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

Focused Low-Intensity Pulsed Ultrasound (FLIPUS) Mitigates Apoptosis of MLO-Y4 Osteocyte-like Cells

Long cytoplasmic processes of osteocytes orchestrate bone activity by integration of biochemical and mechanical signals and regulate load-induced bone adaptation. Low-Intensity Pulsed Ultrasound (LIPUS) is a clinically used technique for fracture healing that delivers mechanical impulses to the damaged bone tissue in a non-invasive and non-ionizing manner. The mechanism of action of LIPUS is still controversially discussed in the scientific community. In this study, the effect of focused LIPUS (FLIPUS) on the survival of starved MLO-Y4 osteocytes was investigated in vitro. Osteocytes stimulated for 10 min with FLIPUS exhibited extended dendrites, which formed frequent connections to neighboring cells and spanned longer distances. The sonicated cells displayed thick actin bundles and experienced increase in expression of connexin 43 (Cx43) proteins, especially on their dendrites, and E11 glycoprotein, which is responsible for the elongation of cellular cytoplasmic processes. After stimulation, expression of cell growth and survival genes as well as genes related to cell-cell communication was augmented. In addition, cell viability was improved after the sonication, and a decrease in ATP release in the medium was observed. In summary, FLIPUS mitigated apoptosis of starved osteocytes, which is likely related to the formation of the extensive dendritic network that ensured cell survival.

Functional Roles of Connexins and Gap Junctions in Osteo-Chondral Cellular Components

Gap junctions (GJs) formed by connexins (Cxs) play an important role in the intercellular communication within most body tissues. In this paper, we focus on GJs and Cxs present in skeletal tissues. Cx43 is the most expressed connexin, participating in the formation of both GJs for intercellular communication and hemichannels (HCs) for communication with the external environment. Through GJs in long dendritic-like cytoplasmic processes, osteocytes embedded in deep lacunae are able to form a functional syncytium not only with neighboring osteocytes but also with bone cells located at the bone surface, despite the surrounding mineralized matrix. The functional syncytium allows a coordinated cell activity through the wide propagation of calcium waves, nutrients and anabolic and/or catabolic factors. Acting as mechanosensors, osteocytes are able to transduce mechanical stimuli into biological signals that spread through the syncytium to orchestrate bone remodeling. The fundamental role of Cxs and GJs is confirmed by a plethora of investigations that have highlighted how up- and downregulation of Cxs and GJs critically influence skeletal development and cartilage functions. A better knowledge of GJ and Cx mechanisms in physiological and pathological conditions might help in developing therapeutic approaches aimed at the treatment of human skeletal system disorders.

Osteocytes regulate bone anabolic response to mechanical loading in male mice via activation of integrin α5

Physical mechanical stimulation can maintain and even increase bone mass. Here, we report an important role of osteocytic integrin α5 in regulating the anabolic response of bone to mechanical loading using an Itga5 conditional gene knockout (cKO) mouse model. Integrin α5 gene deletion increased apoptotic osteocytes and reduced cortical anabolic responses to tibial compression including decreased endosteal osteoblasts and bone formation, and increased endosteal osteoclasts and bone resorption, contributing to the decreased bone area fraction and biomechanical properties, leading to an enlarged bone marrow area in cKO mice. Similar disruption of anabolic responses to mechanical loading was also detected in cKO trabecular bone. Moreover, integrin α5 deficiency impeded load-induced Cx43 hemichannel opening, and production and release of PGE2, an anabolic factor, resulting in attenuated effects of the loading on catabolic sclerostin (SOST) reduction and anabolic β-catenin increase. Together, this study shows an indispensable role of integrin α5 in osteocytes in the anabolic action of mechanical loading on skeletal tissue through activation of hemichannels and PGE2-evoked gene expression. Integrin α5 could act as a potential new therapeutic target for bone loss, especially in the elderly population with impeded mechanical sensitivity.

Connexin hemichannels with prostaglandin release in anabolic function of bone to mechanical loading

Mechanical stimulation, such as physical exercise, is essential for bone formation and health. Here, we demonstrate the critical role of osteocytic Cx43 hemichannels in anabolic function of bone in response to mechanical loading. Two transgenic mouse models, R76W and Δ130–136, expressing dominant-negative Cx43 mutants in osteocytes were adopted. Mechanical loading of tibial bone increased cortical bone mass and mechanical properties in wild-type and gap junction-impaired R76W mice through increased PGE₂, endosteal osteoblast activity, and decreased sclerostin. These anabolic responses were impeded in gap junction/hemichannel-impaired Δ130–136 mice and accompanied by increased endosteal osteoclast activity. Specific inhibition of Cx43 hemichannels by Cx43(M1) antibody suppressed PGE₂ secretion and impeded loading-induced endosteal osteoblast activity, bone formation and anabolic gene expression. PGE₂ administration rescued the osteogenic response to mechanical loading impeded by impaired hemichannels. Together, osteocytic Cx43 hemichannels could be a potential new therapeutic target for treating bone loss and osteoporosis.

Prostaglandin E2 and Connexin 43 crosstalk in the osteogenesis induced by extracorporeal shockwave

As a type of mechanical stimulation, extracorporeal shockwave (ESW) has been widely used in the clinic to treat bone fracture delayed union and non-unions. A large number of studies have shown beneficial effects of ESW in promoting fracture healing by inducing bone regeneration; however, the underlying mechanisms remain unclear. ESW has been shown to induce the production of prostaglandin E2 (PGE2), which is essential for gap junction intercellular communication in response to mechanical stress. Among the 19 known gap junction subunits, connexin43 (Cx43) is the most prevalent for mediating the response of mechanical stress. However, to our knowledge, the effect of ESW on Cx43 expression has not been reported before. Herein, we propose that a crosstalk between PGE2 and Cx43 is involved in the enhancement of osteogenesis induced by ESW. We review the currently available data to propose an unrevealed, but important mechanism via which ESW treatment affects osteogenic differentiation of bone marrow stromal cells.

Glucocorticoid Suppresses Connexin 43 Expression by Inhibiting the Akt/mTOR Signaling Pathway in Osteoblasts

The inhibition of proliferation or functional alteration of osteoblasts by glucocorticoids (GCs) has been recognized as an important etiology of GC-induced osteoporosis (GIO). Connexin 43 (Cx43) is the most abundant connexin isoform in bone cells and plays important roles in bone remodeling. Despite the important role of Cx43 in bone homeostasis and the prevalence of GIO, the direct action of GCs on Cx43 expression in osteoblasts has been poorly described. The aim of the present study was to evaluate how GCs affect Cx43 expression in osteoblasts. Dexamethasone (Dex) treatment decreased expression of Cx43 RNA and protein in MC3T3-E1 mouse osteoblastic cells. Reduction of Cx43 expression by Dex was dependent on the glucocorticoid receptor (GR), as it was abolished by pretreatment with a GR blocker. Treatment with PTH (1-34), a medication used for GIO management, counteracted the suppression of Cx43 by Dex. Akt or mTOR signaling modulators revealed the involvement of the Akt/mTOR signaling pathway in Dex-induced reduction of Cx43 expression. Moreover, overexpression of Cx43 significantly attenuated Dex-inhibited cell viability and proliferation, as evidenced by MTT and bromodeoxyuridine (BrdU) incorporation assay of MC3T3-E1 cells. To account for possible species or cell type differences, human primary osteoblasts were treated with Dex and similar downregulation of Cx43 by Dex was observed. In addition, immunofluorescent staining for Cx43 further demonstrated an apparent decrease in Dex-treated human osteoblasts, while analysis of lucifer yellow propagation revealed reduced gap junction intercellular communication by Dex. Collectively, these findings indicate that GCs suppress Cx43 expression in osteoblasts via GR and the Akt/mTOR signaling pathway and overexpression of Cx43 may, at least in part, rescue osteoblasts from GC-induced reductions in proliferation.

Communication of cAMP by connexin43 gap junctions regulates osteoblast signaling and gene expression

Connexin43 (Cx43) containing gap junctions play an important role in bone homeostasis, yet little is known about the second messengers communicated by Cx43 among bone cells. Here, we used MC3T3-E1 pre-osteoblasts and UMR106 rat osteosarcoma cells to test the hypothesis that cAMP is a second messenger communicated by bone cells through Cx43 containing gap junctions in a manner that is sufficient to impact osteoblast function. Overexpression of Cx43 markedly enhanced the activity of a cAMP-response element driven transcriptional luciferase reporter (CRE-luc) and increased phospho-CREB and phospho-ERK1/2 levels following expression of a constitutively active Gsα or by treatment with prostaglandin E2 (PGE2), 3-Isobutyl-1-methyl xanthine (IBMX) or forskolin. The Cx43-dependent potentiation of signaling in PGE2 treated cells was not accompanied by a further increase in cAMP levels, suggesting that the cAMP was shared between cells rather than Cx43 enhancing cAMP production. To support this, we developed a novel assay in which one set of cells expressing constitutively active Gsα (donor cells) were co-cultured with a second set of cells expressing a CRE-luc reporter (acceptor cells). Using this assay, activation of a CRE-luc reporter in the acceptor cells was both Cx43- and cell contact-dependent, indicating communication of cAMP among cells. Finally, we showed that Cx43 increased the cAMP-dependent mRNA expression of receptor activator of nuclear factor kappa B ligand (RANKL) and enhanced the repression of the sclerostin mRNA, implying a potential mechanism for the modulation of tissue remodeling. In total, these data demonstrate that Cx43 can communicate cAMP between cells and, more importantly, that the communicated cAMP is sufficient to impact signal transduction cascades and the expression of key bone effector molecules between interconnected cells.

The "connexin" between bone cells and skeletal functions

The processes of bone modeling and remodeling are crucial in the skeleton's functions as a supportive and protective structure, a mineral reservoir, and an endocrine organ. The coordination between bone cell activities (bone formation and bone resorption), necessary to maintain the integrity of the skeleton during these processes, is mediated at least in part by cell-cell and cell-environment interactions across gap junctions and hemichannels. The increasing number of genetically engineered Connexin 43 (Cx43) knockout and missense mouse models have provided insight into the complex and critical roles of Cx43-containing gap junctions and hemichannels in the development and turnover of the skeleton, in differentiation, activity and survival of the bone cell lineages, and in the cellular and molecular mechanisms by which Cx43 functions and assists in mediating cellular responses to stimuli in bone. Cx43 may be an important potential therapeutic target, making it crucial that we continue to gain understanding of the multiple and complex roles of Cx43 in bone.

Gap junction regulation of vascular tone: implications of modulatory intercellular communication during gestation

In the vasculature, gap junctions (GJ) play a multifaceted role by serving as direct conduits for cell-cell intercellular communication via the facilitated diffusion of signaling molecules. GJs are essential for the control of gene expression and coordinated vascular development in addition to vascular function. The coupling of endothelial cells to each other, as well as with vascular smooth muscle cells via GJs, plays a relevant role in the control of vasomotor tone, tissue perfusion and arterial blood pressure. The regulation of cell-signaling is paramount to cardiovascular adaptations of pregnancy. Pregnancy requires highly developed cell-to-cell coupling, which is affected partly through the formation of intercellular GJs by Cx43, a gap junction protein, within adjacent cell membranes to help facilitate the increase of uterine blood flow (UBF) in order to ensure adequate perfusion for nutrient and oxygen delivery to the placenta and thus the fetus. One mode of communication that plays a critical role in regulating Cx43 is the release of endothelial-derived vasodilators such as prostacyclin (PGI2) and nitric oxide (NO) and their respective signaling mechanisms involving second messengers (cAMP and cGMP, respectively) that are likely to be important in maintaining UBF. Therefore, the assertion we present in this review is that GJs play an integral if not a central role in maintaining UBF by controlling rises in vasodilators (PGI2 and NO) via cyclic nucleotides. In this review, we discuss: (1) GJ structure and regulation; (2) second messenger regulation of GJ phosphorylation and formation; (3) pregnancy-induced changes in cell-signaling; and (4) the role of uterine arterial endothelial GJs during gestation. These topics integrate the current knowledge of this scientific field with interpretations and hypotheses regarding the vascular effects that are mediated by GJs and their relationship with vasodilatory vascular adaptations required for modulating the dramatic physiological rises in uteroplacental perfusion and blood flow observed during normal pregnancy.

New insights into the role of connexins in pancreatic islet function and diabetes

Multi-cellular systems require complex signaling mechanisms for proper tissue function, to mediate signaling between cells in close proximity and at distances. This holds true for the islets of Langerhans, which are multicellular micro-organs located in the pancreas responsible for glycemic control, through secretion of insulin and other hormones. Coupling of electrical and metabolic signaling between islet β-cells is required for proper insulin secretion and effective glycemic control. β-cell specific coupling is established through gap junctions composed of connexin36, which results in coordinated insulin release across the islet. Islet connexins have been implicated in both Type-1 and Type-2 diabetes; however a clear link remains to be determined. The goal of this review is to discuss recent discoveries regarding the role of connexins in regulating insulin secretion, the regulation of connexins within the islet, and recent studies which support a role for connexins in diabetes. Further studies which investigate the regulation of connexins in the islet and their role in diabetes may lead to novel diabetes therapies which regulate islet function and β-cell survival through modulation of gap junction coupling.

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.

Dipyridamole-related enhancement of gap junction coupling in the GM-7373 aortic endothelial cells correlates with an increase in the amount of connexin 43 mRNA and protein as well as gap junction plaques

Previous data showed that dipyridamole enhanced gap junction coupling in vascular endothelial and smooth muscle cell lines by a cAMP-dependent mechanism. The present study investigates the level at which dipyridamole affects gap junction coupling. In the GM-7373 endothelial cell line, scrape loading/dye transfer experiments revealed a rapid increase in gap junction coupling induced during the first 6 h of dipyridamole treatment, followed by a slow increase induced by further incubation. Immunostaining analyses showed that the rapid enhancement of gap junction coupling correlated with an increased amount of Cx43 gap junction plaques and a reduced amount of Cx43 containing vesicles, while the amount of Cx43 mRNA or protein was not changed during this period, as found by semiquantitative RT-PCR and Western blot. Additionally, brefeldin A did not block this short-term-induced enhancement of gap junction coupling. Along with the dipyridamole-induced long-term enhancement of gap junction coupling, the amount of Cx43 mRNA and protein additionally to the amount of Cx43 gap junction plaques were increased. Furthermore, the anti-Cx43 antibody detected only two bands at 42 kDa and 44 kDa in control cells and cells treated with dipyridamole for 6 h, while long-term dipyridamole-treated cells showed a third band at 46 kDa. We propose that a dipyridamole-induced cAMP synthesis increased gap junction coupling in the GM-7373 endothelial cell line at different levels: the short-term effect is related to already oligomerised connexins beyond the Golgi apparatus and the long-term effect involves new expression and synthesis as well as posttranslational modification of Cx43.

Gap junction and hemichannel functions in osteocytes

Cell-to-cell and cell-to-matrix communication in bone cells mediated by gap junctions and hemichannels, respectively, maintains bone homeostasis. Gap junctional communication between cells permits the passage of small molecules including calcium and cyclic AMP. This cell-to-cell communication occurs between bone cells including osteoblasts, osteoclasts and osteocytes, and is important in both bone formation and bone resorption. Connexin (Cx) 43 is the predominant gap junction protein in bone cells, and facilitates the communication of cellular signals either through docking of gap junctions between two cells, or through the formation of un-paired hemichannels. Systemic deletion of Cx43 results in perinatal lethality, so conditional deletion models are necessary to study the postnatal role of gap junctions in bone. These models provide the opportunity to determine the role of gap junctions in specific bone cells, notably the osteocyte. In this review, we summarize the key roles that gap junctions and hemichannels in osteocytes play in bone cell response to many stimuli including mechanical loading, intracellular and extracellular stimuli, such as parathyroid hormone, PGE2, plasma calcium levels and pH, as well as in maintaining osteocyte survival.

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.

Connexin43 interacts with βarrestin: a pre-requisite for osteoblast survival induced by parathyroid hormone

Parathyroid hormone (PTH) promotes osteoblast survival through a mechanism that depends on cAMP-mediated signaling downstream of the G protein-coupled receptor PTHR1. We present evidence herein that PTH-induced survival signaling is impaired in cells lacking connexin43 (Cx43). Thus, expression of functional Cx43 dominant negative proteins or Cx43 knock-down abolished the expression of cAMP-target genes and anti-apoptosis induced by PTH in osteoblastic cells. In contrast, cells lacking Cx43 were still responsive to the stable cAMP analog dibutyril-cAMP. PTH survival signaling was rescued by transfecting wild type Cx43 or a truncated dominant negative mutant of βarrestin, a PTHR1-interacting molecule that limits cAMP signaling. On the other hand, Cx43 mutants lacking the cytoplasmic domain (Cx43(Δ245)) or unable to be phosphorylated at serine 368 (Cx43(S368A)), a residue crucial for Cx43 trafficking and function, failed to restore the anti-apoptotic effect of PTH in Cx43-deficient cells. In addition, overexpression of wild type βarrestin abrogated PTH survival signaling in Cx43-expressing cells. Moreover, βarrestin physically associated in vivo to wild type Cx43 and to a lesser extent to Cx43(S368A) ; and this association and the phosphorylation of Cx43 in serine 368 were reduced by PTH. Furthermore, induction of Cx43(S368) phosphorylation or overexpression of wild type Cx43, but not Cx43(Δ245) or Cx43(S368A) , reduced the interaction between βarrestin and the PTHR1. These studies demonstrate that βarrestin is a novel Cx43-interacting protein and suggest that, by sequestering βarrestin, Cx43 facilitates cAMP signaling, thereby exerting a permissive role on osteoblast survival induced by PTH.

Regulation of connexin expression by transcription factors and epigenetic mechanisms

Gap junctions are specialized cell-cell junctions that directly link the cytoplasm of neighboring cells. They mediate the direct transfer of metabolites and ions from one cell to another. Discoveries of human genetic disorders due to mutations in gap junction protein (connexin [Cx]) genes and experimental data on connexin knockout mice provide direct evidence that gap junctional intercellular communication is essential for tissue functions and organ development, and that its dysfunction causes diseases. Connexin-related signaling also involves extracellular signaling (hemichannels) and non-channel intracellular signaling. Thus far, 21 human genes and 20 mouse genes for connexins have been identified. Each connexin shows tissue- or cell-type-specific expression, and most organs and many cell types express more than one connexin. Connexin expression can be regulated at many of the steps in the pathway from DNA to RNA to protein. In recent years, it has become clear that epigenetic processes are also essentially involved in connexin gene expression. In this review, we summarize recent knowledge on regulation of connexin expression by transcription factors and epigenetic mechanisms including histone modifications, DNA methylation, and microRNA. This article is part of a Special Issue entitled: The communicating junctions, roles and dysfunctions.

Reciprocal regulation between proinflammatory cytokine-induced inducible NO synthase (iNOS) and connexin43 in bladder smooth muscle cells

Gap junctions (GJs) play an important role in the control of bladder contractile response and in the regulation of various immune inflammatory processes. Here, we investigated the possible interaction between inflammation and GJs in bladder smooth muscle cells (BSMCs). Stimulation of BSMCs with IL1β and TNFα increased connexin43 (Cx43) expression and function, which was associated with increased phosphorylation of vasodilator-stimulated phosphoprotein. Inhibition of PKA with H89 or down-regulation of CREB with specific siRNAs largely abolished the Cx43-elevating effect. Further analysis revealed that IL1β/TNFα induced NFκB-dependent inducible NO synthase (iNOS) expression. Inhibition of iNOS with G-nitro-l-arginine methyl ester abrogated and an exogenous NO donor mimicked the effect of the cytokines on Cx43. Intraperitoneal injection of LPS into mice also induced bladder Cx43 expression, which was largely blocked by an iNOS inhibitor. Finally, the elevated Cx43 was found to negatively regulate iNOS expression. Dysfunction of GJs with various blockers or down-regulation of Cx43 with siRNA significantly potentiated the expression of iNOS. Fibroblasts from Cx43 knock-out (Cx43(-/-)) mice also displayed a significantly higher response to the cytokine-induced iNOS expression than cells from Cx43 wild-type (Cx43(+/+)) littermates. Collectively, our study revealed a previously unrecognized reciprocal regulation loop between cytokine-induced NO and GJs. Our findings may provide an important molecular mechanism for the symptoms of bladder infection. In addition, it may further our understanding of the roles of GJs in inflammatory diseases.

Low-intensity pulsed ultrasound (LIPUS) and cell-to-cell communication in bone marrow stromal cells

Low-intensity pulsed ultrasound (LIPUS) is an established therapy for fracture repair and has been used widely in the clinics, but its underlying mechanism of action remains unclear. The aim of the current research was to determine the effect of LIPUS on gap junctional cell-to-cell intercellular communication in rat bone marrow stromal cells (BMSC) in vitro and to determine whether the ability of BMSCs to communicate by gap junctions would affect their response to LIPUS. Single or daily-multiple LIPUS treatment at 1.5MHz, 30mW/cm(2), for 20min was applied to BMSC. We demonstrated that BMSC form functional gap junctions and single LIPUS treatment significantly increased the intracellular dye transfer between BMSC. In addition, activated phosphorylation of ERK1/2 and p38 by LIPUS stimulation was diminished when cells were treated with a gap junction inhibitor 18β-glycyrrhetinic acid (18β). We further demonstrated that 18β diminished the significant increase in alkaline phosphatase activity following LIPUS stimulation. These results suggest a potential role of gap junctional cell-to-cell intercellular communication on the effects of LIPUS in BMSC.

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.

Prostaglandin promotion of osteocyte gap junction function through transcriptional regulation of connexin 43 by glycogen synthase kinase 3/beta-catenin signaling

Gap junction intercellular communication in osteocytes plays an important role in bone remodeling in response to mechanical loading; however, the responsible molecular mechanisms remain largely unknown. Here, we show that phosphoinositide-3 kinase (PI3K)/Akt signaling activated by fluid flow shear stress and prostaglandin E(2) (PGE(2)) had a stimulatory effect on both connexin 43 (Cx43) mRNA and protein expression. PGE(2) inactivated glycogen synthase kinase 3 (GSK-3) and promoted nuclear localization and accumulation of beta-catenin. Knockdown of beta-catenin expression resulted in a reduction in Cx43 protein. Furthermore, the chromatin immunoprecipitation (ChIP) assay demonstrated an association of beta-catenin with the Cx43 promoter, suggesting that beta-catenin could regulate Cx43 expression at the level of gene transcription. We have previously reported that PGE(2) activates cyclic AMP (cAMP)-protein kinase A (PKA) signaling and increases Cx43 and gap junctions. Interestingly, the activation of PI3K/Akt appeared to be independent of the activation of PKA, whereas both PI3K/Akt and PKA signaling inactivated GSK-3 and increased beta-catenin translocation. Together, these results suggest that shear stress, through PGE(2) release, activates both PI3K/Akt and cAMP-PKA signaling, which converge through the inactivation of GSK-3, leading to the increase in nuclear accumulation of beta-catenin. beta-Catenin binds to the Cx43 promoter, stimulating Cx43 expression and functional gap junctions between osteocytes.

PTH and PTHrP signaling in osteoblasts

The striking clinical benefit of PTH in osteoporosis began a new era of skeletal anabolic agents. Several studies have been performed, new studies are emerging out and yet controversies remain on PTH anabolic action in bone. This review focuses on the molecular aspects of PTH and PTHrP signaling in light of old players and recent advances in understanding the control of osteoblast proliferation, differentiation and function.

Connexin 43 as a signaling platform for increasing the volume and spatial distribution of regenerated tissue

Gap junction intercellular communication (GJIC) is ubiquitous in the majority of vertebrate cells and is required for the proper development of most tissues. The loss of gap junction-mediated cell-to-cell communication leads to compromised development in many tissues and organs. Because cells constantly interact through gap junctions to coordinate tissue functions and homeostasis, we hypothesized that increasing cell-to-cell communication, via genetically engineering cells to overexpress gap junction proteins, could enhance cell differentiation in the interior regions of 3D tissue equivalents, thereby increasing the ability to regenerate larger and more uniform volumes of tissue. To test this hypothesis, we used bone as a model tissue because of the difficulty in achieving spatially uniform bone regeneration in 3D. In bone marrow stromal cells (BMSC), GJIC and osteogenic differentiation were compromised in 3D cultures relative to 2D monolayers and in the core of 3D cultures relative to the surface. Overexpression of connexin 43 (Cx43) via transduction of BMSCs with a lentivirus overcame this problem, enhancing both the magnitude and spatial distribution of GJIC and osteogenic differentiation markers throughout 3D constructs. Transplantation of cells overexpressing Cx43 resulted in an increased volume fraction and spatial uniformity of bone in vivo, relative to nontransduced BMSCs. Increased GJIC also enhanced the effect of a potent osteoinductive agent (BMP-7), suggesting a synergism between the soluble factor and GJIC. These findings present a platform to improve cell-to-cell communication in 3D and to achieve uniformly distributed tissue regeneration in 3D.

Actions of fibroblast growth factor-8 in bone cells in vitro

The fibroblast growth factors (FGFs) are a group of at least 25 structurally related peptides that are involved in many biological processes. Some FGFs are active in bone, including FGF-1, FGF-2, and FGF-18, and recent evidence indicates that FGF-8 is osteogenic, particularly in mesenchymal stem cells. In the current study, we found that FGF-8 was expressed in rat primary osteoblasts and in osteoblastic UMR-106 and MC3T3-E1 cells. Both FGF-8a and FGF-8b potently stimulated the proliferation of osteoblastic cells, whereas they inhibited the formation of mineralized bone nodules in long-term cultures of osteoblasts and reduced the levels of osteoblast differentiation markers, osteocalcin, and bone sialoprotein. FGF-8a induced the phosphorylation of p42/p44 mitogen-activated protein kinase (MAPK) in osteoblastic cells; however, its mitogenic actions were not blocked by either the MAPK kinase (MEK) inhibitor U-0126 or the PI 3-kinase (PI3K) inhibitor LY-294002. Interestingly, FGF-8a, unlike FGF-8b and other members of the family, inhibited osteoclastogenesis in mouse bone marrow cultures, and this was via a receptor activator of NF-kappaB ligand (RANKL)/osteoprotegerin (OPG)-independent manner. However, FGF-8a did not affect osteoclastogenesis in RAW 264.7 cells (a macrophage cell line devoid of stromal cells) exogenously stimulated by RANKL, nor did it affect mature osteoclast function as assessed in rat calvarial organ cultures and isolated mature osteoclasts. In summary, we have demonstrated that FGF-8 is active in bone cells, stimulating osteoblast proliferation in a MAPK-independent pathway and inhibiting osteoclastogenesis via a RANKL/OPG-independent mechanism. These data suggest that FGF-8 may have a physiological role in bone acting in an autocrine/paracrine manner.

Connexin43 potentiates osteoblast responsiveness to fibroblast growth factor 2 via a protein kinase C-delta/Runx2-dependent mechanism

In this study, we examine the role of the gap junction protein, connexin43 (Cx43), in the transcriptional response of osteocalcin to fibroblast growth factor 2 (FGF2) in MC3T3 osteoblasts. By luciferase reporter assays, we identify that the osteocalcin transcriptional response to FGF2 is markedly increased by overexpression of Cx43, an effect that is mediated by Runx2 via its OSE2 cognate element, but not by a previously identified connexin-responsive Sp1/Sp3-binding element. Furthermore, disruption of Cx43 function with Cx43 siRNAs or overexpression of connexin45 markedly attenuates the response to FGF2. Inhibition of protein kinase C delta (PKCdelta) with rottlerin or siRNA-mediated knockdown abrogates the osteocalcin response to FGF2. Additionally, we show that upon treatment with FGF2, PKCdelta translocates to the nucleus, PKCdelta and Runx2 are phosphorylated and these events are enhanced by Cx43 overexpression, suggesting that the degree of activation is enhanced by increased Cx43 levels. Indeed, chromatin immunoprecipitations of the osteocalcin proximal promoter with antibodies against Runx2 demonstrate that the recruitment of Runx2 to the osteocalcin promoter in response to FGF2 treatment is dramatically enhanced by Cx43 overexpression. Thus, Cx43 plays a critical role in regulating the ability of osteoblasts to respond to FGF2 by impacting PKCdelta and Runx2 function.

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.

Role of gap junction, hemichannels, and connexin 43 in mineralizing in response to intermittent and continuous application of parathyroid hormone

Intermittent administration stimulates bone formation, whereas sustained elevation of parathyroid hormone (PTH) as in hyperparathyroidism stimulates bone resorption. Even though PTH(1-34) is the only anabolic agent clinically approved for the treatment of osteoporosis, the molecular mechanism whereby PTH mediates these opposing effects depending on timing of administration is not well understood. In this study, we sought to determine the involvement of gap junctions and hemichannels, and the protein that forms them, connexin 43 (Cx43), in the effect of PTH(1-34) on osteoblast mineralization. The osteoblast-like cell line MLO-A5 that rapidly mineralizes in culture was used. Intermittent PTH enhances mineralization, whereas continuous PTH inhibits this process. The mineralization was significantly inhibited by 18 beta-glycyrrhetinic acid, an inhibitor known to block gap junctions and hemichannels. When the cells were treated with PTH(1-34), gap junctional coupling was increased; however, the degree of stimulation was similar between intermittent and continuous treatment. The permeabilization to dye was not detected under various intermittent or continuous PTH treatments. On the other hand, the overall level of Cx43 protein increased in response to continuous PTH treatment. In contrast, when the cells were subjected to intermittent treatment overall level of Cx43 was unchanged, but there was an increase of connexons associated with an increase in Cx43 expression on the cell surface. Our results suggest that Cx43 overall expression, connexon formation and cell surface expression are differentially regulated by intermittent and continuous PTH(1-34), implying the involvement of Cx43 and Cx43-forming channels in mediating the effects of PTH on bone formation.

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.

ODDD-linked Cx43 mutants reduce endogenous Cx43 expression and function in osteoblasts and inhibit late stage differentiation

INTRODUCTION

Bone development and modeling requires precise gap junctional intercellular communication (GJIC). Oculodentodigital dysplasia (ODDD) is an autosomal dominant human disease caused by mutations in the gene (GJA1) encoding the gap junction protein, connexin43 (Cx43). The disease is characterized by craniofacial bone deformities and limb abnormalities. It is our hypothesis that Cx43 mutation causes osteoblast dysfunction, which may contribute to the bone phenotype of ODDD.

MATERIALS AND METHODS

We expressed human and mouse ODDD-linked Cx43 mutants in MC3T3-E1 cells and primary mouse osteoblasts by retroviral infection and evaluated their in vitro differentiation as an index of osteoblast function. We compared these findings to the differentiation of osteoblasts isolated from a mouse model of ODDD that harbors a germ line Cx43 mutation and exhibits craniofacial and limb defects mimicking human ODDD. We determined the differentiation status of osteoblasts by analyzing alkaline phosphatase activity and the expression levels of osteoblast markers including bone sialoprotein and osteocalcin.

RESULTS

We showed that ODDD-linked Cx43 mutants are loss-of-function and dominant-negative to co-expressed Cx43 and, furthermore, greatly inhibit functional GJIC in osteoblasts. Surprisingly, the mutants had only a minor effect on osteoblast differentiation when introduced into lineage committed cells. In contrast, osteoblasts isolated from the ODDD mouse model exhibited impaired late stage differentiation.

CONCLUSIONS

Expression of human and mouse ODDD-linked Cx43 mutants failed to significantly impair differentiation in cells predisposed to the osteoblast lineage; however, germ line reduction of Cx43-based GJIC leads to impaired osteoblast differentiation, which may account for the bone phenotypes observed in ODDD patients.

Mechanotransduction in human bone: in vitro cellular physiology that underpins bone changes with exercise

Bone has a remarkable ability to adjust its mass and architecture in response to a wide range of loads, from low-level gravitational forces to high-level impacts. A variety of types and magnitudes of mechanical stimuli have been shown to influence human bone cell metabolism in vitro, including fluid shear, tensile and compressive strain, altered gravity and vibration. Therefore, the current article aims to synthesize in vitro data regarding the cellular mechanisms underlying the response of human bone cells to mechanical loading. Current data demonstrate commonalities in response to different types of mechanical stimuli on the one hand, along with differential activation of intracellular signalling on the other. A major unanswered question is, how do bone cells sense and distinguish between different types of load? The studies included in the present article suggest that the type and magnitude of loading may be discriminated by overlapping mechanosensory mechanisms including (i) ion channels; (ii) integrins; (iii) G-proteins; and (iv) the cytoskeleton. The downstream signalling pathways identified to date appear to overlap with known growth factor and hormone signals, providing a mechanism of interaction between systemic influences and the local mechanical environment. Finally, the data suggest that exercise should emphasize the amount of load rather than the number of repetitions.

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

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

INTRODUCTION

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

MATERIALS AND METHODS

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

RESULTS

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

CONCLUSIONS

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

Association with ZO-1 correlates with plasma membrane partitioning in truncated connexin45 mutants

Zonula occludens-1 (ZO-1), the most abundant known connexin-interacting protein in osteoblastic cells, associates with the carboxyl termini of both Cx43 and Cx45. To learn more about the role of the cormexin-ZO-1 interaction, we analyzed connexin trafficking and function in ROS 17/2.8 cells that were stably transfected either with full length Cx45 or with Cx45 lacking 34 or 37 amino acids on the carboxyl terminus (Cx45t34 or Cx45t37). All three proteins were transported to appositional membranes in the transfected cells: Cx45 and Cx45t34 displayed a punctate appositional membrane-staining pattern, while Cx45t37 staining at appositional membranes was more linear. Expression of Cx45 decreased gap junction communication as assayed by dye transfer, while expression of Cx45t34 or Cx45t37 increased the amount of dye transfer seen in these cells. We found that Cx43, Cx45 and Cx45t34 co-precipitated with ZO-1 in these cells, while Cx45t37 did not. We also found that Cx45t37 was much more soluble in 1% Triton X-100 than the other connexins examined. In addition, Cx45t37 migrated to a fraction of lighter buoyant density on sucrose flotation gradients than Cx43, Cx45, ZO-1 and Cx45t34. As ZO-1 is an actin-binding protein, this suggested that the differences in Cx45t37 solubility might be due to a difference between the interaction of gap junctions and the actin cytoskeleton in the ROS/Cx45t37 and in the other transfected ROS cells. To examine this possibility, the transfected ROS cells were stained with fluorescently labeled phalloidin and demonstrated that there was a notable loss of actin stress fibers in the ROS/Cx45t37 cells. These findings suggest that association with ZO-1 alters the plasma membrane localization of Cx45 by removing it from a lipid raft compartment and rendering it Triton-insoluble, presumably by promoting an interaction with the actin cytoskeleton; they also suggest that Cx45 has a complex binding interaction with ZO-1 that involves either an extended carboxyl terminal domain or two distinct binding sites.

Increasing Gap Junctional Coupling: A Tool for Dissecting the Role of Gap Junctions

Much of our current knowledge about the physiological and pathophysiological role of gap junctions is based on experiments where coupling has been reduced by either chemical agents or genetic modification. This has brought evidence that gap junctions are important in many physiological processes. In a number of cases, gap junctions have been implicated in the initiation and progress of disease, and experimental uncoupling has been used to investigate the exact role of coupling. The inverse approach, i.e., to increase coupling, has become possible in recent years and represents a new way of testing the role of gap junctions. The aim of this review is to summarize the current knowledge obtained with agents that selectively increase gap junctional intercellular coupling. Two approaches will be reviewed: increasing coupling by the use of antiarrhythmic peptide and its synthetic analogs and by interfering with the gating of gap junctional channels.

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.

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.

Regulation of osteoclastogenesis by gap junction communication

Receptor activator of NF-kappaB ligand (RANKL) is crucial in osteoclastogenesis but signaling events involved in osteoclast differentiation are far from complete and other signals may play a role in osteoclastogenesis. A more direct pathway for cellular crosstalk is provided by gap junction intercellular channel, which allows adjacent cells to exchange second messengers, ions, and cellular metabolites. Here we have investigated the role of gap junction communication in osteoclastogenesis in mouse bone marrow cultures. Immunoreactive sites for the gap junction protein connexin 43 (Cx43) were detected in the marrow stromal cells and in mature osteoclasts. Carbenoxolone (CBX) functionally blocked gap junction communication as demonstrated by a scrape loading Lucifer Yellow dye transfer technique. CBX caused a dose-dependent inhibition (significant > or = 90 microM) of the number of tartrate-resistant acid phosphatase (TRAP)-positive multinucleated cells formed in 7- to 8-day marrow cultures stimulated by parathyroid hormone (PTH; 10 nM) or forskolin (FSK; 1 microM). Furthermore, CBX (100 microM) significantly inhibited prostaglandin E2 (PGE2; 10 microM) and 1,25(OH)2-vitamin D3 stimulated osteoclast differentiation in the mouse bone marrow cultures. Consequently, quantitative real-time polymerase chain reaction (PCR) analysis demonstrated that CBX downregulated the expression of osteoclast phenotypic markers, but without having any significant effects on RANK, RANKL, and osteoprotegerin (OPG) mRNA expression. However, the results demonstrated that CBX significantly inhibits RANKL-stimulated (100 ng/ml) osteoclastogenesis in the mouse bone marrow cultures. Taken together, our results suggests that gap junctional diffusion of messenger molecules interacts with signaling pathways downstream RANKL in osteoclast differentiation. Further studies are required to define the precise mechanisms and molecular targets involved.

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.

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.

Regulation of connexin expression

Gap junctions contain cell-cell communicating channels that consist of multimeric proteins called connexins and mediate the exchange of low-molecular-weight metabolites and ions between contacting cells. Gap junctional communication has long been hypothesized to play a crucial role in the maintenance of homeostasis, morphogenesis, cell differentiation, and growth control in multicellular organisms. The recent discovery that human genetic disorders are associated with mutations in connexin genes and experimental data on connexin knockout mice have provided direct evidence that gap junctional communication is essential for tissue functions and organ development. Thus far, 21 human genes and 20 mouse genes for connexins have been identified. Each connexin shows tissue- or cell-type-specific expression, and most organs and many cell types express more than one connexin. Cell coupling via gap junctions is dependent on the specific pattern of connexin gene expression. This pattern of gene expression is altered during development and in several pathological conditions resulting in changes of cell coupling. Connexin expression can be regulated at many of the steps in the pathway from DNA to RNA to protein. However, transcriptional control is one of the most important points. In this review, we summarize recent knowledge on transcriptional regulation of connexin genes by describing the structure of connexin genes and transcriptional factors that regulate connexin expression.

Factors influencing stem cell differentiation into the hepatic lineage in vitro

A major area of research in transplantation medicine is the potential application of stem cells in liver regeneration. This would require well-defined and efficient protocols for directing the differentiation of stem cells into the hepatic lineage, followed by their selective purification and proliferation in vitro. The development of such protocols would reduce the likelihood of spontaneous differentiation of stem cells into divergent lineages upon transplantation, as well as reduce the risk of teratoma formation in the case of embryonic stem cells. Additionally, such protocols could provide useful in vitro models for studying hepatogenesis and liver metabolism. The development of pharmokinetic and cytotoxicity/genotoxicity screening tests for newly developed biomaterials and drugs, could also utilize protocols developed for the hepatic differentiation of stem cells. Hence, this review critically examines the various strategies that could be employed to direct the differentiation of stem cells into the hepatic lineage in vitro.

ZO-1 alters the plasma membrane localization and function of Cx43 in osteoblastic cells

ZO-1 is the major connexin-interacting protein in ROS 17/2.8 (ROS) osteoblastic cells. We examined the role of ZO-1 in Cx43-mediated gap junction formation and function in ROS cells that expressed the connexin-interacting fragment of ZO-1 (ROS/ZO-1dn) cells. Expression of this ZO-1(7-444) fusion protein in ROS cells disrupted the Cx43/ZO-1 interaction and decreased dye transfer by 85%, although Cx43 was retained on the plasma membrane as assessed by surface biotinylation. Fractionation of lysates derived from ROS/ZO-1dn cells on a 5-30% sucrose flotation gradient showed that 40% of the Cx43 floated into these sucrose gradients, whereas none of the Cx43 in ROS cell lysates entered the gradients, suggesting that more Cx43 is associated with lipid rafts in the transfected ROS cells than in lysates derived from untransfected ROS cells. In contrast to the ROS/ZO-1dn cells, ROS cells that over-expressed ZO-1 protein (ROS/ZO-1myc cells) exhibited increased gap junctional permeability and appositional membrane staining for Cx43. These data demonstrate that ZO-1 regulates Cx43-mediated gap junctional communication in osteoblastic cells and alters the membrane localization of Cx43. They suggest that ZO-1-mediated delivery of Cx43 from a lipid raft domain to gap junctional plaques may be an important regulatory step in gap junction formation.

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.

Mechanisms by which exercise improves bone strength

Certain exercises can induce osteogenesis and improve bone strength, yet the biological processes involved in bone mechanotransduction are only beginning to be understood. Several pathways are emerging from current research, including calcium signaling associated with membrane ion channels, adenosine triphosphate signaling, second messengers such as prostaglandins and nitric oxide, and signaling involving mitogen-activated protein kinase. One characteristic of the mechanosensing apparatus that has only recently been studied is the important role of desensitization. Experimental protocols that insert "rest" periods to reduce the effects of desensitization can double anabolic responses to mechanical loading. Exercises that reduce desensitization may provide an effective means to build bone strength.

Connexin43 is overexpressed inApcMin/+-mice adenomas and colocalises with COX-2 in myofibroblasts

The expression of gap junction proteins, connexins, in the intestine and their role in tumorigenesis are poorly characterised. Truncating mutations in the tumour suppressor gene adenomatous polyposis coli (APC) are early and important events, both in inheritable (familial adenomatous polyposis, FAP) and spontaneous forms of intestinal cancer. Multiple intestinal neoplasia (Min) mice, a FAP model with inherited heterozygous mutation in Apc, spontaneously develop numerous intestinal adenomas. We recently reported reduced expression of connexin32 in Paneth cells of Min-mice. We further examine the expression of connexin43 (Cx43) and other connexins as a function of heterozygous and homozygous Apc mutation in normal intestinal tissues and adenomas of Min-mice. Qualitative analysis of connexin mRNA in intestine revealed a similar expression pattern in Min- and wild-type (wt) mice. Connexin26 and connexin40 proteins were found in equal amounts in Min and wt epithelia of large and small intestine, respectively. Interestingly, the connexin43 level was increased in the stroma of Min-mice adenomas, in close proximity to epithelial cells with nuclear beta-catenin staining. Cx43 and COX-2 were located to the same areas of the adenomas, and immunostaining exhibited coexpression in the myofibroblasts. Prostaglandin E2 induces Cx43 expression and COX-2 is the rate-limiting enzyme in the prostaglandin synthesis. However, the COX-2-specific inhibitor, celecoxib, did not reduce Cx43 expression. Although both Cx43 and COX-2 are target genes for beta-catenin, they were overexpressed in stromal cells but not in epithelial tumour cells. We hypothesise that gap junctions may be of importance in the transfer of signals between epithelium and stroma.

Strategies for directing the differentiation of stem cells into the osteogenic lineage in vitro

A major area in regenerative medicine is the application of stem cells in bone reconstruction and bone tissue engineering. This will require well-defined and efficient protocols for directing the differentiation of stem cells into the osteogenic lineage, followed by their selective purification and proliferation in vitro. The development of such protocols would reduce the likelihood of spontaneous differentiation of stem cells into divergent lineages on transplantation, as well as reduce the risk of teratoma formation in the case of embryonic stem cells. Additionally, such protocols could provide useful in vitro models for studying osteogenesis and bone development, and facilitate the genetic manipulation of stem cells for therapeutic applications. The development of pharmokinetic and cytotoxicity/genotoxicity screening tests for bone-related biomaterials and drugs could also use protocols developed for the osteogenic differentiation of stem cells. This review critically examines the various strategies that could be used to direct the differentiation of stem cells into the osteogenic lineage in vitro.

Redefining the structure of the mouse connexin43 gene: selective promoter usage and alternative splicing mechanisms yield transcripts with different translational efficiencies

The connexin43 (cx43) gene was originally described as consisting of two exons, one coding for most of the 5'-untranslated region (5'-UTR), and the other for the protein sequence and 3'-UTR. We now report that in mouse four additional exons are expressed, all coding for novel 5'-UTRs. Altogether, we found nine different cx43 mRNA species (GenBank accession numbers NM010288, and AY427554 through AY427561) generated by differential promoter usage and alternative splicing mechanisms. The relative abundance of these different mRNAs varied with the tissue source. In addition, the different transcripts showed varying translational efficiencies in several cell lines, indicating the presence of cis-RNA elements that regulate cx43 translation. We propose that it is the promoter driving the expression of the cx43 gene that determines exon choice in the downstream splicing events in a cell-type-dependent fashion. This in turn will affect the translation efficiency of the transcript orchestrating the events that lead to the final expression profile of cx43. Since a similar organization of the cx43 gene was also observed in rat it is likely that the complex regulation of cx43 expression involving transcription, splicing and translation mechanisms is a common trait conserved during evolution.