Eiko Sakai

Osteoclasts are the primary bone-resorbing cells responsible for maintaining skeletal homeostasis through balanced bone remodeling. Accumulating evidence suggests that oxidative stress is an emerging regulator of osteoclast differentiation, metabolic reprogramming, and bone resorption, arising from excessive reactive oxygen species (ROS) and insufficient antioxidant defenses. Dysregulated redox signaling contributes to osteoporosis, rheumatoid arthritis, periodontitis, periapical lesions, and other osteolytic disorders. This review summarizes recent advances in the molecular mechanisms linking oxidative stress to osteoclast metabolism, focusing on ROS sources, mitochondrial dysfunction, redox-sensitive signaling pathways, endogenous antioxidant systems, and emerging antioxidant-based therapies. Oxidative stress regulates osteoclastogenesis through interconnected signaling networks, including NF-κB, MAPKs, PI3K/Akt, AMPK, mTOR, NFATc1, and Keap1/Nrf2. Under both physiological and pathological conditions, osteoclast function is regulated by mitochondrial dynamics, mitophagy, and epigenetic mechanisms. Although both synthetic and natural antioxidants have been shown to reduce pathological osteoclast activity in preclinical studies, limitations in bioavailability, target specificity, and long-term efficacy hinder clinical translation. Advances in multi-omics technologies, single-cell analyses, biomarker discovery, precision medicine, and targeted antioxidant delivery may facilitate the development of effective therapies for oxidative stress-related bone disorders.

Regulatory T cells (Tregs) maintain peripheral immune tolerance and are involved in various oral diseases with markedly different characteristics. In the oral field, Tregs have largely been discussed through reports on their distribution and numbers, and interpretations of whether their role is protective or pathogenic. Current identification markers do not directly assess the suppressive function, and reports on this function remain limited. This review focuses on the suppressive function of Tregs in the organization of oral diseases. Cells may express FOXP3 without exerting stable suppressive function, and the suppressive capacity of the same FOXP3⁺ cells may vary with TSDR status and the inflammatory environment. In Sjögren’s disease, oral lichen planus, periodontitis, and peri-implantitis, inflammation does not resolve even when Treg numbers are increased or maintained. In contrast, Treg suppressive function is enhanced in oral squamous cell carcinoma. Medication-related osteonecrosis of the jaw differs in that a reduction in Tregs induced by the causative drugs is itself what governs the condition. Attending to the functional state allows these conditions to be organized coherently and supports a therapeutic conception of Treg control that differs in direction according to the disease: weakening the function where it is enhanced, and restoring it where it is impaired.

Osteoclasts are the primary bone-resorbing cells responsible for maintaining skeletal homeostasis through balanced bone remodeling. Accumulating evidence suggests that oxidative stress is an emerging regulator of osteoclast differentiation, metabolic reprogramming, and bone resorption, arising from excessive reactive oxygen species (ROS) and insufficient antioxidant defenses. Dysregulated redox signaling contributes to osteoporosis, rheumatoid arthritis, periodontitis, periapical lesions, and other osteolytic disorders. This review summarizes recent advances in the molecular mechanisms linking oxidative stress to osteoclast metabolism, focusing on ROS sources, mitochondrial dysfunction, redox-sensitive signaling pathways, endogenous antioxidant systems, and emerging antioxidant-based therapies. Oxidative stress regulates osteoclastogenesis through interconnected signaling networks, including NF-κB, MAPKs, PI3K/Akt, AMPK, mTOR, NFATc1, and Keap1/Nrf2. Under both physiological and pathological conditions, osteoclast function is regulated by mitochondrial dynamics, mitophagy, and epigenetic mechanisms. Although both synthetic and natural antioxidants have been shown to reduce pathological osteoclast activity in preclinical studies, limitations in bioavailability, target specificity, and long-term efficacy hinder clinical translation. Advances in multi-omics technologies, single-cell analyses, biomarker discovery, precision medicine, and targeted antioxidant delivery may facilitate the development of effective therapies for oxidative stress–related bone disorders.

Growing evidence suggests that persistent oral infectious diseases (OIDs) contribute to systemic disease, highlighting the importance of understanding their pathogenic mechanisms. Conventional dental treatments, primarily mechanical debridement, surgical intervention, or antimicrobial therapy, often struggle to fully control inflammation or prevent progressive tissue destruction. The nucleotide-binding oligomerization domain, leucine-rich repeat, and pyrin domain-containing 3 (NLRP3) inflammasome is a key regulator of innate immunity, mediating the maturation of proinflammatory cytokines (IL-1β and IL-18) and the pyroptosis-inducing protein gasdermin D. Dysregulated or excessive activation of NLRP3 contributes to the initiation and progression of major oral diseases, including periodontitis, peri-implantitis, pulpitis, and oral mucosal inflammation. Despite growing interest in NLRP3, comprehensive and up-to-date reviews integrating its pathogenic mechanisms and therapeutic potential remain limited. This review summarizes current and past evidence on the role of the NLRP3 inflammasome in oral disease development, highlights emerging pharmacological strategies, and outlines future research directions. Existing studies demonstrate that microbial components and danger signals from injured tissues activate NLRP3, thereby amplifying inflammation, tissue degradation, and bone resorption. Preclinical studies indicate that inflammasome inhibitors and several natural compounds reduce tissue damage; however, their clinical translation remains limited. These findings emphasize the need for deeper understanding of NLRP3-mediated pathways, with translational and clinical research offering promising therapeutic opportunities for oral diseases.

Although osteoclasts play crucial roles in the skeletal system, the mechanisms that underlie oxidative stress during osteoclastogenesis remain unclear. The transcription factor Nrf2 and its suppressor, Keap1, function as central mediators of oxidative stress. To further elucidate the function of Nrf2/Keap1-mediated oxidative stress regulation in osteoclastogenesis, DNA microarray analysis was conducted in this study using wild-type (WT), Keap1 knockout (Keap1 KO), and Nrf2 knockout (Nrf2 KO) osteoclasts. Principal component analysis showed that 403 genes, including Nqo1, Il1f9, and Mmp12, were upregulated in Keap1 KO compared with WT osteoclasts, whereas 24 genes, including Snhg6, Ccdc109b, and Wfdc17, were upregulated in Nrf2 KO compared with WT osteoclasts. Moreover, 683 genes, including Car2, Calcr, and Pate4, were upregulated in Nrf2 KO cells compared to Keap1 KO cells. Functional analysis by Gene Ontology and Kyoto Encyclopedia of Genes and Genomes pathway analysis showed upregulated genes in Nrf2 KO osteoclasts were mostly enriched in oxidative phosphorylation. Furthermore, GeneMANIA predicted the protein–protein interaction network of novel molecules such as Rufy4 from genes upregulated in Nrf2 KO osteoclasts. Understanding the complex interactions between these molecules may pave the way for developing promising therapeutic strategies against bone metabolic diseases caused by increased osteoclast differentiation under oxidative stress.

Background: Rheumatoid arthritis (RA) is an autoimmune disease characterized by immune cell-mediated joint inflammation and subsequent osteoclast-dependent bone destruction. Collagen antibody-induced arthritis (CAIA) is a useful mouse model for examining the inflammatory mechanisms in human RA. Previously, we identified the novel gene Rab44, which is a member of the large Rab GTPase family and is highly expressed in immune-related cells and osteoclasts. Methods: In this study, we induced CAIA in Rab44-knockout (KO) mice to investigate the effects of Rab44 on inflammation, cell filtration, and bone destruction. Results: Compared with wild-type (WT) mice, Rab44-KO mice showed reduced inflammation in arthritis under CAIA-inducing conditions. Rab44-KO CAIA mice exhibited reduced cell filtration in the radiocarpal joints. Consistent with these findings, Rab44-KO CAIA mice showed decreased mRNA levels of arthritis-related marker genes including genes for inflammation, cartilage turnover, bone formation, and bone absorption markers. Rab44-KO CAIA mice exhibited predominant infiltration of M2-type macrophages at inflammatory sites and reduced bone loss compared to WT CAIA mice. Conclusions: These results indicate that Rab44 deficiency reduces the progression of inflammation in CAIA in mice.

Osteoclasts are bone-resorbing multinucleated giant cells formed by the fusion of monocyte/macrophage lineages. Various small GTPases are involved in the multinucleation and differentiation of osteoclasts. However, the roles of small GTPases regulatory molecules in osteoclast differentiation remain unclear. In the present study, we examined the role of Dennd2c, a putative guanine nucleotide exchange factor for Rab GTPases, in osteoclast differentiation. Knockdown of Dennd2c promoted osteoclast differentiation, resorption, and expression of osteoclast markers. Morphologically, Dennd2c knockdown induced the formation of larger osteoclasts with several protrusions. In contrast, overexpression of Dennd2c inhibited the multinucleation and differentiation of osteoclasts, bone resorption, and the expression of osteoclast markers. Dennd2c-overexpressing macrophages exhibited spindle-shaped mononuclear cells and long thin protrusions. Treatment of Dennd2c-overexpressing cells with the Cdc42 inhibitor ML-141 or the Rac1 inhibitor 6-thio-GTP prevented protrusion formation. Moreover, treatment of Dennd2c-overexpressing cells with the actin polymerization inhibitor latrunculin B restored multinucleated and TRAP-positive osteoclast formation. These results indicate that Dennd2c negatively regulates osteoclast differentiation and multinucleation by modulating protrusion formation in macrophages.

Rufy4, a protein belonging to the RUN and FYVE domain-containing protein family, participates in various cellular processes such as autophagy and intracellular trafficking. However, its role in osteoclast-mediated bone resorption remains uncertain. In this study, we investigated the expression and role of the Rufy4 gene in osteoclasts using small interfering RNA (siRNA) transfection and gene overexpression systems. Our findings revealed a significant increase in Rufy4 expression during osteoclast differentiation. Silencing Rufy4 enhanced osteoclast differentiation, intracellular cathepsin K levels, and formation of axial protrusive structures but suppressed bone resorption. Conversely, overexpressing wild-type Rufy4 in osteoclasts hindered differentiation while promoting podosome formation and bone resorption. Similarly, overexpression of a Rufy4 variant lacking the RUN domain mimics the effects of Rufy4 knockdown, significantly increasing intracellular cathepsin K levels, promoting osteoclastogenesis, and elongated axial protrusions formation, yet inhibiting bone resorption. These findings indicate that Rufy4 plays a critical role in osteoclast differentiation and bone resorption by regulating the cytoskeletal organization through its RUN domain. Our study provides new insights into the molecular mechanisms governing osteoclast activity and underscores Rufy4’s potential as a novel therapeutic target for bone disorders characterized by excessive bone resorption.

Rheumatoid arthritis (RA) is an autoimmune disease characterized by immune cell-mediated joint inflammation and subsequent osteoclast-dependent bone destruction. Collagen antibody-induced arthritis (CAIA) is a useful mouse model for examining the inflammatory mechanisms in human RA. Previously, we identified the novel gene Rab44, which is a member of the large Rab GTPase family and is highly expressed in immune-related cells and osteoclasts. In this study, we induced CAIA in Rab44-knockout (KO) mice to investigate the effects of Rab44 on inflammation, cell filtration, and bone destruction. Compared with wild-type (WT) mice, Rab44 KO mice showed reduced inflammation in arthritis under CAIA-inducing conditions. Rab44 KO CAIA mice exhibited reduced cell filtration in the radiocarpal joints. Consistent with these findings, Rab44-KO CAIA mice showed decreased mRNA levels of arthritis-related marker genes including inflammation, cartilage turnover, bone formation, and bone absorption markers. Rab44-KO CAIA mice exhibited predominant infiltration of M2-type macrophages at inflammatory sites and reduced bone loss compared to WT CAIA mice. These results indicate that Rab44 deficiency reduces the progression of inflammation in CAIA in mice.

The development of microscopies, to observe microstructures as they are, has been achieved through the accumulations of efforts of numerous motivated scientists and engineers. Clear images of microstructures in tissues and cells provide insights into their mechanism and improve our understanding of the tissue.