Novel approaches to promote healing of bone loss in inflammatory arthritis

Rheumatoid arthritis (RA) leads to bone loss by activating osteoclasts (OCs) to resorb bone while suppressing the ability of osteoblasts (OBs) to build bone. Importantly, patients with RA develop systemic osteopenia/osteoporosis that is not well controlled by current therapeutic agents. Therefore, it is critical to develop new agents that are anabolic for bone in the setting of inflammatory arthritis. Previously, we identified the adaptor protein Schnurri-3 (SHN3) as a potent inhibitor of bone formation. Mice lacking SHN3 develop a progressive increase in bone mass, as SHN3 deletion enhances OB function. Intriguingly, RA patients express SHN3 in cells within inflamed synovium and in serum, and SHN3 expression is induced in vitro in OBs and in synoviocytes (FLS) isolated from RA patients in response to the RA-associated cytokines TNF and IL-17A. Notably, our preliminary data show that SHN3-deficiency can protect from inflammation-induced bone loss in vitro and in vivo. Therefore, inhibition of SHN3 is an attractive mechanism to promote bone formation to treat the local and systemic bone loss that accompanies RA. Finally, we have developed a bone-specific recombinant adeno-associated virus (rAAV) that targets SHN3 and could prevent or treat bone loss in RA. Aim 1 will test whether SHN3-deficiency can limit the development of osteoporosis and bone erosion in a TNF- induced RA model by augmenting OB function, and can promote healing of articular erosions in a serum transfer- induced arthritis model. Additionally, we will test the hypothesis that SHN3-deficiency in OBs prevents the suppression of osteogenesis by the RA-associated cytokines TNF and IL-17A, while its deficiency in FLS suppresses production of inflammatory cytokines and WNT antagonists that lead to inhibition of OB differentiation. Aim 2 will determine the molecular mechanisms by which SHN3 deficiency protects from inflammation-induced suppression of osteogenesis to identify novel targets promoting bone formation. We will examine how the RA-associated cytokines TNF and IL-17A induce upregulation of SHN3 transcripts via the NF- kB pathway and stabilization of phosphorylated SHN3 via the ERK MAPK pathway, resulting in suppression of the WNT/b-catenin pathway and OB differentiation. Aim 3 will determine whether bone-specific rAAV-mediated silencing of SHN3 in vivo prevents inflammation-induced bone loss in RA. Systemic bone loss and articular erosion will be quantified in mouse models of RA treated with the bone-specific rAAV carrying a SHN3 silencer. Transcriptomic profiling will be performed in AAV-transduced OB-lineage cells isolated from RA mouse models to identify potential molecular effectors in the SHN3 pathway that promote healing of inflammation-induced bone loss. Successful completion of this work will provide proof-of-principle that SHN3-deficiency can augment bone formation at sites of inflammation-induced bone loss in RA. Understanding how SHN3 inhibition protects from suppression of OB differentiation in this setting and identifying potential novel regulators in the SHN3 pathway should provide important therapeutic targets for inflammatory arthritis.