Fate of connexin43 in cardiac tissue harbouring a disease-linked connexin43 mutant

SIRT1 activation and its effect on intercalated disc proteins as a way to reduce doxorubicin cardiotoxicity

According to the World Health Organization, the neoplasm is one of the main reasons for morbidity and mortality worldwide. At the same time, application of cytostatic drugs like an independent type of cancer treatment and in combination with surgical methods, is often associated with the development of cardiovascular complications both in the early and in the delayed period of treatment. Doxorubicin (DOX) is the most commonly used cytotoxic anthracycline antibiotic. DOX can cause both acute and delayed side effects. The problem is still not solved, as evidenced by the continued activity of researchers in terms of developing approaches for the prevention and treatment of cardiovascular complications. It is known, the heart muscle consists of cardiomyocytes connected by intercalated discs (ID), which ensure the structural, electrical, metabolic unity of the heart. Various defects in the ID proteins can lead to the development of cardiovascular diseases of various etiologies, including DOX-induced cardiomyopathy. The search for ways to influence the functioning of ID proteins of the cardiac muscle can become the basis for the creation of new therapeutic approaches to the treatment and prevention of cardiac pathologies. SIRT1 may be an interesting cardioprotective variant due to its wide functional significance. SIRT1 activation triggers nuclear transcription programs that increase the efficiency of cellular, mitochondrial metabolism, increases resistance to oxidative stress, and promotes cell survival. It can be assumed that SIRT1 can not only provide a protective effect at the cardiomyocytes level, leading to an improvement in mitochondrial and metabolic functions, reducing the effects of oxidative stress and inflammatory processes, but also have a protective effect on the functioning of IDs structures of the cardiac muscle.

Towards a Better Understanding of Genotype-Phenotype Correlations and Therapeutic Targets for Cardiocutaneous Genes: The Importance of Functional Studies above Prediction

Genetic variants in gene-encoding proteins involved in cell−cell connecting structures, such as desmosomes and gap junctions, may cause a skin and/or cardiac phenotype, of which the combination is called cardiocutaneous syndrome. The cardiac phenotype is characterized by cardiomyopathy and/or arrhythmias, while the skin particularly displays phenotypes such as keratoderma, hair abnormalities and skin fragility. The reported variants associated with cardiocutaneous syndrome, in genes DSP, JUP, DSC2, KLHL24, GJA1, are classified by interpretation guidelines from the American College of Medical Genetics and Genomics. The genotype−phenotype correlation, however, remains poorly understood. By providing an overview of variants that are assessed for a functional protein pathology, we show that this number (n = 115) is low compared to the number of variants that are assessed by in silico algorithms (>5000). As expected, there is a mismatch between the prediction of variant pathogenicity and the prediction of the functional effect compared to the real functional evidence. Aiding to improve genotype−phenotype correlations, we separate variants into ‘protein reducing’ or ‘altered protein’ variants and provide general conclusions about the skin and heart phenotype involved. We conclude by stipulating that adequate prognoses can only be given, and targeted therapies can only be designed, upon full knowledge of the protein pathology through functional investigation.

Connexin 43 contributes to phenotypic robustness of the mouse skull

BACKGROUND

We compared skull shape and variation among genetically modified mice that exhibit different levels of connexin43 (Cx43) channel function, to determine whether Cx43 contributes to craniofacial phenotypic robustness. Specifically, we used two heterozygous mutant mouse models (G60S/+ and I130T/+) that, when compared to their wildtype counterparts, have an ~80% and ~50% reduction in Cx43 function, respectively.

RESULTS

Both mutant strains showed significant differences in skull shape compared to wildtype littermates and while these differences were more severe in the G60S/+ mouse, shape differences were localized to similar regions of the skull in both mutants. However, increased skull shape variation was observed in G60S/+ mutants only. Additionally, covariation of skull structures was disrupted in the G60S/+ mutants only, indicating that while a 50% reduction in Cx43 function is sufficient to cause a shift in mean skull shape, the threshold for Cx43 function for disrupting craniofacial phenotypic robustness is lower.

CONCLUSIONS

Collectively, our results indicate Cx43 can contribute to phenotypic robustness of the skull through a nonlinear relationship between Cx43 gap junctional function and phenotypic outcomes.

Intercalated discs: cellular adhesion and signaling in heart health and diseases

Intercalated discs (ICDs) are highly orchestrated structures that connect neighboring cardiomyocytes in the heart. Three major complexes are distinguished in ICD: desmosome, adherens junction (AJ), and gap junction (GJ). Desmosomes are major cell adhesion junctions that anchor cell membrane to the intermediate filament network; AJs connect the actin cytoskeleton of adjacent cells; and gap junctions metabolically and electrically connect the cytoplasm of adjacent cardiomyocytes. All these complexes work as a single unit, the so-called area composita, interdependently rather than individually. Mutation or altered expression of ICD proteins results in various cardiac diseases, such as ARVC (arrhythmogenic right ventricular cardiomyopathy), dilated cardiomyopathy, and hypotrophy cardiomyopathy, eventually leading to heart failure. In this article, we first review the recent findings on the structural organization of ICD and their functions and then focus on the recent advances in molecular pathogenesis of the ICD-related heart diseases, which include two major areas: i) the ICD gene mutations in cardiac diseases, and ii) the involvement of ICD proteins in signal transduction pathways leading to myocardium remodeling and eventual heart failure. These major ICD-related signaling pathways include Wnt/β-catenin pathway, p38 MAPK cascade, Rho-dependent serum response factor (SRF) signaling, calcineurin/NFAT signaling, Hippo kinase cascade, etc., which are differentially regulated in pathological conditions.

MiR-21, MiR-29a, GATA4, and MEF2c Expression Changes in Endothelin-1 and Angiotensin II Cardiac Hypertrophy Stimulated Isl-1+Sca-1+c-kit+ Porcine Cardiac Progenitor Cells In Vitro

Cost- and time-intensive porcine translational disease models offer great opportunities to test drugs and therapies for pathological cardiac hypertrophy and can be supported by porcine cell culture models that provide further insights into basic disease mechanisms. Cardiac progenitor cells (CPCs) residing in the adult heart have been shown to differentiate in vitro into cardiomyocytes and could contribute to cardiac regeneration. Therefore, it is important to evaluate their changes on the cellular level caused by disease. We successfully isolated Isl1⁺Sca1⁺cKit⁺ porcine CPCs (pCPCs) from pig hearts and stimulated them with endothelin-1 (ET-1) and angiotensin II (Ang II) in vitro. We also performed a cardiac reprogramming transfection and tested the same conditions. Our results show that undifferentiated Isl1⁺Sca1⁺cKit⁺ pCPCs were significantly upregulated in GATA4, MEF2c, and miR-29a gene expressions and in BNP and MCP-1 protein expressions with Ang II stimulation, but they showed no significant changes in miR-29a and MCP-1 when stimulated with ET-1. Differentiated Isl1⁺Sca1⁺cKit⁺ pCPCs exhibited significantly higher levels of MEF2c, GATA4, miR-29a, and miR-21 as well as Cx43 and BNP with Ang II stimulation. pMx-MGT-transfected Isl1⁺Sca1⁺cKit⁺ pCPCs showed significant elevations in MEF2c, GATA4, and BNP expressions when stimulated with ET-1. Our model demonstrates that in vitro stimulation leads to successful Isl1⁺Sca1⁺cKit⁺ pCPC hypertrophy with upregulation of cardiac remodeling associated genes and profibrotic miRNAs and offers great possibilities for further investigations of disease mechanisms and treatment.

Mice harbouring an oculodentodigital dysplasia-linked Cx43 G60S mutation have severe hearing loss

Given the importance of connexin43 (Cx43) function in the central nervous system and sensory organ processing we proposed that it would also be crucial in auditory function. To that end, hearing was examined in two mouse models of oculodentodigital dysplasia that globally express GJA1 (Cx43) mutations resulting in mild or severe loss of Cx43 function. Although Cx43I130T/+ mutant mice with ∼50% Cx43 channel function did not have any hearing loss, Cx43G60S/+ mutant mice with ∼20% Cx43 channel function had severe hearing loss. There was no evidence of inner ear sensory hair cell loss, suggesting that the Cx43-linked hearing loss lies downstream in the auditory pathway. Since evidence suggests that Cx26 function is essential for hearing and may be protective against noise-induced hearing loss, we challenged Cx43I130T/+ mice with a loud noise and found that they had similar susceptibility to noise-induced hearing loss as controls suggesting that decreased Cx43 function does not sensitize the mice for environmentally-induced hearing loss. Taken together, this study suggests that Cx43 plays an important role in baseline hearing and is essential for auditory processing.

Connexin43 Mutant Patient-Derived Induced Pluripotent Stem Cells Exhibit Altered Differentiation Potential

We present for the first time the generation of induced pluripotent stem cells (iPSCs) from a patient with a connexin-linked disease. The importance of gap junctional intercellular communication in bone homeostasis is exemplified by the autosomal dominant developmental disorder oculodentodigital dysplasia (ODDD), which is linked to mutations in the GJA1 (Cx43) gene. ODDD is characterized by craniofacial malformations, ophthalmic deficits, enamel hypoplasia, and syndactyly. In addition to harboring a Cx43 p.V216L mutation, ODDD iPSCs exhibit reduced Cx43 mRNA and protein abundance when compared to control iPSCs and display impaired channel function. Osteogenic differentiation involved an early, and dramatic downregulation of Cx43 followed by a slight upregulation during the final stages of differentiation. Interestingly, osteoblast differentiation was delayed in ODDD iPSCs. Moreover, Cx43 subcellular localization was altered during chondrogenic differentiation of ODDD iPSCs compared to controls and this may have contributed to the more compact cartilage pellet morphology found in differentiated ODDD iPSCs. These studies highlight the importance of Cx43 expression and function during osteoblast and chondrocyte differentiation, and establish a potential mechanism for how ODDD-associated Cx43 mutations may have altered cell lineages involved in bone and cartilage development. © 2017 American Society for Bone and Mineral Research.

Dissection of Thrombospondin-4 Domains Involved in Intracellular Adaptive Endoplasmic Reticulum Stress-Responsive Signaling

Thrombospondins are a family of stress-inducible secreted glycoproteins that underlie tissue remodeling. We recently reported that thrombospondin-4 (Thbs4) has a critical intracellular function, regulating the adaptive endoplasmic reticulum (ER) stress pathway through activating transcription factor 6α (Atf6α). In the present study, we dissected the domains of Thbs4 that mediate interactions with ER proteins, such as BiP (Grp78) and Atf6α, and the domains mediating activation of the ER stress response. Functionally, Thbs4 localized to the ER and post-ER vesicles and was actively secreted from cardiomyocytes, as were the type III repeat (T3R) and TSP-C domains, while the LamG domain localized to the Golgi apparatus. We also mutated the major calcium-binding motifs within the T3R domain of full-length Thbs4, causing ER retention and secretion blockade. The T3R and TSP-C domains as well as wild-type Thbs4 and the calcium-binding mutant interacted with Atf6α, induced an adaptive ER stress response, and caused expansion of intracellular vesicles. In contrast, overexpression of a related secreted oligomeric glycoprotein, Nell2, which lacks only the T3R and TSP-C domains, did not cause these effects. Finally, deletion of Atf6α abrogated Thbs4-induced vesicular expansion. Taken together, these data identify the critical intracellular functional domains of Thbs4, which was formerly thought to have only extracellular functions.

Manipulating Cx43 expression triggers gene reprogramming events in dermal fibroblasts from oculodentodigital dysplasia patients

Oculodentodigital dysplasia (ODDD) is primarily an autosomal dominant disorder linked to over 70 GJA1 gene [connexin43 (Cx43)] mutations. For nearly a decade, our laboratory has been investigating the relationship between Cx43 and ODDD by expressing disease-linked mutants in reference cells, tissue-relevant cell lines, 3D organ cultures and by using genetically modified mouse models of human disease. Although salient features of Cx43 mutants have been revealed, these models do not necessarily reflect the complexity of the human context. To further overcome these limitations, we have acquired dermal fibroblasts from two ODDD-affected individuals harbouring D3N and V216L mutations in Cx43, along with familial controls. Using these ODDD patient dermal fibroblasts, which naturally produce less GJA1 gene product, along with RNAi and RNA activation (RNAa) approaches, we show that manipulating Cx43 expression triggers cellular gene reprogramming. Quantitative RT-PCR, Western blot and immunofluorescent analysis of ODDD patient fibroblasts show unusually high levels of extracellular matrix (ECM)-interacting proteins, including integrin α5β1, matrix metalloproteinases as well as secreted ECM proteins collagen-I and laminin. Cx43 knockdown in familial control cells produces similar effects on ECM expression, whereas Cx43 transcriptional up-regulation using RNAa decreases production of collagen-I. Interestingly, the enhanced levels of ECM-associated proteins in ODDD V216L fibroblasts is not only a consequence of increased ECM gene expression, but also due to an apparent deficit in collagen-I secretion which may further contribute to impaired collagen gel contraction in ODDD fibroblasts. These findings further illuminate the altered function of Cx43 in ODDD-affected individuals and highlight the impact of manipulating Cx43 expression in human cells.

Mutations in cardiovascular connexin genes

Connexins (Cxs) form a family of transmembrane proteins comprising 21 members in humans. Cxs differ in their expression patterns, biophysical properties and ability to combine into homomeric or heteromeric gap junction channels between neighbouring cells. The permeation of ions and small metabolites through gap junction channels or hemichannels confers a crucial role to these proteins in intercellular communication and in maintaining tissue homeostasis. Among others, Cx37, Cx40, Cx43, Cx45 and Cx47 are found in heart, blood and lymphatic vessels. Mutations or polymorphisms in the genes coding for these Cxs have not only been implicated in cardiovascular pathologies but also in a variety of other disorders. While mutations in Cx43 are mostly linked to oculodentodigital dysplasia, Cx47 mutations are associated with Pelizaeus-Merzbacher-like disease and lymphoedema. Cx40 mutations are principally linked to atrial fibrillation. Mutations in Cx37 have not yet been described, but polymorphisms in the Cx37 gene have been implicated in the development of arterial disease. This review addresses current knowledge on gene mutations in cardiovascular Cxs systematically and links them to alterations in channel properties and disease.

Atrial fibrillation-linked germline GJA5/connexin40 mutants showed an increased hemichannel function

Mutations in GJA5 encoding the gap junction protein connexin40 (Cx40) have been linked to lone atrial fibrillation. Some of these mutants result in impaired gap junction function due to either abnormal connexin localization or impaired gap junction channels, which may play a role in promoting atrial fibrillation. However, the effects of the atrial fibrillation-linked Cx40 mutants on hemichannel function have not been studied. Here we investigated two atrial fibrillation-linked germline Cx40 mutants, V85I and L221I. These two mutants formed putative gap junction plaques at cell-cell interfaces, with similar gap junction coupling conductance as that of wild-type Cx40. Connexin deficient HeLa cells expressing either one of these two mutants displayed prominent propidium iodide-uptake distinct from cells expressing wild-type Cx40 or other atrial fibrillation-linked Cx40 mutants, I75F, L229M, and Q49X. Propidium iodide-uptake was sensitive to [Ca2+]o and the hemichannel blockers, carbenoxolone, flufenamic acid and mefloquine, but was not affected by the pannexin 1 channel blocking agent, probenecid, indicating that uptake is most likely mediated via connexin hemichannels. A gain-of-hemichannel function in these two atrial fibrillation-linked Cx40 mutants may provide a novel mechanism underlying the etiology of atrial fibrillation.

Myogenic bladder defects in mouse models of human oculodentodigital dysplasia

To date, over 65 mutations in the gene encoding Cx43 (connexin43) have been linked to the autosomal-dominant disease ODDD (oculodentodigital dysplasia). A subset of these patients experience bladder incontinence which could be due to underlying neurogenic deterioration or aberrant myogenic regulation. BSMCs (bladder smooth muscle cells) from wild-type and two Cx43 mutant lines (Cx43(G60S) and Cx43(I130T)) that mimic ODDD exhibit a significant reduction in total Cx43. Dye transfer studies revealed that the G60S mutant was a potent dominant-negative inhibitor of co-expressed Cx43, a property not equally shared by the I130T mutant. BSMCs from both mutant mouse strains were defective in their ability to contract, which is indicative of phenotype changes due to harbouring the Cx43 mutants. Upon stretching, Cx43 levels were significantly elevated in controls and mutants containing BSMCs, but the non-muscle myosin heavy chain A levels were only reduced in cells from control mice. Although the Cx43(G60S) mutant mice showed no difference in voided urine volume or frequency, the Cx43(I130T) mice voided less frequently. Thus, similar to the diversity of morbidities seen in ODDD patients, genetically modified mice also display mutation-specific changes in bladder function. Furthermore, although mutant mice have compromised smooth muscle contraction and response to stretch, overriding bladder defects in Cx43(I130T) mice are likely to be complemented by neurogenic changes.

Syndromic and non-syndromic disease-linked Cx43 mutations

There are now at least 14 distinct diseases linked to germ line mutations in the 21 genes that encode the connexin (Cx) family of gap junction proteins. This review focuses on the links between germ-line mutations in the gene encoding Cx43 (GJA1) and the human disease termed oculodentodigital dysplasia (ODDD). This disease is clinically characterized by soft tissue fusion of the digits, abnormal craniofacial bone development, small eyes and loss of tooth enamel. However, the disease is considerably more complex and somewhat degenerative as patients often suffer from other syndromic effects that include incontinence, glaucoma, skin diseases and neuropathies that become more pronounced during aging. The challenge continues to be understanding how distinct Cx43 gene mutations cause such a diverse range of tissue phenotypes and pathophysiological changes while other Cx43-rich organs are relatively unaffected. This review will provide an overview of many of these studies and distill some themes and outstanding questions that need to be addressed in the coming years.

Connexin diversity in the heart: insights from transgenic mouse models

Cardiac conduction is mediated by gap junction channels that are formed by connexin (Cx) protein subunits. The connexin family of proteins consists of more than 20 members varying in their biophysical properties and ability to combine with other connexins into heteromeric gap junction channels. The mammalian heart shows regional differences both in connexin expression profile and in degree of electrical coupling. The latter reflects functional requirements for conduction velocity which needs to be low in the sinoatrial and atrioventricular nodes and high in the ventricular conduction system. Over the past 20 years knowledge of the biology of gap junction channels and their role in the genesis of cardiac arrhythmias has increased enormously. This review focuses on the insights gained from transgenic mouse models. The mouse heart expresses Cx30, 30.2, 37, 40, 43, 45, and 46. For these connexins a variety of knock-outs, heart-specific knock-outs, conditional knock-outs, double knock-outs, knock-ins and overexpressors has been studied. We discuss the cardiac phenotype in these models and compare Cx expression between mice and men. Mouse models have enhanced our understanding of (patho)-physiological implications of Cx diversity in the heart. In principle connexin-specific modulation of electrical coupling in the heart represents an interesting treatment strategy for cardiac arrhythmias and conduction disorders.

Cerebral ischemic injury is enhanced in a model of oculodentodigital dysplasia

Oculodentodigital dysplasia (ODDD) is a rare autosomal dominant disease that results in visible developmental anomalies of the limbs, face, eyes and teeth. Recently analysis of human connexin43 (Cx43) DNA sequences has revealed a number of different missense, duplication and frame shift mutations resulting in this phenotype. A mouse model of this disorder has been created with a missense point mutation of the glycine amino acid at position 60 to serine (G60S). Heterozygote +/G60S mice exhibit a similar ODDD phenotype as observed in humans. In addition to the malformations listed above, ODDD patients often have neurological findings. In the brain, Cx43 is highly expressed in astrocytes and has been shown to play a role in neuroprotection. We were interested in determining the effect of the +/G60S mutation following stroke. Four days after middle cerebral artery occlusion the volume of infarct was larger in mice with the +/G60S mutation. In astrocyte-neuron co-cultures, exposure to glutamate also resulted in greater cellular death in the +/G60S mutants. Protein levels of Cx43 in the mutant mouse were found to be reduced when compared to the normal tissue. Cx43 protein was observed as a continual line of small punctate aggregates in the plasma membrane with increased intracellular localization, which is distinct from the larger plaques seen in the normal mouse astrocytes. Functionally, primary +/G60S astrocytes exhibited reduced gap junctional coupling and increased hemichannel activity, which may underlie the mechanism of increased damage during stroke. This article is part of the Special Issue Section entitled 'Current Pharmacology of Gap Junction Channels and Hemichannels'.

Cytidine diphosphate choline improves the outcome of cardiac arrest vs epinephrine in rat model

OBJECTIVES

Cytidine diphosphate choline (CDP-choline) is a cholinergic agent that can both stimulate the cholinergic pathway and increase blood pressure. We aimed to investigate the effects of CDP-choline on the outcome of cardiac arrest in comparison with epinephrine.

METHODS

This was a randomized prospective animal study. Cardiac arrest was induced by asphyxia in 45 rats. After 7 minutes of asphyxia, resuscitation was attempted. The rats were allocated to different groups treated with 2 mL/kg saline, 100 μg/kg epinephrine, or 250 mg/kg CDP-choline. The hemodynamic parameters were monitored for 2 hours after resuscitation, and cardiac function was evaluated by echocardiography 2 hours after resuscitation. The hearts were harvested at the end of monitoring for histologic evaluation.

RESULTS

Epinephrine and CDP-choline improved the rate of return of spontaneous circulation and blood pressure during cardiopulmonary resuscitation; however, postresuscitation cardiac function in the CDP-choline and placebo groups was better than in the epinephrine group. Compared with the epinephrine group, less myocardial and mitochondrial injury was observed by electron microscopy in the CDP-choline and placebo groups; the level of superoxide dismutase and malondialdehyde indicated less peroxidative injury in the CDP-choline and placebo groups. Cytidine diphosphate choline and placebo also preserved connexin 43 when compared with epinephrine.

CONCLUSION

When administered during resuscitation, CDP-choline increased the rate of return of spontaneous circulation similarly to epinephrine. In addition, it did not increase the severity of myocardial injury and postresuscitation myocardial dysfunction, whereas epinephrine appeared to be harmful.

Autosomal recessive GJA1 (Cx43) gene mutations cause oculodentodigital dysplasia by distinct mechanisms

Oculodentodigital dysplasia (ODDD) is mainly an autosomal dominant human disease caused by mutations in the GJA1 gene, which encodes the gap junction protein connexin43 (Cx43). Surprisingly, there have been two autosomal recessive mutations reported that cause ODDD: a single amino acid substitution (R76H) and a premature truncation mutation (R33X). When expressed in either gap junctional intercellular communication (GJIC)-deficient HeLa cells or Cx43-expressing NRK cells, the R76H mutant trafficked to the plasma membrane to form gap junction-like plaques, whereas the R33X mutant remained diffusely localized throughout the cell, including the nucleus. As expected, the R33X mutant failed to form functional channels. In the case of the R76H mutant, dye transfer studies in HeLa cells and electrical conductance analysis in GJIC-deficient N2a cells revealed that this mutant could form functional gap junction channels, albeit with reduced macroscopic and single channel conductance. Alexa 350 dye transfer studies further revealed that the R76H mutant had no detectable negative effect on the function of co-expressed Cx26, Cx32, Cx37 or Cx40, whereas the R33X mutant exhibited significant dominant or trans-dominant effects on Cx43 and Cx40 as manifested by a reduction in wild-type connexin gap junction plaques. Taken together, our results suggest that the trans-dominant effect of R33X together with its complete inability to form a functional channel may explain why patients harboring this autosomal recessive R33X mutant exhibit greater disease burden than patients harboring the R76H mutant.

Evolving form to fit function: cardiomyocyte intercalated disc and transverse-tubule membranes

The vertebrate cardiac myocyte has evolved a highly organized cellular membrane architecture and cell-cell contacts in order to effectively transmit precisely timed and homogeneous depolarizing waves without failure (>2 billion times/human life span). Two unique specialized membrane domains, the intercalated disc and the transverse tubule (T-tubule), function to ensure the rapid and coordinated propagation of the action potential throughout the heart. Based on their critical roles in structure, signaling, and electric inter- and intracellular communication, it is not surprising that dysfunction in these membrane structures is associated with aberrant vertebrate physiology, resulting in potentially fatal congenital and acquired disease. This chapter will review the fundamental components of cardiomyocyte intercalated disc and transverse-tubule membranes with a focus on linking dysfunction in these membranes with human cardiovascular disease.

Structure and functional studies of N-terminal Cx43 mutants linked to oculodentodigital dysplasia

Mutations in the connexin-43 gap junction protein cause the developmental disease known as oculodentodigital dysplasia. Structure and function approaches are used to demonstrate that the nature of the missense mutation in the amino-terminal domain of connexin-43 governs the mechanism that leads to loss of connexin-43 function.

Mutations in the gene encoding connexin-43 (Cx43) cause the human development disorder known as oculodentodigital dysplasia (ODDD). In this study, ODDD-linked Cx43 N-terminal mutants formed nonfunctional gap junction–like plaques and exhibited dominant-negative effects on the coupling conductance of coexpressed endogenous Cx43 in reference cell models. Nuclear magnetic resonance (NMR) protein structure determination of an N-terminal 23–amino acid polypeptide of wild-type Cx43 revealed that it folded in to a kinked α-helical structure. This finding predicted that W4 might be critically important in intramolecular and intermolecular interactions. Thus we engineered and characterized a W4A mutant and found that this mutant formed a regular, nonkinked α-helix but did not form functional gap junctions. Furthermore, a G2V variant peptide of Cx43 showed a kinked helix that now included V2 interactions with W4, resulting in the G2V mutant forming nonfunctional gap junctions. Also predicted from the NMR structures, a G2S mutant was found to relieve these interactions and allowed the protein to form functional gap junctions. Collectively, these studies suggest that the nature of the mutation conveys loss of Cx43 function by distinctly different mechanisms that are rooted in the structure of the N-terminal region.

Characterization of gap junction proteins in the bladder of Cx43 mutant mouse models of oculodentodigital dysplasia

Oculodentodigital dysplasia (ODDD) is a rare developmental disease resulting from germline mutations in the GJA1 gene that encodes the gap junction protein connexin43 (Cx43). In addition to the classical ODDD symptoms that affect the eyes, teeth, bone and digits, in some cases ODDD patients have reported bladder impairments. Thus, we chose to characterize the bladder in mutant mouse models of ODDD that harbor two distinct Cx43 mutations, G60S and I130T. Histological assessment revealed no difference in bladder detrusor wall thickness in mutant compared to littermate control mice. The overall localization of Cx43 in the lamina propria and detrusor also appeared to be similar in the bladders of mutant mice with the exception that the G60S mice had more instances of intracellular Cx43. However, both mutant mouse lines exhibited a significant reduction in the phosphorylated P1 and P2 isoforms of Cx43, while only the I130T mice exhibited a reduction in total Cx43 levels. Interestingly, Cx26 levels and distribution were not altered in mutant mice as it was localized to intracellular compartments and restricted to the basal cell layers of the urothelium. Our studies suggest that these two distinct genetically modified mouse models of ODDD probably mimic patients who lack bladder defects or other factors, such as aging or co-morbidities, are necessary to reveal a bladder phenotype.

Cardiac connexins, mutations and arrhythmias

PURPOSE OF REVIEW

Connexins are the pore forming subunits of gap junction channels. They are essential for cardiac action potential propagation. Connexins are modified at the transcriptional or posttranslational levels under pathological states such as cardiac hypertrophy or ischemia, thus contributing to the arrhythmogenic substrate. However, the relation between nucleotide substitutions in the connexin gene and the occurrence of cardiac arrhythmias remains largely unexplored.

RECENT FINDINGS

Recent studies have reported an association between nucleotide substitutions in the connexin40 (Cx40) and connexin43 (Cx43) genes (GJA5 and GJA1, respectively) and cardiac arrhythmias. Of note, however, germline mutations in Cx43 are considered causative of oculodentodigital dysplasia, a pleiotropic syndrome wherein cardiac manifestations are notoriously absent.

SUMMARY

Here, we review some of the current knowledge on the association between cardiac connexins and inherited arrhythmias.

Connexin43 phosphorylation in brain, cardiac, endothelial and epithelial tissues

Gap junctions, composed of proteins from the connexin family, allow for intercellular communication between cells in essentially all tissues. There are 21 connexin genes in the human genome and different tissues express different connexin genes. Most connexins are known to be phosphoproteins. Phosphorylation can regulate connexin assembly into gap junctions, gap junction turnover and channel gating. Given the importance of gap junctions in development, proliferation and carcinogenesis, regulation of gap junction phosphorylation in response to wounding, hypoxia and other tissue insults is proving to be critical for cellular response and return to homeostasis. Connexin43 (Cx43) is the most widely and highly expressed gap junction protein, both in cell culture models and in humans, thus more research has been done on it and more reagents to it are available. In particular, antibodies that can report Cx43 phosphorylation status have been created allowing temporal examination of specific phosphorylation events in vivo. This review is focused on the use of these antibodies in tissue in situ, predominantly looking at Cx43 phosphorylation in brain, heart, endothelium and epithelium with reference to other connexins where data is available. These data allow us to begin to correlate specific phosphorylation events with changes in cell and tissue function. This article is part of a Special Issue entitled: The Communicating junctions, composition, structure and characteristics.

The G60S connexin43 mutant regulates hair growth and hair fiber morphology in a mouse model of human oculodentodigital dysplasia

Patients expressing mutations in the gene encoding the gap junction protein Cx43 suffer from a disease called oculodentodigital dysplasia (ODDD). Patients with ODDD are often reported to develop hair that is dry, dull, sparse, and slow growing. To evaluate the linkage between Cx43 and hair growth, structure, and follicle density we employed a mouse model of ODDD that harbors a Cx43 G60S point mutant. Regionally sparse and overall dull hair were observed in mutant mice compared with their wild-type (WT) littermates. However, histological analysis of overall hair follicle density in mutant and WT mice did not reveal any significant differences. After epilation, mutant mouse hair grew back slower, and hair growth was asynchronous. In addition, ultrastructural scanning electron microscopic imaging of hair fibers taken from mutant mice and two patients harboring the G143S mutation revealed severe cuticle weathering. Nodule formation was also observed in the proximal region of hair fibers taken from mutant mice. These results suggest that the G60S mutant mouse model mimics the hair phenotype found in at least some ODDD patients and suggests an important role for Cx43 in hair regeneration, growth, and cuticle formation.

Atrial tachycardia/fibrillation in the connexin 43 G60S mutant (Oculodentodigital dysplasia) mouse

Atrial fibrillation (AF), the most common cardiac arrhythmia seen in general practice, can be promoted by conduction slowing. Cardiac impulse conduction depends on gap junction channels, which are composed of connexins (Cxs). While atrial Cx40 and Cx43 are equally expressed, AF studies have primarily focused on Cx40 reductions. The G60S Cx43 mutant (Cx43(G60S/+)) mouse model of Oculodentodigital dysplasia has a 60% reduction in Cx43 in the atria. Cx43(G60S/+) mice were compared with Cx40-deficient (Cx40(-/-)) mice to determine the role of Cxs in atrial tachycardia/fibrillation (AT/F). Intracardiac electrophysiological studies were done in 6-mo-old male C57BL/6 Cx43(G60S/+) mutant, littermate (Cx43(+/+)), Cx40(-/-), and C57BL/6 wild-type (WT) mice. AT/F induction used an extra stimulus during sinus rhythm, programmed electrical stimulation, or burst pacing (1-ms pulses, 50-Hz, 400-ms train) in the absence and presence of carbachol (CCh). Atrial effective refractory periods did not differ between strains. Cx43(G60S/+) mice were more susceptible to induction of sustained AT/F (duration >2 min, 9 of 12; maximum >35 min) compared with Cx43(+/+) mice (3 of 11; χ(2) = 5.24; P = 0.02). CCh enhanced sustained AT/F susceptibility in WT (from 1 of 12 without, to 7 of 10 with CCh; χ(2) = 8.98; P < 0.01) but not in Cx40(-/-) mice (1 of 13 without vs. 2 of 9 with CCh; χ(2) = 0.95; P = NS). The pattern of epicardial recordings during AT/F in Cx43(G60S/+) mice was left preceding right, with left atrial fractionated activation patterns consistent with clinical observations of AF. In conclusions, while Cx43(G60S/+) mice had severe AT/F, Cx40(-/-) mice were resistant to CCh-induced AT/F.

Cx43 suppresses mammary tumor metastasis to the lung in a Cx43 mutant mouse model of human disease

Gap junctions, the channels formed by the connexin (Cx) family of proteins, are responsible for direct intercellular communication. Although connexins are considered as tumor suppressors, their overall role in cancer onset, progression and metastasis is somewhat controversial. This study uses a novel Cx43 mutant mouse model (G60S mice) and cross-breeding strategies to determine the role of Cx43 in all stages of breast tumorigenesis. G60S mice were cross-bred with ErbB2 overexpressing mice, and spontaneous and 7,12-dimethylbenz[α]anthracene (DMBA)-induced tumor development was evaluated. Mice were killed when tumors reached ∼1 cm(3) or when mice showed signs of critical illness. In both spontaneous and DMBA studies, onset of palpable tumors was delayed in G60S mice compared with mice in control groups. Moreover, while tumors from control mice reached the size threshold, most DMBA-exposed Cx43 mutant mice were killed prematurely because of labored breathing, independent of the presence of a palpable tumor. Reduced Cx43 levels in Cx43 mutant mice were accompanied by extensive mammary gland hyperplasia. Lung histology revealed that all Cx43 mutant mice exhibited mammaglobin-positive mammary gland metastases to the lung, and the number of metastases was increased by threefold in Cx43 mutant mice on treatment with DMBA. Thus, while reduced levels of Cx43 delayed the onset of palpable tumors, normal Cx43 levels inhibited mammary gland tumor metastasis to the lungs. Understanding the mechanisms of how Cx43, which is expressed primarily in myoepithelial cells, inhibits mammary gland tumor metastasis is critical as Cx43 is assessed as a candidate for therapeutic intervention.

A dominant connexin43 mutant does not have dominant effects on gap junction coupling in astrocytes

Dominant mutations in GJA1, the gene encoding the gap junction protein connexin43 (Cx43), cause oculodentodigital dysplasia (ODDD), a syndrome affecting multiple tissues, including the central nervous system (CNS). We investigated the effects of the G60S mutant, which causes a similar, dominant phenotype in mice (Gja1(Jrt/+)). Astrocytes in acute brain slices from Gja1(Jrt/+) mice transfer sulforhodamine-B comparably to that in their wild-type (WT) littermates. Further, astrocytes and cardiomyocytes cultured from Gja1(Jrt/+) mice showed a comparable transfer of lucifer yellow to those from WT mice. In transfected cells, the G60S mutant formed gap junction (GJ) plaques but not functional channels. In co-transfected cells, the G60S mutant co-immunoprecipitated with WT Cx43, but did not diminish GJ coupling as measured by dual patch clamp. Thus, whereas G60S has dominant effects, it did not appreciably reduce GJ coupling.

A new perspective on intercalated disc organization: implications for heart disease

Adherens junctions and desmosomes are intercellular adhesive junctions and essential for the morphogenesis, differentiation, and maintenance of tissues that are subjected to high mechanical stress, including heart and skin. The different junction complexes are organized at the termini of the cardiomyocyte called the intercalated disc. Disruption of adhesive integrity via mutations in genes encoding desmosomal proteins causes an inherited heart disease, arrhythmogenic right ventricular cardiomyopathy (ARVC). Besides plakoglobin, which is shared by adherens junctions and desmosomes, other desmosomal components, desmoglein-2, desmocollin-2, plakophilin-2, and desmoplakin are also present in ultrastructurally defined fascia adherens junctions of heart muscle, but not other tissues. This mixed-type of junctional structure is termed hybrid adhering junction or area composita. Desmosomal plakophilin-2 directly interacts with adherens junction protein alphaT-catenin, providing a new molecular link between the cadherin-catenin complex and desmosome. The area composita only exists in the cardiac intercalated disc of mammalian species suggesting that it evolved to strengthen mechanical coupling in the heart of higher vertebrates. The cross-talk among different junctions and their implication in the pathogenesis of ARVC are discussed in this review.

Decreased levels of Cx43 gap junctions result in ameloblast dysregulation and enamel hypoplasia in Gja1Jrt/+ mice

Coordinated differentiation of the ameloblast cell layer is essential to enamel matrix protein deposition and subsequent mineralization. It has been hypothesized that this process is governed by Cx43-based gap junctional intercellular communication as oculodentodigital dysplasia (ODDD) patients harboring autosomal-dominant mutations in Cx43 exhibit enamel defects typically resulting in early adulthood tooth loss. To assess the role of Cx43 in tooth development we employ a mouse model of ODDD that harbors a G60S Cx43 mutant, Gja1(Jrt)/+, and appears to exhibit tooth abnormalities that mimic the human disease. We found that total Cx43 plaques at all stages of ameloblast differentiation, as well as within the supporting cell layers, were greatly reduced in Gja1(Jrt)/+ incisors compared to wild-type littermate controls. To characterize the Gja1(Jrt)/+ mouse tooth phenotype, mice were sacrificed prior to tooth eruption (postnatal day 7), weaning (postnatal day 21), and adulthood (2 months postnatal). A severely disorganized Gja1(Jrt)/+ mouse ameloblast layer and abnormal accumulation of amelogenin were observed at stages when the cells were active in secretion and mineralization. Differences in enamel thickness became more apparent after tooth eruption and incisor exposure to the oral cavity suggesting that enamel integrity is compromised, leading to rapid erosion. Additional analysis of incisors from mutant mice revealed that they were longer with a thicker dentin layer than their wild-type littermates, which may reflect a mechanical stress response to the depleted enamel layer. Together, these data show that reduced levels of Cx43 gap junctions result in ameloblast dysregulation, enamel hypoplasia, and secondary tissue responses.

Tyrosine-dependent basolateral targeting of human connexin43-eYFP in Madin-Darby canine kidney cells can be disrupted by the oculodentodigital dysplasia mutation L90V

Polarized membrane sorting of connexin 43 (Cx43) has not been well-characterized. Based on the presence of a putative sorting signal, YKLV(286-289), within its C-terminal cytoplasmic domain, we hypothesized that Cx43 is selectively expressed on the basolateral surface of Madin-Darby canine kidney (MDCK) cells in a tyrosine-dependent manner. We generated stable MDCK cell lines expressing human wild-type and mutant Cx43-eYFP, and analyzed the membrane localization of Cx43-eYFP within polarized monolayers using confocal microscopy and selective surface biotinylation. We found that wild-type Cx43-eYFP was selectively targeted to the basolateral membrane domain of MDCK cells. Substitution of alanine for Y286 disrupted basolateral targeting of Cx43-eYFP. Additionally, substitution of a sequence containing the transferrin receptor internalization signal, LSYTRF, for PGYKLV(284-289) also disrupted basolateral targeting. Taken together, these results indicate that Y286 in its native amino acid sequence is necessary for targeting Cx43-eYFP to the basolateral membrane domain of MDCK cells. To determine whether the F52dup or L90V oculodentodigital dysplasia-associated mutations could affect polarized sorting of Cx43-eYFP, we analyzed the expression of these Cx43-eYFP mutant constructs and found that the L90V mutation disrupted basolateral expression. These findings raise the possibility that some oculodentodigitial dysplasia-associated mutations contribute to disease by altering polarized targeting of Cx43.

Sex-dependent gene regulatory networks of the heart rhythm

Expression level, control, and intercoordination of 66 selected heart rhythm determinant (HRD) genes were compared in atria and ventricles of four male and four female adult mice. We found that genes encoding various adrenergic receptors, ankyrins, ion channels and transporters, connexins, cadherins, plakophilins, and other components of the intercalated discs form a complex network that is chamber dependent and differs between the two sexes. In addition, most HRD genes in atria had higher expression in males than in females, while in ventricles, expression levels were mostly higher in females than in males. Moreover, significant chamber differences were observed between the sexes, with higher expression in atria than ventricles for males and higher expression in ventricles than atria for females. We have ranked the selected genes according to their prominence (new concept) within the HRD gene web defined as extent of expression coordination with the other web genes and stability of expression. Interestingly, the prominence hierarchy was substantially different between the two sexes. Taken together, these findings indicate that the organizational principles of the heart rhythm transcriptome are sex dependent, with the newly introduced prominence analysis allowing identification of genes that are pivotal for the sexual dichotomy.

Oogenesis defects in a mutant mouse model of oculodentodigital dysplasia

The essential role of connexin43 (Cx43) during oogenesis has been demonstrated by the severe germ cell deficiency and arrested folliculogenesis observed in Cx43 knockout mice. Recently, another mutant mouse strain became available (Gja1(Jrt)/+) that carries the dominant loss-of-function Cx43 mutation, Cx43(G60S). Gja1(Jrt)/+ mice display features of the human disease oculodentodigital dysplasia (ODDD), which is caused by mutations in the GJA1 gene. We used this new mutant strain to study how a disease-linked Cx43 mutant affects oogenesis. We found that female mutant mice are subfertile with significantly reduced mating success and small litters. The phosphorylated species of the Cx43 protein are reduced in the mutant ovaries in association with impaired trafficking and assembly of gap junctions in the membranes of granulosa cells, confirming that the mutant protein acts dominantly on its wild-type counterpart. Correspondingly, although starting with a normal abundance of germ cells, ovaries of the mutant mice contain significantly fewer pre-ovulatory follicles and do not respond to superovulation by gonadotropins, which is at least partially the result of reduced proliferation and increased apoptosis of granulosa cells. We conclude that the Gja1(Jrt) mutation has a dominant negative effect on Cx43 function in the ovary, rendering the females subfertile. Given these findings, closer examination of reproductive function in ODDD human females is warranted.

A dominant loss-of-function GJA1 (Cx43) mutant impairs parturition in the mouse

Expression of GJA1 (commonly known as connexin43 or Cx43), a major myometrial gap junction protein, is upregulated before the onset of delivery, suggesting an essential role for Cx43-mediated gap junctional intercellular communication (GJIC) in normal uterine contraction during parturition. To determine how a disease-linked Cx43 mutation affects myometrial function, we studied a mutant mouse model carrying an autosomal dominant mutation (Gja1(Jrt)) in the gene encoding Cx43 that displays features of the human genetic disease oculodentodigital dysplasia. We found that Cx43 level, specifically the phosphorylated species of the protein, is significantly reduced in the myometrium of the mutant mice (Gja1(Jrt)/+), as revealed by Western blotting and immunostaining. Patch-clamp electrophysiological measurements demonstrated that coupling between myometrial smooth muscle cells is reduced to <15% of wild-type, indicating that the mutant protein acts dominantly on its wild-type counterpart. The phosphorylated species of Cx43 in the mutant myometrium failed to increase prior to parturition as well as in response to exogenous estrogen. Correspondingly, in vitro experiments with uterine strips revealed weaker contraction of the mutant myometrium and reduced responsiveness to oxytocin, providing an explanation for the prolonged gestation and presence of suffocated fetuses in the uteri that were observed in some of the mutant mice. We conclude that the Gja1(Jrt) mutation has a dominant-negative effect on Cx43 function in the myometrium, severely reducing GJIC, leading to impaired parturition.