Inhibitors of the major cysteinyl proteinase (GP57/51) impair host cell invasion and arrest the intracellular development of Trypanosoma cruzi in vitro

Peptidyl diazomethane (PDAM) derivatives, a class of irreversible inhibitors for cysteine proteinase, were screened for the ability to impair Trypanosoma cruzi invasion and intracellular development in primary cultures of heart muscle cells (HMC). T. cruzi GP57/51, a purified cysteinyl proteinase, and the substrate Z-Phe-Arg-NHMec were used to determine inhibition rate constants (k'+2) by continuous kinetic assays. The k'+2 values ranged from 25,400 to 2,800. The best inhibitors of GP57/51 had bulky hydrophobic residues in the P1 position (in addition to P2), the S1 sub-site specificity of the enzyme being thus similar to mammalian cathepsin L. The effects of these PDAM on parasite infectivity were then investigated. The ability to invade HMC was markedly impaired when trypomastigotes were briefly exposed to 10 microM of Z-(S-Bzl)Cys-Phe-CHN2. Striking effects were observed when PDAM were added to HMC cultures that had been previously infected with trypomastigotes: Z-(S-Bzl)Cys-Phe-CHN2 with an IC50 of 0.4 microM, and less markedly Z-Phe-Phe-CHN2 and Z-Tyr-Phe-CHN2 (or Z-Phe-Tyr-CHN2) blocked amastigote replication as well as their transformation into trypomastigotes, thereby arresting intracellular development. Bz-Phe-Gly-CHN2, in contrast, failed to display antiparasite activity. Direct characterization of the target cysteinyl proteinase was sought, by incubating viable amastigotes or infected HMC with Z-[125I]Tyr-Phe-CHN2. Affinity labeling implicated GP57/51 as the major cysteinyl proteinase target for this probe. We propose that T. cruzi intracellular development is critically dependent on GP57/51 (cruzipain). Selective inhibitors for this cysteinyl proteinase may have therapeutic potential.

Further characterization of Trypanosoma cruzi GP57/51 as the major antigen expressed by differentiating epimastigotes

Study of Trypanosoma cruzi metacyclogenesis under chemically defined conditions showed that the expression of a group of acidic polypeptides with a molecular weight of 45-50 kDa is markedly increased on adhesion of epimastigotes to the culture vessels. Immunochemical analysis revealed that these developmentally regulated polypeptides are structurally related to, and possibly homologous with, a major T. cruzi antigen, namely, GP57/51, a glycoprotein that has recently been characterized as a cysteine proteinase. The differentiation of epimastigotes into metacyclic trypomastigotes is accompanied by a 2- to 5-fold reduction in the concentration of GP57/51 antigen as determined by radioimmunoassays using monoclonal antibodies. Two-dimensional polyacrylamide gel electrophoretic analysis of metabolically labelled parasites indicated that this antigen is synthesized as a precursor with a molecular weight of 60 kDa, which is then processed to a level of 45-50 kDa via the formation of at least one intermediate processing product. The observation that expression of GP57/51-related products increases during epimastigote differentiation suggests an important role for this proteinase during the life cycle of T. cruzi.

Structural and functional identification of GP57/51 antigen of Trypanosoma cruzi as a cysteine proteinase

Purified GP57/51, a Trypanosoma cruzi glycoprotein earlier identified as a major antigen in infected humans, was subjected to N-terminal sequence analysis. Alignment of the first 30 amino acids revealed that its N-terminal region is virtually identical to that reported for a cysteine-proteinase isolated from the Tulahuen strain, including the presence of active site cysteine at position 25. The finding of serine at position 24 of GP57/51 (Y strain) has further increased the homology between this protozoan antigen with other members of the eukaryotic family of cysteine proteases, including human cathepsin L. Functional analysis of GP57/51 indicated that the antigen is indeed an active thiol proteinase, which is active across a wide pH range (5-7.5). This was shown using either human IgG or gelatin substrates co-polymerized into polyacrylamide gels prepared for electrophoresis, and also by enzyme assays peformed with the synthetic substrate Z-phe-arg-NMec. The enzyme was activated by thiol containing reagents, and was strongly inhibited by low concentrations of E-64 (IC50 0.1 microM), cystatin (IC50 1 microM), leupeptin (IC50 0.1 microM) and antipain (IC50 0.1 microM). Monoclonal antibodies directed against distinct epitopes of GP57/51 absorbed the hydrolytic activity from purified preparations, demonstrating that the antigenic and enzymatic activities were indeed expressed by the same molecular entities. The subcellular localization of immunoreactive molecules was investigated by electron microscopy; immunogold staining was conspicuously found in vesicles belonging to the endosomal-lysosomal system, in tissue culture trypomastigotes as well as in epimastigotes. The possibility that this highly antigenic protease is actively secreted and/or leaked out of damaged parasites is under investigation; its release to tissues and to the circulation may contribute to pathology, considering that it (i) can degrade proteins across a wide pH range and (ii) stimulates immune T cells from chronic chagasic patients.

Chagas' disease: serodiagnosis with purified Gp25 antigen

Development of a highly specific test system for the diagnosis of Chagas' disease (CD) was sought using Gp25, a surface glycoprotein recently isolated from Trypanosoma cruzi culture forms. Radioimmunoprecipitation assays were performed to screen 567 sera for IgG antibodies to Gp25. Correct diagnosis was attained in 97.8% of the 321 sera collected from chagasic patients in several endemic areas of South America. Sera from patients with various clinical forms of chronic disease displayed similar levels of antibodies (Abs) to Gp25. Moreover, there was little cross-reactivity when assayed against 246 sera from non-chagasic individuals, including 105 samples from individuals infected with unrelated trypanosomatids. Cross-reactivity was found in two such sera; these were used to identify a minor protein contaminant as the nonspecific antigen. Residual cross-reactive molecules were eliminated from Gp25 by affinity purification on Concanavalin A (Con A) columns. We recommend this antigen-antibody system for use in routine screening of blood donors.