β(IV)-Spectrin regulates TREK-1 membrane targeting in the heart

Phosphorylation of cardiac sodium channel at Ser571 anticipates manifestations of the aging myopathy

Diastolic dysfunction and delayed ventricular repolarization are typically observed in the elderly, but whether these defects are intimately associated with the progressive manifestation of the aging myopathy remains to be determined. In this regard, aging in experimental animals is coupled with increased late Na+ current (INa,L) in cardiomyocytes, raising the possibility that INa,L conditions the modality of electrical recovery and myocardial relaxation of the aged heart. For this purpose, aging male and female wild-type (WT) C57Bl/6 mice were studied together with genetically engineered mice with phosphomimetic (gain of function, GoF) or ablated (loss of function, LoF) mutations of the sodium channel Nav1.5 at Ser571 associated with, respectively, increased and stabilized INa,L. At ∼18 mo of age, WT mice developed prolonged duration of the QT interval of the electrocardiogram and impaired diastolic left ventricular (LV) filling, defects that were reversed by INa,L inhibition. Prolonged repolarization and impaired LV filling occurred prematurely in adult (∼5 mo) GoF mutant mice, whereas these alterations were largely attenuated in aging LoF mutant animals. Ca2+ transient decay and kinetics of myocyte shortening/relengthening were delayed in aged (∼24 mo) WT myocytes, with respect to adult cells. In contrast, delayed Ca2+ transients and contractile dynamics occurred at adult stage in GoF myocytes and further deteriorated in old age. Conversely, myocyte mechanics were minimally affected in aging LoF cells. Collectively, these results document that Nav1.5 phosphorylation at Ser571 and the late Na+ current modulate the modality of myocyte relaxation, constituting the mechanism linking delayed ventricular repolarization and diastolic dysfunction.NEW & NOTEWORTHY We have investigated the impact of the late Na current (INa,L) on cardiac and myocyte function with aging by using genetically engineered animals with enhanced or stabilized INa,L, due to phosphomimetic or phosphoablated mutations of Nav1.5. Our findings support the notion that phosphorylation of Nav1.5 at Ser571 prolongs myocardial repolarization and impairs diastolic function, contributing to the manifestations of the aging myopathy.

Emerging Signaling Regulation of Sinoatrial Node Dysfunction

PURPOSE OF REVIEW

The sinoatrial node (SAN), the natural pacemaker of the heart, is responsible for generating electrical impulses and initiating each heartbeat. Sinoatrial node dysfunction (SND) causes various arrhythmias such as sinus arrest, SAN block, and tachycardia/bradycardia syndrome. Unraveling the underlying mechanisms of SND is of paramount importance in the pursuit of developing effective therapeutic strategies for patients with SND. This review provides a concise summary of the most recent progress in the signaling regulation of SND.

RECENT FINDINGS

Recent studies indicate that SND can be caused by abnormal intercellular and intracellular signaling, various forms of heart failure (HF), and diabetes. These discoveries provide novel insights into the underlying mechanisms SND, advancing our understanding of its pathogenesis. SND can cause severe cardiac arrhythmias associated with syncope and an increased risk of sudden death. In addition to ion channels, the SAN is susceptible to the influence of various signalings including Hippo, AMP-activated protein kinase (AMPK), mechanical force, and natriuretic peptide receptors. New cellular and molecular mechanisms related to SND are also deciphered in systemic diseases such as HF and diabetes. Progress in these studies contributes to the development of potential therapeutics for SND.

TREK channels in Mechanotransduction: a Focus on the Cardiovascular System

Mechano-electric feedback is one of the most important subsystems operating in the cardiovascular system, but the underlying molecular mechanism remains rather unknown. Several proteins have been proposed to explain the molecular mechanism of mechano-transduction. Transient receptor potential (TRP) and Piezo channels appear to be the most important candidates to constitute the molecular mechanism behind of the inward current in response to a mechanical stimulus. However, the inhibitory/regulatory processes involving potassium channels that operate on the cardiac system are less well known. TWIK-Related potassium (TREK) channels have emerged as strong candidates due to their capacity for the regulation of the flow of potassium in response to mechanical stimuli. Current data strongly suggest that TREK channels play a role as mechano-transducers in different components of the cardiovascular system, not only at central (heart) but also at peripheral (vascular) level. In this context, this review summarizes and highlights the main existing evidence connecting this important subfamily of potassium channels with the cardiac mechano-transduction process, discussing molecular and biophysical aspects of such a connection.

Spectrin-Based Regulation of Cardiac Fibroblast Cell-Cell Communication

Cardiac fibroblasts (CFs) maintain the fibrous extracellular matrix (ECM) that supports proper cardiac function. Cardiac injury induces a transition in the activity of CFs to promote cardiac fibrosis. CFs play a critical role in sensing local injury signals and coordinating the organ level response through paracrine communication to distal cells. However, the mechanisms by which CFs engage cell-cell communication networks in response to stress remain unknown. We tested a role for the action-associated cytoskeletal protein βIV-spectrin in regulating CF paracrine signaling. Conditioned culture media (CCM) was collected from WT and βIV-spectrin deficient (qv4J) CFs. WT CFs treated with qv4J CCM showed increased proliferation and collagen gel compaction compared to control. Consistent with the functional measurements, qv4J CCM contained higher levels of pro-inflammatory and pro-fibrotic cytokines and increased concentration of small extracellular vesicles (30-150 nm diameter, exosomes). Treatment of WT CFs with exosomes isolated from qv4J CCM induced a similar phenotypic change as that observed with complete CCM. Treatment of qv4J CFs with an inhibitor of the βIV-spectrin-associated transcription factor, STAT3, decreased the levels of both cytokines and exosomes in conditioned media. This study expands the role of the βIV-spectrin/STAT3 complex in stress-induced regulation of CF paracrine signaling.

POPDC1 scaffolds a complex of adenylyl cyclase 9 and the potassium channel TREK-1 in heart

The establishment of macromolecular complexes by scaffolding proteins is key to the local production of cAMP by anchored adenylyl cyclase (AC) and the subsequent cAMP signaling necessary for cardiac functions. We identify a novel AC scaffold, the Popeye domain-containing (POPDC) protein. The POPDC family of proteins is important for cardiac pacemaking and conduction, due in part to their cAMP-dependent binding and regulation of TREK-1 potassium channels. We show that TREK-1 binds the AC9:POPDC1 complex and copurifies in a POPDC1-dependent manner with AC9 activity in heart. Although the AC9:POPDC1 interaction is cAMP-independent, TREK-1 association with AC9 and POPDC1 is reduced upon stimulation of the β-adrenergic receptor (βAR). AC9 activity is required for βAR reduction of TREK-1 complex formation with AC9:POPDC1 and in reversing POPDC1 enhancement of TREK-1 currents. Finally, deletion of the gene-encoding AC9 (Adcy9) gives rise to bradycardia at rest and stress-induced heart rate variability, a milder phenotype than the loss of Popdc1 but similar to the loss of Kcnk2 (TREK-1). Thus, POPDC1 represents a novel adaptor for AC9 interactions with TREK-1 to regulate heart rate control.

Spectrins and human diseases

Spectrin, as one of the major components of a plasma membrane-associated cytoskeleton, is a cytoskeletal protein composed of the modular structure of α and β subunits. The spectrin-based skeleton is essential for preserving the integrity and mechanical characteristics of the cell membrane. Moreover, spectrin regulates a variety of cell processes including cell apoptosis, cell adhesion, cell spreading, and cell cycle. Dysfunction of spectrins is implicated in various human diseases including hemolytic anemia, neurodegenerative diseases, ataxia, heart diseases, and cancers. Here, we briefly discuss spectrins function as well as the clinical manifestations and currently known molecular mechanisms of human diseases related to spectrins, highlighting that strategies for targeting regulation of spectrins function may provide new avenues for therapeutic intervention for these diseases.

Two-Pore-Domain Potassium (K2P-) Channels: Cardiac Expression Patterns and Disease-Specific Remodelling Processes

Two-pore-domain potassium (K_2P-) channels conduct outward K⁺ currents that maintain the resting membrane potential and modulate action potential repolarization. Members of the K_2P channel family are widely expressed among different human cell types and organs where they were shown to regulate important physiological processes. Their functional activity is controlled by a broad variety of different stimuli, like pH level, temperature, and mechanical stress but also by the presence of lipids or pharmacological agents. In patients suffering from cardiovascular diseases, alterations in K_2P-channel expression and function have been observed, suggesting functional significance and a potential therapeutic role of these ion channels. For example, upregulation of atrial specific K_2P3.1 (TASK-1) currents in atrial fibrillation (AF) patients was shown to contribute to atrial action potential duration shortening, a key feature of AF-associated atrial electrical remodelling. Therefore, targeting K_2P3.1 (TASK-1) channels might constitute an intriguing strategy for AF treatment. Further, mechanoactive K_2P2.1 (TREK-1) currents have been implicated in the development of cardiac hypertrophy, cardiac fibrosis and heart failure. Cardiovascular expression of other K_2P channels has been described, functional evidence in cardiac tissue however remains sparse. In the present review, expression, function, and regulation of cardiovascular K_2P channels are summarized and compared among different species. Remodelling patterns, observed in disease models are discussed and compared to findings from clinical patients to assess the therapeutic potential of K_2P channels.

Electrophysiological and Molecular Mechanisms of Sinoatrial Node Mechanosensitivity

The understanding of the electrophysiological mechanisms that underlie mechanosensitivity of the sinoatrial node (SAN), the primary pacemaker of the heart, has been evolving over the past century. The heart is constantly exposed to a dynamic mechanical environment; as such, the SAN has numerous canonical and emerging mechanosensitive ion channels and signaling pathways that govern its ability to respond to both fast (within second or on beat-to-beat manner) and slow (minutes) timescales. This review summarizes the effects of mechanical loading on the SAN activity and reviews putative candidates, including fast mechanoactivated channels (Piezo, TREK, and BK) and slow mechanoresponsive ion channels [including volume-regulated chloride channels and transient receptor potential (TRP)], as well as the components of mechanochemical signal transduction, which may contribute to SAN mechanosensitivity. Furthermore, we examine the structural foundation for both mechano-electrical and mechanochemical signal transduction and discuss the role of specialized membrane nanodomains, namely, caveolae, in mechanical regulation of both membrane and calcium clock components of the so-called coupled-clock pacemaker system responsible for SAN automaticity. Finally, we emphasize how these mechanically activated changes contribute to the pathophysiology of SAN dysfunction and discuss controversial areas necessitating future investigations. Though the exact mechanisms of SAN mechanosensitivity are currently unknown, identification of such components, their impact into SAN pacemaking, and pathological remodeling may provide new therapeutic targets for the treatment of SAN dysfunction and associated rhythm abnormalities.

Negative Influence by the Force: Mechanically Induced Hyperpolarization via K2P Background Potassium Channels

The two-pore domain K_2P subunits form background (leak) potassium channels, which are characterized by constitutive, although not necessarily constant activity, at all membrane potential values. Among the fifteen pore-forming K_2P subunits encoded by the KCNK genes, the three members of the TREK subfamily, TREK-1, TREK-2, and TRAAK are mechanosensitive ion channels. Mechanically induced opening of these channels generally results in outward K⁺ current under physiological conditions, with consequent hyperpolarization and inhibition of membrane potential-dependent cellular functions. In the past decade, great advances have been made in the investigation of the molecular determinants of mechanosensation, and members of the TREK subfamily have emerged among the best-understood examples of mammalian ion channels directly influenced by the tension of the phospholipid bilayer. In parallel, the crucial contribution of mechano-gated TREK channels to the regulation of membrane potential in several cell types has been reported. In this review, we summarize the general principles underlying the mechanical activation of K_2P channels, and focus on the physiological roles of mechanically induced hyperpolarization.

Ca2+/calmodulin kinase II-dependent regulation of βIV-spectrin modulates cardiac fibroblast gene expression, proliferation, and contractility

Fibrosis is a pronounced feature of heart disease and the result of dysregulated activation of resident cardiac fibroblasts (CFs). Recent work identified stress-induced degradation of the cytoskeletal protein β_IV-spectrin as an important step in CF activation and cardiac fibrosis. Furthermore, loss of β_IV-spectrin was found to depend on Ca²⁺/calmodulin-dependent kinase II (CaMKII). Therefore, we sought to determine the mechanism for CaMKII-dependent regulation of β_IV-spectrin and CF activity. Computational screening and MS revealed a critical serine residue (S2250 in mouse and S2254 in human) in β_IV-spectrin phosphorylated by CaMKII. Disruption of β_IV-spectrin/CaMKII interaction or alanine substitution of β_IV-spectrin Ser2250 (β_IV-S2254A) prevented CaMKII-induced degradation, whereas a phosphomimetic construct (β_IV-spectrin with glutamic acid substitution at serine 2254 [β_IV-S2254E]) showed accelerated degradation in the absence of CaMKII. To assess the physiological significance of this phosphorylation event, we expressed exogenous β_IV-S2254A and β_IV-S2254E constructs in β_IV-spectrin-deficient CFs, which have increased proliferation and fibrotic gene expression compared with WT CFs. β_IV-S2254A but not β_IV-S2254E normalized CF proliferation, gene expression, and contractility. Pathophysiological targeting of β_IV-spectrin phosphorylation and subsequent degradation was identified in CFs activated with the profibrotic ligand angiotensin II, resulting in increased proliferation and signal transducer and activation of transcription 3 nuclear accumulation. While therapeutic delivery of exogenous WT β_IV-spectrin partially reversed these trends, β_IV-S2254A completely negated increased CF proliferation and signal transducer and activation of transcription 3 translocation. Moreover, we observed β_IV-spectrin phosphorylation and associated loss in total protein within human heart tissue following heart failure. Together, these data illustrate a considerable role for the β_IV-spectrin/CaMKII interaction in activating profibrotic signaling.

Regulation of Cardiac Conduction and Arrhythmias by Ankyrin/Spectrin-Based Macromolecular Complexes

The cardiac conduction system is an extended network of excitable tissue tasked with generation and propagation of electrical impulses to signal coordinated contraction of the heart. The fidelity of this system depends on the proper spatio-temporal regulation of ion channels in myocytes throughout the conduction system. Importantly, inherited or acquired defects in a wide class of ion channels has been linked to dysfunction at various stages of the conduction system resulting in life-threatening cardiac arrhythmia. There is growing appreciation of the role that adapter and cytoskeletal proteins play in organizing ion channel macromolecular complexes critical for proper function of the cardiac conduction system. In particular, members of the ankyrin and spectrin families have emerged as important nodes for normal expression and regulation of ion channels in myocytes throughout the conduction system. Human variants impacting ankyrin/spectrin function give rise to a broad constellation of cardiac arrhythmias. Furthermore, chronic neurohumoral and biomechanical stress promotes ankyrin/spectrin loss of function that likely contributes to conduction disturbances in the setting of acquired cardiac disease. Collectively, this review seeks to bring attention to the significance of these cytoskeletal players and emphasize the potential therapeutic role they represent in a myriad of cardiac disease states.

Severe Form of ßIV-Spectrin Deficiency With Mitochondrial Dysfunction and Cardiomyopathy-A Case Report

ßIV-spectrin is a protein of the spectrin family which is involved in the organization of the cytoskeleton structure and is found in high quantity in the axon initial segment and the nodes of Ranvier. Together with ankyrin G, ßIV-spectrin is responsible for the clustering of KCNQ2/3-potassium channels and NaV-sodium channels. Loss or reduction of ßIV-spectrin causes a destabilization of the cytoskeleton and an impairment in the generation of the action potential, which leads to neuronal degeneration. Furthermore, ßIV-spectrin has been described to play an important role in the maintenance of the neuronal polarity and of the diffusion barrier. ßIV-spectrin is also located in the heart where it takes an important part in the structural organization of ion channels and has also been described to participate in cell signaling pathways through binding of transcription factors. We describe two patients with a severe form of ßIV-spectrin deficiency. Whole-exome sequencing revealed the homozygous stop mutation c.6016C>T (p.R2006^*) in the SPTBN4 gene. The phenotype of these patients is characterized by profound psychomotor developmental arrest, respiratory insufficiency and deafness. Additionally one of the patients presents with cardiomyopathy, optical nerve atrophy, and mitochondrial dysfunction. This is the first report of a severe form of ßIV-spectrin deficiency with hypertrophic cardiomyopathy and mitochondrial dysfunction.

Pharmacologic Approach to Sinoatrial Node Dysfunction

The spontaneous activity of the sinoatrial node initiates the heartbeat. Sino-atrial node dysfunction (SND) and sick sinoatrial (sick sinus) syndrome are caused by the heart's inability to generate a normal sinoatrial node action potential. In clinical practice, SND is generally considered an age-related pathology, secondary to degenerative fibrosis of the heart pacemaker tissue. However, other forms of SND exist, including idiopathic primary SND, which is genetic, and forms that are secondary to cardiovascular or systemic disease. The incidence of SND in the general population is expected to increase over the next half century, boosting the need to implant electronic pacemakers. During the last two decades, our knowledge of sino-atrial node physiology and of the pathophysiological mechanisms underlying SND has advanced considerably. This review summarizes the current knowledge about SND mechanisms and discusses the possibility of introducing new pharmacologic therapies for treating SND.

Mechanosensitive TREK-1 two-pore-domain potassium (K2P) channels in the cardiovascular system

TWIK-related K⁺ channel (TREK-1) two-pore-domain potassium (K_2P) channels mediate background potassium currents and regulate cellular excitability in many different types of cells. Their functional activity is controlled by a broad variety of different physiological stimuli, such as temperature, extracellular or intracellular pH, lipids and mechanical stress. By linking cellular excitability to mechanical stress, TREK-1 currents might be important to mediate parts of the mechanoelectrical feedback described in the heart. Furthermore, TREK-1 currents might contribute to the dysregulation of excitability in the heart in pathophysiological situations, such as those caused by abnormal stretch or ischaemia-associated cell swelling, thereby contributing to arrhythmogenesis. In this review, we focus on the functional role of TREK-1 in the heart and its putative contribution to cardiac mechanoelectrical coupling. Its cardiac expression among different species is discussed, alongside with functional evidence for TREK-1 currents in cardiomyocytes. In addition, evidence for the involvement of TREK-1 currents in different cardiac arrhythmias, such as atrial fibrillation or ventricular tachycardia, is summarized. Furthermore, the role of TREK-1 and its interaction partners in the regulation of the cardiac heart rate is reviewed. Finally, we focus on the significance of TREK-1 in the development of cardiac hypertrophy, cardiac fibrosis and heart failure.

βIV-Spectrin/STAT3 complex regulates fibroblast phenotype, fibrosis, and cardiac function

Increased fibrosis is a characteristic remodeling response to biomechanical and neurohumoral stress and a determinant of cardiac mechanical and electrical dysfunction in disease. Stress-induced activation of cardiac fibroblasts (CFs) is a critical step in the fibrotic response, although the precise sequence of events underlying activation of these critical cells in vivo remain unclear. Here, we tested the hypothesis that a βIV-spectrin/STAT3 complex is essential for maintenance of a quiescent phenotype (basal nonactivated state) in CFs. We reported increased fibrosis, decreased cardiac function, and electrical impulse conduction defects in genetic and acquired mouse models of βIV-spectrin deficiency. Loss of βIV-spectrin function promoted STAT3 nuclear accumulation and transcriptional activity, and it altered gene expression and CF activation. Furthermore, we demonstrate that a quiescent phenotype may be restored in βIV-spectrin-deficient fibroblasts by expressing a βIV-spectrin fragment including the STAT3-binding domain or through pharmacological STAT3 inhibition. We found that in vivo STAT3 inhibition abrogates fibrosis and cardiac dysfunction in the setting of global βIV-spectrin deficiency. Finally, we demonstrate that fibroblast-specific deletion of βIV-spectrin is sufficient to induce fibrosis and decreased cardiac function. We propose that the βIV-spectrin/STAT3 complex is a determinant of fibroblast phenotype and fibrosis, with implications for remodeling response in cardiovascular disease (CVD).

Supporting the heart: Functions of the cardiomyocyte's non-sarcomeric cytoskeleton

The non-contractile cytoskeleton in cardiomyocytes is comprised of cytoplasmic actin, microtubules, and intermediate filaments. In addition to providing mechanical support to these cells, these structures are important effectors of tension-sensing and signal transduction and also provide networks for the transport of proteins and organelles. The majority of our knowledge on the function and structure of these cytoskeletal networks comes from research on proliferative cell types. However, in recent years, researchers have begun to show that there are important cardiomyocyte-specific functions of the cytoskeleton. Here we will discuss the current state of cytoskeletal biology in cardiomyocytes, as well as research from other cell types, that together suggest there is a wealth of knowledge on cardiac health and disease waiting to be uncovered through exploration of the complex signaling networks of cardiomyocyte non-sarcomeric cytoskeletal proteins.

STAT3: a link between CaMKII-βIV-spectrin and maladaptive remodeling?

βIV-Spectrin, along with ankyrin and Ca2+/calmodulin-dependent kinase II (CaMKII), has been shown to form local signaling domains at the intercalated disc, while playing a key role in the regulation of Na+ and K+ channels in cardiomyocytes. In this issue of the JCI, Unudurthi et al. show that under chronic pressure overload conditions, CaMKII activation leads to βIV-spectrin degradation, resulting in the release of sequestered STAT3 from the intercalated discs. This in turn leads to dysregulation of STAT3-mediated gene transcription, maladaptive remodeling, fibrosis, and decreased cardiac function. Overall, this study presents interesting findings regarding the role of CaMKII and βIV-spectrin under physiological as well as pathological conditions.

βIV-Spectrin regulates STAT3 targeting to tune cardiac response to pressure overload

Heart failure (HF) remains a major source of morbidity and mortality in the US. The multifunctional Ca2+/calmodulin-dependent kinase II (CaMKII) has emerged as a critical regulator of cardiac hypertrophy and failure, although the mechanisms remain unclear. Previous studies have established that the cytoskeletal protein βIV-spectrin coordinates local CaMKII signaling. Here, we sought to determine the role of a spectrin-CaMKII complex in maladaptive remodeling in HF. Chronic pressure overload (6 weeks of transaortic constriction [TAC]) induced a decrease in cardiac function in WT mice but not in animals expressing truncated βIV-spectrin lacking spectrin-CaMKII interaction (qv3J mice). Underlying the observed differences in function was an unexpected differential regulation of STAT3-related genes in qv3J TAC hearts. In vitro experiments demonstrated that βIV-spectrin serves as a target for CaMKII phosphorylation, which regulates its stability. Cardiac-specific βIV-spectrin-KO (βIV-cKO) mice showed STAT3 dysregulation, fibrosis, and decreased cardiac function at baseline, similar to what was observed with TAC in WT mice. STAT3 inhibition restored normal cardiac structure and function in βIV-cKO and WT TAC hearts. Our studies identify a spectrin-based complex essential for regulation of the cardiac response to chronic pressure overload. We anticipate that strategies targeting the new spectrin-based "statosome" will be effective at suppressing maladaptive remodeling in response to chronic stress.

The two-pore domain potassium channel TREK-1 mediates cardiac fibrosis and diastolic dysfunction

Cardiac two-pore domain potassium channels (K2P) exist in organisms from Drosophila to humans; however, their role in cardiac function is not known. We identified a K2P gene, CG8713 (sandman), in a Drosophila genetic screen and show that sandman is critical to cardiac function. Mice lacking an ortholog of sandman, TWIK-related potassium channel (TREK-1, also known Kcnk2), exhibit exaggerated pressure overload-induced concentric hypertrophy and alterations in fetal gene expression, yet retain preserved systolic and diastolic cardiac function. While cardiomyocyte-specific deletion of TREK-1 in response to in vivo pressure overload resulted in cardiac dysfunction, TREK-1 deletion in fibroblasts prevented deterioration in cardiac function. The absence of pressure overload-induced dysfunction in TREK-1-KO mice was associated with diminished cardiac fibrosis and reduced activation of JNK in cardiomyocytes and fibroblasts. These findings indicate a central role for cardiac fibroblast TREK-1 in the pathogenesis of pressure overload-induced cardiac dysfunction and serve as a conceptual basis for its inhibition as a potential therapy.

NBCe1 Na+-HCO3- cotransporter ablation causes reduced apoptosis following cardiac ischemia-reperfusion injury in vivo

AIM

To investigate the hypothesis that cardiomyocyte-specific loss of the electrogenic NBCe1 Na⁺-HCO₃^- cotransporter is cardioprotective during in vivo ischemia-reperfusion (IR) injury.

METHODS

An NBCe1 (Slc4a4 gene) conditional knockout mouse (KO) model was prepared by gene targeting. Cardiovascular performance of wildtype (WT) and cardiac-specific NBCe1 KO mice was analyzed by intraventricular pressure measurements, and changes in cardiac gene expression were determined by RNA Seq analysis. Response to in vivo IR injury was analyzed after 30 min occlusion of the left anterior descending artery followed by 3 h of reperfusion.

RESULTS

Loss of NBCe1 in cardiac myocytes did not impair cardiac contractility or relaxation under basal conditions or in response to β-adrenergic stimulation, and caused only limited changes in gene expression patterns, such as those for electrical excitability. However, following ischemia and reperfusion, KO heart sections exhibited significantly fewer apoptotic nuclei than WT sections.

CONCLUSION

These studies indicate that cardiac-specific loss of NBCe1 does not impair cardiovascular performance, causes only minimal changes in gene expression patterns, and protects against IR injury in vivo .

CaMKII-dependent late Na+ current increases electrical dispersion and arrhythmia in ischemia-reperfusion

The mechanisms underlying Ca²⁺/calmodulin-dependent protein kinase II (CaMKII)-induced arrhythmias in ischemia-reperfusion (I/R) are not fully understood. We tested the hypothesis that CaMKII increases late Na⁺ current ( I_Na,L) via phosphorylation of Na_v1.5 at Ser⁵⁷¹ during I/R, thereby increasing arrhythmia susceptibility. To test our hypothesis, we studied isolated, Langendorff-perfused hearts from wild-type (WT) mice and mice expressing Na_v channel variants Na_v1.5-Ser571E (S571E) and Na_v1.5-Ser571A (S571A). WT hearts showed a significant increase in the levels of phosphorylated CaMKII and Na_v1.5 at Ser⁵⁷¹ [p-Na_v1.5(S571)] after 15 min of global ischemia (just before the onset of reperfusion). Optical mapping experiments revealed an increase in action potential duration (APD) and APD dispersion without changes in conduction velocity during I/R in WT and S571E compared with S571A hearts. At the same time, WT and S571E hearts showed an increase in spontaneous arrhythmia events (e.g., premature ventricular contractions) and an increase in the inducibility of reentrant arrhythmias during reperfusion. Pretreatment of WT hearts with the Na⁺ channel blocker mexiletine (10 μM) normalized APD dispersion and reduced arrhythmia susceptibility during I/R. We conclude that CaMKII-dependent phosphorylation of Na_v1.5 is a crucial driver for increased I_Na,L, arrhythmia triggers, and substrate during I/R. Selective targeting of this CaMKII-dependent pathway may have therapeutic potential for reducing arrhythmias in the setting of I/R. NEW & NOTEWORTHY Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) phosphorylation of Na_v1.5 at Ser⁵⁷¹ leads to a prolongation of action potential duration (APD), increased APD dispersion, and increased arrhythmia susceptibility after ischemia-reperfusion in isolated mouse hearts. Genetic ablation of the CaMKII-dependent phosphorylation site Ser⁵⁷¹ on Na_v1.5 or low-dose mexiletine (to inhibit late Na⁺ current) reduced APD dispersion, arrhythmia triggers, and ventricular tachycardia inducibility.

T-type calcium channels functionally interact with spectrin (α/β) and ankyrin B

This study describes the functional interaction between the Cav3.1 and Cav3.2 T-type calcium channels and cytoskeletal spectrin (α/β) and ankyrin B proteins. The interactions were identified utilizing a proteomic approach to identify proteins that interact with a conserved negatively charged cytosolic region present in the carboxy-terminus of T-type calcium channels. Deletion of this stretch of amino acids decreased binding of Cav3.1 and Cav3.2 calcium channels to spectrin (α/β) and ankyrin B and notably also reduced T-type whole cell current densities in expression systems. Furthermore, fluorescence recovery after photobleaching analysis of mutant channels lacking the proximal C-terminus region revealed reduced recovery of both Cav3.1 and Cav3.2 mutant channels in hippocampal neurons. Knockdown of spectrin α and ankyrin B decreased the density of endogenous Cav3.2 in hippocampal neurons. These findings reveal spectrin (α/β) / ankyrin B cytoskeletal and signaling proteins as key regulators of T-type calcium channels expressed in the nervous system.

Spectrin-based pathways underlying electrical and mechanical dysfunction in cardiac disease

INTRODUCTION

In the heart, pathways that transduce extracellular environmental cues (e.g. mechanical force, inflammatory stress) into electrical and/or chemical signals at the cellular level are critical for the organ-level response to chronic biomechanical/neurohumoral stress. Specifically, a diverse array of membrane-bound receptors and stretch-activated proteins converge on a network of intracellular signaling cascades that control gene expression, protein translation, degradation and/or regulation. These cellular reprogramming events ultimately lead to changes in cell excitability, growth, proliferation, and/or survival. Areas covered: The actin/spectrin cytoskeleton has emerged as having important roles in not only providing structural support for organelle function but also in serving as a signaling 'superhighway,' linking signaling events at/near the membrane to distal cellular domains (e.g. nucleus, mitochondria). Furthermore, recent work suggests that the integrity of the actin/spectrin cytoskeleton is critical for canonical signaling of pathways involved in cellular response to stress. This review discusses these emerging roles for spectrin and consider implications for heart function and disease. Expert commentary: Despite growth in our understanding of the broader roles for spectrins in cardiac myocytes and other metazoan cells, there remain important unanswered questions, the answers to which may point the way to new therapies for human cardiac disease patients.

Ankyrins and Spectrins in Cardiovascular Biology and Disease

Ankyrins are adaptor proteins critical for the expression and targeting of cardiac membrane proteins, signaling molecules, and cytoskeletal elements. Findings in humans and animal models have highlighted the in vivo roles for ankyrins in normal physiology and in cardiovascular disease, most notably in cardiac arrhythmia. For example, human ANK2 loss-of-function variants are associated with a complex array of electrical and structural phenotypes now termed “ankyrin-B syndrome,” whereas alterations in the ankyrin-G pathway for Na_v channel targeting are associated with human Brugada syndrome. Further, both ankyrin-G and -B are now linked with acquired forms of cardiovascular disease including myocardial infarction and atrial fibrillation. Spectrins are ankyrin-associated proteins and recent studies support the critical role of ankyrin-spectrin interactions in normal cardiac physiology as well as regulation of key ion channel and signaling complexes. This review will highlight the roles of ankyrins and spectrins in cardiovascular physiology as well as illustrate the link between the dysfunction in ankyrin- and spectrin-based pathways and disease.

Stretch-activated potassium currents in the heart: Focus on TREK-1 and arrhythmias

This review focuses on the role and the molecular candidates of the cardiac stretch-activated potassium current (SAK). The functional properties of the two-pore domain potassium (K_2P) channel TREK-1, a major candidate for the cardiac SAK, are analyzed and the molecular mechanism of stretch-activation in K_2P potassium channels is discussed. Furthermore, the functional modulation of TREK-1 by different cardiac interaction partners, as well as evidence for the functional role of the stretch-dependent TREK-1 and its putative subunits in the heart is reviewed. In addition, we summarize the recent evidence that TREK-1 is involved in the pathogenesis of human cardiac arrhythmias.

The Popeye Domain Containing Genes and their Function in Striated Muscle

The Popeye domain containing (POPDC) genes encode a novel class of cAMP effector proteins, which are abundantly expressed in heart and skeletal muscle. Here, we will review their role in striated muscle as deduced from work in cell and animal models and the recent analysis of patients carrying a missense mutation in POPDC1. Evidence suggests that POPDC proteins control membrane trafficking of interacting proteins. Furthermore, we will discuss the current catalogue of established protein-protein interactions. In recent years, the number of POPDC-interacting proteins has been rising and currently includes ion channels (TREK-1), sarcolemma-associated proteins serving functions in mechanical stability (dystrophin), compartmentalization (caveolin 3), scaffolding (ZO-1), trafficking (NDRG4, VAMP2/3) and repair (dysferlin) or acting as a guanine nucleotide exchange factor for Rho-family GTPases (GEFT). Recent evidence suggests that POPDC proteins might also control the cellular level of the nuclear proto-oncoprotein c-Myc. These data suggest that this family of cAMP-binding proteins probably serves multiple roles in striated muscle.

Dysfunction of the β2-spectrin-based pathway in human heart failure

β2-Spectrin is critical for integrating membrane and cytoskeletal domains in excitable and nonexcitable cells. The role of β2-spectrin for vertebrate function is illustrated by dysfunction of β2-spectrin-based pathways in disease. Recently, defects in β2-spectrin association with protein partner ankyrin-B were identified in congenital forms of human arrhythmia. However, the role of β2-spectrin in common forms of acquired heart failure and arrhythmia is unknown. We report that β2-spectrin protein levels are significantly altered in human cardiovascular disease as well as in large and small animal cardiovascular disease models. Specifically, β2-spectrin levels were decreased in atrial samples of patients with atrial fibrillation compared with tissue from patients in sinus rhythm. Furthermore, compared with left ventricular samples from nonfailing hearts, β2-spectrin levels were significantly decreased in left ventricle of ischemic- and nonischemic heart failure patients. Left ventricle samples of canine and murine heart failure models confirm reduced β2-spectrin protein levels. Mechanistically, we identify that β2-spectrin levels are tightly regulated by posttranslational mechanisms, namely Ca(2+)- and calpain-dependent proteases. Furthermore, consistent with this data, we observed Ca(2+)- and calpain-dependent loss of β2-spectrin downstream effector proteins, including ankyrin-B in heart. In summary, our findings illustrate that β2-spectrin and downstream molecules are regulated in multiple forms of cardiovascular disease via Ca(2+)- and calpain-dependent proteolysis.

Two-Pore K+ Channel TREK-1 Regulates Sinoatrial Node Membrane Excitability

Background

Two‐pore K⁺ channels have emerged as potential targets to selectively regulate cardiac cell membrane excitability; however, lack of specific inhibitors and relevant animal models has impeded the effort to understand the role of 2‐pore K⁺ channels in the heart and their potential as a therapeutic target. The objective of this study was to determine the role of mechanosensitive 2‐pore K⁺ channel family member TREK‐1 in control of cardiac excitability.

Methods and Results

Cardiac‐specific TREK‐1–deficient mice (αMHC‐Kcnk^f/f) were generated and found to have a prevalent sinoatrial phenotype characterized by bradycardia with frequent episodes of sinus pause following stress. Action potential measurements from isolated αMHC‐Kcnk2^f/f sinoatrial node cells demonstrated decreased background K⁺ current and abnormal sinoatrial cell membrane excitability. To identify novel pathways for regulating TREK‐1 activity and sinoatrial node excitability, mice expressing a truncated allele of the TREK‐1–associated cytoskeletal protein β_IV‐spectrin (qv^4J mice) were analyzed and found to display defects in cell electrophysiology as well as loss of normal TREK‐1 membrane localization. Finally, the β_IV‐spectrin/TREK‐1 complex was found to be downregulated in the right atrium from a canine model of sinoatrial node dysfunction and in human cardiac disease.

Conclusions

These findings identify a TREK‐1–dependent pathway essential for normal sinoatrial node cell excitability that serves as a potential target for selectively regulating sinoatrial node cell function.

Genetic Deletion of TREK-1 or TWIK-1/TREK-1 Potassium Channels does not Alter the Basic Electrophysiological Properties of Mature Hippocampal Astrocytes In Situ

We have recently shown that a linear current-to-voltage (I-V) relationship of membrane conductance (passive conductance) reflects the intrinsic property of K⁺ channels in mature astrocytes. While passive conductance is known to underpin a highly negative and stable membrane potential (V_M) essential for the basic homeostatic function of astrocytes, a complete repertoire of the involved K⁺ channels remains elusive. TREK-1 two-pore domain K⁺ channel (K_2P) is highly expressed in astrocytes, and covalent association of TREK-1 with TWIK-1, another highly expressed astrocytic K_2P, has been reported as a mechanism underlying the trafficking of heterodimer TWIK-1/TREK-1 channel to the membrane and contributing to astrocyte passive conductance. To decipher the individual contribution of TREK-1 and address whether the appearance of passive conductance is conditional to the co-expression of TWIK-1/TREK-1 in astrocytes, TREK-1 single and TWIK-1/TREK-1 double gene knockout mice were used in the present study. The relative quantity of mRNA encoding other astrocyte K⁺ channels, such as K_ir4.1, K_ir5.1, and TREK-2, was not altered in these gene knockout mice. Whole-cell recording from hippocampal astrocytes in situ revealed no detectable changes in astrocyte passive conductance, V_M, or membrane input resistance (R_in) in either kind of gene knockout mouse. Additionally, TREK-1 proteins were mainly located in the intracellular compartments of the hippocampus. Altogether, genetic deletion of TREK-1 alone or together with TWIK-1 produced no obvious alteration in the basic electrophysiological properties of hippocampal astrocytes. Thus, future research focusing on other K⁺ channels may shed light on this long-standing and important question in astrocyte physiology.

The Popeye domain containing protein family--A novel class of cAMP effectors with important functions in multiple tissues

Popeye domain containing (Popdc) proteins are a unique family, which combine several different properties and functions in a surprisingly complex fashion. They are expressed in multiple tissues and cell types, present in several subcellular compartments, interact with different classes of proteins, and are associated with a variety of physiological and pathophysiological processes. Moreover, Popdc proteins bind the second messenger cAMP with high affinity and it is thought that they act as a novel class of cAMP effector proteins. Here, we will review the most important findings about the Popdc family, which accumulated since its discovery about 15 years ago. We will be focussing on Popdc protein interaction and function in striated muscle tissue. However, as a full picture only emerges if all aspects are taken into account, we will also describe what is currently known about the role of Popdc proteins in epithelial cells and in various types of cancer, and discuss these findings with regard to their relevance for cardiac and skeletal muscle.

An Adaptable Spectrin/Ankyrin-Based Mechanism for Long-Range Organization of Plasma Membranes in Vertebrate Tissues

Ankyrins are membrane-associated proteins that together with their spectrin partners are responsible for micron-scale organization of vertebrate plasma membranes, including those of erythrocytes, excitable membranes of neurons and heart, lateral membrane domains of columnar epithelial cells, and striated muscle. Ankyrins coordinate functionally related membrane transporters and cell adhesion proteins (15 protein families identified so far) within plasma membrane compartments through independently evolved interactions of intrinsically disordered sequences with a highly conserved peptide-binding groove formed by the ANK repeat solenoid. Ankyrins are coupled to spectrins, which are elongated organelle-sized proteins that form mechanically resilient arrays through cross-linking by specialized actin filaments. In addition to protein interactions, cellular targeting and assembly of spectrin/ankyrin domains also critically depend on palmitoylation of ankyrin-G by aspartate-histidine-histidine-cysteine 5/8 palmitoyltransferases, as well as interaction of beta-2 spectrin with phosphoinositide lipids. These lipid-dependent spectrin/ankyrin domains are not static but are locally dynamic and determine membrane identity through opposing endocytosis of bulk lipids as well as specific proteins. A partnership between spectrin, ankyrin, and cell adhesion molecules first emerged in bilaterians over 500 million years ago. Ankyrin and spectrin may have been recruited to plasma membranes from more ancient roles in organelle transport. The basic bilaterian spectrin-ankyrin toolkit markedly expanded in vertebrates through gene duplications combined with variation in unstructured intramolecular regulatory sequences as well as independent evolution of ankyrin-binding activity by ion transporters involved in action potentials and calcium homeostasis. In addition, giant vertebrate ankyrins with specialized roles in axons acquired new coding sequences by exon shuffling. We speculate that early axon initial segments and epithelial lateral membranes initially were based on spectrin-ankyrin-cell adhesion molecule assemblies and subsequently served as "incubators," where ion transporters independently acquired ankyrin-binding activity through positive selection.

Voltage-Gated Sodium Channel Phosphorylation at Ser571 Regulates Late Current, Arrhythmia, and Cardiac Function In Vivo

BACKGROUND

Voltage-gated Na(+) channels (Nav) are essential for myocyte membrane excitability and cardiac function. Nav current (INa) is a large-amplitude, short-duration spike generated by rapid channel activation followed immediately by inactivation. However, even under normal conditions, a small late component of INa (INa,L) persists because of incomplete/failed inactivation of a subpopulation of channels. Notably, INa,L is directly linked with both congenital and acquired disease states. The multifunctional Ca(2+)/calmodulin-dependent kinase II (CaMKII) has been identified as an important activator of INa,L in disease. Several potential CaMKII phosphorylation sites have been discovered, including Ser571 in the Nav1.5 DI-DII linker, but the molecular mechanism underlying CaMKII-dependent regulation of INa,L in vivo remains unknown.

METHODS AND RESULTS

To determine the in vivo role of Ser571, 2 Scn5a knock-in mouse models were generated expressing either: (1) Nav1.5 with a phosphomimetic mutation at Ser571 (S571E), or (2) Nav1.5 with the phosphorylation site ablated (S571A). Electrophysiology studies revealed that Ser571 regulates INa,L but not other channel properties previously linked to CaMKII. Ser571-mediated increases in INa,L promote abnormal repolarization and intracellular Ca(2+) handling and increase susceptibility to arrhythmia at the cellular and animal level. Importantly, Ser571 is required for maladaptive remodeling and arrhythmias in response to pressure overload.

CONCLUSIONS

Our data provide the first in vivo evidence for the molecular mechanism underlying CaMKII activation of the pathogenic INa,L. Relevant for improved rational design of potential therapies, our findings demonstrate that Ser571-dependent regulation of Nav1.5 specifically tunes INa,L without altering critical physiological components of the current.

Regulation of TWIK-related potassium channel-1 (Trek1) restitutes intestinal epithelial barrier function

The disruption of epithelial barrier integrity is an important factor in the pathogenesis of various immune disorders. However, the restitution of the compromised barrier functions is difficult. This study investigates the regulation of TWIK-related potassium channel-1 (Trek1) in the restitution of intestinal epithelial barrier functions. The human colon epithelial cell line T84 was cultured in monolayers and used to observe epithelial barrier functions in vitro. An intestinal allergy mouse model was created. Cytokine levels were determined by enzyme-linked immunosorbent assay and western blotting. The results showed that Trek1 deficiency induced T84 monolayer barrier disruption. Allergic responses markedly suppressed the expression of Trek1 in the intestinal epithelia via activating the mitogen-activated protein kinase pathways and increasing the expression of histone deacetylase-1. The inhibition of histone deacetylase-1 by sodium butyrate or the administration of a butyrate-producing probiotic (Clostridium butyricum) restored the intestinal epithelial barrier functions and markedly enhanced the effect of antigen-specific immunotherapy. The data suggest that Trek1 is required for the maintenance of intestinal epithelial barrier integrity. Allergic responses induce an insufficiency of Trek1 expression in the intestinal epithelia. Trek1 expression facilitates the restoration of intestinal epithelial barrier functions in an allergic environment.

The potassium current carried by TREK-1 channels in rat cardiac ventricular muscle

We studied the potassium current flowing through TREK-1 channels in rat cardiac ventricular myocytes. We separated the TREK-1 current from other current components by blocking most other channels with a blocker cocktail. We tried to inhibit the TREK-1 current by activating protein kinase A (PKA) with a mixture of forskolin and isobutyl-methylxanthine (IBMX). Activation of PKA blocked an outwardly rectifying current component at membrane potentials positive to -40 mV. At 37 °C, application of forskolin plus IBMX reduced the steady-state outward current measured at positive voltages by about 52 %. Application of the potassium channel blockers quinidine or tetrahexylammonium also reduced the steady-state outward current by about 50 %. Taken together, our results suggest that the increase in temperature from 22 to 37 °C increased the TREK-1 current by a factor of at least 5 and that the average density of the TREK-1 current in rat cardiomyocytes at 37 °C is about 1.5 pA/pF at +30 mV. The contribution of TREK-1 to the action potential was assessed by using a dynamic patch clamp technique. After subtraction of simulated TREK-1 currents, action potential duration at 50 or 90 % repolarisation was increased by about 12 %, indicating that TREK-1 may be functionally important in rat ventricular muscle. During sympathetic stimulation, inhibition of TREK-1 channels via PKA is expected to prolong the action potential primarily in subendocardial myocytes; this may decrease the transmural dispersion of repolarisation and thus may serve to prevent the occurrence of arrhythmias.

Ankyrin-G coordinates intercalated disc signaling platform to regulate cardiac excitability in vivo

RATIONALE

Nav1.5 (SCN5A) is the primary cardiac voltage-gated Nav channel. Nav1.5 is critical for cardiac excitability and conduction, and human SCN5A mutations cause sinus node dysfunction, atrial fibrillation, conductional abnormalities, and ventricular arrhythmias. Further, defects in Nav1.5 regulation are linked with malignant arrhythmias associated with human heart failure. Consequently, therapies to target select Nav1.5 properties have remained at the forefront of cardiovascular medicine. However, despite years of investigation, the fundamental pathways governing Nav1.5 membrane targeting, assembly, and regulation are still largely undefined.

OBJECTIVE

Define the in vivo mechanisms underlying Nav1.5 membrane regulation.

METHODS AND RESULTS

Here, we define the molecular basis of an Nav channel regulatory platform in heart. Using new cardiac-selective ankyrin-G(-/-) mice (conditional knock-out mouse), we report that ankyrin-G targets Nav1.5 and its regulatory protein calcium/calmodulin-dependent kinase II to the intercalated disc. Mechanistically, βIV-spectrin is requisite for ankyrin-dependent targeting of calcium/calmodulin-dependent kinase II-δ; however, βIV-spectrin is not essential for ankyrin-G expression. Ankyrin-G conditional knock-out mouse myocytes display decreased Nav1.5 expression/membrane localization and reduced INa associated with pronounced bradycardia, conduction abnormalities, and ventricular arrhythmia in response to Nav channel antagonists. Moreover, we report that ankyrin-G links Nav channels with broader intercalated disc signaling/structural nodes, as ankyrin-G loss results in reorganization of plakophilin-2 and lethal arrhythmias in response to β-adrenergic stimulation.

CONCLUSIONS

Our findings provide the first in vivo data for the molecular pathway required for intercalated disc Nav1.5 targeting/regulation in heart. Further, these new data identify the basis of an in vivo cellular platform critical for membrane recruitment and regulation of Nav1.5.

Inhibition of a TREK-like K+ channel current by noradrenaline requires both β1- and β2-adrenoceptors in rat atrial myocytes

AIMS

Noradrenaline plays an important role in the modulation of atrial electrophysiology. However, the identity of the modulated channels, their mechanisms of modulation, and their role in the action potential remain unclear. This study aimed to investigate the noradrenergic modulation of an atrial steady-state outward current (IKss).

METHODS AND RESULTS

Rat atrial myocyte whole-cell currents were recorded at 36°C. Noradrenaline potently inhibited IKss (IC50 = 0.90 nM, 42.1 ± 4.3% at 1 µM, n = 7) and potentiated the L-type Ca(2+) current (ICaL, EC50 = 136 nM, 205 ± 40% at 1 µM, n = 6). Noradrenaline-sensitive IKss was weakly voltage-dependent, time-independent, and potentiated by the arachidonic acid analogue, 5,8,11,14-eicosatetraynoic acid (EYTA; 10 µM), or by osmotically induced membrane stretch. Noise analysis revealed a unitary conductance of 8.4 ± 0.42 pS (n = 8). The biophysical/pharmacological properties of IKss indicate a TREK-like K(+) channel. The effect of noradrenaline on IKss was abolished by combined β1-/β2-adrenoceptor antagonism (1 µM propranolol or 10 µM β1-selective atenolol and 100 nM β2-selective ICI-118,551 in combination), but not by β1- or β2-antagonist alone. The action of noradrenaline could be mimicked by β2-agonists (zinterol and fenoterol) in the presence of β1-antagonist. The action of noradrenaline on IKss, but not on ICaL, was abolished by pertussis toxin (PTX) treatment. The action of noradrenaline on ICaL was mediated by β1-adrenoceptors via a PTX-insensitive pathway. Noradrenaline prolonged APD30 by 52 ± 19% (n = 5; P < 0.05), and this effect was abolished by combined β1-/β2-antagonism, but not by atenolol alone.

CONCLUSION

Noradrenaline inhibits a rat atrial TREK-like K(+) channel current via a PTX-sensitive mechanism involving co-operativity of β1-/β2-adrenoceptors that contributes to atrial APD prolongation.