Interleukin-1beta increases the functional expression of connexin 43 in articular chondrocytes: evidence for a Ca2+-dependent mechanism

Orbital shaking conditions augment human nasoseptal cartilage formation in 3D culture

Introduction: This study aimed to determine whether a dynamic orbital shaking culture system could enhance the cartilage production and viability of bioengineered nasoseptal cartilage. Methods: Human nasal chondrocytes were seeded onto nanocellulose-alginate biomaterials and cultured in static or dynamic conditions for 14 days. Quantitative polymerase chain reaction for chondrogenic gene expression (type 2 collagen, aggrecan and SOX9) was performed, demonstrating a transient rise in SOX9 expression at 1 and 7 days of culture, followed by a rise at 7 and 14 days in Aggrecan (184.5-fold increase, p < 0.0001) and Type 2 Collagen (226.3-fold increase, p = 0.049) expression. Samples were analysed histologically for glycosaminoglycan content using Alcian blue staining and demonstrated increased matrix formation in dynamic culture. Results: Superior cell viability was identified in the dynamic conditions through live-dead and alamarBlue assays. Computational analysis was used to determine the shear stress experienced by cells in the biomaterial in the dynamic conditions and found that the mechanical stimulation exerted was minimal (fluid shear stress <0.02 mPa, fluid pressure <48 Pa). Conclusion: We conclude that the use of an orbital shaking system exerts biologically relevant effects on bioengineered nasoseptal cartilage independently of the expected thresholds of mechanical stimulation, with implications for optimising future cartilage tissue engineering efforts.

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.

Mesenchymal stromal cells donate mitochondria to articular chondrocytes exposed to mitochondrial, environmental, and mechanical stress

Articular cartilage has limited healing capacity and no drugs are available that can prevent or slow the development of osteoarthritis (OA) after joint injury. Mesenchymal stromal cell (MSC)-based regenerative therapies for OA are increasingly common, but questions regarding their mechanisms of action remain. Our group recently reported that although cartilage is avascular and relatively metabolically quiescent, injury induces chondrocyte mitochondrial dysfunction, driving cartilage degradation and OA. MSCs are known to rescue injured cells and improve healing by donating healthy mitochondria in highly metabolic tissues, but mitochondrial transfer has not been investigated in cartilage. Here, we demonstrate that MSCs transfer mitochondria to stressed chondrocytes in cell culture and in injured cartilage tissue. Conditions known to induce chondrocyte mitochondrial dysfunction, including stimulation with rotenone/antimycin and hyperoxia, increased transfer. MSC-chondrocyte mitochondrial transfer was blocked by non-specific and specific (connexin-43) gap-junction inhibition. When exposed to mechanically injured cartilage, MSCs localized to areas of matrix damage and extended cellular processes deep into microcracks, delivering mitochondria to chondrocytes. This work provides insights into the chemical, environmental, and mechanical conditions that can elicit MSC-chondrocyte mitochondrial transfer in vitro and in situ, and our findings suggest a new potential role for MSC-based therapeutics after cartilage injury.

ATP transporters in the joints

Extracellular adenosine triphosphate (ATP) plays a central role in a wide variety of joint diseases. ATP is generated intracellularly, and the concentration of the extracellular ATP pool is determined by the regulation of its transport out of the cell. A variety of ATP transporters have been described, with connexins and pannexins the most commonly cited. Both form intercellular channels, known as gap junctions, that facilitate the transport of various small molecules between cells and mediate cell-cell communication. Connexins and pannexins also form pores, or hemichannels, that are permeable to certain molecules, including ATP. All joint tissues express one or more connexins and pannexins, and their expression is altered in some pathological conditions, such as osteoarthritis (OA) and rheumatoid arthritis (RA), indicating that they may be involved in the onset and progression of these pathologies. The aging of the global population, along with increases in the prevalence of obesity and metabolic dysfunction, is associated with a rising frequency of joint diseases along with the increased costs and burden of related illness. The modulation of connexins and pannexins represents an attractive therapeutic target in joint disease, but their complex regulation, their combination of gap-junction-dependent and -independent functions, and their interplay between gap junction and hemichannel formation are not yet fully elucidated. In this review, we try to shed light on the regulation of these proteins and their roles in ATP transport to the extracellular space in the context of joint disease, and specifically OA and RA.

Inflammatory Amplification: A Central Tenet of Uterine Transition for Labor

In preparation for delivery, the uterus transitions from actively maintaining quiescence during pregnancy to an active parturient state. This transition occurs as a result of the accumulation of pro-inflammatory signals which are amplified by positive feedback interactions involving paracrine and autocrine signaling at the level of each intrauterine cell and tissue. The amplification events occur in parallel until they reach a certain threshold, ‘tipping the scale’ and contributing to processes of uterine activation and functional progesterone withdrawal. The described signaling interactions all occur upstream from the presentation of clinical labor symptoms. In this review, we will: 1) describe the different physiological processes involved in uterine transition for each intrauterine tissue; 2) compare and contrast the current models of labor initiation; 3) introduce innovative models for measuring paracrine inflammatory interactions; and 4) discuss the therapeutic value in identifying and targeting key players in this crucial event for preterm birth.

miR-382-3p suppressed IL-1β induced inflammatory response of chondrocytes via the TLR4/MyD88/NF-κB signaling pathway by directly targeting CX43

miR-382-3p has been reported to be upregulated in synovial membrane in knee osteoarthritis (OA). Nevertheless, its role in OA remains largely unknown. The aim of this study was to investigate the specific function and mechanisms of miR-382-3p in the course of OA. In this study, human OA chondrocytes were pretreated with interleukin-1β (IL-1β) at 5 ng/ml for 12 hr to stimulate inflammatory response and matrix metalloproteinases (MMPs) expression in chondrocytes. Meanwhile, miR-382-3p was downregulated in IL-1β-stimulated chondrocytes. In addition, we found that miR-382-3p directly interacts with connexin 43 (CX43) and attenuates the increase of cytochrome c oxidase polypeptide II, inducible nitric oxide synthase, and MMP-1/13 that is induced by IL-1β. Furthermore, our observations indicated that miR-382-3p inhibited the expression of Toll-like receptor 4 (TLR4), Myeloid differentiation primary response 88 (MyD88) and nuclear factor κB (NF-κB) in IL-1β-stimulated chondrocytes, while CX43 overexpression could partly reverse these decreases. In conclusion, miR-382-3p participated in OA may through the TLR4/MyD88/NF-κB signaling pathway by directly targeting CX43.

Targeting of chondrocyte plasticity via connexin43 modulation attenuates cellular senescence and fosters a pro-regenerative environment in osteoarthritis

Osteoarthritis (OA), a chronic disease characterized by articular cartilage degeneration, is a leading cause of disability and pain worldwide. In OA, chondrocytes in cartilage undergo phenotypic changes and senescence, restricting cartilage regeneration and favouring disease progression. Similar to other wound-healing disorders, chondrocytes from OA patients show a chronic increase in the gap junction channel protein connexin43 (Cx43), which regulates signal transduction through the exchange of elements or recruitment/release of signalling factors. Although immature or stem-like cells are present in cartilage from OA patients, their origin and role in disease progression are unknown. In this study, we found that Cx43 acts as a positive regulator of chondrocyte-mesenchymal transition. Overactive Cx43 largely maintains the immature phenotype by increasing nuclear translocation of Twist-1 and tissue remodelling and proinflammatory agents, such as MMPs and IL-1β, which in turn cause cellular senescence through upregulation of p53, p16^INK4a and NF-κB, contributing to the senescence-associated secretory phenotype (SASP). Downregulation of either Cx43 by CRISPR/Cas9 or Cx43-mediated gap junctional intercellular communication (GJIC) by carbenoxolone treatment triggered rediferentiation of osteoarthritic chondrocytes into a more differentiated state, associated with decreased synthesis of MMPs and proinflammatory factors, and reduced senescence. We have identified causal Cx43-sensitive circuit in chondrocytes that regulates dedifferentiation, redifferentiation and senescence. We propose that chondrocytes undergo chondrocyte-mesenchymal transition where increased Cx43-mediated GJIC during OA facilitates Twist-1 nuclear translocation as a novel mechanism involved in OA progression. These findings support the use of Cx43 as an appropriate therapeutic target to halt OA progression and to promote cartilage regeneration.

Biochemical alterations in inflammatory reactive chondrocytes: evidence for intercellular network communication

Chondrocytes are effectively involved in the pathophysiological processes of inflammation in joints. They form cellular processes in the superficial layer of the articular cartilage and form gap junction coupled syncytium to facilitate cell-to-cell communication. However, very little is known about their physiological cellular identity and communication. The aim with the present work is to evaluate the physiological behavior after stimulation with the inflammatory inducers interleukin-1β and lipopolysaccharide. The cytoskeleton integrity and intracellular Ca²⁺ release were assessed as indicators of inflammatory state. Cytoskeleton integrity was analyzed through cartilage oligomeric matrix protein and actin labeling with an Alexa 488-conjugated phalloidin probe. Ca²⁺ responses were assessed through the Ca²⁺ sensitive fluorophore Fura-2/AM. Western blot analyses of several inflammatory markers were performed. The results show reorganization of the actin filaments. Glutamate, 5-hydoxytryptamine, and ATP evoked intracellular Ca²⁺ release changed from single peaks to oscillations after inflammatory induction in the chondrocytes. The expression of toll-like receptor 4, the glutamate transporters GLAST and GLT-1, and the matrix metalloproteinase-13 increased. This work demonstrates that chondrocytes are a key part in conditions that lead to inflammation in the cartilage. The inflammatory inducers modulate the cytoskeleton, the Ca²⁺ signaling, and several inflammatory parameters. In conclusion, our data show that the cellular responses to inflammatory insults from healthy and inflammatory chondrocytes resemble those previously observed in astrocyte and cardiac fibroblasts networks.

Joint diseases: from connexins to gap junctions

Connexons form the basis of hemichannels and gap junctions. They are composed of six tetraspan proteins called connexins. Connexons can function as individual hemichannels, releasing cytosolic factors (such as ATP) into the pericellular environment. Alternatively, two hemichannel connexons from neighbouring cells can come together to form gap junctions, membrane-spanning channels that facilitate cell-cell communication by enabling signalling molecules of approximately 1 kDa to pass from one cell to an adjacent cell. Connexins are expressed in joint tissues including bone, cartilage, skeletal muscle and the synovium. Indicative of their importance as gap junction components, connexins are also known as gap junction proteins, but individual connexin proteins are gaining recognition for their channel-independent roles, which include scaffolding and signalling functions. Considerable evidence indicates that connexons contribute to the function of bone and muscle, but less is known about the function of connexons in other joint tissues. However, the implication that connexins and gap junctional channels might be involved in joint disease, including age-related bone loss, osteoarthritis and rheumatoid arthritis, emphasizes the need for further research into these areas and highlights the therapeutic potential of connexins.

Enhanced Expression of Contractile-Associated Proteins and Ion Channels in Preterm Delivery Model Mice With Chronic Odontogenic Porphyromonas Gingivalis Infection

Inflammation and infection have been reported to induce preterm delivery. We have studied the relationship between inflammation and various ion channels, including the L-type Ca(2+) channel and P2X7 receptor, during acute inflammation of the pregnant rat uterus induced by lipopolysaccharides. Recently, we found that mice with odontogenic Porphyromonas gingivalis (P.g, an important odontogenic pathogen) infection delivered at day 18.3 of gestation (vs. day 20.5 in normal mice). The purpose of this study was to investigate the expression of myometrial contractile-associated proteins inducing contractions and confirm that these mice are useful as a model for preterm delivery induced by chronic inflammation. We examined the expression of the oxytocin receptor, connexin 43, prostaglandin F receptors, L-type Ca(2+) channel, and P2X7 receptor in the myometrium at day 18 of gestation by real-time PCR and western blot analyses. We also measured TNF-α and IL-1β levels in the blood serum, placenta, fetal membrane and myometrium on the same day. mRNA expression of the oxytocin receptor, connexin 43, prostaglandin F receptors, L-type Ca(2+) channel, and P2X7 receptor was elevated by 5.4, 3.2, 2.4, 2.5, and 1.7 fold, respectively, in the P.g-infected mice. Protein levels of the oxytocin receptor and connexin 43 also increased. Serum levels of TNF-α and IL-1β were elevated, showing that systemic inflammation continued during pregnancy. IL-1β levels in the placenta and fetal membrane also increased, suggesting inflammatory reactions were induced. Thus, mice with odontogenic infection may be useful as a model of chronic inflammation-induced preterm delivery.

Toll-Like Receptor 4 Is an Essential Upstream Regulator of On-Time Parturition and Perinatal Viability in Mice

An inflammatory response is instrumental in the physiological process of parturition but the upstream signals initiating inflammation are undefined. Because endogenous ligands for Toll-like receptor 4 (TLR4) are released in late gestation, we hypothesized that on-time labor requires TLR4 signaling, to trigger a cytokine and leukocyte response and accelerate the parturition cascade. In pregnant TLR4-deficient (Tlr4-/-) mice, average gestation length was extended by 13 hours and increased perinatal mortality was seen compared with wild-type controls. Quantification of cytokine and uterine activation gene expression showed that late gestation induction of Il1b, Il6, Il12b, and Tnf expression seen in control placenta and fetal membranes was disrupted in Tlr4-/- mice, and accompanied by a transient delay in expression of uterine activation genes, including prostaglandin F receptor, oxytocin receptor, and connexin-43. Leukocyte populations were altered before birth in TLR4-deficient females, with fewer neutrophils and macrophages in the placenta, and fewer dendritic cells and more regulatory T cells in the myometrium. Administration of TLR4 ligand lipopolysaccharide to pregnant wild-type mice induced cytokine expression and fetal loss, whereas Tlr4-/- pregnancies were protected. The small molecule TLR4 antagonist (+)-naloxone increased mean duration of gestation by 16 hours in wild-type mice. Collectively, these data demonstrate that TLR4 is a key upstream regulator of the inflammatory response acting to drive uterine activation and control the timing of labor. Because causal pathways for term and preterm labor converge with TLR4, interventions to manipulate TLR4 signaling may have therapeutic utility for women at risk of preterm labor, or in postterm pregnancy.

Coupled cell networks are target cells of inflammation, which can spread between different body organs and develop into systemic chronic inflammation

Several organs in the body comprise cells coupled into networks. These cells have in common that they are excitable but do not express action potentials. Furthermore, they are equipped with Ca²⁺ signaling systems, which can be intercellular and/or extracellular. The transport of small molecules between the cells occurs through gap junctions comprising connexin 43. Examples of cells coupled into networks include astrocytes, keratinocytes, chondrocytes, synovial fibroblasts, osteoblasts, connective tissue cells, cardiac and corneal fibroblasts, myofibroblasts, hepatocytes, and different types of glandular cells. These cells are targets for inflammation, which can be initiated after injury or in disease. If the inflammation reaches the CNS, it develops into neuroinflammation and can be of importance in the development of systemic chronic inflammation, which can manifest as pain and result in changes in the expression and structure of cellular components. Biochemical parameters of importance for cellular functions are described in this review.

Connexin43 Mediated Delivery of ADAMTS5 Targeting siRNAs from Mesenchymal Stem Cells to Synovial Fibroblasts

Osteoarthritis is a joint-destructive disease that has no effective cure. Human mesenchymal stem cells (hMSCs) could offer therapeutic benefit in the treatment of arthritic diseases by suppressing inflammation and permitting tissue regeneration, but first these cells must overcome the catabolic environment of the diseased joint. Likewise, gene therapy also offers therapeutic promise given its ability to directly modulate key catabolic factors that mediate joint deterioration, although it too has limitations. In the current study, we explore an approach that combines hMSCs and gene therapy. Specifically, we test the use of hMSC as a vehicle to deliver ADAMTS5 (an aggrecanase with a key role in osteoarthritis)-targeting siRNAs to SW982 synovial fibroblast-like cells via connexin43 containing gap junctions. Accordingly, we transduced hMSCs with ADAMTS5-targeting shRNA or non-targeted shRNA, and co-cultured them with synovial fibroblasts to allow delivery of siRNAs from hMSC to synovial fibroblasts. We found that co-culture of hMSCs-shRNA-ADAMTS5 and synovial fibroblasts reduced ADAMTS5 expression relative to co-culture of hMSCs-shRNA-control and synovial fibroblasts. Furthermore, ADAMTS5 was specifically reduced in the synovial fibroblasts populations as determined by fluorescence-activated cell sorting, suggesting transfer of the siRNA between cells. To test if Cx43-containing gap junctions are involved in the transfer of siRNA, we co-cultured hMSCs-shRNA-ADAMTS5 cells with synovial fibroblasts in which connexin43 was knocked down. Under these conditions, ADAMTS5 levels were not inhibited by co-culture, indicating that connexin43 mediates the delivery of siRNA from hMSCs to synovial fibroblasts. In total, our findings demonstrate that hMSCs can function as donor cells to host and deliver siRNAs to synovial fibroblasts via connexin43 gap junction in vitro. These data may have implications in the combination of hMSCs and gene therapy to treat diseases like osteoarthritis, in vivo.

Identification of shoulder osteoarthritis biomarkers: comparison between shoulders with and without osteoarthritis

BACKGROUND

The biologic factors associated with shoulder osteoarthritis (OA) have not been elucidated. The purpose of this study was to investigate osteoarthritic biomarkers of the shoulder. To our knowledge, this is the first study to analyze shoulder cartilage for OA-associated genes and to examine human shoulder cartilage for a possible biomarker, connexin 43 (Cx43).

MATERIALS AND METHODS

Cartilage from 16 osteoarthritic and 10 nonosteoarthritic humeral heads was assessed for expression of the following genes by real-time polymerase chain reaction: types I, II, and X collagen; matrix metalloproteinases (MMPs); tissue inhibitors of MMP (TIMPs); interleukins; versican; cyclooxygenase 2 (Cox-2); inducible nitric oxide synthase (iNOS); tumor necrosis factor α (TNF-α); aggrecanase 2 (ADAMTS5); and Cx43.

RESULTS

In osteoarthritic shoulders, Cx43, Cox-2, versican, collagen type I, ADAMTS5, MMP-3, and TNF-α expressions were significantly increased compared with controls. TIMP-3 and iNOS trended toward significance, with robust expression in osteoarthritic shoulders and low expression in nonosteoarthritic shoulders. In osteoarthritic shoulders, gene expression of Cx43, ADAMTS5, collagen type I, Cox-2, versican, and TIMP-3 showed predominance (85-, 33-, 13-, 12-, 11.5-, and 3-fold increases, respectively) relative to nonosteoarthritic controls. Spearman correlation analysis showed significant correlations between Cx43 and collagen (types I, II, and X), MMP-9, TIMP-2 and TIMP-3, versican, Cox-2, iNOS, and ADAMTS5.

CONCLUSIONS

Certain genes are markedly upregulated in osteoarthritic shoulders compared with nonosteoarthritic shoulders, with Cx43, Cox-2, versican, collagen type I, ADAMTS5, MMP-3, and TNF-α expression being significantly increased. These genes might be useful biomarkers for examining shoulder OA.

CLINICAL RELEVANCE

Identification of osteoarthritic biomarkers can help us better understand shoulder OA and build the foundation for future research on disease progression and treatments.

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.

Pannexin 1 involvement in bladder dysfunction in a multiple sclerosis model

Bladder dysfunction is common in Multiple Sclerosis (MS) but little is known of its pathophysiology. We show that mice with experimental autoimmune encephalomyelitis (EAE), a MS model, have micturition dysfunction and altered expression of genes associated with bladder mechanosensory, transduction and signaling systems including pannexin 1 (Panx1) and Gja1 (encoding connexin43, referred to here as Cx43). EAE mice with Panx1 depletion (Panx1^−/−) displayed similar neurological deficits but lesser micturition dysfunction compared to Panx1^+/+ EAE. Cx43 and IL-1β upregulation in Panx1^+/+ EAE bladder mucosa was not observed in Panx1^−/− EAE. In urothelial cells, IL-1β stimulation increased Cx43 expression, dye-coupling, and p38 MAPK phosphorylation but not ERK1/2 phosphorylation. SB203580 (p38 MAPK inhibitor) prevented IL-1β-induced Cx43 upregulation. IL-1β also increased IL-1β, IL-1R-1, PANX1 and CASP1 expression. Mefloquine (Panx1 blocker) reduced these IL-1β responses. We propose that Panx1 signaling provides a positive feedback loop for inflammatory responses involved in bladder dysfunction in MS.

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

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

REPAIR OF ARTICULAR CARTILAGE INJURY

Articular cartilage defects heal poorly and lead to consequences as osteoarthritis. Clinical experience has indicated that no existing medication would substantially promote the healing process, and the cartilage defect requires surgical replacement. Allograft decays quickly for multiple reasons including the preparation process and immune reaction, and the outcome is disappointing. The extreme shortage of sparing in articular cartilage has much discouraged the use of autograft, which requires modification. The concept that constructs a chondral or osteochondral construct for the replacement of injured native tissue introduces that of tissue engineering. Limited number of cells are expanded either in vitro or in vivo, and resided temporally on a scaffold of biomaterial, which also acts as a vehicle to transfer the cells to the recipient site. Three core elements constitute this technique: the cell, a biodegradable scaffold, and an environment suitable for cells to present their proposed activity. Modern researches have kept updating those elements for a better performance of such cultivation of living tissue.

Pannexin 3 regulates intracellular ATP/cAMP levels and promotes chondrocyte differentiation

Pannexin 3 (Panx3) is a new member of the gap junction pannexin family, but its expression profiles and physiological function are not yet clear. We demonstrate in this study that Panx3 is expressed in cartilage and regulates chondrocyte proliferation and differentiation. Panx3 mRNA was expressed in the prehypertrophic zone in the developing growth plate and was induced during the differentiation of chondrogenic ATDC5 and N1511 cells. Panx3-transfected ATDC5 and N1511 cells promoted chondrogenic differentiation, but the suppression of endogenous Panx3 inhibited differentiation of ATDC5 cells and primary chondrocytes. Panx3-transfected ATDC5 cells reduced parathyroid hormone-induced cell proliferation and promoted the release of ATP into the extracellular space, possibly by action of Panx3 as a hemichannel. Panx3 expression in ATDC5 cells reduced intracellular cAMP levels and the activation of cAMP-response element-binding, a protein kinase A downstream effector. These Panx3 activities were blocked by anti-Panx3 antibody. Our results suggest that Panx3 functions to switch the chondrocyte cell fate from proliferation to differentiation by regulating the intracellular ATP/cAMP levels.

Interleukin-1beta increases gap junctional communication among synovial fibroblasts via the extracellular-signal-regulated kinase pathway

BACKGROUND INFORMATION

The gap junction protein, Cx43 (connexin 43), has been implicated in the aetiology of osteoarthritis. Studies have revealed that the size and number of gap junctions increase in synovial biopsies from patients with osteoarthritis. Furthermore, pharmacological inhibition of Cx43 function has been shown to reduce IL-1beta (interleukin-1beta)-induced metalloproteinase production by synovial fibroblasts in vitro.

RESULTS

In the present study, we examined the link between IL-1beta and Cx43 function. We demonstrated that treatment of a rabbit synovial fibroblast cell line with IL-1beta markedly increased the level of the Cx43 protein in a concentration- and time-dependent manner. The impact on Cx43 protein levels appeared to occur post-transcriptionally, as mRNA levels are unaffected by IL-1beta administration. Additionally, we showed by fluorescence microscopy that IL-1beta alters the cellular distribution of Cx43 to cell-cell junctions and is concomitant with a striking increase in gap junction communication. Furthermore, we demonstrated that the increase in Cx43 protein, and the associated change in protein localization and gap junction communication following IL-1beta treatment, are dependent upon activation of the ERK (extracellular-signal-regulated kinase) signalling cascade.

CONCLUSION

These data show that IL-1beta acts through the ERK signalling cascade to alter the expression and function of Cx43 in synovial fibroblasts.

Repair of articular cartilage defects: review and perspectives

Articular cartilage defects heal poorly and lead to catastrophic degenerative arthritis. Clinical experience has indicated that no existing medication substantially promotes the healing process and the cartilage defect requires surgical replacement, preferably with an autograft. However, there is a shortage of articular cartilage that can be donated for autografting. A review of previous unsuccessful experiences reveals the reason for the current strategy to graft cartilage defects with regenerated cartilage. Autologous cartilage regeneration is a cell-based therapy in which autogenous chondrocytes or other chondrogenic cells are cultured to constitute cartilaginous tissue according to the principles of tissue engineering. Current studies are concentrating on improving such techniques from the three elements of tissue engineering, namely the cells, biomaterial scaffolds, and culture conditions. Some models of articular cartilage regeneration have yielded good repair of cartilage defects, in animal models and clinical settings, but the overall results suggest that there is room for improvement of this technique before its routine clinical application. Autologous cartilage regeneration remains the mainstay for repairing articular cartilage defects but more studies are required to optimize the efficacy of regeneration. A more abundant supply of more stable cells, i.e. capable of maintaining the phenotype of chondrogenesis, has to be identified. Porous scaffolds of biocompatible, biodegradable materials that maintain and support the presentation of the chondrogenic cells need to be fabricated. If the cells are not implanted early to allow their in vivo constitution of cartilage, a suitable in vitro cultivation method has to be devised for a consistent yield of regenerative cartilage.

Inflammatory processes in preterm and term parturition

A role for the pro-inflammatory cytokines interleukin (IL)-1beta, IL-6, IL-8 and tumor necrosis factor alpha (TNF-alpha) is evident in term and preterm delivery, and this is independent of the presence of infection. All uterine tissues progress through a staged transformation near the end of pregnancy that leads from relative uterine quiescence and maintenance of pregnancy to the activation of the uterus that prepares it for the work of labour and production of stimulatory molecules that trigger the onset of labour and delivery. The uterus is activated by pro-inflammatory cytokines through stimulation of the expression and production of uterine activation proteins (UAPs). One of these actions is the stimulation of prostaglandin (PG) synthesis. Particularly important for labour is PGF(2alpha) and its receptor, PTGFR. In addition, pro-inflammatory cytokines are able to increase the synthesis of matrix metalloproteinases (MMPs), vascular endothelial growth factor (VEGF) and the progesterone receptor C isoform, which leads to decreased tissue progesterone responsiveness. Some of these effects are replicated by PGF(2alpha), suggesting that it may act via its receptor to amplify the direct actions of cytokines. In turn, VEGF may enhance leukocyte recruitment to the uterus, and MMP-9 may promote activation of inactive pro-form cytokines. Pro-inflammatory cytokines also decrease the activity of 11beta-hydroxysteroid dehydrogenase, which likely increases intrauterine cortisol concentrations. In turn, cortisol may drive PG synthesis. Together these feed-forward mechanisms activate the uterus, trigger the production of uterine contractile stimulants and lead to labour and delivery.

Opposite regulation of connexin33 and connexin43 by LPS and IL-1α in spermatogenesis

The gap junction proteins, connexins (Cxs), are present in the testis, and among them, Cx43 play an essential role in spermatogenesis. In the present study, we investigated the testicular expression and regulation of another Cx, Cx33, previously described as a negative regulator of gap junction communication. Cx33 mRNA was present in testis and undetectable in heart, liver, ovary, and uterus. In the mature testis, Cx33 was specifically immunolocalized in the basal compartment of the seminiferous tubules, whereas Cx43 was present in both seminiferous tubule and interstitial compartments. During stages IX and X of spermatogenesis, characterized by Sertoli cell phagocytosis of residual bodies, Cx43 was poorly expressed within seminiferous tubules, while Cx33 signal was strong. To evaluate the role of phagocytosis in the control of Cx33 and Cx43 expression, the effect of LPS was analyzed in the Sertoli cell line 42GPA9. We show herein that phagocytosis activation by LPS concomitantly stimulated Cx33 and inhibited Cx43 mRNA levels. These effects appear to have been mediated through IL-1α, because the exposure of Sertoli cells to the IL-1 receptor antagonist partly reversed these effects. IL-1α enhanced and reduced, respectively, the levels of Cx33 and Cx43 mRNA in a time- and dose-dependent manner. These data reveal that Cx33 and Cx43 genes are controlled differently within the testis and suggest that these two Cxs may exert opposite and complementary effects on spermatogenesis.

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.

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.

Connexin 43 mediated gap junctional communication enhances breast tumor cell diapedesis in culture

Introduction

Metastasis involves the emigration of tumor cells through the vascular endothelium, a process also known as diapedesis. The molecular mechanisms regulating tumor cell diapedesis are poorly understood, but may involve heterocellular gap junctional intercellular communication (GJIC) between tumor cells and endothelial cells.

Method

To test this hypothesis we expressed connexin 43 (Cx43) in GJIC-deficient mammary epithelial tumor cells (HBL100) and examined their ability to form gap junctions, establish heterocellular GJIC and migrate through monolayers of human microvascular endothelial cells (HMVEC) grown on matrigel-coated coverslips.

Results

HBL100 cells expressing Cx43 formed functional heterocellular gap junctions with HMVEC monolayers within 30 minutes. In addition, immunocytochemistry revealed Cx43 localized to contact sites between Cx43 expressing tumor cells and endothelial cells. Quantitative analysis of diapedesis revealed a two-fold increase in diapedesis of Cx43 expressing cells compared to empty vector control cells. The expression of a functionally inactive Cx43 chimeric protein in HBL100 cells failed to increase migration efficiency, suggesting that the observed up-regulation of diapedesis in Cx43 expressing cells required heterocellular GJIC. This finding is further supported by the observation that blocking homocellular and heterocellular GJIC with carbenoxolone in co-cultures also reduced diapedesis of Cx43 expressing HBL100 tumor cells.

Conclusion

Collectively, our results suggest that heterocellular GJIC between breast tumor cells and endothelial cells may be an important regulatory step during metastasis.

Directing stem cell differentiation into the chondrogenic lineage in vitro

A major area in regenerative medicine is the application of stem cells in cartilage tissue engineering and reconstructive surgery. This requires well-defined and efficient protocols for directing the differentiation of stem cells into the chondrogenic 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 chondrogenesis and cartilaginous tissue biology. The development of pharmacokinetic and cytotoxicity/genotoxicity screening tests for cartilage-related biomaterials and drugs could also utilize protocols developed for the chondrogenic differentiation of stem cells. Hence, this review critically examines the various strategies that could be used to direct the differentiation of stem cells into the chondrogenic lineage in vitro.

Increased intercellular communication through gap junctions may contribute to progression of osteoarthritis

Our aim was to support the hypothesis of a specific association between gap junctions in synovial tissue and the presence of osteoarthritis, as evidenced by differences between osteoarthritis and non-osteoarthritis synovia in the number of gap junctions, the amount of gap-junction protein, and the amount of enzymatic activity produced through a pathway mediated by gap-junction intercellular communication. An average of 4.41 gap junctions were found per 100 cells counted in the osteoarthritis synovia, compared with 1.00 in the controls. The amount of the gap-junction protein connexin 43 in synovial lining cells was approximately 50% greater in patients with osteoarthritis. Synovial lining cells from patients with osteoarthritis produced matrix metalloproteinases constitutively and, at higher levels, in response to stimulation by interleukin-1 beta. In both cases, intercellular communication through gap junctions was shown to be critical to the ability of the cells to secrete matrix metalloproteinases. Overall, the results indicated that gap junctions between synovial lining cells were altered significantly in patients with osteoarthritis, as a consequence of the disease process or as part of the causal chain. In either case, gap junctions seem to be a rational therapeutic target.

Intercellular calcium communication regulates platelet aggregation and thrombus growth

The ability of platelets to form stable adhesion contacts with other activated platelets (platelet cohesion or aggregation) at sites of vascular injury is essential for hemostasis and thrombosis. In this study, we have examined the mechanisms regulating cytosolic calcium flux during the development of platelet-platelet adhesion contacts under the influence of flow. An examination of platelet calcium flux during platelet aggregate formation in vitro demonstrated a key role for intercellular calcium communication (ICC) in regulating the recruitment of translocating platelets into developing aggregates. We demonstrate that ICC is primarily mediated by a signaling mechanism operating between integrin alpha IIb beta 3 and the recently cloned ADP purinergic receptor P2Y12. Furthermore, we demonstrate that the efficiency by which calcium signals are propagated within platelet aggregates plays an important role in dictating the rate and extent of thrombus growth.

Mechanically induced ATP release from human osteoblastic cells

Extracellular ATP is a widespread autocrine/paracrine signal since many animal cells release ATP in the extracellular medium; often this release is mechanosensitive, but the molecular mechanism is still unclear. The involvement of vesicular release, conductive channels, or ABC transporters has been suggested in different cell types. We investigated the mechanism of ATP release in human HOBIT osteoblastic cells, in which mechanical stimulation induced intercellular calcium waves sustained by both cell-to-cell coupling through gap junctions and ATP release. In this study we employed a luciferin-luciferase bioluminescence assay to measure the amount of ATP released under different stimulatory conditions. Given the role of connexin hemichannels in favoring passive NAD(+) transport [Bruzzone, S., et al. (2001) FASEB J. 15, 10-12], the involvement of connexin hemichannels as putative ATP transporters was initially investigated. In HOBIT cells overexpressing connexin43 the amount of nucleotide released under basal and stimulated conditions was similar to non-transfected cells, ruling out a major involvement of connexin hemichannels in ATP transport. In nontransfected HOBIT cells mechanical stimulations induced by medium displacement and hypotonic stress consistently enhanced ATP efflux. Cytochalsin D treatment did not alter basal and stimulated ATP release, while elevated cAMP levels consistently reduced efflux in both cases. ATP released by hypotonic stress and medium displacement evoked intracellular Ca(2+) transients in fura2-loaded HOBIT cells, indicating that different mechanical stimuli activate physiological cell responses.

Effects of cAMP on intercellular coupling and osteoblast differentiation

Bone-forming cells are organized in a multicellular network interconnected by gap junctions. Direct intercellular communication via gap junctions is an important component of bone homeostasis, coordinating cellular responses to external signals and promoting osteoblast differentiation. The cAMP pathway, a major intercellular signal transduction mechanism, regulates osteoblastic function and metabolism. We investigated the effects of this second messenger on junctional communication and on the expression of differentiation markers in human HOBIT osteoblastic cells. Increased levels of cAMP induce posttranslational modifications (i.e., phosphorylations) of connexin43 and enhancement of gap junction assembly, resulting in an increased junctional permeance to Lucifer yellow and to a positive modulation of intercellular Ca(2+) waves. Increased intercellular communication, however, was accompanied by a parallel decrease of alkaline phosphatase activity and by an increase of osteocalcin expression. cAMP-dependent stimulation of cell-to-cell coupling induces a complex modulation of bone differentiation markers.