Semaphorin 3A (Sema3A) is a diffusible axonal chemorepellent that has an important role in axon guidance. Previous studies have demonstrated that Sema3a(-/-) mice have multiple developmental defects due to abnormal neuronal innervations. Here we show in mice that Sema3A is abundantly expressed in bone, and cell-based assays showed that Sema3A affected osteoblast differentiation in a cell-autonomous fashion. Accordingly, Sema3a(-/-) mice had a low bone mass due to decreased bone formation. However, osteoblast-specific Sema3A-deficient mice (Sema3acol1(-/-) and Sema3aosx(-/-) mice) had normal bone mass, even though the expression of Sema3A in bone was substantially decreased. In contrast, mice lacking Sema3A in neurons (Sema3asynapsin(-/-) and Sema3anestin(-/-) mice) had low bone mass, similar to Sema3a(-/-) mice, indicating that neuron-derived Sema3A is responsible for the observed bone abnormalities independent of the local effect of Sema3A in bone. Indeed, the number of sensory innervations of trabecular bone was significantly decreased in Sema3asynapsin(-/-) mice, whereas sympathetic innervations of trabecular bone were unchanged. Moreover, ablating sensory nerves decreased bone mass in wild-type mice, whereas it did not reduce the low bone mass in Sema3anestin(-/-) mice further, supporting the essential role of the sensory nervous system in normal bone homeostasis. Finally, neuronal abnormalities in Sema3a(-/-) mice, such as olfactory development, were identified in Sema3asynasin(-/-) mice, demonstrating that neuron-derived Sema3A contributes to the abnormal neural development seen in Sema3a(-/-) mice, and indicating that Sema3A produced in neurons regulates neural development in an autocrine manner. This study demonstrates that Sema3A regulates bone remodelling indirectly by modulating sensory nerve development, but not directly by acting on osteoblasts.

SignificanceProstate cancer is one of most common cancers in men worldwide, and osteoblastic bone metastasis is frequently observed in prostate cancer patients. However, the mechanisms responsible for the predominantly osteoblastic phenotype have not been fully elucidated. Cancer-secreted microRNAs (miRNAs) were recently shown to be significant in the modification of the tumor microenvironment. Here, hsa-miR-940, which was highly secreted by prostate cancer cells, promoted osteogenic differentiation of human mesenchymal stem cells in vitro, and induced extensive osteoblastic lesions in the bone metastatic microenvironment in vivo. Our study provides a demonstration that osteoblastic bone metastasis can be induced by miRNAs secreted by cancer cells in the bone microenvironment.

Bone homeostasis is maintained by the balance between osteoblastic bone formation and osteoclastic bone resorption. Osteoclasts are multinucleated cells that are formed by mononuclear preosteoclast fusion. Fat-soluble vitamins such as vitamin D are pivotal in maintaining skeletal integrity. However, the role of vitamin E in bone remodeling is unknown. Here, we show that mice deficient in α-tocopherol transfer protein (Ttpa(-/-) mice), a mouse model of genetic vitamin E deficiency, have high bone mass as a result of a decrease in bone resorption. Cell-based assays indicated that α-tocopherol stimulated osteoclast fusion, independent of its antioxidant capacity, by inducing the expression of dendritic-cell-specific transmembrane protein, an essential molecule for osteoclast fusion, through activation of mitogen-activated protein kinase 14 (p38) and microphthalmia-associated transcription factor, as well as its direct recruitment to the Tm7sf4 (a gene encoding DC-STAMP) promoter. Indeed, the bone abnormality seen in Ttpa(-/-) mice was rescued by a Tm7sf4 transgene. Moreover, wild-type mice or rats fed an α-tocopherol-supplemented diet, which contains a comparable amount of α-tocopherol to supplements consumed by many people, lost bone mass. These results show that serum vitamin E is a determinant of bone mass through its regulation of osteoclast fusion.

ADF is considered especially suitable for cases with massive OPLL and preoperative kyphotic alignment of the cervical spine, although it leads to a higher incidence of surgery-related complications compared with LAMP.