Glycosidases ofEntamoeba histolytica

Suppression of hyaluronidase reduces invasion and establishment of Haemonchus contortus larvae in sheep

Haemonchus contortus is a hematophagous endoparasite of small ruminants, which is responsible for huge economic losses in livestock sector. Hyaluronidase produced by infective larvae of H. contortus can degrade hyaluronic acid present in the host’s abomasal tissue. Thus, it facilitates larval tissue invasion and early establishment. We herein explored this ability of hyaluronidase in H. contortus, and tested whether hyaluronidase is utilized as a virulence factor by H. contortus while establishing the infection. We first successfully blocked the hyaluronidase gene in L3 larvae by RNA interference (RNAi), which was subsequently confirmed by qPCR, enzymatic activity, and immunohistochemistry assays. Using these larvae we then conducted in vivo and in vitro assays on sheep to assess the effects of hyaluronidase suppression on larval invasion and establishment of infection. The in vivo assay showed a significant drop in worm burden in siRNA treated group in comparison to control group. During in vitro assay we applied an ovine ex vivo model where siRNA treated group of larvae showed significantly reduced invasion of the abomasal tissue explants as compared to control group. These findings indicate that hyaluronidase plays a key role in host’s tissue invasion and larval establishment, and it is used as a virulence factor by H. contortus while establishing the infection. As an invasive virulence molecule, its functional research is thus conducive to the prevention of haemonchosis.

The pathogenesis of amoebiasis

Amoebiasis, the infection of humans with Entamoeba histolytica, has a worldwide distribution; humans are the main reservoir and source of infection(1), although some other primates can also be infected. The motile trophozoite of E. histolytica (Fig. 1) lives in the lumen of the large intestine where it multiplies and eventually differentiates into cysts which are shed in the faeces and are responsible for transmission of infection. Two forms of amoebiasis are recognized: luminal amoebiasis where no clinical signs or symptoms are apparent, and invasive amoebiasis where the trophozoites invade the intestinal mucosa to produce dysentery or amoeboma, and can spread in blood to give extraintestinal lesions such as liver abscess. Isoenzyme markers for pathogenic and non-pathogenic types of E. histolytica are well documented, but there is some debate (see Parasitology Today, vol. 3, 37-43) about whether the two types represent completely separate entities or if they can change from one type to the other under certain circumstances (Box 1). Nonpathogenic types produce no apparent symptoms; in this article Adolfo Martínez-Palomo discusses the pathology associated with pathogenic types.

Evidence that hyaluronidase is not involved in tissue invasion of the protozoan parasite Entamoeba histolytica

As previous reports suggested that a hyaluronidase is involved in tissue invasion of Entamoeba histolytica, we searched for such an activity in trophozoite extracts. A hyaluronidase activity was not detectable in long-term cultures or in amoebae freshly passaged through a gerbil liver, as evidenced by four different techniques.

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 effects of Entamoeba histolytica lysates on human colonic mucins

Detergent lysates of Entamoeba histolytica trophozoites contained high levels of beta-N acetyl-D-glucosaminidase, beta-N acetyl-D-galactosaminidase and alpha-D-galactosidase activity, and lower but significant levels of five other glycosidases. Although these activities should have been capable of largely degrading the oligosaccharide side-chains of human colonic mucin, in fact only about one third of high MW mucin was degraded in 72 h and trypsin alone produced a similar effect. There was no evidence that these glycosidases were excreted and we conclude that they are unlikely to represent significant virulence factors for E. histolytica.

Secretory hydrolases of Entamoeba histolytica

Cells of Entamoeba histolytica grown over a period of four days contained NADP+-dependent alcohol dehydrogenase exclusively inside the cells. No activity of this enzyme could be found in the growth medium after harvesting the cells. Under the same conditions, acid phosphatase, beta-N-acetylglucosaminidase, esterase, alpha-glucosidase, and different amylases of the parasite were found both inside the cells and in the medium. The activities present in the cell homogenate and in the medium before and after growth of the amoebas were partially separated by gel filtration on Sephadex G150 and G75, respectively. The comparison of the elution diagrams revealed that NADP+-dependent alcohol dehydrogenase, acid phosphatase, esterase, and amylases occurred as multiple forms inside the cells. These activities, as well as beta-N-acetylglucosaminidase and alpha-glucosidase, were released into the extracellular environment to a different degree. The enzymes originating from the parasite were identified and distinguished from those of the ingredients of the growth medium according to their molecular mass and pH optimum. Furthermore, the amoebic origin of the secreted enzymes was shown on the basis of their inhibition by antibodies prepared against the supernatant fraction of the homogenate.

Separation and characterization of four different amylases of Entamoeba histolytica. I. Purification and properties

Cell homogenate of Entamoeba histolytica trophozoites was investigated for amylolytic activity against various biogenic and synthetic substrates. After gel filtration of the cell homogenate on Sephadex G-150, six partly separated amylases (I to VI) differing in their substrate specificities were detected using maltose, amylose, amylopectin, 4-nitrophenyl alpha-glucoside and 4-nitrophenyl alpha-maltotetraoside. All enzymes are able to degrade amylose, amylopectin, glycogen and biogenic malto-oligosaccharides. Since amylase I and II, which accepted maltose as substrate, were found in fresh (cell-free) medium containing calf serum, the possibility cannot be excluded that these enzymes originate from the medium and therefore are not associated with E. histolytica trophozoites. Amylases III to VI, which were not found in fresh medium, were further purified by isoelectric focusing and chromatographic procedures using DEAE, CM ion exchange materials and Con A Sepharose 4B. pH, temperature optima and relative molecular masses were determined.

Separation and characterization of four different amylases of Entamoeba histolytica. II. Characterization of amylases

Purified E. histolytica amylases III to VI were characterized by their hydrolytic behaviour towards 4-nitrophenyl alpha-malto-oligosaccharides, malto-oligosaccharides, amylose, amylopectin, glycogen and Y-cyclodextrin. The influence of specific inhibitors on the amylase activity of E. histolytica was examined and compared with typical alpha- and beta-amylases. Amylases III and IV showed alpha-glucosidase and glucosyltransferase activity by cleaving terminal non-reducing glucose from pNPG1 (III, IV) and pNPG2 to pNPG7 (III). Both enzymes were able to cleave malto-oligosaccharides and glucopolysaccharides to a large number of malto-oligosaccharides. Also transglucosidation reactions were observed, but maltose was not hydrolysed. Amylase V showed exoamylase-like properties by preferentially cleaving maltose units from the non-reducing end of synthetic and biogenic malto-oligosaccharides by a multiple-attack mechanism. Amylase VI was characterized as an alpha-amylase, showing great similarities with porcine pancreatic alpha-amylase in the hydrolysis pattern of 4-nitrophenyl alpha-malto-oligosaccharides and glucopolysaccharides. With biogenic malto-oligosaccharides amylase VI showed a transglucosidation reaction.