Differences in expression of Wnt antagonist Dkk1 in healthy versus pathological bone samples

Wnt/-catenin signalling components was shown to affect bone cells function including chondrocytes.Secreted Dkk1, a potent osteogenesis inhibiting factor mediates bone loss in diseased bones by suppressing the biological actions of Wnt proteins. In addition, increased Dkk1 signalling inhibits chondrogenesis in new bone formation. Recent findings also show there exists a cross-talk between the chondrocytes and the cells of the osteoblast lineage, which are the most affected cell types in muskuloskeletal disorders. This study investigated whether spatial expression of Dkk1 is confined to only osteoblasts, osteocytes or chondrocytes. The second objective was to detect a difference in the Dkk1 expression pattern in healthy subjects when compared to pathological state. To elucidate the cell specificity of Dickkopf-1 (Dkk1) in healthy bones, samples from female Sprague-Dawley rats were tested against two different antibodies with the two most widely accepted visualization system (ABC and Envision). The findings show Dkk1 specificity predominantly for osteoblasts, chondrocytes and osteocytes depending upon the antibody used. In addition, Dkk1 expression was evaluated in different cells of human osteoarthritis (OA) and rheumatoid arthritis (OA) patients. Its overexpression in pathologic state also suggests the role of Dkk1 in bone formation. This is scientifically and clinically important in studying the effect of Dkk1 in bone healing and in designing treatments for patients with compromised bone status. Taking into consideration the paradigm that cartilage and subchondral bone behave as an interconnected functional unit, normalization of cell behaviour in one compartment may have benefits in both tissues. Lay description Proteins reflect the genetic material of life forms. Cells interact and regulate adjacent cells and cellular matrix through protein expression. Immunohistochemical identification reflects this regulation; therefore false positive signals can be misleading. This study provide a quality assurance of described antibodies showing that whether expression of Dkk1 protein is genuine in cartilage cells (chondrocytes) or bone cells (osteoblasts and osteocytes). Such statement can influence the interpretation of results and thus planning for future therapeutic strategies by targeting the wrong cells. The study shows the cellular specificity of Dkk1 against two widely used antibodies and the available visualization systems. In addition the study also presents its expression patterns in healthy versus pathological tissue samples thereby supporting the role of Dkk1 in bone remodeling. This paper not only aims at establishing a guideline to show which antibody could be combined with what visualization system to provide dependable results and avoid misleading interpretations but also directs to the potential role of the canonical Wnt signaling in bone remodeling.