Isolation and characterization of gap junctions in the osteoblastic MC3T3-E1 cell line

The Effect of Nano-Silica Gel on Biological Activity of Osteoblasts and Expression of Insulin-Like and Growth Factor-2

This study assessed the effect nano-silica gel material on bioactivity of osteoblasts and expression of IGF-2. Methods: Silica gel nanoparticles (Nanjing Kike Company) were divided according to their concentrations as follows; 0 μg/mL a control group with cells without nanoparticle treatment, 25 μg/mL as group 1, 50 μg/mL as group 2, and 100 μg/mL as group 3. The transmission electron microscope was used to measure morphology, while particle size analyzer was used to measure particle size, and potential analyzer measured Zeta potential, and MTT measured proliferation.Moreover, ALP kit was used to measure ALP activity, and Alizarin red staining measured formation of wild flower nodules, while RT-PCR was used to measure expression of IGF-2. Results: The shape of silica gel nanoparticles was spherical, with uniform particle size distribution, and particle size was between 50-800 nm. The average particle size was 383 nm, and Zeta potential was -12.3. The growth rate of control group and group 1 was relatively close (t = 0.95, P = 0.37), and growth rate of groups 2 and 3 was higher than control (group t2 = 5.63, P < 0.05, group t3 = 10.38, P < 0.05). The value-added rate for group 3 was higher than group 2 (t = 4.41, P < 0.05). Group 1 had higher activity than control group (t = 10.29, P < 0.05) and lower activity than group 3 (t = 9.85, P < 0.05) which had higher activity than group 2 (t = 4.16, P < 0.05). Groups 1, 2, and 3 induced the growth of osteoblasts, promoted calcium salt deposition, and produced red mineralized nodules where the cells converged. The formation of mineralized nodules obviously depended on concentration of silica nanoparticles. Group 1 had higher IGF-2 expression than control (t = 19.99, P < 0.05) and lower level than group 2 (t = 16.69, P < 0.05). Silica gel nanoparticles promoted MC3T3-E1 cell proliferation and differentiation. The mechanism of action may be that, silica gel nanoparticles accelerate the growth of ALP activity and osteoblast extracellular matrix mineralization by promoting the level of IGF-2. The production of chemical nodules accelerates the proliferation and differentiation of osteoblasts.

Strain uses gap junctions to reverse stimulation of osteoblast proliferation by osteocytes

Identifying mechanisms by which cells of the osteoblastic lineage communicate in vivo is complicated by the mineralised matrix that encases osteocytes, and thus, vital mechanoadaptive processes used to achieve load-bearing integrity remain unresolved. We have used the coculture of immunomagnetically purified osteocytes and primary osteoblasts from both embryonic chick long bone and calvariae to examine these mechanisms. We exploited the fact that purified osteocytes are postmitotic to examine both their effect on proliferation of primary osteoblasts and the role of gap junctions in such communication. We found that chick long bone osteocytes significantly increased basal proliferation of primary osteoblasts derived from an identical source (tibiotarsi). Using a gap junction inhibitor, 18β-glycyrrhetinic acid, we also demonstrated that this osteocyte-related increase in osteoblast proliferation was not reliant on functional gap junctions. In contrast, osteocytes purified from calvarial bone failed to modify basal proliferation of primary osteoblast, but long bone osteocytes preserved their proproliferative action upon calvarial-derived primary osteoblasts. We also showed that coincubated purified osteocytes exerted a marked inhibitory action on mechanical strain-related increases in proliferation of primary osteoblasts and that this action was abrogated in the presence of a gap junction inhibitor. These data reveal regulatory differences between purified osteocytes derived from functionally distinct bones and provide evidence for 2 mechanisms by which purified osteocytes communicate with primary osteoblasts to coordinate their activity.

Rheological properties, biocompatibility and in vivo performance of new hydrogel-based bone fillers

Three different heterologous substitutes for bone regeneration, manufactured with equine-derived cortical powder, cancellous chips and demineralized bone matrix granules, were compared in vitro and in vivo.

Osteocyte differentiation is regulated by extracellular matrix stiffness and intercellular separation

Osteocytes are terminally differentiated bone cells, derived from osteoblasts, which are vital for the regulation of bone formation and resorption. ECM stiffness and cell seeding density have been shown to regulate osteoblast differentiation, but the precise cues that initiate osteoblast-osteocyte differentiation are not yet understood. In this study, we cultured MC3T3-E1 cells on (A) substrates of different chemical compositions and stiffnesses, as well as, (B) substrates of identical chemical composition but different stiffnesses. The effect of cell separation was investigated by seeding cells at different densities on each substrate. Cells were evaluated for morphology, alkaline phosphatase (ALP), matrix mineralisation, osteoblast specific genes (Type 1 collagen, Osteoblast specific factor (OSF-2)), and osteocyte specific proteins (dentin matrix protein 1 (DMP-1), sclerostin (Sost)). We found that osteocyte differentiation (confirmed by dendritic morphology, mineralisation, reduced ALP, Col type 1 and OSF-2 and increased DMP-1 and Sost expression) was significantly increased on soft collagen based substrates, at low seeding densities compared to cells on stiffer substrates or those plated at high seeding density. We propose that the physical nature of the ECM and the necessity for cells to establish a communication network contribute substantially to a concerted shift toward an osteocyte-like phenotype by osteoblasts in vitro.

Gap junctions and hemichannels in signal transmission, function and development of bone

Gap junctional intercellular communication (GJIC) mediated by connexins, in particular connexin 43 (Cx43), plays important roles in regulating signal transmission among different bone cells and thereby regulates development, differentiation, modeling and remodeling of the bone. GJIC regulates osteoblast formation, differentiation, survival and apoptosis. Osteoclast formation and resorptive ability are also reported to be modulated by GJIC. Furthermore, osteocytes utilize GJIC to coordinate bone remodeling in response to anabolic factors and mechanical loading. Apart from gap junctions, connexins also form hemichannels, which are localized on the cell surface and function independently of the gap junction channels. Both these channels mediate the transfer of molecules smaller than 1.2kDa including small ions, metabolites, ATP, prostaglandin and IP(3). The biological importance of the communication mediated by connexin-forming channels in bone development is revealed by the low bone mass and osteoblast dysfunction in the Cx43-null mice and the skeletal malformations observed in occulodentodigital dysplasia (ODDD) caused by mutations in the Cx43 gene. The current review summarizes the role of gap junctions and hemichannels in regulating signaling, function and development of bone cells. This article is part of a Special Issue entitled: The Communicating junctions, composition, structure and characteristics.

Role of connexin43 hemichannels in mechanical stress-induced ATP release in human periodontal ligament cells

BACKGROUND AND OBJECTIVE

Our previous studies showed that mechanical stress could induce ATP release in human periodontal ligament (HPDL) cells. By signaling through P2 purinergic receptors, ATP increased the expression and the synthesis of osteopontin and RANKL. In this study, the mechanism of stress-induced ATP release was investigated.

MATERIAL AND METHODS

Continuous compressive forces were applied on cultured HPDL cells. The ATP released was measured using luciferin-luciferase bioluminescence. The expression of gap-junction proteins was examined using RT-PCR and western blot analysis. The opening of hemichannels was demonstrated by cellular uptake of a fluorescent dye, 5(6)-carboxyfluorescein, which is known to penetrate hemichannels. Intracellular signal transduction was investigated using inhibitors and antagonists.

RESULTS

Mechanical stress induced the release of ATP into the culture medium, which was attenuated by carbenoxolone, a nonspecific gap-junction inhibitor. Addition of meclofenamic acid sodium salt, a connexin43 inhibitor, inhibited ATP release by mechanical stress. Knockdown of connexin43 expression by small interfering RNA reduced the amount of ATP released by mechanical stress, suggesting the role of connexin43 hemichannels. In addition, intracellular Ca(2+) blockers could also inhibit mechanical stress-induced ATP release and the opening of the gap junction.

CONCLUSION

Our study demonstrated the involvement of gap-junction hemichannels, especially connexin43, in the stress-induced ATP-release mechanism. Furthermore, this mechanism may be regulated by the intracellular Ca(2+) signaling pathway. These results suggest an important role of gap-junction hemichannels in the function and behavior of HPDL cells.

Attenuated response to in vivo mechanical loading in mice with conditional osteoblast ablation of the connexin43 gene (Gja1)

INTRODUCTION

In vitro data suggest that gap junctional intercellular communication mediated by connexin43 (Cx43) plays an important role in bone cell response to mechanical stimulation. We tested this hypothesis in vivo in a model of genetic deficiency of the Cx43 gene (Gja1).

MATERIALS AND METHODS

Four-month-old female mice with a conditional Gja1 ablation in osteoblasts (ColCre;Gja1(-/flox)), as well as wildtype (Gja1(+/flox)) and heterozygous equivalent (Gja1(-/flox)) littermates (eight per genotype), were subjected to a three-point bending protocol for 5 d/wk for 2 wk. Microstructural parameters and dynamic indices of bone formation were estimated on sections of loaded and control contralateral tibias.

RESULTS

ColCre;Gja1(-/flox) mice had significantly thinner cortices, but larger marrow area and total cross-sectional area in the tibial diaphysis, compared with the other groups. The ColCre;Gja1(-/flox) mice needed approximately 40% more force to generate the required endocortical strain. In Gja1(+/flox) mice, the loading regimen produced abundant double calcein labels at the endocortical surface, whereas predominantly single labels were seen in ColCre;Gja1(-/flox) mice. Accordingly, mineral apposition rate and bone formation rate were significantly lower (54.8% and 50.2%, respectively) in ColCre;Gja1(-/flox) relative to Gja1(+/flox) mice. Intermediate values were found in Gja1(-/flox) mice.

CONCLUSIONS

Gja deficiency results in thinner but larger tibial diaphyses, resembling changes occurring with aging, and it attenuates the anabolic response to in vivo mechanical loading. Thus, Cx43 plays an instrumental role in this adaptive response to physical stimuli.

Functional characterization of oculodentodigital dysplasia-associated Cx43 mutants

Oculodentodigital dysplasia (ODDD) is associated with at least 28 connexin43 (Cx43) mutations. We characterized four of these mutants; Q49K, L90V, R202H, and V216L. Populations of these GFP-tagged mutants were transported to the cell surface in Cx43-negative HeLa cells and Cx43-positive NRK cells. Dual patch-clamp functional analysis in N2A cells demonstrated that channels formed by each mutant have dramatically reduced conductance. Dye-coupling analysis revealed that each mutant exhibits a dominant-negative effect on wild-type Cx43. Since ODDD patients display skeletal abnormalities, we examined the effect of three other Cx43 mutants previously shown to exert dominant-negative effects on wild-type Cx43 (G21R, G138R, and G60S) in neonatal calvarial osteoblasts. Differentiation was unaltered by expression of these mutants as alkaline phosphatase activity and extent of culture mineralization were unchanged. This suggests that loss-of-function Cx43 mutants are insufficient to deter committed osteoblasts from their normal function in vitro. Thus, we hypothesize that the bone phenotype of ODDD patients may result from disrupted gap junctional intercellular communication earlier in development or during bone remodeling.

The function of connexin 43 on the differentiation of rat bone marrow cells in culture

Connexin (Cx) 43-mediated gap-junctional intercellular communication (GJC) mainly regulates the osteoblastic differentiation, but much of the function of Cx43 on the differentiation of bone marrow cells is unclear. This study is aimed to clarify relationship between the differentiation of rat bone marrow cells and the function of Cx43. Bone marrow cells derived from four-week-old Wistar strain rats were grown in the presence and absence of 18-alpha-glycyrrhetinic acid (AGA, 100 muM) to inhibit Cx43-mediated GJC. Expression of Cx43 gene and protein, and the level of intracellular cyclic adenosine monophosphate (cAMP) were determined as the assessment of the function in Cx43-mediated GJC, and alkaline phosphatase (ALP) activity and mineralization were measured as the assessment of osteoblastic differentiation. The Cx43 gene expression was first observed at 2 days, but under the condition in which rat bone marrow cells were treated with AGA, there was no significant effect on the Cx43 gene expression. By administrating AGA to rat bone marrow cells, all parameters of maturation but the Cx43 gene expression significantly decreased. The results of this experiment suggest that Cx43-mediated GJC plays a critical role in rat bone marrow cells, progress toward maturation.

Connexin 32 and 43 gap junctions differentially modulate tenocyte response to cyclic mechanical load

Gap junctions allow rapid exchange of ions and small metabolites between cells. They can occur between connective tissue cells, and in tendons there are two prominent types, composed of connexin 32 or 43. These form distinct networks - tenocyte rows are linked by both longitudinally, but only by connexin 43 laterally. We hypothesised that the junctions had different roles in cell response to mechanical loading, and measured the effects of inhibitors of gap junction function on secretion of collagen by tenocyte cultures exposed to mechanical strain. Chicken tendon fibroblasts were exposed to cyclic tensile loading in the presence or absence of general gap junction inhibitors (halothane or the biomimetic peptide gap27), or antisense oligonucleotides to chicken connexin 32 or 43. Untreated cultures increased collagen secretion by around 25% under load. Halothane eliminated this response but caused cell damage. Gap27 peptide reduced secretion but maintained loading effects - strained cultures secreting more collagen than unstrained. Antisense downregulation showed major differences between connexins: antisense 32 reduced, and antisense 43 increased, collagen secretion. In both cases loading effects were maintained. This shows that (i) gap junctional integration of signals is important in load response of tenocyte populations - mechanotransduction occurs in individual cells but integration of signals markedly enhances it and (ii) communication via connexin 32 and 43 have differential effects on the load response, with connexin 32 being stimulatory and connexin 43 being inhibitory. Cells coordinate and control their response to mechanical signals at least in part by differential actions of these two types of gap junction.

The SOX9 transcription factor in the human disc: decreased immunolocalization with age and disc degeneration

STUDY DESIGN

Human intervertebral disc anulus tissue was obtained in a prospective study of immunolocalization of SOX9, a protein that plays a role in chondrogenesis and Type II collagen expression. The Human Subjects Institutional Review Board approved experimental studies. Discs were obtained from surgical specimens and from control donors.

OBJECTIVES

To determine whether SOX9 could be detected in discs of Thompson Grades I-IV using immunohistochemistry and to quantify the percentage of cells with SOX9 expression.

SUMMARY OF BACKGROUND DATA

SOX9 is involved with cell-specific activation of COL2A1 in chondrocytes. Recent studies have used adenoviral delivery vectors expressing SOX9 to infect a chondroblastic cell line and human disc cells; SOX9 and Type II collagen production increased. The AdSOX9 virus has also been injected directly into rabbit discs in which disc architecture was preserved for 5 weeks. Despite current interest in SOX9 for gene therapy, there have been few studies of SOX9 in normal or degenerated discs.

METHODS

Discs from 12 normal donors and 25 surgical subjects 15-76 years old were examined for SOX9 immunolocalization. Eight Thompson Grade I discs, 7 Grade II discs, 10 Grade III discs, and 12 Grade IV discs were studied.

RESULTS

In Thompson Grade I discs, SOX9 was uniformly localized throughout the anulus and in some cells of the nucleus. However, in discs from adult donors, anulus cells were present that showed no SOX9 localization, although neighboring cells might be positive. Mean percent localization was 74% for Grade II discs, 69% for Grade III, and 71.6% for Grade IV. Cervical sites showed significantly greater localization than lumbar sites.

CONCLUSIONS

Findings showed a uniform expression of SOX9 in the newborn healthy anulus. With aging and disc degeneration, some anulus cells no longer express this transcription product. These observations suggest that the loss of expression of SOX9 in some disc cells may play a role indisc aging and disc degeneration by resulting in decreased expression and production of Type II collagen.

Expression of connexin 43 mRNA in microisolated murine osteoclasts and regulation of bone resorption in vitro by gap junction inhibitors

Several studies have demonstrated that connexin 43 (Cx43) mediates signals important for osteoblast function and osteogenesis. The role of gap junctional communication in bone resorption is less clear. We have investigated the expression of Cx43 mRNA in osteoclasts and bone resorption cultures and furthermore, the functional importance of gap junctional communication in bone resorption. RT-PCR analysis demonstrated Cx43 mRNA expression in mouse bone marrow cultures and in osteoclasts microisolated from the marrow cultures. Cx43 mRNA was also expressed in bone resorption cultures with osteoclasts and osteoblasts/stromal cells incubated for 48h on devitalized bone slices. An up-regulation of Cx43 mRNA was detected in parathyroid (PTH)-stimulated (0.1 nM) bone resorption. Two inhibitors of gap junction communication, 18alpha-glycyrrhetinic acid (30 microM) and oleamide (100 microM), significantly inhibited PTH- and 1,25-(OH)(2)D(3)-stimulated osteoclastic pit formation. In conclusion, our data indicate a functional role for gap junction communication in bone resorption.

Fluid shear stress remodels expression and function of junctional proteins in cultured bone cells

We tested the hypothesis that fluid shear stress (tau) modifies the expression, function, and distribution of junctional proteins [connexin (Cx)43, Cx45, and zona occludens (ZO)-1] in cultured bone cells. Cell lines with osteoblastic (MC3T3-E1 cells) and osteocytic (MLO-Y4 cells) phenotypes were exposed to tau-values of 5 or 20 dyn/cm(2) for 1-3 h. Immunostaining indicated that at 5 dyn/cm(2), the distribution of Cx43, Cx45, and ZO-1 was moderately disrupted at cell membranes; at 20 dyn/cm(2), disruption was more severe. Intercellular coupling was significantly decreased at both shear stress levels. Western blots showed the downregulation of membrane-bound Cx43 and ZO-1 and the upregulation of cytosolic Cx43 and Cx45 at different levels of shear stress. Similarly, Northern blots revealed that expression of Cx43, Cx45, and ZO-1 was selectively up- and downregulated in response to different shear stress levels. These results indicate that in cultured bone cells, fluid shear stress disrupts junctional communication, rearranges junctional proteins, and determines de novo synthesis of specific connexins to an extent that depends on the magnitude of the shear stress. Such disconnection from the bone cell network may provide part of the signal whereby the disconnected cells or the remaining network initiate focal bone remodeling.

Cultured periodontal ligament fibroblasts express diverse connexins

Recent studies have suggested multiple functions of periodontal ligament fibroblasts (PDLFs) which may relate to the permeability of gap junctions composed of various types of connexins (Cxs). At present, 15 types of Cxs are known to exist, and six of their antibodies, anti-Cx26, Cx32, Cx37, Cx40, Cx43, and Cx45 are commercially available. This study aims to examine which types of Cxs are expressed in cultured PDLFs by an immunohistochemical method, western blotting, and RT-PCR. The study confirmed the expressions of Cx32, Cx40, Cx43, and Cx45 in PDLFs, while Cx26 and Cx37 were not detected. Considering previous reports, Cx32 may relate to the secretory function, and Cx40 and Cx45 to the contractile function of PDLFs, however, a function for Cx43 has not been specified. In the immunohistochemical examination, different localizations of Cx40/43 and Cx32/45 were established. The former were observed punctately, suggesting that a large part of Cx40/43 may exist in the cell membrane and construct gap junctions. In contrast, the latter were observed uniformly in all the cells, indicating that they are present both in the cell membrane and in the cytoplasm of the cells.

Inhibition of gap junction intercellular communication by extremely low-frequency electromagnetic fields in osteoblast-like models is dependent on cell differentiation

Electromagnetic fields have been used to augment the healing of fractures because of its ability to increase new bone formation. The mechanism of how electromagnetic fields can promote new bone formation is unknown, although the interaction of electromagnetic fields with components of the plasma membrane of cells has been hypothesized to occur in bone cells. Gap junctions occur among bone forming cells, the osteoblasts, and have been hypothesized to play a role in new bone formation. Thus it was investigated whether extremely low-frequency (ELF) magnetic fields alter gap junction intercellular communication in the pre-osteoblastic model, MC3T3-E1, and the well-differentiated osteoblastic model, ROS 17/2.8. ELF magnetic field exposure systems were designed to be used for an inverted microscope stage and for a tissue culture incubator. Using these systems, it was found that magnetic fields over a frequency range from 30 to 120 Hz and field intensities up to 12.5 G dose dependently decreased gap junction intercellular communication in MC3T3-E1 cells during their proliferative phase of development. The total amount of connexin 43 protein and the distribution of connexin 43 gap junction protein between cytoplasmic and plasma membrane pools were unaltered by treatment with ELF magnetic fields. Cytosolic calcium ([Ca(2+)](i)) which can inhibit gap junction communication, was not altered by magnetic field exposure. Identical exposure conditions did not affect gap junction communication in the ROS 17/2.8 cell line and when MC3T3-E1 cells were more differentiated. Thus ELF magnetic fields may affect only less differentiated or pre-osteoblasts and not fully differentiated osteoblasts. Consequently, electromagnetic fields may aid in the repair of bone by effects exerted only on osteoprogenitor or pre-osteoblasts.

Morphologic and molecular evidence for gap junctions and connexin 43 and 45 expression in annulus fibrosus cells from the human intervertebral disc

Data are presented which provide evidence for gap junction formation and connexin (Cx) 43 and 45 gene expression in human intervertebral disc cells in vivo and in vitro. These findings in cells from the annulus are important in conjunction with the well-recognized loss of disc cells during aging and disc degeneration. As a result of this loss of cells, cell-cell communication, which we propose is an important, but as yet poorly understood, mechanism which links and coordinates cellular function throughout the entire population of disc cells, is also disrupted. These studies provide additional information on the fundamental cell biology of the disc cell and provide an additional framework for understanding aging, degeneration and potential repair of the human disc.

Inhibition of gap-junctional communication induces the trans-differentiation of osteoblasts to an adipocytic phenotype in vitro

Osteoblasts and adipocytes are thought to differentiate from a common stromal progenitor cell. These two phenotypically mature cell types show a high degree of plasticity, which can be observed when cells are grown under specific culture conditions. Gap junctions are abundant among osteoblastic cells in vivo and in vitro, whereas they are down-regulated during adipogenesis. Gap junctional communication (GJC) modulates the expression of genes associated with the mature osteoblastic phenotype. Inhibition of GJC utilizing 18-alpha-glycyrrhetinic acid (AGRA) blocks the maturation of pre-osteoblastic cells in vitro. Moreover, cytoplasmic lipid droplets are detectable at the end of the culture period, suggesting that GJC inhibition may favor an adipocytic phenotype. We used several human osteoblastic cell lines, as well as bone-derived primary osteoblastic cells, to show that confluent cultures of human osteoblastic cells grown under osteogenic conditions developed an adipocytic phenotype after 3 days of complete inhibition of GJC using AGRA or oleamide, two dissimilar nontoxic reversible inhibitors. Development of an adipogenic phenotype was confirmed by the accumulation of triglyceride droplets and the increase in mRNA expression of the adipocytic markers peroxisome proliferator-activated receptor gamma2 and lipoprotein lipase. Glycyrrhizic acid, a noninhibitory AGRA analog, or alpha-bromopalmitate, a nondegradable fatty acid, had no effect. Modulation of skeletal GJC may represent a new pharmacological target by which inhibition of marrow adipogenesis can take place with the parallel enhancement of osteoblastogenesis, thus providing a novel therapeutic approach to the treatment of human age-related osteopenic diseases and postmenopausal osteoporosis.

Gap-junctional communication is required for the maturation process of osteoblastic cells in culture

Osteoblastic cells in long-term culture undergo a phenotypic maturation process leading to extracellular matrix (ECM) production and bone nodule (BN) formation. Cell-to-cell communication via gap junctions (GJC) can be detected between osteoblastic cells within 24 h of plating. We evaluated, in long-term cultures of osteoblastic cells, the effect of inhibiting GJC on the phenotypic maturation process and the expression of specific genes associated with this process. MC3T3-E1 cells were plated, and, after 24 h (day 0), cells were exposed to 18-alpha-glycyrrhetinic acid (AGA), a nontoxic reversible inhibitor of GJC. GJC, alkaline phosphatase (AP) activity, BN formation, and the relative level of transcripts encoding osteocalcin (OC), bone sialoprotein (bSP), osteopontin (OP), collagen alpha1 type I (alpha1ICol), and elongation factor-1a (EF1a) were evaluated on day 0 and every 4-7 days thereafter through day 30. GJC was assessed by fluorescent dye transfer. Gene expression was analyzed by northern blot and semiquantitative reverse transcription-polymerase chain reaction. GJC was detectable at day 0 and increased with time in culture. AGA (100 micromol/L) strongly inhibited GJC at all timepoints tested. Moreover, AGA-exposed cells showed a dose-dependent decrease in AP activity and a delay in the appearance of BN. This delayed phenotypic expression coincided with an inhibitory effect on the expression of the osteoblast-specific genes OC and bSP. Expression of alpha1ICol mRNA was also affected, but to a lesser extent, whereas OP and EF1a were not affected. Similar results were obtained with oleamide, an additional reversible inhibitor of GJC. In contrast, cells exposed to either vehicle or 100 micromol/L glycyrrhizic acid (a noninhibitory glycoside of 18-beta-glycyrrhetinic acid) were indistinguishable from untreated cells for all parameters evaluated. We conclude that GJC inhibition interferes with the maturation process of osteoblastic cells in culture, possibly by affecting signals regulating the expression of genes involved in the maturation/differentiation of the osteoblastic phenotype.

Gap-junctional communication mediates parathyroid hormone stimulation of mineralization in osteoblastic cultures

Previously we showed that physiological levels of parathyroid hormone (PTH) can increase the mineralization of extracellular matrix (ECM) by osteoblast-like cells in vitro. In this study, we assess the role of gap-junctional intercellular communication (GJC) in the PTH-enhanced mineralization of ECM in MC3T3-E1 cells, a murine culture model of osteoblastic differentiation. Messenger RNA and protein for connexin 43 (Cx43), the major component of MC3T3-E1 gap junctions, and GJC increased as the cells progressed toward a mature phenotype. Immunocytochemistry showed accumulation of Cx43 at the area of close contact between cells. The timing of the PTH treatment that increased matrix mineralization in these cells coincided with the highest expression of Cx43 and GJC. Administration of 18-alpha-glycyrrhetinic acid (AGA) promptly blocked GJC in cultures of MC3T3-E1 cells in a dose-dependent and reversible manner at all times tested during the culture period. Treatment with AGA, but not with an inactive analog, reversed the PTH-induced ECM mineralization. These data suggest that GJC mediates anabolic actions of PTH related to osteoblast-mediated mineralization.

Sex steroids and bone

Sex steroids are essential for skeletal development and the maintenance of bone health throughout adult life, and estrogen deficiency at menopause is a major pathogenetic factor in the development of osteoporosis in postmenopausal women. The mechanisms by which the skeletal effects of sex steroids are mediated remain incompletely understood, but in recent years there have been considerable advances in our knowledge of how estrogens and, to a lesser extent androgens, influence bone modeling and remodeling in health and disease. New insights into estrogen receptor structure and function, recent discoveries about the development and activity of osteoclasts, and lessons learned from human and animal genetic mutations have all contributed to increased understanding of the skeletal effects of estrogen, both in males and females. Studies of untreated and treated osteoporosis in postmenopausal women have also contributed to this knowledge and have provided unequivocal evidence for the potential of high-dose estrogen therapy to have anabolic skeletal effects. The development of selective estrogen receptor modulators has provided a new approach to the prevention of osteoporosis and other major diseases of menopause and has implications for the therapeutic use of other steroid hormones, including androgens. Further elucidation of the mechanisms by which sex steroids affect bone thus has the potential to improve the clinical management not only of osteoporosis, both in men and women, but also of a number of other diseases related to sex hormone status.

Osteoblastic networks with deficient coupling: differential effects of magnetic and electric field exposure

A gap junction-deficient cell line was utilized to test whether intercellular coupling plays a significant role in modulating the influence of biophysical stimuli such as extracellular electrical currents. ROS 17/2.8 cells, an osteosarcoma cell line, along with a control transfected cell line and a connexin 43-gap junction-deficient cell line, were exposed to a time-changing magnetic flux (30 Hz, 1.8 milliTesla) sufficient to induce an electric field in the cultures on the order of 2 mV/m. Field exposure inhibited cell growth independent of gap junctional coupling, while alkaline phosphatase activity was found to be dependent on gap junctional coupling. These findings can be interpreted to suggest that magnetic and electric field exposures have differential effects on cell cultures, with magnetic field exposure inhibiting cell growth through a mechanism independent of gap junctional coupling, while the alteration in enzyme activity appears to be stimulated by the induced electric field in a gap junction-dependent manner.

Gap junctions and biophysical regulation of bone cell differentiation

Physical signals, in particular mechanical loading, are clearly important regulators of bone turnover. Indeed, the structural success of the skeleton is due in large part to the bone's capacity to recognize some aspect of its functional environment as a stimulus for achievement and retention of a structurally adequate morphology. However, while the skeleton's ability to respond to its mechanical environment is widely accepted, identification of a reasonable mechanism through which a mechanical "load" could be transformed to a signal relevant to the bone cell population has been elusive. In addition, the downstream response of bone cells to load-induced signals is unclear. In this work, we review evidence suggesting that gap junctional intercellular communication (GJIC) contributes to mechanotransduction in bone and, in so doing, contributes to the regulation of bone cell differentiation by biophysical signals. In this context, mechanotransduction is defined as transduction of a load-induced biophysical signal, such as fluid flow, substrate deformation, or electrokinetic effects, to a cell and ultimately throughout a cellular network. Thus, mechanotransduction would include interactions of extracellular signals with cellular membranes, generation of intracellular second messengers, and the propagation of these messengers, or signals they induce, through a cellular network. We propose that gap junctions contribute largely to the propagation of intracellular signals.

Differentiation of human fetal osteoblastic cells and gap junctional intercellular communication

Gap junctional channels facilitate intercellular communication and in doing so may contribute to cellular differentiation. To test this hypothesis, we examined gap junction expression and function in a temperature-sensitive human fetal osteoblastic cell line (hFOB 1.19) that when cultured at 37 degrees C proliferates rapidly but when cultured at 39.5 degrees C proliferates slowly and displays increased alkaline phosphatase activity and osteocalcin synthesis. We found that hFOB 1.19 cells express abundant connexin 43 (Cx43) protein and mRNA. In contrast, Cx45 mRNA was expressed to a lesser degree, and Cx26 and Cx32 mRNA were not detected. Culturing hFOB 1. 19 cells at 39.5 degrees C, relative to 37 degrees C, inhibited proliferation, increased Cx43 mRNA and protein expression, and increased gap junctional intercellular communication (GJIC). Blocking GJIC with 18alpha-glycyrrhetinic acid prevented the increase in alkaline phosphatase activity resulting from culture at 39.5 degrees C but did not affect osteocalcin levels. These results suggest that gap junction function and expression parallel osteoblastic differentiation and contribute to the expression of alkaline phosphatase activity, a marker for fully differentiated osteoblastic cells.

Toward a unifying theory of bone remodeling

A theory is developed to resolve several inconsistencies between current concepts and observations about bone remodeling. For example, the observation that remodeling increases both when mechanical loading is excessively low, that is, in a disuse state, and when it is excessively high, producing substantial fatigue damage, is contrary to the widely held assumption that a signal generated by osteocytes in proportion to mechanical loading stimulates bone lining cells to activate remodeling. The new theory resolves this disparity by assuming that lining cells are inclined to activate remodeling unless restrained by an inhibitory signal, and that the mechanically provoked osteocytic signal serves this inhibitory function. Consequently, remodeling is elevated when signal generation declines due to reduced loading, or when signal generation or transmission is interrupted by damage due to excessive loading. Otherwise, remodeling is kept at a relatively low level by inhibitory signals produced through physiologic loading. Furthermore, the inhibitory signal is postulated to be identical to that proposed by Marotti as the mechanism for conversion of osteoblasts to osteocytes, and responsible for the diminishment of apposition rate during refilling of osteonal basic multicellular units. Consequently, a single, mechanically derived signal, produced in the osteocytic syncytium, may control osteoblast and bone lining cell functions, and thereby a variety of important phenomena in bone biology.

Inhibiting gap junctional intercellular communication alters expression of differentiation markers in osteoblastic cells

Gap junctional intercellular communication (GJIC) may contribute to cellular differentiation. To examine this possibility in bone cells we examined markers of cellular differentiation, including alkaline phosphatase, osteocalcin, and osteopontin, in ROS17/2.8 cells (ROS), a rat osteoblastic cell line expressing phenotypic characteristics of fully differentiated osteoblasts. We utilized ROS rendered communication deficient either by stable transfection with antisense cDNA to connexin 43 (Cx43), the predominant gap junction protein in bone (RCx16 cells), or by overexpression of Cx45, a gap junction protein not normally expressed in ROS (ROS/Cx45 cells). Both RCx16 and ROS/Cx45 cells displayed reduced dye coupling and Cx43 protein expression relative to ROS, control transfectants, and ROS/Cx45tr, ROS cells expressing carboxylterminal truncated Cx45. Steady-state mRNA levels for osteocalcin as well as alkaline phosphatase activity, two markers of osteoblastic differentiation, were also reduced in poorly coupled RCx16 and ROS/Cx45 cells. On the other hand, steady-state mRNA levels for osteopontin increased slightly in RCx16 and ROS/Cx45 cells. These results suggest that GJIC at least partly contributes to the regulation of expression of markers of osteoblastic differentiation.

Serial passage of MC3T3-E1 cells alters osteoblastic function and responsiveness to transforming growth factor-beta1 and bone morphogenetic protein-2

The murine-derived clonal MC3T3-E1 cell is a well-studied osteoblast-like cell line. To understand the effects of serial passages on its cellular function, we examined changes in cell morphology, gap junctional intercellular communication (GJIC), proliferation, and osteoblastic function between early passage (<20) and late passage (>65) cells. MC3T3-E1 cells developed an elongated, spindle shape after multiple passages. Intercellular communication decreased significantly (33%) in late vs. early passage cells. Transforming growth factor-beta1 (TGF-beta1) stimulated cell proliferation in early passage cells and induced c-fos expression, while it inhibited proliferation in late passage cells. Using alkaline phosphatase (ALP) activity and osteocalcin (OC) secretion as markers for osteoblastic function and differentiation, we demonstrated that both markers were significantly reduced after multiple cell passages. Bone morphogenetic protein-2 (BMP-2) significantly enhanced ALP activity and OC secretion in early passage cells while TGF-beta1 exerted an opposite effect. Both BMP-2 and TGF-beta1 had minimal effects on late passage cells. We conclude that serial passage alters MC3T3-E1 cell morphology, and significantly diminishes GJIC, osteoblastic function, TGF-beta1-mediated cell proliferation, and responsiveness to TGF-beta1 and BMP-2. Cell passage numbers should be clearly defined in functional studies involving MC3T3-E1 cells.

TNF-alpha and IL-1beta suppress N-cadherin expression in MC3T3-E1 cells

Excessive production of tumor necrosis factor (TNF) and interleukin-1 (IL-1) secondary to estrogen deficiency have been implicated as the cause of osteoporosis in postmenopausal woman. These cytokines appear to stimulate osteoclast precursor proliferation and activate mature osteoclast formation directly and possibly indirectly via osteoblasts. To investigate the other possible roles that these cytokines may play in stimulating the bone resorption process, we examined the effect of TNF-alpha and IL-1beta on cell-cell adhesion molecules, cadherins, in osteoblastic MC3T3-E1 cells. In this study, we investigated cadherin expression and the effect of TNF-alpha, IL-1beta, and parathyroid hormone (PTH) on the expression of cadherins in MC3T3-E1 cells. Confluent cultures of MC3T3-E1 cells were challenged with recombinant human TNF-alpha (1-100 U/ml), recombinant human IL-1beta (1-100 ng/ml) and human PTH(1-34) (1-100 ng/ml), respectively. The results show that MC3T3-E1 cells express functional cadherin molecules, N-cadherin and OB-cadherin. TNF-alpha (10-100 U/ml) and IL-1beta (10-100 ng/ml) suppressed N-cadherin without changing OB-cadherin expression, while PTH (1-100 ng/ml) had no effect on cadherin expression. These results raise the possibility that TNF-alpha and IL-1beta may compromise the cell-cell adhesion of osteoblasts which cover the bone surface. The ensuing compromised cell-cell adhesion of osteoblasts may in turn facilitate the direct adhesion of osteoclasts on the calcified bone matrix surface. These results implicate an indirect role for osteoblasts in the promotion of bone resorption by TNF-alpha and IL-1beta.

Anabolic or catabolic responses of MC3T3-E1 osteoblastic cells to parathyroid hormone depend on time and duration of treatment

We have investigated signaling (cAMP) and anabolic responses (mineralization of extracellular matrix [ECM]) to parathyroid hormone (PTH) in long-term (30 days) cultures of MC3T3-E1 cells, a murine model of osteoblast differentiation. Expression of PTH/PTH-related peptide receptor (PTH1R) mRNA is detected early and remains relatively constant for 2 weeks with somewhat higher levels observed during the second half of the culture period. In contrast to the relatively stable PTH1R mRNA expression, the cAMP response to PTH varies markedly with no response at day 5 and a marked response (80-fold versus control) by day 10. Responsiveness to PTH remains elevated with fluctuations of 30- to 80-fold stimulation throughout the remainder of the culture period. The timing and duration of PTH treatment to achieve in vitro mineralization of ECM was evaluated. When continuous PTH treatment was initiated before day 20, mineralization decreased. If continuous PTH treatment began on or after day 20, mineralization was unaffected. However, if treatment began on day 20 and then stopped on day 25, mineralization on day 30 was increased 5-fold. This mineralization response to intermittent PTH was confirmed in primary cultures of murine and human osteoblastic cells. These data provide a potential basis for understanding the differential responses to PTH (anabolic versus catabolic) and indicate the developmental temporal variance of anabolic and catabolic responses. Since cAMP signaling was relatively unchanged during this interval (day 10-30) and stimulation of adenylate cyclase only partially mimicked the PTH effect on increased mineralization, other signaling pathways are likely to be involved in order to determine the specific anabolic response to short-term PTH treatment during the differentiation process.

Gap junctional intercellular communication contributes to the contraction of rat osteoblast populated collagen lattices

The contraction of native collagen lattices by resident mesenchymal cells mimics the organization of collagen during development and repair. Lattice contraction is cell density dependent, suggesting that cell-to-cell communications may contribute to the process. This possibility was investigated by comparing lattice contraction by four rat osteoblastic cell lines: ROS 17/2.8 cells (ROS); ROS transfected with an antisense cDNA sequence of the gap junctional protein connexin 43 (RCx16); ROS transfected with connexin 45 cDNA, a connexin not normally expressed in ROS cells (ROS/Cx45); and ROS transfected with cDNA encoding carboxy-terminal truncated Cx45 (ROS/Cx45tr). The cell coupling indices, which reflect gap junctional communication, were quantitated by the fluorescent dye scrape loading. ROS cells were well coupled (index 3.0), ROS/Cx45tr were better coupled (index 4.2), ROS/Cx45 were poorly coupled (index 1.7), and RCx16 showed no coupling (index 1.1). As determined by immunoblotting, the level of connexin 43 protein was increased in both ROS/Cx45tr and ROS/Cx45 cell lines compared with ROS cells, while the level in RCx16 cells was reduced. ROS populated collagen lattices (PCLs) contracted significantly more at day 5 (177 mm2 to 67 mm2) than ROS/Cx45tr (84 mm2), ROS/Cx45 (108 mm2), or RCx16 (114 mm2). Myosin ATPase activity, which is required for lattice contraction, was equivalent in all four cell lines, indicating that it was not responsible for inhibiting PCL contraction. ROS cells in collagen appeared elongated compared with the other cell lines which were more rounded. These experiments suggest gap junctional communication contributes to PCL contraction by resident osteoblasts.

Gap junctional communication modulates gene expression in osteoblastic cells

Bone-forming cells are organized in a multicellular network interconnected by gap junctions. In these cells, gap junctions are formed by connexin43 (Cx43) and connexin45 (Cx45). Cx43 gap junctions form pores that are more permeable to negatively charged dyes such as Lucifer yellow and calcein than are Cx45 pores. We studied whether altering gap junctional communication by manipulating the relative expression of Cx43 and Cx45 affects the osteoblast phenotype. Transfection of Cx45 in cells that express primarily Cx43 (ROS 17/2.8 and MC3T3-E1) decreased both dye transfer and expression of osteocalcin (OC) and bone sialoprotein (BSP), genes pivotal to bone matrix formation and calcification. Conversely, transfection of Cx43 into cells that express predominantly Cx45 (UMR 106-01) increased both cell coupling and expression of OC and BSP. Transient cotransfection of promoter-luciferase constructs and connexin expression vectors demonstrated that OC and BSP gene transcription was down-regulated by Cx45 cotransfection in ROS 17/2. 8 and MC3T3-E1 cells, in association with a decrease in dye coupling. Conversely, cotransfection of Cx43 in UMR 106-01 cells up-regulated OC and BSP gene transcription. Activity of other less specific osteoblast promoters, such as osteopontin and osteonectin, was less sensitive to changes in gap junctional communication. Thus, altering gap junctional permeability by manipulating the expression of Cx43 and Cx45 in osteoblastic cells alters transcriptional activity of osteoblast-specific promoters, presumably via modulation of signals that can diffuse from cell to cell. A communicating intercellular network is required for the full elaboration of a differentiated osteoblastic phenotype.

Parathyroid hormone effect on cell-to-cell communication in stromal and osteoblastic cells

We characterized the formation and regulation of the gap junction in calvarial osteoblasts and in a series of subtypes from marrow stromal cells. The stromal cells included osteogenic, chondro-osteogenic, and endothelial cells. The cell coupling was measured by using fluorescence dye injected into single cells, and its migration to neighboring cells was measured. The functional coupling of cells was highly expressed by the osteoblastic cells. This process is mediated through fast changes in intracellular Ca+2 levels. Calcium ionophore (A 23,187) demonstrated an uncoupling effect on the cells. In addition, the exposure of the cells to the parathyroid hormone increased the formation of the gap junction complex; the highest level was demonstrated in the osteoblastic cells.

Human osteoblasts express a repertoire of cadherins, which are critical for BMP-2-induced osteogenic differentiation

Direct cell-cell interactions are fundamental for tissue development and differentiation. We have studied the expression and function of cadherins in human osteoblasts during in vitro differentiation. Using reverse transcription-polymerase chain reaction and mRNA hybridization, we found that human trabecular bone osteoblasts (HOBs), osteoprogenitor marrow stromal cells (BMCs), and the osteogenic sarcoma lines, SaOS-2 and MG-63, expressed mRNA for cadherin-11 (C11) and N-cadherin (N-cad). HOBs and BMCs also expressed low levels of cadherin-4 (C4) mRNA. C11 was the most abundant cadherin protein present in human osteoblasts, and its expression was unaffected by bone morphogenetic protein-2 (BMP-2) treatment of either BMCs or HOBs. Likewise, N-cad mRNA did not change during BMP-2 incubation. Conversely, C4 protein, undetectable in transformed cell lines, was down-regulated by BMP-2 treatment of normal cells. Both C11 and C4 were localized to sites of cell-cell contact in both HOBs and BMCs, colocalized with beta-catenin, and bands corresponding to cadherins were coimmunoprecipitated by a beta-catenin antibody, findings indicative of functional cadherins. A decapeptide containing the HAV motif of human N-cad partially inhibited Ca2+-dependent cell-cell adhesion and completely prevented BMP-2-induced stimulation of alkaline phosphatase activity by BMCs. Thus, human osteoblasts and their progenitor cells express a repertoire of multiple cadherins. Cadherin-mediated cell-to-cell adhesion is critical for normal human osteoblast differentiation.

Cyclic stretch enhances gap junctional communication between osteoblastic cells

Mechanical loading is essential to maintain skeletal integrity. Because gap junctions in bone are affected by mechanical factors, we studied whether stretch, an anabolic stimulus for osteoblasts, modulates direct intercellular communication in these cells. Gap junctional communication during stretch was assessed using a newly developed method, the "parachute assay," which allows monitoring of dye diffusion without disruption of the plasma membrane. Application of cyclic stretch for 2 or 24 h to well-coupled ROS 17/2.8 cells resulted in a 56.5% and 30.4% increase in dye coupling, respectively, compared with resting conditions. Stretch increased dye diffusion less dramatically (12.4% compared with unstimulated cells) in the poorly coupled UMR 106-01 cells. The stretch-induced increase of cell coupling was abolished in the presence of the gap junctional inhibitor, heptanol. Steady-state mRNA levels of connexin43 (Cx43), the gap junction protein that mediates cell-to-cell diffusion of negatively charged dyes between osteoblasts, were not different between control and stretched ROS 17/2.8 or UMR 106-01 cultures after various periods of cyclic stretch. However, phosphorylated forms of Cx43 protein were more abundant in stretched ROS 17/2.8 than in controls. This was associated with increased punctate Cx43-specific immunostain at appositional membranes of stretched cells. Thus, cyclic stretch increases gap junctional communication between osteoblastic cells by modulating intracellular localization of Cx43.

Regulation of connexin43 expression and function by prostaglandin E2 (PGE2) and parathyroid hormone (PTH) in osteoblastic cells

Connexin43 (Cx43) forms gap junctions that mediate intercellular communication between osteoblasts. We have examined the effects of prostaglandin E2 (PGE2) and parathyroid hormone (PTH) on gap junctional communication in the rat osteogenic sarcoma cells UMR 106-01. Incubation with either PGE2 or PTH rapidly (within 30 min) increased transfer of negatively charged dyes between UMR 106-01 cells. This stimulatory effect lasted for at least 4 h. Both PGE2 and PTH increased steady-state levels of Cx43 mRNA, but only after 2-4 h of incubation. Transfection with a Cx43 gene construct linked to luciferase showed that this effect of PTH was the result of transcriptional upregulation of Cx43 promoter. Stimulation of dye coupling and Cx43 gene transcription were reproduced by forskolin and 8Br-cAMP. Exposure to PGE2 for 30 min increased Cx43 abundance at appositional membranes in UMR 106-01, whereas total Cx43 protein levels increased only after 4-6 h of incubation with either PGE2 or PTH. Inhibition of protein synthesis by cycloheximide did not affect this early stimulation of dye coupling, but it significantly inhibited the sustained effect of PTH and forskolin on cell coupling. In summary, both PTH and PGE2, presumably through cAMP production, enhance gap junctional communication in osteoblastic cell cultures via two mechanisms: initial rapid redistribution of Cx43 to the cell membrane, and later stimulation of Cx43 gene expression. Modulation of intercellular communication represents a novel mechanism by which osteotropic factors regulate the activity of bone forming cells.

Parathyroid hormone up-regulation of connexin 43 gene expression in osteoblasts depends on cell phenotype

Accumulating evidence indicates that gap junctions, primarily composed of connexin 43 (Cx43), are distributed extensively throughout bone. We have previously reported that in osteoblastic cells parathyroid hormone (PTH) increases both the steady-state levels of transcripts for Cx43 and gap-junctional intercellular communication in a process involving cyclic adenosine monophosphate (cAMP). We now present data showing that the mechanism of stimulation of Cx43 gene expression by PTH involves an increased rate of Cx43 gene transcription without affecting Cx43 transcript stability in UMR 106 osteoblastic cells. Activation of the protein kinase C pathway is not involved in this process. Inhibiting translation consistently decreases the PTH-mediated stimulation of Cx43 gene expression at all the times we tested (1-3 h). However, this effect is only partial, demonstrating that de novo protein synthesis is required for full stimulation. PTH increases the steady-state levels of Cx43 mRNA in several osteoblastic cell lines, albeit to different levels. We were unable to detect PTH stimulation in ROS 17/2.8 osteoblastic cells, suggesting that the effect of PTH on Cx43 gene expression may depend on the developmental state of the cell along the osteoblastic differentiation pathway. In the MC3T3-E1 preosteoblastic cell line, we find that PTH increases Cx43 gene expression in proliferating and maturing osteoblastic cells, but not in nondividing, differentiated osteoblasts, where the basal level of Cx43 gene expression is elevated. Unlike PTH, the osteotropic hormones 1,25-dihydroxyvitamin D3 and 17beta-estradiol do not appear to affect Cx43 gene expression in UMR 106 osteoblastic cells.

Down-regulation of gap junctional intercellular communication between osteoblastic MC3T3-E1 cells by basic fibroblast growth factor and a phorbol ester (12-O-tetradecanoylphorbol-13-acetate)

To address the relation between osteoblast growth and cell-to-cell communication, we examined the effects of basic fibroblast growth factor (bFGF) and 12-O-tetradecanoylphorbol-13-acetate (TPA), both potent stimulators of osteoblastic proliferation, on gap junctional intercellular communication between osteoblastic MC3T3-E1 cells. The level of intercellular communication was estimated by a photobleaching method. TPA inhibited the degree of intercellular communication in two different time-dependent manners. The early (< 1 h) inhibition by TPA was consistent with an increase in the phosphorylation of connexin 43 (Cx43). The later inhibition was caused by reduction in the total amount of Cx43 on the plasma membrane, due to the decrease in the level of Cx43 transcripts. These qualitative and quantitative modulations by TPA were inhibited by a selective inhibitor of protein kinase C, GF109203X. bFGF also attenuated the gap junctional intercellular communication. However, short exposure (< 5 h) to bFGF did not affect the communication. The fact that the growth factor immediately stimulated the phosphorylation of Cx43 indicates that the phosphorylation site(s) affected by bFGF was not involved in the inhibition of communication. The decrease in the intercellular communication level was detected by the longer exposure (> 8 h) to bFGF and paralleled the decline in the Cx-mRNA level. This inhibitory effect of bFGF was abolished by the addition of a tyrosine kinase inhibitor, herbimycin A. Thus, gap junctional intercellular communication between osteoblasts was down-regulated by osteoblastic mitogens through different mechanisms of the modulation of Cx43.

Transforming growth factor-beta, osteogenin, and bone morphogenetic protein-2 inhibit intercellular communication and alter cell proliferation in MC3T3-E1 cells

Intercellular communication by gap junctions has been implicated to function in the control of cell growth and differentiation in osseous tissues-processes which are regulated, in part, by peptide growth factors, including transforming growth factor-beta (TGF-beta) and the bone morphogenetic proteins (BMPs). Using the osteoblastic cell line MC3T3-E1, we tested the hypothesis that the effects of TGF-beta and BMPs on cell proliferation may be correlated to changes in intercellular communication. In a series of proliferation assays, MC3T3-E1 cells were cultured in the presence of bone morphogenetic protein-2 (BMP-2) or TGF-beta for up to 48 hr. Proliferation of cells during the linear log phase (days 2 to 4) was assessed by 3H-thymidine (3H-TdR) incorporation. After times ranging from 6 to 48 hr, BMP-2 significantly inhibited uptake of 3H-TdR at doses of 50-800 ng/ml. Similarly, TGF-beta inhibited uptake of 3H-TdR at doses of 2-32 ng/ml. In a separate group of experiments, intercellular communication through gap junctions was demonstrated by cell-cell transfer of the fluorescent tracer, lucifer yellow, after microinjection. One series of experiments showed that the gap junctional intercellular communication (GJIC) of cells, incubated for 48 hr in the presence of the higher dose of osteogenin (OG) (5.0 vs. 0.5 microgram/ml) or higher dose of TGF-beta (2.0 vs. 0.2 ng/ml), was significantly inhibited compared to control. In another series of experiments, time and dose dependent effects of BMP-2 and TGF-beta on GJIC were investigated. In the time course experiments (3, 6, 12, 24, and 48 hr), TGF-beta (2.0 ng/ml) demonstrated a statistically significant effect in inhibiting GJIC as early as 6 hr, while BMP-2 (50 ng/ml) inhibited GJIC after 24 and 48 hr of treatment. The dose-dependent effects of BMP-2 and TGF-beta on cell couplings, determined at 48 hr, showed significant inhibitory effects with BMP-2 at 25 and 50 ng/ml and with TGF-beta at 2 and 4 ng/ml. The cell count results and injection study performed at 12 hr, at a fixed cell density, confirmed that the inhibitory effect was not due to differences in cell density. The 50% effective inhibitory concentrations (EC50) calculated for BMP-2 and TGF-beta at 48 hr, showed no dose correlation between proliferation and GJIC, suggesting that these two events are independent occurrences. Additionally, marked morphological change was observed in the cells treated with TGF-beta. The observation may suggest that TGF-beta may have effects upon cytoskeletal elements in osseous tissues.

Gap junctional intercellular communication contributes to hormonal responsiveness in osteoblastic networks

To evaluate whether intercellular coupling via connexin43 gap junction channels modulates hormonal responsiveness of cells in contact, we have created osteoblastic cell lines deficient in connexin43. Osteoblastic ROS 17/2.8 cells were transfected with a plasmid containing an antisense cDNA construct to rat connexin43. Control transfection did not alter cell-to-cell coupling nor connexin43 mRNA or protein expression relative to nontransfected ROS 17/2.8 cells. In contrast, stable transfection with an antisense connexin43 cDNA resulted in two clones, RCx4 and RCx16, which displayed significant decreases in connexin43 mRNA and protein expression and were dramatically deficient in cell-to-cell coupling. Phenotypically, all transfectants retained osteoblastic characteristics. However, cells rendered connexin43-deficient through antisense transfection displayed a dramatic attenuation in the cAMP response to parathyroid hormone. Alterations in hormonal responses were not due to changes in parathyroid hormone receptor number or binding kinetics nor to alterations in adenylyl cyclase activity. These results indicate that gap junctions may be required for mediating hormonal signals. Furthermore, these experiments support a regulatory role for connexin43-mediated intercellular communication in the modulation of hormonal responses within elaborately networked bone cells.

Regulation of gap junction intercellular communication by pH in MC3T3-E1 osteoblastic cells

Gap junction intercellular communication (GJIC) may be related to coordinating the function of osteoblasts during bone mineralization. Since an alkaline pH supports mineral deposition while an acidic pH promotes mineral dissolution, it was investigated whether GJIC is altered by changes in extracellular pH (pHo) Functional GJIC was assessed by fluorescent dye transfer after microinjection, and connexin protein abundance was examined by immunoprecipitation and immunoblotting in MC3T3-E1 cells, a model of osteoblast-like cells. The percent of cells coupled by GJIC was found to be 40.7% (24 of 59 injected cells) at pH 6.9, 72.2% (26 of 36) at pH 7.2, and 92.8% (26 of 28) at pH 7.6. A decrease in GJIC was detectable by 30-60 minutes of exposure to a pHo of 6.9. Decreased gap junction communication was also found in cells after 3, 8, and 24 h of incubation in a bicarbonate-CO2 system at an ambient pH of 6.9. Connexin protein abundance experiments showed that at after exposure to a pH of 6.9 for 2.75 h, the specific band(s) at 41-43 kD were fainter compared with these same band(s) at pH 7.2 and 7.6. There was no significant difference in band densities at pH 7.2 and 7.6. Determination of intracellular pH (pHi) showed that it was similar to pHo after 2.75 h of incubation at each ambient pH. When pHi was clamped at 6.9 or 7.2, there was a time-dependent decrease in the gap junction coupling frequency at a pHi of 6.9 when pHo was 7.2. Steady-state mRNA levels were decreased at pHo 6.9 but were unchanged at either pHo 7.2 or 7.6. Our conclusions are that (1) longer incubations ( > or = 2.75 h) at low pHo decrease GJIC which in part may be due to a decrease in connexin protein abundance perhaps as a result of a decrease in connexin steady-state mRNA expression; (2) GJIC inhibition or augmentation found at low and high pHo, respectively, suggests that gating of the GJ channel by pH may also occur; (3) pho-induced alterations in GJIC in the MC3T3-E1 osteoblastic model are related to concomitant changes in pHi.