Isolated primary osteocytes express functional gap junctions in vitro

Osteocytes: central conductors of bone biology in normal and pathological conditions

Osteocytes are the most abundant and longest-living cells in the adult skeleton. For a long time, osteocytes were considered static and inactive cells, but in recent years, it has been suggested that they represent the key responder to various stimuli that regulate bone formation and remodelling as well as one of the key endocrine regulators of bone metabolism. Osteocytes respond to mechanical stimuli by producing and secreting several signalling molecules, such as nitric oxide and prostaglandin E(2) , that initiate local bone remodelling. Moreover, they can control bone formation by modulating the WNT signalling pathway, an essential regulator of cell fate and commitment, as they represent the main source of sclerostin, a negative regulator of bone formation. Osteocytes can also act as an endocrine organ by releasing fibroblast growth factor 23 and several other proteins (DMP-1, MEPE, PHEX) that regulate phosphate metabolism. It has been demonstrated that various bone diseases are associated with osteocyte abnormalities, although it is not clear if these changes are the direct cause of the pathology or if they are secondary to the pathological changes in the bone microenvironment. Thus, a better understanding of these cells could offer exciting opportunities for new advances in the prevention and management of different bone diseases.

Mef2c deletion in osteocytes results in increased bone mass

Myocyte enhancer factors 2 (MEF2) are required for expression of the osteocyte bone formation inhibitor Sost in vitro, implying these transcription factors in bone biology. Here, we analyzed the in vivo function of Mef2c in osteocytes in male and female mice during skeletal growth and aging. Dmp1-Cre-induced Mef2c deficiency led to progressive decreases in Sost expression by 40% and 70% in femoral cortical bone at 3.5 months and 5 to 6 months of age. From 2 to 3 months onward, bone mass was increased in the appendicular and axial skeleton of Mef2c mutant relative to control mice. Cortical thickness and long bone and vertebral trabecular density were elevated. To assess whether the increased bone mass was related to the decreased Sost expression, we characterized 4-month-old heterozygous Sost-deficient mice. Sost heterozygotes displayed similar increases in long bone mass and density as Mef2c mutants, but the relative increases in axial skeletal parameters were mostly smaller. At the cellular level, bone formation parameters were normal in 3.5-month-old Mef2c mutant mice, whereas bone resorption parameters were significantly decreased. Correspondingly, cortical expression of the anti-osteoclastogenic factor and Wnt/β-catenin target gene osteoprotegerin (OPG) was increased by 70% in Mef2c mutant males. Furthermore, cortical expression of the Wnt signaling modulators Sfrp2 and Sfrp3 was strongly deregulated in both sexes. In contrast, heterozygous Sost deficient males displayed mildly increased osteoblastic mineral apposition rate, but osteoclast surface and cortical expression of osteoclastogenic regulators including OPG were normal and Sfrp2 and Sfrp3 were not significantly changed. Together, our data demonstrate that Mef2c regulates cortical Sfrp2 and Sfrp3 expression and is required to maintain normal Sost expression in vivo. Yet, the increased bone mass phenotype of Mef2c mutants is not directly related to the reduced Sost expression. We identified a novel function for Mef2c in control of adult bone mass by regulation of osteoclastic bone resorption.

Isolation of mouse osteocytes using cell fractionation for gene expression analysis

Osteocytes are the terminally differentiated cells of the osteoblastic lineage embedded within the mineralized bone matrix. T: hey have been identified as key players in mechanotransduction and in mineral and phosphate homeostasis. In addition, they appear to have a role in mediating bone formation, since they secrete the bone formation inhibitor sclerostin. In contrast to osteoblasts and osteoclasts, which reside on the bone surface, it has been difficult to isolate and analyze cellular and molecular properties of osteocytes due to their specific location inside the "hard" mineralized bone compartment. This chapter describes a method to isolate osteocytes from newborn mouse calvaria and adult mouse long bone, followed by immediate total RNA extraction allowing to selectively study osteocytic versus osteoblastic gene expression by quantitative real-time polymerase chain reaction (qPCR). The osteocyte-enriched cell fraction isolated by this method can further be purified by FACS and selectively expresses osteocytic marker genes, such as Dmp1 and Sost.

Upregulated expression of connexin43 in spinal ligament fibroblasts derived from patients presenting ossification of the posterior longitudinal ligament

STUDY DESIGN

A case-control study was conducted.

OBJECTIVE

To investigate different expressions of connexin43 (Cx43) between spinal ligament fibroblasts from patients with ossification of the posterior longitudinal ligament (OPLL) and non-OPLL patients and demonstrate knockdown of Cx43 protein expression by RNA interference inhibiting expression of osteoblast-specific genes in OPLL cells.

SUMMARY OF BACKGROUND DATA

The OPLL is characterized by ectopic bone formation in spinal ligaments. Some evidence indicates that ligament fibroblasts from OPLL patients have osteogenic characteristics. However, the relevant cellular signaling pathways remain unclear.

METHODS

Twenty patients presenting with OPLL and 18 non-OPLL patients underwent anterior decompression between January 2008 and June 2009. Specimens of the posterior longitudinal ligament were collected intraoperatively. Tissue fragment cell culture was performed. Inverted phase contrast microscopy and hematoxylin-eosin staining were used to observe cell morphology. The mouse antivimentin antibody was used to identify the cultured cells via immunocytochemistry and immunofluorescence. The messenger RNA expression of osteoblast-specific genes of osteocalcin (OCN), alkaline phosphatase (ALP), and type I collagen (COL I) were detected in OPLL and non-OPLL cells by semiquantitative reverse transcription-polymerase chain reaction. The protein expression of Cx43 was detected via Western blotting. And then, after 72 hours, when RNA interference against Cx43 was performed in OPLL cells, expression of the indexes mentioned earlier was compared again between the transfection group and the nontransfection group.

RESULTS

Cultivated cells were observed 7 to 10 days after cell culture. Hematoxylin-eosin staining showed fusiform and multiangular star morphologies, large and elliptical cell nuclei, and ill-defined cell appearances. Immunocytochemistry and immunofluorescence exhibited positive results of vimentin staining. The messenger RNA expressions of OCN, ALP, and COL I and protein expressions of Cx43 from OPLL fibroblasts were greater than those from non-OPLL cells, and the difference was significant. Furthermore, knockdown of Cx43 protein expression inhibited the messenger RNA expressions of OCN, ALP, and COL I remarkably in the transfection group compared with the nontransfection group, 72 hours after RNA interference targeting Cx43 was performed in OPLL cells.

CONCLUSION

Tissue fragment culture of the cervical posterior longitudinal ligament provided a successful fibroblast culture, showing good adherence and subculture. The cultured fibroblasts from OPLL patients exhibited osteogenic characteristics, in which Cx43 played an important role.

Evidence for osteocyte regulation of bone homeostasis through RANKL expression

Osteocytes embedded in bone have been postulated to orchestrate bone homeostasis by regulating both bone-forming osteoblasts and bone-resorbing osteoclasts. We find here that purified osteocytes express a much higher amount of receptor activator of nuclear factor-κB ligand (RANKL) and have a greater capacity to support osteoclastogenesis in vitro than osteoblasts and bone marrow stromal cells. Furthermore, the severe osteopetrotic phenotype that we observe in mice lacking RANKL specifically in osteocytes indicates that osteocytes are the major source of RANKL in bone remodeling in vivo.

Mechanical strain induces Cx43 expression in spinal ligament fibroblasts derived from patients presenting ossification of the posterior longitudinal ligament

Ossification of the posterior longitudinal ligament (OPLL) is characterized by ectopic bone formation in spinal ligaments. Some evidence indicates that mechanical strain can lead to the development of OPLL, although the signaling mechanism is not fully understood. Connexin43 (Cx43), a gap-junction protein, has been shown to be of particular importance in bone formation. We hypothesized that Cx43 may play an important role in the signal transmission induced by mechanical strain during the development of OPLL. To explore this possibility, we cultured fibroblasts from spinal ligaments of OPLL and non-OPLL patients and preloaded mechanical stretch onto the cells via a Flexercell 4000 Tension Plus system. We evaluated expression changes in osteocalcin (OCN), alkaline phosphatase (ALP), type I collagen (COL I) and Cx43 via semi-quantitative RT-PCR and western blotting at 12 and 24 h after mechanical strain application in contrast to static conditions. We observed a significant gene up-regulation of OCN, ALP and COL I and Cx43 protein in OPLL cells after mechanical strain application, but no changes in non-OPLL cells. Notably, after RNA interference targeting Cx43 was performed in OPLL cells, we found that there were no significant changes in the expressions of OCN, ALP, COL I and Cx43 after the mechanical strain was applied for 24 h. Thus, we propose that the increase in Cx43 expression induced by mechanical strain in OPLL cells plays an important role in the progression of OPLL.

Osteocyte Wnt/beta-catenin signaling is required for normal bone homeostasis

Beta-Catenin-dependent canonical Wnt signaling plays an important role in bone metabolism by controlling differentiation of bone-forming osteoblasts and bone-resorbing osteoclasts. To investigate its function in osteocytes, the cell type constituting the majority of bone cells, we generated osteocyte-specific beta-catenin-deficient mice (Ctnnb1(loxP/loxP); Dmp1-Cre). Homozygous mutants were born at normal Mendelian frequency with no obvious morphological abnormalities or detectable differences in size or body weight, but bone mass accrual was strongly impaired due to early-onset, progressive bone loss in the appendicular and axial skeleton with mild growth retardation and premature lethality. Cancellous bone mass was almost completely absent, and cortical bone thickness was dramatically reduced. The low-bone-mass phenotype was associated with increased osteoclast number and activity, whereas osteoblast function and osteocyte density were normal. Cortical bone Wnt/beta-catenin target gene expression was reduced, and of the known regulators of osteoclast differentiation, osteoprotegerin (OPG) expression was significantly downregulated in osteocyte bone fractions of mutant mice. Moreover, the OPG levels expressed by osteocytes were higher than or comparable to the levels expressed by osteoblasts during skeletal growth and at maturity, suggesting that the reduction in osteocytic OPG and the concomitant increase in osteocytic RANKL/OPG ratio contribute to the increased number of osteoclasts and resorption in osteocyte-specific beta-catenin mutants. Together, these results reveal a crucial novel function for osteocyte beta-catenin signaling in controlling bone homeostasis.

Vitamin K promotes mineralization, osteoblast-to-osteocyte transition, and an anticatabolic phenotype by {gamma}-carboxylation-dependent and -independent mechanisms

The vitamin K family members phylloquinone (vitamin K1) and the menaquinones (vitamin K2) are under study for their roles in bone metabolism and as potential therapeutic agents for skeletal diseases. We have investigated the effects of two naturally occurring homologs, phytonadione (vitamin K1) and menatetrenone (vitamin K2), and those of the synthetic vitamin K, menadione (vitamin K3), on human primary osteoblasts. All homologs promoted in vitro mineralization by these cells. Vitamin K1-induced mineralization was highly sensitive to warfarin, whereas that induced by vitamins K2 and K3 was less sensitive, implying that gamma-carboxylation and other mechanisms, possibly genomic actions through activation of the steroid xenobiotic receptor, are involved in the effect. The positive effect on mineralization was associated with decreased matrix synthesis, evidenced by a decrease from control in expression of type I collagen mRNA, implying a maturational effect. Incubation in the presence of vitamin K2 or K3 in a three-dimensional type I collagen gel culture system resulted in increased numbers of cells with elongated cytoplasmic processes resembling osteocytes. This effect was not warfarin sensitive. Addition of calcein to vitamin K-treated cells revealed vitamin K-dependent deposition of mineral associated with cell processes. These effects are consistent with vitamin K promoting the osteoblast-to-osteocyte transition in humans. To test whether vitamin K may also act on mature osteocytes, we tested the effects of vitamin K on MLO-Y4 cells. Vitamin K reduced receptor activator of NF-kappaB ligand expression relative to osteoprotegerin by MLO-Y4 cells, an effect also seen in human cultures. Together, our findings suggest that vitamin K promotes the osteoblast-to-osteocyte transition, at the same time decreasing the osteoclastogenic potential of these cells. These may be mechanisms by which vitamin K optimizes bone formation and integrity in vivo and may help explain the net positive effect of vitamin K on bone formation.

Strontium ranelate treatment of human primary osteoblasts promotes an osteocyte-like phenotype while eliciting an osteoprotegerin response

SUMMARY

The effect of strontium ranelate (SR) on human osteoblast differentiation was tested. SR induced osteoblastic proliferation, in vitro mineralization, and increased the expression of osteocyte markers. SR also elicited an osteoprotegerin (OPG) secretory response. We conclude that SR promotes the osteoblast maturation and osteocyte differentiation while promoting an additional antiresorptive effect.

INTRODUCTION

SR is a new treatment for osteoporosis that reduces the risk of hip and vertebral fractures in postmenopausal women. This study sought to investigate the extent, to which SR modulates human osteoblast differentiation.

METHODS

Adult human primary osteoblasts (NHBC) were exposed to SR under mineralizing conditions in long-term cultures. Osteoblast differentiation status was investigated by cell-surface phenotypic analysis. Expression of genes associated with osteoblast/osteocyte differentiation was examined using real-time RT-PCR. Secreted OPG was assayed by enzyme-linked immunosorbent assay.

RESULTS

SR significantly increased osteoblast replication. SR time- and dose-dependently induced an osteocyte-like phenotype, as determined by cell surface alkaline phosphatase and STRO-1 expression. SR at 5 mM or greater dramatically increased in vitro mineralization. In parallel, mRNA levels of dentin matrix protein (DMP)-1 and sclerostin were higher under SR treatment, strongly suggestive of the presence of osteocytes. SR also increased the OPG/RANKL ratio throughout the culture period, consistent with an effect to inhibit osteoblast-induced osteoclastogenesis.

CONCLUSIONS

This study suggests that SR can promote osteoblast maturation and an osteocyte-like phenotype. Coupled with its effect on the OPG/RANKL system, these findings are consistent with in vivo effects in patients receiving SR for the treatment of osteoporosis.

Do osteocytes contribute to bone mineral homeostasis? Osteocytic osteolysis revisited

Osteocytes are cells buried in the bone matrix. They largely contribute to the regulation of bone remodeling in response to mechanical and microenvironmental changes. Much has been recognized in recent years regarding the role of osteocytes in bone homeostasis, nevertheless their ability to directly contribute to mineral equilibrium has been neglected. In the light of the renewed interest in their biology, we revisited the literature and discuss experimental evidence favoring the hypothesis that osteocytes are able to remove and replace the bone matrix according to the systemic needs of the body. We also reviewed reports against this theory, thus providing current views of what is known so far on the ability of osteocytes to mobilize bone mineral. This re-examination of osteocytic osteolysis might stimulate new interest and open new perspectives in osteocyte biology and in the cellular mechanisms that control bone homeostasis.

IGF-I regulates tight-junction protein claudin-1 during differentiation of osteoblast-like MC3T3-E1 cells via a MAP-kinase pathway

Insulin-like growth factor I (IGF-I) is expressed in many tissues, including bone, and acts on the proliferation and differentiation of osteoblasts as an autocrine/paracrine regulator. Tight-junction proteins have been detected in osteoblasts, and direct cell-to-cell interactions may modulate osteoblast function with respect, for example, to gap junctions. In order to investigate the regulation of expression of tight-junction molecules and of function during bone differentiation, osteoblast-like MC3T3-E1 cells and osteocyte-like MLO-Y4 cells were treated with IGF-I. In both MC3T3-E1 cells and MLO-Y4 cells, the tight-junction molecules occludin, claudin-1, -2, and -6, and the gap-junction molecule connexin 43 (Cx43) were detected by reverse transcription with polymerase chain reaction. In MC3T3-E1 cells but not MLO-Y4 cells, mRNAs of claudin-1, -2, and -6, Cx43, and type I collagen, and proteins of claudin-1 and Cx43 were increased after treatment with IGF-I. Such treatment significantly decreased paracellular permeability in MC3T3-E1 cells. The expression of claudin-1 in MC3T3-E1 cells after IGF-I treatment was mainly upregulated via a mitogen-activated protein (MAP)-kinase pathway and, in part, modulated by a PI3-kinase pathway, whereas Cx43 expression and the mediated gap-junctional intercellular communication protein did not contribute to the upregulation. Furthermore, in MC3T3-E1 cells during wound healing, upregulation of claudin-1 was observed together with an increase of IGF-I and type I collagen. These findings suggest that the induction of tight-junction protein claudin-1 and paracellular permeability during the differentiation of osteoblast-like MC3T3-E1 cells after treatment with IGF-I is regulated via a MAP-kinase pathway, but not with respect to gap junctions.

Osteocytes, mechanosensing and Wnt signaling

The majority of bone cell biology focuses on activity on the surface of the bone with little attention paid to the activity that occurs below the surface. However, with recent new discoveries, osteocytes, cells embedded within the mineralized matrix of bone, are becoming the target of intensive investigation. In this article, the distinctions between osteoblasts and their descendants, osteocytes, are reviewed. Osteoblasts are defined as cells that make bone matrix and osteocytes are thought to translate mechanical loading into biochemical signals that affect bone (re)modeling. Osteoblasts and osteocytes should have similarities as would be expected of cells of the same lineage, yet these cells also have distinct differences, particularly in their responses to mechanical loading and utilization of the various biochemical pathways to accomplish their respective functions. For example, the Wnt/beta-catenin signaling pathway is now recognized as an important regulator of bone mass and bone cell functions. This pathway is important in osteoblasts for differentiation, proliferation and the synthesis bone matrix, whereas osteocytes appear to use the Wnt/beta-catenin pathway to transmit signals of mechanical loading to cells on the bone surface. New emerging evidence suggests that the Wnt/beta-catenin pathway in osteocytes may be triggered by crosstalk with the prostaglandin pathway in response to loading which then leads to a decrease in expression of negative regulators of the pathway such as Sost and Dkk1. The study of osteocyte biology is becoming an intense area of research interest and this review will examine some of the recent findings that are reshaping our understanding of bone/bone cell biology.

Mechanically stimulated osteocytes regulate osteoblastic activity via gap junctions

The strong correlation between a bone's architectural properties and the mechanical forces that it experiences has long been attributed to the existence of a cell that not only detects mechanical load but also structurally adapts the bone matrix to counter it. One of the most likely cellular candidates for such a "mechanostat" is the osteocyte, which resides within the mineralized bone matrix and is perfectly situated to detect mechanically induced signals. However, as osteocytes can neither form nor resorb bone, it has been hypothesized that they orchestrate mechanically induced bone remodeling by coordinating the actions of cells residing on the bone surface, such as osteoblasts. To investigate this hypothesis, we developed a novel osteocyte-osteoblast coculture model that mimics in vivo systems by permitting us to expose osteocytes to physiological levels of fluid shear while shielding osteoblasts from it. Our results show that osteocytes exposed to a fluid shear rate of 4.4 dyn/cm(2) rapidly increase the alkaline phosphatase activity of the shielded osteoblasts and that osteocytic-osteoblastic physical contact is a prerequisite. Furthermore, both functional gap junctional intercellular communication and the mitogen-activated protein kinase, extracellular signal-regulated kinase 1/2 signaling pathway are essential components in the osteoblastic response to osteocyte communicated mechanical signals. By utilizing other nonosteocytic coculture models, we also show that the ability to mediate osteoblastic alkaline phosphatase levels in response to the application of fluid shear is a phenomena unique to osteocytes and is not reproduced by other mesenchymal cell types.