Arthritis is the number one cause of disability in the United States. OA, the most common form of arthritis, is a noninflammatory degenerative joint disease characterized by dysfunction of articular chondrocytes, articular cartilage degradation, osteophyte formation, and subchondral sclerosis.(25
) OA affects nearly 21 million people in the United States. It is estimated that 80% of the population will have radiographic evidence of OA by age 65, although only 60% of those will be symptomatic.(26
) The progression of OA is slow and eventually results in destruction and total loss of articular cartilage of various joints, including fingers, knees, hips, and spine. The disease process leads to limitation of joint movement, joint deformity, joint stiffness, inflammation, and severe pain. Whereas there are several strategies to reduce symptoms and/or decelerate disease progression,(27
) there are few therapeutic approaches for OA patients. Treatments for OA include nonsteroidal anti-inflammatory drugs and local injections of glucocorticoid, and in severe cases, joint replacement surgery. Currently, there is limited information about the cellular and/or molecular events that occur during articular cartilage degeneration. Therefore, understanding these events would have a tremendous impact on the development of more effective therapeutic interventions.
A genetic contribution to OA has been suggested in several epidemiologic studies.(28
) Genome-wide scans, fine-scale mapping, and candidate gene association analyses have identified several loci that may be associated with hip OA.(28
) One such locus was identified by two separate genome-wide scans for familial OA susceptibility,(28
) and finer mapping suggested a peak linkage signal at the D2S2284 (2q31.1) region.(31
) Further single nucleotide polymorphism (SNP) analysis of eight candidate genes in this region showed an association of hip OA with a functional SNP of the Frzb
) A haplotype coding for substitutions of two highly conserved arginine residues (Arg200Trp and Arg324Gly) in Frzb
is a strong risk factor for primary hip OA with an OR of 4.1 (p
) A role for the same alleles/haplotypes in generalized radiographic OA(3
) and in hip OA(11
) has also been reported in other studies in white populations.
encodes secreted frizzled-related protein 3 (sFRP3) that antagonizes the signaling of Wnt ligands through frizzled receptors.(1
) It has been recently reported that the Arg324Gly substitution of sFRP3 is associated with the development of OA in affected patients.(2
) Further-more, Frzb
KO mice are more susceptible to chemically induced OA.(4
) These observations suggest that activation of β-catenin signaling may be associated with the development of OA, although no direct evidence has been reported. In this study, we showed for the first time that conditional activation of the β-catenin
gene in articular chondrocytes in adult mice leads to OA-like articular cartilage destruction associated with accelerated chondrocyte differentiation, suggesting that β-catenin signaling plays a critical role in OA pathogenesis. This finding is consistent with reports from human genetic association studies in which patients with Frzb
gene mutation (Arg324Gly substitution of the sFRP3 protein) have a higher frequency of OA occurrence. Because β-catenin cAct mice show spontaneous OA lesion in articular cartilage, it suggests that β-catenin may play a central role in OA development caused by Frzb
mutations or other mechanisms that lead to activation of β-catenin signaling.
In this study, we found that mRNA expression of Bmp2
was significantly increased in articular chondrocytes and articular cartilage tissues (5- to 6-fold increase) derived from β-catenin cAct mice. Gene expression analysis also showed that expression of chondrocyte differentiation marker genes, regulated by BMP-2 such as Alp, Oc
, and colX
, were also significantly increased in articular chondrocytes derived from β-catenin cAct mice. It has been reported that BMP-2 induces de novo osteophyte formation in the normal murine knee joint.(33
) These observations suggest that β-catenin may regulate chondrocyte maturation and osteophyte formation in part through a Bmp2
-dependent mechanism. In this study, we also found that the expression of Mmp-13
mRNA was increased in articular chondrocytes derived from β-catenin cAct mice. MMP-13 is a potent enzyme that degrades cartilage matrix with preference for type II collagen, and the expression of MMP-13 is upregulated in human OA knee joints.(35
) The transgenic mice expressing constitutively active Mmp-13
show changes in the OA-like phenotype,(37
) suggesting a close relationship between Mmp-13
and cartilage destruction in OA. In the future, we will further study the regulatory mechanisms of β-catenin with respect to the expression of Bmp2
in articular chondrocytes.
To determine changes in Wnt signaling, we examined expression of Wnt ligands and Wnt antagonist in articular chondrocytes in which β-catenin signaling is activated. It is known that Wnt1, Wnt3a, Wnt4, Wnt7a, and Wnt7b are involved in canonical Wnt signaling and Wnt5 and Wnt11 are involved in noncanonical Wnt signaling.(38
) In this study, we found that expression of Wnt1, Wnt3a
, and Wnt7a
was significantly reduced and expression of sFRP2
was significantly increased, suggesting that there is a negative feed-back regulation in genes involved in canonical Wnt signaling. In contrast, expression of Wnt5
was significantly increased, suggesting that activation of β-catenin signaling upregulates noncanonical Wnt signaling in articular chondrocytes.
The mechanism underlying β-catenin-induced OA is that β-catenin promotes articular chondrocyte maturation, which is consistent with the role of β-catenin in the developing growth plate. In , the β-catenin-positive cells in the resting zone have lost their flattened phenotype, suggesting that these cells are undergoing maturation as a result of increased β-catenin within the cells. In addition, β-catenin-positive cells are closer to the articular surface, possibly as a result of more efficient Cre-recombination caused by better tamoxifen penetration in superficial versus deep layers.
Recently, we found that selective inhibition of β-catenin signaling in chondrocytes causes delay of growth plate chondrocyte maturation and articular cartilage destruction in Col2a1-ICAT
) We also found that cell apoptosis of articular chondrocytes is significantly increased in these transgenic mice.(39
) These findings suggest that β-catenin signaling plays a critical role in prevention of articular chondrocytes from undergoing apoptosis under normal physiological conditions.
In summary, in this study, we showed for the first time that conditional activation of the β-catenin gene in articular chondrocytes in adult mice leads to premature chondrocyte differentiation and the development of an OA-like phenotype. Our studies have provided novel and definitive evidence about the role of β-catenin signaling in articular chondrocyte function and OA pathogenesis.