Proteases of protozoan parasites

Exploring Aspartic Protease Inhibitor Binding to Design-Selective Antimalarials

Selectivity is a major issue in the development of drugs targeting pathogen aspartic proteases. Here, we explore the selectivity-determining factors by studying specifically designed malaria aspartic protease (plasmepsin) open-flap inhibitors. Metadynamics simulations are used to uncover the complex binding/unbinding pathways of these inhibitors and describe the critical transition states in atomistic resolution. The simulation results are compared with experimentally determined enzymatic activities. Our findings demonstrate that plasmepsin inhibitor selectivity can be achieved by targeting the flap loop with hydrophobic substituents that enable ligand binding under the flap loop, as such a behavior is not observed for several other aspartic proteases. The ability to estimate the selectivity of compounds before they are synthesized is of considerable importance in drug design; therefore, we expect that our approach will be useful in selective inhibitor designs against not only aspartic proteases but also other enzyme classes.

Overexpression of Plasmodium falciparum M1 Aminopeptidase Promotes an Increase in Intracellular Proteolysis and Modifies the Asexual Erythrocytic Cycle Development

Plasmodium falciparum, the most virulent of the human malaria parasite, is responsible for high mortality rates worldwide. We studied the M1 alanyl-aminopeptidase of this protozoan (PfA-M1), which is involved in the final stages of hemoglobin cleavage, an essential process for parasite survival. Aiming to help in the rational development of drugs against this target, we developed a new strain of P. falciparum overexpressing PfA-M1 without the signal peptide (overPfA-M1). The overPfA-M1 parasites showed a 2.5-fold increase in proteolytic activity toward the fluorogenic substrate alanyl-7-amido-4-methylcoumarin, in relation to the wild-type group. Inhibition studies showed that overPfA-M1 presented a lower sensitivity against the metalloaminopeptidase inhibitor bestatin and to other recombinant PfA-M1 inhibitors, in comparison with the wild-type strain, indicating that PfA-M1 is a target for the in vitro antimalarial activity of these compounds. Moreover, overPfA-M1 parasites present a decreased in vitro growth, showing a reduced number of merozoites per schizont, and also a decrease in the iRBC area occupied by the parasite in trophozoite and schizont forms when compared to the controls. Interestingly, the transgenic parasite displays an increase in the aminopeptidase activity toward Met-, Ala-, Leu- and Arg-7-amido-4-methylcoumarin. We also investigated the potential role of calmodulin and cysteine proteases in PfA-M1 activity. Taken together, our data show that the overexpression of PfA-M1 in the parasite cytosol can be a suitable tool for the screening of antimalarials in specific high-throughput assays and may be used for the identification of intracellular molecular partners that modulate their activity in P. falciparum.

Bioinformatics Analysis and Functional Prediction of Transmembrane Proteins in Entamoeba histolytica

Entamoeba histolytica is an invasive, pathogenic parasite causing amoebiasis. Given that proteins involved in transmembrane (TM) transport are crucial for the adherence, invasion, and nutrition of the parasite, we conducted a genome-wide bioinformatics analysis of encoding proteins to functionally classify and characterize all the TM proteins in E. histolytica. In the present study, 692 TM proteins have been identified, of which 546 are TM transporters. For the first time, we report a set of 141 uncharacterized proteins predicted as TM transporters. The percentage of TM proteins was found to be lower in comparison to the free-living eukaryotes, due to the extracellular nature and functional diversification of the TM proteins. The number of multi-pass proteins is larger than the single-pass proteins; though both have their own significance in parasitism, multi-pass proteins are more extensively required as these are involved in acquiring nutrition and for ion transport, while single-pass proteins are only required at the time of inciting infection. Overall, this intestinal parasite implements multiple mechanisms for establishing infection, obtaining nutrition, and adapting itself to the new host environment. A classification of the repertoire of TM transporters in the present study augments several hints on potential methods of targeting the parasite for therapeutic benefits.

Expression and secretion of the Giardia duodenalis variant surface protein 9B10A by transfected trophozoites causes damage to epithelial cell monolayers mediated by protease activity

Giardia duodenalis is the protozoan parasite responsible for most cases of parasitic diarrhea worldwide. The pathogenic mechanisms of giardiasis have not yet been fully characterized. In this context parasite's excretory/secretory products have been related to the damage induced by the parasite on enterocytes. Among these is the Variable Surface Proteins (VSPs) family involved in antigenic variation and in the induction of protective response. In proteomic analyses carried out to identify the proteases with high molecular weight secreted by Giardia trophozoites during the initial phase of interaction with IEC-6 cell monolayers we identified the VSP9B10A protein. In silico bioinformatics analyses predicted a central region in residues 324-684 displaying the catalytic triad and the substrate binding pocket of cysteine proteases. The analysis of the effect of the VSP9B10A protein on epithelial cell monolayers using trophozoites that were transfected with a plasmid carrying the vsp9b10a gene sequence under the control of a constitutive promoter showed that transfected trophozoites expressing the VSP9B10A protein caused cytotoxic damages on IEC-6 and MDCK cell monolayers. This was characterized by loss of cell-cell contacts and cell detachment from the substrate while no damage was observed with trophozoites that did not express the VSP9B10A protein. The same cytotoxic effect was detected when IEC-6 cell monolayers were incubated only with supernatants from co-cultures of IEC-6 cell monolayers with VSP9B10A transfected trophozoites and this effect was not observed when transfected trophozoites were incubated with a monospecific polyclonal antibody anti-VSP9B10A previous to interaction with IEC-6 monolayers. These results demonstrate that the VSP9B10A protein secreted upon interaction with epithelial cells caused damage in these cells. Thus this protein might be considered as a conditional virulence factor candidate. To our knowledge this is the first report on the proteolytic activity from a Giardia VSP opening new research lines on these proteins.

The gp63 Gene Cluster Is Highly Polymorphic in Natural Leishmania (Viannia) braziliensis Populations, but Functional Sites Are Conserved

GP63 or leishmanolysin is the major surface protease of Leishmania spp. involved in parasite virulence and host cell interaction. As such, GP63 is a potential target of eventual vaccines against these protozoa. In the current study we evaluate the polymorphism of gp63 in Leishmania (Viannia) braziliensis isolated from two sets of American tegumentary leishmaniasis (ATL) cases from Corte de Pedra, Brazil, including 35 cases diagnosed between 1994 and 2001 and 6 cases diagnosed between 2008 and 2011. Parasites were obtained from lesions by needle aspiration and cultivation. Genomic DNA was extracted, and 405 bp fragments, including sequences encoding the putative macrophage interacting sites, were amplified from gp63 genes of all isolates. DNA amplicons were cloned into plasmid vectors and ten clones per L. (V.) braziliensis isolate were sequenced. Alignment of cloned sequences showed extensive polymorphism among gp63 genes within, and between parasite isolates. Overall, 45 different polymorphic alleles were detected in all samples, which could be segregated into two clusters. Cluster one included 25, and cluster two included 20 such genotypes. The predicted peptides showed overall conservation below 50%. In marked contrast, the conservation at segments with putative functional domains approached 90% (Fisher’s exact test p<0.0001). These findings show that gp63 is very polymorphic even among parasites from a same endemic focus, but the functional domains interacting with the mammalian host environment are conserved.

Classico-molecular targeting of oligopeptidase B, cysteine protease and variable surface glycoprotein (VSG) genes of Trypanosoma evansi

Trypanosomosis or Surra can rightly be attributed as the most economically important vector-borne haemoprotozoan disease encountering India. Surra infected chronic cases show almost similar types of signs and symptoms often confusing it with other haemoprotozoan infections, thereby, making it prerequisite for the development of aspecific and sensitive technique for its detection in susceptible animals. Blood microscopy and serology suffers from the hands of lack of sensitivity and specificity thereby leaving molecular detection techniques as one of the promising alternative. Alongside, there is utmost need for exploring of new molecular gene targets for the development of a putative alternative for diagnosis and immunoprophylaxsis. The present communication describes the identification and amplification of oligopeptidase B, cysteine protease and variable surface glycoprotein genes of T. evansi so as to exploit them in future as potential candidates for immune protection and/or molecular detection.

Fighting malaria: structure-guided discovery of nonpeptidomimetic plasmepsin inhibitors

Plasmepsins (Plms) are aspartic proteases involved in the degradation of human hemoglobin by Plasmodium falciparum. Given that the parasite needs the resulting amino acid building blocks for its growth and development, plasmepsins are an important antimalarial drug target. Over the past decade, tremendous progress has been achieved in the development of inhibitors of plasmepsin using two strategies: structure-based drug design (SBDD) and structure-based virtual screening (SBVS). Herein, we review the inhibitors of Plms I-IV developed by SBDD or SBVS with a particular focus on obtaining selectivity versus the human Asp proteases cathepsins and renin and activity in cell-based assays. By use of SBDD, the flap pocket of Plm II has been discovered and constitutes a convenient handle to obtain selectivity. In SBVS, activity against Plms I-IV and selectivity versus cathepsins are not always taken into account. A combination of SBVS, SBDD, and molecular dynamics simulations opens up opportunities for future design cycles.

Immunity against selected piscine flagellates

This discussion is on immune response to Amyloodinium ocellatum, Cryptobia salmositica, Trypanoplasma borreli and Trypanosoma carassii. Piscidin and histone-like proteins enhance innate resistance to Amyloodinium. Fish that are naturally resistant to Cryptobia and Trypanoplasma can be bred. Cryptobia resistance in charr is controlled by a dominant Mendelian locus and protection is via the Alternative Pathway of Complement Activation. Studies on Cryptobia-tolerant charr may lead to production of transgenic Cryptobia-tolerant salmon. Innate response to T. borreli is associated with NO in macrophages. Transferrin regulates resistance and carp have been bred for transferrin genotypes. Recovered fish are protected from homologous challenge, and complement fixing antibodies are crucial in protection. Studies on antigens in T. carassii may lead to a vaccine. There are two vaccines against cryptobiosis; a single dose of the attenuated vaccine protects salmonids. On challenge fish inoculated with the metalloprotease-DNA vaccine do not have the disease and they recover faster.

Novel approaches for the identification of inhibitors of leishmanial dipeptidylcarboxypeptidase

INTRODUCTION

Leishmaniasis imposes a substantial burden of mortality and morbidity affecting 12 million globally and continues to be a neglected tropical disease. Control of the disease is mainly based on chemotherapy, which relies on a handful of drugs with serious limitations. Over the last decade, target-based drug discovery is also being employed in addition to the random screening of compounds. Leishmanial dipeptidylcarboxypeptidase (LDCP), an angiotensin converting enzyme (ACE) related metallopeptidase, has been recently identified as a novel drug target for antileishmanial chemotherapy.

AREAS COVERED

This article examines dipeptidylcarboxypeptidase (DCP) of Leishmania donovani and of other sources from the international literature regarding their biochemical and structural characterization in comparison to mammalian ACE. Furthermore, the author discusses the identification of LdDCP specific inhibitors by virtual screening and their effect on parasite multiplication. Finally, the review looks ahead at areas for further exploration of DCP inhibitors in Leishmania chemotherapy.

EXPERT OPINION

The first step in targeted screening is to identify a suitable drug target and its validation followed by its use in high throughput screening of compounds. Limited studies on LDCP inhibitors have established a good correlation between parasite enzyme inhibition and their biological activity. This suggests that there is a potential for LDCP inhibitors as new antileishmanial drugs.

CPDB: cysteine protease annotation database in Leishmania species

There has been a revival of interest in Cysteine protease for Visceral Leishmaniasis (VL) attributed to massive outbreaks of leishmaniasis in the tropical region. The cysteine protease database (CPDB) was designed to find data related to cysteine protease (CP) of different species of Leishmania and Trypanosoma brucei in a single platform. This has reflected in substantial increase in the submission of Leishmania genome sequences to NCBI (National Center for Biotechnology Information) database. The CPDB database aims to provide a summary of data analysis, such as physiochemical and molecular properties, proteolytic cleavage sites, classification into functional families using SVMProt and other ExPASy tools. The main aim of this database is to provide different protein inhibitors of cysteine protease groups that were collected from literature and make available their 3-D structures through JMol with JAVA platform. These CP inhibitors are freely downloadable and also have added links for functional analyses of other proteins, which is helpful for users. All this information in CPDB, a single platform, will prove to be of great help for researchers who are involved in drug discovery and analysis of other physiochemical and molecular properties of the protein.

AVAILABILITY

the database is available for free at.

Molecular cloning and expression analysis of peptidase genes in the fish-pathogenic scuticociliate Miamiensis avidus

Background

Parasite peptidases have been actively studied as vaccine candidates or drug targets for prevention or treatment of parasitic diseases because of their important roles for survival and/or invasion in the host. Like other parasites, the facultative histophagous ciliate Miamiensis avidus would possess peptidases that are closely associated with the invasion into the host tissue and survival in the host.

Results

The 17 genes encoding peptidases, including seven cathepsin-like cysteine peptidases, four serine carboxypeptidases, a eukaryotic aspartyl protease family protein, an ATP-dependent metalloprotease FtsH family protein, three leishmanolysin family proteins and a peptidase family M49 protein were identified from a Miamiensis avidus cDNA library by BLAST X search. Expression of genes encoding two cysteine peptidases, three leishmanolysin-like peptidases and a peptidase family M49 protein was up-regulated in the cell-fed ciliates compared to the starved ciliates. Especially, one cysteine peptidase (MaPro 4) and one leishmanolysin-like peptidase (MaPro 14) were transcribed more than 100-folds in the cell-fed ciliates.

Conclusions

The genetic information and transcriptional characteristics of the peptidases in the present results would be helpful to elucidate the role of peptidases in the invasion of scuticociliates into their hosts.

Cryptic parasite revealed improved prospects for treatment and control of human cryptosporidiosis through advanced technologies

Cryptosporidium is an important genus of parasitic protozoa of humans and other vertebrates and is a major cause of intestinal disease globally. Unlike many common causes of infectious enteritis, there are no widely available, effective vaccine or drug-based intervention strategies for Cryptosporidium, and control is focused mainly on prevention. This approach is particularly deficient for infections of severely immunocompromised and/or suppressed, the elderly or malnourished people. However, cryptosporidiosis also presents a significant burden on immunocompetent individuals, and can, for example have lasting effects on the physical and mental development of children infected at an early age. In the last few decades, our understanding of Cryptosporidium has expanded significantly in numerous areas, including the parasite life-cycle, the processes of excystation, cellular invasion and reproduction, and the interplay between parasite and host. Nonetheless, despite extensive research, many aspects of the biology of Cryptosporidium remain unknown, and treatment and control are challenging. Here, we review the current state of knowledge of Cryptosporidium, with a focus on major advances arising from the recently completed genome sequences of the two species of greatest relevance in humans, namely Cryptosporidium hominis and Cryptosporidium parvum. In addition, we discuss the potential of next-generation sequencing technologies, new advances in in silico analyses and progress in in vitro culturing systems to bridge these gaps and to lead toward effective treatment and control of cryptosporidiosis.

HIV Protease Inhibitors: Effect on the Opportunistic Protozoan Parasites

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

Immunological and therapeutic strategies against salmonid cryptobiosis

Salmonid cryptobiosis is caused by the haemoflagellate, Cryptobia salmositica. Clinical signs of the disease in salmon (Oncorhynchus spp.) include exophthalmia, general oedema, abdominal distension with ascites, anaemia, and anorexia. The disease-causing factor is a metalloprotease and the monoclonal antibody (mAb-001) against it is therapeutic. MAb-001 does not fix complement but agglutinates the parasite. Some brook charr, Salvelinus fontinalis cannot be infected (Cryptobia-resistant); this resistance is controlled by a dominant Mendelian locus and is inherited. In Cryptobia-resistant charr the pathogen is lysed via the Alternative Pathway of Complement Activation. However, some charr can be infected and they have high parasitaemias with no disease (Cryptobia-tolerant). In infected Cryptobia-tolerant charr the metalloprotease is neutralized by a natural antiprotease, alpha2 macroglobulin. Two vaccines have been developed. A single dose of the attenuated vaccine protects 100% of salmonids (juveniles and adults) for at least 24 months. Complement fixing antibody production and cell-mediated response in vaccinated fish rise significantly after challenge. Fish injected with the DNA vaccine initially have slight anaemias but they recover and have agglutinating antibodies. On challenge, DNA-vaccinated fish have lower parasitaemias, delayed peak parasitaemias and faster recoveries. Isometamidium chloride is therapeutic against the pathogen and its effectiveness is increased after conjugation to antibodies.

Protective immunization against Tetrahymena sp. infection in guppies (Poecilia reticulata)

Systemic tetrahymenosis constitutes a serious problem in guppy (Poecilia reticulata) production worldwide and no therapeutic solution is available for this disease. Three immunization trials were conducted, testing the effectiveness of different Tetrahymena preparations applied by intraperitoneal injection (IP) with or without Freund's complete adjuvant (FCA) and with or without booster dose. In trial 1, immunization with the pathogenic Tet-NI 6 lysate and live attenuated Tet-NI 1 did not provide significant protection from infection, although infection rates were significantly lower in the Tet-NI 6-immunized group than in controls. In trial 2, mortality in Tet-NI 6 + FCA-immunized fish was 10%, significantly lower than in all other treatment groups, including Tet-NI 6 lysate, live attenuated Tet-NI 1 and controls (77, 67 and 73%, respectively). In trial 3, the lowest mortality rates were obtained in the Tet-NI 6 + FCA + booster-immunized group (15%). These levels were lower but not significantly different from the non-boostered Tet-NI 6-immunized group (28%) and the groups immunized with Tet-NI 1, with and without booster (32 and 34%, respectively). Mortality in these four groups was significantly lower than in controls, including adjuvant- and PBS-injected groups (72 and 81%, respectively). Body homogenates of immunized fish immobilized Tetrahymena in-vitro, as compared to no or very little immobilization in controls. Lysozyme levels in the Tet-NI 6 + FCA + booster group were significantly higher than in all other treatments in trial 2 and controls in trial 3. There was no significant difference in anti-protease activity among the differently immunized fish. We conclude that immunization with Tetrahymena lysates in FCA confers a high degree of protection from infection, suggesting this preparation as a basis for vaccine development.

Involvement of serine proteases in the excystation and metacystic development of Entamoeba invadens

Although the functions of cysteine proteases involved in the pathogenicity and differentiation of Entamoeba histolytica have been demonstrated, little is known about the functions of serine proteases. We examined the involvement of serine proteases in amoebic excystation and metacystic development using inhibitors specific for serine proteases. Entamoeba invadens IP-1 strain was used as the model of excystation and metacystic development of E. histolytica. Four serine protease inhibitors, phenylmethanesulfonyl fluoride (PMSF), 4-(2-aminoethyl) bezensulfonylfluoride hydrochloride, 3, 4-dichloroisocoumarin, and N-tosyl-phe-chloromethylketone, decreased the number of metacystic amoebae in a dose-dependent manner, without showing cytotoxicity to cysts. PMSF inhibited not only the increase but also the development of metacystic amoebae as determined by the change of nucleus number from four- to one-nucleate amoebae. The protease activity in cyst lysates was also inhibited by PMSF and the band of protease on gelatin sodium dodecyl sulfate polyacrylamide gel electrophoresis was weaker than controls when treated with PMSF. Three serine protease families, S28 (three types), S9 (two), and S26 (one) were retrieved from the database of E. invadens. Phylogenetic analysis revealed that amebic enzymes from the serine protease families formed different clades from those from other organisms. The expression levels of these serine proteases in cysts 5 h after the induction of excystation as assessed by real-time reverse transcriptase polymerase chain reaction (RT-PCR) were higher than those observed prior to induction assayed by real-time RT-PCR; the increase in one type of S9 (named S9-3) expression was the highest. The expression of S9 enzymes also increased from cysts to trophozoites higher than the other family serine proteases. Thus, the results show that Entamoeba uses their serine proteases in the excystation and metacystic development, which leads to successful infection.

Searching for virulence factors in the non-pathogenic parasite to humans Leishmania tarentolae

Leishmania protozoa are obligate intracellular parasites that reside in the phagolysosome of host macrophages and cause a large spectrum of pathologies to humans known as leishmaniases. The outcome of the disease is highly dependent on the parasite species and on its ascribed virulence factors and the immune status of the host. Characterization of the genome composition of non-pathogenic species could ultimately open new horizons in Leishmania developmental biology and also the disease monitoring. Here, we provide evidence that the lizard non-pathogenic to humans Leishmania tarentolae species expresses an Amastin-like gene, cysteine protease B (CPB), lipophosphoglycan LPG3 and the leishmanolysin GP63, genes well-known for their potential role in the parasite virulence. These genes were expressed at levels comparable to those in L. major and L. infantum both at the level of mRNA and protein. Alignment of the L. tarentolae proteins with their counterparts in the pathogenic species demonstrated that the degree of similarity varied from 59% and 60% for Amastin, 89% for LPG3 and 71% and 68% for CPB, in L. major and L. infantum, respectively. Interestingly, the A2 gene, expressed specifically by the L. donovani complex which promotes visceralization, was absent in L. tarentolae. These findings suggest that the lack of pathogenicity in L. tarentolae is not associated with known virulence genes such as LPG3, CPB, GP63 and Amastin, and that other factors either unique to L. tarentolae or missing from this species may be responsible for the non-pathogenic potential of this lizard parasite.

Purification and subcellular localization of a secreted 75 kDa Trypanosoma cruzi serine oligopeptidase

An extracellular serine peptidase was purified 460-fold from Trypanosoma cruzi epimastigotes culture supernatant with (NH(4))(2)SO(4) precipitation followed by affinity chromatography aprotinin-agarose and continuous elution electrophoresis, yielding a total recovery of 65%. The molecular mass of the active enzyme estimated by reducing and non-reducing SDS-PAGE was about 75kDa. The optimal pH and temperature of this glycosylated peptidase were 8.0 and 37 degrees C using alpha-N-rho-tosyl-L-arginine-methyl ester (L-TAME) as substrate. The enzyme did not hydrolyze polypeptide substrates but was active against short peptide substrates containing arginine at the P1 site, in both ester and amide bonds. The peptidase was inhibited by TPCK and TCLK but not by other protease inhibitors suggesting that the enzyme belongs to the serine peptidase class. Interestingly, the enzyme seems to demonstrate some metal dependence since its activity was reduced by 1,10-phenanthroline, calcium and zinc ions. Rabbit anti-T. cruzi extracellular serine peptidase antiserum was used to show that the enzyme was restricted to intracellular structures, including the flagellar pocket, plasma membrane and cytoplasmic vesicles resembling reservosomes. These results suggest that the serine oligopeptidase is secreted into the extracellular environment through the flagellar pocket and the intracellular location could suggest its participation in certain proteolysis events in reservosomes. These findings show that this peptidase is a novel T. cruzi serine oligopeptidase, which differs not only from other peptidases described in the same parasite but also in other species of Trypanosoma.

Oligopeptidase B from Leishmania amazonensis: molecular cloning, gene expression analysis and molecular model

Serine oligopeptidases of trypanosomatids are emerging as important virulence factors and therapeutic targets in trypanosome infections. A complete open reading frame of oligopeptidase B from Leishmania amazonensis was amplified with polymerase chain reaction with gradient annealing temperatures using primers designed for the oligopeptidase B gene from L. major. The 2,196-bp fragment coded for a protein of 731 amino acids with a predicted molecular mass of 83.49 KDa. The encoded protein (La_OpB) shares a 90% identity with oligopeptidases of L. major and L. infantum, 84% with L. braziliensis, and approximately 62% identity with Trypanosoma peptidases. The oligopeptidase B gene is expressed in all cycle stages of L. amazonensis. The three dimensional model of La_OpB was obtained by homology modeling based on the structure of prolyl oligopeptidases. We mapped a La_OpB model that presents a greater negative charge than prolyl oligopeptidases; our results suggest a difference in the S2 subsite when compared to oligopeptidases B from Trypanosoma and bacterial oligopeptidases B. The La_OpB model serves as a starting point for its exploration as a potential target source for a rational chemotherapy.

Leishmania and the leishmaniases: a parasite genetic update and advances in taxonomy, epidemiology and pathogenicity in humans

Leishmaniases remain a major public health problem today despite the vast amount of research conducted on Leishmania pathogens. The biological model is genetically and ecologically complex. This paper explores the advances in Leishmania genetics and reviews population structure, taxonomy, epidemiology and pathogenicity. Current knowledge of Leishmania genetics is placed in the context of natural populations. Various studies have described a clonal structure for Leishmania but recombination, pseudo-recombination and other genetic processes have also been reported. The impact of these different models on epidemiology and the medical aspects of leishmaniases is considered from an evolutionary point of view. The role of these parasites in the expression of pathogenicity in humans is also explored. It is important to ascertain whether genetic variability of the parasites is related to the different clinical expressions of leishmaniasis. The review aims to put current knowledge of Leishmania and the leishmaniases in perspective and to underline priority questions which 'leishmaniacs' must answer in various domains: epidemiology, population genetics, taxonomy and pathogenicity. It concludes by presenting a number of feasible ways of responding to these questions.

Effects of serine protease inhibitors on viability and morphology of Leishmania (Leishmania) amazonensis promastigotes

To investigate the importance of serine proteases in Leishmania amazonensis promastigotes, we analyzed the effects of classical serine protease inhibitors and a Kunitz-type inhibitor, obtained from sea anemone Stichodactyla helianthus (ShPI-I), on the viability and morphology of parasites in culture. Classical inhibitors were selected on the basis of their ability to inhibit L. amazonensis serine proteases, previously described. The N-tosyl-L: -phenylalanine chloromethyl ketone (TPCK) and benzamidine (Bza) inhibitors, which are potential Leishmania proteases inhibitors, in all experimental conditions reduced the parasite viability, with regard to time dependence. On the other hand, N-tosyl-lysine chloromethyl ketone (TLCK) did not significantly affect the parasite viability, as it was poor Leishmania enzymes inhibitor. Ultrastructural analysis demonstrated that both Bza and TPCK induced changes in the flagellar pocket region with membrane alteration, including bleb formation. However, TPCK effects were more pronounced than those of Bza in Leishmania flagellar pocket in plasma membrane, and intracellular vesicular bodies was visualized. ShPI-I proved to be a powerful inhibitor of L. amazonensis serine proteases and the parasite viability. The ultrastructural alterations caused by ShPI-I were more dramatic than those induced by the classical inhibitors. Vesiculation of the flagellar pocket membrane, the appearance of a cytoplasmic vesicle that resembles an autophagic vacuole, and alterations of promastigotes shape resulted.

Oligopeptidase B from L. amazonensis: molecular cloning, gene expression analysis and molecular model

Serine oligopeptidases of trypanosomatids are emerging as important virulence factors and therapeutic targets in trypanosome infections. A complete open reading frame of oligopeptidase B from Leishmania amazonensis was amplified with polymerase chain reaction with gradient annealing temperatures using primers designed for the oligopeptidase B gene from L. major. The 2,196-bp fragment coded for a protein of 731 amino acids with a predicted molecular mass of 83.49 KDa. The encoded protein (La_OpB) shares a 90% identity with oligopeptidases of L. major and L. infantum, 84% with L. braziliensis, and approximately 62 identity with Trypanosoma peptidases. The oligopeptidase B gene is expressed in all cycle stages of L. amazonensis. The three dimensional model of La_OpB was obtained by homology modeling based on the structure of prolyl oligopeptidases. We mapped a La_OpB model that presents a greater negative charge than prolyl oligopeptidases; our results suggest a difference in the S2 subsite when compared to oligopeptidases B from Trypanosoma and bacterial oligopeptidases B. The La_OpB model serves as a starting point for its exploration as a potential target source for a rational chemotherapy.

Scuticociliate proteinases may modulate turbot immune response by inducing apoptosis in pronephric leucocytes

The role of proteinases of the histiophagous ciliate Philasterides dicentrarchi, purified by affinity chromatography in bacitracin-Sepharose, on apoptosis (programmed cell death) of turbot pronephric leucocytes (PL) was investigated. The results showed that more than 90% of proteinases purified by bacitracin-Sepharose were cysteine proteinases, which lacked significant caspase-3-like activity and generated three main gelatinolytic bands of molecular weights 36, 45 and 77 kDa as determined by gelatine-SDS-PAGE and immunoblot. Viability of PL cells after 24 h stimulation with P. dicentrarchi cysteine proteinases did not differ from that of non-stimulated cells. Apoptosis was confirmed by: (i) caspase activity, (ii) DNA fragmentation, and (iii) nucleus fragmentation. The caspase-3-like activity in PL incubated for 4h in the presence of 125, 250 and 500 microg/ml of proteinases increased in a dose-dependent fashion. The PL DNA was fragmented following 24-h exposure to P. dicentrarchi cysteine proteinases and characteristic DNA ladders consisting of multimers of approximately 180-200 pb were produced. Morphological changes, such as chromatin condensation and nucleus fragmentation, were observed under fluorescence microscopy after DAPI staining of the PL cells incubated with cysteine proteinase-incubated for 24 h. The results suggest that the pathogenic scuticociliate P. dicentrarchi may induce host leucocyte programmed cell death via the production of cysteine proteinases, as a mechanism of pathogenesis and evasion of the turbot innate immune response.

Blastocystis ratti induces contact-independent apoptosis, F-actin rearrangement, and barrier function disruption in IEC-6 cells

Blastocystis is an enteric protozoan purportedly associated with numerous clinical cases of diarrhea, flatulence, vomiting, and other gastrointestinal symptoms. Despite new knowledge of Blastocystis cell biology, genetic diversity, and epidemiology, its pathogenic potential remains controversial. Numerous clinical and epidemiological studies either implicate or exonerate the parasite as a cause of intestinal disease. Therefore, the aim of this study was to investigate the pathogenic potential of Blastocystis by studying the interactions of Blastocystis ratti WR1, an isolate of zoonotic potential, with a nontransformed rat intestinal epithelial cell line, IEC-6. Here, we report that B. ratti WR1 induces apoptosis in IEC-6 cells in a contact-independent manner. Furthermore, we found that B. ratti WR1 rearranges F-actin distribution, decreases transepithelial resistance, and increases epithelial permeability in IEC-6 cell monolayers. In addition, we found that the effects of B. ratti on transepithelial electrical resistance and epithelial permeability were significantly abrogated by treatment with metronidazole, an antiprotozoal drug. Our results suggest for the first time that Blastocystis-induced apoptosis in host cells and altered epithelial barrier function might play an important role in the pathogenesis of Blastocystis infections and that metronidazole has therapeutic potential in alleviating symptoms associated with Blastocystis.

Dipeptide vinyl sultams: synthesis via the Wittig-Horner reaction and activity against papain, falcipain-2 and Plasmodium falciparum

The synthesis of phosphonate derivatives of N-phenyl- and N-benzyl-gamma- and delta-sultams, and their application in the Wittig-Horner reaction with N-Boc-L-phenylalanine aldehyde to afford E- and Z-isomers, are described. These compounds were further processed to provide five dipeptide vinyl sultams, which were found to be inactive against papain at concentrations up to 50 microM. In contrast, vinyl sultams demonstrated weak activity against recombinant falcipain-2 and Plasmodium falciparum W2.

Potencies of human immunodeficiency virus protease inhibitors in vitro against Plasmodium falciparum and in vivo against murine malaria

Parasite resistance to antimalarial drugs is a serious threat to human health, and novel agents that act on enzymes essential for parasite metabolism, such as proteases, are attractive targets for drug development. Recent studies have shown that clinically utilized human immunodeficiency virus (HIV) protease inhibitors can inhibit the in vitro growth of Plasmodium falciparum at or below concentrations found in human plasma after oral drug administration. The most potent in vitro antimalarial effects have been obtained for parasites treated with saquinavir, ritonavir, or lopinavir, findings confirmed in this study for a genetically distinct P. falciparum line (3D7). To investigate the potential in vivo activity of antiretroviral protease inhibitors (ARPIs) against malaria, we examined the effect of ARPI combinations in a murine model of malaria. In mice infected with Plasmodium chabaudi AS and treated orally with ritonavir-saquinavir or ritonavir-lopinavir, a delay in patency and a significant attenuation of parasitemia were observed. Using modeling and ligand docking studies we examined putative ligand binding sites of ARPIs in aspartyl proteases of P. falciparum (plasmepsins II and IV) and P. chabaudi (plasmepsin) and found that these in silico analyses support the antimalarial activity hypothesized to be mediated through inhibition of these enzymes. In addition, in vitro enzyme assays demonstrated that P. falciparum plasmepsins II and IV are both inhibited by the ARPIs saquinavir, ritonavir, and lopinavir. The combined results suggest that ARPIs have useful antimalarial activity that may be especially relevant in geographical regions where HIV and P. falciparum infections are both endemic.

Giardia and Cryptosporidium join the 'Neglected Diseases Initiative'

Giardia and Cryptosporidium are ubiquitous enteric protozoan pathogens that infect humans, domestic animals and wildlife worldwide. Both pathogens are significant causes of diarrhea and nutritional disorders in institutional and community settings. They are also significant waterborne pathogens. In developing regions of the world, Giardia and Cryptosporidium constitute part of the complex group of parasitic, bacterial and viral diseases that impair the ability to achieve full potential and impair development and socio-economic improvements. All diseases included in the WHO Neglected Diseases Initiative have a common link with poverty and, as the current view is to take a comprehensive approach to all these diseases, both Giardia and Cryptosporidium were included in 2004. Our current state of knowledge of Giardia and Cryptosporidium is summarized here, and some important questions are raised that need to be addressed if control strategies are to be effective.

Gene discovery in Plasmodium vivax through sequencing of ESTs from mixed blood stages

Despite the significance of Plasmodium vivax as the most widespread human malaria parasite and a major public health problem, gene expression in this parasite is poorly understood. To accelerate gene discovery and facilitate the annotation phase of the P. vivax genome project, we have undertaken a transcriptome approach to study gene expression in the mixed blood stages of a P. vivax field isolate. Using a cDNA library constructed from purified blood stages, we have obtained single-pass sequences for approximately 21,500 expressed sequence tags (ESTs), the largest number of transcript tags obtained so far for this species. Cluster analysis revealed that the library is highly redundant, resulting in 5407 clusters. Clustered ESTs were searched against public protein databases for functional annotation, and more than one-third showed a significant match, the majority of these to Plasmodium falciparum proteins. The most abundant clusters were to genes encoding ribosomal proteins and proteins involved in metabolism, consistent with the predominance of trophozoites in the field isolate sample. In spite of the scarcity of other parasite stages in the field isolate, we could identify genes that are expressed in rings, schizonts and gametocytes. This study should facilitate our understanding of the gene expression in P. vivax asexual stages and provide valuable data for gene prediction and annotation of the P. vivax genome sequence.

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.

A zymographic study of metalloprotease activities in extracts and extracellular secretions of Leishmania (Viannia) braziliensis strains

Proteolytic activities of 5 strains of Leishmania (Viannia) braziliensis isolated from Brazilian and Colombian patients, presenting distinct clinical manifestations, were characterized and compared using whole-promastigote extracts and extracellular secretions. Zymographic assays concerning whole-cell extracts and supernatants resulted in the detection of high molecular weight bands, ranging from 50 to 125 kDa. Proteolytic activities from both whole-cell extracts and supernatants were optimal in a pH range 5.5 to 9.0 for all analysed strains. Such protease activities were inhibited when 10 mM 1,10-phenanthroline was assayed, strongly suggesting that the enzymes responsible for hydrolysis of the substrate belong to the metalloproteases class. Distinct profiles of metalloproteases were observed among the studied L. (V.) braziliensis strains. Differences among the microorganisms might be related to the geographical origin of the strains and/or to the clinical presentation.

P. falciparum pro-histoaspartic protease (proHAP) protein peptides bind specifically to erythrocytes and inhibit the invasion process in vitro

Plasmodium falciparum histoaspartic protease (HAP) is an active enzyme involved in haemoglobin degradation. HAP is expressed as an inactive 51-kDa zymogen and is cleaved into an active 37-kDa enzyme. It has been proposed that this kind of protease might be implicated in the parasite's invasion of erythrocytes; however, this protein's role during invasion has still to be determined. Synthetic peptides derived from the HAP precursor (proHAP) were tested in erythrocyte binding assays to identify their possible function in the invasion process. Two proHAP high-activity binding peptides (HABPs) specifically bound to erythrocytes; these peptides were numbered 30609 (101LKNYIKESVKLFNKGLTKKS120) and 30610 (121YLGSEFDNVELKDLANVLSF140 ). The binding of these two peptides was saturable, presenting nanomolar affinity constants. These peptides interacted with 26- and 45-kDa proteins on the erythrocyte surface; the nature of these receptor sites was studied in peptide binding assays using enzyme-treated erythrocytes. The HABPs showed greater than 90% merozoite invasion inhibition in in vitro assays. Goat serum containing proHAP polymeric peptide antibodies inhibited parasite invasion in vitro .

Characterization of an extracellular serine protease of Leishmania (Leishmania) amazonensis

A serine protease was purified 942-fold from culture supernatant of L. amazonensis promastigotes using (NH4)2SO4 precipitation followed by affinity chromatography on aprotinin-agarose and continuous elution electrophoresis by Prep Cell, yielding a total recovery of 61%. The molecular mass of the active enzyme estimated by SDS-PAGE under conditions of reduction was 56 kDa and 115 kDa under conditions of non-reduction, suggesting that the protease is a dimeric protein. Additionally, it was found to be a non-glycosylated enzyme, with a pI of 5.0. The optimal pH and temperature of the enzyme were 7.5 and 28 degrees C respectively, using alpha-N-rho-tosyl-L-arginine-methyl ester (L-TAME) as substrate. Assays of thermal stability indicated that 61% of the enzyme activity was preserved after 1 h of pre-treatment at 42 degrees C. Haemoglobin, bovine serum albumin (BSA), ovalbumin, fibrinogen, collagen, gelatin and peptide substrates containing arginine in an ester bond and amide substrates containing hydrophobic residues at the P1 site were hydrolysed by this extracellular protease. The insulin beta-chain was also hydrolysed by the enzyme and many peptidic bonds were susceptible to the protease action, and 4 of them (L11-V12, E3-A14, L15-Y16 and Y16-L17) were identified. Inhibition studies suggested that the enzyme belongs to the serine protease class inhibited by calcium and manganese and activated by zinc. These findings show that this enzyme of L. amazonensis is a novel serine protease, which differs from all known flagellate proteases characterized.

Subcellular localization of an intracellular serine protease of 68 kDa in Leishmania (Leishmania) amazonensis promastigotes

Here we report the subcellular localization of an intracellular serine protease of 68 kDa in axenic promastigotes of Leishmania (Leishmania) amazonensis, using subcellular fractionation, enzymatic assays, immunoblotting, and immunocytochemistry. All fractions were evaluated by transmission electron microscopy and the serine protease activity was measured during the cell fractionation procedure using alpha-N-r-tosyl-L-arginine methyl ester (L-TAME) as substrate, phenylmethylsulphone fluoride (PMSF) and L-1-tosylamino-2-phenylethylchloromethylketone (TPCK) as specific inhibitors. The enzymatic activity was detected mainly in a membranous vesicular fraction (6.5-fold enrichment relative to the whole homogenate), but also in a crude plasma membrane fraction (2.0-fold). Analysis by SDS-PAGE gelatin under reducing conditions demonstrated that the major proteolytic activity was found in a 68 kDa protein in all fractions studied. A protein with identical molecular weight was also recognized in immunoblots by a polyclonal antibody against serine protease (anti-SP), with higher immunoreactivity in the vesicular fraction. Electron microscopic immunolocalization using the same polyclonal antibody showed the enzyme present at the cell surface, as well as in cytoplasmic membranous compartments of the parasite. Our findings indicate that the internal location of this serine protease in L. amazonensis is mainly restricted to the membranes of intracellular compartments resembling endocytic/exocytic elements.

Cysteine-protease activity elicited by Ca2+ stimulus in Plasmodium

Bloodstage malaria parasites require proteolytic activity for key processes as invasion, hemoglobin degradation and merozoite escape from red blood cells (RBCs). We investigated by confocal microscopy the presence of cysteine-protease activity elicited by calcium stimulus in Plasmodium chabaudi and Plasmodium falciparum in free trophozoites or for the later parasite within RBC using fluorescence resonance energy transfer (FRET) peptides. Peptide probes access, to either free or intraerythrocytic parasites, was also tested by selecting a range of fluorescent peptides (653-3146 Da molecular mass) labeled with Abz or FITC. In the present work we show that Ca2+ stimulus elicited by treatment with either melatonin, thapsigargin, ionomicin or nigericin, promotes an increase of substrate hydrolysis, which was blocked by the specific cysteine-protease inhibitor E-64 and the intracellular Ca2+ chelator, BAPTA. When parasites were treated with cytoplasmic Ca2+ releasing compounds, a cysteine-protease was labeled in the parasite cytoplasm by the fluorescent specific irreversible inhibitor, Ethyl-Eps-Leu-Tyr-Cap-Lys(Abz)-NH2, where Ethyl-Eps is Ethyl-(2S,3S)-oxirane-2,3-dicarboxylate. In summary, we demonstrate that P. chabaudi and P. falciparum have a cytoplasmic dependent cysteine-protease activity elicited by Ca2+.

Leishmania amazonensis: early proteinase activities during promastigote-amastigote differentiation in vitro

Leishmania proteinase activity is known as parasite differentiation marker, and has been considered relevant for leishmanial survival and virulence. These properties suggest that Leishmania proteinases can be promising targets for development of anti-leishmania drugs. Here, we analyze the activities of four proteinases during the early phase of the Leishmania amazonensis promastigotes differentiation into amastigotes induced by heat shock. We have examined activities of cysteine-, metallo-, serine-, and aspartic-proteinase by hydrolysis of specific chromogenic substrates at pH 5.0 and at the optimal pH for each enzyme. Our results show that metallo-, serine-, and aspartic-proteinases activities were down-regulated during the shock-induced transformation of promastigotes into amastigotes. In contrast, cysteine-proteinase activity increased concomitantly with the promastigote differentiation. Immunocytochemical localization using two anti-cysteine-proteinase monospecific rabbit antibodies detected the enzyme in several cell compartments of both parasite stages. Our results show different proteinase activity modulation and expression during the early phases of the shock-induced parasite transformation.

Leishmania (Leishmania) amazonensis: purification and characterization of a promastigote serine protease

Pathogenic protozoan proteases play crucial roles in the host-parasite interaction, and its characterization contributes to the understanding of protozoan disease mechanisms. A Leishmania amazonensis promastigote protease was purified 36-fold, using aprotinin-agarose affinity chromatography and gel filtration high performance liquid chromatography, yielding a total recovery of 49%. The molecular mass of active enzyme obtained from native gel filtration HPLC and SDS-PAGE under conditions of reduction and non-reduction was 68 kDa, suggesting that the enzyme may exist as a monomer. The protease isoelectric point (pI) was around 4.45 and, as demonstrated by deglycosylation assay, it did not have any carbohydrate content. The optimal pH and temperature of the enzyme were 8.0 and 28 degrees C, respectively, determined using alpha-N-rho-tosyl-L-arginyl-methyl ester (L-TAME) as substrate. Assays of thermal stability indicated that 50% of the enzymatic activity was preserved after 4 min of pre-treatment at 42 degrees C and after 24 h of pre-treatment at 37 degrees C, both in the absence of substrate. Hemoglobin, bovine serum albumin (BSA), ovalbumin, and both gelatin and peptide substrates containing arginine in ester bound were hydrolyzed by 68 kDa protease. The insulin beta-chain was also hydrolyzed by the protease, and four peptidic bonds (L11-V12, E13-A14, L15-Y16, and Y16-L17) were susceptible to the 68-kDa protease action. Inhibition studies suggested that the enzyme belonged to a serine protease class inhibited by calcium ions and activated by manganese ions. These findings demonstrate that the L. amazonensis 68-kDa serine protease differs from those of other protozoan parasites.

Cysteine proteinase activities in the fish pathogenPhilasterides dicentrarchi(Ciliophora: Scuticociliatida)

This study investigated protease activities in a crude extract andin vitroexcretion/secretion (E/S) products ofPhilasterides dicentrarchi, a ciliate fish parasite causing economically significant losses in aquaculture. Gelatin/SDS–PAGE analysis (pH 4, reducing conditions) detected 7 bands with gelatinolytic activity (approximate molecular weights 30–63 kDa) in the crude extract. The banding pattern observed in analysis of E/S products was practically identical, except for 1 low-molecular-weight band detected in the crude extract but not in the E/S products. In assays with synthetic peptidep-nitroanilide substrates, the crude extract hydrolysed substrates characteristic of cysteine proteases, namely Z-Arg-Arg pNA, Bz-Phe-Val-Arg pNA and Z-Phe-Arg pNA. These activities were strongly inhibited by the cysteine protease inhibitor E-64 and by Ac-Leu-Val-Lys aldehyde, a potent inhibitor of cysteine proteases of the cathepsin B protease subfamily. The proteases present in the crude extract degraded both type-I collagen and haemoglobinin vitro, consistent with roles in tissue invasion and nutrition respectively. Again, E-64 completely (collagen) or markedly (haemoglobin) inhibited this degradation. Finally, the histolytic activity of the ciliate in turbot fibroblast monolayers was strongly reduced in the presence of E-64, confirming the importance of secreted cysteine proteinases in the biology ofPhilasterides dicentrarchi.