Increased association of ZO-1 with connexin43 during remodeling of cardiac gap junctions

Ischemia impairs the association between connexin 43 and M3 subtype of acetylcholine muscarinic receptor (M3-mAChR) in ventricular myocytes

We used Western blot analysis to examine the expression of connexin 43 and M2/M3 acetylcholine muscarinic receptors (mAChR) and their interaction in ventricular myocytes from control and the ischemic heart. We confirmed that the connexin 43 and M2/ M3-mAChR were expressed in ventricular myocytes. Moreover, we showed that M3-mAChR was expressed in non-glycosylated (72 kDa) and glycosylated forms (115 kDa). Immunostaining showed that connexin 43 is closely associated with M3-mAChR in parts of cell membranes of myocytes. Immunoprecipitation of lysate of cardiac myocytes with M2/M3-mAChR antibody pulled down a 44 kDa protein recognized by connexin 43 antibody. Ischemia increased the expression of M3-mAChR in myocytes. The ischemiainduced increase in the M3-mAChR expression was specific because ischemia did not affect the expression of M1, M2, M4 and M5- mAChR in the heart. On the other hand, ischemia decreased the expression of connexin 43 in myocardium. We also examined the effect of ischemia on the interaction between M2/M3-mAChR and connexin 43. Ischemia suppressed the association of M3-mAChR with connexin 43 but did not affect the association of connexin 43 with M2-mAChR. Administration of choline before ischemia not only partially restored the expression of connexin 43 but also attenuated the ischemia-induced suppression of the association between connexin 43 and M3-mAChR. We conclude that connexin 43 interacts with M2/M3-mAChR and that ischemia specifically impairs the association between M3-mAChR and connexin 43.

Connexin43 levels are increased in mouse neural crest cells exposed to homocysteine

BACKGROUND

Elevated homocysteine levels during embryonic development can result in neural tube and cardiovascular defects. The mechanisms that underlie the toxic effect of homocysteine are largely unknown.

METHODS

We cultured mouse neural tube explants to study the effects of homocysteine on the migratory behavior of neural crest cells and on the levels of the gap junction protein Connexin43 (Cx43) and the actin- and Cx43-interacting protein ZO-1.

RESULTS

Homocysteine exposure resulted in a significantly augmented maximal migration distance (MMD). The level of Cx43 immunolabeling was 2 times higher in the cytoplasm and cell protrusions of neural crest cells in homocysteine-treated cultures than in control cultures. Furthermore, colocalization of Cx43 and ZO-1 was increased in neural crest cell protrusions by this treatment.

CONCLUSION

Increased Cx43 levels were previously shown to result in abnormal embryonic development. Our data raises the hypothesis that the embryotoxic effects of homocysteine may be mediated in part by its effects on Cx43 expression level and gap junction function in neural crest cells.

Side-to-side linking of myocardial cells in hypertrophic cardiomyopathy: whole heart microscopic observation with tangential sections

By cross-section or longitudinal section, it is difficult to investigate longitudinal features of myocardial cells in the whole heart. Here, introducing the use of tangential sections to obtain longitudinal aspect of myocardial cells in any part of myocardium, the authors evaluated myocardium in the left ventricle in 10 normal hearts and four hearts with hypertrophic cardiomyopathy (HCM). Tangential sections were obtained by peeling the superficial layer of myocardium. After peeling the whole surface, secondary deep layer was peeled. These procedures were repeated more than five times through the wall. Intercalated discs (ICD) were observed immunohistochemically with anti-N-cadherin and antidesmoplakin. In normal hearts, myocardial cells were cut longitudinally and ran parallel in tangential sections. They linked end-to-end with simple and regular ICD with average lengths of 120-130 microm and average sarcomere numbers of 56-65. In HCM hearts, many myocardial cells were cut almost longitudinally running approximately parallel in tangential sections. Myocardial cells frequently showed side-to-side linking characterized by skewed ICD, indistinct ICD counterparts, and longitudinally arranged ICD. Two young HCM hearts had circle-shaped ICD and vacuole-like structures highlighted by immunostaining for N-cadherin, which were actually extracellular structures comparable with irregular side-to-side linking. It is considered that side-to-side linking of myocardial cells is a characteristic microscopic feature in HCM rather than myocardial disarray.

Gap junctional communication in the male reproductive system

Male fertility is a highly controlled process that allows proliferation, meiosis and differentiation of male germ cells in the testis, final maturation in the epididymis and also requires functional male accessory glands: seminal vesicles, prostate and corpus cavernosum. In addition to classical endocrine and paracrine controls, mainly by gonadotropins LH and FSH and steroids, there is now strong evidence that all these processes are dependent upon the presence of homocellular or heterocellular junctions, including gap junctions and their specific connexins (Cxs), between the different cell types that structure the male reproductive tract. The present review is focused on the identification of Cxs, their distribution in the testis and in different structures of the male genital tract (epididymis, seminal vesicle, prostate, corpus cavernosum), their crucial role in the control of spermatogenesis and their implication in the function of the male accessory glands, including functional smooth muscle tone. Their potential dysfunctions in some testis (spermatogenic arrest, seminoma) and prostate (benign hyperplasia, adenocarcinoma) diseases and in the physiopathology of the human erectile function are also discussed.

Alterations of phosphorylation state of connexin 43 during hypoxia and reoxygenation are associated with cardiac function

Gap junctions formed by connexins mediate cell-cell communication by electrical and chemical coupling. Recently, it has been shown that alterations in the phosphorylation state of the connexins result in functional alteration of cell-cell communication through gap junctions. Therefore, we focused on the association of alterations of phosphorylation state of connexin 43 (Cx43) with cardiac function in vivo. Rat hearts were transferred to Langendorff apparatus and submitted to hypoxia and then reoxygenated. In the control heart, Cx43 was phosphorylated and located at the intercalated disk. When the hearts were subjected to hypoxia, Cx43 at gap junctions was dephosphorylated and changed its localization to the entire plasma membrane. The area of cardiomyocytes stained with anti-phosphorylated Cx43 antibody was decreased in a time-dependent manner. Immunoblot data supported the decrease of phosphorylated Cx43 during hypoxia. ZO-1 did not change its localization at the intercalated disk during the hypoxic period. We also found that the area occupied by dephosphorylated Cx43 was correlated with the decrease of percent of rate-pressure product. These data indicate that dephosphorylation and redistribution of Cx43 is an early sign of cardiac injury after hypoxia. Detection of dephosphorylated Cx43 may serve as a diagnostic tool for examining ischemic heart disease.

Connexin43 PDZ2 binding domain mutants create functional gap junctions and exhibit altered phosphorylation

Connexin43 (Cx43) is the most abundantly expressed gap junction protein. The C-terminal tail of Cx43 is important for regulation of gap junctions via phosphorylation of specific tyrosine and serine residues and through interactions with cellular proteins. The C-terminus of Cx43 has been shown to interact with the PDZ2 domain of the tight and adherens junction associated zona occludens 1 (ZO-1) protein. Analysis of the PDZ2 binding domain of Cx43 indicated that positions -3 and -2, and the final hydrophobic amino acid at the C-terminus, are critical for ZO-1 binding. In addition, the C-termini of connexins 40 and 45, but not Cx32, interacted with ZO-1. To evaluate the functional significance of the Cx43-ZO-1 interaction, Cx43 wild type (Cx43wt) and mutants lacking either the C-terminal hydrophobic isoleucine (Cx43deltaI382) or the last five amino acids (Cx43delta378-382), required for ZO-1 binding in vitro, were introduced into a Cx43-deficient MDCK cell line. In vitro binding studies and coimmunoprecipitation assays indicated that these Cx43 mutants failed to interact with ZO-1. Confocal and deconvolution microscopy revealed that a fraction of Cx43wt colocalized with ZO-1 at the plasma membrane. A similar colocalization pattern was observed for the Cx43deltaI382 and Cx43 delta378-382 mutants, which were translocated to the plasma membrane and formed functional gap junction channels. The wt and mutant Cx43 appeared to have similar turnover rates. However, the P2 and P3 phosphoisoforms of the Cx43 mutants were significantly reduced compared to Cx43wt. These studies indicated that the interaction of Cx43 with ZO-1 may contribute to the regulation of Cx43 phosphorylation.

Developmental regulation and expression of the zebrafish connexin43 gene

We cloned and sequenced the zebrafish (Danio rerio) connexin43 (Cx43alpha1) gene. The predicted protein sequence shows a high degree of sequence conservation. Transcript analyses revealed multiple transcription start sites and a potential alternative transcript encoding a N-terminally truncated Cx43alpha1 protein. Maternal Cx43alpha1 transcripts were detected, with zygotic expression initiated before gastrulation. In situ hybridization revealed many Cx43alpha1 expression domains, including the notochord and brain, heart and vasculature, many resembling patterns seen in mammalian embryos. Of interest, a reporter construct under control of the mouse Cx43alpha1 promoter was observed to drive green fluorescent protein expression in zebrafish embryos in domains mimicking the native Cx43alpha1 expression pattern in fish and mice. Sequence comparison between the mouse and zebrafish Cx43alpha1 promoter sequences showed the conservation of several transcription factor motifs, which otherwise shared little overall sequence homology. The conservation of protein sequence and developmental gene regulation would suggest that Cx43alpha1 gap junctions are likely to have conserved roles in vertebrate embryonic development.

Connexin43 deficiency causes dysregulation of coronary vasculogenesis

The connexin43 knockout (Cx43alpha1 KO) mouse dies at birth from outflow obstruction associated with infundibular pouches. To elucidate the origin of the infundibular pouches, we used microarray analysis to investigate gene expression changes in the pouch tissue. We found elevated expression of many genes encoding markers for vascular smooth muscle (VSM), endothelial cells, and fibroblasts, cell types that are epicardially derived and essential for coronary vasculogenesis. This was accompanied by increased expression of VEGF and genes in the TGFbeta and VEGF/Notch/Eph cell-signaling pathways known to regulate vasculogenesis/angiogenesis. Using immunohistochemistry and a VSM lacZ reporter gene, we confirmed an abundance of ectopic VSM and endothelial cells in the infundibular pouch and in some regions of the right ventricle forming secondary pouches. This was associated with distinct thinning of the compact myocardium. TUNEL labeling showed increased apoptosis in the pouch tissue, in agreement with the finding of altered expression of many apoptotic genes. Defects in vascular remodeling were indicated by a marked reduction in the branching complexity of the distal coronary arteries. In the near term KO mouse, we also observed a profusion of large coronary vascular plexuses subepicardially. This was associated with elevated epicardial expression of VEGF and abnormal epicardial cell morphology. Together, these observations indicate that dysregulated coronary vasculogenesis plays a pivotal role in formation of the infundibular pouches and suggests an essential role for Cx43alpha1 gap junctions in coronary vasculogenesis and vascular remodeling.

Zonula occludens-1 alters connexin43 gap junction size and organization by influencing channel accretion

Regulation of gap junction (GJ) organization is critical for proper function of excitable tissues such as heart and brain, yet mechanisms that govern the dynamic patterning of GJs remain poorly defined. Here, we show that zonula occludens (ZO)-1 localizes preferentially to the periphery of connexin43 (Cx43) GJ plaques. Blockade of the PDS95/dlg/ZO-1 (PDZ)-mediated interaction between ZO-1 and Cx43, by genetic tagging of Cx43 or by a membrane-permeable peptide inhibitor that contains the Cx43 PDZ-binding domain, led to a reduction of peripherally associated ZO-1 accompanied by a significant increase in plaque size. Biochemical data indicate that the size increase was due to unregulated accumulation of gap junctional channels from nonjunctional pools, rather than to increased protein expression or decreased turnover. Coexpression of native Cx43 fully rescued the aberrant tagged-connexin phenotype, but only if channels were composed predominately of untagged connexin. Confocal image analysis revealed that, subsequent to GJ nucleation, ZO-1 association with Cx43 GJs is independent of plaque size. We propose that ZO-1 controls the rate of Cx43 channel accretion at GJ peripheries, which, in conjunction with the rate of GJ turnover, regulates GJ size and distribution.

Patch-clamp study reveals that the importance of connexin43-mediated gap junctional communication for ovarian folliculogenesis is strain specific in the mouse

Genetic ablation of connexin37 (Cx37) or connexin43 (Cx43), the two gap junction proteins expressed by mouse ovarian granulosa cells, has been shown to result in impaired follicle development. We used patch-clamp techniques to evaluate quantitatively the contribution of these connexins to gap junctional intercellular communication (GJIC) among granulosa cells. The coupling conductance derived from a voltage step-induced capacitive current transient was used as a measure of GJIC in cultured granulosa cells. Using this method, we determined that the conductance of wild-type (84.1 +/- 28.6 nS; n = 6) and Cx37-deficient granulosa cells (83.7 +/- 6.4 nS; n = 11) does not differ significantly (P = 0.35), suggesting a limited contribution, if any, of Cx37 to granulosa cell coupling. In contrast, the conductance between granulosa cells of Cx43-deficient mice (2.6 +/- 0.8 nS; n = 5) was not significantly different from that of single, isolated wild-type granulosa cells (2.5 +/- 0.7 nS, n = 5; P = 0.83), indicating that Cx43-deficient granulosa cells were not electrically coupled. A direct measurement of transjunctional conductance between isolated granulosa cell pairs using a dual patch-clamp technique confirmed this conclusion. Interestingly, a partial rescue of folliculogenesis was observed when the Cx43-null mutation in C57BL/6 mice was crossed into the CD1 strain, and capacitive current measurement demonstrated that this rescue was not due to reestablishment of GJIC. These results demonstrate that folliculogenesis is impaired in the absence of GJIC between granulosa cells, but they also indicate that the severity is dependent on genetic background, a phenomenon that cannot be attributed to the expression of additional connexins.

Connexin 43 interacts with zona occludens-1 and -2 proteins in a cell cycle stage-specific manner

Gap junction channels play an important role in cell growth control, secretion and embryonic development. Gap junctional communication and channel assembly can be regulated by protein-protein interaction with kinases and phosphatases. We have utilized tandem mass spectrometry (MS/MS) sequence analysis as a screen to identify proteins from cell lysates that interact with the C-terminal cytoplasmic region of connexin 43 (Cx43). MS/MS analysis of tryptic fragments yielded several proteins including zona occludens-1 (ZO-1), a structural protein previously identified to interact with Cx43, and ZO-2, a potential novel interacting partner. We confirmed the interaction of ZO-2 with Cx43 by using a combination of fusion protein "pull down," co-immunoprecipitation, and co-localization experiments. We show that the C-terminal region of Cx43 is necessary for interaction with the PDZ2 domain of ZO-2. Far Western analysis revealed that ZO-2 can directly bind to Cx43 independent of other interacting partners. Immunofluorescence studies indicate that both ZO-1 and ZO-2 can co-localize with Cx43 within the plasma membrane at apparent gap junctional structures. We examined Cx43 interaction with ZO-1 and ZO-2 at different stages of the cell cycle and found that Cx43 had a strong preference for interaction with ZO-1 during G0, whereas ZO-2 interaction occurred approximately equally during G0 and S phases. Since essentially all of the Cx43 in G0 cells is assembled into Triton X-100-resistant junctions, Cx43-ZO-1 interaction may contribute to their stability.

Quantitative analysis of ZO-1 colocalization with Cx43 gap junction plaques in cultures of rat neonatal cardiomyocytes

The gap junction (GJ) is an aggregate of intercellular channels that facilitates cytoplasmic interchange of ions, second messengers, and other molecules of less than 1000 Da between cells. In excitable organs such as heart and brain, GJs configure extended intercellular pathways for stable and long-term propagation of action potential. In a previous study in adult rat heart, we have shown that the Drosophila disks-large related protein ZO-1 shows low to moderate colocalization at myocyte borders with the GJ protein Cx43. In the present study, we detail a protocol for characterizing the pattern and level of colocalization of ZO-1 with Cx43 in cultures of neonatal myocytes at the level of individual GJ plaques. The data indicate that ZO-1 shows on average a partial 26.6% overlap (SD = 11.3%) with Cx43 GJ plaques. There is a strong positive correlation between GJ plaque size and area of ZO-1 colocalization, indicating that the level of associated ZO-1 scales with the area of the GJ plaque. Qualitatively, the most prominent colocalization occurs at the plaque perimeter. These studies may provide insight into the presently unknown biological function of ZO-1 interaction with Cx43.

Imaging the cardiovascular system: seeing is believing

From the basic light microscope through high-end imaging systems such as multiphoton confocal microscopy and electron microscopes, microscopy has been and will continue to be an essential tool in developing an understanding of cardiovascular development, function, and disease. In this review we briefly touch on a number of studies that illustrate the importance of these forms of microscopy in studying cardiovascular biology. We also briefly review a number of imaging modalities such as computed tomography (CT), magnetic resonance imaging (MRI), ultrasound, and positron emission tomography (PET) that, although they do not fall under the realm of microscopy, are imaging modalities that greatly complement microscopy. Finally we examine the role of proper imaging system calibration and the potential importance of calibration in understanding biological tissues, such as the cardiovascular system, that continually undergo deformation in response to strain.

Connexin43 Associated with an N-cadherin-containing Multiprotein Complex Is Required for Gap Junction Formation in NIH3T3 Cells

Previous studies have indicated an intimate linkage between gap junction and adherens junction formation. It was suggested this could reflect the close membrane-membrane apposition required for junction formation. In NIH3T3 cells, we observed the colocalization of connexin43 (Cx43alpha1) gap junction protein with N-cadherin, p120, and other N-cadherin-associated proteins at regions of cell-cell contact. We also found that Cx43alpha1, N-cadherin, and N-cadherin-associated proteins were coimmunoprecipitated by antibodies to either Cx43alpha1, N-cadherin, or various N-cadherin-associated proteins. These findings suggest that Cx43alpha1 and N-cadherin are coassembled in a multiprotein complex containing various N-cadherin-associated proteins. Studies using siRNA knockdown indicated that cell surface expression of Cx43alpha1 required N-cadherin, and conversely, N-cadherin cell surface expression required Cx43alpha1. Pulse-chase labeling and cell surface biotinylation experiments indicated that in the absence of N-cadherin, Cx43alpha1 cell surface trafficking is blocked. Surprisingly, siRNA knockdown of p120, an N-cadherin-associated protein known to modulate cell surface turnover of N-cadherin, reduced N-cadherin cell surface expression without altering Cx43alpha1 expression. These observations suggest that in contrast to the coregulated cell surface trafficking of Cx43alpha1 and N-cadherin, N-cadherin turnover at the cell surface may be regulated independently of Cx43alpha1. Functional studies showed gap junctional communication is reduced and cell motility inhibited with N-cadherin or Cx43alpha1 knockdown, consistent with the observed loss of both gap junction and cadherin contacts with either knockdown. Overall, these studies indicate that the intracellular coassembly of connexin and cadherin is required for gap junction and adherens junction formation, a process that likely underlies the intimate association between gap junction and adherens junction formation.

News and views in Histochemistry and Cell Biology

Advances in histochemical methodology and ingenious applications of novel and improved methods continue to confirm the standing of morphological means and approaches in research efforts, and contribute significantly to increasing our knowledge about structures and functions in all areas of the life sciences from cell biology to pathology. Reports published during recent months documenting this progress are summarized in the present review.

Structural Changes in the Carboxyl Terminus of the Gap Junction Protein Connexin43 Indicates Signaling between Binding Domains for c-Src and Zonula Occludens-1

Regulation of cell-cell communication by the gap junction protein connexin43 can be modulated by a variety of connexin-associating proteins. In particular, c-Src can disrupt the connexin43 (Cx43)-zonula occludens-1 (ZO-1) interaction, leading to down-regulation of gap junction intercellular communication. The binding sites for ZO-1 and c-Src correspond to widely separated Cx43 domains (approximately 100 residues apart); however, little is known about the structural modifications that may allow information to be transferred over this distance. Here, we have characterized the structure of the connexin43 carboxyl-terminal domain (Cx43CT) to assess its ability to interact with domains from ZO-1 and c-Src. NMR data indicate that the Cx43CT exists primarily as an elongated random coil, with two regions of alpha-helical structure. NMR titration experiments determined that the ZO-1 PDZ-2 domain affected the last 19 Cx43CT residues, a region larger than that reported to be required for Cx43CT-ZO-1 binding. The c-Src SH3 domain affected Cx43CT residues Lys-264-Lys-287, Ser-306-Glu-316, His-331-Phe-337, Leu-356-Val-359, and Ala-367-Ser-372. Only region Lys-264-Lys-287 contains the residues previously reported to act as an SH3 binding domain. The specificity of these interactions was verified by peptide competition experiments. Finally, we demonstrated that the SH3 domain could partially displace the Cx43CT-PDZ-2 complex. These studies represent the first structural characterization of a connexin domain when integrated in a multimolecular complex. Furthermore, we demonstrate that the structural characteristics of a disordered Cx43CT are advantageous for signaling between different binding partners that may be important in describing the mechanism of channel closure or internalization in response to pathophysiological stimuli.

Fusion of GFP to the carboxyl terminus of connexin43 increases gap junction size in HeLa cells

The pattern of gap junctional coupling between cells is thought to be important for the proper function of many types of tissues. At present, little is known about the molecular mechanisms that control the size and distribution of gap junctions. We addressed this issue by expressing connexin43 (Cx43) constructs in HeLa cells, a connexin-deficient cell line. HeLa cells expressing exogenously introduced wild-type Cx43 formed small, punctate gap junctions. By contrast, cells expressing Cx43-GFP formed large, sheet-like gap junctions. These results suggest that the GFP tag, which is fused to the carboxyl terminus of Cx43, alters gap junction size by masking the carboxyl terminal amino acids of Cx43 that comprise a zonula occludins-1 (ZO-1) binding site. We are currently testing this hypothesis using deletion and dominant-negative constructs that directly target the interaction between Cx43 and ZO-1.

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.

Association of ARVCF with zonula occludens (ZO)-1 and ZO-2: binding to PDZ-domain proteins and cell-cell adhesion regulate plasma membrane and nuclear localization of ARVCF

ARVCF, an armadillo-repeat protein of the p120(ctn) family, associates with classical cadherins and is present in adherens junctions, but its function is poorly understood. Here, we show that ARVCF interacts via a C-terminal PDZ-binding motif with zonula occludens (ZO)-1 and ZO-2. ARVCF and ZO-1 partially colocalize in the vicinity of the apical adhesion complex in polarized epithelial Madin-Darby canine kidney cells. ARVCF, ZO-1, and E-cadherin form a complex and are recruited to sites of initial cell-cell contact in sparse cell cultures. E-cadherin binding and plasma membrane localization of ARVCF require the PDZ-binding motif. Disruption of cell-cell adhesion releases ARVCF from the plasma membrane and an increased fraction of the protein localizes to the nucleus. Nuclear localization of ARVCF also requires the PDZ-binding motif and can be mediated by the PDZ domains of ZO-2. Thus, the interaction of ARVCF with distinct PDZ-domain proteins determines its subcellular localization. Interactions with ZO-1 and ZO-2, in particular, may mediate recruitment of ARVCF to the plasma membrane and the nucleus, respectively, possibly in response to cell-cell adhesion cues.

Tyrosine-phosphorylated and Nonphosphorylated Sodium Channel β1 Subunits Are Differentially Localized in Cardiac Myocytes

Voltage-gated sodium channel alpha and beta subunits expressed in mammalian heart are differentially localized to t-tubules and intercalated disks. Sodium channel beta subunits are multifunctional molecules that participate in channel modulation and cell adhesion. Reversible, receptor-mediated changes in beta1 tyrosine phosphorylation modulate its ability to recruit and associate with ankyrin. The purpose of the present study was to test our hypothesis that tyrosine-phosphorylated beta1 (pYbeta1) and nonphosphorylated beta1 subunits may be differentially localized in heart and thus interact with different cytoskeletal and signaling proteins. We developed an antibody that specifically recognizes pYbeta1 and investigated the differential subcellular localization of beta1 and pYbeta1 in mouse ventricular myocytes. We found that pYbeta1 colocalized with connexin-43, N-cadherin, and Nav1.5 at intercalated disks but was not detected at the t-tubules. Anti-pYbeta1 immunoprecipitates N-cadherin from heart membranes and from cells transfected with beta1 and N-cadherin in the absence of other sodium channel subunits. pYbeta1 does not associate with ankyrinB in heart membranes. N-cadherin and connexin-43 associate with Nav1.5 in heart membranes as assessed by co-immunoprecipitation assays. We propose that sodium channel complexes at intercalated disks of ventricular myocytes are composed of Nav1.5 and pYbeta1 and that these complexes are in close association with both N-cadherin and connexin-43. beta1 phosphorylation appears to regulate its localization to differential subcellular domains.

Defective epidermal barrier in neonatal mice lacking the C-terminal region of connexin43

More than 97% of mice in which the C-terminal region of connexin43 (Cx43) was removed (designated as Cx43K258stop) die shortly after birth due to a defect of the epidermal barrier. The abnormal expression of Cx43K258stop protein in the uppermost layers of the epidermis seems to perturb terminal differentiation of keratinocytes. In contrast to Cx43-deficient mice, neonatal Cx43K258stop hearts show no lethal obstruction of the right ventricular outflow tract, but signs of dilatation. Electrocardiographies of neonatal hearts reveal repolarization abnormalities in 20% of homozygous Cx43K258stop animals. The very rare adult Cx43K258stop mice show a compensation of the epidermal barrier defect but persisting impairment of cardiac function in echocardiography. Female Cx43K258stop mice are infertile due to impaired folliculogenesis. Our results indicate that the C-terminally truncated Cx43K258stop mice lack essential functions of Cx43, although the truncated Cx43 protein can form open gap junctional channels.

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.

Nkx2-5 mutation causes anatomic hypoplasia of the cardiac conduction system

Heterozygous mutations of the cardiac transcription factor Nkx2-5 cause atrioventricular conduction defects in humans by unknown mechanisms. We show in KO mice that the number of cells in the cardiac conduction system is directly related to Nkx2-5 gene dosage. Null mutant embryos appear to lack the primordium of the atrioventricular node. In Nkx2-5 haploinsufficiency, the conduction system has half the normal number of cells. In addition, an entire population of connexin40(-)/connexin45(+) cells is missing in the atrioventricular node of Nkx2-5 heterozygous KO mice. Specific functional defects associated with Nkx2-5 loss of function can be attributed to hypoplastic development of the relevant structures in the conduction system. Surprisingly, the cellular expression of connexin40, the major gap junction isoform of Purkinje fibers and a putative Nkx2-5 target, is unaffected, consistent with normal conduction times through the His-Purkinje system measured in vivo. Postnatal conduction defects in Nkx2-5 mutation may result at least in part from a defect in the genetic program that governs the recruitment or retention of embryonic cardiac myocytes in the conduction system.

A proposed role for ZO-1 in targeting connexin 43 gap junctions to the endocytic pathway

Gap junctions are intercellular channels organized in plaque that directly link adjacent cells. Connexins (Cx), the constitutive proteins of gap junctions are associated with several partner proteins (cytoskeletal, anchoring) which could participate in plaque formation and degradation. Coimmunoprecipitation and indirect immunofluorescence analyses showed that ZO-1, a tight junction-associated protein, was linked to Cx43 in the testis. By using gamma-hexachlorocyclohexane (HCH), known to induce gap junction endocytosis, we demonstrated that endocytosis increased Cx43/ZO-1 association within the cytoplasm of treated Sertoli cells. In control cells, the two proteins were present, as expected, at the plasma membrane level, but poorly colocalized. The increased intracytoplasmic Cx43/ZO-1 complex was associated with a shift towards increased levels of Cx43 P1 and P2 isoforms. The HCH induced Cx43 hyperphosphorylation was abolished by the ERK inhibitor PD98059 suggesting that this effect could be mediated through activation of the ERK pathway. These data strongly support a novel role for ZO-1 in the turnover of Cx43 during gap junction plaque endocytosis.

Diversity in protein–protein interactions of connexins: emerging roles

Gap junctions, specialised membrane structures that mediate cell-to-cell communication in almost all tissues, are composed of channel-forming integral membrane proteins termed connexins. The activity of these intercellular channels is closely regulated, particularly by intramolecular modifications as phosphorylations of proteins by protein kinases, which appear to regulate the gap junction at several levels, including assembly of channels in the plasma membrane, connexin turnover as well as directly affecting the opening and closure ("gating") of channels. The regulation of membrane channels by protein phosphorylation/dephosphorylation processes commonly requires the formation of a multiprotein complex, where pore-forming subunits bind to auxiliary proteins (e.g. scaffolding proteins, catalytic and regulatory subunits), that play essential roles in channel localisation and activity, linking signalling enzymes, substrates and effectors into a structure frequently anchored to the cytoskeleton. The present review summarises the up-to-date progress regarding the proteins capable of interacting or at least of co-localising with connexins and their functional importance.

Targeted activation of c-Jun N-terminal kinase in vivo induces restrictive cardiomyopathy and conduction defects

The stress-activated protein kinase, c-Jun N-terminal kinase (JNK), has been implicated in the process of cardiac hypertrophy and apoptosis, yet the specific roles of JNK in heart failure are unclear. To determine the effects of JNK activation in intact heart, we established transgenic animals using a Cre/loxP-mediated gene switch approach to achieve targeted expression of an upstream activator, mitogen-activated protein kinase kinase 7 (D) (MKK7D), in ventricular myocytes. MKK7D expression led to significant JNK activation, robust induction of the fetal gene program, and contractile dysfunction. The animals died approximately 7 weeks after birth with signs of congestive heart failure. Doppler mode echocardiography revealed a marked stiffening of JNK-activated hearts that was associated with the remodeling of specific extracellular matrix components. Gene expression analysis of MKK7D hearts revealed up-regulation of transforming growth factor beta signaling, offering a potential molecular mechanism underlying changes in extracellular matrix composition. In addition, we demonstrated that JNK activation led to specific loss of connexin 43 protein and gap junctions without affecting the expression or localization of other key intercalated disc proteins. This specific and localized gap junction remodeling resulted in significant slowing of ventricular electrical conduction in JNK-activated hearts. These results represent the first characterization of JNK-mediated cardiac pathology in vivo and support an important role for JNK signaling in specific aspects of cardiac remodeling in the pathogenesis of cardiac disease.

Les MAGUK : au-delà de l’accrochage des canaux ioniques

A family of anchoring proteins named MAGUK (for membrane associated guanylate kinase) has emerged as a key element in the organization of protein complexes in specialized membrane regions. These proteins are characterized by the presence of multipe protein-protein interaction domains including PDZ and SH3 domains. The MAGUK family comprises the post-synaptic density 95 (PSD-95) protein and closely related molecules such as chapsyn-110, synapse-associated protein 102 (SAP-102), and SAP-97. These are located either on the pre- and/or post-synaptic sides of synapses or at cell-cell adhesion sites of epithelial cells. MAGUK proteins interact with glutamate receptors and various ionic channels. For instance, an interaction has been reported between the first two PDZ domains of MAGUK proteins and several channels via a consensus sequence Thr/Ser-X-Val/Leu usually located at their carboxy terminus. The role of these anchoring proteins in channel function is not fully understood. MAGUK proteins enhance the current density by increasing the number of functional channels to the sarcolemma. They can also facilitate signaling between channels and several enzymes or G protein-dependent signaling pathways. In the heart also, MAGUK proteins are abundantly expressed and they interact with various channels including Shaker Kv1.5 and connexins.

Effects of 18beta-glycyrrhetinic acid on the junctional complex and steroidogenesis in rat adrenocortical cells

Cellular junctions play important roles in cell differentiation, signal transduction, and cell function. This study investigated their function in steroid secretion by adrenal cells. Immunofluorescence staining revealed the presence of gap junctions and adherens junctions between adrenal cells. The major gap junction protein, connexin43, was seen as a linear dotted pattern of the typical gap junction plaques, in contrast to alpha-, beta-, and gamma-catenin, which were seen as continuous, linear staining of cell-cell adherens junction. Treatment with 18beta-glycyrrhetinic acid, a gap junction inhibitor, reduced the immunoreactivity of these proteins in a time- and dose-dependent manner, and caused the gap junction and adherens junction to separate longitudinally from the cell-cell contact sites, indicating the structural interdependency of these two junctions. Interestingly, 18beta-glycyrrhetinic acid stimulated a two- to three-fold increase in steroid production in these adrenal cells lacking intact cell junctions. These data raise the question of the necessity for cell communication for the endocrine function of adrenal cells. Pharmacological analyses indicated that the steroidogenic effect of 18beta-glycyrrhetinic acid was partially mediated by extracellular signal-related kinase and calcium/calmodulin-dependent kinase, a pathway distinct from the protein kinase A signaling pathway already known to mediate steroidogenesis in adrenal cells.

Evidence for long-lasting cholinergic control of gap junctional communication between adrenal chromaffin cells

We investigated long-lasting interactions that may occur between two forms of intercellular signaling: cholinergic synaptic transmission and gap junction-mediated coupling in the rat adrenal medulla. The junctional coupling between chromaffin cells was studied during reduced or blocked synaptic transmission in adrenal slices. First, cholinergic synaptic activity was reduced by pharmacological treatment. Bath-application of the nicotinic receptor antagonists hexamethonium, the oxystilbene derivative F3, or alpha-bungarotoxin, acting at distinct neuronal-like postsynaptic nicotinic acetylcholine receptors (nAChRs), significantly increased the incidence of Lucifer yellow passage (dye coupling) between chromaffin cells (p > 0.7 in treated slices vs p = 0.4 in controls). Dye coupling was associated with an elevated macroscopic conductance of the junctional current measured by dual patch-clamp. Pharmacological inhibition of protein trafficking from the trans-Golgi network to the plasma membrane by either brefeldin A or nocodazole pretreatment prevented the effects of nAChR antagonists on dye coupling. Interestingly, this upregulation of gap junction-mediated coupling in response to reduced synaptic activity is of physiological relevance, because it is found in the newborn rat, in which cholinergic synaptic transmission has not yet matured. This mechanism may also be of importance in pathological conditions, because chronic blockade of synaptic transmission after surgical denervation of the adrenal gland also resulted in increased dye coupling between chromaffin cells. In conclusion, our pharmacological, physiological, and pathological data concur to demonstrate that gap junction-mediated intercellular communication between chromaffin cells undergoes persistent adaptation in response to impairment of synaptic activity. These results strongly suggest that gap junctional communication between chromaffin cells is under tonic inhibitory control exerted by cholinergic synaptic inputs.

Lens connexins alpha3Cx46 and alpha8Cx50 interact with zonula occludens protein-1 (ZO-1)

Connexin alpha1Cx43 has previously been shown to bind to the PDZ domain-containing protein ZO-1. The similarity of the carboxyl termini of this connexin and the lens fiber connexins alpha3Cx46 and alpha8Cx50 suggested that these connexins may also interact with ZO-1. ZO-1 was shown to be highly expressed in mouse lenses. Colocalization of ZO-1 with alpha3Cx46 and alpha8Cx50 connexins in fiber cells was demonstrated by immunofluorescence and by fracture-labeling electron microscopy but showed regional variations throughout the lens. ZO-1 was found to coimmunoprecipitate with alpha3Cx46 and alpha8Cx50, and pull-down experiments showed that the second PDZ domain of ZO-1 was involved in this interaction. Transiently expressed alpha3Cx46 and alpha8Cx50 connexins lacking the COOH-terminal residues did not bind to the second PDZ domain but still formed structures resembling gap junctions by immunofluorescence. These results indicate that ZO-1 interacts with lens fiber connexins alpha3Cx46 and alpha8Cx50 in a manner similar to that previously described for alpha1Cx43. The spatial variation in the interaction of ZO-1 with lens gap junctions is intriguing and is suggestive of multiple dynamic roles for this association.

The organization of adherens junctions and desmosomes at the cardiac intercalated disc is independent of gap junctions

Adherens junctions and desmosomes are responsible for mechanically coupling myocytes in the heart and are found closely apposed to gap junction plaques at the intercalated discs of cardiomyocytes. It is not known whether loss of cardiac gap junctions, such as described in cardiac disease states, may influence the expression patterns of other intercalated disc-associated proteins. We investigated whether the major cardiac gap junction protein connexin43 (Cx43) may be responsible for regulating adherens junctions, desmosomes and their associated catenins, in terms of abundance and localization at the intercalated discs of cardiomyocytes. In order to study the effect of loss of cardiac gap junctions on the intercalated disc-associated proteins, we used a combination of immunoblotting, immunofluorescence with confocal microscopy and electron microscopy to evaluate heart tissue from mice with cardiac-specific conditional knockout of Cx43. We found that the cardiac adherens junctions, desmosomes and their associated catenins, as well as vinculin and ZO-1, maintain their normal abundance, structural appearance and localization in the absence of Cx43. We conclude from these data that Cx43 is not required for the organization of the cell adhesion junctions and their associated catenins at the intercalated disc in the adult cardiac myocyte.

Tight junction proteins

A fundamental function of epithelia and endothelia is to separate different compartments within the organism and to regulate the exchange of substances between them. The tight junction (TJ) constitutes the barrier both to the passage of ions and molecules through the paracellular pathway and to the movement of proteins and lipids between the apical and the basolateral domains of the plasma membrane. In recent years more than 40 different proteins have been discovered to be located at the TJs of epithelia, endothelia and myelinated cells. This unprecedented expansion of information has changed our view of TJs from merely a paracellular barrier to a complex structure involved in signaling cascades that control cell growth and differentiation. Both cortical and transmembrane proteins integrate TJs. Among the former are scaffolding proteins containing PDZ domains, tumor suppressors, transcription factors and proteins involved in vesicle transport. To date two components of the TJ filaments have been identified: occludin and claudin. The latter is a protein family with more than 20 members. Both occludin and claudins are integral proteins capable of interacting adhesively with complementary molecules on adjacent cells and of co-polymerizing laterally. These advancements in the knowledge of the molecular structure of TJ support previous physiological models that exhibited TJ as dynamic structures that present distinct permeability and morphological characteristics in different tissues and in response to changing natural, pathological or experimental conditions.

Sharing signals: connecting lung epithelial cells with gap junction channels

Gap junction channels enable the direct flow of signaling molecules and metabolites between cells. Alveolar epithelial cells show great variability in the expression of gap junction proteins (connexins) as a function of cell phenotype and cell state. Differential connexin expression and control by alveolar epithelial cells have the potential to enable these cells to regulate the extent of intercellular coupling in response to cell stress and to regulate surfactant secretion. However, defining the precise signals transmitted through gap junction channels and the cross talk between gap junctions and other signaling pathways has proven difficult. Insights from what is known about roles for gap junctions in other systems in the context of the connexin expression pattern by lung cells can be used to predict potential roles for gap junctional communication between alveolar epithelial cells.