofluorometrically by use of fluorogenic substrates. The main findings were: 1) axenic L. (L.) amazonensis promastigotes were destroyed by nanomolar concentrations of DPPE 1.2 (IC50 = 2.13 nM); 2) intracellular parasites were killed by DPPE 1.2 (IC50 = 128.35 nM), and the drug displayed 10-fold less toxicity to macrophages (CC50 = 1,267 nM); 3) one month after intralesional injection of DPPE 1.2 infected BALB/c mice showed a significant decrease of foot lesion size and a reduction of 97% of parasite burdens when compared to controls that received PBS; 4) DPPE 1.2 inhibited the cysteine protease activity of L. (L.) amazonensis amastigotes and more significantly the cathepsin B activity.

Conclusions/Significance

The present results demonstrated that DPPE 1.2 can destroy L. (L.) amazonensis in vitro and in vivo at concentrations that are non toxic to the host. We believe these findings support the potential use of DPPE 1.2 as an alternative choice for the chemotherapy of leishmaniasis.

Leishmaniasis is an important public health problem with an estimated annual incidence of 1.5 million of new human cases of cutaneous leishmaniasis and 500,000 of visceral leishmaniasis. Treatment of the diseases is limited by toxicity and parasite resistance to the drugs currently in use, validating the need to develop new leishmanicidal compounds. We evaluated the killing by the palladacycle complex DPPE 1.2 of Leishmania (Leishmania) amazonensis, an agent of human cutaneous leishmaniasis in the Amazon region, Brazil. DPPE 1.2 destroyed promastigotes of L. (L.) amazonensis in vitro at nanomolar concentrations, whereas intracellular amastigotes were killed at drug concentrations 10-fold less toxic than those displayed to macrophages. L. (L.) amazonensis-infected BALB/c mice treated by intralesional injection of DPPE 1.2 exhibited a significant decrease of foot lesion sizes and a 97% reduction of parasite burdens when compared to untreated controls. Additional experiments indicated the inhibition of the cathepsin B activity of L. (L.) amazonensis amastigotes by DPPE 1.2. Further studies are needed to explore the potential of DPPE 1.2 as an additional option for the chemotherapy of leishmaniasis.

Cysteine peptidases of kinetoplastid parasites

We review Clan CA Family C1 peptidases of kinetoplastid parasites (Trypanosoma and Leishmania) with respect to biochemical and genetic diversity, genomic organization and stage-specificity and control of expression. We discuss their contributions to parasite metabolism, virulence and pathogenesis and modulation of the host's immune response. Their applications as vaccine candidates and diagnostic markers as well as their chemical and genetic validation as drug targets are also summarized.

Cysteine proteases as potential antigens in antiparasitic DNA vaccines

Cysteine proteases in parasites are potent inducers of vertebrate host immune responses and may under certain circumstances take part in the pathogen's immune evasion strategies. These capacities place these parasite molecules as interesting candidate antigens in antiparasitic vaccines for use in vertebrates. Parasite cysteine proteases are able to skew the Th1/Th2 profile in mammals towards a response which allows sustainable parasite burdens in the host. DNA vaccines are also able to skew the Th1/Th2 profile by different administration techniques and the use of cysteine proteases in these genetic immunizations open perspectives for manipulation of the host immune response towards higher protection.

Aziridine-2,3-dicarboxylate-based cysteine cathepsin inhibitors induce cell death in Leishmania major associated with accumulation of debris in autophagy-related lysosome-like vacuoles

The papain-like cysteine cathepsins expressed by Leishmania play a key role in the life cycle of these parasites, turning them into attractive targets for the development of new drugs. We previously demonstrated that two compounds of a series of peptidomimetic aziridine-2,3-dicarboxylate [Azi(OBn)(2)]-based inhibitors, Boc-(S)-Leu-(R)-Pro-(S,S)-Azi(OBn)(2) (compound 13b) and Boc-(R)-Leu-(S)-Pro-(S,S)-Azi(OBn)(2) (compound 13e), reduced the growth and viability of Leishmania major and the infection rate of macrophages while not showing cytotoxicity against host cells. In the present study, we characterized the mode of action of inhibitors 13b and 13e in L. major. Both compounds targeted leishmanial cathepsin B-like cysteine cathepsin cysteine proteinase C, as shown by fluorescence proteinase activity assays and active-site labeling with biotin-tagged inhibitors. Furthermore, compounds 13b and 13e were potent inducers of cell death in promastigotes, characterized by cell shrinkage, reduction of mitochondrial transmembrane potential, and increased DNA fragmentation. Transmission electron microscopic studies revealed the enrichment of undigested debris in lysosome-like organelles participating in micro- and macroautophagy-like processes. The release of digestive enzymes into the cytoplasm after rupture of membranes of lysosome-like vacuoles resulted in the significant digestion of intracellular compartments. However, the plasma membrane integrity of compound-treated promastigotes was maintained for several hours. Taken together, our results suggest that the induction of cell death in Leishmania by cysteine cathepsin inhibitors 13b and 13e is different from mammalian apoptosis and is caused by incomplete digestion in autophagy-related lysosome-like vacuoles.

Cathepsin B-like and cell death in the unicellular human pathogen Leishmania

In several studies reporting cell death (CD) in lower eukaryotes and in the human protozoan parasite Leishmania, proteolytic activity was revealed using pan-caspase substrates or inhibitors such as carbobenzoxy-valyl-alanyl-aspartyl-[O-methyl]-fluoromethylketone (Z-VAD-FMK). However, most of the lower eukaryotes do not encode caspase(s) but MCA, which differs from caspase(s) in its substrate specificity and cannot be accountable for the recognition of Z-VAD-FMK. In the present study, we were interested in identifying which enzyme was capturing the Z-VAD substrate. We show that heat shock (HS) induces Leishmania CD and leads to the intracellular binding of Z-VAD-FMK. We excluded binding and inhibition of Z-VAD-FMK to Leishmania major metacaspase (LmjMCA), and identified cysteine proteinase C (LmjCPC), a cathepsin B-like (CPC) enzyme, as the Z-VAD-FMK binding enzyme. We confirmed the specific interaction of Z-VAD-FMK with CPC by showing that Z-VAD binding is absent in a Leishmania mexicana strain in which the cpc gene was deleted. We also show that parasites exposed to various stress conditions release CPC into a soluble fraction. Finally, we confirmed the role of CPC in Leishmania CD by showing that, when exposed to the oxidizing agent hydrogen peroxide (H(2)O(2)), cpc knockout parasites survived better than wild-type parasites (WT). In conclusion, this study identified CPC as the substrate of Z-VAD-FMK in Leishmania and as a potential additional executioner protease in the CD cascade of Leishmania and possibly in other lower eukaryotes.

Dissimilar peptidase production by avirulent and virulent promastigotes of Leishmania braziliensis: inference on the parasite proliferation and interaction with macrophages

In the present paper, we have analysed the cellular and extracellular proteolytic activity profiles in 2 distinct Leishmania braziliensis strains: a recently isolated (virulent) and a laboratory-adapted (avirulent) strain. Quantitative and qualitative differences on the peptidase expression were observed in both strains. For instance, low-molecular mass acidic cysteine peptidase activities were detected exclusively in the virulent strain. Similarly, metallopeptidase activities were mainly produced by L. braziliensis virulent promastigotes. Interestingly, metallo- and cysteine peptidase activities were drastically reduced after several in vitro passages of the virulent strain. Western blotting, flow cytometry and fluorescence microscopy analyses were performed to detect homologous of the major leishmania metallopeptidase (gp63) and cysteine peptidase (cpb) in virulent and avirulent strains of L. braziliensis. Our results revealed that the virulent strain produced higher amounts of gp63 and cpb molecules, detected both in the surface and cytoplasm regions, than the avirulent counterpart. Metallo- (1,10-phenanthroline and EGTA) and cysteine peptidase (E-64) inhibitors arrested the growth of L. braziliensis virulent strain in a dose-dependent manner, as well as the association index with peritoneal murine macrophages. Conversely, these peptidase inhibitors did not affect either the proliferation or the cellular interaction of the avirulent strain. Corroborating these findings, the pre-treatment of the virulent strain with both anti-peptidase antibodies promoted a prominent reduction in the interaction with macrophages, while the association index of the avirulent strain to macrophage was only slightly diminished. Moreover, the spent culture medium from virulent strain significantly enhanced the association index between avirulent strain and macrophages, and this effect was reversed by 1,10-phenanthroline. Collectively, the results presented herein suggest that peptidases participate in several crucial processes of L. braziliensis.

Cysteine proteinase type III is protective against Leishmania infantum infection in BALB/c mice and highly antigenic in visceral leishmaniasis individuals

Visceral leishmaniasis is the most acute form of leishmaniasis and vaccination is the best approach to control it. One of the major groups of virulence factors in Leishmania belongs to cysteine proteinase family. In this study, for the first time, the protective potential of Leishmania infantum cysteine proteinase type III (CPC) by using a prime-boost strategy is evaluated in BALB/c mice. The experiment was carried out in three groups of mice. Vaccinated group was primed with pcDNA-cpc and boosted with rCPC-DHFR in combination with CpG motif and Montanide 720 as adjuvant. Control groups received pcDNA and rDHFR or PBS. The ratio of IgG2a/IgG1, nitric oxide concentration and IFN-gamma induction in vaccinated group is significantly higher than controls. Furthermore, the parasite load of vaccinated group is significantly lower than controls. In addition, sera reactivity of visceral leishmaniasis individuals was examined and showed considerable reactivities toward rCPC in comparison with cutaneous leishmaniasis. The achieved result is highly encouraging the use of cysteine proteinases types I, II and III as vaccine candidate against visceral leishmaniasis.

Transgenic Leishmania and the immune response to infection

Genetic manipulation of single-celled organisms such as the Leishmania parasite enables in depth analysis of the consequences of genotypic change on biological function. In probing the immune responses to infection, use of transgenic Leishmania has the potential to unravel both the contribution of the parasite to the infection process and the cellular interactions and mechanisms that characterize the innate and adaptive immune responses of the host. Here, we briefly review recent technical advances in parasite genetics and explore how these methods are being used to investigate parasite virulence factors, elucidate immune regulatory mechanisms and contribute to the development of novel therapeutics for the leishmaniases. Recent developments in imaging technology, such as bioluminescence and intravital imaging, combined with parasite transfection with fluorescent or enzyme-encoding marker genes, provides a rich opportunity for novel assessment of intimate, real-time host-parasite interactions at a previously unexplored level. Further advances in transgenic technology, such as the introduction of robust inducible gene cassettes for expression in intracellular parasite stages or the development of RNA interference methods for down-regulation of parasite gene expression in the host, will further advance our ability to probe host-parasite interactions and unravel disease-promoting mechanisms in the leishmaniases.

Molecular and biochemical characterization of a cathepsin B-like protease family unique to Trypanosoma congolense

Cysteine proteases have been shown to be essential virulence factors and drug targets in trypanosomatids and an attractive antidisease vaccine candidate for Trypanosoma congolense. Here, we describe an important amplification of genes encoding cathepsin B-like proteases unique to T. congolense. More than 13 different genes were identified, whereas only one or two highly homologous genes have been identified in other trypanosomatids. These proteases grouped into three evolutionary clusters: TcoCBc1 to TcoCBc5 and TcoCBc6, which possess the classical catalytic triad (Cys, His, and Asn), and TcoCBs7 to TcoCBs13, which contains an unusual catalytic site (Ser, Xaa, and Asn). Expression profiles showed that members of the TcoCBc1 to TcoCBc5 and the TcoCBs7 to TcoCBs13 groups are expressed mainly in bloodstream forms and localize in the lysosomal compartment. The expression of recombinant representatives of each group (TcoCB1, TcoCB6, and TcoCB12) as proenzymes showed that TcoCBc1 and TcoCBc6 are able to autocatalyze their maturation 21 and 31 residues, respectively, upstream of the predicted start of the catalytic domain. Both displayed a carboxydipeptidase function, while only TcoCBc1 behaved as an endopeptidase. TcoCBc1 exhibited biochemical differences regarding inhibitor sensitivity compared to that of other cathepsin B-like proteases. Recombinant pro-TcoCBs12 did not automature in vitro, and the pepsin-matured enzyme was inactive in tests with cathepsin B fluorogenic substrates. In vivo inhibition studies using CA074Me (a cell-permeable cathepsin B-specific inhibitor) demonstrated that TcoCB are involved in lysosomal protein degradation essential for survival in bloodstream form. Furthermore, TcoCBc1 elicited an important immune response in experimentally infected cattle. We propose this family of proteins as a potential therapeutic target and as a plausible antigen for T. congolense diagnosis.

Molecular genetic analysis of purine nucleobase transport in Leishmania major

Leishmania major and all other parasitic protozoa are unable to synthesize purines de novo and are therefore reliant upon uptake of preformed purines from their hosts via nucleobase and nucleoside transporters. L. major expresses two nucleobase permeases, NT3 that is a high affinity transporter for purine nucleobases and NT4 that is a low affinity transporter for adenine. nt3((-/-)) null mutant promastigotes were unable to replicate in medium containing 10 microM hypoxanthine, guanine, or xanthine and replicated slowly in 10 microM adenine due to residual low affinity uptake of that purine. The NT3 transporter mediated the uptake of the anti-leishmanial drug allopurinol, and the nt3((-/-)) mutants were resistant to killing by this drug. Expression of the NT3 permease was profoundly downregulated at the protein but not the mRNA level in stationary phase compared with logarithmic phase promastigotes. The nt4((-/-)) null mutant was quantitatively impaired in survival within murine bone marrow-derived macrophages. Extensive efforts to generate an nt3((-/-))/nt4((-/-)) dual null mutant were not successful, suggesting that one of the two nucleobase permeases must be retained for robust growth of the parasite. The phenotypes of these null mutants underscore the importance of purine nucleobase transporters in the Leishmania life cycle and pharmacology.

Studies on the CPA cysteine peptidase in the Leishmania infantum genome strain JPCM5

Background

Visceral leishmaniasis caused by members of the Leishmania donovani complex is often fatal in the absence of treatment. Research has been hampered by the lack of good laboratory models and tools for genetic manipulation. In this study, we have characterised a L. infantum line (JPCM5) that was isolated from a naturally infected dog and then cloned. We found that JPCM5 has attributes that make it an excellent laboratory model; different stages of the parasite life cycle can be studied in vitro, it is accessible to genetic manipulation and it has retained its virulence. Furthermore, the L. infantum JPCM5 genome has now been fully sequenced.

Results

We have further focused our studies on LiCPA, the L. infantum homologue to L. mexicana cysteine peptidase CPA. LiCPA was found to share a high percentage of amino acid identity with CPA proteins of other Leishmania species. Two independent LiCPA-deficient promastigote clones (ΔLicpa) were generated and their phenotype characterised. In contrast to L. mexicana CPA-deficient mutants, both clones of ΔLicpa were found to have significantly reduced virulence in vitro and in vivo. Re-expression of just one LiCPA allele (giving ΔLicpa::CPA) was sufficient to complement the reduced infectivity of both ΔLicpa mutants for human macrophages, which confirms the importance of LiCPA for L. infantum virulence. In contrast, in vivo experiments did not show any virulence recovery of the re-expressor clone ΔLicpaC1::CPA compared with the CPA-deficient mutant ΔLicpaC1.

Conclusion

The data suggest that CPA is not essential for replication of L. infantum promastigotes, but is important for the host-parasite interaction. Further studies will be necessary to elucidate the precise roles that LiCPA plays and why the re-expression of LiCPA in the ΔLicpa mutants complemented the gene deletion phenotype only in in vitro and not in in vivo infection of hamsters.

Leishmania species: evidence for transglutaminase activity and its role in parasite proliferation

Albeit transglutaminase (TGase) activity has been reported to play crucial physiological roles in several organisms including parasites; however, there was no previous report(s) whether Leishmania parasites exhibit this activity. We demonstrate herein that TGase is functionally active in Leishmania parasites by using labeled polyamine that becomes conjugated into protein substrates. The parasite enzyme was about 2- to 4-fold more abundant in Old World species than in New World ones. In L. amazonensis, comparable TGase activity was found in both promastigotes and amastigotes. TGase activity in either parasite stage was optimal at the basic pH, but the enzyme in amastigote lysates was more stable at higher temperatures (37-55 degrees C) than that in promastigote lysates. Leishmania TGase differs from mouse macrophage (M Phi) TGase in two ways: (1) the parasite enzyme is Ca(2+)-independent, whereas the mammalian TGase depends on the cation for activity, and (2) major protein substrates for L. amazonensis TGase were found within the 50-75 kDa region, while those for the M Phi TGase were located within 37-50 kDa. The potential contribution of TGase-catalyzed reactions in promastigote proliferation was supported by findings that standard inhibitors of TGase [e.g., monodansylcadaverine (MDC), cystamine (CS), and iodoacetamide (IodoA)], but not didansylcadaverine (DDC), a close analogue of MDC, had a profound dose-dependent inhibition on parasite growth. Myo-inositol-1-phosphate synthase and leishmanolysin (gp63) were identified as possible endogenous substrates for L. amazonensis TGase, implying a role for TGase in parasite growth, development, and survival.

Cloning and characterization of angiotensin converting enzyme related dipeptidylcarboxypeptidase from Leishmania donovani

We report the first identification, gene cloning, recombinant expression and biochemical characterization of an angiotensin converting enzyme (ACE) related dipeptidylcarboxypeptidase (DCP) in a protozoan parasite. The mammalian counterpart of this enzyme, peptidyl dipeptidase A (a carboxyl dipeptidase) also known as ACE leads to the cleavage of angiotensin I to produce a potent vasopressor. The catalytic enzyme activity of its Escherichia coli DCP counter part can be inhibited by the antihypertensive drug captopril, suggesting that this class of enzymes constitutes a novel target for drugs and vaccines. By utilizing a DNA microarray expression profiling approach, we identified a gene encoding a DCP enzyme for the kinetoplast protozoan Leishmania donovani (LdDCP) that was differentially expressed in promastigote and amastigote stages of the parasite life cycle. Both RNA and protein levels of LdDCP are higher in axenic amastigotes compared to promastigotes. Immuno-fluorescence analysis revealed the cytosolic expression of the protein. Primary structure analysis of LdDCP revealed the presence of an active Zn binding site. When expressed in E. coli, the recombinant enzyme showed carboxy-dipeptidase activity with synthetic substrates. Replacement of two histidine and one glutamic acid at positions 466, 470 and 467, respectively, with alanine residues in its active site resulted in loss of enzyme activity. Captopril, an ACE specific inhibitor was able both to reduce significantly LdDCP enzyme activity and to inhibit promastigote growth. Both its cytosolic location and close homology to DCPs from bacterial species suggests a role in parasite nutrition. Further, identification of LdDCP now provides an opportunity to investigate Leishmania peptidases for their potential as drug and vaccine targets.

EhCP112 is an Entamoeba histolytica secreted cysteine protease that may be involved in the parasite-virulence

EhCP112 is an Entamoeba histolytica protease that together with the EhADH112 protein forms the EhCPADH complex involved in trophozoite virulence. Here, we produced the recombinant EhCP112 and studied its relationships with extracellular matrix components and with target cells. A DNA fragment containing the pro-peptide and the mature enzyme was expressed in bacteria as an active enzyme (rEhCP112), whereas the full gene containing the signal peptide, the pro-peptide and the mature enzyme expressed a non-active protein. The fragment only with the mature enzyme was not expressed. rEhCP112 purified by affinity columns digested azocasein and had a strong autoproteolytic activity. Four hours after purification the protein appeared degraded. Anti-tag antibodies, monoclonal antibodies against the EhCP112 and sera from human patients with amoebiasis recognized rEhCP112. rEhCP112 digested gelatin, collagen type I, fibronectin and haemoglobin; it destroyed MDCK cell monolayers and bound to red blood cells. The native EhCP112 was poorly expressed in a virulence-deficient mutant, and in the wild-type clone it was located in secreted vesicles, forming the EhCPADH complex. Altogether these results show that EhCP112 is a molecule able to disrupt cell monolayers and digest proteins of the extracellular matrix and haemoglobin, and it is secreted by the trophozoites.

Immunopathogenesis of infection with the visceralizing Leishmania species

Human leishmaniasis is a spectral disease that includes asymptomatic self-resolving infection, localized skin lesions, and progressive visceral leishmaniasis. With some overlap, visceral and cutaneous leishmaniasis are usually caused by different species of Leishmania. This review focuses on host responses to infection with the species that cause visceral leishmaniasis, as they contrast with species causing localized cutaneous leishmaniasis. Data from experimental models document significant differences between host responses to organisms causing these diverse syndromes. The visceralizing Leishmania spp. cause localized organ-specific immune responses that are important determinants of disease outcome. Both the Leishmania species causing cutaneous and those causing visceral leishmaniasis require a Type 1 immune response to undergo cure in mouse models. However, during progressive murine infection with the visceralizing Leishmania sp., the Type 1 response is suppressed at least in part by TGF-beta and IL-10 without type 2 cytokine production. This contrasts with the cutaneous species L. major, in which a Type 2 response suppresses type 1 cytokines and leads to murine disease progression. Population and family studies are beginning to elucidate human genetic determinants predisposing to different outcomes of Leishmania infection. These studies should eventually result in a better understanding of the immunopathogenesis and the spectrum of human leishmaniasis.

Carboxydipeptidase activities of recombinant cysteine peptidases

The recombinant cysteine peptidases, cruzain from Trypanosoma cruzi and CPB2.8DeltaCTE from Leishmania mexicana, are cathepsin L-like and characteristically endopeptidases. In this study, we characterized the carboxydipeptidase activities of these enzymes and compared them with those of human recombinant cathepsin B and cathepsin L. The analysis used the internally quenched fluorescent peptide Abz-FRFK*-OH and some of its analogues, where Abz is ortho-aminobenzoic acid and K* is (2,4-dinitrophenyl)-epsilon-NH2-lysine. These peptides were demonstrated to be very sensitive substrates, due to the strong quenching effect of K* on the fluorescence of the Abz group. The carboxydipeptidase activity of cruzain was shown to be very similar to that of cathepsin B, while that of CPB2.8DeltaCTE is closer to the carboxydipeptidase activity of cathepsin L. The S2 subsite architecture of cruzain and the nature of the amino acid at the P2 position of the substrates determine its carboxydipeptidase activity and gives further and direct support to the notion that the carboxydipeptidase activity of the papain family cysteine peptidases rely on the S2-P2 interaction [Nägler D. K., Tam, W., Storer, A.C., Krupa, J.C., Mort, J.S. & Menard, R. (1999) Biochemistry38, 4868-4874]. Cruzain and CPB2.8DeltaCTE presented a broad pH-range for both the endo- and exo-peptidase activities, although the later is approximately one order of magnitude lower. This feature, that is not common in related mammalian cysteine peptidases, is consistent with the enzymes being exposed to different environmental conditions and having different locations during parasite development.

Functional conservation of a natural cysteine peptidase inhibitor in protozoan and bacterial pathogens

Cysteine peptidase inhibitor genes (ICP) of the chagasin family have been identified in protozoan (Leishmania mexicana and Trypanosoma brucei) and bacterial (Pseudomonas aeruginosa) pathogens. The encoded proteins have low sequence identities with each other and no significant identity with cystatins or other known cysteine peptidase inhibitors. Recombinant forms of each ICP inhibit protozoan and mammalian clan CA, family C1 cysteine peptidases but do not inhibit the clan CD cysteine peptidase caspase 3, the serine peptidase trypsin or the aspartic peptidases pepsin and thrombin. The functional homology between ICPs implies a common evolutionary origin for these bacterial and protozoal proteins.

Genetic characterization of glucose transporter function in Leishmania mexicana

Both insect and mammalian life cycle stages of Leishmania mexicana take up glucose and express all three isoforms encoded by the LmGT glucose transporter gene family. To evaluate glucose transporter function in intact parasites, a null mutant line has been created by targeted disruption of the LmGT locus that encompasses the LmGT1, LmGT2, and LmGT3 genes. This deltalmgt null mutant exhibited no detectable glucose transport activity. The growth rate of the deltalmgt knockout in the promastigote stage was reduced to a rate comparable with that of WT cells grown in the absence of glucose. deltalmgt cells also exhibited dramatically reduced infectivity to macrophages, demonstrating that expression of LmGT isoforms is essential for viability of amastigotes. Furthermore, WT L. mexicana were not able to grow as axenic culture form amastigotes if glucose was withdrawn from the medium, implying that glucose is an essential nutrient in this life cycle stage. Expression of either LmGT2 or LmGT3, but not of LmGT1, in deltalmgt null mutants significantly restored growth as promastigotes, but only LmGT3 expression substantially rescued amastigote growth in macrophages. Subcellular localization of the three isoforms was investigated in deltalmgt cells expressing individual LmGT isoforms. Using anti-LmGT antiserum and GFP-tagged LmGT fusion proteins, LmGT2 and LmGT3 were localized to the cell body, whereas LmGT1 was localized specifically to the flagellum. These results establish that each glucose transporter isoform has distinct biological functions in the parasite.

Activation of TGF-beta by Leishmania chagasi: importance for parasite survival in macrophages

TGF-beta is a potent regulatory cytokine that suppresses expression of inducible NO synthase and IFN-gamma, and suppresses Th1 and Th2 cell development. We examined whether functionally active TGF-beta is present in the local environment surrounding the invading protozoan Leishmania chagasi. Our prior data showed that TGF-beta levels are significantly increased in L. chagasi-infected mice. In the current study, we found TGF-beta was also abundant in bone marrows of humans with acute visceral leishmaniasis but not in those of uninfected controls. Furthermore, L. chagasi infection caused an increase in biologically active TGF-beta in human macrophage cultures without changing the total TGF-beta. Therefore, we investigated the means through which leishmania could augment activated but not total TGF-beta. Incubation of latent TGF-beta with Leishmania sp. promastigotes caused active TGF-beta to be released from the latent complex. In contrast, the nonpathogenic protozoan Crithidia fasciculata could not activate TGF-beta. TGF-beta activation by leishmania was prevented by inhibitors of cysteine proteases and by the specific cathepsin B inhibitor CA074. Physiologic concentrations of TGF-beta inhibited killing of intracellular L. chagasi in macrophages, although the phagocytosis-induced respiratory burst remained intact. In contrast, supraphysiologic concentrations of TGF-beta had no effect on parasite survival. We hypothesize that the combined effect of abundant TGF-beta stores at extracellular sites during infection, and the ability of the parasite to activate TGF-beta in its local environment, leads to high levels of active TGF-beta in the vicinity of the infected macrophage. Locally activated TGF-beta could, in turn, enhance parasite survival through its effects on innate and adaptive immune responses.

The Leishmania mexicana cysteine protease, CPB2.8, induces potent Th2 responses

We have previously identified that Leishmania mexicana cysteine proteases (CPs) are virulence factors. We have now produced a recombinant L. mexicana CP, CPB2.8, which has similar enzymatic activity to native enzyme. Inoculation of CPB2.8 (< or =5 microg) into the footpads of BALB/c mice not only up-regulated mRNA transcripts for IL-4 and IL-4 production in the draining popliteal lymph nodes, but also polarized splenocyte anti-CD3 stimulated responses toward a Th2 bias as measured by increased IL-5 production compared with controls. In agreement with promoting a Th2 response, CPB2.8 also induced strong specific IgE responses in treated mice as well as increasing whole IgE levels. Inhibition of the enzyme activity of CPB2.8 by treatment with E-64 ablated the enzyme's ability to induce IgE. Significantly, infection of mice with CPB-deficient parasites failed to stimulate production of IgE, unlike infection with wild-type parasites. Furthermore, enzymatically active (<0.1 U/ml) but not E-64-inactivated CPB2.8 was able to proteolytically cleave CD23 and CD25, although not B220 or CD4 from murine lymphocytes. These properties are similar to those demonstrated by the house dust mite allergen Der p I and provide an explanation for the immunomodulatory activity of the CPB2.8 virulence factor. Vaccination with CPB2.8 enhanced L. mexicana lesion growth compared with control animals. Nevertheless, vaccination with IL-12 and CPB2.8 resulted in a degree of protection associated with inhibition of lesion growth and a Th1 response. Thus, CPB2.8 is a potent Th2-inducing molecule capable of significant vaccine potential if administered with a suitable adjuvant.

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.

Cell death in Leishmania induced by stress and differentiation: programmed cell death or necrosis?

Unicellular organisms, such as the protozoan parasite Leishmania, can be stimulated to show some morphological and biochemical features characteristic of mammalian apoptosis. This study demonstrates that under a variety of stress conditions such as serum deprivation, heat shock and nitric oxide, cell death can be induced leading to genomic DNA fragmentation into oligonucleosomes. DNA fragmentation was observed, without induction, in the infectious stages of the parasite, and correlated with the presence of internucleosomal nuclease activity, visualisation of 45 to 59 kDa nucleases and detection of TUNEL-positive nuclei. DNA fragmentation was not dependent on active effector downstream caspases nor on the lysosomal cathepsin L-like enzymes CPA and CPB. These data are consistent with the presence of a caspase-independent cell death mechanism in Leishmania, induced by stress and differentiation that differs significantly from metazoa.

Functional analysis of cathepsin B-like cysteine proteases from Leishmania donovani complex. Evidence for the activation of latent transforming growth factor beta

Cathepsin B-like genes from Leishmania donovani and Leishmania chagasi have been isolated and characterized. It is a single gene, which is constitutively expressed in all the life cycle stages of the parasite. Studies using cathepsin B-specific inhibitor treatment suggested that cathepsin B does not seem to play a role in the promastigote stages of the parasite, however it aids in the parasite survival within the host macrophages. Antisense mRNA inhibition of cathepsin B gene also revealed that it plays an important role in the parasite survival within the host macrophages. Furthermore, for the first time, we have shown that Leishmania whole cell lysates as well as the recombinant cathepsin B protein cleaved human recombinant latent transforming growth factor (TGF)-beta1 into a mature peptide releasing the latency associated protein, in a cell-free incubation system. Mink lung epithelial cell growth inhibition assay revealed that the cleaved TGF-beta1 was biologically active, suggesting that Leishmania cathepsin B can cleave latent TGF-beta1 into mature and active form. These results suggest that cathepsin B plays an important role in Leishmania survival within the host macrophages by activating latent TGF-beta1.

Successful therapy of lethal murine visceral leishmaniasis with cystatin involves up-regulation of nitric oxide and a favorable T cell response

The virulence of Leishmania donovani in mammals depends at least in part on cysteine proteases because they play a key role in CD4(+) T cell differentiation. A 6-fold increase in NO production was observed with 0.5 microM chicken cystatin, a natural cysteine protease inhibitor, in IFN-gamma-activated macrophages. In a 45-day BALB/c mouse model of visceral leishmaniasis, complete elimination of spleen parasite burden was achieved by cystatin in synergistic activation with a suboptimal dose of IFN-gamma. In contrast to the case with promastigotes, cystatin and IFN-gamma inhibited the growth of amastigotes in macrophages. Although in vitro cystatin treatment of macrophages did not induce any NO generation, significantly enhanced amounts of NO were generated by macrophages of cystatin-treated animals. Their splenocytes secreted soluble factors required for the induction of NO biosynthesis, and the increased NO production was paralleled by a concomitant increase in antileishmanial activity. Moreover, splenocyte supernatants treated with anti-IFN-gamma or anti-TNF-alpha Abs suppressed inducible NO generation, whereas i.v. administration of these anticytokine Abs along with combined therapy reversed protection against infection. mRNA expression and flow cytometric analysis of infected spleen cells suggested that cystatin and IFN-gamma treatment, in addition to greatly reducing parasite numbers, resulted in reduced levels of IL-4 but increased levels of IL-12 and inducible NO synthase. Not only was this treatment curative when administered 15 days postinfection, but it also imparted resistance to reinfection. These studies provide a promising alternative for protection against leishmaniasis with a switch of CD4(+) differentiation from Th2 to Th1, indicative of long-term resistance.

Stable transformation of trypanosomatids through targeted chromosomal integration of the selectable marker gene encoding blasticidin S deaminase

The susceptibilities of the protozoan parasites Leishmania mexicana and Trypanosoma brucei to the nucleoside antibiotic blasticidin S were assessed. A concentration of 10 microg ml(-1) was sufficient to cause cell death within 72 h of L. mexicana promastigotes and bloodstream forms of T. brucei in vitro. The gene encoding blasticidin S deaminase (BSD) was therefore incorporated into cassettes for targeting to the cysteine proteinase C locus of L. mexicana (CPC::BSD) and the tubulin locus of T. brucei (tub::RAD51-BSR). Following transfection of mutant parasites that contained other well-established selectable marker genes (HYG, NEO, BLE, PAC and SAT), clones resistant to 10 microg ml(-1) blasticidin S were shown by PCR and Southern blotting to have integrated the cassettes by homologous recombination. The results confirm that BSD can be used as a selectable marker gene for targeted chromosomal integration during genetic manipulations of trypanosomatids.

Cysteine protease inhibitors as chemotherapy: lessons from a parasite target

Papain family cysteine proteases are key factors in the pathogenesis of cancer invasion, arthritis, osteoporosis, and microbial infections. Targeting this enzyme family is therefore one strategy in the development of new chemotherapy for a number of diseases. Little is known, however, about the efficacy, selectivity, and safety of cysteine protease inhibitors in cell culture or in vivo. We now report that specific cysteine protease inhibitors kill Leishmania parasites in vitro, at concentrations that do not overtly affect mammalian host cells. Inhibition of Leishmania cysteine protease activity was accompanied by defects in the parasite's lysosome/endosome compartment resembling those seen in lysosomal storage diseases. Colocalization of anti-protease antibodies with biotinylated surface proteins and accumulation of undigested debris and protease in the flagellar pocket of treated parasites were consistent with a pathway of protease trafficking from flagellar pocket to the lysosome/endosome compartment. The inhibitors were sufficiently absorbed and stable in vivo to ameliorate the pathology associated with a mouse model of Leishmania infection.

The Leishmania mexicana proteasome

As a start to understanding the importance of intracellular proteolysis in the protozoon Leishmania mexicana, the parasite proteasome has been purified and characterised. The L. mexicana proteasome is similar to proteasomes from other eukaryotes. It is soluble, and the 20S form has a mass of around 670 kDa, composed of at least 10 distinct subunits in the 22 to 32 kDa size range. The molecular mass of the L. mexicana proteasome increases to 1200 kDa in the presence of adenosine-5'-triphosphate, consistent with there being a 26S proteasome in the parasite. The purified 20S proteasome has activity towards substrates with hydrophobic, basic and acidic P, residues, and is sensitive to a range of peptide aldehyde inhibitors, as well as the proteasome-specific inhibitor lactacystin. The peptide aldehydes are able to arrest parasite growth in vitro with the same relative effectiveness as against the purified proteasome activity. The parasite population arrests with an increased 4N DNA content, indicating that, in part, the essential nature of the proteasome for L. mexicana proliferation is due to a role in the parasite cell cycle. Surprisingly, lactacystin is a relatively inefficient inhibitor of L. mexicana growth in vitro.

The lysosomal cysteine proteases

A significant number of exciting papain-like cysteine protease structures have been determined by crystallographic methods over the last several years. This trove of data allows for an analysis of the structural features that empower these molecules as they efficiently carry out their specialized tasks. Although the structure of the paradigm for the family, papain, has been known for twenty years, recent efforts have reaped several structures of specialized mammalian enzymes. This review first covers the commonalities of architecture and purpose of the papain-like cysteine proteases. From that broad platform, each of the lysosomal enzymes for which there is an X-ray structure (or structures) is then examined to gain an understanding of what structural features are used to customize specificity and activity. Structure-based design of inhibitors to control pathological cysteine protease activity will also be addressed.

Leishmania: overexpression and comparative structural analysis of the stage-regulated meta 1 gene

The meta 1 gene of Leishmania major is upregulated in metacyclic promastigotes and encodes an 11.5-kDa protein with no significant similarities to other proteins in the existing databases. In this paper, we characterize the homologous meta 1 genes in L. amazonensis and L. donovani. Proteins encoded by this gene in all three species present a high degree of identity. The meta 1 gene cannot be replaced by gene targeting in L. major, suggesting an essential role for the protein, at least in promastigotes. Overexpression of the meta 1 protein in L. amazonensis generates parasites that are more virulent than wild-type organisms in vivo.

Proteases of protozoan parasites

Proteolytic enzymes seem to play important roles in the life cycles of all medically important protozoan parasites, including the organisms that cause malaria, trypanosomiasis, leishmaniasis, amebiasis, toxoplasmosis, giardiasis, cryptosporidiosis and trichomoniasis. Proteases from all four major proteolytic classes are utilized by protozoans for diverse functions, including the invasion of host cells and tissues, the degradation of mediators of the immune response and the hydrolysis of host proteins for nutritional purposes. The biochemical and molecular characterization of protozoan proteases is providing tools to improve our understanding of the functions of these enzymes. In addition, studies in multiple systems suggest that inhibitors of protozoan proteases have potent antiparasitic effects. This review will discuss recent advances in the identification and characterization of protozoan proteases, in the determination of the function of these enzymes, and in the evaluation of protease inhibitors as potential antiprotozoan drugs.

Cysteine protease inhibitors as chemotherapy for parasitic infections

Analysis of the evolution, localization and biologic function of papain family cysteine proteases in metazoan and protozoan parasites has provided important and often surprising insights into the biochemistry and cellular function of this diverse enzyme family. Furthermore, the relative lack of redundancy of cysteine proteases in parasites compared to their mammalian hosts makes them attractive targets for the development of new antiparasitic chemotherapy. The treatment of experimental models of parasitic diseases with cysteine protease inhibitors has provided an important 'proof of concept' for the use of cysteine protease inhibitors in vivo. Evidence has now accumulated that cysteine protease inhibitors can selectively arrest replication of a microbial pathogen without untoward toxicity to the host. Furthermore, this can be achieved with reasonable dosing schedules and oral administration of the drug. Initial studies have confirmed the efficacy of cysteine protease inhibitors in treatment of Trypanosoma cruzi, Plasmodium falciparum and Leishmania major. Work on Trypanosoma brucei, the agent of African trypanosomiasis, is preliminary but also promising. Target validation studies have shown that biotinylated or radiolabeled irreversible inhibitors specifically bind to the cysteine protease targets thought to represent the major activity within the parasite. In the case of T. cruzi, the effect of inhibitors appears to be predominantly in blocking protease processing. Transfection studies using variant constructs have supported this model. Finally, the generation of null mutants for the multiple protease genes in Leishmania mexicana has provided the first genetic support for the key role of this enzyme family in parasite virulence. Safety studies in rodents and analysis of uptake of inhibitors by parasites and host cells suggest that the selectivity of inhibitors for the parasite targets may reside in the lack of redundancy of parasite proteases, the higher concentration of host proteases in intracellular compartments, and differential uptake of inhibitors by parasites. Attempts to elicit resistance to cysteine protease inhibitors in parasite cultures suggest that mechanisms of induced resistance are independent of resistance to the traditional antiparasitic agents. This suggests that cysteine protease inhibitors may provide an alternative to traditional therapy in drug-resistant organisms.

Recent advances in identifying and validating drug targets in trypanosomes and leishmanias

The unique aspects of the biochemistry of trypanosomatids make rational drug design an attractive approach, but targets must be selected carefully. Genetic manipulation provides a valuable means of mimicking loss of function attributable to therapeutic intervention, but caution must be exercised when interpreting such data with respect to target validation.