Subtype-specific beta-adrenoceptor signaling pathways in the heart and their potential clinical implications

Tripartite interactions of PKA catalytic subunit and C-terminal domains of cardiac Ca2+ channel may modulate its β-adrenergic regulation

BACKGROUND

The β-adrenergic augmentation of cardiac contraction, by increasing the conductivity of L-type voltage-gated CaV1.2 channels, is of great physiological and pathophysiological importance. Stimulation of β-adrenergic receptors (βAR) activates protein kinase A (PKA) through separation of regulatory (PKAR) from catalytic (PKAC) subunits. Free PKAC phosphorylates the inhibitory protein Rad, leading to increased Ca2+ influx. In cardiomyocytes, the core subunit of CaV1.2, CaV1.2α1, exists in two forms: full-length or truncated (lacking the distal C-terminus (dCT)). Signaling efficiency is believed to emanate from protein interactions within multimolecular complexes, such as anchoring PKA (via PKAR) to CaV1.2α1 by A-kinase anchoring proteins (AKAPs). However, AKAPs are inessential for βAR regulation of CaV1.2 in heterologous models, and their role in cardiomyocytes also remains unclear.

RESULTS

We show that PKAC interacts with CaV1.2α1 in heart and a heterologous model, independently of Rad, PKAR, or AKAPs. Studies with peptide array assays and purified recombinant proteins demonstrate direct binding of PKAC to two domains in CaV1.2α1-CT: the proximal and distal C-terminal regulatory domains (PCRD and DCRD), which also interact with each other. Data indicate both partial competition and possible simultaneous interaction of PCRD and DCRD with PKAC. The βAR regulation of CaV1.2α1 lacking dCT (which harbors DCRD) was preserved, but subtly altered, in a heterologous model, the Xenopus oocyte.

CONCLUSIONS

We discover direct interactions between PKAC and two domains in CaV1.2α1. We propose that these tripartite interactions, if present in vivo, may participate in organizing the multimolecular signaling complex and fine-tuning the βAR effect in cardiomyocytes.

GPER-1 Rapid Regulation Influences p-Akt Expression to Resist Stress-Induced Injuries in a Sex-Specific Manner

G protein-coupled estrogen receptor 1 (GPER-1) has gained recognition for its role in conferring cardioprotection. However, the extent to which GPER-1 exerts equally important effects in both sexes remains unclear. The study found similar expressions of GPER-1 in rat heart apex in both sexes. In male rats, administering epinephrine (Epi) at a dose of 31.36 microg/100 g resulted in a rapid decline in cardiac function, accompanied by a sharp increase in bax/bcl-2 levels. In contrast, female rats did not display significant changes in cardiac function under the same conditions. Additionally, compared to the injection of Epi alone (at a dose of 15.68 microg/100 g), the administration of G15 (GPER-1 antagonist) further decreased cardiac function in both male and female rats. However, it only increased mortality and lung coefficient in male rats. Conversely, G1 (GPER-1 agonist) administration improved cardiac function in both sexes. Notably, the apex of the male heart exhibited lower levels of inhibitory G protein (Galphai). Furthermore, female and male rats treated with Epi displayed elevated phosphorylated protein kinase B (p-Akt). Compared to their respective Epi groups, the administration of G15 increased p-Akt levels in female rat hearts but decreased them in male rat hearts. Conversely, the administration of G1 decreased p-Akt levels in females but rapidly increased them in male rats. Our study uncovers the vital role of GPER-1 in protecting against stress-induced heart injuries in a sex-specific manner. These findings hold immense potential for advancing targeted cardiac therapies and enhancing outcomes for both females and males.

Functional and pathological roles of adenylyl cyclases in various diseases

Adenylyl cyclases (ADCYs) produce the second messengers cAMP, which is crucial for a number of cellular activities. There are ten isoforms in the mammalian ADCY family including nine transmembrane adenylyl cyclases (tmAC) and one soluble adenylyl cyclase (sAC/ADCY10). There have been numerous studies demonstrating the importance of ADCYs in the development of a wide range of diseases, including cardiovascular disease, neurological disease, liver disease, and tumors. The classification, structure and regulation of ADCYs are discussed in this overview, which is followed by an analysis of how ADCYs are involved in various disorders and how they are used as a therapeutic tool. Our objective is to get a more thorough understanding of ADCYs to aid future study and provide novel ideas for the treatment of particular diseases.

Microtubule-Mediated Regulation of β2AR Translation and Function in Failing Hearts

BACKGROUND

β1AR (beta-1 adrenergic receptor) and β2AR (beta-2 adrenergic receptor)-mediated cyclic adenosine monophosphate signaling has distinct effects on cardiac function and heart failure progression. However, the mechanism regulating spatial localization and functional compartmentation of cardiac β-ARs remains elusive. Emerging evidence suggests that microtubule-dependent trafficking of mRNP (messenger ribonucleoprotein) and localized protein translation modulates protein compartmentation in cardiomyocytes. We hypothesized that β-AR compartmentation in cardiomyocytes is accomplished by selective trafficking of its mRNAs and localized translation.

METHODS

The localization pattern of β-AR mRNA was investigated using single molecule fluorescence in situ hybridization and subcellular nanobiopsy in rat cardiomyocytes. The role of microtubule on β-AR mRNA localization was studied using vinblastine, and its effect on receptor localization and function was evaluated with immunofluorescent and high-throughput Förster resonance energy transfer microscopy. An mRNA protein co-detection assay identified plausible β-AR translation sites in cardiomyocytes. The mechanism by which β-AR mRNA is redistributed post-heart failure was elucidated by single molecule fluorescence in situ hybridization, nanobiopsy, and high-throughput Förster resonance energy transfer microscopy on 16 weeks post-myocardial infarction and detubulated cardiomyocytes.

RESULTS

β1AR and β2AR mRNAs show differential localization in cardiomyocytes, with β1AR found in the perinuclear region and β2AR showing diffuse distribution throughout the cell. Disruption of microtubules induces a shift of β2AR transcripts toward the perinuclear region. The close proximity between β2AR transcripts and translated proteins suggests that the translation process occurs in specialized, precisely defined cellular compartments. Redistribution of β2AR transcripts is microtubule-dependent, as microtubule depolymerization markedly reduces the number of functional receptors on the membrane. In failing hearts, both β1AR and β2AR mRNAs are redistributed toward the cell periphery, similar to what is seen in cardiomyocytes undergoing drug-induced detubulation. This suggests that t-tubule remodeling contributes to β-AR mRNA redistribution and impaired β2AR function in failing hearts.

CONCLUSIONS

Asymmetrical microtubule-dependent trafficking dictates differential β1AR and β2AR localization in healthy cardiomyocyte microtubules, underlying the distinctive compartmentation of the 2 β-ARs on the plasma membrane. The localization pattern is altered post-myocardial infarction, resulting from transverse tubule remodeling, leading to distorted β2AR-mediated cyclic adenosine monophosphate signaling.

Gαs is dispensable for β-arrestin coupling but dictates GRK selectivity and is predominant for gene expression regulation by β2-adrenergic receptor

β-arrestins play a key role in G protein-coupled receptor (GPCR) internalization, trafficking, and signaling. Whether β-arrestins act independently of G protein-mediated signaling has not been fully elucidated. Studies using genome-editing approaches revealed that whereas G proteins are essential for mitogen-activated protein kinase activation by GPCRs., β-arrestins play a more prominent role in signal compartmentalization. However, in the absence of G proteins, GPCRs may not activate β-arrestins, thereby limiting the ability to distinguish G protein from β-arrestin-mediated signaling events. We used β2-adrenergic receptor (β2AR) and its β2AR-C tail mutant expressed in human embryonic kidney 293 cells wildtype or CRISPR-Cas9 gene edited for Gαs, β-arrestin1/2, or GPCR kinases 2/3/5/6 in combination with arrestin conformational sensors to elucidate the interplay between Gαs and β-arrestins in controlling gene expression. We found that Gαs is not required for β2AR and β-arrestin conformational changes, β-arrestin recruitment, and receptor internalization, but that Gαs dictates the GPCR kinase isoforms involved in β-arrestin recruitment. By RNA-Seq analysis, we found that protein kinase A and mitogen-activated protein kinase gene signatures were activated by stimulation of β2AR in wildtype and β-arrestin1/2-KO cells but absent in Gαs-KO cells. These results were validated by re-expressing Gαs in the corresponding KO cells and silencing β-arrestins in wildtype cells. These findings were extended to cellular systems expressing endogenous levels of β2AR. Overall, our results support that Gs is essential for β2AR-promoted protein kinase A and mitogen-activated protein kinase gene expression signatures, whereas β-arrestins initiate signaling events modulating Gαs-driven nuclear transcriptional activity.

Regulation of cardiac function by cAMP nanodomains

Cyclic adenosine monophosphate (cAMP) is a diffusible intracellular second messenger that plays a key role in the regulation of cardiac function. In response to the release of catecholamines from sympathetic terminals, cAMP modulates heart rate and the strength of contraction and ease of relaxation of each heartbeat. At the same time, cAMP is involved in the response to a multitude of other hormones and neurotransmitters. A sophisticated network of regulatory mechanisms controls the temporal and spatial propagation of cAMP, resulting in the generation of signaling nanodomains that enable the second messenger to match each extracellular stimulus with the appropriate cellular response. Multiple proteins contribute to this spatiotemporal regulation, including the cAMP-hydrolyzing phosphodiesterases (PDEs). By breaking down cAMP to a different extent at different locations, these enzymes generate subcellular cAMP gradients. As a result, only a subset of the downstream effectors is activated and a specific response is executed. Dysregulation of cAMP compartmentalization has been observed in cardiovascular diseases, highlighting the importance of appropriate control of local cAMP signaling. Current research is unveiling the molecular organization underpinning cAMP compartmentalization, providing original insight into the physiology of cardiac myocytes and the alteration associated with disease, with the potential to uncover novel therapeutic targets. Here, we present an overview of the mechanisms that are currently understood to be involved in generating cAMP nanodomains and we highlight the questions that remain to be answered.

Effects of Carvedilol and Thyroid Hormones Co-administration on Apoptotic and Survival Proteins in the Heart After Acute Myocardial Infarction

Cellular death and survival signaling plays a key role in the progress of adverse cardiac remodeling after acute myocardial infarction (AMI). Therapeutic strategies, such as co-treatment with beta-blocker carvedilol and thyroid hormones (THs), give rise to new approaches that can sustain the cellular homeostasis after AMI. Therefore, we sought to investigate the effects of carvedilol and TH co-administration on apoptosis and survival proteins and on cardiac remodeling after AMI. Male Wistar rats were distributed in 5 groups as follows: sham-operated group (SHAM), infarcted group (MI), infarcted plus carvedilol group (MI+C), infarcted plus TH group (MI+TH), and infarcted plus carvedilol and TH co-treatment group (MI+C+TH). Echocardiographic analysis was performed, and hearts were collected for western blot evaluation. The MI group presented systolic posterior wall thickness loss, an increase in the wall tension index, and an increase in atrial natriuretic peptide tissue levels than the SHAM group. However, in the MI+C+TH group, these parameters were equally to the SHAM group. Moreover, whereas the MI group showed Bax protein expression elevated in relation to the SHAM group, the MI+C+TH group presented Bax reduction and also Akt activation compared with the MI group. In addition, the MI+TH group revealed beta-1 adrenergic receptor (β1AR) upregulation compared with the MI and MI+C groups, whereas the MI+C+TH group presented lower levels of β1AR in relation to the SHAM and MI+TH groups. In conclusion, we suggest that carvedilol and TH co-administration may mediate its cardioprotective effects against adverse cardiac remodeling post-AMI through the Bax reduction, Akt activation, and β1AR decrease.

Reconstitution of β-adrenergic regulation of CaV1.2: Rad-dependent and Rad-independent protein kinase A mechanisms

L-type voltage-gated Ca_V1.2 channels crucially regulate cardiac muscle contraction. Activation of β-adrenergic receptors (β-AR) augments contraction via protein kinase A (PKA)-induced increase of calcium influx through Ca_V1.2 channels. To date, the full β-AR cascade has never been heterologously reconstituted. A recent study identified Rad, a Ca_V1.2 inhibitory protein, as essential for PKA regulation of Ca_V1.2. We corroborated this finding and reconstituted the complete pathway with agonist activation of β1-AR or β2-AR in Xenopus oocytes. We found, and distinguished between, two distinct pathways of PKA modulation of Ca_V1.2: Rad dependent (∼80% of total) and Rad independent. The reconstituted system reproduces the known features of β-AR regulation in cardiomyocytes and reveals several aspects: the differential regulation of posttranslationally modified Ca_V1.2 variants and the distinct features of β1-AR versus β2-AR activity. This system allows for the addressing of central unresolved issues in the β-AR-Ca_V1.2 cascade and will facilitate the development of therapies for catecholamine-induced cardiac pathologies.

Pharmacological Heart Failure Therapy in Children: Focus on Inotropic Support

Pediatric heart failure is a clinical syndrome, which needs to be distinctly defined and the pathophysiological consequences considered. Pharmacological treatment depends on the disease- and age-specific myocardial characteristics. Acute and chronic low cardiac output is the result of an inadequate heart rate (rhythm), myocardial contractility, preload and afterload, and also ventriculo-ventricular interaction, synchrony, atrio-ventricular and ventricular-arterial coupling. The treatment of choice is curing the cause of heart failure, if possible.Acute HF therapy is still based to the use of catecholamines and inodilators. The cornerstone of chronic HF treatment consists of blocking the endogenous, neuro-humoral axis, in particular the adrenergic and renin-angiotensin-aldosterone system.Before neprilysin inhibitors are used in young children, their potential side-effect for inducing Alzheimer disease needs to be clarified. The focus of the current review is put on the differential use of the inotropic drugs as epinephrine, norepinephrine, dopamine and dobutamine, and also the inodilators milrinone and levosimendan. Considering effects and side-effects of any cardiac stimulating treatment strategy, co-medication with ß-blockers, angiotensin converting inhibitors (ACEIs), angiotensin blockers (ARBs) and mineralocorticoid receptor antagonists (MRAs) is not a contradiction, but a senseful measure, even still during the acute inotropic treatment.Missing sophisticated clinical trials using accurate entry criteria and clinically relevant endpoints, there is especially in cardiovascular diagnosis and treatment of young children a compromise of evidence-based versus pathophysiology-based procedures. But based on the pharmacological and pathophysiological knowledge a hypothesis-driven individualized treatment is already currently possible and therefore indicated.

Imaging cAMP nanodomains in the heart

3′-5′-cyclic adenosine monophosphate (cAMP) is a ubiquitous second messenger that modulates multiple cellular functions. It is now well established that cAMP can mediate a plethora of functional effects via a complex system of local regulatory mechanisms that result in compartmentalized signalling. The use of fluorescent probes to monitor cAMP in intact, living cells have been instrumental in furthering our appreciation of this ancestral and ubiquitous pathway and unexpected details of the nano-architecture of the cAMP signalling network are starting to emerge. Recent evidence shows that sympathetic control of cardiac contraction and relaxation is achieved via generation of multiple, distinct pools of cAMP that lead to differential phosphorylation of target proteins localized only tens of nanometres apart. The specific local control at these nanodomains is enabled by a distinct signalosome where effectors, targets, and regulators of the cAMP signal are clustered. In this review, we focus on recent advances using targeted fluorescent reporters for cAMP and how they have contributed to our current understanding of nanodomain cAMP signalling in the heart. We briefly discuss how this information can be exploited to design novel therapies and we highlight some of the questions that remain unanswered.

Whole-Cell cAMP and PKA Activity are Epiphenomena, Nanodomain Signaling Matters

Novel targeted fluorescent biosensors provide key insights into very local nanodomains of cAMP and PKA activity, and how they respond differently to β-adrenergic activation in cardiac myocytes. This unique spatiotemporal detail in living cells is not available with biochemical measurements of total cellular cAMP and PKA, and provides unique physiological insights.

Pathological cardiac hypertrophy: the synergy of adenylyl cyclases inhibition in cardiac and immune cells during chronic catecholamine stress

Response to stressors in our environment and daily lives is an adaptation conserved through evolution as it is beneficial in enhancing the survival and continuity of humans. Although stressors have evolved, the drastic physiological response they elicit still remains unchanged. The chronic secretion and circulation of catecholamines to produce physical responses when they are not required may result in pathological consequences which affect cardiac function drastically. This review seeks to point out the probable implication of chronic stress in inducing an inflammation disorder in the heart. We discussed the likely synergy of a G protein-independent stimuli signaling via β₂-adrenergic receptors in both cardiomyocytes and immune cells during chronic catecholamine stress. To explain this synergy, we hypothesized the possibility of adenylyl cyclases having a regulatory effect on G protein-coupled receptor kinases. This was based on the negative correlations they exhibit during normal cardiac function and heart failures. As such, the downregulation of adenylyl cyclases in cardiomyocytes and immune cells during chronic catecholamine stress enhances the expressions of G protein-coupled receptor kinases. In addition, we explain the maladaptive roles played by G protein-coupled receptor kinase and extracellular signal-regulated kinase in the synergistic cascade that pathologically remodels the heart. Finally, we highlighted the therapeutic potentials of an adenylyl cyclases stimulator to attenuate pathological cardiac hypertrophy (PCH) and improve cardiac function in patients developing cardiac disorders due to chronic catecholamine stress.

Knockout of adenylyl cyclase isoform 5 or 6 differentially modifies the β1-adrenoceptor-mediated inotropic response

Although only β₂-adrenergic receptors (βAR) dually couple with stimulatory G protein (G_s) and inhibitory G protein (G_i), inactivation of G_i enhances both β₁AR and β₂AR responsiveness. We hypothesize that G_i restrains spontaneous adenylyl cyclase (AC) activity independent of receptor activation. Subcellular localization of the AC5/6 subtypes varies contributing to the compartmentation of βAR signaling. The primary objectives were to determine: (1) if β₁AR-mediated inotropic responses were dependent upon either AC5 or AC6; (2) if intrinsic G_i inhibition is AC subtype selective and (3) the role of phosphodiesterases (PDE) 3/4 to regulate β₁AR responsiveness. β₁AR-mediated increases in contractile force and cAMP accumulation in cardiomyocytes were measured from wild type, AC5 and AC6 knockout (KO) mice, with or without pertussis toxin (PTX) pretreatment to inactivate G_i and/or after selective inhibition of PDEs 3/4. Noradrenaline potency at β₁ARs was increased in AC6 KO. PDE4 inhibition increased noradrenaline potency in wild type and AC5 KO, but not AC6 KO. PTX increased noradrenaline potency only in wild type but increased the maximal β₁AR response in all mouse strains. PDE3 inhibition increased noradrenaline potency only in AC5 KO that was treated prior with PTX. β₁AR-evoked cAMP accumulation was increased more by PDE4 inhibition than PDE3 inhibition in wild type and AC5 KO that was amplified by G_i inhibition. These data indicate that β₁AR-mediated inotropic responses are not dependent upon either AC5 or AC6 alone. Inactivation of G_i enhanced β₁AR-mediated inotropic responses despite not coupling to G_i, consistent with G_i exerting a tonic receptor independent inhibition upon AC5/6. PDE4 seems the primary regulator of β₁AR signaling through AC6 in wild type. AC6 KO results in a reorganization of β₁AR compartmentation characterized by signaling through AC5 regulated by G_i, PDE3 and PDE4 that maintains normal contractile function.

Pediatric heart failure therapy: why β1-receptor blocker, tissue ACE-I and mineralocorticoid-receptor-blocker?

Pediatric heart failure (HF) treatment lagged behind the knowledge of pharmacological research and evidence-based clinical experience in adults. Considering the lack of prospective, double blind randomized studies in children, the review is focused on the preferred indication of specific β1-adrenoreceptor blockers (ARB), mineralocorticoid antagonists and tissue angiotensin-converting enzyme inhibitors (ACE-I). Our recommendations are based on the specificity in children, the effectiveness and the side-effect profile of HF-drugs, the receptor-physiological knowledge and the negative results of the few pediatric HF studies with an "evidence study label". In the interest of our pediatric patients, effective HF treatment has not longer to be postponed by balancing between evidence-based versus pathophysiology-based approach. At our institution, bisoprolol, lisinopril and spironolactone (BLS) are used treating HF in patients with left-right shunt lesions, reduced ejection fraction as well as during the inter-stage after HLHS-Hybrid approach. Chronic use of diuretics and fluid restriction is avoided, if always possible; intravascular volume deficiency stimulates further the neurohumoral axis. Pediatric HF needs to be treated with a strategy respecting the variable pathophysiology and the differences of receptor physiology between children and adult patients. The personalized treatment can be easily proofed by the surrogate parameters as heart rate, breath pattern, weight gain and image-derived parameters as well as biomarkers. Effective HF-therapy is also the basis for novel regenerative strategies in particular for young children with "end-stage" HF avoiding cardiac transplant or death.

Chemogenetic Approach Using Ni(II) Complex-Agonist Conjugates Allows Selective Activation of Class A G-Protein-Coupled Receptors

Investigating individual G-protein-coupled receptors (GPCRs) involved in various signaling cascades can unlock a myriad of invaluable physiological findings. One of the promising strategies for addressing the activity of each subtype of receptor is to design chemical turn-on switches on the target receptors. However, valid methods to selectively control class A GPCRs, the largest receptor family encoded in the human genome, remain limited. Here, we describe a novel approach to chemogenetically manipulate activity of engineered class A GPCRs carrying a His₄ tag, using metal complex-agonist conjugates (MACs). This manipulation is termed coordination tethering. With the assistance of coordination bonds, MACs showed 10-100-fold lower EC₅₀ values in the engineered receptors, compared with wild-type receptors. Such coordination tethering enabled selective activation of β₂-adrenoceptors and muscarinic acetylcholine receptors, without loss of natural receptor responses, in living mammalian cells, including primary cultured astrocytes. Our generalized, modular chemogenetic approach should facilitate more precise control and deeper understanding of individual GPCR signaling pathways in living systems.

Caveolins as Regulators of Stress Adaptation

Caveolins have been recognized over the past few decades as key regulators of cell physiology. They are ubiquitously expressed and regulate a number of processes that ultimately impact efficiency of cellular processes. Though not critical to life, they are central to stress adaptation in a number of organs. The following review will focus specifically on the role of caveolin in stress adaptation in the heart, brain, and eye, three organs that are susceptible to acute and chronic stress and that show as well declining function with age. In addition, we consider some novel molecular mechanisms that may account for this stress adaptation and also offer potential to drive the future of caveolin research.

The type VI adenylyl cyclase protects cardiomyocytes from β-adrenergic stress by a PKA/STAT3-dependent pathway

BACKGROUND

The type VI adenylyl cyclase (AC6) is a main contributor of cAMP production in the heart. The amino acid (aa) sequence of AC6 is highly homologous to that of another major cardiac adenylyl cyclase, AC5, except for its N-terminus (AC6-N, aa 1-86). Activation of AC6, rather than AC5, produces cardioprotective effects against heart failure, while the underlying mechanism remains to be unveiled. Using an AC6-null (AC6^-/-) mouse and a knockin mouse with AC6-N deletion (AC6 ^ΔN/ΔN), we aimed to investigate the cardioprotective mechanism of AC6 in the heart.

METHODS

Western blot analysis and immunofluorescence staining were performed to determine the intracellular distribution of AC6, AC6-ΔN (a truncated AC6 lacking the first 86 amino acids), and STAT3 activation. Activities of AC6 and AC6-ΔN in the heart were assessed by cAMP assay. Apoptosis of cardiomyocytes were evaluated by the TUNEL assay and a propidium iodine-based survival assay. Fibrosis was examined by collagen staining.

RESULTS

Immunofluorescence staining revealed that cardiac AC6 was mainly anchored on the sarcolemmal membranes, while AC6-ΔN was redistributed to the sarcoplasmic reticulum. AC6^ΔN/ΔN and AC6^-/- mice had more apoptotic myocytes and cardiac remodeling than WT mice in experimental models of isoproterenol (ISO)-induced myocardial injury. Adult cardiomyocytes isolated from AC6^ΔN/ΔN or AC6^-/- mice survived poorly after exposure to ISO, which produced no effect on WT cardiomyocytes under the condition tested. Importantly, ISO treatment induced cardiac STAT3 phosphorylation/activation in WT mice, but not in AC6^ΔN/ΔN and AC6^-/- mice. Pharmacological blockage of PKA-, Src-, or STAT3- pathway markedly reduced the survival of WT myocytes in the presence of ISO, but did not affect those of AC6^ΔN/ΔN and AC6^-/- myocytes, suggesting an important role of AC6 in mediating cardioprotective action through the activation of PKA-Src-STAT3-signaling.

CONCLUSIONS

Collectively, AC6-N controls the anchorage of cardiac AC6 on the sarcolemmal membrane, which enables the coupling of AC6 with the pro-survival PKA-STAT3 pathway. Our findings may facilitate the development of novel therapies for heart failure.

G protein, phosphorylated-GATA4 and VEGF expression in the hearts of transgenic mice overexpressing β1- and β2-adrenergic receptors

β₁- and β₂-adrenergic receptors (ARs) regulate cardiac contractility, calcium handling and protein phosphorylation. The present study aimed to examine the expression levels of vascular endothelial growth factor A (VEGF-A) and several G proteins, and the phosphorylation of transcription factor GATA binding protein 4 (GATA4), by western blot analysis, using isolated hearts from 6 month-old transgenic (TG) mice that overexpress β₁AR or β₂AR. Cardiac contractility/relaxation and heart rate was increased in both β₁AR TG and β₂AR TG mouse hearts compared with wild type; however, no significant differences were observed between the β₁- and β₂AR TG mouse hearts. Protein expression levels of inhibitory guanine nucleotide-binding protein (Gi) 2, Gi3 and G-protein-coupled receptor kinase 2 were upregulated in both TG mice, although the upregulation of Gi2 was more prominent in the β₂AR TG mice. VEGF-A expression levels were also increased in both TG mice, and were highest in the β₁AR TG mice. In addition, the levels of phosphorylated-GATA4 expression were increased in β₁- and β₂AR TG mice. In conclusion, the present study demonstrated that cardiac contractility/relaxation and heart rate is increased in β₁AR TG and β₂AR TG mice, and indicated that this increase may be related to the overexpression of G proteins and G-protein-associated proteins.

Distinct physiological effects of β1- and β2-adrenoceptors in mouse ventricular myocytes: insights from a compartmentalized mathematical model

The β1- and β2-adrenergic signaling systems play different roles in the functioning of cardiac cells. Experimental data show that the activation of the β1-adrenergic signaling system produces significant inotropic, lusitropic, and chronotropic effects in the heart, whereas the effects of the β2-adrenergic signaling system is less apparent. In this paper, a comprehensive compartmentalized experimentally based mathematical model of the combined β1- and β2-adrenergic signaling systems in mouse ventricular myocytes is developed to simulate the experimental findings and make testable predictions of the behavior of the cardiac cells under different physiological conditions. Simulations describe the dynamics of major signaling molecules in different subcellular compartments; kinetics and magnitudes of phosphorylation of ion channels, transporters, and Ca2+ handling proteins; modifications of action potential shape and duration; and [Ca2+]i and [Na+]i dynamics upon stimulation of β1- and β2-adrenergic receptors (β1- and β2-ARs). The model reveals physiological conditions when β2-ARs do not produce significant physiological effects and when their effects can be measured experimentally. Simulations demonstrated that stimulation of β2-ARs with isoproterenol caused a marked increase in the magnitude of the L-type Ca2+ current, [Ca2+]i transient, and phosphorylation of phospholamban only upon additional application of pertussis toxin or inhibition of phosphodiesterases of type 3 and 4. The model also made testable predictions of the changes in magnitudes of [Ca2+]i and [Na+]i fluxes, the rate of decay of [Na+]i concentration upon both combined and separate stimulation of β1- and β2-ARs, and the contribution of phosphorylation of PKA targets to the changes in the action potential and [Ca2+]i transient.

Cardiac physiologic regulation of sub-type specific adrenergic receptors in transgenic mice overexpressing β1- and β2-adrenergic receptors

Objective

Combination of β₁-adrenergic receptor (AR) blockade and β₂-AR activation might be a potential novel therapy for treating heart failure. However, use of β-AR agonists and/or antagonists in the clinical setting is controversial because of the lack of information on cardiac inotropic or chronotropic regulation by AR signaling.

Methods

In this study, we performed hemodynamic evaluation by examining force frequency response (FFR), Frank-Starling relationship, and response to a non-selective β-AR agonist (isoproterenol) in hearts isolated from 6-month-old transgenic (TG) mice overexpressing β₁- and β₂-ARs (β₁- and β₂-AR TG mice, respectively).

Results

Cardiac physiologic consequences of β₁- and β₂-AR overexpression resulted in similar maximal response to isoproterenol and faster temporary decline of positive inotropic response in β₂-AR TG mice. β₁-AR TG mice showed a pronounced negative limb of FFR, whereas β₂-AR TG mice showed high stimulation frequencies with low contractile depression during FFR. In contrast, Frank-Starling relationship was equally enhanced in both β₁- and β₂-AR TG mice.

Conclusion

Hemodynamic evaluation performed in the present showed a difference in β₁- and β₂-AR signaling, which may be due to the difference in the desensitization of β₁- and β₂-ARs.

Differences in the control of basal L-type Ca(2+) current by the cyclic AMP signaling cascade in frog, rat, and human cardiac myocytes

β-adrenergic receptors (β-ARs) mediate the positive inotropic effects of catecholamines by cAMP-dependent phosphorylation of the L-type Ca(2+) channels (LTCCs), which provide Ca(2+) for the initiation and regulation of cell contraction. The overall effect of cAMP-modulating agents on cardiac calcium current (I Ca,L) and contraction depends on the basal activity of LTCCs which, in turn, depends on the basal activities of key enzymes involved in the cAMP signaling cascade. Our current work is a comparative study demonstrating the differences in the basal activities of β-ARs, adenylyl cyclase, phosphodiesterases, phosphatases, and LTCCs in the frog and rat ventricular and human atrial myocytes. The main conclusion is that the basal I Ca,L, and consequently the contractile function of the heart, is secured from unnecessary elevation of its activity and energy consumption at the several "checking-points" of cAMP-dependent signaling cascade and the loading of these "checking-points" may vary in different species and tissues.

"Nihilism" of chronic heart failure therapy in children and why effective therapy is withheld

Major advances in chronic heart failure (cHF) therapy have been achieved and documented in adult patients, while research regarding the mechanisms and therapy of cHF in children has lagged behind. Based on receptor physiological studies and pharmacological knowledge, treatment with specific ß1-adrenergic receptor blocker (ARB), tissue angiotensin-converting enzyme inhibitor (ACE-I), and mineralocorticoid antagonists have to be recommended in children despite lack of sufficient data derived from prospective randomized studies. At our institution, bisoprolol, lisinopril, and spironolactone have been firmly established to treat systolic cHF, hypoplastic left heart syndrome (HLHS) following hybrid approach and congenital left-right shunt diseases, latest in patients where surgery has to be delayed. Chronic therapy with long-acting diuretics and fluid restriction are not advocated because short-term effects are achieved at the expense of further neuro-humoral stimulation. It remains unclear why diuretics are recommended although evidence-based studies, documenting long-term benefit, are missing. However, that is true for all currently used drugs for pediatric cHF.

CONCLUSION

This review focuses on the prevailing "nihilism" of cHF therapy in children with the goal to encourage physicians to treat pediatric cHF with a rationally designed therapy, which combines available agents that have been shown to improve survival in adult patients with cHF. Because of the lack of clinical trials, which generate the needed evidence, surrogate variables like heart and respiratory rate, weight gain, image-derived data, and biomarkers should be monitored and used instead. The recommended pharmacological therapy for systolic heart failure is also provided as the basis for utilizing reversible pulmonary arterial banding (PAB) as a novel strategy in young children with dilative cardiomyopathy (DCM) with preserved right ventricular function.

WHAT IS KNOWN

• Heart failure (HF) in children is a serious public health concern. • HF has numerous etiologies, but unspecific symptoms. • HF interplays among neuro-humoral, and molecular abnormalities. • Pediatric cHF-therapy is currently based on loop-diuretics, fluid restriction and digoxin. What is New: • Cardiac function analysis has to include cardiac synchrony and VVI. • Considering enormous potential of cardiac regeneration, therapy has to extend with selective ß1-ARB, tissue ACE-I and mineralocorticoid blockers, loop-diuretics avoided as ever possible. • Inhibition of the endogenous neuro-humoral stimulation is monitored by surrogate parameters as heart and breath rate and systolic and diastolic blood pressure. • Advocated HF therapy serves for regenerative strategies as reversible Pulmonary Artery Banding in DCM.

Tracking GPCR biosynthesis and degradation using a nonradioactive pulse chase methodology

The β2-adrenergic receptor (β2AR) is a prototypical member of the G protein-coupled receptor (GPCR) superfamily of proteins and is one of the best-characterized GPCRs due to its role in several important physiological systems. Because of limited availability of high quality antibodies against GPCRs, much of the work done on β2AR took advantage of heterologous expression systems. Overexpressed proteins may overwhelm the cellular regulatory machinery leading potentially to responses distinct from the native protein. To address this issue we generated a stable cell line with a tetracycline-inducible β2AR tagged with a FLAG epitope, such that we are able to control the quantity of receptor produced. This allows us to induce a discrete pulse of FLAG-β2AR transcription and translation allowing us to follow the complete life cycle of the protein from synthesis as an immature protein to degradation. We show that such limited pulses of receptor expression lead to signaling phenotypes that more closely reflect endogenous signaling events.

Applying high-performance computing in drug discovery and molecular simulation

In recent decades, high-performance computing (HPC) technologies and supercomputers in China have significantly advanced, resulting in remarkable achievements. Computational drug discovery and design, which is based on HPC and combines pharmaceutical chemistry and computational biology, has become a critical approach in drug research and development and is financially supported by the Chinese government. This approach has yielded a series of new algorithms in drug design, as well as new software and databases. This review mainly focuses on the application of HPC to the fields of drug discovery and molecular simulation at the Chinese Academy of Sciences, including virtual drug screening, molecular dynamics simulation, and protein folding. In addition, the potential future application of HPC in precision medicine is briefly discussed.

Pathophysiology of sepsis-related cardiac dysfunction: driven by inflammation, energy mismanagement, or both?

Sepsis is a systemic inflammatory response that follows bacterial infection. Cardiac dysfunction is an important consequence of sepsis that affects mortality and has been attributed to either elevated inflammation or suppression of both fatty acid and glucose oxidation and eventual ATP depletion. Moreover, cardiac adrenergic signaling is compromised in septic patients and this aggravates further heart function. While anti-inflammatory therapies are important for the treatment of the disease, administration of anti-inflammatory drugs did not improve survival in septic patients. This review article summarizes findings on inflammatory and other mechanisms that are triggered in sepsis and affect cardiac function and mortality. Particularly, it focuses on the effects of the disease in metabolic pathways, as well as in adrenergic signaling and the potential interplay of the latter with inflammation. It is suggested that therapeutic approaches should include combination of anti-inflammatory treatments, stimulation of energy production, and restoration of adrenergic signaling in the heart.

High concentration of epigallocatechin-3-gallate increased the incidences of arrhythmia and diastolic dysfunction via β2-adrenoceptor

Epigallocatechin-3-gallate (EGCG) is the major and most potent representative in green tea, which has been proved to modulate myocardial contractility. Whether EGCG has some negative effects on cardiac function is not known. In the present study, we investigated the effects of EGCG at different doses on cardiac contraction and explored whether β2 -adrenoceptor (β2 AR) was involved in EGCG-induced cardiac effects. Isolated rat hearts were mounted on the Langendorff system and perfused with different concentrations of EGCG in low or normal calcium Krebs-Henseleit (KH) buffer. The contraction of hearts was measured. Ventricular myocytes were cultured with EGCG and isoprenaline (ISO, 10(-7) M) for 12 h. ICI118,551 (55 nM) was used to inhibit β2 AR. Cardiomyocyte shortening, viability, and responsiveness to ISO (10(-9) M) were measured. EGCG dose dependently enhanced contractility of perfused heart in low calcium KH buffer. In the normal calcium KH buffer, EGCG at low dose (20 μM) increased heart contraction, while at high dose (50 μM), it increased the incidences of arrhythmia and diastolic dysfunction. In isolated ventricular myocytes, EGCG at the concentration of 0.001 to 1.0 μΜ did not affect their contraction. However, the responsiveness to ISO and the survival of myocytes were increased by EGCG (0.01 μM). The increased responsiveness was partially abolished by ICI118,551. The data obtained in this study demonstrated that EGCG at low dose conferred cardioprotection, yet at high dose increased the incidences of arrhythmia and diastolic dysfunction. β2 AR was involved in EGCG-induced cardiac effects.

A compartmentalized mathematical model of the β1-adrenergic signaling system in mouse ventricular myocytes

The β₁-adrenergic signaling system plays an important role in the functioning of cardiac cells. Experimental data shows that the activation of this system produces inotropy, lusitropy, and chronotropy in the heart, such as increased magnitude and relaxation rates of [Ca²⁺]_i transients and contraction force, and increased heart rhythm. However, excessive stimulation of β₁-adrenergic receptors leads to heart dysfunction and heart failure. In this paper, a comprehensive, experimentally based mathematical model of the β₁-adrenergic signaling system for mouse ventricular myocytes is developed, which includes major subcellular functional compartments (caveolae, extracaveolae, and cytosol). The model describes biochemical reactions that occur during stimulation of β₁-adrenoceptors, changes in ionic currents, and modifications of Ca²⁺ handling system. Simulations describe the dynamics of major signaling molecules, such as cyclic AMP and protein kinase A, in different subcellular compartments; the effects of inhibition of phosphodiesterases on cAMP production; kinetics and magnitudes of phosphorylation of ion channels, transporters, and Ca²⁺ handling proteins; modifications of action potential shape and duration; magnitudes and relaxation rates of [Ca²⁺]_i transients; changes in intracellular and transmembrane Ca²⁺ fluxes; and [Na⁺]_i fluxes and dynamics. The model elucidates complex interactions of ionic currents upon activation of β₁-adrenoceptors at different stimulation frequencies, which ultimately lead to a relatively modest increase in action potential duration and significant increase in [Ca²⁺]_i transients. In particular, the model includes two subpopulations of the L-type Ca²⁺ channels, in caveolae and extracaveolae compartments, and their effects on the action potential and [Ca²⁺]_i transients are investigated. The presented model can be used by researchers for the interpretation of experimental data and for the developments of mathematical models for other species or for pathological conditions.

Integrating multi-scale data to create a virtual physiological mouse heart

While the virtual physiological human (VPH) project has made great advances in human modelling, many of the tools and insights developed as part of this initiative are also applicable for facilitating mechanistic understanding of the physiology of a range of other species. This process, in turn, has the potential to provide human relevant insights via a different scientific path. Specifically, the increasing use of mice in experimental research, not yet fully complemented by a similar increase in computational modelling, is currently missing an important opportunity for using and interpreting this growing body of experimental data to improve our understanding of cardiac function. This overview describes our work to address this issue by creating a virtual physiological mouse model of the heart. We describe the similarities between human- and mouse-focused modelling, including the reuse of VPH tools, and the development of methods for investigating parameter sensitivity that are applicable across species. We show how previous results using this approach have already provided important biological insights, and how these can also be used to advance VPH heart models. Finally, we show an example application of this approach to test competing multi-scale hypotheses by investigating variations in length-dependent properties of cardiac muscle.

Cardiac alpha1-adrenergic receptors: novel aspects of expression, signaling mechanisms, physiologic function, and clinical importance

Adrenergic receptors (AR) are G-protein-coupled receptors (GPCRs) that have a crucial role in cardiac physiology in health and disease. Alpha1-ARs signal through Gαq, and signaling through Gq, for example, by endothelin and angiotensin receptors, is thought to be detrimental to the heart. In contrast, cardiac alpha1-ARs mediate important protective and adaptive functions in the heart, although alpha1-ARs are only a minor fraction of total cardiac ARs. Cardiac alpha1-ARs activate pleiotropic downstream signaling to prevent pathologic remodeling in heart failure. Mechanisms defined in animal and cell models include activation of adaptive hypertrophy, prevention of cardiac myocyte death, augmentation of contractility, and induction of ischemic preconditioning. Surprisingly, at the molecular level, alpha1-ARs localize to and signal at the nucleus in cardiac myocytes, and, unlike most GPCRs, activate "inside-out" signaling to cause cardioprotection. Contrary to past opinion, human cardiac alpha1-AR expression is similar to that in the mouse, where alpha1-AR effects are seen most convincingly in knockout models. Human clinical studies show that alpha1-blockade worsens heart failure in hypertension and does not improve outcomes in heart failure, implying a cardioprotective role for human alpha1-ARs. In summary, these findings identify novel functional and mechanistic aspects of cardiac alpha1-AR function and suggest that activation of cardiac alpha1-AR might be a viable therapeutic strategy in heart failure.

βArrestins in cardiac G protein-coupled receptor signaling and function: partners in crime or "good cop, bad cop"?

βArrestin (βarr)-1 and -2 (βarrs) (or Arrestin-2 and -3, respectively) are universal G protein-coupled receptor (GPCR) adapter proteins expressed abundantly in extra-retinal tissues, including the myocardium. Both were discovered in the lab of the 2012 Nobel Prize in Chemistry co-laureate Robert Lefkowitz, initially as terminators of signaling from the β-adrenergic receptor (βAR), a process known as functional desensitization. They are now known to switch GPCR signaling from G protein-dependent to G protein-independent, which, in the case of βARs and angiotensin II type 1 receptor (AT1R), might be beneficial, e.g., anti-apoptotic, for the heart. However, the specific role(s) of each βarr isoform in cardiac GPCR signaling and function (or dysfunction in disease), remain unknown. The current consensus is that, whereas both βarr isoforms can desensitize and internalize cardiac GPCRs, they play quite different (even opposing in certain instances) roles in the G protein-independent signaling pathways they initiate in the cardiovascular system, including in the myocardium. The present review will discuss the current knowledge in the field of βarrs and their roles in GPCR signaling and function in the heart, focusing on the three most important, for cardiac physiology, GPCR types (β1AR, β2AR & AT1R), and will also highlight important questions that currently remain unanswered.

Regulation of cardiac L-type Ca²⁺ channel CaV1.2 via the β-adrenergic-cAMP-protein kinase A pathway: old dogmas, advances, and new uncertainties

In the heart, adrenergic stimulation activates the β-adrenergic receptors coupled to the heterotrimeric stimulatory Gs protein, followed by subsequent activation of adenylyl cyclase, elevation of cyclic AMP levels, and protein kinase A (PKA) activation. One of the main targets for PKA modulation is the cardiac L-type Ca²⁺ channel (CaV1.2) located in the plasma membrane and along the T-tubules, which mediates Ca²⁺ entry into cardiomyocytes. β-Adrenergic receptor activation increases the Ca²⁺ current via CaV1.2 channels and is responsible for the positive ionotropic effect of adrenergic stimulation. Despite decades of research, the molecular mechanism underlying this modulation has not been fully resolved. On the contrary, initial reports of identification of key components in this modulation were later refuted using advanced model systems, especially transgenic animals. Some of the cardinal debated issues include details of specific subunits and residues in CaV1.2 phosphorylated by PKA, the nature, extent, and role of post-translational processing of CaV1.2, and the role of auxiliary proteins (such as A kinase anchoring proteins) involved in PKA regulation. In addition, the previously proposed crucial role of PKA in modulation of unstimulated Ca²⁺ current in the absence of β-adrenergic receptor stimulation and in voltage-dependent facilitation of CaV1.2 remains uncertain. Full reconstitution of the β-adrenergic receptor signaling pathway in heterologous expression systems remains an unmet challenge. This review summarizes the past and new findings, the mechanisms proposed and later proven, rejected or disputed, and emphasizes the essential issues that remain unresolved.

GRK2 blockade with βARKct is essential for cardiac β2-adrenergic receptor signaling towards increased contractility

BACKGROUND

β1- and β2-adrenergic receptors (ARs) play distinct roles in the heart, e.g. β1AR is pro-contractile and pro-apoptotic but β2AR anti-apoptotic and only weakly pro-contractile. G protein coupled receptor kinase (GRK)-2 desensitizes and opposes βAR pro-contractile signaling by phosphorylating the receptor and inducing beta-arrestin (βarr) binding. We posited herein that GRK2 blockade might enhance the pro-contractile signaling of the β2AR subtype in the heart. We tested the effects of cardiac-targeted GRK2 inhibition in vivo exclusively on β2AR signaling under normal conditions and in heart failure (HF).

RESULTS

We crossed β1AR knockout (B1KO) mice with cardiac-specific transgenic mice expressing the βARKct, a known GRK2 inhibitor, and studied the offspring under normal conditions and in post-myocardial infarction (MI). βARKct expression in vivo proved essential for β2AR-dependent contractile function, as β2AR stimulation with isoproterenol fails to increase contractility in either healthy or post-MI B1KO mice and it only does so in the presence of βARKct. The main underlying mechanism for this is blockade of the interaction of phosphodiesterase (PDE) type 4D with the cardiac β2AR, which is normally mediated by the actions of GRK2 and βarrs on the receptor. The molecular "brake" that PDE4D poses on β2AR signaling to contractility stimulation is thus "released". Regarding the other beneficial functions of cardiac β2AR, βARKct increased overall survival of the post-MI B1KO mice progressing to HF, via a decrease in cardiac apoptosis and an increase in wound healing-associated inflammation early (at 24 hrs) post-MI. However, these effects disappear by 4 weeks post-MI, and, in their place, upregulation of the other major GRK in the heart, GRK5, is observed.

CONCLUSIONS

GRK2 inhibition in vivo with βARKct is absolutely essential for cardiac β2AR pro-contractile signaling and function. In addition, β2AR anti-apoptotic signaling in post-MI HF is augmented by βARKct, although this effect is short-lived.

β-Adrenergic receptor antagonism in mice: a model for pediatric heart disease

Children with heart failure are treated with similar medical therapy as adults with heart failure. In contrast to adults with heart failure, these treatment regiments are not associated with improved outcomes in children. Recent studies have demonstrated age-related pathophysiological differences in the molecular mechanisms of heart failure between children and adults. There are no animal models of pediatric cardiomyopathy to allow mechanistic studies. The purpose of the current experiments was to develop a mouse model of pediatric heart disease and test whether the influence of β-adrenergic receptor (β-AR) antagonism could be modeled in this system. We hypothesized that isoproterenol treatment of young mice would provide a model system of cardiac pathology, and that nonselective β-AR blockade would provide benefit in adult, but not young, mice, similar to clinical trial data. We found that isoproterenol treatment (through osmotic minipump implantation) of young and adult mice produced similar degrees of cardiac hypertrophy and recapitulated several age-related molecular abnormalities in human heart failure, including phospholamban phosphorylation and β-AR expression. We also found that nonselective β-AR blockade effectively prevented pathological cardiac growth and collagen expression in the adult but not young mice, and that selective β1-AR blockade was effective in both young and adult isoproterenol-treated mice. In conclusion, we have developed the first model system for β-AR-mediated pediatric heart disease. Furthermore, we have generated novel data suggesting beneficial effects of selective β1-AR blockade in the pediatric heart.

Regulation of cyclic adenosine monophosphate release by selective β2-adrenergic receptor stimulation in human terminal failing myocardium before and after ventricular assist device support

BACKGROUND

Response to catecholamines is blunted in terminal heart failure due to β-receptor downregulation and uncoupling from adenylyl cyclase (AC). Improved myocardial responsiveness to catecholamines after ventricular assist device (VAD) support is associated with upregulation of β1-adrenergic receptors (β1-ARs). Little is known about the regulation of AC and β2-AR coupling after VAD; moreover β2-AR stimulation during VAD was claimed to induce myocardial recovery.

METHODS

We analyzed in VAD-supported human myocardium the regulation of AC activity upon β1-AR and selective β2-AR stimulation in 8 non-failing hearts (NF) and 17 paired samples of VAD patients. AC messenger RNA was measured by TaqMan. AC was stimulated via β2-AR using clenbuterol (β2-AR agonist) and bisoprolol (β1-AR blocker). Organ bath experiments were done with trabeculae from both ventricles. Samples were stratified according to chronic or acute heart failure history.

RESULTS

Isoprenaline-induced AC activity was downregulated (p < 0.001) pre-VAD and increased significantly (p < 0.05) after unloading (mean ± standard deviation pmole/mg/min) in NF (47.9 ± 14.9), pre-VAD (24.35 ± 13.3), and post-VAD (50.04 ± 50.25). Forskolin stimulation revealed significant (p < 0.05) upregulation of AC activity during VAD, especially in acutely failing hearts (NF, 192.1 ± 68.7; pre-VAD, 191.1 ± 60.4; post-VAD, 281.5 ± 133). However, forskolin stimulation relative to isoprenaline-induced inotropy remained reduced before and after VAD compared with NF. The selective stimulation of β2-AR did not reveal influence of VAD support on β2-AR-AC coupling. Stimulation of ventricular trabeculae by > 100 μmole/liter clenbuterol revealed negative inotropic responses.

CONCLUSIONS

VAD does not influence β2-AR coupling to AC stimulation. Elevated response to catecholamines after VAD support is influenced by β1-AR upregulation and modulation of AC activity. Restoration of β-adrenergic responsiveness was restricted to acutely failing hearts.

CaV1.2 signaling complexes in the heart

L-type Ca(2+) channels (LTCCs) are essential for generation of the electrical and mechanical properties of cardiac muscle. Furthermore, regulation of LTCC activity plays a central role in mediating the effects of sympathetic stimulation on the heart. The primary mechanism responsible for this regulation involves β-adrenergic receptor (βAR) stimulation of cAMP production and subsequent activation of protein kinase A (PKA). Although it is well established that PKA-dependent phosphorylation regulates LTCC function, there is still much we do not understand. However, it has recently become clear that the interaction of the various signaling proteins involved is not left to completely stochastic events due to random diffusion. The primary LTCC expressed in cardiac muscle, CaV1.2, forms a supramolecular signaling complex that includes the β2AR, G proteins, adenylyl cyclases, phosphodiesterases, PKA, and protein phosphatases. In some cases, the protein interactions with CaV1.2 appear to be direct, in other cases they involve scaffolding proteins such as A kinase anchoring proteins and caveolin-3. Functional evidence also suggests that the targeting of these signaling proteins to specific membrane domains plays a critical role in maintaining the fidelity of receptor mediated LTCC regulation. This information helps explain the phenomenon of compartmentation, whereby different receptors, all linked to the production of a common diffusible second messenger, can vary in their ability to regulate LTCC activity. The purpose of this review is to examine our current understanding of the signaling complexes involved in cardiac LTCC regulation.

Sex differences in repolarization and slow delayed rectifier potassium current and their regulation by sympathetic stimulation in rabbits

Slow delayed rectifier potassium current (IKs) is important in action potential (AP) repolarization and repolarization reserve. We tested the hypothesis that there are sex-specific differences in IKs, AP, and their regulation by β-adrenergic receptors (β-AR's) using whole-cell patch-clamp. AP duration (APD90) was significantly longer in control female (F) than in control male (M) myocytes. Isoproterenol (ISO, 500 nM) shortened APD90 comparably in M and F, and was largely reversed by β1-AR blocker CGP 20712A (CGP, 300 nM). Inhibition of IKs with chromanol 293B (10 μM) resulted in less APD prolongation in F at baseline (3.0 vs 8.9 %, p < 0.05 vs M) and even in the presence of ISO (5.4 vs 20.9 %, p < 0.05). This suggests that much of the ISO-induced APD abbreviation in F is independent of IKs. In F, baseline IKs was 42 % less and was more weakly activated by ISO (19 vs 68 % in M, p < 0.01). ISO enhancement of IKs was comparably attenuated by CGP in M and F. After ovariectomy, IKs in F had greater enhancement by ISO (72 %), now comparable to control M. After orchiectomy, IKs in M was only slightly enhanced by ISO (23 %), comparable to control F. Pretreatment with thapsigargin (to block SR Ca release) had bigger impact on ISO-induced APD shortening in F than that in M (p < 0.01). In conclusion, we found that there are sex differences in IKs, AP, and their regulation by β-AR's that are modulated by sex hormones, suggesting the potential for sex-specific antiarrhythmic therapy.

Maprotiline treatment differentially influences cardiac β-adrenoreceptors expression under normal and stress conditions

Alterations in cardiac function were observed in antidepressants treated patients and published in several clinical reports. These detected changes could be either a consequence of the treatment or of depression itself, which has already been proved to be a risk factor in heart diseases. In order to determine a possible influence of chronic treatment with norepinephrinergic reuptake inhibitor, maprotiline, on the heart, we investigated gene expression of cardiac β-adrenoceptors both in controls and in animals with signs of depression. The rats were divided into two groups, unstressed controls and those exposed to chronic unpredictable mild stress (CUMS). The groups were further divided into two subgroups, one receiving daily intraperitoneal injections of vehicle (sterile water) and another one maprotiline (10 mg/kg) for four weeks. Tissue samples were collected after the last application. Gene expression of cardiac β1- and β2-adrenoceptor was determined using Real-time RT-PCR analysis. Our results show that in control animals expression of both adrenoreceptors was decreased in the right atria after 4 weeks of maprotiline application. Contrary, the same treatment led to a significant increase in expression of cardiac β1-adrenoceptor in the stressed rats, with no change in the characteristics of β2-adrenoceptor. Our findings might reflect the that molecular mechanisms are underlying factors involved in the development of cardiovascular diseases linked with antidepressant treatment.

MARCH2 promotes endocytosis and lysosomal sorting of carvedilol-bound β(2)-adrenergic receptors

The β₂-adrenergic receptor antagonist carvedilol recruits MARCH2, a unique E3 ubiquitin ligase, to promote receptor endocytosis and lysosomal trafficking.

Lysosomal degradation of ubiquitinated β₂-adrenergic receptors (β₂ARs) serves as a major mechanism of long-term desensitization in response to prolonged agonist stimulation. Surprisingly, the βAR antagonist carvedilol also induced ubiquitination and lysosomal trafficking of both endogenously expressed β₂ARs in vascular smooth muscle cells (VSMCs) and overexpressed Flag-β₂ARs in HEK-293 cells. Carvedilol prevented β₂AR recycling, blocked recruitment of Nedd4 E3 ligase, and promoted the dissociation of the deubiquitinases USP20 and USP33. Using proteomics approaches (liquid chromatography–tandem mass spectrometry), we identified that the E3 ligase MARCH2 interacted with carvedilol-bound β₂AR. The association of MARCH2 with internalized β₂ARs was stabilized by carvedilol and did not involve β-arrestin. Small interfering RNA–mediated down-regulation of MARCH2 ablated carvedilol-induced ubiquitination, endocytosis, and degradation of endogenous β₂ARs in VSMCs. These findings strongly suggest that specific ligands recruit distinct E3 ligase machineries to activated cell surface receptors and direct their intracellular itinerary. In response to β blocker therapy with carvedilol, MARCH2 E3 ligase activity regulates cell surface β₂AR expression and, consequently, its signaling.

Chronic activation of the G protein-coupled receptor 30 with agonist G-1 attenuates heart failure

G protein-coupled receptor (GPR) 30 is a novel estrogen receptor. Recent studies suggest that activation of the GPR30 confers rapid cardioprotection in isolated rat heart. It is unknown whether chronic activation of GPR30 is beneficial or not for heart failure. In this study we investigated the cardiac effect of sustained activation or inhibition of GPR30. Female Sprague–Dawley rats were divided into 7 groups #2Q1: sham surgery (Sham), bilateral ovariectomy (OVX), OVX+estrogen (E₂), OVX+isoproterenol (ISO), OVX+ISO+G-1, OVX+ISO+E₂+G15, OVX+ISO+E₂. ISO (85 mg/kg×17 day, sc) was given to make the heart failure models. G-1(120 µg/kg·d×14 day) was used to activate GPR30 and G15 (190 µg/kg·d×14 day) was used to inhibit GPR30. Concentration of brain natriuretic peptide in serum, masson staining in isolated heart, contractile function and the expression of β₁ and β₂- adrenergic receptor (AR) of ventricular myocytes were also determined. Our data showed that ISO treatment led to heart failure in OVX rats. G-1 or E₂ treatment decreased concentration of brain natriuretic peptide, reduced cardiac fibrosis, and enhanced contraction of the heart. Combined treatment with β₁ (CGP20712A) and β₂-AR (ICI118551) antagonist abolished the improvement of myocardial function induced by G-1. We also found that chronic treatment with G-1 normalized the expression of β₁-AR and increased the expression of β₂-AR. Our results indicate that chronic activation of the GPR30 with its agonist G-1 attenuates heart failure by normalizing the expression of β₁-AR and increasing the expression of β₂-AR.

Phosphoinositide 3-kinase γ protects against catecholamine-induced ventricular arrhythmia through protein kinase A-mediated regulation of distinct phosphodiesterases

BACKGROUND

Phosphoinositide 3-kinase γ (PI3Kγ) signaling engaged by β-adrenergic receptors is pivotal in the regulation of myocardial contractility and remodeling. However, the role of PI3Kγ in catecholamine-induced arrhythmia is currently unknown.

METHODS AND RESULTS

Mice lacking PI3Kγ (PI3Kγ(-/-)) showed runs of premature ventricular contractions on adrenergic stimulation that could be rescued by a selective β(2)-adrenergic receptor blocker and developed sustained ventricular tachycardia after transverse aortic constriction. Consistently, fluorescence resonance energy transfer probes revealed abnormal cAMP accumulation after β(2)-adrenergic receptor activation in PI3Kγ(-/-) cardiomyocytes that depended on the loss of the scaffold but not of the catalytic activity of PI3Kγ. Downstream from β-adrenergic receptors, PI3Kγ was found to participate in multiprotein complexes linking protein kinase A to the activation of phosphodiesterase (PDE) 3A, PDE4A, and PDE4B but not of PDE4D. These PI3Kγ-regulated PDEs lowered cAMP and limited protein kinase A-mediated phosphorylation of L-type calcium channel (Ca(v)1.2) and phospholamban. In PI3Kγ(-/-) cardiomyocytes, Ca(v)1.2 and phospholamban were hyperphosphorylated, leading to increased Ca(2+) spark occurrence and amplitude on adrenergic stimulation. Furthermore, PI3Kγ(-/-) cardiomyocytes showed spontaneous Ca(2+) release events and developed arrhythmic calcium transients.

CONCLUSIONS

PI3Kγ coordinates the coincident signaling of the major cardiac PDE3 and PDE4 isoforms, thus orchestrating a feedback loop that prevents calcium-dependent ventricular arrhythmia.

Myocardial β(2) -adrenoceptor gene delivery promotes coordinated cardiac adaptive remodelling and angiogenesis in heart failure

BACKGROUND AND PURPOSE

We investigated whether β(2) -adrenoceptor overexpression could promote angiogenesis and improve blood perfusion and left ventricular (LV) remodeling of the failing heart.

EXPERIMENTAL APPROACH

We explored the angiogenic effects of β(2) -adrenoceptor overexpression in a rat model of post-myocardial infarction (MI) heart failure (HF). Cardiac adenoviral-mediated β(2) -adrenoceptor overexpression was obtained via direct intramyocardial injection 4-weeks post-MI. Adenovirus(Ad)-GFP and saline injected rats served as controls. Furthermore, we extended our observation to β(2) -adrenoceptor -/- mice undergoing MI.

KEY RESULTS

Transgenes were robustly expressed in the LV at 2 weeks post-gene therapy, whereas their expression was minimal at 4-weeks post-gene delivery. In HF rats, cardiac β(2) -adrenoceptor overexpression resulted in enhanced basal and isoprenaline-stimulated cardiac contractility at 2-weeks post-gene delivery. At 4 weeks post-gene transfer, Ad-β(2) -adrenoceptor HF rats showed improved LV remodeling and cardiac function. Importantly, β(2) -adrenoceptor overexpression was associated with a markedly increased capillary and arteriolar length density and enhanced in vivo myocardial blood flow and coronary reserve. At the molecular level, cardiac β(2) -adrenoceptor gene transfer induced the activation of the VEGF/PKB/eNOS pro-angiogenic pathway. In β(2) -adrenoceptor-/- mice, we found a ~25% reduction in cardiac capillary density compared with β(2) -adrenoceptor+/+ mice. The lack of β(2) -adrenoceptors was associated with a higher mortality rate at 30 days and LV dilatation, and a worse global cardiac contractility compared with controls.

CONCLUSIONS AND IMPLICATION

β(2) -Adrenoceptors play an important role in the regulation of the angiogenic response in HF. The activation of VEGF/PKB/eNOS pathway seems to be strongly involved in this mechanism.

Different expression of adrenoceptors and GRKs in the human myocardium depends on heart failure etiology and correlates to clinical variables

Downregulation of β(1)- adrenergic receptors (β(1)-ARs) and increased expression/function of G-protein-coupled receptor kinase 2 (GRK2) have been observed in human heart failure, but changes in expression of other ARs and GRKs have not been established. Another unresolved question is the incidence of these compensatory mechanisms depending on heart failure etiology and treatment. To analyze these questions, we quantified the mRNA/protein expressions of six ARs (α(1A), α(1B), α(1D), β(1), β(2), and β(3)) and three GRKs (GRK2, GRK3, and GRK5) in left (LV) and right ventricle (RV) from four donors, 10 patients with ischemic cardiomyopathy (IC), 14 patients with dilated cardiomyopathy (DC), and 10 patients with nonischemic, nondilated cardiopathies (NINDC). We correlated the changes in the expressions of ARs and GRKs with clinical variables such as left ventricular ejection fraction (LVEF) and left ventricular end-systolic and left ventricular end-diastolic diameter (LVESD and LVEDD, respectively). The main findings were 1) the expression of the α(1A)-AR in the LV positively correlates with LVEF; 2) the expression of GRK3 and GRK5 inversely correlates with LVESD and LVEDD, supporting previous observations about a protective role for both kinases in failing hearts; and 3) β(1)-AR expression is downregulated in the LV and RV of IC, in the LV of DC, and in the RV of NINDC. This difference, better than an increased expression of GRK2 (not observed in IC), determines the lower LVEF in IC and DC vs. NINDC.

β2-adrenoceptor transfection enhances contractile reserve of isolated rat ventricular myocytes exposed to chronic isoprenaline stimulation by improving β1-adrenoceptor responsiveness

CONTEXT

Heart failure (HF) is a progressive deterioration in heart function associated with overactivity of the sympathetic nervous system. Elevated sympathetic nervous system activity down regulates the β-adrenergic signal system, suppressing β-adrenoceptors (β-ARs)-mediated contractile support in the failing heart.

OBJECTIVE

We investigated the effects of β(2)-AR gene transfer on shortening amplitude of isolated ventricular myocytes under chronic exposure to isoprenaline (ISO), and further determine the contributions of β(1)-AR and β(2)-AR to the contraction.

MATERIALS AND METHODS

Cardiomyocytes were isolated from adult rat hearts and then transfected with β(2)-AR gene using an adenovirus vector. Four hours after the infection, cardiomyocytes were treated with ISO for another 24 hours to imitate high levels of circulating catecholamines in HF. Western blotting was performed to measure myocardial protein expression of β(2)-AR. Video-based edge-detection system was used to evaluate basal and ISO-stimulated shortening amplitudes of cardiomyocytes.

RESULTS

β(2)-AR gene transfer increased β(2)-AR protein content. Chronic ISO stimulation produced a negative inotropic response, whereas acute ISO stimulation showed a positive inotropic response. β(2)-AR gene transfer had no significant effects on shortening amplitude of cardiomyocytes under normal conditions, but enhanced the blunted contraction of cardiomyocytes under pathological conditions induced by chronic ISO stimulation, and the effect was inhibited by β(1)-AR antagonist, CGP 20712A, instead of β(2)-AR antagonist, ICI 118,551.

DISCUSSION AND CONCLUSIONS

We conclude that β(2)-AR gene transfer in isolated ventricular myocytes under chronic ISO stimulation improves cellular contraction, and the beneficial effects might be mediated by improving β(1)-adrenoceptor responsiveness.