Phenotypic Characterization of Peripheral Osteoclast Precursors their Lineage Relation to Macrophages and Dendritic Cells and their Population Dynamics Influenced by Parathyroid Hormone and Inflammatory Signals.

Osteoclasts are unique multinuclear cells that are highly specialized for resorbing bone tissue and therefore play crucial roles in bone remodeling. The identification of bona fide populations of osteoclast precursors (OCPs) is crucial to understand their biology in homeostatic and pathological conditions. We identified a bone marrow precursor with high osteoclastogenic activity (Jacquin et al., 2006) and extended these findings to test their clonogenic potential and bone resorptive capacity at the single cell level. We also identified the phenotype of peripheral blood and spleen OCPs. In chapter III, we showed that bone marrow and peripheral OCPs gave rise to macrophages and dendritic cells (in addition to osteoclasts) when stimulated with differential cytokines, suggesting a common developmental pathway among these cells. We developed a fluorescent reporter mouse model to study osteoclast migration and differentiation in vivo. Transplant studies showed that bone marrow OCPs home back to the bone marrow and migrated to the spleen while circulating in peripheral blood. Interestingly, spleen precursors were able to migrate to bone marrow and also engrafted in the spleen. In addition, transferred OCPs differentiated to fluorescent osteoclasts associated to bone surfaces. These experiments suggest that the bone marrow and spleen precursors are developmentally related. Osteoclasts are critically involved in skeletal function such as bone remodeling, fracture repair, and in pathological bone resorption associated with inflammatory conditions. We challenged mice with LPS and found that peripheral OCPs increased their number and resorptive capacity. In addition, we studied how chronic inflammation modulated OCPs in arthritic (hTNF) mice, and found that peripheral precursors were increased in number. OCPs derived from CathepsinK-Cre-tdTomatoFP recipient mice and transferred to hTNF, engrafted to localize sites of inflammation and differentiated to RFP+ osteoclasts associated to bone surfaces in diarthrodial joints. These experiments suggest that inflammatory signals modulate osteoclast precursor migration, distribution, resorptive capacity and differentiation. Finally, we studied the effect of parathyroid hormone (PTH) on osteoclast precursor populations in the bone marrow. We found that intermittent administration of PTH to mice increased the frequency of these populations in the bone marrow and increased their ability to form osteoclast in vitro