Osteoporosis, a disorder characterised by porous & weak bones, is a major hazard to skeletal health. As the foundation of the human body, bones provide critical structural support. As bone mass decreases, it not only weakens this support but also lowers general function, resulting in a lower quality of life.

With an ageing population and a rise in osteoporosis cases, the burden on healthcare resources for long-term care is clear. As a result, there is a need to understand the mechanisms that cause osteoporosis and develop effective targeted therapeutics to mitigate its long-term effects.

Osteoblasts and osteoclasts are two types of cells that play critical roles in bone tissue maintenance and remodelling. Osteoblasts are bone-forming cells that synthesise and deposit new bone tissue, whereas osteoclasts break down and remove old or damaged bone tissue.

Increased proportion of osteoclasts causes bone mass loss in situations such as osteoporosis, rheumatoid arthritis (joint inflammation), and bone metastases (cancer that has spread to the bones).

Osteoclasts develop from the development of macrophages or monocytes, two types of immune cells. Suppressing osteoclast differentiation could thus be used as a therapeutic technique to prevent bone loss. However, the precise molecular pathways driving the complicated process of bone remodelling are unknown.

In a new groundbreaking study, Professor Tadayoshi Hayata, Mr. Takuto Konno, and Ms. Hitomi Murachi from Tokyo University of Science, along with their co-workers, delved deeper into the molecular regulation of osteoclast differentiation.

Receptor activator of nuclear factor kappa B ligand (RANKL) stimulation induces the differentiation of macrophages into osteoclasts. Further, bone morphogenetic protein (BMP) and transforming growth factor (TGF)-b signalling pathways have been implicated in the regulation of RANKL-mediated osteoclast differentiation.

In the current study, the researchers sought to investigate the role of Ctdnep1 - a phosphatase (an enzyme that removes phosphate groups) that has been reported to suppress BMP and TGF-b signalling.

Giving further insight into their work set to be published on July 30, 2024, in Volume 719 of Biochemical and Biophysical Research Communications, Prof. Hayata states, "RANKL functions as an 'accelerator' for osteoclast cell differentiation. Driving a car requires not only the accelerator but also the brakes. Here, we find that Ctdnep1 functions as a 'brake' on osteoclast cell differentiation."

First, the researchers examined the expression of Ctdnep1 in mouse-derived macrophages treated with RANKL and untreated control cells. They noted that Ctdnep1 expression remained unchanged in response to RANKL stimulation.

However, it localised in the cytoplasm in granular form in the macrophages and differentiated into osteoclasts, distinct from its normal peri-nuclear localisation in other cell types, indicating its cytoplasmic function in osteoclast differentiation.

Further, Ctdnep1 knockdown (downregulation of gene expression) increased tartrate-resistant acid phosphatase-positive (TRAP) osteoclasts; wherein TRAP is a marker for differentiated osteoclasts.

Additionally, Ctdnep1 knockdown led to an increase in the expression of crucial differentiation markers including 'Nfatc1', a RANKL-induced master transcription factor for osteoclast differentiation. These results support the 'brake function' of Ctdnep1, whereby, it negatively regulates osteoclast differentiation.

Moreover, Ctdnep1 knockdown also led to increased absorption of calcium phosphate, suggestive of the suppressive role of Ctdnep1 in bone resorption.

Lastly, while, Ctdnep1 knockdown did not alter BMP and TGF-b signaling, cells deficient in Ctdnep1 showed elevated levels of phosphorylated (activated) proteins downstream of the RANKL signaling pathway.

These findings suggest that the suppressive effect of Ctdnep1 in osteoclast differentiation may not be mediated by BMP and TGF-b signaling, but, through the negative regulation of RANKL signaling and Nfatc1 protein levels.

Overall, these findings provide novel insights into the process of osteoclast differentiation and reveal potential therapeutic targets which can be pursued to develop treatments that address bone loss due to excessive osteoclast activity.

In addition to diseases characterised by bone loss, Ctdnep1 has also been reported as a causative factor in medulloblastoma - a childhood brain tumour. The authors are, therefore, optimistic that their research can be extended to other human diseases beyond bone metabolism.