Physiologic and cardiac roles of beta-arrestins

β-Arrestin as a Therapeutic Target in Heart Failure

Heart failure is a major source of morbidity and mortality, driven, in part, by maladaptive sympathetic hyperactivity in response to poor cardiac output. Current therapies target β-adrenergic and angiotensin II G protein-coupled receptors to reduce adverse cardiac remodeling and improve clinical outcomes; however, there is a pressing need for new therapeutic approaches to preserve cardiac function. β-arrestin is a multifunctional protein which has come under analysis in recent years as a key player in G protein-coupled receptor signal transduction and a potential therapeutic target in heart failure. β-arrestin attenuates β-adrenergic and angiotensin II receptor signaling to limit the deleterious response to excessive sympathetic stimulation while simultaneously transactivating cardioprotective signaling cascades that preserve cardiac structure and function in response to injury. β-arrestin signaling may provide unique advantages compared to classic heart failure treatment approaches, but a number of challenges currently limit clinical applications. In this review, we discuss the role and functions of β-arrestin and the current attempts to develop G protein-coupled receptor agonists biased towards β-arrestin activation. Furthermore, we examine the functional diversity of cardiac β-arrestin isotypes to explore key considerations in the promise of β-arrestin as a pharmacotherapeutic target in heart failure.

The relationship between β1 -adrenergic and M2 -muscarinic receptor autoantibodies and hypertrophic cardiomyopathy

NEW FINDINGS

What is the central question of this study? The concentrations of β₁ -adrenergic receptor and M₂ -muscarinic receptor autoantibodies in hypertrophic cardiomyopathy (HCM) patients and the relationship between the cardiac autoantibodies and clinical manifestations of HCM have rarely been reported. What is the main finding and its importance? We found that the concentrations of the two autoantibodies in HCM patients were significantly higher than those in control subjects. Furthermore, we found that the concentrations of the two autoantibodies could reflect myocardial injury and diastolic dysfunction in HCM patients to some extent and might be involved in the occurrence of arrhythmia. These findings might be valuable in exploration of the mechanisms of occurrence and progression of HCM.

ABSTRACT

Increasing attention is being given to the role of immunological mechanisms in the development of heart failure. The purpose of this study was to investigate the concentration of serum β₁ -adrenergic receptor autoantibody (β₁ -AAb) and M₂ -muscarinic receptor autoantibody (M₂ -AAb) in patients with hypertrophic cardiomyopathy (HCM), and the relationship between β₁ -AAb, M₂ -AAb and clinical indices. One hundred and thirty-four patients with HCM were recruited consecutively into the HCM group. Forty healthy subjects were assigned as the normal controls (NCs). Serum samples were collected to measure the concentrations of β₁ -AAb and M₂ -AAb by enzyme-linked immunosorbent assay. The clinical data of HCM patients were collected. The serum concentrations of β₁ -AAb and M₂ -AAb of HCM patients were significantly higher than those of NCs. In HCM patients, those with a left atrial diameter ≥50 mm or moderate-to-severe mitral regurgitation had significantly higher concentrations of the two autoantibodies. Patients with a history of syncope had higher concentrations of β₁ -AAb. Female patients and patients with a family history of sudden cardiac death or atrial fibrillation had higher concentrations of M₂ -AAb. Maximal wall thickness, interventricular septum thickness and resting left ventricular outflow tract gradient were positively correlated with log β₁ -AAb or log M₂ -AAb in HCM patients. In conclusion, the serum concentrations of β₁ -AAb and M₂ -AAb of HCM patients were significantly higher than those of NCs. Being female, syncope, a family history of sudden death, atrial fibrillation, left atrial diameter ≥50 mm, moderate-to-severe mitral regurgitation, maximal wall thickness, interventricular septum thickness and resting left ventricular outflow tract gradient may affect the concentrations of the two autoantibodies.

Proteinase‑activated receptor 2 deficiency is a protective factor against cardiomyocyte apoptosis during myocardial ischemia/reperfusion injury

Previous studies have established that proteinase‑​activated receptor 2 (PAR2) activation protects against myocardial ischemia/reperfusion injury (MI/RI). However, the role of PAR2 deficiency in MI/RI remains unclear. The aim of the present study was to examine the effect of PAR2 deficiency on cardiomyocyte apoptosis and to clarify the potential molecular mechanisms for its protective effect against MI/RI. Using a mouse model of MI/RI, cardiac function was evaluated by echocardiography, infarct size was assessed by triphenyltetrazolium chloride staining, and myocardial cell apoptosis was measured by terminal deoxynucleotide transferase‑mediated dUTP nick end‑labeling staining. Annexin V/propidium iodide staining, and expression of Bcl‑2 and cleaved PARP were determined to assess apoptosis in myocardial H9c2 cells exposed to hypoxia/reoxygenation (H/R) injury‑simulating MI/RI. Phosphorylated ERK1/2, JNK, and p38 MAPK protein expression levels were analyzed by western blotting. The findings indicated that PAR2 deficiency markedly reduced cardiomyocyte apoptosis in the MI/RI mouse model, as well as in myocardial H9c2 cells exposed to H/R. Furthermore, PAR2 knockdown clearly prevented phosphorylation of ERK1/2 and JNK in myocardial H9c2 cells. The results revealed that PAR2 deficiency alleviated MI/RI‑associated apoptosis by inhibiting phosphorylation of ERK1/2 and JNK. Therefore, targeted PAR2 silencing may be a potential therapeutic approach for alleviation of MI/RI.

Regulation of cardiac fibroblast-mediated maladaptive ventricular remodeling by β-arrestins

Cardiac fibroblasts (CF) play a critical role in post-infarction remodeling which can ultimately lead to pathological fibrosis and heart failure. Recent evidence demonstrates that remote (non-infarct) territory fibrosis is a major mechanism for ventricular dysfunction and arrhythmogenesis. β-arrestins are important signaling molecules involved in β-adrenergic receptor (β-AR) desensitization and can also mediate signaling in a G protein independent fashion. Recent work has provided evidence that β-arrestin signaling in the heart may be beneficial, however, these studies have primarily focused on cardiac myocytes and their role in adult CF biology has not been well studied. In this study, we show that β-arrestins can regulate CF biology and contribute to pathological fibrosis. Adult male rats underwent LAD ligation to induce infarction and were studied by echocardiography. There was a significant decline in LV function at 2–12 weeks post-MI with increased infarct and remote territory fibrosis by histology consistent with maladaptive remodeling. Collagen synthesis was upregulated 2.9-fold in CF isolated at 8 and 12 weeks post-MI and β-arrestin expression was significantly increased. β-adrenergic signaling was uncoupled in the post-MI CF and β-agonist-mediated inhibition of collagen synthesis was lost. Knockdown of β-arrestin1 or 2 in the post-MI CF inhibited transformation to myofibroblasts as well as basal and TGF-β-stimulated collagen synthesis. These data suggest that β-arrestins can regulate CF biology and that targeted inhibition of these signaling molecules may represent a novel approach to prevent post-infarction pathological fibrosis and the transition to HF.

Therapeutic Targets for Treatment of Heart Failure: Focus on GRKs and β-Arrestins Affecting βAR Signaling

Heart failure (HF) is a heart disease that is classified into two main types: HF with reduced ejection fraction (HFrEF) and HF with preserved ejection fraction (HFpEF). Both types of HF lead to significant risk of mortality and morbidity. Pharmacological treatment with β-adrenergic receptor (βAR) antagonists (also called β-blockers) has been shown to reduce the overall hospitalization and mortality rates and improve the clinical outcomes in HF patients with HFrEF but not HFpEF. Although, the survival rate of patients suffering from HF continues to drop, the management of HF still faces several limitations and discrepancies highlighting the need to develop new treatment strategies. Overstimulation of the sympathetic nervous system is an adaptive neurohormonal response to acute myocardial injury and heart damage, whereas prolonged exposure to catecholamines causes defects in βAR regulation, including a reduction in the amount of βARs and an increase in βAR desensitization due to the upregulation of G protein-coupled receptor kinases (GRKs) in the heart, contributing in turn to the progression of HF. Several studies show that myocardial GRK2 activity and expression are raised in the failing heart. Furthermore, β-arrestins play a pivotal role in βAR desensitization and, interestingly, can mediate their own signal transduction without any G protein-dependent pathway involved. In this review, we provide new insight into the role of GRKs and β-arrestins on how they affect βAR signaling regarding the molecular and cellular pathophysiology of HF. Additionally, we discuss the therapeutic potential of targeting GRKs and β-arrestins for the treatment of HF.

Subcellular Organization of GPCR Signaling

G protein-coupled receptors (GPCRs) comprise a large and diverse class of signal-transducing receptors that undergo dynamic and isoform-specific membrane trafficking. GPCRs thus have an inherent potential to initiate or regulate signaling reactions from multiple membrane locations. This review discusses emerging insights into the subcellular organization of GPCR function in mammalian cells, focusing on signaling transduced by heterotrimeric G proteins and β-arrestins. We summarize recent evidence indicating that GPCR-mediated activation of G proteins occurs not only from the plasma membrane (PM) but also from endosomes and Golgi membranes and that β-arrestin-dependent signaling can be transduced from the PM by β-arrestin trafficking to clathrin-coated pits (CCPs) after dissociation from a ligand-activated GPCR.

Endocrinology and the brain: corticotropin-releasing hormone signaling

Corticotropin-releasing hormone (CRH) is a key player of basal and stress-activated responses in the hypothalamic-pituitary-adrenal axis (HPA) and in extrahypothalamic circuits, where it functions as a neuromodulator to orchestrate humoral and behavioral adaptive responses to stress. This review describes molecular components and cellular mechanisms involved in CRH signaling downstream of its G protein-coupled receptors (GPCRs) CRHR1 and CRHR2 and summarizes recent findings that challenge the classical view of GPCR signaling and impact on our understanding of CRHRs function. Special emphasis is placed on recent studies of CRH signaling that revealed new mechanistic aspects of cAMP generation and ERK1/2 activation in physiologically relevant contexts of the neurohormone action. In addition, we present an overview of the pathophysiological role of the CRH system, which highlights the need for a precise definition of CRHRs signaling at molecular level to identify novel targets for pharmacological intervention in neuroendocrine tissues and specific brain areas involved in CRH-related disorders.

Cardioprotective Angiotensin-(1-7) Peptide Acts as a Natural-Biased Ligand at the Angiotensin II Type 1 Receptor

Hyperactivity of the renin-angiotensin-aldosterone system through the angiotensin II (Ang II)/Ang II type 1 receptor (AT1-R) axis constitutes a hallmark of hypertension. Recent findings indicate that only a subset of AT1-R signaling pathways is cardiodeleterious, and their selective inhibition by biased ligands promotes therapeutic benefit. To date, only synthetic biased ligands have been described, and whether natural renin-angiotensin-aldosterone system peptides exhibit functional selectivity at AT1-R remains unknown. In this study, we systematically determined efficacy and potency of Ang II, Ang III, Ang IV, and Ang-(1-7) in AT1-R-expressing HEK293T cells on the activation of cardiodeleterious G-proteins and cardioprotective β-arrestin2. Ang III and Ang IV fully activate similar G-proteins than Ang II, the prototypical AT1-R agonist, despite weaker potency of Ang IV. Interestingly, Ang-(1-7) that binds AT1-R fails to promote G-protein activation but behaves as a competitive antagonist for Ang II/Gi and Ang II/Gq pathways. Conversely, all renin-angiotensin-aldosterone system peptides act as agonists on the AT1-R/β-arrestin2 axis but display biased activities relative to Ang II as indicated by their differences in potency and AT1-R/β-arrestin2 intracellular routing. Importantly, we reveal Ang-(1-7) a known Mas receptor-specific ligand, as an AT1-R-biased agonist, selectively promoting β-arrestin activation while blocking the detrimental Ang II/AT1-R/Gq axis. This original pharmacological profile of Ang-(1-7) at AT1-R, similar to that of synthetic AT1-R-biased agonists, could, in part, contribute to its cardiovascular benefits. Accordingly, in vivo, Ang-(1-7) counteracts the phenylephrine-induced aorta contraction, which was blunted in AT1-R knockout mice. Collectively, these data suggest that Ang-(1-7) natural-biased agonism at AT1-R could fine-tune the physiology of the renin-angiotensin-aldosterone system.

Anti-calreticulin antibodies and calreticulin in sera of patients diagnosed with dilated or hypertrophic cardiomyopathy

Distinct cellular level of the Ca²⁺-binding chaperone calreticulin (CRT) is essential for correct embryonal cardiac development and postnatal function. However, CRT is also a potential autoantigen eliciting formation of antibodies (Ab), whose role is not yet clarified. Immunization with CRT leads to cardiac injury, while overexpression of CRT in cardiomyocytes induces dilated cardiomyopathy (DCM) in animals. Hence, we analysed levels of anti-CRT Ab and calreticulin in the sera of patients with idiopatic DCM and hypertrophic cardiomyopathy (HCM). ELISA and immunoblot using human recombinant CRT and Pepscan with synthetic, overlapping decapeptides of CRT were used to detect anti-CRT Ab. Serum CRT concentration was tested by ELISA. Significantly increased levels of anti-CRT Ab of isotypes IgA (p < 0.001) and IgG (p < 0.05) were found in patients with both DCM (12/34 seropositive for IgA, 7/34 for IgG) and HCM (13/38 seropositive for IgA, 11/38 for IgG) against healthy controls (2/79 for IgA, 1/79 for IgG). Titration analysis in seropositive DCM and HCM patients documented anti-CRT Ab detected at 1/1600 dilution for IgG and 1/800 for IgA (and IgA1) and at least at 1/200 dilution for IgA2, IgG1, IgG2 and IgG3. Pepscan identified immunogenic CRT epitopes recognized by IgA and IgG Ab of these patients. Significantly increased levels of CRT relative to healthy controls were found in sera of patients with HCM (p < 0.01, 5/19). These data extend the knowledge of seroprevalence of anti-CRT Ab and CRT, and suggest possible involvement of autoimmune mechanisms directed to CRT in some forms of cardiomyopathies, which are clinically heterogeneous.

Molecular Mechanisms Linking Autonomic Dysfunction and Impaired Cardiac Contractility in Critical Illness

OBJECTIVES

Molecular mechanisms linking autonomic dysfunction with poorer clinical outcomes in critical illness remain unclear. We hypothesized that baroreflex dysfunction alone is sufficient to cause cardiac impairment through neurohormonal activation of (nicotinamide adenine dinucleotide phosphate oxidase dependent) oxidative stress resulting in increased expression of G-protein-coupled receptor kinase 2, a key negative regulator of cardiac function.

DESIGN

Laboratory/clinical investigations.

SETTING

University laboratory/medical centers.

SUBJECTS

Adult rats; wild-type/nicotinamide adenine dinucleotide phosphate oxidase subunit-2-deficient mice; elective surgical patients.

INTERVENTIONS

Cardiac performance was assessed by transthoracic echocardiography following experimental baroreflex dysfunction (sino-aortic denervation) in rats and mice. Immunoblots assessed G-protein-coupled receptor recycling proteins expression in rodent cardiomyocytes and patient mononuclear leukocytes. In surgical patients, heart rate recovery after cardiopulmonary exercise testing, time/frequency measures of parasympathetic variables were related to the presence/absence of baroreflex dysfunction (defined by spontaneous baroreflex sensitivity of <6 ms mm Hg). The associations of baroreflex dysfunction with intraoperative cardiac function and outcomes were assessed.

MEASUREMENTS AND MAIN RESULTS

Experimental baroreflex dysfunction in rats and mice resulted in impaired cardiac contractility and upregulation of G-protein-coupled receptor kinase 2 expression. In mice, genetic deficiency of gp91 nicotinamide adenine dinucleotide phosphate oxidase subunit-2 prevented upregulation of G-protein-coupled receptor kinase 2 expression in conditions of baroreflex dysfunction and preserved cardiac function. Baroreflex dysfunction was present in 81 of 249 patients (32.5%) and was characterized by lower parasympathetic tone and increased G-protein-coupled receptor kinase 2 expression in mononuclear leukocytes. Baroreflex dysfunction in patients was also associated with impaired intraoperative cardiac contractility. Critical illness and mortality were more frequent in surgical patients with baroreflex dysfunction (relative risk, 1.66 [95% CI, 1.16-2.39]; p = 0.006).

CONCLUSIONS

Reduced baroreflex sensitivity is associated with nicotinamide adenine dinucleotide phosphate oxidase subunit-2-mediated upregulation of G-protein-coupled receptor kinase 2 expression in cardiomyocytes and impaired cardiac contractility. Autonomic dysfunction predisposes patients to the development of critical illness and increases mortality.

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.

β-Arrestin drives MAP kinase signalling from clathrin-coated structures after GPCR dissociation

β-arrestins critically regulate G protein-coupled receptor (GPCR) signaling, not only 'arresting' the G protein signal but also modulating endocytosis and initiating a discrete G protein-independent signal via MAP kinase^1–3. Despite enormous recent progress toward understanding biophysical aspects of arrestin function^4,5, its cell biology remains relatively poorly understood. Two key tenets underlie the present dogma: (1) β-arrestin accumulates in clathrin-coated structures (CCSs) exclusively in physical complex with its activating GPCR, and (2) MAP kinase activation requires endocytosis of formed GPCR - β-arrestin complexes^6–9. We show here, using β1-adrenergic receptors, that β-arrestin-2 (Arrestin 3) accumulates robustly in CCSs after dissociating from its activating GPCR and transduces the MAP kinase signal from CCSs. Moreover, inhibiting subsequent endocytosis of CCSs enhances the clathrin and β-arrestin -dependent MAP kinase signal. These results demonstrate β-arrestin 'activation at a distance', after dissociating from its activating GPCR, and signaling from CCSs. We propose a β-arrestin signaling cycle that is catalytically activated by the GPCR and energetically coupled to the endocytic machinery.

Regulation of cellular oxidative stress and apoptosis by G protein-coupled receptor kinase-2; The role of NADPH oxidase 4

Cardiac myocyte oxidative stress and apoptosis are considered important mechanisms for the development of heart failure (HF). Chronic HF is characterized by increased circulating catecholamines to augment cardiac output. Long-term stimulation of myocardial β-adrenergic receptors (β-ARs) is deleterious in cardiac myocytes, however, the potential mechanisms underlying increased cell death are unclear. We hypothesize that GRK2, a critical regulator of myocardial β-AR signaling, plays an important role in mediating cellular oxidative stress and apoptotic cell death in response to β-agonist stimulation. Stimulation of H9c2 cells with a non-selective β-agonist, isoproterenol (Iso) caused increased oxidative stress and apoptosis. There was also increased Nox4 expression, but no change in Nox2, the primary NADPH isoforms and major sources of ROS generation in cardiac myocytes. Adenoviral-mediated overexpression of GRK2 led to similar increases in ROS production and apoptosis as seen with Iso stimulation. These increases in oxidative stress were abolished by pre-treatment with the non-specific Nox inhibitor, apocynin, or siRNA knockdown of Nox4. Adenoviral-mediated expression of a GRK2 inhibitor prevented ROS production and apoptosis in response to Iso stimulation. β-Arrestins are signaling proteins that function downstream of GRK2 in β-AR uncoupling. Adenoviral-mediated overexpression of β-arrestins increased ROS production and Nox4 expression. Chronic β-agonist stimulation in mice increased Nox4 expression and apoptosis compared to PBS or AngII treatment. These data demonstrate that GRK2 may play an important role in regulating oxidative stress and apoptosis in cardiac myocytes and provides an additional novel mechanism for the beneficial effects of cardiac-targeted GRK2 inhibition to prevent the development of HF.

Long-Acting Beta Agonists Enhance Allergic Airway Disease

Asthma is one of the most common of medical illnesses and is treated in part by drugs that activate the beta-2-adrenoceptor (β₂-AR) to dilate obstructed airways. Such drugs include long acting beta agonists (LABAs) that are paradoxically linked to excess asthma-related mortality. Here we show that LABAs such as salmeterol and structurally related β₂-AR drugs such as formoterol and carvedilol, but not short-acting agonists (SABAs) such as albuterol, promote exaggerated asthma-like allergic airway disease and enhanced airway constriction in mice. We demonstrate that salmeterol aberrantly promotes activation of the allergic disease-related transcription factor signal transducer and activator of transcription 6 (STAT6) in multiple mouse and human cells. A novel inhibitor of STAT6, PM-242H, inhibited initiation of allergic disease induced by airway fungal challenge, reversed established allergic airway disease in mice, and blocked salmeterol-dependent enhanced allergic airway disease. Thus, structurally related β₂-AR ligands aberrantly activate STAT6 and promote allergic airway disease. This untoward pharmacological property likely explains adverse outcomes observed with LABAs, which may be overcome by agents that antagonize STAT6.

Alpha1a-Adrenoceptor Genetic Variant Triggers Vascular Smooth Muscle Cell Hyperproliferation and Agonist Induced Hypertrophy via EGFR Transactivation Pathway

α_1a Adrenergic receptors (α_1aARs) are the predominant AR subtype in human vascular smooth muscle cells (SMCs). α_1aARs in resistance vessels are crucial in the control of blood pressure, yet the impact of naturally occurring human α_1aAR genetic variants in cardiovascular disorders remains poorly understood. To this end, we present novel findings demonstrating that 3D cultures of vascular SMCs expressing human α_1aAR-247R (247R) genetic variant demonstrate significantly increased SMC contractility compared with cells expressing the α_1aAR-WT (WT) receptor. Stable expression of 247R genetic variant also triggers MMP/EGFR-transactivation dependent serum- and agonist-independent (constitutive) hyperproliferation and agonist-dependent hypertrophy of SMCs. Agonist stimulation reduces contractility Using pathway-specific inhibitors we determined that the observed hyperproliferation of 247R-expressing cells is triggered via β-arrestin1/Src/MMP-2/EGFR/ERK-dependent mechanism. MMP-2-specific siRNA inhibited 247R-triggered hyperproliferation indicating MMP-2 involvement in 247R-triggered hyperproliferation in SMCs. β-arrestin1-specific shRNA also inhibited 247R-triggered hyperproliferation but did not affect hypertrophy in 247R-expressing SMCs, indicating that agonist-dependent hypertrophy is independent of β-arrestin1. Our data reveal that in different cardiovascular cells the same human receptor genetic variant can activate alternative modulators of the same signaling pathway. Thus, our findings in SMCs demonstrate that depending on the type of cells expressing the same receptor (or receptor variant), different target-specific inhibitors could be used to modulate aberrant hyperproliferative or hypertrophic pathways in order to restore normal phenotype.

Introduction: G Protein-coupled Receptors and RGS Proteins

Here, we provide an overview of the role of regulator of G protein-signaling (RGS) proteins in signaling by G protein-coupled receptors (GPCRs), the latter of which represent the largest class of cell surface receptors in humans responsible for transducing diverse extracellular signals into the intracellular environment. Given that GPCRs regulate virtually every known physiological process, it is unsurprising that their dysregulation plays a causative role in many human diseases and they are targets of 40-50% of currently marketed pharmaceuticals. Activated GPCRs function as GTPase exchange factors for Gα subunits of heterotrimeric G proteins, promoting the formation of Gα-GTP and dissociated Gβγ subunits that regulate diverse effectors including enzymes, ion channels, and protein kinases. Termination of signaling is mediated by the intrinsic GTPase activity of Gα subunits leading to reformation of the inactive Gαβγ heterotrimer. RGS proteins determine the magnitude and duration of cellular responses initiated by many GPCRs by functioning as GTPase-accelerating proteins (GAPs) for specific Gα subunits. Twenty canonical mammalian RGS proteins, divided into four subfamilies, act as functional GAPs while almost 20 additional proteins contain nonfunctional RGS homology domains that often mediate interaction with GPCRs or Gα subunits. RGS protein biochemistry has been well elucidated in vitro, but the physiological functions of each RGS family member remain largely unexplored. This book summarizes recent advances employing modified model organisms that reveal RGS protein functions in vivo, providing evidence that RGS protein modulation of G protein signaling and GPCRs can be as important as initiation of signaling by GPCRs.

Bioluminescent tools for the analysis of G-protein-coupled receptor and arrestin interactions

New protein-based bioluminescent probes for monitoring GPCR interaction with β-arrestin are presented.

β-Arrestins regulate human cardiac fibroblast transformation and collagen synthesis in adverse ventricular remodeling

Cardiac fibroblasts (CFs) produce and degrade the myocardial extracellular matrix and are critical in maladaptive ventricular remodeling that can result in heart failure (HF). β-Arrestins are important signaling molecules involved in β-adrenergic receptor (β-AR) desensitization and can also mediate signaling in a G protein-independent fashion. We hypothesize that β-arrestins play an important role in the regulation of adult human CF biology with regard to myofibroblast transformation, increased collagen synthesis, and myocardial fibrosis which are important in the development of HF. β-Arrestin1 & 2 expression is significantly upregulated in adult human CF isolated from failing left ventricles and β-AR signaling is uncoupled with loss of β-agonist-mediated inhibition of collagen synthesis versus normal control CF. Knockdown of either β-arrestin1 or 2 restored β-AR signaling and β-agonist mediated inhibition of collagen synthesis. Overexpression of β-arrestins in normal CF led to a failing phenotype with increased baseline collagen synthesis, impaired β-AR signaling, and loss of β-agonist-mediated inhibition of collagen synthesis. β-Arrestin knockdown in failing CF diminished TGF-β stimulated collagen synthesis and also inhibited ERK phosphorylation. Overexpression of β-arrestins in normal CF increased basal ERK1/2 and Smad2/3 phosphorylation and enhanced TGF-β-stimulated collagen synthesis. This was prevented by pre-treatment with a MEK1/2 inhibitor. Enhanced β-arrestin signaling appears to be deleterious in CF by promoting a pro-fibrotic phenotype via uncoupling of β-AR signaling as well as potentiating ERK and Smad signaling. Targeted inhibition of β-arrestins in CF may represent a therapeutic strategy to prevent maladaptive myocardial fibrosis.

Targeting anti-beta-1-adrenergic receptor antibodies for dilated cardiomyopathy

Anti-beta-1-adrenergic receptor antibodies (anti-β1AR Abs) have long been implicated in the pathogenesis of dilated cardiomyopathy (DCM). It is believed that these autoantibodies bind to and constitutively stimulate the β1AR to promote pathological cardiac remodelling and β1AR desensitization and downregulation. The prevalence of anti-β1AR Abs in patients with DCM ranges from 26% to 60%, and the presence of these autoantibodies correlates with a poor prognosis. Several small studies have shown improvements in functional status, haemodynamics, and biomarkers of heart failure upon removal or neutralization of these antibodies from the sera of affected patients. Traditionally, removal of anti-β1AR Abs required immunoadsorption therapy with apheresis columns directed against human immunoglobulins (Igs) and subsequent i.v. Ig infusion, thereby essentially performing a plasma exchange transfusion. However, recent advances have allowed the development of small peptides and nucleotide sequences that specifically target and neutralize anti-β1AR Abs, providing a hopeful avenue for future drug development to treat DCM. Herein, we briefly review the clinical literature of therapy directed against anti-β1AR Abs and highlight the opportunity for further research and development in this area.

Intervention effect of Compound Sophorae Flavescentis decoction on DOR-β-arrestin1-Bcl-2 signal transduction pathway in rats with ulcerative colitis

AIM: To explore the intervention effect of Compound Sophorae Flavescentis decoction on the DOR-β-arrestin1-Bcl-2 signal transduction pathway in rats with ulcerative colitis.

METHODS: Eighty-four Sprague-Dawley rats were equally and randomly divided into normal control group, model group, mesalazine group, high-, medium-, and low-dose Compound Sophorae Flavescentis decoction groups. Except for the normal control group, ulcerative colitis was induced with trinitrobenzene sulfonic acid (TNBS) in rats of other groups. After induction of ulcerative colitis, bloody stools, mental state and diarrhea were observed and recorded daily. Two rats in the model group were randomly selected and executed on day 3 for observing pathological changes in the colon tissue. Rats in the mesalazine group and Compound Sophorae Flavescentis decoction groups were lavaged with mesalazine solution and Compound Sophorae Flavescentis decoction for 15 d, while those in the normal control group and model group were given equal volume of distilled water for the same duration. On day 16, the remaining rats were executed to detect histopathological changes in the colon tissue and the mRNA and protein expression of DOR, β-arrestin1 and Bcl-2 in the colon tissue by real-time PCR and immunohistochemistry, respectively.

RESULTS: The mRNA and protein expression of DOR, β-arrestin1 and Bcl-2 differed significantly among each group (P < 0.05). Compared to the normal control group, the mRNA and protein expression of DOR, β-arrestin1 and Bcl-2 protein was significantly increased (24.11 ± 12.61 vs 11.88 ± 5.90, 38.90 ± 5.30 vs 14.34 ± 8.97, 23.57 ± 9.96 vs 9.68 ± 3.94, all P < 0.05) in the colon mucosa in the model group. Compared to the model group, the mRNA and protein expression of DOR, β-arrestin1 and Bcl-2 in the colon mucosa was significantly decreased in the mesalazine group and Compound Sophorae Flavescentis decoction groups; however, the mRNA and protein expression of DOR, β-arrestin1 and Bcl-2 did not differ significantly between the mesalazine group and Compound Sophorae Flavescentis decoction groups.

CONCLUSION: The expression of DOR, β-arrestin1 and Bcl-2 is elevated in ulcerative colitis. The DOR-β-arrestin1-Bcl-2 signal transduction pathway may be involved in the pathogenesis of ulcerative colitis, and Compound Sophorae Flavescentis decoction may have a significant therapeutic effect against ulcerative colitis.

Visualizing G protein-coupled receptor signalsomes using confocal immunofluorescence microscopy

The heptahelical G protein-coupled receptors (GPCRs) receive and transmit a wide range of extracellular stimuli and induce a wide array of cellular responses by activating signaling kinases. It has become increasingly evident that the agonist-stimulated GPCR complexed with the adaptor protein, β-arrestin, serves as a focal point to recruit, activate, and target kinases to discrete subcellular compartments. This chapter describes a protocol to visualize the changes in the subcellular distribution of activated extracellular signal-regulated kinases 1 and 2 (ERK1/2) when induced by the angiotensin II type 1a receptor.

New perspectives regarding β(2) -adrenoceptor ligands in the treatment of asthma

In the last two decades several significant changes have been proposed in the receptor theory that describes how ligands can interact with G protein-coupled receptors (GPCRs). Here we briefly summarize the evolution of receptor theory and detail recent prominent advances. These include: (i) the existence of spontaneously active GPCRs that are capable of signalling even though they are unoccupied by any ligand; (ii) the discovery of ligands that can inactivate these spontaneously active receptors; (iii) the notion that a ligand may simultaneously activate more than one GPCR signalling pathway; and (iv) the notion that certain ligands may be able to preferentially direct receptor signalling to a specific pathway. Because the data supporting these receptor theory ideas are derived primarily from studies using artificial expression systems, the physiological relevance of these new paradigms remains in question. As a potential example of how these new perspectives in receptor theory relate to drug actions and clinical outcomes, we discuss their relevance to the recent controversy regarding the chronic use of β(2) -adrenoceptor agonists in the treatment of asthma.

IQGAP1 regulates ERK1/2 and AKT signalling in the heart and sustains functional remodelling upon pressure overload

AIMS

The Raf-MEK1/2-ERK1/2 (ERK1/2-extracellular signal-regulated kinases 1/2) signalling cascade is crucial in triggering cardiac responses to different stress stimuli. Scaffold proteins are key elements in coordinating signalling molecules for their appropriate spatiotemporal activation. Here, we investigated the role of IQ motif-containing GTPase-activating protein 1 (IQGAP1), a scaffold for the ERK1/2 cascade, in heart function and remodelling in response to pressure overload.

METHODS AND RESULTS

IQGAP1-null mice have unaltered basal heart function. When subjected to pressure overload, IQGAP1-null mice initially develop a compensatory hypertrophy indistinguishable from that of wild-type (WT) mice. However, upon a prolonged stimulus, the hypertrophic response develops towards a thinning of left ventricular walls, chamber dilation, and a decrease in contractility, in an accelerated fashion compared with WT mice. This unfavourable cardiac remodelling is characterized by blunted reactivation of the foetal gene programme, impaired cardiomyocyte hypertrophy, and increased cardiomyocyte apoptosis. Analysis of signalling pathways revealed two temporally distinct waves of both ERK1/2 and AKT phosphorylation peaking, respectively, at 10 min and 4 days after aortic banding in WT hearts. IQGAP1-null mice show strongly impaired phosphorylation of MEK1/2-ERK1/2 and AKT following 4 days of pressure overload, but normal activation of these kinases after 10 min. Pull-down experiments indicated that IQGAP1 is able to bind the three components of the ERK cascade, namely c-Raf, MEK1/2, and ERK1/2, as well as AKT in the heart.

CONCLUSION

These data demonstrate, for the first time, a key role for the scaffold protein IQGAP1 in integrating hypertrophy and survival signals in the heart and regulating long-term left ventricle remodelling upon pressure overload.

Differential regulation of two palmitoylation sites in the cytoplasmic tail of the beta1-adrenergic receptor

S-Palmitoylation of G protein-coupled receptors (GPCRs) is a prevalent modification, contributing to the regulation of receptor function. Despite its importance, the palmitoylation status of the β(1)-adrenergic receptor, a GPCR critical for heart function, has never been determined. We report here that the β(1)-adrenergic receptor is palmitoylated on three cysteine residues at two sites in the C-terminal tail. One site (proximal) is adjacent to the seventh transmembrane domain and is a consensus site for GPCRs, and the other (distal) is downstream. These sites are modified in different cellular compartments, and the distal palmitoylation site contributes to efficient internalization of the receptor following agonist stimulation. Using a bioorthogonal palmitate reporter to quantify palmitoylation accurately, we found that the rates of palmitate turnover at each site are dramatically different. Although palmitoylation at the proximal site is remarkably stable, palmitoylation at the distal site is rapidly turned over. This is the first report documenting differential dynamics of palmitoylation sites in a GPCR. Our results have important implications for function and regulation of the clinically important β(1)-adrenergic receptor.

Protease-activated receptors and myocardial infarction

Protease-activated receptors (PARs) are widely expressed within the heart. They are activated by a myriad of proteases, including coagulation proteases. In vitro studies showed that activation of PAR-1 and PAR-2 on cardiomyocytes induced hypertrophy. In addition, PAR-1 stimulation on cardiac fibroblasts induced proliferation. Genetic and pharmacologic approaches have been used to investigate the role of the different PARs in cardiac ischemia/reperfusion (I/R) injury. In mice and rats, PAR-1 is reported to play a role in inflammation, infarct size, and remodeling after cardiac I/R injury. However, there are notable differences between the effect of a deficiency in PAR-1 and inhibition of PAR-1. For instance, inhibition of PAR-1 reduced infarct size whereas there was no effect of a deficiency of PAR-1. These differences maybe due to off-target effects of the inhibitor or PAR-4 compensation of PAR-1 deficiency. Similarly, a deficiency of PAR-2 was associated with reduced cardiac inflammation and improved heart function after I/R injury, whereas pharmacologic activation of PAR-2 was found to be protective due to increased vasodilatation. These differences maybe due to different signaling responses induced by an endogenous protease versus an exogenous agonist peptide. Surprisingly, PAR-4 deficiency resulted in increased cardiac injury and increased mortality after I/R injury. In contrast, a pharmacological study indicated that inhibition of PAR-4 was cardioprotective. It is possible that the major cellular target of the PAR-4 inhibitor is platelets, which have been shown to contribute to inflammation in the injured heart, whereas PAR-4 signaling in cardiomyocytes may be protective. These discrepant results between genetic and pharmacological approaches indicate that further studies are needed to determine the role of different PARs in the injured heart.

Role of the Drosophila non-visual ß-arrestin kurtz in hedgehog signalling

The non-visual ß-arrestins are cytosolic proteins highly conserved across species that participate in a variety of signalling events, including plasma membrane receptor degradation, recycling, and signalling, and that can also act as scaffolding for kinases such as MAPK and Akt/PI3K. In Drosophila melanogaster, there is only a single non-visual ß-arrestin, encoded by kurtz, whose function is essential for neuronal activity. We have addressed the participation of Kurtz in signalling during the development of the imaginal discs, epithelial tissues requiring the activity of the Hedgehog, Wingless, EGFR, Notch, Insulin, and TGFβ pathways. Surprisingly, we found that the complete elimination of kurtz by genetic techniques has no major consequences in imaginal cells. In contrast, the over-expression of Kurtz in the wing disc causes a phenotype identical to the loss of Hedgehog signalling and prevents the expression of Hedgehog targets in the corresponding wing discs. The mechanism by which Kurtz antagonises Hedgehog signalling is to promote Smoothened internalization and degradation in a clathrin- and proteosomal-dependent manner. Intriguingly, the effects of Kurtz on Smoothened are independent of Gprk2 activity and of the activation state of the receptor. Our results suggest fundamental differences in the molecular mechanisms regulating receptor turnover and signalling in vertebrates and invertebrates, and they could provide important insights into divergent evolution of Hedgehog signalling in these organisms.

Non-visual β-arrestins are key proteins involved in plasma membrane receptor internalization, recycling, and signalling. The activity of β-arrestins is generally linked to seven-transmembrane receptors, but in vertebrates they can also participate in many other signalling pathways. Consistently, β-arrestins play important roles during vertebrate development and are implicated in a variety of human pathologies. Here we take advantage of the fruit fly model to analyse the genetic requirements of the unique fly non-visual β-arrestin (kurtz) in signalling during the development of imaginal epithelia. To our surprise, we find that the complete elimination of kurtz has no major consequences in imaginal cells. Our data suggest that insect epithelial cells have evolved arrestin-independent mechanisms to control receptor turnover and signalling, so arrestin function has become less critical. On the other hand, in contrast to previous reports in vertebrates, we find that the over-expression of Kurtz blocks Hedgehog signalling by promoting the internalization and degradation of the transductor Smoothened. We suggest that such differences are based on the specific requirement of the primary cilia for Hedgehog signalling in most vertebrates. These results could provide important insights into divergent modes of membrane receptor regulation and Hedgehog signalling in vertebrates and invertebrates.

When simple agonism is not enough: emerging modalities of GPCR ligands

Recent advances in G protein-coupled receptors have challenged traditional definitions of agonism, antagonism, affinity and efficacy. The discovery of agonist functional selectivity and receptor allosterism has meant researchers have an expanded canvas for designing and discovering novel drugs. Here we describe modes of agonism emerging from the discovery of functional selectivity and allosterism. We discuss the concept of ago-allosterism, where ligands can initiate signaling by themselves and influence the actions of another ligand at the same receptor. We introduce the concept of dualsteric ligands that consist of distinct elements which bind to each of the orthosteric and an allosteric domain on a single receptor to enhance subtype selectivity. Finally, the concept that efficacy should be defined by the activity of an endogenous ligand will be challenged by the discovery that some ligands act as 'super-agonists' in specific pathways or at certain receptor mutations.

Calcium-sensing receptor: a sensor and mediator of ischemic preconditioning in the heart

As a G protein-coupled receptor, the extracellular Ca(2+)-sensing receptor (CaSR) responds to changes not only in extracellular Ca(2+), but also to many other ligands. CaSR has been found to be expressed in the hearts and cardiovascular system. In this study, we confirmed that CaSR is expressed in mouse cardiomyocytes and showed that it is predominantly localized in caveolae. The goal of this study was to investigate whether CaSR plays a cardioprotective role in ischemic preconditioning (IPC). Hearts from C57BL/6J mice (male, 12-16 wk) were perfused in the Langendorff mode and subjected to the following treatments: 1) control perfusion; 2) perfusion with a specific CaSR antagonist, NPS2143; 3) IPC (four cycles of 5 min of global ischemia and 5 min of reperfusion); or 4) perfusion with NPS2143 before and during IPC. Following these treatments, hearts were subjected to 20 min of no-flow global ischemia and 120 min of reperfusion. Compared with control, IPC significantly improved postischemic left ventricular functional recovery and reduced infarct size. Although NPS2143 perfusion alone did not change the hemodynamic function and did not change the extent of postischemic injury, NPS2143 treatment abolished cardioprotection of IPC. Through immunoblot analysis, it was demonstrated that IPC significantly increased the levels of phosphorylated ERK1/2, AKT, and GSK-3β, which were also prevented by NPS2143 treatment. Taken together, the distribution of CaSR in caveolae along with NPS2143-blockade of IPC-induced cardioprotective signaling suggest that the activation of CaSR during IPC is cardioprotective by a process involving caveolae.

GSK-3alpha directly regulates beta-adrenergic signaling and the response of the heart to hemodynamic stress in mice

The glycogen synthase kinase-3 (GSK-3) family of serine/threonine kinases consists of 2 highly related isoforms, alpha and beta. Although GSK-3beta has an important role in cardiac development, much remains unknown about the function of either GSK-3 isoform in the postnatal heart. Herein, we present what we believe to be the first studies defining the role of GSK-3alpha in the mouse heart using gene targeting. Gsk3a(-/-) mice over 2 months of age developed progressive cardiomyocyte and cardiac hypertrophy and contractile dysfunction. Following thoracic aortic constriction in young mice, we observed enhanced hypertrophy that rapidly transitioned to ventricular dilatation and contractile dysfunction. Surprisingly, markedly impaired beta-adrenergic responsiveness was found at both the organ and cellular level. This phenotype was reproduced by acute treatment of WT cardiomyocytes with a small molecule GSK-3 inhibitor, confirming that the response was not due to a chronic adaptation to LV dysfunction. Thus, GSK-3alpha appears to be the central regulator of a striking range of essential processes, including acute and direct positive regulation of beta-adrenergic responsiveness. In the absence of GSK-3alpha, the heart cannot respond effectively to hemodynamic stress and rapidly fails. Our findings identify what we believe to be a new paradigm of regulation of beta-adrenergic signaling and raise concerns given the rapid expansion of drug development targeting GSK-3.

Arresting a transient receptor potential (TRP) channel: beta-arrestin 1 mediates ubiquitination and functional down-regulation of TRPV4

β-Arrestins, originally discovered to desensitize activated G protein-coupled receptors, (aka seven-transmembrane receptors, 7TMRs) also mediate 7TMR internalization and G protein-independent signaling via these receptors. More recently, several regulatory roles of β-arrestins for atypical 7TMRs and non-7TM receptors have emerged. Here, we uncover an entirely novel regulatory role of β-arrestins in cross-talk between the angiotensin receptor (AT1aR) and a member of the transient receptor potential (TRP) ion channel family, TRPV4. AT1aR and TRPV4 form a constitutive complex in the plasma membrane, and angiotensin stimulation leads to recruitment of β-arrestin 1 to this complex. Surprisingly, angiotensin stimulation results in ubiquitination of TRPV4, a process that requires β-arrestin 1, and subsequently to internalization and functional down-regulation of TRPV4. β-Arrestin 1 interacts with, and acts as an adaptor for AIP4, an E3 ubiquitin ligase responsible for TRPV4 ubiquitination. Thus, our data provide the first evidence of a functional link between β-arrestins and TRPV4 and uncovers an entirely novel mechanism to maintain appropriate intracellular Ca²⁺ concentration to avoid excessive Ca²⁺ signaling.

Wumeiwan treatment alters the expression of β2AR, β-arrestin 2 and NF-κB p65 in rat ulcerative colitis

AIM: To investigate the effects of Wumeiwan treatment on the expression of β2-adrenergic receptor (β2AR), β-arrestin 2, NF-κB p65 in ulcerative colitis in rats.

METHODS: Sprague-Dawley rats were randomly divided into four groups: control group, model group, mesalazine group and Wumeiwan group. Ulcerative colitis was induced in rats with 2, 4, 6-trinitrobenzenesulfonic acid. The control group and model group were administered intragastrically with 3 mL of normal saline, while the mesalazine group and Wumeiwan group were intragastrically given mesalazine at a dose of 50 mg/100 g body weight and 3 mL of Wumeiwan (0.515 g/mL), respectively. All rats were treated for 15 d. Spleen and colon tissue samples were taken to detect the expression of β2AR, β-arrestin 2 and NF-κB p65 by Western blot and immunohistochemistry.

RESULTS: The expression levels of β2AR and β-arrestin 2 were significantly lower in the model group than in the control group (12.54% ± 1.28% vs 15.28% ± 1.71%, 12.67% ± 1.42% vs 15.28% ± 1.58%). Treatment with either Wumeiwan or mesalazine significantly increased the expression of β2AR and β-arrestin 2 (16.27% ± 1.40%, 16.18% ± 1.12%; 17.05% ± 1.48%, 16.77% ± 1.40%), and there were no significant differences in the expression levels of β2AR and β-arrestin 2 between the Wumeiwan and mesalazine groups. The expression level of NF-κB p65 was significantly higher in the model group than in the control group (17.79% ± 1.24% vs 13.82% ± 1.13%). Treatment with either Wumeiwan or mesalazine significantly decreased the expression of NF-κB p65 (16.61% ± 1.42%, 15.39% ± 1.21%), and there was no significant difference in the expression level of NF-κB p65 between the Wumeiwan and mesalazine groups.

CONCLUSION: Both Wumeiwan and mesalazine have significant efficacy in the treatment of ulcerative colitis in rats.

Modulation of mu opioid receptor desensitization in peripheral sensory neurons by phosphoinositide 3-kinase gamma

G protein-coupled opioid receptors undergo desensitization after prolonged agonist exposure. Recent in vitro studies of mu-opioid receptor (MOR) signaling revealed an involvement of phosphoinositide 3-kinases (PI3K) in agonist-induced MOR desensitization. Here we document a specific role of the G protein-coupled class IB isoform PI3Kgamma in MOR desensitization in mice and isolated sensory neurons. The tail-withdrawal nociception assay evidenced a compromised morphine-induced tolerance of PI3Kgamma-deficient mice compared to wild-type animals. Consistent with a role of PI3Kgamma in MOR signaling, PI3Kgamma was expressed in a subgroup of small-diameter dorsal root ganglia (DRG) along with MOR and the transient receptor potential vanilloid type 1 (TRPV1) receptor. In isolated DRG acute stimulation of MOR blocked voltage-gated calcium currents (VGCC) in both wild-type and PI3Kgamma-deficient DRG neurons. By contrast, following long-term opioid administration the attenuating effect of MOR was strongly compromised in wild-type DRG but not in PI3Kgamma-deficient DRG. Our results uncover PI3Kgamma as an essential modulator of long-term MOR desensitization and tolerance development induced by chronic opioid treatment in sensory neurons.