Several carcinoembryonic antigens (CD66) serve as receptors for gonococcal opacity proteins

Vitronectin binds to the gonococcal adhesin OpaA through a glycosaminoglycan molecular bridge

Several bacterial pathogens including Neisseria gonorrhoeae bind the human serum glycoprotein vitronectin. We aimed at defining the gonococcal receptor for vitronectin. Ligand blots demonstrated that vitronectin bound specifically to the heparin-binding outer-membrane protein OpaA, but that coating OpaA with the sulphated polysaccharide heparin was required for the interaction to occur. Bound vitronectin could be dissociated from OpaA-heparin-vitronectin complexes by the addition of excess heparin, indicating that sulphated polysaccharides provided the main linkage between the two proteins. Binding assays with intact micro-organisms substantiated the requirement of sulphated polysaccharides such as heparin and dextran sulphate for the efficient binding of vitronectin to OpaA+ gonococci. This was underscored by the increased binding of vitronectin to gonococci that had been preincubated with saturating concentrations of dextran sulphate, as opposed to the inhibition of vitronectin binding observed when bacteria were incubated simultaneously with vitronectin and saturating concentrations of dextran sulphate. Binding assays with dextran sulphates of various sizes indicated that vitronectin binding correlated with the size of the polysaccharide rather than with the amount of OpaA produced by the bacteria. The inability of zero-length cross-linking agents to couple vitronectin to OpaA provided further evidence that sulphated polysaccharides formed the linkage between vitronectin and OpaA. Infection experiments demonstrated that proteoglycan-deficient Chinese hamster ovary cells efficiently internalized dextran sulphate/vitronectin-coated gonococci, suggesting that soluble sulphated polysaccharides could substitute for cell surface glycosaminoglycans in the internalization process. On the basis of our results, we propose a novel mechanism of vitronectin binding in which sulphated polysaccharides act as molecular bridges, linking the glycosaminoglycan-binding sites of vitronectin and gonococcal OpaA.

CD66 receptor specificity exhibited by neisserial Opa variants is controlled by protein determinants in CD66 N-domains

Neisseria gonorrhoeae strain MS11 is able to express 11 different opacity (Opa) proteins on its outer surface. A number of these Opa proteins have been shown to function as adhesins through binding of CD66 receptors present on human cells. CD66 antigens, or carcinoembryonic antigen family members, constitute a family of glycoproteins belonging to the immunoglobulin superfamily. Opa variants recognize this class of receptors in a differential manner such that certain Opa variants recognize up to four different CD66 receptors (CD66a, -c, -d, and -e), whereas others recognize only two (CD66a and -e) or none. We explored the basis for this receptor tropism in the present study. Our data show that glycoforms of CD66e and deglycosylated CD66e are recognized by gonococci in an Opa-specific manner. Binding by Opa variants of recombinant N-terminal domains of CD66 receptors expressed in Escherichia coli reflected the adherence specificities of Opa variants to HeLa cells expressing native CD66 molecules. These data indicate that recognition of CD66 receptors by Opa variants is mediated by the protein backbone of the CD66 N-domains. Furthermore, by using chimeric constructs between different CD66 N-domains we identified distinct binding regions on the CD66e N-domain for specific groups of Opa variants, suggesting that the differential recognition of CD66 receptors by Opa variants is dictated by the presence of specific binding regions on the N-domain of the receptor.

Entry of OpaA+ gonococci into HEp-2 cells requires concerted action of glycosaminoglycans, fibronectin and integrin receptors

Heparan sulphate proteoglycans are increasingly implicated as eukaryotic cell surface receptors for bacterial pathogens. Here, we report that Neisseria gonorrhoeae adheres to proteoglycan receptors on HEp-2 epithelial cells but that internalization of the bacterium by this cell type requires the serum glycoprotein fibronectin. Fibronectin was shown to bind specifically to gonococci producing the OpaA adhesin. Binding assays with fibronectin fragments located the bacterial binding site near the N-terminal end of the molecule. However, none of the tested fibronectin fragments supported gonococcal entry into the eukaryotic cells; a 120 kDa fragment carrying the cell adhesion domain with the amino acid sequence RGD even inhibited the fibronectin-mediated uptake of MS11-OpaA. This inhibition could be mimicked by an RGD-containing hexapeptide and by alpha 5 beta 1 integrin-specific antibodies, suggesting that interaction of the central region of fibronectin with integrin receptors facilitated bacterial uptake. Fibronectin was unable to promote gonococcal entry into HEp-2 cells that had been treated with the enzyme heparinase III, which degrades the glycosaminoglycan side-chains of proteoglycan receptors. On the basis of these results, we propose a novel cellular uptake pathway for bacteria, which involves the binding of the pathogen to glycosaminoglycans that, in turn, act as co-receptors facilitating fibronectin-mediated bacterial uptake through integrin receptors. In this scenario, fibronectin would act as a molecular bridge linking to Opa-proteoglycan complex with host cell integrin receptors.

Interaction of invasive bacteria with host signaling pathways

Many bacterial pathogens exploit mammalian cell functions in order to promote their adherence to or uptake by host cells. Recent work has led to the identification of some of the bacterial and mammalian proteins involved in these processes. Although specific mechanisms differ among pathogens, a common aspect appears to be regulation of signaling pathways that control the actin cytoskeleton.

Vitronectin-dependent invasion of epithelial cells by Neisseria gonorrhoeae involves alpha(v) integrin receptors

Binding of vitronectin (VN) to Neisseria gonorrhoeae expressing the heparan sulfate proteoglycan (HSPG) specific Opa50 protein was recently shown to trigger bacterial internalization into distinct epithelial cell lines. We have investigated the role of VN-binding integrin receptors and protein kinase C (PKC) in VN-triggered bacterial uptake. Blocking integrin function by RGDS peptides or by antibodies specific to alpha(v)beta5 or alpha(v)beta3 resulted in an abrogation of VN-triggered bacterial internalization. Moreover, inhibitors of PKC were found to block VN-triggered uptake. The essential role of alpha(v) integrins and the presumable involvement of PKC in VN-triggered gonococcal uptake are discussed.

Neisseria meningitidis producing the Opc adhesin binds epithelial cell proteoglycan receptors

Neisseria meningitidis possesses a repertoire of surface adhesins that promote bacterial adherence to and entry into mammalian cells. Here, we have identified heparan sulphate proteoglycans as epithelial cell receptors for the meningococcal Opc invasin. Binding studies with radiolabelled heparin and heparin affinity chromatography demonstrated that Opc is a heparin binding protein. Subsequent binding experiments with purified 35SO4-labelled epithelial cell proteoglycan receptors and infection assays with epithelial cells that had been treated with heparitinase to remove glycosaminoglycans confirmed that Opc-expressing meningococci exploit host cell-surface proteoglycans to gain access to the epithelial cell interior. Unexpectedly, Opa28-producing meningococci lacking Opc also bound proteoglycans. These bacteria also bound CEA receptors in contrast to the Opc-expressing phenotype, suggesting that Opa28 may possess domains with specificity for different receptors. Opa/Opc-negative meningococci did not bind either proteoglycan or CEA receptors. Using a set of genetically defined mutants with different lipopolysaccharide (LPS) and capsular phenotype, we were able to demonstrate that surface sialic acids interfere with the Opc-proteoglycan receptor interaction. This effect may provide the molecular basis for the reported modulatory effect of capsule and LPS on meningococcal adherence to and entry into various cell types.

Using the yeast two-hybrid system to identify human epithelial cell proteins that bind gonococcal Opa proteins: intracellular gonococci bind pyruvate kinase via their Opa proteins and require host pyruvate for growth

Neisseria gonorrhoeae opacity-associated (Opa) proteins are a family of outer membrane proteins involved in gonococcal adherence to and invasion of human cells. We wanted to identify additional roles for Opa in the infectious process and used the yeast two-hybrid system to identify human epithelial cell proteins that interact with Opa proteins. Although this system has been used successfully to identify many types of interacting proteins, it has not been used to screen a human cell cDNA library for binding partners of a prokaryotic outer membrane protein. Therefore, we were also interested in exploring the versatility of the yeast two-hybrid system in identifying bacteria-host interactions. Using OpaP from strain F62SF as bait, we screened a HeLa cell cDNA library for Opa-interacting proteins (OIPs). We identified five different OIPs, designated OIP1-OIP5, two of which are homologous to human proteins--thyroid hormone receptor interacting protein (TRIP6) and pyruvate kinase isoenzyme M2 (PK). In the studies presented here, we investigated the interaction between Opa proteins and PK in more depth. Opa-PK interactions were confirmed by in vitro and in vivo assays independent of the yeast two-hybrid system. Escherichia coli expressing six different Opa proteins from gonococcal strain FA1090 all bound more PK than Opa-negative E. coli in in vitro binding assays. Using anti-PK antibody and fluorescence microscopy, we showed that human epithelial cell PK co-localizes with intracellular Opa+ gonococci and E. coli expressing Opa proteins. Using a mutant of N. gonorrhoeae unable to grow on pyruvate or lactate, it appears that intracellular pyruvate is essential for gonococcal growth and survival. These results suggest a novel mechanism in bacterial pathogenesis, i.e. the requirement for direct molecular interaction with a host metabolic enzyme (PK) for the acquisition of an essential intracellular carbon source and growth substrate (pyruvate). These results demonstrate that the yeast two-hybrid system is a valuable tool for identifying biologically relevant interactions between bacteria and host proteins, providing valuable leads for further investigations into novel mechanisms of bacterial pathogenesis.

Natural proteoglycan receptor analogs determine the dynamics of Opa adhesin-mediated gonococcal infection of Chang epithelial cells

Many bacterial pathogens possess a complex machinery for the induction and/or secretion of factors that promote their uptake by mammalian cells. We searched for the molecular basis of the 60- to 90-min lag time in the interaction of Neisseria gonorrhoeae carrying the heparin-binding Opa adhesin with Chang epithelial cells. Infection assays in the presence of chloramphenicol demonstrated that the Opa-mediated gonococcal infection of Chang cells required bacterial protein synthesis when the microorganisms were derived from GC agar but not when grown in liquid media. Further analysis indicated that contact with agar ingredients rather than the growth state of the microorganisms determined the infection dynamics. DEAE chromatography of GC agar extracts and sodium dodecyl sulfate-polyacrylamide gel electrophoresis analyses and testing of collected fractions in infection assays identified negatively charged high-molecular-weight polysaccharides in the agar as inhibitors of the cellular infection. Electron microscopy showed that agar-grown gonococci were surrounded by a coat of alcian blue-positive material, probably representing accreted polysaccharides. Similar antiphagocytic material was isolated from bovine serum, indicating that in biological fluids gonococci producing the heparin-binding Opa adhesin may become covered with externally derived polysaccharides as well. Binding assays with gonococci and epithelial proteoglycan receptors revealed that polysaccharides derived from agar or serum compete with the proteoglycans for binding of the heparin-binding Opa adhesin and thus act as receptor analogs. Growth of gonococci in a polysaccharide-free environment resulted in optimal proteoglycan receptor binding and rapid bacterial entry into Chang cells. The recognition that gonococci with certain phenotypes can recruit surface polysaccharides that determine in vitro infection dynamics adds a different dimension to the well-recognized biological significance of genetic variation for this pathogen.