Connexin make-up of endothelial gap junctions in the rat pulmonary artery as revealed by immunoconfocal microscopy and triple-label immunogold electron microscopy

Connexin Expression in Human Minor Salivary Glands: An Immunohistochemical Microscopy Study

Connexins (Cxs) are transmembrane proteins involved in the formation of hemichannels and gap junctions (GJs). GJs are involved in various physiological functions, including secretion in glandular tissue. It has been demonstrated that Cx26, Cx32, and Cx43 are mainly expressed in glands, but no data are available in human salivary glands to date. The aim of our study was to investigate the presence and the localization of Cxs in human minor labial salivary glands. Immunofluorescence and immunoelectron microscopy were employed to evaluate the Cx26, Cx32, and Cx43 protein in human labial salivary gland biopsies (hLSGBs). RT-PCR was also used to detect their mRNA expression. Cx expression was found at both the mRNA and protein levels in all hLSGBs analysed. Cxs were observed at the level of the duct and acinar cells, as well as in myoepithelial cells. The localization of the three Cx types was very similar, suggesting colocalization of these Cxs in the same connexons. These results demonstrated the presence of Cxs in human salivary glands for the first time. Moreover, the few samples with primary Sjögren’s Syndrome analysed only by immunofluorescence showed an alteration of the Cx expression, indicating that these proteins could be involved in salivary gland dysfunctions.

New Insights into Pulmonary Hypertension: A Role for Connexin-Mediated Signalling

Pulmonary hypertension is a serious clinical condition characterised by increased pulmonary arterial pressure. This can lead to right ventricular failure which can be fatal. Connexins are gap junction-forming membrane proteins which serve to exchange small molecules of less than 1 kD between cells. Connexins can also form hemi-channels connecting the intracellular and extracellular environments. Hemi-channels can mediate adenosine triphosphate release and are involved in autocrine and paracrine signalling. Recently, our group and others have identified evidence that connexin-mediated signalling may be involved in the pathogenesis of pulmonary hypertension. In this review, we discuss the evidence that dysregulated connexin-mediated signalling is associated with pulmonary hypertension.

Levels of Gap Junction Proteins in Coronary Arterioles and Aorta of Hamsters Exposed to the Cold and During Hibernation and Arousal

There are marked changes in vascular dynamics during prolonged periods in the cold, entrance into hibernation, and arousal to euthermy. Cell-to-cell communication through gap junction channels plays a pivotal role in the control of vasomotor function. Multiple gap junction proteins are expressed in blood vessels, including connexins 37 (Cx37), 40 (Cx40), 43 (Cx43), and 45 (Cx45). Using immunolabeling techniques combined with confocal microscopy, we quantitated the levels of these connexins in coronary arterioles and the thoracic aorta of the golden hamster in four physiological conditions: normal control animals at euthermy; cold-exposed animals (before entrance into hibernation); during hibernation; and after 2-hr arousal from hibernation. In all groups, Cx37 was localized between endothelial cells of the aorta and Cx40 was observed between endothelial cells of coronary arterioles and the aorta. Cx43 was confined to smooth muscle cells of the aorta. Labeling for Cx45 was detected in the endothelium of the ascending aorta. The expression of Cx37 was significantly reduced in cold-exposed, hibernating, and aroused animals. Immunolabeling for Cx40 was increased in the coronary arteriolar endothelium of the cold-exposed group compared with normal controls, hibernating, and aroused animals, perhaps to facilitate intercellular communication during the prolonged circulatory changes to vascular dynamics required to maintain core temperature during cold adaptation. Cx40 expression was unchanged in the aorta. Cx43 immunoexpression in the aorta remained constant under all conditions examined. These changes in connexin expression did not occur during the rapid circulatory changes associated with arousal from hibernation.

Expression and role of connexin-based gap junctions in pulmonary inflammatory diseases

Connexins are transmembrane proteins that can generate intercellular communication channels known as gap junctions. They contribute to the direct movement of ions and larger cytoplasmic solutes between various cell types. In the lung, connexins participate in a variety of physiological functions, such as tissue homeostasis and host defence. In addition, emerging evidence supports a role for connexins in various pulmonary inflammatory diseases, such as asthma, pulmonary hypertension, acute lung injury, lung fibrosis or cystic fibrosis. In these diseases, the altered expression of connexins leads to disruption of normal intercellular communication pathways, thus contributing to various pathophysiological aspects, such as inflammation or tissue altered reactivity and remodeling. The present review describes connexin structure and organization in gap junctions. It focuses on connexins in the lung, including pulmonary bronchial and arterial beds, by looking at their expression, regulation and physiological functions. This work also addresses the issue of connexin expression alteration in various pulmonary inflammatory diseases and describes how targeting connexin-based gap junctions with pharmacological tools, synthetic blocking peptides or genetic approaches, may open new therapeutic perspectives in the treatment of these diseases.

Involvement of gap junctions between smooth muscle cells in sustained hypoxic pulmonary vasoconstriction development: a potential role for 15-HETE and 20-HETE

In response to hypoxia, the pulmonary artery normally constricts to maintain optimal ventilation-perfusion matching in the lung, but chronic hypoxia leads to the development of pulmonary hypertension. The mechanisms of sustained hypoxic pulmonary vasoconstriction (HPV) remain unclear. The aim of this study was to determine the role of gap junctions (GJs) between smooth muscle cells (SMCs) in the sustained HPV development and involvement of arachidonic acid (AA) metabolites in GJ-mediated signaling. Vascular tone was measured in bovine intrapulmonary arteries (BIPAs) using isometric force measurement technique. Expression of contractile proteins was determined by Western blot. AA metabolites in the bath fluid were analyzed by mass spectrometry. Prolonged hypoxia elicited endothelium-independent sustained HPV in BIPAs. Inhibition of GJs by 18β-glycyrrhetinic acid (18β-GA) and heptanol, nonspecific blockers, and Gap-27, a specific blocker, decreased HPV in deendothelized BIPAs. The sustained HPV was not dependent on Ca(2+) entry but decreased by removal of Ca(2+) and by Rho-kinase inhibition with Y-27632. Furthermore, inhibition of GJs decreased smooth muscle myosin heavy chain (SM-MHC) expression and myosin light chain phosphorylation in BIPAs. Interestingly, inhibition of 15- and 20-hydroxyeicosatetraenoic acid (HETE) synthesis decreased HPV in deendothelized BIPAs. 15-HETE- and 20-HETE-stimulated constriction of BIPAs was inhibited by 18β-GA and Gap-27. Application of 15-HETE and 20-HETE to BIPAs increased SM-MHC expression, which was also suppressed by 18β-GA and by inhibitors of lipoxygenase and cytochrome P450 monooxygenases. More interestingly, 15,20-dihydroxyeicosatetraenoic acid and 20-OH-prostaglandin E2, novel derivatives of 20-HETE, were detected in tissue bath fluid and synthesis of these derivatives was almost completely abolished by 18β-GA. Taken together, our novel findings show that GJs between SMCs are involved in the sustained HPV in BIPAs, and 15-HETE and 20-HETE, through GJs, appear to mediate SM-MHC expression and contribute to the sustained HPV development.

Endothelial and smooth muscle cell ion channels in pulmonary vasoconstriction and vascular remodeling

The pulmonary circulation is a low resistance and low pressure system. Sustained pulmonary vasoconstriction and excessive vascular remodeling often occur under pathophysiological conditions such as in patients with pulmonary hypertension. Pulmonary vasoconstriction is a consequence of smooth muscle contraction. Many factors released from the endothelium contribute to regulating pulmonary vascular tone, while the extracellular matrix in the adventitia is the major determinant of vascular wall compliance. Pulmonary vascular remodeling is characterized by adventitial and medial hypertrophy due to fibroblast and smooth muscle cell proliferation, neointimal proliferation, intimal, and plexiform lesions that obliterate the lumen, muscularization of precapillary arterioles, and in situ thrombosis. A rise in cytosolic free Ca(2+) concentration ([Ca(2+)]cyt) in pulmonary artery smooth muscle cells (PASMC) is a major trigger for pulmonary vasoconstriction, while increased release of mitogenic factors, upregulation (or downregulation) of ion channels and transporters, and abnormalities in intracellular signaling cascades are key to the remodeling of the pulmonary vasculature. Changes in the expression, function, and regulation of ion channels in PASMC and pulmonary arterial endothelial cells play an important role in the regulation of vascular tone and development of vascular remodeling. This article will focus on describing the ion channels and transporters that are involved in the regulation of pulmonary vascular function and structure and illustrating the potential pathogenic role of ion channels and transporters in the development of pulmonary vascular disease.

Structure and composition of pulmonary arteries, capillaries, and veins

The pulmonary vasculature comprises three anatomic compartments connected in series: the arterial tree, an extensive capillary bed, and the venular tree. Although, in general, this vasculature is thin-walled, structure is nonetheless complex. Contributions to structure (and thus potentially to function) from cells other than endothelial and smooth muscle cells as well as those from the extracellular matrix should be considered. This review is multifaceted, bringing together information regarding (i) classification of pulmonary vessels, (ii) branching geometry in the pulmonary vascular tree, (iii) a quantitative view of structure based on morphometry of the vascular wall, (iv) the relationship of nerves, a variety of interstitial cells, matrix proteins, and striated myocytes to smooth muscle and endothelium in the vascular wall, (v) heterogeneity within cell populations and between vascular compartments, (vi) homo- and heterotypic cell-cell junctional complexes, and (vii) the relation of the pulmonary vasculature to that of airways. These issues for pulmonary vascular structure are compared, when data is available, across species from human to mouse and shrew. Data from studies utilizing vascular casting, light and electron microscopy, as well as models developed from those data, are discussed. Finally, the need for rigorous quantitative approaches to study of vascular structure in lung is highlighted.

Neutrophil intercellular communication in acute lung injury. Emerging roles of microparticles and gap junctions

A hallmark of acute inflammation involves the recruitment of polymorphonuclear leukocytes (neutrophils) to infected or injured tissues. The processes underlying this recruitment are complex, and include multiple mechanisms of intercellular communication between neutrophils and the inflamed tissue. In recent studies of the systemic and pulmonary vasculature, interest has increased in novel forms of intercellular communication, such as microparticle exchange and gap junctional intercellular communication. To understand the roles of these novel forms of communication in the onset, progression, and resolution of inflammatory lung injury (such as acute respiratory distress syndrome), we review the literature concerning the contributions of microparticle exchange and gap junctional intercellular communication to neutrophil-alveolar crosstalk during pulmonary inflammation. By focusing on these cell-cell communications, we aim to demonstrate significant gaps of knowledge and identify areas of considerable need for further investigations of the processes of acute lung inflammation.

Endothelial control of vasodilation: integration of myoendothelial microdomain signalling and modulation by epoxyeicosatrienoic acids

Endothelium-derived epoxyeicosatrienoic acids (EETs) are fatty acid epoxides that play an important role in the control of vascular tone in selected coronary, renal, carotid, cerebral and skeletal muscle arteries. Vasodilation due to endothelium-dependent smooth muscle hyperpolarization (EDH) has been suggested to involve EETs as a transferable endothelium-derived hyperpolarizing factor. However, this activity may also be due to EETs interacting with the components of other primary EDH-mediated vasodilator mechanisms. Indeed, the transfer of hyperpolarization initiated in the endothelium to the adjacent smooth muscle via gap junction connexins occurs separately or synergistically with the release of K(+) ions at discrete myoendothelial microdomain signalling sites. The net effects of such activity are smooth muscle hyperpolarization, closure of voltage-dependent Ca(2+) channels, phospholipase C deactivation and vasodilation. The spatially localized and key components of the microdomain signalling complex are the inositol 1,4,5-trisphosphate receptor-mediated endoplasmic reticulum Ca(2+) store, Ca(2+)-activated K(+) (KCa), transient receptor potential (TRP) and inward-rectifying K(+) channels, gap junctions and the smooth muscle Na(+)/K(+)-ATPase. Of these, TRP channels and connexins are key endothelial effector targets modulated by EETs. In an integrated manner, endogenous EETs enhance extracellular Ca(2+) influx (thereby amplifying and prolonging KCa-mediated endothelial hyperpolarization) and also facilitate the conduction of this hyperpolarization to spatially remote vessel regions. The contribution of EETs and the receptor and channel subtypes involved in EDH-related microdomain signalling, as a candidate for a universal EDH-mediated vasodilator mechanism, vary with vascular bed, species, development and disease and thus represent potentially selective targets for modulating specific artery function.

Role of connexins in infantile hemangiomas

The circulatory system is one of the first systems that develops during embryogenesis. Angiogenesis describes the formation of blood vessels as a part of the circulatory system and is essential for organ growth in embryogenesis as well as repair in adulthood. A dysregulation of vessel growth contributes to the pathogenesis of many disorders. Thus, an imbalance between pro- and antiangiogenic factors could be observed in infantile hemangioma (IH). IH is the most common benign tumor during infancy, which appears during the first month of life. These vascular tumors are characterized by rapid proliferation and subsequently slower involution. Most IHs regress spontaneously, but in some cases they cause disfigurement and systemic complications, which requires immediate treatment. Recently, a therapeutic effect of propranolol on IH has been demonstrated. Hence, this non-selective β-blocker became the first-line therapy for IH. Over the last years, our understanding of the underlying mechanisms of IH has been improved and possible mechanisms of action of propranolol in IH have postulated. Previous studies revealed that gap junction proteins, the connexins (Cx), might also play a role in the pathogenesis of IH. Therefore, affecting gap junctional intercellular communication is suggested as a novel therapeutic target of propranolol in IH. In this review we summarize the current knowledge of the molecular processes, leading to IH and provide new insights of how Cxs might be involved in the development of these vascular tumors.

Heterogeneous expression of endothelial connexin (Cx) 37, Cx40, and Cx43 in rat large veins

Gap junctions are clusters of transmembrane protein channels for intercellular communication and are composed of connexin (Cx). The vascular endothelial cells express Cx37, Cx40, and Cx43. We herein examined the spatial distribution of the endothelial connexins Cx37, Cx40, and Cx43 in rat large veins including the cranial vena cava, thoracic section of the caudal vena cava, and abdominal section of the caudal vena cava. We also examined the mean size of the endothelial cells and quantified the protein expression levels of the endothelial connexins. We found that the large veins heterogeneously expressed Cx37, Cx40, and Cx43 as follows: Cx40 > Cx37 > > Cx43 in the cranial vena cava, Cx37 > Cx43 > > Cx40 in the thoracic section of the caudal vena cava, and Cx40 > Cx43 > > Cx37 in the abdominal section of the caudal vena cava. Double immunostaining of two of the endothelial connexins revealed that the gap-junction plaques were composed of various combinations of endothelial connexins. The mean size of the endothelial cells was large, moderate, or small in the cranial vena cava, the abdominal section of the caudal vena cava, or the thoracic section of the caudal vena cava, respectively. The heterogeneity of the endothelial cells of the rat large veins in terms of the connexin expression suggests that the endothelial cells are differently coupled in the large veins. The present data are useful for investigating, for example, disease-related alterations in expression of endothelial connexins in large veins.

Vascular smooth muscle modulates endothelial control of vasoreactivity via reactive oxygen species production through myoendothelial communications

Background

Endothelial control of vascular smooth muscle plays a major role in the resulting vasoreactivity implicated in physiological or pathological circulatory processes. However, a comprehensive understanding of endothelial (EC)/smooth muscle cells (SMC) crosstalk is far from complete. Here, we have examined the role of gap junctions and reactive oxygen species (ROS) in this crosstalk and we demonstrate an active contribution of SMC to endothelial control of vasomotor tone.

Methodology/Principal Findings

In small intrapulmonary arteries, quantitative RT-PCR, Western Blot analyses and immunofluorescent labeling evidenced connexin (Cx) 37, 40 and 43 in EC and/or SMC. Functional experiments showed that the Cx-mimetic peptide targeted against Cx 37 and Cx 43 (^37,43Gap27) (1) reduced contractile and calcium responses to serotonin (5-HT) simultaneously recorded in pulmonary arteries and (2) abolished the diffusion in SMC of carboxyfluorescein-AM loaded in EC. Similarly, contractile and calcium responses to 5-HT were decreased by superoxide dismutase and catalase which, catabolise superoxide anion and H₂O₂, respectively. Both Cx- and ROS-mediated effects on the responses to 5-HT were reversed by L-NAME, a NO synthase inhibitor or endothelium removal. Electronic paramagnetic resonance directly demonstrated that 5-HT-induced superoxide anion production originated from the SMC. Finally, whereas 5-HT increased NO production, it also decreased cyclic GMP content in isolated intact arteries.

Conclusions/Significance

These data demonstrate that agonist-induced ROS production in SMC targeting EC via myoendothelial gap junctions reduces endothelial NO-dependent control of pulmonary vasoreactivity. Such SMC modulation of endothelial control may represent a signaling pathway controlling vasoreactivity under not only physiological but also pathological conditions that often implicate excessive ROS production.

The remodeling of connexin in the hypertrophied right ventricular in pulmonary arterial hypertension and the effect of a dual ET receptor antagonist (bosentan)

Studies on the role of connexins (Cxs) in the pathogenesis of right ventricular (RV) hypertrophy in pulmonary arterial hypertension (PAH) have not been reported to date. Therefore, we established a rat model of PAH induced by monocrotaline (MCT), and they were randomized to three groups: Control, MCT, and MCT+bosentan. Through electromicroscopy, in the control group, the gap junctions were long and frequent in intercalated disks, and short and rare at the sites of side-side cell junctions. In the MCT group, the opposite distribution was detected. In the MCT+bosentan group, the distribution of gap junctions was similar to that in the control group. Using immunoconfocal microscopy, most of the Cx43 staining was aggregated at the cell termini, and staining was weak at the sites of side-side cell junctions in the control group. However, the distribution of Cx43 was opposite in the MCT group. In the MCT+bosentan group, the result was similar to that in the control group. Therefore, perturbation of connexin distribution may be associated with RV hypertrophy. Improving the distribution of Cx43 in RV myocardium may be one of the mechanisms of a dual ET receptor antagonist partly reversing the RV hypertrophy.

Biological and biophysical properties of vascular connexin channels

Intercellular channels formed by connexin proteins play a pivotal role in the direct movement of ions and larger cytoplasmic solutes between vascular endothelial cells, between vascular smooth muscle cells, and between endothelial and smooth muscle cells. Multiple genetic and epigenetic factors modulate connexin expression levels and/or channel function, including cell-type-independent and cell-type-specific transcription factors, posttranslational modifications, and localized membrane targeting. Additionally, differences in protein-protein interactions, including those between connexins, significantly contribute to both vascular homeostasis and disease progression. The biophysical properties of the connexin channels identified in the vasculature, those formed by Cx37, Cx40, Cx43 and/or Cx45 proteins, are discussed in this chapter in the physiological and pathophysiological context of vessel function.

Up-regulation of connexin43 correlates with increased synthetic activity and enhanced contractile differentiation in TGF-beta-treated human aortic smooth muscle cells

Up-regulation of the gap-junctional protein connexin43 (Cx43) in arterial smooth muscle cells (SMCs) features in response to injury and in atherosclerosis, in parallel with phenotypic transition to the synthetic state. TGF-beta1 is known to have a role in SMC differentiation and extracellular matrix (ECM) synthesis, key characteristics of phenotypic state. Here, we set out to examine the effects of TGF-beta1 on Cx43-gap junction expression in relation to SMC differentiation, ECM synthesis and growth. Cx43 expression was analysed by immunoconfocal microscopy and Western blotting in primary human aortic SMCs treated with TGF-beta1 over a 48-h period, with assessment of gap-junctional communication by cell-to-cell transfer of microinjected ethidium bromide. In parallel, synthetic activity was analysed by Northern blotting for ECM components alpha-1(I) and alpha1(III) procollagen transcripts, contractile differentiation was assessed by immunoconfocal microscopy and Western blotting of the markers smooth muscle alpha-actin, calponin and smooth muscle heavy chain isoform 1 (SM1), and growth was measured by BrdU incorporation. Our results demonstrate that TGF-beta1 significantly up-regulates Cx43 expression and intercellular communication, in concert with increased expression of alpha-actin, calponin and SM1. Concomitant with contractile protein expression, ECM synthesis was increased rather than decreased, TGF-beta1 inducing a significant up-regulation of both procollagen transcripts. These effects were independent of growth. We conclude that in human aortic SMCs, TGF-beta1 treatment leads to up-regulation of Cx43-mediated gap-junctional communication and increased synthetic activity yet, somewhat paradoxically, also enhanced contractile differentiation.

Vascular gap junctions and implications for hypertension

Four connexin (Cx) molecules, namely Cx37, Cx40, Cx43 and Cx45, are expressed in the gap junctions that exist within and between the cellular layers of arteries. Endothelial cells are well coupled by large gap junctions expressing Cx37, Cx40 and, to a lesser extent, Cx43, whose expression may be more subject to regulation by physical factors. Smooth muscle cells are more heterogeneously coupled by gap junctions that are small and rare. The identity of the Cx expressed in the media may vary among different arteries. Myoendothelial gap junctions are small and more common in resistance arteries with fewer layers of smooth muscle cells. Given the small size of these gap junctions and the rapid turnover rate of Cxs, homocellular coupling in the media and heterocellular coupling between the cell layers may be subject to more dynamic control than coupling in the endothelium. Vascular gap junctions have been implicated in a number of vasomotor responses that may regulate vascular tone and blood pressure. These include the mechanism of action of the vasodilator, endothelium-derived hyperpolarizing factor (EDHF), the myogenic constriction to intramural pressure increase, the spontaneous or agonist-induced vasomotion of arteries and arterioles and the spreading vasodilation and constriction observed in microcirculatory networks. Few data are available on Cx expression in the media of resistance arteries during hypertension. Changes in the expression of Cx43 described in the media of the aorta of hypertensive rats vary with the hypertensive model studied and are likely to represent adaptations to structural changes in the vascular wall. In contrast, in the endothelium of the caudal and mesenteric arteries of spontaneously hypertensive rats, expression of Cxs is significantly decreased compared with arteries from normotensive rats and this decrease is reversed by inhibitors of the renin-angiotensin system. During hypertension, the activity of EDHF is decreased in the mesenteric artery, but this occurs much later than the initial increase in blood pressure and the decrease in endothelial Cxs, suggesting that changes in EDHF may not be causally related to hypertension or to the changes in endothelial Cxs. Upregulation of the myogenic response and the incidence of vasomotion has been reported in hypertension. Little is currently known of the effects of hypertension on spreading vasomotor responses. Deletion of specific Cxs in genetically modified mice is complicated by neonatal lethality or coordinate regulation and compensatory changes in the remaining Cxs. Nevertheless, mice in which Cx40 has been deleted are hypertensive and spreading vasodilatory responses are significantly impaired. Determination of a role for specific Cxs in the control of blood pressure must await the development of animals in which Cx expression can be modulated in a more complex temporal and tissue-specific manner.

Angiotensin-converting enzyme inhibition restores endothelial but not medial connexin expression in hypertensive rats

OBJECTIVE AND DESIGN

Remodelling in the media and decreases in connexin (Cx) expression and size of endothelial cells occur in the caudal artery of spontaneously hypertensive rats (SHR). The objective of this study was to determine whether similar changes are found in the aorta and whether effects in both aorta and caudal artery are present in the pre-hypertensive period or can be reversed by antihypertensive treatment.

METHODS AND RESULTS

In the aorta of SHR, there was no difference in endothelial cell size although Cxs 37 and 40 were decreased, compared with normotensive Wistar-Kyoto rats. Cxs 37 and 43 were also reduced in the media. These differences were not apparent in pre-hypertensive SHR. Inhibition of angiotensin-converting enzyme (ACE) in SHR decreased blood pressure and restored Cx expression in the endothelium of both aorta and caudal artery. The decreased endothelial cell size in the caudal artery or the reduced Cxs in the media of the aorta of SHR were unaffected by ACE inhibition.

CONCLUSION

We conclude that cellular coupling is reduced in the endothelium of arteries of SHR, but this can be restored by inhibition of the renin-angiotensin system. Decreased cellular coupling in the media or decreased endothelial size in SHR were not reversed by this antihypertensive treatment.

Shear stress-induced upregulation of connexin 43 expression in endothelial cells on upstream surfaces of rat cardiac valves

Endothelial expression of the gap junction proteins, connexin (Cx) 37, Cx40, and Cx43, varies within the vascular network. While previous studies suggest that shear stress may upregulate Cx43, it is not well understood if shear stress affects the expression of all endothelial connexins and to what extent. Endothelial cells on the upstream and downstream surfaces of cardiac valves are subjected to considerably different intensities of shear stress. We therefore reasoned that we could determine the extent hemodynamic forces affect the expression of Cx37, Cx40, and Cx43 by comparing their immunohistochemical distribution on the upstream and downstream surfaces of rat cardiac valves. We found 70- to 200-fold greater expression of Cx43 in the endothelial cells on the upstream than on the downstream surfaces. However, Cx37 was expressed almost equally in the endothelial cells on upstream and downstream surfaces, and Cx40, a major connexin in most vascular endothelial cells, was not detected on either surface. In addition to the heterogeneity in Cx43 expression, endothelial cells on the upstream surface were 35% to 65% smaller than those on the corresponding downstream surface. These results suggest that shear stress may affect endothelial cell size and Cx43 expression but not Cx37 expression.

Adenoviral delivery of human connexin37 induces endothelial cell death through apoptosis

Gap junction channels formed of connexins directly link the cytoplasm of adjacent cells and have been implicated in intercellular signaling that may regulate the functions of vascular cells. To facilitate connexin manipulation and analysis of their roles in adult endothelial cells, we developed adenoviruses containing the vascular connexins (Cx37, Cx40, and Cx43). We infected cultured human umbilical vein endothelial cells with control or connexin adenoviruses. Connexin expression was verified by immunoblotting and immunofluorescence. Infection with the Cx37 adenovirus (but not control or other connexin adenoviruses) led to a dose-dependent death of the endothelial cells that was partially antagonized by the gap junction blocker alpha-glycyrrhetinic acid and altered the intercellular transfer of Lucifer yellow and neurobiotin. Cell morphology, Annexin V and TUNEL staining, and caspase 3 assays all implicated apoptosis in the cell death. These data suggest that connexin-specific alterations of intercellular communication may modulate endothelial cell growth and death.

Regulation of connexin biosynthesis, assembly, gap junction formation, and removal

Gap junctions (GJs) are the only known cellular structures that allow a direct transfer of signaling molecules from cell-to-cell by forming hydrophilic channels that bridge the opposing membranes of neighboring cells. The crucial role of GJ-mediated intercellular communication (GJIC) for coordination of development, tissue function, and cell homeostasis is now well documented. In addition, recent findings have fueled the novel concepts that connexins, although redundant, have unique and specific functions, that GJIC may play a significant role in unstable, transient cell-cell contacts, and that GJ hemi-channels by themselves may function in intra-/extracellular signaling. Assembly of these channels is a complicated, highly regulated process that includes biosynthesis of the connexin subunit proteins on endoplasmic reticulum membranes, oligomerization of compatible subunits into hexameric hemi-channels (connexons), delivery of the connexons to the plasma membrane, head-on docking of compatible connexons in the extracellular space at distinct locations, arrangement of channels into dynamic, spatially and temporally organized GJ channel aggregates (so-called plaques), and coordinated removal of channels into the cytoplasm followed by their degradation. Here we review the current knowledge of the processes that lead to GJ biosynthesis and degradation, draw comparisons to other membrane proteins, highlight novel findings, point out contradictory observations, and provide some provocative suggestive solutions.

Heart and head defects in mice lacking pairs of connexins

Gene ablation studies in mice have revealed roles for gap junction proteins (connexins) in heart development. Of the 20 connexins in vertebrates, four are expressed in developing heart: connexin37 (Cx37), connexin40 (Cx40), connexin43 (Cx43), and connexin45 (Cx45). Although each cardiac connexin has a different pattern of expression, some heart cells coexpress multiple connexins during cardiac morphogenesis. Since different connexins could have overlapping functions, some developmental phenotypes may only become evident when more than one connexin is ablated. In this study, we interbred Cx40(-/-) and Cx43(-/-) mice to generate mice lacking both Cx40 and Cx43. Cx40(-/-)Cx43(-/-) mice die around embryonic day 12.5 (E12.5), much earlier than either Cx40(-/-) or Cx43(-/-) mice, and they exhibit malformed hearts with ventricles that are abnormally rotated, suggesting a looping defect. Some Cx40(-/-)Cx43(-/-) animals also develop head defects characteristic of exencephaly. In addition, we examined mice lacking both Cx40 and Cx37 and found a high incidence of atrial and ventricular septal defects at birth. These results provide further evidence for the importance of gap junctions in embryonic development. Moreover, ablating different pairs of cardiac connexins results in distinct heart defects, suggesting both common and unique functions for Cx40, Cx43, and Cx37 during cardiac morphogenesis.

Impedance measurements and connexin expression in human detrusor muscle from stable and unstable bladders

Three of this month's Scientific Discovery papers highlight the importance of collaboration in delivering high quality scientific research. As scientific technology increases in power and cost, and specific areas of interest become more specialized, it is becoming more difficult to cover all aspects of a completeresearch story. Collaborating with other experts in the field or other fields, including industry, allows strong scientific proof to be generated for the hypothesis and aims. Building strong collaborative,inter-disciplinary, multi-institutional, international groups with academic and industrial partners is the way forward for all discovery. We look forward to publishing more of these collaborative papersin future issues of the BJU International.

OBJECTIVES

To test the hypothesis that intercellular electrical coupling is altered in human detrusor smooth muscle from patients with unstable bladders.

MATERIALS AND METHODS

Human detrusor biopsy samples were obtained from patients with stable and unstable bladders. Intracellular electrical impedance was measured with alternating current (20 Hz-300 kHz) across the ends of detrusor strips in an oil-gap, after correcting for extracellular space resistance. Gap junctions were identified by localization of connexins (Cx), specifically Cx45, Cx43 and Cx40 transcripts, using immunoconfocal microscopy.

RESULTS

Total intracellular resistivity was greater in strips from unstable than from stable bladders (median 1246 vs 817 Omega.cm). The increase was attributed to an increase in junctional resistance; cytoplasmic resistance was unchanged. Cx43 was localized to a submucosal layer and to connective tissue; Cx40 label was confined to endothelial cells of blood vessels. Cx45 labelling was localized to detrusor bundles and appeared to be less marked in samples from unstable bladders. Semi-quantitative analysis of Northern blots showed that Cx45 expression in unstable was less than that in stable bladders.

CONCLUSIONS

These data suggest that intercellular coupling is reduced in detrusor from unstable bladders. Cx45 was localized to the detrusor layer, with Cx 43 more evident in the suburothelial mucosa. Cx45 labelling was less intense in detrusor samples from unstable bladders. These results are consistent with reduced gap junction coupling in detrusor from unstable bladders.

Decreased intercellular dye-transfer and downregulation of non-ablated connexins in aortic endothelium deficient in connexin37 or connexin40

Vascular endothelial cells are coupled by gap junctions that permit cell-to-cell transfer of small molecules, including signals that may be important for vasomotor responses. Connexin37 (Cx37) and connexin40 (Cx40) are the predominant gap-junction proteins present in mouse endothelium. We examined the effect of eliminating Cx37, Cx40, or both, on interendothelial communication in mouse aorta. Intercellular transfer of biocytin and [2-(4-nitro-2,1,3-benzoxadiazol-7-yl)aminoethyl]trimethylammonium (NBD-TMA) was used to assess gap-junction-mediated coupling. Ablation of Cx40 generally had a greater effect on dye-transfer than ablation of Cx37. The effect of Cx40 ablation on dye-transfer was age dependent. There was a 27-fold reduction in biocytin transfer in embryonic Cx40-/- aortic endothelium, a much larger change than in aortas of 6-7-week-old Cx40-/- animals, which showed a 3.5-fold reduction. By contrast, there was no reduction in biocytin transfer in embryonic Cx37-/- endothelium. Embryonic aortas lacking both Cx37 and Cx40 showed a complete loss of endothelial dye-transfer. Surprisingly, elimination of Cx40 resulted in up to a 17-fold drop in endothelial Cx37 on western blots, whereas deletion of Cx37 reduced endothelial Cx40 up to 4.2-fold. By contrast, in the medial layer, both Cx37 and Cx43 increased approximately fourfold in Cx40-/- aortas. Declines in non-ablated endothelial connexins were not mediated by changes in connexin mRNA levels, suggesting a post-transcriptional effect. Our results indicate that Cx37 and Cx40 are the only functional connexins expressed in mouse aortic endothelium and are collectively crucial for endothelial communication. Furthermore, Cx37 and Cx40 are codependent on each other for optimal expression in vascular endothelium.

Vascular abnormalities in mice lacking the endothelial gap junction proteins connexin37 and connexin40

Cells within the vascular wall are coupled by gap junctions, allowing for direct intercellular transfer of low molecular weight molecules. Although gap junctions are believed to be important for vascular development and function, their precise roles are not well understood. Mice lacking either connexin37 (Cx37) or connexin40 (Cx40), the predominant gap junction proteins present in vascular endothelium, are viable and exhibit phenotypes that are largely non-blood vessel related. Since Cx37 and Cx40 are coexpressed in endothelial cells and could overlap functionally, some roles of junctional communication may only be revealed by the elimination of both connexins. In this study, we interbreed Cx37 and Cx40 knockout mice to generate Cx37-/- Cx40-/- animals and show that they display severe vascular abnormalities and die perinatally. Cx37-/- Cx40-/- animals exhibit localized hemorrhages in skin, testis, gastrointestinal tissues, and lungs, with pronounced blood vessel dilatation and congestion occurring in some areas. Vascular anomalies were particularly striking in testis and intestine. In testis, abnormal vascular channels were present, with these channels coalescing into a cavernous, endothelium-lined blood pool resembling a hemangioma. These results provide evidence of a critical role for endothelial gap junction-mediated communication in the development and/or functional maintenance of segments of the mouse vasculature.

Connexin37 is the major connexin expressed in the media of caudal artery

OBJECTIVE

To determine the connexins (Cxs) involved in intercellular coupling within vascular muscle, the present study has quantified mRNA and protein expression for Cx37, Cx40, Cx43, and Cx45 in the caudal artery (CA) and thoracic aorta (ThA) of the rat.

METHODS AND RESULTS

Real-time polymerase chain reaction and immunohistochemistry identified Cx37 as the most abundantly expressed Cx in the CA, with fine punctate staining observed in the media. Conversely, mRNA for Cx43 was 40-fold greater in the ThA than in the CA, with punctate staining in the endothelium and media of the ThA but confined to the endothelium in the CA. Western blotting confirmed the differences in the relative amounts of Cx43 between the 2 vessels. For both arteries, Cx45 was expressed to a lesser degree in the media but not in the endothelium, whereas Cx40 was found only in the endothelium. Cx37, Cx40, and Cx43 were expressed in the endothelium of both vessels, although the density of Cx40 plaques was significantly greater in the CA.

CONCLUSIONS

The demonstration of Cx37 as the dominant Cx in the media of the CA highlights the potential heterogeneity in Cx involvement in vascular smooth muscle.

Gap junctions and connexin expression in human suburothelial interstitial cells

OBJECTIVE

To determine whether suburothelial interstitial cells of the human bladder express gap junctions, and if so, to establish their extent and composition, using immunocytochemistry, confocal microscopy and electron microscopy.

MATERIALS AND METHODS

Bladder tissue was obtained at cystectomy; the tissue was: (i) frozen for cryosectioning and Northern blot analysis; (ii) fixed and embedded for standard thin-section electron microscopy; and (iii) processed using low-denaturation conditions in Lowicryl for immunogold-label electron microscopy. Cryosections were immunofluorescently labelled using antibodies against connexins 43, 40 and 45, vimentin, desmin and c-Kit ligand, and examined by confocal microscopy. Double labelling was used to determine the spatial relationship of labelling for connexin43 with that of vimentin and desmin. Thin-section electron microscopy was used to investigate interstitial cell ultrastructure and permit unequivocal identification of gap junctions, and immunogold labelling of Lowicryl sections was applied to localize connexin43.

RESULTS

Immunoconfocal microscopy showed prominent labelling for the gap junction protein, connexin43, in a suburothelial band of cells that was also strongly positive for vimentin. The connexin43/vimentin-positive cells showed only weak labelling for desmin and c-Kit ligand, and were immunonegative for connexins 40 and 45. Northern blotting showed a corresponding abundance of connexin43 transcript in the mucosal layer but not the detrusor layer of the bladder wall. Electron microscopy revealed abundant gap junctions, recognized by their pentalaminar structure, between the cell processes of interstitial cells in the suburothelial zone. That these interstitial cell gap junctions were the source of the connexin43 immunolabelling observed by immunoconfocal microscopy was confirmed by immunogold labelling in sections of Lowicryl-embedded tissue examined by electron microscopy.

CONCLUSION

A network of interstitial cells, extensively linked by connexin43-containing gap junctions, is located beneath the urothelium in human bladder. As gap junctions provide pathways for direct cell-to-cell communication, the interstitial cellular network may operate as a functional syncytium, integrating signals and responses in the bladder wall.

Differential expression of connexin43 in foetal, adult and tumour-associated human brain endothelial cells

Connexin43 (Cx43), the main protein constituting the gap junctions between astrocytes, has previously been demonstrated in endothelial cells of somatic vessels where the intercellular coupling that it provides plays a role in endothelial proliferation and migration. In this study, Cx43 expression was analysed in human brain microvascular endothelial cells of the cortical plate of 18-week foetal telencephalon, in adult cerebral cortex and glioma (astrocytomas). The study was carried out by immunocytochemistry utilizing a Cx43 monoclonal antibody and a polyclonal antibody anti-GLUT1 (glucose transporter isoform 1) to identify the endothelial cells and to localize Cx43. Endothelial Cx43 is differently expressed in the cortical plate, cerebral cortex and astrocytoma. Within the cortical plate and tumour, Cx43 is highly expressed in microvascular endothelial cells whereas it is virtually absent in the cerebral cortex microvessels. The high expression of the gap junction protein in developing brain, as well as in brain tumours, may be related to the growth status of the microvessels during brain and tumour angiogenesis. The lack of endothelial Cx43 in the cerebral cortex is in agreement with the characteristics of the mature brain endothelial cells that are sealed by tight junctions. In conclusion, the results indicate that endothelial Cx43 expression is developmentally regulated in the normal human brain and suggest that it is controlled by the microenvironment in both normal and tumour-related conditions.

Cell-cell interactions in vascular development

Research into areas as divergent as hemangiopoiesis and cardiogenesis as well as investigations of diseases such as cancer and diabetic retinopathy have converged to form the face of research in vascular development today. This convergence of disparate topics has resulted in rapid advances in many areas of vascular research. The focus of this review has been the role of cell-cell interactions in the development of the vascular system, but we have included discussions of pathology where the mechanism of disease progression may have parallels with developmental processes. A number of intriguing questions remain unanswered. For example, what triggers abnormal angiogenesis in the disease state? Are the mechanisms similar to those that control developmental neovascularization? Perhaps the difference in development in angiogenesis versus in disease is context driven, that is, an adult versus an embryonic organism. If this is the case, can the controls that curtail developmental vessel formation be applied in pathologies? Can cell-cell interactions be targeted as a control point for new vessel formation? For instance, can perivascular cells be stimulated or eliminated to result in increased vessel stability or instability, respectively? If the hypothesis that mural cell association is required for vessel stabilization is accurate, are there mechanisms to promote or inhibit mural cell recruitment and differentiation as needed? These and other questions lie in wait for the next generation of approaches to discern the mechanisms and the nature of the cell-cell interactions and the influence of the microenvironment on vascular development.

Decreased endothelial size and connexin expression in rat caudal arteries during hypertension

OBJECTIVES

Hypertension is accompanied by endothelial dysfunction. The present study has investigated endothelial cell morphology and connexin expression in the caudal artery of the rat during the development of hypertension.

METHODS

A significant increase in systolic blood pressure was detected from 9 weeks of age in spontaneously hypertensive male rats (SHR) compared to normotensive Wistar-Kyoto (WKY) rats, reaching a maximum by 11-12 weeks of age. Immunohistochemistry was used to quantify cell size and expression of connexins (Cxs) 37, 40 and 43 in the endothelium of prehypertensive (3-week-old) and hypertensive (12-week-old) rats.

RESULTS

At 12 weeks, the size of endothelial cells and the expression of all three Cxs per endothelial cell were significantly less in SHR than WKY rats. At 3 weeks, there was no significant difference in cell size nor in the expression of Cxs 37 or 43; however, expression of Cx40 was significantly lower in SHR than in WKY rats. Between 3 and 12 weeks in WKY rats, there was no change in endothelial cell size, nor in the expression of Cxs 37, 40 and 43. In SHR, both cell size and Cx expression per endothelial cell were significantly decreased during the same developmental period, with a significant decrease in the density of Cx40 plaques.

CONCLUSION

The development of hypertension in the SHR is accompanied by significant decreases in endothelial cell size and expression of Cx40, which may contribute to the endothelial dysfunction present in hypertension.

Multiple connexins localized to individual gap-junctional plaques in human myometrial smooth muscle

The synchronous contractions of the uterus in labour depend on electrical coupling of myometrial smooth muscle cells by gap junctions. In the human myometrium, gap junctions are scarce in the non-pregnant uterus, but become abundant at term in preparation for labour. We have previously demonstrated that in the human myometrium at term, three different gap-junctional proteins are expressed, connexins 43, 45, and 40. These connexins are known to have distinctive functional capacities in in vitro expression systems but whether, in the human myometrium in vivo, they are co-assembled into the same gap junction or form different types of gap junction has previously been unclear. By applying triple immunogold labelling to sections of Lowicryl-embedded tissue for electron microscopy, together with complementary immunoconfocal microscopy, we demonstrate here that connexins 43, 45, and 40 are commonly present as mixtures within the same gap-junctional plaque. While all gap junctions contain connexin43, the relative signal for each connexin type varies between individual junctions. The presence within single gap-junctional plaques of three different connexins, each with the potential for conferring distinctive channel properties, suggests an inherent versatility for modulation of smooth muscle cell intercellular communication properties during human parturition.

Regional differentiation of desmin, connexin43, and connexin45 expression patterns in rat aortic smooth muscle

The gap-junctional protein, connexin43, is differentially expressed in vascular smooth muscle cells (SMCs) according to phenotype. Previous studies suggest that desmin-negative SMCs are characterized by high levels of connexin43, whereas desmin-positive SMCs (of a more contractile phenotype) typically have low connexin43 levels. In this study, we examine systematically the inverse relationship between connexin43 and desmin in SMCs of defined regions of the rat aortic media and determine whether additional connexin isotypes are expressed and contribute to this relationship. Immunoconfocal microscopy demonstrated that (1) the inverse relationship between connexin43 and desmin expression holds true for the media of sequential aortic zones, with 1 exception, the ascending aorta, and (2) an additional vascular connexin, connexin45, is expressed by aortic SMCs. Examination of connexin43, connexin45, and desmin expression in sequential aortic zones reveals 3 SMC subpopulations. The first, predominating in the aortic arch and thoracic aorta, is desmin negative and contains high connexin43 levels; the second, predominating in the abdominal aorta and iliac artery, is desmin positive and contains low connexin43 levels; and the third, which is restricted to the ascending aorta, is desmin positive and expresses high connexin43 levels. Connexin45 levels are high in the ascending aorta but low in the other aortic segments. In para-aortic veins, a fourth SMC subpopulation appears, one that is desmin positive and contains connexin45 but not connexin43. These results demonstrate that a diversity of connexin expression patterns characterizes distinctive subpopulations of medial SMCs in situ with a potential to contribute to regional differentiation of vascular function.

Immunocytochemical analysis of connexin expression in the healthy and diseased cardiovascular system

Gap junctions play essential roles in the normal function of the heart and arteries, mediating the spread of the electrical impulse that stimulates synchronized contraction of the cardiac chambers, and contributing to co-ordination of activities between cells of the arterial wall. In common with other multicellular systems, cardiovascular tissues express multiple connexin isotypes that confer distinctive channel properties. This review highlights how state-of-the-art immunocytochemical and cellular imaging techniques, as part of a multidisciplinary approach in gap junction research, have advanced our understanding of connexin diversity in cardiovascular cell function in health and disease. In the heart, spatially defined patterns of expression of three connexin isotypes-connexin43, connexin40, and connexin45-underlie the precisely orchestrated patterns of current flow governing the normal cardiac rhythm. Derangement of gap junction organization and/or reduced expression of connexin43 are associated with arrhythmic tendency in the diseased human ventricle, and high levels of connexin40 in the atrium are associated with increased risk of developing atrial fibrillation after coronary by-pass surgery. In the major arteries, endothelial gap junctions may simultaneously express three connexin isotypes, connexin40, connexin37, and connexin43; underlying medial smooth muscle, by contrast, predominantly expresses connexin43, with connexin45 additionally expressed at restricted sites. In normal arterial smooth muscle, the abundance of connexin43 gap junctions varies according to vascular site, and shows an inverse relationship with desmin expression and positive correlation with the quantity of extracellular matrix. Increased connexin43 expression between smooth muscle cells is closely linked to phenotypic transformation in early human coronary atherosclerosis and in the response of the arterial wall to injury. Current evidence thus suggests that gap junctions in both their guises, as pathways for cell-to-cell signaling in the vessel wall and as pathways for impulse conduction in the heart, contribute to the initial pathogenesis and eventual clinical manifestation of human cardiovascular disease.

Gap junction remodeling in hypertrophied left ventricles of aortic-banded rats: prevention by angiotensin II type 1 receptor blockade

Remodeling of gap-junctional organization in hypertrophied left ventricle (LV) in response to pressure overload in rats induced by abdominal aorta banding was investigated by immunoconfocal and electron microscopy. Eight to 12 weeks after banding, rats developed significant LV hypertrophy. In contrast to control LV myocytes, which showed connexin43 (Cx43) labeling largely confined to the intercalated disks, LV myocytes from aortic-banded rats showed dispersion of punctate Cx43 labeling over the entire cell surface. In LV tissues sectioned longitudinally, the proportion of Cx43 label at the intercalated disk decreased significantly (control, 0.87 v aortic-banded, 0.62). En-face views of intercalated disks of hypertrophied myocardium revealed a reduction of Cx43 gap junctions in the disk center, giving rise to a significant decrease in the proportion of the disk occupied by gap-junctional membrane (control, 0.32 v aortic-banded, 0.24). Electron microscopy of hypertrophied LV tissue revealed that Cx43-containing gap junctions were frequently displaced from their usual locations to form side-to-side contacts distant from the disk, and also appeared as annular profiles. In aortic-banded rats treated with the angiotensin II (AII) type 1 receptor (AT1) antagonist, losartan (10 mg/kg/day, 11 weeks) not only LV hypertrophy, but also the gap junction disorganization was markedly reduced. These results suggest that LV hypertrophy induced by pressure overload is associated with Cx43 gap junction disorganization and that AII may play an important role either directly or indirectly in gap-junctional remodeling.

Heterogeneous control of blood flow amongst different vascular beds

The control and maintenance of vascular tone is due to a balance between vasoconstrictor and vasodilator pathways. Vasomotor responses to neural, metabolic and physical factors vary between vessels in different vascular beds, as well as along the same bed, particularly as vessels become smaller. These differences result from variation in the composition of neurotransmitters released by perivascular nerves, variation in the array and activation of receptor subtypes expressed in different vascular beds and variation in the signal transduction pathways activated in either the vascular smooth muscle or endothelial cells. As the study of vasomotor responses often requires pre-existing tone, some of the reported heterogeneity in the relative contributions of different vasodilator mechanisms may be compounded by different experimental conditions. Biochemical variations, such as the expression of ion channels, connexin subtypes and other important components of second messenger cascades, have been documented in the smooth muscle and endothelial cells in different parts of the body. Anatomical variations, in the presence and prevalence of gap junctions between smooth muscle cells, between endothelial cells and at myoendothelial gap junctions, between the two cell layers, have also been described. These factors will contribute further to the heterogeneity in local and conducted responses.

Age-related alteration of gap junction distribution and connexin expression in rat aortic endothelium

We investigated endothelial gap junctions and their three component connexins, connexin37 (Cx37), Cx40, and Cx43, during growth and senescence in rat aorta by en face immunoconfocal microscopy and electron microscopy. Gap junction spots labeled by specific antisera against Cx37, Cx40, and Cx43 were quantified at 1 day, 7 days, 28 days, 16 months, and > or =20 months of age, and the relationship between the connexins was examined by co-localization analysis. At birth, all three connexins were abundantly expressed; the number and total area of connexin spots then declined within 1 week (p<0.05 for each connexin). From 1 week, each connexin showed a distinct temporal expression pattern. Whereas Cx43 signal decreased progressively, Cx37 signal fluctuated in a downward trend. By contrast, Cx40 maintained an abundant level until > or =20 months of age (> or =20 months vs. 28 days, p<0.05 for number and total connexin signal area). These patterns were associated with changes in endothelial cell morphology. Double-label analysis showed that the extent of co-localization of connexins to the same gap junctional spot was age-dependent [>70% at birth and 28 days old; <70% at later stages (p<0.05)]. We conclude that expression of the three connexins in aortic endothelium is age-related, implying specific intercellular communication requirements during different stages after birth.

Biosynthesis and structural composition of gap junction intercellular membrane channels

Gap junction channels assemble as dodecameric complexes, in which a hexameric connexon (hemichannel) in one plasma membrane docks end-to-end with a connexon in the membrane of a closely apposed cell to provide direct cell-to-cell communication. Synthesis, assembly, and trafficking of the gap junction channel subunit proteins referred to as connexins, largely appear to follow the general secretory pathway for membrane proteins. The connexin subunits can assemble into homo-, as well as distinct hetero-oligomeric connexons. Assembly appears to be based on specific signals located within the connexin polypeptides. Plaque formation by the clustering of gap junction channels in the plane of the membrane, as well as channel degradation are poorly understood processes that are topics of current research. Recently, we tagged connexins with the autofluorescent reporter green fluorescent protein (GFP), and its cyan (CFP), and yellow (YFP) color variants and combined this reporter technology with single, and dual-color, high resolution deconvolution microscopy, computational volume rendering, and time-lapse microscopy to examine the detailed organization, structural composition, and dynamics of gap junctions in live cells. This technology provided for the first time a realistic, three-dimensional impression of gap junctions as they appear in the plasma membranes of adjoining cells, and revealed an excitingly detailed structural organization of gap junctions never seen before in live cells. Here, I summarize recent progress in areas encompassing the synthesis, assembly and structural composition of gap junctions with a special emphasis on the recent results we obtained using cell-free translation/ membrane-protein translocation, and autofluorescent reporters in combination with live-cell deconvolution microscopy.

Multiple connexin expression in regenerating arterial endothelial gap junctions

Endothelial cells form gap junctions that, according to vessel type, may be composed of up to 3 types of connexin, connexin37, connexin40, and connexin43. Although changes in connexin expression have been linked to growth and injury in cultured endothelial cells, information on connexin expression in regenerating endothelium in situ is lacking. We investigated gap junction distribution and expression of all 3 endothelial connexins during healing in rat carotid artery after denudation injury. En face viewing of the vascular luminal surface by means of immunoconfocal microscopy was used to examine the spatial and temporal expression pattern of the endothelial connexins. Gap junction spots labeled by specific antisera against connexin37, connexin40, and connexin43 were quantified 7, 14, and 28 days after injury, and the relations among the connexins were examined by using colocalization analysis. Complementary electron microscopy was also conducted. After injury, the regenerating endothelium initially expressed small, sparse gap junctions, the numbers of which progressively increased to values equivalent to those of controls. Although connexin40 gap-junctional spot size and area returned to uninjured levels by 28 days after injury, connexin37 and connexin43 spot size and area exceeded those of the uninjured artery (P<0.05). Double-label analysis showed that even though colocalization of connexins to the same gap-junctional spot is a common feature, the extent of colocalization was time dependent (>80% in the intact artery at postinjury day 28 and <70% at postinjury days 7 and 14, P<0.01). We conclude that distinct alterations in expression of the 3 connexins are associated with regeneration of the arterial endothelium in situ, implying different intercellular communication requirements during the various phases of the healing process.

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.