Background MDC15 (or ADAM15) is a member of the family of membrane-bound disintegrin-metalloproteinases (a disintegrin and metalloproteinase: ADAM or MDC).1 Some ADAMs are involved in (patho-) physiological important processes such as remodelling of the extracellular matrix (e.g. proteolytic activity of the “aggrecanases”)2 and the shedding of membrane-bound cytokines (e.g. TNF-alpha cleavage by TACE).3 By in situ hybridization we could show that MDC15 is markedly up-regulated in osteoarthritic (OA) cartilage.4 However, mRNA expression in normal articular cartilage was below detection level, suggesting an involvement of MDC15 in catabolic processes of the cartilage matrix.
Objectives In the present study we examined the expression of MDC15 on protein level in OA cartilage compared to normal articular cartilage.
Methods The expression of MDC15 on protein level was analysed on 33 OA cartilage specimens and 14 normal tissue samples using immunohistochemistry. For this purpose a polyclonal antiserum was generated in rabbits by immunisation with a synthetic cyclized peptide (15-mer) derived from the sequence containing the RGD-motive of the disintegrin domain of MDC15. The specificity of the affinity-purified antibody was tested using Western Blot analysis on COS-7 cell lysates that were transfected with a full-length MDC15 cDNA. Immunostaining revealed a single protein band of around 100 kD in the transfected but not in the non-transfected COS-7 cells.
Results In OA cartilage MDC15 was strongly expressed in all samples analysed. MDC15 expression did not exhibit any preference for certain zones in OA cartilage. Analogous to the in situ hybridization studies, the protein could be detected already in the early stages of OA (Mankin grades 2–7). Less damaged cartilage of peripheral non-weight bearing areas also displayed strong signals well above those observed in normal aged cartilage. Strong expression of MDC15 was also present in tissues that were severely damaged (Mankin grades >7). In addition, MDC15 was found in secondary formed cartilage of osteophytes as well as metaplastic cartilage in the opened bone marrow (areas of complete cartilage erosion). In contrary, normal cartilage exhibited no or very low expression of MDC15.
Conclusion In this work we demonstrate that MDC15 expression is upregulated in OA cartilage not only on mRNA level but also on protein level. Human MDC15 is the only ADAM that contains an RGD sequence in its disintegrin domain by which it can interact with integrins5 thus interfering with cell-cell or cell-matrix interactions. Both its proteolytic potential as well as integrin-binding properties suggest that MDC15 plays an important role in cartilage remodelling in pathophysiological processes. Taken together, the proposed functions of MDC15 and its upregulation in OA, make this interesting multidomain molecule a potential candidate as a new therapeutic target.
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