Abstract Collagen X, encoded by the Col10a1 gene, is expressed specifically by hypertrophic chondrocytes (HCs) in the cartilage. Mutations of the Col10a1 gene in humans, which increase the endoplasmic reticulum (ER) stress in HCs, lead to metaphyseal chondrodysplasia type Schmid (MCDS). However, the mechanism of how HCs respond to ER stress to affect bone development and growth is not entirely understood. In this study, we generated a new mouse model harboring a 13 bp deletion (13del) in the endogenous Col10a1 gene. These 13del-MCDS mice display all the known clinical characteristics of human MCDS, as well as the molecular and cellular changes within the growth plate cartilage. For the first time, we found that triggering ER stress leads to activation of hypoxia and HIFs pathways in vivo, promoting cellular survival in HCs of the 13del-MCDS mice. To test whether the activated hypoxia and HIFs pathways are triggered by ER stress or extrinsic factors, we generated WT/GFP-MCDS chimera mice. Our findings showed that the activation of hypoxia and the hypoxic pathway is an intrinsic response to ER stress, which may partly occur through activation of ATF4 in HCs of mice. In parallel, p53 pathway changes and autophagy-associated protein-quality-control responses were observed in spatial association with altered chondrocyte differentiation and survival. These findings reveal coordinated engagement of multiple stress-response pathways during the adaptive response of hypertrophic chondrocytes to chronic ER stress.
Abstract Osteoarthritis is a whole-joint disease in which cartilage loss, synovial inflammation, subchondral bone remodelling and pain-related neural changes co-evolve, yet structural improvement does not reliably translate into pain relief. This Review centres prostaglandin E 2 (PGE 2 ) skeletal interoception in the translational framework for 15-hydroxyprostaglandin dehydrogenase (15-PGDH) inhibition. In skeletal interoception, PGE 2 produced in mechanically stressed bone and osteochondral tissues is sensed by prostaglandin E receptor 4 (EP4)-positive sensory afferents, relayed to central circuits and translated into altered sympathetic norepinephrine output. Because 15-PGDH catabolises PGE 2 , its inhibition may prolong the persistence and spatial reach of this interoceptive cue while also acting locally on chondrocyte state. This combined local and interoceptive action offers a plausible mechanism for cartilage repair and pain modulation, but may also create compartment-specific risk when prolonged PGE 2 persists in synovium or subchondral bone. We compare 15-PGDH inhibition with upstream synthesis blockade and receptor-specific strategies, and propose that development should be organised around three checkpoints: cartilage-enriched exposure, durability under mechanical load and structure–pain coherence. Current evidence supports cautious development rather than broad generalisation; clinical translation will require compartment-resolved pharmacokinetics/pharmacodynamics, EP-receptor attribution and early surveillance for synovial, osteochondral and pain-system liabilities.
Abstract Erythropoietin-producing hepatocellular (EPH) receptors, the largest receptor tyrosine kinase family, have been shown to play a crucial role in normal physiological processes and pathologies. EPHA2, encoded by EPHA2 gene, a member of the EPH receptor family, has been implicated in cataracts and various bone tumors, including osteosarcoma, chondrosarcoma, and bone metastases of breast cancer. However, no association between the EPHA2 gene and genetic skeletal disorders has been reported to date. Here, we identified a Chinese family with a diagnosis of dysosteosclerosis, presenting with significant increase in lumbar spine bone mineral density, multifocal sclerotic lesions throughout the body, sclerotic skull, platyspondyly and sandwich vertebrae, and demonstrated compound heterozygous mutations in EPHA2 gene (c.890 C > T, p.Thr297Met; c.988dupT, p.Ser330PhefsX51). We observed a reduction in numerous sclerotic lesions in patients during the five-year follow-up. To systematically investigate the molecular mechanisms of pathogenesis, we employed a range of methodologies, including in vitro cellular assays, genome-wide association studies analysis, induced pluripotent stem cells models and Epha2 knockout mice models. We have determined that loss-of-function mutations in EPHA2 gene lead to dysosteosclerosis through enhancing mesenchymal stem cells osteogenic differentiation and downregulating the p38-MAPK pathway, leading to decreased osteoclast differentiation. Consequently, we proposed and validated EPHA2 as a viable therapeutic target for osteoporosis by constructing femoral fracture mice models and ovariectomized mice models. In summary, this study identifies bi-allelic loss-of-function mutations in EPHA2 gene lead to dysosteosclerosis and its underlying mechanisms, and suggests EPHA2 as a potential therapeutic target for osteoporosis.
Tumor necrosis factor receptor-associated factor 6 (TRAF6) is a pivotal adaptor molecule in the receptor activator of nuclear factor-κB (RANK) and its ligand (RANKL) signaling pathways, which are essential for osteoclastogenesis. In this study, we identified WD40 repeat-containing protein 23 (WDR23), also known as DDB1-CUL4 associated factor 11 (DCAF11), as a novel binding partner of TRAF6. Our findings demonstrate that WDR23/DCAF11 acts as a negative feedback regulator of RANK/RANKL-induced osteoclastogenesis by promoting the autophagy-dependent degradation of TRAF6. Notably, RANKL induced the upregulation of WDR23 expression during osteoclastogenesis. WDR23 physically interacted with the TRAF domain of TRAF6 via the WD40 repeat domains 1 and 2 of WDR23, resulting in reduced TRAF6 protein stability by its autophagy-dependent degradation during osteoclastogenesis. By modulating TRAF6 protein levels, WDR23 attenuated RANKL signaling cascades, including nuclear factor-κB and mitogen-activated protein kinases, thereby downregulating the expression of osteoclastogenic markers, such as nuclear factor of activated T-cell c1, tartrate-resistant acid phosphatase, dendritic cell-specific transmembrane protein, V-ATPase subunit d2 and cathepsin K. Conversely, WDR23 knockdown or deficiency enhanced RANKL-induced osteoclastogenesis by preventing the autophagy-dependent degradation of TRAF6. WDR23-deficient mice exhibit an osteoporotic bone phenotype characterized by elevated osteoclast formation and reduced bone mass. Collectively, these results establish WDR23 as a key negative feedback regulator of RANKL-induced osteoclastogenesis via autophagy-mediated TRAF6 degradation and underscore its potential as a therapeutic target for bone disorders associated with aberrant osteoclast formation and function.
Utilizing key developmental cues and refining their orchestrating role in degeneration represents a promising strategy for understanding and treating intervertebral disc (IVD) degeneration, a major cause of chronic lower back pain. Here, we focus on notochordal cells (NCs), which originate from the embryonic notochord and reside in the developing nucleus pulposus. These distinctly vacuolated cells exhibit robust regenerative effects and hold promise for new therapeutic approaches. Dogs, like humans, suffer from the consequences of IVD degeneration. As the IVD matures and degenerates, NCs are replaced by smaller non-vacuolated NP cells (NPCs). The dog was employed as a model to capture, at the single-cell level, the heterogeneity of resident cells by studying the nucleus pulposus tissue at three stages (i.e., juvenile, young adult and degenerate adult). Here, we integrated transcriptomic data with repressive histone H3 lysine 27 trimethylation (H3K27me3) profiles at the single-cell level to assess changes in chromatin states and gene expression across this IVD degeneration-associated cell phenotypic transition. H3K27me3 enrichment on key genes involved in IVD development and homeostasis, such as Brachyury (TBXT), aligns with the observed attenuation during ageing and degeneration seen in both dog and human IVDs. This study further demonstrates that eliminating repressive histone marks, together with CRISPR-mediated gene transactivation, enhances TBXT gene expression in human NPCs derived from degenerated aged discs. Our findings underscore how extensive insights gained through single-cell omics can lead to the identification of crucial cellular cues that may enable degenerate NPCs to regain a healthier phenotype.
