Gap junction assembly: PTX-sensitive G proteins regulate the distribution of connexin43 within cells

A new side-effect of sufentanil: increased monocyte-endothelial adhesion

BACKGROUND

Opioids have been identified by the World Health Organization to be 'indispensable for the relief of pain and suffering'. Side-effects, such as nausea, vomiting, postoperative delirium, and effects on breathing, of opioids have been well investigated; however, the influence of opioids on monocyte-endothelial adherence has never been reported. Therefore, we explored the effects of representative opioids, fentanyl, sufentanil, and remifentanil, on monocyte-endothelial adherence and the underlying mechanisms.

METHODS

We built a cell adhesion model with U937 monocytes and human umbilical vein endothelial cells (HUVECs). Two kinds of connexin43 (Cx43) channel inhibitors, 18-α-GA and Gap 27, were used to alter Cx43 channel function in U937 monocytes and HUVECs, respectively, to determine the effects of Cx43 channels on U937-HUVEC adhesion. Subsequently, the effects of fentanyl, sufentanil and remifentanil on Cx43 channel function and U937-HUVEC adhesion were explored.

RESULTS

When fentanyl, sufentanil and remifentanil acted on monocytes or endothelial cells, their effects on monocyte-endothelial adherence differed. When acting on U937 monocytes, sufentanil significantly increased U937-HUVEC adhesion which was associated with reduced release of ATP from Cx43 channels, while fentanyl and remifentanil did not have these influences. Although sufentanil could also inhibit Cx43 channel function in HUVECs, it had no effect on ATP release from HUVECs or U937-HUVECs adhesion.

CONCLUSIONS

We demonstrated that sufentanil application increases monocyte-endothelial adherence which was associated with reduced release of ATP from Cx43 channels in monocytes. This side-effect of sufentanil should be considered seriously by clinicians.

Antigen-loaded monocyte administration induces potent therapeutic antitumor T cell responses

Efficacy of dendritic cell (DC) cancer vaccines is classically thought to depend on their antigen-presenting cell (APC) activity. Studies show, however, that DC vaccine priming of cytotoxic T lymphocytes (CTLs) requires the activity of endogenous DCs, suggesting that exogenous DCs stimulate antitumor immunity by transferring antigens (Ags) to endogenous DCs. Such Ag transfer functions are most commonly ascribed to monocytes, implying that undifferentiated monocytes would function equally well as a vaccine modality and need not be differentiated to DCs to be effective. Here, we used several murine cancer models to test the antitumor efficacy of undifferentiated monocytes loaded with protein or peptide Ag. Intravenously injected monocytes displayed antitumor activity superior to DC vaccines in several cancer models, including aggressive intracranial glioblastoma. Ag-loaded monocytes induced robust CTL responses via Ag transfer to splenic CD8+ DCs in a manner independent of monocyte APC activity. Ag transfer required cell-cell contact and the formation of connexin 43-containing gap junctions between monocytes and DCs. These findings demonstrate the existence of an efficient gap junction-mediated Ag transfer pathway between monocytes and CD8+ DCs and suggest that administration of tumor Ag-loaded undifferentiated monocytes may serve as a simple and efficacious immunotherapy for the treatment of human cancers.

Polyamines preserve connexin 43-mediated gap junctional communication during intracellular hypercalcemia and acidosis

Changes in the regulation, formation, and gating of connexin-based gap junction channels occur in various disorders. It has been shown that H and Ca are involved in the regulation of gap junctional communication. Ischemia-induced intracellular acidification and Ca overload lead to closure of gap junctions and inhibit an exchange by ions and small molecules throughout the network of cells in the heart, brain, and other tissues. In this study, we examined the role of the polyamines in the regulation of connexin 43 (Cx43)-based gap junction channels under elevated intracellular concentrations of hydrogen ([H]i) and calcium ([Ca]i) ions. Experiments, conducted in Novikoff and A172 human glioblastoma cells, which endogenously express Cx43, showed that polyamines prevent downregulation of Cx43-mediated gap junctional communication caused by elevated [Ca]i and [H]i, accompanying ischemic and other pathological conditions. siRNA knockdown of Cx43 significantly reduces gap junctional communication, indicating that Cx43 gap junctions are the targets for spermine regulation.

Astrocytes in neuroprotection and neurodegeneration: The role of connexin43 and pannexin1

The World Health Organization has predicted that by 2040 neurodegenerative diseases will overtake cancer to become the world's second leading cause of death after cardiovascular disease. This has sparked the development of several European and American brain research initiatives focusing on elucidating the underlying cellular and molecular mechanisms of neurodegenerative diseases. Connexin (Cx) and pannexin (Panx) membrane channel proteins are conduits through which neuronal, glial, and vascular tissues interact. In the brain, this interaction is highly critical for homeostasis and brain repair after injury. Understanding the molecular mechanisms by which these membrane channels function, in health and disease, might be particularly influential in establishing conceptual frameworks to develop new therapeutics against Cx and Panx channels. This review focuses on current insights and emerging concepts, particularly the impact of connexin43 and pannexin1, under neuroprotective and neurodegenerative conditions within the context of astrocytes.

Gap junction modulation and its implications for heart function

Gap junction communication (GJC) mediated by connexins is critical for heart function. To gain insight into the causal relationship of molecular mechanisms of disease pathology, it is important to understand which mechanisms contribute to impairment of gap junctional communication. Here, we present an update on the known modulators of connexins, including various interaction partners, kinases, and signaling cascades. This gap junction network (GJN) can serve as a blueprint for data mining approaches exploring the growing number of publicly available data sets from experimental and clinical studies.

Modulation of connexin signaling by bacterial pathogens and their toxins

Inherent to their pivotal tasks in the maintenance of cellular homeostasis, gap junctions, connexin hemichannels, and pannexin hemichannels are frequently involved in the dysregulation of this critical balance. The present paper specifically focuses on their roles in bacterial infection and disease. In particular, the reported biological outcome of clinically important bacteria including Escherichia coli, Shigella flexneri, Yersinia enterocolitica, Helicobacter pylori, Bordetella pertussis, Aggregatibacter actinomycetemcomitans, Pseudomonas aeruginosa, Citrobacter rodentium, Clostridium species, Streptococcus pneumoniae, and Staphylococcus aureus and their toxic products on connexin- and pannexin-related signaling in host cells is reviewed. Particular attention is paid to the underlying molecular mechanisms of these effects as well as to the actual biological relevance of these findings.

Increased interaction of connexin43 with zonula occludens-1 during inhibition of gap junctions by G protein-coupled receptor agonists

Astrocytes are extensively coupled through gap junctions (GJs) that are composed of channels mostly constituted by connexin43 (Cx43). This astroglial gap junctional intercellular communication (GJIC) allows propagation of ions and signaling molecules critical for neuronal activity and survival. It is drastically inhibited by a short-term exposure to endothelin-1 (ET-1) or to sphingosine-1-phosphate (S1P), both compounds being inflammatory mediators acting through activation of GTP-binding protein-coupled receptors (GPCRs). Previously, we have identified the GTPases G(i/o) and Rho as key actors in the process of S1P-induced inhibition. Here, we asked whether similar mechanisms underlied the effects of ET-1 and S1P by investigating changes in the phosphorylation status of Cx43 and in the molecular associations of Cx43 with zonula occludens (ZO) proteins and occludin. We showed that the inhibitory effect of ET-1 on GJIC was entirely dependent on the activation of G(i/o) but not on Rho and Rho-associated kinase. Both ET-1 and S1P induced dephosphorylation of Cx43 located at GJs through a process mediated by G(i/o) and calcineurin. Thanks to co-immunoprecipitation approaches, we found that a population of Cx43 (likely junctional Cx43) was associated to ZO-1-ZO-2-occludin multiprotein complexes and that acute treatments of astrocytes with ET-1 or S1P induced a G(i/o)-dependent increase in the amount of Cx43 linked to these complexes. As a whole, this study identifies a new mechanism of GJIC regulation in which two GPCR agonists dynamically alter interactions of Cx43 with its molecular partners.

Connexin43 phosphorylation: structural changes and biological effects

Vertebrate gap junctions, composed of proteins from the connexin gene family, play critical roles in embryonic development, co-ordinated contraction of excitable cells, tissue homoeostasis, normal cell growth and differentiation. Phosphorylation of connexin43, the most abundant and ubiquitously expressed connexin, has been implicated in the regulation of gap junctional communication at several stages of the connexin 'life cycle', including hemichannel oligomerization, export of the protein to the plasma membrane, hemichannel activity, gap junction assembly, gap junction channel gating and connexin degradation. Consistent with a short (1-5 h) protein half-life, connexin43 phosphorylation is dynamic and changes in response to activation of many different kinases. The present review assesses our current understanding of the effects of phosphorylation on connexin43 structure and function that in turn regulate gap junction biology, with an emphasis on events occurring in heart and skin.

RhoA GTPase and F-actin dynamically regulate the permeability of Cx43-made channels in rat cardiac myocytes

Gap junctions are clusters of transmembrane channels allowing a passive diffusion of ions and small molecules between adjacent cells. Connexin43, the main channel-forming protein expressed in ventricular myocytes, can associate with zonula occludens-1, a scaffolding protein linked to the actin cytoskeleton and to signal transduction molecules. The possible influence of Rho GTPases, major regulators of cellular junctions and of the actin cytoskeleton, in the modulation of gap junctional intercellular communication (GJIC) was examined. The activation of RhoA by cytoxic necrotizing factor 1 markedly enhanced GJIC, whereas its specific inhibition by the Clostridium botulinum C3 exoenzyme significantly reduced it. RhoA activity affects GJIC without major cellular redistribution of junctional plaques or changes in the Cx43 phosphorylation pattern. As these GTPases frequently act via the cortical cytoskeleton, the importance of F-actin in the modulation of GJIC was investigated by means of agents interfering with actin polymerization. Cytoskeleton stabilization by phalloidin slowed down the kinetics of channel rundown in the absence of ATP, whereas its disruption by cytochalasin D rapidly and markedly reduced GJIC despite ATP presence. Cytoskeleton stabilization by phalloidin markedly reduced the consequences of RhoA activation or inactivation. This mechanism appears to be the first described capable to both up- or down-regulate GJIC through RhoA activation or, conversely, inhibition. The inhibition of Rho downstream kinase effectors had no effect on GJIC. The present results provide further insight into the gating and regulation of junctional channels and identify a new downstream target for the small G-protein RhoA.

Genetic nanomedicine and tissue engineering

This article describes current investigative directions in nanotechnology-based medicine. It discusses cell sheet engineering, the molecular design of self-assembling peptide nanomaterials; the reconstitution of membrane protein systems on giant liposomes as artificial cell models, a biochemically engineered molecular communication system, and the use of split-reporter reconstitution analysis to image biomolecules in living cells and animals.

Galphao/i and Galphas signaling function in parallel with the MSP/Eph receptor to control meiotic diapause in C. elegans

BACKGROUND

A conserved biological feature of sexual reproduction in animals is that oocytes arrest in meiotic prophase and resume meiosis in response to extraovarian signals. In C. elegans, sperm trigger meiotic resumption by means of the major sperm protein (MSP) signal. MSP promotes meiotic resumption by functioning as an ephrin-signaling antagonist and by counteracting inhibitory inputs from the somatic gonadal sheath cells.

RESULTS

By using a genome-wide RNAi screen in a female-sterile genetic background, we identified 17 conserved genes that maintain meiotic arrest in the absence of the MSP signal. In vitro binding experiments show that MSP promotes oocyte mitogen-activated protein kinase activation and meiotic maturation in part through direct interaction with the VAB-1 Eph receptor. Four conserved proteins, including a disabled protein (DAB-1), a vav family GEF (VAV-1), a protein kinase C (PKC-1), and a STAM homolog (PQN-19), function with the VAB-1 Eph/MSP receptor in oocytes. We show that antagonistic Galphao/i and Galphas signaling pathways function in the soma to regulate meiotic maturation in parallel to the VAB-1 pathway. Galphas activity is necessary and sufficient to promote meiotic maturation, which it does in part by antagonizing inhibitory sheath/oocyte gap-junctional communication.

CONCLUSIONS

Our findings show that oocyte Eph receptor and somatic cell G protein signaling pathways control meiotic diapause in C. elegans, highlighting contrasts and parallels between MSP signaling in C. elegans and luteinizing hormone signaling in mammals.

Mini-dystrophin expression down-regulates overactivation of G protein-mediated IP3 signaling pathway in dystrophin-deficient muscle cells

We present here evidence for the enhancement of an inositol 1,4,5-trisphosphate (IP3) mediated calcium signaling pathway in myotubes from dystrophin-deficient cell lines (SolC1(−)) as compared to a cell line from the same origin but transfected with mini-dystrophin (SolD(+)). With confocal microscopy, we demonstrated that calcium rise, induced by the perifusion of a solution containing a high potassium concentration, was higher in SolC1(−) than in SolD(+) myotubes. The analysis of amplitude and kinetics of the calcium increase in SolC1(−) and in SolD(+) myotubes during the exposure with SR Ca²⁺ channel inhibitors (ryanodine and 2-APB) suggested the presence of two mechanisms of SR calcium release: (1) a fast SR calcium release that depended on ryanodine receptors and (2) a slow SR calcium release mediated by IP3 receptors. Detection analyses of mRNAs (reverse transcriptase [RT]-PCR) and proteins (Western blot and immunolocalization) demonstrated the presence of the three known isoforms of IP3 receptors in both SolC1(−) and SolD(+) myotubes. Furthermore, analysis of the kinetics of the rise in calcium revealed that the slow IP3-dependent release may be increased in the SolC1(−) as compared to the SolD(+), suggesting an inhibitory effect of mini-dystrophin in this signaling pathway. Upon incubation with pertussis toxin (PTX), an inhibitory effect similar to that of the IP3R inhibitor (2-APB) was observed on K⁺-evoked calcium release. This result suggests the involvement of a Gi protein upstream of the IP3 pathway in these stimulation conditions. A hypothetical model is depicted in which both Gi protein and IP3 production could be involved in K⁺-evoked calcium release as well as a possible interaction with mini-dystrophin. Our findings demonstrate the existence of a potential relationship between mini-dystrophin and SR calcium release as well as a regulatory role of mini-dystrophin on intracellular signaling.

Pharmacology of gap junctions. New pharmacological targets for treatment of arrhythmia, seizure and cancer?

Intercellular communication in many organs is maintained via intercellular gap junction channels composed of connexins, a large protein family with a number of isoforms. This gap junction intercellular communication (GJIC) allows the propagation of action potentials (e.g., in brain, heart), and the transfer of small molecules which may regulate cell growth, differentiation and function. The latter has been shown to be involved in cancer growth: reduced GJIC often is associated with increased tumor growth or with de-differentiation processes. Disturbances of GJIC in the heart can cause arrhythmia, while in brain electrical activity during seizures seems to be propagated via gap junction channels. Many diseases or pathophysiological conditions seem to be associated with alterations of gap junction protein expression. Thus, depending on the target disease opening or closure of gap junctions may be of interest, or alteration of connexin expression. GJIC can be affected acutely by changing gap junction conductance or--more chronic--by altering connexin expression and membrane localisation. This review gives an overview on drugs affecting GJIC.

Gap junction channel gating

Over the last two decades, the view of gap junction (GJ) channel gating has changed from one with GJs having a single transjunctional voltage-sensitive (V(j)-sensitive) gating mechanism to one with each hemichannel of a formed GJ channel, as well as unapposed hemichannels, containing two, molecularly distinct gating mechanisms. These mechanisms are termed fast gating and slow or 'loop' gating. It appears that the fast gating mechanism is solely sensitive to V(j) and induces fast gating transitions between the open state and a particular substate, termed the residual conductance state. The slow gating mechanism is also sensitive to V(j), but there is evidence that this gate may mediate gating by transmembrane voltage (V(m)), intracellular Ca(2+) and pH, chemical uncouplers and GJ channel opening during de novo channel formation. A distinguishing feature of the slow gate is that the gating transitions appear to be slow, consisting of a series of transient substates en route to opening and closing. Published reports suggest that both sensorial and gating elements of the fast gating mechanism are formed by transmembrane and cytoplamic components of connexins among which the N terminus is most essential and which determines gating polarity. We propose that the gating element of the slow gating mechanism is located closer to the central region of the channel pore and serves as a 'common' gate linked to several sensing elements that are responsive to different factors and located in different regions of the channel.

Ilimaquinone inhibits gap-junctional communication prior to Golgi fragmentation and block in protein transport

Brefeldin A and ilimaquinone are compounds known to affect Golgi structure and function. In particular, the transport of proteins is blocked either at the level of exit from endoplasmic reticulum (brefeldin) or at cis-Golgi (ilimaquinone). Brefeldin caused a slow decrease in gap-junctional communication and a slow loss of all phosphorylated forms of connexin43 in hamster and rat fibroblasts, while ilimaquinone caused an abrupt decrease in gap-junctional communication and rapid loss of only the slowest migrating phosphorylated connexin43 band (P2). Ilimaquinone caused these effects prior to any significant Golgi fragmentation, especially in hamster fibroblasts. Concurrently, ilimaquinone minimally affected protein secretion, while brefeldin caused an instantaneous decrease. These results show that ilimaquinone inhibits gap-junctional communication in connexin43-expressing cells by a mechanism not dependent on Golgi fragmentation or block in protein transport.

Gap junctions assemble in the presence of cytoskeletal inhibitors, but enhanced assembly requires microtubules

The role of cytoskeletal elements in gap junction (GJ) assembly has been studied using Novikoff hepatoma cells treated with cytochalasin B (CB) to disrupt actin filaments or with colchicine or nocodazole to disrupt microtubules. After 60 min of cell reaggregation, freeze-fracture was used to evaluate quantitatively the "initiation," "maturation," and "growth" phases of GJ assembly. The development of junctional permeability to fluorescent dyes was also analyzed. The only effects of CB on the structure or permeability of the developing junctions involved an elongation of GJ aggregates and a small decrease in formation plaque areas. Colchicine (but not the inactive form, lumicolchicine) prevented the enhancement of GJ growth by cholesterol, but its effect on basal growth was equivocal. Nocodazole inhibited the growth of GJ, even under basal conditions, without an effect on initiation. Nocodazole also blocked the forskolin-enhanced increase in the growth of GJs and, in living MDCK cells, reduced the movement of transport intermediates containing green fluorescent protein-tagged connexin43. Thus, neither actin filaments nor microtubules appear to restrict GJ assembly by anchoring intramembrane GJ proteins, nor are they absolutely required for functional GJs to form. However, microtubules are necessary for enhanced GJ growth and likely for facilitating connexin trafficking under basal conditions.