Colin D. Robertson

Axenic cultivation and characterization of Leishmania mexicana amastigote-like forms

A new method is described which has made possible the long-term axenic cultivation of Leishmania mexicana amastigote-like forms in Schneiders Drosophila medium supplemented with 20% (v/v) foetal calf serum. Unlike previous methods, it utilizes direct culture of parasites obtained from the lesions of infected animals rather than adaptation of promastigotes in vitro. Ultrastructural (possession of megasomes), biochemical (cysteine proteinase activity and gelatin SDS-PAGE banding pattern) and infectivity (in vivo) data are presented which show the close similarity of the cultured forms to lesion amastigotes. The axenically cultured forms grew optimally at a temperature of 32-33 degrees C, providing further evidence for their amastigote nature. It was found that adjustment of the pH of the growth medium to 5.4 was required in order to retain the amastigote morphology of the cultured parasites. This supports the notion that leishmanial amastigotes are acidophiles.

A developmentally regulated cysteine proteinase gene of Leishmania mexicana

We have isolated a gene encoding a previously unreported class of trypanosomatid cysteine proteinase (CP) from the protozoan parasite Leishmania mexicana. The single‐copy gene (Imcpa) has several unusual features that distinguish it from CP genes cloned from the related species Trypanosoma brucei and Trypanosoma cruzi. These include a shorter C‐terminal extension of only 10 amino acids and a three‐amino‐acid insertion, GlyValMet, close to the predicted N‐terminus of the mature protein. Northern blot analysis showed that the gene is expressed in all life‐cycle stages but at higher levels in the amastigote stage in the mammal and in stationary phase promastigote cultures which contain the infective meta‐cyclic form of the parasite. A precursor protein of 38 kDa was detected in amastigotes and stationary phase promastigotes with antisera specific to the LmCPa pro‐region, but was barely detectable in early log‐phase promastigotes. Anti‐central domain antisera recognized the 38 kDa precursor and 24 and 27 kDa proteins. The major CPs of L. mexicana amastigotes, previously designated types A, B and C, were not detected with the antisera, suggesting that the gene codes for a previously uncharacterized CP in L. mexicana. The 24 kDa protein detected by the antiserum has no activity towards gelatin but apparently hydrolyses the peptide substrate BzPhe‐ValArgAMC. The relative levels of the 24 and 27 kDa proteins vary between the different life‐cycle stages. The results indicate that expression of this CP is regulated at both the RNA and protein level.

Characterisation of three groups of cysteine proteinases in the amastigotes of Leishmania mexicana mexicana

The multiple cysteine proteinases characteristic of the amastigote forms of Leishmania mexicana mexicana have been shown to be of three types. The groups of enzymes are distinguished by their substrate specificities and physical properties and have been purified free from other proteinases and most other proteins. One group (A) comprises at least four enzymes that bind to Con A. The cysteine proteinases comprising the other two groups (B and C) were separated by ion exchange chromatography. These last two groups of enzymes show different specificities towards a range of peptidyl aminomethylcoumarins. Notably, the two group C proteinases are more active towards compounds with a basic amino acid in the P1 position, whereas the three enzymes of group B are as active towards substrates with tyrosine in this position. All the cysteine proteinases show preference for compounds with bulky amino acids at positions P2 and P3 and all are equally susceptible to a range of inhibitors characteristically active against cysteine proteinases. Other proteinases present in amastigotes of L. m. mexicana were shown to differ significantly from the cysteine proteinases with respect to their substrate preferences and susceptibility to inhibitors.

The specificity of trichomonad cysteine proteinases analysed using fluorogenic substrates and specific inhibitors.

The multiple cysteine proteinases of Trichomonas vaginalis and Tritrichomonas foetus, both those retained intracellularly and those released, were separated using gelatin-SDS-PAGE, and their activity towards a range of 15 fluorogenic peptidyl aminomethylcoumarins determined together with their relative sensitivity to inhibitors. Three types of enzyme were apparent in T. vaginalis: (i) an 86-kDa enzyme active only on Z-Arg-Arg-NHMec; (ii) a 54-kDa proteinase which was most active on Z-Phe-Arg-NHMec but also able to hydrolyse N-t-Boc-Val-Leu-Lys-NHMec, Suc-Ala-Phe-Lys-NHMec, H-Pro-Phe-Arg-NHMec and Z-Arg-Arg-NHMec; and (iii) a group of six enzymes which preferentially hydrolysed substrates with bulky residues at the P2 and P3 positions. N-t-Boc-Val-Leu-Lys-NHMec and H-Leu-Val-Tyr-NHMec were the best substrates for the latter group. The 86-kDa proteinase was inactivated by E-64, but only at high concentrations, and was relatively insensitive to the peptidyl diazomethanes. The other proteinases were inhibited by low concentrations of E-64 and by Z-Phe-Ala-CHN2, and to a lesser extent by Z-Phe-Phe-CHN2. Differences between the proteinases of T. foetus were also demonstrated. All of them were active on Z-Arg-Arg-NHMec, but their activity towards other substrates varied. Three predominantly extracellular proteinases (25, 27 and 34 kDa), hydrolysed Z-Arg-Arg-NHMec specifically. Other proteinases (apparent Mr of 20,000 and 32,000) hydrolysed a number of other substrates, with the 32-kDa enzyme having greater activity towards N-t-Boc-Val-Leu-Lys-NHMec and H-Leu-Val-Tyr-NHMec than towards Z-Arg-Arg-NHMec. At a high concentration (270 microM), E-64 inhibited all of the T. foetus enzymes, but lower concentrations were less effective, with the 18-kDa proteinase being particularly insensitive. Z-Phe-Ala-CHN2 and Z-Phe-Phe-CHN2 were relatively poor inhibitors. The results demonstrate that the proteinases of both species are a heterogeneous group with respect to specificity, and have highlighted significant differences between the enzymes of T. vaginalis and T. foetus. The information on the specificities will be useful for assessing the features required in proteinase inhibitors if they are to be of potential value as antitrichomonal agents.

Parasite cysteine proteinases

Cysteine proteinases of protozoan and helminth parasites are considered to have a high potential as targets for novel antiparasite agents. This has stimulated research on the enzymes and a large body of data has now been accumulated. This Perspective provides an overview of the current situation, with recent advances being highlighted. Emphasis is given to the Type I cysteine proteinases of trypanosomatid protozoa, which are atypical in having an extra C-terminal domain, and the asparaginyl endopeptidase ofSchistosoma, for which the only homologues known are those in plants. The locations of parasite cysteine proteinases are described, with emphasis on the extralysosomal sites, and the putative roles of the enzymes in host-parasite interactions, including parasite nutrition and host invasion, are discussed. The major advances being made in elucidating cysteine proteinase function in protozoan parasites through genetic manipulation, including targeted gene deletion, are presented, with the Type I cysteine proteinases ofLeishmania being used as an example. The importance of this approach in the validation of the enzymes as drug targets is discussed. The current status of attempts to exploit parasite cysteine proteinases with drugs is presented, and future prospects are outlined.

Analysis of the proteinases of Trypanosoma brucei.

A method comprising enzyme separation by SDS-PAGE and subsequent use of peptidyl aminomethylcoumarins as substrates has been used to study proteinases of the protozoan parasite Trypanosoma brucei. The application of this method has allowed investigation of the substrate specificities of individual proteinases in cell lysates without the need for enzyme purification. The results show that T. brucei contains a group of cysteine proteinases, probably four in number, with substrate and inhibitor specificities similar to those of cathepsin L. A second group of proteinases, larger enzymes with significantly different substrate specificities and sensitivity to inhibitors, was also detected. Peptidyl diazomethanes inhibited the cysteine proteinases and also parasite growth, offering promise that peculiarities in the substrate specificity of trypanosomal cysteine proteinases could be exploited by compounds of this type.

Cathepsin B-like cysteine proteases of Leishmania mexicana.

A group of Leishmania mexicana cysteine proteases that differ from those previously found in this protozoon are described. The enzymes characteristically have a preference for peptidyl substrates with a phenylalanyl-valyl-arginyl moiety, do not hydrolyse gelatin in substrate-sodium dodecyl sulphate polyacrylamide gels, are stimulated by thiol-reducing agents and are sensitive to inhibitors specific for cysteine proteases. They have unusual solubility properties that indicate that the enzymes are amphiphilic proteins. Two of the cysteine proteases have been purified from L. mexicana amastigotes and shown to have molecular masses of 31 and 33 kDa. Their N-terminal amino acid sequences are very similar and show high homology to the mammalian cysteine protease, cathepsin B.

Identification and isolation of three proteasome subunits and their encoding genes from Trypanosoma brucei

We have determined peptide sequences of three Trypanosoma brucei proteasome subunit proteins by mass spectrometry of tryptic digests of the proteins purified by two-dimensional (2-D) polyacrylamide gel electrophoresis. Three genes identified by the sequence of their cDNA encode the peptides identified in these three proteins. The three proteins predicted from the gene sequences have significant similarity to other known proteasome subunits and represent an alpha6 type subunit (TbPSA6), and two beta-type subunits belonging to the beta1-type (TbPSB1) and beta2 type (TbPSB2). The sequences of both beta-subunits predict formation of catalytically active subunits through proteolytic processing. The prediction is supported by the presence in each of the two beta-subunits of a tryptic peptide that has the correctly processed N-terminus that creates the threonine nucleophile of the mature protein. This peptide cannot be generated by trypsin because of the required cleavage of a glycine-threonine bond. It is thus likely that there are at least two catalytically active beta-subunits, TbPSB1 and TbPSB2, present in the mature 20S proteasome from T. brucei.

Multiple high activity cysteine proteases of Leishmania mexicana are encoded by the Imcpb gene array.

The interrelationship of the multiple cysteine proteases (CPs) found characteristically at high activity in Leishmania mexicana amastigotes has been investigated. The mature forms of the five enzymes of groups B and C, which have subtly different substrate preferences, are the same size. Enzymically deglycosylated group A CP proteins also have the same molecular mass. Proteases of all three groups are specifically recognized by antisera raised against the group B or group C CPs. In addition, CPs of groups A, B and C have highly similar N-terminal amino acid sequences. The consensus sequence matches that predicted from the sequenced Imcpb gene, which occurs in a tandem array of over ten similar genes. Thus, the results are consistent with the groups A, B and C CPs being products of different Imcpb genes within the array, the different genes encoding CPs with identical N-termini, but with limited amino acid substitutions within the mature enzyme accounting for the different properties of the CPs. Evidence is also presented to indicate membrane-association of proteolytically active but less processed forms of Imcpb products.

Expression of Cysteine Proteinases by Metacyclic Promastigotes of Leishmania mexicana

ABSTRACT. The expression of cysteine proteinases by metacyclic promastigotes of Leishmania mexicana was investigated using gelatin polyacrylamide gel electrophoresis. Two prominent bands were detected which distinguished metacyclics from multiplicative promastigotes, lacking detectable cysteine proteinase activity, and amastigotes, with a distinct banding pattern composed of multiple enzymes. A correlation between relative activity of the metacyclic‐specific bands and the prevalence of metacyclics was found both during the growth cycle in vitro as metacyclogenesis occurred, and by comparison of stationary phase populations from consecutive subpassages in vitro. Irreversible inhibition of the metacyclic activities using N‐benzyloxycarbonyl‐phenylalanyl‐alanyl diazomethane did not inhibit metacyclic to amastigote transformation in vitro. These activities provide a useful biochemical marker for the metacyclic promastigotes of L. mexicana.