Enhancement of connexin 43 expression increases proliferation and differentiation of an osteoblast-like cell line

Role of Cx43 on the Bone Cell Generation, Function, and Survival

The presence of gap junction intercellular communication structures in bone cells has been known since the early 1970s, further confirmed by Doty and Marotti at the structural level in the 1980-1990s. Work by Civitelli, Donahue, and others showed the expression of Cx43 at the mRNA and protein levels in all bone cell types: osteoclasts (bone resorbing cells), osteoblasts (bone forming cells), and osteocytes (mature osteoblasts embedded in the bone matrix that regulate the function of both osteoclasts and osteoblasts). While Cx45, Cx46, and Cx37 were also shown to be expressed in bone cells, most studies have focused on Cx43, the most abundant member of the connexin (Cx) family of proteins expressed in bone. The role of Cx43 has been shown to be related to the formation of gap junction intercellular channels, to unopposed hemichannels, and to channel independent functions of the molecule. Cx43 participates in the response of bone cells to pharmacological, hormonal, and mechanical stimuli, and it is involved in the skeletal phenotype with old age. Human and murine studies have shown that mutations of Cx43 lead to oculodentodigital dysplasia and craniometaphyseal dysplasia, both conditions associated with abnormalities in the skeleton. However, whereas substantial advances have been made on the skeletal role of Cx43, further research is needed to understand the basis for the effects of mutated Cx43 and potential ways to prevent the effects of these mutations on bone.

Osteocytes Exposed to Titanium Particles Inhibit Osteoblastic Cell Differentiation via Connexin 43

Periprosthetic osteolysis (PPO) induced by wear particles is the most severe complication of total joint replacement; however, the mechanism behind PPO remains elusive. Previous studies have shown that osteocytes play important roles in wear-particle-induced osteolysis. In this study, we investigated the effects of connexin 43 (Cx43) on the regulation of osteocyte-to-osteoblast differentiation. We established an in vivo murine model of calvarial osteolysis induced by titanium (Ti) particles. The osteolysis characteristic and osteogenesis markers in the osteocyte-selective Cx43 (CKO)-deficient and wild-type (WT) mice were observed. The calvarial osteolysis induced by Ti particles was partially attenuated in CKO mice. The expression of β-catenin and osteogenesis markers increased significantly in CKO mice. In vitro, the osteocytic cell line MLO-Y4 was treated with Ti particles. The co-culturing of MLO-Y4 cells with MC3T3-E1 osteoblastic cells was used to observe the effects of Ti-treated osteocytes on osteoblast differentiation. When Cx43 of MLO-Y4 cells was silenced or overexpressed, β-catenin was detected. Additionally, co-immunoprecipitation detection of Cx43 and β-catenin binding in MLO-Y4 cells and MC3T3-E1 cells was performed. Finally, β-catenin expression in MC3T3-E1 cells and osteoblast differentiation were evaluated after 18α-glycyrrhetinic acid (18α-GA) was used to block the intercellular communication of Cx43 between MLO-Y4 and MC3T3-E1 cells. Ti particles increased Cx43 expression and decreased β-catenin expression in MLO-Y4 cells. The silencing of Cx43 increased the β-catenin expression, and the over-expression of Cx43 decreased the β-catenin expression. In the co-culture model, Ti treatment of MLO-Y4 cells inhibited the osteoblastic differentiation of MC3T3-E1 cells and Cx43 silencing in MLO-Y4 cells attenuated the inhibitory effects on osteoblastic differentiation. With Cx43 silencing in the MLO-Y4 cells, the MC3T3-E1 cells, co-cultured alongside MLO-Y4, displayed decreased Cx43 expression, increased β-catenin expression, activation of Runx2, and promotion of osteoblastic differentiation in vitro co-culture. Finally, Cx43 expression was found to be negatively correlated to the activity of the Wnt signaling pathway, mostly through the Cx43 binding of β-catenin from its translocation to the nucleus. The results of our study suggest that Ti particles increased Cx43 expression in osteocytes and that osteocytes may participate in the regulation of osteoblast function via the Cx43 during PPO.

HSPB7 oppositely regulates human mesenchymal stromal cell-derived osteogenesis and adipogenesis

BACKGROUND

Recent evidence suggests that accumulation of marrow adipose tissue induced by aberrant lineage allocation of bone marrow-derived mesenchymal stromal cells (BMSCs) contributes to the pathophysiologic processes of osteoporosis. Although master regulators of lineage commitment have been well documented, molecular switches between osteogenesis and adipogenesis are largely unknown.

METHODS

HSPB7 gene expression during osteogenic and adipogenic differentiation of BMSCs was evaluated by qPCR and Western blot analyses. Lentiviral-mediated knockdown or overexpression of HSPB7 and its deletion constructs were used to assess its function. The organization of cytoskeleton was examined by immunofluorescent staining. ALP activity, calcium assay, Alizarin Red S staining and Oil Red O staining were performed in vitro during osteoblast or adipocyte differentiation. SB431542 and Activin A antibody were used to identify the mechanism of Activin A in the regulation of osteogenic differentiation in BMSCs.

RESULTS

In this study, we identified HSPB7 capable of oppositely regulating osteogenic and adipogenic differentiation of BMSCs. HSPB7 silencing promoted adipogenesis while reducing osteogenic differentiation and mineralization. Conversely, overexpression of HSPB7 strongly enhanced osteogenesis, but no effect was observed on adipogenic differentiation. Deletion of the N-terminal or C-terminal domain of HSPB7 led to decreased osteoblastic potency and mineralization. Mechanistically, our data showed that Activin A is a downstream target participating in HSPB7 knockdown-mediated osteogenic inhibition.

CONCLUSIONS

Our findings suggest that HSPB7 plays a positive role in driving osteoblastic differentiation, and with the capability in maintaining the osteo-adipogenesis balance. It holds great promise as a potential therapeutic target in the treatment of bone metabolic diseases.

Connexin 43 Hemichannels Regulate Osteoblast to Osteocyte Differentiation

Connexin 43 (Cx43) is the predominant connexin subtype expressed in osteocytes. Osteocytes, accounting for 90%–95% of total bone cells, function as orchestrators coordinating balanced activity between bone-resorbing osteoclasts and bone-forming osteoblasts. In this study, two newly developed osteocytic cell lines, OCY454 and IDG-SW3, were used to determine the role of Cx43 gap junctions and hemichannels (HCs) in the regulation of osteoblast to osteocyte differentiation. We found that the Cx43 level was substantially increased during the differentiation of IDG-SW3 cells and is also much higher than that of OCY454 cells. We knocked down Cx43 expression using the lentiviral CRISPR/Cas9 approach and inhibition of Cx43 HCs using Cx43 (E2) antibody in IDG-SW3 cells. Cx43 knockdown (KD) or Cx43 HC inhibition decreased gene expression for osteoblast and osteocyte markers, including alkaline phosphatase, type I collagen, dentin matrix protein 1, sclerostin, and fibroblast growth factor 23, whereas increasing the osteoclastogenesis indicator and the receptor activator of nuclear factor kappa-B ligand (RANKL)/osteoprotegerin (OPG) ratio at early and late differentiation stages. Moreover, mineralization was remarkably attenuated in differentiated Cx43-deficient IDG-SW3 cells compared to ROSA26 control. The conditioned medium collected from fully differentiated IDG-SW3 cells with Cx43 KD promoted osteoclastogenesis of RAW264.7 osteoclast precursors. Our results demonstrated that Cx43 HCs play critical roles in osteoblast to osteocyte differentiation process and regulate osteoclast differentiation via secreted factors.

A 3D, Dynamically Loaded Hydrogel Model of the Osteochondral Unit to Study Osteocyte Mechanobiology

Osteocytes are mechanosensitive cells that orchestrate signaling in bone and cartilage across the osteochondral unit. The mechanisms by which osteocytes regulate osteochondral homeostasis and degeneration in response to mechanical cues remain unclear. This study introduces a novel 3D hydrogel bilayer composite designed to support osteocyte differentiation and bone matrix deposition in a bone-like layer and to recapitulate key aspects of the osteochondral unit's complex loading environment. The bilayer hydrogel is fabricated with a soft cartilage-like layer overlaying a stiff bone-like layer. The bone-like layer contains a stiff 3D-printed hydrogel structure infilled with a soft, degradable, cellular hydrogel. The IDG-SW3 cells embedded within the soft hydrogel mature into osteocytes and produce a mineralized collagen matrix. Under dynamic compressive strains, near-physiological levels of strain are achieved in the bone layer (≤ 0.08%), while the cartilage layer bears the majority of the strains (>99%). Under loading, the model induces an osteocyte response, measured by prostaglandin E2, that is frequency, but not strain, dependent: a finding attributed to altered fluid flow within the composite. Overall, this new hydrogel platform provides a novel approach to study osteocyte mechanobiology in vitro in an osteochondral tissue-mimetic environment.

Cx43 promotes SHF-DPCs proliferation in the hair follicle of Albas cashmere goats from anagen to telogen

Connexin 43 (Cx43), known to form gap junction transmembrane channels between the cytoplasm of two adjacent cells, plays a key role in physiological functions, such as regulating cell growth, differentiation, and maintaining tissue homeostasis. Cashmere goat is an important farm animal that provides cashmere, which was produced by secondary hair follicles (SHF), for human consumption; however, there is no report about the role of Cx43 on the growth and development of SHF in cashmere goat. In this study, we investigated the effect of Cx43 on proliferation secondary hair follicle dermal papilla cells (SHF-DPCs) in Albas cashmere goat. In SHF-DPCs, Cx43 overexpression promoted cell proliferation and upregulated the expression of IGF-1, whereas Cx43 knockdown was associated with the opposite effects. These results suggested that Cx43 may promote cell proliferation by inducing IGF-1. Overall, our research not only contributes to a better understanding of the mechanism of the growth and development of SHF in cashmere goat, but also shed light on cashmere quality control in the future.

Connexin 43 plays a role in proliferation and migration of pulmonary arterial fibroblasts in response to hypoxia

Pulmonary hypertension (PH) is a disease associated with vasoconstriction and remodelling of the pulmonary vasculature. Pulmonary artery fibroblasts (PAFs) play an important role in hypoxic-induced remodelling. Connexin 43 (Cx43) is involved in cellular communication and regulation of the pulmonary vasculature. Using both in vitro and in vivo models of PH, the aims of this study were to (i) investigate the role of Cx43 in hypoxic-induced proliferation and migration of rat PAFs (rPAFs) and rat pulmonary artery smooth muscle cells (rPASMCs) and (ii) determine whether Cx43 expression is dysregulated in the rat sugen5416/hypoxic model of PH. The role of Cx43 in hypoxic-induced proliferation and migration was investigated using Gap27 (a pharmacological inhibitor of Cx43) or genetic knockdown of Cx43 using siRNA. Cx43 protein expression was increased by hypoxia in rPAFs but not rPASMCs. Hypoxic exposure, in the presence of serum, resulted in an increase in proliferation of rPAFs but not rPASMCs. Hypoxic exposure caused migration of rPAFs but not rPASMCs. Phosphorylation of p38 mitogen-activated protein kinase (MAPK) and ERK1/2 were increased by hypoxia in rPAFs. The effects of hypoxia on proliferation, migration and MAPK phosphorylation in rPAFs were attenuated in the presence of Gap27 or Cx43 siRNA. Cx43 protein expression was increased in sugen5416/hypoxic rat lung; this increased expression was not observed in sugen5416/hypoxic rats treated with the MAPK pathway inhibitor GS-444217. In conclusion, Cx43 is involved in the proliferation and migration of rPAFs in response to hypoxia via the MAPK signalling pathway.

Expression and functional regulation of gap junction protein connexin 43 in dermal mesenchymal stem cells from psoriasis patients

BACKGROUND

Psoriasis is a chronic recurrent inflammatory disease. Mesenchymal stem cells (MSCs) can regulate the inflammatory microenvironment, thereby controlling the proliferation, differentiation, and migration of immune cells. Connexin 43(Cx43), a key gap junction protein, has been shown to form gap junctions for communication between neighboring cells.

OBJECTIVE

We investigated the expression of Cx43 in dermal mesenchymal stem cells (DMSCs) derived from psoriasis patients and explored the relationship between the Cx43-mediated gap junction intercellular communication (GJIC) and DMSCs.

METHODS

Human DMSCs were isolated and propagated in adherent culture. Quantitative real-time reverse transcription PCR and western blot and immunofluorescence were used to detect the expression and localization of Cx43 in DMSCs. Fluorescence redistribution after photobleaching was performed to assess adjacent DMSCs GJIC. CCK8 was used to detect the proliferation of DMSCs before and after gap junction blocker (18α-glycyrrhetinic acid; AGA) treatment. Cell energy metabolism was analyzed with an energy metabolism analyzer.

RESULTS

Cx43 was located in the cytoplasm and cytomembrane, as well as partially in the nucleus of DMSCs. The expression of Cx43 in psoriasis DMSCs was higher than that in control samples and the gap junction function was enhanced. In addition, the glycolysis and mitochondrial respiration of psoriasis DMSCs were also enhanced. However, AGA inhibited the expression of Cx43, attenuated GJIC function, and inhibited the proliferation of DMSCs.

CONCLUSIONS

Our results indicated that the expression of Cx43 in DMSCs from psoriasis lesions is increased and that the inhibition of Cx43 leads to the inhibition of both GJIC and DMSCs proliferation.

The Effects of Fluoride on the Gap-Junctional Intercellular Communication of Rats' Osteoblast

The gap junction protein plays an important role in the bone formation and alteration of these proteins leading to cause bone development. Aim to determine the effects of different concentration of fluoride on gap-junctional intercellular communication (GJIC) related genes and proteins in the rats' osteoblast cells. We treated the osteoblast cells with various concentrations (0, 0.01, 0.1, 0.5, and 1.0 mM) NaF for 24 and 72 h. We used the scrape loading and dye transfer technique to research the intracellular connectivity. Moreover, the mRNA expression levels of connexin 43 (Cx43), connexin45 (Cx45), collagen I, and osteocalcin (OCN) were analyzed by qRT-PCR, the protein expression levels of connexin43 (Cx43) were analyzed by western blotting and immunofluorescence. Our results suggested that the osteoblast proliferations were decreased in the 0.5 and 1 mM NaF groups, after 24 and 72 treatments. The scrape loading and dye transfer experiment showed that the GJIC were increased in the 0.01 mM NaF group and decreased in the 0.5 and 1 mM NaF groups. In addition, the mRNA expressions of Cx43, Cx45, and OCN, and the protein expressions of Cx43 were increased in the 0.01 mM NaF group and decreased in the 0.5 and 1 mM NaF groups. In summary, these results suggest that the low concentration NaF is good for the GJIC, but the high concentration NaF damages the GJIC.

Cell junctions and oral health

The oral cavity and its appendices are exposed to considerable environmental and mechanical stress. Cell junctions play a pivotal role in this context. Among those, gap junctions permit the exchange of compounds between cells, thereby controlling processes such as cell growth and differentiation. Tight junctions restrict paracellular transportation and inhibit movement of integral membrane proteins between the different plasma membrane poles. Adherens junctions attach cells one to another and provide a solid backbone for resisting to mechanistical stress. The integrity of oral mucosa, normal tooth development and saliva secretion depend on the proper function of all these types of cell junctions. Furthermore, deregulation of junctional proteins and/or mutations in their genes can alter tissue functioning and may result in various human disorders, including dental and periodontal problems, salivary gland malfunction, hereditary and infectious diseases as well as tumorigenesis. The present manuscript reviews the role of cell junctions in the (patho)physiology of the oral cavity and its appendices, including salivary glands.

Blocking connexin 43 accelerates corneal healing and improves tissue remodeling during the healing of diabetic rat corneas: A histological and immunohistochemical study

Connexin 43 (Cx43) is a potential target for accelerating wound healing. This study aimed at evaluating the therapeutic efficiency of topical application of Gap27, a Cx43 mimetic peptide, on corneal tissue reorganization during wound healing in streptozocin-induced Diabetes in albino rats and its effect on the infiltration of inflammatory cells. Fifty adult male albino Wistar rats were divided equally into two groups: non-diabetic and diabetic. Twenty rats from each group were subjected to corneal injury: 10 untreated and 10 treated with Gap27. The remaining five rats from each group served as negative controls (intact corneas). All rats were sacrificed 3 days after injury. Histological studies were performed to assess signs of cell degeneration, the infiltration of inflammatory cells. Histomorphometric studies were performed to quantify the expression of Cx43. Gap27 promoted corneal wound healing in non-diabetic and diabetic rats. It reduced mononuclear cell infiltration and improved corneal tissue remodeling. However, minor structural changes were still seen in diabetic corneas after treatment with Gap27. Blocking Cx43 was a valuable tool to restore corneal tissue structure, reduce the infiltration of inflammatory cells in non-diabetic and diabetic rats.

Cartilage regeneration and ageing: Targeting cellular plasticity in osteoarthritis

Ageing processes play a major contributing role for the development of Osteoarthritis (OA). This prototypic degenerative condition of ageing is the most common form of arthritis and is accompanied by a general decline, chronic pain and mobility deficits. The disease is primarily characterized by articular cartilage degradation, followed by subchondral bone thickening, osteophyte formation, synovial inflammation and joint degeneration. In the early stages, osteoarthritic chondrocytes undergo phenotypic changes that increase cell proliferation and cluster formation and enhance the production of matrix-remodelling enzymes. In fact, chondrocytes exhibit differentiation plasticity and undergo phenotypic changes during the healing process. Current studies are focusing on unravelling whether OA is a consequence of an abnormal wound healing response. Recent investigations suggest that alterations in different proteins, such as TGF-ß/BMPs, NF-Kß, Wnt, and Cx43, or SASP factors involved in signalling pathways in wound healing response, could be directly implicated in the initiation of OA. Several findings suggest that osteoarthritic chondrocytes remain in an immature state expressing stemness-associated cell surface markers. In fact, the efficacy of new disease-modifying OA drugs that promote chondrogenic differentiation in animal models indicates that this may be a drug-sensible state. In this review, we highlight the current knowledge regarding cellular plasticity in chondrocytes and OA. A better comprehension of the mechanisms involved in these processes may enable us to understand the molecular pathways that promote abnormal repair and cartilage degradation in OA. This understanding would be advantageous in identifying novel targets and designing therapies to promote effective cartilage repair and successful joint ageing by preventing functional limitations and disability.

The Effects of Hierarchical Micro/Nano-Structured Titanium Surface on Osteoblast Proliferation and Differentiation Under Diabetic Conditions

PURPOSE

The aim of the present study was to mimic the hierarchical structure of bone tissues by simple sandblasting/acid-etching and anodization to investigate the effects of such surface characteristics on proliferation and differentiation of osteoblasts in high glucose concentrations. By the way, the effects of high glucose levels on osteoblast functions were tested.

METHODS

MC3T3-E1 cells cultured on sand-blasted and acid-etched (SLA) surface and nano-modified SLA (NMSLA) surface were subjected to normal serum (NS) and diabetic serum (DS), respectively. The surface characteristics were evaluated by scanning electron microscopy. Cell proliferation was assessed using MTT assay. The levels of alkaline phosphatase (ALP) activity and mineralization were measured and compared. Real-time polymerase chain reaction was applied to detect the expression levels of osteogenic genes.

RESULTS

NMSLA significantly increased cell proliferation at time points ranging from 3 to 7 days under both serums. Cells cultured on NMSLA surfaces displayed significantly higher ALP activities and mineralization. The expression levels of Runx2 (indicates runt-related protein 2), collagen I (COL1), and osteocalcin (OCN) were notably increased on NMSLA surface compared with SLA surface. Moreover, we found that high glucose increased osteoblast proliferation but decreased differentiation of osteoblast slightly.

CONCLUSION

The hierarchical micro/nano-structured titanium surface has a favorable biocompatibility on simultaneously improving osteoblast proliferation and differentiation in diabetic serum.

Bone morphogenetic protein 2 regulates cell-cell communication by down-regulating connexin43 expression in luteinized human granulosa cells

STUDY QUESTION

Does bone morphogenetic protein 2 (BMP2) regulate connexin43 (Cx43) and modulate cell-cell communication in luteinized human granulosa cells?

SUMMARY ANSWER

BMP2 decreases gap junction intercellular communication (GJIC) of luteinized human granulosa cells by down-regulating Cx43 expression through an activin receptor-like kinase (ALK)2/ALK3-mediated Sma- and Mad-related protein (SMAD)-dependent signaling pathway.

WHAT IS KNOWN ALREADY

BMP2 and its putative receptors are highly expressed in the human corpus luteum and are involved in the process of luteolysis. Cx43-coupled gap junctions play a critical role in the development and maintenance of corpus luteum.

STUDY DESIGN DURATION

This is a laboratory study conducted over a 1-year period. At least three independent experiments with three replicates were conducted and the experimental samples were compared with the appropriate vehicle controls for all of the inhibition-approach, concentration-dependent or time-course studies.

PARTICIPANTS/MATERIALS, SETTING, METHODS

SVOG cell line (immortalized human granulosa-lutein cells derived from in vitro fertilization patients in an academic research center) was used as the study model. The changes of Cx43 expression and levels of phosphorylated SMAD1/5/8 protein were evaluated after exposure to recombinant human BMP2. Real-time quantitative PCR and Western blot analysis were used to examine the specific mRNA and protein levels, respectively. The BMP/TGF-β type I receptor inhibitors (Dorsomorphin, DMH-1 and SB431542) and target depletion small interfering RNAs (ALK2, ALK3, ALK6 and SMAD4) were used to investigate the underlying molecular mechanisms. A scrape loading and dye transfer assay was used to evaluate the GJIC between the SVOG cells.

MAIN RESULTS AND THE ROLE OF CHANCE

Treatment with BMP2 down-regulated the expression of Cx43 and decreased the GJIC activity, whereas it increased the phosphorylated SMAD1/5/8 protein in SVOG cells (P < 0.05). These biological effects were abolished by pre-treatment with the BMP type I receptor inhibitors, Dorsomorphin and DMH-1 (P < 0.05), but not SB431542. Additionally, the individual or concomitant small interfering RNA-mediated knockdown of ALK2 and ALK3, but not ALK6 attenuated the BMP2-induced increases in phosphorylated SMAD1/5/8 and down-regulation of Cx43 expression (P < 0.05). The knockdown of SMAD4 completely abolished the BMP2-induced down-regulation of Cx43 expression (P < 0.05).

LIMITATIONS REASONS FOR CAUTION

This experimental study was conducted in an in vitro cell culture system, and may not reflect a realistic intra-ovarian environment.

WIDER IMPLICATIONS OF THE FINDINGS

Our results suggested that BMP2 may be involved in the local modulation of cell-cell communication in the luteal phase. This study also represents the first comprehensive research of molecular mechanisms of BMP2 in the down-regulation Cx43 in luteinized human granulosa cells. Such data may provide valuable insights into ovarian physiology and benefit the development of potential therapeutic methods for patients suffering from luteal insufficiency.

LARGE SCALE DATA

N/A.

STUDY FUNDING AND COMPETING INTEREST(s)

This research was supported by an operating grant from the China-Canadian Joint Health Research Initiative Grants Program to P.C.K. Leung and J.Z. Sheng. The authors declare no competing interest with the contents of this article.

Electronic Cigarette Smoke Impairs Normal Mesenchymal Stem Cell Differentiation

Electronic cigarettes (e-cigarettes) are promoted as low-risk alternatives to combustible cigarettes. However, the effects of chronic inhalation of potential toxicants emitted by ecigarettes remain largely unexamined. It is conceivable that smoking-induced chronic diseases result in cellular injury, in the absence of effective repair by stem cells. This study evaluates the effect of cigarette and e-cigarette aerosol extracts on the survival and differentiation of bone marrow-derived mesenchymal stem cells (MSCs). MSC growth and osteogenic differentiation were examined after exposure to smoke extracts. Data revealed detrimental effects of both cigarette and e-cigarette extracts on MSC morphology and growth. Levels and activity of alkaline phosphatase, an osteogenic marker, decreased and induction of osteoblastic differentiation was impaired. Both smoke extracts prevented osteogenic differentiation from progressing, evident by decreased expression of terminal osteogenic markers and mineralization. Elevated levels of reactive oxygen species (ROS) were detected in cells exposed to smoke extracts. Moreover, decreased differentiation potential was concomitant with severe down-regulation of Connexin 43 expression, leading to the loss of gap junction-mediated communication, which together with elevated ROS levels, could explain decreased proliferation and loss of differentiation potential. Hence, e-cigarettes present similar risk as combustible cigarettes with respect to tissue repair impairment.

Connexin43 Mutant Patient-Derived Induced Pluripotent Stem Cells Exhibit Altered Differentiation Potential

We present for the first time the generation of induced pluripotent stem cells (iPSCs) from a patient with a connexin-linked disease. The importance of gap junctional intercellular communication in bone homeostasis is exemplified by the autosomal dominant developmental disorder oculodentodigital dysplasia (ODDD), which is linked to mutations in the GJA1 (Cx43) gene. ODDD is characterized by craniofacial malformations, ophthalmic deficits, enamel hypoplasia, and syndactyly. In addition to harboring a Cx43 p.V216L mutation, ODDD iPSCs exhibit reduced Cx43 mRNA and protein abundance when compared to control iPSCs and display impaired channel function. Osteogenic differentiation involved an early, and dramatic downregulation of Cx43 followed by a slight upregulation during the final stages of differentiation. Interestingly, osteoblast differentiation was delayed in ODDD iPSCs. Moreover, Cx43 subcellular localization was altered during chondrogenic differentiation of ODDD iPSCs compared to controls and this may have contributed to the more compact cartilage pellet morphology found in differentiated ODDD iPSCs. These studies highlight the importance of Cx43 expression and function during osteoblast and chondrocyte differentiation, and establish a potential mechanism for how ODDD-associated Cx43 mutations may have altered cell lineages involved in bone and cartilage development. © 2017 American Society for Bone and Mineral Research.

Connexin30.3 is expressed in mouse embryonic stem cells and is responsive to leukemia inhibitory factor

The expression of 19 connexin (Cx) isoforms was observed in the mouse embryonic stem (ES) cell line, EB3. Their expression patterns could be classified into either pluripotent state-specific, differentiating stage-specific, or non-specific Cxs. We focused on Cx30.3 as typical of the first category. Cx30.3 was pluripotent state-specific and upregulated by leukemia inhibitory factor (LIF), a specific cytokine that maintains the pluripotent state of ES cell, via a Jak signaling pathway. Cx30.3 protein was localized to both the cell membrane and cytosol. The dynamic movement of Cx30.3 in the cell membrane was suggested by the imaging analysis by means of overexpressed Cx30.3-EGFP fusion protein. The cytosolic portion was postulated to be a ready-to-use Cx pool. The Cx30.3 expression level in ES cell colonies dramatically decreased immediately after their separation into single cells. It was suggested that mRNA for Cx30.3 and Cx30.3 protein might be decomposed more rapidly than mRNA for Cx43 and Cx43 protein, respectively. These indicate possible involvement of Cx30.3 in the rapid formation and/or decomposition of gap junctions; implying a functional relay between Cx30.3 and other systems such as adhesion proteins.

The Role of Pannexin 3 in Bone Biology

Cell-cell and cell-matrix communications play important roles in both cell proliferation and differentiation. Gap junction proteins mediate signaling communication by exchanging small molecules and dramatically stimulating intracellular signaling pathways to determine cell fate. Vertebrates have 2 gap junction families: pannexins (Panxs) and connexins (Cxs). Unlike Cxs, the functions of Panxs are not fully understood. In skeletal formation, Panx3 and Cx43 are the most abundantly expressed gap junction proteins from each family. Panx3 is induced in the transient stage from the proliferation and differentiation of chondrocytes and osteoprogenitor cells. Panx3 regulates both chondrocyte and osteoblast differentiation via the activation of intracellular Ca²⁺ signaling pathways through multiple channel activities: hemichannels, endoplasmic reticulum (ER) Ca²⁺ channels, and gap junctions. Moreover, Panx3 also inhibits osteoprogenitor cell proliferation and promotes cell cycle exit through the inactivation of Wnt/β-catenin signaling and the activation of p21. Panx3-knockout (KO) mice have more severe skeletal abnormalities than those of Cx43-KO mice. A phenotypic analysis of Panx3-KO mice indicates that Panx3 regulates the terminal differentiation of chondrocytes by promoting vascular endothelial growth factor (VEGF) and matrix metalloproteinase (MMP) 13. Based on the generation of Panx3^-/-; Cx43^-/- mice, Panx3 is upstream of Cx43 in osteogenesis. Panx3 promotes Cx43 expression by regulating Wnt/β-catenin signaling and osterix expression. Further, although Panx3 can function in 3 ways, Cx43 cannot function through the ER Ca²⁺ channel, only via the hemichannels and gap junction routes. In this review, we discuss the current knowledge regarding the roles of Panx3 in skeletal formation and address the potential for new therapies in the treatment of diseases and pathologies associated with Panx3, such as osteoarthritis (OA).

Connexin 43 Acts as a Proapoptotic Modulator in Cisplatin-Induced Auditory Cell Death

AIMS

Gap junction coupling is known to play a role in intercellular communication by the Good Samaritan effect or bystander effect. Nonjunctional connexins (Cxs) may also play certain gap junction-independent roles in cell death or survival. The purpose of the present study was to investigate the role of junctional and nonjunctional Cxs in ototoxic drug-induced auditory cell death by focusing on Cx43 in the cochlea.

RESULTS

Nonjunctional Cx43 conditions were prepared by low confluence culture (5 × 10³/cm²) or a trafficking inhibitor, brefeldin A (BFA), in auditory cells, and short lengthened Cx43s with amino-terminal (NT; amino acids 1-256) or carboxy-terminal (CT; amino acids 257-382) were transfected into Cx-deficient HeLa cells to avoid gap junction formation. Knockdown of nonchannel Cx43 (small interfering RNA [siRNA]) inhibited Cis-diamminedichloroplatinum (cisplatin)-induced cell death regardless of gap junction formation; however, a gap junction blocker, 18 alpha-glycyrrhetinic acid (18α-GA), showed inhibitory effect only under the junctional condition. BFA did not show any additive influence on the inhibitory effect of siRNA Cx43. Shortened Cx43-transfected HeLa cells also resulted in a significant increase in cell death under cisplatin. In the animal studies with cisplatin-treated rats, hearing thresholds of auditory brainstem response were significantly preserved by a gap junction blocker, carbenoxolone, showing much more preserved stereocilia of hair cells in scanning electron microscopic findings. Innovation and Conclusion: Cx43 plays a proapoptotic role in cisplatin-induced auditory cell death in both junctional and nonjunctional conditions. Targeting the Cx-mediated signaling control may be helpful in designing new therapeutic strategies for drug-induced ototoxicity. Antioxid. Redox Signal. 25, 623-636.

Cell seeding density is a critical determinant for copolymer scaffolds-induced bone regeneration

Constructs intended for bone tissue engineering (TE) are influenced by the initial cell seeding density. Therefore, the objective of this study was to determine the effect of bone marrow stromal stem cells (BMSCs) density loaded onto copolymer scaffolds on bone regeneration. BMSCs were harvested from rat's bone marrow and cultured in media with or without osteogenic supplements. Cells were seeded onto poly(l‐lactide‐co‐ε‐caprolactone) [poly(LLA‐co‐CL)] scaffolds at two different densities: low density (1 × 10⁶ cells/scaffold) or high density (2 × 10⁶ cells/scaffold) using spinner modified flasks and examined after 1 and 3 weeks. Initial attachment and spread of BMSC onto the scaffolds was recorded by scanning electron microscopy. Cell proliferation was assessed by DNA quantification and cell differentiation by quantitative real‐time reverse transcriptase‐polymerized chain reaction analysis (qRT‐PCR). Five‐millimeter rat calvarial defects (24 defects in 12 rats) were implanted with scaffolds seeded with either low or high density expanded with or without osteogenic supplements. Osteogenic supplements significantly increased cell proliferation (p < 0.001). Scaffolds seeded at high cell density exhibited higher mRNA expressions of Runx2 p = 0.001, Col1 p = 0.001, BMP2 p < 0.001, BSP p < 0.001, and OC p = 0.013. More bone was formed in response to high cell seeding density (p = 0.023) and high seeding density with osteogenic medium (p = 0.038). Poly (LLA‐co‐CL) scaffolds could be appropriate candidates for bone TE. The optimal number of cells to be loaded onto scaffolds is critical for promoting Extracellular matrix synthesis and bone formation. Cell seeding density and osteogenic supplements may have a synergistic effect on the induction of new bone. © 2015 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 103A: 3649–3658, 2015.

Connexins and pannexins in the skeleton: gap junctions, hemichannels and more

Regulation of bone homeostasis depends on the concerted actions of bone-forming osteoblasts and bone-resorbing osteoclasts, controlled by osteocytes, cells derived from osteoblasts surrounded by bone matrix. The control of differentiation, viability and function of bone cells relies on the presence of connexins. Connexin43 regulates the expression of genes required for osteoblast and osteoclast differentiation directly or by changing the levels of osteocytic genes, and connexin45 may oppose connexin43 actions in osteoblastic cells. Connexin37 is required for osteoclast differentiation and its deletion results in increased bone mass. Less is known on the role of connexins in cartilage, ligaments and tendons. Connexin43, connexin45, connexin32, connexin46 and connexin29 are expressed in chondrocytes, while connexin43 and connexin32 are expressed in ligaments and tendons. Similarly, although the expression of pannexin1, pannexin2 and pannexin3 has been demonstrated in bone and cartilage cells, their function in these tissues is not fully understood.

The role of gap junctions and mechanical loading on mineral formation in a collagen-I scaffold seeded with osteoprogenitor cells

Fracture nonunions represent one of many large bone defects where current treatment strategies fall short in restoring both form and function of the injured tissue. In this case, the use of a tissue-engineered scaffold for promoting bone healing offers an accessible and easy-to-manipulate environment for studying bone formation processes in vitro. We have previously shown that mechanical prestimulation using confined compression of differentiating osteoblasts results in an increase in mineralization formed in a 3D collagen-I scaffold. This study builds on this knowledge by evaluating the short and long-term effects of blocking gap junction-mediated intercellular communication among osteogenic cells on their effectiveness to mineralize collagen-I scaffolds in vitro, and in the presence and absence of mechanical stimulation. In this study, confined compression was applied in conjunction with octanol (a general communication blocker) or 18-α-glycerrhetinic acid (AGA, a specific gap junction blocker) using a modified FlexCell plate to collagen-I scaffolds seeded with murine embryonic stem cells stimulated toward osteoblast differentiation using beta-glycerol phosphate. The activity, presence, and expression of osteoblast cadherin, connexin-43, as well as various pluripotent and osteogenic markers were examined at 5-30 days of differentiation. Fluorescence recovery after photobleaching, immunofluorescence, viability, histology assessments, and reverse-transcriptase polymerase chain reaction assessments revealed that inhibiting communication in this scaffold altered the lineage and function of differentiating osteoblasts. In particular, treatment with communication inhibitors caused reduced mineralization in the matrix, and dissociation between connexin-43 and integrin α5β1. This dissociation was not restored even after long-term recovery. Thus, in order for this scaffold to be considered as an alternative strategy for the repair of large bone defects, cell-cell contacts and cell-matrix interactions must remain intact for osteoblast differentiation and function to be preserved. This study shows that within this 3D scaffold, gap junctions are essential in osteoblast response to mechanical loading, and are essential structures in producing a significant amount and organization of mineralization in the matrix.

Connexins: junctional and non-junctional modulators of proliferation

Mounting evidence indicates that dysregulation of gap junctions and their structural subunits-connexins-often occurs in, and sometimes causes, a variety of proliferative disorders, including cancer. Connexin-mediated regulation of cell proliferation is complex and may involve modulation of gap junction intercellular communication (GJIC), hemichannel signalling, or gap junction-independent paths. However, the exact mechanisms linking connexins to proliferation remain poorly defined and a number of contradictory studies report both pro- and anti-proliferative effects, effects that often depend on the cell or tissue type or the microenvironment. The present review covers junctional and non-junctional regulation of proliferation by connexins, with a particular emphasis on their association with cancer.

Hemodynamic forces regulate developmental patterning of atrial conduction

Anomalous action potential conduction through the atrial chambers of the heart can lead to severe cardiac arrhythmia. To date, however, little is known regarding the mechanisms that pattern proper atrial conduction during development. Here we demonstrate that atrial muscle functionally diversifies into at least two heterogeneous subtypes, thin-walled myocardium and rapidly conducting muscle bundles, during a developmental window just following cardiac looping. During this process, atrial muscle bundles become enriched for the fast conduction markers Cx40 and Nav1.5, similar to the precursors of the fast conduction Purkinje fiber network located within the trabeculae of the ventricles. In contrast to the ventricular trabeculae, however, atrial muscle bundles display an increased proliferation rate when compared to the surrounding myocardium. Interestingly, mechanical loading of the embryonic atrial muscle resulted in an induction of Cx40, Nav1.5 and the cell cycle marker Cyclin D1, while decreasing atrial pressure via in vivo ligation of the vitelline blood vessels results in decreased atrial conduction velocity. Taken together, these data establish a novel model for atrial conduction patterning, whereby hemodynamic stretch coordinately induces proliferation and fast conduction marker expression, which in turn promotes the formation of large diameter muscle bundles to serve as preferential routes of conduction.

Connexin43 is required for the maintenance of multipotency in skin-derived stem cells

Expression of the gap junction protein, connexin43 (Cx43), begins early during embryogenesis and is maintained in many different cell types. Several stem cell populations have been shown to express Cx43 and to form functional gap junctions. While it is clear that Cx43 is critical to the function of many organs, whether the same is true for stem cells has not been clearly demonstrated. Recently, stem cells isolated from newborn mouse skin were shown to form oocyte-like cells (OLCs) in vitro, hence the present study focussed on the role Cx43 plays in the proliferation and differentiation of these cells. The stem cells express Cx43 and those from knockout mice (Cx43 KO) exhibited significantly reduced cell-cell coupling. Loss of Cx43 reduced the rate of cellular migration [Cx43 KO, 1.57±0.65 radial cell units (RCU); wildtype (WT), 5.57±0.37 RCU] but increased the proliferation rate of the stem cells (Cx43 KO, 29.40%±2.02%; WT, 12.76%±1.50%). The expression of the pluripotency markers OCT4 and Nanog were found to be reduced in the Cx43 KO population, suggesting an inhibition of differentiation potential. To test the differentiation ability, the stem cells were induced to form neuronal cell types in vitro. While both the WT and KO cells were able to form GFAP-positive astrocytic cells, only WT stem cells were able to form βIII tubulin-positive neurons. Similarly, the ability of the stem cells to form OLCs was ablated by the loss of Cx43. These data reveal a role for Cx43 in maintaining multipotency within the skin-derived stem cell population.

An intact connexin43 is required to enhance signaling and gene expression in osteoblast-like cells

The cytoplasmic C-terminus of connexin43 (Cx43) interacts with numerous signaling complexes. We hypothesize that signal complex docking to the Cx43 C-terminus (CT) is required to propagate the molecules being shared by gap junctions. We have previously shown that Cx43 impacts the responsiveness of osteoblasts to FGF2 in a PKCδ- and ERK-dependent manner, converging on Runx2 activity. Here, we mapped the interaction domain of Cx43 and PKCδ to amino acids 243-302 of the Cx43 CT by GST pulldown assay. Using Runx2-responsive luciferase reporter assays, a Cx43 deletion construct (Cx43 S244Stop), which lacks the C-terminus (amino acids 244-382), failed to support the Cx43-dependent potentiation of transcription following FGF2 treatment in MC3T3 osteoblast-like cells. Similarly, overexpression of Cx43 S244Stop could not mimic the ability of the full length Cx43 to stimulate expression of osteoblast genes. In contrast to full length Cx43, overexpression of just the Cx43 CT (amino acids 236-382) inhibited both transcription from a Runx2 reporter and signaling via PKCδ and ERK. Inhibition of signaling by the CT did not occur in HeLa cells, which lack endogenous Cx43. In summary, the data support a model in which an intact Cx43 is required for both signal propagation/permeability (i.e., channel function) and local recruitment of signaling complexes to the CT (i.e., docking function) in order to mediate its cellular effects. Further, while the CT alone has channel independent activity, it is opposing to the effect of overexpression of the full length Cx43 channel in this cell context.

Shifting paradigms on the role of connexin43 in the skeletal response to mechanical load

Gap junctions (GJs) are membrane-spanning channels that allow for the movement of small molecules across cell membranes. Connexin43 (Cx43) is the predominant GJ protein in bone. In vitro studies suggest that gap junctional intercellular communication (GJIC) sensitizes bone cells to mechanical signals. Additionally, mechanical signals detected by osteocytes are communicated to osteoblasts via GJIC, and osteocytic Cx43 hemichannels release anabolic factors, such as PGE2 and ATP, in response to mechanical load. These findings and others have led to near consensus among researchers in the field that GJIC, hemichannels or connexins facilitate the anabolic response of bone to mechanical load and, in their absence, bone would be less sensitive to load. However, recent in vivo evidence suggests the opposite is true. Studies from our laboratory and others demonstrate that Cx43-deficient mice have an increased anabolic response to mechanical load and are protected against the catabolic effects of mechanical unloading. These developments suggest a paradigm shift in our understanding of connexins, GJIC, and mechanotransduction in bone. That is, inhibiting bone cell Cx43 expression or GJIC has a beneficial effect on bone's response to its mechanical environment, preserving bone during unloading and enhancing its formation during loading. Here, we review literature in support of this hypothesis and suggest a mechanism by which Cx43, through interaction with WNT/β-catenin signaling, moderates both arms of bone remodeling.

Gap junctional regulation of signal transduction in bone cells

The role of gap junctions, particularly that of connexin43 (Cx43), has become an area of increasing interest in bone physiology. An abundance of studies have shown that Cx43 influences the function of osteoblasts and osteocytes, which ultimately impacts bone mass acquisition and skeletal homeostasis. However, the molecular details underlying how Cx43 regulates bone are only coming into focus and have proven to be more complex than originally thought. In this review, we focus on the diverse molecular mechanisms by which Cx43 gap junctions and hemichannels regulate cell signaling pathways, gene expression, mechanotransduction and cell survival in bone cells. This review will highlight key signaling factors that have been identified as downstream effectors of Cx43 and the impact of these pathways on distinct osteoblast and osteocyte functions.

Synergistic effects of orbital shear stress on in vitro growth and osteogenic differentiation of human alveolar bone-derived mesenchymal stem cells

Cellular behavior is dependent on a variety of physical cues required for normal tissue function. In order to mimic native tissue environments, human alveolar bone-derived mesenchymal stem cells (hABMSCs) were exposed to orbital shear stress (OSS) in a low-speed orbital shaker. The synergistic effects of OSS on proliferation and differentiation of hABMSCs were investigated. In particular, we induced the osteoblastic differentiation of hABMSCs cultured in the absence of OM by exposing hABMSCs to OSS (0.86-1.51 dyne/cm(2)). Activation of Cx43 was associated with exposure of hABMSCs to OSS. The viability of cells stimulated for 10, 30, 60, 120, and 180 min/day increased by approximately 10% compared with that of control. The OSS groups with stimulation of 10, 30, and 60 min/day had more intense mineralized nodules compared with the control group. In quantification of vascular endothelial growth factor (VEGF) and bone morphogenetic protein-2 (BMP-2) protein, VEGF protein levels under stimulation for 10, 60, and 180 min/day and BMP-2 levels under stimulation for 60, 120, and 180 min/day were significantly different compared with those of the control. In conclusion, the results indicated that exposing hABMSCs to OSS enhanced their differentiation and maturation.

Inhibition of GSK-3β rescues the impairments in bone formation and mechanical properties associated with fracture healing in osteoblast selective connexin 43 deficient mice

Connexin 43 (Cx43) is the most abundant gap junction protein in bone and is required for osteoblastic differentiation and bone homeostasis. During fracture healing, Cx43 is abundantly expressed in osteoblasts and osteocytes, while Cx43 deficiency impairs bone formation and healing. In the present study we selectively deleted Cx43 in the osteoblastic lineage from immature osteoblasts through osteocytes and tested the hypothesis that Cx43 deficiency results in delayed osteoblastic differentiation and impaired restoration of biomechanical properties due to attenuated β-catenin expression relative to wild type littermates. Here we show that Cx43 deficiency results in alterations in the mineralization and remodeling phases of healing. In Cx43 deficient fractures the mineralization phase is marked by delayed expression of osteogenic genes. Additionally, the decrease in the RankL/ Opg ratio, osteoclast number and osteoclast size suggest decreased osteoclast bone resorption and remodeling. These changes in healing result in functional deficits as shown by a decrease in ultimate torque at failure. Consistent with these impairments in healing, β-catenin expression is attenuated in Cx43 deficient fractures at 14 and 21 days, while Sclerostin (Sost) expression, a negative regulator of bone formation is increased in Cx43cKO fractures at 21 days, as is GSK-3β, a key component of the β-catenin proteasomal degradation complex. Furthermore, we show that alterations in healing in Cx43 deficient fractures can be rescued by inhibiting GSK-3β activity using Lithium Chloride (LiCl). Treatment of Cx43 deficient mice with LiCl restores both normal bone formation and mechanical properties relative to LiCl treated WT fractures. This study suggests that Cx43 is a potential therapeutic target to enhance fracture healing and identifies a previously unknown role for Cx43 in regulating β-catenin expression and thus bone formation during fracture repair.

The regulation of runt-related transcription factor 2 by fibroblast growth factor-2 and connexin43 requires the inositol polyphosphate/protein kinase Cδ cascade

Connexin43 (Cx43) plays a critical role in osteoblast function and bone mass accrual, yet the identity of the second messengers communicated by Cx43 gap junctions, the targets of these second messengers and how they regulate osteoblast function remain largely unknown. We have shown that alterations of Cx43 expression in osteoblasts can impact the responsiveness to fibroblast growth factor-2 (FGF2), by modulating the transcriptional activity of runt-related transcription factor 2 (Runx2). In this study, we examined the contribution of the phospholipase Cγ1/inositol polyphosphate/protein kinase C delta (PKCδ) cascade to the Cx43-dependent transcriptional response of MC3T3 osteoblasts to FGF2. Knockdown of expression and/or inhibition of function of phospholipase Cγ1, inositol polyphosphate multikinase, which generates inositol 1,3,4,5-tetrakisphosphate (InsP₄) and InsP₅, and inositol hexakisphosphate kinase 1/2, which generates inositol pyrophosphates, prevented the ability of Cx43 to potentiate FGF2-induced signaling through Runx2. Conversely, overexpression of phospholipase Cγ1 and inositol hexakisphosphate kinase 1/2 enhanced FGF2 activation of Runx2 and the effect of Cx43 overexpression on this response. Disruption of these pathways blocked the nuclear accumulation of PKCδ and the FGF2-dependent interaction of PKCδ and Runx2, reducing Runx2 transcriptional activity. These data reveal that FGF2-signaling involves the inositol polyphosphate cascade, including inositol hexakisphosphate kinase (IP6K), and demonstrate that IP6K regulates Runx2 and osteoblast gene expression. Additionally, these data implicate the water-soluble inositol polyphosphates as mediators of the Cx43-dependent amplification of the osteoblast response to FGF2, and suggest that these low molecular weight second messengers may be biologically relevant mediators of osteoblast function that are communicated by Cx43-gap junctions.

Human articular chondrocytes express multiple gap junction proteins: differential expression of connexins in normal and osteoarthritic cartilage

Osteoarthritis (OA) is the most common joint disease and involves progressive degeneration of articular cartilage. The aim of this study was to investigate if chondrocytes from human articular cartilage express gap junction proteins called connexins (Cxs). We show that human chondrocytes in tissue express Cx43, Cx45, Cx32, and Cx46. We also find that primary chondrocytes from adults retain the capacity to form functional voltage-dependent gap junctions. Immunohistochemistry experiments in cartilage from OA patients revealed significantly elevated levels of Cx43 and Cx45 in the superficial zone and down through the next approximately 1000 μm of tissue. These zones corresponded with regions damaged in OA that also had high levels of proliferative cell nuclear antigen. An increased number of Cxs may help explain the increased proliferation of cells in clusters that finally lead to tissue homeostasis loss. Conversely, high levels of Cxs in OA cartilage reflect the increased number of adjacent cells in clusters that are able to interact directly by gap junctions as compared with hemichannels on single cells in normal cartilage. Our data provide strong evidence that OA patients have a loss of the usual ordered distribution of Cxs in the damaged zones and that the reductions in Cx43 levels are accompanied by the loss of correct Cx localization in the nondamaged areas.

Osteoblast and osteocyte-specific loss of Connexin43 results in delayed bone formation and healing during murine fracture healing

Connexin43 (Cx43) plays an important role in osteoblastic differentiation in vitro, and bone formation in vivo. Mice with osteoblast/osteocyte-specific loss of Cx43 display decreased gap junctional intercellular communication (GJIC), bone density, and cortical thickness. To determine the role of Cx43 in fracture healing, a closed femur fracture was induced in Osteocalcin-Cre+; Cx43(flox/flox) (Cx43cKO) and Cre-; Cx43(flox/flox) (WT) mice. We tested the hypothesis that loss of Cx43 results in decreased bone formation and impaired healing following fracture. Here, we show that osteoblast and osteocyte-specific deletion of Cx43 results in decreased bone formation, bone remodeling, and mechanical properties during fracture healing. Cx43cKO mice display decreased bone volume, total volume, and fewer TRAP+ osteoclasts. Furthermore, loss of Cx43 in mature osteoblasts and osteocytes results in a significant decrease in torsional rigidity between 21 and 35 days post-fracture, compared to WT mice. These studies identify a novel role for the gap junction protein Cx43 during fracture healing, suggesting that loss of Cx43 can result in both decreased bone formation and bone resorption. Therefore, enhancing Cx43 expression or GJIC may provide a novel means to enhance bone formation during fracture healing.

Age-related changes in gap junctional intercellular communication in osteoblastic cells

Aging demonstrates deleterious effects upon the skeleton which can predispose an individual to osteoporosis and related fractures. Despite the well-documented evidence that aging decreases bone formation, there remains little understanding whereby cellular aging alters skeletal homeostasis. We, and others, have previously demonstrated that gap junctions--membrane-spanning channels that allow direct cell-to-cell conductance of small signaling molecules--are critically involved in osteoblast differentiation and skeletal homeostasis. We examined whether the capacity of rat osteoblastic cells to form gap junctions and respond to known modulators of gap junction intercellular communication (GJIC) was dependent on the age of the animal from which they were isolated. We observed no effect of age upon osteoblastic Cx43 mRNA, protein or GJIC. We also examined age-related changes in PTH-stimulated GJIC. PTH demonstrated age-dependent effects upon GJIC: Osteoblastic cells from young rats increased GJIC in response to PTH, whereas there was no change in GJIC in response to PTH in osteoblastic cells from mature or old rats. PTH-stimulated GJIC occurred independently of changes in Cx43 mRNA or protein expression. Cholera toxin significantly increased GJIC in osteoblastic cells from young rats compared to those from mature and old rats. These data demonstrate an age-related impairment in the capacity of osteoblastic cells to generate functional gap junctions in response to PTH, and suggest that an age-related defect in G protein-coupled adenylate cyclase activity at least partially contributes to decreased PTH-stimulated GJIC.

Fibroblasts regulate osteoblasts through gap junctional communication

BACKGROUND AIMS

Fibroblasts are present in most tissues of the body, playing an active role in the regulation of homeostasis in such tissues. While fibroblast heterotypic interactions are acknowledged in the regeneration of tissues such as skin and periodontal ligament, their role in bone regeneration is far from being understood. We hypothesized that fibroblasts could influence osteoblasts, and as connexin 43 is the predominant connexin in both cell types, we speculated that those heterotypic interactions could occur through gap junctional communication (GjC).

METHODS

Direct co-cultures of human mesenchymal stromal cell (hMSC)-derived osteoblasts and human dermal fibroblasts (hDFb) were established in the presence and absence of the GjC inhibitor α-glycyrrhetinic acid. Communication between osteoblasts and hDFb via GjC was verified by transference of the gap junction-permeable dye calcein-AM. Cell proliferation was assessed by dsDNA quantification, while osteogenic differentiation was evaluated by measuring alkaline phosphatase (ALP) activity and the expression of osteogenic markers by real-time polymerase chain reaction (PCR).

RESULTS

The amount of calcein-AM transferred between the different cell types decreased when α-glycyrrhetinic acid was used. While the proliferation of the hMSC-derived osteoblasts was not affected by the presence of the hDFb, the level of osteogenic markers such as ALP activity and osteocalcin in transcripts in osteoblasts was severely diminished. This effect was partially reversed by adding α-glycyrrhetinic acid to the co-cultures.

CONCLUSIONS

The results strongly suggest that fibroblasts regulate osteoblast behavior partially through GjC. This information could be critical for predicting the outcome of strategies aimed at promoting bone regeneration as, for example, in bone tissue-engineering approaches.

Cell-based therapy for prevention and reversal of myocardial remodeling

Although pharmacological and interventional advances have reduced the morbidity and mortality of ischemic heart disease, there is an ongoing need for novel therapeutic strategies that prevent or reverse progressive ventricular remodeling following myocardial infarction, the process that forms the substrate for ventricular failure. The development of cell-based therapy as a strategy to repair or regenerate injured tissue offers extraordinary promise for a powerful anti-remodeling therapy. In this regard, the field of cell therapy has made major advancements in the past decade. Accumulating data from preclinical studies have provided novel insights into stem cell engraftment, differentiation, and interactions with host cellular elements, as well as the effectiveness of various methods of cell delivery and accuracy of diverse imaging modalities to assess therapeutic efficacy. These findings have in turn guided rationally designed translational clinical investigations. Collectively, there is a growing understanding of the parameters that underlie successful cell-based approaches for improving heart structure and function in ischemic and other cardiomyopathies.

ERK acts in parallel to PKCδ to mediate the connexin43-dependent potentiation of Runx2 activity by FGF2 in MC3T3 osteoblasts

The gap junction protein, connexin43 (Cx43), plays an important role in skeletal biology. Previously, we have shown that Cx43 can enhance the signaling and transcriptional response to fibroblast growth factor 2 (FGF2) in osteoblasts by increasing protein kinase C-δ (PKCδ) activation to affect Runx2 activity. In the present study, we show by luciferase reporter assays that the ERK signaling cascade acts in parallel to PKCδ to modulate Runx2 activity downstream of the Cx43-dependent amplification of FGF2 signaling. The PKCδ-independent activation of ERK by FGF2 was confirmed by Western blotting, as was the Cx43-dependent enhancement of ERK activation. Consistent with our prior observations for PKCδ, flow cytometry analyses show that Cx43 overexpression enhances the percentage of phospho-ERK-positive cells in response to FGF2, supporting the notion that shared signals among gap junction-coupled cells result in the enhanced response to FGF2. Western blots and luciferase reporter assays performed on osteoblasts cultured under low-density and high-density conditions revealed that cell-cell contacts are required for Cx43 to amplify ERK activation and gene transcription. Similarly, inhibition of gap junctional communication with the channel blocker 18β-glycyrrhetinic acid attenuates the Cx43-dependent enhancement of Runx2-transcriptional activity. In total, these data underscore the importance of cell-cell communication and activation of the ERK and PKCδ pathways in the coordination of the osteoblast response to FGF2 among populations of osteoblasts.

The transcriptional activity of osterix requires the recruitment of Sp1 to the osteocalcin proximal promoter

The transcription factor osterix (Osx/Sp7) is required for osteogenic differentiation and bone formation in vivo. While Osx can act at canonical Sp1 DNA-binding sites and/or interact with NFATc1 to cooperatively regulate transcription in some osteoblast promoters, little is known about the molecular details by which Osx regulates osteocalcin (OCN) transcription. We previously identified in the OCN proximal promoter a minimal C/T-rich motif, termed OCN-CxRE (connexin-response element) that binds Sp1 and Sp3 in a gap junction-dependent manner. In the present study, we hypothesized that Osx could act via this non-canonical Sp1/Sp3-binding element to regulate OCN transcription. OCN promoter luciferase reporter assays show that Osx alone is an insufficient activator that requires Sp1, but not Sp3, to synergistically stimulate OCN promoter activity. Moreover, promoter deletion analyses demonstrate that both the Sp1/Sp3-binding OCN-CxRE (-70 to -57) and the -92 to -87 region of the OCN proximal promoter are critical for Osx/Sp1 synergistic activities. Our data show that Sp1 influences Osx activity by enhancing Osx occupancy on the OCN promoter, perhaps via physical interactions between the two transcription factors. Finally, alteration of the expression of the gap junction protein connexin43 modulates the recruitment of both Sp1 and Osx to the OCN promoter. In total, our data are strongly in support of Sp1 as an essential transcription factor required for Osx recruitment and transactivation of the OCN promoter. Further, these data lend insight into a mechanism by which alteration of connexin43 impacts osteogenesis in vitro and in vivo.

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.

Pannexin 3 functions as an ER Ca(2+) channel, hemichannel, and gap junction to promote osteoblast differentiation

Pannexin 3 functions as an essential protein for Ca²⁺ and ATP transport and cell–cell communication during osteoblast differentiation

The pannexin proteins represent a new gap junction family. However, the cellular functions of pannexins remain largely unknown. Here, we demonstrate that pannexin 3 (Panx3) promotes differentiation of osteoblasts and ex vivo growth of metatarsals. Panx3 expression was induced during osteogenic differentiation of C2C12 cells and primary calvarial cells, and suppression of this endogenous expression inhibited differentiation. Panx3 functioned as a unique Ca²⁺ channel in the endoplasmic reticulum (ER), which was activated by purinergic receptor/phosphoinositide 3-kinase (PI3K)/Akt signaling, followed by activation of calmodulin signaling for differentiation. Panx3 also formed hemichannels that allowed release of ATP into the extracellular space and activation of purinergic receptors with the subsequent activation of PI3K–Akt signaling. Panx3 also formed gap junctions and propagated Ca²⁺ waves between cells. Blocking the Panx3 Ca²⁺ channel and gap junction activities inhibited osteoblast differentiation. Thus, Panx3 appears to be a new regulator that promotes osteoblast differentiation by functioning as an ER Ca²⁺ channel and a hemichannel, and by forming gap junctions.

Voluntary exercise has long-term in vivo protective effects on osteocyte viability and bone strength following ovariectomy

Osteocytes are recognized as having a pivotal role in bone tissue homeostasis, and stimuli that increase osteocyte death result in decreased bone tissue quality. Previous in vitro studies have shown that mechanical stimulation prevents osteocyte death; however, in vivo evidence of this protective effect is limited. The aim of this study was to investigate if mechanical stimulation provided by voluntary exercise reduces osteocyte death caused by estrogen deficiency. Thirty-two female Wistar rats (5 months old) were either sacrificed as baseline controls (BSL, n = 7), ovariectomized or sham-operated and housed in cages with a voluntary running wheel (OVXEX, n = 7; SHAMEX, n = 6), or ovariectomized or sham-operated and housed in standard cages of equivalent size (OVXSED, n = 6; SHAMSED, n = 6) and sacrificed at age 14 months. Histomorphometric analysis of femur mid-diaphysis cortical bone revealed a significantly higher osteocyte number (N.Ot) and lower empty lacunae number (N.Lc) in both the OVXEX and SHAMEX groups compared to their SED counterparts. Intracortical porosity (Po.Ar) was also lower in both EX groups compared to their SED counterparts and significantly correlated with N.Lc (r = 0.616; P < 0.001). Three-point bending testing showed a significantly higher Young's modulus and ultimate stress in OVXEX compared to OVXSED and significant correlations between N.Lc and both yield stress (r = -0.376, P < 0.05) and ultimate stress (r = -0.369, P < 0.05) and between intracortical porosity and bone ultimate stress (r = -0.451, P < 0.05). Our results show that voluntary exercise prevented osteocyte death and that this protective effect was associated with increases in femur ultimate stress, which could be partially explained by decreases in Po.Ar.

Connexin 43 expression at an early stage in dog mandibles by β-TCP

β-TCP was implanted into bone defects of dog mandibles, and gene expression profiles were examined using DNA microarray. An implant drill was used to make bone defects, and then β-TCP was filled into bone defects. All specimens were taken out, total RNA was isolated, and levels were analyzed using Affymetrix GeneChip. Higher mRNA levels of connexin 43 (Cx43) and Cx45 were observed in β-TCP-implanted samples compared with controls. The enhancement of Cx43 and Cx45 by β-TCP was confirmed by RT-PCR and real-time RT-PCR. Since Cx43 is known to express in bone-forming regions and is involved in osteogenesis through gap junctional intercellular bone-cell communication (GJIB), immunohistochemical staining was also examined and demonstrated Cx43 protein expression was increased in β-TCP-implanted bone. Cx43 plays a role in osteogenesis through GJIB; therefore, the stimulation of Cx43 expression by β-TCP might be a mechanism of accelerating wound healing and bone formation.

Role of neural-cadherin in early osteoblastic differentiation of human bone marrow stromal cells cocultured with human umbilical vein endothelial cells

In our previous studies, roles of gap junction and vascular endothelial growth factor in the cross-talking of human bone marrow stromal cells (HBMSCs) and human umbilical vein endothelial cells (HUVECs) have been extensively studied. The present study focused on the investigation of the roles of neural (N)-cadherin in early differentiation of HBMSCs in direct-contact cocultures with HUVECs for 24 and 48 h. Quantitative real-time polymerase chain reaction, immunofluorescence, Western blot, as well as functional studies were applied to perform the studies at both protein and gene levels. Results showed that cocultured cells expressed much higher N-cadherin than monocultured cells after 24 and 48 h of culture. We observed that N-cadherin concentrated in the membrane of cocultured HBMSCs (co-HBMSCs) while distributed within the cytoplasm of monocultured HBMSCs, which indicated that the cell-cell adhesion was improved between cocultured cells. In addition, more beta-catenin was found to translocate into the cocultured cells nuclei and more T cell factor-1 (TCF-1) were detected in cocultured cells than in the monocultured cells. Moreover, mRNA levels of early osteoblastic markers including alkaline phosphatase (ALP) and type I collagen (Col-I) of co-HBMSCs were significantly upregulated, whereas neutralization of N-cadherin led to a downregulation of ALP and Col-I in both of the HBMSCs and co-HBMSCs compared with untreated cells. Taking our findings together it can be concluded that cocultures of HBMSCs with HUVECs increased N-cadherin expression and improved cell-cell adhesion. Whether this applies only to osteoprogenitor cells or to all the cell types in the culture will need to be determined by further studies. Subsequently, signaling transduction might be induced with the participation of beta-catenin and TCF-1. With the N-cadherin-mediated cell-cell adhesion and signaling transductions, the early osteoblastic differentiation of co-HBMSCs was significantly upregulated.