The Role of Osteoclasts in Osteoblasts Regulation

Objectives: Many diseases associated with bone loss, such as osteoporosis and periodontitis, are due to imbalance between bone formation by osteoblasts and bone destruction by osteoclasts, which in turn depends on the coupling between osteoclasts and osteoblasts. Osteoblasts are known to produce critical osteoclast regulators, RANKL and M-CSF; however the effects of osteoclasts on osteoblasts are not well understood. The aim of this project is to examine if osteoclasts affect osteoblast differentiation.

Methods: Mouse bone marrow cells were cultured with ascorbic acid (AA, 50 µg/ml), with or without RANKL (50 ng/ml). Alkaline phosphatase (ALP) and tartrate-resistant acid phosphatase (TRAP) were used as markers for osteoblast and osteoclast differentiation respectively. Image analysis was employed to quantify the percentage of area covered by ALP-positive cells, the size of osteoblastic nodules and density of ALP staining.

Results: Treatment of bone marrow cells with AA for 7 days resulted in formation of ALP-positive osteoblast nodules that covered 18 ± 6% of the well, and the formation of few osteoclasts. Co-treatment of bone marrow cells with AA and RANKL further stimulated osteoclastogenesis, resulting in the formation of 100 ± 50 osteoclasts/cm2, generally imbedded within osteoblastic nodules, which covered 14 ± 5% of the well. We analyzed if the presence of osteoclasts affects the size or appearance of individual osteoblastic nodules. Image analysis demonstrated that osteoclasts were generally present in larger nodules. Interestingly, when nodules of similar size were compared, the density of ALP staining was significantly higher in the nodules containing osteoclasts. These data suggest that osteoclasts produce local factors that stimulate ALP expression by osteoblasts.

Conclusions: Out data demonstrate that osteoclasts locally stimulate osteoblast differentiation. This finding may lead to new approaches for the treatment of bone diseases associated with imbalances in bone remodeling.