Activation of Raf/MEK/ERK/cPLA2 Signaling Pathway Is Essential for Chlamydial Acquisition of Host Glycerophospholipids

Reduced display of tumor necrosis factor receptor I at the host cell surface supports infection with Chlamydia trachomatis

The obligate intracellular human pathogenic bacterium Chlamydia trachomatis has evolved multiple mechanisms to circumvent the host immune system. Infected cells exhibit a profound resistance to the induction of apoptosis and down-regulate the expression of major histocompatibility complex class I and class II molecules to evade the cytotoxic effect of effector immune cells. Here we demonstrate the down-regulation of tumor necrosis factor receptor 1 (TNFR1) on the surface of infected cells. Interestingly, other members of the TNFR family such as TNFR2 and CD95 (Fas/Apo-1) were not modulated during infection, suggesting a selective mechanism underlying surface reduction of TNFR1. The observed effect was not due to reduced expression since the overall amount of TNFR1 protein was increased in infected cells. TNFR1 accumulated at the chlamydial inclusion and was shed by the infected cell into the culture supernatant. Receptor shedding depended on the infection-induced activation of the MEK-ERK pathway and the metalloproteinase TACE (TNFalpha converting enzyme). Our results point to a new function of TNFR1 modulation by C. trachomatis in controlling inflammatory signals during infection.

Intracellular interleukin-1alpha mediates interleukin-8 production induced by Chlamydia trachomatis infection via a mechanism independent of type I interleukin-1 receptor

Chlamydia trachomatis infection induces a wide array of inflammatory cytokines and chemokines, which may contribute to chlamydia-induced pathologies. However, the precise mechanisms by which Chlamydia induces cytokines remain unclear. Here we demonstrate that the proinflammatory cytokine interleukin-1alpha (IL-1alpha) plays an essential role in chlamydial induction of the chemokine IL-8. Cells deficient in IL-1alpha expression or IL-1alpha-competent cells treated with IL-1alpha-specific small interfering RNA failed to produce IL-8 in response to chlamydial infection. However, neutralization of extracellular IL-1alpha or blockade of or deficiency in type I IL-1 receptor (IL-1RI) signaling did not affect chlamydial induction of IL-8 in cells capable of producing IL-1alpha. These results suggest that IL-1alpha can mediate the chlamydial induction of IL-8 via an intracellular mechanism independent of IL-1RI, especially during the early stage of the infection cycle. This conclusion is further supported by the observations that expression of a transgene-encoded full-length IL-1alpha fusion protein in the nuclei enhanced IL-8 production and that nuclear localization of chlamydia-induced precursor IL-1alpha correlated with chlamydial induction of IL-8. Thus, we have identified a novel mechanism for chlamydial induction of the chemokine IL-8.

Rottlerin inhibits chlamydial intracellular growth and blocks chlamydial acquisition of sphingolipids from host cells

We report that rottlerin, a plant-derived compound known to inhibit various mammalian kinases, profoundly inhibited chlamydial growth in cell culture with a minimal inhibition concentration of 1 microM. The inhibition was effective even when rottlerin was added as late as the middle stage of chlamydial infection cycle, against multiple Chlamydia species, and in different host cell lines. Pretreatment of host cells with rottlerin prior to infection also blocked chlamydial growth, suggesting that rottlerin targets host factors. Moreover, rottlerin did not alter the chlamydial infection rate and did not directly target chlamydial protein synthesis and secretion. The rottlerin-mediated inhibition of chlamydial replication and inclusion expansion correlated well with the rottlerin-induced blockade of host cell sphingolipid trafficking from the Golgi apparatus into chlamydial inclusions. These studies not only allowed us to identify a novel antimicrobial activity for rottlerin but also allowed us to uncover a potential mechanism for rottlerin inhibition of chlamydial growth.

Effect of host fatty acid-binding protein and fatty acid uptake on growth of Chlamydia trachomatis L2

Chlamydia trachomatis is an obligate intracellular bacterium and acquires both building blocks and energy from host cells for growth. The fatty acid-binding protein (FABP) plays an important role in uptake of long-chain fatty acids (LCFA) and energy metabolism by eukaryotic cells. The roles of FABP and LCFA in chlamydial infection were evaluated. Infection of liver cells with chlamydial organisms promoted fatty acid uptake by the infected cells, suggesting that LCFA may benefit chlamydial growth. Introduction of FABP into the liver cells not only enhanced fatty acid uptake, but also increased chlamydial intravacuolar replication and maturation. The FABP-enhanced chlamydial intracellular growth was dependent on the host cell uptake of fatty acids. These results have demonstrated that C. trachomatis can productively infect liver cells and utilize FABP-transported LCFA for its own biosynthesis.

P2X receptors-mediated cytosolic phospholipase A2activation in primary afferent sensory neurons contributes to neuropathic pain

Activation of P2X(3) and P2X(2/3) receptors (P2X(3)R/P2X(2/3)R), ionotropic ATP receptor subtypes, in primary sensory neurons is involved in neuropathic pain, a debilitating chronic pain that occurs after peripheral nerve injury. However, the underlying mechanisms remain unknown. We investigated the role of cytosolic phospholipase A(2) (cPLA(2)) as a downstream molecule that mediates the P2X(3)R/P2X(2/3)R-dependent neuropathic pain. We found that applying ATP to cultured dorsal root ganglion (DRG) neurons increased the level of Ser505-phosphorylated cPLA(2) and caused translocation of Ser505-phosphorylated cPLA(2) to the plasma membrane. The ATP-induced cPLA(2) activation was inhibited by a selective antagonist of P2X(3)R/P2X(2/3)R and by a selective inhibitor of cPLA(2). In the DRG in vivo, the number of cPLA(2)-activated neurons was strikingly increased after peripheral nerve injury but not after peripheral inflammation produced by complete Freund's adjuvant. Pharmacological blockade of P2X(3)R/P2X(2/3)R reversed the nerve injury-induced cPLA(2) activation in DRG neurons. Moreover, administering the cPLA(2) inhibitor near the DRG suppressed nerve injury-induced tactile allodynia, a hallmark of neuropathic pain. Our results suggest that P2X(3)R/P2X(2/3)R-dependent cPLA(2) activity in primary sensory neurons is a key event in neuropathic pain and that cPLA(2) might be a potential target for treating neuropathic pain.

The extracellular signal-regulated kinase/mitogen-activated protein kinase pathway induces the inflammatory factor interleukin-8 following Chlamydia trachomatis infection

Diseases associated with Chlamydia infection, such as pelvic inflammatory disease and ectopic pregnancy, are due to inflammation-mediated tissue damage and scarring that occur after chronic or repeated infections. The inflammatory chemokine interleukin-8 (IL-8) is produced by Chlamydia-infected cells through an endogenous mechanism of activation, independent of soluble factors in the supernatant. The host signaling pathways necessary for this response are not understood, but the mitogen-activated protein kinase (MAPK) extracellular signal-regulated kinase (ERK) has been shown to be activated at similar times as IL-8 mRNA up-regulation. The purpose of this study was to elucidate the MAPK pathways necessary to induce the endogenous IL-8 response to Chlamydia trachomatis infection of epithelial cells. IL-8 induced by infection with C. trachomatis L2 was shown to be dependent on ERK and independent of p38 and Jun N-terminal MAPK by use of chemical inhibitors of the signaling pathways. Persistent ERK activation during IL-8 mRNA production at 24 h postinfection was necessary to maintain the response. C. trachomatis serovar D also induced IL-8 in an ERK-dependent manner. We concluded that IL-8 induced during infection of epithelial cells is dependent on continual activation of ERK by C. trachomatis.

Characterization of hypothetical proteins Cpn0146, 0147, 0284 & 0285 that are predicted to be in the Chlamydia pneumoniae inclusion membrane

BACKGROUND

Although more than 100 Chlamydia pneumoniae hypothetical proteins have been predicted to be inclusion membrane proteins, only a few have been experimentally demonstrated to be in the inclusion membrane. Using antibodies raised with fusion proteins, we characterized four such hypothetical proteins encoded by two gene clusters (Cpn0146-147 and Cpn0284-285) in the C. pneumoniae genome.

RESULTS

Cpn0146 and 0147 were detected in the inclusion membrane while Cpn0284 and 0285 inside inclusion and mainly associated with reticulate bodies although all four proteins contain an N-terminal bi-lobed hydrophobic region, a signature motif assigned to inclusion membrane proteins. These four hypothetical proteins were only detected in cells infected with C. pneumoniae but not other chlamydial species, with Cpn0147 at 6 hours and Cpn0146, 0284 & 0285 at 24 hours after infection. Cpn0146 & 147 but not Cpn0284 and 285 co-localized with a host cell endoplasmic reticulum marker, a property known to be possessed by some chlamydial inclusion membrane proteins, when expressed in the host cell cytosol via transgenes. However, the endoplasmic reticulum localization of the C. pneumoniae inclusion membrane proteins did not result in inhibition of the subsequent C. pneumoniae infection.

CONCLUSION

The hypothetical proteins Cpn0146 & 0147 were localized in the C. pneumoniae inclusion membrane while Cpn0284 & 0285 within the inclusion although all four were predicted to be Inc proteins, suggesting the need to experimentally characterize the predicted Inc proteins.

Localization of the hypothetical protein Cpn0585 in the inclusion membrane of Chlamydia pneumoniae-infected cells

Cpn0585, encoded by a hypothetical open reading frame in Chlamydia pneumoniae genome, was detected in the inclusion membrane during C. pneumoniae infection using both polyclonal and monoclonal antibodies raised with Cpn0585 fusion protein. The anti-Cpn0585 antibodies specifically recognized the endogenous Cpn0585 without cross-reacting with IncA (a known inclusion membrane protein of C. pneumoniae) or other control antigens. A homologue of Cpn0585 in the C. caviae species (encoded by the ORF CCA00156) was also localized in the inclusion membrane of the C. caviae-infected cells. The Cpn0585 protein became detectable 24h while CCA00156 as early as 8h after infection. Once expressed, both proteins remained in the inclusion membrane throughout the rest of infection course.

Localization of the hypothetical protein Cpn0797 in the cytoplasm of Chlamydia pneumoniae-infected host cells

Using antibodies raised with chlamydial fusion proteins, we have localized a protein encoded by the hypothetical open reading frame Cpn0797 in the cytoplasm of Chlamydia pneumoniae-infected host cells. The anti-Cpn0797 antibodies specifically recognized Cpn0797 protein without cross-reacting with either CPAFcp or Cpn0796, the only two proteins known to be secreted into the host cell cytosol by C. pneumoniae organisms. Thus, Cpn0797 represents the third C. pneumoniae protein secreted into the host cell cytosol experimentally identified so far.

The obligate intracellular pathogen Chlamydia trachomatis targets host lipid droplets

Lipid droplets (LDs) are ubiquitous but poorly understood neutral-lipid-rich eukaryotic organelles that may participate in functions as diverse as lipid homeostasis, membrane traffic, and signaling . We report that infection with the obligate intracellular pathogen Chlamydia trachomatis, the causative agent of trachoma and many sexually transmitted diseases , leads to the accumulation of neutral-lipid-rich structures with features of LDs at the cytoplasmic surface of the bacteria-containing vacuole. To identify bacterial factors that target these organelles, we screened a collection of yeast strains expressing GFP-tagged chlamydial ORFs and identified several proteins with tropism for eukaryotic LDs. We determined that three of these LD-associated (Lda) proteins are translocated into the mammalian host and associate with neutral-lipid-rich structures. Furthermore, the stability of one Lda protein is dependent on binding to LDs, and pharmacological inhibition of LD formation negatively impacted chlamydial replication. These results suggest that C. trachomatis targets LDs to enhance its survival and replication in infected cells. The co-option of mammalian LD function by a pathogenic bacterium represents a novel mechanism of eukaryotic organelle subversion and provides unique research opportunities to explore the function of these understudied organelles.

Hypothetical protein Cpn0308 is localized in the Chlamydia pneumoniae inclusion membrane

The hypothetical protein encoded by Chlamydia pneumoniae open reading frame cpn0308 was detected in inclusion membranes of C. pneumoniae-infected cells using antibodies raised with Cpn0308 fusion proteins. The anti-Cpn0308 antibodies did not cross-react with IncA, a known C. pneumoniae inclusion membrane protein, although the anti-Cpn0308 antibody staining overlapped with the anti-IncA antibody labeling. The labeling of the inclusion membrane by the anti-Cpn0308 antibody was specifically blocked by the Cpn0308 but not IncA fusion proteins. The Cpn0308 antigen was detectable 24 h after infection and remained in the inclusion membrane throughout the infection course.

The hypothetical protein CT813 is localized in the Chlamydia trachomatis inclusion membrane and is immunogenic in women urogenitally infected with C. trachomatis

Using antibodies raised with chlamydial fusion proteins, we have localized a protein encoded by hypothetical open reading frame CT813 in the inclusion membrane of Chlamydia trachomatis. The detection of the C. trachomatis inclusion membrane by an anti-CT813 antibody was blocked by the CT813 protein but not unrelated fusion proteins. The CT813 protein was detected as early as 12 h after chlamydial infection and was present in the inclusion membrane during the entire growth cycle. All tested serovars from C. trachomatis but not other chlamydial species expressed the CT813 protein. Exogenously expressed CT813 protein in HeLa cells displayed a cytoskeleton-like structure similar to but not overlapping with host cell intermediate filaments, suggesting that the CT813 protein is able to either polymerize or associate with host cell cytoskeletal structures. Finally, women with C. trachomatis urogenital infection developed high titers of antibodies to the CT813 protein, demonstrating that the CT813 protein is not only expressed but also immunogenic during chlamydial infection in humans. In all, the CT813 protein is an inclusion membrane protein unique to C. trachomatis species and has the potential to interact with host cells and induce host immune responses during natural infection. Thus, the CT813 protein may represent an important candidate for understanding C. trachomatis pathogenesis and developing intervention and prevention strategies for controlling C. trachomatis infection.

Multifunctional analysis of Chlamydia-specific genes in a yeast expression system

Our understanding of how obligate intracellular pathogens co-opt eukaryotic cellular functions has been limited by their intractability to genetic manipulation and by the abundance of pathogen-specific genes with no known functional homologues. In this report we describe a gene expression system to characterize proteins of unknown function from the obligate intracellular bacterial pathogen Chlamydia trachomatis. We have devised a homologous recombination-based cloning strategy to construct an ordered array of Saccharomyces cerevisiae strains expressing all Chlamydia-specific genes. These strains were screened to identify chlamydial proteins that impaired various yeast cellular functions or that displayed tropism towards eukaryotic organelles. In addition, to identify bacterial factors that are secreted into the host cell, recombinant chlamydial proteins were screened for reactivity towards antisera raised against vacuolar membranes purified from infected mammalian cells. We report the identification of 34 C. trachomatis proteins that impact yeast cellular functions or are tropic for a range of eukaryotic organelles including mitochondria, nucleus and cytoplasmic lipid droplets, and a new family of Chlamydia-specific proteins that are exported from the parasitopherous vacuole. The versatility of molecular manipulations and protein expression in yeast allows for the rapid construction of comprehensive protein expression arrays to explore the function of pathogen-specific gene products from microorganisms that are difficult to genetically manipulate, grow in culture or too dangerous for routine analysis in the laboratory.

Profiling of human antibody responses to Chlamydia trachomatis urogenital tract infection using microplates arrayed with 156 chlamydial fusion proteins

The available chlamydial genome sequences have made it possible to comprehensively analyze host responses to all chlamydial proteins, which is essential for further understanding of chlamydial pathogenesis and development of effective chlamydial vaccines. Microplates arrayed with 156 Chlamydia trachomatis fusion proteins were used to evaluate antibody responses in women urogenitally infected with C. trachomatis. Based on both the antibody recognition frequency and titer, seven chlamydial antigens encoded by open reading frames (ORFs) CT089, CT147, CT226, CT681, CT694, CT795, and CT858, respectively, were identified as relatively immunodominant; six of these are encoded by hypothetical ORFs. Antibody binding to these chlamydial fusion proteins was blocked by C. trachomatis-infected but not by normal HeLa cell lysates or irrelevant bacterial lysates. These results have revealed novel immune-reactive chlamydial antigens, not only indicating that the hypothetical ORF-encoded proteins are expressed during chlamydial infection in humans but also providing the proof of principle that the fusion protein-based approach can be used to profile human immune responses to chlamydial infection at the whole-genome scale.

Bacterial invasion via lipid rafts

Accumulating reports document the use by pathogens of cholesterol-enriched lipid microdomains, often called lipid rafts, as cell surface platforms to interact, bind and possibly enter into host cells. The challenge is now to understand what could be the functional role of these domains during pathogen invasion. Are they hijacked as general clustering devices for cellular binding sites and/or do they have other roles? In particular, is their cell signalling capacity activated and used by pathogens? In reverse, could lipid rafts activate bacterial mechanisms required for invasion? These issues will be discussed after an introduction on the current view on lipid rafts.

Activation of lipid metabolism contributes to interleukin-8 production during Chlamydia trachomatis infection of cervical epithelial cells

Chlamydia trachomatis infection is the most common cause of bacterial sexually transmitted diseases. Infection of the urogenital tract by C. trachomatis causes chronic inflammation and related clinical complications. Unlike other invasive bacteria that induce a rapid cytokine/chemokine production, chlamydial infection induces delayed inflammatory response and proinflammatory chemokine production that is dependent on bacterial growth. We present data here to show that the lipid metabolism required for chlamydial growth contributes to Chlamydia-induced proinflammatory chemokine production. By gene microarray profiling, validated with biochemical studies, we found that C. trachomatis LGV2 selectively upregulated PTGS2 (COX2) and PTGER4 (EP4) in cervical epithelial HeLa 229 cells. COX2 is an enzyme that catalyzes the rate-limiting step of arachidonic acid conversion to prostaglandins, including prostaglandin E2 (PGE2) and other eicosanoids, whereas EP4 is a subtype of cell surface receptors for PGE2. We show that Chlamydia infection induced COX2 protein expression in both epithelial cells and peripheral blood mononuclear cells and promoted PGE2 release. Exogenous PGE2 was able to induce interleukin-8 release in HeLa 229 epithelial cells. Finally, we demonstrated that interleukin-8 induction by Chlamydia infection or PGE2 treatment was dependent on extracellular signal-regulated kinase/mitogen-activated protein activity. Together, these data demonstrate that the host lipid remodeling process required for chlamydial growth contributes to proinflammatory chemokine production. This study also highlights the importance of maintaining a balanced habitat for parasitic pathogens as obligate intracellular organisms.

Up-Regulation of the JAK/STAT1 Signal Pathway duringChlamydia trachomatisInfection

Chlamydia trachomatis infection is the most common cause of sexually transmitted disease, leading to female pelvic inflammatory disease and infertility. The disease process has been linked to cellular response to this bacterial pathogen. This obligate intracellular pathogen infects macrophages, fibroblast cells, and epithelial and endothelial cells. We show in this study that infection of cervical epithelial cells, the primary target of Chlamydia trachomatis, leads to up-regulation and activation of the JAK/STAT signal pathway. Specifically, Chlamydia trachomatis infection of HeLa 229 cells selectively induces STAT1, STAT2, and IFN-stimulated transcription factor 3gamma expression and promotes STAT1 activation. The up-regulation of STAT1 is dependent on bacterial replication, because treatment of infected cells with antibiotics prevents STAT1 up-regulation. By analysis of the gene transcriptional and cytokine expression profiles of host cells combined with the use of neutralizing Abs, we show that IFN-beta production is critical for STAT1 induction in epithelial cells. Finally, we demonstrate that the host up-regulates STAT1 to restrict bacterial infection, because Chlamydia propagates more efficiently in STAT1-null or STAT1 knockdown cells, whereas Chlamydia growth is inhibited in cells with up-regulated STAT1 expression. This study demonstrates that the infected cells up-regulate the host innate antimicrobial response to chlamydial infection. It also highlights the importance of cellular response by nonimmune cells in host clearance of chlamydial infection.

NF-κB Activation Is Not Required forChlamydia trachomatisInhibition of Host Epithelial Cell Apoptosis

Chlamydia trachomatis, an obligate intracellular bacterial species, is known to inhibit host cell apoptosis. However, the chlamydial antiapoptotic mechanism is still not clear. Because NF-kappaB activation is antiapoptotic, we tested the potential role of NF-kappaB activation in chlamydial antiapoptotic activity in the current study. First, no obvious NF-kappaB activation was detected in the chlamydia-infected cells when these cells were resistant to apoptosis induced via either the intrinsic or extrinsic apoptosis pathways. Second, inhibition of NF-kappaB activation with pharmacologic reagents failed to block the chlamydial antiapoptotic activity. Finally, NF-kappaB p65 gene deletion did not prevent chlamydia from inhibiting host cell apoptosis. These observations together have demonstrated that NF-kappaB activation is not required for the chlamydial antiapoptotic activity.

Human antibody responses to a Chlamydia-secreted protease factor

We have previously identified a chlamydia-secreted protein (designated chlamydial proteasome/protease-like activity factor, or CPAF) in the cytosol of chlamydia-infected cells. Although CPAF is known to degrade host transcription factors required for major histocompatibility complex antigen expression in cultured cells, it is not clear whether CPAF is produced and maintains similar functions in humans infected with chlamydial organisms. We now report that CPAF does not preexist in chlamydial organisms and that CPAF synthesis requires live organism replication in cultured cells. Mice inoculated with live, but not mice inoculated with dead, chlamydial organisms produced a strong antibody response to CPAF, correlating CPAF-specific antibody production with CPAF synthesis in animals. Sera from women diagnosed with Chlamydia trachomatis cervicitis displayed higher levels of antibodies to CPAF than to either chlamydial major outer membrane protein or heat shock protein 60, suggesting that CPAF is both produced and immunogenic during human chlamydial infection.

Chlamydia and apoptosis: life and death decisions of an intracellular pathogen

The chlamydiae are important obligate intracellular prokaryotic pathogens that, each year, are responsible for millions of human infections involving the eye, genital tract, respiratory tract, vasculature and joints. The chlamydiae grow in cytoplasmic vesicles in susceptible host cells, which include the mucosal epithelium, vascular endothelium, smooth muscle cells, circulating monocytes and recruited or tissue-specific macrophages. One important pathogenic strategy that chlamydiae have evolved to promote their survival is the modulation of programmed cell death pathways in infected host cells. The chlamydiae can elicit the induction of host cell death, or apoptosis, under some circumstances and actively inhibit apoptosis under others. This subtle pathogenic mechanism highlights the manner in which these highly successful pathogens take control of infected cells to promote their own survival - even under the most adverse circumstances.

C. trachomatis-infection accelerates metabolism of phosphatidylcholine derived from low density lipoprotein but does not affect phosphatidylcholine secretion from hepatocytes

Background

Chlamydia trachomatis is a prevalent sexually transmitted disease and the leading cause of infectious blindness in developing nations. It was not known if C. trachomatis-infection influenced metabolism of lipoprotein-derived phospholipids. Nor was it known if C. trachomatis-infection altered phosphatidylcholine (PC) secretion from hepatocytes. In the current study, low density lipoprotein (LDL)-derived [methyl-³H]PC metabolism was examined in L929 cells infected with C. trachomatis to determine if PC derived from LDL could serve as a potential source of PC trafficked to C. trachomatis. In addition, release of endogenously synthesized [methyl-³H]PC into the medium was examined in rat liver hepatocytes infected with C. trachomatis to determine if infection altered PC secretion.

Results

L929 cells 20 h post infection exhibited a 39% (p < 0.05) reduction in radioactivity in PC but total radioactivity incorporation was unaltered compared to controls. Lysophosphatidyl [methyl-³H]choline (LPC) and aqueous [methyl-³H]choline metabolites were elevated 3.6-fold (p < 0.05) and 16.5-fold (p < 0.05), respectively, in C. trachomatis-infected cells and this was due to a 51% increase (p < 0.05) in calcium-dependent phospholipase A₂ activity. Hepatocytes 22 h post infection then incubated for 16 h with [methyl-³H]choline showed elevated [methyl-³H]PC biosynthesis but [methyl-³H]PC secreted into the medium was unaltered compared to controls. In contrast, both cellular and medium lyso [methyl-³H]PC were elevated in C. trachomatis-infected cells.

Conclusion

This study is the first to show that metabolism of LDL-derived PC is accelerated in C. trachomatis infection and to support the notion that LDL-delivered PC may potentially serve as a source of PC trafficked to Chlamydia. In addition, C. trachomatis-infection does not inhibit PC secretion from hepatocytes indicating that the pool of newly synthesized PC destined for lipoprotein secretion may differ from the pool of PC used for C. trachomatis membrane biosynthesis.