Escherichia coli interaction with human brain microvascular endothelial cells induces signal transducer and activator of transcription 3 association with the C-terminal domain of Ec-gp96, the outer membrane protein A receptor for invasion

SARS-CoV-2 nonstructural protein 6 triggers endoplasmic reticulum stress-induced autophagy to degrade STING1

ABBREVIATIONS

aa: amino acid; ATF6: activating transcription factor 6; ATG5: autophagy related 5; CCPG1: cell cycle progression 1; CFTR: CF transmembrane conductance regulator; cGAMP: cyclic GMP-AMP; CGAS: cyclic GMP-AMP synthase; CHX: cycloheximide; Co-IP: co-immunoprecipitation; CQ: chloroquine; EIF2A/eIF2α: eukaryotic translation initiation factor 2A; EIF2AK3/PERK: eukaryotic translation initiation factor 2 alpha kinase 3; ER: endoplasmic reticulum; ERN1/IRE1: endoplasmic reticulum to nucleus signaling 1; GFP: green fluorescent protein; HSPA5/GRP78: heat shock protein family A (Hsp70) member 5; HSV-1: herpes simplex virus type 1; IFIT1: interferon induced protein with tetratricopeptide repeats 1; IFNB1/IFN-β: interferon beta 1; IRF3: interferon regulatory factor 3; ISG15: ISG15 ubiquitin like modifier; MAP1LC3B/LC3B: microtubule associated protein 1 light chain 3 beta; MAP3K7/TAK1: mitogen-activated protein kinase kinase kinase 7; MAVS: mitochondrial antiviral signaling protein; MOI: multiplicity of infection; NFKB/NF-κB: nuclear factor kappa B; NSP6: non-structural protein 6; Δ106-108: deletion of amino acids 106-108 in NSP6 of SARS-CoV-2; Δ105-107: deletion of amino acids 105-107 in NSP6 of SARS-CoV-2; RETREG1/FAM134B: reticulophagy regulator 1; RIGI/DDX58: RNA sensor RIG-I; SQSTM1/p62: sequestosome 1; STING1: stimulator of interferon response cGAMP interactor 1; TBK1: TANK binding kinase 1.

DH5α Outer Membrane-Coated Biomimetic Nanocapsules Deliver Drugs to Brain Metastases but not Normal Brain Cells via Targeting GRP94

Receptor-mediated vesicular transport has been extensively developed to penetrate the blood-brain barrier (BBB) and has emerged as a class of powerful brain-targeting delivery technologies. However, commonly used BBB receptors such as transferrin receptor and low-density lipoprotein receptor-related protein 1, are also expressed in normal brain parenchymal cells and can cause drug distribution in normal brain tissues and subsequent neuroinflammation and cognitive impairment. Here, the endoplasmic reticulum residing protein GRP94 is found upregulated and relocated to the cell membrane of both BBB endothelial cells and brain metastatic breast cancer cells (BMBCCs) by preclinical and clinical investigations. Inspired by that Escherichia coli penetrates the BBB via the binding of its outer membrane proteins with GRP94, avirulent DH5α outer membrane protein-coated nanocapsules (Omp@NCs) are developed to cross the BBB, avert normal brain cells, and target BMBCCs via recognizing GRP94. Embelin (EMB)-loaded Omp@EMB specifically reduce neuroserpin in BMBCCs, which inhibits vascular cooption growth and induces apoptosis of BMBCCs by restoring plasmin. Omp@EMB plus anti-angiogenic therapy prolongs the survival of mice with brain metastases. This platform holds the translational potential to maximize therapeutic effects on GRP94-positive brain diseases.

Lpp of Escherichia coli K1 inhibits host ROS production to counteract neutrophil-mediated elimination

Escherichia coli (E. coli) is the most common Gram-negative bacterial organism causing neonatal meningitis. The pathogenesis of E. coli meningitis, especially how E. coli escape the host immune defenses, remains to be clarified. Here we show that deletion of bacterial Lpp encoding lipoprotein significantly reduces the pathogenicity of E. coli K1 to induce high-degree of bacteremia necessary for meningitis. The Lpp-deleted E. coli K1 is found to be susceptible to the intracellular bactericidal activity of neutrophils, without affecting the release of neutrophil extracellular traps. The production of reactive oxygen species (ROS), representing the primary antimicrobial mechanism in neutrophils, is significantly increased in response to Lpp-deleted E. coli. We find this enhanced ROS response is associated with the membrane translocation of NADPH oxidase p47^phox and p67^phox in neutrophils. Then we constructed p47^phox knockout mice and we found the incidence of bacteremia and meningitis in neonatal mice induced by Lpp-deleted E. coli is significantly recovered by p47^phox knockout. Proteomic profile analysis show that Lpp deficiency induces upregulation of flagellar protein FliC in E. coli. We further demonstrate that FliC is required for the ROS induction in neutrophils by Lpp-deleted E. coli. Taken together, these data uncover the novel role of Lpp in facilitating intracellular survival of E. coli K1 within neutrophils. It can be inferred that Lpp of E. coli K1 is able to suppress FliC expression to restrain the activation of NADPH oxidase in neutrophils resulting in diminished bactericidal activity, thus protecting E. coli K1 from the elimination by neutrophils.

Biomimetic Lipopolysaccharide-Free Bacterial Outer Membrane-Functionalized Nanoparticles for Brain-Targeted Drug Delivery

The blood-brain barrier (BBB) severely blocks the intracranial accumulation of most systemic drugs. Inspired by the contribution of the bacterial outer membrane to Escherichia coli K1 (EC-K1) binding to and invasion of BBB endothelial cells in bacterial meningitis, utilization of the BBB invasion ability of the EC-K1 outer membrane for brain-targeted drug delivery and construction of a biomimetic self-assembled nanoparticle with a surface featuring a lipopolysaccharide-free EC-K1 outer membrane are proposed. BBB penetration of biomimetic nanoparticles is demonstrated to occur through the transcellular vesicle transport pathway, which is at least partially dependent on internalization, endosomal escape, and transcytosis mediated by the interactions between outer membrane protein A and gp96 on BBB endothelial cells. This biomimetic nanoengineering strategy endows the loaded drugs with prolonged circulation, intracranial interstitial distribution, and extremely high biocompatibility. Based on the critical roles of gp96 in cancer biology, this strategy reveals enormous potential for delivering therapeutics to treat gp96-overexpressing intracranial malignancies.

GP96 Drives Exacerbation of Secondary Bacterial Pneumonia following Influenza A Virus Infection

Influenza A virus (IAV) infection predisposes the host to secondary bacterial pneumonia, known as a major cause of morbidity and mortality during influenza virus epidemics. Analysis of interactions between IAV-infected human epithelial cells and Streptococcus pneumoniae revealed that infected cells ectopically exhibited the endoplasmic reticulum chaperone glycoprotein 96 (GP96) on the surface. Importantly, efficient pneumococcal adherence to epithelial cells was imparted by interactions with extracellular GP96 and integrin α_V, with the surface expression mediated by GP96 chaperone activity. Furthermore, abrogation of adherence was gained by chemical inhibition or genetic knockout of GP96 as well as addition of RGD peptide, an inhibitor of integrin-ligand interactions. Direct binding of extracellular GP96 and pneumococci was shown to be mediated by pneumococcal oligopeptide permease components. Additionally, IAV infection induced activation of calpains and Snail1, which are responsible for degradation and transcriptional repression of junctional proteins in the host, respectively, indicating increased bacterial translocation across the epithelial barrier. Notably, treatment of IAV-infected mice with the GP96 inhibitor enhanced pneumococcal clearance from lung tissues and ameliorated lung pathology. Taken together, the present findings indicate a viral-bacterial synergy in relation to disease progression and suggest a paradigm for developing novel therapeutic strategies tailored to inhibit pneumococcal colonization in an IAV-infected respiratory tract.

Cell entry of a host-targeting protein of oomycetes requires gp96

The animal-pathogenic oomycete Saprolegnia parasitica causes serious losses in aquaculture by infecting and killing freshwater fish. Like plant-pathogenic oomycetes, S. parasitica employs similar infection structures and secretes effector proteins that translocate into host cells to manipulate the host. Here, we show that the host-targeting protein SpHtp3 enters fish cells in a pathogen-independent manner. This uptake process is guided by a gp96-like receptor and can be inhibited by supramolecular tweezers. The C-terminus of SpHtp3 (containing the amino acid sequence YKARK), and not the N-terminal RxLR motif, is responsible for the uptake into host cells. Following translocation, SpHtp3 is released from vesicles into the cytoplasm by another host-targeting protein where it degrades nucleic acids. The effector translocation mechanism described here, is potentially also relevant for other pathogen-host interactions as gp96 is found in both animals and plants.

Escherichia coli K1 utilizes host macropinocytic pathways for invasion of brain microvascular endothelial cells

Eukaryotic cells utilize multiple endocytic pathways for specific uptake of ligands or molecules, and these pathways are commonly hijacked by pathogens to enable host cell invasion. Escherichia coli K1, a pathogenic bacterium that causes neonatal meningitis, invades the endothelium of the blood-brain barrier, but the entry route remains unclear. Here, we demonstrate that the bacteria trigger an actin-mediated uptake route, stimulating fluid phase uptake, membrane ruffling and macropinocytosis. The route of uptake requires intact lipid rafts as shown by cholesterol depletion. Using a variety of perturbants we demonstrate that small Rho GTPases and their downstream effectors have a significant effect on bacterial invasion. Furthermore, clathrin-mediated endocytosis appears to play an indirect role in E. coli K1 uptake. The data suggest that the bacteria effect a complex interplay between the Rho GTPases to increase their chances of uptake by macropinocytosis into human brain microvascular endothelial cells.

The effects of cytotoxic necrotizing factor 1 expression in the uptake of Escherichia coli K1 by macrophages and the onset of meningitis in newborn mice

Macrophages are a permissive niche for E. coli K1 multiplication for which the interaction of the bacterial outer membrane protein A and its cognate receptor CD64 are critical. Using in vitro immunofluorescence and live microscopy with ex vivo macrophage cultures from RFP-Lifeact mice, we show that cytotoxic necrotizing factor 1 (CNF1) secreted by E. coli K1 sequesters cellular actin toward microspike formation, thereby limiting actin availability for OmpA-mediated bacterial invasion. Surprisingly, the observed effects of CNF1 occur despite the absence of 67-kDa laminin receptor in macrophages. Concomitantly, the CNF1 deletion mutant of E. coli K1 (Δcnf1) invades macrophages and the brains of newborn mice in greater numbers compared to wild-type. However, the Δcnf1 strain induces less severe pathology in the brain. These results suggest a novel role for CNF1 in limiting E. coli K1 entry into macrophages while exacerbating disease severity in the brains of newborn mice.

Host-pathogen interactions in bacterial meningitis

Bacterial meningitis is a devastating disease occurring worldwide with up to half of the survivors left with permanent neurological sequelae. Due to intrinsic properties of the meningeal pathogens and the host responses they induce, infection can cause relatively specific lesions and clinical syndromes that result from interference with the function of the affected nervous system tissue. Pathogenesis is based on complex host–pathogen interactions, some of which are specific for certain bacteria, whereas others are shared among different pathogens. In this review, we summarize the recent progress made in understanding the molecular and cellular events involved in these interactions. We focus on selected major pathogens, Streptococcus pneumonia, S. agalactiae (Group B Streptococcus), Neisseria meningitidis, and Escherichia coli K1, and also include a neglected zoonotic pathogen, Streptococcus suis. These neuroinvasive pathogens represent common themes of host–pathogen interactions, such as colonization and invasion of mucosal barriers, survival in the blood stream, entry into the central nervous system by translocation of the blood–brain and blood–cerebrospinal fluid barrier, and induction of meningeal inflammation, affecting pia mater, the arachnoid and subarachnoid spaces.

Pertussis Toxin Exploits Specific Host Cell Signaling Pathways for Promoting Invasion and Translocation of Escherichia coli K1 RS218 in Human Brain-derived Microvascular Endothelial Cells

Pertussis toxin (PTx), an AB5 toxin and major virulence factor of the whooping cough-causing pathogen Bordetella pertussis, has been shown to affect the blood-brain barrier. Dysfunction of the blood-brain barrier may facilitate penetration of bacterial pathogens into the brain, such as Escherichia coli K1 (RS218). In this study, we investigated the influence of PTx on blood-brain barrier permissiveness to E. coli infection using human brain-derived endothelial HBMEC and TY10 cells as in vitro models. Our results indicate that PTx acts at several key points of host cell intracellular signaling pathways, which are also affected by E. coli K1 RS218 infection. Application of PTx increased the expression of the pathogen binding receptor gp96. Further, we found an activation of STAT3 and of the small GTPase Rac1, which have been described as being essential for bacterial invasion involving host cell actin cytoskeleton rearrangements at the bacterial entry site. In addition, we showed that PTx induces a remarkable relocation of VE-cadherin and β-catenin from intercellular junctions. The observed changes in host cell signaling molecules were accompanied by differences in intracellular calcium levels, which might act as a second messenger system for PTx. In summary, PTx not only facilitates invasion of E. coli K1 RS218 by activating essential signaling cascades; it also affects intercellular barriers to increase paracellular translocation.

Identification of minimum carbohydrate moiety in N-glycosylation sites of brain endothelial cell glycoprotein 96 for interaction with Escherichia coli K1 outer membrane protein A

Bacterial meningitis is a serious central nervous system infection and Escherichia coli K1 (E. coli K1) is one of the leading etiological agents that cause meningitis in neonates. Outer membrane protein A (OmpA) of E. coli K1 is a major virulence factor in the pathogenesis of meningitis, and interacts with human brain microvascular endothelial cells (HBMEC) to cross the blood-brain barrier. Using site-directed mutagenesis, we demonstrate that two N-glycosylation sites (NG1 and NG2) in the extracellular domain of OmpA receptor, Ecgp96 are critical for bacterial binding to HBMEC. E. coli K1 invasion assays using CHO-Lec1 cells that express truncated N-glycans, and sequential digestion of HBMEC surface N-glycans using specific glycosidases showed that GlcNAc1-4GlcNAc epitopes are sufficient for OmpA interaction with HBMEC. Lack of NG1 and NG2 sites in Ecgp96 inhibits E. coli K1 OmpA induced F-actin polymerization, phosphorylation of protein kinase C-α, and disruption of transendothelial electrical resistance required for efficient invasion of E. coli K1 in HBMEC. Furthermore, the microvessels of cortex and hippocampus of the brain sections of E. coli K1 infected mice showed increased expression of glycosylated Ecgp96. Therefore, the interface of OmpA and GlcNAc1-4GlcNAc epitope interaction would be a target for preventative strategies against E. coli K1 meningitis.

Listeria monocytogenes triggers the cell surface expression of Gp96 protein and interacts with its N terminus to support cellular infection

Listeria monocytogenes is an intracellular food-borne pathogen causing listeriosis in humans. This bacterium deploys an arsenal of virulence factors that act in concert to promote cellular infection. Bacterial surface proteins are of primary importance in the process of host cell invasion. They interact with host cellular receptors, inducing/modulating specific cellular responses. We previously identified Vip, a Listeria surface protein covalently attached to the bacterial cell wall acting as a key virulence factor. We have shown that Vip interacts with Gp96 localized at the surface of host cells during invasion and that this interaction is critical for a successful infection in vivo. To better understand the importance of Vip-Gp96 interaction during infection, we aimed to characterize this interaction at the molecular level. Here we demonstrate that, during infection, L. monocytogenes triggers the cellular redistribution of Gp96, inducing its exposure at the cell surface. Upon infection, Gp96 N-terminal domain is exposed to the extracellular milieu in L2071 fibroblasts and interacts with Vip expressed by Listeria. We identified Gp96 (Asp(1)-Leu(170)) as sufficient to interact with Vip; however, we also showed that the region Tyr(179)-Leu(390) of Gp96 is important for the interaction. Our findings unravel the Listeria-induced surface expression of Gp96 and the topology of its insertion on the plasma membrane and improve our knowledge on the Vip-Gp96 interaction during Listeria infection.

Regulation of Toll-like receptor 2 interaction with Ecgp96 controls Escherichia coli K1 invasion of brain endothelial cells

The interaction of outer membrane protein A (OmpA) with its receptor, Ecgp96 (a homologue of Hsp90β), is critical for the pathogenesis of Escherichia coli K1 meningitis. Since Hsp90 chaperones Toll-like receptors (TLRs), we examined the role of TLRs in E. coli K1 infection. Herein, we show that newborn TLR2(-/-) mice are resistant to E. coli K1 meningitis, while TLR4(-/-) mice succumb to infection sooner. In vitro, OmpA+ E. coli infection selectively upregulates Ecgp96 and TLR2 in human brain microvascular endothelial cells (HBMEC), whereas OmpA- E. coli upregulates TLR4 in these cells. Furthermore, infection with OmpA+ E. coli causes Ecgp96 and TLR2 translocate to the plasma membrane of HBMEC as a complex. Immunoprecipitation studies of the plasma membrane fractions from infected HBMEC reveal that the C termini of Ecgp96 and TLR2 are critical for OmpA+ E. coli invasion. Knockdown of TLR2 using siRNA results in inefficient membrane translocation of Ecgp96 and significantly reduces invasion. In addition, the interaction of Ecgp96 andTLR2 induces a bipartite signal, one from Ecgp96 through PKC-α while the other from TLR2 through MyD88, ERK1/2 and NF-κB. This bipartite signal ultimately culminates in the efficient production of NO, which in turn promotes E. coli K1 invasion of HBMEC.

Campylobacter jejuni-mediated induction of CC and CXC chemokines and chemokine receptors in human dendritic cells

Campylobacter jejuni is a leading worldwide bacterial cause of human diarrheal disease. Although the specific molecular mechanisms of C. jejuni pathogenesis have not been characterized in detail, host inflammatory responses are thought to be major contributing factors to the resulting typical acute colitis. The intestinal mucosal chemokine response is particularly important in the initial stages of bacterium-induced gut inflammation. Chemokines attract blood phagocytes and lymphocytes to the site of infection and regulate immune cell maturation and the development of localized lymphoid tissues. The production of chemokines by dendritic cells (DCs) following Campylobacter infection has not yet been analyzed. In the current study, we infected human monocyte-derived DCs with C. jejuni to examine the production of key proinflammatory chemokines and chemokine receptors. The chemokines, including CC families (macrophage inflammatory protein 1α [MIP-1α], MIP-1β, RANTES) and CXC families (growth-related oncogene α [GRO-α], IP-10, and monokine induced by gamma interferon [MIG]), were upregulated in Campylobacter-infected DCs. Chemokine receptors CCR6 and CCR7, with roles in DC trafficking, were also induced in Campylobacter-infected DCs. Further, Campylobacter infection stimulated the phosphorylation of P38, P44/42, and stress-activated protein kinase/Jun N-terminal kinase (SAPK/JNK) mitogen-activated protein kinases (MAPKs) in DCs. NF-κB activation was specifically involved in chemokine induction in DCs infected with C. jejuni. Additionally, STAT3 was significantly increased in Campylobacter-infected DCs compared to that in uninfected DCs. These results suggest that DCs play a significant role in the initiation and modulation of the inflammatory response by enlisting monocytes, neutrophils, and T lymphocytes during human intestinal infection with Campylobacter.

IQGAP1 mediates the disruption of adherens junctions to promote Escherichia coli K1 invasion of brain endothelial cells

The transcellular entry of Escherichia coli K1 through human brain microvascular endothelial cells (HBMEC) is responsible for tight junction disruption, leading to brain oedema in neonatal meningitis. Previous studies demonstrated that outer membrane protein A (OmpA) of E. coli K1 interacts with its receptor, Ecgp96, to induce PKC-α phosphorylation, adherens junction (AJ) disassembly (by dislodging β-catenin from VE-cadherin), and remodelling of actin in HBMEC. We report here that IQGAP1 mediates β-catenin dissociation from AJs to promote actin polymerization required for E. coli K1 invasion of HBMEC. Overexpression of C-terminal truncated IQGAP1 (IQΔC) that cannot bind β-catenin prevents both AJ disruption and E. coli K1 entry. Of note, phospho-PKC-α interacts with the C-terminal portion of Ecgp96 as well as with VE-cadherin after IQGAP1-mediated AJ disassembly. HBMEC overexpressing either C-terminal truncated Ecgp96 (Ecgp96Δ200) or IQΔC upon infection with E. coli showed no interaction of phospho-PKC-α with Ecgp96. These data indicate that the binding of OmpA to Ecgp96 induces PKC-α phosphorylation and association of phospho-PKC-α with Ecgp96, and then signals IQGAP1 to detach β-catenin from AJs. Subsequently, IQGAP1/β-catenin bound actin translocates to the site of E. coli K1 attachment to promote invasion.

IbeA and OmpA of Escherichia coli K1 exploit Rac1 activation for invasion of human brain microvascular endothelial cells

Meningitis-causing Escherichia coli K1 internalization of the blood-brain barrier is required for penetration into the brain, but the host-microbial interactions involved in E. coli entry of the blood-brain barrier remain incompletely understood. We show here that a meningitis-causing E. coli K1 strain RS218 activates Rac1 (GTP-Rac1) of human brain microvascular endothelial cells (HBMEC) in a time-dependent manner. Both activation and bacterial invasion were significantly inhibited in the presence of a Rac1 inhibitor. We further showed that the guanine nucleotide exchange factor Vav2, not β-Pix, was involved in E. coli K1-mediated Rac1 activation. Since activated STAT3 is known to bind GTP-Rac1, the relationship between STAT3 and Rac1 was examined in E. coli K1 invasion of HBMEC. Downregulation of STAT3 resulted in significantly decreased E. coli invasion compared to control HBMEC, as well as a corresponding decrease in GTP-Rac1, suggesting that Rac1 activation in response to E. coli is under the control of STAT3. More importantly, two E. coli determinants contributing to HBMEC invasion, IbeA and OmpA, were shown to affect both Rac1 activation and their association with STAT3. These findings demonstrate for the first time that specific E. coli determinants regulate a novel mechanism of STAT3 cross talk with Rac1 in E. coli K1 invasion of HBMEC.

Outer membrane protein A and OprF: versatile roles in Gram-negative bacterial infections

Outer membrane protein A (OmpA) is an abundant protein of Escherichia coli and other enterobacteria and has a multitude of functions. Although the structural features and porin function of OmpA have been well studied, its role in the pathogenesis of various bacterial infections has emerged only during the last decade. The four extracellular loops of OmpA interact with a variety of host tissues for adhesion to and invasion of the cell and for evasion of host-defense mechanisms when inside the cell. This review describes how various regions present in the extracellular loops of OmpA contribute to the pathogenesis of neonatal meningitis induced by E. coli K1 and to many other functions. In addition, the function of OmpA-like proteins, such as OprF of Pseudomonas aeruginosa, is discussed.

Escherichia coli K1 invasion of human brain microvascular endothelial cells

The pathogenic Escherichia coli strain E. coli K1 is a primary causative agent of neonatal meningitis. Understanding how these bacteria cross the blood-brain barrier is vital to develop therapeutics. Here, we describe the use of live-cell imaging techniques to study E. coli K1 interactions with cellular markers following infection of human brain microvascular endothelial cells, a model system of the blood-brain barrier. We also discuss optimization of endothelial cell transfection conditions using nonviral transfection technique, bacterial labeling techniques, and in vitro assays to screen for fluorescent bacteria that retain their ability to invade host cells.

Mechanisms of Candida albicans trafficking to the brain

During hematogenously disseminated disease, Candida albicans infects most organs, including the brain. We discovered that a C. albicans vps51Δ/Δ mutant had significantly increased tropism for the brain in the mouse model of disseminated disease. To investigate the mechanisms of this enhanced trafficking to the brain, we studied the interactions of wild-type C. albicans and the vps51Δ/Δ mutant with brain microvascular endothelial cells in vitro. These studies revealed that C. albicans invasion of brain endothelial cells is mediated by the fungal invasins, Als3 and Ssa1. Als3 binds to the gp96 heat shock protein, which is expressed on the surface of brain endothelial cells, but not human umbilical vein endothelial cells, whereas Ssa1 binds to a brain endothelial cell receptor other than gp96. The vps51Δ/Δ mutant has increased surface expression of Als3, which is a major cause of the increased capacity of this mutant to both invade brain endothelial cells in vitro and traffic to the brain in mice. Therefore, during disseminated disease, C. albicans traffics to and infects the brain by binding to gp96, a unique receptor that is expressed specifically on the surface of brain endothelial cells.

During hematogenously disseminated infection, the fungus Candida albicans is carried by the bloodstream to virtually all organs in the body, including the brain. C. albicans infection of the brain is a significant problem in premature infants with disseminated candidiasis. To infect the brain, C. albicans must adhere to and invade the endothelial cells that line cerebral blood vessels. These endothelial cells express unique proteins on their surface that are not expressed by endothelial cells of other vascular beds. Here, we show that C. albicans infects the brain by binding to gp96, a heat shock protein that is uniquely expressed on the surface of brain endothelial cells. Gp96 is bound by the C. albicans Als3 invasin, which induces the uptake of this organism by brain endothelial cells. The C. albicans Ssa1 invasin also mediates fungal uptake by brain endothelial cells, but does so by binding to a receptor other than gp96. Thus, during hematogenously disseminated infection, C. albicans traffics to and infects the brain by binding to gp96, a receptor that is expressed specifically on the surface of brain endothelial cells.

Bacterial virulence in the moonlight: multitasking bacterial moonlighting proteins are virulence determinants in infectious disease

Men may not be able to multitask, but it is emerging that proteins can. This capacity of proteins to exhibit more than one function is termed protein moonlighting, and, surprisingly, many highly conserved proteins involved in metabolic regulation or the cell stress response have a range of additional biological actions which are involved in bacterial virulence. This review highlights the multiple roles exhibited by a range of bacterial proteins, such as glycolytic and other metabolic enzymes and molecular chaperones, and the role that such moonlighting activity plays in the virulence characteristics of a number of important human pathogens, including Staphylococcus aureus, Streptococcus pyogenes, Streptococcus pneumoniae, Helicobacter pylori, and Mycobacterium tuberculosis.

Elevated inflammatory response in caveolin-1-deficient mice with Pseudomonas aeruginosa infection is mediated by STAT3 protein and nuclear factor kappaB (NF-kappaB)

Caveolin-1 (Cav-1), an important composition protein within the flask-shaped membrane invaginations termed caveolae, may play a role in host defense against infections. However, the phenotype in Pseudomonas aeruginosa-infected cav1 knock-out (KO) mice is still unresolved, and the mechanism involved is almost entirely unknown. Using a respiratory infection model, we confirmed a crucial role played by Cav-1 in host defense against this pathogen because Cav-1 KO mice showed increased mortality, severe lung injury, and systemic dissemination as compared with wild-type (WT) littermates. In addition, cav1 KO mice exhibited elevated inflammatory cytokines (IL-6, TNF-α, and IL-12a), decreased phagocytic ability of macrophages, and increased superoxide release in the lung, liver, and kidney. We further studied relevant cellular signaling processes and found that STAT3 and NF-κB are markedly activated. Our data revealed that the Cav-1/STAT3/NF-κB axis is responsible for a dysregulated cytokine response, which contributes to increased mortality and disease progression. Moreover, down-regulating Cav-1 in cell culture with a dominant negative strategy demonstrated that STAT3 activation was essential for the translocation of NF-κB into the nucleus, confirming the observations from cav1 KO mice. Collectively, our studies indicate that Cav-1 is critical for inflammatory responses regulating the STAT3/NF-κB pathway and thereby impacting P. aeruginosa infection.

Involvement of Src tyrosine kinase in Escherichia coli invasion of human brain microvascular endothelial cells

Invasion of brain microvascular endothelial cells is a prerequisite for successful crossing of the blood-brain barrier by Escherichia coli (E. coli), but the underlying mechanism remains unclear. Here we showed activation of Src tyrosine kinase in E. coli K1 invasion of human brain microvascular endothelial cells (HBMEC). E. coli invasion of HBMEC and the E. coli-induced rearrangement of actin filaments were blocked by Src inhibitors. Overexpression of dominant-negative Src in HBMEC significantly attenuated E. coli invasion and the concomitant actin filaments rearrangement. Furthermore, E. coli K1-triggered phosphatidylinositol 3-kinase (PI3K) activation in HBMEC was effectively blocked by Src inhibitors and dominant-negative Src. These results demonstrated the involvement of Src and its interaction with PI3K in E. coli K1 invasion of HBMEC.

Nitric oxide/cGMP signalling induces Escherichia coli K1 receptor expression and modulates the permeability in human brain endothelial cell monolayers during invasion

Escherichia coli K1 invasion of human brain microvascular endothelial cells (HBMEC) mediated by outer membrane protein A (OmpA) results in the leakage of HBMEC monolayers. Despite the influence of nitric oxide (NO) in endothelial cell tight junction integrity, its role in E. coli-induced HBMEC monolayer permeability is poorly defined. Here, we demonstrate that E. coli invasion of HBMEC stimulates NO production by increasing the inducible nitric oxide synthase (iNOS) expression. Exposure to NO-producing agents enhanced the invasion of OmpA(+)E. coli and thereby increased the permeability of HBMEC. OmpA(+)E. coli-induced NO production lead to increased generation of cGMP and triggered the expression of OmpA receptor, Ec-gp96 in HBMEC. Pre-treatment of HBMEC with iNOS inhibitors or by introducing siRNA to iNOS, but not to eNOS or cGMP inhibitors abrogated the E. coli-induced expression of Ec-gp96. Overexpression of the C-terminal truncated Ec-gp96 in HBMEC prevented NO production and its downstream effector, cGMP generation and consequently, the invasion of OmpA(+)E. coli. NO/cGMP production also activates PKC-alpha, which is previously shown to be involved in HBMEC monolayer leakage. These results indicate that NO/cGMP signalling pathway plays a novel role in OmpA(+)E. coli invasion of HBMEC by enhancing the surface expression of Ec-gp96.