SUMMARY
Osteoarthritis (OA) is a debilitating disease characterized by loss of joint space, degeneration of cartilage at
articular surfaces, remodeling of bone and other joint tissues, and inflammation. Although it is the major cause
of disability in the aged population, not a single disease modifying drug is available. The functions and
identities of the biological processes that pose vulnerabilities to OA and need to be targeted therapeutically are
not known! We propose elevated NOD/RIPK2 signaling in joint tissues confers heightened susceptibility to OA.
Our analyses of 151 families with dominantly inherited OA yielded 7 novel OA-susceptibility alleles affecting 6
components of the NOD/RIPK2 signaling pathway. This pathway utilizes intracellular NOD receptors to sense
damage-associated molecular patterns and signals via RIPK2 to elicit tissue-specific responses. We studied
one disease allele, Ripk2Asn104Asp, in depth. Functional analyses indicate the variant protein drives elevated
pathway signaling. Mice carrying the dominant Ripk2104Asp allele, introduced by genome editing, have
significantly increased sensitivity to post-traumatic OA, and aged Ripk2104Asp mice have severe spontaneous
OA. We hypothesize regulation of Nod/Ripk2 signaling is a central component of the homeostatic signaling
networks and cell processes that maintain the synovial joint.
The initiating factors for OA are unknown. Although chronic systemic inflammation has been proposed as an
OA susceptibility factor, unoperated young Ripk2104Asp mice have elevated expression of OA-associated and
inflammatory markers in their knees without coincident elevation of markers of systemic inflammation. In Aim 1
we use a genetically modified mouse that permits conditional expression of the dominant Ripk2104Asp allele in
selected cell types to identify cells in which Ripk2 signaling affects OA, thus testing our hypothesis that
localized aberrant Nod/Ripk2 signaling restricted to the joint is sufficient to augment susceptibility to OA.
Mouse models of aging-dependent OA are rare and are needed to discover the changes that indicate early
stages of OA. Using mice that constitutively express the Ripk2104Asp allele, in Aim 2 we test the hypotheses that
elevated Ripk2 signaling: i) promotes OA-associated gene expression of multiple joints, and ii) specifically
causes accelerated onset of histologically recognizable age-dependent OA in the weight-bearing knee. RNA-
seq analyses will be used to uncover changes in gene expression patterns and cell populations in the joint that
parallel development of OA. Aim 3 tests the hypothesis that the p.Asn104Asp substitution enhances
ubiquitination of Ripk2, which leads to prolonged or enhanced signaling. In Aim 4 we test whether therapeutic
attenuation of Ripk2 signaling in Ripk2104Asp or WT mice can reduce sensitivity or severity of acute injury. The
work will have direct clinical impact, informing efforts to identify biomarkers of susceptibility or early stages of
disease and efforts to design and develop therapies for OA.
