Localization of cardiac L-type Ca(2+) channels to a caveolar macromolecular signaling complex is required for beta(2)-adrenergic regulation

Cardiac L-type calcium channel regulation by Leucine-Rich Repeat-Containing Protein 10

L-type calcium channels (LTCCs), the major portal for Ca2+ entry into cardiomyocytes, are essential for excitation-contraction coupling and thus play a central role in regulating overall cardiac function. LTCC function is finely tuned by multiple signaling pathways and accessory proteins. Leucine-rich repeat-containing protein 10 (LRRC10) is a little studied cardiomyocyte-specific protein recently identified as a modulator of LTCCs. LRRC10 exerts a remarkable effect on LTCC function, more than doubling L-type Ca2+ current (ICa,L) amplitude in a heterologous expression system by altering the gating of the channels without changing their surface membrane expression. Genetic ablation of LRRC10 expression in mouse and zebrafish hearts leads to a significant reduction in ICa,L density and a slowly progressive dilated cardiomyopathy in mice. Rare sequence variants of LRRC10 have been identified in dilated cardiomyopathy and sudden unexplained nocturnal cardiac death syndrome, but these variants have not been clearly linked to disease. Nevertheless, the DCM-associated variant, I195T, converted LRRC10 from a ICa,L potentiator to a ICa,L suppressor, thus illustrating the wide dynamic range of LRRC10-mediated ICa,L regulation. This review focuses on the contemporary knowledge of LTCC modulation by LRRC10 and discusses potential directions for future investigations.

Electrocontractile remodeling of isolated cardiomyocytes induced during early-stage hypercholesterolemia

Hypercholesterolemia is one of the most important risk factors for cardiovascular diseases. However, it is mostly associated with vascular dysfunction and atherosclerotic lesions, while evidence of direct effects of hypercholesterolemia on cardiomyocytes and heart function is still incomplete and controversial. In this study, we assessed the direct effects of hypercholesterolemia on heart function and the electro-contractile properties of isolated cardiomyocytes. After 5 weeks, male Swiss mice fed with AIN-93 diet added with 1.25% cholesterol (CHO), developed an increase in total serum cholesterol levels and cardiomyocytes cholesterol content. These changes led to altered electrocardiographic records, with a shortening of the QT interval. Isolated cardiomyocytes displayed a shortening of the action potential duration with increased rate of depolarization, which was explained by increased IK, reduced ICa.L and altered INa voltage-dependent inactivation. Also, reduced diastolic [Ca2+]i was found with preserved adrenergic response and cellular contraction function. However, contraction of isolated hearts is impaired in isolated CHO hearts, before and after ischemia/reperfusion, although CHO heart was less susceptible to arrhythmic contractions. Overall, our results demonstrate that early hypercholesterolemia-driven increase in cellular cholesterol content is associated with direct modulation of the heart and cardiomyocytes' excitability, Ca2+ handling, and contraction.

WITHDRAWN: Caveolar Compartmentalization is Required for Stable Rhythmicity of Sinus Nodal Cells and is Disrupted in Heart Failure

The authors have withdrawn their manuscript owing to technical concerns merged during peer review. Therefore, the authors do not wish this work to be cited as a reference. If you have any questions, please contact the corresponding author.

Relationship between ion currents and membrane capacitance in canine ventricular myocytes

Current density, the membrane current value divided by membrane capacitance (Cm), is widely used in cellular electrophysiology. Comparing current densities obtained in different cell populations assume that Cm and ion current magnitudes are linearly related, however data is scarce about this in cardiomyocytes. Therefore, we statistically analyzed the distributions, and the relationship between parameters of canine cardiac ion currents and Cm, and tested if dividing original parameters with Cm had any effect. Under conventional voltage clamp conditions, correlations were high for IK1, moderate for IKr and ICa,L, while negligible for IKs. Correlation between Ito1 peak amplitude and Cm was negligible when analyzing all cells together, however, the analysis showed high correlations when cells of subepicardial, subendocardial or midmyocardial origin were analyzed separately. In action potential voltage clamp experiments IK1, IKr and ICa,L parameters showed high correlations with Cm. For INCX, INa,late and IKs there were low-to-moderate correlations between Cm and these current parameters. Dividing the original current parameters with Cm reduced both the coefficient of variation, and the deviation from normal distribution. The level of correlation between ion currents and Cm varies depending on the ion current studied. This must be considered when evaluating ion current densities in cardiac cells.

cAMP signaling: a remarkably regional affair

Louis Pasteur once famously said 'in the fields of observation chance favors only the prepared mind'. Much of chance is being in the right place at the right time. This is particularly true in the crowded molecular environment of the cell where being in the right place is often more important than timing. Although Brownian motion argues that enzymes will eventually bump into substrates, this probability is greatly enhanced if both molecules reside in the same subcellular compartment. However, activation of cell signaling enzymes often requires the transmission of chemical signals from extracellular stimuli to intracellular sites of action. This review highlights new developments in our understanding of cAMP generation and the 3D utilization of this second messenger inside cells.

Structural bases of inhibitory mechanism of CaV1.2 channel inhibitors

The voltage-gated calcium channel CaV1.2 is essential for cardiac and vessel smooth muscle contractility and brain function. Accumulating evidence demonstrates that malfunctions of CaV1.2 are involved in brain and heart diseases. Pharmacological inhibition of CaV1.2 is therefore of therapeutic value. Here, we report cryo-EM structures of CaV1.2 in the absence or presence of the antirheumatic drug tetrandrine or antihypertensive drug benidipine. Tetrandrine acts as a pore blocker in a pocket composed of S6II, S6III, and S6IV helices and forms extensive hydrophobic interactions with CaV1.2. Our structure elucidates that benidipine is located in the DIII-DIV fenestration site. Its hydrophobic sidechain, phenylpiperidine, is positioned at the exterior of the pore domain and cradled within a hydrophobic pocket formed by S5DIII, S6DIII, and S6DIV helices, providing additional interactions to exert inhibitory effects on both L-type and T-type voltage gated calcium channels. These findings provide the structural foundation for the rational design and optimization of therapeutic inhibitors of voltage-gated calcium channels.

The cardiac conduction system: History, development, and disease

The heart is the first organ to form during embryonic development, establishing the circulatory infrastructure necessary to sustain life and enable downstream organogenesis. Critical to the heart's function is its ability to initiate and propagate electrical impulses that allow for the coordinated contraction and relaxation of its chambers, and thus, the movement of blood and nutrients. Several specialized structures within the heart, collectively known as the cardiac conduction system (CCS), are responsible for this phenomenon. In this review, we discuss the discovery and scientific history of the mammalian cardiac conduction system as well as the key genes and transcription factors implicated in the formation of its major structures. We also describe known human diseases related to CCS development and explore existing challenges in the clinical context.

Cardiac contraction and relaxation are regulated by distinct subcellular cAMP pools

Cells interpret a variety of signals through G-protein-coupled receptors (GPCRs) and stimulate the generation of second messengers such as cyclic adenosine monophosphate (cAMP). A long-standing puzzle is deciphering how GPCRs elicit different physiological responses despite generating similar levels of cAMP. We previously showed that some GPCRs generate cAMP from both the plasma membrane and the Golgi apparatus. Here we demonstrate that cardiomyocytes distinguish between subcellular cAMP inputs to elicit different physiological outputs. We show that generating cAMP from the Golgi leads to the regulation of a specific protein kinase A (PKA) target that increases the rate of cardiomyocyte relaxation. In contrast, cAMP generation from the plasma membrane activates a different PKA target that increases contractile force. We further validated the physiological consequences of these observations in intact zebrafish and mice. Thus, we demonstrate that the same GPCR acting through the same second messenger regulates cardiac contraction and relaxation dependent on its subcellular location.

Caveolae-associated cAMP/Ca2+-mediated mechano-chemical signal transduction in mouse atrial myocytes

Caveolae are tiny invaginations in the sarcolemma that buffer extra membrane and contribute to mechanical regulation of cellular function. While the role of caveolae in membrane mechanosensation has been studied predominantly in non-cardiomyocyte cells, caveolae contribution to cardiac mechanotransduction remains elusive. Here, we studied the role of caveolae in the regulation of Ca2+ signaling in atrial cardiomyocytes. In Langendorff-perfused mouse hearts, atrial pressure/volume overload stretched atrial myocytes and decreased caveolae density. In isolated cells, caveolae were disrupted through hypotonic challenge that induced a temporal (<10 min) augmentation of Ca2+ transients and caused a rise in Ca2+ spark activity. Similar changes in Ca2+ signaling were observed after chemical (methyl-β-cyclodextrin) and genetic ablation of caveolae in cardiac-specific conditional caveolin-3 knock-out mice. Acute disruption of caveolae, both mechanical and chemical, led to the elevation of cAMP level in the cell interior, and cAMP-mediated augmentation of protein kinase A (PKA)-phosphorylated ryanodine receptors (at Ser2030 and Ser2808). Caveolae-mediated stimulatory effects on Ca2+ signaling were abolished via inhibition of cAMP production by adenyl cyclase antagonists MDL12330 and SQ22536, or reduction of PKA activity by H-89. A compartmentalized mathematical model of mouse atrial myocytes linked the observed changes to a microdomain-specific decrease in phosphodiesterase activity, which disrupted cAMP signaling and augmented PKA activity. Our findings add a new dimension to cardiac mechanobiology and highlight caveolae-associated cAMP/PKA-mediated phosphorylation of Ca2+ handling proteins as a novel component of mechano-chemical feedback in atrial myocytes.

Physiological functions of caveolae in endothelium

Endothelial caveolae are essential for a wide range of physiological processes and have emerged as key players in vascular biology. Our understanding of caveolar biology in endothelial cells has expanded dramatically since their discovery revealing critical roles in mechanosensation, signal transduction, eNOS regulation, lymphatic transport, and metabolic disease progression. Furthermore, caveolae are involved in the organization of membrane domains, regulation of membrane fluidity, and endocytosis which contribute to endothelial function and integrity. Additionally, recent advances highlight the impact of caveolae-mediated signaling pathways on vascular homeostasis and pathology. Together, the diverse roles of caveolae discussed here represent a breadth of cellular functions presenting caveolae as a defining feature of endothelial form and function. In light of these new insights, targeting caveolae may hold potential for the development of novel therapeutic strategies to treat a range of vascular diseases.

Muscarinic Receptor Stimulation Does Not Inhibit Voltage-dependent Ca2+ Channels in Rat Adrenal Medullary Chromaffin Cells

Adrenal medullary chromaffin (AMC) and sympathetic ganglion cells are derived from the neural crest and show a similar developmental path. Thus, these two cell types have many common properties in membrane excitability and signaling. However, AMC cells function as endocrine cells while sympathetic ganglion cells are neurons. In rat sympathetic ganglion cells, muscarinic M1 and M4 receptors mediate excitation and inhibition via suppression of M-type K+ channels and suppression of voltage-dependent Ca2+ channels, respectively. On the other hand, M1 receptor stimulation in rat AMC cells also produces excitation by suppressing TWIK-related acid sensitive K+ (TASK) channels. However, whether M4 receptors are coupled with voltage-dependent Ca2+ channel suppression is unclear. We explore this issue electrophysiologically and biochemically. Electrical stimulation of nerve fibers in rat adrenal glands trans-synaptically increased the Ca2+ signal in AMC cells. This electrically evoked increased Ca2+ signal was not altered during muscarine-induced increase in Ca2+ signal, whereas it decreased significantly during a GABA-induced increase, due to a shunt effect of increased Cl- conductance. The whole-cell current recordings revealed that voltage-dependent Ca2+ currents in AMC cells were suppressed by adenosine triphosphate, but not by muscarinic agonists. The fractionation analysis and immunocytochemistry indicated that CaV1.2 Ca2+ channels and M4 receptors are located in the raft and non-raft membrane domains, respectively. We concluded that muscarinic stimulation in rat AMC cells does not produce voltage-dependent Ca2+ channel inhibition. This lack of muscarinic inhibition is at least partly due to physical separation of voltage-dependent Ca2+ channels and M4 receptors in the plasma membrane.

Regulation of the Ca2+ Channel CaV1.2 Supports Spatial Memory and Its Flexibility and LTD

Widespread release of norepinephrine (NE) throughout the forebrain fosters learning and memory via adrenergic receptor (AR) signaling, but the molecular mechanisms are largely unknown. The β2 AR and its downstream effectors, the trimeric stimulatory Gs-protein, adenylyl cyclase (AC), and the cAMP-dependent protein kinase A (PKA), form a unique signaling complex with the L-type Ca2+ channel (LTCC) CaV1.2. Phosphorylation of CaV1.2 by PKA on Ser1928 is required for the upregulation of Ca2+ influx on β2 AR stimulation and long-term potentiation induced by prolonged theta-tetanus (PTT-LTP) but not LTP induced by two 1-s-long 100-Hz tetani. However, the function of Ser1928 phosphorylation in vivo is unknown. Here, we show that S1928A knock-in (KI) mice of both sexes, which lack PTT-LTP, express deficiencies during initial consolidation of spatial memory. Especially striking is the effect of this mutation on cognitive flexibility as tested by reversal learning. Mechanistically, long-term depression (LTD) has been implicated in reversal learning. It is abrogated in male and female S1928A knock-in mice and by β2 AR antagonists and peptides that displace β2 AR from CaV1.2. This work identifies CaV1.2 as a critical molecular locus that regulates synaptic plasticity, spatial memory and its reversal, and LTD.SIGNIFICANCE STATEMENT We show that phosphorylation of the Ca2+ channel CaV1.2 on Ser1928 is important for consolidation of spatial memory and especially its reversal, and long-term depression (LTD). Identification of Ser1928 as critical for LTD and reversal learning supports the model that LTD underlies flexibility of reference memory.

Functional LTCC-β2AR Complex Needs Caveolin-3 and Is Disrupted in Heart Failure

BACKGROUND

Beta-2 adrenergic receptors (β2ARs) but not beta-2 adrenergic receptors (β1ARs) form a functional complex with L-type Ca2+ channels (LTCCs) on the cardiomyocyte membrane. However, how microdomain localization in the plasma membrane affects the function of these complexes is unknown. We aim to study the coupling between LTCC and β adrenergic receptors in different cardiomyocyte microdomains, the distinct involvement of PKA and CAMKII (Ca2+/calmodulin-dependent protein kinase II) and explore how this functional complex is disrupted in heart failure.

METHODS

Global signaling between LTCCs and β adrenergic receptors was assessed with whole-cell current recordings and western blot analysis. Super-resolution scanning patch-clamp was used to explore the local coupling between single LTCCs and β1AR or β2AR in different membrane microdomains in control and failing cardiomyocytes.

RESULTS

LTCC open probability (Po) showed an increase from 0.054±0.003 to 0.092±0.008 when β2AR was locally stimulated in the proximity of the channel (<350 nm) in the transverse tubule microdomain. In failing cardiomyocytes, from both rodents and humans, this transverse tubule coupling between LTCC and β2AR was lost. Interestingly, local stimulation of β1AR did not elicit any change in the Po of LTCCs, indicating a lack of proximal functional interaction between the two, but we confirmed a general activation of LTCC via β1AR. By using blockers of PKA and CaMKII and a Caveolin-3-knockout mouse model, we conclude that the β2AR-LTCC regulation requires the presence of caveolin-3 and the activation of the CaMKII pathway. By contrast, at a cellular "global" level PKA plays a major role downstream β1AR and results in an increase in LTCC current.

CONCLUSIONS

Regulation of the LTCC activity by proximity coupling mechanisms occurs only via β2AR, but not β1AR. This may explain how β2ARs tune the response of LTCCs to adrenergic stimulation in healthy conditions. This coupling is lost in heart failure; restoring it could improve the adrenergic response of failing cardiomyocytes.

Cholesterol-load evokes robust calcium response in macrophages: An early event toward cholesterol-induced macrophage death

Macrophages in atherosclerotic lesions accumulate large amounts of unesterified cholesterol. Excess cholesterol load leads to cell death of macrophages, which is associated with the progression of atherosclerotic lesions. Calcium depletion in the endoplasmic reticulum (ER) and subsequent pro-apoptotic aberrant calcium signaling are key events in cholesterol-induced macrophage death. Although these concepts imply cytoplasmic calcium events in cholesterol-loaded macrophages, the mechanisms linking cholesterol accumulation to cytoplasmic calcium response have been poorly investigated. Based on our previous finding that extracellularly applied cholesterol evoked robust calcium oscillations in astrocytes, a type of glial cells in the brain, we hypothesized that cholesterol accumulation in macrophages triggers cytoplasmic calcium elevation. Here, we showed that cholesterol application induces calcium transients in THP-1-derived and peritoneal macrophages. Inhibition of inositol 1,4,5-trisphosphate receptors (IP3Rs) and l-type calcium channels (LTCCs) prevented cholesterol-induced calcium transients and ameliorated cholesterol-induced macrophage death. These results suggest that cholesterol-induced calcium transients through IP3Rs and LTCCs are crucial mechanisms underlying cholesterol-induced cell death of macrophages.

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.

The interplay of inflammation, exosomes and Ca2+ dynamics in diabetic cardiomyopathy

Diabetes mellitus is one of the prime risk factors for cardiovascular complications and is linked with high morbidity and mortality. Diabetic cardiomyopathy (DCM) often manifests as reduced cardiac contractility, myocardial fibrosis, diastolic dysfunction, and chronic heart failure. Inflammation, changes in calcium (Ca²⁺) handling and cardiomyocyte loss are often implicated in the development and progression of DCM. Although the existence of DCM was established nearly four decades ago, the exact mechanisms underlying this disease pathophysiology is constantly evolving. Furthermore, the complex pathophysiology of DCM is linked with exosomes, which has recently shown to facilitate intercellular (cell-to-cell) communication through biomolecules such as micro RNA (miRNA), proteins, enzymes, cell surface receptors, growth factors, cytokines, and lipids. Inflammatory response and Ca²⁺ signaling are interrelated and DCM  has been known to adversely affect many of these signaling molecules either qualitatively and/or quantitatively. In this literature review, we have demonstrated that Ca²⁺ regulators are tightly controlled at different molecular and cellular levels during various biological processes in the heart. Inflammatory mediators, miRNA and exosomes are shown to interact with these regulators, however how these mediators are linked to Ca²⁺ handling during DCM pathogenesis remains elusive. Thus, further investigations are needed to understand the mechanisms to restore cardiac Ca²⁺ homeostasis and function, and to serve as potential therapeutic targets in the treatment of DCM.

β subunits of voltage-gated calcium channels in cardiovascular diseases

Calcium signaling is required in bodily functions essential for survival, such as muscle contractions and neuronal communications. Of note, the voltage-gated calcium channels (VGCCs) expressed on muscle and neuronal cells, as well as some endocrine cells, are transmembrane protein complexes that allow for the selective entry of calcium ions into the cells. The α1 subunit constitutes the main pore-forming subunit that opens in response to membrane depolarization, and its biophysical functions are regulated by various auxiliary subunits-β, α2δ, and γ subunits. Within the cardiovascular system, the γ-subunit is not expressed and is therefore not discussed in this review. Because the α1 subunit is the pore-forming subunit, it is a prominent druggable target and the focus of many studies investigating potential therapeutic interventions for cardiovascular diseases. While this may be true, it should be noted that the direct inhibition of the α1 subunit may result in limited long-term cardiovascular benefits coupled with undesirable side effects, and that its expression and biophysical properties may depend largely on its auxiliary subunits. Indeed, the α2δ subunit has been reported to be essential for the membrane trafficking and expression of the α1 subunit. Furthermore, the β subunit not only prevents proteasomal degradation of the α1 subunit, but also directly modulates the biophysical properties of the α1 subunit, such as its voltage-dependent activities and open probabilities. More importantly, various isoforms of the β subunit have been found to differentially modulate the α1 subunit, and post-translational modifications of the β subunits further add to this complexity. These data suggest the possibility of the β subunit as a therapeutic target in cardiovascular diseases. However, emerging studies have reported the presence of cardiomyocyte membrane α1 subunit trafficking and expression in a β subunit-independent manner, which would undermine the efficacy of β subunit-targeting drugs. Nevertheless, a better understanding of the auxiliary β subunit would provide a more holistic approach when targeting the calcium channel complexes in treating cardiovascular diseases. Therefore, this review focuses on the post-translational modifications of the β subunit, as well as its role as an auxiliary subunit in modulating the calcium channel complexes.

Role of Ca2+ in healthy and pathologic cardiac function: from normal excitation-contraction coupling to mutations that cause inherited arrhythmia

Calcium (Ca²⁺) ions are a key second messenger involved in the rhythmic excitation and contraction of cardiomyocytes throughout the heart. Proper function of Ca²⁺-handling proteins is required for healthy cardiac function, whereas disruption in any of these can cause cardiac arrhythmias. This comprehensive review provides a broad overview of the roles of Ca²⁺-handling proteins and their regulators in healthy cardiac function and the mechanisms by which mutations in these proteins contribute to inherited arrhythmias. Major Ca²⁺ channels and Ca²⁺-sensitive regulatory proteins involved in cardiac excitation-contraction coupling are discussed, with special emphasis on the function of the RyR2 macromolecular complex. Inherited arrhythmia disorders including catecholaminergic polymorphic ventricular tachycardia, long QT syndrome, Brugada syndrome, short QT syndrome, and arrhythmogenic right-ventricular cardiomyopathy are discussed with particular emphasis on subtypes caused by mutations in Ca²⁺-handling proteins.

The interaction of membrane estradiol receptors and metabotropic glutamate receptors in adaptive and maladaptive estradiol-mediated motivated behaviors in females

Estrogen receptors were initially identified as intracellular, ligand-regulated transcription factors that result in genomic change upon ligand binding. However, rapid estrogen receptor signaling initiated outside of the nucleus was also known to occur via mechanisms that were less clear. Recent studies indicate that these traditional receptors, estrogen receptor α and estrogen receptor β, can also be trafficked to act at the surface membrane. Signaling cascades from these membrane-bound estrogen receptors (mERs) can rapidly alter cellular excitability and gene expression, particularly through the phosphorylation of CREB. A principal mechanism of neuronal mER action has been shown to occur through glutamate-independent transactivation of metabotropic glutamate receptors (mGlu), which elicits multiple signaling outcomes. The interaction of mERs with mGlu has been shown to be important in many diverse functions in females, including driving motivated behaviors. Experimental evidence suggests that a large part of estradiol-induced neuroplasticity and motivated behaviors, both adaptive and maladaptive, occurs through estradiol-dependent mER activation of mGlu. Herein we will review signaling through estrogen receptors, both "classical" nuclear receptors and membrane-bound receptors, as well as estradiol signaling through mGlu. We will focus on how the interactions of these receptors and their downstream signaling cascades are involved in driving motivated behaviors in females, discussing a representative adaptive motivated behavior (reproduction) and maladaptive motivated behavior (addiction).

Intercommunication between Voltage-Gated Calcium Channels and Estrogen Receptor/Estrogen Signaling: Insights into Physiological and Pathological Conditions

Voltage-gated calcium channels (VGCCs) and estrogen receptors are important cellular proteins that have been shown to interact with each other across varied cells and tissues. Estrogen hormone, the ligand for estrogen receptors, can also exert its effects independent of estrogen receptors that collectively constitute non-genomic mechanisms. Here, we provide insights into the VGCC regulation by estrogen and the possible mechanisms involved therein across several cell types. Notably, most of the interaction is described in neuronal and cardiovascular tissues given the importance of VGCCs in these electrically excitable tissues. We describe the modulation of various VGCCs by estrogen known so far in physiological conditions and pathological conditions. We observed that in most in vitro studies higher concentrations of estrogen were used while a handful of in vivo studies used meager concentrations resulting in inhibition or upregulation of VGCCs, respectively. There is a need for more relevant physiological assays to study the regulation of VGCCs by estrogen. Additionally, other interacting receptors and partners need to be identified that may be involved in exerting estrogen receptor-independent effects of estrogen.

Modulation of L-type calcium channels in Alzheimer's disease: A potential therapeutic target

Calcium plays a fundamental role in various signaling pathways and cellular processes in the human organism. In the nervous system, voltage-gated calcium channels such as L-type calcium channels (LTCCs) are critical elements in mediating neurotransmitter release, synaptic integration and plasticity. Dysfunction of LTCCs has been implicated in both aging and Alzheimer's Disease (AD), constituting a key component of calcium hypothesis of AD. As such, LTCCs are a promising drug target in AD. However, due to their structural and functional complexity, the mechanisms by which LTCCs contribute to AD are still unclear. In this review, we briefly summarize the structure, function, and modulation of LTCCs that are the backbone for understanding pathological processes involving LTCCs. We suggest targeting molecular pathways up-regulating LTCCs in AD may be a more promising approach, given the diverse physiological functions of LTCCs and the ineffectiveness of LTCC blockers in clinical studies.

Probing spatiotemporal PKA activity at the ryanodine receptor and SERCA2a nanodomains in cardomyocytes

Spatiotemporal regulation of subcellular protein kinase A (PKA) activity for precise substrate phosphorylation is essential for cellular responses to hormonal stimulation. Ryanodine receptor 2 (RyR2) and (sarco)endoplasmic reticulum calcium ATPase 2a (SERCA2a) represent two critical targets of β adrenoceptor (βAR) signaling on the sarcoplasmic reticulum membrane for cardiac excitation and contraction coupling. Using novel biosensors, we show that cardiac β1AR signals to both RyR2 and SERCA2a nanodomains in cardiomyocytes from mice, rats, and rabbits, whereas the β2AR signaling is restricted from these nanodomains. Phosphodiesterase 4 (PDE4) and PDE3 control the baseline PKA activity and prevent β2AR signaling from reaching the RyR2 and SERCA2a nanodomains. Moreover, blocking inhibitory G protein allows β2AR signaling to the RyR2 but not the SERCA2a nanodomains. This study provides evidence for the differential roles of inhibitory G protein and PDEs in controlling the adrenergic subtype signaling at the RyR2 and SERCA2a nanodomains in cardiomyocytes.

Pathophysiology and molecular mechanism of caveolin involved in myocardial protection strategies in ischemic conditioning

Multiple pathophysiological pathways are activated during the process of myocardial injury. Various cardioprotective strategies protect the myocardium from ischemia, infarction, and ischemia/reperfusion (I/R) injury through different targets, yet the clinical translation remains limited. Caveolae and its structure protein, caveolins, have been suggested as a bridge to transmit damage-preventing signals and mediate the protection of ultrastructure in cardiomyocytes under pathological conditions. In this review, we first briefly introduce caveolae and caveolins. Then we review the cardioprotective strategies mediated by caveolins through various pathophysiological pathways. Finally, some possible research directions are proposed to provide future experiments and clinical translation perspectives targeting caveolin based on the investigative evidence.

Subcellular trafficking and endocytic recycling of KATP channels

Sarcolemmal/plasmalemmal ATP-sensitive K+ (KATP) channels have key roles in many cell types and tissues. Hundreds of studies have described how the KATP channel activity and ATP sensitivity can be regulated by changes in the cellular metabolic state, by receptor signaling pathways and by pharmacological interventions. These alterations in channel activity directly translate to alterations in cell or tissue function, that can range from modulating secretory responses, such as insulin release from pancreatic β-cells or neurotransmitters from neurons, to modulating contractile behavior of smooth muscle or cardiac cells to elicit alterations in blood flow or cardiac contractility. It is increasingly becoming apparent, however, that KATP channels are regulated beyond changes in their activity. Recent studies have highlighted that KATP channel surface expression is a tightly regulated process with similar implications in health and disease. The surface expression of KATP channels is finely balanced by several trafficking steps including synthesis, assembly, anterograde trafficking, membrane anchoring, endocytosis, endocytic recycling, and degradation. This review aims to summarize the physiological and pathophysiological implications of KATP channel trafficking and mechanisms that regulate KATP channel trafficking. A better understanding of this topic has potential to identify new approaches to develop therapeutically useful drugs to treat KATP channel-related diseases.

Förster Resonance Energy Transfer-Based Single-Cell Imaging Reveals Piezo1-Induced Ca2+ Flux Mediates Membrane Ruffling and Cell Survival

A mechanosensitive ion channel, Piezo1 induces non-selective cation flux in response to various mechanical stresses. However, the biological interpretation and underlying mechanisms of cells resulting from Piezo1 activation remain elusive. This study elucidates Piezo1-mediated Ca²⁺ influx driven by channel activation and cellular behavior using novel Förster Resonance Energy Transfer (FRET)-based biosensors and single-cell imaging analysis. Results reveal that extracellular Ca²⁺ influx via Piezo1 requires intact caveolin, cholesterol, and cytoskeletal support. Increased cytoplasmic Ca²⁺ levels enhance PKA, ERK, Rac1, and ROCK activity, which have the potential to promote cancer cell survival and migration. Furthermore, we demonstrate that Piezo1-mediated Ca²⁺ influx upregulates membrane ruffling, a characteristic feature of cancer cell metastasis, using spatiotemporal image correlation spectroscopy. Thus, our findings provide new insights into the function of Piezo1, suggesting that Piezo1 plays a significant role in the behavior of cancer cells.

Aging Alters the Formation and Functionality of Signaling Microdomains Between L-type Calcium Channels and β2-Adrenergic Receptors in Cardiac Pacemaker Cells

Heart rate is accelerated to match physiological demands through the action of noradrenaline on the cardiac pacemaker. Noradrenaline is released from sympathetic terminals and activates β1-and β2-adrenergic receptors (ΑRs) located at the plasma membrane of pacemaker cells. L-type calcium channels are one of the main downstream targets potentiated by the activation of β-ARs. For this signaling to occur, L-type calcium channels need to be located in close proximity to β-ARs inside caveolae. Although it is known that aging causes a slowdown of the pacemaker rate and a reduction in the response of pacemaker cells to noradrenaline, there is a lack of in-depth mechanistic insights into these age-associated changes. Here, we show that aging affects the formation and function of adrenergic signaling microdomains inside caveolae. By evaluating the β1 and β2 components of the adrenergic regulation of the L-type calcium current, we show that aging does not alter the regulation mediated by β1-ARs but drastically impairs that mediated by β2-ARs. We studied the integrity of the signaling microdomains formed between L-type calcium channels and β-ARs by combining high-resolution microscopy and proximity ligation assays. We show that consistent with the electrophysiological data, aging decreases the physical association between β2-ARs and L-type calcium channels. Interestingly, this reduction is associated with a decrease in the association of L-type calcium channels with the scaffolding protein AKAP150. Old pacemaker cells also have a reduction in caveolae density and in the association of L-type calcium channels with caveolin-3. Together the age-dependent alterations in caveolar formation and the nano-organization of β2-ARs and L-type calcium channels result in a reduced sensitivity of the channels to β2 adrenergic modulation. Our results highlight the importance of these signaling microdomains in maintaining the chronotropic modulation of the heart and also pinpoint the direct impact that aging has on their function.

Mechanisms of Lian-Gui-Ning-Xin-Tang in the treatment of arrhythmia: Integrated pharmacology and in vivo pharmacological assessment

BACKGROUND

Lian-Gui-Ning-Xin-Tang (LGNXT), a classical traditional Chinese medicine (TCM) formula, has been widely used in clinical practice and has shown satisfactory efficacy in the treatment of arrhythmias. However, its mechanism of action in the treatment of arrhythmias is still unknown. Moreover, the complex chemical composition and therapeutic targets of LGNXT pose a challenge in pharmacological research.

PURPOSE

To analyze the active compounds and action mechanisms of LGNXT for the treatment of arrhythmias.

METHODS

Here, we used an integrated pharmacology approach to identify the potential active compounds and mechanisms of action of LGNXT in treating arrhythmias. Potential active compounds in LGNXT were identified using ultra-performance liquid chromatography-quadrupole-time-of-flight mass spectrometry (UPLC-Q-TOF/MS) and the potential related targets of these compounds were predicted using an integrated in silico approach. The obtained targets were mapped onto relevant databases to identify their corresponding pathways, following the experiments that were conducted to confirm whether the presumptive results of systemic pharmacology were correct.

RESULTS

Eighty-three components were identified in herbal materials and in animal plasma using UPLC-Q-TOF/MS and were considered the potential active components of LGNXT. Thirty key targets and 57 Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways were identified as possible targets and pathways involved in LGNXT-mediated treatment using network pharmacology, with the cyclic adenosine monophosphate (cAMP)/protein kinase A (PKA)/Ca2+ system pathway being the most significantly affected. This finding was validated using an adrenaline (Adr)-induced rat model of arrhythmias. Pretreatment with LGNXT delayed the occurrence, shortened the duration, and reduced the severity of arrhythmias. LGNXT exerted antiarrhythmic effects by inhibiting cAMP, PKA, CACNA1C, and RyR2.

CONCLUSIONS

The findings of this study revealed that preventing intracellular Ca2+ overload and maintaining intracellular Ca2+ homeostasis may be the primary mechanisms of LGNXT in alleviating arrhythmias. Thus, we suggest that the β-adrenergic receptor (AR)/cAMP/PKA/Ca2+ system signaling hub may constitute a promising molecular target for the development of novel antiarrhythmic therapeutic interventions. Additionally, we believe that the approach of investigation of the biological effects of a multi-herbal formula by the combination of metabolomics and network pharmacology, as used in this study, could serve as a systematic model for TCM research.

Caveolin-3 and Arrhythmias: Insights into the Molecular Mechanisms

Caveolin-3 is a muscle-specific protein on the membrane of myocytes correlated with a variety of cardiovascular diseases. It is now clear that the caveolin-3 plays a critical role in the cardiovascular system and a significant role in cardiac protective signaling. Mutations in the gene encoding caveolin-3 cause a broad spectrum of clinical phenotypes, ranging from persistent elevations in the serum levels of creatine kinase in asymptomatic humans to cardiomyopathy. The influence of Caveolin-3(CAV-3) mutations on current density parallels the effect on channel trafficking. For example, mutations in the CAV-3 gene promote ventricular arrhythmogenesis in long QT syndrome 9 by a combined decrease in the loss of the inward rectifier current (I_K1) and gain of the late sodium current (I_Na-L). The functional significance of the caveolin-3 has proved that caveolin-3 overexpression or knockdown contributes to the occurrence and development of arrhythmias. Caveolin-3 overexpression could lead to reduced diastolic spontaneous Ca²⁺ waves, thus leading to the abnormal L-Type calcium channel current-induced ventricular arrhythmias. Moreover, CAV-3 knockdown resulted in a shift to more negative values in the hyperpolarization-activated cyclic nucleotide channel 4 current (I_HCN4) activation curve and a significant decrease in I_HCN4 whole-cell current density. Recent evidence indicates that caveolin-3 plays a significant role in adipose tissue and is related to obesity development. The role of caveolin-3 in glucose homeostasis has attracted increasing attention. This review highlights the underlining mechanisms of caveolin-3 in arrhythmia. Progress in this field may contribute to novel therapeutic approaches for patients prone to developing arrhythmia.

Nanoscale Organization, Regulation, and Dynamic Reorganization of Cardiac Calcium Channels

The architectural specializations and targeted delivery pathways of cardiomyocytes ensure that L-type Ca²⁺ channels (CaV1.2) are concentrated on the t-tubule sarcolemma within nanometers of their intracellular partners the type 2 ryanodine receptors (RyR2) which cluster on the junctional sarcoplasmic reticulum (jSR). The organization and distribution of these two groups of cardiac calcium channel clusters critically underlies the uniform contraction of the myocardium. Ca²⁺ signaling between these two sets of adjacent clusters produces Ca²⁺ sparks that in health, cannot escalate into Ca²⁺ waves because there is sufficient separation of adjacent clusters so that the release of Ca²⁺ from one RyR2 cluster or supercluster, cannot activate and sustain the release of Ca²⁺ from neighboring clusters. Instead, thousands of these Ca²⁺ release units (CRUs) generate near simultaneous Ca²⁺ sparks across every cardiomyocyte during the action potential when calcium induced calcium release from RyR2 is stimulated by depolarization induced Ca²⁺ influx through voltage dependent CaV1.2 channel clusters. These sparks summate to generate a global Ca²⁺ transient that activates the myofilaments and thus the electrical signal of the action potential is transduced into a functional output, myocardial contraction. To generate more, or less contractile force to match the hemodynamic and metabolic demands of the body, the heart responds to β-adrenergic signaling by altering activity of calcium channels to tune excitation-contraction coupling accordingly. Recent accumulating evidence suggests that this tuning process also involves altered expression, and dynamic reorganization of CaV1.2 and RyR2 channels on their respective membranes to control the amplitude of Ca²⁺ entry, SR Ca²⁺ release and myocardial function. In heart failure and aging, altered distribution and reorganization of these key Ca²⁺ signaling proteins occurs alongside architectural remodeling and is thought to contribute to impaired contractile function. In the present review we discuss these latest developments, their implications, and future questions to be addressed.

NRSF-GNAO1 Pathway Contributes to the Regulation of Cardiac Ca2+ Homeostasis

BACKGROUND

During the development of heart failure, a fetal cardiac gene program is reactivated and accelerates pathological cardiac remodeling. We previously reported that a transcriptional repressor, NRSF (neuron restrictive silencer factor), suppresses the fetal cardiac gene program, thereby maintaining cardiac integrity. The underlying molecular mechanisms remain to be determined, however.

METHODS

We aim to elucidate molecular mechanisms by which NRSF maintains normal cardiac function. We generated cardiac-specific NRSF knockout mice and analyzed cardiac gene expression profiles in those mice and mice cardiac-specifically expressing a dominant-negative NRSF mutant.

RESULTS

We found that cardiac expression of Gαo, an inhibitory G protein encoded in humans by GNAO1, is transcriptionally regulated by NRSF and is increased in the ventricles of several mouse models of heart failure. Genetic knockdown of Gnao1 ameliorated the cardiac dysfunction and prolonged survival rates in these mouse heart failure models. Conversely, cardiac-specific overexpression of GNAO1 in mice was sufficient to induce cardiac dysfunction. Mechanistically, we observed that increasing Gαo expression increased surface sarcolemmal L-type Ca2+ channel activity, activated CaMKII (calcium/calmodulin-dependent kinase-II) signaling, and impaired Ca2+ handling in ventricular myocytes, which led to cardiac dysfunction.

CONCLUSIONS

These findings shed light on a novel function of Gαo in the regulation of cardiac Ca2+ homeostasis and systolic function and suggest Gαo may be an effective therapeutic target for the treatment of heart failure.

Compartmentalized cAMP signaling in cardiac ventricular myocytes

Activation of different receptors that act by generating the common second messenger cyclic adenosine monophosphate (cAMP) can elicit distinct functional responses in cardiac myocytes. Selectively sequestering cAMP activity to discrete intracellular microdomains is considered essential for generating receptor-specific responses. The processes that control this aspect of compartmentalized cAMP signaling, however, are not completely clear. Over the years, technological innovations have provided critical breakthroughs in advancing our understanding of the mechanisms underlying cAMP compartmentation. Some of the factors identified include localized production of cAMP by differential distribution of receptors, localized breakdown of this second messenger by targeted distribution of phosphodiesterase enzymes, and limited diffusion of cAMP by protein kinase A (PKA)-dependent buffering or physically restricted barriers. The aim of this review is to provide a discussion of our current knowledge and highlight some of the gaps that still exist in the field of cAMP compartmentation in cardiac myocytes.

Calcineurin in the heart: New horizons for an old friend

Calcineurin, also known as PP2B or PPP3, is a member of the PPP family of protein phosphatases that also includes PP1 and PP2A. Together these three phosphatases carryout the majority of dephosphorylation events in the heart. Calcineurin is distinct in that it is activated by the binding of calcium/calmodulin (Ca2+/CaM) and therefore acts as a node for integrating Ca2+ signals with changes in phosphorylation, two fundamental intracellular signaling cascades. In the heart, calcineurin is primarily thought of in the context of pathological cardiac remodeling, acting through the Nuclear Factor of Activated T-cell (NFAT) family of transcription factors. However, calcineurin activity is also essential for normal heart development and homeostasis in the adult heart. Furthermore, it is clear that NFAT-driven changes in transcription are not the only relevant processes initiated by calcineurin in the setting of pathological remodeling. There is a growing appreciation for the diversity of calcineurin substrates that can impact cardiac function as well as the diversity of mechanisms for targeting calcineurin to specific sub-cellular domains in cardiomyocytes and other cardiac cell types. Here, we will review the basics of calcineurin structure, regulation, and function in the context of cardiac biology. Particular attention will be given to: the development of improved tools to identify and validate new calcineurin substrates; recent studies identifying new calcineurin isoforms with unique properties and targeting mechanisms; and the role of calcineurin in cardiac development and regeneration.

Mechanisms of cAMP compartmentation in cardiac myocytes: experimental and computational approaches to understanding

The small diffusible second messenger 3',5'-cyclic adenosine monophosphate (cAMP) is found in virtually every cell in our bodies, where it mediates responses to a variety of different G protein coupled receptors (GPCRs). In the heart, cAMP plays a critical role in regulating many different aspects of cardiac myocyte function, including gene transcription, cell metabolism, and excitation-contraction coupling. Yet, not all GPCRs that stimulate cAMP production elicit the same responses. Subcellular compartmentation of cAMP is essential to explain how different receptors can utilize the same diffusible second messenger to elicit unique functional responses. However, the mechanisms contributing to this behaviour and its significance in producing physiological and pathological responses are incompletely understood. Mathematical modelling has played an essential role in gaining insight into these questions. This review discusses what we currently know about cAMP compartmentation in cardiac myocytes and questions that are yet to be answered.

Mechanisms and Regulation of Cardiac CaV1.2 Trafficking

During cardiac excitation contraction coupling, the arrival of an action potential at the ventricular myocardium triggers voltage-dependent L-type Ca²⁺ (Ca_V1.2) channels in individual myocytes to open briefly. The level of this Ca²⁺ influx tunes the amplitude of Ca²⁺-induced Ca²⁺ release from ryanodine receptors (RyR2) on the junctional sarcoplasmic reticulum and thus the magnitude of the elevation in intracellular Ca²⁺ concentration and ultimately the downstream contraction. The number and activity of functional Ca_V1.2 channels at the t-tubule dyads dictates the amplitude of the Ca²⁺ influx. Trafficking of these channels and their auxiliary subunits to the cell surface is thus tightly controlled and regulated to ensure adequate sarcolemmal expression to sustain this critical process. To that end, recent discoveries have revealed the existence of internal reservoirs of preformed Ca_V1.2 channels that can be rapidly mobilized to enhance sarcolemmal expression in times of acute stress when hemodynamic and metabolic demand increases. In this review, we provide an overview of the current thinking on Ca_V1.2 channel trafficking dynamics in the heart. We highlight the numerous points of control including the biosynthetic pathway, the endosomal recycling pathway, ubiquitination, and lysosomal and proteasomal degradation pathways, and discuss the effects of β-adrenergic and angiotensin receptor signaling cascades on this process.

Cyclic nucleotide phosphodiesterase inhibitors as therapeutic interventions for cystic fibrosis

Cystic Fibrosis (CF) lung disease results from mutations in the CFTR anion channel that reduce anion and fluid secretion by airway epithelia. Impaired secretion compromises airway innate defence mechanisms and leads to bacterial colonization, excessive inflammation and tissue damage; thus, restoration of CFTR function is the goal of many CF therapies. CFTR channels are activated by cyclic nucleotide-dependent protein kinases. The second messengers 3'5'-cAMP and 3'5'-cGMP are hydrolysed by a large family of cyclic nucleotide phosphodiesterases that provide subcellular spatial and temporal control of cyclic nucleotide-dependent signalling. Selective inhibition of these enzymes elevates cyclic nucleotide levels, leading to activation of CFTR and other downstream effectors. Here we examine members of the PDE family that are likely to regulate CFTR-dependent ion and fluid secretion in the airways and discuss other actions of PDE inhibitors that can influence cyclic nucleotide-regulated mucociliary transport, inflammation and bronchodilation. Finally, we review PDE inhibitors and the potential benefits they could provide as CF therapeutics.

Compartmentation of β2 -adrenoceptor stimulated cAMP responses by phosphodiesterase types 2 and 3 in cardiac ventricular myocytes

BACKGROUND AND PURPOSE

In cardiac myocytes, cyclic AMP (cAMP) produced by both β₁ - and β₂ -adrenoceptors increases L-type Ca²⁺ channel activity and myocyte contraction. However, only cAMP produced by β₁ -adrenoceptors enhances myocyte relaxation through phospholamban-dependent regulation of the sarco/endoplasmic reticulum Ca²⁺ ATPase 2 (SERCA2). Here we have tested the hypothesis that stimulation of β₂ -adrenoceptors produces a cAMP signal that is unable to reach SERCA2 and determine what role, if any, phosphodiesterase (PDE) activity plays in this compartmentation.

EXPERIMENTAL APPROACH

The cAMP responses produced by β₁ -and β₂ -adrenoceptor stimulation were studied in adult rat ventricular myocytes using two different fluorescence resonance energy transfer (FRET)-based biosensors, the Epac2-camps, which is expressed uniformly throughout the cytoplasm of the entire cell and the Epac2-αKAP, which is targeted to the SERCA2 signalling complex.

KEY RESULTS

Selective activation of β₁ - or β₂ -adrenoceptors produced cAMP responses detected by Epac2-camps. However, only stimulation of β₁ -adrenoceptors produced a cAMP response detected by Epac2-αKAP. Yet, stimulation of β₂ -adrenoceptors was able to produce a cAMP signal detected by Epac2-αKAP in the presence of selective inhibitors of PDE2 or PDE3, but not PDE4.

CONCLUSION AND IMPLICATIONS

These results support the conclusion that cAMP produced by β₂ -adrenoceptor stimulation was not able to reach subcellular locations where the SERCA2 pump is located. Furthermore, this compartmentalized response is due at least in part to PDE2 and PDE3 activity. This discovery could lead to novel PDE-based therapeutic treatments aimed at correcting cardiac relaxation defects associated with certain forms of heart failure.

β-Adrenergic control of sarcolemmal CaV1.2 abundance by small GTPase Rab proteins

The number and activity of Ca_v1.2 channels in the cardiomyocyte sarcolemma tunes the magnitude of Ca²⁺-induced Ca²⁺ release and myocardial contraction. β-Adrenergic receptor (βAR) activation stimulates sarcolemmal insertion of Ca_V1.2. This supplements the preexisting sarcolemmal Ca_V1.2 population, forming large "superclusters" wherein neighboring channels undergo enhanced cooperative-gating behavior, amplifying Ca²⁺ influx and myocardial contractility. Here, we determine this stimulated insertion is fueled by an internal reserve of early and recycling endosome-localized, presynthesized Ca_V1.2 channels. βAR-activation decreased Ca_V1.2/endosome colocalization in ventricular myocytes, as it triggered "emptying" of endosomal Ca_V1.2 cargo into the t-tubule sarcolemma. We examined the rapid dynamics of this stimulated insertion process with live-myocyte imaging of channel trafficking, and discovered that Ca_V1.2 are often inserted into the sarcolemma as preformed, multichannel clusters. Similarly, entire clusters were removed from the sarcolemma during endocytosis, while in other cases, a more incremental process suggested removal of individual channels. The amplitude of the stimulated insertion response was doubled by coexpression of constitutively active Rab4a, halved by coexpression of dominant-negative Rab11a, and abolished by coexpression of dominant-negative mutant Rab4a. In ventricular myocytes, βAR-stimulated recycling of Ca_V1.2 was diminished by both nocodazole and latrunculin-A, suggesting an essential role of the cytoskeleton in this process. Functionally, cytoskeletal disruptors prevented βAR-activated Ca²⁺ current augmentation. Moreover, βAR-regulation of Ca_V1.2 was abolished when recycling was halted by coapplication of nocodazole and latrunculin-A. These findings reveal that βAR-stimulation triggers an on-demand boost in sarcolemmal Ca_V1.2 abundance via targeted Rab4a- and Rab11a-dependent insertion of channels that is essential for βAR-regulation of cardiac Ca_V1.2.

β-Adrenergic control of sarcolemmal CaV1.2 abundance by small GTPase Rab proteins

The number and activity of Cav1.2 channels in the cardiomyocyte sarcolemma tunes the magnitude of Ca2+-induced Ca2+ release and myocardial contraction. β-Adrenergic receptor (βAR) activation stimulates sarcolemmal insertion of CaV1.2. This supplements the preexisting sarcolemmal CaV1.2 population, forming large "superclusters" wherein neighboring channels undergo enhanced cooperative-gating behavior, amplifying Ca2+ influx and myocardial contractility. Here, we determine this stimulated insertion is fueled by an internal reserve of early and recycling endosome-localized, presynthesized CaV1.2 channels. βAR-activation decreased CaV1.2/endosome colocalization in ventricular myocytes, as it triggered "emptying" of endosomal CaV1.2 cargo into the t-tubule sarcolemma. We examined the rapid dynamics of this stimulated insertion process with live-myocyte imaging of channel trafficking, and discovered that CaV1.2 are often inserted into the sarcolemma as preformed, multichannel clusters. Similarly, entire clusters were removed from the sarcolemma during endocytosis, while in other cases, a more incremental process suggested removal of individual channels. The amplitude of the stimulated insertion response was doubled by coexpression of constitutively active Rab4a, halved by coexpression of dominant-negative Rab11a, and abolished by coexpression of dominant-negative mutant Rab4a. In ventricular myocytes, βAR-stimulated recycling of CaV1.2 was diminished by both nocodazole and latrunculin-A, suggesting an essential role of the cytoskeleton in this process. Functionally, cytoskeletal disruptors prevented βAR-activated Ca2+ current augmentation. Moreover, βAR-regulation of CaV1.2 was abolished when recycling was halted by coapplication of nocodazole and latrunculin-A. These findings reveal that βAR-stimulation triggers an on-demand boost in sarcolemmal CaV1.2 abundance via targeted Rab4a- and Rab11a-dependent insertion of channels that is essential for βAR-regulation of cardiac CaV1.2.

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.

Chronic Naltrexone Therapy Is Associated with Improved Cardiac Function in Volume Overloaded Rats

PURPOSE

Myocardial opioid receptors were demonstrated in animals and humans and seem to colocalize with membranous and sarcolemmal calcium channels of the excitation-contraction coupling in the left ventricle (LV). Therefore, this study investigated whether blockade of the cardiac opioid system by naltrexone would affect cardiac function and neurohumoral parameters in Wistar rats with volume overload-induced heart failure.

METHODS

Volume overload in Wistar rats was induced by an aortocaval fistula (ACF). Left ventricular cardiac opioid receptors were identified by immunohistochemistry and their messenger ribonucleic acid (mRNA) as well as their endogenous ligand mRNA quantified by real-time polymerase chain reaction (RT-PCR). Following continuous delivery of either the opioid receptor antagonist naltrexone or vehicle via minipumps (n = 5 rats each), hemodynamic and humoral parameters were assessed 28 days after ACF induction. Sham-operated animals served as controls.

RESULTS

In ACF rats mu-, delta-, and kappa-opioid receptors colocalized with voltage-gated L-type Ca2+ channels in left ventricular cardiomyocytes. Chronic naltrexone treatment of ACF rats reduced central venous pressure (CVP) and left ventricular end-diastolic pressure (LVEDP), and improved systolic and diastolic left ventricular functions. Concomitantly, rat brain natriuretic peptide (rBNP-45) and angiotensin-2 plasma concentrations which were elevated during ACF were significantly diminished following naltrexone treatment. In parallel, chronic naltrexone significantly reduced mu-, delta-, and kappa-opioid receptor mRNA, while it increased the endogenous opioid peptide mRNA compared to controls.

CONCLUSION

Opioid receptor blockade by naltrexone leads to improved LV function and decreases in rBNP-45 and angiotensin-2 plasma levels. In parallel, naltrexone resulted in opioid receptor mRNA downregulation and an elevated intrinsic tone of endogenous opioid peptides possibly reflecting a potentially cardiodepressant effect of the cardiac opioid system during volume overload.

Junctophilin-2 tethers T-tubules and recruits functional L-type calcium channels to lipid rafts in adult cardiomyocytes

AIM

In cardiomyocytes, transverse tubules (T-tubules) associate with the sarcoplasmic reticulum (SR), forming junctional membrane complexes (JMCs) where L-type calcium channels (LTCCs) are juxtaposed to Ryanodine receptors (RyR). Junctophilin-2 (JPH2) supports the assembly of JMCs by tethering T-tubules to the SR membrane. T-tubule remodelling in cardiac diseases is associated with downregulation of JPH2 expression suggesting that JPH2 plays a crucial role in T-tubule stability. Furthermore, increasing evidence indicate that JPH2 might additionally act as a modulator of calcium signalling by directly regulating RyR and LTCCs. This study aimed at determining whether JPH2 overexpression restores normal T-tubule structure and LTCC function in cultured cardiomyocytes.

METHODS AND RESULTS

Rat ventricular myocytes kept in culture for 4 days showed extensive T-tubule remodelling with impaired JPH2 localization and relocation of the scaffolding protein Caveolin3 (Cav3) from the T-tubules to the outer membrane. Overexpression of JPH2 restored T-tubule structure and Cav3 relocation. Depletion of membrane cholesterol by chronic treatment with methyl-β-cyclodextrin (MβCD) countered the stabilizing effect of JPH2 overexpression on T-tubules and Cav3. Super-resolution scanning patch-clamp showed that JPH2 overexpression greatly increased the number of functional LTCCs at the plasma membrane. Treatment with MβCD reduced LTCC open probability and activity. Proximity ligation assays showed that MβCD did not affect JPH2 interaction with RyR and the pore-forming LTCC subunit Cav1.2, but strongly impaired JPH2 association with Cav3 and the accessory LTCC subunit Cavβ2.

CONCLUSIONS

JPH2 promotes T-tubule structural stability and recruits functional LTCCs to the membrane, most likely by directly binding to the channel. Cholesterol is involved in the binding of JPH2 to T-tubules as well as in the modulation of LTCC activity. We propose a model where cholesterol and Cav3 support the assembly of lipid rafts which provide an anchor for JPH2 to form JMCs and a platform for signalling complexes to regulate LTCC activity.

Tissue-specific adrenergic regulation of the L-type Ca2+ channel CaV1.2

Ca²⁺ influx through the L-type Ca²⁺ channel Cav1.2 triggers each heartbeat. The fight-or-flight response induces the release of the stress response hormone norepinephrine to stimulate β-adrenergic receptors, cAMP production, and protein kinase A activity to augment Ca²⁺ influx through Ca_v1.2 and, consequently, cardiomyocyte contractility. Emerging evidence shows that Ca_v1.2 is regulated by different mechanisms in cardiomyocytes compared to neurons and vascular smooth muscle cells.

Recent Advances in β2-Agonists for Treatment of Chronic Respiratory Diseases and Heart Failure

β₂-Adrenoceptor (β₂-AR) agonists are widely used as bronchodilators. The emerge of ultralong acting β₂-agonists is an important breakthrough in pulmonary medicine. In this review, we will provide mechanistic insights into the application of β₂-agonists in asthma, chronic obstructive pulmonary disease (COPD), and heart failure (HF). Recent studies in β-AR signal transduction have revealed opposing functions of the β₁-AR and the β₂-AR on cardiomyocyte survival. Thus, β₂-agonists and β-blockers in combination may represent a novel strategy for HF management. Allosteric modulation and biased agonism at the β₂-AR also provide a theoretical basis for developing drugs with novel mechanisms of action and pharmacological profiles. Overlap of COPD and HF presents a substantial clinical challenge but also a unique opportunity for evaluation of the cardiovascular safety of β₂-agonists. Further basic and clinical research along these lines can help us develop better drugs and innovative strategies for the management of these difficult-to-treat diseases.

Compartmentalized β1-adrenergic signalling synchronizes excitation-contraction coupling without modulating individual Ca2+ sparks in healthy and hypertrophied cardiomyocytes

AIMS

β-adrenergic receptors (βARs) play pivotal roles in regulating cardiac excitation-contraction (E-C) coupling. Global signalling of β1ARs up-regulates both the influx of Ca2+ through sarcolemmal L-type Ca2+ channels (LCCs) and the release of Ca2+ from the sarcoplasmic reticulum (SR) through the ryanodine receptors (RyRs). However, we recently found that β2AR stimulation meditates 'offside compartmentalization', confining β1AR signalling into subsarcolemmal nanodomains without reaching SR proteins. In the present study, we aim to investigate the new question, whether and how compartmentalized β1AR signalling regulates cardiac E-C coupling.

METHODS AND RESULTS

By combining confocal Ca2+ imaging and patch-clamp techniques, we investigated the effects of compartmentalized βAR signalling on E-C coupling at both cellular and molecular levels. We found that simultaneous activation of β2 and β1ARs, in contrast to global signalling of β1ARs, modulated neither the amplitude and spatiotemporal properties of Ca2+ sparks nor the kinetics of the RyR response to LCC Ca2+ sparklets. Nevertheless, by up-regulating LCC current, compartmentalized β1AR signalling synchronized RyR Ca2+ release and increased the functional reserve (stability margin) of E-C coupling. In circumstances of briefer excitation durations or lower RyR responsivity, compartmentalized βAR signalling, by increasing the intensity of Ca2+ triggers, helped stabilize the performance of E-C coupling and enhanced the Ca2+ transient amplitude in failing heart cells.

CONCLUSION

Given that compartmentalized βAR signalling can be induced by stress-associated levels of catecholamines, our results revealed an important, yet unappreciated, heart regulation mechanism that is autoadaptive to varied stress conditions.

Caveolae as Potential Hijackable Gates in Cell Communication

Caveolae are membrane microdomains described in many cell types involved in endocytocis, transcytosis, cell signaling, mechanotransduction, and aging. They are found at the interface with the extracellular environment and are structured by caveolin and cavin proteins. Caveolae and caveolins mediate transduction of chemical messages via signaling pathways, as well as non-chemical messages, such as stretching or shear stress. Various pathogens or signals can hijack these gates, leading to infectious, oncogenic and even caveolin-related diseases named caveolinopathies. By contrast, preclinical and clinical research have fallen behind in their attempts to hijack caveolae and caveolins for therapeutic purposes. Caveolae involvement in human disease is not yet fully explored or understood and, of all their scaffold proteins, only caveolin-1 is being considered in clinical trials as a possible biomarker of disease. This review briefly summarizes current knowledge about caveolae cell signaling and raises the hypothesis whether these microdomains could serve as hijackable “gatekeepers” or “gateways” in cell communication. Furthermore, because cell signaling is one of the most dynamic domains in translating data from basic to clinical research, we pay special attention to translation of caveolae, caveolin, and cavin research into clinical practice.

Discovery and characterization of a monogenetic insult, caveolin-3-V37L, that precipitated oligo-proteomic perturbations governing repolarization reserve

BACKGROUND

Caveolin-3 (Cav-3) is an essential scaffolding protein for caveolae formation in cardiomyocytes and targets multiple long QT syndrome (LQTS)-associated ion channels. Mutations in CAV3 have caused an LQT3-like accentuation in late sodium current, I_Na (Nav1.5). Here, we characterize a novel CAV3-V37L variant and determine whether it is the substrate for the patient's LQTS.

METHODS

The proband was a 39-year-old female with drug-induced, sudden cardiac arrest (SCA) with profound QT prolongation (QTc > 600 ms). Genetic testing revealed a rare CAV3-V37L variant of uncertain significance (VUS). Whole-cell patch clamp technique was used to measure I_Ks, I_Kr, I_Na, and I_{Ca, L} currents co-expressed with either CAV3-WT or CAV3-V37L in TSA201 cells and to measure the action potential duration (APD) in control human induced pluripotent stem cells-derived cardiomyocytes (hiPSC-CMs) overexpressed with CAV3-WT or CAV3-V37L.

RESULTS

CAV3-V37L did not affect Nav1.5 late current. Instead, CAV3-V37L resulted in 1) I_{Ca, L} with slower inactivation, a 1.5 fold increase in peak I_{Ca, L} current density and a 1.1 fold increase in I_{Ca, L} persistent current, 2) dramatically reduced I_Ks peak current density by 74.9%, 3) significantly reduced I_Kr peak current density by 31.1%, and 4) significantly prolonged the APD in hiPSC-CMs.

CONCLUSIONS

These functional validation assays enabled the promotion of CAV3-V37L from VUS status to a likely pathogenic variant. Although Nav1.5 was spared, this monogenetic insult precipitated an oligo-proteomic impact with a concomitant gain-of-function of I_{Ca, L} and loss-of-function of both I_Ks and I_Kr culminating in a marked prolongation of the cardiomyocyte's action potential duration.

β2 Adrenergic Receptor Complexes with the L-Type Ca2+ Channel CaV1.2 and AMPA-Type Glutamate Receptors: Paradigms for Pharmacological Targeting of Protein Interactions

Formation of signaling complexes is crucial for the orchestration of fast, efficient, and specific signal transduction. Pharmacological disruption of defined signaling complexes has the potential for specific intervention in selected regulatory pathways without affecting organism-wide disruption of parallel pathways. Signaling by epinephrine and norepinephrine through α and β adrenergic receptors acts on many signaling pathways in many cell types. Here, we initially provide an overview of the signaling complexes formed between the paradigmatic β₂ adrenergic receptor and two of its most important targets, the L-type Ca²⁺ channel Ca_V1.2 and the AMPA-type glutamate receptor. Importantly, both complexes contain the trimeric G_s protein, adenylyl cyclase, and the cAMP-dependent protein kinase, PKA. We then discuss the functional implications of the formation of these complexes, how those complexes can be specifically disrupted, and how such disruption could be utilized in the pharmacological treatment of disease.

Regulation of Membrane Calcium Transport Proteins by the Surrounding Lipid Environment

Calcium ions (Ca2+) are major messengers in cell signaling, impacting nearly every aspect of cellular life. Those signals are generated within a wide spatial and temporal range through a large variety of Ca2+ channels, pumps, and exchangers. More and more evidences suggest that Ca2+ exchanges are regulated by their surrounding lipid environment. In this review, we point out the technical challenges that are currently being overcome and those that still need to be defeated to analyze the Ca2+ transport protein-lipid interactions. We then provide evidences for the modulation of Ca2+ transport proteins by lipids, including cholesterol, acidic phospholipids, sphingolipids, and their metabolites. We also integrate documented mechanisms involved in the regulation of Ca2+ transport proteins by the lipid environment. Those include: (i) Direct interaction inside the protein with non-annular lipids; (ii) close interaction with the first shell of annular lipids; (iii) regulation of membrane biophysical properties (e.g., membrane lipid packing, thickness, and curvature) directly around the protein through annular lipids; and (iv) gathering and downstream signaling of several proteins inside lipid domains. We finally discuss recent reports supporting the related alteration of Ca2+ and lipids in different pathophysiological events and the possibility to target lipids in Ca2+-related diseases.

Cardiomyocyte Membrane Structure and cAMP Compartmentation Produce Anatomical Variation in β2AR-cAMP Responsiveness in Murine Hearts

Cardiomyocytes from the apex but not the base of the heart increase their contractility in response to β₂-adrenoceptor (β₂AR) stimulation, which may underlie the development of Takotsubo cardiomyopathy. However, both cell types produce comparable cytosolic amounts of the second messenger cAMP. We investigated this discrepancy using nanoscale imaging techniques and found that, structurally, basal cardiomyocytes have more organized membranes (higher T-tubular and caveolar densities). Local membrane microdomain responses measured in isolated basal cardiomyocytes or in whole hearts revealed significantly smaller and more short-lived β₂AR/cAMP signals. Inhibition of PDE4, caveolar disruption by removing cholesterol or genetic deletion of Cav3 eliminated differences in local cAMP production and equilibrated the contractile response to β₂AR. We conclude that basal cells possess tighter control of cAMP because of a higher degree of signaling microdomain organization. This provides varying levels of nanostructural control for cAMP-mediated functional effects that orchestrate macroscopic, regional physiological differences within the heart.

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Cardiomyocyte membrane organization varies in degree between regions of the heart

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Differences in structural organization affect adrenergic signaling via β₂AR

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Reduced organization allows β₂AR-cAMP to influence contractility in myocardial apex

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Variability in cell structure may allow differential response of heart regions