Molecular analysis of the Gal/GalNAc adhesin of Entamoeba histolytica

Expression, purification, and evaluation of recombinant LecA as a candidate for an amebic colitis vaccine

Entamoeba histolytica, which causes amebic colitis and liver abscess, is considered a major enteric pathogen in residents and travelers to developing countries where the disease is endemic. Interaction of this protozoan parasite with the intestine is mediated through the binding of the trophozoite stage to intestinal mucin and epithelium via a galactose and N-acetyl-d-galactosamine (Gal/GalNAc) lectin comprised of a disulfide linked heavy (ca. 180 kDa) and light chain (ca. 35 kDa) and a noncovalently bound intermediate subunit (ca. 150 kDa). Our efforts to develop a vaccine against this pathogen have focused on an internal 578 amino acid fragment, designated LecA, located within the cysteine-rich region of the heavy chain subunit because: (i) it is a major target of adherence-blocking antibodies of seropositive individuals and (ii) vaccination with his-tagged LecA provides protection in animal models. We developed a purification process for preparing highly purified non-tagged LecA using a codon-optimized gene expressed in Escherichia coli. The process consisted of: (i) cell lysis, collection and washing of inclusion bodies; (ii) solubilization and refolding of denatured LecA; and (iii) a polishing gel filtration step. The purified fragment existed primarily as a random coil with β-sheet structure, contained low endotoxin and nucleic acid, was highly immunoreactive, and elicited antibodies that recognized native lectin and that inhibited in vitro adherence of trophozoites to CHO cells. Immunization of CBA mice with LecA resulted in significant protection against cecal colitis. Our procedure yields sufficient amounts of highly purified LecA for future studies on stability, immunogenicity, and protection with protein-adjuvant formulations.

Current and future perspectives on the chemotherapy of the parasitic protozoa Trichomonas vaginalis and Entamoeba histolytica

Trichomonas vaginalis and Entamoeba histolytica are clinically important protozoa that affect humans. T. vaginalis produces sexually transmitted infections and E. histolytica is the causative agent of amebic dysentery. Metronidazole, a compound first used to treat T. vaginalis in 1959, is still the main drug used worldwide to treat these pathogens. It is essential to find new biochemical differences in these organisms that could be exploited to develop new antiprotozoal chemotherapeutics. Recent findings associated with T. vaginalis and E. histolytica biochemistry and host–pathogen interactions are surveyed. Knowledge concerning the biochemistry of these parasites is serving to form the foundation for the development of new approaches to control these important human pathogens.

Stable expression of Gal/GalNAc lectin of Entamoeba histolytica in transgenic chloroplasts and immunogenicity in mice towards vaccine development for amoebiasis

Chloroplast genetic engineering offers several advantages, including high levels of transgene expression, transgene containment via maternal inheritance and multigene engineering in a single transformation event. Entamoeba histolytica infects 50 million people, causing about 100,000 deaths annually, but there is no approved vaccine against this pathogen. LecA, a potential target for blocking amoebiasis, was expressed for the first time in transgenic plants. Stable transgene integration into chloroplast genomes and homoplasmy were confirmed by polymerase chain reaction and Southern blot analyses. LecA expression was evaluated by Western blots and quantified by enzyme-linked immunosorbent assay (up to 6.3% of total soluble protein or 2.3 mg LecA/g leaf tissue). Subcutaneous immunization of mice with crude extract of transgenic leaves resulted in higher immunoglobulin G titres (up to 1:10,000) than in previous reports. An average yield of 24 mg of LecA per plant should produce 29 million doses of vaccine antigen per acre of transgenic plants. Such high levels of expression and immunogenicity should facilitate the development of a less expensive amoebiasis vaccine.

Structure and function of the Entamoeba histolytica Gal/GalNAc lectin

Gal/GalNAc lectin is a novel multifunctional virulence factor of the human parasite Entamoeba histolytica. The native protein is a 260-kDa heterodimer consisting of a type 1 membrane protein disulfide bonded to a lipid-anchored protein. Each subunit has several isoforms that may form functionally different heterodimers, analogous to the integrin family of proteins. Recently a second 150-kDa Gal/GalNAc lectin has been identified in E. histolytica that associates with the 260-kDa lectin. The functions of the 260-kDa lectin have been characterized using specific monoclonal antibodies. This lectin plays roles in many of the critical aspects of this parasite's pathogenicity including adherence, cytolysis, invasion, resistance to lysis by complement, and also perhaps encystment. Current knowledge regarding both the structure and function of this unique multifunctional virulence factor are discussed.

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.

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.

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.