Trabecular bone remodelling under pathological conditions based on biochemical and mechanical processes involved in BMU activity

In adulthood, bone tissue is continuously renewed by processes governed by basic multicellular units composed of osteocytes, osteoclasts and osteoblasts, which are subjected to local mechanical loads. Osteocytes are known to be integrated mechanosensors that regulate the activation of the osteoclasts and osteoblasts involved in bone resorption and apposition processes, respectively. After collagen tissue apposition, a process of collagen mineralisation takes place, gradually increasing the effective stiffness of bone. This study presents a new model based on physicochemical parameters involved in spongy bone remodelling under pathological conditions. Our model simulates the transient evolution of both geometry and effective Young's modulus of the trabeculae, also taking turnover into account. Various loads were applied on a trabecula in order to determine the evolution of bone volume fraction under pathological conditions. A parametric study performed on the model showed that one key parameter here is the kinetic constant of hydroxyapatite crystallisation. We subsequently tested our model on a pathological case approaching osteoporosis, involving a decrease in the number of viable osteocytes present in bone. The model converges to a lower value ( ? 5%) for bone volume fraction than with a normal quantity of osteocytes. This useful tool offers new perspectives for predicting bone remodelling deficits on a local scale in patients with pathological conditions such as osteoporosis and in bedridden patients, as well as for astronauts subjected to weightlessness in space.     

Central control of bone remodeling by neuromedin U

Bone remodeling, the function affected in osteoporosis, the most common of bone diseases, comprises two phases: bone formation by matrix-producing osteoblasts and bone resorption by osteoclasts. The demonstration that the anorexigenic hormone leptin inhibits bone formation through a hypothalamic relay suggests that other molecules that affect energy metabolism in the hypothalamus could also modulate bone mass. Neuromedin U (NMU) is an anorexigenic neuropeptide that acts independently of leptin through poorly defined mechanisms. Here we show that Nmu-deficient (Nmu-/-) mice have high bone mass owing to an increase in bone formation; this is more prominent in male mice than female mice. Physiological and cell-based assays indicate that NMU acts in the central nervous system, rather than directly on bone cells, to regulate bone remodeling. Notably, leptin- or sympathetic nervous system-mediated inhibition of bone formation was abolished in Nmu-/- mice, which show an altered bone expression of molecular clock genes (mediators of the inhibition of bone formation by leptin). Moreover, treatment of wild-type mice with a natural agonist for the NMU receptor decreased bone mass. Collectively, these results suggest that NMU may be the first central mediator of leptin-dependent regulation of bone mass identified to date. Given the existence of inhibitors and activators of NMU action, our results may influence the treatment of diseases involving low bone mass, such as osteoporosis.     

ATP and UTP at low concentrations strongly inhibit bone formation by osteoblasts: a novel role for the P2Y2 receptor in bone remodeling

There is increasing evidence that extracellular nucleotides act on bone cells via multiple P2 receptors. The naturally-occurring ligand ATP is a potent agonist at all receptor subtypes, whereas ADP and UTP only act at specific receptor subtypes. We have reported that the formation and resorptive activity of rodent osteoclasts are stimulated powerfully by both extracellular ATP and its first degradation product, ADP, the latter acting at nanomolar concentrations, probably via the P2Y1 receptor subtype. In the present study, we investigated the actions of ATP, ADP, adenosine, and UTP on osteoblastic function. In 16-21 day cultures of primary rat calvarial osteoblasts, ADP and the selective P2Y1 agonist 2-methylthioADP were without effect on bone nodule formation at concentrations between 1 and 125 microM, as was adenosine. However, UTP, a P2Y2 and P2Y4 receptor agonist, known to be without effect on osteoclast function, strongly inhibited bone nodule formation at concentrations >or= 1 microM. ATP was inhibitory at >or= 10 microM. Rat osteoblasts express P2Y2, but not P2Y4 receptor mRNA, as determined by in situ hybridization. Thus, the low-dose effects of extracellular nucleotides on bone formation and bone resorption appear to be mediated via different P2Y receptor subtypes: ADP, signalling through the P2Y1 receptor on both osteoclasts and osteoblasts, is a powerful stimulator of osteoclast formation and activity, whereas UTP, signalling via the P2Y2 receptor on osteoblasts, blocks bone formation by osteoblasts. ATP, the 'universal' agonist, can simultaneously stimulate resorption and inhibit bone formation. These findings suggest that extracellular nucleotides could function locally as important negative modulators of bone metabolism, perhaps contributing to bone loss in a number of pathological states.     

Serotonin and fluoxetine modulate bone cell function in vitro

Recent studies have proposed a role for serotonin and its transporter in regulation of bone cell function. In the present study, we examined the in vitro effects of serotonin and the serotonin transporter inhibitor fluoxetine "Prozac" on osteoblasts and osteoclasts. Human mononuclear cells were differentiated into osteoclasts in the presence of serotonin or fluoxetine. Both compounds affected the total number of differentiated osteoclasts as well as bone resorption in a bell-shaped manner. RT-PCR on the human osteoclasts demonstrated several serotonin receptors, the serotonin transporter, and the rate-limiting enzyme in serotonin synthesis, tryptophan hydroxylase 1 (Tph1). Tph1 expression was also found in murine osteoblasts and osteoclasts, indicating an ability to produce serotonin. In murine pre-osteoclasts (RAW264.7), serotonin as well as fluoxetine affected proliferation and NFkappaB activity in a biphasic manner. Proliferation of human mesenchymal stem cells (MSC) and primary osteoblasts (NHO), and 5-HT2A receptor expression was enhanced by serotonin. Fluoxetine stimulated proliferation of MSC and murine preosteoblasts (MC3T3-E1) in nM concentrations, microM concentrations were inhibitory. The effect of fluoxetine seemed direct, probably through 5-HT2 receptors. Serotonin-induced proliferation of MC3T3-E1 cells was inhibited by the PKC inhibitor (GF109203) and was also markedly reduced when antagonists of the serotonin receptors 5-HT2B/C or 5-HT2A/C were added. Serotonin increased osteoprotegerin (OPG) and decreased receptor activator of NF-kappaB ligand (RANKL) secretion from osteoblasts, suggesting a role in osteoblast-induced inhibition of osteoclast differentiation, whereas fluoxetine had the opposite effect. This study further describes possible mechanisms by which serotonin and the serotonin transporter can affect bone cell function.     

A unified model for the action of leptin on bone turnover

Leptin has been advocated as a centrally acting factor responsible for inhibiting accumulation of bone mass. However, recent investigations unequivocally establish leptin as a local (autocrine) factor expressed by osteoblasts. Exogenously added leptin causes osteoblastic cell proliferation and differentiation, while also rendering osteoblasts more efficacious in terms of mineralization. Leptin acts as an anti-apoptotic agent, and augments messages responsible for the remodelling of bone tissue, i.e., mRNAs for osteoprotegerin (OPG) and the interleukin IL-6. Furthermore, leptin message is readily expressed in osteoblasts subjected to mechanical strain. In this respect, osteoblasts, which are unilaterally stretched proliferate and differentiate, a phenomenon being potentiated by exposure of the cells to differentiating humoral factors. This article discusses a unified model of dually acting leptin through the central nervous system and the mechanostat principle applied to osteoblasts. The proposed model may account for the finely tuned bone homeostasis maintained within rather narrow limits, depending on exposure to humoral factors and the prevailing mechanostat usage mode.     

microRNAs在骨形成中的作用——运动调控骨代谢的新机制

microRNAs(miRNAs)是一类具有组织或发育阶段特异性的小分子、非编码单链RNA,通过转录后与靶基因特定序列结合来发挥其调控作用. 作为骨中的最重要的两种重要细胞--成骨细胞和破骨细胞,其代谢平衡与骨形成密切相关.研究发现,miRNAs在调节成骨细胞和破骨细胞分化及功能发挥上具有重要作用,并且运动训练可通过调节miRNAs进而调控骨细胞分化. 一般来说,适宜强度运动训练可上调某些miRNAs表达来促进成骨细胞或破骨细胞分化及功能;当失重或过量运动时,则会产生抑制作用. 本文就miRNAs调控干细胞向成骨细胞和破骨细胞分化及功能发挥的分子生物学机制以及运动训练调节与骨代谢相关miRNAs表达的研究进展进行综述.     

Boning up on autophagy: The role of autophagy in skeletal biology

From an evolutionary perspective, the major function of bone is to provide stable sites for muscle attachment and affording protection of vital organs, especially the heart and lungs (ribs) and spinal cord (vertebrae and intervertebral discs). However, bone has a considerable number of other functions: serving as a store for mineral ions, providing a site for blood cell synthesis and participating in a complex system-wide endocrine system. Not surprisingly, bone and cartilage cell homeostasis is tightly controlled, as is the maintenance of tissue structure and mass. While a great deal of new information is accruing concerning skeletal cell homeostasis, one relatively new observation is that the cells of bone (osteoclasts osteoblasts and osteocytes) and cartilage (chondrocytes) exhibit autophagy. The focus of this review is to examine the significance of this process in terms of the functional demands of the skeleton in health and during growth and to provide evidence that dysregulation of the autophagic response is involved in the pathogenesis of diseases of bone (Paget disease of bone) and cartilage (osteoarthritis and the mucopolysaccharidoses). Delineation of molecular changes in the autophagic process is uncovering new approaches for the treatment of diseases that affect the axial and appendicular skeleton.     

Boning up on autophagy

From an evolutionary perspective, the major function of bone is to provide stable sites for muscle attachment and affording protection of vital organs, especially the heart and lungs (ribs) and spinal cord (vertebrae and intervertebral discs). However, bone has a considerable number of other functions: serving as a store for mineral ions, providing a site for blood cell synthesis and participating in a complex system-wide endocrine system. Not surprisingly, bone and cartilage cell homeostasis is tightly controlled, as is the maintenance of tissue structure and mass. While a great deal of new information is accruing concerning skeletal cell homeostasis, one relatively new observation is that the cells of bone (osteoclasts osteoblasts and osteocytes) and cartilage (chondrocytes) exhibit autophagy. The focus of this review is to examine the significance of this process in terms of the functional demands of the skeleton in health and during growth and to provide evidence that dysregulation of the autophagic response is involved in the pathogenesis of diseases of bone (Paget disease of bone) and cartilage (osteoarthritis and the mucopolysaccharidoses). Delineation of molecular changes in the autophagic process is uncovering new approaches for the treatment of diseases that affect the axial and appendicular skeleton.     

Osteotropic effects by the neuropeptides calcitonin gene-related peptide, substance P and vasoactive intestinal peptide

Immunohistochemical phenotypic characterization of skeletal nerve fibers has demonstrated the expression of a restricted number of neuropeptides, including calcitonin gene-related peptide (CGRP), substance P (SP) and vasoactive intestinal peptide (VIP). According to the neuro-osteological hypothesis, such neuropeptides can be released and exert paracrine biological effects on bone cells present close to the nerve endings expressing these signaling molecules. The existence of such interplay is most convincingly shown by the hypothalamic control of bone formation, in the case of leptin stimulation of hypothalamic nuclei mediated by the sympathetic nervous system and inhibitory beta-adrenergic receptors on osteoblasts. In addition to these receptors, osteoblasts and osteoclasts express functional receptors for CGRP, SP and VIP, which can regulate both bone formation and bone resorption. The evidence for these observations is summarized in the present paper.     

TNAP, TrAP, ecto-purinergic signaling, and bone remodeling

Bone remodeling is a process of continuous resorption and formation/mineralization carried out by osteoclasts and osteoblasts, which, along with osteocytes, comprise the bone multicellular unit (BMU). A key component of the BMU is the bone remodeling compartment (BRC), isolated from the marrow by a canopy of osteoblast-like lining cells. Although much progress has been made regarding the cytokine-dependent and hormonal regulation of bone remodeling, less attention has been placed on the role of extracellular pH (pH(e)). Osteoclastic bone resorption occurs at acidic pH(e). Furthermore, osteoclasts can be regarded as epithelial-like cells, due to their polarized structure and ability to form a seal against bone, isolating the lacunar space. The major ecto-phosphatases of osteoclasts and osteoblasts, acid and alkaline phosphatases, both have ATPase activity with pH optima several units different from neutrality. Furthermore, osteoclasts and osteoblasts express plasma membrane purinergic P2 receptors that, upon activation by ATP, accelerate bone osteoclast resorption and impair osteoblast mineralization. We hypothesize that these ecto-phosphatases help regulate [ATP](e) and localized pH(e) at the sites of bone resorption and mineralization by pH-dependent ATP hydrolysis coupled with P2Y-dependent regulation of osteoclast and osteoblast function. Furthermore, osteoclast cellular HCO3(-), formed as a product of lacunar V-ATPase H(+) secretion, is secreted into the BRC, which could elevate BRC pH(e), in turn affecting osteoblast function. We will review the existing data addressing regulation of BRC pH(e), present a hypothesis regarding its regulation, and discuss the hypothesis in the context of the function of proteins that regulate pH(e).     

Extracellular matrix networks in bone remodeling

Bones are constantly remodeled throughout life to maintain robust structure and function. Dysfunctional remodeling can result in pathological conditions such as osteoporosis (bone loss) or osteosclerosis (bone gain). Bone contains 100s of extracellular matrix (ECM) proteins and the ECM of the various bone tissue compartments plays essential roles directing the remodeling of bone through the coupled activity of osteoclasts (which resorb bone) and osteoblasts (which produce new bone). One important role for the ECM is to serve as a scaffold upon which mineral is deposited. This scaffold is primarily type I collagen, but other ECM components are involved in binding of mineral components. In addition to providing a mineral scaffolding role, the ECM components provide structural flexibility for a tissue that would otherwise be overly rigid. Although primarily secreted by osteoblast-lineage cells, the ECM regulates cells of both the osteoblast-lineage (such as progenitors, mature osteoblasts, and osteocytes) and osteoclast-lineage (including precursors and mature osteoclasts), and it also influences the cross-talk that occurs between these two oppositional cells. ECM influences the differentiation process of mesenchymal stem cells to become osteoblasts by both direct cell-ECM interactions as well as by modulating growth factor activity. Similarly, the ECM can influence the development of osteoclasts from undifferentiated macrophage precursor cells, and influence osteoclast function through direct osteoclast cell binding to matrix components. This comprehensive review will focus on how networks of ECM proteins function to regulate osteoclast- and osteoblast-mediated bone remodeling. The clinical significance of these networks on normal bone and as they relate to pathologies of bone mass and geometry will be considered. A better understanding of the dynamic role of ECM networks in regulating tissue function and cell behavior is essential for the development of new treatment approaches for bone loss.     

Interaction between osteoblast and osteoclast: impact in bone disease

The intercellular communication between osteoblasts and osteoclasts is crucial to bone homeostasis. Since Rodan and Martin proposed the control of osteoclasts by osteoblasts in the 1980s, many factors have been isolated from osteoblasts and shown to regulate the differentiation and function of osteoclasts. However, the mechanism by which osteoblasts regulate osteoclasts during bone remodelling is still unclear. On the other hand, it is well accepted that many metabolic bone diseases are associated with the disruption of the communication between osteoblast and osteoclasts. Thus, this review focuses on the cross-talk between osteoblasts and osteoclasts and its impact in bone disease.     

Lactoferrin promotes bone growth

We have demonstrated bovine or human lactoferrin to be an anabolic factor in skeletal tissue. In vitro, lactoferrin stimulates the proliferation of bone forming cells, osteoblasts, and cartilage cells at physiological concentrations (above 0.1 microg/ml). The magnitude of this effect exceeds that observed in response to other skeletal growth factors such as IGF-1 and TGFbeta. DNA synthesis is also stimulated in a bone organ culture system likely reflecting the proliferation of cells of the osteoblast lineage. Lactoferrin is also a potent osteoblast survival factor. In TUNEL and DNA fragmentation assays, lactoferrin decreased apoptosis, induced by serum withdrawal, by up to 70%. In addition, lactoferrin has powerful effects on bone resorbing cells, osteoclasts, decreasing osteoclast development at concentrations > 1 microg/ml in a murine bone marrow culture system. However, lactoferrin did not alter bone resorption in calvarial organ culture, suggesting that it does not influence mature osteoclast function. In vivo, local injection of lactoferrin in adult mice resulted in increased calvarial bone growth, with significant increases in bone area and dynamic histomorphometric indices of bone formation after only 5 injections. Taken together, these data demonstrate that the naturally-occurring glycoprotein lactoferrin is anabolic to bone in vivo, an effect which is consequent upon its potent proliferative and anti-apoptotic actions in osteoblasts, and its ability to inhibit osteoclastogenesis. Lactoferrin may therefore have a physiological role in bone growth, and a potential therapeutic role in osteoporosis.     

神经调节：瘦素调控骨代谢过程中的重要角色

肥胖可以增加骨密度、减少骨质疏松的发生。而大多数肥胖者血清瘦素水平升高,并与肥胖的程度呈正相关。研究证实,来源于脂肪组织的瘦素可直接作用于骨细胞和软骨细胞,进而影响其增殖分化。瘦素通过抑制脑5-羟色胺（serotonin,5-HT）的合成、神经肽Y（neuropeptide Y,NPY）的表达等,并经由交感神经系统（sympathetic nervous system,SNS）介导的成骨细胞内p2肾上腺素能受体（B2-adrenergic receptor,Adrl32）信号通路共同调节骨代谢。这些工作为治疗骨质疏松、骨性关节炎等的新策略提供了理论基础。     

Role of vascular endothelial cells in bone biology

Bone development and remodeling depend on complex interactions between bone-forming osteoblasts, bone-degrading osteoclasts, and other cells present within the bone microenvironment. Balanced control of bone formative and degradative processes is normally carefully maintained in the adult skeleton but becomes uncoupled in the course of aging or in various pathological disease states. Systemic regulators of bone metabolism and local mediators, including matrix molecules, cytokines, prostaglandins, leukotrienes, and other autocrine or paracrine factors, regulate the recruitment, differentiation, and function of cells participating in bone formation and turnover. Although some of these interactions are now understood, many yet remain to be elucidated. Recent studies have begun exploring in detail how vascular endothelial cells and their products function in bone physiology. The findings are revealing that bone vascular endothelial cells may be members of a complex communication network in bone which operates between endothelial cells, osteoblasts, osteoclasts, macrophages, stromal cells, and perhaps other cell types found in bone as well. Therefore, multiple systemic and locally produced signals may be received, transduced, and integrated by individual cells and then propagated by the release from these cells of further signals targeted to other members of the bone cell network. In this manner, bone cell activities may be continuously coordinated to afford concerted actions and rapid responses to physiological changes. The bone microvasculature may play a pivotal role in these processes, both in linking circulatory and local signals with cells of the bone microenvironment and in actively contributing itself to the regulation of bone cell physiology. Thus, skeletal homeostasis and the coupling observed between bone resorption and bone formation during normal bone remodeling may be manifestations of this dynamic interactive communication network, operating via diverse signals not only between osteoblasts and osteoclasts but between many cell types residing within bone. © 1994 Wiley-Liss, Inc.     

Osteoimmunology: interactions of the immune and skeletal systems

Bone is a dynamic tissue that provides mechanical support, physical protection, and enables movement. Bone also serves as a storage site for minerals and is where blood cells are produced. Bone homeostasis is regulated by the balance between bone formation and resorption, and involves the coordinated action of osteoblasts and osteoclasts. Osteoblasts are bone-forming cells that secrete organic matrix molecules, while osteoclasts are derived from hematopoietic precursors and resorb bone matrix. Although osteoblasts and osteoclasts are the major regulators of bone metabolism and are regulated by the local microenvironment, it has recently come to be appreciated that skeletal system homeostasis is greatly influenced by components of the immune system. For example, some pathological bone resorption observed under inflammatory conditions has been shown to be due, in part, to direct and indirect effects of activated T cells on osteoclasts. In this regard, we would like to review current progress and perspectives in "osteoimmunology", an interdisciplinary research principle governing the cross-talk between the bone and immune systems. Better understanding of how the osteoimmune system operates in normal and pathological situations is likely to lay the groundwork for future therapies for the variety of diseases that affect both bone and the immune system.     

HIF-1α在骨组织细胞代谢及骨疾病中的调控作用

低氧诱导因子-1α(HIF-1α)是调节细胞对低氧应答的关键因子,可在氧含量降低时被激活,能够调节氧代谢、糖酵解等多种生理活动。骨代谢主要包括骨形成和骨吸收作用,均受到氧浓度等多种因素的调控。HIF-1α在细胞代谢、骨组织生理及病理过程的调控中起着重要的作用,能够增加骨组织的低氧耐受能力,调节骨形成和矿化过程。该文主要综述了HIF-1α对成骨细胞、破骨细胞、骨髓间充质干细胞、软骨细胞等骨组织细胞的调控,对骨血管形成过程的影响,以及对肿瘤骨转移、股骨头坏死、异位骨化等病理过程的调节作用,为探讨HIF-1α对骨代谢的调控和相关疾病的治疗提供参考。     

A new member of tumor necrosis factor ligand family, ODF/OPGL/TRANCE/RANKL, regulates osteoclast differentiation and function

Osteoclasts, the multinucleated giant cells that resorb bone, develop from monocyte-macrophage lineage cells. Osteoblasts or bone marrow stromal cells have been suggested to be involved in osteoclastic bone resorption. The recent discovery of new members of the tumor necrosis factor (TNF) receptor-ligand family has elucidated the precise mechanism by which osteoblasts/stromal cells regulate osteoclast differentiation and function. Osteoblasts/stromal cells express a new member of the TNF-ligand family "osteoclast differentiation factor(ODF)/osteoprotegerin ligand (OPGL)/TNF-related activation-induced cytokine (TRANCE)/receptor activator of NF-kB ligand (RANKL)" as a membrane associated factor. Osteoclast precursors which possess RANK, a TNF receptor family member, recognize ODF/OPGL/TRANCE/RANKL through cell-to-cell interaction with osteoblasts/stromal cells, and differentiate into osteoclasts in the presence of macrophage colony-stimulating factor. Mature osteoclasts also express RANK, and their bone-resorbingactivity is also induced by ODF/OPGL/TRANCE/RANKL which osteoblasts/stromal cells possess. Osteoprotegerin (OPG)/osteoclastogenesis inhibitory factor (OCIF)/TNF receptor-like molecule 1 (TR1) is a soluble decoy receptor for ODF/OPGL/TRANCE/RANKL. Activation of NF-kB and c-Jun N-terminal kinase through the RANK-mediated signaling system appears to be involved in differentiation and activation of osteoclasts.     

miR-214对骨形成的抑制作用

越来越多的研究表明microRNA广泛参与骨代谢的调控,调节骨髓间充质干细胞、成骨及破骨细胞的增殖及分化,调控骨形成与骨吸收之间的平衡,在维持骨代谢平衡中发挥重要作用。近年来有研究报道老年性骨质疏松、绝经后骨质疏松均与miR-214的高表达有关。miR-214通过靶向作用于Osterix、ATF-4、FGFR1、Pten以及LZTS1等基因调控骨髓间充质干细胞、成骨细胞以及破骨细胞等骨组织细胞的增殖及分化,进而抑制骨形成,促进骨吸收。本文主要综述了miR-214对骨髓间充质干细胞、成骨细胞以及破骨细胞分化的调控作用,旨在探讨miR-214对骨形成的抑制作用,为骨质疏松等骨疾病的诊断及治疗提供理论依据。     

Neural regulation of bone remodeling: Identifying novel neural molecules and pathways between brain and bone

The metabolism and homeostasis of the skeletal system have historically been considered to be associated with the endocrine system. However, this view has been expanded with the recognition of several neural pathways playing important roles in the regulation of bone metabolism via central relays. In particular, bone metabolism and homeostasis have been reported to be precisely modulated by the central neural signaling. Initiated by the finding of leptin, the axis of neural regulation on bone expands rapidly. The semaphorin–plexin system plays an important role in the cross-talk between osteoclasts and osteoblasts; a complex system has also been identified and includes neuropeptide Y and cannabinoids. These findings facilitate our understanding of the central neuropeptides and neural factors in the modulation of bone metabolism and homeostasis, and these neuronal pathways also represent an area of research scenario that identifies the novel regulation between brain and bone. These regulatory mechanisms correlate with other homeostatic networks and demonstrate a more intricate and synergetic bone biology than previously envisioned. As such, this review summarizes the current knowledge of the neural regulation of bone metabolism and homeostasis, as well as its role in skeletal diseases and discusses the emerging challenges presented in this field.