Semun Seong

Jung Ha Kim, Kabsun Kim, Inyoung Kim, Semun Seong, Wonbong Lim, Jeong-Tae Koh & Nacksung Kim. BMB Reports 2026;59:235-41. https://doi.org/10.5483/BMBRep.2025-0239

Osteoclasts are bone‐resorbing cells, and understanding the pathways involved in osteoclast differentiation and activation is essential for developing treatments for bone diseases such as osteoporosis. Since osteoclast differentiation is regulated by signaling pathways such as Akt, MAPK, and NF‐κB, it is important to identify their regulators. Makorin ring finger protein 1 (MKRN1), a conserved member of the ring finger protein family, is closely linked to the regulation of Akt and AMP‐activated protein kinase (AMPK), important signaling pathways in osteoclastogenesis. However, its specific role in osteoclasts has not been revealed. Mkrn1 was overexpressed using a retrovirus or silenced using small interfering RNA (siRNA) in bone marrow‐derived macrophages (BMMs). Subsequently, osteoclast differentiation was induced using receptor activator of nuclear factor kappa B (NF‐κB) ligand (RANKL) and assessed via tartrate‐resistant acid phosphatase (TRAP) staining. Additionally, the role of Mkrn1 in osteoclastogenesis was confirmed using Mkrn1 ‐deficient cells. Western blot analysis was employed to evaluate Mkrn1‐associated Akt and AMPK activation. The bone phenotype of Mkrn1 ‐deficient mice was investigated through bone analysis. This study revealed that Mkrn1 significantly influences RANKL‐induced osteoclastogenesis. Overexpression of Mkrn1 in BMMs enhanced osteoclast differentiation by promoting Akt phosphorylation and inhibiting AMPK phosphorylation upon RANKL stimulation. Conversely, siRNA‐mediated downregulation of MKRN1 reduced Akt phosphorylation and increased AMPK phosphorylation, impairing osteoclastogenesis. Furthermore, in vivo studies using Mkrn1 knockout mice revealed a phenotype with increased bone volume. Our findings establish Mkrn1 as a positive regulator of osteoclast differentiation, highlighting its potential as a therapeutic target for bone diseases characterized by excessive bone resorption.

Multiple small GTPases play crucial roles in bone homeostasis by regulating the differentiation and function of bone cells, including osteoclasts and osteoblasts. Here, we investigated whether developmentally regulated GTP-binding protein 2 (Drg2), a subfamily of the GTPase superfamily, could affect bone mass by regulating osteoclast and osteoblast differentiation. Downregulation of Drg2 using siRNA in bone marrow-derived macrophages inhibited osteoclast differentiation and function and Rac1 activation in vitro. Comparatively, Drg2 downregulation in calvarial-derived osteoprogenitor cells enhanced osteoblast differentiation and function in vitro. Rac1 activation was also suppressed by Drg2 downregulation in osteoprogenitor cells. Both osteoclast and osteoblast differentiation regulated by Drg2 downregulation were restored by suppressing Rac1 activity. Drg2-deficient mice showed increased bone mass due to a dramatic reduction in osteoclast numbers without significantly affecting the number of osteoblasts. Furthermore, Drg2 downregulation strongly inhibited RANKL-induced bone loss in vivo. In summary, Drg2 contributes to bone homeostasis by regulating the differentiation and function of osteoclasts and osteoblasts through Rac1 activation. In particular, the effect of Drg2 on osteoclasts is strong enough to regulate bone mass in vivo; therefore, Drg2 has significant potential for use as a therapeutic target in bone loss-related diseases.

Several members of the transforming growth factor beta (TGF‐β) superfamily regulate the proliferation, differentiation, and function of bone‐forming osteoblasts and bone‐resorbing osteoclasts. However, it is still unknown whether Nodal, a member of the TGF‐β superfamily, serves a function in bone cells. In this study, we found that Nodal did not have any function in osteoblasts but instead negatively regulated osteoclast differentiation. Nodal inhibited RANKL‐induced osteoclast differentiation by downregulating the expression of pro‐osteoclastogenic genes, including c‐fos, Nfatc1, and Blimp1, and upregulating the expression of antiosteoclastogenic genes, including Bcl6 and Irf8. Nodal activated STAT1 in osteoclast precursor cells, and STAT1 downregulation significantly reduced the inhibitory effect of Nodal on osteoclast differentiation. These findings indicate that Nodal activates STAT1 to downregulate or upregulate the expression of pro‐osteoclastogenic or antiosteoclastogenic genes, respectively, leading to the inhibition of osteoclast differentiation. Moreover, the inhibitory effect of Nodal on osteoclast differentiation contributed to the reduction of RANKL‐induced bone loss in vivo.

Human monocyte chemoattractant protein‐1 (MCP‐1) in mice has two orthologs, MCP‐1 and MCP‐5. MCP‐1, which is highly expressed in osteoclasts rather than in osteoclast precursor cells, is an important factor in osteoclast differentiation. However, the roles of MCP‐5 in osteoclasts are completely unknown. In this study, contrary to MCP‐1, MCP‐5 was downregulated during receptor activator of nuclear factor kappa B ligand (RANKL)‐induced osteoclast differentiation and was considered an inhibitory factor in osteoclast differentiation. The inhibitory role of MCP‐5 in osteoclast differentiation was closely related to the increase in Ccr5 expression and the inhibition of IκB degradation by RANKL. Transgenic mice expressing MCP‐5 controlled by Mx‐1 promoter exhibited an increased bone mass because of a decrease in osteoclasts. This result strongly supported that MCP‐5 negatively regulated osteoclast differentiation. MCP‐5 also prevented severe bone loss caused by RANKL.

Nano-formulatedCrkIIsiRNA improves bone microenvironmentviasimultaneous regulation of the formation and function of both osteoclasts and osteoblasts.

Neurogenin 1 (Ngn1) belongs to the basic helix–loop–helix (bHLH) transcription factor family and plays important roles in specifying neuronal differentiation. The present study aimed to determine whether forced Ngn1 expression contributes to bone homeostasis. Ngn1 inhibited the p300/CREB-binding protein-associated factor (PCAF)-induced acetylation of nuclear factor of activated T cells 1 (NFATc1) and runt-related transcription factor 2 (Runx2) through binding to PCAF, which led to the inhibition of osteoclast and osteoblast differentiation, respectively. In addition, Ngn1 overexpression inhibited the TNF-α- and IL-17A-mediated enhancement of osteoclast differentiation and IL-17A-induced osteoblast differentiation. These findings indicate that Ngn1 can serve as a novel therapeutic agent for treating ankylosing spondylitis with abnormally increased bone formation and resorption.

The LIM-homeodomain transcription factor Lmx1b plays a key role in body pattern formation during development. Although Lmx1b is essential for the normal development of multiple tissues, its regulatory mechanism in bone cells remains unclear. Here, we demonstrated that Lmx1b negatively regulates bone morphogenic protein 2 (BMP2)-induced osteoblast differentiation. Overexpressed Lmx1b in the osteoblast precursor cells inhibited alkaline phosphatase (ALP) activity and nodule formation, as well as the expression of osteoblast maker genes, including runt-related transcription factor 2 (Runx2), alkaline phosphatase (Alpl), bone sialoprotein (Ibsp), and osteocalcin (Bglap). Conversely, the knockdown of Lmx1b in the osteoblast precursors enhanced the osteoblast differentiation and function. Lmx1b physically interacted with and repressed the transcriptional activity of Runx2 by reducing the recruitment of Runx2 to the promoter region of its target genes. In vivo analysis of BMP2-induced ectopic bone formation revealed that the knockdown of Lmx1b promoted osteogenic differentiation and bone regeneration. Our data demonstrate that Lmx1b negatively regulates osteoblast differentiation and function through regulation of Runx2 and provides a molecular basis for therapeutic targets for bone diseases.

Activating transcription factor 3 (ATF3) has been identified as a negative regulator of osteoblast differentiation in in vitro study. However, it was not associated with osteoblast differentiation in in vivo study. To provide an understanding of the discrepancy between the in vivo and in vitro findings regarding the function of ATF3 in osteoblasts, we investigated the unidentified roles of ATF3 in osteoblast biology. ATF3 enhanced osteoprotegerin (OPG) production, not only in osteoblast precursor cells, but also during osteoblast differentiation and osteoblastic adipocyte differentiation. In addition, ATF3 increased nodule formation in immature osteoblasts and decreased osteoblast-dependent osteoclast formation, as well as the transdifferentiation of osteoblasts to adipocytes. However, all these effects were reversed by the OPG neutralizing antibody. Taken together, these results suggest that ATF3 contributes to bone homeostasis by regulating the differentiation of various cell types in the bone microenvironment, including osteoblasts, osteoclasts, and adipocytes via inducing OPG production.