Periodontitis promotes bacterial extracellular vesicle-induced neuroinflammation in the brain and trigeminal ganglion.

Gram-negative bacteria derived extracellular vesicles (EVs), also known as outer membrane vesicles, have attracted significant attention due to their pathogenic roles in various inflammatory diseases. We recently demonstrated that EVs secreted by the periodontopathogen Aggregatibacter actinomycetemcomitans (Aa) can cross the blood-brain barrier (BBB) and that their extracellular RNA cargo can promote the secretion of proinflammatory cytokines, such as IL-6 and TNF-α, in the brain. To gain more insight into the relationship between periodontal disease (PD) and neuroinflammatory diseases, we investigated the effect of Aa EVs in a mouse model of ligature-induced PD. When EVs were administered through intragingival injection or EV-soaked gel, proinflammatory cytokines were strongly induced in the brains of PD mice. The use of TLR (Toll-like receptor)-reporter cell lines and MyD88 knockout mice confirmed that the increased release of cytokines was triggered by Aa EVs via TLR4 and TLR8 signaling pathways and their downstream MyD88 pathway. Furthermore, the injection of EVs through the epidermis and gingiva resulted in the direct retrograde transfer of Aa EVs from axon terminals to the cell bodies of trigeminal ganglion (TG) neurons and the subsequent activation of TG neurons. We also found that the Aa EVs changed the action potential of TG neurons. These findings suggest that EVs derived from periodontopathogens such as Aa might be involved in pathogenic pathways for neuroinflammatory diseases, neuropathic pain, and other systemic inflammatory symptoms as a comorbidity of periodontitis.

Use of Bacterial Extracellular Vesicles for Gene Delivery to Host Cells.

Extracellular vesicles (EVs), which are nanosized membranous particles secreted from both prokaryotic and eukaryotic cells, can deliver various biological molecules, such as nucleic acids, proteins, and lipids, into recipient cells. However, contrary to what is known about eukaryotic EVs, whether bacterial EVs (bEVs) can be used as transporters for bioactive molecules is becoming a hot area of research. In this study, we electroporated enhanced green fluorescent protein (EGFP) genes and precursor microRNA of Cel-miR-39 (pre-Cel-miR-39) from isolated bEVs of Escherichia coli and Lactobacillus reuteri. The EGFP plasmid, synthetic EGFP RNA, and pre-Cel-miR-39 were successfully delivered into the murine microglial BV2 cells via bEVs. PCR and confocal microscopy analysis confirmed the transfer of the EGFP plasmid and RNA. The bEV-delivered exogenous pre-Cel-miR-39 was further processed into the mature form of Cel-miR-39; its incorporation into Ago2-a major component of the RNA-induced silencing complex-was assessed using RNA-immunoprecipitation-PCR. Taken together, bEVs can be used as vehicles to deliver genetic materials and for novel biotechnological applications, such as gene transfer and mRNA vaccines.

Delivery of Periodontopathogenic Extracellular Vesicles to Brain Monocytes and Microglial IL-6 Promotion by RNA Cargo.

Gram-negative bacterial extracellular vesicles (EVs), also known as outer membrane vesicles (OMVs), are secreted from bacterial cells and have attracted research attention due to their role in cell-to-cell communication. During OMV secretion, a variety of cargo such as extracellular RNA (exRNA) is loaded into the OMV. The involvement of exRNAs from a range of bacteria has been identified in several diseases, however, their mechanism of action has not been elucidated. We have recently demonstrated that OMVs secreted by the periodontopathogen Aggregatibacter actinomycetemcomitans can cross the blood-brain barrier (BBB) and that its exRNA cargo could promote the secretion of proinflammatory cytokines in the brain. However, it was unclear whether the brain immune cells could actually take up bacterial OMVs, which originate outside of the brain, in an appropriate immune response. In the present study, using monocyte-specific live CX3CR1-GFP mice, we visualized OMV-colocalized meningeal macrophages and microglial cells into which bacterial OMVs had been loaded and intravenously injected through tail veins. Our results suggested that meningeal macrophages uptake BBB-crossed OMVs earlier than do cortex microglia. BV2 cells (a murine microglia cell line) and exRNAs were also visualized after OMV treatment and their proinflammatory cytokine levels were observed. Interleukin (IL)-6 and NF-κB of BV2 cells were activated by A. actinomycetemcomitans exRNAs but not by OMV DNA cargo. Altogether, these findings indicate that OMVs can successfully deliver exRNAs into brain monocyte/microglial cells and cause neuroinflammation, implicating a novel pathogenic mechanism in neuroinflammatory diseases.