Active site mapping, biochemical properties and subcellular localization of rhodesain, the major cysteine protease of Trypanosoma brucei rhodesiense

Structural insights into the protease-like antigen Plasmodium falciparum SERA5 and its noncanonical active-site serine

The sera genes of the malaria-causing parasite Plasmodium encode a family of unique proteins that are maximally expressed at the time of egress of parasites from infected red blood cells. These multi-domain proteins are unique, containing a central papain-like cysteine-protease fragment enclosed between the disulfide-linked N- and C-terminal domains. However, the central fragment of several members of this family, including serine repeat antigen 5 (SERA5), contains a serine (S596) in place of the active-site cysteine. Here we report the crystal structure of the central protease-like domain of Plasmodium falciparum SERA5, revealing a number of anomalies in addition to the putative nucleophilic serine: (1) the structure of the putative active site is not conducive to binding substrate in the canonical cysteine-protease manner; (2) the side chain of D594 restricts access of substrate to the putative active site; and (3) the S(2) specificity pocket is occupied by the side chain of Y735, reducing this site to a small depression on the protein surface. Attempts to determine the structure in complex with known inhibitors were not successful. Thus, despite having revealed its structure, the function of the catalytic domain of SERA5 remains an enigma.

Role of AP-1 in developmentally regulated lysosomal trafficking in Trypanosoma brucei

African trypanosomes are the causative agents of human trypanosomiasis (sleeping sickness). The pathogenic stage of the parasite has unique adaptations to life in the bloodstream of the mammalian host, including upregulation of endocytic and lysosomal activities. We investigated stage-specific requirements for cytoplasmic adaptor/clathrin machinery in post-Golgi apparatus biosynthetic sorting to the lysosome using RNA interference silencing of the Tbmu1 subunit of adaptor complex 1 (AP-1), in conjunction with immunolocalization, kinetic analyses of reporter transport, and quantitative endocytosis assays. Tbmu1 silencing was lethal in both stages, indicating a critical function(s) for the AP-1 machinery. Transport of soluble and membrane-bound secretory cargoes was Tbmu1 independent in both stages. In procyclic parasites, trafficking of the lysosomal membrane protein, p67, was disrupted, leading to cell surface mislocalization. The lysosomal protease trypanopain was also secreted, suggesting a transmembrane-sorting receptor for this soluble hydrolase. In bloodstream trypanosomes, both p67 and trypanopain trafficking were unaffected by Tbmu1 silencing, suggesting that AP-1 is not necessary for biosynthetic lysosomal trafficking. Endocytosis in bloodstream cells was also unaffected, indicating that AP-1 does not function at the flagellar pocket. These results indicate that post-Golgi apparatus sorting to the lysosome is critically dependent on the AP-1/clathrin machinery in procyclic trypanosomes but that this machinery is not necessary in bloodstream parasites. We propose a simple model for stage-specific default secretory trafficking in trypanosomes that is consistent with the behavior of other soluble and glycosylphosphatidylinositol-anchored cargos and which is influenced by upregulation of endocytosis in bloodstream parasites as an adaptation to life in the mammalian bloodstream.

Cathepsin L-like genes of Trypanosoma vivax from Africa and South America--characterization, relationships and diagnostic implications

We characterized sequences from genes encoding cathepsin L-like (CatL-like) cysteine proteases from African and South American isolates of Trypanosoma vivax and T. vivax-like organisms, and evaluated their suitability as genetic markers for population structure analysis and diagnosis. Phylogenetic analysis of sequences corresponding to CatL-like catalytic domains revealed substantial polymorphism, and clades of sequences (TviCatL1-9) were separated by large genetic distances. TviCatL1-4 sequences were from cattle isolates from West Africa (Nigeria and Burkina Faso) and South America (Brazil and Venezuela), which belonged to the same T. vivax genotype. T. vivax-like genotypes from East Africa showed divergent sequences, including TviCatL5-7 for isolates from Mozambique and TviCatL8-9 for an isolate from Kenya. Phylogenetic analysis of CatL-like gene data supported the relationships among trypanosome species reflected in the phylogenies based on the analysis of small subunit (SSU) of ribosomal RNA gene sequence data. The discovery of different CatL-like sequences for each genotype, defined previously by ribosomal DNA data, indicate that these sequences provide useful targets for epidemiological and population genetic studies. Regions in CatL-like sequences shared by all T. vivax genotypes but not by other trypanosomes allowed the establishment of a specific and sensitive diagnostic PCR for epidemiological studies in South America and Africa.

Synthesis of macrocyclic trypanosomal cysteine protease inhibitors

The importance of cysteine proteases in parasites, compounded with the lack of redundancy compared to their mammalian hosts makes proteases attractive targets for the development of new therapeutic agents. The binding mode of K11002 to cruzain, the major cysteine protease of Trypanosoma cruzi was used in the design of conformationally constrained inhibitors. Vinyl sulfone-containing macrocycles were synthesized via olefin ring-closing metathesis and evaluated against cruzain and the closely related cysteine protease, rhodesain.

A parasite cysteine protease is key to host protein degradation and iron acquisition

Cysteine proteases of the Clan CA (papain) family are the predominant protease group in primitive invertebrates. Cysteine protease inhibitors arrest infection by the protozoan parasite, Trypanosoma brucei. RNA interference studies implicated a cathepsin B-like protease, tbcatB, as a key inhibitor target. Utilizing parasites in which one of the two alleles of tbcatb has been deleted, the key role of this protease in degradation of endocytosed host proteins is delineated. TbcatB deficiency results in a decreased growth rate and dysmorphism of the flagellar pocket and the subjacent endocytic compartment. Western blot and microscopic analysis indicate that deficiency in tbcatB results in accumulation of both host and parasite proteins, including the lysosomal marker p67. A critical function for parasitism is the degradation of host transferrin, which is necessary for iron acquisition. Substrate specificity analysis of recombinant tbcatB revealed the optimal peptide cleavage sequences for the enzyme and these were confirmed experimentally using FRET-based substrates. Degradation of transferrin was validated by SDS-PAGE and the specific cleavage sites identified by N-terminal sequencing. Because even a modest deficiency in tbcatB is lethal for the parasite, tbcatB is a logical target for the development of new anti-trypanosomal chemotherapy.

RNA interference of Trypanosoma brucei cathepsin B and L affects disease progression in a mouse model

We investigated the roles played by the cysteine proteases cathepsin B and cathepsin L (brucipain) in the pathogenesis of Trypansoma brucei brucei in both an in vivo mouse model and an in vitro model of the blood–brain barrier. Doxycycline induction of RNAi targeting cathepsin B led to parasite clearance from the bloodstream and prevent a lethal infection in the mice. In contrast, all mice infected with T. brucei containing the uninduced Trypanosoma brucei cathepsin B (TbCatB) RNA construct died by day 13. Induction of RNAi against brucipain did not cure mice from infection; however, 50% of these mice survived 60 days longer than uninduced controls. The ability of T. b. brucei to cross an in vitro model of the human blood–brain barrier was also reduced by brucipain RNAi induction. Taken together, the data suggest that while TbCatB is the more likely target for the development of new chemotherapy, a possible role for brucipain is in facilitating parasite entry into the brain.

African trypanosomiasis, or sleeping sickness, is caused by the single-cell parasite Trypanosoma brucei (T. brucei). Two parasite-derived enzyme proteins have been hypothesized to play an important role in the viability of the parasite or its ability to produce disease in the human host. Utilizing RNA interference that blocks the production of these proteins in the parasite, we show that elimination of parasite cathepsin B cures infection in mice. RNAi of the second enzyme protein, brucipain, results in the prolongation of life of half the infected mice, but does not cure. Further experiments carried out in a culture system show that brucipain facilitates the migration of parasites across a model of the blood–brain barrier. This suggests that while brucipain is not necessary for the viability of the organisms, it may play a role in infection by allowing parasites to reach the central nervous system and produce the severe second stage of sleeping sickness.

The trypanosome transcriptome is remodelled during differentiation but displays limited responsiveness within life stages

BACKGROUND

Trypanosomatids utilise polycistronic transcription for production of the vast majority of protein-coding mRNAs, which operates in the absence of gene-specific promoters. Resolution of nascent transcripts by polyadenylation and trans-splicing, together with specific rates of mRNA turnover, serve to generate steady state transcript levels that can differ in abundance across several orders of magnitude and can be developmentally regulated. We used a targeted oligonucleotide microarray, representing the strongly developmentally-regulated T. brucei membrane trafficking system and approximately 10% of the Trypanosoma brucei genome, to investigate both between-stage, or differentiation-dependent, transcriptome changes and within-stage flexibility in response to various challenges.

RESULTS

6% of the gene cohort are developmentally regulated, including several small GTPases, SNAREs, vesicle coat factors and protein kinases both consistent with and extending previous data. Therefore substantial differentiation-dependent remodeling of the trypanosome transcriptome is associated with membrane transport. Both the microarray and qRT-PCR were then used to analyse transcriptome changes resulting from specific gene over-expression, knockdown, altered culture conditions and chemical stress. Firstly, manipulation of Rab5 expression results in co-ordinate changes to clathrin protein expression levels and endocytotic activity, but no detectable changes to steady-state mRNA levels, which indicates that the effect is mediated post-transcriptionally. Secondly, knockdown of clathrin or the variant surface glycoprotein failed to perturb transcription. Thirdly, exposure to dithiothreitol or tunicamycin revealed no evidence for a classical unfolded protein response, mediated in higher eukaryotes by transcriptional changes. Finally, altered serum levels invoked little transcriptome alteration beyond changes to expression of ESAG6/7, the transferrin receptor.

CONCLUSION

While trypanosomes regulate mRNA abundance to effect the major changes accompanying differentiation, a given differentiated state appears transcriptionally inflexible. The implications of the absence of a transcriptome response in trypanosomes for both virulence and models of life cycle progression are discussed.

The LAMP-like protein p67 plays an essential role in the lysosome of African trypanosomes

RNAi knockdown was employed to study the function of p67, a lysosome-associated membrane protein (LAMP)-like type I transmembrane lysosomal glycoprotein in African trypanosomes. Conditional induction of p67 dsRNA resulted in specific approximately 90% reductions in de novo p67 synthesis in both mammalian bloodstream and procyclic insect-stage parasites. Bloodstream cell growth was severely retarded with extensive death after > 24 h of induction. Biosynthetic trafficking of residual p67, and of the soluble lysosomal protease trypanopain, were unimpaired. Endocytosis of tomato lectin, a surrogate receptor-mediated cargo, was only mildly impaired (approximately 20%), but proper lysosomal targeting was unaffected. p67 ablation had dramatic effects on lysosomal morphology with gross enlargement (four- to fivefold) and internal membrane profiles reminiscent of autophagic vacuoles. Ablation of p67 expression rendered bloodstream trypanosomes refractory to lysis by human trypanolytic factor (TLF), a lysosomally activated host innate immune mediator. Similar effects on lysosomal morphology and TLF sensitivity were also obtained by two pharmacological agents that neutralize lysosomal pH--chloroquine and bafilomycin A1. Surprisingly, however, lysosomal pH was not affected in ablated cells suggesting that other physiological alterations must account for increased resistance to TLF. These results indicate p67 plays an essential role in maintenance of normal lysosomal structure and physiology in bloodstream-stage African trypanosomes.

Discovery of trypanocidal thiosemicarbazone inhibitors of rhodesain and TbcatB

Human African trypanosomiasis (HAT) is caused by the protozoan parasite Trypanosoma brucei. The cysteine proteases of T. brucei have been shown to be crucial for parasite replication and represent an attractive point for therapeutic intervention. Herein we describe the synthesis of a series of thiosemicarbazones and their activity against the trypanosomal cathepsins TbcatB and rhodesain, as well as human cathepsins L and B. The activity of these compounds was determined against cultured T. brucei, and specificity was assessed with a panel of four mammalian cell lines.

A determination of the steady state lysosomal pH of bloodstream stage African trypanosomes

The lysosomal/endosomal system of African trypanosomes is developmentally regulated and is important in the pathogenesis associated with infection of the mammalian bloodstream. Long considered to be a target for drug development, the internal pH of the lysosome has been variously reported to range from <5.0 to >6.0. We have refined a flow cytometric technique using a pH-sensitive probe that specifically targets the lysosome, tomato lectin:Oregon Green 488 conjugate. The probe is delivered to the lysosome with fidelity, where it is shielded against external pH. Measurement of fluorescent output in the presence and absence of lysomotropic agent (NH(4)Cl) then allows precise titration of steady state lysosomal pH (4.84+/-0.23). Using bafilomycin A1 to inhibit acidification we demonstrate that this method is responsive to pharmacological perturbation of lysosomal physiology. This work should facilitate future studies of the lysosomal function in African trypanosomiasis, as well as other parasitic protozoa.

Dipeptidyl-alpha,beta-epoxyesters as potent irreversible inhibitors of the cysteine proteases cruzain and rhodesain

The dipeptidyl epoxyesters 3 and 4 are potent, irreversible inhibitors of cruzain and rhodesain.

Role of the Trypanosoma brucei natural cysteine peptidase inhibitor ICP in differentiation and virulence

ICP is a chagasin-family natural tight binding inhibitor of Clan CA, family C1 cysteine peptidases (CPs). We investigated the role of ICP in Trypanosoma brucei by generating bloodstream form ICP-deficient mutants (Deltaicp). A threefold increase in CP activity was detected in lysates of Deltaicp, which was restored to the levels in wild type parasites by re-expression of the gene in the null mutant. Deltaicp displayed slower growth in culture and increased resistance to a trypanocidal synthetic CP inhibitor. More efficient exchange of the variant surface glycoprotein (VSG) to procyclin during differentiation from bloodstream to procyclic form was observed in Deltaicp, a phenotype that was reversed in the presence of synthetic CP inhibitors. Furthermore, we showed that degradation of anti-VSG IgG is abolished when parasites are pretreated with synthetic CP inhibitors, and that parasites lacking ICP degrade IgG more efficiently than wild type. In addition, Deltaicp reached higher parasitemia than wild type parasites in infected mice, suggesting that ICP modulates parasite infectivity. Taken together, these data suggest that CPs of T. brucei bloodstream form play a role in surface coat exchange during differentiation, in the degradation of internalized IgG and in parasite infectivity, and that their function is regulated by ICP.

Squamous cell carcinoma antigen 1 is an inhibitor of parasite-derived cysteine proteases

The physiological significance of the squamous cell carcinoma antigens 1 (SCCA1) and SCCA2, members of the ovalbumin serpin family, remains unresolved. In this study, we examined whether SCCA1 or SCCA2 inhibits protozoa- or helminth-derived cysteine proteases. SCCA1, but not SCCA2, potently inhibited the cysteine protease activities of CPB2.8 from Leishmania mexicana, cruzain from Trypanosoma cruzi, rhodesain from Trypanosoma brucei rhodesience, and cathepsin L2 from Fasciola hepatica. The inhibitory activities of SCCA1 were due to its resistance to cleavage by the cysteine proteases. The findings indicate that induction of cysteine protease inhibitors might be a novel defense mechanism against parasite development.

The propeptide of cruzipain--a potent selective inhibitor of the trypanosomal enzymes cruzipain and brucipain, and of the human enzyme cathepsin F

Papain-like cysteine proteases of pathogenic protozoa play important roles in parasite growth, differentiation and host cell invasion. The main cysteine proteases of Trypanosoma cruzi (cruzipain) and of Trypanosoma brucei (brucipain) are validated targets for the development of new chemotherapies. These proteases are synthesized as precursors and activated upon removal of the N-terminal prodomain. Here we report potent and selective inhibition of cruzipain and brucipain by the recombinant full-length prodomain of cruzipain. The propeptide did not inhibit human cathepsins S, K or B or papain at the tested concentrations, and moderately inhibited human cathepsin V. Human cathepsin F was very efficiently inhibited (K(i) of 32 pm), an interesting finding indicating that cruzipain propeptide is able to discriminate cathepsin F from other cathepsin L-like enzymes. Comparative structural modeling and analysis identified the interaction between the beta1p-alpha3p loop of the propeptide and the propeptide-binding loop of mature enzymes as a plausible cause of the observed inhibitory selectivity.

The cysteine proteinase inhibitor Z-Phe-Ala-CHN2 alters cell morphology and cell division activity of Trypanosoma brucei bloodstream forms in vivo

Background

Current chemotherapy of human African trypanosomiasis or sleeping sickness relies on drugs developed decades ago, some of which show toxic side effects. One promising line of research towards the development of novel anti-trypanosomal drugs are small-molecule inhibitors of Trypanosoma brucei cysteine proteinases.

Results

In this study, we demonstrate that treatment of T. brucei-infected mice with the inhibitor, carbobenzoxy-phenylalanyl-alanine-diazomethyl ketone (Z-Phe-Ala-CHN₂), alters parasite morphology and inhibits cell division. Following daily intra-peritoneal administration of 250 mg kg^-1 of Z-Phe-Ala-CHN₂ on days three and four post infection (p.i.), stumpy-like forms with enlarged lysosomes were evident by day five p.i. In addition, trypanosomes exposed to the inhibitor had a 65% greater protein content than those from control mice. Also, in contrast to the normal 16% of parasites containing two kinetoplasts – a hallmark of active mitosis, only 4% of trypanosomes exposed to the inhibitor were actively dividing, indicating cell cycle-arrest.

Conclusion

We suggest that inhibition of endogenous cysteine proteinases by Z-Phe-Ala-CHN₂ depletes the parasite of essential nutrients necessary for DNA synthesis, which in turn, prevents progression of the cell cycle. This arrest then triggers differentiation of the long-slender into short-stumpy forms.

Programmed cell death in African trypanosomes

Until recently it had generally been assumed that apoptosis and other forms of programmed cell death evolved during evolution of the metazoans to regulate growth and development in these multicellular organisms. However, recent research is adding strength to the original phenotypic observations described almost a decade ago which indicated that some parasitic protozoa may have evolved a cell death pathway analogous to the process described as apoptosis in metazoa. Here we explore the implications of a programmed cell death pathway in the African tsetse-transmitted trypanosomes.

Lima APC, Kim YV, Lonsdale-Eccles JD, Fukuma T, Scharfstein J, Grab DJ. Blood-brain barrier traversal by African trypanosomes requires calcium signaling induced by parasite cysteine protease

In this study we investigated why bloodstream forms of Trypanosoma brucei gambiense cross human brain microvascular endothelial cells (BMECs), a human blood-brain barrier (BBB) model system, at much greater efficiency than do T. b. brucei. After noting that T. b. gambiense displayed higher levels of cathepsin L-like cysteine proteases, we investigated whether these enzymes contribute to parasite crossing. First, we found that T. b. gambiense crossing of human BMECs was abrogated by N-methylpiperazine-urea-Phe-homopheylalanine-vinylsulfone-benzene (K11777), an irreversible inhibitor of cathepsin L-like cysteine proteases. Affinity labeling and immunochemical studies characterized brucipain as the K11777-sensitive cysteine protease expressed at higher levels by T. b. gambiense. K11777-treated T. b. gambiense failed to elicit calcium fluxes in BMECs, suggesting that generation of activation signals for the BBB is critically dependant on brucipain activity. Strikingly, crossing of T. b. brucei across the BBB was enhanced upon incubation with brucipain-rich supernatants derived from T. b. gambiense. The effects of the conditioned medium, which correlated with ability to evoke calcium fluxes, were canceled by K11777, but not by the cathepsin B inhibitor CA074. Collectively, these in vitro studies implicate brucipain as a critical driver of T. b. gambiense transendothelial migration of the human BBB.

Novel antitrypanosomal agents

Trypanosomes are the causative agents of Chagas' disease in Central and South America and sleeping sickness in sub-Saharan Africa. The current chemotherapy of the human trypanosomiases relies on only six drugs, five of which were developed > 30 years ago. In addition, these drugs display undesirable toxic side effects and the emergence of drug-resistant trypanosomes has been reported. Therefore, the development of new drugs in the treatment of Chagas' disease and sleeping sickness is urgently required. This article summarises the recent progress in identifying novel lead compounds for antitrypanosomal chemotherapy. Particular emphasis is placed on those agents showing promising, selective antitrypanosomal activity.

Substrate profiling of cysteine proteases using a combinatorial peptide library identifies functionally unique specificities

The substrate specificities of papain-like cysteine proteases (clan CA, family C1) papain, bromelain, and human cathepsins L, V, K, S, F, B, and five proteases of parasitic origin were studied using a completely diversified positional scanning synthetic combinatorial library. A bifunctional coumarin fluorophore was used that facilitated synthesis of the library and individual peptide substrates. The library has a total of 160,000 tetrapeptide substrate sequences completely randomizing each of the P1, P2, P3, and P4 positions with 20 amino acids. A microtiter plate assay format permitted a rapid determination of the specificity profile of each enzyme. Individual peptide substrates were then synthesized and tested for a quantitative determination of the specificity of the human cathepsins. Despite the conserved three-dimensional structure and similar substrate specificity of the enzymes studied, distinct amino acid preferences that differentiate each enzyme were identified. The specificities of cathepsins K and S partially match the cleavage site sequences in their physiological substrates. Capitalizing on its unique preference for proline and glycine at the P2 and P3 positions, respectively, selective substrates and a substrate-based inhibitor were developed for cathepsin K. A cluster analysis of the proteases based on the complete specificity profile provided a functional characterization distinct from standard sequence analysis. This approach provides useful information for developing selective chemical probes to study protease-related pathologies and physiologies.

Aziridine-2,3-dicarboxylate inhibitors targeting the major cysteine protease of Trypanosoma brucei as lead trypanocidal agents

The protozoan parasite Trypanosoma brucei causes Human African trypanosomiasis, which is fatal if left untreated. Due to the toxicity of currently used drugs and emerging drug resistance, there is an urgent need for novel therapies. The major trypanosome papain-like cysteine protease expressed by the parasite (e.g., rhodesain in T. b. rhodesiense) is considered an important target for the development of new trypanocidal drugs. Series of aziridine-2,3-dicarboxylate-based cysteine protease inhibitors have been tested, most of them inhibiting rhodesain in the low micromolar range. Among these, only dibenzyl aziridine-2,3-dicarboxylates display trypanocidal activity being equipotent to the drug eflornithine. The Leu-Pro-containing aziridinyl tripeptides 13a-f are the most promising as they are not cytotoxic to macrophages up to concentrations of 125microM.

Proteomics meets microbiology: technical advances in the global mapping of protein expression and function

The availability of complete genome sequences for a large number of pathogenic organisms has opened the door for large-scale proteomic studies to dissect both protein expression/regulation and function. This review highlights key proteomic methods including two-dimensional gel electrophoresis, reference mapping, protein expression profiling and recent advances in gel-free separation techniques that have made a significant impact on the resolution of complex proteomes. In addition, we highlight recent developments in the field of chemical proteomics, a branch of proteomics aimed at functionally profiling a proteome. These techniques include the development of activity-based probes and activity-based protein profiling methods as well as the use of synthetic small molecule libraries to screen for pharmacological tools to perturb basic biological processes. This review will focus on the applications of these technologies to the field of microbiology.

High Throughput Substrate Specificity Profiling of Serine and Cysteine Proteases Using Solution-phase Fluorogenic Peptide Microarrays

Proteases regulate numerous biological processes with a degree of specificity often dictated by the amino acid sequence of the substrate cleavage site. To map protease/substrate interactions, a 722-member library of fluorogenic protease substrates of the general format Ac-Ala-X-X-(Arg/Lys)-coumarin was synthesized (X=all natural amino acids except cysteine) and microarrayed with fluorescent calibration standards in glycerol nanodroplets on glass slides. Specificities of 13 serine proteases (activated protein C, plasma kallikrein, factor VIIa, factor IXabeta, factor XIa and factor alpha XIIa, activated complement C1s, C1r, and D, tryptase, trypsin, subtilisin Carlsberg, and cathepsin G) and 11 papain-like cysteine proteases (cathepsin B, H, K, L, S, and V, rhodesain, papain, chymopapain, ficin, and stem bromelain) were obtained from 103,968 separate microarray fluorogenic reactions (722 substrates x 24 different proteases x 6 replicates). This is the first comprehensive study to report the substrate specificity of rhodesain, a papain-like cysteine protease expressed by Trypanasoma brucei rhodesiense, a parasitic protozoa responsible for causing sleeping sickness. Rhodesain displayed a strong P2 preference for Leu, Val, Phe, and Tyr in both the P1=Lys and Arg libraries. Solution-phase microarrays facilitate protease/substrate specificity profiling in a rapid manner with minimal peptide library or enzyme usage.

Peptidyl allyl sulfones: a new class of inhibitors for clan CA cysteine proteases

A new series of peptidyl allyl sulfone inhibitors was discovered while trying to synthesize epoxy sulfone inhibitors from vinyl sulfones using basic oxidizing conditions. The various dipeptidyl allyl sulfones were evaluated with calpain I, papain, cathepsins B and L, cruzain and rhodesain and found to be potent inhibitors. In comparison to the previously developed class of vinyl sulfone inhibitors, the novel dipeptidyl allyl sulfones were more potent inhibitors than the corresponding dipeptidyl vinyl sulfones. It was observed that the stereochemistry of the vinyl sulfone precursor played a role in the potency of the dipeptidyl allyl sulfone inhibitor.

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.

A cathepsin B-like protease is required for host protein degradation in Trypanosoma brucei

Identification and analysis of Clan CA (papain) cysteine proteases in primitive protozoa and metazoa have suggested that this enzyme family is more diverse and biologically important than originally thought. The protozoan parasite Trypanosoma brucei is the etiological agent of African sleeping sickness. The cysteine protease activity of this organism is a validated drug target as first recognized by the killing of the parasite with the diazomethane inhibitor Z-Phe-Ala-CHN(2) (where Z is benzyloxycarbonyl). Whereas the presumed target of this inhibitor was rhodesain (also brucipain, trypanopain), the major cathepsin L-like cysteine protease of T. brucei, genomic analysis has now identified tbcatB, a cathepsin B-like cysteine protease as a possible inhibitor target. The mRNA of tbcatB is more abundantly expressed in the bloodstream versus the procyclic form of the parasite. Induction of RNA interference against rhodesain did not result in an abnormal phenotype in cultured T. brucei. However, induction of RNA interference against tbcatB led to enlargement of the endosome, accumulation of fluorescein isothiocyanate-transferrin, defective cytokinesis after completion of mitosis, and ultimately the death of cultured parasites. Therefore, tbcatB, but not rhodesain, is essential for T. brucei survival in culture and is the most likely target of the diazomethane protease inhibitor Z-Phe-Ala-CHN(2) in T. brucei.

Synthesis and Structure−Activity Relationships of Parasiticidal Thiosemicarbazone Cysteine Protease Inhibitors againstPlasmodium falciparum,Trypanosoma brucei, andTrypanosoma cruzi

We have synthesized a library of thiosemicarbazones and screened them against three parasitic cysteine proteases, cruzain, falcipain-2, and rhodesain, and against the respective parasite sources of these three proteases, Trypanosoma cruzi, Plasmodium falciparum, and Trypanosoma brucei. The screens identified compounds that were effective against the enzymes and the parasites but also some compounds that were parasiticidal despite a lack of activity against the proteases. Several compounds were effective in killing all tested parasites. These promising lead compounds were tested for general toxicity in mice, and only one produced observable toxicity after 62 h. Our results suggest that thiosemicarbazones represent validated drug leads that kill several species of protozoan parasites through the inhibition of cysteine proteases as well as other novel targets.

Identification and biochemical characterization of vivapains, cysteine proteases of the malaria parasite Plasmodium vivax

Cysteine proteases play important roles in the life cycles of malaria parasites. Cysteine protease inhibitors block haemoglobin hydrolysis and development in Plasmodium falciparum, suggesting that the cysteine proteases of this major human pathogen, termed falcipains, are appropriate therapeutic targets. To expand our understanding of plasmodial proteases to Plasmodium vivax, the other prevalent human malaria parasite, we identified and cloned genes encoding the P. vivax cysteine proteases, vivapain-2 and vivapain-3, and functionally expressed the proteases in Escherichia coli. The vivapain-2 and vivapain-3 genes predicted papain-family cysteine proteases, which shared a number of unusual features with falcipain-2 and falcipain-3, including large prodomains and short N-terminal extensions on the catalytic domain. Recombinant vivapain-2 and vivapain-3 shared properties with the falcipains, including acidic pH optima, requirements for reducing conditions for activity and hydrolysis of substrates with positively charged residues at P1 and Leu at P2. Both enzymes hydrolysed native haemoglobin at acidic pH and the erythrocyte cytoskeletal protein 4.1 at neutral pH, suggesting similar biological roles to the falcipains. Considering inhibitor profiles, the vivapains were inhibited by fluoromethylketone and vinyl sulphone inhibitors that also inhibited falcipains and have demonstrated potent antimalarial activity.

Genetic variability in cysteine protease genes ofHaemonchus contortus

To increase the existent genetic variability in cysteine proteases, a polymorphism study was performed inHaemonchus contortusby comparing 2 different strains of the parasite: North American (NA) and Spanish (SP) strains. For this purpose, the polymorphism of 5 previously reported genes (AC-1,AC-3,AC-4,AC-5andGCP-7) were analysed by PCR–SSCP and sequencing procedures. Based on the SSCP results, a total of 20 different alleles were identified for the 5lociassessed. Exceptlocus AC-5, all thelociwere polymorphic.Loci AC-1,AC-3,AC-4andGCP-7showed 5, 8, 2 and 4 alleles, respectively. The allelic frequencies ranged from 0·0070 to 0·8560 and were significantly different between strains. In addition, nucleotide diversity analyses showed a significant variation within and between strains. The variations in the nucleotide sequence of the different alleles were translated in some cases into changes in the amino acid sequence. Evidence of genetic variability in cysteine proteases from two different strains ofH. contortusfor the same set of genes had not been previously reported.

Heterologous expression of maize (Zea mays L.) Mir1 cysteine proteinase in eukaryotic and prokaryotic expression systems

Several heterologous expression systems were tested for their ability to express a unique maize cysteine proteinase Mir1. A baculovirus-based expression system using Trichoplusia ni larvae as host resulted in the expression of Mir1 that was correctly processed and exhibited proteinase activity. Expression in Escherichia coli resulted in accumulation of Mir1, but it had limited solubility and enzymatic activity. Large quantities of Mir1 were produced when Pichia pastoris was used as the host, but the enzyme was insoluble and inactive.

Synthesis and evaluation of isatins and thiosemicarbazone derivatives against cruzain, falcipain-2 and rhodesain

While commercial isatins were practically inactive against the target proteases, thiosemicarbazone derivatives were found to be active. The most active compound from the series displayed an inhibitory IC(50) value of 1 microM against rhodesain. One thiosemicarbazone was found to be active against all three proteases with inhibitory IC(50) values of 10 microM or less. A combination of N-benzylation and appropriate substitution on the aromatic portion of the isatin scaffold was generally found to be beneficial especially against cruzain for ketone inhibitors.

Stage-specific translational efficiency and protein stability regulate the developmental expression of p37, an RNA binding protein from Trypanosoma brucei

We have previously characterized two novel RNA binding proteins, p34 and p37, from Trypanosoma brucei. Their sequences do not show significant homology to other proteins but are highly homologous to one another. The p34 and p37 proteins are developmentally regulated, with p34 the predominant protein in the procyclic stage and p37 nearly exclusively expressed in the bloodstream cells. In vivo metabolic labeling of procyclic cells showed that p34 and p37 were differentially translated, with levels of p34 approximately fourfold higher than p37. The newly synthesized p34 and p37 exhibited differential stability in the procyclic stage. In vitro analysis confirmed this observation and further suggested that this differential stability may be due to a trypsin-like cysteine protease activity in procyclic extracts that selectively degraded the p37 protein. Taken together, these results indicate that the developmental regulation of the T. brucei RNA binding protein, p37, occurs at both translational and post-translational levels.

Glycosylphosphatidylinositol-dependent protein trafficking in bloodstream stage Trypanosoma brucei

We have previously demonstrated that glycosylphosphatidylinositol (GPI) anchors strongly influence protein trafficking in the procyclic insect stage of Trypanosoma brucei (M. A. McDowell, D. A. Ransom, and J. D. Bangs, Biochem. J. 335:681-689, 1998), where GPI-minus variant surface glycoprotein (VSG) reporters have greatly reduced rates of endoplasmic reticulum (ER) exit but are ultimately secreted. We now demonstrate that GPI-dependent trafficking also occurs in pathogenic bloodstream trypanosomes. However, unlike in procyclic trypanosomes, truncated VSGs lacking C-terminal GPI-addition signals are not secreted but are mistargeted to the lysosome and degraded. Failure to export these reporters is not due to a deficiency in secretion of these cells since the N-terminal ATPase domain of the endogenous ER protein BiP is efficiently secreted from transgenic cell lines. Velocity sedimentation experiments indicate that GPI-minus VSG dimerizes similarly to wild-type VSG, suggesting that degradation is not due to ER quality control mechanisms. However, GPI-minus VSGs are fully protected from degradation by the cysteine protease inhibitor FMK024, a potent inhibitor of the major lysosomal protease trypanopain. Immunofluorescence of cells incubated with FMK024 demonstrates that GPI-minus VSG colocalizes with p67, a lysosomal marker. These data suggest that in the absence of a GPI anchor, VSG is mistargeted to the lysosome and subsequently degraded. Our findings indicate that GPI-dependent transport is a general feature of secretory trafficking in both stages of the life cycle. A working model is proposed in which GPI valence regulates progression in the secretory pathway of bloodstream stage trypanosomes.

Structure-activity relationships for inhibition of cysteine protease activity and development of Plasmodium falciparum by peptidyl vinyl sulfones

The Plasmodium falciparum cysteine proteases falcipain-2 and falcipain-3 appear to be required for hemoglobin hydrolysis by intraerythrocytic malaria parasites. Previous studies showed that peptidyl vinyl sulfone inhibitors of falcipain-2 blocked the development of P. falciparum in culture and exerted antimalarial effects in vivo. We now report the structure-activity relationships for inhibition of falcipain-2, falcipain-3, and parasite development by 39 new vinyl sulfone, vinyl sulfonate ester, and vinyl sulfonamide cysteine protease inhibitors. Levels of inhibition of falcipain-2 and falcipain-3 were generally similar, and many potent compounds were identified. Optimal antimalarial compounds, which inhibited P. falciparum development at low nanomolar concentrations, were phenyl vinyl sulfones, vinyl sulfonate esters, and vinyl sulfonamides with P(2) leucine moieties. Our results identify independent structural correlates of falcipain inhibition and antiparasitic activity and suggest that peptidyl vinyl sulfones have promise as antimalarial agents.

The endocytic apparatus of the kinetoplastida. Part II: machinery and components of the system

Endocytic systems within eukaryotic cells are a diverse set of intracellular transport pathways responsible for uptake, recycling, interaction with the exocytic system and degradation of molecules. Each of these pathways requires the interaction of distinct protein components that function in macromolecule sorting, control of transport rates and in membrane biogenesis. In the second of two articles on kinetoplastida endocytosis, the endocytic system in Trypanosoma brucei is considered as a model, and the molecules that control this system and the protein components of the endocytic pathway are discussed. We also consider novel mechanisms for sorting that have been proposed to operate in trypanosomes.

The kinetoplastida endocytic apparatus. Part I: a dynamic system for nutrition and evasion of host defences

The endocytic system of kinetoplastid parasites is a highly polarized membrane network focused on the flagellar pocket localized at one end of the cell. When first characterized, the endosomal network was envisioned as a simple system for uptake of extracellular material by fluid-phase or receptor-mediated mechanisms. Subsequently, it has become clear that the kinetoplastid endosomal system has an active and vital role in avoiding the host immune system and virulence, as well as providing the basic functions to fulfil cellular nutritional requirements. In two reviews, recent advances in the definition and comprehension of kinetoplastida endocytosis are discussed and, in Trypanosoma brucei in particular as the more developed experimental system. In Part 1, the endocytic system is considered in context of the surface molecules and their potential roles in virulence.

Developmentally regulated trafficking of the lysosomal membrane protein p67 in Trypanosoma brucei

p67 is a lysosomal type I membrane glycoprotein of Trypanosoma brucei. In procyclic stage cells p67 trafficks to the lysosome without modification, but in the bloodstream stage Golgi processing adds poly-N-acetyllactosamine to N-glycans. In both stages proteolytic fragmentation occurs in the lysosome, but turnover is approximately nine times faster in bloodstream cells. Trafficking of wildtype p67 and mutants missing the cytoplasmic (p67ΔCD) or cytoplasmic/transmembrane domains (p67ΔTM) was monitored by pulse-chase,surface biotinylation and immunofluorescence. Overexpressed wildtype p67 trafficks normally in procyclics, but some leaks to the cell surface suggesting that the targeting machinery is saturable. p67ΔCD and p67ΔTM are delivered to the cell surface and secreted, respectively. The membrane/cytoplasmic domains function correctly in procyclic cells when fused to GFP indicating that these domains are sufficient for stage-specific lysosomal targeting. In contrast, p67 wildtype and deletion reporters are overwhelmingly targeted to the lysosome and degraded in bloodstream cells. These findings suggest that either redundant developmentally regulated targeting signals/machinery are operative in this stage or that the increased endocytic activity of bloodstream cells prevents export of the deletion reporters.

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.

Secretory pathway of trypanosomatid parasites

The Trypanosomatidae comprise a large group of parasitic protozoa, some of which cause important diseases in humans. These include Trypanosoma brucei (the causative agent of African sleeping sickness and nagana in cattle), Trypanosoma cruzi (the causative agent of Chagas' disease in Central and South America), and Leishmania spp. (the causative agent of visceral and [muco]cutaneous leishmaniasis throughout the tropics and subtropics). The cell surfaces of these parasites are covered in complex protein- or carbohydrate-rich coats that are required for parasite survival and infectivity in their respective insect vectors and mammalian hosts. These molecules are assembled in the secretory pathway. Recent advances in the genetic manipulation of these parasites as well as progress with the parasite genome projects has greatly advanced our understanding of processes that underlie secretory transport in trypanosomatids. This article provides an overview of the organization of the trypanosomatid secretory pathway and connections that exist with endocytic organelles and multiple lytic and storage vacuoles. A number of the molecular components that are required for vesicular transport have been identified, as have some of the sorting signals that direct proteins to the cell surface or organelles in the endosome-vacuole system. Finally, the subcellular organization of the major glycosylation pathways in these parasites is reviewed. Studies on these highly divergent eukaryotes provide important insights into the molecular processes underlying secretory transport that arose very early in eukaryotic evolution. They also reveal unusual or novel aspects of secretory transport and protein glycosylation that may be exploited in developing new antiparasite drugs.