Altered expression of cruzipain and a cathepsin B-like target in a Trypanosoma cruzi cell line displaying resistance to synthetic inhibitors of cysteine-proteinases

Update on relevant trypanosome peptidases: Validated targets and future challenges

Trypanosoma cruzi, the agent of the American Trypanosomiasis, Chagas disease, and Trypanosoma brucei gambiense and Trypanosoma brucei rhodesiense, the agents of Sleeping sickness (Human African Trypanosomiasis, HAT), as well as Trypanosoma brucei brucei, the agent of the cattle disease nagana, contain cysteine, serine, threonine, aspartyl and metallo peptidases. The most abundant among these enzymes are the cysteine proteases from the Clan CA, the Cathepsin L-like cruzipain and rhodesain, and the Cathepsin B-like enzymes, which have essential roles in the parasites and thus are potential targets for chemotherapy. In addition, several other proteases, present in one or both parasites, have been characterized, and some of them are also promising candidates for the developing of new drugs. Recently, new inhibitors, with good selectivity for the parasite proteasomes, have been described and are very promising as lead compounds for the development of new therapies for these neglected diseases. This article is part of a Special Issue entitled: "Play and interplay of proteases in health and disease".

Calpains of Leishmania braziliensis: genome analysis, differential expression, and functional analysis

BACKGROUND

Calpains are proteins belonging to the multi-gene family of calcium-dependent cysteine peptidases that undergo tight on/off regulation, and uncontrolled proteolysis of calpains is associated with severe human pathologies. Calpain orthologues are expanded and diversified in the trypanosomatids genome.

OBJECTIVES

Here, we characterised calpains in Leishmania braziliensis, the main causative agent of cutaneous leishmaniasis in Brazil.

METHODS/FINDINGS

In total, 34 predicted calpain-like genes were identified. After domain structure evaluation, reverse transcription-quantitative polymerase chain reaction (RT-qPCR) during in vitro metacyclogenesis revealed (i) five genes with enhanced expression in the procyclic stage, (ii) one augmented gene in the metacyclic stage, and (iii) one procyclic-exclusive transcript. Western blot analysis revealed that an antibody against a consensus-conserved peptide reacted with multiple calpain-like proteins, which is consistent with the multi-gene family characteristic. Flow cytometry and immunocytochemistry analyses revealed the presence of calpain-like molecules mainly in the cytoplasm, to a lesser extent in the plasma membrane, and negligible levels in the nucleus, which are all consistent with calpain localisation. Eventually, the calpain inhibitor MDL28170 was used for functional studies revealing (i) a leishmaniostatic effect, (ii) a reduction in the association index in mouse macrophages, (iii) ultra-structural alterations conceivable with autophagy, and (iv) an enhanced expression of the virulence factor GP63.

CONCLUSION

This report adds novel insights into the domain structure, expression, and localisation of L. braziliensis calpain-like molecules.

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.

In vitro selection of Phytomonas serpens cells resistant to the calpain inhibitor MDL28170: alterations in fitness and expression of the major peptidases and efflux pumps

The species Phytomonas serpens is known to express some molecules displaying similarity to those described in trypanosomatids pathogenic to humans, such as peptidases from Trypanosoma cruzi (cruzipain) and Leishmania spp. (gp63). In this work, a population of P. serpens resistant to the calpain inhibitor MDL28170 at 70 µ m (MDLR population) was selected by culturing promastigotes in increasing concentrations of the drug. The only relevant ultrastructural difference between wild-type (WT) and MDLR promastigotes was the presence of microvesicles within the flagellar pocket of the latter. MDLR population also showed an increased reactivity to anti-cruzipain antibody as well as a higher papain-like proteolytic activity, while the expression of calpain-like molecules cross-reactive to anti-Dm-calpain (from Drosophila melanogaster) antibody and calcium-dependent cysteine peptidase activity were decreased. Gp63-like molecules also presented a diminished expression in MDLR population, which is probably correlated to the reduction in the parasite adhesion to the salivary glands of the insect vector Oncopeltus fasciatus. A lower accumulation of Rhodamine 123 was detected in MDLR cells when compared with the WT population, a phenotype that was reversed when MDLR cells were treated with cyclosporin A and verapamil. Collectively, our results may help in the understanding of the roles of calpain inhibitors in trypanosomatids.

Decoding the anti-Trypanosoma cruzi action of HIV peptidase inhibitors using epimastigotes as a model

Background

Aspartic peptidase inhibitors have shown antimicrobial action against distinct microorganisms. Due to an increase in the occurrence of Chagas' disease/AIDS co-infection, we decided to explore the effects of HIV aspartic peptidase inhibitors (HIV-PIs) on Trypanosoma cruzi, the etiologic agent of Chagas' disease.

Methodology and Principal Findings

HIV-PIs presented an anti-proliferative action on epimastigotes of T. cruzi clone Dm28c, with IC₅₀ values ranging from 0.6 to 14 µM. The most effective inhibitors, ritonavir, lopinavir and nelfinavir, also had an anti-proliferative effect against different phylogenetic T. cruzi strains. The HIV-PIs induced some morphological alterations in clone Dm28c epimastigotes, as reduced cell size and swollen of the cellular body. Transmission electron microscopy revealed that the flagellar membrane, mitochondrion and reservosomes are the main targets of HIV-PIs in T. cruzi epimastigotes. Curiously, an increase in the epimastigote-into-trypomastigote differentiation process of clone Dm28c was observed, with many of these parasites presenting morphological alterations including the detachment of flagellum from the cell body. The pre-treatment with the most effective HIV-PIs drastically reduced the interaction process between epimastigotes and the invertebrate vector Rhodnius prolixus. It was also noted that HIV-PIs induced an increase in the expression of gp63-like and calpain-related molecules, and decreased the cruzipain expression in epimastigotes as judged by flow cytometry and immunoblotting assays. The hydrolysis of a cathepsin D fluorogenic substrate was inhibited by all HIV-PIs in a dose-dependent manner, showing that the aspartic peptidase could be a possible target to these drugs. Additionally, we verified that ritonavir, lopinavir and nelfinavir reduced drastically the viability of clone Dm28c trypomastigotes, causing many morphological damages.

Conclusions and Significance

The results contribute to understand the possible role of aspartic peptidases in T. cruzi physiology, adding new in vitro insights into the possibility of exploiting the use of HIV-PIs in the clinically relevant forms of the parasite.

Calpains: potential targets for alternative chemotherapeutic intervention against human pathogenic trypanosomatids

The treatment for both leishmaniasis and trypanosomiasis, which are severe human infections caused by trypanosomatids belonging to Leishmania and Trypanosoma genera, respectively, is extremely limited because of concerns of toxicity and efficacy with the available anti-protozoan drugs, as well as the emergence of drug resistance. Consequently, the urgency for the discovery of new trypanosomatid targets and novel bioactive compounds is particularly necessary. In this context, the investigation of changes in parasite gene expression between drug resistant/sensitive strains and in the up-regulation of virulence-related genes in infective forms has brought to the fore the involvement of calpain-like proteins in several crucial pathophysiological processes performed by trypanosomatids. These studies were encouraged by the publication of the complete genome sequences of three human pathogenic trypanosomatids, Trypanosoma brucei, Trypanosoma cruzi and Leishmania major, which allowed in silico analyses that in turn directed the identification of numerous genes with interesting chemotherapeutic characteristics, including a large family of calpain-related proteins, in which to date 23 genes were assigned as calpains in T. brucei, 40 in T. cruzi and 33 in L. braziliensis. In the present review, we intend to add to these biochemical/biological reports the investigations performed upon the inhibitory capability of calpain inhibitors against human pathogenic trypanosomatids.

Use of cysteine-reactive small molecules in drug discovery for trypanosomal disease

INTRODUCTION

The roles of cysteine protease (CP) enzymes in the biochemistry and infectivity of the three trypanosomal parasitic infections, Chagas' disease, leishmaniasis and human African trypanosomiasis, which have been elucidated over the last three decades are summarized. Inhibitors of these enzymes, which act through trapping the active site cysteine with an electrophilic warhead, hold huge potential as therapeutic agents but the promise of these has yet to be realized in clinical studies. The article addresses aspects that ought to be considered in order to develop orally active CP inhibitors that are safe and effective therapies for trypanosomiasis.

AREAS COVERED

This article reviews learnings from CP research in the trypanosomal field and recent advances in developing cysteine protease inhibitors (CPIs) of human cathepsin K, a related enzyme. Considerations such as intra- and extracellular localization of the CPs, off-target activities against human cathepsin enzymes, basic versus neutral and potential pro-drug inhibitors are reviewed. A description of odanacatib, a cathepsin K inhibitor currently in late stage development, is made to illustrate the attributes of a clinically viable CPI.

EXPERT OPINION

The emerging role of CPs in a wide array of parasitic diseases is highlighted with the vision that CP inhibitors could become the 'β-lactams' of anti-parasitic treatments in the coming decades. New CPI research will see the optimization of intra- and extracellular enzyme targeting, reduction of off-target activities and better understanding of pharmacokinetic-pharmacodynamic interactions which will all lead to compounds with much improved efficacy and viability as clinical therapies.

The Trypanosoma cruzi protease cruzain mediates immune evasion

Trypanosoma cruzi is the causative agent of Chagas' disease. Novel chemotherapy with the drug K11777 targets the major cysteine protease cruzain and disrupts amastigote intracellular development. Nevertheless, the biological role of the protease in infection and pathogenesis remains unclear as cruzain gene knockout failed due to genetic redundancy. A role for the T. cruzi cysteine protease cruzain in immune evasion was elucidated in a comparative study of parental wild type- and cruzain-deficient parasites. Wild type T. cruzi did not activate host macrophages during early infection (<60 min) and no increase in ∼P iκB was detected. The signaling factor NF-κB P65 colocalized with cruzain on the cell surface of intracellular wild type parasites, and was proteolytically cleaved. No significant IL-12 expression occurred in macrophages infected with wild type T. cruzi and treated with LPS and BFA, confirming impairment of macrophage activation pathways. In contrast, cruzain-deficient parasites induced macrophage activation, detectable iκB phosphorylation, and nuclear NF-κB P65 localization. These parasites were unable to develop intracellularly and survive within macrophages. IL 12 expression levels in macrophages infected with cruzain-deficient T. cruzi were comparable to LPS activated controls. Thus cruzain hinders macrophage activation during the early (<60 min) stages of infection, by interruption of the NF-κB P65 mediated signaling pathway. These early events allow T. cruzi survival and replication, and may lead to the spread of infection in acute Chagas' disease.

The peptidases of Trypanosoma cruzi: digestive enzymes, virulence factors, and mediators of autophagy and programmed cell death

Trypanosoma cruzi, the agent of the American Trypanosomiasis, Chagas disease, contains cysteine, serine, threonine, aspartyl and metallo peptidases. The most abundant among these enzymes is cruzipain, a cysteine proteinase expressed as a mixture of isoforms, some of them membrane-bound. The enzyme is an immunodominant antigen in human chronic Chagas disease and seems to be important in the host/parasite relationship. Inhibitors of cruzipain kill the parasite and cure infected mice, thus validating the enzyme as a very promising target for the development of new drugs against the disease. In addition, a 30kDa cathepsin B-like enzyme, two metacaspases and two autophagins have been described. Serine peptidases described in the parasite include oligopeptidase B, a member of the prolyl oligopeptidase family involved in Ca(2+)-signaling during mammalian cell invasion; a prolyl endopeptidase (Tc80), against which inhibitors are being developed, and a lysosomal serine carboxypeptidase. Metallopeptidases homologous to the gp63 of Leishmania spp. are present, as well as two metallocarboxypeptidases belonging to the M32 family, previously found only in prokaryotes. The proteasome has properties similar to those of other eukaryotes, and its inhibition by lactacystin blocks some differentiation steps in the life cycle of the parasite. This article is part of a Special Issue entitled: Proteolysis 50 years after the discovery of lysosome.

Arrested growth of Trypanosoma cruzi by the calpain inhibitor MDL28170 and detection of calpain homologues in epimastigote forms

In this paper, we aimed to explore the effects of the calpain inhibitor III (MDL28170) and to detect calpain-like molecules (CALPs) in epimastigote forms of Trypanosoma cruzi isolate Dm28c. MDL28170 at 70 μM promoted a powerful reduction in the growth rate after 48 h. The IC50 value was calculated to be 31·7 μM. This inhibitor promoted an increase in the cellular volume, but not cell lysis, resulting in a trypanostatic effect. T. cruzi CALPs presented a strong cross-reactivity with anti-Drosophila melanogaster calpain and anti-cytoskeleton-associated protein from Trypanosoma brucei antibodies, and labelling was found mainly intracellularly. Furthermore, an 80 kDa reactive protein was detected by Western blotting assays. No significant cross-reactivity was found with anti-human brain calpain antibody. The expression of CALPs was decreased in cells kept for long periods in axenic cultures in comparison to a strain recently isolated from mice, as well as in MDL28170-treated cells, the latter being paralleled by an increased expression of cruzipain. Different levels of CALPs expression were also detected in distinct phylogenetic lineages, like Y strain (lineage TCI), Dm28c (TCII) and INPA6147 strain (Z3 zymodeme). These results may contribute for the investigation of the functions of CALPs in trypanosomatids.

Protease expression analysis in recently field-isolated strains ofTrypanosoma cruzi: a heterogeneous profile of cysteine protease activities between TC I and TC II major phylogenetic groups

Protease expression among TCI and TCII field isolates was analysed. Gelatin-containing gels revealed hydrolysis bands with molecular masses ranging from 45 to 66 kDa. The general protease expression profile showed that TCII isolates presented higher heterogeneity compared to TCI. By utilizing protease inhibitors, we showed that all active proteases at acid pH are cysteine-proteases and all proteases active at alkaline pH are metalloproteases. However, the expression of cruzipain, theT. cruzimajor cysteine-protease, did not reproduce a heterogeneous TCII cysteine zymogram profile. Dendogram analyses based on presence/absence matrices of proteases and cruzipain bands showed a TCI separation from the TCII group with 50–60% similarity. We suggest that the observed cysteine protease diversification contributes to differential host infection between TCI and II genotypes.

Ultrastructural alterations induced by the neolignan dihydrobenzofuranic eupomatenoid-5 on epimastigote and amastigote forms of Trypanosoma cruzi

Eupomatenoid-5, a compound isolated from leaves of Piper regnellii var. pallescens, showed antiprotozoal activity against the epimastigote proliferative stages and intracellular amastigote forms of Trypanosoma cruzi Y strain. Eupomatenoid-5, at 7.0 microg/ml (50% growth inhibition concentration) produced morphological changes in epimastigote forms of the parasite, such as intense cytoplasmic vacuolization, mitochondrial swelling, kinetoplast alteration, presence of myelin-like figures, and mitochondrial damage, as observed by transmission electron microscopy. In amastigote forms in LLCMK(2) cells, at 7.0-microg/ml concentration, the compound induced a decrease in the number of cells with internalized parasites and in the number of internalized parasites per LLCMK(2) cell, compared with untreated cells. Furthermore, intense cytoplasmic vacuolization and autophagic vacuoles were observed in T. cruzi intracellular amastigotes after 72 h of incubation. Scanning electron microscopy confirmed the alterations in the shape of the parasites.

Deletion of copies of the gene encoding old yellow enzyme (TcOYE), a NAD(P)H flavin oxidoreductase, associates with in vitro-induced benznidazole resistance in Trypanosoma cruzi

Old yellow enzyme (OYE) is a NAD(P)H flavin oxidoreductase that in Trypanosoma cruzi (TcOYE) catalyzes prostaglandin PGF2alpha synthesis and reduction of some trypanocidal drugs. We performed DNA microarray analysis and it revealed that the levels of transcription of the TcOYE gene were six-fold lower in a T. cruzi population with in vitro-induced resistance to benznidazole (BZ) (17LER) than in the wild-type (17WTS). Further we investigated the TcOYE levels in 15 T. cruzi strains and clones that were either susceptible or naturally resistant to BZ and nifurtimox, or had in vivo-selected resistance to BZ. Northern blot and real-time RT-PCR analyses confirmed our finding that TcOYE transcription levels were lower in 17LER than in 17WTS. In contrast, we detected no differences in TcOYE transcription levels between other T. cruzi samples. All T. cruzi strains contained four copies of TcOYE gene, except 17LER that contained only one. A 42kDa TcOYE protein was detected in all T. cruzi strains tested. The expression of this protein was similar for all samples, with the exception of 17LER for which the protein was nearly seven-fold less expressed. The chromosomal location of the TcOYE gene and the polymorphisms detected in TcOYE nucleotide and amino acid sequences of the T. cruzi strains are associated with the zymodeme but not with drug-resistance phenotype. Our data show that one of the mechanisms conferring in vitro-induced BZ resistance to T. cruzi correlates with deletion of copies of the TcOYE gene. In contrast, the in vivo and natural resistance to BZ are mediated by different mechanisms.

Activity of Neolignans Isolated from Piper regnellii (MIQ.) C. DC. var. pallescens (C. DC.) YUNCK against Trypanosoma cruzi

The in vitro antiproliferative effects of 4 neolignans purified from the ethyl-acetate extract from leaves of Piper regnellii (MIQ.) C. DC. var. pallescens (C. DC.) YUNCK against Trypanosoma cruzi were investigated. These isolated compounds were identified through spectral analyses of UV, EI-MS, 1H-, 13C-NMR, H-H COSY, gNOE, HETCOR, and HMBC. The compounds eupomatenoid-5, eupomatenoid-6, and conocarpan showed considerable activity against epimastigote forms of T. cruzi, with 50% inhibition concentrations (IC50) of 7.0, 7.5, and 8.0 microg/ml respectively. After methylation, these compounds showed a lessened inhibitory activity to the growth of the protozoan, suggesting that loss of the hydroxyl group from their molecules reduces the activity. The compound eupomatenoid-3 showed lower activity than the hexane fraction. Eupomatenoid-5 was significantly more active than benznidazole, the antiparasitic drug of choice for treatment of Chagas' disease. The crude extract, hexane fraction, and eupomatenoid-5 caused no lysis in sheep blood at concentrations which inhibit the growth of epimastigote forms. The compound eupomatenoid-5 showed low cytotoxic effects against Vero cells. These results provide new perspectives on the development of novel drugs obtained from natural products with trypanocidal activity. However, the extracts and active compound isolated from P. regnellii var. pallescens should be further studied in animal models for in vivo efficacy.

Specific human antibodies do not inhibit Trypanosoma cruzi oligopeptidase B and cathepsin B, and immunoglobulin G enhances the activity of trypomastigote-secreted oligopeptidase B

Trypanosoma cruzi expresses oligopeptidase B and cathepsin B that have important functions in the interaction with mammalian host cells. In this study, we demonstrated that sera from both chagasic rabbits and humans have specific antibodies to highly purified native oligopeptidase B and cathepsin B. Levels of antibodies to cathepsin B were higher than those observed to oligopeptidase B by absorbance values recorded upon ELISA. We next showed that 90% and 30% of sera from individuals with mucocutaneous leishmaniasis have antibodies that recognize oligopeptidase B and cathepsin B as antigens, respectively. In addition, 55% and 40% of sera from kala-azar patients have antibodies to oligopeptidase B and cathepsin B, respectively. Sera from malaria patients did not recognize the proteases as antigens. Despite high levels of specific antibodies, sera from T. cruzi-infected patients did not inhibit the activities of either oligopeptidase B or cathepsin B. Furthermore, sera or IgG purified from either infected or non-infected individuals enhanced the enzymatic activity of the secreted oligopeptidase B. Oligopeptidase B secreted by trypomastigotes and cathepsin B released upon parasite lysis retain their enzymatic activities and may be associated with Chagas' disease pathogenesis by hydrolyzing host proteins and inducing host immune responses.

Chagasin, the endogenous cysteine-protease inhibitor of Trypanosoma cruzi, modulates parasite differentiation and invasion of mammalian cells

Chagasin is a Trypanosoma cruzi protein that was recently characterized as a tight-binding inhibitor of papain-like cysteine proteases (CPs). Considering that parasite virulence and morphogenesis depend on the endogenous activity of lysosomal CPs of the cruzipain family, we sought to determine whether chagasin and cruzipain interact in the living cell. Ultrastructural studies showed that chagasin and cruzipain both localize to the Golgi complex and reservosomes (lysosome-like organelles), whereas free chagasin was found in small intracellular vesicles, suggesting that chagasin trafficking pathways might intersect with those of cruzipain. Taking advantage of the fact that sodium dodecyl sulphate and beta-mercaptoethanol prevent binding between the isolated proteins but do not dismantle preformed cruzipain-chagasin complexes, we obtained direct evidence that chagasin-cruzipain complexes are indeed formed in epimastigotes. Chagasin transfectants (fourfold increase in CP inhibitory activity) displayed low rates of differentiation (metacyclogenesis) and exhibited increased resistance to a synthetic CP inhibitor. These phenotypic changes were accompanied by a drastic reduction of soluble cruzipain activity and by upregulated secretion of cruzipain-chagasin molecular complexes. Analysis of six T. cruzi strains revealed that expression levels of cruzipain and chagasin are variable, but the molar ratios are fairly stable ( approximately 50:1) in most strains, with the exception of the G strain (5:1), which is poorly infective. On the same vein, we found that trypomastigotes overexpressing chagasin are less infective than wild-type parasites in vitro. The deficiency of chagasin overexpressers is caused by lower activity of membrane-associated CPs, because membranes recovered from wild-type trypomastigotes restored infectivity and this effect was nullified by the CP inhibitor E-64. In summary, our studies suggest that chagasin regulates the endogenous activity of CP, thus indirectly modulating proteolytic functions that are essential for parasite differentiation and invasion of mammalian cells.

Six Trypanosoma cruzi strains characterized by specific gene expression patterns

The intracellular parasite Trypanosoma cruzi, the causative agent of Chagas disease, is known to comprise heterogeneous populations. One possibility to explain the obviously distinct phenotypes of different T. cruzi strains is differential expression of particular genes. This could result in environmental adaptations of the parasite within host organs, leading to distinct clinical symptoms. With the aim of identifying differentially expressed genes, we examined different T. cruzi strains by suppression subtractive hybridization analysis. The isolated clones were sequenced and Blasted for sequence-homology with known T. cruzi genes. A stage-specific glycoprotein (82gp), an 85-kDa protein with homology to heat-shock proteins, a beta-tubulin gene, a hexosetransporter, a dehydrogenase/ prostaglandin F2alpha-synthase and a cathepsin B-like protease were identified. The expression of these genes was analyzed by RT-PCR. Diverse expression patterns were detected for different T. cruzi strains, but no specific correlation between the gene expression and the classification of groups could be found. We discuss the presumed importance of these T. cruzi gene expression patterns for future strategies of molecular therapy of Chagas disease. For pathological studies, other parameters such as distinct gene/antigen expression could also be of interest, because they probably likewise correlate with distinct phenotypes.

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.

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.

Congopain from Trypanosoma congolense: drug target and vaccine candidate

Trypanosomes are the etiological agents of human sleeping sickness and livestock trypanosomosis (nagana), which are major diseases in Africa. Their cysteine proteases (CPs), which are members of the papain family, are expressed during the infective stages of the parasites' life cycle. They are suspected to act as pathogenic factors in the mammalian host, where they also trigger prominent immune responses. Trypanosoma congolense, a major pathogenic species in livestock, possesses at least two families of closely related CPs, named CP1 and CP2. Congopain, a CP2-type of enzyme, shares structural and functional resemblances with cruzipain from T. cruzi and with mammalian cathepsin L. Like CPs from other Trypanosomatids, congopain might be an attractive target for trypanocidal drugs. Here we summarise the current knowledge in the two main areas of research on congopain: first, the biochemical properties of congopain were characterised and its substrate specificity was determined, as a first step towards drug design; second, the possibility was being explored that inhibition of congopain by host-specific antibodies may mitigate the pathology associated with trypanosome infection.

A critical review on Chagas disease chemotherapy

In this "Critical Review" we made a historical introduction of drugs assayed against Chagas disease beginning in 1912 with the works of Mayer and Rocha Lima up to the experimental use of nitrofurazone. In the beginning of the 70s, nifurtimox and benznidazole were introduced for clinical treatment, but results showed a great variability and there is still a controversy about their use for chronic cases. After the introduction of these nitroheterocycles only a few compounds were assayed in chagasic patients. The great advances in vector control in the South Cone countries, and the demonstration of parasite in chronic patients indicated the urgency to discuss the etiologic treatment during this phase, reinforcing the need to find drugs with more efficacy and less toxicity. We also review potential targets in the parasite and present a survey about new classes of synthetic and natural compounds studied after 1992/1993, with which we intend to give to the reader a general view about experimental studies in the area of the chemotherapy of Chagas disease, complementing the previous papers of Brener (1979) and De Castro (1993).

Trypanosoma cruzi prolyl oligopeptidase Tc80 is involved in nonphagocytic mammalian cell invasion by trypomastigotes

Trypanosoma cruzi is an intracellular protozoan parasite able to invade a wide variety of mammalian cells. To have access to the target organs/cells, the parasite must cross the basal laminae and the extracellular matrix (ECM). We previously characterized an 80-kDa proteinase (Tc80) secreted by the infective trypomastigotes that hydrolyzes native collagens and might be involved in infection by degrading ECM components. Here, we present evidence indicating a role for Tc80 in the invasion of nonphagocytic cells. Tc80 was classified as a member of the prolyl oligopeptidase (POP) family of serine proteases and was also found to hydrolyze fibronectin. Selective inhibitors for POP Tc80 were synthesized that blocked parasite entry into cells. Blockage occurred when trypomastigotes were preincubated with irreversible inhibitors but not after host cell preincubation, and the blockage correlated with inhibition of POP Tc80 activity in treated parasites. These data and the enzyme location inside a vesicular compartment close to the flagellar pocket, a specialized domain in endocytosis/exocytosis, strongly suggest a role for POP Tc80 in the maturation of parasite protein(s) and/or, after secretion, in a local action on parasite or host cell/ECM components required for invasion.

Identification, characterization and localization of chagasin, a tight-binding cysteine protease inhibitor inTrypanosoma cruzi

Lysosomal cysteine proteases from mammalian cells and plants are regulated by endogenous tight-binding inhibitors from the cystatin superfamily. The presence of cystatin-like inhibitors in lower eukaryotes such as protozoan parasites has not yet been demonstrated, although these cells express large quantities of cysteine proteases and may also count on endogenous inhibitors to regulate cellular proteolysis. Trypanosoma cruzi, the causative agent of Chagas’ heart disease, is a relevant model to explore this possibility because these intracellular parasites rely on their major lysosomal cysteine protease (cruzipain) to invade and multiply in mammalian host cells. Here we report the isolation, biochemical characterization, developmental stage distribution and subcellular localization of chagasin, an endogenous cysteine protease inhibitor in T. cruzi. We used high temperature induced denaturation to isolate a heat-stable cruzipain-binding protein (apparent molecular mass, 12 kDa) from epimastigote lysates. This protein was subsequently characterized as a tight-binding and reversible inhibitor of papain-like cysteine proteases. Immunoblotting indicated that the expression of chagasin is developmentally regulated and inversely correlated with that of cruzipain. Gold-labeled antibodies localized chagasin to the flagellar pocket and cytoplasmic vesicles of trypomastigotes and to the cell surface of amastigotes. Binding assays performed by probing living parasites with fluorescein (FITC)-cruzipain or FITC-chagasin revealed the presence of both inhibitor and protease at the cell surface of amastigotes. The intersection of chagasin and cruzipain trafficking pathways may represent a checkpoint for downstream regulation of proteolysis in trypanosomatid protozoa.