THE ROLE OF CADHERIN IN THE GENERATION OF MULTINUCLEATED OSTEOCLASTS FROM MONONUCLEAR PRECURSORS IN MURINE MARROW

A critical step in bone resorption is the fusion of mononuclear osteoc last precursors to form multinucleated osteoclasts. However, little is known of the molecular mechanisms that are responsible for this impor tant process. Since the expression of proteins in the cadherin family of hemophilic calcium-dependent cell adhesion molecules is involved in the fusion process for certain other cells, we examined their role in osteoclast formation. Immunohistochemical examination of human and mo use bone using monoclonal antibodies to human and mouse E-cadherin cle arly demonstrated positive staining in osteoclasts. N- and P-cadherin were not detected. In cultures of murine marrow mononuclear cells in w hich osteoclasts form by cell fusion, E-cadherin expression determined by Western blotting reached the highest levels as fusion was taking p lace. Expression of E-cadherin gene fragment was also detected in the marrow cultures by polymerase chain reaction. To study the functional role of E-cadherin expression in osteoclastic differentiation, neutral izing monoclonal antibodies were examined for their effects on osteocl ast formation. The antibodies decreased the number of tartrate-resista nt acid phosphatase (a marker of murine osteoclast)-positive multinucl eated cell (TRAP-positive MNC) by inhibiting the fusion of mononuclear osteoclast precursors, but not proliferation of these cells or their attachment to plastic dish surfaces. This inhibitory effect was revers ible. Furthermore, synthetic peptides containing the cell adhesion rec ognition sequence of cadherins also decreased TRAP-positive MNC format ion. The antibodies and peptides inhibited not only osteoclast formati on but also bone resorption. Antibodies to other types of cadherins an d control rat IgG had no effects in these culture systems. Our finding s suggest that E-cadherin expression may be involved in fusion (differ entiation) of hemopoietic osteoclast precursors into mature multinucle ated osteoclasts.