Osteoclast precursors do not express CD68: results from CD68 promoter-driven RANK transgenic mice

Macrophages and osteoclasts are thought to derive from CD68 lineage marker-positive common myeloid precursors. We used the CD68 promoter to drive an inducible receptor activator of NF-κB (iRANK) construct that selectively activates RANK signaling in myeloid cells in vivo. The cytoplasmic portion of RANK was fused to a mutant FK506 binding domain, which selectively binds the chemical inducer of dimerization AP20187 and initiates signaling. iRANK mRNA was expressed in macrophages isolated from peritoneal cavity, spleen-, and bone marrow-derived myeloid cells. Unexpectedly, AP20187 did not induce osteoclast formation in spleen- and bone marrow-derived myeloid cells. However, AP20187-dependent RANK signaling induced ERK1/2 phosphorylation and mRNA expression of MMP9 and CathepsinK in peritoneal macrophages. Importantly, CD68 was not expressed until day 3 and day 5 in bone marrow and spleen myeloid cells, respectively. Contrary to dogma, osteoclast precursors do not express the lineage marker CD68.