Hells protects mitochondrial integrity via Nr2f2 suppression during osteoclast differentiation

Osteoclasts are specialized bone-resorbing cells essential for skeletal homeostasis. While chromatin remodelers play critical roles in gene regulation, their specific contributions to osteoclastogenesis remain largely unexplored. In particular, the chromatin remodeler Hells has not been previously investigated in osteoclast differentiation. To identify key chromatin remodelers involved in osteoclast differentiation, we performed RNA-seq analysis and compared the differentially expressed remodelers with publicly available human osteoclast RNA-seq data. Among them, Hells was significantly upregulated, which was validated by RT-qPCR and Western blot analysis. To investigate the functional role of Hells, we assessed the effects of Hells KD via siRNA on osteoclast differentiation using TRAP staining, as well as on mitochondrial function by measuring membrane potential, ATP levels, mitochondrial biogenesis, and mitophagy. Bioinformatic analyses were conducted to characterize the Hells-dependent transcriptome. ChIP-qPCR and FAIRE-qPCR were employed to evaluate Hells binding and chromatin accessibility at the Nr2f2 promoter, respectively. Rescue experiments using the Nr2f2 inhibitor and Nr2f2-specific siRNA confirmed the regulatory axis between Hells and Nr2f2 during osteoclastogenesis. Furthermore, baicalin hydrate was applied at varying concentrations to examine its effects on Hells expression, osteoclast differentiation, and mitochondrial integrity. Lastly, we performed ectopic expression of Hells under conditions where its expression was suppressed by baicalin hydrate treatment. Hells expression increased early during osteoclast differentiation, and KD of Hells impaired this process, accompanied by downregulation of key osteoclastogenic markers. Notably, the Hells-dependent transcriptome highlighted mitochondria as the most vulnerable organelle. Mechanistically, Hells directly represses Nr2f2 transcription, thereby maintaining mitochondrial quality and quantity through coordinated regulation of biogenesis and mitophagy. Inhibition of Nr2f2 restored mitochondrial function and osteoclast differentiation in Hells-deficient cells, confirming the presence of a crucial Hells–Nr2f2 axis in osteoclastogenesis. Furthermore, the effect of baicalin hydrate on osteoclast differentiation depended on Hells expression and correlated with mitochondrial integrity. Our study identifies Hells as a key epigenetic regulator of osteoclast differentiation, acting through direct repression of Nr2f2 to ensure proper mitochondrial biogenesis and function. This Hells–Nr2f2 axis establishes a vital connection between nuclear epigenetic regulation and mitochondrial homeostasis, highlighting a promising therapeutic target for osteoclast-related disorders.