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

Disruption of gap junctions reduces biomarkers of decidualization and angiogenesis and increases inflammatory mediators in human endometrial stromal cell cultures

CONTEXT

Uterine decidualization is critical to embryonic implantation and sustained pregnancy.

OBJECTIVE

To evaluate the role of gap junction intercellular communications and connexin (Cx) proteins in the morphological and biochemical differentiation of decidualized human endometrial stromal cell (ESC) cultures.

DESIGN

Translational cell biological study.

SETTING

Academic medical center.

PATIENTS

Endometrial tissue was provided by five healthy reproductive age women on no hormonal medication, undergoing laparoscopy in the early proliferative phase of the menstrual cycle.

INTERVENTIONS

Endometrial biopsy under general anesthesia, establishment and decidualization of ESC with 10 nM 17β-estradiol, 100 nM progesterone and 0.5 mM dibutyryl-cAMP (E/P/c), and manipulation of gap junctions in vitro via a combination of pharmacological or transgenic approaches.

MAIN OUTCOME MEASURES

Decidualized ESC evaluated morphologically for epithelioid transformation, gap junctions by dye diffusion and Cx43, prolactin, VEGF and IL-6 expression by RT-PCR, Western and ELISA methods.

RESULTS

Cx43 accumulation and functional gap junctions between decidualized ESC increase concomitantly with morphological differentiation following E/P/c treatment. Disruption of gap junctions using pharmacological inhibitors or Cx43 shRNA prevents morphological differentiation and inhibits prolactin and VEGF secretion. By contrast, IL-6 secretion from decidualized ESC is augmented by both approaches.

CONCLUSIONS

The findings suggest that decidualized ESC function as a coordinated secretory organ to regulate embryonic implantation via intercellular cooperation mediated by gap junctions. When adjacent cells can communicate through these junctions, decidual prolactin and VEGF secretion appears to be optimized for vascular development of the placental bed. Conversely, when intercellular communications are disrupted, angiogenesis is impaired and an inflammatory state is induced.

Inhibition of osteoblastic differentiation by warfarin and 18-α-glycyrrhetinic acid

Anticoagulation therapy with vitamin K antagonists such as warfarin is widely used to prevent and treat stroke in patients with chronic atrial fibrillation or mechanical heart valves. Because vitamin K is an essential factor for ggg-carboxylation of osteocalcin, vitamin K antagonists might cause bone loss. Although the association between warfarin use and bone metabolism is still controversial, several studies show that bone mineral density is decreased and fracture risk is increased with warfarin therapy. Meanwhile, attenuation of gap junctional communication (GJC) by warfarin is reported in rat liver epithelial cells. However, the effect of warfarin on osteoblasts, in which GJC is important for osteoblastic differentiation, remains unknown. Here we investigated whether warfarin has an inhibitory effect on osteoblastic differentiation using an osteoblastic cell line (C2C12). Warfarin and 18-α-glycyrrhetinic acid (AGA), which is known as a nontoxic reversible GJC inhibitor, had the same effect on osteoblastic differentiation. Warfarin and AGA inhibited the bone morphogenetic protein (BMP)2-induced mRNA levels of alkaline phosphatase (ALP), collagen I α1, osteocalcin (OC) and osterix, which are specific markers for osteoblastic differentiation, in a dose-dependent manner. Moreover, the activities of OC- and ALP-luciferase reporters, which are induced by BMP2, and the transcriptional activity of Runx2 on OC and ALP promoters were inhibited by warfarin and AGA. The amount and activity of ALP induced by BMP2 were also decreased by warfarin and AGA. These results suggest that warfarin and AGA, a GJC inhibitor, have an inhibitory effect on osteoblastic differentiation.

Enhanced osteoclastic resorption and responsiveness to mechanical load in gap junction deficient bone

Emerging evidence suggests that connexin mediated gap junctional intercellular communication contributes to many aspects of bone biology including bone development, maintenance of bone homeostasis and responsiveness of bone cells to diverse extracellular signals. Deletion of connexin 43, the predominant gap junction protein in bone, is embryonic lethal making it challenging to examine the role of connexin 43 in bone in vivo. However, transgenic murine models in which only osteocytes and osteoblasts are deficient in connexin 43, and which are fully viable, have recently been developed. Unfortunately, the bone phenotype of different connexin 43 deficient models has been variable. To address this issue, we used an osteocalcin driven Cre-lox system to create osteoblast and osteocyte specific connexin 43 deficient mice. These mice displayed bone loss as a result of increased bone resorption and osteoclastogenesis. The mechanism underlying this increased osteoclastogenesis included increases in the osteocytic, but not osteoblastic, RANKL/OPG ratio. Previous in vitro studies suggest that connexin 43 deficient bone cells are less responsive to biomechanical signals. Interestingly, and in contrast to in vitro studies, we found that connexin 43 deficient mice displayed an enhanced anabolic response to mechanical load. Our results suggest that transient inhibition of connexin 43 expression and gap junctional intercellular communication may prove a potentially powerful means of enhancing the anabolic response of bone to mechanical loading.

Promotion of osteoblast differentiation in 3D biomaterial micro-chip arrays comprising fibronectin-coated poly(methyl methacrylate) polycarbonate

Due to the architecture of solid body tissues including bone, three-dimensional (3D) in vitro microenvironments appear favorable, since herein cell growth proceeds under more physiological conditions compared to conventional 2D systems. In the present study we show that a 3D microenvironment comprising a fibronectin-coated PMMA/PC-based micro-chip promotes differentiation of primary human osteoblasts as reflected by the densely-packed 3D bone cell aggregates and expression of biomarkers indicating osteoblast differentiation. Morphogenesis and fluorescence dye-based live/dead staining revealed homogenous cell coverage of the microcavities of the chip array, whereat cells showed high viability up to 14 days. Moreover, Azur II staining proved formation of uniform sized multilayered aggregates, exhibiting progressive intracellular deposition of extracellular bone matrix constituents comprising fibronectin, osteocalcin and osteonectin from day 7 on. Compared to 2D monolayers, osteoblasts grown in the 3D chip environment displayed differential mostly higher gene expression for osteocalcin, osteonectin, and alkaline phosphatase, while collagen type I remained fairly constant in both culture environments. Our results indicate that the 3D microenvironment, based on the PMMA biomaterial chip array promotes osteoblast differentiation, and hereby renders a promising tool for tissue-specific in vitro preconditioning of osteoblasts designated for clinically-oriented bone augmentation or regeneration.

Premature osteoblast clustering by enamel matrix proteins induces osteoblast differentiation through up-regulation of connexin 43 and N-cadherin

In recent years, enamel matrix derivative (EMD) has garnered much interest in the dental field for its apparent bioactivity that stimulates regeneration of periodontal tissues including periodontal ligament, cementum and alveolar bone. Despite its widespread use, the underlying cellular mechanisms remain unclear and an understanding of its biological interactions could identify new strategies for tissue engineering. Previous in vitro research has demonstrated that EMD promotes premature osteoblast clustering at early time points. The aim of the present study was to evaluate the influence of cell clustering on vital osteoblast cell-cell communication and adhesion molecules, connexin 43 (cx43) and N-cadherin (N-cad) as assessed by immunofluorescence imaging, real-time PCR and Western blot analysis. In addition, differentiation markers of osteoblasts were quantified using alkaline phosphatase, osteocalcin and von Kossa staining. EMD significantly increased the expression of connexin 43 and N-cadherin at early time points ranging from 2 to 5 days. Protein expression was localized to cell membranes when compared to control groups. Alkaline phosphatase activity was also significantly increased on EMD-coated samples at 3, 5 and 7 days post seeding. Interestingly, higher activity was localized to cell cluster regions. There was a 3 fold increase in osteocalcin and bone sialoprotein mRNA levels for osteoblasts cultured on EMD-coated culture dishes. Moreover, EMD significantly increased extracellular mineral deposition in cell clusters as assessed through von Kossa staining at 5, 7, 10 and 14 days post seeding. We conclude that EMD up-regulates the expression of vital osteoblast cell-cell communication and adhesion molecules, which enhances the differentiation and mineralization activity of osteoblasts. These findings provide further support for the clinical evidence that EMD increases the speed and quality of new bone formation in vivo.

Regulation of gap junction function and Connexin 43 expression by cytochrome P450 oxidoreductase (CYPOR)

Cytochrome P450 oxidoreductase (CYPOR) is a microsomal electron-transferring enzyme containing both FAD and FMN as co-factors, which provides the reducing equivalents to various redox partners, such as cytochromes P450 (CYPs), heme oxygenase (HO), cytochrome b(5) and squalene monooxygenase. Human patients with severe forms of CYPOR mutation show bone defects such as cranio- and humeroradial synostoses and long bone fractures, known as Antley-Bixler-like Syndrome (ABS). To elucidate the role of CYPOR in bone, we knocked-down CYPOR in multiple osteoblast cell lines using RNAi technology. In this study, knock-down of CYPOR decreased the expression of Connexin 43 (Cx43), known to play a critical role in bone formation, modeling, and remodeling. Knock-down of CYPOR also decreased Gap Junction Intercellular Communication (GJIC) and hemichannel activity. Promoter luciferase assays revealed that the decrease in expression of Cx43 in CYPOR knock-down cells was due to transcriptional repression. Primary osteoblasts isolated from bone specific Por knock-down mice calvariae confirmed the findings in the cell lines. Taken together, our study provides novel insights into the regulation of gap junction function by CYPOR and suggests that Cx43 may play an important role(s) in CYPOR-mediated bone defects seen in patients.

Connexin 43 is a potential prognostic biomarker for ewing sarcoma/primitive neuroectodermal tumor

Connexins (Cxs) are building unit proteins of gap junctions (GJs) that are prognostic markers in carcinomas. To investigate the role of Cx in Ewing sarcoma (EWS)/primitive neuroectodermal tumor (PNET), we examined the expression of Cx43 and Cx26 in 36 EWS/PNETs and found (1) cytoplasmic Cx43 reactivity in 28/36 (78%) cases. (2) Cx43 score was significantly correlated with overall survival (P = .025). The average scores for patients alive and dead at 3 years are 46.08 and 96.98 (P = .004) at 5 years are 46.06 and 96.42 (P = .002). (3) Metastasis had a significant effect on the overall survival (P = .003). (4) Cytoplasmic Cx26 reactivity was detected in 2 of 36 (6%) patients who died with metastasis. Our results suggest a possible oncogenic and prognostic role for Cx43 and Cx26 in EWS/PNET. The lack of membranous immunoreactivity suggests that the effect of Cx in EWS/PNET is via a GJ function-independent mechanism.

Flow perfusion culture of human mesenchymal stem cells on coralline hydroxyapatite scaffolds with various pore sizes

Bone grafts are widely used in orthopaedic reconstructive surgery, but harvesting of autologous grafts is limited due to donor site complications. Bone tissue engineering is a possible alternative source for substitutes, and to date, mainly small scaffold sizes have been evaluated. The aim of this study was to obtain a clinically relevant substitute size using a direct perfusion culture system. Human bone marrowderived mesenchymal stem cells were seeded on coralline hydroxyapatite scaffolds with 200 μm or 500 μm pores, and resulting constructs were cultured in a perfusion bioreactor or in static culture for up to 21 days and analysed for cell distribution and osteogenic differentiation using histological stainings, alkaline phosphatase activity assay, and real-time RT-PCR on bone markers. We found that the number of cells was higher during static culture at most time points and that the final number of cells was higher in 500 μm constructs as compared with 200 μm constructs. Alkaline phosphatase enzyme activity assays and real time RT-PCR on seven osteogenic markers showed that differentiation occurred primarily and earlier in statically cultured constructs with 200 μm pores compared with 500 μm ones. Adhesion and proliferation of the cells was seen on both scaffold sizes, but the vitality and morphology of cells changed unfavorably during perfusion culture. In contrast to previous studies using spinner flask that show increased cellularity and osteogenic properties of cells when cultured dynamically, the perfusion culture in our study did not enhance the osteogenic properties of cell/scaffold constructs. The statically cultured constructs showed increasing cell numbers and abundant osteogenic differentiation probably because of weak initial cell adhesion due to the surface morphology of scaffolds. Our conclusion is that the specific scaffold surface microstructure and culturing system flow dynamics has a great impact on cell distribution and proliferation and on osteogenic differentiation, and the data presented warrant careful selection of in vitro culture settings to meet the specific requirements of the scaffolds and cells, especially when natural biomaterials with varying morphology are used.

Gap Junctions and Biophysical Regulation of Bone Cells

Communication between osteoblasts, osteoclasts, and osteocytes is integral to their ability to build and maintain the skeletal system and respond to physical signals. Various physiological mechanisms, including nerve communication, hormones, and cytokines, play an important role in this process. More recently, the important role of direct, cell-cell communication via gap junctions has been established. In this review, we demonstrate the integral role of gap junctional intercellular communication (GJIC) in skeletal physiology and bone cell mechanosensing.

Rescuing loading induced bone formation at senescence

The increasing incidence of osteoporosis worldwide requires anabolic treatments that are safe, effective, and, critically, inexpensive given the prevailing overburdened health care systems. While vigorous skeletal loading is anabolic and holds promise, deficits in mechanotransduction accrued with age markedly diminish the efficacy of readily complied, exercise-based strategies to combat osteoporosis in the elderly. Our approach to explore and counteract these age-related deficits was guided by cellular signaling patterns across hierarchical scales and by the insight that cell responses initiated during transient, rare events hold potential to exert high-fidelity control over temporally and spatially distant tissue adaptation. Here, we present an agent-based model of real-time Ca(2+)/NFAT signaling amongst bone cells that fully described periosteal bone formation induced by a wide variety of loading stimuli in young and aged animals. The model predicted age-related pathway alterations underlying the diminished bone formation at senescence, and hence identified critical deficits that were promising targets for therapy. Based upon model predictions, we implemented an in vivo intervention and show for the first time that supplementing mechanical stimuli with low-dose Cyclosporin A can completely rescue loading induced bone formation in the senescent skeleton. These pre-clinical data provide the rationale to consider this approved pharmaceutical alongside mild physical exercise as an inexpensive, yet potent therapy to augment bone mass in the elderly. Our analyses suggested that real-time cellular signaling strongly influences downstream bone adaptation to mechanical stimuli, and quantification of these otherwise inaccessible, transient events in silico yielded a novel intervention with clinical potential.

Biofunctionalization of dispense-plotted hydroxyapatite scaffolds with peptides: quantification and cellular response

Hydroxyapatite (HA) ceramic is a widely used synthetic bone substitute material for the regeneration of bone defects. We manufactured HA scaffolds with adjustable pore sizes and pore geometry by dispense-plotting. In addition, we attached peptides covalently onto the HA surface and are able to simultaneously quantify the amount of covalently attached and adsorbed peptide down to the picomolar range with a novel fluorescence-based detection method. In cell culture assays with stromal bone marrow cells, we observed a positive effect of biofunctionalization on cell differentiation after 21 days of culture when comparing the scaffold functionalized with the RGD motif containing adhesion peptide to an unmodified scaffold.

Effect of endothelin-1 on osteoblastic differentiation is modified by the level of connexin43: comparative study on calvarial osteoblastic cells isolated from Cx43+/- and Cx43+/+ mice

Bone is a dynamic tissue that undergoes a precise remodeling process involving resorptive osteoclastic cells and bone-forming osteoblastic (OB) cells. The functional imbalance of either of these cell types can lead to severe skeletal diseases. The proliferation and differentiation of OB cells play a major role in bone development and turnover. These cellular processes are coordinated by connexin43 (Cx43)-based gap-junctional intercellular communication (GJIC) and by soluble factors such as endothelin-1 (ET-1). We have used the Cx43 heterozygous (Cx43(+/-)) murine model to study the possible cross-talk between Cx43 and ET-1 in cultured calvarial OB cells. On microcomputed tomographic analysis of 3-day-old pups, Cx43(+/-) mice showed hypomineralized calvaria in comparison with their Cx43(+/+) littermates. Characterization of cultured OB cells clearly demonstrated the effect of the partial deletion of the Cx43 gene on its expression, on GJIC, and subsequently on OB differentiation. In this model, ET-1 (10(-8) M) lost its mitogenic action in Cx43(+/-) OB cells compared with Cx43(+/+) cells. Moreover, a correlation between the inhibition of cell differentiation by ET-1 and the decreased amount and function of Cx43 was found in Cx43(+/+) OB cells but not in their Cx43(+/-) counterparts. Thus, as Cx43 is linked to OB differentiation, our data indicate that this mitogenic ET-1 peptide has pronounced effects on fully differentiated OB cells. With respect to roles in mechanotransduction and OB differentiation, Cx43 might modulate osteoblastic sensitivity to soluble factors.

beta-Tricalcium phosphate promotes cell proliferation, osteogenesis and bone regeneration in intrabony defects in dogs

OBJECTIVE

This study investigates the effect of the new synthetic bone grafting material, high pure-phase beta-tricalcium phosphate (Cerasorb(1) M, granule size 500-1000microm), on the osteogenesis process and proliferation marker in bone marrow stromal cells (BMSCs) and its regenerative effect in the periodontal intrabony defects in dogs.

DESIGN

The effect of Cerasorb(1) M (20 and 40mgml(-1) for 1 and 2 weeks) on the proliferation rate of BMSCs was assessed by 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl-2H-tetrazolium bromide (MTT) assay on the proliferating cell nuclear antigen (PCNA) by immunoblotting and on alkaline phosphatase level by colourimetric assay. The regenerative effect of Cerasorb(1) M in the periodontal intrabony defects in dogs was investigated by histological and immunohistochemical analysis after 3 and 6 months of grafting.

RESULTS

Incubation of BMSCs with Cerasorb(1) M for 2 weeks led to significant increase in cell proliferation rate, which was associated with increased PCNA. Cerasorb(1) M significantly increased the production of alkaline phosphatase as a marker for the osteogenic stromal lineage and for differentiation and bone formation in BMSCs after 2 weeks. In the histological features and immunohistochemical analysis of PCNA of the intrabony defects in dogs augmented with Cerasorb(1) M, osteoid tissue with a plate-like structure and cellular mesenchymal proliferation besides osteoid islands joined by bridges were observed after 3 months. Six months after the implantation, the Cerasorb(1) M granules were replaced by abundant new plate-like bone besides PCNA-enriched, small, oval-shaped mononuclear cells and multinucleated-giant cells that were attached to newly formed bones. No remains of the Cerasorb(1) M granules could be seen after 3 and 6 months with the newly formed plate-like bones and no histological sign of inflammatory reaction or formation of foreign-body granulomas.

CONCLUSION

Cerasorb(1) M may induce cell proliferation via induction of PCNA that may induce early osteogenesis and bone formation. Cerasorb(1) M regenerated the bone completely in intrabony defects and that this regeneration was highly associated with PCNA expression in different cell lineage.

Expression of connexins in normal and neoplastic canine bone tissue

Previous studies showed that intercellular communication by gap junctions has a role in bone formation. The main connexin involved in the development, differentiation, and regulation of bone tissue is connexin (Cx) 43. In addition, Cx46 is also expressed, mostly localized within the trans-Golgi region. Alterations in the expression pattern and aberrant location of these connexins are associated with oncogenesis, demonstrating a deficient gap junctional intercellular communication (GJIC) capacity in neoplastic tissues. In this study, we evaluated normal and neoplastic bone tissues regarding the expression of Cx43 and Cx46 by immunofluorescence, gene expression of these connexins by real-time PCR, and their correlation with cell proliferation index and deposition of collagen. Fourteen neoplastic bone lesions, including 13 osteosarcomas and 1 multilobular tumor of bone, were studied. The mRNA levels of Cx43 were similar between normal and neoplastic bone tissue. In normal bone tissue, the Cx43 protein was found mainly in the intercellular membranes. However, in all bone tumors studied here, the Cx43 was present in both cell membranes and also aberrantly in the cytoplasm. Regarding only tumor samples, we determined a possible inverse correlation between Cx43 expression and cellular proliferation, although a positive correlation between Cx43 expression and collagen deposition was also noted. In contrast, Cx46 had lower levels of expression in neoplastic bone tissues when compared with normal bone and was found retained in the perinuclear region. Even though there are differences between these two connexins regarding expression in neoplastic versus normal tissues, we concluded that there are differences regarding the subcellular location of these connexins in normal and neoplastic dog bone tissues and suggest a possible correlation between these findings and some aspects of cellular proliferation and possibly differentiation.

Gap junctions and osteoblast-like cell gene expression in response to fluid flow

Bone formation occurs in vivo in response to mechanical stimuli, but the signaling pathways involved remain unclear. The ability of bone cells to communicate with each other in the presence of an applied load may influence the overall osteogenic response. The goal of this research was to determine whether inhibiting cell-to-cell gap junctional communication between bone-forming cells would affect the ensemble cell response to an applied mechanical stimulus in vitro. In this study, we investigated the effects of controlled oscillatory fluid flow (OFF) on osteoblastic cells in the presence and the absence of a gap-junction blocker. MC3T3-E1 Clone 14 cells in a monolayer were exposed to 2 h of OFF at a rate sufficient to create a shear stress of 20 dyne/cm(2) at the cell surface, and changes in steady-state mRNA levels for a number of key proteins known to be involved in osteogenesis were measured. Of the five proteins investigated, mRNA levels for osteopontin (OPN) and osteocalcin were found to be significantly increased 24 h postflow. These experiments were repeated in the presence of 18 beta-glycyrrhetinic acid (BGA), a known gap-junction blocker, to determine whether gap-junction intercellular communication is necessary for this response. We found that the increase in OPN mRNA levels is not observed in the presence of BGA, suggesting that gap junctions are involved in the signaling process. Interestingly, enzyme linked immunosorbent assay data showed that levels of secreted OPN protein increased 48 h postflow and that this increase was unaffected by the presence of intact gap junctions.

Gap junction intercellular communication: a review of a potential platform to modulate craniofacial tissue engineering

Defects in craniofacial tissues, resulting from trauma, congenital abnormalities, oncologic resection or progressive deforming diseases, may result in aesthetic deformity, pain and reduced function. Restoring the structure, function and aesthetics of craniofacial tissues represents a substantial clinical problem in need of new solutions. More biologically-interactive biomaterials could potentially improve the treatment of craniofacial defects, and an understanding of developmental processes may help identify strategies and materials that can be used in tissue engineering. One such strategy that can potentially advance tissue engineering is cell-cell communication. Gap junction intercellular communication is the most direct way of achieving such signaling. Gap junction communication through connexin-mediated junctions, in particular connexin 43 (Cx43), plays a major role bone development. Given the important role of Cx43 in controlling development and differentiation, especially in bone cells, controlling the expression of Cx43 may provide control over cell-to-cell communication and may help overcome some of the challenges in craniofacial tissue engineering. Following a review of gap junctions in bone cells, the ability to enhance cell-cell communication and osteogenic differentiation via control of gap junctions is discussed, as is the potential utility of this approach in craniofacial tissue engineering.

The role of gap junctions in megakaryocyte-mediated osteoblast proliferation and differentiation

Gap junctions (GJs) are membrane-spanning channels that facilitate intercellular communication by allowing small signaling molecules (e.g. calcium ions, inositol phosphates, and cyclic nucleotides) to pass from cell to cell. Over the past two decades, many studies have described a role for GJ intercellular communication (GJIC) in the proliferation and differentiation of many cells, including bone cells. Recently, we reported that megakaryocytes (MKs) enhance osteoblast (OB) proliferation by a juxtacrine signaling mechanism. Here we determine whether this response is facilitated by GJIC. First we demonstrate that MKs express connexin 43 (Cx43), the predominant GJ protein expressed by bone cells, including OBs. Next, we provide data showing that MKs can communicate with OBs via GJIC, and that the addition of two distinct GJ uncouplers, 18alpha-glycyrrhetinic acid (alphaGA) or oleamide, inhibits this communication. We then demonstrate that inhibiting MK-mediated GJIC further enhances the ability of MKs to stimulate OB proliferation. Finally, we show that while culturing MKs with OBs reduces gene expression of several differentiation markers/matrix proteins (type I collagen, osteocalcin, and alkaline phosphatase), reduces alkaline phosphatase enzymatic activity, and decreases mineralization in OBs, blocking GJIC does not result in MK-induced reductions in OB gene expression, enzymatic levels, or mineralized nodule formation. Overall, these data provide evidence that GJIC between MKs and OBs is functional, and that inhibiting GJIC in MK-OB cultures enhances OB proliferation without apparently altering differentiation when compared to similarly treated OB cultures. Thus, these observations regarding MK-OB GJIC inhibition may provide insight regarding potential novel targets for anabolic bone formation.

Connexin 43 is required for the anti-apoptotic effect of bisphosphonates on osteocytes and osteoblasts in vivo

Connexin (Cx)43 is required for inhibition of osteocyte and osteoblast apoptosis by bisphosphonates in vitro. Herein, we evaluated its requirement for the in vivo actions of bisphosphonates using mice in which Cx43 was deleted specifically from osteocytes and osteoblasts (Cx43(DeltaOb-Ot/-) mice). Effective removal of Cx43 was confirmed by the presence of the deleted form of the gene and by reduced mRNA and protein expression in osteoblastic cells and bones obtained from Cx43(DeltaOb-Ot/-) mice. The amino-bisphosphonate alendronate (2.3 micromol/kg/d) was injected daily into 5-mo-old female mice (n = 6-11) for 31 days, starting 3 days before implantation of pellets releasing the glucocorticoid prednisolone (2.1 mg/kg/d). Cx43(DeltaOb-Ot/-) mice and their littermates (Cx43(fl/-), Cx43(DeltaOb-Ot/+), and Cx43(fl/+)) gained bone with similar kinetics and exhibited identical bone mass from 2 to 4.5 mo of age, indicating that Cx43 deletion from osteocytes and mature osteoblasts does not impair bone acquisition. In addition, prednisolone induced a similar increase in osteocyte and osteoblast apoptosis in Cx43(DeltaOb-Ot/-) or in control Cx43(fl/-) littermates. However, whereas alendronate prevented prednisolone-induced apoptosis in control Cx43(fl/-) mice, it was ineffective in Cx43(DeltaOb-Ot/-) mice. In contrast, alendronate inhibited glucocorticoid-induced bone loss in both type of animals, suggesting that inhibition of resorption is the predominant effect of alendronate against the early phase of glucocorticoid-induced bone loss. Taken together with earlier in vitro evidence, these findings show that Cx43 is required for the anti-apoptotic effect of bisphosphonates on osteocytes and osteoblasts.

Collagen nanofibres are a biomimetic substrate for the serum-free osteogenic differentiation of human adipose stem cells

Electrospinning has recently gained widespread attention as a process capable of producing nanoscale fibres that mimic native extracellular matrix. In this study, we compared the osteogenic differentiation behaviour of human adipose stem cells (ASCs) on a 3D nanofibre matrix of type I rat tail collagen (RTC) and a 2D RTC collagen-coated substrate, using a novel serum-free osteogenic medium. The serum-free medium significantly enhanced the numbers of proliferating cells in culture, compared to ASCs in traditional basal medium containing 10% animal serum, highlighting a potential clinical role for in vitro stem cell expansion. Osteogenic differentiation behaviour was assessed at days 7, 14 and 21 using quantitative real-time RT-PCR analysis of the osteogenic genes collagen I (Coll I), alkaline phosphatase (ALP), osteopontin (OP), osteonectin (ON), osteocalcin (OC) and core-binding factor-alpha (cbfa1). All genes were upregulated (>one-fold) in ASCs cultured on nanofibre scaffolds over 2D collagen coatings by day 21. Synthesis of mineralized extracellular matrix on the scaffolds was assessed on day 21 with Alizarin red staining. These studies demonstrate that 3D nanoscale morphology plays a critical role in regulating cell fate processes and in vitro osteogenic differentiation of ASCs under serum-free conditions.

Final report on the safety assessment of Glycyrrhetinic Acid, Potassium Glycyrrhetinate, Disodium Succinoyl Glycyrrhetinate, Glyceryl Glycyrrhetinate, Glycyrrhetinyl Stearate, Stearyl Glycyrrhetinate, Glycyrrhizic Acid, Ammonium Glycyrrhizate, Dipotassium Glycyrrhizate, Disodium Glycyrrhizate, Trisodium Glycyrrhizate, Methyl Glycyrrhizate, and Potassium Glycyrrhizinate

Glycyrrhetinic Acid and its salts and esters and Glycyrrhizic Acid and its salts and esters are cosmetic ingredients that function as flavoring agents or skin-conditioning agents - miscellaneous or both. These chemicals may be isolated from licorice plants. Glycyrrhetinc Acid is described as at least 98% pure, with 0.6% 24-OH-Glycyrrhetinic Acid, not more than 20 mu g/g of heavy metals and not more than 2 mu g/g of arsenic. Ammonium Glycyrrhizate has been found to be at least 98% pure and Dipotassium Glycyrrhizate has been found to be at least 95% pure. Glycyrrhetinic Acid is used in cosmetics at concentrations of up to 2%; Stearyl Glycyrrhetinate, up to 1%; Glycyrrhizic Acid, up to 0.1%; Ammonium Glycyrrhizate, up to 5%; Dipotassium Glycyrrhizate, up to 1%; and Potassium Glycyrretinate, up to 1%. Although Glycyrrhizic Acid is poorly absorbed by the intestinal tract, it may be hydrolyzed to Glycyrrhetinic Acid by a beta -glucuronidase produced by intestinal bacteria. Glycyrrhetinic Acid and Glycyrrhizic Acid bind to rat and human albumin, but do not absorb well into tissues. Glycyrrhetinic Acid and Glycyrrhizic Acid and metabolites are mostly excreted in the bile, with very little excreted in urine. Dipotassium Glycyrrhizate was undetectable in the receptor chamber when tested for transepidermal permeation through pig skin. Glycyrrhizic Acid increased the dermal penetration of diclofenac sodium in rat skin. Dipotassium Glycyrrhizate increased the intestinal absorption of calcitonin in rats. In humans, Glycyrrhetinic Acid potentiated the effects of hydrocortisone in the skin. Moderate chronic or high acute exposure to Glycyrrhizic Acid, Ammonium Glycyrrhizate, and their metabolites have been demonstrated to cause transient systemic alterations, including increased potassium excretion, sodium and water retention, body weight gain, alkalosis, suppression of the renin-angiotensis-aldosterone system, hypertension, and muscular paralysis; possibly through inhibition of 11beta -hydroxysteroid dehydrogenase-2 (11beta -OHSD2) in the kidney. Glycyrrhetinic Acid and its derivatives block gap junction intracellular communication in a dose-dependent manner in animal and human cells, including epithelial cells, fibroblasts, osteoblasts, hepatocytes, and astrocytes; at high concentrations, it is cytotoxic. Glycyrrhetinic Acid and Glycyrrhizic Acid protect liver tissue from carbon tetrachloride. Glycyrrhizic Acid has been used to treat chronic hepatitis, inhibiting the penetration of the hepatitis A virus into hepatocytes. Glycyrrhetinic Acid and Glycyrrhizic Acid have anti-inflammatory effects in rats and mice. The acute intraperitoneal LD(50) for Glycyrrhetinic Acid in mice was 308 mg/kg and the oral LD(50) was > 610 mg/kg. The oral LD(50) in rats was reported to be 610 mg/kg. Higher LD(50) values were generally reported for salts. Little short-term, subchronic, or chronic toxicity was seen in rats given ammonium, dipotassium, or disodium salts of Glycyrrhizic Acid. Glycyrrhetinic Acid was not irritating to shaved rabbit skin, but was considered slightly irritating in an in vitro test. Glycyrrhetinic Acid inhibited the mutagenic activity of benzo[a]pyrene and inhibited tumor initiation and promotion by other agents in mice. Glycyrrhizic Acid inhibited tumor initiation by another agent, but did not prevent tumor promotion in mice. Glycyrrhizic Acid delayed mortality in mice injected with Erlich ascites tumor cells, but did not reduce the mortality rate. Ammonium Glycyrrhizate was not genotoxic in in vivo and in vitro cytogenetics assays, the dominant lethal assay, an Ames assay, and heritable translocation tests, except for possible increase in dominant lethal mutations in rats given 2000 mg/kg day(-1) in their diet. Disodium Glycyrrhizate was not carcinogenic in mice in a drinking water study at exposure levels up to 12.2 mg/kg day(-1) for 96 weeks. Glycyrrhizate salts produced no reproductive or developmental toxicity in rats, mice, golden hamsters, or Dutch-belted rabbits, except for a dose-dependent increase (at 238.8 and 679.9 mg/kg day(-1)) in sternebral variants in a study using rats. Sedation, hypnosis, hypothermia, and respiratory depression were seen in mice given 1250 mg/kg Glycyrrhetinic Acid intraperitoneally. Rats fed a powdered diet containing up to 4% Ammonium Glycyrrhizate had no treatment related effects in motor function tests, but active avoidance was facilitated at 4%, unaffected at 3%, and depressed at 2%. In a study of 39 healthy volunteers, a no effect level of 2 mg/kg/day was determined for Glycyrrhizic Acid given orally for 8 weeks. Clinical tests in seven normal individuals given oral Ammonium Glycyrrhizate at 6 g/day for 3 days revealed reduced renal and thermal sweat excretion of Na+ and K+, but carbohydrate and protein metabolism were not affected. Glycyrrhetinic Acid at concentrations up to 6% was not a skin irritant or a sensitizer in clinical tests. Neither Glycyrrhizic Acid, Ammonium Glycyrrhizate, nor Dipotassium Glycyrrhizate at 5% were phototoxic agents or photosensitizers. Birth weight and maternal blood pressure were unrelated to the level of consumption of Glycyrrhizic Acid in 1049 Finnish women with infants, but babies whose mother consumed > 500 mg/wk were more likely to be born before 38 weeks. The Cosmetic Ingredient Review (CIR) Expert Panel noted that the ingredients in this safety assessment are not plant extracts, powders, or juices, but rather are specific chemical species that may be isolated from the licorice plant. Because these chemicals may be isolated from plant sources, however, steps should be taken to assure that pesticide and toxic metal residues are below acceptable levels. The Panel advised the industry that total polychlorobiphenyl (PCB)/pesticide contamination should be limited to not more than 40 ppm, with not more than 10 ppm for any specific residue, and that toxic metal levels must not contain more than 3 mg/kg of arsenic (as As), not more than 0.002% heavy metals, and not more than 1 mg/kg of lead (as Pb). Although the Panel noted that Glycyrrhizic Acid is cytotoxic at high doses and ingestion can have physiological effects, there is little acute, short-term, subchronic, or chronic toxicity and it is expected that these ingredients would be poorly absorbed through the skin. These ingredients are not considered to be irritants, sensitizers, phototoxic agents, or photosensitizers at the current maximum concentration of use. Accordingly, the CIR Expert Panel concluded that these ingredients are safe in the current practices of use and concentration. The Panel recognizes that certain ingredients in this group are reportedly used in a given product category, but the concentration of use is not available. For other ingredients in this group, information regarding use concentration for specific product categories is provided, but the number of such products is not known. In still other cases, an ingredient is not in current use, but may be used in the future. Although there are gaps in knowledge about product use, the overall information available on the types of products in which these ingredients are used and at what concentration indicate a pattern of use. Within this overall pattern of use, the Expert Panel considers all ingredients in this group to be safe.

Interaction between human umbilical vein endothelial cells and human osteoprogenitors triggers pleiotropic effect that may support osteoblastic function

Osteogenesis occurs in striking interaction with angiogenesis. There is growing evidence that endothelial cells are involved in the modulation of osteoblast differentiation. We hypothesized that primary human umbilical vein endothelial cells (HUVEC) should be able to modulate primary human osteoprogenitors (HOP) function in an in vitro co-culture model. In a previous study we demonstrated that a 3 day to 3 week co-culture stimulates HOP differentiation markers such as Alkaline Phosphatase (ALP) activity and mineralization. In the present study we addressed the effects induced by the co-culture on HOP within the first 48 hours. As a prerequisite, we validated a method based on immuno-magnetic beads to separate HOP from HUVEC after co-culture. Reverse transcription-real time quantitative PCR studies demonstrated up-regulation of the ALP expression in the co-cultured HOP, confirming previous results. Surprisingly, down-regulation of runx2 and osteocalcin was also shown. Western blot analysis revealed co-culture induced down-regulation of Connexin43 expression in both cell types. Connexin43 function may be altered in co-cultured HOP as well. Stimulation of the cAMP pathway was able to counterbalance the effect of the co-culture on the ALP activity, but was not able to rescue runx2 mRNA level. Co-culture effect on HOP transcriptome was analyzed with GEArray cDNA microarray showing endothelial cells may also modulate HOP extracellular matrix production. In accordance with previous work, we propose endothelial cells may support initial osteoblastic proliferation but do not alter the ability of the osteoblasts to produce extracellular mineralizing matrix.

18alpha-glycyrrhetinic acid induces phenotypic changes of skeletal muscle cells to enter adipogenesis

The importance of connexins is implicated in proliferation and differentiation of cells. In skeletal muscle cells, connexin43 (Cx43) has been identified as the major connexin, and gap-junctional communication mediated by connexins has been shown to be required for their myogenic differentiation. In addition, inhibition of connexin function has been shown to induce transdifferentiation of osteoblasts to an adipocytic phenotype. In the present study, we examined whether the inhibition of connexin function could induce phenotypic changes in skeletal muscle cells. Treatment of skeletal muscle cells with an inhibitor of connexin function, 18alpha-glycyrrhetinic acid (AGRA), resulted in a reduction in the number of MyoD-positive cells and complete inhibition of myotube formation, concomitantly with an increase in the number of C/EBPalpha-positive cells. AGRA-treated cells cultured in adipogenic differentiation medium could give rise to mature adipocytes that express both PPARgamma and C/EBPalpha. The presence of AGRA during adipogenic differentiation did not inhibit adipogenesis of skeletal muscle cells. AGRA treatment did not affect Cx43 expression in skeletal muscle cells but reduced its phosphorylation. These results indicate that inhibition of connexin function induces phenotypic changes of skeletal muscle cells to enter adipogenesis.

Endothelin1-induced Ca(2+) mobilization is altered in calvarial osteoblastic cells of Cx43(+/- ) mice

During bone remodeling, osteoblastic (OB) cells have a central role leading to the production of extracellular matrix and its subsequent mineralization. As revealed by human physiopathologies, the OB differentiation process is essential for the control of calcium metabolism and normal bone formation. Moreover, accumulating data in the field of bone development suggest that connexin 43 (Cx43)-mediated gap junctional communication plays an important role in OB differentiation and function. Since Ca(2+) has a central role in OB physiology, the aim of the present study was to investigate the hypothetical involvement of Cx43 in OB calcium homeostasis. We performed measurements of intracellular calcium activity ([Ca(2+)]( i )) by a cytofluorimetric method using Fluo-4 as a calcium indicator and endothelin-1 (ET-1) as a physiological calcium-mobilizing factor on cultured OB cells isolated from calvaria of Cx43(+/-) and Cx43(+/+) mice. Partial deletion of the Cx43 gene induced a significant decrease in the [Ca(2+)]( i ) rise elicited by ET-1. This reduction was not correlated to a decrease or a modification of ET receptor subtype expression as assessed by real-time reverse-transcription polymerase chain reaction. Pharmacological investigations led us to demonstrate that the significant difference in [Ca(2+)]( i ) peak amplitude during the ET-1 action was associated with decreased calcium influx involving L-type voltage-sensitive calcium channels, whereas calcium release from intracellular stores and implication of phospholipase C were not affected by the reduced expression of Cx43. In conclusion, our data demonstrate for the first time that the Cx43 level of expression and/or function is able to modulate the [Ca(2+)]( i ) mobilization in OB cells.

Gap-junctional communication is required for mitotic clonal expansion during adipogenesis

OBJECTIVE

Gap-junctional communication (GJC) plays critical roles in cell growth and differentiation. Several studies have demonstrated the involvement of GJC in myogenesis and osteogenesis; however, the role of GJC in adipogenesis has not been fully studied. Thus, we investigated the role of GJC in adipogenesis.

RESEARCH METHODS AND PROCEDURES

3T3-L1 preadipocytes were differentiated in the presence of gap junction inhibitor, 18-alpha-glycyrrhetinic acid (AGA), and accumulation of cytoplasmic triglycerides was measured. 3T3-L1 cells were transfected with 100 nM small interfering RNA duplexes targeting connexin (Cx) 43. The mRNA levels of CCAAT/enhancer-binding protein (C/EBP) alpha, peroxisome proliferator-activated receptor gamma, glucose transporter 4, C/EBPbeta, and Cx43 were measured by real-time polymerase chain reaction. The protein levels of C/EBPbeta were quantitated by Western blotting. The cell proliferation was measured by counting cell numbers, and DNA synthesis was measured by bromodeoxyuridine incorporation.

RESULTS

AGA inhibited adipocyte differentiation dose-dependently. The lipid accumulation and the mRNA levels of C/EBPalpha, peroxisome proliferator-activated receptor gamma, and glucose transporter 4 were markedly reduced in AGA-treated adipocytes. The mRNA levels of C/EBPbeta did not decrease; however, C/EBPbeta [liver-enriched transcriptional activator protein (LAP)] expression and the C/EBPbeta (LAP)-to-C/EBP [liver-enriched transcriptional inhibitory protein (LIP)] ratio were reduced by AGA treatment. The increase in both cell number and DNA synthesis, which occurs during mitotic clonal expansion, was reduced by AGA in a dose-dependent fashion. The major component of gap junctions in 3T3-L1 cells was Cx43. Down-regulation of Cx43 using small interfering RNA reduced the expression of C/EBPbeta (LAP) and inhibited adipogenesis.

DISCUSSION

Our data suggest that GJC plays some important roles in adipogenesis through inhibiting mitotic clonal expansion and modulating C/EBPbeta (LAP) expression.

Extracellular matrix formation and mineralization on a phosphate-free porous bioactive glass scaffold using primary human osteoblast (HOB) cells

Sol-gel derived bioactive glasses of the 70S30C (70mol% SiO2, 30mol% CaO) composition have been foamed to produce 3D bioactive scaffolds with hierarchical interconnected pore morphologies similar to trabecular bone. The aim of this study was to investigate primary human osteoblast response to porous bioactive glass scaffolds. The scaffolds supported osteoblast growth and induced differentiation, within the 3-week culture period, as depicted by enhanced ALPase enzymatic activity, without the addition of supplementary factors such as ascorbic acid, beta-glycerophosphate and dexamethasone. This is the first time this has been observed on a bioactive glass that does not contain phosphate. Deposition of extracellular matrix was also confirmed by enhanced production of the extracellular matrix protein collagen type I. SEM showed indications of mineralized bone nodule formation without the addition of growth factors. The 70S30C bioactive glass scaffolds therefore fulfil many of the criteria for an ideal scaffold for bone tissue engineering applications.

Inhibition of gap junction communication at ectopic Eph/ephrin boundaries underlies craniofrontonasal syndrome

Mutations in X-linked ephrin-B1 in humans cause craniofrontonasal syndrome (CFNS), a disease that affects female patients more severely than males. Sorting of ephrin-B1–positive and –negative cells following X-inactivation has been observed in ephrin-B1^+/− mice; however, the mechanisms by which mosaic ephrin-B1 expression leads to cell sorting and phenotypic defects remain unknown. Here we show that ephrin-B1^+/− mice exhibit calvarial defects, a phenotype autonomous to neural crest cells that correlates with cell sorting. We have traced the causes of calvarial defects to impaired differentiation of osteogenic precursors. We show that gap junction communication (GJC) is inhibited at ectopic ephrin boundaries and that ephrin-B1 interacts with connexin43 and regulates its distribution. Moreover, we provide genetic evidence that GJC is implicated in the calvarial defects observed in ephrin-B1^+/− embryos. Our results uncover a novel role for Eph/ephrins in regulating GJC in vivo and suggest that the pleiotropic defects seen in CFNS patients are due to improper regulation of GJC in affected tissues.

Mutations inephrin-B1 associated with X-linked CFNS result in aberrant formation of facial bone structures.Ephrin-B1 interacts with gap junctions to influence the cell sorting events that underlie the CFNS deformations.

Endothelin-1 inhibits human osteoblastic cell differentiation: Influence of connexin-43 expression level

Gap junctional intercellular communication (GJIC) permits coordinated cellular activities during developmental and differentiation processes. In bone, the involvement of the gap junctional protein, connexin-43 (Cx43), and of GJIC in osteoblastic differentiation and mineralization of the extracellular matrix has been previously demonstrated. Former studies have shown that endothelin-1 (ET-1) was also implicated in the control of osteoblastic proliferation and differentiation. However, depending on the cellular models, ET-1 has been shown to decrease or increase osteoblastic differentiation markers. As no data were available on the ET-1 effect on GJIC and Cx43 expression in osteoblastic cells, we analyzed here the possible crosstalk between Cx43 and ET-1 in a human cell line (hFOB 1.19) which displays different Cx43 expression levels and phenotypes when cultured at 33.5 or 39 degrees C. The presence of ET-1 (10(-8) M) for 2-12 days of culture did not significantly alter the proliferation rate of hFOB cells whatever their phenotype. In contrast, ET-1 induced a differential inhibitory effect on the biochemical differentiation markers (alkaline phosphatase activity and osteocalcin expression) with a significant reduction in the differentiated phenotype at 39 degrees C, whereas no effects were measured at 33.5 degrees C. The inhibitory effect was linked to a decrease of GJIC and of Cx43 both at transcriptional and protein levels. Altogether, our results suggest that Cx43 expression level could influence the action of ET-1 on human osteoblastic cell differentiation. Our data also indicate that the gap junctional protein could play a pivotal role in the response of osteoblasts to mitogenic factors implicated in bone pathologies.

Effects of sinusoidal electromagnetic field on structure and function of different kinds of cell lines

This study investigated that whether a 2 mT, 60 Hz, sinusoidal electromagnetic field (EMF) alters the structure and function of cells. This research compared the effects of EMF on four kinds of cell lines: hFOB 1.19 (fetal osteoblast), T/G HA-VSMC (aortic vascular smooth muscle cell), RPMI 7666 (B lymphoblast), and HCN-2 (cortical neuronal cell). Over 14 days, cells were exposed to EMF for 1, 3, or 6 hours per day (hrs/d). The results pointed to a cell type-specific reaction to EMF exposure. In addition, the cellular responses were dependent on duration of EMF exposure. In the present study, cell proliferation was the trait most sensitive to EMF. EMF treatment promoted growth of hFOB 1.19 and HCN-2 compared with control cells at 7 and 14 days of incubation. When the exposure time was 3 hrs/d, EMF enhanced the proliferation of RPMI 7666 but inhibited that of T/G HA- VSMC. On the other hand, the effects of EMF on cell cycle distribution, cell differentiation, and actin distribution were unclear. Furthermore, we hardly found any correlation between EMF exposure and gap junctional intercellular communication in hFOB 1.19. This study revealed that EMF might serve as a potential tool for manipulating cell proliferation.

Mesenchymal stem cell ingrowth and differentiation on coralline hydroxyapatite scaffolds

Culture of osteogenic cells on a porous scaffold could offer a new solution to bone grafting using autologous human mesenchymal stem cells (hMSC) from the patient. We compared coralline hydroxyapatite scaffolds with pore sizes of 200 and 500 microm for expansion and differentiation of hMSCs. We cultivated the hMSC statically or in spinner flasks for 1, 7, 14 and 21 days and found that the 200-microm pore scaffolds exhibited a faster rate of osteogenic differentiation than did the 500-microm pore scaffolds as shown by an alkaline phosphatase activity assay and real-time reverse transcriptase polymerase chain reaction for 10 osteogenic markers. The 500-microm scaffolds had increased proliferation rates and accommodated a higher number of cells (shown by DNA content, scanning electron microscopy and fluorescence microscopy). Thus the porosity of a 3D microporous biomaterial may be used to steer hMSC in a particular direction. We found that dynamic spinner flask cultivation of hMSC/scaffold constructs resulted in increased proliferation, differentiation and distribution of cells in scaffolds. Therefore, spinner flask cultivation is an easy-to-use inexpensive system for cultivating hMSCs on small to intermediate size 3D scaffolds.

Low peak bone mass and attenuated anabolic response to parathyroid hormone in mice with an osteoblast-specific deletion of connexin43

Connexin43 (Cx43) is involved in bone development, but its role in adult bone homeostasis remains unknown. To overcome the postnatal lethality of Cx43 null mutation, we generated mice with selective osteoblast ablation of Cx43, obtained using a Cx43fl allele and a 2.3-kb fragment of the alpha1(I) collagen promoter to drive Cre in osteoblasts (ColCre). Conditionally osteoblast-deleted ColCre;Cx43-/fl mice show no malformations at birth, but develop low peak bone mass and remain osteopenic with age, exhibiting reduced bone formation and defective osteoblast function. By both radiodensitometry and histology, bone mineral content increased rapidly and progressively in adult Cx43+/fl mice after subcutaneous injection of parathyroid hormone (PTH), an effect significantly attenuated in ColCre;Cx43-/fl mice, with Cx43-/fl exhibiting an intermediate response. Attenuation of PTH anabolic action was associated with failure to increase mineral apposition rate in response to PTH in ColCre;Cx43-/fl, despite an increased osteoblast number, suggesting a functional defect in Cx43-deficient bone-forming cells. In conclusion, lack of Cx43 in osteoblasts leads to suboptimal acquisition of peak bone mass, and hinders the bone anabolic effect of PTH. Cx43 represents a potential target for modulation of bone anabolism.

Mechanically stimulated osteocytes regulate osteoblastic activity via gap junctions

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

Sustained stromal stem cell self-renewal and osteoblastic differentiation during aging

We have reported the isolation of a unique subpopulation of human stromal cells from bone marrow termed marrow-isolated adult multilineage inducible (MIAMI) cells. The expression of embryonic stem cell markers SSEA-4, Oct-4, Rex-1, and telomerase reverse transcriptase indicates the developmentally immature status of these cells. They resemble primitive stem cells in their capacity to differentiate, at least in vitro, into mature-like cells from all three germ layers. MIAMI cells are characterized by a unique molecular profile that distinguishes them from other marrow stromal cell populations. Although the frequency of MIAMI cells, among all marrow nucleated cells, decreases from 0.01% at age 3 to 0.0018% at age 45, their numbers remain unchanged after age 45. The level of expression of the markers characteristic of MIAMI cells remains constant independent of age and gender. In long-term in vitro expansion experiments aging increased the population doubling time by about 30%, whereas specific in vitro differentiation of MIAMI cells toward osteoblastic cells was unaffected. Because the oxygen tension in bone marrow ranges from 1% to 7%, we examined the role of oxygen tension in regulating the capacity of MIAMI cells to self-renew and maintain their pluripotentiality during long-term culture. Low oxygen tension upregulated mRNAs for primitive embryonic stem cell markers. Our results suggest that maintaining developmentally primitive human cells in vitro at low oxygen tension is more physiologic and favors stemness. For osteoblastic differentiation, gap-junctional communication mediated by connexin43 is required. Its inhibition not only blocked osteoblastic differentiation but stimulated the adipocytic differentiation.

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.

Use of Natural Coralline Biomaterials As Reinforcing and Gas-Forming Agent for Developing Novel Hybrid Biomatrices: Microarchitectural and Mechanical Studies

This paper describes the first attempt in fabrication of three-dimensional macroporous composites of chitosan and natural coralline material with pore sizes of 300-400 microm, exceeding the upper pore size limit of 250 microm obtained with freeze-dried chitosan-based scaffolds. Natural coral particulates of less than 20 microm, which is mainly composed of calcium carbonate (CaCO3), was simultaneously used as reinforcing phase and gas-forming agent to obtain a structure with large pores and improved mechanical and biological properties. The reaction between the coralline material and the acidic chitosan polymer solvent, which produced carbon dioxide, was rapidly stopped by the subsequent thermally induced phase separation technique, leaving coralline particulates in the polymeric structure. Scaffolds containing five different proportions of coralline material (0, 25, 50, 75, and 100 wt%) were investigated. The coralline-chitosan weight ratio was studied for its effects on the physical properties of the scaffolds. The relation between scaffold microarchitecture and mechanical properties was assessed with scanning electron microscope (SEM), along with micro-CT imaging and compression testing. The scaffolds were used in bone marrow cell culturing experiments to assess the effect of composition on cell behavior through cell-material interaction and morphological observation by SEM. Higher coralline concentration increased the pore wall thickness and favored large pore formation. Varying the coralline particulate to chitosan polymer ratio from 0 to 75 wt% increased the average pore size from 80 microm to 400 microm while the porosity decreased from 91% to 78%. The compressive modulus was improved proportionally with the coralline content, and the 75 wt% composites had a significantly higher modulus than other chitosan-based scaffold groups. More cells were observed on scaffolds with higher coralline content. The cell culture experiments indicated that the scaffolds containing coralline material might have a high cell affinity, since it allowed fast cell attachment and spreading.

Gap junctions in skeletal development and function

Gap junctions play a critical role in the coordinated function and activity of nearly all of the skeletal cells. This is not surprising, given the elaborate orchestration of skeletal patterning, bone modeling and subsequent remodeling, as well as the mechanical stresses, strains and adaptive responses that the skeleton must accommodate. Much remains to be learned regarding the role of gap junctions and hemichannels in these processes. A common theme is that without connexins none of the cells of bone function properly. Thus, connexins play an important role in skeletal form and function.

Membranous osteogenesis system modeled with KUSA-A1 mature osteoblasts

Several stromal cells were established from murine bone marrow cultures. One of the KUSA subclones, KUSA-A1 cells, displays osteogenic characteristics in vitro and in vivo. The calcium deposition, osteocalcin release, and parathyroid hormone (PTH) responsiveness of KUSA-A1 cells indicate that they are mature osteoblasts or osteocytes. Bone had formed in subcutaneous tissue 1 week after subcutaneous injection of cells into immunodeficient mice. The osteogenesis by KUSA-A1 was not mediated by chondrogenesis and thus was considered to be membranous ossification. These unique characteristics of KUSA-A1 cells provide an opportunity to analyze the process of membranous ossification in detail.

Cell-cell interactions in regulating osteogenesis and osteoblast function

Endochondral bone formation requires an elaborate interplay among autocrine, paracrine, and endocrine signals, positional cues, and cell-cell contacts to mediate the complex three-dimensional architecture and function of the skeleton. Embryonic bone development occurs by migration, aggregation, and condensation of immature mesenchymal progenitor cells to form the cartilaginous anlage. Upon vascular invasion, the cartilaginous scaffold is colonized and subsequently mineralized by osteoblasts. Likewise, bone remodeling in the adult skeleton is a dynamic process that requires coordinated cellular activities among osteoblasts, osteocytes, and osteoclasts to maintain bone homeostasis. This review examines the role of cell-cell interactions mediated by adherens junctions formed by cadherins and communicative gap junctions formed by connexins in regulating bone development and osteogenic function.

Effect of fiber diameter on spreading, proliferation, and differentiation of osteoblastic cells on electrospun poly(lactic acid) substrates

Electrospinning is a promising method to construct fused-fiber biomaterial scaffolds for tissue engineering applications, but the efficacy of this approach depends on how substrate topography affects cell function. Previously, it has been shown that linear, parallel raised features with length scales of 0.5-2 microm direct cell orientation through the phenomenon of contact guidance, and enhance phenotypic markers of osteoblastic differentiation. To determine how the linear, random raised features produced by electrospinning affect proliferation and differentiation of osteoprogenitor cells, poly(lactic acid) and poly(ethylene glycol)-poly(lactic acid) diblock copolymers were electrospun with mean fiber diameters of 0.14-2.1 microm onto rigid supports. MC3T3-E1 osteoprogenitor cells cultured on fiber surfaces in the absence of osteogenic factors exhibited a lower cell density after 7 and 14 days of culture than cells cultured on spin-coated surfaces, but cell density increased with fiber diameter. However, in the presence of osteogenic factors (2 mM beta-glycerophosphate, 0.13 mM L-ascorbate-2-phosphate), cell density after 7 and 14 days of culture on fiber surfaces was comparable to or exceeded spin-coated controls, and alkaline phosphatase activity after 14 days was comparable. Examination of cell morphology revealed that cells grown on fibers had smaller projected areas than those on planar surfaces. However, cells attached to electrospun substrates of 2.1 microm diameter fibers exhibited a higher cell aspect ratio than cells on smooth surfaces. These studies show that topographical factors designed into biomaterial scaffolds can regulate spreading, orientation, and proliferation of osteoblastic cells.

Cell-to-cell interactions in bone

Bone development (modeling) occurs by migration, aggregation, and condensation of immature osteo/chondroprogenitor cells to form the cartilaginous anlage. This process requires precisely controlled cell-cell interactions. Likewise, bone remodeling in the adult skeleton is a dynamic process that requires coordinated cellular activities among osteoblasts, osteocytes, and osteoclasts to maintain bone homeostasis. The cooperative nature of both bone modeling and remodeling requires tightly regulated mechanisms of intercellular recognition and communication that permit the cells to sort and migrate, synchronize activity, equalize hormonal responses, and diffuse locally generated signals. Osteoblasts and osteocytes achieve these interactions through cadherin-based adherens junctions as well as by formation of communicating junctions, gap junctions. This review examines the current knowledge of how direct cell-to-cell interactions modulate osteoblast function.

Developmental expression patterns and regulation of connexins in the mouse mammary gland: expression of connexin30 in lactogenesis

The mammary gland reaches a fully differentiated phenotype at lactation, a stage characterized by the abundant expression of beta-casein. We have investigated the expression and regulation of gap junction proteins (connexins, Cx) during the various developmental stages of mouse mammary gland. Immunohistochemical analysis, with specific antibodies, reveals that Cx26 and Cx32 are expressed and confined to the cell borders of luminal epithelial cells in all developmental stages of the gland. Cx26 and Cx32 expression, at the mRNA and protein levels, increases in pregnancy and peaks in lactation. Whereas Cx43 mRNA decreases in pregnancy and lactation, the functional activity of Cx43 protein, which has been localized to myoepithelial cells, is regulated (through phosphorylation) during pregnancy and peaks during lactation. Cx30 mRNA and proteins have, for the first time, been detected in mammary gland epithelia. Using reverse transcription/polymerase chain reaction and sequencing techniques, we show that Cx30 is abundant in pregnant and lactating mammary gland. Cx30 protein levels have not been detected in the mammary gland prior to day 15 of pregnancy, whereas maximum expression occurs at the onset of lactation. In mouse mammary cells in culture, Cx30 is epithelial-cell-specific and is induced by lactogenic hormones. These data identify a novel player in mammary differentiation and suggest a potential role for Cx30 in the fully differentiated gland.

Development of mice with osteoblast-specific connexin43 gene deletion

Genetic deficiency of Cx43 in vivo causes skeletal developmental defects, osteoblast dysfunction and perinatal lethality. To determine the role of Cx43 in the adult skeleton, we developed two models of osteoblast-specific Cx43 gene deletion using Cre mediated replacement of a "floxed" Cx43 allele with a LacZ reporter gene. Cre recombinase expression in osteoblasts was driven by either the osteocalcin OG2 promoter or the 2.3 kb fragment of the Colalpha1(I) promoter. Homozygous Cx43(fl/fl) mice, in which the Cx43 coding region is flanked by two loxP sites, were crossed with Cre expressing mice in a heterozygous Cx43-null background [Cx43(+/-); Colalpha1(I)-Cre or Cx43(+/-); OG2-Cre]. Cx43 gene ablation was demonstrated in tissues by selective X-gal staining of cells lining the endosteal surface, and in cultured osteoblastic cells from calvaria using different approaches. Although no LacZ expression was observed in proliferating calvaria cells, before osteoblast differentiation begins, post-proliferative cells isolated from conditional knockout mice [Cx43(fl/-); Colalpha1(I)-Cre or Cx43(fl/-); OG2-Cre] developed strong LacZ expression as they differentiated, in parallel to a progressive disappearance of Cx43 mRNA and protein abundance relative to controls. Selective Cre mediated Cx43 gene inactivation in bone forming cells will be useful to determine the role of Cx43 in adult skeletal homeostasis and overcome the perinatal lethality of the conventional null model.

Gap junctions regulate extracellular signal-regulated kinase signaling to affect gene transcription

Osteoblasts are highly coupled by gap junctions formed by connexin43. Overexpression of connexin45 in osteoblasts results in decreased chemical and electrical coupling and reduces gene transcription from connexin response elements (CxREs) in the osteocalcin and collagen Ialpha1 promoters. Here, we demonstrate that transcription from the gap junction-dependent osteocalcin CxRE is regulated by extracellular signal-regulated protein kinase (ERK) and phosphatidylinositol 3-kinase (PI3K) cascades. Overexpression of a constitutively active mitogen-activated protein kinase kinase (MEK), Raf, or Ras can increase transcription more than twofold of the CxRE, whereas inhibition of MEK or PI3K can decrease transcription threefold from the osteocalcin CxRE. Importantly, disruption of gap junctional communication by overexpression of connexin45 or treatment with pharmacological inhibitors of gap junctions results in reduced Raf, ERK, and Akt activation. The consequence of attenuated gap junction-dependent signal cascade activation is a decrease in Sp1 phosphorylation by ERK, resulting in decreased Sp1 recruitment to the CxRE and inhibited gene transcription. These data establish that ERK/PI3K signaling is required for the optimal elaboration of transcription from the osteocalcin CxRE, and that disruption of gap junctional communication attenuates the ability of cells to respond to an extracellular cue, presumably by limiting the propagation of second messengers among adjacent cells by connexin43-gap junctions.

Marrow-isolated adult multilineage inducible (MIAMI) cells, a unique population of postnatal young and old human cells with extensive expansion and differentiation potential

We report here the isolation of a population of non-transformed pluripotent human cells from bone marrow after a unique expansion/selection procedure. This procedure was designed to provide conditions resembling the in vivo microenvironment that is home for the most-primitive stem cells. Marrow-adherent and -nonadherent cells were co-cultured on fibronectin, at low oxygen tension, for 14 days. Colonies of small adherent cells were isolated and further expanded on fibronectin at low density, low oxygen tension with 2% fetal bovine serum. They expressed high levels of CD29, CD63, CD81, CD122, CD164, hepatocyte growth factor receptor (cMet), bone morphogenetic protein receptor 1B (BMPR1B), and neurotrophic tyrosine kinase receptor 3 (NTRK3) and were negative for CD34, CD36, CD45, CD117 (cKit) and HLADR. The embryonic stem cell markers Oct-4 and Rex-1, and telomerase were expressed in all cultures examined. Cell-doubling time was 36 to 72 hours, and cells have been expanded in culture for more than 50 population doublings. This population of cells was consistently isolated from men and women of ages ranging from 3- to 72-years old. Colonies of cells expressed numerous markers found among embryonic stem cells as well as mesodermal-, endodermal- and ectodermal-derived lineages. They have been differentiated to bone-forming osteoblasts, cartilage-forming chondrocytes, fat-forming adipocytes and neural cells and to attachment-independent spherical clusters expressing genes associated with pancreatic islets. Based on their unique characteristics and properties, we refer to them as human marrow-isolated adult multilineage inducible cells, or MIAMI cells. MIAMI cells proliferate extensively without evidence of senescence or loss of differentiation potential and thus may represent an ideal candidate for cellular therapies of inherited or degenerative diseases.

A novel function of N-cadherin and Connexin43: marked enhancement of alkaline phosphatase activity in rat calvarial osteoblast exposed to sulfated hyaluronan

In this study, we examined the interaction of the osteoblast which forms bone and sulfated hyaluronan (SHya). For the purpose of the creation of a new functional polysaccharide, we introduced a sulfate group in hyaluronan (Hya) of high molecular weight, and SHya of high molecular weight could be obtained for the first time. When rat calvarial osteoblast (rOB) cells were cultured with a high concentration of SHya, they formed aggregated spheroids after 4h and the spheroids grew to about 200microm after 24h. We examined the expression of cell adhesion molecules in order to clarify the mechanism of aggregate formation. The N-cadherin (N-cad) and Connexin43 (Cx43) expression level of rOB cells cultured with SHya remarkably increased after 2h. A difference in the expression of Integrin beta1 (Intbeta1) could not be observed between the SHya addition and control group. The alkaline phosphatase (ALPase) activity of rOB cells cultured with SHya after 8h was significantly enhanced in comparison with control. Therefore, the sulfate group of SHya seems to enhance expression of cell adhesion protein such as N-cad and Cx43, resulting in aggregate formation and further remarkable induction of the ALPase activity of rOB cells.

Defective endochondral ossification in mice with strongly compromised expression of JunB

Functional analysis in mice has established an absolute requirement of JunB, a member of the AP-1 transcription factor family, during early embryonic development. To investigate the role of JunB during mid and late gestation and postnatally Ubi-junB transgenic mice were used to generate two junB–/– Ubi-junB mutant lines, in which embryonic lethality was rescued but strongly reduced JunB expression in several adult tissues was observed. Mutant mice from both rescue lines were growth retarded and shared significantly reduced longitudinal bone growth. Mutant long bones were characterised by reduced numbers of growth plate chondrocytes and a severe osteoporosis. Decreased JunB levels in epiphysal growth plate chondrocytes and bone lining osteoblasts correlated with deregulated expression of Cyclin A, Cyclin D1 and p16INK4a, key regulators of cell cycle control. Furthermore, junB–/– Ubi-junB bone marrow stromal cells were unable to differentiate into bone forming osteoblasts in vitro. Our data demonstrate that JunB plays a crucial role in endochondral ossification by regulating proliferation and function of chondrocytes and osteoblasts.