α1-adrenoceptors regulate only the caveolae-located subpopulation of cardiac K(V)4 channels

Caveolin-3 and Caveolae regulate ventricular repolarization

RATIONALE

Flask-shaped invaginations of the cardiomyocyte sarcolemma called caveolae require the structural protein caveolin-3 (Cav-3) and host a variety of ion channels, transporters, and signaling molecules. Reduced Cav-3 expression has been reported in models of heart failure, and variants in CAV3 have been associated with the inherited long-QT arrhythmia syndrome. Yet, it remains unclear whether alterations in Cav-3 levels alone are sufficient to drive aberrant repolarization and increased arrhythmia risk.

OBJECTIVE

To determine the impact of cardiac-specific Cav-3 ablation on the electrophysiological properties of the adult mouse heart.

METHODS AND RESULTS

Cardiac-specific, inducible Cav3 homozygous knockout (Cav-3KO) mice demonstrated a marked reduction in Cav-3 expression by Western blot and loss of caveolae by electron microscopy. However, there was no change in macroscopic cardiac structure or contractile function. The QTc interval was increased in Cav-3KO mice, and there was an increased propensity for ventricular arrhythmias. Ventricular myocytes isolated from Cav-3KO mice exhibited a prolonged action potential duration (APD) that was due to reductions in outward potassium currents (Ito, Iss) and changes in inward currents including slowed inactivation of ICa,L and increased INa,L. Mathematical modeling demonstrated that the changes in the studied ionic currents were adequate to explain the prolongation of the mouse ventricular action potential. Results from human iPSC-derived cardiomyocytes showed that shRNA knockdown of Cav-3 similarly prolonged APD.

CONCLUSION

We demonstrate that Cav-3 and caveolae regulate cardiac repolarization and arrhythmia risk via the integrated modulation of multiple ionic currents.

Kv4.2 phosphorylation by PKA drives Kv4.2 - KChIP2 dissociation, leading to Kv4.2 out of lipid rafts and internalization

Sympathetic regulation of the Kv4.2 transient outward potassium current is critical for the acute electrical and contractile response of the myocardium under physiological and pathological conditions. Previous studies have suggested that KChIP2, the key auxiliary subunit of Kv4 channels, is required for the sympathetic regulation of Kv4.2 current densities. Of interest, Kv4.2 and KChIP2, and key components mediating acute sympathetic signaling transduction are present in lipid rafts, which are profoundly involved in regulation of I_to densities in rat ventricular myocytes. However, little is known about the mechanisms of Kv4.2-raft association and its connection with acute sympathetic regulation. With the aid of high-resolution fluorescent microscope, we demonstrate that KChIP2 assists Kv4.2 localization in lipid rafts in HEK293 cells. Moreover, PKA-mediated Kv4.2 phosphorylation, the downstream signaling event of acute sympathetic stimulation, induced dissociation between Kv4.2 and KChIP2, resulting in Kv4.2 shifting out of lipid rafts in KChIP2-expressed HEK293．The mutation that mimics Kv4.2 phosphorylation by PKA similarly disrupted Kv4.2 interaction with KChIP2 and also decreased the surface stability of Kv4.2. The attenuated Kv4.2-KChIP2 interaction was also observed in native neonatal rat ventricular myocytes (NRVMs) upon acute adrenergic stimulation with phenylephrine (PE). Furthermore, PE accelerated internalization of Kv4.2 in native NRVMs, but disruption of lipid rafts dampens this reaction. In conclusion, KChIP2 contributes to targeting Kv4.2 to lipid rafts. Acute adrenergic stimulation induces Kv4.2 - KChIP2 dissociation, leading to Kv4.2 out of lipid rafts and internalization, reinforcing the critical role of Kv4.2-lipid raft association in the essential physiological response of I_to to acute sympathetic regulation.

Caveolin-3 is required for regulation of transient outward potassium current by angiotensin II in mouse atrial myocytes

Angiotensin II (AngII) is a key mediator of the renin-angiotensin system and plays an important role in the regulation of cardiac electrophysiology by affecting various cardiac ion currents, including transient outward potassium current, Ito. AngII receptors and molecular components of Ito, Kv4.2 and Kv4.3 channels, have been linked to caveolae structures. However, their functional interaction and the importance of such proximity within 50- to 100-nm caveolar nanodomains remain unknown. To address this, we studied the mechanisms of Ito regulation by AngII in atrial myocytes of wild-type (WT) and cardiac-specific caveolin-3 (Cav3) conditional knockout (Cav3KO) mice. We showed that in WT atrial myocytes, a short-term (2 h) treatment with AngII (5 µM) significantly reduced Ito density. This effect was prevented 1) by a 30-min pretreatment with a selective antagonist of AngII receptor 1 (Ang1R) losartan (2 µM) or 2) by a selective inhibition of protein kinase C (PKC) by BIM1 (10 µM). The effect of AngII on Ito was completely abolished in Cav3-KO mice, with no change in a baseline Ito current density. In WT atria, Ang1Rs co-localized with Cav3, and the expression of Ang1Rs was significantly decreased in Cav3KO in comparison with WT mice, whereas no change in Kv4.2 and Kv4.3 protein expression was observed. Overall, our findings demonstrate that Cav3 is involved in the regulation of Ang1R expression and is required for the modulation of Ito by AngII in mouse atrial myocytes.NEW & NOTEWORTHY Angiotensin II receptor 1 is associated with caveolae and caveolar scaffolding protein caveolin-3 in mouse atrial myocytes that is required for the regulation of Ito by angiotensin II. Downregulation of caveolae/caveolin-3 disrupts this regulation and may be implicated in pathophysiological atrial remodeling.

Cell-Adhesion Properties of β-Subunits in the Regulation of Cardiomyocyte Sodium Channels

Voltage-gated sodium (Nav) channels drive the rising phase of the action potential, essential for electrical signalling in nerves and muscles. The Nav channel α-subunit contains the ion-selective pore. In the cardiomyocyte, Nav1.5 is the main Nav channel α-subunit isoform, with a smaller expression of neuronal Nav channels. Four distinct regulatory β-subunits (β1–4) bind to the Nav channel α-subunits. Previous work has emphasised the β-subunits as direct Nav channel gating modulators. However, there is now increasing appreciation of additional roles played by these subunits. In this review, we focus on β-subunits as homophilic and heterophilic cell-adhesion molecules and the implications for cardiomyocyte function. Based on recent cryogenic electron microscopy (cryo-EM) data, we suggest that the β-subunits interact with Nav1.5 in a different way from their binding to other Nav channel isoforms. We believe this feature may facilitate trans-cell-adhesion between β1-associated Nav1.5 subunits on the intercalated disc and promote ephaptic conduction between cardiomyocytes.

Long QT syndrome caveolin-3 mutations differentially modulate Kv 4 and Cav 1.2 channels to contribute to action potential prolongation

KEY POINTS

Mutations in the caveolae scaffolding protein, caveolin-3 (Cav3), have been linked to the long QT type 9 inherited arrhythmia syndrome (LQT9) and the cause of underlying action potential duration prolongation is incompletely understood. In the present study, we show that LQT9 Cav3 mutations, F97C and S141R, cause mutation-specific gain of function effects on Ca_v 1.2-encoded L-type Ca²⁺ channels responsible for I_Ca,L and also cause loss of function effects on heterologously expressed K_v 4.2 and K_v 4.3 channels responsible for I_to . A computational model of the human ventricular myocyte action potential suggests that the major ionic current change causing action potential duration prolongation in the presence of Cav3-F97C is the slowly inactivating I_Ca,L but, for Cav3-S141R, both increased I_Ca,L and increased late Na⁺ current contribute equally to action potential duration prolongation. Overall, the LQT9 Cav3-F97C and Cav3-S141R mutations differentially impact multiple ionic currents, highlighting the complexity of Cav3 regulation of cardiac excitability and suggesting mutation-specific therapeutic approaches.

ABSTRACT

Mutations in the CAV3 gene encoding caveolin-3 (Cav3), a scaffolding protein integral to caveolae in cardiomyocytes, have been associated with the congenital long-QT syndrome (LQT9). Initial studies demonstrated that LQT9-associated Cav3 mutations, F97C and S141R, increase late sodium current as a potential mechanism to prolong action potential duration (APD) and cause LQT9. Whether these Cav3 LQT9 mutations impact other caveolae related ion channels remains unknown. We used the whole-cell, patch clamp technique to characterize the effect of Cav3-F97C and Cav3-S141R mutations on heterologously expressed Ca_v 1.2+Ca_v β_2cN4 channels, as well as K_v 4.2 and K_v 4.3 channels, in HEK 293 cells. Expression of Cav3-S141R increased I_Ca,L density without changes in gating properties, whereas expression of Cav3-F97C reduced Ca²⁺ -dependent inactivation of I_Ca,L without changing current density. The Cav3-F97C mutation reduced current density and altered the kinetics of I_Kv4.2 and I_Kv4.3 and also slowed recovery from inactivation. Cav3-S141R decreased current density and also slowed activation kinetics and recovery from inactivation of I_Kv4.2 but had no effect on I_Kv4.3 . Using the O'Hara-Rudy computational model of the human ventricular myocyte action potential, the Cav3 mutation-induced changes in I_to are predicted to have negligible effect on APD, whereas blunted Ca²⁺ -dependent inactivation of I_Ca,L by Cav3-F97C is predicted to be primarily responsible for APD prolongation, although increased I_Ca,L and late I_Na by Cav3-S141R contribute equally to APD prolongation. Thus, LQT9 Cav3-associated mutations, F97C and S141R, produce mutation-specific changes in multiple ionic currents leading to different primary causes of APD prolongation, which suggests the use of mutation-specific therapeutic approaches in the future.

Myocyte membrane and microdomain modifications in diabetes: determinants of ischemic tolerance and cardioprotection

Cardiovascular disease, predominantly ischemic heart disease (IHD), is the leading cause of death in diabetes mellitus (DM). In addition to eliciting cardiomyopathy, DM induces a ‘wicked triumvirate’: (i) increasing the risk and incidence of IHD and myocardial ischemia; (ii) decreasing myocardial tolerance to ischemia–reperfusion (I–R) injury; and (iii) inhibiting or eliminating responses to cardioprotective stimuli. Changes in ischemic tolerance and cardioprotective signaling may contribute to substantially higher mortality and morbidity following ischemic insult in DM patients. Among the diverse mechanisms implicated in diabetic impairment of ischemic tolerance and cardioprotection, changes in sarcolemmal makeup may play an overarching role and are considered in detail in the current review. Observations predominantly in animal models reveal DM-dependent changes in membrane lipid composition (cholesterol and triglyceride accumulation, fatty acid saturation vs. reduced desaturation, phospholipid remodeling) that contribute to modulation of caveolar domains, gap junctions and T-tubules. These modifications influence sarcolemmal biophysical properties, receptor and phospholipid signaling, ion channel and transporter functions, contributing to contractile and electrophysiological dysfunction, cardiomyopathy, ischemic intolerance and suppression of protective signaling. A better understanding of these sarcolemmal abnormalities in types I and II DM (T1DM, T2DM) can inform approaches to limiting cardiomyopathy, associated IHD and their consequences. Key knowledge gaps include details of sarcolemmal changes in models of T2DM, temporal patterns of lipid, microdomain and T-tubule changes during disease development, and the precise impacts of these diverse sarcolemmal modifications. Importantly, exercise, dietary, pharmacological and gene approaches have potential for improving sarcolemmal makeup, and thus myocyte function and stress-resistance in this ubiquitous metabolic disorder.

Electrophysiology and metabolism of caveolin-3-overexpressing mice

Caveolin-3 (Cav-3) plays a critical role in organizing signaling molecules and ion channels involved in cardiac conduction and metabolism. Mutations in Cav-3 are implicated in cardiac conduction abnormalities and myopathies. Additionally, cardiac-specific overexpression of Cav-3 (Cav-3 OE) is protective against ischemic and hypertensive injury, suggesting a potential role for Cav-3 in basal cardiac electrophysiology and metabolism involved in stress adaptation. We hypothesized that overexpression of Cav-3 may alter baseline cardiac conduction and metabolism. We examined: (1) ECG telemetry recordings at baseline and during pharmacological interventions, (2) ion channels involved in cardiac conduction with immunoblotting and computational modeling, and (3) baseline metabolism in Cav-3 OE and transgene-negative littermate control mice. Cav-3 OE mice had decreased heart rates, prolonged PR intervals, and shortened QTc intervals with no difference in activity compared to control mice. Dobutamine or propranolol did not cause significant changes between experimental groups in maximal (dobutamine) or minimal (propranolol) heart rate. Cav-3 OE mice had an overall lower chronotropic response to atropine. The expression of Kv1.4 and Kv4.3 channels, Nav1.5 channels, and connexin 43 were increased in Cav-3 OE mice. A computational model integrating the immunoblotting results indicated shortened action potential duration in Cav-3 OE mice linking the change in channel expression to the observed electrophysiology phenotype. Metabolic profiling showed no gross differences in VO2, VCO2, respiratory exchange ratio, heat generation, and feeding or drinking. In conclusion, Cav-3 OE mice have changes in ECG intervals, heart rates, and cardiac ion channel expression. These findings give novel mechanistic insights into previously reported Cav-3 dependent cardioprotection.

Autocrine A2 in the T-system of ventricular myocytes creates transmural gradients in ion transport: a mechanism to match contraction with load?

Transmural heterogeneities in Na/K pump current (IP), transient outward K(+)-current (Ito), and Ca(2+)-current (ICaL) play an important role in regulating electrical and contractile activities in the ventricular myocardium. Prior studies indicated angiotensin II (A2) may determine the transmural gradient in Ito, but the effects of A2 on IP and ICaL were unknown. In this study, myocytes were isolated from five muscle layers between epicardium and endocardium. We found a monotonic gradient in both Ip and Ito, with the lowest currents in ENDO. When AT1Rs were inhibited, EPI currents were unaffected, but ENDO currents increased, suggesting endogenous extracellular A2 inhibits both currents in ENDO. IP- and Ito-inhibition by A2 yielded essentially the same K0.5 values, so they may both be regulated by the same mechanism. A2/AT1R-mediated inhibition of IP or Ito or stimulation of ICaL persisted for hours in isolated myocytes, suggesting continuous autocrine secretion of A2 into a restricted diffusion compartment, like the T-system. Detubulation brought EPI IP to its low ENDO value and eliminated A2 sensitivity, so the T-system lumen may indeed be the restricted diffusion compartment. These studies showed that 33-50% of IP, 57-65% of Ito, and a significant fraction of ICaL reside in T-tubule membranes where they are transmurally regulated by autocrine secretion of A2 into the T-system lumen and activation of AT1Rs. Increased AT1R activation regulates each of these currents in a direction expected to increase contractility. Endogenous A2 activation of AT1Rs increases monotonically from EPI to ENDO in a manner similar to reported increases in passive tension when the ventricular chamber fills with blood. We therefore hypothesize load is the signal that regulates A2-activation of AT1Rs, which create a contractile gradient that matches the gradient in load.

Interaction of membrane/lipid rafts with the cytoskeleton: impact on signaling and function: membrane/lipid rafts, mediators of cytoskeletal arrangement and cell signaling

The plasma membrane in eukaryotic cells contains microdomains that are enriched in certain glycosphingolipids, gangliosides, and sterols (such as cholesterol) to form membrane/lipid rafts (MLR). These regions exist as caveolae, morphologically observable flask-like invaginations, or as a less easily detectable planar form. MLR are scaffolds for many molecular entities, including signaling receptors and ion channels that communicate extracellular stimuli to the intracellular milieu. Much evidence indicates that this organization and/or the clustering of MLR into more active signaling platforms depends upon interactions with and dynamic rearrangement of the cytoskeleton. Several cytoskeletal components and binding partners, as well as enzymes that regulate the cytoskeleton, localize to MLR and help regulate lateral diffusion of membrane proteins and lipids in response to extracellular events (e.g., receptor activation, shear stress, electrical conductance, and nutrient demand). MLR regulate cellular polarity, adherence to the extracellular matrix, signaling events (including ones that affect growth and migration), and are sites of cellular entry of certain pathogens, toxins and nanoparticles. The dynamic interaction between MLR and the underlying cytoskeleton thus regulates many facets of the function of eukaryotic cells and their adaptation to changing environments. Here, we review general features of MLR and caveolae and their role in several aspects of cellular function, including polarity of endothelial and epithelial cells, cell migration, mechanotransduction, lymphocyte activation, neuronal growth and signaling, and a variety of disease settings. This article is part of a Special Issue entitled: Reciprocal influences between cell cytoskeleton and membrane channels, receptors and transporters. Guest Editor: Jean Claude Hervé.

Adrenergic regulation of cardiac ionic channels: role of membrane microdomains in the regulation of kv4 channels

The heart must constantly adapt its activity to the needs of the body. In any potentially dangerous or physically demanding situation the activated sympathetic nervous system leads a very fast cardiac response. Under these circumstances, α1-adrenergic receptors activate intracellular signaling pathways that finally phosphorylate the caveolae-located subpopulation of Kv4 channels and reduce the transient outward K(+) current (Ito) amplitude. This reduction changes the shape of the cardiac action potential and makes the plateau phase to start at higher voltages. This means that there are more calcium ions entering the myocyte and the result is an increase in the strength of the contraction. However, an excessive reduction of Ito could dangerously prolong action potential duration and this could cause arrhythmias when the heart rate is high. This excessive current reduction does not occur because there is a second population of Ito channels located in non-caveolar membrane rafts that are not accessible for α1-AR mediated regulation. Thus, the location of the components of a given transduction signaling pathway in membrane domains determines the correct and safe behavior of the heart. This article is part of a Special Issue entitled: Reciprocal influences between cell cytoskeleton and membrane channels, receptors and transporters. This article is part of a Special Issue entitled: Reciprocal influences between cell cytoskeleton and membrane channels, receptors and transporters. Guest Editor: Jean Claude Hervé.

Remodelling of human atrial K+ currents but not ion channel expression by chronic β-blockade

Chronic β-adrenoceptor antagonist (β-blocker) treatment in patients is associated with a potentially anti-arrhythmic prolongation of the atrial action potential duration (APD), which may involve remodelling of repolarising K(+) currents. The aim of this study was to investigate the effects of chronic β-blockade on transient outward, sustained and inward rectifier K(+) currents (I(TO), I(KSUS) and I(K1)) in human atrial myocytes and on the expression of underlying ion channel subunits. Ion currents were recorded from human right atrial isolated myocytes using the whole-cell-patch clamp technique. Tissue mRNA and protein levels were measured using real time RT-PCR and Western blotting. Chronic β-blockade was associated with a 41% reduction in I(TO) density: 9.3 ± 0.8 (30 myocytes, 15 patients) vs 15.7 ± 1.1 pA/pF (32, 14), p < 0.05; without affecting its voltage-, time- or rate dependence. I(K1) was reduced by 34% at -120 mV (p < 0.05). Neither I(KSUS), nor its increase by acute β-stimulation with isoprenaline, was affected by chronic β-blockade. Mathematical modelling suggested that the combination of I(TO)- and I(K1)-decrease could result in a 28% increase in APD(90). Chronic β-blockade did not alter mRNA or protein expression of the I(TO) pore-forming subunit, Kv4.3, or mRNA expression of the accessory subunits KChIP2, KChAP, Kvβ1, Kvβ2 or frequenin. There was no reduction in mRNA expression of Kir2.1 or TWIK to account for the reduction in I(K1). A reduction in atrial I(TO) and I(K1) associated with chronic β-blocker treatment in patients may contribute to the associated action potential prolongation, and this cannot be explained by a reduction in expression of associated ion channel subunits.

Caveolae create local signalling domains through their distinct protein content, lipid profile and morphology

Compartmentation of signalling allows multiple stimuli to achieve diverse cellular responses with only a limited pool of second messengers. This spatial control of signalling is achieved, in part, by cellular structures which bring together elements of a particular cascade. One such structure is the caveola, a flask-shaped lipid raft. Caveolae are well-recognised as signalosomes, platforms for assembly of signalling complexes of receptors, effectors and their targets, which can facilitate efficient and specific cellular responses. Here we extend this simple model and present evidence to show how the protein and lipid profiles of caveolae, as well as their characteristic morphology, define their roles in creating local signalling domains in the cardiac myocyte. This article is part of a Special Issue entitled "Local Signaling in Myocytes."

Lipid microdomains and the regulation of ion channel function

Many types of ion channel localize to cholesterol and sphingolipid-enriched regions of the plasma membrane known as lipid microdomains or 'rafts'. The precise physiological role of these unique lipid microenvironments remains elusive due largely to difficulties associated with studying these potentially extremely small and dynamic domains. Nevertheless, increasing evidence suggests that membrane rafts regulate channel function in a number of different ways. Raft-enriched lipids such as cholesterol and sphingolipids exert effects on channel activity either through direct protein-lipid interactions or by influencing the physical properties of the bilayer. Rafts also appear to selectively recruit interacting signalling molecules to generate subcellular compartments that may be important for efficient and selective signal transduction. Direct interaction with raft-associated scaffold proteins such as caveolin can also influence channel function by altering gating kinetics or by affecting trafficking and surface expression. Selective association of ion channels with specific lipid microenvironments within the membrane is thus likely to be an important and fundamental regulatory aspect of channel physiology. This brief review highlights some of the existing evidence for raft modulation of channel function.