Secreted gingipains from Porphyromonas gingivalis induce microglia migration through endosomal signaling by protease-activated receptor 2.

Much attention has been paid to the connection between periodontitis and Alzheimer's disease (AD). We previously showed that infection of P. gingivalis, one of major periodontal pathogen causing periodontitis, induced migration and inflammatory responses in murine microglia through gingipain-induced activation of protease-activated receptor 2 (PAR2). In this study, we have attempted to clarify effects of secreted gingipains on cell migration of human microglial cell line, cleavage sites of PAR2 by gingipains and subsequent signaling pathways. P. gingivalis culture supernatant induced migration and membrane ruffling, which is necessary for microglia migration, in human microglial cell line HMC3 cells through PAR2. These effects were mainly mediated by gingipains, because cell migration and membrane ruffling were dramatically inhibited by treatment with gingipain inhibitors. Furthermore, pharmacological and genetic inhibition of Src kinase and β-arrestin, which are important for the internalization of G protein-coupled receptors, significantly inhibited P. gingivavlis culture supernatant-induced membrane ruffling in HMC3 cells. After treatment with P. gingivalis culture supernatant in Flag-PAR2-HA transfected HEK293T cells, Flag was removed from the cell surface, and HA was detected in the cytosol, indicating the internalization of PAR2. Furthermore, the phosphorylation level of ERK1/2 increased in PAR2-transfected HEK293T cells after treatment with P. gingivalis culture supernatant. The gingipain inhibitors, Src kinase inhibitor and β-arrestin knockdown suppressed PAR2 internalization and ERK1/2 phosphorylation. These observations suggest that secreted gingipains from P. gingivalis induce Src- and β-arrestin-dependent internalization of PAR2 and further activate the ERK1/2 pathway to promote migration of microglia. PAR2 are activated by the tethered ligands exposed by cleavage of extracellular N-terminal of PAR2. We also estimated potential gingipain cleavage sites in PAR2 and exposed tethered ligands, which are required for PAR2 internalization and membrane ruffling. The identified mechanism in this study might contribute to the retrogression of sporadic AD in patients after infection with P. gingivalis.