Diminished zonula occludens-1 expression in the failing human heart

Altered Expression of Zonula occludens-1 Affects Cardiac Na+ Channels and Increases Susceptibility to Ventricular Arrhythmias

Zonula occludens-1 (ZO-1) is an intracellular scaffolding protein that orchestrates the anchoring of membrane proteins to the cytoskeleton in epithelial and specialized tissue including the heart. There is clear evidence to support the central role of intracellular auxiliary proteins in arrhythmogenesis and previous studies have found altered ZO-1 expression associated with atrioventricular conduction abnormalities. Here, using human cardiac tissues, we identified all three isoforms of ZO-1, canonical (Transcript Variant 1, TV1), CRA_e (Transcript Variant 4, TV4), and an additionally expressed (Transcript Variant 3, TV3) in non-failing myocardium. To investigate the role of ZO-1 on ventricular arrhythmogenesis, we generated a haploinsufficient ZO-1 mouse model (ZO-1^+/-). ZO-1^+/- mice exhibited dysregulated connexin-43 protein expression and localization at the intercalated disc. While ZO-1^+/- mice did not display abnormal cardiac function at baseline, adrenergic challenge resulted in rhythm abnormalities, including premature ventricular contractions and bigeminy. At baseline, ventricular myocytes from the ZO-1^+/- mice displayed prolonged action potential duration and spontaneous depolarizations, with ZO-1^+/- cells displaying frequent unsolicited (non-paced) diastolic depolarizations leading to spontaneous activity with multiple early afterdepolarizations (EADs). Mechanistically, ZO-1 deficient myocytes displayed a reduction in sodium current density (I_Na) and an increased sensitivity to isoproterenol stimulation. Further, ZO-1 deficient myocytes displayed remodeling in I_Ca current, likely a compensatory change. Taken together, our data suggest that ZO-1 deficiency results in myocardial substrate susceptible to triggered arrhythmias.

Understanding the molecular basis of cardiomyopathy

Inherited cardiomyopathies are a major cause of mortality and morbidity worldwide and can be caused by mutations in a wide range of proteins located in different cellular compartments. The present review is based on Dr. Ju Chen's 2021 Robert M. Berne Distinguished Lectureship of the American Physiological Society Cardiovascular Section, in which he provided an overview of the current knowledge on the cardiomyopathy-associated proteins that have been studied in his laboratory. The review provides a general summary of the proteins in different compartments of cardiomyocytes associated with cardiomyopathies, with specific focus on the proteins that have been studied in Dr. Chen's laboratory.

Decrease of Pdzrn3 is required for heart maturation and protects against heart failure

Heart failure is the final common stage of most cardiopathies. Cardiomyocytes (CM) connect with others via their extremities by intercalated disk protein complexes. This planar and directional organization of myocytes is crucial for mechanical coupling and anisotropic conduction of the electric signal in the heart. One of the hallmarks of heart failure is alterations in the contact sites between CM. Yet no factor on its own is known to coordinate CM polarized organization. We have previously shown that PDZRN3, an ubiquitine ligase E3 expressed in various tissues including the heart, mediates a branch of the Planar cell polarity (PCP) signaling involved in tissue patterning, instructing cell polarity and cell polar organization within a tissue. PDZRN3 is expressed in the embryonic mouse heart then its expression dropped significantly postnatally corresponding with heart maturation and CM polarized elongation. A moderate CM overexpression of Pdzrn3 (Pdzrn3 OE) during the first week of life, induced a severe eccentric hypertrophic phenotype with heart failure. In models of pressure-overload stress heart failure, CM-specific Pdzrn3 knockout showed complete protection against degradation of heart function. We reported that Pdzrn3 signaling induced PKC ζ expression, c-Jun nuclear translocation and a reduced nuclear ß catenin level, consistent markers of the planar non-canonical Wnt signaling in CM. We then show that subcellular localization (intercalated disk) of junction proteins as Cx43, ZO1 and Desmoglein 2 was altered in Pdzrn3 OE mice, which provides a molecular explanation for impaired CM polarization in these mice. Our results reveal a novel signaling pathway that controls a genetic program essential for heart maturation and maintenance of overall geometry, as well as the contractile function of CM, and implicates PDZRN3 as a potential therapeutic target for the prevention of human heart failure.

Tight junction structure, function, and assessment in the critically ill: a systematic review

BACKGROUND

Epithelial and endothelial barrier integrity, essential for homeostasis, is maintained by cellular boarder structures known as tight junctions (TJs). In critical illness, TJs may become disrupted, resulting in barrier dysfunction manifesting as capillary leak, pulmonary edema, gut bacterial translocation, and multiple organ failure. We aim to provide a clinically focused overview of TJ structure and function and systematically review and analyze all studies assessing markers of endothelial and epithelial TJ breakdown correlated with clinical outcomes in critically ill humans.

METHODS

We systematically searched MEDLINE, EMBASE, and PubMed. Additional articles were identified by targeted searches. We included studies that looked at the relationship between biomarkers of endothelial or epithelial TJ structure or function and critical illness. Results were qualitatively analyzed due to sample size and heterogeneity.

RESULTS

A total of 5297 abstracts met search criteria, of which 150 articles met requirements for full text review. Of these, 30 studies met inclusion criteria. Fifteen of the 30 reports investigated proteins of endothelial tight junctions and 15 investigated epithelial TJ markers, exclusively in the gastrointestinal epithelium. No studies investigated TJ-derived proteins in primary cardiac or pulmonary pathology.

CONCLUSIONS

TJ integrity is essential for homeostasis. We identified multiple studies that indicate TJs are disrupted by critical illness. These studies highlight the significance of barrier disruption across many critical disease states and correlate TJ-associated markers to clinically relevant outcomes. Further study on the role of multiple tissue-specific claudins, particularly in the setting of respiratory or cardiac failure, may lead to diagnostic and therapeutic advances.

SYSTEMATIC REVIEW REGISTRATION

This systematic review is registered in the PROSPERO database: CRD42017074546 .

At the heart of inter- and intracellular signaling: the intercalated disc

Proper cardiac function requires the synchronous mechanical and electrical coupling of individual cardiomyocytes. The intercalated disc (ID) mediates coupling of neighboring myocytes through intercellular signaling. Intercellular communication is highly regulated via intracellular signaling, and signaling pathways originating from the ID control cardiomyocyte remodeling and function. Herein, we present an overview of the inter- and intracellular signaling that occurs at and originates from the intercalated disc in normal physiology and pathophysiology. This review highlights the importance of the intercalated disc as an integrator of signaling events regulating homeostasis and stress responses in the heart and the center of several pathophysiological processes mediating the development of cardiomyopathies.

Connexin43 and zonula occludens-1 are targets of Akt in cardiomyocytes that correlate with cardiac contractile dysfunction in Akt deficient hearts

While deletion of Akt1 results in a smaller heart size and Akt2^-/- mice are mildly insulin resistant, Akt1^-/-/Akt2^-/- mice exhibit perinatal lethality, indicating a large degree of functional overlap between the isoforms of the serine/threonine kinase Akt. The present study aimed to determine the cooperative contribution of Akt1 and Akt2 on the structure and contractile function of adult hearts. To generate an inducible, cardiomyocyte-restricted Akt2 knockout (KO) model, Akt2^flox/flox mice were crossed with tamoxifen-inducible MerCreMer transgenic (MCM) mice and germline Akt1^-/- mice to generate the following genotypes:Akt1^+/+; Akt2^flox/flox (WT), Akt2^flox/flox; α-MHC-MCM (iAkt2 KO), Akt1^-/-, and Akt1^-/-; Akt2^flox/flox; α-MHC-MCM mice (Akt1^-/-/iAkt2 KO). At 28 days after the first tamoxifen injection, Akt1^-/-/iAkt2 KO mice developed contractile dysfunction paralleling increased atrial and brain natriuretic peptide (ANP and BNP) levels, and repressed mitochondrial gene expression. Neither cardiac fibrosis nor apoptosis were detected in Akt1^-/-/iAkt2 KO hearts. To explore potential molecular mechanisms for contractile dysfunction, we investigated myocardial microstructure before the onset of heart failure. At 3 days after the first tamoxifen injection, Akt1^-/-/iAkt2 KO hearts showed decreased expression of connexin43 (Cx43) and connexin-interacting protein zonula occludens-1 (ZO-1). Furthermore, Akt1/2 silencing significantly decreased both Cx43 and ZO-1 expression in cultured neonatal rat cardiomyocytes in concert with reduced beating frequency. Akt1 and Akt2 are required to maintain cardiac contraction. Loss of Akt signaling disrupts gap junction protein, which might precipitate early contractile dysfunction prior to heart failure in the absence of myocardial remodeling, such as hypertrophy, fibrosis, or cell death.

Decreased mRNA levels of cardiac Cx43 and ZO1 in sudden cardiac death related to coronary atherosclerosis: a pilot study

Sudden cardiac death (SCD) is the most frequent cause of sudden unexplained death in forensic practice. The most common cause of SCD is coronary artery disease related to coronary atherosclerosis. Previous study suggested the possible application of connexin 43 (Cx43) and zonula occludens-1 (ZO1) immunostaining in the early diagnosis of myocardial ischemia. However, there appears to be insufficient data with regard to their mRNA levels. The present study investigated the cardiac mRNA levels of Cx43 and ZO1, using forensic autopsy materials consisting of 41 control cases without any disease or structural abnormality of the heart (group 1), 32 deaths due to acute ischemic heart disease related to coronary atherosclerosis without apparent myocardial necrosis (group 2), and 29 traumatic deaths with coronary atherosclerosis (group 3). Ten candidate reference genes were evaluated in the left ventricles of 10 forensic autopsy cases. EEF1A1, PPIA, TPT1, and RPL13A were identified as the most stable reference genes. Using these validated reference genes, mRNA levels of Cx43 and ZO1 were examined in the bilateral ventricles and atria of the heart. Relative mRNA quantification demonstrated decreased calibrated normalized relative quantity (CNRQ) values of Cx43 and ZO1 in bilateral ventricles of group 2. When using one conventional reference gene (GAPDH or ACTB) for normalization, nearly no difference was detected among the three groups. These findings indicate that ventricular gap junction remodeling may be a key contributor to rhythm disturbances. Analysis of cardiac Cx43 and ZO1 using real-time PCR is useful in diagnosis of SCD, and validation of reference genes is crucial.

Chronic hypoxia down-regulates tight junction protein ZO-2 expression in children with cyanotic congenital heart defect

Aims

Tight junction protein zonula occludens protein 2 (ZO‐2) is a member of the membrane‐associated guanylate kinases protein family known to be expressed at tight junctions of epithelial and endothelial cells and at adherens junctions (AJs) in cardiomyocytes. Little is known about ZO‐2 expression and function in the human heart. Here, we examined the hypothesis that chronic hypoxia down‐regulates ZO‐2 expression in human myocardium and cultured rat cardiomyocytes.

Methods and results

Patients with a diagnosis of cyanotic (n = 10) or acyanotic (n = 10) Tetralogy of Fallot undergoing surgical repair were used to examine ZO‐2 messenger RNA and protein expression by real time‐PCR, immunohistochemistry, and western blotting. A model of cultured rat cardiomyocytes was used to measure ZO‐2 and AJ proteins levels in response to hypoxia and to investigate ZO‐2 cellular localization. We showed that ZO‐2 is expressed in myocardial tissue in acyanotic and cyanotic children with congenital heart defects. ZO‐2 was specifically down‐regulated in cyanotic myocardium at both the messenger RNA and protein levels when compared with acyanotic patients. This specific down‐regulation can be mimicked in cultured rat cardiomyocytes by treating them with hypoxic conditions confirming that ZO‐2 gene down‐regulation is specifically due to cyanosis. Furthermore, in addition to its cytoplasmic expression, ZO‐2 showed nuclear expression in cultured rat cardiomyocytes suggesting potential role in transcription regulation.

Conclusions

Hypoxia down‐regulates ZO‐2 expression in both cyanotic patient's myocardium and cultured rat cardiomyocytes. This down‐regulation suggest an involvement of ZO‐2 in cardiac remodelling of AJs in cyanotic children and may explain the greater susceptibility of cyanotic patients to corrective heart surgery.

Exploring transcriptomic diversity in muscle revealed that cellular signaling pathways mainly differentiate five Western porcine breeds

Background

Among transcriptomic studies, those comparing species or populations can increase our understanding of the impact of the evolutionary forces on the differentiation of populations. A particular situation is the one of short evolution time with breeds of a domesticated species that underwent strong selective pressures. In this study, the gene expression diversity across five pig breeds has been explored in muscle. Samples came from: 24 Duroc, 33 Landrace, 41 Large White dam line, 10 Large White sire line and 39 Piétrain. From these animals, 147 muscle samples obtained at slaughter were analyzed using the porcine Agilent 44 K v1 microarray.

Results

A total of 12,358 genes were identified as expressed in muscle after normalization and 1,703 genes were declared differential for at least one breed (FDR < 0.001). The functional analysis highlighted that gene expression diversity is mainly linked to cellular signaling pathways such as the PI3K (phosphoinositide 3-kinase) pathway. The PI3K pathway is known to be involved in the control of development of the skeletal muscle mass by affecting extracellular matrix - receptor interactions, regulation of actin cytoskeleton pathways and some metabolic functions. This study also highlighted 228 spots (171 unique genes) that differentiate the breeds from each other. A common subgroup of 15 genes selected by three statistical methods was able to differentiate Duroc, Large White and Piétrain breeds.

Conclusions

This study on transcriptomic differentiation across Western pig breeds highlighted a global picture: mainly signaling pathways were affected. This result is consistent with the selection objective of increasing muscle mass. These transcriptional changes may indicate selection pressure or simply breed differences which may be driven by human selection. Further work aiming at comparing genetic and transcriptomic diversities would further increase our understanding of the consequences of human impact on livestock species.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-2259-9) contains supplementary material, which is available to authorized users.

Inhibition of PKC-θ preserves cardiac function and reduces fibrosis in streptozotocin-induced diabetic cardiomyopathy

BACKGROUND AND PURPOSE

T-cell infiltration, interstitial fibrosis and cardiac dysfunction have been observed in diabetic patients with cardiovascular diseases. PKC-θ is crucial for the activation of mature T-cells. We hypothesized that inhibition of PKC-θ might protect diabetic hearts through inhibition of T-cell stimulation and maintenance of tight junction integrity.

EXPERIMENTAL APPROACH

A model of type 1 diabetes was induced by streptozotocin (STZ) (50 mg kg(-1) for 5 days) in male C57BL/6J wild-type (WT) mice and Rag1 knockout (KO) mice which lack mature lymphocytes. A cell-permeable selective PKC-θ peptide inhibitor (PI) was administered i.p. (0.2 mg kg(-1) ·day(-1) ) for 4 weeks (first phase) and 2 weeks (second phase). At the end of the 11th week, cardiac contractile force was measured in isolated perfused hearts. Cardiac morphology and fibrosis were determined. Phosphorylation of PKC-θ at Tyr(358) , infiltrated T-cells and tight junction protein ZO-1 within the hearts were detected, using immunohistochemcial techniques.

KEY RESULTS

PI did not affect high blood glucose level in both WT and Rag1 KO diabetic mice. Diabetes induced cardiac fibrosis in WT mice but not in Rag1 KO mice. PI attenuated cardiac fibrosis and improved cardiac contractility of WT diabetic hearts. PI decreased expression of phosphorylated PKC-θ, reduced the infiltration of T-cells and increased ZO-1 expression within WT diabetic hearts.

CONCLUSION AND IMPLICATIONS

Inhibition of PKC-θ improves cardiac function and reduces cardiac fibrosis in WT mice with streptozotocin-induced diabetes. Mature T-cells play a key role in pathophysiology of diabetic cardiomyopathy.

Characterization of the connexin45 carboxyl-terminal domain structure and interactions with molecular partners

Mechanisms underlying the initiation and persistence of lethal cardiac rhythms are of significant clinical and scientific interests. Gap junctions are principally involved in forming the electrical connections between myocytes, and changes in distribution, density, and properties are consistent characteristics in arrhythmic heart disease. Therefore, understanding the structure and function of gap junctions during normal and abnormal impulse propagation are essential in the control of arrhythmias. For example, Cx45 is predominately expressed in the specialized myocytes of the impulse generation and conduction system. In both ventricular and atrial human working myocytes, Cx45 is present in very low quantities. However, a reduction in Cx43 coupled with an increased Cx45 protein levels within the ventricles have been observed after myocardial infarction and end-stage heart failure. Cx45 may influence electrical and/or metabolic coupling as a result of pathophysiological overexpression. Our goal was to identify mechanisms that could cause cellular coupling to be different between the cardiac connexins. Based upon the conserved transmembrane and extracellular loop segments, our focus was on identifying features within the divergent cytoplasmic portions. Here, we biophysically characterize the carboxyl-terminal domain of Cx45 (Cx45CT). Purification revealed the possibility of oligomeric species, which was confirmed by analytical ultracentrifugation experiments. Sedimentation equilibrium and circular dichroism studies of different Cx45CT constructs identified one region of α-helical structure (A333-N361) that mediates CT dimerization through hydrophobic contacts. Interestingly, the binding affinity of Cx45CT dimerization is 1000-fold stronger than Cx43CT dimerization. Cx45CT resonance assignments were also used to identify the binding sites and affinities of molecular partners involved in the Cx45 regulation; although none disrupted dimerization, many of these proteins interacted within one intrinsically disordered region (P278-P285). This domain has similarities with other cardiac connexins, and we propose they constitute a master regulatory domain, which contains overlapping molecular partner binding, cis-trans proline isomerization, and phosphorylation sites.

Polarity proteins as regulators of cell junction complexes: implications for breast cancer

The epithelium of multicellular organisms possesses a well-defined architecture, referred to as polarity that coordinates the regulation of essential cell features. Polarity proteins are intimately linked to the protein complexes that make the tight, adherens and gap junctions; they contribute to the proper localization and assembly of these cell-cell junctions within cells and consequently to functional tissue organization. The establishment of cell-cell junctions and polarity are both implicated in the regulation of epithelial modifications in normal and cancer situations. Uncovering the mechanisms through which cell-cell junctions and epithelial polarization are established and how their interaction with the microenvironment directs cell and tissue organization has opened new venues for the development of cancer therapies. In this review, we focus on the breast epithelium to highlight how polarity and cell-cell junction proteins interact together in normal and cancerous contexts to regulate major cellular mechanisms such as migration. The impact of these proteins on epigenetic mechanisms responsible for resetting cells toward oncogenesis is discussed in light of increasing evidence that tissue polarity modulates chromatin function. Finally, we give an overview of recent breast cancer therapies that target proteins involved in cell-cell junctions.

Differential expression of PDZ domain-containing proteins in human diseases - challenging topics and novel issues

The general features of the PDZ domain structure and functions have been extensively studied during the last decade. PDZ domains are generally present in proteins that are involved in multiple interactions to assemble functional protein complexes that control key cellular processes. One of the best characterized functions of PDZ domain-containing proteins is control of epithelial cell polarity and cell-cell contacts. In the present review, we summarize the current knowledge on regulation of expression of certain PDZ polarity proteins localized at the intercellular junctions. In addition, we provide a critical overview of recent findings regarding the role of these proteins during development of human diseases. Complete understanding of these issues is valuable for the design of novel therapeutic intervention for common pathologies, such as cancer.

The connexin43 carboxyl terminus and cardiac gap junction organization

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

The tight junction protein ZO-2 and Janus kinase 1 mediate intercellular communications in vascular smooth muscle cells

Recent evidence points to a multifunctional role of ZO-2, the tight junction protein of the MAGUK (membrane-associated guanylate kinase-like) family. Though ZO-2 has been found in cell types lacking tight junction structures, such as vascular smooth muscle cells (VSMC), little is known about ZO-2 function in these cells. We provide evidence that ZO-2 mediates specific homotypic cell-to-cell contacts between VSMC. Using mass spectrometry we found that ZO-2 is associated with the non-receptor tyrosine kinase Jak1. By generating specific ZO-2 constructs we further found that the N-terminal fragment of ZO-2 molecule is responsible for this interaction. Adenovirus-based expression of Jak1 inactive mutant demonstrated that Jak1 mediates ZO-2 tyrosine phosphorylation. By means of RNA silencing, expression of Jak1 mutant form and fluorescently labeled ZO-2 fusion protein we further specified that active Jak1, but not Jak1 inactive mutant, mediates ZO-2 localization to the sites of intercellular contacts. We identified the urokinase receptor uPAR as a pre-requisite for these cellular events. Functional requirement of the revealed signaling complex for VSMC network formation was confirmed in experiments using Matrigel and in contraction assay. Our findings imply involvement of the ZO-2 tight junction independent signaling complex containing Jak1 and uPAR in VSMC intercellular communications. This mechanism may contribute to vascular remodeling in occlusive cardiovascular diseases and in arteriogenesis.

Mechanisms of gap junction traffic in health and disease

Gap junctions (GJs) allow direct communication between cells. In the heart, GJs mediate the electrical coupling of cardiomyocytes and as such dictate the speed and direction of cardiac conduction. A prominent feature of acquired structural heart disease is remodeling of GJ protein expression and localization concomitant with increased susceptibility to lethal arrhythmias, leading many to hypothesize that the two are causally linked. Detailed understanding of the cellular mechanisms that regulate GJ localization and function within cardiomyocytes may therefore uncover potential therapeutic strategies for a significant clinical problem. This review will outline our current understanding of GJ cell biology with the intent of highlighting cellular mechanisms responsible for GJ remodeling associated with cardiac disease.

Oxidant stress derails the cardiac connexon connection

Connexin 43 (Cx43) is the major protein component of gap junctions that electrically couple cardiomyocytes at the intercalated disc. Oxidant stress, reduced Cx43 expression, and altered subcellular localization are present in many forms of structural heart disease. These changes in Cx43 lead to alterations in electrical conduction in the ventricle and predispose to lethal cardiac arrhythmias. In their study in this issue of the JCI, Smyth et al. tested the hypothesis that oxidant stress perturbs connexon forward trafficking along microtubules to gap junctions (see the related article beginning on page 266). Failing human ventricular myocardium exhibited a reduction in Cx43 and the microtubule-capping protein EB1 at intercalated discs. Oxidant stress in the adult mouse heart reduced N-cadherin, EB1, and Cx43 colocalization. In HeLa cells and neonatal mouse ventricular myocytes, peroxide exposure displaced EB1 from the plus ends of microtubules and altered microtubule dynamics. Mutational disruption of the EB1-tubulin interaction mimicked the effects of oxidant stress, including a reduction in surface Cx43 expression. These data provide important new molecular insights into the regulation of Cx43 at gap junctions and may identify targets for preservation of cellular coupling in the diseased heart.

The tight junction protein ZO-2 mediates proliferation of vascular smooth muscle cells via regulation of Stat1

AIMS

Recent evidence suggests that the zonula occludens protein 2 (ZO-2) might have additional cellular functions, beyond regulation of paracellular permeability of epithelial and endothelial cells. Deregulation of ZO-2 in response to ischaemia, hypertensive stress, and vascular injury implies its involvement in cardiovascular disorders, most likely via regulating the functional behaviour of vascular smooth muscle cells (VSMC). However, a role of ZO-2 in VSMC biology has yet to be established. Our study was designed to understand the specific functions of ZO-2 in human VSMC.

METHODS AND RESULTS

The expression of ZO-2 and Stat1 upon vascular injury was studied using ex vivo organ culture of coronary arteries combined with immunohistochemistry. ZO-2 silencing in human primary VSMC was achieved by means of lentiviral gene transfer. Cell proliferation was assessed by analysing DNA synthesis and by cell counting. Stat1 expression was examined using immunoblotting, immunocytochemistry, TaqMan, and fluorescence activated cell sorting (FACS) analysis. Functional relevance of Stat1 up-regulation was studied using a Stat1 promoter-luciferase reporter assay and intracellular microinjections of a Stat1 specific antibody. ZO-2 was highly expressed in the media and neointima of dilated but not of control arteries, whereas expression of the transcription factor Stat1 was inversely regulated upon injury. Analysis of VSMC with down-regulated ZO-2 revealed increased expression of Stat1 in these cells, whereas Stat1 phosphorylation was not affected. Stat1 up-regulation in VSMC with ZO-2 silencing resulted in a coordinate activation of Stat1-specific genes and consequently led to inhibition of cell proliferation. This effect was restored by microinjection of a Stat1 neutralising antibody.

CONCLUSION

Our data suggest that the tight junction protein ZO-2 is involved in regulation of VSMC growth control upon vascular injury that is mediated by the transcription factor Stat1. Our findings point to a novel function of ZO-2 in VSMC and implicate ZO-2 as a novel important molecular target in pathological states of vascular remodelling in cardiovascular diseases.

The second PDZ domain of zonula occludens-1 is dispensable for targeting to connexin 43 gap junctions

Zonula occludens (ZO)-1 is emerging as a central player in the control of gap junction (GJ) dynamics. Previously the authors reported that ZO-1 localizes preferentially to the periphery of Cx43 GJs. How ZO-1 arrives at GJ edges is unknown, but this targeting might involve we established interaction between the Cx43 C-terminus and the PDZ2 domain of ZO-1. Here the show that despite blocking the canonical PDZ2-mediated interaction by fusion of GFP to the C-terminus of Cx43, ZO-1 continued to target to domains juxtaposed with the edges of GJs comprised solely of tagged Cx43. This edge-association was not abolished by deletion of PDZ2 from ZO-1, as mutant ZO-1 also targeted to the periphery of GJs composed of either tagged or untagged Cx43. Additionally, ZO-2 was found colocalized with ZO-1 at GJ edges. These data demonstrate that ZO-1 targets to GJ edges independently of several known PDZ2-mediated interactions, including ZO-1 homodimerization, heterodimerization with ZO-2, and direct ZO-1 binding to the C-terminal residues of Cx43.

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

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

Remodelling of gap junctions and connexin expression in diseased myocardium

Gap junctions form the cell-to-cell pathways for propagation of the precisely orchestrated patterns of current flow that govern the regular rhythm of the healthy heart. As in most tissues and organs, multiple connexin types are expressed in the heart: connexin43 (Cx43), Cx40 and Cx45 are found in distinctive combinations and relative quantities in different, functionally-specialized subsets of cardiac myocyte. Mutations in genes that encode connexins have only rarely been identified as being a cause of human cardiac disease, but remodelling of connexin expression and gap junction organization are well documented in acquired adult heart disease, notably ischaemic heart disease and heart failure. Remodelling may take the form of alterations in (i) the distribution of gap junctions and (ii) the amount and type of connexins expressed. Heterogeneous reduction in Cx43 expression and disordering in gap junction distribution feature in human ventricular disease and correlate with electrophysiologically identified arrhythmic changes and contractile dysfunction in animal models. Disease-related alterations in Cx45 and Cx40 expression have also been reported, and some of the functional implications of these are beginning to emerge. Apart from ventricular disease, various features of gap junction organization and connexin expression have been implicated in the initiation and persistence of the most common form of atrial arrhythmia, atrial fibrillation, though the disparate findings in this area remain to be clarified. Other major tasks ahead focus on the Purkinje/working ventricular myocyte interface and its role in normal and abnormal impulse propagation, connexin-interacting proteins and their regulatory functions, and on defining the precise functional properties conferred by the distinctive connexin co-expression patterns of different myocyte types in health and disease.

N-cadherin haploinsufficiency affects cardiac gap junctions and arrhythmic susceptibility

Cardiac-specific deletion of the murine gene (Cdh2) encoding the cell adhesion molecule, N-cadherin, results in disassembly of the intercalated disc (ICD) structure and sudden arrhythmic death. Connexin 43 (Cx43)-containing gap junctions are significantly reduced in the heart after depleting N-cadherin, therefore we hypothesized that animals expressing half the normal levels of N-cadherin would exhibit an intermediate phenotype. We examined the effect of N-cadherin haploinsufficiency on Cx43 expression and susceptibility to induced arrhythmias in mice either wild-type or heterozygous for the Cx43 (Gja1)-null allele. An increase in hypophosphorylated Cx43 accompanied by a modest decrease in total Cx43 protein levels was observed in the N-cadherin heterozygous mice. Consistent with these findings N-cadherin heterozygotes exhibited increased susceptibility to ventricular arrhythmias compared to wild-type mice. Quantitative immunofluorescence microscopy revealed a reduction in size of large Cx43-containing plaques in the N-cadherin heterozygous animals compared to wild-type. Gap junctions were further decreased in number and size in the N-cad/Cx43 compound heterozygous mice with increased arrhythmic susceptibility compared to the single mutants. The scaffold protein, ZO-1, was reduced at the ICD in N-cadherin heterozygous cardiomyocytes providing a possible explanation for the reduction in Cx43 plaque size. These data provide further support for the intimate relationship between N-cadherin and Cx43 in the heart, and suggest that germline mutations in the human N-cadherin (Cdh2) gene may predispose patients to increased risk of cardiac arrhythmias.

Gap junction remodelling in human heart failure is associated with increased interaction of connexin43 with ZO-1

Aims

Remodelling of gap junctions, involving reduction of total gap junction quantity and down-regulation of connexin43 (Cx43), contributes to the arrhythmic substrate in congestive heart failure. However, little is known of the underlying mechanisms. Recent studies from in vitro systems suggest that the connexin-interacting protein zonula occludens-1 (ZO-1) is a potential mediator of gap junction remodelling. We therefore examined the hypothesis that ZO-1 contributes to reduced expression of Cx43 gap junctions in congestive heart failure.

Methods and results

Left ventricular myocardium from healthy control human hearts (n = 5) was compared with that of explanted hearts from transplant patients with end-stage congestive heart failure due to idiopathic dilated cardiomyopathy (DCM; n = 5) or ischaemic cardiomyopathy (ICM; n = 5). Immunoconfocal and immunoelectron microscopy showed that ZO-1 is specifically localized to the intercalated disc of cardiomyocytes in control and failing ventricles. ZO-1 protein levels were significantly increased in both DCM and ICM (P = 0.0025), showing a significant, negative correlation to Cx43 levels (P = 0.0029). There was, however, no significant alteration of ZO-1 mRNA (P = 0.537). Double immunolabelling demonstrated that a proportion of ZO-1 label is co-localized with Cx43, and that co-localization of Cx43 with ZO-1 is significantly increased in the failing ventricle (P = 0.003). Interaction between the two proteins was confirmed by co-immunoprecipitation. The proportion of Cx43 that co-immunoprecipitates with ZO-1 was significantly increased in the failing heart.

Conclusion

Our findings suggest that ZO-1, by interacting with Cx43, plays a role in the down-regulation and decreased size of Cx43 gap junctions in congestive heart failure.