Activation of host cell protein kinase C by enteropathogenic Escherichia coli

Gut feelings: enteropathogenic E. coli (EPEC) interactions with the host

Enteropathogenic Escherichia coli (EPEC) is a gram-negative bacterial pathogen that adheres to human intestinal epithelial cells, resulting in watery, persistent diarrhea. It subverts the host cell cytoskeleton, causing a rearrangement of cytoskeletal components into a characteristic pedestal structure underneath adherent bacteria. In contrast to other intracellular pathogens that affect the actin cytoskeleton from inside the host cytoplasm, EPEC remains extracellular and transmits signals through the host cell plasma membrane via direct injection of virulence factors by a "molecular syringe," the bacterial type III secretion system. One injected factor is Tir, which functions as the plasma membrane receptor for EPEC adherence. Tir directly links extracellular EPEC through the epithelial membrane and firmly anchors it to the host cell actin cytoskeleton, thereby initiating pedestal formation. In addition to stimulating actin nucleation and polymerization in the host cell, EPEC activates several other signaling pathways that lead to tight junction disruption, inhibition of phagocytosis, altered ion secretion, and immune responses. This review summarizes recent developments in our understanding of EPEC pathogenesis and discusses similarities and differences between EPEC pedestals, focal contacts, and Listeria monocytogenes actin tails.

Exploitation of host cells by enteropathogenic Escherichia coli

Microbial pathogens have evolved many ingenious ways to infect their hosts and cause disease, including the subversion and exploitation of target host cells. One such subversive microbe is enteropathogenic Escherichia coli (EPEC). A major cause of infantile diarrhea in developing countries, EPEC poses a significant health threat to children worldwide. Central to EPEC-mediated disease is its colonization of the intestinal epithelium. After initial adherence, EPEC causes the localized effacement of microvilli and intimately attaches to the host cell surface, forming characteristic attaching and effacing (A/E) lesions. Considered the prototype for a family of A/E lesion-causing bacteria, recent in vitro studies of EPEC have revolutionized our understanding of how these pathogens infect their hosts and cause disease. Intimate attachment requires the type III-mediated secretion of bacterial proteins, several of which are translocated directly into the infected cell, including the bacteria's own receptor (Tir). Binding to this membrane-bound, pathogen-derived protein permits EPEC to intimately attach to mammalian cells. The translocated EPEC proteins also activate signaling pathways within the underlying cell, causing the reorganization of the host actin cytoskeleton and the formation of pedestal-like structures beneath the adherent bacteria. This review explores what is known about EPEC's subversion of mammalian cell functions and how this knowledge has provided novel insights into bacterial pathogenesis and microbe-host interactions. Future studies of A/E pathogens in animal models should provide further insights into how EPEC exploits not only epithelial cells but other host cells, including those of the immune system, to cause diarrheal disease.

Enteropathogenic Escherichia coli dephosphorylates and dissociates occludin from intestinal epithelial tight junctions

Enteropathogenic Escherichia coli (EPEC) increases tight junction permeability in part by phosphorylating the 20 kDa myosin light chain (MLC20) that induces cytoskeletal contraction. The impact of this enteric pathogen on specific tight junction (TJ) proteins has not been investigated. We examined the effect of EPEC infection on occludin localization and phosphorylation in intestinal epithelial cells. After infection by EPEC, a progressive shift of occludin from a primarily TJ-associated domain to an intracellular compartment occurred, as demonstrated by immunofluorescent staining. A reverse in the ratio of phosphorylated to dephosphorylated occludin accompanied this morphological change. Eradication of EPEC with gentamicin resulted in the normalization of occludin localization and phosphorylation. The serine/threonine phosphatase inhibitor, calyculin A, prevented these events. The EPEC-associated decrease in transepithelial electrical resistance, a measure of TJ barrier function, returned to baseline after gentamicin treatment. Non-pathogenic E. coli, K-12, did not induce these changes. Transformation of K-12 with the pathogenicity island of EPEC, however, conferred the phenotype of wild-type EPEC. Deletion of specific EPEC genes encoding proteins involved in EPEC type III secretion markedly attenuated these effects. These findings suggest that EPEC-induced alterations in occludin contribute to the pathophysiology associated with this infection.

Enteropathogenic Escherichia coli (EPEC) attachment to epithelial cells: exploiting the host cell cytoskeleton from the outside

Enteropathogenic Escherichia coli (EPEC), a leading cause of human infantile diarrhoea, is the prototype for a family of intestinal bacterial pathogens that induce attaching and effacing (A/E) lesions on host cells. A/E lesions are characterized by localized effacement of the brush border of enterocytes, intimate bacterial attachment and pedestal formation beneath the adherent bacteria. As a result of some recent breakthrough discoveries, EPEC has now emerged as a fascinating paradigm for the study of host-pathogen interactions and cytoskeletal rearrangements that occur at the host cell membrane. EPEC uses a type III secretion machinery to attach to epithelial cells, translocating its own receptor for intimate attachment, Tir, into the host cell, which then binds to intimin on the bacterial surface. Studies of EPEC-induced cytoskeletal rearrangements have begun to provide clues as to the mechanisms used by this pathogen to subvert the host cell cytoskeleton and signalling pathways. These findings have unravelled new ways by which pathogenic bacteria exploit host processes from the cell surface and have shed new light on how EPEC might cause diarrhoea.

Citrobacter rodentium espB is necessary for signal transduction and for infection of laboratory mice

Citrobacter rodentium is the causative agent of transmissible murine colonic hyperplasia and contains a locus of enterocyte effacement (LEE) similar to that found in enteropathogenic Escherichia coli (EPEC). EPEC espB is necessary for intimate attachment and signal transduction between EPEC and cultured cell monolayers. Mice challenged with wild-type C. rodentium develop a mucosal immunoglobulin A response to EspB. In this study, C. rodentium espB has been cloned and its nucleotide sequence has been determined. C. rodentium espB was found to have 90% identity to EPEC espB. A nonpolar insertion mutation in C. rodentium espB was constructed and used to replace the chromosomal wild-type allele. The C. rodentium espB mutant exhibited reduced cell association and had no detectable fluorescent actin staining activity on cultured cell monolayers. The C. rodentium espB mutant also failed to colonize laboratory mice following experimental inoculation. The espB mutation could be complemented with a plasmid-encoded copy of the gene, which restored both cell association and fluorescent actin staining activity, as well as the ability to colonize laboratory mice. These studies indicate that espB is necessary for signal transduction and for colonization of laboratory mice by C. rodentium.

Host cell death due to enteropathogenic Escherichia coli has features of apoptosis

Enteropathogenic Escherichia coli (EPEC) is a cause of prolonged watery diarrhea in children in developing countries. The ability of EPEC to kill host cells was investigated in vitro in assays using two human cultured cell lines, HeLa (cervical) and T84 (colonic). EPEC killed epithelial cells as assessed by permeability to the vital dyes trypan blue and propidium iodide. In addition, EPEC triggered changes in the host cell, suggesting apoptosis as the mode of death; such changes included early expression of phosphatidylserine on the host cell surface and internucleosomal cleavage of host cell DNA. Genistein, an inhibitor of tyrosine kinases, and wortmannin, an inhibitor of host phosphatidylinositol 3-kinase, markedly increased EPEC-induced cell death and enhanced the features of apoptosis. EPEC-induced cell death was contact dependent and required adherence of live bacteria to the host cell. A quantitative assay for EPEC-induced cell death was developed by using the propidium iodide uptake method adapted to a fluorescence plate reader. With EPEC, the rate and extent of host cell death were less that what has been reported for Salmonella, Shigella, and Yersinia, three other genera of enteric bacteria known to cause apoptosis. However, rapid apoptosis of the host cell may not favor the pathogenic strategy of EPEC, a mucosa-adhering, noninvasive pathogen.

Enteropathogenic Escherichia coli: cellular harassment

The mechanisms by which enteropathogenic Escherichia coli (EPEC) mediates diarrhea remain a mystery. Recently a number of interesting and at times surprising results have come from studying EPEC interactions with host cells. Identification and characterization of bacterial factors, including Tir, EspA, EspB and EspD, and host responses have expanded our grasp of the diverse effects of EPEC on host cells.

Enteropathogenic E. coli, Salmonella, and Shigella: masters of host cell cytoskeletal exploitation

Bacterial pathogens have evolved numerous strategies to exploit their host's cellular processes so that they can survive and persist. Often, a bacterium must adhere very tightly to the cells and mediate its effects extracellularly, or it must find a way to invade the host's cells and survive intracellularly. In either case, the pathogen hijacks the host's cytoskeleton. The cytoskeleton provides a flexible framework for the cell and is involved in mediating numerous cellular functions, from cell shape and structure to programmed cell death. Altering the host cytoskeleton is crucial for mediating pathogen adherence, invasion, and intracellular locomotion. We highlight recent advances in the pathogenesis of enteropathogenic Escherichia coli, Salmonella Typhimurium, and Shigella flexneri. Each illustrates how bacterial pathogens can exert dramatic effects on the host cytoskeleton.

Putting E. coli on a pedestal: a unique system to study signal transduction and the actin cytoskeleton

Enteropathogenic Escherichia coli (EPEC) subverts host signalling pathways and the cytoskeleton during infection, resulting in disease characterized by diarrhoea. Recent studies have revolutionized our understanding of the infection process by showing that this bacterium inserts its own receptor into the plasma membrane overlying the host actin cytoskeleton. The reorganized actin forms a pedestal-like structure with the bacterium at the tip. This review discusses the mechanism of infection and pedestal formation and how this system might be a powerful tool for studying actin dynamics at the plasma membrane.

Expression of the EspB protein of enteropathogenic Escherichia coli within HeLa cells affects stress fibers and cellular morphology

The EspB protein of enteropathogenic Escherichia coli (EPEC) is essential for the signaling events that lead to the accumulation of actin beneath intimately attached bacteria, a process that is known as the attaching and effacing effect. EspB is targeted to the host cell cytoplasm by a type III secretion apparatus. To determine the effect of intracellular EspB on the host cell cytoskeleton, we transfected HeLa cells with a plasmid containing the espB gene under the control of an inducible eukaryotic promoter. A HeLa cell clone that expressed espB mRNA and EspB protein after induction was selected for further study. The expression of EspB in these cells caused a dramatic change in cell morphology and a marked reduction in actin stress fibers. Cells expressing EspB were significantly impaired in their ability to support invasion by EPEC and Salmonella typhimurium. However, the expression of EspB within host cells could not compensate for the lack of EspB expression by an espB mutant strain of EPEC to restore attaching and effacing activity. These studies suggest that EspB is a cytoskeletal toxin that is translocated to the host cell cytoplasm, where it causes a redistribution of actin.

The EspB protein of enteropathogenic Escherichia coli is targeted to the cytoplasm of infected HeLa cells

The EspB protein of enteropathogenic Escherichia coli (EPEC) is exported via a type III secretion apparatus. EspB is critical for signaling the host cell and for the development of the attaching and effacing lesion characteristic of EPEC infection. We used cellular fractionation and confocal laser scanning microscopy to determine the cellular location of EspB during infection of HeLa cells. Both methods indicated that EspB is targeted to the cytoplasm of infected cells. Using mutants, we found that EspB targeting to the host cell cytoplasm requires the type III secretion apparatus and the secreted proteins EspA and EspD, but not intimin. These results provide insights into the function of the type III secretion apparatus of EPEC and the functions of the Esp proteins.

Increased levels of intracellular calcium are not required for the formation of attaching and effacing lesions by enteropathogenic and enterohemorrhagic Escherichia coli

Elevated concentrations of intracellular calcium ([Ca]i) have been implicated as an important signalling event during attaching and effacing (A/E) lesion formation by enteropathogenic Escherichia coli (EPEC). The highly localized nature of the cytoskeletal and cell surface alterations occurring during A/E lesion formation suggests that there should be equally localized EPEC-induced signalling events. To analyze further the calcium responses to infection of HEp-2 cells by EPEC, we employed calcium-imaging fluorescence microscopy, which allows both temporal and spatial measurements of [Ca]i in live cells. Using this imaging technique, not only were we unable to detect any significant elevation in [Ca]i at sites of A/E EPEC adhesion, but, with several different classical EPEC and enterohemorrhagic E. coli (EHEC) strains and three different infection procedures, each of which resulted in extensive A/E bacterial adhesion, we were unable to detect any significant alterations in [Ca]i in infected cells compared to uninfected cells. In addition, chelation of intracellular free calcium with bis-(aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid (BAPTA) did not, as previously reported, prevent A/E lesion formation. We conclude that increased [Ca]i are not required for A/E lesion formation by EPEC and EHEC.

Signal transduction pathways involved in enterohemorrhagic Escherichia coli-induced alterations in T84 epithelial permeability

Enterohemorrhagic Escherichia coli (EHEC) infection is associated with watery diarrhea and can lead to complications, including hemorrhagic colitis and the hemolytic-uremic syndrome. The mechanisms by which these organisms produce diarrheal disease remain to be elucidated. Changes in T84 epithelial cell electrophysiology were examined following EHEC infection. T84 cell monolayers infected with EHEC O157:H7 displayed a time-dependent decrease in transepithelial resistance. Increases in the transepithelial flux of both [3H]mannitol and 51Cr-EDTA accompanied the EHEC-induced decreases in T84 resistance. Altered barrier function induced by EHEC occurred at the level of the tight junction since immunofluorescent staining of the tight-junction-associated protein ZO-1 was disrupted when examined by confocal microscopy. Decreased resistance induced by EHEC involved a protein kinase C (PKC)-dependent pathway as the highly specific PKC inhibitor, CGP41251, abrogated the EHEC-induced drop in resistance. PKC activity was also increased in T84 cells infected with EHEC. Calmodulin and myosin light chain kinase played a role in EHEC-induced resistance changes as inhibition of these effector molecules partially reversed the effects of EHEC on barrier function. These studies demonstrate that intracellular signal transduction pathways activated following EHEC infection link the increases in T84 epithelial permeability induced by this pathogen.

Diarrheagenic Escherichia coli

Escherichia coli is the predominant nonpathogenic facultative flora of the human intestine. Some E. coli strains, however, have developed the ability to cause disease of the gastrointestinal, urinary, or central nervous system in even the most robust human hosts. Diarrheagenic strains of E. coli can be divided into at least six different categories with corresponding distinct pathogenic schemes. Taken together, these organisms probably represent the most common cause of pediatric diarrhea worldwide. Several distinct clinical syndromes accompany infection with diarrheagenic E. coli categories, including traveler's diarrhea (enterotoxigenic E. coli), hemorrhagic colitis and hemolytic-uremic syndrome (enterohemorrhagic E. coli), persistent diarrhea (enteroaggregative E. coli), and watery diarrhea of infants (entero-pathogenic E. coli). This review discusses the current level of understanding of the pathogenesis of the diarrheagenic E. coli strains and describes how their pathogenic schemes underlie the clinical manifestations, diagnostic approach, and epidemiologic investigation of these important pathogens.