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

Connexins, important players in the dissemination of prostate cancer cells

Over the past 50years, increasing experimental evidences have established that connexins (Cxs) and gap junctional intercellular communication (GJIC) ensure an important role in both the onset and development of cancerous processes. In the present review, we focus on the impact of Cxs and GJIC during the development of prostate cancer (PCa), from the primary growth mainly localized in acinar glands and ducts to the distant metastasis mainly concentrated in bone. As observed in several other types of solid tumours, Cxs and especially Cx43 exhibit an ambivalent role with a tumour suppressor effect in the early stages and, conversely, a rather pro-tumoural profile for most of invasion and dissemination steps to secondary sites. We report here the current knowledge on the function of Cxs during PCa cells migration, cytoskeletal dynamics, proteinases activities and the cross talk with the surrounding stromal cells in the microenvironment of the tumour and the bones. In addition, we discuss the role of Cxs in the bone tropism even if the prostate model is rarely used to study the complete sequence of cancer dissemination compared to breast cancer or melanoma. Even if not yet fully understood, these recent findings on Cxs provide new insights into their molecular mechanisms associated with progression and bone targeted behaviour of PCa. This article is part of a Special Issue entitled: Gap Junction Proteins edited by Jean Claude Herve.

Biological responses of osteocytic connexin 43 hemichannels to simulated microgravity

Connexin 43 (Cx43) hemichannels and gap junctions in osteocytes are responsive to mechanical loading, which is important for bone formation and remodeling. However, the mechanism of these Cx43-forming channels in the process of mechanical unloading is still not very clear. In this study, unloading caused by weightlessness was simulated by using a random position machine (RPM). Osteocytic MLO-Y4 cells were subjected to 2 h of RPM treatment, and levels of Cx43 mRNA and total and cell surface expressed protein were determined by quantitative real-time PCR, western blotting, and biotinylation analysis. Although mRNA was elevated by RPM, total protein level of Cx43 was not altered; however, surface biotinylated Cx43 was significantly reduced. Interestingly, RPM promoted the retention of Cx43 in the Golgi apparatus detected by co-immunofluorescence with antibodies against Cx43 and 58 K Golgi marker protein. Dye uptake assay showed that hemichannels were induced open after RPM for 2 h. Consistently, prostaglandin E₂ release was increased and this increase was completely attenuated with the treatment of a Cx43 hemichannel blocking antibody. Together, this study demonstrates increased activity of Cx43 hemichannels to RPM, and active Cx43 hemichannels with prostaglandin E₂ release are likely to module biological function under simulated weightless conditions. © 2017 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 35:1195-1202, 2017.

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.

microRNAs and connexins in bone: interaction and mechanisms of delivery

PURPOSE OF REVIEW

To describe the current knowledge on the cross-talk between connexins and microRNAs (miRs) in bone cells.

RECENT FINDINGS

Connexins play a crucial role on bone development and maintenance, and disruptions in their abundance or localization can affect how bone perceives and responds to mechanical, hormonal, and pharmacological stimuli. Connexin expression can be modified by miRs, which modulate connexin mRNA and protein levels. Recently, different manners by which miRs and connexins can interact in bone have been identified, including mechanisms that mediate miR exchange between cells in direct contact through gap junctions, or between distant cells via extracellular vesicles (EVs).

SUMMARY

We bring to light the relationship between miRs and connexins in bone tissue, with special focus on regulatory effects of miRs and connexins on gene expression, as well as the mechanisms that mediate miR exchange between cells in direct contact through gap junctions, or between distant cells via EVs.

Blockage of hemichannels alters gene expression in osteocytes in a high magneto-gravitational environment

Osteocytes, the most abundant cells in bone, are highly responsive to external environmental changes. We tested how Cx43 hemichannels which mediate the exchange of small molecules between cells and extracellular environment impact genome wide gene expression under conditions of abnormal gravity and magnetic field. To this end, we subjected osteocytic MLO-Y4 cells to a high magneto-gravitational environment and used microarray to examine global gene expression and a specific blocking antibody was used to assess the role of Cx43 hemichannels. While 3 hr exposure to abnormal gravity and magnetic field had relatively minor effects on global gene expression, blocking hemichannels significantly impacted the expression of a number of genes which are involved in cell viability, apoptosis, mineral absorption, protein absorption and digestion, and focal adhesion. Also, blocking of hemichannels enriched genes in multiple signaling pathways which are enaged by TGF-beta, Jak-STAT and VEGF. These results show the role of connexin hemichannels in bone cells in high magneto-gravitational environments.

Osteoblast Differentiation and Bone Matrix Formation In Vivo and In Vitro

We review the characteristics of osteoblast differentiation and bone matrix synthesis. Bone in air breathing vertebrates is a specialized tissue that developmentally replaces simpler solid tissues, usually cartilage. Bone is a living organ bounded by a layer of osteoblasts that, because of transport and compartmentalization requirements, produce bone matrix exclusively as an organized tight epithelium. With matrix growth, osteoblasts are reorganized and incorporated into the matrix as living cells, osteocytes, which communicate with each other and surface epithelium by cell processes within canaliculi in the matrix. The osteoblasts secrete the organic matrix, which are dense collagen layers that alternate parallel and orthogonal to the axis of stress loading. Into this matrix is deposited extremely dense hydroxyapatite-based mineral driven by both active and passive transport and pH control. As the matrix matures, hydroxyapatite microcrystals are organized into a sophisticated composite in the collagen layer by nucleation in the protein lattice. Recent studies on differentiating osteoblast precursors revealed a sophisticated proton export network driving mineralization, a gene expression program organized with the compartmentalization of the osteoblast epithelium that produces the mature bone matrix composite, despite varying serum calcium and phosphate. Key issues not well defined include how new osteoblasts are incorporated in the epithelial layer, replacing those incorporated in the accumulating matrix. Development of bone in vitro is the subject of numerous projects using various matrices and mesenchymal stem cell-derived preparations in bioreactors. These preparations reflect the structure of bone to variable extents, and include cells at many different stages of differentiation. Major challenges are production of bone matrix approaching the in vivo density and support for trabecular bone formation. In vitro differentiation is limited by the organization and density of osteoblasts and by endogenous and exogenous inhibitors.

BMP Signaling Mediated by BMPR1A in Osteoclasts Negatively Regulates Osteoblast Mineralization Through Suppression of Cx43

Osteoblasts and osteoclasts are well orchestrated through different mechanisms of communication during bone remodeling. Previously, we found that osteoclast-specific disruption of one of the BMP receptors, Bmpr1a, results in increased osteoblastic bone formation in mice. We hypothesized that BMPR1A signaling in osteoclasts regulates production of either membrane bound proteins or secreted molecules that regulated osteoblast differentiation. In our current study, we co-cultured wild-type osteoblasts with either control osteoclasts or osteoclasts lacking BMPR1A signaling activity. We found that loss of Bmpr1a in osteoclasts promoted osteoblast mineralization in vitro. Further, we found that the expression of Cx43/Gja1 in the mutant osteoclasts was increased, which encoded for one of the gap junction proteins connexin 43/gap junction alpha 1. Knockdown of Gja1 in the mutant osteoclasts for Bmpr1a reduced osteoblastic mineralization when co-cultured. Our findings suggest that GJA1 may be one of the downstream targets of BMPR1A signaling in osteoclasts that mediates osteoclast-osteoblast communication during bone remodeling. J. Cell. Biochem. 118: 605-614, 2017. © 2016 Wiley Periodicals, Inc.

Concise Review: In Vitro Formation of Bone-Like Nodules Sheds Light on the Application of Stem Cells for Bone Regeneration

: Harnessing the differentiation of stem cells into bone-forming cells represents an intriguing avenue for the creation of functional skeletal tissues. Therefore, a profound understanding of bone development and morphogenesis sheds light on the regenerative application of stem cells in orthopedics and dentistry. In this concise review, we summarize the studies deciphering the mechanisms that govern osteoblast differentiation in the context of in vitro formation of bone-like nodules, including morphologic and molecular events as well as cellular contributions to mineral nucleation, occurring during osteogenic differentiation of stem cells. This article also highlights the limitations of current translational applications of stem cells and opportunities to use the bone-like nodule model for bone regenerative therapies.

SIGNIFICANCE

Harnessing the differentiation of stem cells into bone-forming cells represents an intriguing avenue for the creation of functional skeletal tissues. Therefore, a profound understanding of bone development and morphogenesis sheds light on the regenerative application of stem cells in orthopedics and dentistry. In this concise review, studies deciphering the mechanisms that govern osteoblast commitment and differentiation are summarized. This article highlights the limitations of current translational applications of stem cells and the opportunities to use the bone-like nodule model for bone regenerative therapies.

Cx43-Dependent Skeletal Phenotypes Are Mediated by Interactions between the Hapln1a-ECM and Sema3d during Fin Regeneration

Skeletal development is a tightly regulated process and requires proper communication between the cells for efficient exchange of information. Analysis of fin length mutants has revealed that the gap junction protein Connexin43 (Cx43) coordinates cell proliferation (growth) and joint formation (patterning) during zebrafish caudal fin regeneration. Previous studies have shown that the extra cellular matrix (ECM) protein Hyaluronan and Proteoglycan Link Protein1a (Hapln1a) is molecularly and functionally downstream of Cx43, and that hapln1a knockdown leads to reduction of the glycosaminoglycan hyaluronan. Here we find that the proteoglycan aggrecan is similarly reduced following Hapln1a knockdown. Notably, we demonstrate that both hyaluronan and aggrecan are required for growth and patterning. Moreover, we provide evidence that the Hapln1a-ECM stabilizes the secreted growth factor Semaphorin3d (Sema3d), which has been independently shown to mediate Cx43 dependent phenotypes during regeneration. Double knockdown of hapln1a and sema3d reveal synergistic interactions. Further, hapln1a knockdown phenotypes were rescued by Sema3d overexpression. Therefore, Hapln1a maintains the composition of specific components of the ECM, which appears to be required for the stabilization of at least one growth factor, Sema3d. We propose that the Hapln1a dependent ECM provides the required conditions for Sema3d stabilization and function. Interactions between the ECM and signaling molecules are complex and our study demonstrates the requirement for components of the Hapln1a-ECM for Sema3d signal transduction.

The Renal Connexome and Possible Roles of Connexins in Kidney Diseases

Connexins are membrane-spanning proteins that allow for the formation of cell-to-cell channels and cell-to-extracellular space hemichannels. Many connexin subtypes are expressed in kidney cells. Some mutations in connexin genes have been linked to various human pathologies, including cardiovascular, neurodegenerative, lung, and skin diseases, but the exact role of connexins in kidney disease remains unclear. Some hypotheses about a connection between genetic mutations, endoplasmic reticulum (ER) stress, and the unfolded protein response (UPR) in kidney pathology have been explored. The potential relationship of kidney disease to abnormal production of connexin proteins, mutations in their genes together with ER stress, or the UPR is still a matter of debate. In this scenario, it is tantalizing to speculate about a possible role of connexins in the setting of kidney pathologies that are thought to be caused by a deregulated podocyte protein expression, the so-called podocytopathies. In this article, we give examples of the roles of connexins in kidney (patho)physiology and propose avenues for further research concerning connexins, ER stress, and UPR in podocytopathies that may ultimately help refine drug treatment.

Cardiomyocyte-specific overexpression of the ubiquitin ligase Wwp1 contributes to reduction in Connexin 43 and arrhythmogenesis

Gap junctions (GJ) are intercellular channels composed of connexin subunits that play a critical role in a diverse number of cellular processes in all tissue types. In the heart, GJs mediate electrical coupling between cardiomyocytes and display mislocalization and/or downregulation in cardiac disease (a process known as GJ remodeling), producing an arrhythmogenic substrate. The main constituent of GJs in the ventricular myocardium is Connexin 43 (Cx43), an integral membrane protein that is rapidly turned over and shows decreased expression or function with age. We hypothesized that Wwp1, an ubiquitin ligase whose expression in known to increase in aging-related pathologies, may regulate Cx43 in vivo by targeting it for ubiquitylation and degradation and yield tissue-specific Cx43 loss of function phenotypes. When Wwp1 was globally overexpressed in mice under the control of a β-actin promoter, the highest induction of Wwp1 expression was observed in the heart which was associated with a 90% reduction in cardiac Cx43 protein levels, left ventricular hypertrophy (LVH), and the development of lethal ventricular arrhythmias around 8weeks of age. This phenotype was completely penetrant in two independent founder lines. Cardiomyocyte-specific overexpression of Wwp1 confirmed that this phenotype was cell autonomous and delineated Cx43-dependent and -independent roles for Wwp1 in arrhythmogenesis and LVH, respectively. Using a cell-based system, it was determined that Wwp1 co-immunoprecipitates with and ubiquitylates Cx43, causing a decrease in the steady state levels of Cx43 protein. These findings offer new mechanistic insights into the regulation of Cx43 which may be exploitable in various gap junctionopathies.

The effect of mechanical stimulation on mineralization in differentiating osteoblasts in collagen-I scaffolds

Developing a viable and functional bone scaffold in vitro that is capable of surviving and bearing mechanical load in vivo requires an understanding of the cell biology of osteoprogenitor cells, particularly how they are influenced by mechanical stimulation during cell differentiation and maturation. In this study, mechanical load was applied using a modified FlexCell plate to impart confined compression to collagen-I scaffolds seeded with undifferentiated murine embryonic stem cells. The activity, presence, and expression of osteoblast-cadherin (OB-Cad) and connexin-43, as well as various pluripotent and osteogenic markers were examined at 5-30 days of differentiation as cells were stimulated to differentiate to osteoblasts with and without applied mechanical load. Fluorescence recovery after photobleaching, immunofluorescence, viability, von Kossa, and real-time polymerase chain reaction assessments revealed that mechanical prestimulation of this cell-seeded scaffold altered the expression of OB-Cad and connexin-43 and resulted in significant differences in the structure and organization of mineralization present in the collagen matrix. Specifically, cells in gels that were loaded for 40 h after 5 days of differentiation and then left to fully differentiate for 30 days produced a highly structured honeycomb-shaped mineralization in the matrix; an outcome that was previously shown to be indicative of late osteoblast/early osteocyte activity. This study highlights the potential of mechanical load to accelerate differentiation and enhance osteoblast communication and function during the differentiation process, and highlights a time point of cell differentiation within this scaffold to apply load in order to most effectively transduce a mechanical signal.

1,25-Dihydroxyvitamin D3 and extracellular calcium promote mineral deposition via NPP1 activity in a mature osteoblast cell line MLO-A5

While vitamin D supplementation is common, the anabolic mechanisms that improve bone status are poorly understood. Under standard mineralising conditions including media ionised calcium of 1.1 mM, 1,25-dihydroxyvitamin D3 (1,25D) enhanced differentiation and mineral deposition by the mature osteoblast/pre-osteocyte cell line, MLO-A5. This effect was markedly increased with a higher ionised calcium level (1.5 mM). Gene expression analyses revealed that 1,25D-induced mineral deposition was associated with induction of Enpp1 mRNA, coding for nucleotide pyrophosphatase phosphodiesterase 1 (NPP1) and NPP1 protein levels. Since MLO-A5 cells express abundant alkaline phosphatase that was not further modified by 1,25D treatment or exposure to increased calcium, this finding suggested that the NPP1 production of pyrophosphate (PPi) may provide alkaline phosphatase with substrate for the generation of inorganic phosphate (Pi). Consistent with this, co-treatment with Enpp1 siRNA or a NPP1 inhibitor, PPADS, abrogated 1,25D-induced mineral deposition. These data demonstrate that 1,25D stimulates osteoblast differentiation and mineral deposition, and interacts with the extracellular calcium concentration. 1,25D regulates Enpp1 expression, which presumably, in the context of adequate tissue non-specific alkaline phosphatase activity, provides Pi to stimulate mineralisation. Our findings suggest a mechanism by which vitamin D with adequate dietary calcium can improve bone mineral status.

Intracellular Cleavage of the Cx43 C-Terminal Domain by Matrix-Metalloproteases: A Novel Contributor to Inflammation?

The coordination of tissue function is mediated by gap junctions (GJs) that enable direct cell-cell transfer of metabolic and electric signals. GJs are formed by connexin (Cx) proteins of which Cx43 is most widespread in the human body. Beyond its role in direct intercellular communication, Cx43 also forms nonjunctional hemichannels (HCs) in the plasma membrane that mediate the release of paracrine signaling molecules in the extracellular environment. Both HC and GJ channel function are regulated by protein-protein interactions and posttranslational modifications that predominantly take place in the C-terminal domain of Cx43. Matrix metalloproteases (MMPs) are a major group of zinc-dependent proteases, known to regulate not only extracellular matrix remodeling, but also processing of intracellular proteins. Together with Cx43 channels, both GJs and HCs, MMPs contribute to acute inflammation and a small number of studies reports on an MMP-Cx43 link. Here, we build further on these reports and present a novel hypothesis that describes proteolytic cleavage of the Cx43 C-terminal domain by MMPs and explores possibilities of how such cleavage events may affect Cx43 channel function. Finally, we set out how aberrant channel function resulting from cleavage can contribute to the acute inflammatory response during tissue injury.

Biological Events in Periodontal Ligament and Alveolar Bone Associated with Application of Orthodontic Forces

Orthodontic force-induced stresses cause dynamic alterations within the extracellular matrix and within the cytoskeleton of cells in the periodontal ligament and alveolar bone, mediating bone remodelling, ultimately enabling orthodontic tooth movement. In the periodontal ligament and alveolar bone, the mechanically induced tensile strains upregulate the expression of osteogenic genes resulting in bone formation, while mechanically induced compressive strains mediate predominantly catabolic tissue changes and bone resorption. In this review article we summarize some of the currently known biological events occurring in the periodontal ligament and in the alveolar bone in response to application of orthodontic forces and how these facilitate tooth movement.

Physiological mechanisms and therapeutic potential of bone mechanosensing

Skeletal loading is an important physiological regulator of bone mass. Theoretically, mechanical forces or administration of drugs that activate bone mechanosensors would be a novel treatment for osteoporotic disorders, particularly age-related osteoporosis and other bone loss caused by skeletal unloading. Uncertainty regarding the identity of the molecular targets that sense and transduce mechanical forces in bone, however, has limited the therapeutic exploitation of mechanosesning pathways to control bone mass. Recently, two evolutionally conserved mechanosensing pathways have been shown to function as "physical environment" sensors in cells of the osteoblasts lineage. Indeed, polycystin-1 (Pkd1, or PC1) and polycystin-2 (Pkd2, or PC2' or TRPP2), which form a flow sensing receptor channel complex, and TAZ (transcriptional coactivator with PDZ-binding motif, or WWTR1), which responds to the extracellular matrix microenvironment act in concert to reciprocally regulate osteoblastogenesis and adipogenesis through co-activating Runx2 and a co-repressing PPARγ activities. Interactions of polycystins and TAZ with other putative mechanosensing mechanism, such as primary cilia, integrins and hemichannels, may create multifaceted mechanosensing networks in bone. Moreover, modulation of polycystins and TAZ interactions identify novel molecular targets to develop small molecules that mimic the effects of mechanical loading on bone.

The protective effect of lycopene intake on bone loss in ovariectomized rats

Antioxidant lycopene supplementation has been shown to decrease oxidative stress and have beneficial effects on bone health. However, it remains unclear whether lycopene exerts its beneficial effect on bone metabolism through mitigation of oxidative stress in vivo. The aim of this study was to investigate whether lycopene intake protects against bone loss by reducing oxidative stress in ovariectomized rats. Female Sprague-Dawley 6-week-old rats were ovariectomized and randomly divided into four groups according to the lycopene content of their diet: 0, 50, 100, and 200 ppm. The tibial bone mineral density (BMD) in the 50, 100, and 200 ppm groups was significantly higher than that in the 0 ppm group. Serum and urinary bone resorption marker levels were significantly lower in the 50, 100, and 200 ppm groups than in the 0 ppm group. There was no significant difference in systemic oxidative stress markers among all groups. However, systemic oxidative stress levels were inversely correlated with the tibial BMD. Our findings suggest that lycopene intake significantly inhibits bone loss by suppressing bone resorption in ovariectomized rats. Further studies are necessary to clarify the effect of lycopene on oxidative stress in local tissues such as bone tissue.

Differential impact of adenosine nucleotides released by osteocytes on breast cancer growth and bone metastasis

Extracellular ATP has been shown to either inhibit or promote cancer growth and migration; however, the mechanism underlying this discrepancy remained elusive. Here we demonstrate the divergent roles of ATP and adenosine released by bone osteocytes on breast cancers. We showed that conditioned media (CM) collected from osteocytes treated with alendronate (AD), a bisphosphonate drug, inhibited the migration of human breast cancer MDA-MB-231 cells. Removal of the extracellular ATP by apyrase in CM abolished this effect, suggesting the involvement of ATP. ATP exerted its inhibitory effect through the activation of purinergic P2X receptor signaling in breast cancer cells evidenced by the attenuation of the inhibition by an antagonist, oxidized ATP, as well as knocking down P2X7 with small interfering RNA (siRNA), and the inhibition of migration by an agonist, BzATP. Intriguingly, ATP had a biphasic effect on breast cancer cells-lower dosage inhibited but higher dosage promoted its migration. The stimulatory effect on migration was blocked by an adenosine receptor antagonist, MRS1754, ARL67156, an ecto-ATPase inhibitor, and A2A receptor siRNA, suggesting that in contrast to ATP, adenosine, a metabolic product of ATP, promoted migration of breast cancer cells. Consistently, non-hydrolyzable ATP, ATPγS, only inhibited but did not promote cancer cell migration. ATP also had a similar inhibitory effect on the Py8119 mouse mammary carcinoma cells; however, adenosine had no effect owing to the absence of the A2A receptor. Consistently, ATPγS inhibited, whereas adenosine promoted anchorage-independent growth of MDA-MB-231 cells. Our in vivo xenograft study showed a significant delay of tumor growth with the treatment of ATPγS. Moreover, the extent of bone metastasis in a mouse intratibial model was significantly reduced with the treatment of ATPγS. Together, our results suggest the distinct roles of ATP and adenosine released by osteocytes and the activation of corresponding receptors P2X7 and A2A signaling on breast cancer cell growth, migration and bone metastasis.

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.

Connexin43 enhances the expression of osteoarthritis-associated genes in synovial fibroblasts in culture

Background

Recent work has shown that the gap junction protein connexin43 (Cx43) is upregulated in cells of the joint during osteoarthritis (OA). Here we examined if the OA-associated increase in Cx43 expression impacts the function of synovial fibroblasts by contributing to the production of catabolic and inflammatory factors that exacerbate joint destruction in arthritic disease.

Methods

Using rabbit and human synovial fibroblast cell lines, we examined the effects of Cx43 overexpression and Cx43 siRNA-mediated knockdown on the gene expression of OA-associated matrix metalloproteinases (MMP1 and MMP13), aggrecanases (ADAMTS4 and ADAMTS5), and inflammatory factors (IL1, IL6 and PTGS2) by quantitative real time RT-PCR. We examined collagenase activity in conditioned media of cultured synovial cells following Cx43 overexpression. Lastly, we assessed the interplay between Cx43 and the NFκB cascade by western blotting and gene expression studies.

Results

Increasing Cx43 expression enhanced the gene expression of MMP1, MMP13, ADAMTS4, ADAMTS5, IL1, IL6 and PTGS2 and increased the secretion of collagenases into conditioned media of cultured synovial fibroblasts. Conversely, knockdown of Cx43 decreased expression of many of these catabolic and inflammatory genes. Modulation of Cx43 expression altered the phosphorylation of the NFκB subunit, p65, and inhibition of NFκB with chemical inhibitors blocked the effects of increased Cx43 expression on the mRNA levels of a subset of these catabolic and inflammatory genes.

Conclusions

Increasing or decreasing Cx43 expression alone was sufficient to alter the levels of catabolic and inflammatory genes expressed by synovial cells. The NFκB cascade mediated the effect of Cx43 on the expression of a subset of these OA-associated genes. As such, Cx43 may be involved in joint pathology during OA, and targeting Cx43 expression or function may be a viable therapeutic strategy to attenuate the catabolic and inflammatory environment of the joint during OA.

Electronic supplementary material

The online version of this article (doi:10.1186/1471-2474-15-425) contains supplementary material, which is available to authorized users.

Nonvascular transport distraction osteogenesis in bone formation and regeneration. Is it an accidental phenomenon?

PURPOSE

To explore the osteogenic mechanism of nonvascular transport distraction osteogenesis (NTDO) by constructing mandibular defects in dogs.

METHODS

Sixty adult dogs were randomly divided into three groups with 20 dogs in each group. Canine mandibular defect models of NTDO were constructed. Animals were euthanized 1, 4 and 12 weeks after distraction, and the transport disc and surrounding tissue were collected and fixed. Histochemical staining using hematoxylin and eosin (H&E) and electron microscopic observations were used to examine bone regeneration.

RESULTS

Distraction bone regeneration was observed in the distraction gap and around the transport disc, and osseous connections had formed between new bone and the transport disc after one week. Osteoclasts gathered around the transport disc, and bone absorption pit formation could be seen. After 4 weeks of distraction, the new bone around the transport disc was close to maturity with thick sclerostin on the middle of the transport disc. After 12 weeks the new bone and the transport disc were fully integrated, and were difficult to distinguish by H&E staining and electron microscopy.

CONCLUSIONS

Canine mandibular defects were successfully repaired by NTDO resulting in ideal new bone formation and fully recovered mandibular physiological function. The surrounding tissues, including musculoskeletal tissues, the periosteum and other soft tissues and the nonvascular transport disc, together contribute to bone regeneration and neovascularization in NTDO.

Regulation of hemichannels and gap junction channels by cytokines in antigen-presenting cells

Autocrine and paracrine signals coordinate responses of several cell types of the immune system that provide efficient protection against different challenges. Antigen-presenting cells (APCs) coordinate activation of this system via homocellular and heterocellular interactions. Cytokines constitute chemical intercellular signals among immune cells and might promote pro- or anti-inflammatory effects. During the last two decades, two membrane pathways for intercellular communication have been demonstrated in cells of the immune system. They are called hemichannels (HCs) and gap junction channels (GJCs) and provide new insights into the mechanisms of the orchestrated response of immune cells. GJCs and HCs are permeable to ions and small molecules, including signaling molecules. The direct intercellular transfer between contacting cells can be mediated by GJCs, whereas the release to or uptake from the extracellular milieu can be mediated by HCs. GJCs and HCs can be constituted by two protein families: connexins (Cxs) or pannexins (Panxs), which are present in almost all APCs, being Cx43 and Panx1 the most ubiquitous members of each protein family. In this review, we focus on the effects of different cytokines on the intercellular communication mediated by HCs and GJCs in APCs and their impact on purinergic signaling.

Fluid flow-induced calcium response in osteoclasts: signaling pathways

Intracellular calcium oscillation and its downstream signaling in osteoclasts is believed to play critical roles in regulating bone resorption. Our previous study demonstrated that fluid shear stress (FSS) induced more calcium responsive peaks in the late differentiated osteoclasts than the early ones. In this paper, the signaling pathways of FSS-induced calcium response for the osteoclasts in different differentiation stages were studied. RAW264.7 macrophage cells were induced to differentiate into osteoclasts with the conditioned medium from MC3T3-E1 osteoblasts. Furthermore pharmacological agents were added to block the specific signaling pathways. Finally the cells were exposed to FSS at different levels (1 or 10 dyne/cm(2)) after being induced for 4 or 8 days. The results showed that the mechanosensitive, cation-selective channels, phospholipase C (PLC) and endoplasmic reticulum constituted the major signaling pathway for mechanical stimulation-induced calcium response in osteoclasts. Extracellular calcium or ATP involved with calcium oscillation in a FSS magnitude-dependent manner. This pathway study may help to give insight into the molecular mechanism of mechanical stimulation-regulated bone remodeling.

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.

Temozolomide resistance in glioblastoma cells occurs partly through epidermal growth factor receptor-mediated induction of connexin 43

Glioblastoma Multiforme (GBM) is an aggressive adult primary brain tumor with poor prognosis. GBM patients develop resistance to the frontline chemotherapy, temozolomide (TMZ). As the connexins (Cx) have been shown to have a complex role in GBM, we investigated the role of Cx43 in TMZ resistance. Cx43 was increased in the TMZ-resistant low passage and cell lines. This correlated with the data in The Cancer Genome Atlas. Cx43 knockdown, reporter gene assays, chromatin immunoprecipitation assay, real-time PCR and western blots verified a role for Cx43 in TMZ resistance. This occurred by TMZ-resistant GBM cells being able to activate epidermal growth factor receptor (EGFR). In turn, EGFR activated the JNK-ERK1/2-AP-1 axis to induce Cx43. The increased Cx43 was functional as indicated by gap junctional intercellular communication among the resistant GBM cells. Cell therapy could be a potential method to deliver drugs, such as anti-EGF to tumor cells. Similar strategies could be used to reverse the expression of Cx43 to sensitize GBM cells to TMZ. The studies showed the potential for targeting EGF in immune therapy. These agents can be used in conjunction with stem cell therapy to treat GBM.

Gap junctional communication in osteocytes is amplified by low intensity vibrations in vitro

The physical mechanism by which cells sense high-frequency mechanical signals of small magnitude is unknown. During exposure to vibrations, cell populations within a bone are subjected not only to acceleratory motions but also to fluid shear as a result of fluid-cell interactions. We explored displacements of the cell nucleus during exposure to vibrations with a finite element (FE) model and tested in vitro whether vibrations can affect osteocyte communication independent of fluid shear. Osteocyte like MLO-Y4 cells were subjected to vibrations at acceleration magnitudes of 0.15 g and 1 g and frequencies of 30 Hz and 100 Hz. Gap junctional intracellular communication (GJIC) in response to these four individual vibration regimes was investigated. The FE model demonstrated that vibration induced dynamic accelerations caused larger relative nuclear displacement than fluid shear. Across the four regimes, vibrations significantly increased GJIC between osteocytes by 25%. Enhanced GJIC was independent of vibration induced fluid shear; there were no differences in GJIC between the four different vibration regimes even though differences in fluid shear generated by the four regimes varied 23-fold. Vibration induced increases in GJIC were not associated with altered connexin 43 (Cx43) mRNA or protein levels, but were dependent on Akt activation. Combined, the in silico and in vitro experiments suggest that externally applied vibrations caused nuclear motions and that large differences in fluid shear did not influence nuclear motion (<1%) or GJIC, perhaps indicating that vibration induced nuclear motions may directly increase GJIC. Whether the increase in GJIC is instrumental in modulating anabolic and anti-catabolic processes associated with the application of vibrations remains to be determined.

Beta-1 integrin is important for the structural maintenance and homeostasis of differentiating fiber cells

β1-Integrin is a heterodimeric transmembrane protein that has roles in both cell-extra-cellular matrix and cell-cell interactions. Conditional deletion of β1-integrin from all lens cells during embryonic development results in profound lens defects, however, it is less clear whether this reflects functions in the lens epithelium alone or whether this protein plays a role in lens fibers. Thus, a conditional approach was used to delete β1-integrin solely from the lens fiber cells. This deletion resulted in two distinct phenotypes with some lenses exhibiting cataracts while others were clear, albeit with refractive defects. Analysis of "clear" conditional knockout lenses revealed that they had profound defects in fiber cell morphology associated with the loss of the F-actin network. Physiological measurements found that the lens fiber cells had a twofold increase in gap junctional coupling, perhaps due to differential localization of connexins 46 and 50, as well as increased water permeability. This would presumably facilitate transport of ions and nutrients through the lens, and may partially explain how lenses with profound structural abnormalities can maintain transparency. In summary, β1-integrin plays a role in maintaining the cellular morphology and homeostasis of the lens fiber cells.

Negative influence of a long-term high-fat diet on murine bone architecture

A correlation between obesity and bone metabolism is strongly assumed because adipocytes and osteoblasts originate from the same precursor cells and their differentiation is conversely regulated by the same factors. It is controversially discussed if obesity protects bone or leads to loss of bone mass. Thus, the aim of the present study was to investigate the influence of diet-induced mild obesity (11% increased body weight compared to control) on bone microstructure in mice. Four-week-old male C57BL/6J mice received a high-fat diet (HFD, 60% kcal from fat) and were analyzed by means of dual X-ray absorptiometry, histological methods, real-time RT-PCR, and transmission electron microscopy in comparison to control animals (10% kcal from fat). The cancellous bone mass, collagen 1α1 expression, amount of osteoid, and cohesion of cells via cell-to-cell contacts decreased in HFD mice whereas the bone mineral density and the amount of osteoblasts and osteoclasts were not modified. The amount of apoptotic osteocytes was increased in HFD mice in comparison to controls. We conclude that moderately increased body weight does not protect bone architecture from age-dependent degeneration. By contrast, bone microstructure is negatively affected and reduced maintenance of cell-cell contacts may be one of the underlying mechanisms.

Elevated Intracellular Ca(2+) Signals by Oxidative Stress Activate Connexin 43 Hemichannels in Osteocytes

Elevated oxidative stress (OS) during aging leads to bone loss. OS increases intracellular Ca(2+) ([Ca(2+)]i), resulting in cellular damage and death. We show earlier that Cx43 hemichannels open in response to OS, which serves as a protective mechanism for osteocytes. However, the underlying mechanism is unknown. Here, we found that treatment with H2O2 increased [Ca(2+)]i in osteocytes with [Ca(2+)]i being primarily derived from an extracellular Ca(2+) source. Hemichannel opening induced by OS was inhibited by the depletion of [Ca(2+)]i with BAPTA-AM, a Ca(2+) chelator, suggesting that [Ca(2+)]i influenced the activity of Cx43 hemichannels. Conversely, blockade of hemichannels had no effect on [Ca(2+)]i. A biotinylation assay showed that cell surface-expressed Cx43 was increased by OS, which could be inhibited by BAPTA-AM, suggesting that [Ca(2+)]i is necessary for Cx43 migration to the cell surface in response to OS. Together, these data suggest that increased hemichannel activity induced by OS was likely to be caused by elevated [Ca(2+)]i through increased Cx43 on the cell surface.

14-3-3θ facilitates plasma membrane delivery and function of mechanosensitive connexin 43 hemichannels

Intracellular signaling in osteocytes activated by mechanical loading is important for bone formation and remodeling. These signaling events are mediated by small modulators released from Cx43 hemichannels (HC). We have recently shown that integrin α5 senses the mechanical stimulation and induces the opening of Cx43 HC; however, the underlying mechanism is unknown. Here, we show that both Cx43 and integrin α5 interact with 14-3-3θ, and this interaction is required for the opening of Cx43 HC upon mechanical stress. The absence of 14-3-3θ prevented the interaction between Cx43 and integrin α5, and blocked HC opening. Furthermore, it decreased the transport of Cx43 and integrin α5 from the Golgi apparatus to the plasma membrane. Mechanical loading promoted the movement of Cx43 to the surface which was associated not only with an increase in 14-3-3θ levels but also its interaction with Cx43 and integrin α5. This stimulatory effect on forward transport by mechanical loading was attenuated in the absence of 14-3-3θ and the majority of the Cx43 accumulated in the Golgi. Disruption of the Golgi by brefeldin A reduced the association of Cx43 and integrin α5 with 14-3-3θ, further suggesting that the interaction is likely to occur in the Golgi. Together, these results define a previously unidentified, scaffolding role of 14-3-3θ in assisting the delivery of Cx43 and integrin α5 to the plasma membrane for the formation of mechanosensitive HC in osteocytes.

Neurological manifestations of oculodentodigital dysplasia: a Cx43 channelopathy of the central nervous system?

The coordination of tissue function is mediated by gap junctions (GJs) that enable direct cell–cell transfer of metabolic and electric signals. GJs are formed by connexins of which Cx43 is most widespread in the human body. In the brain, Cx43 GJs are mostly found in astroglia where they coordinate the propagation of Ca²⁺ waves, spatial K⁺ buffering, and distribution of glucose. Beyond its role in direct intercellular communication, Cx43 also forms unapposed, non-junctional hemichannels in the plasma membrane of glial cells. These allow the passage of several neuro- and gliotransmitters that may, combined with downstream paracrine signaling, complement direct GJ communication among glial cells and sustain glial-neuronal signaling. Mutations in the GJA1 gene encoding Cx43 have been identified in a rare, mostly autosomal dominant syndrome called oculodentodigital dysplasia (ODDD). ODDD patients display a pleiotropic phenotype reflected by eye, hand, teeth, and foot abnormalities, as well as craniofacial and bone malformations. Remarkably, neurological symptoms such as dysarthria, neurogenic bladder (manifested as urinary incontinence), spasticity or muscle weakness, ataxia, and epilepsy are other prominent features observed in ODDD patients. Over 10 mutations detected in patients diagnosed with neurological disorders are associated with altered functionality of Cx43 GJs/hemichannels, but the link between ODDD-related abnormal channel activities and neurologic phenotype is still elusive. Here, we present an overview on the nature of the mutants conveying structural and functional changes of Cx43 channels and discuss available evidence for aberrant Cx43 GJ and hemichannel function. In a final step, we examine the possibilities of how channel dysfunction may lead to some of the neurological manifestations of ODDD.

Two tyrosine-based sorting signals in the Cx43 C-terminus cooperate to mediate gap junction endocytosis

Three tyrosine-based sorting signals in the gap junction protein connexin 43 were identified, two of which function cooperatively as adaptor protein complex-2 binding sites. The analyses provide a molecular model for clathrin to efficiently internalize large plasma membrane structures and suggest a mechanism for regulating constitutive versus acute gap junction internalization.

Gap junction (GJ) channels that electrically and chemically couple neighboring cells are formed when two hemichannels (connexons) of apposed cells dock head-on in the extracellular space. Remarkably, docked connexons are inseparable under physiological conditions, and we and others have shown that GJs are internalized in whole, utilizing the endocytic clathrin machinery. Endocytosis generates double-membrane vesicles (annular GJs or connexosomes) in the cytoplasm of one of the apposed cells that are degraded by autophagosomal and, potentially, endo/lysosomal pathways. In this study, we investigated the structural motifs that mediate Cx43 GJ endocytosis. We identified three canonical tyrosine-based sorting signals of the type “YXXΦ” in the Cx43 C-terminus, two of which function cooperatively as AP-2 binding sites. We generated a set of green fluorescent protein–tagged and untagged Cx43 mutants that targeted these two sites either individually or together. Mutating both sites completely abolished Cx43-AP-2/Dab2/clathrin interaction and resulted in increased GJ plaque size, longer Cx43 protein half-lives, and impaired GJ internalization. Interestingly, Dab2, an accessory clathrin adaptor found earlier to be important for GJ endocytosis, interacts indirectly with Cx43 via AP-2, permitting the recruitment of up to four clathrin complexes per Cx43 protein. Our analyses provide a mechanistic model for clathrin's efficient internalization of large plasma membrane structures, such as GJs.

Connexin 43 channels protect osteocytes against oxidative stress-induced cell death

The increased osteocyte death by oxidative stress (OS) during aging is a major cause contributing to the impairment of bone quality and bone loss. However, the underlying molecular mechanism is largely unknown. Here, we show that H₂O₂ induced cell death of primary osteocytes and osteocytic MLO-Y4 cells, and also caused dose-dependent decreased expression of gap junction and hemichannel-forming connexin 43 (Cx43). The decrease of Cx43 expression was also demonstrated with the treatment of other oxidants, rotenone and menadione. Antioxidant reversed the effects of oxidants on Cx43 expression and osteocyte cell death. Cx43 protein was also much lower in the osteocytes from 20-month-old as opposed to the 5-week-old or 20-week old mice. Dye transfer assay showed that H₂O₂ reduced the gap junction intercellular communication (GJIC). In contrast to the effect on GJIC, there was a dose-dependent increase of hemichannel function by H₂O₂, which was correlated with the increased cell surface expression of Cx43. Cx43(E2) antibody, an antibody that specifically blocks Cx43 hemichannel activity but not gap junctions, completely blocked dye uptake induced by H₂O₂ and further exacerbated H₂O₂-induced osteocytic cell death. In addition, knockdown of Cx43 expression by small interfering RNA (siRNA) increased the susceptibility of the cells to OS-induced death. Together, our study provides a novel cell protective mechanism mediated by osteocytic Cx43 channels against OS.

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.

Osteoimmunology: the study of the relationship between the immune system and bone tissue

Bone tissue is a highly regulated structure, which plays an essential role in various physiological functions. Through autocrine and paracrine mechanisms, bone tissue is involved in hematopoiesis, influencing the fate of hematopoietic stem cells. There are a number of molecules shared by bone cells and immune system cells indicating that there are multiple connections between the immune system and bone tissue. In order to pool all the knowledge concerning both systems, a new discipline known under the term «osteoimmunology» has been developed. Their progress in recent years has been exponential and allowed us to connect and increase our knowledge in areas not seemingly related such as rheumatoid erosion, postmenopausal osteoporosis, bone metastases or periodontal disease. In this review, we have tried to summarize the most important advances that have occurred in the last decade, especially in those areas of interest related to rheumatology.

Antibodies targeting extracellular domain of connexins for studies of hemichannels

Hemichannels are transmembrane channels composed of either a connexin or pannexin hexamer. The docking of the extracellular domains of connexin hemichannels contributed by neighboring cells forms a gap junction channel that joins the cytoplasm of adjacent cells. Connexins are expressed ubiquitously in different organs, but some subtypes are expressed exclusively in certain tissues and tumors. Both gap junction channels and hemichannels participate in diverse physiological and pathological responses. However, the lack of specific reagents that inhibit only gap junction channels or hemichannels is a challenge that makes it different to discern the specific roles of either channel. Fortunately, the available information regarding the connexin sequence, secondary and tertiary structure, and their biochemical and physiological properties permits the development of strategies to block exclusively the hemichannel activity exclusively, with no effect on gap junction activity. This task is accomplished through the use of specifics antibodies that target the extracellular sites of desired connexin subtype. However, the underlying mechanism of how antibodies targeting extracellular connexin epitopes actually inhibit hemichannels remains unknown. Although these antibodies are being used for detecting and blocking of hemichannels in normal and tumor cells, they can also be potentially used for tissue-specific treatment and drug delivery in clinical applications. In this article, we will first review the literature concerning the structure of connexins and the unique properties of extracellular loop domains of the connexins. Furthermore, we will discuss briefly the development of connexin (Cx) 43(E2) antibody, a specific antibody which detects the second extracellular loop of Cx43 and specifically prevents the opening of Cx43 hemichannels. We will then summarize the reported studies of specific reagents used for the inhibition of connexin hemichannels including antibodies developed against extracellular loop domains. This article is part of the Special Issue Section entitled 'Current Pharmacology of Gap Junction Channels and Hemichannels'.

The effect of osteocyte apoptosis on signalling in the osteocyte and bone lining cell network: a computer simulation

Osteocytes play a critical role in the regulation of bone remodelling by translating strain due to mechanical loading into biochemical signals transmitted through the interconnecting lacuno-canalicular network to bone lining cells (BLCs) on the bone surface. This work aims to examine the effects of disruption of that intercellular communication by simulation of osteocyte apoptosis in the bone matrix. A model of a uniformly distributed osteocyte network has been developed that simulates the signalling through the network to the BLCs based on strain level. Bi-directional and asymmetric communication between neighbouring osteocytes and BLCs is included. The effect of osteocyte apoptosis is examined by preventing signalling at and through the affected cells. The simulation shows that apoptosis of only 3% of the osteocyte cells leads to a significant reduction in the peak signal at the BLCs. Furthermore, experiments with the model confirm how important the location and density of the apoptotic osteocytes are to the signalling received at the bone surface. With 5% and 9% osteocyte apoptosis, the mean peak BLC levels were reduced by 25% and 37% respectively. Such a significant reduction in the signal at the BLCs may explain a possible mechanism that leads to the increased remodelling and eventual bone loss observed with osteoporosis. More generally, it provides a unique framework for a broader exploration of the role of osteocyte and bi-directional and asymmetric cell-cell communication in mechanotransduction, and the effects of disruption to that communication.

Osteocyte regulation of bone mineral: a little give and take

Osteocytes actively participate in almost every phase of mineral handling by bone. They regulate the mineralisation of osteoid during bone formation, and they are also a major RANKL-producing cell. Osteocytes are thus able to liberate bone mineral by regulating osteoclast differentiation and activity in response to a range of stimuli, including bone matrix damage, bone disuse and mechanical unloading, oestrogen deficiency, high-dose glucocorticoid and chemotherapeutic agents. At least some of these activities may be regulated by the osteocyte-secreted product, sclerostin. There is also mounting evidence that in addition to regulating phosphate homeostasis systemically, osteocytes contribute directly to calcium homeostasis in the mature skeleton. Osteocyte cell death and the local loss of control of bone mineralisation may be the cause of focal hypermineralisation of bone and osteopetrosis, as seen in aging and pathology. The sheer number of osteocytes in bone means that "a little give and take" in terms of regulation of bone mineral content translates into a powerful whole organism effect.

Connexin43 and pannexin1 channels in osteoblasts: who is the "hemichannel"?

Osteoblasts sense and respond to mechanical stimuli in a process involving influx and release of large ions and signaling molecules. Unapposed gap junction hemichannels formed of connexin43 (Cx43) have been proposed as a major route for such exchange, in particular for release of ATP and prostaglandin E₂ (PGE₂) in osteocytes. However, we have found that Cx43-null osteoblasts have unaltered, mechanically induced PGE₂ release and ATP-induced YoPro dye uptake. In contrast, PGE₂ release in response to fluid shear stress is abolished in P2X₇ receptor (P2X₇R)-null osteoblasts, and ATP-induced dye uptake is attenuated following treatment of wild-type cells with a P2X₇R or Pannexin1 (Panx1) channel blocker. These data indicate that Panx1 channels, in concert with P2X₇R, likely form a molecular complex that performs the hemichannel function in osteoblast mechanosignaling.

[Gap junctions and bone remodeling]

Gap junctions are specialized plasma membrane structures consisting of transmembrane channels that directly link the cytoplasms of adjoining cells and mediate the reciprocal exchange of ions and low molecular weight molecules (<1200 Da). Structural studies have demonstrated that each gap junctional channel is formed by the extracellular interaction of two hemi-channels (connexons). Each connexon is a hexameric assembly of protein subunits (connexins), which delineate an aqueous pore. Connexins are homologous proteins encoded by a multigene family and are named according to their predicted molecular weight. Connexin 43, widely distributed in different cell types, is the main gap junction protein expressed in human bone cells, although Cx45 and Cx46 have been reported to be expressed as well. Bone remodeling requires coordinated activity among osteoblasts and osteoclasts. Osteoblasts (bone forming cells) are derived from mesenchymal stem cells, and osteoclasts (bone resorbing cells), are multinucleated cells of monocyte/macrophage origin. Here, we review what is known regarding the structure of gap junctions and the mechanisms regulating bone remodeling, and discuss the evidence suggesting that gap junctional intercellular communication contributes to the bone remodeling.

Gap junction proteins on the move: connexins, the cytoskeleton and migration

Connexin43 (Cx43) has roles in cell-cell communication as well as channel independent roles in regulating motility and migration. Loss of function approaches to decrease Cx43 protein levels in neural cells result in reduced migration of neurons during cortical development in mice and impaired glioma tumor cell migration. In other cell types, correlations between Cx43 expression and cell morphology, adhesion, motility and migration have been noted. In this review we will discuss the common themes that have been revealed by a detailed comparison of the published results of neuronal cells with that of other cell types. In brief, these comparisons clearly show differences in the stability and directionality of protrusions, polarity of movement, and migration, depending on whether a) residual Cx43 levels remain after siRNA or shRNA knockdown, b) Cx43 protein levels are not detectable as in cells from Cx43(-/-) knockout mice or in cells that normally have no endogenous Cx43 expression, c) gain-of-function approaches are used to express Cx43 in cells that have no endogenous Cx43 and, d) Cx43 is over-expressed in cells that already have low endogenous Cx43 protein levels. What is clear from our comparisons is that Cx43 expression influences the adhesiveness of cells and the directionality of cellular processes. These observations are discussed in light of the ability of cells to rearrange their cytoskeleton and move in an organized manner. This article is part of a Special Issue entitled: The Communicating junctions, roles and dysfunctions.