Heterogeneity of ATP-sensitive K+ channels in cardiac myocytes: enrichment at the intercalated disk.

Ventricular ATP-sensitive potassium (K(ATP)) channels link intracellular energy metabolism to membrane excitability and contractility. Our recent proteomics experiments identified plakoglobin and plakophilin-2 (PKP2) as putative K(ATP) channel-associated proteins. We investigated whether the association of K(ATP) channel subunits with junctional proteins translates to heterogeneous subcellular distribution within a cardiac myocyte. Co-immunoprecipitation experiments confirmed physical interaction between K(ATP) channels and PKP2 and plakoglobin in rat heart. Immunolocalization experiments demonstrated that K(ATP) channel subunits (Kir6.2 and SUR2A) are expressed at a higher density at the intercalated disk in mouse and rat hearts, where they co-localized with PKP2 and plakoglobin. Super-resolution microscopy demonstrate that K(ATP) channels are clustered within nanometer distances from junctional proteins. The local K(ATP) channel density, recorded in excised inside-out patches, was larger at the cell end when compared with local currents recorded from the cell center. The K(ATP) channel unitary conductance, block by MgATP and activation by MgADP, did not differ between these two locations. Whole cell K(ATP) channel current density (activated by metabolic inhibition) was ∼40% smaller in myocytes from mice haploinsufficient for PKP2. Experiments with excised patches demonstrated that the regional heterogeneity of K(ATP) channels was absent in the PKP2 deficient mice, but the K(ATP) channel unitary conductance and nucleotide sensitivities remained unaltered. Our data demonstrate heterogeneity of K(ATP) channel distribution within a cardiac myocyte. The higher K(ATP) channel density at the intercalated disk implies a possible role at the intercellular junctions during cardiac ischemia.

Deletion of the last five C-terminal amino acid residues of connexin43 leads to lethal ventricular arrhythmias in mice without affecting coupling via gap junction channels.

The cardiac intercalated disc harbors mechanical and electrical junctions as well as ion channel complexes mediating propagation of electrical impulses. Cardiac connexin43 (Cx43) co-localizes and interacts with several of the proteins located at intercalated discs in the ventricular myocardium. We have generated conditional Cx43D378stop mice lacking the last five C-terminal amino acid residues, representing a binding motif for zonula occludens protein-1 (ZO-1), and investigated the functional consequences of this mutation on cardiac physiology and morphology. Newborn and adult homozygous Cx43D378stop mice displayed markedly impaired and heterogeneous cardiac electrical activation properties and died from severe ventricular arrhythmias. Cx43 and ZO-1 were co-localized at intercalated discs in Cx43D378stop hearts, and the Cx43D378stop gap junction channels showed normal coupling properties. Patch clamp analyses of isolated adult Cx43D378stop cardiomyocytes revealed a significant decrease in sodium and potassium current densities. Furthermore, we also observed a significant loss of Nav1.5 protein from intercalated discs in Cx43D378stop hearts. The phenotypic lethality of the Cx43D378stop mutation was very similar to the one previously reported for adult Cx43 deficient (Cx43KO) mice. Yet, in contrast to Cx43KO mice, the Cx43 gap junction channel was still functional in the Cx43D378stop mutant. We conclude that the lethality of Cx43D378stop mice is independent of the loss of gap junctional intercellular communication, but most likely results from impaired cardiac sodium and potassium currents. The Cx43D378stop mice reveal for the first time that Cx43 dependent arrhythmias can develop by mechanisms other than impairment of gap junction channel function.

Remodeling of the cardiac sodium channel, connexin43, and plakoglobin at the intercalated disk in patients with arrhythmogenic cardiomyopathy.

BACKGROUND: Arrhythmogenic cardiomyopathy (AC) is closely associated with desmosomal mutations in a majority of patients. Arrhythmogenesis in patients with AC is likely related to remodeling of cardiac gap junctions and increased levels of fibrosis. Recently, using experimental models, we also identified sodium channel dysfunction secondary to desmosomal dysfunction. OBJECTIVE: To assess the immunoreactive signal levels of the sodium channel protein NaV1.5, as well as connexin43 (Cx43) and plakoglobin (PKG), in myocardial specimens obtained from patients with AC. METHODS: Left and right ventricular free wall postmortem material was obtained from 5 patients with AC and 5 controls matched for age and sex. Right ventricular septal biopsies were taken from another 15 patients with AC. All patients fulfilled the 2010 revised Task Force Criteria for the diagnosis of AC. Immunohistochemical analyses were performed using antibodies against Cx43, PKG, NaV1.5, plakophilin-2, and N-cadherin. RESULTS: N-cadherin and desmoplakin immunoreactive signals and distribution were normal in patients with AC compared to controls. Plakophilin-2 signals were unaffected unless a plakophilin-2 mutation predicting haploinsufficiency was present. Distribution was unchanged compared to that in controls. Immunoreactive signal levels of PKG, Cx43, and NaV1.5 were disturbed in 74%, 70%, and 65% of the patients, respectively. CONCLUSIONS: A reduced immunoreactive signal of PKG, Cx43, and NaV1.5 at the intercalated disks can be observed in a large majority of the patients. Decreased levels of Nav1.5 might contribute to arrhythmia vulnerability and, in the future, potentially could serve as a new clinically relevant tool for risk assessment strategies.

Remodeling of mechanical junctions and of microtubule-associated proteins accompany cardiac connexin43 lateralization.

BACKGROUND: Desmosomes and adherens junctions provide mechanical continuity between cardiac cells, whereas gap junctions allow for cell-cell electrical/metabolic coupling. These structures reside at the cardiac intercalated disc (ID). Also at the ID is the voltage-gated sodium channel (VGSC) complex. Functional interactions between desmosomes, gap junctions, and VGSC have been demonstrated. Separate studies show, under various conditions, reduced presence of gap junctions at the ID and redistribution of connexin43 (Cx43) to plaques oriented parallel to fiber direction (gap junction "lateralization"). OBJECTIVE: To determine the mechanisms of Cx43 lateralization, and the fate of desmosomal and sodium channel molecules in the setting of Cx43 remodeling. METHODS: Adult sheep were subjected to right ventricular pressure overload (pulmonary hypertension). Tissue was analyzed by quantitative confocal microscopy and by transmission electron microscopy. Ionic currents were measured using conventional patch clamp. RESULT: Quantitative confocal microscopy demonstrated lateralization of immunoreactive junctional molecules. Desmosomes and gap junctions in lateral membranes were demonstrable by electron microscopy. Cx43/desmosomal remodeling was accompanied by lateralization of 2 microtubule-associated proteins relevant for Cx43 trafficking: EB1 and kinesin protein Kif5b. In contrast, molecules of the VGSC failed to reorganize in plaques discernable by confocal microscopy. Patch-clamp studies demonstrated change in amplitude and kinetics of sodium current and a small reduction in electrical coupling between cells. CONCLUSIONS: Cx43 lateralization is part of a complex remodeling that includes mechanical and gap junctions but may exclude components of the VGSC. We speculate that lateralization results from redirectionality of microtubule-mediated forward trafficking. Remodeling of junctional complexes may preserve electrical synchrony under conditions that disrupt ID integrity.

Sodium current deficit and arrhythmogenesis in a murine model of plakophilin-2 haploinsufficiency.

AIMS: The shRNA-mediated loss of expression of the desmosomal protein plakophilin-2 leads to sodium current (I(Na)) dysfunction. Whether pkp2 gene haploinsufficiency leads to I(Na) deficit in vivo remains undefined. Mutations in pkp2 are detected in arrhythmogenic right ventricular cardiomyopathy (ARVC). Ventricular fibrillation and sudden death often occur in the 'concealed phase' of the disease, prior to overt structural damage. The mechanisms responsible for these arrhythmias remain poorly understood. We sought to characterize the morphology, histology, and ultrastructural features of PKP2-heterozygous-null (PKP2-Hz) murine hearts and explore the relation between PKP2 abundance, I(Na) function, and cardiac electrical synchrony. METHODS AND RESULTS: Hearts of PKP2-Hz mice were characterized by multiple methods. We observed ultrastructural but not histological or gross anatomical differences in PKP2-Hz hearts compared with wild-type (WT) littermates. Yet, in myocytes, decreased amplitude and a shift in gating and kinetics of I(Na) were observed. To further unmask I(Na) deficiency, we exposed myocytes, Langendorff-perfused hearts, and anaesthetized animals to a pharmacological challenge (flecainide). In PKP2-Hz hearts, the extent of flecainide-induced I(Na) block, impaired ventricular conduction, and altered electrocardiographic parameters were larger than controls. Flecainide provoked ventricular arrhythmias and death in PKP2-Hz animals, but not in the WT. CONCLUSIONS: PKP2 haploinsufficiency leads to I(Na) deficit in murine hearts. Our data support the notion of a cross-talk between desmosome and sodium channel complex. They also suggest that I(Na) dysfunction may contribute to generation and/or maintenance of arrhythmias in PKP2-deficient hearts. Whether pharmacological challenges could help unveil arrhythmia risk in patients with mutations or variants in PKP2 remains undefined.

A connexin40 mutation associated with a malignant variant of progressive familial heart block type I.

BACKGROUND: Progressive familial heart block type I (PFHBI) is a hereditary arrhythmia characterized by progressive conduction disturbances in the His-Purkinje system. PFHBI has been linked to genes such as SCN5A that influence cardiac excitability but not to genes that influence cell-to-cell communication. Our goal was to explore whether nucleotide substitutions in genes coding for connexin proteins would associate with clinical cases of PFHBI and if so, to establish a genotype-cell phenotype correlation for that mutation. METHODS AND RESULTS: We screened 156 probands with PFHBI. In addition to 12 sodium channel mutations, we found a germ line GJA5 (connexin40 [Cx40]) mutation (Q58L) in 1 family. Heterologous expression of Cx40-Q58L in connexin-deficient neuroblastoma cells resulted in marked reduction of junctional conductance (Cx40-wild type [WT], 22.2±1.7 nS, n=14; Cx40-Q58L, 0.56±0.34 nS, n=14; P<0.001) and diffuse localization of immunoreactive proteins in the vicinity of the plasma membrane without formation of gap junctions. Heteromeric cotransfection of Cx40-WT and Cx40-Q58L resulted in homogenous distribution of proteins in the plasma membrane rather than in membrane plaques in ≈50% of cells; well-defined gap junctions were observed in other cells. Junctional conductance values correlated with the distribution of gap junction plaques. CONCLUSIONS: Mutation Cx40-Q58L impairs gap junction formation at cell-cell interfaces. This is the first demonstration of a germ line mutation in a connexin gene that associates with inherited ventricular arrhythmias and emphasizes the importance of Cx40 in normal propagation in the specialized conduction system.

The formin FRL1 (FMNL1) is an essential component of macrophage podosomes.

Podosomes are highly dynamic actin-rich adhesion structures in cells of myeloid lineage and some transformed cells. Unlike transformed mesenchymal cell types, podosomes are the sole adhesion structure in macrophage and thus mediate all contact with adhesion substrate, including movement through complex tissues for immune surveillance. The existence of podosomes in inflammatory macrophages and transformed cell types suggest an important role in tissue invasion. The proteome, assembly, and maintenance of podosomes are emerging, but remain incompletely defined. Previously, we reported a formin homology sequence and actin assembly activity in association with macrophage beta-3 integrin. In this study we demonstrate by quantitative reverse transcriptase polymerase chain reaction and Western blotting that the formin FRL1 is specifically upregulated during monocyte differentiation to macrophages. We show that the formin FRL1 localizes to the actin-rich cores of primary macrophage podosomes. FRL1 co-precipitates with beta-3 integrin and both fixed and live cell fluorescence microscopy show that endogenous and overexpressed FRL1 selectively localize to macrophage podosomes. Targeted disruption of FRL1 by siRNA results in reduced cell adhesion and disruption of podosome dynamics. Our data suggest that FRL1 is responsible for modifying actin at the macrophage podosome and may be involved in actin cytoskeleton dynamics during adhesion and migration within tissues.