Seven transmembrane receptors: something old, something new

Receptors for hormones, neurotransmitters, drugs, sensory stimuli and many other agents represent the gateway to cellular metabolism and activity. They regulate virtually all physiological processes in mammals. Yet as recently as 40 years ago their very existence was still in question. One class of receptors, those coupled to G proteins (also known as GPCRs or seven transmembrane receptors) comprise by far the largest group (approx. 1000), and are the most important target of clinically used drugs. Here I provide a very personal retrospective of research over the past 35 years which ultimately led to the identification, purification, reconstitution and cloning of the adrenergic receptors; the discovery of their homology with the seven transmembrane spanning visual light receptor rhodopsin and the realization that there was a large gene family of G protein coupled receptors; the elucidation of the molecular mechanisms of receptor desensitization and signalling through G protein-coupled receptor kinases and beta-arrestins; and the appreciation that the structure, signalling, and regulatory mechanisms of the receptors are all highly conserved across the large receptor superfamily.

Relative opioid efficacy is determined by the complements of the G protein-coupled receptor desensitization machinery

G protein-coupled receptor regulation by G protein-coupled receptor kinases and beta-arrestins can lead to desensitization and subsequent internalization of the receptor. In in vitro and cellular systems, beta-arrestins do not seem to play a major role in regulating micro opioid receptor (microOR) responsiveness. Removal of the betaarrestin2 (betaarr2) gene in mice leads paradoxically to enhanced and prolonged microOR-mediated antinociception. The betaarr2 knockout (betaarr2-KO) mice also fail to develop morphine antinociceptive tolerance in the hot-plate test, further indicating that the betaarr2 protein plays an essential role in microOR regulation in vivo. In this study, the contribution of betaarr2 to the regulation of the microOR was examined in both human embryonic kidney 293 cells and in betaarr2-KO mice after treatment with several opiate agonists. A green fluorescent protein tagged betaarr2 was used to assess receptor-betaarr2 interactions in living cells. Opiate agonists that induced robust betaarr2-green fluorescent protein translocation produced similar analgesia profiles in wild-type and betaarr2-KO mice, whereas those that do not promote robust betaarr2 recruitment, such as morphine and heroin, produce enhanced analgesia in vivo. In this report, we present a rationale to explain the seemingly paradoxical relationship between beta-arrestins and microOR regulation wherein morphine-like agonists fail to promote efficient internalization and resensitization of the receptor.

G protein-coupled receptor kinase 5 regulates airway responses induced by muscarinic receptor activation

G protein-coupled receptors (GPCRs) transduce extracellular signals into intracellular events. The waning responsiveness of GPCRs in the face of persistent agonist stimulation, or desensitization, is a necessary event that ensures physiological homeostasis. GPCR kinases (GRKs) are important regulators of GPCR desensitization. GRK5, one member of the GRK family, desensitizes central M(2) muscarinic receptors in mice. We questioned whether GRK5 might also be an important regulator of peripheral muscarinic receptor responsiveness in the cardiopulmonary system. Specifically, we wanted to determine the role of GRK5 in regulating muscarinic receptor-mediated control of airway smooth muscle tone or regulation of cholinergic-induced bradycardia. Tracheal pressure, heart rate, and tracheal smooth muscle tension were measured in mice having a targeted deletion of the GRK5 gene (GRK5(-/-)) and littermate wild-type (WT) control mice. Both in vivo and in vitro results showed that the airway contractile response to a muscarinic receptor agonist was not different between GRK5(-/-) and WT mice. However, the relaxation component of bilateral vagal stimulation and the airway smooth muscle relaxation resulting from beta(2)-adrenergic receptor activation were diminished in GRK5(-/-) mice. These data suggest that M(2) muscarinic receptor-mediated opposition of airway smooth muscle relaxation is regulated by GRK5 and is, therefore, excessive in GRK5(-/-) mice. In addition, this study shows that GRK5 regulates pulmonary responses in a tissue- and receptor-specific manner but does not regulate peripheral cardiac muscarinic receptors. GRK5 regulation of airway responses may have implications in obstructive airway diseases such as asthma or chronic obstructive pulmonary disease.

Desensitization of the isolated .beta.2-adrenergic receptor by .beta.-adrenergic receptor kinase, cAMP-dependent protein kinase, and protein kinase C occurs via distinct molecular mechanisms

Exposure of beta 2-adrenergic receptors (beta 2ARs) to agonists causes a rapid desensitization of the receptor-stimulated adenylyl cyclase response. Phosphorylation of the beta 2AR by several distinct kinases plays an important role in this desensitization phenomenon. In this study, we have utilized purified hamster lung beta 2AR and stimulatory guanine nucleotide binding regulatory protein (Gs), reconstituted in phospholipid vesicles, to investigate the molecular properties of this desensitization response. Purified hamster beta 2AR was phosphorylated by cAMP-dependent protein kinase (PKA), protein kinase C (PKC), or beta AR kinase (beta ARK), and receptor function was determined by measuring the beta 2AR-agonist-promoted Gs-associated GTPase activity. At physiological concentrations of Mg2+ (less than 1 mM), receptor phosphorylation inhibited coupling to Gs by 60% (PKA), 40% (PKC), and 30% (beta ARK). The desensitizing effect of phosphorylation was, however, greatly diminished when assays were performed at concentrations of Mg2+ sufficient to promote receptor-independent activation of Gs (greater than 5 mM). Addition of retinal arrestin, the light transduction component involved in the attenuation of rhodopsin function, did not enhance the uncoupling effect of beta ARK phosphorylation of beta 2AR when assayed in the presence of 0.3 mM free Mg2+. At concentrations of Mg2+ ranging between 0.5 and 5.0 mM, however, significant potentiation of beta ARK-mediated desensitization was observed upon arrestin addition. At a free Mg2+ concentration of 5 mM, arrestin did not potentiate the inhibition of receptor function observed on PKA or PKC phosphorylation. These results suggest that distinct pathways of desensitization exist for the receptor phosphorylated either by PKA or PKC or alternatively by beta ARK.

beta-Arrestin1 modulates lymphoid enhancer factor transcriptional activity through interaction with phosphorylated dishevelled proteins

One aspect of the function of the beta-arrestins is to serve as scaffold or adapter molecules coupling G-protein coupled receptors (GPCRs) to signal transduction pathways distinct from traditional second messenger pathways. Here we report the identification of Dishevelled 1 and Dishevelled 2 (Dvl1 and Dvl2) as beta-arrestin1 (betaarr1) interacting proteins. Dvl proteins participate as key intermediates in signal transmission from the seven membrane-spanning Frizzled receptors leading to inhibition of glycogen synthase kinase-3beta (GSK-3beta), stabilization of beta-catenin, and activation of the lymphoid enhancer factor (LEF) transcription factor. We find that phosphorylation of Dvl strongly enhances its interaction with betaarr1, suggesting that regulation of Dvl phosphorylation and subsequent interaction with betaarr1 may play a key role in the activation of the LEF transcription pathway. Because coexpression of the Dvl kinases, CK1epsilon and PAR-1, with Dvl synergistically activates LEF reporter gene activity, we reasoned that coexpression of betaarr1 with Dvl might also affect LEF-dependent gene activation. Interestingly, whereas betaarr1 or Dvl alone leads to low-level stimulation of LEF (2- to 5-fold), coexpression of betaarr1 with either Dvl1 or Dvl2 leads to a synergistic activation of LEF (up to 16-fold). Additional experiments with LiCl as an inhibitor of GSK-3beta kinase activity indicate that the step affected by betaarr1 is upstream of GSK-3beta and most likely at the level of Dvl. These results identify betaarr1 as a regulator of Dvl-dependent LEF transcription and suggest that betaarr1 might serve as an adapter molecule that can couple Frizzled receptors and perhaps other GPCRs to these important transcription pathways.

Binding of the beta2 adrenergic receptor to N-ethylmaleimide-sensitive factor regulates receptor recycling

Following agonist stimulation, most G protein-coupled receptors become desensitized and are internalized, either to be degraded or recycled back to the cell surface. What determines the fate of a specific receptor type after it is internalized is poorly understood. Here we show that the rapidly recycling beta2 adrenergic receptor (beta2AR) binds via a determinant including the last three amino acids in its carboxyl-terminal tail to the membrane fusion regulatory protein, N-ethylmaleimide-sensitive factor (NSF). This is documented by in vitro overlay assays and by cellular coimmunoprecipitations. Receptors bearing mutations in any of the last three residues fail to interact with NSF. After stimulation with the agonist isoproterenol, a green fluorescent protein fusion of NSF colocalizes with the wild type beta2AR but not with a tail-mutated beta2AR. The beta2AR-NSF interaction is required for efficient internalization of the receptors and for their recycling to the cell surface. Mutations in the beta2AR tail that ablate NSF binding reduce the efficiency of receptor internalization upon agonist stimulation. Upon subsequent treatment of cells with the antagonist propranolol, wild type receptors return to the cell surface, while tail-mutated receptors remain sequestered. Thus, the direct binding of the beta2AR to NSF demonstrates how, after internalization, the fate of a receptor is reliant on a specific interaction with a component of the cellular membrane-trafficking machinery.

beta -Arrestin-mediated recruitment of the Src family kinase Yes mediates endothelin-1-stimulated glucose transport

The insulin and the endothelin type A (ETA) receptor both can couple into the heterotrimeric G protein alpha(q/11) (Galpha(q/11)), leading to Galpha(q/11) tyrosine phosphorylation, phosphatidylinositol 3-kinase activation, and subsequent stimulation of glucose transport. In this study, we assessed the potential role of Src kinase in ET-1 signaling to glucose transport in 3T3-L1 adipocytes. Src kinase inhibitor PP2 blocked ET-1-induced Src kinase activity, Galpha(q/11) tyrosine phosphorylation, and glucose transport stimulation. To determine which Src family kinase member was involved, we microinjected anti-c-Src, -c-Fyn, or -c-Yes antibody into these cells and found that only anti-c-Yes antibody blocked GLUT4 translocation (70% decreased). Overexpression or microinjection of a dominant negative mutant (K298M) of Src kinase also inhibited ET-1-induced Galpha(q/11) tyrosine phosphorylation and GLUT4 translocation. In co-immunoprecipitation experiments, we found that beta-arrestin 1 associated with the ETA receptor in an agonist-dependent manner and that beta-arrestin 1 recruited Src kinase to a molecular complex that included the ETA receptor. Microinjection of beta-arrestin 1 antibody inhibited ET-1- but not insulin-stimulated GLUT4 translocation. In conclusion, 1) the Src kinase Yes can induce tyrosine phosphorylation of Galpha(q/11) in response to ET-1 stimulation, and 2) beta-arrestin 1 and Src kinase form a molecular complex with the ETA receptor to mediate ET-1 signaling to Galpha(q/11) with subsequent glucose transport stimulation.

Augmentation of cardiac contractility mediated by the human beta(3)-adrenergic receptor overexpressed in the hearts of transgenic mice

BACKGROUND

Stimulation of beta(1)- and beta(2)-adrenergic receptors (ARs) in the heart results in positive inotropy. In contrast, it has been reported that the beta(3)AR is also expressed in the human heart and that its stimulation leads to negative inotropic effects.

METHODS AND RESULTS

To better understand the role of beta(3)ARs in cardiac function, we generated transgenic mice with cardiac-specific overexpression of 330 fmol/mg protein of the human beta(3)AR (TGbeta(3) mice). Hemodynamic characterization was performed by cardiac catheterization in closed-chest anesthetized mice, by pressure-volume-loop analysis, and by echocardiography in conscious mice. After propranolol blockade of endogenous beta(1)- and beta(2)ARs, isoproterenol resulted in an increase in contractility in the TGbeta(3) mice (30%), with no effect in wild-type mice. Similarly, stimulation with the selective human beta(3)AR agonist L-755,507 significantly increased contractility in the TGbeta(3) mice (160%), with no effect in wild-type mice, as determined by hemodynamic measurements and by end-systolic pressure-volume relations. The underlying mechanism of the positive inotropy incurred with L-755,507 in the TGbeta(3) mice was investigated in terms of beta(3)AR-G-protein coupling and adenylyl cyclase activation. Stimulation of cardiac membranes from TGbeta(3) mice with L-755,507 resulted in a pertussis toxin-insensitive 1.33-fold increase in [(35)S]GTPgammaS loading and a 1.6-fold increase in adenylyl cyclase activity.

CONCLUSIONS

Cardiac overexpression of human beta(3)ARs results in positive inotropy only on stimulation with a beta(3)AR agonist. Overexpressed beta(3)ARs couple to G(s) and activate adenylyl cyclase on agonist stimulation.

beta-Arrestin-mediated ADP-ribosylation factor 6 activation and beta 2-adrenergic receptor endocytosis

beta-Arrestins are multifunctional adaptor proteins known to regulate internalization of agonist-stimulated G protein-coupled receptors by linking them to endocytic proteins such as clathrin and AP-2. Here we describe a previously unappreciated mechanism by which beta-arrestin orchestrates the process of receptor endocytosis through the activation of ADP-ribosylation factor 6 (ARF6), a small GTP-binding protein. Involvement of ARF6 in the endocytic process is demonstrated by the ability of GTP-binding defective and GTP hydrolysis-deficient mutants to inhibit internalization of the beta(2)-adrenergic receptor. The importance of regulation of ARF6 function is shown by the ability of the ARF GTPase-activating protein GIT1 to inhibit and of the ARF nucleotide exchange factor, ARNO, to enhance receptor endocytosis. Endogenous beta-arrestin is found in complex with ARNO. Upon agonist stimulation of the receptor, beta-arrestin also interacts with the GDP-liganded form of ARF6, thereby facilitating ARNO-promoted GTP loading and activation of the G protein. Thus, the agonist-driven formation of a complex including beta-arrestin, ARNO, and ARF6 provides a molecular mechanism that explains how the agonist-stimulated receptor recruits a small G protein necessary for the endocytic process and controls its activation.

Regulation of receptor fate by ubiquitination of activated beta 2-adrenergic receptor and beta-arrestin

Although trafficking and degradation of several membrane proteins are regulated by ubiquitination catalyzed by E3 ubiquitin ligases, there has been little evidence connecting ubiquitination with regulation of mammalian G protein (heterotrimeric guanine nucleotide-binding protein)-coupled receptor (GPCR) function. Agonist stimulation of endogenous or transfected beta2-adrenergic receptors (beta2ARs) led to rapid ubiquitination of both the receptors and the receptor regulatory protein, beta-arrestin. Moreover, proteasome inhibitors reduced receptor internalization and degradation, thus implicating a role for the ubiquitination machinery in the trafficking of the beta2AR. Receptor ubiquitination required beta-arrestin, which bound to the E3 ubiquitin ligase Mdm2. Abrogation of beta-arrestin ubiquitination, either by expression in Mdm2-null cells or by dominant-negative forms of Mdm2 lacking E3 ligase activity, inhibited receptor internalization with marginal effects on receptor degradation. However, a beta2AR mutant lacking lysine residues, which was not ubiquitinated, was internalized normally but was degraded ineffectively. These findings delineate an adapter role of beta-arrestin in mediating the ubiquitination of the beta2AR and indicate that ubiquitination of the receptor and of beta-arrestin have distinct and obligatory roles in the trafficking and degradation of this prototypic GPCR.

Classical and new roles of beta-arrestins in the regulation of G-protein-coupled receptors

In the classical model of G-protein-coupled receptor (GPCR) regulation, arrestins terminate receptor signalling. After receptor activation, arrestins desensitize phosphorylated GPCRs, blocking further activation and initiating receptor internalization. This function of arrestins is exemplified by studies on the role of arrestins in the development of tolerance to, but not dependence on, morphine. Arrestins also link GPCRs to several signalling pathways, including activation of the non-receptor tyrosine kinase SRC and mitogen-activated protein kinase. In these cascades, arrestins function as adaptors and scaffolds, bringing sequentially acting kinases into proximity with each other and the receptor. The signalling roles of arrestins have been expanded even further with the discovery that the formation of stable receptor-arrestin complexes initiates photoreceptor apoptosis in Drosophila, leading to retinal degeneration. Here we review our current understanding of arrestin function, discussing both its classical and newly discovered roles.

Beta-Arrestins: new roles in regulating heptahelical receptors' functions

The last few years have seen a marked expansion in appreciation of the diversity of roles played by the betaArrestins in regulating GPCR functions. Originally discovered as molecules that desensitize such receptors, the roles of betaArrestins have expanded to include acting as signalling adapters or intermediates that recruit other key molecules to the GPCRs in an agonist-regulated fashion. For example, interactions with components of the endocytic machinery, such as clathrin, the adapter protein AP-2 and the N-ethylmaleimide sensitive fusion protein (NSF), demonstrate the ability of betaArrestins to act as adapters to facilitate the clathrin-mediated endocytosis of certain members of the GPCR family. BetaArrestins have also been shown to serve as signalling molecules. The Ras-dependent activation of ERK1/2 may involve the betaArrestin-dependent recruitment of c-Src to the beta2-adrenergic receptor (beta2-AR). More recently, betaArrestins have been shown to act as molecular scaffolds that coordinate the assembly of certain MAP kinase complexes that lead to the stimulation of either ERK1/2 or JNK3. Finally, long-term accumulation of arrestin-rhodopsin complexes, in photoreceptor cells has been shown to trigger apoptosis.

Regulation of myocardial betaARK1 expression in catecholamine-induced cardiac hypertrophy in transgenic mice overexpressing alpha1B-adrenergic receptors

OBJECTIVES

Using a transgenic mouse model of myocardial-targeted overexpression of the wild-type alpha1B adrenergic receptor (AR) (Tg alpha43), we studied the role of the betaAR kinase (betaARK1) in the evolution of myocardial hypertrophy and its transition to heart failure (HF).

BACKGROUND

Increased myocardial expression of betaARK1 has been shown to be associated with HF and certain models of hypertrophy.

METHODS

Tg alpha43 mice and their nontransgenic littermate controls were treated with the alpha1AR agonist phenylephrine (PE) for 3, 7 or 14 days to characterize the cardiac consequences.

RESULTS

Nontransgenic littermate control mice treated for 14 days with PE display cardiac hypertrophy with no increase in betaARK1 expression. However, Tg alpha43 animals show a reduced tolerance to 14-day PE treatment, demonstrated by reduced survival and severe cardiac hypertrophy. Moreover, PE treatment for three and seven days in Tg alpha43 mice resulted in an exaggerated hypertrophic response accompanied by significant cardiac biochemical abnormalities that are normally associated with HF, including fetal gene expression, reduced betaAR density and enhanced betaARK1 expression. We also found reduced myocardial stores of the sympathetic neurotransmitter neuropeptide Y.

CONCLUSIONS

These data suggest that PE-treated Tg alpha43 mice have chronic activation of the cardiac sympathetic nervous system, which may be responsible for the appearance of apparent maladaptive hypertrophy with an evolution towards HF and sudden death. Thus, the cardiac phenotypes found in these mice are not the direct result of enhanced alpha1B AR signaling and suggest that betaARK1 is a key molecule in the transition of myocardial hypertrophy to HF.

Epidermal growth factor (EGF) receptor-dependent ERK activation by G protein-coupled receptors: a co-culture system for identifying intermediates upstream and downstream of heparin-binding EGF shedding

"Transactivation" of epidermal growth factor receptors (EGFRs) in response to activation of many G protein-coupled receptors (GPCRs) involves autocrine/paracrine shedding of heparin-binding EGF (HB-EGF). HB-EGF shedding involves proteolytic cleavage of a membrane-anchored precursor by incompletely characterized matrix metalloproteases. In COS-7 cells, alpha(2A)-adrenergic receptors (ARs) stimulate ERK phosphorylation via two distinct pathways, a transactivation pathway that involves the release of HB-EGF and the EGFR and an alternate pathway that is independent of both HB-EGF and the EGFR. We have developed a mixed culture system to study the mechanism of GPCR-mediated HB-EGF shedding in COS-7 cells. In this system, alpha(2A)AR expressing "donor" cells are co-cultured with "acceptor" cells lacking the alpha(2A)AR. Each population expresses a uniquely epitope-tagged ERK2 protein, allowing the selective measurement of ERK activation in the donor and acceptor cells. Stimulation with the alpha(2)AR selective agonist UK14304 rapidly increases ERK2 phosphorylation in both the donor and the acceptor cells. The acceptor cell response is sensitive to inhibitors of both the EGFR and HB-EGF, indicating that it results from the release of HB-EGF from the alpha(2A)AR-expressing donor cells. Experiments with various chemical inhibitors and dominant inhibitory mutants demonstrate that EGFR-dependent activation of the ERK cascade after alpha(2A)AR stimulation requires Gbetagamma subunits upstream and dynamin-dependent endocytosis downstream of HB-EGF shedding and EGFR activation, whereas Src kinase activity is required both for the release of HB-EGF and for HB-EGF-mediated ERK2 phosphorylation.

Identification of a motif in the carboxyl terminus of beta -arrestin2 responsible for activation of JNK3

Accumulating evidence indicates that the beta-arrestins act as scaffold molecules that couple G-protein-coupled receptors to mitogen-activated protein (MAP) kinase signaling pathways. Recently, we identified the c-Jun N-terminal kinase 3 (JNK3) as a beta-arrestin2-interacting protein in yeast-two hybrid and co-immunoprecipitation studies. Beta-arrestin2 acts as a scaffold to enhance signaling to JNK3 stimulated by overexpression of the MAP3 kinase ASK1 or by agonist activation of the angiotensin 1A receptor. Whereas beta-arrestin2 is a very strong activator of JNK3 signaling, beta-arrestin1 is very weak in this regard. The data also indicate that the specific step enhanced by beta-arrestin2 involves phosphorylation of JNK3 by the MAP2 kinase MKK4. We reasoned that defining the region (or domain) in beta-arrestin2 responsible for high level JNK3 activation would provide insight into the mechanism by which beta-arrestin2 enhances the activity of this signaling pathway. Using chimeric beta-arrestins, we have determined that sequences in the carboxyl-terminal region of beta-arrestin2 are important for the enhancement of JNK3 phosphorylation. More detailed analysis of the carboxyl-terminal domains of the beta-arrestins indicated that beta-arrestin2, but not beta-arrestin1, contains a sequence (RRSLHL) highly homologous to the conserved docking motif present in many MAP kinase-binding proteins. Replacement of the beta-arrestin2 RRS residues with the corresponding KP residues present in beta-arrestin1 dramatically reduced both JNK3 interaction and enhancement of JNK3 phosphorylation. Conversely, replacement of the KP residues in beta-arrestin1 with RRS significantly increased both JNK3 binding and enhancement of JNK3 phosphorylation. These results delineate a mechanism by which beta-arrestin2 functions as a scaffold protein in the JNK3 signaling pathway and implicate the conserved docking site in beta-arrestin2 as an important factor in binding JNK3 and stimulating the phosphorylation of JNK3 by MKK4.

Cardiac beta ARK1 inhibition prolongs survival and augments beta blocker therapy in a mouse model of severe heart failure

Chronic human heart failure is characterized by abnormalities in beta-adrenergic receptor (betaAR) signaling, including increased levels of betaAR kinase 1 (betaARK1), which seems critical to the pathogenesis of the disease. To determine whether inhibition of betaARK1 is sufficient to rescue a model of severe heart failure, we mated transgenic mice overexpressing a peptide inhibitor of betaARK1 (betaARKct) with transgenic mice overexpressing the sarcoplasmic reticulum Ca(2+)-binding protein, calsequestrin (CSQ). CSQ mice have a severe cardiomyopathy and markedly shortened survival (9 +/- 1 weeks). In contrast, CSQ/betaARKct mice exhibited a significant increase in mean survival age (15 +/- 1 weeks; P < 0.0001) and showed less cardiac dilation, and cardiac function was significantly improved (CSQ vs. CSQ/betaARKct, left ventricular end diastolic dimension 5.60 +/- 0.17 mm vs. 4.19 +/- 0.09 mm, P < 0.005; % fractional shortening, 15 +/- 2 vs. 36 +/- 2, P < 0.005). The enhancement of the survival rate in CSQ/betaARKct mice was substantially potentiated by chronic treatment with the betaAR antagonist metoprolol (CSQ/betaARKct nontreated vs. CSQ/betaARKct metoprolol treated, 15 +/- 1 weeks vs. 25 +/- 2 weeks, P < 0.0001). Thus, overexpression of the betaARKct resulted in a marked prolongation in survival and improved cardiac function in a mouse model of severe cardiomyopathy that can be potentiated with beta-blocker therapy. These data demonstrate a significant synergy between an established heart-failure treatment and the strategy of betaARK1 inhibition.

Regulation of membrane targeting of the G protein-coupled receptor kinase 2 by protein kinase A and its anchoring protein AKAP79

The beta2 adrenergic receptor (beta2AR) undergoes desensitization by a process involving its phosphorylation by both protein kinase A (PKA) and G protein-coupled receptor kinases (GRKs). The protein kinase A-anchoring protein AKAP79 influences beta2AR phosphorylation by complexing PKA with the receptor at the membrane. Here we show that AKAP79 also regulates the ability of GRK2 to phosphorylate agonist-occupied receptors. In human embryonic kidney 293 cells, overexpression of AKAP79 enhances agonist-induced phosphorylation of both the beta2AR and a mutant of the receptor that cannot be phosphorylated by PKA (beta2AR/PKA-). Mutants of AKAP79 that do not bind PKA or target to the beta2AR markedly inhibit phosphorylation of beta2AR/PKA-. We show that PKA directly phosphorylates GRK2 on serine 685. This modification increases Gbetagamma subunit binding to GRK2 and thus enhances the ability of the kinase to translocate to the membrane and phosphorylate the receptor. Abrogation of the phosphorylation of serine 685 on GRK2 by mutagenesis (S685A) or by expression of a dominant negative AKAP79 mutant reduces GRK2-mediated translocation to beta2AR and phosphorylation of agonist-occupied beta2AR, thus reducing subsequent receptor internalization. Agonist-stimulated PKA-mediated phosphorylation of GRK2 may represent a mechanism for enhancing receptor phosphorylation and desensitization.

Alterations in cardiac adrenergic signaling and calcium cycling differentially affect the progression of cardiomyopathy

The medical treatment of chronic heart failure has undergone a dramatic transition in the past decade. Short-term approaches for altering hemodynamics have given way to long-term, reparative strategies, including beta-adrenergic receptor (betaAR) blockade. This was once viewed as counterintuitive, because acute administration causes myocardial depression. Cardiac myocytes from failing hearts show changes in betaAR signaling and excitation-contraction coupling that can impair cardiac contractility, but the role of these abnormalities in the progression of heart failure is controversial. We therefore tested the impact of different manipulations that increase contractility on the progression of cardiac dysfunction in a mouse model of hypertrophic cardiomyopathy. High-level overexpression of the beta(2)AR caused rapidly progressive cardiac failure in this model. In contrast, phospholamban ablation prevented systolic dysfunction and exercise intolerance, but not hypertrophy, in hypertrophic cardiomyopathy mice. Cardiac expression of a peptide inhibitor of the betaAR kinase 1 not only prevented systolic dysfunction and exercise intolerance but also decreased cardiac remodeling and hypertrophic gene expression. These three manipulations of cardiac contractility had distinct effects on disease progression, suggesting that selective modulation of particular aspects of betaAR signaling or excitation-contraction coupling can provide therapeutic benefit.

Expanding roles for beta-arrestins as scaffolds and adapters in GPCR signaling and trafficking

beta-arrestins play previously unsuspected and important roles as adapters and scaffolds that localize signaling proteins to ligand-activated G-protein-coupled receptors. As with the paradigmatic role of the beta-arrestins in uncoupling receptors from G proteins (desensitization), these novel functions involve the interaction of beta-arrestin with phosphorylated heptahelical receptors. beta-arrestins interact with at least two main classes of signaling proteins. First, interaction with molecules such as clathrin, AP-2 and NSF directs the clathrin-mediated internalization of G-protein-coupled receptors. Second, interaction with molecules such as Src, Raf, Erk, ASK1 and JNK3 appears to regulate several pathways that result in the activation of MAP kinases. These recent discoveries indicate that the beta-arrestins play widespread roles as scaffolds and/or adapter molecules that organize a variety of complex signaling pathways emanating from heptahelical receptors. It is likely that additional roles for the beta-arrestins remain to be discovered.

New mechanisms in heptahelical receptor signaling to mitogen activated protein kinase cascades

Activation of classical second messenger cascades cannot fully explain the recently appreciated roles of heptahelical, or G-protein coupled receptors (GPCRs), in stimulation of mitogen activated protein kinase (MAPK) cascades. Rather, several distinct signaling mechanisms appear to contribute to GPCR-mediated MAPK activation. These include transactivation of the Epidermal Growth Factor Receptor (EGFR) via the autocrine/paracrine release of EGF-like ligands at the cell surface and scaffolding of MAPK cascades. A significant advance in the understanding of how GPCRs activate MAPK cascades is the discovery that beta-arrestin, a protein well known for its roles in both receptor desensitization and internalization, serves as a scaffolding protein for at least two GPCR stimulated MAPK cascades, the extracellular signal regulated kinase (ERK) cascade and the c-jun N-terminal kinase 3 (JNK3) cascade. Together, these novel mechanisms of GPCR-mediated MAPK regulation may permit GPCRs in specific situations to control the temporal and spatial activity of MAPKs and thereby determine the consequences of GPCR stimulation with respect to transcriptional activation, cell proliferation and apoptosis.

In vivo ventricular gene delivery of a beta-adrenergic receptor kinase inhibitor to the failing heart reverses cardiac dysfunction

BACKGROUND

Genetic manipulation to reverse molecular abnormalities associated with dysfunctional myocardium may provide novel treatment. This study aimed to determine the feasibility and functional consequences of in vivo beta-adrenergic receptor kinase (betaARK1) inhibition in a model of chronic left ventricular (LV) dysfunction after myocardial infarction (MI).

METHODS AND RESULTS

Rabbits underwent ligation of the left circumflex (LCx) marginal artery and implantation of sonomicrometric crystals. Baseline cardiac physiology was studied 3 weeks after MI; 5x10(11) viral particles of adenovirus was percutaneously delivered through the LCx. Animals received transgenes encoding a peptide inhibitor of betaARK1 (Adeno-betaARKct) or an empty virus (EV) as control. One week after gene delivery, global LV and regional systolic function were measured again to assess gene treatment. Adeno-betaARKct delivery to the failing heart through the LCx resulted in chamber-specific expression of the betaARKct. Baseline in vivo LV systolic performance was improved in Adeno-betaARKct-treated animals compared with their individual pre-gene delivery values and compared with EV-treated rabbits. Total beta-AR density and betaARK1 levels were unchanged between treatment groups; however, beta-AR-stimulated adenylyl cyclase activity in the LV was significantly higher in Adeno-betaARKct-treated rabbits compared with EV-treated animals.

CONCLUSIONS

In vivo delivery of Adeno-betaARKct is feasible in the infarcted/failing heart by coronary catheterization; expression of betaARKct results in marked reversal of ventricular dysfunction. Thus, inhibition of betaARK1 provides a novel treatment strategy for improving the cardiac performance of the post-MI heart.

Activation and targeting of extracellular signal-regulated kinases by beta-arrestin scaffolds

Using both confocal immunofluorescence microscopy and biochemical approaches, we have examined the role of beta-arrestins in the activation and targeting of extracellular signal-regulated kinase 2 (ERK2) following stimulation of angiotensin II type 1a receptors (AT1aR). In HEK-293 cells expressing hemagglutinin-tagged AT1aR, angiotensin stimulation triggered beta-arrestin-2 binding to the receptor and internalization of AT1aR-beta-arrestin complexes. Using red fluorescent protein-tagged ERK2 to track the subcellular distribution of ERK2, we found that angiotensin treatment caused the redistribution of activated ERK2 into endosomal vesicles that also contained AT1aR-beta-arrestin complexes. This targeting of ERK2 reflects the formation of multiprotein complexes containing AT1aR, beta-arrestin-2, and the component kinases of the ERK cascade, cRaf-1, MEK1, and ERK2. Myc-tagged cRaf-1, MEK1, and green fluorescent protein-tagged ERK2 coprecipitated with Flag-tagged beta-arrestin-2 from transfected COS-7 cells. Coprecipitation of cRaf-1 with beta-arrestin-2 was independent of MEK1 and ERK2, whereas the coprecipitation of MEK1 and ERK2 with beta-arrestin-2 was significantly enhanced in the presence of overexpressed cRaf-1, suggesting that binding of cRaf-1 to beta-arrestin facilitates the assembly of a cRaf-1, MEK1, ERK2 complex. The phosphorylation of ERK2 in beta-arrestin complexes was markedly enhanced by coexpression of cRaf-1, and this effect is blocked by expression of a catalytically inactive dominant inhibitory mutant of MEK1. Stimulation with angiotensin increased the binding of both cRaf-1 and ERK2 to beta-arrestin-2, and the association of beta-arrestin-2, cRaf-1, and ERK2 with AT1aR. These data suggest that beta-arrestins function both as scaffolds to enhance cRaf-1 and MEK-dependent activation of ERK2, and as targeting proteins that direct activated ERK to specific subcellular locations.

Dual modulation of cell survival and cell death by beta(2)-adrenergic signaling in adult mouse cardiac myocytes

The goal of this study was to determine whether beta(1)-adrenergic receptor (AR) and beta(2)-AR differ in regulating cardiomyocyte survival and apoptosis and, if so, to explore underlying mechanisms. One potential mechanism is that cardiac beta(2)-AR can activate both G(s) and G(i) proteins, whereas cardiac beta(1)-AR couples only to G(s). To avoid complicated crosstalk between beta-AR subtypes, we expressed beta(1)-AR or beta(2)-AR individually in adult beta(1)/beta(2)-AR double knockout mouse cardiac myocytes by using adenoviral gene transfer. Stimulation of beta(1)-AR, but not beta(2)-AR, markedly induced myocyte apoptosis, as indicated by increased terminal deoxynucleotidyltransferase-mediated UTP end labeling or Hoechst staining positive cells and DNA fragmentation. In contrast, beta(2)-AR (but not beta(1)-AR) stimulation elevated the activity of Akt, a powerful survival signal; this effect was fully abolished by inhibiting G(i), G(beta gamma), or phosphoinositide 3 kinase (PI3K) with pertussis toxin, beta ARK-ct (a peptide inhibitor of G(beta gamma)), or LY294002, respectively. This indicates that beta(2)-AR activates Akt via a G(i)-G(beta gamma)-PI3K pathway. More importantly, inhibition of the G(i)-G(beta gamma)-PI3K-Akt pathway converts beta(2)-AR signaling from survival to apoptotic. Thus, stimulation of a single class of receptors, beta(2)-ARs, elicits concurrent apoptotic and survival signals in cardiac myocytes. The survival effect appears to predominate and is mediated by the G(i)-G(beta gamma)-PI3K-Akt signaling pathway.

-Arrestin 1 and 2 differentially regulate heptahelical receptor signaling and trafficking

The two widely coexpressed isoforms of beta-arrestin (termed beta arrestin 1 and 2) are highly similar in amino acid sequence. The beta-arrestins bind phosphorylated heptahelical receptors to desensitize and target them to clathrin-coated pits for endocytosis. To better define differences in the roles of beta-arrestin 1 and 2, we prepared mouse embryonic fibroblasts from knockout mice that lack one of the beta-arrestins (beta arr1-KO and beta arr2-KO) or both (beta arr1/2-KO), as well as their wild-type (WT) littermate controls. These cells were analyzed for their ability to support desensitization and sequestration of the beta(2)-adrenergic receptor (beta(2)-AR) and the angiotensin II type 1A receptor (AT(1A)-R). Both beta arr1-KO and beta arr2-KO cells showed similar impairment in agonist-stimulated beta(2)-AR and AT(1A)-R desensitization, when compared with their WT control cells, and the beta arr1/2-KO cells were even further impaired. Sequestration of the beta(2)-AR in the beta arr2-KO cells was compromised significantly (87% reduction), whereas in the beta arr1-KO cells it was not. Agonist-stimulated internalization of the AT(1A)-R was only slightly reduced in the beta arr1-KO but was unaffected in the beta arr2-KO cells. In the beta arr1/2-KO cells, the sequestration of both receptors was dramatically reduced. Comparison of the ability of the two beta-arrestins to sequester the beta(2)-AR revealed beta-arrestin 2 to be 100-fold more potent than beta-arrestin 1. Down-regulation of the beta(2)-AR was also prevented in the beta arr1/2-KO cells, whereas no change was observed in the single knockout cells. These findings suggest that sequestration of various heptahelical receptors is regulated differently by the two beta-arrestins, whereas both isoforms are capable of supporting receptor desensitization and down-regulation.

Arrestins as signaling molecules involved in apoptotic pathways: a real eye opener

Recent data suggest that internalized receptor and arrestin complexes are actively involved in signal transduction. Miller and Lefkowitz discuss evidence from the Drosophila visual system that suggests that intracellular rhodopsin and arrestin2 complexes induce apoptosis. Experiments with activated mammalian G protein-coupled receptor and arrestin complexes point to a mechanism by which proliferative or proapoptotic signals can be mediated largely independent from G protein activation.

Prospects for cardiovascular research

Cardiovascular disease is the greatest threat to human life and health. The past decade has seen remarkable progress in clinical and basic cardiovascular research, and many areas of opportunity are promising. The pace of current progress in clinical and basic research is such that remarkable improvement in the quality and length of life for those at risk for cardiovascular disease is likely.

beta 1-adrenergic receptor association with PSD-95. Inhibition of receptor internalization and facilitation of beta 1-adrenergic receptor interaction with N-methyl-D-aspartate receptors

The beta(1)-adrenergic receptor (beta(1)AR) is the most abundant subtype of beta-adrenergic receptor in the mammalian brain and is known to potently regulate synaptic plasticity. To search for potential neuronal beta(1)AR-interacting proteins, we screened a rat brain cDNA library using the beta(1)AR carboxyl terminus (beta(1)AR-CT) as bait in the yeast two-hybrid system. These screens identified PSD-95, a multiple PDZ domain-containing scaffolding protein, as a specific binding partner of the beta(1)AR-CT. This interaction was confirmed by in vitro fusion protein pull-down and blot overlay experiments, which demonstrated that the beta(1)AR-CT binds specifically to the third PDZ domain of PSD-95. Furthermore, the full-length beta(1)AR associates with PSD-95 in cells, as determined by co-immunoprecipitation experiments and immunofluorescence co-localization studies. The interaction between beta(1)AR and PSD-95 is mediated by the last few amino acids of the beta(1)AR, and mutation of the beta(1)AR carboxyl terminus eliminated the binding and disrupted the co-localization of the beta(1)AR and PSD-95 in cells. Agonist-induced internalization of the beta(1)AR in HEK-293 cells was markedly attenuated by PSD-95 co-expression, whereas co-expression of PSD-95 has no significant effect on either desensitization of the beta(1)AR or beta(1)AR-induced cAMP accumulation. Furthermore, PSD-95 facilitated the formation of a complex between the beta(1)AR and N-methyl-d-aspartate receptors, as assessed by co-immunoprecipitation. These data reveal that PSD-95 is a specific beta(1)AR binding partner that modulates beta(1)AR function and facilitates physical association of the beta(1)AR with synaptic proteins, such as the N-methyl-d-aspartate receptors, which are known to be regulated by beta(1)AR stimulation.

Direct binding of activated c-Src to the beta 3-adrenergic receptor is required for MAP kinase activation

Both beta(2)- and beta(3)-adrenergic receptors (ARs) are able to activate the extracellular signal-regulated kinase (ERK) pathway. We previously showed that c-Src is required for ERK activation by beta(2)AR and that it is recruited to activated beta(2)AR through binding of the Src homology 3 (SH3) domain to proline-rich regions of the adapter protein beta-arrestin1. Despite the absence of sites for phosphorylation and beta-arrestin binding, ERK activation by beta(3)AR still requires c-Src. Agonist activation of beta(2)AR, but not beta(3)AR, led to redistribution of green fluorescent protein-tagged beta-arrestin to the plasma membrane. In beta-arrestin-deficient COS-7 cells, beta-agonist-dependent co-precipitation of c-Src with the beta(2)AR required exogenous beta-arrestin, but activated beta(3)AR co-precipitated c-Src in the absence or presence of beta-arrestin. ERK activation and Src co-precipitation with beta(3)AR also occurred in adipocytes in an agonist-dependent and pertussis toxin-sensitive manner. Protein interaction studies show that the beta(3)AR interacts directly with the SH3 domain of Src through proline-rich motifs (PXXP) in the third intracellular loop and the carboxyl terminus. ERK activation and Src co-precipitation were abolished in cells expressing point mutations in these PXXP motifs. Together, these data describe a novel mechanism of ERK activation by a G protein-coupled receptor in which the intracellular domains directly recruit c-Src.

Mu-opioid receptor desensitization by beta-arrestin-2 determines morphine tolerance but not dependence

Morphine is a powerful pain reliever, but also a potent inducer of tolerance and dependence. The development of opiate tolerance occurs on continued use of the drug such that the amount of drug required to elicit pain relief must be increased to compensate for diminished responsiveness. In many systems, decreased responsiveness to agonists has been correlated with the desensitization of G-protein-coupled receptors. In vitro evidence indicates that this process involves phosphorylation of G-protein-coupled receptors and subsequent binding of regulatory proteins called beta-arrestins. Using a knockout mouse lacking beta-arrestin-2 (beta arr2-/-), we have assessed the contribution of desensitization of the mu-opioid receptor to the development of morphine antinociceptive tolerance and the subsequent onset of physical dependence. Here we show that in mice lacking beta-arrestin-2, desensitization of the mu-opioid receptor does not occur after chronic morphine treatment, and that these animals fail to develop antinociceptive tolerance. However, the deletion of beta-arrestin-2 does not prevent the chronic morphine-induced up-regulation of adenylyl cyclase activity, a cellular marker of dependence, and the mutant mice still become physically dependent on the drug.

Beta-arrestin 2: a receptor-regulated MAPK scaffold for the activation of JNK3

beta-Arrestins, originally discovered in the context of heterotrimeric guanine nucleotide binding protein-coupled receptor (GPCR) desensitization, also function in internalization and signaling of these receptors. We identified c-Jun amino-terminal kinase 3 (JNK3) as a binding partner of beta-arrestin 2 using a yeast two-hybrid screen and by coimmunoprecipitation from mouse brain extracts or cotransfected COS-7 cells. The upstream JNK activators apoptosis signal-regulating kinase 1 (ASK1) and mitogen-activated protein kinase (MAPK) kinase 4 were also found in complex with beta-arrestin 2. Cellular transfection of beta-arrestin 2 caused cytosolic retention of JNK3 and enhanced JNK3 phosphorylation stimulated by ASK1. Moreover, stimulation of the angiotensin II type 1A receptor activated JNK3 and triggered the colocalization of beta-arrestin 2 and active JNK3 to intracellular vesicles. Thus, beta-arrestin 2 acts as a scaffold protein, which brings the spatial distribution and activity of this MAPK module under the control of a GPCR.

Platelet-derived growth factor receptor association with Na(+)/H(+) exchanger regulatory factor potentiates receptor activity

Platelet-derived growth factor (PDGF) is a potent mitogen for many cell types. The PDGF receptor (PDGFR) is a receptor tyrosine kinase that mediates the mitogenic effects of PDGF by binding to and/or phosphorylating a variety of intracellular signaling proteins upon PDGF-induced receptor dimerization. We show here that the Na(+)/H(+) exchanger regulatory factor (NHERF; also known as EBP50), a protein not previously known to interact with the PDGFR, binds to the PDGFR carboxyl terminus (PDGFR-CT) with high affinity via a PDZ (PSD-95/Dlg/Z0-1 homology) domain-mediated interaction and potentiates PDGFR autophosphorylation and extracellular signal-regulated kinase (ERK) activation in cells. A point-mutated version of the PDGFR, with the terminal leucine changed to alanine (L1106A), cannot bind NHERF in vitro and is markedly impaired relative to the wild-type receptor with regard to PDGF-induced autophosphorylation and activation of ERK in cells. NHERF potentiation of PDGFR signaling depends on the capacity of NHERF to oligomerize. NHERF oligomerizes in vitro when bound with PDGFR-CT, and a truncated version of the first NHERF PDZ domain that can bind PDGFR-CT but which does not oligomerize reduces PDGFR tyrosine kinase activity when transiently overexpressed in cells. PDGFR activity in cells can also be regulated in a NHERF-dependent fashion by stimulation of the beta(2)-adrenergic receptor, a known cellular binding partner for NHERF. These findings reveal that NHERF can directly bind to the PDGFR and potentiate PDGFR activity, thus elucidating both a novel mechanism by which PDGFR activity can be regulated and a new cellular role for the PDZ domain-containing adapter protein NHERF.

Functional consequences of altering myocardial adrenergic receptor signaling

From the ability to successfully manipulate the mouse genome has come important transgenic and gene-targeted knockout models that impact many areas of biomedical research. Genetically engineered mouse models geared toward the study of cardiovascular regulation have recently been described and provide powerful tools to study normal and compromised cardiac physiology. The genetic manipulation of the adrenergic receptor (AR) signaling system in the heart, including its regulation by desensitizing kinases, has shed light on the role of this signaling pathway in the regulation of cardiac contractility. One major finding, supported by several mouse models, is that in vivo contractility can be enhanced via alteration of myocardial AR signaling. Thus genetic manipulation of this critical receptor system in the heart represents a novel therapeutic approach for improving function of the failing heart.

Adenovirus-mediated genetic manipulation of the myocardial beta-adrenergic signaling system in transplanted hearts

OBJECTIVES

Ex vivo perfusion of the cardiac allograft during organ procurement is an ideal environment for adenoviral vectors with transgenes that target improving graft contractility. One such target is the beta-adrenergic receptor-signaling system, in which alterations in transgenic mice have elucidated novel means to improve the function of the heart in vivo. The purpose of the current study was to determine the functional consequences of beta-adrenergic receptor manipulation in a rabbit model of cardiac allograft transplantation.

METHODS

New Zealand White rabbits weighing 3 kg served as recipients to 1-kg outbred donors. Donor hearts were arrested and harvested, and 1 of 3 adenoviral constructs was administered into the aortic root perfusing the graft. Transgenes delivered encoded either the human beta(2)-adrenergic receptor, a peptide inhibitor of beta-adrenergic receptor densensitization, or the marker transgene beta-galactosidase.

RESULTS

Five days after cervical heterotopic transplantation, left ventricular performance was measured on a Langendorff apparatus. A moderate pattern of rejection was seen in all grafts. Biventricular myocyte expression of beta-galactosidase was observed, and beta(2)-adrenergic receptor density was elevated 10-fold in grafts that received adeno-beta(2)-adrenergic receptor. Left ventricular systolic and diastolic performance was significantly increased in grafts transfected with either adeno-beta(2)-adrenergic receptor or adeno-beta-adrenergic receptor densensitization compared with control grafts that received adeno-beta-galactosidase.

CONCLUSIONS

Ex vivo adenovirus-mediated gene transfer is feasible in a rabbit allograft model and, more important, genetic manipulation of beta-adrenergic receptor signaling either by increasing beta(2)-adrenergic receptor density or blocking endogenous receptor desensitization improves graft function acutely in this allograft model.

The GIT family of ADP-ribosylation factor GTPase-activating proteins. Functional diversity of GIT2 through alternative splicing

We recently characterized a novel protein, GIT1, that interacts with G protein-coupled receptor kinases and possesses ADP-ribosylation factor (ARF) GTPase-activating protein activity. A second ubiquitously expressed member of the GIT protein family, GIT2, has been identified in data base searches. GIT2 undergoes extensive alternative splicing and exists in at least 10 and potentially as many as 33 distinct forms. The longest form of GIT2 is colinear with GIT1 and shares the same domain structure, whereas one major splice variant prominent in immune tissues completely lacks the carboxyl-terminal domain. The other 32 potential variants arise from the independent alternative splicing of five internal regions in the center of the molecule but share both the amino-terminal ARF GTPase-activating protein domain and carboxyl-terminal domain. Both the long and short carboxyl-terminal variants of GIT2 are active as GTPase-activating proteins for ARF1, and both also interact with G protein-coupled receptor kinase 2 and with p21-activated kinase-interacting exchange factors complexed with p21-activated kinase but not with paxillin. Cellular overexpression of the longest variant of GIT2 leads to inhibition of beta(2)-adrenergic receptor sequestration, whereas the shortest splice variant appears inactive. Although GIT2 shares many properties with GIT1, it also exhibits both structural and functional diversity due to tissue-specific alternative splicing.

Inhibition of spontaneous beta 2-adrenergic activation rescues beta 1-adrenergic contractile response in cardiomyocytes overexpressing beta 2-adrenoceptor

Cardiac-specific overexpression of the human beta(2)-adrenergic receptor (AR) in transgenic mice (TG4) enhances basal cardiac function due to ligand-independent spontaneous beta(2)-AR activation. However, agonist-mediated stimulation of either beta(1)-AR or beta(2)-AR fails to further enhance contractility in TG4 ventricular myocytes. Although the lack of beta(2)-AR response has been ascribed to an efficient coupling of the receptor to pertussis toxin-sensitive G(i) proteins in addition to G(s), the contractile response to beta(1)-AR stimulation by norepinephrine and an alpha(1)-adrenergic antagonist prazosin is not restored by pertussis toxin treatment despite a G(i) protein elevation of 1.7-fold in TG4 hearts. Since beta-adrenergic receptor kinase, betaARK1, activity remains unaltered, the unresponsiveness of beta(1)-AR is not caused by betaARK1-mediated receptor desensitization. In contrast, pre-incubation of cells with anti-adrenergic reagents such as muscarinic receptor agonist, carbachol (10(-5)m), or a beta(2)-AR inverse agonist, ICI 118,551 (5 x 10(-7)m), to abolish spontaneous beta(2)-AR signaling, both reduce the base-line cAMP and contractility and, surprisingly, restore the beta(1)-AR contractile response. The "rescued" contractile response is completely reversed by a beta(1)-AR antagonist, CGP 20712A. Furthermore, these results from the transgenic animals are corroborated by in vitro acute gene manipulation in cultured wild type adult mouse ventricular myocytes. Adenovirus-directed overexpression of the human beta(2)-AR results in elevated base-line cAMP and contraction associated with a marked attenuation of beta(1)-AR response; carbachol pretreatment fully revives the diminished beta(1)-AR contractile response. Thus, we conclude that constitutive beta(2)-AR activation induces a heterologous desensitization of beta(1)-ARs independent of betaARK1 and G(i) proteins; suppression of the constitutive beta(2)-AR signaling by either a beta(2)-AR inverse agonist or stimulation of the muscarinic receptor rescues the beta(1)-ARs from desensitization, permitting agonist-induced contractile response.

The superfamily of heptahelical receptors

Despite a growing appreciation of functional analogies between visual and hormonal signalling systems in the early 1980s, the discovery of the close structural relationship between rhodopsin and the beta2-adrenergic receptor, and of the existence of a larger 'superfamily' of such receptors, came as a total surprise. Here I provide a personal perspective on events leading up to and flowing from this exciting discovery that opened up a vast field of research.

alpha-Actinin is a potent regulator of G protein-coupled receptor kinase activity and substrate specificity in vitro

G protein-coupled receptor kinases (GRKs) phosphorylate G protein-coupled receptors, thereby terminating receptor signaling. Herein we report that alpha-actinin potently inhibits all GRK family members. In addition, calcium-bound calmodulin and phosphatidylinositol 4,5-bisphosphate (PIP2), two regulators of GRK activity, coordinate with alpha-actinin to modulate substrate specificity of the GRKs. In the presence of calmodulin and alpha-actinin, GRK5 phosphorylates soluble, but not membrane-incorporated substrates. In contrast, in the presence of PIP2 and alpha-actinin, GRK5 phosphorylates membrane-incorporated, but not soluble substrates. Thus, modulation of alpha-actinin-mediated inhibition of GRKs by PIP2 and calmodulin has profound effects on both GRK activity and substrate specificity.

Preservation of myocardial beta-adrenergic receptor signaling delays the development of heart failure after myocardial infarction

When the heart fails, there is often a constellation of biochemical alterations of the beta-adrenergic receptor (betaAR) signaling system, leading to the loss of cardiac inotropic reserve. betaAR down-regulation and functional uncoupling are mediated through enhanced activity of the betaAR kinase (betaARK1), the expression of which is increased in ischemic and failing myocardium. These changes are widely viewed as representing an adaptive mechanism, which protects the heart against chronic activation. In this study, we demonstrate, using in vivo intracoronary adenoviral-mediated gene delivery of a peptide inhibitor of betaARK1 (betaARKct), that the desensitization and down-regulation of betaARs seen in the failing heart may actually be maladaptive. In a rabbit model of heart failure induced by myocardial infarction, which recapitulates the biochemical betaAR abnormalities seen in human heart failure, delivery of the betaARKct transgene at the time of myocardial infarction prevents the rise in betaARK1 activity and expression and thereby maintains betaAR density and signaling at normal levels. Rather than leading to deleterious effects, cardiac function is improved, and the development of heart failure is delayed. These results appear to challenge the notion that dampening of betaAR signaling in the failing heart is protective, and they may lead to novel therapeutic strategies to treat heart disease via inhibition of betaARK1 and preservation of myocardial betaAR function.

GIT proteins, A novel family of phosphatidylinositol 3,4, 5-trisphosphate-stimulated GTPase-activating proteins for ARF6

ADP-ribosylation factor (ARF) proteins are key players in numerous vesicular trafficking events ranging from the formation and fusion of vesicles in the Golgi apparatus to exocytosis and endocytosis. To complete their GTPase cycle, ARFs require a guanine nucleotide-exchange protein to catalyze replacement of GDP by GTP and a GTPase-activating protein (GAP) to accelerate hydrolysis of bound GTP. Recently numerous guanine nucleotide-exchange proteins and GAP proteins have been identified and partially characterized. Every ARF GAP protein identified to date contains a characteristic zinc finger motif. GIT1 and GIT2, two members of a new family of G protein-coupled receptor kinase-interacting proteins, also contain a putative zinc finger motif and display ARF GAP activity. Truncation of the amino-terminal region containing the zinc finger motif prevented GAP activity of GIT1. One zinc molecule was found associated per molecule of purified recombinant ARF-GAP1, GIT1, and GIT2 proteins, suggesting the zinc finger motifs of ARF GAPs are functional and should play an important role in their GAP activity. Unlike ARF-GAP1, GIT1 and GIT2 stimulate hydrolysis of GTP bound to ARF6. Accordingly we found that the phospholipid dependence of the GAP activity of ARF-GAP1 and GIT proteins was quite different, as the GIT proteins are stimulated by phosphatidylinositol 3,4, 5-trisphosphate whereas ARF-GAP1 is stimulated by phosphatidylinositol 4,5-bisphosphate and diacylglycerol. These results suggest that although the mechanism of GTP hydrolysis is probably very similar in these two families of ARF GAPs, GIT proteins might specifically regulate the activity of ARF6 in cells in coordination with phosphatidylinositol 3-kinase signaling pathways.

beta-arrestin1 interacts with the catalytic domain of the tyrosine kinase c-SRC. Role of beta-arrestin1-dependent targeting of c-SRC in receptor endocytosis

beta-Arrestins can act as adapter molecules, coupling G-protein-coupled receptors to proteins involved in mitogenic as well as endocytic pathways. We have previously identified c-SRC as a molecule that is rapidly recruited to the beta2-adrenergic receptor in a beta-arrestin1-dependent manner. Recruitment of c-SRC to the receptor appears to be involved in pathways leading to receptor internalization and mitogen-activated protein kinase activation. This recruitment of c-SRC to the receptor involves an interaction between the amino-terminal proline-rich region of beta-arrestin1 and the Src homology 3 (SH3) domain of c-SRC, but deletion of the proline-rich domain does not totally ablate the interaction. We have found that a major interaction also exists between beta-arrestin1 and the catalytic or kinase domain (SH1) of c-SRC. We therefore hypothesized that a catalytically inactive mutant of the isolated catalytic subunit, SH1(kinase dead) (SH1(KD)), would specifically block those cellular actions of c-SRC that are mediated by beta-arrestin1 recruitment to the G-protein-coupled receptor. In contrast, the majority of cellular phosphorylations catalyzed by c-SRC, which do not involve interaction with the SH1 domain, would be predicted to be unaffected. The SH1(KD) mutant did indeed block beta2-adrenergic receptor internalization and receptor-stimulated tyrosine phosphorylation of dynamin, actions previously shown to be c-SRC-dependent. In contrast, SAM-68 and whole cell tyrosine phosphorylation by c-SRC was unaffected, indicating that the SH1(KD) mutant did not inhibit c-SRC tyrosine kinase activity in general. These results not only clarify the nature of the beta-arrestin1/c-SRC interaction but also implicate beta-arrestin1 as an important mediator of receptor internalization by recruiting tyrosine kinase activity to the cell surface to phosphorylate key endocytic intermediates, such as dynamin.

Assembly of an A kinase-anchoring protein-beta(2)-adrenergic receptor complex facilitates receptor phosphorylation and signaling

Phosphorylation of G-protein-coupled receptors by second-messenger-stimulated kinases is central to the process of receptor desensitization [1-3]. Phosphorylation of the beta(2)-adrenergic receptor (beta(2)-AR) by protein kinase A (PKA), in addition to uncoupling adenylate cyclase activation, is obligatory for receptor-mediated activation of mitogen-activated protein kinase (MAP kinase) cascades [4] [5]. Although mechanisms for linking G-protein-coupled receptor kinases to the activated receptor are well established, analogous mechanisms for targeting second messenger kinases to the beta(2)-AR at the plasma membrane have not been elucidated. Here we show that the A-kinase-anchoring protein, AKAP79/150, co-precipitates with the beta(2)-AR in cell and tissue extracts, nucleating a signaling complex that includes PKA, protein kinase C (PKC) and protein phosphatase PP2B. The anchoring protein directly and constitutively interacts with the beta(2)-AR and promotes receptor phosphorylation following agonist stimulation. Functional studies show that PKA anchoring is required to enhance beta(2)-AR phosphorylation and to facilitate downstream activation of the MAP kinase pathway. This defines a role for AKAP79/150 in the recruitment of second-messenger-regulated signaling enzymes to a G-protein-coupled receptor.

The beta(2)-adrenergic receptor mediates extracellular signal-regulated kinase activation via assembly of a multi-receptor complex with the epidermal growth factor receptor

Many G protein-coupled receptors (GPCRs) activate MAP kinases by stimulating tyrosine kinase signaling cascades. In some systems, GPCRs stimulate tyrosine phosphorylation by inducing the "transactivation" of a receptor tyrosine kinase (RTK). The mechanisms underlying GPCR-induced RTK transactivation have not been clearly defined. Here we report that GPCR activation mimics growth factor-mediated stimulation of the epidermal growth factor receptor (EGFR) with respect to many facets of RTK function. beta(2)-Adrenergic receptor (beta(2)AR) stimulation of COS-7 cells induces EGFR dimerization, tyrosine autophosphorylation, and EGFR internalization. Coincident with EGFR transactivation, isoproterenol exposure induces the formation of a multireceptor complex containing both the beta(2)AR and the "transactivated" EGFR. beta(2)AR-mediated EGFR phosphorylation and subsequent beta(2)AR stimulation of extracellular signal-regulated kinase (ERK) 1/2 are sensitive to selective inhibitors of both EGFR and Src kinases, indicating that both kinases are required for EGFR transactivation. beta(2)AR-dependent signaling to ERK1/2, like direct EGF stimulation of ERK1/2 activity, is sensitive to inhibitors of clathrin-mediated endocytosis, suggesting that signaling downstream of both the EGF-activated and the GPCR-transactivated EGFRs requires a productive engagement of the complex with the cellular endocytic machinery. Thus, RTK transactivation is revealed to be a process involving both association of receptors of distinct classes and the interaction of the transactivated RTK with the cells endocytic machinery.

Role of endocytosis in the activation of the extracellular signal-regulated kinase cascade by sequestering and nonsequestering G protein-coupled receptors

Acting through a number of distinct pathways, many G protein-coupled receptors (GPCRs) activate the extracellular signal-regulated kinase (ERK)/mitogen-activated protein kinase (MAPK) cascade. Recently, it has been shown that in some cases, clathrin-mediated endocytosis is required for GPCR activation of the ERK/MAPK cascade, whereas in others it is not. Accordingly, we compared ERK activation mediated by a GPCR that does not undergo agonist-stimulated endocytosis, the alpha(2A) adrenergic receptor (alpha(2A) AR), with ERK activation mediated by the beta(2) adrenergic receptor (beta(2) AR), which is endocytosed. Surprisingly, we found that in COS-7 cells, ERK activation by the alpha(2A) AR, like that mediated by both the beta(2) AR and the epidermal growth factor receptor (EGFR), is sensitive to mechanistically distinct inhibitors of clathrin-mediated endocytosis, including monodansylcadaverine, a mutant dynamin I, and a mutant beta-arrestin 1. Moreover, we determined that, as has been shown for many other GPCRs, both alpha(2A) and beta(2) AR-mediated ERK activation involves transactivation of the EGFR. Using confocal immunofluorescence microscopy, we found that stimulation of the beta(2) AR, the alpha(2A) AR, or the EGFR each results in internalization of a green fluorescent protein-tagged EGFR. Although beta(2) AR stimulation leads to redistribution of both the beta(2) AR and EGFR, activation of the alpha(2A) AR leads to redistribution of the EGFR but the alpha(2A) AR remains on the plasma membrane. These findings separate GPCR endocytosis from the requirement for clathrin-mediated endocytosis in EGFR transactivation-mediated ERK activation and suggest that it is the receptor tyrosine kinase or another downstream effector that must engage the endocytic machinery.

Intracoronary adenovirus-mediated delivery and overexpression of the beta(2)-adrenergic receptor in the heart : prospects for molecular ventricular assistance

BACKGROUND

Genetic modulation of ventricular function may offer a novel therapeutic strategy for patients with congestive heart failure. Myocardial overexpression of beta(2)-adrenergic receptors (beta(2)ARs) has been shown to enhance contractility in transgenic mice and reverse signaling abnormalities found in failing cardiomyocytes in culture. In this study, we sought to determine the feasibility and in vivo consequences of delivering an adenovirus containing the human beta(2)AR cDNA to ventricular myocardium via catheter-mediated subselective intracoronary delivery.

METHODS AND RESULTS

Rabbits underwent percutaneous subselective catheterization of either the left or right coronary artery and infusion of adenoviral vectors containing either a marker transgene (Adeno-betaGal) or the beta(2)AR (Adeno-beta(2)AR). Ventricular function was assessed before catheterization and 3 to 6 days after gene delivery. Both left circumflex- and right coronary artery-mediated delivery of Adeno-beta(2)AR resulted in approximately 10-fold overexpression in a chamber-specific manner. Delivery of Adeno-betaGal did not alter in vivo left ventricular (LV) systolic function, whereas overexpression of beta(2)ARs in the LV improved global LV contractility, as measured by dP/dt(max), at baseline and in response to isoproterenol at both 3 and 6 days after gene delivery.

CONCLUSIONS

Percutaneous adenovirus-mediated intracoronary delivery of a potentially therapeutic transgene is feasible, and acute global LV function can be enhanced by LV-specific overexpression of the beta(2)AR. Thus, genetic modulation to enhance the function of the heart may represent a novel therapeutic strategy for congestive heart failure and can be viewed as molecular ventricular assistance.

Multiple endocytic pathways of G protein-coupled receptors delineated by GIT1 sensitivity

Recently, we identified a GTPase-activating protein for the ADP ribosylation factor family of small GTP-binding proteins that we call GIT1. This protein initially was identified as an interacting partner for the G protein-coupled receptor kinases, and its overexpression was found to affect signaling and internalization of the prototypical beta(2)-adrenergic receptor. Here, we report that GIT1 overexpression regulates internalization of numerous, but not all, G protein-coupled receptors. The specificity of the GIT1 effect is not related to the type of G protein to which a receptor couples, but rather to the endocytic route it uses. GIT1 only affects the function of G protein-coupled receptors that are internalized through the clathrin-coated pit pathway in a beta-arrestin- and dynamin-sensitive manner. Furthermore, the GIT1 effect is not limited to G protein-coupled receptors because overexpression of this protein also affects internalization of the epidermal growth factor receptor. However, constitutive agonist-independent internalization is not regulated by GIT1, because transferrin uptake is not affected by GIT1 overexpression. Thus, GIT1 is a protein involved in regulating the function of signaling receptors internalized through the clathrin pathway and can be used as a diagnostic tool for defining the endocytic pathway of a receptor.

Hybrid transgenic mice reveal in vivo specificity of G protein-coupled receptor kinases in the heart

G protein-coupled receptor kinases (GRKs) phosphorylate activated G protein-coupled receptors, including alpha(1B)-adrenergic receptors (ARs), resulting in desensitization. In vivo analysis of GRK substrate selectivity has been limited. Therefore, we generated hybrid transgenic mice with myocardium-targeted overexpression of 1 of 3 GRKs expressed in the heart (GRK2 [commonly known as the beta-AR kinase 1], GRK3, or GRK5) with concomitant cardiac expression of a constitutively activated mutant (CAM) or wild-type alpha(1B)AR. Transgenic mice with cardiac CAMalpha(1B)AR overexpression had enhanced myocardial alpha(1)AR signaling and elevated heart-to-body weight ratios with ventricular atrial natriuretic factor expression denoting myocardial hypertrophy. Transgenic mouse hearts overexpressing only GRK2, GRK3, or GRK5 had no hypertrophy. In hybrid transgenic mice, enhanced in vivo signaling through CAMalpha(1B)ARs, as measured by myocardial diacylglycerol content, was attenuated by concomitant overexpression of GRK3 but not GRK2 or GRK5. CAMalpha(1B)AR-induced hypertrophy and ventricular atrial natriuretic factor expression were significantly attenuated with either concurrent GRK3 or GRK5 overexpression. Similar GRK selectivity was seen in hybrid transgenic mice with wild-type alpha(1B)AR overexpression concurrently with a GRK. GRK2 overexpression was without effect on any in vivo CAM or wild-type alpha(1B)AR cardiac phenotype, which is in contrast to previously reported in vitro findings. Furthermore, endogenous myocardial alpha(1)AR mitogen-activated protein kinase signaling in single-GRK transgenic mice also exhibited selectivity, as GRK3 and GRK5 desensitized in vivo alpha(1)AR mitogen-activated protein kinase responses that were unaffected by GRK2 overexpression. Thus, these results demonstrate that GRKs differentially interact with alpha(1B)ARs in vivo such that GRK3 desensitizes all alpha(1B)AR signaling, whereas GRK5 has partial effects and, most interestingly, GRK2 has no effect on in vivo alpha(1B)AR signaling in the heart.