Some binding properties of the envelope of Porphyromonas gingivalis to hemoglobin

An outer membrane vesicle specific lipoprotein promotes Porphyromonas gingivalis aggregation on red blood cells

Porphyromonas gingivalis uses a variety of mechanisms to actively interact with and promote the hydrolysis of red blood cells (RBCs) to obtain iron in the form of heme. In this study, we investigated the function of lipoprotein PG1881 which was previously shown to be up-regulated during subsurface growth and selectively enriched on outer membrane vesicles (OMVs). Our results show that wildtype strain W83 formed large aggregates encompassing RBCs whereas the PG1881 deletion mutant remained predominately as individual cells. Using a PG1881 antibody, immunofluorescence revealed that the wildtype strain's aggregation to RBCs involves an extracellular matrix enriched with PG1881. Our findings discover that RBCs elicit cell aggregation and matrix formation by P. gingivalis and that this process is promoted by an OMV-specific lipoprotein. We propose this strategy is advantageous for nutrient acquisition as well as dissemination from the oral cavity and survival of this periodontal pathogen.

Hemoglobin binding activity and hemoglobin-binding protein of Prevotella nigrescens

Prevotella nigrescens, lacking siderophores was found to bind to the hemoproteins. The binding was observed also in the envelope which was prepared by sonication of the cell. The binding occurred in the pH-dependent manner; the binding was observed below neutral pHs of the incubation mixtures but only slightly observed in the neutral and alkaline pHs. Furthermore, hemoglobin bound to the envelope was dissociated at high pHs buffers. Maximum amounts of hemoglobin bound to 1 mg envelope was 51.2 μg. Kd for the reaction at pH 5.0 was 2.1 × 10^-10M (210 pM). From the dot blot assay, hemoglobin could bind to a protein solubilized from the envelope by a detergent, referred to as hemoglobin-binding protein (HbBP), then it was purified by the sequential procedures of ion exchange chromatography, affinity chromatography and isoelectric focusing. Molecular weight and isoelectric point of the HbBP were 46 kDa and 6.1, respectively.

Characterization of binding and utilization of hemoglobin by Prevotella nigrescens

The ability of Prevotella nigrescens to utilize and bind to hemoglobin was investigated. Growth studies showed that P. nigrescens was able to utilize hemoglobin efficiently as an iron source. Binding of P. nigrescens to hemoglobin was demonstrated by dot blot assay. Heat and trypsin treatments of the bacteria led to a decrease in activity. Globin gave nearly complete inhibition of activity. Additionally, lactoferrin partially inhibited activity. In contrast, transferrin, cytochrome C and catalase exerted little or no inhibitory effect. Although the sugars tested did not affect activity, several of the amino acids tested, including arginine, cysteine, histidine and lysine, inhibited activity. In a solid phase assay, 41-, 56- and 59-kDa proteins of P. nigrescens reacted with hemoglobin. These results suggest that P. nigrescens utilizes hemoglobin for growth and 41-, 56- and 59-kDa proteins may be involved in hemoglobin binding.

Human lactoferrin binds and removes the hemoglobin receptor protein of the periodontopathogen Porphyromonas gingivalis

Porphyromonas gingivalis possesses a hemoglobin receptor (HbR) protein on the cell surface as one of the major components of the hemoglobin utilization system in this periodontopathogenic bacterium. HbR is intragenically encoded by the genes of an arginine-specific cysteine proteinase (rgpA), lysine-specific cysteine proteinase (kgp), and a hemagglutinin (hagA). Here, we have demonstrated that human lactoferrin as well as hemoglobin have the abilities to bind purified HbR and the cell surface of P. gingivalis through HbR. The interaction of lactoferrin with HbR led to the release of HbR from the cell surface of P. gingivalis. This lactoferrin-mediated HbR release was inhibited by the cysteine proteinase inhibitors effective to the cysteine proteinases of P. gingivalis. P. gingivalis could not utilize lactoferrin for its growth as an iron source and, in contrast, lactoferrin inhibited the growth of the bacterium in a rich medium containing hemoglobin as the sole iron source. Lactoferricin B, a 25-amino acid-long peptide located at the N-lobe of bovine lactoferrin, caused the same effects on P. gingivalis cells as human lactoferrin, indicating that the effects of lactoferrin might be attributable to the lactoferricin region. These results suggest that lactoferrin has a bacteriostatic action on P. gingivalis by binding HbR, removing it from the cell surface, and consequently disrupting the iron uptake system from hemoglobin.

Binding and utilization of myoglobin by Porphyromonas gingivalis

Myoglobin was found to bind reversibly to the envelope of Porphyromonas gingivalis in a pH-dependent manner; the binding took place below neutral pHs of the incubation mixtures and myoglobin bound released from the envelope at high pHs. The amounts of myoglobin bound to 1 mg of the envelope at pH 5.0 per min under the presence of sufficient myoglobin were 1.4 microg. K(d) for the reaction at pH 5.0 was 2.2 x 10(-10) M. From the dot blot assay, myoglobin obviously bound to hemoglobin-binding protein (HbBP) of P. gingivalis, however, the amounts of myoglobin that bound to HbBP were half those of hemoglobin. One of the fractions, separated by gel filtration, of the digested materials of myoglobin by the detergent-solubilized envelope containing proteinases was found to support the growth of P. gingivalis in the iron source-depleted medium.

Genetic analyses of proteolysis, hemoglobin binding, and hemagglutination of Porphyromonas gingivalis. Construction of mutants with a combination of rgpA, rgpB, kgp, and hagA

Porphyromonas gingivalis produces arginine-specific cysteine proteinase (Arg-gingipain, RGP) and lysine-specific cysteine proteinase (Lys-gingipain, KGP) in the extracellular and cell-associated forms. Two separate genes (rgpA and rgpB) and a single gene (kgp) have been found to encode RGP and KGP, respectively. We constructed rgpA rgpB kgp triple mutants by homologous recombination with cloned rgp and kgp DNA interrupted by drug resistance gene markers. The triple mutants showed no RGP or KGP activity in either cell extracts or culture supernatants. The culture supernatants of the triple mutants grown in a rich medium had no proteolytic activity toward bovine serum albumin or gelatin derived from human type I collagen. Moreover, the mutants did not grow in a defined medium containing bovine serum albumin as the sole carbon/energy source. These results indicate that the proteolytic activity of P. gingivalis toward bovine serum albumin and gelatin derived from human type I collagen appears to be attributable to RGP and KGP. The hemagglutinin gene hagA of P. gingivalis possesses the adhesin domain regions responsible for hemagglutination and hemoglobin binding that are also located in the C-terminal regions of rgpA and kgp. A rgpA kgp hagA triple mutant constructed in this study exhibited no hemagglutination using sheep erythrocytes or hemoglobin binding activity, as determined by a solid-phase binding assay with horseradish peroxidase-conjugated human hemoglobin, indicating that the adhesin domains seem to be particularly important for P. gingivalis cells to agglutinate erythrocytes and bind hemoglobin, leading to heme acquisition.

Life below the gum line: pathogenic mechanisms of Porphyromonas gingivalis

Porphyromonas gingivalis, a gram-negative anaerobe, is a major etiological agent in the initiation and progression of severe forms of periodontal disease. An opportunistic pathogen, P. gingivalis can also exist in commensal harmony with the host, with disease episodes ensuing from a shift in the ecological balance within the complex periodontal microenvironment. Colonization of the subgingival region is facilitated by the ability to adhere to available substrates such as adsorbed salivary molecules, matrix proteins, epithelial cells, and bacteria that are already established as a biofilm on tooth and epithelial surfaces. Binding to all of these substrates may be mediated by various regions of P. gingivalis fimbrillin, the structural subunit of the major fimbriae. P. gingivalis is an asaccharolytic organism, with a requirement for hemin (as a source of iron) and peptides for growth. At least three hemagglutinins and five proteinases are produced to satisfy these requirements. The hemagglutinin and proteinase genes contain extensive regions of highly conserved sequences, with posttranslational processing of proteinase gene products contributing to the formation of multimeric surface protein-adhesin complexes. Many of the virulence properties of P. gingivalis appear to be consequent to its adaptations to obtain hemin and peptides. Thus, hemagglutinins participate in adherence interactions with host cells, while proteinases contribute to inactivation of the effector molecules of the immune response and to tissue destruction. In addition to direct assault on the periodontal tissues, P. gingivalis can modulate eucaryotic cell signal transduction pathways, directing its uptake by gingival epithelial cells. Within this privileged site, P. gingivalis can replicate and impinge upon components of the innate host defense. Although a variety of surface molecules stimulate production of cytokines and other participants in the immune response, P. gingivalis may also undertake a stealth role whereby pivotal immune mediators are selectively inactivated. In keeping with its strict metabolic requirements, regulation of gene expression in P. gingivalis can be controlled at the transcriptional level. Finally, although periodontal disease is localized to the tissues surrounding the tooth, evidence is accumulating that infection with P. gingivalis may predispose to more serious systemic conditions such as cardiovascular disease and to delivery of preterm infants.

The binding and utilization of hemoglobin by Prevotella intermedia

Prevotella intermedia, a putative periodontopathic microorganism, requires iron for growth. Hemoglobin can be a major source of iron for bacterial growth in vivo since it is present in the crevicular fluid collected from periodontitis sites. Experiments studying the growth of P. intermedia in iron-depleted Todd-Hewitt broth supplemented with human hemoglobin showed that the bacteria were able to utilize human hemoglobin as a source of iron. The uptake of iron from hemoglobin by P. intermedia appears to be initiated by the binding of hemoglobin to the bacteria as shown by direct binding studies using 125I-labeled human hemoglobin. Scatchard analysis of saturation binding data revealed that 125I-labeled human hemoglobin had a dissociation constant (Kd) of 2.53 x 10(-8) M for the receptor on P. intermedia. Binding of labeled hemoglobin to P. intermedia was competitively inhibited by unlabeled human hemoglobin showing that the binding was specific. The ability of bovine hemoglobin, but not hemin or non-hemoglobin heme-containing compounds, to inhibit binding competitively suggested that the globin moiety of the hemoglobin molecule is recognized by the hemoglobin binding receptors.

Haemoglobin receptor protein is intragenically encoded by the cysteine proteinase-encoding genes and the haemagglutinin-encoding gene of Porphyromonas gingivalis

The obligately anaerobic bacterium Porphyromonas gingivalis produces characteristic black-pigmented colonies on blood agar. It is thought that the black pigmentation is caused by haem accumulation and is related to virulence of the microorganism. P. gingivalis cells expressed a prominent 19 kDa protein when grown on blood agar plates. Analysis of its N-terminal amino acid sequence indicated that the 19 kDa protein was encoded by an internal region (HGP15 domain) of an arginine-specific cysteine proteinase (Arg-gingipain, RGP)-encoding gene (rgp1) and was also present in genes for lysine-specific cysteine proteinases (prtP and kgp) and a haemagglutinin (hagA) of P. gingivalis. The HGP15 domain protein was purified from an HGP15-overproducing Escherichia coli and was found to have the ability to bind to haemoglobin in a pH-dependent manner. The anti-HGP15 antiserum reacted with the 19 kDa haemoglobin-binding protein in the envelope of P. gingivalis. P. gingivalis wild-type strain showed pH-dependent haemoglobin adsorption, whereas its non-pigmented mutants that produced no HGP15-related proteins showed deficiency in haemoglobin adsorption. These results strongly indicate a close relationship among HGP15 production, haemoglobin adsorption and haem accumulation of P. gingivalis.

Isolation and characterization of a hemin-regulated gene, hemR, from Porphyromonas gingivalis

An hemR (hemin-regulated) gene from Porphyromonas gingivalis ATCC 53977 has been isolated and characterized. This gene is present downstream from the prtT gene, previously cloned in this laboratory. In addition, another putative gene, ORF1, was identified between hemR and prtT. The complete nucleotide sequences of ORF1 and hemR were determined, and the deduced amino acid sequence of ORF1 and HemR proteins corresponded to 16- and 48-kDa proteins, respectively. The amino termini of the HemR protein exhibited significant homology with iron-regulated, TonB-dependent outer membrane receptor proteins from various bacteria, while the carboxyl terminus of the HemR protein displayed almost complete identity with a P. gingivalis PrtT protease domain. PCR analyses confirmed the existence of such extensive homology between the carboxyl termini of both the prtT and hemR genes on the P. gingivalis chromosome. Northern blots indicated that ORF1 was part of a 1.0-kb mRNA and was positively regulated by hemin levels. On the other hand, the hemR gene was apparently a part of a 3.0-kb polycistronic message and was negatively regulated at the transcriptional level by hemin. Primer extension analysis of the hemR gene revealed that the transcription start site was at a C residue located within ORF1. An examination of HemR::lacZ constructs in both Escherichia coli and P. gingivalis confirmed hemin repression of hemR expression in both organisms. Moreover, the HemR protein expressed in E. coli was detected by an antiserum from a periodontitis patient heavily colonized with P. gingivalis but not by serum from a periodontally healthy patient or by antisera against hemin-grown P. gingivalis cells. Therefore, it is likely that the 48-kDa HemR protein can be expressed only under hemin-restricted conditions. These results suggest that we have isolated a hemin-regulated gene, hemR, which encodes a 48-kDa protein that may be a TonB-dependent outer membrane protein.

Binding of hemoglobin to the envelope of Porphyromonas gingivalis and isolation of the hemoglobin-binding protein

The binding activity of the Porphyromonas gingivalis envelope and hemoglobin was examined over a wide range of pH values from 4.5 to 9.0. The binding activity in low-pH buffers was much higher than that at high pH; the optimum pHs for the binding were found to be 4.5 and 5.0. Since the hemoglobin bound to the envelope was found to dissociate in the pH 8.5 and 9.0 buffers, the binding is reversible. We hypothesized that hemoglobin-binding protein (HbBP), responsible for the binding to hemoglobin, exists in the envelope and confirmed its presence by dot blot determination with peroxidase-conjugated hemoglobin. Then we attempted to isolate HbBP from the solubilized (by a detergent) materials of the envelope by affinity chromatography. The molecular mass of HbBP was 19 kDa, and the isoelectric point was 4.3.

Binding of hemoglobin by Porphyromonas gingivalis

In this study, we investigated whether Porphyromonas gingivalis can bind hemoglobin as an initial step in the acquisition of heme from hemoglobin. The binding of human hemoglobin by P. gingivalis cells was determined using [3H]hemoglobin. Hemoglobin binding occurred rapidly, reversibly and specifically. A Scatchard analysis of the binding data generated a linear plot, indicating a single population of binding proteins. The apparent Kd was 1.0 +/- 0.19 x 10(-6) M and there were 3.2 +/- 0.76 x 10(4) binding sites per cell. Hemoglobin binding was inhibited by unlabeled human hemoglobin but not by hemin and protoporphyrin IX. The binding was only partially inhibited by human serum albumin, transferrin, lactoferrin, catalase and cytochrome c. These results suggest that the ligand recognized by the binding protein may not be the heme moiety. The binding of hemoglobin considerably increased when the organisms were grown under hemin-limited conditions. Hemoglobin bound to outer membrane proteins extracted from P. gingivalis cells on a dot blot binding assay and binding ability was lost after heating bacterial proteins. These results suggest that P. gingivalis cells interact with human hemoglobin through specific binding sites on their surfaces as a preliminary step in iron acquisition.