The neutrophil–osteogenic cell axis promotes bone destruction in periodontitis

The immune-stromal cell interactions play a key role in health and diseases. In periodontitis, the most prevalent infectious disease in humans, immune cells accumulate in the oral mucosa and promote bone destruction by inducing receptor activator of nuclear factor-κB ligand (RANKL) expression in osteogenic cells such as osteoblasts and periodontal ligament cells. However, the detailed mechanism underlying immune–bone cell interactions in periodontitis is not fully understood. Here, we performed single-cell RNAsequencing analysis on mouse periodontal lesions and showed that neutrophil–osteogenic cell crosstalk is involved in periodontitis-induced bone loss. The periodontal lesions displayed marked infiltration of neutrophils, and in silico analyses suggested that the neutrophils interacted with osteogenic cells through cytokine production. Among the cytokines expressed in the periodontal neutrophils, oncostatin M (OSM) potently induced RANKL expression in the primary osteoblasts, and deletion of the OSM receptor in osteogenic cells significantly ameliorated periodontitis-induced bone loss. Epigenomic data analyses identified the OSM-regulated RANKL enhancer region in osteogenic cells, and mice lacking this enhancer showed decreased periodontal bone loss while maintaining physiological bone metabolism. These findings shed light on the role of neutrophils in bone regulation during bacterial infection, highlighting the novel mechanism underlying osteoimmune crosstalk.

Identification of an intronic enhancer regulating RANKL expression in osteocytic cells

The bony skeleton is continuously renewed throughout adult life by the bone remodeling process,in which old or damaged bone is removed by osteoclasts via largely unknown mechanisms.Osteocytes regulate bone remodeling by producing the osteoclast differentiation factor RANKL(encoded by the TNFSF11 gene).However,the precise mechanisms underlying RANKL expression in osteocytes are still elusive.Here,we explored the epigenomic landscape of osteocytic cells and identified a hitherto-undescribed osteocytic cell-specific intronic enhancer in the TNFSF11 gene locus.Bioinformatics analyses showed that transcription factors involved in cell death and senescence act on this intronic enhancer region.Single-cell transcriptomic data analysis demonstrated that cell death signaling increased RANKL expression in osteocytic cells.Genetic deletion of the intronic enhancer led to a high-bone-mass phenotype with decreased levels of RANKL in osteocytic cells and osteoclastogenesis in the adult stage,while RANKL expression was not affected in osteoblasts or lymphocytes.These data suggest that osteocytes may utilize a specialized regulatory element to facilitate osteoclast formation at the bone surface to be resorbed by linking signals from cellular senescence/death and RANKL expression.