Identification of cell cycle–arrested quiescent osteoclast precursors in vivo

Abstract

Osteoclasts are multinucleated cells that resorb bone. Although osteoclasts originate from the monocyte/macrophage lineage, osteoclast precursors are not well characterized in vivo. The relationship between proliferation and differentiation of osteoclast precursors is examined in this study using murine macrophage cultures treated with macrophage colony-stimulating factor (M-CSF) and receptor activator of NF-κB (RANK) ligand (RANKL). Cell cycle–arrested quiescent osteoclast precursors (QuOPs) were identified as the committed osteoclast precursors in vitro. In vivo experiments show that QuOPs survive for several weeks and differentiate into osteoclasts in response to M-CSF and RANKL. Administration of 5-fluorouracil to mice induces myelosuppression, but QuOPs survive and differentiate into osteoclasts in response to an active vitamin D3 analogue given to those mice. Mononuclear cells expressing c-Fms and RANK but not Ki67 are detected along bone surfaces in the vicinity of osteoblasts in RANKL-deficient mice. These results suggest that QuOPs preexist at the site of osteoclastogenesis and that osteoblasts are important for maintenance of QuOPs.

Vitamin D endocrine system and osteoclasts

“…More than 80% of nuclei in those osteoclasts were BrdU negative. 49 These results suggest that osteoclasts are formed from QOPs in both RANKL À / À mice and op/op mice. QOPs are expected to express RANK and c-Fms, but not TRAP or Ki67.…”

“…The expression of cyclins and cyclin-dependent kinases (Cdks) was suppressed, whereas that of p27 KIP1 , a Cdk inhibitor, was upregulated in the precursors during their differentiation into osteoclasts. 49 Neither these precursors nor osteoclasts expressed Ki67, a cell proliferation marker. Therefore, these osteoclast precursors were named 'cell cycle-arrested quiescent osteoclast precursors' (QOPs).…”

“…Mizoguchi et al 49 reported that cell cycle progression and subsequent cell cycle arrest in osteoclast progenitors were required for their differentiation into direct osteoclast precursors ( Figure 5). The expression of cyclins and cyclin-dependent kinases (Cdks) was suppressed, whereas that of p27 KIP1 , a Cdk inhibitor, was upregulated in the precursors during their differentiation into osteoclasts.…”

“…More than 70% of nuclei in RANKL-induced osteoclasts were negative for BrdU. 49 op/op mice were also given BrdU and were injected with M-CSF for 7 days. Many osteoclasts appeared in the bone in op/op mice injected with M-CSF.…”

Induction of osteoclast progenitors in inflammatory conditions: key to bone destruction in arthritis

“…Circulating long-lived quiescent lineagecommitted OCPs (QOPs) were characterized as CD115 lo RANK hi cells, with almost no expression of myeloid markers CD11b, F4/80 and Gr-1, and were also present in very low numbers in bone marrow [35]. In response to different osteoclastogenic stimuli QOPs had the potential to differentiate into osteoclasts in vitro and in vivo [35,36].…”