Recent studies have reported a relationship between the abundance of epicardial adipose tissue (EAT) and the risk of cardiovascular diseases including atrial fibrillation (AF). However, the underlying mechanisms are unknown. The aim of this study was to examine the effects of the secretome of human EAT on the histological properties of the myocardium. Samples of EAT and subcutaneous adipose (SAT), obtained from 39 patients undergoing coronary bypass surgery, were analysed and tested in an organo-culture model of rat atria to evaluate the fibrotic properties of human fat depots. The EAT secretome induced global fibrosis (interstitial and peripheral) of rat atria in organo-culture conditions. Activin A was highly expressed in EAT compared with SAT and promoted atrial fibrosis, an effect blocked using neutralizing antibody. In addition, Activin A levels were enhanced in patients with low left-ventricular function. In sections of human atrial and ventricular myocardium, adipose and myocardial tissues were in close contact, together with fibrosis. This study provides the first evidence that the secretome from EAT promotes myocardial fibrosis through the secretion of adipo-fibrokines such as Activin A.

Rationale:: The cardiac sodium channel Na _v 1.5 plays a key role in excitability and conduction. The 3 last residues of Na _v 1.5 (Ser-Ile-Val) constitute a PDZ-domain binding motif that interacts with the syntrophin–dystrophin complex. As dystrophin is absent at the intercalated discs, Na _v 1.5 could potentially interact with other, yet unknown, proteins at this site. Objective:: The aim of this study was to determine whether Na _v 1.5 is part of distinct regulatory complexes at lateral membranes and intercalated discs. Methods and Results:: Immunostaining experiments demonstrated that Na _v 1.5 localizes at lateral membranes of cardiomyocytes with dystrophin and syntrophin. Optical measurements on isolated dystrophin-deficient mdx hearts revealed significantly reduced conduction velocity, accompanied by strong reduction of Na _v 1.5 at lateral membranes of mdx cardiomyocytes. Pull-down experiments revealed that the MAGUK protein SAP97 also interacts with the SIV motif of Na _v 1.5, an interaction specific for SAP97 as no pull-down could be detected with other cardiac MAGUK proteins (PSD95 or ZO-1). Furthermore, immunostainings showed that Na _v 1.5 and SAP97 are both localized at intercalated discs. Silencing of SAP97 expression in HEK293 and rat cardiomyocytes resulted in reduced sodium current ( I _Na ) measured by patch-clamp. The I _Na generated by Na _v 1.5 channels lacking the SIV motif was also reduced. Finally, surface expression of Na _v 1.5 was decreased in silenced cells, as well as in cells transfected with SIV-truncated channels. Conclusions:: These data support a model with at least 2 coexisting pools of Na _v 1.5 channels in cardiomyocytes: one targeted at lateral membranes by the syntrophin-dystrophin complex, and one at intercalated discs by SAP97.

Background—: Brugada syndrome (BrS) is caused mainly by mutations in the SCN5A gene, which encodes the α-subunit of the cardiac sodium channel Na _v 1.5. However, ≈20% of probands have SCN5A mutations, suggesting the implication of other genes. MOG1 recently was described as a new partner of Na _v 1.5, playing a potential role in the regulation of its expression and trafficking. We investigated whether mutations in MOG1 could cause BrS. Methods and Results—: MOG1 was screened by direct sequencing in patients with BrS and idiopathic ventricular fibrillation. A missense mutation p.Glu83Asp (E83D) was detected in a symptomatic female patient with a type-1 BrS ECG but not in 281 controls. Wild type (WT)- and mutant E83D-MOG1 were expressed in HEK Na _v 1.5 stable cells and studied using patch-clamp assays. Overexpression of WT-MOG1 alone doubled sodium current (I _Na ) density compared to control conditions ( P <0.01). In contrast, overexpression of mutant E83D alone or E83D+WT failed to increase I _Na ( P <0.05), demonstrating the dominant-negative effect of the mutant. Microscopy revealed that Na _v 1.5 channels failed to properly traffic to the cell membrane in the presence of the mutant. Silencing endogenous MOG1 demonstrated a 54% decrease in I _Na density. Conclusions—: Our results support the hypothesis that dominant-negative mutations in MOG1 can impair the trafficking of Na _v 1.5 to the membrane, leading to I _Na reduction and clinical manifestation of BrS. Moreover, silencing MOG1 reduced I _Na , demonstrating that MOG1 is likely to be important in the surface expression of Na _v 1.5 channels. All together, our data support MOG1 as a new susceptibility gene for BrS.

Cardiac myocytes are characterized by distinct structural and functional entities involved in the generation and transmission of the action potential and the excitation-contraction coupling process. Key to their function is the specific organization of ion channels and transporters to and within distinct membrane domains, which supports the anisotropic propagation of the depolarization wave. This review addresses the current knowledge on the molecular actors regulating the distinct trafficking and targeting mechanisms of ion channels in the highly polarized cardiac myocyte. In addition to ubiquitous mechanisms shared by other excitable cells, cardiac myocytes show unique specialization, illustrated by the molecular organization of myocyte-myocyte contacts, e.g., the intercalated disc and the gap junction. Many factors contribute to the specialization of the cardiac sarcolemma and the functional expression of cardiac ion channels, including various anchoring proteins, motors, small GTPases, membrane lipids, and cholesterol. The discovery of genetic defects in some of these actors, leading to complex cardiac disorders, emphasizes the importance of trafficking and targeting of ion channels to cardiac function. A major challenge in the field is to understand how these and other actors work together in intact myocytes to fine-tune ion channel expression and control cardiac excitability.

Cholesterol is an important determinant of cardiac electrical properties. However, underlying mechanisms are still poorly understood. Here, we examine the hypothesis that cholesterol modulates the turnover of voltage-gated potassium channels based on previous observations showing that depletion of membrane cholesterol increases the atrial repolarizing current I _Kur . Whole-cell currents and single-channel activity were recorded in rat adult atrial myocytes (AAM) or after transduction with hKv1.5-EGFP. Channel mobility and expression were studied using fluorescence recovery after photobleaching (FRAP) and 3-dimensional microscopy. In both native and transduced-AAMs, the cholesterol-depleting agent MβCD induced a delayed (≈7 min) increase in I _Kur ; the cholesterol donor LDL had an opposite effect. Single-channel recordings revealed an increased number of active Kv1.5 channels upon MβCD application. Whole-cell recordings indicated that this increase was not dependent on new synthesis but on trafficking of existing pools of intracellular channels whose exocytosis could be blocked by both N -ethylmaleimide and nonhydrolyzable GTP analogues. Rab11 was found to coimmunoprecipitate with hKv1.5-EGFP channels and transfection with Rab11 dominant negative (DN) but not Rab4 DN prevented the MβCD-induced I _Kur increase. Three-dimensional microscopy showed a decrease in colocalization of Kv1.5 and Rab11 in MβCD-treated AAM. These results suggest that cholesterol regulates Kv1.5 channel expression by modulating its trafficking through the Rab11-associated recycling endosome. Therefore, this compartment provides a submembrane pool of channels readily available for recruitment into the sarcolemma of myocytes. This process could be a major mechanism for the tuning of cardiac electrical properties and might contribute to the understanding of cardiac effects of lipid-lowering drugs.