Porphyromonas gingivalis, an important periodontal pathogen, infects primary gingival epithelial cells (GECs). Despite the large number of bacteria that replicate inside the GECs, the host cell remains viable. We demonstrate that P. gingivalis triggers rapid and reversible surface phosphatidylserine exposure through a mechanism requiring caspase activation. However, after 1 day of infection, the bacteria no longer induce phosphatidylserine externalization and instead protect infected cells against apoptosis. Infection exerts its effect at the level of mitochondria, as P. gingivalis also blocks depolarization of the mitochondrial transmembrane potential and cytochrome c release. Interestingly, protein kinase B/Akt is phosphorylated during infection, which can be blocked with the phosphatidylinositol 3-kinase (PI3K) inhibitor LY294002. Suppression of the PI3K/Akt pathway following staurosporine treatment results in mitochondrial-membrane depolarization, cytochrome c release, DNA fragmentation, and increased apoptosis of infected GECs. Thus, P. gingivalis stimulates early surface exposure of phosphatidylserine, which could downmodulate the inflammatory response, while also promoting host cell survival through the PI3K/Akt pathway.
Ageing is associated with a decrease in the ability of cells to cope with environmental challenges. This is due partly to the attenuation of a primordial stress response, the so-called heat shock (HS) response, which induces the expression of heat shock proteins (HSPs), composed of chaperones and proteases. The attenuation of the HS response during ageing may be responsible for the accumulation of damaged proteins as well as abnormal regulation of cell death. Maintenance of the HS response by repeated mild heat stress causes anti-ageing hormetic effects on cells and organisms. Here, we describe the molecular mechanism and the state of the HS response as well as the role of specific HSPs during ageing, and discuss the possibility of hormetic modulation of ageing and longevity by repeated mild stress.
Porphyromonas gingivalis, an important periodontal pathogen, is an effective colonizer of oral tissues. The organism successfully invades, multiplies in, and survives for extended periods in primary gingival epithelial cells (GECs). It is unknown whether P. gingivalis resides in the cytoplasm of infected cells throughout the infection or can spread to adjacent cells over time. We developed a technique based on flow cytofluorometry and fluorescence microscopy to study propagation of the organism at different stages of infection of GECs. Results showed that P. gingivalis spreads cell to cell and that the amount of spreading increases gradually over time. There was a very low level of propagation of bacteria to uninfected cells early in the infection (3 h postinfection), but there were 20-fold and 45-fold increases in the propagation rate after 24 h and 48 h, respectively, of infection. Immunofluorescence microscopy of infected cells suggested that intercellular translocation of P. gingivalis may be mediated through actin-based membrane protrusions, bypassing the need for release of bacteria into extracellular medium. Consistent with these observations, cytochalasin D treatment of infected cells resulted in significant inhibition of bacterial spreading. This study shows for the first time that P. gingivalis disseminates from cell to cell without passing through the extracellular space. This mechanism of spreading may allow P. gingivalis to colonize oral tissues without exposure to the humoral immune response.
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