Bone formation during development and growth relies on a number of molecular signalling pathways and their complex interactions [18
]. Increasing evidence supports the concept that similar pathways are important during cartilage and bone pathology, particularly with regard to new bone formation. These pathways include bone morphogenetic protein (BMP), wingless-type like (WNT), hedgehog, fibroblast growth factors, notch and parathyroid hormone-like peptide signalling.
The potential roles played by BMP and WNT signalling in the process of ankylosis in SpA were recently studied in various animal models. Our group has used the spontaneous arthritis model in ageing male DBA/1 mice to study molecular mechanisms of ankylosing enthesitis [19
]. These immunologically normal mice develop oligoarthritis, especially in the toes of the hind limbs, from the age of 12 weeks onward after grouped caging of males from different litters. The disease process is not characterized by primary synovitis but rather by entheseal cell proliferation, cartilage and bone differentiation, leading to peripheral joint ankylosis through orthotopic endochondral bone formation. The model also presents with dactylitis and destructive onychoperiostitis, which are well recognized features of human PsA. This model also has its limitations. Entheseal new cartilage and bone formation are only seen in peripheral joints and not in the spine. Inflammation with infiltration of immune populations into the joint tissues is only of short duration and does not appear to become a chronic process. These features are in contrast to what is commonly seen in SpA. Nevertheless, the model allows one to study molecular mechanisms of new tissue formation and may provide some information about the relationship between inflammation and ankylosis.
BMPs were originally identified as protein factors that can induce an ectopic cascade of endochondral bone formation in vivo
, and are members of the transforming growth factor-β superfamily. We demonstrated that different BMPs are expressed during the process of ankylosis in male DBA/1 mice [20
]. BMP2 is typically found in proliferating cells and entheseal cells that commit their differentiation fate to chondrogenesis. BMP7 is recognized in prehypertrophic chondrocytes, whereas BMP6 is associated with hypertrophic chondrocytes.
In the spontaneous ankylosing enthesitis model, systemic over-expression of noggin, a BMP antagonist with broad ligand affinity, inhibited the incidence, and clinical and histomorphological severity of arthritis in a dose-dependent manner in both preventive and therapeutic experiments [20
]. Progenitor cells committing to chondrogenic differentiation were recognized as BMP target cells. The histomorphological and molecular analysis of the experiments strongly suggested that BMPs play a role in these initial phases of the disease process.
However, the process of entheseal endochondral bone formation is highly regulated at different stages. Endogenous noggin is expressed in prehypertrophic and hypertrophic chondrocytes and appears to play a role in reducing some BMP signals in the replacement of hypertrophic chondrocytes by bone. A reduction in these endogenous noggin levels in noggin haplo-insufficient mice was associated with slower progression of ankylosis without affecting the initial stages of the disease [21
]. These data are consistent with the complex role played by the BMP signalling pathway and its antagonists as regulators of endochondral bone formation, with different effects at distinct stages [18
Interestingly, in a recent study, presented as an abstract, the investigators used a similar strategy to inhibit BMP signalling in aggrecan-induced spondylitis [22
]. As our group demonstrated for peripheral arthritis, over-expression of noggin resulted in reduced spinal ankylosis, a feature of this murine disease model. Different BMPs were found at similar disease stages, and the target cells in this model appeared to be identical to those in our earlier work. We also described such BMP target cells in human entheseal lesions of the Achilles' tendon insertion [20
Another study identified dickkopf (DKK)1, an antagonist of the WNT signalling pathway, as a potential key regulator of the balance between erosive joint destruction and new bone formation in inflammatory arthritis. Diarra and coworkers [23
] demonstrated that inhibition of DKK1 with specific antibodies changed the histomorphological appearance of arthritis in human TNF transgenic mice and other models, such as collagen-induced and glucose-6-phosphate isomerase-induced arthritis. The anti-DKK treated mice exhibited osteophyte formation, which was absent in control antibody treated mice. Dkk1
is a TNF target gene through p38 mitogen-activated protein kinase. Inhibition of DKK1 results in higher osteoprotegerin levels, which block the activation of osteoclasts and hence bone erosion. In addition, bone formation appears to be directly enhanced by stimulating WNT signalling both in vitro
and in vivo
Both observations, blocking BMPs to inhibit ankylosis and a WNT antagonist to stimulate it, albeit in different models, raise questions about the potential interactions or primary roles of these specific pathways. As mentioned above, BMPs were originally identified as proteins that can induce endochondral bone formation. In our studies, we identified BMP2 as an early mediator of chondrogenesis in ankylosing enthesopathy. Similar observations were reported in other models of chondrogenesis and osteogenesis. Tsuji and coworkers [24
] demonstrated that limb-specific BMP2 knockout mice develop a normal skeleton but fail to maintain bone growth and homeostasis in the limb after birth. Limb-specific osteoporosis and spontaneous fractures occur, and the natural healing process is absent. In addition, these limb-specific BMP2 knockout mice fail to heal fractures in a fracture model [24
]. The authors hypothesize that before birth loss of BMP2 in the limb can be compensated for by other BMPs, whereas this seems no longer the case postnatally. These findings indicate that developmental and postnatal processes may have many similarities but can be different at the molecular level. BMPs also play a critical role in the development of osteophytes in models of osteoarthritis [25
The effects of WNT signalling on bone formation appear more complex. WNTs are a family of glycoproteins with an array of functions during development, growth, tissue homeostasis and disease. Some of the WNT ligands, in particular WNT3A and WNT10B, are associated with direct membranous bone formation during development and growth, most likely by activation of the so-called canonical WNT signalling pathway in which the nuclear translocation of β-catenin acts as a downstream mediator [26
]. The roles of WNTs in endochondral bone formation are more difficult to understand. WNT3A and WNT7A have been shown to inhibit chondrogenesis in endochondral bone formation in developmental models [26
]. Other ligands, WNT5A and WNT5B, appear to play opposite roles in determining the pace of chondrocyte differentiation [27
The complex and contrasting effects of WNT proteins are further highlighted by studies of intracellular mediator β-catenin. Over-expression of a constitutively active form of this molecule in developing skeletal elements, mimicking enhanced WNT signalling, inhibited the early stages of chondrogenesis, whereas over-expression in later stages stimulated maturation of the chondrocytes and bone formation [28
]. These observations are in accordance with a study in which the progression of BMP2-induced endochondral bone formation was found to be dependent on β-catenin [29
Taken together, current evidence therefore suggests that WNTs are most important in the later stages of endochondral bone formation. WNTs signals stimulate progenitor cells into the bone lineage and may inhibit early cartilage differentiation. This negative effect on chondrogenic differentiation may also be important postnatally, because WNTs appear to have a negative effect on articular cartilage homeostasis. For instance, mice that are deficient in the secreted WNT antagonist frizzled related protein (FRZB) develop more severe cartilage damage in osteoarthritis models, which is associated with enhanced WNT signalling and increased expression of WNT target genes [30
]. Specific activation of β-catenin in articular cartilage in a genetic mouse model also leads to an osteoarthritic phenotype [31
]. Surprisingly, the same group also reported that lack of β-catenin in vivo
leads to loss of articular cartilage [32
Based upon these data, we hypothesize that BMP family members are critical in the early phases of ankylosis in SpA and that WNT signalling through β-catenin plays a crucial supportive role in this process, in particular in the progression of endochondral bone formation (Figure ).
Figure 1 Roles of BMPs and WNTs in endochondral bone formation. (a) Physiological endochondral bone formation is stimulated by bone morphogenetic proteins (BMPs). Wingless-type like (WNT) signaling plays a supportive role in relation to BMPs. However, some WNTs (more ...)