Cytoskeletal rearrangements and the functional role of T-plastin during entry of Shigella flexneri into HeLa cells

Pathogenic diversification of the gut commensal Providencia alcalifaciens via acquisition of a second type III secretion system

Providencia alcalifaciens is a Gram-negative bacterium found in various water and land environments and organisms, including insects and mammals. Some P. alcalifaciens strains encode gene homologs of virulence factors found in pathogenic Enterobacterales members, such as Salmonella enterica serovar Typhimurium and Shigella flexneri. Whether these genes are pathogenic determinants in P. alcalifaciens is not known. In this study, we investigated P. alcalifaciens-host interactions at the cellular level, focusing on the role of two type III secretion systems (T3SS) belonging to the Inv-Mxi/Spa family. T3SS1b is widespread in Providencia spp. and encoded on the chromosome. A large plasmid that is present in a subset of P. alcalifaciens strains, primarily isolated from diarrheal patients, encodes for T3SS1a. We show that P. alcalifaciens 205/92 is internalized into eukaryotic cells, lyses its internalization vacuole, and proliferates in the cytosol. This triggers caspase-4-dependent inflammasome responses in gut epithelial cells. The requirement for the T3SS1a in entry, vacuole lysis, and cytosolic proliferation is host cell type-specific, playing a more prominent role in intestinal epithelial cells than in macrophages or insect cells. In a bovine ligated intestinal loop model, P. alcalifaciens colonizes the intestinal mucosa and induces mild epithelial damage with negligible fluid accumulation in a T3SS1a- and T3SS1b-independent manner. However, T3SS1b was required for the rapid killing of Drosophila melanogaster. We propose that the acquisition of two T3SS has allowed P. alcalifaciens to diversify its host range, from a highly virulent pathogen of insects to an opportunistic gastrointestinal pathogen of animals.

Pathogenic diversification of the gut commensal Providencia alcalifaciens via acquisition of a second type III secretion system

Providencia alcalifaciens is a Gram-negative bacterium found in a wide variety of water and land environments and organisms. It has been isolated as part of the gut microbiome of animals and insects, as well as from stool samples of patients with diarrhea. Specific P. alcalifaciens strains encode gene homologs of virulence factors found in other pathogenic members of the same Enterobacterales order, such as Salmonella enterica serovar Typhimurium and Shigella flexneri. Whether these genes are also pathogenic determinants in P. alcalifaciens is not known. Here we have used P. alcalifaciens 205/92, a clinical isolate, with in vitro and in vivo infection models to investigate P. alcalifaciens -host interactions at the cellular level. Our particular focus was the role of two type III secretion systems (T3SS) belonging to the Inv-Mxi/Spa family. T3SS 1b is widespread in Providencia spp. and encoded on the chromosome. T3SS 1a is encoded on a large plasmid that is present in a subset of P. alcalifaciens strains, which are primarily isolates from diarrheal patients. Using a combination of electron and fluorescence microscopy and gentamicin protection assays we show that P. alcalifaciens 205/92 is internalized into eukaryotic cells, rapidly lyses its internalization vacuole and proliferates in the cytosol. This triggers caspase-4 dependent inflammasome responses in gut epithelial cells. The requirement for the T3SS 1a in entry, vacuole lysis and cytosolic proliferation is host-cell type specific, playing a more prominent role in human intestinal epithelial cells as compared to macrophages. In a bovine ligated intestinal loop model, P. alcalifaciens colonizes the intestinal mucosa, inducing mild epithelial damage with negligible fluid accumulation. No overt role for T3SS 1a or T3SS 1b was seen in the calf infection model. However, T3SS 1b was required for the rapid killing of Drosophila melanogaster . We propose that the acquisition of two T3SS by horizontal gene transfer has allowed P. alcalifaciens to diversify its host range, from a highly virulent pathogen of insects to an opportunistic gastrointestinal pathogen of animals.

Pathophysiology of Enteropathogenic Escherichia coli-induced Diarrhea

Enteropathogenic Escherichia coli (EPEC) are important diarrheal pathogens of infants and young children. Since the availability of molecular diagnosis methods, we now have new insights into the incidence and prevalence of these infections. Recent epidemiological studies indicate that atypical EPEC (aEPEC) are seen more frequently than typical EPEC (tEPEC) worldwide, including in both endemic diarrhea and diarrhea outbreaks. Therefore, it is important to further characterize the pathogenicity of these emerging strains. The virulence mechanisms and pathophysiology of the attaching and effacing lesion (A/E) and the type-three-secretion-system (T3SS) are complex but well-studied. A/E strains use their pool of locus of enterocyte effacement (LEE)-encoded and non-LEE-encoded effector proteins to subvert and modulate cellular and barrier properties of the host. However, the exact mechanisms of diarrhea in EPEC infection are not completely understood. From the clinical perspective, there is a need for fast, easy, and inexpensive diagnostic methods to define optimal treatment and prevention for children in endemic areas. In this article, we present a review of the classification of EPEC, epidemiology, pathogenesis of the disease caused by these bacteria, determinants of virulence, alterations in signaling, determinants of colonization vs. those of disease, and the limited information we have on the pathophysiology of EPEC-induced diarrhea. This article combines peer-reviewed evidence from our own studies and the results of an extensive literature search in the databases PubMed, EMBASE, and Scopus.

Plastin 3 in health and disease: a matter of balance

For a long time, PLS3 (plastin 3, also known as T-plastin or fimbrin) has been considered a rather inconspicuous protein, involved in F-actin-binding and -bundling. However, in recent years, a plethora of discoveries have turned PLS3 into a highly interesting protein involved in many cellular processes, signaling pathways, and diseases. PLS3 is localized on the X-chromosome, but shows sex-specific, inter-individual and tissue-specific expression variability pointing towards skewed X-inactivation. PLS3 is expressed in all solid tissues but usually not in hematopoietic cells. When escaping X-inactivation, PLS3 triggers a plethora of different types of cancers. Elevated PLS3 levels are considered a prognostic biomarker for cancer and refractory response to therapies. When it is knocked out or mutated in humans and mice, it causes osteoporosis with bone fractures; it is the only protein involved in actin dynamics responsible for osteoporosis. Instead, when PLS3 is upregulated, it acts as a highly protective SMN-independent modifier in spinal muscular atrophy (SMA). Here, it seems to counteract reduced F-actin levels by restoring impaired endocytosis and disturbed calcium homeostasis caused by reduced SMN levels. In contrast, an upregulation of PLS3 on wild-type level might cause osteoarthritis. This emphasizes that the amount of PLS3 in our cells must be precisely balanced; both too much and too little can be detrimental. Actin-dynamics, regulated by PLS3 among others, are crucial in a lot of cellular processes including endocytosis, cell migration, axonal growth, neurotransmission, translation, and others. Also, PLS3 levels influence the infection with different bacteria, mycosis, and other pathogens.

"Omics" in Human Colostrum and Mature Milk: Looking to Old Data with New Eyes

Human Milk (HM) is the best source for newborn nutrition until at least six months; it exerts anti-inflammatory and anti-infective functions, promotes immune system formation and supports organ development. Breastfeeding could also protect from obesity, diabetes and cardiovascular disease. Furthermore, human colostrum (HC) presents a peculiar role in newborn support as a protective effect against allergic and chronic diseases, in addition to long-term metabolic benefits. In this review, we discuss the recent literature regarding "omics" technologies and growth factors (GF) in HC and the effects of pasteurization on its composition. Our aim was to provide new evidence in terms of transcriptomics, proteomics, metabolomics, and microbiomics, also in relation to maternal metabolic diseases and/or fetal anomalies and to underline the functions of GF. Since HC results are so precious, particularly for the vulnerable pre-terms category, we also discuss the importance of HM pasteurization to ensure donated HC even to neonates whose mothers are unable to provide. To the best of our knowledge, this is the first review analyzing in detail the molecular pattern, microbiota, bioactive factors, and dynamic profile of HC, finding clinical correlations of such mediators with their possible in vivo effects and with the consequent impact on neonatal outcomes.

Comparative proteomic analysis of milk-derived exosomes in human and bovine colostrum and mature milk samples by iTRAQ-coupled LC-MS/MS

Exosomes are membranous vesicles found in biological fluids with important functions. However, milk-derived exosome proteins from humans and bovines have not been studied in detail. The advanced iTRAQ proteomic approach was used to analyze milk-derived exosomes in human and bovine colostrum and mature milk samples. A total of 920 milk exosome proteins were identified and quantified. Among these, 575 differentially expressed exosome proteins (P<0.05) were found. Multivariate analysis, gene ontology (GO) annotation and the KEGG pathway were used to interpret the identified proteins. The major biological processes involved were: response to stimulus (22%), localization (16%), establishment of localization (14%), and cellular component organization (14%). Cellular components engaged in intracellular (31%) and intracellular part (31%). The most prevalent molecular function mainly touched upon binding (52%). Milk exosome proteins participated in several KEGG pathways containing ribosome, regulation of actin cytoskeleton, glycolysis/gluconeogenesis, leukocyte transendothelial migration, aminoacyl-tRNA biosynthesis, pentose phosphate pathway, galactose metabolism and fatty acid biosynthesis. These results provide important information on human and bovine milk exosomes, and increase knowledge on the proteomes of these exosomes across different lactation stages, which could provide potential directions for newborn milk powder, biological markers and functional foods.

Contribution of Crk adaptor proteins to host cell and bacteria interactions

The Crk adaptor family of proteins comprises the alternatively spliced CrkI and CrkII isoforms, as well as the paralog Crk-like (CrkL) protein, which is encoded by a different gene. Initially thought to be involved in signaling during apoptosis and cell adhesion, this ubiquitously expressed family of proteins is now known to play essential roles in integrating signals from a wide range of stimuli. In this review, we describe the structure and function of the different Crk proteins. We then focus on the emerging roles of Crk adaptors during Enterobacteriaceae pathogenesis, with special emphasis on the important human pathogens Salmonella, Shigella, Yersinia, and enteropathogenic Escherichia coli. Throughout, we remark on opportunities for future research into this intriguing family of proteins.

Molecular mechanisms of host cytoskeletal rearrangements by Shigella invasins

Pathogen-induced reorganization of the host cell cytoskeleton is a common strategy utilized in host cell invasion by many facultative intracellular bacteria, such as Shigella, Listeria, enteroinvasive E. coli and Salmonella. Shigella is an enteroinvasive intracellular pathogen that preferentially infects human epithelial cells and causes bacillary dysentery. Invasion of Shigella into intestinal epithelial cells requires extensive remodeling of the actin cytoskeleton with the aid of pathogenic effector proteins injected into the host cell by the activity of the type III secretion system. These so-called Shigella invasins, including IpaA, IpaC, IpgB1, IpgB2 and IpgD, modulate the actin-regulatory system in a concerted manner to guarantee efficient entry of the bacteria into host cells.

Intracellular Vibrio parahaemolyticus escapes the vacuole and establishes a replicative niche in the cytosol of epithelial cells

Vibrio parahaemolyticus is a globally disseminated Gram-negative marine bacterium and the leading cause of seafood-borne acute gastroenteritis. Pathogenic bacterial isolates encode two type III secretion systems (T3SS), with the second system (T3SS2) considered the main virulence factor in mammalian hosts. For many decades, V. parahaemolyticus has been studied as an exclusively extracellular bacterium. However, the recent characterization of the T3SS2 effector protein VopC has suggested that this pathogen has the ability to invade, survive, and replicate within epithelial cells. Herein, we characterize this intracellular lifestyle in detail. We show that following internalization, V. parahaemolyticus is contained in vacuoles that develop into early endosomes, which subsequently mature into late endosomes. V. parahaemolyticus then escapes into the cytoplasm prior to vacuolar fusion with lysosomes. Vacuolar acidification is an important trigger for this escape. The cytoplasm serves as the pathogen's primary intracellular replicative niche; cytosolic replication is rapid and robust, with cells often containing over 150 bacteria by the time of cell lysis. These results show how V. parahaemolyticus successfully establishes an intracellular lifestyle that could contribute to its survival and dissemination during infection.

IMPORTANCE

The marine bacterium V. parahaemolyticus is the leading cause worldwide of seafood-borne acute gastroenteritis. For decades, the pathogen has been studied exclusively as an extracellular bacterium. However, recent results have revealed the pathogen's ability to invade and replicate within host cells. The present study is the first characterization of the V. parahaemolyticus' intracellular lifestyle. Upon internalization, V. parahaemolyticus is contained in a vacuole that would in the normal course of events ultimately fuse with a lysosome, degrading the vacuole's contents. The bacterium subverts this pathway, escaping into the cytoplasm prior to lysosomal fusion. Once in the cytoplasm, it replicates prolifically. Our study provides new insights into the strategies used by this globally disseminated pathogen to survive and proliferate within its host.

Pathogens hijack the epigenome: a new twist on host-pathogen interactions

Pathogens have evolved strategies to promote their survival by dramatically modifying the transcriptional profile and protein content of the host cells they infect. Modifications of the host transcriptome and proteome are mediated by pathogen-encoded effector molecules that modulate host cells through a variety of different mechanisms. Recent studies highlight the importance of the host chromatin and other epigenetic regulators as targets of pathogens. Host gene regulatory mechanisms may be targeted through cytoplasmic signaling, directly by pathogen effector proteins, and possibly by pathogen RNA. Although many of these changes are short-lived and persist only during the course of infection, several studies indicate that pathogens are able to induce long-term, heritable changes that are essential to pathogenesis of infectious diseases and persistence of pathogens within their hosts. In this review, we discuss how pathogens modulate the epigenome of host cells, a new and flourishing avenue of host-pathogen interaction studies.

Serological Identification of Endothelial Antigens Predominantly Recognized in Kawasaki Disease Patients by Recombinant Expression Cloning

We showed IgG immune response against endothelial antigens in sera obtained from convalescent Kawasaki disease (KD) patients after recovery from the disease during the follow-up period using serological analysis of recombinant cDNA expression library (SEREX) methodology. We identified 46 antigens represented by 69 clones by immunoscreening of a cDNA expression library from tumor necrosis factor-alpha (TNFalpha) treated human umbilical vein endothelial cells (HUVEC) with sera from 4 KD patients. They included ubiquitin pathway proteins, transcriptional factors, signal transduction molecules, metabolic enzymes, cytoskeletal proteins, an adhesion molecule, and a cell cycle protein. By serological survey using phage plaque assay, sera from 5 non-KD patients were rarely reactive with the antigens. Among the antigens, tropomyosin was most frequently isolated (18 of 69 clones). Seventeen of the 18 clones were identified using KD3 serum. The second most frequently isolated antigen was also a cytoskeletal protein, called Tplastin (3 of 69 clones).

Inducible expression and pathophysiologic functions of T-plastin in cutaneous T-cell lymphoma

A molecular feature of Sézary syndrome (SS) is the abnormal expression of T-plastin by malignant T cells. Herein, we investigated the molecular mechanisms involved in T-plastin synthesis and the functions of this actin-binding protein, with a special interest in chemoresistance and migration. We confirm the specific expression of T-plastin in peripheral blood lymphocytes (PBLs) from SS patients and its total absence in PBLs from patients with mycosis fungoides, inflammatory cutaneous or hematologic diseases, and from healthy volunteers. Only 3 of 4 SS patients did constitutively express T-plastin. To assess whether T-plastin expression was inducible, T-plastin-negative PBLs were stimulated by phorbol 12-myristate 13-acetate and ionomycin. Our results demonstrate that T-plastin synthesis was induced in negative PBLs from SS patients, other studied patients, and healthy volunteers. Both constitutive and calcium-induced T-plastin expression was down-regulated by calcineurin inhibitors and involved nuclear factor of activated T cells transcription pathway. Constitutive T-plastin expression in SS was associated with resistance to etoposide-induced apoptosis and cell migration toward chemokines (TARC/CCL17, IP-10). In conclusion, T-plastin is a marker restricted to malignant lymphocytes from SS patients and plays a role for cell survival and migration. This opens new strategies for the treatment of SS advanced stages.

Functional domains and motifs of bacterial type III effector proteins and their roles in infection

A key feature of the virulence of many bacterial pathogens is the ability to deliver effector proteins into eukaryotic cells via a dedicated type three secretion system (T3SS). Many bacterial pathogens, including species of Chlamydia, Xanthomonas, Pseudomonas, Ralstonia, Shigella, Salmonella, Escherichia and Yersinia, depend on the T3SS to cause disease. T3SS effectors constitute a large and diverse group of virulence proteins that mimic eukaryotic proteins in structure and function. A salient feature of bacterial effectors is their modular architecture, comprising domains or motifs that confer an array of subversive functions within the eukaryotic cell. These domains/motifs therefore represent a fascinating repertoire of molecular determinants with important roles during infection. This review provides a snapshot of our current understanding of bacterial effector domains and motifs where a defined role in infection has been demonstrated.

Domains of the Shigella flexneri type III secretion system IpaB protein involved in secretion regulation

Type III secretion systems (T3SSs) are key determinants of virulence in many Gram-negative bacterial pathogens. Upon cell contact, they inject effector proteins directly into eukaryotic cells through a needle protruding from the bacterial surface. Host cell sensing occurs through a distal needle "tip complex," but how this occurs is not understood. The tip complex of quiescent needles is composed of IpaD, which is topped by IpaB. Physical contact with host cells initiates secretion and leads to assembly of a pore, formed by IpaB and IpaC, in the host cell membrane, through which other virulence effector proteins may be translocated. IpaB is required for regulation of secretion and may be the host cell sensor. It binds needles via its extreme C-terminal coiled coil, thereby likely positioning a large domain containing its hydrophobic regions at the distal tips of needles. In this study, we used short deletion mutants within this domain to search for regions of IpaB involved in secretion regulation. This identified two regions, amino acids 227 to 236 and 297 to 306, the presence of which are required for maintenance of IpaB at the needle tip, secretion regulation, and normal pore formation but not invasion. We therefore propose that removal of either of these regions leads to an inability to block secretion prior to reception of the activation signal and/or a defect in host cell sensing.

Characterization of L-plastin interaction with beta integrin and its regulation by micro-calpain

Recent evidences suggest that plastin/fimbrin is more than a simple actin cross-linking molecule. In this context and based on the fact that other members of the same family interact with transmembrane proteins, such as integrins, we have investigated a possible interaction between L-plastin and integrins. By combining coimmunoprecipitation of endogenous proteins and in vitro techniques based on solid phase and solution assays, we demonstrate that L-plastin is an additional binding partner for the beta-chain of integrin and confirmed that both proteins display some colocalization. We then show that L-plastin binds to the cytoplasmic domain of beta1 integrin and to beta1 and beta2 peptides. Using recombinant L-plastin domains, we demonstrate that the integrin-binding sites are not located in NH(2) terminal part of L-plastin but rather in the two actin-binding domains. Using pull-down, cross-linking experiments, and enzyme-linked immunosorbent assay, we show that the L-plastin/integrin complex is regulated by mu-calpain cleavage and is not directly dissociated by calcium. Indeed, despite the ability of calpain to cleave both proteins, only the cleavage of beta integrin hindered the formation of the L-plastin/integrin complex. We discuss these results in the light of the three-dimensional structure of the actin-binding domains of L-plastin.

The extreme C terminus of Shigella flexneri IpaB is required for regulation of type III secretion, needle tip composition, and binding

Type III secretion systems (T3SSs) are widely distributed virulence determinants of Gram-negative bacteria. They translocate bacterial proteins into host cells to manipulate them during infection. The Shigella T3SS consists of a cytoplasmic bulb, a transmembrane region, and a hollow needle protruding from the bacterial surface. The distal tip of mature, quiescent needles is composed of IpaD, which is topped by IpaB. Physical contact with host cells initiates secretion and leads to assembly of a pore, formed by IpaB and IpaC, in the host cell membrane, through which other virulence effector proteins may be translocated. IpaB is required for regulation of secretion and may be the host cell sensor. However, its mode of needle association is unknown. Here, we show that deletion of 3 or 9 residues at the C terminus of IpaB leads to fast constitutive secretion of late effectors, as observed in a DeltaipaB strain. Like the DeltaipaB mutant, mutants with C-terminal mutations also display hyperadhesion. However, unlike the DeltaipaB mutant, they are still invasive and able to lyse the internalization vacuole with nearly wild-type efficiency. Finally, the mutant proteins show decreased association with needles and increased recruitment of IpaC. Taken together, these data support the notion that the state of the tip complex regulates secretion. We propose a model where the quiescent needle tip has an "off" conformation that turns "on" upon host cell contact. Our mutants may adopt a partially "on" conformation that activates secretion and is capable of recruiting some IpaC to insert pores into host cell membranes and allow invasion.

Small-molecule type III secretion system inhibitors block assembly of the Shigella type III secreton

Type III secretion systems (T3SSs) are essential virulence devices for many gram-negative bacteria that are pathogenic for plants, animals, and humans. They serve to translocate virulence effector proteins directly into eukaryotic host cells. T3SSs are composed of a large cytoplasmic bulb and a transmembrane region into which a needle is embedded, protruding above the bacterial surface. The emerging antibiotic resistance of bacterial pathogens urges the development of novel strategies to fight bacterial infections. Therapeutics that rather than kill bacteria only attenuate their virulence may reduce the frequency or progress of resistance emergence. Recently, a group of salicylidene acylhydrazides were identified as inhibitors of T3SSs in Yersinia, Chlamydia, and Salmonella species. Here we show that these are also effective on the T3SS of Shigella flexneri, where they block all related forms of protein secretion so far known, as well as the epithelial cell invasion and induction of macrophage apoptosis usually demonstrated by this bacterium. Furthermore, we show the first evidence for the detrimental effect of these compounds on T3SS needle assembly, as demonstrated by increased numbers of T3S apparatuses without needles or with shorter needles. Therefore, the compounds generate a phenocopy of T3SS export apparatus mutants but with incomplete penetrance. We discuss why this would be sufficient to almost completely block the later secretion of effector proteins and how this begins to narrow the search for the molecular target of these compounds.

The C-terminus of IpaC is required for effector activities related to Shigella invasion of host cells

Invasion plasmid antigen C (IpaC) is secreted by the Shigella flexneri type III secretion system (TTSS) as an essential trigger of epithelial cell invasion. At the molecular level, IpaC possesses a distinct functional organization. The IpaC C-terminal region between amino acids 319 and 345 is predicted to form a coiled-coil structure. Such alpha-helical motifs appear to be a recurring structural theme among TTSS components. Together with IpaB, this IpaC region is also required for the formation of translocon pores in target cell membranes. In contrast, mutations within the C-terminal tail of IpaC (defined by residues 345-363) have no effect on contact hemolysis (a putative measure of translocon pore formation), but they can contribute significantly to IpaC's ability to trigger S. flexneri entry into cultured cells. Here we describe the molecular dissection of the IpaC C-terminus and how changes in this region affect selected virulence-related activities. IpaC invasion function requires its immediate C-terminus and this general region may be involved in its ability to trigger actin nucleation. In contrast, IpaC could not be shown to interact directly with Cdc42, a host GTPase closely tied to Shigella invasion.

The type III secretion system needle tip complex mediates host cell sensing and translocon insertion

Type III secretion systems (T3SSs) are essential virulence determinants of many Gram-negative bacterial pathogens. The Shigella T3SS consists of a cytoplasmic bulb, a transmembrane region and a hollow 'needle' protruding from the bacterial surface. Physical contact with host cells initiates secretion and leads to assembly of a pore, formed by IpaB and IpaC, in the host cell membrane, through which proteins that facilitate host cell invasion are translocated. As the needle is implicated in host cell sensing and secretion regulation, its tip should contain components that initiate host cell contact. Through biochemical and immunological studies of wild-type and mutant Shigella T3SS needles, we reveal tip complexes of differing compositions and functional states, which appear to represent the molecular events surrounding host cell sensing and pore formation. Our studies indicate that the interaction between IpaB and IpaD at needle tips is key to host cell sensing, orchestration of IpaC secretion and its subsequent assembly at needle tips. This allows insertion into the host cell membrane of a translocation pore that is continuous with the needle.

Host responses to secreted Shigella virulence factors

Shigella and related enteropathogens deliver effector molecules into the cytoplasm of epithelial cells and macrophages via a type III secretion system. Epithelial cells respond to contact with Shigella by rearranging the cytoskeleton, which leads to uptake of the bacterium. Apart from several cytoskeletal proteins, this process involves the recruitment and activation of kinases, and the small GTPase rho. Macrophages infected with Shigella undergo apoptosis and release mature IL-1beta, a pro-inflammatory cytokine. This apoptotic pathway requires caspase-1 (IL-1beta-converting enzyme). Pro-inflammatory macrophage apoptosis triggers acute shigellosis and might be relevant in other infectious diseases.

Host cell invasion mediated by Trypanosoma cruzi surface molecule gp82 is associated with F-actin disassembly and is inhibited by enteroinvasive Escherichia coli

The target cell F-actin disassembly, induced by a Ca2+-signaling Trypanosoma cruzi factor of unknown molecular identity, has been reported to promote parasite invasion. We investigated whether the metacyclic trypomastigote stage-specific surface molecule gp82, a Ca2+-signal-inducing molecule implicated in host cell invasion, displayed the ability to induce actin cytoskeleton disruption, using a recombinant protein (J18) containing the full-length gp82 sequence fused to GST. J18, but not GST, induced F-actin disassembly in HeLa cells, significantly reducing the number as well as the length of stress fibers. The number of cells with typical stress fibers scored approximately 70% in untreated and GST-treated cells, as opposed to approximately 30% in J18-treated samples, which also showed decreased F-actin content. J18, but not GST, inhibited approximately 6-fold the HeLa cell entry of enteroinvasive Escherichia coli (EIEC), which depends on actin cytoskeleton. Not only were fewer cells infected with bacteria in the presence of J18, there were also fewer bacteria per cell. The inhibitory activity of J18 was Ca2+ dependent. In co-infection experiments, preincubation of HeLa cells with EIEC drastically reduced gp82-dependent internalization of T. cruzi metacyclic forms. All these data, plus the finding that gp82-mediated penetration of metacyclic forms was associated with disrupted HeLa cell cytoskeletal architecture, indicate that gp82 promotes parasite invasion by disassembling the cortical actin cytoskeleton.

PtdIns5P activates the host cell PI3-kinase/Akt pathway during Shigella flexneri infection

The virulence factor IpgD, delivered into nonphagocytic cells by the type III secretion system of the pathogen Shigella flexneri, is a phosphoinositide 4-phosphatase generating phosphatidylinositol 5 monophosphate (PtdIns5P). We show that PtdIns5P is rapidly produced and concentrated at the entry foci of the bacteria, where it colocalises with phosphorylated Akt during the first steps of infection. Moreover, S. flexneri-induced phosphorylation of host cell Akt and its targets specifically requires IpgD. Ectopic expression of IpgD in various cell types, but not of its inactive mutant, or addition of short-chain penetrating PtdIns5P is sufficient to induce Akt phosphorylation. Conversely, sequestration of PtdIns5P or reduction of its level strongly decreases Akt phosphorylation in infected cells or in IpgD-expressing cells. Accordingly, IpgD and PtdIns5P production specifically activates a class IA PI 3-kinase via a mechanism involving tyrosine phosphorylations. Thus, S. flexneri parasitism is shedding light onto a new mechanism of PI 3-kinase/Akt activation via PtdIns5P production that plays an important role in host cell responses such as survival.

Shigella flexneri infection is dependent on villin in the mouse intestine and in primary cultures of intestinal epithelial cells

Villin is an actin-binding protein present in intestinal and kidney brush borders. Villin has been shown to present in vitro Ca(2+)-dependent bundling and severing F-actin properties. The study of villin knock-out mice allowed us to show that while bundling of F-actin microfilaments is unaffected, this protein is important for the reorganization of the actin cytoskeleton elicited by various signals during both physiological and pathological conditions. Here, we studied the role of villin during infection by Shigella flexneri, the causative agent of bacillary dysentery. This bacterium induces the reorganization of the host actin cytoskeleton to penetrate into epithelial cells and spread from cell to cell. In vivo, we show that unlike newborn vil+/+ mice, which are sensitive to Shigella invasion, resulting in a destructive inflammatory response of the intestinal mucosa following intragastric inoculation, newborn vil-/- mice appear fully resistant to infection. Using primary cultures of intestinal epithelial cells derived from vil+/+ or vil -/- mice, we demonstrate that villin plays an essential role in S. flexneri entry and cell-to-cell dissemination. Villin expression is thus critical for Shigella infection through its ability to remodel the actin cytoskeleton.

Francisella tularensis enters macrophages via a novel process involving pseudopod loops

Intracellular bacterial pathogens employ a variety of strategies to invade their eukaryotic host cells. From an ultrastructural standpoint, the processes that bacteria employ to invade their host cells include conventional phagocytosis, coiling phagocytosis, and ruffling/triggered macropinocytosis. In this paper, we describe a novel process by which Francisella tularensis, the agent of tularemia, enters host macrophages. F. tularensis is a remarkably infectious facultative intracellular bacterial parasite--as few as 10 bacteria can cause life-threatening disease in humans. However, the ultrastructure of its uptake and the receptor mechanisms that mediate its uptake have not been reported previously. We have used fluorescence microscopy and electron microscopy to examine the adherence and uptake of a virulent recent clinical isolate of F. tularensis, subspecies tularensis, and the live vaccine strain (LVS), subspecies holarctica, by human macrophages. We show here that both strains of F. tularensis enter human macrophages by a novel process of engulfment within asymmetric, spacious pseudopod loops, a process that differs ultrastructurally from all previously described uptake mechanisms. We demonstrate also that adherence and uptake of F. tularensis by macrophages is strongly dependent upon complement receptors and upon serum with intact complement factor C3 and that uptake requires actin microfilaments. These findings have significant implications for understanding the intracellular biology and virulence of this extremely infectious pathogen.

Ataxin-2 and huntingtin interact with endophilin-A complexes to function in plastin-associated pathways

Spinocerebellar ataxia type 2 is an inherited neurodegenerative disorder that is caused by an expanded trinucleotide repeat in the SCA2 gene, encoding a polyglutamine stretch in the gene product ataxin-2. Although evidence has been provided that ataxin-2 is involved in RNA metabolism, the physiological function of ataxin-2 remains unclear. Here, we demonstrate that ataxin-2 interacts with two members of the endophilin family, endophilin-A1 and endophilin-A3. To elucidate the physiological implications of these interactions, we exploited yeast as a model system and discovered that expression of ataxin-2 as well as both endophilin proteins is toxic for yeast lacking the SAC6 gene product fimbrin, a protein involved in actin filament organization and endocytotic processes. Intriguingly, expression of huntingtin, another polyglutamine protein interacting with endophilin-A3, was also toxic in Deltasac6 yeast. These effects can be suppressed by simultaneous expression of one of the two human fimbrin orthologs, L- or T-plastin. Moreover, we have discovered that ataxin-2 associates with L- and T-plastin and that overexpression of ataxin-2 leads to accumulation of T-plastin in mammalian cells. Thus, our findings suggest an interplay between ataxin-2, endophilin proteins and huntingtin in plastin-associated cellular pathways.

Plastins: versatile modulators of actin organization in (patho)physiological cellular processes

Many actin-binding proteins are expressed in eukaryotic cells. These polypeptides assist in stabilizing and rearranging the organization of the actin cytoskeleton in response to external stimuli, or during cell migration and adhesion. Here we review a particular set of actin-binding proteins called plastins. Plastins (also called fimbrins) belong to a subclass of actin-binding proteins known as actin bundling proteins. Three isoforms have been characterized in mammals: T-plastin is expressed in cells from solid tissue, whereas L-plastin occurs predominantly in hematopoietic cells. The third isoform, I-plastin, is specifically expressed in the small intestine, colon and kidney. These proteins share the unique property of cross-linking actin filaments into tight bundles. Although plastins are primarily involved in regulation of the actin cytoskeleton, they possess some unique features. For instance, they are implicated in invasion by pathogenic bacteria such as Shigella flexneri and Salmonella typhimurium. Also, L-plastin plays an important role in leukocyte function. T-plastin, on the other hand, is possibly involved in DNA repair. Finally, both T- and L-plastin are implicated in several diseases, and L-plastin is considered to be a valuable marker for cancer.

Actin-filament cross-linking protein T-plastin increases Arp2/3-mediated actin-based movement

Increasing evidence suggests that actin cross-linking or bundling proteins might not only structure the cortical actin cytoskeleton but also control actin dynamics. Here, we analyse the effects of T-plastin/T-fimbrin, a representative member of an important actin-filament cross-linking protein by combining a quantitative biomimetic motility assay with biochemical and cell-based approaches. Beads coated with the VCA domain of the Wiskott/Aldrich-syndrome protein (WASP) recruit the actin-nucleating Arp2/3 complex, polymerize actin at their surface and undergo movement when placed in cell-free extracts. T-Plastin increased the velocity of VCA beads 1.5 times, stabilized actin comets and concomitantly displaced cofilin, an actin-depolymerizing protein. T-Plastin also decreased the F-actin disassembly rate and inhibited cofilin-mediated depolymerization of actin filaments in vitro. Importantly, a bundling-incompetent variant comprising the first actin-binding domain (ABD1) had similar effects. In cells, this domain induced the formation of long actin cables to which other actin-regulating proteins were recruited. Altogether, these results favor a mechanism in which binding of ABD1 controls actin turnover independently of cross-link formation. In vivo, this activity might contribute to the assembly and maintenance of the actin cytoskeleton of plasma-membrane protrusions.

Shigella infections in children: new insights

Shigellosis , the acute enteric infection caused by bacteria of the genus Shigella , has a worldwide distribution with an estimated annual incidence of 164.7 million cases, of which 163.2 million occur in developing countries, and 1.1 million deaths. Sixty-nine percent of all episodes and 61 percent of all Shigella -related deaths involve children younger than 5 years old. In the United States, 10,000 to 15,000 cases of shigellosis are reported each year. Although usually confined to the colonic mucosa, shigellosis sometimes can cause extraintestinal complications. Recent publications have shed light on the clinical characteristics of Shigella -induced bacteremia, surgical complications, urogenital symptoms, and neurologic manifestations, and on the unique manifestations in the neonatal period. The mainstay of treatment of shigellosis in children is correction of the fluid and electrolyte loss, which often is achieved by the administration of oral rehydration solutions. Appropriate antibiotic therapy shortens the duration of both clinical symptoms and fecal excretion of the pathogen. However, the increasing antimicrobial resistance of shigellae worldwide constitutes a major problem. Regarding the pathophysiology of shigellosis and its complications, recent data not only elucidated the molecular mechanisms involved but also linked manifestations of disease to the interplay of bacterial virulence factors and host responses. The improved understanding of the pathophysiology is hoped to lead to innovative therapeutic approaches against shigellosis and new generations of vaccine candidates.

Invasive ability of Escherichia coli O18 isolated from swine neonatal diarrhea

Neonatal diarrhea occurred at two swine breeding farms in Hokkaido. Ten piglets aged 2 to 4 days were examined. Grossly, significant changes were confined to the small intestine. The mucous membrane was muddy and thickened. The intraluminal contents from the jejunum to the colon were liquid and yellow. In the small intestine, numerous Gram-negative bacilli preferentially adhered to the apex of villi. The mucosa was erosive with villous atrophy. There were bacilli also in the lamina propria and in the cytoplasm of degenerated enterocytes. Nonhemolytic Escherichia coli strains, belonging to serogroup E. coli O18 and possessing K88 fimbriae, were isolated from the small intestine. They could not be classified into any of the diarrheagenic E. coli groups because of the absence of genes of LT, STh, STp, VT1, VT2, eae, invE, and ipaH. After inoculation of the isolates on HEp-2 cells, some bacilli were engulfed by cytoplasmic projections resembling membrane ruffles and subsequently were localized in cytoplasmic vacuoles or free in the cytoplasm. These findings support the view that the present E. coli O18 is a new invasive strain enteropathogenic to piglets.

Rafts can trigger contact-mediated secretion of bacterial effectors via a lipid-based mechanism

Infection by the Gram-negative bacterial pathogen Shigella flexneri depends on its ability to invade host cells. Bacterial engulfment requires a functional type III secretion system (TTSS) allowing the translocation into host cells of bacterial effectors that activate cell-signaling cascades. We demonstrated previously that specialized lipid membrane domains enriched in cholesterol and sphingolipids (rafts) are involved during early steps of invasion, namely in binding and host cell entry. In this study, we addressed the issue of contact-mediated secretion by the TTSS. We show that contact-mediated and TTSS-induced hemolysis depend on the presence of cholesterol on the host cell surface. We found that purified detergent resistant membranes were able to activate TTSS. Finally, we found that artificial liposomes, devoid of proteins, were able to activate the TTSS but only when their composition mimicked that of lipid rafts. Altogether, these data indicate that specific lipid packing can trigger contact-mediated secretion by S. flexneri.

Structure of the Actin Crosslinking Core of Fimbrin

Filamentous actin is organized into bundles and orthogonal networks by the fimbrin/alpha-actinin superfamily of F-actin crosslinking proteins. The crystal structure of the Arabidopsis thaliana and Schizosaccharomyces pombe fimbrin cores provides the first description of a functional F-actin crosslinking protein and highlights the compact and distinctly asymmetric organization of the fimbrin molecule, in which the two actin binding domains present distinct surfaces to solvent. The mapping of functionally important residues onto the structure affords new insights into the binding process and provides additional constraints which must be accommodated by models for F-actin binding and crosslinking. Most strikingly, this work provides unique insight into the mechanistic features of conditional-lethal mutants and their extragenic suppressors, which highlight conformational and dynamic properties required for fimbrin function. These results underscore the power of jointly considering structural and genetic suppressor data for obtaining unexpected and biologically relevant mechanistic information.

Abl tyrosine kinases are required for infection by Shigella flexneri

Infection by the opportunistic bacterial pathogen Shigella flexneri stimulates tyrosine phosphorylation of host cell proteins, but the kinases involved and their effects on the regulation of cell signaling pathways during bacterial entry remain largely undefined. Here, we demonstrate a requirement for the Abl family of tyrosine kinases during Shigella internalization. Family members Abl and Arg are catalytically activated upon Shigella infection, accumulate at the site of bacterial entry, and are required for efficient bacterial uptake, as internalization is blocked upon targeted deletion of these kinases or treatment with a specific pharmacological inhibitor. We identify the adapter protein Crk as a target for Abl kinases during Shigella uptake, and show that a phosphorylation-deficient Crk mutant significantly inhibits bacterial uptake. Moreover, we define a novel signaling pathway activated during Shigella entry that links Abl kinase phosphorylation of Crk to activation of the Rho family GTPases Rac and Cdc42. Together, these findings reveal a new role for the Abl kinases, and suggest a novel approach to treatment of Shigella infections through inhibition of host cell signaling pathways.

Phosphoinositide signaling disorders in human diseases

Phosphoinositides (PIs) play an essential role in diverse cellular functions. Their intracellular level is strictly regulated by specific PI kinases, phosphatases and phospholipases. Recent discoveries indicate that dysfunctions in the control of their level often lead to pathologies. This review will focus on some human diseases whose etiologies involve PI-metabolizing enzymes. The role of PTEN (phosphatase and tensin homolog deleted on chromosome ten) in cancer, the impact of the Src homology 2-containing inositol-5-phosphatase phosphatases in acute myeloid leukemia or diabetes, the involvement of myotubularin family members in genetic diseases and the implication of OCRL1 in Lowe syndrome will be emphasized. We will also review how some bacterial pathogens have evolved strategies to specifically manipulate the host cell PI metabolism to efficiently infect them.

Src kinase has a central role in in vitro cellular internalization of Staphylococcus aureus

Traditionally recognized as an extracellular pathogen, the Gram-positive bacterium Staphylococcus aureus can also be internalized by a variety of cell types in vitro. Internalization is known to involve binding of the host extracellular protein fibronectin to the bacterium, recognition of the fibronectin-coated bacterium by the fibronectin-binding integrin alpha5beta1 on the host cell surface, and integrin-mediated internalization. Here we examine elements of mammalian cell signalling pathways involved in S. aureus internalization. The mouse fibroblast cell line GD25, in which the gene encoding the beta1 integrin subunit is inactivated, has been complemented with a beta1 integrin cDNA encoding a tyrosine (Y) to phenylalanine (F) mutation in each of the two beta1 integrin intracellular NPXY motifs. This cell line, GD25beta1 A Y783/795F, is defective in migration on fibronectin coated surfaces and intracellular signalling activities involving the tyrosine kinase Src. GD25beta1 A Y783/795F cells have a decreased ability to internalize S. aureus compared to GD25beta1 A cells expressing wild-type beta1 integrins. Furthermore, using mouse embryo fibroblasts in which different members of the Src family kinases are genetically inactivated, we demonstrate that optimal internalization is dependent on expression of Src kinase. Interferon, which has been implicated in repression of the effects of the viral homologue of Src inhibits internalization of S. aureus indicating that internalization may be blocked by inhibitors of Src kinase function. We then demonstrate that Src family kinase specific inhibitors effectively block S. aureus internalization into HeLa cells leading to the conclusion that a function unique to Src is required for optimal internalization of S. aureus in vitro.

Phagocytosis and intracellular fate of Aspergillus fumigatus conidia in alveolar macrophages

Aspergillus fumigatus is the most prevalent airborne fungal pathogen responsible for fatal invasive aspergillosis in immunocompromised patients. Upon arrival in the lung alveolus, conidia of A. fumigatus are phagocytosed by alveolar macrophages, the major phagocytic cells of the lung. Engulfment and intracellular trafficking of A. fumigatus conidia in alveolar macrophages of two different origins, the murine cell line MH-S and human pulmonary alveolar macrophages, were analyzed by electron microscopy and immunofluorescence. Phagocytosis of A. fumigatus conidia required actin polymerization and phosphatidylinositol 3-kinase activity. Fusion of A. fumigatus phagosomes with early and late endosomes was shown by immunolabeling with specific markers for the transferrin receptor, early endosome antigen, and Rab7. Maturation of A. fumigatus phagolysosomes was monitored by using a fixable acidotropic probe, LysoTracker Red DND-99, and an anti-cathepsin D antibody. Bafilomycin A-induced inhibition of lysosomal acidification abolished the conidial killing by the macrophages. These data suggest that the maturation of A. fumigatus phagosomes results from fusion with the compartments of the endocytic pathway and that the killing of conidia depends on phagolysosome acidification. A model for the phagocytosis of A. fumigatus conidia by alveolar macrophages is proposed on the basis of these results.

Conversion of PtdIns(4,5)P(2) into PtdIns(5)P by the S.flexneri effector IpgD reorganizes host cell morphology

Phosphoinositides play a central role in the control of several cellular events including actin cytoskeleton organization. Here we show that, upon infection of epithelial cells with the Gram-negative pathogen Shigella flexneri, the virulence factor IpgD is translocated directly into eukaryotic cells and acts as a potent inositol 4-phosphatase that specifically dephosphorylates phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P(2)] into phosphatidylinositol 5-monophosphate [PtdIns(5)P] that then accumulates. Transfection experiments indicate that the transformation of PtdIns(4,5)P(2) into PtdIns(5)P by IpgD is responsible for dramatic morphological changes of the host cell, leading to a decrease in membrane tether force associated with membrane blebbing and actin filament remodelling. These data provide the molecular basis for a new mechanism employed by a pathogenic bacterium to promote membrane ruffling at the entry site.

Initial steps of Shigella infection depend on the cholesterol/sphingolipid raft-mediated CD44-IpaB interaction

Shigellosis is an acute inflammatory bowel disease caused by the enteroinvasive bacterium SHIGELLA: Upon host cell-Shigella interaction, major host cell signalling responses are activated. Deciphering the initial molecular events is crucial to understanding the infectious process. We identified a molecular complex involving proteins of both the host, CD44 the hyaluronan receptor, and Shigella, the invasin IpaB, which partitions during infection within specialized membrane microdomains enriched in cholesterol and sphingolipids, called rafts. We also document accumulation of cholesterol and raft-associated proteins at Shigella entry foci. Moreover, we report that Shigella entry is impaired after cholesterol depletion using methyl-beta-cyclodextrin. Finally, we find that Shigella is less invasive in sphingosid-based lipid-deficient cell lines, demonstrating the involvement of sphingolipids. Our results show that rafts are implicated in Shigella binding and entry, suggesting that raft-associated molecular machineries are engaged in mediating the cell signalling response required for the invasion process.

Chlamydia trachomatis induces remodeling of the actin cytoskeleton during attachment and entry into HeLa cells

To elucidate the host cell machinery utilized by Chlamydia trachomatis to invade epithelial cells, we examined the role of the actin cytoskeleton in the internalization of chlamydial elementary bodies (EBs). Treatment of HeLa cells with cytochalasin D markedly inhibited the internalization of C. trachomatis serovar L2 and D EBs. Association of EBs with HeLa cells induced localized actin polymerization at the site of attachment, as visualized by either phalloidin staining of fixed cells or the active recruitment of GFP-actin in viable infected cells. The recruitment of actin to the specific site of attachment was accompanied by dramatic changes in the morphology of cell surface microvilli. Ultrastructural studies revealed a transient microvillar hypertrophy that was dependent upon C. trachomatis attachment, mediated by structural components on the EBs, and cytochalasin D sensitive. In addition, a mutant CHO cell line that does not support entry of C. trachomatis serovar L2 did not display such microvillar hypertrophy following exposure to L2 EBs, which is in contrast to infection with serovar D, to which it is susceptible. We propose that C. trachomatis entry is facilitated by an active actin remodeling process that is induced by the attachment of this pathogen, resulting in distinct microvillar reorganization throughout the cell surface and the formation of a pedestal-like structure at the immediate site of attachment and entry.

Shigella flexneri Interactions with the Basolateral Membrane Domain of Polarized Model Intestinal Epithelium: Role of Lipopolysaccharide in Cell Invasion and in Activation of the Mitogen-Activated Protein Kinase ERK

An early step governing Shigella flexneri pathogenesis is the invasion of the colonic epithelium from the basolateral surface followed by disruption of the colonic epithelial barrier. Despite recent insight into S. flexneri-host interactions, much remains to be determined regarding the nature of the initial contact between S. flexneri and the host epithelial basolateral membrane domain. Since the lipopolysaccharide (LPS) is located at the outermost part of the bacterial membrane, we considered that this component might be used by S. flexneri to attach to the basolateral surface of the intestinal epithelium and promote a proinflammatory response. Therefore, polarized human T84 intestinal epithelial cells were infected from the basolateral surface with either wild-type S. flexneri or one of its isogenic LPS-defective strains with mutations in either rfc, rfaL, or galU. We found that both adherence to and internalization into the basolateral surface of a polarized intestinal epithelium with S. flexneri were highly dependent on the length of the LPS (i.e., rfc > rfaL > galU). Furthermore, the addition of the anti-inflammatory LPS (RsDPLA) considerably decreased the invasion profile of wild-type S. flexneri by nearly 50%. Since LPS is associated with host inflammation, we further examined whether this molecule was involved in Shigella-induced inflammatory events. We found that S. flexneri LPS plays an important role in mediating epithelial-derived signaling, which leads to directed migration of polymorphonuclear leukocytes across model intestinal epithelium. This signaling most likely involves the activation of the mitogen-activated protein kinase extracellular regulated kinase. Thus, our findings have important implications on the understanding of the mechanisms by which S. flexneri can elicit mucosal inflammation.

Phagocytosis of bacterial pathogens: implications in the host response

Phagocytosis of bacterial pathogens is at the heart of the pathogenesis of infections. Pathogens have evolved a large array of strategies to escape the deleterious effect of phagocytosis by professional phagocytes among which avoiding phagocytosis, killing the phagocytes or surviving inside them are the most 'popular' solutions. Bacterial pathogens are also using induction of phagocytic entry into non-professional phagocytic cells, such as epithelial cells, as a strategy of survival and multiplication. We have taken enteroinvasive micro-organisms such as Yersinia, Shigella and Salmonella as a paradigm of the significance of phagocytosis/antiphagocytosis in the development of an infection and on the elicitation of the host response.

GTPases of the Rho subfamily are required for Brucella abortus internalization in nonprofessional phagocytes: direct activation of Cdc42

Members of the genus Brucella are intracellular alpha-Proteobacteria responsible for brucellosis, a chronic disease of humans and animals. Little is known about Brucella virulence mechanisms, but the abilities of these bacteria to invade and to survive within cells are decisive factors for causing disease. Transmission electron and fluorescence microscopy of infected nonprofessional phagocytic HeLa cells revealed minor membrane changes accompanied by discrete recruitment of F-actin at the site of Brucella abortus entry. Cell uptake of B. abortus was negatively affected to various degrees by actin, actin-myosin, and microtubule chemical inhibitors. Modulators of MAPKs and protein-tyrosine kinases hampered Brucella cell internalization. Inactivation of Rho small GTPases using clostridial toxins TcdB-10463, TcdB-1470, TcsL-1522, and TcdA significantly reduced the uptake of B. abortus by HeLa cells. In contrast, cytotoxic necrotizing factor from Escherichia coli, known to activate Rho, Rac, and Cdc42 small GTPases, increased the internalization of both virulent and non-virulent B. abortus. Expression of dominant-positive Rho, Rac, and Cdc42 forms in HeLa cells promoted the uptake of B. abortus, whereas expression of dominant-negative forms of these GTPases in HeLa cells hampered Brucella uptake. Cdc42 was activated upon cell contact by virulent B. abortus, but not by a noninvasive isogenic strain, as proven by affinity precipitation of active Rho, Rac, and Cdc42. The polyphasic approach used to discern the molecular events leading to Brucella internalization provides new alternatives for exploring the complexity of the signals required by intracellular pathogens for cell invasion.

IpaC from Shigella and SipC from Salmonella possess similar biochemical properties but are functionally distinct

Invasion plasmid antigen C (IpaC) is secreted via the type III secretion system (TTSS) of Shigella flexneri and serves as an essential effector molecule for epithelial cell invasion. The only homologue of IpaC identified thus far is Salmonella invasion protein C (SipC/SspC), which is essential for enterocyte invasion by Salmonella typhimurium. To explore the biochemical and functional relatedness of IpaC and SipC, recombinant derivatives of both proteins were purified so that their in vitro biochemical properties could be compared. Both proteins were found to: (i) enhance the entry of wild-type S. flexneri and S. typhimurium into cultured cells; (ii) interact with phospholipid membranes; and (iii) oligomerize in solution; however, IpaC appeared to be more efficient in carrying out several of the biochemical properties examined. Overall, the data indicate that purified IpaC and SipC are biochemically similar, although not identical with respect to their in vitro activities. To extend these observations, complementation analyses were conducted using S. flexneri SF621 and S. typhimurium SB220, neither of which is capable of invading epithelial cells because of non-polar null mutations in ipaC and sipC respectively. Interestingly, both ipaC and sipC restored invasiveness to SB220 whereas only ipaC restored invasiveness to SF621, suggesting that SipC lacks an activity possessed by IpaC. This functional difference is not at the level of secretion because IpaC and SipC are both secreted by SF621 and it does not appear to be because of SipC dependency on this native chaperone as coexpression of sipC and sicA in SF621 still failed to restore detectable invasiveness. Taken together, the data suggest that IpaC and SipC differ in either their ability to be translocated into host cells or in their function as effectors of host cell invasion. Because IpaB shares significant sequence homology with the YopB translocator of Yersinia species, the ability for IpaC and SipC to associate with this protein was explored as a potential indicator of translocation function. Both proteins were found to bind to purified IpaB with an apparent dissociation constant in the nanomolar range, suggesting that they may differ with respect to effector function. Interestingly, whereas SB220 expressing sipC behaved like wild-type Salmonella, in that it remained within its membrane-bound vacuole following entry into host cells, SB220 expressing ipaC was found in the cytoplasm of host cells. This observation indicates that IpaC and SipC are responsible for a major difference in the invasion strategies of Shigella and Salmonella, that is, they escape into the host cell cytoplasm. The implications of the role of each protein's biochemistry relative to its in vivo function is discussed.