Bone homeostasis is regulated by communication between bone-forming mature osteoblasts (mOBs) and bone-resorptive mature osteoclasts (mOCs). However, the spatial–temporal relationship and mode of interaction in vivo remain elusive. Here we show, by using an intravital imaging technique, that mOB and mOC functions are regulated via direct cell–cell contact between these cell types. The mOBs and mOCs mainly occupy discrete territories in the steady state, although direct cell–cell contact is detected in spatiotemporally limited areas. In addition, a pH-sensing fluorescence probe reveals that mOCs secrete protons for bone resorption when they are not in contact with mOBs, whereas mOCs contacting mOBs are non-resorptive, suggesting that mOBs can inhibit bone resorption by direct contact. Intermittent administration of parathyroid hormone causes bone anabolic effects, which lead to a mixed distribution of mOBs and mOCs, and increase cell–cell contact. This study reveals spatiotemporal intercellular interactions between mOBs and mOCs affecting bone homeostasis in vivo.

Bone metabolism is regulated by the cooperative activity between bone-forming osteoblasts and bone-resorbing osteoclasts. However, the mechanisms mediating the switch between the osteoblastic and osteoclastic phases have not been fully elucidated. Here, we identify a specific subset of mature osteoblast-derived extracellular vesicles that inhibit bone formation and enhance osteoclastogenesis. Intravital imaging reveals that mature osteoblasts secrete and capture extracellular vesicles, referred to as small osteoblast vesicles (SOVs). Co-culture experiments demonstrate that SOVs suppress osteoblast differentiation and enhance the expression of receptor activator of NF-κB ligand, thereby inducing osteoclast differentiation. We also elucidate that the SOV-enriched microRNA miR-143 inhibits Runt-related transcription factor 2, a master regulator of osteoblastogenesis, by targeting the mRNA expression of its dimerization partner, core-binding factor β. In summary, we identify SOVs as a mode of cell-to-cell communication, controlling the dynamic transition from bone-forming to bone-resorbing phases in vivo.

Osteoclastic bone resorption and osteoblastic bone formation/replenishment are closely coupled in bone metabolism. Anabolic parathyroid hormone (PTH), which is commonly used for treating osteoporosis, shifts the balance from osteoclastic to osteoblastic, although it is unclear how these cells are coordinately regulated by PTH. Here, we identify a serine protease inhibitor, secretory leukocyte protease inhibitor (SLPI), as a critical mediator that is involved in the PTH-mediated shift to the osteoblastic phase. Slpi is highly upregulated in osteoblasts by PTH, while genetic ablation of Slpi severely impairs PTH-induced bone formation. Slpi induction in osteoblasts enhances its differentiation, and increases osteoblast–osteoclast contact, thereby suppressing osteoclastic function. Intravital bone imaging reveals that the PTH-mediated association between osteoblasts and osteoclasts is disrupted in the absence of SLPI. Collectively, these results demonstrate that SLPI regulates the communication between osteoblasts and osteoclasts to promote PTH-induced bone anabolism.

Bone homeostasis is maintained by a balance in activity between bone-resorbing osteoclasts and bone-forming osteoblasts. Shifting the balance toward bone resorption causes osteolytic bone diseases such as rheumatoid arthritis and periodontitis. Osteoclast differentiation is regulated by receptor activator of nuclear factor B ligand (RANKL), which, under some pathological conditions, is produced by T and B lymphocytes and synoviocytes. However, the mechanism underlying bone destruction in other diseases is little understood. Bone destruction caused by cholesteatoma, an epidermal cyst in the middle ear resulting from hyperproliferation of keratinizing squamous epithelium, can lead to lethal complications. In this study, we succeeded in generating a model for cholesteatoma, epidermal cyst-like tissue, which has the potential for inducing osteoclastogenesis in mice. Furthermore, an in vitro coculture system composed of keratinocytes, fibroblasts, and osteoclast precursors was used to demonstrate that keratinocytes stimulate osteoclast differentiation through the induction of RANKL in fibroblasts. Thus, this study demonstrates that intercellular communication between keratinocytes and fibroblasts is involved in the differentiation and function of osteoclasts, which may provide the molecular basis of a new therapeutic strategy for cholesteatoma-induced bone destruction. Bone is a highly dynamic organ in which homeostasis is maintained by a balance in activity between bone-resorbing osteoclasts and bone-forming osteoblasts. An imbalance between osteoclastic and osteoblastic activity causes various skeletal disorders. Osteoclasts are multinucleate cells that differentiate from mononuclear macrophage/monocyte-lineage hematopoietic precursor cells, which are attracted to the bloodstream by factors, including sphingsine-1 phosphate (1, 2). Osteoclast differentiation and function are regulated by macrophage colony-stimulating factor (M-CSF) and receptor activator of nuclear factor B ligand (RANKL) (3-5). Under physiological conditions, osteoblasts or osteocytes are the source of RANKL (6-8). RANKL-mediated osteoclastogenesis also plays an important role in bone destruction in inflammatory bone diseases such as rheumatoid arthritis (RA) or periodontal diseases. In such pathological states, synoviocytes or immune cells, such as B or T lymphocytes, infiltrating into the area are proposed to be the major sources of RANKL (9-11). In RA, infiltration of T helper 17 (Th17) cells is observed in the synovium. Th17 cells indirectly induce RANKL expression in synoviocytes by producing interleukin-17 (IL-17) (12), and Th17 cells themselves express RANKL on their plasma membrane (13). Thus, in many bone destructive diseases, the mechanisms of bone destruction are becoming clarified in detail. However, there are still certain bone destructive diseases whose underlying mechanisms remain to be elucidated. Cholesteatoma, a benign entity arising in temporal bone, is one such disease where our understanding is limited.Cholesteatoma is an epid...

III-V-based diluted magnetic semiconductor (DMS) GaGdN/ AlGaN multiple quantum disks (MQDisks) were fabricated on Si (001) substrates with native silicon oxides by RF-plasmaassisted molecular-beam epitaxy (RF-MBE). It was found that the degree of the c-axis orientation of GaGdN/AlGaN MQDisks and the Ga atomic configuration around Gd atom were improved due to the growth of AlGaN disks, comparing with the GaGdN nanorods grown without AlGaN disks. Additionally, GaGdN/AlGaN MQDisks showed larger saturation magnetisation than that of the GaGdN nanorods. With increasing the height of AlGaN disk, saturation magnetisation was increased.