Mono-Heteromeric Configurations of Gap Junction Channels Formed by Connexin43 and Connexin45 Reduce Unitary Conductance and Determine both Voltage Gating and Metabolic Flux Asymmetry

Cx45 regulation by kinases and impact of expression in heart failure

Phosphorylation plays a crucial role in connexin regulation by modulating gap junction intercellular communication (GJIC), localization, stability, and interactions with signaling proteins. Few kinases are known to phosphorylate Cx45, and their target residues remain unknown. A phosphorylation screen identified several Cx45-targeting kinases activated in heart disease, among which c-Src was found by mass spectroscopy to phosphorylate residues Y324 and Y356. Unlike Cx43, c-Src phosphorylation of Cx45 did not impair GJIC, alter junctional localization, or affect interactions with cytoskeletal proteins β-tubulin, Drebrin, and ZO-1. In LA-25 cells where Cx43 is internalized after temperature sensitive activation of v-Src, expression of Cx45 unexpectedly maintained Cx43 at the plasma membrane. Phospho-specific antibodies helped identify that while Cx43 had a tyrosine phosphorylation pattern favoring turnover, the serine phosphorylation pattern was conducive for GJIC. Furthermore, in a rat model of heart failure, Cx45 was expressed in the ventricle and co-localized with Cx43, leading to altered dye coupling indicative of a shift toward Cx45-like channel permeability. Altogether, our data suggests that in heart failure, c-Src activation on its own would not have an adverse effect on Cx45 function and that aberrant Cx45 expression helps Cx43 transport to and maintain at the intercalated disc. Yet the dominant effect of Cx45 in heteromeric channels could ultimately make Cx45 a key driver of cardiac dysfunction. Finally, the observation that Cx45-mediated coupling remains functional even in the same pathological environment where Cx43-mediated communication is inhibited suggests that kinase regulation of connexins is isoform-specific and not universally predictable.

Long Noncoding RNA HOTAIR Functions as a Competitive Endogenous RNA to Regulate Connexin43 Remodeling in Atrial Fibrillation by Sponging MicroRNA-613

Several studies have indicated that long noncoding RNAs (lncRNAs)-HOX transcript antisense RNA (HOTAIR) is involved in some cardiovascular diseases by regulating gene expression as a competitive endogenous RNA (ceRNA). GJA1 encoding Cx43 is one potential target gene of microRNA-613 (miR-613). Meanwhile, there is a potential target regulatory relationship between HOTAIR and miR-613. The present study is aimed at investigating whether HOTAIR functions as a ceRNA to regulate the Cx43 expression in atrial fibrillation (AF) by sponging miR-613. The expressions of HOTAIR, miR-613, and Cx43 were detected in the right atrial appendages of 45 patients with heart valve disease, including 23 patients with chronic AF. The HOTAIR overexpressed and underexpressed HL-1 cell model were constructed to confirm the effect of HOTAIR on Cx43. Then, the Cx43 expression was detected to testify the interplay between HOTAIR and miR-613 after cotransfecting HOTAIR and miR-613. Furthermore, luciferase assays were performed to verify that HOTAIR could regulate Cx43 remolding as a ceRNA by sponging miR-613. The expression of HOTAIR and Cx43 was significantly downregulated in chronic AF group. HOTAIR regulated positively the Cx43 expression in HL-1 cells. The upregulated effect of HOTAIR on the Cx43 expression could be remarkably attenuated by miR-613. Moreover, the inhibitory effect of miR-613 on the Cx43 expression could be obviously mitigated by HOTAIR. At last, luciferase assays confirmed HOTAIR functioned as a ceRNA in the Cx43 expression by sponging miR-613. Our study suggests that HOTAIR, functioning as a ceRNA by sponging miR-613, is an important contributor to Cx43 remolding in AF.

Connexins-Based Hemichannels/Channels and Their Relationship with Inflammation, Seizures and Epilepsy

Connexins (Cxs) are a family of 21 protein isoforms, eleven of which are expressed in the central nervous system, and they are found in neurons and glia. Cxs form hemichannels (connexons) and channels (gap junctions/electric synapses) that permit functional and metabolic coupling between neurons and astrocytes. Altered Cx expression and function is involved in inflammation and neurological diseases. Cxs-based hemichannels and channels have a relevance to seizures and epilepsy in two ways: First, this pathological condition increases the opening probability of hemichannels in glial cells to enable gliotransmitter release, sustaining the inflammatory process and exacerbating seizure generation and epileptogenesis, and second, the opening of channels favors excitability and synchronization through coupled neurons. These biological events highlight the global pathological mechanism of epilepsy, and the therapeutic potential of Cxs-based hemichannels and channels. Therefore, this review describes the role of Cxs in neuroinflammation and epilepsy and examines how the blocking of channels and hemichannels may be therapeutic targets of anti-convulsive and anti-epileptic treatments.

Anisotropic Panglial Coupling Reflects Tonotopic Organization in the Inferior Colliculus

Astrocytes and oligodendrocytes in different brain regions form panglial networks and the topography of such networks can correlate with neuronal topography and function. Astrocyte-oligodendrocyte networks in the lateral superior olive (LSO)-an auditory brainstem nucleus-were found to be anisotropic with a preferred orientation orthogonally to the tonotopic axis. We hypothesized that such a specialization might be present in other tonotopically organized brainstem nuclei, too. Thus, we analyzed gap junctional coupling in the center of the inferior colliculus (IC)-another nucleus of the auditory brainstem that exhibits tonotopic organization. In acute brainstem slices obtained from mice, IC networks were traced employing whole-cell patch-clamp recordings of single sulforhodamine (SR) 101-identified astrocytes and concomitant intracellular loading of the gap junction-permeable tracer neurobiotin. The majority of dye-coupled networks exhibited an oval topography, which was preferentially oriented orthogonal to the tonotopic axis. Astrocyte processes showed preferentially the same orientation indicating a correlation between astrocyte and network topography. In addition to SR101-positive astrocytes, IC networks contained oligodendrocytes. Using Na⁺ imaging, we analyzed the capability of IC networks to redistribute small ions. Na⁺ bi-directionally diffused between SR101-positive astrocytes and SR101-negative cells-presumably oligodendrocytes-showing the functionality of IC networks. Taken together, our results demonstrate that IC astrocytes and IC oligodendrocytes form functional anisotropic panglial networks that are preferentially oriented orthogonal to the tonotopic axis. Thus, our data indicate that the topographic specialization of glial networks seen in IC and LSO might be a general feature of tonotopically organized auditory brainstem nuclei.

Intramolecular signaling in a cardiac connexin: Role of cytoplasmic domain dimerization

Gap junctions, composed of connexins, mediate electrical coupling and impulse propagation in the working myocardium. In the human heart, the spatio-temporal regulation and distinct functional properties of the three dominant connexins (Cx43, Cx45, and Cx40) suggests non-redundant physiological roles for each isoform. There are substantial differences in gating properties, expression, and trafficking among these isoforms, however, little is known about the determinants of these different phenotypes. To gain insight regarding these determinants, we focused on the carboxyl-terminal (CT) domain because of its importance in channel regulation and large degree of sequence divergence among connexin family members. Using in vitro biophysical experiments, we identified a structural feature unique to Cx45: high affinity (K_D~100nM) dimerization between CT domains. In this study, we sought to determine if this dimerization occurs in cells and to identify the biological significance of the dimerization. Using a bimolecular fluorescence complementation assay, we demonstrate that the CT domains dimerize at the plasma membrane. By inhibiting CT dimerization with a mutant construct, we show that CT dimerization is necessary for proper Cx45 membrane localization, turnover, phosphorylation status, and binding to protein partners. Furthermore, CT dimerization is needed for normal intercellular communication and hemichannel activity. Altogether, our results demonstrate that CT dimerization is a structural feature important for correct Cx45 function. This study is significant because discovery of how interactions mediated by the CT domains can be modulated would open the door to strategies to ameliorate the pathological effects of altered connexin regulation in the failing heart.