Homologous cysteine proteinases of pathogenic and nonpathogenic Entamoeba histolytica. Differences in structure and expression

Proteases from Entamoeba spp. and Pathogenic Free-Living Amoebae as Virulence Factors

The standard reference for pathogenic and nonpathogenic amoebae is the human parasite Entamoeba histolytica; a direct correlation between virulence and protease expression has been demonstrated for this amoeba. Traditionally, proteases are considered virulence factors, including those that produce cytopathic effects in the host or that have been implicated in manipulating the immune response. Here, we expand the scope to other amoebae, including less-pathogenic Entamoeba species and highly pathogenic free-living amoebae. In this paper, proteases that affect mucin, extracellular matrix, immune system components, and diverse tissues and cells are included, based on studies in amoebic cultures and animal models. We also include proteases used by amoebae to degrade iron-containing proteins because iron scavenger capacity is currently considered a virulence factor for pathogens. In addition, proteases that have a role in adhesion and encystation, which are essential for establishing and transmitting infection, are discussed. The study of proteases and their specific inhibitors is relevant to the search for new therapeutic targets and to increase the power of drugs used to treat the diseases caused by these complex microorganisms.

Host-Parasite interactions in Entamoeba histolytica and Entamoeba dispar: what have we learned from their genomes?

Invasive amoebiasis caused by Entamoeba histolytica is a major global health problem. Virulence is a rare outcome of infection, occurring in fewer than 1 in 10 infections. Not all strains of the parasite are equally virulent, and understanding the mechanisms and causes of virulence is an important goal of Entamoeba research. The sequencing of the genome of E. histolytica and the related avirulent species Entamoeba dispar has allowed whole-genome-scale analyses of genetic divergence and differential gene expression to be undertaken. These studies have helped elucidate mechanisms of virulence and identified genes differentially expressed in virulent and avirulent parasites. Here, we review the current status of the E. histolytica and E. dispar genomes and the findings of a number of genome-scale studies comparing parasites of different virulence.

In silico analysis of EST and genomic sequences allowed the prediction of cis-regulatory elements for Entamoeba histolytica mRNA polyadenylation

In most eukaryotic cells, the poly(A) tail at the 3'-end of messenger RNA (mRNA) is essential for nuclear export, translatability, stability and transcription termination. Poly(A) tail formation involves multi-protein complexes that interact with specific sequences in 3'-untranslated region (3'-UTR) of precursor mRNA (pre-mRNA). Here we have performed a computational analysis of a large EST and genomic sequences collection from Entamoeba histolytica, the protozoan parasite responsible for human amoebiasis, to identify conserved elements that could be involved in pre-mRNA polyadenylation. Results evidenced the presence of an AU-rich domain corresponding to the consensus UA(A/U)UU polyadenylation signal or variants, the cleavage and polyadenylation site that is generally denoted by U residue and flanked by two U-rich tracts, and a novel A-rich element. This predicted array was validated through the analysis of genomic sequences and predicted mRNA folding of genes with known polyadenylation site. The molecular organization of pre-mRNA 3'-UTR cis-regulatory elements appears to be roughly conserved through evolutionary scale, whereas the polyadenylation signal seems to be species-specific in protozoan parasites and the novel A-rich element is unique for the primitive eukaryote E. histolytica. To our knowledge, this paper is the first work about the identification of potential pre-mRNA 3'-UTR cis-regulatory sequences through in silico analysis of large sets of cDNA and genomic sequences in a protozoan parasite.

Cysteine proteinases of microorganisms and viruses

This review considers properties of secreted cysteine proteinases of protozoa, bacteria, and viruses and presents information on the contemporary taxonomy of cysteine proteinases. Literature data on the structure and physicochemical and enzymatic properties of these enzymes are reviewed. High interest in cysteine proteinases is explained by the discovery of these enzymes mostly in pathogenic organisms. The role of the proteinases in pathogenesis of several severe diseases of human and animals is discussed.

Structure and content of the Entamoeba histolytica genome

The intestinal parasite Entamoeba histolytica is one of the first protists for which a draft genome sequence has been published. Although the genome is still incomplete, it is unlikely that many genes are missing from the list of those already identified. In this chapter we summarise the features of the genome as they are currently understood and provide previously unpublished analyses of many of the genes.

The role of extracellular cysteine proteinases in pathogenesis of Entamoeba histolytica invasion

The extracellular cysteine proteinases of Entamoeba histolytica have been implicated as important virulence factors in the pathogenesis of amebiasis and play a key role in tissue invasion and disruption of host defenses. These proteinases have attracted considerable interest as targets for novel therapeutic agents and as vaccine candidates. Here, Xuchu Que and Sharon Reed highlight some of the more recent findings, focusing in particular on functional and structural features of the extracellular cysteine proteinases of E. histolytica.

The intestinal protozoan parasite Entamoeba histolytica contains 20 cysteine protease genes, of which only a small subset is expressed during in vitro cultivation

Cysteine proteases are known to be important pathogenicity factors of the protozoan parasite Entamoeba histolytica. So far, a total of eight genes coding for cysteine proteases have been identified in E. histolytica, two of which are absent in the closely related nonpathogenic species E. dispar. However, present knowledge is restricted to enzymes expressed during in vitro cultivation of the parasite, which might represent only a subset of the entire repertoire. Taking advantage of the current E. histolytica genome-sequencing efforts, we analyzed databases containing more than 99% of all ameba gene sequences for the presence of cysteine protease genes. A total of 20 full-length genes was identified (including all eight genes previously reported), which show 10 to 86% sequence identity. The various genes obviously originated from two separate ancestors since they form two distinct clades. Despite cathepsin B-like substrate specificities, all of the ameba polypeptides are structurally related to cathepsin L-like enzymes. None of the previously described enzymes but 7 of the 12 newly identified proteins are unique compared to cathepsins of higher eukaryotes in that they are predicted to have transmembrane or glycosylphosphatidylinositol anchor attachment domains. Southern blot analysis revealed that orthologous sequences for all of the newly identified proteases are present in E. dispar. Interestingly, the majority of the various cysteine protease genes are not expressed in E. histolytica or E. dispar trophozoites during in vitro cultivation. Therefore, it is likely that at least some of these enzymes are required for infection of the human host and/or for completion of the parasite life cycle.

Entamoeba histolytica cysteine proteinases disrupt the polymeric structure of colonic mucin and alter its protective function

The adherent mucous gel layer lining the colonic epithelium is the first line of host defense against invasive pathogens, such as Entamoeba histolytica. The mucous layer prevents the attachment of amoeba to the colonic epithelium by trapping and aiding in the expulsion of the parasite. Disruption of the mucous layer is thought to occur in invasive amebiasis, and the mechanism by which the parasite overcomes this barrier is not known. The aim of this study was to characterize the specific interactions occurring between E. histolytica secreted cysteine proteinases and colonic mucin as a model to examine the initial events of invasive amebiasis. E. histolytica secreted products were examined for mucinase activity utilizing mucin metabolically labeled with [(35)S]cysteine as a substrate. Cysteine proteinases degraded mucin in a time- and dose-dependent manner. A significant reduction (>50%) in high-molecular-weight mucin with altered buoyant density was observed when degraded mucin was analyzed by Sepharose 4B column chromatography, sodium dodecyl sulfate-polyacrylamide gel electrophoresis and autoradiography, and CsCl density gradient centrifugation. Mucinase activity was eliminated by the specific cysteine protease inhibitor trans-epoxysuccinyl-L-leucylamido-(4-guanidino)butane and was independent of glycosidase activity. Moreover, the degraded mucin was 38% less effective than native mucin at inhibiting amebic adherence to target epithelial cells. These results are the first to show that E. histolytica cysteine proteinases alter the protective function of the mucous barrier by disrupting the structure of the MUC2 polymer. Mechanistically, the parasite achieves this via proteolytic degradation of the terminal cysteine-rich domains.

Pore-forming polypeptides of the pathogenic protozoon Naegleria fowleri

The free-living amoeboflagellate and potential human pathogen Naegleria fowleri causes the often fatal disease primary amoebic meningoencephalitis. The molecular repertoire responsible for the cytolytic and tissue-destructive activity of this amoeboid protozoon is largely unknown. We isolated two pore-forming polypeptides from extracts of highly virulent trophozoites of N. fowleri by measuring their membrane-permeabilizing activity. N-terminal sequencing and subsequent molecular cloning yielded the complete primary structures and revealed that the two polypeptides are isoforms. Both polypeptides share similar structural properties with antimicrobial and cytolytic polypeptides of the protozoon Entamoeba histolytica (amoebapores) and of cytotoxic natural killer (NK) and T cells of human (granulysin) and pig (NK-lysin), all characterized by a structure of amphipathic alpha-helices and an invariant framework of cysteine residues involved in disulfide bonds. In contrast to the aforementioned proteins, the Naegleria polypeptides both are processed from large precursor molecules containing additional isoforms of substantial sequence divergence. Moreover, biochemical characterization of the isolated polypeptides in combination with mass determination showed that they are N-glycosylated and variably processed at the C terminus. The biological activity of the purified polypeptides of Naegleria was examined toward human cells and bacteria, and it was found that these factors, named naegleriapores, are active against both types of target cells, which is in good agreement with their proposed biological role as a broad-spectrum effector molecule.

Comparative molecular modelling identifies a common putative IgE epitope on cysteine protease allergens of diverse sources

BACKGROUND

Previous approaches for studying common allergenic epitopes have mainly focused on sequence comparisons, which unfortunately yield little or no information on the shape of the epitope which is the most important determinant of cross-reactivity.

OBJECTIVES

The aim of this study was to investigate the structural basis for cross-reactivity between a previously identified immunodominant epitope of the house dust mite allergen Der p 1 (Leu147-Gln160) and the corresponding epitopes on other allergens that are either taxonomically closely related (i.e. cysteine proteases of other mite species) or representing evolutionary conserved structures (i.e. plant, human and parasite cysteine proteases).

METHODS

We carried out comparative molecular modelling on a range of cysteine proteases, including those of other mite species (Der f 1 and Eur m 1), human (cathepsins B, K, L, S and O), plants (papain, chymopapain and actinidin) and parasites (cruzain, cathepsin L-like Leishmania protease, Entamoeba ACP1 protease and Schistosoma Q26534, Q11003 and cathepsin L proteases).

RESULTS

Our study shows that all the cysteine proteases investigated here display an epitope corresponding to that previously identified on Der p 1, but with varying shapes and degree of accessibility. It appears that the core of the epitope on these homologous cysteine proteases consists of a centrally located conserved Tyr residue flanked on either sides by accessible amino acids.

CONCLUSION

Therefore, these cysteine proteases seem to use similar accessible structures, which may form the basis for the rational design of generic epitope-directed treatment strategies for controlling allergic diseases.

Pathogenesis of intestinal amebiasis: from molecules to disease

In spite of a wealth of knowledge on the biochemistry and cellular and molecular biology of Entamoeba histolytica, little has been done to apply these advances to our understanding of the lesions observed in patients with intestinal amebiasis. In this review, the pathological and histological findings in acute amebic colitis are related to the molecular mechanisms of E. histolytica pathogenicity described to date. Infection of the human colon by E. histolytica produces focal ulceration of the intestinal mucosa, resulting in dysentery (diarrhea with blood and mucus). Although a complete picture has not yet been achieved, the basic mechanisms involved in the production of focal lytic lesions include complex multifactorial processes in which lectins facilitate adhesion, proteases degrade extracellular matrix components, porins help nourish the parasite and may also kill incoming polymorphonuclear leukocytes and macrophages, and motility is used by the parasite to invade deeper layers of the colon. In addition, E. histolytica has developed mechanisms to modulate the immune response during acute infection. Nevertheless, much still needs to be unraveled to understand how this microscopic parasite has earned its well-deserved histolytic name.

Pore-forming peptides of Entamoeba dispar. Similarity and divergence to amoebapores in structure, expression and activity

Amoebapore, a 77-residue peptide with pore-forming activity from the human pathogen Entamoeba histolytica, is implicated in the killing of phagocytosed bacteria and in the cytolytic reaction of the amoeba against host cells. Previously, we structurally and functionally characterized three amoebapore isoforms in E. histolytica but recognized only one homolog in the closely related but non-pathogenic species Entamoeba dispar. Here, we identified two novel amoebapore homologs from E. dispar by molecular cloning. Despite strong resemblance of the primary structures of the homologs, molecular modeling predicts a species-specific variance between the peptide structures. Parallel isolation from trophozoite extracts of the two species revealed a lower amount of pore-forming peptides in E. dispar and substantially higher activity of the major isoform from E. histolytica towards natural membranes than that from E. dispar. Differences in abundance and activity of the lytic polypeptides may have an impact on the pathogenicity of amoebae.

A DNA sequence corresponding to the gene encoding cysteine proteinase 5 in Entamoeba histolytica is present and positionally conserved but highly degenerated in Entamoeba dispar

Cysteine proteinases of Entamoeba histolytica are considered to be one of the most important classes of molecules responsible for the parasite's ability to destroy human tissues. Interestingly, one particular cysteine proteinase, located on the surface of E. histolytica trophozoites and designated cysteine proteinase 5 (CP5), is not expressed in the closely related but nonpathogenic species Entamoeba dispar. By comparing the E. histolytica and E. dispar genomic loci containing the gene for CP5 (cp5), it was found that the position of cp5 within the genomic context is conserved between the two organisms, but that the gene is highly degenerated in E. dispar, as it contains numerous nucleotide exchanges, insertions, and deletions, resulting in multiple stop codons within the cp5 reading frame. An alignment of all available orthologous E. histolytica and E. dispar DNA sequences suggested that cp5 started to degenerate in E. dispar coincidently when the two organisms began to diverge from a common ancestor.

Purification and biochemical characterization of a novel cysteine protease of Entamoeba histolytica

Cysteine proteases are important virulence factors of Entamoeba histolytica, the causative agent of amoebiasis. A novel cysteine protease from parasite extracts was purified 15-fold by a procedure including concanavalin A-Sepharose, hydroxylapatite and DEAE-Sepharose chromatography. The purification resulted in the obtainment of an homogeneous protein with a molecular mass of 66 kDa on native PAGE. In 10% SDS/PAGE, three bands of 60, 54 and 50 kDa were evident. Each of the three specific mouse antisera raised against these proteins showed cross-reactivity with the three bands obtained from the purified eluate. The N-terminal sequencing of the first 10 amino acids from the three proteins showed 100% identity. These results support the hypothesis of a common precursor for the 60, 54 and 50-kDa proteins. Protease activity of the purified enzyme was demonstrated by electrophoresis in a gelatine-acrylamide copolymerized gel. Its activity was quantified by cleaving a synthetic fluorogenic peptide substrate such as N-carbobenzyloxy-arginyl-arginyl-7-amido-4-methylcoumarin. The optimum pH for the protease activity was 6.5; however, enzymatic activity was observed between pH 5 and pH 7.5. Typical of cysteine proteases, the enzyme was inhibited by 4-[(2S, 3S)-carboxyoxiran-2-ylcarbonyl-L-leucylamido]butylg uanidine and iodoacetamide, and activated by free sulfhydryl groups. The cellular location of the enzyme was examined on trophozoites before and after contact with red blood cells using indirect immunofluorescence and cellular fractionation. The 60-kDa cysteine protease translocated to the amoebic surface upon the interaction of trophozoites with red blood cells. This result provided evidence for participation of the 60-kDa protease in erythrophagocytosis.

Virulence factors of Entamoeba histolytica

Recent studies have increased our knowledge of Entamoeba histolytica cell biology and gene regulation. In the ameba, dominant-negative mutations in the Gal/GalNAc lectin affect adhesion and cytolysis, whereas mutations in meromyosin affect cytoskeletal function. Studying these mutant proteins has improved our understanding of the role of these proteins in E. histolytica virulence. The characterization of the CP5 cysteine protease and the induction of apoptosis in host target cells has led to a better comprehension of the mechanisms by which trophozoites can lyse target cells.

The cysteine-rich region of the Entamoeba histolytica adherence lectin (170-kilodalton subunit) is sufficient for high-affinity Gal/GalNAc-specific binding in vitro

Adherence of Entamoeba histolytica trophozoites to colonic mucin, epithelium, and other target cells is mediated by the amebic Gal/GalNAc lectin. We constructed in vitro expression vectors containing full-length (residues 1 to 1280), cysteine-poor (1 to 353 and 1 to 480), and cysteine-rich (356 to 1143 and 480 to 900) fragments of the gene encoding the heavy subunit of the adherence lectin, hgl2. In vitro transcription followed by translation using a nuclease-treated rabbit reticulocyte lysate system was carried out. Immunoreactivity of in vitro-translated Hgl2 was confirmed by immunoprecipitation with lectin-specific monoclonal antibodies (MAbs) 1G7 and 8A3, which recognize linear epitopes. Protein disulfide isomerase (PDI) refolding of Hgl2 enhanced immunoreactivity (P < 0.05) with the conformationally dependent MAb 3F4. Binding of PDI-refolded full-length (P < 0.001) and cysteine-rich (P = 0.005) Hgl2 to CHO cells was galactose dependent and competitively inhibited by native hololectin (50% inhibitory concentration of 39.6 ng/ml). The cysteine-poor region (1 to 353) did not bind CHO cells. Both full-length (1 to 1280) and cysteine-rich (356 to 1143) Hgl2 bound the glyconeoconjugate GalNAc(19)BSA in a GalNAc-specific manner. The smaller cysteine-rich fragment (480 to 900) also exhibited GalNAc-specific binding but to a lesser extent (P < 0.05) than residues 1 to 1280 and 356 to 1143. Neither the cysteine-poor fragment (1 to 480), luciferase (protein control), nor control translation reactions (without hgl2 lectin mRNA) bound GalNAc(19)BSA. Binding to GalNAc(19)BSA was shown to be dependent on the concentration of GalNAc(19)BSA coated in each well or (35)S-lectin added (K(D) = 0.85 +/- 0.37 pM). Binding was competitively inhibited by the terminal GalNAc-containing glycoprotein asialofetuin (P < 0.005). Taken together, these data provide direct evidence that the cysteine-rich region of the Gal/GalNAc lectin heavy subunit contains one or more carbohydrate-binding domains.

Down regulation of Entamoeba histolytica virulence by monoxenic cultivation with Escherichia coli O55 is related to a decrease in expression of the light (35-kilodalton) subunit of the Gal/GalNAc lectin

Entamoeba histolytica virulence is related to a number of amebic components (lectins, cysteine proteinases, and amebapore) and host factors, such as intestinal bacterial flora. Trophozoites are selective in their interactions with bacteria, and the parasite recognition of glycoconjugates plays an important role in amebic virulence. Long-term monoxenic cultivation of pathogenic E. histolytica trophozoites, strains HK-9 or HM-1:IMSS, with Escherichia coli serotype O55, which binds strongly to the Gal/GalNAc amebic lectin, markedly reduced the trophozoites' adherence and cytopathic activity on cell monolayers of baby hamster kidney (BHK) cells. Specific probes prepared from E. histolytica lectin genes as well as antibodies directed against the light (35-kDa) and heavy (170-kDa) subunits of the Gal/GalNAc lectin revealed a decrease in the transcription and expression of the light subunit in trophozoites grown monoxenically with E. coli O55. This effect was not observed when E. histolytica was grown with E. coli 346, a mannose-binding type I pilated bacteria. Our results suggest that the light subunit of the amebic lectin is involved in the modulation of parasite adherence and cytopathic activity.

Proteases of protozoan parasites

Proteolytic enzymes seem to play important roles in the life cycles of all medically important protozoan parasites, including the organisms that cause malaria, trypanosomiasis, leishmaniasis, amebiasis, toxoplasmosis, giardiasis, cryptosporidiosis and trichomoniasis. Proteases from all four major proteolytic classes are utilized by protozoans for diverse functions, including the invasion of host cells and tissues, the degradation of mediators of the immune response and the hydrolysis of host proteins for nutritional purposes. The biochemical and molecular characterization of protozoan proteases is providing tools to improve our understanding of the functions of these enzymes. In addition, studies in multiple systems suggest that inhibitors of protozoan proteases have potent antiparasitic effects. This review will discuss recent advances in the identification and characterization of protozoan proteases, in the determination of the function of these enzymes, and in the evaluation of protease inhibitors as potential antiprotozoan drugs.

Specific and reversible inhibition of Entamoeba histolytica cysteine-proteinase activities by Zn2+: implications for adhesion and cell damage

BACKGROUND

Cysteine-proteinases are thought to play an important role in E. histolytica pathogenicity. Although effective blockage of proteolytic activities can be obtained with several known inhibitors, the high cellular toxicity of most of the inhibitors precludes experimentation with live cells or animal models. Specific cysteine-proteinase inhibitors that could be utilized in studies of virulence are of great need in the field of amebiasis.

METHODS

Cysteine-proteinase activities were determined in trophozoite lysates by azocasein degradation and after PAGE and gelatin zymograms. Inhibition of the activities was assessed in the presence of 0.01-2.5 mM concentrations of divalent cations of the IIB and VIII series such as Zn, Cd, Hg, Ni, and Co. Reversibility was induced with 25 mM L-cysteine or 50 mM L-histidine and by metal chelation with 5 mM phenantroline. The inhibitory effect of ZnCl2 was tested with live cells in fibronectin-binding and cytotoxicity assays.

RESULTS

ZnCl2 specifically inhibited cysteine-proteinase activities in trophozoite lysates in a concentration-dependent manner. Additionally, 1.0-2.5 mM ZnCl2 blocked proteolysis in more than 70%. This inhibition was completely reverted by L-cysteine, L-histidine, or phenantroline. Similar results were obtained by analyzing individual cysteine-proteinase activities separated in gelatin zymograms. At these concentrations, ZnCl2 significantly interfered with trophozoite adhesion, thus making amebas deficient in substrate degradation and cell damage.

CONCLUSIONS

ZnCl2 is an effective inhibitor of amebic cysteine-proteinases. Its low toxicity at relatively high concentrations, high solubility, and low cost make it adequate for live cell experimentation and animal models of amebic virulence.

Royal Society of Tropical Medicine and Hygiene Meeting at Manson House, London, 19 February 1998. Amoebic disease. Entamoeba histolytica and E. dispar: comparison of molecules considered important for host tissue destruction

Entamoeba histolytica and E. dispar are genetically distinct but closely related protozoan species. Both colonize the human gut but only E. histolytica is able to invade tissues and cause disease. Comparison of the 2 species may help to elucidate the specific mechanisms involved in the pathogenicity of E. histolytica. During the last few years, various amoeba molecules considered to be important for pathogenic tissue invasion have been identified and characterized, such as a galactose-inhibitable surface lectin, pore-forming peptides and cysteine proteinases. This review summarizes present knowledge about the structure and function of these molecules, with emphasis on the differences between E. histolytica and E. dispar.

Molecular characterization of an exceptionally acidic lysozyme-like protein from the protozoon Entamoeba histolytica

The protozoan parasite Entamoeba histolytica contains a second antibacterial protein with lysozyme-like properties. The newly recognized bacteriolytic protein was purified from extracts of amoebic trophozoites to allow amino-terminal sequencing. Subsequent molecular cloning revealed that it is an isoform of the amoeba lysozyme described previously but also demonstrated a substantial sequence divergence of the two forms. As lysozymes typically are basic proteins, the novel amoebic protein differs markedly in having a pI of 4.5. There is no significant similarity of both amoeba lysozymes with any bacteriolytic protein of other organisms reported so far; however, striking sequence identity is found with predicted gene products of unknown function derived from the bacteria-feeding nematode Caenorhabditis elegans.

Isolation and molecular characterization of a surface-bound proteinase of Entamoeba histolytica

Major pathogenic functions of Entamoeba histolytica involved in destruction of host tissues are the degradation of extracellular matrix proteins mediated by secreted cysteine proteinases and contact-dependent killing of host cells via membrane-active factors. A soluble protein with an affinity for membranes was purified from amoebic extracts to apparent homogeneity. N-terminal sequencing and subsequent molecular cloning of the factor revealed that it is a member of the cysteine proteinase family of E. histolytica, which we termed CP5. Further experiments with the purified protein showed that it has potent proteolytic activity that is abrogated in the presence of inhibitors specific for cysteine proteinases. The enzyme firmly associates with membranes retaining its proteolytic activity and it produces cytopathic effects on cultured monolayers. A model of the three-dimensional structure of CP5 revealed the presence of a hydrophobic patch that may account for the potential of the protein to associate with membranes. Immunocytochemical localization of the enzyme to the surface of the amoeba in combination with the recent finding that the gene encoding CP5 is missing in the closely related but non-pathogenic Entamoeba dispar suggests a potential role of the protein in host tissue destruction of E. histolytica.

Riboprinting: a tool for the study of genetic diversity in microorganisms

Classical morphology-based methods of taxonomic and phylogenetic analysis are inadequate in many groups of structurally simple eukaryotes. Molecular methods can generate data independently of the complexity of the organisms' morphology. Riboprinting is one such technique, and involves restriction enzyme analysis of polymerase chain reaction amplified small subunit ribosomal RNA genes. The utility of the method is illustrated with examples from several genera of intestinal and bloodstream parasites. Among the applications of riboprinting are the detection of cryptic genetic variation within species, organism misidentifications and culture mix-ups, independent verification of DNA sequences, and the rapid generation of data useful in phylogenetic analyses.

Protozoa. Amebiasis

The intestinal protozoan parasite Entamoeba histolytica causes amebic dysentery and amebic liver abscess, and ranks third worldwide among parasitic causes of death. The application of molecular techniques to the study of this organism have led to major advances in understanding the pathophysiology of amebic infection. This article reviews what is currently known about the pathogenesis, clinical manifestations, diagnosis, and treatment of amebiasis.

A homologue of the cysteine proteinase gene (ACP1 or Eh-CPp3) of pathogenic Entamoeba histolytica is present in non-pathogenic E. dispar strains

One of the three cysteine proteinase genes, ACP1 (or CP 3), has been reported to be missing in non-pathogenic strains of Entamoeba histolytica (or Entamoeba dispar as recently labeled). Unexpectedly, a gene fragment very similar in its sequence (95% homology) to ACP1 of pathogenic strains was obtained by use of the polymerase chain reaction from genomic DNA and cDNA of various cloned non-pathogenic strains as well as in 23 clinical isolates from asymptomatic carriers. The finding of the ACP1 homologue in non-pathogenic or E. dispar strains rules out the proposed use of its absence for diagnostic purposes.

Recent advances in amebiasis

Advancements in our understanding of amebiasis have been rapid over the decade that I have followed this field. What was identified morphologically for years as Entamoeba histolytica has been redescribed with modern techniques as a complex of two species, the commensal parasite E. dispar and the pathogenic parasite E. histolytica that is the cause of colitis and liver abscess. Antigen detection tests are now available for the rapid detection in stool of the pathogenic species E. histolytica. New understandings of the importance of luminal as well as tissue-active antimebic medications in the treatment of invasive disease have been reached. The groundwork is being laid for an understanding of the protective immune responses to infection, and at the lab bench DNA transfection of the parasite has opened studies of pathogenesis to genetic analysis. While necessarily an incomplete sketch of the field, I have attempted here to highlight some recent and important developments of interest to clinicians and microbiologists.

Sequence, characterization and localization of a cysteine proteinase cathepsin L in Schistosoma mansoni

A cDNA encoding Schistosoma mansoni cathepsin L was isolated from a cDNA library and sequenced. Alignment of the proposed amino-acid sequence with known members of cathepsin L shows highest homologies with sequences from mouse and rat. An expression plasmid was constructed in Escherichia coli to produce recombinant schistosome cathepsin L with an extension of six histidines at its N terminus. Using antibodies raised against the purified fusion protein, two polypeptide bands with approx. molecular masses of 38 and 31 kDa were identified in a schistosome extract. By use of specific radioiodinated inhibitors, a radioactively labeled protein could be detected at 31 kDa, suggesting that this is the active mature enzyme. The larger protein of 38 kDa did not react with the inhibitor, indicating that it represents the inactive precursor molecule. Immunohistological experiments revealed that the proteinase is localized in structures associated with the reproductive system of females and with the subtegumental region of the gynecophoric canal of males. However, Northern blot hybridization demonstrates that more transcripts are present in female parasites than in males. Genomic Southern blotting suggests that schistosome cathepsin L is expressed from a single-copy gene.

Role of the Entamoeba histolytica cysteine proteinase in amebic liver abscess formation in severe combined immunodeficient mice

Evidence from in vitro studies suggest that the Entamoeba histolytica cysteine proteinase plays a role in the tissue lysis and cytopathic effects seen in invasive amebiasis. We used affinity-purified antibodies against a recombinant E. histolytica cysteine proteinase to demonstrate that the proteinase is present extracellularly in amebic liver abscesses in mice with severe combined immunodeficiency (SCID mice). Treatment of E. histolytica trophozoites with specific cysteine proteinase inhibitor E-64 blocked or greatly reduced liver abscess formation at 48 h in SCID mice. Our study suggests an important role for a functional cysteine proteinase in amebic liver abscess formation.

Molecular cloning of a 30-kilodalton lysine-rich surface antigen from a nonpathogenic Entamoeba histolytica strain and its expression in a pathogenic strain

A monoclonal antibody (MAb), 318-28, that specifically reacts with a 30-kDa antigen present on membrane surfaces of all nonpathogenic (NP) Entamoeba histolytica strains tested and which did not react with pathogenic (P) strains was used for the isolation of the cDNA coding for this antigen from an expression library of an NP E. histolytica strain. The deduced amino acid composition was rich in lysine residues (14.5%), with some sequence similarity to a polyadenylate-binding protein. Southern and Northern (RNA) blot analyses, as well as amplifications of DNA segments by PCR, indicate that a very similar gene (identity of 96.5%) exists in P strains of E. histolytica. Unexpectedly, the NP-specific antigen was also identified by MAb 318-28 on the surfaces of a cloned, xenically cultivated and well-characterized P strain (BNI:0591) that was recently isolated from a human liver abscess. Binding of the MAb, both to the cell surfaces and to Western blots (immunoblots), was abolished, however, upon axenization of the BNI:0591 cultures. Oligonucleotide primers, designed to anneal only to specific DNA sequences of the NP 30-kDa protein gene copy, amplified a DNA segment from P strain BNI:0591 which was identical in sequence to that of the NP 30-kDa protein gene. Our findings indicate that a P strain of E. histolytica can possess and express, under certain growth conditions, an antigen that is usually detected only in NP strains.

Adherence and cytotoxicity of Entamoeba histolytica or how lectins let parasites stick around

Images

Isolation and characterization of a species-specific multicopy DNA sequence from Entamoeba histolytica

A genomic library of Entamoeba histolytica (pathogenic strain HM-1:IMSS) was screened to detect repetitive DNA clones other than those from the highly abundant ribosomal DNA (rDNA). One such clone (HMc) had a 2.3 kb insert which hybridized with the main genome and not the rDNA circle. Southern hybridization of E. histolytica genomic DNA, digested with EcoR I and probed with HMc, showed multiple bands. The banding pattern was identical in all axenic pathogenic strains tested. Differences, however, existed when the banding pattern of a pathogenic strain was compared with that of a non-pathogenic strain. HMc was present in about 25-30 copies per genome in strain HM-1:IMSS. Nucleotide sequence analysis of HMc revealed a partial open reading frame which hybridized with a 1.35 kb poly A+ transcript in Northern blots. The deduced amino acid sequence did not, however, show significant homology with known proteins. The HMc sequence was found only in E. histolytica as it hybridized with 5 different axenic strains of E. histolytica but did not recognize other closely related species of Entamoeba. It has thus the potential to be used as a species-specific DNA probe.

Cloning of a protease gene family of Fasciola hepatica by the polymerase chain reaction

Degenerate oligonucleotide primers derived from conserved cysteine protease sequences were used in the reverse transcription polymerase chain reaction to amplify seven different cysteine protease cDNA clones, Fcp1-7, from RNA isolated from adult Fasciola hepatica. Five of the amplified F. hepatica sequences showed homology to the cathepsin L type and two were more related to the cathepsin B type. Southern blot analysis suggests that some members of this protease gene family are present in multiple copies. Northern blot analysis revealed differences in the levels of steady state mRNA expression for some of these proteases. The 5' and the 3' regions of Fcp1 were amplified using the rapid amplification of cDNA ends PCR protocol (RACE-PCR) and an additional clone was obtained by screening a lambda gt10 cDNA library using Fcp1 as a probe. The Fcp1 cDNA fragment was also subcloned in the expression vector pGEX and expressed as a glutathione-S-transferase (GST) fusion protein in Escherichia coli. Antibodies, raised in rabbits against the GST:Fcp1 fusion protein, were used in western blot analysis to examine expression in different life-cycle stages of F. hepatica. In extracts from adult and immature parasites, the immune serum recognised predominantly two proteins of 30 kDa and 38 kDa. In other parasite stages, proteins of different molecular weight were recognised by the anti-GST:Fcp1 antiserum, indicating stage-specific gene expression or processing of Fcp1. In gelatine substrate gel analysis, strong proteolytic activity could be detected at 30 kDa, but not at 38 kDa, suggesting that the 30 kDa protein represents the mature enzyme and the 38 kDa protein the proenzyme.

Purification and immunologic characterization of a 30-kDa cysteine proteinase of Entamoeba histolytica

A 30-kDa cysteine proteinase was purified from extracts of axenically grown trophozoites of Entamoeba histolytica strain HM1:IMSS. The purification procedure involved two consecutive chromatographic steps. Sequence analysis revealed high similarity with histolysin and with other 27-kDa cysteine proteinase. Western-blot analysis using F(ab')2 fragments of a polyclonal antibody raised against the purified enzyme revealed that when the amebic extract was prepared in the absence of proteinase inhibitors there were many positive bands ranging in relative molecular weight from 115 to 12.5 kDa, but when the extract was prepared in the presence of proteinase inhibitors there was only a single 30-kDa positive band. Similar results were obtained with immunoprecipitates. This phenomenon would suggest the formation of multimer aggregates of the 30-kDa cysteine proteinase after partial proteolysis.

Unusual gene organization in the protozoan parasite Entamoeba histolytica

We have analyzed three independent genomic loci of the protozoan parasite Entamoeba histolytica that contain coding regions for the iron-containing superoxide dismutase, the pore-forming peptide, and the galactose-inhibitable lectin. All of the three structural genes were found to be closely linked unidirectionally to other coding sequences. The intergenic regions did not exceed 1,350 nucleotides. Nuclear run-on data demonstrated that at least the galactose-inhibitable lectin gene is transcribed in a monocistronic fashion. Comparison of the genomic sequences described here with several others reported previously for E. histolytica revealed a number of invariable peculiarities for the gene organization of this parasite: (i) Coding sequences are not interrupted by introns; (ii) 5' untranslated regions are rather short and transcription starts at the consensus sequences ATTCA or ATCA; (iii) an unusual TATA-motif is located about 30 nucleotides upstream of the start of transcription and comprises the sequence TATTTAAA, which reveals protein binding activity as determined by gel retardation assays; (iv) the conserved pentanucleotide motif TAA/TTT is found within the relatively short 3' untranslated regions and functions putatively as the transcription termination signal; and (v) a stretch of up to 12 pyrmidine residues is located at the end of transcribed sequences.

Cell signalling and motility in Entamoeba histolytica

Entamoeba histolytica crawls as a polarized cell following external stimuli, with the translocation of signals modifying extracellular matrix interactions and the amoeba cytoskeleton. Nancy Guillén here describes how the gliding of E. histolytica cells requires the activity of the actomyosin complex, and how actomyosin functions related to motility are necessary for pathogenesis and for amoebal escape from the host immune response.

A family of transcripts (K2) of Entamoeba histolytica contains polymorphic repetitive regions with highly conserved elements

Sera from patients with invasive amebiasis were used to identify a cDNA clone (K2p-1) encoding a commonly recognized, repeat-containing antigen of the pathogenic Entamoeba histolytica HM-1:IMSS. K2p-1 was used to isolate 3 cDNA clones (K2 clones); one K2p-1 related clone from the same pathogenic E. histolytica strain and 2 from the nonpathogenic E. histolytica strain SAW-142. The nucleotide sequence and predicted amino acid sequence revealed a closely related family of transcripts differing mainly in the extent and arrangement of an internal region consisting of tandemly arranged repetitive elements. The repetitive units encoding either 12 or 8 amino acids were found to be highly conserved in all the K2 clones analyzed so far, suggesting that the repeat motifs perform functions common to both pathogenic and nonpathogenic E. histolytica. The genomic organization of the K2 genes was different when compared in pathogenic and nonpathogenic E. histolytica and may therefore be used to discriminate between pathogenic and nonpathogenic E. histolytica strains.

A redescription of Entamoeba histolytica Schaudinn, 1903 (Emended Walker, 1911) separating it from Entamoeba dispar Brumpt, 1925

Explaining the low incidence of invasive disease (10%) in humans infected with Entamoeba histolytica has occupied the attention of generations of both clinical and nonclinical investigators. One possible explanation would be the existence of two morphologically identical species-one an invasive pathogen, the other noninvasive. This was first proposed by Brumpt in 1925, but his explanation was virtually ignored until 1978 when the first of several publications appeared suggesting that E. histolytica did indeed consist of two species. We have reexamined Brumpt's claim in light of recent biochemical, immunological and genetic studies and conclude that the data derived from these investigations provide unequivocal evidence supporting his hypothesis. With this in mind, we redescribe the invasive parasite retaining the name Entamoeba histolytica Schaudinn, 1903 (Emended Walker, 1911), and set it apart from the noninvasive parasite described by Brumpt, Entamoeba dispar Brumpt, 1925.

A new test for infection by Entamoeba histolytica

Entamoeba histolytica is a common parasite of the human intestine, and in a significant percentage of cases it invades the tissues. Virulent strains of the organism produce the proteinose histolysain, which almost certainly facilitates the invasive behaviour. Histolysoin has been isolated, a selective assay developed, and specific antisera raised against the enzyme. As described here by Alfredo Luaces, Lyda Osorio and Alan Barren, the ENZYMEBA test for infection by E. histolytica combines the specificities of both antibodies and enzyme assay to achieve a high degree of sensitivity and selectivity. Circulating antibodies to histolysain were detected in a solid-phase enzyme immunoassay, the results suggesting that a humoral immune response is induced by histolysain during the formation of liver abscess.

Phagocytosis and proteinase activity are not related to pathogenicity of E. histolytica

To examine the relationship between phagocytosis, proteinase activity and pathogenicity of axenically grown trophozoites of E. histolytica strain HM-1:IMSS four different cultures were used: (1) a culture preserved in our laboratory for over 4 years, which lost its pathogenicity 3 years ago; (2) a culture passaged several times through hamster liver, which lost its pathogenicity recently; (3) a highly virulent culture supplied by another laboratory; and (4) amebas recovered from hamster liver abscesses caused by culture 3. Phagocytosis was measured as erythrophagocytosis. Proteinase activity was determined on azocasein. Pathogenicity was defined as the capacity to cause liver abscesses in hamsters. A negative correlation was found between phagocytic activity and pathogenicity, since amebas unable to cause liver abscesses had the highest phagocytic activity, whereas those recovered from liver abscesses had the lowest phagocytic activity. The percent of phagocytic amebas showed wide variations through a 2-month observation period, with no change in amebic pathogenicity. No correlation was found between the level of proteinase activity and pathogenicity. It is concluded that neither phagocytosis nor proteinase activity is an adequate marker of amebic pathogenicity.

Entamoeba histolytica as a model for the primitive eukaryotic cell

Entamoeba histolytica is a structurally simple eukaryote lacking mitochondria, peroxisomes and a well-developed Golgi apparatus, also in its biochemistry, it deviates substantially from the more complex eukoryotes. These features have alternatively been interpreted as archaic, ie. the ancestor of Entamoeba branched off before the primitive eukaryotic cell obtained proto-mitochondria, or as regressive, ie. Entamoeba has lost its mitochondria in the course of its adaptation to a parasitic life style. Tilly Bakker-Grunwald and Claudia Wöstmann favor the first interpretation and discuss in which respects E. histolytica may serve as a model for the primitive eukaryote.

Molecular characterization of the cDNA coding for translation elongation factor-2 of pathogenic Entamoeba histolytica

To investigate the humoral immune response of patients with amoebic dysentry against Entamoeba histolytica, immunoglobulin G (IgG)-immunopositive cDNA clones from the pathogenic strain SFL-3 were examined. A large part of the IgG-positive cDNA clones obtained with one serum encoded highly conserved intracellular proteins. A clone was found that was homologous to translation elongation factor-2 (EF-2). Sequence analysis of the EF-2 cDNA showed 63.6% amino acid sequence identity with the human homologue. The deduced protein sequence has a length of 840 amino acid residues with a molecular mass of 93.3 kD. The 3' and 5' untranslated regions of the mRNA are relatively short as shown for other genes of E. histolytica. A genomic clone was used to analyze the region upstream of the translation initiation codon. The codon distribution of EF-2 and other published E. histolytica sequences reflects the high A/T content. The codons for different amino acids are biased to a widely differing extent.

Detection of pathogenic Entamoeba histolytica DNA in liver abscess fluid by polymerase chain reaction

A sensitive method for detection of pathogenic Entamoeba histolytica DNA in drained fluids from liver abscess patients, using the polymerase chain reaction (PCR), has been developed. The PCR employs oligonucleotide primers specific for the gene encoding the 30 kDa molecule of pathogenic E. histolytica. Liver abscess fluids (19 samples), from 14 patients with a presumptive amebic liver abscess, were examined microscopically and by the PCR method. Only two of the 19 samples were positive microscopically, whereas all 19 samples tested positive by PCR. This technique can be used to confirm the diagnosis of amebic liver abscess.