The BMP antagonists follistatin and gremlin in normal and early osteoarthritic cartilage: an immunohistochemical study.

OBJECTIVE: Bone morphogenic protein (BMP) activities are controlled in part by antagonists. In human osteoarthritic (OA) cartilage, the BMP antagonists follistatin and gremlin are increased but differentially regulated. Using the OA dog model, we determined if these BMP antagonists were produced at different stages during the disease process by comparing their in situ temporal and spatial distribution.

METHODS: Dogs were sacrificed at 4, 8, 10 and 12 weeks after surgery; normal dogs served as control. Cartilage was removed, differentiating fibrillated and non-fibrillated areas. Immunohistochemistry and morphometric analyses were performed for follistatin, gremlin, BMP-2/4 and IL-1beta. Growth factor-induced gremlin expression was assessed in dog chondrocytes.

RESULTS: Follistatin and gremlin production were very low in normal cartilage. Gremlin was significantly up-regulated in both non-fibrillated and fibrillated areas at 4 weeks, and only slightly increased with disease progression. Follistatin showed a time-dependent increased level in the non-fibrillated areas with significance reached at 8-12 weeks; in the fibrillated areas significant high levels were seen as early as 4 weeks. In the whole cartilage, follistatin and IL-1beta temporal production showed similar patterns; this was also true for gremlin and BMP-2/4, though BMP-2/4 production was already high in the normal dogs. Interestingly, data revealed that basic fibroblast growth factor (bFGF) could be another factor increasing gremlin expression early in the disease process. Comparison between superficial and deep zones revealed similar patterns for follistatin and IL-1beta in the superficial zone only; gremlin and BMP-2/4 had similar patterns in both zones.

CONCLUSION: Data show, for the first time, different spatial and temporal production of gremlin and follistatin in cartilage during OA progression. These findings may reflect different roles for each antagonist in this disease.