Cardiac-specific ablation of synapse-associated protein SAP97 in mice decreases potassium currents but not sodium current

The Kir2.1E299V mutation increases atrial fibrillation vulnerability while protecting the ventricles against arrhythmias in a mouse model of short QT syndrome type 3

AIMS

Short QT syndrome type 3 (SQTS3) is a rare arrhythmogenic disease caused by gain-of-function mutations in KCNJ2, the gene coding the inward rectifier potassium channel Kir2.1. We used a multidisciplinary approach and investigated arrhythmogenic mechanisms in an in-vivo model of de-novo mutation Kir2.1E299V identified in a patient presenting an extremely abbreviated QT interval and paroxysmal atrial fibrillation.

METHODS AND RESULTS

We used intravenous adeno-associated virus-mediated gene transfer to generate mouse models, and confirmed cardiac-specific expression of Kir2.1WT or Kir2.1E299V. On ECG, the Kir2.1E299V mouse recapitulated the QT interval shortening and the atrial-specific arrhythmia of the patient. The PR interval was also significantly shorter in Kir2.1E299V mice. Patch-clamping showed extremely abbreviated action potentials in both atrial and ventricular Kir2.1E299V cardiomyocytes due to a lack of inward-going rectification and increased IK1 at voltages positive to -80 mV. Relative to Kir2.1WT, atrial Kir2.1E299V cardiomyocytes had a significantly reduced slope conductance at voltages negative to -80 mV. After confirming a higher proportion of heterotetrameric Kir2.x channels containing Kir2.2 subunits in the atria, in-silico 3D simulations predicted an atrial-specific impairment of polyamine block and reduced pore diameter in the Kir2.1E299V-Kir2.2WT channel. In ventricular cardiomyocytes, the mutation increased excitability by shifting INa activation and inactivation in the hyperpolarizing direction, which protected the ventricle against arrhythmia. Moreover, Purkinje myocytes from Kir2.1E299V mice manifested substantially higher INa density than Kir2.1WT, explaining the abbreviation in the PR interval.

CONCLUSION

The first in-vivo mouse model of cardiac-specific SQTS3 recapitulates the electrophysiological phenotype of a patient with the Kir2.1E299V mutation. Kir2.1E299V eliminates rectification in both cardiac chambers but protects against ventricular arrhythmias by increasing excitability in both Purkinje-fiber network and ventricles. Consequently, the predominant arrhythmias are supraventricular likely due to the lack of inward rectification and atrial-specific reduced pore diameter of the Kir2.1E299V-Kir2.2WT heterotetramer.

The network of cardiac KIR2.1: its function, cellular regulation, electrical signaling, diseases and new drug avenues

The functioning of the human heart relies on complex electrical and communication systems that coordinate cardiac contractions and sustain rhythmicity. One of the key players contributing to this intricate system is the KIR2.1 potassium ion channel, which is encoded by the KCNJ2 gene. KIR2.1 channels exhibit abundant expression in both ventricular myocytes and Purkinje fibers, exerting an important role in maintaining the balance of intracellular potassium ion levels within the heart. And by stabilizing the resting membrane potential and contributing to action potential repolarization, these channels have an important role in cardiac excitability also. Either gain- or loss-of-function mutations, but also acquired impairments of their function, are implicated in the pathogenesis of diverse types of cardiac arrhythmias. In this review, we aim to elucidate the system functions of KIR2.1 channels related to cellular electrical signaling, communication, and their contributions to cardiovascular disease. Based on this knowledge, we will discuss existing and new pharmacological avenues to modulate their function.

Ion channel trafficking implications in heart failure

Heart failure (HF) is recognized as an epidemic in the contemporary world, impacting around 1%-2% of the adult population and affecting around 6 million Americans. HF remains a major cause of mortality, morbidity, and poor quality of life. Several therapies are used to treat HF and improve the survival of patients; however, despite these substantial improvements in treating HF, the incidence of HF is increasing rapidly, posing a significant burden to human health. The total cost of care for HF is USD 69.8 billion in 2023, warranting a better understanding of the mechanisms involved in HF. Among the most serious manifestations associated with HF is arrhythmia due to the electrophysiological changes within the cardiomyocyte. Among these electrophysiological changes, disruptions in sodium and potassium currents' function and trafficking, as well as calcium handling, all of which impact arrhythmia in HF. The mechanisms responsible for the trafficking, anchoring, organization, and recycling of ion channels at the plasma membrane seem to be significant contributors to ion channels dysfunction in HF. Variants, microtubule alterations, or disturbances of anchoring proteins lead to ion channel trafficking defects and the alteration of the cardiomyocyte's electrophysiology. Understanding the mechanisms of ion channels trafficking could provide new therapeutic approaches for the treatment of HF. This review provides an overview of the recent advances in ion channel trafficking in HF.

The intercalated disc: a unique organelle for electromechanical synchrony in cardiomyocytes

The intercalated disc (ID) is a highly specialized structure that connects cardiomyocytes via mechanical and electrical junctions. Although described in some detail by light microscopy in the 19th century, it was in 1966 that electron microscopy images showed that the ID represented apposing cell borders and provided detailed insight into the complex ID nanostructure. Since then, much has been learned about the ID and its molecular composition, and it has become evident that a large number of proteins, not all of them involved in direct cell-to-cell coupling via mechanical or gap junctions, reside at the ID. Furthermore, an increasing number of functional interactions between ID components are emerging, leading to the concept that the ID is not the sum of isolated molecular silos but an interacting molecular complex, an "organelle" where components work in concert to bring about electrical and mechanical synchrony. The aim of the present review is to give a short historical account of the ID's discovery and an updated overview of its composition and organization, followed by a discussion of the physiological implications of the ID architecture and the local intermolecular interactions. The latter will focus on both the importance of normal conduction of cardiac action potentials as well as the impact on the pathophysiology of arrhythmias.

Subcellular diversity of Nav1.5 in cardiomyocytes: distinct functions, mechanisms and targets

In cardiomyocytes, the rapid depolarisation of the membrane potential is mediated by the α-subunit of the cardiac voltage-gated Na⁺ channel (Na_V 1.5), encoded by the gene SCN5A. This ion channel allows positively charged Na⁺ ions to enter the cardiomyocyte, resulting in the fast upstroke of the action potential and is therefore crucial for cardiac excitability and electrical propagation. This essential role is underscored by the fact that dysfunctional Na_V 1.5 is associated with high risk for arrhythmias and sudden cardiac death. However, development of therapeutic interventions regulating Na_V 1.5 has been limited due to the complexity of Na_V 1.5 structure and function and its diverse roles within the cardiomyocyte. In particular, research from the last decade has provided us with increased knowledge on the subcellular distribution of Na_V 1.5 as well as the proteins which it interacts with in distinct cardiomyocyte microdomains. We here review these insights, detailing the potential role of Na_V 1.5 within subcellular domains as well as its dysfunction in the setting of arrhythmia disorders. We furthermore provide an overview of current knowledge on the pathways involved in (microdomain-specific) trafficking of Na_V 1.5, and their potential as novel targets. Unravelling the complexity of Na_V 1.5 (dys)function may ultimately facilitate the development of therapeutic strategies aimed at preventing lethal arrhythmias. This is not only of importance for pathophysiological conditions where sodium current is specifically decreased within certain subcellular regions, such as in arrhythmogenic cardiomyopathy and Duchenne muscular dystrophy, but also for other acquired and inherited disorders associated with Na_V 1.5.

A Novel DLG1 Variant in a Family with Brugada Syndrome: Clinical Characteristics and In Silico Analysis

BACKGROUND

Brugada syndrome (BrS) is an inherited primary channelopathy syndrome associated to sudden cardiac death. Overall, variants have been identified in eighteen genes encoding for ion channel subunits and seven genes for regulatory proteins. Recently, a missense variant in DLG1 has been found within a BrS phenotype-positive patient. DLG1 encodes for synapse associated protein 97 (SAP97), a protein characterized by the presence of multiple domains for protein-protein interactions including PDZ domains. In cardiomyocytes, SAP97 interacts with Nav1.5, a PDZ binding motif of SCN5A and others potassium channel subunits.

AIM OF THE STUDY

To characterize the phenotype of an Italian family with BrS syndrome carrying a DLG1 variant.

METHODS

Clinical and genetic investigations were performed. Genetic testing was performed with whole-exome sequencing (WES) using the Illumina platform. According to the standard protocol, a variant found by WES was confirmed in all members of the family by bi-directional capillary Sanger resequencing. The effect of the variant was investigated by using in silico prediction of pathogenicity.

RESULTS

The index case was a 74-year-old man with spontaneous type 1 BrS ECG pattern that experienced syncope and underwent ICD implantation. WES of the index case, performed assuming a dominant mode of inheritance, identified a heterozygous variant, c.1556G>A (p.R519H), in the exon 15 of the DLG1 gene. In the pedigree investigation, 6 out of 12 family members had the variant. Carriers of the gene variant all had BrS ECG type 1 drug induced and showed heterogeneous cardiac phenotypes with two patients experiencing syncope during exercise and fever, respectively. The amino acid residue #519 lies near a PDZ domain and in silico analysis suggested a causal role for the variant. Modelling of the resulting protein structure predicted that the variant disrupts an H-bond and a likelihood of being pathogenic. As a consequence, it is likely that a conformational change affects protein functionality and the modulating role on ion channels.

CONCLUSIONS

A DLG1 gene variant identified was associated with BrS. The variant could modify the formation of multichannel protein complexes, affecting ion channels to specific compartments in cardiomyocytes.

Involvement of SAP97 anchored multiprotein complexes in regulating cardiorenal signaling and trafficking networks

SAP97 is a member of the MAGUK family of proteins, but unlike other MAGUK proteins that are selectively expressed in the CNS, SAP97 is also expressed in peripheral organs, like the heart and kidneys. SAP97 has several protein binding cassettes, and this review will describe their involvement in creating SAP97-anchored multiprotein networks. SAP97-anchored networks localized at the inner leaflet of the cell membrane play a major role in trafficking and targeting of membrane G protein-coupled receptors (GPCR), channels, and structural proteins. SAP97 plays a major role in compartmentalizing voltage gated sodium and potassium channels to specific cellular compartments of heart cells. SAP97 undergoes extensive alternative splicing. These splice variants give rise to different SAP97 isoforms that alter its cellular localization, networking, signaling and trafficking effects. Regarding GPCR, SAP97 binds to the β₁-adrenergic receptor and recruits AKAP5/PKA and PDE4D8 to create a multiprotein complex that regulates trafficking and signaling of cardiac β₁-AR. In the kidneys, SAP97 anchored networks played a role in trafficking of aquaporin-2 water channels. Cardiac specific ablation of SAP97 (SAP97-cKO) resulted in cardiac hypertrophy and failure in aging mice. Similarly, instituting transverse aortic constriction (TAC) in young SAP97 c-KO mice exacerbated TAC-induced cardiac remodeling and dysfunction. These findings highlight a critical role for SAP97 in the pathophysiology of a number of cardiac and renal diseases, suggesting that SAP97 is a relevant target for drug discovery.

Molecular stratification of arrhythmogenic mechanisms in the Andersen Tawil Syndrome

Andersen Tawil Syndrome (ATS) is a rare inheritable disease associated with loss-of-function mutations in KCNJ2, the gene coding the strong inward rectifier potassium channel Kir2.1, which forms an essential membrane protein controlling cardiac excitability. ATS is usually marked by a triad of periodic paralysis, life-threatening cardiac arrhythmias and dysmorphic features, but its expression is variable and not all patients with a phenotype linked to ATS have a known genetic alteration. The mechanisms underlying this arrhythmogenic syndrome are poorly understood. Knowing such mechanisms would be essential to distinguish ATS from other channelopathies with overlapping phenotypes and to develop individualized therapies. For example, the recently suggested role of Kir2.1 as a countercurrent to sarcoplasmic calcium reuptake might explain the arrhythmogenic mechanisms of ATS and its overlap with catecholaminergic polymorphic ventricular tachycardia (CPVT). Here we summarize current knowledge on the mechanisms of arrhythmias leading to sudden cardiac death in ATS. We first provide an overview of the syndrome and its pathophysiology, from the patient´s bedside to the protein, and discuss the role of essential regulators and interactors that could play a role in cases of ATS. The review highlights novel ideas related to some post-translational channel interactions with partner proteins that might help define the molecular bases of the arrhythmia phenotype. We then propose a new all-embracing classification of the currently known ATS loss-of-function mutations according to their position in the Kir2.1 channel structure and their functional implications. We also discuss specific ATS pathogenic variants, their clinical manifestations and treatment stratification. The goal is to provide a deeper mechanistic understanding of the syndrome toward the development of novel targets and personalized treatment strategies.

Genomic and Non-Genomic Regulatory Mechanisms of the Cardiac Sodium Channel in Cardiac Arrhythmias

Nav1.5 is the predominant cardiac sodium channel subtype, encoded by the SCN5A gene, which is involved in the initiation and conduction of action potentials throughout the heart. Along its biosynthesis process, Nav1.5 undergoes strict genomic and non-genomic regulatory and quality control steps that allow only newly synthesized channels to reach their final membrane destination and carry out their electrophysiological role. These regulatory pathways are ensured by distinct interacting proteins that accompany the nascent Nav1.5 protein along with different subcellular organelles. Defects on a large number of these pathways have a tremendous impact on Nav1.5 functionality and are thus intimately linked to cardiac arrhythmias. In the present review, we provide current state-of-the-art information on the molecular events that regulate SCN5A/Nav1.5 and the cardiac channelopathies associated with defects in these pathways.

Initiation and entrainment of multicellular automaticity via diffusion limited extracellular domains

Electrically excitable cells often spontaneously and synchronously depolarize in vitro and in vivo preparations. It remains unclear how cells entrain and autorhythmically activate above the intrinsic mean activation frequency of isolated cells with or without pacemaking mechanisms. Recent studies suggest that cyclic ion accumulation and depletion in diffusion-limited extracellular volumes modulate electrophysiology by ephaptic mechanisms (nongap junction or synaptic coupling). This report explores how potassium accumulation and depletion in a restricted extracellular domain induces spontaneous action potentials in two different computational models of excitable cells without gap junctional coupling: Hodgkin-Huxley and Luo-Rudy. Importantly, neither model will spontaneously activate on its own without external stimuli. Simulations demonstrate that cells sharing a diffusion-limited extracellular compartment can become autorhythmic and entrained despite intercellular electrical heterogeneity. Autorhythmic frequency is modulated by the cleft volume and potassium fluxes through the cleft. Additionally, inexcitable cells can suppress or induce autorhythmic activity in an excitable cell via a shared cleft. Diffusion-limited shared clefts can also entrain repolarization. Critically, this model predicts a mechanism by which diffusion-limited shared clefts can initiate, entrain, and modulate multicellular automaticity in the absence of gap junctions.

Remodeling of Ion Channel Trafficking and Cardiac Arrhythmias

Both inherited and acquired cardiac arrhythmias are often associated with the abnormal functional expression of ion channels at the cellular level. The complex machinery that continuously traffics, anchors, organizes, and recycles ion channels at the plasma membrane of a cardiomyocyte appears to be a major source of channel dysfunction during cardiac arrhythmias. This has been well established with the discovery of mutations in the genes encoding several ion channels and ion channel partners during inherited cardiac arrhythmias. Fibrosis, altered myocyte contacts, and post-transcriptional protein changes are common factors that disorganize normal channel trafficking during acquired cardiac arrhythmias. Channel availability, described notably for hERG and K_V1.5 channels, could be another potent arrhythmogenic mechanism. From this molecular knowledge on cardiac arrhythmias will emerge novel antiarrhythmic strategies.

Mog1 knockout causes cardiac hypertrophy and heart failure by downregulating tbx5-cryab-hspb2 signalling in zebrafish

AIMS

MOG1 is a small protein that can bind to small GTPase RAN and regulate transport of RNA and proteins between the cytoplasm and nucleus. However, the in vivo physiological role of mog1 in the heart needs to be fully defined.

METHODS

Mog1 knockout zebrafish was generated by TALEN. Echocardiography, histological analysis, and electrocardiograms were used to examine cardiac structure and function. RNA sequencing and real-time RT-PCR were used to elucidate the molecular mechanism and to analyse the gene expression. Isoproterenol was used to induce cardiac hypertrophy. Whole-mount in situ hybridization was used to observe cardiac morphogenesis.

RESULTS

Mog1 knockout zebrafish developed cardiac hypertrophy and heart failure (enlarged pericardium, increased nppa and nppb expression and ventricular wall thickness, and reduced ejection fraction), which was aggravated by isoproterenol. RNAseq and KEGG pathway analyses revealed the effect of mog1 knockout on the pathways of cardiac hypertrophy, dilatation and contraction. Mechanistic studies revealed that mog1 knockout decreased expression of tbx5, which reduced expression of cryab and hspb2, resulting in cardiac hypertrophy and heart failure. Overexpression of cryab, hspb2 and tbx5 rescued the cardiac oedema phenotype of mog1 KO zebrafish. Telemetry electrocardiogram monitoring showed QRS and QTc prolongation and a reduced heart rate in mog1 knockout zebrafish, which was associated with reduced scn1b expression. Moreover, mog1 knockout resulted in abnormal cardiac looping during embryogenesis because of the reduced expression of nkx2.5, gata4 and hand2.

CONCLUSION

Our data identified an important molecular determinant for cardiac hypertrophy and heart failure, and rhythm maintenance of the heart.

Structural and Functional Characterization of a Nav1.5-Mitochondrial Couplon

RATIONALE

The cardiac sodium channel NaV1.5 has a fundamental role in excitability and conduction. Previous studies have shown that sodium channels cluster together in specific cellular subdomains. Their association with intracellular organelles in defined regions of the myocytes, and the functional consequences of that association, remain to be defined.

OBJECTIVE

To characterize a subcellular domain formed by sodium channel clusters in the crest region of the myocytes and the subjacent subsarcolemmal mitochondria.

METHODS AND RESULTS

Through a combination of imaging approaches including super-resolution microscopy and electron microscopy we identified, in adult cardiac myocytes, a NaV1.5 subpopulation in close proximity to subjacent subsarcolemmal mitochondria; we further found that subjacent subsarcolemmal mitochondria preferentially host the mitochondrial NCLX (Na+/Ca2+ exchanger). This anatomic proximity led us to investigate functional changes in mitochondria resulting from sodium channel activity. Upon TTX (tetrodotoxin) exposure, mitochondria near NaV1.5 channels accumulated more Ca2+ and showed increased reactive oxygen species production when compared with interfibrillar mitochondria. Finally, crosstalk between NaV1.5 channels and mitochondria was analyzed at a transcriptional level. We found that SCN5A (encoding NaV1.5) and SLC8B1 (which encode NaV1.5 and NCLX, respectively) are negatively correlated both in a human transcriptome data set (Genotype-Tissue Expression) and in human-induced pluripotent stem cell-derived cardiac myocytes deficient in SCN5A.

CONCLUSIONS

We describe an anatomic hub (a couplon) formed by sodium channel clusters and subjacent subsarcolemmal mitochondria. Preferential localization of NCLX to this domain allows for functional coupling where the extrusion of Ca2+ from the mitochondria is powered, at least in part, by the entry of sodium through NaV1.5 channels. These results provide a novel entry-point into a mechanistic understanding of the intersection between electrical and structural functions of the heart.

The p.P888L SAP97 polymorphism increases the transient outward current (Ito,f) and abbreviates the action potential duration and the QT interval

Synapse-Associated Protein 97 (SAP97) is an anchoring protein that in cardiomyocytes targets to the membrane and regulates Na⁺ and K⁺ channels. Here we compared the electrophysiological effects of native (WT) and p.P888L SAP97, a common polymorphism. Currents were recorded in cardiomyocytes from mice trans-expressing human WT or p.P888L SAP97 and in Chinese hamster ovary (CHO)-transfected cells. The duration of the action potentials and the QT interval were significantly shorter in p.P888L-SAP97 than in WT-SAP97 mice. Compared to WT, p.P888L SAP97 significantly increased the charge of the Ca-independent transient outward (I_to,f) current in cardiomyocytes and the charge crossing Kv4.3 channels in CHO cells by slowing Kv4.3 inactivation kinetics. Silencing or inhibiting Ca/calmodulin kinase II (CaMKII) abolished the p.P888L-induced Kv4.3 charge increase, which was also precluded in channels (p.S550A Kv4.3) in which the CaMKII-phosphorylation is prevented. Computational protein-protein docking predicted that p.P888L SAP97 is more likely to form a complex with CaMKII than WT. The Na⁺ current and the current generated by Kv1.5 channels increased similarly in WT-SAP97 and p.P888L-SAP97 cardiomyocytes, while the inward rectifier current increased in WT-SAP97 but not in p.P888L-SAP97 cardiomyocytes. The p.P888L SAP97 polymorphism increases the I_to,f, a CaMKII-dependent effect that may increase the risk of arrhythmias.

GRK5 Controls SAP97-Dependent Cardiotoxic β1 Adrenergic Receptor-CaMKII Signaling in Heart Failure

RATIONALE

Cardiotoxic β₁ adrenergic receptor (β₁AR)-CaMKII (calmodulin-dependent kinase II) signaling is a major and critical feature associated with development of heart failure. SAP97 (synapse-associated protein 97) is a multifunctional scaffold protein that binds directly to the C-terminus of β₁AR and organizes a receptor signalosome.

OBJECTIVE

We aim to elucidate the dynamics of β₁AR-SAP97 signalosome and its potential role in chronic cardiotoxic β₁AR-CaMKII signaling that contributes to development of heart failure.

METHODS AND RESULTS

The integrity of cardiac β₁AR-SAP97 complex was examined in heart failure. Cardiac-specific deletion of SAP97 was developed to examine β₁AR signaling in aging mice, after chronic adrenergic stimulation, and in pressure overload hypertrophic heart failure. We show that the β₁AR-SAP97 signaling complex is reduced in heart failure. Cardiac-specific deletion of SAP97 yields an aging-dependent cardiomyopathy and exacerbates cardiac dysfunction induced by chronic adrenergic stimulation and pressure overload, which are associated with elevated CaMKII activity. Loss of SAP97 promotes PKA (protein kinase A)-dependent association of β₁AR with arrestin2 and CaMKII and turns on an Epac (exchange protein directly activated by cAMP)-dependent activation of CaMKII, which drives detrimental functional and structural remodeling in myocardium. Moreover, we have identified that GRK5 (G-protein receptor kinase-5) is necessary to promote agonist-induced dissociation of SAP97 from β₁AR. Cardiac deletion of GRK5 prevents adrenergic-induced dissociation of β₁AR-SAP97 complex and increases in CaMKII activity in hearts.

CONCLUSIONS

These data reveal a critical role of SAP97 in maintaining the integrity of cardiac β₁AR signaling and a detrimental cardiac GRK5-CaMKII axis that can be potentially targeted in heart failure therapy. Graphical Abstract: A graphical abstract is available for this article.

Abnormal myocardial expression of SAP97 is associated with arrhythmogenic risk

Synapse-associated protein 97 (SAP97) is a scaffolding protein crucial for the functional expression of several cardiac ion channels and therefore proper cardiac excitability. Alterations in the functional expression of SAP97 can modify the ionic currents underlying the cardiac action potential and consequently confer susceptibility for arrhythmogenesis. In this study, we generated a murine model for inducible, cardiac-targeted Sap97 ablation to investigate arrhythmia susceptibility and the underlying molecular mechanisms. Furthermore, we sought to identify human SAP97 (DLG1) variants that were associated with inherited arrhythmogenic disease. The murine model of cardiac-specific Sap97 ablation demonstrated several ECG abnormalities, pronounced action potential prolongation subject to high incidence of arrhythmogenic afterdepolarizations and notable alterations in the activity of the main cardiac ion channels. However, no DLG1 mutations were found in 40 unrelated cases of genetically elusive long QT syndrome (LQTS). Instead, we provide the first evidence implicating a gain of function in human DLG1 mutation resulting in an increase in Kv4.3 current (I_to) as a novel, potentially pathogenic substrate for Brugada syndrome (BrS). In conclusion, DLG1 joins a growing list of genes encoding ion channel interacting proteins (ChIPs) identified as potential channelopathy-susceptibility genes because of their ability to regulate the trafficking, targeting, and modulation of ion channels that are critical for the generation and propagation of the cardiac electrical impulse. Dysfunction in these critical components of cardiac excitability can potentially result in fatal cardiac disease.NEW & NOTEWORTHY The gene encoding SAP97 (DLG1) joins a growing list of genes encoding ion channel-interacting proteins (ChIPs) identified as potential channelopathy-susceptibility genes because of their ability to regulate the trafficking, targeting, and modulation of ion channels that are critical for the generation and propagation of the cardiac electrical impulse. In this study we provide the first data supporting DLG1-encoded SAP97's candidacy as a minor Brugada syndrome susceptibility gene.

Dystrophin and calcium current are decreased in cardiomyocytes expressing Cre enzyme driven by αMHC but not TNT promoter

The Cre/lox system is a potent technology to control gene expression in mouse tissues. However, cardiac-specific Cre recombinase expression alone can lead to cardiac alterations when no loxP sites are present, which is not well understood. Many loxP-like sites have been identified in the mouse genome that might be Cre sensitive. One of them is located in the Dmd gene encoding dystrophin, a protein important for the function and stabilization of voltage-gated calcium (Ca_v1.2) and sodium (Na_v1.5) channels, respectively. Here, we investigate whether Cre affects dystrophin expression and function in hearts without loxP sites in the genome. In mice expressing Cre under the alpha-myosin heavy chain (MHC-Cre) or Troponin T (TNT-Cre) promoter, we investigated dystrophin expression, Na_v1.5 expression, and Ca_v1.2 function. Compared to age-matched MHC-Cre^- mice, dystrophin protein level was significantly decreased in hearts from MHC-Cre⁺ mice of more than 12-weeks-old. Quantitative RT-PCR revealed decreased mRNA levels of Dmd gene. Unexpectedly, calcium current (I_CaL), but not Na_v1.5 protein expression was altered in those mice. Surprisingly, in hearts from 12-week-old and older TNT-Cre⁺ mice, neither I_CaL nor dystrophin and Na_v1.5 protein content were altered compared to TNT-Cre^-. Cre recombinase unpredictably affects cardiac phenotype, and Cre-expressing mouse models should be carefully investigated before experimental use.

A Distinct Pool of Nav1.5 Channels at the Lateral Membrane of Murine Ventricular Cardiomyocytes

Background: In cardiac ventricular muscle cells, the presence of voltage-gated sodium channels Na_v1.5 at the lateral membrane depends in part on the interaction between the dystrophin–syntrophin complex and the Na_v1.5 C-terminal PDZ-domain-binding sequence Ser-Ile-Val (SIV motif). α1-Syntrophin, a PDZ-domain adaptor protein, mediates the interaction between Na_v1.5 and dystrophin at the lateral membrane of cardiac cells. Using the cell-attached patch-clamp approach on cardiomyocytes expressing Na_v1.5 in which the SIV motif is deleted (ΔSIV), sodium current (I_Na) recordings from the lateral membrane revealed a SIV-motif-independent I_Na. Since immunostaining has suggested that Na_v1.5 is expressed in transverse (T-) tubules, this remaining I_Na might be carried by channels in the T-tubules. Of note, a recent study using heterologous expression systems showed that α1-syntrophin also interacts with the Na_v1.5 N-terminus, which may explain the SIV-motif independent I_Na at the lateral membrane of cardiomyocytes.

Aim: To address the role of α1-syntrophin in regulating the I_Na at the lateral membrane of cardiac cells.

Methods and Results: Patch-clamp experiments in cell-attached configuration were performed on the lateral membranes of wild-type, α1-syntrophin knockdown, and ΔSIV ventricular mouse cardiomyocytes. Compared to wild-type, a reduction of the lateral I_Na was observed in myocytes from α1-syntrophin knockdown hearts. Similar to ΔSIV myocytes, a remaining I_Na was still recorded. In addition, cell-attached I_Na recordings from lateral membrane did not differ significantly between non-detubulated and detubulated ΔSIV cardiomyocytes. Lastly, we obtained evidence suggesting that cell-attached patch-clamp experiments on the lateral membrane cannot record currents carried by channels in T-tubules such as calcium channels.

Conclusion: Altogether, these results suggest the presence of a sub-pool of sodium channels at the lateral membrane of cardiomyocytes that is independent of α1-syntrophin and the PDZ-binding motif of Na_v1.5, located in membrane domains outside of T-tubules. The question of a T-tubular pool of Na_v1.5 channels, however, remains open.

The Cardiac Sodium Channel and Its Protein Partners

Activation of the electrical signal and its transmission as a depolarizing wave in the whole heart requires highly organized myocyte architecture and cell-cell contacts. In addition, complex trafficking and anchoring intracellular machineries regulate the proper surface expression of channels and their targeting to distinct membrane domains. An increasing list of proteins, lipids, and second messengers can contribute to the normal targeting of ion channels in cardiac myocytes. However, their precise roles in the electrophysiology of the heart are far from been extensively understood. Nowadays, much effort in the field focuses on understanding the mechanisms that regulate ion channel targeting to sarcolemma microdomains and their organization into macromolecular complexes. The purpose of the present section is to provide an overview of the characterized partners of the main cardiac sodium channel, Na_V1.5, involved in regulating the functional expression of this channel both in terms of trafficking and targeting into microdomains.

Cardiac Arrhythmias as Manifestations of Nanopathies: An Emerging View

A nanodomain is a collection of proteins localized within a specialized, nanoscale structural environment, which can serve as the functional unit of macroscopic physiologic processes. We are beginning to recognize the key roles of cardiomyocyte nanodomains in essential processes of cardiac physiology such as electrical impulse propagation and excitation–contraction coupling (ECC). There is growing appreciation of nanodomain dysfunction, i.e., nanopathy, as a mechanistic driver of life-threatening arrhythmias in a variety of pathologies. Here, we offer an overview of current research on the role of nanodomains in cardiac physiology with particular emphasis on: (1) sodium channel-rich nanodomains within the intercalated disk that participate in cell-to-cell electrical coupling and (2) dyadic nanodomains located along transverse tubules that participate in ECC. The beat to beat function of cardiomyocytes involves three phases: the action potential, the calcium transient, and mechanical contraction/relaxation. In all these phases, cell-wide function results from the aggregation of the stochastic function of individual proteins. While it has long been known that proteins that exist in close proximity influence each other’s function, it is increasingly appreciated that there exist nanoscale structures that act as functional units of cardiac biophysical phenomena. Termed nanodomains, these structures are collections of proteins, localized within specialized nanoscale structural environments. The nano-environments enable the generation of localized electrical and/or chemical gradients, thereby conferring unique functional properties to these units. Thus, the function of a nanodomain is determined by its protein constituents as well as their local structural environment, adding an additional layer of complexity to cardiac biology and biophysics. However, with the emergence of experimental techniques that allow direct investigation of structure and function at the nanoscale, our understanding of cardiac physiology and pathophysiology at these scales is rapidly advancing. Here, we will discuss the structure and functions of multiple cardiomyocyte nanodomains, and novel strategies that target them for the treatment of cardiac arrhythmias.

Transcriptomic analyses of murine ventricular cardiomyocytes

Mice are used universally as model organisms for studying heart physiology, and a plethora of genetically modified mouse models exist to study cardiac disease. Transcriptomic data for whole-heart tissue are available, but not yet for isolated ventricular cardiomyocytes. Our lab therefore collected comprehensive RNA-seq data from wildtype murine ventricular cardiomyocytes as well as from knockout models of the ion channel regulators CASK, dystrophin, and SAP97. We also elucidate ion channel expression from wild-type cells to help forward the debate about which ion channels are expressed in cardiomyocytes. Researchers studying the heart, and especially cardiac arrhythmias, may benefit from these cardiomyocyte-specific transcriptomic data to assess expression of genes of interest.

Cardiac Kir2.1 and NaV1.5 Channels Traffic Together to the Sarcolemma to Control Excitability

RATIONALE

In cardiomyocytes, Na_V1.5 and Kir2.1 channels interact dynamically as part of membrane bound macromolecular complexes.

OBJECTIVE

The objective of this study was to test whether Na_V1.5 and Kir2.1 preassemble during early forward trafficking and travel together to common membrane microdomains.

METHODS AND RESULTS

In patch-clamp experiments, coexpression of trafficking-deficient mutants Kir2.1^Δ314-315 or Kir2.1^R44A/R46A with wild-type (WT) Na_V1.5^WT in heterologous cells reduced inward sodium current compared with Na_V1.5^WT alone or coexpressed with Kir2.1^WT. In cell surface biotinylation experiments, expression of Kir2.1^Δ314-315 reduced Na_V1.5 channel surface expression. Glycosylation analysis suggested that Na_V1.5^WT and Kir2.1^WT channels associate early in their biosynthetic pathway, and fluorescence recovery after photobleaching experiments demonstrated that coexpression with Kir2.1 increased cytoplasmic mobility of Na_V1.5^WT, and vice versa, whereas coexpression with Kir2.1^Δ314-315 reduced mobility of both channels. Viral gene transfer of Kir2.1^Δ314-315 in adult rat ventricular myocytes and human induced pluripotent stem cell-derived cardiomyocytes reduced inward rectifier potassium current and inward sodium current, maximum diastolic potential and action potential depolarization rate, and increased action potential duration. On immunostaining, the AP1 (adaptor protein complex 1) colocalized with Na_V1.5^WT and Kir2.1^WT within areas corresponding to t-tubules and intercalated discs. Like Kir2.1^WT, Na_V1.5^WT coimmunoprecipitated with AP1. Site-directed mutagenesis revealed that Na_V1.5^WT channels interact with AP1 through the Na_V1.5^Y1810 residue, suggesting that, like for Kir2.1^WT, AP1 can mark Na_V1.5 channels for incorporation into clathrin-coated vesicles at the trans-Golgi. Silencing the AP1 ϒ-adaptin subunit in human induced pluripotent stem cell-derived cardiomyocytes reduced inward rectifier potassium current, inward sodium current, and maximum diastolic potential and impaired rate-dependent action potential duration adaptation.

CONCLUSIONS

The Na_V1.5-Kir2.1 macromolecular complex pre-assembles early in the forward trafficking pathway. Therefore, disruption of Kir2.1 trafficking in cardiomyocytes affects trafficking of Na_V1.5, which may have important implications in the mechanisms of arrhythmias in inheritable cardiac diseases.

Ion Channel Trafficking: Control of Ion Channel Density as a Target for Arrhythmias?

The shape of the cardiac action potential (AP) is determined by the contributions of numerous ion channels. Any dysfunction in the proper function or expression of these ion channels can result in a change in effective refractory period (ERP) and lead to arrhythmia. The processes underlying the correct targeting of ion channels to the plasma membrane are complex, and have not been fully characterized in cardiac myocytes. Emerging evidence highlights ion channel trafficking as a potential causative factor in certain acquired and inherited arrhythmias, and therapies which target trafficking as opposed to pore block are starting to receive attention. In this review we present the current evidence for the mechanisms which underlie precise control of cardiac ion channel trafficking and targeting.

Conditional knockout of Fgf13 in murine hearts increases arrhythmia susceptibility and reveals novel ion channel modulatory roles

The intracellular fibroblast growth factors (iFGF/FHFs) bind directly to cardiac voltage gated Na⁺ channels, and modulate their function. Mutations that affect iFGF/FHF-Na⁺ channel interaction are associated with arrhythmia syndromes. Although suspected to modulate other ionic currents, such as Ca²⁺ channels based on acute knockdown experiments in isolated cardiomyocytes, the in vivo consequences of iFGF/FHF gene ablation on cardiac electrical activity are still unknown. We generated inducible, cardiomyocyte-restricted Fgf13 knockout mice to determine the resultant effects of Fgf13 gene ablation. Patch clamp recordings from ventricular myocytes isolated from Fgf13 knockout mice showed a ~25% reduction in peak Na⁺ channel current density and a hyperpolarizing shift in steady-state inactivation. Electrocardiograms on Fgf13 knockout mice showed a prolonged QRS duration. The Na⁺ channel blocker flecainide further prolonged QRS duration and triggered ventricular tachyarrhythmias only in Fgf13 knockout mice, suggesting that arrhythmia vulnerability resulted, at least in part, from a loss of functioning Na⁺ channels. Consistent with these effects on Na⁺ channels, action potentials in Fgf13 knockout mice, compared to Cre control mice, exhibited slower upstrokes and reduced amplitude, but unexpectedly had longer durations. We investigated candidate sources of the prolonged action potential durations in myocytes from Fgf13 knockout mice and found a reduction of the transient outward K⁺ current (I_to). Fgf13 knockout did not alter whole-cell protein levels of Kv4.2 and Kv4.3, the I_to pore-forming subunits, but did decrease Kv4.2 and Kv4.3 at the sarcolemma. No changes were seen in the sustained outward K⁺ current or voltage-gated Ca²⁺ current, other candidate contributors to the increased action potential duration. These results implicate that FGF13 is a critical cardiac Na⁺ channel modulator and Fgf13 knockout mice have increased arrhythmia susceptibility in the setting of Na⁺ channel blockade. The unanticipated effect on I_to revealed new FGF13 properties and the unexpected lack of an effect on voltage-gated Ca²⁺ channels highlight potential compensatory changes in vivo not readily revealed with acute Fgf13 knockdown in cultured cardiomyocytes.

Central role for GSK3β in the pathogenesis of arrhythmogenic cardiomyopathy

Arrhythmogenic cardiomyopathy (ACM) is characterized by redistribution of junctional proteins, arrhythmias, and progressive myocardial injury. We previously reported that SB216763 (SB2), annotated as a GSK3β inhibitor, reverses disease phenotypes in a zebrafish model of ACM. Here, we show that SB2 prevents myocyte injury and cardiac dysfunction in vivo in two murine models of ACM at baseline and in response to exercise. SB2-treated mice with desmosome mutations showed improvements in ventricular ectopy and myocardial fibrosis/inflammation as compared with vehicle-treated (Veh-treated) mice. GSK3β inhibition improved left ventricle function and survival in sedentary and exercised Dsg2^mut/mut mice compared with Veh-treated Dsg2^mut/mut mice and normalized intercalated disc (ID) protein distribution in both mutant mice. GSK3β showed diffuse cytoplasmic localization in control myocytes but ID redistribution in ACM mice. Identical GSK3β redistribution is present in ACM patient myocardium but not in normal hearts or other cardiomyopathies. SB2 reduced total GSK3β protein levels but not phosphorylated Ser 9-GSK3β in ACM mice. Constitutively active GSK3β worsens ACM in mutant mice, while GSK3β shRNA silencing in ACM cardiomyocytes prevents abnormal ID protein distribution. These results highlight a central role for GSKβ in the complex phenotype of ACM and provide further evidence that pharmacologic GSKβ inhibition improves cardiomyopathies due to desmosome mutations.

Ion channel macromolecular complexes in cardiomyocytes: roles in sudden cardiac death

The movement of ions across specific channels embedded on the membrane of individual cardiomyocytes is crucial for the generation and propagation of the cardiac electric impulse. Emerging evidence over the past 20 years strongly suggests that the normal electric function of the heart is the result of dynamic interactions of membrane ion channels working in an orchestrated fashion as part of complex molecular networks. Such networks work together with exquisite temporal precision to generate each action potential and contraction. Macromolecular complexes play crucial roles in transcription, translation, oligomerization, trafficking, membrane retention, glycosylation, post-translational modification, turnover, function, and degradation of all cardiac ion channels known to date. In addition, the accurate timing of each cardiac beat and contraction demands, a comparable precision on the assembly and organizations of sodium, calcium, and potassium channel complexes within specific subcellular microdomains, where physical proximity allows for prompt and efficient interaction. This review article, part of the Compendium on Sudden Cardiac Death, discusses the major issues related to the role of ion channel macromolecular assemblies in normal cardiac electric function and the mechanisms of arrhythmias leading to sudden cardiac death. It provides an idea of how these issues are being addressed in the laboratory and in the clinic, which important questions remain unanswered, and what future research will be needed to improve knowledge and advance therapy.

Role of connexins and pannexins in cardiovascular physiology

Connexins and pannexins form connexons, pannexons and membrane channels, which are critically involved in many aspects of cardiovascular physiology. For that reason, a vast number of studies have addressed the role of connexins and pannexins in the arterial and venous systems as well as in the heart. Moreover, a role for connexins in lymphatics has recently also been suggested. This review provides an overview of the current knowledge regarding the involvement of connexins and pannexins in cardiovascular physiology.

Pathogenesis of Arrhythmogenic Cardiomyopathy

Arrhythmogenic cardiomyopathy (ACM) is a primary myocardial disease. It is characterized by frequent ventricular arrhythmias and increased risk of sudden cardiac death typically arising as an early manifestation before the onset of significant myocardial remodelling. Myocardial degeneration, often confined to the right ventricular free wall, with replacement by fibrofatty scar tissue, develops in many patients. ACM is a familial disease but genetic penetrance can be low and disease expression is highly variable. Inflammation might promote disease progression. It also appears that exercise increases disease penetrance and accelerates its development. More than 60% of probands harbour mutations in genes that encode desmosomal proteins, which has raised the possibility that defective cell-cell adhesion might play a role in disease pathogenesis. Recent advances have implicated changes in the canonical wingless-type mouse mammary tumour virus integration site (Wnt)/β-catenin and Hippo signalling pathways and defects in forwarding trafficking of ion channels and other proteins to the intercalated disk in cardiac myocytes. In this review we summarize the current understanding of the pathogenesis of ACM and highlight future research directions.

Protein assemblies of sodium and inward rectifier potassium channels control cardiac excitability and arrhythmogenesis

The understanding of how cardiac ion channels function in the normal and the diseased heart has greatly increased over the last four decades thanks to the advent of patch-clamp technology and, more recently, the emergence of genetics, as well as cellular and molecular cardiology. However, our knowledge of how these membrane-embedded proteins physically interact with each other within macromolecular complexes remains incomplete. This review focuses on how the main cardiac inward sodium channel (NaV1.5) and the strong inward rectifier potassium channel (Kir2.1) function within macromolecular complexes to control cardiac excitability. It has become increasingly clear that these two important ion channel proteins physically interact with multiple other protein partners and with each other from early stages of protein trafficking and targeting through membrane anchoring, recycling, and degradation. Recent findings include compartmentalized regulation of NaV1.5 channel expression and function through a PDZ (postsynaptic density protein, Drosophila disc large tumor suppressor, and zonula occludens-1 protein) domain-binding motif, and interaction of caveolin-3 with Kir2.1 and ankyrin-G as a molecular platform for NaV1.5 signaling. At the cardiomyocyte membrane, NaV1.5 and Kir2.1 interact through at least two distinct PDZ domain-scaffolding proteins (synapse-associated protein-97 and α1-syntrophin), thus modulating reciprocally their cell-surface expression at two different microdomains. Emerging evidence also shows that inheritable mutations in plakophilin-2, ankyrin-G, dystrophin, syntrophin, synapse-associated protein-97, and caveolin-3, among others, modify functional expression and/or localization in the cardiac cell of NaV1.5, Kir2.1 or both to give rise to arrhythmogenic diseases. Unveiling the mechanistic underpinnings of macromolecular interactions should increase our understanding of inherited and acquired arrhythmogenic cardiac diseases and may lead to advances in therapy.