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.

Efficacy of an orally active small-molecule inhibitor of RANKL in bone metastasis

Bone is one of the preferred sites for the metastasis of malignant tumours,such as breast cancer,lung cancer and malignant melanoma.Tumour cells colonizing bone have the capacity to induce the expression of receptor activator of nuclear factor-κB ligand(RANKL),which promotes osteoclast differentiation and activation.Tumour-induced osteoclastic bone resorption leads to a vicious cycle between tumours and bone cells that fuels osteolytic tumour growth,causing bone pain and hypercalcaemia.Furthermore,RANKL contributes to bone metastasis by acting as a chemoattractant to bone for tumour cells that express its receptor,RANK.Thus inhibition of the RANKL–RANK pathway is a promising treatment for bone metastasis,and a human monoclonal anti-RANKL antibody,denosumab,has been used in the clinic.However,orally available drugs targeting RANKL must be developed to increase the therapeutic benefits to patients.Here we report the efficacy of the small-molecule RANKL inhibitor AS2676293 in treating bone metastasis using mouse models.Oral administration of AS2676293 markedly inhibited bone metastasis of human breast cancer cells MDA-MB-231-5a-D-Luc2 as well as tumour-induced osteolysis.AS2676293 suppressed RANKLmediated tumour migration in the transwell assay and inhibited bone metastasis of the murine cell line B16F10,which is known not to trigger osteoclast activation.Based on the results from this study,RANKL inhibition with a small-molecule compound constitutes a promising therapeutic strategy for treating bone metastasis by inhibiting both osteoclastic bone resorption and tumour migration to bone.

Simultaneous augmentation of muscle and bone by locomomimetism through calcium-PGC-1αsignaling

Impaired locomotion has been extensively studied worldwide because those afflicted with it have a potential risk of becoming bedridden.Physical exercise at times can be an effective remedy for frailty,but exercise therapy cannot be applied in all clinical cases.Medication is safer than exercise,but there are no drugs that reinforce both muscle and bone when administered alone.Multiple medications increase the risk of adverse events;thus,there is a need for individual drugs targeting both tissues.To this end,we established a novel sequential drug screening system and identified an aminoindazole derivative,locamidazole(LAMZ),which promotes both myogenesis and osteoblastogenesis while suppressing osteoclastogenesis.Administration of this drug enhanced locomotor function,with muscle and bone significantly strengthened.Mechanistically,LAMZ induced Mef2c and PGC-1αin a calcium signaling–dependent manner.As this signaling is activated upon physical exercise,LAMZ mimics physical exercise.Thus,LAMZ is a promising therapeutic drug for locomotor diseases,including sarcopenia and osteoporosis.