Key protein identified to prevent bone mass loss in osteoporosis

Osteoporosis, a condition characterized by porous and fragile bones, poses a significant threat to skeletal health. Bones, as the primary structural support of the human body, provide vital support. When bone mass decreases, it not only impairs this support, but also impairs overall function, leading to a decreased quality of life.

As the incidence of osteoporosis increases in the aging population, there is an increasing burden on healthcare resources for long-term care. It is therefore necessary to understand the mechanisms that contribute to the development of osteoporosis and to develop effective targeted treatments to minimize its long-term impact.

Osteoblasts and osteoclasts are two types of cells that play a key role in the maintenance and remodeling of bone tissue. While osteoblasts are bone-forming cells responsible for the synthesis and deposition of new bone tissue, osteoclasts are bone-degrading cells involved in the breakdown and removal of old or damaged bone tissue.

An increase in the proportion of osteoclasts leads to bone loss in conditions such as osteoporosis, rheumatoid arthritis (inflammation of the joints), and bone metastases (cancer that has spread to the bones). Osteoclasts arise from the differentiation of macrophages or monocytes, which are types of immune cells.

Thus, inhibition of osteoclast differentiation may serve as a therapeutic strategy to prevent bone loss. However, the precise molecular mechanisms regulating the complex process of bone remodeling remain unclear.

In a new study, Professor Tadayoshi Hayata, Mr. Takuto Konno and Ms. Hitomi Murachi from Tokyo University of Science, together with colleagues, delved into the molecular regulation of osteoclast differentiation. Stimulation with receptor activator of nuclear factor kappa B ligand (RANKL) induces differentiation of macrophages into osteoclasts.

In addition, bone morphogenetic protein (BMP) and transforming growth factor (TGF)-β signaling pathways have been implicated in the regulation of RANKL-mediated osteoclast differentiation. In the current study, the researchers aimed to investigate the role of Ctdnep1, a phosphatase (an enzyme that removes phosphate groups) that has been reported to suppress BMP and TGF-β signaling pathways.

The study is published in the journal Biochemical and Biophysical Research Communications.

Professor Hayata states: "RANKL acts as an 'accelerator' for osteoclast differentiation. Driving a car requires not only an accelerator but also brakes. Here, we found that Ctdnep1 acts as a 'brake' in osteoclast differentiation."

The researchers first examined Ctdnep1 expression in RANKL-treated mouse macrophages and untreated control cells. They observed that Ctdnep1 expression did not change in response to RANKL stimulation. However, it was localized to the cytoplasm in a granular form in macrophages and differentiated into osteoclasts, distinct from its normal perinuclear localization in other cell types, indicating its cytoplasmic function in osteoclast differentiation.

Furthermore, knockdown of Ctdnep1 (downregulation of gene expression) resulted in an increase in the number of osteoclasts positive for tartrate-resistant acid phosphatase (TRAP), where TRAP is a marker of differentiated osteoclasts.

Knockout of Ctdnep1 resulted in increased expression of key differentiation markers, including "Nfatc1," a master transcription factor induced by RANKL for osteoclast differentiation. These results support a "brake function" of Ctdnep1, whereby it negatively regulates osteoclast differentiation. Moreover, knockout of Ctdnep1 also resulted in increased calcium phosphate absorption, suggesting a suppressive role for Ctdnep1 in bone resorption.

Finally, although Ctdnep1 knockout did not alter BMP and TGF-β signaling, Ctdnep1-deficient cells showed increased levels of phosphorylated (activated) proteins, which are products of the RANKL signaling pathway. These results suggest that the inhibitory effect of Ctdnep1 on osteoclast differentiation may not be mediated through BMP and TGF-β signaling, but through downregulation of the RANKL signaling pathway and Nfatc1 protein levels.

Overall, these results provide new insights into the osteoclast differentiation process and identify potential therapeutic targets that could be used to develop treatments to reduce bone loss due to osteoclast overactivity. In addition to diseases characterized by bone loss, Ctdnep1 has also been identified as a causative factor in medulloblastoma, a childhood brain tumor. The authors are optimistic that their research can be extended to other human diseases beyond bone metabolism.

Professor Hayata concludes: "Our results suggest that Ctdnep1 is required to prevent excessive osteoclastogenesis. These results may further expand our knowledge of how the phosphorylation-dephosphorylation network controls osteoclast differentiation and may provide new therapeutic strategies for the treatment of bone diseases associated with excessive osteoclast activity."