Bone resorption stimulated by lipopolysaccharides from Bacteroides, Fusobacterium and Veillonella, and by the lipid A and the polysaccharide part of Fusobacterium lipopolysaccharide

Reexamining the role of Fusobacterium nucleatum subspecies in clinical and experimental studies

The Gram-negative anaerobic species Fusobacterium nucleatum was originally described as a commensal organism from the human oral microbiome. However, it is now widely recognized as a key inflammophilic pathobiont associated with a wide variety of oral and extraoral diseases. Historically, F. nucleatum has been classified into four subspecies that have been generally considered as functionally interchangeable in their pathogenic potential. Recent studies have challenged this notion, as clinical data reveal a highly biased distribution of F. nucleatum subspecies within disease sites of both inflammatory oral diseases and various malignancies. This review details the historical basis for the F. nucleatum subspecies designations and summarizes our current understanding of the similarities and distinctions between these organisms to provide important context for future clinical and laboratory studies of F. nucleatum.

Alveolar Bone Protection by Targeting the SH3BP2-SYK Axis in Osteoclasts

Periodontitis is a bacterially induced chronic inflammatory condition of the oral cavity where tooth-supporting tissues including alveolar bone are destructed. Previously, we have shown that the adaptor protein SH3-domain binding protein 2 (SH3BP2) plays a critical role in inflammatory response and osteoclastogenesis of myeloid lineage cells through spleen tyrosine kinase (SYK). In this study, we show that SH3BP2 is a novel regulator for alveolar bone resorption in periodontitis. Micro-CT analysis of SH3BP2-deficient (Sh3bp2 ^-/- ) mice challenged with ligature-induced periodontitis revealed that Sh3bp2 ^-/- mice develop decreased alveolar bone loss (male 14.9% ± 10.2%; female 19.0% ± 6.0%) compared with wild-type control mice (male 25.3% ± 5.8%; female 30.8% ± 5.8%). Lack of SH3BP2 did not change the inflammatory cytokine expression and osteoclast induction. Conditional knockout of SH3BP2 and SYK in myeloid lineage cells with LysM-Cre mice recapitulated the reduced bone loss without affecting both inflammatory cytokine expression and osteoclast induction, suggesting that the SH3BP2-SYK axis plays a key role in regulating alveolar bone loss by mechanisms that regulate the bone-resorbing function of osteoclasts rather than differentiation. Administration of a new SYK inhibitor GS-9973 before or after periodontitis induction reduced bone resorption without affecting inflammatory reaction in gingival tissues. In vitro, GS-9973 treatment of bone marrow-derived M-CSF-dependent macrophages suppressed tartrate-resistant acid phosphatase (TRAP)-positive osteoclast formation with decreased mineral resorption capacity even when GS-9973 was added after RANKL stimulation. Thus, the data suggest that SH3BP2-SYK is a novel signaling axis for regulating alveolar bone loss in periodontitis and that SYK can be a potential therapeutic target to suppress alveolar bone resorption in periodontal diseases. © 2019 American Society for Bone and Mineral Research. © 2019 American Society for Bone and Mineral Research.

Immunological Pathways Triggered by Porphyromonas gingivalis and Fusobacterium nucleatum: Therapeutic Possibilities?

Porphyromonas gingivalis (P. gingivalis) and Fusobacterium nucleatum (F. nucleatum) are Gram-negative anaerobic bacteria possessing several virulence factors that make them potential pathogens associated with periodontal disease. Periodontal diseases are chronic inflammatory diseases of the oral cavity, including gingivitis and periodontitis. Periodontitis can lead to tooth loss and is considered one of the most prevalent diseases worldwide. P. gingivalis and F. nucleatum possess virulence factors that allow them to survive in hostile environments by selectively modulating the host's immune-inflammatory response, thereby creating major challenges to host cell survival. Studies have demonstrated that bacterial infection and the host immune responses are involved in the induction of periodontitis. The NLRP3 inflammasome and its effector molecules (IL-1β and caspase-1) play roles in the development of periodontitis. We and others have reported that the purinergic P2X7 receptor plays a role in the modulation of periodontal disease and intracellular pathogen control. Caspase-4/5 (in humans) and caspase-11 (in mice) are important effectors for combating bacterial pathogens via mediation of cell death and IL-1β release. The exact molecular events of the host's response to these bacteria are not fully understood. Here, we review innate and adaptive immune responses induced by P. gingivalis and F. nucleatum infections and discuss the possibility of manipulations of the immune response as therapeutic strategies. Given the global burden of periodontitis, it is important to develop therapeutic targets for the prophylaxis of periodontopathogen infections.

Emerging roles of immunostimulatory oral bacteria in periodontitis development

Periodontitis is a common dental disease which results in irreversible alveolar bone loss around teeth, and subsequent tooth loss. Previous studies have focused on bacteria that damage the host and the roles of commensals to facilitate their colonization. Although some immune responses targeting oral bacteria protect the host from alveolar bone loss, recent studies show that particular host defense responses to oral bacteria can induce alveolar bone loss. Host-damaging and immunostimulatory oral bacteria cooperatively induce bone loss by inducing gingival damage followed by immunostimulation. In mouse models of experimental periodontitis induced by either Porphyromonas gingivalis or ligature, γ-proteobacteria accumulate and stimulate host immune responses to induce host damage. Here we review the differential roles of individual bacterial groups in promoting bone loss through the induction of host damage and immunostimulation.

Molecular mechanism of the bifunctional role of lipopolysaccharide in osteoclastogenesis

Lipopolysaccharide (LPS), a common bacteria-derived product, has long been recognized as a key factor implicated in periodontal bone loss. However, the precise cellular and molecular mechanisms by which LPS induces bone loss still remains controversial. Here, we show that LPS inhibited osteoclastogenesis from freshly isolated osteoclast precursors but stimulated osteoclast formation from those pretreated with RANKL in vitro in tissue culture dishes, bone slices, and a co-culture system containing osteoblasts, indicating that RANKL-mediated lineage commitment is a prerequisite for LPS-induced osteoclastogenesis. Moreover, the RANKL-mediated lineage commitment is long term, irreversible, and TLR4-dependent. LPS exerts the dual function primarily by modulating the expression of NFATc1, a master regulator of osteoclastogenesis, in that it abolished RANKL-induced NFATc1 expression in freshly isolated osteoclast precursors but stimulated its expression in RANKL-pretreated cells. In addition, LPS prolonged osteoclast survival by activating the Akt, NF-kappaB, and ERK pathways. Our current work has not only unambiguously defined the role of LPS in osteoclastogenesis but also has elucidated the molecular mechanism underlying its complex functions in osteoclast formation and survival, thus laying a foundation for future delineation of the precise mechanism of periodontal bone loss.

Direct effects of metabolic products and sonicated extracts of Porphyromonas gingivalis 2561 on osteogenesis in vitro

It is well documented that oral microorganisms play a significant role in the initiation and progression of periodontal disease. By using various in vitro models, it has been shown that some bacteria considered periodontal pathogens or their products can stimulate bone resorption and some other parameters of osteoblast-like cell activity. However, the effects of these organisms and their products on osteogenesis itself are not known. This study was undertaken to determine the direct effects of metabolic products and sonicated extracts of Porphyromonas gingivalis on bone formation in the chick periosteal osteogenesis model. Cultures of P. gingivalis 2561 were grown under standard anaerobic culture conditions. The spent medium was collected, and following centrifugation, sonicated bacterial extracts were prepared from the bacterial pellet. These were added in various proportions to the chick periosteal osteogenesis cultures. Sonicated extracts were further fractionated into five molecular-size ranges and similarly tested. Parameters of osteogenesis, including alkaline phosphatase activity, calcium and Pi accumulation, and collagen synthesis, were measured on 6-day-old cultures. Compared with controls devoid of bacterial products, osteogenesis was inhibited significantly in cultures treated with either conditioned medium or extracts obtained from P. gingivalis. Various amounts of inhibitory activity were observed in the different ultrafiltration molecular-size fractions, with very profound inhibitory effects observed in the < 5-kDa range. Histological observations indicated the presence of cells, some bone, and/or new fibrous connective tissue at all concentrations, indicating that toxicity was not a factor. These results suggest that periodontal pathogens such as P. gingivalis might contribute to the bone loss in periodontal diseases not only by stimulating resorption but, possibly, by inhibiting bone formation directly.

Biology of asaccharolytic black-pigmented Bacteroides species

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Isolation of a major cell envelope protein from Fusobacterium nucleatum

A major, heat-modifiable cell envelope protein was identified in Fusobacterium nucleatum FDC 364 by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. This protein, designated HM-1, had apparent molecular weights of 38,500 and 50,000 when heated in sodium dodecyl sulfate at 50 and 100 degrees C, respectively. Whole cells were labeled with 125I, and the results suggested that the HM-1 protein may be exposed on the bacterial surface. The HM-1 protein was isolated in association with the peptidoglycan by extraction of whole cells or cell envelopes with 2% sodium dodecyl sulfate at 55 degrees C. Heating the peptidoglycan-HM-1 protein complex in the detergent at 100 degrees C resulted in the quantitative release of the protein. Isoelectric focusing experiments and amino acid analysis revealed that the HM-1 protein had a basic character and was moderately hydrophilic. Various strains of F. nucleatum as well as three oral fusiform isolates contained a serologically related protein. The abundance and location of the HM-1 protein in F. nucleatum suggest that it has the potential to participate in cell surface-related interactions of this bacterium.

Utilization of fructose and ribose in lipopolysaccharide synthesis by Veillonella parvula

Veillonella parvula, which cannot ferment or incorporate most sugars, incorporated radioactivity from [14C]ribose and [14C]fructose into cellular lipopolysaccharide (LPS) in the presence of lactate as an energy source. It was shown that virtually all of the fructose carbon which was assimilated into LPS material appeared in hydrophilic LPS components, and almost none was assimilated into fatty acid LPS components. The assimilation of lactate carbon into LPS in the presence of fructose was shifted from the hydrophilic toward the fatty acid components.