Guanabenz inhibits alveolar bone resorption in a rat model of periodontitis

Increase of sympathetic activity has been known to exacerbate osteoporosis through promotion of bone resorption. However, it is largely unknown about involvement of sympathetic activity in exacerbation of periodontitis. In this study, we investigated whether α2-adrenergic receptor (α2-AR) agonist guanabenz which decreases sympathetic activity, attenuates alveolar bone resorption in rats having high sympathetic activity with periodontitis. Volumes of residual alveolar bone and attachment levels in periodontium were examined using micro-computed tomography and hematoxylin-eosin staining, respectively. Furthermore, osteoclast numbers per bone surface and osteoclast surface per bone surface were measured using tartrate-resistant acid phosphatase staining. To examine the suppressive effects of guanabenz on pro-inflammatory cytokines, expression levels of tyrosine hydroxylase (TH), TNF-α, IL1-β, and IL-6 in periodontium were measured using immunohistostaining. Administration of guanabenz attenuated loss of alveolar bone and attachment levels in rats having high sympathetic activity. Furthermore, its administration suppressed osteoclast numbers in rats having high sympathetic activity. TH, TNF-α, IL-1β, and IL-6 positive cells in periodontium in rats treated with guanabenz for 12 weeks, were lower than those in control rats having high sympathetic activity. This study demonstrated administration of α2-AR agonist guanabenz attenuates alveolar bone resorption through decrease of sympathetic activity in rats.

Suppression of alveolar bone resorption by salubrinal in a mouse model of periodontal disease

AIMS

The relationship between stress to endoplasmic reticulum (ER) and periodontitis has been known, and ER stress induced by Porphyromonas gingivalis results in the loss of alveolar bone. Salubrinal is a small synthetic compound and attenuates ER stress through inhibition of de-phosphorylation of eukaryotic translation initiation factor 2 alpha (eIF2α). In this study, we examined whether salubrinal attenuates periodontitis in a mouse model of experimental periodontal disease.

MATERIALS AND METHODS

We evaluated loss of alveolar bone and attachment levels in periodontium using micro-computed tomography (μCT) and hematoxylin-eosin (HE) staining, respectively. Furthermore, we measured osteoclast numbers using tartrate-resistant acid phosphatase (TRAP) staining and osteoblast numbers using HE staining for bone resorption and for bone formation, respectively. To examine the inhibitory effects of salubrinal against pro-inflammatory cytokines, we measured TNF-α and IL1-β score in periodontium using immunohistostaining.

KEY FINDINGS

The results revealed that salubrinal suppressed loss of alveolar bone and attachment levels in periodontium induced by periodontitis. It decreased osteoclast numbers and increased osteoblasts. It also suppressed the expression levels of TNF-α in periodontium.

SIGNIFICANCE

These results show that salubrinal alleviates periodontitis through suppression of alveolar bone resorption and the pro-inflammatory cytokine, and promotion of the bone formation. Since salubrinal has been shown to have these beneficial effects for periodontal disease, it may provide a novel therapeutic possibility for the disease.

Contribution of Glucosylceramide Synthase to the Proliferation of Mouse Osteoblasts

BACKGROUND/AIM

Glycosphingolipids are known to be involved in bone metabolism. However, their roles and regulatory mechanisms in osteoblast proliferation are largely unknown. In this study, we examined the effects of inhibitors of glucosylceramide synthase (GCS), which is responsible for the generation of all glycosphingolipids, on osteoblast proliferation.

MATERIALS AND METHODS

We analyzed the expression of glycosphingolipids and cell growth in MC3T3-E1 mouse osteoblast cells treated with the GCS inhibitors miglustat, D-PDMP and D-PPMP. We also conducted microarray analysis and RNA interference to identify genes involved in cell growth regulated by GCS.

RESULTS

Glycosphingolipids GD1a and Gb4 expressed in MC3T3-E1 cells, were suppressed by GCS inhibitors. Furthermore, the proliferation of MC3T3-E1 cells was suppressed by the inhibitors. Using microarray analysis, we predicted nine genes (Fndc1, Acta2, Igfbp5, Cox6a2, Cth, Mymk, Angptl6, Mab21l2, and Igsf10) suppressed by all three inhibitors. Furthermore, partial silencing of Angptl6 by RNA interference reduced MC3T3-E1 cell growth.

CONCLUSION

These results show that GCS regulates proliferation through Angptl6 in osteoblasts.

Orthodontic tooth movement-activated sensory neurons contribute to enhancing osteoclast activity and tooth movement through sympathetic nervous signalling

OBJECTIVES

Orthodontic tooth movement (OTM) increases sympathetic and sensory neurological markers in periodontal tissue. However, the relationship between the sympathetic and sensory nervous systems during OTM remains unclear. Therefore, the present study investigated the relationship between the sympathetic and sensory nervous systems activated by OTM using pharmacological methods.

MATERIALS AND METHODS

We compared the effects of sympathectomy and sensory nerve injury during OTM in C57BL6/J mice. Capsaicin (CAP) was used to induce sensory nerve injury. Sympathectomy was performed using 6-hydroxydopamine. To investigate the effects of a β-agonist on sensory nerve injury, isoproterenol (ISO) was administered to CAP-treated mice. Furthermore, to examine the role of the central nervous system in OTM, the ventromedial hypothalamic nucleus (VMH) was ablated using gold thioglucose.

RESULTS

Sensory nerve injury and sympathectomy both suppressed OTM and decreased the percent of the alveolar socket covered with osteoclasts (Oc.S/AS) in periodontal tissue. Sensory nerve injury inhibited increases in OTM-induced calcitonin gene-related peptide (CGRP) immunoreactivity (IR), a marker of sensory neurons, and tyrosine hydroxylase (TH) IR, a marker of sympathetic neurons, in periodontal tissue. Although sympathectomy did not decrease the number of CGRP-IR neurons in periodontal tissue, OTM-induced increases in the number of TH-IR neurons were suppressed. The ISO treatment restored sensory nerve injury-inhibited tooth movement and Oc.S/AS. Furthermore, the ablation of VMH, the centre of the sympathetic nervous system, suppressed OTM-induced increases in tooth movement and Oc.S/AS.

CONCLUSIONS

The present results suggest that OTM-activated sensory neurons contribute to enhancements in osteoclast activity and tooth movement through sympathetic nervous signalling.

Effects of a β2-adrenergic receptor blocker on experimental periodontitis in spontaneously hypertensive rats

AIMS

Recent studies have reported a relationship between periodontal disease and hypertension, and previous evidence suggests that the sympathetic nervous system plays an important role in the control of bone metabolism. This study sought to evaluate the effect of the beta-2 adrenergic receptor (β2-AR) blocker butoxamine on experimental periodontitis in a rat model.

MATERIALS AND METHODS

Wistar-Kyoto and spontaneously hypertensive rats (n = 6 per group) were orally administered butoxamine 1 mg/kg/day and experimental periodontitis was induced by applying an orthodontic ligature wire. The rats were sacrificed after 4 weeks and the residual alveolar bone was measured using micro-computed tomography (micro-CT) imaging analysis software for histological analysis.

KEY FINDINGS

Micro-CT imaging analysis showed a higher ratio of residual alveolar bone, BV/TV, and Tb.N in both Wistar-Kyoto and spontaneously hypertensive rats treated with butoxamine compared with the corresponding control rats. In histological analysis, compared with the Wistar-Kyoto and spontaneously hypertensive rat control groups, the corresponding butoxamine-treated groups showed a lower ratio of attachment level, lower values of osteoclast number and surface.

SIGNIFICANCE

β2-AR blockers maintained the alveolar bone mass and attachment level by suppressing osteoclast activity. Thus, β2-AR blockers may be effective in preventing periodontitis.

Suppression of tooth movement-induced sclerostin expression using β-adrenergic receptor blockers

OBJECTIVE

Regulation of bone metabolism by the sympathetic nervous system has recently been clarified. Tooth movement is increased by increased bone metabolic turnover due to sympathetic activation. This study aimed to compare the effects of the β-adrenergic receptor (β-AR) blockers atenolol (β1-AR blocker), butoxamine (β2-AR blocker) and propranolol (non-selective β-AR blocker) on tooth movement in spontaneously hypertensive rats (SHR) with sympathicotonia.

MATERIALS AND METHODS

Spontaneously hypertensive rats were divided into the following four groups: an SHR control group and groups treated with 0.1 mg/kg atenolol, 1 mg/kg butoxamine or 1 mg/kg propranolol (n = 6 rats/group). Atenolol, butoxamine or propranolol was administered daily to each treatment group, and orthodontic force was applied using a closed-coil spring. Finally, immunohistochemical analysis was performed for receptor activator of nuclear factor kappa-B ligand (RANKL) and sclerostin (SOST).

RESULTS

Atenolol, butoxamine and propranolol inhibited tooth movement and increased maxillary alveolar bone volume. Histological analysis revealed that these β-AR blockers decreased osteoclast activity on the compression side. Furthermore, immunohistochemical analysis revealed that atenolol, butoxamine and propranolol decreased the number of RANKL- and SOST-positive osteocytes on the compression side.

CONCLUSIONS

β-AR blockers decreased tooth movement and downregulated SOST in osteocytes, accompanied by increasing alveolar bone resorption.

Co-cultured spheroids of human periodontal ligament mesenchymal stem cells and vascular endothelial cells enhance periodontal tissue regeneration

Introduction

Human periodontal ligament mesenchymal stem cells (hPDLMSCs) have been known that they play important roles in homeostasis and regeneration of periodontal tissues. Additionally, spheroids are superior to monolayer-cultured cells. We investigated the characteristics and potential of periodontal tissue regeneration in co-cultured spheroids of hPDLMSCs and human umbilical vein endothelial cells (HUVECs) in vitro and in vivo.

Methods

Co-cultured spheroids were prepared with cell ratios of hPDLMSCs: HUVECs = 1:1, 1:2, and 2:1, using microwell chips. Real-time polymerase chain reaction (PCR) analysis, Enzyme-Linked Immuno Sorbent Assay (ELISA), and nodule formation assay were performed to examine the properties of co-cultured spheroids. Periodontal tissue defects were prepared in the maxillary first molars of rats and subjected to transplantation assay.

Results

The expression levels of stemness markers, vascular endothelial growth factor (VEGF), osteogenesis-related genes were up-regulated in co-cultured spheroids, compared with monolayer and spheroid-cultured hPDLMSCs. The nodule formation was also increased in co-cultured spheroids, compared with monolayer and spheroid cultures of hPDLMSCs. Treatment with co-cultured spheroids enhanced new cementum formation after 4 or 8 weeks of transplantation, although there was no significant difference in the new bone formation between co-cultured spheroids and hPDLMSC spheroids.

Conclusions

We found that co-cultured spheroids enhance the periodontal tissue regeneration. Co-cultured spheroids of hPDLMSCs and HUVECs may be a useful therapy that can induce periodontal tissue regeneration.

Deletion of Gb3 Synthase in Mice Resulted in the Attenuation of Bone Formation via Decrease in Osteoblasts

Glycosphingolipids are known to play a role in developing and maintaining the integrity of various organs and tissues. Among glycosphingolipids, there are several reports on the involvement of gangliosides in bone metabolism. However, there have been no reports on the presence or absence of expression of globo-series glycosphingolipids in osteoblasts and osteoclasts, and the involvement of their glycosphingolipids in bone metabolism. In the present study, we investigated the presence or absence of globo-series glycosphingolipids such as Gb3 (globotriaosylceramide), Gb4 (globoside), and Gb5 (galactosyl globoside) in osteoblasts and osteoclasts, and the effects of genetic deletion of Gb3 synthase, which initiates the synthesis of globo-series glycosphingolipids on bone metabolism. Among Gb3, Gb4, and Gb5, only Gb4 was expressed in osteoblasts. However, these glycosphingolipids were not expressed in pre-osteoclasts and osteoclasts. Three-dimensional micro-computed tomography (3D-μCT) analysis revealed that femoral cancellous bone mass in Gb3 synthase-knockout (Gb3S KO) mice was lower than that in wild type (WT) mice. Calcein double labeling also revealed that bone formation in Gb3S KO mice was significantly lower than that in WT mice. Consistent with these results, the deficiency of Gb3 synthase in mice decreased the number of osteoblasts on the bone surface, and suppressed mRNA levels of osteogenic differentiation markers. On the other hand, osteoclast numbers on the bone surface and mRNA levels of osteoclast differentiation markers in Gb3S KO mice did not differ from WT mice. This study demonstrated that deletion of Gb3 synthase in mice decreases bone mass via attenuation of bone formation.

Conditioned medium from rat dental pulp reduces the number of osteoclasts via attenuation of adhesiveness in osteoclast precursors

Dental pulp is known to play crucial roles in homeostasis of teeth and periodontal tissue. Although resorption of bone around the roots of nonvital teeth is occasionally observed in clinical practice, little is known about the role of dental pulp in osteoclastogenesis. Here we evaluated the effects of conditioned medium (CM) from rat dental pulp on osteoclastogenesis. It was found that the CM reduced the number of tartrate-resistant acid phosphatase (TRAP)-positive multinucleated osteoclasts, but did not alter the mRNA levels of nuclear factor of activated T-cells, cytoplasmic 1 and TRAP. To further understand the mechanism behind these results, we evaluated the effects of CM on osteoclast precursors and found that the CM removed cell processes, resulting in a significant reduction in the number of attached cells and an increase in the number of freely floating cells. Furthermore, the CM suppressed the mRNA levels of focal adhesion kinase and paxillin, which are involved in cell adhesiveness and spreading. Collectively, the present results show that CM from dental pulp serves as an inhibitor of osteoclastogenesis by reducing the number and adhesiveness of osteoclast precursors, suggesting novel therapeutic applicability for osteoporosis.

Suppression of osteoclastogenesis via α2-adrenergic receptors

The sympathetic nervous system is known to regulate osteoclast development. However, the involvement of α2-adrenergic receptors (α2-ARs) in osteoclastogenesis is not well understood. In the present study, their potential role in osteoclastogenesis was investigated. Guanabenz, clonidine and xylazine were used as agonists of α2-ARs, while yohimbine and idazoxan were employed as antagonists. Using RAW264.7 pre-osteoclast and primary bone marrow cells, the mRNA expression of the osteoclast-related genes nuclear factor of activated T-cells, cytoplasmic 1 (NFATc1), tartrate-resistant acid phosphatase (TRAP) and cathepsin K was evaluated following induction with receptor activator of nuclear factor κB ligand (RANKL). TRAP staining was also conducted to assess effects on osteoclastogenesis in mouse bone marrow cells in vitro. Administration of 5–20 µM guanabenz (P<0.01, for RANKL-only treatment), 20 µM clonidine (P<0.05, for RANKL-only treatment) and 20 µM xylazine (P<0.05, for RANKL-only treatment) attenuated RANKL-induced upregulation of NFATc1, TRAP and cathepsin K mRNA. Furthermore, the reductions in these mRNAs by 10 µM guanabenz and 20 µM clonidine in the presence of RANKL were attenuated by 20 µM yohimbine or idazoxan (P<0.05). The administration of 5–20 µM guanabenz (P<0.01, for RANKL-only treatment) and 10–20 µM clonidine (P<0.05, for RANKL-only treatment) also decreased the number of TRAP-positive multi-nucleated osteoclasts. Collectively, the present study demonstrates that α2-ARs may be involved in the regulation of osteoclastogenesis.

Bidirectional communication between sensory neurons and osteoblasts in an in vitro coculture system

Recent studies have revealed that the sensory nervous system is involved in bone metabolism. However, the mechanism of communication between neurons and osteoblasts is yet to be elucidated. In this study, we investigated the signaling pathways between sensory neurons of the dorsal root ganglion (DRG) and the osteoblast-like MC3T3-E1 cells using an in vitro coculture system. Our findings indicate that signal transduction from DRG-derived neurons to MC3T3-E1 cells is suppressed by antagonists of the AMPA receptor and the NK₁ receptor. Conversely, signal transduction from MC3T3-E1 cells to DRG-derived neurons is suppressed by a P2X₇ receptor antagonist. Our results suggest that these cells communicate with each other by exocytosis of glutamate, substance P in the efferent signal, and ATP in the afferent signal.

Increase in RANKL: OPG Ratio in Synovia of Patients with Temporomandibular Joint Disorder

Although a recent study suggested the involvement of RANKL and osteoprotegerin (OPG) in the pathogenesis of bone-destructive disease, no study has focused on the RANKL:OPG ratio in the synovial fluid of patients with temporomandibular joint (TMJ) disorder. This communication reports on the concentrations of RANKL and OPG in synovial fluid from TMJ patients and healthy control individuals. In contrast to an unchanged concentration of RANKL, a strong decrease in the concentration of OPG was detected in the synovial fluid from patients with TMJ internal derangement. Treatment with the synovial fluid of osteoarthritis (OA) patients resulted in the high production of osteoclast-like cells from blood mononuclear cells in vitro, as well as in pit formation in dentin slices. The addition of anti-RANKL IgG or OPG attenuated OA-synovial fluid-induced osteoclast formation, suggesting that the increase in the RANKL:OPG ratio in the microenvironment of the joint has the potential to induce osteoclastogenesis in TMJ osteoarthritis.

α1B -Adrenoceptor signalling regulates bone formation through the up-regulation of CCAAT/enhancer-binding protein δ expression in osteoblasts

BACKGROUND AND PURPOSE

The sympathetic nervous system regulates bone remodelling, in part, through ß2 -adrenoceptor signalling. However, the physiological role of α1 -adrenoceptor signalling in bone in vivo remains unclear. Therefore, to obtain a deeper understanding of bone remodelling by the sympathetic nervous system, we investigated the role of α1B -adrenoceptor signalling in bone metabolism.

EXPERIMENTAL APPROACH

Prazosin, a nonspecific α1 -adrenoceptor antagonist, was administered for 2 weeks in C57BL6 mice, and efficacy was evaluated by bone microarchitecture using microcomputed tomography and determination of bone formation by fluorescent labelling of bone. We also compared the bone phenotype of α1B -adrenoceptor null mice (α1B (-/-) ) with that of wild-type littermates.

KEY RESULTS

We demonstrated that the systemic administration of prazosin decreased bone formation. In addition, α1B -adrenoceptor-deficient mice had a lower bone mass due to decreased bone formation but did not exhibit any changes in bone-resorbing activity. Furthermore, stimulation with phenylephrine, a non-specific α1 -adrenoceptor agonist, increased the expression of the transcriptional factor CCAAT/enhancer-binding protein δ (Cebpd) in MC3T3-E1 osteoblastic cells. The overexpression of Cebpd induced cellular proliferation in MC3T3-E1 cells, whereas the silencing of Cebpd suppressed it.

CONCLUSIONS AND IMPLICATIONS

Taken together, these results suggested that α1B -adrenoceptor signalling is required for bone formation and regulated cellular proliferation through a mechanism relevant to the up-regulation of Cebpd in osteoblasts and, thus, provide new evidence for the physiological importance of α1B -adrenoceptor signalling in bone homeostasis.

α1B-Adrenergic receptor signaling controls circadian expression of Tnfrsf11b by regulating clock genes in osteoblasts

Circadian clocks are endogenous and biological oscillations that occur with a period of <24 h. In mammals, the central circadian pacemaker is localized in the suprachiasmatic nucleus (SCN) and is linked to peripheral tissues through neural and hormonal signals. In the present study, we investigated the physiological function of the molecular clock on bone remodeling. The results of loss-of-function and gain-of-function experiments both indicated that the rhythmic expression of Tnfrsf11b, which encodes osteoprotegerin (OPG), was regulated by Bmal1 in MC3T3-E1 cells. We also showed that REV-ERBα negatively regulated Tnfrsf11b as well as Bmal1 in MC3T3-E1 cells. We systematically investigated the relationship between the sympathetic nervous system and the circadian clock in osteoblasts. The administration of phenylephrine, a nonspecific α1-adrenergic receptor (AR) agonist, stimulated the expression of Tnfrsf11b, whereas the genetic ablation of α_1B-AR signaling led to the alteration of Tnfrsf11b expression concomitant with Bmal1 and Per2 in bone. Thus, this study demonstrated that the circadian regulation of Tnfrsf11b was regulated by the clock genes encoding REV-ERBα (Nr1d1) and Bmal1 (Bmal1, also known as Arntl), which are components of the core loop of the circadian clock in osteoblasts.

Summary: This study demonstrates that the circadian regulation of TNFRSF11B is regulated by the clock genes Nr1d1 and Arntl, which are components of the core loop of the circadian clock in osteoblasts, providing a molecular mechanism for the control of bone remodelling by circadian rhythms.

Modulation of osteoblast differentiation and bone mass by 5-HT2A receptor signaling in mice

Recent studies reported that serotonin (5-hydroxytryptamine, 5-HT) may be an endogenous paracrine and/or autocrine factor that is used for intercellular communication in bone cells and between multiple organs regulating bone homeostasis. In the present study, we showed that the administration of MDL11939, a selective 5-HT2A receptor antagonist, reduced bone mass in mice. The loss of bone mass in MDL11939-treated mice was associated with impaired bone formation in vivo, as demonstrated by the lower expression of osterix (Osx) and osteocalcin than that in vehicle-treated mice. On the other hand, no significant differences were observed in osteoclast numbers between MDL11939- and vehicle-treated mice. The pharmacological blockade of 5-HT2A receptor signaling significantly decreased alkaline phosphatase activity in osteoblastic cells. In addition, the knockdown of the 5-HT2A receptor by a siRNA treatment decreased Osx, but not Runx2 gene expression in MC3T3-E1 cells. These results suggest that 5-HT2A receptor signaling mediated bone mass by regulating osteoblast differentiation.

Shear stress-induced Ca(2+) elevation is mediated by autocrine-acting glutamate in osteoblastic MC3T3-E1 cells

Mechanical loading is an important regulatory factor in bone homeostasis. Neurotransmitters, such as glutamate and ATP, are known to be released from osteoblasts, but their roles have been less studied. In this study, we investigated the role of transmitter release in mechanotransduction. To identify from where transmitters were released, focal fluid flow was applied to a single cell of MC3T3-E1, mouse calvaria-derived osteoblastic cell line, by using a glass micropipette. Intracellular Ca(2+) elevation induced by the focal shear stress was eliminated by either GdCl3, a mechanosensing channel inhibitor, or removal of extracellular Ca(2+). On the other hand, the focal shear stress-induced Ca(2+) elevation was also significantly suppressed by inositol triphosphate receptor antagonist or vesicular release inhibitors. These results suggest that not only mechanosensitive channel-mediated Ca(2+) influx but also some autocrine transmitters are involved in mechanotransduction. Additionally, glutamate receptor antagonists, but not ATP receptor antagonist, suppressed most of the focal shear stress-induced Ca(2+) elevation. Therefore, it is suggested that glutamate is released from osteoblasts following the activation of mechanosensitive Ca(2+) channels and acts in an autocrine manner. The glutamate release may have a significant role in the initial event of mechanotransduction in bone tissue.

β-adrenergic receptor signaling regulates Ptgs2 by driving circadian gene expression in osteoblasts

The sympathetic nervous system modulates bone remodeling and mediates the expression of core clock genes in part through the β-adrenergic receptor (β-AR) in osteoblasts. In this study, we show that in MC3T3-E1 osteoblastic cells that isoproterenol (Iso), a non-selective β-AR agonist, upregulated the transcriptional factor Nfil3, and induced rhythmic mRNA expression of prostaglandin-endoperoxide synthase 2 (Ptgs2, also known as Cox2). The rhythmic effects of Iso on Ptgs2 expression were mediated by interplay between the Per2 and Bmal1 clock genes in osteoblasts. In addition, Ptgs2 was significantly decreased in bone after continuous Iso treatment. Overexpression of Nfil3 decreased Ptgs2 expression in MC3T3-E1 cells. Knockdown of Nfil3 upregulated the expression of Ptgs2 in MC3TC-E1 cells, indicating that Nfil3 negatively regulated Ptgs2 in osteoblasts. Furthermore, Iso acutely induced the expression Nfil3 and increased the binding of Nfil3 to the Ptgs2 promoter in MC3T3-E1 cells. These results suggest that Iso-mediated induction of Nfil3 in osteoblasts regulates the expression of Ptgs2 by driving the expression of circadian clock genes. These findings provide new evidence for a physiological role of circadian clockwork in bone metabolism.

Selective β2-adrenergic Antagonist Butoxamine Reduces Orthodontic Tooth Movement

Recently, involvement of the sympathetic nervous system in bone metabolism has attracted attention. β2-Adrenergic receptor (β2-AR) is presented on osteoblastic and osteoclastic cells. We previously demonstrated that β-AR blockers at low dose improve osteoporosis with hyperactivity of the sympathetic nervous system via β2-AR blocking, while they may have a somewhat inhibitory effect on osteoblastic activity at high doses. In this study, the effects of butoxamine (BUT), a specific β2-AR antagonist, on tooth movement were examined in spontaneously hypertensive rats (SHR) showing osteoporosis with hyperactivity of the sympathetic nervous system. We administered BUT (1 mg/kg) orally, and closed-coil springs were inserted into the upper-left first molar. After sacrifice, we calculated the amount of tooth movement and analyzed the trabecular microarchitecture and histomorphometry. The distance in the SHR control was greater than that in the Wistar-Kyoto rat group, but no significant difference was found in the SHR treated with BUT compared with the Wistar-Kyoto rat control. Analysis of bone volume per tissue volume, trabecular number, and osteoclast surface per bone surface in the alveolar bone showed clear bone loss by an increase of bone resorption in SHR. In addition, BUT treatment resulted in a recovery of alveolar bone loss. Furthermore, TH-immunoreactive nerves in the periodontal ligament were increased by tooth movement, and BUT administration decreased TH-immunoreactive nerves. These results suggest that BUT prevents alveolar bone loss and orthodontic tooth movement via β2-AR blocking.

α1-adrenergic receptor signaling in osteoblasts regulates clock genes and bone morphogenetic protein 4 expression through up-regulation of the transcriptional factor nuclear factor IL-3 (Nfil3)/E4 promoter-binding protein 4 (E4BP4)

Several studies have demonstrated that the α1-adrenergic receptor (AR) plays an important role in regulating cell growth and function in osteoblasts. However, the physiological role of α1-AR signaling in bone metabolism is largely unknown. In this study, the stimulation of phenylephrine (PHE), a nonspecific α1-AR agonist, increased the transcriptional factor Nfil3/E4BP4 and led to the rhythmic expression of bone morphogenetic protein 4 (Bmp4) in MC3T3-E1 osteoblastic cells. We also showed that Bmp4 mRNA expression peaked in bone near zeitgeber time 8 in a 24-h rhythm. Furthermore, the expression of Nfil3 and Bmp4 displayed a circadian pattern with opposing phases, which suggested that Nfil3 repressed the expression of the Bmp4 gene during a circadian cycle. On a molecular level, both loss-of-function and gain-of-function experiments demonstrated that Nfil3/E4BP4 negatively regulated Bmp4 expression in osteoblasts. Furthermore, the systemic administration of PHE increased the expression of Nfil3 mRNA in bone, whereas it decreased that of Bmp4 mRNA. The expression of Bmp4 mRNA was decreased significantly by exposure to PHE, and this was concomitant with the increase in Nfil3 binding to the D-box-containing Bmp4 promoter region in MC3T3-E1 cells, which indicates that the expression of Nfil3 by α1-AR signaling can bind directly to the Bmp4 promoter and inhibit Bmp4 expression in osteoblasts. Our results suggest that α1-AR signaling regulates clock genes and Bmp4 expression in osteoblasts. Moreover, α1-AR signaling negatively regulated Bmp4 expression by up-regulating the transcriptional factor Nfil3/E4BP4 in osteoblasts.

Glucocorticoids mediate circadian timing in peripheral osteoclasts resulting in the circadian expression rhythm of osteoclast-related genes

Circadian rhythms are prevalent in bone metabolism. However, the molecular mechanisms involved are poorly understood. Recently, we suggested that output signals from the suprachiasmatic nucleus (SCN) are transmitted from the master circadian rhythm to peripheral osteoblasts through β-adrenergic and glucocorticoid signaling. In this study, we examined how the master circadian rhythm is transmitted to peripheral osteoclasts and the role of clock gene in osteoclast. Mice were maintained under 12-hour light/dark periods and sacrificed at Zeitgeber times 0, 4, 8, 12, 16 and 20. mRNA was extracted from femur (cancellous bone) and analyzed for the expression of osteoclast-related genes and clock genes. Osteoclast-related genes such as cathepsin K (CTSK) and nuclear factor of activated T-cells, cytoplasmic 1 (NFATc1) showed circadian rhythmicity like clock genes such as period 1 (PER1), PER2 and brain and muscle Arnt-like protein 1 (BMAL1). In an in vitro study, not β-agonist but glucocorticoid treatment remarkably synchronized clock and osteoclast-related genes in cultured osteoclasts. Chromatin immunoprecipitation (ChIP) assay showed the interaction between BMAL1 proteins and promoter region of CTSK and NFATc1. To examine whether endogenous glucocorticoids influence the osteoclast circadian rhythms, mice were adrenalectomized (ADX) and maintained under 12-hour light/dark periods at least two weeks before glucocorticoid injection. A glucocorticoid injection restarted the circadian expression of CTSK and NFATc1 in ADX mice. These results suggest that glucocorticoids mediate circadian timing to peripheral osteoclasts and osteoclast clock contributes to the circadian expression of osteoclast-related genes such as CTSK and NFATc1.

Acidosis inhibits mineralization in human osteoblasts

Osteoblasts and osteoclasts maintain bone volume. Acidosis affects the function of these cells including mineral metabolism. We examined the effect of acidosis on the expression of transcription factors and mineralization in human osteoblasts in vitro. Human osteoblasts (SaM-1 cells) derived from the ulnar periosteum were cultured with α-MEM containing 50 μg/ml ascorbic acid and 5 mM β-glycerophosphate (calcifying medium). Acidosis was induced by incubating the SaM-1 cells in 10 % CO₂ (pH approximately 7.0). Mineralization, which was augmented by the calcifying medium, was completely inhibited by acidosis. Acidosis depressed c-Jun mRNA and increased osteoprotegerin (OPG) production in a time-dependent manner. Depressing c-Jun mRNA expression using siRNA increased OPG production and inhibited mineralization. In addition, depressing OPG mRNA expression with siRNA enhanced mineralization in a dose-dependent manner. Acidosis or the OPG protein strongly inhibited mineralization in osteoblasts from neonatal mice. The present study was the first to demonstrate that acidosis inhibited mineralization, depressed c-Jun mRNA expression, and induced OPG production in human osteoblasts. These results suggest that OPG is involved in mineralization via c-Jun in human osteoblasts.

Noradrenaline stimulates cell proliferation by suppressing potassium channels via G(i/o) -protein-coupled α(1B) -adrenoceptors in human osteoblasts

BACKGROUND AND PURPOSE

Recent studies demonstrated that the sympathetic nervous system regulates bone metabolism via β(2) -adrenoceptors. Although α-adrenoceptors are also expressed in osteogenic cells, their functions in bone metabolism have been less studied. We previously demonstrated that noradrenaline suppressed potassium currents via α(1B) -adrenoceptors in the human osteoblast SaM-1 cell line. The aim of this study was to investigate the signal transduction pathway and the physiological role of noradrenaline in human osteoblasts in more detail.

EXPERIMENTAL APPROACH

To investigate signal transduction through α(1B) -adrenoceptors, we used whole-cell patch clamp recording and Ca fluorescence imaging. Potassium channels regulate membrane potential and cell proliferation activity in non-excitable cells, so we evaluated cell proliferation activity by BrdU incorporation and WST assay.

KEY RESULTS

In SaM-1 cells, bath-applied noradrenaline elevated intracellular Ca(2+) concentration and this effect was abolished by both chloroethylclonidine, an α(1B) -adrenoceptor antagonist, and U73122, a PLC inhibitor. However, the inhibitory effect of noradrenaline on whole-cell current was unaffected by U73122. In contrast, in cells pretreated with either Pertussis toxin, a G(i/o) -protein-coupled receptor inhibitor, or gallein, a Gβγ-protein inhibitor, the inhibitory effect of noradrenaline on whole-cell current was significantly suppressed. Noradrenaline-induced enhancement of cell proliferation was inhibited by CsCl, a non-selective potassium channel blocker, gallein and H89, a PKA inhibitor, but not by U73122.

CONCLUSIONS AND IMPLICATIONS

Noradrenaline facilitated cell proliferation by regulation of potassium currents in human osteoblasts via G(i/o) -protein-coupled α(1B) -adrenoceptors, not via coupling to Gq-proteins.

Store-operated calcium entry induced by activation of Gq-coupled alpha1B adrenergic receptor in human osteoblast

Recent studies have revealed that the sympathetic nervous system is involved in bone metabolism. We previously reported that noradrenaline (NA) suppressed K(+) currents via Gi/o protein-coupled alpha1B-adrenergic receptor (α1B-AR) in human osteoblast SaM-1 cells. Additionally, it has been demonstrated that the intracellular Ca(2+) level ([Ca(2+)]i) was increased by NA via α1B-AR. In this study, we investigated the signal pathway of NA-induced [Ca(2+)]i elevation by using Ca(2+) fluorescence imaging in SaM-1 cells. NA-induced [Ca(2+)]i elevation was suppressed by pretreatment with a PLC inhibitor, U73122. This suggested that the [Ca(2+)]i elevation was mediated by Gq protein-coupled α1B-AR. On the other hand, NA-induced [Ca(2+)]i elevation was completely abolished in Ca(2+)-free solution, which suggested that Ca(2+) influx is the predominant pathway of NA-induced [Ca(2+)]i elevation. Although the inhibition of K(+) channel by NA caused membrane depolarization, the [Ca(2+)]i elevation was not affected by voltage-dependent Ca(2+) channel blockers, nifedipine and mibefradil. Meanwhile, NA-induced [Ca(2+)]i elevation was abolished following activation of store-operated Ca(2+) channel by thapsigargin. Additionally, the [Ca(2+)]i elevation was suppressed by store-operated channel inhibitors, 2-APB, flufenamate, GdCl3 and LaCl3. These results suggest that Ca(2+) influx through store-operated Ca(2+) channels plays a critical role in the signal transduction pathway of Gq protein-coupled α1B-AR in human osteoblasts.

β-Adrenergic signaling stimulates osteoclastogenesis via reactive oxygen species

Sympathetic signaling regulates bone resorption through receptor activator of nuclear factor-κB ligand (RANKL) expression via the β-adrenergic receptor (β-AR) on osteoblasts. Reactive oxygen species (ROS) are known as one type of osteoclast regulatory molecule. Here we show that an antioxidant, α-lipoic acid (α-LA), treatment prevent the β-adrenergic signaling-induced bone loss by suppressing osteoclastogenesis, and sympathetic signaling directly regulates osteoclastogenesis through β2-AR expressed on osteoclasts via intracellular ROS generation. In an in vitro study, the β-AR agonist isoprenaline increased intracellular ROS generation in osteoclasts prepared from bone marrow macrophages (BBMs) and RAW 264.7 cells. Isoprenaline enhanced osteoclastogenesis through β2-AR expressed on BMMs and RAW 264.7 cells. The antioxidant α-LA inhibited isoprenaline-enhanced osteoclastogenesis. Isoprenaline increased the expression of osteoclast-related genes such as nuclear factor of activated T cells, cytoplasmic, calcineurin-dependent 1, tartrate-resistant acid phosphatase, and cathepsin K on osteoclasts. α-LA also inhibited isoprenaline-induced increases of these gene expressions. These in vitro results led to the hypothesis that β-adrenergic signaling directly stimulates osteoclastogenesis via ROS generation. In an in vivo study, isoprenaline treatment alone caused oxidative damage in local bone and reduced bone mass because of an increase in bone resorption, and, in α-LA-treated mice, isoprenaline did not increase tibial osteoclast number even though the RANKL-to-osteoprotegerin ratio increased. These in vitro and in vivo results indicate that β-adrenergic signaling, at least in part, directly stimulates osteoclastogenesis through β2-AR on osteoclasts via ROS generation.

Neopterin and Cytokines in Hereditary Dystonia and Parkinson's Disease

Both neopterin and biopterin concentrations in cerebrospinal fluid from patients with Parkinson's disease, in which the nigrostriatal dopamine neurons degenerate, were lower than those from age-matched older control subjects. However, the decrease in biopterin was more marked than that in neopterin, resulting in the increase in the neopterin/ biopterin ratio in Parkinson's disease. These results suggests that neopterin in cerebrospinal fluid in Parkinson's disease may partly be derived from immunoactivated glial cells, besides catecholamine or serotonin n eurons including nigrostriatal dopamine neurons. In accordance to this hypothesis, cytokines (TNF-α, IL-1, IL-2 , IL-6, EGF, TGF-α, TGF-β1) were found to be increased in the striatum and/or in cerebrospinal fluid. The increment of cytokines in the brain in Parkinson's disease may be related to the mechanism of neurodegeneration of dopaminergic neurons in Parkinson's disease . In contrast to Parkinson's disease, in hereditary progressive dystonia/ dopa-responsive dystonia, which is a dopamine deficiency caused by mutations in GTP cyclohydrolase I without neuronal cell death (Segawa's disease), neopterin and biopterin in cerebrospinal fluid decreases in parallel owing to the decreased activity in GTP cyclohydrolase I .

Dose effects of butoxamine, a selective β2-adrenoceptor antagonist, on bone metabolism in spontaneously hypertensive rat

Recent studies have shown that osteoblasts and osteoclasts express β2-adrenoceptor, and increased sympathetic nervous activity causes bone loss via an increase in osteoclastic bone resorption and a decrease in osteoblastic bone formation. We previously demonstrated that non-selective β-adrenoceptor antagonist propranolol at low doses (0.1 and 1mg/kg), but not at a higher dose (10mg/kg), prevented a decrease in bone mass and an increase in bone fragility in spontaneously hypertensive rat (SHR), an animal model of osteoporosis with hyperactivity of the sympathetic nervous system, without affecting blood pressure. In the present study, the dose effects of butoxamine, a selective β2-adrenoceptor antagonist, on bone metabolism were examined in SHR by analysis of microcomputed tomography, bone histomorphometry, biomechanical testing and plasma biochemistry. Treatment of SHR with butoxamine at 0.1, 1 and 10mg/kg (per os) for 12 weeks increased bone mass indices and biomechanical parameters of strength and toughness of the lumbar vertebrae, suggesting antiosteoporotic activity. Butoxamine dose-dependently decreased osteoclast number and surface per bone surface with decreases in plasma tartrate-resistant acid phosphatase-5b level, a biochemical index of osteoclastic activity. On the other hand, histomorphometry indices of bone formation and plasma osteocalcin concentration reflecting osteoblastic activity were increased in SHR treated with butoxamine at 0.1 and 1mg/kg, but not at 10mg/kg. These results suggest that β-adrenoceptor antagonists at a low dose may improve osteoporosis with hyperactivity of the sympathetic nervous system via β2-adrenoceptor blocking action, while they may have a somewhat inhibitory effect on osteoblastic activity at a high dose.

Experimental tooth movement-induced osteoclast activation is regulated by sympathetic signaling

Experimental tooth movement (ETM) changes the distribution of sensory nerve fibers in periodontal ligament and the bone architecture through the stimulation of bone remodeling. As the sympathetic nervous system is involved in bone remodeling, we examined whether ETM is controlled by sympathetic signaling or not. In male mice, elastic rubber was inserted between upper left first molar (M1) and second molar (M2) for 3 or 5 days. Nerve fibers immunoreactive for not only sensory neuromarkers, such as calcitonin gene-related peptide (CGRP), but also sympathetic neuromarkers, such as tyrosine hydroxylase (TH) and neuropeptide Y (NPY) were increased in the periodontal ligament during ETM. To elucidate the effect of the sympathetic signal mediated by ETM, mice were intraperitoneally injected with a β-antagonist, propranolol (PRO: 20 μg/g/day), or a β-agonist, isoproterenol (ISO: 5 μg/g/day) from 7 days before ETM. PRO treatment suppressed the amount of tooth movement by 12.9% in 3-day ETM and by 32.2% in 5-day ETM compared with vehicle treatment. On the other hand, ISO treatment increased it. Furthermore, ETM remarkably increased the osteoclast number on the bone surface (alveolar socket) (Oc.N/BS) in all drug treatments. PRO treatment suppressed Oc.N/BS by 39.4% in 3-day ETM, while ISO treatment increased it by 32.1% in 3-day ETM compared with vehicle treatment. Chemical sympathectomy using 6-hydroxydopamine (6-OHDA: 250 μg/g) showed results similar to those for PRO treatment in terms of both the amount of tooth movement and osteoclast parameters. Our data showed that blockade of sympathetic signaling inhibited the tooth movement and osteoclast increase induced by ETM, and stimulation of sympathetic signaling accelerated these responses. These data suggest that the mechano-adaptive response induced by ETM is controlled by sympathetic signaling through osteoclast activation.

Comparison of β-adrenergic and glucocorticoid signaling on clock gene and osteoblast-related gene expressions in human osteoblast

Most living organisms exhibit circadian rhythms that are generated by endogenous circadian clocks, the master one being present in the suprachiasmatic nuclei (SCN). Output signals from the SCN are believed to transmit standard circadian time to peripheral tissue through sympathetic nervous system and humoral routes. Therefore, the authors examined the expression of clock genes following treatment with the β-adrenergic receptor agonist, isoprenaline, or the synthetic glucocorticoid, dexamethasone, in cultured human osteoblast SaM-1 cells. Cells were treated with 10(-6) M isoprenaline or 10(-7) M dexamethasone for 2 h and gene expressions were determined using real-time polymerase chain reaction (PCR) analysis. Treatment with isoprenaline or dexamethasone induced the circadian expression of clock genes human period 1 (hPer1), hPer2, hPer3, and human brain and muscle Arnt-like protein 1 (hBMAL1). Isoprenaline or dexamethasone treatment immediately increased hPer1 and hPer2 and caused circadian oscillation of hPer1 and hPer2 with three peaks within 48 h. hPer3 expression had one peak after isoprenaline or dexamethasone treatment. hBMAL expression had two peaks after isoprenaline or dexamethasone treatment, the temporal pattern being in antiphase to that of the other clock genes. Dexamethasone treatment delayed the oscillation of all clock genes for 2-6 h compared with isoprenaline treatment. The authors also examined the expression of osteoblast-related genes hα-1 type I collagen (hCol1a1), halkaline phosphatase (hALP), and hosteocalcin (hOC). Isoprenaline induced oscillation of hCol1a1, but not hALP and hOC. On the other hand, dexamethasone induced oscillation of hCol1a1 and hALP, but not hOC. Isoprenaline up-regulated hCol1a1 expression, but dexamethasone down-regulated hCol1a1 and hALP expression in the first phase.

Hydrocortisone inhibits cellular proliferation by downregulating hepatocyte growth factor synthesis in human osteoblasts

Glucocorticoids have multiple systemic effects that may influence bone metabolism but also directly affect osteoblasts by decreasing their proliferation. Using human osteoblastic SaM-1 cells, we examined whether the effects of hydrocortisone on cellular proliferation are mediated by hepatocyte growth factor (HGF). Human osteoblasts constitutively express both HGF and c-Met, its receptor. Hydrocortisone decreased the gene and protein expression of HGF as well as proliferation in SaM-1 cells. These hydrocortisone (0.01-1 µM)-induced decreases in HGF synthesis and cellular proliferation occurred in a concentration-dependent manner. However, no hydrocortisone (0.01-1 µM)-induced decrease in cellular proliferation was observed in human osteosarcoma-derived cells (HOS and SaOS-2), which are not able to produce HGF. In the cellular proliferation in SaM-1 cells, the decrease was blocked concentration-dependently by exogenously applied HGF (0.01-3 ng/ml). Furthermore, SU11274 (1 µM), a highly specific inhibitor of c-Met, suppressed the proliferation of SaM-1 cells, but not HOS cells. From these results, we concluded that hydrocortisone inhibits the proliferation of SaM-1 cells by interrupting the autocrine/paracrine loop via the downregulation of HGF synthesis.

Inhibitory effect of chlorpromazine on RANKL-induced osteoclastogenesis in mouse bone marrow cells

Chlorpromazine (CPZ), the first widely used phenothiazine tranquilizer, is shown to inhibit the action of intracellular calmodulin (CaM) and bone resorption in vivo and in vitro. In this study, CPZ (0.63 - 10 µM) dose-dependently inhibited the formation of tartrate-resistant acid phosphatase (TRAP) staining-positive osteoclast-like cells in mouse bone marrow cells (BMCs) treated with 1α,25(OH)(2)D(3) (10 nM) or soluble receptor activator of nuclear factor-κB ligand (s-RANKL) (20 ng/ml). Expressions of mRNA for the nuclear factor of activated T-cells c1 (NFATc1), a key regulator of osteoclast differentiation; dendritic cell-specific transmembrane protein (DC-STAMP), an essential protein for cell-cell fusion; and characteristic markers of osteoclasts such as TRAP, cathepsin K, carbonic anhydrase II, and calcitonin receptor in BMCs were up-regulated by s-RANKL and decreased by the addition of CPZ (5 µM) or the selective CaM antagonist W7, but not the inactive analog W5. The general CaM kinase (CaMK) inhibitor KN-93 and CaM-dependent phosphatase calcineurin inhibitor FK-506 also inhibited s-RANKL-induced osteoclastogenesis. Phenothiazines such as CPZ, trifluoperazine (TFPZ), and promethazine (PMZ) inhibited s-RANKL-induced osteoclast-like cell formation in mouse BMCs. Osteoclastogenesis inhibitory effects decreased in the order of TFPZ, CPZ, PMZ, depending on their anti-CaM potency. These findings suggest that CPZ inhibits RANKL-induced osteoclastogenesis by its anti-CaM action.

Continuous treatment with a low-dose β-agonist reduces bone mass by increasing bone resorption without suppressing bone formation

The sympathetic nervous system regulates bone remodeling through the β-adrenergic receptor (β-AR). However, the systemic roles of adrenergic actions on bone remodeling through the β-AR are largely unknown. In this study, we examined the dose effect of continuous treatment with isoprenaline, a nonspecific β-AR agonist, on bone remodeling. Male C57BL/6J mice were intrasubcutaneously administrated with four different doses (5, 25, 50, or 100 μg/g daily) of isoprenaline or vehicle using an osmotic pump for 2 weeks. The region of high-turnover cancellous bone was analyzed by microcomputed tomography (μCT). Continuous isoprenaline treatment caused a ~35.7% decline in the femoral cancellous bone volume fraction (BV/TV) at all doses (5-100 μg/g daily). Furthermore, continuous isoprenaline treatment weakened the bone mechanical properties in the trunk of lumbar vertebra 4 (L4). These parameters did not show significant differences between doses. Histomorphometric analysis revealed that isoprenaline doses of 50 μg/g daily or less did not significantly inhibit bone formation parameters, such as bone formation rate (BFR) and mineral surface/bone surface (MS/BS). Only the highest dose (100 μg/g daily) of isoprenaline significantly inhibited BFR and MS/BS. On the other hand, osteoclast number/bone surface (Oc.N/BS) was enhanced approximately 2.4-fold and osteoclast surface/bone surface (Oc.S/BS) was increased 2.0-fold by all doses of continuous isoprenaline treatment. The osteoclast parameters plateaued at the lowest dose (5 μg/g daily) of continuous isoprenaline treatment. These results indicate that chronic stimulation of β-AR with low-dose agonist treatment induces bone loss mainly via enhanced bone resorption.

[Control of bone remodeling by nervous system. Regulation of bone metabolism by peripheral nervous system]

Bone is richly innervated with adrenergic and peptidergic nerve terminals, and these play important roles in bone remodeling. Recent study confirmed a functional nerve-osteoblastic interplay by using an in vitro co-culture model comprising mouse osteoblastic cells and neurite-spouting mouse superior cervical ganglia. This indicates that nerve-osteoblastic cell cross-talk can occur for the dynamic regulation of local bone metabolism by peripheral nervous system. This review summarized both in vivo and in vitro evidences implicating peripheral neuron action in bone metabolism.

Effects of capsaicin-induced sensory denervation on bone metabolism in adult rats

Bone metabolism has recently been revealed to be under nerve regulation. In this study, the integrity of the sensory innervation contributing to bone metabolism was examined by capsaicin-induced sensory neuron lesions. Eight-week-old male Wistar strain rats in a modeling phase of skeletal growth were divided into four groups (8 rats per group) and treated with capsaicin at one of three different doses (37.5, 75, 150 mg/kg) or vehicle, subcutaneously. Five weeks later, high-dose (150 mg/kg) capsaicin treatment had reduced trabecular bone volume (BV/TV) due to increased trabecular separation (Tb.Sp) in the proximal tibia and the modification of mechanical properties such as strength, ductility, and toughness toward increasing bone fragility in the trunk of the sixth lumbar vertebrae (L6). Moderate-dose (75 mg/kg) capsaicin treatment had no significant effect on trabecular BV/TV or bone mechanical properties but increased Tb.Sp as seen high-dose capsaicin treatment. Bone histomorphometry showed osteoclast number (Oc.N/BS) and surface (Oc.S/BS) were increased in both the moderate-dose and high-dose capsaicin treatment groups. High-dose capsaicin significantly increased the level of tartrate-resistant acid phosphatase form 5b (TRAP 5b) in plasma, a systemic bone resorption marker, but had no influence on plasma osteocalcin concentration, a bone formation marker, suggesting that capsaicin-induced sensory nerve denervation increased bone resorption but had no influence on bone formation. Low-dose (37.5mg/kg) capsaicin had no influence on bone remodeling. These results suggest that sensory nerve innervation contributes to the maintenance of trabecular bone mass and its mechanical properties by inhibiting bone resorption.

Demonstration of direct neurite-osteoclastic cell communication in vitro via the adrenergic receptor

There is currently great interest in the bone metabolism induced by the sympathetic nerve system. Recently, direct neurite-osteoblastic cell communication was demonstrated using an in vitro co-culture model comprising neurite-sprouting murine superior cervical ganglia and MC3T3-E1 osteoblast-like cells. In the present study, we examined whether the direct nerve-osteoclastic cell communication was present in an in vitro co-culture model comprising cultured murine superior cervical ganglia and mouse osteoclast-like cells. RAW264.7 cells treated with receptor activator of NF-kappaB ligand were used as osteoclast-like cells. We found that the addition of scorpion venom (SV) elicited neurite activation via intracellular Ca(2+) mobilization and, after a lag period, osteoclastic Ca(2+) mobilization in the co-culture. SV did not have any direct effect on the osteoclastic cells in the absence of the neurites. The addition of an alpha(1)-adrenergic receptor (AR) antagonist, prazosin, concentration-dependently prevented the osteoclastic activation that resulted as a consequence of neural activation by SV. We also found that alpha(1)-adrenergic receptor agonists evoked transient Ca(2+) mobilization and gene expression of interleukin-6 in osteoclastic cells. These results demonstrate that osteoclastic activation occurs via alpha(1)-AR in osteoclastic cells as a direct response to neuronal activation.

Effect of bone morphogenetic protein-2 (BMP-2) or troglitazone, as an inducer of osteogenic cells or adipocytes, on differentiation of a bone marrow mesenchymal progenitor cell line established from temperature-sensitive (ts) simian virus (SV) 40 T-antigen gene transgenic mice

TBR31-2 is one of the bone marrow stromal cell lines. Differentiation toward osteogenic cells and calcification was observed when TBR31-2 cells were cultured for 4 weeks. Bone morphogenetic protein-2 (BMP-2) stimulated alkaline phosphatase (ALP) activity in a dose- and time-dependent manner. On the other hand, troglitazone increased oil droplet accumulation in a dose-dependent manner. In the presence of BMP-2, an increase of expression in osteogenic cell differentiation marker genes and a decrease of expression in adipocyte differentiation marker genes were observed with the exception of the induced expression of peroxisome proliferator-activated receptor gamma (PPARgamma), however, troglitazone, a ligand of PPARgamma treatment exhibited the opposite tendency. Interestingly, treatment with both BMP-2 and troglitazone resulted in a decrease of ALP activity and an increase of oil droplet accumulation. Reverse tanscription-polymerase chain reaction (RT-PCR) analysis also indicated that osteogenic differentiation markers decreased and that adipocyte differentiation markers increased. Thus, when the cells were cultured with BMP-2, osteogenic differentiation was enhanced while the expression of PPARgamma was maintained, and the addition of troglitazone caused a significant number of differentiated cells into adipocytes. These findings indicate that BMP-2 enhanced osteogenic differentiation and the expression of adipogenic transcription factor (PPARgamma) followed by osteogenic differentiation without activation of PPARgamma by its ligand.

Electrophysiological properties of a novel Ca(2+)-activated K(+) channel expressed in human osteoblasts

Intracellular Ca(2+) mobilization plays important roles in cell survival, proliferation, and differentiation of osteoblasts. In this study, we identified a novel type of Ca(2+)-activated K(+) channel in human osteoblasts and investigated its physiological roles. Using RT-PCR methods and single-channel analysis in the patch-clamp technique, we found that BK and IK channels were genetically expressed in human osteoblasts and had electrophysiological properties similar to those reported previously for the channels in other organs (conductance, voltage dependence, and sensitivity to intracellular Ca(2+)). Taking advantage of the fact that ATP induces elevation of the intracellular Ca(2+) concentration in human osteoblasts, we successfully demonstrated that ATP-induced hyperpolarization was effectively inhibited by the IK channel blockers charybdotoxin and clotrimazole and by a P2 purinergic receptor antagonist, suramin, but not by the BK channel blockers tetraethylammonium chloride and iberiotoxin under the current-clamp mode of whole-cell clamp. The present study is the first to demonstrate the electrophysiological properties and functional expression of IK channels in human osteoblasts, findings which suggest that IK channels are regulators of membrane potential that give rise to intracellular Ca(2+) mobilization by physiological stimulation.

Pharmacological topics of bone metabolism: the physiological function of the sympathetic nervous system in modulating bone resorption

The vertebrate skeleton is richly innervated with adrenergic and peptidergic nerve terminals, and these play important roles in bone remodeling. Recent studies have generally shown that increased sympathetic nervous activity causes bone loss via an increase in bone resorption and a decrease in bone formation. Increased bone resorption is based on the stimulation of both osteoclast formation and osteoclast activity. These effects are associated with beta(2)-adrenergic activity toward both osteoblastic and osteoclastic cells. Such findings indicate that beta-blockers may be effective against osteoporosis, in which case there is increased sympathetic activity. This review summarizes evidence obtained both in vitro and in vivo implicating sympathetic neuron action in bone resorption.

Lysophosphatidic acid-stimulated interleukin-6 and -8 synthesis through LPA1 receptors on human osteoblasts

Using human osteoblastic SaM-1 cells, we investigated the effects of lysophosphatidic acid (LPA) on the production of interleukin (IL)-6 and IL-8, molecules which are capable of stimulating the development of osteoclasts from their haematopoietic precursors, and examined the signal transduction systems involved in their effect on these cells. These human osteoblasts constitutively expressed endothelial differentiation genes (Edg)-2 and Edg-4, which are LPA receptors. LPA increased gene and protein expression of IL-6 and IL-8 in SaM-1 cells. The expression of IL-6 and IL-8 mRNAs was maximal at 1-3h, and the increase in IL-6 and IL-8 synthesis in response to lysophosphatidic acid (1-10 microM) occurred in a concentration-dependent manner. These increases were blocked by Ki16425, an Edg-2/7 antagonist. In addition, LPA caused an increase in the intracellular Ca(2+) concentration ([Ca(2+)](i)), which was inhibited by pretreatment with Ki16425 or 2-aminoethoxy-diphenylborate (2-APB), an inositol 1,4,5-triphosphate (IP(3)) receptor (IP(3)R) blocker. The pretreatment of SaM-1 cells with U-73122, a phospholipase C (PLC) inhibitor, and 2-APB also inhibited the increase in IL-6 and IL-8 synthesis in response to LPA. These findings suggest that extracellular LPA-induced IL-6 and IL-8 synthesis occurred through Edg-2 (LPA(1) receptor) and the activation of PLC and IP(3)-mediated intracellular calcium release in SaM-1 cells.

Inhibitory effect of NPY on isoprenaline-induced osteoclastogenesis in mouse bone marrow cells

The effect of neuropeptide Y (NPY), a co-transmitter with noradrenaline in peripheral sympathetic nerve fibers, on the osteoclastogenesis in mouse bone marrow cell cultures treated with isoprenaline, a beta-adrenergic receptor (beta-AR) agonist, was examined. The mouse bone marrow cells constitutively expressed mRNAs for the NPY-Y1 receptor and beta2-AR. NPY inhibited the formation of osteoclast-like cells induced by isoprenaline but not that by 1alpha,25-dihydroxyvitamin D3 (1alpha,25(OH)2D3) or soluble receptor activator of nuclear factor-kappaB ligand (RANKL); and it suppressed the production of RANKL and cyclic AMP (cAMP) increased by isoprenaline but not those increased by 1alpha,25(OH)2D3. NPY also inhibited osteoclastogenesis induced by forskolin, an activator of adenylate cyclase; however, it did not inhibit that induced by exogenously supplied dibutyryl cAMP, a cell-permeable cAMP analog that activates cAMP-dependent protein kinase. These results demonstrate that NPY inhibited the isoprenaline-induced osteoclastogenesis by blocking the agonist-elicited increases in the production of cAMP and RANKL in mouse bone marrow cells, suggesting an interaction between NPY and beta-AR agonist in bone resorption.

The role of the sympathetic nervous system in controlling bone metabolism

Experimental studies have generally shown that increased sympathetic nervous activity causes bone loss via an increase in bone resorption and a decrease in bone formation. Increased bone resorption is based on the stimulation of both osteoclast formation and osteoclast activity. These effects are associated with beta2-adrenergic activity towards both osteoblastic and osteoclastic cells. Decreased bone formation is based on the inhibition of osteoblastic activity through beta2-adrenergic receptors on osteoblasts. Such findings indicate that beta-blockers may be effective against osteoporosis, in which case there is increased sympathetic activity. In fact, in a population-based, case-control study, the current use of beta-blockers has been demonstrated to be associated with a reduced risk of fractures. These clinical studies suggest that pharmacological blockade of the beta-adrenergic system is beneficial to the human skeleton. In another prospective study, however, no association between beta-blocker use and fracture risk was shown in perimenopausal and older women. To confirm this important new therapeutic avenue to prevent bone loss, the relationship between the pharmacological effectiveness of beta-blockers and the pathogenesis of osteoporosis must be explored in detail.