Regulatory mechanism of osteoclast differentiation in osteoimmunology

Objective: Receptor activator of NF-κB ligand (RANKL) and macrophage-colony stimulating factor (M-CSF) have been thought to be essential and sufficient for the induction of osteoclast differentiation. RANKL is a TNF family cytokine essential for the differentiation of osteoclasts, while M-CSF provides a survival signal for this lineage. Methods: See below. Results: NFAT family of transcription factors are well characterized in the context of T cell activation, but we revealed that NFATc1 integrates the RANKL signaling and represents a master regulator of osteoclastogenesis (Dev Cell, 3: 889, 2002). The induction of NFATc1 depends on TRAF6 and c-Fos, which are also known to be essential factors for osteoclastogenesis. NFATc1 induction is also dependent on calcium signaling and calcium-regulated phosphatase, calcineurin. However, it was unclear how RANKL activates the calcium signal during osteoclastogenesis. Therefore, we analyzed the function of FcRγ and DAP12 in the skeletal system, both of which are adaptor proteins containing immunoreceptor tyrosine activation motif (ITAM), important for calcium signal in the immune system. Interestingly, mice lacking both FcRγ and DAP12 exhibit severe osteopetrosis owing to the defective differentiation of osteoclasts. In osteoclast precursor cells, FcRγ and DAP12 associate with multiple immunoreceptors such as OSCAR, PIR-A, TREM-2 or SIRPβ1 and induces the phosphorylation of PLCγ through Syk kinases, thus leading to the activation of calcium signaling (Nature, 428: 758, 2004). Conclusion: We have established the importance of the ITAM-mediated "costimulatory signals" for RANKL in osteoclast differentiation, which is analogous to the role of costimulatory signals in the immune system. Thus, we provide another example in which immune and skeletal systems are regulated by a similar mechanism, underscoring the importance of osteoimmunology. Our results show that RANKL and M-CSF are not sufficient for osteoclastogenesis and this process is finely regulated by a number of immunoreceptors.