Primary structure and partial characterization of a life-cycle-regulated cysteine protease from Trypanosoma (Nannomonas) congolense

Host Immune Responses and Immune Evasion Strategies in African Trypanosomiasis

Parasites, including African trypanosomes, utilize several immune evasion strategies to ensure their survival and completion of their life cycles within their hosts. The defense factors activated by the host to resolve inflammation and restore homeostasis during active infection could be exploited and/or manipulated by the parasites in an attempt to ensure their survival and propagation. This often results in the parasites evading the host immune responses as well as the host sustaining some self-inflicted collateral tissue damage. During infection with African trypanosomes, both effector and suppressor cells are activated and the balance between these opposing arms of immunity determines susceptibility or resistance of infected host to the parasites. Immune evasion by the parasites could be directly related to parasite factors, (e.g., antigenic variation), or indirectly through the induction of suppressor cells following infection. Several cell types, including suppressive macrophages, myeloid-derived suppressor cells (MDSCs), and regulatory T cells have been shown to contribute to immunosuppression in African trypanosomiasis. In this review, we discuss the key factors that contribute to immunity and immunosuppression during T. congolense infection, and how these factors could aid immune evasion by African trypanosomes. Understanding the regulatory mechanisms that influence resistance and/or susceptibility during African trypanosomiasis could be beneficial in designing effective vaccination and therapeutic strategies against the disease.

Cysteine peptidases of kinetoplastid parasites

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

Expression, purification and characterisation of two variant cysteine peptidases from Trypanosoma congolense with active site substitutions

Congopain, the major cysteine peptidase of Trypanosoma congolense is an attractive candidate for an anti-disease vaccine and target for the design of specific inhibitors. A complicating factor for the inclusion of congopain in a vaccine is that multiple variants of congopain are present in the genome of the parasite. In order to determine whether the variant congopain-like genes code for peptidases with enzymatic activities different to those of congopain, two variants were cloned and expressed. Two truncated catalytic domain variants were recombinantly expressed in Pichia pastoris. The two expressed catalytic domain variants differed slightly from one another in substrate preferences and also from that of C2 (the recombinant truncated form of congopain). Surprisingly, a variant with the catalytic triad Ser(25), His(159) and Asn(175) was shown to be active against classical cysteine peptidase substrates and inhibited by E-64, a class-specific cysteine protease inhibitor. Both catalytic domain clones and C2 had pH optima of either 6.0 or 6.5 implying that these congopain-like proteases are likely to be expressed and active in the bloodstream of the host animal.

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.

Trypanosome hydrolases and the blood-brain barrier

African trypanosomes cross the blood-brain barrier, but how they do so remains an area of speculation. We propose that proteases, such as the trypanopains and oligopeptidases that are released by trypanosomes, could mediate in this process. The trypanosomes also possess cell-surface-associated acid phosphatases that could play a role in invasion similar to that in advancing cancer cells. Such enzymes, perhaps acting in concert, have the potential to cause tissue degradation and ease the passage of the trypanosomes through various tissues in the host, including the blood-brain barrier.

Functional expression of the catalytic domains of two cysteine proteinases from Trypanosoma congolense

The catalytic domains of two closely related cysteine proteinases (CP1 and CP2) from Trypanosoma congolense, referred to as C1 and C2, were expressed as proforms in Escherichia coli (C1) and in the baculovirus system (C1 and C2). While the bacterial expression system did not allow recovery of active C1, the baculovirus system led to secretion of inactive zymogens which could be processed at acidic pH into mature enzymes. Active C1 and C2 were purified from serum-free culture supernatants by anion-exchange chromatography and characterised. Their kinetic parameters and pH activity profiles confirmed the relatedness between C2 and native CP2 (congopain). These properties also underline major functional differences between C1 and C2, that appear to relate to discrete but essential sequence differences. It is likely that these two enzymes perform distinct roles in vivo, in the parasite and/or in the host-parasite relationships.

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

Cysteine protease activity of African trypanosome parasites is a target for new chemotherapy using synthetic protease inhibitors. To support this effort and further characterize the enzyme, we expressed and purified rhodesain, the target protease of Trypanosoma brucei rhodesiense (MVAT4 strain), in reagent quantities from Pichia pastoris. Rhodesain was secreted as an active, mature protease. Site-directed mutagenesis of a cryptic glycosylation motif not previously identified allowed production of rhodesain suitable for crystallization. An invariable ER(A/V)FNAA motif in the pro-peptide sequence of rhodesain was identified as being unique to the genus Trypanosoma. Antibodies to rhodesain localized the protease in the lysosome and identified a 40-kDa protein in long slender forms of T. b. rhodesiense and all life-cycle stages of T. b. brucei. With the latter parasite, protease expression was five times greater in short stumpy trypanosomes than in the other stages. Radiolabeled active site-directed inhibitors identified brucipain as the major cysteine protease in T. b. brucei. Peptidomimetic vinyl sulfone and epoxide inhibitors designed to interact with the S2, S1 and S' subsites of the active site cleft revealed differences between rhodesain and the related trypanosome protease cruzain. Using fluorogenic dipeptidyl substrates, rhodesain and cruzain had acid pH optima, but unlike some mammalian cathepsins retained significant activity and stability up to pH 8.0, consistent with a possible extracellular function. S2 subsite mapping of rhodesain and cruzain with fluorogenic peptidyl substrates demonstrates that the presence of alanine rather than glutamate at S2 prevents rhodesain from cleaving substrates in which P2 is arginine.

Immunisation of cattle with cysteine proteinases of Trypanosoma congolense: targetting the disease rather than the parasite

In order to test the hypothesis that trypanosome cysteine proteinases (CPs) contribute to pathology of trypanosomosis, cattle were immunised with CP1 and/or CP2, the major CPs of Trypanosoma congolense, and subsequently challenged with T. congolense. Immunisation had no effect on the establishment of infection and the development of acute anaemia. However, immunised cattle, unlike control cattle, maintained or gained weight during infection. Their haematocrit and leukocyte counts showed a tendency to recovery after 2-3 months of infection. Cattle immunised with CP2 mounted early and prominent IgG responses to CPs and to the variable surface glycoprotein following challenge. Thus trypanosome CPs may play a role in anaemia and immunosuppression; conversely, anti-CP antibody may modulate the trypanosome-induced pathology.

Susceptibility and resistance to Trypanosoma congolense infections

We have put emphasis on recent findings in experimental Trypanosoma congolense infections in highly susceptible BALB/c and relatively resistant C57Bl/6 mice. Based on various analyses, it has been shown that a major difference in resistance to T. congolense infections is expressed early in infection at the macrophage level. A novel plastic-adherent Thy1.2(+) suppressor lymphocyte, which in absolute synergy with a Thy 1.2(-) cell exerts its suppression via interleukin-10 and interferon-gamma opens up an exciting new field of research.

Expression of foreign proteins in Trypanosoma congolense

An expression vector was constructed to express foreign genes in Trypanosoma congolense. The foreign gene and a neomycin phosphotransferase (NPT) gene are flanked by glutamate and alanine rich protein (GARP) gene processing signals and their expression is driven by a ribosomal RNA gene promoter. The plasmid is not maintained as an episome in T. congolense, but the NPT gene permits selection of cells in which the plasmid has integrated into the genome. We used this plasmid to express luciferase, green fluorescent protein and a surface protein of Trypanosoma brucei, glycine-proline-glutamate glutamate threonine procyclic acidic repetitive protein (GPEET PARP). The plasmid-derived GPEET PARP is expressed on the surface of procyclic T. congolense and comigrates on a polyacrylamide gel with native GPEET PARP from T. brucei procyclic cells. We also attempted to use the plasmid to overexpress a previously identified T. congolense cysteine protease. The plasmid-derived cysteine protease mRNA species occurs in the transfected cells, but we were unable to detect increased levels of protein or protease activity.

Cysteine protease inhibitors as chemotherapy for parasitic infections

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

A comparison of the enzymatic properties of the major cysteine proteinases from Trypanosoma congolense and Trypanosoma cruzi

Congopain and cruzipain, the major cysteine proteinases from Trypanosoma congolense and Trypanosoma cruzi, were compared for their activities towards a series of new, sensitive fluorogenic substrates of the papain family of cysteine proteinases and for their sensitivity to inhibition by cystatins and related biotinylated peptidyl diazomethanes. Low Ki values, in the 10 pM range, were found for the interaction of both proteinases with natural cystatin inhibitors. The kinetic constants for the hydrolysis of cystatin-derived substrates, and the inhibition by related diazomethanes were essentially identical. Unlike cathepsins B and L, the related mammal papain family proteinases, congopain and cruzipain accomodate a prolyl residue in P2'. Substrates having the sequence VGGP from P2 to P2' were hydrolysed by both congopain and cruzipain with a k(cat)/Km greater than 4.10(3) mM(-1) s(-1). Irreversible diazomethane inhibitors, deduced from the unprime sequence of cystatin-derived substrates, inhibited the two parasite proteinases. N-terminal labelling of diazomethanes with a biotin group did not alter the rate of inhibition significantly, which provides a useful tool for examining the distribution of these enzymes in the parasite and in the host. Despite their similar activities on cystatin-derived substrates, congopain and cruzipain had significantly different pH-activity profiles when assayed with a cystatin-derived substrate. They were correlated with structural differences, especially at the presumed S2 subsites.

Leishmania major: comparison of the cathepsin L- and B-like cysteine protease genes with those of other trypanosomatids

Cysteine proteases play important roles in the pathogenesis of several parasitic infections and have been proposed as targets for the structure-based strategy of drug design. As a first step toward applying this strategy to design inhibitors as antiparasitic agents for leishmaniasis, we have isolated and sequenced the full-length clones of two cysteine protease genes from Leishmania major. One of the genes is structurally similar to the cathepsin L-like family and the other is similar to the cathepsin B-like family of cysteine proteases. The L. major cathepsin L-like sequence has a proregion that shares high sequence similarity with other cathepsin L sequences but not cathepsin B sequences and has a proline/threonine-rich C-terminal extension. The cathepsin L-like gene occurs in multiple copies, whereas there may be only one copy of the cathepsin B-like gene. Northern blot analyses show that both genes are expressed in the promastigote and amastigote stages, and pulse field gel electrophoresis revealed that the cathepsin L- and B-like genes are each found on two nonhomologous chromosomes. The L. major L-like amino acid sequence is 75% identical to the L. mexicana sequence, 74% identical to the L. pifanoi sequence, 47% identical with the Trypanosoma cruzi sequence, 47% identical with the T. congolense sequence, and 45% identical with the T. brucei sequence. L. major is one of two trypanosomatid species for which a cathepsin B-like gene has been identified and sequenced; its amino acid sequence is 82% identical to the one from L. mexicana. Tree inference based on distance and parsimony methods of kinetoplastid cathepsin L proteins yielded independent support for phylogenetic hypotheses inferred from analyses of ribosomal RNA genes. Because the cathepsin L locus has a high level of phylogenetic signal with respect to trypanosomatid taxa, this locus has great potential utility for investigating the evolutionary history of trypanosomatids and related organisms.

Proteases from Trypanosoma brucei brucei. Purification, characterisation and interactions with host regulatory molecules

African trypanosomes contain proteases that may be released into the bloodstream of their infected hosts. This paper describes a novel, combined isolation of a cysteine proteinase (called trypanopain-Tb) and a serine oligopeptidase (which we call oligopeptidase-Tb) from Trypanosoma brucei brucei, as well as a comparison of the activities of these two enzymes against several host regulatory molecules. The enzymes differed in various respects. Firstly, purified trypanopain-Tb was shown to readily cleave proteins such as gelatin maximally at acidic pH. In contrast, oligopeptidase-Tb, which is optimally active at alkaline pH, did not hydrolyse proteins larger than 4 kDa. However, it readily hydrolysed various polypeptides, including neurotensin and atrial natriuretic factor. The interaction of the two enzymes with mammalian protease inhibitors also differed. Cystatins and alpha2-macroglobulin effectively inhibited trypanopain-Tb, with the Ki values for cystatin C and low-molecular-mass kininogen (approximately 10(-11) M) predicting, that trypanopain-Tb is likely to be effectively controlled by these inhibitors if released into the host bloodstream. In contrast, oligopeptidase-Tb was not inhibited by serpins or (a2-macroglobulin, suggesting that it may remain active if released into the host bloodstream. In support of these in vitro results, the blood of trypanosome-infected rats displayed no trypanopain-Tb-like activity, but exhibited high oligopeptidase-Tb-like activity. Thus, while trypanopain-Tb seems likely to be confined to an intracellular role within the parasite, oligopeptidase-Tb has the potential to remain active in the host bloodstream and so contribute directly to pathogenesis.