In this study, we demonstrate that BMP signaling affects both the initiation and the progression of ankylosis in a model of ankylosing enthesitis and spondyloarthropathy. BMP signaling has been studied in a mouse model of degenerative joint disease (30
). Ectopic bone formation in this disorder may be a consequence of structural damage and is likely to be part of the body’s response to local injury, either as an attempt to increase joint stability or as a secondary effect of growth factors trying to preserve joint homeostasis (31
). Our data provide evidence that deregulation of specific embryonic signaling pathways can also contribute to pathology. Indeed, several arguments indicate that the process of enthesial bone formation leading to ankylosis, as seen in spondyloarthropathies and in this model, is different from osteophytes in osteoarthritis. First, progressive ankylosis in both spine and peripheral joints is a major contributor to disability. Second, ankylosing enthesitis presents frequently as clinically symptomatic arthritis. Third, in contrast to osteoarthritis, heterotopic endochondral bone formation takes place in tissue that is considered one of the primary disease targets (3
). Therefore, inhibition of ankylosis may be an important and specific therapeutic target in spondyloarthropathy (11
To address the mechanisms driving joint ankylosis, we have used a spontaneous mouse model of arthritis. Although it was originally reported as another model of rheumatoid arthritis (23
), subsequent studies involving microscopic analysis have clearly demonstrated that this model is characterized by ankylosing enthesitis (25
) rather than destructive synovitis. This model is particularly attractive since arthritis occurs in association with environmental factors such as stress that have also been implicated in spondyloarthropathy (24
). It also shows specific features of psoriatic arthritis (25
) that have not been reported in other models of arthritis. The high incidence and reproducibility of arthritis in this model, in contrast to other models of ankylosis (35
), highlight its usefulness for mechanistic studies in the process of ankylosis in spondyloarthropathy. As no animal model completely mimics human disease, different questions can be raised regarding the relevance of this model to human spondyloarthropathy. First, the absence of spine pathology in this model can at least partially be explained by differences in weight bearing between rodents and humans. So far, there are no indications that ectopic bone formation in spine and in peripheral joints is distinct. Both processes mimic predominantly endochondral bone formation (5
). Second, the inflammatory reaction associated with the proliferative and metaplastic process at the enthesis is still poorly defined. We have demonstrated that dactylitis or local inflammation at the enthesis, characterized by the presence of neutrophils and some mononuclear cells, precedes or coincides with connective tissue proliferation and differentiation (25
). Remarkably, Nordling et al. demonstrated that anti-idiotypic anticollagen type II antibody treatment may prevent the onset of arthritis (37
). Autoimmunity toward components of fibrocartilage has been proposed by different authors as directly relevant for the development of spondyloarthropathy (5
). Autoantibodies toward type II collagen have also been demonstrated in spondyloarthropathy patients (39
). Interestingly, we have also shown that inhibition of IFN-γ delays the onset of disease (26
). However, in contrast to the effect of noggin, no therapeutic effect of anti–IFN-γ strategies was seen once symptoms had occurred (26
). The finding that the disease process is apparently T cell independent seems to be in contrast with the proposed role for T cells in both psoriatic arthritis (40
) and ankylosing spondylitis (5
). In these experiments, however, T cell receptors αβ–/–
were used separately. Therefore, a role for T cells in the development of autoimmunity toward fibrocartilage components in this model cannot be excluded. Taken together, all these data support the concept of juxtaposed stages in the disease that are coupled but partially independent. Our model therefore seems representative of the process of ankylosis in spondyloarthropathy, which can be considered as a specific therapeutic target.
The somewhat surprising effects in the early stages of the disease highlight the importance of BMP signaling. This pathway is an essential part of the complex network regulating skeletal development (14
). Endochondral bone formation is initiated by mesenchymal cell condensations. Cells within these condensations undergo chondrogenic differentiation, progressively acquiring the phenotypes of proliferating, prehypertrophic, and hypertrophic chondrocytes. In the later stages, the cartilaginous elements are replaced by bone (13
). Studies in developmental models suggest that fine-tuned balances between BMPs and inhibitors such as noggin appear to influence different stages of endochondral bone formation. Retroviral over- and misexpression of noggin in the developing chick limb revealed a dual role for BMP signaling in early stages (41
). Both cell condensation and differentiation of chondroprogenitor cells into chondrocytes were inhibited effectively, depending on the time of infection.
Various challenges regarding the role of BMPs in ankylosing enthesitis remain. First, understanding of the relative contribution of specific BMPs and inhibitors to the pathological cascade needs to be refined. Our data suggest an early role for BMP2 and later involvement of BMP6 and BMP7. The presence of additional ligands has not been studied. However, regardless of their presence, noggin has been demonstrated as an antagonist of the BMP2/4, the BMP5/6/7, and the growth and differentiation factor 5/6/7 groups (42
). Therefore, noggin overexpression is likely to change the overall balance in BMP signaling in our model. Second, the identification of factors leading to the activation of the BMP-signaling pathway is of critical importance to understanding the links between inflammation and bone formation, but the exact mechanism remains unknown. We and other groups have demonstrated that specific BMPs, including BMP2 and BMP6, are upregulated by proinflammatory cytokines in cell populations obtained from the joint, providing circumstantial evidence that inflammatory changes and BMP expression are linked (43
). Other explanations include increased expression of BMP receptors or BMP-signaling enhancers (46
), through genetic or environmental factors, that lead to enhanced activation of the smad pathway.
It is well known that the biological response to BMPs is dependent on the target tissue (19
). Injection of recombinant BMP2 into the knee joints of mice resulted in osteophyte formation in areas adjacent to the bone and did not result in cartilage formation in the synovium itself (47
). Adenoviral infection of periost with BMP2 in vivo stimulated endochondral bone formation at the application site (48
In summary, our findings support the concept that BMP signaling is an attractive therapeutic target for achieving disease modification in spondyloarthropathy. Symptom control by inhibition of inflammation may not be sufficient to stop the structural progression of disease and the resulting disability. Therefore, specific molecular targets involved in cartilage and bone formation, including the BMPs and their transduction machinery, may provide a complementary or alternative therapeutic approach in patients with spondyloarthropathies. In the complex network of cell differentiation and lineage commitment, molecular networks may be crucial, and single-gene approaches to treating diseases may not be sufficient.