OSTEOPONTIN EXPRESSION BY OSTEOCLAST AND OSTEOBLAST PROGENITORS IN THE MURINE BONE-MARROW - DEMONSTRATION OF ITS REQUIREMENT FOR OSTEOCLASTOGENESIS AND ITS INCREASE AFTER OVARIECTOMY

Osteoclast development requires: cell-to-cell contact between hematopo ietic osteoclast progenitors and bone marrow stromal/osteoblastic supp ort cells. Based on this, we hypothesized that osteopontin, an adhesio n protein produced by osteoclasts and osteoblasts, plays a role in ost eoclastogenesis. Using in situ hybridization, we demonstrate that cell s expressing the osteopontin messenger RNA (mRNA) appear after 3 days of culturing murine bone marrow cells. The number of these cells incre ases thereafter, reaching a peak on day 5. In the same cultures, cells expressing alkaline phosphatase (AP) or tartrate resistant acid phosp hatase (TRAP), phenotypic markers for osteoblastic and osteoclast-like cells, respectively, appeared subsequent to the appearance of the ast eopontin-positive cells. By means of a combination of in situ hybridiz ation and histostaining, it was shown that the osteopontin mRNA was lo calized in 30-50% of the AP-positive or the TRAP-positive, as well as in nonspecific esterase (NSE)-positive, cells. The number of cells exp ressing both the osteopontin mRNA and either one of the three phenotyp ic markers was significantly increased in bone marrow cultures from es trogen-deficient mice, as compared with controls. Conversely, the numb er of all three populations of double positive cells was decreased in cultures treated with a specific antimouse rabbit osteopontin antibody or an RGD peptide. These findings indicate that osteopontin is expres sed during the early stages of the differentiation of osteoclast and o steoblast progenitors in the bone marrow and that its cell adhesion pr operties are required for osteoclastogenesis.