Modulation of gap-junction-dependent arterial relaxation by ascorbic acid

AIMS

To investigate whether ascorbic acid (AA) can influence endothelium-dependent relaxation by modulating the spread of endothelial hyperpolarization through the arterial wall via gap junctions.

METHODS

Force development and membrane potential were monitored by myography and sharp electrode techniques in isolated rabbit iliac arteries.

RESULTS

AA prevented the ability of the gap junction blocker 2-aminoethoxydiphenyl borate to inhibit endothelium-dependent relaxations and subintimal smooth muscle hyperpolarizations evoked by cyclopiazonic acid in the presence of nitric oxide (NO) synthase and cyclooxygenase blockade. AA also prevented the ability of a connexin-mimetic peptide targeted against Cx37 and Cx40 (37,40Gap 26) to attenuate the transmission of endothelial hyperpolarization to subintimal smooth muscle, and a peptide targeted against Cx43 (43Gap 26) to attenuate the spread of subintimal hyperpolarization to subadventitial smooth muscle and the associated mechanical relaxation. Parallel studies with endothelium-denuded preparations demonstrated that AA and cyclopiazonic acid both depressed relaxation evoked by the NO donor MAHMA NONOate.

CONCLUSIONS

The data suggest that AA can modulate arterial function through a previously unrecognized ability to preserve electrotonic signalling via myoendothelial and homocellular smooth muscle gap junctions under conditions where cell coupling is depressed. Underlying mechanisms do not involve amplification of 'residual' NO activity by AA.

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

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

Endothelin1-induced Ca(2+) mobilization is altered in calvarial osteoblastic cells of Cx43(+/- ) mice

During bone remodeling, osteoblastic (OB) cells have a central role leading to the production of extracellular matrix and its subsequent mineralization. As revealed by human physiopathologies, the OB differentiation process is essential for the control of calcium metabolism and normal bone formation. Moreover, accumulating data in the field of bone development suggest that connexin 43 (Cx43)-mediated gap junctional communication plays an important role in OB differentiation and function. Since Ca(2+) has a central role in OB physiology, the aim of the present study was to investigate the hypothetical involvement of Cx43 in OB calcium homeostasis. We performed measurements of intracellular calcium activity ([Ca(2+)]( i )) by a cytofluorimetric method using Fluo-4 as a calcium indicator and endothelin-1 (ET-1) as a physiological calcium-mobilizing factor on cultured OB cells isolated from calvaria of Cx43(+/-) and Cx43(+/+) mice. Partial deletion of the Cx43 gene induced a significant decrease in the [Ca(2+)]( i ) rise elicited by ET-1. This reduction was not correlated to a decrease or a modification of ET receptor subtype expression as assessed by real-time reverse-transcription polymerase chain reaction. Pharmacological investigations led us to demonstrate that the significant difference in [Ca(2+)]( i ) peak amplitude during the ET-1 action was associated with decreased calcium influx involving L-type voltage-sensitive calcium channels, whereas calcium release from intracellular stores and implication of phospholipase C were not affected by the reduced expression of Cx43. In conclusion, our data demonstrate for the first time that the Cx43 level of expression and/or function is able to modulate the [Ca(2+)]( i ) mobilization in OB cells.

Gene Transfer of Connexin43 Mutants Attenuates Coupling in Cardiomyocytes

Modification of electrical conduction would be a useful principle to recruit in preventing or treating certain arrhythmias, notably ventricular tachycardia (VT). Here we pursue a novel gene transfer approach to modulate electrical conduction by reducing gap junctional intercellular communication (GJIC) and hence potentially modify the arrhythmia substrate. The ultimate goal is to develop a nondestructive approach to uncouple zones of slow conduction by focal gene transfer. Lentiviral vectors encoding connexin43 (Cx43) internal loop mutants were produced and studied in vitro. Transduction of neonatal rat ventricular myocytes (NRVMs) revealed the expected subcellular localization of the mutant gene product. Fluorescent dye transfer studies showed a significant reduction of GJIC in NRVMs that had been genetically modified. Additionally, adjacent mutant gene-modified NRVMs displayed delayed calcium transients, indicative of electrical uncoupling. Multi-site optical mapping of action potential (AP) propagation in gene-modified NRVM monolayers revealed a 3-fold slowing of conduction velocity (CV) relative to nontransduced NRVMs. In conclusion, lentiviral vector–mediated gene transfer of Cx43 mutants reduced GJIC in NRVMs. Electrical charge transfer was also reduced as evidenced by delayed calcium transients in adjacent NRVMs and reduced CV in NRVM monolayers. These data validate a molecular tool that opens the prospect for gene transfer targeting gap junctions as an approach to modulate cardiac conduction.

Combined effects of reduced connexin 43, depressed active generator properties and energetic stress on conduction disturbances in canine failing myocardium

To show that reductions in connexin43 (Cx43) can contribute, in association with electrophysiological alterations identified from unipolar recordings, to conduction disturbances in a realistic model of heart failure, canines were subjected to chronic rapid pacing (240/min for 4 weeks) and progressive occlusion of the left coronary circumflex artery (LCx) by an ameroid constrictor. Alterations identified from 191 epicardial recordings included abrupt activation delay, functional block, ST segment potential elevation, and reduced maximum negative slope (-dV/dt (max)). The LCx territory was divided into apical areas with depressed conduction velocity (LCx1: 0.06 +/- 0.04 m/s, mean +/- SD) and basal areas with relatively preserved conduction (LCx2: 0.28 +/- 0.01 m/s). Subepicardial Cx43 immunoblot measurements (percent of corresponding healthy heart measurements) were reduced in LCx1 ( approximately 40%) and LCx2 ( approximately 60%). In addition, -dV/dt (max) was significantly depressed (-3.8 +/- 3.3 mV/ms) and ST segment potential elevated (23.3 +/- 14.6 mV) in LCx1 compared to LCx2 (-9.5 +/- 3.4 mV/ms and 0.3 +/- 1.4 mV). Anisotropic conduction, Cx43 and ST segment potential measurements from the left anterior descending coronary artery territory, and interstitial collagen from all regions were similar to the healthy. Thus, moderate Cx43 reduction to "clinically relevant" levels can, in conjunction with regional energetic stress and depression of sarcolemmal active generator properties, provide a substrate for conduction disturbances.

The unstoppable connexin43 carboxyl-terminus: new roles in gap junction organization and wound healing

Intercellular connectivity mediated by gap junctions (GJs) composed of connexin43 (Cx43) is critical to the function of excitable tissues such as the heart and brain. Disruptions to Cx43 GJ organization are thought to be a factor in cardiac arrhythmias and are also implicated in epilepsy. This article is based on a presentation to the 4th Larry and Horti Fairberg Workshop on Interactive and Integrative Cardiology and summarizes the work of Gourdie and his lab on Cx43 GJs in the heart. Background and perspective of recently published studies on the function of Cx43-interacting protein zonula occludens-(ZO)-1 in determining the organization of GJ plaques are provided. In addition how a peptide containing a PDZ-binding sequence of Cx43, developed as part of the work on cardiac GJ organization is also described, which has led to evidence for novel and unexpected roles for Cx43 in modulating healing following tissue injury.

Acute downregulation of connexin43 at wound sites leads to a reduced inflammatory response, enhanced keratinocyte proliferation and wound fibroblast migration

Experimental downregulation of connexin43 (Cx43) expression at skin wound sites appears to markedly improve the rate and quality of healing, but the underlying mechanisms are currently unknown. Here, we have compared physiological and cell biological aspects of the repair process with and without Cx43 antisense oligodeoxynucleotide treatment. Treated wounds exhibited accelerated skin healing with significantly increased keratinocyte and fibroblast proliferation and migration. In vitro knockdown of Cx43 in a fibroblast wound-healing model also resulted in significantly faster healing, associated with increased mRNA for TGF-beta1, and collagen alpha1 and general collagen content at the wound site. Treated wounds showed enhanced formation of granulation tissue and maturation with more rapid angiogenesis, myofibroblast differentiation and wound contraction appeared to be advanced by 2-3 days. Recruitment of both neutrophils and macrophages was markedly reduced within treated wounds, concomitant with reduced leukocyte infiltration. In turn, mRNA levels of CC chemokine ligand 2 and TNF-alpha were reduced in the treated wound. These data suggest that, by reducing Cx43 protein with Cx43-specific antisense oligodeoxynucleotides at wound sites early in the skin healing process repair is enhanced, at least in part, by accelerating cell migration and proliferation, and by attenuating inflammation and the additional damage it can cause.

Irsogladine maleate potentiates the effects of nitric oxide on activation of cAMP signalling pathways and suppression of mesangial cell mitogenesis

BACKGROUND AND PURPOSE

Deficiency in nitric oxide (NO) is a major factor leading to deterioration and progression of certain glomerular diseases. Agents enhancing NO availability and potentiality are renoprotective. Irsogladine maleate (IM), an anti-ulcer drug, is reported to improve gastric blood flow via NO-dependent mechanisms. We, therefore, asked whether and how IM interacted with NO on glomerular mesangial cells.

EXPERIMENTAL APPROACH

Mesangial cells were exposed to IM and NO donors. Activation of cAMP signalling pathways was assessed by intracellular cAMP, phosphorylation of VASP, activation of the cAMP response element (CRE) and expression of CRE-regulated proteins.

KEY RESULTS

IM alone did not affect cell proliferation. However, it greatly enhanced the growth-inhibitory effect of NO donor S-nitroso-N-acetylpenicillamine (SNAP). IM acted synergistically with NO on suppression of mitogen-activated protein kinase activation, induction of gap junction protein connexin43, increase of intracellular cAMP, and phosphorylation of VASP. With the use of the CRE-SEAP-based reporting system, IM and SNAP cooperatively activated cAMP response elements (CRE). A similar activation of cAMP was induced by IM with two different NO donors, the sGC activator Bay 41-2272 and the cGMP analogue 8-bromo-cGMP. The effects of SNAP and IM on cAMP activation were mimicked by phosphodiesterase 3 (PDE3) and PDE4 inhibitors. In addition, IM markedly augmented cytokine-induced expression of iNOS, production of NO and activation of CRE.

CONCLUSION AND IMPLICATIONS

The effects of NO were greatly potentiated by IM through synergistic activation of cAMP pathway. Combined therapy with IM and NO may be developed for certain renal diseases.

Propagation of the cardiac impulse in the diabetic rat heart: reduced conduction reserve

Diabetes mellitus is a growing epidemic with severe cardiovascular complications. Although much is known about mechanical and electrical cardiac dysfunction in diabetes, few studies have investigated propagation of the electrical signal in the diabetic heart and the associated changes in intercellular gap junctions. This study was designed to investigate these issues, using hearts from control and diabetic rats. Diabetic conditions were induced by streptozotocin (STZ), given i.v. 7-14 days before experiments. Optical mapping with the voltage-sensitive dye di-4-ANEPPS, using hearts perfused on a Langendorff apparatus, showed little change in baseline conduction velocity in diabetic hearts, reflecting the large reserve of function. However, both the gap junction uncoupler heptanol (0.5-1 mM) and elevated potassium (9 mM, to reduce cell excitability) produced a significantly greater slowing of impulse propagation in diabetic hearts than in controls. The maximal action potential upstroke velocity (an index of the sodium current) and resting potential was similar in single ventricular myocytes from control and diabetic rats, suggesting similar electrical excitability. Immunoblotting of connexin 43 (Cx43), a major gap junction component, showed no change in total expression. However, immunofluorescence labelling of Cx43 showed a significant redistribution, apparent as enhanced Cx43 lateralization. This was quantified and found to be significantly larger than in control myocytes. Labelling of two other gap junction proteins, N-cadherin and beta-catenin, showed a (partial) loss of co-localization with Cx43, indicating that enhancement of lateralized Cx43 is associated with non-functional gap junctions. In conclusion, conduction reserve is smaller in the diabetic heart, priming it for impaired conduction upon further challenges. This can desynchronize contraction and contribute to arrhythmogenesis.

Primary cultures of chick osteocytes retain functional gap junctions between osteocytes and between osteocytes and osteoblasts

The inaccessibility of osteocytes due to their embedment in the calcified bone matrix in vivo has precluded direct demonstration that osteocytes use gap junctions as a means of intercellular communication. In this article, we report successfully isolating primary cultures of osteocytes from chick calvaria, and, using anti-connexin 43 immunocytochemistry, demonstrate gap junction distribution to be comparable to that found in vivo. Next, we demonstrate the functionality of the gap junctions by (1) dye coupling studies that showed the spread of microinjected Lucifer Yellow from osteoblast to osteocyte and between adjacent osteocytes and (2) analysis of fluorescence replacement after photobleaching (FRAP), in which photobleaching of cells loaded with a membrane-permeable dye resulted in rapid recovery of fluorescence into the photobleached osteocyte, within 5 min postbleaching. This FRAP effect did not occur when cells were treated with a gap junction blocker (18alpha-glycyrrhetinic acid), but replacement of fluorescence into the photobleached cell resumed when it was removed. These studies demonstrate that gap junctions are responsible for intercellular communication between adjacent osteocytes and between osteoblasts and osteocytes. This role is consistent with the ability of osteocytes to respond to and transmit signals over long distances while embedded in a calcified matrix.

Myoblast proliferation and syncytial fusion both depend on connexin43 function in transfected skeletal muscle primary cultures

Muscles are formed by fusion of individual postmitotic myoblasts to form multinucleated syncytial myotubes. The process requires a well-coordinated transition from proliferation, through migratory alignment and cycle exit, to breakdown of apposed membranes. Connexin43 protein and cell-cycle inhibitor levels are correlated, and gap junction blockers can delay muscle regeneration, so a coordinating role for gap junctions has been proposed. Here, wild-type and dominant-negative connexin43 variants (wtCx43, dnCx43) were introduced into rat myoblasts in primary culture through pIRES-eGFP constructs that made transfected cells fluoresce. GFP-positive cells and vitally-stained nuclei were counted on successive days to reveal differences in proliferation, and myotubes were counted to reveal differences in fusion. Individual transfected cells were injected with Cascade Blue, which permeates gap junctions, mixed with FITC-dextran, which requires cytoplasmic continuity to enter neighbouring cells. Myoblasts transfected with wtCx43 showed more gap-junctional coupling than GFP-only controls, began fusion sooner as judged by the incidence of cytoplasmic coupling, and formed more myotubes. Myoblasts transfected with dnCx43 remained proliferative for longer than either GFP-only or wtCx43 myoblasts, showed less coupling, and underwent little fusion into myotubes. These results highlight the critical role of gap-junctional coupling in myotube formation.

Gating properties of heterotypic gap junction channels formed of connexins 40, 43, and 45

Connexins (Cxs) 40, 43, and 45 are expressed in many different tissues, but most abundantly in the heart, blood vessels, and the nervous system. We examined formation and gating properties of heterotypic gap junction (GJ) channels assembled between cells expressing wild-type Cx40, Cx43, or Cx45 and their fusion forms tagged with color variants of green fluorescent protein. We show that these Cxs, with exception of Cxs 40 and 43, are compatible to form functional heterotypic GJ channels. Cx40 and Cx43 hemichannels are unable or effectively impaired in their ability to dock and/or assemble into junctional plaques. When cells expressing Cx45 contacted those expressing Cx40 or Cx43 they readily formed junctional plaques with cell-cell coupling characterized by asymmetric junctional conductance dependence on transjunctional voltage, V(j). Cx40/Cx45 heterotypic GJ channels preferentially exhibit V(j)-dependent gating transitions between open and residual states with a conductance of approximately 42 pS; transitions between fully open and closed states with conductance of approximately 52 pS in magnitude occur at substantially lower ( approximately 10-fold) frequency. Cx40/Cx45 junctions demonstrate electrical signal transfer asymmetry that can be modulated between unidirectional and bidirectional by small changes in the difference between holding potentials of the coupled cells. Furthermore, both fast and slow gating mechanisms of Cx40 exhibit a negative gating polarity.

[The role of connexin 43 gene in acupuncture analgesia]

OBJECTIVE

To investigate the possible relationship between the analgesic effect of acupuncture and connexin 43.

METHODS

Connexin 43 gene knock-out mice were divided into 4 groups: a wide type (WT) control group, a WT acupuncture group, a heterozygous (HT) control group and HT acupuncture group. Hot-plate test and writhing response induced by acetic acid were used for investigating the analgesic effect of acupuncture.

RESULTS

There was no significant difference in the basic pain threshold value between HT and WT mice (P > 0.05). Acupuncture could significantly increase the pain threshold value, prolong the latency period of writhing body and decrease the number of writhing body as compared with pre-acupuncture in WT and HT mice (P < 0.01 or P < 0.05). The pain threshold, latency period of writhing and number of writhing body in HT mice were less than WT mice post-acupuncture (P < 0.05).

CONCLUSION

Connexin 43 gene knock-out might partially inhibit the analgesic effect of acupuncture, suggesting that connexin 43 is possibly related with meridians and the effect of acupuncture.

The function of connexin 43 on the differentiation of rat bone marrow cells in culture

Connexin (Cx) 43-mediated gap-junctional intercellular communication (GJC) mainly regulates the osteoblastic differentiation, but much of the function of Cx43 on the differentiation of bone marrow cells is unclear. This study is aimed to clarify relationship between the differentiation of rat bone marrow cells and the function of Cx43. Bone marrow cells derived from four-week-old Wistar strain rats were grown in the presence and absence of 18-alpha-glycyrrhetinic acid (AGA, 100 muM) to inhibit Cx43-mediated GJC. Expression of Cx43 gene and protein, and the level of intracellular cyclic adenosine monophosphate (cAMP) were determined as the assessment of the function in Cx43-mediated GJC, and alkaline phosphatase (ALP) activity and mineralization were measured as the assessment of osteoblastic differentiation. The Cx43 gene expression was first observed at 2 days, but under the condition in which rat bone marrow cells were treated with AGA, there was no significant effect on the Cx43 gene expression. By administrating AGA to rat bone marrow cells, all parameters of maturation but the Cx43 gene expression significantly decreased. The results of this experiment suggest that Cx43-mediated GJC plays a critical role in rat bone marrow cells, progress toward maturation.

Many faces of drebrin: from building dendritic spines and stabilizing gap junctions to shaping neurite-like cell processes

In this review we consider the multiple functions of developmentally regulated brain protein (drebrin), an actin-binding protein, in the formation of cellular polarity in different cell types. Drebrin has a well-established role in the morphogenesis, patterning and maintenance of dendritic spines in neurons. We have recently shown that drebrin also stabilizes Connexin-43 containing gap junctions at the plasma membrane. The latest literature and our own data suggest that drebrin may be broadly involved in shaping cell processes and in the formation of stabilized plasma membrane domains, an effect that is likely to be of crucial significance for formation of cell polarity in both neuronal and non-neuronal types.

Molecular dynamics and in vitro analysis of Connexin43: A new 14-3-3 mode-1 interacting protein

The interaction of cellular proteins with the gap junction protein Connexin43 (Cx43) is thought to form a dynamic scaffolding complex that functions as a platform for the assembly of signaling, structural, and cytoskeletal proteins. A high stringency Scansite search of rat Cx43 identified the motif containing Ser373 (S373) as a 14-3-3 binding site. The S373 motif and the second best mode-1 motif, containing Ser244 (S244), are conserved in rat, mouse, human, chicken, and bovine, but not in Xenopus or zebrafish Cx43. Docking studies of a mouse/rat 14-3-3 homology model with the modeled phosphorylated S373 or S244 peptide ligands or their serine-to-alanine mutants, S373A or S244A, revealed that the pS373 motif facilitated a greater number of intermolecular contacts than the pS244 motif, thus supporting a stronger 14-3-3 binding interaction with the pS373 motif. The alanine substitution also reduced more than half the number of intermolecular contacts between 14-3-3 and the S373 motif, emphasizing the phosphorylation dependence of this interaction. Furthermore, the ability of the wild-type or the S244A GST-Cx43 C-terminal fusion protein, but not the S373A fusion protein, to interact with either 14-3-3 or 14-3-3zeta in GST pull-down experiments clearly demonstrated that the S373 motif mediates the direct interaction between Cx43 and 14-3-3 proteins. Blocking growth factor-induced Akt activation and presumably any Akt-mediated phosphorylation of the S373 motif in ROSE 199 cells did not prevent the down-regulation of Cx43-mediated cell-cell communication, suggesting that an Akt-mediated interaction with 14-3-3 was not involved in the disruption of Cx43 function.

Connexins act as tumor suppressors in three-dimensional mammary cell organoids by regulating differentiation and angiogenesis

Connexins are tumor suppressors, and human breast connexin 26 (Cx26) and connexin 43 (Cx43) gap junctions are often down-regulated in breast cancer. We previously showed that Cx26 and Cx43 overexpressed in MDA-MB-231 breast cancer cells inhibited tumor growth in vivo but not in two-dimensional cultures. In the current study, we show that overexpression of Cx26 or Cx43 has tumor-suppressive properties in a three-dimensional environment such that they reduced anchorage-independent cell growth and induced partial redifferentiation of three-dimensional organoids of MDA-MB-231 cells. Importantly, the majority of exogenous connexins did not localize to the cell-cell interface or rescue gap junctional intercellular communication (GJIC) as assessed by dye transfer, providing evidence of a GJIC-independent mechanism of mammary tumor suppression. To further elucidate the mechanisms involved in connexin-induced three-dimensional redifferentiation of tumor cells, we examined whether connexin expression has a role in epithelial to mesenchymal transition (EMT). Cx26 and Cx43 reduced cell migration, increased cytokeratin 18 expression, and decreased vimentin levels, indicating a shift from a mesenchymal towards an epithelial phenotype. In addition, we examined the role of connexins in angiogenesis by probing an angiogenesis antibody array with conditioned media from three-dimensional MDA-MB-231 cultures. This revealed that connexin overexpression regulated various angiogenesis-linked proteins. Furthermore, secreted factors from connexin overexpressing cells inhibited endothelial cell tubulogenesis and migration, and xenografts of Cx43 overexpressing MDA-MB-231 cells showed reduced tumor angiogenesis. In summary, Cx26 and Cx43 inhibit the malignant properties of MDA-MB-231 cells via GJIC-independent mechanisms, including regulation of EMT and angiogenesis.

Relative contributions of connexins 40 and 43 to atrial impulse propagation in synthetic strands of neonatal and fetal murine cardiomyocytes

Atrial tissue expresses both connexin 40 (Cx40) and 43 (Cx43) proteins. To assess the relative roles of Cx40 and Cx43 in atrial electrical propagation, we synthesized cultured strands of atrial myocytes derived from mice with genetic deficiency in Cx40 or Cx43 expression and measured propagation velocity (PV) by high-resolution optical mapping of voltage-sensitive dye fluorescence. The amount of Cx40 and/or Cx43 in gap junctions was measured by immunohistochemistry and total or sarcolemmal Cx43 or Cx40 protein by immunoblotting. Progressive genetic reduction in Cx43 expression decreased PV from 34+/-6 cm/sec in Cx43(+/+) to 30+/-8 cm/sec in Cx43(+/-) and 19+/-11 cm/sec in Cx43(-/-) cultures. Concomitantly, the cell area occupied by Cx40 immunosignal in gap junctions decreased from 2.0+/-1.6% in Cx43(+/+) to 1.7+/-0.5% in Cx43(+/-) and 1.0+/-0.2% in Cx43(-/-) strands. In contrast, progressive genetic reduction in Cx40 expression increased PV from 30+/-2 cm/sec in Cx40(+/+) to 40+/-7 cm/sec in Cx40(+/-) and 45+/-10 cm/sec in Cx40(-/-) cultures. Concomitantly, the cell area occupied by Cx43 immunosignal in gap junctions increased from 1.2+/-0.9% in Cx40(+/+) to 2.8+/-1.4% in Cx40(+/-) and 3.1+/-0.6% in Cx40(-/-) cultures. In accordance with the immunostaining results, immunoblots of the Triton X-100-insoluble fraction revealed an increase of Cx43 in gap junctions in extracts from Cx40-ablated atria, whereas total cellular Cx43 remained unchanged. Our results suggest that the relative abundance of Cx43 and Cx40 is an important determinant of atrial impulse propagation in neonatal hearts, whereby dominance of Cx40 decreases and dominance of Cx43 increases local propagation velocity.

Direct measurement of cyclic AMP diffusion and signaling through connexin43 gap junctional channels

Gap junctions (GJ) are clusters of transmembrane channels that allow direct cell-to-cell transfer of ions and small molecules. The GJ-permeant signaling molecule cAMP is of particular interest because of its numerous cellular effects. However, to assess the biological relevance of GJ-mediated cAMP transfer, quantitative aspects must be determined. Here we employed cAMP indicators based on fluorescence resonance energy transfer (FRET) to study propagation of cAMP signals to neighbor cells through connexin43 (Cx43)-based gap junctions in Rat-1 cells quantitatively. Intracellular cAMP levels were selectively raised in single cells by either photorelease of caged cAMP or stimulation of G(s)-coupled receptors. cAMP elevations spread to adjacent cells within seconds in a Cx43-dependent manner. We determined that Rat-1 cells follow cAMP rises in surrounding monolayer cells to approx. 40% in amplitude. This degree of cAMP transfer sufficed to evoke a well-characterized response to cAMP in neighbor cells, i.e. the PKA-mediated phosphorylation of the ER transcription factor in A431 carcinoma cells. We conclude that contacting cells can cooperatively regulate cAMP-sensitive processes via gap junctional diffusion of cAMP.

Connexin 32 and 43 gap junctions differentially modulate tenocyte response to cyclic mechanical load

Gap junctions allow rapid exchange of ions and small metabolites between cells. They can occur between connective tissue cells, and in tendons there are two prominent types, composed of connexin 32 or 43. These form distinct networks - tenocyte rows are linked by both longitudinally, but only by connexin 43 laterally. We hypothesised that the junctions had different roles in cell response to mechanical loading, and measured the effects of inhibitors of gap junction function on secretion of collagen by tenocyte cultures exposed to mechanical strain. Chicken tendon fibroblasts were exposed to cyclic tensile loading in the presence or absence of general gap junction inhibitors (halothane or the biomimetic peptide gap27), or antisense oligonucleotides to chicken connexin 32 or 43. Untreated cultures increased collagen secretion by around 25% under load. Halothane eliminated this response but caused cell damage. Gap27 peptide reduced secretion but maintained loading effects - strained cultures secreting more collagen than unstrained. Antisense downregulation showed major differences between connexins: antisense 32 reduced, and antisense 43 increased, collagen secretion. In both cases loading effects were maintained. This shows that (i) gap junctional integration of signals is important in load response of tenocyte populations - mechanotransduction occurs in individual cells but integration of signals markedly enhances it and (ii) communication via connexin 32 and 43 have differential effects on the load response, with connexin 32 being stimulatory and connexin 43 being inhibitory. Cells coordinate and control their response to mechanical signals at least in part by differential actions of these two types of gap junction.

Connexin43 is required for production of the aqueous humor in the murine eye

Connexin43 is a major component of the gap junctions between pigmented and non-pigmented cells of the double-layered epithelium in the ciliary body of the eye. We directly tested the hypothesis that gap junctions play a crucial role in the production of the aqueous humor by inactivating the GJA1 (connexin43) gene in the pigmented epithelium with cre-loxP technology. To accomplish this, we crossed a line expressing cre recombinase driven by the nestin promoter and a line with floxed connexin43 alleles. Resultant lines exhibited loss of connexin43 from the pigmented epithelium, iris, retinal pigment epithelium and the lens. We observed plasma proteins in the aqueous humor and pathological changes consistent with a loss of intraocular pressure. As the ciliary body is responsible for aqueous humor production, these data support the hypothesis that the gap junctions between pigmented and non-pigmented epithelium are necessary for production of the aqueous humor that is in turn required for the generation of normal intraocular pressure and nourishment of the postnatal lens. The loss of connexin43 expression in the iris correlated with a separation of the posterior pigmented epithelium from the anterior myoepithelium and with meiosis, possibly resulting from a loss of function of the dilator pupillae.

ATP release from activated neutrophils occurs via connexin 43 and modulates adenosine-dependent endothelial cell function

Extracellular ATP liberated during hypoxia and inflammation can either signal directly on purinergic receptors or can activate adenosine receptors following phosphohydrolysis to adenosine. Given the association of polymorphonuclear leukocytes (PMNs) with adenine-nucleotide/nucleoside signaling in the inflammatory milieu, we hypothesized that PMNs are a source of extracellular ATP. Initial studies using high-performance liquid chromatography and luminometric ATP detection assays revealed that PMNs release ATP through activation-dependent pathways. In vitro models of endothelial barrier function and neutrophil/endothelial adhesion indicated that PMN-derived ATP signals through endothelial adenosine receptors, thereby promoting endothelial barrier function and attenuating PMN/endothelial adhesion. Metabolism of extracellular ATP to adenosine required PMNs, and studies addressing these metabolic steps revealed that PMN express surface ecto-apyrase (CD39). In fact, studies with PMNs derived from cd39(-/-) mice showed significantly increased levels of extracellular ATP and lack of ATP dissipation from their supernatants. After excluding lytic ATP release, we used pharmacological strategies to reveal a potential mechanism involved in PMN-dependent ATP release (eg, verapamil, dipyridamole, brefeldin A, 18-alpha-glycyrrhetinic acid, connexin-mimetic peptides). These studies showed that PMN ATP release occurs through connexin 43 (Cx43) hemichannels in a protein/phosphatase-A-dependent manner. Findings in human PMNs were confirmed in PMNs derived from induced Cx43(-/-) mice, whereby activated PMNs release less than 15% of ATP relative to littermate controls, whereas Cx43 heterozygote PMNs were intermediate in their capacity for ATP release (P<0.01). Taken together, our results identify a previously unappreciated role for Cx43 in activated PMN ATP release, therein contributing to the innate metabolic control of the inflammatory milieu.

Low peak bone mass and attenuated anabolic response to parathyroid hormone in mice with an osteoblast-specific deletion of connexin43

Connexin43 (Cx43) is involved in bone development, but its role in adult bone homeostasis remains unknown. To overcome the postnatal lethality of Cx43 null mutation, we generated mice with selective osteoblast ablation of Cx43, obtained using a Cx43fl allele and a 2.3-kb fragment of the alpha1(I) collagen promoter to drive Cre in osteoblasts (ColCre). Conditionally osteoblast-deleted ColCre;Cx43-/fl mice show no malformations at birth, but develop low peak bone mass and remain osteopenic with age, exhibiting reduced bone formation and defective osteoblast function. By both radiodensitometry and histology, bone mineral content increased rapidly and progressively in adult Cx43+/fl mice after subcutaneous injection of parathyroid hormone (PTH), an effect significantly attenuated in ColCre;Cx43-/fl mice, with Cx43-/fl exhibiting an intermediate response. Attenuation of PTH anabolic action was associated with failure to increase mineral apposition rate in response to PTH in ColCre;Cx43-/fl, despite an increased osteoblast number, suggesting a functional defect in Cx43-deficient bone-forming cells. In conclusion, lack of Cx43 in osteoblasts leads to suboptimal acquisition of peak bone mass, and hinders the bone anabolic effect of PTH. Cx43 represents a potential target for modulation of bone anabolism.

Connexin 43-mediated modulation of polarized cell movement and the directional migration of cardiac neural crest cells

Connexin 43 knockout (Cx43α1KO) mice have conotruncal heart defects that are associated with a reduction in the abundance of cardiac neural crest cells (CNCs) targeted to the heart. In this study, we show CNCs can respond to changing fibronectin matrix density by adjusting their migratory behavior,with directionality increasing and speed decreasing with increasing fibronectin density. However, compared with wild-type CNCs, Cx43α1KO CNCs show reduced directionality and speed, while CNCs overexpressing Cx43α1 from the CMV43 transgenic mice show increased directionality and speed. Altered integrin signaling was indicated by changes in the distribution of vinculin containing focal contacts, and altered temporal response of Cx43α1KO and CMV43 CNCs to β1 integrin function blocking antibody treatment. High resolution motion analysis showed Cx43α1KO CNCs have increased cell protrusive activity accompanied by the loss of polarized cell movement. They exhibited an unusual polygonal arrangement of actin stress fibers that indicated a profound change in cytoskeletal organization. Semaphorin 3A, a chemorepellent known to inhibit integrin activation, was found to inhibit CNC motility, but in the Cx43α1KO and CMV43 CNCs, cell processes failed to retract with semaphorin 3A treatment. Immunohistochemical and biochemical analyses suggested close interactions between Cx43α1,vinculin and other actin-binding proteins. However, dye coupling analysis showed no correlation between gap junction communication level and fibronectin plating density. Overall, these findings indicate Cx43α1 may have a novel function in mediating crosstalk with cell signaling pathways that regulate polarized cell movement essential for the directional migration of CNCs.

Role of the N-terminus in permeability of chicken connexin45.6 gap junctional channels

Previous studies have shown that gap junctional channels formed from the lens connexins Cx50 (or its chicken orthologue, Cx45.6) and Cx43 exhibit marked differences in transjunctional voltage gating and unitary conductance. In the present study, we used the negatively charged dye, Lucifer Yellow (LY), to examine and compare quantitative differences in dye transfer between pairs of HeLa cells stably transfected with Cx45.6 or Cx43. Our results show that Cx45.6 gap junctional channels are three times less permeable to LY than Cx43 channels. Replacement of the N-terminus of Cx45.6 with the corresponding domain of Cx43 increased LY permeability, reduced the transjunctional voltage (V(j)) gating sensitivity, and reduced the unitary conductance of Cx45.6-43N gap junctional channels. Further experiments, using a series of Alexa probes that had similar net charge but varied in size showed that the Cx45.6-43N had a significantly higher permeability for the two largest Alexa dyes than Cx45.6. These data suggest that the N-terminus plays a critical role in determining many of biophysical properties of Cx45.6 gap junctional channels, including molecular permeability and voltage gating.