Connexin40 and connexin43 determine gating properties of atrial gap junction channels

Department of Pharmacology, SUNY Upstate Medical University, 750 East Adams Street, Syracuse, NY 13210, USA.
Journal of Molecular and Cellular Cardiology (Impact Factor: 4.66). 05/2009; 48(1):238-45. DOI: 10.1016/j.yjmcc.2009.05.014
Source: PubMed


While ventricular gap junctions contain only Cx43, atrial gap junctions contain both Cx40 and Cx43; yet the functional consequences of this co-expression remain poorly understood. We quantitated the expression of Cx40 and Cx43 and their contributions to atrial gap junctional conductance (g(j)). Neonatal murine atrial myocytes showed similar abundances of Cx40 and Cx43 proteins, while ventricular myocytes contained at least 20 times more Cx43 than Cx40. Since Cx40 gap junction channels are blocked by 2 mM spermine while Cx43 channels are unaffected, we used spermine block as a functional dual whole cell patch clamp assay to determine Cx40 contributions to cardiac g(j). Slightly more than half of atrial g(j) and <or=20% of ventricular g(j) were inhibited. In myocytes from Cx40 null mice, the inhibition of ventricular g(j) was completely abolished, and the block of atrial g(j) was reduced to <20%. Compared to ventricular gap junctions, the transjunctional voltage (V(j))-dependent inactivation of atrial g(j) was reduced and kinetically slowed, while the V(j)-dependence of fast and slow inactivation was unchanged. We conclude that Cx40 and Cx43 are equally abundant in atrium and make similar contributions to atrial g(j). Co-expression of Cx40 accounts for most, but not all, of the differences in the V(j)-dependent gating properties between atrium and ventricle that may play a role in the genesis of slow myocardial conduction and arrhythmias.

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Available from: Eric C Beyer, Sep 11, 2014
    • "Of note, altered expression and/or distribution of connexin43 (Cx43), which is the major component of gap junctions (GJs) connecting working CMs [14], has been described in cardiac rhythm disturbances, such as atrial fibrillation (AF) and tachycardia-associated remodeling [15]. To date, different groups have used tachypaced CMs as in vitro models to study AF [16] and GJ remodeling [17]. "
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    ABSTRACT: Communication between cardiomyocytes depends upon Gap Junctions (GJ). Previous studies have demonstrated that electrical stimulation induces GJ remodeling and modifies histone acetylases (HAT) and deacetylases (HDAC) activities, although these two results have not been linked. The aim of this work was to establish whether electrical stimulation modulates GJ-mediated cardiac cell-cell communication by acetylation-dependent mechanisms. Field stimulation of HL-1 cardiomyocytes at 0.5Hz for 24hours significantly reduced Connexin43 (Cx43) expression and cell-cell communication. HDAC activity was down-regulated whereas HAT activity was not modified resulting in increased acetylation of Cx43. Consistent with a post-translational mechanism, we did not observe a reduction in Cx43 mRNA in electrically stimulated cells, while the proteasomal inhibitor MG132 maintained Cx43 expression. Further, the treatment of paced cells with the HAT inhibitor Anacardic Acid maintained both the levels of Cx43 and cell-cell communication. Finally, we observed increased acetylation of Cx43 in the left ventricles of dogs subjected to chronic tachypacing as a model of abnormal ventricular activation. In conclusion, our findings suggest that altered electrical activity can regulate cardiomyocyte communication by influencing the acetylation status of Cx43. Copyright © 2015. Published by Elsevier Ltd.
    Journal of Molecular and Cellular Cardiology 08/2015; DOI:10.1016/j.yjmcc.2015.08.001 · 4.66 Impact Factor
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    • "Cell homogenates were prepared 48 h after transfection with connexin DNA as described by Gong et al. [15]. Immunoblots were performed as described earlier [5]. The protein concentrations of homogenates were determined using the method of Bradford (Bio-Rad, Richmond, CA) [16]. "
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    ABSTRACT: Several Cx40 mutants have been identified in patients with atrial fibrillation (AF). We have been working to identify physiological or cell biological abnormalities of several these human mutants that might explain how they contribute to disease pathogenesis. Wild type (wt) Cx40 or four different mutants (P88S, G38D, V85I, and L229M) were expressed by transfection of communication-deficient HeLa cells or HL-1 cardiomyocytes. Biophysical channel properties and the sub-cellular localization and protein levels of Cx40 were characterized. Wild type Cx40 and all mutants except P88S formed gap junction plaques and induced significant gap junctional conductances. The functional mutants showed only modest alterations of single channel conductances or gating by trans-junctional voltage as compared to wtCx40. However, immunoblotting indicated that the steady state levels of G38D, V85I, and L229M were reduced relative to wtCx40; most strikingly, G38D was only 20 - 31% of wild type levels. After inhibition of protein synthesis with cycloheximide, G38D (and to a lesser extent the other mutants) disappeared much faster than wtCx40. Treatment with the proteasomal inhibitor, epoxomicin, greatly increased levels of G38D and restored the abundance of gap junctions and the extent of intercellular dye transfer. Thus, G38D, V85I, and L229M are functional mutants of Cx40 with small alterations of physiological properties, but accelerated degradation by the proteasome. These findings suggest a novel mechanism (protein instability) for the pathogenesis of AF due to a connexin mutation and a novel approach to therapy (protease inhibition).
    Journal of Molecular and Cellular Cardiology 06/2014; 74. DOI:10.1016/j.yjmcc.2014.06.010 · 4.66 Impact Factor
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    • "For double-labeling experiments, cells were incubated simultaneously with both mouse anti-mCherry monoclonal and rabbit anti-GFP antibodies (Abcam). Immunoblots were performed as described earlier [10] using cell homogenates prepared 48 h after transfection with connexin DNA as described by Gong et al. [11]. For immunofluorescence microscopy, cultured cells were fixed in methanol/acetone (1:1), stained as previously described [8], and imaged using a Zeiss Axioplan 2 microscope or Leica TCS SP2 laser-scanning confocal microscope. "
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    ABSTRACT: Mutations of Cx40 (GJA5) have been identified in people with lone chronic atrial fibrillation including G38D and M163V which were found in the same patient. We used dual whole cell patch clamp procedures to examine the transjunctional voltage (Vj) gating and channel conductance properties of these two rare mutants. Each mutant exhibited slight alterations of Vj gating properties and increased the gap junction channel conductance (γj) by 20-30 pS. While co-expression of the two mutations had similar effects on Vj gating, it synergistically increased γj by 50%. Unlike WTCx40 or M163V, G38D induced activity of a dominant 271 pS hemichannel.
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