Osteoarthritis (OA) is the most common form of arthritis and a leading cause of disability in adults. While aging is an important risk factor for the development of OA, the molecular mechanisms responsible for OA in the context of aging remain unclear. Here, we propose to test the hypothesis that DNA DSBs in articular chondrocytes promote cell cycle re-entry and induce both IKK?/NF-?B and IRF signaling downstream of STING to mediate a pro-inflammatory secretory phenotype that leads to cartilage degeneration and inflammation in surrounding joint tissues. We show that IKK?/NF-?B signaling increases with age in the articular chondrocytes of wild type mice and that, by 24 months, these mice develop a knee joint phenotype exhibiting early hallmarks of OA. We also show that sustained IKK? activation in the chondrocytes of young mice greatly accelerates the onset of this age-related OA phenotype likely through paracrine actions on surrounding cells via a proinflammatory secretory program consisting of numerous cytokines, chemokines, growth factors, and MMPs that can alter proliferation and viability of neighboring cells, remodel the cartilage ECM, and initiate an innate immune response. DNA double-strand breaks (DSBs) are known to induce IKK?/NF-?B signaling and we provide evidence here of DNA DSBs in aged murine articular chondrocytes. We also provide gene expression data showing that IRF and Interferon signaling are among the most significantly affected pathways in aged articular chondrocytes. DNA DSBs can induce both NF-?B and IRF signaling through Stimulator of interferon genes, or STING. Thus, we hypothesize that STING may be upstream of both NF-?B and IRF to produce a proinflammatory secretory phenotype in chondrocytes with DSBs. In the first Aim of this proposal, we will test the effects of chondrocyte-specific DNA DSBs on chondrocyte function and OA development using AcanCreERT2/+; R26I-PpoI mice where DSBs are produced specifically in chondrocytes upon administration of tamoxifen. We will determine how chondrocytes respond to this genotoxic stress and also whether persistent DSBs are capable of promoting OA. In the second Aim, we will test whether STING is specifically involved in the activation of IKK?/NF-?B and IRF signaling following induction of DSBs and in the onset of age-related OA using in vitro cell culture methods as well as an in vivo STING knockout model in combination with NF-?B and IRF endogenous reporters. Results from these studies will provide valuable insight to the effects of DNA DSBs on chondrocyte fate and also on the identity of upstream activators of NF-?B in aged chondrocytes, potentially leading to the identification of a novel mechanism in which to inhibit IKK?/NF-?B signaling and inflammation in OA chondrocytes.
Osteoarthritis (OA) is the most common cause of disability among aging adults and is a significant clinical challenge and public health burden as there is currently no treatment to prevent disease progression or induce cartilage regeneration. Chronic, low-grade inflammation in the joint is now recognized as a key contributor to OA pathogenesis, however, the source of this inflammation and its role in joint degeneration is currently unclear. The experiments outlined in this proposal are designed to examine whether aging chondrocytes are the initial source of pro-inflammatory factors due to DNA double-strand breaks and elevated activity of a protein known to initiate pro-inflammatory signaling in response to damaged DNA. Whether inhibition of this protein could be a viable therapeutic approach to preventing the onset or progression of disease will be investigated.
