Electron microscope studies of experimental Entamoeba histolytica infection in the guinea pig. II. Early cellular and vascular changes accompanying invasion of the lamina propria

Infectious diarrhea: Cellular and molecular mechanisms

Diarrhea caused by enteric infections is a major factor in morbidity and mortality worldwide. An estimated 2-4 billion episodes of infectious diarrhea occur each year and are especially prevalent in infants. This review highlights the cellular and molecular mechanisms underlying diarrhea associated with the three classes of infectious agents, i.e., bacteria, viruses and parasites. Several bacterial pathogens have been chosen as model organisms, including Vibrio cholerae as a classical example of secretory diarrhea, Clostridium difficile and Shigella species as agents of inflammatory diarrhea and selected strains of pathogenic Escherichia coli (E. coli) to discuss the recent advances in alteration of epithelial ion absorption. Many of the recent studies addressing epithelial ion transport and barrier function have been carried out using viruses and parasites. Here, we focus on the rapidly developing field of viral diarrhea including rotavirus, norovirus and astrovirus infections. Finally we discuss Giardia lamblia and Entamoeba histolytica as examples of parasitic diarrhea. Parasites have a greater complexity than the other pathogens and are capable of creating molecules similar to those produced by the host, such as serotonin and PGE(2). The underlying mechanisms of infectious diarrhea discussed include alterations in ion transport and tight junctions as well as the virulence factors, which alter these processes either through direct effects or indirectly through inflammation and neurotransmitters.

Interaction between pathogenic amebas and fibronectin: substrate degradation and changes in cytoskeleton organization

Invasion of human tissues by the parasitic protozoan Entamoeba histolytica is a multistep process involving, as a first step, the recognition of surface molecules on target tissues by the amebas or trophozoites. This initial contact is followed by the release of proteolytic and other activities that lyse target cells and degrade the extracellular matrix. In other parasitic diseases, as well as in certain cancers, the interaction of invasive organisms or cells with fibronectin (FN) through specific receptors has been shown to be the initial step in target cell recognition. Interaction with FN triggers the release of proteolytic activities necessary for the effector cell migration and invasion. Here, we describe the specific interaction of Entamoeba histolytica trophozoites with FN, and identify a 37-kD membrane peptide as the putative receptor for FN. The interaction between the parasite and FN leads to a response reaction that includes the secretion of proteases that degrade the bound FN and the rearrangement of amebic actin into "adhesion plates" at sites of contact with FN-coated surfaces. The kinetics of the interaction was determined by measuring the binding of soluble 125I-FN to the trophozoites and visualization of the bound protein using specific antibodies. Degradation of FN was measured by gel electrophoresis and the release of radioactivity into the incubation medium. Focal degradation of FN was visualized as black spots under the trophozoites at contact sites with fluorescent FN. We conclude that the interaction of E. histolytica with FN occurs through a specific surface receptor. The interaction promotes amebic cytoskeleton changes and release of proteases from the parasite. The binding and degradation of extracellular matrix components may facilitate the migration and penetration of amebas into tissues, causing the lesions seen in human hosts.

The major neutral proteinase of Entamoeba histolytica

FPLC anion-exchange and chromatofocusing chromatography were used to purify the major neutral proteinase from secretions of axenically cultured Entamoeba histolytica trophozoites. HM-1 strain trophozoites, which were more proteolytically active than the less virulent HK-9 strain, were used for purification of the enzyme. It is a thiol proteinase with a subunit Mr of approximately 56,000, a neutral pH optimum, and a pI of 6. The importance of this enzyme in extraintestinal amoebiasis is suggested by its ability to degrade a model of connective tissue extracellular matrix as well as purified fibronectin, laminin, and type I collagen. The enzyme caused a loss of adhesion of mammalian cells in culture, probably because of its ability to degrade anchoring proteins. Experiments with a peptide substrate and inhibitors indicated that the proteinase preferentially binds peptides with arginine at P-1. It is also a plasminogen activator, and could thus potentiate host proteinase systems.