Noradrenaline Synergistically Enhances Porphyromonas gingivalis LPS and OMV-Induced Interleukin-1β Production in BV-2 Microglia Through Differential Mechanisms

Infection with <i>Porphyromonas gingivalis</i> (<i>Pg</i>), which is a major periodontal pathogen, causes a large number of systemic diseases based on chronic inflammation such as diabetes and Alzheimer's disease (AD). However, it is not yet fully understood how <i>Pg</i> can augment local systemic immune and inflammatory responses during progression of AD. There is a strong association between depression and elevated levels of inflammation. Noradrenaline (NA) is a key neurotransmitter that modulates microglial activation during stress conditions. In this study, we have thus investigated the regulatory mechanisms of NA on the production of interleukin-1<i>β</i> (IL-1<i>β</i>) by microglia following stimulation with <i>Pg</i> virulence factors, lipopolysaccharide (LPS), and outer membrane vesicles (OMVs). NA (30-1000 nM) significantly enhanced the mRNA level, promoter activity, and protein level of IL-1<i>β</i> up to 20-fold in BV-2 microglia following treatment with <i>Pg</i> LPS (10 μg/mL) and OMVs (150 μg of protein/mL) in a dose-dependent manner. Pharmacological studies have suggested that NA synergistically augments the responses induced by <i>Pg</i> LPS and OMVs through different mechanisms. AP-1 is activated by the <i>β</i>₂ adrenergic receptor (A<i>β</i>₂R)-mediated pathway. NF-<i>κ</i>B, which is activated by the <i>Pg</i> LPS/toll-like receptor 2-mediated pathway, is required for the synergistic effect of NA on the <i>Pg</i> LPS-induced IL-1<i>β</i> production by BV-2 microglia. Co-immunoprecipitation combined with Western blotting and the structural models generated by AlphaFold2 suggested that cross-coupling of NF-<i>κ</i>B p65 and AP-1 c-Fos transcription factors enhances the binding of NF-<i>κ</i>B p65 to the I<i>κ</i>B site, resulting in the synergistic augmentation of the IL-1<i>β</i> promoter activity. In contrast, OMVs were phagocytosed by BV-2 microglia and then activated the TLR9/p52/RelB-mediated pathway. The A<i>β</i>₂R/Epac-mediated pathway, which promotes phagosome maturation, may be responsible for the synergistic effect of NA on the OMV-induced production of IL-1<i>β</i> in BV-2 microglia. Our study provides the first evidence that NA synergistically enhances the production of IL-1<i>β</i> in response to <i>Pg</i> LPS and OMVs through distinct mechanisms.