Effects of chemically modified heparin on Chlamydia trachomatis serovar L2 infection of eukaryotic cells in culture

Sulfated motifs in heparan sulfate inhibit Streptococcus pneumoniae adhesion onto fibronectin and attenuate corneal infection

A large number of bacterial pathogens bind to host extracellular matrix (ECM) components. For example, many Gram-negative and Gram-positive pathogens express binding proteins for fibronectin (FN) on their cell surface. Mutagenesis studies of bacterial FN-binding proteins have demonstrated their importance in pathogenesis in preclinical animal models. However, means to draw on these findings to design therapeutic approaches that specifically target FN-bacteria interactions have not been successful because bacterial pathogens can elaborate several FN-binding proteins and also because FN is an essential protein and likely a nondruggable target. Here we report that select heparan compounds potently inhibit Streptococcus pneumoniae infection of injured corneas in mice. Using intact heparan sulfate (HS) and heparin (HP), heparinase-digested fragments of HS, HP oligosaccharides, and chemically or chemoenzymatically modified heparan compounds, we found that inhibition of S. pneumoniae corneal infection by heparan compounds is not mediated by simple charge effects but by a selective sulfate group. Removal of 2-O-sulfates significantly inhibited the ability of HP to inhibit S. pneumoniae corneal infection, whereas the addition of 2-O-sulfates to heparosan (H) significantly increased H's ability to inhibit bacterial corneal infection. Proximity ligation assays indicated that S. pneumoniae attaches directly to FN fibrils in the corneal epithelial ECM and that HS and HP specifically inhibit this binding interaction in a 2-O-sulfate-dependent manner. These data suggest that heparan compounds containing 2-O-sulfate groups protect against S. pneumoniae corneal infection by inhibiting bacterial attachment to FN fibrils in the subepithelial ECM of injured corneas. Moreover, 2-O-sulfated heparan compounds significantly inhibited corneal infection in immunocompromised hosts, by a clinical keratitis isolate of S. pneumoniae, and also when topically administered in a therapeutic manner. These findings suggest that the administration of nonanticoagulant 2-O-sulfated heparan compounds may represent a plausible approach to the treatment of S. pneumoniae keratitis.

Chlamydia trachomatis Cell-to-Cell Spread through Tunneling Nanotubes

Tunneling nanotubes (TNTs) are transient cellular connections that consist of dynamic membrane protrusions. They play an important role in cell-to-cell communication and mediate the intercellular exchanges of molecules and organelles. TNTs can form between different cell types and may contribute to the spread of pathogens by serving as cytoplasmic corridors. We demonstrate that Chlamydia (C.) trachomatis-infected human embryonic kidney (HEK) 293 cells and other cells form TNT-like structures through which reticulate bodies (RBs) pass into uninfected cells. Observed TNTs have a life span of 1 to 5 h and contain microtubules, which are essential for chlamydial transfer. They can bridge distances of up to 50 μm between connecting neighboring cells. Consistent with the biological role for TNTs, we show that C. trachomatis spread also occurs under conditions in which the extracellular route of chlamydial entry into host cells is blocked. Based on our findings, we propose that TNTs play a critical role in the direct, cell-to-cell transmission of chlamydia. IMPORTANCE Intracellular bacterial pathogens often undergo a life cycle in which they parasitize infected host cells in membranous vacuoles. Two pathways have been described by which chlamydia can exit infected host cells: lytic cell destruction or exit via extrusion formation. Whether direct, cell-to-cell contact may also play a role in the spread of infection is unknown. Tunneling nanotubes (TNTs) interconnect the cytoplasm of adjacent cells to mediate efficient communication and the exchange of material between them. We used Chlamydia trachomatis and immortalized cells to analyze whether TNTs mediate bacterial transmission from an infected donor to uninfected acceptor cells. We show that chlamydia-infected cells build TNTs through which the intracellular reticulate bodies (RBs) of the chlamydia can pass into uninfected neighboring cells. Our study contributes to the understanding of the function of TNTs in the cell-to-cell transmission of intracellular pathogens and provides new insights into the strategies by which chlamydia spreads among multicellular tissues.

A FACS-Based Genome-wide CRISPR Screen Reveals a Requirement for COPI in Chlamydia trachomatis Invasion

The invasion of Chlamydia trachomatis, an obligate intracellular bacterium, into epithelial cells is driven by a complex interplay of host and bacterial factors. To comprehensively define the host genes required for pathogen invasion, we undertook a fluorescence-activated cell sorting (FACS)-based CRISPR screen in human cells. A genome-wide loss-of-function library was infected with fluorescent C. trachomatis and then sorted to enrich for invasion-deficient mutants. The screen identified heparan sulfate, a known pathogen receptor, as well as coatomer complex I (COPI). We found that COPI, through a previously unappreciated role, promotes heparan sulfate cell surface presentation, thereby facilitating C. trachomatis attachment. The heparan sulfate defect does not fully account for the resistance of COPI mutants. COPI also promotes the activity of the pathogen's type III secretion system. Together, our findings establish the requirement for COPI in C. trachomatis invasion and the utility of FACS-based CRISPR screening for the elucidation of host factors required for pathogen invasion.

•

FACS-based CRISPR screen to identify host factors required for C. trachomatis invasion

•

Candidate genes comprise heparan sulfate biosynthesis, actin remodeling, and COPI

•

COPI regulates heparan sulfate cell surface presentation and C. trachomatis attachment

•

COPI is also required for efficient C. trachomatis T3SS translocation

Fluorescence-Reported Allelic Exchange Mutagenesis Reveals a Role for Chlamydia trachomatis TmeA in Invasion That Is Independent of Host AHNAK

Development of approaches to genetically manipulate Chlamydia is fostering important advances in understanding pathogenesis. Fluorescence-reported allelic exchange mutagenesis (FRAEM) now enables the complete deletion of specific genes in C. trachomatis L2. We have leveraged this technology to delete the coding sequences for a known type III effector. The evidence provided here indicates that CT694/CTL0063 is a virulence protein involved in chlamydial invasion. Based on our findings, we designate the gene product corresponding to ct694-ctl0063translocated membrane-associated effector A (TmeA). Deletion of tmeA did not impact development of intracellular chlamydiae. However, the absence of TmeA manifested as a decrease in infectivity in both tissue culture and murine infection models. The in vitro defect was reflected by impaired invasion of host cells. TmeA binds human AHNAK, and we demonstrate here that AHNAK is transiently recruited by invading chlamydiae. TmeA, however, is not required for endogenous AHNAK recruitment. TmeA also impairs AHNAK-dependent actin bundling activity. This TmeA-mediated effect likely does not explain impaired invasion displayed by the tmeA strain of Chlamydia, since AHNAK-deficient cells revealed no invasion phenotype. Overall, our data indicate the efficacy of FRAEM and reveal a role of TmeA during chlamydial invasion that manifests independently of effects on AHNAK.

Glycosaminoglycans are involved in bacterial adherence to lung cells

Background

Lower respiratory infections are among the top ten causes of death worldwide. Since pathogen to cell adhesion is a crucial step in the infection progress, blocking the interaction between eukaryotic receptors and bacterial ligands may enable the pathogenesis process to be stopped. Cell surface glycosaminoglycans (GAGs) are known to be mediators in the adhesion of diverse bacteria to different cell types, making it of interest to examine their involvement in the attachment of various pathogenic bacteria to lung cells, including epithelial cells and fibroblasts.

Methods

The function of cell surface GAGs in bacterial adhesion was studied by reducing their levels through inhibiting their biosynthesis and enzymatic degradation, as well as in binding competition experiments with various species of GAGs. The participation of the different bacterial adhesins in attachment was evaluated through competition with two peptides, both containing consensus heparin binding sequences. Blocking inhibition assays using anti-syndecans and the enzymatic removal of glypicans were conducted to test their involvement in bacterial adhesion. The importance of the fine structure of GAGs in the interaction with pathogens was investigated in competition experiments with specifically desulfated heparins.

Results

The binding of all bacteria tested decreased when GAG levels in cell surface of both lung cells were diminished. Competition experiments with different types of GAGs showed that heparan sulfate chains are the main species involved. Blocking or removal of cell surface proteoglycans evidenced that syndecans play a more important role than glypicans. The binding was partially inhibited by peptides including heparin binding sequences. Desulfated heparins also reduced bacterial adhesion to different extents depending on the bacterium and the sulfated residue, especially in fibroblast cells.

Conclusions

Taken together, these data demonstrate that the GAG chains of the cell surface are involved in the adhesion of bacterial adhesins to lung cells. Heparan sulfate seems to be the main species implicated, and binding is dependent on the sulfation pattern of the molecule. These data could facilitate the development of new anti-infective strategies, enabling the development of new procedures for blocking the interaction between pathogens and lung cells more effectively.

Role of glycosaminoglycans in infectious disease

Glycosaminoglycans (GAGs) have been shown to bind to a wide variety of microbial pathogens, including viruses, bacteria, parasites, and fungi in vitro. GAGs are thought to promote pathogenesis by facilitating pathogen attachment, invasion, or evasion of host defense mechanisms. However, the role of GAGs in infectious disease has not been extensively studied in vivo and therefore their pathophysiological significance and functions are largely unknown. Here we describe methods to directly investigate the role of GAGs in infections in vivo using mouse models of bacterial lung and corneal infection. The overall experimental strategy is to establish the importance and specificity of GAGs, define the essential structural features of GAGs, and identify a biological activity of GAGs that promotes pathogenesis.

Characterization of the interaction between the chlamydial adhesin OmcB and the human host cell

In a previous study, we reported that the OmcB protein from Chlamydia pneumoniae mediates adhesion of the infectious elementary body to human HEp-2 cells by interacting with heparin/heparan sulfate-like glycosaminoglycans (GAGs) via basic amino acids located in the first of a pair of XBBXBX heparin-binding motifs (K. Moelleken and J. H. Hegemann, Mol. Microbiol. 67:403-419, 2008). In the present study, we show that the basic amino acid at position 57 (arginine) in the first XBBXBX motif, the basic amino acid at position 61 (arginine) in the second motif, and another amino acid (lysine 69) C terminal to it play key roles in the interaction. In addition, we show that discrimination between heparin-dependent and -independent adhesion by C. trachomatis OmcBs is entirely dependent on three variable amino acids in the so-called variable domain C terminal to the conserved XBBXBX motif. Here, the predicted conformational change in the secondary structure induced by the proline at position 66 seems to be crucial for heparin recognition. Finally, we performed neutralization experiments using different anti-heparan sulfate antibodies to gain insight into the nature of the GAGs recognized by OmcB. The results suggest that C. trachomatis serovar L2 OmcB interacts with 6-O-sulfated domains of heparan sulfate, while C. pneumoniae OmcB apparently interacts with domains of heparan sulfate harboring a diverse subset of O-sulfations.

Endosulfatases SULF1 and SULF2 limit Chlamydia muridarum infection

The first step in attachment of Chlamydia to host cells is thought to involve reversible binding to host heparan sulfate proteoglycans (HSPGs), polymers of variably sulfated repeating disaccharide units coupled to diverse protein backbones. However, the key determinants of HSPG structure that are involved in Chlamydia binding are incompletely defined. A previous genome-wide Drosophila RNAi screen suggested that the level of HSPG 6-O sulfation rather than the identity of the proteoglycan backbone maybe a critical determinant for binding. Here, we tested in mammalian cells whether SULF1 or SULF2, human endosulfatases, which remove 6-O sulfates from HSPGs, modulate Chlamydia infection. Ectopic expression of SULF1 or SULF2 in HeLa cells, which decreases cell surface HSPG sulfation, diminished C. muridarum binding and decreased vacuole formation. ShRNA depletion of endogenous SULF2 in a cell line that primarily expresses SULF2 augmented binding and increased vacuole formation. C. muridarum infection of diverse cell lines resulted indownregulation of SULF2 mRNA. In a murine model of acute pneumonia, mice genetically deficient in both endosulfatases or in SULF2 alone demonstrated increased susceptibility to C. muridarum lung infection. Collectively, these studies demonstrate that the level of HSPG 6-O sulfation is a critical determinant of C. muridarum infection in vivo and that 6-O endosulfatases are previously unappreciated modulators of microbial pathogenesis.

Attachment of Chlamydia trachomatis L2 to host cells requires sulfation

Chlamydia trachomatis is a pathogen responsible for a prevalent sexually transmitted disease. It is also the most common cause of infectious blindness in the developing world. We performed a loss-of-function genetic screen in human haploid cells to identify host factors important in C. trachomatis L2 infection. We identified and confirmed B3GAT3, B4GALT7, and SLC35B2, which encode glucuronosyltransferase I, galactosyltransferase I, and the 3'-phosphoadenosine 5'-phosphosulfate transporter 1, respectively, as important in facilitating Chlamydia infection. Knockout of any of these three genes inhibits Chlamydia attachment. In complementation studies, we found that the introduction of functional copies of these three genes into the null clones restored full susceptibility to Chlamydia infection. The degree of attachment of Chlamydia strongly correlates with the level of sulfation of the host cell, not simply with the amount of heparan sulfate. Thus, other, as-yet unidentified sulfated macromolecules must contribute to infection. These results demonstrate the utility of screens in haploid cells to study interactions of human cells with bacteria. Furthermore, the human null clones generated can be used to investigate the role of heparan sulfate and sulfation in other settings not limited to infectious disease.

Effect of heparin, fucoidan and other polysaccharides on adhesion of enterohepatic helicobacter species to murine macrophages

Helicobacter species have been isolated and cultured from both the gastric and enterohepatic niches of the gastrointestinal tract and are associated with a wide spectrum of diseases. Some members of the enterohepatic Helicobacter species (EHS), which include Helicobacter bilis, Helicobacter hepaticus and Helicobacter pullorum, are associated with chronic inflammatory and proliferative bowel inflammation, hepatitis and in experimental murine studies with hepatic cancer. The present study aimed to explore if polysulphated polysaccharides can prevent adhesion of EHS to the murine macrophage cell line J774A.1. A competitive binding assay showed that heparin and heparan sulphate at a concentration of 1.25 mg/ml reduced binding of H. hepaticus and H. pullorum to the host cells, but not H. bilis. Of the tested Helicobacter spp, the highest inhibition by heparin was demonstrated for H. pullorum (P < 0.01), the most hydrophilic strain. Partially or completely de-sulphated heparin derivatives lost the ability to inhibit adherence of EHS, indicating the importance of sulphated groups of heparin. The most efficient inhibitor of EHS binding to macrophages was fucoidan, which reduced bacterial adhesion of the three enterohepatic Helicobacter species to a greater extent than heparin, 60-90% inhibition vs 30-70% inhibition by heparin. Identification of receptors that EHS ligands bind to is important for understanding the development of infection and may provide a rational target to prevent infection and therapy.

Uncivil engineers: Chlamydia, Salmonella and Shigella alter cytoskeleton architecture to invade epithelial cells

The obligate intracellular bacterial pathogen Chlamydia trachomatis is a major cause of blindness and sexually transmitted diseases. Like the enteric pathogens Salmonella and Shigella, Chlamydia injects effector proteins into epithelial cells to initiate extensive remodeling of the actin cytoskeleton at the bacterial attachment site, which culminates in the engulfment of the bacterium by plasma membrane extensions. Numerous Salmonella and Shigella effectors promote this remodeling by activating Rho GTPases and tyrosine kinase signaling cascades and by directly manipulating actin dynamics. Recent studies indicate that similar host-cell alterations occur during Chlamydia invasion, but few effectors are known. The identification of additional Chlamydia effectors and the elucidation of their modes of function are critical steps towards an understanding of how this clinically important pathogen breaches epithelial surfaces and causes infection.

Susceptibility of Chlamydia trachomatis to the excipient hydroxyethyl cellulose: pH and concentration dependence of antimicrobial activity

Hydroxyethyl cellulose (HEC) is used as a neutral excipient in microbicides used against sexually transmitted pathogens. However, HEC inhibits the infection of cervical epithelial cells by Chlamydia trachomatis at pH 5 in a concentration-dependent manner. At pH 7, infection is inversely dependent on the concentration of HEC, possibly due to pH-dependent calcium sequestration.

The Chlamydia outer membrane protein OmcB is required for adhesion and exhibits biovar-specific differences in glycosaminoglycan binding

Chlamydia pneumoniae, an obligate intracellular human pathogen, causes a number of respiratory diseases. We explored the role of the conserved OmcB protein in C. pneumoniae infections, using yeast display technology. (i) Yeast cells presenting OmcB were found to adhere to human epithelial cells. (ii) Pre-incubation of OmcB yeast cells with heparin, but not other glycosaminoglycans (GAGs), abrogated adhesion. (iii) Pre-treatment of the target cells with heparinase inhibited adherence, and GAG-deficient CHO cell lines failed to bind OmcB yeast. (iv) A heparin-binding motif present near the N-terminus of OmcB is required for host cell binding. (v) Pre-treatment of chlamydial elementary bodies (EBs) with anti-OmcB antibody or pre-incubation of target cells with recombinant OmcB protein reduced infectivity upon challenge with C. pneumoniae. (vi) Adhesion of fluorescently labelled EBs to epithelial or endothelial cells was abrogated by prior addition of heparin or OmcB protein. Thus, C. pneumoniae OmcB is an adhesin that binds heparan sulphate-like GAGs. OmcB from Chlamydia trachomatis serovar L1 also adheres to human cells in a heparin-dependent way, unlike its counterpart from serovar E. We show that a single position in the OmcB sequence determines heparin dependence/independence, and variations there may reflect differences between the two serovars in cell tropism and disease pattern.

Interaction of Chlamydia trachomatis with mammalian cells is independent of host cell surface heparan sulfate glycosaminoglycans

The hypothesis that host cell surface heparan sulfate is required to promote chlamydial infection was tested using a cell line (CHO-18.4) containing a single retroviral insertion and the concomitant loss of heparan sulfate biosynthesis. Tests of chlamydial infectivity of heparan sulfate-deficient CHO-18.4 cells and parental cells, CHO-22, demonstrated that both were equally sensitive to infection by Chlamydia trachomatis serovars L2 and D. These data do not support the hypothesis and demonstrate that host cell surface heparan sulfate does not serve an essential functional role in chlamydial infectivity.

Selectively desulfated heparin inhibits P-selectin-mediated adhesion of human melanoma cells

Accumulating evidence has suggested that one of the mechanisms by which heparin inhibits metastasis is by blocking the P-selectin-based interaction of platelets with tumor cells. Here we demonstrate that the sulfate groups at C6/N and especially C6, but not C2 and C3, of heparin play a critical role in P-selectin recognition and that 2-O,3-O-desulfated heparin can block P-selectin-mediated A375 human melanoma cell adhesion. Our findings show that chemical modification of heparin, especially 2-O,3-O-desulfation, may result in a therapeutic agent that is anti-metastatic because it blocks unwanted P-selectin-dependent adhesion but that lacks dose-limiting anticoagulant effects.