Gap junction connexon configuration in rapidly frozen myocardium and isolated intercalated disks

Mechanisms of Connexin Regulating Peptides

Gap junctions (GJ) and connexins play integral roles in cellular physiology and have been found to be involved in multiple pathophysiological states from cancer to cardiovascular disease. Studies over the last 60 years have demonstrated the utility of altering GJ signaling pathways in experimental models, which has led to them being attractive targets for therapeutic intervention. A number of different mechanisms have been proposed to regulate GJ signaling, including channel blocking, enhancing channel open state, and disrupting protein-protein interactions. The primary mechanism for this has been through the design of numerous peptides as therapeutics, that are either currently in early development or are in various stages of clinical trials. Despite over 25 years of research into connexin targeting peptides, the overall mechanisms of action are still poorly understood. In this overview, we discuss published connexin targeting peptides, their reported mechanisms of action, and the potential for these molecules in the treatment of disease.

Potassium channels in the Cx43 gap junction perinexus modulate ephaptic coupling: an experimental and modeling study

It was recently demonstrated that cardiac sodium channels (Nav1.5) localized at the perinexus, an intercalated disc (ID) nanodomain associated with gap junctions (GJ), may contribute to electrical coupling between cardiac myocytes via an ephaptic mechanism. Impairment of ephaptic coupling by acute interstitial edema (AIE)-induced swelling of the perinexus was associated with arrhythmogenic, anisotropic conduction slowing. Given that Kir2.1 has also recently been reported to localize at intercalated discs, we hypothesized that Kir2.1 channels may reside within the perinexus and that inhibiting them may mitigate arrhythmogenic conduction slowing observed during AIE. Using gated stimulated emission depletion (gSTED) and stochastic optical reconstruction microscopy (STORM) super-resolution microscopy, we indeed find that a significant proportion of Kir2.1 channels resides within the perinexus. Moreover, whereas Nav1.5 inhibition during AIE exacerbated arrhythmogenic conduction slowing, inhibiting Kir2.1 channels during AIE preferentially increased transverse conduction velocity-decreasing anisotropy and ameliorating arrhythmia risk compared to AIE alone. Comparison of our results with a nanodomain computer model identified enrichment of both Nav1.5 and Kir2.1 at intercalated discs as key factors underlying the experimental observations. We demonstrate that Kir2.1 channels are localized within the perinexus alongside Nav1.5 channels. Further, targeting Kir2.1 modulates intercellular coupling between cardiac myocytes, anisotropy of conduction, and arrhythmia propensity in a manner consistent with a role for ephaptic coupling in cardiac conduction. For over half a century, electrical excitation in the heart has been thought to occur exclusively via gap junction-mediated ionic current flow between cells. Further, excitation was thought to depend almost exclusively on sodium channels with potassium channels being involved mainly in returning the cell to rest. Here, we demonstrate that sodium and potassium channels co-reside within nanoscale domains at cell-to-cell contact sites. Experimental and computer modeling results suggest a role for these channels in electrical coupling between cardiac muscle cells via an ephaptic mechanism working in tandem with gap junctions. This new insight into the mechanism of cardiac electrical excitation could pave the way for novel therapies against cardiac rhythm disturbances.

Sodium channels in the Cx43 gap junction perinexus may constitute a cardiac ephapse: an experimental and modeling study

It has long been held that electrical excitation spreads from cell-to-cell in the heart via low resistance gap junctions (GJ). However, it has also been proposed that myocytes could interact by non-GJ-mediated “ephaptic” mechanisms, facilitating propagation of action potentials in tandem with direct GJ-mediated coupling. We sought evidence that such mechanisms contribute to cardiac conduction. Using super-resolution microscopy, we demonstrate that Na_v1.5 is localized within 200 nm of the GJ plaque (a region termed the perinexus). Electron microscopy revealed close apposition of adjacent cell membranes within perinexi suggesting that perinexal sodium channels could function as an ephapse, enabling ephaptic cell-to-cell transfer of electrical excitation. Acute interstitial edema (AIE) increased intermembrane distance at the perinexus and was associated with preferential transverse conduction slowing and increased spontaneous arrhythmia incidence. Inhibiting sodium channels with 0.5 μM flecainide uniformly slowed conduction, but sodium channel inhibition during AIE slowed conduction anisotropically and increased arrhythmia incidence more than AIE alone. Sodium channel inhibition during GJ uncoupling with 25 μM carbenoxolone slowed conduction anisotropically and was also highly proarrhythmic. A computational model of discretized extracellular microdomains (including ephaptic coupling) revealed that conduction trends associated with altered perinexal width, sodium channel conductance, and GJ coupling can be predicted when sodium channel density in the intercalated disk is relatively high. We provide evidence that cardiac conduction depends on a mathematically predicted ephaptic mode of coupling as well as GJ coupling. These data suggest opportunities for novel anti-arrhythmic therapies targeting noncanonical conduction pathways in the heart.

The connexin43 carboxyl terminus and cardiac gap junction organization

The precise spatial order of gap junctions at intercalated disks in adult ventricular myocardium is thought vital for maintaining cardiac synchrony. Breakdown or remodeling of this order is a hallmark of arrhythmic disease of the heart. The principal component of gap junction channels between ventricular cardiomyocytes is connexin43 (Cx43). Protein-protein interactions and modifications of the carboxyl-terminus of Cx43 are key determinants of gap junction function, size, distribution and organization during normal development and in disease processes. Here, we review data on the role of proteins interacting with the Cx43 carboxyl-terminus in the regulation of cardiac gap junction organization, with particular emphasis on Zonula Occludens-1. The rapid progress in this area suggests that in coming years we are likely to develop a fuller understanding of the molecular mechanisms causing pathologic remodeling of gap junctions. With these advances come the promise of novel approach to the treatment of arrhythmia and the prevention of sudden cardiac death. This article is part of a Special Issue entitled: The Communicating junctions, composition, structure and characteristics.

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.

The cardiac muscle cell

The cardiac myocyte is the most physically energetic cell in the body, contracting constantly, without tiring, 3 billion times or more in an average human lifespan. By coordinating its beating activity with that of its 3 billion neighbours in the main pump of the human heart, over 7,000 litres of blood are pumped per day, without conscious effort, along 100,000 miles of blood vessels. A detailed picture of the membrane organisation of the cardiac muscle cell underpins our understanding of how the electrical impulse, generated within the heart, stimulates coordinated contraction of the cardiac chambers. This article highlights, with the aid of modern cellular imaging methods, key components of the membrane machinery responsible for coupling electrical excitation and contraction in the cardiomyocyte, focusing on plasma membrane/sarcoplasmic reticulum and plasma membrane/plasma membrane junctions. BioEssays 22:188-199, 2000.

Dispersed and aggregated gap junction channels identified by immunogold labeling of freeze-fractured membranes

An indirect immunogold labeling technique was applied to replicas of freeze-fractured membranes of rapidly frozen unfixed cells. The endogenous gap junction protein Cx43 of BICR/M1Rk rat mammary tumor cells was preferentially identified in quasi-crystalline gap junction plaques as were the transfected connexins Cx40, Cx43, and Cx45 in HeLa (human cervical carcinoma) cells. With this method we also detected contact areas with dispersed gap junction channels which are the only structural correlation for endogenous Cx45 in HeLa wild-type cells where no gap junction plaques exist. In double-transfected HeLa cells a colocalization of Cx40 and Cx43 was occasionally detected in quasi-crystalline gap junction plaques, whereas in contact areas with dispersed particles only one Cx type was present. Our results indicate that functional gap junction channels exist outside the quasi-crystalline plaques.

Disturbed connexin43 gap junction distribution correlates with the location of reentrant circuits in the epicardial border zone of healing canine infarcts that cause ventricular tachycardia

BACKGROUND

Slow, nonuniform conduction caused by abnormal gap-junctional coupling of infarct-related myocardium is thought to be a component of the arrhythmogenic substrate. The hypothesis that changes in gap-junctional distribution in the epicardial border zone (EBZ) of healing canine infarcts define the locations of reentrant ventricular tachycardia (VT) circuits was tested by correlating activation maps of the surviving subepicardial myocardial layer with immunolocalization of the principal gap-junctional protein, connexin43 (Cx43).

METHODS AND RESULTS

The EBZ overlying 4-day-old anterior infarcts in three dogs with inducible VT and three noninducible dogs was mapped with a high-resolution electrode array and systematically examined by standard histology and confocal immunolocalization of Cx43. The thickness of the EBZ was significantly less in the hearts with (538 +/- 257 microns) than without (840 +/- 132 microns; P < .05) VT. At the interface with the underlying necrotic cells, the EBZ myocardium showed a marked disruption of gap-junctional distribution, with Cx43 labeling abnormally arrayed longitudinally along the lateral surfaces of the cells. In the EBZ of all hearts, the disrupted Cx43 labeling extended part of the way to the epicardial surface, with the most superficial epicardial myocytes having the normal transversely orientated pattern. Only in the hearts with inducible VT did the disorganization extend through the full thickness of the surviving layer at sites correlating with the location of the central common pathways of the figure-of-8 reentrant VT circuits.

CONCLUSIONS

Altered gap-junctional distribution is part of the early remodeling of myocardium after infarction, and by defining the location of the common central pathway of the reentrant VT circuits, it may be a determinant of VT susceptibility.

Gap junctions in the adult cerebral cortex: regional differences in their distribution and cellular expression of connexins

Gap junctions are membrane channels that mediate electrical and metabolic coupling between adjacent cells. Immunocytochemical analysis by using a panel of anti-connexin antibodies, as well as electron microscopy of thin sections and freeze-fracture replicas, has shown that gap junctions and their constituent proteins are abundant in the cerebral cortex of the adult rat. Their frequency and distribution vary in different cortical regions, which may reflect differences in the cellular and functional organization of these areas of the cortex. Gap junctions were identified between glial cells and, less frequently, between neuronal elements. Heterologous junctions were also identified between astrocytes and oligodendrocytes and between neurons and glia; the latter category included abundant junctions between astrocytic processes and neurons. Double-antibody labelling experiments in tissue sections and in acutely dissociated cells showed that connexin 32 was expressed in neurons and oligodendrocytes, whereas connexin 43, widely believed to be expressed only in astrocytes, was also localized in a population of cortical neurons. These results show that gap junctions can provide a major nonsynaptic means of communication between cortical cell types.

Microscopy of the gap junction: a historical perspective

Gap junctions were discovered more than three decades ago, and since this time, enormous strides have been made in understanding their structure and function. This article summarises the part played by microscopy, within the context of multidisciplinary research, in the historical development of our knowledge of the gap junction.

Myocardial gap junction organization in ischemia and infarction

Ischemia causes an increase in myocardial resistivity and a decrease in conduction velocity, thereby enhancing cardiac contractile dysfunction and arrhythmic tendency. Myocardial gap junctions, as principal determinants of conduction velocity, may, therefore, be expected to be deranged in ischemia. Despite a lack of consensus, attempts at correlating gap junction ultrastructural morphology with functional state have revealed the component connexons of gap junctions in freeze-fractured myocardium to be in multiple small hexagonal arrays, tending to become randomly distributed and compacted under uncoupling conditions. Further hypoxic uncoupling causes ultrastructural damage and a reduction in gap-junctional surface area. Immunohistochemical detection of connexin43 gap junctions in chronically ischemic non-infarcted human myocardium demonstrates a reduction in junctional surface area within a normal number of intercalated disks per myocyte, and with a normal distribution of junction sizes. In healed canine infarction there are smaller and fewer gap junctions in the fibrotic myocardium adjacent to infarcts, with reductions in overall gap-junctional content and the proportion of side-to-side vs. end-to-end intercellular connections. Immunohistochemical examination of intact human ventricular myocardium shows the myocytes immediately abutting healed infarcts to have connexin43 gap junctions spread longitudinally over the cell surfaces, and not in discrete transversely orientated intercalated disks as in normal myocardium. Early after canine infarction, and before fibrotic healing, the connexin43 gap junctions in myocytes abutting the infarct show disorganization similar to that described in healed human infarcts, suggesting that this disturbance is an early pathophysiological cellular response, and not simply due to later fibrotic distortion. Such changes in gap-junctional organization in myocardial ischemia and infarction may be implicated in the elusive link between subcellular structure, contractile dysfunction and arrhythmogenesis.

Spatial organization of cardiac gap junctions can affect access resistance

In the heart, gap junctions electrically couple myocytes together. Electron- and light-microscope-based analyses have revealed that cardiac gap junctions show a variety of organizational patterns. At the level of gap-junctional channel aggregates, freeze fracture has demonstrated diverse channel packing arrangements in the membranes of different myocardial tissues. Ultrastructural and immunohistochemical studies have shown variation and specialization in the 3-dimensional spatial distribution of gap junctional contacts between different types of myocardial cells. Here, we estimate the access resistance of various configurations of gap junctions using physical principles and explore how certain of these specializations in gap-junctional organization may influence access resistance, a potentially important determinant of electrical conductance between coupled myocardial cells.

Structure--function relationships in gap junctions

Gap junctions are metabolic and electrotonic pathways between cells and provide direct cooperation within and between cellular nets. They are among the cellular structures most frequently investigated. This chapter primarily addresses aspects of the assembly of the gap junction channel, considering the insertion of the protein into the membrane, the importance of phosphorylation of the gap junction proteins for coupling modulation, and the formation of whole channels from two hemichannels. Interactions of gap junctions with the subplasmalemmal cytoplasm on the one side and with tight junctions on the other side are closely considered. Furthermore, reviewing the significance and alterations of gap junctions during development and oncogenesis, respectively, including the role of adhesion molecules, takes up a major part of the chapter. Finally, the literature on gap junctions in the central nervous system, especially between astrocytes in the brain cortex and horizontal cells in the retina, is summarized and new aspects on their structure-function relationship included.

Multiple structural types of gap junctions in mouse lens

Gap junctions in the epithelium and superficial fiber cells from young mice were examined in lenses prepared by rapid-freezing, and processed for freeze-substitution and freeze-fracture electron microscopy. There appeared to be three structural types of gap junction: one type between epithelial cells and two types between fiber cells. Epithelial gap junctions seen by freeze-substitution were approximately 20 nm thick and consistently associated with layers of dense material lying along both cytoplasmic surfaces. Fiber gap junctions, in contrast, were 15–16 nm (type 1) or 17–18 nm thick (type 2), and had little associated cytoplasmic material. Type 1 fiber gap junctions were extensive in flat expanses of cell membrane and had a thin, discontinuous central lamina, whereas type 2 fiber gap junctions were associated with the ball-and-socket domains and exhibited a dense, continuous central lamina. Both types of fiber gap junction had a diffuse arrangement of junctional intramembrane particles, whereas particles and pits of epithelial gap junctions were in a tight, hexagonal configuration. The type 2 fiber gap junctions, however, had a larger particle size (approximately 9 nm) than the type 1 (approximately 7.5 nm). In addition, a large number of junctional particles typified the E-faces of both fiber types but not the epithelial type of gap junction. Gap junctions between fiber and epithelial cells had structural features of type 1 fiber gap junctions.(ABSTRACT TRUNCATED AT 250 WORDS)

Intercellular junctions and the application of microscopical techniques: the cardiac gap junction as a case model

Intercellular junctions are fundamental to the interactions between cells. By means of these junctions, the activities of the individual cells that make up tissues are co-ordinated, enabling each tissue system to function as an integrated whole. In this review, the work of the authors on one specific type of junction--the cardiac gap junction--is presented as a case model to illustrate how the application of a range of microscopical methods, as part of a multidisciplinary approach, can help extend our understanding of cell junctions and their functions. In the heart, gap junctions form the low-resistance pathways for rapid impulse conduction and propagation, enabling synchronous stimulation of myocyte contraction. Gap junctions also form pathways for direct intercellular communication, a function of particular importance for morphogenetic signalling during development. The work discussed demonstrates some of the applications of techniques in electron microscopy, immunocytochemistry and confocal scanning laser microscopy to the understanding of the structural basis of the function of gap junctions in the normal adult heart, the developing heart and the diseased heart. Freeze-fracture electron microscopy of heart tissue prepared by rapid freezing techniques, in which excision-related structural damage to the cells is minimized or avoided, makes it possible to deduce the structure of the functioning gap junction in vivo. Gap junctions in hearts that are beating normally in the living animal until the very instant of freezing consist of connexons (transmembrane channels) organized in a quasi-crystalline arrangement, not a 'random' arrangement as proposed in the original hypothesis on the structural correlates of gap junction function. Alterations in connexon arrangement occur in response to ischaemia and hypoxia, though the relationship of these to gap-junctional permeability is indirect. To obtain probes for mapping the distribution of gap junctions in cardiac tissue, polyclonal antisera to synthetic peptides matching portions of the sequence of connexin43, the major gap-junctional protein reported in the heart, were raised. The specificity of the antisera was confirmed by dot blotting, Western blotting and by immunogold labelling of isolated gap junctions. One antiserum (that raised to residues 131-142) was found to be particularly effective as a cytochemical probe. An immunofluorescence labelling procedure for use with confocal scanning laser microscopy was developed to enable the three-dimensional precision mapping of gap junctions through thick slices of cardiac tissue.(ABSTRACT TRUNCATED AT 400 WORDS)

Expression of connexin43 gap junctions between cultured vascular smooth muscle cells is dependent upon phenotype

The smooth muscle cell is the predominant cell type of the arterial media. In the adult vascular system, smooth muscle cells are found primarily in the contractile phenotype, but following injury or during atherosclerotic plaque formation the secretory synthetic phenotype is expressed. Recently it has been shown that gap junction connexin43 messenger RNA levels are six times higher in cultured smooth muscle cells in the synthetic phenotype than in intact aorta. We have modulated rabbit aortic smooth muscle cells in culture between the synthetic phenotype and one resembling the contractile phenotype, and correlated gap junction expression with phenotype. A dual labelling technique with antibodies against smooth muscle myosin and a synthetic peptide constructed to match a portion of the connexin43 gap junction protein was used for these experiments. Gap junctions are numerous between synthetic phenotype cells but few are observed between contractile cells. Rat aortic smooth muscle cells were also cultured and the growth and structure of gap junctions followed in the synthetic phenotype by use of freeze-fracture electron microscopy and immunohistochemical techniques. Junctional plaques are similar in structure to those observed in cardiac muscle, their size and number increasing with time in culture. The increased numbers of gap junctions between synthetic phenotype smooth muscle cells may be important during vessel development, following injury, or in atherosclerotic plaque formation.

Robert Feulgen Prize Lecture. Distribution and role of gap junctions in normal myocardium and human ischaemic heart disease

In the heart, individual cardiac muscle cells are linked by gap junctions. These junctions form low resistance pathways along which the electrical impulse flows rapidly and repeatedly between all the cells of the myocardium, ensuring their synchronous contraction. To obtain probes for mapping the distribution of gap junctions in cardiac tissue, polyclonal antisera were raised to three synthetic peptides, each matching different cytoplasmically exposed portions of the sequence of connexin43, the major gap-junctional protein reported in the heart. The specificity of each antiserum for the peptide to which it was raised was established by dot blotting. New methods were developed for isolating enriched fractions of gap junctions from whole heart and from dissociated adult myocytes, in which detergent-treatment and raising the temperature (potentially damaging steps in previously described techniques) are avoided. Analysis of these fractions by SDS-polyacrylamide gel electrophoresis revealed major bands at 43 kDa (matching the molecular mass of connexin43) and at 70 kDa. Western blot experiments using our antisera indicated that both the 43-kDa and the 70-kDa bands represent cardiac gap-junctional proteins. Pre-embedding immunogold labelling of isolated gap junctions and post-embedding immunogold labelling of Lowicryl-embedded whole tissue demonstrated the specific binding of the antibodies to ultrastructurally defined gap junctions. One antiserum (raised to residues 131-142) was found to be particularly effective for cytochemical labelling. Using this antiserum for immunofluorescence labelling in combination with confocal scanning laser microscopy enabled highly sensitive detection and three-dimensional mapping of gap junctions through thick slices of cardiac tissue. By means of the serial optical sectioning ability of the confocal microscope, images of the entire gap junction population of complete en face-viewed disks were reconstructed. These reconstructions reveal the presence of large junctions arranged as a peripheral ring around the disk, with smaller junctions in an interior zone: an arrangement that may facilitate efficient intercellular transfer of current. By applying our immunolabelling techniques to tissue from hearts removed from transplant patients with advanced ischaemic heart disease, we have demonstrated that gap junction distribution between myocytes at the border zone of healed infarcts is markedly disordered. This abnormality may contribute to the genesis of reentrant arrhythmias in ischaemic heart disease.

Gap junction morphology of retinal horizontal cells is sensitive to pH alterations in vitro

Isolated goldfish retinae were incubated in NaHCO3-reduced solutions, a treatment known to lower intracellular pH and to decrease gap-junction-mediated coupling between cells. The morphology of the gap junctions of horizontal cells examined by means of freeze-fracture replicas and ultrathin sections displays alterations after such treatment. The gap-junctional particles aggregate into dense clusters or crystalline arrays, whereas controls (pH 7.5) display a loose arrangement of particles. Incubation in NaHCO3-reduced solution leads to the appearance, in ultrathin sections, of prominent, electron-dense material beneath the gap-junctional membranes. Both effects, the increasing density of particles and the appearance of electron-dense material, are reversible. The application of dopamine, which uncouples horizontal cells, and its antagonist haloperidol produce less clear-cut effects on particle density in vitro.

Direct cell-cell communication in the blood-forming system

In mammals, bone marrow is the principal tissue where blood is formed during adult life. Paracrine factors are generally considered to control this process but there is considerable evidence that gap junctions are present in haemopoietic tissues. Gap junctions have been implicated in developmental and patterning roles, and we set out to characterize the cells which are coupled, and to provide evidence for their role(s) in blood cell formation. Direct cell-cell communication, shown by dye-transfer, occurs between haemopoietic cells and certain stromal cells. In culture these stromal cells form a mat in which they retain their dye-coupling properties. Freeze-fracture electron microscopy confirms that this coupling is via gap junctions. When haemopoietic cells are cultured on top of these mats dye spreads upwards from the stromal cells into the haemopoietic cells above. Experiments in which haemopoietic cells were cultured alone, with stromal cell conditioned medium, or in direct contact with stromal cell underlays, were therefore carried out. The results of these experiments provide evidence that gap junctional communication may be playing a vital role in maintaining populations of precursor cells which would otherwise differentiate into end cells, leading to the ultimate demise of the system.

Structural changes in cardiac gap junctions after hypoxia and reoxygenation: a quantitative freeze-fracture analysis

Isolated rat hearts were subjected to increasing periods of hypoxia with or without subsequent reoxygenation and the gap-junctional particle configuration was followed quantitatively. Irregular contractions were prevented by K(+)-arrest; glucose, counteracting the effects of hypoxia, was omitted. Hyperkalemia alone and a maximum of 20 min of hypoxia do not produce reorganization of the gap-junctional particles normally forming multiple hexagonally packed arrays separated by smooth aisles. After 30 min of hypoxia, the aisles disappear in a proportion of the junctions, thereby increasing the particle density from 9400 +/- 800/microns2 to 10,200 +/- 900/microns2. After 40 min of hypoxia, the normal configuration is no longer found and numerous junctions are arranged as uninterrupted hexagonal lattices. The particles are further condensed to 11,600 +/- 900/microns2. Following reoxygenation after both 30 and 40 min of hypoxia, the proportion of crystalline gap junctions dramatically augments and the mean particle density has further increased significantly. Corresponding thin sections show irreversible cell damage. When reoxygenation is performed with a control solution containing normal levels of K+ and glucose, the particle density does not increase substantially in comparison to the respective 30- and 40-min hypoxic periods. In both groups, the gap junctions display either a normal, a crystalline or an intermediate configuration with crystalline margins and loose centers. The gap-junctional reorganization during hypoxia essentially represents a particle condensation, while the mean center-to-center distances between the particles and pits remain constant. Furthermore, the reappearance of normal gap junctions after reoxygenation appears to depend on glucose availability.

Structural characteristics of gap junctions. I. Channel number in coupled and uncoupled conditions

Gap junctions between crayfish lateral axons were studied by combining anatomical and electrophysiological measurements to determine structural changes associated during uncoupling by axoplasmic acidification. In basal conditions, the junctional resistance, Rj, was approximately 60-80 k omega and the synapses appeared as two adhering membranes; 18-20-nm overall thickness, containing transverse densities (channels) spanning both membranes and the narrow extracellular gap (4-6 nm). In freeze-fracture replicas, the synapses contained greater than 3 X 10(3) gap junction plaques having a total of approximately 3.5 X 10(5) intramembrane particles. "Single" gap junction particles represented approximately 10% of the total number of gap junction particles present in the synapse. Therefore, in basal conditions, most of the gap junction particles were organized in plaques. Moreover, correlations of the total number of gap junction particles with Rj suggested that most of the junctional particles in plaques corresponded to conducting channels. Upon acidification of the axoplasm to pH 6.7-6.8, the junctional resistance increased to approximately 300 k omega and action potentials failed to propagate across the septum. Morphological measurements showed that the total number of gap junction particles in plaques decreased approximately 11-fold to 3.1 X 10(4) whereas the number of single particles dispersed in the axolemmae increased significantly. Thin sections of these synapses showed that the width of the extracellular gap increased from 4-6 nm in basal conditions to 10-20 nm under conditions where axoplasmic pH was 6.7-6.8. These observations suggest that single gap junction particles dispersed in the synapse most likely represent hemi-channels produced by the dissasembly of channels previously arranged in plaques.

Light-dependent dynamics of gap junctions between horizontal cells in the retina of the crucian carp

The dynamics of gap junctions between outer horizontal cells or their axon terminals in the retina of the crucian carp were investigated during light and dark adaptation by use of ultrathin-section and freeze-fracture electron microscopy. Light adaptation was induced by red light, while dark adaptation took place under ambient dark conditions. The two principal findings were: (1) The density of connexons within an observed gap junction is high in dark-adapted retina, and low in light-adapted retina. This, respectively, may reflect the coupled and uncoupled state of the gap junction. (2) The size of individual gap junctions is larger in light- than in dark-adapted retinae. Whereas the overall area occupied by gap junctions is reduced with dark adaptation, the percentage of small and very small gap junctions increases dramatically. A lateral shift of connexons in the gap junctional membrane is strongly suggested by these reversible processes of densification and dispersion. Two additional possibilities of gap junction modulation are discussed: (1) the de novo formation of very small gap junctions outside the large ones in the first few minutes of dark adaptation, and (2) the rearrangement of a portion of the very large gap junctions. The idea that the cytoskeleton is involved in such modulatory processes is corroborated by thin-section observations.

Gap junction ultrastructure in rat liver parenchymal cells after in vivo ischemia

The ultrastructure of gap junctions between rat liver parenchymal cells has been studied after in vivo ischemia, with and without subsequent blood reflow. Freeze fracture replicas were analysed by electron microscopic observation, optical diffraction and morphometric analysis. In control specimens gap junction connexons were widely dispersed and arranged in nearly random fashion over nearly the whole junctional area, with only minute spots of hexagonal connexon arrangement. An ischemic period of 30 min, from which the vast majority of cells are capable of recovery after restoration of the blood supply, usually entails only a slight enlargement of the areas of hexagonally arranged connexons. After 120 min of ischemia without reflow, which results in necrosis of most parenchymal cells, all gap junctions showed a completely hexagonal arrangement of connexons. The numerical density of connexons after 30 and 120 min of ischemia without reflow was significantly higher than in controls, whereas after 30 min of ischemia followed by 2 h of reflow the numerical density had returned to control levels. A fully hexagonal arrangement of gap junction connexons, as occurs after longer periods of ischemia, seems to be related to irreversible cell damage and presumably to metabolic uncoupling of cells. This was preceded by an increase in the numerical density of connexons, which is probably a reversible phenomenon.

Membrane structure in ultrarapidly frozen, unpretreated, freeze-fractured myocardium

Ultrarapid freezing has been applied to monitor the structure of the freeze-fractured myocardial sarcolemma. Our two goals were to demonstrate that large areas of membrane can be preserved free of visible ice crystal damage and, thus, be amenable to quantitative analysis and to compare the structure of directly frozen myocardial membranes with conventionally prepared tissue. The E face was most affected by lack of chemical pretreatment. First, our laboratory reported an increase in E face particle density from 379 +/- 30/micron 2 in conventional fixed tissue to 489 +/- 18/micron 2 in unpretreated tissue. Discrete arrays of 12-15 nm particles on the E face were a striking feature of the unfixed sarcolemma. However, P face intramembrane particle (IMP) density remained unchanged from previous estimates in fixed tissue. Specialized regions of the sarcolemma were enhanced in ultrarapidly frozen tissue. Particle domains of the adherens junctions were very prominent in forming a cap alongside the gap junctions. Both the P and E faces of the gap junctions were highly ordered into hexagonal arrays. Caveolae in the membrane were infrequent in both P and E faces.

Complementarity of particles and pits in freeze-fractured hepatic and cardiac gap junctions

Particles and pits of freeze-fractured gap junctions are considered as complementary structures despite the frequent observations of more regular and closer spacings of pits, ascribed to plastic deformation of particle arrays. Recently, however, the noncomplementarity of pits and particles in Purkinje fibers has been reported. To ascertain the relationship between both structures, gap junctions from fixed, cryoprotected liver and myocardium were investigated using spacing and density measurements and complementary replicas. In hepatocyte gap junctions, the center-to-center distances (mean +/- SD) among pits, 9.57 +/- 1.49 nm, and particles, 9.70 +/- 1.77 nm, are not significantly different. Density determinations yielded a slightly higher value for the pits, (11,510 +/- 830)/microns 2, than for the particles, (11,230 +/- 950)/microns 2. In the myocardium, the spacing of the regularly arrayed pits, 9.55 +/- 1.33 nm, barely exceeds the value of 9.44 +/- 1.62 nm for the particles, which show some clustering. However, the packing density for the pits, (10,090 +/- 740)/microns 2, appears a little higher than that of the particles, (9,890 +/- 920)/microns 2. As density and spacing measurements provided no decisive answers, the positions of individual pits and particles of complementary junctional faces were recorded on transparent sheets and compared. In this fashion, a one-to-one correspondence between particles and pits could be established, while small discrepancies may be attributed to plastic deformation. Moreover, the co-linearity of pits and particles may be suggested by the observation of a platinum grain in the center of many pits.

Structure and distribution of gap junctions in lens epithelium and fiber cells

We report a comparative study of gap junctions in lens epithelia of frog, rabbit, rat and human, using a "double mounting" method for freeze-fracture electron microscopy. The gap junctions on the narrow sides of hexagonal cortical fiber cells of various species were also studied with the same technique. Gap junctions were commonly present between epithelial cells of the entire undifferentiated epithelium, between fiber cells on both wide and narrow sides, and between epithelial cells and fiber cells. Structural diversity of gap junctions, based on connexon arrangements, was evident in lens epithelia among the four species studied. Gap junctions with random arrays of connexons were found predominantly in frog lens epithelium, while the crystalline and striated configurations were mainly observed in the epithelia of human and rat, and of rabbit, respectively. On the other hand, there was no structural variation of gap junctions observed on either wide or narrow sides of lens fiber cells from any species studied. Only the random-type gap junction was found. However, the distribution of gap junctions was unique on the narrow sides. There was a single row of junctional plaques along the middle of the narrow sides, whereas the wide sides showed an uneven distribution pattern. The gap junctions between epithelial cells and fiber cells had a random packing of connexons.

Modification of gap junctions in cells transformed by a temperature-sensitive mutant of Rous sarcoma virus

Prompted by our observation that a reduction in junctional permeance is one of the earlier events in the process of neoplastic transformation of a cell line by Rous sarcoma virus, we analyzed the gap junctions from these cells to determine if the basis of the reduction is a loss of junctional channels. The cells (normal rat kidney, or NRK) are infected with a temperature-sensitive mutant of Rous sarcoma virus, allowing one easily to manipulate the cells into and out of the transformed state, and hence also to manipulate the junctional permeance. Using freeze-fracture electron microscopy, we found that the number and size of the junctions did not change in parallel with the permeance changes we had previously characterized. There is, however, a significant rearrangement of the junctional particles to a more random configuration when the cells are transformed and a reversal to the more ordered pattern when the cells are shifted back to the normal phenotype. These changes do parallel the changes in junctional permeance. We conclude that the permeance of existing junctional channels is modified and that the change in permeance may involve a change in the interaction of the junctional channels with each other and/or the surrounding lipid domain.

Quantitative gap junction alterations in mammalian heart cells quickly frozen or chemically fixed after electrical uncoupling

The gap junction morphology was quantified in freeze-fracture replicas prepared from rat auricles that had been either quickly frozen at 6 K or chemically fixed by glutaraldehyde, in a state of normal cell-to-cell conduction or in a state of electrical uncoupling. The general appearance of the gap junctions was similar after both preparative procedures. A quantitative analysis of three gap junctional dimensions provided the following measurements in the quickly frozen conducting auricles (mean +/- SD): P-face particles' diameter 8.27 +/- 0.74 nm (n = 5709), P-face particles' center-to-center distance 10.78 +/- 2.12 nm (n = 4800), and E-face pits' distance 9.99 +/- 2.19 nm (n = 1600). Corresponding values obtained from chemically fixed tissues were decreased by about 3% for the particle's diameter and about 5% for the particles' and pits' distances. Electrical uncoupling by the action of either 1 mM 2-4-dinitrophenol (DNP), or 3.5 mM n-Heptan-1-ol (heptanol), induced a decrease of the particle's diameter, which amounted to -0.69 +/- 0.01 nm (mean +/- SE) in the quickly frozen preparations and -0.71 +/- 0.01 nm in the chemically fixed ones. The particles' distance was decreased by -0.96 +/- 0.04 nm in the quickly frozen samples and by -0.90 +/- 0.03 nm in the chemically fixed ones and the E-face pits' distance was similarly reduced. All differences were statistically significant (P less than 0.001 for all dimensions). Electrical recoupling after the heptanol effect promoted a return of these gap junctional dimensions towards normal values, which was about 50% complete within 20 min. It is concluded that very similar morphological alterations of the gap junctional structure are induced in the mammalian heart by different treatments promoting electrical uncoupling and that these conformational changes appear independently of the preparative procedure. The suggestion that the observed decrease of the particles' diameter is genuinely related to the closing mechanism of the unit cell-to-cell channel set in their centers is thus confirmed.

Cytoplasmic surface ultrastructures of cardiac gap junctions as revealed by quick-freeze, deep-etch replicas

Rapid-freeze, deep-etch, rotary-shadow replica studies were performed to examine the cytoplasmic surface membrane of the cardiac gap junctions of rats, mice, and guinea pigs. In quick-frozen fresh cardiac muscles, while the nonjunctional cytoplasmic surfaces were covered with filamentous materials, the cytoplasmic surface membrane continuous with freeze-fractured gap junction plaques were relatively free of such filaments and revealed particulate patterns. After brief rinsing in high K buffer, gap junction membranes showed granular substructures resembling a tiled surface made of round tiles of various sizes. After prolonged rinsing for more than 20 min, however, cytoplasmic surfaces of gap junctions became less particulate but rather smooth. The particulate substructures observed in the rapid-freeze deep-etch replicas may correspond to the fuzzy cytoplasmic layer in thin sections and serine protease sensitive peptide moiety in sodium dodecyl sulfate-polyacrylamide gel electrophoresis reported in isolated cardiac gap junction pellets. These cytoplasmic components, which are absent in liver gap junctions, seem to be specific in cardiac and neural gap junctions and may be related to the large electrical current passed by these junctions.

Gap junction structures after experimental alteration of junctional channel conductance

Gap junctions are known to present a variety of different morphologies in electron micrographs and x-ray diffraction patterns. This variation in structure is not only seen between gap junctions in different tissues and organisms, but also within a given tissue. In an attempt to understand the physiological meaning of some aspects of this variability, gap junction structure was studied following experimental manipulation of junctional channel conductance. Both physiological and morphological experiments were performed on gap junctions joining stage 20-23 chick embryo lens epithelial cells. Channel conductance was experimentally altered by using five different experimental manipulations, and assayed for conductance changes by observing the intercellular diffusion of Lucifer Yellow CH. All structural measurements were made on electron micrographs of freeze-fracture replicas after quick-freezing of specimens from the living state; for comparison, aldehyde-fixed specimens were measured as well. Analysis of the data generated as a result of this study revealed no common statistically significant changes in the intrajunctional packing of connexons in the membrane plane as a result of experimental alteration of junctional channel conductance, although some of the experimental manipulations used to alter junctional conductance did produce significant structural changes. Aldehyde fixation caused a dramatic condensation of connexon packing, a result not observed with any of the five experimental uncoupling conditions over the 40-min time course of the experiments.

Dynamics of gap junctions between horizontal cells in the goldfish retina

Experimental alterations of gap junctions between outer horizontal cells have been demonstrated in freeze-fracture replicas of goldfish retina. The alterations consisted predominantly of an increase of connexon densities and a decrease in the variability of the arrangement of connexons. They were observed i. in dark adapted retinae, ii. in animals with crushed optic nerves, iii. in picrotoxin- and bicuculline-treated animals. Since experiment i. is characterized by a depolarization of the horizontal cell, and experiment iii. was shown by others to result in uncoupling of horizontal cells, we conclude that the functional connectivity of horizontal cells might be correlated with the structure of gap junctions. An interesting detail is the differentiated reaction of axonal and perikaryal gap junctions on dark adaptation or blindness: whereas normally the axonal gap junctions are less densely packed, they increase their connexon density in darkness or blindness much more than the perikaryal gap junctions.

Inhibited intercellular communication as a mechanistic link between teratogenesis and carcinogenesis

Teratogenesis and carcinogenesis share many characteristics, leading to the speculation that they may also share pathogenic mechanisms. Direct intercellular communication mediated by membrane junctions is known to occur between a variety of cells and may play an important role in the control of cell growth and differentiation. Inhibition of junctional communication may be a mechanism common to both teratogenesis and carcinogenesis whereby cells and tissues are diverted from their normal differentiation paths. The multistage model of carcinogenesis predicts that the irreversibly initiated cell is at least partially regulated by the surrounding cells of a tissue, and that the initiated cell remains inactive until stimulated to proliferate by a tumor promotor. Tumor promoters may release the initiated cell from control of the surrounding tissue by interrupting intercellular communication, since many tumor promoters have now been shown to interfere with junctional communication in cultured mammalian cells. Furthermore, many tumorigenic cells have compromised junctional communication abilities. Similarly, it has been reasoned that the cells of an embryo must be able to communicate with each other to define tissue specificity and pattern formation, and to coordinate morphogenetic events. Many studies have chronicled alterations in junctional communication that occur coincident with major developmental events and some studies suggest that junctional communication may be modified at boundaries of morphogenetic fields. A recent in vivo study has provided evidence that inhibition of junctional communication may interfere with embryonic development, and several teratogens are known to interrupt junctional communication in mammalian cells in culture. These observations suggest that inhibition of junctional intercellular communication may be a shared mechanism of carcinogenesis and teratogenesis.

Intercalated discs of mammalian heart: a review of structure and function

Intercalated discs are exceptionally complex entities, and possess considerable functional significance in terms of the workings of the myocardium. Examination of different species and heart regions indicates that the original histological term has become out-moded; it is likely, however, that all such complexes will continue to fall under the generic heading of 'intercalated discs'. The membranes of the intercalated discs establish specific associations with a variety of intracellular and extracellular structures, as well as with numerous types of proteins and glycoproteins. Characterization of discs and their components has already brought together a large number of research disciplines, including microscopy, cytochemistry, morphometry, cell isolation and culture, cell fractionation, cryogenics, immunology, biochemistry, and electrophysiology. The continued dissection of substance and function of intercalated discs will depend on such interdisciplinary approaches. The intercalated disc component which continues to attract the greatest amount of interest is the so-called gap junction. All indications thus far point to a great deal of inherent lability in the architecture of the gap junction. There is thus considerable potential for the creation of artefact while preserving and observing gap junctions, and this problem will doubtless continue to hamper the understanding of their functions. A question of special interest concerns whether the gap junctions of intercalated discs are required for transfer of electrical excitation between cells, or maintain cell-to-cell adhesion, or in fact subserve both electrical and structural phenomena. Two schools of thought exist with respect to cell-to-cell coupling in the heart. One proposes that low-resistance junctions in the discs mediate electrical coupling, whereas the other supports the possibility of coupling across ordinary high-resistance membranes. Thus the intercalated discs continue to be a source of controversy, just as they have been since they were originally discovered in heart muscle over a century ago.