Antigens in electron-dense granules from Entamoeba histolytica as possible markers for pathogenicity

Diagnostic parameters of serological ELISA for invasive amoebiasis, using antigens preserved without enzymatic inhibitors

Amoebiasis is the third cause of death due to parasites in the world. Although, numerous serodiagnostic and salivary tests have been developed, the majority of these assays lack sensitivity in endemic zones to detect acute amoebic liver abscess. The two main limiting factors to develop reliable assays are the high levels of anti-amoeba antibodies in populations living in endemic zones, and the proteolysis of amoebic extracts even treated with inhibitors. Our group reported a method to preserve amoebic antigens without using enzymatic inhibitors (IC:MC fraction) that shows stability for years. Here we describe the development of a serologic ELISA to diagnose amoebiasis made with IC: MC antigens, and its validation for clinical use in endemic areas. In our study, we included sera from 66 patients diagnosed with acute amoebic liver abscess and 33 volunteers living in an endemic area for amoebiasis. Our assay was compared with an indirect haemagglutination assay (IHA) an ELISA elaborated with antigens derived from untreated trophozoites. The ELISA made with IC: MC antigens presented more reproducibility compared to other assays. Sera from 95% ALA patients showed a positive value. The ELISA (IC: MC) detected 97% of patients with ALA compared to an 81% using IHA. The parameters of ELISA (vs. IHA) were Sensitivity 98% (81%), Specificity 96% (97%), Positive predictive value 98% (96%), Negative predictive value 96% (73%) and Accuracy 98% (87%). A negative serologic test does not rule out the diagnosis of invasive amoebiasis. The ELISA made with antigens preserved without using enzymatic inhibitors has valuable serodiagnostic value to diagnose acute amoebic liver abscess, even in populations living in endemic zones of amoebiasis carrying antibodies against amoebas. In conclusion, ELISA-IC:MC presented better diagnostic parameters than IHA although a negative serologic test does not rule out acute invasive amoebiasis.

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 interaction: parasite-derived and -induced proteases that degrade human extracellular matrix

Parasitic protozoa are among the most important pathogens worldwide. Diseases such as malaria, leishmaniasis, amoebiasis, giardiasis, trichomoniasis, and trypanosomiasis affect millions of people. Humans are constantly threatened by infections caused by these pathogens. Parasites engage a plethora of surface and secreted molecules to attach to and enter mammalian cells. The secretion of lytic enzymes by parasites into host organs mediates critical interactions because of the invasion and destruction of interstitial tissues, enabling parasite migration to other sites within the hosts. Extracellular matrix is a complex, cross-linked structure that holds cells together in an organized assembly and that forms the basement membrane lining (basal lamina). The extracellular matrix represents a major barrier to parasites. Therefore, the evolution of mechanisms for connective-tissue degradation may be of great importance for parasite survival. Recent advances have been achieved in our understanding of the biochemistry and molecular biology of proteases from parasitic protozoa. The focus of this paper is to discuss the role of protozoan parasitic proteases in the degradation of host ECM proteins and the participation of these molecules as virulence factors. We divide the paper into two sections, extracellular and intracellular protozoa.

Molecular nature of virulence in Entamoeba histolytica

For many years virulence of pathogenic Entamoeba histolytica has been attributed to the capacity of the parasite to destroy tissues through the expression and/or secretion of various molecules. Such view is supported mainly by in vitro experimentation, whereas data obtained by using animal models of the disease have clearly demonstrated that the host's inflammatory response is primarily responsible for tissue damage. This review analyzes the content and/or activity of some of the presumed toxic amebic molecules present in amebic strains with different degrees of virulence compared to various parasite in vitro functions that are supposed to correlate with in vivo virulence. The analysis suggests that amebic virulence is primarily determined by the parasite's capacity to adapt and survive the aerobic conditions present in animal tissues. This initial episode in the host-parasite relationship is an absolute requirement for the further development of tissue lesions, which result from the concerted action of many molecules derived from both, the host and the parasite.

Expression and function of a family of transmembrane kinases from the protozoan parasite Entamoeba histolytica

The signaling proteome of Entamoeba histolytica is made of transmembrane kinases (TMKs) that are rarely found in unicellular eukaryotes. There are 90 TMK genes reported for E. histolytica, and these have been grouped into nine distinct families based on motifs present on both extracellular and kinase domains. Of these, the B1 family was chosen for further analysis. Genomic sequencing revealed the presence of 28 members belonging to this family. Genes corresponding to the majority of these were truncated and not considered for further analysis. Only five members were full length and contained both extracellular and cytosolic kinase domains. BLAST analysis revealed the presence of homologs of these B1 TMKs in the nonpathogenic Entamoeba dispar. However, the ligand binding domains of the orthologous B1 TMKs of the two species showed considerable divergence, indicating the possibility of a correlation with the pathogenic potential of the organism. Only two of the five full-length copies (B1.I.1 and B1.I.2) were expressed in E. histolytica under the culture conditions used. Antisera generated against the extracellular domain of B1.I.1 stained the cell surface, particularly the areas of contact between the trophozoites. Staining was also seen in the frontal and posterior regions of the motile amoeba. An amoebic cell line expressing a truncated version of the B1.I.1 that lacked the kinase domain was generated. Inducible expression of the truncated TMK resulted in a decrease in cellular proliferation and an increase in sensitivity to serum starvation. Our data indicate that the B1.I class of TMKs is involved in parasite proliferation.

The beta-N-acetylhexosaminidase of Entamoeba histolytica is composed of two homologous chains and has been localized to cytoplasmic granules

We have purified a beta-N-acetylhexosaminidase from trophozoites of Entamoeba histolytica to homogeneity. In SDS-PAGE, the enzyme yielded a single protein band at an apparent M(r) of 64,000. The elution behaviour of the native enzyme upon molecular sieve chromatography corresponded to a molecular mass of approximately 132,000 suggesting that the enzyme is a dimer. Upon sedimentation velocity centrifugation, hexosaminidase activity sedimented at 12S, implying aggregation to a higher molecular mass complex with an apparent M(r) of approximately 400,000. Based on the N-terminal sequence of the purified enzyme and on data extracted from the E. histolytica genomic data base, we amplified and cloned two genes (EhHEXA and EhHEXB) coding for two presumptive, highly similar hexosaminidase chains which we designated as Ehhexalpha and Ehhexbeta. Northern blot analysis indicated that the two genes were expressed to a similar level, and Western blotting with chain-specific antisera showed that the trophozoites synthesize both proteins. By cell fractionation, the hexosaminidase was found to be a major component of cytoplasmic granules; these contain tissue-destructive factors and are released after collagen-induced exocytosis to the cell surface. In agreement with this observation, immunocytochemistry with an antiserum cross-reacting with both hexosaminidase chains revealed strong fluorescence in surface patches, which we interpret as released granules, and in vesicles throughout the cell. Its localization in cytoplasmic granules strengthens the notion that the hexosaminidase complex may contribute to amoebic pathogenicity.

Cysteine proteinases and the pathogenesis of amebiasis

Amebiasis is a major cause of morbidity and mortality throughout the tropical world. Entamoeba histolytica is now recognized as a separate species from the morphologically identical E. dispar, which cannot invade. Cysteine proteinases are a key virulence factor of E. histolytica and play a role in intestinal invasion by degrading the extracellular matrix and circumventing the host immune response through cleavage of secretory immunoglobulin A (sIgA), IgG, and activation of complement. Cysteine proteinases are encoded by at least seven genes, several of which are found in E. histolytica but not E. dispar. A number of new animal models, including the formation of liver abscesses in SCID mice and intestinal infection in human intestinal xenografts, have proven useful to confirm the critical role of cysteine proteinases in invasion. Detailed structural analysis of cysteine proteinases should provide further insights into their biochemical function and may facilitate the design of specific inhibitors which could be used as potential chemotherapeutic agents in the future.

Cysteine proteinases and the pathogenesis of amebiasis

Amebiasis is a major cause of morbidity and mortality throughout the tropical world. Entamoeba histolytica is now recognized as a separate species from the morphologically identical E. dispar, which cannot invade. Cysteine proteinases are a key virulence factor of E. histolytica and play a role in intestinal invasion by degrading the extracellular matrix and circumventing the host immune response through cleavage of secretory immunoglobulin A (sIgA), IgG, and activation of complement. Cysteine proteinases are encoded by at least seven genes, several of which are found in E. histolytica but not E. dispar. A number of new animal models, including the formation of liver abscesses in SCID mice and intestinal infection in human intestinal xenografts, have proven useful to confirm the critical role of cysteine proteinases in invasion. Detailed structural analysis of cysteine proteinases should provide further insights into their biochemical function and may facilitate the design of specific inhibitors which could be used as potential chemotherapeutic agents in the future.

Partial characterization of a 36-kDa antigen of Entamoeba histolytica and its recognition by sera from patients with amoebiasis

A 36-kDa antigen of axenically grown pathogenic Entamoeba histolytica (HM1-IMSS) was eluted from the sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE)-resolved crude amoebic extract antigens. The immunoreactivity of this partially purified 36-kDa antigen with monoclonal antibody (MoAb) 3D(10) altered significantly (P<0.01) after heat and trypsin treatment but remained unaltered after treatment with sodium metaperiodate (P0.5), thereby indicating the protein nature of the epitope recognized by MoAb 3D(10). The epitope was found to be localized on the surface as well as in the cytoplasm of the E. histolytica trophozoites with the majority of it in the cytoplasm. In addition, this epitope was also found to be present on the cyst form of the parasite. The 36-kDa molecule was recognized by the sera from 29 (85%) of the 34 patients with amoebic liver abscess and five (83%) of the six patients with amoebic colitis. No serum samples from asymptomatic cyst passers, from patients with non-amoebic hepatic or intestinal disorders and apparently healthy subjects had antibodies that reacted with this 36-kDa molecule. The immune responses in man to this 36-kDa amoebic molecule indicate a potential specific role for this molecule in invasive amoebiasis.

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.

In vitro secretion of metallo-protease (200 kDa) by the pathogenic piscine haemoflagellate, Cryptobia salmositica Katz, and stimulation of protease production by collagen

Cryptobia salmositica cultured in minimum essential medium secreted metallo-protease, and a significantly higher amount of the protease was found in media supplemented with either type I or type IV collagen. The enhancement of the protease secretion by type I collagen was dose-dependent. Using haemoglobin (substrate) SDS-PAGE, the secreted protease was detected as a single clear band (about 200 kDa). The 200 kDa metallo-protease was also detected on the C. salmositica surface membrane and the amount was increased by incubating the parasite with collagen. Collagen as a specific substrate may enhance the role of the C. salmositica metallo-protease in the disease process in infected fish.

Electron probe analysis and biochemical characterization of electron-dense granules secreted by Entamoeba histolytica

The interaction of Entamoeba histolytica with collagen induces the intracellular formation and release of electron-dense granules (EDGs) containing collagenase activity which are important in the pathogenicity of this parasite. Purified EDGs contain at least 25 polypeptides with acidic pIs, nine gelatinase activities, small molecules, including inorganic phosphate (Pi), pyrophosphate (PP) and other elements, including Na, Mg, S, Cl, K, Ca and Fe as measured by scanning transmission electron microscopy. Six of these polypeptides with apparent molecular weights of 108, 106, 104, 97, 68 and 59 kDa and two protease activities with apparent molecular weights of 40 and 85 kDa were detected exclusively in the EDGs and were not observed in total trophozoite extracts. Actin was also detected in the EDGs. Therefore, EDGs are a complex of mainly cationic proteins, which contains numerous proteolytic activities, actin and small molecules such as P(i), PP and cations.

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.

Carbohydrate epitopes of Entamoeba histolytica cell surface glycoproteins are major targets of the human humoral response

The antigens of Entamoeba histolytica recognized by antibodies in 11 individual sera from patients treated for amebic liver abscess were determined both by immunoprecipitation of metabolically-radiolabeled whole trophozoite proteins and by immunoblotting.Collectively, twenty-s even antigens ranging from 167 to 21 kDa were detected in immunoblots of whole trophozoite extracts; eight of these were recognized by all tested patient sera. Immunoprecipitation studies also revealed a complex amebic antigenic profile. Of a total of twenty immunoprecipitated polypeptides (from 200 to 24 kDa), seventeen were uniquely recognized by the patient sera. Eight of these seventeen antigens were immunoprecipitated by most immune sera. The cellular localization of trophozoite antigens was determined by analyzing plasma membrane and soluble cytosol fractions. Plasma membranes contained virtually as many antigenic moieties as the total trophozoite extract; in contrast, the soluble fraction was antigenically less complex. Mild periodate oxidation of plasma membrane antigens indicated that surface glycoproteins are highly immunogenic for the human host and that antibodies to their carbohydrate epitopes are a major component of the total response of most patients.

Entamoeba histolytica: electron-dense granule secretion, collagenase activity and virulence are altered in the cytoskeleton mutant BG-3

HM-1:IMSS, a pathogenic strain of Entamoeba histolytica, and its mutant BG-3, identified by resistance to cytochalasin D, were tested for their capacity to: (i) secrete electron-dense granules; (ii) adhere and digest native type I collagen gels; and (iii) produce liver abscesses in new-born hamsters. The results demonstrate that the mutant has low adherence to collagen, low electron-dense granule secretion and collagenolytic activity, and low capacity to produce liver lesions in vivo, compared with the parental strain HM1:IMSS.

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 monoclonal antibody for distinction of invasive and noninvasive clinical isolates of Entamoeba histolytica

Approximately 10% of the world population is infected with Entamoeba histolytica, but only 10% of the carriers develop symptomatic amebiasis. This discrepancy could be explained by the genotypic differences between the morphologically indistinguishable invasive and noninvasive strains of E. histolytica currently identified by zymodeme analysis, a technique that is unsuitable for routine diagnostic laboratories. Here we report the production of a monoclonal antibody against E. histolytica and its use in an immunofluorescence assay to identify invasive isolates cultured from stool samples of infected patients in several regions where amebiasis is endemic: Bangladesh, Colombia, and Mexico. After testing a total of 88 E. histolytica isolates, the correlation between zymodeme characterization and the immunofluorescence assay with the invasive isolate-specific monoclonal antibody was 100%. The epitope detected by the invasive isolate-specific monoclonal antibody resides in a previously undescribed internal protein with molecular masses of 84 and 81 kDa in axenic and polyxenic E. histolytica strains, respectively.

Purification and biochemical properties of calmodulin in Entamoeba histolytica and its distribution during secretion of electron-dense granules

1. Calmodulin (CaM) was detected during secretion of electron-dense granules by Entamoeba histolytica trophozoites with immunofluorescence. 2. It was purified to apparent homogeneity by chromatography with a yield of 2.26 micrograms of calmodulin/mg of protozoan protein. Purity was established by gel electrophoresis. 3. The parasite calmodulin has properties characteristic of calmodulin isolated from other eukaryotes: an apparent molecular weight of 19 or 17 kDa in presence of EGTA or CaCl2, respectively, activation in a calcium dependent manner of bovine heart cyclic nucleotide phosphodiesterase, and its UV spectrum.

Determination of taxonomic status of pathogenic and nonpathogenic Entamoeba histolytica zymodemes using isoenzyme analysis

Entamoeba histolytica infection results in either asymptomatic colonization or invasion of host tissues leading generally to clinical symptoms. Zymodemes studies have demonstrated a correlation between isoenzyme profiles and clinical presentation. Thus, strains have been attributed to pathogenic or nonpathogenic groups according to their zymodeme. To determine the taxonomic relationship of these two groups, the isoenzyme profiles of 14 loci of 38 E. histolytica strains (pathogenic and nonpathogenic) and seven strains of other species of the same genus were analyzed. Genetic distance analysis clearly demonstrates the existence of two separate groups within the species E. histolytica.

Entamoeba histolytica: is conversion of "nonpathogenic" amebae to the "pathogenic" form a real phenomenon?

Entamoeba histolytica isolates have been shown to fall into two groups based on isoenzyme analysis. These groupings ("pathogenic" and "nonpathogenic") correlate well with the clinical course of the infection. A controversy exists over whether isoenzyme patterns are stable or whether under certain circumstances an isolate can convert from one form to the other. Resolution of this uncertainty is of importance since the nonpathogenic pattern has never been observed in amebae isolated from cases of active disease. This implies that, if the patterns are stable, carriers of amebae with this nonpathogenic pattern may never develop invasive disease. Although we set out to study isoenzyme conversion, we have been unable to replicate the two published accounts of this phenomenon. We have examined all of the variables proposed to be involved in the triggering of conversion, both individually and in combination. In none of the experiments was an alteration in the isoenzyme pattern observed. We now believe that isoenzyme patterns are stable and that all available evidence, other than the reported conversions, points to pathogenic and nonpathogenic E. histolytica being distinct species.

Entamoeba histolytica: erythrophagocytosis, collagenolysis, and liver abscess production as virulence markers

High rates of erythrophagocytosis and collagenolysis in vitro have been regarded as indicative of virulence in vivo of Entamoeba histolytica trophozoites. In the present study, the erythrophagocytic index and the collagenolytic activity of 3 axenic lines of E. histolytica, strain HM1:IMSS, were measured. The 3 lines shared the same pathogenic zymodeme but showed clear-cut differences in the extent of liver damage induced in hamsters. A direct correlation between collagenolysis in vitro and the size of liver abscesses produced by each line of E. histolytica trophozoites was found. In contrast, the line with the highest erythrophagocytic index produced small amoebic abscesses in hamsters, whereas the line with a relatively low erythrophagocytic index produced the largest liver lesions. It is concluded that the extent of collagenolytic activity is a better marker of virulence of E. histolytica cultured under axenic conditions than is erythrophagocytosis.

Possible role of calmodulin in the secretion of Entamoeba histolytica electron-dense granules containing collagenase

Entamoeba histolytica cells secrete electron-dense granules (EDGs) that have collagenase activity. To study the possible involvement of calmodulin (CaM) on EDG secretion, the effect of several CaM antagonists (TFP, R24571, W-7, W-5, dibucaine and DL-propranolol) was tested on this cellular function. Except for W-5 and dibucaine, the rest of these compounds inhibited EDG secretion. Transmission electron microscopy of collagen-activated trophozoites showed numerous EDGs located in or near the surface membrane. In contrast, trophozoites incubated with TFP showed no EDGs. Protein kinase C inhibitors (H-7, ML-9) had no effect on EDG secretion, suggesting that CaM antagonists acted by selectively inhibiting CaM. These results suggest that a CaM-dependent process is involved in EDG secretion.