Cellular invasion byStaphylococcus aureusreveals a functional link between focal adhesion kinase and cortactin in integrin-mediated internalisation

Staphylococcal Osteomyelitis: Disease Progression, Treatment Challenges, and Future Directions

Osteomyelitis is an inflammatory bone disease that is caused by an infecting microorganism and leads to progressive bone destruction and loss. The most common causative species are the usually commensal staphylococci, with Staphylococcus aureus and Staphylococcus epidermidis responsible for the majority of cases. Staphylococcal infections are becoming an increasing global concern, partially due to the resistance mechanisms developed by staphylococci to evade the host immune system and antibiotic treatment. In addition to the ability of staphylococci to withstand treatment, surgical intervention in an effort to remove necrotic and infected bone further exacerbates patient impairment. Despite the advances in current health care, osteomyelitis is now a major clinical challenge, with recurrent and persistent infections occurring in approximately 40% of patients. This review aims to provide information about staphylococcus-induced bone infection, covering the clinical presentation and diagnosis of osteomyelitis, pathophysiology and complications of osteomyelitis, and future avenues that are being explored to treat osteomyelitis.

Staphylococcal Adhesion and Host Cell Invasion: Fibronectin-Binding and Other Mechanisms

Opportunistic bacteria from the genus Staphylococcus can cause life-threatening infections such as pneumonia, endocarditis, bone and joint infections, and sepsis. This pathogenicity is closely related to their capacity to bind directly to the extracellular matrix or to host cells. Adhesion is indeed the first step in the formation of biofilm or the invasion of host cells, which protect the bacteria from the host immune system and facilitate chronic infection. Adhesion relies on the expression of a repertoire of surface proteins called adhesins, notably microbial surface components recognizing adhesive matrix molecules. In this short review, we discuss the main pathway (FnBP-Fn-α5β1 integrin), as well as alternatives, through which Staphylococcus aureus adheres to and then invades non-professional phagocytic cells. We then examine the corresponding mechanisms for coagulase negative staphylococci. There is currently a little understanding of the molecular mechanisms that lead to internalization. Filling this gap in the literature would therefore be an important step toward limiting the duration of staphylococci infections in clinical practice.

Staphylococcus carnosus: from starter culture to protein engineering platform

Since the 1950s, Staphylococcus carnosus is used as a starter culture for sausage fermentation where it contributes to food safety, flavor, and a controlled fermentation process. The long experience with S. carnosus has shown that it is a harmless and "food grade" species. This was confirmed by the genome sequence of S. carnosus TM300 that lacks genes involved in pathogenicity. Since the development of a cloning system in TM300, numerous genes have been cloned, expressed, and characterized and in particular, virulence genes that could be functionally validated in this non-pathogenic strain. A secretion system was developed for production and secretion of industrially important proteins and later modified to also enable display of heterologous proteins on the surface. The display system has been employed for various purposes, such as development of live bacterial delivery vehicles as well as microbial biocatalysts or bioadsorbents for potential environmental or biosensor applications. Recently, this surface display system has been utilized for display of peptide and protein libraries for profiling of protease substrates and for generation of various affinity proteins, e.g., Affibody molecules and scFv antibodies. In addition, by display of fragmented antigen-encoding genes, the surface expression system has been successfully used for epitope mapping of antibodies. Reviews on specific applications of S. carnosus have been published earlier, but here we provide a more extensive overview, covering a broad range of areas from food fermentation to sophisticated methods for protein-based drug discovery, which are all based on S. carnosus.

Candida albicans: The Ability to Invade Epithelial Cells and Survive under Oxidative Stress Is Unlinked to Hyphal Length

In its hyphal form, Candida albicans invades epithelial and endothelial cells by two distinct mechanisms: active penetration and induced endocytosis. The latter is dependent on a reorganization of the host cytoskeleton (actin/cortactin recruitment), whilst active penetration does not rely on the host's cellular machinery. The first obstacle for the fungus to reach deep tissues is the epithelial barrier and this interaction is crucial for commensal growth, fungal pathogenicity and host defense. This study aimed to characterize in vitro epithelial HeLa cell invasion by four different isolates of C. albicans with distinct clinical backgrounds, including a C. albicans SC5314 reference strain. All isolates invaded HeLa cells, recruited actin and cortactin, and induced the phosphorylation of both Src-family kinases (SFK) and cortactin. Curiously, L3881 isolated from blood culture of a patient exhibited the highest resistance to oxidative stress, although this isolate showed reduced hyphal length and displayed the lowest cell damage and invasion rates. Collectively, these data suggest that the ability of C. albicans to invade HeLa cells, and to reach and adapt to the host's blood, including resistance to oxidative stress, may be independent of hyphal length.

Mapping the recognition domains of pneumococcal fibronectin-binding proteins PavA and PavB demonstrates a common pattern of molecular interactions with fibronectin type III repeats

Colonization of mucosal respiratory surfaces is a prerequisite for the human pathobiont Streptococcus pneumoniae (the pneumococcus) to cause severe invasive infections. The arsenal of pneumococcal adhesins interacts with a multitude of extracellular matrix proteins. A paradigm for pneumococci is their interaction with the adhesive glycoprotein fibronectin, which facilitates bacterial adherence to host cells. Here, we deciphered the molecular interaction between fibronectin and pneumococcal fibronectin-binding proteins (FnBPs) PavA and PavB respectively. We show in adherence and binding studies that the pneumococcal interaction with fibronectin is a non-human specific trait. PavA and PavB target at least 13 out of 15 type III fibronectin domains as demonstrated in ligand overlay assays, surface plasmon resonance studies and SPOT peptide arrays. Strikingly, both pneumococcal FnBPs recognize similar peptides in targeted type III repeats. Structural comparisons revealed that the targeted type III repeat epitopes cluster on the inner strands of both β-sheets forming the fibronectin domains. Importantly, synthetic peptides of FnIII¹ , FnIII⁵ or FnIII¹⁵ bind directly to FnBPs PavA and PavB respectively. In conclusion, our study suggests a common pattern of molecular interactions between pneumococcal FnBPs and fibronectin. The specific epitopes recognized in this study can potentially be tested as antimicrobial targets in further scientific endeavours.

The expression of fibronectin is significantly suppressed in macrophages to exert a protective effect against Staphylococcus aureus infection

Background

Fibronectin (Fn) plays a major role in the attachment of Staphylococcus aureus to host cells by bridging staphylococcal fibronectin-binding proteins (FnBPs) and cell-surface integrins. A previous study demonstrated that the phagocytosis of S. aureus by macrophages is enhanced in the presence of exogenous Fn. We recently found that FnBPs overexpression also enhances phagocytic activity. The effect of S. aureus infection on the expression of macrophage Fn was investigated.

Result

The level of Fn secreted by monocytes (THP-1), macrophages, human lung adenocarcinoma (A549) cells, and hepatocellular carcinoma (HepG2) cells in response to S. aureus infection was determined by Western blotting and it was significantly suppressed only in macrophages. The activation of signaling pathways associated with Fn regulation in macrophages and HepG2 cells was also investigated by Western blotting. Erk was activated in both macrophages and HepG2 cells, whereas Src-JNK-c-Jun signaling was only activated in macrophages. A significant decrease in macrophage viability was observed in response to S. aureus infection in the presence of exogenous Fn.

Conclusion

The Src-JNK-c-Jun signaling pathway was activated in macrophages in response to S. aureus infection and resulted in the suppression of Fn expression. This suppression may play a protective role in macrophages against S. aureus infection. This study provides the first demonstration that Fn is suppressed in macrophages by S. aureus infection.

Electronic supplementary material

The online version of this article (doi:10.1186/s12866-017-1003-9) contains supplementary material, which is available to authorized users.

A fibronectin-binding protein (FbpA) of Weissella cibaria inhibits colonization and infection of Staphylococcus aureus in mammary glands

Staphylococcus aureus (S. aureus) is a frequent cause of infections in both humans and animals. Probiotics are known to inhibit colonization of pathogens on host tissues. However, mechanisms for the inhibition are still elusive due to complex host-microbe and microbe-microbe interactions. Here, we show that reduced abilities of S. aureus to infect mammary glands in the presence of Weissella cibaria (W. cibaria) were correlated with its poor adherence to mammary epithelial cells. Such inhibition by W. cibaria isolates was at least partially attributed to a fibronectin-binding protein (FbpA) on this lactic acid bacterium. Three W. cibaria isolates containing fbpA had higher inhibitory abilities than other three LAB isolates without the gene. The fbpA-deficient mutant of W. cibaria isolate LW1, LW1ΔfbpA, lost the inhibitory activity to reduce the adhesion of S. aureus to mammary epithelial cells and was less able to reduce the colonization of S. aureus in mammary glands. Expression of FbpA to the surface of LW1ΔfbpA reversed its inhibitory activities. Furthermore, addition of purified FbpA inhibited S. aureus biofilm formation. Our results suggest that W. cibaria FbpA hinders S. aureus colonization and infection through interfering with the S. aureus invasion pathway mediated by fibronectin-binding proteins and inhibiting biofilm formation of S. aureus.

Reduced Innate Immune Response to a Staphylococcus aureus Small Colony Variant Compared to Its Wild-Type Parent Strain

Background:Staphylococcus aureus (S. aureus) small colony variants (SCVs) can survive within the host intracellular milieu and are associated with chronic relapsing infections. However, it is unknown whether host invasion rates and immune responses differ between SCVs and their wild-type counterparts. This study used a stable S. aureus SCV (WCH-SK2^SCV) developed from a clinical isolate (WCH-SK2^WT) in inflammation-relevant conditions. Intracellular infection rates as well as host immune responses to WCH-SK2^WT and WCH-SK2^SCV infections were investigated. Method: NuLi-1 cells were infected with either WCH-SK2^WT or WCH-SK2^SCV, and the intracellular infection rate was determined over time. mRNA expression of cells infected with each strain intra- and extra-cellularly was analyzed using a microfluidic qPCR array to generate an expression profile of thirty-nine genes involved in the host immune response. Results: No difference was found in the intracellular infection rate between WCH-SK2^WT and WCH-SK2^SCV. Whereas, extracellular infection induced a robust pro-inflammatory response, intracellular infection elicited a modest response. Intracellular WCH-SK2^WT infection induced mRNA expression of TLR2, pro-inflammatory cytokines (IL1B, IL6, and IL12) and tissue remodeling factors (MMP9). In contrast, intracellular WCH-SK2^SCV infection induced up regulation of only TLR2. Conclusions: Whereas, host intracellular infection rates of WCH-SK2^SCV and WCH-SK2^WT were similar, WCH-SK2^SCV intracellular infection induced a less widespread up regulation of pro-inflammatory and tissue remodeling factors in comparison to intracellular WCH-SK2^WT infection. These findings support the current view that SCVs are able to evade host immune detection to allow their own survival.

The remarkably multifunctional fibronectin binding proteins of Staphylococcus aureus

Staphylococcus aureus expresses two distinct but closely related multifunctional cell wall-anchored (CWA) proteins that bind to the host glycoprotein fibronectin. The fibronectin binding proteins FnBPA and FnBPB comprise two distinct domains. The C-terminal domain comprises a tandem array of repeats that bind to the N-terminal type I modules of fibronectin by the tandem β-zipper mechanism. This causes allosteric activation of a cryptic integrin binding domain, allowing fibronectin to act as a bridge between bacterial cells and the α5β1 integrin on host cells, triggering bacterial uptake by endocytosis. Variants of FnBPA with polymorphisms in fibronectin binding repeats (FnBRs) that increase affinity for the ligand are associated with strains that infect cardiac devices and cause endocarditis, suggesting that binding affinity is particularly important in intravascular infections. The N-terminal A domains of FnBPA and FnBPB have diverged into seven antigenically distinct isoforms. Each binds fibrinogen by the 'dock, lock and latch' mechanism characteristic of clumping factor A. However, FnBPs can also bind to elastin, which is probably important in adhesion to connective tissue in vivo. In addition, they can capture plasminogen from plasma, which can be activated to plasmin by host and bacterial plasminogen activators. The bacterial cells become armed with a host protease which destroys opsonins, contributing to immune evasion and promotes spreading during skin infection. Finally, some methicillin-resistant S. aureus (MRSA) strains form biofilm that depends on the elaboration of FnBPs rather than polysaccharide. The A domains of the FnBPs can interact homophilically, allowing cells to bind together as the biofilm accumulates.

Proteomic discovery of host kinase signaling in bacterial infections

Protein phosphorylation catalyzed by protein kinases acts as a reversible molecular switch in signal transduction, providing a mechanism for the control of protein function in cellular processes. During microbial infection, cellular signaling essentially contributes to immune control to restrict the dissemination of invading pathogens within the host organism. However, pathogenic microbes compete for the control of host signaling to create a beneficial environment for successful invasion and infection. Although efforts to achieve a better understanding of the host–pathogen interaction and its molecular consequences have been made, there is urgent need for a comprehensive characterization of infection‐related host signaling processes. System‐wide and hypothesis‐free analysis of phosphorylation‐mediated host signaling during host–microbe interactions by mass spectrometry (MS)‐based methods is not only promising in view of a greater understanding of the pathogenesis of the infection but also may result in the identification of novel host targets for preventive or therapeutic intervention. Here, we review state‐of‐the‐art MS‐based techniques for the system‐wide identification and quantitation of protein phosphorylation and compare them to array‐based phosphoprotein analyses. We also provide an overview of how phosphoproteomics and kinomics have contributed to our understanding of protein kinase‐driven phosphorylation networks that operate during host–microbe interactions.

The Microtubule Plus End Tracking Protein TIP150 Interacts with Cortactin to Steer Directional Cell Migration

Cell migration is orchestrated by dynamic interactions of microtubules with the plasma membrane cortex. How these interactions facilitate these dynamic processes is still being actively investigated. TIP150 is a newly characterized microtubule plus end tracking protein essential for mitosis and entosis (Ward, T., Wang, M., Liu, X., Wang, Z., Xia, P., Chu, Y., Wang, X., Liu, L., Jiang, K., Yu, H., Yan, M., Wang, J., Hill, D. L., Huang, Y., Zhu, T., and Yao, X. (2013) Regulation of a dynamic interaction between two microtubule-binding proteins, EB1 and TIP150, by the mitotic p300/CBP-associated factor (PCAF) orchestrates kinetochore microtubule plasticity and chromosome stability during mitosis. J. Biol. Chem. 288, 15771-15785; Xia, P., Zhou, J., Song, X., Wu, B., Liu, X., Li, D., Zhang, S., Wang, Z., Yu, H., Ward, T., Zhang, J., Li, Y., Wang, X., Chen, Y., Guo, Z., and Yao, X. (2014) Aurora A orchestrates entosis by regulating a dynamic MCAK-TIP150 interaction. J. Mol. Cell Biol. 6, 240-254). Here we show that TIP150 links dynamic microtubules to steer cell migration by interacting with cortactin. Mechanistically, TIP150 binds to cortactin via its C-terminal tail. Interestingly, the C-terminal TIP150 proline-rich region (CT150) binds to the Src homology 3 domain of cortactin specifically, and such an interaction is negatively regulated by EGF-elicited tyrosine phosphorylation of cortactin. Importantly, suppression of TIP150 or overexpression of phospho-mimicking cortactin inhibits polarized cell migration. In addition, CT150 disrupts the biochemical interaction between TIP150 and cortactin in vitro, and perturbation of the TIP150-cortactin interaction in vivo using a membrane-permeable TAT-CT150 peptide results in an inhibition of directional cell migration. We reason that a dynamic TIP150-cortactin interaction orchestrates directional cell migration via coupling dynamic microtubule plus ends to the cortical cytoskeleton.

Allosteric Regulation of Fibronectin/α5β1 Interaction by Fibronectin-Binding MSCRAMMs

Adherence of microbes to host tissues is a hallmark of infectious disease and is often mediated by a class of adhesins termed MSCRAMMs (Microbial Surface Components Recognizing Adhesive Matrix Molecules). Numerous pathogens express MSCRAMMs that specifically bind the heterodimeric human glycoprotein fibronectin (Fn). In addition to roles in adhesion, Fn-binding MSCRAMMs exploit physiological Fn functions. For example, several pathogens can invade host cells by a mechanism whereby MSCRAMM-bound Fn bridges interaction with α₅β₁ integrin. Here, we investigate two Fn-binding MSCRAMMs, FnBPA (Staphylococcus aureus) and BBK32 (Borrelia burgdorferi) to probe structure-activity relationships of MSCRAMM-induced Fn/α₅β₁integrin activation. Circular dichroism, fluorescence resonance energy transfer, and dynamic light scattering techniques uncover a conformational rearrangement of Fn involving domains distant from the MSCRAMM binding site. Surface plasmon resonance experiments demonstrate a significant enhancement of Fn/α₅β₁ integrin affinity in the presence of FnBPA or BBK32. Detailed kinetic analysis of these interactions reveal that this change in affinity can be attributed solely to an increase in the initial Fn/α₅β₁ on-rate and that this rate-enhancement is dependent on high-affinity Fn-binding by MSCRAMMs. These data implicate MSCRAMM-induced perturbation of specific intramolecular contacts within the Fn heterodimer resulting in activation by exposing previously cryptic α₅β₁ interaction motifs. By correlating structural changes in Fn to a direct measurement of increased Fn/α₅β₁ affinity, this work significantly advances our understanding of the structural basis for the modulation of integrin function by Fn-binding MSCRAMMs.

The Periodontal Pathogen Porphyromonas gingivalis Preferentially Interacts with Oral Epithelial Cells in S Phase of the Cell Cycle

Porphyromonas gingivalis, a key periodontal pathogen, is capable of invading a variety of cells, including oral keratinocytes, by exploiting host cell receptors, including alpha-5 beta-1 (α5β1) integrin. Previous studies have shown that P. gingivalis accelerates the cell cycle and prevents apoptosis of host cells, but it is not known whether the cell cycle phases influence bacterium-cell interactions. The cell cycle distribution of oral keratinocytes was characterized by flow cytometry and BrdU (5-bromo-2-deoxyuridine) staining following synchronization of cultures by serum starvation. The effect of cell cycle phases on P. gingivalis invasion was measured by using antibiotic protection assays and flow cytometry, and these results were correlated with gene and surface expression levels of α5 integrin and urokinase plasminogen activator receptor (uPAR). There was a positive correlation (R = 0.98) between the number of cells in S phase and P. gingivalis invasion, the organism was more highly associated with cells in S phase than with cells in G2 and G1 phases, and S-phase cells contained 10 times more bacteria than did cells that were not in S phase. Our findings also show that α5 integrin, but not uPAR, was positively correlated with cells in S phase, which is consistent with previous reports indicating that P. gingivalis invasion of cells is mediated by α5 integrin. This study shows for the first time that P. gingivalis preferentially associates with and invades cells in the S phase of the cell cycle. The mechanism of targeting stable dividing cells may have implications for the treatment of periodontal diseases and may partly explain the persistence of this organism at subgingival sites.

Human Metapneumovirus Is Capable of Entering Cells by Fusion with Endosomal Membranes

Human metapneumovirus (HMPV), a member of the Paramyxoviridae family, is a leading cause of lower respiratory illness. Although receptor binding is thought to initiate fusion at the plasma membrane for paramyxoviruses, the entry mechanism for HMPV is largely uncharacterized. Here we sought to determine whether HMPV initiates fusion at the plasma membrane or following internalization. To study the HMPV entry process in human bronchial epithelial (BEAS-2B) cells, we used fluorescence microscopy, an R18-dequenching fusion assay, and developed a quantitative, fluorescence microscopy assay to follow virus binding, internalization, membrane fusion, and visualize the cellular site of HMPV fusion. We found that HMPV particles are internalized into human bronchial epithelial cells before fusing with endosomes. Using chemical inhibitors and RNA interference, we determined that HMPV particles are internalized via clathrin-mediated endocytosis in a dynamin-dependent manner. HMPV fusion and productive infection are promoted by RGD-binding integrin engagement, internalization, actin polymerization, and dynamin. Further, HMPV fusion is pH-independent, although infection with rare strains is modestly inhibited by RNA interference or chemical inhibition of endosomal acidification. Thus, HMPV can enter via endocytosis, but the viral fusion machinery is not triggered by low pH. Together, our results indicate that HMPV is capable of entering host cells by multiple pathways, including membrane fusion from endosomal compartments.

Human metapneumovirus (HMPV) is a paramyxovirus that causes severe lower respiratory tract infections. HMPV infection is initiated by the viral surface fusion (F) glycoprotein. HMPV F attaches to cellular receptors, including RGD-binding integrins, and catalyzes virus membrane fusion with cellular membranes during virus entry. Although most paramyxoviruses enter cells by coupling receptor binding to membrane fusion at the cell surface, the entry mechanism for HMPV is largely uncharacterized. In this study, we sought to determine the cellular site of HMPV fusion. We show that HMPV particles are internalized by clathrin-mediated endocytosis and fuse with endosomal membranes. Furthermore, HMPV engages RGD-binding integrins for endosomal trafficking and full virus membrane fusion with intracellular membranes, suggesting that HMPV uses integrins to facilitate movement into target cells rather than as a trigger for fusion at the cell surface. Inhibition of endosomal acidification had only a modest strain-specific effect, suggesting that low pH exposure is not required for HMPV fusion. These results expand knowledge of mechanisms of HMPV entry and suggest new potential therapeutic interventions against this medically important virus.

Genome-Wide Association Study of Staphylococcus aureus Carriage in a Community-Based Sample of Mexican-Americans in Starr County, Texas

Staphylococcus aureus is the number one cause of hospital-acquired infections. Understanding host pathogen interactions is paramount to the development of more effective treatment and prevention strategies. Therefore, whole exome sequence and chip-based genotype data were used to conduct rare variant and genome-wide association analyses in a Mexican-American cohort from Starr County, Texas to identify genes and variants associated with S. aureus nasal carriage. Unlike most studies of S. aureus that are based on hospitalized populations, this study used a representative community sample. Two nasal swabs were collected from participants (n = 858) 11–17 days apart between October 2009 and December 2013, screened for the presence of S. aureus, and then classified as either persistent, intermittent, or non-carriers. The chip-based and exome sequence-based single variant association analyses identified 1 genome-wide significant region (KAT2B) for intermittent and 11 regions suggestively associated with persistent or intermittent S. aureus carriage. We also report top findings from gene-based burden analyses of rare functional variation. Notably, we observed marked differences between signals associated with persistent and intermittent carriage. In single variant analyses of persistent carriage, 7 of 9 genes in suggestively associated regions and all 5 top gene-based findings are associated with cell growth or tight junction integrity or are structural constituents of the cytoskeleton, suggesting that variation in genes associated with persistent carriage impact cellular integrity and morphology.

Spheres of influence: Porphyromonas gingivalis outer membrane vesicles

Outer membrane vesicles (OMVs) are asymmetrical single bilayer membranous nanostructures produced by Gram-negative bacteria important for bacterial interaction with the environment. Porphyromonas gingivalis, a keystone pathogen associated with chronic periodontitis, produces OMVs that act as a virulence factor secretion system contributing to its pathogenicity. Despite their biological importance, the mechanisms of OMV biogenesis have not been fully elucidated. The ~14 times more curvature of the OMV membrane than cell outer membrane (OM) indicates that OMV biogenesis requires energy expenditure for significant curvature of the OMV membrane. In P. gingivalis, we propose that this may be achieved by upregulating the production of certain inner or outer leaflet lipids, which causes localized outward curvature of the OM. This results in selection of anionic lipopolysaccharide (A-LPS) and associated C-terminal domain (CTD) -family proteins on the outer surface due to their ability to accommodate the curvature. Deacylation of A-LPS may further enable increased curvature leading to OMV formation. Porphyromonas gingivalis OMVs that are selectively enriched in CTD-family proteins, largely the gingipains, can support bacterial coaggregation, promote biofilm development and act as an intercessor for the transport of non-motile bacteria by motile bacteria. The P. gingivalis OMVs are also believed to contribute to host interaction and colonization, evasion of immune defense mechanisms, and destruction of periodontal tissues. They may be crucial for both micro- and macronutrient capture, especially heme and probably other assimilable compounds for its own benefit and that of the wider biofilm community.

Virulence and the presence of aminoglycoside resistance genes of Staphylococcus haemolyticus strains isolated from clinical specimens

We examined thirty methicillin-resistant Staphylococcus haemolyticus isolates cultured from clinical specimens for antibiotic resistance, various important interactions of the bacteria with epithelial cells and putative virulence determinants. All strains were resistant to oxacillin and carried the mecA gene. Aminocyclitol-3′-phosphotransferase (aph(3′)-IIIa) gene encoding nucleotidyltransferases was detected in 43 %, aminocyclitol-6′-acetyltransferase-aminocyclitol-2″-phosphotransferase (aac(6′)/aph(2″)) gene encoding bifunctional acetyltransferases/phosphotransferases in 33 %, aminocyclitol-4′-adenylyltransferase (ant(4′)-Ia) gene encoding phosphotransferases in 20 %. The coexistence of resistance to methicillin and aminoglycosides was investigated in multi-resistant strains. Coexisting (aac(6′)/aph(2″)) and (aph(3′)-IIIa) genes were detected in 33 % of isolates, whereas 63 % of isolates had at least one of these genes. All strains revealed adherence ability and most of them (63 %) were invasive to epithelial cells. Electron microscopy revealed that the bacteria were found in vacuoles inside the cells. We observed that the contact of the bacteria with host epithelial cells is a prerequisite to their cytotoxicity at 5 h-incubation. Culture supernatant of the strains induced a low effect of cytotoxicity at the same time of incubation. Cell-free supernatant of all isolates expressed cytotoxic activity which caused destruction of HEp-2 cells at 24 h. None of the strains was cytotonic towards CHO cells. Among thirty strains, 27 % revealed lipolytic activity, 43 % produced lecithinase and 20 % were positive for proteinase activity. Analyses of cellular morphology and DNA fragmentation exhibited typical characteristic features of those undergoing apoptosis. The Pearson linear test revealed positive correlations between the apoptotic index at 24 h and percentage of cytotoxicity. Our results provided new insights into the mechanisms contributing to the development of S. haemolyticus-associated infections. The bacteria adhered and invaded to non-professional phagocytes. The invasion of epithelial cells by S. haemolyticus could be similar to phagocytosis that requires polymerization of the actin cytoskeleton. The process is inhibited by cytochalasin D. Moreover, they survived within the cells by residing in membrane bound compartments and induced apoptotic cell death.

Staphylococcus aureus keratinocyte invasion is mediated by integrin-linked kinase and Rac1

Staphylococcus aureus is a major component of the skin microbiota and causes a large number of serious infections. S. aureus first interacts with epidermal keratinocytes to breach the epidermal barrier through mechanisms not fully understood. By use of primary keratinocytes from mice with epidermis-restricted Ilk gene inactivation and control integrin-linked kinase (ILK)-expressing littermates, we investigated the role of ILK in epidermal S. aureus invasion. Heat-killed, but not live, bacteria were internalized to Rab5- and Rab7-positive phagosomes, and incubation with keratinocyte growth factor increased their uptake 2.5-fold. ILK-deficient mouse keratinocytes internalized bacteria 2- to 4-fold less efficiently than normal cells. The reduced invasion by live S. aureus of ILK-deficient cells was restored in the presence of exogenous, constitutively active Rac1. Thus, Rac1 functions downstream from ILK during invasion. Further, invasion by S. aureus of Rac1-deficient cells was 2.5-fold lower than in normal cells. Paradoxically, staphylococcal cutaneous penetration of mouse skin explants with ILK-deficient epidermis was 35-fold higher than that of normal skin, indicating defects in epidermal barrier function in the absence of ILK. Thus, we identified an ILK-Rac1 pathway essential for bacterial invasion of keratinocytes, and established ILK as a key contributor to prevent invasive staphylococcal cutaneous infection.

The integrin adhesome: from genes and proteins to human disease

The adhesive interactions of cells with their environment through the integrin family of transmembrane receptors have key roles in regulating multiple aspects of cellular physiology, including cell proliferation, viability, differentiation and migration. Consequently, failure to establish functional cell adhesions, and thus the assembly of associated cytoplasmic scaffolding and signalling networks, can have severe pathological effects. The roles of specific constituents of integrin-mediated adhesions, which are collectively known as the 'integrin adhesome', in diverse pathological states are becoming clear. Indeed, the prominence of mutations in specific adhesome molecules in various human diseases is now appreciated, and experimental as well as in silico approaches provide insights into the molecular mechanisms underlying these pathological conditions.

Nonprofessional phagocytic cell receptors involved in Staphylococcus aureus internalization

Staphylococcus aureus is a successful human and animal pathogen. The majority of infections caused by this pathogen are life threatening, primarily because S. aureus has developed multiple evasion strategies, possesses intracellular persistence for long periods, and targets the skin and soft tissues. Therefore, it is very important to understand the mechanisms employed by S. aureus to colonize and proliferate in these cells. The aim of this review is to describe the recent discoveries concerning the host receptors of nonprofessional phagocytes involved in S. aureus internalization. Most of the knowledge related to the interaction of S. aureus with its host cells has been described in professional phagocytic cells such as macrophages. Here, we showed that in nonprofessional phagocytes the α 5 β 1 integrin host receptor, chaperons, and the scavenger receptor CD36 are the main receptors employed during S. aureus internalization. The characterization and identification of new bacterial effectors and the host cell receptors involved will undoubtedly lead to new discoveries with beneficial purposes.

Serine phosphorylation of cortactin is required for maximal host cell invasion by Campylobacter jejuni

BACKGROUND

Campylobacter jejuni causes acute disease characterized by severe diarrhea containing blood and leukocytes, fever, and abdominal cramping. Disease caused by C. jejuni is dependent on numerous bacterial and host factors. C. jejuni invasion of the intestinal epithelial cells is seen in both clinical samples and animal models indicating that host cell invasion is, in part, necessary for disease. C. jejuni utilizes a flagellar Type III Secretion System (T3SS) to deliver the Campylobacter invasion antigens (Cia) to host cells. The Cia proteins modulate host cell signaling leading to actin cytoskeleton rearrangement necessary for C. jejuni host cell invasion, and are required for the development of disease.

RESULTS

This study was based on the hypothesis that the C. jejuni CiaD effector protein mediates Erk 1/2 dependent cytoskeleton rearrangement. We showed that CiaD was required for the maximal phosphorylation of Erk 1/2 by performing an immunoblot with a p-Erk 1/2 specific antibody and that Erk 1/2 participates in C. jejuni invasion of host cells by performing the gentamicin protection assay in the presence and absence of the PD98059 (a potent inhibitor of Erk 1/2 activation). CiaD was also found to be required for the maximal phosphorylation of cortactin S405 and S418, as judged by immunoblot analysis. The response of human INT 407 epithelial cells to infection with C. jejuni was evaluated by confocal microscopy and scanning electron microscopy to determine the extent of membrane ruffling. This analysis revealed that CiaD, Erk 1/2, and cortactin participate in C. jejuni-induced membrane ruffling. Finally, cortactin and N-WASP were found to be involved in C. jejuni invasion of host cells using siRNA to N-WASP, and siRNA to cortactin, coupled with the gentamicin protection assay.

CONCLUSION

We conclude that CiaD is involved in the activation of Erk 1/2 and that activated Erk 1/2 facilitates C. jejuni invasion by phosphorylation of cortactin on serine 405 and 418. This is the first time that cortactin and N-WASP have been shown to be involved in C. jejuni invasion of host cells. These data also provide a mechanistic basis for the requirement of Erk 1/2 in C. jejuni-mediated cytoskeletal rearrangement.

Interplay of host-pathogen microvesicles and their role in infectious disease

The release of extracellular vesicles, whether MVs (microvesicles) or exosomes, from host cells or intracellular pathogens is likely to play a significant role in the infection process. Host MVs may fuse with pathogen surfaces to deliver host complement regulatory proteins. They may also deliver cytokines that enhance invasion. Decoy functions are also possible. Whereas host MVs may direct pathogens away from their target cells, pathogen MVs may in turn redirect complement membrane-attack complexes away from their target pathogen. An understanding of the mechanisms of this interplay, bringing about both immune evasion and enhanced invasion, will help to direct future research with a view to rendering pathogens more susceptible to immune attack or in improving drug efficacy. It should also be possible to use MVs or exosomes isolated directly from the pathogens, or from the cells infected with pathogens, to provide alternative vaccination strategies.

Glycobiology of the ocular surface: mucins and lectins

Glycosylation is an important and common form of posttranscriptional modification of proteins in cells. During the last decade, a vast array of biological functions has been ascribed to glycans because of a rapid evolution in glycomic technologies. Glycogenes that are highly expressed at the human ocular surface include families of glycosyltransferases, proteoglycans, and glycan degradation proteins, as well as mucins and carbohydrate-binding proteins, such as the galectins. On the apical glycocalyx, mucin O-glycans promote boundary lubrication, prevent bacterial adhesion and endocytic activity, and maintain epithelial barrier function through interactions with galectins. The emerging roles attributed to glycans are contributing to the appreciation of their biological capabilities at the ocular surface.

Iron-regulated surface determinant B (IsdB) promotes Staphylococcus aureus adherence to and internalization by non-phagocytic human cells

Staphylococcus aureus is a human pathogen that causes invasive and recurring infections. The ability to internalize into and persist within host cells is thought to contribute to infection. Here we report a novel role for the well-characterized iron-regulated surface determinant B (IsdB) protein which we have shown can promote adhesion of 293T, HeLa cells and platelets to immobilized bacteria independently of its ability to bind haemoglobin. IsdB bound to the active form of the platelet integrin αIIb β3 , both on platelets and when the integrin was expressed ectopically in CHO cells. IsdB also promoted bacterial invasion into human cells. This was clearly demonstrated with bacteria lacking fibronectin-binding proteins (FnBPs), which are known to promote invasion in the presence of fibronectin. However, IsdB also contributed significantly to invasion by cells expressing FnBPs in the presence of serum. Thus IsdB appears to be able to interact with the broader family of integrins that bind ligands with the RGD motif and to act as a back up mechanism to promote interactions with mammalian cells.

Integrin α5β1-fimbriae binding and actin rearrangement are essential for Porphyromonas gingivalis invasion of osteoblasts and subsequent activation of the JNK pathway

BACKGROUND

Chronic periodontitis is an infectious disease of the periodontium, which includes the gingival epithelium, periodontal ligament and alveolar bone. The signature clinical feature of periodontitis is resorption of alveolar bone and subsequent tooth loss. The Gram-negative oral anaerobe, Porphyromonas gingivalis, is strongly associated with periodontitis, and it has been shown previously that P. gingivalis is capable of invading osteoblasts in a dose- and time-dependent manner resulting in inhibition of osteoblast differentiation and mineralization in vitro. It is not yet clear which receptors and cytoskeletal components mediate the invasive process, nor how the signaling pathways and viability of osteoblasts are affected by bacterial internalization. This study aimed to investigate these issues using an in vitro model system involving the inoculation of P. gingivalis ATCC 33277 into primary osteoblast cultures.

RESULTS

It was found that binding between P. gingivalis fimbriae and integrin α5β1 on osteoblasts, and subsequent peripheral condensation of actin, are essential for entry of P. gingivalis into osteoblasts. The JNK pathway was activated in invaded osteoblasts, and apoptosis was induced by repeated infections.

CONCLUSIONS

These observations indicate that P. gingivalis manipulates osteoblast function to promote its initial intracellular persistence by prolonging the host cell life span prior to its intercellular dissemination via host cell lysis. The identification of molecules critical to the interaction between P. gingivalis and osteoblasts will facilitate the development of new therapeutic strategies for the prevention of periodontal bone loss.

Integrin-mediated internalization of Staphylococcus aureus does not require vinculin

Background

Disease manifestations of Staphylococcus aureus are connected to the fibronectin (Fn)-binding capacity of these Gram-positive pathogens. Fn deposition on the surface of S. aureus allows engagement of α5β1 integrins and triggers uptake by host cells. For several integrin- and actin-associated cytoplasmic proteins, including FAK, Src, N-WASP, tensin and cortactin, a functional role during bacterial invasion has been demonstrated. As reorganization of the actin cytoskeleton is critical for bacterial entry, we investigated whether vinculin, an essential protein linking integrins with the actin cytoskeleton, may contribute to the integrin-mediated internalization of S. aureus.

Results

Complementation of vinculin in vinculin -/- cells, vinculin overexpression, as well as shRNA-mediated vinculin knock-down in different eukaryotic cell types demonstrate, that vinculin does not have a functional role during the integrin-mediated uptake of S. aureus.

Conclusions

Our results suggest that vinculin is insignificant for the integrin-mediated uptake of S. aureus despite the critical role of vinculin as a linker between integrins and F-actin.

Integrins and small GTPases as modulators of phagocytosis

Phagocytosis is the mechanism whereby cells engulf large particles. This process has long been recognized as a critical component of the innate immune response, which constitutes the organism's defense against microorganisms. In addition, phagocytic internalization of apoptotic cells or cell fragments plays important roles in tissue homeostasis and remodeling. Phagocytosis requires target interactions with receptors on the plasma membrane of the phagocytic cell. Integrins have been identified as important mediators of particle clearance, in addition to their well-established roles in cell adhesion, migration and mechanotransduction. Indeed, these ubiquitously expressed proteins impart phagocytic capacity to epithelial, endothelial and mesenchymal cell types. The importance of integrins in particle internalization is emphasized by the ability of microbial and viral pathogens to exploit their signaling pathways to invade host cells, and by the wide variety of disorders that arise from abnormalities in integrin-dependent phagocytic uptake.

Differential adhesion and invasion by Staphylococcus aureus of epithelial cells derived from different anatomical sites

Staphylococcus aureus can invade epithelial cells, and the host-cell receptor α(5)β(1) integrin is thought to mediate this process. The aim of this study was to investigate S. aureus invasion of epithelial cell lines derived from oral (H357), skin (UP) and nasopharyngeal (Detroit 562) sites and to determine whether any differences were due to the levels of α(5)β(1) integrin expressed. While the adhesion and invasion of two S. aureus strains were similar in both oral and skin-derived keratinocytes, this was markedly reduced in the nasopharyngeal cell line, despite it expressing similar levels of α(5)β(1). While this might be explainable on the basis of availability of cell receptor, adhesion to and invasion of H357 and UP cells by S. aureus were enhanced when the epithelial cells were in suspension rather than on a surface, and levels of α(5) integrin subunit mRNA were also increased. Detroit 562 cells exhibited a similar α(5) gene upregulation, but this did not result in enhanced adhesion and invasion of S. aureus. The Detroit 562 cells also showed reduced adhesion to fibronectin compared with the other cell types. This, and the low S. aureus invasion, may result from reduced α(5)β(1) integrin activity or from variation in an as-yet-unidentified additional receptor or accessory molecule. These studies shed further light on the mechanisms of S. aureus invasion of human cells.

Cortactin as a target for FAK in the regulation of focal adhesion dynamics

Background

Efficient cell movement requires the dynamic regulation of focal adhesion (FA) formation and turnover. FAs are integrin-associated sites of cell attachment and establish linkages to the cellular actin cytoskeleton. Cells without focal adhesion kinase (FAK), an integrin-activated tyrosine kinase, exhibit defects in FA turnover and cell motility. Cortactin is an actin binding adaptor protein that can influence FA dynamics. FAK and cortactin interact, but the cellular role of this complex remains unclear.

Principal Findings

Using FAK-null fibroblasts stably reconstituted with green fluorescent protein (GFP) tagged FAK constructs, we find that FAK activity and FAK C-terminal proline-rich region 2 (PRR2) and PRR3 are required for FA turnover and cell motility. Cortactin binds directly to FAK PRR2 and PRR3 sites via its SH3 domain and cortactin expression is important in promoting FA turnover and GFP-FAK release from FAs. FAK-cortactin binding is negatively-regulated by FAK activity and associated with cortactin tyrosine phosphorylation. FAK directly phosphorylates cortactin at Y421 and Y466 and over-expression of cortactin Y421, Y466, and Y482 mutated to phenylalanine (3YF) prevented FAK-enhanced FA turnover and cell motility. However, phospho-mimetic cortactin mutated to glutamic acid (3YE) did not affect FA dynamics and did not rescue FA turnover defects in cells with inhibited FAK activity or with PRR2-mutated FAK that does not bind cortactin.

Conclusions

Our results support a model whereby FAK-mediated FA remodeling may occur through the formation of a FAK-cortactin signaling complex. This involves a cycle of cortactin binding to FAK, cortactin tyrosine phosphorylation, and subsequent cortactin-FAK dissociation accompanied by FA turnover and cell movement.

Cell invasion by Neisseria meningitidis requires a functional interplay between the focal adhesion kinase, Src and cortactin

Entry of Neisseria meningitidis (the meningococcus) into human brain microvascular endothelial cells (HBMEC) is mediated by fibronectin or vitronectin bound to the surface protein Opc forming a bridge to the respective integrins. This interaction leads to cytoskeletal rearrangement and uptake of meningococci. In this study, we determined that the focal adhesion kinase (FAK), which directly associates with integrins, is involved in integrin-mediated internalization of N. meningitidis in HBMEC. Inhibition of FAK activity by the specific FAK inhibitor PF 573882 reduced Opc-mediated invasion of HBMEC more than 90%. Moreover, overexpression of FAK mutants that were either impaired in the kinase activity or were not capable of autophosphorylation or overexpression of the dominant-negative version of FAK (FRNK) blocked integrin-mediated internalization of N. meningitidis. Importantly, FAK-deficient fibroblasts were significantly less invaded by N. meningitidis. Furthermore, N. meningitidis induced tyrosine phosphorylation of several host proteins including the FAK/Src complex substrate cortactin. Inhibition of cortactin expression by siRNA silencing and mutation of critical amino acid residues within cortactin, that encompass Arp2/3 association and dynamin binding, significantly reduced meningococcal invasion into eukaryotic cells suggesting that both domains are critical for efficient uptake of N. meningitidis into eukaryotic cells. Together, these results indicate that N. meningitidis exploits the integrin signal pathway for its entry and that FAK mediates the transfer of signals from activated integrins to the cytoskeleton. A cooperative interplay between FAK, Src and cortactin then enables endocytosis of N. meningitidis into host cells.

Cortactin is involved in the entry of Coxiella burnetii into non-phagocytic cells

BACKGROUND

Cortactin is a key regulator of the actin cytoskeleton and is involved in pathogen-host cell interactions. Numerous pathogens exploit the phagocytic process and actin cytoskeleton to infect host cells. Coxiella burnetii, the etiologic agent of Q fever, is internalized by host cells through a molecular mechanism that is poorly understood.

METHODOLOGY/PRINCIPAL FINDING

Here we analyzed the role of different cortactin motifs in the internalization of C. burnetii by non-phagocytic cells. C. burnetii internalization into HeLa cells was significantly reduced when the cells expressed GFP-cortactin W525K, which carries a mutation in the SH3 domain that renders the protein unable to bind targets such as N-WASP. However, internalization was unaffected when the cells expressed the W22A mutant, which has a mutation in the N-terminal acidic region that destroys the protein's ability to bind and activate Arp2/3. We also determined whether the phosphorylation status of cortactin is important for internalization. Expression of GFP-cortactin 3F, which lacks phosphorylatable tyrosines, significantly increased internalization of C. burnetii, while expression of GFP-cortactin 3D, a phosphotyrosine mimic, did not affect it. In contrast, expression of GFP-cortactin 2A, which lacks phosphorylatable serines, inhibited C. burnetii internalization, while expression of GFP-cortactin SD, a phosphoserine mimic, did not affect it. Interestingly, inhibitors of Src kinase and the MEK-ERK kinase pathway blocked internalization. In fact, both kinases reached maximal activity at 15 min of C. burnetii infection, after which activity decreased to basal levels. Despite the decrease in kinase activity, cortactin phosphorylation at Tyr421 reached a peak at 1 h of infection.

CONCLUSIONS/SIGNIFICANCE

Our results suggest that the SH3 domain of cortactin is implicated in C. burnetii entry into HeLa cells. Furthermore, cortactin phosphorylation at serine and dephosphorylation at tyrosine favor C. burnetii internalization. We present evidence that ERK and Src kinases play a role early in infection by this pathogen.

Host cell kinases, α5 and β1 integrins, and Rac1 signalling on the microtubule cytoskeleton are important for non-typable Haemophilus influenzae invasion of respiratory epithelial cells

Non-typable Haemophilus influenzae (NTHi) is a common commensal of the human nasopharynx, but causes opportunistic infection when the respiratory tract is compromised by infection or disease. The ability of NTHi to invade epithelial cells has been described, but the underlying molecular mechanisms are poorly characterized. We previously determined that NTHi promotes phosphorylation of the serine-threonine kinase Akt in A549 human lung epithelial cells, and that Akt phosphorylation and NTHi cell invasion are prevented by inhibition of phosphoinositide 3-kinase (PI3K). Because PI3K-Akt signalling is associated with several host cell networks, the purpose of the current study was to identify eukaryotic molecules important for NTHi epithelial invasion. We found that inhibition of Akt activity reduced NTHi internalization; differently, bacterial entry was increased by phospholipase Cγ1 inhibition but was not affected by protein kinase inhibition. We also found that α5 and β1 integrins, and the tyrosine kinases focal adhesion kinase and Src, are important for NTHi A549 cell invasion. NTHi internalization was shown to be favoured by activation of Rac1 guanosine triphosphatase (GTPase), together with the guanine nucleotide exchange factor Vav2 and the effector Pak1. Also, Pak1 might be associated with inactivation of the microtubule destabilizing agent Op18/stathmin, to facilitate microtubule polymerization and NTHi entry. Conversely, inhibition of RhoA GTPase and its effector ROCK increased the number of internalized bacteria. Src and Rac1 were found to be important for NTHi-triggered Akt phosphorylation. An increase in host cyclic AMP reduced bacterial entry, which was linked to protein kinase A. These findings suggest that NTHi finely manipulates host signalling molecules to invade respiratory epithelial cells.

Intracellular Staphylococcus aureus: live-in and let die

Staphylococcus aureus uses a plethora of virulence factors to accommodate a diversity of niches in its human host. Aside from the classical manifestations of S. aureus-induced diseases, the pathogen also invades and survives within mammalian host cells.The survival strategies of the pathogen are as diverse as strains or host cell types used. S. aureus is able to replicate in the phagosome or freely in the cytoplasm of its host cells. It escapes the phagosome of professional and non-professional phagocytes, subverts autophagy, induces cell death mechanisms such as apoptosis and pyronecrosis, and even can induce anti-apoptotic programs in phagocytes. The focus of this review is to present a guide to recent research outlining the variety of intracellular fates of S. aureus.

Integrin β1 mediates vaccinia virus entry through activation of PI3K/Akt signaling

Vaccinia virus has a broad range of infectivity in many cell lines and animals. Although it is known that the vaccinia mature virus binds to cell surface glycosaminoglycans and extracellular matrix proteins, whether additional cellular receptors are required for virus entry remains unclear. Our previous studies showed that the vaccinia mature virus enters through lipid rafts, suggesting the involvement of raft-associated cellular proteins. Here we demonstrate that one lipid raft-associated protein, integrin β1, is important for vaccinia mature virus entry into HeLa cells. Vaccinia virus associates with integrin β1 in lipid rafts on the cell surface, and the knockdown of integrin β1 in HeLa cells reduces vaccinia mature virus entry. Additionally, vaccinia mature virus infection is reduced in a mouse cell line, GD25, that is deficient in integrin β1 expression. Vaccinia mature virus infection triggers the activation of phosphatidylinositol 3-kinase (PI3K)/Akt signaling, and the treatment of cells with inhibitors to block P13K activation reduces virus entry in an integrin β1-dependent manner, suggesting that integrin β1-mediates PI3K/Akt activation induced by vaccinia virus and that this signaling pathway is essential for virus endocytosis. The inhibition of integrin β1-mediated cell adhesion results in a reduction of vaccinia virus entry and the disruption of focal adhesion and PI3K/Akt activation. In summary, our results show that the binding of vaccinia mature virus to cells mimics the outside-in activation process of integrin functions to facilitate vaccinia virus entry into HeLa cells.

Adherence and invasion of streptococci to eukaryotic cells and their role in disease pathogenesis

Streptococcal adhesion, invasion, intracellular trafficking, dissemination, and persistence in eukaryotic cells have a variety of implications in the infection pathogenesis. While cell adhesion establishes the initial host contact, adhering bacteria exploit the host cell for their own benefit. Internalization into the host cell is an essential step for bacterial survival and subsequent dissemination and persistence, thus playing a key role in the course of infection. This chapter summarizes the current knowledge about the diverse mechanisms of streptococcal adhesion to and invasion into different eukaryotic cells and the impact on dissemination and persistence which is reflected by consequences for the pathogenesis of streptococcal infections.

The signaling pathway of Campylobacter jejuni-induced Cdc42 activation: Role of fibronectin, integrin beta1, tyrosine kinases and guanine exchange factor Vav2

Background

Host cell invasion by the foodborne pathogen Campylobacter jejuni is considered as one of the primary reasons of gut tissue damage, however, mechanisms and key factors involved in this process are widely unclear. It was reported that small Rho GTPases, including Cdc42, are activated and play a role during invasion, but the involved signaling cascades remained unknown. Here we utilised knockout cell lines derived from fibronectin^-/-, integrin-beta1^-/-, focal adhesion kinase (FAK)^-/- and Src/Yes/Fyn^-/- deficient mice, and wild-type control cells, to investigate C. jejuni-induced mechanisms leading to Cdc42 activation and bacterial uptake.

Results

Using high-resolution scanning electron microscopy, GTPase pulldowns, G-Lisa and gentamicin protection assays we found that each studied host factor is necessary for induction of Cdc42-GTP and efficient invasion. Interestingly, filopodia formation and associated membrane dynamics linked to invasion were only seen during infection of wild-type but not in knockout cells. Infection of cells stably expressing integrin-beta1 variants with well-known defects in fibronectin fibril formation or FAK signaling also exhibited severe deficiencies in Cdc42 activation and bacterial invasion. We further demonstrated that infection of wild-type cells induces increasing amounts of phosphorylated FAK and growth factor receptors (EGFR and PDGFR) during the course of infection, correlating with accumulating Cdc42-GTP levels and C. jejuni invasion over time. In studies using pharmacological inhibitors, silencing RNA (siRNA) and dominant-negative expression constructs, EGFR, PDGFR and PI3-kinase appeared to represent other crucial components upstream of Cdc42 and invasion. siRNA and the use of Vav1/2^-/- knockout cells further showed that the guanine exchange factor Vav2 is required for Cdc42 activation and maximal bacterial invasion. Overexpression of certain mutant constructs indicated that Vav2 is a linker molecule between Cdc42 and activated EGFR/PDGFR/PI3-kinase. Using C. jejuni mutant strains we further demonstrated that the fibronectin-binding protein CadF and intact flagella are involved in Cdc42-GTP induction, indicating that the bacteria may directly target the fibronectin/integrin complex for inducing signaling leading to its host cell entry.

Conclusion

Collectively, our findings led us propose that C. jejuni infection triggers a novel fibronectin→integrin-beta1→FAK/Src→EGFR/PDGFR→PI3-kinase→Vav2 signaling cascade, which plays a crucial role for Cdc42 GTPase activity associated with filopodia formation and enhances bacterial invasion.

Major host factors involved in epithelial cell invasion of Campylobacter jejuni: role of fibronectin, integrin beta1, FAK, Tiam-1, and DOCK180 in activating Rho GTPase Rac1

Host cell entry by the food-borne pathogen Campylobacter jejuni has been reported as one of the primary reasons of tissue damage in infected humans, however, molecular invasion mechanisms and cellular factors involved in this process are widely unclear. Here we used knockout cell lines derived from fibronectin^−/−, integrin beta1^−/−, and focal adhesion kinase (FAK)^−/− deficient mice and corresponding wild-type (WT) controls, to study C. jejuni-induced signaling cascades involved in the bacterial invasion process. Using high resolution scanning electron microscopy, GTPase pull-downs, G-LISA, and gentamicin protection assays we found that each of these host cell factors is indeed required for activation of the small Rho GTPase member Rac1 and maximal host cell invasion of this pathogen. Interestingly, membrane ruffling, tight engulfment of bacteria and invasion were only seen during infection of WT control cells, but not in fibronectin^−/−, integrin beta1^−/−, and FAK^−/− knockout cell lines. We also demonstrate that C. jejuni activates FAK autophosphorylation activity at Y-397 and phosphorylation of Y-925, which is required for stimulating two downstream guanine exchange factors, DOCK180 and Tiam-1, which are upstream of Rac1. Small interfering (si) RNA studies further show that DOCK180 and Tiam-1 act cooperatively to trigger Rac1 activation and C. jejuni invasion. Moreover, mutagenesis data indicate that the bacterial fibronectin-binding protein CadF and the intact flagellum are involved in Rho GTPase activation and host cell invasion. Collectively, our results suggest that C. jejuni infection of host epithelial target cells hijacks a major fibronectin → integrin beta1 → FAK → DOCK180/Tiam-1 signaling cascade, which has a crucial role for Rac1 GTPase activity and bacterial entry into host target cells.

Bartonella henselae engages inside-out and outside-in signaling by integrin β1 and talin1 during invasome-mediated bacterial uptake

The VirB/D4 type IV secretion system (T4SS) of the bacterial pathogen Bartonella henselae (Bhe) translocates seven effector proteins (BepA-BepG) into human cells that subvert host cellular functions. Two redundant pathways dependent on BepG or the combination of BepC and BepF trigger the formation of a bacterial uptake structure termed the invasome. Invasome formation is a multi-step process consisting of bacterial adherence, effector translocation, aggregation of bacteria on the cell surface and engulfment, and eventually, complete internalization of the bacterial aggregate occurs in an F-actin-dependent manner. In the present study, we show that Bhe-triggered invasome formation depends on integrin-β1-mediated signaling cascades that enable assembly of the F-actin invasome structure. We demonstrate that Bhe interacts with integrin β1 in a fibronectin- and VirB/D4 T4SS-independent manner and that activated integrin β1 is essential for both effector translocation and the actin rearrangements leading to invasome formation. Furthermore, we show that talin1, but not talin2, is required for inside-out activation of integrin β1 during invasome formation. Finally, integrin-β1-mediated outside-in signaling by FAK, Src, paxillin and vinculin is necessary for invasome formation. This is the first example of a bacterial entry process that fully exploits the bi-directional signaling capacity of integrin receptors in a talin1-specific manner.

Serine phosphorylation of cortactin controls focal adhesion kinase activity and cell scattering induced by Helicobacter pylori

Cell migration and invasion require the coordinated regulation of cytoskeletal architectural changes by signaling factors, including the actin-binding protein cortactin. Bacterial and viral pathogens subvert these signaling factors to promote their uptake, spread and dissemination. We show that the gastric pathogen Helicobacter pylori (Hp) targets cortactin by two independent processes leading to its tyrosine dephosphorylation and serine phosphorylation to regulate cell scattering and elongation. The phosphorylation status of cortactin dictates its subcellular localization and signaling partners. Upon infection, cortactin was found to interact with and stimulate the kinase activity of focal adhesion kinase (FAK). This interaction required the SH3 domain and phosphorylation of cortactin at serine 405 and a proline-rich sequence in FAK. Using Hp as a model, this study unravels a previously unrecognized FAK activation pathway. We propose that Hp targets cortactin to protect the gastric epithelium from excessive cell lifting and ensure sustained infection in the stomach.

The phosphoinositide-3-kinase-Akt signaling pathway is important for Staphylococcus aureus internalization by endothelial cells

Internalization of Staphylococcus aureus in bovine endothelial cells (BEC) is increased by tumor necrosis factor alpha stimulation and NF-κB activation. Because the phosphoinositide-3-kinase (PI3K)-Akt signaling pathway also modulates NF-κB activity, we considered whether the internalization of S. aureus by BEC is associated with the activity of PI3K and Akt. We found a time- and multiplicity of infection-dependent phosphorylation of Akt on Ser473 in BEC infected with S. aureus. This phosphorylation was inhibited by LY294002 (LY), indicating the participation of PI3K. Inhibition of either PI3K with LY or wortmannin, or Akt with SH-5, strongly reduced the internalization of S. aureus. Transfection of BEC with a dominant-negative form of the Akt gene significantly decreased S. aureus internalization, whereas transfection with the constitutively active mutant increased the number of internalized bacterium. Inhibition of PDK1 activity with OSU-03012 did not affect the level of S. aureus internalization, demonstrating that phosphorylation of Akt on Thr308 is not important for this process. Compared to the untreated control, the adherence of S. aureus to the surface of BEC was unaltered when cells were transfected or incubated with the pharmacological inhibitors. Furthermore, Akt activation by internalized S. aureus triggered a time-dependent phosphorylation of glycogen synthase kinase-3α (GSK-3α) on Ser21 and GSK-3β on Ser9 that was partially inhibited with SH-5. Finally, treatment of BEC with LY prior to S. aureus infection inhibited the NF-κB p65 subunit phosphorylation on Ser536, indicating the involvement of PI3K. These results suggest that PI3K-Akt activity is important for the internalization of S. aureus and phosphorylation of GSK-3α, GSK-3β, and NF-κB.

Integrin-mediated uptake of fibronectin-binding bacteria

Invasion of mammalian cells via cell adhesion molecules of the integrin family is a common theme in bacterial pathogenesis. Whereas some microorganisms directly bind to integrins, other pathogens such as Staphylococcus aureus indirectly engage these receptors via fibronectin-binding proteins (FnBPs). In this review, we summarize the structure-function relationship of FnBPs and the current view of the role of these proteins during pathogenesis in vivo. A major focus will be on recent findings on the role of cholesterol- and sphingolipid-rich membrane microdomains for integrin-initiated uptake of fibronectin-binding bacteria and the surprising inhibitory function of caveolin-1 in this process. The detailed mechanistic understanding of host cell invasion by fibronectin-binding S. aureus can not only serve as a paradigm for other fibronectin-binding pathogenic bacteria, but might also reveal the physiological regulation of endocytosis of ligand-occupied integrins.

Role of fibronectin-binding proteins A and B in in vitro cellular infections and in vivo septic infections by Staphylococcus aureus

Fibronectin-binding protein A (FnBPA) and FnBPB are important adhesins for Staphylococcus aureus infection. We constructed fnbA and/or fnbB mutant strains from S. aureus SH1000, which possesses intact rsbU, and studied the role of these adhesins in in vitro and in vivo infections. In intravenous infection, all fnb mutants caused a remarkable reduction in the colonization rate in kidneys and the mortality rate of mice. fnbB mutant caused a more severe decrease in body weight than that caused by fnbA mutant. Serum levels of interleukin-6 and nuclear factor κB (NF-κB) activation in spleen cells were remarkably reduced in fnbA or fnbA fnbB mutant infections; however, there was no significant reduction in fnbB mutant infections. In in vitro cellular infection, FnBPA was shown to be indispensable for adhesion to and internalization by nonprofessional phagocytic cells upon ingestion by inflammatory macrophages and NF-κB activation. However, both FnBPs were required for efficient cellular responses. The results showed that FnBPA is more important for in vitro and in vivo infections; however, cooperation between FnBPA and FnBPB is indispensable for the induction of severe infection resulting in septic death.

Invasion of eukaryotic cells by Borrelia burgdorferi requires β(1) integrins and Src kinase activity

Lyme disease, caused by the bacterium Borrelia burgdorferi, is the most widespread tick-borne infection in the northern hemisphere that results in a multistage disorder with concomitant pathology, including arthritis. During late-stage experimental infection in mice, B. burgdorferi evades the adaptive immune response despite the presence of borrelia-specific bactericidal antibodies. In this study we asked whether B. burgdorferi could invade fibroblasts or endothelial cells as a mechanism to model the avoidance from humorally based clearance. A variation of the gentamicin protection assay, coupled with the detection of borrelial transcripts following gentamicin treatment, indicated that a portion of B. burgdorferi cells were protected in the short term from antibiotic killing due to their ability to invade cultured mammalian cells. Long-term coculture of B. burgdorferi with primary human fibroblasts provided additional support for intracellular protection. Furthermore, decreased invasion of B. burgdorferi in murine fibroblasts that do not synthesize the β(1) integrin subunit was observed, indicating that β(1)-containing integrins are required for optimal borrelial invasion. However, β(1)-dependent invasion did not require either the α(5)β(1) integrin or the borrelial fibronectin-binding protein BBK32. The internalization of B. burgdorferi was inhibited by cytochalasin D and PP2, suggesting that B. burgdorferi invasion required the reorganization of actin filaments and Src family kinases (SFK), respectively. Taken together, these results suggest that B. burgdorferi can invade and retain viability in nonphagocytic cells in a process that may, in part, help to explain the phenotype observed in untreated experimental infection.

Caveolin limits membrane microdomain mobility and integrin-mediated uptake of fibronectin-binding pathogens

Staphylococcus aureus, which is a leading cause of hospital-acquired infections, binds via fibronectin to integrin α5β1, a process that can promote host colonization in vivo. Integrin engagement induces actin cytoskeleton rearrangements that result in the uptake of S. aureus by non-professional phagocytic cells. Interestingly, we found that fibronectin-binding S. aureus trigger the redistribution of membrane microdomain components. In particular, ganglioside GM1 and GPI-linked proteins were recruited upon integrin β1 engagement, and disruption of membrane microdomains blocked bacterial internalization. Several membrane-microdomain-associated proteins, such as flotillin-1 and flotillin-2, as well as caveolin, were recruited to sites of bacterial attachment. Whereas dominant-negative versions of flotillin-2 did not affect bacterial attachment or internalization, cells deficient for caveolin-1 (Cav1(-/-)) showed increased uptake of S. aureus and other Fn-binding pathogens. Recruitment of membrane microdomains to cell-associated bacteria was unaltered in Cav1(-/-) cells. However, fluorescence recovery after photobleaching (FRAP) revealed an enhanced mobility of membrane-microdomain-associated proteins in the absence of caveolin-1. Enhanced membrane microdomain mobility and increased uptake of S. aureus was repressed by expression of wild-type caveolin-1, but not caveolin-1 G83S, which harbors a point mutation in the caveolin scaffolding domain. Similarly, chemical or physical stimulation of membrane fluidity led to increased uptake of S. aureus. These results highlight a crucial role for caveolin-1 in negative regulation of membrane microdomain mobility, thereby affecting endocytosis of bacteria-engaged integrins. This process might not only limit host cell invasion by integrin-binding bacterial pathogens, but might also be physiologically relevant for integrin-mediated cell adhesion.

Listeria monocytogenes internalin and E-cadherin: from bench to bedside

Listeria monocytogenes is a Gram-positive bacterium responsible for a severe infection associated with different clinical features (gastroenteritis, meningoencephalitis, and abortion in pregnant women). These pathologies are caused by the unusual capacity of the bacterium to cross three host barriers during infection and to invade nonphagocytic cells. To invade host cells, Listeria uses two proteins, InlA and InlB, which have specific receptors on the host-cell surface, E-cadherin and Met, respectively. Here, we discuss the specificity of the InlA-E-cadherin interaction, the signaling cascade activated on E-cadherin engagement by InlA, and the role of InlA and E-cadherin in the breaching of host barriers and the dissemination of the infection.

Polymeric immunoglobulin receptor-mediated invasion of Streptococcus pneumoniae into host cells requires a coordinate signaling of SRC family of protein-tyrosine kinases, ERK, and c-Jun N-terminal kinase

Streptococcus pneumoniae are commensals of the human nasopharynx with the capacity to invade mucosal respiratory cells. PspC, a pneumococcal surface protein, interacts with the human polymeric immunoglobulin receptor (pIgR) to promote bacterial adherence to and invasion into epithelial cells. Internalization of pneumococci requires the coordinated action of actin cytoskeleton rearrangements and the retrograde machinery of pIgR. Here, we demonstrate the involvement of Src protein-tyrosine kinases (PTKs), focal adhesion kinase (FAK), extracellular signal-regulated kinase (ERK), and c-Jun N-terminal kinase (JNK) but not p38 mitogen-activated protein kinases (MAPK) in pneumococcal invasion via pIgR. Pharmacological inhibitors of PTKs and MAPKs and genetic interference with Src PTK and FAK functions caused a significant reduction of pIgR-mediated pneumococcal invasion but did not influence bacterial adhesion to host cells. Furthermore, pneumococcal ingestion by host cells induces activation of ERK1/2 and JNK. In agreement with activated JNK, its target molecule and DNA-binding protein c-Jun was phosphorylated. We also show that functionally active Src PTK is essential for activation of ERK1/2 upon pneumococcal infections. In conclusion, these data illustrate the importance of a coordinated signaling between Src PTKs, ERK1/2, and JNK during PspC-pIgR-mediated uptake of pneumococci by host epithelial cells.