JNK bond regulation: why do mammalian hearts invest in connexin43?

The Cardiac Gap Junction has Discrete Functions in Electrotonic and Ephaptic Coupling

Connexin43-formed gap junctions have long been thought to contribute to cardiac conduction in the mammalian ventricle by providing direct electrotonic connectivity between the cytoplasms of neighboring cardiomyocytes. However, accumulating data from studies of non-mammalian hearts, Connexin 43 (Cx43) knockout mice and human Cx43 mutations have raised questions as to whether gap junctions are the sole means by which electrical coupling between cardiomyocytes is accomplished. Computational and experimental work over the last decade have indicated that intercellular propagation of action potentials in the heart may involve both electrotonic and ephaptic contributions-in what has been dubbed "mixed-mode" conduction. Interestingly, the Cx43 gap junction may provide a common structural platform in mammals that facilitates the operation of these two mechanisms. In addition to gap junction channels functioning in an electrotonic role, the perinexus region at the edge of gap junctions may be provide a niche for voltage-gated sodium channels from neighboring cells to be in sufficiently close proximity to enable ephaptic transmission of action potential. A novel role has recently been identified in this potential ephaptic mechanism for inter-membrane adhesion mediated by the beta subunit (beta1/Scn1b) of the sodium channel. The new perspective of the operational redundancy that is built into cardiac electrical connectivity could provide new understanding of arrhythmia mechanisms and holds the promise for new approach to anti-arrhythmic therapy. Anat Rec, 302:93-100, 2019. © 2018 Wiley Periodicals, Inc.

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

AIMS

More than 40 mutations in the GJA1 gene encoding connexin43 (Cx43) have been linked to oculodentodigital dysplasia (ODDD), a pleiotropic, autosomal dominant disorder. We hypothesized that even with a significant reduction in the levels of Cx43 in a mutant mouse model of ODDD (Gja1(Jrt/+)) harbouring a G60S mutation (Cx43(G60S)), cardiomyocyte function may only be moderately compromised given that a majority of mutant mice typically survive.

METHODS AND RESULTS

Western blotting and quantitative reverse transcriptase-polymerase chain reaction in conjunction with immunofluorescence were used to assess the expression and localization of Cx43 in hearts and cultured cardiomyocytes from wild-type and Gja1(Jrt/+) mice. Dye-coupling and dual whole cell patch-clamp recordings were also used to assess the gap junction channel status in cultured cardiomyocytes from wild-type and mutant mice. Cardiac tissue from adult Gja1(Jrt/+) mice revealed a 60-80% reduction in Cx43 protein with a preferential loss of the highly phosphorylated forms of Cx43. Compensation via the up-regulation of Cx40 or Cx45 was not observed. Immunofluorescent analysis of cultured cardiomyocytes revealed a trafficking defect, with a decrease in Cx43 plaques and a large population of Cx43 being retained in the Golgi apparatus. However, cultured cardiomyocytes from mutant mice remained beating with a 50% decrease in coupling conductance.

CONCLUSION

These results suggest that the Cx43(G60S) mutant impairs normal trafficking and function of co-expressed Cx43 with no dramatic effect on cardiomyocyte function, suggesting that Cx43 is biosynthesized in excess of an essential need.

Zonula occludens-1 and connexin 43 expression in the failing human heart

Focal disorganization of gap junctional distribution and down-regulation of the major gap junctional protein connexin 43 are typical features of myocardial remodelling in the failing human heart. Increasing evidence indicates that connexin 43 interacts with zonula-occludens-1 (ZO-1), and it has recently been shown that ZO-1 promotes the formation and growth of gap junctional plaques. In the present study, distribution patterns of ZO-1 and connexin 43 were studied in normal and in heart failure patients using double-label immunohistochemistry and confocal microscopy. ZO-1 was found to be co-localized with connexin 43 at intercalated disks. Importantly, in patients with heart failure due to dilated or ischaemic cardimyopathy, areas of diminished connexin 43 expression were characterized by a markedly reduced ZO-1 staining. Based on these data it is concluded that in patients with heart failure, down-regulation of ZO-1 matches the diminished expression levels of connexin 43, suggesting that ZO-1 plays an important role in gap junction formation and gap junction plaque stability.

The unstoppable connexin43 carboxyl-terminus: new roles in gap junction organization and wound healing

Intercellular connectivity mediated by gap junctions (GJs) composed of connexin43 (Cx43) is critical to the function of excitable tissues such as the heart and brain. Disruptions to Cx43 GJ organization are thought to be a factor in cardiac arrhythmias and are also implicated in epilepsy. This article is based on a presentation to the 4th Larry and Horti Fairberg Workshop on Interactive and Integrative Cardiology and summarizes the work of Gourdie and his lab on Cx43 GJs in the heart. Background and perspective of recently published studies on the function of Cx43-interacting protein zonula occludens-(ZO)-1 in determining the organization of GJ plaques are provided. In addition how a peptide containing a PDZ-binding sequence of Cx43, developed as part of the work on cardiac GJ organization is also described, which has led to evidence for novel and unexpected roles for Cx43 in modulating healing following tissue injury.

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

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

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

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

Ischemic preconditioning preserves connexin 43 phosphorylation during sustained ischemia in pig hearts in vivo

During myocardial ischemia, connexin 43 (Cx43) is dephosphorylated in vitro, and the subsequent opening of gap junctions formed by two opposing Cx43 hexamers was suggested to propagate ischemia/reperfusion injury. Reduction of infarct size (IS) by ischemic preconditioning (IP) involves activation of protein kinase C (PKC) and p38 mitogen activated protein kinase (MAPK), both of which can phosphorylate Cx43. We now studied in anesthetized pigs whether IP impacts on Cx43 phosphorylation by measuring the density of non-phosphorylated and total Cx43 (confocal laser) during normoperfusion and 90-min ischemia in non-preconditioned and preconditioned hearts. Co-localization of PKCalpha, p38MAPKalpha, and p38MAPKbeta with Cx43 and the activity of p38MAPK were assessed. IP by 10 min ischemia and 15 min reperfusion reduced IS. Non-phosphorylated Cx43 remained unchanged during ischemia in preconditioned hearts, while it increased from 35+/-3 to 75+/-8 AU (P<0.05) in non-preconditioned hearts. Co-localization of PKCalpha, p38MAPKalpha, and p38MAPKbeta with Cx43 during ischemia increased only in preconditioned hearts. While the ischemia-induced increase in p38MAPKalpha activity was comparable in preconditioned and non-preconditioned hearts, p38MAPKbeta activity was increased only in preconditioned hearts. Blockade of p38MAPK by SB203580 attenuated the IS-reduction and the increased p38MAPK-Cx43 co-localization by IP. We conclude that IP increases co-localization of protein kinases with Cx43 and preserves phosphorylation of Cx43 during ischemia.

Development of the cardiac pacemaking and conduction system

The heartbeat is initiated and coordinated by a heterogeneous set of tissues, collectively referred to as the pacemaking and conduction system (PCS). While the structural and physiological properties of these specialized tissues has been studied for more than a century, distinct new insights have emerged in recent years. The tools of molecular biology and the lessons of modern embryology are beginning to uncover the mechanisms governing induction, patterning and developmental integration of the PCS. In particular, significant advances have been made in understanding the developmental biology of the fast conduction network in the ventricles--the His-Purkinje system. Although this progress has largely been made by using animal models such as the chick and mouse, the insights gained may help explain cardiac disease in humans, as well as lead to new treatment strategies.