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

Highly porous novel chondro-instructive bioactive glass scaffolds tailored for cartilage tissue engineering

Cartilage injuries remain challenging since the regenerative capacity of cartilage is extremely low. The aim was to design a novel type of bioactive glass (BG) scaffold with suitable topology that allows the formation of cartilage-specific extracellular matrix (ECM) after colonization with chondrogenic cells for cartilage repair. Highly porous scaffolds with interconnecting pores consisting of 100 % BG were manufactured using a melting, milling, sintering and leaching technique. Scaffolds were colonized with porcine articular chondrocytes (pAC) and undifferentiated human mesenchymal stromal cells (hMSC) for up to 35 days. Scaffolds displayed high cytocompatibility with no major pH shift. Scanning electron microscopy revealed the intimate pAC-scaffold interaction with typical cell morphology. After 14 days MSCs formed cell clusters but still expressed cartilage markers. Both cell types showed aggrecan, SOX9 gene and protein expression, cartilage proteoglycan and sulfated glycosaminoglycan synthesis for the whole culture time. Despite type II collagen gene expression could not anymore be detected at day 35, protein synthesis was visualized for both cell types during the whole culturing period, increasing in pAC and declining after day 14 in hMSC cultures. The novel BG scaffold was stable, cytocompatible and cartilage-specific protein synthesis indicated maintenance of pAC's differentiated phenotype and chondro-instructive effects on hMSCs.

Effects of wollastonite on proliferation and differentiation of human bone marrow-derived stromal cells in PHBV/wollastonite composite scaffolds

In this study, the effects of wollastonite on proliferation and differentiation of human bone marrow-derived stromal cells (hBMSCs) have been investigated based on a polyhydroxybutyrate-co-hydroxyvalerate (PHBV)/ wollastonite (W) composite scaffolds system. Cell morphology, proliferation, and differentiation were measured. The results showed that the incorporation of wollastonite benefited hBMSCs adhesion, proliferation, and differentiation rate. In addition, an increase of proliferation and differentiation rate was observed when the wollastonite content in the PHBV/W composite scaffolds increased from 10 to 20 wt%. Based on our previous studies on PHBV/W composite discs, the differentiation measurements in this paper further proved that the wollastonite itself can stimulate the hBMSCs to differentiate toward osteoblasts without any osteogenic medium, and the ionic products (Ca and Si) released from wollastonite might contribute to this advantage. It is also suggested that the osteogenic differentiation of the hBMSCs can be affected by adjusting the wollastonite content in the composite scaffolds.

Hormonal, pH, and calcium regulation of connexin 43-mediated dye transfer in osteocytes in chick calvaria

Gap junctional intercellular communication among osteocytes in chick calvaria, their natural 3D environment, was examined using FRAP analysis. Cell-cell communication among osteocytes in chick calvaria was mediated by Cx43 and was regulated by extracellular pH, extracellular calcium ion concentration, and PTH.

INTRODUCTION

The intercellular network of communication among osteocytes is mediated by gap junctions. Gap junctional intercellular communication (GJIC) is thought to play an important role in integration and synchronization of bone remodeling. We hypothesized that extracellular pH (pH(o)) and extracellular calcium ion concentration ([Ca2+](e)), both of which are dynamically altered by osteoclasts during bone remodeling, affect GJIC among osteocytes. Using fluorescence replacement after photobleaching (FRAP) analysis, we examined the effect of changes in pH(o) and [Ca2+](e) and addition of PTH on GJIC in osteocytes in chick calvaria. Additionally, we examined the role of intracellular calcium on the regulation of GJIC among osteocytes.

MATERIALS AND METHODS

Anti-Connexin43 (Cx43) immunolabeling was used to localize gap junctions in chick calvaria. GJIC among osteocytes in chick calvariae was assessed using FRAP.

RESULTS

Cx43 immunoreactivity was detected in most of the osteocyte processes. FRAP analysis showed dye-coupling among osteocytes in chick calvariae. In untreated osteocytes, fluorescence intensity recovered 43.7 +/- 2.2% within 5 min after photobleaching. Pretreatment of osteocytes with 18 alpha-GA, a reversible inhibitor of GJIC, significantly decreased fluorescence recovery to 10.7 +/- 2.2%. When pH(o) was decreased from 7.4 to 6.9, fluorescence recovery significantly decreased from 43.3 +/- 2.9% to 19.7 +/- 2.3%. Conversely, when pH(o) was increased from 7.4 to 8.0, fluorescence recovery was significantly increased to 61.9 +/- 4.5%. When [Ca2+](e) was increased from 1 to 25 mM, fluorescence recovery was significantly decreased from 47.0 +/- 6.1% to 16.1 +/- 2.1%. In bone fragments exposed to 1.0-10 nM rPTH for 3 h, replacement of fluorescence was significantly increased to 60.7 +/- 7.2%. Chelating intracellular calcium ions affected GJIC regulation by [Ca2+](e) and PTH.

CONCLUSIONS

Our study of cell-cell communication between osteocytes in chick calvaria showed for the first time that GJIC among osteocytes is regulated by the extracellular environment and by hormonal stimulation during bone remodeling. This method may be more biologically relevant to living bone than current methods.

Mefloquine effects on the lens suggest cooperative gating of gap junction channels

Mefloquine (MFQ) selectively blocks exogenously expressed gap junction channels composed of Cx50 but not Cx46. The purpose of the current study was to evaluate MFQ effects on wild-type (WT) mouse lenses that express both Cx50 and Cx46 in their outer shell of differentiating fibers (DFs). Lenses in which Cx46 was knocked into both Cx50 alleles (KI) were used as controls; MFQ had no effect on coupling in these lenses. When WT lenses were exposed to MFQ, the DF coupling conductance decreased significantly, suggesting that Cx50 contributes about 57% of the coupling conductance in DF and Cx46 contributes 43%. Remarkably, in the presence of MFQ, the 43% of the channels that remained open did not gate closed in response to a reduction in pH, whereas in the absence of MFQ, the same pH change caused all the DF channels to gate closed. Since MFQ is a selective blocker of Cx50 channels, it appears that Cx46 channels lack pH-mediated gating in the absence of functional Cx50 channels but are pH-sensitive in the presence of Cx50 channels. These results suggest the two types of channels interact and gate cooperatively.

Osteoblast response to zirconia-hybridized pyrophosphate-stabilized amorphous calcium phosphate

Calcium phosphate bioceramics, such as hydroxyapatite, have long been used as bone substitutes because of their proven biocompatibility and bone binding properties in vivo. Recently, a zirconia-hybridized pyrophosphate-stabilized amorphous calcium phosphate (Zr-ACP) has been synthesized, which is more soluble than hydroxyapatite and allows for controlled release of calcium and phosphate ions. These ions have been postulated to increase osteoblast differentiation and mineralization in vitro. The focus of this work is to elucidate the physicochemical properties of Zr-ACP and to measure cell response to Zr-ACP in vitro using a MC3T3-E1 mouse calvarial-derived osteoprogenitor cell line. Cells were cultured in osteogenic medium and mineral was added to culture at different stages in cell maturation. Culture in the presence of Zr-ACP showed significant increases in cell proliferation, alkaline phosphatase activity (ALP), and osteopontin (OPN) synthesis, whereas collagen synthesis was unaffected. In addition, calcium and phosphate ion concentrations and medium pH were found to transiently increase with the addition of Zr-ACP, and are hypothesized to be responsible for the osteogenic effect of Zr-ACP.

Acidosis inhibits bone formation by osteoblasts in vitro by preventing mineralization

The negative effect of acidosis on the skeleton has been known for almost a century. Bone mineral serves an important pathophysiologic role as a reserve of hydroxyl ions to buffer systemic protons if the kidneys and lungs are unable to maintain acid-base balance within narrow physiologic limits. Extracellular hydrogen ions are now thought to be the primary activation signal for osteoclastic bone resorption, and osteoclasts are very sensitive to small changes in pH within the pathophysiologic range. Herein, we investigated the effects of acidosis on osteoblast function by using mineralized bone nodule-forming primary osteoblast cultures. Osteoblasts harvested from neonatal rat calvariae were cultured up to 21 days in serum-containing medium, with ascorbate, beta-glycerophosphate and dexamethasone. pH was manipulated by addition of 5 to 30 mmol/L HCl and monitored by blood gas analyzer. Abundant, matrix-containing mineralized nodules formed in osteoblast cultures at pH 7.4, but acidification progressively reduced mineralization of bone nodules, with complete abolition at pH 6.9. Osteoblast proliferation and collagen synthesis, assessed by 3H-thymidine and 3H-proline incorporation, respectively, were unaffected by pH in the range 7.4 to 6.9; no effect of acidification on collagen ultrastructure and organization was evident. The apoptosis rate of osteoblasts, assessed by the enrichment of nucleosomes in cell lysates, was also unaffected by pH within this range. However, osteoblast alkaline phosphatase activity, which peaked strongly near pH 7.4, was reduced eight-fold at pH 6.9. Reducing pH to 6.9 also downregulated messenger ribonucleic acid (mRNA) for alkaline phosphatase, but upregulated mRNA for matrix Gla protein, an inhibitor of mineralization. The same pH reduction is associated with two-and four-fold increases in Ca2+ and PO4(3-) solubility for hydroxyapatite, respectively. Our results show that acidosis exerts a selective, inhibitory action on matrix mineralization that is reciprocal with the osteoclast activation response. Thus, in uncorrected acidosis, the deposition of alkaline mineral in bone by osteoblasts is reduced, and osteoclast resorptive activity is increased in order to maximize the availability of hydroxyl ions in solution to buffer protons.

Growth behavior, matrix production, and gene expression of human osteoblasts in defined cylindrical titanium channels

The purpose of the current study was to investigate the effect of different diameters of cylindrical titanium channels on human osteoblasts. Titanium samples having continuous drill channels with diameters of 300, 400, 500, 600, and 1000 microm were put into osteoblast cell cultures that were isolated from 12 adult human trauma patients. Cell migration into the drill channels was investigated by transmitted-light microscopy. The DNA content in the drill channels was measured photometrically, collagen type I production was analyzed by enzyme-linked immunosorbent assay (ELISA) and osteocalcin gene expression by reverse transcriptase-polymerase chain reaction (RT-PCR). Formation of mineralized tissue was assessed by microradiographs of histological sections. Within 20 days, cells grew an average of 838 microm (+/-128 microm) into the drill channels with a diameter of 600 microm and were significantly faster (p < 0.05) than in all other channels. Cells produced significantly more osteocalcin messenger RNA (mRNA) in 600-microm channels (p < 0.05) than they did in 1000-microm channels and demonstrated the highest osteogenic differentiation. The channel diameter did not influence collagen type I production. The highest cell density was found in 300-microm channels (p < 0.05). The DNA content of the channels linearly decreased with increasing channel diameters. After 40 days of culture, the proportion of mineralized tissue at the mouth section amounted to 6% in 300-microm channels and to 9-11% in 400-600-microm channels. In 1000-microm channels, only traces of mineralization were detected. Our data suggest that the diameter of cylindrical titanium channels has a significant effect on migration, gene expression, and mineralization of human osteoblasts.

Mechanism of pH regulation of connexin 43 expression in MC3T3-E1 cells

Gap junction (GJ) expression and function allowing cell-cell communication among osteoblasts may be important in new bone formation. An alkaline milieu stimulates mineralization, while extracellular acidification leads to demineralization of bone. It was previously demonstrated that alkaline pH increases, while acid pH decreases GJ intercellular communication by an increase in steady-state GJ connexin 43 (Cx43) mRNA and protein expression. At pH 7.6, transcription of new Cx43 mRNA was significantly higher than that at pH 6.9 but not significantly different at pH 7.2, as assessed by nuclear run-on assay. Transcription of new Cx43 mRNA was higher at pH 7.2 compared to that at pH 6.9. Although Cx43 mRNA half-life tended to be longer at pH 7.6, analysis of variance did not yield a significant difference of the Cx43 mRNA half-life at any of the pHs tested. Likewise, the half-life of Cx43 protein at pHs of 6.9, 7.2, and 7.6 was not significantly different. Plasma membrane and cytosolic Cx43 fractions were proportionately similar at pH 7.2 and 6.9. Thus, the decrease in Cx43 mRNA at low pH compared to high pH is due to a decrease in the transcription rate of Cx43 but not due to an alteration of message stability. The early uncoupling of gap junctions by low pH found previously does not appear to be due to changes in the half-life nor distribution of Cx43 protein between cytosolic and plasma membrane compartments.

Effect of HUVEC on human osteoprogenitor cell differentiation needs heterotypic gap junction communication

Bone development and remodeling depend on complex interactions between bone-forming osteoblasts and other cells present within the bone microenvironment, particularly vascular endothelial cells that may be pivotal members of a complex interactive communication network in bone. Our aim was to investigate the interaction between human umbilical vein endothelial cells (HUVEC) and human bone marrow stromal cells (HBMSC). Cell differentiation analysis performed with different cell culture models revealed that alkaline phosphatase activity and type I collagen synthesis were increased only by the direct contact of HUVEC with HBMSC. This "juxtacrine signaling" could involve a number of different heterotypic connexions that require adhesion molecules or gap junctions. A dye coupling assay with Lucifer yellow demonstrated a functional coupling between HUVEC and HBMSC. Immunocytochemistry revealed that connexin43 (Cx43), a specific gap junction protein, is expressed not only in HBMSC but also in the endothelial cell network and that these two cell types can communicate via a gap junctional channel constituted at least by Cx43. Moreover, functional inhibition of the gap junction by 18alpha-glycyrrhetinic acid treatment or inhibition of Cx43 synthesis with oligodeoxyribonucleotide antisense decreased the effect of HUVEC cocultures on HBMSC differentiation. This stimulation could be mediated by the intercellular diffusion of signaling molecules that permeate the junctional channel.

Effect of seeding osteoprogenitor cells as dense clusters on cell growth and differentiation

One approach for forming tissue equivalents involves seeding of cells into porous scaffolds followed by culture in vitro. Within this paradigm, the strategy by which cells are initially seeded may dictate the ultimate properties of the tissue equivalent. In particular, low cell densities may suffer from poor intercellular communication, whereas high densities may result in an unfavorable microenvironment due to transport limitations. A third alternative is to seed cells as dense clusters, which might benefit from intercellular contact without the high nutrient demand. To test this approach, planar substrates were seeded with 10(4) osteoprogenitor marrow stromal cells either as a diffuse subconfluent dispersion (2.6 x 10(3) cells/cm(2)) or as a single dense cluster (8 x 10(4) cells/cm(2)). In this study, the densely clustered cells demonstrated significantly diminished cell growth and collagen synthesis. However, a significantly higher level of alkaline phosphatase activity--a measure of bone-forming potential--and moderately more mineralization were observed with these dense cultures. These findings show that clustering can enhance the differentiation phase while diminishing the proliferating phase of these diploid cells without requiring large cell numbers. Thus, this seeding strategy may improve the quality of engineered tissues.

Bioactive glass stimulates in vitro osteoblast differentiation and creates a favorable template for bone tissue formation

In this study, we have investigated the behavior of fetal rat osteoblasts cultured on bioactive glasses with 55 wt% silica content (55S) and on a bioinert glass (60S) used either in the form of granules or in the form of disks. In the presence of Bioglass granules (55 wt% silica content), phase contrast microscopy permitted step-by-step visualization of the formation of bone nodules in contact with the particles. Ultrastructural observations of undecalcified sections revealed the presence of an electron-dense layer composed of needle-shaped crystals at the periphery of the material that seemed to act as a nucleating surface for biological crystals. Furthermore, energy dispersive X-ray (EDX) analysis and electron diffraction patterns showed that this interface contains calcium (Ca) and phosphorus (P) and was highly crystalline. When rat bone cells were cultured on 55S disks, scanning electron microscopic (SEM) observations revealed that cells attached, spread to all substrata, and formed multilayered nodular structures by day 10 in culture. Furthermore, cytoenzymatic localization of alkaline phosphatase (ALP) and immunolabeling with bone sialoprotein antibody revealed a positive staining for the bone nodules formed in cultures on 55S. In addition, the specific activity of ALP determined biochemically was significantly higher in 55S cultures than in the controls. SEM observations of the material surfaces after scraping off the cell layers showed that mineralized bone nodules remained attached on 55S surfaces but not on 60S. X-ray microanalysis indicated the presence of Ca and P in this bone tissue. The 55S/bone interfaces also were analyzed on transverse sections. The interfacial analysis showed a firm bone bonding to the 55S surface through an intervening apatite layer, confirmed by the X-ray mappings. All these results indicate the importance of the surface composition in supporting differentiation of osteogenic cells and the subsequent apposition of bone matrix allowing a strong bond of the bioactive materials to bone.

Intra- and intercellular Ca(2+)-transient propagation in normal and high glucose solutions in ROS cells during mechanical stimulation

Experiments using confocal laser microscopy on the rat osteosarcoma cell line (ROS 17/2.8) indicate that mechanical stimulation elicits pronounced [Ca2+](i)transients in the MS (mechanically stimulated) cell, which then propagate to the NB (neighbouring) cells. Experiments with Ca(2+)-free solutions or gadolinium suggest that Ca(2+)-influx through stretch-sensitive channels is required. When intracellular stores are depleted with thapsigargin, mechanical stimulation was able to evoke a Ca(2+)transient of reduced amplitude that disappeared entirely after subsequent blocking of Ca(2+)-influx. Heptanol inhibited intercellular propagation of the Ca(2+)transient, demonstrating the involvement of gap junctions in the propagation of the Ca(2+)transient in ROS cells. PKC activation has only a small inhibitory effect, while inhibition of PKC or tyrosine kinase was ineffective. PKA activation reduced the amplitude of the [Ca2+](i)-rise in NB cells, and decreased the percentage of responsive cells. Cells grown in 50mM glucose for 72h presented only a very limited decrease of the Ca(2+)-rise during mechanical stimulation in the MS and NB cells compared to control conditions. PKC downregulation in high glucose did not modulate this effect. The results of our experiments indicate that PKC or sustained high glucose concentrations do not affect gap junctional communication in ROS cells, while activation of PKA has an inhibitory effect. This might indicate that osteoblastic dysfunction in diabetes could be directly related to the high glucose concentrations and not to inhibition of the intercellular communication.

Interactions of bioactive glasses with osteoblasts in vitro: effects of 45S5 Bioglass, and 58S and 77S bioactive glasses on metabolism, intracellular ion concentrations and cell viability

In a cell culture model of murine osteoblasts three particulate bioactive glasses were evaluated and compared to glass (either borosilicate or soda-lime-silica) particles with respect to their effect on metabolic activity, cell viability, changes in intracellular ion concentrations, proliferation and differentiation. 45S5 Bioglass caused extra- and intracellular alkalinization, a rise in [Ca2+]i and [K+]i, a small plasma membrane hyperpolarization, and an increase in lactate production. Glycolytic activity was also stimulated when cells were not in direct contact with 45S5 Bioglass particles but communicated with them only through the medium. Similarly, raising the pH of culture medium enhanced lactate synthesis. 45S5 Bioglass had no effect on osteoblast viability and, under most conditions, did not affect either proliferation or differentiation. Bioactive glasses 58S and 77S altered neither the ion levels nor enhanced metabolic activity. It is concluded that: (1) some bioactive glasses exhibit well-defined effects in osteoblasts in culture which are accessible to experimentation; (2) 45S5 Bioglass causes marked external and internal alkalinization which is, most likely, responsible for enhanced glycolysis and, hence, cellular ATP production; (3) changes in [H+] could contribute to alternations in concentrations of other intracellular ions; and (4) the rise in [Ca2+]i may influence activities of a number of intracellular enzymes and pathways. It is postulated that the beneficial effect of 45S5 on in vivo bone growth and repair may be due to some extent to alkalinization, which in turn increases collagen synthesis and crosslinking, and hydroxyapatite formation.

Antisense oligonucleotide blockade of connexin expression during embryonic bone formation: evidence of functional compensation within a multigene family

Prior studies in our laboratory demonstrated the presence of gap junction proteins (connexins) throughout intramembranous bone formation [Minkoff et al. (1994) Anat Embryol 190:231-241]. In addition, two members of the connexin family of gap junction proteins, connexin 43 (Cx43; Gj alpha 1) and connexin 45 (Cx45; Gj alpha 6), were found by Civitelli et al. [1993; J Clin Invest 91:1888-1896] to be associated, specifically, with osteogenesis. Recently, however, a null mutation in the gene encoding Gj alpha 1 in mice has been produced by Reaume et al. [1995; Science 267:1831-1834]. Gj alpha 1 null homozygotes survived to term but died at birth of heart abnormalities. Examination of the null homozygous embryos, surprisingly, did not reveal overt histological or anatomical abnormalities in any organ system other than the heart. In view of this, the present investigation was initiated in order to evaluate bone formation under conditions in which the expression of Gj alpha 1 and Gj alpha 6, the connexins specifically associated with osteogenesis, had been perturbed, individually as well as in combination. An in vitro system employing organ cultures of dissociated embryonic chick mandibular mesenchyme was employed. Mesenchyme was cultured in the presence and absence of sense and antisense oligodeoxynucleotides (ODN), ranging in length from 15 to 24 mer and containing sequences that included the initiation codon of Gj alpha 1 and of Gj alpha 6. In cultures of mesenchyme, grown for 6 to 13 days in the presence of the combined antisense ODNs to Gj alpha 1 and Gj alpha 6, bone formation was markedly reduced or absent. By contrast, in cultures grown in medium containing the combination of corresponding sense ODNs to both Gj alpha 1 and Gj alpha 6, bone formation was evident. In addition, when cultures were grown in the presence of antisense or sense ODNs to either Gj alpha 1 or Gj alpha 6, individually, bone formation was seen. Immunohistochemical analysis of connexin expression revealed intense immunoreactive signal to Gj alpha 1 and Gj alpha 6 in bone of the control explants, in which no ODNs were present; in those cultures in which either Gj alpha 1 and Gj alpha 6 antisense ODNs were present, however, the expression of the respective connexin protein was either significantly reduced or absent. Further, in those explants in which Gj alpha 1 expression was blocked, immunoreactive signal to Gj alpha 6 appeared to have been amplified in regions of developing bone. These results suggest that, in avian osteogenic tissue, when Gj alpha 1 protein expression has been impeded another related connexin protein (Gj alpha 6) may subserve the functions of the missing connexin. The findings of this study, therefore, support the hypothesis that, within the connexin gene family, functional compensation can occur.

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.

Insulin-like growth factor I rapidly enhances acid efflux from osteoblastic cells

Insulin-like growth factor I (IGF-I) is thought to stimulate bone resorption indirectly through a primary effect on osteoblasts, which in turn activate osteoclasts by as-yet-unidentified mechanisms. Small decreases in extracellular pH (pHo) dramatically increase the resorptive activity of osteoclasts. Our purpose was to characterize the effect of IGF-I on acid production by osteoblastic cells. When confluent, UMR-106 osteoblast-like cells and rat calvarial cells acidified the compartment beneath them. Superfusion with IGF-I caused a further decrease in pHo. To investigate the mechanism, we monitored acid efflux from subconfluent cultures. IGF-I rapidly increased net efflux of H+ equivalents in a concentration-dependent manner. IGF-II (10 nM) evoked a smaller response than IGF-I (10 nM). The response to IGF-I was partially dependent on extracellular Na+, but not glucose, and exhibited little if any desensitization. Wortmannin, an inhibitor of phosphatidylinositol 3-kinase, abolished the response to IGF-I but not to parathyroid hormone. Thus IGF-I enhances acid efflux from osteoblastic cells, via a signaling pathway dependent on activation of phosphatidylinositol 3-kinase. In vivo, acidification of the compartment between the osteogenic cell layer and the bone matrix may affect diverse processes, including mineralization and osteoclastic bone resorption.

Extracellular pH modulates the activity of cultured human osteoblasts

The effect of medium pH on the activity of cultured human osteoblasts was investigated in this study. Osteoblasts derived from explants of human trabecular bone were grown to confluence and subcultured. The first-pass cells were incubated in Hepes-buffered media at initial pHs adjusted from 7.0 to 7.8. Osteoblast function was evaluated by measuring lactate production, alkaline phosphatase activity, proline hydroxylation, DNA content, and thymidine incorporation. Changes in medium pH were determined from media pHs recorded at the beginning and end of the final 48 h incubation period. As medium pH increased through pH 7.6, collagen synthesis, alkaline phosphatase activity, and thymidine incorporation increased. DNA content increased from pH 7.0 to 7.2, plateaued from pH 7.2 to 7.6, and increased again from pH 7.6 to 7.8. The changes in the medium pH were greatest at pHs 7.0 and 7.8, modest at pHs 7.4 and 7.6, and did not change at 7.2, suggesting that the pHs are migrating towards pH 7.2. Lactate production increased at pH 7.0 but remained constant from 7.2 to 7.8. These results suggest that in the pH range from 7.0-7.6 the activity of human osteoblasts increases with increasing pH, that this increase in activity does not require an increase in glycolytic activity, and that pH 7.2 may be the optimal pH for these 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.

Electrical signal transmission and gap junction regulation in a bone cell network: a cable model for an osteon

A cable model is formulated to estimate the spatial distribution of intracellular electric potential and current, from the cement line to the lumen of an osteon, as the frequency of the loading and the conductance of the gap junction are altered. The model predicts that the characteristic diffusion time for the spread of current along the membrane of the osteocytic processes, 0.03 sec, is nearly the same as the predicted pore pressure relaxation time in Zeng et al. (Annals of Biomedical Engineering. 1994) for the draining of the bone fluid into the osteonal canal. This approximate equality of characteristic times causes the cable to behave as a high-pass, low-pass filter cascade with a maximum in the spectral response for the intracellular potential at approximately 30 Hz. This behavior could be related to the experiments of Rubin and McLeod (Osteoporosis, Academic Press, 1996) which show that live bone appears to be selectively responsive to mechanical loading in a specific frequency range (15-30 Hz) for several species.