SGK1 and the control of periodontal inflammation

Abstract Macrophage polarization enables macrophages adapt to various microenvironmental cues and adopt a spectrum of functional phenotypes. These are generally classified into classically activated (pro-inflammatory, M1) and alternatively activated (pro-healing, M2) categories. The M1/M2 balance determines the extent and severity of inflammatory responses and tissue damage. However, the signaling pathway(s) and cues driving macrophage polarization in the oral cavity's infectious milieu are largely unknown. Yes-associated protein (YAP) and transcriptional co-activator with PDZ-binding motif (TAZ) are transcriptional co-activators implicated in inflammation regulation and macrophage polarization in a context-dependent manner. Despite the observation of aberrant YAP1/TAZ expression in gingival tissues from patients with periodontitis, the triggers and mechanisms of YAP1/TAZ signaling in regulating macrophage polarization in periodontium remain unclear. Our previous R01 study unveiled the suppressive role of serum- and glucocorticoid- inducible kinase (SGK) 1 in the immune response to the oral pathogen. Using Porphyromonas gingivalis (Pg) as a model organism, in this renewal project, we aim to investigate the regulation of YAP1 by SGK1 in Pg-stimulated macrophages and its subsequent impact on macrophage polarization, thereby revealing a novel regulatory module, SGK1-YAP1, in periodontal inflammation. Preliminary results showed that (i) SGK1 inhibition remarkably decreases YAP1 phosphorylation and increases its nuclear retention in Pg-stimulated macrophages; (ii) Pg infection enhances YAP1 phosphorylation and promotes its retention in the cytoplasm, and the ubiquitin-proteasome inhibitor further augments YAP1 expression; (iii) YAP1 inhibition suppresses macrophage polarization towards M1 phenotypes; and (iv) sgk1 deficiency increases nuclear YAP1 expression and the M1/M2 ratio in the gingival tissue from mice subjected to repeated Pg infection. These findings lead to our hypothesis that YAP1 cytoplasmic retention and expression are controlled by SGK1 in response to Pg challenge. This control mechanism limits macrophage polarization towards M1 phenotypes, thereby preventing tissue damage and periodontal inflammation. To test this hypothesis, we will (i) determine and characterize the regulation of YAP1 by SGK1 in innate immune cells in response to Pg; (ii) functionally dissect SGK1-YAP1 signaling pathways in macrophage polarization upon Pg challenge; (iii) assess the effect of SGK1- YAP1 signaling in vivo on macrophage polarization and disease process induced by Pg using murine infection models. Successful completion of this study will identify SGK1 as a novel regulator of YAP1/TAZ signaling, functioning through dual mechanisms of phosphorylation and ubiquitination. For the first time, we will delineate the alternative macrophage polarization mediated by Pg via SGK1-YAP1 signaling. This work not only advances our understanding of macrophage polarization in periodontium but also could inform studies aimed at identifying novel targets to restore macrophage subset balance, thus ameliorating/preventing Pg-induced tissue destruction.