This study presents morphological and biochemical evidence of programmed cell death (PCD) in Entamoeba histolytica induced by exposure of trophozoites to the aminoglycoside antibiotic G418. Morphological characteristics of PCD, including cell shrinkage, reduced cellular volume, nuclear condensation, DNA fragmentation and vacuolization were observed, with preservation of trophozoite membrane integrity. PCD is orchestrated biochemically by alterations in intracellular ion fluxes. In G418-treated trophozoites, overproduction of reactive oxygen species (ROS), decreased intracellular K + , increased cytosolic calcium, and decreased intracellular pH levels were observed. However, externalization of phosphatidylserine was not detected. These results suggest that amoebae can undergo PCD under stress conditions, and that this PCD shares several properties with PCD reported in mammals and in a variety of unicellular organisms. INTRODUCTIONEntamoeba histolytica, the causal agent of amoebiasis, is a protozoan parasite that resides in the colon of infected humans. The invasive trophozoites adhere to mucus and epithelial cells, proliferate by binary fusion, and release proteolytic factors that destroy the intestinal mucosa, resulting in amoebic dysentery. In one in 10 patients with intestinal E. histolytica infection, the trophozoites migrate through the portal vein to the liver and give rise to amoebic abscesses, the main cause of death by this parasite (Espinosa-Cantellano & Martínez-Palomo, 2000). The apoptosis of host cells such as macrophages induced by contact with E. histolytica trophozoites has been widely studied, and it is considered an important feature of the host-parasite relationship (Ragland et al., 1994;Berninghausen & Leippe, 1997).Programmed cell death (PCD) has been considered a critical mechanism of development, differentiation and control of cellular proliferation in metazoans. However, increasing evidence indicates that PCD is also present in unicellular organisms. Forms of PCD such as apoptosis, apoptosis-like processes and necrosis-like processes have been identified in several bacteria (Lewis, 2000), yeast (Madeo et al., 1999), the slime mould Dictyostelium discoideum (Cornillon et al., 1994), the dinoflagellate Peridinium gatunense (Vardi et al., 1999), the euglenoid Euglena gracilis (Scheuerlein et al., 1995), the ciliate Tetrahymena thermophila (Christensen et al., 1995), and the protozoan parasites Trypanosoma, Leishmania (Nguewa et al., 2004) and Plasmodium (Al-Olayan et al., 2002). Recently, results reported by Ramos et al. (2007) have suggested the induction of an apoptotic-like process by nitric oxide species in E. histolytica. Apoptosis is the result of a genetic program that induces cellular and biochemical changes, including caspase activation, externalization of phosphatidylserine (PS), an increase in intracellular Ca 2+ and mitochondrial dysfunction, as well as physical changes such as cell shrinkage, alteration in cell volume, cytoplasmic blebbing and vacuolization, chromatin conden...
Actin has been described in all eukaryotic cells as the major microfilament cytoskeletal protein. Although prokaryotic cells do not have a cytoskeleton, proteins related to the latter have been found in different prokaryotic species. We have found prokaryotic actin-related proteins in the enterobacterium Escherichia coli and in the cyanobacteria Anabaena cylindrica and Anabaena variabilis. They were identified by the following criteria: (1) by cross-reaction with a fluorescent conjugated anti-actin (rat-brain) mAb by Western blot analysis (in total cellular extracts); (2) specific binding of acetone powder and soluble cellular extracts to DNase 1; and (3) specific binding of cells and total cellular extracts to phalloidin. In E. coli, specific binding of phalloidin labelled with rhodamine to cells was detected by spectrofluorometry. In total cellular extracts, three bands of 60, 43 and 35 kDa were weakly recognized by the mAb by Western blot analysis; this recognition increased when phalloidin was added to the extracts. Furthermore, three polypeptides of 60 kDa were isolated by binding to DNase I, showing pl values of 6.7, 6.65 and 6.6, less acidic than all reported actin pl values. In A. cylindrica and A. variabilis, specific binding of phalloidin labelled with rhodamine to cells was also detected by spectrofluorometry. In total and soluble cellular extracts, the mAb recognized two bands of 45 and 40 kDa by Western blot analysis, but only the first was purified by binding to DNase I, and it showed three isoforms of pl values 6.8, 6.5 and 6.4. These results suggest the presence, in prokaryotes, of proteins with similar biochemical characteristics to eukaryotic actin.
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