Neuromodulation in bone homeostasis and injury repair

Identification of Functionally Distinct Mx1+αSMA+ Periosteal Skeletal Stem Cells.

The enkephalinergic osteoblast.

Bone homeostasis encompasses two interrelated aspects: bone remodeling and cartilage metabolism. Disruption of bone homeostasis can lead to the development of metabolic bone diseases such as osteoporosis and osteoarthritis. The maintenance of bone homeostasis is a complex process that does not solely rely on the functions of the bone tissue itself. In fact, bone tissue is not an isolated entity; it is closely connected to other tissues in the body via exosomes. Within this interconnectivity, exosomes derived from both bone and non-bone cells interfere with each other, forming a complex regulatory network. Therefore, with cell origin as the guiding principle, we have delineated the bone regulatory network of exosomes, elaborated on the specific roles and regulatory mechanisms of exosomes derived from common cell types (cells within the skeletal microenvironment, stem cells from extra-osseous tissues, vascular-derived cells, muscle-derived cells, and neurogenic cells) in bone formation, bone resorption, and cartilage metabolism. We have also discussed the challenges faced in the field of exosome research related to bone homeostasis, unveiled the critical role of exosomes in maintaining bone homeostasis, and proposed that exosomes could serve as highly valuable therapeutic targets for metabolic bone diseases.

Adult bone mass is maintained through a balance of the activities of osteoblasts and osteoclasts. Although Notch signaling has been shown to maintain bone homeostasis by controlling the commitment, differentiation, and function of cells in both the osteoblast and osteoclast lineages, the precise mechanisms by which Notch performs such diverse and complex roles in bone physiology remain unclear. By using a transgenic approach that modified the expression of delta‐like 1 (DLL1) or Jagged1 (JAG1) in an osteoblast‐specific manner, we investigated the ligand‐specific effects of Notch signaling in bone homeostasis. This study demonstrated for the first time that the proper regulation of DLL1 expression, but not JAG1 expression, in osteoblasts is essential for the maintenance of bone remodeling. DLL1‐induced Notch signaling was responsible for the expansion of the bone‐forming cell pool by promoting the proliferation of committed but immature osteoblasts. However, DLL1‐Notch signaling inhibited further differentiation of the expanded osteoblasts to become fully matured functional osteoblasts, thereby substantially decreasing bone formation. Osteoblast‐specific expression of DLL1 did not alter the intrinsic differentiation ability of cells of the osteoclast lineage. However, maturational arrest of osteoblasts caused by the DLL1 transgene impaired the maturation and function of osteoclasts due to a failed osteoblast‐osteoclast coupling, resulting in severe suppression of bone metabolic turnover. Taken together, DLL1‐mediated Notch signaling is critical for proper bone remodeling as it regulates the differentiation and function of both osteoblasts and osteoclasts. Our study elucidates the importance of ligand‐specific activation of Notch signaling in the maintenance of bone homeostasis. J. Cell. Physiol. 232: 2569–2580, 2017. © 2016 The Authors. Journal of Cellular Physiology Published by Wiley Periodicals Inc.

Growing evidence has shown that adipose tissue accumulation is associated with bone metabolism. Arguably, adipokines, secretory proteins of adipose tissue, are involved in bone homeostasis including development and remodeling. The classical adipokine leptin regulates bone formation negatively and bone resorption positively via central nervous system. Leptin also stimulates directly osteoblastogenesis and inhibits osteoclastogenesis through stimulation of osteoprotegerin and inhibition of receptor activator for nuclear factor IoB ligand (RANKL) expression in osteoblasts. Another major adipokine, adiponectin, and its receptor are expressed in osteoblasts. Adiponectin stimulates the proliferation, differentiation, and mineralization of osteoblastic cells, and indirectly stimulates osteoclast differentiation via enhancement of RANKL and inhibition of osteoprotegerin expression in osteoblasts, whereas directly inhibits osteoclast activity and bone resorption. Altogether, adipose tissue via adipokines plays a crucial role in the maintenance of bone homeostasis. These findings are reviewed herein. Adipobiology 2011; 3: 39-50.

The bone represents surprisingly dynamic structures that are subject to constant remodeling by the concerted action of bone-forming osteoblasts and bone-resorbing osteoclasts - two cell subsets of distinct developmental origin that are key in maintaining skeletal integrity throughout life. In general, abnormal bone remodeling due to dysregulated bone resorption and formation is an early event in the manifestation of various human bone diseases, such as osteopetrosis/osteoporosis and arthritis. But bone remodeling is also closely interrelated with lympho-hematopoietic homeostasis, as the bone marrow niche is formed by solid and trabecular bone structures that provide a framework for the long-term maintenance and differentiation of HSCs (>blood lineage cells and osteoclasts) and MSCs (>osteoblasts). Numerous studies in mice and humans have implicated innate and adaptive immune cells in the dynamic regulation of bone homeostasis, but despite considerable clinical relevance, the exact mechanisms of such immuno-bone interplay have remained incompletely understood. This holds particularly true for CD4+ regulatory T (Treg) cells expressing the lineage specification factor Foxp3: Foxp3+ Treg cells have been shown to play an indispensable role in maintaining immune homeostasis, but may also exert critical non-immune functions, which includes the control of metabolic and regenerative processes, as well as the differentiation of HSCs and function of osteoclasts. Here, we summarize our current knowledge on the T cell/bone interplay, with a particular emphasis on our own efforts to dissect the role of Foxp3+ Treg cells in bone and hematopoietic homeostasis, employing experimental settings of gain- and loss-of-Treg cell function. These data make a strong case that Foxp3+ Treg cells impinge on lympho-hematopoiesis through indirect mechanisms, i.e., by acting on osteoclast development and function, which translates into changes in niche size. Furthermore, we propose that, besides disorders that involve inflammatory bone loss, the modulation of Foxp3+ Treg cell function in vivo may represent a suitable approach to reinstate bone homeostasis in non-autoimmune settings of aberrant bone remodeling.

The importance of neuropeptide Y (NPY) and Y2 receptors in the regulation of bone and energy homeostasis has recently been demonstrated. However, the contributions of the other Y receptors are less clear. Here we show that Y1 receptors are expressed on osteoblastic cells. Moreover, bone and adipose tissue mass are elevated in Y1(-/-) mice with a generalized increase in bone formation on cortical and cancellous surfaces. Importantly, the inhibitory effects of NPY on bone marrow stromal cells in vitro are absent in cells derived from Y1(-/-) mice, indicating a direct action of NPY on bone cells via this Y receptor. Interestingly, in contrast to Y2 receptor or germ line Y1 receptor deletion, conditional deletion of hypothalamic Y1 receptors in adult mice did not alter bone homeostasis, food intake, or adiposity. Furthermore, deletion of both Y1 and Y2 receptors did not produce additive effects in bone or adiposity. Thus Y1 receptor pathways act powerfully to inhibit bone production and adiposity by nonhypothalamic pathways, with potentially direct effects on bone tissue through a single pathway with Y2 receptors.

骨重建和骨再生对于保持骨骼完整性和维持矿物质稳态至关重要。T淋巴细胞为适应性免疫的关键成员，通过产生一系列细胞因子和生长因子，在骨重建和骨再生过程中起着举足轻重的作用。在生理状态下，T淋巴细胞通过与间充质干细胞、成骨细胞、破骨细胞的交互作用参与骨稳态的维持；在病理状态下，T淋巴细胞通过与雌激素、糖皮质激素、甲状旁腺激素的协作参与不同类型骨质疏松的病理过程；在损伤后修复的骨折愈合过程中，T淋巴细胞在炎症血肿期、骨痂形成期和骨重建期发挥了不同的作用。因此，靶向T淋巴细胞成为调节骨稳态的潜在策略。本文综述了T细胞免疫参与骨重建和骨再生的研究进展及相关机制，以期为靶向T淋巴细胞调控骨重建和骨再生提供科学依据。

Aryl hydrocarbon receptor catabolic activity in bone metabolism is osteoclast dependent in vivo

European meeting “P2 receptors: new targets for the treatment of osteoporosis”

Potential Role of Pancreatic and Enteric Hormones in Regulating Bone Turnover

Cardiovascular disease (CVD) and vitamin D deficiency are extremely prevalent worldwide (Holick 2007; World Health Organization 2011). The potential link between vitamin D deficiency and CVD‐related death in both healthy and diseased populations is a growing area of translational research. Historically, the traditional role of vitamin D in maintaining calcium homeostasis and mineral metabolism has been extensively studied (Lieben and Carmeliet 2013). Vitamin D, via its classical genomic transcriptional activity, is known to affect the physiological function of a number of target organs such as the heart, bone, and kidneys (Lieben et al. 2011). Furthermore, as highlighted in a paper published recently in Physiological Resports (Foong et al. 2014), there appears to be a link between vitamin D deficiency and respiratory symptoms in chronic lung disease. In addition, a number of nongenomic, rapid action roles of vitamin D are now known to affect tissue function, which fall outside the traditional role of regulating mineral metabolism (Gniadecki 1998; Brown et al. 1999; Norman et al. 2002; Haussler et al. 2011). While calcium and phosphate have long been regarded as the major players in the interactions observed between mineral metabolism dysfunction and CVD‐related deaths, (Peacock 2010; Bolland et al. 2011; Brini et al. 2013), these newer nongenomic roles outside of the traditional target organs suggest that vitamin D itself may have intrinsic actions outside the realm of mineral metabolism (Gniadecki 1998; Brown et al. 1999; Norman et al. 2002; Haussler et al. 2011). Vitamin D encompasses a group of fat‐soluble prohormones which can be obtained by the body through both the skin via sunlight exposure and through intestinal absorption of dietary sources and supplements (Holick 2007). Although each source contributes to the overall maintenance of vitamin D levels, exposure to sunlight is the greatest resource for the synthesis of this prohormone in …

Osteoclasts are major actors in the maintenance of bone homeostasis. The full functional maturation of osteoclasts from monocyte lineage cells is essential for the degradation of old/damaged bone matrix. Diuron is one of the most frequently encountered herbicides, particularly in water sources. However, despite a reported delayed ossification invivo, its impact on bone cells remains largely unknown. The objectives of this study were to first better characterize osteoclastogenesis by identifying genes that drive the differentiation of monocyte progenitors into osteoclasts and to evaluate the toxicity of diuron on osteoblastic and osteoclastic differentiation in vitro. We performed chromatin immunoprecipitation (ChIP) against H3K27ac followed by ChIP-sequencing (ChIP-Seq) and RNA-sequencing (RNA-Seq) at different stages of differentiation of monocytes into active osteoclasts. Differentially activated super-enhancers and their potential target genes were identified. Then to evaluate the toxicity of diuron on osteoblasts and osteoclasts, we performed RNA-Seq and functional tests during in vitro osteoblastic and osteoclastic differentiation by exposing cells to different concentrations of diuron. The combinatorial study of the epigenetic and transcriptional remodeling taking place during differentiation has revealed a very dynamic epigenetic profile that supports the expression of genes vital for osteoclast differentiation and function. In total, we identified 122 genes induced by dynamic super-enhancers at late days. Our data suggest that high concentration of diuron () affects viability of mesenchymal stem cells (MSCs) in vitro associated with a decrease of bone mineralization. At a lower concentration (), an inhibitory effect was observed in vitro on the number of osteoclasts derived from monocytes without affecting cell viability. Among the diuron-affected genes, our analysis suggests a significant enrichment of genes targeted by pro-differentiation super-enhancers, with an odds ratio of 5.12 (). Exposure to high concentrations of diuron decreased the viability of MSCs and could therefore affect osteoblastic differentiation and bone mineralization. This pesticide also disrupted osteoclasts maturation by impairing the expression of cell-identity determining genes. Indeed, at sublethal concentrations, differences in the expression of these key genes were mild during the course of in vitro osteoclast differentiation. Taken together our results suggest that high exposure levels of diuron could have an effect on bone homeostasis. https://doi.org/10.1289/EHP11690.

Bone remodeling is essential for maintaining bone health. The imbalance between bone formation and bone resorption leads to bone diseases such as osteoporosis. Connexin43 (Cx43) is a gap junction molecule that plays an important role in bone homeostasis. The present study investigates the morphological characteristics of bone trabeculae and the distribution of Cx43 in bone cells using osteoporotic rat models to explore the relationship between osteoporosis and bone remodeling. Female Sprague–Dawley rats were divided into three groups: sham, ovarectomy with food deprivation (OVX+diet), and ovarectomy with steroid administration (OVX+steroid) for 3 and 12 months to induce osteoporosis. The lumbar vertebrae were processed for histomorphometric and immunohistochemical evaluation of the trabeculae and the distribution of Cx43 in bone cells. The data showed a significant reduction in trabecular bone in both osteoporotic groups. After 12 months, the OVX+diet treatment resulted in reduced mineralization and an increase in unmineralized bone. The percentage of alkaline phosphatase-positive areas in the OVX+diet vertebrae was lower at 12 months compared to the sham group. A significant increase in tartrate-resistant acid phosphatase (TRAP) positive osteoclasts was observed in the OVX+diet group. Both osteoporotic groups showed a decrease in Cx43-positive osteoblasts areas. An increase in the number of osteoclasts positive for Cx43 was detected in the OVX+diet group. The changes in Cx43 distribution in bone cells, together with trabecular mineralization, suggest that Cx43 may play a role in the progression of osteoporosis and could be a valuable target to improve bone remodeling.

Coordination between the nervous and innate immune systems to maintain bone homeostasis is largely uncharacterized. The present study investigated the sensory-immune interaction in resting alveolar bone and healing socket by surgical sensory denervation. Bone histomorphometry and immunohistochemistry showed that sensory denervation resulted in moderate suppression of bone remodeling, with a proinflammatory milieu manifested by increased neutrophil recruitment and possible alternations in macrophage phenotypes along the resting bone surface. This denervation effect intensified when bone remodeling was triggered by tooth extraction, as revealed by disrupted temporospatial variations in macrophage subpopulations and neutrophil infiltration, which were closely associated with a dramatic decline in socket bone filling and residual ridge height. Antagonism of calcitonin gene-related peptide (CGRP) brought about similar antianabolic and proinflammatory effects as sensory denervation, suggesting that sensory nerves may monitor the bony milieu by CGRP. Depletion of macrophages, rather than neutrophils, ruled out CGRP effects, illustrating that macrophages were the primary immune mechanism that linked sensory innervation, innate immunity, and bone. The data support that sensory innervation is required for control of innate immune responses and maintenance of bone homeostasis. Sensory neuropeptides, such as CGRP, are a possible target for the development of proanabolic treatments in bone disease by modulating innate immune responses.