Immune signals in the context of secondary osteoporosis

Bone homeostasis is maintained by a balance between bone resorption by osteoclasts and bone formation by osteoblasts, and alterations in bone metabolism can lead to diseases such as osteoporosis. Inter-cellular and intra-cellular signaling, originating from the immune system, the largest source of cell-derived regulatory signals, are involved in these processes. Immune-competent cells such as macrophages and lymphocytes deliver cell-cell signaling through soluble factors such as cytokines and through direct contact with the cells. Such immunological signals to the bone are transmitted primarily through osteoblasts or direct stimulation of osteoclasts to induce osteoclast maturation or bone resorption, which may in turn lead to the disequilibrium of bone metabolism. Inflammatory diseases such as rheumatoid arthritis are good examples of such a process, in which immunological signals play a central role in the pathogenesis of the accompanying secondary osteoporosis. We will achieve a better understanding of the pathogenesis of bone metabolism in osteoporosis through immune signaling, and thereby develop improved therapeutic strategies for these conditions.     

ATPase pumps in osteoclasts and osteoblasts

Osteoblasts, osteocytes and osteoclasts are specialised cells of bone that play crucial roles in the formation, maintenance and resorption of bone matrix. Bone formation and resorption critically depend on optimal intracellular calcium and phosphate homeostasis and on the expression and activity of plasma membrane transport systems in all three cell types. Osteotropic agents, mechanical stimulation and intracellular pH are important parameters that determine the fate of bone matrix and influence the activity, expression, regulation and cell surface abundance of plasma membrane transport systems. In this paper the role of ATPase pumps is reviewed in the context of their expression in bone cells, their contribution to ion homeostasis and their relation to other transport systems regulating bone turnover.     

Voltage-gated K+ channel KCNQ1 regulates insulin secretion in MIN6 β-cell line

Bone homeostasis is maintained by a dynamic balance between bone resorption by osteoclasts and bone formation by osteoblasts. Since excessive osteoclast activity is implicated in pathological bone resorption, understanding the mechanism underlying osteoclast differentiation, function and survival is of both scientific and clinical importance. Osteoclasts are monocyte/macrophage lineage cells with a short life span that undergo rapid apoptosis, the rate of which critically determines the level of bone resorption in vivo. However, the molecular basis of rapid osteoclast apoptosis remains obscure. Here we report the role of a BH3-only protein, Noxa (encoded by the Pmaip1 gene), in bone homeostasis using Noxa-deficient mice. Among the Bcl-2 family members, Noxa was selectively induced during osteoclastogenesis. Mice lacking Noxa exhibit a severe osteoporotic phenotype due to an increased number of osteoclasts. Noxa deficiency did not have any effect on the number of osteoclast precursor cells or the expression of osteoclast-specific genes, but led to a prolonged survival of osteoclasts. Furthermore, adenovirus-mediated Noxa overexpression remarkably reduced bone loss in a model of inflammation-induced bone destruction. This study reveals Noxa to be a crucial regulator of osteoclast apoptosis, and may provide a molecular basis for a new therapeutic approach to bone diseases.     

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

Bone mass is regulated by various molecules including endogenous factors as well as exogenous factors, such as nutrients and pollutants. Aryl hydrocarbon receptor (AhR) is known as a dioxin receptor and is responsible for various pathological and physiological processes. However, the role of AhR in bone homeostasis remains elusive because the cell type specific direct function of AhR has never been explored in vivo. Here, we show the cell type specific function of AhR in vivo in bone homeostasis. Systemic AhR knockout (AhRKO) mice exhibit increased bone mass with decreased resorption and decreased formation. Meanwhile, osteoclast specific AhRKO (AhR^ΔOc/ΔOc) mice have increased bone mass with reduced bone resorption, although the mice lacking AhR in osteoblasts have a normal bone phenotype. Even under pathological conditions, AhR^ΔOc/ΔOc mice are resistant to sex hormone deficiency-induced bone loss resulting from increased bone resorption. Furthermore, 3-methylcholanthrene, an AhR agonist, induces low bone mass with increased bone resorption in control mice, but not in AhR^ΔOc/ΔOc mice. Taken together, cell type specific in vivo evidence for AhR functions indicates that osteoclastic AhR plays a significant role in maintenance of bone homeostasis, suggesting that inhibition of AhR in osteoclasts can be beneficial in the treatment of osteoporosis.     

Mechanisms influencing bone metabolism in chronic illness

Bone is permanently renewed by the coordinated actions of bone-resorbing osteoclasts and bone-forming osteoblasts, which model and remodel bone structure during growth and adult life. The origin of osteoblastic cells (osteoblasts, osteocytes and bone-lining cells) differs from that of osteoclasts, but both cell groups communicate with each other using cytokines and cell-cell contact as to optimally maintain bone homeostasis. This communication in many ways uses the same players as the communication between cells in the immune system. During acute life-threatening illness massive bone resorption is the rule, while bone formation is suppressed. During chronic illness, the balance between bone formation and bone resorption also shifts, frequently resulting in decreased bone mass and density. Several factors may contribute to the osteopenia that accompanies chronic illness, the most important being undernutrition and low body weight, inflammatory cytokines, disorders of the neuroendocrine axis (growth hormone/IGF-1 disturbances, thyroid and gonadal deficiency), immobilization, and the long-term use of glucocorticoids. Their combined effects not only influence the generation and activity of all bone cells involved, but probably also regulate their life span by apoptotic mechanisms. Osteopenia or even osteoporosis and bone fragility, and before puberty also decreased linear growth and lower peak bone mass are therefore frequent consequences of chronic illnesses.     

Synaptotagmin VII regulates bone remodeling by modulating osteoclast and osteoblast secretion

Maintenance of bone mass and integrity requires a tight balance between resorption by osteoclasts and formation by osteoblasts. Exocytosis of functional proteins is a prerequisite for the activity of both cells. In the present study, we show that synaptotagmin VII, a calcium sensor protein that regulates exocytosis, is associated with lysosomes in osteoclasts and bone matrix protein-containing vesicles in osteoblasts. Absence of synaptotagmin VII inhibits cathepsin K secretion and formation of the ruffled border in osteoclasts and bone matrix protein deposition in osteoblasts, without affecting the differentiation of either cell. Reflecting these in vitro findings, synaptotagmin VII-deficient mice are osteopenic due to impaired bone resorption and formation. Therefore, synaptotagmin VII plays an important role in bone remodeling and homeostasis by modulating secretory pathways functionally important in osteoclasts and osteoblasts.     

Nutritional factors and bone homeostasis: synergistic effect with zinc and genistein in osteogenesis

Bone homeostasis is regulated through osteoclasts and osteoblasts. Osteoporosis, which is induced with its accompanying decrease in bone mass with increasing age, is widely recognized as a major public health problem. Bone loss may be due to decreased osteoblastic bone formation and increased osteoclastic bone resorption. There is growing evidence that nutritional and food factors may play a part in the prevention of bone loss with aging and have been to be worthy of notice in the prevention of osteoporosis. Zinc, an essential trace element, or genistein, which are contained in soybeans, has been shown to have a stimulatory effect on osteoblastic bone formation and an inhibitory effect on osteoclastic bone resorption, thereby increasing bone mass. These factors have an effect on protein synthesis and gene expression, which are related to bone formation in osteoblastic cells and bone resorption in osteoclastic cells. The combination of zinc and genistein is found to reveal the synergistic effect on bone anabolic effect. The oral administration of those factors has been shown to prevent on bone loss in ovariectomized rats, an animal model for osteoporosis, indicating a role in the prevention of osteoporosis. Supplemental intake of ingredient with the combination of zinc and genistein has been shown to have a preventive effect on osteoporosis in human subjects, suggesting a role in the prevention of bone loss.     

Identification of osteopontin in isolated rabbit osteoclasts.

Bone remodeling is a complex process coupling bone formation and resorption. Osteoblasts, the bone-forming cells, are known to produce various bone matrix proteins and cytokines; however, little is known about protein factors produced by osteoclasts or bone-resorbing cells. A method utilizing the high affinity of osteoclasts for tissue culture dishes was developed to isolate a large number of pure osteoclasts from rabbit long bones. A cDNA library was then constructed from these isolated osteoclasts, and differential cDNA screening was performed between osteoclasts and spleen cells. Two clones representing osteoclast-specific clones, named OC-1 and OC-2, were isolated. By Northern blot analysis, OC-1 was expressed in osteoclasts and in kidneys, whereas OC-2 was specific for osteoclasts. OC-1 was found to encode osteopontin from its nucleotide sequence, and therefore, osteopontin may have other functions for osteoclastic bone resorption besides osteoclast attachment to bone.     

Tyrosine kinases Btk and Tec regulate osteoclast differentiation by linking RANK and ITAM signals

Certain autoimmune diseases result in abnormal bone homeostasis, but association of immunodeficiency with bone is poorly understood. Osteoclasts, which derive from bone marrow cells, are under the control of the immune system. Differentiation of osteoclasts is mainly regulated by signaling pathways activated by RANK and immune receptors linked to ITAM-harboring adaptors. However, it is unclear how the two signals merge to cooperate in osteoclast differentiation. Here we report that mice lacking the tyrosine kinases Btk and Tec show severe osteopetrosis caused by a defect in bone resorption. RANK and ITAM signaling results in formation of a Btk(Tec)/BLNK(SLP-76)-containing complex and PLCgamma-mediated activation of an essential calcium signal. Furthermore, Tec kinase inhibition reduces osteoclastic bone resorption in models of osteoporosis and inflammation-induced bone destruction. Thus, this study reveals the importance of the osteoclastogenic signaling complex composed of tyrosine kinases, which may provide the molecular basis for a new therapeutic strategy.     

Screening of Korean medicinal plants for possible osteoclastogenesis effects in vitro

Bone undergoes continuous remodeling through bone formation and resorption, and maintaining the balance for skeletal rigidity. Bone resorption and loss are generally attributed to osteoclasts. Differentiation of osteoclasts is regulated by receptor activator of nuclear factor NF-kB ligand (RANKL), a member of tumor necrosis factor family. When the balance is disturbed, pathological bone abnormality ensues. Through the screening of traditional Korean medicinal plants, the effective molecules for inhibition and stimulation of RANKL-induced osteoclast differentiation in mouse bone marrow macrophages were identified. Among 222 methanol extracts, of medicinal plants, 10 samples exhibited ability to induce osteoclast differentiation. These include Dryobalanops aromatica, Euphoria longana, Lithospermum erythrorhizon, Prunus mume, Prunus nakaii, and Polygonatum odoratum. In contrast, Ailanthus altissima, Curcuma longa, Solanum nigrum, Taraxacum platycarpa, Trichosanthes kirilowii, and Daphne genkwa showed inhibitory effects in RANKL-induced osteoclast differentiation.     

Coupling factors and exosomal packaging microRNAs involved in the regulation of bone remodelling

Bone remodelling is a continuous process by which bone resorption by osteoclasts is followed by bone formation by osteoblasts to maintain skeletal homeostasis. These two forces must be tightly coordinated not only quantitatively, but also in time and space, and its malfunction leads to diseases such as osteoporosis. Recent research focusing on the cross‐talk and coupling mechanisms associated with the sequential recruitment of osteoblasts to areas where osteoclasts have removed bone matrix have identified a number of osteogenic factors produced by the osteoclasts themselves. Osteoclast‐derived factors and exosomal‐containing microRNA (miRNA) can either enhance or inhibit osteoblast differentiation through paracrine and juxtacrine mechanisms, and therefore may have a central coupling role in bone formation. Entwined with angiocrine factors released by vessel‐specific endothelial cells and perivascular cells or pericytes, these factors play a critical role in angiogenesis–osteogenesis coupling essential in bone remodelling.     

Membrane trafficking in osteoblasts and osteoclasts: new avenues for understanding and treating skeletal diseases

The endocytic and exocytic/secretory pathways are two major intracellular membrane trafficking routes that regulate numerous cellular functions in a variety of cell types. Osteoblasts and osteoclasts, two major bone cells responsible for bone remodeling and homeostasis, are no exceptions. During the past few years, emerging evidence has pinpointed a critical role for endocytic and secretory pathways in osteoblast and osteoclast differentiation and function. The endosomal membrane provides a platform to integrate bone tropic signals of hormones and growth factors in osteoblasts. In osteoclasts, endocytosis, followed by transcytosis, of degraded bone matrix promotes bone resorption. Secretory pathways, especially lysosome secretion, not only participate in bone matrix deposition by osteoblasts and degradation of mineralized bone matrix by osteoclasts; they may also be involved in the coupling of bone resorption and bone formation during bone remodeling. More importantly, mutations in genes encoding regulatory factors within the endocytic and secretory pathways have been identified as causes for bone diseases. Identification of the molecular mechanisms of these genes in bone cells may provide new therapeutic targets for skeletal disorders.     

Osteoclast lineage and function

Osteoclasts are members of the monocyte/macrophage lineage and are formed by cellular fusions from their mononuclear precursors. Their differentiation is regulated by a number of other cells and their products, especially by RANKL and M-CSF. The resorbing osteoclasts are polarized and show specific plasma membrane domains. Polarization and bone resorption need a continuous membrane trafficking and modulation of the cytoskeleton. The most characteristic feature of osteoclasts is their unique capacity to dissolve crystalline hydroxyapatite by targeted secretion of HCl into the extracellular resorption lacuna. Organic matrix is degraded by enzymes like cathepsin K and the degradation products are transcytosed through the cell for secretion. Dissolution of hydroxyapatite releases large amounts of soluble calcium, phosphate and bicarbonate. Removal of these ions apparently involves the vesicular pathways and direct ion transport via different ion exchangers, channels and pumps. Detailed molecular knowledge of osteoclast differentiation and function has helped us to identify several target molecules and develop specific treatments to inhibit pathological bone resorption in various skeletal diseases.     

Gelsolin-independent podosome formation in dendritic cells

Podosomes, important structures for adhesion and extracellular matrix degradation, are claimed to be involved in cell migration. In addition, podosomes are also reported to be of importance in tissue remodelling, e.g., in osteoclast-mediated bone resorption. Podosomes are highly dynamic actin-filament scaffolds onto which proteins important for their function, such as matrix metallo-proteases and integrins, attach. The dynamics of the podosomes require the action of many proteins regulating actin assembly and disassembly. One such protein, gelsolin, which associates to podosomes, has been reported to be important for podosome formation and function in osteoclasts. However, podosome-like structures have been reported in gelsolin-deficient dendritic cells, but the identity of these structures was not confirmed, and their dynamics and function was not investigated. Like many other cells, dendritic cells of the immune system also form matrix degrading podosomes. In the present study, we show that dendritic cells form podosomes independently of gelsolin, that there are no major alterations in their dynamics of formation and disassembly, and that they exhibit matrix-degrading function. Furthermore, we found that gelsolin is not required for TLR4-induced podosome disassembly. Thus, the actin cytoskeleton of podosomes involved in dendritic cell extracellular matrix degradation appears to be regulated differently than the cytoskeleton in podosomes of osteoclasts mediating bone resorption.     

Skeletal tissue and transforming growth factor beta

Normal skeletal growth results from a balance between the processes of bone matrix synthesis and resorption. These activities are regulated by both systemic and local factors. Bone turnover is dynamic, and skeletal growth must be maintained throughout life. Although many growth promoters are associated with bone matrix, it is enriched particularly with transforming growth factor beta (TGF-beta) activity. Experimental evidence indicates that TGF-beta regulates replication and differentiation of mesenchymal precursor cells, chondrocytes, osteoblasts, and osteoclasts. Recent studies further suggest that TGF-beta activity in skeletal tissue may be controlled at multiple levels by other local and systemic agents. Consequently, the intricate mechanisms by which TGF-beta regulates bone formation are likely to be fundamental to understanding the processes of skeletal growth during development, maintenance of bone mass in adult life, and healing subsequent to bone fracture.     

The osteoprotective role of USP26 in coordinating bone formation and resorption

Bone homeostasis is maintained through a balance of bone formation by osteoblasts and bone resorption by osteoclasts. Ubiquitin-specific proteases (USPs) are involved in regulating bone metabolism by preserving bone formation or antagonizing bone resorption. However, the specific USPs that maintain bone homeostasis by orchestrating bone formation and bone resorption simultaneously are poorly understood. Here, we identified USP26 as a previously unknown regulator of bone homeostasis that coordinates bone formation and resorption. Mechanistically, USP26 stabilizes β-catenin to promote the osteogenic activity of mesenchymal cells (MSCs) and impairs the osteoclastic differentiation of bone myelomonocytes (BMMs) by stabilizing inhibitors of NF-κBα (IκBα). Gain-of-function experiments revealed that Usp26 supplementation significantly increased bone regeneration in bone defects in aged mice and decreased bone loss resulting from ovariectomy. Taken together, these data show the osteoprotective effect of USP26 via the coordination of bone formation and resorption, suggesting that USP26 represents a potential therapeutic target for osteoporosis.Subject terms: Deubiquitylating enzymes, Deubiquitylating enzymes, Endocrine system and metabolic diseases, Immunopathogenesis     

Osteoclast-derived activity in the coupling of bone formation to resorption

The cells of bone and the immune system communicate by means of soluble and membrane-bound cytokines and growth factors. Through local signalling mechanisms, cells of the osteoblast lineage control the formation and activity of osteoclasts and, therefore, the resorption of bone. Both T and B lymphocytes produce activators and inhibitors of osteoclast formation. A local 'coupling factor' linking bone resorption to subsequent formation in the bone multicellular unit (BMU) has long been proposed as the key regulator of the bone remodelling process, but never identified. There is evidence in support of the view that the coupling mechanism is dependent on growth factors released from the bone matrix during resorption, or is generated from maturing osteoblasts. We argue that osteoclasts contribute in important ways to the transiently activated osteoclast, and stimulate osteoblast lineage cells to begin replacing the resorbed bone in each BMU.