Innate immune sensors detect nucleic acid from viral and bacterial infections to clear infection, and also detect endogenous nucleic acid from stressed or dying cells. Toll-like receptors have long been known to detect nucleic acid, while nucleic acid sensors within the cytosol have only recently been discovered. Activation of cytosolic DNA sensor pathways has been associated with autoimmune disease in which DNA from apoptotic cells accumulates. In addition, endogenous sources of DNA within cells can activate these pathways, including oxidized, "damaged" DNA that accrues with aging and can escape degradation, as well as DNA derived from replication of endogenous retroelements within the human genome. Despite the importance of these pathways, little is known about their role in cell types other than macrophages and dendritic cells. Data from our laboratory demonstrate a potentially important role for cytosolic DNA sensor pathways in bone that may provide insight into the bone loss occurring in aging and in certain autoimmune diseases. Several cytosolic DNA sensors signal through an ER-associated protein, stimulator of interferon genes (STING), including interferon-inducible protein 16 (IFI16/p204) and cyclic GMP-AMP synthase (cGAS), resulting in the production of type I interferons and proinflammatory cytokines. The cytosolic DNA sensor AIM2 does not signal through STING, but instead coordinates the assembly of an inflammasome complex, resulting in IL-1b production. We demonstrate that cytosolic DNA sensors are expressed in both osteoclast (OC) and osteoblast (OB)-lineage cells and that STING deficient mice are osteopenic, whereas AIM2 deficient mice accrue bone in long bones. Furthermore, STING and AIM2 differentially modulate the bone phenotype in a mouse model of arthritis in which cytoplasmic DNA accumulates due to deficiency in DNaseII. We hypothesize that the STING and AIM2 pathways regulate OC and/or OB differentiation/function.
Aim 1 will determine the cell-intrinsic role of the STING and AIM2 pathways in OC differentiation and/or function. Part A will determine the role of STING in osteoclastogenesis through regulation of the inhibitor of OC function, A20, or through production of cytokines. Part B will determine inflammasome-dependent or independent roles of AIM2 in OC differentiation/function. We will explore the role of these pathways in OBs as an alternative approach. Finally, in Aim 2 we will determine the impact of the STING and AIM2 pathways on expression of macrophage-derived cytokines/factors that regulate bone remodeling. This proposal explores the entirely novel hypothesis that cytosolic DNA sensors and their ligands regulate bone remodeling and may be relevant to pathologic bone remodeling in aging and autoimmunity.
This grant explores the novel hypothesis that innate immune cytosolic DNA sensor pathways regulate cells within bone to alter bone volume. Discovery of new pathways linking the skeletal and immune systems will provide critical insights that may expand the potential targets available for the treatment of bone destruction in inflammatory diseases and in aging, and may identify new anabolic pathways to build bone.