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.